Research ArticleEmissions of SO3 from a Coal-Fired Fluidized Bed underNormal and Staged Combustion
Wasi Z Khan1 Bernard M Gibbs2 and Assem Ayaganova1
1 Department of Chemical Engineering Nazarbayev University 53 Kabanbay Batyr Avenue Astana 010000 Kazakhstan2Department of Fuel and Energy University of Leeds Leeds LS2 9JT UK
Correspondence should be addressed to Wasi Z Khan wasikhannuedukz
Received 26 February 2013 Accepted 28 March 2013
Academic Editors N Fontanals F Long and K L Smalling
Copyright copy 2013 Wasi Z Khan et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
This paper reports the measurements of SO3emissions with and without limestone under unstaged and staged fluidized-bed
combustion carried out on a 03 times 03m2 and 2m high stainless-steel combustor at atmospheric pressure The secondary air wasinjected 100 cm above the distributor SO
3emissions were monitored for staging levels of 85 15 70 30 and 60 40 equivalent to a
primary aircoal ratio (PACR) of sim086 075 and 067 Experiments were carried out at 0ndash60 excess air level 1-2ms fluidizingvelocity 800ndash850∘C bed temperature and 20ndash30 cm bed height During unstaged combustion runs SO
3emissions were monitored
for awide range ofCaS ratios from05 to 13However for the staged combustion runs theCaS ratiowas fixed at 3 SO3was retained
to a lesser extent than SO2 suggesting that SO
2reacts preferentially with CaO and that SO
3is involved in the sulphation process
to a lesser degree The SO3emissions were found to be affected by excess air whereas the fluidizing velocity and bed temperature
had little effect SO3was depressed on the addition of limestone during both the staged and unstaged operations and the extent of
the reduction was higher under staged combustion
1 Introduction
The presence of SO3in flue gas corrodes the equipment
and ducts of combustion system and therefore needs tobe removed [1] In order to control emissions of SO
3
more studies on its formation and dissociation are requiredunder air-fired and oxy-fired combustion conditions Thesimulation study of Zheng and Furimsky [2] shows that SO
3
emissions would be unaffected during oxy-fuel combustionbeing governed only by oxygen concentration The kineticsof reactions occurring in the combustor were studied byBurdett et al [3] using a TGA microbalance They proposedthe following mechanisms for the formation of SO
3
11 SO2SO3Homogeneous Gas Phase Reaction SO
2may be
oxidized to SO3by two reactions
SO2+O2997888rarr SO
3+O (1)
M + SO2+O 997888rarr SO
3+M (2)
where M is a chaperon third body molecule The largetemperature dependence of reactions (1) and (2) ensuresthat the rate of production falls rapidly with decreasing gastemperature and in fact 90ndash95 of SO
3is formed in the
bed and freeboard and the remaining 5ndash10 in the regionbetween the freeboard and sampling point SO
3increases
sharply with temperature but the homogeneous reactioncannot account for all the SO
3produced
12 Heterogeneous Catalysis of SO2on Bed Particles and
Heat Transfer Surfaces In a coal burning combustor amore effective catalytic material iron oxide is present in flyash While the SO
3formation in this process is important
the experimental data is insufficient to quantify the SO3
formationDennis and Hayhurst [4] used an 80mm diameter
fluidized-bed combustor and mass spectrometer for measur-ing the concentration of SO
3 They confirmed the amount of
SO3formed at atmospheric pressure to be very low andmuch
less than the equilibrium concentration The rate measured
2 ISRN Environmental Chemistry
was 100 times faster than expected for oxidation in the gasphase
Willium and Gibbs [5] measured SO2and SO
3emissions
from a 03m2 fluidized-bed combustor They reported thatthe higher the excess air level the lower the SO
2emissions (on
removal of the dilution effect SO2increased with an increase
in excess air reaching a limiting value of 1300 ppm above 30excess air) and that SO
3emissions increased slightly with the
increase in excess air Barnes [6] also observed the similareffect of excess air on SO
2and SO
3emissions Willium and
Gibbs [5] and Barnes [6] found that the higher the sulphurcontent of fuel the higher the SO
2emissions while the SO
3
was unaffected by the fuel sulphur content [5] Coal fed tothe bed caused SO
2and SO
3emissions to increase than when
fed to the surface [5] SO3shows a weak dependence on bed
temperature [5 6]Barnes [6] studied the effect of sand particle size flu-
idizing velocity and bed depth on SO2and SO
3emissions
Barnesrsquo findings indicate that fine sand (0300mm) produceshigh SO
2and SO
3emissions Increasing fluidizing velocity
from 1 to 2ms caused reduced formation of SO2in the
bed and freeboard An increase in bed depth increased SO3
emission a deep bed (30 cm) and fine sand resulted in a slightdecrease in SO
2emission
Oxygen availability and fluidizing characteristics withinthe bed also affect SO
3formation Ahn et al [7] found that
for pulverized coal concentrations of SO3and SO
2were
significantly higher for oxy-fired conditions as comparedto air-fired conditions In circulating fluidized bed SO
3
concentrations were notably higher for oxy-fired conditionstoo For higher sulfur coal SO
3concentrations were 4ndash6
times greater on averageTheir findings contradict the findingof Barnes [6]
Hindiyarti et al [8] investigated the reaction of SO3with
H O and OH radical The revised rate constant calculatedby them suggests that SO
3and O reaction is found to be
insignificant during most conditions According to themSO3+H is the major consumption reaction for SO
3
Stanger and Wall [9] reviewed published work on SO3
concentrations and emissions under oxy-fuel firing Theirconclusion is that the conversion of SO
2to SO
3is consider-
ably variableWillium andGibbs [5] found that coal char had a very sig-
nificant removal effect on SO3emissions because SO
3above
the bed was 50 greater than in the exit Barnes [6] said thatunburnt char does not have a major effect on SO
3as carbon
carryover increases under the given operating conditionsHer results showed that the quantity of inert particles (30 cmdeep bed) resulted in an increase in heterogeneous catalyticreaction of SO
2to form SO
3
Burdett et al [3] carried out experiments in a microbal-ance to study the effect of limestone on SO
3emissions
According to Burdett et al [3] the reaction of CaO O2 and
SO2 in order to yield CaSO
4(reaction (3)) must occur in two
separate steps Two possibilities exist either SO2reacts with
CaO and CaSO3is formed which is then oxidized or else the
formation of SO3in gas phase or on a stone surface is followed
by an attack on the CaO Consider the following
CaO + 12O2+ SO2997888rarr CaSO
4(3)
Route 1
CaO + SO2997888rarr CaSO
3 (4)
CaSO3+1
2O2997888rarr CaSO
4(5)
Route (2)
SO2+1
2O2997888rarr SO
3(6)
SO3+ CaO 997888rarr CaSO
4 (7)
It is not possible due to the lack of available experimentaldata to say which of these mechanisms is operative undergiven conditions although both may be important A mostinteresting comparison between the SO
119909level detected with
and without limestone (proposed by Burdett et al [10]) isthe SO
2SO3ratio Without limestone the ratio they found
was 220033 (or 67 1) and with limestone it increased to35015 (or 230 1) It is clear that SO
3is depressed to a
greater extent than SO2on the addition of limestone and
they attribute this to the higher reactivity of SO3compared
with that of SO2 Burdett [11] and Burdett et al [10] have
assumed that SO2oxidizes to SO
3in the particles at a rate
dependent on local SO2and O
2concentrations with SO
3
diffusing through theCaSO4shell and reactingwithCaOThe
rate of formation of CaSO4may be linked to different rates
of production of SO3at different locations within the stone
At a high oxygen level oxidation increases preferentially atthe edge of a particle High utilization is achieved when SO
2
diffusion in the interior of the stone ismaximized and this inturn implies a low SO
3formation rate Barnes [6] reported a
decrease in SO2conversion to SO
3with oxygen concentration
and an increase with SO2concentration
Fieldes et al [12] reported the achievement of a highfractional sulphation (036) when coal is burnt in the bedIt appears that the fraction of the sulphur gas phase whichis SO3 has a substantial effect on the fractional sulphation
of the limestone Ash also appears to remove SO3selectively
The mechanism of this hypothesis supposes that the directreaction of CaO with SO
3is faster than a reaction via
the CaSO3intermediate This paper examines the factors
responsible for formation and reduction of SO2
and SO3
from a coal-fired fluidized bed under varying operatingconditions
2 Apparatus and Procedure
The main features of the fluidized-bed combustor and ancil-laries are presented in Figure 1 The bed consisted of silicasand of mean size 0700mm Fluidizing air was supplied by afan and metered and introduced through a distributor plateFor staged combustion the secondary air was introducedinto the combustor through a stainless-steel pipe 100 cmabove the bed surface In staged combustion mode the totalcombustion air is separated into a primary air stream supplied
ISRN Environmental Chemistry 3
Table 1 Typical analysis of Linby and Daw Mill coal
to fluidize the bed and a secondary air stream injected abovethe bed to complete the combustion For example in 70 30staging 30 of the total air is injected as secondary air
The bed was preheated by a propane burner that was fixedabove the bed and the fluidizing airflow rate was adjustedto the lowest level to minimize heating time Coal was fedin the combustor when the bed temperature reached 550∘CWhen the bed temperature reached 800∘C the desired coalfeed rate was adjusted to a constant value the propane burnerwas switched off and the fluidizing air was adjusted to therequired levelThe bed temperature was maintained constantby using an adjustable cooling coil with circulating waterConcentrations of O
2 CO CO
2 and SO
2were recorded
continuously by an ADC-RF infrared gas analyzerThe experiments were carried out at bed temperatures
of 800ndash850∘C fluidizing velocities of 1-2ms and excess airlevels of 0ndash60 Static bed height was 20ndash30 cm Two typesof coal bituminous Linby and Daw Mill of 3ndash16mm (large)diameter in size and two types of limestone Ballidone andPenrith of lt3mm mean diameter in size were used Inboth cases the coal was premixed with the limestone andfed overbed at 42 cm above the distributor (see Table 1 forproximate and ultimate analyses of the coal and Table 2 forchemical composition of the limestone) The CaS ratio was3 1 mole per mole or otherwise as indicated Three levels ofstaging (15ndash40 secondary air) were used to investigate theeffect of fluidizing velocity bed temperature and excess airon SO
3reduction during air staging
3 Sampling of SO3
An SSLMEL SO3analyzer developed by Severn Science
LabsMarchwood Eng Labs was used for continuous mon-itoring of the SO
3 A detailed account of the principles and
operating procedures of an SSLMEL analyzer can be foundin Jackson et al [13] and Hotchkiss et al [14]
The representative sample of flue gas was extracted fromthe sample point located near the exit of flue gas to the cyclone(200 cm above the distributor) where the gas temperaturewas around 550∘CUnder these conditions the use of a quartzsampling probe was found to be adequate This enabledthe extraction of the acid-containing gas directly into thefilter-contactor of the SO
3analyzer The temperature of the
sampled gas was maintained (by keeping the length of thetube as short as possible) at a value in excess of the aciddewpoint and below the temperature at which significantdissociation to SO
3occurred accurate determination of the
acid gas content in the gas could then be made preciselyover a significant period of time This procedure effectivelyeliminates any interaction between the SO
3in the gas sample
and other species within the sampling probe itself
4 Results and Discussion
41 SO3Emissions without Limestone under Unstaged Com-
bustion The effect of some operating variables on theSO3emissions under unstaged combustion is presented in
Figure 2 The results are corrected to 5 oxygen in the flueequivalentThe flue gas SO
3emissions ranged from 5 to 10 for
the Linby coal SO3emissions decrease with excess air when
corrected for dilution increase slightly with excess air reachlimiting values and then gradually decrease The fluidizingvelocity also affects emissions to some extent The effect ofbed height on SO
3emissions was associated with the size
of sand particles As the fine sand produces more reducingenvironment the oxygen stoichiometry influences the ratesof oxidation of SO
2 and as a result the generation of SO
3
is reduced The SO3emissions were also less sensitive to
change in bed temperatureThe rise in emissionswas typically0510∘C
In another set of experiments the Daw Mill Coal wastested for SO
3emissions The flue gas SO
3emission ranged
from 4 to 195 ppm for Dawmill coal Changing the operatingparameters resulted in a maximum change in SO
3emission
of 13 ppm Increasing the bed depth resulted in higherSO3emission the fluidizing velocity also affected emission
depending on the size of bedmaterial usedThe SO3emission
increased as the bed size was varied from coarse to fine withthe bed depth SO
3emissions were weakly dependent on
temperature typically rising 07 ppm10∘C
42 Comparison to ReportedWork Dennis and Hayhurst [4]have reported that the SO
3formation under atmospheric
pressure was very low (eg mole fraction of SO3in off gas
= 7 times10minus5 for mole fraction of SO2gas entering a bed = 23
times10minus3 at 875∘C) The oxidation rate in the gas phase was 100
times faster than expected An Eley-Rideal mechanism was
4 ISRN Environmental Chemistry
Screw feederLS hopper
Secondary air
Rotary valve
Plenum
DC motor
Primary airPG
Blower
Air
Hot vertical probeTC 5
Sample probeCyclone
TC 6
Stack
Propane
Burner
Air
TC 4
TC 3
TC 2TC 1
Ash and spentsorbentcollector
Coal hopper
N2
PG = pressure gauge
TC = thermocouple
CWinCWout
Figure 1 Main features of fluidized-bed combustor and ancillaries
121110
987654
0 1 2 3 4 5 6 7 8 9 10 11 12
SO3
emiss
ion
(ppm
)
Oxygen concentration in the fuel gas ()
Fluidizing velocity 10 ms 30 cm bed heightFluidizing velocity 15 ms 20 cm bed heightFluidizing velocity 20 ms 20 cm bed height
Figure 2 Influence of fluidizing velocity excess air and bedheight on SO
3emissions during unstaged combustion at 850∘C bed
temperature and coarse sand (corrected to 5 in flue equivalent)
proposed in which O2and SO
2competitively chemisorb on
the surface and the rate of reaction is controlled by gas-phasemolecule of SO
2reacting with adsorbed O atom
Willium and Gibbs [5] have tested many coal types forSO3concentration without limestone in 750ndash900∘C temper-
ature range Their findings suggest that ash (having tracesof Ca Mg Na K etc) is the principle removing species ofSO3 In another experiment when pure SO
2was introduced
the SO3reacted with added char at 850∘C in the absence of
oxygen to give SO3of 7 vpm in the outlet which suggests that
char is important in the removal of SO3 He also observed a
50 reduction in SO3in the freeboardAccording toWillium
the reduction was due to the reaction of SO3with unburnt
charSO3emissions are dependent on the oxygen and sulfur
dioxide concentrations and were found to follow a similartrend Willium and Gibbs [5] found that in contrast tothe effect on SO
2emissions fine coal produced lower SO
3
emissions In this study SO3emissions were slightly higher
when fine sand was used and tended to increase with bedheight This suggests that unburnt char does not have asignificant effect on SO
3emissions The results of this study
indicate that the amount of particles in the bed could have asignificant effect on SO
3emissions resulting in an increase
in the heterogeneous catalytic reaction of SO2to form SO
3
as the quantity of bed particles increases Higher bed heighttherefore will also result in high SO
3emissions The oxygen
concentration and fluidizing velocity will also affect SO3
formation
43 SO3Emissions with Limestone under Unstaged Combus-
tion SO3emissions decrease in the presence of limestone
and the reduction is temperature sensitive The SO3reduc-
tions were less sensitive than the reductions achieved for SO2
at similar conditions [15 16] At a temperature around 850∘Cthe SO
3reductions were only 28 of the SO
2reductions but
ISRN Environmental Chemistry 5
30272421181512
9630
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
SO3
redu
ctio
n (
)
CaS molar ratio
10 excess air30 excess air48 excess air
Figure 3 Influence of limestone addition on SO3reduction during
unstaged combustion 25 cmheight 10ms fluidizing velocity 850∘Cbed temperature and 12 cm limestone addition height
322824201612
840
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
SO3
redu
ctio
n (
)
CaS molar ratio
40 excess air60 excess air80 excess air
Figure 4 Influence of limestone addition on SO3reduction during
unstaged combustion 25 cmheight 10ms fluidizing velocity 850∘Cbed temperature and 42 cm limestone addition height as well ascoarse sand
at 800∘C the reductions reached 70 of the SO2reduction
level The results corresponding to the operating conditionsare shown graphically in Figures 3 and 4 It should be notedthat the SO
3reduction shown in Figure 3 was obtained
when limestone was injected 12 cm above the distributor andFigure 4 represents the results when limestone was injected42 cm above the distributor
At a temperature of 850∘C some of the SO2will always be
converted to SO3via reaction (6) The conversion decreases
with oxygen concentration and increases with sulfur dioxideconcentration An increase in temperature enhances the rateof SO3formation SO
3can react with CaO to formCaSO
4via
reaction (7) The rate of this reaction is temperature depen-dent Yilmaz et al [17] studied the thermal dissociation of SO
3
in the range of 800ndash1200∘C under atmospheric pressure Atthe location in the flame where the net SO
3formation rate is
zero he determined a rate constant of 69times 1010 cm3molminus1 sminus1
2422201816141210
86420
0 2 4 6 8 10 12 14 16 18
SO3
redu
ctio
n (
)
CaS molar ratio
Limestone injection height 42 cm excess air 32Limestone injection height 42 cm excess air 60
Figure 5 Influence of limestone addition on SO3level in the flue
under staged combustion
for SO3+ N2rarr SO
3+ O + N
2 that was consistent with
other flame results A high temperature lowers the reactionrate Therefore at a high temperature more SO
3is produced
but less will be consumed in sulphation As a result a largerdecrease in SO
3emissions is observed at lower temperature
The effect of excess air can also be seen in these graphsSO3emissions have been found to increase with excess air
but upon removing the dilution effect the increase is withina narrow range indicating that there could be an optimumreduction at a particular excess air beyond which the SO
3
reduction decreases An increase in the fluidizing velocity haslittle effect on the overall reduction of SO
3emissions
During another set of experiments Penrith limestonewas added to the Daw Mill coal It was observed that SO
3
emissions were decreased in the presence of limestone andthe reduction was temperature dependent At the highertemperature of 850∘C the SO
3reductions were 18ndash20 of
the SO2reduction but at 800∘C the SO
3reductions reached
55 of the SO2reduction level Figure 5 shows the results of
this set
44 Comparison to Reported Work (Conducted on Microbal-ance or Small Bed of 36ndash78mm ID) Burdett et al [3] havereported that the reaction between limestone and sulfuroxides is highly sensitive to changes in O
2 SO and SO
3
concentrations Absorption of SO3by the coal ash cannot be
quantified on themicrobalance and themicrobalance resultsare not applicable to fluidized combustor
Fieldes et al [12] have reported that extent of SO2oxida-
tion to SO3varied with SO
2and O
2concentration The coal
combustion test showed that the lower SO3concentrations
are due to its selective removal by ash They had tested avariety of limestone and in all the cases the mole fraction ofCaO converted to CaSO
4was affected by inlet oxygen in the
same way as Penrith limestoneThibault et al [18] have conducted experiments on a
small (6mm) fixed bed packed with CaO particle They havetested two grain size of the sorbent and reported that forefficient capture of SO
3a small grain size and openmacropore
structure are essential
6 ISRN Environmental Chemistry
45 Comparison to ReportedWork (Conducted on Pilot Scale)Burdett et al [19] have reported fractional conversion of SO
2
to SO3decreased from about 15 in the limestone-free case
to around 035 when the limestone and alkaline ash werepresent which was due to the greater reaction of SO
3with
limestone compared with ash the absorption occurring bothin the bed itself and in the freeboard
Burdett et al [10] have reported that combustion of a 3sulfur coal in a bed burning at 900∘C generated 33 vpm ofSO3and proposed that the effect of O
2on sulphation capacity
results from the formation of SO3within the pores of the
stone
46 SO3Emissions without Limestone under Staged Combus-
tion Merryman and Levy [20] have conducted staged exper-iments on a quartz tube methane burner producing stablemethane-H
2S flame within desired fuel-air ratio without a
sorbent presence under staged combustion conditions Theyhave reported that when the remaining excess air was injectedinto these gases the maximum amount of SO
3formed was
greater than formed when this additional air was includedwith the initial combustion air the overall excess of airbeing the same in both cases The experimental conditionsof Merryman and Levy do not match with our fluidized bedtherefore their results are not comparable with this study
During this study the SO3emissions under staged com-
bustion without limestone could not be monitored exten-sively due to malfunctioning of SO
3analyzer
47 SO3Emissions with Limestone under Staged Combustion
The concentration of SO3emissions at 15ms and 20 excess
air was 170 ppm which decreased to 55 ppm in the presenceof limestone The SO
3emissions at 7030 staged (15ms
850∘C) combustion (without limestone) were similar to thoseof unstaged combustion (without limestone) However it wasobserved that in the presence of limestone staged combus-tion results in a higher reduction of SO
3than unstaged
Figure 6 gives the SO3emissions as a function of PACR The
emissions at 15 secondary air were 15 ppm and increased to72 ppm at 45 secondary air It is clear that SO
3is depressed
on the addition of limestone during both the unstaged andstaged operations and the extent of reduction was higherunder staged combustion
Figure 6 shows that the maximum removal of SO3occurs
at a lower staging levels of 8515 and as the bed becomesmore substoichiometric the rate of SO
3removal decreases
This trend indicates the formation of SO3in the freeboard
which bypasses the limestone and appears in the flue Thisincrease in SO
3reduction with the in-bed air ratio is in
agreement with Barnes [6] findingsThe results of the staged combustion test with Daw Mill
coal in the presence of Penrith limestone indicate that SO3
emissions varied little with changes in excess air Howeverif excess air is coupled with fluidizing velocity then it hadsome effects on the emissions At higher velocity of 2ms thechange was up to 4 ppmThe concentration of SO
3emissions
at 15ms and 30 excess air was 20 ppm which decreased to
8
6
4
2
006 08 1 12 14 16
20 excess air
SO3
emiss
ion
(ppm
)
Primary air to coal ratio
Figure 6 Influence of PACR on SO3emission at 15ms fluidizing
velocity and coarse sand
5
45
4
35
3
25
20 5 10 15 20 25 30 35
SO3
emiss
ion
(ppm
)
Secondary air ()
Figure 7 Influence of staging SO3emission in the flue secondary
air injection height 100 cm above the distributor excess air 30
8 ppm under staged combustion in the presence of limestoneThe results of Daw Mill coal test are shown in Figure 7
It should be noted that there is no published work onSO3emissions under staged combustion conditions with or
without limestone on any scale Therefore the results of thisstudy could not be compared
5 Conclusion
The experimental data shows that during unstaged com-bustion without limestone SO
3emissions are dependent
on oxygen and SO2concentration SO
3emissions increase
slightly with excess air reaching a limiting value and thenslowly decrease SO
3emissions are less sensitive to change
in bed temperature However the fluidizing velocity and bedheight affect the emissions
In the presence of limestone SO3emissions are reduced
during both staged and unstaged operations and the reduc-tion is temperature sensitive However during staged com-bustion the reduction is enhanced As staged fluidized-bed combustion is a proven technique to reduce NO
119909and
SO2emissions therefore it should be possible to operate a
fluidized-bed combustor under a stagedmodewith limestoneto keep SO
2 SO3 and NO
119909emissions to a minimum
ISRN Environmental Chemistry 7
References
[1] Y Mitsui N Imada H Kikkawa and A Katagawa ldquoStudyof Hg and SO
3behavior in flue gas of oxy-fuel combustion
systemrdquo International Journal of Greenhouse Gas Control vol5 pp S143ndashS150 2011
[2] L Zheng and E Furimsky ldquoAssessment of coal combustion inO2+CO2by equilibrium calculationsrdquo Fuel Processing Technol-
ogy vol 81 no 1 pp 23ndash34 2003[3] N A Burdett R C Hotchkiss and R B Fields ldquoSO
3formation
and retention in coal fired fluidized bed combustorsrdquo AIChESymposium Series vol 57 pp M1ndashM11 1979
[4] J S Dennis and A N Hayhurst ldquoThe formation of SO3in a
fluidized bedrdquo Combustion and Flame vol 72 no 3 pp 241ndash258 1988
[5] P T Willium and B M Gibbs ldquoThe emissions of SO2and
SO3from fluidized bedsrdquo in Proceedings of the 4th International
Conference on Fluidization pp 443ndash450 Kashikojima Japan1983
[6] J P Barnes Abatement of nitric oxide emission from a coal-fixedfluidized bed combustor [PhD thesis] Department of Fuel andEnergy University of Leeds Leeds UK 1988
[7] J Ahn R Okerlund A Fryb and E G Eddingsa ldquoSulfurtrioxide formation during oxy-coal combustionrdquo InternationalJournal of Greenhouse Gas Control vol 5 pp S127ndashS135 2011
[8] L Hindiyarti P Glarborg and P Marshall ldquoReactions of SO3
with the OH radical pool under combustion conditionsrdquoJournal of Physical Chemistry A vol 111 no 19 pp 3984ndash39912007
[9] R Stanger and T Wall ldquoSulphur impacts during pulverisedcoal combustion in oxy-fuel technology for carbon capture andstoragerdquo Progress in Energy and Combustion Science vol 37 no1 pp 69ndash88 2011
[10] N A Burdett R C Hotchkiss R P Hensel and R B FieldsldquoCoal devolatilization and emission of SO
3in a fluidized bed
combustionrdquo in Proceedings of the Fluidized Combustion Con-ference vol 2 pp 424ndash442 Capetown South Africa January1981
[11] N A Burdett ldquoThe mechanism of the sulphation of limestoneduring fluidized bed desulphurizationrdquo in Proceedings of theInstitute of Energy Symposium Series vol 4 London UK 1980
[12] R B Fieldes N A Burdett J F Davidson and J F T ldquoReactionof sulphur dioxide with limestone particles the influence ofsulphur trioxiderdquo Transactions of the Institution of ChemicalEngineers vol 57 no 4 pp 276ndash280 1979
[13] P J Jackson D A Hilton and J H Buddery ldquoContinuousmeasurements of sulphuric acid vapour in combustion gasesusing a portable automatic monitorrdquo Journal of the Institute ofEnergy vol 54 pp 124ndash135 1981
[14] R C Hotchkiss P J Jackson and D A Hilton ldquoA portableautomatic monitor for continuously measuring sulphuric acidvapor in combustion gasesrdquo in Proceedings of the Symposium onInstrumentation and Control for Fossil-Energy Processes ANL-81-62 Conf 810607 Paper A026 San Francisco Calif USA1981
[15] W Z Khan and B M Gibbs ldquoThe influence of air staging in thereduction of SO
2by limestone in a fluidized bed combustorrdquo
Fuel vol 74 no 6 pp 800ndash805 1995[16] W Z Khan and BMGibbs ldquoAn approach to estimate the depth
of oxidizing and reducing regions in a fluidized bed combustorwith staged combustionrdquo Fuel vol 75 no 7 pp 899ndash906 1996
[17] A Yilmaz L Hindiyarti A D Jensen P Glarborg and PMarshall ldquoThermal dissociation of SO
3at 1000ndash1400Krdquo Journal
of Physical Chemistry A vol 110 no 21 pp 6654ndash6659 2006[18] J D Thibault F R Steward and D M Ruthve ldquoThe kinetics
of absorption of SO3in calcium and magnesium oxidesrdquo The
Canadian Journal of Chemical Engineering vol 60 pp 796ndash8011982
[19] N A Burdett ldquoThe inhibition of the limestone sulphation pro-cess during fluidized bed combustionmdashA theoretical approachrdquoJournal of the Institute of Energy pp 198ndash208 1983
[20] E L Merryman and A Levy ldquoEnhanced SO3emissions from
staged combustionrdquo Symposium (International) on Combustionvol 17 no 1 pp 727ndash736 1979
was 100 times faster than expected for oxidation in the gasphase
Willium and Gibbs [5] measured SO2and SO
3emissions
from a 03m2 fluidized-bed combustor They reported thatthe higher the excess air level the lower the SO
2emissions (on
removal of the dilution effect SO2increased with an increase
in excess air reaching a limiting value of 1300 ppm above 30excess air) and that SO
3emissions increased slightly with the
increase in excess air Barnes [6] also observed the similareffect of excess air on SO
2and SO
3emissions Willium and
Gibbs [5] and Barnes [6] found that the higher the sulphurcontent of fuel the higher the SO
2emissions while the SO
3
was unaffected by the fuel sulphur content [5] Coal fed tothe bed caused SO
2and SO
3emissions to increase than when
fed to the surface [5] SO3shows a weak dependence on bed
temperature [5 6]Barnes [6] studied the effect of sand particle size flu-
idizing velocity and bed depth on SO2and SO
3emissions
Barnesrsquo findings indicate that fine sand (0300mm) produceshigh SO
2and SO
3emissions Increasing fluidizing velocity
from 1 to 2ms caused reduced formation of SO2in the
bed and freeboard An increase in bed depth increased SO3
emission a deep bed (30 cm) and fine sand resulted in a slightdecrease in SO
2emission
Oxygen availability and fluidizing characteristics withinthe bed also affect SO
3formation Ahn et al [7] found that
for pulverized coal concentrations of SO3and SO
2were
significantly higher for oxy-fired conditions as comparedto air-fired conditions In circulating fluidized bed SO
3
concentrations were notably higher for oxy-fired conditionstoo For higher sulfur coal SO
3concentrations were 4ndash6
times greater on averageTheir findings contradict the findingof Barnes [6]
Hindiyarti et al [8] investigated the reaction of SO3with
H O and OH radical The revised rate constant calculatedby them suggests that SO
3and O reaction is found to be
insignificant during most conditions According to themSO3+H is the major consumption reaction for SO
3
Stanger and Wall [9] reviewed published work on SO3
concentrations and emissions under oxy-fuel firing Theirconclusion is that the conversion of SO
2to SO
3is consider-
ably variableWillium andGibbs [5] found that coal char had a very sig-
nificant removal effect on SO3emissions because SO
3above
the bed was 50 greater than in the exit Barnes [6] said thatunburnt char does not have a major effect on SO
3as carbon
carryover increases under the given operating conditionsHer results showed that the quantity of inert particles (30 cmdeep bed) resulted in an increase in heterogeneous catalyticreaction of SO
2to form SO
3
Burdett et al [3] carried out experiments in a microbal-ance to study the effect of limestone on SO
3emissions
According to Burdett et al [3] the reaction of CaO O2 and
SO2 in order to yield CaSO
4(reaction (3)) must occur in two
separate steps Two possibilities exist either SO2reacts with
CaO and CaSO3is formed which is then oxidized or else the
formation of SO3in gas phase or on a stone surface is followed
by an attack on the CaO Consider the following
CaO + 12O2+ SO2997888rarr CaSO
4(3)
Route 1
CaO + SO2997888rarr CaSO
3 (4)
CaSO3+1
2O2997888rarr CaSO
4(5)
Route (2)
SO2+1
2O2997888rarr SO
3(6)
SO3+ CaO 997888rarr CaSO
4 (7)
It is not possible due to the lack of available experimentaldata to say which of these mechanisms is operative undergiven conditions although both may be important A mostinteresting comparison between the SO
119909level detected with
and without limestone (proposed by Burdett et al [10]) isthe SO
2SO3ratio Without limestone the ratio they found
was 220033 (or 67 1) and with limestone it increased to35015 (or 230 1) It is clear that SO
3is depressed to a
greater extent than SO2on the addition of limestone and
they attribute this to the higher reactivity of SO3compared
with that of SO2 Burdett [11] and Burdett et al [10] have
assumed that SO2oxidizes to SO
3in the particles at a rate
dependent on local SO2and O
2concentrations with SO
3
diffusing through theCaSO4shell and reactingwithCaOThe
rate of formation of CaSO4may be linked to different rates
of production of SO3at different locations within the stone
At a high oxygen level oxidation increases preferentially atthe edge of a particle High utilization is achieved when SO
2
diffusion in the interior of the stone ismaximized and this inturn implies a low SO
3formation rate Barnes [6] reported a
decrease in SO2conversion to SO
3with oxygen concentration
and an increase with SO2concentration
Fieldes et al [12] reported the achievement of a highfractional sulphation (036) when coal is burnt in the bedIt appears that the fraction of the sulphur gas phase whichis SO3 has a substantial effect on the fractional sulphation
of the limestone Ash also appears to remove SO3selectively
The mechanism of this hypothesis supposes that the directreaction of CaO with SO
3is faster than a reaction via
the CaSO3intermediate This paper examines the factors
responsible for formation and reduction of SO2
and SO3
from a coal-fired fluidized bed under varying operatingconditions
2 Apparatus and Procedure
The main features of the fluidized-bed combustor and ancil-laries are presented in Figure 1 The bed consisted of silicasand of mean size 0700mm Fluidizing air was supplied by afan and metered and introduced through a distributor plateFor staged combustion the secondary air was introducedinto the combustor through a stainless-steel pipe 100 cmabove the bed surface In staged combustion mode the totalcombustion air is separated into a primary air stream supplied
ISRN Environmental Chemistry 3
Table 1 Typical analysis of Linby and Daw Mill coal
to fluidize the bed and a secondary air stream injected abovethe bed to complete the combustion For example in 70 30staging 30 of the total air is injected as secondary air
The bed was preheated by a propane burner that was fixedabove the bed and the fluidizing airflow rate was adjustedto the lowest level to minimize heating time Coal was fedin the combustor when the bed temperature reached 550∘CWhen the bed temperature reached 800∘C the desired coalfeed rate was adjusted to a constant value the propane burnerwas switched off and the fluidizing air was adjusted to therequired levelThe bed temperature was maintained constantby using an adjustable cooling coil with circulating waterConcentrations of O
2 CO CO
2 and SO
2were recorded
continuously by an ADC-RF infrared gas analyzerThe experiments were carried out at bed temperatures
of 800ndash850∘C fluidizing velocities of 1-2ms and excess airlevels of 0ndash60 Static bed height was 20ndash30 cm Two typesof coal bituminous Linby and Daw Mill of 3ndash16mm (large)diameter in size and two types of limestone Ballidone andPenrith of lt3mm mean diameter in size were used Inboth cases the coal was premixed with the limestone andfed overbed at 42 cm above the distributor (see Table 1 forproximate and ultimate analyses of the coal and Table 2 forchemical composition of the limestone) The CaS ratio was3 1 mole per mole or otherwise as indicated Three levels ofstaging (15ndash40 secondary air) were used to investigate theeffect of fluidizing velocity bed temperature and excess airon SO
3reduction during air staging
3 Sampling of SO3
An SSLMEL SO3analyzer developed by Severn Science
LabsMarchwood Eng Labs was used for continuous mon-itoring of the SO
3 A detailed account of the principles and
operating procedures of an SSLMEL analyzer can be foundin Jackson et al [13] and Hotchkiss et al [14]
The representative sample of flue gas was extracted fromthe sample point located near the exit of flue gas to the cyclone(200 cm above the distributor) where the gas temperaturewas around 550∘CUnder these conditions the use of a quartzsampling probe was found to be adequate This enabledthe extraction of the acid-containing gas directly into thefilter-contactor of the SO
3analyzer The temperature of the
sampled gas was maintained (by keeping the length of thetube as short as possible) at a value in excess of the aciddewpoint and below the temperature at which significantdissociation to SO
3occurred accurate determination of the
acid gas content in the gas could then be made preciselyover a significant period of time This procedure effectivelyeliminates any interaction between the SO
3in the gas sample
and other species within the sampling probe itself
4 Results and Discussion
41 SO3Emissions without Limestone under Unstaged Com-
bustion The effect of some operating variables on theSO3emissions under unstaged combustion is presented in
Figure 2 The results are corrected to 5 oxygen in the flueequivalentThe flue gas SO
3emissions ranged from 5 to 10 for
the Linby coal SO3emissions decrease with excess air when
corrected for dilution increase slightly with excess air reachlimiting values and then gradually decrease The fluidizingvelocity also affects emissions to some extent The effect ofbed height on SO
3emissions was associated with the size
of sand particles As the fine sand produces more reducingenvironment the oxygen stoichiometry influences the ratesof oxidation of SO
2 and as a result the generation of SO
3
is reduced The SO3emissions were also less sensitive to
change in bed temperatureThe rise in emissionswas typically0510∘C
In another set of experiments the Daw Mill Coal wastested for SO
3emissions The flue gas SO
3emission ranged
from 4 to 195 ppm for Dawmill coal Changing the operatingparameters resulted in a maximum change in SO
3emission
of 13 ppm Increasing the bed depth resulted in higherSO3emission the fluidizing velocity also affected emission
depending on the size of bedmaterial usedThe SO3emission
increased as the bed size was varied from coarse to fine withthe bed depth SO
3emissions were weakly dependent on
temperature typically rising 07 ppm10∘C
42 Comparison to ReportedWork Dennis and Hayhurst [4]have reported that the SO
3formation under atmospheric
pressure was very low (eg mole fraction of SO3in off gas
= 7 times10minus5 for mole fraction of SO2gas entering a bed = 23
times10minus3 at 875∘C) The oxidation rate in the gas phase was 100
times faster than expected An Eley-Rideal mechanism was
4 ISRN Environmental Chemistry
Screw feederLS hopper
Secondary air
Rotary valve
Plenum
DC motor
Primary airPG
Blower
Air
Hot vertical probeTC 5
Sample probeCyclone
TC 6
Stack
Propane
Burner
Air
TC 4
TC 3
TC 2TC 1
Ash and spentsorbentcollector
Coal hopper
N2
PG = pressure gauge
TC = thermocouple
CWinCWout
Figure 1 Main features of fluidized-bed combustor and ancillaries
121110
987654
0 1 2 3 4 5 6 7 8 9 10 11 12
SO3
emiss
ion
(ppm
)
Oxygen concentration in the fuel gas ()
Fluidizing velocity 10 ms 30 cm bed heightFluidizing velocity 15 ms 20 cm bed heightFluidizing velocity 20 ms 20 cm bed height
Figure 2 Influence of fluidizing velocity excess air and bedheight on SO
3emissions during unstaged combustion at 850∘C bed
temperature and coarse sand (corrected to 5 in flue equivalent)
proposed in which O2and SO
2competitively chemisorb on
the surface and the rate of reaction is controlled by gas-phasemolecule of SO
2reacting with adsorbed O atom
Willium and Gibbs [5] have tested many coal types forSO3concentration without limestone in 750ndash900∘C temper-
ature range Their findings suggest that ash (having tracesof Ca Mg Na K etc) is the principle removing species ofSO3 In another experiment when pure SO
2was introduced
the SO3reacted with added char at 850∘C in the absence of
oxygen to give SO3of 7 vpm in the outlet which suggests that
char is important in the removal of SO3 He also observed a
50 reduction in SO3in the freeboardAccording toWillium
the reduction was due to the reaction of SO3with unburnt
charSO3emissions are dependent on the oxygen and sulfur
dioxide concentrations and were found to follow a similartrend Willium and Gibbs [5] found that in contrast tothe effect on SO
2emissions fine coal produced lower SO
3
emissions In this study SO3emissions were slightly higher
when fine sand was used and tended to increase with bedheight This suggests that unburnt char does not have asignificant effect on SO
3emissions The results of this study
indicate that the amount of particles in the bed could have asignificant effect on SO
3emissions resulting in an increase
in the heterogeneous catalytic reaction of SO2to form SO
3
as the quantity of bed particles increases Higher bed heighttherefore will also result in high SO
3emissions The oxygen
concentration and fluidizing velocity will also affect SO3
formation
43 SO3Emissions with Limestone under Unstaged Combus-
tion SO3emissions decrease in the presence of limestone
and the reduction is temperature sensitive The SO3reduc-
tions were less sensitive than the reductions achieved for SO2
at similar conditions [15 16] At a temperature around 850∘Cthe SO
3reductions were only 28 of the SO
2reductions but
ISRN Environmental Chemistry 5
30272421181512
9630
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
SO3
redu
ctio
n (
)
CaS molar ratio
10 excess air30 excess air48 excess air
Figure 3 Influence of limestone addition on SO3reduction during
unstaged combustion 25 cmheight 10ms fluidizing velocity 850∘Cbed temperature and 12 cm limestone addition height
322824201612
840
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
SO3
redu
ctio
n (
)
CaS molar ratio
40 excess air60 excess air80 excess air
Figure 4 Influence of limestone addition on SO3reduction during
unstaged combustion 25 cmheight 10ms fluidizing velocity 850∘Cbed temperature and 42 cm limestone addition height as well ascoarse sand
at 800∘C the reductions reached 70 of the SO2reduction
level The results corresponding to the operating conditionsare shown graphically in Figures 3 and 4 It should be notedthat the SO
3reduction shown in Figure 3 was obtained
when limestone was injected 12 cm above the distributor andFigure 4 represents the results when limestone was injected42 cm above the distributor
At a temperature of 850∘C some of the SO2will always be
converted to SO3via reaction (6) The conversion decreases
with oxygen concentration and increases with sulfur dioxideconcentration An increase in temperature enhances the rateof SO3formation SO
3can react with CaO to formCaSO
4via
reaction (7) The rate of this reaction is temperature depen-dent Yilmaz et al [17] studied the thermal dissociation of SO
3
in the range of 800ndash1200∘C under atmospheric pressure Atthe location in the flame where the net SO
3formation rate is
zero he determined a rate constant of 69times 1010 cm3molminus1 sminus1
2422201816141210
86420
0 2 4 6 8 10 12 14 16 18
SO3
redu
ctio
n (
)
CaS molar ratio
Limestone injection height 42 cm excess air 32Limestone injection height 42 cm excess air 60
Figure 5 Influence of limestone addition on SO3level in the flue
under staged combustion
for SO3+ N2rarr SO
3+ O + N
2 that was consistent with
other flame results A high temperature lowers the reactionrate Therefore at a high temperature more SO
3is produced
but less will be consumed in sulphation As a result a largerdecrease in SO
3emissions is observed at lower temperature
The effect of excess air can also be seen in these graphsSO3emissions have been found to increase with excess air
but upon removing the dilution effect the increase is withina narrow range indicating that there could be an optimumreduction at a particular excess air beyond which the SO
3
reduction decreases An increase in the fluidizing velocity haslittle effect on the overall reduction of SO
3emissions
During another set of experiments Penrith limestonewas added to the Daw Mill coal It was observed that SO
3
emissions were decreased in the presence of limestone andthe reduction was temperature dependent At the highertemperature of 850∘C the SO
3reductions were 18ndash20 of
the SO2reduction but at 800∘C the SO
3reductions reached
55 of the SO2reduction level Figure 5 shows the results of
this set
44 Comparison to Reported Work (Conducted on Microbal-ance or Small Bed of 36ndash78mm ID) Burdett et al [3] havereported that the reaction between limestone and sulfuroxides is highly sensitive to changes in O
2 SO and SO
3
concentrations Absorption of SO3by the coal ash cannot be
quantified on themicrobalance and themicrobalance resultsare not applicable to fluidized combustor
Fieldes et al [12] have reported that extent of SO2oxida-
tion to SO3varied with SO
2and O
2concentration The coal
combustion test showed that the lower SO3concentrations
are due to its selective removal by ash They had tested avariety of limestone and in all the cases the mole fraction ofCaO converted to CaSO
4was affected by inlet oxygen in the
same way as Penrith limestoneThibault et al [18] have conducted experiments on a
small (6mm) fixed bed packed with CaO particle They havetested two grain size of the sorbent and reported that forefficient capture of SO
3a small grain size and openmacropore
structure are essential
6 ISRN Environmental Chemistry
45 Comparison to ReportedWork (Conducted on Pilot Scale)Burdett et al [19] have reported fractional conversion of SO
2
to SO3decreased from about 15 in the limestone-free case
to around 035 when the limestone and alkaline ash werepresent which was due to the greater reaction of SO
3with
limestone compared with ash the absorption occurring bothin the bed itself and in the freeboard
Burdett et al [10] have reported that combustion of a 3sulfur coal in a bed burning at 900∘C generated 33 vpm ofSO3and proposed that the effect of O
2on sulphation capacity
results from the formation of SO3within the pores of the
stone
46 SO3Emissions without Limestone under Staged Combus-
tion Merryman and Levy [20] have conducted staged exper-iments on a quartz tube methane burner producing stablemethane-H
2S flame within desired fuel-air ratio without a
sorbent presence under staged combustion conditions Theyhave reported that when the remaining excess air was injectedinto these gases the maximum amount of SO
3formed was
greater than formed when this additional air was includedwith the initial combustion air the overall excess of airbeing the same in both cases The experimental conditionsof Merryman and Levy do not match with our fluidized bedtherefore their results are not comparable with this study
During this study the SO3emissions under staged com-
bustion without limestone could not be monitored exten-sively due to malfunctioning of SO
3analyzer
47 SO3Emissions with Limestone under Staged Combustion
The concentration of SO3emissions at 15ms and 20 excess
air was 170 ppm which decreased to 55 ppm in the presenceof limestone The SO
3emissions at 7030 staged (15ms
850∘C) combustion (without limestone) were similar to thoseof unstaged combustion (without limestone) However it wasobserved that in the presence of limestone staged combus-tion results in a higher reduction of SO
3than unstaged
Figure 6 gives the SO3emissions as a function of PACR The
emissions at 15 secondary air were 15 ppm and increased to72 ppm at 45 secondary air It is clear that SO
3is depressed
on the addition of limestone during both the unstaged andstaged operations and the extent of reduction was higherunder staged combustion
Figure 6 shows that the maximum removal of SO3occurs
at a lower staging levels of 8515 and as the bed becomesmore substoichiometric the rate of SO
3removal decreases
This trend indicates the formation of SO3in the freeboard
which bypasses the limestone and appears in the flue Thisincrease in SO
3reduction with the in-bed air ratio is in
agreement with Barnes [6] findingsThe results of the staged combustion test with Daw Mill
coal in the presence of Penrith limestone indicate that SO3
emissions varied little with changes in excess air Howeverif excess air is coupled with fluidizing velocity then it hadsome effects on the emissions At higher velocity of 2ms thechange was up to 4 ppmThe concentration of SO
3emissions
at 15ms and 30 excess air was 20 ppm which decreased to
8
6
4
2
006 08 1 12 14 16
20 excess air
SO3
emiss
ion
(ppm
)
Primary air to coal ratio
Figure 6 Influence of PACR on SO3emission at 15ms fluidizing
velocity and coarse sand
5
45
4
35
3
25
20 5 10 15 20 25 30 35
SO3
emiss
ion
(ppm
)
Secondary air ()
Figure 7 Influence of staging SO3emission in the flue secondary
air injection height 100 cm above the distributor excess air 30
8 ppm under staged combustion in the presence of limestoneThe results of Daw Mill coal test are shown in Figure 7
It should be noted that there is no published work onSO3emissions under staged combustion conditions with or
without limestone on any scale Therefore the results of thisstudy could not be compared
5 Conclusion
The experimental data shows that during unstaged com-bustion without limestone SO
3emissions are dependent
on oxygen and SO2concentration SO
3emissions increase
slightly with excess air reaching a limiting value and thenslowly decrease SO
3emissions are less sensitive to change
in bed temperature However the fluidizing velocity and bedheight affect the emissions
In the presence of limestone SO3emissions are reduced
during both staged and unstaged operations and the reduc-tion is temperature sensitive However during staged com-bustion the reduction is enhanced As staged fluidized-bed combustion is a proven technique to reduce NO
119909and
SO2emissions therefore it should be possible to operate a
fluidized-bed combustor under a stagedmodewith limestoneto keep SO
2 SO3 and NO
119909emissions to a minimum
ISRN Environmental Chemistry 7
References
[1] Y Mitsui N Imada H Kikkawa and A Katagawa ldquoStudyof Hg and SO
3behavior in flue gas of oxy-fuel combustion
systemrdquo International Journal of Greenhouse Gas Control vol5 pp S143ndashS150 2011
[2] L Zheng and E Furimsky ldquoAssessment of coal combustion inO2+CO2by equilibrium calculationsrdquo Fuel Processing Technol-
ogy vol 81 no 1 pp 23ndash34 2003[3] N A Burdett R C Hotchkiss and R B Fields ldquoSO
3formation
and retention in coal fired fluidized bed combustorsrdquo AIChESymposium Series vol 57 pp M1ndashM11 1979
[4] J S Dennis and A N Hayhurst ldquoThe formation of SO3in a
fluidized bedrdquo Combustion and Flame vol 72 no 3 pp 241ndash258 1988
[5] P T Willium and B M Gibbs ldquoThe emissions of SO2and
SO3from fluidized bedsrdquo in Proceedings of the 4th International
Conference on Fluidization pp 443ndash450 Kashikojima Japan1983
[6] J P Barnes Abatement of nitric oxide emission from a coal-fixedfluidized bed combustor [PhD thesis] Department of Fuel andEnergy University of Leeds Leeds UK 1988
[7] J Ahn R Okerlund A Fryb and E G Eddingsa ldquoSulfurtrioxide formation during oxy-coal combustionrdquo InternationalJournal of Greenhouse Gas Control vol 5 pp S127ndashS135 2011
[8] L Hindiyarti P Glarborg and P Marshall ldquoReactions of SO3
with the OH radical pool under combustion conditionsrdquoJournal of Physical Chemistry A vol 111 no 19 pp 3984ndash39912007
[9] R Stanger and T Wall ldquoSulphur impacts during pulverisedcoal combustion in oxy-fuel technology for carbon capture andstoragerdquo Progress in Energy and Combustion Science vol 37 no1 pp 69ndash88 2011
[10] N A Burdett R C Hotchkiss R P Hensel and R B FieldsldquoCoal devolatilization and emission of SO
3in a fluidized bed
combustionrdquo in Proceedings of the Fluidized Combustion Con-ference vol 2 pp 424ndash442 Capetown South Africa January1981
[11] N A Burdett ldquoThe mechanism of the sulphation of limestoneduring fluidized bed desulphurizationrdquo in Proceedings of theInstitute of Energy Symposium Series vol 4 London UK 1980
[12] R B Fieldes N A Burdett J F Davidson and J F T ldquoReactionof sulphur dioxide with limestone particles the influence ofsulphur trioxiderdquo Transactions of the Institution of ChemicalEngineers vol 57 no 4 pp 276ndash280 1979
[13] P J Jackson D A Hilton and J H Buddery ldquoContinuousmeasurements of sulphuric acid vapour in combustion gasesusing a portable automatic monitorrdquo Journal of the Institute ofEnergy vol 54 pp 124ndash135 1981
[14] R C Hotchkiss P J Jackson and D A Hilton ldquoA portableautomatic monitor for continuously measuring sulphuric acidvapor in combustion gasesrdquo in Proceedings of the Symposium onInstrumentation and Control for Fossil-Energy Processes ANL-81-62 Conf 810607 Paper A026 San Francisco Calif USA1981
[15] W Z Khan and B M Gibbs ldquoThe influence of air staging in thereduction of SO
2by limestone in a fluidized bed combustorrdquo
Fuel vol 74 no 6 pp 800ndash805 1995[16] W Z Khan and BMGibbs ldquoAn approach to estimate the depth
of oxidizing and reducing regions in a fluidized bed combustorwith staged combustionrdquo Fuel vol 75 no 7 pp 899ndash906 1996
[17] A Yilmaz L Hindiyarti A D Jensen P Glarborg and PMarshall ldquoThermal dissociation of SO
3at 1000ndash1400Krdquo Journal
of Physical Chemistry A vol 110 no 21 pp 6654ndash6659 2006[18] J D Thibault F R Steward and D M Ruthve ldquoThe kinetics
of absorption of SO3in calcium and magnesium oxidesrdquo The
Canadian Journal of Chemical Engineering vol 60 pp 796ndash8011982
[19] N A Burdett ldquoThe inhibition of the limestone sulphation pro-cess during fluidized bed combustionmdashA theoretical approachrdquoJournal of the Institute of Energy pp 198ndash208 1983
[20] E L Merryman and A Levy ldquoEnhanced SO3emissions from
staged combustionrdquo Symposium (International) on Combustionvol 17 no 1 pp 727ndash736 1979
to fluidize the bed and a secondary air stream injected abovethe bed to complete the combustion For example in 70 30staging 30 of the total air is injected as secondary air
The bed was preheated by a propane burner that was fixedabove the bed and the fluidizing airflow rate was adjustedto the lowest level to minimize heating time Coal was fedin the combustor when the bed temperature reached 550∘CWhen the bed temperature reached 800∘C the desired coalfeed rate was adjusted to a constant value the propane burnerwas switched off and the fluidizing air was adjusted to therequired levelThe bed temperature was maintained constantby using an adjustable cooling coil with circulating waterConcentrations of O
2 CO CO
2 and SO
2were recorded
continuously by an ADC-RF infrared gas analyzerThe experiments were carried out at bed temperatures
of 800ndash850∘C fluidizing velocities of 1-2ms and excess airlevels of 0ndash60 Static bed height was 20ndash30 cm Two typesof coal bituminous Linby and Daw Mill of 3ndash16mm (large)diameter in size and two types of limestone Ballidone andPenrith of lt3mm mean diameter in size were used Inboth cases the coal was premixed with the limestone andfed overbed at 42 cm above the distributor (see Table 1 forproximate and ultimate analyses of the coal and Table 2 forchemical composition of the limestone) The CaS ratio was3 1 mole per mole or otherwise as indicated Three levels ofstaging (15ndash40 secondary air) were used to investigate theeffect of fluidizing velocity bed temperature and excess airon SO
3reduction during air staging
3 Sampling of SO3
An SSLMEL SO3analyzer developed by Severn Science
LabsMarchwood Eng Labs was used for continuous mon-itoring of the SO
3 A detailed account of the principles and
operating procedures of an SSLMEL analyzer can be foundin Jackson et al [13] and Hotchkiss et al [14]
The representative sample of flue gas was extracted fromthe sample point located near the exit of flue gas to the cyclone(200 cm above the distributor) where the gas temperaturewas around 550∘CUnder these conditions the use of a quartzsampling probe was found to be adequate This enabledthe extraction of the acid-containing gas directly into thefilter-contactor of the SO
3analyzer The temperature of the
sampled gas was maintained (by keeping the length of thetube as short as possible) at a value in excess of the aciddewpoint and below the temperature at which significantdissociation to SO
3occurred accurate determination of the
acid gas content in the gas could then be made preciselyover a significant period of time This procedure effectivelyeliminates any interaction between the SO
3in the gas sample
and other species within the sampling probe itself
4 Results and Discussion
41 SO3Emissions without Limestone under Unstaged Com-
bustion The effect of some operating variables on theSO3emissions under unstaged combustion is presented in
Figure 2 The results are corrected to 5 oxygen in the flueequivalentThe flue gas SO
3emissions ranged from 5 to 10 for
the Linby coal SO3emissions decrease with excess air when
corrected for dilution increase slightly with excess air reachlimiting values and then gradually decrease The fluidizingvelocity also affects emissions to some extent The effect ofbed height on SO
3emissions was associated with the size
of sand particles As the fine sand produces more reducingenvironment the oxygen stoichiometry influences the ratesof oxidation of SO
2 and as a result the generation of SO
3
is reduced The SO3emissions were also less sensitive to
change in bed temperatureThe rise in emissionswas typically0510∘C
In another set of experiments the Daw Mill Coal wastested for SO
3emissions The flue gas SO
3emission ranged
from 4 to 195 ppm for Dawmill coal Changing the operatingparameters resulted in a maximum change in SO
3emission
of 13 ppm Increasing the bed depth resulted in higherSO3emission the fluidizing velocity also affected emission
depending on the size of bedmaterial usedThe SO3emission
increased as the bed size was varied from coarse to fine withthe bed depth SO
3emissions were weakly dependent on
temperature typically rising 07 ppm10∘C
42 Comparison to ReportedWork Dennis and Hayhurst [4]have reported that the SO
3formation under atmospheric
pressure was very low (eg mole fraction of SO3in off gas
= 7 times10minus5 for mole fraction of SO2gas entering a bed = 23
times10minus3 at 875∘C) The oxidation rate in the gas phase was 100
times faster than expected An Eley-Rideal mechanism was
4 ISRN Environmental Chemistry
Screw feederLS hopper
Secondary air
Rotary valve
Plenum
DC motor
Primary airPG
Blower
Air
Hot vertical probeTC 5
Sample probeCyclone
TC 6
Stack
Propane
Burner
Air
TC 4
TC 3
TC 2TC 1
Ash and spentsorbentcollector
Coal hopper
N2
PG = pressure gauge
TC = thermocouple
CWinCWout
Figure 1 Main features of fluidized-bed combustor and ancillaries
121110
987654
0 1 2 3 4 5 6 7 8 9 10 11 12
SO3
emiss
ion
(ppm
)
Oxygen concentration in the fuel gas ()
Fluidizing velocity 10 ms 30 cm bed heightFluidizing velocity 15 ms 20 cm bed heightFluidizing velocity 20 ms 20 cm bed height
Figure 2 Influence of fluidizing velocity excess air and bedheight on SO
3emissions during unstaged combustion at 850∘C bed
temperature and coarse sand (corrected to 5 in flue equivalent)
proposed in which O2and SO
2competitively chemisorb on
the surface and the rate of reaction is controlled by gas-phasemolecule of SO
2reacting with adsorbed O atom
Willium and Gibbs [5] have tested many coal types forSO3concentration without limestone in 750ndash900∘C temper-
ature range Their findings suggest that ash (having tracesof Ca Mg Na K etc) is the principle removing species ofSO3 In another experiment when pure SO
2was introduced
the SO3reacted with added char at 850∘C in the absence of
oxygen to give SO3of 7 vpm in the outlet which suggests that
char is important in the removal of SO3 He also observed a
50 reduction in SO3in the freeboardAccording toWillium
the reduction was due to the reaction of SO3with unburnt
charSO3emissions are dependent on the oxygen and sulfur
dioxide concentrations and were found to follow a similartrend Willium and Gibbs [5] found that in contrast tothe effect on SO
2emissions fine coal produced lower SO
3
emissions In this study SO3emissions were slightly higher
when fine sand was used and tended to increase with bedheight This suggests that unburnt char does not have asignificant effect on SO
3emissions The results of this study
indicate that the amount of particles in the bed could have asignificant effect on SO
3emissions resulting in an increase
in the heterogeneous catalytic reaction of SO2to form SO
3
as the quantity of bed particles increases Higher bed heighttherefore will also result in high SO
3emissions The oxygen
concentration and fluidizing velocity will also affect SO3
formation
43 SO3Emissions with Limestone under Unstaged Combus-
tion SO3emissions decrease in the presence of limestone
and the reduction is temperature sensitive The SO3reduc-
tions were less sensitive than the reductions achieved for SO2
at similar conditions [15 16] At a temperature around 850∘Cthe SO
3reductions were only 28 of the SO
2reductions but
ISRN Environmental Chemistry 5
30272421181512
9630
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
SO3
redu
ctio
n (
)
CaS molar ratio
10 excess air30 excess air48 excess air
Figure 3 Influence of limestone addition on SO3reduction during
unstaged combustion 25 cmheight 10ms fluidizing velocity 850∘Cbed temperature and 12 cm limestone addition height
322824201612
840
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
SO3
redu
ctio
n (
)
CaS molar ratio
40 excess air60 excess air80 excess air
Figure 4 Influence of limestone addition on SO3reduction during
unstaged combustion 25 cmheight 10ms fluidizing velocity 850∘Cbed temperature and 42 cm limestone addition height as well ascoarse sand
at 800∘C the reductions reached 70 of the SO2reduction
level The results corresponding to the operating conditionsare shown graphically in Figures 3 and 4 It should be notedthat the SO
3reduction shown in Figure 3 was obtained
when limestone was injected 12 cm above the distributor andFigure 4 represents the results when limestone was injected42 cm above the distributor
At a temperature of 850∘C some of the SO2will always be
converted to SO3via reaction (6) The conversion decreases
with oxygen concentration and increases with sulfur dioxideconcentration An increase in temperature enhances the rateof SO3formation SO
3can react with CaO to formCaSO
4via
reaction (7) The rate of this reaction is temperature depen-dent Yilmaz et al [17] studied the thermal dissociation of SO
3
in the range of 800ndash1200∘C under atmospheric pressure Atthe location in the flame where the net SO
3formation rate is
zero he determined a rate constant of 69times 1010 cm3molminus1 sminus1
2422201816141210
86420
0 2 4 6 8 10 12 14 16 18
SO3
redu
ctio
n (
)
CaS molar ratio
Limestone injection height 42 cm excess air 32Limestone injection height 42 cm excess air 60
Figure 5 Influence of limestone addition on SO3level in the flue
under staged combustion
for SO3+ N2rarr SO
3+ O + N
2 that was consistent with
other flame results A high temperature lowers the reactionrate Therefore at a high temperature more SO
3is produced
but less will be consumed in sulphation As a result a largerdecrease in SO
3emissions is observed at lower temperature
The effect of excess air can also be seen in these graphsSO3emissions have been found to increase with excess air
but upon removing the dilution effect the increase is withina narrow range indicating that there could be an optimumreduction at a particular excess air beyond which the SO
3
reduction decreases An increase in the fluidizing velocity haslittle effect on the overall reduction of SO
3emissions
During another set of experiments Penrith limestonewas added to the Daw Mill coal It was observed that SO
3
emissions were decreased in the presence of limestone andthe reduction was temperature dependent At the highertemperature of 850∘C the SO
3reductions were 18ndash20 of
the SO2reduction but at 800∘C the SO
3reductions reached
55 of the SO2reduction level Figure 5 shows the results of
this set
44 Comparison to Reported Work (Conducted on Microbal-ance or Small Bed of 36ndash78mm ID) Burdett et al [3] havereported that the reaction between limestone and sulfuroxides is highly sensitive to changes in O
2 SO and SO
3
concentrations Absorption of SO3by the coal ash cannot be
quantified on themicrobalance and themicrobalance resultsare not applicable to fluidized combustor
Fieldes et al [12] have reported that extent of SO2oxida-
tion to SO3varied with SO
2and O
2concentration The coal
combustion test showed that the lower SO3concentrations
are due to its selective removal by ash They had tested avariety of limestone and in all the cases the mole fraction ofCaO converted to CaSO
4was affected by inlet oxygen in the
same way as Penrith limestoneThibault et al [18] have conducted experiments on a
small (6mm) fixed bed packed with CaO particle They havetested two grain size of the sorbent and reported that forefficient capture of SO
3a small grain size and openmacropore
structure are essential
6 ISRN Environmental Chemistry
45 Comparison to ReportedWork (Conducted on Pilot Scale)Burdett et al [19] have reported fractional conversion of SO
2
to SO3decreased from about 15 in the limestone-free case
to around 035 when the limestone and alkaline ash werepresent which was due to the greater reaction of SO
3with
limestone compared with ash the absorption occurring bothin the bed itself and in the freeboard
Burdett et al [10] have reported that combustion of a 3sulfur coal in a bed burning at 900∘C generated 33 vpm ofSO3and proposed that the effect of O
2on sulphation capacity
results from the formation of SO3within the pores of the
stone
46 SO3Emissions without Limestone under Staged Combus-
tion Merryman and Levy [20] have conducted staged exper-iments on a quartz tube methane burner producing stablemethane-H
2S flame within desired fuel-air ratio without a
sorbent presence under staged combustion conditions Theyhave reported that when the remaining excess air was injectedinto these gases the maximum amount of SO
3formed was
greater than formed when this additional air was includedwith the initial combustion air the overall excess of airbeing the same in both cases The experimental conditionsof Merryman and Levy do not match with our fluidized bedtherefore their results are not comparable with this study
During this study the SO3emissions under staged com-
bustion without limestone could not be monitored exten-sively due to malfunctioning of SO
3analyzer
47 SO3Emissions with Limestone under Staged Combustion
The concentration of SO3emissions at 15ms and 20 excess
air was 170 ppm which decreased to 55 ppm in the presenceof limestone The SO
3emissions at 7030 staged (15ms
850∘C) combustion (without limestone) were similar to thoseof unstaged combustion (without limestone) However it wasobserved that in the presence of limestone staged combus-tion results in a higher reduction of SO
3than unstaged
Figure 6 gives the SO3emissions as a function of PACR The
emissions at 15 secondary air were 15 ppm and increased to72 ppm at 45 secondary air It is clear that SO
3is depressed
on the addition of limestone during both the unstaged andstaged operations and the extent of reduction was higherunder staged combustion
Figure 6 shows that the maximum removal of SO3occurs
at a lower staging levels of 8515 and as the bed becomesmore substoichiometric the rate of SO
3removal decreases
This trend indicates the formation of SO3in the freeboard
which bypasses the limestone and appears in the flue Thisincrease in SO
3reduction with the in-bed air ratio is in
agreement with Barnes [6] findingsThe results of the staged combustion test with Daw Mill
coal in the presence of Penrith limestone indicate that SO3
emissions varied little with changes in excess air Howeverif excess air is coupled with fluidizing velocity then it hadsome effects on the emissions At higher velocity of 2ms thechange was up to 4 ppmThe concentration of SO
3emissions
at 15ms and 30 excess air was 20 ppm which decreased to
8
6
4
2
006 08 1 12 14 16
20 excess air
SO3
emiss
ion
(ppm
)
Primary air to coal ratio
Figure 6 Influence of PACR on SO3emission at 15ms fluidizing
velocity and coarse sand
5
45
4
35
3
25
20 5 10 15 20 25 30 35
SO3
emiss
ion
(ppm
)
Secondary air ()
Figure 7 Influence of staging SO3emission in the flue secondary
air injection height 100 cm above the distributor excess air 30
8 ppm under staged combustion in the presence of limestoneThe results of Daw Mill coal test are shown in Figure 7
It should be noted that there is no published work onSO3emissions under staged combustion conditions with or
without limestone on any scale Therefore the results of thisstudy could not be compared
5 Conclusion
The experimental data shows that during unstaged com-bustion without limestone SO
3emissions are dependent
on oxygen and SO2concentration SO
3emissions increase
slightly with excess air reaching a limiting value and thenslowly decrease SO
3emissions are less sensitive to change
in bed temperature However the fluidizing velocity and bedheight affect the emissions
In the presence of limestone SO3emissions are reduced
during both staged and unstaged operations and the reduc-tion is temperature sensitive However during staged com-bustion the reduction is enhanced As staged fluidized-bed combustion is a proven technique to reduce NO
119909and
SO2emissions therefore it should be possible to operate a
fluidized-bed combustor under a stagedmodewith limestoneto keep SO
2 SO3 and NO
119909emissions to a minimum
ISRN Environmental Chemistry 7
References
[1] Y Mitsui N Imada H Kikkawa and A Katagawa ldquoStudyof Hg and SO
3behavior in flue gas of oxy-fuel combustion
systemrdquo International Journal of Greenhouse Gas Control vol5 pp S143ndashS150 2011
[2] L Zheng and E Furimsky ldquoAssessment of coal combustion inO2+CO2by equilibrium calculationsrdquo Fuel Processing Technol-
ogy vol 81 no 1 pp 23ndash34 2003[3] N A Burdett R C Hotchkiss and R B Fields ldquoSO
3formation
and retention in coal fired fluidized bed combustorsrdquo AIChESymposium Series vol 57 pp M1ndashM11 1979
[4] J S Dennis and A N Hayhurst ldquoThe formation of SO3in a
fluidized bedrdquo Combustion and Flame vol 72 no 3 pp 241ndash258 1988
[5] P T Willium and B M Gibbs ldquoThe emissions of SO2and
SO3from fluidized bedsrdquo in Proceedings of the 4th International
Conference on Fluidization pp 443ndash450 Kashikojima Japan1983
[6] J P Barnes Abatement of nitric oxide emission from a coal-fixedfluidized bed combustor [PhD thesis] Department of Fuel andEnergy University of Leeds Leeds UK 1988
[7] J Ahn R Okerlund A Fryb and E G Eddingsa ldquoSulfurtrioxide formation during oxy-coal combustionrdquo InternationalJournal of Greenhouse Gas Control vol 5 pp S127ndashS135 2011
[8] L Hindiyarti P Glarborg and P Marshall ldquoReactions of SO3
with the OH radical pool under combustion conditionsrdquoJournal of Physical Chemistry A vol 111 no 19 pp 3984ndash39912007
[9] R Stanger and T Wall ldquoSulphur impacts during pulverisedcoal combustion in oxy-fuel technology for carbon capture andstoragerdquo Progress in Energy and Combustion Science vol 37 no1 pp 69ndash88 2011
[10] N A Burdett R C Hotchkiss R P Hensel and R B FieldsldquoCoal devolatilization and emission of SO
3in a fluidized bed
combustionrdquo in Proceedings of the Fluidized Combustion Con-ference vol 2 pp 424ndash442 Capetown South Africa January1981
[11] N A Burdett ldquoThe mechanism of the sulphation of limestoneduring fluidized bed desulphurizationrdquo in Proceedings of theInstitute of Energy Symposium Series vol 4 London UK 1980
[12] R B Fieldes N A Burdett J F Davidson and J F T ldquoReactionof sulphur dioxide with limestone particles the influence ofsulphur trioxiderdquo Transactions of the Institution of ChemicalEngineers vol 57 no 4 pp 276ndash280 1979
[13] P J Jackson D A Hilton and J H Buddery ldquoContinuousmeasurements of sulphuric acid vapour in combustion gasesusing a portable automatic monitorrdquo Journal of the Institute ofEnergy vol 54 pp 124ndash135 1981
[14] R C Hotchkiss P J Jackson and D A Hilton ldquoA portableautomatic monitor for continuously measuring sulphuric acidvapor in combustion gasesrdquo in Proceedings of the Symposium onInstrumentation and Control for Fossil-Energy Processes ANL-81-62 Conf 810607 Paper A026 San Francisco Calif USA1981
[15] W Z Khan and B M Gibbs ldquoThe influence of air staging in thereduction of SO
2by limestone in a fluidized bed combustorrdquo
Fuel vol 74 no 6 pp 800ndash805 1995[16] W Z Khan and BMGibbs ldquoAn approach to estimate the depth
of oxidizing and reducing regions in a fluidized bed combustorwith staged combustionrdquo Fuel vol 75 no 7 pp 899ndash906 1996
[17] A Yilmaz L Hindiyarti A D Jensen P Glarborg and PMarshall ldquoThermal dissociation of SO
3at 1000ndash1400Krdquo Journal
of Physical Chemistry A vol 110 no 21 pp 6654ndash6659 2006[18] J D Thibault F R Steward and D M Ruthve ldquoThe kinetics
of absorption of SO3in calcium and magnesium oxidesrdquo The
Canadian Journal of Chemical Engineering vol 60 pp 796ndash8011982
[19] N A Burdett ldquoThe inhibition of the limestone sulphation pro-cess during fluidized bed combustionmdashA theoretical approachrdquoJournal of the Institute of Energy pp 198ndash208 1983
[20] E L Merryman and A Levy ldquoEnhanced SO3emissions from
staged combustionrdquo Symposium (International) on Combustionvol 17 no 1 pp 727ndash736 1979
Figure 1 Main features of fluidized-bed combustor and ancillaries
121110
987654
0 1 2 3 4 5 6 7 8 9 10 11 12
SO3
emiss
ion
(ppm
)
Oxygen concentration in the fuel gas ()
Fluidizing velocity 10 ms 30 cm bed heightFluidizing velocity 15 ms 20 cm bed heightFluidizing velocity 20 ms 20 cm bed height
Figure 2 Influence of fluidizing velocity excess air and bedheight on SO
3emissions during unstaged combustion at 850∘C bed
temperature and coarse sand (corrected to 5 in flue equivalent)
proposed in which O2and SO
2competitively chemisorb on
the surface and the rate of reaction is controlled by gas-phasemolecule of SO
2reacting with adsorbed O atom
Willium and Gibbs [5] have tested many coal types forSO3concentration without limestone in 750ndash900∘C temper-
ature range Their findings suggest that ash (having tracesof Ca Mg Na K etc) is the principle removing species ofSO3 In another experiment when pure SO
2was introduced
the SO3reacted with added char at 850∘C in the absence of
oxygen to give SO3of 7 vpm in the outlet which suggests that
char is important in the removal of SO3 He also observed a
50 reduction in SO3in the freeboardAccording toWillium
the reduction was due to the reaction of SO3with unburnt
charSO3emissions are dependent on the oxygen and sulfur
dioxide concentrations and were found to follow a similartrend Willium and Gibbs [5] found that in contrast tothe effect on SO
2emissions fine coal produced lower SO
3
emissions In this study SO3emissions were slightly higher
when fine sand was used and tended to increase with bedheight This suggests that unburnt char does not have asignificant effect on SO
3emissions The results of this study
indicate that the amount of particles in the bed could have asignificant effect on SO
3emissions resulting in an increase
in the heterogeneous catalytic reaction of SO2to form SO
3
as the quantity of bed particles increases Higher bed heighttherefore will also result in high SO
3emissions The oxygen
concentration and fluidizing velocity will also affect SO3
formation
43 SO3Emissions with Limestone under Unstaged Combus-
tion SO3emissions decrease in the presence of limestone
and the reduction is temperature sensitive The SO3reduc-
tions were less sensitive than the reductions achieved for SO2
at similar conditions [15 16] At a temperature around 850∘Cthe SO
3reductions were only 28 of the SO
2reductions but
ISRN Environmental Chemistry 5
30272421181512
9630
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
SO3
redu
ctio
n (
)
CaS molar ratio
10 excess air30 excess air48 excess air
Figure 3 Influence of limestone addition on SO3reduction during
unstaged combustion 25 cmheight 10ms fluidizing velocity 850∘Cbed temperature and 12 cm limestone addition height
322824201612
840
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
SO3
redu
ctio
n (
)
CaS molar ratio
40 excess air60 excess air80 excess air
Figure 4 Influence of limestone addition on SO3reduction during
unstaged combustion 25 cmheight 10ms fluidizing velocity 850∘Cbed temperature and 42 cm limestone addition height as well ascoarse sand
at 800∘C the reductions reached 70 of the SO2reduction
level The results corresponding to the operating conditionsare shown graphically in Figures 3 and 4 It should be notedthat the SO
3reduction shown in Figure 3 was obtained
when limestone was injected 12 cm above the distributor andFigure 4 represents the results when limestone was injected42 cm above the distributor
At a temperature of 850∘C some of the SO2will always be
converted to SO3via reaction (6) The conversion decreases
with oxygen concentration and increases with sulfur dioxideconcentration An increase in temperature enhances the rateof SO3formation SO
3can react with CaO to formCaSO
4via
reaction (7) The rate of this reaction is temperature depen-dent Yilmaz et al [17] studied the thermal dissociation of SO
3
in the range of 800ndash1200∘C under atmospheric pressure Atthe location in the flame where the net SO
3formation rate is
zero he determined a rate constant of 69times 1010 cm3molminus1 sminus1
2422201816141210
86420
0 2 4 6 8 10 12 14 16 18
SO3
redu
ctio
n (
)
CaS molar ratio
Limestone injection height 42 cm excess air 32Limestone injection height 42 cm excess air 60
Figure 5 Influence of limestone addition on SO3level in the flue
under staged combustion
for SO3+ N2rarr SO
3+ O + N
2 that was consistent with
other flame results A high temperature lowers the reactionrate Therefore at a high temperature more SO
3is produced
but less will be consumed in sulphation As a result a largerdecrease in SO
3emissions is observed at lower temperature
The effect of excess air can also be seen in these graphsSO3emissions have been found to increase with excess air
but upon removing the dilution effect the increase is withina narrow range indicating that there could be an optimumreduction at a particular excess air beyond which the SO
3
reduction decreases An increase in the fluidizing velocity haslittle effect on the overall reduction of SO
3emissions
During another set of experiments Penrith limestonewas added to the Daw Mill coal It was observed that SO
3
emissions were decreased in the presence of limestone andthe reduction was temperature dependent At the highertemperature of 850∘C the SO
3reductions were 18ndash20 of
the SO2reduction but at 800∘C the SO
3reductions reached
55 of the SO2reduction level Figure 5 shows the results of
this set
44 Comparison to Reported Work (Conducted on Microbal-ance or Small Bed of 36ndash78mm ID) Burdett et al [3] havereported that the reaction between limestone and sulfuroxides is highly sensitive to changes in O
2 SO and SO
3
concentrations Absorption of SO3by the coal ash cannot be
quantified on themicrobalance and themicrobalance resultsare not applicable to fluidized combustor
Fieldes et al [12] have reported that extent of SO2oxida-
tion to SO3varied with SO
2and O
2concentration The coal
combustion test showed that the lower SO3concentrations
are due to its selective removal by ash They had tested avariety of limestone and in all the cases the mole fraction ofCaO converted to CaSO
4was affected by inlet oxygen in the
same way as Penrith limestoneThibault et al [18] have conducted experiments on a
small (6mm) fixed bed packed with CaO particle They havetested two grain size of the sorbent and reported that forefficient capture of SO
3a small grain size and openmacropore
structure are essential
6 ISRN Environmental Chemistry
45 Comparison to ReportedWork (Conducted on Pilot Scale)Burdett et al [19] have reported fractional conversion of SO
2
to SO3decreased from about 15 in the limestone-free case
to around 035 when the limestone and alkaline ash werepresent which was due to the greater reaction of SO
3with
limestone compared with ash the absorption occurring bothin the bed itself and in the freeboard
Burdett et al [10] have reported that combustion of a 3sulfur coal in a bed burning at 900∘C generated 33 vpm ofSO3and proposed that the effect of O
2on sulphation capacity
results from the formation of SO3within the pores of the
stone
46 SO3Emissions without Limestone under Staged Combus-
tion Merryman and Levy [20] have conducted staged exper-iments on a quartz tube methane burner producing stablemethane-H
2S flame within desired fuel-air ratio without a
sorbent presence under staged combustion conditions Theyhave reported that when the remaining excess air was injectedinto these gases the maximum amount of SO
3formed was
greater than formed when this additional air was includedwith the initial combustion air the overall excess of airbeing the same in both cases The experimental conditionsof Merryman and Levy do not match with our fluidized bedtherefore their results are not comparable with this study
During this study the SO3emissions under staged com-
bustion without limestone could not be monitored exten-sively due to malfunctioning of SO
3analyzer
47 SO3Emissions with Limestone under Staged Combustion
The concentration of SO3emissions at 15ms and 20 excess
air was 170 ppm which decreased to 55 ppm in the presenceof limestone The SO
3emissions at 7030 staged (15ms
850∘C) combustion (without limestone) were similar to thoseof unstaged combustion (without limestone) However it wasobserved that in the presence of limestone staged combus-tion results in a higher reduction of SO
3than unstaged
Figure 6 gives the SO3emissions as a function of PACR The
emissions at 15 secondary air were 15 ppm and increased to72 ppm at 45 secondary air It is clear that SO
3is depressed
on the addition of limestone during both the unstaged andstaged operations and the extent of reduction was higherunder staged combustion
Figure 6 shows that the maximum removal of SO3occurs
at a lower staging levels of 8515 and as the bed becomesmore substoichiometric the rate of SO
3removal decreases
This trend indicates the formation of SO3in the freeboard
which bypasses the limestone and appears in the flue Thisincrease in SO
3reduction with the in-bed air ratio is in
agreement with Barnes [6] findingsThe results of the staged combustion test with Daw Mill
coal in the presence of Penrith limestone indicate that SO3
emissions varied little with changes in excess air Howeverif excess air is coupled with fluidizing velocity then it hadsome effects on the emissions At higher velocity of 2ms thechange was up to 4 ppmThe concentration of SO
3emissions
at 15ms and 30 excess air was 20 ppm which decreased to
8
6
4
2
006 08 1 12 14 16
20 excess air
SO3
emiss
ion
(ppm
)
Primary air to coal ratio
Figure 6 Influence of PACR on SO3emission at 15ms fluidizing
velocity and coarse sand
5
45
4
35
3
25
20 5 10 15 20 25 30 35
SO3
emiss
ion
(ppm
)
Secondary air ()
Figure 7 Influence of staging SO3emission in the flue secondary
air injection height 100 cm above the distributor excess air 30
8 ppm under staged combustion in the presence of limestoneThe results of Daw Mill coal test are shown in Figure 7
It should be noted that there is no published work onSO3emissions under staged combustion conditions with or
without limestone on any scale Therefore the results of thisstudy could not be compared
5 Conclusion
The experimental data shows that during unstaged com-bustion without limestone SO
3emissions are dependent
on oxygen and SO2concentration SO
3emissions increase
slightly with excess air reaching a limiting value and thenslowly decrease SO
3emissions are less sensitive to change
in bed temperature However the fluidizing velocity and bedheight affect the emissions
In the presence of limestone SO3emissions are reduced
during both staged and unstaged operations and the reduc-tion is temperature sensitive However during staged com-bustion the reduction is enhanced As staged fluidized-bed combustion is a proven technique to reduce NO
119909and
SO2emissions therefore it should be possible to operate a
fluidized-bed combustor under a stagedmodewith limestoneto keep SO
2 SO3 and NO
119909emissions to a minimum
ISRN Environmental Chemistry 7
References
[1] Y Mitsui N Imada H Kikkawa and A Katagawa ldquoStudyof Hg and SO
3behavior in flue gas of oxy-fuel combustion
systemrdquo International Journal of Greenhouse Gas Control vol5 pp S143ndashS150 2011
[2] L Zheng and E Furimsky ldquoAssessment of coal combustion inO2+CO2by equilibrium calculationsrdquo Fuel Processing Technol-
ogy vol 81 no 1 pp 23ndash34 2003[3] N A Burdett R C Hotchkiss and R B Fields ldquoSO
3formation
and retention in coal fired fluidized bed combustorsrdquo AIChESymposium Series vol 57 pp M1ndashM11 1979
[4] J S Dennis and A N Hayhurst ldquoThe formation of SO3in a
fluidized bedrdquo Combustion and Flame vol 72 no 3 pp 241ndash258 1988
[5] P T Willium and B M Gibbs ldquoThe emissions of SO2and
SO3from fluidized bedsrdquo in Proceedings of the 4th International
Conference on Fluidization pp 443ndash450 Kashikojima Japan1983
[6] J P Barnes Abatement of nitric oxide emission from a coal-fixedfluidized bed combustor [PhD thesis] Department of Fuel andEnergy University of Leeds Leeds UK 1988
[7] J Ahn R Okerlund A Fryb and E G Eddingsa ldquoSulfurtrioxide formation during oxy-coal combustionrdquo InternationalJournal of Greenhouse Gas Control vol 5 pp S127ndashS135 2011
[8] L Hindiyarti P Glarborg and P Marshall ldquoReactions of SO3
with the OH radical pool under combustion conditionsrdquoJournal of Physical Chemistry A vol 111 no 19 pp 3984ndash39912007
[9] R Stanger and T Wall ldquoSulphur impacts during pulverisedcoal combustion in oxy-fuel technology for carbon capture andstoragerdquo Progress in Energy and Combustion Science vol 37 no1 pp 69ndash88 2011
[10] N A Burdett R C Hotchkiss R P Hensel and R B FieldsldquoCoal devolatilization and emission of SO
3in a fluidized bed
combustionrdquo in Proceedings of the Fluidized Combustion Con-ference vol 2 pp 424ndash442 Capetown South Africa January1981
[11] N A Burdett ldquoThe mechanism of the sulphation of limestoneduring fluidized bed desulphurizationrdquo in Proceedings of theInstitute of Energy Symposium Series vol 4 London UK 1980
[12] R B Fieldes N A Burdett J F Davidson and J F T ldquoReactionof sulphur dioxide with limestone particles the influence ofsulphur trioxiderdquo Transactions of the Institution of ChemicalEngineers vol 57 no 4 pp 276ndash280 1979
[13] P J Jackson D A Hilton and J H Buddery ldquoContinuousmeasurements of sulphuric acid vapour in combustion gasesusing a portable automatic monitorrdquo Journal of the Institute ofEnergy vol 54 pp 124ndash135 1981
[14] R C Hotchkiss P J Jackson and D A Hilton ldquoA portableautomatic monitor for continuously measuring sulphuric acidvapor in combustion gasesrdquo in Proceedings of the Symposium onInstrumentation and Control for Fossil-Energy Processes ANL-81-62 Conf 810607 Paper A026 San Francisco Calif USA1981
[15] W Z Khan and B M Gibbs ldquoThe influence of air staging in thereduction of SO
2by limestone in a fluidized bed combustorrdquo
Fuel vol 74 no 6 pp 800ndash805 1995[16] W Z Khan and BMGibbs ldquoAn approach to estimate the depth
of oxidizing and reducing regions in a fluidized bed combustorwith staged combustionrdquo Fuel vol 75 no 7 pp 899ndash906 1996
[17] A Yilmaz L Hindiyarti A D Jensen P Glarborg and PMarshall ldquoThermal dissociation of SO
3at 1000ndash1400Krdquo Journal
of Physical Chemistry A vol 110 no 21 pp 6654ndash6659 2006[18] J D Thibault F R Steward and D M Ruthve ldquoThe kinetics
of absorption of SO3in calcium and magnesium oxidesrdquo The
Canadian Journal of Chemical Engineering vol 60 pp 796ndash8011982
[19] N A Burdett ldquoThe inhibition of the limestone sulphation pro-cess during fluidized bed combustionmdashA theoretical approachrdquoJournal of the Institute of Energy pp 198ndash208 1983
[20] E L Merryman and A Levy ldquoEnhanced SO3emissions from
staged combustionrdquo Symposium (International) on Combustionvol 17 no 1 pp 727ndash736 1979
Figure 3 Influence of limestone addition on SO3reduction during
unstaged combustion 25 cmheight 10ms fluidizing velocity 850∘Cbed temperature and 12 cm limestone addition height
322824201612
840
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
SO3
redu
ctio
n (
)
CaS molar ratio
40 excess air60 excess air80 excess air
Figure 4 Influence of limestone addition on SO3reduction during
unstaged combustion 25 cmheight 10ms fluidizing velocity 850∘Cbed temperature and 42 cm limestone addition height as well ascoarse sand
at 800∘C the reductions reached 70 of the SO2reduction
level The results corresponding to the operating conditionsare shown graphically in Figures 3 and 4 It should be notedthat the SO
3reduction shown in Figure 3 was obtained
when limestone was injected 12 cm above the distributor andFigure 4 represents the results when limestone was injected42 cm above the distributor
At a temperature of 850∘C some of the SO2will always be
converted to SO3via reaction (6) The conversion decreases
with oxygen concentration and increases with sulfur dioxideconcentration An increase in temperature enhances the rateof SO3formation SO
3can react with CaO to formCaSO
4via
reaction (7) The rate of this reaction is temperature depen-dent Yilmaz et al [17] studied the thermal dissociation of SO
3
in the range of 800ndash1200∘C under atmospheric pressure Atthe location in the flame where the net SO
3formation rate is
zero he determined a rate constant of 69times 1010 cm3molminus1 sminus1
2422201816141210
86420
0 2 4 6 8 10 12 14 16 18
SO3
redu
ctio
n (
)
CaS molar ratio
Limestone injection height 42 cm excess air 32Limestone injection height 42 cm excess air 60
Figure 5 Influence of limestone addition on SO3level in the flue
under staged combustion
for SO3+ N2rarr SO
3+ O + N
2 that was consistent with
other flame results A high temperature lowers the reactionrate Therefore at a high temperature more SO
3is produced
but less will be consumed in sulphation As a result a largerdecrease in SO
3emissions is observed at lower temperature
The effect of excess air can also be seen in these graphsSO3emissions have been found to increase with excess air
but upon removing the dilution effect the increase is withina narrow range indicating that there could be an optimumreduction at a particular excess air beyond which the SO
3
reduction decreases An increase in the fluidizing velocity haslittle effect on the overall reduction of SO
3emissions
During another set of experiments Penrith limestonewas added to the Daw Mill coal It was observed that SO
3
emissions were decreased in the presence of limestone andthe reduction was temperature dependent At the highertemperature of 850∘C the SO
3reductions were 18ndash20 of
the SO2reduction but at 800∘C the SO
3reductions reached
55 of the SO2reduction level Figure 5 shows the results of
this set
44 Comparison to Reported Work (Conducted on Microbal-ance or Small Bed of 36ndash78mm ID) Burdett et al [3] havereported that the reaction between limestone and sulfuroxides is highly sensitive to changes in O
2 SO and SO
3
concentrations Absorption of SO3by the coal ash cannot be
quantified on themicrobalance and themicrobalance resultsare not applicable to fluidized combustor
Fieldes et al [12] have reported that extent of SO2oxida-
tion to SO3varied with SO
2and O
2concentration The coal
combustion test showed that the lower SO3concentrations
are due to its selective removal by ash They had tested avariety of limestone and in all the cases the mole fraction ofCaO converted to CaSO
4was affected by inlet oxygen in the
same way as Penrith limestoneThibault et al [18] have conducted experiments on a
small (6mm) fixed bed packed with CaO particle They havetested two grain size of the sorbent and reported that forefficient capture of SO
3a small grain size and openmacropore
structure are essential
6 ISRN Environmental Chemistry
45 Comparison to ReportedWork (Conducted on Pilot Scale)Burdett et al [19] have reported fractional conversion of SO
2
to SO3decreased from about 15 in the limestone-free case
to around 035 when the limestone and alkaline ash werepresent which was due to the greater reaction of SO
3with
limestone compared with ash the absorption occurring bothin the bed itself and in the freeboard
Burdett et al [10] have reported that combustion of a 3sulfur coal in a bed burning at 900∘C generated 33 vpm ofSO3and proposed that the effect of O
2on sulphation capacity
results from the formation of SO3within the pores of the
stone
46 SO3Emissions without Limestone under Staged Combus-
tion Merryman and Levy [20] have conducted staged exper-iments on a quartz tube methane burner producing stablemethane-H
2S flame within desired fuel-air ratio without a
sorbent presence under staged combustion conditions Theyhave reported that when the remaining excess air was injectedinto these gases the maximum amount of SO
3formed was
greater than formed when this additional air was includedwith the initial combustion air the overall excess of airbeing the same in both cases The experimental conditionsof Merryman and Levy do not match with our fluidized bedtherefore their results are not comparable with this study
During this study the SO3emissions under staged com-
bustion without limestone could not be monitored exten-sively due to malfunctioning of SO
3analyzer
47 SO3Emissions with Limestone under Staged Combustion
The concentration of SO3emissions at 15ms and 20 excess
air was 170 ppm which decreased to 55 ppm in the presenceof limestone The SO
3emissions at 7030 staged (15ms
850∘C) combustion (without limestone) were similar to thoseof unstaged combustion (without limestone) However it wasobserved that in the presence of limestone staged combus-tion results in a higher reduction of SO
3than unstaged
Figure 6 gives the SO3emissions as a function of PACR The
emissions at 15 secondary air were 15 ppm and increased to72 ppm at 45 secondary air It is clear that SO
3is depressed
on the addition of limestone during both the unstaged andstaged operations and the extent of reduction was higherunder staged combustion
Figure 6 shows that the maximum removal of SO3occurs
at a lower staging levels of 8515 and as the bed becomesmore substoichiometric the rate of SO
3removal decreases
This trend indicates the formation of SO3in the freeboard
which bypasses the limestone and appears in the flue Thisincrease in SO
3reduction with the in-bed air ratio is in
agreement with Barnes [6] findingsThe results of the staged combustion test with Daw Mill
coal in the presence of Penrith limestone indicate that SO3
emissions varied little with changes in excess air Howeverif excess air is coupled with fluidizing velocity then it hadsome effects on the emissions At higher velocity of 2ms thechange was up to 4 ppmThe concentration of SO
3emissions
at 15ms and 30 excess air was 20 ppm which decreased to
8
6
4
2
006 08 1 12 14 16
20 excess air
SO3
emiss
ion
(ppm
)
Primary air to coal ratio
Figure 6 Influence of PACR on SO3emission at 15ms fluidizing
velocity and coarse sand
5
45
4
35
3
25
20 5 10 15 20 25 30 35
SO3
emiss
ion
(ppm
)
Secondary air ()
Figure 7 Influence of staging SO3emission in the flue secondary
air injection height 100 cm above the distributor excess air 30
8 ppm under staged combustion in the presence of limestoneThe results of Daw Mill coal test are shown in Figure 7
It should be noted that there is no published work onSO3emissions under staged combustion conditions with or
without limestone on any scale Therefore the results of thisstudy could not be compared
5 Conclusion
The experimental data shows that during unstaged com-bustion without limestone SO
3emissions are dependent
on oxygen and SO2concentration SO
3emissions increase
slightly with excess air reaching a limiting value and thenslowly decrease SO
3emissions are less sensitive to change
in bed temperature However the fluidizing velocity and bedheight affect the emissions
In the presence of limestone SO3emissions are reduced
during both staged and unstaged operations and the reduc-tion is temperature sensitive However during staged com-bustion the reduction is enhanced As staged fluidized-bed combustion is a proven technique to reduce NO
119909and
SO2emissions therefore it should be possible to operate a
fluidized-bed combustor under a stagedmodewith limestoneto keep SO
2 SO3 and NO
119909emissions to a minimum
ISRN Environmental Chemistry 7
References
[1] Y Mitsui N Imada H Kikkawa and A Katagawa ldquoStudyof Hg and SO
3behavior in flue gas of oxy-fuel combustion
systemrdquo International Journal of Greenhouse Gas Control vol5 pp S143ndashS150 2011
[2] L Zheng and E Furimsky ldquoAssessment of coal combustion inO2+CO2by equilibrium calculationsrdquo Fuel Processing Technol-
ogy vol 81 no 1 pp 23ndash34 2003[3] N A Burdett R C Hotchkiss and R B Fields ldquoSO
3formation
and retention in coal fired fluidized bed combustorsrdquo AIChESymposium Series vol 57 pp M1ndashM11 1979
[4] J S Dennis and A N Hayhurst ldquoThe formation of SO3in a
fluidized bedrdquo Combustion and Flame vol 72 no 3 pp 241ndash258 1988
[5] P T Willium and B M Gibbs ldquoThe emissions of SO2and
SO3from fluidized bedsrdquo in Proceedings of the 4th International
Conference on Fluidization pp 443ndash450 Kashikojima Japan1983
[6] J P Barnes Abatement of nitric oxide emission from a coal-fixedfluidized bed combustor [PhD thesis] Department of Fuel andEnergy University of Leeds Leeds UK 1988
[7] J Ahn R Okerlund A Fryb and E G Eddingsa ldquoSulfurtrioxide formation during oxy-coal combustionrdquo InternationalJournal of Greenhouse Gas Control vol 5 pp S127ndashS135 2011
[8] L Hindiyarti P Glarborg and P Marshall ldquoReactions of SO3
with the OH radical pool under combustion conditionsrdquoJournal of Physical Chemistry A vol 111 no 19 pp 3984ndash39912007
[9] R Stanger and T Wall ldquoSulphur impacts during pulverisedcoal combustion in oxy-fuel technology for carbon capture andstoragerdquo Progress in Energy and Combustion Science vol 37 no1 pp 69ndash88 2011
[10] N A Burdett R C Hotchkiss R P Hensel and R B FieldsldquoCoal devolatilization and emission of SO
3in a fluidized bed
combustionrdquo in Proceedings of the Fluidized Combustion Con-ference vol 2 pp 424ndash442 Capetown South Africa January1981
[11] N A Burdett ldquoThe mechanism of the sulphation of limestoneduring fluidized bed desulphurizationrdquo in Proceedings of theInstitute of Energy Symposium Series vol 4 London UK 1980
[12] R B Fieldes N A Burdett J F Davidson and J F T ldquoReactionof sulphur dioxide with limestone particles the influence ofsulphur trioxiderdquo Transactions of the Institution of ChemicalEngineers vol 57 no 4 pp 276ndash280 1979
[13] P J Jackson D A Hilton and J H Buddery ldquoContinuousmeasurements of sulphuric acid vapour in combustion gasesusing a portable automatic monitorrdquo Journal of the Institute ofEnergy vol 54 pp 124ndash135 1981
[14] R C Hotchkiss P J Jackson and D A Hilton ldquoA portableautomatic monitor for continuously measuring sulphuric acidvapor in combustion gasesrdquo in Proceedings of the Symposium onInstrumentation and Control for Fossil-Energy Processes ANL-81-62 Conf 810607 Paper A026 San Francisco Calif USA1981
[15] W Z Khan and B M Gibbs ldquoThe influence of air staging in thereduction of SO
2by limestone in a fluidized bed combustorrdquo
Fuel vol 74 no 6 pp 800ndash805 1995[16] W Z Khan and BMGibbs ldquoAn approach to estimate the depth
of oxidizing and reducing regions in a fluidized bed combustorwith staged combustionrdquo Fuel vol 75 no 7 pp 899ndash906 1996
[17] A Yilmaz L Hindiyarti A D Jensen P Glarborg and PMarshall ldquoThermal dissociation of SO
3at 1000ndash1400Krdquo Journal
of Physical Chemistry A vol 110 no 21 pp 6654ndash6659 2006[18] J D Thibault F R Steward and D M Ruthve ldquoThe kinetics
of absorption of SO3in calcium and magnesium oxidesrdquo The
Canadian Journal of Chemical Engineering vol 60 pp 796ndash8011982
[19] N A Burdett ldquoThe inhibition of the limestone sulphation pro-cess during fluidized bed combustionmdashA theoretical approachrdquoJournal of the Institute of Energy pp 198ndash208 1983
[20] E L Merryman and A Levy ldquoEnhanced SO3emissions from
staged combustionrdquo Symposium (International) on Combustionvol 17 no 1 pp 727ndash736 1979
45 Comparison to ReportedWork (Conducted on Pilot Scale)Burdett et al [19] have reported fractional conversion of SO
2
to SO3decreased from about 15 in the limestone-free case
to around 035 when the limestone and alkaline ash werepresent which was due to the greater reaction of SO
3with
limestone compared with ash the absorption occurring bothin the bed itself and in the freeboard
Burdett et al [10] have reported that combustion of a 3sulfur coal in a bed burning at 900∘C generated 33 vpm ofSO3and proposed that the effect of O
2on sulphation capacity
results from the formation of SO3within the pores of the
stone
46 SO3Emissions without Limestone under Staged Combus-
tion Merryman and Levy [20] have conducted staged exper-iments on a quartz tube methane burner producing stablemethane-H
2S flame within desired fuel-air ratio without a
sorbent presence under staged combustion conditions Theyhave reported that when the remaining excess air was injectedinto these gases the maximum amount of SO
3formed was
greater than formed when this additional air was includedwith the initial combustion air the overall excess of airbeing the same in both cases The experimental conditionsof Merryman and Levy do not match with our fluidized bedtherefore their results are not comparable with this study
During this study the SO3emissions under staged com-
bustion without limestone could not be monitored exten-sively due to malfunctioning of SO
3analyzer
47 SO3Emissions with Limestone under Staged Combustion
The concentration of SO3emissions at 15ms and 20 excess
air was 170 ppm which decreased to 55 ppm in the presenceof limestone The SO
3emissions at 7030 staged (15ms
850∘C) combustion (without limestone) were similar to thoseof unstaged combustion (without limestone) However it wasobserved that in the presence of limestone staged combus-tion results in a higher reduction of SO
3than unstaged
Figure 6 gives the SO3emissions as a function of PACR The
emissions at 15 secondary air were 15 ppm and increased to72 ppm at 45 secondary air It is clear that SO
3is depressed
on the addition of limestone during both the unstaged andstaged operations and the extent of reduction was higherunder staged combustion
Figure 6 shows that the maximum removal of SO3occurs
at a lower staging levels of 8515 and as the bed becomesmore substoichiometric the rate of SO
3removal decreases
This trend indicates the formation of SO3in the freeboard
which bypasses the limestone and appears in the flue Thisincrease in SO
3reduction with the in-bed air ratio is in
agreement with Barnes [6] findingsThe results of the staged combustion test with Daw Mill
coal in the presence of Penrith limestone indicate that SO3
emissions varied little with changes in excess air Howeverif excess air is coupled with fluidizing velocity then it hadsome effects on the emissions At higher velocity of 2ms thechange was up to 4 ppmThe concentration of SO
3emissions
at 15ms and 30 excess air was 20 ppm which decreased to
8
6
4
2
006 08 1 12 14 16
20 excess air
SO3
emiss
ion
(ppm
)
Primary air to coal ratio
Figure 6 Influence of PACR on SO3emission at 15ms fluidizing
velocity and coarse sand
5
45
4
35
3
25
20 5 10 15 20 25 30 35
SO3
emiss
ion
(ppm
)
Secondary air ()
Figure 7 Influence of staging SO3emission in the flue secondary
air injection height 100 cm above the distributor excess air 30
8 ppm under staged combustion in the presence of limestoneThe results of Daw Mill coal test are shown in Figure 7
It should be noted that there is no published work onSO3emissions under staged combustion conditions with or
without limestone on any scale Therefore the results of thisstudy could not be compared
5 Conclusion
The experimental data shows that during unstaged com-bustion without limestone SO
3emissions are dependent
on oxygen and SO2concentration SO
3emissions increase
slightly with excess air reaching a limiting value and thenslowly decrease SO
3emissions are less sensitive to change
in bed temperature However the fluidizing velocity and bedheight affect the emissions
In the presence of limestone SO3emissions are reduced
during both staged and unstaged operations and the reduc-tion is temperature sensitive However during staged com-bustion the reduction is enhanced As staged fluidized-bed combustion is a proven technique to reduce NO
119909and
SO2emissions therefore it should be possible to operate a
fluidized-bed combustor under a stagedmodewith limestoneto keep SO
2 SO3 and NO
119909emissions to a minimum
ISRN Environmental Chemistry 7
References
[1] Y Mitsui N Imada H Kikkawa and A Katagawa ldquoStudyof Hg and SO
3behavior in flue gas of oxy-fuel combustion
systemrdquo International Journal of Greenhouse Gas Control vol5 pp S143ndashS150 2011
[2] L Zheng and E Furimsky ldquoAssessment of coal combustion inO2+CO2by equilibrium calculationsrdquo Fuel Processing Technol-
ogy vol 81 no 1 pp 23ndash34 2003[3] N A Burdett R C Hotchkiss and R B Fields ldquoSO
3formation
and retention in coal fired fluidized bed combustorsrdquo AIChESymposium Series vol 57 pp M1ndashM11 1979
[4] J S Dennis and A N Hayhurst ldquoThe formation of SO3in a
fluidized bedrdquo Combustion and Flame vol 72 no 3 pp 241ndash258 1988
[5] P T Willium and B M Gibbs ldquoThe emissions of SO2and
SO3from fluidized bedsrdquo in Proceedings of the 4th International
Conference on Fluidization pp 443ndash450 Kashikojima Japan1983
[6] J P Barnes Abatement of nitric oxide emission from a coal-fixedfluidized bed combustor [PhD thesis] Department of Fuel andEnergy University of Leeds Leeds UK 1988
[7] J Ahn R Okerlund A Fryb and E G Eddingsa ldquoSulfurtrioxide formation during oxy-coal combustionrdquo InternationalJournal of Greenhouse Gas Control vol 5 pp S127ndashS135 2011
[8] L Hindiyarti P Glarborg and P Marshall ldquoReactions of SO3
with the OH radical pool under combustion conditionsrdquoJournal of Physical Chemistry A vol 111 no 19 pp 3984ndash39912007
[9] R Stanger and T Wall ldquoSulphur impacts during pulverisedcoal combustion in oxy-fuel technology for carbon capture andstoragerdquo Progress in Energy and Combustion Science vol 37 no1 pp 69ndash88 2011
[10] N A Burdett R C Hotchkiss R P Hensel and R B FieldsldquoCoal devolatilization and emission of SO
3in a fluidized bed
combustionrdquo in Proceedings of the Fluidized Combustion Con-ference vol 2 pp 424ndash442 Capetown South Africa January1981
[11] N A Burdett ldquoThe mechanism of the sulphation of limestoneduring fluidized bed desulphurizationrdquo in Proceedings of theInstitute of Energy Symposium Series vol 4 London UK 1980
[12] R B Fieldes N A Burdett J F Davidson and J F T ldquoReactionof sulphur dioxide with limestone particles the influence ofsulphur trioxiderdquo Transactions of the Institution of ChemicalEngineers vol 57 no 4 pp 276ndash280 1979
[13] P J Jackson D A Hilton and J H Buddery ldquoContinuousmeasurements of sulphuric acid vapour in combustion gasesusing a portable automatic monitorrdquo Journal of the Institute ofEnergy vol 54 pp 124ndash135 1981
[14] R C Hotchkiss P J Jackson and D A Hilton ldquoA portableautomatic monitor for continuously measuring sulphuric acidvapor in combustion gasesrdquo in Proceedings of the Symposium onInstrumentation and Control for Fossil-Energy Processes ANL-81-62 Conf 810607 Paper A026 San Francisco Calif USA1981
[15] W Z Khan and B M Gibbs ldquoThe influence of air staging in thereduction of SO
2by limestone in a fluidized bed combustorrdquo
Fuel vol 74 no 6 pp 800ndash805 1995[16] W Z Khan and BMGibbs ldquoAn approach to estimate the depth
of oxidizing and reducing regions in a fluidized bed combustorwith staged combustionrdquo Fuel vol 75 no 7 pp 899ndash906 1996
[17] A Yilmaz L Hindiyarti A D Jensen P Glarborg and PMarshall ldquoThermal dissociation of SO
3at 1000ndash1400Krdquo Journal
of Physical Chemistry A vol 110 no 21 pp 6654ndash6659 2006[18] J D Thibault F R Steward and D M Ruthve ldquoThe kinetics
of absorption of SO3in calcium and magnesium oxidesrdquo The
Canadian Journal of Chemical Engineering vol 60 pp 796ndash8011982
[19] N A Burdett ldquoThe inhibition of the limestone sulphation pro-cess during fluidized bed combustionmdashA theoretical approachrdquoJournal of the Institute of Energy pp 198ndash208 1983
[20] E L Merryman and A Levy ldquoEnhanced SO3emissions from
staged combustionrdquo Symposium (International) on Combustionvol 17 no 1 pp 727ndash736 1979
[1] Y Mitsui N Imada H Kikkawa and A Katagawa ldquoStudyof Hg and SO
3behavior in flue gas of oxy-fuel combustion
systemrdquo International Journal of Greenhouse Gas Control vol5 pp S143ndashS150 2011
[2] L Zheng and E Furimsky ldquoAssessment of coal combustion inO2+CO2by equilibrium calculationsrdquo Fuel Processing Technol-
ogy vol 81 no 1 pp 23ndash34 2003[3] N A Burdett R C Hotchkiss and R B Fields ldquoSO
3formation
and retention in coal fired fluidized bed combustorsrdquo AIChESymposium Series vol 57 pp M1ndashM11 1979
[4] J S Dennis and A N Hayhurst ldquoThe formation of SO3in a
fluidized bedrdquo Combustion and Flame vol 72 no 3 pp 241ndash258 1988
[5] P T Willium and B M Gibbs ldquoThe emissions of SO2and
SO3from fluidized bedsrdquo in Proceedings of the 4th International
Conference on Fluidization pp 443ndash450 Kashikojima Japan1983
[6] J P Barnes Abatement of nitric oxide emission from a coal-fixedfluidized bed combustor [PhD thesis] Department of Fuel andEnergy University of Leeds Leeds UK 1988
[7] J Ahn R Okerlund A Fryb and E G Eddingsa ldquoSulfurtrioxide formation during oxy-coal combustionrdquo InternationalJournal of Greenhouse Gas Control vol 5 pp S127ndashS135 2011
[8] L Hindiyarti P Glarborg and P Marshall ldquoReactions of SO3
with the OH radical pool under combustion conditionsrdquoJournal of Physical Chemistry A vol 111 no 19 pp 3984ndash39912007
[9] R Stanger and T Wall ldquoSulphur impacts during pulverisedcoal combustion in oxy-fuel technology for carbon capture andstoragerdquo Progress in Energy and Combustion Science vol 37 no1 pp 69ndash88 2011
[10] N A Burdett R C Hotchkiss R P Hensel and R B FieldsldquoCoal devolatilization and emission of SO
3in a fluidized bed
combustionrdquo in Proceedings of the Fluidized Combustion Con-ference vol 2 pp 424ndash442 Capetown South Africa January1981
[11] N A Burdett ldquoThe mechanism of the sulphation of limestoneduring fluidized bed desulphurizationrdquo in Proceedings of theInstitute of Energy Symposium Series vol 4 London UK 1980
[12] R B Fieldes N A Burdett J F Davidson and J F T ldquoReactionof sulphur dioxide with limestone particles the influence ofsulphur trioxiderdquo Transactions of the Institution of ChemicalEngineers vol 57 no 4 pp 276ndash280 1979
[13] P J Jackson D A Hilton and J H Buddery ldquoContinuousmeasurements of sulphuric acid vapour in combustion gasesusing a portable automatic monitorrdquo Journal of the Institute ofEnergy vol 54 pp 124ndash135 1981
[14] R C Hotchkiss P J Jackson and D A Hilton ldquoA portableautomatic monitor for continuously measuring sulphuric acidvapor in combustion gasesrdquo in Proceedings of the Symposium onInstrumentation and Control for Fossil-Energy Processes ANL-81-62 Conf 810607 Paper A026 San Francisco Calif USA1981
[15] W Z Khan and B M Gibbs ldquoThe influence of air staging in thereduction of SO
2by limestone in a fluidized bed combustorrdquo
Fuel vol 74 no 6 pp 800ndash805 1995[16] W Z Khan and BMGibbs ldquoAn approach to estimate the depth
of oxidizing and reducing regions in a fluidized bed combustorwith staged combustionrdquo Fuel vol 75 no 7 pp 899ndash906 1996
[17] A Yilmaz L Hindiyarti A D Jensen P Glarborg and PMarshall ldquoThermal dissociation of SO
3at 1000ndash1400Krdquo Journal
of Physical Chemistry A vol 110 no 21 pp 6654ndash6659 2006[18] J D Thibault F R Steward and D M Ruthve ldquoThe kinetics
of absorption of SO3in calcium and magnesium oxidesrdquo The
Canadian Journal of Chemical Engineering vol 60 pp 796ndash8011982
[19] N A Burdett ldquoThe inhibition of the limestone sulphation pro-cess during fluidized bed combustionmdashA theoretical approachrdquoJournal of the Institute of Energy pp 198ndash208 1983
[20] E L Merryman and A Levy ldquoEnhanced SO3emissions from
staged combustionrdquo Symposium (International) on Combustionvol 17 no 1 pp 727ndash736 1979
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![Page 3: Emissions of SO from a Coal-Fired Fluidized Bed under Normal …downloads.hindawi.com/journals/isrn.environmental... · 2014-03-26 · Fieldes et al. [12] reported the achievement](https://reader034.documents.pub/reader034/viewer/2022043019/5f3b1a240e3ab800d01a7a4d/html5/thumbnails/3.jpg)
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