018525 REMOVALS Reduction, modification and valorisation of sludge SPECIFIC TARGETED RESEARCH OR INNOVATION PROJECT PRIORITY 1.1.6.3 Global change and ecosystems, D6.1 Influence of pyrolysis extent on synthesis gas composition and yield Due date of report: 31 December 2007 Actual submission date: 20 March 2008 Revision 2 actual submission date: 30 June 2009 Start date of project: 1 JUL 2006 Duration: 3 YEARS Organisation name of lead constructor for this deliverable: TECHNICAL UNIVERSITY OF LODZ Revision 2 Project co-funded by the European Commission within the Sixth Framework Programme (2002-2006) Dissemination Level PU Public PP Restricted to other programme participants (including the Commission Services) RE Restricted to a group specified by the consortium (including the Commission Services) CO Confidential, only for members of the consortium (including the Commission Services) x
Transcript
018525
REMOVALS
Reduction, modification and valorisation of sludge SPECIFIC TARGETED RESEARCH OR INNOVATION PROJECT PRIORITY 1.1.6.3 Global change and ecosystems,
D6.1 Influence of pyrolysis extent on synthesis gas composition and yield
Due date of report: 31 December 2007 Actual submission date: 20 March 2008
Revision 2 actual submission date: 30 June 2009 Start date of project: 1 JUL 2006 Duration: 3 YEARS Organisation name of lead constructor for this deliverable: TECHNICAL UNIVERSITY OF LODZ Revision 2
Project co-funded by the European Commission within the Sixth Framework Programme (2002-2006) Dissemination Level
PU Public PP Restricted to other programme participants (including the Commission Services) RE Restricted to a group specified by the consortium (including the Commission Services) CO Confidential, only for members of the consortium (including the Commission Services) x
1. INTRODUCTION 3
2. EXPERIMENTAL EQUIPMENT AND CALIBRATION 3
3. INFLUENCE OF SEWAGE SLUDGE ORIGIN AND COMPOSITION ON PYROLYSIS PROCESS 6
3.1. TG analysis 7 3.2. MS-TG analysis 8 3.3. Elemental analysis and balance of elements 11
4. COMPARISON OF THERMAL DECOMPOSITION OF SEWAGE SLUDGE IN ARGON AND CO2 ATMOSPHERE 13
5. EFFECT OF PYROLYSIS TEMPERATURE ON CHAR YIELD 13
6. INFLUENCE OF HEATING RATE ON PYROLYSIS EXTENT 17
7. RATE OF CHAR GASIFICATION IN CO2 18
8. INFLUENCE OF CATALYTIC ADDITIVES ON PYROLYSIS EXTENT 20
8.1. CaO additive 20 8.2. Dolomite additive 23
9. CONCLUSIONS 25
REFERENCES 25
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1. INTRODUCTION
The pyrolysis and gasification of sewage sludge are currently being investigated as an alternative to the problem of sewage sludge disposal. These two processes present certain advantages over the most widely used combustion or co-combustion technologies. The flue gas volume decreases significantly when changing from combustion to gasification [1] and the product of gasification - synthesis gas, is a useful raw material for chemical industries apart from being used as clean fuel.
Pyrolysis is the process of thermal decomposition of organic substances in an inert (oxygen free) atmosphere, at temperatures below 800°C. During the pyrolysis sewage sludge undergoes a sequence of physical and chemical steps, starting with the drying at about 100°C. The major products that are formed during the thermal degradation of the sludge in an inert atmosphere are the following: • non-condensable gas (at normal conditions) containing mainly carbon monoxide, carbon
dioxide, hydrogen, methane and several other hydrocarbons at smaller concentrations, • tars and/or oil (liquid fraction), • carbonaceous material (char) consisting mainly of carbon and minerals.
In gasification process the primary pyrolysis products, condensable vapours and non-condensable gases and mainly char undergo secondary reactions with gasification reagent forming additional amounts of synthesis gas. In fact, pyrolysis and gasification are two separate or partly overlapping stages (depending on the gasifying agent used) because the conversion of char occurs at higher temperatures when most of thermal decomposition processes are finished. It is well known that gasification is influenced by the reactivity of chars [2] which depends on process conditions during pyrolysis of the thermo-chemical conversion. This problem was extensively studied for coal and biomass gasification [3,4]. However, only limited knowledge of the effect of sewage sludge pyrolysis conditions on the yield, composition and reactivity of char is available in literature [5].
The aim of the present work was to extend knowledge on the influence of pyrolysis conditions (temperature, heating rate and addition of catalysts) on the yield of gas, liquid and especially solid fraction (char). Sewage sludges from different wastewater treatment plants in Poland, Spain and Czech Republic were used. The char reactivity in the subsequent gasification by CO2 was also measured and compared.
2. EXPERIMENTAL EQUIPMENT AND CALIBRATION
In order to carry out studies on pyrolysis and gasification processes the experimental set-up shown in Fig. 1 was assembled. The Mettler-Toledo TGA/SDTA851 LF mg) thermobalance (temperature range 20÷1100°C, sample mass up to 5000 was coupled with a QMS 200 Balzers ThermoStar Mass Spectrometer and a Varian CP-3800 gas chromatograph equipped with a thermostated micro-valve for gas sampling. The gas supplying system consisting of three calibrated mass flow meters (Brooks model 4850S) was set to control the feed gas flow rate and composition. The content of C, H, N, S and O in the raw samples of sewage sludge and in the samples at different stages of the process was determined by a CE Instruments NA 2500 elementary analyser.
The TG-MS-GC system provides extensive information on thermal decomposition of a solid substrate and the evolved gaseous products. The analytical methods applied result in a complete picture of the ongoing reactions during the process – mass loss (TG), overall reaction rate (DTG), and formation rate of evolved gases (MS-GC).
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TG
MS
THERMOBALANCETHERMOSTAT
REACTIVE GAS
GASEOUSPRODUCTS
MASSSPECTROMETER
MS
TG
INOUT
OUT
IN
SAMPLE
GASSUPPLYING
SYSTEM
OUT
IN
GASCHROMATOGRAPH
GC
Fig. 1. Scheme of the TG-MS-GC system
In order to use results obtained from the TG-MS-GC system for semi-quantitative analysis of reaction products the MS profiles recorded as relative intensity must be converted into the composition of evolved gases and then to formation rate, which can be compared with the rate of a sample mass loss (DTG curve). Two calibration methods of MS signals were proposed:
• External standard – a binary mixture of inert gas (Ar) and CO or CO2 or H2 or CH4 was prepared outside the TG-MS system using precise mass flow meters set on specified flow rates for each component.
• Internal standard – the calibration gas was produced or consumed by chemical reaction occurring in the thermo-balance. On the basis of sample mass loss measured in TG and reaction stoichiometry the molar fraction of produced or consumed gas can be calculated and compared with MS signal.
For external standard method the relationship between gas mole fraction of individual components and relative intensity of MS signals (RI) can be described with high accuracy by the straight lines [6]. The slopes of calibration lines for all investigated components are given in Table 1.
Table 1. The slopes (ax) of calibration lines calculated on the basis of different methods
gas H 2 CH4 H2O CO CO2
m/z 2 16 18 28 44 external standard 1.011 1.358 - 1.003 0.826 internal standard 0.959 - 0.980 1.142 0.840
For the internal standard method the CuO reduction in CO or H2 atmosphere was chosen [7]:
( ) ( ) ( ) ( )gCOsCugCOsCuO 2+→+ (1)
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( ) ( ) ( ) ( )gOHsCugHsCuO 22 +→+ (2)
The amount of evolved (CO2 or H2O) and consumed gases (H2 or CO) can be easily calculated from the mass loss determined from TG measurement and stoichiometric balance.
In Fig. 2 the DTG curves practically overlap with MS signals in the whole range of temperatures, which confirms a linear character of the calibration curve. The mole fraction of calibrated gas on the thermobalance outlet was calculated from the equation:
Ar
ArAr
S
SS
O
CuO
x
M Q
M Q
M m DTG
yρρ
β
+= (3)
where: DTG – differential of TG signal (%/K), mCuO – initial sample mass (mg), β - heating rate (K/min), M – molecular weight (mg/mol), Q – inlet gas flow rate (ml/min), ρ - gas density (mg/ml), subscripts: S = CO or H2, Ar – argon, O – atomic oxygen. Making comparison of calibrated gas mole fraction with relative intensity of m/z signal a slope of the calibration lines coefficient can be calculated (see Table 1 for results).
100 200 300 400
-0,3
-0,2
-0,1
0,0
0,1
0,2
-0,9
-0,6
-0,3
0,0
0,3
0,6
m/z = 28
DTG
, -dm
/dT,
%/K
Temperature, oC
m/z = 44DTG
MS
, relative intensity, RIm
/z=x ,%
(a)
100 200 300 400-0,3
-0,2
-0,1
0,0
0,1
0,2
0,3
-2
-1
0
1
2
m/z=2
MS
, relative intensity, RIm
/z=x , %
DTG
, -dm
/dT,
%/K
Temperature, oC
DTG
m/z=18
(b)
Fig. 2. Comparison of DTG curve and relative intensity of m/z signals for CuO reduction in CO (a) and H2 (b) atmosphere
The slopes of calibration lines calculated on the basis of both methods are similar, except for CO where the difference is higher (about 14%). For further analysis the slope values from internal standard method were accepted as more accurate.
The calibration of MS signals was verified by using gas chromatography. The analyses were carried out on the Refinery Gas System C based on Varian 3800 GC design for samples containing hydrogen, non-condensable gases (nitrogen, carbon monoxide, carbon dioxide, oxygen) and hydrocarbon isomers of C1 through C5. An important feature the GC system used is capability of three concurrently operating detectors (two TCD and FID). An example of GC trace obtained for a standard gas mixture is shown in Figure 3.The GC system was also used for an online analysis of the gas leaving the TG oven during thermal treatment of sewage sludge samples.
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TCD -He TCD -Ar
FID -He
RETENTION TIME, min
Met
hane
Etha
ne
Ethy
lene
Prop
ane
Prop
ylen
e
iso-
But
ane
n-B
utan
e
1-B
uthy
lene
t-2-B
uthy
lene
iso-
But
hyle
nec-
2-B
uthy
lene
H2
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2
2 3 4 5 6 7 8 9 10 11 12 0 1 2 3
CO
N2
Met
hane
ArEthy
lene
Etha
ne
CO
2
0
Fig. 3. Chromatograms of standard gas analysis on VARIAN 3800 GC
3. INFLUENCE OF SEWAGE SLUDGE ORIGIN AND COMPOSITION ON PYROLYSIS PROCESS
Samples used in this study were collected from 8 sources comprising municipal, rural, and industrial wastewater treatment plants located in Poland, Czech Republic and Spain. The list of sludge samples and their origin is given in Table 2.
Before testing in the TG-MS-GC system the samples were dried in the air at 110°C for 12 hours in an oven to achieve moisture content of about 5 wt%, then ground and sieved to obtain a uniform size fraction less than 40 mm, which enabled constant chemical and physical characteristics in the whole volume of the sample.
Table 2. List of samples
Sample Type of WTP Type of sludge
GOS-Lodz (GOS) Poland municipal digested Kostrzyn (KOS) Poland industrial (paper factory) digested Slonsk (SLO) Poland rural raw Constanti (CON) Spain industrial (food industry) digested Reus (REU) Spain municipal digested Prague (PRA) Czech Republic municipal digested Kalisz (KAN) Poland municipal digested Kalisz (KAW) Poland municipal digested, limed
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3.1. TG analysis
In TG experiments the sample weight of approximately 20 mg was used for each experiment under non-isothermal conditions. The sample was heated up from ambient temperature to 1000ºC at the heating rate of 20ºC/min. The experiments were carried out in argon at the flow rate of 50 cm3/min. Then, the sample was cooled to the 900ºC and held at this temperature when the flowing gas composition was changed to a mixture of argon and carbon dioxide with the molar ratio 1 to 1. In these conditions char formed at earlier stages of the process was gasified. The TG and DTG results were used to characterize the pyrolysis behaviour of the sludge samples, to determine their composition (proximate analysis) as well as to provide data for kinetic analysis and modelling. Fig. 4 shows a typical TG curve and the idea of estimation of the sample composition.
0 15 30 45 60 75 900
1040
50
60
70
80
90
100
0
200
400
600
800
1000
Atmosphere:Ar 50%CO2 50%Ar 100% ash
char
volatiles
TG, %
Time, min
moisture
Temperature, oC
Fig. 4. TG curve and proximate analysis of GOS sample
Proximate analysis of sewage sludge composition showed significant differences in volatile matter and ash content owing to a more or less complete digestion process and the original waste water. The lowest ash content was obtained for CON and SLO samples. The CON sample was obtained from a wastewater treatment plant for food industry and SLO was taken from a small rural wastewater treatment plant. The latter was not subjected to any stabilization process. Low ash content means high heating value of the sample, although this parameter was not determined.
Table 3. Proximate analysis of samples
moisture volatiles char ash sample % of initial mass
Figure 5 shows experimental data of sewage sludge pyrolysis in the form of TG and DTG curves. It should be noted that for all samples, the pyrolysis occurred mainly in temperature range of 200-600ºC after drying as indicated by a DTG peak within 180ºC. As can be seen from Table 3,
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the mass loss corresponding to the drying varied from 2% to 7% of the total mass loss. For most samples, the decomposition takes place in three overlapping steps: the first step with the maximum rate at 290-300ºC, the second at 380-400ºC, and the third at about 450ºC.
Above 600°C a slow and gradual mass loss of the sample is observed. The decomposition at such a high temperature is due to the degradation of calcium carbonate and autooxidation of char caused by carbon dioxide and water released from the minerals contained in ash [6,7].
Although small variances occur in DTG characteristics such as peak height and temperature, the pyrolysis behaviour of all samples is very similar. Differences may be attributed to different fractional compositions of samples and their different reactivities.
200 400 600 800 10000,00
0,05
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0,15
0,20
0,25
0,30
0,35
0,40
0 200 400 600 800 100030
40
50
60
70
80
90
100
DTG
, %/o C
Temperature, oC
GOS SLO KOS CON REU PRA KAN KAW
TG, %
Temperature, oC
Fig. 5. TG and DTG profiles of the sewage sludge pyrolysis
3.2. MS-TG analysis
MS analyses were carried out by sampling every 6 seconds the gas phase evolved during pyrolysis. Evolved gases of m/z of less than 100 were monitored and the evolution profiles were synchronized with TG mass loss profiles by taking into account the delay between TG and MS determined in series of preliminary experiments. The gas samples evolved at the maximum intensities of mass losses were also analysed by GC for a better identification of released compounds. Fig. 6a shows typical evolution profiles of various gas products during sewage sludge pyrolysis. This figure illustrates relative intensity of selected m/z signals (RI – the ratio of specific m/z current to total ion current) along with a DTG curve for this experiment. As an example, a chromatogram of the gas sample evolved at 700°C is shown in Fig. 6b. The GC analysis indicates the evolution of CO, CO2, H2 as well as small amounts of methane and other lighter hydrocarbons.
The ion intensities measured in MS were converted to the units which describe the rate of mass loss using the following equation:
Arx
xArArxx M m a
M Q RIMS
0βρ
= (4)
where: RIx – relative intensity of MS signal for component x (% of the sum of all MS signals recorded), ax – slope of the calibration line given in Table 1.
Finally, the recalculated profiles were normalized to the measured sample mass taking into account that in a low temperature range (up to about 150ºC) as well as at very high temperatures
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(above 900ºC) gas was the only product of pyrolysis, so that the DTG profile had to coincide with the sum of all individual MS profiles recalculated to DTG units according to Eq. (4).
200 400 600 800 10000,00
0,04
0,08
0,12
0,16
0,20
0,0
0,5
1,0
1,5
2,0
2,5(a)
MS
, relative intensity, RIm
/z=x , %
Temperature, oC
DTG
, -dm
/dT,
%/K
DTG
m/z = 2 m/z = 16 m/z = 18 m/z = 28 m/z = 44
CO
2
H2
Met
hane
Ethy
lene
Ar
5
N2
10
Prop
ylen
eC
O C4 Hydrocarbons
15 200
RETENTION TIME, min
(b)
Fig. 6. DTG curve and relative intensity of selected MS profiles (a) and Gas chromatogram of the gas phase
evolved and sampled at 700°C (b) during the pyrolysis of the GOS sewage sludge
For mass balance of sewage sludge pyrolysis process the following products from TG-MS analysis were taken into account: H2 (m/z = 2), H2O (18), CO (28), CO2 (44), CH4 (16) C2H6 (27), C3H8 (41), C4H10 (55), CH3OH (31), C2H5OH (46) C6H6 (78), SO2 (48) and H2S (34). For all components, for which calibration was not performed, the ax value obtained for CH4 was used.
Figure 7 shows a comparison of DTG curve and the formation rate of gas products for all investigated samples. DTG signal and the curve of total gas production rate overlap, except for the temperature range 200 - 500°C, where the gas production rate is distinctly smaller than the rate of sample mass loss. This difference can be explained by the formation of liquid product (tars and oils) in this temperature range [8-10], which left the sample but was not detected by MS. The rate of tar production can be evaluated by subtracting the rate of gas production curve from the DTG signal. On the basis of the area under curves in Fig. 7 the composition of all products formed during the whole process of pyrolysis was calculated for all samples (Table 4).
Table 4. Composition of products formed during the whole process of pyrolysis
H2 H2O CO2 CO CH4 C2H6 C3H8 C4H10 CH3OH C6H6 SO2 H2S Total gas Sample
Fig. 7. Comparison of DTG signal and formation rate of major gas products and tars for pyrolysis process of all investigated sludge samples
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The estimated amount of liquid product formed during pyrolysis of different samples of sewage sludge was in the range of 20-40 wt% of initial sample mass. It is worth noting that the fraction of liquid product in the volatiles (see Table 5) is an individual feature of the sludge and it is probably related to wastewater treatment technology, amount of ash and its composition. Generally, liquid product is major fraction of volatiles except for KOS, KAW and CON samples for which a relatively low yield of liquid product was observed. The first two samples are the product of industrial waste treatment process, so they may contain more elements, which are potential tar breaking catalysts. It is clear that in the case of the KAW sample low liquid product yield was a result of high calcium content, which is added in the form of CaO for hygienisation and stabilization of the raw sludge.
Table 5. Estimated yield of gaseous and liquid products of pyrolysis
Volatiles Gas Liquid Liquid content in volatiles Sample
% of initial sample mass % GOS 44.6 16.4 28.2 63.2 KOS 52.4 32.5 19.9 37.6 SLO 58.9 19.7 39.2 66.6 CON 62.1 30.0 32.1 51.7 REU 53.0 20.8 32.2 60.8 PRA 42.3 21.5 24.7 58.3 KAN 52.6 13.0 38.9 74.0 KAW 48.8 29.5 19.3 39.5
3.3. Elemental analysis and balance of elements The content of C, H, N, S and O in the raw samples of sewage sludge and in samples after
heating up to 1000ºC was determined by elementary analysis. These results are given in Table 6.
Table 6. Elemental analysis of raw samples (0) and after pyrolysis (P)
N C H S O* Ash Total H/C O/C Sample % of raw sample mass mole fraction
On the basis of data from Tables 4 and 6, atomic composition of three basic products formed during the pyrolysis for all investigated samples was calculated and presented in Fig. 8. These data in the form of H/C ratio are given in Table 7.
As can be seen in Table 7, the atomic H/C ratio for most dried sewage sludge samples is about
1.8 with the exception of GOS (lower H/C ratio) and SLO (much higher). Atomic hydrogen is released from the sample mainly as water. Molecular hydrogen is present in the gas phase only at above 600ºC. Even if molecular hydrogen is also formed at lower temperatures it is probably converted into water in the water gas shift reaction. The composition of liquid product seems to be dependent on the sample origin, although we need to remember that the quantitative analysis of gas phase is not very accurate and the data listed in Table 7 can be imprecise as well.
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4. COMPARISON OF THERMAL DECOMPOSITION OF SEWAGE SLUDGE IN ARGON AND CO2 ATMOSPHERE
In order to compare typical pyrolysis and CO2 gasification processes a dried sewage sludge sample was heated in an inert (Ar) and reactive (CO2) atmosphere to the temperature of 1000°C at constant heating rate (20ºC/min). Figure 9 shows DTG and evolved gas profiles for both experiments. As can be seen in the figure, both the DTG and MS profiles for pyrolysis and gasification are very similar up to 550°C. It means that below this temperature the type of gaseous atmosphere has no impact on the process rate and composition of gaseous products.
Fig. 9. Comparison of DTG (a) and MS (b) profiles for thermal decomposition of GOS sewage sludge sample in Ar (solid lines) and CO2 (dash lines)
5. EFFECT OF PYROLYSIS TEMPERATURE ON CHAR YIELD
The aim of the work described in next chapters was to study the influence of different pyrolysis conditions (e.g. temperature, heating rate and additives) on the yield, composition and reactivity in CO2 of solid residue obtained from sewage sludge pyrolysis. This may be useful information about the effect of operation conditions on subsequent combustion, activation (production of adsorbents) or gasification of the pyrolysed char.
The study was performed using a calibrated TG-MS-GC system as well as elemental analysis. Sewage sludge produced in a Lodz wastewater treatment plant (GOS) was used as a starting material in the pyrolysis experiments. This sample differed in composition from the GOS samples studied before. After drying in air at 110°C for 12 hours, the sample contained 5.8 wt% moisture and 37.8 wt% ash (the previous sample had 42.7 wt% ash).
The results of TG-MS-GC analysis of GOS sewage sludge heated in argon at the rate of 20°C/min are presented in Figs. 10-12. As can be seen, fast devolatilisation of the sample occurs between 200 and 500°C. In this temperature range all liquid products (tars and oil) and carbon rich solid product are formed. Carbon oxides which are involved in the autooxidation process are main gaseous products formed in a high-temperature region (see Fig. 12) where slower decomposition of the solid residue is observed.
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0 200 400 600 8000
20
40
60
80
100
CharMass loss curve
ASH
VOLATILESGAS PRODUCTS
TARS AND OIL
Cun
ulat
ieve
mas
s lo
ss, %
Temperature, oC
Fig. 10. Results of TG-MS-GC analysis of GOS sample pyrolysed at different temperatures
0 200 400 600 8000,00
0,05
0,10
0,15
0,20
Total mass loss Gas products formation Tar formation
Rat
e, %
mas
s/o C
Temperature, oC Fig. 11. Rates of total mass loss and formation of liquid and gaseous products during GOS sewage sludge
pyrolysis at different temperatures
600 700 800 9000,000
0,005
0,010
0,015
0,020
Carbon oxide Carbon dioxide water Hydrogen Methane
Rat
e of
form
atio
n, %
mas
s/o C
Temperature, oC
Fig. 12. Rate of formation of main gaseous products during GOS sewage sludge pyrolysis at different temperatures
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Table 8 summarizes the results of elemental analysis of dried sewage sludge and solid residue obtained by pyrolysis performed in the thermobalance. The sewage sludge samples were heated at the constant rate of 20°C/min up to the final temperature listed in the first column of Table 8. Results of the elemental analysis of solid residue are also given in Fig. 13. As can be seen, the solid product of pyrolysis obtained at temperatures above 500°C contains, beside not analyzed atoms, mainly atomic carbon and some amount of oxygen, which is probably a component of the mineral layer (ash). Sulphur, nitrogen and hydrogen are also present. Above 500°C the atomic carbon content in the solid residue decreases slowly with the final pyrolysis temperature. This carbon is released in the form of carbon oxides. However, the main mass loss of the sample at these temperatures is caused by a release of oxygen as H2O and CO2 (see Fig. 12).
Table 8. Characterization of sewage sludge and solid remaining after pyrolysis at different temperatures
a final heating temperature of the sample b volatile matter subjected to gasification in CO2 at 900°C
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20
40
60
80
100
ASH CONTENTBY PROXIMATE ANALYSIS
TOTAL SAMPLE MASS
OTHER ELEMENTS
SULPHUR + NITROGEN
OXYGEN
HYDROGEN
CARBON
Ato
mic
com
posi
tion,
%
Temperature, oC
Fig. 13. Change in atomic composition of solid residue during pyrolysis of GOS sewage sludge sample as a function of temperature
Pyrolysis of the GOS sewage sludge yields solid residue with considerable ash content
ranging from 74 wt%, when the pyrolysis is finished at 500°C, to 84 wt% at 900°C. Even at 900°C, the rate of mass loss (DTG curve in Fig. 11) is not equal to zero, so that pyrolysis does not seem to
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be complete. This means, the higher is the pyrolysis temperature the more complete is the carbonisation (lower yields in carbonaceous materials and relatively higher ash content).
Figure 14 shows the TG profiles recorded during thermal decomposition of three different sewage sludge samples. In these experiments the samples were heated with the constant rate of 10°C/min up to 900°C in argon, and then at this temperature the gas was changed to carbon dioxide to initiate the gasification of pyrolysis char.
0 20 40 60 80 100 120 130
20
40
60
80
100
GOS SLO KOS
TG
, %
Time, min
Gasification in CO2
Devolatilisation in Ar
900 oC
Fig. 14. TG profiles for pyrolysis and subsequent gasification of chars at 900°C
(heating rate 10°C/min)
Comparison of char reactivity obtained from different samples is shown in Fig. 15 where conversion of char is plotted as a function of time. The char conversion α(t) in the gasification reaction was defined as:
)(m)(k
k
mtmt
ΔΔ−Δ
=α (5)
where Δmk = m0 – mk, Δm(t) = m0 – m(t), and m0 denotes the initial mass of char, m(t) the sample mass at any time and mk the mass of ash.
0 200 400 600 8000,0
0,2
0,4
0,6
0,8
1,0
Con
vers
ion
Time, s
GOS SLO KOS
Fig. 15. Reactivity of pyrolysis chars at 900°C
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As can be seen in Fig. 15, char reactivity in CO2 depends on the sewage sludge origin. This may be attributed to different amounts of ash and carbon in the raw sample. The bigger amount of ash is present in the sewage sludge, the higher is the gasification rate.
6. INFLUENCE OF HEATING RATE ON PYROLYSIS EXTENT
Figure 16 shows a comparison of TG and DTG profiles for pyrolysis conducted at different heating rates. It is worth noting that there are no significant differences between these curves, the displacements observed are due to thermal inertia of the sample.
200 400 600 800 1000
50
60
70
80
90
100
200 400 600 800 1000
-0,2
-0,1
0,0
0,1
DTG
, %/C
Temperature, oC
5oC/min 10oC/min 20oC/min 40oC/min 150oC/min
Temperature, oC
TG, %
Fig. 16. TG and DTG profiles for pyrolysis conducted at different heating rates
Figure 17 shows the formation rate of selected gas products as a function of temperature for different heating rates. The composition of major products formed during the whole process were calculated and given in Table 9. As it was expected, there were no significant differences in the composition of gas products formed at different heating rates.
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0,01
0,02
0,03
0,04
200 400 600 8000,00
0,01
0,02
0,03
(b)
100 oC/min 50 oC/min 20 oC/min 10 oC/min
DTG
, %/o C
Temperature, oC
(a)
DTG
, %/o C
Temperature, oC Figure 17. Formation rate of selected gaseous products in thermal treatment of sewage sludge at different
heating rates for GOS sample, (a) – CO, (b) – CO2
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Table 9. Composition of products formed during the whole pyrolysis process at different heating rates for GOS sample
H2 H2O CO2 CO CH4 C2H6 C3H8 C4H10 CH3OH tars solids (char+ash) Heating rate mass fraction %
After pyrolysis the inert atmosphere of Ar was changed to CO2 and char was gasified under isothermal conditions at 900°C. Figure 18 shows TG curves for gasification experiments. It turned out that heating rate in the pyrolysis process had an insignificant influence on produced char quantity and reactivity – all curves in Fig. 18 overlap.
0 10 20 30 40 50 6
86
88
90
92
94
96
98
100
0
100 oC/min 50 oC/min 20 oC/min 10 oC/min
TG, %
Time, min Fig. 18. TG curves for gasification of char produced at different heating rates
7. RATE OF CHAR GASIFICATION IN CO2
Pyrolysed char used in the gasification experiments had been previously produced outside the TG-MS system, in a fixed-bed reactor by heating a dried sewage sludge sample at 1000°C for 4 hours in argon flowing through the reactor. About 30 mg of solid residue removed from the fixed-bed reactor was placed in a TG crucible and heated in inert argon atmosphere to the fixed gasification temperature. After reaching this temperature CO2 flow was initiated and kept for a time period necessary to complete the process. Gasification was performed at four different temperatures and TG results are presented in Fig. 19. Figure 19b shows the initial stage of experiments in a magnified scale.
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0 2 4 6 8 10 12 14
90
92
94
96
98
100
102
104TG
, %
Time, h
800oC 850oC 900oC 950oC
(a)
0,40 0,45 0,50 0,55 0,60
100
101
102
103
TG, %
Time, h
800oC 850oC 900oC 950oC
(b)
Fig. 19. TG profiles for gasification of pyrolysis char at different temperatures
As can be seen in Fig. 19, the rate of gasification depends strongly on temperature. It took about 14 hours to complete the gasification at 800°C, whereas at 950°C the process was over in less than 1 hour time.
Figure 19 shows an increase of mass sample at the beginning of the gasification process between 25-30 min, and thereafter the expected mass loss was observed. Moreover, in the experiment conducted at 950°C an increase of sample mass was also observed at the final stage of gasification process. This phenomenon is caused by the adsorption of carbon dioxide on the ash surface and equilibrium reactions of CaO present in the ash with CO2:
CaO + CO2 CaCO3 (6)
More details about this reaction and its influence on pyrolysis and gasification processes are given in chapter 8.1. In order to separate processes of CO2 adsorption and gasification, an additional experiment was performed in the thermobalance. First, the char was exposed to CO2 at the temperature in which the gasification reaction was very slow (700°C). Then, the temperature was increased rapidly to 950°C to observe gasification. The temperature program and TG curve are shown in Fig. 20. The increase of the sample mass at the first stage of experiment is significant – almost 5 wt% was observed.
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4
90
95
100
105
0
300
600
900
1200
1500
gasification -15.2%
TG, %
Time, h
adsorption/ oxidation +4.7% Tem
perature, oC
Fig. 20. TG profile for experiment on pyrolysis char with a modified temperature program
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8. INFLUENCE OF CATALYTIC ADDITIVES ON PYROLYSIS EXTENT
8.1. CaO additive Calcium, in the form of oxide or hydroxide, is often applied in the final stage of sewage
sludge processing for hygienisation. The factors which influence biocidality of the liming process are a high alkaline reaction and increased temperature caused by the exothermal reaction of calcium oxide hydration [9]. Sludge liming also enables stabilisation and enhances dehydration [10]. Moreover, it has been demonstrated that, similarly to coal pyrolysis [11,12], the addition of CaO has also a great impact on thermal decomposition of the sewage sludge.
In our study a sample of dried (about 97% dry mass) and ground sewage sludge (GOS) was mixed in different proportions with calcium oxide and heated at a constant heating rate (20°C/min) in thermobalance in the inert atmosphere of Ar to the temperature 1000°C. The list of experiments performed is given in Table 9.
Figure 21 shows changes of sewage sludge mass in samples containing various amounts of CaO. The smallest mass loss (53.6%) was observed for the pyrolysis of a sewage sludge sample without addition of CaO. After this process solid residue was composed of inert ash and pyrolysis char which underwent gasification process in the gaseous atmosphere of CO2 (experiment GOS_CO2). Total mass loss after the gasification was 62.8% of the initial sample mass. When CaO was added to the sewage sludge, an increase in mass loss was observed in the inert atmosphere. Using the mixture of CaO and sewage sludge in 1:1 mass ratio, entire carbon from the sample was converted into gas and liquid fractions. Total mass loss of 62.4 wt% was observed in this experiment (CaO50) which is the same as for sewage sludge without CaO in CO2 atmosphere.
200 400 600 800 1000
40
50
60
70
80
90
100
Temperature, oC
TG
, %
GOS CaO10 CaO25 CaO50 GOS_CO2
200 400 600 800 1000
-0,20
-0,15
-0,10
-0,05
0,00
DTG
, %/o C
(b)
Temperature, oC
(a)
Fig. 21. Comparison of TG (a) and DTG (b) curves for all experiments
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One may explain this observation by adsorption of CO2 evolved during pyrolysis in the inert atmosphere, onto CaO and subsequent CO2 desorption and simultaneous dry gasification at higher temperatures. For all experiments a qualitative analysis of gaseous products was performed (see Fig. 22). The compositions of gaseous, liquid and solid products are given in Table 11.
200 400 600 800 10000.00
0.04
0.08
0.12
0.16
0.20
H2
H2O CO
CO2
CH4
other
tars
DTG
Temperature, oC
DTG
, -dm
/dT,
%/K
(a)
200 400 600 800 1000
0.00
0.04
0.08
0.12
0.16
0.20
Temperature, oCD
TG, -
dm/d
T, %
/K
(b)
200 400 600 800 10000.00
0.04
0.08
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0.16
0.20
Temperature, oC
DTG
, -dm
/dT,
%/K
(c)
200 400 600 800 1000
0.00
0.04
0.08
0.12
0.16
0.20
Temperature, oC
DTG
, -dm
/dT,
%/K
(d)
Fig. 22. Analysis of gas products for samples: GOS (a), CaO10 (b), CaO25 (c) and CaO50 (d)
Tab. 11. Composition of products formed during the whole pyrolysis process of GOS sample and mixtures of GOS and CaO samples
H2 H2O CO2 CO CH4 C2H6 C3H8 C4H10 CH3OH C6H6 SO2 H2S tars solid Sample % initial mass of sewage sludge
The production rates of main gaseous products for different experiments are compared in Fig. 23. While increasing the fraction of CaO in the initial sludge sample, an insignificant decrease in the production rate of water vapour in the temperature range of 150-300°C (CaO hydration reaction by water vapour produced in the pyrolysis process) and a significant increase in the production rate
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at 450°C (dehydration) were observed. A similar situation occurred for carbon dioxide: the whole amount of CO2 produced in pyrolysis was used for the carbonization of CaO (250-500°C), and it was released at higher temperatures (750°C). CO2 released at these temperatures caused the auto-gasification of pyrolysis char, resulting in a significant increase in carbon monoxide production rate. As a result, a further decrease in the sample mass was observed. Similar processes, but to a lesser extent, were observed for the samples without the addition of CaO. These phenomena were due to the presence of CaO in every sewage sludge sample (from 3 to 15% of dry basis) [13].
CaO, which acts as a catalyst of gasification process, under CO2 atmosphere at the temperature 750°C occurs in the form of CaCO3. Therefore, we do not observe a DTG peak in this temperature region as it is common for inert atmosphere.
200 400 600 800 1000
0,02
0,04
0,06
0,08
0,10
200 400 600 800 1000
0,01
0,02
0,03
0,04
0,05
0,06
0,07
200 400 600 800 10000,0000
0,0005
0,0010
0,0015
0,0020
200 400 600 800 10000,00
0,02
0,04
0,06
0,08
0,10
0,12
Temperature, oC
DTG
, %/o C
GOS CaO10 CaO25 CaO50
Temperature, oC
DTG
, %/o C
CO2
H2
H2O
DTG
, %/o C
Temperature, oC
DTG
, %/o C
CO
Temperature, oC
Fig. 23. Effect of CaO content in the sludge sample on the formation rate of major gas products
Apart from the aforementioned chemical reactions, other unidentified processes catalyzed by Ca and its compounds occur. With an increase in CaO content in the initial sludge sample, an increase in water vapour emission in the process of pyrolysis was observed, which cannot be attributed to the hydration and dehydration reactions of CaO. Simultaneously, a decrease in tar production and an increase of H2 emission are observed.
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8.2. Dolomite additive TG curves for pyrolysis of the GOS sample without any additives and with the addition of
dolomite CaMg(CO3)2 are compared in Fig. 24. Finely ground dolomite (the fraction with a diameter less than 10 μm) was mixed with sewage sludge in a mass ratio 1:3. The samples were heated in the thermobalance at constant heating rate 20 K/min and in the inert gaseous atmosphere of Ar.
As one may deduce from Fig. 24, the total mass loss in both experiments was the same. However, one must take into account, that the mass of sewage sludge in the sample with the addition of dolomite is only 75% of the total sample mass. The mass loss of the sample with dolomite was caused partly by thermal decomposition of sewage sludge, and partly by the decomposition of dolomite which at temperatures above 600°C lost its mass with the emission of CO2. The total mass loss of dolomite in the temperature range from 600 to 1000°C amounts to 45.4%. Considering that at temperatures below 600°C the mass loss in the experiment with a mixture of the sludge/dolomite sample was a result of the thermal decomposition of sewage sludge, TG curve in Fig. 24 can be rescaled to the reference mass of sewage sludge only (blue line).
200 400 600 800 1000
40
50
60
70
80
90
100
TG, %
Temperature,°C
GOS GOS with dolomite GOS in the presence of dolomite
Fig. 24. TG curves for GOS sample without additives and with 25% addition of dolomite
The sample with a sludge/dolomite mixture of mass 75.4 mg used in the experiment comprised 55.6 mg of sewage sludge. Assuming that the mass loss of dolomite in the mixture with sewage sludge was the same as for pure dolomite (45.4%) the mass of sewage sludge after pyrolysis may be calculated. According to these calculations, this mass amounts to 24.5 mg which means that the mass loss of sewage sludge amounts to 55.8% – higher than for mass loss in the experiments with sewage sludge without additives (53.6%).
During the process of pyrolysis with dolomite the composition of gaseous products was analyzed by a mass spectrometer. The results of this analysis are presented in Fig. 25 and Fig. 26. At temperatures below 600°C the main products were H2O and CO2 and, in smaller quantities, CO and CH4. At higher temperatures mainly CO and CO2 were formed. During the pyrolysis of the sludge/dolomite mixture much CO2 was formed due to the decomposition of dolomite. Great quantities of CO are also characteristic of this case which appears in parallel with CO2, being the result of pyrolysis char auto-gasification. These phenomena explain why more sludge was converted into gaseous fuels if the sludge/dolomite mixture for the pyrolysis process was used.
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Figure 26 shows hydrogen production rates in pyrolysis for samples with and without dolomite addition. It can be noticed that H2 is formed in measurable quantities only above 400°C reaching a maximum at 600-650°C. In the case of the sludge/dolomite mixture sample a second peak also appeared above 800°C. This peak was caused by the release of H2 from char, which was connected with auto-gasification reactions. A total quantity of H2 released from pyrolysis for the sludge/dolomite mixture was about 15% higher than for sewage sludge without any additives.
200 400 600 800 10000,00
0,04
0,08
0,12
0,16
0,20
0,24
H2
H2O CO CO2
CH4 other
tars DTG
Temperature, °C
DTG
, -dm
/dT,
%/K
Fig. 25. Analysis of gaseous products for mixture sludge/dolomite sample
200 400 600 800 10000,0000
0,0003
0,0006
0,0009
0,0012
0,0015
GOS GOS + dolomite
Temperature, oC
DTG
, -dm
/dT,
%/K
Fig. 26. Rate of H2 production for pyrolysis of sewage sludge without and with additive of dolomite
Due to the fact that dolomite is stable at temperatures below 600°C we can exactly compare the quantities of gaseous products formed in both experiments at these temperatures. At this stage of the process tars are formed. Relevant data are given in Table 12. Analysis of these data shows that the use of dolomite as an additive has an influence on the reduction of quantities of tars produced (an observable decrease from 23.2 to 17.3%) and in parallel increases the quantities of gaseous fuel produced.
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Table 12. Amounts of products formed during the pyrolysis process at temperatures below 600°C for sewage sludge with and without addition of dolomite
Components GOS GOS + dolomite
% initial mass of sewage sludge
H2 0.1 - H2O 13.6 17.4 CO 1.7 1.3 CO2 5.3 8.6 CH4 1.0 1.1 other gases 1.5 1.6
• The amount of char formed during pyrolysis of various sludge samples depends on sludge origin and composition.
• The TG curve for pyrolysis and gasification in CO2 atmosphere overlap up to 600°C, which means that these two processes occur at the same stages.
• The gasification process starts at temperatures above 600°C.
• There is an insignificant effect of the heating rate (in the range from 5 to 150°C/min) on pyrolysis extent.
• CaO and dolomite are good catalysts in the processes of sewage sludge pyrolysis and gasification. Adding CaO or dolomite to the sludge sample we can achieve a decrease of residual mass, increase of CO and H2 production and decrease of tar production.
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[10] Adegoroye A., Paterson N., Li X., Morgan T., Herod A.A., Dugwell D.R., KandiYoti R., The characterisation of tars produced during the gasification of sewage sludge in a spouted bed reactor, Fuel, 83 (2004) 1949-1960.
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[12] Morris M., Waldheim L., Energy recovery from solid waste fuels using advanced gasification technology, Waste Management, 18 (1998) 557-564.
[13] Tingyu Z., Shouyu Z., Jiejie H., Yang W., Effect of calcium oxide on pyrolysis of coal in a fluidized bed, Fuel Processing Technology, 64 (2000) 271-284.
[14] Khan M.R., Fuel. Sci. Technol. Int., 5 (2) (1987) 185-231. [15] Bień J.B., Osady ściekowe. Teoria i praktyka, Wydawnictwo Politechniki Częstochowskiej,
Mat. Międzynarodowego Seminarium Szkoleniowego nt. Podstawy oraz praktyka przeróbki i zagospodarowania osadów, LEM s.c., Kraków 1998 (in Polish).
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