© CSIR 2010 Slide 1www.csir.co.za
Fluidised bed gasification of high-ash South African coals
A.D. Engelbrecht, B.C. North, B.O. Oboirien, R.C. Everson and H.W.P.J. Neomagus
July 2013
© CSIR 2010 Slide 2
Outline of presentation
• Coal gasification
• Coal gasification reactors
• Fluidised bed gasification
• Characterisation of high-ash SA coals
• Pilot-scale fluidised bed gasification of high SA coals
• Gasifier modelling
• Sensitivity analysis
• Conclusions and recommendations
© CSIR 2010 Slide 3
Products from combustion and gasification
© CSIR 2010 Slide 4
Products from combustion and gasification
© CSIR 2010 Slide 5
Worldwide Gasification Capacity & Planned Growth Cumulative by YearLimited oil and gas reserves with abundant coal reserves
ChinaIndiaUSASouth Africa and Australia
Increase in cost of oil and gas
Need to diversify energy resources
Greenhouse gas emission regulations
Drivers for investment in coal gasification
© CSIR 2010 Slide 6
Source: DOE NETL Database, 2010
Worldwide Gasification Capacity & Planned Growth Cumulative by Year
© CSIR 2010 Slide 7
End Use Applications of Syngas
Source: DOE NETL Database, 2010
Source: DOE NETL Database, 2010
© CSIR 2010 Slide 8Source: DOE NETL Database, 2007
Worldwide Gasification Capacity & Planned Growthby Region Source: DOE NETL Database, 2010
© CSIR 2010 Slide 9
Gasification by Technology
Source: DOE NETL Database, 2007
Source: DOE NETL Database, 2010
© CSIR 2010 Slide 10
Coal gasification reactors
© CSIR 2010 Slide 11
KBR TRIGTM gasifier
Projects:
§ Kemper County, USA, 550 MW IGCC, start-up 2014, lignite
§ Dongguan, China ,120 MW IGCC, start-up 2014, high-ash bituminous coal
§ Inner Mongolia, China, 35 000 Nm3/h syngas for ethylene glycol, start-up 2015, high-ash lignite
Standpipe
J-Valve
Riser
Upper Mixing Zone
Lower Mixing Zone
Startup Burner
Second Separation
Device
Seal Leg
First Separation
Device
Syngas
CoalAir
SteamOxygen
Ash
© CSIR 2010 Slide 12
GTI U-gas® fluidised bed gasifier
Projects:
§ Shadong, China, 28 000 Nm3/h syngas for methanol, start-up 2008, high-ash coal wash plant waste.
§ Henan, China, 45 000 Nm3/h syngas for ammonia, start-up 2014, high-ash sub-bituminous coal
gas
char
oxygen + steam
oxygen + steam
ash agglomerates
coal
high pressure
steam
steam
© CSIR 2010 Slide 13
Coal properties important for fluidised bed gasification
Coal properties
Requiredfor FBG
SA coalin future
Fit
Moisture < 7.5 % < 6 %
Ash 15 - 50 % 25-50 %
AFT > 1400 °C > 1400 °C
Caking index low <2 ?
Reactivity high med-low ?
Size 1- 4 mm Fines
Volatiles >20 % ± 25 %
© CSIR 2010 Slide 14
Information on selected coals
Colliery New Vaal Matla Grootegeluk Duvha Location of mine Free State
Province Mpumalanga Province
Limpopo Province
Mpumalanga Province
Coal field Sasolburg Highveld Waterburg Witbank Production rate (Mt/a)1 15.2 14.0 15.0 16.0 Started production 1985 1979 1985 1979 Expected lifetime (yr)1 30 - 40 40 - 50 40 - 50 30 - 40 Coal preparation Washed Raw coal Washed Raw coal Receiving Power Station Lethabo Matla Matimba Duvha Power station rating(GWe) 3.6 3.6 3.6 3.5 1 2005
© CSIR 2010 Slide 15
Coal characterization parameters
New Vaal Matla Grootegeluk Duvha
Calorific value (MJ/kg) 15.1 18.6 19.8 21.0 Ash content (%) 40.4 33.4 34.9 32.5 Carbon (%) (maf)* 79.10 80.2 81.8 89.3 Vitrinite reflectance (%) 0.53 0.64 0.68 0.75 Surface Area (m2/g) 7.01 2.08 <1 < 1 Porosity (%) 3.2 1.3 1.5 1.1 Free swelling index (FSI)
0 0 1 0
Roga index (RI) 0 0 10 0 AFT (° C) + 1600 1500 1500 1600 * Dry ash free + Ash fusion temperature
© CSIR 2010 Slide 16
Rank classification system using vitrinite reflectance(Du Cann)
© CSIR 2010 Slide 17
Coal characterization parameters
New Vaal Matla Grootegeluk Duvha
Calorific value (MJ/kg) 15.1 18.6 19.8 21.0 Ash content (%) 40.4 33.4 34.9 32.5 Carbon (%) (maf)* 79.10 80.2 81.8 89.3 Vitrinite reflectance (%) 0.53 0.64 0.68 0.75 Surface Area (m2/g) 7.01 2.08 <1 < 1 Porosity (%) 3.2 1.3 1.5 1.1 Free swelling index (FSI)
0 0 1 0
Roga index (RI) 0 0 10 0 AFT (° C) + 1600 1500 1500 1600 * Dry ash free + Ash fusion temperature
© CSIR 2010 Slide 18
Coal reactivityConversion of coals at 950°C in 100% CO2 at 87.5 kPa
0.00.10.20.30.40.50.60.70.80.91.0
0 50 100 150 200 250 300Time, t (min)
Con
vers
ion,
X (-
)
New Vaal
Matla
Grootegeluk
Duvha
© CSIR 2010 Slide 19© CSIR 2006 www.csir.co.zaSlide 19
Specification of fluidised bed gasifier pilot plant
Operating pressure : Atmospheric (90 kPa)Bed dimensions : 0.2 m X 0.2 m (square)Freeboard dimensions : 0.4 m X 0.4 m (square)Furnace height : 2m bed & 2m freeboardDynamic bed height : < 1.5 mCoal feedrate : 15 - 35 kg/h
Coal particle size : < 4 mmCoal calorific value : > 10 MJ/kgAir flow rate : 15 - 30 Nm3/hOxygen flowrate : 4 - 10 kg/hSteam flowrate : 10 - 20 kg/hBed temperature : 900 -1000 °C
Reactant temperature : 200 – 300 °CFluidising velocity : 1.5 – 2.5 m/s
© CSIR 2010 Slide 20© CSIR 2006 www.csir.co.zaSlide 20
Fluidised bed gasifier pilot plant
T
T
T
T
T
T
T
P
Gasifier
Coal feed
Hea
t exc
hang
er
FD fan Flare
Cyclone
ID fan
Air
Boiler
Bed char
Cyclone char
Steam
Gas cleaning
Steampurge
PFeedwater
Water to drain
LPG
Water cooled screw Oxygen
storage
Oxygenrotameter
Orificeplate
Scrubber effluentWater pump
Buffer tank
Wat
er q
uenc
h sc
rubb
er
Gas to analysers
SteamRotameter
P
TT
TT
TT
TT
TT
TT
TT
P
Gasifier
Coal feed
Hea
t exc
hang
er
FD fan Flare
Cyclone
ID fan
Air
Boiler
Bed char
Cyclone char
Steam
Gas cleaning
Steampurge
PPFeedwater
Water to drain
LPG
Water cooled screw Oxygen
storage
Oxygenrotameter
Orificeplate
Scrubber effluentWater pump
Buffer tank
Wat
er q
uenc
h sc
rubb
er
Gas to analysers
SteamRotameter
PP
© CSIR 2010 Slide 21
Fluidised bed gasifier fixed carbon conversions
30
40
50
60
70
80
90
0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
Vitrinite random reflectance (%)
Fixe
d ca
rbon
con
vers
ion
(%)
950 C925 C
925 °C
950 °C
New Vaal
Matla
Grootegeluk
Duvha
© CSIR 2010 Slide 22
Fluidised bed gasifier gas quality
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
Vitrinite random reflectance (%)
Gas
CV
(MJ/N
m3 )
950 C925 C
New VaalMatla
Grootegeluk
Duvha
Nitrogen dilution
Homogenous gas combustion
Lower fixed carbon conversion
© CSIR 2010 Slide 23
Coal gasification reactors
© CSIR 2010 Slide 24
Pilot-scale fluidised bed gasification testsoxygen enriched air + steam
Ten pilot-scale fluidised bed (FBG) gasification tests were carried out on two high-ash South African coals (six test on each coal) using oxygen enriched air and steam as the gasification agents.
FBG operating conditions
Bed temperature (875 - 975 °C)Residence time (15 -55 min)Fluidising velocity (1.5 – 2.0 m/s)Oxygen enrichment level (33 - 37 %)
© CSIR 2010 Slide 25© CSIR 2006 www.csir.co.zaSlide 25
Experimental resultsEffect of bed temperature on fixed carbon conversion
20
30
40
50
60
70
80
90
100
850 875 900 925 950 975 1000Mid bed temperature (ºC)
Fixe
d ca
rbon
con
vers
ion
(%)
New Vaal Grootegeluk
Superficial gas velocity (m/s) : 1.25 - 1.84
τ = 35 minutes
τ = 25 minutes
Test 5
Test 1
Test 2
Test 3
Test 1Test 3
© CSIR 2010 Slide 26© CSIR 2006 www.csir.co.zaSlide 26
Experimental resultsEffect of residence time on fixed carbon conversion
50
55
60
65
70
75
80
85
90
95
100
10 15 20 25 30 35 40 45 50 55Average residence time (min)
Fixe
d ca
rbon
con
vers
ion(
%)
New Vaal Grootegeluk
Temperature = 943 °C
Temperature = 980 °CTest 3
Test 4
Test 5Test 2 Test 3
Test 4
© CSIR 2010 Slide 27
Experimental results
Air
Oxygen enriched air
H2 - 22.7CO - 17.6CH4 - 1.2
© CSIR 2010 Slide 28
FBG syngas flare
© CSIR 2010 Slide 29© CSIR 2006 www.csir.co.zaSlide 29
Experimental resultsSummary of fluidised bed coal gasification tests
Fixed carbon conversion increases with:q Fluidised bed temperatureq Residence time of char in the gasifierq Steam concentration of the reacting gasses in the gasifierq Reactivity index of the coals tested
Calorific value of the gas increases with:q Level of oxygen enrichment of the gasification airq Fixed carbon conversion
Carbon content of the elutriated char is higher than the bed char :q There is little or no oxygen present in freeboardq The concentration of CO2 and H2O are lower in the freeboard than the bed
resulting in lower gasification ratesq The concentrations of CO and H2 are higher , which inhibits the rates of the
gasification reactions q The temperature of the freeboard is lowerq The residence time of particles in the freeboard is lower than in the bed
© CSIR 2010 Slide 30
Fluidised bed gasifier modelling
O2 flow & temperature
Steam flow & temperature
Coal flow & analysis
Bed ash flow & carbon content
Air flow & temperature
Fly ash flow & carbon content
Syngas Flow
CO2
H2
CO
CH4
H2O
N2
FBG
Temperature
Gasifier dimension
Dynamic bed height
© CSIR 2010 Slide 31© CSIR 2006 www.csir.co.zaSlide 31
Fluidised bed gasifier modellingMotivation
q Existing available fluidised bed coal gasifier models are in somecases not suitable for simulating the fluidised bed gasification ofhigh-ash South African coals.
q An in-depth understanding of the chemical and physical processesthat take place in a fluidised bed coal gasifier can be obtainedduring model development.
q The model can be used for process analysis, design, optimisationand scale-up.
q The model can be extended later to other gasification processes such as:
• Circulating fluidised bed gasifiers• Moving bed gasifiers• Entrained flow gasifiers• Dual fluidised bed gasifiers
© CSIR 2010 Slide 32© CSIR 2006 www.csir.co.zaSlide 32
Fluidised bed gasifier modellingObjectives
q Develop a fluidised bed coal gasifier model that is based on fluidised bed hydrodynamics, coal devolatilisation, rates of chemical reactions and rates of heat and mass transfer in the gasifier.
q Test the predictive capability of the model by comparing model predictions with the pilot plant data.
q Study the effect of major operating variables on gasifier performance
© CSIR 2010 Slide 33
Fluidised bed gasifier modellingModel types
1. Mass and energy balance models
§ Conservation of mass and energy§ More unknowns than equations§ Assumptions regarding unknowns are required:
Ø Fixed carbon conversionØ CH4 and CO in the gas
2. Equilibrium models
§ Equilibrium relationships are added § Only fixed carbon conversion assumption is required§ Reactions are assumed to have reached equilibrium
3. Rate models
§ Conservation of mass and energy§ Bubbling fluidised bed hydrodynamics § Kinetics of gas-phase reaction (homogenous)§ Kinetics of char-gas reactions (heterogeneous)§ Heat and mass transfer rates in the gasifier§ Coal pyrolysis
© CSIR 2010 Slide 34© CSIR 2006 www.csir.co.za
Slide 34
Fluidised bed gasifier modellingModel equations for (FBGRM)
S u b -m o d e l C o r r e l a ti o n E q u a ti o n
H y d ro d y n a m ic s
M i n im u m flu i d is in g v e l o c it y G e l d a rt ( B .1 .1 .1 a )
B u b b le d i a m e te r M o ri a n d W e n ( 4 .3 ) D a r lt o n ( B .1 .1 .1 a )
B u b b le v e lo c i ty D H ( 4 .4 ) B u b b le f ra c t io n D H ( 4 .6 ) S l u g fra c t io n O rm is to n ( 4 .8 )
D e v o la t il isa t io n C S I R b e n c h - sc a l e ( B .2 .8 ) L o i so n a n d C h a u v i n ( 4 .1 2 )
H e t e ro g e n e o u s
re a c t io n s
C h a r c o m b u s ti o n L a N a u z e a n d Ju n g ( 4 .1 7 ) F i e ld ( 4 .1 9 )
C h a r-s te a m g a s i fi c a ti o n Jo h n so n ( 4 .2 5 ) L H (4 .5 5
C h a r-C O 2 g a s i fi c a ti o n A r r h e n i u s e q u a t io n ( 4 .5 8 ) C h a r-H 2 g a s if ic a t io n Jo h n so n ( 4 .3 0 )
H o m o ge n e o u s
re a c t io n s
N o n - c a ta l y ti c W G S R 3 K a r im a n d M o h i n d ra ( 4 .3 5 ) C a ta l y ti c W G S R W e n a n d T se n g ( 4 .3 8 ) C O c o m b u s t io n H a s l a m ( 4 .4 0 ) H 2 c o m b u sti o n H a s l a m ( 4 .4 1 ) C H 4 c o m b u s ti o n H a s l a m ( 4 .4 0 )
I n te r -p h a s e m a ss
t ra n sf e r
E m u l s io n t o b u b b le t ra n sf e r
b y d if fu s i o n
S it a n d G r a c e ( 4 .4 4 ) W e n a n d F a n ( B .1 .5 .1 d )
N e t fl o w D e r iv e d f ro m Y a n ( B .2 .1 4 )
In t e r- p h a se h e a t t ra n sf e r
P a rt ic l e to e m u l s io n R a n z a n d M a r sh a ll ( 4 .4 6 )
E m u l s io n to b u b b l e K u n i i a n d L e v e n s p ie l ( 4 .4 9 )
A t tri ti o n M a ( 4 .5 0 ) E l u t ria t io n G e l d a rt ( B .1 .6 )
F re e b o a rd v o i d fr a c ti o n H o ri o ( 4 .5 2 ) T ra n sp o r t a n d t h e rm o d y n a m ic p r o p e rt ie s A p p e n d i x (B .2 .6 ) s e v e ra l
© CSIR 2010 Slide 35
Fluidised bed gasifier modellingSub models developed by CSIR and NWU
βCO
CO0CO X)(1)p
RTE
exp(kdtdX
2
2
2−
−=
Char - CO2 reaction : Arrhenius equation
Char - H2O reaction : Langmuir-Hinshelwood
Coal devolatilisation
TGA - NWU
2 “ Ø reactor
CSIR( )
=
−
− gvfvevdv
.
0111cv120bv202
av010
nQ
VVV 1
1
OH
CO
H
2
2
2
© CSIR 2010 Slide 36
Fluidised bed gasfier modellingModel development
q Sub-models are combined into an overall gasifier model by carrying differential mass and energy balances over the reactor.
q Mass and energy balances produce 18 first order ODE’s with the reactor height (Z) being the independent variable.
q Equation were solve using Matlab with computational times varying between 5 and 10 minutes
© CSIR 2010 Slide 37© CSIR 2006 www.csir.co.zaSlide 37
Fluidised bed gasifier modellingParameter estimation
Parameter New Vaal Grootegeluk
fL - Relative reactivity factor for the char-H2Oreaction (-) 24.64 1.04
Ln(ko) - Ln (pre-exponential factor of the char-CO2reaction)(s-1) 11.29 16.07
fwgs- Catalytic water-gas shift reaction rateparameter (-) 1.2 1.0
Q - Fluidised bed heat losses (MJ/h) 28.93 40.43
Estimated model parameters for New Vaal and Grootegeluk coals
q Model parameters for New Vaal and Grootegeluk coals were estimated using 7 test on each coal (5 tests O2 enrichment, 1 test air/steam and 1 test oxygen/steam)
q Sum of squared deviation between measured and predicted performance variables were minimized for each coal
© CSIR 2010 Slide 38© CSIR 2006 www.csir.co.zaSlide 38
Fluidised bed gasifier modellingModel validation
Temperature – New Vaal
Fixed carbon conversion – New Vaal
© CSIR 2010 Slide 39© CSIR 2006 www.csir.co.zaSlide 39
Fluidised bed gasifier modellingModel validation
Gas calorific value – New Vaal
q For temperature and fixed carbon the predictive capability of the model was validated within 5 % and for calorific value of the gas within 10 %.
q The predictive capability is considered acceptable for preliminary design, optimisation and scale-up purposes
© CSIR 2010 Slide 40© CSIR 2006 www.csir.co.zaSlide 40
Fluidised bed gasifier modellingModel output
Gas concentration as a function of bed height
© CSIR 2010 Slide 41© CSIR 2006 www.csir.co.zaSlide 41
Fluidised bed gasifier modellingModel output
Temperature as a function of bed height
© CSIR 2010 Slide 42© CSIR 2006 www.csir.co.zaSlide 42
Fluidised bed gasifier modellingModel output
Fixed carbon converted by combustion and gasification as a function of bed height
© CSIR 2010 Slide 43© CSIR 2006 www.csir.co.zaSlide 43
Fluidised bed gasifier modellingModel output
Void fraction of the bed as a function of bed height
© CSIR 2010 Slide 44© CSIR 2006 www.csir.co.zaSlide 44
Fluidised bed gasifier modellingSensitivity analysis
Reactant pre-heat temperature
3
4
5
6
7
8
200 250 300 350 400 450 500 550 600
(Air + oxygen + steam) temperature (°C)
Dry
gas
cal
orifi
c va
lue
(MJ/N
m3 )
30
40
50
60
70
80
90
100
Fixe
d ca
rbon
con
vers
ion
(%)
Gas
ifica
tion
effic
ienc
y (%
)
Dry gas calorific value Fixed carbon conversion Gasification efficiency
Mid bed temperature - 981 °CDynamic bed height - 1.2 mOxygen in enriched air - 37 %Superficial gas velocity - 1.87 m/s
© CSIR 2010 Slide 45© CSIR 2006 www.csir.co.zaSlide 45
Fluidised bed gasifier modellingSensitivity analysis
Dynamic bed height
3.00
4.00
5.00
6.00
7.00
8.00
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2
Dynamic bed height (m)
Dry
gas
cal
orifi
c va
lue
(MJ/N
m3 )
30
40
50
60
70
80
90
100
Fixe
d ca
rbon
con
vers
ion
(%)
Gas
ifica
tion
effic
ienc
y (%
)
Dry gas calorific value Fixed carbon conversion Gasification efficiency
Mid bed temperature - 981 °CAir + O2 + steam temperature - 274 °COxygen in enriched air - 37 %Superficial gas velocity - 1.87 m/s
© CSIR 2010 Slide 46
Summary and conclusions
§ Four high-ash South African coals were gasified in an atmospheric pressure fluidised bed gasifier
§ No agglomeration and clinkering of the bed was observed
§ Fixed carbon conversion limited to 85 % (secondary combustor is required)
© CSIR 2010 Slide 47
Gasifier
Coal &sorbent
To steam circuit
Pulse gas
Gascooler
Cyclone
Gasfilter
Gas turbine& generator
Combustor
Waste heatrecovery
To steam circuit
Gas tostack
Air
FD fan
Condenser
Steam turbine& generator
Air from heaterAsh
Tosteamcircuit
Pressurelet down
Steam
Char
Air
Cooler
Tosteamcircuit
Compressor
Solids toCFBC
IntercoolerReheater Air
Air heater
Air to CFBC
Steam circuit
Air
Solids
Air
Tosteamcircuit
Fluidisingair
CFBC
BHEL gasifier
© CSIR 2010 Slide 48
Summary and conclusions
§ Four high-ash South African coals were gasified in an atmospheric pressure fluidised bed gasifier
§ No agglomeration and clinkering of the bed was observed
§ Fixed carbon conversion limited to 85 % (secondary combustor is required)
§ Calorific value of the gas during air blown gasification is limited to less than 4.0 MJ/Nm3 due to homogenous gas combustion.
© CSIR 2010 Slide 49
Recommendations
§ Fluidised bed gasification of high-ash South African coals at higher pressures is investigated
§ Evaluation of the following options for the gasification of high-ash South African coals
Ø KBR-TRIG and GTI U- Gas gasifiers
© CSIR 2010 Slide 50
Commercial fluidised bed gasifier designs
gas
char
oxygen + steam
oxygen + steam
ash agglomerates
coal
high pressure
steam
steamStandpipe
J-Valve
Riser
Upper Mixing Zone
Lower Mixing Zone
Startup Burner
Second Separation
Device
Seal Leg
First Separation
Device
Syngas
CoalAir
SteamOxygen
Ash
KBR-TRIG GTI – U-gas
© CSIR 2010 Slide 51
Recommendations
§ Fluidised bed gasification of high-ash South African coals at higher pressures is investigated
§ Evaluation of the following options for the gasification of high-ash South African coals
Ø KBR-TRIG and GTI U- Gas gasifiers
Ø The dual fluidised bed gasification
© CSIR 2010 Slide 52
Dual fluidised bed gasifier designs
© CSIR 2010 Slide 53
Recommendations
§ Fluidised bed gasification of high-ash South African coals at higher pressures is investigated
§ Evaluation of the following options for the gasification of high-ash South African coal
Ø KBR-TRIG and GTI U- Gas gasifiers
Ø The dual fluidised bed gasification
Ø Chemical looping gasification
© CSIR 2010 Slide 54
Chemical looping fluidised bed gasifier
© CSIR 2010 Slide 55
Thank You