Modeling of black liquor
gasification
Rikard Gebart, LTU
Per Carlsson, ETC
Magnus Marklund, ETC
1
Black liquor
•By-product from chemical Kraft
pulping
• ~8.1 Mton dry biomass (~40 TWh/y)
•1/3 water, 1/3 combustibles and 1/3
ash
•Conventionally combusted in recovery
boilers to recover heat and chemicals
•Highly viscous fluid
•Heating value ~12 MJ/kg (Oil: ~42
MJ/kg)
•Gasification of black liquor and catalytic
conversion of syngas could replace about
25% of Swedish gasoline and diesel
demand if all black liquor is gasified
2
BLACK LIQUOR
COOLING
WATER
OXYGEN AND
ATOMIZING MEDIA
GAS COOLER
REACTOR
RAW GAS
QUENCH
GREEN
LIQUOR
3
CONDENSATE
WEAKWASH
Courtesy of Chemrec AB
COOL DRY RAW
GAS
Experimental measurements
with a suction probe in the
lower part of the reactor
4
MEASURING POSITION
Species
CO2 (% mol)
CO (% mol)
H2 (% mol)
CH4 (% mol)
H2 S (% mol)
COS (ppm mol)
5
Probe measurements
After CCC
33.9±0.3
28.7±0.2
34.3±0.2
1.36±0.07
1.65±0.04
468±22
33.6±0.2
28.5±0.2
34.8±0.1
1.44±0.07
1.71±0.02
122±5
Conceptual model for conversion
Spray burner
Initial droplet
Drying
2.3 m
Refractory lining
Dry solids
Devolatilization
Char
Char gasification
Smelt
Smelt formation
Sampling probe
0.6 m
6
Reactor inlet
BL
O2
aOH(l)
NaOH(l)
Na2C
Na2CO3(l)
O3(l)
Na2S(l
Na2S(l)
)
Na2SO
Na2SO4(l)
4(l)
H2O(l)
Volatil
Volatiles
es
C(s)
CH4
H2
NaOH(l)
Conversion steps
Volatiles
Na2CO3(l)
CO2
C(s)
Na2S(l)
CO
Na2SO4(l)
NaOH(l)
H2S
C(s)
Na2CO3(l)
Na2SO4(l)
Na2S(l)
NaOH(l)
Na2S(l)
Na2CO3(l)
7
H2 + ½O2 → H2O
CH4 + ½O2 → CO + 2H2
CH4 + H2O → CO + 3H2
CO + H2O ↔ CO2 + H2
C + H2O → CO+H2
C + CO2 → 2CO
C + ½Na2SO4 →½Na2S + CO2
Reactor outlet
A few words about tars
• Benzene concentration was below 100 ppm
which indicates a very low tar content
• This has later been confirmed in dedicated tar
measurements
• Tar species are typical for high temperature
formation (i.e. multi-ring tars like phenantrene)
• The heaviest compound in the pyrolysis model is
methane
• Equivalent to assuming infinitely fast cracking of
tars that are formed during pyrolysis
8
CFD model
• Multi-phase flow formulation with discrete
particles and continuous gas phase (EulerLagrange)
• Turbulence modeled with Reynolds averaged
equations and an eddy viscosity model
• Heat transfer from convection and radiation
• Iterative solution with alternating gas and droplet
phase computations
• Droplet size distribution from experimental
measurements
9
Particle phase
• Particles are assumed to be spherical
• Heat transfer is directly computed from the gas
phase and radiation solution
• Step 1: Drying can be directly computed from
the heat transfer to the droplet
• Step 2: Devolatilisation consists of two parts
– Volatile composition is computed beforehand from the
fuel analysis enforcing element conservation
– Devolatilisation rate is computed from an Arrhenius
expression taken from literature (rate depends on
droplet temperature)
10
Particle phase/2
• Step 3: Char gasification is limited by mass
transfer at the droplet surface
– Mass transfer rate is computed from the gas phase
solution at the droplet surface
• Step 4: Smelt formation is limited by mass
transfer at the droplet surface
11
Gas phase reactions
• First attempt to model with the Jones & Lindstedt
4-step scheme over-predicted methane
destruction
• Comparison with a detailed kinetic model (GRIMECH) showed large errors in the Jones &
Lindstedt model at reducing conditions and
temperatures below about 1200 C
• Modified model with an assumed infinitely slow
methane conversion for T < 1400 K agreed well
with GRI-MECH and experiments
12
Carlsson, P., Iisa, K., & Gebart, R. (2011). Computational Fluid Dynamics Simulations of Raw Gas
Composition from a Black Liquor Gasifier—Comparison with Experiments. Energy & Fuels, 25(9),
4122–4128. doi:10.1021/ef2003798
Assumed volatile
composition that
conserves elements
(8 simple assumptions
and solution of an
overdetermined system
of equations)
13
All heats of reaction add up to the experimental
heating value (energy is conserved)
14
15
16
17
18
19
38.0
Variable
Pressure
Black liquor mass flow rate
Black liquor pre-heat temperature
O2 mass flow rate
λ
Normalized gas residence time
Variations in
Unit equivalence ratio
bar
28
28
28
kg/h 870 870 870
°C
140 140 140
kg/h 232 254 265
0.396 0.434 0.452
1.019
1 0.991
-
36.0
H2
34.0
CO2
32.0
30.0
28.0
Data from:
Carlsson, P., Wiinikka, H., Marklund, M., Grönberg, C.,
Pettersson, E., Lidman, M., & Gebart, R. (2010).
Experimental investigation of an industrial scale black
liquor gasifier. 1. The effect of reactor operation
parameters on product gas composition. Fuel, 89(12),
4025–4034. doi:10.1016/j.fuel.2010.05.003
20
CO
CO2
26.0
H2
CH4
24.0
3.0
2.5
2.0
CO
CH4
1.5
1.0
0.5
0.38 0.40 0.42 0.44 0.46 0.48
λ
38.0
Variable
Pressure
Black liquor mass flow rate
Black liquor pre-heat temperature
O2 mass flow rate
λ
Normalized gas residence time
Variations in
Unit equivalence ratio
bar
28
28
28
kg/h 870 870 870
140 140 140
°C
kg/h 232 254 265
0.396 0.434 0.452
1.019
1 0.991
-
36.0
H2
34.0
CO2
32.0
30.0
28.0
CO
CO2
26.0
H2
CH4
24.0
3.0
2.5
2.0
CO
CH4
1.5
1.0
0.5
0.38 0.40 0.42 0.44 0.46 0.48
λ
21
38.00
Variable
Pressure
Black liquor mass flow rate
Black liquor pre-heat temperature
O2 mass flow rate
λ
Normalized gas residence time
Variations in
Unit
pressure
16
21
28
bar
kg/h 487 645 870
140 140 140
°C
kg/h 151 196 254
0.460 0.451 0.434
0.977 0.989
1
-
CO2
36.00
34.00
H2
32.00
30.00
28.00
CO
CO2
26.00
H2
CH4
24.00
1.50
1.25
CO
CH4
1.00
0.75
0.50
14 16 18 20 22 24 26 28 30
Pressure
22
38.00
Variable
Pressure
Black liquor mass flow rate
Black liquor pre-heat temperature
O2 mass flow rate
λ
Normalized gas residence time
Variations in
Unit
pressure
16
21
28
bar
kg/h 487 645 870
140 140 140
°C
kg/h 151 196 254
0.460 0.451 0.434
0.977 0.989
1
-
CO2
36.00
34.00
H2
32.00
30.00
28.00
CO
CO2
26.00
H2
CH4
24.00
1.50
1.25
CO
CH4
1.00
0.75
0.50
14 16 18 20 22 24 26 28 30
Pressure
23
Conclusions
• Simplified model yields results that agree well
with experiments
– Absolute values for syngas composition
– Trends when the process parameters are changed
• Based on this it is assumed that scale up effects
should also be predicted reasonably well
24
Future work
• Idealized experiments; distribution of Na2SO4-Na2S-H2S
and time resolved measurement of the pyrolysis gas
composition for high heating rates
• Turbulence models and chemistry
• Investigation of numerical errors from finite number of
particles
• Comparison of experimental flame dynamics with
simulations
• Tar measurements
• Tar modeling if necessary
25