A STEADY STATE MODEL FOR PREDICTING PERFORMANCE OF...

Freiberg 2014 20/5/2014 - Dresden 1/25

International Freiburg Conference on IGCC & XtL

Dresden, Germany, 18-22 May 2014

A STEADY STATE MODEL FOR PREDICTING PERFORMANCE OF SMALL-SCALE UPDRAFT

COAL GASIFIERS

Giorgio Cau1, Vittorio Tola1, Alberto Pettinau2

1DIMCM, Dept of Mechanical, Chemical and Materials Engineering, University of Cagliari, Italy 2Sotacarbo S.p.A. – c/o Grande Miniera di Serbariu – Carbonia, Italy

email:[email protected]


Introduction

Ø  Coal gasification is increasing its importance for both power generation and production of chemicals and clean fuels (hydrogen, methanol, DME and others).

Ø  Integrated with advanced CC and FC will make it possible to achieve efficiency exceeding 50-55%. Large scale IGCC can already achieve efficiency as high as 43-46% with very low emissions.

Ø  R&D activities are focussing on medium- and small-scale industrial applications.

Ø  Small-scale fixed bed gasifiers are commonly fed by air. Air blown gasifier generates a syngas with a low LHV (about 5-6 MJ/kg), high N2 (40-45%) and low hydrogen fractions.


Gasification processes

Ø  Gasification processes are very complex, involving: drying, devolatilisation, pyrolysis, gasification and combustion.

Ø  Influenced by kinetics and fluid-dynamical effects. Ø  Thermodynamic equilibrium only for a known temperature.

Ø  Equilibrium models can predict accurately performance of fluidised-bed and entrained-bed (temperature distribution almost constant).

Ø  In the fixed-bed gasifier drying, devolatilisation, pyrolysis take place at a lower temperature in the heat transfer zone. Moisture and volatiles are generally not involved in the gasification reactions.

Ø  In fixed-bed gasifiers equilibrium models are barely able to accurately predict performance and syngas composition leading to large uncertainties in the reaction temperature, even though reactants residence time inside the gasifier is higher than reaction time.


Simplified scheme of fixed bed gasification process

Coal preheating and drying

Coal devolatilization

Char gasification and combustion

Steam and oxidant preheating

ash

coal syngas

gasification agents

water inlet

water outlet

water outlet

verti

cal

trans

latio

n

rotate

water inlet

coal feed

gas offtake

coal feed

air + steam

ash

water jacket

water inlet

water outlet

water outlet

verti

cal

trans

latio

n

rotate

water inlet

coal feed

gas offtake

coal feed

air + steam

ash

water jacket


Gasification modeling

Ø  A computer simulation model for predicting fixed bed gasifiers performance, has been developed at the DIMCM using ASPEN Plus® commercial software package.

Ø  The ASPEN model schematizes the gasifier into several different zones: coal prehating and drying, devolatilisation, gasification, combustion, steam and oxidant preheating.

Ø  The model evaluates the mass and energy balance in each zone and the syngas main characteristics for given coal composition and coal, steam and oxidant mass flows.

Ø  In the gasification and combustion zone the model calculates the syngas composition and the equilibrium temperature minimizing the Gibbs free energy.

Ø  The model calculates the syngas temperature considering the “countecurrent” heat exchange processes between syngas and coal inside the different sections, imposing a suitable ΔT.


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DRYING

DEC - COAL DEC - VOL

REACTOR

A sh TAR

C oal

F ixed Carbon

C har

A sh S team

A ir

R aw S yngas Coal M oisture

V olatile Matter G ases W ater

H ot Syngas

S yngas

ASH - COOL

Simplified scheme of the Aspen Plus-based gasification model

Simplified scheme of the Aspen-Plus fixed bed gasifier using


Main input data for the Aspen-Plus model

Ø  Proximate coal analysis (fixed carbon, volatile matter, moisture and ash);

Ø  Ultimate coal analysis (carbon, hydrogen, sulphur, nitrogen, oxygen, moisture and ash);

Ø  Lower Heating Value of the coal;

Ø  Volatile matter and volatile gases composition;

Ø  Oxidant and steam characteristics;

Ø  Carbon conversion rate;

Ø  Thermal energy losses and other energy losses;

Ø  Temperature difference between syngas and coal inside the different sections.


Input data for the ASPEN model

Proximate Analysis (%Wt) Fixed carbon 31.33 Volatile matter 41.00 Ash 10.02 Moisture 17.64

Ultimate Analysis (%Wt) Carbon 48.56 Hydrogen 5.96 Sulphur 0.18 Nitrogen 0.50 Chlorine 0.05 Oxygen 17.14 Ash 10.02 Moisture 17.64 Lower Heating Value (MJ/kg) 19.95

Volatile matter (%Wt) Water 15.00 TAR 20.00 Volatile gases 65.00 Volatile gases (%Vol) CO 19.78 CO2 4.20 H2 66.11 CH4 9.92

Operating conditions Steam/coal mass ratio 0.154 Steam conditions (°C/MPa) 250/0.14 Air/coal mass ratio 2.4 Air conditions (°C/MPa) 250/0.14 Gasifier energy losses (%) 0.0-5.0

Usibelli coal


Variable parameters

Ø  Coal flow rate;

Ø  Air/coal and Steam/coal mass ratios;

Ø  Thermal energy losses;

Ø  Carbon conversion rate;

Ø  Rate of TAR and ash in the syngas;

Ø  Rate of air not reacted in the gasification and combustion section;

Ø  Volatile matter composition.

Sensitivity analysis


Gasification and syngas Temp., LHV and ηCG vs air/coal mass ratio

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3

Air coal mass ratio α

100200300400500600700800900

Syng

as e

xitin

g te

mpe

ratu

re (°

C) 600

800

1000

1200

1400

1600

Gas

ifica

tion

tem

pera

ture

(°C

)

3

3.5

4

4.5

5

5.5

6

6.5

Syng

as L

HV

(MJ/

kg)

0.7

0.75

0.8

0.85

0.9

0.95

Col

d ga

s ef

ficie

ncy

Syngas exit temperatureGasification temperatureSyngas Lower Heating ValueCold Gas Efficiency

An increase of air/coal mass ratio leads to a higher gass. and syngas temperature. For α higher than 2 combustion reactions prevails against gassification reactions and temperatures increase remarkably.

An increase of air/coal mass ratio leads to a lower LHV, expecially for an α value for that combustion reactions prevail against gassification reactions.

Maximum cold gas efficiency can be obtained for an α value equal to 2.


Syngas composition vs air/coal mass ratio

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3


0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

Syng

as c

ompo

sitio

n (d

ry) CO

CO2H2CH4N2

An increase of air/coal mass ratio leads to a higher nitrogen molar fraction.

An increase of air/coal mass ratio leads to a lower hydrogen molar fraction due to combustion reactions.

For an air/coal mass ratio value equal to 2 a maximum value for the CO molar fraction and a minimum value for the CO2 molar fraction is reached.


Gasification and syngas Temp., LHV and ηCG vs steam/coal mass ratio

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

Steam/coal mass ratio µ

100200300400500600700800900

Syng

as e

xitin

g te

mpe

ratu

re (°

C) 600

800

1000

1200

1400

1600

Gas

ifica

tion

tem

pera

ture

(°C

)

3

3.5

4

4.5

5

Syng

as L

HV

(MJ/

kg)

0.7

0.75

0.8

0.85

0.9

0.95

Col

d ga

s ef

ficie

ncy


An increase of steam/coal mass ratio leads to a slightly lower gassification temperature.

Syngas temperature is almost constant with the steam/coal mass ratio despite a lower gassification temperature due to the higher heat capacity of steam.

An increase of the steam/coal mass ratio leads to a lower LHV and despite an higher syngas mass flow also to a slightly lower cold gas efficiency.


Syngas composition vs steam/coal mass ratio

0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3


0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

Syng

as c

ompo

sitio

n (d

ry) CO

CO2H2CH4N2

An increase of steam/coal mass ratio leads to a lower nitrogen molar fraction.

An increase of steam/coal mass ratio favours shift conversion reaction leading to lower CO molar fraction and higher H2 and CO2 molar fractions.


Gasification and syngas Temp., LHV and ηCG vs steam/coal and air/coal mass ratios

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3


100200300400500600700800900

Syng

as e

xitin

g te

mpe

ratu

re (°

C) 600

800

1000

1200

1400

1600

Gas

ifica

tion

tem

pera

ture

(°C

)

3

3.5

4

4.5

5

5.5

6

6.5

Syng

as L

HV

(MJ/

kg)

0.7

0.75

0.8

0.85

0.9

0.95

Col

d ga

s ef

ficie

ncy


0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3


100200300400500600700800900

Syng

as e

xitin

g te

mpe

ratu

re (°

C) 600

800

1000

1200

1400

1600

Gas

ifica

tion

tem

pera

ture

(°C

)

3

3.5

4

4.5

5

Syng

as L

HV

(MJ/

kg)

0.7

0.75

0.8

0.85

0.9

0.95

Col

d ga

s ef

ficie

ncy



Gasification and syngas Temp., LHV and ηCG vs thermal energy losses

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Thermal energy losses (%LHV)

500

550

600

650

700

750

Syng

as e

xitin

g te

mpe

ratu

re (°

C) 1000

1100

1200

1300

1400

1500

Gas

ifica

tion

tem

pera

ture

(°C

)

4.5

4.6

4.7

4.8

4.9

5

Syng

as L

HV

(MJ/

kg)

0.7

0.75

0.8

0.85

0.9

Col

d ga

s ef

ficie

ncy


An increase of thermal energy losses leads to a remarkable reduction of gassification temperature and consequentely of syngas temperature.

An increase of thermal energy losses leads to a slightly lower LHV, and a consequentely reduced cold gas efficiency.


Syngas composition vs thermal energy losses

0 0.1 0.2 0.3 0.4 0.5

Thermal energy losses (%LHV)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

Syng

as c

ompo

sitio

n (d

ry) CO

CO2H2CH4N2

No substantial variations in syngas composition are reported increasing thermal energy losses.

Due to lower gassification temperature shift reacion is favoured leading to a slight increase of hydrogen and CO2 molar fraction and a reduction of CO.


Gasification and syngas Temp., LHV and ηCG vs carbon conversion rate

80 82 84 86 88 90 92 94 96 98 100

Carbon conversion rate (%)

450

500

550

600

650

700

Syng

as e

xitin

g te

mpe

ratu

re (°

C) 1000

1100

1200

1300

1400

1500

Gas

ifica

tion

tem

pera

ture

(°C

)

4

4.2

4.4

4.6

4.8

5

Syng

as L

HV

(MJ/

kg)

0.65

0.7

0.75

0.8

0.85

0.9

Col

d ga

s ef

ficie

ncy


An increase of carbon conversion rate leads to a lower gassification temperature. Also the syngas temperature is reduced.

An increase of carbon conversion rate allows to reach a higher LHV and consequently a higher cold gas efficiency.


Syngas composition vs carbon conversion rate

80 82 84 86 88 90 92 94 96 98 100

Carbon conversion rate (%)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

Syng

as c

ompo

sitio

n (d

ry) CO

CO2H2CH4N2

An increase of carbon conversion rate reduces nitrogen molar fraction for dilution.

The increase of hydrogen and CO molar fraction causes the increase of LHV reported in the previuos slide.


Gasifier performance vs Volatile matter composition

TAR/volatiles/H2O 25/55/20 25/60/15 25/65/10 20/65/15 20/70/10 20/75/05 15/70/15 15/75/10 15/80/05 10/75/15 10/80/10 10/85/05

Volatile gases (% vol) CO 0,1853 0,1940 0,2008 0,1978 0,2044 0,2097 0,2018 0,2081 0,2131 0,2059 0,2119 0,2166 CO2 0,0393 0,0411 0,0426 0,0420 0,0434 0,0445 0,0428 0,0441 0,0452 0,0437 0,0449 0,0460 H2 0,6897 0,7259 0,7545 0,6611 0,6971 0,7262 0,5936 0,6376 0,6730 0,5234 0,5759 0,6180 CH4 0,0857 0,0390 0,0021 0,0992 0,0552 0,0196 0,1618 0,1102 0,0687 0,2269 0,1673 0,1194

TAR/volatiles/H2O 25/55/20 25/60/15 25/65/10 20/65/15 20/70/10 20/75/05 15/70/15 15/75/10 15/80/05 10/75/15 10/80/10 10/85/05

Syngas composition (dry)

CO 0,2252 0,2238 0,2224 0,2160 0,2149 0,2145 0,2068 0,2063 0,2055 0,1960 0,1960 0,1959 CO2 0,0502 0,0520 0,0539 0,0573 0,0589 0,0599 0,0635 0,0647 0,0661 0,0710 0,0718 0,0728 H2 0,1977 0,2173 0,2362 0,1945 0,2142 0,2326 0,1709 0,1916 0,2114 0,1469 0,1685 0,1891 CH4 0,0152 0,0077 0,0004 0,0196 0,0119 0,0046 0,0322 0,0240 0,0162 0,0454 0,0367 0,0283 N2 0,4923 0,4802 0,4686 0,4932 0,4811 0,4698 0,5066 0,4939 0,4818 0,5202 0,5070 0,4944 O2 0,0132 0,0128 0,0125 0,0132 0,0129 0,0126 0,0136 0,0132 0,0129 0,0139 0,0136 0,0132 Ar 0,0058 0,0057 0,0056 0,0059 0,0057 0,0056 0,0060 0,0059 0,0057 0,0062 0,0060 0,0059 H2S 0,0004 0,0004 0,0004 0,0004 0,0004 0,0004 0,0004 0,0004 0,0004 0,0004 0,0004 0,0004 COS 0,0000 0,0000 0,0000 0,0000 0,0000 0,0000 0,0001 0,0001 0,0000 0,0001 0,0001 0,0001 H2O 0,1230 0,1124 0,1018 0,1199 0,1092 0,0993 0,1270 0,1163 0,1057 0,1336 0,1229 0,1123 H2/CO 0,8777 0,9708 1,0621 0,9004 0,9968 1,0845 0,8262 0,9288 1,0284 0,7494 0,8596 0,9656 LHV [MJ/kg] 4,7422 4,7945 4,8465 4,7341 4,7864 4,8397 4,6728 4,7254 4,7779 4,6101 4,6631 4,7159 Cold gas efficiency 0,8139 0,8229 0,8318 0,8132 0,8222 0,8313 0,8034 0,8124 0,8214 0,7933 0,8024 0,8115 Gass. Temp. [#C] 1038,7 981,1 923,4 1078,6 1021,1 984,2 1176,4 1119,0 1061,5 1274,7 1217,5 1160,1 Syngas Temp. [°C] 503,0 464,2 425,6 606,8 489,8 465,2 593,0 554,3 515,6 657,7 619,0 580,3


Sotacarbo pilot plant

gasification section

- fixed-bed up-draft gasification

- syngas outlet temp.: ~ 350-400 °C

- ash discharge: dry

- internal diameter: 300 mm

- internal heigh: 2000 mm

- coal particle size: 5-15 mm

- gasif. agents: air and steam

- operating pressure: 0.14 MPa


Gasifier performance and Syngas composition for the air-blown gasifier

Gasifier performance *

Coal mass flow (kg/h) 24.0

Air mass flow (kg/h) 57.6

Steam mass flow (kg/h) 3.7

Syngas mass flow (kg/h) 83.0

Mean gasification temperature (°C) 1352

Syngas exit temperature (°C) 786

Syngas LHV (MJ/kg) 4.74

Cold gas efficiency (%) 81.5

Dry Syngas/coal flow ratio (Nm3/kg) 2.91

Syngas composition (%vol)

CO 19.58 CO2 4.47 H2 16.54 CH4 1.72 N2 43.41 O2 1.16 Ar 0.51 H2S + COS 0.04 H2O 12.56 Gasifier energy losses (%) 0.0

* Reference to Sotacarbo pilot updraft gasifier

Adiabatic gasifier


Gasifier performance and Syngas composition for the air-blown gasifier

Gasifier performance *

Coal mass flow (kg/h) 24.0

Air mass flow (kg/h) 57.6

Steam mass flow (kg/h) 3.7

Syngas mass flow (kg/h) 83.0

Mean gasification temperature (°C) 1079

Syngas exit temperature (°C) 606

Syngas LHV (MJ/kg) 4.73

Cold gas efficiency (%) 81.3

Dry Syngas/coal flow ratio (Nm3/kg) 2.93

Syngas composition (%vol)

CO 19.01 CO2 5.04 H2 17.11 CH4 1.72 N2 43.41 O2 1.16 Ar 0.51 H2S + COS 0.04 H2O 11.99 Gasifier energy losses (%) 5.0

* Reference to Sotacarbo pilot updraft gasifier

Gasifier energy losses = 5% of mcoal*LHV


Experimental and calculated syngas composition and properties

Experimental and calculated syngas composition and properties at reference conditions for the up-draft gasifier

experimental adiabatic 5% thermal energy losses

Raw syngas composition (% vol dry basis) CO 0,2368 0,2239 0,2160 CO2 0,0771 0,0510 0,0573 H2 0,1779 0,1892 0,1945 CH4 0,0173 0,0197 0,0196 N2 0,4729 0,4964 0,4932 O2 0,0173 0,0132 0,0132 Ar * 0,0059 0,0059 H2S + COS 0,0003 0,004 0,0004 H2/CO 0,7513 0,8450 0,9004 Raw syngas properties (dry basis) Mass flow (kg/h) 79,67 74,95 75,33 Volume flow (Nm3/h) 72,90 69,86 70,31 LHV (MJ/kg) 5,14 5,43 5,38 Syngas Temperature (°C) * 787 607 Main gasifier performance Mean gasification temperature (°C) 993 1353 1079 Gasifier yield (Nm3/kg) 3,04 2,91 2,93

Coal mass flow increase of 6%

Rate of air not reacted in the gasification and combustion section from 10 to 14%


Experimental and calculated syngas composition and properties

experimental adiabatic %% thermal energy losses

adiabatic %% thermal energy losses

Raw syngas composition (% vol dry basis) CO 0,2368 0,2239 0,2160 0,2387 0,2295 CO2 0,0771 0,0510 0,0573 0,0398 0,0469 H2 0,1779 0,1892 0,1945 0,2034 0,2093 CH4 0,0173 0,0197 0,0196 0,0200 0,0198 N2 0,4729 0,4964 0,4932 0,4743 0,4708 O2 0,0173 0,0132 0,0132 0,0177 0,0176 Ar * 0,0059 0,0059 0,0056 0,0056 H2S + COS 0,0003 0,004 0,0004 0,004 0,0004 H2/CO 0,7513 0,8450 0,9004 0,8492 0,9085 Raw syngas properties (dry basis) Mass flow (kg/h) 79,67 74,95 75,33 75,90 76,34 Volume flow (Nm3/h) 72,90 69,85 70,31 73,13 73,68 LHV (MJ/kg) 5,14 5,43 5,38 5,87 5,81 Syngas Temperature (°C) * 787 607 568 373 Main gasifier performance Mean gasification temperature (°C) 993 1353 1079 1174 878 Gasifier yield (Nm3/kg) 3,04 2,91 2,93 2,87 2,90


Conclusions

Ø  The computational model developed using Aspen Plus simulates fixed bed gasifier performance accurately.

Ø  Good agreement between gasifier performance calculated by the model and experimental results.

Ø  The influence of the main process parameters has been also evaluated. The study shows that gasifier performance is extremely sensitive to air/coal and steam/coal mass ratios and to thermal energy losses and carbon conversion rate.

Ø  Accuracy of performance prediction could be improved introducing a devolitilization model and optimising adjustable parameters as internal minimum ΔT between syngas and coal, amount and distribution of thermal energy losses, tar and ash flow rate in the syngas, and carbon conversion rate.

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