Fluidised bed gasification of high-ash South African coals ...

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Fluidised bed gasification of high-ash South African coals: An experimental and modelling

A.D. Engelbrecht, B.C. North, B.O. Oboirien, R.C. Evers on and H.W.P.J. Neomagus

MAY 2012

Outline of presentation

�Coal gasification reactors

�Fluidised bed coal gasification

�Fluidised bed pilot plant at CSIR

�Pilot-scale coal gasification tests results

�Fluidised bed gasifier modelling

� 15 MW FB gasifier design

�Summary and conclusions

Coal gasification reactors

Entrained flow gasifierEntrained flow gasifierAdvantages:•Very low tar and CH 4 conc.•Caking coals•High pressure operation

Disadvantages:•Reduced refractory life•Short residence times•High oxygen consumption

Moving bed gasifierAdvantages:•High gasification efficiency•Long residence time•High ash coals

Disadvantages:•Coal 8 – 50 mm•Produces tar and CH 4

•Rotating grate

Fluidised bed gasifierAdvantages:•Coal 0 - 5 mm•High ash coal•Low tar and CH 4 conc.

Disadvantages:•Lower gasification efficiency•Mixed flow reactor•Bed agglomeration

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

Oxygen + steam

High pressuresteam

KBR TRIGTM transport gasifier

Standpipe

Mixing

Startup

Burner

Second

Separation

Device

Separation

Device

Syngas

CoalAir

SteamOxygen

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

Coal properties important for fluidised bed gasific ation

Coal properties

Requiredfor FBG

SA coalin future

Moisture < 7.5 % < 6 %

Ash 15 - 50 % 25-50 %

AFT > 1400 °C > 1400 °C

Caking index <1 <1

Reactivity high med-low

Size 1- 4 mm Fines

Volatiles >20 % ± 25 %

Proximate and ultimate analysis of New Vaal and Groote geluk coals

New Vaal Grootegeluk

Proximate analysis:

Calorific value (MJ/kg) 14.8 21.4

Ash content (%) 40.7 31.7

Moisture (%) 5.7 1.9

Volatile matter (%) 20.5 28.3

Fixed carbon (%) 33.1 38.1

Total sulphur (%) 0.84 1.17

Ultimate analysis:

Carbon (%) 39.25 52.93

Hydrogen (%) 3.45 4.11

Nitrogen (%) 0.90 1.19

Sulphur (%) 0.84 1.17

Oxygen (%) 9.16 7.00

Reflectance analysis:

Vitrinite reflectance (%) 0.55 0.71

Fluidised bed gasifier pilot plant

Bed area: 200mm X 200mmFurnace height: 4000mmRating: 140 kWCoal feedrate: 20 -30 kg/h

Experimental Program

� Twelve 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 ms-1)� Oxygen enrichment level (32 - 37 %)

Experimental results

Oxygen enriched air

H2 - 22.7CO - 17.6CH4 - 1.2

FBG syngas flare

Fluidised bed gasifier modelling

Bottom ash

Fly ash

Syngas

Fluidised bed gasifier models can be used for:

� Design, optimisation and scale-up

� Accessing start-up and shut-down conditions

� Adaptive control

� Trouble shooting

1. Mass and energy balance models

� Conservation of mass and energy� More unknowns than equations� Assumptions regarding unknowns are required:

� Fixed carbon conversion� 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. Kinetic 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

Comprehensive Simulator of Fluidised and Moving Beds (CeSFaMB)

� Development of CeSFaMB started in 1989 at the University of Sheffield (UK) by Professor Mario De-Souza Santos

� CeSFaMB can be used for the simulation of :

� Fluidised bed combustors, gasifiers and dryers� Moving bed gasifiers (updraft and downdraft)� Entrained flow gasifiers and combustors

� CeSFaMB was obtained by North West University and the CSIR under an academic license

CeSFaMB sub-models and correlations

Sub-model Correlation used by CeSFaMB

Hydrodynamics:Minimum fluidising velocity (Umf)Bubble diameter (db)Bubble rise velocity (m/s)Bubble fraction (-)

Wen and Yu Mori and Wen Davidson and Harrison Davidson and Harrison

Reaction rates:Gas-solid (combustion and gasification)Gas combustionWater gas shift

Yoon et al , Johnson Villenski and Hezeman Franks

Mass transfer coefficientsBubble - emulsionEmulsion- solid

Sit and Grace La Nauze et al

Heat transfer coefficientsBubble - emulsionEmulsion- solid

Kunii and Levenspiel Kunii and Levenspiel

Devolatilisation Loison and Chauvin

Elutriation Wen and Chen

CeSFaMB inputs

Gasifier design inputs:

� Gasifier diameter, height and thermal conductivity of insulation in the bed

� Gasifier diameter, height and thermal conductivity of insulation in the freeboard

� Height at which coal is fed into the gasifier

� Height at which reactants (air, oxygen and steam) are injected into the gasifier

� Height of syngas withdrawal from the gasifier

� Number and diameter of holes in the distributor for reactant injection

Operation inputs:

� Coal feedrate and analysis (proximate, ultimate, CV, size, sphericity, density)

� Feedrate of air, oxygen and steam

� Temperature of air, oxygen and steam

� Gasifier pressure

� Fluidised bed height (dynamic)

CeSFaMB outputs

� Bed and freeboard temperatures ( i.e. temperature profile through the

gasifier)

� Fixed and total carbon conversions

� Syngas output flow and composition

� Concentration of all gasses as a function of height in the gasifier

� Superficial gas velocity as a function of height in the gasifier

� Bubble diameter and bubble velocities as a function of bed height

� Rate of all reactions as function of height in the gasifier

CeSFaMB model parameters

Model parameters

� Pre-exponential factor (ko3) for the steam- char reaction ( C + H2O => CO + H2 )

� Pre-exponential factor (ko4) for the the carbon dioxide – char reaction ( C + CO2 => 2 CO )

� Pre-exponential factor (ko5) for the the hydrogen-char reaction ( C + 2H2 => CH4 )

� Pre-exponential factor (ko41) for the water-gas shift reaction ( CO + H2O => CO2 + H2 )

Rate equationRate equation

ioi PXF

dX)()exp(

CeSFaMB model parameters

Rate equationParameter New Vaal Grootegeluk

Pre-exponential factors (s -1)

C+H2O �� CO + H2: k03 13 600 49

C + CO2 �� 2CO: k04 210 6

C + H2 �� CH4: k05 3.1E-07 4.0E-07

CO + H2O �� CO2 + H2: k41 78 25

CeSFaMB predictive capability

Rate equationDeviation between measured and predicted output variables (%)

New Vaal Grootegeluk

Mid bed temperature 2.1 2.4

Fixed carbon conversion 3.6 3.4

Gasifier exit temperature 3.0 13.4

CO 6.3 11.0

H2 11.7 39.5

CH4 38.5 27.1

CO2 4.6 8.6

15 MW FBG design

15 MW FBG output variables

Rate equationOutput variables New Vaal

Mid bed temperature (°°°°C) 948

Gasifier exit temperature (°°°°C) 931

CO (%) 19.80

H2 (%) 24.3

CH4 (%) 1.2

CO2 (%) 18.62

Calorific value (MJ/Nm 3) 6.2

Fixed carbon conversion (%) 93.2

Gas concentration profiles in the gasifier

Concentration of gasses in the gasifier as a functi on of height

0 1 2 3 4 5 6 7 8 9 10

Gasifier heigh t (m )

C+H2O ----> H2+CO

C+CO2 ----> 2COCO+H2O ---->H2+CO2

H2+O2---> H2O

CO+O2---> CO2

C+O2--->CO+CO2

CH4+O2--->H2+CO2

Bubble velocity and bubble diameter in the bed

Rate equation

Summary and conclusions

� Two high-ash South African coals were successfully gasified in a pilot-scale fluidised bed gasifier

� The fixed carbon conversion in the fluidised bed gasifier increases with an increase in coal reactivity, temperature and residence time of char particles in the gasifier

� The deviation between measured values and values predicted by the model can partially be attributed to the elemental mass balance non-closures produced by CeSFaMB

� For a scaled-up 15 MW fluidised bed gasifier CeSFaMB predicts a significant increases in fixed carbon conversion

� A higher fixed carbon conversion is possible due to due to the increase in residence time and the absence of bed slugging

Thank You

Fluidised bed gasification of high-ash South African coals ...

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