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

Transcription

1 Fluidised bed gasification of high-ash South African coals: An experimental and modelling study A.D. Engelbrecht, B.C. North, B.O. Oboirien, R.C. Everson and H.W.P.J. Neomagus MAY CSIR 2010 Slide 1

2 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 CSIR 2010 Slide 2

3 Coal gasification reactors Moving bed gasifier Advantages: High gasification efficiency Long residence time High ash coals Fluidised bed gasifier Advantages: Coal 0-5 mm High ash coal Low tar and CH 4 conc. Entrained flow gasifier Advantages: Very low tar and CH 4 conc. Caking coals High pressure operation Disadvantages: Coal 8 50 mm Produces tar and CH 4 Rotating grate Disadvantages: Lower gasification efficiency Mixed flow reactor Bed agglomeration Disadvantages: Reduced refractory life Short residence times High oxygen consumption CSIR 2010 Slide 3

4 GTI U-gas fluidised bed gasifier High pressure steam coal char steam Gas Projects: Shadong, China, Nm 3 /h syngas for methanol, start-up 2008, high-ash coal wash plant waste. Henan, China, Nm 3 /h syngas for ammonia, start-up 2014, high-ash sub-bituminous coal Oxygen + steam Oxygen + steam char CSIR 2010 Slide 4

5 KBR TRIG TM transport gasifier First Separation Device Syngas Projects: Riser Second Separation Device Kemper County, USA, 550 MW IGCC, start-up 2014, lignite Startup Burner Upper Mixing Zone Coal Air Lower Mixing Zone Steam Oxygen Seal Leg J-Leg Syngas Standpipe Dongguan, China,120 MW IGCC, start-up 2014, high-ash bituminous coal Inner Mongolia, China, Nm 3 /h syngas for ethylene glycol, start-up 2015, high-ash lignite CSIR 2010 Slide 5

6 Coal properties important for fluidised bed gasification Coal properties Required for FBG Moisture < 7.5 % < 6 % SA coal in future Fit Ash % % AFT > 1400 C > 1400 C Caking index <1 <1 Reactivity high med-low Size 1-4 mm Fines Volatiles >20 % ± 25 % CSIR 2010 Slide 6

7 Proximate and ultimate analysis of New Vaal and Grootegeluk coals Proximate analysis: New Vaal Grootegeluk Calorific value (MJ/kg) Ash content (%) Moisture (%) Volatile matter (%) Fixed carbon (%) Total sulphur (%) Ultimate analysis: Carbon (%) Hydrogen (%) Nitrogen (%) Sulphur (%) Oxygen (%) Reflectance analysis: Vitrinite reflectance (%) CSIR 2010 Slide 7

8 Fluidised bed gasifier pilot plant Bed area: 200mm X 200mm Furnace height: 4000mm Rating: 140 kw Coal feedrate: kg/h CSIR 2010 Slide 8

9 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 ( C) Residence time (15-55 min) Fluidising velocity ( ms -1 ) Oxygen enrichment level (32-37 %) CSIR 2010 Slide 9

10 Experimental results CSIR 2010 Slide 10

11 Experimental results CSIR 2010 Slide 11

12 Experimental results H CO CH Oxygen enriched air Air CSIR 2010 Slide 12

13 FBG syngas flare CSIR 2010 Slide 13

14 Fluidised bed gasifier modelling Syngas FBG Fly ash Coal O2 Air Steam Bottom ash CSIR 2010 Slide 14

15 Fluidised bed gasifier modelling Fluidised bed gasifier models can be used for: Design, optimisation and scale-up Accessing start-up and shut-down conditions Adaptive control Trouble shooting CSIR 2010 Slide 15

16 Fluidised bed gasifier modelling 1. Mass and energy balance models Conservation of mass and energy More unknowns than equations Assumptions regarding unknowns are required: Fixed carbon conversion CH 4 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 CSIR 2010 Slide 16

17 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 CSIR 2010 Slide 17

18 CeSFaMB sub-models and correlations Sub-model Hydrodynamics: Minimum fluidising velocity (U mf ) Bubble diameter (d b ) Bubble rise velocity (m/s) Bubble fraction (-) Reaction rates: Gas-solid (combustion and gasification) Gas combustion Water gas shift Mass transfer coefficients Bubble - emulsion Emulsion- solid Heat transfer coefficients Bubble - emulsion Emulsion- solid Devolatilisation Elutriation Correlation used by CeSFaMB Wen and Yu Mori and Wen Davidson and Harrison Davidson and Harrison Yoon et al, Johnson Villenski and Hezeman Franks Sit and Grace La Nauze et al Kunii and Levenspiel Kunii and Levenspiel Loison and Chauvin Wen and Chen CSIR 2010 Slide 18

19 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) CSIR 2010 Slide 19

20 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 CSIR 2010 Slide 20

21 CeSFaMB model parameters Rate equation Rate equation dx dt = k oi E exp( RT i ) F ( X ) P n j Model parameters Pre-exponential factor (ko 3 ) for the steam- char reaction ( C + H 2 O => CO + H 2 ) Pre-exponential factor (ko 4 ) for the the carbon dioxide char reaction ( C + CO 2 => 2 CO ) Pre-exponential factor (ko 5 ) for the the hydrogen-char reaction ( C + 2H 2 => CH 4 ) Pre-exponential factor (ko 41 ) for the water-gas shift reaction ( CO + H 2 O => CO 2 + H 2 ) CSIR 2010 Slide 21

22 CeSFaMB model parameters Rate equation Parameter New Vaal Grootegeluk Pre-exponential factors (s -1 ) C+H 2 O CO + H 2 : k C + CO 2 2CO: k C + H 2 CH4: k E E-07 CO + H 2 O CO 2 + H 2 : k CSIR 2010 Slide 22

23 CeSFaMB predictive capability Deviation between measured Rate equation and predicted New Vaal Grootegeluk output variables (%) Mid bed temperature Fixed carbon conversion Gasifier exit temperature CO H CH CO CSIR 2010 Slide 23

24 15 MW FBG design CSIR 2010 Slide 24

25 15 MW FBG output variables Output variables Rate equation New Vaal Mid bed temperature ( C) 948 Gasifier exit temperature ( C) 931 CO (%) H 2 (%) 24.3 CH 4 (%) 1.2 CO 2 (%) Calorific value (MJ/Nm 3 ) 6.2 Fixed carbon conversion (%) 93.2 CSIR 2010 Slide 25

26 Gas concentration profiles in the gasifier Concentration of gasses in the gasifier as a function of height C +H2O ----> H2+C O 40 Gas concentration (%) H2+O 2---> H2O C O +O 2---> C O 2 C H4+O 2--->H2+C O 2 C +C O > 2C O C +O 2--->C O +C O 2 C O +H2O ---->H2+C O 2 O 2 H2O CO 2 CO H2 CH Gasifier height (m ) CSIR 2010 Slide 26

27 Bubble velocity and bubble diameter in the bed Rate equation CSIR 2010 Slide 27

28 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 CSIR 2010 Slide 28

29 Thank You CSIR 2010 Slide 29