Department of Mechanical Engineering, University of Cagliari Piazza d Armi, Cagliari, Italia

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1 Department of Mechanical Engineering, University of Cagliari Piazza d Armi, Cagliari, Italia CCT 2009 Fourth International Conference on Clean Coal Technologies for Our Future 18/21 May 2009 Dresden Germany PERFORMANCE ANALYSIS OF UPDRAFT COAL GASIFIERS FED BY OXYGEN WITH STEAM, CO 2 OR RECIRCULATED SYNGAS MIXTURES Giorgio Cau, Daniele Cocco, Fabio Serra, Vittorio Tola 1/18

2 Introduction Coal gasification is increasing its importance for both power generation and for the production of chemicals and clean fuels (hydrogen, methanol, DME and others). 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. R&D activities are focussing on medium- and small-scale industrial applications. Small-scale fixed bed gasifiers commonly fed by air. Air blown gasifier generates a syngas with a low LHV (about 6-7 MJ/kg), high N 2 (40-45%) and low hydrogen fractions. 2/18

3 Research project Department of Mechanical Engineering of the University of Cagliari with project partner ENEA. Research project concerning the effect of substituting N 2 with CO 2 in small-scale fixed bed gasifiers. Production of a syngas mainly composed by H 2 and CO 2 devoted to the production of pure hydrogen. Closely related to research project called CO.HY.GEN. for developing integrated gasification processes for the combined production of hydrogen and electricity, in small-to-medium scale commercial plants.(sotacarbo, Ansaldo Ricerche S.p.A. and ENEA). 3/18

4 Gasification processes Gasification processes are very complex, involving: drying, devolatilisation, pyrolysis, gasification and combustion. Influenced by kinetics and fluid-dynamical effects. Thermodynamic equilibrium only for a known temperature. Equilibrium models can predict accurately performance of fluidised-bed and entrained-bed (temperature distribution almost constant). 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. 4/18

5 Simplified scheme of fixed bed gasification process coal syngas Coal preheating and drying Simplified scheme of the fixed bed gasification process Coal devolatilization Char gasification and combustion Steam and oxidant preheating ash gasification agents 5/18

6 ASPEN simulation model Computer simulation model for predicting fixed bed gasifiers performance, developed using ASPEN Plus. The ASPEN model schematizes the gasifier into several different zones: coal prehating and drying, devolatilisation, gasification, combustion, steam and oxidant preheating. 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. In the gasification and combustion zone the model calculates the syngas composition and the equilibrium temperature minimizing the Gibbs free energy. The model calculates the syngas temperature considering the countecurrent heat exchange processes between syngas and coal inside the different sections, imposing a suitable ΔT. 6/18

7 ASPEN simplified scheme COAL SYNGAS DRYING MOISTURE SYN-GAS COALHOT MX-SYNG Simplified scheme of the Aspen Plus-based gasification model DEC-COAL Q DEC-VOL VOLATILE FIXED-C WATER GASES TAR ASH MX-CHAR Q-LOSSES CHAR REACTOR CHAR-HT ASH-COLD HOT-GAS RGIBBS PRE-HT OXIDANT STEAM ASH-HOT 7/18

8 Input data for the ASPEN model commercial coal Proximate Analysis (%Wt) Fixed carbon Volatile matter Ash Moisture 8.00 Ultimate Analysis (%Wt) Carbon Hydrogen 3.71 Sulphur 0.55 Nitrogen 1.50 Chlorine 0.05 Oxygen 5.35 Ash Moisture 8.00 Lower Heating Value (MJ/kg) 25.4 Volatile matter (%Wt) Water TAR Volatile gases Volatile gases (%Vol) CO 9.50 CO H CH Operating conditions Steam/coal mass ratio 0.34 Steam conditions ( C/bar) 120/1.30 Air/coal mass ratio 2.43 Air conditions ( C/bar) 120/1.30 Gasifier energy losses (%) 0.0 8/18

9 Gasifier performance and Syngas composition for the air-blown gasifier Gasifier performance * Coal mass flow (kg/h) 700 Air mass flow (kg/h) 1700 Steam mass flow (kg/h) 238 Syngas mass flow (kg/h) 2540 Mean gasification temperature ( C) 737 Syngas exit temperature ( C) 518 Syngas LHV (MJ/kg) 6.22 Cold gas efficiency (%) 88.5 Syngas/coal flow ratio (Nm 3 /kg) 3.35 Syngas composition (%vol) CO CO H N CH H 2 S 0.10 COS 0.01 Ar 0.49 H 2 O 4.89 Gasifier energy losses (%) 0.0 * Reference to Sotacarbo pilot updraft gasifier 9/18

10 Substitution of Nitrogen with another diluent 1. Introduction of CO 2 2. Introduction of a further amount of steam 3. Recirculation of a fraction of syngas 1 SPR NSR Steam CO2 Syngas Reference case NSR Nitrogen Substitution Ratio SPR Syngas Production Ratio 10/18

11 Temperature vs NSR 1400 ~ 1300 Temperature ( C) Syngas temperature Mean gasification temperature 0.70 Steam CO2 Syngas Reference case 0.86 ~ NSR 11/18

12 Syngas LHV vs NSR Syngas Lower Heating Value (MJ/kg) Steam CO2 Syngas Reference case NSR 12/18

13 Cold gas efficiency vs NSR η CG = m syng LHV syng m coal LHV coal Cold gas efficiency Steam CO2 Syngas Reference case NSR 13/18

14 0.6 Syngas molar fraction vs NSR (steam) WET 0.6 DRY 0.5 CO CO2 H2 0.5 CH4 H2O CO CO2 Syngas composition (wet) Syngas composition (dry) H2 CH4 H2O NSR NSR 14/18

15 0.6 WET Syngas molar fraction vs NSR (CO 2 ) 0.6 DRY Syngas composition (wet) CO CO2 H2 CH4 H2O Syngas composition (dry) CO CO2 H2 CH4 H2O NSR NSR 15/18

16 0.6 WET Syngas molar fraction vs NSR (syngas) 0.6 DRY Syngas composition (wet) CO CO2 H2 CH4 H2O Syngas composition (dry) CO CO2 H2 CH4 H2O NSR NSR 16/18

17 Conclusions Steam, CO 2 and syngas are suitable temperature moderating agents in substitution of N 2 using pure oxygen instead of air.their use can improve syngas characteristics. Pure oxygen-blown gasifier (without any diluent) allows to reach highest LHV syngas, but too high temperature inside the gasifier. The addition of CO 2 or steam reduces syngas LHV (lower values of CO and H 2 ) and increases syngas mass flow. Syngas recirculation doesn t affect significatively LHV and mass flow. The addition of CO 2 leads to highest values of cold gas efficiency (best solution from an energetic point of view), but to a syngas with very low H 2 content which hampers pure hydrogen production. To produce hydrogen best solution appears to be dilution with steam. 17/18

18 Acknowledgments This work was carried out in the framework of the research projects Development and validation of a simulation model of gasification processes in fixed bed gasifiers using mixtures of CO 2 and O 2 funded by ENEA and Research and Development Program for near zero emission technologies of coal utilization in distributed power micro-generation funded by the Italian Ministry for Universities and Scientific Research (MIUR). 18/18

19 Department of Mechanical Engineering, University of Cagliari Piazza d Armi, Cagliari, Italia EXTRA SLIDE PROCESS ANALYSIS AND PERFORMANCE EVALUATION OF UPDRAFT COAL GASIFIER Vittorio Tola, Giorgio Cau 19/18

20 Research project Department of Mechanical Engineering of the University of Cagliari with project partner ENEA. Research project concerning the effect of substituting N 2 with CO 2 in small-scale fixed bed gasifiers. Closely related to research project called CO.HY.GEN. for developing integrated gasification processes for the combined production of hydrogen and electricity, in small-to-medium scale commercial plants.(sotacarbo, Ansaldo Ricerche S.p.A. and ENEA). Development of a pilot plant recently built at the Sotacarbo Research Centre in Sardinia. 20/18

21 Input data for the ASPEN model Proximate coal analysis (fixed carbon, volatile matter, moisture and ash); Ultimate coal analysis (carbon, hydrogen, sulphur, nitrogen, oxygen, moisture and ash); Lower Heating Value of the coal; Volatile matter and volatile gases composition; Oxidant and steam characteristics; Thermal energy removed by water jacket and other energy losses; Carbon conversion rate. 21/18

22 Pilot gasifier coal feed water outlet vertical translation water outlet ash rotate water inlet gas offtake coal feed water jacket water inlet air + steam Simplified scheme of the 700 kg/h fixed bed pilot gasifier Proximate Analysis LS coal (%Wt) Fixed carbon Volatile matter Ash Moisture 8.00 Ultimate Analysis (%Wt) Carbon Hydrogen 3.71 Sulphur 0.55 Nitrogen 1.5 Chlorine 5.35 Oxygen 0.05 Ash Moisture 8.00 Lower Heating Value (MJ/kg) 25.4 Volatile matter (%Wt) Water TAR Volatile gases Volatile gases (%Vol) CO 9.73 CO H CH Operating conditions Steam/coal mass ratio 0.34 Steam conditions ( C/bar) 120/1.30 Air/coal mass ratio 2.43 Air conditions ( C/bar) 120/1.30 Gasifier energy losses (%) 0.0 Coal composition and gasifier operating parameters 22/18

23 Gasification and syngas temperature and LHV vs steam/coal and air/coal mass ratios SOUTH AFRICAN COAL Syngas exiting temperature ( C) Syngas exiting temperature Gasification temperature Syngas Lower Heating Value Gasification temperature ( C) Syngas LHV (MJ/kg) Syngas exiting temperature ( C) Syngas exiting temperature Gasification temperature Syngas Lower Heating Value Gasification temperature ( C) Syngas LHV (MJ/kg) Steam/coal mass flow ratio μ Air/coal mass flow ratio α 23/18

24 Syngas molar fraction vs steam/coal and air/coal mass ratios Syngas molar fraction (dry) CO CO 2 H 2 N 2 CH 4 H 2 S Synags molar fraction (dry) CO CO 2 H 2 N 2 CH 4 H 2 S Steam/coal mass flow ratio μ Air/coal mass flow ratio α 24/18

25 Gasification and syngas temperature and LHV vs thermal energy losses and ΔT Syngas exiting temperature ( C) Syngas exiting temperature Gasification temperature Syngas Lower Heating Value Gasification temperature ( C) Syngas LHV (MJ/kg) Syngas exiting temperature ( C) Syngas exiting temperature Gasification temperature Syngas Lower Heating Value Gasification temperature ( C) Syngas LHV (MJ/kg) Thermal energy losses (%LHV) Temperature difference between syngas and coal ( C) 25/18

26 Gasification and syngas temperature and LHV vs steam/coal mass ratio South African coal Syngas exiting temperature ( C) Syngas exit temperature Gasification temperature Syngas Lower Heating Value Gasification temperature ( C) Syngas LHV (MJ/kg) Steam/coal mass flow ratio μ 26/18

27 Gasification and syngas temperature and LHV vs air/coal mass ratio South African coal Syngas exiting temperature ( C) Syngas exit temperature Gasification temperature Syngas Lower Heating Value Gasification temperature ( C) Syngas LHV (MJ/kg) Air/coal mass flow ratio α 27/18

28 Gasification and syngas temperature and LHV vs thermal energy losses South African coal Syngas exiting temperature ( C) Syngas exit temperature Gasification temperature Syngas Lower Heating Value Gasification temperature ( C) Syngas LHV (MJ/kg) Thermal energy losses (%LHV) 28/18

29 Gasification and syngas temperature and LHV vs temperature difference South African coal Syngas exiting temperature ( C) Syngas exit temperature Gasification temperature Syngas Lower Heating Value Gasification temperature ( C) Syngas LHV (MJ/kg) Temperature difference between syngas and coal ( C) 29/18