Gasification of Biomass and SulfurContaining Carbonaceous Fuels Majid Charmchi Department of Mechanical Engineering University of Massachusetts - Lowell 1
Introduction Biomass: Black Liquor Gasification (BLG) What is Black Liquor? Heat and Chemical Recovery: Current Recovery Process (Kraft Recovery Boiler) Gasification Recovery Process (BLG) Advantages of BLG Process Petroleum Coke Gasification (PCG) Considering the current environmental issues, is there a future for sulfur-containing low grade fuels? Could be a similarity between PCG and BLG? 2
What is Black Liquor? 3
Current Kraft Recovery Cycle 4
BL Gasification Recovery Cycle 5
BLG Effect on Emission Efficiency of a typical Recovery Boiler is 64% (power production < 15%) Efficiency of a typical BLG is 74% The industry s vision is to become a net exporter of electricity Approximately 11% reduction in GHG emissions US paper production is about 100 million tones per year Approximate energy content in BL: North America 1.35 EJ World 2.4 EJ 6
Petroleum Coke Pet coke has: high heating value (~14,000 Btu/lb) low ash levels (less than 0.5%) low moisture (~ 8%) There is plenty of pet coke (over 60 MT/yr most at coastal refineries) Pet coke contains: high sulfur levels (~ 4-7%) low grindability (Hardgrove index HGI ~ 35-70) some potentially harmful heavy metals, such as Vanadium, Mercury, Nickel, etc. As a fuel source, It is limited due to: It does not burn easily Its direct burning has harmful emissions 7
Pet Coke & Black Liquor* Compositions *North American Wood Species Typical Pet Coke Composition % Wt 76-89 0-0.1 2-7 3-4 1.2-2.5 5.5-15 0-0.6 Aqueous Phase %Wt Na2CO3 18 NaHS 2 H2O 80 500-2000 250-450 50-250 Kraft Black Liquor Composition % dry Sodium 17-25 Potassium 0.1-2 Chloride 0.2-2 Carbon 34-39 Oxygen 33-38 Sulfur 3-7 Hydrogen 3-5 Nitrogen 0.04-0.2 Moisture Ash Ash Elements ppm Vanadium Nickel Iron There is a close similarity between concentrated black liquor and a slurry mixture of pet coke and green liquor 8
Objectives of BLG & PCG Processing Aqueous Black Liquor to Obtain: Combustible gases (mainly CO, H2 and CH4) with: High Heating Values (HHV) over 100 Btu/scf H2/CO ratio 1 Sulfide-rich melt -- sulfate reduction efficiency of Na2S/(Na2S + Na2SO4) 93% Gasification of Pet Coke to: Upgrading low-value petroleum coke to clean fuel gas or synthesis gas Recover elemental sulfur and vanadium concentrate 9
Gasification Technology: R/D and Commercialization History Numerous techniques are developed for gasifying black liquor, coal and other carbonaceous fuels In 1983 EPRI listed 22 coal gasification processes Recently, only 9 technologies are listed by NETL as being in use or under R&D activity and all focused on coal gasification Initially, over 20 BLG technologies were developed From that list, only two are currently under further development; none is fully commercialized Gasification on Molten Salt Pool: In 1980-85 period, a series of research efforts on both bench-scale and pilot-scale, funded by DOE, were conducted. These comprehensive tests have clearly illustrated that gasification of Kraft mill black liquor and pet coke/green liquor slurry is technically feasible, 10 economically attractive, and environmentally cleaner
The Concept: Pilot-Scale Gasifier Gasification on Molten Salt Pool At start up: 70 % Na2CO3 30% Na2S 11
Laboratory-Scale Molten Salt Gasifier 12
The Gasification Process The Stages of Gasification Process Atomization High pressure liquid, splash plate, or steam assisted atomization Drying Devolatilization (Pyrolysis) Swelling Drying is aided by mixing and droplets swelling The temperature range of devolatilization is fairly narrow (~250450 oc) Droplet heating is rapid droplets of about 1mm in size reach above 450 oc before devolatilization is complete Swelling of the particles during devolatilization enhances external transport of oxygen during gasification Devolatilization rate in oxidizing environment tend to be faster Over half of the carbon in the solid would remain as residue after devolatilization and arrive to the gasification zone 13
The Gasification Process On molten salt pool (the gasification zone) Turbulent mixing with molten salt and air bubbles Carbon oxidation (produces mainly CO, H2 and heat) Positive aspect of the inorganic salt pool The inorganic matters increase the rate of char gasification The presence of Na2SO4 and Na2S enhance carbon oxidation through the sulfate/sulfide cycle that transports oxygen to the pool surface The top area of the salt pool provides a surface for air/carbon contact, and it protects the sulfide from re-oxidization Important reactions on the molten salt: Reduction of the oxidized forms of sulfur (mainly sulfate to sulfide) 4C + Na2SO4 Na2S + 4CO 2C + Na2SO4 Na2S + 2CO2 In absence of carbon and in case of excess oxygen (not likely), the sulfide may oxidize 2O2 + Na2S Na2SO4 14
BLG: Reduction of sulfate with CO reaction (Comparison of test data with equilibrium theory) Source: Kelleher et al. 15
Bench-Scale Gasification Test Results (BLG Case Study) Case A Predried BL solids injected into the salt pool Case B Direct injection of wet BL into the salt pool Case C Wet BL is sprayed into the hot gas (drying zone) above the salt pool Feed (wt. % solids) 98 64.7 64.7 Gasification temperature (oc) 925 997 992 Product gas composition, dry basis (vol. %) CO2 8.5 16.6 13.7 CO 20.6 7.4 14.3 H2 11 10.4 15.1 CH4 1.2 1.4 2.5 C2 s 0.1 0.05 0.5 N2 56.5 62.8 53-0.3 0.2 4322 (116) 2607 (70) 4795 (129) 100 64.7 96.5 H2S HHV, dry basis, kj/m3 (Btu/scf) Smelt sulfur reduction (%) Source: A.L. Kohl 16
Pet Coke Gasification Test Results (PCG in Air and in O2/steam ) Gasification in air Source: A.L. Kohl & J. A. Ashworth Gasification in O2/steam 17
Conclusions Molten salt gasification process provides a means of converting sulfur containing biomass fuel and low-value petroleum coke to clean fuel gas The process recovers: inorganic pulping chemicals in BLG Elemental sulfur and vanadium in PCG The presence of molten salt pool serve an important role as catalysts in gasification of fixed carbon Almost any carbonaceous material, e.g., pet coke, residual oil, coal, and wood can be converted to clean fuel gas Gasification in pure O2/steam provides hydrogen rich product gas with medium-btu HHV 18
Challenges and Barriers Capital cost Market penetration (conservative industries) Technical issues Corrosion problem (presence of alkali metals) Advances in refractory material Slurry atomization spray system design (critical drying zone issue) Process scaling Moving from small scale R/D tests to full scale commercial size gasifier 19
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. E. Larson, S. Consonni, and R. Katofsky, A Cost-Benefit Assessment of Biomass Gasification Power Generation in the Pulp and Paper Industry, Final Report, Prepared by Navigant Inc., Princeton University, and Politecnico di Milano, 2003 A. L. Kohl, Black Liquor Gasification, The Canadian Journal of Chemical Engineering Vol.64, 1986 E. G. Kelleher, Gasification of Kraft Black Liquor and Use of the Products in Combined Cycle Cogeneration, Final Report Phase II, DOE/CS/40341-T5, 1985 E. G. Kelleher and A. L. Kohl, Black Liquor Gasification Technology, Chemical Engineering Technology in Forest Product Processing, AIChE, Vol.2, P40, 1988 A. L. Kohl and J. C. Newcomb, Effect of Pressure on Smelt-Water Explosions, TAPPI Journal, P82, September 1986 P. J. Hurley, Energy Balances for Alternative Kraft Recovery Systems, Chemical Eng. Progress, P43, 1980 A. L. Kohl, R. B. Harty and J. G. Johanson, The Molten Salt Coal Gasification Process, Chemical Eng. Progress, P73, 1978 R. E. Henry and H. K. Fauske, Nucleation Processes in Large Scale Vapor Explosions, ASME Journal of Heat Transfer, P 280, 1979 D. Stelman, A. J. Darnell, J. R. Christie, and S. J. Yosim, Air Oxidation of Graphite in Molten Salts, The Electrochemical Society Inc., p. 299, 1976 D. R. Stull and H. Profet, JANAF Thermodynamic Tables, 2nd Ed., NSRDS-NB537, National Bureau of Standards, 1971 Electric Power Research Institute (EPRI) Report AP-3109, Coal Gasification Systems: A Guide to Status, Applications, and Economics," Prepared by Synthetic Fuels Associates, Inc. June 1983. DOE report, "Molten Salt Coal Gasification Process Development Unit," Phase 2 Final Report, Rockwell Inter national, DOE/ET/10296-l96, April 1982 A. L. Kohl, R, B. Harty, J. G. Johanson, and L. M. Naph thali, "The Molten Salt Coal Gasification Process," Chem. Eng. Prog. Vol. 74, p73, 1978 A. L. Kohl and J. A. Ashworth, "Process Upgrades Coke to Gas," Hydrocarbon Processing, pp. 97-100, July 1983 20 A. L. Kohl and F. C. Riesenfeld, "Gas Purification," 4th ed. Gulf Publishing Co. Houston Texas, 1985