Biomass Gasification, Gas Cleaning, and Gas Fermentation

[PHOTO] BEI gasifier at BCRFA pilot-scale 25 kg/h fluidized bed, oxygen/steam blown gasifier and syngas cleaning system was developed to convert switchgrass into clean syngas. The system is rated for operation at gage pressures up to 1 bar. The reactor vessel incorporated a novel guard heating system to simulate near-adiabatic operation of large commercial-scale gasifiers, and was effective for gasification temperatures up to 900 °C. After removing particulate from the gas stream via cyclones, a warm-gas cleaning operation based on oil scrubbing was used to remove tars. Sulfur compounds were removed via solid-phase adsorption. Ammonia was removed by water scrubbing.

[ILLUSTRATION]Feed system and fluidized bed gasifier

The feed system and fluidized bed gasifier at Iowa State University.

Baseline gasification tests with steam and oxygen were conducted at equivalence ratios (ER) between 0.21 and 0.38 using switchgrass as fuel. Measurements on the raw and cleaned syngas included permanent gas composition, C2 hydrocarbons, water, heavy and light tars, gasification residues (char and ash), hydrogen sulfide (H2S), carbonyl sulfide (COS), carbon disulfide (CS2), ammonia (NH3), and the first reported measurements of hydrogen cyanide (HCN) for oxygen/steam blown gasification. Heavy tars were removed with high efficiency by the method employed, although more difficult to remove light tars reduced overall tar removal efficiency to less than 80%. The sulfur scrubbing system demonstrated 99.9% removal efficiency, resulting in less than 200 ppb of H2S in the cleaned gas. The NH3 scrubbing system also accomplished greater than 99.9% removal efficiency, resulting in final NH3 concentrations of less than 1 ppm.

References

Timmer, K. J., Brown, R. C. (2019) Transformation of char carbon during bubbling fluidized bed gasification of biomass, Fuel 242, 837-845, DOI: 10.1016/j.fuel. 2019.01.039.

Broer, K.M., Brown, R.C. (2016) The role of char and tar in determining the gas-phase partitioning of nitrogen during biomass gasification, Applied Energy 158, 474-483.

Broer, K.M., Brown, R.C. (2016) The effect of equivalence ratio on partitioning of nitrogen during biomass gasification, Energy & Fuels 30, 407-413.

Ou, L., Li, B., Dang, Q., Jones, S., Brown, R. C., Wright, M. M. (2016) Understanding the uncertainty in economic feasibility of transportation fuel production via biomass gasification and mixed alcohol synthesis, Energy Technology 4, 441-448.

Broer, K.M., Woolcock, P.J., Johnston, P. A., and Brown, R.C. (2015) Steam/oxygen gasification system for the production of clean syngas from switchgrass, Fuel 140, 282-292.

Broer, K. M., P. A. Johnston, A. Haag and R. C. Brown (2015) Resolving inconsistencies in measurements of hydrogen cyanide in syngas, Fuel 140: 97-101.

Woolcock, P., Brown, R. C. (2013) A review of cleaning technologies for biomass-derived syngas, Biomass and Bioenergy 52, 54–84.

Gas Fermentation

A variety of microorganisms are capable of performing syngas fermentation. Based on the end products, those microorganisms can be

Hydrogenogens and acetogens. The hydrogenogens produce H2 from proton reduction coupled with CO oxidation to CO2, which is also referred to as the biological water-gas shift reactions.

CO + H2O → CO2 + 2H+ + 2e → CO2 + H2.

The above reactions are catalyzed by two key enzymes: nickel–CO dehydrogenase (Ni-CODH) and CO-induced hydrogenase.

Acetogens are facultative autotrophs capable of CO/H2/CO2 metabolism via the Wood–Ljungdahl (WL) pathway. The overall stoichiometry of the pathway is given by:

2CO2+4H2+ATP→Acetyl‐CoA+2H2O+nATP

where n is the ATP conservation coefficient; acetyl-CoA serves as a central intermediate and an ATP source, which is utilized to produce metabolites such as acetate and ethanol, with the supply of electron donors (H2 or CO).

Most acetogens produce acetate as the sole end product. Some organisms are also capable of converting syngas to other products such as ethanol, butanol, butyrate, and 2,3-butanediol but the yields are usually low. Native acetogens have been engineered to divert carbon flow to the desired end products with improved yields.

References

Sathish, A., Sharma, A., Gable, P., Brown, R., Skiadas, I., Zhiyou, W. (2019) A novel bulk-gas-to-atomized-liquid reactor for enhanced mass transfer efficiency and its application to syngas fermentation, Chemical Engineering Journal 370, 60-70, DOI:  10.1016/j.cej.2019.03.183.

Shen, Y., Brown, R., Wen, Z. (2016) Syngas fermentation of Clostridium carboxidivorans P7 in a horizontal rotating packed bed biofilm reactor with enhanced ethanol production, Applied Energy 187, 585-594.

Choi, D.-W., DiSpirito, A. A., Chipman, D. C., and Brown, R. C. (2011) Hybrid processing, in Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals, and Power, Brown, R.C. (Ed.), Wiley & Sons, pp. 280-299.

Choi, D., Chipman, D., Bents, S., Brown, R. (2010) A techno-economic analysis of poly-hydroxyalkanoates and hydrogen production from syngas fermentation of gasified biomass, Applied Biochemistry and Biotechnology 160, 1032-1046.



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