(79a) Towards Hybrid Biomass and Coal Processes for Satisfying Current Transportation Fuel Demands

Concerns about declining world oil production coinciding with the industrial growth of developing nations such as China and India have strongly motivated research in process systems utilizing biomass and coal to produce liquid fuels. One such process involves (a) gasification of the feedstock to produce a synthesis gas containing mostly CO, CO2, H2, H2O, and hydrocarbons followed by (b) conversion of the CO and H2 to a wide range of paraffins and olefins via a Fischer-Tropsch (FT) mechanism [1-5]. These hydrocarbons are then upgraded through a series of hydrotreating, hydrocracking, isomerization, reformation, and alkylation [5] steps to produce gasoline, diesel, and kerosene in quantities that reflect the transportation requirements of the United States.

Almost all current gasification systems suffer from limited thermal conversion efficiency of the resulting fuel gas from the production of CO2 from gasification [6-9]. Of great concern in the above process is the handling of the CO2 that is removed through (1) an acid gas removal stage prior to FT synthesis and (2) the post-combustion flue gas stream. The decision to sequester or vent the CO2 has a dramatic effect on the economic feasibility of the process as the capital and operating costs associated with isolation and compression of the CO2 can be substantial. A recent patent for the H2Car process [1] which suggests a novel alternative to CO2 sequestration could provide a means for CO2 handling that could dramatically enhance the process economics using a CO2 recycle loop to minimize the amount of feedstock required to produce a given quantity of transportation fuels. The CO2 is selectively removed from the acid gas removal stage and is recycled back to the system to produce more CO via the reverse water-gas-shift reaction. The H2Car process has a number of simplifying assumptions that can be alleviated. A thorough analysis on the processing of the flue gas from post-combustion streams needs to be investigated as well.

An additional limitation of current analysis of FT transportation fuel production is the approach to modeling gasification. Gasification of biomass or coal is generally assumed to occur in a nominal fashion with the syngas output is dictated by the effluent acquired by running the reactor at a given temperature, equivalence ratio (for oxygen/air blown gasification), or steam to biomass ratio (for steam gasification) [1-5]. Moreover, if current analyses were to alter the conditions at which the gasification takes place, an additional set of experimental runs would be required to verify the syngas output for a given set of input conditions. The lack of a gasifier model in current FT transportation fuel systems precludes the discovery an optimal set of operating conditions that would minimize the overall cost of gasoline, diesel, and kerosene production. To address this issue, we have developed two non-linear optimization (NLP) models for biomass and coal gasification that will predict the output flow rate and composition of the synthesis gas, as well as the proportion of unreacted char. The parameters of the gasifier models were calculated using a preliminary NLP designed to minimize the difference between the vapor phase mole fractions of the model and multiple case studies [6-9].

Using the gasification models, we subsequently explore the idea of a CO2 recycle loop that will be converted back to CO using either a non-renewable or renewable source of H2. We will significantly expand on the proposed H2Car process by removing all simplifying assumptions and incorporating a power integration scheme that will produce the necessary electricity and steam that the process requires. We have developed eight process flowsheets that will produce a mixture of gasoline, diesel, and kerosene in proportion to current United States demand using either (1) biomass or (2) coal, a (i) low-temperature or (ii) high-temperature Fischer-Tropsch process, and (a) a non-renewable (i.e., natural gas) or (b) renewable (i.e., wind, solar, nuclear) source of H2. A steady-state simulation for each of the process flowsheets is conducted using ASPEN PLUS and appropriate cost functions for each unit in the flowhseet are developed based on appropriate sizing calculations. A heat exchanger network is then developed for each flowsheet that sequentially minimizes (a) the total utility cost, (b) the total number of heat exchangers, and (c) the total heat exchanger area [10]. We finally provide a complete economic analysis of each process flowsheet and discuss the impact of future technological advancements to the results of this analysis.

[1] Agrawal, R.A., Singh, N.R., Ribeiro, F.H., and Delgass, W.N. Sustainable fuel for the transportation sector. PNAS. 104(12):4828-4833, 2007.

[2] Kreutz, T.G., Larson, E.D., Liu, G., and Williams, R.H. Fischer-Tropsch Fuels from Coal and Biomass. 25th International Pittsburg Coal Conference. 2008.

[3] Sudiro, M. and Bertucco, A. Synthetic Fuels by a Limited CO2 Emission Process Which Uses Both Fossil and Solar Energy. Energy Fuels, 21(6):3668-3675, 2007.

[4] Sudiro, M., Bertucco, A., Ruggeri, F., and Fontana, M. Improving Process Performances in Coal Gasification for Power and Synfuel Production. Energy Fuels, 22:3894-3901, 2008.

[5] Bechtel, ?Aspen Process Flowsheet Simulation Model of a Battelle Biomass-Based Gasification, Fischer-Tropsch Liquefaction and Combined-Cycle Power Plant,? DE-AC22-93PC91029-16, US Dept. of Energy, Pittsburgh, Pennsylvania, May 1998.

[6] van der Drift, A., van Doorn, J., and Vermeulen, J.W. Ten residual fuels for circulating fluidized-bed gasification. Biomass Bioenergy, 20:45?56, 2001.

[7] Huang, J., Fang, Y., Chen, H., and Wang, Y. Coal Gasification in a Pressurized Fluidized Bed. Energy Fuels, 17:1474-1479, 2003.

[8] Xiao, R., Zhang, M., Jin, B., and Huang, Y. High-Temperature Air/Steam-Blown Gasification of Coal in a Pressurized Spout-Fluid Bed. Energy Fuels, 20:715-720, 2006.

[9] Nikoo, M.B. and Mahinpey, N. Simulation of biomass gasification in a fluidized bed reactor using ASPEN PLUS. Biomass Bioenergy, 32:1245?1254, 2008.

[10] C. A. Floudas. Nonlinear and Mixed-Integer Optimization. Oxford University Press:

New York, 1995.