(10e) Techno-Economic Analysis of Effects of Alternative Gasification Configurations Assumptions On Lignocellulosic Biomass to Ethanol Systems

Abstract The conversion of biomass to liquid fuels plays an important role, as part of a sustainable energy portfolio, for managing oil shortage and environmental problems being confronted today. Currently, lignocellulosic biomass gasification technologies have been developed to the point of large-scale demonstrations. However, widespread commercialization of large-scale biomass gasifiers and their integration with syngas conversion to ethanol technologies have not been realized because of technical and economic challenges. With these technologies still in the early development phase, system modeling and economic analysis is a powerful tool to provide insights for decision making about the research and development priorities of these technologies. Techno-economic assessments have been conducted for ethanol production from biomass systems. As an important process in a gasifier based biomass-to-fuel system, gasifiers with different configurations or operating conditions affect not only the gasifier equipment cost and size, but also impact downstream processes. The purpose of this study is to examine alternative gasification configurations and determine their effects on the overall biomass-to-ethanol performance and economics, and thus to identify potential opportunities for system improvement and optimization.

In this study, an indirectly-heated gasifier based wood chips to ethanol system was investigated. The system consists of five major processes: (1) gasification, (2) syngas purification, (3) syngas adjustment, (4) mixed alcohol synthesis and product purification, and (5) steam cycle. In this system, wood chips are dried and gasified to produce syngas. The syngas is cleaned and the H2/CO ratio is adjusted. The syngas is then converted to mixed alcohols and separated into products. Unconverted syngas is used for processes fuel and electricity production. Gasifier capacity was assumed to be 2000 dry tonne/day wood chips. Process models of systems with alternative gasification configurations were developed in CHEMCAD. Excel spreadsheet models were developed to estimate the capital and operating costs of different systems. In this study, the major configuration assumptions for the gasification process are investigated in this study, including operating temperatures, pressures, and different raw syngas compositions. Two gasification temperatures of 1600 oF and 1800 oF were considered. Gasification pressures were assumed to be 23 psi to 147 psi. Two different indirectly-heated gasifiers, generating syngas with H2:CO ratios of 0.6 and 1.7, respectively, were considered to study the impact of raw syngas composition.

The process design basis and modeling approach are described in the paper. Major performance and cost results of case studies are listed and analyzed. The effects of cases with alternative gasification configuration assumptions are compared and their results are discussed, including carbon conversion efficiencies, power consumption and output, and final product yields. The cost analysis results include estimated capital and operating costs, and ethanol selling prices. By comparing the performance and cost results of alternative configurations, potential opportunities for improving the performance and costs of gasification based biomass-to-ethanol systems are identified. As an example, results given in this work indicate that high pressure gasifier operation results in an ethanol selling price that is decreased by approximately 20% due to the cost savings associated with syngas compression. The impacts of alternative gasification temperatures and different raw syngas compositions are also discussed. By identifying those changes that have the greatest impacts, insights for decision making on research and development priorities of these technologies are obtained. Research can be more effectively directed toward achieving economic viability by focusing research in the key areas and setting processing milestones to measure technical progress.

Keywords: Biomass, Gasification, Syngas, Mixed Alcohol