Effects of Alternative Mixed Alcohol Synthesis Configurations On Ethanol Production by Indirectly-Heated Gasification of Lignocellulosic Biomass

As a sustainable energy source, the conversion of biomass to alternative liquid fuels plays an important role in solving oil shortage and environmental problems being confronted today. Currently, these applications are mostly met through the conversion of grain products into fuel-grade ethanol that can be blended with gasoline. However, while these forms of biomass can supply significant quantities of transportation fuels, other more widely available biomass forms, specifically lignocellulosic biomass forms, also need to be used to have a major impact on domestic supplies of transportation fuel. At this time, lignocellulosic biomass gasification technologies have been developed to the point of large-scale demonstrations. However, widespread commercialization of large-scale biomass gasifiers or their integration with ethanol synthesis technologies has not been realized because of technical and economic challenges. With these technologies still in early development phase, system modeling and economic analysis should be implemented to provide insights for decision making about the research and development of these technologies. Techno-economic assessments have been conducted for ethanol production from biomass systems. Limited sensitivity analyses were performed to evaluate the impacts of alternative configuration assumptions on overall system performance and costs. The purpose of this study is to investigate the technical and economic sensitivities of biomass gasification based ethanol production systems related to alternative mixed alcohol synthesis configurations, and thus to identify potential opportunities for the system improvement and optimization.

In this study, a systematic analysis method is used to investigate the impacts of alternative mixed alcohol synthesis configurations on the ethanol production from indirectly-heated gasification of biomass. The system consists of five major processes: (1) gasification, (2) syngas purification, (3) syngas adjustment, (4) mixed alcohol synthesis and product purification, (5) steam cycle. In this system, wood chips are converted to syngas in the gasifier. 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 are used as fuels for processes and electricity production. Alternative mixed alcohol synthesis configurations with difference process assumptions are examined in this study. The effects of different methanol recycle ratios, different syngas recycle ratios, and alternative recycle syngas injection locations on the system performance and costs are evaluated. For each configuration, a process model is developed in CHEMCAD to estimate heat and material balances. Cost analyses were implemented based on inputs from the process models.

The simulation results are analyzed and discussed. The major performance analysis results include 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. Results analysis indicated that with the methanol recycle increasing from 0% to 90%, the ethanol selling price decreases approximately 14% resulting from the decrease of the ethanol yield. The impacts of syngas recycle ratios and recycle syngas injection locations are also discussed. 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. By identifying those changes that have the greatest impacts, insights for decision making on research and development priorities of these technologies are provided. 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.

In this paper, the process design basis and modeling approach are described. Major performance and cost results of case studies are listed and analyzed. The results of alternative configurations are compared and the impacts of different process assumptions are discussed.

Keywords: Biomass, Gasification, Syngas, Mixed Alcohol