Pilot-Scale Comparison of Steam Gasification with Herbaceous and Woody Feedstocks

Production of renewable transportation fuels from biomass has become an important priority to reduce the nation's dependence upon imported oil and minimize the impact upon the environment. However, a sustainable biofuels economy will require conversion processes that utilize the lignocellulosic components of plants so as not to compete with food production. To this end, the National Renewable Energy Laboratory's gasification research focuses on developing processes that use a variety of feedstocks, such as agricultural residues, waste from forest thinning, and energy crops. Biomass gasification combined with catalytic synthesis has the potential to meet these criteria. This approach can potentially produce alcohols or Fischer-Tropsch fuels from a variety of biomass sources.

Key to the economic viability of gasification/catalytic synthesis processes is the production of a syngas with low concentrations of products which interfere with the catalytic step, such as methane, tars and sulfur compounds. Developing an understanding of the formation of these species as a function of feedstock is critical for evaluating and optimizing gasification. In this study, steam gasification was conducted on a ten to twenty kilogram per hour (pilot) scale of mixed hardwoods, corn stover, switchgrass and wheat straw. Process conditions such as steam-to-biomass ratio and temperature were investigated and their effects upon product formation. Real-time measurements of gaseous products were gathered, including sulfur compounds. The chemical composition of the tars formed was monitored in real time using a Molecular Beam Mass Spectrometer (MBMS). The process and unit operations, operational lessons-learned, and results from the experiments as they relate to the formation of undesirable products will be discussed.