No abstract

Natural gas has the potential to increase the biofuel production output by combining gas- and biomass-to-liquids (GBTL) processes followed by naphtha and diesel fuel synthesis via Fischer-Tropsch (FT). This study reflects on the use of commercial-ready configurations of GBTL technologies and the environmental impact of enhancing biofuels with natural gas. The autothermal and steam-methane reforming processes for natural gas conversion and the gasification of biomass for FT fuel synthesis are modeled to estimate system well-to-wheel emissions and compare them to limits established by U.S. renewable fuel mandates. We show that natural gas can enhance FT biofuel production by reducing the need for water-gas shift (WGS) of biomass-derived syngas to achieve appropriate H2/CO ratios. Specifically, fuel yields are increased from less than 60 gallons per ton to over 100 gallons per ton with increasing natural gas input. However, GBTL facilities would need to limit natural gas use to less than 19.1% on a LHV energy basis (7.83 wt %) to avoid exceeding the emissions limits established by the Renewable Fuels Standard (RFS2) for clean, advanced biofuels. This effectively constitutes a blending limit that constrains the use of natural gas for enhancing the biomass-to-liquids (BTL) process.

This work investigates the potential to integrate a biomass combustor with an existing geothermal power plant. The motivation is to identify the most cost-effective approach to boost the geothermal turbine power output using heat from the biomass combustor to superheat the geothermal steam upstream of the turbine inlet. Different alternative integration configurations were identified and simulated using Aspen Plus software to evaluate their performance in terms of incremental power output and efficiency. Of the three different alternatives proposed, only one of them looked promisingthis configuration uses the saturated well-steam for partial preheating of the combustion air. The most promising integration options are compared on the basis of their levelized cost of electricity. The key conclusion is that one should use low-grade heat for low-level heating (well-steam for air preheating) and high-grade heat from the flue gas for steam superheating. Also, the quantity and quality of biomass available dictate the hybrid configuration selected. A proper design of the steam turbine (higher efficiency at higher steam inlet temperatures) is also necessary to enhance the performance of the hybrid geothermal−biomass power plant.