Thermodynamic driving forces for postgasification carbon deposition

Abstract

The product stream from a biomass gasifier is at a sufficiently high temperature (roughly 1100°C) to be able to undergo further chemical reactions if not immediately quenched. If the product stream is allowed to cool at a slower rate, there is the possibility of solid carbon formation, which can foul surfaces in contact with the cooling product gas. The goal of this work is to assess the thermodynamic driving forces for solid carbon formation via global equilibrium calculations, identifying conditions under which carbon formation is favored or disfavored. Global equilibrium calculations are performed using conditions representative of biomass gasifiers. Operating variables explored include the steam/carbon mass ratio, system pressure, and the amount of carbon lost and removed as char after the gasifier (% char). Both by varying the steam/carbon ratio and the % char, the temperature at which solid carbon is predicted to form and deposit on surfaces (Tdep) varies by hundreds of degrees Celsius, because varying both these operating conditions directly affects the atomic carbon/oxygen ratio of the product stream. There is a direct correlation between the atomic C/O ratio and Tdep. Varying the steam/carbon ratio and the % char also strongly changes the predicted gas-phase product compositions. The effect of pressure upon Tdep and gas-phase compositions is also explored. These results have implications for optimizing the composition of the product stream so as to not only preclude carbon formation but also maximize the yield from further downstream processes such as catalytic reactions to produce liquid fuels. © 2010 American Institute of Chemical Engineers Environ Prog, 2010