Atom economy or product yield to determine optimal gasification conditions in biomass-to-olefins biorefinery

Abstract

Biorefinery design uses various metrics for achieving biomass conversion with process efficiency and biochemical selectivity. A common optimization strategy targets the product yield expecting favorable waste reductions. As a green-chemistry metric, atom economy offers a novel approach to achieving mass efficiency by simultaneously computing products and waste. A comparison between the product yield and atom economy presumes different optimal operating conditions. This study applied atom economy to a biomass-to-olefins process to elucidate metric differences. An initial revision of the stoichiometric conditions determined the ideal product and waste generations. A realistic simulation predicted the mass flows to olefins via gasification, methanol synthesis, and methanol-to-olefins conversion. The gasifier operating conditions were modified to clarify interactions among the atom economy, reaction temperature, and steam-to-biomass mass ratio (S/B). The optimal atom-economic and product-yield conditions differed; a lower gasification temperature and atom-economic (S/B) ratio enhanced the olefins-to-waste ratio. The atom economy indicated that CO2 and H2O are inefficient transport molecules for releasing oxygen in biomass. Both wastes consume at best 33% g/g of feedstock carbon or hydrogen, complying to olefin and intermediate H/C ratios. A comprehensive revision of the energy performance is required to complement the atom economy as a performance indicator.