Biogas Valorization from a Process Synthesis Perspective: Heat and Work Integration to Maximize CO2 Conversion

Abstract

Biogas is often considered as a source of renewable energy, for heat and power production. However, biogas has greater promise as a source of concentrated CO2 in addition to methane, making it a rich supply of carbon and hydrogen for the generation of fuel and chemicals. In this work, we use the concept of attainable region in the enthalpy-Gibbs free energy space to identify opportunities for effective biogas valorization that maximizes the conversion of CO2. The AR concept allows us to study a chemical process without knowing the exact reaction mechanism that the species in the process use. Deriving Material Balance equations that relate a reactive process's output species to its input species is sufficient to identify process limits and explore opportunities to optimize its performance in terms of material, energy, and work. The conversion of biogas to valuable products is currently done in two steps; the high temperature and endothermic reformer step, followed by the low temperature exothermic synthesis step. We demonstrate, using Aspen Simulation, that energy integration, both heat and work, between the two steps is crucial to achieving a substantial amount of CO2 conversion. We also show how a heat pump configuration can be utilized to integrate energy between the reformer and synthesis steps against the temperature gradient by integrating external renewable energy.