Combustion versus Gasification in Power- and Biomass-to-X Processes: An Exergetic Analysis.

Residual biomass is a promising carbon feedstock for the production of electricity-based organic chemicals and fuels since, unlike carbon dioxide captured from point sources or air, it also has a valuable energy input. Biomass can be converted into an intermediate stream suitable for Power-to-X processes mainly via combustion or gasification. Such combined processes are generally called biohybrid or Power- and Biomass-to-X processes. To investigate the potential of biomass utilization in Power- and Biomass-to-X processes and identify inherent efficiency differences between these pathways, we model the process units with simple mass and energy balances considering empirical parameters for the key process units and perform an exergetic analysis. The analysis is conducted for several molecules of interest for the chemical and transport sectors with different C:H:O ratios, i.e., methane, methanol, dimethyl ether, and dodecane. For all considered products, the Power- and Biomass-to-X processes with biomass gasification, either with pure oxygen or steam as oxidizing agents, have a significantly higher (∼15-20 percentage points) exergy efficiency. This difference is mainly due to the lower exergy loss for water electrolysis since a lower amount of hydrogen is needed and to the higher exergy efficiency of the gasification unit compared to that of the combustion unit. Therefore, gasification-based Power- and Biomass-to-X processes have clear thermodynamic advantages in the ideal case. These conclusions obtained with the simple models are confirmed by modeling a Power- and Biomass-to-Methanol process in detail, also accounting for practical factors such as side reactions, incomplete reactant conversion, and ash formation.