Combustion versus\nGasification in Power- and Biomass-to-X\nProcesses: An Exergetic Analysis

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