Macro algae to carbon negative methanol: Techno-economic assessment, carbon sequestration potential, and benchmarking through hydrothermal carbonization and sorbent enhanced processing

This manuscript introduces novel process schemes targeting methanol with carbon-negative potential from wet biogenic waste. It integrated hydrothermal carbonization and in-situ CO2 capture sorption-enhanced processing through energy, environmental, and economic evaluations using dedicated reactor modelling, process simulation, sizing, and cash flow analysis. Key configurations include two autothermal baseline scenarios, in addition to two other electrified configurations, namely direct (electrical heating) and indirect (via electrolyzer), introduced to improve biogenic carbon use in methanol production. A conventional biomass-to-methanol (BtM) pathway adapted to wet biogenic waste without carbon capture serves as a benchmark. The baseline scenarios present the best energy and carbon removal trade-off. High-pressure operations in baseline result in lower negative GHG emissions, while mild pressure promotes improved energy efficiency and carbon conversion. Electrification, in both cases, enhances the carbon conversion and methanol yield at the expense of poor process energy efficiency and significant GHG emissions. The grid emission factor was shown to be critical in achieving the desired GHG emissions. Baseline scenarios stay negative across a large range exceeding 1000 g-CO2eq/kWh, whereas direct and indirect achieve neutral carbon footprints at 22.3 and 383 g-CO2eq/kWh, respectively. The baseline and electrified scenarios require more capital due to their dual responsibilities in producing methanol and absorbing CO2, but they have lower operational expenses covered by carbon income than the benchmark. The benchmark has the most competitive levelized cost of production (653.32 €/t), followed by baselines and then electrified scenarios. High electricity and carbon prices make baseline scenarios more appealing than benchmark and electrified alternatives.