Design, modeling, and optimization of flexible hydrogen-based power-to-X processes

Power-to-X (PtX) and Power- and Biomass-to-X (PBtX) processes are promising alternatives to conventional production processes of bulk chemicals and fuels, which generally rely on fossil feedstocks. These processes can utilize renewable energy sources and sustainable feedstocks such as carbon dioxide and residual biomass to synthesize electricity-based products. Nevertheless, substantial challenges must be addressed to improve their efficiency and economic viability. This dissertation addresses some of these challenges by investigating PtX and PBtX processes that synthesize some key carbon-based chemicals and fuels using hydrogen produced via low-temperature water electrolysis. In particular, after reviewing the suitability of PtX processes for process integration and flexible operation, several ways of utilizing residual biomass in PBtX processes, as well as some key trade-offs between flexible operation and intermediate storage technologies, are investigated, all being crucial aspects for improving process efficiency and viability. Concerning the utilization of biomass in PBtX processes, two key thermochemical processing options, i.e., combustion and gasification, are investigated and compared. With relatively simple models, the thermodynamic advantages of biomass gasification over combustion for all considered products “X” are shown. The key trends are then confirmed for a Power- and Biomass-to-Methanol process modeled in detail. Nevertheless, the use of these relatively simple models could lead to misleading conclusions for other process comparisons, e.g., of kerosene production pathways, if the models are not adequately representative. When investigating the trade-off between flexible operation and intermediate storage, a Power-to-Methanol (PtMeOH) case study is optimized with respect to its design and scheduling, simultaneously. The optimization results show that enabling flexible operation of the entire process can reduce the methanol production cost. In contrast, the need for intermediate storage depends on the economic assumptions, as its investment cost may not be recovered through the savings in the operating costs. This trade-off is further investigated by optimizing the PtMeOH process with intermediate storage under electricity price scenario uncertainty. While enabling flexible operation of all process units consistently lowers the production cost for given process designs, no clear pattern emerges concerning the need for intermediate storage: different process designs with and without storage can result in similar production costs when flexible operation of all process units is enabled and scheduling is optimized. Still, both process design and scheduling should be optimized simultaneously. As these optimization problems often suffer from numerical intractability, a complexity reduction approach based on feature-based scenario clustering and degrees of freedom reduction is investigated. This dissertation advances the understanding of hydrogen-based PtX and PBtX processes and highlights some key efficiency and economic drivers to improve their competitiveness against fossil-based alternatives. Also, some limitations in the applied and developed methods, specifically related to process model representativeness and computational speed-up consistency, are identified, thus indicating the need for further investigation.