The application of renewable energy power generation technologies in power-to-methanol projects will become viable only if the chemical industry will make processes for methanol production more flexible, thus adapting them to the intermittent and variable nature of wind and solar power. This is the main conclusion of a study conducted by scientists of the University of Oxford, in which the authors explained that the methanol industry is currently dominated by conventional chemical processes that require a constant, rigid power supply that can only be ensured by conventional energy sources.
By contrast, adopting more flexible chemical processes would allow power-to-methanol plant operators to adjust their demand to the variable pattern of renewable energy while incorporating demand-side management to balance power generation and end-use load. “To date, the limitation in renewable chemical production is largely addressed by expensive energy storage, particularly in the form of compressed hydrogen (H2) to level renewable power output,” the U.K. scientists explained, adding that this solution is expensive and has, still, several technical issues.
For their case study, the researchers considered a methanol plant that acquires CO2 by carbon capture from a point source and H2 from electrolysis, with a methanol synthesis loop centered on a catalytic reactor and a distillation-based step for product purification. Two different analyses for full electrification of the facility were made, with PV and wind power generation, respectively, with the option of having a conventional back-up power source in both cases. ” In this system, the variability of renewable energy supply is tackled by a combination of an H2-based energy storage system and process flexibility enabled by the storage of process materials and energy,” the academics specified.
The system was sized for the production of 400,000 tonnes of methanol per year for two different locations – Norderney, which is an island off the North Sea coast of Germany and has significant wind potential; and Kramer Junction, which is located in the Mojave Desert, in California, and has very high solar radiation levels. It was designed in a way that, when the renewable power is in surplus, excess hydrogen produced via electrolysis and raw methanol generated via synthetic reactor can be produced and stored in two different storage subsystems. “When the power deficit is too large, the stored H2 can additionally be fed to the fuel cells to sustain a minimum level of operation of the methanol plant,” the group explained, adding that back-up power is only a failsafe measure to ensure sustainable production and the CO2 storage subsystem handles load mismatch between carbon capture and methanol synthesis. Furthermore, a heat storage unit facilitates the integration between methanol synthesis and carbon capture.
The viability of this solution, according to the scientists, is heavily dependent on the dispatchable energy price and the degree of process flexibility. The potential benefits of flexible methanol production, they went on to say, are possible when a 100% renewable power supply is ensured, with most of the savings coming from the removal of the H2-based energy storage system or from very cheap renewable energy, as in the case of Kramer Junction, in which the high costs of H2 storage are offset by the use of PV. “The optimization result confirms that the process flexibility would lead to reduced curtailment, which represents improved utilization of renewable energy,” the authors of the study stated. “Note, however, that the flexible operation for the most economical scenario in both locations incurs a higher renewable generation cost despite the reduction of the overall cost.”
They further explained that the estimated levelized cost of methanol (LCOMeOH) for the Kramer Junction case without flexibility is around 17% higher than that of the system in Norderney, and this is due to the surplus solar power needed to handle the high variability of solar. When flexibility in the chemical processes is introduced, however, solar has the potential to reduce the project's LCOMeOH by 34%, while for the wind-powered project, this percentage stands at 22%. “By introducing process flexibility, the excess generation is greatly circumvented,” the Oxford team stated.
The decisive factors in the application of this model are the costs of the core flexible chemical process units and the storage cost for intermediate products in the chemical processes. Furthermore, it requires the over-sizing of the flexible process units. “In a sense, process flexibility is a competing design to process integration; its implementation requires an analysis on a case-by-case basis,” the academics affirmed. “Nevertheless, if the range of cost reduction (20-35%) brought by implementing process flexibility as demonstrated in this work can be attained, its potential gains would most likely exceed the opportunity-cost of energy integration for many chemical processes.”
The findings of the research were described in the paper Power-to-methanol: The role of process flexibility in the integration of variable renewable energy into chemical production, published in Energy Conversion and Management.
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