Integration of electrolysis with pyrolysis: effects of carbon conversion in methanol production

The decarbonization of the industrial and transport sectors requires the supply of green chemicals and fuels, which most likely will be derived from novel processes converting biomass and/or renewable electricity. For instance, biomass pyrolysis can be integrated with electrolysis to maximize carbon conversion into methanol (MeOH), while efficiently using excess power from fluctuating energy sources. However, the effective conversion of carbon and power consumption may be conflicting objectives in the design of methanol production plants integrated with electrolysis. Although several conversion routes and plant designs have been proposed recently, the trade-offs between carbon conversion, power consumption and efficiency were seldom evaluated and discussed. Thus, this research investigates the influence of two different electrolysis systems (steam and co-electrolysis) on the performance of methanol synthesis from straw pyrolysis.  These designs are benchmarked against a base scenario of methanol production without electrolysis aid. The results show that carbon conversions above 80% can only be achieved in scenarios including electrolysis systems. In addition, the electrolysis system can increase exergy efficiency up to 70%, 18 percentage points higher than the base case. However, this may require a significant increase in power consumption per kg of methanol, up to 19.2 MJ/kg (0.96 J/_JMeOH,_LHV) for 97% carbon conversion, which can be reduced in the co-electrolysis design (17.7 MJ/kg – 0.89 J/J_MeOH,_LHV) due to its lower steam consumption compared with steam electrolysis. These trade-offs relationships can be considered in the optimization of biomass to methanol plants for different fuel costs, electricity prices and emissions.