Reducing the lifecycle carbon emissions of rice straw-to-methanol for alternative marine fuel through self-generation and renewable electricity

The advancement of maritime decarbonization has accelerated the adoption of alternative marine fuels, particularly methanol. However, its widespread adoption encounters three primary obstacles: pricing, availability, and carbon accounting. Given the current state of technological and industrial developments, designing technical routes that comply with carbon emission restrictions is a primary concern for all project stakeholders. Therefore, based on a 100,000 tons/year biomass-to-methanol (BTM) plant located in the eastern coast of China, four integrated routes were designed and simulated to obtain steam, electricity and water balances. Energy and exergy analysis were then performed. Finally, a carbon emission assessment (calculated as equivalent carbon dioxide 'CO2eq') was conducted with 30 scenarios. Results indicated that the carbon emissions during rice straw production stage, including carbon emissions from methane and nitrous oxide emissions from rice fields, were 0.2020 and 0.5563 kg CO2eq/kg straw-dry using value and energy allocations, respectively. Excluding rice cultivation and harvesting, compared to the 2021 baseline level of 94 gCO2eq/MJ set by the European Union's Renewable Energy Directive III (EU RED III), the carbon emission intensity of methanol decreased by 75.4 %, 76.2 %, 71.5 % and 74.5 %, respectively for route 1 to 4, satisfying the EU-mandated carbon emission limit. Sensitivity analysis showed that reducing rice straw consumption and energy consumption of rice straw pulverizing, drying, and shaping could effectively reduce carbon emissions. Overall, given the carbon emission limit, for a BTM plants in plan or under-construction, this study suggests to provide external renewable electricity, or to configure self-generation utilizing by-product fuel gas and syngas.