Decarbonisation Technology August 2022 issue

Biomass, BECCS and electrolysis for climate-neutral liquid fuels Synthetic e-fuels, biofuels, and BECCS can be ‘carbon-negative’ and therefore have a valuable role to play in a ‘net-zero’ energy system Stephen B. Harrison sbh4 Consulting

T he energy transition has many geopolitical, economic, and environmental drivers. Principle drivers include diversification of energy supply, avoidance of dependence on fragile fossil fuel supply chains, avoiding price spikes in traded commodities, and mitigating climate change. Synthetic e-fuels and carbon dioxide (CO₂) utilisation from bioenergy with carbon capture and storage (BECCS) related to biofuels can be part of the solution. The recycling of atmospheric CO₂ into synthetic fuels using renewable energy offers a solution with no net CO₂ emissions. Renewable synthetic liquid fuels will therefore play a key role in the energy transition alongside green hydrogen, as traditional refined products are challenged by fossil-free energy vectors. Carbon accounting and credible climate- neutral claims The production of liquid fuels from biomass can be carbon neutral or carbon negative. Greenhouse gas (GHG) emissions that emanate directly from production are referred to as Scope 1 emissions. However, in a full lifecycle analysis of the environmental impact, it is important to go beyond production of the fuel and consider GHG emissions from the use of the fuel: referred to as Scope 3 emissions. For example, ammonia and hydrogen yield no CO₂ emissions when used. On the other hand, synthetic e-fuels or synthetic methanol do emit CO₂ when burned to release its energy value. So-called Scope 2 emissions, which are generated by inputs to the process such as power generation, must also be accounted for, and all three must be considered for a valid ‘carbon negative’ declaration.

Furthermore, we must think beyond carbon neutrality to ‘climate neutrality’, meaning the CO₂ equivalence of methane emissions must be considered. For example, biogas, biomethane, and renewable LNG are all low-carbon energy vectors, but if there are methane leaks that result from their production or distribution, they can have a very negative environmental impact. Per tonne of emissions to the atmosphere, methane is a much more potent GHG than CO₂. The mechanics and principles of ‘carbon accounting’ and ‘life cycle analysis’ are well documented in ISO standards, and these can be followed to justify the use of labels such as ‘climate neutral’ or ‘carbon negative’ for certain fuels. For example, these standards give guidance on how the substitution of fossil fuel usage or the avoidance of alternative biomass decomposition pathways can have a positive effect on carbon accounting calculations. Some biomass-related pathways to produce energy vectors have the potential to be carbon negative or GHG emissions negative. According to the EU Renewable Energy Directive, certain modes of biomethane production from biogas are regarded as carbon negative. Annex VI declares numerical values for the climate impact of biomethane production from various digester technologies and feedstocks for heat and power or mobility applications. In certain scenarios, there are significant carbon-negative impacts of producing and using renewable biomethane. Another example would be the gasification of biomass to make syngas and the conversion of that syngas to gasoline, either via methanol and methanol-to-gasoline process (MTG) or via Fischer-Tropsch (FT). This pathway could be