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
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The 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 carbon negative if the CO2 from the biomass gasification process is captured and permanently sequestered, referred to as BECCS. However, the overall life cycle of that pathway must consider the CO2 emissions from the use of methanol or liquid fuel. Furthermore, if the gasification is optimised for hydrogen production using reforming within the gasification process, and subsequent water gas shift reactions and BECCS are involved in sequestering the CO2 emissions from the process, we can produce a fuel that has zero emissions when used.
In the case of other lower temperature biomass thermolysis processes, such as pyrolysis, we often yield solid carbon as char in addition to producing liquid fuels similar to heavy fuel oil. Locking carbon into biochar is also regarded as carbon negative, and in the EU, the regulations allow for carbon credits through this pathway.
Limitations to scaling up biofuels and the use of MSW as an alternative feedstock
Biomass gasification to yield syngas is a viable techno-economic pathway to methanol and other liquid fuels. However, the difficulty of securing mass-scale biomass feedstock has limited scale-up and has acted as a bottleneck for biofuels. The planting of energy crops to displace food production and deforestation to make way for energy crops must be avoided if biofuels are to be a sustainable part of our future.
Biomass collection and use are therefore limited to regions with significant agricultural waste, such as the central Californian valley, where almond shells or pruning clippings from orange groves are abundant. Other notable examples include managed forests, such as in Canada or northern Europe, where saw-mill wastes can be used as pelletised woodchips.
The use of municipal solid waste (MSW) as an alternative feedstock to biomass is technically possible. Both have similar moisture content and handling properties. MSW is generally around 50% biomass, even after sorting out the green and paper fractions. The residual content is often plastics from packaging that are also hydrocarbons, like biomass.
Gasification technologies have been used to process biomass and MSW. See Figure 1 for a generic plasma gasifier representation. Some have even made the bridge from MSW to biomass. For example, the InEnTec plasma gasifier has been used on more than 13 MSW gasification projects since 1995. Aetemis is also planning to deploy the InEnTec plasma gasifier for a biomass-to-hydrogen gasification process in the US using feedstock signed for walnut, almond, and pistachio nut waste from Californian farms with 20-year supply contracts now signed. A life cycle analysis study has concluded that this is a carbon-negative process due to avoidance of CO2 emissions from crop waste burning on the farms. The Plagazi system, which is designed to process landfill waste, or MSW, also uses a plasma gasification reactor at the heart of its process.
Captured CO2 & synthetic e-fuels as a solution
An effective solution to the biomass feedstock issues is the use of captured CO2 and synthetic e-fuels. An alternative pathway to synthetic liquid hydrocarbons is through electrolysis. This can be using a solid oxide electrolyser cell (SOEC) system with CO2 feed to yield syngas, a conventional polymer electrolyte membrane (PEM), or an alkaline electrolyser to make hydrogen and then convert it to hydrocarbons with the addition of CO2 and further processing. For example, hydrogen and CO2 can be converted to methanol using a hydrogenation process over copper and zinc-oxide catalysts. Alternatively, the CO2 can be reduced to carbon monoxide (CO) to form syngas in combination with hydrogen.
In the case of electrolysis, significant amounts of electrical power must be consumed. For the process to be carbon neutral, this must be renewable power from solar, wind, or hydro schemes. Nuclear power is low-carbon, but the debate is open as to whether it is a ‘sustainable’ mode of power generation or not. These are effectively the Scope 1 emissions.
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