Decarbonisation Technology August 2022 issue

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

Solid waste

Syngas, ~35% H

Note: – For the reaction stoichiometry shown, methane is used as an example hydrocarbon.

Char, slag, ash

Plasma gasification of solid hydrocarbons, eg waste

Carbon feedstock

Municipal solid waste, dried waste water treatment sludge, biomass, waste paper, tyres, etc Hydrocarbon + 0 2 ➔  2CO + 4H 2 Hydrocarbon + H²O  ➔CO+ 3H 2 Hydrocarbon + 20 2  ➔CO 2 + 2H 2 0 CO, CO 2 , char, slag and ash Close to atmospheric pressure ~1,000°C

Target chemical reactions Additional side reactions Carbon produced as Product gas pressure Product gas temp

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. Figure 1 Plasma-based gasification of waste or biomass


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