Decarbonisation Technology - February 2024 Issue

Producing synthetic fuels from renewable feeds A comprehensive review of renewable processes and feedstocks used in the production of renewable or synthetic fuels

Scott Sayles, Robert Ohmes, Pattabhi Raman Narayanan and Jessica Hofmann Becht

T he method of synthetic fuel production is dependent on the ability to meet the requirements of a circular economy. The feed used to produce the synthetic fuels determines how it fits into the circular economy and within the carbon lifecycle. For example, the use of a renewable feed, such as woody waste, produces a renewable fuel that is consumed and has a carbon value that is about net zero ​ (Ohmes, et al., 2022a), ​ (Ohmes, et al., 2022b)​, ​(Jacob-Lopes, et al., 2022). Many of the processes used to produce renewable fuels are identical or very similar to those used for fossil fuels. The critical difference is that both the feedstock and the energy used in the conversion processes must be certified as renewable. The main processes to convert renewable feedstocks into these renewable or synthetic fuel products are discussed.

Renewable and synthetic fuels • Biogas • Renewable diesel • Biodiesel •  Renewable jet and sustainable aviation fuel (SAF)

• Hydrogen • Ammonia • Methanol

Renewable feedstocks A range of renewable feeds such as seed oils and animal fats and biomass such as woody waste can be used to produce renewable or synthetic fuels, as summarised below. Plant (seed) oils and animal fats Triglycerides make up the majority of feeds, which are either hydroprocessed or cracked (thermally or catalytically) to produce renewable fuels. To drive sustainability, plant oils are increasingly byproducts from the production of seed oils or seed oil processing and not from sources that compete with food products. Used cooking oil is a preferred feedstock for this reason. Typical plant seed oils and animal fats are shown in Table 1 (Sayles & Ohmes, 2022). Residual biomass feedstocks Residual biomass feedstocks cover a wide range of feeds from waste and residue sources. Typical sources include filtered oil and grease (FOG) from waste systems, palm oil mill extract (POME), woody waste, and biomass from microbes. The common factor is that biomass is a waste product, not usable for food or other uses. The advantage is that the waste is repurposed to

Renewable feedstocks • Plant oils and animal fats • Sustainable biomass

Renewable processes • Anaerobic digestion • Gasification • Pyrolysis • Hydrothermal liquefaction (HTL) • Pretreatment • Hydroprocessing • Fluid catalytic cracking (FCC)

• Hydrogen production • Ammonia synthesis • Methanol production

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