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
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The 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.
• Plant oils and animal fats
• Sustainable biomass
• Anaerobic digestion
• Hydrothermal liquefaction (HTL)
• Fluid catalytic cracking (FCC)
• Hydrogen production
• Ammonia synthesis
• Methanol production
Renewable and synthetic fuels
• Renewable diesel
• Renewable jet and sustainable aviation fuel (SAF)
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 become a usable product as a renewable source of carbon. Some wastes, such as POME, are suitable for direct hydroprocessing. However, woody wastes require preprocessing using pyrolysis or HTL before they can be hydrotreated (IEA Bioenergy, 2022).
Renewable processes convert raw feeds into usable transportation fuels and chemical feedstocks. Some processes are required for initial upgrading (pretreating and pyrolysis), while others produce the finished product (Basu, 2018):
Œ Anaerobic digestion: Anaerobic digestion, as shown in Figure 1, is a series of biological processes in which micro-organisms break down biodegradable material in the absence of oxygen. Typical feedstocks include food waste and animal manure. The main product is biogas, which is then dried and compressed for downstream use. Biogas typically consists of 45-85% methane (CH4) and 25-50% carbon dioxide (CO2). The biogas can be upgraded to biomethane or renewable natural gas (RNG)by removing the CO2 along with water vapour and other trace contaminants. The process also produces digestate, a valuable organic fertiliser.
Gasification: Biomass can be fed into a gasifier, as shown in Figure 2, producing syngas for downstream conversion to fuels or chemicals.
ŽPyrolysis: Thermal processing upgrades biomass to a product suitable for direct combustion or further upgrading. The removal of carbon increases the remaining hydrocarbon hydrogen content via thermal (pyrolysis) or catalytic (FCC) processes. Pyrolysis thermally decomposes the organic feed material, leaving a large portion as a residual solid (coke) and the remaining product as gas and liquid.
The pyrolysis process typically requires temperatures of 450-600°C in an oxygen-depleted environment. Before pyrolysis, the biomass feedstock is prepared by drying in order to reduce moisture content, size reduction to improve the uniformity of heat transfer, and sometimes torrefaction to enhance the energy density of the biomass.
The resulting pyrolysis oil (or bio-oil) is a dark, viscous liquid with a high oxygen content (about 35-40 wt%), which includes unique properties like non-volatility, corrosiveness, and immiscibility with fossil fuels (USDA, 2023). Its elemental composition is approximately 40-55 wt% carbon, 6-10 wt% hydrogen, 35-40 wt% oxygen, 0-3 wt% nitrogen, and less than 1 wt% sulphur, contributing to a lower heating value of 16-19 MJ/kg. The high oxygen content leads to thermal instability and a tendency for the oil to polymerise when exposed to air. Despite these challenges, pyrolysis oil is considered a promising renewable energy source and an alternative to fossil fuels, particularly in applications like chemical manufacturing, engines, turbines, and boilers.
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