Biofuels syncrude pathway for producing SAF from waste
A waste-to-fuels process converts refuse derived fuel to syncrude that can be readily refined and co-processed to form SAF using conventional refining.
Candice Carrington and Mohammed Navedkhan
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In 2020, the aviation industry’s global economic impact was estimated to be about $3.5 trillion, equivalent to 4% of global gross domestic product (GDP) (ATAG, 2020), consuming approximately 300 million tons of aviation jet fuel per year worldwide. Moreover, despite its median emissions profile, between gasoline and diesel, conventional jet fuel is envisaged to be the fastest-growing source of transportation emissions due to the continued growth of the aviation sector. Clearly, decoupling the expansion of the aviation industry from its emissions is vital to achieve global net-zero targets.
Sustainable aviation fuel (SAF) has been promulgated as a preferred aviation fuel that will remain indispensable until at least the 2050s, particularly for medium- and long-haul flights, as outlined in the ‘Clean Skies for Tomorrow’ initiative (ATAG, 2020).
The essence of an emissions-free drop-in aviation fuel like SAF is that it must be synthesised from renewable and carbon-sink sources such as biomass, organic waste, and municipal solid waste. These sources absorb carbon from the atmosphere and store it as biogenic carbon over their lifetimes. Thus, when this carbon is incorporated into fuels and, following combustion, emitted back into the atmosphere, it creates a closed-loop cycle of energy production with no net carbon emissions into the atmosphere. Other aspects of the sustainability of SAF over its lifecycle are also critical, such as avoiding land-use change and diverting land and water away from food crops, primary forests, protected areas, and biodiverse grasslands and peatlands that serve as significant carbon sinks.
In the UK, the government has set out to focus on future-proofing the UK aviation industry, worth about £12 billion to the economy, under the Ten-Point Plan for a Green Industrial Revolution in the country. A key element of this plan is the Green Fuels, Green Skies (GFGS) Competition, a £15 million undertaking to support the production of SAF in the UK. The aim is to support the early-stage development of a UK SAF industry, such as the pre-FEED or FEED stages of the projects, with an emphasis on the development of first-of-a-kind (FOAK) commercial SAF facilities in the UK. The target of the Ten-Point Plan is to achieve a SAF mandate of up to 10% SAF blending by 2030 and up to 75% SAF blending by 2050. This will include an option to review and increase the targets at regular intervals until and beyond 2050 via a Renewable Fuels Transport Fuel Obligation (RTFO) instrument (see Figure 1).
As part of the GFGS competition, Protos Biofuels Project was selected to perform a concept/pre-FEED study to evaluate the feasibility of producing sustainable synthetic crude (syncrude) that can be processed in a conventional refinery to produce SAF. Petrofac was engaged for the pre-FEED study. Protos Biofuels is a novel waste-to-fuels process that can accept refuse-derived feedstock (RDF)sourced from a mix of waste management companies and aggregators within the UK.
In a typical gas-to-liquids (GTL) process focused on transport fuels, syngas made from fossil sources is transformed into a heavy wax product via thermo-catalytic processes, including Fischer–Tropsch (FT). The wax is then hydrocracked to produce a mix of gaseous and liquid hydrocarbons, which can be fractionated to form aviation fuel, gasoline, diesel, and other products.
When syngas is produced from biomass or waste, it may be converted into low-carbon liquid fuels, creating a sustainable fuel source with substantial potential for emission reduction. In the same spirit, the RDF feedstock in the Protos Biofuels Syncrude process is gasified to produce a stream of syngas containing predominantly hydrogen (H2) and carbon monoxide (CO). Advanced Biofuels Solutions (ABSL) RadGas gasification technology was selected as the RDF gasification process for the syncrude synthesis. It produces a relatively clean syngas by breaking down the tar formed in gasification via a two-stage process. For conversion of syngas to syncrude, a gas-to-liquids process using an FT reaction mechanism with negligible wax production was chosen from among the various FT technologies that were reviewed to avoid the need for a hydrocracker to break down the wax prior to refining. The Protos Biofuel Syncrude process has flexibility in modifying the process conditions for FT technology to increase the yield of SAF components at the expense of other fractions but with a higher Capex. A middle-road design of optimised Capex vs SAF production capacity was chosen. As an estimate, from a feedstock of around 152,000 tonnes of refuse-derived fuel per annum, approximately 11,000 tonnes of syncrude can be produced with a high yield of SAF.
Key aspects of Protos Biofuels Syncrude process
The Protos Biofuels Syncrude process has four essential steps to produce syncrude, with two additional ancillary steps for handling emissions and effluents. These are listed below and represented in Figure 2.
Essential steps are:
• Refuse derived fuel (RDF) feedstock reception from suppliers and processing for contaminants removal
• RadGas gasification unit with syngas gas clean-up and compression
• Gas-to-liquids unit employing FT process
• Syncrude stabilisation.
Ancillary steps include:
• CO2 capture, treatment, and export for sequestration
• Wastewater treatment and disposal.
The nature of RDF is such that the feedstock varies in quality across batches, even from the same supplier or within the same batch. Variations, specifically the composition in terms of carbon and other elements, can be seasonal and dependent on the source of the waste, whether it be municipal solid waste (MSW), commercial and industrial, or construction waste.
Solid feedstock preparation is a standard requirement for gasification processes to ensure the feedstock size, distribution, moisture content, and metal levels are controlled within the design limits for optimal operation of the gasifier. This is performed in the feedstock processing stage. RDF is shredded, screened, subjected to removal of ferrous and other metals, and then dried before it is routed to the gasification step. Some of the physical variability of the RDF, as mentioned above, is controlled and mixed in the feedstock processing. However, there are other compositional challenges, as described below.
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