Molecular magic – turning waste into wonder fuels

Sustainable fuel and other valuable products like biofuels and e-fuels offer great promise in the challenge to reduce our reliance on fossil fuels. 

With feedstocks ranging from biomass to captured carbon dioxide the feedstocks, and the techniques driving the production of these wonder fuels are diverse. We spoke to Candice Carrington, a Senior Refinery Consultant, about how the team’s refining expertise is helping to unlock opportunities for the production of sustainable aviation fuels (SAF) and other valuable products produced from waste

Over the past few years, our Consulting team in Woking, UK, has supported an increasing number of projects that seek to produce sustainable liquid fuels from the recycling of waste materials, like used cooking oils, biomass, and unrecycled household refuse as well as captured carbon dioxide and low-carbon hydrogen. 

Every project seeks to produce valuable products like biofuels and e-fuel through a dizzying array of techniques. One area of particular interest has been the production of Sustainable Aviation Fuels (SAF) – a product that offers a sustainable alternative to traditional jet fuel – and one that will be central to decarbonising the aviation 
industry.

We talked to Candice Carrington, Senior Refinery Consultant, who is deploying her many years of oil and gas refinery experience to support these potentially groundbreaking projects.

Why SAF?

We all acknowledge the impact of rising greenhouse gas emissions, with rising global temperatures (due to the rise in atmospheric greenhouse gas concentrations, including CO2) resulting in increasingly severe natural disasters, rising sea levels, and changing weather patterns across the globe. Measures to slow the rise in CO2 emissions to mitigate further excessive warming will require a many-pronged approach: capturing and sequestering CO2, reforestation, and the use of lower-carbon fuels all form part of this mix. 

This is where Sustainable Aviation Fuel (SAF) comes in. Flying aircraft on hydrogen or electricity is possible, but for now, is not viable for long haul and many passenger flights. Short to medium-term, SAF can provide the answer.

Many pathways to SAF production

Though similar in composition to conventional jet fuels, SAF’s end-to-end manufacturing process emits less greenhouse gas emissions^[1] than its fossil-based predecessors. Our team has worked on projects that look at a range of sustainable fuel projects, each drawing on different feedstocks and natural resources, from used cooking oil (UCO), waste wood, waste tyres and waste plastics, unrecycled household waste to sewage sludge and captured carbon dioxide. All have been proven as technically viable in producing lower-carbon fuels; with a couple of pathways with plants operating commercially, while others are at various stages of development.

A recipe for success – used cooking oil

Of the established routes for producing SAF, Hydro processed Esters and Fatty Acids to Synthesised Paraffinic Kerosene (HEFA SPK), produced using non-food vegetable oils, and waste materials like UCO and animal fats is one the most proven and readily implementable. One reason is because the oil feedstocks have a good proportion of molecules with the same numbers of carbon atoms as those found in fossil-based jet fuel.

The right ingredients

Creating a product that can blend into its fossil-based equivalent is a question of carefully arranging its ingredients. To explain, HEFA involves running the oil feedstock through a series of catalytic reactors to hydrogenate its fatty acids and esters and remove unwanted components like oxygen. Then, using additional catalytic and distillation processes, the chains of hydrocarbons can be adjusted until the required fuel properties are achieved. The result is a fuel product comparable in composition to fossil-based jet fuel. 

This chemistry has been used in traditional petroleum refining for close to 80 years, which is another reason the HEFA pathway is readily implementable. The process has only been adapted for a different recipe of molecules. Pre-treatment of the feedstock to eliminate impurities is critical, but considering the food processing industry has huge experience in this area, early adopters have successfully implemented this technology within refineries with relative ease. It is hence, considered a low-risk route with many operating references and many options for technology licensors. 

An essential element for the HEFA and most other SAF pathways is the reliable supply of renewable or low carbon hydrogen.

Benefits of UCO to SAF

The benefits of UCO-based SAF are many. Not only does it alleviate the need for new fossil fuel extraction which contributes to lower end-to-end GHG emissions, but it also diverts potentially huge volumes of waste oil from being burnt or flushed away (which has an environmental impact)^[2]. And for many regions, it also reduces the reliance on imported fossil fuels, particularly for regions without access to domestic supplies of fossil fuels, aiding energy security.

Other routes to SAF

But SAF production from UCO is just one solution, and for many, it will be difficult to secure a secure and continuous supply of UCO because the volumes are limited. 

Hence, Biomass-based fuels and eFuels (fuels produced using electricity from renewable sources, water, and CO2) are fast-emerging alternatives, bringing the reality of a ‘climate-neutral’ fuel source even closer - not just for aviation, but also for shipping and other fuel-hungry sectors. 

Understanding eFuels

eFuels are produced from combining captured CO2 with hydrogen to produce a synthetic gas (syngas) as a precursor. Syngas has been a mainstay in the chemical industry for over 70 years, produced from gasification of coal or petroleum or reforming of natural gas or other hydrocarbons, all on a fossil-based feedstock. A modern twist involves innovative technologies using captured CO2 (from industry as part of wider decarbonisation measures) and renewable or green hydrogen (produced using renewable energy), which are chemically ‘shifted’ to form syngas. The resulting eFuels can be blended with traditional fuels, to certain limits, enabling introduction into existing systems without engine modification. 

Biomass – a bright future

The use of biomass as a source of SAF is extremely attractive for those regions that have access to significant supplies of biomass, such as from forestry industries or sustainably produced, non-food crops. The key is that biomass contains a high proportion of carbon and hydrogen, the very atoms needed in SAF. 

Biomass is already an established feedstock for the power industry where it can undergo ‘gasification’ to produce syngas. This syngas requires additional processing to remove by-products and impurities such as tars: it is the precursor for producing eFuels like eSAF.

Non-recyclable waste material from municipal waste, half of which is typically from biological sources, has also been touted as a viable feedstock that can similarly undergo ‘gasification’ to generate syngas, which is also cleaned up and used for eSAF production.

Sewage works

Sewage sludge is another interesting biomass feedstock and our work with Firefly Green Fuels has gained understandable interest in the media.  It is the byproduct of sewage treatment works that contains high-carbon content as well as high water content. Because it is wet, it offers a perfect opportunity to use a process called hydrothermal liquefaction (a little like pressure-cooking the feedstock!) to break the feedstock into a biocrude oil which can be processed in a similar way to fossil-based fuel to produce SAF.

In the UK there has been much outcry and media attention about sewage sludge flowing into rivers and water ways during periods of heavy rain, causing significant harm to the aquatic life and the wider environment, so this technology would deliver a timely solution to many problems.

Brewing up alternatives

Another interesting opportunity involves the fermentation of biomass into ethanol to produce SAF via an alcohol-to-jet pathway. The chemistry used is like that already used to produce LPG, gasoline, or diesel from ethanol. 

The challenge is managing the economics around the supply and demand of biomass-based ethanol. The route favours land-rich regions that make ethanol from crops (corn starch) such as the USA and Brazil which together produce close to 80% of the world’s ethanol. With this too comes the challenge of land use, deforestation, and the diversion of natural resources such as water needed in its production. 

However, recent developments show that gas fermentation can be used to convert captured CO2 and hydrogen into ethanol and, as such, removing the reliance on ethanol from crops alone.

Methanol routes are also yielding good results. Here CO2 and hydrogen are used as the basis for methanol production, using distillation and catalysts to create water and active building blocks that can be used to make SAF, eGasoline and eDiesel. Methanol has many applications, offers a high yield of jet range material and is gaining increasing attention as an alternative fuel.

A fast-moving sector

At the time of writing there are eight qualified SAF under the ASTM D7566 standard. However, innovations in processing technologies are being made all the time, opening the door for many other routes to SAF with new ones being qualified every year. 

The key challenges for the supply chain and investors are time and cost. The pace at which these first-of-a-kind projects are developing is fast, but this comes with risk – with few able to afford to be first adopters in the absence of operating references. 

Competition for resources will be a future challenge. The supply chain, including feedstock suppliers, technology providers, engineering contractors, OEMs, regulators, transportation and shipping companies and others may face bottlenecks with the number of projects required to progress at the same time. This too will require investment to increase the number of skilled personnel and the production of physical equipment to meet the coming need. 

But with increased focus on this potential challenge and with determined collaboration and creativity, it should be possible to address these challenges to turn our waste into tomorrow’s wonder fuels.

Sources:

Bioukfuels.co.uk

[1] Studies show that SAF emits fewer Greenhouse Gases (GHGs) than fossil fuels - and may result in other environmental benefits, such as reduced contrail formation. Though it still produces carbon emissions, SAF can deliver an 80% reduction in carbon emissions across its lifecycle, so it is a technology here to stay.

[2] One tablespoon of waste cooking oil improperly disposed of can cover half an acre of flat water, causing the water to be less rich in oxygen and harming aquatic life below