In March, April and May of 2020, countries around the world imposed some of the strictest mass limitations on individual freedom in recent history. These lockdowns were an attempt to halt the spread of the rapidly spreading novel coronavirus, Covid-19. For the first time in modern history, a true pandemic had wrapped around the globe but also for the first time, modern medicine (and modern technology) were to be tested against contagion. Factories were closed, cities deserted, and planes grounded. The stalled global economy resulted in reduced carbon emissions. According to a paper in the May 2020 issue of Nature Climate Change, daily global CO2 emissions decreased by 17% in April 2020 versus the previous year. At the absolute peak, emissions in individual countries decreased by 26% on average.
Air travel plummets
Air travel was one of the worst sectors affected, and during those three months of 2020 – daily air traffic was down 75%. According to the Global Carbon Project, this corresponded to a 60% decline in emission from aviation. Aviation normally contributes somewhere between 2.5 and 3% of all man-made greenhouse gasemissions. It sounds small, but when compared to the estimated 26% of emissions that emerge from agricultural efforts the outsize impact of aviation becomes clear.
But the pandemic is not the way we want to reduce greenhouse gas emissions. The effects were too temporary, and too costly. The UK Government alone expects to spend close to £400 billion on efforts to contain Covid, and alleviate the burden on individuals and the economy and crashing the global economy is a poor response to any crisis…
At the same time the world is opening up again. A year on from the height of lockdown, IATA data shows air cargo demand in April 2021 outperformed even April 2019 by 12%. And although Revenue Passenger Kilometres (a metric to measure passenger demand) in April were still down 65.4%, this was growing 3% month on month.
The world begins to move again.
The biggest ever demand-side impact on aviation failed to solve its carbon problem. As we take to the skies again to see family, travel for work, and build ties many of us can’t help but wonder whether we need some new approaches to make it more sustainable. Approaches such as a US based innovation that requires no changes in behaviour, no newly designed aircraft engines, and no exotic hydrogen-based system. It is a creation of the National Renewable Energy Laboratory (NREL) in Colorado, which is itself part of the US Department of Energy. Here a team has been working on a process to “convert wet waste carbon to meet jet fuel specifications”.
This is an elegantly academic way of saying they have been taking some really nasty by-products of industrial food processing that would otherwise rot in landfill such as slaughterhouse waste and dairy waste and repurposed them as aviation fuel. This is then blended with existing fuels in ever-improving ratios. This process is something that environmental engineer Derek Vardon knows well. His journey begins with joining the navy straight out of high school, where he got his first taste of engineering. Derek then went to the university of Illinois in Urbana Champaign to study environmental engineering. At that time biofuels were around they were not as mature or relevant a technology as they now have the potential to be. “I got involved with a lot of research, focusing on algae for wastewater treatment. And soon that turned into algae for biofuels and spent my research career really focusing on that waste and energy and environmental impact intersection,” says Derek.
Derek approached the NREL lab for a placement post-graduation and found that it had aims beyond pure science and research.The US Department of Energy has been looking at converting food waste biomass into fuels for over five years. And that’s where we will begin… because the food waste problem is very real. According to the Food and Agriculture Organisation of the United Nations, an estimated one-third of all food produced for human consumption in the world goes to waste – about 1.3 billion tonnes per year.
Tackling food waste
Food waste occurs throughout the supply chain, from the farm itself, all the way to the household. It represents enough calories to potentially feed every undernourished person on the planet. Target 12.3 of the Sustainable Development Goals calls for halving per capita global food waste at retail and consumer levels by 2030. But setting aside the humanitarian ethics of the situation, there is an environmental cost to this. Of the 26% of human greenhouse gas emissions that come from agriculture, about 24% come from food waste. That’s 6% of all human greenhouse gas emissions resulting from food loss and waste. Methane is many times more potent than CO2 as a greenhouse gas, most estimates putting it at 20-30 times worse and so this results in more than twice the emissions generated by aviation.
This is why Derek, the National Renewable Energy Laboratory and the US Department of Energy are convinced that this wasted fuel source that should no longer be ignored. “We actually looked at when you look at the biological processes taking place, you can actually stop before methane. And so these micro-organisms are doing just what we do as humans, they’re chewing on the carbohydrates that are in food waste. There’s a lot of protein, a lot of fats and lipids,” he explains.
The biological web at work. “And so this technology uses a mixture of naturally occurring micro-organisms,” says Derek. But instead of making methane it produces volatile fatty acids – VFAs. These are short chain carboxylic acids that are products from fermentation. “But we’ve also been working with industry partners that have moved the technology forward into the demonstration and pilot and scale to be able to take food waste and other wet waste resources, and ferment and separate the short chain carboxylic acids,” says Derek.
It is a bit of a joke they have in the laboratory, as these volatile fatty acids are not actually that volatile, they’re fairly stable, and liquid at room temperature. “But they smell. They smell really bad,” laughs Derek. “Our work primarily in my team has been focusing on how to catalytically then with non-living chemistry, use a solid catalyst very similar to the catalyst that’s in your automobile tailpipe that can convert, you know your engine gases, and oxidise those.”
Meeting specifications
The process developed at the National Renewable Energy Laboratory turns these volatile fatty acids into finished jet fuel. And a lot of the effort goes into to trying to understand how to tune and tailor the chemistry – to take the short molecules and make them longer, as well as removing oxygen from them, to make sure they are compatible with existing jet engines. Meeting all of the rigorous specifications required for certified jet fuel is critical. Derek explains that his team has been developing catalysts for two main chemical transformations. “The first one is called a ketonisation catalyst. And it takes two volatile fatty acids, and actually turns them into one molecule.”
Conveniently these small molecules can then be stitched together to bring them into the appropriate boiling point range for a jet fuel. Over the last five years, in consortium with other research organisations, they have been looking to tune and tailor the catalysts and reaction conditions to turn as many carbon molecules as possible into these elongated ketones.Making this first chemical transformation as efficient as possible. Even better is that these smell good! “The ketones that that form, when they’re coupled together, are nice, sweet smelling, and actually quite pleasant. I don’t recommend using your nose as a chemical detection system,” says Derek.
The organic ketone liquid is separated from and sits on top of the water that is left over from this process. “And once that longer carbon backbone is formed, the final catalytic step we work on is then how do you remove oxygen fully and add hydrogen so that the fuel now is comprised fully of just carbon and hydrogen.”
In other words, a highly energy dense liquid hydrocarbon fuel. After that they send a sample off to the University of Dayton’s research institute to analyse the fuel properties of the hydrocarbon solution. Various properties are analysed such as the fuel volatility, viscosity, energy density… but also things like the flammability and the flashpoint.
The flashpoint is the lowest temperature at which a liquid (usually a petroleum product) will form a vapour in the air near its surface that will “flash,” or briefly ignite, on exposure to an open flame. The flash point is a general indication of the flammability or combustibility of a liquid. “We take that, and then go back to the first step of the actual fermentation itself. And try to understand how we can tweak the process, tailor the chemistry or modify the biology to really get more carbon into the desirable range of jet fuel products that we’re trying to make.”
Biofuel mix increases
For this project in particular the initial target was to achieve a liquid that could be blended in with a fossil jet fuel at a 10% level, and still meet all of the fuel property specifications that were set out by the governing body ASTM International. “Very quickly, we were able to meet that goal and then we’d say OK, well, how high can you increase the renewable content, because ideally, right now, the industry standard that’s certified and qualified, where you could fly it in a commercial aeroplane, they have set the limit at 50%, could be renewable.”
In the last few months, Derek and the team claim to have been able to push this further to 70%. Calculating the impact of this on the greenhouse gas emissions is a fiendishly complex discipline involving lifecycle and sustainability analysis that is constantly being updated. Regarding the process itself, they need to consider the chemicals used to produce the sustainable aviation fuel, what utilities are required and how the electricity used to run a heater or a pump was produced… how green the energy grid is in the US.
The original feedstock also needs to be taken into account, how it is typically produced and handled. Perhaps different feedstock was used at different time and for this the US Environmental Protection Agency develops standardised frameworks allowing the comparison of various types of waste.
Does the landfill that the waste was bound for have methane capture and combustion technology? That needs to be taken into account for any of these measurements.
But taking all that into account, and after double checking and triple checking with the EPA Derek and his team are blown away by the carbon reduction that they are getting. “Looking at food waste as a feedstock, when we’re able to turn that into sustainable aviation fuel, we’re able to get actually a negative carbon footprint because of how significant methane is as a greenhouse gas relative to carbon dioxide. And so we baselined this overall process and estimated 150% reduction, and the carbon intensity footprint of just our sustainable aviation fuel made from food waste. And so when we’re able to look at the blend limit of 70%, you would actually have a negative footprint from the bio based content, and then even the 30% of the fuel was still fossil… the math behind it, they would actually offset one another. So you could get to a carbon Zero Based footprint. For a, you know, for a blended fuel that had a mixture of both fossil and bio based.”
Derek says that this isn’t to say that we can just burn this fuel and solve all of our environmental woes and points to the reduce, reuse and recycle philosophy, but he is optimistic about this carbon solution. “It is I think, promising of how you can try to engineer carbon flows within systems to make better use of that carbon, and prevent more of it from going into the high impact greenhouse gas emissions, and still trying to meet that societal need of energy for transportation.”
The future of sustainable aviation fuel is partly in the chemistry, and partly… in the quantity. The goal from autumn to spring is to start making barrel quantities of this fuel for ASTM testing and certification. Once the fuel is certified, any institution could use it and benefit from it, as long as they are capable of meeting the chemistry and process requirements.
“If we could put a plane in the sky on food waste in the next three years, I’d say that’s definitely our goal.”
