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Flying is an activity that emits greenhouse gases, and the main culprit is aviation turbine fuel — a kerosene. Therefore, a global search is on to replace kerosene, at least partly, with a more environment-friendly fuel. The search has yielded a class of plant-based fuels that have now come to be known as ‘drop-in sustainable aviation fuels’ or SAF, where ‘drop-in’ refers to functional similarity to fossil fuels. As Aaditya Khanal and Mohammad Shahriar of University of Texas, Tyler, observe, the carbon dioxide production during SAF combustion in aircraft engines “is roughly equivalent to the carbon dioxide absorbed by the plants to produce the biomass”. So, we have a solution at hand, right? Wrong.
The problem
The problem is to do with economics. SAF has been known about for years. The first commercial flight with 50 per cent SAF blend was KLM’s Boeing 737-400 between Amsterdam and Paris, carrying 171 passengers, on June 29, 2011. Since then, according to Khanal and Shahriar, over 2,500 commercial passenger flights of 22 different airlines have used 50 per cent SAF derived from jatropha, cooking oil, camelina and sugar cane. Yet, the consumption of SAF is less than a per cent of total jet fuel.
Average global SAF production from 2013 to 2015 was 0.29 million litres per year, which rose to 6.45 million litres per year from 2016 to 2018. Additionally, annual global SAF production was projected to reach 8 billion litres by 2032. Sounds like much, but this is really like offering a banana to a hungry elephant.
The first-generation SAF is useless because it is made from edible oils needed for food. The second-generation SAF — produced from jatropha, castor, pongamia pinnata and so on — is the one under consideration, because the third-generation SAF, produced from photosynthetic algae, emits more greenhouse gases than it saves.
The conventional process for producing second-generation SAF is known as HEFA ( hydroprocessed esters and fatty acids), which calls for the removal of oxygen present in fatty acids in plant oils, by adding hydrogen. Now you know the problem — hydrogen. The process needs energy and one must also consider the land-use change needed to grow crops for SAF, and the water and fertiliser consumption. Overall, SAF is two to five times costlier than conventional jet fuel.
The solution
In a research paper published in the preprint server bioRxiv.org, Timothy Sheppard, et al, suggest a new technology called ‘electromicrobial production’ (EMP) of SAF, which, as the name suggests, uses microbes. “Production of hydrocarbons using electrically powered microbes employing fatty acid synthesis-based production of alkanes could be an efficient means to produce drop-in replacement jet fuels using renewable energy,” the authors say. These microbes “have an extraordinary ability to manufacture organic compounds using electricity as the primary source of metabolic energy”, they say. This process uses light, atmospheric carbon dioxide and electricity.
Traditionally, engineered cyanobacteria are used for microbial production, but they are difficult to engineer. Sheppard points to a better microbe, Vibrio natriegens, capable of ‘extracellular electron uptake’ (EEU).
There are two ways of getting microbes to produce biomass. One is hydrogen oxidation, where the microbes consume hydrogen to produce biomass. The second — EEU — involves delivery of electrons into cells, either through a diffusible intermediary such as water-soluble quinones, or through direct electrical contact with an anode.
“We believe the time is right to start scaling up production of jet fuels with EMP,” say the authors of the paper. They believe that “hydrogen-mediated EMP” is a slightly more efficient method, but are also working on EEU as a viable alternative.
Peer comment
Asked for a response to the paper, Dr Anjan Ray, Director, Indian Institute of Petroleum, Dehradun (which is also engaged in the development of SAF) told Quantum: “Conceptually, it is rather exciting to imagine an electromicrobial system, as described by the authors, with the theoretical conversion efficiencies indicated.”
However, Dr Ray cautioned against undue optimism. The results in the paper indicate only the theoretical possibilities, not the practical limitations, of the proposed process of carbon fixation, wherein carbon dioxide provides the carbon source and electrical energy provides the metabolic energy for the conversion of carbon dioxide, by the microbe, into alkanes and terpenoids in the desired jet fuel range, he said.
Observing that Sheppard’s paper describes the best-case scenarios, which are impressive, Dr Ray said that the probability of practically achieving such efficiencies is not evident at this time. “I expect this to be a long haul of several years, if not decades,” he said.
He further observed that the “efficiencies only indicate the extent of energy conversion, not the kinetics (the rate at which such conversion happens over time)”. Electromicrobiological kinetics can vary widely, so a lot more research would be needed before one can be sure of producing an adequate volume of fuel.
“In essence, it is too early to comment on the chances of commercial success for this route or a timeline for such success — but the proposed pathway is potentially exciting and disruptive,” Dr Ray said.
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