Thursday, April 22, 2021

How food waste could be turned into climate-friendly jet fuel

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For many uses, liquid fuels remain the most convenient source of energy – airplanes and large ships are obvious examples. It is possible to produce biofuels for these applications. But so far no one has been able to do it competitively, leaving fossil fuels as the dominant option.

In one new paper, a group of American researchers therefore looked into the possibility of converting food waste into jet fuel. Chemically, the results are excellent, producing a material that can be mixed with some standard jet fuel to meet all regulatory standards. Economically, the situation is not that great, operating only at prices that prevailed more than five years ago. But the fact that the waste would otherwise put methane into the atmosphere, as it breaks down more than makes up for the carbon dioxide produced by the jet fuel in the mixture. A carbon price could therefore change the equation.

The work here focuses on what’s known as “wet waste,” which includes things like food waste, animal manure, and sewage. As you might expect, we produce a lot of this material, with the authors estimating its total energy content to be roughly equivalent to 10 billion gallons of jet fuel each year. Due to the amount of water present, it is extremely energy intensive to convert this waste directly into any type of fuel because the water has to be discarded. However, it is possible to place the waste in an oxygen-free environment and have the bacteria convert it to methane.

What the authors focus on is interrupting this process. If you grow the bacteria under the right conditions, they won’t completely break down the longer complex fats. Instead, they will stop at a point where much of the carbon in these cells is in the form of relatively short molecules four to eight carbons long. These usually have two oxygen attached to one end of the carbon chain, making them weak acids.

Chemically, it is possible to make these molecules react in a way where two of the weak acids merge into a single molecule, releasing water and a single molecule of carbon dioxide in the process. The resulting molecule is now almost twice the length (two four-carbon molecules would fuse to form one seven-carbon molecule and release the other carbon as CO2). And that brings the length back to the neighborhood of the typical hydrocarbon in jet fuel.

The longer molecule still has oxygen attached, and there are two ways to get rid of it. One is a simple reaction with hydrogen and a cheap catalyst, which releases oxygen as water. An alternative is an additional fusion of another weak acid molecule, creating a more complicated branched structure. (This process also requires a reaction with hydrogen to convert the substance to a pure hydrocarbon when completed.) Researchers have demonstrated that with the right catalysts, these two reactions work extremely well and produce a mixture of hydrocarbons with properties similar to those of jet fuel.

So researchers now had a process. Feed the waste into a bacterial digester, prevent bacteria from producing methane, and isolate short fatty acids from the digester. Then run them through a few reactions and take out a mixture of hydrocarbons that can be used as fuel.

Of course, there are quite specific requirements regarding what constitutes jet fuel, designed by aviation authorities to ensure the safety of flights and operations on the ground. And the fuels made by these two processes differed from standard jet fuel in some critical aspects, like flash point and freezing point, which determine the behavior of the fuel in response to high and low temperatures, respectively.

It was not a problem if these biofuels were kept below 10 percent of the total jet fuel mixture. But that would be problematic if you wanted to make a mixture that was mostly biofuel. However, the two different reactions produced products that differed in opposite ways (one produced a higher flash point liquid, the other a lower flash point liquid, for example). Thus, by mixing the two, it was possible to create a jet fuel mixture containing more than 70% biofuel.

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