Human waste could solve aviation's biggest problem

Rotten food scraps and vats of sewage typically do not make the cut when it comes to the list of what makes a glamorous, jet-setting lifestyle. But researchers say that transforming these otherwise discarded “wet waste” materials into biofuel could be the future of environmentally friendly flying.

In a study published Monday in the journal Proceedings of the National Academy of Sciences, a team of researchers led by the U.S. National Renewable Energy Laboratory describe how to convert organic waste into paraffin, a combustible hydrocarbon used in aviation fuel. Ultimately, their new formulation may pave the way for a greener jet fuel and a more eco-friendly aviation industry that doesn’t rely on developing an electric jet.

What’s new — The new approach presented in this paper is a break from traditional production methods for biofuels like ethanol or biodiesel. It uses a chemical process to efficiently remove excess water from so-called “wet waste,” which can include food scraps, and then isolate the kinds of combustible materials needed to make fuel. The scientists report in the paper that such a waste-derived fuel could cut aviation emission levels by 165 percent.

Why it matters — The aviation industry contributes 2.5 percent of all the planet’s greenhouse gases. The U.S. contributes to that emissions dump with the staggering 21 billion gallons of jet fuel it consumes each year as a result of aviation.

Aviation is part of the transportation industry, which is one of the three highest emissions sectors. The other two include energy use in industry (e.g. textile or car manufacturing), and energy use for generating electricity and heat. Within the transportation industry, aviation is the second largest contributor of emissions after highway traffic.

The airline industry, for its part, isn’t ignorant of the problem. For instance, in 2018 United Airlines set a goal of 50 percent carbon reduction before 2050. But the airline also acknowledged meeting this goal ultimately depends on innovations like this one.

In this paper, the scientists propose a circular manufacturing process — essentially, they argue that because humans will relieve themselves on airlines, that waste will, with this production method in place, power their future flights. The paper’s authors argue their process has the potential to account for 20 percent of the U.S.’s jet fuel consumption, bringing the aviation industry one step closer to reaching the coveted “net-zero” for fuel consumption.

Here’s the background — If you thought aviation’s carbon footprint was problematic, food waste’s carbon footprint is even worse. Globally, food waste accounts for 6 percent of greenhouse emissions. In this accounting, food waste includes both spoiled food and uneaten scraps.

Combined with other forms of “wet waste,” which also includes human waste, animal manure, and wastewater sludge, these byproducts have the potential to change the game when it comes to the production of biofuels. But there’s one small problem.

Unlike fats and oils (like those used to fry french fries) often used to generate biodiesel, “wet waste” — as the name suggests — has a high water content that makes it difficult to process using traditional approaches. As a result, using typical processing techniques which rely on heat and pressure to transform biomaterials from a solid or liquid into usable gas won’t work on “wet waste.”

Instead of resigning themselves to simply collect natural methane bubbling off this unused waste, the research team behind this new work decided to develop a new chemical conversion pipeline that could change the game.

“While renewable natural gas targets an enormous US market, producing liquid hydrocarbon fuels from wet waste offers the potential to address the challenge of decarbonizing the aviation sector,” write the authors in the study.

What they did — To better capture the potential of these waste products, the researchers had to first interrupt their natural fermentation process before they began manufacturing methane. Disrupting this process allowed the researchers a window of opportunity to manipulate the chain length of the carbon molecules created by the rotting waste. Specifically, they did this to generate short-chain and medium-chain carboxylic acids, which are precursors for biofuels collectively known as “volatile fatty acids” (VFAs). These fatty acids are derived from intestinal gut microbes breaking down and fermenting indigestible foods.

After isolating these VFAs, the team used a process called ketonization to “elongate the carbon backbone of the VFAs” by removing oxygen in the form of water and carbon dioxide. This process led to the creation of both typical paraffin-rich hydrocarbons and isoparaffin-rich hydrocarbons (a slightly different kind of paraffin known for having a lower freezing point than traditional jet fuel).

What they discovered — In laboratory tests, the new fuel appeared to have minimal impurities or chemical leftovers from the waste not useful for fuel creation. The researchers also found that blending together both paraffin-derived and isoparaffin-derived fuels achieved a fuel blend that produced 34 percent less soot than traditional fossil fuels.

In the study, the researchers make the compelling point that diverting food waste from landfills to create sustainable aviation fuel could reduce greenhouse gas emissions up to 165 percent compared with conventional fossil jet fuels. A techno-economic analysis also found that such a fuel could be produced cheaply — at only $2.50 per gallon.

What’s next — Right now, the team has only tested a few hundred milliliters of their sustainable fuel — far less than the amount needed to run a prop-plane, let alone a commercial jet. But the researchers are excited about the possibilities this technique could open for the world of green aviation.

Abstract: With the increasing demand for net-zero sustainable aviation fuels (SAF), new conversion technologies are needed to process waste feedstocks and meet carbon reduction and cost targets. Wet waste is a low-cost, prevalent feedstock with the energy potential to displace over 20% of US jet fuel consumption; however, its complexity and high moisture typically relegates its use to methane production from anaerobic digestion. To overcome this, methanogenesis can be arrested during fermentation to instead produce C2 to C8 volatile fatty acids (VFA) for catalytic upgrading to SAF. Here, we evaluate the catalytic conversion of food waste–derived VFAs to produce n-paraffin SAF for near-term use as a 10 vol% blend for ASTM “Fast Track” qualification and produce a highly branched, isoparaffin VFA-SAF to increase the renewable blend limit. VFA ketonization models assessed the carbon chain length distributions suitable for each VFA-SAF conversion pathway, and food waste–derived VFA ketonization was demonstrated for >100 h of time on stream at approximately theoretical yield. Fuel property blending models and experimental testing determined normal paraffin VFA-SAF meets 10 vol% fuel specifications for “Fast Track.” Synergistic blending with isoparaffin VFA-SAF increased the blend limit to 70 vol% by addressing flashpoint and viscosity constraints, with sooting 34% lower than fossil jet. Techno-economic analysis evaluated the major catalytic process cost-drivers, determining the minimum fuel selling price as a function of VFA production costs. Life cycle analysis determined that if food waste is diverted from landfills to avoid methane emissions, VFA-SAF could enable up to 165% reduction in greenhouse gas emissions relative to fossil jet.