From Waste to Jet Fuel: The Next Frontier for RNG

Nick

Have you ever looked out the window of a commercial flight, uh, right as those massive engines spool up for takeoff, and wondered how on earth we are ever going to decarbonize that?

Emily Nguyen 4

Oh, all the time. Because, I mean, you can put batteries in a passenger car, or even a local delivery delivery truck, but a fully loaded long haul jet? The physics just hit a wall. Weight is everything.

Nick

Right, exactly. Welcome back to Digest This, Unpacking Our Sustainable Future. I am Nick.

Emily Nguyen 4

And I am Emily. Today, we are taking a deep dive into the massive challenge of sustainable aviation fuel, or SAF, and how renewable molecules, especially renewable natural gas and methane conversion, could redefine the energy transition.

Nick

Yeah, because aviation is one of the hardest sectors to abate. According to the Alternative Fuels Data Center, aviation accounts for two percent of all global carbon dioxide emissions and twelve percent of all carbon dioxide emissions from transportation as a whole.

Emily Nguyen 4

And that is why the International Civil Aviation Organization, or ICAO, set up the CORSIA framework. CORSIA basically caps net carbon dioxide emissions from international aviation at 2020 levels all the way through 2035. And ICAO reports that over 360,000 commercial flights have already flown on SAF blends across 46 different airports, mostly in the United States and Europe.

Nick

Which sounds like a lot, right? 360,000 flights! But when you look at the total global volume, we are barely scratching the surface. Which brings us to the federal Sustainable Aviation Fuel Grand Challenge.

Emily Nguyen 4

Right, the Grand Challenge was announced back in 2021 by the U.S. Department of Energy alongside the Department of Transportation and the Department of Agriculture. And the targets are, er, well, staggering. They want domestic SAF production to scale from where it was in 2021, which was just 5 million gallons.

Nick

5 million gallons.

Emily Nguyen 4

To 3 billion gallons per year by 2030, and then 35 billion gallons per year by 2050.

Nick

Wait, wait, let us stop on those numbers for a second. According to EPA data collected under the Renewable Fuel Standard, U.S. SAF consumption went from 5 million gallons in 2021 to 15.84 million gallons in 2022, and reached 24.5 million gallons in 2023.

Emily Nguyen 4

Which is growth, sure, but going from 24.5 million gallons to 35 billion gallons by 2050 is a 1,400 fold expansion in under three decades.

Nick

I, I, I mean, every time I buy a flight online, I see that little pop up box asking if I want to pay three dollars or five dollars for a carbon offset flight. But voluntary passenger offsets do not put actual liquid hydrocarbons into an airport hydrant system. You cannot offset your way out of the fundamental physics of jet fuel consumption.

Emily Nguyen 4

No, you need physical gallons of drop in fuel that meet incredibly strict chemical standards.

Emily Nguyen 4

So how do you actually make a non petroleum fuel that can mix seamlessly into an airplane engine without causing it to, you know, fail at 30,000 feet?

Nick

That is where the rulebook comes in. Non petroleum jet fuel quality standards are governed by ASTM D7566. That specification dictates the required fuel properties for synthetic hydrocarbons and outlines approved SAF feedstocks and allowable blend limits with conventional Jet A, usually capped between 10 percent and 50 percent.

Emily Nguyen 4

And once it is blended, it actually gets re certified under ASTM D1655, which is the standard specification for conventional aviation turbine fuel. That re certification is what lets it move through existing supply chains as a drop in fuel.

Nick

And right now, almost all the SAF commercially available comes from just one approved pathway under ASTM D7566 Annex A2.

Emily Nguyen 4

HEFA, right? Hydroprocessed Esters and Fatty Acids.

Nick

Exactly. According to the Department of Energy, all three currently operating commercial scale SAF plants in the U.S. use the HEFA pathway. You have World Energy in Paramount, California, which started back in 2016 supplying LAX. Then Neste began supplying San Francisco International Airport in 2020, expanding to other California airports and places like Aspen and Telluride. And Montana Renewables partnered with Shell at a facility in 2023.

Emily Nguyen 4

And HEFA works by taking triglyceride feedstocks like used cooking oil, yellow grease, animal fats, or tallow, breaking down the fatty acid chains through hydroprocessing, and then hydrocracking them into jet fuel.

Nick

Which is great, except for one huge problem. The feedstock ceiling.

Emily Nguyen 4

Right! If you collected every single gallon of used cooking oil and renderable animal tallow in North America, it still would not give you enough raw material to hit 35 billion gallons of jet fuel. We talked about this ceiling previously when looking at liquid fats. You hit a wall.

Nick

Plus, people wonder, why not just co process fats inside existing petroleum refineries? Well, ASTM D1655 Annex A1 allows co processing of biomass feedstocks or Fischer Tropsch biocrude at traditional oil refineries, but it limits that co processing to a maximum of 5 percent. You cannot just flood a petroleum refinery with 30 percent waste grease without compromising refinery catalysts and engine seals.

Emily Nguyen 4

So HEFA is maxed out, co processing is capped at 5 percent, which means if we are going to meet this Grand Challenge, we have to look far beyond waste oils and fats.

Nick

And that is where the whole synthetic pathway ecosystem opens up.

Emily Nguyen 4

Yeah, if you look at the technical specifications in ASTM D7566, there are several other approved Annex pathways. For instance, Annex A5 covers Alcohol to Jet Synthetic Paraffinic Kerosene, or ATJ SPK. That process takes starchy or cellulosic ethanol or isobutanol, dehydrates it, oligomerizes it, and hydrotreats it into jet fuel at up to a 50 percent blend limit.

Nick

And then you have Fischer Tropsch pathways, like Annex A1 and Annex A4. Gasifying municipal solid waste, forestry residue, or agricultural waste into syngas, carbon monoxide and hydrogen, and then synthesizing synthetic kerosene.

Emily Nguyen 4

There are even more exotic options on the books. In 2020, ASTM approved Annex A6 for Catalytic Hydrothermolysis Synthesized Kerosene, which converts clean free fatty acids or lipids using high temperature water reactions. And Annex A7 covers Hydrocarbon HEFA, which converts triglyceride oils from a specific high growth alga called Botryococcus braunii, though that one is capped at a 10 percent blend limit.

Nick

And do not forget Annex A3, Hydroprocessed Fermented Sugars to Synthetic Isoparaffins, HFS SIP. That pathway ferments sugars derived from corn stover or herbaceous biomass into specific hydrocarbons using engineered microbes, limited to a 10 percent blend.

Emily Nguyen 4

Think about that! You take corn stalks, extract the cellulosic sugars, feed them to microbes, and those microbes literally output fuel molecules that match jet fuel chemistry.

Nick

Uh, but, okay, let us look at the energy balance here. If you take ethanol, dehydrate it, heat it up, force it through multi step chemical reactions like oligomerization and hydrotreatment to build long chain hydrocarbons, you lose energy at every single step. Is it really energy efficient compared to starting directly from a gas syngas mixture?

Emily Nguyen 4

Well, it depends on the carbon intensity of your inputs and process energy. Direct syngas via Fischer Tropsch gives you a direct path from gas to liquid, but gasification facilities for solid waste require massive capital expenditures. Alcohol to Jet, on the other hand, can leverage the existing multi billion gallon ethanol infrastructure. So it is a tradeoff between chemical yield efficiency and supply chain readiness.

Nick

Which brings us back to renewable natural gas and methane. Because methane, CH4, is already a compressed gaseous carbon molecule ready for upgrading.

Emily Nguyen 4

This is where the intersection of waste management and advanced fuels gets really fascinating. Recent research highlighted by the Department of Energy showcases work coming out of the National Renewable Energy Laboratory, or NREL.

Nick

Right, NREL scientists developed a process that takes wet waste streams, like dairy manure, food waste, sewage sludge, and inedible greases, and ferments them in anaerobic digesters to produce volatile fatty acids.

Emily Nguyen 4

Then they use specialized catalysts to build those volatile fatty acids into energy dense hydrocarbons suitable for aviation. And according to NREL's analysis published by DOE, this wet waste pathway produces a drop in fuel with a carbon footprint up to one hundred sixty five percent smaller than conventional Jet A.

Nick

Wait, 165 percent smaller? That means it is net carbon negative.

Emily Nguyen 4

Exactly! Because methane is a greenhouse gas that is over 20 times more potent than carbon dioxide at trapping heat in the atmosphere. By capturing that methane from manure lagoons or food waste before it escapes, and turning it into jet fuel, you divert more greenhouse gas emissions from entering the air than are released when the plane burns the fuel.

Nick

That completely flips the narrative on waste. A dairy farm or municipal wastewater plant isn't just treating an environmental liability or trying to figure out where to dispose of sludge. They are sitting on a feedstock for high value, low carbon molecules.

Emily Nguyen 4

And it shows how methane derived pathways are evolving. Recent coverage in SAF Magazine and industry reporting, like Infinium's recent announcement of their mSAF platform, shows companies looking at converting renewable natural gas, biogas, and flare gas directly into low carbon jet fuel using electrified steam methane reforming and Fischer Tropsch synthesis.

Nick

Instead of just burning renewable natural gas to heat a building or generate power, you use electrified reforming powered by clean energy to convert that methane into syngas, preserving more carbon to turn into liquid aviation fuel.

Emily Nguyen 4

It is such a cool convergence. Ten years ago, if you talked about agricultural manure management and commercial jet engines, nobody would have put them in the same sentence. Now, organic waste chemistry is bridging the two.

Nick

So let us talk about how this fuel actually gets to the plane. A common question people ask is, why don't airlines or airports just buy pure SAF and blend it right there at the airport fuel farm?

Emily Nguyen 4

Oh, because that would be an operational and logistical nightmare! The Department of Energy's Alternative Fuels Data Center points out that blending fuel at an airport requires separate storage tanks, specialized blending equipment, additional technical staff, and massive liability insurance.

Nick

Right. It is far more cost effective to do all the blending and quality testing upstream at fuel terminals or refineries, certify the batch as ASTM D1655, and then send it down the pipeline.

Emily Nguyen 4

And because it is fully drop in, that certified fuel moves through common carrier pipelines, barges, and tank trucks, straight into the airport hydrant system. The airport receives the exact same pipeline fuel it always has, and the aircraft engines require zero modifications.

Nick

Which is critical for execution, but it raises an interesting dynamic in the market. Right now, almost every major airline is signing long term off take agreements for future SAF output. But there is still a noticeable gap between those announced agreements and the actual speed of capital deployment for commercial scale facilities.

Emily Nguyen 4

Which brings us to the big question for the energy transition: as CORSIA mandates kick in and global emissions targets draw closer, will synthetic pathways like ATJ, Fischer Tropsch, and methane conversion, scale quickly enough to supply the billions of gallons needed?

Nick

Or will a fuel supply crunch force regulators and airlines into tough choices? What is clear is that renewable natural gas and methane are no longer just pipeline replacements for fossil gas. They are flexible, multi use renewable carbon building blocks.

Emily Nguyen 4

Whether it is pipeline gas, industrial process heat, heavy transport, or liquid jet fuel, the future of RNG is defined by options. The value comes from how many critical jobs renewable methane can perform across the economy.

Nick

To recap our sources today, check out the U.S. Department of Energy Alternative Fuels Data Center, reports from NREL, and coverage from SAF Magazine. That is all for today on Digest This. Thanks for listening!

Emily Nguyen 4

See you next time!