Airlines Want to Make Flight More Sustainable. How Will They Do It?

by Leigh Krietsch Boerner (Chemical & Engineering News) Air travel produces millions of metric tons of carbon dioxide each year. New technologies could help airlines reduce their emissions and meet their sustainability goals -- ... Airlines are trying to reduce their emissions in several ways. Probably the most common is switching from traditional fossil-derived jet fuels to ones that are made from renewable sources and have lower emissions during production. Airlines are also looking to new materials and coating technologies to make planes lighter, more aerodynamic, and more resistant to wear and tear. Meanwhile, a few airlines, such as United, think they can get to carbon neutrality while reintroducing supersonic flight.

"For short-haul flights, there's some encouraging movement for zero-emission planes, such as those running on electricity," Pavlenko (Nikita Pavlenko, senior fuels researcher for the ICCT) says. "But for everything else, it comes down to what fuel you can switch to."

The goal of using sustainable aviation fuels, or SAFs, is to reduce the amount of greenhouse gases emitted during the lifetime of the fuels, from production to combustion, compared with current petroleum-based jet fuels. The scale of that reduction depends on the process used to make the fuel and the carbon source. And even though some SAFs boast significant emission reductions, few are made at large scale.

Most companies are looking instead at drop-in fuels, or fuels that can work with existing jet engines. They need to have similar properties to jet fuel, including their energy released when burned, performance at low temperatures, and flow. These specifications ensure that the fuels will behave the same way fossil-derived jet fuel does in an airplane's engine, Pavlenko says.

Most SAFs need to be blended 50:50 with Jet A-1. And while there's interest in SAFs that don't need to be blended, none have been commercially approved, according to Pavlenko.

There are three main ways to make SAFs: from hydroprocessed esters and fatty acids (HEFA), Fischer-Tropsch synthetic paraffinic kerosene (FT-SPK), and alcohol-to-jet synthetic paraffinic kerosene (ATJ-SPK). All three can be used at similar blend levels, around 50%.

To make HEFA fuels, industrial scientists remove the oxygens from molecules in unused vegetable oils, or waste fats, oils, and greases. Then they treat the mixtures with hydrogen to yield burnable hydrocarbons the right length for jet fuel, usually between 10 and 20 carbons long, according to the ICCT. Compared with Jet A and Jet A-1, these fuels are the most cost competitive SAF technology, Pavlenko says.

To make Fischer-Tropsch-SPK, scientists oxidize a wide variety of plant and human wastes and residues to make synthesis gas, a mixture of H₂and carbon monoxide. Adding a catalyst—usually iron, cobalt, or ruthenium—to this gas triggers Fischer-Tropsch synthesis, which produces hydrocarbons.

The sources for alcohol-to-jet-SPK are crops such as sugarcane and corn, plant and agriculture wastes, and in some cases industrial flue gases. Generally, scientists convert these feedstocks to ethanol or isobutyl alcohol and then upgrade the alcohols to long-chain kerosene by removing water, treating them with hydrogen, and combining short-chain hydrocarbons to form longer ones.

No fuel companies currently produce Fischer-Tropsch-SPKs, so the fuel currently in production with the lowest lifetime GHG emissions is HEFA, depending on the feedstocks.

The company closest to commercial production of Fischer-Tropsch-SPKs is Fulcrum BioEnergy in Pleasanton, California. This company's technology uses municipal solid waste, also known as trash, to make jet fuel. Fulcrum plans to start producing biofuel in the last quarter of this year at its plant just east of Reno, Nevada, Vice President Barraza says. The company plans to build eight other plants with a total production of about 1.5 billion L per year, enough to meet the needs of United and other partners, which include Cathay Pacific Airways and Japan Airlines, Barraza says.

Powering airplanes with waste is certainly the dream for sustainable flight, and a viable one, United's Robinson ( Aaron Robinson, senior manager of environmental strategy and sustainability for United Airlines) says. "You could power all of United using just 20% of US landfill waste."

But the reality is that relative to total amounts of jet fuel, airlines aren't using a lot of SAFs right now. "It's less than 0.1% globally," Pavlenko says. READ MORE PDF

Excerpt from AgWeb: Southwest Airlines, Delta, JetBlue and United have all made short- and long-term commitments to bio-based fuels. The six pathways that have been approved to make SAF include wood residue, animal fats, algae, cooking oil, vegetable oil, sugars, alcohol, ethanol or greases.

The chemical and physical characteristics of SAF are almost identical to those of the conventional fuels, Sarisky-Reed (Valerie Sarisky-Reed, acting director of the Bioenergy Technologies Office) says.

The continued airline pledges for SAF, and the connected renewable diesel, are fueling the argument U.S. soybeans could soon see increased domestic demand, says Peter Meyer, head of grain and oilseed analytics, S&P Global Platts.

Currently, Meyer says only a few plants in the U.S. are producing SAF, but several are in the development stage. Many include partnerships with some of the largest energy companies in the world. Sustainable Aviation Fuel Plants