On November 14, Alaska Airlines used a 20% blend of cellulosic-based jet fuel on a flight from Seattle to Washington DC. Feedstock for the jet fuel was forestry wastes that were converted into sugars at Washington State University which leads the Northwest Advanced Renewables Alliance (NARA). NARA, a consortium of 32 organizations from industry, academia and government laboratories is supported by a $39.6 million grant from the Department of Agriculture’s Institute of Food and Agriculture (NIFA), to develop a sustainable biojet fuel from forest residuals. The five year program is known as “Wood to Wing.” www.nararenewables.org
After harvesting timber, forest residues end up in what are known as slash piles and are typically burned. For the above test flight, some of this forest slash was converted into fermentable sugars at Washington State University The process involved chemical and thermal pretreatment to breakdown the slash biomass into its cellulose, hemicellulose and lignin constituents. Enzymatic hydrolysis was then used to further breakdown the cellulosic components into their C5 and C6 sugars.
The sugars were then transported to Gevo Inc.’s fermentation facility in St. Joseph, Missouri where, using Gevo’s proprietary enzyme, they were converted into isobutanol, a four carbon alcohol. Gevo, a member of the NARA consortium, shipped the resultant isobutanol to its jointly operated (with South Hampton Resources) biorefinery in Silsbee, Texas. Here the isobutanol was converted into jet fuel, designated as ATJ (Alcohol to Jet fuel). The standard jet fuel ASTM D7566 specification was specifically revised in April, 2016 to include ATJ as derived from any renewable isobutanol, regardless of carbohydrate feedstock whether it be cellulosic, corn, or sugar.
The technology for converting isobutanol into ATJ involves several steps. First it is dehydrated into isobutylene, which is then oligomerized into C8, C12 and C16 olefins. The final step involves hydrogenation of the olefins to saturate the double bonds and then distillation to recover a C12 – C16 kerosene mixture, called Synthetic Paraffinic Kerosene (SPK). SPK specifications are acceptable for use as jet fuel; i.e. ATJ
The interest in using renewable jet fuels is driven by the fact that aviation is responsible for 12 % of CO2 emissions from all transportation sources. While this pales in comparison to the 74 % from road transport, it is still a significant amount that warrants attention. In fact, the International Air Transport Association (IATA) has set a target of capping net aviation CO2 emissions from 2020 and a reduction of 50% by 2050 relative to 2005 levels. Part of their strategy in achieving these goals is the deployment of sustainable low-carbon fuels. (www.iata.org)
According to the Air Transport Action Group (www.atag.org/facts-and-figures.html) if commercial aviation were to get 6 % of its fuel supply from renewables by 2020, its overall carbon footprint would be reduced by 5 %.
While there is still a significant amount of development to make it a commercial reality, NARA’s Wood to Wing program is a start in helping to achieve IATA’s ambitious goals in reducing aviation’s carbon emissions. The following figures from NARA’s website illustrate the overall concept of an integrated biorefinery to convert the forest slash or any biomass feedstock into aviation fuel.