From Concept to Commercialization: NREL’s Sustainable Aviation Fuels Program

- By Zia Abdullah

The National Renewable Energy Laboratory supports the U.S. DOE Bioenergy Technologies Office with fundamental and applied research, development, and deployment (RD&D) designed to advance technologies to produce biobased energy, fuels, chemicals and products from initial conception to demonstration stages.

NREL’s program is organized along six science and technology (S&T) research platforms, focused on pretreatment and biological conversion of biomass feed; catalytic conversion of biomass feed or intermediate streams; coproduct poly- mers and bioproducts production; lignin valorization; waste gas valorization; and algae biofuels and bioproducts. Supporting these S&T platforms is a crosscutting strategic and sustainability analysis platform, which provides process economics and sustainability metrics to help inform S&T research priorities. Projects in each S&T platform are integrated into feedstock–toproduct “pathways” that support the DOE sustainable aviation fuel (SAF) and materials and chemicals CO2 reduction goals. The technology pathways are deployed through the market impact platform in col- laboration with industry partners and other stakeholders.

Commercial aircraft being sold today will likely still be in operation for the next 30 years. Our program, therefore, targets development of SAF, which is completely fungible and drop-in to the existing commercial aviation fueling systems, as per ASTM 7566. Our approach is to make SAF blendstocks that comprise only those molecule classes, (cyclo-alkanes, n-alkanes, iso-alkanes, and aromatics) and that already exist in Jet-A, therefore resulting in a fully fungible fuel that meets the safety, performance, and operability requirements as per ASTM D4054 and ASTM D1655, and can be seamlessly used with existing infrastructure and airplanes.

Refinery Integration

Targeted feedstocks for our SAF pathways range from CO2, wet waste and aquatic species, to municipal solid waste, to herbaceous and woody terrestrial biomass. With some preprocessing, which we collaborate on with our partner, the U.S. DOE Idaho National Laboratory, this very broad range of feedstocks can be converted to a form suitable for feeding into reactors for chemical or biological conversion to intermediates, and then further upgraded. Example intermediate streams include Fischer Tropsch (FT) waxes, alcohols, bio-oils & biocrudes, acids, sugars, ketones, oils and lignin streams.

Our approach is to develop biological and chemical conversion technologies that can produce these intermediates from renewable feedstocks, and further upgrade these intermediates into streams that can be processed to make SAF blendstocks through existing petroleum refineries.

The U.S. has approximately 100 billion gallons per year of petroleum hydrotreater and 85 billion gallons of fluid catalytic cracking capacity. A significant portion of these refinery assets may be underutilized if light-duty transportation undergoes significant electrification, and gasoline demand eventually starts reducing. Leveraging these petroleum refinery assets may significantly reduce capital costs for SAF production. Partnering with the petroleum industry may also allow an incremental transition to renewables by blending fossil and renewable streams. Refinery integration will also allow utilization of existing personnel skills in safety, equipment operation, fuel finishing, quality assurance and quality control, fuel blending, fuel branding and industry knowhow. There are some challenges associated with refinery integration, however. Petroleum refineries operate at very large scales (a typical refinery in the U.S. may produce 500,000 barrels per day (b/d) of fuels), whereas biorefinery streams are much smaller (around 1,500 to 3,000 b/d of product). Therefore, it would be difficult to match smaller renewable streams to equipment designed to process at much larger scale. This is where concepts such as blending of renewable and fossil streams may help, until production of renewable feeds can be scaled up. Renewable streams are also generally very chemically different from petroleum streams. For example, most renewable streams listed above contain oxygen, which may make them incompatible with materials of construction used in petroleum refineries. Oxygen from renewable streams must be removed completely, and processes to achieve this are often highly exothermic and produce excessive water, both issues that refinery operations may not be designed for and are topics of our research program.

Technology Development

NREL has a pipeline of SAF technologies at different stages of development and timelines to market. Some examples are as follows.

Waste biomass pyrolysis to SAF using catalytic pyrolysis. In this process, nonfood biomass is pyrolyzed and then the pyrolysis gas is upgraded in a fixed bed catalytic reactor (Figure 1). Condensation of the pyrolysis gas and separation of streams produces a stabilized bio-oil that can be upgraded in conventional refinery hydrotreaters to SAF blend stock, and a renewable chemical coproducts stream (acetone and 2-butanone). The fossilbased petrochemical industry maximizes profit by coproducing fuels and chemicals, and this process will offer similar coproduction opportunities for biorefineries. If regenerative agriculture is used to produce biomass for this process, carbon can be stored in the soil, potentially producing carbon-negative fuels.

Wet waste to SAF using arrested anerobic digestion. In this project, arrested anerobic digestion is used to produce volatile fatty acids (VFA) from food industry waste. These VFAs are then catalytically upgraded to ketones, when can be upgraded to SAF blendstocks. Testing has shown that these blendstocks can potentially be blended at very high ratios (potentially up to 70%) and still meet all the required properties for jet fuel. This technology has the ability not only to use wet waste from the food industry, but also eventually wet waste from farms and agriculture.

Corn stover to SAF. The current DOEFOA project with SAFFiRE Renewables is. This is an example of market deployment in partnership with industry. In this project, NREL’s Deacytelation Mechanical Refining technology is used to along with enzymatic hydrolysis and fermentation to produce ethanol, which is upgraded to SAF blendstocks.

In addition to the nearer-term DOE Funding Opportunity Announcement projects with industry partners, NREL has very promising earlier-stage technologies in the pipeline, including algae to SAF and coproducts, lignin deconstruction and upgrading to SAF, syngas upgrading to SAF or high-octane blendstocks, carboxylic acids to SAF, and also multiple routes that can use renewable electricity and CO2 to produce intermediates for upgrading to SAF. Each of these technologies will eventually be licensed to industry partners for deployment to meet the administration’s goals to produce at least 3 billion gallons of SAF by 2030, and to completely decarbonize U.S. commercial aviation by producing 35 billion gallons of SAF by 2050.

Author: Zia Abdullah Bioenergy Laboratory Program Manager National Renewable energy Laboratory