Commercial Aircraft Propulsion and Energy Systems Research: Reducing Global Carbon Emissions (2016)

June 01, 2016

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by Committee on Propulsion and Energy Systems to Reduce Commercial Aviation Carbon Emissions; Aeronautics and Space Engineering Board; Division on Engineering and Physical Sciences; National Academies of Sciences, Engineering, and Medicine (National Academies Press) The primary human activities that release carbon dioxide (CO2) into the atmosphere are the combustion of fossil fuels (coal, natural gas, and oil) to generate electricity, the provision of energy for transportation, and as a consequence of some industrial processes. Although aviation CO2 emissions only make up approximately 2.0 to 2.5 percent of total global annual CO2 emissions, research to reduce CO2 emissions is urgent because (1) such reductions may be legislated even as commercial air travel grows, (2) because it takes new technology a long time to propagate into and through the aviation fleet, and (3) because of the ongoing impact of global CO2 emissions.

Commercial Aircraft Propulsion and Energy Systems Research develops a national research agenda for reducing CO2 emissions from commercial aviation. This report focuses on propulsion and energy technologies for reducing carbon emissions from large, commercial aircraft— single-aisle and twin-aisle aircraft that carry 100 or more passengers—because such aircraft account for more than 90 percent of global emissions from commercial aircraft. Moreover, while smaller aircraft also emit CO2, they make only a minor contribution to global emissions, and many technologies that reduce CO2 emissions for large aircraft also apply to smaller aircraft.

As commercial aviation continues to grow in terms of revenue-passenger miles and cargo ton miles, CO2 emissions are expected to increase. To reduce the contribution of aviation to climate change, it is essential to improve the effectiveness of ongoing efforts to reduce emissions and initiate research into new approaches. READ MORE

Excerpt from book: This chapter (on alternative sustainable aviation fuels) looks at alternative jet fuels that have lower carbon emissions than conventional petroleum-based fuels over the entire life cycle of the fuels. It discusses the challenges associated with their development and commercialization and outlines key needs for achieving significant production and use of drop-in sustainable jet fuels produced from feedstocks other than petroleum (see Box 5-1).

If such commercialization takes place, aviation has the opportunity to significantly lower the net carbon emissions from aviation, potentially in a more aggressive and timely fashion than can be reasonably achieved with improved operations, infrastructure, and aircraft.

This reduction can also be achieved without impacting the time frame or suitability of other potential carbon-lowering approaches.

Much has been accomplished over the last decade to validate the qualification, production, and usage
of lower net carbon fuels. Some versions of these fuels are on the cusp of commercialization. However,
many research, development, demonstration, and deployment challenges remain in moving these fuels to significant production and mainstream usage. This chapter addresses those challenges and related
research projects.

It is not feasible for the aviation industry to switch from conventional jet fuel to a different fuel type,
nor are there readily identifiable, feasible, lower-carbon alternative fuel types that could be introduced in a reasonable time frame. Many entities have validated the technical viability of producing synthetic jet
fuel (or jet fuel blending components) from a wide range of hydrocarbon sources other than petroleum,
using a broad range of biochemical and thermochemical conversion processes. Several approaches to
producing synthetic drop-in jet fuels have demonstrated not only a lower life-cycle carbon footprint than
conventional petroleum-based jet fuel, but also other elements of sustainability—for example, social,
environmental, or economic. This report refers to such fuels as sustainable alternative jet fuels (SAJF, see Box 5.2).

A wide array of organizations has been working for the last decade to support the development of
SAJF and to create a framework by which such fuels can enter the marketplace. The SAJF community in
the United States now includes a broad coalition federal agencies, state and local constituents, operators
of aircraft powered by gas turbines (commercial, military, business, and general aviation), engine and
aircraft manufacturers, some members of the petroleum industry, academia, nongovernmental
organizations, and various public–private partnership efforts. READ MORE

TABLE 5.1 Approved Alternative Fuel Production Pathways
Name (from ASTM D7566 Annex)

Description

Qualification Date

Blend Limitation

(%)

A1: FT-SPK a

Fischer-Tropsch conversion of syngas to synthetic paraffinic kerosene

September 2009

A2: HEFA-SPK b

Hydroprocessed esters and fatty acids (lipids from plant and animal sources) to synthetic paraffinic
kerosene

July 2011

A3: HFS-SIP c

Hydroprocessed fermented sugars to synthesized isoparaffins

June 2014

A4: FT-SPK/A d

Fischer-Tropsch conversion of syngas to synthetic paraffinic kerosene and aromatics

November 2015

A5: ATJ-SPK e

Thermochemical conversion of alcohols (isobutanol only initially) to paraffinic kerosene

April 2016

a In this process, syngas (a mixture of carbon monoxide and hydrogen) is processed in a Fischer-
Tropsch catalytic reactor to produce a mix of longer-chain paraffinic hydrocarbons which are subsequently converted into jet fuel with typical refinery finishing processes. Common methods of
producing syngas include gasification of solid forms of hydrocarbons (e.g., biomass residues, municipal
solid waste, coal, or combinations thereof) and the conversion of natural gas or biogas into syngas (e.g.,
via steam methane reforming). Gasification entails processing feedstock in a high-energy, reduced oxygen environment such that the feedstock does not combust but is thermally deconstructed into its constituent elements (hydrogen, carbon monoxide, nitrogen, water, methane, hydrogen sulfide, carbon dioxide, and other compounds). The gasifier output must be cleaned of particulate matter, sulfur, and other impurities. Syngas is also produced as a byproduct of various industrial processes.

b Waste fats, oils, and greases or plant derived oils can be cleaned and treated with hydrogen to
produce jet fuel. Some sources of plant-derived oils, such as soybeans, are so expensive that they
exacerbate the challenge of producing cost-competitive SAJF. Other options are potentially more
competitive. Examples include waste fats, oils, and grease and non-food crops, especially those grown on
land that is not suitable for growing food crops, are potentially more competitive.

c Biomass feedstocks can be converted to sugars using a variety of pre-treatment technologies. Microorganisms have been developed that will convert the sugars directly into an iso-paraffin for blending with jet fuel.

d This process is similar to FT-SPK, but it includes the addition of production methods that also
produce aromatic hydrocarbons.

e Alcohols can be converted to pure hydrocarbons in the jet fuel range through a process of
dehydration, oligomerization, hydrogenation, and fractionation.

...

... SAJF development efforts are being supported by federal, state, and local
government agencies that recognize the potential benefits:
 Environmental Benefits—Lowering Emissions Around Airports. SAJF blending components typically contain less sulfur than conventional jet fuel. As a result, their use will likely reduce emissions of oxides of sulfur (SOx), and because SOx is a precursor for secondary particulate matter, such emissions will also be reduced. SAJF blending components typically contain lower levels of aromatics (specifically, polycyclic aromatics), which improves combustion characteristics. Owing to this and other factors, SAJF tests have shown general reductions in aerosol emissions, particles, and black carbon.10
10 Virent, Inc. “Virent Bio-Jet Provides more than 50% Reduction in Particulate Matter Emissions,” last update January 6, 2016, http://www.virent.com/news/virent-bio-jet-provides-more-than-50-reduction-in-particulate-matteremissions. READ MORE