For Part 1 of this discussion, click here.
The health hazards, loss of IQ points, and associated costs of lead (Pb) fuel emissions leaves only one option for the General Aviation (GA) fleet: stop using leaded fuel. In order to accomplish this task, GA has several options to consider, including Avgas alternatives such as renewable biofuel, fleet-wide modification or to continue searching for a “drop-in” replacement that will meet or exceed the current engine specifications. Considering the severe cost of fleet-wide modification, it has become a last-resort option as all other avenues are explored.
Biofuel Avgas Alternatives
Because petroleum is a finite natural resource, a long-term solution is to replace Avgas and other petroleum-based fuel with renewable energy. One type of renewable energy is biomass, which is converted into bio-oil and then biofuel. Biomass has been considered from many different crops and each are classified by generation. First generation biofuel was made from sugarcane, sugar, beet, maize and rapeseed, but the use of these crops proved to be unsustainable because biofuel production drew on resources needed for food, and subsequently raised food prices. Second generation biofuel was made from wood, organic waste and food crop waste, which did not impact food production, but these crops had the limitation of year-round availability and high conversion costs. Third generation biofuel shows promise by using microalgae as biomass, which does not share resources with our food supply and can be produced year-round.
Microalgae produce more oil than oilseed crops and can be processed in various ways to produce several different types of fuel. With thermo-chemical production, microalgae can produce oil and gas, while biochemical production results in ethanol, biodiesel, and biohydrogen (Demirbas, 2010). According to Brennan and Owende (2009) bio-oil is created through the thermo-chemical process of pyrolysis, which supports large-scale production of biofuel and has the potential to eventually replace petroleum. Biomass already supplies approximately 13% of the world primary energy supply, and as production methods become more efficient bioenergy is expected to replace a greater amount of petroleum each year, providing 25-33% of global energy by 2050 (Hossain and Davies, 2013).
According to Demirbas (2010) there is potential for large-scale production of microalgae through the use of raceway ponds and tubular photobioreactors, however, microalgae production has not matched theoretical claims of oil yields. Limitations on the ability to supply nutrients and CO2 may inhibit large-scale production, and may become more restrictive as production capacity nears 10 billion gallons per year (Pate, Klise and Wu, 2011). Improvements are needed in the growing and harvesting of microalgae to reduce costs and enhance the production of algal biomass. With such a large infrastructure and dependence on petroleum, it is unknown if these improvements will allow microalgae production to compete and replace petroleum-based fuel completely.
While bioethanol is not a prime candidate for use in the aviation industry, and biodiesel can be used in limited quantities with kerosene as a fuel extender, the efficiency of hydrogen biofuel is worth a second look. Hydrogen can be produced by algae under specific conditions, such as direct and indirect photolysis, and ATP-driven hydrogen-production (Demirbas, 2010). Liquid hydrogen (LH2) powered aircraft boast a much lower fuel weight, which decreases operating costs and improves efficiency. The trade-off is higher pricing for LH2 and the increased frequency of contrail formation, with these aircraft expected to enter into commercial service around 2040 (Yilmaz, Ilbas, Tastan and Tahran, 2012).
Because of the prohibitive cost of modifying the entire fleet of piston-engine aircraft, the general aviation sector has been searching for a “drop-in” solution. A true “drop-in” solution would allow the aircraft to operate on the avgas alternative without any modifications. To support the reduction in Pb emissions, the Federal Aviation Administration (FAA) has set a goal of 2018 for the procurement of an avgas alternative that is usable in most piston-engine aircraft (FAA, 2013).
Currently, the FAA has entered Phase 2 of the Piston Aviation Fuel Initiative (PAFI), a program designed to evaluate potential avgas alternatives for suitability as a drop-in replacement for 100LL. Phase 1 included assessments in emissions and toxicology, production and distribution, and performance in worst-case conditions. The FAA has selected two fuel prospects, Swift Fuels and Shell, to continue Phase 2 testing at the engine and aircraft level with the purpose of being adopted across as much of the existing fleet as possible. According to the FAA,”…the PAFI process is not intended to be a barrier to entry for proposed fuels but rather is designed to enable the most promising fuels to undergo the necessary independent peer review and data collection necessary to gain broad based industry, regulatory, and consumer acceptance leading to production and sale across the entire aviation marketplace.” (FAA, n.d.).
While the well-known industry giant Shell submitted a promising fuel formulation, Swift Fuels, established in 2005, also advanced with their UL102, an “all-hydrocarbon” unleaded 102 Motor octane aviation gasoline that meets ASTM D7719. With Phase 2 testing of the PAFI set to continue for the next couple of years, GA’s era of leaded fuel is finally coming to an end. The environmentally-friendly, high-performance unleaded avgas alternatives of the future will prove a wise choice for generations to come. Generations who will be, quite literally, smarter than the last.
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Brennan, L. & Owende, P. (2009). Biofuels from microalgae- A review of technologies for production, processing, and extractions of biofuels and co-products. Renewable and Sustainable Energy Reviews, 14, 557-577.
Demirbas, A. (2010). Use of algae as biofuel sources. Energy Conversion and Management, 51, 2738-2749.
Federal Aviation Administration. (n.d.). White Paper. Piston Aviation Fuel Initiative.
Federal Aviation Administration. (2013). FAA Issues Request for Unleaded Replacements for General Aviation Gasoline (Avgas).
Hossain, A. K. & Davies, P. A. (2013). Pyrolysis liquids and gasses as alternative fuels in internal combustion engines- A review. Renewable and Sustainable Energy Reviews, 21, 165-189.
Pate, R., Klise, G., & Wu, B. (2011). Resource demand implications for US algae biofuels production scale-up. Applied Energy, 88, 3377-3388.
Yilmaz, I., Ilbas, M., Tastan, M., Tarhan, C. (2012). Investigation of hydrogen usage in aviation industry. Energy Conversion and Management, 63, 63-69.