To date, three pathways have been approved for aviation biofuels:

• Fischer-Tropsch Synthetic Paraffinic Kerosene (SPK). Feedstocks including cellulosic biomass and municipal wastes are gasified. The resulting syngas (CO and H₂) is then processed with heat and catalysts to produce olefins or hydrocarbons that can be converted into fuel. SPK is approved for use up to 50% when mixed with conventional jet fuel.

• Hydroprocessed Esters And Fatty Acids (HEFA). Fatty acid esters and free fatty acids (e.g. bio-oils and fats) are fed into a reactor catalytically de-oxygenated. Further hydroprocessing and fractionation produces jet range hydrocarbon molecules. The process can also produce diesel fuel as well as light (naphtha) hydrocarbons. HEFA is approved for use for up to 50% when mixed with conventional jet fuel.

• Synthesized Iso-Parafins (SIP). Pretreatment breaks down feedstocks to yield plant sugars, which are then fermented by microorganisms into a terpene that is subsequently hydrotreated into SIP. More commonly known as farnesane, SIP is a liquid hydrocarbon that can be used as a portion of jet fuel as well as diesel fuel. SIP is approved for use for up to 10% when mixed with conventional jet fuel.

Three other pathways are currently in the approval process:

• Green Diesel (High Freeze Point HEFA). The process to produce HEFA from fatty acid esters and free fatty acids also produces a diesel fraction. The approval of the higher freeze point HEFA fraction would lower the cost of the fuel. However, the blend percentage will likely be lower than 50% percent.

• Alcohol-to-Jet (ATJ). For cellulosics and municipal and agricultural wastes, pretreatment breaks down plant matter to access plant sugars. Microorganisms have been engineered to process sugars or industrial waste gas (carbon monoxide) to alcohols (ethanol, n-butanol, iso-butanol). Oxygen and other molecules are removed to make olefins. The olefins are then oligomerized and hydroprocessed to make jet and diesel range molecules. Varying processes are being vetted by the OEMs. Approval for some of the processes is expected in 2015.

• Hydrotreated Depolymerized Cellulosic Jet (HDCJ). Cellulosics, municipal and agricultural wastes are subjected to high temperatures and pressure in an oxygen-starved environment (pyrolysis), transforming it into synthesis gas, biochar, and bio-oil. The bio-oil is upgraded to liquid hydrocarbons including jet fuel and diesel fuel using hydroprocessing technology. Biochar can be used as a soil conditioner with carbon storage capability. A hydrotreated depolymerized cellulosic jet fuel is being reviewed by the OEMs for approval.

Another possible pathway is also emerging: hydrothermal liquefaction. Here, wet organic matter (cellulosics; municipal and agricultural wastes; oils and fats) streams through a chamber and is subjected to high temperatures and pressure in a flow process yielding bio-oil that is upgraded to liquid hydrocarbons including jet fuel and diesel fuel using hydroprocessing technology. The approval process has yet to be initiated.

The roadmap notes that Abu Dhabi has the potential to supply domestic feedstocks from salt-tolerant halophyte plants that can be irrigated with seawater, inland planted forest areas, and municipal and agricultural wastes.

Abu Dhabi has a limited but real capacity to provide biofeedstocks for drop-in fuels. Supplies of freshwater and arable lands are constrained, so Abu Dhabi’s current agricultural sector would not be a sustainable source of dedicated energy crops unless sustainable alternative water sources such as recycled water from effluent could be utilized. However, residues from agricultural operations could supply feedstocks, as could municipal wastes. The use of innovative agricultural techniques using seawater irrigation and desert lands not currently considered arable to grow salt-tolerant halophyte plants could produce substantial feedstock supplies. Converting inland planted forests to commercial species capable of producing fuel feedstocks could help address management challenges.

Algaes and macroalgaes could also be considered here although meeting strict sustainability principles and ensuring commercial viability may be a challenge. Fats from animal tallows are used in some regions to produce fuel and energy. The slaughter industry in Abu Dhabi is not currently large enough to generate sufficient tallow supplies. However, as the industry grows it could in future produce viable quantities of feedstock. Used cooking oil could supply limited amounts of biodiesel for ground transportation in the UAE, and could potentially supplement other supplies of oil and fats in hydroprocessing facilities. A sustainable means to collect these feedstocks from a broader region could make animal tallows and used cooking oil a more significant feedstock option in the future.

—“BIOjet Abu Dhabi”

The roadmap explores how a supply chain can be established in the UAE through the exploration of sustainable feedstocks, new infrastructure requirements and necessary policy frameworks.

The BIOjet Abu Dhabi roadmap builds on local research undertaken by the Sustainable Bioenergy Research Consortium, led by the Masdar Institute, whose flagship project is the Integrated Seawater Energy and Agriculture System (ISEAS). ISEAS is an initiative to develop a unique form of agriculture, producing food and energy products on traditionally non-arable desert land irrigated with seawater. The pilot ISEAS facility is currently under construction within Masdar City.

The report concludes that the aviation biofuel mix in current jet fuel supplies can grow in a phased manner from small batches to blending in the entire fuel stream. Takreer, the refining arm of Abu Dhabi National Oil Company and exclusive supplier of fuels to Abu Dhabi, will play a vital role in biorefinery development.

The report also finds that development of a full Abu Dhabi aviation biofuel supply chains will require significant financial investment that in turn will require public policy support.