Options for biojet fuel
A refiner reviews the available technologies for introducing drop-in jet fuel production. Aviation turbine fuel (ATF), or jet fuel, is a specialised type of petroleum–based fuel used to power aircraft.
JITENDRA KUMAR Satyarthi, CHIRANJEEVi Thota, Gokak Dattatraya Tammanna Shastry and
Poyyamani Swaminatha VISWANATHAN
Bharat Petroleum Corporation
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It is generally of a higher quality than fuels used in less critical applications such as heating or road transport. It is colourless to straw coloured and is a complex blend of hydrocarbons, such as normal paraffins, isoparaffins, cyclic and aromatic hydrocarbons. It is obtained by the direct distillation of petroleum crude, as well as from the hydrocracking of heavier crude fractions. Tables 1 and 2 list the chemical composition and various properties of jet fuel used for commercial airlines. A product of petroleum refining, which belongs to the middle distillate group, it has a distillation range between 150°C and 300°C, but not generally above 250°C.
About 25-40% of the operating costs of an airline are on fuel expenditure. Annual jet fuel demand is over five million barrels per day. Airlines alone spent $140 billion on jet fuel in 2010. That cost was expected to reach $200 billion in 2012. Therefore, airline companies, as the largest users of jet fuel, are exposed to price risks owing to extreme volatility in prices worldwide. Besides these, there is environmental concern about the air pollution due to greenhouse gas emissions. The high cost of ATF, along with volatility in petroleum prices and pressure to reduce carbon emissions, are the main factors propelling researchers and industry to look for alternative renewable fuel options for the aviation industry.
Due to the stringent regulation of jet fuel and high investment in current jet engine technology as well as the supply infrastructure, the aviation industry wants no solution other than a drop-in fuel alternative to petroleum jet fuel. Therefore, biojet fuel technology should fulfil the following requirements to have any significant impact:
• Flexibility in feedstock processing, because even the same type of feedstock from renewable sources can vary in composition; for instance, vegetable oils can have different levels of unsaturation and acid content
• Should meet jet fuel specifications, especially freezing point, thermal stability and energy content
• Should be a drop-in replacement for petroleum-based jet fuel
• Should be environmentally acceptable
• Production, operating and maintenance costs should be competitive with petroleum jet fuel.
In the last few years, significant developments have been observed towards the use of biojet fuel. Besides providing an alternative and sustainable option, it greatly helps in reducing carbon emissions because the use of biojet fuel recycles CO2 and gives a carbon-neutral alternative. Significantly, there has been approval of the specifications for Fischer-Tropsch-based and hydrotreated renewal jet fuels by ASTM and the commercial trials of biojet fuel by several airlines, including Lufthansa, Finnair, Interjet, Aeroméxico, Thomson Airways, Air France, Air China, Alaska Airlines, Thai Airways and Jetstar.
Biojet fuels are nearly chemically identical to conventional petroleum jet fuels and they match petroleum jet fuel for performance. There are various routes and approaches by which biojet fuels can be prepared, and some important routes are discussed here.
Biomass to jet fuel (Fischer-Tropsch process)
In this process, a synthetic gas (syngas) consisting of carbon monoxide and hydrogen is first produced by gasifying the biomass using heat, oxygen and steam. This syngas is converted into a paraffinic wax by the Fischer-Tropsch (FT) process. Finally, the paraffinic wax is selectively cracked and isomerised to produce biojet fuel. As syngas (CO+H2) can be produced from any source, whether renewable (biomass) or non-renewable (coal, natural gas), the gasification step and further steps are not affected by the starting material. Therefore, fuel produced by this process is similar to that obtained from non-renewable feeds.
Feedstocks that could be used for gasification include any plant biomass containing sugar, starch, cellulose or other complex carbohydrates. Besides these feedstocks, municipal, agricultural and forestry wastes, switchgrass, sorghum, miscanthus, willow, poplar or macroalgae can also be used.
The FT process is commercially proven for the conversion of coal and natural gas into non-renewable liquid fuels as coal-to-liquid or gas-to-liquid processes. Synthetic jet fuel produced by Sasol using CTL technology is the first synthetic jet fuel to gain approval for use in commercial airlines. Instead of coal, if biomass is used to produce syngas, this process provides renewable jet fuel, and CO2 savings can be more than 80% on a lifecycle basis. This type of renewable fuel also has ASTM approval in its new D7566 standard when used as a 50% blend with conventional jet fuel.
These fuels offer big advantages, as they involve production from completely renewable feedstocks and therefore offer an opportunity for significant CO2 reduction. Compared to conventional fossil fuel, and according to some analyses focusing on road diesel BTL fuel, CO2 savings can exceed more than 80% on a lifecycle basis. Most of the steps used in BTL, such as the gasification of biomass, FT processing and hydroprocessing, are commercially available and biojet production has been demonstrated by several companies, such as Choren in Germany, Syntroleum in the US, Sasol in Brasil, and Neste Oil and Stora Enso in Finland. Also, Uhde, together with a number of French companies, has announced a small pilot-scale plant. However, large-scale production is still required to finally assess biojet’s potential and to reduce its production costs.
Hydroprocessed renewable jet fuel
Hydroprocessed renewable jet fuel (HRJ) or hydroprocessed esters and fatty acids (HEFA) are produced by hydroprocessing vegetable oils or animal fats in a process currently used in petroleum refining. In this, oxygen is first removed from triglycerides by hydrodeoxygenation/decarboxylation and double bonds are saturated by the addition of hydrogen to produce long straight-chain hydrocarbons. Vegetables and fat are triglycerides, which mostly contain fatty acids with carbon numbers in the range C14 to C20, but jet fuel contains hydrocarbons in the range C8 to C16. These straight hydrocarbons mostly fall into the diesel range and are converted into jet fuel by selective cracking and isomerisation (see Figure 1). As cracking is involved, more hydrogen is consumed compared to the production of green diesel for road transportation.
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