Technology Section

There are many sustainable aviation fuel (SAF) conversion platforms at different stages of technological development. The different technologies can covert a range of biomass feedstocks into different types of jet fuel, with varying blend percentages. Currently, four technologies are certified (by ASTM) to supply commercial aviation, see the table below. Although these technologies have been around for some time now, the installed production capacity is limited.
There have been some flights with Alcohol-to-Jet (AtJ) and Synthesized Iso-Paraffinic (SIP) fuel, but 90% of the biofuel flights to date were powered with Hydrotreated Esters and Fatty Acids (HEFA) fuel. Besides the four existing technologies, there is a strong drive to develop technologies that offer better economic and sustainability performance. Below we will discuss the nine technology platforms that are closest to commercial scale deployment.

table-technology

ASTM certified conversion platforms

Fischer-Tropsch (FT)

How it works

ft-chart

The Fischer-Tropsch platform can use any carbon rich material as a feedstock. The process starts with a thermochemical conversion process that forms synthesis gas (or syngas, a mixture of H2 and CO) by reacting any carbon-rich material with air or steam under high pressures and temperature. Before the syngas can be further used the gas is often led through a clean-up step in which contaminations are removed. The syngas is then further processed through the Fischer-Tropsch synthesis process by feeding pure syngas, at controlled pressure and temperature, over a catalyst that assists in the formation of the desired hydrocarbon molecules. The longer the reaction runs, the longer the chain length of the hydrocarbons. Usually the product is a FT wax, which needs further hydrocracking and distillation to get a range of end products, jet being one of them.

The Fischer-Tropsch technology can be divided into two pathways. The Fischer-Tropsch Synthetic Paraffinic Kerosene, or FT-SPK, and is approved for commercial use by ASTM in a 50% blend. The fuel contains only alkanes, no aromatics. A similar process producing aromatic hydrocarbons (FT SPK/A) has been approved by ASTM as well. Due to the presence of aromatics in this fuel, it can be a 100% replacement of fossil fuels. At this moment, FT SPK/A is the only technological pathway that produces a fuel which can be used in a 100% blend.

Hydrotreated esters and fatty acids (HEFA)

How it works

hefa-chart

The HEFA process starts with vegetable oil which is extracted from oil crops or animal fats. The feedstock is pretreated to remove impurities, such as phosphorus or metals. The vegetable oil reacts with hydrogen (hydrotreating to remove the oxygen and split the triglyceride into separate hydrocarbons. The hydrocarbons are then cracked in the presence of hydrogen to produce the desired chain lengths. The HEFA technology can produce two separate products; Hydrotreated Renewable Jet (HRJ) and Hydrotreated Renewable Diesel (HRD). When optimizing for HRD the distillation step is rather simple, only the very light ends are separated while the remaining mix of hydrocarbons is then sold as HRD. A facility dedicated for HRJ production does an extra cracking and distillates step to separate the Jet molecules which can be separately sold as HRJ.

The HRJ produced with the HEFA technology is certified by ASTM up to a maximum of 50% in a blend with conventional jet A1. The renewable diesel (HRD) produced with this technology, could also be sold as jet fuel and is in the ASTM certification process in a smaller blend (up to 10%). This process is expected to be finalized by 2017. As most of the HEFA facilities are optimized for HRD production, HRD certification would lead to a substantial increase in production capacity around the world.

Alcohol to jet (ATJ)

How it works

atj-chart

The Alcohol to Jet (ATJ) pathway starts with (any) alcohol. Ethanol is a large commodity and is readily available. Other alcohols of interest are propanol and (iso)butanol. These alcohols are usually obtained through fermentation of sugars or starches. Also significant effort has been put into alcohols form cellulosic material. In the AtJ process the alcohol molecules are dehydrated (-OH removal) and oligomerized (joining the molecules together) to end up with a mixture of hydrocarbons of different chain lengths. The remaining oxygen and double bonds are removed through hydrogenation followed by a distillation step to separate the fractions.

As with the Fischer-Tropsch pathway, the ATJ technology can be divided in two pathways. The synthetic paraffinic kerosene (SPK) and the synthetic aromatic kerosene (SKA) pathway. The SPK pathway has been certified by ASTM in march 2016 up to a 30% blend percentage. The SKA pathway produce fuels containing aromatics and can therefore go for a 100% blend, a pure substitute. The SKA pathway has not been approved by ASTM so far.

Synthesized Iso-Paraffins (SIP)

How it works

sip-chart

In the SIP pathway sugars are fed to a biological platform (microbes or yeast), these organisms produce straight hydrocarbons. There are multiple early stage R&D activities working on this pathway, but only one company has taken SIP production to a commercial scale: Amyris. The yeast that Amyris uses is genetically modified to have the organism’s metabolism produce a C15 hydrocarbon without oxygen (farnesene). This C15 molecule is a versatile platform molecule (e.g. precursor to an anti-malaria drug, fuels, chemicals). A hydrogenation step is used to remove the double bonds from the farnesene.

The SIP pathway is approved by ASTM in a 10% blend. Because this jet component is not a mixture of different hydrocarbons it does not meet some of the jet fuel specifications as stand-alone molecule (e.g. boiling range and density), therefore the blend allowance is lower than some of the other pathways.

Not yet ASTM certified conversion platforms

Hydrotreated depolymerized cellulosic jet (HDCJ)

How it works

hdjc-chart

The hydrotreated depolymerized cellulosic jet pathway creates fuels out of lignocellulosic biomass (e.g. forest residues, agri-waste). The lignocellulosic biomass is pretreated before thermochemical conversion takes place. This results in a biocrude which can be upgraded using conventional refinery equipment into different hydrocarbons. The thermochemical conversions can be either hydrothermal liquefaction (HTL) or pyrolysis. In both the technologies the biomass is very rapidly heated to a high temperature (250 – 500°C). In the presence of a catalysator this leads to the formation of the biocrude. The main difference is that the pyrolysis technology produces a biocrude with a lot more oxygen left in the hydrocarbons. Furthermore, pyrolysis can only process dry biomass, while HTL can process biomass with a water content up to 80%. Pyrolysis therefore needs an extra drying step during the pretratement process, consequently increasing the energy required to decompose the biomass. The oxygen left in the biocrude (and remaining double bonds) is removed with hydrotreatment, after which a distillation step yields the different fractions (lights, gasoline, jet, diesel). As the last two steps of the process are very similar to the refining of crude oil they can take place in a traditional oil refinery.

The pyrolysis development is further than the HTL technological development. However, the HTL technology has more potential due to the ability to use wet feedstocks and lower oxygen content in the biocrude. The HDCJ pathway has not been ASTM certified so far.

Aqueous phase reforming (APR)

How it works

apr-chart

Aqueous phase reforming is a technology that uses sugars as feedstock. It is possible to use lignocellulosic feedstocks, then first a hydrolysis step takes place to break down the cellulosic material into sugars and other degradation products generated from the deconstruction of the biomass. These products are fed into the APR process, in which deoxygenation takes place. This results in a mixture of chemical intermediates including hydrocarbons, alcohols and aromatics. These intermediate products can be condensated to create longer hydrocarbons. A final step of hydrotreatment is done to remove the remaining oxygen out of the jet fuel. Jet fuel is produced next to PX and a multitude of other chemicals.

The APR process is currently in the process of ASTM approval. Virent is developing this technology along two seperate pathways; Hydrodeoxygenated Synthesized Kerosene (SK) consisting of C9 – C16 paraffins and Hydrodeoxygenated Synthesized Aromatic Kerosene (SAK) consisting of C9 – C11 aromatics.

Catalytic Hydrothermolysis Jet (CHJ)

How it works

chj-chart

The CHJ process uses any kind of oil and fats as feedstock. The technology is developed and patented by Applied Research Associates (ARA) in cooperation with Chevron Lummus Global (CLG). The feedstock is converted into an intermediate bio oil by using catalytic hydrothermolysis. In this process, the feedstock is fed to a catalysator, using super critical water the oils and fats are converted into the bio oil. The bio oil contains a variety of hydrocarbons including paraffins and aromatics. As this oil contains aromatics, no blending with fossil jet A1 is needed. The bio oil can be hydrotreated and distillated similar to conventional oil processing.

Currently the CHJ pathway has not been approved by ASTM, however the certification process has started. ARA and CLG provided the US Navy with 150.000 gallons of jet and diesel to demonstrate the fuels capabilities.

Co-processing vegetable oil with conventional oil crude

How it works

co-chart

Co-processing is a conversion pathway, based on co-mingling fossil based crude oil with vegetable oil before processing it in a conventional refinery. The vegetable and mineral oil are hydrogenated into different products as with conventional refining. The vegetable oil, or ‘green molecules’ get into all the output products and can be allocated on mass balance to one or several output products.

This process has not been certified by ASTM, it is developed by Petrobras in collaboration with Boeing and NASA and started in 2007. When the oil price went down Petrobras put the activities on hold.