Biofuel - Dypdykk fra Tim Robinson

Tim Robinson
Greener biofuels?

Research conducted by the German Aerospace Center DLR appears to show that emissions from aircraft powered by some blends of biofuel may have environmental advantages over conventional fuels. BILL READ FRAeS reports.

In recent years, there has been increased interest in the development of renewable biofuels as ‘drop in’ alternatives to conventional fossil fuels with blends of biofuels and conventional fuels being successfully tested on commercial aircraft. Meanwhile, the German Aerospace Center DLR (Deutsches Zentrum für Luft- und Raumfahrt) has been conducting research into the environmental effects of biofuels and has come up with some interesting conclusions.

Soot and contrails

NASA DC-8 test aircraft and HU-25C Guardian. (NASA)

Aircraft jet engines emit hot exhaust gases and particles of soot. If the atmosphere is sufficiently cold and moist, these hot particles will act as condensation nuclei for small droplets and ice crystals. These crystals can remain for several hours in cold and humid conditions at altitudes from 8 to 12kg and form high level contrail cirrus clouds. These artificially generated long thin clouds can remain in the sky for long periods of time and are believed to be a factor in influencing the Earth’s environment. Any initiative which can reduce soot particle emissions can therefore also reduce the climatic impact of contrails.

In 2013-2014, NASA conducted a series of test flights at the Armstrong Flight Research Center in Edwards, California to analyse the effects of alternative fuels on engine performance, emissions and aircraft-generated contrails at altitudes flown by commercial airliners. To assist with the flight tests, which were part of the Alternative Fuel Effects on Contrails and Cruise Emissions Study (ACCESS), NASA enlisted the help of the German Aerospace Center DLR (Deutsches Zentrum für Luft- und Raumfahrt) and the National Research Council (NRC) of Canada. DLR and NASA had worked together before, conducting joint projects into atmospheric research for the past 18 years, including air traffic management, low-noise and low-emission flying.

The four aircraft involved in the NASA emissions tests. (NASA)

Conducted in May 2014, the test flights were carried out by four aircraft - a four-engined NASA DC-8 research aircraft and three alternate trailing aircraft, consisting of NASA’s own HU-25C Guardian measuring aircraft, a CT-133 from the NRC and the DLR’s Falcon 20-E5 converted business jet research aircraft.

The tests involved flying NASA's DC-8 at altitudes up to 40,000 feet. The DC-8's four CFM56 engines were powered either by normal JP-8 jet fuel or a 50-50 blend of JP-8 with a renewable biofuel of hydroprocessed esters and fatty acids (HEFA) produced from Camelina plant oil. The trio of research aircraft took turns flying behind the DC-8 at distances ranging from 300ft to over than 20 miles to take measurements of ice particles and engine emissions and to study contrail formation as the different fuels were burned. To ensure that they were only sampling the exhaust plume from a particular engine, the chase aircraft had to fly as close as 30-150m behind each engine and directly in the plume (see Youtube video https://www.youtube.com/watch?time_continue=5&v=TtcUqIWmTBM)

The DRL Falcon takes samples from the exhaust plume of NASA’s DC-8 test aircraft. (NASA)

This was the first time that the amount of soot particles emitted by jet engines burning biofuel-blended fuel had been measured in the environmental conditions prevailing in flight. The conclusion from the tests, which were published in a paper written by Rich Moore from NASA and Hans Schlager from the DLR Institute of Atmospheric Physics in the scientific journal Nature earlier this year (http://www.nature.com/nature/journal/v543/n7645/full/nature21420.html?foxtrotcallback=true), were that a 50% conventional/biofuel blend could, depending of different flying conditions, produce 50% less black carbon by number and up to 70% by mass.

Specifications standards

Interior of the DLR Falcon test aircraft. Operated by a crew of specialists from the DLR Institute of Atmospheric Physics in Germany and DLR Flight Operations in the US, DLR’s test aircraft was equipped with over a dozen instruments to analyse the exhaust particles and gases emitted by the engines of the preceding DC-8. (DLR)

While this would appear to be an encouraging start for biofuels, there are other hurdles to cross. To be approved for use in commercial aircraft, a new fuel needs first to be certificated. Bio kerosene in Europe must conform to DefStan 91-91 specifications which, for alternative fuels, are similar to the US ASTM D7566 approval process. Before any biofuel/kerosene blend may be used in commercial aviation, three analyses have to be performed:  

- An ASTM D1655 analysis (which covers kerosene in general) of the conventional kerosene before blending

- An ASTM D7566 analysis (which covers alternative fuels and blends with conventional kerosene) of the bio kerosene before blending

- An analysis of the blend, which includes elements of the parameters in both ASTM D7566 and ASTM D1655, plus some additional ones.

However, the DefStan 91-91 specifications have restrictions on the maximum amount of bio-synthetic kerosene that can be mixed with conventional kerosene (50% for FT- and HEFA-kerosene, 10% for SIP fuel). The content of bio-synthetic kerosene must not exceed the maximum percentage permitted by ASTM D7566 and has to meet the same parameters as conventional ASTM D1655 kerosene, plus some additional ones.

The aromatic factor

A major factor currently limiting maximum biofuel blend ratios is aromatics content. Aromatics are necessary to preserve the tightness of fuel systems but have negative effect on fuel burn and emissions. The addition of aromatics to a fuel blend will alter the properties of the blend. ASTM D7566 requires a minimum aromatics content for a blend of 8% but several bio-fuels have virtually zero aromatics content. For blends of these bio fuels, the aromatic content must come from the conventional kerosene which is limited by ASTM D1655 to a maximum value of 25%. Thus, any blend with more than 68% of bio kerosene must have an aromatics content of below 8% and will be off-spec.

Complicated and expensive

Jet-A kerosene currently used to power jet aircraft is comprised of several hundred substances which makes an analysis of its combustion very complicated. A blend of biofuel and conventional kerosene will share some properties with those of normal kerosene which means that not all parameters required for the combustion process have to be completely re-examined. The fewer individual components that a fuel possesses, the easier it is to define the chemical and physical processes that occur during its combustion. The aim with new ‘designer fuel’ blends is therefore to use substances with as few components as possible, so that combustion properties can be optimised and polluting emissions minimised.

According to the DLR Institute of Combustion Technology, which specialises in the certification of new fuels, a full ASTM D1655 analysis takes around 20 man hours to perform and will require specialised and expensive equipment. A problem with testing new fuels is that every blend will need an expensive analysis which will make it very expensive compared to its volume. The unit cost of blended fuel will also vary depending on its blending ratio. For example, to blend 200,000 tons of bio kerosene at 50% requires 200,000 tons of conventional kerosene while to blend 200,000 tons of bio kerosene at 5% would require a logistically impracticable 3.8m tons of conventional kerosene.

Ground tests

Engine on test rig at Lufthansa Technik Hamburg. (DLR)

Following the NASA tests, DLR has been conducting further research on its own. In June 2017, DLR published details of additional ground tests it had conducted which looked at the chemical and physical properties of particular biofuels and biofuel blends, according to source, production process and approval status. Conducted as part of the European Union-funded 'High Biofuel Blends in Aviation' (HBBA) study (http://www.hbba.eu/study/index.html#pf68) the tests involved researchers from DLR and the Bundeswehr Research Institute for Materials, Fuels and Lubricants using a CFM56 engine fitted on special test rig at Lufthansa Technik in Hamburg.

The tests compared three different fuels: pure biofuel, a 50/50 biofuel/conventional fuel blend and conventional kerosene. The exhaust gas stream from the engine was analysed by probes installed in a tunnel behind the engine. For each test run, the engine was run for 10 minutes at minimum idle, five minutes at flight idle, five minutes at cruise power, three minutes at climb power and one 1 minute at take-off power.

Blending analyses

The DLR ground tests used five different conventional kerosenes with a broad range of properties which were used for blending with six different examples of bio kerosene derived from different sources. The bio kerosenes used were:

- Fischer-Tropsch synthesized paraffinic kerosene (SPK) coal to liquid from Sasol

- Hydrotreated esters of fatty acids (HEFA) SPK from UOP

- Synthesized iso-paraffins (SIP) fuel from Total/Amyris

- Alcohol-to-jet (ATJ)-SPK from Gevo

- ATJ-SKA (synthesized kerosene with aromates) from Swedish Biofuels

- Catalytic hydrothermolysis (CH) kerosene from ARA

Each of the conventional and biofuel fuels had a range of parameters (density, specific energy, viscosity, freezing point, smoke point, sulphur content and aromatics content) and many of the blends tested included a mix of conventional and biofuels with similar properties.

Material effects

Another factor to be considered when analysing different bio kerosene blends is their effect on different materials - an important factor when considering how they will be stored and transferred.  An analysis was conducted on the chemical and physical reaction of different neat biofuels on ceramic, metal, alloy and synthetic materials, including elastomer O-ring seals made from nitrile-butadiene rubber (NBR), fluorosilicone rubber (FVMQ) and fluorocarbon rubber (FKM). As the effect of fuel on seal tightness is generally attributed to their aromatics content, tests were also conducted with aromatics added to aromatics-free bio kerosenes. Tests were also carried out in which the elastomer materials were first exposed to the conventional fuel with the highest aromatics content and then exposed to aromatics-free bio kerosenes, simulating a situation where an aircraft has been operated on conventional kerosene and is then exposed to bio kerosene. The tests showed NBR was adversely affected in both mass and volume by the fuels, particularly by those with a higher aromatic content, FVMQ was affected to a lesser extent and FKM hardly at all.

Emissions tests

There were fewer and smaller particle soot emissions for the SIP blends, particularly at climb and take-off high power settings. (EU HBBA Report)

Consideration of emissions is not part of the ASTM fuel certification process, since emissions vary depending on the engine the fuel is burned in. However, following the earlier NASA flight tests, DLR was interested in seeing how the different blends of biofuels performed in emissions tests. An initial set of tests was conducted in November 2013 using 10% and 20% blends of farnesane non aromatic SIP biofuel, followed by tests on neat CH kerosene in November 2016. The results were mixed. The SIP fuel showed little different on CO2 and NOx emissions between SIP fuel blends and conventional fuel but fewer and smaller particle soot emissions for the SIP blends, particularly at climb and take-off high power settings. However, the tests of the CH fuel was less positive with CO2, NOx and particle emissions similar or ever higher than conventional fuel.

More NASA tests

NASA DC8 before a test flight. (NASA)

Further tests are planned. In early 2018, NASA’s DC-8 will take part in a series of research flights in Germany, as part of DLR's ECLIF (Emission and Climate Impact of alternative Fuel) own project. In these tests, researchers will be investigating how the composition of different alternative fuels influences the emissions and the climate-relevant properties of contrails.

But is it sustainable?

IATA says that there is more to green fuels than just emissions. (IATA)

However, while initial test results look generally promising for the environmental properties of biofuels, airlines considering using renewal fuels must pass an additional test - that of sustainability. This is defined in the IATA Guidance Material for Biojet Fuel Management (https://www.iata.org/publications/Documents/guidance-biojet-management.pdf) as: ‘conserving an ecological balance by avoiding depletion of natural resources. While airlines may use any biojet fuel that meets certification criteria, those certifications (ASTM D7566, DefStan 91-91, etc) do not guarantee sustainability; they only describe the physical properties of the fuel itself … Simply using biofuels does not necessarily reduce overall carbon emissions; a biofuel must demonstrate a net carbon reduction through a lifecycle analysis (LCA), which is an essential element of sustainability certification.’

These analyses look at how the feedstocks for the biofuel are created and includes such factors as lifecycle carbon emissions, fertiliser and pesticide management, direct and indirect land use change, waste management, authorised land categories, invasive species controls and water, air and soil considerations.

If aviation biofuels are to be more environmentally friendly, they must be proved to be greener both in the air and on the ground.