Sustainability of supply or the planet: a review of potential drop-in alternative aviation fuels

Abstract

The development of kerosene-like drop-in alternative aircraft fuels can be categorised into two groups, depending on whether the product increases supply security or provides a reduced environmental footprint. This paper uncovers this relationship through a review of commercially available process technologies (Fischer Tropsch and hydroprocessing) to produce alternative fuels, lifecycle results and recent flight test campaigns, before evaluating the prospects for future fuel development. Supply may be improved through the conversion of coal (with carbon sequestration) or natural gas using the Fischer Tropsch process. Refinement of these alternative fossil fuels, however, provides comparable total life cycle emissions to Jet A-1. The hydroprocessing of biomass feedstock provides for a reduced environmental footprint—approximately 30% reduction for sustainable cultivated feedstock, when blended 50/50 with conventional jet fuel. However, securing supply is a significant issue. Considering aviation is responsible for 2.6% of global CO₂ emissions, converting 6% of arable land (representing 0.95% of the earth surface) to supply a 50/50 blend, thus offsetting 0.78% of global CO₂ emissions, seems impractical based upon the current land use scenario. Furthermore, ground based sectors have significant environmental footprints compared to aviation, yet require little pre-processing of feedstock (i.e. power generation can burn raw feedstock), thus presenting a better biomass opportunity cost.

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Broader context

Aviation in the context of the environment is often seen in the public eye as being responsible for considerable environmental detriment. This, coupled with a limited supply of conventional jet fuel, has resulted in the sector searching for alternative fuels. Recent activity has demonstrated incident free operation—without airframe or engine modification—of fuel refined from both gas and biomass feedstocks. Unfortunately, diversification away from crude oil does not necessarily improve both environmental credentials and supply security. Considering the sector's environmental footprint, the best case test flight achieved a 30% reduction in emissions. The reduction, however, only equates to a potential saving of 0.78% of global CO₂ emissions. Realising this saving across the sector would require 6% of the world's arable land or 3.2% of global surface area if feedstock (Jatropha) was grown on marginal land. Refinement of gas or coal (Fischer Tropsch process) provides for sufficient feedstock volumes to diversify supply, however, the process is at best comparable to a modern crude oil refinery in terms of well to wake emissions. It is anticipated that market forces will allocate feedstock to sectors which provide the highest level of abatement at the lowest cost, meaning that aviation will need to compete to secure fuel.

1. Introduction

The jet fuel, which has evolved from the illuminating kerosene used by Whittle to Jet A/Jet A-1, the product used in today's airliners, has been largely refined from crude oil feedstock. In recent times, however, the aviation industry has come under the public spotlight in terms of emissions and their contribution to climate change. The industry's reliance on a single product has also raised concerns over future supply security and operational cost. These factors have led to much recent interest in the development of product produced from alternative sources. The asset intensive nature of the aviation industry, however, limits solutions for even the newest commercial aircraft (i.e. A380) to a backwards compatible kerosene-like drop-in alternative fuel. A drop-in product may be defined as one that is fully fungible with today's Jet A-1 and can be incorporated into the existing jet fuel specification.

Advancement in process technology is providing alternative means of producing traditionally distilled crude products, through the conversion of coal, gas or biomass. The processing techniques provide for the production of liquid hydrocarbons which have similar properties to Jet A-1, thus providing a number of potential drop-in alternative products. Since 1999, coal gasification and synthesis have demonstrated the potential to provide Johannesburg International Airport with a secure supply of aviation fuel.¹ More recently, conversion of gas and biomass feedstocks is beginning to show potential for providing increased supply security or an improved environmental profile, respectively. These feedstocks have been used to supply product for numerous flight test programs over the past year.^2–4

Recently, the media has highlighted a number of full scale proof of concept flight tests, which involved operating a single engine on an alternative product blended with Jet A-1. The campaigns have demonstrated incident free operation of alternative drop-in fuels, along with highlighting the readiness level and commercialisation of process technology. The flights, however, have raised numerous issues, most notably being biomass feedstock availability and sustainability. This paper shall explore these test campaigns and other recent work by examining the two drivers leading to the development of alternative aviation fuel, namely supply security and the sector’s carbon footprint. The review shall begin, however, by examining the crucial requirement of a drop-in jet fuel and how these pathway drivers fit in with expected future fuel requirements.

2. Aviation fuel requirements

Illuminating kerosene, the product chosen by Frank Whittle to fuel his gas turbine research, has evolved to become the standard fuel for the aviation industry. The evolution and, hence, composition of jet fuels has been largely dictated by the aircraft operational envelope and has evolved largely through safety requirements and security of supply. Two of the more apparent technical requirements for increased range high altitude operations included fuel energy density and flow characteristics (i.e. freeze and viscosity). A less obvious requirement dictated by advancements in airframe and engine systems is fuel thermal stability, which can be defined as the tendency of the fuel to breakdown and form gum and/or carbon deposits in the fuel system at elevated temperature.⁵

More recently, in addition to technical fuel requirements and supply security, concerns regarding the environmental footprint of aviation are beginning to become an important issue, especially with airport expansion and up-coming emissions trading schemes. Despite this, the aviation sector is inherently cautious in moving away from crude oil derived jet fuel to other sources which may provide greener benefits, largely due to inherent uncertainty of moving to an unfamiliar product. To aid the development of fuels which meet the minimum requirements for aviation, a fuel approval process has been created. The process is driver independent in that all fuels must always first satisfy the key requirement laid down in the fuel approval process.

2.1 The approval process

The above-mentioned operational fuel requirements (and many others) have led to the creation of a quality control specification which aims to ensure that different refinement techniques and crude sources are indistinguishable to aircraft operators. The specification controls product quality through a series of simple lab based tests to quickly establish if crude derived kerosene meets a range of predetermined values.⁶ Simple lab tests, however, are not a direct measure of gas turbine performance parameters (i.e. fuel spray atomisation quality or thermal stability), but instead provide confidence built on extensive operational experience.

Production of a drop-in product refined from non-traditional petroleum sources, such as coal, gas or biomass, therefore raises concern as, although the product may meet the specification pass off tests, the actual performance in a gas turbine is unknown. This is due to the fact that the specification provides for quality control of crude product only and does not provide the complete picture in terms of assessing the suitability of an alternative aviation fuel. For example, residual heavy metals, not currently tested for within the specification, in a certain biomass product may damage the gas turbine hot section or the lack of a certain hydrocarbon group may affect the fuel system performance.⁷

In order to establish the suitability of a drop-in product, fit for purpose analysis, as well as specification property pass off testing, needs to be conducted. Fit for purpose testing involves establishing how the fuel actually performs in the airframe and engine fuel systems. It involves assessing, for example, materials and additive compatibility, seal swell and lubricity. Specification and fit for purpose testing forms the initial step of the fuel approval protocol.⁷

Once it is established that the fuel has no negative impact on specification and fit for purpose properties, fuel and engine system component testing (i.e. fuel pumps, lines and hot section) and/or full scale engine endurance testing may be required before engine manufacturer approval is given. Once approval is granted, the product may be entered into the fuel specification. The fuel approval process is illustrated in Fig. 1.

Fig. 1 Gas turbine fuel approval protocol.

The approval protocol has been drafted from nearly 14 years of experience gained through the ‘special case certification’ of coal derived synthetic product given to the South African based Sasol Petroleum Company.¹

The fuel approval mechanism—which has been documented in the ASTM D-4054 draft procedure⁸—is currently being applied to the generic approval of synthetic kerosene produced via hydroprocessing. The ASTM approval process is based upon the refinery technique employed, thus effectively creating a recipe to produce a drop-in alternative fuel. Such approval is possible, as product produced through either the Fischer Tropsch or hydroprocessing technologies are feedstock independent.^3,9 Technical endorsement of gas derived synthetic kerosene by the ASTM aviation fuel subcommittee occurred on the 24th June 2009,¹⁰ with full approval granted on the 29th September 2009 through ASTM D7566. The document provides criteria for production, distribution and use of synthetic kerosene refined from coal, gas or biomass through the Fischer Tropsch process.^11,12 Generic approval of hydroprocessing technology is expected during 2010.¹⁰ Endorsement of alternative production routes paves the way for feedstock diversification, thus providing potential for increased supply and/or environmental benefits. Product availability through increased quantity of supply is an apparent inherent requirement of a drop-in fuel, with environmental pressures likely to impose lifecycle emission obligations in the near future.

2.2 Feedstock availability

Unlike the automotive industry, where individual countries can focus on providing independent fuelling options (e.g. Brazil—ethanol), the global nature of the aviation industry requires a global secure supply of fuel which is indistinguishable to the many airframe and engine configurations. To date, the sector has achieved this by relying almost entirely on crude oil, with the only exception being in Johannesburg, South Africa.

The supply of conventional crude is, however, limited, and may be examined using Hubbert peak oil theory.¹³ Hubbert proposed that on discovery of a new oil field, supply from that field would continue to increase with infrastructure installation until a maximum output is reached. Extraction would continue at this rate until the field pressure decreases, resulting in a reduced extraction rate. Superimposing current reserves with predicted findings yields a normal distribution. The point at which maximum production is reached is entitled ‘peak oil’. However, the date at which this occurs is difficult to quantify. On one hand, the oil industry claims that sufficient reserves (discovered, undiscovered and unconventional) exist to meet global demand for decades, while others claim that production has or is about to peak. A recent paper estimates that “remaining resources appear to be sufficient to meet demand up to 2030”.¹⁴ It is certain, however, that once peak production is reached, demand will exceed supply, leading to price rise, product security concerns and increased resource diversification.

Besides the pressures of peak oil on crude derived product, it should be recognised that kerosene represents only 10 to 15% of the overall cut of a crude oil barrel. Kerosene fits between the heavier cut of diesel and the lighter cut of gasoline; therefore, the future availability of jet fuel has a strong interdependency on the developments in the automotive sector. The aviation sector’s reliance on a single depletable reserve therefore presents a considerable issue. Efforts to diversify supply away from crude oil represent a significant driving force behind the development of alternative aviation fuels.

Environmental pressures through the introduction of trading schemes and taxes also have the potential to create supply security issues, as economics begin to push other sectors towards cleaner fuel sources. For example, diesel oil, which inherently has poor environmental credentials, provides a cheap source of fuel for the commercial shipping sector. If, however, environmental penalties were imposed, it is expected that the shipping sector would begin looking for cleaner sources of fuel, namely kerosene, thereby creating increased demand for the limited available crude oil cut (i.e. peak cut).

Refinement of biomass product as an alternative feedstock has been demonstrated to a large extent in the automotive industry. Biofuels have the potential to provide emission savings and, unlike fossil feedstocks, are a renewable resource and thus are theoretically not subject to peak supply concerns. Cultivation of sufficient energy crop to support the sector, however, has raised numerous issues regarding land use allocation, including food crop displacement and land use reallocation (i.e. the clearing of forests to site cropland).¹⁵

Creating sufficient alternative product volume to wean aviation off kerosene through supply diversification is not an easy task considering an average of 6.8 million barrels of product was consumed per day in 2007. Forecasts predicted this value will increase to nearly 7.2 million barrels per day in 2012,¹⁶ although the current recession is expected to delay consumption forecasts by a few years. The forecasts, however, highlight perhaps the most important requirement to consider when evaluating the potential of a candidate fuel: that the feedstock must be available in sufficient quantities to allow for refinement of product in adequate quantities to support the aviation sector. A second important factor to consider is that commercialisation is steering the majority of feedstock and, hence, product to the automotive or petrochemical market. Thus, the aviation industry will need to compete to secure these hydrocarbons.

To date, increasing supply has involved refinement of other fossil fuels at a similar environmental expense (best case), while environmentally focused fuel programs have been based on the refinement of limited biomass feedstocks, thus creating supply issues. This has demonstrated an incompatibility between diversifying supply and environmental credentials. The analysis of an alterative fuel's environmental credentials—which is expected to become a key requirement of an alternative fuel, largely due to political pressures and environmental legislation—is achieved through an assessment of emissions from well to wake.

2.3 Lifecycle emissions

An emerging driver behind alternative fuel development is aimed at reducing the environmental impact of the sector. Historically, airframe and engine manufacturers have continued to reduce fuel burn and, hence, corresponding emissions through technological advances, such as the integration of composites and improved aerodynamics. Fleet renewal programs introduce these saving into the sector at approximately 1.2% per annum.¹⁷ Unfortunately, sector growth is outpacing the technological developments and their introduction, meaning that aviation's relatively low contribution of 2.6% to CO₂ globally (2006) is likely to increase to 3.2% by 2020.¹⁸ This fact has placed the sector under considerable public scrutiny over the past few years, perhaps none more so than the high profile case regarding the expansion plans of Heathrow Airport in the UK.

Environmental pressures have created numerous research efforts, which have thus far focused on producing alternative fuels from biomass feedstock. The biomass pathway has been driven by the realisation that combustion of a drop-in fuel will produce a similar emission profile to a conventionally fuelled aircraft. This is due to the fact that in order to satisfy the approval protocol requirements, a candidate drop-in fuel must be compositionally similar to Jet A-1. Therefore, given the similar chemistry, if an alternative fuel is to provide greener credentials than conventional refined crude, emission savings must be made during the extraction and refinery stages of fuel production.

Lifecycle analysis aims to establish the total emissions over a product's lifetime (i.e. well to wake) and is usually measured in total CO₂ equivalent emissions (CO₂e). This comprises the CO₂ released over the lifecycle, in addition to non-CO₂ greenhouse gases. The other gases included are N₂O and CH₄, which relate to agricultural emissions, extraction and transportation of product.¹⁹Fig. 2 depicts the typical steps involved in the refinement of fossil fuels (a) and biomass (b).

Fig. 2 Lifecycle analysis steps; (a) fossil fuels, (b) biomass.²¹

One of the earlier works on emission lifecycle analyses, which focused on evaluating alternative aviation fuel production techniques, was provided by Wong (MIT).²⁰ Another recent piece of work was conducted by Vera-Morales¹⁹ through the UK Government funded OMEGA Program. Comparison between the literature, however, is difficult, as no defined methodology exists to take account of the fact that results are highly dependent on the choice of boundaries and assumptions, in particular, the maturity of the processing technology and/or the cultivation techniques employed. In order to introduce consistency, the work of Wong,²⁰ which has since been incorporated into the US PARTNER Program's Life Cycle Analysis,²¹ and that of Vera-Morales, were made to be internally self consistent through collaboration between the research groups. Results of both these works are presented later in the paper.

Observing that a drop-in fuel must satisfy the fuel approval requirements, the remainder of this paper examines the relationship between the drive for increasing supply security and that behind reducing the sector’s environmental profile. It is noted from above that focus on ensuring supply must inherently be a requirement of an alternative fuel, while environmental drivers are expected to become fuel requirements through increased political pressure and economic incentives.

3. Sustainability of supply

Limited crude oil reserves have provided a key driver behind research efforts focusing on the diversification of supply to ensure supply security. The key requirement to secure supply is adequate feedstock and, hence, despite the literature highlighting a vast number of different synthetic refining techniques—from indirect liquefaction to more exotic processes, such as the production of hydrocarbons using micro-organisms (i.e. sugar based fermentation routes)—few of these processes represent near term options to provide sufficient product. In fact, the only processing pathway which is providing for supply diversification on a commercial scale is the Fischer Tropsch process.

3.1 Processing pathways

Direct conversion of feedstock into product (also known as liquefaction) is considered to be the most energy efficient route. However, at present, commercial technology is based upon indirect conversion.²² This route requires the production of syngas from suitable feedstock, which is then fed into a liquid conversion process. Depending on the feedstock used, the process is entitled ‘anything to liquid’ (xTL), where ‘x’ is ‘c’ (coal), ‘g’ (gas) or ‘b’ (biomass).

Syngas, namely a gaseous mixture of hydrogen and carbon monoxide, may be derived from coal, gas or biomass. Gasification technology is used to produce syngas from coal or biomass via the partial oxidation of the solid fuel.²³ This technology is in commercial operation at Sasol's cTL South African plant. Steam reforming and/or partial oxidation is used to create syngas from a methane feedstock. Depending on the type of feedstock, the creation of syngas is either a net consumer (coal) or producer (natural gas) of water. A review of different syngas production technologies has been carried out by Keshav and Basu.²⁴

The Fischer Tropsch (FT) process principally involves carbon chain building,²³ requiring a synthesis gas (syngas) feedstock. Depending on product requirements, catalyst is usually selected to favour the production of long chain paraffins (alkanes), as shown in eqn (1)²³ below,

The synthetic crude yield is then upgraded (hydroprocessing—cracking and separation) to produce the commercial product, thereby allowing the refiner to effectively design a fuel based upon the desired chain lengths. Depending on the refinery process employed, it is possible to develop synthetic kerosene suitable for aviation. The process is shown below in Fig. 3.

Fig. 3 Synthetic product production process.²⁵

Experience gained through the operation of commercial cTL and gTL plants is expected to enhance bTL technology development and, hence, environmental benefits. This, however, may be hindered by the availability and transportation of the very large volumes of biomass feedstock required, due to the inherently low energy density and corresponding lower carbon yields of biomass feedstock.²⁶

3.2 Lifecycle analysis

The lifecycle results taken from the PARTNER program (continuation of Wong's work) are illustrated in Fig. 4. Biomass derived fuel is included for completeness and shall be discussed later in this paper.

Fig. 4 CO₂ equivalent lifecycle analysis.²¹

Conventional (i.e. crude) and unconventional (i.e. sands and shale) refinement of oil into product is labelled as ‘Jet Fuel’ in the above Figure. Further processing during crude refinement produces an ultra low sulfur fuel, identified above as ‘ULS Jet Fuel’. ‘F-T fuel’ refers to synthetic kerosene produced via the Fischer Tropsch process, using coal, gas or biomass as feedstock. An alternative product produced through the hydroprocessing of biomass oils is entitled ‘HRJ’ or ‘Hydroprocessed Renewable Jet’ Fuel.²¹ Refinement of product using hydroprocessing is introduced later in this work. For simplicity, it is assumed that combustion CO₂ is comparable across fuel types as, compositionally, all fuels are comprised of similar hydrocarbons (i.e. drop-in fuels). The tank to wake CO₂ released during biofuel combustion is assumed to be similar to that absorbed during plant growth, thus resulting in approximately zero net atmospheric change.^20,21 Note that carbon capture techniques (CCS) involve sequestration and storing of the CO₂ produced during the FT synthesis process.

The results clearly demonstrate that refinement of feedstock readily available in sufficient quantity to provide secure supply (i.e. unconventional oils and other fossil fuels) produces similar lifecycle emissions to that of conventional crude oil refinement.

3.3 Recent activity

Sasol, despite being the only petrochemical company certified to produce a synthetic product for commercial aviation at the time of writing, is not alone in the industry. Over the past year, the media has highlighted numerous full scale, proof of concept test flights. These campaigns have involved operating a single engine on a synthetic fuel, produced from coal, gas or biomass, blended with Jet A-1. Based on the previous discussion, the test campaigns may be categorised depending on their program objective, i.e. to demonstrate incident free operation and the level of technological readiness of product, which either provides for supply diversification or improved environmental credentials. Research which has involved the commercialisation and/or testing of alternative products available in sufficient quantities to provide ensured supply is reviewed below.

Sasol Petroleum (cTL). Air traffic to South Africa, notably Johannesburg International Airport (JIA), has been increasing substantially since 1994.²⁷ The airport's ability to meet this increasing demand was restricted by the supply of jet fuel. Two product sources that supplied JIA, however, had volumetric and logistical constraints that meant both were predicted to meet maximum practical output levels in 1998. The situation of continually increasing air traffic with insufficient fuel supply created an urgent need to secure jet fuel supplies for JIA.¹

Political circumstances, therefore, led to a ‘special case’ approval given to the South African based Sasol Petroleum Company to supply alternative synthetic product produced from a coal based feedstock to JIA. The certification, granted in 1999, allowed for a maximum of 50% synthetic product to be mixed with normal crude derived Jet A-1. More recently (2008), further approval has been granted for Sasol to supply a fully synthetic product to JIA.¹ A consequence of Sasol's efforts to seek approval of synthetic kerosene was the development of a fuel approval blueprint. The experience gained has paved the way for the industry to allow faster approval of alternative drop-in product and, thus, entry in to service.^7,28

Shell Petroleum (gTL). Application of Fischer Tropsch technology to convert stranded gas fields to liquefied product has created considerable interest as a means of securing supply. The technological readiness of gas derived alternative fuel was first demonstrated by the Shell, Rolls Royce and Airbus research consortium. The consortium operated an Airbus A380 from Filton (UK) to Toulouse (France) on 37.4% synthetic product (1st Feb 2008). The test flight generated valuable data which supported the evaluation of gTL kerosene through the fuel approval mechanism (ASTM 4054).³

Generic certification of synthetic kerosene through ASTM D7566 provided approval for Qatar Airways to operate a commercial flight using a 50/50 blend of synthetic kerosene and Jet A-1. The Airbus A340-600 aircraft, powered by four Rolls Royce Engines, operated out of Gatwick (UK) to Doha (Qatar). Unlike other experimental flight campaigns, all engines on this commercial flight were fuelled with the alternative blend.^11,29

The consortium cited fuel diversity, the production of a premium product and its environmental impact as key drivers.³⁰ Specifically, synthetic kerosene may improve range-payload options due to a slight increase in energy density (43.6 MJ kg⁻¹, gTL blend) compared to a typical Jet A-1 (43.2 MJ kg⁻¹).^3,11 Improved local airport air quality is also expected, as combustion yields a reduction in sulfur dioxide and particulate matter (PM) emissions.²⁹

The airline is expected to operate regular commercial flights once the supply of gTL kerosene has be secured through the Shell–Qatar Petroleum Pearl Project, which is scheduled for completion by the end of 2010. Supply of gTL kerosene is anticipated to be available from 2012, after the 12 month ramp-up period required to bring the plant online.¹¹

United States Air Force. Similar to South Africa, the United States has considerable coal reserves, but limited access to domestic crude oil (2.4% of proven global reserves³¹). Limited crude resources require the US military to rely on foreign oil reserves, thus creating potential supply issues due to political pressure. Reliance on foreign oil reserves has been recognised by the US Air Force as an operational weakness, thus resulting in considerable recent effort to identify a suitable domestic crude oil alternative through the Assured Fuels Initiative.³² Practicalities in terms of ensuring sufficient product availability and an option for domestic feedstock supply resulted in the Air Force sourcing synthetic fuel produced via the Fischer Tropsch process.

In order to introduce synthetic product, the US Air Force needs to ensure the operational reliability of its fleet. Similar to the Sasol certification case, this requires many hours of component and engine testing, along with a final certification flight. The first aircraft to be approved for operational readiness was the B-52 in August 2007. The B-52 has since been followed by several other transport and fighter platforms.³³ Initially, certification was based on fossil derived synthetic fuels produced by Syntroleum (cTL) and Shell (gTL). In the near–mid term, it is expected that domestic coal supplies will likely satisfy the Air Force’s fuel requirements; however, legislation capping pollution levels of an alternative fuel to that emitted by current product (JP-8) may cause issues. The Air Force, however, has stated that it is interested in assessing other feedstocks, including biomass, to ensure supply and increase its environmental credentials.³²

AAFEX. More recently in the United States, the AAFEX III campaign in Palm Dale California (19th Jan–7th Feb) focused on measuring the effect of fossil derived FT fuel (Sasol—cTL and Shell—gTL) on engine and APU emissions. The experimental platform utilised two of four CFM56-2C1 engines fitted to a NASA DC-8 research aircraft. The campaign involved measurements both close to the exhaust plane and at two downstream locations, thus providing data for the evolution of PM and gaseous emissions.

The aforementioned activity, which has demonstrated the capability of increasing supply, is achieved without improving lifecycle emissions below that of conventional jet fuel refinement and combustion.

4. Environmental sustainability

The environmental profile of the aviation sector has provided an additional driver to develop a greener drop-in alternative fuel. To date, the development of such fuels has involved the conversion of biomass, instead of fossil based feedstock into a liquid drop-in fuel. Technological readiness, however, is significantly lower than the fossil pathways, with production capability far from a commercial stage.

4.1 Processing pathways

Experience gained through the refinement of fossil feedstock using the Fischer Tropsch process (cTL and gTL) provides a stepping stone towards the cleaner bTL technology. The identification or development of sufficient biomass feedstock and the lower technological readiness of the process, however, present significant hurdles to be overcome.

A technology at a higher readiness level, recently used to produce fuel for a series of test flights, is based on the hydroprocessing of vegetable oils, which are broadly a triglyceride mixture. Hydroprocessing is employed in the conventional refinery process to remove undesirable materials, including nitrogen, sulfur and residual metals (hydrotreatment) and break down carbon chain lengths (hydrocracking).³⁴ The process is illustrated in Fig. 5.

Fig. 5 Hydroprocessing of vegetable oil.^34,35

The biomass oil based synthetic jet fuel production process involves removing oxygen molecules and other undesirable materials (i.e. heavy metals) from the triglyceride,^26,36 through hydrotreatment. Subsequent selective hydrocracking and rearrangement of the atomic structure (isomerisation) yields lighter hydrocarbons suited for aviation (Jet A-1: C8–C16 ²⁶). Aromatics are blended into the synthetic product to yield a Jet A-1 drop-in replacement.

The process, however, is more economically suited to producing diesel, as most triglyceride producing plants have a natural carbon number closer to that of diesel (between C14–C20 ²⁶), thus eliminating the need for selective hydrocracking. Biologically engineering feedstock to produce a carbon range more suitable for aviation may improve the economics of this process in the future. However, simply obtaining enough feedstock to supply the aviation market is a significant issue.

4.2 Lifecycle analysis

Fuel sourced from biomass feedstock has, until recently, been thought to provide substantial emission savings, when compared to petroleum derived products. The simple reasoning behind this is that emissions released during biofuel combustion are absorbed from the atmosphere throughout plant growth, thus resulting in a zero net field–wake emissions footprint. It is important, however, to consider the whole process involved in producing the biomass feedstock. For example, increasing feedstock prices (palm oil has doubled in the last three years) provides an economic incentive to acquire additional land to site plantations. Clearing land to keep up with demand releases previously trapped CO₂ back into the atmosphere, thus disturbing the carbon balance.¹⁵ Considering that biofuel crops typically have poor CO₂ absorption rates compared with the original growth, it is quite likely that there will be a net increase in atmospheric CO₂. Depending on the original growth, it is probable that the released CO₂ will be higher than if a traditional fossil fuel was combusted.

The work of Wong²⁰ and that of the PARTNER Program²¹ demonstrated that land use change through the cultivation of biomass has the potential to release significant greenhouse gas (GHG) emissions.³⁷ The GHG emissions resulting from biomass cultivation is largely dependent on previous land usage, as illustrated via numerous scenarios considered in the PARTNER program. The first scenario (LUC P0) assumes that no land use change occurs, meaning that all emissions result from production, refining and transportation. The second scenario involves the conversion of previously logged forest to palm plantation (LUC P1). Scenario three involves the conversion of tropical rainforest to palm plantation fields. The final scenario involves the conversion of peatland rainforest to palm plantations. The land use change scenarios are summarised in Table 1 and presented in Fig. 6.

Fig. 6 Land use change scenarios: palm oil.²¹

Table 1 Land use change scenarios²¹

Soy oil to HRJ fuel		Palm oil to HRJ fuel
LUC S0	No land use change	LUC P0	No land use change
LUC S1	Grass land conversion to soybean field	LUC P1	Logged over forest conversion to palm plantation field
LUC S2	Worldwide conversion of non-crop land	LUC P2	Tropical rainforest conversion to palm plantation field
LUC S3	Tropical rainforest conversion to soybean field	LUC P3	Peatland rainforest conversion to palm plantation field

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The adapted Fig. 6 illustrates CO₂e lifecycle emissions for a hydroprocessed renewable jet fuel based on a palm oil feedstock. Conventional crude derived jet fuel is included for comparison. The ‘biomass credit’ shown in the Fig. 6 legend represents the CO₂e absorbed during plant growth, while the effect of ‘land use change’ on CO₂e emissions is summarised through the introduced scenarios (Table 1). ‘Other’ refers to the CO₂e emissions not related to land use change, as illustrated in Fig. 4.

On review of the lifecycle results presented earlier (Fig. 4), the work clearly demonstrates that sustainable farming is crucial in yielding any well to wake emission reductions below that of a fossil fuel product. The research also shows that it is the land use change required for feedstock cultivation that defines a good or bad biomass and not the cultivated crop.† The variability of lifecycle emissions based on cultivation practices presents a considerable issue when analysing the environmental benefits of refining feedstock sourced from different geographical locations.

The literature demonstrates that although emissions may be reduced through the introduction of an alternative product, care must be taken to uncover the entire ‘well to wake’ profile before assessing the product’s true environmental impact. Unless sustainable biomass feedstock can be sourced and grown on marginal land without significant clearing, the current state of the art is likely to produce similar emissions to that produced via conventional crude refinement and combustion.

4.3 Recent activity

Diversification away from petroleum derived products in an effort to improve environmental credentials has been a key driver behind the pursuit of biomass feedstock in recent test flight campaigns. In each case, the airlines sourced either readily available biofuel in the marketplace or selected suitable feedstock and had it converted into a test fuel component for blending. As with the supply security categorised campaigns, the biomass flight programs demonstrated both incident free operation and process technological readiness through blend operation on a single engine. Research which may be categorised as improving the environmental footprint of aviation is reviewed below.

Virgin Atlantic. Richard Branson's airline, Virgin Atlantic, was the first commercial carrier to successfully demonstrate that flight on alternative biomass derived jet fuel was possible.³⁸ The non-revenue flight between London and Amsterdam on the 23rd February 2008 was operated with one of the four Boeing 747-400 engines (General Electric) fuelled with a blend of Jet A-1 and 20% coconut and babassu palm oil.

Virgin stated that the feedstock was chosen carefully so that it did not compete with food production. However, the airline received considerable backlash over the sustainability of their test flight.^39–41 Boeing has, in fact, provided a disclaimer stating that the FAME‡ fuel used in the Virgin flight was the only suitable fuel available at the time and that it did not consider FAME a viable option for aviation. The test flight, however, did prove a key objective in successfully demonstrating alternative fuel operation and perhaps paved the way for more recent developments.

Air New Zealand. Armed with the knowledge of the importance of sustainable fuel selection, Air New Zealand defined three feedstock selection criteria. The criteria required biomass and hence the refined product not only to satisfy economical and technical factors, but also social responsibility.² The latter factor was demonstrated through the selection of a suitable feedstock which did not compete with food consumption. Social responsibility feeds back into the environmental footprint of the alterative product, as demonstrated by the presented lifecycle results;^20,21 unsustainable farming practices (i.e. clearing forest to plant crops) have a high probability of increased lifecycle CO₂e emissions compared to conventionally refined crude product.

In the case of Air New Zealand, Jatropha—a plant which, when grown under favourable conditions, produces high oil content inedible seeds^42,43—was identified as a feedstock which matched the key selection criteria. Hydroprocessing techniques were employed to convert the seed oil into a drop-in jet fuel, which was then blended with 50% Jet A-1. The test flight, which took place on the 20th December 2008 between Auckland and Wellington (New Zealand) involved one of the airline’s Boeing 747-400 aircraft. The non-revenue flight was fuelled so that one of the four Rolls Royce engines operated on the 50/50 fuel blend.

Engine parameter analysis from the test flight has shown a potential fuel burn saving of 1.2%.⁴⁴ However, limited flight data exists to extrapolate this conclusion (i.e. single test flight). The result, however, is supported by data from UOP LLC, which shows that the energy content of a Jatropha based hydroprocessed fuel is 44.3 MJ kg⁻¹, compared to 43.2 MJ kg⁻¹ (typical value) for Jet A-1.^3,45 Regardless, the Air New Zealand flight successfully demonstrated that extracted oil, in this case from Jatropha feedstock, could be converted into a drop-in jet fuel through hydroprocessing.

Continental Airlines. The technological readiness of hydroprocessing technology was further demonstrated through the Continental Airlines flight program, which operated a twin engine aircraft (Boeing 737-800) out of Houston on the 7th January 2009. One of the two engines was powered using a 50/50 blend of conventional jet fuel and a mixture of Jatropha (47.5%) and algae (2.5%).

The airline became the first to demonstrate operation of an algae feedstock, which has been cited in the literature as having the potential to provide a higher yield than land based crops.⁴⁶ Potential algae yields are, however, masked by much uncertainty, as will be discussed later in this work.

Japan Airlines. Japan Airlines has also demonstrated the feedstock independence of hydroprocessed jet fuel through the incident free operating of one of its Boeing 747-300 aircraft out of Tokyo. The round trip test flight, which departed on the 30th January 2009, operated one of four engines on a 50/50 blend of conventional jet fuel and hydroprocessed biomass feedstock, consisting of Camelina§ (42%), Jatropha (8%) and algae (<0.5%).

The success of the recent test campaigns has not only highlighted gas turbine biomass product compatibility, but also demonstrates the technological readiness (expected 2010 approval) and feedstock independence of the hydrotreatment process.

5. Discussion

The technological readiness of refinery techniques required to produce drop-in alternative product, which satisfy the baseline specifications and performance characteristics outlined in the fuel approval protocol, has clearly been demonstrated. Unfortunately, efforts which focus on increasing supply through the conversion of fossil feedstock fail to yield a direct lifecycle benefit when compared to Jet A-1 refinement.

Lifecycle analysis demonstrates that emissions produced during the cTL process are significantly higher than that of conventional jet fuel, raising concern regarding the environment and the cost of carbon in the future marketplace. In fact, the literature reports that Sasol's efforts to secure supply—thereby ensure continually operation of JIA—are responsible for creating the world's largest single source of CO₂.⁴⁷ The emission footprint, however, is by no means exclusively related to aviation fuel. The advent of cTL carbon sequestration (best suited for such concentrated emission sources) may improve Sasol's environmental credentials. However, lifecycle research shows that emissions would remain higher than traditionally refined crude product.²⁰ Research through the OMEGA program also reports that fuel-energy cycle (i.e. MJ input per MJ product) requirements would be over eight times higher than traditional crude refinement.¹⁹

The use of natural gas as feedstock provides for cleaner application of the Fischer Tropsch process, with lifecycle analysis demonstrating that expected emissions are no higher than a modern crude refinery complex. Natural gas feedstock, therefore, provides for supply diversification at substantial production capacity without an environmental penalty and is hence considered key to reducing the dependence of crude derived kerosene.

The literature reports that tank to wake emissions resulting from the combustion of Fischer Tropsch fuels are expected to be lower than that of a conventionally fuelled aircraft, largely due to the increased energy density and almost nil sulfur content of a Fischer Tropsch fuel. Improved thermal stability is also expected to provide for efficiency improvements in future engine designs. Experimental work conducted by the USAF using a T63 gas turbine engine (helicopter) has also demonstrated that increasing the percentage blend of a Fischer Tropsch product in neat JP-8 reduced both the PM count and size.⁴⁸ Particulate matter exposure can be harmful to humans,⁴⁹ thus reduction in local airport PM concentration may provide significant local health benefits. Reduced PM at cruise altitude may also reduce cirrus formation, which is suspected to be induced indirectly by particle emissions acting as cloud condensation nuclei. A significant degree of uncertainty, however, masks the relationship between partial emissions and cloud formation.^50,51 It is expected that these physical–chemical fuel benefits will be evident in other production techniques, which closely match the final hydrocarbon composition of a Fischer Tropsch fuel.

Lifecycle emission reductions below that of conventional crude derived kerosene requires diversification away from fossil feedstock. Introduction of biomass feedstock into the Fischer Tropsch process provides, on paper, at least, for significant emission saving. However, the process technology is currently at a pre-commercial stage. The flight test campaigns, therefore, focused their research efforts on identifying biomass feedstock which could be refined using technology at a higher readiness level. The environmental profile of each test flight campaign is illustrated in Fig. 7.¹⁹ The Figure shows that for the latest test campaigns, the hydroprocessing of sustainable oils provided a little over 30% reduction in total emissions.

Fig. 7 Lifecycle GHG emissions.⁵⁶

Despite the potential emission reductions and optimism regarding the availability of biomass in some of the literature,⁵² an evaluation of land requirements demonstrates the issue of refining an environmentally sustainable fuel in sufficient volumes to support the aviation sector. Fig. 7 demonstrates a clear distinction between fossil and biomass feedstocks, with the earlier being available in sufficient volumes to augment crude oil supplies, thus providing security of supply. The latter, despite providing environmental benefits, has raised concerns in terms of the production capability and resource utilisation,^15,36 especially considering the vast feedstock requirements for even a two hour test flight. For example, the Virgin Atlantic cocktail of coconut and babassu palm oil required the equivalent of 150,000 coconuts to produce the 5% biofuel burned¶ during the one hour crossing of the North Sea.³⁶

Considering the best case arable land||Jatropha dry seed yield of 4–5 T ha⁻¹ yr^−1 42 as an example, 6% of available arable land (which represents 0.95% of the world's surface) is required to supply the sector with a 50/50 blend of hydroprocessed product mixed with conventional jet fuel.** If Jatropha is cultivated on marginal land, the required global surface area increases to 3.2%. At present, the industry makes up only 2.6% of global CO₂ emissions, hence it seems impractical, even unethical (through the resulting offsetting of food production), to allocate such land volumes to aviation, especially considering that the best case test flights only demonstrate a 30% reduction in CO₂ emissions, meaning that the above example would reduce global CO₂ emissions by 0.78%. If market forces come to dominate the alternative fuel market, as is expected through the emissions trading schemes, feedstocks will first be allocated to other industries (i.e. power generation) that require less processing (i.e. energy), hence yielding greater returns and increased emissions savings.

In order to secure biomass for the aviation industry, thereby ensuring supply of sufficient volume and reducing the sector’s environmental footprint, a higher feedstock yield or waste feedstock is required. Interest in the former has been dominated by algae, largely due to claims of considerable lipid yields (biofuel feedstock), high growth rates and low land requirements.⁴⁶ The literature reports oil yields from anywhere between 30 ⁵³ to 200 ⁵⁴ times the amount of oil per unit of land area, compared to terrestrial oil crops. Despite these claims, the majority of high growth species have low lipid yields. Conversely, high lipid yields are characteristic of slow growing species.⁴⁶ The literature reports that numerous techniques involving non-lethal lipid extraction may improve yield.⁴⁶ However, much of this work is at an embryonic stage. Algae as a feedstock is thus in its infancy, with much research required to assess species and cultivation practices to establish whether the high yield requirements for a drop-in aviation fuel are obtainable.

Biomass feedstock left over from cultivation practices or the forestry industry, for example, has been highlighted in the literature as potential feedstock for greener aviation fuel.⁵² Conversion of this waste biomass product through the Fischer Tropsch process (bTL) has the potential to yield further reductions in the emissions footprint, compared to hydroprocessing.^20,21 The effect of waste feedstock removal needs to be quantified, as feedstock reuse helps to offset fertiliser requirements in agricultural practices. The technology, however, is still in the pre-commercial phase with a pilot plant operation in Germany. Despite the future potential of this technology, as with hydroprocessing, sustainable feedstock needs to be available in sufficient volumes so that environmental benefits are realised on an industrial scale. The more omnivorous nature of the bTL process, however, allows for a much wider range of feedstocks (including cellulose) to be processed, whereas hydroprocessing relies on the more limited supply of vegetable oils. However, as commented by Rutherford,⁵⁵ the aviation sector will have to compete with other transportation modes and/or sectors to secure this supply.

The energy required to convert feedstock into product raises a question regarding the opportunity cost, and hence the appropriateness of using feedstock to produce aviation fuel. For example, hydroprocessing requires selective hydrocracking and isomerisation to yield a product suitable for aviation, while due to the natural carbon lengths of most feedstocks, a diesel product could be produced without the need for hydrocracking (i.e. hydrotreatment only). The production of diesel is therefore expected to yield lower lifecycle emissions, thus demonstrating the need to consider an energy balance for fuel production. Fig. 8 summarises work conducted through the OMEGA program, which illustrates the required energy input to produce 1 MJ of product.

Fig. 8 Lifecycle energy usage (MJ/MJ product).¹⁹

If substantial aviation emission savings can not be achieved using biomass feedstock, an efficient market and ‘cap and trade’ system should logically reallocate the feedstock to another industry and/or sector, based on the highest level of abatement at the lowest cost. In other words, product should ideally be allocated based upon lifecycle emission savings, along with the energy opportunity cost to yield these emission savings. Practically, however, in order to ensure supply security to all industries, a compromise is expected to be made between long term supply stability and the environmental impact.

6. Conclusion

Research into the development of alternative aviation fuels is being driven by ensured supply and/or the sector’s environmental footprint. The technological readiness of refinery technology demonstrated in the flight test campaigns shows that at present, product may be refined from different fossil or biomass feedstocks using the Fischer Tropsch or hydroprocessing technologies, respectively. The former case provides for supply diversification, as sufficient volumes of feedstock are available to support the sector. The second case provides for emission reductions, provided sustainable biomass is sourced. Unfortunately, cultivating sufficient feedstock to support just the commercial aviation industry with a 50/50 blend is calculated to require 6% of the world's available arable land. If sufficient feedstock was cultivated and could be secured by the aviation sector, it is expected this would save approximately 30% of CO₂ emissions, or 0.78% of total global CO₂ emissions. The opportunity cost, therefore, to reduce aviation emissions by powering the world’s fleet on 50% biomass derived product is unacceptably high, especially considering that there are lower cost alternatives which require substantially less post-processing and, hence, yield greater lifecycle benefits (i.e. power generation).

In the near term, it is therefore expected that drop-in alternative fuels appearing in the marketplace will be refined from either gas or coal feedstock, thus ensuring a sufficient volume of product to augment crude derived Jet A-1. Fossil fuel supplies are, however, limited, meaning that crude, gas and coal reserves will eventually become uneconomical to extract—although supplies of gas will outlast petroleum, proven coal reserves are more plentiful than both natural gas and oil. Economic incentives through the introduction of emission trading schemes are expected to favour the uptake of gTL, as opposed to cTL technology. Such incentives are also expected to drive the economic case for the long term development of biomass derived fuel, which has been demonstrated to provide a lower upstream footprint. Therefore, despite current biomass feedstocks being impractical in terms of yielding large scale emission reductions for aviation, it is expected that the sector will continue to search for and develop alternative high yield per tonne biomass feedstocks, which are available in sufficient quantities to provide for supply diversification and safe operation at the best opportunity cost. It should be realised, however, that large scale emission reductions of a drop-in fuel are only possible upstream of the combustion process. Thus, a reduced emission profile requires the identification of sustainable feedstock and upstream technological developments and/or improvements. Once such a feedstock is identified, it is expected that advancements in the Fischer Tropsch process will provide the sector with both supply security and a reduced lifecycle profile.

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Footnotes

† Certain feedstock crops have a higher yield per hectare, thus all else being equal, such crops could be considered as ‘better’ energy crops.

‡ Fatty acid methyl ester—this contaminate is currently limited to 5 ppm in the jet fuel specification.

§ A weed like flowering plant which does not directly complete with food consumption

¶ 22 tons of total fuel burn (5.5 tons per engine), equating to 1.1 tons or 5% biomass product (20% blend in one engine)

|| Jatropha yield on marginal land is expected to be significantly less than arable land, with the literature reporting figures as low as 1.5 T ha⁻¹ yr⁻¹ (0.5 T oil ha⁻¹ yr⁻¹, given 34% of seed mass content oil).^36,37

** Commercial aviation fuel consumption: 6.8 million barrels per day (2007)¹⁶

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