High Biofuel Blends in Aviation (HBBA)
The HBBA study was performed in accordance with tender ENER/C2/2012/ 420-1 “High Biofuel Blends in Aviation”. The purpose of this research was to analyse the properties of bio kerosene blends with various samples of conventional kerosene, with a focus on blends with high percentages of bio kerosene.
To learn more about the study, you can also view the relevant presentations delivered at the HBBA Study and BioJetMap Workshop in Brussels on 11. February 2015:
- Background and Fuels used [Download]
- Properties of Bio Kerosene Blends [Download]
- Material Compatibility [Download]
- Effects on emissions [Download]
- Conclusions [Download]
The issue of using high biofuel content blends in aviation might seem to be of merely academic relevance given that the share of bio kerosene of the overall aviation kerosene market is not expected to be more than a few per cent in the next ten years. However, for the practical introduction of bio kerosene it will be relevant to know about high biofuel blends even at an early stage in order to minimize blending and logistical costs.
The reason here is that for every bio kerosene blend three analyses have to be performed before it may be used in commercial aviation:
- An ASTM D1655 analysis of the conventional kerosene before blending
- An ASTM D7566 analysis of the neat bio kerosene before blending
- An analysis of the blend, which is described in ASTM D7566, but in practice is an analysis of the ASTM D1655 parameters, plus some additional ones.
It takes about 20 man hours to perform a full ASTM D1655 analysis, and requires use of specialized and expensive equipment. The cost of such an analysis is therefore thousands of Euros. In the case of the first two analyses this cost is independent of the blend ratio, and will typically be performed for large batches of thousands of tons, so the cost per ton is only a few Euros, which is a normal cost element of selling and shipping kerosene. In the case of the analysis after blending, however, the cost impact per ton of biofuel is crucially dependent on the blend ratio. If blend ratios of only a few per cent are used, the cost for the analysis will be incurred for selling only a few tons of bio kerosene, leading to very high costs per ton. At high blending ratios, on the other hand, these costs will not be an issue. In addition, use of low blend ratios imply that large volumes of conventional kerosene have to be transported to the blending point, making logistics complex and expensive, as well as potentially creating environmentally undesirable extra transports of the conventional kerosene.
For a producer or blender of biofuels it will therefore be important to know how much bio kerosene can possibly be blended, and how blending percentages can be maximized, even while the bio kerosene market is at an early stage. It will also be relevant for governments and communities involved in the planning of logistics and blending capacities.
The specification relevant for bio kerosene in Europe is DefStan 91-91, which however for alternative fuels mirrors the US ASTM approval process. Thus, for practical purposes, the specifications of interest for this study are ASTM D1655 and ASTM D7566. Of these, ASTM D1655 covers kerosene in general, whereas ASTM D7566 specifically covers alternative fuels and its blending with conventional kerosene. According to these specifications, the maximum amount of bio-synthetic kerosene that can be mixed with conventional kerosene is currently restricted by two factors:
- The requirement that the content of bio-synthetic kerosene does not exceed the maximum percentage permitted by ASTM D7566.
- The requirement that the blend has to meet the same parameters as conventional ASTM D1655 kerosene, plus some additional ones
The first requirement is an arbitrary one, based solely on caution. It is the explicit intention of ASTM to eventually relax this restriction. The second requirement however is based on technical considerations – every specification parameter of ASTM D1655 is there for a reason, and this reason will not go away with the introduction of biofuels. Even when the formal maximum limit for synthetic kerosene will be removed by ASTM, the maximum bio kerosene content possible will be limited by the ability of the bio kerosene to meet the ASTM D1655 parameters when blended with a given conventional kerosene.
However, as ASTM D1655 specifies minimum or/and maximum values for fuel parameters rather than defined values, conventional kerosene properties cover a rather broad range. The maximum possible blend ratio for bio kerosene therefore does not only depend on the bio kerosene, but also on the conventional kerosene. This is because conventional kerosene that is comfortably within specification limits can be used to compensate unfavorable properties of a neat bio kerosene, and still produce an on-spec blend.
For this study, therefore, a total of five different conventional kerosene samples, covering a broad range of properties, were used for blending with bio kerosene. The range and distribution of properties observable for conventional kerosene, and details of the conventional kerosenes used in this study, are described in chapter 2 and appendix 1 of the study.
For the biofuels to be analysed in this study, the original intention had been to primarily use three different samples of HEFA bio kerosene, and in addition investigate only a very limited amount of other kinds of bio kerosene blends, as it was assumed that only HEFA would be available in sufficient volume to permit an extensive blending programme. However, as the project progressed it became evident that development of alternative fuels was progressing faster than originally assumed, and samples for most of the relevant production processes were actually available. At the same time evidence showed that different HEFA samples would be very similar to each other, such that analyzing three different samples would merely produce three sets of basically the same results. The scope of the study was therefore modified such that only one HEFA sample was used, and instead samples of bio kerosene from a variety of pathways were included. A description of the bio kerosenes used in this study is given in chapter 3 and appendix 2. In addition, chapter 3 gives a technical description of the production pathways and the certification status for all bio kerosene production pathways either already certified or undergoing ASTM certififcation, including these pathways for which no samples could be obtained for inclusion in the analytical part of the study.
The results of the blending analyses are presented in chapter 4 and appendix 3. The topic of interest here was the relationship between parameters of the blend and the blend ratio. For some parameters this relationship is linear, such that e.g. blending neat bio kerosene with a parameter value of 80 and neat conventional kerosene with a parameter value of 70, using a blending ration of 50%/50%, results in a blend with a parameter value of 75. Such relationships, which in particular were typically observed for volume or mass related parameters (like density or sulfur content), are straightforward and pose no particular challenges from a blending point of view. However, for other parameters the relationship is non-linear and more complex, and we feel that it is with regard to these parameters that this study will be of particular use for practical blenders and users of bio kerosene. For two parameters, lubricity and freezing point, we even found cases where the parameter value of the blend actually went beyond the range defined by the parameters of the two original fuels.
Further lab analysis
One major factor currently limiting maximum biofuel blend ratios is aromatics content. This is because ASTM D7566 requires a minimum aromatics content for the blend of 8%, but several of the bio kerosene production pathways yield a fuel with virtually zero aromatics content. For these fuels, all aromatics must come from the conventional kerosene. However, aromatics content of conventional kerosene is limited by ASTM D1655 to a maximum value of 25%, so any blend with more than 68% of bio kerosene must have an aromatics content of below 8%, and hence be off-spec (To be precise, there are two alternative ways of measuring aromatics content, ASTM D1319 and ASTM D6379. If the first is used, minimum aromatics content of the blend is 8% and maximum aromatics content is 25%. If the second is used, the respective figures are 8.4% and 26.5%). Moreover, as is shown in chapter 2, the typical aromatics content of conventional kerosene is well below the maximum figure, hence the practical limit for blend ratios is well below 68%.
There is potentially a simple way around this obstacle, which is by adding aromatics. It is to be assumed that this route will be pursued in the future, and indeed one of the fuels currently up for ASTM approval consists almost solely of aromatics and is explicitly designed as such a blending com-ponent (see section 3.9). However, the addition of aromatics to the fuel will in itself alter the properties of the blend, hence it was considered relevant for future blending applications to assess what these effects are likely to be. Accordingly, for each of the three bio kerosenes concerned (FT-kerosene, HEFA, ATJ) two high-level blends were produced, and then aromatics were added to increase their content to the minimum value required by ASTM D 7566. The results of this research are described in chapter 5 and appendix 4.
Materials compatibility tests
In addition to analyzing the properties of the various bio kerosene blends, the study also investigated the influence of the different synthetic fuels on the elastomers of which the seals in fuels systems typically are composed. This analysis of materials compatibility was conducted on seals from Nitrile-Butadiene Rubber, Fluorosilicone Rubber and Fluorocarbon Rubber. For these tests no blends were used; instead, the elastomer material was exposed to the neat bio kerosenes, as well as for reference to the various conventional fuels used in this study.
The effect of fuel on seal tightness is generally attributed to the aromatics content. To verify the effect of aromatics on elastomers, and to investigate the role of different kinds of bio kerosene, aromatics were systematically added to the aromatics-free bio kerosenes, and the tests repeated.
In a final step, the elastomer materials were first exposed to the conventional fuel with the highest aromatics content, and subsequently exposed to the neat aromatics-free bio kerosenes, simulating a situation where an aircraft has been operated on conventional kerosene and is then exposed to bio kerosene.
Aircraft engine emissions tests
In addition to the safe functioning of a bio kerosene, which is extensively investigated during the ASTM approval process, another technical aspect of bio kerosenes is their emissions behavior. Consideration of emissions is not part of the ASTM fuel certification process, and indeed it would be very difficult for ASTM to include emissions in a fuel specification, since emissions are primarily dependent on the engine the fuel is burned in. Emission measurements therefore are not a required part of the ASTM process, and little emissions data is typically presented in the research reports submitted to ASTM. All the same, some fuels will burn cleaner in a given engine than others. It is therefore of interest to see whether biofuels will lead to an improvement of the emissions of a given engine. For that reason, emissions tests were included in the program of this study.
A first set of tests was conducted in November 2013, using farnesane. The results of this test are presented in section 7.1. However, lack of availability then postponed further testing until November 2016, when emission measurements for CH kerosene were conducted. This is a fully synthetic kerosene including aromatics, permitting the test to be conducted using unblended bio kerosene. The results of this test are presented in section 7.2. These tests showed that - except for specially designed aromatic material as described in section 3.9. - a bio kerosene with a similar content of aromatics as fossil kerosene also will have a similiar amount of emissions, demonstrating that the aromatic compounds play a key role in particulate emissions. The detailed conclusions from the emissions tests are presented in section 7.3.
The conclusions of this study are presented in chapter 8. Section 8.1 explores the results of the study by fuel properties, discussing which properties are expected to be critical for future blend ratios of bio kerosene, but also discussing properties which are not likely to be critical for blending but where the relationship between the blend ratio and the property was unexpected. The latter are not relevant from a bio kerosene blending point of view, but since these phenomena are as yet unexplained they are potentially of interest for more fundamental research. Section 8.2 explores the same results by fuel, discussing which role the individual kinds of bio kerosene are likely to play in future blending activities.
One particularly critical property is aromatics content. It is critical not only because several bio kerosene production pathways result in fuel that is virtually aromatics-free, but also because the role of aromatics is a two-faced one, with aromatics being currently necessary to preserve the tightness of fuel systems but on the other hand being undesirable from a fuel burn and emissions point of view. This specific role of aromatics is discussed in section 8.3.