An Investigation of the Usage of Biofuelsin the Aerospace Sector

Roger Antony Francis, 996765255

University of Toronto Institute for Aerospace Studies

4925 Dufferin Street, Toronto, Ontario, M3H 5T6, Canada

Sustainable Aviation Final Report

April 2014

I. Introduction

At the beginning of the jet age, the biggest drivers of aircraft design were speed and luxury. Over time, efficiency has come

to bear the most influence in the design of new aircraft and propulsion units, primarily for the purpose of reducing green house

gas emissions. In addition to the implementation of more aerodynamic and lighter aircraft, todays gas turbines are highly

efficient (as high as 99% combustion efficiency [1]). Furthermore, the drive towards efficiency has even changed how air traffic

controllers direct planes as they fly into and out of airports. Despite all this, the one factor that has remained constant over the

years, is the fuel that is used.

While the aerospace industry has taken significant steps in improving the efficiency of aircraft -there has been a 70%

improvement in fuel efficiency since the 1970s [2], current technology can only take us so far. Most of these improvements were

obtained through years of optimizing the current wing and tube design, with subsequent attempts to extract further efficiencies

proving more and more difficult. As such, there are few options left that the aviation sector can implement, to reduce emissions

in the near to medium term without requiring extensive changes to aircraft and infrastructure. Furthermore, with gas turbine

technology not expected to change dramatically for the next decade, the best solution is to develop green fuels, that can be

used in the current generation of gas turbines.

II. Motivation

The aviation sector currently produces around 2% of the worlds man-made CO2, or about 12% of CO2 emissions from

transportation based sources [2]. Given the forecasted growth of the aerospace sector, the Air Transport Action Group (ATAG)

has developed a set of ambitious targets aimed at limiting its climate impact, while enabling the sector to continue to be a key

vehicle for economic growth. The targets include: improving fleet fuel efficiency by 1.5% per year until 2020; capping net

aviation emissions from 2020; and to halve aviation CO2 emissions by 2050, compared to 2005 [2].

Simultaneously, a number of governing bodies such as the European Committee for Standardization (CEN), British Ministry

of Defense (MoD) and the American Society for Testing and Materials (ASTM), have developed standards that jet fuels need to

meet for performance and safety criteria. This means that the implementation of biofuels would need to provide a reduction in

emissions, while also meeting jet fuel standards. In order for biofuels to be sucessful for the long term, they need to demonstrate

three important criteria; clean and sustainable and economical.

A. Clean

The usage of biofuels is expected to result in a large reduction in CO2 emissions, when compared to fossil fuels across their life-

cycle. This is because the CO2 released through combustion is neglected, as it is considered to be equivalent to the CO2 absorbed

by the plant during its growth. However, there are emissions produced during the production of biofuels; the equipment needed

to grow the crops, transportation of raw goods, refinement of the fuel and so on. As such, we need to ensure that the appropiate

feedstock is chosen to give us the best greenhouse gas (GHG) savings. Accordingly, analysis of Camelina feedstock for jet fuel

use has shown an 84% reduction in life-cycle emissions [3].

B. Sustainable

As with the implementation of biofuels in any sector, there has been a lot of unease about the exploitation of food lands for

producing biofuel based crops; other items like food price issues, land and water use, and pollution, have also been of great

concern. Fortunately, because of the strict requirements that aviation fuels need to meet, many of first-generation fuels (that

were food based) were unusable. They did not meet the fuel specifications such as UK MoD Def Stan 91-91 or US ASTM

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D1655 in terms of combustion properties, energy content and material compatibility. Consequently, the aviation sector has been

looking at a wide range of non-food crops and other sources of biofuels.

There is also the additional factor of yield and the amount of land use for producing biofuels. A report released in 2010 by

the US Federal Aviation Administration (FAA) estimated that as much as 100M hectares of land (USA) would be required to

meet the requirements of 100% biojet usage at 2008 levels [4].

C. Economical

Economic feasbility is one of the most important factors that will determine the longevity of biofuels. Biodiesel is currently

much more expensive to make than traditional Jet-A1. The price of neat biodiesel is currently around 387 cents [5], which is

way above the current price of 298 cents for Jet-A1 as of April 2, 2014 [6].

The two most expensive aspects of biodiesel production are land and farming capital costs. Farming costs account for 47%

of the total cost of production [5], which means that one can expect the price of biodiesel to be quite volatile as both yield and

farming mechanisms change. Consequently, without any drivers from the government such as subsidies and carbon targets, the

adoption rate of biodiesel will be very low.

III. Study

Given the large economic costs in refining pure biojet and its implementation, along with the uncertainity about the long

term effects of the usage of biofuels; the first goal of the aerospace industry is to practically replace 6% of fossil based fuels

by 2020 with bio-based jet fuel [3]. The topic of importance is then to find out what percentage blends we are looking at to

be sustainable, economical and clean. This report will focus on work done by various researchers on trying to obtain optimal

blends of bio-based jet fuel that satisfy the requirements. Studies by Canmet-Energy and the National Research Council (NRC)

have demonstrated that fuels with certain chemical properties -like Fatty Acid Methyl Esters (FAME), have a strong potential

to be drop-in fuels when blended appropriately with petroleum based jet fuel [7] & [8].

IV. Current Fuels

Jet fuel is a mixture of a number of different hydrocarbons. As such, one cannot expect to write up a single CxHy formula

hoping to describe the fuel. Having said that, most of the fuel molecules for Jet-A1 and Jet-A - twomost widely used commercial

fuels, have a carbon chain distribution that is between C8 and C16 [9].

The chemical composition of jet fuel is a complex mixture of alkanes ( 65%), mono and poly-aromatics ( 20%) andcycloalkanes or naphthenes ( 15%) [10]. Due to its high aromatic content, jet fuel has a high hydrogen to carbon ratio of about1.94, which also gives it a high heat to weight ratio. Additionally, aromatics are known to cause fuel seals to swell, helping

prevent any potential fuel leakages. Consequentially, there is currently a requirement that jet fuel have at least 8% aromatic

content by volume [11].

Jet fuel also contains small amounts of Sulphur, Nitrogen and Oxygen, which can all arise from the crude oil from which

fuel was produced. The presence of these substances has been known to have an effect on properties like NOx formation, fuel

stability, lubricity and emissions [9].

V. Pure Biofuel Properties and their Production Process

Fatty Acid Methyl Ester (FAME) is produced when it is prepared through a process called trans-esterification, from veg-

etable oils or animal fats. Biofuels obtained from Rapeseed oil have the acronym RME (Rapeseed Methyl Ester), similarly

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Figure 1: Different methods of Biokerosene production. Branches 1 and 2 result in FAME based fuels. Branch 3 results in Bio-SPKs.

those from Camelina are called CME (Camelina Methyl Ester) and so on. The biggest advantage with RME and CME blends

with petroleum based jet fuel, is that they can be used in gas turbines without significant modifications. It should be mentioned

that from now on, pure FAMEs will be refered to as biodiesel, while blends of biodiesel and petrolium based jet fuel will be

refered to as biokerosene.

In addition to trans-esterification, there is another method of producing biokerosene (as indicated in Figure 1); this second

process involves a series of chemical processes that convert the bio-derived oils to a Bio-Synthetic Paraffinic Kerosene (Bio-

SPK). While the end product is a Bio-SPK fuel that contains the same types of molecules that are typically found in conventional

petroleum derived jet fuel, it has very low yields and is much more expensive than trans-esterification [12]. Furthermore, given

that the majority of the biofuels industry has chosen to implement some form of the trans-esterification process, it will be the

focus for this report.

There are three principal chemical differences between biodiesel and petroleum-derived jet fuel. Unlike petrolium derived

jet fuel, biodiesel contains oxygen, has a low nitrogen content and the length of its carbon chains is significantly affected by its

feedstock. These differences in chemical composition affect the properties of biodiesel, some of which are critical to aviation.

Table 1 shows the composition of the fatty acid profiles of several feedstocks. While there is a significant difference in

composition between the feedstocks, we see that they all consist of mixtures of the same FAME species (C16 to C20). It is this

difference in the composition, that results in the eventual variation of fuels physical properties. Palm biodiesel for instance,

has a higher pour point value of 15C compared to Rapeseed biodiesel of -10C.

Biodiesel has a number of advantages in comparisition to traditional petrolium based jet fuel. They inlcude a higher flash

point (safer handling of the fuel), reduced toxicity and increased lubricity. Biodiesel also does not contain any sulfur or aromatic

species, which often leads to toxic emissions. Furthermore, Biodiesel has also been shown to have a lower sooting propensity.

The disadvantages of biodiesels include poor cold temperature properties and oxidative instability of the fuel when stored.

This is primarily due to the carbon chain length and the number of C=C double bonds in each of the five biodiesel components

[13]. Saturated fatty acids (C-C single bond) condense at temperatures much higher than those with one or more C=C double

bonds, so biodiesel fuels with large fractions of saturated fatty acids become gel-like and do not flow properly at low operating

temperatures or in cold climates. In contrast, biodiesel fuels with multiple C=C double bonds flow well at low temperatures,

but react much more rapidly than those with high saturated fractions, when they are stored for any length of time. Another

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Table 1: Fatty acid composition of different biodiesels, and number of Carbon double bonds. Number of Carbon double bonds is usedto indicate low temperature properties [14].

aspect of biofuels is that a majority of biofuel feedstocks have a carbon range that is outside that of petrolium based jet fuel.

Consequentially, this means that when we blend biodiesel with petrolium based jet fuel, we must avoid large changes to the

average chemical composition.

VI. FAME Blended Biofuel Properties

A number of authors like Alberto et al. and Baezewksi et al. have conducted tests on how properties of FAME based fuels

change, as the blended biodiesel ratio changes. Alberto et al. studied blends of 5%, 10%, 20% and 100% Camelina(CAM)

and Babossa(BBK) by volume [15] and then subsequently studied blends of 5%, 10%, 20% and 100% Palm Kernel(PBK) and

Coconut oil(CBK) by volume [16]; Baezewksi et al. studied Rapeseed blends of 5%, 10%, 20% by volume [17].

A. Density

Knowing the variation of density as the biodiesel blend changes, is very important. This is because density is known to directly

correlate to factors such as viscosity, surface tension and heating value. Additionally, the volumetric and gravimetric energy

density can usually be predicted by fuel density. The distinction between the two, is the amount of energy stored in a system

on a per unit volume, or mass basis respectively. Generally, less dense jet fuels have a higher gravimetric energy content, and

more dense jet fuels have a higher volumetric energy content [11].

The preference towards a higher volumetric or gravitational energy content depends on the application of the aircraft. For

aircraft that take off with their fuel tanks full (as most military aircraft do), a fuel with a high volumetric energy content

maximizes the energy that can be stored in a fixed volume, and thus provides the longest flight range. For the commercial

aviation sector however, the answer is not so obvious. This is because most airlines do not fill their fuel tanks full before each

flight. Instead, they take on enough fuel to reach their intended destination, plus an adequate safety margin. In this situation, it

is more advantageous to use a less dense fuel with a high gravimetric energy content to minimize fuel weight.

Table 2 demonstrates the change in density for the varying blends of biodiesel, with its petrolium equivalent. We see that

all the blends fit within the limits fixed in the ASTM and MoD standard (775 to 840 kg m-3 [11]). Only the pure biodiesels

have a density value outside the required range. Overall, the density was shown by Alberto et al. and Baezewksi et al. to be an

additive property. This could be explained through the fact that when the fuels are blended, there is no change in the volume

occupied by each molecule. These can be said to be reasonable assumptions considering the large number of atoms in each

molecule, the similarity of the molecules, and lack of any chemical reactions between them.

While the density of blends was linear with the blending ratio, the effect of blending was different for each biodiesel. CAM

was found to have the largest effect on the density, while CBK had the smallest. This particular trend is also inline with the

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Table 2: Density changes for different niodiesels(feedstocks) and blends (CAM-Camelina and BBK-Babossa [15],CBK-Coconut Oiland PBK-Palm Oil) along with changing fuel composition [16].

change in chemical composition of the biokerosenes. The imposition of the linear relationship forces the blending percentage

to stay under 39% for CAM, and 45% on average for the other biodiesels (in order to stay within density limits).

B. Fluidity at Low Temperatures

Another important requirement of jet fuel, is the ablility to handle low temperatures without solidifying. In addition to the

usage of fuels in colder regions on the ground, an increasing number of fights are using polar routes to reduce flight times. On

these routes, the fuel temperature can go as low as -32C [18]. This means that at low temperatures, the lack of viscosity could

have some dangerous consequences, especially at flameout conditions.

Table 3 demonstrates the change in the cloud and pour point for various feedstocks. Cloud point is defined as the tem-

perature at which dissolved solids are no longer completely soluble, precipitating as a second phase and giving the fluid a

cloudy appearance [11]. The pour point is defined as the temperature at which a fluid becomes semi-solid, and loses its flow

characteristics [11].

It should be mentioned that for jet fuels, the fuel freezing point is not the most important fluidity property, since it does

not dictate fuel flow to the pump inlets. Because jet fuel is a mixture of many different hydrocarbon molecules, each with its

own freezing point, jet fuel does not become solid at one temperature as water does. As the fuel is cooled, the hydrocarbon

components with the highest freezing points solidify first, forming wax crystals. Further cooling causes hydrocarbons with

lower freezing points to solidify. Thus, as the fuel cools, it changes from a homogenous liquid, to a liquid containing a few

hydrocarbon (wax) crystals, to a slush of fuel and hydrocarbon crystals, and finally to a near-solid block of hydrocarbon wax.

Because the freezing point is defined as the temperature at which the last wax crystal melts, the freezing point of jet fuel is well

above the temperature at which it completely solidifies. Consequently, the critical condition of pumpability or flowability for

cold fuels towards or into pump inlets, is governed by the pour point of the fuel. Generally, the pour point is 10C to 15C

lower than the freezing point of the fuel [9].

From Table 3 we see that there is some inconsistency between the cloud/pour point values at zero blends. This is primarily

because of the fact that the composition(and thus its properties) of the fuels are highly determinant on factors such as distillation

and crude oil properties at its origin. The important item to note though is the fact that as we increase the blending ratio, the

cloud/pour point both take a turn for the worse. This means that for safety concerns, only small percentage blends can be

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Table 3: Cloud/Pour point values for various feedstocks and blends (CAM-Camelina and BBK-Babossa) [15], RME-Rapeseed [17].

used. Another issue is also the fact that this particular percentage will be unique to each feedstock. From the data above and

keeping in mind the possibility of cold temperature exposure, it seems like the best feedstock to use is Rapeseed, as it allowed

the highest blend percentage while still staying around -40C mark.

C. Operating Conditions

The usage of biokerosene will require a number of changes to the metering systems within the combustor of a gas turbine. This

is because the difference in fuel properties can have a significant effect on heating value and fuel atomization droplet sizes.

While the equipment itself may not need to be changed, it does require the amount of air or fuel that is being input, to be

modulated differently.

Table 4 shows the air and fuel mass flow rates required to create the same power output condition, for a small can combustor.

The air and fuel mass flow rates were metered based on the energy content of the fuel, so as to obtain a combustor power output

at 6 kW, while maintaining a global equivalence ratio of = 0.47 [19]. It should be noted that a typical combustor is often

under a range of equivalence ratios ( = 0.1 to = 2.5) depending on its type and power condition (idle 7% power, takeoff

100% power). From Table 4 we can see that the biodiesels (RME and PME) require a larger fuel and air flow rate, given their

lower energy content as opposed to Jet-A1. The differences in the fuel or air flow rate are small because the combustion system

studied here is quite small. With a power output of 6 kW, this combustor is 250 times less powerful than Pratt and Whitneys

PT6 turboshaft engine. As such, for full size systems, one can begin to comprehend the magnitude of the changes that would

be required to maintain the optimal performance of the combustor.

For the 50% blended fuels case, we see that the change in the fuel/air mass flow rate is halfway within that of the pure

petrolium and bio-based fuel. As such, it seems reasonable to expect that by restricting the blending ratios to smaller blends,

we should be able to find the optimal value that helps improve emissions, while also not requiring very large operational

changes. The reason for not requiring large operational changes is because the increase (or reduction) in momentum caused

by the change in air/fuel flow rate, can affect the airflow patterns within the combustor. This may have adverse side effects,

especially for older combustors that werent specifically designed for this.

D. Sooting Propensity

As was mentioned before, FAME based biofuels are different from traditional fuels in a number of ways. One way in which

they differ, is in their oxygen content.

A number of studies, both experimental and numerical have shown that oxygenated fuel additives can reduce soot formation

in engines [20]. The same soot reduction is observed when biodiesel is blended with conventional jet fuel [17]. The reason for

this is because the oxygen allows for a more complete combustion, resulting in a higher flame temperature, but consequently

higher NOx production (which is a strong function of temperature). This increase in NOx is attributed to differences in both the

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Table 4: Operating conditions for a can combustor for biokerosene blends (RME-Rapseed and PME-Palm Oil) [19].

Figure 2: Smoke points of fossil kerosenes (K1 and K2), and blends with Babassu biokerosene (BBK), Palm Kernel biokerosene (PBK),Coconut biokerosene (CBK), Camelina Methyl Ester (CAM), and Linseed Methyl Ester (LIN) [24].

physical and chemical oxidation mechanisms between biodiesel and jet fuel [21]. The reduction of soot is important because it

is measured in the form of volatile and nonvolatile particle materials from the gas turbine exhaust. These reading are then used

for an accurate estimation of the potential impacts of airport activities on local air quality, atmospheric processes, and climate

change [22].

Due to the reduction in soot formation, studies have found that particulate matter emissions from a gas turbine can fall by as

much as 40%, when jet fuel is blended with oxygenated fuels [23]. A good indication of soot formation is the smoke point of a

fuel. This parameter is defined as the height (in millimetres) of the highest flame produced without smoking soot breakthrough,

when the fuel is burned in a specific test lamp. Figure 2 shows the trends for the smoke point for a variety of fuels. In Figure 2,

K1 is a kerosene directly obtained from straight-run atmospheric distillation cut, and K2 is commercial Jet-A1.

Despite the fact that each blending ratio and feedstock had different effects on the smoke point, the results provide a

quantification of the ability of biokerosenes to reduce the sooting tendency of jet fuel. All tested biokerosenes showed significant

reductions of sooting tendency. Among them, biokerosene made from Palm kernel oil was shown to be the most effective (24%

reduction when blended with K1 at 20% vol.), while Camelina and Linseed oils showed in general to have a lower effectiveness

towards reducing the sooting tendency [24]. This result was also shown to be true by Llamas et al. [15]. Llamas et al. also

noted that the sooting propensity tended to follow a linear relationship, with increasing smoke points at higher biodiesel blends.

Figure 3 depicts the spatial distribution of soot in a premixed flame by Tran et al. [21]. The contour plot of Figure 3 further

helps us to see where exactly soot is present in the flame. It should be noted that a small blend percentage has very small effect

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Figure 3: Spatial distribution of soot volume fraction for various Rapeseed blending ratios [21].

on changing soot production. We only start to see a significant reduction starting at blends of 15%. Having said that, of all the

blends (0% to 30%) that were tested, the 20% biodiesel blend was found to have both the lowest soot concentration and soot

volume fraction [21]; indicating that the optimal blend potentially lies in the 15% to 20% range.

VII. Performance and Emissions

An important aspect with the usage of biokerosene, is understanding the effect it has on the overall performance of a gas

turbine. Gas turbines are characterised by a number of statistics, they include static thrust, thrust-specific fuel consumption

(TSFC) and the exhaust concentrations of NOx and CO (represented by EICO and EINOx). The TSFC is an efficiency factor

used to to characterize an engines fuel efficiency, and it defined as the mass of fuel burned by an engine in one hour, divided

by the thrust that the engine produces [25].

A. Static Thrust and TSFC

Figure 4 shows the static thrust production for a combination of pure and blended biodiesel with jet fuel, for a small scale gas

turbine [26]. When Habib et al. tested their fuels, Soy Methyl Ester (SME) had the lowest energy density with 37 MJ/kg,

incomparision to 41.6 MJ/Kg of Jet-A1 [26]. The rest of the biodiesels (Rapeseed Methyl Ester (RME) = 37.3 MJ/Kg,

Camelina Methyl Ester (CME) = 37.4MJ/Kg), had similar values. While the energy denisty values for the biodiesels were in

line with what other papers have described, the energy denisty for pure Jet-A1 was much lower. Tran et al. described Jet-A1 as

having 46.7 MJ/kg [21] while Llamas et al. described Jet-A1 as having 47.4 MJ/Kg [24]. Despite this difference, the results

of Habib et al. should still hold, albeit one should expect a more pronounced difference in results between pure petrolium and

biodiesel than depicted.

Figure 4 shows that for the gas turbine running on Jet-A1, a static thrust of 410 lbf (2040N) was produced at the maximum

recommended operating speed [26]. We also see that the static thrust increases almost linearly with the engine speed for all

fuels.

From Figure 4 we see that both SME and CME, do not suffer as much for both the blended and pure biodiesel case,

incomparision to Jet-A1. For the most part, the B50 blends seem to be capable of performing on par with the pure Jet-A1 case.

The static thrust developed with the pure biodiesel was also comparable to that of Jet-A1, with a maximum drop in thrust of 6%

(Please keep in mind the point mentioned earlier where energy density of Jet-A1 closer to biodiesel than usual) [26]. Unlike

SME and CME, the usage of RME had a much stronger detrimental effect on static thrust. A reason for this is possibly because

of the energy content difference between the biodiesel and jet fuel in that particular case.

It should also be mentioned that while the principles of operation for a small-scale turbine are the same as that for a larger

gas turbine, there are certain issues (and subsequent solutions) associated with a larger sized turbine that might cause a slight

deviation from the above results. However, even within the relm of small-scale gas turbines, there seems to be some discrepency

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Figure 4: Static thrust as a function of engine RPM for Jet-A and blends of: (a) SME, (b) CME, (c) RME [26].

within these results. The Baylor Institute for Air Science [27] tested blends of RME biodiesel in Jet-A1 (up to 20% by volume)

in a modified gas turbine engine. No difference in the performance or fuel consumption was found between pure Jet-A1 and

biokerosene for the same power output. French [28] operated the same gas turbine used by Habib et al. on a 25% blend of SME

biodiesel, and found that the static thrust for the biokerosene fuel was 8% less than that of Jet-A1 [28].

For the TSFC, Habib et al. showed that the variation of TSFC with RPM for SME and CME biodiesel was slightly lower

at low engine speeds, and insignificant at higher speeds. This is because at higher RPMs, the lack of energy input is made up

for by the combination of the amount of fuel that is input into the combustor, and interior airflow conditions. The TSFC of

all 50% biodiesel blends, except that of RME, was found to not markedly differ from the TSFC of Jet-A1 [26]. The lack of

performance by RME in this particular situation is possibly because of the source from which the fuel was obtained. Given the

data presented here, and in other papers, one should expect RME to perform atleast inline with the other biodiesels like CME.

B. CO and NOx Emissions

Low emissions of CO and NOx is of primary importance, if biokerosene is to be sucessfully implemented in the aviation sector.

In the early 1980s, the International Civil Aviation Organization (ICAO) adopted standards for controlling emissions from

aircraft engines, through an engine certification scheme. Through this scheme, they estabilished limits for emissions of CO

and NOx during an aircrafts landing and takeoff cycle (LTO). Overtime, the ICAO has imposed an average reduction in the

emissions of NOx of 1% every year [29].

For this particular case, when measuring the impact of NOx or CO, the emissions index was used. The emission index

signifies the mass of pollutant emitted per unit time, per unit thrust generated.

C. CO Emissions

Figure 5 shows the variation of CO emissions with the equivalence ratio. From Figure 5 we see that the CO emissions decreased

with the equivalence ratio for all fuels tested. This trend is due to the fact that with an increasing equivalence ratio, the com-

bustion process transitioned from incomplete to complete. This consequently means that CO (which is a product of incomplete

combustion) decreased, while CO2 (is a product of complete combustion) increased. Additionally, with higher temperatures in

the combustor (associated with improving completeness of the combustion process), the number of quenching reactions went

down, further reducing CO production. The reason for the difference in CO emissions between the biodiesel and neat jet fuel,

was due to the oxygen content in the biokerosene. As was mentioned before, the presense of oxygen allows for a more complete

combustion and higher temperatures inside the combustor. Habib et al. [26] confirmed these effects experimentally, when they

studied the individual concentrations of CO and CO2 inside the combustor. They found out that as the CO2 content in the

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Figure 5: Thrust-specific carbon monoxide emissions index as a function of engine RPM for Jet-A and various blends of: (a) SME, (b)CME and (c) RME [26].

exhaust gases increased, the CO content decreased [26].

Similar results were also obtained by Chong [19] for his test of Rapeseed and Palm Oil biodiesel. Unlike Habib et al.

[26], Chong studied a larger equivalence ratio range of 0.25 to 0.85. In his study, he found out that the emissions of CO only

decrease until an equivalence ratio of 0.45, after which it starts increasing. This is because combustion remains incomplete

for all equivalence ratios that is outside stoichometry, independant of whether we have a higher or lower equivalence ratio than

stoichometry. Since CO is a product of incomplete production, the closer we are to stoichometric, the lower the EICO is and

vice versa.

The results shown in Figure 5 indicate that more CO was produced per unit thrust per unit time for Jet-A1, than for

biodiesel and biokerosene, at the same equivalence ratio and throttle setting. Similar observations were made by Krishna in his

experiments with Soy and Rapeseed based biodiesel [30]. For all biokerosenes, the 50% blend produced a lower EICO value

than the pure biodiesel. While it was expected that the biodiesels would produce lower CO emissions, it was suprising to see

the 50% blend outperform its biodiesel counterpart.

D. NOx Emissions

As with CO, the thrust-specific emissions index was computed by Habib et al. [26], to present the variation of NOx with the

equivalence ratio. As shown in Figure 6, more NOx pollutant was produced per unit thrust for Jet-A1, than for the biodiesels

and 50% blends at the same throttle setting. It should also be mentioned that NOx production is unlike CO, in that it is produced

in high temperature regions, which generally only occur during a complete combustion process.

The reduction in NOx emissions for biodiesel and its blends, was in part due to the lack of nitrogen-bound components in

the Soy and Camelina biodiesel. Furthermore, Habib et al. [26] also found that the turbine inlet temperatures were comparable

for all the fuels; indicating that the temperatures inside the combustor may not have changed much, for changes in feedstock or

blending ratio. As such, the authors surmised that the reduction in NOx was not due to a change in the thermal mechanism. This

result is quite suprising, since the majority of NOx emissions for gas turbines is generally produced via the thermal mechanism.

From Figure 6 we see that the addition of biodiesel resulted in lower NOx per unit thrust than jet fuel. We also see that both

Soy and Camelina biodiesel had EINO values, that were very close to the Soy and Camelina 50% blend EINO values. This was

not the case for Rapeseed though, where we see that there is a sizable difference in EINO values between the pure biodiesel

and 50% blend.

The lack of an EINO improvement for both Soy and Camelina from a 50% to 100% blend, could be due to the fact that the

temperatures in the combustor did not go high enough(at least locally) for the production of NOx to ramp up. Both the fact that

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Figure 6: Thrust-specific Nitric Oxide emissions index with engine RPM for Jet-A, and various blends of: (a) SME, (b) CME and (c)RME [26].

turbine inlet temperatures did not change by much, and the lower NOx emissions of the 50% blend fuel (compared to the pure

biodiesel), is quite contradictory to what was generally expected. Having said that, Glaude et al. [31] demonstrated a similar

set of results when they studied the adabatic flame temperature for a number of biokerosene fuels. Glaude et al. found the

adabatic flame temperature of varying feedstocks and blends to be off at most by 85K, to petrolium based jet fuel for a variety

of power conditions [31].

A similar study was conducted Catalanotti et al. on a modern annular combustor. Catalanotti et al. used CFD modeling to

predict the effect of biofuels on the emissions of NOx [32]. In this CFD model, the biofuel was represented by the surrogate

fuel Methyl Buthanoate, which has very similar properties to FAME based fuels. Catalanotti et al. also demonstrated that

the majority of NOx production occured in the post flame area, where the gas temperature was high. By further studying the

concentration of radicals in the combustor, Catalanotti et al. were able to discern that the quantity of NOx produced through

non-thermal processes, was under 10% of the total NOx formed in the engine [32].

Figure 7 shows the predicted EINOx values for the various fuels tested by Catalanotti et al. [32] We see that the EINOxvalues for both the biodiesel and the blends, were much lower than that of conventional jet fuel. In this particular case, the pure

biodiesel generated NOx values that were on average 28% lower than the 50% blend [32]. The observed difference in predicted

NOx emissions, was attributed to the disparity in the flame location (due to the usage of a biodiesel blend), and a different

concentration of radicals that were important in the NOx formation process. The disagreement in results between Catalanotti et

al. [32] and Habib et al. [26], is an indication of our lack of understanding in this particular field, and means that more work

needs to be done.

VIII. Conclusion

Considerable evaluations and studies have been conducted, in order to create fuels that provide acceptable engine perfor-

mance, compatibility with aircraft systems, oxidative stability and proper fuel performance throughout all aircraft operating

environments. Furthermore, the recent certification extention by the ASTM to allow biofuels blends of up to 50%, will further

encourage aviation companies to explore this avenue.

As was mentioned before, the choice of the appropiate feedstock to be used as a blend depends on a number of factors.

Coconut oil and Palm oil were shown to have the smallest effect on density for a 20% blend. Given that density is an additive

property, this advantage is still expected to stand for larger blends. Moreover, in terms of low temperature performance,

Rapeseed was shown to have better properties (atleast at low blends). For sooting propensity, Coconut Oil and Palm Oil were

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University of Toronto Institute of Aerospace Studies

Figure 7: Comparisons of CFD generated NOx emissions for various fuels[32].

shown to have the lowest sooting propensity of all the feedstocks that were tested.

The emissions of NOx and CO were also established for neat Jet-A1, Rapeseed, Palm Oil and their blends with Jet-A1. The

results showed that the emissions of NOx for biodiesels, were consistently lower than that of Jet-A1. Furthermore, of the three

feedstocks that were studied for CO and NOx emissions, Soy Oil and Camelina were both shown to be good choices. Even

though Palm Oil and Coconut Oil were not tested for CO and NOx emissions, we should expect them to behave like Soy, since

they have a similar chemical makeup [7]. It should also be mentioned that while we showed certain feedstocks to have strong

technical properties (engineering wise), there are other factors such as economics or agricultural issues that may force another

feedstock (or a combination of a class) to be chosen.

While the feedstocks studied all showed qualities that made them good choices, the unfortunate fact that most of these

tests came from different people with different testing methodologies, means that we need to be careful before coming up with

definitive conclusions. As such, these results provide us with a strong short list of feedstocks. A good followup to much of the

research would be for a single group of researchers to study a number of important features such as density, lubricity, material

compatibility, oxidative stability and the subsequent NOx and CO emissions on a full scale aviation gas turbine.

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References[1] Arthur H. Lefebvre, D. R. B., Gas Turbine Combustion: Alternative Fuels and Emissions, CRC Press, September 2010.

[2] ATAG, Beginners Guide To Aviation Industry, Printed, 2010.

[3] Hileman, J. I., Ortiz, D. S., Bartis, J. T., Wong, H. M., Donohoo, P. E., Weiss, M. A., and Waitz, I. A., Near-Term Feasibility of

Alternative Jet Fuels. Tech. rep., RAND Corporation, Santa Monica, Calif., 2009.

[4] Stratton, R. W., wong, H. M., and Hileman, J. I., Life Cycle Greenhouse Gas Emissions from Alternative Jet Fuels, Tech. rep.,

Massachusetts Institute of Technology, 2010.

[5] Agusdinata, D. B., Zhao, F., Ileleji, K., and DeLaurentis, D., Life Cycle Assessment of Potential Biojet Fuel Production in the United

States, Environmental Science & Technology, Vol. 45, No. 21, 2011, pp. 91339143.

[6] Jet A1 price, http://www.indexmundi.com/commodities/?commodity=jet-fuel Retrieved March 2nd, 2014.

[7] Al-Sabawi, M., Chen, J., and Ng, S., Fluid Catalytic Cracking of Biomass-Derived Oils and Their Blends with Petroleum Feedstocks:

A Review, Energy & Fuels, Vol. 26, No. 9, 2012, pp. 53555372.

[8] Al-Sabawi, M. and Chen, J., Hydroprocessing of Biomass-Derived Oils and Their Blends with Petroleum Feedstocks: A Review,

Energy & Fuels, Vol. 26, No. 9, 2012, pp. 53735399.

[9] Briddle, T., Handbook of Aviation Fuel Properties, Coordinating Research Council, 3650 Mansell Road, Alpharetta GA 30022, 635th

ed., 2004.

[10] Dagaut, P. a., Chemical Kinetic Study of the Effect of a Biofuel Additive on Jet-A1 Combustion, The Journal of Physical Chemistry

A, Vol. 111, No. 19, 2007, pp. 39924000, PMID: 17253673.

[11] ASTM, S., Standard specification for aviation turbine fuels., ASTM International, West Conshohocken, Pennsylvania., astm d1655-09a

ed., 2009.

[12] Rosillo-Calle, F., Thran, D., Seiffert, M., and Teelucksingh, S., The Potential Role of Biofuels in Commercial Air Transport - Biojet-

fuel. Tech. rep., Imperial College London, 2012.

[13] Gerpen, J. V., Biodiesel processing and production, Fuel Processing Technology, Vol. 86, No. 10, 2005, pp. 1097 1107, Biodiesel

Processing and Production.

[14] Atabani, A., Silitonga, A., Ong, H., Mahlia, T., Masjuki, H., Badruddin, I. A., and Fayaz, H., Non-edible vegetable oils: A critical eval-

uation of oil extraction, fatty acid compositions, biodiesel production, characteristics, engine performance and emissions production,

Renewable and Sustainable Energy Reviews, Vol. 18, No. 0, 2013, pp. 211 245.

[15] Llamas, A., Al-Lal, A.-M., Hernandez, M., Lapuerta, M., and Canoira, L., Biokerosene from Babassu and Camelina Oils: Production

and Properties of Their Blends with Fossil Kerosene, Energy & Fuels, Vol. 26, No. 9, 2012, pp. 59685976.

[16] Llamas, A., Garca-Martnez, M., Al-Lal, A.-M., Canoira, L., and Lapuerta, M., Biokerosene from coconut and palm kernel oils:

Production and properties of their blends with fossil kerosene, Fuel, Vol. 102, No. 0, 2012, pp. 483 490, Special Section: {ACS}Clean Coal.

[17] Szczawinski, P. and K., B., Investigation properties of rapeseed oil methyl esters/aviation turbine fuel Jet A-1 blends, Journal of

KONES, Vol. 18, No. 1, 2011, pp. 15 22.

[18] Ekstrand, C., Polar Routes, Flight Operations, Vol. 16, 2001.

[19] C.T.Chong, Combustion properties of Alternative liquid fuels, Ph.D. thesis, University of Toronto, Feburary 2011.

[20] Barrientos, E. J., Lapuerta, M., and Boehman, A., Group additivity in soot formation for the example of C-5 oxygenated hydrocarbon

fuels, Combustion and Flame, Vol. 160, No. 8, 2013, pp. 1484 1498.

[21] Tran, M. K., Dunn-Rankin, D., and Pham, T. K., Characterizing sooting propensity in biofuel diesel flames, Combustion and Flame,

Vol. 159, No. 6, 2012, pp. 2181 2191.

13 of 14

University of Toronto Institute of Aerospace Studies

[22] Timko, M. T., Knighton, W. B., Onasch, T. B., Northway, M. J., Jayne, J. T., Canagaratna, M. R., Herndon, S. C., Wood, E. C., and

Miake-Lye, R. C., Gas Turbine Engine EmissionsPart II: Chemical Properties of Particulate Matter, Journal of Engineering for Gas

Turbines and Power, Vol. 132, No. 6, 03 2010, pp. 061505061505.

[23] Westbrook, C. K., Pitz, W. J., and Curran, H. J., Chemical Kinetic Modeling Study of the Effects of Oxygenated Hydrocarbons on Soot

Emissions from Diesel Engines, The Journal of Physical Chemistry A, Vol. 110, No. 21, 2006, pp. 69126922, PMID: 16722706.

[24] Llamas, A., Lapuerta, M., Al-Lal, A.-M., and Canoira, L., Oxygen Extended Sooting Index of FAME Blends with Aviation Kerosene,

Energy & Fuels, Vol. 27, No. 11, 2013, pp. 68156822.

[25] Blakey, S., Rye, L., and Wilson, C. W., Aviation gas turbine alternative fuels: A review, Proceedings of the Combustion Institute,

Vol. 33, No. 2, 2011, pp. 2863 2885.

[26] Habib, Z., Parthasarathy, R., and Gollahalli, S., Performance and emission characteristics of biofuel in a small-scale gas turbine

engine, Applied Energy, Vol. 87, No. 5, 2010, pp. 1701 1709.

[27] for Air Science, B. I., Renewable aviation fuels development center. Development of bio-based fuels for aircraft turbine engines, Tech.

rep., Baylor Institute for Air Science, 1998.

[28] K., F., Recycled fuel performance in the SR-30 gas turbine. Annual conference proceedings- American society for engineering edu-

cation; 683, John Brown University, American Society for Engineering Education, Siloam Springs, AR 72761, 2003, p. 683.

[29] Bonn, Statement from the ICAO to the twentieth Session of the UNFCCC Subsidary body from Scientific and Technological Advice.

2004.

[30] Krishhna, C., Performance of the Capstone C30 Microturbine on Biodiesel Blends, Energy sciences and technology/energy resources

division, Brookhaven National Laboratory, 2007.

[31] Glaude, P.-A., Fournet, R., Bounaceur, R., and Molire, M., Adiabatic flame temperature from biofuels and fossil fuels and derived

effect on {NOx} emissions, Fuel Processing Technology, Vol. 91, No. 2, 2010, pp. 229 235.[32] Shafagh, I., Wilson, C. W., Hughes, K. J., Catalanotti, E., Liu, Z., and Pourkashanian, M., Experimental and Modeling Studies of the

Oxidation of Surrogate Bio-Aviation Fuels, Journal of Engineering for Gas Turbines and Power, Vol. 134, No. 4, 01 2012, pp. 041501

041501.

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University of Toronto Institute of Aerospace Studies