Biofuels applications as fuels for internal combustion engines
The growth and industrialisation of the world's economies has resulted in the massive increase of fossil fuel use. The majority of this usage has been through burning fuels such as petrol (gasoline) and diesel in internal combustion engines within motor vehicles. Therefore providing alternatives to these fuels has been crucial and biofuels have provided a promising option. At the same time the development and use of diesel engines has seen major gains due to the higher engine efficiencies and lower emissions of diesel fuel compared with gasoline. Biodiesel has become the most promising alternative to conventional fossil derived diesel fuel. It is very attractive due its benefits on the environment in terms of emissions and the energy security it can provide to many countries.
Although raw vegetable oils can be used within diesel engines they pose many problems. The viscosity of the vegetable oil is too high resulting in blocked parts within the motor vehicles. Incomplete combustion of the oil is dominant resulting in the formation of carbon deposits and increased emissions of unburned hydrocarbons. The lubricating oil within the engine loses its effectiveness as it is mixed with the raw vegetable oils resulting in shorter engine life cycles and higher long term motoring costs.
Transesterification of the raw vegetable oils seems to be the most accepted from of producing a useful fuel (biodiesel). The vegetable oils are converted to their methyl esters which are then used as fuel. This fuel has comparable properties to conventional diesel but also contains significant levels of oxygen (approximately 10%). This presence of oxygen allows for better combustion and results in lower carbon monoxide and unburned hydrocarbon emissions compared with conventional diesel fuel. The lack of sulfur presence in the biodiesel means lower sulphur based emissions however this is balanced out by the fact that NOx emissions are greater. This is due to the higher temperatures within the combustion chamber of the engine due to improved combustion and better oxygen availability of the biodiesel. Biodiesel also seems to possess inherent lubrication characteristics resulting in lower engine wear and lower levels of carbon deposits within the engine.
The ultimate property of any commodity is its price; biodiesel prices are higher than conventional diesel throughout the world making its use unfeasible. However production using waste oils from various industries allows feedstock prices to be reduced and ease prices on food markets simultaneously. This aspect along with the fact that economies of scale are yet to be developed provides a scope for lower biodiesel prices. Finally a very appealing solution is to tax fuels according to their environmental impact, this would most definitely solve the cost problem making biodiesel more feasible than conventional diesel fuel in the future.
It is a well known fact that as the industrialisation of the world is increasing the use of transport has become crucial and almost impossible to live without. This increasing and sustained use of transport (mostly motor vehicles) has led to a massive growth in the consumption of petroleum based fuels. Petroleum based fuels are finite and obtained from lacking reserves. According to predictions the levels of fossil fuels remaining for use at current levels is 148 years for coal, 61 years for gas and 43 years for oil1. This is only for current levels of use and future increases in demand are not considered.
Furthermore this sustained usage of fossil fuels has had a significant and lasting effect on the environment. Burning of fossil fuels has resulted in the emission of greenhouse gasses such as carbon dioxide (CO2) to increase significantly, levels of CO2 have raised steeply from 315ppm in 1958 to 378ppm in 2004. This is approximately an increase of 20% within the last 46 years alone, compared with the fact that CO2 levels varied only between 7ppm during the 800 years from 1000 to 1800 A.D2. The increase in greenhouse gasses has led to an increase in surface temperatures and changing weather patterns, this has had devastating effects on certain parts of the world from entire ecosystems being destroyed to large cultures being displaced. More severe seasons are being experienced and decreases in human health have also been noted. This is due to the fact that burning of various fossil fuels has led to the release of many pollutants of which some have been acknowledged and recognised more than others.
The well recognised pollutants have been regulated by many governments around the world while other pollutants have been left largely unnoticed. Regulated pollutants within the European Union include carbon monoxide (CO), nitrogen oxides (NOx), unburned fuel hydrocarbons (HC) and particulates which are less than 10 microns in size (PM10). It has been noted that particles <5 microns in diameter reach the alveoli in human lungs and deposit there. These particulates consist of a carbon based core onto which approximately 18,000 high molecular weight organic compounds are adsorbed4. It has been suggested that the intake of these particulates in humans has led to an increase in cardiovascular diseases, immune system disorders and carcinogenicity. This has also been acknowledged by the United Nations environment programme which believes that particulate emissions pose a major threat to world health5. These may be a serious problem but what may be a larger cause for concern are the unregulated pollutants emitted from exhaust gasses. These pollutants form a large group of compounds which include volatile organic compounds (VOCs) such as BTEX (benzene, toluene, ethylbenzene and xylene) and other polycyclic aromatic hydrocarbons (PAH). The VOCs can enter the body via the air easily and cause negative long term health effects such as increased carcinogenicity and myeloid leukaemia even in low concentrations6. Although benzene levels are regulated at <5g/m3 in Europe the levels of xylene and toluene are not which have very similar effects. This poses a serious cause for concern especially in urban areas which experience high levels of traffic; these places have significantly high levels of VOCs in the atmosphere causing the frequency of related diseases to increase considerably over time.
Another aspect of fossil fuel use is the fact that it is concentrated within certain regions of the world where there are many developing countries with rapidly expanding and emerging economies. These nations are gradually hesitant to export oil due to their rapid expansion and need for fuel. Hence the reliance and dependency on them to provide others with fuel has potential for disaster and is a possible cause for conflicts, this is especially true when "peak oil" has been reached i.e. when half of the earth's supply of crude oil has been exhausted. Prices are inevitably going to rise resulting in severe discontent amongst heavy end users such as transport companies and those earning a living via the use of motor vehicles. The question remains whether developed nations economies would be able to cope with these price rises as seen during the summer of 2008 when oil prices had peaked at $147/barrel. The growth of developing economies would not be the only cause for concern, political indifferences and instabilities within certain nations could lead to a sudden fall in supply which would have major implications for users in other parts of the world.
Furthermore it has been estimated that the demand for fossil fuels will continually rise as a result of many factors such as population increases, etc. Transport energy usage alone is expected to rise by 2% over the next few decades i.e. transport energy usage in 2030 will be 80% higher than it was in 20023. This poses a serious problem in reducing the problems discussed above.
To overcome these problems many potential solutions have been suggested of which some are for the short term while others long term. Solutions include increased use of renewable forms of energy such as solar, wind and tidal power, increased energy efficiency, change in human habits, etc. However a promising alternative offered is the aspect of biofuel which could potentially limit the changes needed to be made to current lifestyles whilst still providing sustainable energy for use in motor vehicles in the long term.
Biofuels produced currently offer alternatives for vehicles running on petrol or diesel engines. However diesel engines are more efficient in terms of miles per gallon and lower emissions. Also as the cost of both types of fuel is currently almost the same it seems more economical for end users to go with diesel powered cars. These vehicles over the years have developed significantly and offer the same sort of power/torque as most petrol powered cars. It seems diesel engines may be the way forward therefore this report provides a focus on the alternatives for conventional fossil based diesel fuel.
2. Concept of Biofuel
Biofuel is any type of renewable fuel which is derived from biomass; it is different to fossil fuel which is derived from long dead biological material. Biofuels include bioethanol, biodiesel, other types of alcohols such as methanol, etc. Biofuels come in all three solid, liquid and gaseous forms although liquid and gaseous forms are most dominant. This is due to the fact that the majority if not all of biofuel usage is destined for use in motor vehicles with internal combustion engines. These engines need fuel which burns cleanly i.e. forms little carbon deposits or other solid particulate waste during the combustion process. This prevents engine damage and allows them to conform to air pollution targets.
Biofuel formation has been gradually changing over time, first generation biofuels which followed the idea of creating fuel from biomass are fuels derived from sugar, starch, animal fats and vegetable oils using common technologies such as fermentation (using micro-organisms such as yeast) and transesterification processes (explained in later sections). However as plants such as corn, canola and sugarcane are used the effect on food prices has become a cause for concern. This has been observed within the USA where approximately 11% of total corn produce only produced 3 billion gallons of bioethanol7. The removal of grain supplies to food markets inevitably results in higher food prices which has significant effects on those within developing poor nations. Therefore the need to change source is crucial and second generation biofuels provide a slightly more acceptable form of fuel.
Unlike first generation biofuels lignocellulosic biomass is used to create second generation biofuels i.e. non food crops such as grass species, agricultural waste and trees are utilized. However the need to covert the cellulose into useful forms such as sugars forms a problem and newer technologies are required. These newer technologies include processes in which the cellulosic biomass can be converted via 2 main routes; biochemical and thermochemical. Biochemical routes include the use of micro-organisms which break the cellulose down into sugars which are then fermented to produce bioethanol or another route where micro-organisms convert the cellulose into biogas or biohydrogen which are gaseous fuels. Thermochemical routes include processes such as fast-pyrolysis where the biomass is heated to high temperatures (approximately 500-600oC) in conditions where there is no oxygen present to form a heavy liquid type fuel. This is used in this form or is then refined to produce different grades of fuel depending on requirements. Another thermochemical route is gasification, this process converts biomass at high temperatures (>700oC) to form a gaseous mixture known as syngas. Using this as a fuel in combustion engines is significantly more efficient than petroleum based fuels as it can be combusted at higher temperatures. Also the syngas can be further processed to form hydrogen or methanol which can be used as energy in various applications. These thermochemical routes are energy intensive but process integrations are continually improving allowing them to be more efficient. An overview of the biofuel production methods discussed above is displayed below in figure 1.
According a report by the World Energy Council the proportions of BTL (biomass to liquid) fuels as mentioned above are set to increase considerably by 2050. The report estimates that given a set of scenarios, BTL may account for up to 50% of fuel which replaces general diesel use and 50% replacement of diesel usage in passenger vehicles. This along with the fact that BTL plant production efficiencies will increase has the potential to reduce petroleum based fuel use by up to 22.4% in vehicles by 20509. This is summarised in Table 1 below.
This report also states that cellulosic ethanol can potentially replace 21.6% of fossil fuel use. Another interesting projection shown is that biofuels form a much larger reduction in fossil fuel use compared with other potential solutions studied such as hybrids which will only contribute to a 5% reduction and other technologies such as hydrogen fuel cell vehicles and electric vehicles.
More promising is the development and production of third generation biofuels. Unlike second generation biofuels which are concerned with advancing the steps of converting biomass to fuel, third generation fuels are developed with advancing the source of biomass used. The production of biomass is crucial, methods such as genetically engineering existing crops to yield high energy crop are employed. Other aspects such as plant compositions are considered such as levels of material which may need processing. A noted development in this area is the work carried out by the Agricultural Research Service (ARS) in the USA where scientists have developed sorghum (grass species/grain) which has low levels of lignin10. This allows for easier conversion into cellulosic ethanol. More work carried out on the same crop has looked at other factors which include drought resistance and growth in acidic environments. This is a significant advancement as the growth of plants in acidic environments would ease the pressure on food prices as the supply of arable land would increase resulting in the elimination of a trade off between food producing and fuel producing land for crops11. Another promising advancement is the development of plants which contain specific enzymes that convert the biomass into fuel. Scientists at the Michigan State University in the USA have done just this on corn crops. The leaves and stalks of the plant contain the enzyme cellulose allowing the plant to be converted from biomass to fuel more easily12. Although all the mentioned advancements result in better crops another advantage is the time taken for the production of fuel from these plants. Some of these plants may be grown much quicker and processed more efficiently allowing for greater overall efficiencies from existing resources such as arable land.
Finally it is worth mentioning fourth generation biofuels, these are yet to be commercially produced but the theory has now been established and shows massive potential. Fourth generation biofuels promise the same advantages as third generation fuels along with the ability to capture massive amounts of CO2 from the atmosphere. An example of this includes the biological modification of eucalyptus trees by 2 teams of American and Taiwanese scientists. The trees are able to produce low lignin crop and at the same time hold the ability to absorb up to 3 times more CO2 than normal eucalyptus trees. These crops can be processed via more established technologies from second generation fuels such as fast-pyrolysis, gasification, etc. However the excess CO2 can be stored with the use of other carbon capture technologies to result in a large net negative carbon fuel. This is summarised in the figure 2 below;
3. Raw Vegetable Oils as Fuels
Ever since Dr Rudolf Diesel invented the diesel engine more than 110 years ago there has been significant interest in using vegetable oils as fuels. Dr Diesel himself had a vision for these types of fuels, amongst his many experiments one stood out and was displayed at the World Exhibition in Paris in 1900. This showed an engine which was running entirely on peanut oil13. However after his unexpected death in 1913 the development of this type of fuel for the diesel engine faded away. The diesel engine has gone great lengths since then with massive improvements made to the original design such as incorporating the use of fuel injection systems, turbo's, etc. with the engine in most modern vehicles. However modern engines are nearly all designed only to be used with petroleum based diesel fuel.
Vegetable oils are liquid fuels similar to conventional diesel but obtained from renewable sources, mainly crops of the oilseed variety. The use and development of this type of fuel in recent times has been heavily influenced by local environments. Different crops used are soybean in the USA, palm oil in Asian countries such as Indonesia and Malaysia, sunflower oils in Europe and coconut oil in the Philippines, i.e. local crops are almost always used.
The main advantages are fairly obvious and stated previously such as the removal of CO2 from the atmosphere by the vegetable oil producing crop and energy security. However other advantages include the relatively easy production of vegetable oil from the oilseed crop hence potentially lower end price plus lower input energy requirements. Another possible advantage is the ability to fix nitrogen into the soil of the oil producing crop; this is a very similar principle to the removal of CO2 from the atmosphere as mentioned before.
Different vegetable oils have different chemical structures consisting of various organic compounds such as simple straight chain compounds to more complex proteins and fat-soluble compounds. Petroleum based diesel has a chemical structure consisting of simple long chain carbon based compounds (usually between 12-18 carbon atoms long). Therefore the physical properties of vegetable oils are fairly different to diesel and are a cause for concern if used to replace conventional diesel in modern diesel engines. These properties are summarised in tables 2-3 below. Table 2 outlines the properties of conventional diesel fuel; each property is measured according to The American Society for Testing and Materials (ASTM) standard and has been allocated a test number. This allows the vegetable oils (table 3) to be compared fairly accurately with the conventional diesel.
As seen in the data the viscosity of the vegetable oils is much higher than conventional diesel, this poses a major problem in most modern diesel engines as fuel injection systems are used. The vegetable oil cannot be pumped efficiently and cannot be sprayed into the engine similar to conventional diesel. The vegetable oil would enter the combustion chamber in the engine as large drops instead of a spray leading to a reduction in the surface area of the fuel. This leads to incomplete combustion and the formation of carbon deposits within the engine, blocked fuel injectors and sticky piston rings. Furthermore the unburned vegetable oil has the potential to mix with the lubricating oil in the engine causing it to dilute and eventually lose its effectiveness15, 16. All of these problems are likely to result in a very short engine life and ask questions regarding the cost of using this fuel in the long term. Also another observed fact is that the vegetable oils are less volatile than conventional diesel, this along with the viscosity issue means cold starting is almost impossible in some countries such northern parts of Europe, Canada, etc. due to the cold climates.
However a positive aspect is that the vegetable oils contain lower levels of sulphur (test D129), i.e. the potential to produce unwanted sulphur based emissions is reduced.
Finally in most modern countries especially Europe and the USA, for a fuel to be acceptable it has to meet the environmental and energy security requirements. The operating performance cannot be sacrificed; even small changes in performance can result in a significant number of users to ditch the product. Therefore vegetable oils cannot be used in the current form as an alternative fuel; however changes/modifications could provide an answer. There is the possibility to modify both the fuel and the existing engines but certain factors have to be taken into account first. Existing engines have been developed over many years with billions of pounds spent on research and development by many car manufacturers. The result has been the production of very complex and sophisticated engines with crucial complementary parts attached to them such as turbos, superchargers, etc. Even small modifications in these engines to allow them to be more user friendly with vegetable oils would implement huge costs such changing parts in all existing road vehicles, agricultural vehicles, etc. making it economically unfeasible.
Therefore the best solution lies in modifying the vegetable oils to produce a fuel which can work contentedly with existing diesel engine motor vehicles. The production of this fuel is explained later with the end result being a well known form of fuel known as biodiesel.
As discussed previously, the use of raw vegetable oils as fuels in engines is very impractical and not suitable for the long term. Therefore the formation of a more acceptable fuel has been necessary, i.e. the processing/modification of vegetable oil to a more useful form. This form of fuel is better referred to as biodiesel. Vegetable oils can be modified in a selected few processes to form a fuel with lower viscosity and better combustion properties. These processes are outlined below.
4.1 Biodiesel Production
Vegetable oils are lipids which are mainly made up of triglycerides molecules (tri-esters of glycerol) and free fatty acids (long alkyl chain carboxylic acids). The triglycerides can form di-glycerides or mono-glycerides if one or two of the fatty acid chains are substituted with hydroxyl groups17. This is summarised in the figure below.
There are various methods of processing the vegetable oils into biodiesel which is also known chemically as an alkyl ester.
Transesterification is the name given to the process which converts these lipids into alkyl esters with the use of an alcohol and without first isolating the free fatty acids19. It is as follows:
Triglycerides (Vegetable Oil) + Alcohol ? Glycerine + Alkyl Ester
The main purpose of transesterification of the vegetable oil to its alkyl ester (biodiesel) is to lower the viscosity. The final product (biodiesel) of this process is totally miscible with petroleum based diesel in any proportion7. The reaction is reversible and can be influenced with the use of a catalyst. Generally the reaction takes place at approximately 65oC and 2atm17.
Maximising the yield of the biodiesel is of significant interest as it would help improve the efficiency of the process making the fuel more environmentally acceptable. Therefore investigations have been carried out by various researchers and several factors affecting the transesterification have been noted. An observation made is that a compromise between different factors is most definitely needed as each factor has an effect on the others.
Different types of catalyst can be used which are acid-based, alkali-based or enzymes. Acid-based catalysts usually give a higher yield but involve slower reactions and require higher temperatures than alkali-based catalysts20. Alkali-based catalysts can give very high yields (>98%) in short periods of time (30mins) even at low concentrations21. However these have their drawbacks such as the production of large amounts of soap instead of biodiesel when there are higher quantities of free fatty acids (FFA) present in the vegetable oil. Also the removal of these catalysts is more difficult adding to the final overall cost of the process. A study comparing the acid/alkali catalysts also summarises the result of yield for different catalysts versus temperature below.
Enzyme catalysts form no by-products which is appealing for environmentally conscience industries. They are not affected by other factors such as water content of the vegetable oils or FFA levels. Reaction temperatures are much lower but the process requires a long time to produce the desired product (biodiesel). Also the actual enzymes are generally very costly and mostly economically unfeasible. Finally it should be noted that yield levels can be very high as demonstrated by a study which claims to have produced yields of 98%23. Another factor noted is the reaction temperatures, most studies conclude that the reaction will eventually complete at room temperature. However changes in temperature are heavily influenced by the molar ratios of alcohol to oil therefore various setups are used for the process. The most significant factor is the water and FFA content of the vegetable oil, the presence of these lead to the formation of soaps, loss of catalyst effectiveness and the consumption of catalysts. This in turn reduces the yield of the biodiesel24. This is shown by work carried out by researchers at the Banaras Hindu University in India and shown in the figures below.
This is a process which converts one substance into another with the use of high temperatures and pressures, sometimes also referred to as thermal cracking. Vegetable oil can be processed via this method however the production of biodiesel is very limited. The majority of products formed via this reaction are more suitable for petrol engines therefore commercial production of biodiesel is not usually carried via this route. However improvements in this process could potentially provide the biodiesel which will partially replace the use of petroleum diesel in the near future26.
A micro-emulsion is a mixture of two normally immiscible compounds which have been bought together with the use of a surfactant. The difference between an emulsion and micro-emulsion is that the micro-emulsion is produced without the need for conditions of high shear i.e. formation is done upon the simple mixing of the separate components. Vegetable oils can be mixed with compounds such as alcohols, water and other solvents to form micro-emulsions. This mixture when sprayed into the combustion chamber of the engine has much greater combustion properties than vegetable oil. This is because the low boiling point component such as water, alcohol, etc vaporizes immediately allowing the fuel to atomize and increase the surface area of the fuel27. This solves the problem of the high viscosity vegetable oil which doesn't form spray droplets in the engine as discussed earlier.
4.2 Biodiesel Properties
As a result of the processing methods of vegetable oils into biodiesel the properties of the fuel are significantly changed. The most significant property change observed is the decrease in viscosity. The properties of a vegetable oil, conventional diesel, biodiesel and diesel/biodiesel blend are compared in table 4 below.
The viscosity of the biodiesel is very similar to conventional diesel. The cetane number of the biodiesel is higher than the conventional diesel fuel. The cetane number is a measure of the ignition quality of the fuel where a higher value indicates better ignition properties. It also influences the emissions (both gasses and particulates). Another worthwhile property is the sulphur content which is significantly lower for the biodiesel. Other properties that should be noted are biodiesel fuels contain approximately 10% by weight oxygen which improves the combustion characteristics of the fuel. However biodiesel has lower volumetric heating values, this measures the amount of energy released during combustion for each volumetric unit of fuel. Therefore a greater amount of biodiesel is required to provide the same amount of energy as conventional diesel. The effects of each of these properties are explained further within the following sections of this report.
4.3 Aspects of Biodiesel Use
4.3.1 Net Contributions
Probably the most important factor considered when talking about biofuels is net CO2 emissions. It's no point having a fuel which is more polluting and harmful than what we are currently using. Therefore life cycle analysis has been carried out by many researchers to establish the net CO2 impact of biofuel (biodiesel, bioethanol and others). Biodiesel has net negative CO2 emissions in the majority of studies with results varying according to different authors. This is due to the fact that many factors are taken into account whilst carrying out the life cycle analysis. These include the amount of CO2 produced during the combustion process within the engine, CO2 emissions from biodiesel production industries such as agriculture, processing, etc. and the amount of CO2 actually removed by the plants used to produce biodiesel. One report which looked at biodiesel derived from soyabean oil suggested a net CO2 reduction of 78% when compared with conventional diesel use in North America29. Another more recent study concludes a reduction of CO2 between 57-66% for biodiesel derived from canola oil30.
Another aspect to consider is the net use of energy i.e. the amount of energy used to produce one unit of fuel energy. It has been established that conventional diesel derived from fossil fuels has an efficiency of 83.28% of converting the primary energy source to the final product. Biodiesel is also very similar with an efficiency of 80.55%29. Therefore the production of biodiesel is not resulting is significantly large losses of overall energy. Comparisons within this study are displayed below in figure 6 & 7.
As observed the process energy requirements for both diesel and biodiesel are 0.2009 and 0.2318 MJ per MJ of fuel produced respectively. Both are comparable and the conclusion made is that the differences are insignificant hence the production of biodiesel seems feasible in the long term. Improvements in processing technology are inevitable for biodiesel industries therefore higher efficiencies are expected making this argument only temporary.
4.3.2 Engine Emissions
There are many different types of diesel engines used in motor vehicles therefore measuring biodiesel emissions is expected to be complex. However many projects have been done on this matter and nearly all show the same trend within the results regardless of the type of engine used.
A study carried out on the use of soybean derived biodiesel in 3 modern engines showed a 28-50% reduction in PM and a 38-45% reduction in carbon monoxide (CO) when compared with conventional diesel use. However the levels of NOx showed an increase between 4-13% when compared with conventional diesel31. A later study using biodiesel derived from waste vegetable oils showed reductions in PM by 20%, HC by 40% and CO by 25%. Once again levels of NOx were 24% higher than with conventional diesel use32. Another study which is the latest one carried out looked at the use of biodiesel derived from rapeseed oil. This also concluded with results of reductions in HC by 58% and CO by 34-50%. NOx levels were 24% higher than conventional diesel33.
As seen in the 3 studies the levels of CO emissions are reduced when using biodiesel in diesel engines, this is due to the fact that biodiesel fuels contain a significant level of oxygen and are therefore said to be oxygenated fuels. This allows for the conversion of CO into CO2. As the biodiesel has better combustion characteristics the levels of unburned hydrocarbons (HC) are reduced as incomplete combustion is reduced. Also the higher oxygen content could potentially enhance the oxidation of soot/particulate matter (PM) therefore providing an explanation for the reduced levels of PM emissions34. However a cause for concern is the rise in levels of NOx emissions, this is most probably due to the fact that as combustion is enhanced within the engine due to the improved biodiesel characteristics the temperature within the engine is expected to be higher. The kinetics of NOx formation is dependent on a mechanism which is influenced by the temperature and oxygen availability. Therefore an increase in engine temperature and better oxygen availability from biodiesel is the likely cause for an increase in NOx levels. Although this has a negative aspect which leads to the formation of acid rain it is potentially balanced out by the fact that sulphur based emissions are almost zero due there being negligible levels of sulphur present within the biodiesel to start with. Also the low sulphur content could potentially allow the use of NOx control technology which would otherwise not be used with conventional diesel7. This would enable the engine emissions argument to be completely in favour of biodiesel and allow focus onto other areas where improvements can be made.
Finally it is worth noting that the levels of each type of emission have been observed to be different for biodiesel derived from different sources. Therefore to give a better idea as to whether this is because of different fuels being used in various engines or another factor the following results from a very recent study (2009) are shown below (figures 8, 9 & 10)34. Different biodiesel fuels have been used within the same engine during this study to allow for a good comparison. The fuels used are conventional diesel, waste cooking oil methyl ester (WME), palm oil methyl ester (PME), rapeseed methyl ester (RME), soybean methyl ester (SME) and cottonseed methyl ester (CME). The emissions test was carried out at the same engine speed but various levels of brake mean effective pressure (BMEP) (a measure of the engines ability to produce work at a selected speed).
As seen from the results the levels of emissions are different for various biodiesel fuels. This is a good indication that the use of biodiesels is not as straightforward as it seems, there is potential for some fuels to be less polluting than others. Therefore further analysis and optimisation in this area is required to produce the biofuel.
Although changes in engine performance may be of little concern for environmentalists the majority of car users would be largely put off by fuels offering less power with their current vehicles. As mentioned earlier small changes in performance can result in a significant proportion of car users to ditch the product and also create lots of negative publicity especially in an environment where biofuel has yet to establish itself. Therefore the performance attributes of biodiesel have been of significant interest to both car manufacturers and others concerned with sustainable alternative fuels.
A recent study which looked at the characteristics of soybean derived biodiesel found that the power and torque output of a diesel engine is similar for both biodiesel and conventional diesel use35. This is shown in figure 11 below. However this was not the case with another study which concluded that the power and torque outputs of various biodiesels could not match that of conventional diesel fuel. This study showed that some biofuels could reduce power by up to 20% when compared with the conventional diesel36.The results of this study are outlined in figures 12 & 13.
The differences in the results for both studies might be explainable according to the following reason. Biodiesel has a lower heating value as it contains a significant proportion of oxygen however its density is higher than conventional diesel. Nonetheless it still can mean biodiesel contains approximately 5% less energy per unit volume than conventional diesel7. During the first study the fuel injection pump may have discharged more biodiesel than conventional diesel into the combustion chamber of the engine due to the density difference thus showing no drop in power and torque. Therefore if taken into consideration the results of both studies should be similar, i.e. biodiesel produces less power/torque on a per unit volume basis.
A more accurate method would then be to measure the brake specific fuel consumption (BSFC); this is the consumption of fuel per unit power produced per unit time. The result of this measurement is shown below in figure 14 and is as expected i.e. higher biodiesel consumption.
However this aspect of greater fuel consumption can easily be solved especially in modern engines with injection systems, more fuel can simply be sprayed into the engine to get similar power/torque outputs. Also fuel tanks can be adjusted to meet these requirements. Therefore a much better measure would be to analyse the brake specific energy consumption (BSEC), this factor would eliminate the density issue and would essentially only measure the energy input required to produce per unit power. The results from this outlined in figure 15 below show that biodiesel energy consumption is identical to that of conventional diesel therefore no extra energy is used in biodiesel use making a feasible fossil fuel replacement in diesel engines.
4.3.4 Engine Wear
An engine consists of metallic parts which are sliding across each other consistently. These surfaces are continually grinding against each other resulting in wear. This is usually minimised within engines using lubricating oil which holds the particles such as silica and acids within solution. Therefore looking at the composition of the lubricating oil gives a good analysis of the levels of wear within the engine.
Engines are the heartbeat of all motor vehicles and are usually the most expensive and valuable component of any vehicle. Any damage to the engine by a fuel would be totally unacceptable making the fuel unfeasible in the long run. Therefore biodiesel has to have good characteristics when looking at engine wear. A study carried out on this very aspect showed promising results. A biodiesel/diesel blend of 20:80 was used along with conventional 100% diesel in two new identical engines. After long endurance tests it was concluded that the biodiesel blend fuelled engine formed significantly lower carbon deposits within the engine compared with the conventional diesel tested engine37 38. Another study performed by the same author measured the wear of the vital moving parts within the engine; the results once again outlined the superior characteristics of biodiesel fuel. It stated that the wear of the vital parts was 30% lower when compared with conventional diesel use39. However a cause for concern is the use of rubber seals within engine components. Most diesel engine vehicles use nitrile butadiene rubber seals, these rubbers have been seen to swell and expand when submerged in biodiesel fuel. One study concluded that the rubber seals can actually swell up to 45% over a period of 12 months of exposure to biodiesel fuel40. This is a potential problem as blockages would occur rendering the use of biodiesel useless. However further studies in this area suggest replacing the type of rubber e.g. from nitrile butadiene to fluorocarbon rubbers could solve the problem but further investigation are required. Therefore it can be concluded that the biodiesel properties within engines are superior in terms of lubrication and any problems encountered such as effects on complementary parts (rubbers, etc.) can be overcome by simple replacements.
Finally the ultimate criterion for determining whether biodiesel can actually be used for daily use in motor vehicles is economic feasibility. Fuel, similar to other commodities is driven by cost; higher prices for certain fuels will certainly drive them out of the majority of markets due to there being no demand for the product. Therefore biodiesel has to be cheaper than conventional diesel for there to be uptake by the end users. Currently conventional diesel prices are much lower than biodiesel; this is due to the fact that the feedstock for most of the biodiesel produced is from crops which are edible and that feedstock costs are the major contributor to the final cost (up to 80%)41. The demand for these crops from both food and fuel markets drives prices higher resulting in a high end cost product (biodiesel). The cost of conventional diesel is approximately USD$0.18/l before tax in the USA and USD$0.24/l in Europe. In contrast biodiesel costs are USD$0.6-0.8/l for fuel derived from transesterified vegetable oils, USD$0.9/l for lignocellulosic derived fuel and USD$0.4-0.5/l for biodiesel derived from waste oils41. From this it can be concluded that biodiesel is just not feasible at current prices. However what seems promising is that there is scope for cheaper production of biodiesel from waste oils. Waste oils from industry can reduce the need for edible crops having both advantages in terms of lower feedstock prices for biodiesel but also beneficial for food costs. If taken into account the cost of processing waste oils in water treatment facilities is also reduced. Furthermore if economies of scale are developed i.e. bigger fuel production plants, supply chains, etc. then the end biodiesel price has potential to fall further. Another aspect which can be influenced is taxation, many researchers believe that lower taxes on biodiesel compared to conventional diesel fuel is needed for it to be profitable and encourage more suppliers and producers into the market42. This view on taxation would also be more acceptable to end users as fuels can potentially be taxed according to their net emissions and environmental effects. If this is the case then biodiesel would comfortably outcompete conventional diesel by a large margin in terms of price and be economically feasible.
The use of fossil fuels throughout the last century has had major impacts on our environment and atmosphere. A major contributor to this problem has been the use of internal combustion engines within nearly all aspects of transportation. The use of petrol (gasoline) and diesel has been uncontrollable and an alternative is a major requirement for a sustainable future. Within the last 10-20 years the advances and development in diesel engine technology has meant that diesel use has had significant gains. Diesel powered vehicles provide better efficiencies such as miles per gallon and lower emissions. As a result many see diesel engines as the way forward. Therefore considerable interest has been placed on replacing the diesel used in internal combustion engines without making too many sacrifices.
The use of vegetable oils has been looked at but the problems encountered with this type of fuel are significant. The viscosity of the raw oil is just too high for long term use due to the fact it fails to completely combust within the engine. The result has been high levels of unburned hydrocarbons and carbon monoxide emissions. Furthermore mixing of the raw oil with engine lubricants has meant a loss in the lubrication characteristics ending with short engine lives and higher long term motoring costs which are unacceptable.
Therefore modifications have been carried out to these vegetable oils to form useful fuels (biodiesel) with the most common process employed as transesterification. This process converts the raw vegetable oils which are mostly triglycerides into alkyl esters, mostly methyl esters. As a result the viscosity is reduced allowing the biodiesel to be injected as smaller drops within the engine. This along with the fact that biodiesel also consists of approximately 10% oxygen by volume enhances its combustion properties within the engine. Lower levels of carbon monoxide, unburned hydrocarbons and particulate emissions are therefore produced when compared with conventional diesel fuel. Also the missing presence of sulphur in the biodiesel means sulphur based emissions are a problem of the past but this seems to be balanced out by the increased formation of NOx emissions. These higher levels are mainly due to the increased availability of oxygen within biodiesel and higher engine temperatures arising from enhanced combustion. Analysis of the biodiesel shows that it consumes similar amounts of energy for the amount of work done (measured as brake specific energy consumption) during the combustion process within the engine as that of conventional diesel. Therefore there is no net energy loss whilst using biodiesel making it more acceptable. As biodiesel contains significant amounts of oxygen and hence lower hydrocarbon content it should be noted that it has a lower heating value than conventional diesel. Therefore consumption on a volume basis is higher for biodiesel when compared with conventional diesel fuel. This problem can be overcome easily with the use of bigger fuel tanks and modifications in fuel injection systems therefore more emphasis is placed on comparing energy per unit of work produced as mentioned above.
Biodiesel also seems to be a fuel which prolongs the life of the internal combustion engine. It possess inherent lubrication characteristics which lower engine wear and forms lower levels of carbon deposits on vital moving engine components. However it affects on rubber components such as swelling can be problematic but these can be replaced fairly easily with more suitable forms of rubber.
Finally economic feasibility is probably the most important aspect considered. High prices are inevitably going to make any product not worthwhile in the long run due to the low demand and uptake by end users. Unfortunately biodiesel costs significantly higher (up to USD$0.9/l) than conventional diesel (USD$0.18/l) currently mainly due to the high feedstock prices. Vegetable oils used for production are usually derived from edible plants resulting in demand from food markets. However the use of waste vegetable oils could potentially solve this issue along with the development of economies of scale i.e. larger production plants, bigger supply chains, etc. Another solution which seems to be the biggest factor is taxation, most researchers seems to think that this would solve the price issue in the long run and please end users at the same time. Taxing each fuel in terms of its polluting aspect would make the use of biodiesel feasible in the future.
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