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The world's energy consumption has been increasing progressively since the industrial revolution 1. Global population is continuing to dramatically rise and the increase in economic development, particularly in China and India, over recent decades has contributed to increases in energy consumption 2. Currently, nearly 45 percent of Australia's total energy consumption is accounted for by coal, with oil providing approximately 35 percent, natural gas supplying around 15 percent and 'green' power providing just over 5 percent 2. Alternative fuels are stated to be a potentially viable alternative to the use of fossil fuels. Fossil fuels contribute to almost eighty percent of the total energy used in the world 3, 4. Fossil fuels are non-renewable resources that are limited in their supply and the burning of fossil fuels on a global scale can produce air pollution, such as nitrogen oxides (NOx), release significant amounts of greenhouse gases, such as carbon dioxide (CO2), and contribute to global warming 1, 3, 5, 6. Alternative fuels are considered to have a less adverse effect on the environment, and are stated to be a solution to the problems created by fossil fuels 1. The main difference between fossil derived fuels and alternative fuels is the oxygen content, with alternative fuels having 10-45 wt% oxygen compared to fossil fuels which contain almost none 7. In Australia, the alternative fuels industry is relatively small, but is gradually expanding particularly in regard to ethanol or biodiesel production8. Alternative fuels, such as ethanol and biodiesel may prospectively provide an alternative for global fuel requirements. There are however, a number of drawbacks related to alternative fuels which may inhibit them from completely replacing fossil fuels without technological or genetic advancements 9.
This paper aims to examine the advantages and disadvantages of alternative fuels. The discussion will commence by defining alternative fuels and a number of relevant terms. The analysis will then consider ethanol as an alternative fuel and an overview of ethanol production. Subsequently, biodiesels and an overview of biodiesel production will be examined. Following this, the advantages and disadvantages of alternative fuels will be considered. Lastly, the main arguments of this account will be summarised.
Alternative fuels are a sustainable form of energy, are fuels that have not derived from petroleum and can include alcohols, biofuels, hydrogen, natural gas and propane 1. They are commonly solid, liquid or gas biofuels acquired from biomass, vegetable oil, or generated from agricultural food crops 6, 10. A viable alternative fuel must be economical, supply a net energy gain, be beneficial to the environment, and be able to be produced in considerable volumes with limited detrimental impacts6. The alternative fuels that are currently prevalent throughout the world are ethanol and biodiesel 7, 11, 12. Ethanol also referred to as ethyl alcohol, is a colourless liquid alcohol, less dense than water, with the chemical formula C2H5OH 13. Biodiesel is a carbon-neutral fuel that is a mixture of fatty acid methyl esters 13, 14.
Ethanol is one of the most widely utilized liquid biofuels that can be combined with gasoline to create an ethanol blend fuel, or it can be used in pure form 7, 13. It may be derived by fermenting carbohydrates obtained from natural sugars, starches or cellulosic biomass in plants including sugar cane, corn or straw 7, 15. Generally, this process involves the yeast driven fermentation of glucose in which the energy from the glucose is concentrated in the ethanol and CO2 is released as seen in equation 1 13.
The USA and Brazil are presently the two major producers of bioethanol with corn and sugar cane the primary base plants for the US and Brazil, respectively 16, 17. Current fuel yield for corn ethanol has been demonstrated at 1135 - 1900 L/hectare; to achieve 50 percent of American transport fuel demands, more than 157 percent of USA cropland would need to be utilized 17. In 2005/2006, Australia produced and consumed 41 million litres of ethanol fuel 8. Use and production of ethanol, however has been demonstrated to be increasing 8. In Australia, since 2003, the maximum allowed limit of ethanol in ethanol-petrol blends has been 10 percent 8. In 2007, there were three ethanol production facilities operating in Australia that generated ethanol from grains, such as sorghum and wheat, and from sugar cane, with production capacities of over 150 million litres annually 8.
Biodiesel generation emerged in the late 1990's and has continued to rapidly increase 13. Biodiesels are produced from a variety of seed plant oils, such as soybean, palm or rapeseed oil, or from animal fats 10, 15, 18. They are mostly manufactured through the process of transesterification 18. Transesterification involves the mixing and of an oil with alcohol in the presence of a catalyst, such as sodium hydroxide, which reacts to create biodiesel and glycerine 18. The triglyceridies in the oil or fat are converted into a glycerine molecule and three methyl esters of long chain fatty acid molecules (Figure 1)13.The glycerin by-product is not in market demand due to the excess produced from biodiesel generation and recently been discovered to convert into propylene glycol, which has a large market, and acetol 13. Biodiesel manufactured from soybean produces more than 90 percent more energy than is needed to generate it 13.
Figure : Transesterification of a triglyceride 18.
Biodiesel is manufactured globally, with the USA and Germany dominating the market 20. Around the world, over 1.5 million tonnes of biodiesel is generated annually 15. In 2005/2006 Australia produced and consumed 16 million litres of biodiesel fuel 8. Use and production of biodiesels has increased since8. In Australia biodiesel can be utilized as a replacement for diesel or in a biodiesel-diesel blend of between 5 and 20 percent biodiesel 8. Investigations into utilizing canola or mustard for biodiesel production are occurring in southern Australia while currently, a range of fats and oils are used, such as tallow 8. In Australia, biodiesel production capacity is greater than ethanol production capacity 8.
Advantages and Disadvantages of Alternative Fuels
Alternative fuels such as ethanol and biodiesel have numerous advantages and disadvantages regarding environmental and societal impacts. The replacement of fossil fuels with alternative fuels could result in a decrease of CO2 emissions, a decrease in air pollution, reduce acid rain and decrease global warming3. Additional benefits include sustainability, fuel security, regional development and a decrease in rural poverty 7, 21. The land area required to generate sufficient alternative fuel to meet demands however, may compete directly with agriculture requirements, water use, may cause pollution from the use of herbicides and pesticides, and could result in the destruction of natural habitats and a decrease in biodiversity 3, 21, 22. In developing countries, the expansion of the alternative fuel industry could increase deforestation, decreasing the removal of CO2 from the atmosphere through photosynthesis 12. The prices obtained for the sale of, for example, Australian produced alternative fuels is variable and depends on domestic prices for petrol and diesel, the world prices of oil and the Australian exchange rate 8.
There are several advantages concerning the use of ethanol as an alternative fuel. Carbon dioxide in the atmosphere is removed by the crop used for ethanol generation, resulting in the process being carbon neutral 15. Using ethanol over fossil derived fuels can result in a reduction of CO2 up to 75 percent 24. Higher densities of ethanol fuel and air can be combusted in an engine compared to petroleum due to ethanol's constricted boiling point range and higher latent heat of vaporization (Table 1) 10, 16. Additionally, higher energy density can be achieved in the engine due to a lower stoichiometric air to fuel ratio 10, 16.This may produce enhanced engine efficiency and elevated power outputs in ethanol fuelled vehicles when contrasted to petroleum fuelled vehicles. The use of ethanol as an alternative fuel can result in lower pollution emissions compared to fossil fuels, particularly regarding particulates, alkenes, aromatics and carbon monoxide 13.
There are disadvantages associated with the use of ethanol as an alternative fuel. At present, ethanol is not economically competitive 15, 24. Expensive production costs associated with ethanol can be generated due to the use of water for irrigation, herbicides and pesticides, fertilisers, and machinery maintenance 11. Other expenses may be accrued from wages, insurance, land charges, and depreciation of farm assets 11. Ethanol has a lower amount of energy generated per litre combusted, is more corrosive than gasoline and is toxic to ecosystems 13, 20. The use of pure ethanol is limited in colder climates by their low vapour pressures 13. Ethanol production from crops is stated to cause environmental degradation such as dust and exhaust emissions, displacement of alternative land uses, pollution of surface and ground water systems, increased turbidity in surface waters, salinity and bioaccumulation of some pesticides 9, 11. There are a number of environmental problems related to corn generated ethanol such as denitrification, erosion, and nitrogen leaching 9, 11. Ethanol crops are generally inefficient and the yields vary dramatically between various crops 15, 17. The disadvantages noted above indicate that currently, ethanol is not a viable alternative fuel.
A number of advantages are linked with the use of biodiesel fuel over fossil derived diesel fuel. Biodiesel production is stated to be sustainable, environmentally friendly, nontoxic, and biodegradable 1, 5, 10. Biodiesel's properties are alike the properties of fossil derived diesel, Table 1, suggesting it can be used unmodified in indirect injection diesel engines 5, 16. Table 1 demonstrates that biodiesel has a higher flash point than petroleum-based diesel making it is safer to store and transport 10, 16. Biodiesel burns cleanly, and the emissions produced have fewer pollutants including less carbon monoxide, sulphates and sulphur oxides, hydrocarbons, nitrogen and particulates 1, 5. It also has a small increase in fuel economy and superior lubricity compared to fossil derived diesel, which can reduce engine wear 7; 14.
There are many disadvantages regarding the use of biodiesel as an alternative fuel. Biodiesel production from crops competes with food supply, can detrimentally impact the environmental through nitrogen and phosphorus leaching, and loss of biodiversity 6, 7. Biodiesel generally generates lower power and torque than petroleum-based diesel and can have a higher fuel consumption 10. Biodiesels are also not currently economically competitive 6. They are stated to have higher emissions of nitrous oxides and cold start problems 7. The biodegradability of biodiesel can create problems regarding fuel stability and long-term storage 14. Methyl ester fatty acids deteriorate in conditions with high temperatures, sunlight, oxygen or non-ferrous metals 14. The disadvantages noted above indicate that currently, biodiesel may not be a viable alternative fuel.
Alternative fuels such as ethanol and biodiesel have both advantages and disadvantages regarding impacts on the environment. Ethanol is usually produced through the fermentation of carbohydrates while biodiesel is generally produced by transesterification. Within Australia, the industry of alternative fuels is steadily expanding. Advantages of alternative fuels may include decreased emissions and air pollution, reduced impact on global warming, sustainability, fuel security, regional development and a decrease in rural poverty. Disadvantages of alternative fuels may include land use competition, water use competition, pollution from the use of herbicides and pesticides, deforestation, the destruction of natural habitats, and a decrease in biodiversity. Currently, the production and use of the alternative fuels ethanol and biodiesel do not appear to be viable.