Global energy demand is increasing dramatically with the increasing population, as well as improving living standards. Currently, most of this energy demand (89%) is met by fossil fuels (i.e. crude oil, coal and natural gas), with nuclear and renewable fuels making a small contribution (11%) [123]. Total primary fuel consumption in 2012 was observed to be 70%
higher than in 1987 [123]. As shown in Figure 2-4, current global consumption is equivalent to more than 11 billion tonnes of oil every year, with crude oil reserves diminishing at a rate of 4 billion tonnes per year. If these rates continues, current crude oil reserves will be depleted by 2052 and gas by 2060 [124]. Coal reserves will serve our energy needs for a few more decades, but this form of energy is not suitable for every application, especially in the transportation sector. A similar prediction can also be found in an earlier model [125].
Figure 2-4: The total reserves of coal, natural gas and crude oil and their predicted diminishing trends[124]
Globally, the transportation sector is the second largest energy consuming sector which uses fuel in the form of liquid or gas, second only to the industrial sector. It has experienced steady growth over the last 30 years, due mainly to the increasing number of cars in operation around the world, and with global transportation sector energy use expected to rise by an average of 1.8% per year from 2005 to 2035, it is also the fastest growing energy demand sector in the world. This growing demand for liquid fossil fuel, in conjunction with its limited reserves, is driving the world to look for alternative energy sources which can accommodate future demands. Biofuel is considered as a potential source to meet this demand. According to the International Energy Agency (IEA) 2011 roadmap, 37% of total energy used in the global transportation sector will be replaced by biofuel in 2050.
Figure 2-5: IEA prediction of biofuel demand in 2050
Figure 2-5 displays the projected trend of biofuel demand between the periods of 2010 to 2050. Biofuel is considered to be a renewable, biodegradable, environmentally friendly and energy efficient fuel, which is believed to have the potential to fulfil energy security needs without sacrificing the engines operational performance. Among the different kinds of biofuel, biodiesel and bio-ethanol are the most prominent. Biodiesel is a mixture of alkyl esters of long-chain fatty acids, usually methyl or ethyl esters, obtained by the trans-esterification of triglycerides, obtained mostly from vegetable oils or animal fats. Interest in biodiesel is increasing, due to the rising concern in relation to greenhouse gas emissions.
Biodiesel combustion is neutral in terms of CO2 emission, which means that the combustion of biodiesel liberates the same amount of CO2 as the plant absorbed during photosynthesis. The use of pure biodiesel instead of diesel prevents 2.8 kg of CO2
emissions being released into the atmosphere per kg of biodiesel. The EU reduced 8% of their total greenhouse gas emissions between 1992 and 2012 as a result of their biodiesel use [126]. Biodegradability of biodiesel is twice as fast as diesel, which also prevents soil and ground water pollution. Biodiesel can be used in conventional diesel engines, either as a blend with diesel or as 100% biodiesel (B100), without any engine modification. The lubricity of biodiesel is also higher than diesel, which causes less wear and tear on piston rings, cylinder walls and the precision parts of pumps for fuel injection, which may extend engine life.
Interest in biodiesel is also increasing due to the diversity of its production sources.
Biodiesel can be produced from variety of sources, including vegetable oil, animal fats, domestic waste, industrial waste and microalgae [127-129]. Biodiesel production from edible vegetable oils and animal fats is marked as unsustainable by many studies, because of its growing conflict with human and animal foods [130]. However, this conflict can be largely minimised by choosing 2nd generation biodiesel feedstocks, which are non-edible [123, 131-133]. In addition, 3rd generation biodiesel feedstocks (i.e. microalgae) are considered as the most sustainable biodiesel feedstock because of their high production rate on limited, non-arable land [132, 134]. There is also evidence of environmental benefits from biodiesel production using industrial and domestic waste [135]. Hernandez et al. [135] recovered 94% of residual oil as biodiesel from Alperujo (residue of olive oil industry), which is highly toxic [136] and requires treatment prior to disposal. [137].
Recent literature even indicates that biodiesel can be produced from insects [138].
Apart from the issue of energy security, the use of biodiesel is expanding around the world due to its low emissions. According to a review study [139], well-to-wheel CO2 benefits of biodiesel are in the range of 50–80%, compared to fossil diesel, depending on the feedstock and the production process used. Numerous studies have reported reductions in CO, HC and PM from using biodiesel [117, 140], which is primarily associated with its oxygen content, higher cetane value and the absence of aromatic hydrocarbons and sulfur.
Although biodiesel generally reduces PM emissions, studies have found a decrease in particle size but an increase in particle number (PN) [62, 141, 142], which is significant when determining the health impact of DPM, with toxicological studies suggesting an increase in health effect from smaller particles. Some studies [143] suggest biodiesel decreases PAH emissions, which is beneficial as most PAHs are carcinogenic, however there are also reports of an increase in the soluble organic fraction (SOF) of DPM when using biodiesel [62].
Global biodiesel production has grown steadily over the last two decades and has become one of the fastest growing chemical industries. The average annual growth rate of biodiesel production was about 17% globally and 35% in EU between 2007 and 2012 [126], reaching 22.5 billion litres globally in 2012. The EU is third largest producer of biodiesel after the USA and Brazil, but it consumes highest amount of biodiesel due to government directives which ensure that 2% of the energy used in transportation comes from biofuels.
Figure 2-6 illustrates the increasing trend in biodiesel demand in EU nations from 2002 to 2010. This trend is expected to continue, with many new policies and regulations favouring the use of biodiesel. According to a new EU renewable energy directive (RED2009/28/EC), member states should meet 20% of their total energy demand and 10%
of transport sector energy from renewable sources [144]. Global trends are likely to follow the EU, as many other countries are also starting to promote policies favourable to the industrialisation of biodiesel (i.e. India, Indonesia, Malaysia and other Asian countries) [126].
Figure 2-6: Existing/required production capacity of biodiesel in the EU [126]