English
中文Introduction
Modern societies and industrial sectors utilise large quantities of energy, which is mainly derived from fossil fuels. Among these, the transport sector demands approximately 84 million barrels a day, which has been projected to increase up to 116 million barrels a day by 2030 (Anon, 2007). Since the transport sector uses fossil fuels as their main source, there is a necessity and proper action plan to implement sustainable energy sources to replace the fossil fuels at least to some extent. The bio fuels from the plant origin seems to be an promising source of sustainable fuel with the capability of substituting the fossil fuels alone or in combination with other source in a hybrid system. Therefore, this issue need to be addressed in concise manner to be adopted more and more people.
Overview
The scientists demonstrate that the emission of green house gases such as carbon dioxide, methane and nitrous oxide arise from fossil fuel combustion and changes in land use pattern due to human activities are main source for the climate change (Anon, 2007a). According to Houghton et al. (1990) carbon dioxide released by the burning of fossil fuels is the most prominent component in green house gas emission. In general, the transport sector alone contributes to about 20% of carbon emission; however it goes up to 25% in United Kingdom. The increasing trend in these figures, let policy makers to think seriously to find a sustainable alternative to meet this challenge that was set to reduce the carbon emission by 60% by 2050 (Anon, 2007). It is obvious that transport has become the main driver for increasing global primary oil demand. Simply cutting down the transport won’t be the solution for this problem as mobility become as the essential part in our modern life style, however proper management and possibility for the alternative fuel usage can help to attain the target.
According to Hughes (1991), energy in the UK transport sector is dominated by road transport, which accounts for about 80% of energy used in the sector, three-quarters of which is consumed in private cars. Relatively inexpensive regarding its high energy density, easy to handle with less storage capacity in vehicle and wide available may be the main reasons for the fossil fuels (oil), are being used for the transport. On the other hand, burning fossil fuel in the vehicle engines is thermodynamically inefficient because a considerable percentage of energy is lost as waste rather than turned into kinetic energy of movement. In principal, electricity, as a high-grade energy source, is much more suited to supply the high grade kinetic energy required in transport. However, if the electricity is supplied from conventional thermal power station, there is still a great deal of waste heat generated. This is not the case, of course, if the electricity is generated from a high-grade source, such as hydro power or many of the other renewable energy sources. To improve the energy use efficiency can be driven by improving the efficiency of the motor engines using lighter weight materials, lower drag coefficients, electronic engine controls, lower resistance tyres, and low friction engine coatings (Hughes, 1990). It is clear that no single solution can address this problem. Combined action such as improvement of fuel efficiency, technical improvement, social changes such as car sharing, increased availability and use of public transport, more walking and cycling and introducing new fuel and technology may make cause considerable improvement.
Among the existing alternative, bio fuel have a unique characteristic which can address the predominant issues like renewable, carbon balance and can be cultivated even marginal lands. But when developing bio fuel for the global demand, the alternative should be feasible on technical, social, economical and environment aspect. Since each is unique it’s necessary to assess them individually (ESTU, 1991). According to UK Energy Technology Support Unit the bio fuel is defined as any solid, liquid or gaseous fuels produces from organic materials, either directly from plants or indirectly from industrial, commercial, domestic or agricultural waste. Among these sources plants grown especially as energy sources, have attracted increasing attention in recent years (ESTU, 1991). The main advantages of using bio fuels are sulphur dioxide emissions can be reduced, additional lubricity, reduced unburned hydrocarbons, carbon monoxide and particulate matter in exhaust fumes and can improve engine performance.
The primary component of biomass is carbohydrate (sugars, starch and wood) which must be converted into much more concentrated form such as hydro carbon. Bio fuels are currently produced from the products of conventional food crops such as wheat maize, sugar cane, palm oil and oil seed rape. Sugar from sugar cane/sugar beet and starch from maize/wheat have extensively been used to produce ethanol. When using same materials for food/feed and bio fuel, there are may be some social, economical and environmental impacts. Therefore, attention now focused on the potential of grain sorghum as a replacement for maize and sweet sorghum for sugar cane (ESTU,1991). Recent studies are driven to produce bio fuels from broader range of feedstock including the lignocelluloses in dedicated energy crops such as perennial grasses (Switch grass, Miscanthus reed canary grass and giant reed) since these need minimal cultivation (Lewandowsk et al., 2003). A significant advantage of developing and using dedicated crops and trees for bio fuels is that the plants can be bred for purpose. This could involve development of higher carbon to nitrogen ratios, higher yield of biomass or oil, cell wall lignocellulose characteristics that make the feedstock more amenable for processing.
Genetic modification technology addresses the issues which are faced by cellulosic biomass in refinery process. The celluloses and pre-treated process increase the cost of production than that of corn grain ethanol. The attempt of growing genetically modified plants in order to produce celluloses and hemicelluloses and reduce the need for pre-treatment processes through lignin modification is a promising path to solving this problem. However, it would be more effective once it is incorporated together with other strategies such as increasing plant polysaccharide content and overall biomass without affecting the structural strength and pest and disease resistance (Sticklen, 2008). Recent studies have revealed that there are potential of producing high efficiency bio fuels from micro organisms like E.coli bacteria and sea weeds.
However, algae bio diesel is one of several sustainable sources of bio diesel. Algae bio diesel looks much more promising for the future since it is exempted from the general problems arise with bio diesel sourced from crops. The advantages of producing algae bio fuel are waste water can be effectively utilised, more efficient bio fuels than other plant sourced fuels, fast growing and quick in action, save the land for food production, etc. Algae bio diesel is also called as algal fuel, third generation bio fuel and oilgae (Anon, 2009).
Discussion
Bio refinery converts biomass into liquid transportable fuels or valuable chemicals and other products. Because of the renewable nature of bio mass, a bio refine is an important element in a circular economy which seeks to minimize negative ecological impacts.
The benefits of an integrated bio-refinery are numerous because of the diversification in feed stocks and products. There are currently several different levels of integration in bio-refineries which adds to their sustainability, both economically and environmentally. For example, some bio-refinery concepts solely produce ethanol or biodiesel, whereas other concepts fully incorporate livestock farming or heat and power and other bio based products. Most of these refineries are nearly self-sustaining in respect to energy consumption.
UK Government’s Environmental Audit Committee (EAC) reveals that there are several limitations in using bio fuel since it is a sustainable source. This report also states that there is no carbon neutral bio fuel because the net emission during agriculture practices, manufacturing and distribution generally exceeds the absorption by the plant during photosynthesis. If the existing agriculture land needs to be shared with the bio fuel production, the potential increased usage of fertiliser to compensate the food demand may lead to the increased release nitrous oxide. This will increase green house gases about 300 times more potential than that of carbon dioxide. The global expansion of bio fuel agriculture may also lead to higher food price, environmental impact of habitat loss, soil erosion and water depletion, which may become a severe threat to the future generation (Anon, 2008).
Therefore studies need to be undertaken to address the issue in relation t its sustainability. Especially bio fuel feed stock and conversion technology development must occur with the end use needs in mind that is engine usage efficiency and vice versa. To achieve this, research and development must be integrated across supply chain. The feed stock need to be from variety of sources such as annual food crops, perennial energy crops and forestry, agricultural co-products such as straw marine organism and micro organism. These sources should increase the yield per hectare, while reducing the negative environmental impacts. Application of genetic modification technology will ease the issue in relation to lignocelluloses feed stock problem. Emphasis needs to be given to the synthetic biology, increase techniques in machineries and process in bio-refineries.
Conclusions
It can be concluded that the increasing trend of fossil fuel usage in demand of the busy life style as a main source of fuel for the transport pollute the environment in several ways. Therefore, there is a necessity and urgent action to replace the fossil fuel by a sustainable alternative. Several sustainable alternative energy sources have been extensively studied and in practice to some extend in some part of the world. However, using the bio fuels from the plant origins, especially from the purposely grown plants, is considered as an effective alternative fuel for the sustainable future.
