For the developing countries like Ethiopia to have a sustainable development there is a need to develop rural community so that the disparities in basic life conditions between urban and rural areas can be reduced. Which of course may not be possible without the availability of sustainable energy systems in rural areas. The need for sustainable energy production has prompted new research into the possibility of gasification as a key source of energy production. Biomass technologies, such as biomass gasification, that use locally available resources, would enable poor rural areas to access the electricity produced in a decentralized power plants. It would bring the opportunity to experience an economic and social development, consequently offering more employment to the local people, more opportunities for basic health care and at the end of the day, bringing welfare to the rural communities. In this paper, an attempt is made to give the overall view of the gasification technologies and the advantages and disadvantages of the Biomass gasification energy system.

Key words: Rural development, biomass gasification, and pollution

1. Introduction

Developing countries contain 80 per cent of the world's population but consume only 30 percent of global commercial energy. As energy consumption rises with increase in population and living standards, awareness is growing about the need to expand access to energy in new ways. As recognition grows of the contribution that renewable energy can make- towards rural development, lowering the health costs (linked to air pollution), energy independence and climate change mitigation- renewable energy is shifting from fringe to the mainstream of sustainable development. Current energy sources and patterns of energy use are unsustainable. Continue to consume ever-greater amounts of fossil fuels will cause too much damage to the environment, risk unprecedented climate change and rapidly deplete petroleum resources. Current trends in energy supply and demand will also exacerbate inequity and tensions among nations, tensions that fuel regional conflict. In short, continuing dependence on non-renewable energy sources will put the well being of future generations at risk. By growing reliance on renewable energy sources such as solar energy, wind power and bio energy, all of the problems associated with current energy patterns and trends can be mitigated. However, a formidable set of barriers is limiting the transition to renewable energy sources in most parts of the world.

The sustainable development of countries like Ethiopia and India is possible only with the development of the rural population. The rural people are suffering a lot under the scarcity and non- availability of affordable alternative energy supply. The hope that the rural areas would be electrified by expanding the central electric power grid is too far from being feasible due to economic reasons. Conventional energy sources based on oil, coal and natural gas proven to be highly effective drivers of economic progress, but at the same time, promoting serious damage to the environment and human health. Renewable energy sources currently supply about 15-20% of global world's energy demand. This supply is dominated by traditional biomass, mostly wood used for cooking and heating, especially in Africa, Asia and Latin America. The scattered and remote nature of rural communities makes the extension of grid power very uneconomical. Decentralised power systems, driven by biomass can be viable options for electrifying such remote areas. Ethiopia has enormous biomass potential, which has not yet been well exploited. Farmers can own his own farm biomass based power plant, the power that can be used to meet his farm requirements like pumping water etc.

2. Biomass and Biomass Energy Conversion Systems

2.1 Biomass

Biomass is the term used to name material derived from plants (grass, trees and crops) and animals. Plantal biomass is mainly composed of carbon, oxygen and hydrogen and traces of mineral elements such as nitrogen, potassium, phosphorus, sulphur and some others. Biomass energy is a form of solar energy because it depends on photosynthesis. Green plants transform the energy of sunlight into chemical energy in the chemical bonds of the structural components of biomass; by converting carbon dioxide from the air and water from the ground into energy rich organic compounds mostly sugar cellulose, starch, legnin and also proteins and oils. When burning biomass (extracting the energy stored in the chemical bonds) efficiently, oxygen from the atmosphere combines with the carbon from biomass to produce carbon dioxide and water. Both water and carbon dioxide are the basic compounds that, together with inorganic nutrients from the lithosphere, driven by the solar energy, build up new biomass "organism" through the process known as photosynthesis. Thus biomass is a renewable resource. Biomass from animals comes from animal excreta (waste), which still contains carbon (or organic material) and therefore can be submitted to fermentation (anaerobic/ aerobic digestion), converting it into biogas or light alcohols.
Many bio-energy conversion technologies offer flexibility in choice of feedstock and the manner in which it is produced. In contrast, most agricultural products are subject to rigorous consumer demands in terms of taste, nutritional content, uniformity, organoleptic properties, etc. This flexibility makes it easier to meet simultaneous challenges of producing biomass energy feedstock and meeting environmental objectives. For instance, unlike the case with food crops, there are good possibilities for energy crops to be used to revegetate barren lands, to reclaim water logged or salinated soils, and to stabilize erosion prone land. Biomass energy feedstock when properly managed can both provide habitat and improve biodiversity on previous degraded land.

The externalities of bio-energy, which are not accounted for its cost, are important to be considered as well and can offer benefits compared to fossil fuels. Its carbon neutral character is one of those externalities. Furthermore, biomass has very low sulphur content. It is available to most countries in the world, while fossil fuels need to be imported from a limited number of suppliers. Indigenous production of energy has a macroeconomics as well as employment benefits: it can offer relatively large numbers of unskilled jobs, which can be important for many developing countries. Although there are environmental impacts related to bio energy, it is usually considerably more beneficial in terms of external costs than coal, gas or oil. Coal, diesel oil and natural gas emit respectively about 2240%, 1630% and 1390% as much carbon as released by biomass fuel (Short Rotation Coppice-Wood). It stresses that biomass is far environmentally friendly fuel than fossil fuels.

2.2 Biomass Energy Conversion Systems

Biomass energy conversion systems range from simple, traditional processes to modern, highly efficient technologies. Some of these systems are now commercially and fully available. Others still require certain technical improvements and declining costs and some others need long-term funding to allow sustainable techniques to be developed and to encourage replicability. The "primitive" biomass technology conversion is the direct combustion, which is still widely used in scales ranging from small scale (household uses for cooking or heating) to industrial scale, for heat and/or electricity generation in different scales up to hundreds of MWe. Presently, biomass energy technologies consist of many other conversion technologies used to extract biomass energy and convert it into a more useful form. They can be divided principally in three main groups:

1. Thermo chemical conversion
 Direct combustion
 Gasification
 Pyrolysis

2. Bio-chemical (biological) conversion
 Bio digestion
 Fermentation

3.Physico-chemical (mechanical) conversion.

Biochemical processes refer mainly to the conversion through bio digestion in the absence of air (oxygen) leading to a formation of biogas (mixture of CO and methanol) and fermentation, in an aerobic environment, giving methanol and ethanol, as products.
The most dominant way of extracting biomass energy still is the direct combustion.Nevertheless, direct combustion gives energy low transfer efficiency since it depends in many factors including the reaction conversion rate. An intensive R&D is still underway to improve direct combustion energy efficiency.
Gasification is another thermochemical conversion process, which converts dry biomass into a mixture of fuel gases that can be burnt in internal combustion engines and gas turbines. Actually, air gasification is a thermal process that takes place in a special sealed container in a poor oxygen environment.
Pyrolysis is the process that converts biomass into liquid fuel (bio-crude), solid and some gaseous fractions, in the total absence of air at relatively high temperature (about 500ºC).

Table 1. Feedstock for different Biomass conversion Technologies

3. Biomass Gasification: Background history and Theoretical framework

3.1 Background History
The basic principles of biomass gasification have been known since the late 18th century and commercial applications of the principle first recorded in 1830. By 1850, large parts of London had gaslights and there was an established gas industry manufacturing ‘producer gas’ from coal and biomass fuels. The use of producer gas to run internal combustion engine was first tried around 1881, and because the gas was sucked by the engine from the gasifier, it was referred to as 'suction gas'. Early gasifier designs show many features in common with more modern designs.

By 1920s, producer gas systems were being used to operate trucks and tractors in Europe. While it was demonstrated that it was possible to operate engines with producer gas, it was not convenient or reliable and producer gas systems for operating mobile or stationary engines did not gain acceptability. In the other hand, the advent of petroleum accelerated a decline in the need for producer gas, and it fell from popularity.

Biomass gasification systems reappeared with a force in Europe, Asia, Latin America and Australia during World War II as a result of the scarcity of petroleum fuels. In Europe alone, almost a million gasifier-powered vehicles helped keep basic transport systems running during the war. In most cases, the gasified biomass fuels were either wood or charcoal. After World War II, gasifier systems were generally abandoned, triggered by the re-emergence of convenient and relatively inexpensive liquid fossil fuels.
The publishing of the 'Gengas' by the Swedish Academy of Engineering in the 1950 was a major break through in the promotion of gasification. This classic book on gasification outlines the scientific, technical and commercial information developed during World War II. Much of the information within this book remains relevant today, and is probably the singularly most important book published on gasification.

3.2 Theoretical Framework
Biomass energy has the potential to be "modernised" worldwide, that is, produced and converted efficiently and cost competitively into more convenient forms such as fuel gas and liquids or electricity. A variety of technologies can convert biomass into clean, convenient energy carriers over a range of scales from household, village to large industrial. One of these technologies is gasification through which biomass is converted into fuel gas. It has been ages since gasification has been known, in the late 18th century. Since then, it has proven to be capable of enabling biomass to play a much more significant role in the future than it does presently, especially in the developing countries. It is a fact that raw biomass has several disadvantages as energy source. It is bulky with low energy density (about 16-20 MJ/kg) and, direct combustion is, generally, highly inefficient (as above mentioned) and produces high levels of indoor and outdoor air pollutants.

The goal of modernised biomass energy technologies is to increase the fuel's density while decreasing its emissions during production and use.
Gasification is, as stated before, a thermochemical conversion process that converts biomass into fuel gases. It can be classified in several categories, according to different reference items. Referring to the oxidant specie used for biomass oxidation, it can be regarded as air gasification, the common way of gasifying solid fuels, meaning that it uses oxygen from the air as oxidant; pure oxygen gasification and steam gasification, when using pure oxygen or steam as oxidants.
Another alternatives are a mixture of air/oxygen and steam or (less used) carbon dioxide and hydrogen.
It can also, according to the energy source, be classified as autothermal, meaning that it gets energy from it's self oxidation phase to complete the process; or allothermal, meaning that energy must be supplied to "heat" the distillation phase, through pre-heating of the gasifying agent.
Air biomass gasification, the principal subject of this study, comprises four principal stages determined by chemical changes together with energy flows in form of heat. These four stages can be summarised by the following reactions:

Stage I: Drying
Wet biomass + Heat = dry biomass + H2O
Stage II: Pyrolysis (Distillation)
Biomass + Heat = Pyrolysis gas + Charcoal
Stage III: Combustion (Oxidation)
C + O2 = CO2 + Heat (1)
4H + O2 = 2H2O
CnHm + (n/2+m/4) O2 = nCO2 + m/2 H2O
Stage IV: Reduction
C + CO2 = 2CO
C + H2O = CO + H2
CnHm + nH2O = nCO + (m/2 +n) H2
CnHm + nCO2 = 2nCO + m/2 H2

Fig 1. Biomass cycle

As already stated, through gasification, solid biomass can be an energy source for gas engines and turbines, for electricity production or for "internal combustion engines", even though its calorific value still relatively low. The product is a convenient and modernised fuel gas that can be used as the conventional fuel gas with the advantage of releasing less harmful emissions. Thus, gasification is referred as being the way of adding value to the solid biomass energy. According to De Montfort University, gasification converts biomass into combustible gas with 60-70% of initial energy content, and is a potential energy source for electricity production through combustion engines or turbines.

4.Gasification Technology

Gasification takes place inside a suitable vessel named gasifier which is characterised according to the design of fuel bed and the method in which the solid fuel is brought to contact with the oxidant (air, oxygen, steam, hydrogen, carbon dioxide or various mixture of previous species). According to the fuel gas end use, the gasifier system can be divided into Heat Gasifiers- used for fuelling external burners in boilers, kilns or dryers; and Power Gasifiers - coupled to internal combustion engines for shaft power producing. Additionally, apart from being auto/allothermal or Heat/Power, gasifiers can be classified as i) fixed bed; ii) fluidised bed and iii) entrained bed designs. Although fluidised and entrained bed gasifiers are robust and versatile in their operation, they're, at the same time, generally more difficult to design, build and operate, more expensive and not recommendable for small scale (<1MWe) applications. In the other side, fixed bed gasifiers are the most common, especially in the poor countries due to their simplicity on designing and construction, as well as low investment, operational and maintenance costs.

FBG can be divided into Bubbling (BFBG) and Circulating (CFBG) gasifiers.
BFBG give a good temperature control and high conversion rates, good scale-up-potential, possibility of in-bed catalytic processing, are tolerant to particle size and to fluctuations in feed quantity and moisture. Although their product gas has low tar content, unhappily, it is rich in particulates. CFBG are suitable for fuel capacity higher than 10 MWth. Compared to BFBG, they have the additional advantage of giving high gas quality.

Fig 3. Power Gasifier

5.Economics and Social Implications

This technology is still in its infancy, therefore there are many unanswered questions regarding its economic viability. The only way for this to be investigated is through demonstration plants but these are expensive and require large investment. Companies are not willing to invest in something that has a long or limited pay back; this is where governmental support is key to the growth of the industry.
Which biomass technology is the most economically viable depends on site-specific circumstances. This depicts the type of feedstock that is available and therefore which method of generation is best suited. The transportation of the feedstock has the possibility to incur costs so obviously it makes sense to position plants where minimum transportation is necessary.

There are many beneficial factors to consider. Biomass feedstock production, handling and processing due to the nature of all the materials are practiced in rural areas and so these benefits would be for those areas. These include:
• Rural development through
• Regional economic gain
• Return of investments
• Employment opportunities
• Job creation
• Sustainability
But on the other hand, there are many disadvantages to be considered. There will be increased traffic flow in the area and the various plants will represent a visual intrusion. Furthermore the running of the plants will create noise that may be unacceptable for nearby residents. The plants may even affect local ecology and any by-products and wastes must be removed thus adding to the traffic. Therefore reducing the distance the products travel to their point of use is an important part of this sustainable strategy. Only products requiring a specific form of management should be transported and where possible this should be by rail as residents are not keen on masses of lorries passing in front of their otherwise peaceful landscape.

The emissions from plants that use combustion are always a concern for local residents or anyone who is affected by them. The gases and particulates are often carried by the wind to another area other than that where generation takes place. This poses problems and political issues which need to be addressed. The possible detrimental effect on human health has the potential to rule out the use of such plants, especially when there are other alternatives for generation.

6. Environmental Aspects

Biomass is by no means the only solution to global warming and other problems and the environmental effects of its use should be examined very closely.

For instance the combustion of wastes solves a disposal problem and may offset the use of fossil fuels but the incombustible materials such as ash have to be removed from the site. This incurs a cost to the environment due to pollution from transportation. In addition if care is not taken and a conventional combustion chamber is used (as opposed to a newly designed one that is capable of filtering out harmful emissions) by-products such as particulates and poly-aromatic hydrocarbons (PAHs) can escape to the atmosphere.
On the other hand tree planting on a very large scale (such as the one needed for arable coppicing) helps in the absorption of CO2. If care is taken in the overall processing (harvesting, chipping, transporting, drying and so on) of this biomass feedstock then the net CO2 emissions after it has been used for fuel will be less than the absorbed CO2 thus benefiting the environment.

In defence of biomass use comes methane (!). Methane is a very harmful greenhouse gas as well as causing accidental explosions due to its migration from landfill sites to nearby buildings. Moreover it has 30 times the damaging effect on the environment than that of CO2. The extraction, use of and combustion of methane actually protects against global warming. Even though a by-product of the process is carbon dioxide the net effect to the environment is much smaller.

Finally another issue concerning the environment is the use of ‘set aside’ land for energy crops. SRC has potential as there are only a few stumbling points that may cause some objections; for example the potentially sensitive nature of sites has to be considered - proximity to settlements and roads- as views could be impeded due to the growth rates causing a 3-dimensional woodland type mass in the field. But integrating the area with the surrounding landscape - trees and other features- when establishing new plantations can reduce visual impact. Also
changes in the landscape can be quick during growth and also during harvesting. Additionally the scale of SRC has to be considered to avoid saturation of the landscape by monotonous planting. If farming conforms with regulations about adjacent plots of different tree species and so on most of the above concerns can turn out to be unfounded.

7. Conclusions and recommendations

• Among the various renewable energy sources, bio-resources hold special promise as future fuel and feed stock.
• Biomass is an eco-friendly and an inexhaustible source of energy.
• Sustainable exploitation of biomass as energy has a potential to act as a catalyst for overall sustainable development of rural areas and thereby the development of the country as a whole.
• The bio energy projects should be subsidized, at the community level in rural areas.
• A government agency specially mandated to promote renewable energy development through coordinated efforts should be set up.
• Considerable efforts must be made to establish resource database related to bio energy.
• Suitable policy intervention by the government would be necessary to overcome different barriers to diffusion of renewable energy systems.
• Investment subsidy and tax credits should be provided to reduce the financial burden of the project developers.
• National research institutions should play a key role in promoting, and improving the renewable energy technologies.

References

1. Energy Information Administration (EIA), International Energy Outlook 2001,DOE/EIA-0484(2001), Washington DC, March 2001.
2. Ferrero Gian Luca, Biomass Gasification-version 1, Lior CD-R Collection(Renewable Energy Series), Lior International 2000.
3. Ferrero, G.L., Maniatis, K., Buekens, A. and Bridgwater, A.V. (ed), 1989, Pyrolysis and Gasification (Proceedings -Luxembourg, 1989).
4. Bridgwater, A.V.(ed), 1993, Advances in Thermochemical Biomass Conversion-Vol 1 Blackie Academic & Professional (UK), ISBN 0-7514 0171 4.
5. Engström, F., Overview of Power generation from Biomass, 1999 Gasification Technology Conference, S. Francisco, CA-USA).
6. Hertzog, A.V., Lipman, T.E. and Kammen, D. M,(2000), Renewable Energy Sources (SES Material) .