Biogas based Electricity generation plant

Even today in 2013, over 400 million people in India have no access to electricity. Because of the remoteness of much of India’s un-electrified population, renewable energy can offer an economically viable means of undoing this undone. And what better than a biomass power plant. Today, about 32% of the total primary energy use in the country is still derived from biomass and more than 70% of the country’s population depends upon it for its energy needs in some way or the other. Biomass power generation in India is an industry that attracts investments of over Rs.600 crores every year, generating more than 5000 million units of electricity and yearly employment of more than 10 million man-days in the rural areas. We can divide the biomass power plant market in India into 3 categories. Biogas based power projects Present. The current availability of biomass in India is estimated to be approximately640 million metric tones per annum. This converted to cubic meters of biogas comes to 12800 million cubic meters per annum (@ 20 cubic meters of biogas per metric ton of biomass). • This should lead to both electricity and heat generation of 25600 GWh and 51200 GWh per annum respectively (@ 2 KWh of electricity and 4 KWh of heat energy per cubic meter of biogas). • The Government of India has estimated this availability of biomass to a corresponding potential of about 18000 MW generation capacity. • If the above two statements are combined, it comes to around 4 hrs of electricity generation for 365 days a year. Some case studies displayed at MNRE website talk about 6-7 hours of electricity daily in villages using a 10 KW plant. • A total of around 130 biogas power projects have been installed in the country as on date for feeding power to the grid aggregating to 1176.10 MW of generation capacity. Biogas technology, the generation of a combustible gas from anaerobic biomass digestion, is a well-known technology. There are already millions of biogas plants in operation throughout the world. Whereas using the gas for direct combustion in household stoves or gas lamps is common, producing electricity from biogas is still relatively rare in most developing countries. In Germany and other industrialised countries, power generation is the main purpose of biogas plants; conversion of biogas to electricity has become a standard technology. This document will discuss the potentials, obstacles and necessary framework conditions for the utilisation of biogas for small and medium scale electricity generation in developing countries. This paper will not address the biogas production process in general but focus uniquely on electricity generation. The findings presented here are based mainly on available experience from GTZ-related pilot biogas power plants in different countries. They focus on more or less well-documented existing country cases even though little extensive documentation of practical long term operat-ing experience is available. Besides the cases described here, we know of further examples from other GTZ projects (e.g. Bolivia, Tunisia and the Ivory Coast). However, there is not yet sufficient information concerning these to merit inclusion in this assessment. Biogas is gas resulting from an anaerobic digestion process. A biogas plant can convert animal manure, green plants, waste from agro industry and slaughterhouses into combustible gas. Biogas can be used in similar ways to natural gas in gas stoves, lamps or as fuel for engines. It consists of 50-75% methane, 25-45% carbon dioxide, 2-8% water vapour and traces of O2 N2, NH3 H2 H2S. Compare this with natural gas, which contains 80 to 90% methane. The energy content of the gas depends mainly on its methane content. High methane content is therefore desirable. A certain carbon dioxide and water vapour content is unavoidable, but sulphur content must be minimised - particularly for use in engines. The average calorific value of biogas is about 21-23.5 MJ/m³, so that 1 m³ of biogas corre-sponds to 0.5-0.6 l diesel fuel or about 6 kWh (FNR, 2009). The biogas yield of a plant depends not only on the type of feedstock, but also on the plant design, fermentation temperature and retention time. Maize silage for example - a common feedstock in Germany - yields about 8 times more biogas per tonne than cow manure. In most cases, biogas is used as fuel for combustion engines, which convert it to mechanical energy, powering an electric generator to produce electricity. Appropriate electric generators are available in virtually all countries and in all sizes. The technology is well known and maintenance is simple. In most cases, even universally available 3-phase electric motors can be converted into generators. Technologically far more challenging is the first stage of the generator set: the combustion engine using the biogas as fuel. In theory, biogas can be used as fuel in nearly all types of combustion engines, such as gas engines (Otto motor), diesel engines, gas turbines and Stirling motors etc.

Gas turbines are occasionally used as biogas engines, especially in the US. They are very small and can meet strict exhaust emissions requirements. Small biogas turbines with power outputs of 30-75 kW are available on the market, but are rarely used for small-scale applications in developing countries as they are expensive. Furthermore, due to their spinning at very high speeds and the high operating temperatures, the design and manufacturing of gas turbines is challenging and maintenance requires specific skills. External combustion engines such as Stirling motors have the advantage of being tolerant of fuel composition and quality. They are, however, relatively expensive and characterised by low efficiency. Their use is therefore limited to a number of very specific applications. In most commercially run biogas power plants today, internal combustion motors have become the standard technology either as gas or diesel motors.