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Communication Feature | November 2014

Capstone Micro Turbines: A new renewable source

Landfilling is commonly being developed as a renewable resource of energy through the systematic recovery and utilisation of biogas generated during anaerobic decomposition of municipal solid wastes. In India there is good scope for the development of landfill gas technology as municipal solid waste contains a high proportion of degradable organic matter. Biogas generation from various sources is also seen as a key renewable energy source in the National Energy Policy. Increasingly States like Tamil Nadu and Karnataka have come forward in showing interest in Landfills for their municipal waste and thus opting for land fill gas which can generate electricity in return. In the developed countries landfill gas (LFG) systems are established and a sizeable proportion of renewable energy is generated and utilised from landfills

Landfill gas is reacted during the anaerobic decomposition of organic substance in municipal solid waste (MSW) and commercial and industrial wastes. Depending upon the landfill design and its management, as well as waste composition, compaction, moisture and several other factors, thousands of landfill are available worldwide to collect and utilise this valuable renewable energy source for power generation. If landfill gas is allowed to escape to atmosphere, methane contained within it is a powerful greenhouse gas, 21 times more so than carbon dioxide. Therefore, its prevention of escape to atmosphere and its utilization as a renewable fuel source is a win-win situation.

Landfill gas composition and production rates are primarily affected by the waste that has been deposited in the landfill site. MSW contains 150-250 kg of organic carbon per tonne which micro-organisms convert to landfill gas via anaerobic processes. The gas formation is influenced by a number of factors such as waste composition, landfill storage height and density, air temperature, atmospheric pressure and precipitation levels. Gas production starts one to two years after the waste is deposited in the landfill and lasts 15-25 years. The continuously decreasing gas volume can be compensated by the disposal of additional waste during this period. With a calorific value of 3.5 to 5.5 kWh Nm3 (35-55% methane), landfill gas constitutes a high-value fuel for gas turbines that can be effectively used for power generation.

Microturbines are an emerging landfill gas (LFG) energy recovery technology option with the lowest emission and maximum efficiency, especially at smaller landfills where larger electric generation plants are not generally feasible due to economic factors and lower amounts of LFG. Microturbines may play an important role in future LFG project development, if the technical and economic hurdles facing them can be overcome. There are several Micro turbines installed worldwide running successfully on the low BTU methane generated in the LFG. In coming times Landfills will gather momentum in India, where there is huge municipal waste available along all the major metros round the country and the authorities trying to get rid of MSW.

Methane CH4 capturing benefits

There are two ways in which the problem related to the escape of LFG from landfill could be solved. The first one, commonly used in the past, is the extraction and flare of the LFG. In this way the pressure of the LFG within the landfill is decreased which reduces the escape of LFG from the landfill.

The flare of the LFG also reduces the problem of odor. The main products of flare of LFG are Carbon dioxide and water, which means that the GWP of the released gas has been largely reduced.

The other way is to follow a similar strategy as in the first case except that the gas is not flared but used in an economical way. Though the flare of LFG reduces the environmental impact of the landfill site on the environment, methane has a high calorific value and the flare of LFG represents waste of valuable resources. This influences the number of landfills where LFG is used as the supplementary or primary fuel for the production of electric power to increase.

A fundamental problem in expanding methane recovery efforts in developing countries is the lack of reliable data on solid waste. Most estimates of methane emissions from landfills are based on a "top-down" approach, in which the quantities and types of decomposable waste deposited are estimated and multiplied by assumed rates of methane generation With population growth, economic development, and increased urbanization, methane emissions from landfills in developing countries now account for nearly 40 percent of annual global landfill methane emissions and are expected to increase in the future.

Important factors in this increase are the continuing priority of many developing countries to reduce unmanaged dumping and develop larger, solid waste disposal sites, which typically have higher methane emissions. Table-1 below show the ten most MSW producing cities of India. Although most of the waste generated in developing countries is landfilled, much of it is deposited in open dumps. As these dumps are replaced with covered landfills, methane emissions will increase substantially. However, there are also substantial opportunities for capturing methane emissions in these countries.

In 2000, recognizing the environmental problems associated with MSW, India's Ministry of Environment and Forests required that all organic waste be organized and processed separately and not be dumped into landfills. The ban immediately faced difficulties in enforcement, as a number of municipalities failed to implement the new rules. As a result, organic wastes continued to be dumped at waste sites, leading to significant methane emissions. While the ban was in place, however, LFG recovery and use was not seen as economically viable. Recognizing this problem, the Ministry has begun the process to withdraw the organics ban. In this way, LFG use can provide a new revenue source to help fund the upgrade and improvement of the dump sites toward cleaner, safer sanitary landfills.

The practice of running a LFGE (Land fill gas Energy generation) project mean that only those sites that are closed or about to close are being considered for LFG capture. In the future, with the development of sanitary landfills, LFG management should be considered at the design stage as a way to minimize odors, maximize safety risks and generate revenue through LFGE. Currently, several LFGE projects are in the feasibility stage.

In India Brio Energy Pvt Ltd, a company with dedicated vision to overcome the energy requirements in a renewable way are continuously working towards efficiently using waste biogas generated in landfills and converting it into energy.

The author is Shubham Mishra, MD of Brio Energy Pvt Ltd, an authorised distributor for Capstone Turbine Corporation, USA

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