Landfill Processes

In a landfill site, LFG, which is composed of methane, carbon dioxide, and non-methane organic compound (NMOC), i.e., ethane, butane, hexane, hydrogen sulfide, etc., is generated due to a series of biological processes. Over time, the amount of gas generated increases until such time the site reaches its capacity. Subsequently, the amount of gas generated begins to decrease due to the reduction in the organic material components. Because of the adverse environmental effects of the LFG, this gas should be collected and properly utilized by flaring or electricity generation technologies. In the following subsections, LFG generation and collection processes are discussed.

8.2.1 Calculation of landfill gas generation

Landfill gas generation from MSW can be calculated using the software called LandGEM, which was developed by the US Environmental Protection Agency (LandGEM, 2008). This software is based on a first order decomposition rate equation for quantifying emissions from the decomposition of landfilled waste in MSW landfills, which is shown in the following equation. (Alexander et al., 2005)

From Eq. (8.1), annual methane generation in a year can be calculated. Generally, it is assumed that landfill gas has a composition of 50% CH4 and 50% CO2. Hence, total landfill gas generation may be found by doubling the result from Eq. (8.1). Methane generation rate, k, is a function of factors such as moisture content, availability of

nutrients for methane-generating bacteria, pH, and temperature of the waste mass. The potential methane generation capacity, Lo, depends on the type and composition of the waste placed in the landfill. The Clean Air Act (CAA) default values, which are based on federal regulations for MSW landfills laid out by the CAA for k and Lo, are 0.05 year-1 and 170 m3/ton, respectively (Alexander et al., 2005).

8.2.2 Landfill gas collection

Landfill gas generated by the decomposition of organic materials should be collected in a well designed and managed site due to environmental, health, and energetic considerations. The quantity of gas collected is estimated by multiplying the generated landfill gas by collection efficiency. According to the EPA (1998), collection efficiencies at well-designed landfills typically range from 60 to 85%, with an average of 75%. A very well-designed collection system, i.e., 85% efficiency, should have the following features: a composite bottom liner consisting of a synthetic (plastic) layer over 2 feet (0.6 meter) of clay or similar material; soil cover applied over newly deposited refuse on a daily basis; no significant off-site lateral migration of landfill gas; a comprehensive landfill gas collection system with vertical wells and/or horizontal collectors providing 100% collection system coverage of all areas with waste within a few years after the waste is deposited; a gas collection system which is operating effectively so that all wells are fully functioning (i.e., relatively free of liquids and drawing landfill gas under vacuum) (Stege, 2003).

The GHG emission routes in a well-designed landfill site with a collection system are shown in Fig. 8.1. These routes will be discussed in detail in the following sections.

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