Global warming, which is a specific case of global climate change, refers to the increase in the average temperature of the atmosphere and oceans in recent decades, and the projected continuation of this increase. The drivers of climate change are seen as: changes in the atmospheric concentrations of GHGs and aerosols, land cover, and solar radiation (IPCC, 2007). According to the Intergovernmental Panel on Climate Change (IPCC, 2007), most of the increase in global average temperatures since the mid-20th century is linked to the observed increase in the anthropogenic GHG concentrations.
The four long-lived GHGs, which are released due to human activity, are CO2, CH4, N2O, and halocarbons. The effect of these gases on global warming is assessed using an index called 'global warming potential' (GWP), which is a measure of how much a given mass of GHG contributes to global warming relative to a reference gas (usually CO2) for which the GWP is set to 1. For a 100-year time horizon, GWPs of CO2, CH4, and N2O are reported to be 1, 25, and 298, respectively (IPCC, 2007). Using this index, one can calculate the equivalent CO2 emission by multiplying the emission of a GHG by its GWP.
Municipal solid waste may have significant effects on the production of GHG as well as other environmental problems and human health if it is disposed in landfills where there are no treatments and processes. There are several steps in the production of GHG from waste. Waste is first decomposed by aerobic bacteria until all the oxygen is consumed. Then, organic acids are produced in the absence of oxygen, which is followed by methanogenic state in which organic materials are decomposed
This paper draws on earlier publications of the authors (Colpan et al., 2008a, 2009).
I. Dincer et al. (eds.), Global Warming, Green Energy and Technology,
DOI 10.1007/978-1-4419-1017-2_8, © Springer Science+Business Media, LLC 2010
into CH4 and CO2. The leachate is also produced, which may contaminate the groundwater. There are also explosion risks due to the release of flammable gases, e.g., CH4. To prevent the health and environmental effects of landfills, these sites should be properly designed and operated. For example, while the groundwater may be protected by using liners and leachate collection systems, gas collection, treatment and processing systems must be used to reduce the GHG effect.
Energy may be produced from MSW through technologies, such as incineration, gasification, and generation of biogas and its utilization. Murphy and McKeogh (2004) investigated these technologies and concluded that generation of biogas and its conversion to transport fuel requires the least gate fee. Landfill gas can be converted into fuel and energy forms by direct combustion, chemical energy storage, introducing into the natural gas grid, and electricity generation. Qin et al. (2001) analyzed LFG combustion through experimental and numerical studies. Their experiments include the determination of laminar flame speeds, extinction strain rates, stable species and NO concentrations, and flame structures. Electricity generation from landfill gas can be accomplished by technologies such as the internal combustion engine, gas turbine, the Stirling engine, and fuel cells. Bove and Uber-tini (2006) compared several technologies used to generate electricity from landfill gases, and showed that the internal combustion engine, which is the most widely used technology due to economical reasons, presents the poorest environmental performance. On the contrary, fuel cells are shown to be the cleanest electricity generating systems; however, they are not yet economically advantageous. There are different types of fuel cells and most of them may be fueled by LFG. However, low temperature fuel cells need a reformer to convert the fuel into hydrogen. Additionally, in all fuel cell types, LFG should be cleaned according to the impurity tolerance levels of the fuel cell. Lombardi et al. (2006) compared conventional treatments with the following alternatives: the direct LFG feeding to a fuel cell; the production of a hydrogen-rich gas by means of steam reforming and CO2 capture to feed a stationary FC; and the production of a hydrogen-rich gas by means of steam reforming and CO2 capture to feed a vehicle FC. Their study reveals that LFG reforming to a vehicle FC has the lowest specific greenhouse effect emission. Spiegel et al. (1999) demonstrated the operation of a commercial phosphoric acid fuel cell (PAFC) with LFG. Their system produces up to 137 kW power, 37.1% efficiency at 120 kW, and exceptionally low secondary emissions. Lunghi et al. (2004) conducted life cycle assessment analysis of a molten carbonate fuel cell (MCFC) system for LFG recovery for an evaluation of environmental consequences, and to provide a guide for further environmental impact reduction. Duerr et al. (2007) analyzed a biogas fueled alkaline fuel cell (AFC). They chose the AFC because of its very low freezing point of the potassium hydroxide electrolyte (~ -50 °C).
The SOFC has the highest operating temperature level among the fuel cell types. According to the manufacturing type, i.e., electrolyte, electrode or interconnect supported, the SOFC may operate between 500 and 1000°C, which enables successful integration with other systems. Currently, there is more research on intermediate and low temperature SOFCs since they have better structural integrity and a lower start-up time. Another important advantage of SOFCs over other fuel cell types is that gas mixtures including hydrocarbons may be reformed inside the fuel cell, which is called direct reforming. This feature together with the other advantages of SOFCs makes this fuel cell type very appropriate for production of electricity in landfill sites. A review of the SOFC and its modeling may be found in the paper by Colpan et al. (2008b).
In this study, GHG emissions from an uncontrolled landfill site are compared with those from controlled landfill sites in which flaring, conventional electricity generation technologies such as internal combustion engine and gas turbine, and an emerging technology, the SOFC, are utilized. For this comparison, GHG emission from each technology is first found for each year of its lifetime for a selected case study using the method developed by the authors. Then, the GHG reduction ratio and specific lifetime GHG emission are calculated for each case. Consequently, the most effective technology is determined. It should be noted that GHG emissions are calculated using on-site direct emissions (from flaring, ICE, GT, or SOFC), without taking into consideration the life cycle emissions occurring during manufacture of the infrastructure (engine, flares, cells, pipes), production and delivery of auxiliary materials, auxiliary energy consumption, gas-cleaning treatment, and so on.
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