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Chapter 1 Evaluation of Combined Heat and Power Technologies for Wastewater Treatment Facilities 1.3.4 Carbon/Green House Gas Implications Greenhouse gases (GHGs) are gases present in the Earth’s atmosphere. As their name suggests, GHGs contribute to the greenhouse effect. The greenhouse effect is critical to sustaining life on Earth and is a key mechanism which aids in regulating the Earth’s temperature. The greenhouse effect warms the Earth as GHGs absorb and emit radiation within the thermal infrared range. Greenhouse gases primarily include carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Global warming refers to the acceleration of the greenhouse effect and the rise in the Earth’s temperature brought about by additional GHG emissions that result from human activities. Global Warming Potential (GWP) is a scale that allows the comparison of GHGs in terms of their relative capacities for global warming. By definition, the GWP of CO2 is one. According to the Second Assessment Report, issued by the Intergovernmental Panel on Climate Change (IPCC) in 1995, the GWPs of CH4 and N2O are 21 and 310, respectively. One the advantages of operating a CHP system in conjunction with anaerobic digestion is the opportunity to reduce the POTW’s GHG emissions associated with otherwise purchased electrical power. The term carbon footprint is a generally accepted and convenient way to describe the net GHG emissions caused by any given entity. It is important to understand that an entity is responsible for any GHG emissions emitted on its behalf by a third party. Due to this distinction, GHG emissions are characterized as either direct emissions or indirect emissions and are, to a significant degree, associated with the production of energy. In the case of POTWs, direct emissions are the result of energy production from a source owned or operated by the entity. For example, the CO2 emissions from a natural gas-fired boiler used to produce thermal energy are considered direct emissions. Conversely, indirect emissions are the result of energy production by a third party. For example, the CO2 (and to a lesser extent CH4 and N2O) emissions that result from the generation of electric power by an electric utility and purchased by a POTW are considered indirect emissions attributable to the POTW. The operation of a CHP system can significantly reduce a POTW’s direct and indirect GHG emissions. Direct emissions can be reduced by supplanting natural gas or diesel fuel with digester gas to produce electric power and thermal energy. The production of electricity onsite from digester gas also results in the reduction of indirect emissions as the amount of electricity purchased from a power provider is reduced. The combustion of digester gas, by flaring or operation of a CHP system, does emit GHG emissions, including CO2 and very small amounts of N2O and uncombusted CH4. However, the CO2 emissions (which represent over 99 percent of the total emissions normalized by GWP) are considered neutral and are not counted against the energy producer’s carbon footprint. The reason for this is that the carbon found in digester gas is biogenic, meaning that it is already part of the global carbon cycle. On the other hand, CO2 emissions that result from the combustion of fossil fuels count against the carbon footprint of the responsible party. This is because the carbon found in fossil fuels was previously sequestered inside the earth and subsequently added to the global carbon cycle due to human activities. In other words, the previously sequestered carbon was not “in play.” Carbon that is released into the global carbon cycle by human activity is known as anthropogenic carbon. In contrast to the CO2 emitted during combustion of biogenic fuels like digester gas, N2O and uncombusted CH4 must be accounted for in a facility’s carbon footprint. Case studies of CHP facilities, presented in Chapter 6 of this document, include an accounting for N2O and uncombusted CH4, which contribute to the POTW’s overall carbon footprint. An accounting of CO2 is also provided that, in many cases, is required to be reported separately but not included in the POTW’s overall carbon footprint. 1-8PDF Image | Combined Heat and Power Technologies for Wastewater Facilities
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