Page 177 - Wastewater Solids Incineration Systems

144 Wastewater Solids Incineration Systems

1.2.6 Greenhouse Gases
Greenhouse gases are gases that absorb or trap heat in the atmosphere, contributing
to global warming. Although a certain amount of these gases is necessary to make the
earth habitable, the concentration of GHGs in the atmosphere has been increasing
steadily over the last 100 years and is now having a profound effect on the earth’s cli-
mate and ecosystems. According to the National Oceanic and Atmospheric Adminis-
tration (NOAA), the earth’s average surface temperature has risen by 0.67 to 0.78°C
(1.2 to 1.4°F) since 1900, with the majority occurring since 1970 (0.56°C [1.0°F]). The
current rate of global surface warming has increased to 0.18°C (0.32°F) per decade or
1.8°C (3.2°F) per century. The NOAA also reports that the five warmest years over the
last century occurred in 2005, 2004, 2003, 2002, and 1998 (NOAA, 2005). The National
Research Council (NRC) states that climatic change is occurring over most of the
globe and is evident from the global retreat of glaciers, reduction in the extent of
snow cover, earlier spring melting of lakes and rivers, and an increase in the ocean
surface temperatures and heat content (NRC, 2001).
The most significant GHGs are carbon dioxide (CO ); methane (CH ); nitrous
2 4
oxide (N O); and fluorinated gases, including hydrofluorocarbons (HFCs), perfluoro-
2
carbons (PFCs), and sulfur hexafluoride (SF ). It should be noted that there are also
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chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) that are ozone-
depleting substances (ODSs) as well as being GHGs. The use of these ODSs has been
decreasing since the adoption of the Montreal Protocol and the U.S. Clean Air Act
Amendments of 1990. Although the emissions of the fluorinated gases are relatively
low, these gases can persist in the atmosphere for extremely long periods of time,
hence are very potent GHGs.
Greenhouse gases are ranked according to their global warming potential
(GWP), which is the ratio, on a time-integrated basis, of how much global warming a
particular gas can exert relative to CO . Carbon dioxide is assigned a GWP of one;
2
CH and N O have GWPs of 21 and 310, respectively; and the fluorinated gases have
4 2
GWPs of several thousands. Despite its low GWP, CO is the predominant GHG dri-
2
ving global warming. The U.S. EPA compiles an annual inventory of GHG emissions
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in which the emissions of each gas are calculated in terms of teragrams (10 grams)
of CO equivalents. According to the U.S. EPA, in 2004 CO emissions represented
2 2
84.6% of the total GHG emissions in the United States; CH and N O constituted 7.9%
4 2
and 5.5% of the GHG emissions, respectively; and the fluorinated gases composed
approximately 2.0% (U.S. EPA, 2006).