Another large producer of CO2 emissions is portland cement kilns. For instance, the use of fly ash (a by-product of coal burning in power generation and most common CCP) in the cement-making process could reduce substantial amounts of CO2 emitted by a cement kiln. Worldwide, the production of portland cement alone accounts for 6 to 8 percent of all human generated CO2 greenhouse gases (Huntzinger, Deborah N. and Eatmon, Thomas D., 2009). Portland cement production is not only a source of combustion-related CO2 emissions, but it is also one of the largest sources of industrial process-related emissions in the United States. Combustion related emissions from the U.S. [portland] cement industry were estimated at approximately 36 Tg of CO2 accounting for approximately 3.7 percent of combustion-related emissions in the U.S. industrial sector in 2001 (USGS, 2002)