Date of this Version
Published in Soil Amendments for Sustainability: Challenges and Perspectives, edited by Amitava Rakshit, Binoy Sarkar, & Purushothaman Chirakkuzhyil Abhilash (Boca Raton: CRC, 2019).
Flue gas desulfurization (FGD) gypsum is one of the by-products of a coal-fired power generation plant. Coal is the world’s most abundant and widely distributed fossil fuel. After natural gas, coal is the second primary source of energy to generate electricity globally (more than 25%) and remains a key component of the fuel mix for power generation to meet electricity demand in most of the developing countries. The U.S., China and India are the top coal producers and consumers (for production of electricity from coal sources) in the world (OECD/IEA, 2014; IEA, 2016). However, in the U.S., its contribution to power generation is declining in favor of natural gas and other energy sources due to low natural gas prices, renewable energy standards and environmental activism and regulations.
Coal combustion in power plants generates about 120 million metric tons of coal combustion residues (CCR) annually. These by-products include fly ash, bottom ash, flue gas desulfurization (FGD) material and flue bed combustion ash. According to the American Coal Ash Association (ACAA, 2015), only 61.1 million metric tons of CCR were beneficially used. The 1990 U.S. Clean Air Act Amendments restrict sulfur dioxide (SO2) emissions into the atmosphere from coal-fired facilities, if the coal contains considerable amounts of sulfur (S). To meet the SO2 emission reduction requirements, most of the U.S. coal power plants use the FGD process, and in this process, the gypsum is produced which is known as FGD gypsum.
FGD gypsum is created by forced oxidation scrubbers in coal-fired power plants which remove SO2 emission from the flue gas stream. There are three different scrubbing processes: wet, semi-dry and dry. However, SO2 removal efficiencies are significantly higher in wet scrubbing process (90 to 98%) than semi-dry (80 to 90%) and dry (50 to 60%) processes for calcium-based sorbents (Schnelle and Brown, 2002). In general, a wet scrubbing process first exposes the flue gases to a slurry of hydrated lime, where it reacts with S in the gas to form calcium sulfite (CaSO3). Forcing additional air into the system oxidizes the CaSO3 and converts it into gypsum. The FGD gypsum is also known as recaptured gypsum, byproduct gypsum and synthetic gypsum.
The chemical formula for mined gypsum or FDG gypsum is the same, which is calcium sulfate dihydrate (CaSO4•2H2O). By weight, it is 79% calcium sulfate and 21% water. It contains 23% calcium (Ca) and 18% sulfur (S). However, the amount and types of trace materials and unreacted sorbents found in the gypsum can vary among power plants and among mines. FGD gypsum contains 90 to 99% of purity concentration compared to 66 to 98% concentration in mined gypsum. Production of FDG gypsum has gradually increased in the past several years. According to the ACAA, approximately 33 million metric tons of FDG gypsum was produced in 2015 in the U.S., of which 53% (17.5 million metric tons) was used in building industry and road construction. Less than 2% of the total FGD gypsum production was used in agriculture.