Coal Combustion ByProducts CCB

More than 100 million tons of coal-related residues are generated annually by coal-burning plants [24,25]. These materials have many names—they are referred to as fossil fuel combustion wastes (FFCWs) by the U.S. Environmental Protection Agency (EPA); as coal combustion products (CCPs) by the utility industry, ash marketers, and ash users; and as coal combustion by-products (CCBs) by the U.S. Department of Energy and other federal agencies. These residues become products when utilized and wastes when disposed of [24]. They include fly ash, bottom ash, boiler slag, and flue gas desulfurization (FGD) material (i.e., synthetic gypsum). The fly ash is the fine fraction of the CCBs that is entrained in the flue gas exiting a boiler and is captured by particulate control devices. Bottom ash is the large ash particles that accumulate at the bottom of a boiler. Boiler slag is the molten inorganic material that is collected at the bottom of some boilers and discharged into a water-filled pit where it is quenched and removed as glassy particles. FGD units, which remove sulfur dioxide using calcium-based reagents, generate large quantities of synthetic gypsum, which is a mixture of mainly gypsum (CaSO4) and calcium sulfite (CaSO3) but which can also contain fly ash and unreacted lime or limestone [24,25]. In 2000, CCB production in the United States was 108,050,000 short tons and was comprised of [25]:

• Fly ash—62,810,000 short tons, or 58.1% of the total generated;

• Bottom ash—16,940,000 short tons, or 15.7% of the total generated;

• Boiler slag—2,670,000 short tons, or 2.5% of the total generated;

• FGD material—25,630,000 short tons, or 23.7% of the total generated.

In the United States, approximately 30% of the CCBs are used in a variety of applications, with the remainder being disposed of [25]. The components of the CCBs have different uses because they have distinct chemical and physical properties that make them suitable for specific applications. CCBs are used in cement and concrete; mine backfill, agriculture, blasting grit, and roofing applications; waste stabilization; wallboard production; acid mine drainage control; and as road base/subbase, anti-skid material, fillers, and extenders [24,25].

Globally, CCB use varies significantly. In Europe, more CCBs are used than in the United States; for example, in 1999, 56% of the CCBs were profitably used in Europe compared to about 30% in the United States [25]. The CCBs are used in a number of applications, primarily in concrete, portland cement manufacture, and road construction. Raw materials shortages and favorable state regulations account for higher usage in Europe than in the United States. Countries such as Canada, India, and Japan utilize 27, 13, and 84% of their CCBs, respectively [25]. Canada's usage is similar to that of the United States, CCB usage in India is low due to the relatively large amount of CCBs produced because of the coal's high ash content, and Japan utilizes most of its CCBs due to the high cost of disposal in Japan.

Coal combustion by-products primarily contain elements such as iron, aluminum, magnesium, manganese, calcium, potassium, sodium, and silica, which for the most part are innocuous. CCBs also contain small amounts of trace elements such as arsenic, barium, beryllium, cadmium, cobalt, chromium, copper, nickel, lead, selenium, zinc, and mercury. These elements can be classified as essential nutrients, toxic elements, or priority pollutants and are considered to have some environmental or public health impacts [24]. The risks include potential groundwater contamination of trace elements and above-ground human health impacts through inhalation and ingestion of contaminants released through wind erosion and surface water erosion and runoff [26].

The Resource Conservation and Recovery Act (RCRA) has been the primary statute governing the management and use of CCBs. The EPA was considering some form of Subtitle C regulation (i.e., classify CCBs as hazardous wastes) under the RCRA for CCBs used in mine backfill or for agricultural applications [26]; however, the agency investigated the dangers of CCBs to human health and the environment and concluded that CCBs do not pose sufficient danger to the environment to warrant regulations under RCRA Subtitle C. The EPA does intend to develop national regulations under RCRA Subtitle D (nonhazardous solid waste) or to work with the U.S. Department of the Interior toward modifying existing regulations under the SMCRA when CCBs are placed in landfills or surface impoundments or are used as fill in surface or underground mines [24].

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