Factors Limiting the Use of Coal Combustion Products

Unburned Carbon on Fly Ash
Increasingly strict limits on U.S. NOX emissions have led to the widespread use of low-NOX burners, reburning, and overfire air. Often these strategies impair complete coal combustion and produce high-carbon fly ash, which is unsuitable for use in cement manufacturing. The excessive carbon adsorbs concrete’s air-entraining agent, which impacts the setting process, causing air entrainment problems, and reduces the material’s strength and durability, especially during freeze-thaw conditions. In some cases, careful design and operation of low-NOX combustion systems can mitigate the problem of excessive carbon. Several organizations have also developed processes to separate the unburned carbon from the fly ash or developed cement additives to block adsorption of the air-entraining chemicals by the carbon.
Ammonia Deposition on Fly Ash
Two other NOX-reduction technologies SCR and SNCR—introduce NH3 into the flue gas. Unreacted NH3, referred to as ammonia slip, contaminates the system’s fly ash via adsorption or other mechanisms. This effect can be detrimental to both utilization and disposal of the ash. In the manufacture of building materials, fly ash must meet permissible limits of ammonia content based on exposure limits for employees and product performance. Even if exposure limits are not exceeded, ammonia can present problems for sensitive individuals.
If the ash is placed in a landfill, odor becomes a factor. Further, in landfills, rainwater can leach ammonia along with other components of ash, resulting in violation of groundwater pollution regulations. These effects must be taken into account in establishing SCR/SNCR operations, and several thermal and chemical techniques have been developed to counteract the effects of ammonia slip. One process subjects the fly ash to an additional FBC step to remove both the unburned carbon and the residual ammonia. Another process treats the fly ash with an alkali, such as lime, to release and recover the ammonia. In addition, several methods have been developed to remove ammonia and other pollutants from the wastewater that results from exposure of fly ash in landfills to rainwater.
Recent regulations to control mercury emissions in stack gases could also have a negative effect on the utilization of coal combustion products. One method likely to be widely used for mercury control is the injection of powdered, activated carbon into the flue gas. Currently, the carbon is removed along with the fly ash using standard fly ash removal equipment. This results in ash contaminated with high carbon levels.
Wet FGD processes can also be effective for mercury removal, especially if the mercury in the flue gas is in the oxidized state when it enters the FGD system. There is currently some interest in enhancing these processes to achieve higher levels of mercury removal. One concern is the possibility of FGD by-products containing elevated levels of mercury. However, tests to date indicate that mercury contamination will not be a problem for either the disposal or constructive use of these CUBs. A second concern is the re-release of gaseous mercury during the manufacture of wallboard, cement, and other products that contain FGD CUBs. The Innovations for Existing Plants program of Department of Energy’s National Energy Technology Laboratory is currently investigating the extent to which such rereleases of mercury may occur.