The combustion of carbonaceous fuels by power plants produces a number of pollutants. First, gaseous pollutants such as NO.sub.x and SO.sub.2 are released during the combustion of these fuels. Next, toxic pollutants are carried within the combustion gases resulting from the combustion of carbonaceous fuels. Toxic pollutants include gaseous mercury, complex organics, and halogens, as well as solid inorganic trace elements. Finally, the solid waste materials produced from the combustible materials (bottom ash and fly ash) may eventually result in additional pollution. This pollution source may be generated if the solid waste materials are disposed on or in the ground and contain contaminants in a leachable form such that they may eventually migrate into water supplies.
Presently, separate dedicated systems are utilized for the suppression of each individual pollutant. For instance, groundwater is protected from the ash leachate of solid waste materials by liners and leachate recovery systems.
The removal of SO.sub.2 is typically achieved by a reaction with a calcium-based substance. The byproducts of this reaction are commonly precipitated and then disposed.
Moderate levels of NO.sub.x reduction (e.g., 50-60%) are typically achieved using a low NO.sub.x combustion system. If greater NO.sub.x reductions are required, a system is used that injects ammonia, urea, or related compounds downstream of the combustion zone. If done at 600.degree. to 300.degree. Fahrenheit, a catalyst is required and the process is called selective catalytic reduction.
Solid pollutants (fly ash) are controlled through a variety of techniques. The most common technique is to precipitate the fly ash with an electrostatic precipitator or arrest it with a baghouse.
Recently, slagging combustors have been developed which substantially reduce NO.sub.x and SO.sub.2 emissions. Slagging combustors also have the benefit of converting 60-90% of the noncombustible material in the fuel (the ash) into a vitrified slag which is resistant to leaching. While slagging combustors capture most of the NO.sub.x, SO.sub.2, and particulate (ash) produced by combustion, additional NO.sub.x, SO.sub.2, and particulate controls are needed to achieve very low emissions.
Lignite coke (i.e., activated coke derived from lignite) systems have been employed to remove SO.sub.2, NO.sub.x, and gaseous toxic pollutants. They also capture fly ash. These systems have been used downstream of cyclone boilers and municipal waste garbage incinerators. Because coke consumption becomes unacceptably high if these systems are used as the sole SO.sub.2 and fly ash control, they have been utilized in a staged process which includes a number of expensive devices. For example, for coal fired boilers, an electrostatic precipitator is typically used for fly ash precipitation. Then, a flue gas desulphurization (FGD) apparatus is used to remove SO.sub.2. Because the lignite coke system will remove residual SO.sub.2, the FGD may be a lower cost device capable of only moderate SO.sub.2 removal. Such a system could comprise a suspension of hydrated lime sprayed into the flue gas stream within a reaction section. In this case, another electrostatic precipitator is used for the precipitation of the desulfurized products from the desulphurization apparatus.