Pollutants coal combustion gases

S02 and NOx in flue gas from coal combustion contribute to smog and acid rain. Methods to remove these pollutants include alkaline wet scrubber systems that fix S02 to solid CaS04, and selective catalytic reduction by metal/metal oxide systems of NO/NOz to N2 and steam in the presence of ammonia. Particulate active carbons have also been used in flue gas decontamination, especially as they avoid costly scrubber processes and can operate at lower temperatures. The potential of active carbon fibers in this application has been explored by a... 

The pollutants which increased dramatically in converting from oil/gas to coal combustion are particulate matter and sulfur dioxide. State laws, especially those governing emissions in urban areas, specify limits for both species. They could be met when low sulfur fuel oil or gas was burned, but coal combustion required new or upgraded pollution control devices. To understand the problem of coping with both of these emissions, it is simpler to treat the response to particulate control separate from that of... 

Recently, work on nitrogen oxidation in the combustion of a fuel mixed with air has become extensive in connection with pollution of the environment by toxic combustion products (including NO). An good review of this research was done by A. N. Heiherst and I. M. Vincent,7 and by A. Macek for coal combustion.8 Detailed experimental and theoretical work on the formation of nitrogen oxides in turbulent gas flames was performed by P. Moreau and R. Borghi.9... 

There are several approaches available to a utility to construct a boiler that will meet New Source Performance Standards. These approaches can be classified according to the position in the combustion system at which pollutant control technology is applied. Precombustion control involves removal of sulfur, nitrogen, and ash compounds from the fuel before it is burned. For coal combustion this approach involves the application of coal-cleaning technology. Combustion control relies on modifications to the combustion process itself or the addition of material to the combustion process to reduce pollutant formation or capture the pollutants formed in the combustion chamber. Examples of combustion control include staged combustion, boiler limestone injection, and fluidized-bed combustion with limestone addition. Post-combustion control involves removal of pollutants after they have been formed but before they are released into the atmosphere. Traditionally, flue gas desulfurization has meant the application of postcombustion control either alone or in conjunction with another... 

Although coal combustion produces substantially greater air pollution problems than does oil or natural gas combustion, because of its great abundance in the United States and other countries (such as Russia), there has been renewed interest in developing technology to burn coal more cleanly. However, all fossil fuels consist mainly of hydrocarbons (compounds that contain only carbon and hydrogen), which, upon complete combustion, yield carbon dioxide, a major greenhouse gas. 

Intelligent design of circulating fluidized bed boilers depends on sufficient understanding of the physico-chemical hydrodynamics occurring in the combustors, such as chemical kinetics of coal combustion and pollutant formation, hydrodynamics of gas-solid two phase flow, mixing of gas and solids, distribution of heat released, and heat transfer between immersed... 

Existing pollution control devices can remove as much as 99% of the trace elements (except mercury and selenium) from the combustion gases. Substantial amounts of mercury and, to a lesser extent, selenium are emitted with the combustion gases. There are a number of ways to reduce trace-element emissions due to coal combustion. These pollution control options include switching to coals with a lower trace-element content, selectively mining those parts of coal beds with lower trace-element contents, cleaning the mined coal, and using pollution control devices such as fabric filters, electrostatic precipitators, and combustion gas scrubbers. 

A thermochemical conversion of a solid biofuel into a fuel gas is an additional operation prior to application, which can be justified by a more efficient and flexible use of the cleaner gas. The pollution and flue gas cleaning problems from direct combustion of dirty coal are expected to be reduced by more efficient and environmentally con atible integrated gasification combined cycle (IGCC) processes. This strategy for an advanced utilisation of fossil fuels is also suited for solid biofuels like the relatively clean wood, as it is even more suited for the dirtier cereal straw and related herbaceous bio-materials. 

The environmental aspects of coal combustion have been a major factor in the various processes, and the movement of the fossil fuel base away from petroleum and natural gas to coal has increased the need for effluent/pollutant control for large, fossil-fueled power plants (Argonne, 1990). Very large amounts of coal are consumed in generating electricity and the emissions from power stations and similar industrial sources represent a potential, and considerable, environmental hazard. These power plants and the accompanying flue gas desulfurization processes emit effluents, which often are pollutants, and which by mere contact with the external environment or by (generally) simple atmospheric chemical transformations, may form secondary pollutants that are more harmful than the initial effluent/pollutant. 

Environmental concerns and strict pollution legislation prompted action in the construction of up to date coal-fired boilers and adaptation of existing plants. At the present time, flue gas desulfurization is the only conventional method employed on a commercial scale for reducing sulfur emissions after coal combustion. Over 90% reduction of sulfur dioxide in flue gases can be achieved by this process. Combustion control techniques of the flames will effectively reduce oxides of nitrogen emissions into the atmosphere. 

The air emissions from coal gasification plants u e quite low. In fact, it is possible to design an IGCC power plant whose air emissions are as low as those firom a natural gas combined-cycle power plant. Specific provisions of the Qean Air Act Amendments (CAAA) of 1990 favor coal gasification over direct coal combustion. These provisions include maximum SO2 emissions or "caps" reductions and offsets for SO2/NOX, restriction on trace component hazardous air pollutants, and incentives for coal gasification repowering of existing power plants. 

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