Precombustion Cleaning

there are (1) precombustion cleaning, (2) cleaning during combustion, (3) postcombustion cleaning, and (4) cleaning by conversion. 

Precombustion cleaning involves the removal of any, or at least of a part, of pollution-generating impnrities from coal by physical, chemical, or biological means. A substantial amount of the coal nsed in ntility boilers does receive some form of cleaning before it is burned. The major objective of many of the precombustion cleaning processes is the reduction of the sulfur content (usually pyritic, FeS2, sulfur). The wider use of conventional coal-cleaning processes will allow the sulfur dioxide emissions to be reduced markedly. 

Coal-cleaning processes could achieve trace element rejections of 50%-80% (Luttrell, et al., 2000 Xu et al., 2003). Drying removes the excess moisture and reduces the weight and volume of the coal, rendering it more economical to transport and increases the heating value. Solar drying is 

The removal of mineral matter content including the pyritic sulfur improves the efficiency of power plant it could provide a reduction of up to 40% in SO2 emissions and about 5% in carbon dioxide emissions (Breeze, 2005 lEA, 2008). 

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Precombustion control involves removal of sulfur compounds from fuel prior to combustion. Control during combustion employs techniques to minimize the formation and/or release of SO2 and N0X during the combustion process. Finally, SO2 and N0X can be removed from the combustion flue gas using various postcombustion control methods. This chapter discusses the potential of mitigating acid deposition through precombustion cleaning of coal to remove sulfur compounds. 

Extraction with an organic solvent, such as molten anthracene or a sequential series of solvents, after conventional coal cleaning is also used [20, 21]. Microbial methods of coal desulfurization have also been tested. Precombustion cleaning measures such as these dramatically improve the emission characteristics of coal-burning installations. Coal conversion to a liquid or gaseous fuel permits much greater improvement in the residual precombustion impurities present, but also at greater cost. 

Even the methane present in natural gas is not without its sulfur gas polluting potential if burned directly in the form obtained from the wellhead. This potential is removed by thorough desulfurization using an amine scrub before it is piped to industrial and domestic consumers (Section 7.5). This is another example of a precombustion cleaned fuel. 

There are strong incentives to develop processes for removing sulfur from coal before combustion (precombustion cleaning), during combustion, or after combustion (postcombustion cleaning)... 

Desulfurization of fossil fuels was the subject of an authoritative review by J. B. Hyne (Alberta Sulphur Research Institute). This is a topic of increasing importance as Canada relies more and more on sulfur-containing fuels such as tar sands and heavy oils. Hyne reviewed the present state of the chemistry and technology for both precombustion desulfurization of natural gas and crude oils and postcombustion tailgas clean up of coals and cokes. He clearly identified areas of possible future research such as the high temperature-high pressure chemistry pertaining to in-situ desulfurization processes. 

As costs of precombustion hydrodesulphurisation and post combustion flue gas clean-up have escalated and as environmental regulations have further limited the sulphur dioxide emission rates, there has been a growing interest in technology designed to effect fuel desulphurisation during the combustion process. Desulphurisation during fluidised bed combustion of coal has been a leading technique in these developments. 

The full potential for removing pyritic sulfur from various coals by physical coal cleaning is significant but difficult to achieve. However, SO2 control by precombustion removal of pyrite could be an important S02-emissions reduction strategy. The cleaned coal produced could be used in coal-fired utilities, constructed both pre-and post-NSPS, as well as in industrial boilers. To realize the potential for coal cleaning in actual practice, however, new techniques must be demonstrated in the laboratory and then at the "proof-of-concept" scale (approximately one ton of coal per hour). These new coal beneficiation techniques could be advanced physical-coal-cleaning (PCC) processes, or they could employ microbial desulfurization or chemical desulfurization to remove organic sulfur. These latter processes could be used by themselves or in concert with PCC processes. 

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... 

It is therefore beneficial to pretreat the coal to remove as much sulfur as is practical before combustion, so that the expense of postcombustion desulfurization can be reduced. Combined with the other benefits of coal cleaning, it is evident that precombustion coal treatment is valuable even when it is not sufficient to completely desulfurize the coal by itself. 

Precombustion of sulfur from coal by selective oxidation is an important step in coal cleaning, without the need for a postcombustion cleanup step to remove sulfur oxides. This process, known as oxydesulfurization of coal, is a three-phase gas-liquid-solid system. As an example, a sparged reactor wiU be designed to oxydesulfurize 100 tons/day of coal in an aqueous slurry of coal with air. 

Coal can be utilized in ways that are cleaner and often more energy-efficient than those used in the past. In the various technologies, the cleaning can occur precombustion, during combustion, postcombustion, or by conversion of the coal to another fuel. 

Precombustion capture. This solution is developed in two phases (1) the conversion of the fuel in a mixture of H2 and CO (syngas mixture) through, for example, partial oxidation, steam reforming, or autothermal reforming of hydrocarbons, followed by water-gas shift (WGS), and (2) the separation of CO2 (at 30%-35%) from the H2 that is then fed as clean fuel to turbines. In these cases, the CO2 separation could happen at very high pressures (up to 80 bar of pressure difference) and high temperatures (300°C-700°C).42... 

Generally, precombustion coal cleaning is achieved by the use of physical techniques, some of which have been used for more than a century. Physical cleaning methods typically separate undesirable matter from coal by relying on differences in densities or variations in surface properties. 

Column flotation A precombustion coal cleaning technology in which coal particles attach to air bubbles rising in a vertical column—the coal is then removed at the top of the column. 

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