Desulfurization precombustion

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. 

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... 

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. 

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. 

Also, as precombustion sulfur removal becomes an attractive alternative to flue gas scrubbers, scientists have explored several methods of removing inorganic sulfur from coal and coal chars. Oxidation, microbial oxidation/desulfurization, and halogenation are avenues that are now, and have been, under investigation for some time (Thomas, 1995). 

Until now, much attention has been focused on the development of technologies to control SO2 emissions. The commonly required desulfurization process is used for natural gas, flue gas, oil, and coal. There are two main methods to achieve desulfurization. One method is precombustion desulfurization, such as the production of ultra-low sulfur (ULS) gasoline and diesel fuel, and the other method is postcombustion desulfurization, such as flue gas desulfurization (FGD). 

All these methods are specific to the precombustion removal of inorganic sulfur and are expensive when applied to the large quantities of coal burned in power stations. The organic sulfur retained within the carbonaceous structure represents an appreciable contribution to acid rain and, therefore, these methods may not provide effective desulfurization. 

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