Postcombustion process

Combustion modifications and postcombustion processes are the two major compliance options for NO., emissions available to utilities using coal-fircd boilers. Combustion modifications include low-NO burners (LNBs), overfire air (OFA), reburning, flue gas recirculation (FGR), and operational modifications. Postcombustion processes include selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR). The CCT program has demonstrated innovative technologies in both of these major categories. Combustion modifications offer a less-expensive appiroach. 

Postcombustion processes are designed to capture NO, after it has been produced. In a selective catalytic reduction (SCR) system, ammonia is mixed with flue gas in the presence of a catalyst to transform the NO, into molecular nitrogen and water. In a selective noncatalytic reduction (SNCR) system, a reducing agent, such as ammonia or urea, is injected into the furnace above the combustion zone where it reacts with the NO, to form nitrogen gas and water vapor. Existing postcombustion processes are costly and each has drawbacks. SCR relies on expensive catalysts and experiences problems with ammonia adsorption on the fly ash. SNCR systems have not been proven for boilers larger than 300 MW. 

Schach, M., Schneider, R., Schramm, H. and Repke, J. (2010) Techno-economic analysis of postcombustion processes for the capture of carbon dioxide from power plant flue gas. Industrial and Engineering Chemistry Research, 49 (5), 2363-2370. 

The only commercially proven process for postcombustion separation is an amine-based process. Substantial research is needed to improve postcombustion processes and reduce their costs. Reduction in size and weight of the systems is an important objective when remote and offshore operations are considered. Improving the contact efficiency and mass transfer between the gas phase and the separation chemicals is needed. Novel methods that do not depend on large volumes of expensive chemicals would be very attractive because of industry s concerns about the waste products of the separation process. 

NO Emission Control It is preferable to minimize NO formation through control of the mixing, combustion, and heat-transfer processes rather than through postcombustion techniques such as selective catalytic reduction. Four techniques for doing so, illustrated in Fig. 27-15, are air staging, fuel staging, flue-gas recirculation, and lean premixing. 

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 selective Noncatalytic reduction (SNCR) process is a postcombustion NO reduction technology. NO is reduced through the controlled injection of a reagent, either ammonia or urea, into the combustion products of boiler, heater, or FCC regenerator. This process is typically applied on partial burn applications with a CO boiler (COB). 

A number of vendors offer SNCR technology based on either ammonia or urea. Exxon Mobil Thermal DeNO (TDN) technology is a common SNCR process applied to FCC units. The technology is licensed exclusively to Hamon Research-Cottrell Inc., and has been utilized to achieve postcombustion NO reduction in CO furnaces, thermal oxidizers, overhead regenerators, and power boilers. Thermal... 

The postcombustion systems used at power generating plants and factories are somewhat different. These systems remove nitrogen oxides from the waste gases jlue gases) processes using classified as selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR). Oxides of nitrogen are also removed by some systems... 

Figure 9.1 Basic schematic diagram of (a) postcombustion capture, (b) precombustion decarbonization, and (c) oxy-fuel processes.

Postcombustion Capture Processes with Membrane Separation... 

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. 

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... 

RCP [Recycled Clean Products] A high-temperature process for recycling sold wastes. It combines pyrolysis, smelting, and slag refining with postcombustion in a circulating, fluidized bed. Developed by Von Roll, Switzerland, and operated commercially in Germany since 1997. 

In this section, we discuss the laboratory techniques and production technologies used for the combustion synthesis process. The laboratory studies reveal details of the CS process itself, while the technologies may also include other processing, such as densification of the product by external forces. In both cases, it is necessary to control the green mixture characteristics as well as the reaction conditions. For the production technologies, however, optimization of parameters related to external postcombustion treatment is also necessary in order to produce materials with desired properties. 

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