Selective noncatalytic reduction SNCR

When NO destmction efficiencies approaching 90% are required, some form of post-combustion technology appHed downstream of the combustion 2one is needed to reduce the NO formed during the combustion process. Three post-combustion NO control technologies are utilized selective catalytic reduction (SCR) nonselective catalytic reduction (NSCR) and selective noncatalytic reduction (SNCR). 

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. 

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). 

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... 

Most of the existing processes for nitrogen oxide removal are chemically based requiring high temperature or expensive catalysts. The main techniques involve either selective noncatalytic reduction (SNCR) or selective catalytic reduction (SCR). SNCR uses ammonia for conversion of NO to N2 and H20 at elevated temperatures (550-850 K). SCR can use catalysts such as Ti02 with active coatings of V2Os and WO, . 

The third strategy for minimizing NOx, known as posttreatment, involves removing NOx from the exhaust gases after the NOx has already been formed in the combustion chamber. Two of the most common methods of posttreatment are selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR).7 Wet techniques for posttreatment include oxidation/absorption, oxidation/absorption/reduc-tion, absorption/oxidation, and absorption/reduction. Dry techniques for posttreatment, besides SCR and SNCR, include activated carbon beds, electron beam radiation, and reaction with hydrocarbons. 

Postcombustion NO control options such as selective Noncatalytic reduction (SNCR), SCR, and catalytic oxidation/scrubbing. 

Selective noncatalytic reduction (SNCR) (e.g., Ammonia injection or Thermal DeNOx)... 

Selective noncatalytic reduction (SNCR) Injection of ammonia or urea solution in the flue gas in a "temperature window" from 900°C to 1050°C. 

A key research area is the study of the effect of selective catalytic reduction (SCR), selective noncatalytic reduction (SNCR), dry particulate adsorbent injection, and SOx removal systems on the removal efficiency, pressure drop, and lifetime of the baghouses. The use of fabric filters in PC boilers is increasing every year due to the stringent particulate emission standards. Pennsylvania Power and Light was the first utility to install a bag-house in 1973, and up until 2005, more than 129 RGFFs were installed on 28 GW of power plant capacities [64] (Figure 18.10). 

Several techniques have been considered to decrease NOx emission, such as selective noncatalytic reduction (SNCR), selective catalytic reduction (SCR) with ammonia (NH3) or hydrocarbon, and direct catalytic decomposition of NO. The main disadvantage of the SCR process is the high cost associated with the consumption of reductants. Nevertheless, direct catalytic decomposition of NO without the addition of reducing agents is an effective and economical procedure to decrease NOx emission. Therefore, the direct decomposition of NO into N2 and O2 (2NO = N2 + O2) is the optimal way for NO removal, because the process is simple and there is no necessity for a reductant such as a hydrocarbon, NH3, or urea. 

Among flue gas treatment methods, the selective catalytic reduction (SCR), is best proven and it is used worldwide due to its efficiency, selectivity, and economics (2,5,6). The SCR process is based on the reaction between NOx and ammonia (NH3) or urea (CO(NH2)2), injected into the flue gas stream, to produce harmless water and nitrogen. Selective noncatalytic reduction (SNCR) has also been proposed, through which NOx is selectively reduced in the homogeneous phase by ammonia (or urea), which is introduced into the upper part of the boiler. The major drawback of the SNCR process is constituted by the narrow... 

Selective Noncatalytic Reduction (SNCR). This process involves NHj, either anhydrous or in solution with water, or nrea injection withont a catalyst. A major commercially available SNCR system is the Exxon Thermal DeNO NH3 (anhydrous or aqueous) injectiou system. 

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