SCR-NH3 processes

In this chapter, the development of an unsteady kinetic model of the NH3-SCR process for vanadium-based catalysts is presented. The model was based on the results from an extensive investigation of reactivity, chemistry, catalytic mechanism, and kinetics of the frill NH3-NO/NO2 SCR reacting system over a commercial V205-W03/Ti02 catalyst performed in our laboratories [1-10]. 

Ammonia may likewise be used as reductant for selective catalytic reduction of NOx species. For this application, metal-exchanged zeolite catalysts offer new opportunities to reduce NOx emissions from lean-bum engine via the NH3-SCR process. Iron-exchanged ZSM-5 has received much attention because of its promising activity and stability in the NH3-SCR process. Correlating catalytic activities with the concentration of mononuclear and binuclear Fe species shows that both types of Fe ions and even small metal clusters are active sites for SCR,... 

NH3-SCR Processes for the Emissions from Combustion and Nitric Acid Plants... 

The NH3-SCR process is established as a robust and useful technique for the elimination of NOx from off-gases. Commercially used V205-W03/Ti02 catalysts reduce NO selectively to N2 and H2O by the following reaction ... 

In the SCR process, ammonia, usually diluted with air or steam, is injected through a grid system into the flue/exhaust stream upstream of a catalyst bed (37). The effectiveness of the SCR process is also dependent on the NH3 to NO ratio. The ammonia injection rate and distribution must be controlled to yield an approximately 1 1 molar ratio. At a given temperature and space velocity, as the molar ratio increases to approximately 1 1, the NO reduction increases. At operations above 1 1, however, the amount of ammonia passing through the system increases (38). This ammonia slip can be caused by catalyst deterioration, by poor velocity distribution, or inhomogeneous ammonia distribution in the bed. 

While the development of flue gas clean-up processes has been progressing for many years, a satisfactory process is not yet available. Lime/limestone wet flue gas desulfurization (FGD) scrubber is the most widely used process in the utility industry at present, owing to the fact that it is the most technically developed and generally the most economically attractive. In spite of this, it is expensive and accounts for about 25-35% of the capital and operating costs of a power plant. Techniques for the post combustion control of nitrogen oxides emissions have not been developed as extensively as those for control of sulfur dioxide emissions. Several approaches have been proposed. Among these, ammonia-based selective catalytic reduction (SCR) has received the most attention. But, SCR may not be suitable for U.S. coal-fired power plants because of reliability concerns and other unresolved technical issues (1). These include uncertain catalyst life, water disposal requirements, and the effects of ammonia by-products on plant components downstream from the reactor. The sensitivity of SCR processes to the cost of NH3 is also the subject of some concern. 

The SCR process consists of the reduction of NO (typically 95% NO and 5% NO2 v/v) with NH3. The reaction stoichiometry is usually represented as 4NO + 4NH3 + 02 4N2 + 6H2O. This reaction is selectively effected by the catalyst, since the direct oxidation of ammonia by oxygen is prevented In the case of the treatment of sulfur-containing gas streams, the DcNO reaction is accompanied by the catalytic oxidation of SO2 to SO3 Oxidation of SO2 is highly undesirable because SO3 is known to react with water and residual ammonia to form ammonium sulfates, which can damage the process equipment. 

Orsenigo et al. [47] have proposed an alternative reactor design suitable in principle to exploit NH3 inhibition for minimizing SO3 formation in the SCR process. This is based on the idea of splitting the NOx-containing feed stream in substreams fed separately to the SCR reactor in this way, a portion of the catalyst volume can operate with an excess of ammonia, while the overall NH3/NO feed ratio is still substoichiometric. 

For all of these reasons, a thorough understanding of the NH3 adsorption-desorption phenomena on the catalyst surface is a prerequisite In fact, typical SCR catalysts can store large amounts of ammonia, whose surface evolution becomes the rate-controlling factor of the reactor dynamics. Also, mathematical modeling appears to be even more useful for the analysis and development of unsteady SCR processes than in the case of steady-state operation. 

In this section, the selective catalytic reduction of NOx with NH3 is emphasized. This reaction is the basis of the selective catalytic reduction (SCR) process that removes NOx from oxygen-rich emissions that occur in power plants, waste incinerators, and gas turbines. 

In the SCR process, stochiometric quantities of ammonia (NH3) is injected along with the flue gas over a catalyst at temperatures between 300 and 400 °C to reduce NOx to harmless nitrogen (N2) and water (H2O). It may be emphasized that Indian coals have a high sulphur content and as already mentioned, this leads to the formation of su hur dioxide during combustion. In presence of the catalyst, this SO2 may get oxidised to sulphur trioxide (SO3) which in turn may react with excess NH3 resukiog in the formation of ammonium salts causing... 

Source reduction of NOx from combustion is based on the modification of combustion conditions (mostly temperature). This approach includes EGR. SAC and LNB etc. In SNCR process urea or NH3 is injected into high temperature region (> 900 C) to promote noncatalytic reaction between NH radicals and NOx. Early applications of SNCR mostly used anhydrous or aqueous NHj and suffered from a narrow temperature range. Later, the use of urea has been found to be efTiciem. and now marketed under the trade name of NOxOUT process. The urca-SNCR (NO.vOUT) process can reduce NOx up to 90%, while the reductions rate NOx using typical SNCR ranges 40-75% depending on residence lime, temperature, and mixing condition. SCR process has been widely applied for NOx control in many combustion facilities. SCR process use NH3 or hydrocarbons before a catalyst bed. 

Within current survey, NH3 seems to be the most efneiem additive especially at low temperature. In the conventional SCR process, dominant reaction responsible for NO reduction is RIO, which proceeds at equimolar ratio. 

The objective of this study was to investigate and compare the conversion vs. temperature behavior of promising high-performance zeolite catalysts, NC-301, ZNX, and Cu-ZSM-5, for NOx reduction by NH3 at conditions representative of the primary SCR reactors of the WINCO NWP SCR process. 

Conversion of nitrogen oxides in the presence of NH3 to yield N2 (selective catalytic reduction, SCR) Cu-ZSM-5, Cu-ZSM-ll,Cu-ZSM-12, Cu-P, Cu-SSZ-13, Cu-SAPO-34 Small-pore, highly hydrothermally stable zeolites are currently of increasing interest in the SCR process [58, 71]... 

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. 

The NOx reduction reaction takes place when the gasses pass through the catalyst chamber. Before entering the feed stream to the catalyst chamber, the NH3 or other reductant, such as urea, is mixed with the gasses. The chemical equations for stoichiometric reaction using either anhydrous or aqueous NH3 for an SCR process are ... 

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... 

The SCR process is based on the reduction of NO with NH3 into H2O and N2 according to the following main reactions ... 

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