SCR System Design

Furthermore, in combination with the DOC model described earlier, it is possible to study the effect of N02, formed over the DOC, on the performance of the SCR catalyst (Table II). In the case shown, the extra N02 appears to have a deleterious effect on NOx conversion. Indeed, it is now well known that too much N02 in the vanadia SCR catalyst results in a decrease in performance, while when optimised the N02 leads to a significantly improved performance (Chandler et al., 2000 Gieshoff et al., 2000). Therefore, it is clear to see how the models can be used for SCR system design. 

One regulatory development that will drive SCR system design in Europe and elsewhere is emerging now in Europe Real-World Driving Emissions (RDE). Investigators have found that NOx emissions from LD diesels can be 3-4X higher... 

Many new SCR systems designs are emerging to meet the tightening cold start and deNOx efhciency requirements under low-load urban driving conditions. 

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. 

A vast majority of stractured catalysts and reactors are used in flue gas cleaning. However, due to proprietary reasons, not much information is provided. Babcock Power Environmental reported [65] that in the United States they have over 30000 MWs of SCRs commissioned, in design or evaluation, and under construction. BP s licensor has over 26000 MWs of SCRs operating in Europe. The German company EON operates 44 SCR systems, totaling 12 300 MWs. It should be noted that not all SCRs plants are based upon structured catalysts. 

Question 5 ("Is combustion with air the only chemistry intended at your facility ") can be answered YES in this case, assuming the "facility" being addressed is limited to the incinerator system. Due to the great number of combustion systems in operation, many other resources are available for ensuring safe design and operation of the combustion part of the incinerator facility. However, it should be noted that many combustors now have effluent treatment systems, such as selective catalytic reduction (SCR) systems, that involve intentional chemistry beyond the combustion reaction. 

In addition to the choices between the types of catalysts, the following are also required in designing SCR systems ... 

Mixing of ammonia with flue gas should be carried out with eare because a very low slip is allowed of 5 ppm ammonia at an NO removal efficiency of 80%. SCR systems in the United States are designed for 80-90% NO c removal with an ammonia slip of 10-20 ppm [137]. 

Kaiser R, Rusch K (2007) Designing of SCR Systems for reducing Nitrogen Oxide in Diesel Engines. MTZ 68(12) 1062-1070... 

The choice of system is not obvious. The SCR system has the advantage of a simpler engine design leading to less complexity and lower engine product cost. Since the SCR system does not require cooled EGR, higher engine power output... 

Koltsakis G, Koutoufaris I, Haralampous O, Tourlonias P, Kramer L (2007) Model-based design of combined DPF-SCR systems for passenger cars. Paper presented at the FAD conference 2007... 

To verify this ammonia storage distribution control approach in practice, a two-catalyst SCR system is developed. A controller is designed to control the AdBlue injection such that the ammonia coverage ratio of the upstream catalyst is kept at a higher value and the ammonia coverage ratio of the downstream catalyst is limited under a lower level as presented in Fig. 14.13. 

Devarkonda M et al (2009) Model-based control system design in a urea-SCR aftertreatment system based on NH3 sensor feedback. International Journal of Automotive Technology, 10(6) 653-662... 

The modeling of SCR systems is a particularly challenging task since it involves different physical phenomena where the evolution of multiphase and multicomponent flows take place. First, a urea/water solution is sprayed into the gas stream. The dispersed multicomponent phase then evaporates and suitably mixes with the gas stream before entering the catalyst. A comprehensive understanding of the underlying flow dynamics is therefore mandatory to design an efficient DeNO system which is characterized by a low impact on the engine performances. 

G. Montenegro and A. Onorati. Id thermo-fluid dynamic modelling of de-nox scr systems for diesel engine exhaust gas after-treatment Int. J. of Vehicle Design, 41(No.l/2/3/4) 285-306, 2006. 

Pereda-Ayo B, Duraiswami D, GonzSlez-Velasco JR (2011) Control of NOx storage and reduction in NSR bed for designing combined NSR-SCR systems. Catal. Today 172 66-72... 

R. Morita, R. Honda, Y. Takahashi, Development of oral controlled release preparations, a PVA swelling controlled release system (SCRS). 1. Design of SCRS and its release controlling factor. 7 Control Release, 63 297-304, 2004. 

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