NOX Removal Systems

It can be assumed that in the presence of an excess of oxygen, NO reacts with an equimolar quantity of NH3 to give N2 and H2O. 

The catalysts must be designed so that side reactions such as the oxidation of ammonia by oxygen (Eq. 10-8) and the formation of N2O (Eq. 10-9) are suppressed. The oxidation of SO2 to SO3 must also be avoided. 

The SCR processes have become established in Western Europe, and the required Ti02W205 based honeycomb catalysts are produced by various European catalyst producers under a Japanese license. 

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With the available data generated so far, the percentage loading effect and the effect of residence time could be employed for design of the NOx removal system. 

You are to design the NOx removal system for each unit and determine which of the two systems is the most cost effective for NOx removal. 

The primary form of NOx for a combustion source is nitric oxide (NO). The NOx removal system to be considered is Selective Catalytic Reduction (SCR). 

A NOx removal system may be required to meet a site s emission requirements. The typical system used for this case is a Selective Catalytic Reduction unit (SCR). The SCR unit reduces NOx by reacting armnonia with the flue gas over a catalyst, which yields nitrogen and water vapor. An SCR... 

Dors et al. [49] investigated NOx removal in a hybrid system consisting in a d.c. corona discharge and a molecular sieve placed inside the plasma region. In the corona discharge, all NO converted was oxidized to N02, so the removal rate of NOx was zero. 

Dors, M., Mizeraczyk, J. and Nichipor, G.V. (2004) Influence of ammonia on NOx removal in corona discharge-molecular sieve hybrid system,./. Adv. Oxid. Technol. 7, 142-4. 

Dors, M. and Mizeraczyk, J. (2004) NOx removal from a flue gas in a corona discharge-catalyst hybrid system. Catal. Today 89, 127-33. 

The typical steps for a gas cleanup system aim at particulate removal, sulfur removal, and NOx removal. This is achieved as follows ... 

The NSR technology has been also applied to diesel engines, and is most reliable and attractive method for lean-burn combustion vehicles. Diesel particulate-NOx reduction system (DPNR) method is used to realize the simultaneous and continuous reduction of particulate and NOx is also recommended. This catalyst system is DPF combined with NSR catalyst. Soot on catalyst is removed during NOx reduction by occasional rich engine modification. Many other catalyst systems with NSR catalyst have been also developed. With decreasing S content in fuel and successive development of... 

The simultaneous removal of NOx and soot on Pt-Ba/Al203 NSRC was investigated by Castoldi et al. (2006). They concluded that the presence of soot does not affect the NOx reduction activity of the NSRC, while the soot combustion is enhanced by the presence of N02. This principle has been already utilized by Toyota in the integrated DPNR (diesel particulate and NOx reduction) system (Nakatani et al., 2002). 

These catalysts contained promoters to minimize S02 oxidation. Second-generation systems are based on a combined oxidation catalyst and particulate trap to remove HC and CO, and to alleviate particulate emissions on a continuous basis. The next phase will be the development of advanced catalysts for NOx removal under oxidizing conditions. Low or zero sulfur diesel fuel will be an advantage in overall system development. 

In an economic comparison of abatement systems, a 1991 EPA study indicates that extended absorption to be the most cost-effective method for NOx removal. Selective Reduction matches its performance only in small-capacity plants of about 200 to 250 tonnes per day. Nonselective abatement systems were indicated to be the least cost-effective method of abatement. The results of any comparison depend on the cost of capital versus variable operating costs. A low capital cost for SCR is offset by the ammonia required to remove the NOx. Higher tail gas NOx concentrations make this method less attractive. The investment for extended absorption is partially recovered by increased yield of nitric acid product104. 

Chapter 12 handles the design of a Biochemical Process for NOx Removal from flue gases. The process involves absorption and reaction steps. The analysis of the process kinetics shows that both large G/L interfacial area and small liquid fraction favor the absorption selectivity. Consequently, a spray tower is employed as the main process unit for which a detailed model is built. Model analysis reveals reasonable assumptions, which are the starting point of an analytical model. Then, the values of the critical parameters of the coupled absorber-bioreactor system are found. Sensitivity studies allow providing sufficient overdesign that ensures the purity of the outlet gas stream when faced with uncertain design parameters or with variability of the input stream. 

Overexchanged Fe-MFI catalysts via an oxalate method offer high performance and hydrothermal durability for NOx reduction with iso-butane even in the presence of significant amounts of H2O and SO2 for high temperature deNOx application to the ICEs. However, this approach has hardly been reproducible. On the other hand, the CVD method can prepare such Fe-MFI catalysts on which somewhat lower NOx removal efficiency but still high durability can be achieved for the present reaction system. Not only could the technique be reproducible for the catalyst preparation, but it also depends strongly on the... 

NOx removal at the 80% level or higher can be achieved in favorable cases, but this can fall to 70%, where there is a possibility of significant fouling or poisoning of the catalyst. Deactivation of these and other environmental catalyst systems has recently been reviewed [40]. Often the SCR system is placed downstream of an oxidation catalyst to permit simultaneous removal of NOx, hydrocarbons, and carbon monoxide. 

The performance of the PPR for NOx removal by the Shell low-temperature NOx reduction has been investigated extensively [20]. In the first commercial application of the Shell process with parallel-passage reactors, flue gases of six ethylene cracker furnaces at Rheinische Olefin Werke at Wesseling, Germany, are treated in a PPR system with 120-m catalyst in total to reduce the nitrogen oxide emissions to about 40 ppm v. Since its successful start-up in April 1990, the unit has performed according to expectations... 

The NOx removal process was studied experimentally in a pulsed corona discharge combined with the Ti02 photocatalytic reaction [387]. N02 was found to adsorb easily on the photocatalyst surface, whereas NO was hardly adsorbed. Addition of water vapor enhanced the N02 adsorption. It was concluded that the main role of the plasma-chemical reaction in this system is the oxidation of NO into N02. A considerable part of N02 is adsorbed on the photocatalyst surface, and is transformed to HN03 through photocatalytic reaction with OH. 

A stocktaking of SCR as of today is as follows. SCR is capable of up to 90% NOx removal, but it needs a bulky catalytic reactor with a catalyst load of even up to 1000 m, it needs a system for ammonia storage cuid supply, and analytical equipment for NOx and NH3 at inlet and outlet of the SCR plant. The capital investment and operating costs are quite high DM 100-200 per installed kW. Per installed mW, the catalyst required is 2 m and this will cost DM 60,000. Cost of catalyst comes to 40% of overall cost of installation. This is absurdly high for a catalytic process. By contrast, for a chemical or petrochemical complex, the investment on catalysts is only about 1% of the total investment for the complex. 

The water-gas shift (WGS) reaction is one of the oldest catalytic processes employed in the chemical industry. Recently, there is renewed interest in this reaction because of its relevance for producing pure hydrogen for use in fuel cell power systems. Another reason for the increased interest is the key role of the WGS reaction in automotive exhaust processes, since the hydrogen produced is an effective reductant for NOx removal [1]. New technologies require improvements of the WGS catalyst system, and it is desirable to prepare catalysts with high activity at relatively low temperatures and better stability than the commercial Cu/Zn0/Al203 catalyst. The catalyst properties may... 

Ammonia slip t)T)ically occurs when overstoichiometric amoimts of urea/ammonia are injected. Due to the ammonia storage capacity of the catalyst this is not emitted directly so if an appropriate control action is taken, ammonia slip can be avoided. However, up to today an inexpensive and fast NOx and/or ammonia sensor is not available to accommodate such a control action. Solutions have been proposed in the use of engine maps for predicting the NOx output [9] and/or an additional oxidation catalyst for avoiding ammonia slip [3]. The latter is detrimental to the sulfate emissions and as most of the oxidized ammonia will leave the system as NO, the overall NOx removal efficiency will be lowered. Therefore, it would be convenient if a possible overdose of ammonia will leave the system in the form of nitrogen (N2)- This would solve the SCR control problem. 

Air pollution control devices (gas cleaning devices) (Chapter 23) found in fossil fuel-fired systems (particularly steam electric power facilities) include particulate removal equipment, sulfur oxide (SOx) removal equipment, and nitrogen oxide (NOx) removal equipment. 

Coal can also be converted into gaseous and liquid low-sulfiir fuels. The catalytic converter in an automobile exhaust system reduces NOx to N2 and NH3, and in power plants, NOx removed from the hot stack gases with ammonia ... 

Work is continuing on the combination SCR + DPF system, wherein SCR catalyst is coated onto the DPF. This allows SCR catalyst to be placed on the vehicle without using an added component, and can get the SCR catalyst closer to the engine for faster light-ofif. Numerous reports dating to 2(X)8 show that total NOx removal efficiency is thus improved, with little to no compromise in DPF... 

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