Monoliths SCR catalysts

Kleemann, M., Elsener, M., Koebel, M., et al. (2000) Investigation of the Ammonia Adsorption on Monolithic SCR Catalysts by Transient Response Analysis, Appl. Catal. B, 27, 231. 

Figure 2 Types of monolithic SCR catalysts (a) honeycomb monolith (b) plate-type catalyst (c) coated metal monolith.

Only recently have efforts been devoted to a detailed chemical engineering analysis of the operation and design of monolith SCR catalysts. 

As previously discussed, the physico-chemical phenomena occurring in a monolith SCR catalyst are relatively well understood by now the mathematical models describing such phenomena can then be applied with confidence to the rational design of SCR catalysts and processes. 

In the last few years a growing interest has been focused on unsteady operation of monolith SCR catalysts. This is due in essence to the following reasons. 

Unsteady-State Kinetics of the SCR Reaction. Kinetic studies of the SCR reaction have also been performed under transient conditions. These investigations have been motivated by the fact that in the past few years there is a growing interest concerning unsteady operation of monolith SCR catalysts, due to the following reasons ... 

Inter- and Intraphase Mass Transfer Limitations in the DeNOx Reaction. It is well established that in both laboratory and power plant conditions, extruded monolithic SCR catalysts work under combined intraparticle and external diffusion control because of the high reaction rate and of the laminar flow regime prevailing in the channels of the monolith catalysts. As an example. Figure 12 points out that, for the same reaction conditions, different extents of NO reduction are observed over SCR honeycomb catalysts with identical composition but different channel openings. 

Some aspects of extrusion procedure for monolithic SCR catalyst based on Ti02... 

Fig. 18.5 a Absolute deviations between experimental NOx conversions and model simulation for steady-state runs over the monolith SCR catalyst as a function of temperature and NO2/NOX. GHSV = 100,000 h ... 

Blanco, J., Avila, R, Marzo, L., Suarez, S., and Knapp, C. Low temperature monolithic SCR catalysts for tail gas treatment in nitric acid plants. Stud. Surf. Sci. Catal 2000,130, 1391-1396. 

As with the automotive exhaust converter, the SCR catalyst is designed to handle large flows of gas (e.g. 300 N s for a 300 MW power plant) without causing a significant pressure drop. Figure 10.12 shows a reactor arrangement with about 250 m of catalyst in monolithic form, sufficient for a 300 MW power plant. 

Extruded SCR catalysts based on V205/W03—Ti02 with cell densities of 300cpsi are commercialized and 400 cpsi prototypes are available. Extruded SCR catalyst means that the whole monolith consists of the active mass V205/W03—Ti02, which is enforced by glass fibers and binders. 

The SCR catalysts are used in the form of honeycomb monoliths or plates to guarantee low pressure drops in view oflarge frontal area with parallel channels, high external surface area per unit volume of catalyst, high attrition resistance and low tendency for fly ash plugging. The SCR monoliths and plates are assembled into standard modules and inserted into the reactor to form catalyst layers. 

A simple isothermal pseudo-homogeneous, single-channel, ID model is typically adopted to model a monolith SCR reactor [27, 30, 38, 40-50], which implies uniform conditions over the entire cross-section of the monolith catalysts and accounts only... 

In recent years, several proposals have been made for the numerical simulation of SCR catalysts, primarily of coated monoliths [59, 60], and for control-oriented purposes [61]. 

Tronconi et al. [46] developed a fully transient two-phase 1D + 1D mathematical model of an SCR honeycomb monolith reactor, where the intrinsic kinetics determined over the powdered SCR catalyst were incorporated, and which also accounts for intra-porous diffusion within the catalyst substrate. Accordingly, the model is able to simulate both coated and bulk extruded catalysts. The model was validated successfully against laboratory data obtained over SCR monolith catalyst samples during transients associated with start-up (ammonia injection), shut-down (ammonia... 

The SCR catalyst designed for FCCU regenerator flue gas service is a homogenous monolith, typically made from 1 mm thick material. Some catalysts are extruded clays that receive a wash coat of titanium dioxide before impregnation of the vanadium and tungsten metals. Another type involves painted plates of expanded metal... 

They focus on the ID simulation of an urea SCR system. The system includes a model for N02 production on a DOC, a model for urea injection, urea decomposition and hydrolysis catalyst, a model for a vanadium-type SCR catalyst and a model for NH3 decomposition on a clean-up catalyst. The catalyst models consist of a ID monolith model with global kinetic reactions on the washcoat surface, kinetic parameters have been taken from literature or adjusted to experimental data from literature. The complete model was implemented in AVL BOOST (2006). AVL BOOST is an engine cycle and gas exchange simulation software tool, which allows for the building of a model of the entire engine. 

A first data set was measured on a heavy-duty diesel engine test bench. Extruded monoliths with 300 cpsi, a wall thickness of 0.32 mm and a diameter of 144 mm were used in these runs. By varying the number of monolith catalyst samples it was possible to test different catalyst volumes (25, 32 and 43 L). The complete SCR catalyst setup resulted in SVs ranging from 21,000 to... 

Beretta A, Orsenigo C, Tronconi E, Forzatti P, Berti F. Analysis of plate-type monolith SCR-DeNO(x) catalysts. Kinet Catal 1998 39 646-648. 

Figure 13 Fabrication of individual square metal monoliths into catalyst panels that are inserted into an industrial SCR unit for con-trol of NO, emissions. Similar reactors are used for oxidation of VOCs...

The original SCR catalysts were offered in the form of pellets or spheres and used in clean or low-dust applications. They were soon replaced by parallel-flow catalysts, namely, honeycomb monoliths, plates, and coated metal monoliths, as shown in Fig. 2. In the case of the SCR process, the main advantages of monolith-shaped catalysts with respect to conventional packed beds are as follows ... 

For application of zeolite SCR catalysts a monolith t3T>e reactor can be used, which is however... 

Prior to testing, the freshly loaded catalysts were initially conditioned under the same reacting stream used in the experiments. A steady state denoxing activity of the monoliths was reached after few hours on stream. Several days of operations were instead necessary to reach a steady activity in SO2 oxidation tests over granules. Conditioning of the SCR catalysts for both reactions has been associated to the gradual formation and build-up of surface sulfates from SO2 and water in the gas phase. Such surface sulfates have been shown to enhance the catalyst activity, with different extent for the two reactions in particular, the fraction of surface sulfates adjacent to vanadium sites seem responsible for the promoting effect of DeNOx reaction, while the overall amount of surface sulfates apparently contribute to enhance the SO2 oxidation activity [7]. 

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