Monolith SCR reactor

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... 

Figure 13.9 Validation of the dynamic model ofthe monolith SCR reactor during ESC and ETC tests. All concentrations are normalized by the respective maximum inlet valueduringthe test cycle. Dotted black lines, inlet values solid black lines, outlet measurements gray lines, outlet simulations. Adapted from ref. [62].

As outlined in the section Effects of the Operating Variables, the approach to design and analysis of monolith SCR reactors customarily adopted in the technical literature has been based on simple pseudo-homogeneous models accovmt-ing only for axial concentration gradients. The effects of inter- and intraphase mass transfer limitations were lumped into effective pseudo-first-order rate constants, such as in equation 14, which were specific for each type of catalyst. Such constants actually varied not only with temperature, but included dependences on the gas flow velocity, on the monolith channel geometry, and on the catalyst pore structure as well. 

Unsteady Models of Monolith SCR Reactors. As pointed out already in the section Unsteady-State Kinetics of the SCR Reaction, the growing interest in the dynamic behavior of DeNO systems has originated in recent years a number of studies related to transient operation of monolith SCR reactors. Mathematical modeling appears to be particularly useful for the analysis and development of unsteady SCR processes. 

The kinetic parameters estimated by the experimental data obtained frmn the honeycomb reactor along with the packed bed flow reactor as listed in Table 1 reveal that all the kinetic parameters estimated from both reactors are similar to each other. This indicates that the honeycomb reactor model developed in the present study can directly employ intrinsic kinetic parameters estimated from the kinetic study over the packed-bed flow reactor. It will significantly reduce the efibrt for predicting the performance of monolith and estimating the parameters for the design of the commercial SCR reactor along with the reaction kinetics. 

The vanadium content of some fuels presents an interesting problem. When the vanadium leaves the burner it may condense on the surface of the heat exchanger in the power plant. As vanadia is a good catalyst for oxidizing SO2 this reaction may occur prior to the SCR reactor. This is clearly seen in Fig. 10.13, which shows SO2 conversion by wall deposits in a power plant that has used vanadium-containing Orimulsion as a fuel. The presence of potassium actually increases this premature oxidation of SO2. The problem arises when ammonia is added, since SO3 and NH3 react to form ammonium sulfate, which condenses and gives rise to deposits that block the monoliths. Note that ammonium sulfate formation also becomes a problem when ammonia slips through the SCR reactor and reacts downstream with SO3. 

Figure 1.2. Schematic flow diagram of SCR process and of the trays and monoliths of the SCR reactor.

Tronconni E, Forzatti P. Adequacy of lumped parameter models for SCR reactor with monolith structure. AIChE J 1992 38 201-210. 

Square metal monoliths, as shown in Figure 13, are used in SCR reactors. Catalyst lives of up to more than 10 years are possible, and with proper catalyst management... 

Both in laboratory and power plant conditions the SCR reactor works under combined intraparticlc and external diffusion control, due to the high reaction rate and the laminar flow regime in the monolith channels. 

N. de Nevers, Fluid Mechanics for Chemical Engineers, McGraw-Hill, New York, 1991, p. 386. E. Tronconi and P. Forzatti, Adequacy of lumped parameter models for SCR reactors with monolith structure, AIChE J. 38(2) 20 (1992). 

Monolithic honeycomb have found major application in the selective catalytic reduction process (Fig. 16). The pressure op is about 250-1000 Pa. The SCR reactor asks for a flat velocity profile of the flue gas. Therefore, a layer of dummy honeycombs is installed for flow-straightening purposes. 

DeNOx - Scope of the model analysis was to evaluate on a quantitative basis the effective dependence of the intrinsic activity of the monoliths on the thermal sintering, and separate it from the contributions of inter-phase mass transfer and the effect of morphological modifications on intra-porous diffusion. When excess ammonia is present, as in the case of the experiments herein analyzed, then the Ealy-Rideal kinetic expression which is contained in the model of the SCR reactor reduces to a first order dependence on NO concentration under such conditions, an unique adaptive parameter, kc, accounts for the DeNOx intrinsic activity. Estimation of kc for the three calcined catalysts was obtained by fitting the model to each set of experiments. Input data included the operating conditions, the geometrical... 

Catalyst optimization has reduced the size and the cost of the SCR reactor by a factor of 2-4 and has greatly improved the economics of the SCR process. The SCR monoliths and plates are assembled into standard steel-cased... 

Table 3. Published Mathematical Models of Monolithic SCR- DeNOx Reactors. 

Applications of Steady-State Models to Design of SCR Catalysts and Reactors. The physicochemical phenomena occurring in a monolith SCR catalyst are relatively well understood by now accordingly, the mathematical models describing such phenomena can be profitably applied to the rational design of SCR catalysts and processes, as proposed in the literature studies discussed below. 

Other dynamic models of SCR monolith catalysts have been published in relation to the analysis of reverse-flow DeNO -SCR reactors they are discussed in the following paragraph. 

Progress has been made towards the development of monolith reactor models that predict SCR performance under both steady state and transient operation. Guth-enke et al. [33]. provided a thorough review of SCR reactors. Most of the earlier work in this area was done for the more established Vanadia-based catalysts and involved the use of global kinetic models [81-83]. More recent works by Nova et al. provided detailed transient model for the SCR reaction system on Vanadia-based catalyst [8, 45]. Olsson and coworkers developed both global and detailed kinetic models for NH3-SCR reactions on Cu-ZSM-5 catalysts [14, 15, 49]. More recent works have communicated models for NH3-SCR reactions on Fe-zeoUte catalysts [25, 57, 76, 84]. 

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. 

Figure 10.12. An SCR (selective catalytic reduction) reactor is loaded with monolith assemblies, one of which is shown on the right. Each unit is about 40 x 40 x 50 cm. ...

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. 

Therefore, there is a strong motivation to develop a dynamic model of the SCR monolithic reactor suitable for extended temperature operation and to study the fast SCR reaction in view of future possible applications. In the following, we will focus on these two issues. 

It has been demonstrated that kg can be estimated by analogy with the Graetz-Nusselt problem governing heat transfer to a fiuid in a duct with constant wall temperature (SH= Nut) [30] and that the axial concentration profiles of NO and of N H 3 provided by the 1D model are equivalent and almost superimposed with those of a rigorous multidimensional model of the SCR monolith reactor in the case of square channels and of ER kinetics, which must be introduced to comply with industrial conditions for steady-state applications characterized by substoichiometric NH3 NO feed ratio, that is, a[Pg.401]

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 same model was applied to the simulation of typical transients occurring during the operation of industrial SCR monolith reactors in large power plants. In all cases it was found that the change in NO outlet concentration is considerably delayed with respect to the variation of the inlet NH3 concentration. This is unfavorable for a feedback control system using the ammonia feed as the control variable and makes... 

---