SCR reactor

When a 1 1 mixture of NO and NO2 (i.e., NO2/NOx=0,5) is fed to the SCR reactor at low temperature (200 °C) where the thermodynamic equilibrium between NO and NO2 is severely constrained by kinetics, the NO2 conversion is much greater than (or nearly twice) the NO conversion for all three catalysts. This observation is consistent with the following parallel reactions of the SCR process [6] Reaction (2) is the dominant reaction due to its reaction rate much faster than the others, resulting in an equal conversion of NO and NO2. On the other hand, Reaction (3) is more favorable than Reaction (1), which leads to a greater additional NO2 conversion by Reaction (3) compared with the NO conversion by Reaction... 

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 ammonia is either injected as pure ammonia under pressure or in an aqueous solution at atmospheric pressure. Instead of ammonia, urea can also be used. The challenge of the process is to efficiently remove as much NOx as possible at full conversion of the reductant, as emission of NH3 from the SCR reactor would of course be highly undesirable. 

The combined approach of removing both the sulfur and the NOx from the flue gas is called SNOX (Haldor Topsoe A/S) or DESONOX (Degussa). An example of the setup for this process is shown in Fig. 10.11, where 99% of the NOx is converted in the SCR reactor and the SO2 is converted into sulfuric acid. 

Dust, which is a particular problem, is filtered out of the flue gas by electrostatic precipitators either before (low dust operation) or after the SCR reactor (high dust operation). 

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.

Unreacted NH3 in the flue gas downstream the SCR reactor is referred to as NH3 slip. It is essential to hold the NH3 slip below 5ppm, preferably 2-3 ppm, to minimize the formation of (NH4)2S04 and NH4HS04, which can cause plugging and corrosion of downstream equipment. In order to avoid the ammonia slip, and to limit the direct oxidation of NH3 to N2, the NH3/NO ratio in the feed is typically maintained below the stoichiometric values, e.g. between 0.90 and 0.95. 

The oxidation of NO to N02 can be considered an advantage when combining plasma with selective catalytic reduction, as it is well-known that at low temperature the efficiency of NO reduction by SCR strongly depends on the N02 concentration in the gas. An oxidation pre-treatment of the gas by non-thermal plasma, prior to the SCR reactor greatly enhances the performance of SCR at temperatures below 500 K. Therefore, the PE-SCR technique is very promising for efficient NO reduction. 

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... 

The study of the intra-phase mass transfer in SCR reactors has been addressed by combining the equations for the external field with the differential equations for diffusion and reaction of NO and N H 3 in the intra-porous region and by adopting the Wakao-Smith random pore model to describe the diffusion of NO and NH3 inside the pores [30, 44]. The solution of the model equations confirmed that steep reactant concentration gradients are present near the external catalyst surface under typical industrial conditions so that the internal catalyst effectiveness factor is low [27]. 

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].

If the SCR is placed downstream of an ESP or TSS, the design can take advantage of a cleaner flue gas. This would allow for smaller catalyst volumes using finer pitch catalyst and thus smaller SCR reactors. Problems occur when the ESP or TSS collection efficiency no longer removes the particulates from the flue gas. Not only does the SCR catalyst bed foul, requiring increased run frequency on the soot blowers, the stack opacity will also increase. 

Several units with a PM collection device located upstream of the SCR have seen increased pressure drop from fine particulates accumulating on the catalyst bed. Soot blowers have been partially successful in this application. When the SCR is applied to a CO boiler with limited pressure drop, the SCR has typically been located upstream of PM removal to avoid this problem. Some refiners have chosen to install a spare SCR reactor to provide redundancy due to pressure drop concerns. Others have used a bypass where local regulations allow. 

Once the design parameters are identified, sizing of the SCR catalyst follows. Typically, a multilayer SCR reactor is considered with the flue gas directed in a vertical down flow orientation. Some U.S. refiners have installed bypasses around the... 

FIGURE 17.13 FCCU regenerator flow scheme with SCR reactor and bypass line. (With... 

SCR reactor or installed parallel reactors in a wish-bone design. This was required to meet reliability reqnirements for a 5-year cycle and provide redundancy for any pressnre drop concerns. Both designs are shown in Figures 17.12 and 17.13. 

At elevation, an electric motor driven pallet mover picks the module up at the base frame and sets it in its intended place. The weight of an individual SCR catalyst module is approximately 2000 pounds and therefore requires safe handling. Loading the SCR reactor is labor intensive. The use of a motorized lifting tool increases the efficiency of the crew to complete their task as shown in Figure 17.17. Before the use of this machine, loading a layer of 36-40 modules required 2 days. The pallet mover reduced this time in half. 

Figure 17.18 shows catalyst modules installed inside the SCR reactor. The modules are loaded at the same time the sealing gutters are installed. A small space is allotted between adjacent modules to slide in a narrow steel gutter that creates the seal between itself and the modules pedestal frame at the base. Adjacent modules are not touching rake soot blowers. In this configuration, the distance from the rake soot blowers to the top grid of the catalyst modules is approximately 3 feet. 

Figure 17.20 shows the static mixers inside the duct that create eddies in the flue gas. Ammonia is injected at this location and the guide vanes direct the flue gas downstream toward the SCR reactor. 

FIGURE 17.18 Catalyst modules loaded inside SCR reactor. (With permission from Haldor-Topsoe, Inc.)... 

Fignre 17.24 shows the operating conditions of another commercial FCCU SCR. The inlet fine gas flows into the SCR reactor at 637°F and contains 208 ppm of NO and 20 ppm NO2. After passing through the two layers of SCR catalyst, the outlet NO and NO2 measnres 7 ppm and 5 ppm, respectively. 

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

The removal of N20 in low concentration in flue gases will be fully efficient with the development of catalysts active below 570 K. Moreover, the simultaneous removal of NOx and N20 in the same SCR reactor will also constitute a significant step forward. 

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. 

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. 

The use of the Ljungstroem air heater as SCR reactor is another strategy for NO abatement in power plants that is based on unsteady-state periodic operation [56]. Indeed, catalytic reactors in SCR technology require considerable volumes that are not always available when existing boilers have to be retrofitted. Moreover the high costs of traditional SCR installations favor alternative solutions requiring limited modifications of existing plants. 

---