SCR Development for Enhanced Performance
By Paulo Oliveira and Larry Czarnecki
The phrase “high efficiency” has been used to describe emerging SCR systems for NOx emission control. This term can be interpreted in a number of ways, with an obvious one being high removal efficiency, typically greater than 90 percent.
High removal efficiencies may be necessary for applications with high inlet NOx levels, such as boiler NOx levels greater than 0.50 lb/MMBTU. A similar case can be made when low NOx emissions, less than 0.05 lb/MMBTU, may be required. Another condition occurs with a requirement for a narrow NOx concentration distribution exiting the SCR system.
Another parameter that drives SCR system performance is ammonia slip, the emission of unreacted ammonia from the SCR reactor. Since the use of ammonia is strictly controlled in a SCR system, the removal efficiency for the ammonia reactant is much greater than the NOx efficiency. For a typical SCR system, the ammonia removal efficiency will be more than 99 percent while the NOx removal efficiency is 90%. To achieve ammonia slip limits lower than a typical level of 2 [email protected]%O2, greater SCR system performance capabilities are required.
There may also be cases when the SCR system may be required to perform under a wide range of operating conditions due to variable boiler loads or different types of coal.
SCR SYSTEM PERFORMANCE FACTORS
There are two major components that determine the overall performance of an SCR system, the catalyst and the distribution of reactant gases. The designs of these two components are interrelated, and they are both subject to the flue gas qualities and system arrangements.
The catalyst component parameters are the amount of catalyst necessary to obtain emission control objectives along with the catalyst pitch suitable for the particulate content in the flue gas. The catalyst activity component is determined by its chemical composition, which would be selected to meet both the NOx emission and SO2 oxidation requirements.
The reactant gas distribution entering the catalyst bed is affected by several components. The reactor inlet contains flow distribution devices to direct and evenly distribute the flue gas velocity entering the catalyst. Upstream of the reactor, diluted ammonia vapor is injected into the flue gas stream through an injection grid which is an array of piping lances and nozzles. The design objective of the injection grid is to deliver an ammonia concentration profile uniformly proportional to the NOx concentration profile of the flue gas stream. Mixing devices are often included with the design of the ammonia injection grid (AIG) to assist with achieving this objective.
Those two key SCR system components are inter-related. The NOx removal performance of the SCR system is partially determined by the quality of the ammonia to NOx ratio distribution entering the reactor. The example illustrated in Table 1 shows the potential benefits of improving the ammonia distribution from a 5 percent coefficient of variation (CV) to a 2 percent coefficient of variation. For a given SCR system and catalyst, gains may be realized in higher NOx removal efficiency-lower NOx emissions, lower ammonia slip levels, or longer catalyst lifetimes.
HIGH EFFICIENCY scr system DESIGN
Designs for AIGs supply dilute ammonia-air mixtures through an array of piping equipped with multiple nozzles. Reagent supply headers and flow adjustment valves may also compliment the AIG design such that the overall grid can feature several flow zones. Designs for traditional and High Efficiency AIGs are shown in Figure 1. Traditional grids, with numerous nozzles and valves, can offer tuning capabilities but may be considered difficult to operate and maintain. High Efficiency grids utilize less piping and nozzles than a conventional design. When this simpler AIG is coupled with a gas mixer, they offer excellent ammonia distribution as well as robust performance.
A Comparison: 1
|High efficiency ammonia injection grid (right) compared to the traditional design (left).|
To obtain a proper ammonia distribution, the High Efficiency system also features a number of in-duct mixers collectively known as IsoSwirl mixers. The IsoSwirl mixers, located downstream of the ammonia injection nozzles, take the numerous ammonia flows and intensely mix them with the overall flue gas stream to produce an extremely well-mixed gas with very good distributions. The IsoSwirl mixers consist of two principle components, as shown in Figure 2. The first set of mixers is located adjacent to the ammonia injection grid where they assist in distributing the ammonia gas across exhaust gas path. The second set of mixers is located further downstream initial AIG and mixer assembly and these mixers provide overall gas mixing across the entire duct gas path. This global mixing further distributes NOx and ammonia:NOx concentration profiles.
The effectiveness of the IsoSwirl mixers make this ammonia injection system design well suited for high performance applications and may be the only option viable when very good ammonia to NOx ratio distributions are necessary. The quality of the mixing also provides a very robust system for meeting applications with challenging NOx distributions or ones with varying operating environments. The robust nature also means that the high efficiency design with IsoSwirl™ mixers does not require any tuning. The simple piping-nozzle configuration of the high efficiency design offers less maintenance concerns than a traditional design.
Today Alstom is supplying (in different stages of either design or construction) high efficiency SCR systems for two customers, a 210 MW coal-fired utility in Croatia and four systems for four combined heat and power (CHP) petcoke boilers for a Russian customer. Furthermore, a high efficiency AIG and the IsoSwirl mixing technology can be easily retrofitted to existing Alstom SCR designs or other-OEM equipment. Recent investigations with American utility customers have been already deployed and retrofit assessments are taking place.
FLOW MODEL TESTING
Two types of fluid flow models have been used to study the performance of High Efficiency SCR system studies; physical scale flow and Computational Fluid Dynamics (CFD) models. The objectives of the flow models used in the study include:
- Demonstrating that acceptable flue gas temperatures, velocities, and both NOx and ammonia gas distributions meet acceptable criteria, and
- Defining and locating flow distribution and mixing devices.
Flow models for were created at Alstom Power’s laboratories located in Växjö, Sweden. The flow models were based on a typical 300MW coal-fired utility boiler with a high dust, vertical down flow SCR system positioned between the economizer outlet and the air preheater gas inlet. The exhaust gas duct located upstream of the SCR reactor consisted of a short, horizontal run, immediately after the economizer outlet, followed by a longer, vertical riser run leading to the SCR reactor inlet.
Two different ammonia distribution systems were evaluated in this study. The baseline system was a traditional ammonia injection grid followed by a typical in-duct static mixer. The high efficiency, IsoSwirl™, system was also studied using replica models of the ammonia injection grid and associated mixers. The study examined the performance of both grid-mixer systems in two different arrangements. The first arrangement located the ammonia injection grid close to the economizer outlet while the mixers were located in the vertical riser duct. The second arrangement located both the ammonia injection grids and mixers in the vertical duct, closer to the SCR reactor inlet. These positions are indicated in Figure 3.
Flow MODEL TESTING RESULTS
Results from the physical model gas distribution studies showed that the performance of the IsoSwirl system provided a superior distribution to the traditional system, as shown in Table 2. In addition to the excellent distribution, the performance of the IsoSwirl assembly indicates that it is consistent and independent of location within the SCR system ductwork. The performance of the traditional system was better when the location was placed close to the economizer outlet to allow for additional mixing.
With the AIG located in the reactor inlet, overall system pressure measurements were between the economizer outlet and the SCR reactor inlet and the results are summarized in Table 3.
As expected, better gas distribution coincides with greater draft losses. A good ammonia injection and gas mixer design will balance these issues and strive to maximize distribution while minimizing the corresponding draft losses. The IsoSwirl system delivers substantial gas distribution benefits while the incremental penalty, 1.0 in.w.c. for this study, is well within the typical allowances for many SCR applications.
The CFD model evaluated the effect of simulated NOx profile distributions exiting the economizer outlet on both NOx and ammonia:NOx ratio concentration profiles at the catalyst face. Two levels of economizer outlet NOx distributions were considered with CV values of either 7 percent or 14 percent. The IsoSwirl mixer assembly was studied at arrangements either close to the economizer outlet or close to the reactor inlet, as was done during the physical flow model. An example of the CFD results, based on a 14 percent NOx CV and reactor inlet arrangement is shown in Figure 3 above on page 116.
The CFD model study results indicated that the IsoSwirl mixer improved the NOx distribution by a factor of 10 regardless of the arrangement position of the IsoSwirl assembly. It is worth noting that these two results were obtained without making any tuning adjustments to the ammonia injection grid. With some turning of the ammonia injection grid, the IsoSwirl assembly was able obtain ammonia:NOx CV values <1.2 percent, even with an economizer outlet NOx profile CV of 14 percent. The data from the CFD study is summarized in Table 4.
The results from both the physical and CFD flow models confirmed substantial improvements to NOx distribution values from the economizer outlet to the catalyst face. The measured ammonia:NOx ratio CV values were ≤5% percent for all cases studied, indicating that the ammonia distribution quality was better than the standard 5 percent requirement for many SCR applications. These levels were obtained with an untuned injection grid, indicating that the IsoSwirl™ mixers offer a robust and tuning-free approach to ammonia injection for SCR systems. Low ammonia:NOx ratio CV values ≤2 percent were obtained with a minimally tuned injection grid. This indicates that an SCR system equipped with IsoSwirl™ mixers is capable of obtaining high efficiency performance levels described earlier.
OPTIMZED SCR: A COST- EFECTIVE WAY TO COMPLY
With High Efficiency AIG and IsoSwirl mixer components provides one avenue towards developing an advanced SCR system. Several additional concepts have been combined with the IsoSwirl mixers in with an objective of developing a more cost effective SCR system. These concepts are described below.
Compact Casing Design
Reduction of the height between catalyst layers from a typical space of twelve feet to a shorter distance between catalyst layers, dependent on the catalyst module height and catalyst cleaning requirements. In many examples the distance was reduced to less than nine feet, resulting in a significant reduction in casing height. This offers reductions in reactor materials and reduces material costs. It also has the added synergistic benefit of reducing loadings for support structures, thus reducing material costs further.
Common Wall Arrangement
This feature merges the SCR casing and the SCR inlet duct into a single monolithic structure that shares a common wall. This feature significantly contributes to the compact features of Optimized SCR, reducing material costs and reducing the overall footprint of the integrated unit. The reduced footprint can often be a critically important feature in many applications, such as retrofit or brownfield applications where space may be very limited.
Streamlined Catalyst Loading Facilities
Often, SCR reactor designs have employed a single catalyst loading/unloading door at each catalyst level. These small, single access entryways to the catalyst layers have proven to be “choke points” for replacing catalyst and have contributed to long of plant outage times. A design for a more streamlined catalyst change out method relies on improved access. Larger doors which expose multiple catalyst rows go a long way toward eliminating this bottleneck.
Spare Layer Utilization
SCR reactor design convention often features one or two spare layers. Initially empty, these layers are filled with catalyst at a later date to with an objective aimed at reducing long-term catalyst costs in the future. The Optimized SCR system locates catalyst in the spare‐reserve layer for immediate benefit rather than a three to five year waiting period. This strategy can be used to design a reactor vessel with a smaller footprint with associated lower capital costs. Catalyst management plans can be crafted around planned boiler system maintenance intervals rather than an arbitrary catalyst replacement schedule.
Smaller Reactor Footprint
A smaller reactor footprint offers relatively higher SCR reactor gas velocities which are suitable for sub‐bituminous- and lignite coal‐fired applications. For these fuels, the risk of catalyst plugging and subsequent shutdowns for cleaning outweigh the risks of accelerated catalyst deactivation from erosion. These smaller reactor footprints also minimize reactor material costs. Higher draft loss penalties associated with higher velocities can be offset with the use of larger pitch catalyst. Higher pitch catalyst are also less prone to ash deposition and plugging.
The smaller reactor also translates to a relatively lower catalyst module count per reactor layer. Reactor loading and catalyst handling time is directly proportional to catalyst module count. Since catalyst replacement is performed on a per layer basis, the catalyst maintenance downtime duration is quicker than a comparable conventionally designed SCR reactor.
These features are collectively illustrated in Figure 4 on page 120.
A cost assessment was made comparing the Optimized SCR system against a similar standard SCR system, based on typical design principles. The Optimized SCR offers lower equipment costs through reductions in AIG and catalyst from the IsoSwirl mixers. Other cost benefits are seen from the smaller reactor and the associated equipment, such as a catalyst cleaning equipment.
Substantial material cost savings are noted for the shorter and smaller reactor vessel, and the associated inlet and outlet ducts. This cost saving also includes the materials savings from the Common Wall feature.
Along with the substantial material savings benefits, there is a considerable weight advantage for the Optimized SCR system over the standard design. This would provide an attractive benefit for support structures which are not included in the cost comparison.
These cost benefit comparison is summarized below in Table 5.
As performance demands increase for SCR systems, the design of key components becomes more critical. These demands transcend beyond only NOx removal and must be considered for ammonia slip and emission range profiles. The two chief components affected by these designs are the catalyst and its reactor housing, and the ammonia injection system. Since the catalyst is a static design component and its performance is driven by the quality of the exhaust gas being treated, achieving an adequate ammonia distribution prior to the catalyst is also becoming more demanding.
High performance ammonia systems using efficient IsoSwirl mixers offer a means to obtain excellent ammonia distributions while avoiding system tuning requirements to produce a robust SCR system. It can also be a very effective answer to the more stringent environmental legislations worldwide, making IsoSwirl™ an item of choice for challenging SCR performances.
Latest SCR designs include optimized catalyst loading facilities and reduced reactor size, which combined with proper catalyst management practices prove to be an effective way to save in costs for both CAPEX and OPEX for the end user.
Paulo Oliveira is a former global product marketing manager and Larry Czarnecki is principal process engineer for Alstom Power, Inc.