Designing SCR systems for high-dust applications

A number of countries in Asia are faced with increasingly stringent NOx emissions regulations, with some of the target levels low enough to call for installation of selective catalytic reduction (SCR) systems, capable of reducing NOx by more than 90%. Many of these Asian countries also burn coal with very high ash content, in excess of 30%, which must be taken into consideration in designing and selecting the appropriate SCR technology. A case in point is India, which had no NOx standards until recently but now has stringent emissions standards to regulate NOx emissions from power plants (see Figure 1). NOx levels of 600 mg/Nm3 to as low as 100 mg/Nm3 will have to be achieved in a fairly short period, while also contending with the high ash content of Indian coals, typically in the range 30-45% ash, with a high percentage of abrasive and erosive solids including silica, aluminium oxide, and iron oxide.

SCR system design basics

The SCR process involves the reduction of NOx to nitrogen and water by the reaction of NOx with ammonia with the aid of a catalyst:

4 NO + 4 NH3 + O2 - 4 N2 + 6 H2O

2 NO2 + 4 NH3 + O2 - 3 N2 + 6 H2O

An ammonia-air or ammonia-steam mixture is injected through an injection grid into exhaust gases containing NOx. The flue gases are thoroughly mixed in a turbulent zone, and then pass through the catalyst where the NOx reduction reactions take place. The process is referred to as "selective" because it takes oxygen from nitrogen compounds only and not from carbon, sulphur, or other oxygenated compounds.

Most coal boiler SCR systems operating in high-ash/high-dust environments require detailed knowledge of how fuel ash can impact SCR performance.

SCR systems are typically located between the economiser and the air preheater, upstream of an electrostatic precipitator (ESP) or baghouse. In this, "high-dust", configuration, the flue gas entering the SCR system can have ash levels of over 18% by weight and, depending on the coal being fired, the flue gas can contain highly abrasive and erosive solid particulate species.

The primary benefit of this configuration is that the flue gas temperature (typically 650°F) is optimum for the catalytic NOx reduction reaction and is also above the temperature required to avoid condensation of ammonium salts onto the catalyst. The formation and presence of ammonium salts reduces the effective catalyst surface area by blocking the catalyst pores. Ammonium salt condensation also can foul heat transfer surfaces in the downstream air preheater.

The major drawback of this configuration is that the SCR system components are exposed to high levels of particulates in the flue gas. The abrasive and erosive nature of the particulates can have detrimental effects on the SCR reactor inlet ductwork, large-particle ash (LPA) screens, ammonia injection nozzles, static mixers, turning vanes, and the catalyst itself. These effects can potentially increase maintenance and catalyst management costs.

An alternative configuration, exposing the SCR system to considerably less dust, is to place it downstream of the ESP or baghouse, resulting in an essentially particulate-free flue gas stream. This "low-dust" configuration eliminates the need for LPA screens and minimises abrasion issues, extending catalyst lifetime and reducing maintenance costs.

However, because the SCR system is further downstream in this configuration, the flue gas temperature at the SCR location is generally below about 450°F, and ammonium salt condensation is likely. To avoid formation of ammonium salts, the flue gas must be reheated, for example with an SCR reheat duct burner. If natural gas is available this could typically employ a number of 5-10 MMBtu/h natural gas fired perimeter burners installed across the SCR inlet duct cross-section. Each burner requires 15-20 SCFM natural gas, affecting overall plant efficiency and operating costs.

Even with the reheating the flue gas temperature at the SCR is still lower than with the high-dust configuration, reducing catalyst activity and necessitating additional catalyst volume a given level of NOx reduction.

SCR system designs for coal-fired power plants have generally favoured the high-dust configuration to avoid the negative effects associated with ammonium salt formation. Globally, over the past 25 years or so, SCR systems have been installed on 350+ GW of coal-fired capacity, with some 120+ in the US alone, and in over 85% of these installations the SCR system is deployed in a high-dust configuration.

Dealing with erosion

In the high-dust configuration, ash levels in the flue gas entering the SCR system can be significant, as already noted, but coal ash itself has relatively low erosivity. Other constituents of the flue gas can, however, be much more erosive.

The Vickers Hardness (HV) scale provides an indication of the erosivity of specific materials. The carbonaceous components of bituminous coal are relatively soft, with a hardness of 294 HV.

Certain flue gas constituents, however, can be 2-6 times more erosive. Silica (SiO2) has a hardness of 700-1500 HV and alumina (Al2O3) has a hardness of 1900 HV. These components can be used to define an "erosivity index", Ei, which can be used to quantify the effects of SiO2, Al2O3 and Fe2O3 ash constituents:

Ei = Es + Ef + Ea

where Es = % ash in coal/100 x 1.00 x %SiO2 in ash, Ef = % ash in coal/100 x 0.80 x %Fe2O3 in ash, and Ea = % ash in coal/100 x 1.35 x % Al2O3 in ash.

The erosivity index can help inform SCR design decisions. A high value, for example, can lead engineers to make changes to catalyst type, pitch, design velocities, and sonic horn spacing. Figure 2 illustrates the erosivity index and ash loading associated with several high-dust SCR projects. By comparison, for northeastern Indian coal a low-end erosivity index of 18 can be calculated using very conservative percentages for ash (32%), SiO2 (38%), Fe2O3 (7%) and Al2O3 (8%).

A typical high-dust SCR configuration is shown in Figure 3. Design considerations for specific surfaces subject to erosion and wear are described below.

Large particle ash screens (upstream of ammonia injection grid)

Depending on the properties of the coal being burned, large particle ash (LPA), also known as popcorn ash, can form in the upper convective heat exchanger surfaces of the boiler. The LPA particles - typically 5-10 mm or more - are conveyed in the high-velocity flue gases to the SCR catalyst, resulting in catalyst erosion and reduced NOx emissions removal.

To prevent catalyst damage, LPA screens can be installed to capture the LPA, with subsequent removal in the economiser hopper. Hardened materials such as abrasion resistant (AR) plate should be used to provide inherent screen strength and minimise erosion effects. Wear-resistant coatings including materials made with chromium oxides, tungsten carbides, etc, also can be applied to the screens to reduce erosion and prolong the screen life. An economic analysis is usually warranted to determine if it makes sense to apply the coatings or simply supply an extra screen that would need to be replaced during a later outage - essentially treating the LPA screen as a consumable with a specified replacement schedule. The frequency of screen rapping with discharge to the economiser hopper should also be analysed given the LPA loading and properties.

Ammonia injection grid

The ammonia injection grid (AIG) downstream of the economiser outlet is susceptible to erosion as well. The AIG nozzles protruding into the flue gas stream present an easy target for erosion if abrasive silica and aluminium species are present.

Several protective measures may be needed to limit degradation, such as the severe AIG lance erosion shown in Figure 4. For example: erosion allowances, designing the lances with an additional thickness (1/8") to account for expected wear; using a more wear-resistant material such as 304 stainless steel; and installing wear shields - sections of pipe around each AIG header pipe (Figure 5) penetrating about one foot into the duct - to counteract the effects of high localised velocities near the duct wall where the AIG penetrates into the flue gas.

Flow distribution devices

Flow distribution devices - including turning vanes, static mixing elements, and distribution plates - are installed in the SCR inlet ductwork to ensure homogeneous ammonia distribution and uniform flue gas velocity ahead of the SCR catalyst bed. These duct additions present another high erosion point within the SCR ductwork. Leading edges of turning vanes, flow mixers, and duct transition sections (constrictions, expansions) should be designed using erosion-resistant wear plate and/or extra thickness (1/16") as deemed necessary.

Reactor sizing and CFD modelling

The design size of the catalyst reactor also must consider the erosivity of the intended application. Computational fluid dynamics modelling can help in properly sizing SCR systems for high-dust configurations and in determining appropriate velocity distributions.

SCRs that will be used in high-dust environments should be designed with a larger cross-sectional area such that the catalyst face velocity is at the lower end of the design range (8-10 ft/s). This ensures sufficient contact between flue gas and catalyst to effect the NOx reduction reactions. For low erosivity applications, a typical design velocity through the catalyst is in the 12-18 ft/s range.

CFD modelling is particularly useful in determining velocity distributions to minimise pressure drop and particulate dropout. As important, CFD modelling is used to identify high velocities, and, therefore, expected high wear locations within the SCR and associated ductwork.

The CFD image in Figure 6 indicates areas of high wear, typically where flue gas velocities exceed 80 ft/s. The model also indicates zones of high velocity near the duct wall. Further, the CFD image shows, as expected, relatively higher velocities at the turning vanes. But in addition it identifies an expected high wear area in the vertical duct, immediately after the turning vane. Based on this, designers might consider adding additional erosion protection in this section of ductwork.

CFD modelling also can help evaluate potential operational issues, such as those that might occur due to excessive gaps between catalyst modules. The CFD images in Figure 7 illustrate the high-velocity zones caused by a 1" x 1" gap between catalyst modules. The SCR inlet ductwork flue gas velocity at full load (90 ft/s) and partial load (61 ft/s) are shown on the left and right images, respectively. Such localised spikes can lead to accelerated catalyst deactivation and increased maintenance costs. As noted above, velocities in the catalyst bed should be in the 8-10 ft/s range for fuels with high erosivity indices. To tighten the gaps between the catalyst modules, a sealing plate can be installed, as shown in Figure 8.

Case study: high-dust SCR in Finland

Applying lessons learned from high-dust SCR installations over the past few decades, Amec Foster Wheeler has optimised its designs to increase NOx reduction while reducing ammonia consumption, flue gas pressure loss and required maintenance.

Since 1982 the company has supplied over 120 SCR systems, including the first on a US coal fired power plant (Carney's Point, 1993) and the first on a 100% petcoke fired boiler (Deepwater, 2006) and worked with a wide range of fuels, including PRB.

A current focus is retrofits in Europe, with a high-dust SCR engineering, procurement and construction project recently completed at the Nantaali 3 unit of Tarun Seudun Energiantuotanto Oy (TSE) in Finland (Figure 9).

This 125 MWe coal and biomass fuelled plant is being upgraded to comply with the European Industrial Emissions Directive. The project scope included the design, supply, erection, and commissioning of a new high-dust SCR system complete with catalyst and steel structures, an ammonia- water storage, unloading, and injection and distribution system, as well as flue gas ductwork and new induced draft fans. The new SCR system will reduce the plant's NOx emissions by 70% while maintaining ammonia slip to under 3 ppm.