Challenges for Emissions Catalyst Operation

Catalyst-based systems for control of CO, NOx, and volatile organic compounds (VOC) are strongly affected by the conditions of the turbine exhaust gas. Oxidation catalysts are passive catalysts used to oxidize CO and VOC to CO2. Selective catalytic reduction (SCR) systems are active catalyst systems that require injection of a reagent containing ammonia (NH3) to reduce NOx to N2 and H2O.

Prior to 2001, turbine catalyst systems were designed primarily for steady-state, baseload operation. These systems had generous startup/shutdown windows, normally of 1 to 3 hours, and typically operated above 80% of maximum load. Between 2001 and 2005, the startup/shutdown windows were reduced to 30 to 60 minutes. Today, these systems can be required to start up or shut down in 10 minutes, while at the same time dealing with more frequent cycling, faster ramp rates, and wider load swings than ever before.

As a unit cycles, the temperature, flow rate, and concentration of emissions in the turbine exhaust entering the SCR/CO catalyst system vary. Exhaust temperatures will often depend on the pressure and steam production in the HP steam system and can vary by +/–100F entering the catalyst system. Also, because the lower-load operation introduces lower flow into the same fixed cross-section, the flow distribution differs from baseload, imparting gas velocity, ammonia concentration, and temperature variations at the catalyst inlet.

Turbine exhaust NOx and CO also spike during load changes, which can affect the stack emissions and require greater emissions reduction to maintain compliance during short averaging periods. Maintaining peak SCR system performance and low levels of both stack NOx and ammonia slip through these cycles requires precise control of ammonia injection. In some cases this may require updates to the ammonia flow control logic or even upgrades to the equipment and additional monitoring hardware.

An example of SCR operation with turbine cycling is shown in Figure 5. The variations from cycling operation are most apparent in the ammonia slip (the unreacted ammonia), shown in orange. Variations of 3 to 4 ppm are common. These variations in ammonia slip are most often caused by a lag in response time of the ammonia vaporization and injection system in reacting to changing ammonia demands. Some progress is being made to improve response times, but more improvement is needed.

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In the permitting phase for modern cycling unit sites, permit applications must take into account these variations, but they currently do not. Regulations at the state and federal levels have reduced best available control technology (BACT) guidelines for turbines to between 2 and 5 ppm NOx and 2 to 5 ppm ammonia slip, both with averaging times as little as 30 minutes. In ozone nonattainment areas, air permit limits of 2 ppm NOx and 2 ppm ammonia slip are normal. These BACT limits required by regulations are not practical in cases with cycling operation, as illustrated in Figure 5.

Fast-start combined cycle systems are capable of startup in 10 minutes. Conventional SCR/CO system designs have demonstrated startup in approximately 30 minutes. Though the difference of 20 minutes may seem small, much happens in those 20 minutes, including heating of the SCR catalyst and ammonia vaporization system that are required for optimum performance. Figures 6 and 7 compare a conventional SCR system start (with a 1-hour startup window) to a modern fast-start system (with a 10-minute start requirement).

6. Conventional SCR system startup. Courtesy: Environex Inc. 7. Modern fast-start SCR system startup. Courtesy: Environex Inc. Even with a warm start, the fast-start system struggles to achieve compliance within 15 minutes. In many cases, particularly for cold starts, SCR systems cannot meet, or have difficulty meeting, the 10-minute startup requirement. To bridge this gap between conventional and fast-start requirements, SCR system designs are being modified to remove thermal mass, improve response times of the ammonia injection and continuous emission monitoring systems (CEMS), and use modified catalyst products/designs that can reduce start times. Even with the modifications, 10-minute starts are often not achieved in practice from the SCR/CO system perspective, as shown in Figure 7. Some of these modifications include: ■ Ammonia vaporizers with preheat capability. ■ Smaller ammonia vaporization chambers without packing and with more heat transfer surface to minimize heating time. ■ Catalyst designs that offer improved formulations with wider operating temperature windows. ■ Catalyst frame/seal designs that allow for more rapid temperature ramp rates. ■ CEMS systems and NOx analyzers with faster response capability, as little as 5 seconds compared to 30 to 120 seconds for extractive sampling systems. ■ Improved controls systems that are more capable of analyzing/predicting transients in catalyst system performance. ■ In some cases, where practical, a return to anhydrous ammonia systems to eliminate the need to vaporize the water (up to 80% water by weight) in aqueous ammonia solutions. Modern emissions control systems are required to convert a greater percentage of emissions in engine exhaust, meet lower stack emissions limits, and do so over shorter averaging periods. All of these changes require greater engineering and higher cost for both new and retrofit systems. Operators of modern turbines need to provide additional oversight and maintenance of their emission control systems to ensure they continuously provide reliable performance. Integrated Fast-Start Management Required Higher ammonia slip and potentially greater SO2 conversion in cycling and fast-start units create additional challenges for control of sulfur-bearing deposits in the colder HRSG areas. Low-pressure evaporators and economizers are particularly at risk. Both current cycling and high-cycling fast-start units can be at risk. Units in reserve or standby for rapid start can experience high corrosion levels in fin tubes (Figure 8), as the deposited material tends to be hygroscopic and corrosive in humid environments. More frequent cleaning of gas-side tube surfaces may be required to prevent excessive backpressure and corrosion on HRSG systems. 8. Bad environment. Units in reserve or standby for rapid start can experience high corrosion levels in fin tubes, as shown here, because the deposited material tends to be hygroscopic and corrosive in humid environments. Courtesy: Tetra Engineering Group Inc. Because the response of emissions catalysts to fast startups is a key element in plant performance and longevity, the HRSG, steam cycle, and emissions control systems in cycling and fast-start CCGT plants must be considered as a single system with multiple operational limits and goals. ■ —David S. Moelling, PE (david.moelling@tetra-eng.com) is chief engineer for Tetra Engineering Group Inc. and Daniel W. Ott (daniel.ott@environex.com) is president of Environex Inc.