Stationary SCR Systems

W. Addy Majewski

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Abstract: Selective catalytic reduction (SCR) has been used for decades to control NOx emissions. Extruded homogeneous vanadia-based catalysts in conjunction with ammonia reductant are commonly used in stationary SCR installations. In the SCR reactor, monolithic catalyst elements are assembled into steel-cased square catalyst modules, which are then placed inside the reactor in the form of catalyst layers.

Process Configuration

Selective catalytic reduction (SCR) technology, first developed to reduce NOx from thermal power plants, was commercialized in the 1970s and 1980s. With time, the use of SCR has expanded to other industrial applications, including utiliy boilers, process heaters, gas turbines, and chemical plants. SCR systems for large stationary engines and for marine engines bear certain similarities to the industrial systems—for instance, these applications often share a similar SCR reactor architecture with extruded homogeneous vanadia-based catalysts. Stationary SCR systems were also the starting point for the development of SCR systems for land-based mobile diesel engines.

Three basic SCR system configurations have been used in power plant applications, depending on the location of the SCR unit relative to the dust and SO2 removal units [1155]:

  1. High-dust arrangement (HD): The SCR unit is positioned immediately after the boiler. This configuration is used mostly in coal-fired applications, because the gas flue temperature at this location (300 - 400°C) is optimal for the catalyst activity. Dust removal is typically accomplished in cold electrostatic precipitators downstream of the SCR unit. Low ammonia slip and low SO2 oxidation are required to minimize fouling by ammonium sulfate.
  2. Low-dust arrangement (LD): The SCR unit, positioned downstream of a hot electrostatic precipitator, operates in dust-free gas, resulting in reduced maintenance and less catalyst deterioration. The LD arrangement is limited to applications that are compatible with hot precipitators.
  3. Tail-end arrangement (TE): This is a clean application, where the SCR catalyst is positioned at the end of the process, after most of the dust and the SO2 have been removed. Since there is less SO2 oxidation concern, a more active (more vanadia) and smaller catalyst can be used. However, the flue gas needs to be preheated in a gas-to-gas heat exchanger, the cost of which might outweigh the savings from the smaller catalyst size and from reduced maintenance.

Among these three configurations, the greatest operating experience exists in the HD plants. Typical design specifications for HD SCR systems require a NOx removal efficiency of 80-85%, NH3/NO feed ratios of 0.8-0.85, and NH3 slip below 3-5 ppm [1155].