Selective Catalytic Reduction (SCR)
SCR is a process for controlling emissions of nitrogen oxides from stationary sources. The basic principle of SCR is the reduction of NOx to N2 and H2O by the reaction of NOx and ammonia (NH3) within a catalyst bed. The primary reactions occurring in SCR require oxygen, so that catalyst performance is best at oxygen levels above 2-3%.
Several different catalysts are available for use at different exhaust gas temperatures. In use the longest and most common are base metal catalysts, which typically contain titanium and vanadium oxides, and which also may contain molybdenum, tungsten, and other elements. Base metal catalysts are useful between 450 °F and 800 °F. For high temperature operation (675 °F to over 1100 °F), zeolite catalysts may be used. In clean, low temperature (350-550 °F) applications, catalysts containing precious metals such as platinum and palladium are useful. (Note that these compositions refer to the catalytically active phase only; additional ingredients may be present to give thermal and structural stability, to increase surface area, or for other purposes.)
The mechanical operation of an SCR system is quite simple. It consists of a reactor chamber with a catalyst bed, composed of catalyst modules, and an ammonia handling and injection system, with the ammonia injected into the flue gas upstream of the catalyst. (In some cases, a fluidized bed of catalyst pellets is used.) There are no moving parts. Other than spent catalyst, the SCR process produces no waste products.
In principle, SCR can provide reductions in NOx emissions approaching 100%. (Simple thermodynamic calculations indicate that a reduction of well over 99% is possible at 650 °F.) In practice, commercial SCR systems have met control targets of over 90% in many cases.
Selective Non-Catalytic Reduction (SNCR)
SNCR is a chemical process that changes oxides of nitrogen (NOx) into molecular nitrogen (N2). A reducing agent, typically ammonia or urea, is injected into the combustion/process gases. At suitably high temperatures (1,600 - 2,100 °F), the desired chemical reactions occur. Other chemicals can also be added to improve performance, reduce equipment maintenance, and expand the temperature window within which SNCR is effective.
Conceptually, the SNCR process is quite simple. A gaseous or aqueous reagent of a selected nitrogenous compound is injected into, and mixed with, the hot flue gas in the proper temperature range. The reagent then, without a catalyst, reacts with the NOx in the gas stream, converting it to harmless nitrogen gas and water vapor. SNCR is "selective" in that the reagent reacts primarily with NOx, and not with oxygen or other major components of the flue gas.
No solid or liquid wastes are created in the SNCR process.
In almost all commercial SNCR systems, either ammonia or urea is used as the reagent. Other reagents such as cyanuric acid and hydrazine have also been used. Ammonia may be injected in either anhydrous or aqueous form, and urea, as an aqueous solution.
The principal components of an SNCR system are a reagent storage and injection system, which includes tanks, pumps, injectors, and associated controls, and often NOx continuous emissions monitors. Given the simplicity of these components, installation of SNCR is easy relative to the installation of other NOx control technologies. SNCR retrofits typically do not require extended source shutdowns.
While SNCR performance is specific to each unique application, NOx reduction levels ranging from 30% to over 75% have been reported.
Temperature, residence time, reagent injection rate, reagent-flue gas mixing, and uncontrolled NOx level are important in determining the effectiveness of SNCR. In general, if NOx and reagent are in contact at the proper temperature for a long enough time, then SNCR will be successful at reducing the NOx level.
SNCR will remove the most NOx within a specified temperature range or window. At temperatures below the window, reaction rates are extremely low, so that little or no NOx reduction occurs. On the left side of the curve, the extent of NOx removal increases with increasing temperature because reaction rates increase with temperature. Residence time typically limits the NOx reduction in this range. At the plateau, reaction rates are optimal for NOx reduction. A temperature variation in this range will have only a small effect on NOx reduction.
A further increase in temperature beyond the plateau decreases NOx reduction. On the right side of the curve, the oxidation of reagent becomes a significant path and competes with the NOx reduction reactions for the reagent. A further increase in temperature beyond the right side can actually increase the level of NOx. Although the reduction is less than the optimum, operation on the right side is practiced and recommended to minimize reaction times and byproduct emissions.
The temperature window becomes wider as the residence time increases, thus improving the removal characteristics of the process. Long residence times (>0.5 second) at optimum temperatures promote relatively high NOx reduction performance even with less than optimum mixing or temperatures.
Normal stoichiometric ratio (NSR) is the term used to describe the N/NO molar ratio of the reagent injected to uncontrolled NOx concentrations. If one mole of anhydrous ammonia is injected for each mole of NOx in the flue gas, the NSR is one, as one mole of ammonia will react with one mole of NOx. If one mole of urea is injected into the flue gas for each mole of NOx, the NSR is two. This is because one mole of urea will react with two moles of NOx. For both reagents, the higher the NSR, the greater the level of NOx reduction. Increasing NSR beyond a certain point, however, will have a diminishing effect on NOx reduction, with reagent utilization decreasing beyond this point.