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Selective catalytic reduction (SCR) of NOx by nitrogen compounds, such as ammonia or urea—commonly referred to as simply “SCR”—has been developed for and well proven in industrial stationary applications. The SCR technology was first applied in thermal power plants in Japan in the late 1970s, followed by widespread application in Europe since the mid-1980s. In the USA, SCR systems were introduced for gas turbines in the 1990s, with increasing potential for NOx control from coal-fired powerplants. In addition to coal-fired cogeneration plants and gas turbines, SCR applications also include plant and refinery heaters and boilers in the chemical processing industry, furnaces, coke ovens, as well as municipal waste plants and incinerators. The list of fuels used in these applications includes industrial gases, natural gas, crude oil, light or heavy oil, and pulverized coal .
The application of SCR for mobile diesel engines requires overcoming several problems, which are discussed later. However, SCR remains the only proven catalyst technology capable of reducing diesel NOx emissions to levels required by a number of future emission standards. Urea-SCR has been selected by a number of manufacturers as the technology of choice for meeting the Euro V (2008) and the JP 2005 NOx limits—both equal to 2 g/kWh—for heavy-duty truck and bus engines. First commercial diesel truck applications were launched in 2004 by Nissan Diesel in Japan  and by DaimlerChrysler in Europe.
SCR systems are also being developed in the USA in the context of the 2010 NOx limit of 0.2 g/bhp-hr for heavy-duty engines, as well as the Tier 2 NOx standards for light-duty vehicles. However, the US clean air authorities have voiced concerns about the SCR technology. From the regulatory perspective SCR poses enforcement problems, both in terms of ensuring that the reductant (urea) is available together with diesel fuel throughout the nationwide distribution network, and that it is always timely replenished by vehicle operators.
This paper covers the fundamentals of SCR—reductants, chemical reactions, and catalysts—as well as stationary SCR systems. Development and experience with SCR systems for mobile diesel engines is discussed in the “SCR Systems for Mobile Engines” paper.
Two forms of ammonia may be used in SCR systems: (1) pure anhydrous ammonia, and (2) aqueous ammonia. Anhydrous ammonia is toxic, hazardous, and requires thick-shell, pressurized storage tanks and piping due to its high vapor pressure. Aqueous ammonia, NH3·H2O, is less hazardous and easier to handle. A typical industrial grade ammonia, containing about 27% ammonia and 73% water by weight, has nearly atmospheric vapor pressure at normal temperatures and can be safely transported on highways in the USA and other countries.
A number of chemical reactions occur in the ammonia SCR system, as expressed by Equations (1) to (5). All of these processes represent desirable reactions which reduce NOx to elemental nitrogen. Equation (2) represents the dominant reaction mechanism . Reactions given by Equation (3) through (5) involve nitrogen dioxide reactant. The reaction path described by Equation (5) is very fast. This reaction is responsible for the promotion of low temperature SCR by NO2 . Normally, NO2 concentrations in most flue gases, including diesel exhaust, are low. In some diesel SCR systems, NO2 levels are purposely increased to enhance NOx conversion at low temperatures.
(1)6NO + 4NH3 → 5N2 + 6H2O
(2)4NO + 4NH3 + O2 → 4N2 + 6H2O
(3)6NO2 + 8NH3 → 7N2 + 12H2O
(4)2NO2 + 4NH3 + O2 → 3N2 + 6H2O
(5)NO + NO2 + 2NH3 → 2N2 + 3H2O
It has been found that the above reactions are inhibited by water . Moisture is always present in diesel exhaust and other flue gases. To obtain valid results, water vapor should be always present in laboratory gas tests of SCR processes and in process modeling.
In case the NO2 content has been increased to exceed the NO level in the feed gas, N2O formation pathways are also possible, as shown in Equation (6) and (7) .
(6)8 NO2 + 6 NH3 → 7 N2O + 9 H2O
(7)4 NO2 + 4 NH3 + O2 → 4 N2O + 6 H2O
Undesirable processes occurring in SCR systems include several competitive, nonselective reactions with oxygen, which is abundant in the system. These reactions can either produce secondary emissions or, at best, unproductively consume ammonia. Partial oxidation of ammonia, given by Equations (8) and (9), may produce nitrous oxide (N2O) or elemental nitrogen, respectively. Complete oxidation of ammonia, expressed by Equation (10), generates nitric oxide (NO).
(8)2NH3 + 2O2 → N2O + 3H2O
(9)4NH3 + 3O2 → 2N2 + 6H2O
(10)4NH3 + 5O2 → 4NO + 6H2O
Ammonia can also react with NO2 producing explosive ammonium nitrate (NH4NO3), Equation (11). This reaction, due to its negative temperature coefficient, occurs at low temperatures, below about 100-200°C. Ammonium nitrate may deposit in solid or liquid form in the pores of the catalyst, leading to its temporary deactivation .
(11)2NH3 + 2NO2 + H2O → NH4NO3 + NH4NO2
Ammonium nitrate formation can be avoided by making sure that the temperature never falls below 200°C. The tendency of NH4NO3 formation can also be minimized by supplying into the gas stream less than the precise amount of NH3 necessary for the stoichiometric reaction with NOx (1 to 1 mole ratio).
When the flue gas contains sulfur, as is the case with diesel exhaust, SO2 can be oxidized to SO3 with the following formation of H2SO4 upon reaction with H2O. These reactions are the same as those occurring in the diesel oxidation catalyst. In another reaction, NH3 combines with SO3 to form (NH4)2SO4 and NH4HSO4, Equation (12) and (13), which deposit on and foul the catalyst, as well as piping and equipment. At low exhaust temperatures, generally below 250°C, the fouling by ammonium sulfate may lead to a deactivation of the SCR catalyst .
(12)NH3 + SO3 + H2O → NH4HSO4
(13)2NH3 + SO3 + H2O → (NH4)2SO4
The SCR process requires precise control of the ammonia injection rate. An insufficient injection may result in unacceptably low NOx conversions. An injection rate which is too high results in release of undesirable ammonia to the atmosphere. These ammonia emissions from SCR systems are known as ammonia slip. The ammonia slip increases at higher NH3/NOx ratios. According to the dominant SCR reaction, Equation (2), the stoichiometric NH3/NOx ratio in the SCR system is about 1. Ratios higher than 1 significantly increase the ammonia slip. In practice, ratios between 0.9 and 1 are used, which minimize the ammonia slip while still providing satisfactory NOx conversions. Figure 1 presents an example relationship between the NH3/NOx ratio, NOx conversion, temperature, and ammonia slip . The ammonia slip decreases with increasing temperature, while the NOx conversion in an SCR catalyst may either increase or decrease with temperature, depending on the particular temperature range and catalyst system, as will be discussed later.
In stationary applications, the maximum permitted NH3 slip is usually specified, with a typical specification at 5-10 vppm NH3. These concentrations of ammonia are generally undetectable by the human nose. Optionally, ammonia slip can be also controlled by a guard catalyst (oxidation catalyst) installed downstream of the SCR catalyst.