Selective Catalytic Reduction

W. Addy Majewski

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Abstract: In the selective catalytic reduction (SCR) process, NOx reacts with ammonia to produce nitrogen and water, with urea being commonly used as the ammonia precursor. Different SCR catalysts such as vanadium oxide or metal substituted zeolites have different operating temperature windows and other properties, and must be carefully selected for a particular SCR process. As the NOx conversion rate depends on the NO2:NO ratio, an oxidation catalyst is typically used to increase the NO2 concentration at the SCR inlet. Many SCR systems also include an ammonia slip catalyst to control the emissions of unreacted ammonia.


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. It 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, followed by a growing number of installations for NOx control from coal-fired powerplants. Further SCR applications 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 [203].

The use of SCR systems for NOx control from mobile applications started with marine engines. The large size and steady-state operation patterns of marine units made the adaptation of stationary SCR technology relatively straightforward. The first SCR units were installed in 1989 and 1990 on two Korean 30,000 metric ton carriers [202]. The ships, powered by MAN B&W 2-stroke 8 MW diesel engines, were equipped with ammonia SCR system designed for a 92% NOx reduction. Exhaust gases were passed through the reactor only when the ships were sailing in waters subject to NOx emission regulations. In 1992, in another early marine SCR project, the ferry “Aurora of Helsingborg” that shuttled between Sweden and Denmark was equipped with a urea SCR system [201]. The ferry was powered by a 2.4 MW Wärtsilä type 6R32E engine, and the SCR reactor included three layers of monolithic extruded SCR catalysts and one layer of an oxidation catalyst. The SCR technology has also been considered for locomotive diesel engines [207].

Since the mid-1990s, numerous development projects have been conducted to adapt the SCR technology for diesel truck and car engines. Several early SCR systems for heavy-duty truck engines were developed and tested by the Dutch TNO [200][199][621], while Johnson Matthey was developing their Compact SCR-Trap system—a device comprising a particulate filter (CRT) upstream of an SCR catalyst [981]. Not surprisingly, mobile systems were also being developed by companies with traditional expertise in stationary installations, such as Haldor Topsøe [623] or Argillon (formerly Siemens, now Johnson Matthey) with its automotive SCR system termed SINOx [334][331]. In some of the early tests, the SINOx system was coupled with a particulate filter upstream of the SCR catalyst [980][1172]. Ford developed a light-duty urea SCR system targeting the US EPA Tier 2 Bin 5 emission limits [206][983].

The mobile engine application required overcoming several problems related to the urea dosing technology under transient operating conditions, catalysts optimization, as well as urea infrastructure. Some regulatory authorities—notably the US EPA—were initially skeptical about the SCR compliance path with emission standards, 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. Ultimately, SCR proved to be a more robust emission technology than the main alternative option, NOx adsorbers, and has been widely used in all types of mobile diesel engines.

Since around the mid-2000s, urea-SCR technology has been gradually commercialized for land-based mobile diesel engines. The major steps in this process were:

The following sections cover the fundamentals of SCR—reductants, chemical reactions, and catalysts. A brief overview of SCR installations for NOx control from industrial processes is provided under Stationary SCR Systems. Development and experience with SCR systems for mobile diesel engines is discussed under SCR Systems for Diesel Engines.

Reductants and Catalytic Reactions


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. Typical industrial grade ammonia, containing about 27% ammonia and 73% water by weight, has a 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 that reduce NOx to elemental nitrogen. Equation (2) represents the dominant reaction mechanism [306]. Reactions given by Equation (3) through (5) involve nitrogen dioxide as a reactant. The reaction path described by Equation (5) is very fast. This reaction is responsible for the promotion of low temperature SCR by NO2 [972]. Normally, NO2 concentrations in most flue gases, including diesel exhaust, are low. In diesel SCR systems, NO2 levels are often purposely increased to enhance NOx conversion at low temperatures.

6NO + 4NH3 → 5N2 + 6H2O(1)

4NO + 4NH3 + O2 → 4N2 + 6H2O“standard” SCR reaction(2)

6NO2 + 8NH3 → 7N2 + 12H2O(3)

2NO2 + 4NH3 + O2 → 3N2 + 6H2O(4)

NO + NO2 + 2NH3 → 2N2 + 3H2O“fast” SCR reaction(5)

The above reactions are inhibited by water [974]. 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.

If the NO2 content is increased to exceed the NO concentration in the feed gas, N2O formation pathways are also possible, Equations (6) and (7) [1170].

8 NO2 + 6 NH3 → 7 N2O + 9 H2O(6)

4 NO2 + 4 NH3 + O2 → 4 N2O + 6 H2O(7)

Undesirable processes occurring in SCR systems include 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).

2NH3 + 2O2 → N2O + 3H2O(8)

4NH3 + 3O2 → 2N2 + 6H2O(9)

4NH3 + 5O2 → 4NO + 6H2O(10)

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 [973].

2NH3 + 2NO2 + H2O → NH4NO3 + NH4NO2(11)

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, fouling by ammonium sulfate may lead to deactivation of the SCR catalyst [205].

NH3 + SO3 + H2O → NH4HSO4(12)

2NH3 + SO3 + H2O → (NH4)2SO4(13)

The SCR process requires precise control of the ammonia injection rate. An insufficient injection rate results in unacceptably low NOx conversion. An injection rate that is too high results in the undesirable release of ammonia to the atmosphere. These ammonia emissions from SCR systems, known as ammonia slip, increase with increasing NH3/NOx ratio (abbreviated ANR and also referred to as alpha ratio). 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 ammonia slip. Figure 1 presents an example relationship between the NH3/NOx ratio, NOx conversion, temperature, and ammonia slip [187]. Ammonia slip decreases with increasing temperature, while NOx conversion over an SCR catalyst may either increase or decrease with temperature, depending on the particular temperature range and catalyst system, as will be discussed later.

Figure 1. NOx conversion and ammonia slip for different NH3/NOx ratios

V2O5/TiO2 SCR catalyst, 200 cpsi

Alpha ratios between 0.9 and 1 can be used to minimize ammonia slip while still providing satisfactory NOx conversions. However, in applications with very demanding NOx performance targets, the SCR system must be operated with alpha ratios of ≥ 1. In such cases, ammonia slip can be controlled using a guard catalyst (ammonia oxidation catalyst) positioned downstream of the SCR catalyst.

In stationary applications, the maximum permitted NH3 slip is usually specified, with a typical specification at 5-10 vppm NH3. A 10 ppm NH3 limit is also applicable in some mobile SCR applications. These concentrations of ammonia are generally undetectable by the human nose.

SCR reactions and NOx conversion are also affected by ammonia storage in the catalyst washcoat. SCR catalysts can store substantial amounts of NH3, Figure 2, particularly at lower temperatures [5531]. This stored ammonia is another source of reductant, in addition to gas phase ammonia, which is particularly important in transient engine operation.

Figure 2. Ammonia storage on a Cu-zeolite catalyst

Aged Cu-CHA catalyst. SV = 30,000 1/h. Gas composition: 10% O2, 5% H2O, 8% CO2, 200 ppm NO, 200 ppm NH3.