Selective catalytic reduction
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Selective catalytic reduction (SCR), a process where a gaseous or liquid reductant (most commonly ammonia or urea) is added to the flue or exhaust gas stream and is absorbed onto a catalyst. The reductant reacts with NOx in the exhaust gas to form H2O (water vapor) and N2 (nitrogen gas).
Specialty catalytic converters are required to make an SCR system work, the current options being a vanadium-based catalyst, or a catalyst with zeolites in the washcoat. The performance of Fe- and Cu-exchanged zeolite urea-SCR is approximately equal to that of vanadium urea-SCR if the fraction of the NO2 is 20% to 50% of the total NOx.[1]
Zeolites offer improved durability at high temperatures[2], including an ability to withstand extended operation above 650 °C and brief exposure to temperatures of 750 to 850 °C. Thermal durability is particularly important for mobile SCR applications that incorporate the use of a diesel particulate filter with forced regeneration. Vanadium catalysts are quickly deactivated at temperatures above 600 °C.[2] The reliance on an increased fraction of NO2 within the total NOx to increase low temperature (sub-350 °C) performance of zeolite catalysts typically requires the use of an upstream oxidation catalyst for NO to NO2, which in turn requires the use of fuels that are virtually sulfur-free to allow low-temperature oxidation of NO to NO2 and to prevent formation of sulfate particulate matter. Zeolite catalysts combined with an upstream oxidation catalyst also have a smaller overall volume than vanadium catalysts.[3]
For specific lower temperature applications and applications that use higher sulfur fuels and where space is a less critical concern (power plants and some large ships burning bunker fuel oils as part of their propulsion systems), vanadium catalysts are generally preferred. Vanadium catalysts are much more common than zeolites for controlling NOx emissions from stationary power plants with the sole exception of power plants that use high-temperature gas turbines. SCR catalysts function well only within a relatively narrow temperature window, and process control systems must be programmed to keep the exhaust gas temperature within that range.
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[edit] Reductants
There is considerable discussion about which reductant is best. While ammonia offers slightly better performance, it is toxic and a difficult substance to handle safely. Urea is safer to handle, but not quite as effective. It has, however, to date been a more popular choice for engine manufacturers. In both cases, the reductant must be extremely pure, because the impurities can clog the catalyst. Typically, SCR catalysts require frequent cleaning even with pure reductants, as the reductant can cake the inlet surface of the catalyst, while the exhaust gas stream temperature is too low for the SCR reaction to take place.
Research into reductant technology is continuing. A wide variety of suggestions have been made for alternative reductants, especially ones that have a wide distribution infrastructure in place. Due to the lack of a distribution infrastructure for both ammonia and urea, the United States Environmental Protection Agency has been reluctant to certify any diesel engines fitted with SCR. In Europe, SCR is a common choice for NOx control technology by engine manufacturers, and a variety of ammonia and urea brands are available, such as AdBlue for example.
[edit] Ammonia slip
A common problem with all SCR systems is ammonia slip. The term describes exhaust pipe emissions of ammonia that occur when:
- exhaust gas temperatures are too cold for the SCR reaction to occur.
- the injection device feeds too much reductant into the exhaust gas stream for the amount of NOx.
A variety of strategies have been developed to deal with ammonia slip, including the use of extra catalysts after the SCR catalyst.
[edit] Technical problems with automotive SCR units
In order to ensure that the SCR unit remains free from contaminants, correct materials of construction must be used for both storage and dispensing. Manufacturers of the SCR unit have specified that, without using compatible materials of construction, ions can be passed from the dispensing materials into the porous head on the SCR unit. This can render the SCR unit ineffective and reduce its life expectancy by more than 60%. Equipment which may prove suitable for urea solution is often not compatible with AdBlue, and the common assumption that it is compatible has led to a number of systems failing prematurely.
To ensure that the AdBlue is not affected by incorrect material specification, operators should refer to the DIN70070 standard for production of AdBlue and CEFIC quality control document AUS32.
The biggest issue with SCR is the necessity to tune the SCR system to the engine operating cycle. This requires running the engine through a simulation of the operating cycle of the machine it will be fitted to. The simulation can be run on a dynamometer, or on an actual piece of equipment during its normal work day (data logging). Even at best, data logging tends to be inaccurate, as no two operators will use the equipment in the same way. Even when used for the same general purposes (i.e., a truck delivering goods to stores in a city), small differences in the route such as hills, one-way streets, amount unloaded, etc., can make the engine loads different enough that effectiveness of the system will suffer.
Research into how to defeat this issue is ongoing.
[edit] Power plants
In power stations, the same basic technology is employed for removal of NOx from the flue gas of boilers used in power generation and industry. The major difference being the size and cost of the installation. Both aqueous and anhydrous ammonia as well as urea are used as a reductant. Normally a base metal catalyst is used, in the form of plates or honeycombed blocks fabricated of titanium oxide with proprietary formulations of vanadium, molybdenum, tungsten and other materials.
The SCR unit is generally located between the furnace economizer and the air heater. As in other SCR applications, the temperature of operation is critical. Ammonia slip is also an issue with SCR technology used in power plants.
Other issues which must be considered in using SCR for NOx control in power plants are the formation of ammonium sulfate salts due to the sulfur content of the fuel as well as the undesirable catalyst-caused formation of SO3 from the SO2 and O2 in the flue gas.
A further operational difficulty in coal-fired boilers is the blinding of the catalyst by fly ash from the fuel combustion. This requires the usage of sootblowers, sonic horns and careful design of the ductwork and catalyst materials to avoid plugging by the fly ash.
[edit] See also
[edit] External links
- Selective Catalytic Reduction
- Selective catalytic reduction for NOx control
- The definitive web site for Diesel Emissions information
