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 fuel gas stream and is absorbed onto a catalyst. The reductant reacts with NOx in the fuel gas to form H2O and N2.
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. In some jurisdictions use of a Vanadium based catalyst are frowned on (California) and the Zeolite type preferred, however Vanadium offers better performance.
SCR catalysts function well only within a narrow temperature window, and for OEM installations the Engine Control Unit is specially programmed to keep the exhaust gas temperature in 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 poisonous, and a difficult substance to handle safely. Urea is safer to handle, however not quite as effective, however to date, it has 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, the most popular of which is AdBlue.
[edit] Ammonia Slip
A common problem with all SCR systems is ammonia slip. The term describes tailpipe emissions of ammonia that occur when
- exhaust gas temperatures are too cold for the SCR Reaction to occur.
- Electronic Controller 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 fitting of extra catalysts after the SCR catalyst.
[edit] Technical Problems
In order to ensure that the SCR remains free from contaminants, correct materials of construction must be used for both storage and dispensing. Manufacturers of the SCR have specified that without using compatible materials of construction ions can be passed from the dispensing materials and into the porous head on the SCR. This can render the SCR ineffective and reduce it’s life expectancy by more than 60%. Equipment which may prove suitable for urea solution is often not compatible with AdBlue, and the common assumption 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 an Engine Dyno, or on an actual piece of equipment during its normal work day (Datalogging). Even at best this tends to be inaccurate, as no two operators will use the equipment in the same way, and 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 SO2 and O2 in the flue gas.
A further operational difficulty in solid fuel coal fired boilers is the blinding of the catalyst by ash from the fuel. This requires the usage of sootblowers, sonic horns and careful design of the ductwork and catalyst materials to avoid plugging by the fly ash.
