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NoX
SNCR |
SNCR is probably the abatement technology currently the closest to implementation. It can reduce NOx emissions by 30 to 50% without significant impact on unit performance.
The technology was initially demonstrated in boilers fired by oil or natural gas, but the use of SNCR in coal fired boilers is presently under way. The technology has been demonstrated in 15 utility scale boilers in the United States and Europe (especially in Germany and Austria).
NOx emissions in the flue gas are converted into elemental nitrogen and water by injecting a nitrogen based chemical reagent, most commonly urea (CH2CONH2) or ammonia (NH3; either anhydrous or aqueous). Because the highest NOx reduction is achieved at temperatures between 870 and 1,200 C (1,600 to 2,200 F), the reagent is introduced at the top and backpass of the boiler. Multiple injection locations may be required, especially in case of cycling units; different injection locations are used as the unit operates at a reduced load.
The following technical issues remain to be addressed: Ability to satisfactorily minimize deposition of ammonium bisulfate on the air heater baskets, which plugs them, especially at boilers burning high sulfur coal (above 22.5%) Ammonia contamination of the ash; ammonia is odorous at concentrations as low as 20 ppm.
The cost of retrofitting a boiler with SNCR is US$10 to 20/kW, whereas incorporating SNCR in a new boiler is US$5 to 10/kW. This difference is caused by the cost associated with modifying the existing boiler to install the reagent injection ports. The operating costs associated with the reagent, auxiliary power, and potential adverse O&M impacts are usually on the order of 1 to 2 mills/kWh. Two to five weeks of outage are required to retrofit a boiler with SNCR.
This technology is considered suitable for developing countries that require NOx reduction above and beyond what is achieved by low NOx burners. The suitability and deployment of the combined SO2/NOx control processes in developing countries should be assessed though after they have been demonstrated and commercialized in industrialized countries (3 to 10 years, depending on the process).
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