ABSTRACT
Combustion control techniques are the most cost-effective for control of nitrogen oxide (NOx) emissions from combustion systems. As acceptable levels of NOx emissions are lowered, even these techniques may not be sufficient to meet environmental regulations. Two types of postcombustion NOx control are available: selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR). In SNCR, a reagent, usually ammonia or urea, is injected into the hot thermal oxidizer combustion products. Unlike urea and cyanuric acid, ammonia can be injected either as an anhydrous vapor or an aqueous liquid. With anhydrous injection, pure ammonia liquid is stored under pressure. An electric heater is used to vaporize the liquid. The chemistry of NOx reduction is straightforward. The real technology comes from mixing the reagent with the combustion gas stream. If a reagent molecule does not come in contact with a nitric oxide (NO) molecule, no reaction can take place.
