Nitrogen oxides are air pollutants produced during coal combustion. The rate of NOx formation depends on combustion temperature and available oxygen and can rise significantly if not properly controlled. Consequently, combustion control techniques can influence and minimize the formation of NOx.
The Rawhide Operating Permit issued by the State of Colorado limits the NOx emission rate, which is controlled by the tangentially fired design of Rawhide's boiler and the use of an enhanced low-NOx combustion control system. The tangential boiler design provides improved fuel-air mixing and combustion staging over other boiler designs, resulting in lower NOx emissions. Rawhide's enhanced combustion control (ECC) system utilizes the latest in boiler burner technology, separated over-fire air registers and a computerized control system that continuously monitors combustion variables such as air damper positions, burner tilts, coal feeder speeds and other process parameters and makes adjustments to keep NOx emissions below the permitted rate. The ECC system provides an additional 50 percent reduction in NOx emission over the emission rates generally achievable from the tangential boiler design.