Burner optimisation is usually the first method used to control NOx formation. Optimisation is achieved by modifying boiler-operating conditions. Excess air control, boiler fine tuning and balancing the fuel and air flow to the various burners are in use and continue to be investigated to achieve minimum NOx formation in the burner.
In the excess air control procedure, minimising air (oxygen) flow to the fuel dAuring the initial stages of combustion leads to reduced NOx formation. As the oxygen level is reduced, combustion may become incomplete and the amount of unburned carbon in the ash may increase. In addition, the steam temperature may be decreased. Reducing the oxygen in the primary zones to very low amounts (< 1%) can also lead to high levels of carbon monoxide. The result of these changes can be a reduction in the boiler efficiency, slagging, corrosion and a counteractive overall impact on boiler performance. Potential safety problems, which may result from the use of this technique without a strict control system include, fires in air preheaters and ash hoppers as well as increases in opacity and in rates of waterwall wastage.
Fine tuning the boiler settings include mill balancing, adjusting air registers, air and coal flow balancing, tuning firing configuration and improving the plant control system.
Levy and others (1993) found that controlling the varying burner tilt angles to control steam temperature and changing oxygen flow, mill loading and air register settings during different burner loads can also contribute to reducing NOx formation. They reported that reducing excess air in combination with fine tuning the boiler could achieve as much as 39% less NOx formation. However, they also found that fine tuning the boiler could result in increased heat rate. They concluded that several maintenance and operational issues have to be addressed. These include studying the effects of sustained low oxygen operation on waterwall wastage and the possible damage the burner buckets may sustain as a result of close proximity of the flame fronts.
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