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NOx in Pulverised Coal Combustion

AUTHOR: Stuart C Mitchell
DATE: April 1998

The combustion of pulverised coal in a steam generating boiler is the most widely used process for power generation, supplying over 40% of the electricity worldwide. However, coal-fired plants are a source of NOx (sum of NO and NO2), which are among the major environmental pollutants associated with the formation of acid rain and photochemical smog and with the greenhouse effect through the formation of ozone.

To meet with regulatory emission requirements NOx control technologies, which can be broadly classified as either primary (low NOx burners, overfire air and reburning) or secondary (SCR, SNCR or hybrid SCR/SNCR), continue to be developed and adopted in both existing and new coal-fired plants. Much recent attention has been focused on the development of primary NOx control technologies with the aim of finding the most practical and cost-effective ways of reducing emissions through control of the combustion process.

Most of the NOx emitted from coal-fired power plant is in the form of nitric oxide (NO), the amount of nitrogen dioxide (NO2) emitted being comparitively small (less than 5% of the total NOx emitted). NO originates both from the release of organically bound nitrogen (fuel NO) and from the reaction of nitrogen in the air used in the combustion process (thermal NO, prompt NO), with fuel NO accounting for over 75% of the total NO generated. For most practical cases the fundamental reaction chemistry involved in the formation and destruction of NO is known and the quantity of NO produced is dependent on the influence of the furnace conditions and the properties of the coal being fired.

This report reviews recent literature relating to NOx formation and destruction during PC combustion. Studies investigating the fundamental reaction mechanisms involved in the partitioning of coal nitrogen between the volatiles and char and the production of NO from volatile nitrogen species and char nitrogen are detailed. Findings from laboratory-scale, pilot-scale and full-scale plant studies aimed at establishing the relationships between bsic coal properties and NOx emissions are summarised.