Fuel Nitrogen and NOx formation

Improved techniques for the prediction

High nitrogen content coals may not necessarily be high NO_x generators.

Combustion of fossil fuels will always result in the formation of nitrogen oxides (NO_x). NO_x species can have local and long range environmental consequences such as acid rain and smog formation. Such environmental concerns will increasingly impact on the coal industry and generators, as demonstrated by the NSW governments move to load based licensing.

The nitrogen contents of Australian black coals generally fall in the range 1.5 - 2.0% (daf basis), however, a number of coals have nitrogen contents in excess of 2.0%. Australian coals with high nitrogen contents have already experienced marketing difficulties, in spite of evidence that NO_x formation from such coals is not excessive.

Pulverised coal combustion produces NO_x through various pathways (Figure 1) including thermal NO_x (from N₂ in the combustion air) and fuel NO_x (from nitrogen contained in the volatile matter or in the char). Predictive techniques for NO_x formation from coals are affected by the complexity of the factors influencing NO_x emissions. In particular the lack of a direct relationship between coal N content and the amount of NO_x produced.

Char N can contribute as much as 80% to the NO_x emissions from modern power plants and thus an understanding of the behaviour of char N is central to improving the ability to predict NO_x emissions.

Figure 1: Pathways for NOx formation

Investigations at the CCSD have provided evidence that char nitrogen is mainly converted to oxidised NO, which is then partially reduced to molecular N₂ during diffusion out of the char particle. The relative amounts of NO and N₂ produced is strongly related to combustion stoichiometry, with fuel rich conditions resulting in high conversions to N₂. Further experiments are under way to investigate the effect of gas phase NO on char N conversion.

The Centre’s aim has been to characterise the fate of char N under conditions relevant to pulverised coal combustion. A range of experiments have been conducted in order to measure the efficiency of char N conversion to NO_x for a variety of Australian bituminous coals under varying conditions of fuel-air ratio, temperature, residence time and carbon burnout.

Figure 2: Char N release as a function of burnout

Figure 2 shows the correlation between nitrogen release and carbon burnout. It has been found that the release and conversion of nitrogen is slower than that of carbon, and that the total nitrogen release is proportional to burnout.

Using argon as a carrier gas, an acetylene/oxygen flame, and the exclusion of air nitrogen it has been possible to accurately measure the generation of reduced nitrogen species (HCN and NH₃) and gas phase nitrogen products (N₂ and NO_x species).

The expertise and information from these investigations are available for use by industry.

For more information contact:

Prof Peter Nelson
Education Program Leader
Macquarie University