NOx Emissions

The undesirable emissions of oxides of nitrogen, such as nitric acid and nitrogen dioxide, is a side effect of operating a combustion system

These gases are toxic, smog-inducing, and contribute to climate change through greenhouse effects

Three mechanisms for NOx formationo Thermal (Zeldovitch)o Prompto Fuel

Zeldovitch mechanism predominant at high temperatures (above 1100-1300 degrees), of note when preheating incoming air as this increases flame temperature, occurs when inert nitrogen in air dissociates into N-radicals at high energies

Prompt mechanism is very fast compared to other mechanisms, but generally occurs in lower temperature combustion, and is therefore not as notable in industrial combustion processes. It is most prevalent in fuel-rich processes

Fuel NOx is released by the direct oxidation/decomposition of organic nitrogen in fuels, and is an issue with low-quality fuels such as oil or coal. Higher-quality fuels such as methane do not typically exhibit the Fuel NOx mechanism

Various governmental bodies have established permissible standards for NOx production, for example, ADR79/04 (Australian Design Rule) for light vehicles fuelled by petrol or gas limits NOx production to 0.06-0.08 g/km

Controls on NOx production relevant to gas turbines are:o Reduction of peak combustion temperature by injecting water/steam,

reducing the air preheat, or leaning out the mixtureo Minimising residence time in the combustion chamber by “air staging”, or

having a rich initial combustion stage at lower temperature/high equiv. ratio, then adding air in quick-mix and dilution stages to ensure complete combustion and lower the temperature (RQL combustor)

o Chemical reduction by selecting appropriate fuels or reburning The most prevalent factors are clearly the combustor temperature and the residence

time, for managing NOx emissions