Nitrogen oxides (NOX) formed during the combustion of coal are mainly nitric oxide (NO) and nitrogen dioxide (NO2), together commonly referred to as NOX. NOX formed during combustion is composed predominantly of NO (90-95%) and a lesser amount (5-10%) of NO2. A very small percentage of nitrous oxides (N2O) are also formed (5.5). The nitric oxide slowly oxidizes to nitrogen oxide in the atmosphere. However this reaction is accelerated in the presence of ozone (O3) and reactive organic compounds.

In the presence of sunlight, NOX reacts with reactive organic compounds to form photochemical smog (ground-level ozone) and other strong oxidants that involve the conversion of NO2 to nitric acid and peroxyacetyl nitrate (PAN).

Nitrogen oxides are formed during the combustion of coal by the oxidation of molecular nitrogen in the combustion air and nitrogen compounds contained in the coal.  There are basically three different formation mechanisms for NOX: thermal NOX, prompt NOX and fuel NOX.  

• “Thermal NOX” is formed during coal combustion by oxidation of molecular nitrogen (N2) in the combustion air.  The formation of thermal NOX is highly dependent on gas temperature. Nitrogen is rapidly oxidised to NO and NO2 once gas temperatures rise above 1,700oC. Formation of thermal NOX in a coal-fired boiler is dependent on two conditions occurring simultaneously in the combustion zone: high temperature and an excess of combustion air (NESCAUM 2005).
• “Fuel NOX” is produced by oxidation of nitrogen compounds contained in the coal. The nitrogen contents of Australian black coals generally range from approximately 1.5 to 2% (daf basis).
• “Prompt NOX” is formed at the flame front through the reaction of hydrocarbon radicals, released during volatile evolution, with nitrogen in the combustion air. The contribution of prompt NOX to the overall NOX formation is relatively small (less than 5% of the total NOX formed).

The relationship between NOX formation and coal structure is complex and depends on factors such as coal properties (nitrogen content), devolatilization and combustion, soot and tar formation and combustion, char burn out, the interaction of NO and N2O with char soot and volatiles, as well as burner design and reaction conditions. High nitrogen content coals may not necessarily be high NOX generators as there is no direct relationship between coal N content and the amount of NOX produced.

The effects of boiler type on uncontrolled NOX emissions are shown in Figure 5.3. Cyclone boilers have the highest NOX emissions, followed by wall-fired and then tangentially fired boilers.

Full-load NOX emissions for pre-NSPS coal-fired U.S. utility boiler configurations.

Figure 5.3
Effects of boiler types on uncontrolled NOX emissions
Source: National Energy Technology Laboratory 1996

A review of NOX formation in PF combustion provides the current state of knowledge about NOX formation during PF combustion.