Nitrogen oxides are of environmental concern because they initiate reactions that result in the formation of ozone and acid rain, which can cause health problems, damage buildings, and reduce visibility. The allowable NOx emissions from boilers and HRSGs vary depending on local regulations but are gradually edging toward single-digit values in parts per million volume (ppmv) due to advances in combustion and pollution control technology. The principal nitrogen pollutants generated by boilers, gas turbines, and engines and other combustion equipment are nitric oxide (NO) and nitrogen dioxide (NO2), collectively referred to as NOx and reported as NO2. Once released into the atmosphere, NO reacts to form NO2, which reacts with other pollutants to form ozone (O3). Oxides of nitrogen are produced during the combustion of fossil fuels through the oxidation of atmo­spheric nitrogen and fuel-bound nitrogen. These sources produce three kinds of NOx: fuel NOx, prompt NOx, and thermal NOx.

Fuel NOx is generated when nitrogen in fuel combines with oxygen in combustion air. Gaseous fuels have little fuel-bound nitrogen, whereas coal and oil contain significant amounts. Fuel-bound nitrogen can account for about 50% of total NOx emissions from coal and oil combustion. Most NOx control technologies for industrial boilers reduce thermal NOx and have little impact on fuel NOx, which is economically reduced by fuel treatment methods or by switching to cleaner fuels. Fuel NOx is relatively insensitive to flame temperature but is influenced by oxygen availability.

Prompt NOx results when fuel hydrocarbons break down and recombine with nitrogen in air. Prompt NOx is chemically produced by the reactions that occur during burning; specifically, it forms when intermediate hydrocarbon species react with nitrogen in air instead of oxygen. Prompt NOx, so called because the reaction takes place ahead of the flame tip, accounts for about 15-20 ppm of the NOx formed in the combustion process and is a concern only in low temperature situations.

Thermal NOx forms when atmospheric nitrogen combines with oxygen under intense heat. This rate of formation increases exponentially with an increase in temperature and is directly proportional to oxygen concen­tration. Its formation is well understood and straightforward to control. Keeping the flame temperature low reduces it. Below a certain tempera­ture, thermal NOx is nonexistent, as indicated in Fig. 4.1. Combustion temperature, residence time, turbulence, and excess air are the other factors that affect the formation of thermal NOx. Most NOx is formed in this manner in gas turbines, industrial boilers, and heaters fueled by natural gas, propane, butane, and light fuel oils.

Common boiler fuels in the order of increasing NOx potential are methanol, ethanol, natural gas, propane, butanes, distillate fuel oil, heavy fuel oils, and coal.