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Emissions

Pollutants and Control Techniques
Combustion of fossil fuels - natural gas and oil - in commercial and industrial boilers results in emissions of: carbon dioxide, nitrogen, oxygen, water, carbon monoxide, nitrogen oxide, sulfur oxides, volatile organic compounds, and particulate matter. The latter five products of combustion are considered pollutants and are known to, either directly or indirectly, cause harmful effects on humans and the environment. The following section describes the formation and control of each of the pollutants in commercial and industrial boilers:

Nitrogen Compounds (NOx)
NOx is considered an environmental problem because it initiates reactions that result in the production of ozone and acid rain. Ozone and acid rain can damage fabric, cause rubber to crack, reduce visibility, damage buildings, harm forests and lakes, and cause health problems. By controlling NOx levels, along with the other pollutants, the levels of acid rain and ozone can be reduced.

The principal nitrogen pollutants generated by boilers are nitric oxide (NO) and nitrogen dioxide (NO2), collectively referred to as NOx. The contribution from different NOx sources to the total NOx levels varies among metropolitan areas. In general, the contribution of mobile sources to the total NOx level ranges from 60 to 80 percent. For stationary sources, it ranges between 20 and 40 percent. A significant portion of the NOx from stationary sources can be attributed to residential, commercial, and industrial sources, including industrial boilers. In industrial boilers, NOx is primarily formed in two ways: thermal NOx and fuel NOx. Thermal NOx is formed when nitrogen and oxygen in the combustion air combine with one another at the high temperatures in a flame. Thermal NOx makes up the majority of NOx formed during the combustion of gases and light oils.

Fuel NOx is formed by the reaction of nitrogen in the fuel with oxygen in the combustion air. It is rarely a problem with gaseous fuels. But in oils containing significant amounts of fuel-bound nitrogen, fuel NOx can account for up to 50% of the total NOx emissions. NOx emissions from boilers are influenced by many factors. The most significant factors are flame temperature and the amount of nitrogen in the fuel. Other factors affecting NOx formation are excess air level and combustion air temperature.

While flame temperature primarily affects thermal NOx formation, the amount of nitrogen in the fuel determines the level of fuel NOx emissions. Fuel containing more nitrogen results in higher levels of NOx emissions. Most NOx control technologies for industrial boilers, with inputs less than 100 MMBtu/hr, reduce thermal NOx and have little effect on fuel NOx. Fuel NOx is most economically reduced in commercial and industrial boilers by switching to cleaner fuels (fuels containing less fuel-bound nitrogen), if available.