REDUCTION OF NOX EMISSIONS

Reduction of NOx Emissions from Coal Power Plants

Reduction of NOx Emissions from Coal Power Plants

Nitrogen oxides (NOx) are formed when nitrogen (N2) and oxygen (O2) are combined at high temperatures and pressure during the combustion of fuel. All fuels, such as gasoline, diesel, biodiesel, propane, coal, and ethanol, emit NOx when burned. NOx is the generic name for a group of highly reactive gases that contain varying amounts of NO and NO2 (www.arvinmeritor.com). NOx tends to be colorless and odorless. Common sources of NOx are motor vehicles (49%), electric utilities (27%), and other industrial, commercial and residential sources (19%) that burn fuels.

The EPA estimates that 49% of NOx emissions come from on-road and off-road vehicles, 27% from power generation (electric utilities) and the remaining 24% from industrial, commercial and residential sources. Due to the many compounds that are a part of NOx (predominantly nitrogen dioxide and nitric oxide), the pollutant contributes to a wide variety of health and environmental problems. NOx is also a main component of ground-level ozone and contributes to global warming (www.treepower.org).

Since the passage of the Clean Air Act in 1970, all primary air pollutants have decreased - except NOx, which has increased by 10%. Due to its serious health and environmental impact, the reduction of NOx in our atmosphere has now become a major focus in the fight against air pollution (www.arvinmeritor.com).

Increased environmental performance at a modest cost is one of the drivers for biomass/coal cofiring in utility boilers. Biomass usually has lower sulfur content than coal so cofiring results in a reduction of SOx emissions because of a displacement of sulfur in the fuel blend. Similar reductions are also observed for NOx emissions because the nitrogen content of the cofired biomass fuels is generally lower than the nitrogen content of the coal. Initially, any change in NOx emissions as a result of blending the biomass and coal can be attributed to changing the amount of nitrogen in the fuel blend (www.treepower.org).

This may not be the case, however, in full-scale biomass/coal cofiring applications. Of course, in a full-scale cofiring situation many other engineering factors contribute to NOx formation. Utility boilers are far from isothermal, and adding biomass to a pulverized coal-fired boiler can significantly change the flame structure and characteristics. The addition of biomass has been shown to reduce NOx emissions in most commercial facilities, usually beyond the reductions expected because of a lower overall fuel-bound nitrogen content. The high volatiles content of biomass can effectively establish a fuel-rich zone early in the flame that can reduce NOx emissions. Adding biomass can also reduce flame temperatures, leading to lower levels of thermal NOx. The high moisture content of some biomass may also be effective for NOx reduction at full-scale (www.arvinmeritor.com).

A summary of the measured NOx emissions reduction as a function of the biomass cofiring percentage for a number of full-scale biomass cofiring demonstrations and pilot-scale results is plotted in Figure 1 on a % biomass cofired on a mass basis on a ...