17.2) Nitrogen Analysis

What are the different sources of NO and NO₂ in stack gases?

Nitric oxide (NO) and nitrogen dioxide (NO₂) are the main components of NO_x, which all combustion processes involving atmospheric air can produce.

NO_x emissions contribute to the formation of acid rain and, additionally, can react with hydrocarbons in sunlight to produce a potent respiratory irritant that is commonly called smog.

There are three different sources of NO_x in combustion processes: thermal NO_x, prompt NO_x, and fuel bound NO_x.

In boilers and furnaces which burn fuels low in nitrogen (gas and light oils), thermal NO_x predominates. By burning nitrogen-rich fossil fuels (heavy oils), higher concentrations of fuel NO_x are generated. Typically, nitric oxide (NO) comprises over 95% of the NO_x found in stack gases, however, a significant amount of NO converts to NO₂ in the atmosphere.

Prompt NO_x is a small amount of total NO_x, emissions that occurs during the low temperature stages of combustion. Since NO_x contributes to smog and acid rain, legislation to reduce these emissions has become much tougher.

To a lesser extent there will be formation of NO₃ (nitrogen trioxide), N₂O₄ (dinitrogen tetroxide) and N₂O (commonly known as laughing gas). These form a thermal equilibrium dependent on temperature and pressure, but are not generally present in quantities that cause serious concern.

17.3) Sampling for oxides of nitrogen

The general principles involved are the same as for sulphur dioxide, however the relationship between nitric oxide (NO), nitrogen dioxide (NO₂), dinitrogen tetroxide (N₂O₄), and air (oxygen O₂) is a complex temperature dependent equilibrium.

This makes life difficult for all types of spectrophotometer and hence the basic reason for calling this measurement (NO_x)

Most of the oxides of nitrogen become strong acids in the presence of oxygen and/or water. It is therefore important to keep them in the vapour state. For conventional sample systems this is best achieved by using a heated probe and sample line controlled at temperatures around 130°C. Since the common components of NOx are nitric oxide and nitrogen dioxide, only these two are generally measured. This approximation will be accurate enough for all practical purposes.