Nitrogen oxides emissions standards for domestic gas appliances

Background study
Mr Bob Joynt, Environmental Consultant and Mr Stephen Wu, Combustion Engineering Consultant
Environment Australia, February 2000

NOx and environment

3.1 NOx

NOx refers to the gaseous nitrogen oxide compounds. In the discussions of nitrogen oxide combustion products and their impact to environment, the major nitrogen oxide species of concern are nitric oxide (NO) and nitrogen dioxide (NO2).

Under high temperature combustion conditions, the formation of NO is favoured and consequently, less than 10% of the NOx in typical exhaust is in the form of NO2 (Pereira and Amiridis, 1995). However, a higher percentage of NOx in the form of NO2 has been experienced in domestic applications. NO when it cools down in the atmosphere combines with oxygen in air to form NO2 (Eqn. 1).

Eqn. 1. Equation 1

In warm, sunny days the NO2 breaks down into NO and a nascent oxygen atom (Eqn. 2) which can combine with a molecule of oxygen to form ozone (Eqn. 3). The ozone reacts with NO to yield back NO2 almost as fast as it is formed.

Eqn. 2. Equation 2

Eqn. 3. Equation 3

When volatile organic compounds (VOC) exist in the air, they combine with the NO in the present of sunlight to change it back to NO2. Less NO is then available to remove the nascent oxygen, and hence ozone accumulates, resulting in photochemical smog.

3.2 Environmental effects

The major effects of NOx on the ambient environment are:

Ambient air standards and goals were introduced in the National Environment Protection Measure for Ambient Air Quality (NEPC, 1998a). The one-hour average standard for NO2 is 0.12 ppm, with a goal to exceed this level on no more than one day a year to be achieved within ten years. There is also a one-year average standard of 0.03 ppm NO2 with a goal for no exceedances within ten years.

The health effects of NO2 have been summarised in many places, e.g. by EPAV (1993) and NEPC (1998b), where there is additional discussion of other effects of NOx and the costs and benefits of options for regulation of ambient NOx in Australia.

Indoor air quality studies indicate that substantially higher levels of NO2 and other pollutants can occur inside domestic buildings with gas appliances than outside. Recent Australian studies of indoor NO2 include Nitschke et al (1998), Brown (1998), Keywood et al (1998), Immig et al (1997), Garrett et al (1996) and Steer et al (1991). NOx emitted indoors would be partially removed by sinks before leaving the building.

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