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

Executive summary


Nitrogen oxides (NOx) are some of the common air pollutants that are regulated for the control of health and other adverse effects. For practical purposes, only the two main oxides of nitrogen, nitric oxide (NO) and nitrogen dioxide (NO2) are normally considered. The exhaust gases from fuel gas combustion are expected to contain mainly NO and less than 10% as NO2, but a higher percentage of NO2 in NOx has been experienced in domestic applications.

NO oxidises in the atmosphere to form NO2, which can cause adverse health effects. In sunlight, NOx and hydrocarbons can react to form photochemical smog, which is controlled by controlling emissions of its two precursors.

Natural gas and liquid petroleum gas (LPG) are widely used throughout Australia. Gas is suitable for domestic use because it burns relatively cleanly and can be reticulated. It is used in dwellings for water heating, space heating and cooking. The most significant pollutant emitted to the atmosphere from domestic gas use is NOx. Domestic gas appliances contribute a small but significant proportion of urban NOx emissions. In Australia, gas space heaters use approximately as much energy as gas water heaters and cookers combined (AATSE, 1997).

In Australia, there are industry standards that specify maximum limits for NO2 emissions from cookers and unflued space heaters. These are called up in statutory regulations in each jurisdiction. In other countries there are a variety of statutory and voluntary emission limits. At the present, Environment Australia is working with the gas industry to consider further development of voluntary industry standards for emissions of NOx from domestic gas appliances. This is part of the Commonwealth Government's Clear the Air program in response to the recommendations of the Australian Academy of Technological Sciences and Engineering Inquiry into Urban Air Pollution in Australia (AATSE, 1997).

The present report has reviewed the relevant NOx emissions standards for domestic gas appliances, low NOx technologies, methods of implementing NOx emissions standards and costs to business and end-users of this background study. The need for this study was identified in the initial discussions between Environment Australia and the gas industry.

NOx emissions standards

Australia is one of the few countries that does have emission standards for NO2 or NOx from domestic appliances. Where the literature search was successful in locating other standards, comparisons with the Australian standards were often difficult because of assumptions that have to be made in converting between units. Sometimes the applicability of the standards to particular appliances was not clear as a result of translation problems (e.g., 'air heaters' may mean central heaters, not space heaters, as in Australian usage). In many cases full details of the units used were not known (e.g., the temperature to which gas volumes are standardised). In spite of these difficulties, standards found for natural gas fired domestic appliances is summarised in Table S–1 and Table S–2.

Table S–1: Summary of NOx emissions standards for natural gas fired room heaters
Flued Heaters   ng/J(heat input)
Austria 60 ng(NOx)/J 60
Europe – automatic forced draft burners 170 mg(NOx)/kWh 47
Germany – fan assisted burner 200 mg(NOx)/kWh
150 mg(NOx)/kWh
Poland 60 g(NOx)/GJ 60
Unflued Heaters   ng/J(heat input)
Australia 5 ng(NO2)/J 5 (NO2)
Austria 30 ng(NOx)/J 30
Germany – radiant heater 60 mg(NOx)/kWh 17
Japan 10 ppm(NO2) 5.7 (NO2)
Table S–2: Summary of NOx emissions standards for natural gas fired water heaters.
Flued Heaters   ng/J(heat input)
Austria 60 ng(NOx)/J 60
Belgium – central heating boilers 100 mg(NOx)/m³ (0% O2) 28
Czech Republic
– atmospheric
– fan assisted

200 mg(NOx)/m³ (3% O2)
150 mg(NOx)/m³ (3% O2)

Europe – automatic forced draft burners 170 mg(NOx)/kWh 47
– central heating boiler
– wall mounted boiler

200 mg(NOx)/kWh
200 mg(NOx)/kWh

Japan 60 ppm(NOx) @ 0% O2 34
Poland 35 g(NOx)/GJ 35
USA (California AQMDs)
– residential
– mobile home

40 ng(NOx)/J(output)
50 ng(NOx)/J(output)


The assumptions used in converting between different units are also detailed below. The third columns of the tables contain converted standards in units of ng(NOx)/J or ng(NO2)/J consistent with the specification of the standard in column 2. It is not possible to convert from a NOx basis to an NO2 basis without knowing the applicable NO2/NOx ratio. It is important to note that entries in column 3 expressed on a NOx basis cannot be compared with other entries on an NO2 basis as the conversion may vary by a factor of 10 or more, depending on the NO2/NOx ratio.

Assumptions used in unit conversions in Table S–1 and Table S–2:

This study contributes to Environment Australia's program to improve ambient air quality. It will be important in the formulation of policy and regulations to integrate ambient air considerations with those of indoor air quality, greenhouse effect, energy efficiency and building insulation, noise and pollutants other than NOx.

NOx control technologies and applications

On a theoretical basis, NO is the major species of NOx in the combustion products which is the predominant source of NOx in atmosphere. NO in combustion products can be either thermal NO, fuel NO, or prompt NO.

Thermal NO is formed by the oxidation of molecular nitrogen in combustion air and fuel gases, which is more dependent on the combustion temperature (and hence the fuel-to-air ratio) and less dependent on the oxygen concentration. Fuel NO is formed by the oxidation of nitrogen chemically bound in fuel which is practically absent from reticulated natural gas and LPG. Prompt NO is most frequently observed in fuel (hydrocarbon)-rich flames and at low temperatures which can be reduced by increasing aeration.

For gas combustion which has a high flame temperature (>1550°C), the NO formed should be predominantly thermal NO, with a small fraction as prompt NO. However, high concentrations of NO2 have been experienced in domestic applications.

There are many primary NOx control technologies that can be applied to domestic gas appliances to reduce NOx formation. Some technologies can just achieve the current NOx emission limits. If the emissions limits decrease further, these technologies may require further development or may even not be acceptable anymore. The primary NOx control technologies are compared and summarised in Table S–3.

Table S–3: Comparison of primary NOx control strategies for residential gas appliances*
Primary NOxc control technology Likely lowest NOx (ppm, O2-free) * Likely change in CO emissions * Likely change in thermal efficiency * Technology status for domestic application *
Premixed, high excess air ~ 20 Decrease Decrease Current
Flue-gas recirculation ~ 25 Increase Decrease Not commercialised
Staged combustion ~ 25 Increase Decrease Current
Delayed combustion ~ 25 Increase Decrease Not commercialised
Humidified combustion ~ 25 Increase Decrease Not commercialised
Flame inserts ~ 40 Increase Decrease Current
Thermally active burner ~ 65 Decrease Increase Current
Port-loading reduction ~ 50 Increase Increase Current
Port redesign ~ 45 Decrease Increase Current
Radiant combustion ~ 4 ** Decrease Increase Current
Catalytic combustion ~ 5 Decrease Decrease Not commercialised
Pulse combustion ~ 20 Increase Increase Current

* Some information is based on reports published 2–5 years ago and could be superseded in 1999.
** This low level is achieved by the Bowin technology which has been put into this category by the author. Otherwise, the 'Likely Lowest NOx' would be ~ 10 ppm claimed by the Acotech technology.

Many primary control technologies have been applied commercially to water heaters and air heaters, but few to cooking appliances. Any acceptable NOx control technology should reduce NOx emissions, at the same time maintain or decrease CO and formaldehyde emissions, and maintain or increase thermal efficiency.

Most of the primary low NOx technologies were developed in countries which use natural gas supply pressures = 2 kPa gauge for domestic use. In Australia, most of the country receives a much lower line pressure (e.g., ~ 1.1 kPa gauge in most regions in Victoria) and some of the overseas low NOx technologies may require fan assistance for application under Australian conditions. A possible approach to improve this situation is to raise the gas supply pressure to similar levels used overseas if no change of hardware is required. This will require the involvement and cooperation of Australian gas distribution companies.

Secondary control technologies are expensive, may be of environmental concern, and have not been applied to domestic situations.

Low NOx technology adoption strategies

In the Australian context, limiting NOx emissions from domestic appliances and encouraging the introduction of low NOx technology will be done most easily by continued use of AGA standards. However, if these standards are to be used to address ambient air quality concerns, full cooperation of the government environmental regulators and the gas industry will be required in the development of the standards, which until now have been based on indoor air quality.

In setting NOx emission standards, a clear distinction can be drawn between criteria based on indoor air quality and criteria based on ambient air quality. Australian AGA codes have been based on the former. The intention has been to limit NO2 concentrations indoors, with little concern for NOx emitted through flues to the outside atmosphere. On the other hand, NOx emission regulations for domestic appliances in the USA and at least some parts of Europe have been driven by ambient air quality concerns. Only 'non-attainment areas' for ozone are regulated in the USA, with the objective of reducing photochemical smog. Ozone problems probably do not occur to the same extent in Australia; however a judgement on this is outside the scope of the present study (an overview of Australian ozone levels is given in NEPC (1998b)). It should also be remembered that NOx from gas appliances constitute less than 0.5% of total NOx emissions in Australian capital cities, except for Melbourne, where the figure is 1.75% (Todd et al., 1997).

Various strategies are available for governments to encourage the introduction of low NOx technology in domestic gas appliances. These include 'command and control' regulation, voluntary regulation, subsidies, tax concessions, assistance with promotion, public education and tradable emission permits.

The Australian practice of calling up voluntary industry codes in legislation has worked well in the past and the gas industry appears to be moving towards introducing low NOx technology in flued appliances (Saxby, 1998). However, it may be necessary to demonstrate a clearer link between domestic emissions and indoor or ambient criteria before further steps can be agreed.

Australia does not have an eco-labelling scheme. Present appliance labels indicate energy efficiency levels. Some overseas eco-labels include pollutant emission levels in their criteria and a scheme including NOx levels from domestic gas appliances could be introduced to assist consumers in their choice of products.

Cost to business

The cost to business is a sensitive subject. The indicative cost and lead time expected for modifying an existing overseas low NO model or developing a brand new low NOx model vary much among different types of appliances and different manufacturers. The cost can be:

In Australia, four space heater manufacturers, three water heater manufacturers, one cooking appliance manufacturer and one components manufacturer provided information on this subject. Direct response from overseas was limited, but the information provided by half of the responding local manufacturers was referred to the experience of their overseas parent/subsidiary companies. The information collected from the Australian manufacturers is summarised as follows:

There have been a number of comments and concerns raised by the industry. These are:

Price differentials

Based upon the indicated manufacturing cost of low NOx burners provided by two burner suppliers, the cost of a low NOx burner to end-users is expected to be $A 60–80 approximately. It can be down to $A 40 or less for high volume production.

Two water heater manufacturers expected the price and the installation cost of a low NOx water heater to be higher than that of a conventional model by 5–15% if a natural draft system can be used. One manufacture expected that it would be higher by at least 25% if a fan-assisted combustion system is required. In an exceptional case, Tokyo Gas in Japan has not increased the retailing price of low NOx water heaters to encourage end-users using low NOx water heaters despite the increase in production cost.

In Victoria, the normal retailing price of an ultra low NOx flued space heater was approximately $250, or 17%, higher than a conventional flued space heater from another supplier. Features and performance were not compared but they are believed to contribute to part of the price differential.

In a report in 1997, the retailing prices of low NOx grills under development in Japan and low NOx hotplates under development in Germany were expected to be higher than the conventional ones by less than 20%. The price of low NOx hotplates under development in Austria was expected to be twice as much as the conventional models.

There has been a local concern about little perception of willingness to pay for low NOx technology by consumers. In order to persuade Australian consumers to purchase low emissions appliances, perhaps a possible approach is:

  1. Educational promotion of community awareness of NOx emissions and the effects on environment.
  2. Survey on consumers' awareness of NOx emissions and willingness to pay for low NOx appliances.
  3. Development of NOx emissions standards if the consumers and industry are ready to accept it.

The educational promotion may proceed together with other emissions issues such as greenhouse gases.

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