Mr Bob Joynt, Environmental Consultant and Mr Stephen Wu, Combustion Engineering Consultant
Environment Australia, February 2000
When discussing the formation of NOx found in atmospheric air, one refers specifically to the NOx formed during combustion which is the predominant source of NOx to atmosphere.
The NO2/NO ratio should be negligibly small at typical flame temperatures under thermodynamic equilibrium conditions. Kinetic models also indicate that the conversion of NO to NO2 can be neglected in practical devices (Glassman, 1996). But in practical domestic applications, higher ratios have been experienced. For the convenience of discussion on a theoretical basis, only NO is discussed in this section.
NO can be categorised into the following:
- Thermal NO.
- Fuel NO.
- Prompt NO.
For gas combustion burners such as Bunsen burners and flat flame burners which have a high flame temperature (> 1550°C), the NO formed is predominantly thermal NO, with a small fraction as prompt NO.
Thermal NO is found mainly in the high-temperature post-flame zone. It is formed by the oxidation of molecular nitrogen in combustion air and fuel gases by the extended Zeldovich mechanism:
where the nascent oxygen atom in Eqn. 4 is formed (with a large activation energy) from the H2-O2 radical pool or possibly from the dissociation of O2 (Glassman, 1996).
The hydroxyl (OH) radical in Eqn. 6 may come from the following reaction, which obtains the hydrogen atom from the dissociation of hydrocarbon fuel:
Eqn. 4 is rate-determining. To reduce thermal NO formation, O (nascent oxygen atom) must be reduced. The formation of O, and hence thermal NO, is more dependent on the combustion temperature and less dependent on the oxygen concentration. It increases with temperature. For combustion systems like those obtained on Bunsen and flat flame burners, the temperature, and hence the mixture ratio, is the prime parameter in determining the quantities of thermal NO formed.
Fuel NO is formed by the oxidation of nitrogen chemically bound in fuel. In the production of natural gas and liquid petroleum gas, combustible gaseous nitrogen compounds such as ammonia and amines have been removed to insignificant levels and little or no fuel NO would be formed.
Prompt NO is most frequently observed in fuel-rich flames and at low temperatures, and its formation is found to be relatively independent of temperature. There are three possible sources of prompt NO (Glassman, 1996):
- Non-equilibrium nascent oxygen (O) and hydroxyl (OH) radical concentrations in the reaction zone and burnt gas, which accelerate the rate of thermal NO (Zeldovich) mechanism.
- A reaction sequence initiated by reactions of hydrocarbon radicals, present in and near the reaction zone, with molecular nitrogen (Fenimore prompt NO mechanism):
The nascent N atom can yield NO by reactions such as Eqn. 5 and Eqn. 6, and CN can form NO with a nascent oxygen atom or oxygen molecule.
- Reaction of nascent oxygen (O) with molecular nitrogen to form nitrous oxide (N2O) via the three-body recombination reaction (Eqn. 10) and the subsequent reaction (Eqn. 11) to form NO:
The non-equilibrium O and OH concentration mechanism is more important for non-pre-mixed flames, stirred reactors for lean conditions, or low pressure premixed flames.
The Fenimore prompt NO mechanism is dominant in fuel-rich pre-mixed hydrocarbon combustion.
The nitrous oxide mechanism becomes more important when the fuel-air ratio decreases, when the burnt gas temperature decreases, or when the pressure increases.
At common combustion temperatures, increase in aeration can reduce prompt NO formation.
Despite the favoured formation of NO dictated by thermodynamics and reaction kinetics, high concentrations of NO2 have been experienced in domestic applications, e.g., Glassman (1996) cited that high concentrations of NO2 was reported in the exhaust of range-top burners.
It was observed that NO2 was formed by HO2 and NO in the low-temperature regime of visible flames (Eqn. 12) and suggested that the conversion of NO2 to NO and oxygen in the near-post-flame zone (as given by Eqn. 11) was quenched.
NOx control may be:
- Primary – to reduce NOx formation.
- Secondary – to remove NOx formed.
There are three basic principles of primary NOx control to reduce NOx formation:
- Reduction of high combustion/flame temperature since more NOx will be formed at higher temperatures under thermodynamic equilibrium conditions.
- Reduction of residence time at high combustion temperature to resist the NOx formation approaches thermodynamic equilibrium concentration.
- Reduction of oxygen concentration and hence the nascent oxygen concentration in the high temperature zone as discussed in Section 5.1.2.
It is possible to quench the NOx reactions, obtain the chemical heat release and prevent NOx formation (non-equilibrium Zeldovich mechanism) but in practice efficiency often suffers if quenching is done by adding a non-reacting mass such as water or steam to the system.
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.
The term low NOx technology used in the industry has a broad range in terms of the NOx emission level achieved. In some instances, an emission of 70–80 ppm at 0% O2 on dry basis is regarded as 'low'. In other instances, it may be down to 10–15 ppm or less. The difference, to some extent, could be created by the advance in the development of low NOx technologies and changes of emission regulations in some regions in recent years.
In this report, the original NOx emission figures quoted in the source of information is cited. Therefore the term low NOx technology has a broad meaning, and the actual emission level achieved by a particular technology should be referred to if a comparison between low NOx technologies is required.
The most common combustion system of domestic gas appliances (except ranges) consists of natural gas injection into a venturi (Raghavan and Reuther, 1994). The jet aspirates ambient air, yielding a fuel/air mixture containing ~ 50% to a maximum of 70% of the air required for stoichiometric combustion. The gas mixture is combusted at the burner port(s). A flame is formed and secondary air is drawn to the flame from the surrounding under some control to give an overall excess air level of ~ 40–60%.
A dual flame structure with an inner, fuel-rich and greenish flame followed by an outer, bluish diffusion flame is obtained when the primary air rate is less than 100% of stoichiometric requirement. At a primary air rate > 100% stoichiometric requirement, a single blue premixed flame is obtained.
NOx emission from conventional blue flames (the dual flame structure) is in the order of 100 ppm at 0% O2 on dry basis.
The primary NOx control technologies involve either or both of the following:
- Modification of fuel/air delivery-burner system.
- Modification of gas burner.
The strategies to modify fuel/air delivery-burner systems were summarised by Raghavan and Reuther in 1994 as follows:
- Increasing the primary pre-mixed air from ~ 50% to more than 100%.
- Recirculating combustion exhaust gases into primary combustion air.
- Staging combustion into more than one discrete step, with heat extracted between steps.
- Delaying, distributing, or dispersing fuel/air mixing within the combustion chamber.
- Humidifying fuel gas, combustion air, or the flame.
Increasing the primary premixed air
NOx emissions from blue flames could be reduced from ~ 100 ppm to < 70 ppm (oxygen (O2) free) by increasing the primary air from ~ 50% to ~150% of the stoichiometric air required. Effectively any excess air above 100% stoichiometric dilutes the combustion exhaust and brings down the combustion temperature from a maximum of ~ 1900°C to ~ 1200°C, causing less NOx to be formed.
Lower combustion temperature would result in longer combustion time at high temperature because of slower burning rate. This would encourage NOx formation, but this effect was observed to be secondary and a net decrease in NOx emission would result.
Raghavan and Reuther (1994) listed the following means which had been patented to increase the primary air flow to ~ 50% excess:
- Very large venturi.
- Higher gas- or air-line pressure.
- New design of aspiration such as alternating burner ports fire with primary air < 100% in one port and up to ~ 85% excess air in the adjacent ports to achieve ~ 70 ppm.
- Fan with venturi.
- New burner design to accelerate the velocity of the burning pre-mixture and shorten the residence time besides reducing combustion temperature, with a hemispherical bluff body re-stabilises the flame.
Burners designed for excess primary aeration would have deeper ports and thicker walls than the usual stamped metal burners. Secondary aeration would not be required and could be eliminated by closed combustion chamber or baffles.
Recirculating combustion exhaust gases
Recirculation of flue gases could be achieved by:
The cooled combustion exhaust gases (mainly molecular nitrogen and oxygen, carbon dioxide and water vapour) are mixed with air entering the burner. The recirculated gases dilute the primary air and lowers the oxygen concentration of the air mixture from ~ 21 % by volume to ~ 18%. Consequently the flame temperature is lowered. Research on larger scale applications has demonstrated that NOx could be reduced by ~ 75% when the primary air contains ~ 30% recirculated flue gas.
Ducting of the exhaust gases to the fuel/air delivery system would be required. The combustion chamber and heat exchanger of the appliance may become larger to accommodate the higher total gas flow rate and lower flame temperature to maintain baseline thermal efficiency. The burner may have to be upgraded to light and stabilise the fuel-air-exhaust mixture which is more difficult to ignite and slower in combustion, although the warm mixture (if the exhaust gases are mixed at a few hundred degrees C) would alleviate this to some extent. Another concern is that lower flame temperature and oxygen concentration would favour CO formation.
Raghavan and Reuther (1994) pointed out that recirculation of combustion exhaust gases had been used at industrial scale to reduce NOx emission but not in domestic application, which is still true. Because of the high NOx reduction potential, they felt that domestic application of this strategy should be explored further.
A personal opinion is that recirculation of exhaust gases may find its application in storage water heaters, flued space heaters and ovens. These appliances could have the secondary air flow more or less controlled by an enclosed or semi-enclosed combustion chamber, and a relatively large heat exchanger to cool the exhaust gases. However, recirculation often requires a fan driven system that may have to work at elevated temperatures and this would increase the cost of the appliance and its operation.
Staged combustion can be conducted in two stages, the first is the fuel-rich combustion with < 100% primary aeration and the second is fuel-lean, with inter-stage cooling such as radiant heat loss from a radiant burner, or heat exchange with air or water. In principle, more stages can be used but the design, manufacture and operation will be more complicated and more expensive.
Staging can be achieved by modifying the gas burner or the combustion chamber, or both. The flame temperature at the two stages is lower than the dual flame combustion using the same overall (primary plus secondary) aeration. In a combined approach for a fan-assisted space heater prototype with a radiant burner, a reduction of NOx emission by ~ 75% was reported (Raghavan and Reuther, 1994).
Design and manufacture of staged combustion gas appliances are more complicated and expensive. Many of the components such as channels, flame holder, ignition system, combustion chamber and heat exchanger may have to be increased in number or in physical size. This will increase the manufacturing cost of the appliance.
Staged combustion can be performed with stable flame without fan assistance, which would make this technique attractive, but the problem of increased CO emission and decreased thermal efficiency must be addressed together with NOx reduction.
Different from staged combustion, delaying combustion allows the combustion process to occur continuously rather than at discrete stages, over lower temperatures, to retard NOx formation. This is achieved by dispersion, with slower heat release, over larger volumes and time.
Raghavan and Reuther (1994) cited from the literature four examples of burner design to delay combustion, with one suitable for air heaters and the other for water heaters. They recognised that although this approach was effective to lower NOx emissions (by up to ~ 75%) and amenable to a variety of atmospheric or powered burners, the development had been limited, which could be related to higher CO emissions and lower efficiency. The fuel/air delivery might need to be pressurised, the burner, combustion chamber and heat exchanger might need enlargement, and the ignition system might require improvement.
Humidifying the fuel gas, combustion air or flame
Humidification can be conducted by:
- Spraying water to the combustion air.
- Spraying water to the combustion chamber.
- Spraying steam to the combustion air or fuel gas.
- Spraying steam to the combustion chamber.
Steam dilutes the combustion exhaust in the same way as recirculated combustion exhaust gases. The effect of water is two fold: water evaporates by absorbing a large quantity of heat (latent heat of evaporation) from the combustion system and the steam evolved dilutes the combustion exhaust gases. Both result in cooling the combustion system.
The spraying rate of water to combustion air is restricted by the ambient humidity conditions and the efficiency of water atomisation. The spraying rate of water to the combustion chamber and the spraying rate of steam would depend on flame stability.
The investigation of the humidification for domestic appliances was limited even though the NOx reduction could be up to ~ 50–60% (Raghavan and Reuther, 1994). It has not been attractive probably because the efficiency of the system will decrease with humidification, unless steam in the exhaust gases is condensed and the heat extracted is recoverable. Condensation would complicate the combustion system, create corrosion problem and increase the equipment cost.
Humidification has been used in commercial scale continuous gas turbine operation but not yet in domestic situations. The loss of efficiency in gas turbine application is traded off with the increase in power output by the higher mass flow through the gas turbine.
Raghavan and Reuther (1994) identified the major modifications of gas burners as follows:
- Flame inserts.
- Blue-flame burner redesign.
- Blue-flame burner replacement.
A simple means to reduce flame temperature is to insert a foreign object, such as a solid rod or porous screen, into a blue flame and allow the object to radiate red hot. As part of the heat liberated is transferred by radiation, the flame temperature is reduced and hence the NOx emissions are reduced. The inserts could be made of refractory metals or ceramics.
Raghavan and Reuther (1994) cited five different flame inserts patented for atmospheric burners:
- A ring shaped solid insert for range or water heater burners.
- A rod shaped solid insert for furnace burners.
- A porous screen insert.
- A solid channel insert for furnaces.
- Small solid fin inserts integral with the burner but not in the flame.
A perforated radiant insert for fan-assisted power burner was also illustrated.
From the literature search, Raghavan and Reuther indicated that most flame inserts could achieve a ~ 60% reduction but the CO emissions would typically increase, since the combustion conditions remain the same except at a lower temperature which favours CO formation. Adjusting the position of insert or using secondary-air baffles may alleviate CO formation. Thermal efficiency could be an issue, but it may be overcome depending on the application and design.
Compared to other NOx control techniques, Raghavan and Reuther believed that flame inserts had the least impact on gas appliance component design. However, because of the change in heat transfer and flame shape, heat exchangers, particularly those used in space heaters, might require re-design.
Blue-flame burner redesign
Blue-flame burners could be redesigned either by changing the burner's thermal mass, port loading, or port design to achieve reduced NOx emissions.
- Thermal Mass
Cast iron burners are more'thermal active' than the traditional stamped steel and aluminium burners, and are found to emit less (~ 30%) NOx and CO. This is achieved by dissipating more heat via their high thermal mass (and structure).
Cast iron atmospheric or power burners have been applied to ranges and water heaters to lower NOx (down to < 70 ppm at 0% O2 dry basis). Thermal efficiency was reported to increase slightly (Raghavan and Reuther, 1994).
- Port Loading
NOx emissions depend on port loading – the heat released per port area per time. It was reported that NOx emissions from atmospheric blue flames could be reduced by half if the port loading was reduced by one third. Reducing port loading is achieved by increasing burner size if the same heat input rate is maintained. Thermal efficiency may increase or remain the same, but CO emissions could increase and flashback may occur.
- Port Design
Port spacing determines the extent of flame aeration and interaction, which affect NOx formation. If the heat dissipated by the ports is increased and the secondary aeration of flames is improved, NOx emissions can be reduced.
Raghavan and Reuther (1994) described the Worgas hyperstoichiometric burner as an example. The Worgas burner uses a venturi-burner system with unique port spacing and 80–160% stoichiometric air requirement. The burner is larger than the traditional Bunsen type blue flame burner. It has improved secondary-air entrainment, yielding violet flames with low and uniform temperature distribution. The butterfly-wing flame shape has the aerodynamics designed to bring combustion products back to the flame.
Laboratory results indicated that the Worgas burners could achieve 40 ppm NOx at 3% O2, dry basis, which is equivalent to 45 ppm at 0% O2, dry basis. Thermal efficiency is claimed to be high, and the technology can be used in boilers, instantaneous water heaters, storage water heaters, and room/air heaters (E. Cigarini and G. Tiplady, personal communications).
Blue-flame burner replacement
Blue flame burners have been suggested to be replaced with 'flameless' burners which adopt radiant combustion, catalytic combustion, or pulse combustion.
- Radiant Combustion
Radiant combustion occurs near or within burners which are either porous or ported, and may be fan-assisted. The burners can have different shapes to suit different heat exchangers. In operation, the burners glow in a red-orange colour (> 680°C).
Similar to flame inserts, radiant burners restrict NOx formation by lowering the combustion temperature, but in a better and more complete manner. NOx emission < 25 ppm and CO emission < 50 ppm O2-free have been reported (Raghavan and Reuther, 1994). Facilitated with high excess aeration and reduced port loading, radiant burners could achieve < 10 ppm NOx O2-free. In combination with staged combustion, NOx emissions < 10 ppm O2-free was experienced. With proper location of heat exchangers, higher thermal efficiency can be obtained.
Radiant burners are normally larger than blue-flame burners. Modification of other components is often required. Pressurisation of the fuel/air delivery system and filtering may be required depending upon burner port size. Usually the combustion chamber is reduced but the ignition system would require upgrading. The heat exchanger would have to be relocated closer to the burner.
- Catalytic Combustion
Catalytic combustion may be fully catalytic (or simply catalytic), or partial which is also known as catalytically stabilised (Ro and Scholten, 1997).
In catalytic combustion, a catalyst such as palladium or platinum is used to reduce the activation energy of combustion and allow the fuel gas to be oxidised by air at a low temperature of 500–1000°C. The reaction temperature is maintained low by effective removal of heat liberated from oxidation to the heating medium. Because the reaction temperature is low, Ro and Scholten stated that NOx levels < 5 ppm could be achieved.
In catalytically stabilised combustion, part of the fuel gas is oxidised by catalytic combustion, and the remaining gas is oxidised by homogenous (blue flame) combustion after or during catalytic combustion. Providing heat is removed from the catalytic system, the product gases from catalytic combustion dilute the exhaust gases from the homogenous combustion and lower the overall combustion temperature, and hence NOx emission, in a way similar to flue gas recirculation.
Ro and Scholten compared the performance of boilers using catalytic combustion and catalytically stabilised combustion. They concluded that catalytically stabilised combustion had a higher reliability because it could be operated as a conventional radiant burner even if the catalyst was poisoned and totally de-activated, and the security and control system required for temperature/combustion control would be more easily developed. Catalytic combustion on the other hand, emitted less NOx and CO, and its method of catalyst coating was easier.
In the review performed by Raghavan and Reuther three years earlier than Ro and Scholten, a catalytic burner used in a gas-fired appliance was cited. The burner surface was a matrix of ceramic fibres interspersed with chrome (catalyst) fibres. NOx emission < 15 ppm and CO emission < 10 ppm O2-free were reported.
Catalytic converters similar to those used in automobiles were also cited by Raghavan and Reuther. The converter completed catalytically the combustion of the products from an earlier fuel-rich combustion with more cool air at a temperature < 540°C. NOx emission from this two-staged combustion was lower than that from a second stage combustion which was non-catalytic but conducted at a higher temperature.
Raghavan and Reuther suggested that the requirements of fan-assistance to overcome the problem of low temperatures and low heat fluxes, larger heat-exchange areas, and smaller combustion chamber volumes might be the main draw backs of wide application of catalytic combustion to gas appliances.
- Pulse Combustion
In this mode, combustion occurs intermittently and the combustion gases experience high temperatures for very short time only. Heat transfer from gases to heat exchange surfaces is fast due to high turbulence, which maintains a lower temperature and hence lower NOx emissions.
NOx levels of < 50 ppm were reported, and the technology had been commercialised in residential heating appliances (Raghavan and Reuther, 1994).
The noise level of pulse combustion systems would be high, and this could limit the application of pulse combustion in domestic situations.
To meet the low NOx regulations in Australia, Japan, USA and Europe, there are a number of manufacturers producing low NOx water heaters, air heaters and cooking appliances in these countries. Due to the limitation on time and resources, the following compilation of manufacturers of low NOx appliances and their technologies is indicative rather than exhaustive.
There have been a large variety of primary control technologies applied commercially to water heaters and air heaters, but few to cooking appliances. Besides the economic and regulation factors, perhaps the usage of some cooking appliances such as cooktops makes it difficult to apply some control technologies that have been used in water heaters and air heaters. For example, spillage of cooked food on burners could make the use of porous radiant burners and surface combustion burners difficult on cooktops.
There has been a new technology using glass/ceramic seals for cooktops which separates the burners completely away from the utensils. This type of cooking appliance could adopt radiant burners or surface combustion burners. The appliance is suitable for some styles of cooking such as boiling, but may not be very appropriate for high temperature or high heat transfer cooking such as wok cooking and deep frying with a flat cooktop design.
Tokyo Gas (Japan)
Tokyo Gas provides information on its website about a low noise and NOx (less than 50 ppm at 0% O2, dry basis) domestic water heater using rich-lean premixed combustion at alternate ports which has been sold by Tokyo Gas for outdoor installation. This combustion technology maintains flame stability by fuel-rich (Bunsen) combustion in some ports, and achieves low NOx and CO emissions by lean-combustion in adjacent ports.
The water heater was jointly developed by Tokyo Gas and Gastar Co. Ltd. In 1997, it was planned to replace the conventional water heaters with this new product or similar.
Mr.Shinji Tanaka, Director of Planning of Rinnai Australia, informed that Rinnai in Japan had a low noise and low NOx (less than 60 ppm at 0% O2, dry basis) water heater on the Japanese market. This is similar to that sold by Tokyo Gas but made by another Japanese manufacturer. This practice is possible because it was a joint research and development by a number of companies including Rinnai and Tokyo Gas.
The Gas Research Institute has a summary of its residential low NOx burner projects on its website. The summary indicates that Burnham and GRI were jointly developing a low NOx boiler using staged air combustion with internal flue gas recirculation.
A low NOx level of 29 ppm at 3% O2 (equivalent to 33 ppm at 0% O2, dry basis) was reported. CO was found to be less than 50 ppm air-free.
Empire Comfort (USA)
In the same summary of GRI's residential low NOx burner projects, it is indicated that low NOx wall furnace using staged air combustion with internal flue gas recirculation was co-developed by Empire Comfort and GRI.
The same summary of GRI's residential low NOx burner projects mentions low NOx warm air furnace using staged air combustion with internal flue gas recirculation being co-developed by Trane and GRI.
In the summary, it is claimed that the NOx emission was reduced from 60 ppm to 25 ppm at 3% O2 (equivalent to 68 ppm and 28 ppm at 0% O2, dry basis, respectively); CO was less than 50 ppm, air-free.
DSL Technology (USA)
Low NOx gas burner porous inserts of porosity over 90% (metallic and ceramic) and porous insert gas burners have been marketed by DSL Technologies. These are suitable for burners with low excess air (<130%).
The insert is located at the base of the blue flame from the burner ports, where the temperature is around 500°C. It contains only a fraction of the blue flame, and is prevented from glowing red hot (~ 800°C). It seems that the insert extends the residence time of the flame in the low temperature (~ 500°C) zone but shortens the residence time at the high temperature zone to reduce NO formation.
The concentration of the hydroxyl (OH) radical in blue flames is known to be above equilibrium, with the surplus greater at the base than at the tip of the flames. By prolonging the residence time at low temperature, the OH radical and other chemical species can approach closer to equilibrium concentrations. Hence the NOx formation (by Eqn. 6) is reduced.
The high temperature zone of the blue flame (~ 1600°C) is retained to reduce CO and formaldehyde formation. It is also believed that the insert preheats secondary air before it is entrained into the blue flame. This would reduce quenching, and hence NO2 and CO formation, at the outer edge of the blue flame.
It was claimed that NOx, CO and CH2O emissions could be reduced by 50–80% at 85–100% primary air aeration (Reuther and Billick, 1996; R & D Magazine, 1996). The technology was developed in Gas Research Institute (GRI), USA.
Mr. Bern Connell, the Technical Services Manager of Lennox Australia and Mr. Bill McCullough, Senior Engineer of Lennox USA indicated that Lennox's low NOx domestic air heaters in the USA used a flame impingement-quenching device in each heat exchanger for each burner (personal communications). This device is typically made of Pyrolite (soft and brittle ceramic) or stainless steel.
This technology can meet the Californian regulation of <40 ng/J (heat output) by taking full advantage of all the manufacturing tolerances and testing conditions.
Bradford White (USA)
Bradford White indicates on its website that it has residential storage water heaters (Residential Energy Saver Gas-Upright model) meeting C.E.C. Title 24, South Coast Air Quality Management District and all other district requirements. The low NOx technology adopted is not mentioned but cast iron burner is specified for LP gas heaters and is assumed to be used to achieve the low NOx emissions required.
Mr. Phill Hubbard, Manager of Research and Development of Chef, disclosed that Chef's domestic gas cooking appliances had met the Australian NOx emissions standard by effective design of combustion system which included proper port sizing, flame retention, fuel-aeration ratio and flame impingement (personal communication).
Worgas indicates in its marketing materials and on its website that its low NOx burners has been marketed and used for domestic applications in water heaters, boilers and air/room heaters. The principle of the technology was described in Section 5.5.3.
It is claimed that NOx emissions <40 ppm at 3% O2 (equivalent to 45 ppm at 0% O2) on dry basis can be achieved, and more than 4 millions of these burners have been used on the market.
Bowin mfg. Pty. Ltd. (Australia)
Bowin has been manufacturing a number of ultra-low NOx flued and flueless natural aerated and powered domestic flue heaters using Bowin's patented surface combustion technology. The technology is also applicable to domestic water heaters and cooking appliances (John Joyce, personal communication).
The Bowin low NOx technology is a hybrid of staged-premixed-radiant combustion technology with a major surface combustion preceded by a minor radiant combustion. In the Bowin burner, air and fuel gas are premixed at a ratio greater than or equal to the stoichiometric combustion requirement.
Combustion is maintained at or adjacent to a combustion surface formed from one or more layers of conductive heat resistant material such as nickel based steel mesh with uniform porosity of 20–60% (Australian Patent Document Number: AU-B-64743/90). The porosity provides a flow rate of air-fuel mixture that results in a combustion temperature of 600–900°C and radiant heat transfer that maintains the combustion temperature.
Low NOx (≤ 2 ng/J or ~ 4 ppm at 0% O2 on dry basis) and CO emissions have been achieved (as measured by The Australian Gas and Light Company (AGL)). Further reduction in NOx emission could be achieved by using baffles, barriers walls or enclosed combustion chamber to restrict or prevent cold secondary air contacting the flame before combustion is completed (Australian Patent Document No.: AU-B-16047/92).
Currently Bowin is collaborating with an Australian water heater manufacturer to develop a prototype low NOx water heater using Bowin's technology.
Alzeta Corp (USA)
A low NOx gas fired pre-mix radiant burner with a trade name as Pyrocore/Duratherm has been developed by Alzeta Corp. in the USA. The burners are fabricated from alumina-silica fibres which are formed into either cylinders or flat plates with high porosity. This technology has been used by Alzeta's OEM partner, Nuovi Sistemi Termotecnici in Italy on domestic boilers and instantaneous water heaters (Andy Minden, personal communication).
Global Environmental Solutions (USA)
An infra-red burner for glass-ceramic cooktops was exhibited by Global Environmental Solutions of San Clemente, California, USA in Domotechnica 1993, a major international trade fair for residential appliances. It was claimed to have low NOx and CO emissions.
On the website of Acotech, it is indicated that N.V. Acotech S.A., a joint venture company of Shell and Bekaert in Belgium, has been marketing its metal fibre burner technology with its related companies such as Furigas in Netherlands.
The metal fibre burner products are used for the premixed gas surface combustion. It can be operated in either radiant combustion mode or blue flame surface combustion mode. In the former mode NOx emission < 10 ppm at 0% O2 dry basis is claimed to be achieved. In the latter mode, it is claimed that low NOx levels are achieved at 30% excess air. In general, it is claimed that 30 ppm NOx can be readily achieved. CO emission is claimed to be < 10 ppm. Other advantages such as homogeneous combustion with high modulation rate, high efficiency, low pressure drop, resistance to thermal shock and flashback safety are also claimed.
This technology was used by Stoves (UK) as a rotating metal fibre burner in a residential convection oven. Its application to boilers, condensing boilers and gas stoves was also demonstrated. Major boiler manufacturers such as Vaillant, Viessmann and Buderus in Germany, Radson and Remeha in Holland, and Ecoflam and Baltur in Italy have been using Acotech's technology.
In Europe the gas supply pressure for residential usage is higher than the 1.1 kPa gauge used in many places in Australia. To apply under Australian conditions, fan driven burner system would be required (Koen De Bock, personal communication).
Cramer (1990) mentioned that pulse combustion was used in a low NOx residential furnace introduced by Lennox Industries in the USA.
Empire Comfort Systems (USA)
Cramer (1990) mentioned also that pulse combustion was applied to a residential space heater developed by Empire Comfort Systems.
American Water Heater Company (USA)
American Water Heater Company, owned by Southcorp Water Heaters Australia, has been manufacturing low NOx water heaters for the Californian domestic water heater market (Alan Law, personal communication).
Information on Delta's website reveals that a low NOx domestic hot water generator of brand "Delta" using pre-mix gas burner is on the USA market.
Teledyne Laars (USA)
Teledyne Laars mentions on its website that it has a low NOx (less than 20 ppm) large hot water generator suitable for apartment buildings.
Trianco Heatmaker (USA)
Trianco indicates on its website that all Heatmaker domestic water heaters using its patented sealed combustion technology have very low NOx emissions.
In the Netherlands, Nefit has been selling condensing boilers using closed combustion systems and fully premixed burners to control NOx formation to meet the Dutch and European standards (Heijink, 1994).
Ovenden and Williams (1997) reviewed the possible technical solutions to reduce NOx emissions from cooking burners. In the review, some work on the testing of hotplate and oven burners equipped with drop-over stainless steel "flame rings" by British Gas (UK) and Electrolux (Denmark) was reported. The results indicated a NOx reduction of 17–33% for hotplate burners and 40–60% for oven burners. The CO emission was approximately doubled for hotplate burners and maintained at low levels for oven burners.
Burner port redesign
In Japan, development work on the application of swirl combustion to cooktops/ hotplates to achieve 50 ppm NOx at 0% O2 was reported (Sudo and Nielsen, 1997).
Blue-flame burner replacement
In the report of Committee E to the 20th World Gas Conference, Sudo and Nielsen (1997) mentioned that a fully premixed flat surface ceramic burner underneath a glass/ceramic panel was under development in Austria to achieve NOx emission of 11 ppm at 0% O2.
The burner technology was cited as "fully premixed", but it seems that it is similar to the Bowin technology and hence is included in this category.
Development work in France using metallic fibre burner for cooktops/hotplates to achieve < 20 ppm NOx at 0% O2 was reported (Sudo and Nielsen, 1997).
In Germany, a radiant burner and a ceramic burner for cooktops/ hotplates were under development to achieve 20–30 ppm NOx at 0% O2 (Sudo and Nielsen, 1997).
In the UK, a grill using surface combustion and forced aeration was under development to achieve low NOx emission (Sudo and Nielsen, 1997).
Raghavan and Reuther (1994) summarised the status of different primary NOx control technologies. Their comparison between NOx control technologies for domestic gas appliances is reproduced in Table 15. In general, the primary NOx control technologies can reduce NOx emissions to < 70 ppm O2-free.
|Primary NOx Control Technology||Lowest NOx(ppm, O2-free)||CO Emissions (Change)||Thermal Efficiency (Change)||Technology Status|
|Premixed, high excess air||~ 20||Decrease||Decrease||Current|
|Flue-gas recirculation||~ 25||Increase||Decrease||Emerging|
|Staged combustion||~ 25||Increase||Decrease||Emerging|
|Delayed combustion||~ 25||Increase||Decrease||Emerging|
|Humidified combustion||~ 25||Increase||Decrease||Future|
|Flame inserts||~ 40||Increase||Decrease||Current|
|Thermally active burner||~ 65||Decrease||Increase||Current|
|Port-loading reduction||~ 50||Increase||Increase||Current|
|Port redesign||~ 50||Decrease||Increase||Current|
|Radiant combustion||~ 15||Decrease||Increase||Current|
|Catalytic combustion||~ 10||Decrease||Decrease||Future|
|Pulse Combustion||~ 20||Increase||Increase||Current|
Source: Raghavan and Reuther (1994)
Ro and Scholten (1997) summarised the NOx emissions achieved by various types of burners. The results are reproduced in Figure 1:
Source: Ro and Scholten (1997)
The summary by Ro and Scholten is similar to that by Raghavan and Reuther, except that the emission of catalytic combustion was significantly lower. Definitions of some the burner types are as follows:
- Conventional burner – Burner with the dual flame structure and the primary air rate less than 100% stoichiometric.
- Burner with ceramic bar – Burner with flame insert.
- Water cooled burner – Burner designed for staged combustion.
Combining with the reported emission levels as compiled in Section 5.5, Section 5.6 and Section 5.7, Table 15 is updated as given in Table 16.
|Primary NOx 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 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.
Current regulations have NOx emissions limits set close to the minimum achieved by some technologies. If the NOx emissions limits are going to be set lower in the future, some technologies may require further development or may even not be acceptable anymore. Other technologies that have already achieved lower NOx levels could still be used readily.
In Australia, the gas line pressure for domestic use is not uniform. For example, it is mainly 1.1 kPa gauge in Victoria, but 2.75 kPa in the ACT and some parts of NSW.
There have been two low NOx technologies developed and commercialised in Australia suitable for a gas line pressure of 1.1 kPa, namely the staged surface combustion technology developed by Bowin mfg. Pty. Ltd. for space heaters and water heaters, and the burner port re-design developed by Chef for cooking appliances.
In particular, Bowin has a range of air heaters equipped with low port loading burners which operate with natural aspiration. It also has a design for water heater which utilises natural draught to create a low vacuum inside the combustion chamber. The low vacuum effectively increases the pressure differential across the combustion system to increase the flow of aspiration air to the burner. In addition, Bowin manufactures a number of unflued air heaters with medium or high port loading burners which use warm-air-circulation fans to induce the low vacuum, and flued air heaters with high port loading burners and induced draught fans to create the low vacuum.
Other primary low NOx technologies were developed mainly in Japan, USA and Europe for gas line pressures ≥ 2 kPa gauge. Some of these technologies do not require fan assistance if the gas line pressure is above 2 kPa, but when applied under the low gas line pressure (1.1 kPa) condition in Australia, a fan-assisted combustion system may be required. For example, the low NOx burners developed by Tokyo Gas and Worgas (Section 5.6.7) are expected to be functional without fan assistance at a gas line pressure of 2.75 kPa.
To allow these overseas low NOx technologies to be more readily applicable in Australia, one possible approach is to raise the gas line pressure if this can be done without replacing the existing pipelines and other hardware. Re-setting of pressure regulators and revision of gas consumption and billing calculations will be required. The former is labour intensive and could cost millions of dollars. To assess more accurately the cost to the gas distribution industry and the likeliness of its involvement, the participation and cooperation of Australian gas distribution industry would be required.
NOx can be removed from combustion exhaust gasses in three approaches:
- Selective catalytic reduction (SCR).
- Selective non-catalytic reduction (SNCR).
- Hybrid SNCR/SCR.
These technologies are expensive because consumable reagents and additional NOx removal systems are introduced. Moreover, the additives such as ammonia if not consumed in the process will escape to the atmosphere which would lead to NOx. Until now, applications of secondary control are mostly to power generation and other industrial combustion processes.
In this approach, a reductant such as ammonia vapour is injected into the low temperature (300–400°C) flue gas in the presence of a catalyst such as titanium oxide, zeolite, iron oxide or activated carbon. In the case of using ammonia, ammonia reduces NO and NO2 by the following overall mechanism:
Eqn. 13. 4NO + 4NH3 + O2 → 4N2 + 6H2O
Eqn. 14. 2NO2 + 4NH3 + O2 → 3N2 + 6H2O
This kind of technology can achieve up to 95% NOx removal. However, catalyst can be degraded by sintering, poisoning and other means common to catalyst applications. Another possible form of using catalyst is to reform the fuel gas to CO and H2 before combustion. NOx formation is restricted because CH radicals are absent in the flame.
Without a catalyst, ammonia or urea can still reduce NO, but only at a higher and narrower range of temperature between 900°C and 1100°C (Warnatz et al, 1995). The reduction of NO by urea is by the overall reaction (Eqn. 15).
Eqn. 15. O + 2CO(NH2)2 + O2 → 4N2 + 2CO2 + 4H2O
In boiler applications, reduction of 30–50% can be achieved. Comparing with SCR, SNCR requires higher stoichiometric ratios (3–4 times) than those for SCR.
Combining SNCR injection of reagent into the boiler with SCR catalyst can achieve up to 84% reduction.