Heating without polluting THE AIR

Domestic heater designers could benefit from a new means of combustion

High ozone levels, summer smog in cities, the threat of irreversible changes in the climate: the only answer is a drastic reduction in air pollution. Many critics have latched onto cars and power stations as scapegoats, while overlooking the fact that a third of our energy consumption and the resulting emissions can be attributed to domestic heaters.

Modern technology offers astonishing potential savings in this field, but in practice very few heating systems are optimised. On the contrary, older systems are often not even properly serviced and adjusted.

New laws in Germany, at least, will force householders to modernise their boilers. Those laws on small-scale combustion equipment, which came into effect three years ago, were made more stringent with effect from November 1 1996. Many older systems will not be able to meet them. As a result, a huge market will exist for low-emission burners.

Whenever fossil fuels are burnt, they produce exhaust gases. Besides carbon dioxide (CO2), there are smaller quantities of the poisonous carbon monoxide (CO), various oxides of nitrogen (NOx), and other air-borne pollutants. These gases are led out through the chimney, where they pollute the atmosphere.

But it would be better to prevent the toxic gases being produced in the first place, and that means choosing a completely different method of burning, namely flameless catalytic combustion.

This new heating technology uses a mixture of air and natural gas that is not burnt with a flame but oxidised using a catalyst. The Fraunhofer-Institut fur Solare Energiesysteme ISE (Fraunhofer Institute for Solar Energy Systems) in Freiburg in Germany has

developed such a flameless burner that is currently undergoing practical trials in a pilot scheme.

All conventional burners have the drawback that temperatures of over 1200iC and reactions in the flame front lead to generation of nitrogen oxide and other harmful substances. Catalytic combustion, however, tackles the problem at the root: there is no open flame, and the temperature is kept below 1000iC. Now, scientists at the ISE have succeeded in combining a normal commercial honeycomb catalyst structure with a cooling device in such a way that the catalytic burner can produce a high power output and still function reliably.

Once the catalyst has been preheated, stable and almost 100% complete combustion begins in the fine honeycomb structure. The resulting heat is given up to the flue gas and also by radiation to the cooling plate below it. The water circulating in this plate heats up; at the same time the design ensures that more heat is extracted from the catalyst as the rate of combustion increases. This self-regulating process prevents the material overheating.

The ISE fitted a prototype catalytic burner with 10kW output to a normal commercial boiler. The results of several hundred hours of operation were very encouraging: the NOx values in the flue gas were only about half a milligram/kilowatt hour, conventional flame burners produce 100 to 200 times as much, depending on the design. The CO and unburnt methane emissions were both under 5mg/kWh, close to the detectable limit.

Investigations by the ISE show that the principle of a catalytic burner can be varied for use in other applications: for example, heating the air in tents, caravans, or portable cabins on building sites. Another application for the new technology would be catalytically accelerating chemical reactions in reforming natural gas.

{{Fraunhofer-Institut fur Solare Energiesysteme ISETel: Germany +49 7614 588 213Enter 420}}