High temperature furnaces, such as those used in the metallurgical industries for metal melting and reheating, are notoriously inefficient because of the large amount of heat leaving the furnace in the flue gases. Over the years this heat has been captured by incorporating waste heat recovery systems to preheat the combustion air. This can be done using recuperative burners or more recently, the development of compact, high-effectiveness regenerative burners. The regenerative burners can preheat the air to well over a 1,000°C, producing energy savings of around 60 per cent. However, these levels of air preheat temperatures result in increased flame temperatures, which produce high levels of oxides of nitrogen (NOx).Methods to reduce NOx emissions are well established and can be achieved by either fuel or air staging, or flue gas recirculation (FGR). Modest levels of air staging or FGR can achieve 50 per cent reductions in NOx. A new technology, known as flameless combustion, extends these techniques to high levels, resulting in dramatic reductions in NOx while maintaining efficiency and lower CO2. In addition, flameless combustion burner systems have shown to give uniform heat flux distributions, which can extend plant life, improve product quality and increase production rates.
In a furnace operating in the flameless combustion mode no visible flame exists (Figure 1). The furnace itself effectively acts a chemical reactor with the combustion process filling the high temperature environment. Clearly there are safety concerns, as a conventional flame detection system no longer sees a flame. For this reason, the flameless combustion mode normally operates when the furnace walls are well above the auto-ignition temperate of the fuel. Thus when firing natural gas, the furnace would be at around 800°C, giving a safety margin (below this temperature the burner(s) operate conventionally).
To date, the main applications of flameless combustion have been limited to the iron and steel industries, but the technology can be utilised in other industries. Unfortunately, the wider application is held-back because little is known about the principles governing combustion and heat transfer of the flameless combustion/furnace systems. To meet this dearth of knowledge, and to provide design information to extend the technology, the International Flame Research Foundation (IFRF) has commissioned a semi-industrial scale furnace fired with a pair of regenerative flameless-combustion burners.
The new furnace forms part of the £2M, four-year, development programme funded by the IFRF Members, Corus, Gasunie and Novem. Its flexible design allows for variable heat densities to simulate different product requirements. It is highly instrumented and has excellent access to the combustion chamber allowing both conventional and laser-based instrumentation. These measurements will provide essential data for the development of design guidelines, including zone and CFD mathematical models to simulate the industrial practice.
The new furnace was successfully installed and commissioned in March and the initial trials have demonstrated it is meeting its design objectives. Preliminary results were reported at the IFRF conference - Challenges in the Development of the IFRF High Efficiency Combustion Programme. These showed NOx levels of around 160mg/m3 for air preheats around 1,250°C. These compare with levels around 3,000mg/m3 for a regenerative burner without any NOx control techniques.