Heat recovery from exhaust gases and flameless combustion – this is the magic formula in many high-temperature processes in order to combine optimal fuel usage with low nitrogen oxide emissions. With a new recuperator burner, this technology now also offers advantages in the lower range of outputs under 100 kW. The exhaust gas losses have been almost halved compared to conventional recuperator burners. A number of burners of this type have been undergoing tests since early in 2009 in heat-treatment equipment for screws.

Recuperator burners use the exhaust gas from the combustion chamber to preheat the incoming combustion air. The heat exchanger geometry and surface determine the degree of waste heat recovery. The flow configuration of the exhaust gas presents a challenging optimisation problem, particularly in smaller burners. Researchers at WS Wärmeprozesstechnik GmbH have achieved a larger heat exchanger area and improved heat transfer by distributing the combustion air over a number of individual heat exchangers. The high heat transfer is obtained thanks to the flow of air and exhaust gas in narrow gaps.
The so-called gap-flow burner works on the principle of flameless oxidation. It is possible to maintain combustion without a flame thanks to the carefully designed mixing of combustion gas, combustion air and recirculating exhaust gas. This avoids temperature peaks in the furnace, which are responsible for most of the nitrogen oxide emissions.

Background: Flameless combustion

Experiments carried out on a recuperator burner resulted in a surprising discovery in 1989: at furnace temperatures of 1,000 °C, and with air preheating of 650 °C, the monitoring device for the burner flame ceased to indicate a signal, and no flame could be heard. Nevertheless, the fuel was being burnt completely, and combustion was stable and uniform in the absence of a flame.
The researchers were able to determine the conditions under which flameless combustion is possible in further experiments as part of the research project sponsored by the German Federal Ministry of Economics and Technology. It soon became evident that this process resulted in considerable energy savings and lower emission values. For this reason, the Ministry approved a large number of follow-on projects that gradually expanded the range of possible applications. Today, attention is focused on the use of lean gases and biofuels, and on power plant technology and cogeneration. Equipment was also developed for the reformation of natural gas to yield hydrogen. The results of these research projects are presented in two BINE ‘Projektinfo’ publications from the years 2006 and 2008.