This paper presents the experimental results of High Temperature Air Combustion (HiTAC) investigations with the use of a radiant tube, in order to compare the Regenerative System (RS) with a conventional recuperative system.
For this work a semi-industrial HiTAC test furnace was equipped with the W-shape Radiant Tube. The working length of this tube was around 7,0 m and the diameter was 0,195 m. The radiant tube was operated in sequence with a conventional recuperative system and a Regenerative System. The recuperative burner was mounted in the upper end of the tube. The RS consisted of two burners, equipped with honeycomb ceramic regenerators, mounted to both ends of the radiant tube. The temperature profile of the tube wall was monitored by 74 thermocouples located along the tube. Additional temperatures, flow rates and pressures were measured to assess and compare the energy balance of both systems. Pollutant emissions, including NOx and CO, as well as the exhaust gas composition were measured. The tests were carried out over a wide range of parameters: firing power from 75 kW to 155 kW, furnace temperature from 670°C to 950°C and an oxygen molar fraction in the exhaust gases set at 3%. LPG was used as a fuel in all tests.
Test results show that the temperature profiles along the tube were more uniform when the regenerative system was used. The cross-sectional temperature distribution for the tube was also more uniform. Because of the relatively flat temperature distribution along the tube, more energy from the radiant tube can be emitted using RS in comparison with the conventional recuperative burner, for the same maximum temperatures of the tube. In certain conditions, the increase of energy release can be up to 100%. Energy balance calculations show that the efficiency of the Regenerative System can be up to 25% higher than that of the recuperative system, mainly due to very low temperature of flue gases for RS operation. Although, the preheated air temperature used for combustion was much higher in the case of the regenerative system (in some tests as high as 960°C), the NOx emission was found to be almost the same in both cases.