Thermal Process And Performance Of Double-P Radiant Tube For High Efficiency And Low NOx Emission

At present, low thermal efficiency, uneven temperature distribution of wall and high NOx emissions are main problems of the radiant tubes in the heat treatment industry. To solve the above problems, experiment and numerical simulation method are employed to investigate the thermal process and performance of the double-P radiant tube.According to thermal process analysis of the double-P radiant tube, the mathematical models of gas flow, heat and mass transfer and combustion are established. Then,analyzed the rule of the thermal efficiency, the flue gas recirculation ratio and the uniformity of wall temperature under the varied jet velocity, tube body structure and process parameters. The results show a significant increasing of flue gas circulation ratio with the decrease of the burner nozzle diameter; meanwhile the wall temperature uniformity is also improved. Increasing the diameter of the branch pipe can raise the thermal efficiency and improve temperature uniformity of the wall, and increase the degree of flattening can raise the thermal efficiency, but lead to worse temperature uniformity. Based on the off-stoichiometric combustion principle, the zoned combustion method is proposed to study the zoned combustion mode of air, fuel and air-fuel, thermal process and thermal characteristics of the double-P radiant tube. The results show that the zoned combustion method is advantaged to the original method to temperature uniformity performance. As the second zone nature gas proportion less than 10%, the thermal efficiency, the flue gas recirculation ratio and wall temperature uniformity are better than there in the air zoned combustion mode.Secondly, to test the thermal performance and NOx emissions double P type radiant tube, and to verify the reliability of the mathematical model, the effects of the position of combustor nozzle on the performance of were investigated experimentally. The results show that thermal efficiency of the double-P radiant tube was more than 70%. Wall temperature uniformity is optimal when the nozzle is in the position of three tube center intersection, whereas the NOx emission increases. Move-out of the nozzle position can increase the thermal efficiency, but the temperature uniformity of the wall performs worse. And NOx emission is lowest when nozzle is in the front of flue gas outlet. Later, based on the theory of the off-stoichiometric combustion and thermal performance characteristics of the fuel zoned combustion mode, the fuel two-staged combustor was adopted. Then NOx emissions and wall temperature distribution of double-P radiant tube with the fuel two-staged combustor were investigated by experiment study. The results show that the NOx emissions of double-P radiant tube with the fuel two-staged combustor is 56% lower than the original design, and wall temperature uniformity is also improved.Finally, NOx emission of double-P radiant tube with the fuel three-staged combustor was studied, under the varied air preheating temperature, air consumption coefficient, and secondary air ratio. The results show that the increase air preheating temperature and decreasing of the air consumption coefficient can raise the thermal efficiency but the NOx emission is also raised. And increasing the ratio of secondary air and the second gas can reduce NOx emissions. Then, the orthogonal experiment design method was used to analyze the effect of main parameters. Optimization shows that increasing the secondary fuel nozzle diameter not only reduce NOx emissions and improve the wall temperature uniformity,but also make the thermal efficiency raise. Base on the result of the optimization, a new type of fuel dispersion combustor was developed and studied the effects of the fuel dispersion pipe structure to the thermal performance of double-P radiant tube.Results show the minimum NOx emissions is 45.6 ppmv as the diameter of dispersion pipe is 10 mm, the dispersion hole spacing is 60 mm and the dipersion hole diameter is 5 mm.The research in this thesis can provide theoretical guidance for developing high efficiency and low NOx emission radiant tube, which point out the optimization direction of combustion structure, and provide theoretical and technical support to accelerate the industrial application of double-P radiant tube.