612 / 2019-01-20 21:38:15

Expreimental Study of Pressure Oscillation in Gas Boilers of Ultra-low NOx Combustion

ultra-low NOx combustion; flue gas recirculation; combustion instability; pressure oscillation

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Under some conditions of introducing flue gas recirculation, the boiler has a surge phenomenon, which causes a flameout in severe cases, which seriously affects the safe operation of the boiler. In this paper, the surge phenomenon in the ultra-low NOx fully premixed combustion process of a small semi-industrial combustion test bench with flue gas recirculation system is studied, and the influence of flue gas recirculation rate on the pressure characteristics of the combustion process is studied. The furnace has a diameter of 0.4 m and a length of 2.0 m. While studying the dynamic characteristics of the pressure field in the combustion process, the influence of the mixing uniformity of the recirculated flue gas and air on the surge of the combustion system was analyzed. The results show that with the increase of flue gas recirculation rate, the characteristic frequency of the pressure field of the combustion chamber becomes obvious, and the maximum amplitude of the pressure decreases first and then increases. During the surge process, when the recirculating flue gas and air are not uniformly mixed, the pressure oscillations in the combustion chamber and the flue gas recirculation pipeline tend to be the same in frequency and in phase. When the recirculated flue gas and air are uniformly mixed, the coupling relationship between the pressure oscillations in the combustion chamber and the flue gas recirculation pipeline is significantly weakened. The mixing uniformity of the flue gas and the air has a great influence on the surge phonomenon. The more uniform the mixing is, the larger the range of the flue gas recirculation rate can be adjusted for stable combustion. The characteristic frequency of the combustion chamber is about 20 Hz when the combustion is stable, and the characteristic frequency is about 2 Hz when the surge occurs. During the process from the occurrence of the surge to the flameout, the pressure amplitude of the frequency of 20 Hz gradually decreases while the pressure amplitude of the frequency of 2 Hz increases, finallly, the pressure oscillation with a frequency of 2 Hz is dominant. As the combustion load increases, the limit value of the flue gas recirculation rate corresponding to the surge increases. As load increases, the range of stable combustion is enlarged.