Studies Of Mild Combustion Technology Applied To Natural Gas Boilers

Industrial emissions cause frequent fog and haze, lead to more and more serious atmospheric pollution. Nitrogen oxides, sulfur dioxide and dust particles relaeased from combustion are the main cause of air pullotion. Efficient and low-emission combustion technologies are important means to alleviate the fog and haze. Among these technologies, Moderate or Intense Low-oxygen Dilution (MILD) combustion is a potential and advanced combustion technology due to its special frature. MILD combustion’s application in Gas boilers makes it possible to reduce pollutants, especially NOx emissions. While the complete combustion and uniform temperature distribution in boils can improve thermal efficiency and extend service life of boils. Tt is necessary to perform research on MILD combustion for the purpose of NOx reduction.With Fuel and oxidizer’s high speed jet, Mild combustion has negative conditions in chamber. Strong exhaust gas recirculation is applied in MILD combustion to preheat and dilute fresh reactants before the spontaneous ignition, which facilitates the occurrence of volumetrically distributed reaction, the reduction of peak flame temperature and NOx/CO emissions and, the uniform distribution of temperature across the combustor exit.The present investigation aimed to report the thermodynamic conditions of MILD combustion when used in gas boilers. A zero dimensional model was used to study the self ignition temperature, the concentration of oxygen and gas recirculation ratio of MILD combustion. And the influence of gas recirculation ratio on ignition delay time was also studied. The results showed that, MILD combustion can be realized only when temperature is above ignition temperature and gas recirculation ratio is greater than critical value. Igniton delay time decreases with the increase of gas recirculation ratio and increase with the increase of excess air coefficient.The influence of structure parameters on MILD combustion was obtained by means of numerical simulation. Chamber structure parameters include height, the fuel/air nozzle positions and diameters, the gas outlet chammel size. The simulation results showed that the flow structure is controlled by air jet. The gas entrainment rate and recirculation ratio are increased when air velocity is increased. The increasing of air velocity also causes more dispersed oxygen concentration and more uniform temperature distribution in combustion chamber. While the fuel jet velocity mainly affects the mixing time of air and fuel. The reactant can be fully mixed before reaction when mixing time is short. Therefore high fuel jet velocity is needed. Based on the simulation, an optimization model of combustion chamber was formed.Experimental studies were carried out on the processed combustion chamber aimed to study the MILD combustion experimental performance. NOx emissions of MILD combustion aregreatly reduced compared with diffusion combustion. The NOx emissions have a concentration range of10-17ppm@3%O2, which is less than diffusion combustion. The completeness of reaction and the residence time of flue gas determine the CO emissions. When air coefficient is increased, CO emissions decrease at first and then increase. This work could provide theoretical basis and experimental data for development of gas boilers with MILD combustion.