Numerical And 0.3MWth Experimental Investigation Of MILD-Oxy Combustion

Oxy-fuel combustion technology,so far,is the most promising carbon capture technology onthe domain of fossil fuel combustion,which can effectively decrease the CO2emission in the power plant.Meanwhile,MILD combustion technology is one kind of high-efficient combustion technology,it is considered to achieve both energy efficiency and low pollutant emission.Moreover,a new-style combustion technology is also presented through integrating the Oxy-fuel combusting and the MILD combustion,which is expected to achieved advantages of both two technology.However,about the MILD and MILD-Oxy combustion,some issues are still to be figure out,for example the combustionmechanism,criterion and method for the establishment of MILD condition,etc.And the investigation on these issues is the aim of this dissertation.In the Oxy-fuel combustion of pulverized coal,the high concentration of steam will largely affect the combustion characteristics on physical and chemical sides.A numericalinvestigation was presented on these effects in this dissertation.The results showed that under low oxygen concentration,the effect of heat capacity wasdominant;as a result,increasing the steam concentration would decrease the combustion temperature.In contrast,under high oxygen concentration,the effect of radiation heat transfer wasdominant;while increasing the steam concentration would decrease the combustion temperature.Additionally,increasing the steam and oxygen concentration would promote the gasificationreaction,which tends to decrease the temperature gradient in the reaction zone.The Damkohler number Da is considered as an important criterion for the establishment of MILD combustion.The value of Da should approximate unity for the ideal MILD combustion.This dissertation numerically discussed how the preheating temperature,jet velocity and CO2 dilution affect the value of Da.The results showed that increase the jet velocity can largely decreased the value of Da which would approximate unity as the jet velocity was high enough.The CO2 dilution can also decrease the value of Da.Decreasing the preheating temperature would slightly decrease the value of Da.The steam and oxygen concentration have important effect on the MILD-Oxy combustion of pulverized coal.This dissertation also discussed this effect from the aspect of Damkohler number Da andparticle surface reactions.The numerical results found that increasing the oxygen concentration would increase value of Da for all particle surface reactions,especially for carbon oxidation reaction.It means the high oxygen concentration would drive the MILD combustion to the flame combustion in some extent.Increasing the steam concentration would increase the surface diffusion coefficient of oxygen,and in turn increase the diffusion rate of oxygen molecule on the particle surface.Compare to that in the swirl-flame combustion,the carbon consumption ratio of gasification reaction is much higher in the MILD combustion and it also higher than the ratio of carbon oxidation reaction.The dissertation also presented a pilot-scale experiment study on MILD combustion of pulverized coal with high-velocity jet and low preheating temperature.The results showed that in the MILD combustion,the reaction zone was extremely enlarged and taken by the sparks and flame fragments.The NO emission in the exhaust gas decreased because of the low temperature and the atmosphere with high CO concentration in the MILD combustion.However,the high-temperature preheating still has advantages in terms of establishment MILD combustion,because the combustion stability decrease and both the temperature fluctuation and ignition delay increased near the burner exit under low preheating temperature.A pilot-scale experiment of MILD-Oxy combustion of pulverized coal was achieved under low preheating temperature by increasing the jet velocity,in which an entirely flue gas recycling system was integrated and the working medium was the mixture of flue gas and pure oxygen rather than air.The experiment results showed that the high jet velocity was the key to maintain the MILD-Oxy combustion under low preheating temperature.Similarly,compared to that in the swirl-flame Oxy-fuel combustion,the combustion temperature,temperature gradient and NO emission decreased in the MILD combustion.The air leakage largely increased because of the high jet velocity,which is 23.1%in the MILD-Oxy combustion,and only 8.9%in the swirl-flame oxy-fuel combustion.