Experimental And Numerical Studies On Ignition Promotion And Burnout Inhibition During Blended Coals Combustion

In China,blended-coal combustion technology has been widely used in electric power plants.During combustion process of blended coals,there exist interactions owning to the difference of the combustion characteristic of component coals.On the one hand,the volatile of the high-volatile coal preferentially releases and burns to form a high gas temperature environment,which promotes the devolatilization and ignition of the low-volatile coal.On the other hand,oxygen is rapidly consumed by the high-volatile coal,which delays the char combustion and burnout of the low-volatile coal.Compared with those in the O₂/N₂ atmosphere,some factors such as gas physical property change and char gasification reaction under the high CO₂ concentration condition,affect chemical reaction process,heat and mass transfer.These will have complex influence on the ignition promotion and burnout inhibition.Thus,it necessary to study the ignition promotion and burnout inhibition,wich can provide useful information for the combustion optimization of blended coals.In this paper,experimental study and modelling are performed to investigate the effects of different conditions on ignition promotion and burnout inhibition under the high CO₂ concentration condition,which mainly include the followings:Firstly,the effects of different conditions on ignition promotion and burnout inhibition are investigated under the region of low temperature and any oxygen condition.The volatile combustion increases the promotive and inhibitive effects.Compared with that in the O₂/N₂ mixture,the promotive and inhibitive effects for blended coals is weaker in the O₂/CO₂ mixture;but the inhibitive effect for blended chars is stronger in the O₂/CO₂mixture,because the influence degree of volatile combustion and gas physical properties on char combustion is different.Increasing the O₂ concentration or decreasing the particle size result in decreasing the promotive effect and increasing the inhibitive effect.However,they change slowly when O₂ concentration exceeds 40%.Enhancement of the heating rate increases the intensitiy of promotive effect and decreases the intensitiy of inhibitive effect.Under the high CO₂ concentration condition,H₂O increases the promotive and the inhibitive effects.Increasing H₂O/CO₂ ratio enhances the promotive effect and the inhibitive effect;increasing O₂/H₂O ratio or increasing O₂/CO₂ ratio decrease the promotive effect and increase the inhibitive effect.Under the region of low temperature and any oxygen condition,the ignition temperature of blended coal is lower,the ignition is easier,the ignition promotion is weaker;the burnout temperature of blended coal is lower,the burnout is easier,the burnout inhibition is stronger.Secondly,the effects of char-CO₂ gasification reaction on the promotive and inhibitive effects are invesigated under the region of high temperature and low oxygen condition.Enhancing char-CO₂ gasification reaction increases the promotive effect and decreases the inhibitive effect.Enhancing char-O₂ reaction decreases the promotive and inhibitive effects.Enhancing char-CO₂ gasification and weakening char-O₂ reaction increase both the promotive effect and the inhibitive effect.Under the region of high temperature and low oxygen condition,the ignition temperature is lower,the ignition is easier,the promotion is weaker;the burnout temperature is lower,the burnout is easier,the inhibition is weaker.Then,experimental study and modelling are performed to investigate the effects of char-CO₂ gasification reaction and gas physical property change on the intensities of the promotive and inhibitive effects under the region of high temperature and high oxygen condition.The char-CO₂ gasification reaction is weak in the devolatilization stage,and has little effect on the devolatilization and ignition promotive effect.But in the stage of intense combustion,the char-CO₂ gasification reaction improves the reaction rate of less-volatile coal char,and decreases the inhibitive effect.Compared with that in the air,the promotive effect is weaker,but the inhibitive effect is stronger in the 21%O₂/79%CO₂ environment.Increasing the oxygen concentration or decreasing particle sizes result in increasing the promotive effect but decreasing the inhibitive effect.In addition,the promotive and inhibitive effects achieve a level similar to that in the air when the oxygen concentration increases to approximately 30-35%.Increasing excess oxygen coefficient or the reactor wall temperature weaken the intensities of the promotive and the inhibitive effects.Under the region of high temperature and high oxygen condition,the ignition is easier,the devolatilization and ignition promotion are stronger;the burnout is easier,the char burnout inhibition is weaker.Finally,the effects of ignition promotion and burnout inhibition on the blended coal combustion,and the optimization of the blended coal combustion are carried out based on the established model.Under out-furnace blending method,the ignition promotion can obviously improve the ignition behavior of the blended coal,but the burnout inhibitive effect lowers the burnout degree of less-volatile coal,which has a negative effect on the total burnout degree of the blended coal.In the high CO₂ concentration,increasing the oxygen concentration can enhance the burnout degree of less-volatile coal,and then improves the burnout behavior of the blended coal.The in-furnace blending method is advantageous to improving the burnout degree of the less-volatle coal.Due to the difference of boiler types and burner layouts,the effects of different factors on the burnout characteristic are different.For the tangentially-fired boiler with out-furnace blending method,due to the obvious influence of the residence time on the burnout of the less-volatile coal,the burnout of less-volatile coal sent into the furnace from the middle and upper burners is relatively low,and has a great impact on the total burnout of the blended coal.The less-volatile and blended coals can obtain high burnout degree after adopting the"Zhengbaota"wind distribution.Under in-furnace blending method,the less-volatile and blended coals can obtain high burnout degree when the less-volatile coal is injected into the furnace from the below burners.Under in-and out-furnace blending method,the less-volatile coal is blended with a small proportion of the more-volatile coal into the blended coals,and then the blended coals are injected into the furnace from the below burners.This can not only improve the flame stability but also significantly improve the burnout degree of the less-volatile and blended coals.For the W-flame boiler with out-furnace blending method,the burnout degree of the less-volatile coal corresponding to the burner in the central area of the furnace is relatively low,and has a great impact on the total burnout of the blended coal.In order to weaken the inhibitive effect and improve the burnout of the less-volatile coal,the more secondary air in the back-fire side near the central area but the less secondary air in the face-fire side should be adopted,the over fire air ratio should not be too high,and the concave arch air distribution should be avoided.