Study On The Heat Flow Ignition Of Coal Based On The Semenov's Theory

Coal is the main source of thermal energy generation,and this trend is still lasting in the future.The ignition behavior and reactivity of coal,char,and activated carbon particles are of considerable importance in the design of boilers,control of flame stability,formation and emission of pollutants,flame extinction,the prevention of spontaneous combustion of stockpile coal in mining sites and power plant.This paper is based on Semenov ignition theory(TET)and heat flux TG-DSC analysis technique to study the characteristic temperature(mainly ignition temperature),ignition heat of coal/char combustion,and the heat release rate,heat acceleration and the corresponding conversion rate in the process of coal/char combustion;and comparing the ignition temperature under TGA experimental conditions with the ignition temperature of one-dimensional furnace,to obtain the valuable research results for practical industrial applications.Firstly,a heat diagram was proposed based on the TG-DSC thermal analysis,and a detailed theoretical model of thermal analysis of coal/char was established to analyse the functional relationship between the heat changes and TG analysis;according to the TET theory,the temperature corresponding to the inflection point on the DSC curve is the intensive ignition temperature Tig,and the tangent of Tig which corresponding to the ambient temperature is consistent with the industrial furnace wall temperature.The concept of heat acceleration was firstly proposed to reveal that the intensive ignition temperature is the highest ignition temperature of coal combustion.Secondly,the TG-DSC results show that under the air atmosphere,the peak value of DSC curve increases with heating rates increasing,while the temperature corresponding to the maximum rate of heat release shows a delay time,the temperature corresponding to the maximum acceleration of heat release(Tamax)and the peak value of acceleration(amax)increase as heating rates increase.As the temperature corresponding to the maximum acceleration of heat release(Tamax)decreases and the peak value of acceleration(amax)increases,the activity of coal char increases.The increase of temperature of char prepared would increase the ignition temperature Ti,flame extinction temperature Te,burnout temperature Tb,the intensive ignition temperature Tig and the temperature corresponding to the maximum heat release point TDSCmax.With the increase of coal rank,Ti,Te,Tb,Tig and TDSCmax also both increase.Under the oxy-fuel combustion,as the oxygen concentration increases from 21% to 100%,Tamax trends to decline while amax ascends.In the same oxygen volume concentration(21%),the value of amax in air is greater than that in oxy-fuel,we can obtain that the reactivity of char in air is higher than that in oxy-fuel combustion,while coal rank increases,Tamax ascends and amax declines,indicates that the activity of coal char decreases.The characteristics of heat release of the char samples were studied through the DSC curve.The reaction of the char combustion is divided into three distinct zones by using the inflection points of DSC curve:the ignition zone of char,the burning zone of char and the burnout zone of char.The ignition zone of char contains complex heat changes,for example,the heat of pyrolysis,the heat of capacity and the heat of ignition;the burning zone of char is mainly the heat release of char combustion and the burnout zone of char is the burnout of remaining combustible matter and the endotherm of ash because of heat capacity.A linear relationship is found between the acceleration of heat release of char and the ambient temperature in different reaction zones.Furthermore,the combustion characteristics of different ranks of seven coals were studied by using TG-DSC.The results show that the low-temperature oxidation of pulverized coal samples mainly occur markedly from 150?~200?.With the increase of coal rank,the weight increasement though oxygen absorbed is decreasing.An inverse relationship is found between the heating rate and oxygen absorbing capacity.The maximum heat flow of pulverized coal after oxidation and the heat release acceleration increase with increasing the heating rate.At the same heating rate,as coal rank increases,the heat release acceleration decreases.When coal particle reaches ignition temperature,the weight loss will decrease as the increase of coal rank.On the basis of the weight loss,the amount of air required for ignition of pulverized coal was calculated.The amount of the primary air in homogeneous ignition is the air required for volatile combustion,the amount of the primary air in heterogeneous ignition is including both the volatile combustion and part of the fixed carbon combustion.The primary air ratio calculating by TG-DSC and industrial coal-fired boilers have a certain relationship.As the increase of the content of volatile matter,the primary air ratio is gradually increasing,the value of the primary air ratio obtained from TG-DSC are in the recommended range of the boiler manual.Finally,the combustion characteristics of different ranks of five coals were studied by using one dimensional furnace.The results show that the ignition temperature of pulverized coal decreases with volatile matter increasing,and the ignition temperature of lean coal is higher than bituminous coal.The distance of ignition exhibits the same trends with the ignition temperature,which increases with the rank increasing.Though the heating rate and the flow conditions are different,the difference between the ultimate intensive ignition temperature according to the theoretical calculations of TG-DSC,and the ignition temperature of one dimensional furnace,is about 30~100?.It can be concluded that the variation trend of the ultimate intensive ignition temperature of TG-DSC is the same as the ignition temperature obtained from flowing conditions.