Study On The Characteristics Of Seif-heating And Spontaneous Combustion Of Crop Straws In Storage

Microbial biological oxidation and low temperature chemical oxidation are the important mechanisms for self-heating and spontaneous combustion of biomass fuels.Investigating the characteristics of microbial biological oxidation and low temperature chemical oxidation of crop straws can provide theoretical basis and practical guidance for predicting and preventing self-heating and spontaneous combustion.In the present work,three main crop straws(i.e.,rice straw,wheat straw and corn stalk)in China were taken as the research object,and their exothermic characteristics of microbial activities and self-heating processes were studied by isothermal calorimetry and Dewar flask self-heating experiment.Meanwhile,proximate analysis and thermogravimetric analysis(TGA)are used to explore the effect of microbial oxidation process on fuel properties.Moreover,TGA at slow heating was conducted to investigate the characteristics and kinetics of low temperature oxidation and pyrolysis of crop straws.The results of isothermal calorimetry experiments showed that,the microbial biological oxidation process of straws occurs at three stages: induction,acceleration,and declination and stop period.For straw with an initial moisture content of 40 % ～ 70 %(dry basis),the higher the moisture content of the straws,the shorter the induction period,the faster the rate of microbial biological oxidation,and the more significant the heat production.The heat release curve of microbial biological oxidation of the straws at different temperatures showed that,the microbial biological oxidation of rice straw takes place at the temperatures lower than 60 ℃,and is the most significant at 30 ℃-40 ℃;the microbial biological activity of wheat straw occurs at the temperatures of less than 70 ℃,and microbial biological activity is the strongest and the heat production is the most significant at 40 ℃-50 ℃;corn stalk has the microbial biological oxidation exothermic process at below 50℃,and its microbial biological activity is the most intense at 30℃.The comparison of microbial biological exothermic characteristics between the crop straws showed that,the microbial biological exothermicity of rice straw is the strongest at the temperature of 30 ℃-40 ℃,and that of wheat straw is the strongest at the temperatures higher than 40 ℃.The results of Dewar self-heating experiments showed that,self-heating processes of the straws can be divided into three stages: induction,temperature rising,and temperature decreasing period,and the temperature change pattern of each stage in the sample corresponds to the exothermic characteristics of microbial activity.Meanwhile,for straw with an initial moisture content of 40% ～ 70%(dry basis),the higher the moisture content of the straw,the higher the maximum temperature rise,and the more intensive the self heating.Additionally,the study found that self-heating processes of the straws rely on oxygen availability,and that the higher the oxygen concentration,the more significant the self-heating.The results of fuel characteristics analysis showed that,after the microbial biological oxidation,the moisture and ash contents of the samples increase to different degrees,and the increase is more obvious with increasing the initial moisture content.These reflect that some easily degradable compounds of the straws were consumed to produce water and other substances during microbial biological oxidation.Meanwhile,the results of TGA showed that,the oxidation reactivity of the reacted samples was significantly lower than that of the raw samples,which also confirms that the microbial biological oxidation process causes the consumption of easily degradable substances.The comparison of the TG curves,characteristic temperatures,and kinetic parameters,and the self-ignition risk assessment results from the low temperature oxidation TGA showed that,as compared to wheat straw,rice straw and corn stalk have stronger low temperature oxidation reactivity and higher risk of self-ignition during pulverization.Moreover,combining the microbial oxidation and low temperature oxidation capacity,rice straw and corn stalk were evaluated to have higher risk of self-heating and spontaneous combustion during storage.