The Self-heating Process During Biomass Fuel Storage And Its Impact On Fuel Properties

Spontaneous combustion of biomass fuels is one of the most important disasters during its production,transportation and storage,also is a hot thermal safety problem that needs to be focused on in the process of bio-fuels utilization.Low temperature oxidation is well known to be the main mechanism responsible for the self-heating and spontaneous combustion during bio-fuels storage.Studying the characteristics of low temperature oxidation contributes to providing a theoretical basis for the description of the self-heating process of bio-fuels,and is conductive to the prediction and effective prevention of spontaneous combustion.For this purpose,the self-heating processes and characteristics of stored rice straw and husk were observed by storing the materials with different particle sizes and moisture contents in an adiabatic Dewar-scale reactor and monitoring the evolution of the inside temperatures.Then the proximate analysis,calorific value test,thermogravimetric analysis?TGA?,diffused reflection fourier transform infrared spectroscopy analysis?DRIFTS?and pH test,were conducted as the auxiliary means to comprehensively analyze the composition,physical and chemical characteristics of the biomass fuels,and the difference of combustion characteristics,the evolution trends of surface functional group structures before and after storage reaction.Through systematical comparison,the impact of the self-heating on the properties of the stored biomass fuels was explored.The results of self-heating experiments showed that,when rice straw and husk biomass fuels were stored with different initial conditions,an evident self-heating process has taken place inside the samples.The process includes three stages:induction period,temperature rising period and temperature dropping stage.Keeping the identical particle size,and changing the moisture content by increasing from 40%?on a dry matter basis?to 70%,the induction time gradually shortened,the self-heating rate was accelerated,and the temperature within the sample was gradually increasing.Moreover,with the same moisture content,the smaller the particle size,the faster the rate of self-heating and the higher self-heating peak temperature achived,while the induction time was longer.Meanwhile,when rice straw and husk were stored under the same conditions,the temperature increases inside the rice husk samples were significantly lower than thoes within the straw samples,indicating that,with the same conditions,the thermal stability of rice husk was significantly higher than rice straw.The results of characterizing fuel properties showed that,after the self-heating experiment,moisture and ash contents of the samples were all increased to different degrees,and with the higher initial water content,the more obvious temperature increase,indicating the self-heating process consumed a part of easily degradable materials to produce water,carbon dioxide and other substances.At the same time,TGA of the raw and stored materials showed that the oxidation of rice straw and husk after self-heating was significantly lower than that of the original sample,indicating that the self-heating process caused the losses of degradable materials,such as extractives and hemicellulose,which resulted in a certain losses of the dry matter and energy.In addition,for rice straw,the dry matter loss and energy loss were 8.5-35.5%.However,for rice husk,the dry matter loss and energy loss were lower,with a range of 3.8-13.5%.DRIFTS analysis results showed that after the self-heating experiments,the the band intensities of oxygen-containing and C-H functional groups attached to the surfaces of the samples apparently increased.They mainly include:-OH bond,-CH,-CH₂ and-CH₃ bonds,-C=O bonds?aldehydes and ketones etc.?,-C=O bonds?ester?and-C=O bonds in-COOH,at the same time in the vicinity of 1300-1110cm^-1 slightly appeared the phenoxy structure in the-C-O and-C-H bonds and aromatic alcohols and esters were nearby the 1100-1000cm^-1.All these showed that there was an evidenced process of low temperature oxidation during the storage of rice straw and husk,which produced a large amount of water,aldehyde,ketone,acid,alcohol and other substances.The pH values of the sampels significantly increased after storage,which also indicates the obvious self-heating inside the samples caused by oxidation reactions in the presence of oxygen.