Study On Typical Thermal Hazard And Smoldering Characteristics Of Cotton In Storage Environment

Cotton is the most widely used natural textile fiber for manufacturing apparels,home furniture as well as industrial products all around the world.Cotton has the characteristics of flammability,self-ignition and smoldering,and there are numerous fire hazards in the processing,transportation and storage of cotton,so the fire risk of cotton fire is quite high.Along with the continual increase in cotton yield over the past few decades,cotton bales fires tend to occur frequently,leading to tremendous economic losses and bad social impact.As the raw material of lint cotton production,a large number of unginned cotton are usually piled up in the storage areas of cotton ginning plant.However,unginned cotton contains various combustible impurities and local mildewing,so the fire conditions of unginned cotton are more complicated.Additionally,cotton is usually compressed and packed for storage,so the density of cotton bale is high.However,previous studies on the ignition,combustion and spread of cotton bales fire mainly focused on low-density samples(less than 150kg/m3),thus,the previous research results were unable to represent the cotton bales in real warehouses.Therefore,this work carried out a series of pyrolysis and combustion experiments of unginned cotton and high-density cotton bales.A series of TG-FTIR,DSC and SEM tests were conducted to investigate the influence of mildewing on pyrolysis and oxidation characteristic of cotton.Pyrolysis TG curves show that the pyrolysis temperature and mass loss rate of cotton decrease,but the residual mass increases after mildewing.In addition,activation energy calculation and DSC thermal analysis show that cotton mildewing results in the reduction of the heat absorption at the initial stage of cotton pyrolysis and the decrease of activation energy in air atmosphere,which proves that cotton is more flammable and more dangerous in fire after mildewing.The SEM micrographs show that the morphological structure of cotton fibers was damaged obviously after mildewing,which results in the significant changes in the macroscopic pyrolysis and oxidation characteristics of cotton.TG and DSC tests of cotton leaf,cottonseed and cotton husk in unginned cotton were conducted to compare the differences and similarities of them,and the effects of these impurities on the fire hazard of unginned cotton were discussed.The TG curves in nitrogen and air atmosphere show that the pyrolysis temperature of cotton leaf,cottonseed and cotton husk are lower than that of cotton,so their thermal stability are lower than that of cotton.Among the three kinds of impurities(cotton leaf,cottonseed and cotton husk),the final mass residual of cottonseed is lowest,which is about 20%.In addition,at the initial pyrolysis stage in nitrogen atmosphere,the heat absorption and activation energy of cotton leaf and cottonseed are lower than that of cotton,while the heat absorption and activation energy of cotton husks are higher than that of cotton.All of which indicates that cotton leaf and cottonseed are easier to ignite than cotton,but the ignition of cotton husk is most difficult.DSC thermal analysis in air atmosphere shows that the combustion heat release of cotton leaf,cottonseed and cotton husk are higher than that of cotton.Especially for cotton leaf,the combustion heat release of cotton leaf is about 1.7 times of that of cotton,which will result in the significant increase of fire hazard of the unginned cotton piles.Finally,based on the self-made small combustion wind tunnel,the smoldering spread of high-density cotton bales under the concurrent wind from 0.1 m/s to 2.8 m/s were investigated.It is found that the smoldering spread of cotton bales is very sensitive to the wind velocity.Under the 0.01 m/s wind,the smoldering spread is extremely slow and shows two stages,(?)surface spread until the smoldering front covered the entire surface,(?)the in-depth spread from the surface to internal.With the increase of concurrent wind,surface smoldering and internal smoldering spread take place simultaneously,and the spread rate near the free surface is clearly faster.Also,at the steel wire rope of packing cotton bale,because of the increase of the local density of the cotton bale,surface smoldering spread slows down by about 5-10 cm/h.As the wind velocity increases,both smoldering spread rates on the free surface and inside the cotton increase,but the internal smoldering spread is less affected by the external wind.As the wind velocity increases from 0.01 m/s to 2.8m/s,the peak smoldering temperature increases from 510? to 750?.Moreover,under the wind velocity of 2.8 m/s,the smoldering-to-flaming transition was observed,but the flame could not be sustained.