Study On Combustion Characteristics And Fire Risks Of Rice In Storage Environment

Rice is an important strategic reserve material in China,and the safe storage of rice is of vital importance to the people's livelihood.With the development of society and the advancement of science and technology,most of the granaries in China have been mechanized construction and information management,which has greatly reduced the possibility of fire in the granaries.Presently,the focus of fire safety prevention of granaries is mainly on the safety management of mechanical and electrical equipment and human behavior,and there is a lack of understanding of the combustion behavior of the grains in the granaries.In this paper,a series of multi-scale and multi-influencing factors experimental studies were conducted on the typical grain stored in the granary,that is,rice,and the combustion characteristics and fire spread of rice were analyzed.At first,using thermogravimetric analysis instruments,the crushed rice,rice husks and white rice were subjected to thermogravimetric experiments with multiple heating rates in air.and N2 atmosphere.The thermogravimetric curve in the air atmosphere shows that the maximum weight loss peak of rice appears at about 300?,and the peak shape is high and narrow,which means that it releases a large amount of combustible volatile substances at this temperature.In addition,constant temperature thermogravimetric experiments in air atmosphere were conducted on the single rice.The results showed that when the ambient temperature reached 245?,the rice husk began to crack,and the rice inside the husk was molten and break out of the husk,then bond with rice husk to form carbon.When the temperature is lower than 250?,the single rice is mainly in the dehydration stage.When the temperature reached to 350?,the single rice has begun the first stage of oxidative pyrolysis,and the resulting carbonized material needs to be decomposed at a higher temperature.When the temperature was higher than 500?,the rice began to oxidize the residual carbon in the second stage,and the rice is almost exhausted.Based on the combustion experiment platform within penetrating airflow,two experiments were conducted to study the effects of moisture content and airflow on the combustion characteristics of rice.The experiment found that the water content has great impact on the flammability of rice.In the case of no penetrating airflow or slow airflow,those with a water content of 13%cannot be ignited,while 3%are easily ignited and form violent combustion.Combustion experiments of rice with a water content of 14%within penetrating airflow of different rates showed that,for the ignition conditions given in the experiment,there is a minimum critical air flow velocity(0.6 m/s)for the ignition of the rice.At the same time,the greater the airflow rate,the higher the risk of fire.Once ignited,three stages of hidden fire spread processes below the surface were revealed,(?)Smoldering spread;(?)Flame spread;(?)Horizontal smoldering spread.In addition,the smoldering and flaming rates of rice were calculated separately,and then revealed the variation with spatial location and airflow rate.The experiment indicates that the actual fire development of the rice stack under the airflow may exceed the estimated results of surface observation,so it is necessary to strengthen the temperature monitoring inside the rice stack.Finally,by the experiment of upslope fire spread of rice with wind and external heating flux,it is found that the combustion of the rice only occurred on the surface layer.Because when the rice was on fire,a charring layer with a small porosity was formed on the surface,which not only hindered the heat transfer of external heat flow and burning flame to the deep rice,but also weakened the diffusion of the air into the rice,resulting in combustion stopped on the surface of the rice.Under the condition of continuous ignition and external heating flux,the faster the wind speed,the longer the ignition time of rice.After ignition,the fire spread rate of the rice on the slope showed a deceleration trend.The increase in wind speed makes the fire spread faster.The difference in external heat flux radiation and the carbonation behavior of the rice surface are the main reasons for the special fire combustion and fire spreading behavior of rice.