| Biomass energy,due to its sustainability,environmentally-friendly and wide-sources,is considered as the trend of new energy development.However,the smaller calorific value of biomass energy than other traditional energies leads to a large amount of accumulation in industrial utilization.The self-heating process may occur during biomass storage and transportation,due to the low thermal conductivity of biomass materials,which poses a major risk for spontaneous combustion.As one of the three fast-growing tree species,eucalyptus is widely planted in the world.Eucalyptus bark is used as one of the main fuels for biomass power generation and fire accidents due to the self-heating and spontaneous combustion during storage have been reported.However,there is a lack of research on the self-heating process of eucalyptus bark pile at present.Moisture is an important factor influencing the self-heating process of eucalyptus bark.In addition to endothermic evaporation,it is also a necessary condition for heat release in the low temperature range.Quantitative study on the effect of moisture on the heat release reaction of eucalyptus bark in the low temperature range is imperative.Meanwhile,the research on the pyrolysis gases emission during the self-heating process of eucalyptus bark pile is still lacking.The flammable and combustible gases evolved are also an important fire safety risk.The self-heating process in the industrial eucalyptus bark pile may be affected by extreme weather,for example,the tropical storm.Tropical storm will greatly increase the ambient humidity in a short period of time.Related experiments and fire cases have shown that it will quickly increase the temperatures inside the pile,causing a potential fire safety risk.Therefore,it is necessary to study the influence of ambient humidity variation on the self-heating process of eucalyptus bark piles.It is hard to achieve ambient humidity variation of industrial-scale piles via experimental measurements.And the existing models are lack of mass transfer of moisture evaporation,transportation and boundary convection mass exchange,which cannot reflect the influence of ambient humidity variation.Therefore,it is necessary to develop a self-heating model to study the influence of ambient humidity variation on the self-heating process in the pile.The detailed work and results of this thesis are summarized as follows.The pyrolysis characteristics,evolved gases and sources of eucalyptus bark were studied by using the thermogravimetric(TG)experiment and Fourier Transform Infrared spectroscopy(FTIR).The mass loss rate model of eucalyptus bark pyrolysis process was established and the optimal kinetic parameters of each step were obtained by genetic algorithm(GA).FTIR technology was used to identify the main gases and functional groups generated by pyrolysis of eucalyptus bark,and determine the temperature range generated.And the sources of gas generation were determined by combining the pyrolysis temperature ranges of the three pseudo components.The micro-calorimeter C80 was used to study the exothermicity of Eucalyptus bark samples with different moisture contents in the low temperature range of30-200℃The influence of sample mass was considered in experiments.The experimental results indicated that the sample with the moisture content of 22.08%has a high self-ignition risk.The evaporation model under constant heating rate in a closed container was established.The heat absorbed by evaporation was estimated.Then the reaction heat of the exothermic reaction of eucalyptus bark in the low temperature range was corrected.The calculation results showed that heat absorption from evaporation is almost half of the real heat released by the exothermic reaction.The heat released in the low temperature range reaches a maximum value of923.16J g-1 at 42.08%moisture content,and then decreases.As the moisture content increases,the mechanism of measured heat release of C80 decreasing was analyzed.Evaporation is strengthened in the interval of moisture content of 22.08%to 42.79%,while the low temperature exothermic reactions of Eucalyptus bark samples are suppressed during 42.79%and 77.40%.Considering the impact of ambient conditions on industrial-scale biomass pile,a two-dimensional axisymmetric numerical model of biomass self-heating was established.In the model.the dynamic ambient temperature function was fitted by using the recorded air temperatures.A time-dependent piecewise function was constructed in the model to reflect the ambient humidity variation.A function characterizing the effect of the moisture content on the heat release in the low temperature range was established with the experimental results of C80.An experimental measurement of the industrial-scale eucalyptus bark pile was carried out.The measurement time was 30 days.And the ambient temperature and temperatures inside the pile were recorded.Based on the industrial-scale of the eucalyptus bark pile experiment,the initial conditions and dynamic boundary conditions(ambient air temperature and humidity)were set.The diffusion and convection effects of vapor and the capillary force effect of liquid water were embodied in the model.The processes of mass transfer of moisture evaporation,transportation and convective exchange of vapor in the boundary based on vapor pressure were also considered,which laid a foundation for the study of the effect of ambient humidity variation.The experimental and simulation results presented the same temperature evolution trend with time,which verified the reliability of the model.The reason of the rapid rise of temperature when the ambient humidity increased was analyzed in detail with the evolution of the distribution of liquid water and vapor.By increasing the exothermic rate of the exothermic terms in the model and reducing the boundary convection heat transfer coefficient,the evolution of reactions of the model was analyzed when spontaneous combustion occurred.It was also found that the fluctuation of ambient temperature and the sharp increase of ambient humidity could convert unburned pile(subcritical state)to spontaneous combustion(supercritical state). |
