| The characteristic of high moisture content of lignite reduces the calorific value and limits its exploitation and utilization.In order to save transportation cost and improve the calorific value of coal,lignite needs to be dehydrated firstly.However,on the one hand,the upgrading lignite is easy to absorb moisture,reducing the quality of lignite;on the other hand,there are security risks due to the upgrading lignite in the occurrence of moisture re-absorption,at the same time it will lead to oxidation resulting in spontaneous combustion in the process of coal storage.Therefore,according to the characteristics of easy to absorb moisture and easy oxidation of upgrading lignite,the upgrading lignite of Zhaotong was selected as the research object,and its moisture re-absorption law was studied.The effect of moisture on the diffusion and mass transfer of oxygen in coal and oxygen consumption in coal was studied.The mechanism of moisture on the mass transfer of oxygen was obtained.It has certain guiding significance for the storage,transportation and utilization of lignite and the prevention of spontaneous combustion of coal.First of all,the experimental coal samples have the typical characteristics of lignite,that is,high moisture content,high volatile content and low degree of metamorphism.The surface structure of coal samples is loose,and the tendency of spontaneous combustion is grade Ⅱ.Lignite drying is required to improve the application value because of its high moisture content.Study on the influence of ambient temperature and humidity on lignite moisture re-adsorption,it was found that compared with the ambient temperature,the effect of ambient humidity on moisture re-absorption rate is greater.Based on the Chung-Pfost model,the optimum forecasting models of the equilibrium moisture content of the drying samples was established at 30 ℃,and the specific parameters of coal samples with different moisture content were obtained by using this model.The oxygen consumption rate of coal samples with different moisture content during the process of low-temperature oxidation was studied by using the experimental system for low-temperature oxidation of coal.The pore structure characteristics of coal samples with different moisture content during the process of low-temperature oxidation were analyzed by fractal theory.The influence of water on the diffusion and mass transfer of oxygen in coal was investigated based on the static double volume theory.The results show that the inherent moisture of coal sample has a significant effect on the changes of pore structure.For the coal samples with low moisture content,a large number of pores collapse due to the removal of water from the coal particles.However,the coal oxygen complex reaction of coal samples with higher water content is more severe,because there are some of the excess water will be involved in the complex reaction between coal and oxygen to form an intermediate complex,thus destroying the coal structure,exposing more new surface.The oxygen diffusion coefficient of coal samples with different moisture content was positively correlated with the fractal dimension of coal samples.Finally,the TG-DTG-DSC curves of coal samples with different moisture content were analyzed in the process of oxidation and combustion,and the kinetic parameters were calculated in the process of low-temperature oxidation.Because of the different moisture content in coal,the activation energy in the stage of oxygen absorption and mass gain changes of the whole range of 9.57~10.36 k J?mol-1,compared to the activation energy in the stage of water evaporation and gas desorption(97.61~122.85 k J?mol-1)was decreased greatly,and the pre-exponential factor decreased by at least 5 orders of magnitude.This shows that the main reaction in the stage of oxygen absorption and mass gain is easier than that in the stage of water evaporation and gas desorption,but the reaction rate is greatly reduced.With the increase of water content,the desorption rate of coal sample to water and gas will be faster,and the coal sample may be able to react with oxygen as soon as possible in the stage of water evaporation and gas desorption in the range of lower moisture content of coal samples(moisture content was 8.93 %,11.95 % and 15.13 %).But for the high moisture content of coal sample(moisture content was 18.08 %),because of the high moisture content in the coal samples,they will prevent oxygen from entering the pores of coal by occluding the pore.The coal sample with the water content of 15.13 % has the highest oxidation resistance,and the heat absorption of the coal sample is the smallest in the stage of water evaporation and gas desorption because the heat released by the coal oxygen complex reaction will make up the heat absorbed by the water evaporation.The activation energy of coal samples with water content of 15.13 % was reached to the maximum in the stage of oxygen absorption and mass gain.Due to the early completion of the stage of water evaporation and gas desorption,there is still a part of water in the sample,and this part of water will be removed in the stage of oxygen absorption and mass gain. |
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