Study On Influence Of Variable Temperature Drying Condition On Physical Properties Of Brown Coal

China's rich in brown coal resource,due to the fact that lignite got high water content,high ash content,easy oxidation,and easy storage and transportation.A lot of The utilization of lignite was ineffective,which was inseparable from the dry upgrading of raw coal.Traditional lignite drying and upgrading methods generally adopted a linear heating method,and the drying efficiency and quality of the upgraded coal needed to be improved.Staged temperature change can properly input dry energy for the non-linear change of physical properties in lignite drying,so as to solve the above problems.So the exploration of the step-type drying with increasing temperature and declining temperature and the lignite drying process was of great significance to the development and improvemtnt of lignite drying technology.In this paper,temperature was taken as the cut-in point to research the change of physical properties of lignite under non-linear increasing temperature and declining temperature prograss,which including sore structure and surface functional groups.In this paper,the raw coal of Mengdong and Zhaotong was taken as the research object.The method of combining the fixed-bed variable-temperature drying and rapid moisture determination was used to analyze the lignite drying process.The pore structure of lignite before and after drying was characterized by the adsorption of cryogenic liquid nitrogen in lignite.The surface functional groups of lignite before and after drying were analyzed by infrared spectroscopy and X-ray energy spectroscopy.The aim was to find a variable temperature program with high drying efficiency and good quality.By the combination of fixed-bed variable-temperature drying and rapid determination of moisture,the characteristics of warming and drying of lignite in Mengdong and Zhaotong were studied.It is found that the temperature of Zhaotong lignite drying has the most obvious influence on the temperature-varying drying.The temperature increases,the drying rate and the dehydration rate increase significantly.Mengdong lignite can be completely removed after drying at 300°C for 12 minutes.The drying efficiency at 200-200°C is lower than 200.-300 °C high,100-300 °C and300 °C drying dehydration are mainly occurs in the high temperature section,within the same temperature range,cooling and drying is slightly better than the heating effect.The high temperature-low temperature process effectively destroyed the lignitewater holding structure during the high temperature drying process,weakening the influence of the temperature in the subsequent low temperature and constant temperature drying zone on the lignite;the lignite residence time has little effect on the drying,the residence time increases,and the drying rate increases,but The degree of drying is basically unchanged.This effect showed a trend of weakening as the drying temperature increased;the particle size had almost no effect on the temperature-varying drying,and the drying endpoints of the-1mm,-2mm,-3mm,-6mm,and-13 mm raw coals were roughly coincident after drying at the same variable temperature program.In order to improve the quality of coal with different temperature-variation procedures,the structure of pores and the distribution of surface functional groups were investigated using cryogenic nitrogen adsorption,Fourier infrared spectrometry,and X-ray analysis.The results showed that the adsorption isotherms of six kinds of upgraded coal in Mengdong lignite belonged to No.Type II isotherms and adsorption inflection points exist at a relative pressure of 0.05.The low-temperature drying temperature mainly acts on the micropores and the effect of the cooling-drying on the micropores is stronger than the temperature-rising drying.In the same temperature range,the surface functional groups of the various temperature-improved coals of Zhaotong and Mengdong lignite have little difference.The surface functional groups of different temperature-varying upgraded coals in Mengdong and Zhaotong had different degrees of change compared to the raw coal,but the functional groups of different upgraded coals had no significant difference.The re-adsorption of lignite-refined coal was tested using a dynamic water vapor adsorption instrument and the re-adsorption rate of Mengdong lignite upgraded coal was found to be 300-200°C<200-300°C<300-100°C<100-300°C<200-At100°C<100-200°C,the rate of rehydration with dry-type drying is lower than that with warm-up drying,and the water-holding structure of lignite is more damaged.The thermogravimetric experiment showed that the weight loss rate of the first high temperature and low temperature coal sample was higher than that of the first low temperature and then high temperature drying coal sample.After a large amount of dehydration,the internal pores of the lignite increased(especially micropores),and the volatiles during the pyrolysis process.Easier to escape;calculation of thermodynamic parameters found that the thermal activation energy of each upgraded coal is significantly greater than that of raw coal,and the activation energy ofpyrolysis of temperature-reduced dry upgraded coal is also higher than that of warm-up drying.