Investigation On Lignite Upgrading Characteristics Via Low Temperature Fule Gas Baffle Countercurrent Drying Technology

The high moisture content of lignite not only increases transportation costs,reduces boiler combustion efficiency,but also increases emissions of pollutants（CO₂,SO₂,NO_x）.Therefore,drying and upgrading low-quality lignite is one of the research priorities in the energy field at home and abroad in recent years.The COMBDry drying technology used in this paper has a low outlet temperature for drying products,which is convenient for further cooling to prevent spontaneous combustion.The demand for dry lignite granularity is low and easy to operate;it is conducive to large-scale,simple system and flexible operation.However,little research has been done on the microscopic mechanism of drying of lignite using this drying technique.In this paper,the drying characteristics of lignite on the COMBDry drying test bench,the physical and chemical structure after drying,the re-adsorption of moisture content after drying,and the effects of lignite pyrolysis and combustion reactivity after drying were studied.In this thesis,the drying characteristics of two kinds of Chinese domestic montanite（Hailal HLE and Shengli SL lignite granules）on COMBDry drying equipment were studied.The drying rate、dehydration rate、calorific value heat loss and combustion efficiency was measured by different parameters such as temperature（T_d）and ratio of the flue gas to lignite（R）.Furthermore,the specific values of the different parameters of the cross section at different heights of the COMBDry drying device are intercepted to quantitatively analyze the influence of the temperature（T_d）of the flue gas inlet and ratio of flue gas to lignite（R）on the drying process of the lignite particles.The physical and chemical structural changes during lignite drying were qualitatively analyzed by nitrogen adsorption,FT-IR,Raman and SEM.The results show that the drying process is consistent with random nucleation and subsequent growth mechanisms.Shrinkage and collapse of large pores and partial mesopores occur during the drying process,but the micropores are substantially unchanged.During the drying process,the oxygen-containing functional groups such as carboxyl group（-COOH）and hydroxyl group（-OH）increase at a temperature lower than 210℃.When the temperature is higher than 210℃,the content of these oxygen-containing functional groups decreases,and the decomposition releases such as H₂O and CO₂,but its aromaticity is remarkably increased,and the degree of graphitization of the two lignite particles after drying is lowered.Secondly,the moisture resorption characteristics of lignite particles before and after drying were studied experimentally,and the experimental data were theoretically analyzed.The results show that the pore structure of lignite particles irreversibly collapses during the drying process,which makes the moisture resorption characteristics of lignite after drying have a certain hysteresis;The re-adsorption rate of lignite particles after drying is only related to the drying temperature,but not to the particle size and adsorption conditions.Increasing the drying temperature reduces the moisture resorption rate and equilibrium moisture content（MHC）.The MHC model established by multiple linear regression is suitable for describing the moisture resorption process and can be used for the prediction the MHC of lignite under different drying temperatures.Finally,the pyrolysis and combustion characteristics of lignite particles before and after drying were studied by high temperature horizontal tube furnace and thermogravimetric analyzer.The results show that the amount of HCN and NH₃released from the lignite particles during the removal of the volatiles is significantly increased as the degree of drying increases as the dried lignite particles are compared with the raw coal.The flammability index,burnout index and combustion composite index are reduced.The drying treatment increases the specific surface area and C_f content of the surface of the coal char particles.On this basis,the structural characteristics,surface behavior and combustion reactivity of raw coal char and dry lignite particle coke collected during pyrolysis of dry lignite were characterized.The results show that the degree of drying is large and its reducibility and reactivity are not necessarily high.Therefore,in order to improve the reactivity of coal-based fuels in the coke combustion stage,lignite should be pretreated to an appropriate degree before combustion.