| Lignite is a kind of coal with low coal degree,high moisture content and low heat value.However,due to its large reserves,low mining costs,low sulfur and phosphorus content,which has little pollution to environment,and then the compresensive utilization of lignite has become one of the hot topics in the present study,and the prerequisite for comprehensive utilization of lignite is drying.Thermal drying is one of the main methods for lignite drying because of its high water removing,high safety and reliability,and low costs,which includes Fluidized-bed drying,Microwave drying,hot-air drying,stirring drying and so on.However,few studies have been reported on the thermal drying model and experimental mechanism of lignite.The main purpose of this paper is to establish the thermal drying model of lignite and to explore the mechanism of typical thermal drying on the physicochemical properties of lignite itself,the physical and chemical characteristics of the removing moisture,and the pyrolysis and combustion characteristics of dried lignite.A series of work are performed as following:Firstly,in order to establish the thermal drying model,the moisture contained inlignite was divided into three kinds according to the conbination form of the moisture and coal,and the corresponding energy consumption in the thermal drying process was divided into four kinds.The chemical binding energy of up to several tens of megajoules,and the content of this kind of moisture is small,usually this the energy is not considered;The energy required for water phase changing at room temperature and pressure was set value of 2.4 MJ/kg;The energy required for the moisture overcoming the physical interaction between molecules was quantitative calculated with Lennard-Jones 10-4-3 potential energy equation;The energy required for the moisture contained in the capillary was calculated using the balanced vapor pressure.At the same time,the Intelligent Gravimetric Analysers(IGA)was used to verifie the energy of the thermal drying process.Results show that the calculated results were consistent with the experimental results.Finally,the thermal drying process was divided into three stages according to the model,and the economy of each stage was also analyzed.Secondly,the drying characteristics of lignite in the stirring dryer and the physical and chemical properties of the wastewater generated from the process was analyzed.Results show that with the increasing of drying temperature,the functional groups in lignite decomposed,resulting in the increase of chemical oxygen consumption and total organic carbon content.The main negative ion(F-,Cl-,Br-,PO43-,SO42-,NO3-)and main positive ion(Na+,K+,Ca2+,As3+,Fe2+,Hg2+,Mn2+,Se2+,Zn2+)gradually dissolved into the water from the coal,resulting to the decreasing of its pH value and the surface tension value.At the same time,the drying characteristics of the sludge with lignite additive were also studied.It was found that the addition of lignite could reduce the viscosity of the sludge during the drying process,resulting to the increasing of drying rate and the reduction of the drying energy consumption.Thirdly,in order to improve the drying rate of lignite,the fluidized-bed drying characteristics were studied.Particle size of lignite is one of important factors affecting the characteristics of fluidized-bed drying,and the moisture contained in lignite with different particle size was studied by low temperature Differential Scanning Calorimetry(DSC).The results show that there were three kinds of moisture contained in the lignite:body water,bound water and non-frozen water.During the process of reducing the size of lignite,the body water has a certain amount of removal,while the content of body water and non-frozen water was almost unchanged.Finally,the existence of critical particle size was found in the fluidized-bed drying:when the particle size is larger than the critical value,the drying rate has little relationship with the particle size,and when the particle size is less than the critical value,the drying rate increases with decreasing the partidcle size.The 915 MHz microwave drying has the advantages of high drying rate,low diameter request,while it’s not widely used during the industrial application.The comparative experiment between the 915 MHz microwave drying with high power and hot air drying show that,after microwave drying,the specific surface area and specific volume of the lignite increased,the content of oxygen functional groups contained in the lignite decreased while the ratio of C/O increased,the hydrophobicity enhanced,and the readsorption rate reduced during the storage process.Finally,the main factors affecting the readsorption characteristics were discussed.At the same time,the pyrolysis and combustion characteristics of the lignite after 915 MHz microwave drying and hot air drying were also studied.It was found that,after microwave drying,the activation energy increased during the pyrolysis process.The proportion of gas was low while the proportion of tar was high in the pyrolysis product.In the tar,the proportion of Polycyclic Aromatic Hydrocarbons(PAHs)which is harmful to the environment and human body was low,while the content of benzene series was high.The activation energy increased,and the ignition point increased while the burnout temperature decreased during the combustion process.The overall combustion characteristics improved according to the comprehensive combustion index evaluation. |
