Experimental Study On Microwave Drying And Moisture Migration Characteristics Of Lignite

The lignite in our country is rich featured with high moisture,high volatile,low calorific value and self-ignite easily, thus, it has low efficiency and poor economy when used directly and it needs to be upgrading firstly and the moisture removal will be the essential step before its large scale application. Among the lignite drying technologies,hot air or superheated steam may be used as drying medium, while these traditional way has low heating rate, complicated process and environment pollution. Microwave has the features of selective heating、penetrability、high efficiency and instantaneity, and the moisture in the lignite is polar molecules which has strong ability to absorb microwave.The typical lignite in east of inner Mongolian was selected as the experimental samples, microwave dielectric heating technology was used to explore the characteristics of drying lignite, the pattern of moving and transformation of the water was shown by experiment, and the safety measures of lignite drying process by microwave was presented in the paper. Basic data and theoretical guidance can be provided through the studies of lignite drying by microwave-for the industrial application of lignite.Firstly, the experiments under the different affecting factors (Microwave power, sample particle size, type of coal,sample residual temperature、dehumidification ways) were carried out, and showed that the process of lignite drying by microwave consist of three stages:constant weight period,quickly losing weight period and the slowly losing weight period,the difference between the stages of quickly losing weight and slowly losing weight is the different critical moistures,and the concept of the "saturated power" which is defined as the optimal microwave power for the lignite with specific quality and particles size is presented,the critical moisture would increase and the result of drying would become deteriorative when the power deviate the saturated power; the effects of particle size are different from that of the traditional drying process, the critical moisture of the three particle size are arranged as follows: powdered coal(<0.2mm)<lump coal(8-12mm)<granular coal(1 4mm)corresponding drying effects are powdered coal> lump coal> granular coal). The initial moisture content of coal sample is the main factor of affecting the coal sample heating and weight loss. The higher moisture content, the faster the temperature and weight loss become. Desiccation of coal is decided by coal quality, if the coal quality can easily absorb microwave, the level of desiccation is small; The remained heat of coal sample with at least 60℃ of surface temperature is obvious in dropping moisture content. By simulating the dehumidification device in desiccation system, with the rate of dehumidification increase, the temperature of coal sample becomes higher, and the risk of catching fire is higher, so the rate of dehumidification should be controlled strictly; by contrasting with traditional desiccation and the inside water begins to remove by absorbing microwave only after most of the free water were removed by analyzing the water inside or outside of the coal sample during desiccation. The results showed that the calorific value of coal sample would improve after microwave irradiation and it would increase about 60-65kcal/kg when the moisture decerase by one percentage.The hydroxyl and carboxyl content and oxygen content in coal samples are reduced after drying, which shows that the microwave irradiation can promote coal rank; The porosity of coal samples changes after drying, the total pore volume and total specific surface area increase firstly and then decrease, while the pore volume and total specific surface area for the micro pores show the opposite trends. The moisture return tests of lignite drying show that the smaller the particle size of coal samples, the more obvious the moisture return become, microwave drying can not inhibit the suction of moisture; The ignition temperature is higher than that of conventional drying, about 6℃,for the samples after the microwave drying, and the tendency of spontaneous combustion and self-igition become lower slightly.Microwave power distribution, coal sample temperature, oxygen content and so on are adjusted to reduce the risk of burning on the process of lignite drying. The resu Its show that:intermittent microwave heating method can maintain the temperature of coal sample between 70℃ to 90℃ and the drying process is safer; controlling the surface temperature around 90℃ and the lower oxygen content can both reduce the risk of ignition of lignite and increase the drying effect of lignite.Finally, the removing form of the inside moisture were studied, through the contrastive analysis of moisture content and temperature distribution at different locations in lump lignite, the conclusion shows that some moisture releases quickly in form of liquid or gas, and significant amount of water aggregates or leaves from the lignite with the help of the pump effect and accordingly the obvious rising of pressure inside the lignite. The conclusion makes it possible to collect the water in non evaporative way to reduce the energy consumption of the water evaporation and improve the drying efficiency of microwave. Given this, for the microwave drying by using large particle size coal, lower energy consumption can achive by avoiding the power process and better drying effect can also be expected.