Experimental Research On Drying Behavior Of Coal Slime By Combined Hot-air And Microwave

Coal slime is a by-product of coal washing, and cannot be utilized on large volumetric and temporal scales owing to its high levels moisture content, high viscosity, high ash content and low heat value properties. The main outlet is for civil use, but the consumption is limited. In recent years, coal slime increases drastically due to the fast rise of ratio of coal washing in China, which has been a prominent problem may lead to significant energy dissipation and potential environmental pollution.At present, the preferred way of utilization of coal slime is combustion as fuel for power generation, however, the high quality energy for moisture evaporation is consumed when the raw coal slime are burnt in boilers, even endanger the stable operation of the boilers. Consequently, the drying process of coal slime before burnt is the essential step to lower moisture content and improve heat values, which makes the coal slime meet the requirements as power coal to combust alone. Currently, the drying technologies include combustion gases drying and superheated steam drying, whereas, the conventional drying technologies appear some problems in later period of drying process, such as low efficiency, time-consuming, uneven drying, the difficulty in deep drying, and unsafety operation, which have bad effect on security of the drying system. The new drying technology is urgent needed for the large scale utilization of coal slime because the rapid drying, low energy consumption and deep drying cannot be realized by thermal drying method alone.Microwave heating has several unique features related to selective, volumetric and low temperature heating compared to the traditional approaches. In view of advantages of hot-air and microwave drying technologies, we can pretreat the coal slime with hot-air in the earlier period which can remarkably reduce the energy consumption as the main drying process, and followed by microwave which can shorten the drying time and improve the quality of products. The text is aimed to exploit new technology to upgrade the coal slime and provide essential date for large-scale utilization through the combination of hot-air and microwave drying technologies.Firstly, the hot-air drying experimental studies of single particle and multi-particle coal slime were conducted at low temperature which is less than 200℃. It can be found that the drying process comprised three stages:preheating, constant-rate and decreasing-rate drying periods. Effect of different factors, including temperature and velocity of hot-air, particle size, sample mass and geometrical shape of coal slime on dewatering process of coal slime were studied by orthogonal regression experimental method. The moisture effective diffusivity and activation energy in drying process were estimated, and the Page model could describe the hot-air drying process well through the date fitting.Secondly, Microwave Thermogravimetric Analysis (MTGA) was adopted to carry out an all-round research on the microwave drying performances of coal slime. The microwave drying thermogravimetric characteristics were obtained through the combination of the weight loss curve and the inner temperature changes of coal slime which was measured by optical fiber thermometer not affected in microwave field. The drying process and internal mechanism were revealed according to weight loss curve and temperature variation curve, consequently, the comprehensive microwave drying behavior was obviously present. It was also found that the M-K model could fit microwave drying process well according to dynamic characteristics at different conditions. At last, the analysis of energy consumption in microwave drying process was conducted, and found that the energy consumption was the lowest in rapid-rate stage, however, the energy consumption present an exponential growth in decreasing-rate period when the moisture content of coal slime reached 5％.Finally, the experimental research on combined drying of hot-air and microwave was conducted based on the date of hot-air and microwave drying alone, and it was remarkable that the drying rate improved and the time shortened by combined drying method. The critical points were selected to obtain the characteristics of combined drying through the combined drying experiments of coal slime. According to the analysis of energy consumption in combined drying process, the critical point was the critical moisture content of 15％of hot-air drying which provided significant theoretical direction for the combined drying process design.Furthermore, the text researched the combustion characteristic and moisture re-adsorption behavior of dried coal slime. The results showed that the effect was not obvious on component of dried coal slime except the moisture content, and the change of combustion characteristic was also small. Air humidity was the principle factor which had significant effect on the moisture re-adsorption behavior of coal slime, however, the particle size and drying method had little effect.