Experimental Study The Drying Process Of Quartz Sands Bed In High Temperature Circumstance

Direct combustion is one of most popular utilization method of biomass in China. A certain amount of water is contained in biomass materials, such as agricultural and forest residues, urban d solid waste, and etc. The water affects the ignition, combustion and emission of the burning via the drying process. Therefore, investigation on the drying process of biomass combustion is of great significance for biomass utilization.The two characteristics in drying process of the biomass combustion are its high boundary temperature and its reverse direction of heat and mass transfer (heat transfer from outside to inside and moisture flow from inside tooutside). Front reaction model is usually used to model the drying stage of biomass combustion. Currently, there are some problems in experimental verification of drying models. Firstly in biomass combustion experiments, drying is coupled with combustion and pyrolysis, thus, the validation of drying is not reliable. Secondly, experiments of wood drying and porous medium moisture migration usually occurs at low temperature (<200℃), which differs from the high temperature circumstance of the drying stage in biomass combustion. In this paper, drying process experiments and analysis was applied on drying process in high temperature circumstance. The main contents and results include:1) A quasi-one-dimensional experimental set-up for drying was designed and manufactured based on the relative literatures and the characteristics of drying stage during combustion. This device includes four parts:a high temperature heater, a cylinder vessel with insulation, a temperature acquisition system and an electronic balance. Temperatures at 6 different heights inside the bed and mass of total bed can be recorded during the drying process. In order to ensure the reliability of the experiments, high temperature heater of the device was tested and then improved.2) Quartz sand was chosen as the experimental material. Pre-experiments has been performed to determine the saturated moisture content, evaporation characteristics, uniformity of moisture in the bed of different particle size of quartz sand. Based on these,12 experiments of three different particle size (40-60,80-100, 120-140mesh) and of four moisture contents level (3%,6%,9%,13%), were planed. The planed experiments were performed at the surface temperature of 400 ℃ and the data of inner temperature and mass of bed was processed and analyzed.3) Results showed that the sand bed can be divided into wet zone and dry zone by the drying front. The temperature gradient in wet zone is an order of magnitude smaller than that of in dry zone. The temperature of the drying front increases from about 70 ℃ to nearly 100 ℃ with the increase of the moisture content, which is different from the temperature assumption (100 ℃) in front reaction model. The comparison of different conditions showed that both water evaporation rate and its variation range increase with the increase of moisture content.4) The comparison of water evaporation between calculation and experiments showed that front reaction model is reliable only at occasions of low moisture content and small particle size. The reason of the difference between calculation and experiments is that the structure internal high moisture bed is similar to "film region" in which moisture below the drying front could migrate upwardly by capillary pump and evaporated, and this phenomenon is different from the assumptions in front reaction model.