Research On The Kinetic Model Of Superheated Steam Pneumatic Drying Process Of Lignite

In China, lignite is the abundant energy resource.130.3billion tons of lignite reserves, about13%in national coal reserves have been proven. The high-quality kinds of coal, such as bituminous coal and anthracite, have been fully utilized to match the rapid national economic development. Consequently, lignite has become an important part of energy production and supply. However, because of its low rank characteristics, such as high moisture, high ash and high volatile contents, low calorific value and low ash melting point and so on, there is a lot of problems to be resolved in the lignite utilization, including combustion efficiency and greenhouse gas emissions, as well as transportation costs and storage. The development and utilization of lignite encounters a serious bottleneck. The lignite utilization, such as liquefaction, pyrolysis and gasification, requires low moisture content, say lower than10%. So far, large scale industrial lignite utilization in China has not been actually carried into execution. One of the main reasons is the lack of an effective process and technology to match the requirements of safety, energy conservation, economic and environmental protection in lignite drying. Obviously, the development of an effective drying process and its associated equipment depend on the in-depth understanding on the mechanism and the reliabilities on the approaches of design and calculation..This paper aims at the development on the theory and the process for drying of lignite, which is of high energy strategic significance. The characteristics of lignite and application status on lignite drying were global widely analyzed. It was found that despite the massive effort, for lignite the progress on drying technologies and processes is still insufficient. At present, the emphasis of research is still on the development of reliable theories and the development of proper drying technologies. Because of many advantages, such as safety, energy conservation and environmental protection, the superheated steam drying has been widely taken for a suitable drying approach for lignite. Although some scholars have being performed the researches on the experiments and models, the researches research of lignite superheated steam drying is only in infancy. The prediction of drying process and the calculation of parameters on drying process are generally realized by a heat and mass transfer model. Drying kinetics can be described through a series of drying curves. The drying curves are usually based on experimental determination and theory analysis and, the drying curves related with its drying condition and physical characteristics. However, in practical drying process, the drying conditions are most likely variable. As a result, the drying kinetic model is difficult to apply to the practical drying process prediction. In order to obtain the basic information for the further research of heat and mass transfer, this paper performed the analyses and determinations properties, including the pore-size distribution, proximate analysis (moisture content, ash content, volatile and fixed carbon), elemental analysis (C,H,O,N and S) and calorific value. The testing sample was taken from Inner Mongolia, where reserves a great deal of lignite. It was found that lignite is of the same structure as the common porous media.Because of the physical and chemical properties, the first-line factors to be considered in industrial lignite drying are safety, energy consumption, water consumption, environmental protection, capacity, operating costs and so on. The consequence depends on the selections of drying approach and drying equipment. Based on the analysis of existing drying equipment, including rotary dryer, rotary steam tube dryer, fluid bed dryer and pneumatic dryer, this paper selected pneumatic dryer as the objective dryer for the reasons of simple structure, feasible parameter measurement, and scaling-up and the carried heat and mass transfer mathematical description is also suitable to other fluidized drying method (such as fluidized bed drying, spin pneumatic drying, spray drying, etc.). Combining the advantages of the superheated steam drying approach, a unique drying system, named'Upgrading of Low Degree Coal by Superheated Steam Grading-pulverization Circulated Pneumatic Dryer (SPU)'was proposed in this paper.In order to realize the superheated steam pneumatic drying process of lignite and verify the feasibility a pilot-scale testing system was carried out. This drying system can deal with an input from20to300kg/h. the gas (superheated steam) velocity in the pipe can be varied from10to30m/s with a variable inlet temperature in a range of400～650℃. The testing lignite material is also taken from Chifeng, Inner Mongolia, with an initial moisture content range of30～50%and an average particle size of0～3mm. a number of measuring holes were maken to perform the measurement of superheated steam flow's velocity, pressure and temperature. By taking sample, moisture content, temperature and composition distributions of lignite were also determined. A large number of tests provided a massive data, such as residence time and temperature distribution. These data are valuable and indispensable in the analysis of feasibility for engineering application, particularly in our further steps for the verification of developed drying kinetics and CFD models. The tests results also showed that the drying system provides high quality product and, works with a safety and stable performance.One of the most important preconditions for realization of accurate prediction for lignite superheated steam pneumatic drying process is the development of a mathematical model to describe the parameters variations, such as moisture content, velocity and temperature in the drying process. Radically, drying process is the coupling of momentum, heat and mass transfer. At present, because of simple formation, the widely applied models for mass transfer in drying process usually ignore the inner mass transfer resistance. These models are only suitable for the cases that moisture transfers from the particle surface. It most likely results in large error, when the model is used to calculate the inner moisture transfer process. On the bases of N-S model and discrete phase model, this paper developed a basic CFD model. In the first step, the inner mass transfer resistance was ignored. However, the simulation result showed that covering the influence of inner mass transfer resistance in the drying kinetics model is necessary for lignite drying.Despite the plentiful of achievement on for moisture transfer in porous media, there still a far way to be applied in practical calculations, due to the extreme complicated formulation and difficulties on the determination of key variables, such as porous distribution and average porous dimension. Until now, the simple and reliable method for drying kinetic research is still experiment. The feature of drying kinetics is usually denoted by drying curves. A unique experimental set was carried out to research the drying mechanism and to determine drying curves for both hot gas drying and superheated steam drying. The experiments showed that, different from hot gas drying the superheated steam drying process can be divided by three stages:vapor condensation, constant drying rate and falling drying rate. Consequently, to make drying kinetics complete and accurate, further research was carried out that a drying kinetics model, including steam condensation and inner mass transfer resistance, was proposed.Among the above stages, the first two only connect with surface moisture that can be accurately described by the completed pure water drop model. The influence of inner mass transfer only appears in the falling drying rate stage. It was found that for a certain material with a certain particle dimension, despite different drying conditions, the drying curves show a same variation tendency. By a proper non-dimension treatment, the drying curves can be described by a same mathematical formula. The formula is independent from the drying conditions. Based on this performance, to apply the determined drying curves, which measured under a certain drying condition, this paper proposed a now drying model, named generalized drying model. The key steps are determine the drying rate in constant drying stage, critical moisture content and the relationship of drying rate and critical moisture constant. When the process is in the first two stages, the drying process can be modeled by pure water drop. While, when it is in the falling drying rate stage, the drying process should be described by generalized drying model.Based on the above work, a complete CFD model fro the prediction of lignite superheated team dring and calculation of engineering processes was carried out. Comparason with the pilot experimental results was also performed in this paper. The comparason shwen that this CFD model satisfies engineering requirement, can be applied into the design and system optimization.