Search

Relevance. The need to analyze the current state of the subject of coal drying in order to identify the most effective methods in terms of time and energy costs, as well as environmental safety and improve the quality of fuel. As practice shows, drying is one of the most important stages in coal preparation for combustion, it helps to increase thermal efficiency, reduce emissions and environmental pollution, increase energy capacity and improve combustion stability. Among the main methods of drying coal, drying with evaporation: A) rotary drying; B) drying in a fluidized bed; C) immersion drying with hot oil; D) drying in a microwave oven, as well as drying without evaporation: A) hydrothermal dehydration; B) mechanical/thermal dehydration; C) solvent extraction, can be distinguished. At the same time, each of these methods is constantly evolving and it is required to evaluate the influence of the dynamics of these changes on the key characteristics of drying. Aim. Review and analysis of modern methods of coal drying, the most efficient in terms of time and energy costs. Object. Wet material – coal, subjected to various drying methods. Methods. Search for works on the stated topic, checking the indication of the main drying parameters: time, energy consumption, etc. Based on the data obtained, a comparison of various drying methods was carried out, and optimal methods were recommended. Results. The authors have considered the modern methods of drying coal and assessed their effectiveness. Rotary drying, hot oil immersion drying, mechanical/thermal dehydration and solvent extraction were found to be the most energy efficient methods for drying coal. The paper considers the features of each drying method.

The relevance of the research is dictated by the need to develop mathematical models of microwave heating and MV-drying of wet materials to obtain technologically optimal and cost-effective modes. This publication is a continuation of the articles of the same authors in The Bulletin of the Tomsk Polytechnic University. Geo Assets Engineering, in which using mathematical modeling, the authors have studied in details the process at the first and second stages of drying – the heating stage, when heat exchange between the surface of a wet body and the environment occurs due to radiation and convection, and microwave energy is absorbed by the surface layer owing to its small penetration depth, as well as the stage of constant drying rate. The authors constructed the asymptotic solutions of this nonlinear problem for small and large values ​​of dimensionless time, which are demanded by engineering practice, both for parametric analysis and for performing operational calculations. The approach associated with the consideration of the third stage – the stage of the falling drying rate, is based on determination of the basic drying equation, which provides the relationship between heat transfer and moisture exchange using the Rebinder criterion. The aim of the researchis tostate the problem of the third stage of microwave drying of a wet material – the stage of a falling drying rate, and to implement a theoretical solution to determine temperature field distribution over the layer thickness and the drying rate. The object of the research is a flat layer of wet material – coal, sand, wood, etc. capillary-porous arrays, which are affected by microwave radiation. Such materials have a high dielectric constant and, as a result, very effectively absorb microwave radiation, which is almost 100% converted to thermal energy. The research methodsare associated with mathematical modeling, which are based on the equations of Maxwell’s electrodynamics and heat and moisture transfer by A.V. Lykov. In this article, when assessing the parameters of heat and moisture transfer, heat transfer accounting is conducted in more details than mass transfer. One of the features of this problem is the consideration of materials with a small depth of absorption, whereby the source term in the system of equations for heating is in the boundary condition. The temperature and moisture content of the body to be dried were determined in the mode of the falling drying speed, the calculated analytical ratios for small and large parameters of time were obtained.

The relevance of the research is dictated by the need to develop mathematical models of microwave heating and MW-drying of wet materials to obtain technologically optimal and cost-effective modes. This publication is a continuation of the article by the same authors in «Bulletin of the Tomsk Polytechnic University. Geo Assets Engineering», in which the authors using mathematical modeling, studied in detail the process at the first stage of drying –the heating stage, when heat exchange between the surface of a moist body and the environment is due to radiation and convection – energy is absorbed by the surface layer due to its small depth of penetration. The authors constructed the asymptotic solutions of this nonlinear problem for small and large values ​​of dimensionless time demanded by engineering practice, both for parametric analysis, and for carrying out operational calculations. The approach associated with the consideration of the second stage, the stage of constant drying rate, is based on a more detailed study of heat transfer, and the calculation of the rate of drying is carried out using the approximation dependence of A.V. Lykov. The paper proposes a condition for cross-linking of thermal modes of I and II stages of drying. The aim of the research is the state of the problem of the second stage of microwave drying of wet material – a stage of constant drying rate – and implementation of a theoretical solution to determine the distribution of the temperature field across the layer thickness and the magnitude of the drying rate. The object of the research is a flat layer of wet material – coal, sand, wood, and other capillary-porous arrays, which are affected by microwave radiation. Such materials have a high dielectric constant and, as a result, very effectively absorb microwave radiation, which is almost 100 % converted to thermal energy. The research methods are associated with mathematical modeling, which are based on Maxwell's electrodynamics equations and A.V. Lykov heat and moisture transfer. In this article, the Maxwell problem is solved independently of the problem of heat and mass transfer; therefore, the flux density absorbed by microwave radiation is known. Also, one of the features of this problem is the consideration of materials with a small absorption depth, that is why the source term in the system of equations for heating is in the boundary condition. As a result of the research, the authors involving the stationary temperature field conditions and the constancy of the moisture content flux density over time stated and solved the problem of temperature field distribution across the thickness of the wet plate, which strictly follows the square parabolic law. The drying speed of the II stage and the critical moisture content at the end of the II stage were determined from A,V. Lykov correlation dependencies. The paper introduces the stitching solutions for the I and II stages and the analysis of the constructed solutions.

The relevance of the research is determined by the need to develop mathematical models of microwave heating and MW-drying of wet materials to obtain technologically optimal and cost-effective modes. Due to the complexity and non-linearity of the processes taking place inside wet materials during microwave processing, it is extremely important to build models that would allow one to construct analytically approximate solutions in order to find the main laws and characteristic features of the processes under consideration. The aim of the research is to state the first stage of the microwave drying of wet material – the warm-up phase. The search for analytical solution for the temperature of the layer at different points in time allows us to determine the beginning of the next stage – drying. The object of the research is a flat layer of wet material – coal, sand, wood and other capillary-porous arrays, which are affected by microwave radiation. Such materials have a high dielectric constant and, as a result, they very efficiently absorb microwave radiation, which is almost 100 % converted to thermal energy. The methods are associated with mathematical modeling, which are based on Maxwell electrodynamics equations and heat and moisture transfer of A.V. Lykov. In this article, the Maxwell problem is solved independently on the problem of heat and mass transfer; therefore, the flux density of the absorbed microwave radiation is assumed to be known. Also, one of the features of this problem is the consideration of materials with a small depth of absorption, whereby the source term in the system of equations for heating is in the boundary condition. As a result of the research the problem of heat transfer at a given power source of heat was solved using asymptotic procedures. The authors have obtained approximate analytical solutions for the first of the three characteristic stages of microwave drying under conditions of low penetration depth of microwaves: the warm-up stage of a moist material. The constructed solutions were analyzed.