Mathematical Modeling of the Bark Drying Process

Abstract

Currently, a large amount of wood bark waste is generated at the timber processing enterprises of the Russian Federation, which is not widely used in industry and has a negative impact on the environment. One of the feasible directions for the processing of such waste is its burning in layered furnaces. However, tree bark, due to its high moisture content and extremely heterogeneous fractional composition, belongs to a low-energy, difficult-to-burn fuel. High humidity reduces the energy density of the bark. The extremely heterogeneous fractional composition of tree bark (linear dimensions of random bark chips can vary from millimeters to tens of centimeters) complicates the organization of the burning process. Effective utilization of tree bark is ensured when it is pre-dried to a moisture content of 55–60 % and the size of bark chips not exceeding 100 mm. The purpose of the presented work is to study the temperature field in a dense blown layer of high-moisture bark when drying it with high-temperature flue gases from boiler units. A mathematical model for heating the dense layer of high moisture tree bark has been developed based on the physics analysis. It was done taking into account the assessment of the bark dense layer thickness, which uses the method of calculating the aerodynamic resistance of a transversely blown layer to a gas flow. Numerical experiments were carried out in order to calculate the temperature fields during the heating of a single chip of high moisture tree bark located in the front part of the dense blown layer. The calculation data were compared with the results obtained using the developed model. A procedure that allows calculating the total duration of heating the wet material until the layer is completely dry, the time spent on heating the blown dense layer until the complete release of volatile substances, and the temperature fields in the bark layer and gas flow has been developed on the basis of mathematical modeling of the process of the dense layer heating and thermal preparation of high moisture tree bark for burning. A scheme for preparing high moisture tree bark for burning and a method for determining the characteristic parameters of a two-stage scheme of energy use of high moisture tree bark in layered furnaces of boilers and heat generators are proposed.