Mathematical modelling of woody particles pyrolysis in a fixed bed

Abstract

Рассмотрена задача термического разложения совокупности последовательно расположенных древесных частиц с учетом внешнего тепломассообмена с газовым потоком и внутренних физико-химических процессов (теплопроводность, диффузия, фильтрация, сушка и химическая реакция). Математическая модель строится из субмоделей одиночных частиц, сопряженных по потокам теплоты и массы. Результаты численных расчетов позволяют исследовать динамическое поведение частиц в условиях плотного слоя, что представляет интерес при проектировании малых энергетических установок на биотопливе. The development of new energy technologies requires the improvement of mathematical models to describe the physical and chemical processes taking place in power plants. The process of wood particles fixed-bed pyrolysis is investigated in this paper: this process takes place both in the traditional combustion of wood fuels in fixed-bed boilers and in energotechnology processes aimed at producing combustible gases and chemical products (tar, charcoal). The problem of pyrolysis of a set of successively located wood particles is considered. Each particle is considered as an object with an internal distribution of temperature, pressure and concentrations. A system of equations is constructed for a single particle, including external heat and mass transfer between the particles and the ambient gas flow combined with internal physicochemical processes (heat conduction, diffusion, filtration, drying and chemical brutto-reaction of the organic mass decomposition producing gases and solid residue). The temperature of the gas in the pores of the particles is equal to the temperature of the solid. Using the model of pyrolysis of a single particle, it is possible to reproduce the known experimental data. The mathematical model of a fixed-bed pyrolysis is based on submodels of single particles, conjugated over heat and mass flows. The interaction between the particles composing the layer is reduced to heat fluxes: radiant heat transfer between the surfaces of adjacent particles occurs in the bed, as well as convective heat transfer between the heated gas and particles. The result is that each next particle layer is heated at a smaller temperature difference. On the one hand, the intensity of heat transfer decreases, on the other hand, the efficiency of using heat increases. The results of numerical calculations make it possible to study the dynamic behavior of particles in a fixed bed, which is of interest in the design of small power plants using biofuels.