Mathematical modeling of the wood biomass particles thermal decomposition process

Abstract

Coal power remains one of the main parts of the world energy, which is confirmed by the high level of electricity generation by coal-fired thermal power plants in countries with the most developed industry. However, due to the negative impact of power plants on the environment, in recent years there has been an intensive search for methods and means of reducing the concentration of anthropogenic oxides in flue gases. One of these methods is the combustion of wood biomass in furnace devices. However, it should be said that the problem of slagging of heating surfaces of furnace devices has not been solved. This is due to the lack of a theory describing the full range of thermophysical and thermochemical processes leading to the “sticking” of wood particles during the period of thermal preparation for combustion in boiler furnaces. The purpose of this article is to develop, based on the results of preliminary experiments, a mathematical model of the processes of pyrolysis of wood particles taking into account the formation of liquid pyrolysis products and a theoretical analysis of the time characteristics of the completion of the pyrolysis process of typical particles of wood biomass.

The article presents the results of experimental and theoretical studies of the process of thermal decomposition of individual particles of wood biomass. Based on the experimental results, temperature trends were established in the central region of a wood particle during its heating. It was shown that during the period of thermal decomposition, a complex set of thermochemical reactions with positive and negative thermal effects occurs in a wood particle. It was found that complete pyrolysis of solid wood particles at moderate temperatures (less than 1200 K) is practically impossible under combustion conditions even with very small wood particle sizes (0 < δ < 1 mm). It was also shown that pyrolysis of particles with a characteristic size of about 1 mm is completed in 2–3 s at moderate temperatures.

Based on the results of the experimental and theoretical studies, two temperature regimes were established (low-temperature and high-temperature). Under low-temperature heating conditions, reactions with negative thermal effects usually prevail. Under high-temperature heating conditions, reactions with significant exothermic thermal effects occur in wood particles. A mathematical model was developed to describe heat transfer in wood particles during pyrolysis, taking into account the thermal conductivity tensor. Comparison of theoretical and experimental data showed a good correlation between them. Based on the results of theoretical studies, characteristic times of complete thermal decomposition of a large group of wood particles were established.