Background. The adequacy of CFD modelling results depends to a large extent on the modelling approach adopted and the accuracy of the specified boundary conditions. Modelling of solid fuel combustion is very complicated and, due to high computational and time costs, is impossible without significant simplifications. Thus, it is relevant to develop a CFD model of solid fuel combustion in a fixed bed in furnaces and low-power boilers. The model allows obtaining simulation results with acceptable computational and time costs.
Materials and methods. The authors have proposed an approach to design a CFD model of solid fuel combustion in a fixed bed in furnaces and low-power boilers. It is proposed not to allocate the fuel bed into a separate computational domain and replace it with boundary conditions; in this case, the computational domain should contain only the gas part. For combustion modelling, the authors have applied the mechanism that consists of gas-phase reactions of CO and hydrocarbon oxidation of the released volatiles, and a heterogeneous interaction of solid carbon combustion on the surface of the fuel bed. To estimate the model parameters, the results of tests of an industrial boiler burning RDF have been used. It made it possible to ensure acceptable agreement of the modelling results and the data of measurements on the boiler under different modes of operation.
Results. To oxidize the organic harmful substances in the combustion products of the waste, it is suggested to carry out afterburning of deleterious substances in an additional gas duct located between the furnace and the convective part (gas-water heat exchanger). As a result of modelling the combustion process when changing humidity from 10 to 50 % and excess air coefficient from 1,4 to 2,2, it is established that with an insignificant increase of parameters (humidity up to 30 % and a up to 1,6), conditions of afterburning of harmful substances (presence of combustion products for more than 2 sec at temperatures of 850/1100 °C) are observed. But with further increase of parameters afterburning is not ensured. When the humidity increases up to 50 percent due to low combustion temperature, even when the time of gases in the calculated volume increases, the combustion ends later. With an increase of excess air ratio along with a decrease of temperature of gases, their volume and velocity increase, which leads to a reduction of time of presence of gases in the calculated volume.
Conclusions. The developed CFD model provides an acceptable agreement with the results of measurements on the industrial boiler under different modes of operation, which allows us to consider it adequate. To ensure complete combustion of fuel and subsequent presence of combustion products at temperatures above 850/1100 °C for more than 2 s, it is necessary to observe the combustion mode at the optimum excess air ratio (1,4–1,6) without exceeding this value. At the same time, it is unacceptable to supply material with high humidity for combustion.