The study of the combustion of solid fuels is still crucial for the energy industry since despite the large-scale use of oil and gas, char, or coal, is still widely used in many countries. For this reason, scientists explore the details of combustion processes to improve fuel use and reduce solid fuel consumption. Karchniwy et al., in their research, studied the influence of turbulence on mass transfer in solid fuel combustion using industrial examples to determine its effect on the RANS simulation mode applied to predict char burnout.
The authors begin their work by explaining the stages of solid fuel conversion, and the impact of various parameters and forces on it. For example, the authors talk about the kinetic-diffusion surface reaction rate model, according to which the general reaction rate can be impacted by reaction kinetics and reactant diffusion. However, much of the rationale for the importance of the study by Karchniwy et al. (2021) pay attention to the influence of turbulence, which is in agreement with the theories of Haugen et al., can reduce the overall conversion rate if it promotes particle clustering, as well as increase mass transfer through velocity fluctuations. For this reason, the authors investigate both options of the turbulence impact and the specific parameters of these processes affecting the char conversion by using an industrial-scale boiler and a jet of particles. The study explores the Reynolds Averaged Navier-Stokes (RANS) approach.
The authors divided the research into theoretical and practical parts of the work. The main ideas of this article are that turbulence can reduce the fluid-particle mass transfer if the lifetime of cluster particles is equal to the consumption of any gaseous reactant (Karchniwy et al., 2021). However, when clustering is not important, the transfer rate can increase because of the relative velocity between particles and fluid, which is caused by turbulence. However, although these effects are accounted for in some other models, RANS stimulation greatly overestimates the relative velocity between particles and fluid when considering classical turbulence. At the same time, the authors determined that the most suitable conditions demonstrating the impact of turbulence are “relatively large particles, large-scale facilities, fuel-rich conditions, moderate turbulence intensity, and a low stoichiometrical air-fuel ratio” (Karchniwy et al., 2021, p. 77). Some of these parameters are reflected in practical examples.
Practical verification of these effects on the simplified jet burner demonstrated to the authors the impact of turbulence for fuel mass flow rates, particle sizes, and jet velocities. At the same time, the authors determined that for low temperatures that are characterized by predominant kinetic reactions, the conversion rate of char will not be influenced by a reduction in the mass transfer rate (Karchniwy et al., 2021). At the same time, the effect of turbulence in the industrial scale boiler was not so significant due to the low density of particles. Nevertheless, the authors conclude that, presumably, this effect will be stronger in systems characterized by larger particles. Moreover, Karchniwy et al. (2021) find that, in general, the effect of turbulence should be considered in RANS simulations to predict char burnout accurately. In addition, such conclusions allow the authors to propose direction for future researches aimed at studying the impacts of turbulence on mass transfer in solid fuel combustion in other industrial settings, such as gasifiers or MILD combustors.
Reference
Karchniwy, E., Haugen, N. E., Klimanek, A., Langørgen, Ø., & Sładek, S. (2021). The effect of turbulence on mass transfer in solid fuel combustion: RANS model. Combustion and Flame, 227, 65–78. Web.