A diffusion model for particle mixing in a packed bed of burning solids

• Published in: Fuel (Guildford) Vol. 84; no. 2; pp. 213 - 225
• Main Authors: Yang, Y.B., Lim, C.N., Goodfellow, J., Sharifi, V.N., Swithenbank, J.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 2005; Elsevier
• Subjects: Applied sciences; Biomass; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Fuels; Mathematical modelling; Natural energy; Particle mixing; Wastes; Mathematical modelling; Particle mixing; Biomass; Wastes; Combustion residue; Packed bed; Mixing; Combustion; Furnace; Combustion velocity; Concentration measurement; Temperature measurement; Incineration; Diffusion coefficient
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/j.fuel.2004.09.002

Particle mixing caused by grate movement in a packed bed of solids is an important process for biomass combustion and waste incineration. In this paper, a diffusion model for particle mixing in a burning bed is proposed and the related diffusion coefficient is measured. The diffusion model was incorporated into a combustion model for waste incineration in an actual full-scale bed and numerical calculations were carried to assess the effect of different mixing levels on the burning characteristics of the furnace. In-bed measurement of temperature, oxygen concentration and particle movement was also made using a special electronic device. It is found that the modelled flame front reaches the bed bottom at an earlier stage for a higher level of particle mixing; the average burning rate ranges from 0.05 to 0.13 kg/m 2 s and the mass loss rate for a higher mixing level can be twice of that for a lower mixing level. However, excessive mixing can cause significant delay in ignition or even extinction of the bed combustion; the obtained local air to fuel stoichiometric ratio covers a range from sub-stoichiometric (0.6 for the highest mixing level) to super-stoichiometric (1.6 for the lowest mixing level); the carbon in ash ranges from 3.5 to 10.5%; the most reasonable range of the particle-mixing (diffusion) coefficient is from 1.8 to 6.0 cm 2/min for a full-scale bed, according to the calculation.