Devolatilization of a single fuel particle in a fluidized bed under oxy-combustion conditions. Part B: Modeling and comparison with measurements

Artikel i vetenskaplig tidskrift, 2015

A detailed one-dimensional transient model is developed to describe the conversion of a single fuel particle in O2/N2 and O2/CO2 atmospheres in a fluidized bed (FB). The model takes into account the main relevant phenomena occurring from the addition of a particle to the FB up to the instant when most of the volatiles have been released. The model accounts for the rates of drying, fuel devolatilization, homogeneous combustion of volatiles in a thin flame, heterogeneous combustion of char, and mass and heat transfer, the latter involving the heat transfer from the FB reactor and flame to the particle. The model is used to simulate and explain the experiments given in Part A of the present work, which includes tests with four ranks of coal (from anthracite to lignite) and one type of wood in O2/N2 and O2/CO2 atmospheres with the O2 volume concentration varying in the range of 0–40% at a fixed bed temperature of 1088 K. The predicted history of the temperature of the fuel particle and of the volatiles flame agrees well with the measurements. The simulated results indicate that the heat transfer processes at the particle scale are similar in pure N2 and CO2. The model reveals that only a small amount of heat from the flame is transferred to the fuel particle, explaining why the rate of particle heating is hardly affected by the flame. The decrease in the devolatilization time measured at higher O2 concentration is explained by heterogeneous (char) combustion, which is seen to be significant during the last stages of devolatilization. The model shows that the char combustion is limited by the rate of diffusion of O2 to the particle and justifies the lower heating rate observed in O2/CO2 compared to in O2/N2. A sensitivity analysis shows that the thermal capacity and conductivity of the fuel, as well as the convective heat transfer coefficient, are the most influencing parameters affecting the time of devolatilization.