Transient model for soot formation during the combustion of single coal particles

Abstract A transient mathematical model was developed to describe soot formation during the combustion of single coal particles based on the static semi-empirical model presented by Fletcher and coworkers. Sensitivity analyses of the model parameters show that soot emissivity and mass diffusivity of tar play an important role in predicting soot volume fraction ( f v ) and flame temperature ( T f ). The model was applied to simulate the combustion of single bituminous coal particles with initial diameter (2 r 0 ) of 83 µm in a drop tube furnace and air atmosphere. It was found that soot is only formed within the first ∼ 5 ms after the appearance of the volatile flame. Although most of the soot is oxidized during the volatile flame phase, a small portion of soot still remains during the char combustion. Due to the soot presence, the volatile flame duration is extended by 2.6 ms. Compared with the soot-free flame, the sooting flame has remarkable lower T f and its peak T f value is ∼ 410 K lower. As a consequence, char combustion starts at a temperature that is ∼ 125 K lower than that of the soot-free case. Spatially, the peak f v at 16.6 ms appears at 4.5 r 0 and soot oxidation zone spans to ∼ 10 r 0 . The model was validated by comparing the predicted T f and f v under different O 2 mole fractions ( x O2 ) with recent experimental results reported by Khatami and coworkers. The predicted trends are consistent with those of the experimental results. With increasing x O2 , T f increases, but the increase rate becomes more gradual at a large x O2 . While for f v , a non-monotonic variation is observed, where soot first increases and then decreases with a peak value occurring at x O2 ≈ 40%.