A mechanistic study of coal swelling and char structure evolution during pyrolysis - experiments and model predictions

Research Doctorate - Doctor of Philosophy (PhD)This work presents a systematic study on swelling and char formation during pyrolysis of pulverized coal using both experimental measures and modelling approach. By using the density fraction samples, i.e. Fl.25, Fl.30, Fl.35, Fl.50 and Sl.50, prepared using the sink-float method, transient observations using a single particle reactor (SPR) and the analysis of DTF (drop tube furnace) chars prepared at atmospheric pressure consistently reveal the heterogeneity of the pyrolysis behaviour and char structures from pulverized coal. Particles from the light density fractions (i.e. Fl.25 and Fl.30) experience intensive softening and swelling during heating. Apparent bubbling phenomena have been observed in single particle experiments, which is responsible for the coal swelling. On the contrary, particles from heavy density fraction samples, i.e. Fl .50 and S 1.50, do not exhibit softening and swelling. Correspondingly, the porosity of DTF chars decreases markedly for heavy density fraction samples. It is observed that Group I chars (porous structure) are mainly generated from two light density fraction samples, while Group III chars (solid structure) are yielded from heavy density fractions. The medium density faction sample contains a mixture of different types of chars. The heterogeneity of char characteristics is attributed to the variations in the raw coal properties among different density fractions. The characters of PEFR (pressurized entrained flow reactor) chars prepared at the elevated pressure of 2.0 MPa are examined, and compared with PDTF (pressurized drop tube furnace) and DTF chars. Consistent with previous work, the results suggest that high pressures increase the swelling, the number of bubbles and char porosity, while the population of both the cenospheric char and the solid char decreases at elevated pressures. A mathematical model for coal swelling and char structure formation of single coal particles during devolatilization is developed based on a simplified multi-bubble mechanism. The char formation has been considered as two successive steps: the multibubble stage followed by a single bubble stage. During the multi-bubble stage, the rupture of bubbles is a rate-controlled process, during which the volatile release is governed by the bubble rupture rate. When the cenospheric char structure is formed, the single bubble model applies. During this stage, the bubble rupture is controlled by the wall stress, and the volatiles are released through both bubble ruptures and direct diffusion of volatiles to the particle surface. The sensitivity study has been carried out, based on which the parameters for the present modelling work have been determined. Comparisons of the model predictions with the experimental data show that the present model predicts the experimental trends of the coal swelling and char structure characteristics under different heating conditions. As an advancement of previous work, the model provides a complete description of the char structure evolution process of pulverized coal during pyrolysis. From the standard parent coal properties of densityfraction samples, the present model predicts the heterogeneity of the char structure from the same coal, and estimates the distribution of char types, i.e., the Group I, II and III chars. The model predicted results agree with the experimental measurements. Overall, the experimental observations and the model predictions from this study consistently reveal the heterogeneity of the char characteristics owing to the heterogeneous nature of coal. In addition to the dominant role of coal macerals, the influence of the ash level in coal on char formation is identified, which extends the current understanding of the heterogeneous nature of coal for char formation. In the meantime, conditions under which coal is heated have a significant impact on char formation. High heating rates increase the swelling ratio, from both the experimental observations and the model prediction. Pressure plays a significant role in char formation, and favours the formation of the foam char structures with a high porosity. An optimum pressure range has been predicted, which is consistent with the literature data