Effect of Pyrolysis Intensity on the Reactivity of Coal Char

Abstract

The effect of pyrolysis conditions on char reactivity was studied in this paper. The char samples were prepared in a N2 atmosphere from the vitrinite constituents of two bituminous coals (YX and JJ) and their demineralized samples at different temperatures (800−1200 °C) and for different residence times. The effect of pyrolysis intensity on the carbon microstructure in the resulting char was characterized by the X-ray diffraction (XRD) technique. The reactivity of the char samples prepared under different pyrolysis conditions was measured by isothermal thermogravimetric analysis methods in an air atmosphere. The results show that the reactivity of the coal char decreases with increasing pyrolysis intensity and, when the pyrolysis temperature is high enough, the reactivity of the chars prepared from two coals and their demineralized samples tend to be the same. With increasing pyrolysis intensity, the growth of the carbon microstructure, accompanied by diminishing catalytic capability of minerals, results in the decrease in char reactivity.

Density functional theory molecular modelling and experimental particle kinetics for CO2–char gasification

“…The structural parameters were found to be 2.08 and 1.08 for the inertenite-and vitrinite-rich chars, respectively, which depend on the initial properties of the chars, Equation (8). Since the limiting value for pore growth according to the random pore model is greater than a value of 2 [6,101,102,106,127] both chars can be considered to have experienced mainly pore growth and pore coalescence during the initial stage of the char-CO2 gasification reaction.…”

“…Substantial research have found that the chemical interactions between carbon molecules and reactant gases take place at the edge active carbon atoms for coal based char models and also for non-coal char models [3,28,39,43,60,64,[103][104][105], and Zhang et al found that the carbons in the basal plane of large, fused-ring aromatic domains of chars, were inert [106]. Figure 1 shows results from the HRTEM technique, which were used to derive the char models used here [45].…”

“…Figure 1 shows results from the HRTEM technique, which were used to derive the char models used here [45]. It was postulated that during CO2 gasification, the higher reactive sites such as Cz and Czi, from the 3x3, will preferentially lose H. Anealling during pyrolysis however, may give rise to active sites of lower reactivity as the 3x3 grows to 5x5 sizes, because at high temperatures, and chemically-controlled gasification process, char undergoes thermal annealing in competition with gasification [106][107][108]. This causes the growth of crystallite diameter, La, by the coalescence of crystallites along the a-axis (e.g.…”

“…from 3x3 to 5x5 aromatic fringes) [109] for both graphitisable and non-graphitisable carbons [109]. Zhang et al studied the effects of pyrolysis intensity on the reactivity of coal char in chemically-controlled CO2 gasification experiments and found the largest difference in coal char reactivity was observed in the initial stage of gasification, but, as the process continues, the active sites of high activity were preferentially removed by CO2 or annealed into less active site and the active sites with lower reactivity were left in residual coal char [106]. Suuberg also found that the inherent reactivity of the chars was attributed to the changes in the active site concentrations, which were often thermally induced [110].…”

“…Hence, under high temperature regimes and chemically-controlled gasification process, the char actually undergoes a process of thermal annealing (pyrolysis) in competition with gasification [106][107][108]. With the increase of carbon conversion or when the higher reactive sites in the 3x3 model are removed/annealed, the reactivity, and the number of reactive carbon sites reach similar levels and the difference in char reactivities becomes smaller [106,110].…”