Kinetics of combustion of a pulverized brown coal char between 630 and 2200°K
Measurements have been made of the combustion kinetics of size-graded fractions (89, 49, and 22 μm) of a char prepared from Yallourn brown coal. Reactions were carried out in an entrainment reactor (between 900 and 2200°K) at oxygen partial pressures of about 0.2 and 0.1 atm, and in a fixed-bed reactor (between 630 and 760°K) at about 0.1 atm oxygen. Above 900°K, the 89 and 49 μm fractions were subject to rate control by the coupled processes of pore diffusion and chemical reaction on the pore walls; the rate coefficient, Ra,c (the rate of carbon combustion per unit external area of the particle per (atmosphere of oxygen)0.5), being given by R a , c = 9.3 exp [ − 16 , 200 / ( ℝ T p ) ] , g / [ cm 2 sec ( atm ) 0.5 ] , where Tp is the temperature of the particle in °K and R is in cal/(mol °K). Below 760°K, the 89 and 49 μm fractions were found to react with rate control by chemical reaction alone. Above 900°K rate control for the 22 μm fraction lay intermediate between these two limits. At 1800°K the reactivity of the brown coal char is a factor of 4 higher than that of anthracite, and is 50% higher than that of a char from a low-rank bituminous coal; however, at 770°K the reactivity of the char is one to six orders of magnitude higher than the reactivities of other carbons.
The Reactivity of a Porous Brown Coal Char to Oxygen between 630 and 1812° K
Measurements have been made over the temperature range 630 to 1812 °K of the combustion rates and physical structure of sized fractions (89, 49 and 22 μm) of a brown coal char. Above 900 °K the 89 and 49 μm fractions were found to react with rate control by the coupled processes of pore diffusion and chemical reaction on the pore walls. Below 760 °K these fractions reacted with rate control by chemical reaction alone, as apparently did the 22 μm fraction above 900 °K. The “intrinsic reactivity” Rs of the char to oxygen was given for all three fractions and over the temperature range 630 to 1812 °K, by the empirical relation taking the true order to be zero over the whole temperature range.
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