Rapid devolatilization of pulverized coal

Abstract

The rapid devolatilization of a lignite and a bituminous coal was studied by electrically heating in helium approximately monolayer samples of small particles supported on wire mesh heating elements. The samples were rapidly brought to a desired temperature, held there for a desired time, and then rapidly cooled. Devolatilization rates, measured by weighing samples before and after experiments of known duration, were determined as a function of residence time (0.05–20 sec), temperature (400–1100°C), heating rate (102–104°C/sec), pressure (0.001–100 atm), and particle size (50–1000 μm). Devolatilization kinetics were determined by non-isothermal techniques since substantial reaction occurred during heating even under the most rapid heating rates. Weight loss from both coals was essentially complete within a fraction to a few seconds depending upon temperature, and increased with increasing final temperature up to 900 to 950°C. Weight loss (corrected to its value at a fixed temperature) was found to be independent of pressure, heating rate and particle size for the lignite, i.e., it depended only on temperature and time; but for the bituminous coal it increased with decreasing pressure, decreasing particle size and, to a small extent, increasing heating rate. The general reaction scheme appears to involve thermal decomposition forming volatiles and initiating a sequence of secondary polymerization and char-forming reactions. The kinetics and yields of the primary decomposition are successfully described by a set of independent first-order parallel reactions represented by a Gaussian distribution of activation energies around a mean of 56 kcal/mole for the lignite, with a standard deviation of 11, and 51 kcal/mole for the bituminous coal at 69 atm and 70 μm particle diameter, with a standard deviation of 7. For the bituminous coal it was necessary in addition to allow for pressure- and particle-size-dependent secondary reactions representing competition between char-forming reactions and diffusional escape of volatiles. Attempts to correlate the data in terms of a single first-order reaction lead to an overall activation energy (∼10 kcal/mole) that is considerably lower than the mean activation energy of the multiple-reaction system, and to a different set of kinetic parameters for each set of experimental conditions. Conditions such as lower pressure, smaller particle size, and better particle dispersion which help to diminish the effect of secondary reactions appear to be more important than rapid heating in the production of volatile yields in excess of the volatile content obtained by proximate analysis.