Global rates of devolatilization for various coal types

Abstract

This modeling study reports global rate expressions for transient weight loss and yields of tar and gas for coals across the rank spectrum. A novel kinetic analysis demonstrates that the Distributed Activation Energy Model (DAEM) reliability correlates transient weight loss at various heating rates, and makes reasonably accurate extrapolations for pulverized coal (p.c.) firing conditions. This performance motivates solutions of the DAEM for arbitrary thermal histories, including approximate closed-form solutions for transient weight loss along linear temperature ramps and exponential temperature histories. Our analysis also rigorously defines the rate constants in single first order reactions that give the same devolatilization rate as the DAEM at every instant in any thermal history. For devolatilization during heatup at constant rates, frequency factors increase by a factor of 6 for every order-of-magnitude increase in heating rate, while activation energies remain constant. Throughout convective heating, however, both rate parameters decrease continuously, so that straight lines on Arrhenius diagrams do not represent rate constants for coal devolatilization. Rate variations for different coal ranks are modest by comparison. For ranks from lignite to high-volatile bituminous, rates decrease by up to a factor of 3. Low volatility coals are much more resistant to thermal decomposition; they devolatilize at rates up to 5 times slower than all other lower ranks. Correlations for different coal types from FLASH2, a more detailed devolatilization model, describe separate gas and tar evolution rates, and replace hypothetical ultimate yield parameters by accurate predictions for ultimate tar and gas yields.