Global kinetics of the rate of volatile release from biomasses in comparison to coal

The chemistry involved in the propagation of pulverised biomass flames is not well understood. However, the release of volatiles from the biomass under low temperature heating plays an important role in the flame propagation. All biomass fuels release volatiles at a much lower temperature than coal and the characterization of this volatile release is an important part of a pulverized biomass flame propagation model. Rate of release of volatiles from dry biomass follow three stages: the first stage occurs over 200 – 300oC and accounts for typically 10% of the volatile loss. The second stage is the rapid mass loss of about 70% of the total mass over the temperature range 300 – 400oC. Finally, there is a slow loss of volatiles accounting for about 20% of the volatile loss over the temperature range 400 – 900oC. These three stages were present in the coal samples but at much higher temperatures. Release of volatiles were normalised to the total volatiles which were a much higher proportion of the total mass, typically 80%, than for coal where volatiles were typically 30%. Thus, the rate of release of volatiles for biomass flames is much more important in the flame propagation than it is for coal flames where the rate of char oxidation is important. Stagg’s quick approximation method was used to determine the kinetics for the rate of volatile’s yield for the combined 1st and 2nd stages and 3rd stage separately of volatile release. Biomass samples were found to have less activation energies in comparison with coal samples for the 1st and 2nd stages showing their higher rate for the release of volatiles, whereas the rates were similar for the 3rd stage. A good correlation was found between activation energies and the minimum explosive concentration ’MEC’ determined on the modified Hartmann equipment. Also simple calculations were performed for a number of biomasses based on the elemental and proximate analysis for the characterization of volatiles considering H2, CO and CH4 as main products. TGA analysis involves slow heating and flames have fast heating and it is known that fast heating increases the volatile release. A limiting case of all the fixed carbon being converted to CO was also investigated.