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. All biomass fuels release volatiles at a much lower temperature than coal and the proportion of volatiles is much greater than for coal, typically 80% compared to 30%. Thus, the rate of release of volatiles from biomass fuels is much more important in the pulverised fuel flame propagation than it is for coal, where the rate of char oxidation is more important. The rate of release of volatiles from dry biomass follows three stages: the first stage for typically 10% of the volatiles occurs over 200–300 C, the second stage of about 70% of the total volatile mass occurs over the temperature range 300–400 C. Finally, there is a slow loss of volatiles, accounting for remaining 20% of volatiles, over the temperature range 400–900 C. Stagg’s quick approximation method was used to determine the kinetics for the rate of volatile yield. Biomass samples were found to have lower activation energies and higher rate of release of volatiles in comparison with coal samples, up to 300–400 C. Similar release rates were found for the 3rd stage of volatile release. The release of volatiles at low temperatures potentially makes the biomass pulverised fuel more reactive and one consequence is shown in the measurement of minimum explosion concentration, MEC. A good correlation was found between activation energies and the MEC, determined on the Hartmann equipment. There is currently little understanding of the composition of the volatiles released at low temperatures from biomass, as most publications are for pyrolysis conditions at high temperature. It is possible that the volatiles are a mixture of mainly H2, CO and CH4 and the likely proportions of these were calculated from the elemental and thermo-gravimetric analysis. This was done for a range of biomasses and this showed that the most important volatile gas is likely to be CO and that CH4 yield is very low. This means that the conventional model used in coal combustion of char plus methane combustion is not applicable to biomass combustion.