Development of a Variable Activation Energy Model for Biomass Devolatilization

A thermogravimetric balance is used in this work to characterize different classes of biomass fuels: residues (rice husks, olive cake, cacao shells), woods (poplar, beech, pellets), and grasses (mischantus). The effect of the heating rate is evaluated in the range 10-80 K/min providing significant parameters for the fingerprinting of the fuels. Kinetic parameters are obtained by applying traditional isoconversional methods. The activation energy as a function of the conversion reveals the multistep nature of the biomass devolatilization. Although average values allow the reactivity of different fuels to be compared, a first-order reaction model can hardly predict the biomass devolatilization in the whole range of conversions. A VEB (variable activation energy model for biomass devolatilization) model is developed, based on the results of the kinetic analysis, maintaining a simple kinetic scheme. A good agreement is obtained for the biomass residues in all HR runs in the entire range of temperatures. The multistep mechanism can be studied without assuming any chemical components or pseudocomponents, thus limiting the number of model parameters. Similarities in the optimized VEB curves for the fuels studied in this work give useful generalization parameters for biomass devolatilization modeling.