The influence of size and morphology on devolatilization of biomass particles

Abstract

This modeling study examines the effect of particle morphology on devolatilization of biomass particles at conditions relevant for suspension firing. A model, which can calculate devolatilization times and particle temperatures for both spherical and cylindrical particles of a wide size range is established, and modeling predictions are compared to experimental data from literature with good consistency. The model accounts for the influence of aspect ratio and allows for biomass particles to be modeled as cylinders; a more accurate representation than the classical spherical approach. If a spherical approach is desirable due to limitations in computational costs, it is found to be more accurate to retain the diameter of the cylindrical biomass particle in the calculations rather than to adjust the radius to create a sphere with the same volume as the original particle. An analysis of the relative effect on devolatilization time of relevant physical parameters for three particle sizes (dp = 79 μm, 0.8 mm, and 3.1 mm) is presented, evaluating both kinetic and heat transfer limitations. In general, model results show that the time for full devolatilization varies with more than two orders of magnitude in the particle size range (dp = 0.08–3 mm) relevant for suspension firing. For the relevant gas temperature (Tg = 1300–1900 K) and density (ρ = 400–1000 kg/m3) intervals, the devolatilization times approximately doubles within both interval ranges. Variations in moisture content primarily influence the time for onset of devolatilization, which may affect flame stability in suspension fired boilers.