Predicting the macroscopic combustion characteristics of diverse forms of biomass in p. p. firing

Abstract

This study validates a transient, zero-dimensional analysis for suspension firing of biomass in cylindrical shapes with specified particle density and aspect ratio (AR). The associated heat transfer coefficients determine the thermal histories for devolatilization which govern both ignition delays and volatiles combustion times for raw and torrefied biomass forms. The analysis accurately interprets ignition delays for four woods and two agricultural residues (AgRes), including two torrefied samples. It is also accurate for volatiles combustion times for six woods, two grasses, and two AgRes, including four torrefied samples. Simulations for a single biomass form with uniform moisture level over the ranges of density and AR for all biomass forms display enormous variations in the volatiles combustion times, even though the same devolatilization kinetics were implemented with all sets of particle properties. Char burnout times were insensitive to char oxidation kinetics from 850 to 1500 °C and from 5 to 100% O2. All cases gave burning rates limited by film diffusion of O2, and thermal annealing and ash encapsulation had to be omitted from CBK/E to match the predicted and measured char burnout times. Very large variations in the combustion characteristics of biomass in p. f. firing are inherent, because neither particle density nor AR are uniform throughout any biomass sample. These inhomogeneities are apparent in the spreads in reported ignition delays for different size cuts for a single biomass form, which were half as large as the spreads predicted for the complete ranges of density and AR over all biomass forms.