Numerical modeling results of combustion of fibrous sludge are presented and validated in a series of experiments. Combustion experiments were conducted in a thermogravimetric coupled with Fourier transform infrared spectrometer and gas chromatograph mass spectrometer. Sludge material (open matrix of lignocellulosic fibers with inorganic fillers) was generated in pulp and a paper mill during the de-inking process. Mathematical models were developed for solid- and gas-phase combustion. The mathematical model for the decomposition of solidiphase is based on the following assumptions: (1) rate of combustion determined by oxygen mass transfer, (2) laminar gas flow, and (3) negligible radiation. The combustion of aromatic hydrocarbons formed/released during the combustion process is formulated taking the following assumptions: (1) reaction rates of methyl-naphthalene and naphthalene an relatively fast and thereby constitute the driving force for the initiation of combustion; and (2) kinetics rate data for the oxidation of methyl-naphthalene and naphthalene are equal to those of benzene. Numerical computations compare well with measurements and provide good predictions of the reactivity of the material during the combustion process. Mass fraction remaining at the end of the simulation period was predicted within 2% accuracy. Flue gas combustion simulations have shown acceptable results, however the computed overall reaction rate was over-predicted. Predictions of the behavior of major gaseous species (CO2, O-2, CO and PAK) were reasonable. Simulations also revealed the mechanism of solid biomass combustion to start at the center of the sample and then propagate toward the surface. Such information could not be obtained from experimental data. It was also shown that indenyl may play an important role in the pulp and paper biomass combustion and may be considered as a catalyst for ignition. (C) 2000 Elsevier Science Ltd. All rights reserved.