Pyrolysis characteristics and kinetics of biomass residuals mixtures with lignite

Biomass residues in the Mediterranean region come mainly from agricultural and agro-industrial activities and forest byproducts. The paper presents the results of kinetic parameters and main devolatilisation characteristics of three biomass materials, when these are used either alone or in conjunction with Greek lignite. Namely, olive kernel, forest and cotton residues were pyrolysed in a thermogravimetric analyser, under dynamic conditions. The effect of material particle size and heating rate was investigated both on the pyrolysis behavior and reaction kinetics, over the temperature range of 25-850 degreesC. Furthermore, experiments with blends of lignite and biomass were conducted under the same conditions at the lower heating rate. The biomass materials presented higher thermochemical reactivity than lignite. Their decomposition was successfully modeled by three first-order independent parallel reactions, describing the degradation of hemicellulose, cellulose and lignin. No significant influence of the particle size was detected, both on the devolatilisation characteristics and kinetics. The effect of the heating rate on the pyrolysis behavior was more pronounced for biomass materials rather than lignite. A comparison between slow and fast heating rate tests reveals a small displacement of the DTG profiles to higher temperatures. It was concluded that such 'solid biofuels' could support the combustion of poor coals, because of the faster and in much higher quantity release of volatile compounds. (C) 2003 Elsevier Ltd. All rights reserved.