Fuel devolatilization in packed bed wood combustion

Abstract

Packed bed combustion is the burning of solid fuel particles supported by a grate with the combustion air supplied from below. The combustion process is divided into four main stages: drying, devolatilization, volatiles combustion and char combustion. Biomasses proposed as renewal energy sources, such as wood, have a very high volatile content (∼80%). Therefore mechanistic models developed for the prediction of bed characteristics during biomass combustion must include devolatilization and volatile combustion stages in order to correctly predict combustion behaviour for better emissions control and process efficiency. A novel in-situ sampling method for tar, a major pyrolysis product, was developed that allows its concentration to be measured at various heights within the packed bed and appears to work satisfactorily. A series of experiments on packed bed combustion were conducted in a laboratory 'pot' type combustor. Two different equivalent particle size diameters (2.8 cm and 3.2 cm) of untreated spruce wood and two different airflow rates (0.025 kg/m2s and 0.03 kg/m 2s) were tested at a 22 cm bed height. Although the experimental data show scatter, the measurements indicated that pyrolysis occurred primarily within two particle diameters of the top of the bed, with large amounts of tar and CO and somewhat less CO2 being produced. This research also expanded a numerical model for packed bed combustion of solid fuels with the addition of a simple first order pyrolysis reaction, in which fixed proportions of the products were set as light volatiles of CO and CO2 with the balance as tar. The model results compared well with bed temperature, particle size and density measurement throughout the bed and gas concentration (CO, CO2, O2, and CH4) measurements in the reduction and oxidation zone.