The combustion of biomass follows a sequence of several distinct phases which begin with drying, the subsequent pyrolysis with the release of volatile components that burn in gaseous phase and finally the combustion of carbonaceous resiue, the time consuming step. For small solid fuel boilers, as it is the case of pellet boilers for domestic heating, biomass particles are continuously burned in fixed bed boilers or heaters. In these appliances the furnace is a metallic basket into which fuel particles are thrown and burn. The combustion air is introduced through orifices drilled in this metallic basket although some lateral air entrances are also found. The gaseous flow is a piston flow, the particles fall under gravity on the upper bed surface and as they burn they slowly move downwards until their size is small enough to fall down through the air entrance orifices or are dragged by the upcoming combustion gas flow. So the gaseous current has an up flow movement while the solid particles move downwards. It is assumed that the bed is at uniform temperature and all the average properties of the gaseous flow are constant with the exception of the oxygen concentration that diminishes as the gas flow rises through the bed. The mathematical development of a simple model that allows the calculation of the steady state burning time of a biomass particle in fixed bed, the amount of energy released, the fixed bed size and its particle inventory, is hereby presented. The pedagogical interest of this model is pertinent because it presents, in a synthetic way, the relative importance of the combustion kinetic, the mass transfer mechanism and the reactor fluid dynamics, upon the lifetime of a biochar particle during its combustion process.
A preliminary non-exhaustive study was conducted on the ignition of some ligneous biomass pellets inside a laboratory scale traveling bed furnace. The experiments consisted in the measurement of the ignition time of volatiles released by six different types of pellets, obtained from wood species found in the Portuguese forest, namely Pinus pinaster, Acacia dealbata, Cytisus scoparius, Cistus ladanifer, Paulownia cotevisa and Eucalyptus globulus. The experiments were carried out at corrected furnace temperatures of 359, 381, 403, 424 and 443 °C, using two different pellet sizes and with batches of 6 and 8 g of pellets. The ignition time was determined measuring the time elapsed between placing the batch of pellets on the traveling grate and the volatiles’ ignition moment. Its dependency was linear, and an increase in ignition time with the furnace temperature was verified. Pinus pinaster was the species that presented a higher ignition time and Cytisus scoparius the shortest. For the same pellets size, an increase in the mass of batches led to shorter ignition times.
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