Pyrolysis kinetics of wood pellets in fluidized beds

This paper deals with the pyrolysis of wood pellets with 6 mm diameter in order to get information about the kinetics of their thermal degradation. The determination of the pyrolysis kinetics takes place by means of an optical and a gravimetrical method. The optical method applies UV light absorption by two LEDs of different wavelengths. With this approach it is possible to record the amounts of tar produced online. The gravimetrical method records the mass loss during pyrolysis reaction.

The characterization of the kinetics uses two different approaches. The first one shows the time for 50% value of total mass at different temperatures, while the second method – as many papers in literature – uses the one-component model with first order employing the conversional mass. The second method describes – in contrast to the first method – the trend of mass loss by pyrolysis process at different temperatures.

The combination of the gravimetrical and optical method delivers a rate constant for the thermal degradation of pellets to gas. One of the two methods alone is not able to predict the rate constant for gas, just for tar or combination of tar and gas. The kinetic data presented in this paper are “apparent kinetics” as there is a limitation of the pyrolysis by heat transfer and diffusion. The data in this paper are usable directly to design fluidized bed reactors in opposite to the data with pulver in literature that do not reflect heat transfer limitation within the sample.