Kinetics of thermolysis and burning of solid biofuel particles. Part 2. Thermolysis rate of dry and wet wood and straw pellets
In the previous part of this work, it was substantiated necessity to study the kinetics of solid biofuels thermolysis at their rapid heating up in conditions of the oxidative atmosphere, i.e. in conditions close to that of their combustion in furnaces and boilers, the experimental thermogravimetrical installation and procedure of experiments were described.
This article provides information about the characteristics of wood and straw pellets selected for investigation, a method of experimental data processing, obtained results of thermolysis kinetics for single particles of wood and straw pellets.
Determining of pellet thermolysis rate was carried out at furnace heating to 700°C. Particles of pellets having the diameter of 6 and 8 mm at their weight 0.5 g, at moisture content from 0 to 13%, and ash content from 0.4 to 11.1% were used. As a result of experiments video recordings of particles mass change at their fast heating up in an oxidative atmosphere of the furnace were obtained, as well as visually marked time moments of volatile ignition around the particle and that at the completion of volatile release and burning.
It was found what thermolysis reaction in particles of wood and a straw pellet occurs in layers, with propagation from the surface to the center. At the completion of pellet thermolysis, the coke and ash residue is formed which has geometrical form and size almost the same as that of the initial particle of a pellet.
Volatile release begins with a certain time delay from the moment of sample submission into an oven; this can be explained by the necessity of particle heating up to the temperature of chemical reactions of thermolysis start, at that duration of heating up of wet pellets is longer. In this connection, looking for a dependence of volatile release the time should be divided into two periods: the period of particle heating up with an absence of volatile release and period of intensive volatile release.
It was found that at rapid heating up of wet pellet particles the processes of fuel moisture evaporation and biomass thermolysis with the volatile release are combined in time.
It was proposed the thermolysis reaction rate for dry pellets to determine as volatile mass flow assigned to the original area of the outer particle surface, and for wet pellet as mass flow of evaporated moisture and released volatile to the original area of the outer particle surface. Experimentally found that the instant rate of thermolysis is variable - with the rapid growth at the reaction beginning, relative stability at the maximum values and slowing at the end of a volatile release.
The average thermolysis rates during the period of intensive volatile release for investigated dry and wet wood and straw pellets are in quite a narrow range from 17.3 to 26.7 g/(m2s). For wood and straw pellets increasing pellets length up to 25 mm had no significant impact on the thermolysis rate.
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