Shape and size transformations of biomass particles during combustion

Combustion of individual particles of different woody and agricultural residue biomass have been studied under a laboratory scale rapid-heating apparatus. Particles used were in the size range of 300–1400 mm and weight 0.5–7 mg. A wire mesh element is used to radiatively heat the particle to 1200–1400 K. The apparatus allows a high-speed camera to record the combustion of the individual particles directly. Examination of the resulting video images showed a sequential combustion of volatile matter followed by burn-out of the remaining char for all fuels. Analysis identified differences and patterns in burnout time, combustion behavior and the evolution of char size and shape transformations.

Heterogeneous behavior was observed between the different biomass samples and also among particles within some of the samples. Particles with initial prolate (fibrous) shapes have been observed to become more equant (quasi-spherical) during combustion. Measurement of the particle dimensions during its combustion extracted from the images of the high speed recording have allowed evaluation of the size and shape changes to be mapped. Particle size and shape appears to change only slightly during devolatilization and swelling was rarely observed. Following devolatilization, during the remaining char combustion, more pronounced changes in the size and shape of the particle are apparent. In most cases the shrinking char becomes more rounded as the char particle partially melts and contracts due to surface tension. This transformation is more distinct in some of the biomass samples. Profiles and images of the different fuels examined are presented.