(596b) Structural and Compositional Transformations of Biomass Chars during Slow and Fast Pyrolysis

In this work the physical and chemical transformations of biomass chars during fast pyrolysis, considered as a 2^nd stage of combustion, has been investigated. Seven biomasses containing different amount of ash and organic components were divided into five different particle size fractions from 50 um to 1 mm and reacted at up to 1673 K with high heating rates in a wire-mesh reactor. The fast pyrolysis data was compared with the slow pyrolysis results from the fixed bed in terms of the char yield and burnout.

In order to obtain information on the structural and compositional transformations of the biomass chars from the fast pyrolysis, samples were subjected to elemental analysis, scanning electron microcopy with EDX, FTIR, Raman confocal spectrometry and XRD. The results show that there are significant changes in both the organic and inorganic constituents of the chars.

The scanning electron analysis showed that under high heating rates (> 600 K/s) char particles underwent different types of melting and pores of different size were developed in dependency on the temperature and biomass composition. Samples with high lignin content tended to melt completely and developed micropores, while the samples with the high hemicellulose content softened and transformed into the separated molten carbon skeleton particle with the form similar to the virgin biomass particle and with inner and outer macropores.  The results were validated against the data from the single particle burner (lab-scale entrained flow system), where the particles > 3 mm were pyrolyzed at the operational conditions similar to the wire-mesh reactor.

From the lab-scale experiments on the wire-mesh reactor, it was observed that the final temperature has more significant effect on the char yield than heating rate. The applied variation of holding time from 1 to 4 sec and a particle size fraction from 50 micron up to 1 mm showed that the char yield depends mainly on the biomass type. The char yield from the pyrolysis in a wire-mesh reactor for the particle size fraction from 0.85 mm up to 1 mm is not different from the char yield in a single particle burner for the particle cube sizes of  3 mm and 5 mm. The FTIR analysis showed the loss of main functional groups with the increasing pyrolysis temperature, indicating the raw sample graphitization/crystallization. From the Raman measurement, with the increasing pyrolysis temperature slight changes in graphitization were observed, while more significant changes were seen in the amorphous carbonaceous chars’ structure. The influences of heating rate and temperature on the structure of fast pyrolyzed chars were determined from the XRD measurements.