High-temperature pyrolysis of biomass pellets: The effect of ash melting on the structure of the char residue

The formation of ash during pyrolysis of biomass pellets (corn straw and rice husk) was studied, at high temperature conditions where the formation of tars is minimized. In particular, the influences of the melting ash on the physical and chemical properties of the biomass chars were examined. Cylindrical pellets (9 by 19 mm) were pyrolyzed at three temperatures (1200, 1300, and 1400 °C) and three resident times (10, 20, and 30 min), at moderately high heating rates. The biomass behavior during pyrolysis and the properties of the formed chars were assessed by a variety of experimental techniques. Results show that the char yields decreased with increasing pyrolysis temperature and increasing residence time. The char yields from rice husk were significantly higher than those from corn straw. The ash on the surface of corn straw pellets melted and polymerized, and balls of ash formed and agglomerated, especially at the highest temperature. Molten ash of corn straw contained Ca, O, Al, Mg and Si, forming mainly CaAl₂Si₂O₈, quartz, and cristobalite. In contrast, the surface morphology of rice husk pellets remained intact and the molten ash on the surface of chars consisted mainly of quartz and cristobalite. In the case of corn straw, the melting ash blocked the pore structure and decreased the accessible surface area. To the contrary, in the case of rice husk, the effect of ash melting on its pore structure was negligible and the accessible surface area increased upon pyrolysis. As the pyrolysis temperature increased from 1200 °C to 1400 °C, the carbon structure of the biomass chars became more orderly. Overall, increasing the pyrolysis temperature and duration increased the amount of pyrolysis gas, decreased the char yields and decreased the extent of ash melting and polymerization on the surface of the chars.