Comparison of Rice Husk and Wheat Straw: From Slow and Fast Pyrolysis to Char Combustion

• Published in: Energy & fuels 2013-11, Vol.27 (11), p.7115-7125
• Main Authors: Pottmaier, Daphiny, Costa, Mário, Farrow, Timipere, Oliveira, Amir A. M, Alarcon, Orestes, Snape, Colin
• Format: Article
• Language: English
• Subjects: Combustion; Conversion; Graphitic structure; Pyrolysis; Rate constants; Reaction kinetics; Straw; Wheat
• ISSN: 0887-0624; 1520-5029
• DOI: 10.1021/ef401748e

Thermochemical conversion of world top crops (rice and wheat) has been extensively investigated (TGA, DTF, SEM, XRD, BET, EA), and main insights are discussed in light of materials and process kinetics. Overall, the results show that the rice husk presents lower reactivity than the wheat straw for all thermal processes regardless of the final temperatures (300 °C–1300 °C), residence times (0.6 s–300 min), and atmospheres (100–340 mL·min–1 N2/air). The higher reactivity of wheat straw is attributed not only to higher alkali and ash contents but also to differences in both silica morphology and graphitic structure after pyrolysis. Chars produced from slow pyrolysis present more homogeneous characteristics than those produced from fast pyrolysis. Combustion of the chars from slow pyrolysis (up to 900 °C) show similar kinetic parameters with activation energies, E a, of 101.8 and 101.0 kJ·mol–1 with pre-exponential factor, A, of 4.3 × 107 and 9.6 × 107 min–1 for rice husk and wheat straw, respectively; while chars from fast pyrolysis (up to 1300 °C) show a range of values. Reaction times at 90 wt % loss (min) and rate constants k o (min–1) gives a more clear difference in values even for chars from slow pyrolysis with 12.4 and 0.221 for rice husk and 4.3 and 0.499 for wheat straw, correspondingly. These results are discussed herein according to changes in the physical and chemical characteristics of the nascent chars and, consequently, on their reactivity.