Self-heating effects in the thermogravimetric analysis of wood char oxidation

• Published in: Fuel (Guildford) Vol. 276; p. 118012
• Main Authors: Branca, C., Di Blasi, C.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 15-09-2020; Elsevier BV
• Subjects: Char; Devolatilization; Diffusion rate; Hardwoods; Heat transfer; Heating; High temperature effects; Kinetics; Mass transfer; Oxidation; Pyrolysis; Self-heating; Softwoods; Thermogravimetric analysis; Wood; Oxidation; Kinetics; Wood; Char; Self-heating
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/j.fuel.2020.118012

Thermogravimetric curves are measured, by means of a flexible home-made (HM) system, of the oxidation of wood chars generated from packed-bed pyrolysis at 800 K. A careful control of the sample temperature and a small mass/size (4 mg mass corresponding to 140micron thick particle layer) allow a kinetic regime to be established for heating rates between 5 and 10 K/min. Good quantitative predictions are obtained based on a two-step mechanism, with activation energies affected by the hardwood or softwood origin of chars (125–143 kJ/mol and 186–207 kJ/mol for the devolatilization and oxidation reaction, respectively). The same conditions applied for a standard commercial (SC) thermogravimetric system result into remarkable self-heating leading to uncontrolled ignition and too slow oxygen diffusion rates. Only extremely small sample masses (≤1mg) and slow heating rates (≤5K/min) permit to completely eliminate heat and mass transfer intrusions but the actual limits are strictly related to the specific char reactivity.