Chemisorption Trends of Oxygen on Freshly Pyrolyzed Wood Charcoal

The ignition of wood is a multistage process, with the first major thermo-chemical conversion step being the pyrolytic disintegration of the material, forming a highly reactive intermediate, charcoal. Dependent on the pyrolysis temperature and duration, charcoal can exhibit different properties. In this study, preliminary results from a recently started project, concerned with the impact of the pyrolysis temperature and duration on the reactivity and the propensity for chemisorption of oxygen on freshly formed charcoal, are reported. Milled beech and spruce wood are examined in a Simultaneous Thermal Analyzer (STA) by first being pyrolyzed at six consecutive heat treatment temperatures (HTT), 350 ℃ to 600 ℃, with 50 ℃ increments under nitrogen flow. After cooling to room temperature, a combustion step is performed in air to a maximum temperature of 600 ℃. During this heating step an increase in the mass (TG signal) of the charcoal samples can be seen and at same time the DSC signal already exhibits exothermal events. These exothermal events are considered to be the result of oxygen being chemisorbed on the porous charcoal structure with a high internal surface and one of the reasons for self-ignition of charcoal. By producing wood char at specific HTT followed by a combustion step, the propensity of the charcoal of two of the most used European wood species towards chemisorption of oxygen and heterogeneous combustion are methodically analyzed at a microscale.