Mechanistic Pathway of Carbon Monoxide Off-Gassing from Wood Pellets

The off-gassing of carbon monoxide (CO) from stored wood pellets has been identified as a significant problem, potentially resulting in adverse occupational and residential exposures. The mechanism for the production of CO from wood pellets has not been fully identified. In this study, a multiple step process has been hypothesized. The reaction is initiated by the autoxidation of unsaturated compounds, including fatty acids and terpenes, by molecular oxygen. As a byproduct of these reactions, hydroxyl radicals are formed. Then, the bulk of CO results from the reactions of hemicellulose and hydroxyl radicals. To understand the mechanistic pathway of CO off-gassing, a number of experiments were conducted in which CO was measured and evolved organic compounds were analyzed using gas chromatography mass spectrometry (GC MS). These studies identified a number of short- and long-chain aldehydes from the evolved gases that indicates the autoxidation mechanism. However, there is insufficient mass of these unsaturated compounds in wood to support the observed mass of off-gassed CO. However, autoxidation would form hydroxyl radicals. The role of hydroxyl radicals was investigated using a radical scavenger, and its role in CO production was confirmed Thus, if the autoxidation initiation can be eliminated, then CO off-gassing from pellets would be substantially reduced. Destruction of the reactive compounds with ozone led to a suppression of CO formation, suggesting an approach to process the wood fiber that would result in low or no CO emission wood pellets.