Energy & Fuels, Vol.31, No.8, 8228-8234, 2017
Continuous Ozonolysis Process To Produce Non-CO Off-Gassing Wood Pellets
Prior work on the mechanism of carbon monoxide (CO) formation from stored wood pellets has provided the basis for a continuous process that results in the production of pellets that do not off-gas CO. It had been shown that exposure to ozone eliminated the unsaturated hydrocarbons that autoxidize to produce hydroxyl radicals, which, in turn, react with the hemicellulose to produce CO. To develop a practical process to eliminate the CO formation, a kinetic study of the continuous ozonolysis of wood fiber was conducted using a small materials auger. The reaction was found to follow a pseudo-first-order reaction such that the reduction in CO emissions was linearly proportional to the ozone exposure (concentration x time). The exposure needed to reduce or eliminate the formation of CO from the exposed fiber was around 42 000 ppm min at a flow rate of 0.57 kg/min of fiber or approximately 0.032 g of O-3/kg of fiber to be passivated. The volatile organic compounds produced during the ozonolysis of fiber were analyzed with gas chromatography/mass spectrometry. Aldehydes, such as nonanal and decanal, were identified, indicating that oleic, linoleic, and linolenic acids in the fiber were ozonized. To determine the changes in the wood characteristics following exposure to ozone, thermogravimetric analysis was performed and no major changes to the wood properties were observed. To establish the industrial viability of the process, trials were conducted at scale in a commercial pellet mill. Wood pellets produced through this process showed no measurable CO off-gassing given enough ozone exposure, proving the viability of the process. The fuel properties of the resulting pellets were measured and found that the wood pellets produced from the treated fiber exhibit a similar calorific content but somewhat different moisture and ash contents from the non-treated wood pellets.