Exposure to carbon monoxide off-gassed from stored wood pellets can be a major health problem leading to critical occupational and residential exposures. Hazardous incidents and fatalities have been reported from carbon monoxide exposure related to transportation and storage of wood pellets (Gauthier, 2012). The use of wood pellets is on the rise across the world in view of the high demand for high-density, concerns about climate change, renewable energy, and the long-term decline of fossil fuel resources.
Wood pellets are easy to handle, have a homogeneous quality, and high energy density, which make them competitive with fossil fuels for heating applications. In view of the fact that wood pellet consumption as well as production has risen considerably in the last few years, concerns relating to health issues and safety due to CO exposure resulting from off-gassing from wood pellets are on the rise.
Researchers have mounted studies to analyze the processes that result in formation of carbon monoxide from stored wood pellets. A previous study has shown that autoxidation of unsaturated compounds, such as terpenes and fatty acids is responsible for the production of carbon monoxide from wood pellets. It was also shown that ozonation of wood fiber passivated the reactive hydrocarbons under static conditions.
Unfortunately, the kinetics of continuous ozonolysis reactions has not been determined. Elucidating the kinetics would be required for analyzing the feasibility of a continuous process as demanded in a commercial pellet mill. Researchers led by Professor Philip Hopke at Clarkson University in New York studied the kinetics of continuous ozonolysis of wood fiber and implemented those details to adopt ozonolysis at industrial scales. They showed that a considerable reduction of carbon monoxide emissions achieved at low cost in the pellet production process would eliminate one barrier limiting the expanded use of wood pellets. Their research is published in journal, Energy & Fuels. This work was supported by the New York State Energy Research and Development Authority intend to provide technical assistance to all pellet mills in New York to eliminate CO off-gassing from pellets with the intention of removing the hazard to consumers
The authors observed that the reaction followed a pseudo-first-order reaction implying that the reduction in carbon monoxide emissions was linearly proportional to the ozone exposure. They also observed that the exposure required to minimize or eliminate completely carbon monoxide formation from the exposed fiber was approximately 42000 ppm min at about 0.57kg/min of fiber or about 0.032g of O3/kg of fiber to be passivated.
The research team investigated the volatile organic compounds generated during the ozonolysis of the fiber implementing gas chromatography. They identified aldehydes such as decanal, and nonanal. This indicated that linoleic, oleic, and linolenic acids were ozonized. The authors also performed thermogravimetric analysis in a bid to analyze changes in wood characteristics following exposure to ozone. However, no major changes in the wood characteristics were observed.
In order to establish an industrial viability of the proposed process, the researchers performed trials at scale in a commercial pellet mill (in collaboration with scientists at Queenaire Technologies and Curran Renewable Energy). The pellets produced through this process had no measurable carbon monoxide off-gassing when given sufficient ozone exposure, therefore this provided the viability of the process.
Fuel attributes of the produced pellets were established and indicated that the wood pellets produced from the treated fiber had a similar calorific value content but had different moisture and ash contents from non-treated pellets.
Mohammad Arifur Rahman, Stefania Squizzato, Richard Luscombe-Mills, Patrick Curran, and Philip K. Hopke. Continuous Ozonolysis Process to Produce Non-CO Off-Gassing Wood Pellets. Energy Fuels, volume 31 (2017), pages 8228−8234.Go To Energy & Fuels