Monitoring Chemical Changes for Stored Sawdust from Pine and Spruce Using Gas Chromatography-Mass Spectrometry and Visible-Near Infrared Spectroscopy

Fuel pellets that are very often made of sawdust represent a renewable energy source for heat production and there is a growing interest in these products in a large part of the world. Raw material production in the sawmills and use of sawdust in the pellet mills are out of phase during a large part of the year and for reasons of logistics there is a need for storage of large quantities of the raw material. Long-term storage changes the sawdust and, therefore, processing parameters have to be adapted, making some knowledge of storage time or maturity necessary. An experiment on the industrial storage of pine and spruce sawdust was carried out over a period of 16 weeks. Samples were taken out every week and all samples were analysed by visible-near infrared (vis-NIR) spectroscopy, while some samples were analysed by GC/MS for their composition of fatty- and resin acids. The resulting data were subjected to multivariate data analysis. GC/MS data showed the difference between pine and spruce sawdust and the influence of maturity. This maturity effect could be associated with the decrease in fatty- and resin acids due to auto-oxidative reactions. The first six weeks of storage had no influence on the concentration of fatty- and resin acids. Most of the changes in the amount of fatty- and resin acids occurred in the fresh pine sawdust during weeks six to 12. Since the spruce sawdust had less initial amounts of fatty- and resin acids, the changes were less in absolute terms but the fractional changes were similar to pine. During weeks 13 to 17 the fatty- and resin acids content had stabilised and changes were marginal. Multivariate analysis of the vis-NIR data showed a major effect due to maturity associated with a colour change and also weaker effects of fatty- and resin acid differences. PLS regression was used to predict the storage time with RMSEP values between 10 and 15 days. However, since weather conditions, precipitation and seasonal variation, have a very strong influence on the rate of maturing of sawdust it will be necessary to monitor the stored sawdust to determine the degree of maturity. Because of the large flow rate human eye assessment should be replaced by frequent vis-NIR measurements. The vis-NIR spectra allow prediction of storage time and therefore maturity.