Within-tree Fuel Quality of Loblolly Pine (Pinus taeda)

Abstract

There is a lack of knowledge in the whole tree’s fuel quality and how that quality might change within the tree or between size classes of trees. A sample set of 20 loblolly pines (5 trees within 4 diameter at breast height (DBH) size classes) were sampled at 5 cuts across the trunk and 4 sections of the tree’s crown and limbs with respective height and diameter measurements. Fuel quality in this study specifically measured and compared the proximate analysis, higher heating value, and the ultimate analysis of loblolly pine crowns and ash content, density, higher heating value, and ultimate analysis of loblolly pine bark-free stemwood. Ash content (db.) comparisons and correlations were found to progressively increase on average from the base of the tree (0.36%) to the top of the crown (1.68%). Higher heating value increased from the lowest stemwood section (20.878 MJ/kg) to the highest crown section (21.381MJ/kg) and is significantly larger than all of the stemwood disc sections. It was discovered the current notion of ash’s negative effect on energy content is not supported with the finding of minimal increase of authigenic ash content as it changes across the tree’s total height in both the crown and the stemwood sections. Individual regression results on each tree found a general increase in ash correlated to diameter of the respective crown and stemwood disc sections, not the height. DBH class regression results yielded only the tree’s DBT as the only indicator for predicting ash content and HHV. Ultimate analysis yielded the chemical composition of the loblolly pine samples. In crown sections, Dulong calculations consistently underestimate the HHV of crown samples with a root mean square error (RMSE) of 2.80 MJ/kg and Boie equation estimates the crown sample HHV much better with a RMSE of 1.09 MJ/kg. Double bark thickness in both stemwood and crown regression analysis was shown to be the only significant variable to understand the ash and HHV variability within individual trees. Proportional sections as determined by the tree’s total height and crown length proved to be useful in determining fuel quality changes. The lower half of tree crowns can be utilized as a bioenergy feedstock if harvested with only authigenic ash content is low enough (<1%) for the combustion process. The residue could be gathered and processed into chips or pellets to use in high-valued fuel processes or fossil fuel co-combustion. The results from this study is useful for bioenergy fuel quality purposes and can serve as a baseline for understanding the fuel quality variability between tree components on a per tree or DBH class.