Mercury content in woody biomass and its removal in the torrefaction process

Abstract

Anthropogenic mercury emissions can be reduced by using biomass. However, the mercury content in the biomass varies within a relatively wide range and can be even higher than in coal. This especially relates to waste biomass, including wood residues. In some cases, its use may result in increased emissions. A solution may be subjecting the woody biomass to the torrefaction process before its utilisation.In the paper, samples of woody biomass derived from different parts of a tree were analysed: wood, bark, branches, leaves, needles and roots. The performed studies showed significant differences in mercury content: wood 0.8–5.7 μg kg−1, roots 1.9–6.5 μg kg−1, branches 1.2–10.3 μg kg−1, bark 6.0–43.5 μg kg−1, needles 19.8–29.6 μg kg−1, leaves 40.3–67.3 μg kg−1 (on dry basis). The torrefaction process allowed for a reduction of the mercury content in the woody biomass regardless of the part of the tree from which it was obtained. The effectiveness of mercury removal at 300 °C was between 83 and 99%, with an average of 90%. In light of the results obtained, torrefied biomass should be classified as a very low mercury fuel, below 0.3 μg MJ−1.