Effects of storage on dry matter, energy content and amount of extractives in Norway spruce bark

Abstract Wood bark is one of the major feedstocks in bioenergy and bioeconomy scenarios in Finland. Currently it is used mainly for producing heat and power, which make buffer storage essential due to seasonal variations in the demand for energy. The storage of bark is associated with problems like the occurrence of self-heating, biomass losses, and the reduction of its quality as fuel. Extraction of valuable extractives from the bark before burning is an interesting option when aiming to increase the value of this side-stream. However, the content of extractives starts to decrease immediately after tree felling and this degradation continues during storage. This study examined changes in fuel quality and extractive amounts during the storage of Norway spruce (Picea abies) bark from sawmills. The moisture content of the bark decreased five percent during the storage period of eight weeks. The temperature in the storage pile rose within one week to over 50° and there was evidence of appreciable initial biological activity within the bark. The dry matter losses of spruce bark were on average six % per month. The content of acetone-soluble extractives decreased significantly, being 66.2% of the original amount after eight weeks’ storage. The most significant changes in extractive content occurred within the two first weeks of storage when 30% of the extractives were lost. This means that in order to utilize these valuable compounds, the material should be sent for further processing as soon as possible after debarking.