Solid biofuels from forest - Fuel specification and quality assurance
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Biomass from forestry sector is able to provide an important contribution to meet the government’s targets for increasing bioenergy use. Traditionally it has been stem wood which is used as raw material for energy. For a deeper understanding of trees, knowledge is required not only of the stem wood, but also of the branches and tree tops. The most important fuel qualitative properties of stem wood, bark, branch wood and tree tops were investigated in order to know the potential of Norway spruce biomass available for bioenergy use in Norway. Considerable variations in qualitative properties between stem wood, stem bark and branch wood of Norway spruce among geographical locations and vertically along the stem were observed. The basic density of stem wood was in range 382.8-523.5 kg/m3, of stem bark 273.2-582.0 kg/m3 and of branch wood 243.5-673.0 kg/m3. Basic density of stem wood decreased from the roof base till 20 % of the tree height and afterwards increased towards the tree top. The axial dependence of basic density in stem bark was different than the one in stem wood, more regular, decreasing towards the top. The vertical density gradient of stem bark in the base was roughly 5-10 % steeper to that in tree top. Branch density decreased moderately within the axial direction along the crown. Branch wood had higher basic density than stem wood within a difference between these two densities of 80-216 kg/m3. The basic density of branch wood decreased in the direction from the branch basis to its top. The greatest decrease was found in the first 20 cm of the branch, and then the decrease was minimal. The branch diameter strongly affected the basic density distribution along the branch. There was found relationship between basic density of stem wood, stem bark, branch wood and geographical locations. The highest basic density of all collected biomass samples was observed for the eastern part of South Norway, namely Hobøl site. It was also found that the higher site index the higher basic density in this location. The bark proportion and bark thickness were highly linear to the tree height. Spruce bark originated from the middle part of South Norway, Seljord site had considerably higher bark proportion than bark collected from trees from other two geographical sites. The average moisture content of stem wood and stem bark harvested in summer season increased axially from the base toward tree top, within significantly more pronounced variations on the tree base compare to tree top. Stem bark had relatively higher moisture content compare to stem wood. The moisture content in stem wood was in range 36.0-52.3 %, in stem bark 37.6-62.4 % and in branch wood 29.0-67.9 %. The vertical dependence of moisture content in branch wood, collected in summer season originated from Hobøl site differed more than that in Seljord and Vindafjord site. Characteristic chemical components of stem bark did differ considerably from that of stem wood. Chemical composition of stem bark had higher percentage of lignin and extractives (17 % and 40 %, respectively, the remaining 43 % is holocellulose), compared to stem wood (10 % and 6 %, respectively, the remaining 84 % is holocellulose). There were indications that calorific value of woody-based material was highly affected by its chemical composition. The net calorific value of stem wood was in range 4.82-5.23 kWh/kg, of stem bark 5.11-5.45 kWh/kg and of branch wood 5.33-5.52 kWh/kg. Increased in heating value of Norway spruce stem wood, stem bark and branch wood consequently generated higher content of ash. More ever we found that the ash content of Norway spruce branch wood did vary along the branch, whereas the position of branch in crown did not affect the ash content. The ash content of stem wood was in range 0.17-0.22 %, of stem bark 1.49-2.63 % and of branch wood 1.11-2.49 %. Applied combustion process of twigs performed under oxidative atmosphere resulted in higher residue mass compare to the branch base. Elevated net calorific value of stem bark, and branch wood because of their higher amount of extractives and lignin content, make these materials a valuable energy source for bioenergy industry in Norway.