Effects of storage and geographical location on fuel quality of Norway spruce forest residues

University essay from SLU/Dept. of Energy and Technology

Abstract: Sweden is a forest rich country with a total land area of 40.8 million hectares of which 22.7 million hectares are categorized as productive forest land. More than 50% of this is owned by private owners. The total standing volume on the productive forest land is about 3400 million m3. The dominant forest species are Scots pine and Norway spruce, which constituted about 38% and 27% respectively of the Swedish productive area (Swedish Forest Agency, 2010). This abundant availability of forest biomass plays a prominent role in the Swedish energy supply. According to Swedish Energy Agency (2008) renewable sources meet 43% of the total energy requirements in Sweden, out of which a considerable figure of 23% is fulfilled by wood fuels. Forest residues are a potential source, which can be utilized for bioenergy purpose, and the demand for it has increased in the wood fuel market in recent years (Björheden, 2006). In Sweden, the highest demand for forest residues occurs in the cold winter season since it is mostly used for heat production. Storage is therefore necessary to meet the demand. Storage and handling of forest residues is a complex process. The forest residues which include needles and twigs are vulnerable for varying weather conditions i.e. temperature, wind, humidity, air and precipitation. Furthermore other factors including duration of storage, placement of windrows and the initial quality of the forest residues have an impact on the fuel quality. For all these reasons it becomes necessary to skilfully manage the storage of forest residues and thereby ensuring high fuel quality. The aim of this thesis work was to evaluate and compare the effect of prior summer storage (brown storage) or direct windrows storage of freshly harvested biomass (green storage) on the fuel quality of Norway spruce forest residues at three different geographical locations in Sweden. In this study Norway spruce forest residues were collected at three geographical locations in southern Sweden after storage. At each locality numbers of sites were chosen. Freshly harvested forest residues were then stored at each location by using two storage methods. The first storage method was referred to as brown storage (prior summer stored forest residues). In this method the forest residues were stored in small piles for at least 20 weeks at the clear cut after harvesting and then they were gathered into windrows at landing for further storage. The other method used was referred to green storage (freshly harvested and stored forest residues) where the biomass was gathered to windrows after a few weeks storage in small piles at the clear cut. The investigated fuel characteristics included; moisture content, ash content, calorific value and net calorific value. Moisture content decreased significantly after storage in both green and brown stored forest residues. After one year storage the rate of moisture content was lower in brown stored forest residues as compared to green stored forest residues. Brown forest residues had lower ash content than green stored at the end of the storage period. This was probably due to the amount of needles which was larger in green stored material than in brown stored. After one year storage the calorific value and net calorific value had almost reached the same value irrespectively of storage method. Therefore from energy point of view, it can be concluded that there was no significant difference between green and brown storage methods. However, green storage method has an advantage such as shorter storage duration of forest residues before gathering into windrows than brown storage on the same site. Moreover, it is more time saving and economically profitable compared to brown storage.

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