Sawmills’ by-products as a raw material

Sawmilling processes always produce by-products: chips, sawdust and bark. These material flows are actually very significant: form an unbarked saw log one gets

1. about 45–50 % sawn timber,
2. 28–32 % chips,
3. 10–15 % sawdust
4. 10–12 % bark.

Formation of by-products:

1. Bark is peeled off when the log goes through the debarking machine, before the actual sawing begins
2. Sawdust is generated during sawing, edging, trimming and chipping. There is two types of chips: wet and dry.
3. Wet chips come during the sawing process of timber
4. The dry chips are a product from processes after the kilning.

By-products are an important part of any sawmill’s profitability, because about 15% of sawmills’ revenues come from the by-products.

Most important one for the revenue stream is the chips. Sale of sawdust and bark is not as important, but they are often used in other processes as raw materials or fuel at sawmill’s own plants. Sometimes these commodities are also traded with.

Chips

Over 90% of chips are used for pulp and paper production, the rest is utilized as raw materials for fibreboard and chipboard production. Dry chips are used as a fuel for energy production.

The average density of chips used for pulp production is on average 145–170kg/ mÑ. Chips used for pulp and paper production must be screened to rid too big or small chips. The quality definitions of chips are based on the requirements of the end use: these can be the size of chips and bark content. Impurities such as plastic, sand and stones are not allowed at all.

Sawdust

The further processing of sawdust is split quite evenly between pulp production, energy production and board manufacturing. Unlike chips, sawdust does not require further processing once it has been produced. The most important properties of sawdust for the fibreboard and chipboard production are the grain size and bark content. The loose density of sawdust is 136–153kg/ m3.

When used for energy production the moisture content of sawdust is of great importance. Wet sawdust’s heat of combustion can be improved by mixing it with some dry sawdust. Sawdust produced at trimming and grading plants is especially dry and fine, and thereby it is most suitable for energy use.

Sawdust is nowadays also being used in the making of bioethanol as a fuel for vehicles. Wet sawdust is suitable for this production. In this process the cellulose of sawdust is first split, thereby releasing the sugars with the aid of heat, pressure and enzymes. When ethanol is produced the resulting by-product is lignin, which can be utilized in the power plants when electricity and steam are being generated.

Bark

Bark is used primarily at the mills’ own power plants as a fuel for heat energy. At sawmills the bark generates the heat used for sawmills’ sawing and drying processes. The energy use of bark is dependent on its moisture content and the moisture of the bark depends on how the logs have been transported and stored.

The dry content of bark must be minimum 35%, otherwise it does not pay to burn it. The dry content is established by measuring a bark sample as wet and then also as dry, when the moisture has been evaporated. Finally, one calculates the dry content over the wet content as a percentage. The dry content and heat combustion of bark can be increased by adding on some dry sawdust into it. Bark can also be used for various end uses in landscaping and agriculture, as well as gardens and greenhouses.

When logs are being sold they are measured over bark, meaning that the bark is included in the stumpage price.

If the logs have been measured without bark one can calculate the corresponding over bark measurement, by adding the from the diameter tables the volumes or by adding on the bark functions to the measured diameters.