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Moisture Content

The Effect of Moisture to Energy Content

The vertical axis refers to wood’s energy content in relation to weight (kWh/kg). The horizontal axis describes the moisture content percentage (%).

Fresh tree has about 50–60 % water. Fire wood is classified as of high-quality when its moisture content is 15–16 % or less. Wood chip is of high-quality if its moisture content is 25–30 %. The table illustrates the effect moisture content has on energy content. Source: VTT.

Dry Wood – Multiple Benefits

The importance of dry wood in fuel use cannot be overplayed. Were the wood in chips or logs, energy efficiency decreases rapidly when the wood is moist. Dry wood gives considerably more energy when burned than moist wood.

In large-scale energy trade, energy is bought in kilowatts or megawatts. The buyer pays according to the energy gained from the raw material bought. This is the method for example in power plants which buy wood chips, peat and reed canary grass.

Actors in wood trade should recognize the benefits which are gained when dry wood is sold instead of moist wood. Both buyers and sellers would benefit of this. Why transport expensively wood which contains so much water, instead of transporting a cargo of wood and wood only, thus maximising the economic benefit? Rationalising wood trade and maximising benefits is the interest of everyone.

Measuring wood’s moisture content

The moisture content of wood can be measured with a hygrometer, or with the following method used by large-scale buyers, such as power plants, when measuring the moisture content of wood chips and peat:

Measuring the percentage of moisture

Wood’s initial percentage of moisture can be measured by comparing wood’s wet weight with its dry weight according to the following formula: (wet weight - dry weight) / wet weight x 100

Defining dry weight

  • Place wood chips, small wood sticks or bits of peat in a dish.
  • Weigh the dish with the goods.
  • Put in oven at 105 Celsius degrees

The dish is weight at equal intervals until the weight stops decreasing. For total drying, 16 hours should be enough if the particle size is less than 30 mm. Now the wood’s moisture content should be 0 %.

Example

Wood’s wet weight is 200 g, dry weight is 150 g.

  • Formula: (200-150) g / 200 g x 100 = 25
  • The initial moisture content of the wood was 25 %

In practice, it is useless to burn wood that has the moisture content of 55% or more, because the energy value is as low as 2kWh/kg, which equals the energy loss of a heating boiler. The energy-efficiency of dry wood is more than double than that of wet wood, up to 4–5 kWh/kg.

  

Chip and peat samples in foil dish, going first to weighing and then to oven.


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