# What is the cost of energy in purchased wood – 1 ton of birch wood and humidity 50%?

Purchase made 1 t of fresh birch wood for the price 85 zloty. Wood moisture determined by the method of weighing the sample before and after drying at the temperature of. 105° C until constant weight was reached 50%. Means, that a purchase was made 0,5 t of wood i 0,5 t of water. The calorific value of wood with moisture 50% is 8,2 GJ / t.

Calculations:

The price of energy in wood:

85 PLN / t ÷ 8,2 GJ / t = 10,36 PLN / GJ

If the wood is left to dry to humidity 15% (24 months under the roof) appropriate sizes will be:

ms = 0,51 t
mw = x t
mw/(ms +mw) = 0,15
mw = 0,09 t

After the seasoning period, the weight of the wood will be:

m = ms +mw = 0,51 + 0, 09 = 0,6 t

The calorific value of wood with moisture 15%, according to the table is 15,79 GJ / t,
hence the energy value of our wood is equal:

0,6 t × 15,79 GJ / t = 9,47 GJ

and the price of energy in wood will be:

85 PLN ÷ 9,47 = 8,97 PLN / GJ

Thanks to the drying of the wood, the user has gained 1,39 PLN / GJ of energy, which means 13,4 % less compared to the price on the day of purchase. It is not the whole benefit, because dry wood is used in the boiler or fireplace with efficiency at least 20% higher than wet wood, therefore the benefits will increase even more. We would get by burning wet wood 4,9 GJ / t of heat useful for efficiency 60%, and the cost 1 GJ would be 85 PLN / t: 4,92 GJ / t = 17,28 PLN / GJ.

For dry wood, the appropriate values ​​are 7,58 GJ/t i 11,21 PLN / GJ, and this is already Fr. 35% cheaper when burning dry wood than when burning wet wood.

The method of weighing wood and measuring moisture is the best method to determine the energy value of wood delivered to large boiler houses. The user should pay for the supplied energy, and not for the weight, a significant part of which is water.