Wood chip based power production can increase the electrical efficiency by firing dry wood chips, instead of wet wood chips.
The lower calorific value of wood chips can be calculated with the following formula:
HL = 19MJ/kg x DSw – 2,45 MJ/kg x (1-DS)
The first term in the formula is the calorific value of wood; the second term is the subtraction from the water content. This means, the higher dry substance,the more energy you will get. E.g. if we take 1 kg dry substance of wood, we will have the following lower calorific value.
40%DSw = 2,5 kg wood chips x 6,13 MJ/kg =15,3 MJ
50%DSw = 2 kg wood chips x 8,28 MJ/kg = 16,6 MJ
90%DSw = 1,11 kg wood chips x 16,86 MJ/kg = 18,7 MJ
From this we can see that we get app. 13% more energy by using wood with 90% DSw instead of wood with 50% DSw. Drying the wood chips in a conventional drum dryer before burning them, will consume app. 15%. The steam dryer will use app. 12% to dry the wood chips, but it will return the energy as steam with 4 bars of pressure. This means that by installing a steam dryer on a wood chip based power plant, the consumption of wood chips (for the same amount of electricity produced) will be reduced 10 – 15%.
Alternatively flue gas condensation can be installed, but this can only generate hot water for feed water heating or district heating. This will not produce more electricity. With dry fuel it is possible to increase the electrical efficiency, because it’s possible to reach higher temperatures. Boiler and flue gas cleaning components will be smaller, because less flue gas will be created.
Power plant steam diagrams are quite complicated and detailed.This diagram is very simple, but it shows an example of how an EnerDry steam dryer can be intergrated in a power plant.