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Calculating Kilowatt-hour output of firewood, anyone?


luke_mclachlan
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Hi all! I've read a lot about "universal" kWh/kg values for firewood, with oven dry @ 5.25 kWh/kg and 60% moisture @ 1.68 kWh/kg.

 

Do any of you know the process for measuring the calorific value, expressed in kWh/kg, for firewood. I've Goggled around and found nothing, and science wasn't my subject of choice at uni :confused1:

 

Thanks

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The normal way to measure CV is via a bomb calorimeter, which isn't as exciting as it sounds.

 

Small amount of fuel in a twin-skinned metal box, with water between the two skins.

 

Set light to the fuel and measure how much warmer the water gets.

 

Since we know the volume of water and the specific heat capacity per unit (i.e. how much energy has to be put into water to make it warm up by one degree) we can then work out how much heat has been given off by the burning fuel.

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It would very interesting to see if your testing confirms the usual figures bandied about. Have always heard 4.0-4.1 kWh per KG @ 20% moisture content with some of the softwoods being the highest. Think TCD posted a table for a multitude of species calorific values.

 

I have always taken this as gospel Typical calorific values of fuels

Edited by Woodworks
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Hi all! I've read a lot about "universal" kWh/kg values for firewood, with oven dry @ 5.25 kWh/kg and 60% moisture @ 1.68 kWh/kg.

 

Do any of you know the process for measuring the calorific value, expressed in kWh/kg, for firewood. I've Goggled around and found nothing, and science wasn't my subject of choice at uni :confused1:

 

Thanks

 

It is also possible very reliably to link the calorific value to the moisture content. The amount of energy required per unit to take water in the wood from room temperature to 100 deg C and then vapourise it is fixed, see latent heat capacities.

 

Oddly, it means that the calorific value of wood changes with the temperature it is held at.

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ok guys thank you very much for the responses. I had a feeling that it would be complicated and certainly not something that I could measure on the kitchen scales.

 

The problem I have with the figures from biomassenergycentre, and anywhere else in fact, is that they're missing a source and an explanation of the methodology. Though you never know, they may be spot on!

 

I'm going to have to collaborate with a scientific lab on this one, unless anyone here has a calorimeter and knows how to use it :thumbup1:

 

daltontrees, I see what you mean about calorific value changing according to the temperature it's being held at. This is therefore why higher moisture values correspond to a lower calorific value :thumbup1:

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The reason it changes with moisture content is the huge energy boost needed to boil off the water. The specific heat capacity of water is 4.187 KJ per degree kelvin. So stored at 20c room temp, then to heat the water in the log to 100 is roughly 400kJ. To boil that water off so that it can go up the chimney, the latent heat of evaporation is 2270 kJ per kg. So that same Kg of water will take a further 2270Kj to boil. so roughly 6/7 of heat to boil and 1/7 of heat to get to 100c. Hope that helps.

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daltontrees, I see what you mean about calorific value changing according to the temperature it's being held at. This is therefore why higher moisture values correspond to a lower calorific value :thumbup1:

 

Not quite. The higher the moisture content the lower the calorific value. The higher the holding temperature, the higher the calorific value. The two things are related insofar as driving off misture takes up energy that would otherwise be given off as dry heat. The colder and wetter you start, the more energy is wasted.

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The reason it changes with moisture content is the huge energy boost needed to boil off the water. The specific heat capacity of water is 4.187 KJ per degree kelvin. So stored at 20c room temp, then to heat the water in the log to 100 is roughly 400kJ. To boil that water off so that it can go up the chimney, the latent heat of evaporation is 2270 kJ per kg. So that same Kg of water will take a further 2270Kj to boil. so roughly 6/7 of heat to boil and 1/7 of heat to get to 100c. Hope that helps.

 

Yeah, what he said!

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Not quite. The higher the moisture content the lower the calorific value. The higher the holding temperature, the higher the calorific value. The two things are related insofar as driving off misture takes up energy that would otherwise be given off as dry heat. The colder and wetter you start, the more energy is wasted.

 

I completely failed to realise that part of the calculation, thank you daltontrees!

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