calorific value of wood??

[educated arborist]

It is my understanding that dry wood of any species has a similar heat output when burnt, the only difference being the speed at which it burns.

 

nope, not at all. The wood with the highest calorific output per ODT is a species of pine! yet by volume it is ash. The equasion to work out calorific value relates to mass which is useless when we come to talk about firewood. The forestry commision identified that we should be concerned by how many KJ's/KWh the end user will see from the product.

[Albedo]

nope, not at all. The wood with the highest calorific output per ODT is a species of pine! yet by volume it is ash. The equasion to work out calorific value relates to mass which is useless when we come to talk about firewood. The forestry commision identified that we should be concerned by how many KJ's/KWh the end user will see from the product.

 

I've been doing a bit of googling on this to see what can be found.

 

It seems that the method for obtaining calorific value is to stick a dry sample in a calorimeter and measure the heat emited when burned and as you say this is done by mass so you get an answer in kj/kg

 

The problem for the original poster seems to be that a bomb calorimeter is an expensive bit of kit. There are other types and I'm looking to see if it can be done more cheaply.

 

I'm quoting you because:

 

A. Its not done by equation (initially), its done by a procedure - prep of sample and use of calorimeter... then perhaps some equations.

B. Just wonder why you say the use of mass is useless and that it must be done by volume. As to do the procedure with a calorimeter its done by mass.

 

Also whether the info is deemed useless or not its what the OPs question was.... "Anyone know how to calculate CV?"

 

If he can get on and do it he can get the information which he wants on his own which seems to me to be quite a good idea, hence my interest.

 

smile just added for safety as I have no axe to grind, just interested

[Pete Bannister]

nope, not at all. The wood with the highest calorific output per ODT is a species of pine! yet by volume it is ash. The equasion to work out calorific value relates to mass which is useless when we come to talk about firewood. The forestry commision identified that we should be concerned by how many KJ's/KWh the end user will see from the product.

 

I hate to be a pedant...well ok so no I dont.. but.. are my eyes deceiving me or are you really mixing mass with volume again? This is bound to confuse.

The matter is quite simple. If people want to know how much they are paying for the energy they buy then they shoud buy solid flues such as wood by weight ( as you would with coal) and at a known moisture content. Where's the difuculty in that? Its far easier to measure the weight of a load of firewood than to estimate its volume (becausing of bulking ratios) and, with a hand-held moisture meter and a few minutes you can acurately estimate the total moisture content of the load ( I reckon on + - 5% accuracy). Subtract that from the load weight and you have the 'dry wood' mass (weight). From that you can estimate calorific value from published data and the now know dry biomass. Its not rocket sceince and if folk wished it, they could buy or sell in theoretical units of 'dry wood' or even potential 'thermal units' regardless of moisture content or deceptive bulking ratios.

[Albedo]

The wood with the highest calorific output per ODT is a species of pine! yet by volume it is ash.

 

This is quite a complex subject isn't it.

 

I think I get this bit though.

 

The species of pine referred to is probably less dense than ash therefore if youre gonna put a few grams in the calorimeter (the same weight for the pine and the ash) you'd need a bigger piece of pine than you would for the ash, .... thus making the ash more efficient by volume assuming that its CV was the same or greater than that of the pine.

 

 

That made my head hurt

[firewoodman]

If people want to know how much they are paying for the energy they buy then they shoud buy solid flues such as wood by weight ( as you would with coal) and at a known moisture content. Where's the difuculty in that? Its far easier to measure the weight of a load of firewood than to estimate its volume (becausing of bulking ratios) and, with a hand-held moisture meter and a few minutes you can acurately estimate the total moisture content of the load ( I reckon on + - 5% accuracy)[/i]. [/b]Subtract that from the load weight and you have the 'dry wood' mass (weight). From that you can estimate calorific value from published data and the now know dry biomass. Its not rocket sceince and if folk wished it, they could buy or sell in theoretical units of 'dry wood' or even potential 'thermal units' regardless of moisture content or deceptive bulking ratios.

 

sorry mate, but you cant ( not that i fully understand your statement)

 

quote in bold, i accuratly measure my volume for delivery, not when split fresh, there is no estimation

 

second quote in bold you will most definatly find the wood at the top of the pile is much drier and contains less moisture than the wood in the center of the pile, no matter what the size of the pile is.

 

also if i had to by scales for what ever reason to weigh the wood, then that just pushes up the price of the product for the customer, and i have no available use of a weighbridge.

 

but thanks for your input, its very helpful:thumbup1:

Edited August 12, 2010 by firewoodman

[Peter]

i accuratly measure my volume for delivery, not when split fresh, there is no estimation

 

You cant accurately measure the volume of the wood, unless you can stack it with no air gaps, or you have a way of measuring the air gaps?

 

If you weighed it, on the other hand, you would get an accurate measurement.

[firewoodman]

You cant accurately measure the volume of the wood, unless you can stack it with no air gaps, or you have a way of measuring the air gaps?

 

If you weighed it, on the other hand, you would get an accurate measurement.

 

i see the point now:blushing:

[davetaz]

None of my customers seem to care about what species of firewood they get (so long as it's hardwood!), I don't think they would be bothered about which timber produces most heat, so long as "it does what it says on the tin" and they come back for more.

The firewood buying public need to become a lot more educated about other aspects of firewood before they need to worry about specific calorific values.

Sell well seasoned wood (hard or soft) by volume, bacause that's what they want

[Pete Bannister]

Hi Firewoodman, how do you accurately measure the volume for delivery? Unless you have some special Welsh magic?! I'll stick to my previous view which t'other Peter has also made above.

I agree with you about the variability of moisture content in any batch of timber. The way this is addressed is to measure many samples throughout the batch. That way a statistically significant estimate can be accurately made. The more samples tested and the wider their distribution, the more accurate your estimate will be. This is what hand held moisture meters were made for. In split logs I can estimate a few m3 in less than 2 mins with a simple moisture meter. The readings need to be from below the surface of each piece of timber ( best place is equidistant from the core and the outside or the piece and not too close to end grain ). With baulks or cordwood a similar method is used. I use a 6 mm diam probe on a moisture meter and drill 8 mm holes up to 300 mm long to quickly obtain useful readings to larger sections.

 

On the matter of overhead costs: I take your point, but its dead easy to rig up a balance scales using known weights. Again you don’t have to weigh the entire load if you use a logical and statistically valid sampling method. If the method is sound and if it’s simple it can be safely duplicated. Provided you clearly state your methods for estimating, you shouldn’t get into trouble from Trading Standards Officers. If you can provide accurately valued firewood, then it will carry a preimum if you have the market for it. As you might have guessed, I measure material moisture content in materials as part of my job!

Anorak or what!

[John Barleycorn]

You cant accurately measure the volume of the wood, unless you can stack it with no air gaps, or you have a way of measuring the air gaps?

 

If you weighed it, on the other hand, you would get an accurate measurement.

 

Well, Archimedes managed to figure it a couple of thousand years ago

when he stepped into a bath and noticed that the water level rose — he suddenly understood that the volume of water displaced must be equal to the volume of the part of his body he had submerged. This meant that the volume of irregular objects could be calculated with precision, a previously intractable problem. He is said to have been so eager to share his realisation that he leapt out of his bathtub and ran through the streets of Syracuse naked.