BTU chart accuracy/consistency

[Jeffm1]

As I have compared btu charts on the internet I have noticed a rather wide discrepancy between them. If you have not already seen this yourself, take a look and compare between species you are familiar with. Here are a bunch of pages:

(broken link removed)

http://mb-soft.com/juca/print/firewood.html

http://www.firewood-for-life.com/firewood-btu.html

http://worldforestindustries.com/forest-biofuel/firewood/firewood-btu-ratings/

http://forestry.usu.edu/forest-products/index#menu1
(Click "wood heating" link)

http://www.grit.com/farm-and-garden/best-firewood-btu-zm0z12sozmoo

http://www.offroaders.com/tech/firewood-BTU.htm

(broken link removed)

http://www.capecodfirewood.com/firewood-btu-chart/

http://www.engineeringtoolbox.com/wood-combustion-heat-d_372.html

http://pages.sssnet.com/go2erie/FirewoodChart.htm

I wonder where the data comes from originally. What makes for the variances? Conditions the trees grew in such as soil, weather, etc? Geography? Age of tree?

 

[pernox]

Geography, and therefore the other things you mentioned such as soil, climate, etc probably has a lot to do with it. I often wonder if measurements are conducted at a standard moisture content, as well. At the risk of sounding like a scientific layman, Is the energy carried away in steam accounted for?

 

[Poindexter]

On the one hand a pound of cellulose has a fixed, certain number of BTUs in it no matter how tightly packed it may be.

On the other hand we are trying to keep people warm, and there are dozens of variables in between.

I think the two primary variables for a home owner to control are MC of fuel and flue gas temp.

If your wood is wetter you are going to use more of the BTUs in that pound of cellulose driving out the last of the water so the stick can actually burn.

Flue gas (exhaust) gas temperature should be just high enough that you don't get creosote deposits in your pipe. Higher than that and you are wasting heat up the chimney. Too low and you got to brush the pipe all the time.

The EPA HHV and LHV values were a stab at making that uniform, the High Heat Value supposedly some kind of measure of potential and the Low Heat Value a closer approximation of what really happens.

At the end of the day wood burners "waste" or "lose" or "invest" some of their BTU stash in pumping water vapor and potential creosote up and out the chimney.

 

[Jeffm1]

On the one hand a pound of cellulose has a fixed, certain number of BTUs in it no matter how tightly packed it may be.

On the other hand we are trying to keep people warm, and there are dozens of variables in between.

I think the two primary variables for a home owner to control are MC of fuel and flue gas temp.

If your wood is wetter you are going to use more of the BTUs in that pound of cellulose driving out the last of the water so the stick can actually burn.

Flue gas (exhaust) gas temperature should be just high enough that you don't get creosote deposits in your pipe. Higher than that and you are wasting heat up the chimney. Too low and you got to brush the pipe all the time.

The EPA HHV and LHV values were a stab at making that uniform, the High Heat Value supposedly some kind of measure of potential and the Low Heat Value a closer approximation of what really happens.

At the end of the day wood burners "waste" or "lose" or "invest" some of their BTU stash in pumping water vapor and potential creosote up and out the chimney.

So why such big differences in btu values from chart to chart in what is supposed to be a uniform measurement? My guess is there is quite a bit of variation from sample to sample even though it's the same species. For example, trees that grew in an environment with low amounts of water are going to grow slower and therefore denser and therefore contain more BTUs than the same species that grew in a more moisture rich environment and the tree (wood) grew faster and is not quite as dense.

 

[peakbagger]

My employer tests large biomass power plant boilers on occasion. We don't trust any charts rather we use what is called the "losses method" where we measure or estimate all the heat losses from the boiler and the flue gases plus measure the energy taken out in steam and back calculate the energy input. Its a very complex process. We do take samples of the fuel as a check and input to some intermediate calculations but even if we have a good btu snaphot from testing we cant measure the weight of the fuel going in accurately and despite trying to get well mixed piles, the moisture content varies significantly from the top of the pile to the bottom. The fuel is also going through various decomposition processes from the day its cut so age of the wood also factors in.

As long as moisture content is in the mix regard all these charts as relative values.

 

[Poindexter]

I agree terroir probably makes a difference in cord wood.

Peakbagger has some really good observations about sample inconsistency.

The other one not touched yet in this thread is how do experimentally determine how many BTUs are in that cord?

I could stack a cord of spruce for you easy. You got a tank that will hold 20 million pounds of water, then i will light the fire under the pot and you monitor the water temperature in the tank..

I stopped worrying about it, but the answer to how much wood i need to put up is always more, more more.

For starters I would toss out all the tables that dont list the test conditions on the same webpage as the table of results.

Once you have tossed the shysters, idiots and the liars we can compare test conditions and results simultaneously from there.

 

[Highbeam]

I don't expect that the charts are nearly as sophisticated as you folks think. I expect that it is simply a matter of species density and a constant (say 20%) MC value for all samples and an assumption of a constant btu per lb of wood.

Efficiency of your appliance obviously has nothing to do with the chart.

So all they do is determine the lbs per CF for each of the listed species at 20% MC and then the rest is math.

The density of the sample can vary greatly for several reasons and this is what leads to the variability.

 

[peakbagger]

There are a couple of ASTM tests procedures for determining BTU content. The one I am most familiar with is "bomb calorimetry". It can give very accurate values of the specific sample you are testing. They take the moisture out of the equation by pre-drying to bone dry before running the test. The test gives results on BTU/lb basis so wood density isn't an issue.

The problem is all the errors that stack up in informal estimating. A cord is volume measurement not a weight basis. How tight is it packed? I expect the volume of wood could vary easily by 30%. Even if its identically stacked volume wise we need to account for wood density. Old growth super dense wood is going to have a higher density than aspen. Even if its the same species growing conditions will alter the grain pattern which alters the density. Then the moisture can vary significantly add in more error.

It all comes down to figure out what locally available wood works best for you, get it split into preferably small splits, let it dry under cover in loose stacks for a minimum of two years and burn it with adequate oxygen and temperature to combust it completely.

 

[CTYank]

On the one hand a pound of cellulose has a fixed, certain number of BTUs in it no matter how tightly packed it may be.

Flue gas (exhaust) gas temperature should be just high enough that you don't get creosote deposits in your pipe. Higher than that and you are wasting heat up the chimney. Too low and you got to brush the pipe all the time.

The EPA HHV and LHV values were a stab at making that uniform, the High Heat Value supposedly some kind of measure of potential and the Low Heat Value a closer approximation of what really happens.

At the end of the day wood burners "waste" or "lose" or "invest" some of their BTU stash in pumping water vapor and potential creosote up and out the chimney.

Not really, about the smokepipe deposits. If you have complete combustion, there's nothing to deposit, except maybe a tiny bit of fly ash.

The big limiting factor re low flue temp, without forced draft, is that you need to keep the flue warm enough to draw air. Some forced-draft designs have gotten past that, with flue gases then having most all available heat removed. Richard Hill, UMO ME Prof, has done some work along those lines.

 

[maple1]

Not really, about the smokepipe deposits. If you have complete combustion, there's nothing to deposit, except maybe a tiny bit of fly ash.

The big limiting factor re low flue temp, without forced draft, is that you need to keep the flue warm enough to draw air. Some forced-draft designs have gotten past that, with flue gases then having most all available heat removed. Richard Hill, UMO ME Prof, has done some work along those lines.

Since there is still moisture in the flue gases, you still need to stay above condensation point temperature or you will have a wet mess in there.

 

[peakbagger]

Actually you just hit on the difference between HHV and LHV, HHV takes into account the latent heat of vaporization, LHV doesn't. If the flue pipe is made of stanless and there is drain for the condensed liquids, there is no real issue in allowing the vapors to condense unless the system needs the temperature differential to avoid a fan. If there is a hot fire, the liquid produced is just water but if the boiler is run at partial load without enough air for complete combustion then its a nasty foul liquid. (heat it and cool it a few times and it becomes creosote) The European district heating systems use stainless flues and they have a use for the low temperature water so they install a ID fan and pump out the gases at very low temps. One method used is a direct contact heat exchanger where cool water is sprayed in the flue gases. The water heats up by condensing the hot vapors in the gas. It also saturates the exhaust stream so when the plume hits the cooler air outside the stack, it forms a vapor cloud giving the characteristic plume associated with power plants (which is mostly water vapor). Professor Dick Hill's wood boiler used an ID fan to allow for this subcooling in the heat exchangers ( it also allowed for his innovative top loaded wood feeder)

 

[kennyp2339]

Richard Hill, UMO ME Prof, has done some work along those lines.

A man well ahead of his time, I loved watching his video's.