Wood Moisture Content - Impact on BTU's

[ddahlgren]

The not enough moisture I agree with as some wood that was delivered in the past had some pieces that seemed dry rotted as they weighted nothing compared to normal wood and though dry low moisture were terrible burning. i just got a moisture meter in UPS today a General instruments MMD4E. I have learned a bunch playing with it this morning and the biggest is if you go by looks and feel you might be wrong. Also that checking the end grain very important as the slab side might be 14% and the end grain 25%. I am assuming it is because the water gets out by following the grain so the ends maybe a better indicator. Tomorrow sometime I should have a high quality probe thermometer for the stack as the Rutland one is very slow and i do not believe it anyway.

 

[oldspark]

That was just part of the article, the fact water dont burn covers it and covers it well.

 

[Huntindog1]

Here is some more reference material:

http://mb-soft.com/juca/print/311.html#green

http://www.ruraltech.org/projects/conversions/briggs_conversions/briggs_ch09/chapter09_combined.pdf

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr29.pdf

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

http://www.engineeringtoolbox.com/weigt-wood-d_821.html







http://www.engineeringtoolbox.com/wood-biomass-combustion-heat-d_440.html
Note that

• by volume wet wood has about 85% of the energy of oven-dry wood
• by weight wet wood has less than half - 42% - of the energy of oven-dry wood

One weight unit of wood has enough energy to evaporate 6 weight units of water.

 

[Huntindog1]

Wood Heat Calculator, its does all the work for you just make your selections:

http://www.southeastcleanenergy.org/resources/WoodEnergy.swf

 

[Jon1270]

While wood dries, it shrinks, so you get more wood in the same cord of 8x4x4

Since we're throwing numbers around...

An extreme case would be if you dried Overcup oak until it was very, very dry (Overcup oak shrinks a lot). Going from dead green to 15% MC (dry basis), Overcup oak loses about 7.6% of its volume. Madrone shrinks almost as much as Overcup oak, followed by dogwood, most other oaks, then hickory. Quaking aspen, dried to 15%, loses only about 4.4% volume.

In general, the harder the wood, the more it shrinks as it dries.

 

[Ashful]

A couple of things...

I was hoping you'd chime in. Someone who knows enough to really lay it down.

Since we're throwing numbers around... Overcup oak loses about 7.6% of its volume... Quaking aspen, dried to 15%, loses only about 4.4% volume.

Very interesting, but obscure references. Most people are not losing a significant amount of their wood expenditure to shrinkage, and that's not the primary (or secondary, or tertiary...) factor in generated BTU's.

...the fact water dont burn covers it and covers it well.

Yes, and stop signs are generally red. We both have a solid grasp of the obvious. However, we know that some water is always present, and indeed desirable in our firewood. The question was to quantify what effect it has on BTU output, and perhaps explain why. I am sorry if the topic bores you, but I didn't force you to read it.

 

[Jon1270]

Very interesting, but obscure references. Most people are not losing a significant amount of their wood expenditure to shrinkage, and that's not the primary (or secondary, or tertiary...) factor in generated BTU's.

Agreed that it's not a major factor. In fact, that's what I meant to illustrate.

I'm happy to explain where the numbers come from if you like; I thought it might be a bit over the top to do so here.

 

[oldspark]

"Yes, and stop signs are generally red. We both have a solid grasp of the obvious. However, we know that some water is always present, and indeed desirable in our firewood. The question was to quantify what effect it has on BTU output, and perhaps explain why. I am sorry if the topic bores you, but I didn't force you to read it"
MMMM my answer not good enough for ya, I thought it was covered it lowers the temp of the fire box and you have to burn hotter to remove the moisture.

 

[Adios Pantalones]

"Yes, and stop signs are generally red. We both have a solid grasp of the obvious. However, we know that some water is always present, and indeed desirable in our firewood. The question was to quantify what effect it has on BTU output, and perhaps explain why. I am sorry if the topic bores you, but I didn't force you to read it"
MMMM my answer not good enough for ya, I thought it was covered it lowers the temp of the fire box and you have to burn hotter to remove the moisture.

...and expanding water may require more draft... and smokey burning creates a loss of fuel up the chimney... and the mass of water decreases the total percentage of the load that is fuel... and... and

I think that Joful was interested in a full discussion, and though your simple answer covers some of it, it does not cover all or how.

 

[oldspark]

"If you try to burn green wood, the heat produced by combustion must dry the wood before it will burn, using up a large percentage of the available energy in the process"
Water dont burn

 

[Jon1270]

That settles it. We must find a way to keep trees from soaking up all this water in the first place. Roof the forests!

 

[oldspark]

I'm still struggling with "stop signs are generally red", I thought they were all red.

 

[NE WOOD BURNER]

H2O is necessary in gasification? No?

 

[schlot]

Generally red...well unless they are faded I suppose?

I will ask a question to the subject rather than offer an answer.

Since fire is a chemical reaction between (predominantly) carbon and oxygen, aren't BTU's wasted in the process of releasing the oxygen from the bonds they hold with hydrogen as compared to the free oxygen (or combination of gases) with oxygen in the air?

 

[mellow]

A styrofoam cup won't burn till all the water is boiled out.

 

[Dustin]

Owe my head...

 

[Slow1]

A styrofoam cup won't burn till all the water is boiled out.

Yeah, they tend to melt a bit though - however I have boiled water in a paper cup sitting on an open fire before so your point is valid....

 

[Backwoods Savage]

Geeze, we can tell the weather was not too hot today. Looks like a lot of brains got some exercise...

 

[Adios Pantalones]

H2O is necessary in gasification? No?

No, I don't think so.

 

[jatoxico]

OK so I bet I can confuse this further with my 2 cts.

I think one element that has not yet been mentioned is time. Although a a given mass of wood contains a given amount of BTU's, if heat is given off over too long a time period due to excess agua the consequenses are easily understood. So although only a small amount of the total available BTU are used to remove the water the time is extended for the process.

In addition although the # of BTU consumed may vary only marginally on a percentage between wet and dry wood (I'll accept all the OP's #'s) the amount of water present between a 35% split and a 20% split is substantially more. That amount of water absorbs enough heat to reduce the temperature of the burning fire such that those volatiles present that might burn at higher temps (even in an open fire) are unconsumed and float off unused reducing the amount of actual BTU's produced from the theoretical amount. This, in part is what the OP was thinking, I think .

 

[denjohn]

No one has commented on this .........bumping it, looks to be very informative and objectively addresses the OP's question.

Wood Heat Calculator, its does all the work for you just make your selections: http://www.southeastcleanenergy.org/resources/WoodEnergy.swf​

 

[TradEddie]

No one has commented on this .........bumping it, looks to be very informative and objectively addresses the OP's question.

That looks like an interesting and possibly useful tool, but the point of the OP is that the theoretical difference in heat output from 30%mc to 20%mc is modest (<20%), while the real-world observed difference, in secondary combustion stoves at least, is huge.

TE

 

[Ashful]

...the point of the OP is that the theoretical difference in heat output from 30%mc to 20%mc is modest (<20%), while the real-world observed difference, in secondary combustion stoves at least, is huge.

Ding, ding, ding! A few of you are catching on, now.

 

[Huntindog1]

Being that these are secondary burn tube stoves. Secondary Burn gets your efficiency increased by lets say for example , an additional 30% by burning wasted smoke gases.

So lets just throw the , how much energy it takes to burn a lb of water, out the window.

If you cant get the stove into the secondary burn mode of operation then you just lost the 30% you were supposed to gain by buying a energy efficient EPA stove.

 

[oldspark]

Ding, ding, ding! A few of you are catching on, now.

I got it many posts ago but after burning wood for over 30 years in both EPA and non EPA stoves and knowing first hand how crappy wet wood is to burn I never gave the numbers much thought.