Green Wood Considered Harmful

ewdudley · Jan 29, 2011

"For a number of years I have been familiar with the observation that the quality of wood fuel is a decreasing function of its moisture content. More recently I discovered why the use of the green wood has such disastrous effects, and I became convinced that green wood should be abolished from all 'gasification' wood burning appliances." [With apologies to EWD.]

Or so the story goes. You never know until you try.

I didn't have any green wood, plus I wanted to be sure it was indeed green, so I took a few minutes to go out to fell and split a couple hundred pounds of the real thing this morning. [See photos.]

Now even I will concede you can't burn what isn't lit, so I placed a six-inch bone-dry well-split round over the nozzle , filled the firebox with nice fresh wood, and touched it off.

Well I can't say it burned normally, but with a couple of adjustments it certainly burned hot and clean.

First off I placed a 'throttle' plate over 60% of the nozzle so as to avoid the rookie mistake of too much carburetor.

Next I changed my return temperature setpoint from my normal 71 degC / 160 degF to 80 degC / 176 degF.

Primary air was increased from 1mm to 2mm (whatever that means).

And flue neck setpoint was changed from 145 degC / 293 degF to 175 degC / 347 degF.

The load gasified from the get-go, flue gases were transparent, and rate of heated water production was greater than normal (due to higher flue neck setpoint).

Certainly there was a certain amount of live steam going up the flue, but it hardly seems as disastrous as I would have expected.

ewdudley · Jan 29, 2011

More photos.

Round display is supply minus system load return in kelvins.

Little rectangular display is flue neck in centigrade.

Right hand display is supply in Fahrenheit.

tom in maine · Jan 29, 2011

Great post Eliot!
Anyone who measures stuff in Kelvin is okay with me!!

heaterman · Jan 29, 2011

Please proceed as you describe for the rest of the winter, examine your flue tubes and report back.

Also, bear in mind that wood cut right now has about half the moisture content of wood cut during spring, summer or fall.

Hunderliggur · Jan 29, 2011

heaterman said:
Also, bear in mind that wood cut right now has about half the moisture content of wood cut during spring, summer or fall.

Where is my saw? Going to go cut some big ones right now!

The "harmful" green wood is the pressure treated stuff, the other green wood is just extra work.

Fred61 · Jan 29, 2011

heaterman said:
Please proceed as you describe for the rest of the winter, examine your flue tubes and report back.

Also, bear in mind that wood cut right now has about half the moisture content of wood cut during spring, summer or fall.

Mother nature has it all figured out. She reduces the amount of water in a tree just enough to keep the wood from splitting when it freezes. Water is funny acting stuff. It expands when it is heated and it expands when it freezes. Pretty unique stuff. We're lucky to have it!

ewdudley · Jan 29, 2011

earlll said:
stand downwind from the smoke & cop a smell?

There is no trace of smoke whatsoever, so no smoke to smell.

The gasification jet photo does not do justice the to the true iridescent whitish blue flame. It's burning as bright and clean as you could ask for.

--ewd

ewdudley · Jan 29, 2011

heaterman said:
Please proceed as you describe for the rest of the winter, examine your flue tubes and report back.

I'll leave that as an exercise to the student. But I wouldn't recommend attempting this with a Garn where you have no direct control over water jacket temperature.

sdrobertson · Jan 29, 2011

I can't argue with your testing.....could you describe the throttle plate that you placed over the nozzle. Is this just to slow down the "fuel" going down the nozzle by reducing the size? I'm interested in how it preformed over the entire burn (i.e. after the dry split was used up). Did the one dry split produce enough heat to dry out the rest of the load so that you could keep coals and gasification.

rombi · Jan 29, 2011

oh no you didn't....... This has always been of interest to me as I am a burner of a smoke dragon I can tell you junk wood is more of a mess than green wood but I do not have anyone close to complain.
Right now I am watching wood that was cut 2 weeks ago burn. It is white and mostly water as far as I can tell out or the stack. Throw some junk wood in and watch the black smoke roll and smell it.

Fred61 · Jan 29, 2011

I think you should burn whatever suits you. I learned the value of burning dry wood when the air tight stoves started coming to the US from Sweden and Denmark.That was pre Vermont Castings. I know what I want to burn but I'm keeping it a secret. After reading the above posts I've concluded that talking about which moisture content is correct is about the same as talking politics.

ewdudley · Jan 29, 2011

sdrobertson said:
I can't argue with your testing.....could you describe the throttle plate that you placed over the nozzle. Is this just to slow down the "fuel" going down the nozzle by reducing the size? I'm interested in how it preformed over the entire burn (i.e. after the dry split was used up). Did the one dry split produce enough heat to dry out the rest of the load so that you could keep coals and gasification.

The DP45 has an approximately 1"x9" refractory slot for a nozzle, which works fine for a big hot fire with properly dry fuel. But for my contrived example I needed to sustain a small hot fire long enough to heat the water jacket and the fuel up hot enough to really bake the water out of the green wood. For a small hot fire you need a smaller nozzle to create the mixing, turbulence, and velocity to sustain a nice little gasification jet. So I just laid a piece of mild steel bar stock over about sixty percent of the nozzle slot.

The dry wood was more than a split, it was round that I split that weighed close to ten pounds, which was plenty to get the water jacket up to the 176 degF setpoint. I peeked inside the firebox along the way and by the time the dry wood was turning to charcoal there was plenty of green wood that had dried out around the edges and it was able to maintain steady flue temperatures and blue flame the rest of the way after I increased the primary air setting.

Oh, and did I mention that's black locust? It's the exception that proves the rule. If I had attempted this stunt with oak or maple I'd have boiler with a steaming black mess in it instead of storage tanks with 750 gallons of 185 degF water in them. ;-)

Cheers --ewd

Dave T · Jan 29, 2011

Is the moisture that is boiled out of the wood harmfull to the fire box? That is my question, I would have to guess yes it is harmfull to the boiler and will reduce firebox life. YAY or NAY

heaterman · Jan 29, 2011

EW. I'm confused. What are you attempting to prove with your experiment?

ewdudley · Jan 29, 2011

heaterman said:
EW. I'm confused. What are you attempting to prove with your experiment?

Edsger W. Dijkstra popularized the edict that the 'goto' statement is 'considered harmful' and 'should be abolished'. Which is perfectly true, but nonetheless it's possible to come up with examples of situations where the goto can work nicely.

Likewise, it's perfectly true that green wood should be abolished. Which is not to say it is impossible to burn it cleanly and efficiently. But in coming up with an example of what it takes to successfully burn green wood correctly, we can understand better what it is about green wood that should cause it to be abolished.

Cheers --ewd

Como · Jan 29, 2011

I would not have thought that water itself would be an issue, assuming it is being expelled as vapour.

If you look at some of the high efficiency wood chip boilers they will take chips with a much high water content than is usually considered acceptable for cord wood boilers and presumably still burn them even if you lose a bit of energy expelling the water.

The local School district are installing a wood chip boiler. They are chipping on demand to avoid the freezing issue. I can not imagining that the wood will be cut and split so opportunity for drying would be limited.

But a good idea here, buy only tree lengths cut in winter.

pybyr · Jan 30, 2011

I'd be among the first to challenge blanket statements such as "all wood must be seasoned two years" but I will also be the first to say that my experience confirms that - even with the same species of wood, drier wood allows an efficient boiler to put out _substantially_ more net useful heat, from the same amount of wood, than damp wood.

If you want or need to burn wood that has not had much of a chance to dry, expect to burn significantly more wood (or if your wood supply is limited, end up needing to run your house cooler) end up with more ash, and need to pay much closer attention to the cleanliness of your heat exchanger.

heaterman · Jan 30, 2011

I won't ever say that green wood is impossible to burn. It can and does burn. It will also raise the water temp just like dry wood.

The main issue at stake here is efficiency. A true test would be to weigh in a load of green wood, burn it and measure the water temp rise. Then do the same with the equivalent weight load of 20% wood and see what happens.

Green wood will most definitely leave more residue deposited in the upper chamber of a gasification boiler as temps there are substantially lower due to the evaporation going on. Same principal as an evaporative cooler works with.

All that steam escaping the chimney is latent heat which is wasted as water vapor.

ewdudley · Jan 30, 2011

heaterman said:
I won't ever say that green wood is impossible to burn. It can and does burn. It will also raise the water temp just like dry wood.

The main issue at stake here is efficiency. A true test would be to weigh in a load of green wood, burn it and measure the water temp rise. Then do the same with the equivalent weight load of 20% wood and see what happens.

Green wood will most definitely leave more residue deposited in the upper chamber of a gasification boiler as temps there are substantially lower due to the evaporation going on. Same principal as an evaporative cooler works with.

All that steam escaping the chimney is latent heat which is wasted as water vapor.

It's perhaps more subtle than many appreciate.

First you assume that upper chamber temperatures will be lower, which ignores the fact that for this demonstration the return temperature was raised from the conventional 140 degF to 175 degF. This was critical in assuring that the upper chamber was maintained hot and dry, not steamy and condensing.

Second you assume that latent heat up the chimney is necessarily a deal breaker. The inefficiencies of attempting to burn green wood are because of incomplete combustion, water vapor up the stack is minor factor.

Here's a table to illustrate the point.

For each row of the table we take a pound of perfectly dry wood and then add some water. This gives us wood at a certain percent moisture. The first two columns list percent moisture on dry basis and again on wet basis.

The next column is the number of btus per lb dry matter it takes to vaporize the water products of combustion and send them up the stack. I'm assuming 0.54 lb water combustion products per lb dry matter, 50 degF boiler ambient, and 400 degF stack temperature.

Then comes the number of btus necessary to vaporize the water added to the pound of wood and send it up the stack.

"Net btu per lb dry matter" is bomb calorimeter heat value of a pound of wood (8660 btu) minus the the previous two columns.

"Net btu per lb total weight" is btu per pound of what goes in the firebox. This is irrelevant because it's not the weight of what goes in the firebox that matters. What matters is how many centicords of wood we're putting in the firebox and the what the difference in net heat output per unit volume is as a function of moisture content.

Which brings us to the last column, which shows how much wood it would take at each of the various moisture content levels to equal a unit volume of wood at 20% moisture, dry basis. This ignores the fact that wood shrinks as it drys, but then again 'cords per year' is typically measured according to the size of the stack when it was stacked, not when it was burnt.

And there you have it. Going from 100 parts wood to 20 parts water to 100 parts wood to 40 parts water only costs 3.5% more wood on a dry matter basis, assuming complete combustion. Not 100% more, not 50%, not even 10%.

The key is complete combustion. Excess moisture in a gasification boiler normally causes incomplete combustion, the moisture itself is not such a big deal.

Code:

.
  mc     mc    btu    btu   net btu net btu relative 
 dry    wet   vapor  vapor  per lb  per lb   volume  
basis  basis  burn   boil    dry     total    per    
                            matter  weight  net btu  
  0.00   0.00    705      0    7955    7955    0.967 
 10.00   9.09    705    131    7824    7113    0.983 
 11.00   9.91    705    144    7811    7037    0.985 
 12.00  10.71    705    157    7798    6963    0.987 
 13.00  11.50    705    170    7785    6889    0.988 
 14.00  12.28    705    183    7772    6817    0.990 
 15.00  13.04    705    196    7759    6747    0.992 
 16.00  13.79    705    209    7746    6677    0.993 
 17.00  14.53    705    222    7733    6609    0.995 
 18.00  15.25    705    235    7720    6542    0.997 
 19.00  15.97    705    248    7707    6476    0.998 
 20.00  16.67    705    261    7694    6411    1.000 
 21.00  17.36    705    274    7680    6348    1.002 
 22.00  18.03    705    287    7667    6285    1.003 
 23.00  18.70    705    300    7654    6223    1.005 
 24.00  19.35    705    313    7641    6162    1.007 
 25.00  20.00    705    326    7628    6103    1.009 
 26.00  20.63    705    340    7615    6044    1.010 
 27.00  21.26    705    353    7602    5986    1.012 
 28.00  21.88    705    366    7589    5929    1.014 
 29.00  22.48    705    379    7576    5873    1.016 
 30.00  23.08    705    392    7563    5818    1.017 
 31.00  23.66    705    405    7550    5763    1.019 
 32.00  24.24    705    418    7537    5710    1.021 
 33.00  24.81    705    431    7524    5657    1.023 
 34.00  25.37    705    444    7511    5605    1.024 
 35.00  25.93    705    457    7498    5554    1.026 
 36.00  26.47    705    470    7485    5503    1.028 
 37.00  27.01    705    483    7472    5454    1.030 
 38.00  27.54    705    496    7458    5405    1.032 
 39.00  28.06    705    509    7445    5356    1.033 
 40.00  28.57    705    522    7432    5309    1.035 
 45.00  31.03    705    588    7367    5081    1.044 
 50.00  33.33    705    653    7302    4868    1.054 
 55.00  35.48    705    718    7236    4669    1.063 
 60.00  37.50    705    784    7171    4482    1.073 
 65.00  39.39    705    849    7106    4307    1.083 
 70.00  41.18    705    914    7041    4142    1.093 
 75.00  42.86    705    980    6975    3986    1.103 
 80.00  44.44    705   1045    6910    3839    1.113 
 85.00  45.95    705   1110    6845    3700    1.124 
 90.00  47.37    705   1175    6779    3568    1.135 
 95.00  48.72    705   1241    6714    3443    1.146 
100.00  50.00    705   1306    6649    3324    1.157

heaterman · Jan 30, 2011

So. Simply put, is it safe to say that you're attempting to prove a person can burn green wood if the boiler is operated outside normal parameters?

DaveBP · Jan 30, 2011

Whenever involved in a shootout of generalizations I prefer to eschew obfuscation.

tigermaple · Jan 30, 2011

[Oh, and did I mention that's black locust? It's the exception that proves the rule. If I had attempted this stunt with oak or maple I'd have boiler with a steaming black mess ]
That 'bout says it all.
Ash and hickory will work also. Fresh winter cut ash burns better than 1 year seasoned oak or maple 'round here. Oak really hangs on to its acidy water and makes better furniture and floors than firewood IMO.

Green Wood Considered Harmful

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