Sawing and Drying

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burning wood scraps

1/26/18       
Gary

I have two outdoor woodburners with forced air draft to heat my shop. I notice that the kiln dried wood scraps or briquets smell quite different than normal firewood. They do not smell as good as firewood. I notice they also make more creasote. I would think they would make less since they are so dry. Maybe they sit and smolder more or something. Has anyone else experienced this?

1/26/18       #3: burning wood scraps ...
Keith Newton

Do both have the same size flue and is it straight up?

When you load it, do you cover a hot bed of coals then close down the air and flue damper to slow it down? If you do that, you need to make sure you get ignition on top first, otherwise, you're cooking all the VOCs out, which are condensating out on the way up the flue. I'm no expert, nor physicist but thats my best guess.

1/26/18       #4: burning wood scraps ...
rich c.

A clean burning fire needs the right percentage of oxygen. Lots of little scrap pieces pack much tighter than big chunks of firewood. Very similar thoughts to what Keith is saying, even if you don't choke down the flue, you are still choking down an existing fire till the gasses fire off.

1/27/18       #5: burning wood scraps ...
GeneWengert-WoodDoc

I agree that with very dry wood, the speed of burning and the heat that results is often too much, so there is a reduced amount of oxygen (fresh air vents closed). Creosote is merely unburned wood that is a gas, which then hits a cooler spot somewhere and condenses. It is very flammable, which is the major issue.

The glowing coals do not emit creosote, as the wood gas is already gone from that region. There is only carbon, which unites with oxygen to form carbon dioxide, unless there is a shortage of oxygen, in which case it forms carbon monoxide.

When burning very wet wood, the evaporation of water occurs at 212 F, which is too cool for wood to burn actively. This evaporation is using lots of the heat created, so it is cooling the fire. Oftentimes some of the wood that dries tries to get hot enough for combustion of the gas, but plenty of oxygen is needed. After the water is gone, then the wood heats to 450 F or so and emits wood gas that burns quickly with enough oxygen. Finally, the carbon left reaches 1100 F or so, glows, emits plenty of heat, and forms CO or CO2. If there is no oxygen with the carbon, then we do not get burning and form charcoal.

1/29/18       #6: burning wood scraps ...
Steve

In general wet wood will form creosote in the flue because the fire is not hot enough.
In general a damped down fire will form creosote because the fire is not hot enough.
In my experience a wood stove that has been damped down needs the fire to "roar" at least once a day in the burning season to burn out the creosote in a controlled manner.
Flue fires look like a blow torch out of the top of the flue and if the flue is thin metal it doesn't take much to burn through and cause ignition on adjoining material.

1/29/18       #7: burning wood scraps ...
Gene Wengert-WoodDoc

Creosote, which I unburned wood gas, forms in burning because there is not enough oxygen. This can occur with a hot or cool fire. Damping a stove or furnace means reducing the air (oxygen). With some of the combustibles going up the chimney, there is less heat generated by the fire, so indeed, the fire will then cool off, compared to the fire with adequate or excess oxygen.

1/29/18       #8: burning wood scraps ...
Gary

With my Heatmore and Woodmaster outdoor wood burners, and most brands to my knowledge, there are no dampers in the flue. You open the door, add wood, close the door and walk away. The Heatmore does have dampers on the forced air draft fans but they are less than a perfect science to adjust. When the stove isn't calling for heat the fans shut down effectively closing the "dampers". I assume that's when the creosote forms. When I burn regular firewood no creosote builds up. When I burn kiln dried scraps creosote forms.


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