[Greenbuilding] one more pleasure (or not) of heating with wood

[Frank Tettemer]

On Dec 30, 2010, at 11:24 PM, Reuben Deumling (and Bruce Marshall) wrote:

>  I wonder if it is reasonable to try to estimate how much of this unpleasant experience is due to
>  - technique of the fire laying and stoking person
>  - quality/condition of the firewood
>  - design or condition of the stove

>  + the flue.

That list you've both created is certainly the basics of good combustion, I agree. ( + given an experienced stoking person, which is often the Joker Card ... an unknown.)

Here's my take on good combustion;  It is the burning of wood (or any fuel) so completely and efficiently that the spent gases, downstream of the combustion chamber, are below the EPA levels of safety in both particulate emissions, as well as the release of a minimum standard amount of unburnt fuel gas emissions.  In other words, the burning appliance must heat the fuel to a high temperature, and maintain that temperature in the firebox, mixing oxygen-rich hot air, with the hot wood gases, for a long enough time, that they burn completely before they leave downstream into the much cooler air in the flue. Even stack temperatures of 1000 degrees are too cool to further combust any remaining wood fuel gases. The full combustion should be taking place, ahead of the chimney flue.

A masonry heater has it's claim to fame by forcing the stoking person to attempt hot flash fires, which is the right attitude for ALL wood stoves, in my humble opinion, in order to achieve clean combustion.  However, the masonry stove does not burn cleanly, (with low emissions), UNTIL the firebox liner, (High temperature ceramics), have absorbed enough heat from the warm-up initial burn(S), to contribute to keeping the combustion chamber hot enough to fully burn the wood gas.  Since masonry heaters aim at having lots of mass, (for all the good reason of high BTU absorption and release to the house), that same mass is a big help over-night in the home, but somewhat of a detriment to emission-free burning for the first half hour (to one and a half hours) of the morning's initial firing.  The heater is not burning all that clean initially. (Just like any other initial firing of any other wood stove).

Any stove is burning it's best, after the firebox lining has heated substantially enough to contribute to the burn, instead of keeping the firing chamber below 1000 degrees, by it's initial period of high Delta T, and large heating absorption from the heat of the fire chamber.  Here's the thinking behind that:

Very little of the firewood actually burns, as a solid.  And of that, it is only on the surface area of the wood that there is burning going on.
For this reason, finely-split wood offers up the greatest surface area for combustion. For example, the surface area of a 16" x 6" diameter round of wood is about 300 sq. inches.  If we split that same round of wood, (fanatically!), into 500 pieces of kindling, and stack them is an open, criss-cross pattern, we now have 8000 square inches of surface area to be exposed to oxygen-rich combustion air.  Naturally, it burns much faster, and gives off much more heat.

The analogy here is similar to the combustion in the cylinder of a gasoline or diesel engine.  A teaspoon of liquid fuel simply won't burn, as a single droplet of fuel.  But it downright explodes in it's combustion, if the fuel is atomized into a mist, and compressed by the piston.  The carb or fuel injector has drastically increased the surface area of that one teaspoon of fuel.

So finely split wood burns quicker. And as it heats up the solid wood fuel, in the already hot ceramic combustion chamber, the solid wood gives off it's wood gases, (gasifies).  Once the air temperature in the combustion chamber is hot enough, a few freshly-stoked pieces of wood burst into flame.  And what is really burning now is not solid fuel, but wood gas.  This is the important part in the combustion.  The hotter the wood, the faster it gives off wood gas, I believe.  And if all that gas mixes with rich combustion air, it burns, completely and "cleanly".

My guess is that at least half the wood stoves in use aren't burnt hot enough to fully combust all the wood gas given off in the heating of the solid wood fuel. These are the fires of record, that have contributed to the bad reputation of wood fired appliances, and to the smog in areas around the house. This is what we want to avoid; poor burning. We don't have to give up wood burning to achieve the goal of clean emissions; we have to, instead, burn ALL our firewood as cleanly as possible.

Thus the fairly-recent idea of pre-heating and mixing fresh air (secondary air) precisely into the correct area of the combustion chamber, to fully burn off all, or most of, the as-yet unburned wood gases. This is the step in the combustion process that is missing in most all older cast iron wood stoves.  New, well built stoves, (like a Pacific Energy wood stove), are air-tight and lined with enough replaceable ceramic bricks, walls, and ceramic insulated 'ceilings', that the firebox can get to those very high temperatures needed, that the BTU-absorbing cast-iron liner has trouble reaching. The newer clean-burning stoves inject that hot, fresh air right into small jet holes at the hottest part of the primary fuel combustion, and then provide a super hot area for this secondary combustion to take place, just before the gases leave to enter the flue area.

I'm not a studied wood heat scientist, though. So some of my thinking may not be totally accurate.  I'm offering here, more of a curious back-woods observations over a few decades, fascinated with watching fires burn.  I think my disease is called pyromania, and so far the only ease in my Jones is to keep on wood burning.

My neighbour, John Gulland has the science part of it down, though.  He's a professional, qualified pyromaniac, and has the authority granted over many decades of observation, building&  testing wood stoves, as well as having been hired to write and assist at updating the WETT heat installation codes.  You can read his thoughts on good combustion here:

http://www.woodheat.org/tips/hiperwoodburning.htm

Happy New Year, woodburners,
and may all your fire wood be dry.

Frank Tettemer











-- 
Frank Tettemer
Living Sol ~ Building and Design
www.livingsol.com
613 756 3884