[Greenbuilding] firewood moisture content - a question for Norbert perhaps

bill.allen at verizon.net bill.allen at verizon.net
Sun Dec 18 08:53:04 PST 2011


There has been a lot of discussion about the benefits of "fine split wood".  For this novice, how big is "fine split"?
I'm assuming each log is cut to 16 inches and then split....but how small are you talking?
Thanks,
Bill

-----Original Message-----
From: Frank Tettemer <frank at livingsol.com>
Sender: greenbuilding-bounces at lists.bioenergylists.org
Date: Sat, 17 Dec 2011 19:46:45 
To: <Greenbuilding at bioenergylists.org>
Reply-to: Green Building <greenbuilding at lists.bioenergylists.org>
Subject: Re: [Greenbuilding] firewood moisture content - a question for Norbert
 perhaps

/Does anyone on this list know whether wood burning best practice, as 
outlined and no doubt practiced by several folks here, would 'work' in a 
location where inversions occur? In other words is it conceivable that 
even in 'the wrong' place it would be fine for everyone to burn wood if 
it were just done right/very carefully?
I'd like to think so, but have so far not really found much to help me 
understand this./
...................................................................................................................................

Reuben, I'd like to think so, too; I really would.
I think there are lot of factors against it being fine for everyone to 
burn wood, in any type of large village, town or city.  Especially, in 
an area of frequent inversions.

The main factor I believe is this:
Just about no-one is truly consistent about clean burning.
With the best of intentions, I have periods of burning time in which 
there is a lot of unburnt wood gases, carbon particulates, and tars, 
escaping up the flue pipe, to mix and dilute with the outdoor air.

During the initial fire start-up period is one common period of time.
Starting a fire in a cold firebox, (under 600degrees), guarantees that 
some wood will convert to wood gas and be cooled too quickly to burn 
into open flame. This unburnt wood gas then exits the flue into the 
atmosphere, as a pollutant.
Even building a top-down fire gives off some particulate and unburnt 
wood gas,
though this does shorten the unburnt gas productivity by a long, long shot.

Lighting a well-built fire from the top means that the chimney exhaust 
will be clean, almost immediately, as the firebox begins to heat up, 
since finely splintered kindling, shredded birch bark, and crumpled news 
paper at the top of the fire will be burning cleanly with no dark coals 
to smolder unburnt.  This fairly well-carbureted gas mixture will burn 
brightly and begin warming the flue, which increases the draft in the 
chimney, while warming the firebox lining further. It's not perfect, and 
there will be some black smoke, but the immediate heat of the fine 
combustibles gets things going fairly cleanly.

Next, the upper layer of split wood begins to burn, while warming up the 
thicker wood below it, and increasing the firebox lining's temperature 
even more. Still burning pretty cleanly now, with plenty of combustion 
air, and faster upward speed of the exhaust gases, in the now-warmed-up 
chimney.

As the lower layers of wood in the firebox are heated to the gassing off 
temperature, the released gases are now ignited. They have better odds 
of full ignition, as the firebox temperature rises to the flashpoint 
temperature of the wood gas.  When the firebox linings are heated to 
this temperature, ignition is assured, as long as there is good 
combustion air to mix with it.  The object of the game is to burn only 
fine split wood, until the temperature is high enough to ignite the 
gasification of the larger wood pieces below the fine wood.

In most modern EPA stoves, the art of injecting the unburnt gases, back 
into the hottest spot in the firebox, for a secondary burn, is the place 
where recent technology has helped us woodburners come clean.

By and large though, a village of 1000 people, all burning wood, would 
suffer poor air quality, some days, when there is no breeze to mix and 
dilute the wood smoke. And how does one create the need in all these 
different folks to learn and practice good combustion, which relies so 
much on firewood moisture content, well designed appliances, well 
maintained stoves and chimneys, and the human condition?  A small 
minority of those 1000 people in the village will be motivated and 
curious enough to learn good burning skills, and to practice with 
consistency. Meaning the majority of the fires will be burning poorly at 
times.

I think that the very interest in maintaining steady and even indoor air 
temperatures is exactly the cause of poor burning.  This comes about 
through a generation of peoples' experiences living with central 
heating, modern thermostats, and a variation of only a few degrees in 
indoor air temperature, through out the whole day. This has been 
socialized and normalized as a 'good thing'.

It is much easier to maintain a smoldering fire all day, giving off an 
even heat,
than it is to burn brightly, through the entire burning cycle, and then 
let the fire go out.

For one thing, lighting two fires a day seems like a lot of work. And 
another disadvantage of lighting twice a day, burning hot and clean, and 
letting it go out, is that the temperature swings in many houses will 
oscillate through the day, between ten degrees from high to low.  This 
sometimes requires donning a sweater for a few hours, then taking it 
off, during the next burning cycle, and even having to open a window to 
cool down, at times.

So one has to weigh and balance the pros and cons of consistently 
burning cleanly, against the idea of having perfectly constant air 
temperatures in the house, I think.  Sometimes our lives are such that 
we get what we get, and it's sometimes a smoldering, damped-down fire.

But I'd still like to think that educating a town full of woodburners 
with keen enthusiasm for clean air could make it all more possible.

Frank

Frank Tettemer

Living Sol ~ Building and Design
www.livingsol.com


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