JJHerb@aol.com on sat 3 may 97
Within some limits, the heating value of wood, with the same moisture
content, is about the same on a per pound basis. The biggest problem with
wood fuel is water. The 10 or 15 percent moisture content is water that must
be heated, boiled, raised to kiln temperature, and sent on its way by the
wood you are trying to heat your pots with.
The heat capacity of water is significant. The definition of a BTU is the
heat required to raise the temperature of one pound of water 1 degree (at 72
degrees F.). So, 100 pounds of wood with a 10 percent moisture content will
contain 10 pounds of water. That water has to be heated to boiling (1400 BTU
roughly) and them vaporized (don t have that number in my head but its large
enough to matter), then the vapor has to be heated another 2000 degrees (more
BTU, see your copy of the steam tables). So, you have to carry the water to
the fire box, that s 10 percent more weight, and you have to carry the wood
to heat up that water, vaporize it, and to heat the vapor. This all means
that you have to supply between 15 and 20 percent more wood to get to the
same place in the kiln if the wood has a 10 percent moisture content.
It really doesn t matter from the standpoint of heat what plant fuel you burn
as long as it is dry. You will get about the same heat per pound. The
composition of the ash produced can be very different and that can make a
difference if you are counting on ash deposits to glaze your pots. In
previous discussions there has been indications of ranging from 20 percent of
the ash to 95 percent of the ash. Big differences.
The choice of wood type, for most people, is made for them by what is
available at a reasonable cost. When you need a couple of tons of fuel, cost
per pound becomes the overriding factor. That s why Greeks fire with olive
pit fragments - it s cheap and available. Put any wood you can think of on
your wish list, you will end up taking what you can get cheap. Pallets,
builders off cuts, branch trimmings, anything that s cheap and dry.
Tires, for example, are cheap, easily available, high heat content, low
moisture content. If the people of Philadelphia had been given the choice of
having the Clay Studio fire kilns with tires or use the tires to destroy
I-95, the kilns were an easy choice. Events overtook the tire storage area.
Best of luck in the waste wood adventure
David Hendley on sun 4 may 97
Ditto to Joseph Herbert. DRY is the word for wood kiln fuel.
Variety of wood, with or without bark, and size (within reason) are all
secodary to moisture content when considering heat value.
BTW, in Jaurez Mexico old tires are routinely used to fire brick
kilns. According to a newspaper story I saw last year the pollution is
crossing the border and causing problems in El Paso.