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Rev. 05/2020
Types of Wood
There are different types of wood, classified by species, hard or soft, old or new, dry or
damp, even larger or smaller. All will burn satisfactorily as long as the proper moisture
content and heat load combinations are observed. Therefore, knowing your heat load
demands and your fuel supply characteristics are very important.
Wood as a Fuel
Wood as a fuel contains more moisture than most heating fuels. Therefore, proper
management of the process is more important. Heat is required to evaporate the moisture
from the fuel. Once moisture is evaporated, proper control of the remainder of the
combustion process is also required. At about 600 degrees Fahrenheit the wood will gasify.
At this point the fuel/air mixture is fuel rich. With proper “secondary air” introduction the
fuel/air mixture will approach ideal and result in proper combustion. Your Heatmor furnace
is designed to create this situation. Wood too high in moisture content results in lower
temperatures and unreliable performance.
Stages of Combustion
During the four stages of combustion, wood breaks down into water, smoke and charcoal.
The
first stage
occurs when wood is placed in the furnace. It must be heated to drive off
the moisture. The higher the moisture content, the greater the amount of heat needed
and subsequently lost for heating purposes. The drier the wood, the more rapidly it can be
heated and passed through this first stage of heating the water. When moisture is being
driven from the wood, white smoke may be emitted from the chimney. This is what we call
“steam smoke.” It is mostly water vapor.
In the
second stage
, at 500 degrees Fahrenheit, wood begins to break down chemically.
If this smoke is released but not burned, two-thirds of the energy in the wood will be lost. A
hot fire is needed to burn the smoke.
The
third stage
takes place at temperatures above 1100 degrees Fahrenheit. At this point,
the smoke is burning at 100 percent efficiency, as long as
the proper amounts of oxygen,
temperature and draft are present. If one of these elements is missing, the combustion
will be incomplete. The third stage is the most important stage of wood combustion since
smoke represents two-thirds of the wood heat.
The
fourth stage
takes place after 1100 degrees Fahrenheit to 2000 degrees Fahrenheit
temperatures have been reached. The smoke and gases are completely burned and the
charcoal remains, which represents approximately one-third of the wood heat, and allows
the fire to re-start when required.
When a new charge of wood is loaded, the first stage of combustion begins again. The
charcoal heats the fresh wood until it gets hot enough to react and ignite, and the process
continues. All four stages can take place concurrently but complete combustion requires
proper placement of secondary air and adequate temperatures. This is incorporated in the
design of the Heatmor furnace.
Efficiency Measurements and Types of Fires
There are different ways of expressing efficiency and to correctly compare values.
Combustion Efficiency - Input Based
The amount of fuel that is completely burned compared to the total amount available for
combustion expressed as a percentage.
For example
: An open bonfire; 100 pounds of wood burns and produces three pounds of
ash, which equals 97 percent combustion efficiency. Good combustion efficiency but poor
heating efficiency. No heat got into the house.
