Burning Wood

There is a lot of interest in wood gas and other wood topics in our DIYer group. It’s that time of year where many of us check our stoves and wood supply for the winter.  Diyer John Laswell has been burning wood for a long time, we all know that knowledge plus ‘hands on’ is a great combination. What you learn about the fuel goes a long way in understanding what might be done with it.

I recently read a post about a wood gasifier that burned so hot that it broke down water to hydrogen and oxygen and used it for fuel 🙂  Of course, if that were true we’d look no further for energy, AND water might be considered dangerous stuff. I’m sure I’ll see or hear about someone using magnets in a wood stove sooner or later.

Without further delay, my thanks to DIYer and retired Mechanical Engineer John Laswell for sharing his article here, stay tuned for more installments of this article. George B.

Effect of Moisture on Wood Burning Efficiency, Revised – Installment 1         

By John E. Laswell, Mechanical Engineer – Aug 2008

     Disclosure Statement – the information in this article is mainly the opinion of the author except where websites are shown.  There may be errors in the information and errors in any calculations.  So, the ideas herein presented should not be used to design a heating system for any home or building.  Application of any engineering formulas and principles may be in error.  This article is strictly for the reading enjoyment of the curiously minded individual!!

     Forward, Hello fellow DIYers.  My name is John L. and I live in the hills of scenic, southern Indiana.  I have burnt a wood furnace in my basement for 22 years.  The wood came from my 38 acre farm, mainly from dead or non-timber grade trees.  My wood was cut with a chain saw and hand split for the first nineteen years.  I now have a 27 ton hydraulic log splitter.  I mainly burn oak, hickory, ash, hard maple and dogwood.   For the last three years I have I have been heating one half of my home with a King, 1107J, Blaze King stove.  It is wonderful.  I have installed it in my dining room.  In my opinion this is the best wood stove.  If there is any interest, I would write an article about my experience with buying, installing, operating and the pure enjoyment of heating with the “king”. 

     I am so tired of products that have been “value-engineered” to the point of planned-failure-in-design.  How many times have you been asked when you purchase a fairly simple product if you wanted to buy an extended warranty plan?  Here is some of the modern philosophy: use it up, burn it up or throw it out when it stops working.  I recently bought a pair of trousers where it cost almost as much to alter the length as the cost of the trousers!  Something is wrong!  I know what it is? Products made in China and cost of labor in USA.

     This article comes in three installments:

     First – Introduction, Wood Stove Efficiency and Results

     Second – Determining The Percent Of Moisture In A Wood Sample, Conclusions,                               Energy to Cook-off Moisture, Operating the Wood Stove

     Third – What Is The Value of Burning Wood?, Creosote and Soot – Formation and Need for           Removal, Chimney Fires, Combustion of Wood   

Introduction 

Wood stove experts advise that wood must be ‘well seasoned’ before burning.  Therefore, I asked the following questions.  What is the effect of different levels of moisture in wood on burning and how can the percentage of moisture in wood be practically determined?  I will answer the above questions and more!  You should know that the combustion of wood is a complex process and it occurs in three basic phases: one, the primary phase is where the wood is ignited and the moisture is evaporated.  Two, the secondary phase is where the volatile liquids and gases are expelled.  These products of combustion burn at about 1100 deg F and 50 to 60 % of heating value of wood is contained in these gaseous products.  Three, is the charcoal phase which is a very slow, hot combustion process.  Now, some engineering definitions and assumptions.  A British Thermal Unit (BTU) is the amount of heat required to raise the temperature of one pound of water for one degree Fahrenheit.  So, it would take 180 BTUs to raise one pound of water from 32 to 212 degrees F.  At 212 deg F it takes 973 BTUs to vaporize one pound of water into steam.  It also takes 144 BTUs to melt one pound of ice.  Adding these values up (180 + 973 + 144) totals1297 BTUs of heat to melt one pound of ice in wood and then vaporize that water.  That requires about 1/5 of a pound of burning dry wood fiber.

     For my calculations I assumed that the heating value of dry wood (no moisture) is 7,000 BTUs per pound.  Then, assume I have an arm load of wood at 32 deg F that weights 20 pounds.  Table 1 below shows the effect of different percentages of water in the wood and the amount of heat that is available to produce heat for your intended surroundings and the efficiency in a stove when the water is  “cooking-off”.  The efficiencies for two, new Phase II EPA approved stoves and an “old” conventional stove are also calculated in Table 1.  

Wood Stove Efficiency

When I write about “the efficiency of a wood stove”, I am referring to the stove’s capability to burn a “charge” of  wood and to transfer the wood’s heat energy to the stove’s intended surroundings.   A few factors will determine the stove’s efficiency: the stove’s inherent design, the stove’s chimney, percent of moisture in the wood, maintenance of the stove: firebrick, gaskets, catalyst, etc., the manner in which the stove is “operated” which might include the size of fuel charge, amount of draft and the maintenance of the ash “pit”.

In general the stove’s inherent design can not be changed (metaphor: can not change a car’s cylinders from 4 to 6) .  So, buy a stove with the highest efficiency that meets your specifications and pocket book.  Keep in mind that you are making a long term investment of 20-30 years (metaphor: you need to find that car that gets 40-50 miles per gallon).   The cost of fuel will keep increasing with time.

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The stove’s flue must “draw” properly.  If your house is too “tight” (this calls for an outside air kit), then the draft will be weak and the amount of oxygen reaching the fire-bed will be inadequate for proper combustion which will produce smokey, lazy fires.  If the chimney is too high,  then the draft may be too strong and hard to control. Even small air leaks in the combustion chamber will cause the fire to “burn out” prematurely.

  Assuming that stove, chimney and stove maintenance factors are adequate, then the proper wood for burning will be considered next.  The use of “seasoned” wood is the most important factor in controlling the efficiency of the stove than any other aspect of owning and operating a wood stove.  My engineering calculations are tabulated in Table 1 which quantifies the effect (efficiencies) of various levels of moisture in wood when used in three levels of efficient stoves.  Moisture in wood has three major effects on efficiency: first, the “charge” of wood fiber is diluted with water.  That is a 20 pound arm load of wood that has 20 % moisture, which is considered the maximum level of moisture to qualify as “seasoned wood” by some authorities, has 4 pounds of water (that is two quarts -a pint is a pound the world around) and 16 pounds of wood fiber. 

The second effect of the moisture is that it must be “cooked off” in the form of steam which requires about one fifth pounds of wood for each pound of water.  For our arm load of wood that would be about 0.8 pounds of wood which is equal to 4 % of 20 pounds of wood.   Thus, our 20 pounds of wood has become just 15.2 pounds of usable wood fiber.  Now, consider if I burn this wood in an old-design, wood stove that is 50 % efficient, then you are only heating your intended surroundings with about 7.6 pounds of dry wood fiber, ouch, as Table 1 indicates, 20 % row & last column an efficiency of 38%.  An article on the Principles of Combustion of Wood and three stages of combustion can be found at www.montana.edu/wwwpb/pubs/mt8405.

The third effect of the moisture is the “cooling effect” on the products of combustion as the temperature of combustion is trying to increase from 212 to 575 to1100 deg F (remember how you controlled the flames of a small brush fire with a spray of water?).   The volatile gases / vapors are escaping with the evaporated water while temperatures are below 1100 deg F.  These vapors contain 50 to 60 % of the woods heat value.  Increasing the draft to “cook off” the water, means additional amounts of nitrogen gas (inert) must be heated and lost up the flue.

Table 1

Effect of Moisture in Wood on Heating Efficiency  

Results

Now assuming that we have a great Phase II EPA stove with 82.5 % efficiency in heating, then the efficiency in using wood with 20 % moisture would be about 68 %.  If I were heating with an “old” conventional wood stove with an efficiency of 50% (getting very little secondary burn), then my heating efficiency with wood containing 20 % moisture would be about 38 % which would surprise many wood-stove users that this thermal efficiency is so low!   In other words only about 1/3 of the dry weight of my wood is heating my house!