[Stoves] Practical Stoves- Introducing The Versatile Stove

ajheggie at gmail.com ajheggie at gmail.com
Thu Nov 17 04:36:18 CST 2016


[Default] On Wed, 16 Nov 2016 20:27:05 +0000,"Andreatta, Dale A."
<dandreatta at sealimited.com> wrote:

> An approximate formula I use for burning wood is that the energy content in Joules per gram is 20634-22894*MC where MC is the moisture content on a wet basis, as a decimal (not a percentage).  The 20694 is the lower heating value of bone dry Douglas Fir.  In the second term, 20694 out of the 22894 is the heat no longer available in the wood because it’s replaced by water, and the remaining 2260 is the energy to boil the water itself.  With this formula you’ll have positive energy content up to 90% moisture.  

Where does the 22894J come from?

I know Douglas Fir has a high calorific value because it has a higher
lignin content than hardwood.

The thing you need to consider is the water originally in the log has
a free ride through the system so it contributes to the mass flow, it
has other negative effects over its enthalpy of evaporation, in that
it's mass has to be raised to boiling point  as well as being
evaporated then it is further heated to whatever the temperature the
flame is discarded at, this is why I gave the (rule of thumb) figure
of 2.7MJ/kg for the heat loss from the initial water content.

Typically a fire with high moisture content wood will require excess
air to burn so say you wish to burn 50% mc wood and have 100% excess
air then your massflow per 1 kg of dry wood increases from the
stoichiometric value of approximately 6kg to 12kg.

Now the heat in the wood has to raise double the mass to the
temperature under the pot.

Intuitively the fire burning dry wood will reach a temperature
considerably higher than that burning wet and if this lower
temperature falls below a critical point for the stove then secondary
combustion is quenched and  products of incomplete combustion are
vented, thus there is a further loss of energy in the flame as these
compounds still have chemical energy which is being discarded.

AJH




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