[Stoves] Saving the WBT

[Frank Shields]

Greetings Stovers,

 

All this talk about the ocean water got me thinking about the Water Boiling
Test.

I would like to suggest a new way of testing and reporting results:

 

1)  Procedure

2)  Justification

3)  Calculations

 

Procedure: We take some oven dried wood and place in a pipe. Add both end
caps, loosen one, weigh and place in an oven at ~450c. Then cool and weigh.
The loss in weight is the volatile fraction of the fuel. This is the
fraction that provides the energy to boil water. We determine the energy of
this fraction and that is the energy of the fuel. Keeping track of the fuel
weight we use we determine the total usable volatile-energy. 

 

We put the pot of water on the stove, measure the temperature of the water,
start the fire and monitor the water temperature. We keep the fire going
until the water is at 'simmer' then keep steady for 30 min. Adding no more
fuel we then we manipulate the fire to keep the secondary burn going as long
as possible. Soon as the secondary burn goes out we pull the pot off and
measure the area under the temperature plot for energy that went into the
pot. Energy in the pot / volatile-energy X 100 is the efficiency(?).

 

Justification: When you are boiling water it is only good as long as the
secondary burn is going. When that goes out, even with a glowing stove
below, the water heating process slows way down because, as I learned in
Stove Camp, we need the heat forced hitting the bottom of the pot to stick
to it, go through the pot and heat the water. 

 

Biomass fuel has two types of energy; 1) the tars (C-H-O) that create the
secondary burn and 2) the chars (C-C) that only heat the stove body.
Important for the chars to heat the stove body but there is more than enough
with a good insulated stove and all that extra heat is wasted - not used to
heat the pot. When biomass is heated between 300c to 450c tars of massive
C-H-O structures go to the secondary burn and ALL C > CO2, and all H > H2O
releasing massive energy just at the pot bottom. The C-C bonds (chars) left
need to go C (solid) - CO volatile) forms releasing energy only in the stove
body. The CO (volatile) goes to the secondary burn (adding to the energy of
the tars) to go CO > CO2 releasing a relatively small amount of energy.
Under the best of conditions all the C goes to CO (not CO2) in the stove
body but this is such a small amount of energy compared to the tars
providing ALL their energy to heat the pot I suggest we can ignore (or
estimate) the CO > CO2 added energy. 

 

This being the case if we use only the volatile fraction as the total energy
then once the secondary burn stops all the rest of the material in the stove
body can be ignored. 

 

Calculations: A block of 100 g dried wood contains 44g C, 50g O and 6g H.
Let's say 22g C goes to the secondary as tars to heat the pot and 22g C left
behind to heat the stove. This can be determined (if needed) in the lab
measuring C and H in the biomass and C and H in the char left. The weight
loss in the pipe contains 28.2 % carbon and 7.7 % hydrogen for the starting
energy value figuring all H and all O are included in the tar fraction. 

 

Now we need to use Bond Energy (I need help) to determine the energy value
we give for all the tar carbon going all the way to CO2 and the hydrogen
going all the way to H2O. We sum the Bond Energies in the tars as the Total
Energy of the fuel. Add to it (ignore or estimate) the Bond Energy of the CO
to CO2 in the chars. 

 

Bond Energies: 

C - - O = 360 kj/mol

H - - O = 366 kj/mol

What is C>CO2 and H>H2O? 

 

I realize if one has H2 and O2 that nothing happens until you provide enough
energy (light a match) to break the H-H and O-O bonds to re-create H2O in an
explosion. In this stove case there might be enough of the extra heat in the
stove body to break apart the tars into C and H and O so we can just
calculate them going completely to their end components.  

 

Regards

 

Frank

 

 

     

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Frank Shields

Control Laboratories; Inc.

42 Hangar Way

Watsonville, CA  95076

(831) 724-5422 tel

(831) 724-3188 fax

 <mailto:frank at compostlab.com> frank at compostlab.com

 <http://www.compostlab.com/> www.compostlab.com

 

 

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