[Stoves] Fuel Testing

[Crispin Pemberton-Pigott]

Dear Frank

Your question is a good one:

>My questions are: If you use dry wood to boil
>a pot of water and determine the energy produced...

Determining the effect of the water added is not the same as testing damp 
wood because the distribution of the water is not the same in the samples. 
Dipping would soak the outside and evaporate sooner.

The effect of moisture in the wood seems to be to moderate the production of 
volatiles and perhaps acting as a catalyst (?) to clean up the combustion, 
but sucking heat out as well.  Some of the heat, especially early in the 
cooking, is recovered on the pot surface by condensation.  Later it 
moderates heat production, lowering the heat production and retarding the 
fuel burning rate which is good for simmering.

Some stoves (as we have been discussing) really need moisture in the fuel to 
burn clean and hot, some don't.  A gasifier could be made to work with oven 
dried (0% H20) or kiln dried (6%) or air dried (12%) or partly dried (20%). 
Paul's stoves are built to use ordinary wood not artificial wood.  He could 
change it if it were important to do so.

In order to remove the effect of differences in moisture, the team at 
Aprovecho  standardizes the fuel in a repeatable way.  There are a couple of 
consequences of that.  You mention the heat loss.  But the more we talk 
about it, some benefits are coming out, not just about gasifiers and 
semi-gasifiers not 'running away' but real thermal benefits from having some 
moisture in the fuel.

Suppose the moisture helps increase the flame temperature from 750 to 900 C, 
and there is a claculable energy loss from boiling the water.  The loss to 
the water might be 5 or 10 or 20% of the energy released.  But the heat 
transfer rate (not stove efficiency) is increased by ((273+900)/(273+750))^4 
= 1.72 because of the higher temperature.  A consequent gain in overall heat 
transfer efficiency might far more than compensate for the water heating 
loss.  These things are very stove dependent.  More water, less efficient, 
more water, more efficient.  Huh.

Thus the assessment of _thermal_ potential performance of a fuel does not 
set the limit for the delivery of heat from the fuel to a pot 
(unfortunately).  As my historian brother Andrew used to say, "It is not as 
simple as that."

A collection of data on the performance of a stove + pot combination 
together with the moisture content of the fuel used may in future detect 
relationships that give direction to stove builders.  At this time ProBEC is 
collecting this and other information in Lesotho and Swaziland during its 
baseline surveys.

Best regards
Crispin