[Stoves] RE: Comparing the Charcoal Burning Rocket and Jiko Stoves

[Crispin at newdawn.sz]

Dear Friends

I think we have to have some useful way of reporting stove tests or there is 
going to be confusion, even for people reading about these tests well into 
the future.

The plots of the CO levels for two stoves shown on Dean's chart might give 
people the impression that the data in some way compares these stoves.  As 
there is no CO2 plot done simultaneously, we have no way of knowing how much 
fuel was burned at each stage and thus we have no idea what the combustion 
efficiency was.  Obviously it is too early to talk about heat transfer - it 
was a test of combustion.

This forces me to repeat my old whinge that a high CO level is not 
necessarily bad if you are burning a great deal of fuel cleanly at the time. 
The CO ppm level as a single measure are for measuring room contents, not 
the cleanliness of flames.

We can't tell from the plot if the 'high CO level' in the earlier part of 
the chimney fire indicates bad combustion or just a high rate of fuel 
burning.  Both would give a high ppm.  There isn't any indication of how 
much fuel was used.  Perhaps the Jiko used less fuel total.  Maybe not.  If 
it used more, it might have a more complete combustion of the fuel used 
because for a portion of the time the CO level in the hood was lower.

The essence of the video's claims are first that closing the door on the 
Jiko 'does not increase emissions' which is a remarkable claim seeing as 
cutting off the air to a charcoal fire usually vastly increases the CO/CO2 
ratio by a factor of from 5 to 40.  Remarkable claims require remarkable 
proof.  If the fire indeed decreased in heat output (lower fuel consumption 
rate) and the CO ppm level stayed the same, then the emissions have 
definitely increased - per kg of fuel burned (i.e. less efficient 
combustion).

Second, the video claims later that opening the door on the Jiko 'increases 
the emissions'.  Well...it probably does in grams per minute, but probably 
not in grams per kilogramme of fuel burned except in those cases where the 
fire is tiny and the air is not preheated.  If it is burning more fuel at 
the time perhaps the combustion is better, not worse.  Dean says opening the 
door was an attempt to maintain a boil.  If this was successful in 
increasing the heat output then it IS burning more fuel and one would expect 
to see a rise in CO output (total) along with far more CO2 and more heat. 
That CO rise does not mean the fire is burning less efficiently nor does it 
mean it is 'not burning any cleaner'.

If the rate of fuel consumption varies, as well as the excess air ratio, as 
well as the combustion efficiency, as well as the heat transfer efficiency, 
the charts do not tell us much.

My guess is that the insulated combustion chamber helps burn CO if there is 
sufficient air getting through the fuel bed and the flames do not touch the 
pot.

And in conclusion, I repeat my earlier note about getting the CO produced by 
a fire through the mechanism of taking periodic CO measurements, together 
with the CO/CO2 ratio and fuel burned to calculate the mass of CO produced. 
The COr is done comparing ppm readings to give a fraction, which when 
corrected for molecular weight and multiplied times the mass burned gives 
the CO mass emitted between any two time intervals.  This method does not 
require a hood with mass flow measurements etc. just a homogenous sample 
from undiluted stove output gases.

Andrew, do you agree?

Best regards
Crispin


----- Original Message ----- 
From: "Tom Miles" <tmiles at trmiles.com>
To: <STOVES at LISTSERV.REPP.ORG>
Sent: Friday, June 23, 2006 4:37 AM
Subject: [Stoves] Comparing the Charcoal Burning Rocket and Jiko Stoves


Comparing the Charcoal Burning Rocket and Jiko Stoves
Dean Still and Nordica MacCarty, Aprovecho Research Center, June 22nd, 2006

The charcoal Rocket Stove was compared to the Jiko-type charcoal stove sent
from Ghana.  600 grams of Kingsford charcoal was placed in the stoves under
the Aprovecho emissions hood.  One test was performed on each stove for
preliminary studies.  The stoves are compared in the following graph for
carbon monoxide emissions during a cooking task (to boil 5L of water and
simmer 30 minutes).

See: http://bioenergylists.org/en/charcoalrocketjiko



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