[Stoves] Effect of Charcoal Loading

Mon Oct 28 04:30:26 CDT 2019

Dear Christa

Thank you for popping up.  That was very helpful.  Perhaps Kevin would do well to promote the correct loading and, it if has any effect, the elevation of the surface that holds the fuel.  The only difference I can think of between the new surface (and 3 or 4 rocks to hold it up) is the flow of air around the perimeter oxygenating the CO as it leaves the chamber.

Kevin:
Maybe it is worth experimenting with different gaps at the edge.  To find out if the heat retention in the rocks is helping at all, monitor the temperature of the ground under the original floor.  If it is reduced, we can assume the heat is retained in the system somehow and raises the temperature of the primary air.

Also known as a Mali Stove in original form, there was an improved Malgache designed in Senegal that had a solid plate underneath and the combustion air was deliberately drafted between them, with the charcoal sitting on the upper surface.  The food served at lunch at the GTZ office was cooked on a large Mali Stove and this modification dramatically reduced the fuel consumption - something like 60%.  The contributors were two-fold: the reduction in excess air (far too much flows through both the Senegalese and Zambian original designs) and the preheating of that air.

[cid:image003.png at 01D58D4F.2A9861A0]
This shows a large (420mm square) modified "Mali Stove" with added secondary air preheating and a square of metal on top holding a pot shroud. This combination was quite effective at saving fuel.  Charcoal is expensive and pretty much the only way for the poor to cook in Dakar other than subsidized LPG.

Clean combustion of charcoal requires that the temperature near the outlet be high.  Because of the loss of energy from failing to burn the CO (called a chemical loss) the heat available is reduced.  An examination in October of the performance of a traditional stove in Mongolia burning recently introduced semi-coked coal (which is almost identical to charcoal in analysis) shows a 25% loss of heat energy due to the CO alone. That is a huge waste of fuel for people earning $250 a month.

Xavier Brandao on this list spent some time trying to make a Malgache with the "Senegal" preheated air work well in Benin on a small scale (single domestic pot size).

[cid:image004.jpg at 01D58D50.B0D55A10]
Side by side test: reduced fuel needed from 550 to 325g (-40%). Traditional Malgache on the left with preheated secondary air added, IKJ on the right.

We was not have enough time to optimize things such as the cone angle and full or partial coverage of the preheating surface.  The idea was to use the downdrafting primary air entrance to limited the total flow to the ideal amount.  The concept had earlier been successfully embodied in the Vesto Stove so it works in principle.

[cid:image005.jpg at 01D58D50.B0D55A10]
Primary air rises via the door through the flat bottom of the lower cone, and pre-heated secondary air is vented into the fuel from the side.   This approach to air flow control greatly reduces the CO generated. As an air/power controller, the door is several times too large.

Here is the same unit mounted in a stable body. The traditional stoves are notoriously unstable with a full pot on top.

For good measure, here is a 2000 year old charcoal stove that cooks, roasts and heat water at the same time:

[cid:image009.jpg at 01D58D50.B0D55A10]
This is from Thermodynamics of a brazier cooking system modeled to mimic the lead brazier of a Roman ship by A. Mosyaka, E. Galilib, D. Daniela, I. Rozinskya, B. Rosenc, G. Yossifona, Journal of Archaeological Science: Reports 16 (2017) 19-26

The notable thing about this design is that it heats water using energy normally lost to the local air.  The combined water heating and cooking fuel efficiency it pretty good.  Note that it can be used to roast Indonesian sate (small pieces meat on a wooden skewer) while cooking and water heating. That is the function of the portion on the right.  No modern stove with these functions exists. The authors replicated the general design using steel sheet.

Less than half the available charcoal energy went to the air, the rest heating the water, food or containers.  The stove body itself is a water container though the is evidence of its being used to heat wine.

Regards
Crispin

From: Stoves <stoves-bounces at lists.bioenergylists.org> On Behalf Of Christa Roth (bioenergylist)
Sent: Monday, October 28, 2019 1:29
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: Re: [Stoves] (no subject)

The main reason for these savings is that the initial load of charcoal matters: This effect was documented very well by Sam Bentson and the rest of the team from Aprovecho many years ago:
https://www.researchgate.net/publication/257434335_The_influence_of_initial_fuel_load_on_Fuel_to_Cook_for_batch_loaded_charcoal_cookstoves<https://eur04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.researchgate.net%2Fpublication%2F257434335_The_influence_of_initial_fuel_load_on_Fuel_to_Cook_for_batch_loaded_charcoal_cookstoves&data=02%7C01%7C%7Cc8a0d66310404c31479208d75b680b16%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637078374725974848&sdata=zhAXnTczWlBP8O8Cm1i9dkfSdXe3jub4HPZgJRPLP%2FA%3D&reserved=0>

If only we could convince people to buy the right size charcoal mbaula we would not need to use fillers like rocks and additional grates to displace the charcoal and have people start with a smaller initial load.

Best regards
Christa

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