[Stoves] "Young-adult" TLUD research Re: List of woods for TLUDs?

Mon May 1 11:39:06 CDT 2017

Dear Friends

This subject of controlling the primary and secondary airflow on small stoves is very interesting and goes back a long way.

Ron wrote:
preferring to use one controller for both is that the ratio of primary to secondary should always be the same
Paul wrote:
The point is that the ratio is NOT to always be the same.   Even the different packing of the fuel in to the TLUD can make primary air (PA) flow more easily, creating more gases and needing a change in secondary air (SA) to get optimal performance.

Crispin sez:
Both points are good. It depends on what you want to do with the fuel.

The purpose of being able to vary the secondary air and primary air separately is not only to optimise the combustion conditions for different fuels. It allows one to over-fuel a stove with a batch and burn it at leisure, with a varying power level, and still have good combustion at all times. This is not necessarily done by burning ‘in the same way’ at a different power level.

Here is a Vesto stove with separate primary and secondary air control from December 2002:
[cid:image002.jpg at 01D2C275.9DCD10F0]

The secondary air is provided through the side of the outer sleeve surrounding the combustion chamber:

There was a second adjusting lever with full control over the admission of air:

[cid:image006.jpg at 01D2C275.9DCD10F0]The vertical tab shown half-covering the hole is one of 6.

The control of the primary and secondary was thus separate and one could put the stove into a char making pyrolyser mode creating charcoal, then shortly before the volatiles were burned out, open the primary more and burn it.

The main point of this was to control the firepower while maintaining clean combustion. Although the fuel loaded was ‘too much’ it was prevented from burning not by choking it and having a smoky low fire, but by changing the burning mode from burning volatiles to burning char or any combination in between.

The instructions were to control the primary air to get the desired burn rate and to control the secondary air to minimise smoke.

The problem with the design, according to the marketing people, was training users. In the end, after conducting marketing exercises around Johannesburg, the word was that it was not worth having this level of control. It was easier (and cheaper) to leave out the secondary air controller and live with the consequences. Because the secondary air was partially self-controlled using a down-drafting preheating gas path, there was an element of self-regulation, however in my view it was not as good as having mechanical control over both supplies.

I think this goes one step beyond the idea that the secondary air should be matched to the primary supply. Yes there is going to be an ideal level which is dependent on the architecture. Different combustors are optimal at different excess air levels. Having such control allows one to shift the combustion type completely from a regular fire, to a pyrolyser, to a charcoal burner, while always being able to have the right primary/secondary air split.

The result was pretty good in terms of being able to match the firepower to the task at hand. Here is a 2002 comparative fuel consumption report for an open fire, an SA Mali Stove, a Rocket Stove, an Eco Stove and a Vesto from Swaziland.
[cid:image009.jpg at 01D2C275.9DCD10F0]
The contrast between this approach and the idea of having secondary and primary air vary ‘together’ is this: the ‘vary together’ idea assumes that the combustion type is always the same, with a varying burn rate. The Vesto approach was that one could change the type of combustion process on the fly.

The case of varying together, co-varying the primary and secondary, is implemented well in the Kyrgyzstan Model 4 which has a single air controller for all air, however it is split into two by the fuel bed itself. The depth of the coke on the lower grate and the particle size when it gets there controls the excess air level above it in the combustion zone. Turning down the air supply limits the pyrolysis rate in the fuel bed immediately above the air entrance, simultaneously lowering the rate of airflow through the coke bed. This results in an excess air level that is pretty stable in the combustion zone across a power change or about 4:1 or 5:1.

Air control door in low power position (KG4.3, 2017)

[cid:image014.jpg at 01D2C275.9DCD10F0]

A photo of the pyrolysis zone just inside the air control door.
[cid:image015.jpg at 01D2C275.9DCD10F0]

The ‘trick’ to achieving this is to get the fuel size correct for the volume of fuel involved in the staged combustion processes. It is a pyrolysing gasifier that does not attempt to do what the Vesto did (creating different types of combustion at different times). It only has one mode, and it is continuous. The thing the operator can change is the rate at which gases are produced in each zone. The zones are pyrolysis, semi-coking, coking, and coke gasification. Once established, these zones are self-perpetuating provided there is new fuel available from above. There is no airflow through the fuel in the hopper.

In such a condition, the use of a single air controller can provide ‘near enough’ ideal conditions to completely burn the PM and CO produced by the decomposition of the fuel.

I received a message this morning from Prof Philip Lloyd who just met Altanzul, the Mongolian Masters student at CAU in Beijing. He commented positively on the quality of the work and testing she has been doing investigating the performance of this approach to combustion using different fuel types and particle sizes. He said he was “doubly impressed”.

[cid:image016.jpg at 01D2C275.9DCD10F0]
[cid:image017.png at 01D2C275.9DCD10F0]

Her work is being independently validated by Masters student Lu Sumbane at the North-West University in South Africa where this model was replicated.

Regards
Crispin

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