[Stoves] Gasifier+charcoal burner, a new stove from Dr Nurhuda

Crispin Pemberton-Pigott crispinpigott at outlook.com
Tue Dec 20 11:59:31 CST 2016


Dear Friends of Stove Design

Dr Nurhuda, someone known to this list for his work on TLUD gasifiers, has produced a new stove that has two distinct burning mode with two architectures for supporting the pot.

My informant says that the pot remains clean on the bottom like a gas stove with both fuels: wood pellets and charred pellets created out of the wood ones.

So shared with his permission, here is the latest technology from Bogor (that’s a city).

Here is the combustion chamber for wood pellets. You can see it is filled to a point just below the secondary air holes. The air holes are small and the air jets will not reach to the centre.

[cid:image001.png at 01D25ABB.1DA6F1B0]

A small amount of fine kindling is placed on top of the pellets. The chamber is dropped into the stove body.
[cid:image003.jpg at 01D25AC0.7A4E05C0]

Because the secondary air jets do not reach the centre, a space is needed above the chamber to allow for the gases to complete burning. This is created by having a relatively tall pot support keeping the flames away from the bottom of the pot. This permits the gases to burn completely and eliminate the sooting problem so common with wood fires. See below.

The kindling is ignited and allowed to ignite the pellets.
[cid:image005.jpg at 01D25AC0.7A4E05C0]

When the fire is fulley developed across the top of the fuel bed it looks like this:
[cid:image007.jpg at 01D25AC0.7A4E05C0]
Note that without the pot in place there is a toroidal flow if gases. There is air entering the fire from above in the centre, descending and rolling in a torus (donut)  towards the wall.  The effect is to draw the flames down and heat the top of the pellets. This flow pattern is easier to create and observe when there is a larger vertical distance between the bottom air holes and the top of the fuel, but still, it is visible in this photo.

With the pot in place the centre air flow is blocked so the flames take a different path.
[cid:image009.jpg at 01D25AC0.7A4E05C0]
This appears to be operating at low power.

High power looks like this
[cid:image011.jpg at 01D25AC0.7A4E05C0]
Again, note the vertical distance between the top of the flame and the bottom of the pot. There is space for the PICs to burn completely.

In the very centre of the flame you can see a peaks of yellow. That is the result of the secondary air not reaching the centre. Above it there is sufficient space for the particles of incomplete combustion (that are source of the yellow glow) to burn completely. If you create such a flame and lower the pot, you will get soot accumulation immediately above the fire. If you increase the air jet hole size and reduce their number, you can shorten the flame. It is a designer’s choice.

I was told this next photo is with the shortened fuel chamber installed and the char pellets burning at high power. Here is the stove late in the burn:
[cid:image013.jpg at 01D25AC0.7A4E05C0]
Note the typical colour of a high carbon, low volatiles charcoal-burning flame. The blue is CO burning to CO2. The individual air jets are still visible.

If the power is reduced, the tall pot support ring can be removed and the pot can sit on the lower, fixed supports.

While this is not the first stove I have seen with two combustion chambers for different fuels in the same stove body, it is the first I have seen to use the chambers with different pot rests specific to the flame characteristics. I have seen stoves with two chambers (wood and charcoal) as well as a paraffin burner that fits into the same space. I have seen gas burners that drop into the central space. If the gas runs out, it is converted to wood or charcoal burning.

The concept is well executed here – very professional appearance and the pot stays clean – a matter of major interest to the modern cook.

You may want to see the final residue from the fuel. Here it is:
[cid:image021.jpg at 01D25AC0.7A4E05C0]
There is a small amount of unburned and glowing char with a lot of ash.

I do not yet have a test result using the Central Java cooking sequence so I can’t comment on the fuel consumption. The PM will be extremely low. Nurhuda’s other models of similar construction have an energy efficiency in the high 30’s [%]. This type of device tends to be very efficient at low power.

A test method for reporting the efficiency of the two burn sequences separately and as a set was proposed on this list a couple of years ago in anticipation of this type of product. A pair of separate stoves can also function as a pair with one creating the charcoal while cooking and other carrying on, either immediately or by recovering and cooling the char and burning it later.

Regards
Crispin

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