Crispin Pemberton-Pigott, New Dawn Engineering, Swaziland, December 2005
Dear Friends of Big Stoves
Together with Cecil Cook from New Dawn International and working for ProBEC South I tested the new 'Lion Stove' on the 15th of December 2005. This stove is a replacement for the 'open fire' commonly used for very large pots. Such open fires have a fuel effiency between 1 and 5%. I have seen fires going at a windy site for 2 hours that could not boil 20 litres in one of these pots. The estimated fuel use per 'boil' of 50 litres in these black three-legged globes is 30 to 40 Kg (figures provided by experienced cooks).
The Libhubesi Stove is a large institutional Rocket Stove with preheated air feeding from the 'back' of the combustion chamber. It is made for 70 litre cast iron three-legged pots which dominate the local market when it comes to cooking for the multitudes. The stove requires about 200 bricks, one thin cement slab, 125 Kg of cement and a steel frame to support the 61 Kg pot (empty mass!).
We have finished 4 of these stoves, the inner brickwork of which looks remarkably like a Sphinx or a resting Lion. The women who first used it at Bethany in Swaziland thought it should instead be called "Mngani waMake" which means "Woman's Friend".
As Cecil Cook, our social anthropoligist, is difficult to get hold of we had to use our time sparingly and decided to do a boiling test only to find at least a clear indication of the fuelwood use of this stove while heating water. Previous tests in the factory showed that the flame shape and emissions were very good with a variety of fuels.
The principal reasons for changing from the standard Rocket Stove construction fall into two groups: issues to do with training and dissemination, and technical issues to do with combustion and durability.
Training issues include the difficulty of getting people who we have never met to build a stove with a reasonably predictable performance. This relates to the issue Roger Samson is raising - getting CDM credits based on claims that stoves perform to a certain standard. Stove builders do all sorts of strange things once they are turned loose. Aprovecho, to their great credit, have produced a list of design guidelines which if followed, produces stoves with this sought-after reasonably predictable performance. What builders do when they alone in the bush can vary a good deal from that list, and cooks do all sorts of thing which they "shouldn't". As both the builder and the operator have to be trained, often by third or fourth generation communcators, some problems have arisen for which there are 'design-arounds'.
Three of these are: the removal of the metal shelf separating the fuel from the airflow underneath and placing the fuel on the floor of the airway; the use of materials that do not stand up to the heat or the physical damage caused by jamming fuel into the stove; poorly made insulative bricks or metal with a low heat tolerance); and a high excess air ratio reducing the gas temperature (Delta T) which consumes fuel unnecessarily.
Training and dissemination isses include the low skill level people may have regarding their ability to read a tape measure, divide by two, follow a drawing with dimensions on it, mix of ingredients properly, the unavailability of some suggested components and forgetfulness. Increasing the complexity of a stove increases the chances the builder will want a unrealistic fee, this being prompted by the amount of his 'secret knowledge' he received in training. It is much easier when doing a large scale implementation, to work with a fixed set of components, dimensions based on the brick and a short training session that focuses on one particular stove for a particular pot.
Clearly this is not a one-size-fits-all approach and we make no pretense of that. This is a particular environment with a particular need requiring a particular solution. We are not training builders of 'general' stoves, but ones used at orphan feeding centres. In the beginning it is for World Food Programme neighbourhood care points (NPC) and many others supported privately by Rotary Clubs and faith-based organisations.
Swaziland is small, has a good road system and nowhere is more than 80 miles from Manzini. There are 300 or so WFP NCP's which have one or more 70-litre cast iron pots. There are probably another 300 NCP's at least. Most have 1 or 2 of those pots plus smaller ones. The reason for the consistency is that the No. 25 Falkirk or Best Duty pots are the largest available to the ordinary person.
The stove sits on a 2 meter square slab 75 to 100mm thick. It is 1 metre square sitting approximately in the centre and thus has a cement walkway around it. The Lion is built starting in the middle (which is the combustion chamber) and the builder works outward. When the air inlet, fuel magazine and the clean-out are finished, the surrounding structure is built to knee-height at an appropriate distance. This is the low wall that forms the air preheating chamber. The heat that gets into the combustion chamber bricks and passes through, heats the incoming air.
The wall is topped with a 1 metre square 40mm concrete slab. Because transport in not a big issue, these slabs are produced centrally (like the fired clay face bricks). The steelwork can be made anywhere, but at the moment is available from one workshop. The result is a consistent set of materials. The air inlet, fuel magazine and combustion chamber dimensions are made as per Rocket Stove sizes.
On top of the slab is a steel frame that holds the huge pot-bellied (is that the word?) pot in such a way that it does not bear on the centre of the slab. The weight is transferred to the outside edge so that with time, deterioration of the cement will not affect the stove's structure or performance. The steelwork is surrounded by bricks and an outer wall 1 metre square surrounds everything. When it is finished it is a 1 metre cube.
There are protruding steel loops that rise above the top deck of the stove which allow the pot to be locked or fastened into the stove to prevent theft. This is a serious issue at NCP's because no one lives there all the time. They are run by volunteers who show up daily and there is a high staff turnover. The pots are worth more than $200. A chain can be passed through the loops and the handle loops on the pot, then padlocked. We also tried using a 16mm re-bar which is fed through the pot 'ear' and bent tight around the loop.
The fire is lit in the normal way. The only difference is that there is no metal shelf to slide in and out. This appears not to be a problem even for first time users. The hole size is 165w x 110h and allows for enough wood to be inserted to give 10-15 Kw of heat. Air enters the combustion chamber along the fuel magazine, as well as through the lower air inlet.
In a Rocket Stoves these passages are on the same side of the chamber and all the air rushes to the back in an upward curve. This heats the brickwork at the back to a much higher temperature than at the front as the flame is away from the front surface. (I am calling the fuel magazing side the front.)
In the Lion stove layout, the air entering from below comes from the back so it counteracts the airflow from the fuel magazine, reasonably cancelling the tendency to carry the flame to the back wall. The flames tend to stay in the centre of the combustion chamber, reducing brick heating, increasing flame temperature and promoting good mixing. I have available an MPEG movie of the flames showing this effect.
As the bricks heat up and transfer their heat to the incoming air (which takes a long time) the primary air temperature rises. After 1 hour it is about 20 degrees above ambient and after 2 hours it is about 40 degress higher inside the lower chamber than outside. The preheating of the air contributes to the low CO level.
The excess air ratio (the amount of air flowing through the stove that is above the amount theoretically required for combustion of the fuel) ranges from 100 to 400% during heating to a boil. With a small fire it may be much higher, and we recommend the air flowing through the fuel magazine be blocked with a rag when simmering. The increase in gas temperature is pronounced when the air coming through the fuel is choked - as much as 200 degrees, measured _after_ the pot. It has a very big influence on the heat transfer efficiency.
When the fire is hot the CO level is usually under 2% of the CO2, running from a low of 0.5% to 4%. It is high when stirring the fire as new fuel is added. There is usually no visible smoke. We did one test with wet wood - so wet it would not burn without dry sticks being added - and the smoke level was not affected as long as flames were present.
Water Boiling only - cold start
Pot mass 61 Kg (cast iron)
Water Mass 48.199 litres
Initial temperature 31 deg C
Final temperature 100 deg C
Time to boil 63'45" from lighting the match
Kindling 200gm (grass, twigs, small roots) Fuelwood consumed during the test 3.760 gm in addition to the kindling (air-dried saligna crating and pine with an estimated 12% moisture content). Total, 3960 gm, dry mass 3490 gm with 150gm charcoal remaining).
An accurate measurement of the charcoal remaining in the bottom of the combustion chamber could not be made, so it was estimated to be 150gm which is conservative for a 1 hour 10Kw+ fire.
Percentage of the heat released absorbed into the water (corrected for evaporation of the moisture in the wood and the remaining charcoal) = 22.3%
Percentage of the heat released absorbed into the pot and the water (corrected for evaporation of the moisture in the wood and the remaining charcoal) = 25.8%
It is a pretty good stove: easy to build (1 day) and finish (another day to plaster). It does not use any insulative materials such as sawdust or vermiculite (both usually unavailable). It is very robust and is expected survive daily wear and tear for several years without maintenance.
1. The wood used was rather large and later we got a better temperature rise by splitting 3 inch square pieces into 4 parts so they burned faster. This reduced the excess air ratio to less than 100% and increased the gas temperature. The temperature rise was a consistent 1.25 degrees per minute but could be increased to 2 or more with this technique.
2. The cast iron pot is a difficult shape to heat efficiently because it does not have a large horizontal surface under which to pass the hot gases.
3. The initial water temperature was quite high (a hot day). The fuel used was 57 gm per degree C for the pot and water. This may be a more useful number than the total fuel required. Another number (ignoring the pot) is that it took 1.18 gm of wood (with moisture) to heat 1 litre 1 degree C.
4. There is quite a lot of heat in the pot (about 2.1 MJ) most of which would be recovered in a 'hot start' test - though this will pose an interesting topic for testers to discuss. As the pot cannot be removed from the stove, is it part of the stove? Clearly the fuel required to boil 48 litres when the stove and pot are already hot is going to be less than is indicated by this test. It may in that case have an efficiency in the high 20's.