| Boiling Point No. 20- December 1989 |
by Niek Verhoeven
This is an extended summary of the Masters's thesis of N.A. Verhoeven submitted to The Faculty of Physics, Eindhoven University of Technology, Eindhoven, The Netherlands, February 1989.
The scarcity of fuelwood in developing countries is well known. It has generally been assumed that deforestation is the prime cause of fuel wood scarcity. In some situations substituting fuel wood by other kinds of fuel is a solution. Substituting fuel wood and wood stoves in urban Sahel areas and cities by kerosene and kerosene stoves is one of the options under consideration. This article is a summary of a study done at the Woodburning Stove Group on one type of kerosene stoves: the kerosene wick stove (see picture).
Name: Pet Stove
Manufacturer/country: The Netherlands
Weight (empty): 4.5kg
Tank capacity: 2.2kg
Wick Material: Cotton
Fuel level indicator: None
Number of wicks: 21
It is concerned with the principle of the working of a kerosene wick stove with some test results. A small part will deal with single wick tests which give some understanding of the principle of kerosene wick stove but also raises new questions.
The Woodburning Stove Group (WSG)
WSG started as an informal association of people from the faculties of Physics and Mechanical Engineering at the Eindhoven University of Technology. The group was formed in March 1980, with the aim of developing a set of design rules for efficient wood burning cookstoves for use in the Third World. WSG cooperates with engineers from the Netherlands Organization for Applied Scientific Research (TNO, Apeldoorn), Bio Technology Group at Twente University and a couple of private consultants. At the moment of writing this article the work of WSG in Eindhoven is concentrated on three topics:
(1) the wood burning bakery oven;
(2) the clean combustion of wood;
(3) the kerosene stoves.
The Kerosene Wick Stove Study
The work on kerosene stoves at WSG started in 1983 at the instance of the Energy Assessment Division, of the Energy Department of the World Bank. The main purpose of the work was to provide reliable data on kerosene stoves of diverse designs, as an aid to policy planners for selection of design. A result of the test programme was that none of the commercially available kerosene stoves at that moment had a maximum power output above 2kW (World Bank 1985). In the test programme two major types of stoves, each with its own advantages and disadvantages, were found: the kerosene pressure burner and the kerosene wick stove. It is against this background that the World Bank in 1987 awarded a research & development contract to the Woodburning Stove Group in Eindhoven. The purpose of this project was:
(a) to develop a high-power, low-cost kerosene wick stove;
(b) to use the information obtained during the course of the work to recommend measures to adapt existing stoves. In particular the following specifications of a kerosene wick stove were considered to be suitable for a Sahelian country (Bussmann et al 1987):
· maximum power 4.0 kW
· minimum power 0.7 kW
· thermal efficiency 50 %
· adapted to cooking habits in Sahel (touo preparation, use of spherical pot
· low cost
A more extensive specification regarding requirements for kerosene stoves for the Sahel is given by ET Ferguson in Appendix A (attached).
A direct result of this work was the development and building of a high-power kerosene stove, the Pet Stove. The Pet Stove has the various adjustments suggested by the results of the R&D work. In the report of this R&D work (Bussmann et al 1987) it is clearly stated that the Pet Stove is still a prototype and not yet ready for large-scale production. At that moment a new high-power kerosene stove was discovered on the commercial market, the Thomas Cup 36. In field studies in Burkina Faso (Sulilatu 1988) and in Cape Verde (Bussmann et al 1988) both stoves were tested and showed disappointing results with respect to power output and efficiency. Another problem of the kerosene wick stoves, found during field studies, was that, even if the wicks were protected as suggested by the R&D work, they wear out very quickly (Bussmann, Sangen, Sulilatu and Visser internal communication WSG 1988). On the whole the kerosene wick stove is a promising stove for developing countries, due to the easy maintenance, the low purchase price and the possibility of local manufacturing of the stove by small-scale industries and artisans (Sulilatu 1988 & Sangen 1988).
Principle of Operation of Kerosene Wick Stoves
Although the use of mineral oil and its residues for burning and other purposes has been recorded since ancient times, the major breakthrough in the application of fuel oil for technical purposes is not more than one hundred years old. Kerosene lamps were originally made with cord wicks, like those of candles, but very soon annular wicks with air admittance in the centre proved to reduce the tendency to soot considerably, obviously because the centre, being exposed to a strong radiation of heat, then consisted of thermally stable air instead of unstable hydrocarbons. This construction change can still be found in the different stove designs, whether they consist of a single ring wick or a group of small wicks placed in a ring. Wick burners can be subdivided into open wick burners and range wick burners. This study is concentrated with the latter. Range wick burners were developed to increase the power output of wick burners, while keeping the combustion clean. The principle of the construction is as follows (Prasad et al 1983).
A number of wicks is fixed in a holder such that they can move up and down. Moving up and down causes them to emerge into an annular space formed by two thin-walled concentric perforated steel shells, the flame holders. The distance between the inner and outer flame holder is a little more than the thickness of the wicks, usually around 12 mm. The height of the cylindrical flame holders is about 10 cm. To start the stove, the wicks are turned up and set alight. The draft created by the flames draws ambient air through the small holes in the flame holders into the annular space. At these small holes tiny blue flames can be observed. If the wicks are being turned up to a sufficient height, the top level of the flames gradually rises, eventually filling the whole annular space and emerging from the open top in a stable blue flame. Raising the wicks even more will make the upper part of the blue flame become yellow, which is an indication of cracking of the kerosene in the flame. The cracking of kerosene is mostly accompanied by the production of soot. Soot reduces combustion quality and is therefore undesirable. The heat generated by the reaction of a* and kerosene vapour will, after some time, make the flame holders glow red hot. To keep that heat from radiating away, the burner is usually provided with an outer cover, the wind shield.
As a part of this study six different kerosene wick burners were tested and compared: the Pet Stove, the Thomas Cup 36, the Thomas Cup 24, which are stoves; the Elegance K786, the Zibro Kamin RCA68, which are room heaters; and the Aladin Lamp; which is a lamp. In table 1 only the burners which could function as a stove are included: the Elegance was very easy to rebuild as a stove. The heat transfer efficiency results should be seen as maximum results under very ideal conditions: in the laboratory there was no wind; the used kerosene was free of dirt etc.; the stoves were clean and the wicks were brand new.
The minimum power output of a stove is the lowest power at which a stove still burns stable and is used during the simmering phase of cooking. The minimum power output of the four stoves - both Thomas Cups, Pet Stove and Elegance K786 - was measured. In all cases, except for the Thomas Cup 36, the minimum power output is quite large compared to the blue flame maximum; it is only a factor 2 smaller. This factor is called the turn down ratio. The test with the Thomas Cup 36 - all 36 wicks were used - failed, because there was not stable minimum power. A turn down ratio of 5 with this stove is possible when one only uses the inner 12 wicks (Sulilatu, 1988).
If one divides the minimum power by the number of wicks, in case of the multi wick stove, one will get a nearly identical minimum power output per wick for each stove. This is demonstrated in table 2.
Table 2: Minimum power output per wick
No of wicks
Min power (kW)
Min power per wick (W)
Thomas Cup 24
It is as if there is a lowest minimum power per wick; this leads to the assumption: to get a very low minimum power one better reduce the number of wicks used.
A summary of the results for the Pet Stove is reproduced below. The Pet Stove was the result of the work of the project to develop a high-power kerosene stove, it is still a prototype in the process of improvement. In field studies in Burkina Faso (Sulilatu, 1988) and Cape Verde (Bussmann, 1988) it was tested along with a new commercial stove - Thomas Cup 36 - and both showed disappointing results for power output and efficiency. Another problem was that wicks are used up very quickly. Their conclusion was that "the kerosene wicks stove is a promising stove for developing countries due to the easy maintenance, low purchase price and the possibility of local manufacture by small-scale industries and artisans".
Single Wick Experiments (see Table 3 for results)
To understand the processes in a kerosene wick stove a single wick burner was constructed. The geometry of this single wick burner could be changed for different types of experiments. By using pyrex glass tubes one could observe what happens to a cotton wick and artificially create a draft around the wick. A general conclusion of these experiments is that at small tube lengths the mass rate is increased compared with no tube and at large tube lengths the mass rate has decreased. In Figure 3 the mass rate and the CO/CO2 ratio is presented as a function of the glass tube length for a free wick length of 5mm. Remarkable in this figure is the sudden raise of CO/CO2 ratio and the sudden fall of the mass rate. The reason for the occurence of this phenomena needs further investigation.
At this moment (November 1989) known to WSG no kerosene wick burner is suitable for full cooking purposes for an average Sahel family. The high power kerosene wick stove fails with regard to heat transfer efficiency, safety, combustion quality and power regulation. To be specific the heat transfer efficiency is low, because the burner heads are too large; the safety is bad, because the fire cannot be put out easily; the combustion quality is poor at the lighting and extinguishing of the fire, a large CO production is measured during these periods; the power regulation is insufficient, because a turn down ratio of only 2 is found and 5 is required. Further more most kerosene wick stoves are not suitable for Sahelian round- bottomed pots of sizes 1 to 6. Although the high power kerosene wick stoves like Pet Stove and Thomas Cup 24 and 36 are insufficient they show some promising features for further development. Low power kerosene wick stoves can be used in Sahel for preparing small quantities of food, for boiling tea or other less energy consuming activities. In such a case one does not have to light a large wood fire with its large wood consumption.
The extreme values of the single wick experiments indicate that the building and constructing of a kerosene stove still needs further investigation. If one wants to build a stove with 20 wicks one could get according to the data a stove with a maximum power of 5.32kW and a minimum power of 0.48kW. This hypothetical stove would have a large heat transfer efficiency because it is compact (only 20 wicks).
Requirements for Kerosene Stoves for the Sahel
By Dr. ir. Eric T. Ferguson, September 1988
In the Sahel, substituting LPG or Kerosene for fuelwood and charcoal is an important way of reducing deforestation due to fuelwood consumption. However, no really practical stoves for kerosene are yet available. This note lists preferred requirements for such kerosene stoves; completeness was attempted but certainly not attained. The list sets high demands; it will certainly prove impossible to satisfy all conditions simultaneously. Designers could aim for the closest approach feasible, using the list as a 'check-list' of aspects to be included in the evaluation of the competing designs.
The requirements are formulated as if for one stove. It may be necessary to specify a set of several stoves of increasing sizes, but this can mean that some households would have to buy two stoves instead of one, thus increasing the effective price.
Usage and Performance
· may be either a wick or a pressure-burner stove (each has its advantages and disadvantage)
· suitable for Sahelian round-bottomed pots of sizes 1 to 6. No accurate fitting of the stove to pot needed; in some areas flat-bottom pots are also used and should fit
· an advantage if also suitable for frying (flat or bowl-shaped pan)
· easy lighting, also in a wind: preferably no separate lighting fuel (such as petrol or spirits)
· maximum power sufficient for cooking meals in the largest pots
· low specific energy consumption at high power even m medium wind
· low fuel consumption when simmering (maintaining boiling)
· easy power regulation: turn-down ratio of 4 or preferably more
· no unintended extinguishing of the flames at low power, even in wind
· no very hot outer parts (danger of burns; children! )
· pots and pans can be put on/taken off without getting burned
· good quality combustion; no CO, smoke or smell
· clear indication when fuel is low
· easy filling with fuel, even when hot
· should stand stable on sand or uneven surface
· the pot/pan should stand firmly in/on the stove, especially if the food requires vigorous stirring as for to'
· only simple instructions for use
· errors in handling should not lead to danger lifetime at least several years with daily use; materials used should not age.
Maintenance and Servicing
· simple maintenance and cleaning by housewife, even in the unfavourable conditions of Sahelian households
· should withstand boiling over of food without damage or need for servicing
· mechanical moving parts must move easily, both when hot or cold, even if dirty or poorly oiled or maintained; should not get out of adjustment
· tolerant of sand and dust, both outside and in the fuel
· tolerant mechanical mishandling (dents, breakage) or being left unused for a long time (dirt and rust)
· infrequent replacement of consumable parts (such as wicks), preferably by the owner; minimum maintenance
· no loose parts or items that can be lost
· simple spare parts that any local retailer can sell or fit
· spare parts to use materials or items currently available (e.g. pump, seals for motor vehicles standard wicks)
· maintenace by a dealer should not require complicated tools or training, no accurate adjustment needed
· exchangeability of parts between different models and sizes; no possibility of confusion of parts looking somewhat alike
· no possibility for wrong assembly
· as far as possible local manufacturer in one or more Sahelian countries
· type of manufacturer to correspond to manufacturing techniques and accuracy required
· if possible manufactured by local artisans of the informal sector
· a price that is not more than three times that of a traditional metal stove ("foyer malgache")
· no difficult manufacturing "know-how" that requires extensive training or special equipment
· most materials to be locally available or already normally imported; use of local raw materials (like clay or recuperated metal) is an advantage
· any special parts (e.g. wicks, jets) may if necessary be imported
· support bye multinational maybe an advantage, but monopolistic supply is too vulnerable; second sourcing required.