W. Mühlbauer, A. Esper, E. Stumpf and R. Baumann
Institute for Agricultural Engineering in the Tropics and Subtropics,
Hohenheim University, Stuttgart, Germany

Some photos of the burner

ABSTRACT
INTRODUCTION
PLANT OILS AS COOKING FUEL
PHYSICAL AND CHEMICAL PROPERTIES OF PLANT OIL
COMBUSTION OF PLANT OILS
PLANT OIL COOKING STOVES
OBJECTIVES
PRIMARY RESEARCH RESULTS
CONCLUSIONS
ACKNOWLEDGEMENT
LITERATURE  

Table 1. Physical properties of selected plant oils, kerosene and diesel oil
Figure 1. Function scheme of a wick burner.
Figure 2. Function scheme of a pressure burner.
Figure 3. The original and the modified vaporizer of the primary plant oil stove
Figure 4. At Hohenheim University modified pressure stove

ABSTRACT

In rural areas of tropical and subtropical countries wood is still the main energy source. Steadily rising wood consumption for cooking purposes results in deforestation of large areas creating severe ecological, economical and sociological problems. In order to protect the environment it is urgently required to substitute the use of firewood for cooking purposes. Use of electricity or kerosene is limited mainly to urban areas due to high prices, shortages, uncertain supply, and difficult distribution of these energy sources especially in remote areas. Introduction of fuel-efficient stoves can significantly reduce the firewood consumption. However, the decrease in consumption will soon be compensated by the fast growing population. The use of solar cookers and the utilization of biogas are still limited due to technical and handling problems. Plant oils are a promising alternative energy source offering a variety of economical and ecological advantages.

Investigation indicate, that plant oils can be used in modified kerosene pressure stoves which are available in most developing countries. Since plant oils have high ignition points and high viscosity an admixture of 25 up to 50 % kerosene is required to guarantee continuous combustion. In addition, incomplete combustion causes high emissions, unacceptable from a health viewpoint. Deposits require frequent cleaning or even replacement of nozzle, vaporizer and oil lines of the stove.

At Hohenheim University a modified pressure cooker was developed, which can be operated with physic nut oil and other plant oils. A novel vaporizer was constructed increasing the retention time of the plant oil within the flame considerably. Besides of the starting phase, the cooker can be operated exclusively on plant oil. After ignition with kerosene the cooker burns with a stable blue flame. Visible emissions occur only during start-up and close-down. Nevertheless, further development and optimization especially of vaporizer as well as nozzle and rebounding plate are required.

In rural areas of the tropics and subtropics firewood is still the main energy source for cooking (RADY, 1987). During traditional cooking on open fire only about 10 % of the energy can be utilized. An average per capita consumption of 500 - 700 kilograms of firewood annually sums up to a world-wide consumption of 1 800 million cubic meter of wood per year constantly rising in line with population growth (Anonymous, 1996). The corresponding deforestation has serious ecological consequences such as increased soil erosion. In addition, soil fertility is being diminished and natural water cycle disturbed resulting in desertification, droughts and flooding. Currently, in Africa, Asia and Latin America, some 100 million persons suffer from acute wood scarcity and over 1000 million people face wood shortages (RADY, 1987).

Collection of firewood is a drudgery work mainly carried out by women and children. This often causes severe health problems, especially for children. With less wood available, collecting firewood becomes increasingly difficult and time spent for collection and transportation rises. As a consequence, families have less time for generating income and for household as well as field work, or they must pay steadily rising prices for wood. In many cases the wood necessary for cooking is more expensive than the food itself. Such economic trends lead to a decline in living standards and to increasing malnutrition.

Due to shortage of firewood, dung and plant residues are used more widely as cooking fuel. However, burning these organic materials disturbs the natural nutrient cycle and soils become impoverished and less fertile (KAMMEN, 1995).

Cooking on open fires with firewood, dung or plant residues is often done in poorly ventilated or even closed rooms which leads to serious health risks. Noxious gases cause eye and lung maladies (ANONYMOS, 1992). In Nigeria more than 95 % of the users of exposed fires complain of eye irritations and more than 50 % have respiratory complaints (KERSTEN et al., 1998).

The currently predominant open fire cooking is desperately in need of improvement or even replacement. A more intensive utilization of cooking stoves using fossil fuels like gas or kerosene as energy source can economically and ecologically hardly be recommended. Moreover, most tropical and subtropical countries are already dependent on fossil fuel imports. This often burdens the foreign currency reserves to a great extent. Due to difficult distribution fossil fuels are very expensive especially in remote areas. Since fossil fuels become more scarce in the future, it can be expected that prices will rise furthermore.

Fuel-efficient stoves have an efficiency of 20 - 40 % (KAMMEN, 1995). Even though they burn more efficiently, however, they cannot compensate the rising demand for firewood due to increasing population. The basic problem of the use of firewood still remains. The same applies to charcoal. Compared to firewood charcoal is easier to transport and incinerates producing less smoke, but achieves an over-all efficiency of only 10 - 15 % depending on the manufacturing process (MAYER et al., 1984).

As an alternative to the use of firewood low cost solar boxes and parabolic solar cookers were developed which can be locally produced in tropical and subtropical regions. Yet, both cooker types cannot store energy. Therefore they can only be operated during periods of high insulation. This limits the use of these solar cookers considerably and prevents wide spread dissemination. In addition, the need to operate the cooker in the sun causes acceptance problems.

For large-scale kitchens, e.g. hospitals, kindergartens or schools, high efficient solar cookers offer a promising alternative to traditional cooking methods. Through sufficient heat storage capacity cooking can be done early and late in the day. While the collector is placed outside the stove plate is located within the kitchen, allowing the user to stay inside the room during the cooking process. Such equipment, however, is expensive and not suitable as cooking stoves at family level.

Biogas can be used as cooking fuel but is likewise suitable for large scale operations only where sufficient quantity of organic materials can be assured. The equipment is expensive and requires specialized and well trained personnel for operation.

To overcome the current firewood problems it is an urgent need to develop an alternative cooking method. The use of plant oils as fuel for cooking stoves presents a promising alternative. In the following the state of the art of the plant oil cookers is described. Problems connected with this new cooking method are analyzed and a concept of a new plant oil cooking stove is described.

Introduction of plant oil cookers would bring a variety of socio-economic as well as ecological benefits especially for rural communities in tropical and subtropical countries. The specific benefits may vary in their individual weighting according to local conditions, for example depending on the prices of firewood, kerosene and plant oil.

Among the great variety of oil plants available in tropics and subtropics the oil of the physic nut tree (Jatropha curcas L.) seems to be most suitable for energy applications (GTZ, 1995). Originating in tropical America, the plant grows in diverse regions in Africa and Asia and receives special attention in different international development projects (MÜNCH, 1988, THIROLF, 1996, HELLER, 1996). Most varieties of this plant produce oils, which are toxic to humans and animals so that they are unsuitable for either food or feed. Jatropha plants are frequently used as hedges to protect plantings in gardens and fields from grazing animals. The plant is adapted to extreme growing conditions and is resistant to drought and therefore allows re-cultivation of desert areas. Depending on growing conditions seed production per plant varies between 1.5 and 2.0 kg. Seed production is attributed to precipitation and ranges from 0.4 tons per hectare to over 12 t/ha/a (JONES and Miller, 1992). HENNING reported productivity of Jatropha hedges in Mali from 0.8 - 1.0 kg of seed per meter of fence which is equivalent to 2.5 to 3.5 tons per hectare and year (HENNING, 1992). Assuming an extraction rate of 30 % and an efficiency of the plant oil cooking stove of 50 % the total cooking energy for on person could be covered by 55 liter of Jatropha oil per year. In Mali, this quantity can be produced on an area of about 0.06 ha or with a hedge of 175 m in length. The oil of Jatropha is currently used locally, mostly for the manufacturing of soap, for medical purposes and, experimentally, as a substitute for diesel engine fuel (HENNING, 1995). Use as cooking fuel would enhance the possible applications. In general, however, any plant oil, liquid at ambient temperatures, can be used for exposed flame cooking.

Considering the existing food scarcity in developing countries, utilization of plant oil as cooking fuel should not compete with production or use as food. Next to Jatropha curcas exists a variety of other oil-bearing plants, which may be used as well. Like Jatropha curcas they neither in their cultivation nor in their applications offer any or only very little competition to food production. Examples of those plants are the castor oil plant varieties (Ricinus communis L.), the croton tree (Croton tiglium L.), the toothbrush tree (Salvadora indica Royle), the madia (Madia sativa Mol.), the portia tree (Thespesia polpulnea L.), the babassu palm (Orbignya phalerata Mart.), and the neem tree (Ayadirachta indica Juss.) (GTZ, 1995, FRANKE, 1981, REHM and ESPIG, 1996).

Fuels for cooking stoves can also be extracted from plants which are already being planted in abundance in tropical and subtropical countries like oil palms (Elias guineensis), sunflower seeds (Helianthus annuus L.), cotton seeds (Gossypium) or coconut palms (Cocos nucifera L.) (GERMER, 1996). Even if these oils are edible, they can be used as cooking fuel if the supply exceeds its demand for food. This will be economically feasible if prices for cooking with those plant oils are locally lower than prices for other energy sources like firewood, kerosene or gas.

Traditional methods for oil extracting already exist and can be performed locally. In order to optimize the oil processing, however, efforts have to be made to develop more efficient methods for oil extraction and purification of the oil on small scale level. Utilization of plant oils as cooking fuel strengthens decentralized structures providing employment and income opportunities for rural people. Independent energy supply in both rural and urban regions reduces dependence upon imported fossil fuels. It diminishes or even eliminates time, effort and health problems associated with firewood collection and reduces purchasing cost of wood, respectively. Use of plant oils as fuel secures a long-term supply of cooking energy, which guarantees proper preparation of meals and also provides heat which is necessary for basic hygienic needs such as boiling water.

Next to the plant oil the press cake represents an economic value as well. For non-toxic varieties of Jatropha curcas as well as numerous other oil plants, it can be used as fodder for animals or as high quality fertilizer reducing the costs for buying fodder and mineral fertilizers, respectively. The press cake of toxic varieties is limited to the use as fertilizer.

In addition, using plant oils as fuel could have a number of ecological benefits. Since less wood as well as less dung and plant residues are needed for cooking deforestation will be reduced and more dung and plant residues will remain within the nutrient cycle. As the cookers multiply an increasing demand for plant oils will motivate rural dwellers to cultivate more oil-bearing plants. Since some of these plants grow on marginal land unused tracts could be cultivated and further erosion prevented.

Overall it should be noted that burning of plant oils is carbon dioxide neutral. The emitted CO₂ was originally acquired by the plant from the atmosphere through photosynthesis. Burning plant oils will therefore not contribute to global warming. Also plant oils are likewise bio-degradable and handling is both simple and danger-free.

Plant oils consist mainly of glycerides of fatty acids. Fatty acids are saturated and unsaturated aliphatic monocarbon acids, whose chain length is between 4 and 24 carbon atoms. Amongst plant oils tri-clycerides are the most common. In these all 3 hydroxyl groups of glycerines are replaced by fatty acids.

According to species and variety specific composition of fatty acids are different. The composition influences physical and chemical properties of the plant oil as well as its burning characteristics. In Jatropha oil the predominant fatty acids are oleic acid (C₁₈H₃₄O₂), linoleic acid (C₁₈H₃₂O₂), and palmitic acid (C₁₆H₃₂O₂) with 43,1 %, 34,3 %, and 14,2 % of the total mass, respectively. According to the variety, 0.06% up to 6.7 % of the oil can be free fatty acids. Sulfur and nitrogen are likewise present in amounts of 0.13 % and 0.11 %, respectively (LIDE and FREDRIKSE, 1995, Kollar et al., 1993).

Fossil fuels are mainly composed of hydrocarbons also. In contrast to plant oils the predominant chemical structure in crude petroleum oil are paraffines, olefines and naphtenes (ANOMYMOUS, 1989).

According to the structural differences between plant oil and fossil fuels, differences in physical and chemical properties between those liquids can be derived. Table 1 shows the different properties of some plant oils in comparison to kerosene and diesel oil.

The gross calorific value enables an evaluation of total heating energy generated during the combustion process. Gross calorific values of plant oils are in general about 10 percent lower than the ones of kerosene and diesel oil, respectively. For plant oils, the calorific value mainly depends on the iodine and the saponification value. According to the similarity of gross calorific values plant oils may be considered as substitute for diesel oil or kerosene as fuel.

Due to the low ignition points of about 50 °C fossil fuels can easily be ignited with a match. Due to the high ignition point of plant oils ranging from 300 to 350 °C these fuels have to be pre-heated in order to be vaporized. At temperatures lower than 300 °C plant oil starts to dissociate leaving cracking products. Cracking is likewise encouraged through thermal stresses within the plant oil. A good mixing of the fuel within the vaporizer lowers thermal stresses and diminishes resulting crack products which can cause clogging of nozzle, vaporizer and oil lines of the stove.

Furthermore the viscosity of plant oils is likewise considerably higher than the viscosity of kerosene and diesel oil. To avoid clogging of tubes and nozzle the viscosity of plant oils has to be decreased. Since viscosity reduces with higher temperature pre-heating of plant oil in the cooker has to be considered as well as possible admixture of additives.

Due to different chemical properties, especially the high ignition point and the high viscosity, plant oil show a completely different combustion characteristic compared to fossil fuels. Even though same principles as for kerosene cookers can be applied, a completely different design for the plant oil cooking stoves is necessary.

In the ideal stochiometric combustion process the plant oil as well as other biomass burns completely to carbon dioxide and water. Prerequisite is complete mixing of plant oil with oxygen of surrounding air. In real processes this ideal mixing cannot be achieved. In some regions of the flame excess air is predominant. In other regions a deficiency of air causes air pollutant emission. Harmful substances can be formed like carbon monoxide, different hydrocarbons, and soot. The formation of those depend on the fuel as well as on different characteristics of the combustion process.

Carbon monoxide is generally formed as intermediate product of combustion of carbon to carbon dioxide. For complete oxidation of CO to CO₂ certain retention time within a temperature above 720 °C is required (BAUMBACH, 1993). If this cannot be achieved CO gets emitted. An incomplete oxidizing of plant oil molecules leads to different hydrocarbons. Thermal decomposition of molecules may occur and hydrocarbons can get formed like polycyclic aromatic hydrocarbons, aldehyde, ketone, etc.

Soot is built through thermal decomposition of hydrocarbons, leaving an agglomeration of elemental carbon. Existence of soot in the flame can be detected visually. Since soot emits incident radiation like a blackbody flames with soot burn in a deep or bright yellow flame. In contrast, flames without soot have a transparent, blue flame.

As indicated earlier small amounts of sulfur and nitrogen are present within plant oil also. During combustion these two elements oxidize to sulfur dioxide and nitrogen oxide, respectively. The later one can also be formed through reaction of air nitrogen with oxygen in the burning zone.

Investigations show the immissions of a petroleum-physic nut oil mixture used in a slightly modified kerosene stove within a room with a volume of 20 m³ are very high. For CO they are given to 114 mg/m³ and for NO₂ to 45 mg/m³. Formaldehydes were measured to 400 g/m³ and suspended particles to 2.4 mg/m³, respectively (KOLLAR, 1993). On the other hand immissions of an open wood fire in a kitchen comparable in size are given by 32 - 102 mg/m³ for CO, 145 - 220 m g/m³ for NO₂, 145 - 182 g/m³ for formaldehyde and 4.3 mg/m³ for suspended particulate matter, respectively. The concentration depends on the position in the room (KANDPAL and MAHESHWARI, 1995, KANDPAL et al., 1994).

While immissions of CO, formaldehyde, and suspended particulate matter are within the same range using the plant oil cooker and the open wood fire, respectively, immission of NO₂ is very much higher with the plant oil burner. All these levels, however, are far beyond safe limits stated by the WHO. Safe limits are given to 10 mg/m³ for CO and 60 m g/m³ for NO₂, respectively (KANDPAL and MAHESHWARI, 1995). Since in many countries cooking still takes place in poorly ventilated kitchens, considerable improvements regarding the emissions of the plant oil cooker are required in order to lower the risk of health problems.

In industrialized countries, cooking is mainly done with electricity or gas. Both stove types allow easy control of power output and do not produce any dangerous emissions. In developing countries utilization of these types of cooking energy is limited to urban areas only. In these areas kerosene burners are likewise in use which are quite similar to stoves utilized for camping and other outdoor activities in industrialized countries.

For combustion of kerosene in cooking stoves two basic principles are common practice. There are wick-type burners as well as pressure burners with power output between 0.6 to 3.5 kW.

Wick-type burners utilize the capillary effect of fluids. The amount of fuel which vaporizes at the upper end of the wicks gets drawn out of the tank at the lower end of the wick. The general function scheme of the wick burner is outlined in Figure 1. Open wick cookers burn in a yellow flame under soot formation. Pans need to be placed at a certain distance to the upper end of the wick, to ensure clean operation. This, of course, lowers efficiency of the process and power-output remains comparatively low. In the wick-type cookers with higher power output, air is reacting with combustible vapors within an annular combustion chamber. For kerosene, in this process a stable blue flame is formed.

Figure 1. Function scheme of a wick burner.

The basic parts of a pressure burner are shown in Figure 2. Fuel is stored in a tank in which pressure is induced with a small incorporated hand-pump. Through the pressure the liquid flows into the vaporizer. In the vaporizer fuel is heated, evaporates, and emits in a high velocity jet through a small nozzle into the combustion area. In the free space between nozzle and flame holder the jet mixes with sufficient quantities of ambient air to enable the mixture to burn. With a rebounding plate the flame is kept in the desired place. Since fuel is burned in a continuous process at surrounding conditions this process is called "exposed flame combustion".

Figure 2. Function scheme of a pressure burner.

Generally, pressure burners have higher maximal power output than wick stoves as well as wider regulation ranges. Efficiencies of the cookers are in the range of 0.35 to 0.65 %, while, again, pressure burners have higher efficiencies than wick burners and therefore need less fuel to perform certain cooking tasks (WIECHERT et al., 1987, ANONYMOUS, 1985). The specific consumption of pressure stoves is about 20 to 30 g fuel per kg cooked food while wick stoves need about 30 to 100 g fuel per kg cooked food (ANONYMOUS, 1985).

Kerosene cookers are well established and exist in a broad variety distributed all over the world. Adapting the described principles of kerosene combustion to plant oil requires special attention. As shown in chapter 3, high ignition point of plant oil in connection with extremely high viscosity require special adaptation of the cookers. Only very few attempts were made until now to develop a plant oil cooker (ANDREWS and MKPADI, 1983, PATIL and SINGH, 1991, METZLER, 1996). PATIL investigated the use of Jatropha oil for cooking and heating purposes, METZLER examined the use of non-edible plant oils as fuel.

KOLLER et al. examined in a systematic approach the utilization of plant oil as fuel in modified kerosene stoves (KOLLAR et al., 1993, SCHWENNINGER et al., 1995, GRUNOW et al., 1997). Since viscosity of plant oil is about 30 to 50 times higher than viscosity of kerosene wick-type stoves are not suitable for use with plant oils. The wicks are not able to maintain the oil supply in the same way the oil burns at the upper end of the wick. Consequently the wicks catch fire, and the flame is extinguished.

Further experiments were carried out with different modified pressure kerosene stoves using different plant oils, especially physic nut oil. It was proved that plant oils are suitable as fuels, even though, to guarantee continuous combustion, kerosene of at least 25% had to be admixed. Only coconut oil could be used purely, but could not be recommended since energy output in the utilized cookers was very low.

For ignition of the stove kerosene had to be used in all cases since plant oils have poor ignition characteristics. The second main problem was the presence of residues when the percentage of plant oil was higher than 25% in the plant oil/kerosene mixture (KOLLAR et al., 1993). The residues were produced in different parts of the cooking stove and required frequent cleaning and sometimes even replacing of parts. As already stated in chapter 4, immissions of the cooker calculated for a closed room with a volume of 20 m³ exceeded all acceptable air concentration standards.

The studies mentioned above showed the feasibility of a plant oil cooking stove. However, optimization is required. Since wick-type stoves are not suitable for plant oil as fuel, investigation concentrate on pressure burners.

At the Institute for Agricultural Engineering in the Tropics and Subtropics of Hohenheim University a research project is conducted whose aim is the development of a pressure cooker which can be operated with pure plant oil at most with help of an ignition medium. This pressure cooker is supposed to be introduced later on into both rural and urban setting in tropical and subtropical countries. To achieve a successful introduction into those areas, some criteria of the design have to be met. Economical and socio-cultural factors, local education standards as well as acceptance levels for technology will play equally important roles. The main criteria which are to be met by the design are:

• Emissions should lie below hazardous limits
• Safe operation standards diminish operating risks
• High efficiency as well as use of low processed plant oil reduce operating costs
• Local fabrication with locally available material strengthens decentralized structures
• Simple Maintenance and long life-span increase the economic feasibility
• User friendliness and simple operation enhance the acceptance by the user
• Easy power regulation in a wide range permits fulfillment of different cooking requirements for a family

For further development of the plant oil cooking stove thorough investigations are necessary. Examination of plant oil properties and analysis of the combustion process as well as accurate measurements of stove parameters and emissions are indispensable. In order to overcome the problems associated with the use of plant oil in modified kerosene stoves, a completely new designed plant oil cooking stoves is necessary.

In general, suitability of plant oil or mixtures of plant oils as fuel for cooking stoves depend on their characteristic chemical and physical properties. These properties, like the ones discussed in chapter 3, vary according to the specific variety of the plant. Even literature values differ to a noticeable extent. Therefore supplementary analysis is unavoidable. Dependencies of properties and fuel suitability can be concluded allowing selection of appropriate plant oils and plant oil mixtures as fuels. Chemical and physical properties which have an influence on the combustion characteristics are e.g. net calorific value, boiling point, ignition point, density, iodine value, and saponification value. The viscosity of the plant oil decreases with temperature. For a complete combustion analysis chemical composition as well as impurities of the plant oil need to be known.

As stated earlier production of plant oil as cooking fuel should be preferably performed locally even in remote areas of tropical and subtropical countries. Regarding existing means in those areas the proposed plant oil cooker should be able to use low processed plant oil as fuel, for example crude, cold pressed plant oil. The less processing is needed, the easier dissemination of the plant oil cooking stove will be since production of the oil is easier and cheaper. On the other hand the less processed the oil the more impurities exist within it. Impurities influence burning characteristics and lower storage stability. Specific dependencies are yet not readily known and need to be investigated.

Oxidation of double bounds in unsaturated fatty acids and of free fatty acids, respectively, as well as water content of plant oil cause corrosiveness. Corrosion has a big influence on storage stability of plant oils. It depends on temperature while high temperatures as predominant in various tropical and subtropical countries support storage instability. Oxidation of fatty acids can be prevented through admixture of herbal additives. Water content can be reduced through mechanical extraction.

At certain temperature below boiling temperature plant oils start to dissociate. The cracking products clog the nozzle and leaves the cooker unusable. Cracking is likewise encouraged through thermal stresses within the plant oil. A good mixing of the fuel within the vaporizer as well as pre-heating of the plant oil lowers thermal stresses and diminishes resulting crack products.

The efficiency of the cooker will be determined applying the Water Boiling Test of the World Bank (ANONYMOS, 1985). Efficiency as well as combustion characteristics and emissions depend on different parameters of the cooker, like power, distance between nozzle and cooking pot, temperature of supply air etc. Different geometries and materials of vaporizer, nozzle, flame holder, and rebounding plate of the cooker are likewise considered.

Since plant oils cannot get lighted with a match alone, a special ignition device is needed allowing reduction of ignition time and amount of kerosene required. It might even permit use of pure plant oil only. Admixture of ethanol or other alcohols instead of kerosene is possible. Through pre-heating of the plant oil its ignition characteristics may be improved and emissions in the starting phase decreased.

While fuel is flowing out of the fuel container during combustion process, the pressure in the container lowers continuously. Reaching a minimum pressure the hand-pump needs to be utilized in order to raise the pressure again. This procedure is quite feasible for camping cookers, but should not be applied for household cookers. For the later purpose operation should be possible with little or even without any attention paid by the user. A possible installation of a pressure reduction valve, for example, permits a constant pressure adapted to the specific cooking task. Dependencies of pressure on power and quality of combustion will likewise be investigated more closely.

A test bench is under development which allows a systematic analysis of the combustion process and a continuous measurement of emissions. Operation performance of the cooker as well as long-term behavior and suitability for preparation of typical meals can likewise be studied. This test bench registers all data automatically by a computer controlled data acquisition system. Based on the outcome of these investigations a prototype of the plant oil pressure stove will be developed and tested under local conditions in developing countries.

7. PRIMARY RESEARCH RESULTS

For primary research at Hohenheim University towards construction of a plant oil cooking stove refined physic nut oil as well as sunflower, rape, corn and peanut oil were used. Results for all plant oils were quite similar. At first, investigation about ignition characteristics of different plant oils have been carried out.

Plant oil can not get lighted with a match alone within evaporating dishes since it has a high ignition point. However, mixtures of plant oil with kerosene and gasoline, respectively, are inflammable. Due to low ignition points the later ingredients get inflamed and burn along with parts of the plant oil. Unburned parts of the plant oil as well as cracking products are left in the evaporating dishes.

With utilization of a support medium the surface of the plant oil fluid can be enhanced considerably and ignition can occur. Support mediums have comparable effect like wicks. Different materials have been studied for use as support medium. Fine structured metal grids can not hold enough plant oil to ensure ignition. Cotton and steel wool burn before plant oil inflames while webs made of fiber glass melt during combustion. Fiber glass mats, however, are appropriate as support medium for plant oils. These mats are normally used in the automotive industry. Those mats can get re-ignited numerous times without deterioration. The plant oil burns on ambient air with a yellow flame under the formation of soot.

As by KOLLAR, primary investigation of a plant oil stove were based on a pressure kerosene stove for camping purposes. The "MSR-X-GK" is manufactured by the Mountain Safety Research Company, USA. It can be fueled with kerosene and gasoline, respectively, and its power output ranges from 1.5 kW to 3.3 kW. Modification of this cooker took place in different steps which are recapitulated below.

Since peanut oil is very viscous at low ambient temperatures it was excluded in the following. Mixtures of 25, 50, 70, and 90 % plant oil with kerosene were used for the following experiments. Utilization of gasoline instead of kerosene did not lead to distinct results. Moreover, pure plant oil was tested also. As for nomenclature, a mixture containing 25 % plant oil and 75 % kerosene is called a "25 % mixture".

For ignition of the MSR-X-GK, fuel is discharged in a dish below flame holder and vaporizer. Within the dish a web is placed as support medium to enable ignition of the fuel with a match. After lighting the heat generated by fuel combustion is sufficient to start the cooker.

Ignition of the stove without changes is possible for mixtures up to 50 % plant oil. Even 90 % mixtures can get ignited but need the above described fiber glass mat instead of the web as support medium. Nevertheless, this method of heating up the stove can not be applied for pure plant oil. According to the amount of plant oil in the fuel the ignition time of the stove increased from 45 seconds for the 25% mixture to over 5 minutes for the 90% mixture.

In each case, the cooker burns with a yellow flame under soot production right after start-up. This yellow flame changes into a blue flame after 1 minute or 5 minutes for the 25 % and the 90 % mixture, respectively. The higher the amount of plant oil in the fuel, the more unstable the combustion and the more fluttering of the flame.

For ignition of the stove with pure plant oil an external flame from a gas burner is needed heating up the vaporizer for 30 seconds. The flame of the cooker fueled by pure plant oil is very unstable changing its color continuously between yellow and blue. The nozzle clog up within about 20 minutes.

Since ignition of plant oils occurs at higher temperatures than ignition of kerosene the above described phenomena are explainable. Construction of a new vaporizer enhanced the suitability of the cooker for plant oil mixture as well as for pure plant oil. With the new vaporizer the retention time of plant oil within the flame is increased considerably. The new vaporizer is made from a copper pipe with a diameter of 6 mm. Copper can be formed easily and has a high thermal conductivity. The original and the modified vaporizer are shown in Figure 3.

Figure 3. The original and the modified vaporizer of the primary plant oil stove (BAUMANN, 1997)



To ensure sufficient air supply for the combustion process the number of air holes in the cooker were increased. At this stage of research the cooker was successfully fueled by pure rapeseed methyl ester also. The ignition phase lasted 5 minutes before a stable blue flame developed. Likewise the stove with the new vaporizer burned in a stable blue flame with both the 10 % mixture and the pure plant oil. While the cooker could be started with the 10 % mixture heating up with the gas burner was required for the pure plant oil.

For easier operation this external flame of the gas burner was integrated within the cooker. Therefore a system containing two tanks was designed. One fuel container is filled with pure plant oil while the smaller tank is filled with kerosene. At first the stove is fueled with kerosene. After 2 to 3 minutes operating temperature is reached and the valve at the plant oil tank is opened while the valve at the kerosene tank is closed. Hence, the cooker is fueled by plant oil only. The only attention required during the combustion process is to pump in some more air in the plant oil fuel container whenever the pressure is too low. For closing down operation the stove needs to be fueled with kerosene again enabling a problem-free start the next time. Except of the ignition and the closing phase the cooker burns with a stable blue flame.

An overview over the modified cooker is given in Figure 4.

Figure 4. At Hohenheim University modified pressure stove


This stove can operate for more than 5 hours continuously with the plant oils described above. Even though this result is promising, further research is necessary in order to overcome existing problems. Heavy visible emissions especially during start-up and close-down phase, ignition and cleaning are the main problems. The later one since during closing down operations the nozzle and other parts of the cooker become clogged so the stove has to be cleaned thoroughly before it can be re-ignited. Moreover handling of this preliminary cooker is still rather difficult and not acceptable for broader dissemination.

8. CONCLUSIONS

A pressurized plant oil cooking stove was developed which can be fueled by plant oil alone. Only some kerosene is needed for ignition and closing which both last for no more than 3 minutes. The main problems related to utilization of the cooker are occurrence of emissions and clogging of different parts of the stove. The cooker can be operated with different plant oils like oil of Jatropha curcas. Further investigation is required. Subsequently, a new design of the plant oil cooking stove can be realized and a prototype can be constructed.

Utilization of the proposed cooker can produce in the medium term varied ecological, economical, and sociological benefits in developing countries. It is expected that introduction of the new plant oil cooking stove will be readily acceptable to people in tropical and sub-tropical countries since its operation is quite similar to known kerosene stoves.

The exposed burning of plant oils does not need to be limited to production of heating energy for cooking purposes only. Using the same principle, energy for cooling and lighting can be generated also, e.g. for absorption refrigerators, various items of hospital equipment and lamps.

The authors acknowledge their indebtedness to The Rockefeller Foundation and the German Federal Foundation for the Environment for their financial support of this investigation. The authors are furthermore thankful to the German Agency for Technical Cooperation (GTZ) for their cooperative support.

e-mail to the author: stumpf@ats.uni-hohenheim.de

last modification  21.06.02   by