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development of a household cooking stove
for tropical and subtropical countries
E. Stumpf and
W. Mühlbauer
Institute for Agricultural Engineering in the Tropics and Subtropics
Hohenheim University
Garbenstr. 9, 70599
Stuttgart, Germany
Tel.: +49 711-459 2490, Fax: +49 711-459 3298, e-mail: muehlbauer@ats.uni-hohenheim.de
| Abstract | 3. Prototype of the Plant Oil Stove |
| 1. Introduction | 4. Ackknowledgement |
| 2. Liquid Fuel Cooking Stoves | Literature |
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 utilization of firewood for cooking purposes. Plant oils are a promising alternative energy source offering a variety of economical and ecological advantages. A variety of oil plants originate in developing countries and their oil can be produced locally even in remote areas. Existing cooking stoves for liquid fuels, however, do not allow utilization of plant oils as fuel.
At Hohenheim University a new pressure cooking stove was developed, which can be operated on different pure plant oils like Jatropha oil or Canola oil. According to the chemical, physical and combustion properties of plant oils, a new vaporizer, a new burner head and a new tank as well as a novel start-up device were designed.
Household energy, especially cooking energy, often counts for a big part of the overall energy consumption in many developing countries. In Nepal, for example, over 70 % of the total energy consumed is used for cooking purposes only. In general, wood is still the main energy source in the rural areas of tropical and subtropical countries [1]. Steadily rising firewood consumption for cooking purposes results in deforestation of large areas creating severe ecological problems. In order to protect the environment it is urgently required to utilize alternative methods for cooking purposes. Introduction of fuel-efficient stoves can reduce the firewood consumption significantly [2]. However, decrease in consumption will soon be compensated by the fast growing population.
Electricity is still restricted mainly to urban areas. Use of solar cookers and utilization of biogas are limited because of technical and handling problems [3]. Dissemination of conventional cooking stoves utilizing fossil fuels is restricted due to limited availability of those fuels especially in rural areas. Furthermore, import and subsidization of those fossil fuels burdens the budgets of developing countries.
Utilization of plant oils as cooking fuel presents an interesting alternative to yet known cooking methods and offers a variety of ecological, economic and sociological benefits.
A vast variety of oil plants originate in tropical and subtropical countries. In general, all plant oils liquid at ambient temperatures can be utilized as cooking fuel. Many oil-bearing plants grow on low grade surfaces or in marginal locations, which are unsuiztable for food crops while their oils are often toxic to human beings. Those plants are often cultivated on waste lands in order to prevent further erosion and inhibit desertification. Energetic utilization of their oils will not compete with food production [4], [5]. Exemples of those oil plants are the physic nut tree (Jatropha curcas L.), the castor oil plant varieties (Ricinus communis L.) and the babassú palm (Orbignya phalerata Mart.) [6], [7].
Traditional methods for harvesting the fruits from oil plants and extracting the oil already exist in many regions of tropical and subtropical countries [8]. This local oil production strengthens decentralized structures providing employment and income opportunities for local population and ensures sustainability [9]. A long-term supply of heat energy can be secured. Plant oils are bio-degradable and handling is both simple and free of danger. The burning of plant oils is carbon dioxide neutral since the CO2 was originally acquired by the plants from the atmosphere through photosynthesis. Utilization of the plant oil stove will relieve women from the suffering due to eye and lung diseases caused by open fires in often poorly ventilated rooms. In addition, collection time and purchase costs of fire wood, respectively, will be reduced considerably. Moreover, the presscake as a by-product of the oil processing can be used either as fodder or as high-quality fertilizer.
Kerosene is the most well-known liquid cooking fuel within developing countries
today. Its chemical structure consists of hydrocarbon molecules with chain
lengths of C8 or C10. Plant oils are tri-glycerols of
fatty acids whose chain lengths range mostly from C12 to C18.
In general, plants oils are regarded as suitable substitutes for fossil fuels
since their gross calorific value per volume is only 5 percent lower than the
gross calorific value for kerosene or diesel fuel.
Nevertheless, due to the different chemical structure plant oils have distinct physical and chemical properties and show a different combustion characteristics than kerosene. The flash point, for example, ranges around 180 to 300 °C in comparison with the flash point of kerosene at 80 °C. Moreover, viscosity of plant oils is at least 30 times higher than viscosity of kerosene.
Liquid fuels can be burned in wick and pressure stoves [10]. Generally, production as well as maintenance costs of wick stoves are lower than of pressure stoves. Pressure stoves, however, have higher power outputs and higher efficiencies. In most cases, the power of pressure stoves ranges from 0.8 up to 3.5 kW with an efficiency of 45 – 52 % whereas power output of wick stoves ranges from 0.8 to 2.2 kW with an efficiency of 38 – 47 %. Combustion in pressure stoves is a higher quality combustion producing less emissions. Nevertheless, the emissions of both stove types a very much lower than the hazardous emissions of open wood.
Wick stoves utilize the capillary effect of fluids. However, plant oils cannot be used in common stoves with cotton wicks. Due to their high viscosity, the flow velocity of plant oils in those wicks is very low. Therefore the wicks cannot maintain the oil supply and the flame extinguished consequently. Therefore, investigations on utilization of plant oils as cooking fuel have been focused on combustion in pressure stoves.
In those stoves pressure is induced in a tank through application of a small hand pump. The liquid evaporates in a vaporizer and emits through a nozzle into a burner head where the jet mixes with ambient air. While leaving the burner head through openings the fuel-air-mixture burns in a pre-mixed flame. The power is adjusted with a valve which regulates the fuel flow. The function diagram of a pressure cooking stove is shown in Figure 1.
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Figure 1: Function diagram of a pressure stove
In earlier investigations by other research groups plant oil was used as fuel in modified kerosene pressure stoves. Those stoves needed an admixture of at least 50 % of kerosene to the plant oil in order to perform satisfactorily [12]. Nevertheless, residues of the fuel mixture clogged the vaporizer and left the cookers unusable after short operation time [13].
At the Institute for Agricultural Engineering in the Tropics and Subtropics of Hohenheim University a plant oil pressure stove has been developed which enables continuous operation and repeated ignition with divers pure plant oils. Next to the plant oils, the prototype can also be fueled with plant oil esters, kerosene, diesel fuel as well as gasoline. The prototype is realized as a one-flame cooker and can be produced using simple means and materials which are available in developing countries.
Since handling of the plant oil cooking stove is similar to the known kerosene pressure stoves, it will be easily introduced even in rural areas of developing countries. Regarding power output and efficiency as well as emissions, the plant oil stove is comparable to kerosene stoves. Utilization of plant oils as fuel, however, prevents users from severe operating risks related to the easy inflammation of kerosene.
Research is based on investigation of chemical and physical properties of plant oils at high temperatures which led to a completely new design of the cooking stove. The cooker frame consists of 3 steel tubes with an diameter of 60 mm which are connected among each other. The frame serves as pot support as well as tank. It can get filled through an opening at the upper side of one tube. After filling the tank, the hand-pump is screwed into this opening and a pressure of around 1.2 bar is induced into the tank. Due to this pressure, the fuel flows into the oil line. This oil line has a diameter of 12 mm taking into account the high viscosity of plant oils. The fuel flux is regulated with a valve included within the oil line.
The vaporizer is a stainless steel tube with an inner diameter of 6 mm and is connected to the oil line. Within this tube vaporization of the fuel takes place due to the heat of the cooker flame. Since plant oils have high flash points, retention time of the fuel within the flame is increased considerably in comparison to kerosene stoves. However, during vaporization process, cracking of the plant oil molecules takes place. Hence, recombination products are deposited at the inner wall of the vaporizer and have to be cleaned mechanically. Therefore, the vaporizer can be opened and cleaned with an external brush while the stove is turned off.
The vaporizer is connected to the side of the gas expansion chamber. At the upper side of this chamber the nozzle is screwed in. Therefore, the trajectory of the hot gas flux is changed abruptly within the chamber. Consequently, impurities within the gas flux like small coke particles will be deposited at the bottom part of the gas expansion chamber. The chamber serves moreover as heat accumulator as well as gas buffer in case of uneven evaporation. Within the gas expansion chamber, the nozzle cleaning device is located enabling a cleaning of the nozzle during burning of the stove.
The nozzle has a diameter of 0.4 mm. After leaving the nozzle the stream of vaporized plant oil mixes with ambient air. The fuel-air-mixture is gathered in the gas collection tube of the burner head. The gas collection tube has an inner diameter of 12 mm. Within the burner head complete mixing of the fuel gas with the air as well as spatial distribution takes place. While leaving the openings of the burner head, the fuel-air mixture incinerates and burns.
To ensure a proper flame distribution without lifting off, a flame baffle plate is used which also conducts the flame towards the bottom of the cooking pot.
For incineration of the cooker a new start-up device was designed enabling a low emission incineration with pure plant oils. It is fixed to the stove support right beneath the gas expansion chamber. The start-up device consists of a flame holder as well as a bottom metal sheet with 24 air holes and a ring groove serving as wick holder. The wick is made out of fiber glass mat, since cotton or other organic material would burn itself while being ignited with plant oil.
For starting the cooker plant oil is poured over the wick and incinerated with a match or a lighter. Afterwards, the flame holder is placed above the wick. The plant oil vaporizes on the wick and the fuel gas burns in bluish flame.
The heat of this flame is sufficient for starting the vaporization process within the vaporizer. After the stove has been started, the flames of the start-up device are extinguished.
The prototype is shown in top view in figure 2 and in side view in figure 3 [14].
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| Figure 2: Side view of the plant oil stove prototype |
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| Figure 3: Top view of the plant oil stove prototype |
Current research is carried on optimization the vaporization process in order to enhance the homogeneity of vaporization as well as to lower the occurrence of deposits. Moreover the combustion process of vaporized plant oil is investigated closely. Later on, the prototype will be tested within a field test in a developing country.
The authors acknowledge their indebtedness to the German Federal Foundation for the Environment, Osnabrück (Germany) and The Rockefeller Foundation, New York (USA) for their financial support of this project.
e-mail to the author: stumpf@ats.uni-hohenheim.de
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[6] Heller, J.: Physic Nut - Jatropha curcas L. International Plant Genetic Resources Institute, Rome, Italy, 1996.
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[8] Andrews, G.E. and M.C. Mkapi: Vegetable oil as alternative household fuel to imported kerosene in Africa. Leeds African Studies Bulletin (1996). No. 61, p. 48/52.
[9] Anonym: Oil extraction. UNIFEM, The United Nations Development Fund for Women, New York, N.Y., USA 1987.
[10] Anonym: Test results on kerosene and other stoves for developing countries. World Bank, Washington, DC, USA, 1985.
[11] Lide, D.R. and H.P.R. Frederikse : CRC Handbook of chemistry and physics. CRC Press, Boca Raton, USA, 1995.
[12] Kollar, M. et al.: Intigriertes Herdverbreitungsprogamm: Weiterentwicklung von zwei Petroleumdruckkochern – Verwendungsmöglichkeiten von Pflanzenölen als Brennstoff für Kochherde. Technical University Berlin, Berlin, Germany, 1993.
[13] Kollar, M.: Abgasemissionen und Betriebsverhalten beim Einsatz von Pflanzenölen im Wirbelkammer-Dieselmotor und Kochherd – Ein Beitrag zur Lösung von Energieproblemen in den Tropen und Subtropen. Fortschritt-Berichte Series 15, No. 217. VDI-Verlag, Düsseldorf, Germany, 1999.
[14] Lutz, K.: Personal information. Institute for Agricultural Engineering, Hohenheim University, Stuttgart, Germany, 2000.
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