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How to cite this paperAn. 4. Enc. Energ. Meio Rural 2002
Energy generation from "neat" vegetable oils
Alain Liennard; Daniel Pioch; Nathalie Chirat; Paul Lozano; Gilles Vaitilingom
Physico-chemistry of Processes and Bioenergy Labratory. CIRAD-AMIS / Agri-Food Systems Programme TA 40/16, 73 avenue Jean-Francois Breton, F34398 Montpellier Ceex 5, France. Email pioch@cirad.fr, fax/phone (33) 4 67 61 55 15/ 58 82
ABSTRACT
The main points about the uses of vegetables oils (VO) as fuels are highlighted, based on examples taken from the CIRAD long standing experience. First the ressource is presented (world production, chemical aspects, processing of crude VO, food / non food uses).
Among the two ways for using VO as diesel fuels, neat VO imply to adapt the hardware at various levels according to physico-chemical properties and bench test with engines, in comparison to mineral diesel fuel (especially for ignition delay and cocking, the most critical parameters). Fundamental research results aiming to understand the specific and intriguing behavior of VO when injected in a diesel engine are presented. These scientific and technical results are currently applied to the real world: adaptation of small engines, cars, tractors, up to electric power plants in factories, as well as heat production.
These solutions share the common advantage of requiring limited processing of VO. In other situations it is wise to adapt the fuel properties to existing hardware (esters, catalytic cracking, blends). Thus climatic, social, political and economic aspects of the topic are also briefly discussed with the aim of using neat VO as fuels under the best suitable conditions -not only from the technical stand point- to take advantage of local conditions and driving forces.
Key words: Oleos vegetais, biocarburantes, motores diesel, bioenergia, desenvolvimento rural.
RESUMO
Os principais aspectos no tocante aos oleos vegetais-carburantes serão debatidos com base nos exemplos tirados da longa experiência do CIRAD neste campo.
Apos a apresentação dos recursos (produção de oleos vegetais (OV), aspectos quimicos, refinação, utilização alimentar e não alimentar) serão debatidos os principais meios para a utilização de tais biocarburantes diesel.
A alimentação de motores diretamente por OV não transformados, implica numa adaptação dos equipamentos, em função das propriedades fisicas e quimicas e uma vez realizados os testes comparando-os com o gasoleo em motor (tempo de inflamabilidade, depositos sobre os injetores). Alguns resultados dos trabalhos de pesquisas de tipo fundamental feitas pelo CIRAD são presentados com a finalidade de entender o comportamento singular dos OV injetados num motor diesel.
Estes dados cientificos são aplicados pelo CIRAD fora do laboratorio, na adaptação de pequenos motores fixes, de carros, tratores e até mesmo de geradores fabricas, assim como para a produção de calor.
Todas essas soluçoes técnicas têm a vantagem de não exigir um tratamento complexo dos OV. Mas em outras situaçoes é preferivel adaptar as propriedades dos biocarburantes aos equipamentos existentes (ésters, quebrador catalitico o misturas ). A integração de parâmetros de ordem climatica, geografica, social, politica e econômica, sera debatida de forma resumida, afim de chegar-se à escolha dos OV in natura nas condiçoes mais adaptadas - não apenas limitada aos aspectos técnicos - com o intuito de aproveitar as oportunidades locais e de criar sinergias.
INTRODUCTION
The topic is very wide and it would be difficult to cover all aspects in details under the scope of this communication. Nevertheless we wish to give a broad picture of the advantages and drawbakcs for the uses of vegetable oils as fuel. First we will focus on scientific and technical problems. To achieve this goal we will rely mainly on the activities of CIRAD and former research institutes from which it inherited a long standing experience of more than 50 years.
CIRAD, a French State owned agency, contributes to rural development in tropics and subtropics through research, field experimentation and as a consultant in agriculture and food sectors (annual or perennial crops, forest, arid or humid areas, preservation of natural resources... ).
THE VEGETABLE OILS
Before entering the topic of biofuels, it is useful to introduce the starting biomass. The world yearly production of vegetable and animal oils, close to 110 millions tons, is not in the range of that of energy sources like mineral oil or coal. In addition the gap between the average price of petroleum and that of vegetable oils is in the range of 50%. This means that the competition can be favourable to VO only under special economic conditions specific to a given area.
As a livestock, about 4/5 of the world production of oils and fats is devoted to food uses, the remaining being for non food uses ie animal feed and oleochemicals. Finally the consumption as energy is still very low but shows a fast increase ; for example France used about 350,000 t of rape methyl esters blended to gasoil during year 2000.
Two VO only, namely soybean and palm oils, account for about 40 percent of the total oil and fat production, half the whole production of VO. It is worthwile to point out that these two oils belong to very different agronomic and economic systems :
- Soybean is a highly mechanised annual crop, produced mainly in America (USA, Brazil) and the cake is a valuable product as an animal feed,
- Oil palm ("Dendê" in Brazil) is perennial, harvested manually throughout the year and produced mainly in south-east Asia (Malaysia, Indonesia); in addition, its kernel oil is also a valuable product for the manufacture of surfactants and body care products.
This is an illustration of the specificity of each economic situation one has to face when looking at VO based fuels. Out of these two crops one may find a handful of VO of some importance and many others, used for local or specific uses.
The extraction of VO can be made, according to the case, either with hexane or with a screw press, the last being more flexible for adaptation to at very small scale (50kg/hr to 4t/hr per unit).
The chemical composition of crude VO is made of about 95% of triglycerides (tri-esters of glycerol and fatty acids) but also contains free fatty acids, phopholipids and many other minor components (partial glycerides, waxes, sterol esters, pigments, oxidation products and water). Some of them are not suitable for marketing as food. Because of this, edible VO are usually processed through many steps to get RBD oil (refined, bleached, deodorized) (Figure 1). But the picture is different for fuel purpose and the extent to which VO should be refined is still under investigation, as discussed later [1 - 7].
As a broad idea, among the many parameters, fatty acid composition of triglycerides and iodine value (IV) give a good picture of the chemical composition of a VO [2].
DIRECT USE OF VEGETABLE OILS AS FUELS
It is well known (since Rudolf Diesel himself) that VO and their methyl or ethyl esters can be used in compression ignited engines. Compared to gasoil (petroleum diesel fuel), their physical and chemical properties given in Table 1 are somewhat different according to the type of oil. The first property to check is of course the melting point (and associated low temperature properties). An oil solid at ambiant temperature would require a side tank for gasoil and a switch valve, in addition to several devices to warm up oil tank and pipes.
Choice of convenient parameters
Actually not all standard fuel specifications are suitable to assess the true efficiency of these biofuels - especially cetane number which is linked to the self-ignition ability of hydrocarbons [3].
Thus we prefer to measure the ignition delay (ID) either in a standard engine or in a constant volume combustion chamber, the last being more suitable for research work because it requires a smaller sample volume [4],[5],[6]. As a matter of fact although the cetane number of rapeseed oil is substantially lower than for gasoil (respectively 35.9 and 46.7 measured by ASTM D613 method), the ID of the oil is even better (shorter) compared to gasoil (respectively 2.62 and 2.78ms, measured in a constant volume chamber). Thus ID gives a better picture of the properties of a given oil for fuel purpose.
Properties of vegetable oils as fuels
Generally, the properties are studied by indirect measurements out of an engine and then more accurately characterised in an engine at the bench scale. To achieve these goals, we are conducting fundamental work with the aim of understanding the specific behaviour of VO when they are injected in the combustion chamber : spray granulometry, evapo-stalagmometry, precombustional chemistry, ignition, combustion and exhaust gases [1],[7]. Since triglycerides cannot enter easily the gas phase because of their high molecular weight (average 57 carbon atoms/molecule compared to less than 20 for gasoil) and very high boiling point, there is no doubt that the ignition is linked to chemical reactions occurring at the gas-liquid interface (precombustional chemistry) [8], [9].
Table 2 [7] shows that a larger spray area and a faster evaporation coefficient (pyrolysis) result in a shorter ID (better fuel quality) for coconut oil. As a general picture, ID is generally linked to the percent of saturated fatty acid (low IV) : the more saturated the fatty acids the shorter the ID (especially for coconut and palm oils). Table 3 gives data for a broad range of fatty acid composition of VO, including the main VO traded in the world. The IV of south-american oils that could be available for fuel use are given in Figure 2. This may help for selecting biofuels or at least to anticipate the problems and try to overcome them by a convenient adaptation of the engine for example.
Cocking, another matter of concern when using neat VO as fuels, is often said to come from a high level of phospholipids or from a high viscosity. We have extended experimental evidence that at least the first hypothesis is not true. Phospholipids have mainly a negative effect during the storage because they slowly form insoluble particles settling at the bottom of tanks. Other minor components have virtually no effect (when checked individually as a sole additive) unless they are altogether as in water degummed oils (figure 1). In that case there is a positive synergistic effect on the fuel quality [7].
Thus despite the common sense, fully refined VO are not the best for energy generation, thus one must note the following : refining increases the cost of the biofuel but might have an adverse effect on the performances of the engine. Actually cocking is to a large extent a function of the percentage of unsaturated fatty acids. Apart of applying hydrogenation -a rather complex process- to decrease the IV, we noted that an increase of the temperature of the walls of the combustion chamber helps to decrease cocking and above a given temperature there is no more difference between gasoil and VO. This has also been noted by other authors [10]
It is always possible to run any diesel engine with neat VO at full load for several tens of hours but the outstanding question deals with long term effects: cocking of injector tip and carbon deposit in combustion chamber. These troubles appear to be strongly linked to engine type : direct-injection engines are much more sensitive to fuel properties than indirect-injection ones. This is why most of the applications of VO as diesel fuel in the world concern the last engine type.
As an example let us consider Jatropha Curcas oil, an attractive non edible oil that could be harvested from this crop grown as hedges in ranches in tropics and subtropics : the torque at 2500rpm for an indirect-injection engine (148.5Nm), the specific fuel consomption (265g/kWh) and the global efficiency (36%) are very close to the data for petroleum diesel fuel (150, 232 and 35 respectively) [11].
However direct-injection engines are by far the most widely used except for cars. Because of that, owing to the CIRAD research aim, we also test the biofuels with direct-injection engines. The understanding -although still partial- we have now of the behaviour of biofuels allows us to make only a few changes to original engines :
-new design of the combustion chamber,
-insulation of piston and liners,
-new injector type.
Indeed our main research work is now situated in the area of long term behaviour of direct-injection engines fueled with VO.
Then, after these adaptations, data for exhaust gases of an direct-injection engine are similar to those for other liquid oils like rapeseed or gasoil (Table 4). This is an interesting feature because exhaust emissions are of course another matter of concern for any kind of engine or fuel: carbon monoxide, unburned hydrocarbons and nitrogen oxides along with other harmful chemicals and solid particles.
Then comes the long term durability test (1000hr) with same oil and engine as above before setting up standard values for oil quality and going to the field test. The modified engine had been checked also for wear and dilution of crankcase lubricant every hundred hours (metals content, viscosity). It turns out that the drainage of the lubricant can be made as for diesel fuel.
This allows the engine manufacturer to keep its warranty for using Jatropha Curcas oil as fuel for indirect-injection engines and this is now proven to be convenient also for modified direct-injection engines, as long as the oil meets our standard. Thus this work led to set up quality standard for the oil which must be slightly purified but not fully refined before storage. The full project may also include sizing and testing of the hardware for oil extraction and processing.
Field applications
These research results led to the following applications.
- Adaptation of small engines
Now especially designed for developing countries, a series of Hatz engines (5-8 kW) can work with vegetable oils on small equipment like pumps for irrigation in Africa. Kubota engines fueled with coconut oil are also used to supply drinkable water in the Pacific.
- Adaptation to cars and tractors
As a result of collaborative works, for example John Deere tested before marketing a panel of tractors ran with VO. CIRAD is currently running field applications, in the case of coconut oil, in Pacific islands (pickups of city fleets and private cars). In addition, tractors having modified direct-injection engines are experienced with sunflower oil in south of France (cooperation with the manufacturer New Holland and farmers associations).
- Electric power generation
Following the last example above, in small pacific islands nearby a coprah drying oven, small presses for coconut oil extraction are operated to feed a small electric generator (100 - 200kVA) to power either the press itself and the village nearby [12].
After appropriate adaptation, big diesel engines (over 1000kVA) fed with cotton oil are used for electric power production in large agro-industrial plants in Africa.
- Heat production
Another programme concerns the use of VO in burners in place of fuel oil [13]. The first step was to check the ability of rapeseed and J. Curcas oils as biofuels in standard commercial burners. The second step was to define the modification of standard burners required for an efficient combustion with both domestic fuel oil and VO (dual fuel); one of the modified burners (260kWh) was tested during 200hours for corn drying. Other uses include drying of sugar beet cake for example.
BEST CONDITIONS FOR USING NEAT VO AS FUELS
All the examples listed above share the common advantage of requiring a limited processing of VO and to be suitable for small scale manufacture. They favour the use of the VO by the farmer himself.
Depending upon the case, all situations are not suitable for using neat VO as fuels. Under given circumstances it is wise to adapt the fuel properties to existing hardware. For example when it would be necessary to adapt too many types of engines. In that case it is possible to apply a chemical reaction or to change the composition by blending VO with other components. In this last case the composition of the final fuel may also include ethanol (including 4 % of water) and gasoil, no additive is required in this case except under cold climate. When the components are not miscible as a simple binary mixture (case of alcohols and VO), it is still possible to bring all this together as a microemulsion. This reduces the viscosity compared to VO, but the main drawback lies in the requirement of at least four components including one surfactant. Catalytic cracking may also be applied for production of both gasoline and diesel fuel hydrocarbons having properties very close to those of mineral fuels [14], [15].
Finally, in addition to the above solutions, there are now several research projects focusing on the synthesis of "green" additives from VO for mineral fuels and biofuels [16],[17]
In Europe the price of VO ester is high compared to that of gasoil and governments must give subsides to the farmers for rapeseed production on set-aside land. Actually this is a political solution to the problems coming from the international ruling of food crops.
The main alternatives for using VO as fuels, their technical advantages and drawbacks are summarised in Table 5 and a general scheme shown in Figure 3 shows the main steps in an attempt to choose the best solution. First the need for an alternative energy, the lack of energy or the risk of shortage must be clearly defined. This could be based on environmental problems or any inconvenience in fuel supply or storage, by-products to be valorised, seasonal VO overproduction, unsecurity, remote location..
Then one has to evaluate the type and amount of energy required (fuel for various vehicles, for small or large-scale electric power production, heating). This helps to select the best starting biomass among which are VO.
Then the level of technology available in the area or that could be implemented for energy generation must be defined (oil extraction, laboratory for quality control, process engineering, local chemists, mechanical technicians and engineers, chemicals currently used in nearby plants).
As a first step this work shows several opportunities, including VO, that could be applied from the sole technical side. Then as a second step comes the financial point which requires the estimation of the production cost under various conditions.
Before reaching a conclusion, experts must check the compatibility of each technical solution with local conditions:
- Political: taxes or tax cutting, subsides for environment preservation/pollution and bioenergy development, political stability and safety
- Social and economic: national and local energy market, policy of petroleum companies, retail price of petroleum fuels/actual price without any subsides for long carriage, unemployment and manpower available for harvest, cultural aspects as well
- Climatic: seasonal problems of supply, period of harvest, floods and high sensitivity to climatic changes.
Finally the combination of all these informations helps selecting of the most suitable way that can be neat vegetable oils, believed to be workable from technical, political and economic points of view.
PROSPECT
Environment preservation is a matter of concern and a major political issue. As a result governments are putting more and more pressure and taxes on all kinds of pollution makers (Clean Air Act in US, TGAP-Ecotax in France). All this helps the shift to cleaner energy sources, among which VO have to play a role not only because of their attractive physical and chemical properties or because they grow under various climates but also to shift to closed carbon cycle (30% increase of CO2 in air during the last 2 centuries, 145% increase of CH4).
Although the demand of VO for food is expected to grow fast during the coming 20 years because of demography, intensive farming will still supply more oils. Today governments of industrialized countries are obliged to rule in order to avoid overproduction and market collapse. Also the demand for cakes (soybean, sunflower, coconut) as cattle feed is growing faster than the VO demand and VO may shift from main product to a side-product status, and will possibly face market problems.
In addition many oil bearing fruits or seeds (how much?) are not harvested, especially in the amazonian rainforest.
Thus VO would be still available for non-food uses in the coming years.
From a technical point of view, there is a wide panel of solutions for energy generation from VO, adapted to various local conditions. Owing to the R&D work carried out under partnership by manufacturers and research centers, VO fuels are no more an idea still in the mind of researchers nor a spare solution workable only in a few places in the world. The driving forces listed above (self sufficiency, environment concern, rural development...) will help to take advantage of local conditions (macro and micro levels, agro-industrial plants, small-holders associations, large estates, remote village, regional politics) to develop energy generation from VO.
AKNOWLEDGEMENTS
Authors thank researchers involved in collaborative works with Cirad and from which some results were obtained :
T.W. Ryan III (Southwest Research Institute, Engines and Vehicles Division, San Antonio,(USA); G. Knothe, Oil Chemical Research,National Center for Agricultural Utilization Research, Agricultural Research Service, USDA,Peoria USA); J.L.Vanhemelryck (Faculté des Sciences Appliquées, Unité Thermodynamique,Louvain-La-Neuve, Belgium).
Special thanks to ADEME (Agence Francaise pour le Développement et la Maîtrise de l'Energie) for funding the CIRAD research projects.
SHORT BIBLIOGRAPHY
[1] Vaitilingom G., "Huiles végétales- Biocombustibles Diesel : Influence de la Nature des Huiles et en Particulier de leur Composition en Acides Gras sur la Qualité Carburant" ; PhD Thesis ; Université d'Orléans, France (1992).
[2] R.O. Dunn, G. Knothe and M.O. Bagby, Recent Advances in the Development of Alternative Diesel Fuel from Vegetable Oils and Animal Fats, Recent Res. Devel. Oil Chem. 1:31-56 (1997).
[3] G. Knothe, M.O. Bagby and T.W. Ryan III, Precombustion of Fatty Acids and Esters of Biodiesel. A Possible Explanation for Differing Cetane Numbers, J. Am. Oil Chem. Soc., 75, 1007-1013 (1998).
[4] T. W. Ryan III and B. Stapper, Diesel Fuel Ignition Quality as Determined in a Constant Volume Combustion Bomb, SAE Technical Paper Series 870586 (1987).
[5] T.W. Ryan III and T.J. Callahan, Engine and Constant Volume Bomb Studies of Diesel Ignition and Combustion, SAE Technical Paper Series 881626 (1988).
[6] N. Chirat, P. Lozano, G. Vaitilingom, A. Liennard and D. Pioch, An Efficient Method for the Preparation of Pure Natural Triglycerides by Flash Chromatography and Subseqent Formulation of Model Fuel Samples, J. Am. Oil Chem. Soc., in press.
[7] N. Chirat, Etude de la Qualité de Carburants Dérivés des Huiles Végétales : Approche Méthodologique, PhD Thesis, Université Montpellier II, Montpellier, France (1996).
[8] G. Knothe, M.O. Bagby, T.W. Ryan III, H.G. Wheeler and T.J. Callahan, Semi-Volatile and Volatile Compounds Formed by Degradation of Triglycerides in a Pressurized Reactor, J. Am. Oil Chem. Soc. 69:341-346 (1992).
[9] W. Ryan III and M.O. Bagby, Identification of Chemical Changes Occurring During the Transient Injection of Selected Vegetable Oils, ASAE Technical Paper Series 930933 (1993).
[10] T.W. Ryan III and J. Erwin, Effects of Fuels Properties and Composition on the Temperature Dependent Autoignition of Diesel Fuel Fractions, ASAE Technical Paper Series 922229 (1992).
[11] G. Vaitilingom, A. Liennard, Various Vegetable Oils as Fuels for Diesel and Burners: Jatropha Curcas Particularities. Ed. Teschnishe Universität Graz, Austria. Proceedings International Symposium, Feb., Managua (1997).
[12] Vaitilingom, A. Liennard, P. Courty , Crude coprah oil, a biofuel for diesel generators. More self-reliance and higher incomes. 1st World Conference and Exhibition on Biomass for Energy and Industry, June, Sevilla, Spain (2000).
[13] G. Vaitilingom, Ch. Perilhon, A. Liennard, M. Gandon., Development of rapeseed oil burners for drying and heating, Industrial Crops and Products, 7, 273-279 (1998).
[14] D. Pioch, P. Lozano, M.C. Rasoanantoandro, J. Graille, P. Geneste, A. Guida, Biofuels from Catalytic Cracking of Tropical Vegetable Oils, Oléagineux, 48, 289-292 (1993).
[15] M.C. Rasoanantoandro, Transformation des Huiles Végétales par Voies Thermique et Catalytique, PhD Thesis, Université Montpellier II, Montpellier, France (1986).
[16] G. J. Suppes, M. Goff, M.L. Burkhart, K. Bockwinkel, M.H. Mason,, J.B. Botts and J. A. Heppert, Safer, Less Volatile and Renewable cetane Improvers from Vegetable Oils, Proceedings of the ASAE Spring F&L Meeting, (June 2000), Paris, in press.
[17] J.V. Van Gerpen, S. Soyolu, M. E. Tat, Evaluation of Lubricity of Soybean Oil-Based Additives in Diesel Fuel, Proceedings of the ASAE Meeting, , Toronto (July 1999), in press.