Biofuels and Industrial Products from Jatropha
curcas
Edited by:
G. M. Gübitz
Graz University of Technology, Austria
M. Mittelbach
Karl-Franzens University Graz, Austria
M. Trabi
Graz University of Technology, Austria
Developed from the Symposium
"Jatropha 97"
Managua,
Nicaragua
February 23 – 27, 1997,
sponsored by the Austrian Ministry of
Foreign Affairs
and by Sucher & Holzer Graz
Dbv-Verlag für die Technische Universität Graz
Uhlandgasse 8, A-8010
Graz, Austria
ISBN 3-7041-0242-3
Contents:
1. Cultivation
1.1 Growth and reproduction of Jatropha
curcas
C.L. Aker
Departamento de Biología, Facultad de Ciencias, Universidad
Nacional Autónoma de Nicaragua, León, Nicaragua
Abstract
An exploratory study to detect patterns of variation in
flower, fruit and seed production in one-year-old plants of Jatropha
curcas (Euphorbiaceae) in response to variation in soil moisture and
fertility was conducted during a 12-month period in Nicaragua. The plant's
architecture conforms to Leeuwenberg's model. Flowering tends to be episodic and
responds to variation in rainfall. Nutrient deficiency caused growth and
reproduction to terminate in smaller plants well before the end of the wet
season. Both inflorescence size and the proportion of female flowers vary with
the vigour of modules. Fruit development is often uneven within an
infructescence, the growth of later initiated fruits being delayed until after
maturation of earlier fruits.
1.2 Cultivation of Jatropha curcas as future source of
hydrocarbon and other industrial products
G.D. Sharma, S.N. Gupta, M. Khabiruddin
1.3 Biotechnological approaches to the improvement
of Jatropha curcas
A. da Câmara Machado, N.S. Frick, R. Kremen, H. Katinger, M. Laimer da
Câmara Machado
Institute of Applied Microbiology, University of Agricultural
Sciences, Vienna, Austria
Abstract
Tissue culture protocols for the rapid propagation of selected genotypes
of Jatropha curcas and for further genetic improvement are highly
desirable. These allow the rapid supply of planting material for new plantations
from genotypes selected for their properties, be it yield or resistance aspects.
Initiation of aseptic cultures from seeds after varying storage periods (1 - 3
years) and the multiplication phase have been optimised for a range of different
genotypes from various geographical origins (Nicaragua, Mexico, Cabo Verde,
Santa Lucia (in Nicaragua), Madagascar). Besides the composition of the culture
media one of the key factors was the cutting technique during propagation.
Trials to optimise rooting and acclimatisation are currently underway.
In
parallel experiments to induce somatic embryogenesis from callus and
adventitious shoots from different explants, e.g. leaves, petioles and stems
were carried out. This represents the necessary basis for genetic improvement by
transformation or mutagenesis. Adventitious shoots from leaf discs of the
genotype Santa Lucia were obtained on media supplemented with BA and IBA.
1.4 Planting Jatropha curcas using bare
roots
M.C. de la Siles1, A. Montoya1, W.
Vázquez1, N. Foidl2
1 Departamento de Biomasa
Universidad Nacional de Ingenieria, Managua, Nicaragua
2 Sucher
& Holzer Austria
Abstract
Conventional agriculture uses nurseries in the first phase of
establishing permanent cultures. This is based on many years of experience with
this technique. However, it is the time now to look for alternative methods to
establish nurseries which have lower costs, avoid complicated operations, and
yield plants of better quality. Using bare roots is one such alternative which
is especially appropriate for the cultivation of J. curcas.
1.5 Arthropod fauna associated with Jatropha
curcas in Nicaragua: a synopsis of species, their biology and pest
status
C. Grimm1, J.-M. Maes2
1 Institute of
Forest Entomology, Forest Pathology and Forest Protection, Universität für
Bodenkultur, Vienna, Austria
2 Entomological Museum S.E.A., León,
Nicaragua
Abstract
Pests and beneficial arthropods in physic nut (Jatropha
curcas L., Euphorbiaceae) plantations in Nicaragua are presented. The key
pest is Pachycoris klugii Burmeister (Heteroptera: Scutelleridae), which
damages the developing fruit. Second most frequent true bug is Leptoglossus
zonatus (Dallas) (Het.: Coreidae). Twelve further species of true bugs also
feed on physic nut. Other pests include the stem borer Lagocheirus
undatus (Voet) (Coleoptera: Cerambycidae), grasshoppers, leaf eating beetles
and caterpillars as well as leaf hoppers. Among the beneficial insects
pollinators, predators and parasitoids are found. Their potential to limit
outbreaks of heteropterous pests is discussed.
1.6 Potential of entomopathogenous fungi for the biological
control of true bugs in Jatropha curcas plantations in
Nicaragua
C. Grimm1, F. Guharay2
1 Institute
of Forest Entomology, Forest Pathology and Forest Protection, Universität für
Bodenkultur, Vienna, Austria
2 Proyecto CATIE/INTA-MIP (NORAD),
Managua, Nicaragua
Abstract
Key pests of physic nut Jatropha curcas L.
(Euphorbiaceae) in Nicaragua are Pachycoris klugii Burmeister
(Heteroptera: Scutelleridae) and Leptoglossus zonatus (Dallas)
(Heteroptera: Coreidae), which cause fruit abortion and malformation of seeds.
The potential for biological control by the entomopathogenous fungi Beauveria
bassiana (Bals.) Vuill. and Metarhizium anisopliae (Metsch.) Sorok.
(Deuteromycotina: Hyphomycetes) were assessed in the laboratory obtaining up to
99 % mortality of L. zonatus and 64 % of P. klugii. Both species
of fungus are mass produced in Nicaragua using a two step production system on
rice sterilised in polypropylene bags. Formulations in oil and water were
successfully tested in the field using knapsack sprayers and ultra low volume
applicators.
1.7 Chemical composition of three different varieties of
Jatropha curcas from Mexico
J.A. Rivera-Lorca, J.C. Ku-Vera
1.8 Jatropha curcas: Distribution and uses
in the Yucatán peninsula of Mexico
B. Schmook, L. Serralta-Peraza
El Colegio de la Frontera Sur,
Chetumal, Mexico
Abstract
In many villages of the Yucatan Peninsula Jatropha
curcas trees can be found in the gardens or fields. But there are
differences in the way and intensity people use the seeds or other products from
this plant. The Mayan people may have one tree in their garden, using it
principally for medical purposes and in some cases, when there is a lack of
other seeds (Curcubita sp.), for their traditional dishes. It could be stated
that in comparison to former times there is less use of J. curcas by the
Mayans. The other group of people using the different products of this plant are
the settlers from the centre of Mexico, mainly from the state of Veracruz, who
came to the Peninsula during the 1970’s. For this reason we can find a
concentration of J. curcas in the villages where these settlers live. The
trees are used as living fences, the latex for medical purposes and the seeds
are used for human and in some cases for animal consumption.
2 Biologically active substances
2.1 The anti-inflammatory effect of Jatropha
curcas leaves
R. Staubmann1,2, M. Schubert-Zsilavecz3, A.
Hiermann1, T. Kartnig1
1 Institute of Pharmacognosy,
University of Graz, Austria
2 Institute of Biotechnology, Graz
University of Technology, Austria
3 Institute of Pharmaceutic
Chemistry, University of Graz, Austria
Abstract
Different parts of the plant Jatropha curcas are used
in ethnomedicine of many countries such as India and Mexico and various
indications on the anti-inflammatory effect of extracts of the leaves have been
described in literature. An anti-inflammatory substance was isolated from
Jatropha curcas leaves using the following procedure. The fresh leaves
were minced, degreased with petrol ether and soxhlet extracted with ethyl
acetate. The ethyl acetate extract was separated by column chromatography using
Sephadex LH 20, Silica gel and Fractogel TSK HW40.
All separation steps were
monitored by TLC and the different fractions were tested on their
anti-inflammatory effect by a Carrageenin induced rat-paw-edema-test. The active
fraction obtained from TSK column was analysed by HPLC on a RP-18 column showing
one main substance. After purification by HPLC in a preparative scale, the
isolated substance was analysed by 1H-NMR, 13C-NMR and MS. Further
investigations on the elucidation of the structure are in progress.
2.2 Lectin activity in toxic and non-toxic varieties
of Jatropha curcas using a latex agglutination test
E.M. Aregheore1, H.P.S. Makkar1, K.
Becker2
1 Department of Agricultural Sciences, College of
Education, Warri, Delta State, Nigeria.
2 Institute for Animal
Production in the Tropics and Subtropics, University of Hohenheim, Germany
Abstract
Lectin activity in toxic and non-toxic varieties of
Jatropha curcas seed meal was investigated using a latex agglutination
method. The glycoprotein (ovalbumin) was adsorbed onto the latex beads. The
agglutination test was carried out in the presence of Ca2+,
Mn2+ and Mg2+ ions. Mn2+ ion was found to be
the best. A concentration of 0.286 mM of Mn2+ ion was maintained in
the assay mixture. Lectin activity when expressed as reciprocal of the minimum
quantity (in mg) of Jatropha meal per ml of the assay mixture which produced
agglutination was 2.88 +/- 0.57 and 1.71 +/- 0.00 (mean +/- sd) for the toxic
and the non-toxic varieties respectively, which did not differ significantly (P
> 0.05). Both toxic and non-toxic varieties were subjected to heat
treatments: i) moist heat (MH) treatment (66 % moisture) at 100ºC and 121ºC for
20, 40 and 60 min and 10, 20, 30 and 40 min respectively, and ii) dry heat (DH)
treatment at 130 and 160ºC for 20, 40 and 60 min. The MH treatment at 100ºC and
DH treatments at 130ºC and 160ºC for 60 min did not inactivate lectin in both
varieties. Agglutination of the latex beads was observed at 10 and 20 min of MH
treatment at 121ºC, however agglutination was not observed after 30 min.
These results suggest that in J. curcas meal: i) MH treatment is more
effective than DH treatment in inactivating lectins, ii) lectins can be
completely inactivated using MH treatment at 121ºC for 30 min, and iii) lectins
probably are not the toxic principle in Jatropha meal.
2.3 A study of the hemagglutination properties of three different
varieties of Jatropha curcas from Mexico
J.A. Rivera-Lorca, J.C. Ku-Vera
2.4 Curcain - a protease isolated from the latex of Jatropha
curcas Linn.
S.K. Dutta
2.5 Acute toxicity studies and wound healing response of curcain,
a proteolytic enzyme extract from the latex of Jatropha curcas Linn.
L.K. Natn, S.K. Dutta
3. Biofuels
3.1 Agro-industrial exploitation of Jatropha curcas
N. Foidl, P. Eder
Sucher & Holzer, Austria
Abstract
A variety of products can be produced from the fruits of
J. curcas in an integrated process.
The oil of the J. curcas seeds can
be used after detoxification as edible oil. Or it can be converted into
bio-diesel. The press-cake can be used as fodder (again after detoxification),
and the glycerol phase of the bio-diesel production process can serve as an
insecticide against tick in cattle.
The commercial use of by-products has the
advantage of hardly any waste being produced and of optimised financial
feasibility. A financial evaluation of the integrated process is presented,
which has been carried out using UNIDO’s COMFAR III Expert program. The results
have been obtained making optimistic assumptions about yields, costs and prices,
thus representing a best-case scenario. It turns out that the process is
financially feasible and very profitable.
3.2 Fuel production improves food production: the Jatropha
project in Mali
R. Henning
3.3 Various vegetable oils as fuel for diesel and burners:
Jatropha curcas particularities
G. Vaitilingom, A. Liennard
3.4 Production of ethyl esters as diesel fuel substitute in the
developing countries
M.N. Eisa
3.5 Trans-esterification of Jatropha curcas
oil
E. Zamora1, E. Sobalvarro1, R. Norori1,
G. Foidl2, N. Foidl2
1 Universidad Nacional de
Ìngenieria, Departamento de Biomasa, Managua, Nicaragua
2 Sucher
& Holzer Austria
Abstract
At laboratory level the two-step trans-esterification process
of J. curcas oil was optimised, to obtain via methanolysis (KOH and NaOH
as catalyst) a product with an ester concentration > 99 %.The best results
were obtained with 50 % excess of methanol (of the stoichiometric quantity), 1.3
% (of weight) of KOH / NaOH - relation, 8 : 2, and 75 % of the basic solution
added at the first and 25 % at the second step. Under the same conditions a
two-step process was simulated, working with a mixture of 90 % ester and 10 %
J. curcas oil. The recirculation of the ester permitted the
transformation of a batch process into a continuous one. These experiments were
carried out at laboratory and pilot plant level.
3.6 Production of biogas from Jatropha curcas
fruitshells
O. López1, G. Foidl2, N. Foidl2
1
Universidad Nacional de Ingenieria, Departamento de Biomasa, Managua,
Nicaragua
2 Sucher & Holzer, Austria
Abstract
The anaerobic digestion of J. curcas fruit shells was
investigated at laboratory scale.
The experiment was carried out in an upflow
anaerobic filter with a volume of 23.8 l, the reactor worked at ambient
temperature. The initial organic volumetric loading rate was 0.5 VS/l,d and
steady state 4.0 g VS/l,d, the hydraulic retention time was 3 days. Addition of
NaOH for pH stabilisation was necessary at the beginning, but at steady state it
could be reduced to zero.
At steady state about 2.5 l of biogas l-1d-1 (70 %
methane) were produced, the degradation rate was about 70 - 80 %. The fruit
shells had to be submitted to a pre-treatment to separate the fibres from the
pulp, to avoid the obstruction of the reactor.
3.7 Production of biogas from Jatropha
curcas seeds press cake
R. Staubmann1, G. Foidl2, N. Foidl2,
G.M. Gübitz1, R.M. Lafferty1, V.M. Valencia
Arbizu2, W. Steiner1
1 Institute of
Biotechnology, Graz Technical University, Austria
2 Proyecto
Biomasa, Universidad Nacional de Ingeniería, Managua, Nicaragua
Abstract
Seeds of the tropical plant Jatropha curcas (purge
nut, physic nut) are used for the production of oil. Several methods for oil
extraction have been developed. In all processes, about 50 % of the weight of
the seeds remain as a press cake containing mainly protein and carbohydrates.
Investigations have shown that this residue contains toxic compounds and cannot
be used as animal feed without further processing. Preliminary experiments have
shown that the residue is a good substrate for biogas production. Biogas
formation was studied using a semi-continuous Upflow Anaerobic Sludge Blanket
(UASB) reactor, a contact-process and an anaerobic filter each reactor having a
total volume of 110 litre. A maximum production rate of 3.5 m3 m-3 d-1 was
obtained in the anaerobic filter with a loading rate of 13 kg COD m-3 d-1.
However the UASB reactor and the contact-process were not suitable for using
this substrate. When using an anaerobic filter with J. curcas seed cake
as a substrate 76 % of the COD was degraded and 1 kg degraded COD yielded 355 l
of biogas containing 70 % methane.
3.8 15 years of biodiesel experience in Europe
M. Mittelbach
3.9 Evaluation of European biodiesel-production and use with the
sustainable process index
T. Dielacher, P. Eder, M. Naradoslawsky
3.10 Policy context of biomass fuels and their economics
E. Schaltegger
3.11 Commercial feasibility of biofuels in 1997
B.R.Bonine, C.L. Cantrell, M.E. Bonesteel, N.S. Chong
4 Toxicity
4.1 Phorbol esters of Jatropha curcas - biological
activities and potential applications
M. Wink, C. Koschmieder, M. Sauerwein, F. Sporer
4.2 Rumen digestion of raw, roasted and boiled seeds of
Jatropha curcas from Chiapas, Mexico
J.A. Rivera-Lorca, J.C. Ku-Vera
4.3 Toxicity of Jatropha curcas
seeds
M. Trabi1, G.M. Gübitz1, W. Steiner1, N.
Foidl2
1 Institute of Biotechnology, Graz University of
Technology, Graz, Austria
2 Proyecto Biomasa, Universidad Nacional
de Ingeniería, Managua, Nicaragua
Abstract
The seeds of Jatropha curcas L. contain up to 60 % oil
with a fatty acid pattern similar to that of edible oils. The amino acid
composition, the percentage of essential amino acids and the mineral content of
the press cake can be compared to those of other seeds and press cakes used as
fodder. Due to several different toxic principles including a lectin (curcin),
phorbol esters, saponins, protease inhibitors and phytates neither the seeds nor
the press cake nor the oil of Jatropha curcas can be used for human or
animal nutrition.
Feeding experiments with fish were carried out to determine
the toxicity of different fractions as well as the influence of heat and alkali
treatment on the press cake. The heat treated seeds and seed meal showed less
toxicity than the untreated material, whereas the toxicity of the alcoholic oil
extract did not change after treatment with hot alkali.
4.4 Detoxification of Jatropha curcas press
cake and oil and feeding experiments on fish and mice
H. Gross1 , G. Foidl2, N. Foidl2
1
Universidad Nacional de Ingeniería, Departamento de Biomasa, Managua,
Nicaragua
2 Sucher & Holzer Austria
Abstract
At laboratory scale detoxification treatments were carried
out on J. curcas press cake and oil (to remove curcin and phorbolic
esters), followed by feeding experiments on fish and mice. Feeding fish on
heat-treated only press cake, resulted in 100 % mortality, however after an
additional extraction with ethanol (92 %) they grew without problems. When
feeding mice on a concentrate containing J. curcas meal as protein
source, the best results also were obtained with the ethanol extracted meal, but
the mice were growing slower than the control group fed on soya. Feeding
neutralised J. curcas oil lead to 100 % mortality, whereas the ethanol
extracted one did not provoke any signs of intoxication.
5 Other applications
5.1 Use of enzymes for oil extraction from
Jatropha curcas seeds
E. Winkler1, G.M. Gübitz1, N. Foidl2, R.
Staubmann1, W. Steiner1
1 Institute of Biotechnology,
Graz University of Technology, Austria
2 Proyecto Biomasa, Managua
University of Technology (UNI), Nicaragua
Abstract
Jatropha curcas is a tropical plant widely distributed
in arid areas. The seeds contain about 55 % of oil, which is mainly used for the
production of soap as a fuel and after trans-esterification as biodiesel.
Various methods for recovering of oil from the seeds, including extraction with
organic solvents and water, have been investigated. Compared to hexane
extraction (98 %) the oil extraction using water only yielded 38 % of the total
oil content of the seeds. Using several cell wall degrading enzymes during
aqueous extraction a maximum yield of 86 % was obtained.
The influence of
cellulolytic, hemicellulolytic enzymes as well as proteases was studied.
The
experiments were carried out at different pH-values and temperatures to find out
the optimum for oil recovering using enzymes. Surprisingly the best results (86
%) were obtained using an alkaline protease. Combinations of proteases with
hemicellulases and / or cellulases did not further increase the extraction
yield. The enzyme supported aqueous extraction offers a nontoxic alternative to
common extraction methods using organic solvents with reasonable yields.
5.2 Potential of Jatropha curcas seed meal
as a protein supplement to livestock feed, constraints to its utilisation and
possible strategies to overcome constraints
H.P.S. Makkar, K. Becker
Institute for Animal Production in the
Tropics and Subtropics (480), University of Hohenheim, D-70593 Stuttgart,
Germany
Abstract
Studies in our laboratory have shown that J. curcas
seed meal (with 1 - 2 % residual oil) has 58 - 64 % crude protein (90 % of which
is present in the form of true protein) and levels of essential amino acids
except lysine are higher than for the FAO reference protein. However, the seed
meal from the Cape Verde and Nicaragua varieties has been found to be highly
toxic to fish, rats and chikens while seed meal from a Mexican variety was not.
Although the growth rate of fish fed a diet containing Jatropha seed meal
from the non-toxic variety for 7 days (50 % of fishmeal protein replaced by
Jatropha meal) did not differ significantly from that of the control group,
appearance of mucus in faeces was observed. The crude protein content and amino
acid composition of this non-toxic variety were similar to those of the toxic
Cape Verde or Nicaragua variety. In addition, in experiments with rats the
protein efficiency ratio of the feed containing heat-treated seed meal from the
non-toxic variety was found to be about 86% of that obtained with casein. These
observations suggest that both toxic and non-toxic varieties can be good protein
sources for livestock. However, the seed meal from Jatropha varieties must be
detoxified. The feeding of unheated meal from the non-toxic variety might have
sub-clinical effects which could reduce the performance of animals when fed a
diet containing this seed meal for a longer period of time. Some constraints
identified by us which could restrict the optimum utilisation of seed cake from
both the toxic and non-toxic variety are: very high levels of trypsin inhibitor
activity (21 - 27 mg trypsin inhibited/g DM), lectin (51 - 102 when expressed as
inverse of minimum concentration in mg of Jatropha meal per ml of the assay
which produced hemagglutination) and phytate concentration (9 - 10%). Saponins
were also present at a level of 2.6 -3.4% (as diosgenin equivalent).
Phorbolesters were present in kernels of the toxic variety (2.2 - 2.7 mg/g) but
virtually absent in the Mexican variety (0.11 mg/g). Tannins, cyanogens, amylase
inhibitors and glucosinolates were not detected in any of the varieties. Trypsin
inhibitors and lectins are heat labile and can therefore be destroyed by heat
treatment. The unheated seed cakes from both the toxic and non-toxic variety
were found to have low in vitro rumen degradable nitrogen (IVRDN24h). The
IVRDN24h after heat treatment increased almost 2-fold (38 to 65%). Heat
treatment for the Mexican variety and a combination of heat and chemical (NaOH
and NaOCl) treatments or extraction with 80 - 90 % aqueous ethanol or methanol
for toxic varieties hold promise for detoxification of Jatropha meals.
5.3 Fermentation of Jatropha curcas seeds
and press cake with Rhizopus oryzae
Trabi1, G.M. Gübitz1, W. Steiner1,
N. Foidl2
1 Institute of Biotechnology, Graz University of
Technology, Graz, Austria
2 Proyecto Biomasa, Universidad Nacional
de Ingeniería, Managua, Nicaragua
Abstract
A fungus was isolated from the seeds of Jatropha
curcas L. grown in Nicaragua and identified as Rhizopus oryzae Went
& Prinsen Geerlings.
Seed meal, seed shells and press cake of Jatropha
curcas were used in different concentrations as substrates for fermentations
of Rhizopus oryzae. Regarding the biomass production the fungus grew very
well on seed meal and press cake, whereas the seed shells turned out to be a
poor substrate. Highest production of carbohydrolases was yielded by fermenting
the seed shells in the presence of yeast extract. The fungus produced a wide
spectrum of hydrolytic enzymes suitable to increase the yield of the oil
extraction. Furthermore, degradation of toxic substances by fermenting the seeds
or the press cake with Rhizopus oryzae are thinkable.
The experiments showed
that using the remainings of the oil extraction as substrate for enzyme
production with Rhizopus oryzae could be an alternative to using the
press cake as fodder, especially as long as there is no practical way for its
detoxification.
5.4 Extraction of bioactive substances from Jatropha curcas
L. and bioassays on Zonocerus variegatus, Sesamia calamistis and
Busseola fusca for characterisation of insecticidal properties
L. Mengual
5.5 Pesticidal efficacy of formulated Jatropha curcas oil
on pests of selected field crops
A.D. Solsoloy, T.S. Solsoloy
5.6 Molluscicidal properties of Jatropha curcas against
vector snails of the human parasites Schistosoma mansoni and S.
japonicum
M. Rug, F. Sporer, M. Wink, S.Y. Liu, R. Henning, A. Ruppel
5.7 Development of eri silkworm Samia cynthia
ricini (Boisd.) (Lepidoptera: Saturniidae) on different provenances of
Jatropha curcas leaves
C. Grimm1, A. Somarriba2, N. Foidl3
1
Institute of Forest Entomology, Forest Pathology and Forest Protection,
Universität für Bodenkultur, Vienna, Austria
2 Biomass Project,
UNI-DINOT, Managua, Nicaragua
3 Sucher & Holzer, Graz, Austria
Abstract
Several generations of the eri silkworm Samia cynthia
ricini (Boisd.) (Lepidoptera: Saturniidae) were reared in Nicaragua using
castor (Ricinus communis L., Euphorbiaceae) leaves. Bioassays were
conducted with seven different provenances of physic nut (Jatropha curcas
L., Euphorbiaceae) leaves from Nicaragua, the Cape Verde Islands,
Madagascar, India and Mexico. On five of these no growth of eri silkworms was
observed. The Mexican variety proved a possible food plant of low quality with a
survival rate of only 5%. One Nicaraguan variety from Santa Lucía showed the
highest potential with a survival rate of 21%, but the weight of the shells
obtained was significantly lower than those of silkworm fed with castor leaves.
6 Potential
6.1 Jatropha curcas: potential, limitations and future
research needs
J. Heller
6.2 Politics of energy production in developing countries
H. Danninger, P. Eder
6.3 Pilot project of energetic vectors using agricultural waste
material - ecological and economic justification
S. Andrijasevic, S. Benc, T. Filipan
6.4 Policy of energy production in Bulgaria
A.G. Aladjadjian