Energy Exploitation of Agricultural Residues in Crete

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1 ENERGY EXPLORATION & EXPLOITATION Volume 20 Number Energy Exploitation of Agricultural Residues in Crete D. Vamvuka 1 and T.D. Tsoutsos 2 1 Department of Mineral Sources, Technical University of Crete, Chania, tel: (30 821) 37403, fax: (30 821) 64802, vamvuka@mres.tuc.gr 2 Centre for Renewable Energy Sources (CRES), 19th km Marathon Ave, GR Pikermi, tel: (30 1) , fax: (30 1) , ttsout@cres.gr ABSTRACT The island of Crete is a typical Mediterranean area with a high biomass potential, the major part of which is still unexploited or irrationally exploited, but at the same time has a problematic energy supply during the high touristic season. In this paper the energy content of the biomass potential is estimated, as a parameter to alleviate the energy system of the island. The exploitation of biomass is studied with reference to the following aspects: The major residue production (olive kernel, husks - citrus fruits, grapes), branches (olive tree,citrus tree, grape tree); The qualitative analysis (proximate, ultimate, calorific value, ash analysis) of samples of basic agricultural residues of the Cretan production (vineshoots, olive tree wood and citrus, olive kernel). Keywords: bioenergy sources, byproducts, qualitative analysis, renewable energy, biomass 1. INTRODUCTION During the last two decades, in the island of Crete there is a significant increase in the energy demand due to the change of the consumption patterns, as well as due to its prosperous tourism industry. Nowadays, the major part of this demand is covered by conventional energy sources (diesel, gasoline, and liquid gas). The current conventional power stations of the Public Power Corporation (PPC) in Crete have a capacity of 514,4 MW (June 2000). Additionally, the installed capacity of Renewable Energy (RE) Sources is about 59.7 MW (0.359 MW biomass-to-electricity, June 2000) (various, 1999a; various, 1999b; various, l999c). The major part of this amount (more than 95%) is wind turbine systems. Crete has to face the typical problems of an island: great energy dependence on fuel imports (86%);

2 114 Energy Exploitation of Agricultural Residues in Crete high cost of energy production; big and immediate problem in the power supply due to the limited backup; Iow base load due to the absence of big industrial units; high trends in the increase of the energy demand (8.5% annually); environmental constraints and seasonality of the demand; great RE potential that is inadequately exploited. Biomass-to-energy systems could play important - and potentially synergistic - roles in addressing these challenges. Of the potentially CO 2 -neutral and less polluting energy alternatives that are being seriously considered for large scale implementation, biomass can both (i) readily supply base-load electrical power, and (ii) be converted to liquid transportation fuels - and so it will likely be a key part of the solution. Several plans have been formulated to exploit this renewable potential, suggesting that biomass residues can substitute a large part of conventional fuels. Most of these technologies are mature and can be embodied in the Cretan energy system, contribute in the local/regional development and create new jobs (Tsoutsos et al, 1991; Vamvouka et al, 2000b; Vamvouka, 2001). The Greek legislative framework, as well as the current incentives (law 2244/94 for electricity production, law 2601/98 for the support of private investments including energy, and especially the Operational Programme for Energy ), were proved essential tools to support further penetration of RE technologies in the Cretan energy system. The Greek electricity market is liberated since 19th February 2001 according to the EP Directive 96/92/EC. This study aimed at participating in the better knowledge of the bioenergy sources of the island, assisting (as a next step) in its further exploitation in the medium- and long- term. Figure 1. Production of Olive Products and Byproducts in Crete 2. THE RAW MATERIAL AS ENERGY RESOURCE The exploitation of biomass was studied with reference to the major residue

3 ENERGY EXPLORATION & EXPLOITATION Volume 20 Number production (olive kernel, husks - citrus fruits, grapes), branches (olive tree, citrus tree, grape tree) and forestry biomass (Sdrolias, 1999). Additionally to this amount, about 0.9 million t/y of forest biomass is produced in Crete. These having significant energy content, are consumed by the local industries, or for domestic heating Olive byproducts In Crete, where the cultivation of the olive tree goes back to Minoan times, the average annual production of olive byproducts is about 2.3 million t/y (considerable increasing 8% per year). The byproducts and liquid wastes from the olive processing are depicted in Fig. 1. The most well known byproduct through the years is the kernel, having a thermal value of 3,500-4,000 kcal/kg. Taking into consideration its specific weight ( kg/m 3 ), it is obvious that this is appropriate for the storage of great quantities, to the contrary of the other fuels (wood wastes, etc). Olive kernel in Crete is currently used as a fuel for thermal boilers, furnaces, limekilns, etc. Its annual production is equivalent to ktoe), which amounts to about 1/3 of the total electricity consumed in Crete. Among all byproducts, the kernel-oil refineries have commercialised only olive kernel. The leaves are either wasted in the dump, or used as fertilisers, while the juices are wasted in the aquatic environment, with no significant treatment Citrus byproducts Figure 2. Citrus Products and byproducts in Crete On the other hand, overproduction of citrus products in Crete has made obvious the need for a lower production. Thus, citrus byproducts have decreased accordingly (Fig. 2). The orange husks are used for feeding animals, while the branches are used for heating.

4 116 Energy Exploitation of Agricultural Residues in Crete 2.3. Vine byproducts Figure 3. Vine Products and Byproducts in Crete In addition, the production of vine byproducts has been decreasing since 1993, as Fig. 3 shows. This was due to a disease of the vines, to the increased demand in working potential, as well as due to European agricultural policy. The shoots, which are the main byproduct, are used for heating, while the curls, the hulls and the seeds are used as fertilisers. Nevertheless, 71.7 thousand t/y of citrus byproducts and thousand t/y of vine byproducts are still available. Table 1. Regression analysis of the agricultural woody energy potential in Crete 2.4. Forecasts Table I shows the results of the year 2003 forecasts based on the business-as-usual scenario of the agricultural residues production. The simple relation between biomass and time is given by the single regression equation: Y=at+b (1)

5 ENERGY EXPLORATION & EXPLOITATION Volume 20 Number where: Y: the produced biomass quantities and t: the corresponding year 3. LABORATORY ANALYSIS OF THE CRETAN BIOMASS RESIDUES The major biomass byproducts of Crete were characterised in terms of proximate analysis, ultimate analysis and calorific value, according to ASTM D , ASTM D and ISO 1928 standards, respectively. The proximate analysis consisted in the determination of the moisture, ash and combustible matter contents of each sample, after heating in programmable furnaces under specified conditions. Table 2. Proximate and ultimate analysis of biomass residues (on a dry basis) The ultimate analysis consisted in the determination of the chemical elements C, H, O, N and S, by using LECO analysers of type CHN-600 and S-HF10. The gross calorific value was measured using a bomb calorimeter, type LECO AC-300, while the net calorific value, as received, was calculated by the following expression: Qn=Qg-{[100-(A+W)]*0,45+W}*5,85 (2) where: Qn, Qg: net and gross calorific value, respectively (kcal/kg) A: ash content (%) W: moisture content (%)

6 118 Energy Exploitation of Agricultural Residues in Crete Chemical analysis of ashes included the identification of inorganic oxides via X- ray fluorescence spectrometry (XRF), as well as the identification of trace elements via inductively coupled plasma (ICP) spectroscopy. As can be observed from Table 2, the ash content of these woody materials is low ( % on a dry basis). The content of combustibles in all biomass samples tested is more than 95% on a dry basis, indicating the good quality of these fuels. Carbon content ranges from 47 to 50% and sulfur content is very low ( %), indicating that during combustion SO 2 emissions will be negligible. The net calorific value of these fuels, varying from about to kcavkg, is significantly higher than that of Greek lignites to kcal/kg- (Kavouridis, 1999). Table 3. Chemical analysi of ashes The chemical analysis of ashes formed at 600 C is included in Table 3. As can be seen, up to about 50% of the ash of these biomass samples consists of CaO and K2O. As compared to Greek lignites, the ash is enriched in CaO, K 2 O and P 2 O 5 and is poorer in SiO 2, Al 2 O 3 and TiO 2. The role of these elements during combustion is well known. Calcium, generally, increases the fluid temperature of the ash and decreases the solubility of toxic trace elements in this. On the other hand, Si, Al and K tend to form

7 ENERGY EXPLORATION & EXPLOITATION Volume 20 Number lower melting point compounds during combustion, increasing slagging and fouling phenomena in boilers (Kakaras et al, 1999; Zugarlicke et al, 1994; Obernberger et al, 1999). Also, K, being volatile, forms chlorides, hydroxides and sulphates, which play an important role in the corrosion mechanisms relevant for the boiler section (Baxter, 1998; Obernberger, 1998; Miles, 1996). As far as trace elements are concerned, Fig. 4 indicates that olive wood and citrus wood are enriched in Ni, Co, Mo and Pb. Kernel wood has the highest concentration of Cu, while vine shoots have the highest concentration of Sr. A positive correlation between Ni and Fe 2 O 3 content, from Table 3, is observed in all biomass samples, with the exception of kernel wood. However, all samples show that as the Cu concentration is increased, the Zn concentration in these is decreased. Some other elements, which are not included in Fig. 5, were detected in very small amounts, such as Cd<0.2ppm and U<0.8ppm. From the above heavy metals, Pb, Sr and Zn are ecologically more important. In any case, the presence of high Ca contents in the ash renders the solubility of these elements very small, by increasing the ph of the ash (Vamvuka, 2000a). As a result, the environmental impact on land through the combustion of these fuels will be negligible. Table 4. Woody energy potential in Crete (10 6 Mcal) 4. DISCUSSION AND RECOMMENDATIONS Bioenergy s sustainable use does not emit CO 2. The increased use of this energy source could therefore contribute to achieving the objectives of the Framework

8 120 Energy Exploitation of Agricultural Residues in Crete Convention on Climate Change to stabilise atmospheric concentrations of greenhouse gazes below dangerous levels. Figure 4. Distribution of trace elements in the ashes Direct heating is the most widespread application, but electricity production and biofuels are currently gaining considerable interest among energy market actors. Table 4 contains the estimated energy content of the agricultural residues produced in Crete. Accurate statistics about the supply and the demand of the cellulosic agricultural byproducts are hard to come by, partly because substantial amounts are exploited informally, but also because their use varies greatly from region to region. The use of biomass-to-energy systems has to face high challenges in Crete. For the large scale of these applications the following parameters are essential: The safety as concerns the availability of the source; the existence of efficient subsidies by development laws; the attractive energy tariffs by PPC; in the case of the CHP systems the availability of the user of the produced heat; the financial viability of these investments (a detailed technical and economic analysis is required); the solution of various specific technology problems to achieve high efficiency of the system. Furthermore maximum diffusion should thus be given to the energy problems so that tax incentives, tax penalisation on CO 2 and financial support to practical initiatives in direction of bioenergy will be more generally and favourable accepted. 5. LITERATURE Baxter, L.L., The behaviour of inorganic material in biomass-fired power boilers: field and laboratory experiences, Fuel Processing Technology, 54(1-3), 47.

9 ENERGY EXPLORATION & EXPLOITATION Volume 20 Number Kakaras, E., P. Vourliotis, P. Grammelis and D. Vamvuka, Fossil fuels combined combustion with biomass in a fluidized bed combustor, Mediterranean Combustion Symposium-99, Antalya, Turkey, June 1999, p.941. Kavouridis C., Supportive fuels and homogenization methods to improve lignite quality produced from the Ptolemais-Amyndeon multiseam deposits for power generation purposes. Workshop of Thermie Programme, Thessaloniki, April Miles, T. R., Alkali deposits found in biomass power plants, Research report NREL/TP SAND Vols I and II, National Renewable Energy Laboratory, Golden, Co. Obernberger I., F. Biedermann, W. Widmann and R. Riedl, Concentrations of inorganic elements in biomass fuels and recovery in the different ash fractions, Biomass and Bioenergy, 12(3), 211. Obernberger, I., Decentralized biomass combustion: state of the art and future development, Biomass and Bioenergy, 14(1), 33. Sdrolias, T., Study for the energy exploitation of the Cretan biomass, Diploma Thesis, Technical University of Crete, Chania, pp 68-70, 89, 117, 129. Tsoutsos T., O.S. Umealu and E.G. Koukios, Potential of Biomass in Greece. Present situation and proposed management, in Biomass for energy and industry, eds G. Grassi, G. Gosse and G.dos Santos, Elsevier Applied Science, London - NY, Vol. 1, pp Vamvuka, D., E. Mistakidou, S. Drakonaki, A. Foscolos and E. Kakaras, 2000a. On the quality of ash of a beneficiated lignite, Proceedings of the Open Meeting on Combustion, Ischia. Vamvouka D., T. Sdrolias, M. Christou and T. Tsoutsos, 2000b. CHP systems in Crete using biomass residues, proceedings of the 1 World Conference in biomass, Sevilla, 5-10/6/2000. Vamvuka D., Clean Use of Coals - Low Rank Coal Technologies, ION Publishers various, 1999a. RES in Crete. Guidelines for the investors and the local authorities (in Greek), National Technical University of Athens, Centre for Renewable Energy Sources, Regional Energy Centre of Crete, supported by the ALTENER Programme, pp various, 1999b. RES in Crete. An implementation plan (short presentation), National Technical University of Athens, ALTENER project, p 8. various, 1999c. Integrated resources planning for the island of Crete, SAVE project XVII/ /Z , Centre for Renewable Energy Sources. Zugarlicke, C. J. et al 1994, Proceedings on Coal Blending and Switching to Low S Western Coals, ASME, p.281.