Harvesting of cotton residue for energy production

Transcription

1 PERGAMON Biomass and Bioenergy 16 (1999) 51±59 Harvesting of cotton residue for energy production T.A. Gemtos a, *, Th. Tsiricoglou b a Laboratory of Farm Mechanisation, University of Thessaly, Pedio Areos, Volos, Greece b TEI of Larissa, Larissa, Greece Abstract The possibility of collecting cotton stalks in Greece and using them for energy production was investigated. The production and properties of cotton stalks were studied and a system for collection of the aerial part is proposed as a feasible solution to avoid wet conditions under the local climate. A successful method for collection and packaging of the residue was applied, using conventional but highly advantageous equipment, o ering reduced investment cost and use of existing machinery. The energy required to harvest cotton stalks was measured by an instrumented tractor. The tractor was able to measure the developed forces between tractor and implement, the power absorbed through the PTO, as well as tractor velocity and fuel consumption. The energy consumed for the operation was calculated and when compared to the energy of the biomass collected gave a positive balance. The work proved the feasibility of harvesting cotton stalks using conventional machinery giving the possibility to collect energy material with a total energy content of 500,000 tons of oil equivalent at national level. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Biomass; Cotton stalks; Residue harvesting; Energy 1. Introduction During recent decades, biomass use for energy production has been more and more proposed as a substitute for fossil fuels. Biomass, as a zero CO 2 emission fuel, can o er an immediate solution in the reduction of CO 2 atmosphere content. Although energy crops can o er a basis for larger energy producing plants, the use of crop residues can o er a more immediate source of biomass for energy production in small installations. The economics of a system which produces * Corresponding author. Tel ; Fax: ; gemtos@uth.gr. energy from crop residue highly depends on the cost of collection, transportation and storage of the raw material. It is probably true that specially constructed machinery could give, in the long run, the best results. However, at the initial stages of the residue use the purchase of new equipment would increase the cost of operation with an unknown acceptance by the end user. Therefore the best solution to promote biomass utilisation is to employ, when possible, existing equipment for the collection of the raw material. Cotton is cultivated in Greece in more than 400,000 hectares [14]. It is harvested by cotton pickers between the end of September and the beginning of December leaving stalks in the eld /99/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S (98)

2 52 T.A. Gemtos, T. Tsiricoglou / Biomass and Bioenergy 16 (1999) 51±59 Climatic data in the cotton growing plains of Greece show that there is considerable rainfall at the end of the harvesting period. Greek farmers usually chop the stalks by cotton shredders and the residue is then incorporated into the soil by ploughing. Only a few farmers leave the stalks uncut and drill winter cereals without any tillage [1]. The latter is mostly applied during very wet years. Cotton stalks are a residue, which is left unused for the time being, but has the potential to be used as biomass for energy production or as raw material for other industries. Ebeling and Jenkings [2] studied the physical and chemical properties of di erent crop residues. For cotton stalks the following values were obtained: higher heating value MJ/kg; volatile 65.40%; ash 17.30%; carbon fraction 17.30%. The stoichiometric analysis of the stalks gave C 39.47%, H 5.07%, O 39.14%, N 1.20%, S 0.02%, residue 15.10%. Sumner et al. [3] have reported, the gross heat of combustion to be 18.1 MJ/kg and 18.4 MJ/kg for dry cotton stalks and roots respectively. Sumner et al. [4] have done a study of the parameters needed for the design of a cotton stalk puller. They quoted from Demian an average pulling force to uproot cotton stalks of 903 N with maximum 1188 N and from Colwick an average uprooting force of 489 N. They reported that the cotton stalk mean diameter was 13 mm (12±14 mm). The average pulling force was found [4] to be 317 N (256±373 N). Sumner et al. [5] measured the moisture content of the stalks (average 42.7% wb, range 23±62.3% ) and of the roots (average 60.8% range 53±64.4%). Dry matter yield was 4.42 t/ha with range of 3± 7.04 t/ha. Roots were 23.2% of the whole plant in average with the measured values ranging between 14.3 and 29.1%. In the same experiment the moisture content was found to drop from 50% to under 20% when the stalks were left in the eld, after uprooting, for three weeks. Sumner et al. [4±6] have suggested a method to harvest cotton stalks by uprooting them. They constructed a machine with two rubber wheels turning in opposite directions. The wheels trapped the stalks and, as they turned, uprooted them. The stalks were then left on the ground surface to dry and were collected after remaining there for two weeks. The soil which remained on the roots when uprooted was dried and mostly removed due to the movement of the balling machinery. This drying period permitted the collection of the residue quite free of soil. However, there was no exact estimation of the amount of the soil collected with the residue. Similar processes were also used by Kemp and Matthews [7] and the NIAE in Sudan and by others [4]. Coates [8] reported results of a research of cotton stalks harvesting systems in Arizona, USA. He used two machines that undercut the plants before they pulled them o the ground, leaving most of the root in the soil. One system chopped the material immediately after pulling while the other left it on the ground before chopping or packaging it. Packaging was carried out by round balers or seed cotton module builders. Coates found the e ective cotton stalks yields to range from 2.34 (chopped by ail mower) to 5.7 t/ha (for hand harvested). Soil contamination ranged from 0.9 to 7.7% while total energy required for harvesting ranged from 47.9±52.1 kwh/ha or 8.6±10.9 kwh/t dry matter. In the literature cited cotton stalks were harvested by uprooting them. In most cases, the local climate allowed su cient time for the uprooted plants and the soil stacked on the roots to dry. As a result of these conditions, the cotton residue could be collected dry enough for storage and without signi cant soil impurities. In Greece, however, it is unusual to experience continuous periods of dry weather as required by the previously described procedure. In anticipation of the wet period of the year, farmers plough their elds for the next crop as soon as cotton is picked, so that their elds are not too wet for ploughing. It is well known that ploughing under wet conditions causes compaction that deteriorates the physical properties of soil and adversely a ects crops [9]. In order to investigate the possibility of using cotton stalks for energy production under Greek conditions, a research programme was undertaken. The objectives of the programme were:

3 T.A. Gemtos, T. Tsiricoglou / Biomass and Bioenergy 16 (1999) 51±59 53 (a) to quantify the yield of cotton stalks and its potential in Greece; (b) to study the physical properties of cotton residue; (c) to identify and propose a procedure for harvesting cotton stalks; (d) to investigate the possibilities for storing the material in a simple and economical way; (e) to study the energy budget and the economics of harvesting cotton stalks; and (f) to investigate possible utilisation methods of energy production by burning. The present paper reports some of the results obtained during the project, which was funded by the Greek Ministry of Education. 2. Material and methods 2.1. Theoretical and preliminary tests The size of the stalks, their distribution, their properties in the eld and the yield were studied after seed cotton picking. Rows of 2±10 m length (cotton is cultivated in rows of 0.95 to 1.00 m apart) were studied in di erent elds in central Greece. The diameters of the stalks at soil level and at a height of 0.05 m from the soil level (the height a mower cuts the stalks) and their height were measured. The plants in a row were then uprooted and without any disturbance of the soil stacked on the roots were placed in bags. The bags were weighed in a laboratory. Then the stalks were removed from the bags and samples were taken from di erent plant parts for moisture content determination. They were placed in an oven to dry at 728C for 48 h. The soil was then removed from the roots by washing it with water and its percentage in the total weight was estimated. The Higher Heating Value was estimated from samples taken from freshly uprooted plants. The measurements were made using an IKA 400 adiabatic bomb calorimeter. The material was dried in an oven and divided into roots, stalks and branches and bolls. Then a mill ground it and 1 g samples were formed by a special small, hand operated press. The material was burnt in the oxygen enriched atmosphere of the bomb of the calorimeter. Additionally, uprooting of the stalks was carried out to study the uprooting forces. Uprooting was performed using a small vice and the hydraulic system of a tractor. Each stalk was clamped in the vice and pulled out of the soil by the upward movement of the three-point linkage of the tractor. A mechanical balance did the force measurement and it was recorded by a video camera. Based on observations of the amount of the soil stacked on the roots during eld work, it was decided to investigate the possibility of collecting only the aerial part of the residue, leaving the roots in the eld. It was anticipated that the collected material would be free of soil and with less moisture content. These factors would make its storage easier and its use for energy production by burning more attractive. Based on the analysis and the measurements of the strength of cotton stalks and the friction properties between cotton stalks and mower knife [10] the feasibility of using existing farm machinery was investigated. The cutting resistance encountered by the mower during hay cutting (grasses or legumes) is estimated from Kepner et al. [11]. They quoted from Elfes an average PTO power for cutting mixed hay of 1.9 kw at a speed of 7.9 km/h. Part of this power, 0.89 kw, was due to cutting resistance of the plants. From measurements of cotton stalks strength, carried out during the present investigation [10], the following equation giving the energy required for cutting the stalks as a function of their diameter was found: WORK = *DCUT with r 2 = 0.69 where: WORK is the energy required to cut the stalk in J, DCUT stalk diameter at a height 0.05 cm from the ground in mm. According to this equation a stalk with base diameter of 10 mm requires J to be cut. Based on the plant base diameter distribution found from the eld measurements and assuming a cotton plant population of 100,000 per hectare or 10 plants per meter on the row, the total work which is needed for cutting 2 m of row is

4 54 T.A. Gemtos, T. Tsiricoglou / Biomass and Bioenergy 16 (1999) 51±59 approximately 750 J. Given that only two rows can be cut down by a 1.70 m working width mower, the total power requirements for a working speed of 2 m/s, are 1500 J/s or 1.5 kw. This means that the mower will encounter resistance of the same order cutting two rows of cotton as cutting grass at a width of 1.70 m and the same speed. The di erence will be in the concentration of the load, which in the case of the cotton stalks harvesting, will be only applied on two knives. It was concluded that a mower with reciprocating knives could be used without the risk of any damage, apart from a minor possibility of damage to the knives due to the concentration of the load. To illustrate this, a reciprocating knife hay mower was used to cut cotton stalks of two rows at a speed of about 2 m/s. The machine worked well, with a slight deterioration of the knives. That means that the mower should be used with stronger knives, possibly with teeth. The stalks cut by the mower were left on the soil surface. A rotating head rake was used to collect more material in one row. The rake worked well and collected four rows in one, in one run. Actually only two rows were moved on top of the other two. From the beginning of the project, the use of a hay baler for small square bales which was widely used in the area, was rejected for packaging the cotton stalks. It appeared rather di cult for stalks with high moisture content to be fed into the compression chamber and to be cut by the ram knife. Additionally the material had high density, which would cause heating and destruction of the material. Some preliminary tests showed the di culties in using it. Two additional possibilities were also investigated. The rst was a special machine which gathered vine prunings and packed them into round bales of 0.50 m in diameter and length. In the rst experimental year, the stalks used for the analysis of their properties were fed into the machine. The machine worked well and formed bales of about 10 kg. The bales were loose enough and the moisture content of the stalks dropped from about 40% to under 20% w.b. in less than 20 days without any warming of the material. Bales left outdoors proved that they absorbed water easily after a rainfall but under Greek climatic conditions they dried again within 10 days. These bales could be handled easily by hand and fed into a bunch shape biomass boiler. In the second year, the machine was used to collect material immediately after cutting. It proved that there were a lot of blockages due to the high moisture content of the stalks which blocked the pick-up and made baling impractical. The second machine used successfully was a Claas Rollant 44 round baler with a xed chamber and metallic rollers. The wrapping material was a plastic net. The machine worked well during the second year just after the cutting of the stalks, producing bales with dry matter of about 190 Kg. The bales were 1.20 m in diameter and in length and could be handled by a front tractor end fork lifter and transportable by a platform Field trials A eld application of the method was carried out in 1994 in order to prove the feasibility of harvesting cotton stalks by conventional machinery, assess their performance and measure the energy required. Harvesting was applied in a 1.4 ha eld using the machinery shown in Table 1. The particular eld is typically representative of the area. During eld work the performance of the machinery used was monitored and their e ciency was determined. An instrumented tractor described by Gemtos and Tsiricoglou [12] and Tsiricoglou and Gemtos [13] was used to measure the forces developed during the work, the power Table 1 Farm machinery used for harvesting cotton stalks Machinery Tractor Mower Raker Round bales baler Fork lifter Platform Speci cation Two wheel drive, 50 kw Reciprocating knives, 1.70 m width 1.70 m width Bales 1.2 m in diameter and height, with plastic net wrapping Mounted on tractor Carrying up to 8 bales

5 T.A. Gemtos, T. Tsiricoglou / Biomass and Bioenergy 16 (1999) 51±59 55 Table 2 Size of cotton plants and their distribution in the eld (in mm) Year Diameter Diameter Diameter Diameter Distance Distance Height of Height at soil at at 0.5m at between between plants of level soil height 0.5m plants plants mm plants mm level mm height mm mm mm mm mm mean st dev mean st dev mean st dev mean st dev 1st NR NR NR NR 2nd requirements and the fuel consumption. The tractor had six loading cells (three measuring horizontal forces, two vertical and one side forces) measuring the forces in the space. Additionally a torque and rotation frequency meter was installed on the PTO shaft of the tractor to measure the power absorbed through PTO. Analogue signals after ampli cation were converted to digital by an A/D converter and recorded by a portable PC. Samples were taken at 1000 s/s for each transducer. A fuel discharge meter and a radar type linear velocity meter were installed. Mean values of fuel consumption and speed were given on a liquid crystal display as a mean for each run. Based on the recorded measurements the energy budget of cotton stalks harvesting was determined. 3. Results and discussion 3.1. Theoretical and preliminary results Table 3 Distribution of the stalk diameter at a height of 0.05 m Diameter range Frequencies % Frequencies % in mm 1st year 2nd year 5± ± ± ± ± ± ± UP The results of cotton stalks size, water content and yield, as well as the soil stacked on the roots during uprooting, are summarised in Tables 2±4, for two years of measurements. It is clear that cotton stalks were quite di erent during the two studied years. It is also clear that large amounts of soil were stacked on the roots when pulled by hand. If collected immediately the movement will remove a part of the soil caused by the harvesting equipment. Even if most of the soil is removed a considerable amount will remain and be collected with the plants, which would eventually cause problems in the energy conversion plant. This e ect is indicated by the reported research, where the stalks remained in the eld for long periods not only for the stalks to dry but also the soil, which then would be easily removed. The results of the higher heating value measurements are shown in Table 5. In Tables 5 and 6 the yield of the stalks and the roots are presented. Total yield averaged 3,144 kg/ha, of which 2,547 kg/ha were the aerial part. Mean water content at the end of the harvesting period Table 4 Amount of Soil stack on the cotton roots when uprooted a/a Soil weight on Soil Percent of the soil the root Moisture on the weight of content the whole plant-soil g % Mean Range 200± ± ±79.1

6 56 T.A. Gemtos, T. Tsiricoglou / Biomass and Bioenergy 16 (1999) 51±59 Table 5 Higher heating value of cotton residue (mean values for measurements of several years) plant part Dry matter Higher heating Moisture yield value content kg/ha MJ/Kg % Bowls Stalk and branches Total aerial part Roots Total aerial part was 41.5% for the aerial part and 54.8% for the root. The reported dry matter yields of cotton stalks varied from 2.3 to 5.7 t/ha with water content mean 34.9% and range 23.3±41.4% [8]. Sumner et al. [6] reported yields ranged from 3 to 7 t/ha (including roots) with water contents ranging from 23.2% to 62.3% for stalks and from 53% to 64.4% for roots in the period between mid November till end of February. The Greek results are similar to the results of Sumner et al. [6], showing that a considerable amount of energy could be produced by cotton stalks, which can give about 1250 kg of diesel equivalent per ha or a total of 500,000 tons of oil equivalent for the country. Total Greek energy consumption in 1991 was 22,214,000 tons of oil equivalent [14]. The results of the uprooting force from two years of measurements are shown in Table 7. According to the results obtained, when cotton residue is harvested by uprooting the plants, a large amount of soil will be collected with it unless a rather long drying period in the eld is available. It appears to present a problem with Greek farmers who cannot delay soil tillage for the next crop until the soil stacked on the roots is dry enough to be easily removed. Although during collection of the stalks some of the soil will be removed at any case, a considerable amount will remain stacked. This amount of soil will eventually cause problems in the work of any burner and of any system for biomass conversion Field trial results During eld trials the experiment nished in one day. Fifteen bales were formed weighing 4923 kg of fresh material (2880 kg of dry matter) with a water content at 41.5%. During the work the performance of the equipment was monitored as well as their power consumption. The results on the performance of the machinery as well as their energy consumption during the work are Table 7 Uprooting force for cotton stalks Year a/a Mean diameter Mean diameter Pulling soil level 05 m height Mean mm mm N 1st Mean Range 11.5±11.7 9±10 278±389 2nd Mean Range 13.8± ± ±392 Table 6 Cotton residue production. Samples collected by hand Variable Mean Range Dry matter yield of stalks kg/ha ±3391 Dry matter yield of roots kg/ha ±690 Stalks moisture content % mid November ±64 Root moisture content % mid November ±69.5 Stalks moisture content % mid December ±52.0 Root moisture content % mid December ±62.5 Energy content of dry aerial part MJ/ha 45, ,024±61,038 Energy content of dry root MJ/ha 10, ,508±12,627 Energy content of total dry material MJ/ha 56,762

7 T.A. Gemtos, T. Tsiricoglou / Biomass and Bioenergy 16 (1999) 51±59 57 Table 8 Energy consumed during cotton stalks harvesting Machinery Speed Pulling Power in Fuel Speci c km/h power PTO consumption consumption kw kw l/h g/kwh Hay mower Raker Baler beginning Baler end of cycle Baler mean Transportation Ð given in Tables 8 and 9. Table 8 presents the e ective speed of work, as measured in the eld. Pulling power was calculated from the mean horizontal force developed between tractor and implement and the mean linear velocity of the tractor during the run was measured by the radar. Power from the PTO was calculated from the torque and the rotating frequency of the PTO shaft of the tractor. Fuel consumption was the mean for each run. Speci c fuel consumption was calculated from the fuel consumption and the total useful power produced by the tractor. It is clear from Table 8 that a smaller tractor should have been used for part of the work. This underused power has caused a high speci c fuel consumption. This indicates the di culty of obtaining optimum results during eld work because there is rarely a close t between power requirements of the equipment and the power of the tractor. In Table 9 the performance of the machinery is given. Field e ciency coe cients are generally small. For the mower and the raker Coates [8] found higher e ciencies for the machinery used. Hunt [15] gave the e ciency coe cient range for mowers as 75±89%, for rakers 62±89%, for ail mowers 50±76% and for balers 65±85%. The values of the present work are at the lower end of the range of the literature which is reasonable given the small harvested area. In larger areas, operator experience gained during the work would increase e ciency. The e ciency of the round baler given by Coates is rather high for conventional machines unless a non-stop model was used which was not made clear in the Table 9 Performance of machinery used for cotton stalk harvesting in a eld of 1.4 ha Machine working power theoretical e ective e ciency energy consumption energy consumption energy consumption width required eld eld coe cient based on measured based on measured based on measured measured capacity capacity power requirements fuel consumption fuel consumption by tractor m kw ha/h ha/h MJ/1,4ha MJ/ha MJ/1,4ha mower raker baler mean transport full transport N/A* empty Total Energy of cotton stalks collected: 15 bales *192 kg of dry matter by 18 MJ/kg gives MJ. *Measurements during return were not taken due to the high speed. Values of loaded transportation are taken for the return trip.

8 58 T.A. Gemtos, T. Tsiricoglou / Biomass and Bioenergy 16 (1999) 51±59 paper. In columns seven and nine of Table 9 the energy consumed for harvesting 1.4 ha of cotton stalks is given. In column seven the estimation is based on the net energy consumed by the machinery while the values in column nine include the energy consumed by the tractor and all its parts. It is clear that much more energy is consumed to do a eld work than it is actually required by the machinery. This di erence is much higher as tractor power is not well matched to the equipment size. This mismatch is also apparent from the high speci c fuel consumption (Table 8). In this higher fuel consumption is also included any low e ciencies of the tractor engine due to bad maintenance or settings. However, that energy is a more realistic estimation of the energy consumed for any eld work and should be the base for any assessment of the energy budget of cotton stalk collection. It should be noted that transportation energy is only measured for the loaded platform as the high speed in the return trip could damage the instrumentation. So, a slight overestimation was introduced. Additionally, energy consumption was not estimated during the pre-compression of the baler by the hydraulic system of the tractor and during fork loading and unloading of the bales. Total direct energy consumed for the eld operation is 1407 MJ. In these gures, the indirect energy consumed for the machinery as well as the energy for lubricants, repair and maintenance etc should be added to have an exact energy balance. An analysis of the indirect energy consumed for the machinery used was given by Gemtos and Tsiricoglou [16]. In this work the energy sequested for the construction, maintenance etc, of the machinery used ranged from 521 to 695 MJ for the 1.4 ha giving a total energy consumption of 1929±2103 MJ. The energy content of the collected stalks was 15 bales of 192 kg dry matter/bale and heating value of 18 MJ/kg, yield 51,840 MJ, giving a net energy gain of about 49,800 MJ or 35,571 MJ/ha. The material collected by the baler was 2,880 kg dry matter (15 bales of 192 kg each). With theoretical yield kg the e ciency of collection is 80.8% which is quite satisfactory. In order to investigate the problems encountered during storage, the bales were stored outside and inside a barn. In both cases, the bales were dried to a water content of less than 20% w.b. in 20 days of outdoor storage without any heating of the material. Cotton stalks are covered at the base by a cork layer which prevents moisture loss. An uprooting of the plants without the disturbance of this layer could decrease the moisture loss speed. In our case, the cutting of the stalks and their bending during baling would cause damage to that layer and thus increase moisture loss. Detailed results of storing experiments were given by Gemtos and Tsiricoglou [17]. 4. Conclusions From the present work it can be concluded that under Greek conditions:. uprooting of cotton stalks is not a feasible method of harvesting due to the soil stacked on the roots and the limited period available for the plants to be left in the eld for drying;. cutting of stalks is feasible with the existing hay mowers with reciprocating knives;. the whole work of collection, packaging and transportation of the residue can be ful lled by the existing conventional hay making equipment which gives a great advantage to the method;. the best results were given by the use of a large round baler;. the harvesting operation is energy e ective, giving a net energy of 35,571 MJ/ha;. the bales can be stored safely in or out of doors without heating problems for the material and, equally importantly, with natural drying during the initial storage period. References [1] Gemtos TA, Galanopoulou St, Kavalaris Chr. Wheat establishment after cotton with minimal tillage. European Journal of Agronomy 1997;8:137±47.

9 T.A. Gemtos, T. Tsiricoglou / Biomass and Bioenergy 16 (1999) 51±59 59 [2] Ebeling JM, Jenkins HM. Physical and chemical properties of biomass fuels. Trans ASAE 1985;28(3):898. [3] Sumner HR, Sumner PF, Hammond WC, Monroe GE. Indirect red biomass furnace test and bomb calorimeter determinations. Trans ASAE 1983;26(1):238. [4] Sumner HR, Monroe GE, Hellwig RE. Design elements of a cotton plant puller. Trans ASAE 1984a;27(20): 366. [5] Sumner HR, Hellwig RE, Monroe GE. Harvesting cotton plant residue for fuel. Trans ASAE 1984b;27(4): 968. [6] Sumner HR, Monroe GE, Hellwig RE. Puller for cotton plant stable. ASAE paper SER , [7] Kemp DC, Matthews MDP. Development of a cotton stalk pulling machine. J Agr Engng Res 1982;27:201. [8] Coates W. Harvesting systems for cotton plant residue. Applied Engineering in Agriculture 1996;12(6): 639±644. [9] Chancelor W. Compaction of soil by agricultural equipment. University of California, Division of Agricultural Sciences, Bulletin 1881, [10] Gemtos TA, Tsiricoglou TI. Report on a research programme. TEI of Larissa, Unpublished, [11] Kepner RA, Bainer R, Barger EL. Principles of Farm Machinery, 3rd ed. AVI Publ. Co., p [12] Gemtos TA, Tsiricoglou TI. Design and instrumentation of a farm tractor to measure and record forces developed in mounted tools. Geotechnical Chambers of Greece, Scienti c Issue 1995;(4):89±96 (In Greek). [13] Tsiricoglou ThI, Gemtos TA. Measurement of power requirements of farm machinery which take power through the PTO. Technika Chronika (in press, in Greek), [14] National Statistics Service National Statistics, Athens. [15] Hunt D. Farm power and machinery management. Iowa State University Press, [16] Gemtos TA, Tsiricoglou TI. The economics and energetics of a system of harvesting cotton stalks for energy production. Fourth National Conference for the Renewable Energy Resources of the Institution of Solar 1992;Vol B(BIO):54±65 (in Greek). [17] Gemtos TA, Tsiricoglou TI Cotton residue harvesting and storage in Greece for energy production. 2nd World Renewable Energy Congress. Vol. 3. Pergamon Press.