Mechanical strength testing of stalk materials and compacting energy evaluation

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1 Industrial Crops and Products 11 (2000) Mechanical strength testing of stalk materials and compacting energy evaluation E. Kronbergs * Institute of Mechanics, Lat ia Uni ersity of Agriculture, J. Cakstes bul. 5., Jelga a, LV3001, Lat ia Accepted 8 October 1999 Abstract The great potential of cereal lignocelluloses for use as energy or as an industrial raw material source determines the necessity to investigate the mechanical properties of stalks, as the main parts of any herbaceous material. Annual world production of cereal straw is approximately of the same amount (around 2 billion tons) as wood material production. Latvia as a country with more than 2000 lakes has wide areas, covered with reeds, which are also an important stalk material resource. Stalk cross section and structural studies show that it is a complicated structure. Special test piece preparation and clamping methods, measuring devices, testing procedure for stalk material strength testing have been worked out. Ultimate tensile ( N/mm 2 ) and shear ( N/mm 2 ) strength, modulus of elasticity ( GPa) and shear modulus ( GPa) had been experimentally determined for wheat stalks in order to find methods for mechanical conversion with minimum energy consumption. Stalk biomass compacting methods and energy requirement for that are analysed on the basis of the results from the experiments. It has been stated, that energy for wheat straw pressing ( 40 kj/kg up to pressure 160 MPa) is one order of magnitude less than that for heating of the same mass up to 200 C in the briquetting process ( 360 kj/kg), if solid material has to be obtained. Depending on the target goals for usage, different energy saving compacting technologies can be developed. Thus the analysis of mechanical and physical properties of the stalk materials determines the profit of biomass usage Elsevier Science B.V. All rights reserved. Keywords: Stalk materials; Mechanical strength testing; Compacting energy 1. Introduction Annual world production of cereal straw was 1900 million tons in 1987 (Munck, 1992), but in Latvia the total straw production amount in 1997 was tons. If 20% from the total amount * Tel.: ; fax: address: eriks@inka.cs.llu.lv (E. Kronbergs) could be used for energy production ( t straw), it is equal to heat capacity of t of oil. Straw biomass can be used not only for agricultural purposes and energy production but also as an industrial raw material for paper and particleboard production. Latvia as a country with more than 2000 lakes, has wide areas covered with emergent vegetation-reeds also an important stalk material resource. The aforemen /00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S (99)

2 212 E. Kronbergs / Industrial Crops and Products 11 (2000) tioned stalk materials have to be used for energy and industrial purposes with a good understanding of the agricultural ecosystem function. As it brings about soil impoverishment and erosion, the organic residue removal from the fields must be limited. The biomass amount whose removal has no significant impact on the carbon cycle varies from 20 to 50%. In the rural area, open water systems (rivers and lakes) and wetlands also play an important role in the functioning of the agricultural ecosystem. For ages lake ecosystems have been acting as sinks for collecting organic and minerogenic matter (Bjork, 1988). At the same time, soil cultivation in the catchment area of natural open water systems interacts with water flows. It has been stated that the intense land cultivation over the last 100 years has led to irreversible charge flows (matter loss, mainly alkaline metal cations) from agricultural soils by surface water flows. Matter losses from agricultural areas may amount to more than one ton per hectare a year (Ripl et al., 1994). The application of fertilizers will hardly compensate for this loss, nitrogen and phosphorus fertilizers together make for only 1 2% of this matter loss. So, the intense soil cultivation and surface water flows lead to the acidification and desertification of the catchment area. Overloading by nutrients (N, P, K and Ca) from external sources favours the growth of plants in the lakes and this, in turn, causes intensive accumulation of sediments (sapropel) and peat. Limnologists (Ripl et al., 1994) recommend that for the redevelopment of catchment areas of open water systems solid matter (sludge, sediments, and compost) should be transported in a direction opposite to the water flow. It is also an important biomass application, because the energy requirement for production, packaging, transportation and application of nitrogen, phosphorus and potassium (NPK) fertilizers is BTUs/lb., for N, 7.53 BTUs/lb., for P 2 O 5 and 5.94 BTUs/lb. for K 2 O. It corresponds to the consumption of diesel fuel 2.41 l/kg (N), 0.54 l/kg (P 2 O 5 ) and 0.42 l/kg (K 2 O) (Helsel, 1992). Biomass usage, directly for energy production, or as fertilizer, is an activity with equal importance in agriculture. Dry vegetation materials and their stalks as the main part of any plant material can be used for all previously mentioned purposes, but their mechanical property data and compacting energy requirement have to be investigated. Mechanical properties are important measures for the design of processing and handling equipment. Investigations of compaction properties of such stalk materials are carried out for the development of the most economical transport and storage. 2. Materials and methods Fig. 1. Cross-section of stalks: (a) a wheat stalk, (b) a reed stalk. All stalk material consists of nodes and tubular internodes. The cross-section of wheat stalk with such parts as sclerenhyma, parenchyma and vascular bundles show us the complexity of its structure. Reed stalks have some differences, such as air channels for oxygen supply to roots, because they are aquatic plants (Fig. 1). Stalk cross section can be assumed as an elliptic ring and mea-

3 E. Kronbergs / Industrial Crops and Products 11 (2000) safeguarding rubber tapes 3, glued to the jaws 2. For tensile strength determination, it is recommended that a test piece from a stalk slice should be prepared with the width equal to triple thickness of the stalk material. The slice is bent around the droplet like section member and the end is connected to the main slice part with glue 6. The radius r of the droplet like section member can be determined with inequality: r 1 2 E b 1 (1) Fig. 2. Clamping of the stalk test piece for tensile testing. F is the tensile force. Fig. 3. Schemes of shear stress measurement and compacting experiments: (a) shear stress measurement device, (b) closed die compacting. sured by means of a measuring microscope for calculations of the cross-section area and moments of inertia. Tensile strength measurement is the basic material testing method, also for the investigation of stalk material mechanical properties. The main problem is preparing the stalk test pieces in such a condition, that during the testing procedure for a tensile strength determination at break the test piece breaking occurs between clamping devices. A special tensile test piece preparation for stalk materials has been developed (Fig. 2). The testing machine clamping device frame 1, with self-locking jaws 2, have been supplemented with where is the stalk slice thickness; E is the stalk material modulus of elasticity; b is the stalk material tensile strength at break. The modulus of elasticity has been calculated from Vereschagins equation on the basis of wheat stalk bending elastic deformation measurements under a determinate force. The accuracy of a stalk cross section measurement is important and a measuring microscope has to be used. Fig. 3a shows the stalk ultimate shear stress measuring equipment. Flattened wheat stalk test piece 1, has been clamped between the equipment frame 2, and elastic lever 3, supplemented with strain gauges 4, measurement system and data collecting on the PC. Turning clockwise elastic lever 3, stalk test piece 1 has been sheared. Shear force has been recorded during all test procedure. For material cross section measurement a microscope also has to be used. Shear modulus has been determined for the whole stalk material internodes part in torsion experiments. Compacting experiments have been carried out in a closed die (Fig. 3b) by means of tensile testing machine supplemented with press equipment. Wheat straw stalk material biomass with moisture content of 10% and chopped to different lengths, have been used for compacting. Chopped stalk material 1, in closed die 2, has been compacted by means of piston 3 (Fig. 3b). Force and displacement have been recorded in the compacting process. Maximum compacting pressure 160 Mpa has been achieved. The calculation of area under force and displacement relation curves (Fig. 4) from records, obtained in experiments, led to finding the energy requirement for wheat stalk compact-

4 214 E. Kronbergs / Industrial Crops and Products 11 (2000) ing. Temperature influence on press density was also investigated. These experiments have been carried out on a laboratory hydraulic press. Preliminary stalk biomass was heated until the planned temperature had been achieved, then compacting started. Straw stalk biomass unit (kg) heating to temperature 200 C energy requirement calculations, using Specific Heat value 1.8 kj/kg g rad (as for wood with a temperature 100 C), have been done. The biomass cutting (chopping) energy E c also for stalk biomass unit (kg) can be calculated using the known (Srivastava et al., 1993) equation: E c = E sc L c (2) where E sc is the specific cutting energy per unit mass (J m/kg); L c is the length of stalk cut (m). It has been stated (Persson, 1987), that specific cutterhead energy was not affected by the moisture content, if the energy was calculated on a dry Fig. 4. Force and displacement relation curves. Fig. 5. Compacting energy requirement. matter basis. The value of E c =38 J m/kg (Persson, 1987) can be found for dry stalk material cutting energy requirement calculations. The wheat stalk material cold compacting experiments with two additives have also been carried out. Dried molasses is a well-known binding agent and it was used for the biomass compacting. Admixing of the lake sediment (sapropel) additive as an antislagging agent for fuel briquettes production took place in compacting experiments. Briquettes obtained in the compacting experiments were measured and weighed for calculating the density. 3. Results Ultimate tensile ( N/mm 2 ) and shear ( N/mm 2 ) strength, modulus of elasticity ( kn/mm 2 ) and shear modulus ( GPa) were experimentally determined for wheat stalks in order to find methods for mechanical conversion with minimum energy consumption. The energy requirement for wheat stalk compacting, calculated by integration of the area under force and displacement relation curves (Fig. 4) are reproduced in Fig. 5. Pressing energy consumption in high extent, depends on the stalk material biomass amount, which is in a closed die. On this basis it can be found out, that no more energy is necessary for wheat stalk material (chopped to length 2 cm) compacting up to pressure 160 Mpa than 40 kj/kg, if every briquette obtained is 0.1 kg mass. The density of such compacted coarse chopped stalk material biomass also depends on the amount of mass, which is in a closed die (Fig. 6). The average density 0.7 g/cm 3 has been obtained. The experiments of the next series with heating the same coarse chopped wheat stalk material and pressing it up to the pressure 80 MPa show, that the average density 0.8 g/cm 3 has been obtained (Fig. 7). So with twice reduced pressure and heating stalk material up to temperature 200 C compacting density increased, but not considerably. The energy requirement for heating of the same mass up to temperature 200 C in the compacting process is 360 kj/kg.

5 E. Kronbergs / Industrial Crops and Products 11 (2000) comparison with coarse chopped material. Only fine chopping provide a density for compacted (t=20 C) material 0.85 g/cm 3. The obtained density is higher than the density of oak wood (0.803 g/cm 3 ). Cold compacting of fine chopped wheat stalk material with addition of molasses more than 9% and sapropel more than 15% provide density 1g/ cm 3 without any heating (Fig. 9). Fig. 6. Compacting density (without heating). Fig. 7. Compacting density (coarse chopped, with heating). 4. Discussion Experimentally determined mechanical properties are important measures for the design of processing and handling equipment. The ultimate shear strength of wheat stalks is 14 times less than ultimate tensile strength value. This fact is of main importance for cutting tool design. Stalk material tensile deformations in the cutting process have to be prevented. The ultimate tensile strength of N/mm 2 is higher than tensile strength of pine wood material and this is an argument for stalk material use as a construction material. The described test piece clamping method for tensile strength measurement, provide a reliability of the testing procedure with sample breaking between clamping devices. Ultimate shear stress measurement by means of elastic lever equipped with a strain gauge system can also be recommended as a reliable means for stalk material testing. Fig. 8. Compacting density (fine chopped, with heating). Fine chopped (ground) stalk material, selected through a sieve with 1.5 mm eye, heated and pressed up to pressure 80 MPa enabled us to obtain the higher density 1 g/cm 3 with temperature C (Fig. 8). Additional energy requirement for fine chopping of stalk material, calculated on the basis of Eq. (2), is 25 kj/kg in Fig. 9. Cold compacting density.

6 216 E. Kronbergs / Industrial Crops and Products 11 (2000) Wheat stalk material compacting experiment results let us evaluate the energy requirement for different density increasing methods. If there is no more energy necessary for wheat stalk material (chopped to length 2 cm) compacting up to the pressure of 160 MPa than 40 kj/kg, in ordinary briquetting up to the pressure of 100 MPa then also the energy requirement is less. The average density 0.7 g/cm 3, obtained in such coarse chopped material compacting, is higher than pine wood density (0.498 g/cm 3 ). Without binding agents or heating material in the compacting process, the obtained briquette is not shock resistant. Density increasing up to 1 g/cm 3 allows an increase also in shock resistance of the briquette. Compacting experiments have shown that fine chopping of stalk materials increases the density in the same extent as heating of the mass. Energy requirement calculation results for stalk material fine chopping 25 kj/kg is one order of magnitude less than that for heating of the mass up to 200 C temperature 360 kj/kg. Therefore the stalk material chopping is more preferable than heating. If solid fuel (briquettes or pellets) have to be obtained from stalk material molasses can be used as a binding agent for cold compacting. Sugar production provides annually more than tons of molasses in Latvia. As antislagging agents also are recommended for plant material fuels, lake sediment (sapropel) usage for this purpose is reasonable in Latvia s conditions. Both molasses and sapropel are low cost organic materials and their influence on compacting density is positive. Further research for such fuel combustion properties should be performed to determine the optimum amount of additives for the best combustion. 5. Conclusion Ultimate tensile ( N/mm 2 ) and shear ( N/mm 2 ) strength, modulus of elasticity ( GPa) and shear modulus ( GPa) were experimentally determined for wheat stalks in order to find methods for mechanical conversion with minimum energy consumption. These mechanical properties are important measures for the design of processing and handling equipment, but suggested methods for tensile test piece clamping and ultimate shear stress measurement can be recommended for stalk material testing. Fine chopping of stalk materials significantly influence compacting density and is more preferable to heating, because energy requirement for chopping 25 kj/kg is one order of magnitude less than that for mass heating up to 200 C temperature 360 kj/kg. Molasses and sapropel are low cost organic materials in Latvia conditions and those can be used as binding agent and antislagging additive for stalk material solid fuel production. Acknowledgements The author wishes to express his gratitude to Dr ing. Aivars Strupausis for performing the tensile strength and Modulus of elasticity determination tests, MSc. ing. Aivars Kakitis for performing the shear stress measurement equipment design, colleagues Raimonds Zarins and Ritvars Klapars for carrying out the compacting experiments. References Bjork, S., Redevelopment of Lake Ecosystems-A Case- Study Approach, AMBIO Vol. 17 NO.2: Ripl, W., Pokorny J., Eiseltova M. and Ridgil S., An holistic approach to the structure and function of wetlands, and their degradation. In: M. Eiseltova (Ed.), Restoration of Lake Ecosystems, an Holistic Approach. A Training handbook, pp Helsel, Z.R., Energy and alternatives for fertilizer and pesticide use. In: Fluck, R.C. (Ed.), World Agriculture. Energy in Farm Production, vol. 6. Elsevier, New York, pp Munck, L., The contribution of barley to agriculture today and in the future. Barley Genetics VI, volume II, Barley Research reviews Session and Workshop Summaries. Proceedings of the Sixth International Barley Genetics Symposium, July , Helsingborg, Sweden. Helsingborg, The Organizing Committee of the Nordic Countries, Sixth International Genetics Symposium pp Srivastava, A.K., Goering, C.E., Rohrbach, R.P., Engineering principles of agricultural machines. In: P. De Vore- Hansen (Ed.), ASAE Textbook Number 6. Books and Journals, American Society of Agricultural Engineers, St. Josephs, Michigan,p Persson, S., Mechanics of cutting plant material. An ASAE Monograph Number 7.