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Thermal properties of fractions of corn stover
Construction and Building Materials 210 (2019) 709–712
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Thermal properties of fractions of corn stover Łukasz Czajkowski a, Dawid Wojcieszak b,⇑, Wiesław Olek a, Jacek Przybył b a b
´ University of Life Sciences, ul. Wojska Polskiego 28, 60-637 Poznan ´ , Poland Faculty of Wood Technology, Poznan ´ University of Life Sciences, ul. Wojska Polskiego 50, 60-627 Poznan ´ , Poland Institute of Biosystems Engineering, Poznan
h i g h l i g h t s Specific heat determined for four corn stover fractions. The thermal properties of corn stover fractions should be determined separately. All corn stover fractions are useful for insulating materials production. The applied method ensured high accuracy of measurements of corn stover specific heat.
a r t i c l e
i n f o
Article history: Received 19 August 2018 Received in revised form 18 February 2019 Accepted 11 March 2019 Available online 23 March 2019 Keywords: Corn stover fractions Specific heat Thermal properties Calorimetric method
a b s t r a c t The properties of agriculture by-products make them applicable for insulating materials production. One of the potential source of natural insulating material is corn stover, especially the stalks. In the present study the density, specific heat and volumetric specific heat capacity of individual fractions of corn stover were determined. The separation of the corn fractions allowed to identify the most suitable fractions for insulating panels production. Four separated fractions, i.e. stalks, leaves, husks and cobs, were in focus of the study. The specific heat measurements of selected corn factions were carried out with a water calorimeter. The measurements were made for the oven dry materials which were placed in a heat shrinkable membrane to prepare samples of a cylindrical shape. The obtained values of specific heat and volumetric specific heat capacity confirmed that the thermal properties of corn stover fractions should be determined separately. Only leaves and stalks were characterized by similar values of specific heat and volumetric specific heat capacity. The highest value of specific heat equal to 1723 J/(kgK) was determined for husks. The observed high values of volumetric specific heat capacity determined for all corn stover fractions confirmed their ability of heat accumulation. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Changes in natural environment and the reduction of fossil fuels are contributing to the development of new environmentallyfriendly biomaterials. Low price, annual renewability and recyclability make agricultural by-products an excellent source for natural fibrous materials. Energy consumption reduction during production process, biodegradability, as well as the positive impact on end users cause an increase in the application of biomaterials in the ecological building solutions [1,2]. The properties of agriculture by-products, i.e. low density, high rigidity and good strength properties, make them applicable in building engineering, primarily as insulating materials [3]. It results in finding potential applications of fibrous materials being produced from cereal stover (wheat, barley, corn), nut shells, ⇑ Corresponding author. E-mail address: [email protected] (D. Wojcieszak). https://doi.org/10.1016/j.conbuildmat.2019.03.092 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
hemp, bagasse [4–6]. Moreover, the materials are used to reduce weight and improve insulation properties of standard building materials. The agriculture by-products are often added during production processes of concrete e.g. [7,8] and wood-based panels e.g. [9]. In the last decade the global production of maize grain has increased by 40%. The current world production amounts to 1030 million tons. In 2016 the total production of maize grain in the EU was 60.3 million tons [10]. The production of maize grain results in crop residues, such as stalks, leaves, cobs and husks, which make up 47–50% of the yield of dry mass of maize plants. Due to the scale of the global production of corn the residues are the important, potential raw material for insulation panels production. The structure of corn stover, especially its ‘‘spongy” stalk pith, is one of the reasons to use this feedstock as a natural insulating material. This resulted in a number of preliminary studies investigating thermal properties of the materials produced from corn stover. The obtained results have shown that stover can be used
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Ł. Czajkowski et al. / Construction and Building Materials 210 (2019) 709–712
in the production of insulating panels [11–13]. However, corn stover is the only by-product in cereal production characterized by an inhomogeneous structure consisting of stalks, leaves, husks and cobs. These fractions have different chemical composition, tissue structure and fiber properties [14,15]. Previous studies primarily focused on selected and separated fractions of corn stover. The inhomogeneous structure and complex chemical composition of individual fractions of corn stover needs comprehensive research on the properties of each fraction and their applications for building materials. In addition the application of corn stover as insulating materials requires the knowledge on its primary thermal properties, e.g., specific heat and thermal conductivity. These properties are required for proper modeling and description of heat transfer e.g. [16]. Moreover, the thermal properties and density of the materials are often used to determine thermal diffusivity and volumetric specific heat capacity. Thermal conductivity is the property most often determined for corn stover materials [17,18]. However, there is no data on specific heat for individual fractions, which is crucial for the evaluation of the transient heat transfer [19]. The specific heat of various by-products, including corn stover, was determined by Dupont et al. [20]. However, there is no information on fractions of stover which have been tested. These studies concern experiments in which the investigated materials were grounded in order to homogenize the samples. The grinding operation caused changes in the structure of the materials and air content in the samples. This resulted in unwanted alteration of the properties. The thermal properties are mainly determined for boards made of mixed different fractions of corn stover. The parameters of the production process (temperature and pressure) as well as type of resin and resin load significantly influence thermal properties. Sampathrajan et al. [21] determined the specific heat of low density particleboard made of farm residues. The results obtained for husks and cobs panels as compared to currently produced natural insulating materials (coir, jute, kenaf, wood fiber) were characterized by low specific heat [3]. It could disqualify husks and cobs as potential insulating materials. The porous structure of the corn stalk pith makes it particularly applicable as insulating material. Etuk et al. [22] determined the thermal properties of panels produced separately from whole corn stalks and internal stalk pith. The measured values of specific heat as obtained for the two types of panels were similar. However, the panels which were made of corn stalk were characterized by two times higher values of thermal conductivity as compared to the panel made of pith stalks only. These results clearly indicated the need for selective harvesting and treatment of corn stover as well as the appropriate selection of corn stover fractions e.g. [23].
The objective of the present study was to determine the density, specific heat and volumetric specific heat capacity of individual fractions of corn stover. The separation of the fractions was intended to make measurements providing more reliable data on the thermal property and identifying the most suitable fractions of corn stover for insulating panels production. 2. Materials and methods 2.1. Materials The experimental material was obtained from a farm in which corn was cultivated for grain production (FAO 240). The farm was located in the western part of the Wielkopolska region (Poland). Mature plants have been separated into individual fractions, i.e. stalks, leaves (stalk leaves), husks (cob cover leaves) and cobs without grain. Stalks were cut into parts with 140 mm length, and not divided into the internal (stalk pith) and external (stalk outer layer) fragments. All four fractions were oven dried to the constant mass. The measurements were made at the oven dry conditions in order to avoid an influence of moisture content on the measured specific heat values. 2.2. Experimental procedure The oven dry material was placed in a heat shrinkable membrane to prepare samples of a cylindrical shape with length of ca. 135 mm and a diameter of ca. 75 mm (Fig. 1). The samples were made for each separated fraction and their natural structure was preserved within the samples. The space of voids filled with air was minimized inside the samples. The temperature distribution in each sample was controlled by two type K thermocouples mounted in the center and at the surface. The measurements of specific heat were carried out with a water calorimeter specially designed for bio-materials of low density. The equilibrium temperature of the calorimetric system was determined with the type J thermocouple of the uncertainty of 0.02 K. The application of the type J thermocouple was crucial for minimizing the errors of specific heat measurements. It was already demonstrated that the measurements of the equilibrium temperature of the calorimetric system with the type K thermocouple resulted in five times higher values of the relative error [24]. After the specific heat measurements the thermocouples were dismounted and the samples were again covered with heat shrinkable membrane in order to determine bulk density. The samples were inserted in a calibrated cylinder being filled with water and the volume of each sample was determined by water displacement. The mass and volume of heat shrinkable membrane were
Fig. 1. Tested samples: a) cobs, b) stalks, c) leaves, d) husks.
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determined separately. The bulk density of selected corn stover fractions was calculated as the ratio of mass and volume. The volumetric specific heat capacity (cq) was determined as the product of the specific heat and density e.g. [25]. 2.3. Statistical analysis Results of the specific heat measurements were analyzed using the one-way ANOVA variance analysis, while the significant differences between specific heat values for separated corn stover fractions were determined using the Tukey HDS test for a = 0.05. The absolute error of the specific heat determination was calculated using the total differential method [26]. It was made in order to estimate uncertainties of individual measurements e.g. mass values of sample, water, aluminum calorimeter inner cup with stirring rod, shrinkable membrane, as well initial temperature of a sample and calorimeter with water and equilibrium temperature of the calorimetric system. Moreover, the relative error of measurements was determined to account for the uncertainty of specific heat measurements. The detailed description of the applied method and heat balance of the calorimetric system was presented by Czajkowski et al. [24]. 3. Results and discussion In order to ensure the reproducibility of the measurements six replicates have been performed for each tested fraction of corn stover. The average values of the data used in the heat balance equation, i.e. sample mass, sample initial temperature, equilibrium temperature of the calorimetric system, temperature increase of the calorimetric system are presented in Table 1. The data were used to calculate the specific heat of the examined corn stover fractions in accordance with the heat balance equation of the calorimetric system presented in details by Czajkowski et al. [24]. The average values of the specific heat as well as the absolute and relative error of the measurements for cobs, stalks, leaves and husks are also given in Table 1. Despite of inhomogeneity of the investigated fractions similar mean values of the specific heat were obtained for stalks and leaves 1637 and 1644 J/(kgK). The differences in the specific heat values were negligible for a = 0.05. Therefore, these two fractions can be mixed during production processes of insulation materials, i.e. there is no need to separate these two fractions during harvesting. The highest mean value of specific heat as equal to 1723 J/(kgK) was observed for husks, while the lowest one of 1484 J/(kgK) for cobs. High differences in the specific heat values for husks and cobs would cause different insulating properties of the materials produced from these separated fractions. The obtained results clearly showed that the specific heat has to be determined for the separated fractions. The estimated errors of the specific heat measurements truly depicted that the uncertainty of specific heat measurements increases with a decrease of mass of tested samples. It confirms
the requirement for making measurements with samples of high mass. The highest values of the relative and absolute error i.e. 6.48% and 106.11 J/(kgK) respectively, were obtained for the stalks. This can be explained by natural light and porous structure of stalks which resulted in the lowest mass of the sample. For the other fractions, the relative error did not exceed 5% and it confirmed the reliability of the method to determining the specific heat of by-products. The obtained results of the specific heat being higher than 1400 J/(kgK) confirm the usefulness of such fraction of corn stover for the production of insulating materials e.g. Asdrubali et al. [19]. A comparison of the determined values of the specific heat with the data given for standard synthetic insulation materials (e.g. expanded polystyrene or fiber glass) shows similar or higher values as obtained for all fractions of corn stover [27,28]. The measured values of density, specific heat and volumetric specific heat capacity of the corn stover fractions are presented in Table 2. The similar values of density as well as specific heat were obtained for stalks and leaves confirm that these two fractions can be mixed for insulating materials production. In spite of the lowest value of the specific heat determined for cobs the highest volumetric specific heat capacity was obtained for this fraction. All corn stover fractions are characterized by high volumetric specific heat capacity and their usefulness depends on end-use. High values of volumetric specific heat capacity of corn stover fraction confirm its heat accumulation capabilities, which can be used in energy efficient building solutions. The determined values were also compared to the available data on specific heat of corn stover or corn stover materials reported in the literature. Dupont et al. [20] determined the effect of temperature on the values of specific heat of absolutely dry corn stover. Moreover, the measurements were carried out for ground material which caused alteration in its structure and porosity as compared to native stover. The specific heat of 1610 J/(kgK) was measured at the temperature of 353 K. The obtained value was similar only to the results acquired in the present study for stalks and leaves (Table 1). However, there was no information on fractions being used in the research. The majority of the reported data on specific heat usually concern already manufactured insulating panels. Etuk et al. [22] determined specific heat for panels being made of separated fractions, i.e. whole stalks and stalk piths and obtained values of 1870 and 1979 J/(kgK), respectively. The measured results for panels made of whole stalks are ca. 300 J/(kgK) higher as compared to the values for stalk fraction presented in Table 1. The observed differences can be explained by different moisture content values of the tested materials. Thermal properties of particle boards made of selected corn stover fractions were also determined by Sampathrajan et al. [21]. The boards were produced from husk and cobs and the specific heat was equal to 847 J/(kgK) and 879 J/(kgK), respectively. The values presented in Table 1 are twice as high as specific heat obtained for the particle panels, despite the fact that the boards were characterized by moisture content ranging from 11 to 14%.
Table 1 The measured values of the heat balance equation components with calculated specific heat together with the absolute and relative error of measurements. Corn stover fraction
Cobs Stalks Leaves Husks
kg
Initial temperature of sample; tic °C
Equilibrium temperature of the calorimetric system; te °C
Increase of temperature; DT K
Specific heat; c J/(kgK)
Absolute error; Dc J/(kgK)
Relative error; % %
0.1512 0.0930 0.1262 0.1401
92.58 91.86 91.76 92.32
14.34 13.12 14.34 13.20
1.10 0.74 1.00 1.16
1484 ± 11a 1637 ± 18b 1644 ± 13b 1723 ± 4c
62.79 106.11 78.72 70.95
4.23 6.48 4.79 4.11
Sample mass; m
Note: all presented results are mean values of 6 replicates, mean value of specific heat are supplemented by standard deviation, identical letters (a, b, c) indicated no significant differences (P < 0.05) between mean values of specific heat according to Tuckey’s HDS test (ANOVA).
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Table 2 Thermal properties of corn stover fractions. Corn stover fraction
Cobs Stalks Leaves Husks
References
q
Specific heat; c
kg/m3
J/(kgK)
Volumetric specific heat capacity; cq MJ/(m3K)
257.5 148.3 173.4 206.5
1484 1637 1644 1723
0.382 0.243 0.285 0.356
Density;
The data on specific heat of corn stover as well as corn stover panels are ambiguous due to the differences reported in the literature. Discrepancies between the results reported in different sources make the data uncertain for heat transfer modelling. The determined values of the volumetric specific heat capacity for corn stover are higher than values obtained for polystyrene and glass fiber but lower than the values for wood [29]. The volumetric specific heat capacity values are similar to cork. This confirms the usefulness of all corn stover fractions for heat accumulation in building envelopes. Ahn et al. [30] determined volumetric specific heat capacity with differential scanning calorimeter (DSC) for dry corn stalks and obtained the values ranging from 0.05 to 0.60 MJ/(m3K). It suggests high variation of the results reported by Ahn et al. [30]. It was probably due to low accuracy of the applied DSC method. The results presented in this paper are contained in much smaller range from 0.243 to 0.382 MJ/ (m3K). It suggests that the applied method is more appropriate for characterized thermal properties of hygroscopic natural materials with low density.
4. Conclusions The obtained values of specific heat and volumetric specific heat capacity confirmed that the thermal properties of corn stover fractions should be determined separately. Only leaves and stalks were characterized by similar values of specific heat and volumetric specific heat capacity. Thus, these two fractions can be mixed during production processes of insulation materials. The observed high values of volumetric specific heat capacity determined for all corn stover fractions confirmed their ability to accumulate heat. Therefore, the fractions can be used for production of insulating materials being used for low-energy efficient building and passive houses. The measurement method applied in the present study ensured high accuracy of specific heat determination i.e. the relative error did not exceed 5% for cobs, leaves and husks. The accuracy was only slightly worse for the fraction of the lowest density (lowest mass of sample) i.e. the stalks. It clearly shows that the measurements of the specific heat has to be supplemented by the error analysis and mass of samples should be carefully estimated.
Conflict of interest None.
Acknowledgement The paper was financed within the framework of Ministry of Science and Higher Education programme ‘Regional Initiative of Excellence’ in years 2019-2022, Project No. 005/RID/2018/19.
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Akpan, Comparative study of thermal transport in zea mays straw and zea mays heartwood (cork) boards, Therm. Sci. 14 (2010) 31–38. [23] J.A. Lacey, R.M. Emerson, D.N. Thompson, T.L. Westover, Ash reduction strategies in corn stover facilitated by anatomical and size fractional, Biomass Bioenergy 90 (2016) 173–180. [24] Ł. Czajkowski, W. Olek, J. Weres, R. Guzenda, Thermal properties of woodbased panels: specific heat determination, Wood Sci. Technol. 50 (2016) 537– 545. [25] H. Hens, Building Physics: Heat, Air and Moisture, second ed., Wilhelm & Sohn, Ernst, 2012. [26] J.R. Taylor, An Introduction to Error Analysis. The Study of Uncertainties in Physical Measurements, second ed., University Science Books, Sausalito, 1997. [27] L. Ducoulombier, Z. Lafhaj, Comparative study of hygrothermal properties of five thermal insulation materials, Case Stud. Therm. Eng. 10 (2017) 628–640. [28] A. Kumar, B.M. 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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Thermal properties of fractions of corn stover Łukasz Czajkowski a, Dawid Wojcieszak b,⇑, Wiesław Olek a, Jacek Przybył b a b
´ University of Life Sciences, ul. Wojska Polskiego 28, 60-637 Poznan ´ , Poland Faculty of Wood Technology, Poznan ´ University of Life Sciences, ul. Wojska Polskiego 50, 60-627 Poznan ´ , Poland Institute of Biosystems Engineering, Poznan
h i g h l i g h t s Specific heat determined for four corn stover fractions. The thermal properties of corn stover fractions should be determined separately. All corn stover fractions are useful for insulating materials production. The applied method ensured high accuracy of measurements of corn stover specific heat.
a r t i c l e
i n f o
Article history: Received 19 August 2018 Received in revised form 18 February 2019 Accepted 11 March 2019 Available online 23 March 2019 Keywords: Corn stover fractions Specific heat Thermal properties Calorimetric method
a b s t r a c t The properties of agriculture by-products make them applicable for insulating materials production. One of the potential source of natural insulating material is corn stover, especially the stalks. In the present study the density, specific heat and volumetric specific heat capacity of individual fractions of corn stover were determined. The separation of the corn fractions allowed to identify the most suitable fractions for insulating panels production. Four separated fractions, i.e. stalks, leaves, husks and cobs, were in focus of the study. The specific heat measurements of selected corn factions were carried out with a water calorimeter. The measurements were made for the oven dry materials which were placed in a heat shrinkable membrane to prepare samples of a cylindrical shape. The obtained values of specific heat and volumetric specific heat capacity confirmed that the thermal properties of corn stover fractions should be determined separately. Only leaves and stalks were characterized by similar values of specific heat and volumetric specific heat capacity. The highest value of specific heat equal to 1723 J/(kgK) was determined for husks. The observed high values of volumetric specific heat capacity determined for all corn stover fractions confirmed their ability of heat accumulation. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Changes in natural environment and the reduction of fossil fuels are contributing to the development of new environmentallyfriendly biomaterials. Low price, annual renewability and recyclability make agricultural by-products an excellent source for natural fibrous materials. Energy consumption reduction during production process, biodegradability, as well as the positive impact on end users cause an increase in the application of biomaterials in the ecological building solutions [1,2]. The properties of agriculture by-products, i.e. low density, high rigidity and good strength properties, make them applicable in building engineering, primarily as insulating materials [3]. It results in finding potential applications of fibrous materials being produced from cereal stover (wheat, barley, corn), nut shells, ⇑ Corresponding author. E-mail address: [email protected] (D. Wojcieszak). https://doi.org/10.1016/j.conbuildmat.2019.03.092 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
hemp, bagasse [4–6]. Moreover, the materials are used to reduce weight and improve insulation properties of standard building materials. The agriculture by-products are often added during production processes of concrete e.g. [7,8] and wood-based panels e.g. [9]. In the last decade the global production of maize grain has increased by 40%. The current world production amounts to 1030 million tons. In 2016 the total production of maize grain in the EU was 60.3 million tons [10]. The production of maize grain results in crop residues, such as stalks, leaves, cobs and husks, which make up 47–50% of the yield of dry mass of maize plants. Due to the scale of the global production of corn the residues are the important, potential raw material for insulation panels production. The structure of corn stover, especially its ‘‘spongy” stalk pith, is one of the reasons to use this feedstock as a natural insulating material. This resulted in a number of preliminary studies investigating thermal properties of the materials produced from corn stover. The obtained results have shown that stover can be used
710
Ł. Czajkowski et al. / Construction and Building Materials 210 (2019) 709–712
in the production of insulating panels [11–13]. However, corn stover is the only by-product in cereal production characterized by an inhomogeneous structure consisting of stalks, leaves, husks and cobs. These fractions have different chemical composition, tissue structure and fiber properties [14,15]. Previous studies primarily focused on selected and separated fractions of corn stover. The inhomogeneous structure and complex chemical composition of individual fractions of corn stover needs comprehensive research on the properties of each fraction and their applications for building materials. In addition the application of corn stover as insulating materials requires the knowledge on its primary thermal properties, e.g., specific heat and thermal conductivity. These properties are required for proper modeling and description of heat transfer e.g. [16]. Moreover, the thermal properties and density of the materials are often used to determine thermal diffusivity and volumetric specific heat capacity. Thermal conductivity is the property most often determined for corn stover materials [17,18]. However, there is no data on specific heat for individual fractions, which is crucial for the evaluation of the transient heat transfer [19]. The specific heat of various by-products, including corn stover, was determined by Dupont et al. [20]. However, there is no information on fractions of stover which have been tested. These studies concern experiments in which the investigated materials were grounded in order to homogenize the samples. The grinding operation caused changes in the structure of the materials and air content in the samples. This resulted in unwanted alteration of the properties. The thermal properties are mainly determined for boards made of mixed different fractions of corn stover. The parameters of the production process (temperature and pressure) as well as type of resin and resin load significantly influence thermal properties. Sampathrajan et al. [21] determined the specific heat of low density particleboard made of farm residues. The results obtained for husks and cobs panels as compared to currently produced natural insulating materials (coir, jute, kenaf, wood fiber) were characterized by low specific heat [3]. It could disqualify husks and cobs as potential insulating materials. The porous structure of the corn stalk pith makes it particularly applicable as insulating material. Etuk et al. [22] determined the thermal properties of panels produced separately from whole corn stalks and internal stalk pith. The measured values of specific heat as obtained for the two types of panels were similar. However, the panels which were made of corn stalk were characterized by two times higher values of thermal conductivity as compared to the panel made of pith stalks only. These results clearly indicated the need for selective harvesting and treatment of corn stover as well as the appropriate selection of corn stover fractions e.g. [23].
The objective of the present study was to determine the density, specific heat and volumetric specific heat capacity of individual fractions of corn stover. The separation of the fractions was intended to make measurements providing more reliable data on the thermal property and identifying the most suitable fractions of corn stover for insulating panels production. 2. Materials and methods 2.1. Materials The experimental material was obtained from a farm in which corn was cultivated for grain production (FAO 240). The farm was located in the western part of the Wielkopolska region (Poland). Mature plants have been separated into individual fractions, i.e. stalks, leaves (stalk leaves), husks (cob cover leaves) and cobs without grain. Stalks were cut into parts with 140 mm length, and not divided into the internal (stalk pith) and external (stalk outer layer) fragments. All four fractions were oven dried to the constant mass. The measurements were made at the oven dry conditions in order to avoid an influence of moisture content on the measured specific heat values. 2.2. Experimental procedure The oven dry material was placed in a heat shrinkable membrane to prepare samples of a cylindrical shape with length of ca. 135 mm and a diameter of ca. 75 mm (Fig. 1). The samples were made for each separated fraction and their natural structure was preserved within the samples. The space of voids filled with air was minimized inside the samples. The temperature distribution in each sample was controlled by two type K thermocouples mounted in the center and at the surface. The measurements of specific heat were carried out with a water calorimeter specially designed for bio-materials of low density. The equilibrium temperature of the calorimetric system was determined with the type J thermocouple of the uncertainty of 0.02 K. The application of the type J thermocouple was crucial for minimizing the errors of specific heat measurements. It was already demonstrated that the measurements of the equilibrium temperature of the calorimetric system with the type K thermocouple resulted in five times higher values of the relative error [24]. After the specific heat measurements the thermocouples were dismounted and the samples were again covered with heat shrinkable membrane in order to determine bulk density. The samples were inserted in a calibrated cylinder being filled with water and the volume of each sample was determined by water displacement. The mass and volume of heat shrinkable membrane were
Fig. 1. Tested samples: a) cobs, b) stalks, c) leaves, d) husks.
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determined separately. The bulk density of selected corn stover fractions was calculated as the ratio of mass and volume. The volumetric specific heat capacity (cq) was determined as the product of the specific heat and density e.g. [25]. 2.3. Statistical analysis Results of the specific heat measurements were analyzed using the one-way ANOVA variance analysis, while the significant differences between specific heat values for separated corn stover fractions were determined using the Tukey HDS test for a = 0.05. The absolute error of the specific heat determination was calculated using the total differential method [26]. It was made in order to estimate uncertainties of individual measurements e.g. mass values of sample, water, aluminum calorimeter inner cup with stirring rod, shrinkable membrane, as well initial temperature of a sample and calorimeter with water and equilibrium temperature of the calorimetric system. Moreover, the relative error of measurements was determined to account for the uncertainty of specific heat measurements. The detailed description of the applied method and heat balance of the calorimetric system was presented by Czajkowski et al. [24]. 3. Results and discussion In order to ensure the reproducibility of the measurements six replicates have been performed for each tested fraction of corn stover. The average values of the data used in the heat balance equation, i.e. sample mass, sample initial temperature, equilibrium temperature of the calorimetric system, temperature increase of the calorimetric system are presented in Table 1. The data were used to calculate the specific heat of the examined corn stover fractions in accordance with the heat balance equation of the calorimetric system presented in details by Czajkowski et al. [24]. The average values of the specific heat as well as the absolute and relative error of the measurements for cobs, stalks, leaves and husks are also given in Table 1. Despite of inhomogeneity of the investigated fractions similar mean values of the specific heat were obtained for stalks and leaves 1637 and 1644 J/(kgK). The differences in the specific heat values were negligible for a = 0.05. Therefore, these two fractions can be mixed during production processes of insulation materials, i.e. there is no need to separate these two fractions during harvesting. The highest mean value of specific heat as equal to 1723 J/(kgK) was observed for husks, while the lowest one of 1484 J/(kgK) for cobs. High differences in the specific heat values for husks and cobs would cause different insulating properties of the materials produced from these separated fractions. The obtained results clearly showed that the specific heat has to be determined for the separated fractions. The estimated errors of the specific heat measurements truly depicted that the uncertainty of specific heat measurements increases with a decrease of mass of tested samples. It confirms
the requirement for making measurements with samples of high mass. The highest values of the relative and absolute error i.e. 6.48% and 106.11 J/(kgK) respectively, were obtained for the stalks. This can be explained by natural light and porous structure of stalks which resulted in the lowest mass of the sample. For the other fractions, the relative error did not exceed 5% and it confirmed the reliability of the method to determining the specific heat of by-products. The obtained results of the specific heat being higher than 1400 J/(kgK) confirm the usefulness of such fraction of corn stover for the production of insulating materials e.g. Asdrubali et al. [19]. A comparison of the determined values of the specific heat with the data given for standard synthetic insulation materials (e.g. expanded polystyrene or fiber glass) shows similar or higher values as obtained for all fractions of corn stover [27,28]. The measured values of density, specific heat and volumetric specific heat capacity of the corn stover fractions are presented in Table 2. The similar values of density as well as specific heat were obtained for stalks and leaves confirm that these two fractions can be mixed for insulating materials production. In spite of the lowest value of the specific heat determined for cobs the highest volumetric specific heat capacity was obtained for this fraction. All corn stover fractions are characterized by high volumetric specific heat capacity and their usefulness depends on end-use. High values of volumetric specific heat capacity of corn stover fraction confirm its heat accumulation capabilities, which can be used in energy efficient building solutions. The determined values were also compared to the available data on specific heat of corn stover or corn stover materials reported in the literature. Dupont et al. [20] determined the effect of temperature on the values of specific heat of absolutely dry corn stover. Moreover, the measurements were carried out for ground material which caused alteration in its structure and porosity as compared to native stover. The specific heat of 1610 J/(kgK) was measured at the temperature of 353 K. The obtained value was similar only to the results acquired in the present study for stalks and leaves (Table 1). However, there was no information on fractions being used in the research. The majority of the reported data on specific heat usually concern already manufactured insulating panels. Etuk et al. [22] determined specific heat for panels being made of separated fractions, i.e. whole stalks and stalk piths and obtained values of 1870 and 1979 J/(kgK), respectively. The measured results for panels made of whole stalks are ca. 300 J/(kgK) higher as compared to the values for stalk fraction presented in Table 1. The observed differences can be explained by different moisture content values of the tested materials. Thermal properties of particle boards made of selected corn stover fractions were also determined by Sampathrajan et al. [21]. The boards were produced from husk and cobs and the specific heat was equal to 847 J/(kgK) and 879 J/(kgK), respectively. The values presented in Table 1 are twice as high as specific heat obtained for the particle panels, despite the fact that the boards were characterized by moisture content ranging from 11 to 14%.
Table 1 The measured values of the heat balance equation components with calculated specific heat together with the absolute and relative error of measurements. Corn stover fraction
Cobs Stalks Leaves Husks
kg
Initial temperature of sample; tic °C
Equilibrium temperature of the calorimetric system; te °C
Increase of temperature; DT K
Specific heat; c J/(kgK)
Absolute error; Dc J/(kgK)
Relative error; % %
0.1512 0.0930 0.1262 0.1401
92.58 91.86 91.76 92.32
14.34 13.12 14.34 13.20
1.10 0.74 1.00 1.16
1484 ± 11a 1637 ± 18b 1644 ± 13b 1723 ± 4c
62.79 106.11 78.72 70.95
4.23 6.48 4.79 4.11
Sample mass; m
Note: all presented results are mean values of 6 replicates, mean value of specific heat are supplemented by standard deviation, identical letters (a, b, c) indicated no significant differences (P < 0.05) between mean values of specific heat according to Tuckey’s HDS test (ANOVA).
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Table 2 Thermal properties of corn stover fractions. Corn stover fraction
Cobs Stalks Leaves Husks
References
q
Specific heat; c
kg/m3
J/(kgK)
Volumetric specific heat capacity; cq MJ/(m3K)
257.5 148.3 173.4 206.5
1484 1637 1644 1723
0.382 0.243 0.285 0.356
Density;
The data on specific heat of corn stover as well as corn stover panels are ambiguous due to the differences reported in the literature. Discrepancies between the results reported in different sources make the data uncertain for heat transfer modelling. The determined values of the volumetric specific heat capacity for corn stover are higher than values obtained for polystyrene and glass fiber but lower than the values for wood [29]. The volumetric specific heat capacity values are similar to cork. This confirms the usefulness of all corn stover fractions for heat accumulation in building envelopes. Ahn et al. [30] determined volumetric specific heat capacity with differential scanning calorimeter (DSC) for dry corn stalks and obtained the values ranging from 0.05 to 0.60 MJ/(m3K). It suggests high variation of the results reported by Ahn et al. [30]. It was probably due to low accuracy of the applied DSC method. The results presented in this paper are contained in much smaller range from 0.243 to 0.382 MJ/ (m3K). It suggests that the applied method is more appropriate for characterized thermal properties of hygroscopic natural materials with low density.
4. Conclusions The obtained values of specific heat and volumetric specific heat capacity confirmed that the thermal properties of corn stover fractions should be determined separately. Only leaves and stalks were characterized by similar values of specific heat and volumetric specific heat capacity. Thus, these two fractions can be mixed during production processes of insulation materials. The observed high values of volumetric specific heat capacity determined for all corn stover fractions confirmed their ability to accumulate heat. Therefore, the fractions can be used for production of insulating materials being used for low-energy efficient building and passive houses. The measurement method applied in the present study ensured high accuracy of specific heat determination i.e. the relative error did not exceed 5% for cobs, leaves and husks. The accuracy was only slightly worse for the fraction of the lowest density (lowest mass of sample) i.e. the stalks. It clearly shows that the measurements of the specific heat has to be supplemented by the error analysis and mass of samples should be carefully estimated.
Conflict of interest None.
Acknowledgement The paper was financed within the framework of Ministry of Science and Higher Education programme ‘Regional Initiative of Excellence’ in years 2019-2022, Project No. 005/RID/2018/19.
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