This article was downloaded by: [University Of South Australia Library] On: 11 August 2014, At: 13:24 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Factors affecting mechanical properties of biomass pellet from compost a
A. Zafari & M.H. Kianmehr
a
a
Department of Agrotechnology, College of Abouraihan, University of Tehran, Tehran, Iran Published online: 23 Sep 2013.
To cite this article: A. Zafari & M.H. Kianmehr (2014) Factors affecting mechanical properties of biomass pellet from compost, Environmental Technology, 35:4, 478-486, DOI: 10.1080/09593330.2013.833639 To link to this article: http://dx.doi.org/10.1080/09593330.2013.833639
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2014 Vol. 35, No. 4, 478–486, http://dx.doi.org/10.1080/09593330.2013.833639
Factors affecting mechanical properties of biomass pellet from compost A. Zafari∗ and M.H. Kianmehr Department of Agrotechnology, College of Abouraihan, University of Tehran, Tehran, Iran
Downloaded by [University Of South Australia Library] at 13:24 11 August 2014
(Received 28 December 2012; accepted 23 July 2013 ) Effectiveness of a densification process to create strong and durable bonding in pellets can be determined by testing the mechanical properties such as compressive strength (CS) and durability. Mechanical properties of pellet from composted municipal solid waste were determined at different raw material and densification conditions. Ground compost samples were compressed with three levels of moisture content (35%, 40% and 45% (wb)), piston compaction speed (2, 6 and 10 mm/s), die length (8, 10 and 12 mm) and raw material particle size (0.3, 0.9 and 1.5 mm) into cylindrical pellets utilizing opened-end dies under axial stress from a vertical piston applied by a hydraulic press. The effects of independent variables on mechanical properties were determined using response surface methodology based on Box-Behnken design (BBD). All independent variables affected the durability significantly. However, different piston speed and die length not produce any significant difference on CS of pellets. Also in this research the electron photography method was used to identify the binding mechanism of compost particles. Keywords: compost; pellet; durability; municipal solid waste; response surface methodology
1. Introduction Municipal solid waste (MSW) is produced in great quantity in Iran (about 16 million ton per year) and its management has become a challenge, both economically and environmentally. More than 50% of the MSW generated by the Iran population is organic waste that can be converted into compost.[1] Composting MSW is considered as a method of transferring organic waste materials from landfills to a product, which is suitable for agricultural purposes at relatively lowcost.[2,3] Composting of MSW has the potential to become a beneficial recycling tool for waste management in Iran. One of the major barriers against the use of composts is their handling, application, and storage due to low density. Therefore, these bulky residues need to be densified. A variety of densification systems are considered for producing a uniform format feedstock including (i) pellet mill, (ii) cuber, (iii) screw extruder, (iv) briquette press, (v) roller press, (vi) tablet press, and (vii) agglomerator.[4] Pelletizing is a method of increasing the bulk density of biomass by mechanical pressure. Pellets have low moisture content (MC) (about 12% wet basis (wb)) for safe storage, and a high bulk density (more than 1000 kg/m3 ) for efficient transport and storage. Some of other the benefits of compost pellets are [1] (1) Reducing the conservation space because of densification. ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
(2) Suitable for mechanization and compatible with farmer’s implements for implanting or scattering. (3) Suitable for residential places because of producing no dust and no pollution for environment. (4) More precision with spreaders and reducing manure consumption. (5) Suitable for transporting to long distances. (6) Suitable for planters and no needing to separate operation. (7) Ability of long time conservation. (8) Ability of adding chemical materials for increasing the quality of pellets. There are two kinds of moulding machine available on the market which shape composted into pellets.[5] One kind is the diskpelleter (dry moulding method), and the other is the extruder type (wet moulding method). Moulding machines which are of the diskpelleter type are of three kinds: the roller disk die type, the roller ring die type, and the double die type. Each type has a basic structure of a disk with many holes and a roller or two disks. The compost is fed between the disks and/or roller, and as the disk and/or roller turns, the compost is forced into the holes, producing the pellets. This method is suitable for raw material which has a comparatively low MC of 20–30%. Moulding machines of the extruder type have a barrel into which the raw material is forced by a screw. The material is then compressed into the die installed at the end of the machine, producing the
Downloaded by [University Of South Australia Library] at 13:24 11 August 2014
Environmental Technology pellet. Wet method is suitable for raw material with high MC about 30–50%. For success of the pelletization process, the quality of the densified products must meet the consumer requirements and market standards. Therefore, testing the pellet to estimate the amount of damage that could be observed at the point of utilization in term of strength and durability would help optimize the feed material, pre-conditioning processes, and densification equipment to produce high-quality pellets. The effectiveness of the inter-particle bonds created during the densification process has been measured in terms of Strength. Compression strength (or crushing resistance or hardness) is the maximum crushing load a pellet can withstand before cracking or breaking. The tensile strength of the densified products may be determined using the diametrical compression test. The products may fail (e.g. fracture of a cylindrical pellet into two semi-cylinders) due to the tensile forces resulting from the applied compressive force or stress. The tensile strength is related to the adhesion forces between particles at all contact points in the agglomerate.[6,7] Diametrical compression test is predominantly used for testing tablets in the pharmaceutical industry.[7] Compression strength test simulates the compressive stress due to weight of the top pellets on the lower pellets during storage in bins or silos, crushing of pellets in a screw conveyor. Durability test simulates either mechanical or pneumatic handling. These tests can help control the densification process and, thus, pellet quality in the feed manufacturing industry. In the feed industry, high durability means high-quality pellets. Tumbling can method is used to estimate the pellet quality in terms of pellet durability index (PDI), or, simply percent durability. This test simulates the mechanical handling of pellets and predicts the possible fines produced due to mechanical handling. The physical quality of pellet may vary depending on the raw material properties and the densification process. These variables can be controlled to optimize production efficiency and improve the quality of finished product. So far, various authors analysed the effect of process parameters and biomass properties on the mechanical quality of pellet produced with biomass material. Tabil and Sokhansanj [8] studied the effect of process parameters, such as steam conditioning, die geometry, L/D ratio, die speed, and particle sizes of the biomass, on quality of pellets. Tabil and Sokhansanj [9] evaluated the bulk properties of alfalfa in relation to its compaction characteristics. They reported that pellets from high-quality alfalfa chops were more compressible (higher density) than pellets from low-quality chops. Mani et al. [10] and Samson et al. [11] reviewed the biomass pelleting process and the effect of various process parameters on pellet density and durability. Mani et al. [12] studied the effects of compressive force, particle size, and MC on mechanical properties of biomass pellets from grasses. Kalyan and Morey [13] reviewed the
479
factors that affecting strength and durability of biomass pellets. Tumuluru et al. [14] studied the effect of process conditions, such as die temperature and MC of raw material, on the pellet properties. Theerarattananoon et al. [15] evaluated the physical properties of pellet made from sorghum stalk, corn stover, wheat straw and big bluestem under different raw materials and process condition. The objective of this study was to investigate the effects of MC, speed of piston, length of die and particles size on compression strength and durability of pellets from composted MSW. In order to focus on the pelleting process, pellets were produced under controlled conditions in laboratory scale.
2. Materials and experimental protocols 2.1. Sample preparation Compost was taken from composting factory located at Tehran. The compost was produced from organic matter or biodegradable waste, such as urban waste, animal manure and crop residues using turned windrow composting method. Turned windrow composting is the production of compost in windrows using mechanical aeration that represents a low technology and medium labour approach and produces uniform compost. A chemical analysis of the compost was conducted by chemical analytical laboratory (University of Tehran). The amount of N , P and K in compost was 2.21%, 0.1% and 1.02%, respectively. Also the pH of compost was 8.59. Initial MC of compost was determined using the oven method at 103◦ C for 24 h.[16] The initial MC of compost was 12 ± 1% (wb). There is an optimal range of particle size to obtain granules with acceptable quality. The results of pre-test indicate that sample with particle size larger than 1.5 mm have not ability to produce pellets with acceptable quality because compost samples were ground using a hammer mill with three different hammer mill screen sizes (0.3, 0.9 and 1.5 mm) in order to understand the influence of particle size on mechanical properties of pellets. The samples were wetted by sprinkling water on them to MCs of 35%, 40% and 45% (wb) and stored in a cooler kept at 4◦ C for a minimum of 72 h.
2.2. Experimental protocols 2.2.1. Particle size analysis A compost sample of 100 g was placed in a stack of sieves arranged from the largest to the smallest opening. The sieve series selected were based on the range of particles in the sample. The American series sieve numbers 6, 8, 10, 12, 16, 20, 30, 40, 50, 70 and 100 (sieve sizes: 3.36, 2.38, 2.0, 1.7, 1.2, 0.85, 0.59, 0.43, 0.30, 0.21 and 0.15 mm, respectively) were used. The set of sieves was placed on the sieve shaker. The duration of sieving was 10 min, which was previously
Downloaded by [University Of South Australia Library] at 13:24 11 August 2014
480
Figure 1.
A. Zafari and M.H. Kianmehr
The laboratory equipment used for producing pellets (all dimensions in mm).
determined through trials to be optimal. After sieving, the mass retained on each sieve was weighed. Sieve analysis was repeated three times for each ground sample. The particle size was determined according to ANSI/ASAE standard S319.3 JUL 97.[17] The geometric mean diameter (dgw ) of the sample and geometric standard deviation of particle diameter [Sgw (mm)] were calculated according to the aforementioned standard [Equations (1) and (2)]. (Wi log di ) dgw = log−1 , (1) Wi Wi (log di − log dgw ) 0.5 Sgw = log−1 . (2) Wi There is an optimal range of particle size to obtain granules with acceptable quality. The results of pre-test indicate that sample with particle size larger than 1.5 mm have not ability to produce pellets with acceptable quality because compost samples were ground using a hammer mill with three different hammer mill screen sizes (0.3, 0.9 and 1.5 mm) in order to understand the influence of particle size on mechanical properties of pellets. 2.2.2. Pellet production A hydraulic press and a single pelleter (Figure 1) were used to produce pellet. The pelleter’s cylinder had a precompression section with internal diameter of 18 mm and a length of 200 mm. The dies, placed at the end of cylinder, had a 6 mm hole diameter and different lengths. The die in pelleter is moveable, three dies with 8, 10 and 12 mm length were made and in each test the desired die was replaced. A hydraulic press was used to move plunger. Pressure control, piston speed control and residence time control are important features in this press. The hydraulic press was adapted
with a data recording system for displacement, force and time. Typical pelleting systems include multiple extrusion holes arranged on a ring die. The laboratory die is trying to simulate the extrusion by using only one hole (6 mm) in real scale. The pellets were produced with piston pressure to materials against the die in three different piston speeds (2, 6 and 10 mm/s). The length of die (8, 10 and 12 mm) and piston speed are parameters that determine the residence time. For example in 2 mm/s piston speed the residence time are 4, 5 and 6 s for 8, 10 and 12 mm dies, respectively. The resistance to flow of material is a parameter that control the load applied. In this research, the pellets were produced using a single pelleter and pressure was between 60–70 Mpa. Constant value of compost, about 20 g, was fed to pelleter in each test. The produced pellets were dried at ambient temperature (about 25◦ C and 50% RH) without using dryer and their MC reached to about 12% after about a day. 2.2.3. CS of pellet CS of pellets is determined by diametrical compression test. A pneumatic press used to measuring the CS of pellets (Figure 2). A pellet was placed between two horizontal plates and was compressed radially.[18] A data logger was used for registering of force (F). Force was registered with a data logger at a sampling rate of 1000 readings min−1 . An increasing load is applied at a constant rate, until the test specimen fails by cracking or braking (Figure 3). To reduce the effects on CS due to varying pellet length (L), pellets with equal length were used. Pellet strength values are reported as an average of five measurements. 2.2.4. Durability of pellet Tumbling can method was used to estimate the pellet quality in terms of PDI, or, simply percent durability (Figure 4).[19]
Environmental Technology
Downloaded by [University Of South Australia Library] at 13:24 11 August 2014
Figure 2. The laboratory equipment and operating principle of pellets strength test.
481
2.2.5. Electron photography In order to identify the binding mechanism of the particles of compost, the electron photography method with electron microscope (XL30 model of scanning electron microscopy (SEM) electron microscope, Germany) was used. Electron photography carried out after drying the pellets. A tiny notch was cut in centre of the pellet using a razor blade, and the pellets snapped into two. Care was taken to look at the surface a little distance from the end of the notch. The two parts of the fractured pellet were attached to metal stubs using conductive carbon paste (Leit-C, Neubauer Chemikalien, Germany) that was carefully applied below and on all sites of the sample to prevent electric charging of the specimen. The upper surface was coated with a thin layer of palladium and gold using a sputter coater (SC7680, Polaron, UK). Electron micrographs were recorded using a SEM operated at 12.5–20 kV. Multiple samples were observed for each type of pellet.[20] 2.2.6. Experimental design Response surface methodology based on BBD with four variables was used to study the responses pattern and to understand the influence of MC, speed of piston, die length and particle size on strength and durability of pellets. The experimental design was developed using Design Expert 8.0.7.1, which resulted in 29 tests. The level values of each variable and code investigated in this study are presented
Figure 3.
Example of force changes during CS test of pellets. Table 1. ables.
During tumbling, pellets abrade and produce fines due to impact, and shearing of pellets over each other and over the wall of the tumbling can. After tumbling 500 g of pellets for 10 min at 50 rpm, the pellets are sieved using a sieve size of about 0.8 times the pellet diameter. The PDI or durability is calculated as the ratio of weight after tumbling over the weight before tumbling, multiplied by 100. A detailed procedure can be found at ASABE Standards.[16]
Figure 4.
Experimental range and level of independent variCoded level and range
Independent variables MC (%) Speed of piston, (mm/s) Length of die (mm) Particles size (mm)
Apparatus used to durability test according to ASABE Standard.
Symbol
−1
0
1
X1 X2 X3 X4
35 2 8 0.3
40 6 10 0.9
45 10 12 1.5
482
A. Zafari and M.H. Kianmehr
in Table 1. A second-order quadratic equation was used to describe the effect of independent variables in terms of linear, quadratic and interactions. The proposed model for the response was y = β0 +
m
βi X i +
Downloaded by [University Of South Australia Library] at 13:24 11 August 2014
i=1
m
βij Xi Xj +
i
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Factors affecting mechanical properties of biomass pellet from compost a
A. Zafari & M.H. Kianmehr
a
a
Department of Agrotechnology, College of Abouraihan, University of Tehran, Tehran, Iran Published online: 23 Sep 2013.
To cite this article: A. Zafari & M.H. Kianmehr (2014) Factors affecting mechanical properties of biomass pellet from compost, Environmental Technology, 35:4, 478-486, DOI: 10.1080/09593330.2013.833639 To link to this article: http://dx.doi.org/10.1080/09593330.2013.833639
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2014 Vol. 35, No. 4, 478–486, http://dx.doi.org/10.1080/09593330.2013.833639
Factors affecting mechanical properties of biomass pellet from compost A. Zafari∗ and M.H. Kianmehr Department of Agrotechnology, College of Abouraihan, University of Tehran, Tehran, Iran
Downloaded by [University Of South Australia Library] at 13:24 11 August 2014
(Received 28 December 2012; accepted 23 July 2013 ) Effectiveness of a densification process to create strong and durable bonding in pellets can be determined by testing the mechanical properties such as compressive strength (CS) and durability. Mechanical properties of pellet from composted municipal solid waste were determined at different raw material and densification conditions. Ground compost samples were compressed with three levels of moisture content (35%, 40% and 45% (wb)), piston compaction speed (2, 6 and 10 mm/s), die length (8, 10 and 12 mm) and raw material particle size (0.3, 0.9 and 1.5 mm) into cylindrical pellets utilizing opened-end dies under axial stress from a vertical piston applied by a hydraulic press. The effects of independent variables on mechanical properties were determined using response surface methodology based on Box-Behnken design (BBD). All independent variables affected the durability significantly. However, different piston speed and die length not produce any significant difference on CS of pellets. Also in this research the electron photography method was used to identify the binding mechanism of compost particles. Keywords: compost; pellet; durability; municipal solid waste; response surface methodology
1. Introduction Municipal solid waste (MSW) is produced in great quantity in Iran (about 16 million ton per year) and its management has become a challenge, both economically and environmentally. More than 50% of the MSW generated by the Iran population is organic waste that can be converted into compost.[1] Composting MSW is considered as a method of transferring organic waste materials from landfills to a product, which is suitable for agricultural purposes at relatively lowcost.[2,3] Composting of MSW has the potential to become a beneficial recycling tool for waste management in Iran. One of the major barriers against the use of composts is their handling, application, and storage due to low density. Therefore, these bulky residues need to be densified. A variety of densification systems are considered for producing a uniform format feedstock including (i) pellet mill, (ii) cuber, (iii) screw extruder, (iv) briquette press, (v) roller press, (vi) tablet press, and (vii) agglomerator.[4] Pelletizing is a method of increasing the bulk density of biomass by mechanical pressure. Pellets have low moisture content (MC) (about 12% wet basis (wb)) for safe storage, and a high bulk density (more than 1000 kg/m3 ) for efficient transport and storage. Some of other the benefits of compost pellets are [1] (1) Reducing the conservation space because of densification. ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
(2) Suitable for mechanization and compatible with farmer’s implements for implanting or scattering. (3) Suitable for residential places because of producing no dust and no pollution for environment. (4) More precision with spreaders and reducing manure consumption. (5) Suitable for transporting to long distances. (6) Suitable for planters and no needing to separate operation. (7) Ability of long time conservation. (8) Ability of adding chemical materials for increasing the quality of pellets. There are two kinds of moulding machine available on the market which shape composted into pellets.[5] One kind is the diskpelleter (dry moulding method), and the other is the extruder type (wet moulding method). Moulding machines which are of the diskpelleter type are of three kinds: the roller disk die type, the roller ring die type, and the double die type. Each type has a basic structure of a disk with many holes and a roller or two disks. The compost is fed between the disks and/or roller, and as the disk and/or roller turns, the compost is forced into the holes, producing the pellets. This method is suitable for raw material which has a comparatively low MC of 20–30%. Moulding machines of the extruder type have a barrel into which the raw material is forced by a screw. The material is then compressed into the die installed at the end of the machine, producing the
Downloaded by [University Of South Australia Library] at 13:24 11 August 2014
Environmental Technology pellet. Wet method is suitable for raw material with high MC about 30–50%. For success of the pelletization process, the quality of the densified products must meet the consumer requirements and market standards. Therefore, testing the pellet to estimate the amount of damage that could be observed at the point of utilization in term of strength and durability would help optimize the feed material, pre-conditioning processes, and densification equipment to produce high-quality pellets. The effectiveness of the inter-particle bonds created during the densification process has been measured in terms of Strength. Compression strength (or crushing resistance or hardness) is the maximum crushing load a pellet can withstand before cracking or breaking. The tensile strength of the densified products may be determined using the diametrical compression test. The products may fail (e.g. fracture of a cylindrical pellet into two semi-cylinders) due to the tensile forces resulting from the applied compressive force or stress. The tensile strength is related to the adhesion forces between particles at all contact points in the agglomerate.[6,7] Diametrical compression test is predominantly used for testing tablets in the pharmaceutical industry.[7] Compression strength test simulates the compressive stress due to weight of the top pellets on the lower pellets during storage in bins or silos, crushing of pellets in a screw conveyor. Durability test simulates either mechanical or pneumatic handling. These tests can help control the densification process and, thus, pellet quality in the feed manufacturing industry. In the feed industry, high durability means high-quality pellets. Tumbling can method is used to estimate the pellet quality in terms of pellet durability index (PDI), or, simply percent durability. This test simulates the mechanical handling of pellets and predicts the possible fines produced due to mechanical handling. The physical quality of pellet may vary depending on the raw material properties and the densification process. These variables can be controlled to optimize production efficiency and improve the quality of finished product. So far, various authors analysed the effect of process parameters and biomass properties on the mechanical quality of pellet produced with biomass material. Tabil and Sokhansanj [8] studied the effect of process parameters, such as steam conditioning, die geometry, L/D ratio, die speed, and particle sizes of the biomass, on quality of pellets. Tabil and Sokhansanj [9] evaluated the bulk properties of alfalfa in relation to its compaction characteristics. They reported that pellets from high-quality alfalfa chops were more compressible (higher density) than pellets from low-quality chops. Mani et al. [10] and Samson et al. [11] reviewed the biomass pelleting process and the effect of various process parameters on pellet density and durability. Mani et al. [12] studied the effects of compressive force, particle size, and MC on mechanical properties of biomass pellets from grasses. Kalyan and Morey [13] reviewed the
479
factors that affecting strength and durability of biomass pellets. Tumuluru et al. [14] studied the effect of process conditions, such as die temperature and MC of raw material, on the pellet properties. Theerarattananoon et al. [15] evaluated the physical properties of pellet made from sorghum stalk, corn stover, wheat straw and big bluestem under different raw materials and process condition. The objective of this study was to investigate the effects of MC, speed of piston, length of die and particles size on compression strength and durability of pellets from composted MSW. In order to focus on the pelleting process, pellets were produced under controlled conditions in laboratory scale.
2. Materials and experimental protocols 2.1. Sample preparation Compost was taken from composting factory located at Tehran. The compost was produced from organic matter or biodegradable waste, such as urban waste, animal manure and crop residues using turned windrow composting method. Turned windrow composting is the production of compost in windrows using mechanical aeration that represents a low technology and medium labour approach and produces uniform compost. A chemical analysis of the compost was conducted by chemical analytical laboratory (University of Tehran). The amount of N , P and K in compost was 2.21%, 0.1% and 1.02%, respectively. Also the pH of compost was 8.59. Initial MC of compost was determined using the oven method at 103◦ C for 24 h.[16] The initial MC of compost was 12 ± 1% (wb). There is an optimal range of particle size to obtain granules with acceptable quality. The results of pre-test indicate that sample with particle size larger than 1.5 mm have not ability to produce pellets with acceptable quality because compost samples were ground using a hammer mill with three different hammer mill screen sizes (0.3, 0.9 and 1.5 mm) in order to understand the influence of particle size on mechanical properties of pellets. The samples were wetted by sprinkling water on them to MCs of 35%, 40% and 45% (wb) and stored in a cooler kept at 4◦ C for a minimum of 72 h.
2.2. Experimental protocols 2.2.1. Particle size analysis A compost sample of 100 g was placed in a stack of sieves arranged from the largest to the smallest opening. The sieve series selected were based on the range of particles in the sample. The American series sieve numbers 6, 8, 10, 12, 16, 20, 30, 40, 50, 70 and 100 (sieve sizes: 3.36, 2.38, 2.0, 1.7, 1.2, 0.85, 0.59, 0.43, 0.30, 0.21 and 0.15 mm, respectively) were used. The set of sieves was placed on the sieve shaker. The duration of sieving was 10 min, which was previously
Downloaded by [University Of South Australia Library] at 13:24 11 August 2014
480
Figure 1.
A. Zafari and M.H. Kianmehr
The laboratory equipment used for producing pellets (all dimensions in mm).
determined through trials to be optimal. After sieving, the mass retained on each sieve was weighed. Sieve analysis was repeated three times for each ground sample. The particle size was determined according to ANSI/ASAE standard S319.3 JUL 97.[17] The geometric mean diameter (dgw ) of the sample and geometric standard deviation of particle diameter [Sgw (mm)] were calculated according to the aforementioned standard [Equations (1) and (2)]. (Wi log di ) dgw = log−1 , (1) Wi Wi (log di − log dgw ) 0.5 Sgw = log−1 . (2) Wi There is an optimal range of particle size to obtain granules with acceptable quality. The results of pre-test indicate that sample with particle size larger than 1.5 mm have not ability to produce pellets with acceptable quality because compost samples were ground using a hammer mill with three different hammer mill screen sizes (0.3, 0.9 and 1.5 mm) in order to understand the influence of particle size on mechanical properties of pellets. 2.2.2. Pellet production A hydraulic press and a single pelleter (Figure 1) were used to produce pellet. The pelleter’s cylinder had a precompression section with internal diameter of 18 mm and a length of 200 mm. The dies, placed at the end of cylinder, had a 6 mm hole diameter and different lengths. The die in pelleter is moveable, three dies with 8, 10 and 12 mm length were made and in each test the desired die was replaced. A hydraulic press was used to move plunger. Pressure control, piston speed control and residence time control are important features in this press. The hydraulic press was adapted
with a data recording system for displacement, force and time. Typical pelleting systems include multiple extrusion holes arranged on a ring die. The laboratory die is trying to simulate the extrusion by using only one hole (6 mm) in real scale. The pellets were produced with piston pressure to materials against the die in three different piston speeds (2, 6 and 10 mm/s). The length of die (8, 10 and 12 mm) and piston speed are parameters that determine the residence time. For example in 2 mm/s piston speed the residence time are 4, 5 and 6 s for 8, 10 and 12 mm dies, respectively. The resistance to flow of material is a parameter that control the load applied. In this research, the pellets were produced using a single pelleter and pressure was between 60–70 Mpa. Constant value of compost, about 20 g, was fed to pelleter in each test. The produced pellets were dried at ambient temperature (about 25◦ C and 50% RH) without using dryer and their MC reached to about 12% after about a day. 2.2.3. CS of pellet CS of pellets is determined by diametrical compression test. A pneumatic press used to measuring the CS of pellets (Figure 2). A pellet was placed between two horizontal plates and was compressed radially.[18] A data logger was used for registering of force (F). Force was registered with a data logger at a sampling rate of 1000 readings min−1 . An increasing load is applied at a constant rate, until the test specimen fails by cracking or braking (Figure 3). To reduce the effects on CS due to varying pellet length (L), pellets with equal length were used. Pellet strength values are reported as an average of five measurements. 2.2.4. Durability of pellet Tumbling can method was used to estimate the pellet quality in terms of PDI, or, simply percent durability (Figure 4).[19]
Environmental Technology
Downloaded by [University Of South Australia Library] at 13:24 11 August 2014
Figure 2. The laboratory equipment and operating principle of pellets strength test.
481
2.2.5. Electron photography In order to identify the binding mechanism of the particles of compost, the electron photography method with electron microscope (XL30 model of scanning electron microscopy (SEM) electron microscope, Germany) was used. Electron photography carried out after drying the pellets. A tiny notch was cut in centre of the pellet using a razor blade, and the pellets snapped into two. Care was taken to look at the surface a little distance from the end of the notch. The two parts of the fractured pellet were attached to metal stubs using conductive carbon paste (Leit-C, Neubauer Chemikalien, Germany) that was carefully applied below and on all sites of the sample to prevent electric charging of the specimen. The upper surface was coated with a thin layer of palladium and gold using a sputter coater (SC7680, Polaron, UK). Electron micrographs were recorded using a SEM operated at 12.5–20 kV. Multiple samples were observed for each type of pellet.[20] 2.2.6. Experimental design Response surface methodology based on BBD with four variables was used to study the responses pattern and to understand the influence of MC, speed of piston, die length and particle size on strength and durability of pellets. The experimental design was developed using Design Expert 8.0.7.1, which resulted in 29 tests. The level values of each variable and code investigated in this study are presented
Figure 3.
Example of force changes during CS test of pellets. Table 1. ables.
During tumbling, pellets abrade and produce fines due to impact, and shearing of pellets over each other and over the wall of the tumbling can. After tumbling 500 g of pellets for 10 min at 50 rpm, the pellets are sieved using a sieve size of about 0.8 times the pellet diameter. The PDI or durability is calculated as the ratio of weight after tumbling over the weight before tumbling, multiplied by 100. A detailed procedure can be found at ASABE Standards.[16]
Figure 4.
Experimental range and level of independent variCoded level and range
Independent variables MC (%) Speed of piston, (mm/s) Length of die (mm) Particles size (mm)
Apparatus used to durability test according to ASABE Standard.
Symbol
−1
0
1
X1 X2 X3 X4
35 2 8 0.3
40 6 10 0.9
45 10 12 1.5
482
A. Zafari and M.H. Kianmehr
in Table 1. A second-order quadratic equation was used to describe the effect of independent variables in terms of linear, quadratic and interactions. The proposed model for the response was y = β0 +
m
βi X i +
Downloaded by [University Of South Australia Library] at 13:24 11 August 2014
i=1
m
βij Xi Xj +
i
