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An automatic shake mechanism for the biomass pyrolysis feeding system
Powder Technology 207 (2011) 348–352
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
An automatic shake mechanism for the biomass pyrolysis feeding system Jun Zhang ⁎, Zhen-wei Yuan, Xin-li Wei School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450002, China
a r t i c l e
i n f o
Article history: Received 28 April 2010 Received in revised form 29 October 2010 Accepted 16 November 2010 Available online 24 November 2010 Keywords: Biomass particles Screw feeder Automatic shake mechanism Power consumption
a b s t r a c t Successful feeding is critical to biomass utilization processes such as pyrolysis liquefaction or gasification. The biomass particles in hopper frequently become bridge or adhere to the wall to prevent the feeding system from providing the required uniform and continuous flow of feedstocks. To solve the problem, an automatic shake mechanism was devised which was set in the hopper and driven by the rotation of screw. Experiments were done using the automatic shake mechanism to feed three kinds of sawdust; the total torque requirement of the screw feeder with the automatic shake mechanism in the hopper was predicted and compared with experiment results. The results showed that the biomass particles could be delivered uniformly and the feeding rate had a significant increase with 7% extra power consumption. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Interest in producing liquid fuels from biomass is increasing worldwide for the reason that bio-oil is easy to transport and store. In practical processes, a critical problem is how to feed the biomass feedstocks into the reactors properly. Feeding problems often impede smooth operation in industries [1,2]. Such particle properties as mean size, size distribution, shape, surface roughness, density, moisture content, compressibility, and pliability can all affect the successive feeding. Biomass feeding devices frequently become blocked and do not provide the required uniform and continuous flow of the feedstocks. Furthermore, the reactor often operates at high pressure and/or high temperature. This causes additional challenges in establishing reliable feeding [3–6]. Several kinds of feeders and their combinations used in biomass utilization processes have been reported, such as lock hopper systems, screw feeders, rotary valves, piston feeders, pneumatic feeders, etc. [7,8]. With its simple structure, high efficiency, and easiness for industrialization, screw feeders have been tested for biomass feedstocks feeding. Roberts et al. [9–11] analyzed the volumetric characteristics and mechanics of screw feeders in relation to the bulk solid draw-down characteristics of the feed hopper. Carleton et al. [12] discussed the performance of screw conveyors and screw feeders based on experiments on the effects of screw geometry, speed, fill level and material properties. Bates [13] provided detailed analysis of mechanics and entrained patterns of screw feeders, especially those combined with hoppers. Rautenbach and Schumacher [14] derived a set of parameters by dimensional analysis to calculate the power
consumption and transport capacity and compared two geometrically similar screws. Dai and Grace [15,16] proposed a model to delineate what limits screw feeding in terms of the mechanisms of blockage and to predict torque requirements for biomass materials, and defined blockage mechanisms and plug seal with the aid of tapered and extended sections. The tapered and extended sections increased torque requirements significantly while improving the plug seal to the reactor. However, little information has been focused on the problem that biomass particles in hopper become bridge or adhere to the wall to prevent the screw feeding from providing the required uniform and continuous flow of the feedstock. In this paper, a screw feeder system is described, which is composed of a hopper and two screws. The first screw determines biomass feeding rate, while the second crew simply transports biomass to the reactor. For the unique properties of biomass particles such as low density, big resting-angle and poor mobility, it is easy to form bridge and adhere to the wall in the hopper, leading to instability or feeding breaks. An automatic shake mechanism was devised aiming at obtaining stability and continuity of feeding. Experiments were done using the automatic shake mechanism to feed three kinds of sawdust. In addition, the experimental results were analyzed and compared with the prediction model. The total torque requirement of the screw feeder with the automatic shake mechanism in the hopper was predicted and compared with experimental results. 2. The automatic shake mechanism description and experimental procedure 2.1. The automatic shake mechanism
⁎ Corresponding author. Tel.: +86 3 7167780114. E-mail address: [email protected] (J. Zhang). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.11.018
Shown in Fig. 1 is the feeding system, including a storage hopper, two feeding screws with their motors and frequency controllers. The
J. Zhang et al. / Powder Technology 207 (2011) 348–352
349
Fig. 3. Stick on the wall of storage bin. Fig. 1. Feeding system of biomass pyrolysis equipment.
first screw drills across the root of the biomass storage hopper. The screws are driven by their motors and the motors work under the frequency control. Biomass particles stored in the storage hopper pass the first and second screws in turn, and then flow into the fluidized bed reactor. The first screw determines biomass feeding rate, while the second screw simply transports biomass to the fluidized bed reactor. The storage hopper is composed of a cylinder and a cone, which has larger volumetric capacity than the wedge-shaped hopper. Feeding stability is critical to the biomass pyrolysis liquefaction processes. However, biomass particles in the hopper are easy to form bridge as shown in Fig. 2, or adhere to the wall as shown in Fig. 3. These phenomena could lead to instability or feeding breaks. Therefore, an automatic shake mechanism in the hopper is proposed, as shown in Fig. 4. The mechanism is driven by the rotation of the first screw to reciprocate in the hopper to prevent biomass particles from forming bridge or adhering to the wall and ensure biomass particles flowing into the first screw continuously. The automatic shake mechanism is made up of a spring bar and two astro-legs. Each astro-leg includes six tip balls, six legs, a sleeve and a fixing bolt. The spring bar and the legs are made of spring steel, while the tip balls are made of aluminum. One end of the spring bar is fixed on the cover of the hopper and the other end touches one flight of the first screw. The bar goes forward with the rotation of the first screw until it separates from the flight and rebounds to a position. The two astro-legs are fixed on the spring bar to enhance disturbing effects. The sleeve connected with six legs is fixed to the spring bar with a bolt. The reciprocating motions of the spring bar and two astro-legs prevent biomass particles from
forming bridge while the tip balls strike the wall to-and-fro to prevent biomass particles from adhering to the wall. 2.2. Experimental set up and procedure The biomass pyrolysis liquefaction equipment was built up by feeding screws, fluidized bed reactor, cyclones, condensers, control system and carrier gas circulation system. To prepare for the biomass pyrolysis liquefaction experiments, cold test of this feeding system was taken in advance to validate if it could feed uniformly. Fig. 5 shows the morphology of the biomass particle. The magnification factor of the optical microscopy photo is 12 and the resolution is 800 × 600. It was shown that the size, shape and surface of the biomass particles were irregular. The biomass particle used in the experiment is the sawdust, which was dried at 100 °C for 8 h before experiment; the main physical properties of the three kinds of sawdust after drying are shown in Table 1. In the present tests, the biomass was loosely poured into the hopper and trimmed same high level to secure equilibrium conditions in the bulk material. The biomass density was calculated by weighing biomass particles of per unit volume using a JY1001 electronic balance whose error is 0.1 g. Schematics of the biomass feeding systems used in the experiments are displayed in Fig. 6. The first screw rotated at different speeds, 13.25, 20.75, 29.75 and 38.75 rpm respectively, and the second screw rotating speed was 300 rpm. The rotating speed of the screw was measured using a DT-2858 ratemeter. The period of feeding was measured according to a stopwatch. When biomass particles were delivered into the pre-set container, the weight of the biomass particles were obtained using the DT501K electronic balance within a feeding period of 10 min; the feeding rate could be calculated accordingly. The weigh scale and the ratemeter were connected to the computer to record the weight of biomass fed and the rotational speed of the screw during feeding. A torquemeter of JN-338N with a digital indicator was installed on the transmission shaft of the first screw to measure the torque during the experiments. All experiments were performed 2–5 times to determine the reproducibility and range of flow rates and torques for a given material under experimental conditions. 3. Mechanics and torque analysis for automatic shake mechanism
Fig. 2. Bridge formation of biomass.
The torque generated by the automatic shake mechanism in the hopper was considered. The spring bar goes forward pushed by the movement of the screw flight and rebounds back a position under the rotating condition as shown in Fig. 7. The frictional force exerted by the spring bar on the screw flight resists motion and rotation.
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Fig. 4. Automatic shake mechanism in the hopper.
According to the curvature of beam theories [17], the maximum forces exerted by the spring bar to the screw in the empty condition can be expressed by F=
3πEd40 f 64L3
ð1Þ
where d0, E, L is the diameter, the elastic modulus, the length of the spring bar, respectively. f is the axial distance that the screw flight pushes the spring bar. In the present study, we set f equal to the half of the pitch of the screw to ensure the spring bar shaking range. The insertion length of the spring bar ycan be obtained by calculation and experiment. The maximum torque generated by the automatic shake mechanism in the empty condition can be written as TA = kμFD = 2
ð2Þ
where k is the effect factor of the associated parts of the spring bar, μ is the friction coefficient between the spring bar and the screw flight, and the D is the diameter of the screw flight. The total torque required to drive the screw feeder in the full condition is Ttotal = TA + ð1 + kA ÞTi
ð3Þ
where Ti is the torque without the automatic shake mechanism, which can be calculated according to Dai [15]. kA is the torque coefficient
caused by the automatic shake mechanism due to pressure and friction with the bulk material in the hopper in the full condition, which was obtained by experiments. 4. Results and analysis 4.1. Feeding fluctuation comparison before and after using the automatic shake mechanism The biomass feeding rates with different rotating speeds before using the automatic shake mechanism for the sawdust-2 are shown in Fig. 8, with weighing intervals of 10 min. The four lines in the figure correspond to four rotating speeds of the screw. It can be seen from the figure that the feeding rate fluctuates more and more heavily with the increment of rotating speed of the screw. For the biomass particle has special shape as shown in Fig. 5, and it is easy to absorb moisture and become sticky, the biomass particles stored in hopper frequently form bridge or adhere to the wall to prevent the feeding system from providing the required uniform and continuous flow of feedstocks. The biomass feeding rates with different rotating speeds after using the automatic shake mechanism for the sawdust-2 are shown in Fig. 9. The experimental results show that the feeding rates get to be stable, in the presence of the automatic shake mechanism, the average feeding rate grows up to 40.2 kg/h for the first screw speed of 38.75 rpm while the fluctuation narrows down to 3.0%. The feeding system using the automatic shake mechanism improves the performance of the feeding system significantly, the reasons are that the reciprocating motions of the spring bar and the two astro-legs prevent biomass particles from forming bridge while the tip balls strike the wall to-and-fro to prevent biomass particles from adhering to the wall, leading to a condition of the screws being filled properly by biomass particles and an increase in the volumetric capacity of the screw feeders. The feeding rate of this screw system can be calculated using the formula as follows:
Q = 60 ×
π 2 2 × D −d × s × γ × ψ × n 4
ð4Þ
Table 1 Specifications of the biomass particle.
Fig. 5. Morphology of sawdust under a microscope.
Description
Size (μm)
Moisture (w%)
Density (kg/m3)
Sawdust-1 Sawdust-2 Sawdust-3
200–300 350–450 550–900
5.9 5 4.9
188 170 165
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351
Fig. 6. Schematic of the biomass feeding system.
where Q is theoretical feeding rate (kg/h), D is the external diameter of the screw (m), d is the shaft diameter of the screw (m), s is the screw-pitch (m), γ is the density of biomass (kg/m3), ψ is filling coefficient, and n is the rotating speed (r/min). 4.2. Feeding rate comparison of model predictions with experimental results
Fig. 8. Feeding rate before improvement with different rotating speeds.
denotes the filling level of biomass in the screw. The value ψ = 1.0 means the feeding screw is fully filled. In fact, this condition is not reached even under the function of the automatic shake mechanism.
The experiments of the feeding rate for the sawdust-1, 2 and 3 with different rotating speeds of the screw before and after using the automatic shake mechanism were performed. The results were compared with the model predictions of Eq. (4), shown in Fig. 10. The experimental results show that the three kinds of sawdust feeding using the automatic shake mechanism have higher average feeding rates than before improvement; sawdust-1 has small particle size and high moisture content, which is easier to adhesive to the wall; sawdust-3 has bigger particle size which is more likely to form bridge. Despite the different size, moisture and density of the sawdust sets, the biomass feeding rates displayed fair stability after using the automatic shake mechanism. Table 2 demonstrates the comparison of the fluctuations scope between before and after improvement. It is indicated that the automatic shake mechanism works well, the biomass feeding rate can increase over 70%. However, the average feeding rates after and before improvement are both smaller than the model prediction. Eq. (4) is mainly based on the parameters ψ which
The total torque required to drive the screw feeder is the sum of the torques needed by the automatic shake mechanism at empty condition, by the system in the transportation of bulk biomass without the automatic shake mechanism, and by spring bar and its associated parts at filled condition. The comparison of model predictions with experimental results shows in Fig. 11. Predictions of the torque requirement with the automatic shake mechanism are good agreement with experiments for several sawdust feedings. The torque requirements of the automatic shake mechanism are seen to account for less than 7% of the total torque for different kinds of sawdust tested. The torque coefficient kA = 0.08 can be used to calculate the total torque. The insert length y is 2 mm according to the
Fig. 7. Force and deformation of the spring bar.
Fig. 9. Feeding rate after improvement with different rotating speeds.
4.3. Torque comparison of model predictions with experimental results
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Fig. 10. Feeding rate before and after improvement compared with the model prediction.
Fig. 11. Comparison of predicted and measured average torques.
experiment results to ensure the reciprocating motions range of the spring bar and its associated parts. The prediction torque caused by the spring bar and its associated parts, pressure and friction with the bulk material in the hopper is not considered in the present study due to the complexity, which could not compromise the conclusions significantly.
This mechanism can be used not only in biomass feeding but also in other powder feeding processes.
5. Conclusions In conventional feeding systems, the biomass particles could form bridge and adhere to the wall in the hopper, due to the irregular size and shape, and the stickiness of the biomass particles. The feeding rate fluctuation was so big that it could not meet the requirements of uniform and continuous feeding in biomass pyrolysis operations. However, when the automatic shake mechanism is used, the feeding rate fluctuation is reduced and the feeding capacity can increase over 70%. Three kinds of sawdust were tested to measure the feeding rates. The average feeding rate is greatly improved by the automatic shake mechanism, However, the average feeding rates both after and before improvement are smaller than the model prediction, for the complete fullness condition is not reached under the function of the automatic shake mechanism. The torque required to drive the automatic shake mechanism under the filled condition was predicted and compared with experimental results. The torque requirements of the automatic shake mechanism are seen to account for less than 7% of the total torque, including that needed by the spring bar and its associated parts at the feeding condition. The automatic shake mechanism works well to solve the problem that the biomass particles in hopper become bridge and/or adhere to the wall to prevent the screw feeding from providing the required uniform and continuous flow of the feedstocks.
Table 2 Average feeding rate comparison for the different sawdust. Name of specimen
Fluctuation before (%)
Fluctuation Feeding rate after (%) before (kg/h)
Feeding rate after (kg/h)
Increasing rate (%)
Sawdust-1 Sawdust-2 Sawdust-3
25.1 22.4 24.6
3.5 3.1 3.8
28.23 27.12 25.89
64.3 68.8 70.2
17.18 16.07 15.21
Acknowledgements This work was supported by the Oil Complementary and Alternative Energy Research Team of Henan province, and the Henan Provincial Research Foundation for Basic and Advanced Technology (grant no. 092300410037). The work was also sponsored by China Postdoctoral Foundation. The authors also wish to thank other team members for their help. References [1] M.F. Demirbas, Balat Mustafa, Recent advances on the production and utilization trends of bio-fuels: a global perspective, Energy Convers. Manage. 47 (2006) 2371–2381. [2] J.N. Chheda, J.A. Dumesic, An overview of dehydration, aldol-condensation and hydrogenation processes for production of liquid alkanes from biomass-derived carbohydrates, Catal. Today 123 (2007) 59–70. [3] K.R. Cummer, R.C. Brown, Ancillary equipment for biomass gasification, Biomass Bioenergy 23 (2002) 113–128. [4] KANGBo-sung , LeeKyung Hae , Fast pyrolysis of radiata pine in a bench scale plant with a fluidized bed: influence of a char separation system and reaction conditions on the production of bio-oil, J. Anal. Appl. Pyrol. 76 (2006) 32–37. [5] P. Vriesman, E. Heginuz, K. Sjostrom, Biomass gasification in a laboratory-scale AFBG: influence of the location of the feeding point on the fuel-N conversion, Fuel 79 (2000) 1371–1378. [6] K.R. Cummer, R.C. Brown, Ancillary equipment for biomass gasification, Biomass Bioenergy 23 (2002) 113–128. [7] K.R. Cummer, R.C. Brown, Ancillary equipment for biomass gasification, Biomass Bioenergy 23 (2002) 113–128. [8] X.T. Li, J.R. Grace, C.J. Lim, A.P. Watkinson, H.P. Chen, J.R. Kim, Biomass gasification in a circulating fluidized bed, Biomass Bioenergy 26 (2004) 171–193. [9] A.W. Roberts, Determining screw geometry for specified hopper drawdown performance, Proc. Bulk 2000 Conference, Institution of Mechanical Engineers, London, 1991, pp. 111–116, Oct. [10] A.W. Roberts, Basic Principles of Bulk Solids Storage, Flow and Handling, Institute for Bulk Materials Handling Research, University of Newcastle, 1992. [11] A.W. Roberts, Predicting the volumetric and torque characteristics of screw feeders, Bulk Solids Handl. 16 (1996) 233–244. [12] A. Carleton, J. Miles, F. Valentin, A study of factors affecting the performance of screw conveyors and feeders, Trans. ASME J. Eng. Ind. 91 (1969) 329–334. [13] BatesL. , Entrainment pattern of screw hopper dischargers, Trans. ASME J. Eng. Ind. 91B (1969) 295–302. [14] R. Rautenbach, W. Schumacher, Theoretical and experimental analysis of screw feeders, Bulk Solids Handl. 7 (1987) 675–680. [15] J. Dai, J.R. Grace, A model for biomass screw feeding, Powder Technol. 186 (2008) 40–55. [16] J. Dai, J.R. Grace, Biomass screw feeding with tapered and extended sections, Powder Technol. 186 (2008) 56–64. [17] J.M. Gere, B.J. Goodno, Mechanics of Materials, Cengage Leaning Inc. Publishers, 2009.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
An automatic shake mechanism for the biomass pyrolysis feeding system Jun Zhang ⁎, Zhen-wei Yuan, Xin-li Wei School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450002, China
a r t i c l e
i n f o
Article history: Received 28 April 2010 Received in revised form 29 October 2010 Accepted 16 November 2010 Available online 24 November 2010 Keywords: Biomass particles Screw feeder Automatic shake mechanism Power consumption
a b s t r a c t Successful feeding is critical to biomass utilization processes such as pyrolysis liquefaction or gasification. The biomass particles in hopper frequently become bridge or adhere to the wall to prevent the feeding system from providing the required uniform and continuous flow of feedstocks. To solve the problem, an automatic shake mechanism was devised which was set in the hopper and driven by the rotation of screw. Experiments were done using the automatic shake mechanism to feed three kinds of sawdust; the total torque requirement of the screw feeder with the automatic shake mechanism in the hopper was predicted and compared with experiment results. The results showed that the biomass particles could be delivered uniformly and the feeding rate had a significant increase with 7% extra power consumption. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Interest in producing liquid fuels from biomass is increasing worldwide for the reason that bio-oil is easy to transport and store. In practical processes, a critical problem is how to feed the biomass feedstocks into the reactors properly. Feeding problems often impede smooth operation in industries [1,2]. Such particle properties as mean size, size distribution, shape, surface roughness, density, moisture content, compressibility, and pliability can all affect the successive feeding. Biomass feeding devices frequently become blocked and do not provide the required uniform and continuous flow of the feedstocks. Furthermore, the reactor often operates at high pressure and/or high temperature. This causes additional challenges in establishing reliable feeding [3–6]. Several kinds of feeders and their combinations used in biomass utilization processes have been reported, such as lock hopper systems, screw feeders, rotary valves, piston feeders, pneumatic feeders, etc. [7,8]. With its simple structure, high efficiency, and easiness for industrialization, screw feeders have been tested for biomass feedstocks feeding. Roberts et al. [9–11] analyzed the volumetric characteristics and mechanics of screw feeders in relation to the bulk solid draw-down characteristics of the feed hopper. Carleton et al. [12] discussed the performance of screw conveyors and screw feeders based on experiments on the effects of screw geometry, speed, fill level and material properties. Bates [13] provided detailed analysis of mechanics and entrained patterns of screw feeders, especially those combined with hoppers. Rautenbach and Schumacher [14] derived a set of parameters by dimensional analysis to calculate the power
consumption and transport capacity and compared two geometrically similar screws. Dai and Grace [15,16] proposed a model to delineate what limits screw feeding in terms of the mechanisms of blockage and to predict torque requirements for biomass materials, and defined blockage mechanisms and plug seal with the aid of tapered and extended sections. The tapered and extended sections increased torque requirements significantly while improving the plug seal to the reactor. However, little information has been focused on the problem that biomass particles in hopper become bridge or adhere to the wall to prevent the screw feeding from providing the required uniform and continuous flow of the feedstock. In this paper, a screw feeder system is described, which is composed of a hopper and two screws. The first screw determines biomass feeding rate, while the second crew simply transports biomass to the reactor. For the unique properties of biomass particles such as low density, big resting-angle and poor mobility, it is easy to form bridge and adhere to the wall in the hopper, leading to instability or feeding breaks. An automatic shake mechanism was devised aiming at obtaining stability and continuity of feeding. Experiments were done using the automatic shake mechanism to feed three kinds of sawdust. In addition, the experimental results were analyzed and compared with the prediction model. The total torque requirement of the screw feeder with the automatic shake mechanism in the hopper was predicted and compared with experimental results. 2. The automatic shake mechanism description and experimental procedure 2.1. The automatic shake mechanism
⁎ Corresponding author. Tel.: +86 3 7167780114. E-mail address: [email protected] (J. Zhang). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.11.018
Shown in Fig. 1 is the feeding system, including a storage hopper, two feeding screws with their motors and frequency controllers. The
J. Zhang et al. / Powder Technology 207 (2011) 348–352
349
Fig. 3. Stick on the wall of storage bin. Fig. 1. Feeding system of biomass pyrolysis equipment.
first screw drills across the root of the biomass storage hopper. The screws are driven by their motors and the motors work under the frequency control. Biomass particles stored in the storage hopper pass the first and second screws in turn, and then flow into the fluidized bed reactor. The first screw determines biomass feeding rate, while the second screw simply transports biomass to the fluidized bed reactor. The storage hopper is composed of a cylinder and a cone, which has larger volumetric capacity than the wedge-shaped hopper. Feeding stability is critical to the biomass pyrolysis liquefaction processes. However, biomass particles in the hopper are easy to form bridge as shown in Fig. 2, or adhere to the wall as shown in Fig. 3. These phenomena could lead to instability or feeding breaks. Therefore, an automatic shake mechanism in the hopper is proposed, as shown in Fig. 4. The mechanism is driven by the rotation of the first screw to reciprocate in the hopper to prevent biomass particles from forming bridge or adhering to the wall and ensure biomass particles flowing into the first screw continuously. The automatic shake mechanism is made up of a spring bar and two astro-legs. Each astro-leg includes six tip balls, six legs, a sleeve and a fixing bolt. The spring bar and the legs are made of spring steel, while the tip balls are made of aluminum. One end of the spring bar is fixed on the cover of the hopper and the other end touches one flight of the first screw. The bar goes forward with the rotation of the first screw until it separates from the flight and rebounds to a position. The two astro-legs are fixed on the spring bar to enhance disturbing effects. The sleeve connected with six legs is fixed to the spring bar with a bolt. The reciprocating motions of the spring bar and two astro-legs prevent biomass particles from
forming bridge while the tip balls strike the wall to-and-fro to prevent biomass particles from adhering to the wall. 2.2. Experimental set up and procedure The biomass pyrolysis liquefaction equipment was built up by feeding screws, fluidized bed reactor, cyclones, condensers, control system and carrier gas circulation system. To prepare for the biomass pyrolysis liquefaction experiments, cold test of this feeding system was taken in advance to validate if it could feed uniformly. Fig. 5 shows the morphology of the biomass particle. The magnification factor of the optical microscopy photo is 12 and the resolution is 800 × 600. It was shown that the size, shape and surface of the biomass particles were irregular. The biomass particle used in the experiment is the sawdust, which was dried at 100 °C for 8 h before experiment; the main physical properties of the three kinds of sawdust after drying are shown in Table 1. In the present tests, the biomass was loosely poured into the hopper and trimmed same high level to secure equilibrium conditions in the bulk material. The biomass density was calculated by weighing biomass particles of per unit volume using a JY1001 electronic balance whose error is 0.1 g. Schematics of the biomass feeding systems used in the experiments are displayed in Fig. 6. The first screw rotated at different speeds, 13.25, 20.75, 29.75 and 38.75 rpm respectively, and the second screw rotating speed was 300 rpm. The rotating speed of the screw was measured using a DT-2858 ratemeter. The period of feeding was measured according to a stopwatch. When biomass particles were delivered into the pre-set container, the weight of the biomass particles were obtained using the DT501K electronic balance within a feeding period of 10 min; the feeding rate could be calculated accordingly. The weigh scale and the ratemeter were connected to the computer to record the weight of biomass fed and the rotational speed of the screw during feeding. A torquemeter of JN-338N with a digital indicator was installed on the transmission shaft of the first screw to measure the torque during the experiments. All experiments were performed 2–5 times to determine the reproducibility and range of flow rates and torques for a given material under experimental conditions. 3. Mechanics and torque analysis for automatic shake mechanism
Fig. 2. Bridge formation of biomass.
The torque generated by the automatic shake mechanism in the hopper was considered. The spring bar goes forward pushed by the movement of the screw flight and rebounds back a position under the rotating condition as shown in Fig. 7. The frictional force exerted by the spring bar on the screw flight resists motion and rotation.
350
J. Zhang et al. / Powder Technology 207 (2011) 348–352
Fig. 4. Automatic shake mechanism in the hopper.
According to the curvature of beam theories [17], the maximum forces exerted by the spring bar to the screw in the empty condition can be expressed by F=
3πEd40 f 64L3
ð1Þ
where d0, E, L is the diameter, the elastic modulus, the length of the spring bar, respectively. f is the axial distance that the screw flight pushes the spring bar. In the present study, we set f equal to the half of the pitch of the screw to ensure the spring bar shaking range. The insertion length of the spring bar ycan be obtained by calculation and experiment. The maximum torque generated by the automatic shake mechanism in the empty condition can be written as TA = kμFD = 2
ð2Þ
where k is the effect factor of the associated parts of the spring bar, μ is the friction coefficient between the spring bar and the screw flight, and the D is the diameter of the screw flight. The total torque required to drive the screw feeder in the full condition is Ttotal = TA + ð1 + kA ÞTi
ð3Þ
where Ti is the torque without the automatic shake mechanism, which can be calculated according to Dai [15]. kA is the torque coefficient
caused by the automatic shake mechanism due to pressure and friction with the bulk material in the hopper in the full condition, which was obtained by experiments. 4. Results and analysis 4.1. Feeding fluctuation comparison before and after using the automatic shake mechanism The biomass feeding rates with different rotating speeds before using the automatic shake mechanism for the sawdust-2 are shown in Fig. 8, with weighing intervals of 10 min. The four lines in the figure correspond to four rotating speeds of the screw. It can be seen from the figure that the feeding rate fluctuates more and more heavily with the increment of rotating speed of the screw. For the biomass particle has special shape as shown in Fig. 5, and it is easy to absorb moisture and become sticky, the biomass particles stored in hopper frequently form bridge or adhere to the wall to prevent the feeding system from providing the required uniform and continuous flow of feedstocks. The biomass feeding rates with different rotating speeds after using the automatic shake mechanism for the sawdust-2 are shown in Fig. 9. The experimental results show that the feeding rates get to be stable, in the presence of the automatic shake mechanism, the average feeding rate grows up to 40.2 kg/h for the first screw speed of 38.75 rpm while the fluctuation narrows down to 3.0%. The feeding system using the automatic shake mechanism improves the performance of the feeding system significantly, the reasons are that the reciprocating motions of the spring bar and the two astro-legs prevent biomass particles from forming bridge while the tip balls strike the wall to-and-fro to prevent biomass particles from adhering to the wall, leading to a condition of the screws being filled properly by biomass particles and an increase in the volumetric capacity of the screw feeders. The feeding rate of this screw system can be calculated using the formula as follows:
Q = 60 ×
π 2 2 × D −d × s × γ × ψ × n 4
ð4Þ
Table 1 Specifications of the biomass particle.
Fig. 5. Morphology of sawdust under a microscope.
Description
Size (μm)
Moisture (w%)
Density (kg/m3)
Sawdust-1 Sawdust-2 Sawdust-3
200–300 350–450 550–900
5.9 5 4.9
188 170 165
J. Zhang et al. / Powder Technology 207 (2011) 348–352
351
Fig. 6. Schematic of the biomass feeding system.
where Q is theoretical feeding rate (kg/h), D is the external diameter of the screw (m), d is the shaft diameter of the screw (m), s is the screw-pitch (m), γ is the density of biomass (kg/m3), ψ is filling coefficient, and n is the rotating speed (r/min). 4.2. Feeding rate comparison of model predictions with experimental results
Fig. 8. Feeding rate before improvement with different rotating speeds.
denotes the filling level of biomass in the screw. The value ψ = 1.0 means the feeding screw is fully filled. In fact, this condition is not reached even under the function of the automatic shake mechanism.
The experiments of the feeding rate for the sawdust-1, 2 and 3 with different rotating speeds of the screw before and after using the automatic shake mechanism were performed. The results were compared with the model predictions of Eq. (4), shown in Fig. 10. The experimental results show that the three kinds of sawdust feeding using the automatic shake mechanism have higher average feeding rates than before improvement; sawdust-1 has small particle size and high moisture content, which is easier to adhesive to the wall; sawdust-3 has bigger particle size which is more likely to form bridge. Despite the different size, moisture and density of the sawdust sets, the biomass feeding rates displayed fair stability after using the automatic shake mechanism. Table 2 demonstrates the comparison of the fluctuations scope between before and after improvement. It is indicated that the automatic shake mechanism works well, the biomass feeding rate can increase over 70%. However, the average feeding rates after and before improvement are both smaller than the model prediction. Eq. (4) is mainly based on the parameters ψ which
The total torque required to drive the screw feeder is the sum of the torques needed by the automatic shake mechanism at empty condition, by the system in the transportation of bulk biomass without the automatic shake mechanism, and by spring bar and its associated parts at filled condition. The comparison of model predictions with experimental results shows in Fig. 11. Predictions of the torque requirement with the automatic shake mechanism are good agreement with experiments for several sawdust feedings. The torque requirements of the automatic shake mechanism are seen to account for less than 7% of the total torque for different kinds of sawdust tested. The torque coefficient kA = 0.08 can be used to calculate the total torque. The insert length y is 2 mm according to the
Fig. 7. Force and deformation of the spring bar.
Fig. 9. Feeding rate after improvement with different rotating speeds.
4.3. Torque comparison of model predictions with experimental results
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Fig. 10. Feeding rate before and after improvement compared with the model prediction.
Fig. 11. Comparison of predicted and measured average torques.
experiment results to ensure the reciprocating motions range of the spring bar and its associated parts. The prediction torque caused by the spring bar and its associated parts, pressure and friction with the bulk material in the hopper is not considered in the present study due to the complexity, which could not compromise the conclusions significantly.
This mechanism can be used not only in biomass feeding but also in other powder feeding processes.
5. Conclusions In conventional feeding systems, the biomass particles could form bridge and adhere to the wall in the hopper, due to the irregular size and shape, and the stickiness of the biomass particles. The feeding rate fluctuation was so big that it could not meet the requirements of uniform and continuous feeding in biomass pyrolysis operations. However, when the automatic shake mechanism is used, the feeding rate fluctuation is reduced and the feeding capacity can increase over 70%. Three kinds of sawdust were tested to measure the feeding rates. The average feeding rate is greatly improved by the automatic shake mechanism, However, the average feeding rates both after and before improvement are smaller than the model prediction, for the complete fullness condition is not reached under the function of the automatic shake mechanism. The torque required to drive the automatic shake mechanism under the filled condition was predicted and compared with experimental results. The torque requirements of the automatic shake mechanism are seen to account for less than 7% of the total torque, including that needed by the spring bar and its associated parts at the feeding condition. The automatic shake mechanism works well to solve the problem that the biomass particles in hopper become bridge and/or adhere to the wall to prevent the screw feeding from providing the required uniform and continuous flow of the feedstocks.
Table 2 Average feeding rate comparison for the different sawdust. Name of specimen
Fluctuation before (%)
Fluctuation Feeding rate after (%) before (kg/h)
Feeding rate after (kg/h)
Increasing rate (%)
Sawdust-1 Sawdust-2 Sawdust-3
25.1 22.4 24.6
3.5 3.1 3.8
28.23 27.12 25.89
64.3 68.8 70.2
17.18 16.07 15.21
Acknowledgements This work was supported by the Oil Complementary and Alternative Energy Research Team of Henan province, and the Henan Provincial Research Foundation for Basic and Advanced Technology (grant no. 092300410037). The work was also sponsored by China Postdoctoral Foundation. The authors also wish to thank other team members for their help. References [1] M.F. Demirbas, Balat Mustafa, Recent advances on the production and utilization trends of bio-fuels: a global perspective, Energy Convers. Manage. 47 (2006) 2371–2381. [2] J.N. Chheda, J.A. Dumesic, An overview of dehydration, aldol-condensation and hydrogenation processes for production of liquid alkanes from biomass-derived carbohydrates, Catal. Today 123 (2007) 59–70. [3] K.R. Cummer, R.C. Brown, Ancillary equipment for biomass gasification, Biomass Bioenergy 23 (2002) 113–128. [4] KANGBo-sung , LeeKyung Hae , Fast pyrolysis of radiata pine in a bench scale plant with a fluidized bed: influence of a char separation system and reaction conditions on the production of bio-oil, J. Anal. Appl. Pyrol. 76 (2006) 32–37. [5] P. Vriesman, E. Heginuz, K. Sjostrom, Biomass gasification in a laboratory-scale AFBG: influence of the location of the feeding point on the fuel-N conversion, Fuel 79 (2000) 1371–1378. [6] K.R. Cummer, R.C. Brown, Ancillary equipment for biomass gasification, Biomass Bioenergy 23 (2002) 113–128. [7] K.R. Cummer, R.C. Brown, Ancillary equipment for biomass gasification, Biomass Bioenergy 23 (2002) 113–128. [8] X.T. Li, J.R. Grace, C.J. Lim, A.P. Watkinson, H.P. Chen, J.R. Kim, Biomass gasification in a circulating fluidized bed, Biomass Bioenergy 26 (2004) 171–193. [9] A.W. Roberts, Determining screw geometry for specified hopper drawdown performance, Proc. Bulk 2000 Conference, Institution of Mechanical Engineers, London, 1991, pp. 111–116, Oct. [10] A.W. Roberts, Basic Principles of Bulk Solids Storage, Flow and Handling, Institute for Bulk Materials Handling Research, University of Newcastle, 1992. [11] A.W. Roberts, Predicting the volumetric and torque characteristics of screw feeders, Bulk Solids Handl. 16 (1996) 233–244. [12] A. Carleton, J. Miles, F. Valentin, A study of factors affecting the performance of screw conveyors and feeders, Trans. ASME J. Eng. Ind. 91 (1969) 329–334. [13] BatesL. , Entrainment pattern of screw hopper dischargers, Trans. ASME J. Eng. Ind. 91B (1969) 295–302. [14] R. Rautenbach, W. Schumacher, Theoretical and experimental analysis of screw feeders, Bulk Solids Handl. 7 (1987) 675–680. [15] J. Dai, J.R. Grace, A model for biomass screw feeding, Powder Technol. 186 (2008) 40–55. [16] J. Dai, J.R. Grace, Biomass screw feeding with tapered and extended sections, Powder Technol. 186 (2008) 56–64. [17] J.M. Gere, B.J. Goodno, Mechanics of Materials, Cengage Leaning Inc. Publishers, 2009.