Scholarly article on topic 'Analysis of the Influence of Vortexbinder Dimension on Cyclone Separator Performance in Biomass Gasification System'

Analysis of the Influence of Vortexbinder Dimension on Cyclone Separator Performance in Biomass Gasification System Academic research paper on "Chemical engineering"

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{"Biomass Gasification" / "Cyclone Separator" / "Vortex Finder" / "Collection Efficiency" / Experimental / "CFD Simulation"}

Abstract of research paper on Chemical engineering, author of scientific article — Adi Surjosatyo, Adi Respati, Hafif Dafiqurrohman, Muammar

Abstract Producer gas produced from biomass gasification still contains particulate ash. It is cleaned by using a cyclone separator. Cyclone separator capability to ‘capture’ ash is characterized by its performance known as collection efficiency. One major factor influencing cyclone separator performance is vortex finder dimension. A cyclone separator model with 2D standard configuration with 150 mm diameter has beenmade. Standard configuration model isused to minimize geometrical factors affecting cyclone separator performance other than vortex finder itself. Three variations of length (3/8 D; 5/8 D; 1 D) and three variations of diameter (35; 70; 85 mm) of vortex finder are used to identify their influences on cyclone performance. Experimental study and CFD simulation using ANSYS Fluent were performed. Experimental study aims to find out vortex finder dimension effects on collection efficiency trend, whileCFD simulation goal is to determine the velocity profile of air inside cyclone separator. RNG k-ɛ for swirl dominated flow was chosen as the turbulence model. Velocity magnitude inside cyclone separator is used to predict the collection efficiency trend. Collection efficiency fromexperimentare determined by weighing method, i.e. percentage of total mass of ash captured divided by total mass of ash entering cyclone separator. Experimental results show that increasing vortex finder length and decreasing vortex finder diameter can increase the collection efficiency. In general, increasing vortex finder length and decreasing vortex finder diameter can also increasethe velocity magnitude of air inside cyclone separator. Highest collection efficiency is achieved by using vortex finder with 35 mm (1/4 D) in diameter and 1 D (150 mm) in length.

Academic research paper on topic "Analysis of the Influence of Vortexbinder Dimension on Cyclone Separator Performance in Biomass Gasification System"

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Procedía Engineering 170 (2017) 154-161

Procedía Engineering

www.elsevier.com/locate/procedia

Engineering Physics International Conference, EPIC 2016

Analysis of the Influence of Vortexbinder Dimension on Cyclone Separator Performance in Biomass Gasification System

Adi Surjosatyo*, Adi Respati, Hafif Dafiqurrohman, Muammar

Departement of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, 16424, Indonesia

Abstract

Producer gas produced from biomass gasification still contains particulate ash. It is cleaned by using a cyclone separator. Cyclone separator capability to 'capture' ash is characterized by its performance known as collection efficiency. One major factor influencing cyclone separator performance is vortex finder dimension. A cyclone separator model with 2D standard configuration with 150 mm diameter has beenmade. Standard configuration model isused to minimize geometrical factors affecting cyclone separator performance other than vortex finder itself. Three variations of length (3/8 D; 5/8 D; 1 D) and three variations of diameter (35; 70; 85 mm) of vortex finder are used to identify their influences on cyclone performance. Experimental study and CFD simulation using ANSYS Fluent were performed. Experimental study aims to find out vortex finder dimension effects on collection efficiency trend, while CFD simulation goal is to determine the velocity profile of air inside cyclone separator. RNG k-e for swirl dominated flow was chosen as the turbulence model. Velocity magnitude inside cyclone separator is used to predict the collection efficiency trend. Collection efficiency fromexperimentare determined by weighing method, i.e. percentage of total mass of ash captured divided by total mass of ash entering cyclone separator. Experimental results show that increasing vortex finder length and decreasing vortex finder diameter can increase the collection efficiency. In general, increasing vortex finder length and decreasing vortex finder diameter can also increase the velocity magnitude of air inside cyclone separator. Highest collection efficiency is achieved by using vortex finder with 35 mm (1/4 D) in diameter and 1 D (150 mm) in length.

© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of the Engineering Physics International Conference 2016 Keywords: Biomass Gasification, Cyclone Separator, Vortex Finder, Collection Efficiency, Experimental, CFD Simulation

1. Introduction

Biomass gasification is a thermochemical process that converts biomass into producer gas. Producer gas contains combustible such as CO, H2, CH4, non-combustible gas such as N2, CO2, and the impurities such as tar and ash. Tar and ash can be a crucial issue against pollution and maintenance equipment of gasification system if not handled properly. One of the key components that will be created is a cyclone separator. Cyclone separator has been widely used to clean the gas stream from impurity particles. Cyclone separator is selected because of its simple design, low cost, and no moving parts so the maintenance is relatively easy, as well as high efficiency for solid particle larger than 10 microns [2].

The existing cyclone separator used in gasification systems in Universitas Indonesia (University of Indonesia) is not designed carefully and without proper calculation and simulation. Modification of cyclone separator by extending vortex finder length has been done under the assumption that by using longer vortex finder, the ash will not escape out of the cyclone separator. Hopefully, the results of this study will be taken into consideration to design a new cyclone separator which will be applied to the gasification system in the University of Indonesia.

* Corresponding author. Tel.: +68-21-727-0032; fax: +68-21-727-0033. E-mail address:adisur@eng.ui.ac.id

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of the Engineering Physics International Conference 2016 doi:10.1016/j.proeng.2017.03.036

(a) (b)

Figure 1. Cyclone in Biomass Gasification Universitas Indonesia; (a) Cyclone separator existing, (b) Cyclone separator components

Cyclone Separator separates particles from gas streams by utilizing inertial forces. The flow of incoming gas is forced to rotate and it moves down inside the barrel due to the gravity after which it goes to the bottom of the cone. Because of the shrinkage cross-section area on the bottom of the cone, the gas can be pulled upwards (reverse flow) and exits through the vortex finder. The gas stream rotating in the barrel and cone section is referred as the outer vortex, while the gas flow rotating in the vortex finder is called the inner vortex. The process of separation of the particles from the gas stream occurs in the outer vortex, where the centrifugal force arising from rotational movement, forcing the particles with greater density than gas to be thrown up against the barrel wall.

Performance of cyclone separator is generally influenced by three factors: the particle properties, the dimensions of cyclone (inlet duct, barrel, cone, vortex finder, etc.), and the operating conditions (gas velocity, particle concentration, temperature, etc.). One of cyclone separator standard configuration designed by Lapple (1951) is referred to as 2D. The standard configuration provides ease of designing cyclone separator, where all dimensions of the components of the cyclone are only determined by the diameter of the cyclone itself. Lapple (1951) developed a semi-empirical equation called the cut diameter, that is the diameter of particle that has 50% probability to be captured.

■■1/2

d„r = -^-r (1)

2nNeVi(pp - pg\

2. The object of the study

Some of the pertinent literature about cyclone separator efficiency has been summarized from various sources as follows:

• Vortex Finder Optimum Length

Measurements made by Heumann (1991) [3] show the optimum length of vortex finder is 1.25 times the inlet duct. Decreasing vortex finder length can lower pressure drop, but when the vortex finder is too short (half the height of the inlet duct), some of the gas and solid particles can be directly entered into the vortex finder without experiencing separation, this phenomenon is referred to 'short circuiting' which led to a decrease in collection efficiency [3].

• Particle Short Circuiting and Particle Re-Entrainment

Studies conducted by Gujun, et al (2008) [4] shows there is a phenomenon of particle short circuiting in an area 0.25D below the vortex finder. The phenomenon described as the first escape, while in the 0.5 D above the dust hopper, exists particle re-entrainment called a second escape. Particles that have been separated in the dust hopper, can be introduced back to the gas stream. This phenomenon described as back mixing.

• Natural Vortex Length

Natural vortex length is the length of the outer vortex, measured from the tip vortex finder. Natural vortex length indicates the location where the change in the direction of flow from downward flow to the upward flow appears. Theoretically, a good cyclone separator design has equal natural vortex length and physical length (overall length of barrel and cone minus vortex finder length) [5]. The formula to calculate the natural vortex length according to Alexander (1949) is as follows:

I = 2,3D,

• Effects of Temperature and Pressure

Cyclone separator that operates at high temperatures (293 - 1123 K) shows the difference in cut diameter, which the actual cut diameter is smaller than the calculated cut diameter [9]. It is important to know, because the cyclone separator in this study will be applied to biomass gasification systems, where the producer gas has high temperature, which is around 300oC (573.15 K) [2]. Mi Soo, et.al [6] showed that in general, collection efficiency for particles 10 microns, heavily influenced by pressure and temperature. Effect of pressure and temperature seen opposite each other. The increase in pressure increases the collection efficiency while lowering the temperature increasing the collection efficiency at a certain flow rate.

• Effects of Particle Concentration and Inlet Velocity

Experiment conducted by Zhongli, et al (2008) [7] shows that the collection efficiency increases with the increase in particle concentration. Furthermore, the study investigated the effects of inlet velocity to collection efficiency at very low particle concentration, i.e. the particle concentration of 5 mg/m3, which is usually a cyclone separator operates on a particle concentration about 300 mg/m3.

Prediction of Optimum Inlet Velocity

Particles that hit the wall are considered trapped in the cyclone separator. However, according to a study conducted by Jingxuan, et al (2015) [11], there is a possibility that the particles hitting the wall arebouncing back and escaping from the separation. This can occur if the particles have sufficient energy to bounce. A high inlet velocity gives high energy to the particle, therefore, according to Jingxuan, et al, when the inlet velocity is lower than the critical velocity, the increase in inlet velocity increase collection efficiency.Whereas when the inlet velocity exceeds the critical velocity, inlet velocity increment actually reduce collection efficiency. Furthermore, the term critical velocity used in that study was the maximum-efficiency inlet velocity (MEIV). One of the simple formula to predict VMEIV are as follows:

jfeg^V b/D ) (3)

1 3pg* ) ll - b/D) 0 (3)

3. Methods 3.1 Experimental

In this experiment, standard PVC pipe with diameter of 1 % inches; 2 % inches; and 3 inches act as the vortex finder. Through measurements, the variation of PVC inner diameter are: 35 mm; 70 mm; and 85 mm. While variations of length used are: 3/8 D (56.25 mm); 5/8 D (93.75 mm); and 1 D (150 mm). Vortex finder length is measured from the roof of cyclone.

Figure 2. Experimental set-up

Details from Figure 2:

1. Rice husk ash

2. Strainer

3. Refined rice husk ash

4. Scales, to measure the mass of ash before entering the cyclone

5. Cyclone separator

Controlled variables in this study are: • Blower RPM

6. Vortex finder

7. IDF/ suction blower

8. Dust hopper

9. Scales, to measure the mass of ash in dust hopper

Inverter on suction blower enables to adjust the suction blower rpm. Suction blower rpm is determined by the frequency in the inverter, the frequency used is 1200 and 2000 in accordance with the usual operation of gasifier. The flow rate measurement using rotameter, at a frequency of 1200 results in the air flow rate of 2.4 m3/h, while at frequency of 2000 yields 5.2 m3/h

• Mass of the Rice Husk Ash

The density of rice husk ash through measurement yields153.3 kg/m3. Total mass of ash introduced into the cyclone separator is 40 gr. Mass of ash is weighed on digital scales with maximum measurement capacity of 600 gr and the smallest measurement scale is 0.01 gr. Rice husk ash needs to be refined in order to obtain a homogeneous size, so the calculation of collection efficiency on a mass basis will have better accuracy.

2.2 CFD Simulation

CFD simulations were performed on ANSYS Fluent using RNG k-e for swirl dominated flow as the turbulence model. Simulations were performed with inlet velocity of 0.24 m/sand no particle injection. Therefore, this simulation aims to determine the influence of vortex finder dimensions on velocity profile, especially tangential and axial velocity. Later, the velocity profile will be evaluated to determine its effect on collection efficiency. There are nine variations of the simulated cyclone that are presented in Table 1.

Table 1. Variation of vortex finder dimensions

Cyclone

Vortex Finder

Length

Diameter [mm]

3/8 D 5/8 D 1 D 3/8 D 5/8 D 1 D 3/8 D 5/8 D 1 D

35 35 35 70 70 70 85 85 85

All velocity data are taken by making a horizontal line along the barrel with a distance of 200 mm (Y axis) from the roof of cyclone. The horizontal lines are created on a side view following the Z axis.

Figure 3. Example of velocity profile from side view; and horizontal line on Z axis (yellow line)

Tangential velocity is a velocity component that largely affects the centrifugal force acting on the particle. The greater the tangential velocity, the greater the centrifugal force acting on the particle. It is known that the working principle of cyclone separators utilizes centrifugal force, the centrifugal force has a direction away from the axis of rotation, which means that the particles undergo separation by hitting the wall due to the centrifugal force.

Axial velocity is the velocity component that indicates the direction of the flow, downward or upward flow. Axial velocity can also be used to determine the natural length of the vortex, which is the point where the gas stream changes its direction from downward to upward. By making the Y axis as a reference direction, the value of axial velocity will be positive if the axial flow is upward and negative if the flow is downward. By knowing the location where the flow changes, this can design an appropriate length of vortex finder to avoid particle re-entrainment. Direction of tangential, axial and radial velocity are illustrated in Figure 4.

tangential

Figure 4. Velocity components [Cooper, et al, 1986]

4. Results and Discussion 4.1 Experimental Results

Based on experimental results, it is found that the value of the overall collection efficiency is high, i.e. above 98%. The highest collection efficiency is 99.88% with vortex finder 1D (150 mm) length and 35 mm (1/4 D) diameter at the air flow rate of 5.2 m3/h or inlet velocity of 0.51 m/s. Lowest collection efficiency is 98.55% with vortex finder dimensions are: length 3/8 D (56.25 mm) and diameter (70 mm) at air flow rate of 2.4 m3/h or inlet velocity of 0.24 m/s:

Diameter Vortex Finder 35 mm

Inlet Flowrate [mVs]

Vortec Finder Length

Diameter Vortex Finder 70 mm

101.00%

100.00% C 99.00%

99.30% ^— ,<^^ 99.30% Inlet Flowrate [mVs]

98.68%^-^" —•—2,4

i 98.55% -•-5,2

98.00%

97.00%

3/8 D 5/8 D Vorte< Finder Length ID

101.00% Diameter Vortex Finder 70 mm

Collection Efficiency

99.30%_____ 59.30% Inlet Flowrate [m'/s]

98.68%^^^ 98.55% His'

97.00%

3/8 D 5/8 D ID Vorte< Finder Length

Figure 5. (a) Collection efficiency chart at diameter vortex finder 35 mm, (b) Collection efficiency chart at diameter vortex finder 70 mm, (c) Collection efficiency chart at diameter vortex finder 85 mm

Based on Figure 5, it can be deduced that, the longer the vortex finder is, the higher the collection efficiency is. It is evident, until the vortex finder length equal to the diameter of the cyclone. For vortex finder longer than cyclone diameter, more experiments are needed, because in this study vortex finder length is limited to 150 mm. The increase in air flow rate can also improve collection efficiency. This is in accordance with the formula for calculating the cut-diameter inlet where the higher the velocity is, the higher the collection efficiency is. From all of the data, the length of 1 D vortex finder has higher collection efficiency than the length of 3/8 D and 5/8 D at all conditions.

(a) (b)

Figure 6. (a) Collection efficiency chart at air flow rate 2.4 m3/h, (b) Collection efficiency chart at air flow rate 5.2 m3/h

Based on the chart above, it is known that the diameter of 35 mm and 85 mm have the best collection efficiency. Diameter 70 mm has a significant efficiency improvement when the vortex finder is extended. The interesting thing about the chart above is that both the diameter of 35 mm and 85 mm have collection efficiency that is almost identical. Overall, vortex finder with diameter 70 mm has the lowest collection efficiency. The effect of vortex finder diameter is still unclear, both diameter 35 mm and 85 mm shows high collection efficiency. If the diameter 85 mm is excluded from the result, it may be concluded that decreasing the diameter increases collection efficiency. Or if the diameter 35 mm is excluded from the result, it may be concluded that increasing diameter can increase collection efficiency.

4.2 CFD Simulation Results

• Effects of Vortex Finder Length on Velocity Magnitude

(a) (b)

Figure 7. (a) Velocity chart at diameter vortex finder 35 mm, (b) Velocity chart at diameter vortex finder 70 mm, (c) Velocity

chart at diameter vortex finder 85 mm

Based on the three charts above, the increase in length of vortex finder increases the velocity of the air inside the cyclone. The increase in velocity occurs significantly when the vortex finder length is extended to 1 D.

• Effects of Vortex Finder Diameter on Velocity Magnitude

(a) (b)

Z axis (mm)

Figure 8. (a) Velocity chart at vortex finder length 3/8 D, (b) Velocity chart at vortex finder length 5/8 D, (c) Velocity chart at

vortex finder length 1 D

Based on the three charts above, the increaseof vortex finder diameter decreasesthe velocity of the air inside the cyclone. A significant increase in velocity occurs when the diameter of the vortex finderisreduced to 35 mm.

4.3 Comparison Between Experimental and Simulation Results

Experimental and simulation results showed an increasing trend in collection efficiency along with increasing length and decreasing diameter of the vortex finder. Vortex finder dimension with the highest collection efficiency obtained from the experiment is 1 D in length and 35 mm (0.25 D) in diameter. CFD simulation shows that the vortex finder with dimensions of length and diameter of 1 D and 35 mm, has the highest velocity magnitude. Based on these facts, we conclude that the rising in the length of vortex finder would increases velocity profile of air inside the cyclone, causing the raise in collection efficiency. Increasing diameter of vortex finder lowers the velocity profile of air inside the cyclone, causing a decrease in collection efficiency.

5. Conclusion

Based on the experimental results, vortex finder with diameter of 70 mm has the lowest collection efficiency. Diameter 85 mm has a stable collection efficiency on both the condition of the air flow rate. Diameter 35 mm with length of 1 D has highest collection efficiency among other dimensions of the vortex finder and it applies in both air flow rate. In the air flow rate 2.4 m3/h, diameter 35 mm has a significant increase in collection efficiency by elongation of the vortex finder.

The conclusions that can be obtained from this study are:

1. Based on the experimental results, the longer the vortex finder, the higher the collection efficiency.

2. When vortex finder is too short, it is at risk of short circuiting phenomenon that can lower collection efficiency.

3. When vortex finder is too long, it is at risk of re-entrainment phenomenon that can lower collection efficiency.

4. Vortex finder dimensions with the highest collection efficiency are 35 mm (1/4 D) in diameter and 1 D (150 mm) in length.

5. Vortex finder with diameter of 85 mm, where some of its part 'block' inlet duct, causing the particles hit the outside wall of the vortex finder which resulted in the fall of particles from the loss of momentum to follow the air vortex. However, these phenomena need further study.

6. The longer the vortex finder, the higher the velocity magnitude.

7. Increment on tangential velocity by elongation of vortex finder was noticeable at vortex finder diameter 35 mm.

8. Axial velocity profile is mostly influenced by the diameter of vortex finder.

Acknowledgem ents

To Ministry of Research and Higher Education Indonesia that gives Penelitian Unggulan Perguruan Tinggi Grant for studying this research. Also to Samsul Ma'arif who help in technical and analysis work of this research.

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