Pilot-Scale Experimental Study on Phase Diagrams of Pressurized

Aug 14, 2017 - Pressurized pneumatic conveying plays an important role in large-scale coal gasification systems. Some of the underlying mechanisms aff...
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Pilot-Scale Experimental Study on Phase Diagrams of Pressurized Pneumatic Conveying with Pulverized Coal Peng Lu,*,† Peijie Yang,† Xingwen Zheng,† Xiaoping Chen,‡ and Changsui Zhao‡ †

Key Laboratory of Thermal Environment and Structure of Ministry of Industry and Information, Jiangsu Province Key Laboratory of Aerospace Power System, College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China ‡ Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China ABSTRACT: Pressurized pneumatic conveying plays an important role in large-scale coal gasification systems. Some of the underlying mechanisms affecting the conveying characteristics remain open issues. Phase diagrams can provide key information regarding flow characteristics by revealing the relationship between pressure drop and superficial gas velocity, which can be utilized for flow pattern identification, stability analysis, and system optimization, in an economic and feasible way. The present work experimentally investigated phase diagrams of pressurized pneumatic conveying on a pilot-scale setup, with three types of the most widely used pulverized coal in China selected as the conveyed materials. The effects of superficial gas velocity, solidphase mass flow rate, pipeline layout, particle size, coal type, and moisture content were studied, respectively. The findings will contribute to a better understanding of the underlying mechanism of pressurized pneumatic conveying with pulverized coal, and to a better guidance for design and operation of industrial systems. regarded as the “economic gas velocity” due to the minimum energy loss. In this way, the dense phase flow is positioned on the left side of the economic gas velocity, while the dilute phase flow is on the right side. Besides, Zenz believes that the flow regimes are closely related to the phase diagram, and the flowing state of the materials in the pipe can be qualitatively predicted according to the phase diagram. Moreover, the range of pressure drop and superficial gas velocity can also be estimated when the flow regime is given. In some occasions, to prevent a pneumatic conveying system operating in a dilutephase pattern at a very high gas velocity, which may result in excessive power consumption, pipe attrition, and materials degradation,20 phase diagrams can be used to regulate this system and to keep the pressure drop and conveying velocity as low as possible. However, there are very few literatures available regarding the phase diagrams of pressurized pneumatic conveying with pulverized coal. Due to the vast varieties of pulverized coal in China, how the particle sizes, coal types, moisture content, pipeline layout, and other operating conditions will affect the conveying characteristics, especially the phase diagrams, needs in-depth investigation. Therefore, this paper presents a systematic study into phase diagrams of pressurized pneumatic conveying on a pilot-scale experimental setup, with the most widely used types of pulverized coal in China, aiming to reveal the effects of the above factors. The results lead to a better understanding of the underlying mechanism of pressurized pneumatic conveying.

1. INTRODUCTION Pneumatic conveying is extensively used to transport bulk materials in a variety of industries.1−3 It has many advantages, such as low cost, flexibility of layout, ease of installation and automation, safety, and hygienic. The pipe geometries, operational conditions, and material properties will all affect the flow characteristics.4 Global warming mitigation is facing serious challenges nowadays. Clean coal technology (CCT) in China has been developed to a strategic perspective, in order to save energy, to increase power generation efficiency, and to resolve the atmospheric pollution. The energy structure of China is mainly based on coal, making pressurized entrained-flow coal gasification one of the most promising CCT technologies.5 This technology requires pulverized coal to be pneumatically conveyed to the gasifier at high pressure, usually above 3 MPa,6−9 which differs from traditional pressure conveying systems in terms of flow speed, solid-phase concentration, flow patterns and stability, etc.10 Therefore, it has been extensively studied both experimentally and theoretically in recent years, on the topics of pressure drop prediction,11−13 effects of operating conditions or material properties,14,15 flow stability,6,7,16,17 and flow regimes.8,9,18 Phase diagrams can well describe flow characteristics of pneumatic conveying by exhibiting the relationship between pressure drops and the superficial gas velocity, which provides an economic and feasible method for flow regime identification, stability analysis, and system optimization. It was first proposed by Zenz19 to characterize a specific pneumatic conveying system in terms of dense phase or dilute phase type. In a phase diagram, as the superficial gas velocity decreases, the pressure drop first drops and then rises. The superficial gas velocity corresponding to the minimum pressure drop is, therefore, © XXXX American Chemical Society

Received: June 20, 2017 Revised: August 11, 2017 Published: August 14, 2017 A

DOI: 10.1021/acs.energyfuels.7b01752 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION 3.1. Feasibility of Phase-Diagram-Based Analysis. The phase diagram of 300 μm Inner Mongolia pulverized coal, which shows the relationship between the pressure drop and superficial gas velocity (Vg), is depicted in Figure 2, and the

2.1. Experimental Materials. The key physical properties of the pulverized coals (Inner Mongolia, Datong, and Yanzhou) of China are listed in Table 1.

Table 1. Physical Properties of Conveyed Materials materials

density (kg·m−3)

Inner Mongolia pulverized coal

1400

Datong pulverized coal Yanzhou pulverized coal

1500 1350

mean dp (μm) 52 115 300 73 68

moisture (%) 3.74 2.09 0.77

2.2. Experimental Apparatus. The pilot-scale experimental apparatus of pressurized pneumatic conveying is schematically shown in Figure 1. High pressure nitrogen from the buffer tank is

Figure 2. Phase diagram of 300 μm Inner Mongolia pulverized coal along horizontal pipe.

corresponding flow regimes photos at different superficial gas velocities are displayed in Figure 3. As illustrated in Figure 2,

Figure 1. Pilot-scale experimental setup of pneumatic conveying at high pressure: 1 - motorized valve; 2 - electronic scale; 3 - sending and receiving hopper; 4 - pressurizing gas; 5 - fluidizing gas; 6 complementary gas; 7 - buffer tank; 8 - high-pressure N2 gas supply; 9 - moisture adjustor; 10 - differential pressure transmitter; 11 - ECT (Electric Capacitance Tomography) measurement; 12 - visualization pipe; 13 - data acquisition and control system; 14 - computer. Figure 3. Flow regimes photos of 300 μm Inner Mongolia pulverized coal. divided into pressurizing gas, fluidizing gas, and supplemental gas. The sending hopper adopts the bottom-fluidization and top-discharge arrangement. Pulverized coal in the sending hopper is fluidized by fluidizing gas and enters the conveying pipeline through the accelerating section. Supplemental gas is introduced at the outlet of the sending hopper to enhance the conveying ability and to adjust the solid−gas ratio. Pressurizing gas is used to maintain the pressure in the sending hopper. The pressure of the receiving hopper is controlled by a motor-drive control valve. Both the sending hopper and the receiving hopper have a capacity of 0.648m3. The conveying pipeline, which is made of stainless steel tubes, has a total length of 53.4 m and an inner diameter of 10 mm. The gas volumetric flow rates are measured by metal tube rotameters, and the mass of materials in the hopper is measured by load cells. Differential pressures are acquired by single crystalline silicon resonance transmitters with the precision of 0.075%. The data of differential pressures, mass of pulverized coal, and gas volumetric flow rates are collected and transmitted to the computer through a multichannel sampling system and an A/D converter.

the superficial gas velocity corresponding to the minimum pressure drop (the economic gas velocity) is approximately 7.8 m·s−1. At a relatively higher gas velocity above 7.8 m·s−1, the pressure drop declines with Vg decreasing, due to the reduction of frictional resistance between gas−solid fluids and pipe walls, yet further reducing Vg to below 7.8 m·s−1 will result in an increase of the pressure drop. This is mainly because, when Vg drops to 6.1 m·s−1 or even lower, the majority of particles are not suspended in the carrying gas any longer; instead, they deposit down to the bottom of the pipeline. Accumulation of these particles produces the stratified flow (Figure 3b) and the dune flow (Figure 3c); consequently, the contact area between particles and pipe wall increases, which causes the growth in flow resistance and pressure drop as well. B

DOI: 10.1021/acs.energyfuels.7b01752 Energy Fuels XXXX, XXX, XXX−XXX

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4 and 5 that, at approximately the same solid mass flow rate, the economic gas velocity for pulverized coal with 300 μm is slightly higher, indicating that larger coal particles are more likely to be conveyed in dense-phase form. Figure 6 illustrates that, keeping the solid phase flow rate constant for 52 μm Inner Mongolia pulverized coal, the

The comparison of Figures 2 and 3 indicates that the phase diagram can not only show the relationship of pressure drop and Vg but also reflect the flow characteristics including flow regime transition to a great extent. In other words, the phase diagram can provide a simple and feasible way of characterizing the pneumatic conveying process under high pressure, especially for the pulverized coal with fine particle sizes of less than 100 μm, since the optical observation is severely limited by the opacity of the solid phase resulting from the strong adhesion and electrostatic attraction of particulates to the pipe wall. 3.2. Phase Diagrams of Vertical and Horizontal Pneumatic Conveying. Figure 4 shows the phase diagram

Figure 6. Phase diagram of 52 μm Inner Mongolia pulverized coal along horizontal pipe at different solid flow rates.

horizontal pressure drop first declines and then increases with the decreasing superficial gas velocity, which exhibits a similar trend to that in Figure 4. In comparison, Figure 7 depicts the Figure 4. Phase diagram of 52 μm Inner Mongolia pulverized coal along vertical pipe at different solid flow rates.

of 52 μm Inner Mongolia pulverized coal along the vertical pipe at different solid flow rates. Similar to Figure 2, as the superficial gas velocity decreases, the vertical pressure drop first falls and then rises, at two different solid phase mass flow rates. Higher solid phase mass flow rate leads to a higher vertical pressure drop and a higher economic gas velocity. By contrast, the phase diagram of 300 μm Inner Mongolia pulverized coal along the vertical pipe at different solid flow rates is displayed in Figure 5. Although the trend of the curves is similar to that in Figure 4, it can be seen that, at approximately the same solid phase mass flow rate of 500 kg·h−1, the pressure drop of larger particles is higher than that of smaller ones, and such a difference is more significant when the solid phase mass flow rate increases to about 760−780 kg·h−1. It can be further obtained from Figures

Figure 7. Phase diagram of 300 μm Inner Mongolia pulverized coal along horizontal pipe at different solid flow rates.

Figure 5. Phase diagram of 300 μm Inner Mongolia pulverized coal along vertical pipe at different solid flow rates.

phase diagram of 300 μm Inner Mongolia pulverized coal along the horizontal pipe at two different solid flow rates. Similar to vertical conveying, the horizontal pressure drop of larger particles is still higher than that of smaller ones, and this difference becomes considerably more at a higher solid phase mass flow rate. Additionally, the economic gas velocity is higher for larger particles at the same solid flow rate, since larger particles are more difficult to be carried by gas flow and they tend to fall down onto the bottom of the pipeline, forming stratified or dune flow, as shown in Figure 3. Furthermore, in order to find out the difference between vertical and horizontal pneumatic conveying, phase diagrams of 52 μm Inner Mongolia pulverized coal are plotted in Figure 8, in terms of different solid phase mass flow rates and vertical/horizontal layouts. Obviously, the vertical pressure drop is always higher than the horizontal at the same solid phase mass flow rate, and such a C

DOI: 10.1021/acs.energyfuels.7b01752 Energy Fuels XXXX, XXX, XXX−XXX

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of larger particles. In addition, at the same solid mass flow rate, particle size, and superficial gas velocity, pressure drops of the vertical pipe are always higher than those of the horizontal pipe caused by the gravity effect. Moreover, it should be pointed out that the economic gas velocity of 52 μm seems paradoxically higher than that of 115 μm in Figures 9 and 10. Our previous work indicated that particulates with the average size of 52 μm, which belong to Geldart A particles, feature strong adhesion and electrostatic attraction,8 making it difficult to recognize the flow patterns by direct optical techniques due to the opacity of the solid phase.9 This electrostatic attraction is the main cause for particles agglomeration,21 which suggests that such particulates tend to be agglomerated into large particles in the pneumatic conveying process and, consequently, leads to a higher economic gas velocity than they should have. In addition, due to a larger specific surface area at the same loading, small particles exhibit greater electrostatic effects than bigger ones.22 Most probably for this reason, the economic gas velocity of 52 μm becomes higher than that of 115 μm. 3.4. Influences of Coal Types on the Phase Diagrams. Figure 11 shows the phase diagrams of Datong and Yanzhou

Figure 8. Comparison of phase diagrams for 52 μm Inner Mongolia pulverized coal along vertical and horizontal pipelines.

difference becomes more noticeable with the solid phase mass flow rate increasing due to the gravity. 3.3. Influences of Particle Sizes on the Phase Diagrams. Figures 9 and 10 describe the phase diagrams of

Figure 9. Phase diagrams of Inner Mongolia pulverized coal with different particle sizes along vertical pipeline.

Figure 11. Phase diagrams of pulverized coal of different types along horizontal pipeline.

pulverized coal, respectively, at very close experimental conditions of solid mass flow rate of 430 kg·h−1 and conveying pressure of around 3.0 MPa. The change of pressure drop with superficial gas velocity is similar; however, at the same superficial gas velocity, the pressure drop of Datong pulverized coal is higher, which can be interpreted and analyzed as follows. The pressure drop reflects the flow ability, which is determined by many factors, including density, particle size, moisture content, permeability, apparent morphology, etc. When comparing two different types of coal particles, it is almost impossible to determine how each factor impacts the flow characteristics. For this reason, we employ the Flow Function (FF), which is proposed by Jenike,23 to evaluate the flow ability of different coal types. FF is a comprehensive parameter that can involve the above factors. It is noted from Figure 12 that the flow ability of Yanzhou pulverized coal is better than that of Datong pulverized coal. Furthermore, Figure 13, as a supporting proof, provides the morphologies of Yanzhou and Datong pulverized coal, from which it can be noticed that the latter’s surface is coarser, with many irregular-shaped minor particles sticking on large ones. The relatively smoother surface of Yanzhou pulverized coal is beneficial to reducing the flow resistance. In addition, it is

Figure 10. Phase diagrams of Inner Mongolia pulverized coal with different particle sizes along horizontal pipeline.

Inner Mongolia pulverized coal with different particle sizes, in vertical and horizontal pipes, respectively. As the superficial gas velocity decreases, all the pressure drops decline before they grow again. At the same gas velocity, the pressure drop of larger particles is always higher than that of smaller ones, both in vertical and horizontal pipes, due to the greater resistance loss D

DOI: 10.1021/acs.energyfuels.7b01752 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 2. Comparison of the Physical Properties materials

density (kg/m3)

Yanzhou pulverized coal lignite pulverized coal

1350 1400

mean dp (μm) moisture (%) 68 56

0.77 3.24

of moisture content on the flow characteristics of Yanzhou pulverized coal will be very similar to that in Figure 14, and comparing two moisture contents can uncover the influences of moisture content on the phase diagram, as depicted in Figure 15.

Figure 12. Flow function of pulverized coal of different types.

Figure 15. Phase diagrams of Yanzhou pulverized coal at different moisture content.

Figure 13. SEM micrographs of pulverized coals: (a) Yanzhou coal; (b) Datong coal.

It can be seen from Figure 15 that, despite a similar varying tendency, the pressure drop of the particles with higher moisture content is also higher, even though the upper curve is corresponding to a lower solid mass flow rate. Figure 16 clearly

further noticed that the particle size of Yanzhou pulverized coal has a more uniform distribution, which further contributes to a better flow ability. 3.5. Influences of Moisture Contents on the Phase Diagrams. Prior to the present work, we experimentally investigated the impact of moisture content on the solid flux for another type of lignite pulverized coal with the average particle size of 56 μm, as depicted in Figure 14. It is clearly shown that the solid flux decreases with an increasing moisture content, at five different moisture contents. In other words, higher moisture content results in a worse flow ability. Moreover, Table 2 shows that the lignite pulverized coal has a quite similar physical property to Yanzhou pulverized coal which is used in the present work. It can be therefore inferred that the influence

Figure 16. Flow function of Yanzhou pulverized coal at different moisture content.

illustrates that the flow ability with higher moisture content is worse. With the moisture content increasing, coal particles become wetter and tend to adhere to each other, forming agglomerates by liquid-bridging force, which considerably reduces the flow ability of coal particles in the pipe by increasing the flow resistance and the pressure drop.

Figure 14. Impact of moisture content on the solid flux of the lignite pulverized coal. E

DOI: 10.1021/acs.energyfuels.7b01752 Energy Fuels XXXX, XXX, XXX−XXX

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4. CONCLUSIONS This article focuses on the phase diagrams of pressurized pneumatic conveying of pulverized coal on a pilot-scale experimental setup, aiming to reveal the underlying mechanism for a reliable design and operation of practical coal gasification systems. The three most typical types of coal are selected as the experimental materials. Feasibility of phase-diagram-based analysis is first verified. Afterward, flow diagrams are studied at different solid phase mass flow rates, pipeline layouts, particle sizes, coal types, and moisture contents, respectively, with the conclusions as follows: (1) Both vertical and horizontal pressure drops fall first and then rise with the decreasing superficial gas velocity, and a higher solid mass flow rate leads to a higher economic gas velocity. (2) At the same solid phase mass flow rate, the pressure drop of larger particles is higher than that of smaller ones, and it can be obtained that the economic gas velocity of 300 μm coal particles is slightly higher than that of 52 μm coal particles, suggesting that large particles tend to be conveyed in a dense-phase flow regime. Nevertheless, the economic gas velocity of 52 μm coal particles, which belong to Geldart A particles, is higher than that of 115 μm coal particles, due to strong adhesion, electrostatic attraction, and agglomeration. (3) The flow ability of Yanzhou pulverized coal is better than that of Datong coal. In addition, moisture contents also play an important role on phase diagrams. The flow ability of coal particles with higher moisture content is worse, which consequently leads to a higher pressure drop.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peng Lu: 0000-0002-0938-1351 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by “China Postdoctoral Science Foundation”, No. 2015M571747; “National Natural Science Foundation of China”, No. 51506087; “the Fundamental Research Funds for the Central Universities”, No. NS2015017; and “Foundation of Graduate Innovation Center in NUAA” (No. kfjj20160210; kfjj20170202).

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NOMENCLATURE

dp = average particle size (μm) Gs = solid phase mass flow rate (kg·h−1) L = length of the visualization section (m) ΔP = pressure drop (kPa·m−1) ΔPh = pressure drop of the horizontal pipe (kPa·m−1) ΔPv = pressure drop of the vertical pipe (kPa·m−1) Vg = superficial gas velocity (m·s−1) W = moisture content (%) Ψ = solid flux (kg·s−1·m−2) F

DOI: 10.1021/acs.energyfuels.7b01752 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels (21) Yao, J.; Zhang, Y.; Wang, C. H.; Matsusaka, S.; Masuda, H. Electrostatics of the granular flow in a pneumatic conveying system. Ind. Eng. Chem. Res. 2004, 43, 7181−7199. (22) Yao, J.; Wang, C. H. Granular size and shape effect on electrostatics in pneumatic conveying systems. Chem. Eng. Sci. 2006, 61, 3858−3874. (23) Jenike, A. W. Storage and Flow of Solids. In Bulletin of the University of Utah; University of Utah: Salt Lake City, Utah, 1964; Vol. 53., No. 123.

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DOI: 10.1021/acs.energyfuels.7b01752 Energy Fuels XXXX, XXX, XXX−XXX