Dense-Feeding of Pulverized Coal into the ... - ACS Publications

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Study on dense-feeding of pulverized coal into the entrained-flow gasifier Haifeng Lu, Xiaolei Guo, Yong Jin, and Xin Gong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02020 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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Industrial & Engineering Chemistry Research

Study on dense-feeding of pulverized coal into the entrained-flow gasifier Haifeng Lu, Xiaolei Guo, Yong Jin, Xin Gong∗ Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, Shanghai 200237, PR China ABSTRACT: Reliable feeding of pulverized coal into the entrained-flow gasifier is of great importance for the gasification process because powder feeding properties can affect the gasification performance index and the final product quality directly. With the background of the industrial demonstration of SE gasification technology, this paper focuses on its dense-feeding system which consists the hopper discharge unit and the pneumatic conveying unit. Firstly, the flow properties of pulverized coal in terms of angle of repose, compressibility and Carr’s flowability index were measured using a PT-X powder flow tester. Secondly, operation characteristics of pulverized coal discharged from the hopper were studied, where effects of particle size and hopper pressure were obtained based on analyzing the weight-versus-time curve and the pressure-versus-time curve. Finally, characteristics of dense-phase pneumatic conveying of pulverized coal were investigated by analyzing the system pressure distribution and the gas-solid flow stability. KEYWORDS: dense-feeding; pulverized coal; hopper discharge; pneumatic conveying; flow properties; entrained-flow gasification

Corresponding author. Tel: +86 21 6425 2521. Fax: +86 21 6425 1312

E-mail address: [email protected] (X. Gong). 1

ACS Paragon Plus Environment


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1. Introduction With the development of society and economy, there is an increasing need for energy. Fossil fuels are vital for our global energy needs, accounting for more than 80% of the world primary energy consumption.1 Fossil fuels will continue to dominate the world energy supplies in the 21st century and coal will play a significant role. Coal gasification technology2 appears to be an inevitable choice for chemical production, liquid fuel production, Integrated Gasification Combined Cycle (IGCC) power generation, poly-generation system and so on. The most eminent environmental advantage of coal gasification lies in its inherent reaction features that produce negligible sulfur and nitrogen oxides, as well as other pollutants. The development of advanced gasification technologies is the need of energy safety and sustainable development. Recently, kinds of entrained-flow coal gasification technologies are undergoing rapid industrialization around the world with the characterization of large-scale, high efficiency and clean emissions.3 Among them, the SE gasification technology developed by Sinopec (China Petroleum & Chemical Corporation) and ECUST (East China University of Science and Technology), is promising. Characterized as dry feeding of pulverized coal, membrane wall lining and water quench bath process, the SE gasification technology has many advantages such as wide adaptability of coal, low consumption of raw materials, advantageous performance and low cost in investment and operation. The SE entrained-flow gasifier is designed to operate at the pressure of 4.0 MPa and the temperature of 1400 ℃, with a throughput of 1000 t coal per day. This technology has been industrialized successfully since 2014 in Sinopec Yangzi Petrochemical Company Ltd., located in Nanjing, China, as shown in Figure 1.

2


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Figure 1. SE industrial plant in Sinopec Yangzi Petrochemical Company Ltd. As we known, reliable feeding of pulverized coal into the gasifier is an important issue for the dry coal gasification technology since it can affect the final product quality and the efficiency of the processes.4 In this industry, the dense-phase pneumatic conveying technology is usually adopted to feed pulverized coal into the gasifier. This technology is promising due to much smaller quantities of carrier gas consumption and more efficient power utilization. As compared with the dilute phase transportation, the dense-phase pneumatic conveying with a lower transport velocity can significantly reduce the wear of conveying lines and other mechanical parts. However, it is commonly accepted that handling coal is a major process concern for situations of adherence containing surfaces, excessive aeration in pneumatic conveying systems5, bridging in hoppers6 , electrostatic charging during handling and consolidation during transport or storage7 etc. What’s more, due to the different geographical environment, production techniques and applications, there is a wide range of particle size distribution, different moisture contents and coal types, all of which contribute to the changeability of coal, and as a consequence the processing or handling coal is certainly not straightforward or easily predictable.8 For the entrained-flow pulverized coal gasification, coal is always pulverized to a size in the 3



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order of 90% < 100 µm diameter to make sure the high efficiency of carbon conversion inside the gasifier. Such kind of powder shows poor flowability and strong cohesive forces due to its agglomeration effect, typical of Group C powders. Though intensive research has been carried out on the flow properties and the feeding characteristics of powders9~10, after a throughout investigation of these reports, we came to the conclusion that: (1) most studies were focused on Group A, B or D powders, while there were relatively fewer reports on pulverized coal applied in gasification; (2) most studies were carried out in small, laboratory-scale devices, while industrial-scale data were rarely found. It is therefore concluded that the fundamental study of dense feeding of pulverized coal into the gasifier is very limited and imperfect in comparison with increasing industrial applications of the entrained-flow gasification of pulverized coal. The purpose of this work is to assess on the performance and influencing factors of the dense-feeding system of the entrained-flow gasification. The research was carried out in the industrial demonstration plant of SE gasification technology located in Nanjing, China. The flowability of pulverized coal was first determined in terms of angle of repose, compressibility and Carr's flowability index. Then, discharge characteristics of pulverized coal from the hopper were studied and the effects of particle size and hopper pressure were discussed. Finally, characteristics of dense-phase pneumatic conveying of pulverized coal were investigated by analyzing the system pressure distribution and the gas-solid flow stability. The study in this work can provide a valuable reference for the dense-feeding of pulverized coal into the gasifier during the industrial operations. 2. Experimental setup The research was carried out in a SE gasification industrial plant located in Yangzi 4


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Petrochemical Company Ltd., Nanjing, China. Figure 2 shows the schematic diagram of SE gasification process, where the feeding system is regarded as one of the key technologies and presents challenges for whole process. As shown in this picture, the feeding system of pulverized coal can be simply divided into two parts: the hopper discharge unit where pulverized coal is discharged from atmospheric hopper to lock hopper then to feed hopper, and the pneumatic conveying unit where pulverized coal is conveyed from feed hopper to gasifier.

Figure 2. Schematic diagram of SE entrained-flow pulverized coal gasification process In this work, we focused on both the hopper discharge and the pneumatic conveying units. The core facilities include atmospheric hopper, lock hopper and feed hopper. Their structural parameters are shown as follows: (1) The atmospheric hopper, has an outlet diameter of 300 mm, a half opening angle of 15°, a column with the diameter of 5 m and the height of 21 m, giving the volume about 340 m3. (2) The lock hopper, has an outlet diameter of 300 mm, a half opening angle of 15°, a column with the diameter of 3.2 m and the height of 13 m, giving the volume about 80 m3. (3) The feed hopper, has a half opening angle of 15°, a column with the diameter of 5.2 m and the height of 10 m, giving the volume about 190 m3. The gasifier is currently operating at 60~70% capacity, using CO2 as carrier gas with respect to its production of hydrogen. The 5



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experimental data were taken at different operation times and the experimental materials (named coal A and B) were taken at corresponding times from the SE gasification industrial plant. The operation conditions were provided in Table S1. In particular, pressures were measured by the pressure sensors (Keller, Series 35X), which produced the measurement errors generally lower than ±0.05%. A mass flow meter system ((Model 2019, produced by Thermo Ramsey Company) was used to measure the flow of bulk solids through conveying lines. The system is composed of two independent sensors (the solid concentration sensor DC 13 and the solid velocity sensor DK 13) and a single transmitter, where the concentration sensor and the velocity sensor operate using capacitance technology. Coal mass flow rate was calculated by multiplying solid concentration, solid velocity, and flow cross-sectional area. As can be seen from Table S1, the solid concentration is in the order of 200kg/m3, typical of dense flow. In this case, it will be reasonable to consider the gas velocity equal to the solid velocity in the conveying system as suggested by Geldart and Ling11. Therefore, the slip velocity which is the relative velocity between gas and solid phase, approaches zero in this work. Some physical properties of pulverized coal were measured and listed in Table 1, where particle size distribution was measured using a laser granulometer Mastersizer 2000 with a wet dispersion unit. It is understood that during feeding into the gasifier, there might be some differences in the particle size distribution of the pulverized coal due to the diversities in mill parameters and raw coal properties. These differences will in turn affect the subsequent powder handing processes.12 As reported in Table 1 and Figure 3, both coal A and B cover a certain range of sizes with a mean particle diameter of 14.0µm and 19.8µm, respectively. In comparison, coal A shows smaller particle size and contains more fines. In addition, particle density was determined 6


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by mercury intrusion analysis13, with a value around 1400 kg/m3 for both coal A and coal B. The moisture content was measured by an infrared moisture analyzer MA150. The detected moisture contents of both coals are very small, lower than 1% suggesting a limited influence that can be ignored. 40

6

coal A coal B

coal A coal B

5

30

Volume(%)

Volume, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 3 2

20

10

1 0 0.1

0

1

10

100

1000

200

Particle size(µm)

Particle Size, um

Figure 3. Particle size distribution of pulverized coal The discussion above indicates that the feeding of pulverized coal into the entrained-flow gasifier will be affected by both the operation conditions and the coal properties. In this work, we aim to evaluate the operation performance and influencing factors of the dense-feeding system of pulverized coal in the SE gasification industrial plant. The hopper discharge unit was investigated where the effects of particle size and hopper pressure were discussed, and the pneumatic conveying unit was investigated where the system pressure distribution and the gas-solid flow stability were discussed. Table 1. Physical properties of pulverized coal Moisture

dsv

d10

d50

d90

< 40µm

> 105µm

Material

content (µm)

(µm)

(µm)

(µm)

(%)

(%) (%)

Coal A

14.0

6.4

38.4

108.2

58.68

5.55

0.14 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Coal B

19.8

9.1

74.5

218.6

35.86

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31.80

0.21

3. Results and discussion

3.1 Flow properties of pulverized coal The characterization of powder flow properties is often required for reliable design and proper operation of industrial processes. The flowability determines the powder operation properties with respect to arching in the hopper and jamming during the conveying process in a pipeline14~15. In this work, the flow properties of pulverized coal were characterized in terms of angle of repose, compressibility and Carr's flowability index (CFI). All these parameters were provided in Table 2 measured by the PT-X powder flow tester, produced by Hosokawa Micron Group. Table 2. Flow properties of pulverized coal measured by PT-X powder flow tester Coal A

Coal B

Item Measured value

Index

Measured value

Index

Angle of repose (°)

41.8

16

45.2

15

Compressibility (%)

39.7

2

27.2

12

Spatula angle (°)

58.7

16

59.8

15

Uniformity

7.5

21

10.8

18

CFI

55

60

Angle of repose is one of the primary properties in characterizing the flow behavior of powders. It is the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping. The larger value of angle of repose always corresponds to the weaker powder flowability. In establishing a relation between powder flowability and angle of repose, Geldart et al.16 used 40° as a criteria to classify free-flowing and cohesive powders. As it can be 8


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seen from Table 2, coal A and coal B are the angles of repose of 41.8° and 45.2°, larger than 40° and indicative of some cohesive properties. Compressibility is a measure of the relative volume change of a powder as a response to a pressure (or mean stress) change. It can offer another perspective on powder flowability, where the larger value corresponds the weaker flowability. The compressibility of coal A and coal B are 39.7 and 27.2, indicative of poor flowability and cohesive properties. In comparison of angle of repose and compressibility between coal A and coal B, it seems the relation between different parameters may be not consistent. In view of Leturia et al’s report17 that powders and bulk materials cannot be viewed as invariant entities, it is therefore considered that flow properties of powders cannot be predicted by only one indicator. Krantz et al18 further pointed out that flow properties were dependent upon the stress state and that no single technique was suitable for the full characterization of a powder. Consequently, the connection of several characterization methods is required to ensure a complete understanding of the powder flow properties over a wide range of conditions. In order to more comprehensively and objectively describe the powder flowability, Carr's flowability index was further adopted to characterize the flow properties of pulverized coal, which was defined as the sum of the four items (angle of repose, compressibility, spatula angle and uniformity) as listed in Table 2. The experimental results gave values of Carr's flowability index 55 and 60 for coal A and coal B, respectively. It therefore came to the conclusion that both coals indicated the cohesive characteristics and poor flowability. The flowability of coal A was worse, corresponding to the fact that it had small particle size and contained more fines.

3.2 Hopper discharge In this section, investigation was carried out on coal A and coal B in turn discharged from 9



the atmospheric hopper and then from the lock hopper which operated at atmospheric pressure and high pressure, respectively, and the effects of particle size and hopper pressure were discussed. The information of coal discharged from the atmospheric hopper is relatively simply, which can be determined from the weight-versus-time curve provided by the weighing cells installed on the atmospheric hopper to monitor the mass variation of the powders. As for the lock hopper, the weigh method is ineffective because the hopper pressure can be varied significantly associated with the operation condition of the gasifier, and the pressure change will contribute to the mass variation as well. Therefore, the discharge information of the lock hopper is more commonly obtained by analyzing its pressure-versus-time curve. A typical pressure time series of the lock hopper is plotted in Figure 4, which includes five operation cycles. Generally, the operating cycle of a lock hopper can be divided into four steps: filling, pressurization, discharge and depressurization, where the discharge takes place at the region of the constant high pressure. 5

tion

3

pressuriza

Pressure of lock hopper, MPa

discharge

4 depressurization

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2 1

2

1 0

0

2000

4000

6000

3 8000

4 10000

5 12000

14000

16000

Time, s

Figure 4. Pressure time series of the lock hopper Table S2 lists the experimental details taken from the SE gasification industrial plant. To obtain the statistics, each experiment was repeated five times corresponding to the five operation cycles shown in Figure 4. The first striking observation is that, the hopper pressure has a significant negative effect on the discharge of pulverized coal. Because of the large outlet diameter 10


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(300 mm) and the half opening angle (15°) adopted by the SE industrial hoppers, pulverized coal rarely presents discharge problems when flowing out of the atmospheric hopper, which can be always accomplished rapidly with the discharge time arranged from 90 s to 110 s giving the solid discharge rate around 1000t/h. However, after pressurization in the lock hopper, pulverized coal frequently will not discharge reliably and smoothly. As it can be seen from Table S2, the time of pulverized coal discharge from the lock hopper is ranged from 587 s to 886 s. The worst case is the formation of the static arch during the discharge which can block the flow completely. Comparison indicates that the discharge of pulverized coal under 4.20~4.26 MPa costs about 5~8 times than that under atmospheric pressure. The lock hopper receives pulverized coal at atmospheric pressure and, after suitable pressurization, discharges the pulverized coal under gravity into the feed hopper. Jenike19 thought that during the pressurization process, gas pressure gradients added to the consolidating stresses acting in the solid under its own weight, resulting in an increasing of the bulk density and the solid strength. The higher the strength, the more difficult the solid discharge. Our previous investigation20 on a bench-scale hopper showed that the hopper pressure could be positive if introduced as the “fluidized pressurization” method. However, it is generally by no means guaranteed to achieve the preferred fluidization quality during the pressurization in industry. Actually, in order to quickly increase the hopper pressure, gas is usually introduced from different sections of the lock hopper in the SE gasification industrial plant. Gas, that from the top will contribute to the gas pressure gradient acted on the solid with resultant significant densification and consolidation. As we known, powder cohesion is due to the contribution of different kinds of interparticle forces, such as van der Waals, electrostatic, and capillary forces. It depends closely on the consolidation conditions and will increase significantly 11



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because the number of contact points between the particles as well as the intensity of the contact force increases during the pressurization. As a result, unstable flow always occurs for the pulverized coal discharged from the pressurized hoppers associated with powder consolidation, discontinuous flow and arching phenomena etc. This has become a thorny issue and needs to be solved urgently to ensure the continuous, stable and long period operation of gasification. Further comparison shows that the discharge times of coal A and coal B are not quite different. As discussed above, both coals belong to cohesive powders with a mean particle diameter of 14.0 µm and 19.8 µm, respectively. It is accepted that, for thus fine particles, the discharge becomes especially difficult due to the cohesive interparticle interactions. As for the SE industrial hoppers, aeration was used to promote the discharge of pulverized coal, which improved the discharge and increased the solid discharge rate significantly. Because aeration is more effective in promoting the weaker flowability powders, it is not surprise that coal A and coal B show the approximate discharge time. It should be however noted that, coal A shows higher arching probability compared to coal B. As shown in Table S2, arch was formed for coal A discharged from the pressurized hopper while coal B could be discharged completely with the action of aeration. It agrees with the fact that coal A has small particle size and contains more fines, indicating that weaker powder flowability always corresponds to weaker discharge ability. Figure 5 shows the time series of pressure during the discharge of coal A from the lock hopper with respect to the operation case of arch formation. Both pressures of the lock hopper and the feed hopper were recorded. Once the arch is formed, the pressure of the lock hopper increases immediately, which corresponds to powder compaction and associated internal voidage reduction, resulting in a temporary increase of the gas pressure. On the contrary, the pressure of feed hopper 12


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decreases since the arch blocks its connection with the lock hopper. As a result, a positive pressure gradient develops between the lock hopper and the feed hopper and increases with time. When the pressure difference approaches about 150 kPa, the arch collapses and the discharge continues. Gas inside the lock hopper escapes from the outlet, resulting in the pressure decrease of the lock hopper and the pressure increase of the feed hopper. A new pressure balance will then build after a certain time. As shown in the same picture, two arches were formed in this case based on the analysis of the pressure signals. It should be noted that, the feed hopper is required to operate at almost constant pressure, slightly above that of the gasifier to assist feeding and prevent reverse flow of hot gases and bed materials from the gasifier. Though the arches formed during the discharge can be broken by appropriate means, this will however produce a considerable fluctuation in the pressure of the feed hopper. In Figure 5, the pressure fluctuation of the feed hopper is about 100 kPa. It is surely that any fluctuation in the pressure of the feed hopper will affect the coal mass flow rate directly and consequently result in the unstable operation of the gasifier.21 Thus, it is very important to understand the operation characteristics of the hopper discharge unit and make sure the smooth discharge of the pulverized coal. 4.45

Lock hopper Feed hopper

4.40

Pressure, MPa

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4.35

4.30

4.25

4.20 0

100

200

300

400

500

600

700

800

Time, s

Figure 5. Pressure signals of lock hopper and feed hopper during the discharge of coal A at the 13



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case of arch formation

3.3 Pneumatic conveying In the SE gasification technology, two conveying pipelines with inner diameter of 54 mm are led out from the bottom of the feed hopper, from which the pulverized coal is carried by high pressurized CO2 along a 30-m-long vertical section, a 10-m-long horizontal section, and finally entered the gasifier. Each pipeline is equipped with pressure sensors, solid mass flow rates and regulating valve to monitor and control the powder flow. In this section, dense-phase pneumatic conveying of pulverized coal from the feed hopper to the gasifier was investigated under the operation conditions provided in Table S1. Figure 6 shows the pressure distribution of the feeding system, which is determined with pressure sensors installed along the conveying network: one at the feed hopper, two at the pipeline, respectively at the vertical and the horizontal pipes, and one at the gasifier. The regulating valve, used to adjust the coal mass flow rate, is installed closely outside the feed hopper before the pressure sensor that installed at the vertical pipe. The valve opening is set at designated value to give the desired coal mass flow rate. The pressure difference between the feed hopper and the vertical pipe as shown in Figure 6 can be mostly attributed to the valve pressure drop, which takes up over 70% of the whole system pressure drop that is the difference between the feed hopper and the gasifier, indicating the regulating valve is the main resistance component of the feeding system. The valve, by varying its opening, can regulate the coal mass flow rate effectively. It should be noted that, the coal mass flow rate is more sensitive to the small valve openings associated with the more significant spool erosion. Meanwhile, the energy of solids movement is supplied by the system pressure difference which is regarded as a common mean to regulate the coal mass flow 14


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rate. As a result, to balance the requirements of the flexible control of the coal mass flow rate and the long-period operation of the regulating valve, it is important to match the conveying pressure difference and the valve opening. The comparison between two operation conditions in Table S1 indicates that the system pressure drop can be slightly decrease (0.99 MPa → 0.89 MPa) by increasing the valve opening (21.0%→ 22.3%), which also benefits the energy conservation. The pressure difference between the vertical pipe and the horizontal pipe is used for conveying purpose, to drive the pulverized coal inside the conveying pipeline. This pressure different, made up of the interactions between gas and particles, particles and particles, particles and walls, only takes up about 15% of the whole system pressure drop. It therefore indicates that there is optimizing space of the feeding system in the SE gasification industrial plant. In addition, the pressure difference between the horizontal pipe and the gasifier is mostly from the gas-solids flow through a nozzle, which produces a high speed jet flow to diffuse the powders entering the gasifier. 4.4 Coal A Coal B

4.2

Pressure, MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.0 3.8 3.6 3.4 3.2

Fe

ed h

op pe r

Ve r ti

ca

Ho lp

ip

e

r iz

on

ta l

p ip

Ga si f ier e

Figure 6. Pressure distribution of the feeding system Stability is one of the most important indicators to evaluate the dense-phase pneumatic 15



conveying technology. In gasification process, the stability of the pulverized coal feeding will directly affect the gasifier performance. It is considered that signals during the conveying seem to be random, but in essence include plenty of information. In this work, the feeding stability of the SE gasification technology was investigated by analyzing the typical signals obtained during the conveying process. Figure 7 plots the time series of pressure of feed hopper, of pressure of horizontal pipe, of solid mass flow rate and of solid concentration. It can be seen clearly that the dense-phase pneumatic conveying of pulverized coal in the SE gasification plant is relatively stable, though the pressure signal of the feed hopper shows some periodic features which is worth to minimize to seek for better feeding properties. The pipe pressure maintains at a nearly constant value with small fluctuations. The coal mass flow rate, determined by both the system pressure difference and the valve opening, shows a fluctuation of about 10%. The solid concentrations are 233 kg/m3 and 209 kg/m3 for coal A and B, respectively, typical of dense flow. 3.6

4.5

4.2 4.1 2000

4000

6000

8000

10000 12000 14000

4.5

Coal B

4.4 4.3 4.2 4.1 4.0 0

2000

4000

6000

8000

10000 12000 14000

Pressure of horizontal pipe, MPa

4.3

4.0 0

Coal A

Coal A

4.4

Pressure of feed hopper, MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3.5 3.4 3.3 3.2 0

2000

4000

6000

8000

10000 12000 14000

3.6

Coal B 3.5 3.4 3.3 3.2 0

2000

4000

Time, s

6000

8000

10000 12000 14000

Time, s

16


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300

18000

Coal A

Coal A

16000

270

14000

240

12000 10000 0

2000

4000

6000

8000

10000 12000 14000

18000 Coal B

16000 14000

210 180 0 300

2000

4000

6000

8000

10000 12000 14000

Coal B

270 240 210

12000 10000 0

Solid concentration, kg/m3

solid mass flow rate, kg/h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2000

4000

6000

8000

10000 12000 14000

180 0

2000

4000

Time, s

6000

8000

10000 12000 14000

Time, s

Figure 7. Time series of pressure, solid mass flow rate and solid concentration. The fluctuations of horizontal pressure, solid mass flow rate and solid concentration shown in Figure 7 could be probably attributed to the coal particles accumulation in the conveying system. It is considered that, for the dilute flow powders are usually suspended in the horizontal pipe and conveyed by the carrier gas with a high velocity. However, for the dense flow as depicted in the work, powders could accumulate in the elbows and on the bottom of the horizontal pipe and subsequently be re-entrained in the mainstream causing fluctuations in the pressure of the conveying system and in the mass flow rate of powders. According to the solid concentration profiles, we can divide the flow into three regions: the upper suspended region, the lower sedimentary region, and the transition region in between. It is accepted that there are significant momentum and mass exchanges in the transition region because of the actions of gravity and entrainment, which result in the fluctuations in the pressure, sold mass flow rate and solid concentration, etc. In order to further highlight this issue, the time series of pressure signals in the horizontal pipe were analyzed by adopting the power spectral density (PSD) function. The PSD function which is the magnitude of the Fast Fourier Transform (FFT) square divided by the time period 17



(Pahk, 2006), is defined as follows:

PSD( f ) =

2

2 X (n)

(1)

t2 − t1

where X(n) is the Fast Fourier Transform (FFT), t1 and t2 represent the start and end time. Figure 8 shows the PSD function of horizontal pressure signals. The first striking observation is that, the pressure signals of both coal A and coal B show a peak value of PSD at the small frequency. According to its deification, the peak value of PSD indicates its contribution of the corresponding frequency to the pressure fluctuation. Compared to coal B, coal A shows the larger peak value of PSD, indicating a worse conveying stability. Meanwhile, the PSD functions of both coal A and coal B show multimodal characteristics with some lower peaks at increasing frequencies. This feature indicates that there are multiscale pressure pulsations during the conveying, which are probably caused by the accumulation as well as the subsequent re-entrainment of coal particles in the horizontal pipe as discussed above. 600 Coal A

400 2

Power spectrum density, MPa /Hz

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200 0 6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Coal B

4 2 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Frequency, Hz

Figure 8. The PSD function of horizontal pressure signals for coal A and coal B Furthermore, in order to quantitatively compare the stability of coal A and coal B conveying, 18


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relative standard difference (RSD) and maximum amplitude (M) of the sample were calculated.

Std is the standard difference of the sample expressed as

Std =

1 n ( xi − x)2 ∑ n −1 i =1

where xi represents the time series of the sample and

(2)

x is the average value.

RSD can be calculated by dividing Std to the sample average value Mean.

RSD = Std / Mean

(3)

M is the maximum of the difference between instantaneous value to the average.

M =max xi − x

(4)

Table 3 summarizes the stability parameter including pressures, solid mass flow rate and solid concentration. Comparison indicates that coal A shows weaker conveying features with relatively larger RSD and M values of all the stability parameters. This finding agrees with the weaker flowability of coal A, and can be probably attributed to its poor discharge ability. Table 3. Stability parameter of the feeding system in the SE gasification industrial plant Pressure of feed hopper

Pressure at horizontal pipe

Solid mass flow rate

Solid concentration

Material

RSD

M/MPa

RSD

M/MPa

RSD

M/kg·h-1

RSD

M/kg·m-3

Coal A

0.0079

0.1105

0.0053

0.0812

0.0285

1799

0.0272

26.90

Coal B

0.0078

0.0826

0.0040

0.0527

0.0268

1230

0.0268

22.10

As shown in Figure 7, the pressure signal of the feed hopper shows some periodic features probably related to the periodic discharge of pulverized coal from the lock hopper. The feed hopper is the origin of pneumatic unit, and is also a component of the discharge unit. Figure 9 further plots together the pressure time series of lock hopper, feed pressure and gasifier tighter as a function of time. It can be seen that the pressure of the gasifier keeps almost constant during the 19



whole operation, but the pressure of the feed hopper fluctuates and is linked to the pressure of the lock hopper in essence. It should be noted that any fluctuation in the pressure of the feed hopper will affect the coal mass flow rate directly and consequently result in the unstable operation of the gasifier. It is therefore worth to study on the periodic characteristics of the pressure signal. As shown in the picture, the pressure of feed hopper contains both the short-period, small-range fluctuations and the long-period, large-range fluctuations. The former can be probably attributed to the intensive gas-solid interactions while the latter can be associated with the hopper discharge actions. Combined with what was discussed above, it is accepted that coal A is more cohesive and easy to form arches during the discharge from the pressurized lock hopper. Therefore, compared with coal B, coal A would produce more considerable fluctuations in the pressure of the feed hopper, consequently shows relatively weaker conveying stability. 4.5

3.5

Lock hopper

Feed hopper

arching

3.4

4.3 3.3

Pressure, MPa

Gasifier

4.4

Pressure, MPa

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Page 20 of 25

4.2

4.1

0

3000

6000

9000

12000

3.2 15000

Time, s

Figure 9. Pressure signals of lock hopper, feed hopper and gasifier 4. Conclusions In this paper, dense-feeding of pulverized coal with different particle sizes were investigated in the SE gasification industrial plant. The flow properties of coal A and B were determined by different indexes to provide a more comprehensive understanding of powder flowbility. Both coal 20


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A and B showed the cohesive characteristics and poor flowability, while the flowability of coal A was even worse due to its smaller particle size and more fines. The effects of particle size and hopper pressure on the hopper discharge unit were investigated. The pulverized coal can be discharge smoothly from the atmospheric hopper but may present problems from the lock hopper due its high pressure. The formation of arch during the discharge will produce a considerable fluctuation in the pressure of the feed hopper. The characteristics of dense-phase pneumatic conveying of pulverized coal were investigated by analyzing the system pressure distribution and the gas-solid flow stability. Suggestion is to better match the system pressure drop and the valve opening to balance the requirements of the flexible control of the coal mass flow rate and the long-period operation of the regulating valve. The dense-phase pneumatic conveying of pulverized coal in the SE gasification plant is relatively stable, though the pressure signal of the feed hopper shows some periodic features probably related to the periodic discharge of pulverized coal from the lock hopper which is worth to minimize to seek for better feeding properties. Comparison between different materials indicated that, coal A was more cohesive and formed arches during the discharge from the pressurized lock hopper, which produced considerable fluctuations in the pressure of the feed hopper, consequently affected the conveying stability finally. Acknowledgements This work was supported by the National Natural Science Foundation of China (21206041) and the Fundamental Research Funds for the Central Universities. Supporting Information Available This information is available free of charge via the Internet at http://pubs.acs.org/. References 21



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