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Drying characteristics of low rank coals in a pressurized flash drying system
Accepted Manuscript Title: Drying characteristics of low rank coals in a pressurized flash drying system Authors: In Seop Gwak, You Ra Gwak, Ye Bin Kim, See Hoon Lee PII: DOI: Reference:
S1226-086X(17)30436-7 http://dx.doi.org/10.1016/j.jiec.2017.08.017 JIEC 3566
To appear in: Received date: Revised date: Accepted date:
16-6-2017 4-8-2017 11-8-2017
Please cite this article as: In Seop Gwak, You Ra Gwak, Ye Bin Kim, See Hoon Lee, Drying characteristics of low rank coals in a pressurized flash drying system, Journal of Industrial and Engineering Chemistryhttp://dx.doi.org/10.1016/j.jiec.2017.08.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Drying characteristics of low rank coals in a pressurized flash drying system
In Seop Gwak, You Ra Gwak, Ye Bin Kim, and See Hoon Lee†
Department of Mineral Resources and Energy Engineering, Chonbuk National University, Jeonju, Korea
+ Corresponding Author: Chonbuk National University, Department of Resources and Energy Engineering, Jeonju, Korea Tel: +82-63-270-2362 E-mail : [email protected]
Highlights
► The characteristics of pressurized flash drying of low rank coal were investigated.
► The drying ratio mostly increased with temperatures and pressures. But the drying ratio did not increase at high pressure conditions because of increasing boiling temperature.
► The empirical equation to predict drying ratio was suggested as follows:
𝐷𝑟 = 2.82 × 𝑅 0.63 × 𝑆 0.02 × 𝑃0.28 × 𝑇 0.65
ABSTRACT Drying characteristics of low rank coals were examined and determined using a pressurized flash drying system which consisted of a pressurized feeder, rolled tubes(maximum of 6 m), a cyclone, and a back pressure regulator. The effect of the operating conditions, such as pressure(maximum of 40 bar), gas outlet temperature(maximum of 300 °C), particle sizes(212~300 and 300~355 µm) and residence time(maximum of 1 s) on the drying ratio was investigated and analyzed. The study results show that temperature is a more effective factor than pressure. A correlation equation to predict the drying ratio of coals was suggested based on operation conditions.
Keywords : low rank coal, drying, pressurized flash dryer, correlation equation
1. Introduction Large deposits of low rank coals are available in countries such as Australia, Eastern Europe, North America, Germany, and China[1]. Thus, the role of low rank coals in power generation[2] and in chemical industries[3] is predicted to grow in the long term. Low rank coals constitute about half of all coal deposits and are primarily used to generate electricity[4] because they are relatively inexpensive 20-30% cheaper than high grade coal[5, 6]. Hence, attention has been paid by many countries with insufficient energy resources to using more abundant and cheaper energy resources-an example of such a country would be Korea, which is the 9th largest coal consumer and 3rd largest coal importer[2]. Despite the large reserve and relatively low market price, these low rank coals have not been utilized to nearly the same extent as higher rank coals[7]. The lack of interest in low rank coals is mainly due to their high moisture content and high oxygen functional groups, which leads to strong tendencies for self-ignition during both transportation and storage[8]. High moisture and volumetric matter contents(25-70 wt%) in low rank coals are problematic because they raise transport costs, lower calorific values, and complicate handling and grinding operations[9, 10]. When low rank coals with high moisture contents are used in combustors, they might lower combustion temperatures and generate larger volume of produced gas. This might result in increasing combustor size and construction cost[11]. In addition, power plants for low rank coals consume more coal as compared to those for bituminous coal with same electric power outputs, which increases CO2 emissions[12]. In the case of conventional low rank coal combustion, removing water might use 20-25% of the produced heat and result in a decrease of overall efficiency. Thus, optimizing the drying process increases the efficiency of a lignite power plant by 4-6 % [13]. The amount of lignite used decreases as the efficiency of the power plant increases, and this can reduce the generation of carbon dioxide. There
is a relative reduction in CO2/MWh by 30 % when the water content of coal is reduced from 60 % to 40 % [14]. In order to increase or expand the usage of low rank coals in industrial fields, drying, dehydration, dewatering, which implies removing moisture from natural or industrial materials to a specific moisture content, while ensuring prime product quality, high throughput, and minimal operational costs [15] is essential prior to combustion and gasification. The pre-drying technologies to increase the calorific value of low rank coal as well as improve the energy conversion efficiency have recently attracted the industrial communities[5]. Various types of drying, dehydration, dewatering processes including rotary driers[16], fluidized bed driers[17], mechanical thermal dewatering[18], hydrothermal dewatering[19], solvent dewatering[20], and flash drying[21] have been developed and used for removing moisture from low rank coal. Recently, more complicated drying systems based on self-heat recuperation technology have been developed for advanced energy saving[22, 23]. The rotary drier is mainly composed of steel drum with a slope of 2-5° to the horizontal and roller[24]. The pre-drying occurs while the low rank coal is in contact with flue gas[25] or superheated steam[26, 27]. The fluidized bed drying exhibits a high drying rate due to the high degree of contact between the coal particles and the drying gas[5]. The fluidized bed drying is widely used because of its compact structure, good mixing performance, and high heat and mass transfer rates [23]. Specially, using fluidized bed drying with self-heat recuperation system could reduce energy consumption by nearly 30% lower than that of vapor recompression drying[22, 23]. Compared to fluidized bed drying, mechanical thermal dewatering (MTE) and hydrothermal dewatering (HTD) were operated in fixed bed reactors[28, 29]. These drying processes need higher temperature because the higher temperature in fixed bed drying processes would generate a large driving force and remove more amount of water in low rank coals[23]. Flash drying is another drying process that is performed with a very short contact for a residence time that is less than 1s[24]. A flash dryer does not require a separate transport device because the gas and low rank coal are transported and dried together. In addition, this system is relatively easy to operate and control and has the advantage of low maintenance, construction costs, and easy integration of a flash drying step with a combustor or gasifier, when compared with other dryers[30]. In particular, the
integrated drying gasification combined cycle developed by Hurman Research Pty in Australia and Korea had used flash drying for improving carbon conversion efficiencies [4, 31]. Many previous studies on the low rank coal drying technology have focused on the drying characteristics and the comparisons of different types of dryers that have been operated under atmosphere condition because most of the power plants have been operated under atmosphere conditions. However, novel power plants, such as pressurized pulverized coal boiler, pressurized circulating fluidized bed boiler, and integrated gasification combined cycle (IGCC), have been designed to be operated at higher pressures, up to 40 bar to increase the conversion efficiency [31]. Accordingly, as the drying experiments of low rank coals at high pressure have been studied sparsely, there is no sufficient data or design criteria for the drying characteristics of low rank coals under higher pressure and temperature conditions required for novel power plants. In this study, the flash drying characteristics of low rank coals were investigated in a pressurized flash drying system. Additionally, this study evaluated the effect of the drying temperature, residence time, and particle size on the drying ratios and suggested a correlation equation to predict the drying ratio of low rank coal based on operation conditions.
2. Experimental 2.1. Pressurized flash drying system The pressurized flash drying experiments of low grade coals were conducted in a lab-scale experimental apparatus shown in Fig. 1. The pressurized flash drying system consisted of high pressure mass flow controllers(MFCs), a rolled tube, a pressurized container, a feed hopper, a disk type feeder, an external electric heater, a cyclone, a back pressure regulator, controllers, and auxiliary accessories. The flash dryer system included a rolled 1/4” sus tube(ASTM A 269 TP 316L) placed in an external electric heater. Pneumatic conveying drying with variation in residence time was achieved by modifying the tube lengths(2 m, 4 m, and 6 m). Further, the flash drying temperatures were varied and controlled using an external electric heater, and the pressure conditions were controlled by a back-pressure
regulator. The flow rate(6 m/s) was adjusted to obtain the same inlet volume flow rate across similar pressure conditions. The coal was fed to the roll tube from a feed hopper with a disk feeder(Distek, DW-S06) in a pressurized container. Before the flash drying experiments, the disk feeder calibration was performed in order to inject coal samples at 10 g/min. The high-pressure nitrogen gas was used to inject sample coal at mass flow rate of 10 g/min when the experimental pressure and temperature reached target conditions. Coal samples passed in the flash drier were separated and collected by using cyclone. Pressurized flash drying system can be integrated with a combustor or gasifier. Increasing operation temperature can potentially increase drying efficiency. However, higher operation temperature over the devolatilization temperature of low rank coal can result in the unintended weight loss of low rank coal. Thus, it is necessary to carefully control the operation temperature to prevent the devolatilization of coal samples. An external electric heater was used and controlled to increase and maintain temperatures inside the system used in the study. The average temperatures of gas outlet based on the temperature of the external electric heater were listed in Table 1. It can be seen that the mean gas temperatures at the end of the roll tube reached approximately 300 C, at which the devolatilization of coal could start. Thus, the thermal decomposition temperature was measured using a thermogravimetric analyzer. Fig. 2 shows the thermal decomposition profiles of the low rank coals used in this study. As a result, the thermal decomposition temperatures of coal A sample and coal B sample were measured as 320 C and 290 C, respectively. This is similar to the gas outlet temperature. However, it was very difficult to devolatilize low rank coal because the residence times of low rank coal in the pressurized flash drying system were less than 1 s. Thus, the maximum operation temperature in the pressurized flash drying system was 600 C in this study. Furthermore, the pressure conditions were changed from 1 bar to 40 bar to analyze the flash drying characteristics according to the pressure change. To investigate the effect of residence time on drying ratio at the same temperature and pressure condition, three different length of tubes were used in this study. All experimental conditions are summarized in Table 2.
2.2. Analytical methods
Two different low rank coals were chosen for this study. The raw coals were pulverized and sieved to a size similar to the average particle size in commercial circulating fluidized bed boilers. Table 3 shows the proximate analysis of the coal samples. The proximate analysis was conducted using a TGADSC1 thermogravimeter(LECO) and a TruSpec Elemental Analyzer(LECO). The analyses indicated high oxygen contents even with a dry ash-free basis that resulted in a low heating value of the sample. As shown in Table 3, the average moisture contents of the two coal samples were 21.5 wt% and 37.3 wt%. Nevertheless, the moisture content of each coal sample was different; thus, the moisture contents of the tested samples were analyzed in each experiment. Each coal sample was separated from gas and moisture by a cyclone and was finally sent to a hopper for storage and sampling. The samples were used to calculate the drying ratios. The moisture content can be expressed as follows: 𝑚𝑐 =
𝑤𝑜 − 𝑤𝑑 𝑤𝑜
(1)
where mc denotes the moisture content of each sample, wo denotes the original weight of sample, and wd denotes the weight of the moisture free sample. The process was performed using an oven drier in which each sample was dried at conditions of 106 C for 3 h. The drying ratio(Dr) of each sample was calculated by the following equation: 𝐷𝑟 = (1 −
𝑚𝑑 ) 𝑋 100 𝑚𝑜
(2)
where mo denotes the moisture content of the original coal sample, and md denotes moisture content of the dried coal. Coal is injected with nitrogen at a velocity of 6 m/s. The injection velocity of pneumatic conveying drying should be higher than the terminal velocity shown in Table 4. The discharge velocities of N2 flow at different operation conditions were calculated based on the volumetric flow rate of N2 and gas outlet temperature, as summarized in Table 1. As shown in Table 1, the discharge velocity of flow tended to increase with increased temperature.
3. Results and Discussion Coal drying involves various exchange processes including diffusion, convection, phase change (evaporation), and capillary action in which the driving forces for drying are temperature, pressure and concentration[14]. In order to investigate the effect of temperature on coal drying, experiments were conducted at different conditions (A, B, C, D, and E) using coal A sample with 300-355 µm in a 2 m tube. The experimental results were shown in Fig. 3. It can be seen that the drying ratio always increased with the increasing gas outlet temperature. The drying ratios of coal A sample increased from approximately 20 % to approximately 98 %. As the external heater temperature increased, the heat transfer into coal particles and the evaporation rate of moisture inside the particles were enhanced, and this resulted in an increase in the drying ratio[4]. The experimental results from Kim et al.[4] and this experimental results had a very similar pattern. Additionally, Nikolopoulos et al. [24] indicated that coal grains were heated and dried during pneumatic transportation and concluded that the most intensive drying occurred during the early phase of the process in the first 2-3 m of the dryer. Fig. 4 shows the effect of the operation pressure in flash dryer on the drying ratio of coal A sample with 300-355 µm in a 2 m tube. As observed, the drying ratio increased with increased operation pressure. Since the nitrogen gas velocity for coal transport was fixed in this study, the mass flow rate of nitrogen had to increase with increasing the pressure condition. Increasing the mass flow of nitrogen gas could result in an increase in drying ratio. The mass of the tested coal is fixed at 10 g/min, however it is found that the gas flow increases with increased pressure. Table 4 show that as the pressure increases, the mass ratio of gas to solid increases. Therefore, the drying ratio might increase because of the increase of N2 mass flowrate that caused by higher gas density. As shown in Fig. 4, the drying ratio at condition A had the maximum value and decreased over 20 bar. At high pressure conditions, the boiling temperature of water is higher than that at atmosphere condition (1bar at 100 C to 40bar at 250C). Increasing the boiling temperature might decrease the evaporation rate of moisture inside the coal particles. It means increasing the pressure at lower temperature might disadvantage flash drying. In case of devolatilization experiments at high pressure
conditions, the volatile yield of Australian coal decreased with the increase of pressure[32]. The effects of operation pressures and temperatures on the drying ratio of coal are shown in Fig. 5. It can be seen that the drying ratios increased with increasing both operational conditions. When comparing with the effect of pressures and temperatures, temperature might be more influential than operation pressure. The obtained drying ratios of different particle sizes of coal B sample in a 6 m tube at 40 bar condition were shown in Fig. 6. The moisture removal decreased in the flash dryer as the particle sizes of coal increased. This is because smaller coal particles have larger external surface area that facilitate heat transfer from the drying medium to the center of particles and moisture evaporation from inside the coal particles to the surface [11]. Kim et al. [4] performed the drying experiment based on changing the particle size using 100-2000 µm. As a result, the water content of each particle size distribution showed a difference from 4.92 % to 9.44 %. The two studies findings from Kim et al.[4] and this experiments had a very similar pattern. The effect of residence times on the drying ratio is shown in Fig 7. In the experiment, coal A sample was used with its particle size distribution corresponded to 300-355 µm. Using the same temperature conditions, as anticipated, a graph showing the drying ratio increasing was obtained as the residence time increased. The results of the experiment conducted by Dening et al. [34] showed that the moisture content of the sample decreases as the residence time increased. At condition A, the change in the drying ratio with the change of residence time was the greatest, and as the temperature increased, the effect of the residence time could be identified as small. When the same tube length was used, it was found that a higher the temperature resulted in shorter residence time but better drying ratio. It means that temperature might be a more influential factor. Thus, the optimal temperature and residence time should be considered in order to find optimal drying conditions. Therefore, a multiple regression analysis was performed on the obtained results to derive a correlation equation. Multiple regression analysis is a type of regression analysis that estimates causality between variables using statistical methods. In regression analysis, the independent variables cause an effect, and the dependent variables correspond to the results. A regression analysis involving a dependent variable and two or more independent variables is known as a multiple regression analysis. In order to
make the drying correlation equation, independent variables such as temperature, pressure, residence time, and particle size were used in this study. Additionally, the data from the experiments conducted by Kim et al. [4] were used. As a result, the following equation was obtained: 𝐷𝑟 = 2.82 × 𝑅 0.63 × 𝑆 0.02 × 𝑃0.28 × 𝑇 0.65
(3)
where R denotes residence time (s), S denotes particle size (μm), P denotes pressure (bar), and T denotes temperature (°C). The values obtained using this equation and the experimental results were compared, as shown in Fig 8. The equation was determined as reliable given that the calculated R2 value corresponded to 0.82.
4. Conclusion In this study, the characteristics of pressurized flash drying of low rank coal were investigated. The experiment was performed at a gas outlet temperature of 100-300 °C, a pressure of 1-40 bar, an average particle size range of 256-327.5 μm, and a residence time(tube lengths of 2-6 m) under changing conditions. The results indicated that the drying ratio increased with increased temperature. Moisture was removed to almost 98 %. The drying ratio increased as the pressure increased, while the drying ratio did not increase under a pressure of 1-20 bar at lower temperatures as it was affected by the boiling temperature of water. Additionally, the drying ratio increased as the residence time increased at the same temperature; however, the results indicated that the drying ratio was less affected by the residence time as the temperature increased. A correlation equation to predict the drying ratio of low rank coal under the experimental conditions was developed.
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2017R1D1A3B03033160) and by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) (No. CRC-15-07-KIER).
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List of Figures
Fig. 1. Schematic of flash drying device
Fig. 2. Thermal decomposition profiles of coal samples
Fig. 3. Effect of gas outlet temperature on Dr in Coal A (Tube length : 2m)
Fig. 4. Effect of pressure on Dr in Coal A (Tube length : 2m)
Fig. 5. Dr according to gas outlet temperatures and pressures in Coal A (Tube length : 2m)
Fig. 6. Effect of temperature on Dr with different particle sizes in Coal B (Tube length : 6m)
Fig. 7. Dr with residence times in Coal A (Tube length : 2, 4, 6m)
Calculated Results [% ]
100
90
Coal A (212-300um) Coal A (300-355um) Coal B (212-300um) Coal B (300-355um) Kim et al. [2]
80
R2 = 0.82 70
60 60
70
80
90
Experimental Results [% ]
Fig. 8. Comparison of the experimental and calculated results
100
Table 1 Mean gas outlet temperature and discharge velocities of flow at different conditions Condition
External heater temperature (C)
Mean gas outlet temperature (C) Velocity of flow (m/s)
A
200
104
7.6
B
300
152
8.6
C
400
200
9.5
D
500
251
10.6
E
600
297
11.5
Table 2 Flash drying experiment conditions
Coal A (Moisture Content : 21.7%) Coal Type Coal B (Moisture Content : 37.3%)
Coal particle size (µm)
212-300, 300-355
Drying gas
N2
Pressure (bar)
1, 10, 20, 30, 40
Reactor temperature (°C)
200, 300, 400, 500, 600
Tube length (m)
2, 4, 6
Coal mass rate (g/min)
10
Coal input temperature (C)
25
Table 3 Proximate analysis of coal samples Coal
Moisture (%)
Volatile matter (%)
Fixed Carbon (%)
Ash (%)
Coal A
21.5
35.3
31.3
11.9
Coal B
37.3
27.4
28.8
6.5
Table 4 Mass ratios of gas to solid according to pressurized conditions Pressure (bar) 1 10 20 30 40
Gas/Coal mass ratio (-) 1.375 13.75 27.5 41.25 55
Table 5 dp*, Ut*, Ut according to particle sizes of samples Particle size
Average particle size (µm) dp*
Ut*
Ut
212-300 µm
256
11.87
2.06
1.36 m/s
300-355 µm
327.5
15.18
2.52
1.67 m/s
dp*,Ut*,Ut calculated using equations [33]
S1226-086X(17)30436-7 http://dx.doi.org/10.1016/j.jiec.2017.08.017 JIEC 3566
To appear in: Received date: Revised date: Accepted date:
16-6-2017 4-8-2017 11-8-2017
Please cite this article as: In Seop Gwak, You Ra Gwak, Ye Bin Kim, See Hoon Lee, Drying characteristics of low rank coals in a pressurized flash drying system, Journal of Industrial and Engineering Chemistryhttp://dx.doi.org/10.1016/j.jiec.2017.08.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Drying characteristics of low rank coals in a pressurized flash drying system
In Seop Gwak, You Ra Gwak, Ye Bin Kim, and See Hoon Lee†
Department of Mineral Resources and Energy Engineering, Chonbuk National University, Jeonju, Korea
+ Corresponding Author: Chonbuk National University, Department of Resources and Energy Engineering, Jeonju, Korea Tel: +82-63-270-2362 E-mail : [email protected]
Highlights
► The characteristics of pressurized flash drying of low rank coal were investigated.
► The drying ratio mostly increased with temperatures and pressures. But the drying ratio did not increase at high pressure conditions because of increasing boiling temperature.
► The empirical equation to predict drying ratio was suggested as follows:
𝐷𝑟 = 2.82 × 𝑅 0.63 × 𝑆 0.02 × 𝑃0.28 × 𝑇 0.65
ABSTRACT Drying characteristics of low rank coals were examined and determined using a pressurized flash drying system which consisted of a pressurized feeder, rolled tubes(maximum of 6 m), a cyclone, and a back pressure regulator. The effect of the operating conditions, such as pressure(maximum of 40 bar), gas outlet temperature(maximum of 300 °C), particle sizes(212~300 and 300~355 µm) and residence time(maximum of 1 s) on the drying ratio was investigated and analyzed. The study results show that temperature is a more effective factor than pressure. A correlation equation to predict the drying ratio of coals was suggested based on operation conditions.
Keywords : low rank coal, drying, pressurized flash dryer, correlation equation
1. Introduction Large deposits of low rank coals are available in countries such as Australia, Eastern Europe, North America, Germany, and China[1]. Thus, the role of low rank coals in power generation[2] and in chemical industries[3] is predicted to grow in the long term. Low rank coals constitute about half of all coal deposits and are primarily used to generate electricity[4] because they are relatively inexpensive 20-30% cheaper than high grade coal[5, 6]. Hence, attention has been paid by many countries with insufficient energy resources to using more abundant and cheaper energy resources-an example of such a country would be Korea, which is the 9th largest coal consumer and 3rd largest coal importer[2]. Despite the large reserve and relatively low market price, these low rank coals have not been utilized to nearly the same extent as higher rank coals[7]. The lack of interest in low rank coals is mainly due to their high moisture content and high oxygen functional groups, which leads to strong tendencies for self-ignition during both transportation and storage[8]. High moisture and volumetric matter contents(25-70 wt%) in low rank coals are problematic because they raise transport costs, lower calorific values, and complicate handling and grinding operations[9, 10]. When low rank coals with high moisture contents are used in combustors, they might lower combustion temperatures and generate larger volume of produced gas. This might result in increasing combustor size and construction cost[11]. In addition, power plants for low rank coals consume more coal as compared to those for bituminous coal with same electric power outputs, which increases CO2 emissions[12]. In the case of conventional low rank coal combustion, removing water might use 20-25% of the produced heat and result in a decrease of overall efficiency. Thus, optimizing the drying process increases the efficiency of a lignite power plant by 4-6 % [13]. The amount of lignite used decreases as the efficiency of the power plant increases, and this can reduce the generation of carbon dioxide. There
is a relative reduction in CO2/MWh by 30 % when the water content of coal is reduced from 60 % to 40 % [14]. In order to increase or expand the usage of low rank coals in industrial fields, drying, dehydration, dewatering, which implies removing moisture from natural or industrial materials to a specific moisture content, while ensuring prime product quality, high throughput, and minimal operational costs [15] is essential prior to combustion and gasification. The pre-drying technologies to increase the calorific value of low rank coal as well as improve the energy conversion efficiency have recently attracted the industrial communities[5]. Various types of drying, dehydration, dewatering processes including rotary driers[16], fluidized bed driers[17], mechanical thermal dewatering[18], hydrothermal dewatering[19], solvent dewatering[20], and flash drying[21] have been developed and used for removing moisture from low rank coal. Recently, more complicated drying systems based on self-heat recuperation technology have been developed for advanced energy saving[22, 23]. The rotary drier is mainly composed of steel drum with a slope of 2-5° to the horizontal and roller[24]. The pre-drying occurs while the low rank coal is in contact with flue gas[25] or superheated steam[26, 27]. The fluidized bed drying exhibits a high drying rate due to the high degree of contact between the coal particles and the drying gas[5]. The fluidized bed drying is widely used because of its compact structure, good mixing performance, and high heat and mass transfer rates [23]. Specially, using fluidized bed drying with self-heat recuperation system could reduce energy consumption by nearly 30% lower than that of vapor recompression drying[22, 23]. Compared to fluidized bed drying, mechanical thermal dewatering (MTE) and hydrothermal dewatering (HTD) were operated in fixed bed reactors[28, 29]. These drying processes need higher temperature because the higher temperature in fixed bed drying processes would generate a large driving force and remove more amount of water in low rank coals[23]. Flash drying is another drying process that is performed with a very short contact for a residence time that is less than 1s[24]. A flash dryer does not require a separate transport device because the gas and low rank coal are transported and dried together. In addition, this system is relatively easy to operate and control and has the advantage of low maintenance, construction costs, and easy integration of a flash drying step with a combustor or gasifier, when compared with other dryers[30]. In particular, the
integrated drying gasification combined cycle developed by Hurman Research Pty in Australia and Korea had used flash drying for improving carbon conversion efficiencies [4, 31]. Many previous studies on the low rank coal drying technology have focused on the drying characteristics and the comparisons of different types of dryers that have been operated under atmosphere condition because most of the power plants have been operated under atmosphere conditions. However, novel power plants, such as pressurized pulverized coal boiler, pressurized circulating fluidized bed boiler, and integrated gasification combined cycle (IGCC), have been designed to be operated at higher pressures, up to 40 bar to increase the conversion efficiency [31]. Accordingly, as the drying experiments of low rank coals at high pressure have been studied sparsely, there is no sufficient data or design criteria for the drying characteristics of low rank coals under higher pressure and temperature conditions required for novel power plants. In this study, the flash drying characteristics of low rank coals were investigated in a pressurized flash drying system. Additionally, this study evaluated the effect of the drying temperature, residence time, and particle size on the drying ratios and suggested a correlation equation to predict the drying ratio of low rank coal based on operation conditions.
2. Experimental 2.1. Pressurized flash drying system The pressurized flash drying experiments of low grade coals were conducted in a lab-scale experimental apparatus shown in Fig. 1. The pressurized flash drying system consisted of high pressure mass flow controllers(MFCs), a rolled tube, a pressurized container, a feed hopper, a disk type feeder, an external electric heater, a cyclone, a back pressure regulator, controllers, and auxiliary accessories. The flash dryer system included a rolled 1/4” sus tube(ASTM A 269 TP 316L) placed in an external electric heater. Pneumatic conveying drying with variation in residence time was achieved by modifying the tube lengths(2 m, 4 m, and 6 m). Further, the flash drying temperatures were varied and controlled using an external electric heater, and the pressure conditions were controlled by a back-pressure
regulator. The flow rate(6 m/s) was adjusted to obtain the same inlet volume flow rate across similar pressure conditions. The coal was fed to the roll tube from a feed hopper with a disk feeder(Distek, DW-S06) in a pressurized container. Before the flash drying experiments, the disk feeder calibration was performed in order to inject coal samples at 10 g/min. The high-pressure nitrogen gas was used to inject sample coal at mass flow rate of 10 g/min when the experimental pressure and temperature reached target conditions. Coal samples passed in the flash drier were separated and collected by using cyclone. Pressurized flash drying system can be integrated with a combustor or gasifier. Increasing operation temperature can potentially increase drying efficiency. However, higher operation temperature over the devolatilization temperature of low rank coal can result in the unintended weight loss of low rank coal. Thus, it is necessary to carefully control the operation temperature to prevent the devolatilization of coal samples. An external electric heater was used and controlled to increase and maintain temperatures inside the system used in the study. The average temperatures of gas outlet based on the temperature of the external electric heater were listed in Table 1. It can be seen that the mean gas temperatures at the end of the roll tube reached approximately 300 C, at which the devolatilization of coal could start. Thus, the thermal decomposition temperature was measured using a thermogravimetric analyzer. Fig. 2 shows the thermal decomposition profiles of the low rank coals used in this study. As a result, the thermal decomposition temperatures of coal A sample and coal B sample were measured as 320 C and 290 C, respectively. This is similar to the gas outlet temperature. However, it was very difficult to devolatilize low rank coal because the residence times of low rank coal in the pressurized flash drying system were less than 1 s. Thus, the maximum operation temperature in the pressurized flash drying system was 600 C in this study. Furthermore, the pressure conditions were changed from 1 bar to 40 bar to analyze the flash drying characteristics according to the pressure change. To investigate the effect of residence time on drying ratio at the same temperature and pressure condition, three different length of tubes were used in this study. All experimental conditions are summarized in Table 2.
2.2. Analytical methods
Two different low rank coals were chosen for this study. The raw coals were pulverized and sieved to a size similar to the average particle size in commercial circulating fluidized bed boilers. Table 3 shows the proximate analysis of the coal samples. The proximate analysis was conducted using a TGADSC1 thermogravimeter(LECO) and a TruSpec Elemental Analyzer(LECO). The analyses indicated high oxygen contents even with a dry ash-free basis that resulted in a low heating value of the sample. As shown in Table 3, the average moisture contents of the two coal samples were 21.5 wt% and 37.3 wt%. Nevertheless, the moisture content of each coal sample was different; thus, the moisture contents of the tested samples were analyzed in each experiment. Each coal sample was separated from gas and moisture by a cyclone and was finally sent to a hopper for storage and sampling. The samples were used to calculate the drying ratios. The moisture content can be expressed as follows: 𝑚𝑐 =
𝑤𝑜 − 𝑤𝑑 𝑤𝑜
(1)
where mc denotes the moisture content of each sample, wo denotes the original weight of sample, and wd denotes the weight of the moisture free sample. The process was performed using an oven drier in which each sample was dried at conditions of 106 C for 3 h. The drying ratio(Dr) of each sample was calculated by the following equation: 𝐷𝑟 = (1 −
𝑚𝑑 ) 𝑋 100 𝑚𝑜
(2)
where mo denotes the moisture content of the original coal sample, and md denotes moisture content of the dried coal. Coal is injected with nitrogen at a velocity of 6 m/s. The injection velocity of pneumatic conveying drying should be higher than the terminal velocity shown in Table 4. The discharge velocities of N2 flow at different operation conditions were calculated based on the volumetric flow rate of N2 and gas outlet temperature, as summarized in Table 1. As shown in Table 1, the discharge velocity of flow tended to increase with increased temperature.
3. Results and Discussion Coal drying involves various exchange processes including diffusion, convection, phase change (evaporation), and capillary action in which the driving forces for drying are temperature, pressure and concentration[14]. In order to investigate the effect of temperature on coal drying, experiments were conducted at different conditions (A, B, C, D, and E) using coal A sample with 300-355 µm in a 2 m tube. The experimental results were shown in Fig. 3. It can be seen that the drying ratio always increased with the increasing gas outlet temperature. The drying ratios of coal A sample increased from approximately 20 % to approximately 98 %. As the external heater temperature increased, the heat transfer into coal particles and the evaporation rate of moisture inside the particles were enhanced, and this resulted in an increase in the drying ratio[4]. The experimental results from Kim et al.[4] and this experimental results had a very similar pattern. Additionally, Nikolopoulos et al. [24] indicated that coal grains were heated and dried during pneumatic transportation and concluded that the most intensive drying occurred during the early phase of the process in the first 2-3 m of the dryer. Fig. 4 shows the effect of the operation pressure in flash dryer on the drying ratio of coal A sample with 300-355 µm in a 2 m tube. As observed, the drying ratio increased with increased operation pressure. Since the nitrogen gas velocity for coal transport was fixed in this study, the mass flow rate of nitrogen had to increase with increasing the pressure condition. Increasing the mass flow of nitrogen gas could result in an increase in drying ratio. The mass of the tested coal is fixed at 10 g/min, however it is found that the gas flow increases with increased pressure. Table 4 show that as the pressure increases, the mass ratio of gas to solid increases. Therefore, the drying ratio might increase because of the increase of N2 mass flowrate that caused by higher gas density. As shown in Fig. 4, the drying ratio at condition A had the maximum value and decreased over 20 bar. At high pressure conditions, the boiling temperature of water is higher than that at atmosphere condition (1bar at 100 C to 40bar at 250C). Increasing the boiling temperature might decrease the evaporation rate of moisture inside the coal particles. It means increasing the pressure at lower temperature might disadvantage flash drying. In case of devolatilization experiments at high pressure
conditions, the volatile yield of Australian coal decreased with the increase of pressure[32]. The effects of operation pressures and temperatures on the drying ratio of coal are shown in Fig. 5. It can be seen that the drying ratios increased with increasing both operational conditions. When comparing with the effect of pressures and temperatures, temperature might be more influential than operation pressure. The obtained drying ratios of different particle sizes of coal B sample in a 6 m tube at 40 bar condition were shown in Fig. 6. The moisture removal decreased in the flash dryer as the particle sizes of coal increased. This is because smaller coal particles have larger external surface area that facilitate heat transfer from the drying medium to the center of particles and moisture evaporation from inside the coal particles to the surface [11]. Kim et al. [4] performed the drying experiment based on changing the particle size using 100-2000 µm. As a result, the water content of each particle size distribution showed a difference from 4.92 % to 9.44 %. The two studies findings from Kim et al.[4] and this experiments had a very similar pattern. The effect of residence times on the drying ratio is shown in Fig 7. In the experiment, coal A sample was used with its particle size distribution corresponded to 300-355 µm. Using the same temperature conditions, as anticipated, a graph showing the drying ratio increasing was obtained as the residence time increased. The results of the experiment conducted by Dening et al. [34] showed that the moisture content of the sample decreases as the residence time increased. At condition A, the change in the drying ratio with the change of residence time was the greatest, and as the temperature increased, the effect of the residence time could be identified as small. When the same tube length was used, it was found that a higher the temperature resulted in shorter residence time but better drying ratio. It means that temperature might be a more influential factor. Thus, the optimal temperature and residence time should be considered in order to find optimal drying conditions. Therefore, a multiple regression analysis was performed on the obtained results to derive a correlation equation. Multiple regression analysis is a type of regression analysis that estimates causality between variables using statistical methods. In regression analysis, the independent variables cause an effect, and the dependent variables correspond to the results. A regression analysis involving a dependent variable and two or more independent variables is known as a multiple regression analysis. In order to
make the drying correlation equation, independent variables such as temperature, pressure, residence time, and particle size were used in this study. Additionally, the data from the experiments conducted by Kim et al. [4] were used. As a result, the following equation was obtained: 𝐷𝑟 = 2.82 × 𝑅 0.63 × 𝑆 0.02 × 𝑃0.28 × 𝑇 0.65
(3)
where R denotes residence time (s), S denotes particle size (μm), P denotes pressure (bar), and T denotes temperature (°C). The values obtained using this equation and the experimental results were compared, as shown in Fig 8. The equation was determined as reliable given that the calculated R2 value corresponded to 0.82.
4. Conclusion In this study, the characteristics of pressurized flash drying of low rank coal were investigated. The experiment was performed at a gas outlet temperature of 100-300 °C, a pressure of 1-40 bar, an average particle size range of 256-327.5 μm, and a residence time(tube lengths of 2-6 m) under changing conditions. The results indicated that the drying ratio increased with increased temperature. Moisture was removed to almost 98 %. The drying ratio increased as the pressure increased, while the drying ratio did not increase under a pressure of 1-20 bar at lower temperatures as it was affected by the boiling temperature of water. Additionally, the drying ratio increased as the residence time increased at the same temperature; however, the results indicated that the drying ratio was less affected by the residence time as the temperature increased. A correlation equation to predict the drying ratio of low rank coal under the experimental conditions was developed.
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2017R1D1A3B03033160) and by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) (No. CRC-15-07-KIER).
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List of Figures
Fig. 1. Schematic of flash drying device
Fig. 2. Thermal decomposition profiles of coal samples
Fig. 3. Effect of gas outlet temperature on Dr in Coal A (Tube length : 2m)
Fig. 4. Effect of pressure on Dr in Coal A (Tube length : 2m)
Fig. 5. Dr according to gas outlet temperatures and pressures in Coal A (Tube length : 2m)
Fig. 6. Effect of temperature on Dr with different particle sizes in Coal B (Tube length : 6m)
Fig. 7. Dr with residence times in Coal A (Tube length : 2, 4, 6m)
Calculated Results [% ]
100
90
Coal A (212-300um) Coal A (300-355um) Coal B (212-300um) Coal B (300-355um) Kim et al. [2]
80
R2 = 0.82 70
60 60
70
80
90
Experimental Results [% ]
Fig. 8. Comparison of the experimental and calculated results
100
Table 1 Mean gas outlet temperature and discharge velocities of flow at different conditions Condition
External heater temperature (C)
Mean gas outlet temperature (C) Velocity of flow (m/s)
A
200
104
7.6
B
300
152
8.6
C
400
200
9.5
D
500
251
10.6
E
600
297
11.5
Table 2 Flash drying experiment conditions
Coal A (Moisture Content : 21.7%) Coal Type Coal B (Moisture Content : 37.3%)
Coal particle size (µm)
212-300, 300-355
Drying gas
N2
Pressure (bar)
1, 10, 20, 30, 40
Reactor temperature (°C)
200, 300, 400, 500, 600
Tube length (m)
2, 4, 6
Coal mass rate (g/min)
10
Coal input temperature (C)
25
Table 3 Proximate analysis of coal samples Coal
Moisture (%)
Volatile matter (%)
Fixed Carbon (%)
Ash (%)
Coal A
21.5
35.3
31.3
11.9
Coal B
37.3
27.4
28.8
6.5
Table 4 Mass ratios of gas to solid according to pressurized conditions Pressure (bar) 1 10 20 30 40
Gas/Coal mass ratio (-) 1.375 13.75 27.5 41.25 55
Table 5 dp*, Ut*, Ut according to particle sizes of samples Particle size
Average particle size (µm) dp*
Ut*
Ut
212-300 µm
256
11.87
2.06
1.36 m/s
300-355 µm
327.5
15.18
2.52
1.67 m/s
dp*,Ut*,Ut calculated using equations [33]