Laboratory investigation of drying process of Illinois coals

Powder Technology 225 (2012) 72–85

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Laboratory investigation of drying process of Illinois coals Wei-Cheng Wang ⁎ Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695, USA

a r t i c l e

i n f o

Article history: Received 20 November 2011 Received in revised form 14 March 2012 Accepted 24 March 2012 Available online 30 March 2012 Keywords: Drying rate High moisture coal Drying temperature Coal-fired power plant Specific humidity Relative humidity

a b s t r a c t High moisture coal leads to low power plant efficiency, increased stack emissions of pollutants and maintenance and operational problems when it is used in coal fired power plants. In this study, laboratory experiments and theoretical calculations of the coal drying process were carried out and compared in order to determine proper drying conditions. This research describes several experiments to present the effects of parameters, such as drying temperature, on drying performance. The tests were carried out with three different types of coal — Buckheart, Crown mine and Viper mine, which have initial moisture contents of 23%, 18% and 20%, respectively. The drying tests were performed at 1.25 m/s air velocity and from 43 °C to 60 °C drying temperature. In this paper, the effect of drying temperature on drying rate of different types of coal was studied to obtain information relating to optimal operating conditions. The drying performances of each type of coal can be determined by analyzing the test data and operation conditions. A theoretical model was also utilized for the drying process based on mass balances and conservation of energy. Comparisons were made between experimental and theoretical results. Good agreement with laboratory test results was obtained especially at lower drying temperature. It is shown that this model can be reasonably used to predict the drying performance. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Some coals used in U.S. coal fired power plants have unusually high moisture levels. When this coal is used in coal-fired boilers, the high moisture affects the operation of the power plant, results in the reduction of power plant efficiency, the increase of stack gas emission and station service power. It also affects heat rate, mass rate of emissions and the consumption of water needed for evaporative cooling. Recent research work concerns about the impact of coal moisture content on boiler efficiency and cooling water makeup flow from an evaporative cooling tower [1]. The theoretical analysis and experimental results show that drying the coal from 40 to 25% moisture can reduce makeup water flow rate by 5 to 7% while the average reduction in auxiliary power as fans and mill was reduced by 3.8%. Drying the coal from 37.5 to 31.4% can improve the boiler efficiency by about 2.6% and the net unit rate by 2.7 to 2.8%. The test data also showed that the fuel flow rate was reduced by 10.8% and the flue gas flow rate was reduced by 4%. Another study also shows lower mass emissions of CO2 and SO2 when coal moisture content is low [2]. Besides, research shows that approximately 80% excess air is required to prevent smoke formation for moist coals [3]. For dry coals, only 30% excess air is required. Using less excess air reduces sensible heat ⁎ 911 Oval Drive, Raleigh, NC 27695, USA. Tel.: + 1 919 389 5430; fax: + 1 919 515 7968. E-mail address: [email protected]. 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.03.034

losses with the flue gases, increasing boiler efficiency. Another reason for a higher overall boiler efficiency is the lower flue gas temperature to the stack. In a boiler without coal drying, the flue gas temperature might be 176 °C or higher, but with a dryer this temperature will be closer to 104 °C coming out of a dryer. The overall thermal efficiency increases can amount to 5%–15%, with steam production increases of 50–60%. These studies show the benefits of reducing the coal moisture content in power stations. Energy efficiency in drying can be improved by using recirculating exhaust gases. The air leaving a directly heated air dryer is usually not saturated, so some of the hot exhaust gas can be recirculated to the inlet of the dryer. Because it is still warm, energy is not needed to heat it, increasing the drying efficiency [3]. In addition, power stations generate a large amount of low quality heat which is removed by cooling water from the condenser. Coal drying would be accomplished by both warm air passing through the dryer, and a flow of hot circulating cooling water passing through a heat exchanger located in the dryer. Higher temperature drying can be accomplished if hot flue gas from the boiler or extracted steam from the turbine cycle is used to supplement the thermal energy obtained from the circulating cooling water. The thermal efficiency of the boiler will be increased while thermal pollution is decreased. In an earlier study [4], the authors also presented that low rank coal can be dried in a fluidized bed using low grade waste heat with coal residence time which are short enough to make the drying process practical and economic for power plant use. The approach for doing this is making use of the hot circulating cooling water leaving

W.-C. Wang / Powder Technology 225 (2012) 72–85

the condenser to provide the thermal energy used for coal drying. The temperature of the circulating water leaving the condenser is usually about 49 °C, and this can be used to produce an air stream at approximately 43 °C. Therefore, the coal drying performance, which is drying rates, at this range of temperature was studied to see if the coal can be dried effectively. The drying rates of the coal depend critically on the design and operating conditions of the drying system, which are drying temperature, mass of coal in the reaction, drying air velocity, bed depth, the equilibrium moisture content of the coal, in-bed heat flux and inlet air humidity, but not on fluidized bed bubble behavior or on particle-gas contact [4]. Thomas and Varma [5] used two kinds of fluidized beds-batch and continuous, to dry granular cellular materials. The experimental results show that the critical moisture content depends on the velocities and temperatures of the heating medium, as well as the particle size and mass of solids. With the moisture content of 62 to 66%, the drying rate of these materials is enhanced by an increase in the feed temperature of the air or an increase in its flow rate. It is reduced by an increase in the particle size or an increase in solids inventory. Gao et al. [6] show that inlet air velocity can play a critical role in maintaining proper fluidization and ultimately, uniform drying. Ng and Tan [7] carried out an optimization study of fluidized bed drying, using an industrial-scale fluidized bed dryer. The results show that the drying rates are approximately 10 to 12% higher as the fluidization velocity increased from 1.5 Umf to 2 Umf. On the other hand, the drying temperature is also an important factor during drying process. Higher drying temperatures imply greater driving forces for the heat transfer. Chen et al. [8] conducted an important model for coal drying. His results show that the drying time decreases significantly when the drying temperature increases. In addition, the equilibrium moisture content in the final product may be lower for a higher drying temperature. The results also show that the size of coal particles influences significantly the time required for the particles to reach a steady state. In the report written by Levy et al. [9], the drying rates of two coals (PRB and lignite) were compared at different drying temperatures. The results show that with the same general characteristics, the drying rate of

73

PRB was 14 to 20% lower than lignite. Both of the drying rates were reduced when the drying temperature was decreased. This research deals with three Illinois coals, Buckheart, Crown mine and Viper mine. Typically, the moisture content of Buckheart is 23%, where Crown mine and Viper mine are 18 and 20%, respectively. Three of them are expressed on a dry coal basis, as kg H2O/kg dry coal. If we consider the wet coal basis, as kg H2O/kg wet coal, the moisture content of Buckheart, Crown mine and Viper mine is 19%, 15% and 17%, respectively. An experimental investigation on drying of Buckheart, Crown mine and Viper mine under batch fluidization was carried out in this study. By doing experiments in a lab scale fluidized bed, the drying conditions of the industrial drying equipment can be easily simulated. The performance of coal drying reached in the laboratory will provide useful information for optimum drying operation and for building an optimum dryer in coal-fired power plants. The significant facts that can affect the drying performance are drying temperature, mass of coal in the reaction, drying air velocity, bed depth, inbed heat flux, and inlet air humidity. In this paper, the effect of drying temperature for three different types of coal was studied. Compared with other materials, such as food or nylon particles, coal used in the power station has extremely high moisture content and will cause troubles drying in the industrial dryer. Currently many reports dealing with coal drying process are focused on industrial scale. To offset the operating losses from an industrial dryer because economy-of-scale is not utilized, new techniques must be included to improve operating efficiency and minimize energy requirements. Therefore, a re-evaluation of coal drying process at the laboratory scale, where the entire process can be characterized under a wellcontrolled environment, was necessary. In this study, a lab-scale coal drying using fluidization process under various parameters has been presented. In addition, a theoretical model of the drying process was used in which the air and coal particles are assumed to be at the same temperature and the air–water vapor mixture leaving the bed at the free surface is in equilibrium with the local values of particle moisture. The model was compared with experimental data in different drying conditions. With the completion of experimental work and theoretical analyses, the variation of drying rate versus the

3

17 2

16

4 11

1 5

10

15

14 9

13 6 7

12

8 Fig. 1. Sketch of the experimental apparatus.

1. Filter Bag 2. Water Supply 3. Metal Duct 4. Plexiglas Tube 5. Metal Bed 6. Distributor 7. Plenum 8. Air heater 9. Rotameter 10. Pressure gage 11. Compressed Air 12. Thermocouple #1 (air tem.) 13. Thermocouple #2 (bottom temp.) 14. Thermocouple #1 (middle temp.) 15. Thermocouple #1 (top temp.) 16. Thermocouple #1 (dry bulb temp.) 17. Thermocouple #1 (wet bulb temp.)

74

W.-C. Wang / Powder Technology 225 (2012) 72–85

Table 1 Particle size distribution (p) for Buckheart, Crown mine, and Viper mine. Particle size distribution (cm)

Buckheart

Crown mine

Viper mine

0.00254 b p b 0.0353 0.0353 b p b 0.0419 0.0419 b p b 0.05 0.05 b p b 0.07 0.07 b p b 0.119 0.119 b p b 0.14 0.14 b p b 0.199 0.199 b p b 0.282 0.282 b p b 0.64 Top size (cm)

0.01 0.012 0.005 0.04 0.28 0.08 0.12 0.16 0.31 0.64

0.02 0.01 0.005 0.03 0.16 0.07 0.097 0.22 0.415 0.64

0.015 0.01 0.002 0.06 0.205 0.07 0.15 0.198 0.31 0.64

drying parameters can be found and the prediction of drying process will be certainly obtained. 2. Experimental method 2.1. Experimental setup The drying experiments were performed in a fluidized bed as shown in Fig. 1. The steel bed has a height of 0.38 m and a diameter of 0.15 m. A 1.5 m Plexiglas tube was attached above the metal bed. A 0.1-meter-diameter metal duct was connected to the Plexiglas tube. The duct ended with a filter bag to capture elutriated particles. The

Fig. 2. Bed pressure drop versus velocity — Buckheart, Crown Mine and Viper Mine.

W.-C. Wang / Powder Technology 225 (2012) 72–85

75

Record temp. for each thermocouple

Turn on air compressor Turn on air preheater

Steady State

Load coal into the fluidized bed

Every five minutes

Measure wet and dry bulb temp. to obtain specific humidity

Collect one crucible of coal

Turn off inlet air and all the heaters

Bed temp. consistency

Adjust voltage regulators to remain steady surf. Temp.

Measure the weight and moisture of elutriated coal

Analyze the test results

Put all samples into oven at 110 0C and leave for 6 hours

Fig. 3. Fluidized bed drying test procedure.

By using sieve analysis, where the powder is separated on sieves of different sizes, the particle size distribution of coal is defined in terms of discrete size ranges. Table 1 presents the particle size distributions for the Buckheart, Crown mine, and Viper mine coals. The top size of these three coals is 0.64 cm.

Γ, T2, ω2, ψ2 Air + Vapor

Qheater

mdc + mL

2.3. Minimum fluidization velocity measurement

Qloss

Γ0

Air + Vapor ma, T 1, ω1 Fig. 4. Sketch of control volume.

compressed air used in the experiments flowed through a rotameter and air heater before entering the plenum. The thermocouples inserted through the bed wall were used to measure vertical distribution of bed temperature. A horizontal bundle of eighteen 1.27 cm diameter electric heating elements is used to provide in-bed heating. The heaters are located in the region from 0.08 m to 0.3 m above the distributor and are instrumented with thermocouples to indicate heater surface temperature. By controlling power to the heaters, the heater surface temperature can be operated in a range from 43 °C to 60 °C. At a given heater surface temperature, total heat flux to the bed can be reduced from the maximum by disconnecting selected heaters from the power supply. 2.2. Coal particle size distribution Particle size distribution should be determined to further understand the drying mechanism involved in the drying process.

The flow rates at which a bed is expanded to such a degree that the particles may move within the bed are known as the onset of fluidization or fluidization point, and the bed is referred to as an incipiently fluidized bed. When mixing occurs, because of the high degree of turbulence, temperatures are quickly attained throughout the system. Large instabilities with bubbling and channeling of gas occur when the flow rate is increased above the minimum fluidization velocity [10]. The minimum fluidization velocity (Umf) is the superficial velocity point where the bed pressure drop reaches the maximum value and remains constant [10,11]. It can be obtained from the interception of two straight lines depicted in the pressure drop versus air velocity curve, the slopes of the curve obtained in both the fixed-bed region and fluidized bed region [12]. Fig. 2 shows the pressure drop of bed under different fluidization velocities for the Buckheart, Crown mine and Viper mine coals. These results show that Umf ranged from 1.1 to 1.24 m/s. 2.4. Drying test procedure The drying tests were performed with specific humidity of the inlet air ranging from 0.007 to 0.009. Small samples of the coal were removed and collected from the fluidized bed periodically during the tests and coal moisture content was measured. The complete test procedure used in these experiments is detailed in Fig. 3.

Φ vs Γ (Buckheart)

1 0.9 0.8

y = 1162x4- 580.01x3 + 61.878x2 + 6.6736x - 0.0855

0.7

2 R = 0.9771

0.6

Φ 0.5 0.4 0.3 0.2 0.1 0

0

0.05

0.1

0.15

0.2

Γ, Moisture content (kg water /kg dry mass) Fig. 5. Relative humidity versus coal moisture content — Buckheart.

0.25

0.3

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W.-C. Wang / Powder Technology 225 (2012) 72–85

Φ vs Γ (Crown mine)

1 0.9 0.8 0.7

y = 3260.5x4- 1678.7x3 + 259.41x2- 7.6571x + 0.2049

0.6

2 R = 0.8361

Φ 0.5 0.4 0.3 0.2 0.1 0

0

0.05

0.1

0.15

0.2

0.25

0.3

Γ, Moisture content (kg water /kg dry mass) Fig. 6. Relative humidity versus coal moisture content — Crown mine.

3. Theoretical model When the drying operations are known, drying performance can be predicted by a mathematical model [13]. This model was developed based on the comprehensive understanding of the mechanisms of drying process. The parameters controlled in this simulation were inlet air velocity, inlet air temperature, in-bed heater temperature, specific humidity, initial moisture and mass of dry coal. The four parameters which were calculated as a function of time, are coal moisture content (Γ), exit air temperature (T2), exit air specific humidity (ω2) and exit air relative humidity (ϕ2). The control volume is sketched in Fig. 4. To describe the drying process, several assumptions were made: (a) at any instant of time, the particles and air in the bed are at the same temperature (the particle and exit air temperature will raise as the moisture content of the coal particles decreases) (b) gas and particle properties do not vary with vertical distance in the bed (c) the temperature, flow rate and specific humidity of inlet air remain constant during a test (d) the energy losses on the dryer wall only occur in the interstitial gas phase (e) the solid phase behaves as a perfect mixer (f) all the transfer mechanisms presented in the bubble gas phase are purely convective and unidirectional and (g) the mass of inlet and outlet dry air are equal,

while the water vapor content increases as the air passes through the dryer. The governing equations for the drying process can be written as follows: (1) Conservation of energy [4,13]:

dðmdc udc Þ dðmL uL Þ þ þ ðma ha þmv hv Þ2 −ðma ha þmv hv Þ1 Q heater −Q loss ¼ dt
 dt   h i dT 2 m• a þ uL − ¼ mdc ðC c þ ΓC L Þ ðω2 −ω1 Þ þ ma • C pa ðT 2 −T 1 Þ þ ω2 hg2 −ω1 hg1 dt mdc

ð1Þ

Where Qheater and Qloss are the power and heat loss of the heater; mdc and udc are mass and internal energy of the dry coal; mL and uL are mass and internal energy of the water; (maha + mvhv)2 − (maha + mvhv)1 represents enthalpy change during the drying test; Cc and CL are the specific heats of the coal and water; m• a represents flow rate of the dry air; ω1 and ω2 are the inlet and exit air specific humidity; Cpa is the air specific heat; hg refers to the enthalpy of the saturate vapor; T1 and T2 are inlet and exit air temperature.

Φ vs Γ (Viper mine) 1 0.9 0.8 0.7 0.6 y = 2373.2x4- 1339.3x3 + 221.53x2- 6.4544x + 0.1535

Φ 0.5

2 R = 0.9547

0.4 0.3 0.2 0.1 0

0

0.05

0.1

0.15

0.2

0.25

Γ, Moisture content (kg water /kg dry mass) Fig. 7. Relative humidity versus coal moisture content — Viper mine.

0.3

W.-C. Wang / Powder Technology 225 (2012) 72–85

77

Moisture Content (kg water/ kg dry)

Coal Moisture Versus Time (Buckheart)_dry basis 0.3 51.7

60

0.25 0.2 0.15 0.1

Test R1,R2 Vair,in=1.25m/s Ta,in & Tsurf = 51.7, 60 °C ho = 8" weight: 2.5 kg

0.05 0

0

5

10

15

20

25

30

35

40

45

Time (min) Coal Moisture Versus Time (Crown mine)_dry basis

Moisture Content (kg water/ kg dry)

0.3

Test A1,A2,A3 Vair,in=1.25m/s Ta,in & Tsurf = 43, 51.7, 60 °C ho =8" weight: 2.5kg

0.25

43

51.7

60

0.2 0.15 0.1 0.05 0

0

5

10

15

20

25

30

35

40

45

Time (min)

Moisture Content (kg water/ kg dry)

Coal Moisture Versus Time (Viper mine)_dry basis 0.3

Test V1,V2,V3 Vair,in=1.25m/s Ta,in = 43, 51.7, 60 °C ho =8"

0.25

43

51.7

60

weight: 2.5kg

0.2 0.15 0.1 0.05 0

0

5

10

15

20

25

30

35

40

45

Time (min) Fig. 8. Moisture content versus time — Buckheart, Crown mine, and Viper mine.

The equation of enthalpy can be obtained by data curve fitting of thermodynamic properties table. It can be written as: 3

2

hg ¼−6E  07  T þ 6E  05  T þ 0:4369  Tþ 1061:4 where T is in Celsius and ranges from 25 °C to 700 °C.

(2) Conservation of mass [4]:

ð2Þ mdc :

dΓ þ ma :ðω2 −ω1 Þ ¼ 0 dt

ð3Þ

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W.-C. Wang / Powder Technology 225 (2012) 72–85

Exit Air Temperature Versus Time_buckheart

60

Temperature (°C)

50 40 30 20 10 v=1.25 m/s ho = 8" weight: 2.5kg

0 -10

0

5

51.7 °C 60 °C

10

15

20

25

30

35

40

45

Time (min) Exit Air Temperature Versus Time_Crown mine

60

Temperature (°C)

50 40 30 20 10 0 -10

43 °C 60 °C 51.7 °C

v=1.25 m/s ho = 8" weight: 2.5kg 0

5

10

15

20

25

30

35

40

45

Time (min) Exit Air Temperature Versus Time_Viper mine

60

Temperature (°C)

50 40 30 20 10

v=1.25 m/s ho = 8" weight: 2.5kg

43 °C

0

51.7 °C 60°C

-10 0

5

10

15

20

25

30

35

40

Time (min) Fig. 9. Exit air temperature versus time — Buckheart, Crown mine, and Viper mine.

45

W.-C. Wang / Powder Technology 225 (2012) 72–85 Table 2 Drying rate for Buckheart, Crown mine and Viper mine (unit: kg water/kg dry mass ⁎ min).

79

Table 4 Time required for 80% moisture content reduction (unit: min).

Drying temperature (°C)

Buckheart

Crown mine

Viper mine

Drying temperature (°C)

Buckheart

Crown mine

Viper mine

43 51.7 60

0.004 0.005 0.008

0.013 0.015 0.018

0.017 0.021 0.025

43 51.7 60

39 28 21

22 13 15

32 22 18

Where Γ¼

mL mdc

ð4Þ

and ma represents the mass of the dry air. (3) Equation of specific humidity and relative humidity [14] ω2 ¼

0:622  ϕ2  P g2 P−ϕ2  P g2

ð5Þ

The saturation pressure (Pg2) can be defined by the curve fitting of data from thermodynamic properties table. The equation of saturation pressure is: 3

2

Pg2 ¼ 5E  06  T −0:0011  T þ 0:0999  T−2:5654

ð6Þ

where T is in Celsius and ranges from 25 °C to 700 °C. (4) Equation of relative humidity and coal moisture content The relation between coal moisture content and relative humidity of air leaving the bed ϕ=f(Γ) is given graphically in Fig. 5 for the Buckheart, Fig. 6 for the Crown mine and Fig. 7 for the Viper mine coals. By curve fitting of these data, the equations of relative humidity versus coal moisture content can be acquired. Polynomial functions, which give the best fit for the data, were developed from these relations. For Buckheart, the equation is  4

 3

 2



ϕ ¼ 1162 Γ −580:01 Γ þ 61:878 Γ þ 6:6736 Γ−0:0855

ð7Þ

For Crown mine, the equation is  4

 3

 2



ϕ ¼ 3260:5 Γ −1678:7 Γ þ 259:41 Γ −7:6571 Γ þ 0:2049 ð8Þ For Viper mine, the equation is  4

 3

 2



ϕ ¼ 2373:2 Γ −1339:3 Γ þ 221:53 Γ −6:4544 Γ þ 0:1535 ð9Þ The equations of energy balance, mass balance, the relation of specific and relative humidity, as well as the relation of relative humidity and coal moisture content were used to calculate the four unknowns, which are coal moisture content during drying tests (Γ), exit air temperature (T2), exit specific humidity (ω2), and exit relative humidity (ϕ). Note that by calculating the conduction heat transfer between bed temperature and surrounding temperature, the heat loss

Table 3 Time to reach the equilibrium moisture content (unit: min). Drying temperature (°C)

Buckheart

Crown mine

Viper mine

43 51.7 60

> 45 45 25

35 20 25

35 25 20

in our system is relatively low and was neglected. The initial coal moisture content was used to obtain relative humidity by Eqs. (7)–(9). Through Eq. (5), exit specific humidity can be acquired. Eqs. (1) and (3) were used to calculate the changing rate of exit air temperature and moisture content. After knowing these two values, the numerical method was applied to calculate the following coal moisture content and exit air temperature. This equilibrium model can be compared with the experimental results to see accuracy of the prediction. 4. Experimental results and discussions The drying tests were done over a range of conditions. Both inlet air temperature and surface temperature were the same during a test, with these values ranging from 43 °C to 60 °C. According to the fluidization experiments, the Umf of the Buckheart, Crown mine and Viper mine coals is 1.1 m/s, 1.09 m/s and 1.24 m/s, respectively. To assure the best fluidization performance, the air velocity is operated at 1.25 m/s. The initial bed mass for the Buckheart is 4 kg, which has 0.32 m bed depth. For the Crown mine and Viper mine, the initial mass is 2.5 kg, with 0.2 m bed depth. The initial moisture contents ranged from 18% to 23% (kg water/kg dry coal). All the tests were performed with the coal which had a 0.63 cm top size. 4.1. Moisture reduction curves The extent to which coal can be dried depends on the way the moisture is associated with it, and hence knowledge of the moisture–solid equilibrium is an important aspect when considering drying processes. A coal drying curve is the best characterization of the simultaneous heat and mass transfer between the coal and hot air drying medium [15]. The curve can also be used directly to determine the time required for drying larger batches under the same drying conditions. Fig. 8 shows the moisture content reduction of Buckheart within 45 min of drying time for two different drying temperatures. The moisture contents decreased more rapidly at higher temperature. The same results can be seen in the Crown mine and Viper mine coals. The time to attain equilibrium moisture content is reduced when increasing drying temperature. Diamond et al. [16] had the same results when carrying out lignite drying in a fluidized bed. The drying rate, defined as the slope of the moisture content versus time curves, was shown in Table 2. It is obvious that the drying rate increases with the increasing temperature. Tables 2–4 display the drying performance of three types of coal. These tests were run to determine if the key parameters have the same effect on the drying kinetics for different kinds of coals. They were carried out with the same values of coal top size, settled bed depth, heater temperature, and air velocity. With the same drying temperature and the same gas velocity, Viper mine has the highest drying performance compared with Crown mine and Buckheart. The drying rate of Viper mine was higher than Crown mine by 28% in 60 °C, by 29% in 51.7 °C and by 24% in 43 °C. It is also higher than Buckheart by 68% in 60 °C, by 76% in 51.7 °C and by 76% in 43 °C. For Buckheart and Viper mine, as shown in Table 3, the time required to reach the equilibrium moisture content was decreased when enhancing the drying temperature. Table 4 shows the time required for reducing 80% moisture content. At this point, Crown mine is more preferable than

W.-C. Wang / Powder Technology 225 (2012) 72–85

Specific humidity (0.001 kg/kg dry air)

80

Outlet Specific Humidity Versus Time_Buckheart

0.12

Vair,in=1.25m/s Ta,in = 60 °C

0.1

elutration:17.28% 0.08 0.06 outlet specific humidity saturated air

0.04 0.02 0 0

5

10

15

20

25

30

35

Specific humidity (0.001 kg/kg dry air)

Time (min) Outlet Specific Humidity Versus Time_Crown Mine 0.1 Vair,in=1.25m/s Ta,in = 60 °C elutration:2.78%

0.09 0.08 0.07 0.06 0.05 0.04 0.03

outlet specific humidity saturated air

0.02 0.01 0

0

5

10

15

20

25

30

35

40

45

Time (min) Outlet Specific Humidity Versus Time_Viper Mine Specific humidity (0.001 kg/kg dry air)

0.1 0.09

outlet specific humidity

0.08

saturated air

0.07 0.06 0.05 0.04 0.03 Vair,in=1.25m/s Ta,in = 60 °C elutration:6.28%

0.02 0.01 0 0

5

10

15

20

25

30

35

40

45

Time (min) Fig. 10. Specific humidity versus time — Buckheart, Crown mine, and Viper mine.

Buckheart and Viper mine. It can be seen that the time to reduce most of the moisture content decreased with increasing drying temperature. 4.2. Exit air temperature To determine the equilibrium state of the drying medium, exit gas temperature was measured periodically. The exhaust air temperature can

be used to detect poor fluidization and the homogeneity of the fluidized bed. If the exit air temperature rises more rapidly and is not stabilized, it is an indication that fluidization is incomplete [6]. The exit air temperature changed with time for the Buckheart, Crown mine and Viper mine coals as is shown in Fig. 9. These tests were done by three different drying temperatures: 43 °C, 51.7 °C and 60 °C. Obviously, the exhaust air temperature rises gradually with time. The trend was similar to the

W.-C. Wang / Powder Technology 225 (2012) 72–85

81

1.2

relative humidity

1.0

Data: Buckheart drying tests Model: Boltzmann Equation: y = A2 + (A1-A2)/(1 + exp((x-x0)/dx)) Weighting: y No weighting

0.8

0.6

Chi^2/DoF = 0.00301 R^2 = 0.97646

0.4

A1 A2 x0 dx

0.2

-0.17605 0.96973 0.05692 0.03023

0.0 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Moisture content (kg water/ kg dry mass) 1.2

relative humidity

1.0

0.8 Data: Crown mine drying tests Model: SLogistic2 Equation: y = a/(1 + ((a-y0)/y0)*exp(-4*Wmax*x/a)) Weighting: y No weighting

0.6

0.4 Chi^2/DoF = 0.01317 R^2 = 0.8269 y0 0.07502 a 0.8877 Wmax 7.54446

0.2

0.0 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Moisture content (kg water/ kg dry mass) 1.2

relative humidity

1.0

0.8

Data: Viper mine drying tests Model: SLogistic2 Equation: y = a/(1 + ((a-y0)/y0)*exp(-4*Wmax*x/a)) Weighting: y No weighting

0.6

0.4

Chi^2/DoF = 0.00327 R^2 = 0.95183

0.2

y0 0.03531 a 0.78592 Wmax 8.64374

0.0 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Moisture content (kg water/ kg dry mass) Fig. 11. Relative humidity versus moisture content — Buckheart, Crown mine, and Viper mine.

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W.-C. Wang / Powder Technology 225 (2012) 72–85

Bed, surface and exit temperature vs time_ Buckheart

70

Temperature (°C)

60 50 40 30 20 Vair,in=1.25m/s Ta,in = 60°C ho = 8" weight: 2.5 kg elutration:17.28%

10 0 -10 0

5

Upper bed temp outlet temp, dry bulb outlet temp, wet bulb Lower bed temp Average surface temp

10

15

20

25

30

35

40

45

Time (min) Bed, surface and exit temperature vs time _Crown mine

60

Temperature (°C)

50 40 30 20 Vair,in=1.25m/s Ta,in = 60°C ho = 8" weight: 2.5kg elutration:2.78%

10 0

Upper bed temp outlet temp, dry bulb outlet temp, wet bulb Lower bed temp Average surface temp

-10 0

5

10

15

20

25

30

35

40

45

Time (min) Bed, surface and exit temperature vs time_Viper mine

60

Temperature (°C)

50 40 30 20 10

Vair,in=1.25m/s Ta,in = 60°C ho = 8" weight: 2.5kg elutration:6.28%

0 -10

0

5

Upper bed temp outlet temp, dry bulb outlet temp, wet bulb Lower bed temp Average surface temp

10

15

20

25

30

35

40

45

Time (min) Fig. 12. Bed, surface and exit temperature versus time — Buckheart, Crown mine, and Viper mine.

result obtained by Vitor et al. [17]. The exit air temperature increased more rapidly when drying temperature was increased.

where the saturation humidity ratio Ws* can be defined as: 

W s ¼ 0:62198

4.3. Exit specific humidity The specific humidity W was obtained from [15]

W¼







ð1093−0:556t ÞW s −0:240ðt−t Þ 1093 þ 0:444t−t 

ð10Þ

pws p−pws

ð11Þ

where p is the total mixture pressure and pws is the saturated partial pressure of water vapor. From the thermocouples located at the exit of the fluidized bed, the dry and wet bulb temperatures of the exit air were measured. Using Eq. (10), the specific humidity of the exit air was calculated.

W.-C. Wang / Powder Technology 225 (2012) 72–85

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Simulation results (kg water/kg dry mass)

Moisture reduction 3.00E-01

2.50E-01

2.00E-01 Buckheart Crown mine Viper mine

1.50E-01

1.00E-01

5.00E-02

0.00E+00 0.000

0.050

0.100

0.150

0.200

0.250

0.300

Test result (kg water/kg dry mass) Fig. 13. Comparison between simulation and test results — change in moisture content.

Exit Air Temperature

60

Simulation results (°C)

50 40 30

Buckheart Crown mine Viper mine

20 10 0 -10 -10

0

10

20

30

40

60

50

Test result (°C) Fig. 14. Comparison between simulation and test results — average exit air temperature.

Outlet Specific Humidity

Simulation results (0.001 g/g dry air)

2.00E-02 1.80E-02 1.60E-02

V6

1.40E-02

V3

1.20E-02 Buckheart Crown mine Viper mine

1.00E-02 8.00E-03 6.00E-03 4.00E-03 2.00E-03 0.00E+00 0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

Test result (0.001g/g dry air) Fig. 15. Comparison between simulation and test results — average exit specific humidity.

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W.-C. Wang / Powder Technology 225 (2012) 72–85

Relative Humidity

6.00E-01

Simulation results (%)

5.00E-01

V6 4.00E-01

3.00E-01

Buckheart Crown mine Viper mine

2.00E-01

1.00E-01

0.00E+00 0.000

0.100

0.200

0.300

0.400

0.500

0.600

Test result (%) Fig. 16. Comparison between simulation and test results — average exit relative humidity.

Fig. 10 indicates the variations between specific humidity and saturated air in 60 °C. Note that the elutriation value was defined as the fraction of fine particles, which the terminal velocity is smaller than the operating gas velocity, to the total particles [10,18]. At the beginning, the specific humidity was close to saturated conditions. With the increase of difference between these two, the coal moisture content was decreasing.

The relative humidity (ϕ) can be calculated from [14]

pw μ ¼ pws 1−ð1−μ Þðpws =pÞ

ð12Þ

 W  ϕ ¼ W s t;p 1 þ ð1−ϕÞW s =0:62198

ð13Þ

4.5. Bed and surface temperature The surface temperature was set equal to the inlet air temperature during the test and the average surface temperature is defined as the mean value of the upper and lower surface temperatures. The

where pw is actual partial pressure of water vapor.

Initial Drying rate

3.00E-02

Simulation results (kg/min)

μ¼

where Ws is saturated specific humidity. The relative humidity of air can also be expressed as a function of coal moisture content. Fig. 11 shows the relation between these two. The data were fitted by an exponential or polynomial function. By using these relations, along with the equations of conservation of mass and conservation of energy, the coal drying mathematical model could be developed.

4.4. Relative humidity

ϕ¼

The Degree of Saturation, μ, is

2.50E-02

2.00E-02

R2 A3

1.50E-02

A1

1.00E-02

A5

Buckheart Crown mine Viper mine

V6

V3

V1 5.00E-03

0.00E+00

0

0.005

0.01

0.015

0.02

0.025

Test result (kg/min) Fig. 17. Comparison between simulation and test results — drying rate.

0.03

W.-C. Wang / Powder Technology 225 (2012) 72–85

higher temperature the in-bed heaters have, the lower the moisture content of the coal would be. The power input of the heaters was adjusted continuously to keep the surface temperature stable. Fig. 12 shows the bed, surface and exit temperatures. The bed temperatures measured by the top and bottom thermocouples are very consistent. That means the coal in the bed is well fluidized and mixed. Also, with the increase of the difference between dry and wet bulb temperature at the exit of our system, the coal moisture content was simultaneously reduced. 5. Simulation results and discussion Some cases were run numerically from the experimental conditions and compared with the experimental results. Four parameters, which are moisture content, exit air temperature, exit specific humidity and exit relative humidity, were performed to represent the predicted drying test results for the Buckheart, Crown mine and Viper mine coals. Comparisons for all of the test runs are given in Figs. 13–17. Fig. 13 compares predicted and measured values of moisture reduction, which is the difference between the initial and the end moisture content. Figs. 14 and 15 show the test and simulation results of average exit air temperature and specific humidity. The measured values were the average values obtained from air temperature and humidity measurement downstream of the bed. The average values from the computer simulations were obtained by integrating exit air temperature and specific humidity from the initial to the end. For all types of coals, Figs. 13 and 14 show an excellent agreement between the two results. In Fig. 15, the experimental data of the Viper mine coal with 60 °C drying temperature (V3, V6) have 10–15% error comparing with predicted values. In Fig. 16, the test results were the average relative humidity calculated from Eq. (12), and the predicted results were the average values obtained from the computed data using Eqs. (7)–(9). From this graph, the Viper mine coal with higher drying temperature (V6) has a larger error than the other coals. Fig. 17, which compares measured to predicted initial drying rates in the first 5 min, presents a bias error [9] between the two. The experimental data were larger than the calculated values by 12% to 25% during the first 5 min. The error will be larger when the drying temperature was increased (R2, A3, A5, V3, V6). 6. Conclusion The objective of this research was to use quantitative and qualitative methods for finding the factors which affect the drying performance of three Illinois coals: Buckheart, Crown and Viper mines. Experimental data were obtained to find the effects of drying temperature on drying rate for different kinds of coals. A theoretical model based on all the drying conditions was also developed to predict the drying process and compare to the test results. Systematic analysis of the experimental results leads to the following conclusions: • The moisture content reduction was increased by increasing the drying temperature. The drying rate of Viper Mine has the most significant enhancement when drying temperature increases. • For the Buckheart and Viper mine coals, the time required to reach the equilibrium moisture content was decreased when enhancing the drying temperature. • Based on the results of drying rate, the drying performance of Viper mine was better than Crown mine and Buckheart. If they were based on

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the drying time to reduce the moisture content down to 20%, Crown mine was preferable. • With the smooth increasing of exit air temperature, as well as the consistency of top and bottom bed temperature, the coal in fluidized bed was considered to be well fluidized and mixed. • The coal moisture content was considered to be reduced with the increase of difference between exit specific humidity and saturate air, and also between exit dry and wet bulb temperatures. • By analyzing numerically the relation between relative humidity and moisture content, the mathematical equations used to calculate the drying performance were established. The theoretical model of the drying process in this study is in excellent agreement with the laboratory data.

Acknowledgment This study was supported by grants made possible by the Illinois Department of Commerce and Economic Opportunity through the Office of Coal Development and the Illinois Clean Coal Institute. The author would also like to express the appreciation to Dr. Edward K. Levy and all the members in Energy Research Center, Bethlehem, PA, for their generous assistance.

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