Effect of Cleaning Process on the Combustion Characteristics of Two

Energy & Fuels 2001, 15, 1461-1468

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Effect of Cleaning Process on the Combustion Characteristics of Two Different Rank Coals Mustafa Versan Kok,*,† Cahit Hicyilmaz,‡ and Kazim Esber Ozbas‡ Department of Mining Engineering, Middle East Technical University, 06531, Ankara, Turkey, and Department of Petroleum and Natural Gas Engineering, Middle East Technical University, 06531, Ankara, Turkey Received March 27, 2001. Revised Manuscript Received August 15, 2001

In this research, thermogravimetry (TG/DTG) was used to determine the combustion characteristics of two different rank coals (Tunç bilek and Afs¸ in Elbistan) before and after cleaning process. Applying sink-float process cleaned raw coal samples, and optimum-separating densities for each sample was determined using the criteria of “degree of washability”. The results indicated that coal cleaning was very effective on Tunç bilek sample due to its high rank. TG/DTG analysis of raw and cleaned samples indicated different reaction regions occurring at different temperature intervals. Easy combustibility and long-lasting combustion were the distinctive effects of coal cleaning on raw coals. Kinetic analysis of the samples showed that clean coals require lower activation energies to initiate the combustion process than raw coals.

Introduction The importance of low quality coals such as lignite’s increases with the increasing energy demand all around the world. But, because of the environmental problems, the use of such low quality lignite’s has some restrictions especially around highly populated regions. At this point, mineral processing plays a very important role as cleaning of these lignites to improve their qualities, which is necessary for their usage in different industrial branches. Thermal analysis and associated coupled techniques have been used extensively to elucidate the origin, structure, petrographic and chemical composition, degree of carbonization and different physicochemical properties of peat, lignite, brown, black, and anthracite coal types and bituminous shales. Thermal analysis methods also play an important role in determining the characteristics of coal structure, and its change with increasing temperature. The most important impurities, which exist in the coals, are inorganic matters, sulfur, and the moisture. With the separation of inorganic matter from the coal, ash content of the coal can be decreased and percentage of combustibles can be increased. As a result, heating value of coal increases while transportation, storage, and combustion of unwanted ash are prevented. During the cleaning processes of coals, it is possible to remove some amount of sulfur from the coals. As the inorganic matters, moisture reduces the amount of combustibles and consumes some amounts of heating value during combustion. Therefore, moisture is also being removed from coals. It is possible to reduce the ash contents of coals by applying different coal cleaning methods. The necessities for cleaning of low quality coals can be † ‡

Department of Petroleum and Natural Gas Engineering. Department of Mining Engineering.

technical and/or economical. Cleaning of coal results in a product containing less solid waste with respect to raw coal that is very advantageous for consumers. Transportation cost of a cleaned coal is lower than raw coal due to reduced ash content. Due to the narrower particle size range and quality obtained after cleaning process, the combustion of a cleaned coal is more efficient and therefore causes less dust and smoke emissions which positively effects the environment. Thermal efficiency of a coal increases after cleaning process, which is very advantageous for consumers. That is, because of higher thermal efficiency of cleaned coal with respect to raw coal, consumers can use less amount of coal for their own purposes. Gold1 demonstrated the occurrence of exothermic reactions associated with the production of volatile matter in or near the plastic region of coals studied. He concluded that the temperature and the magnitude of the exothermic peak was strongly affected by the heating rate, sample mass and particle size. Cumming and McLaughlin2 applied thermogravimetry (TG) and derivative thermogravimetry (DTG) to a range of samples and obtained proximate analysis results for a total of fourteen coal samples of widely different properties. They have also established burning and volatile release profile tests. They suggested that volatile release profile test might find application in the fields of coking and gasification, rather than in combustion and it could be basis of system fingerprinting coals. Rosenvold et al.3 analyzed twenty-one bituminous coal samples from Ohio by differential scanning calorimeter and nonisothermal thermogravimetry. Three regions of endothermic activity were distinguished in the DSC scans in an inert (1) Gold, P. I. Thermochimica Acta 1980, 42, 135-142. (2) Cumming, J. W.; McLaughlin, J. Thermochim. Acta 1982, 57, 253-272. (3) Rosenvold, R. J.; Dubow, J. B.; Rajeshwar, K. Thermochim. Acta 1982, 53, 321-332.

10.1021/ef0100707 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/18/2001

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atmosphere. The first peak (25-150 °C) corresponds to devolatilization of the organic matter and a partially resolved endothermic at temperatures above 550 °C probably corresponds to cracking and coking processes after the pyrolysis step. Morgan et al.4 determined coal burning profiles by thermogravimetric analysis. They have claimed that kinetic parameters from Arrhenius plots of profiles cannot readily be related to any specific stage of combustion. But some features of the profiles are clearly related to coal properties, and a correlation exists between unburned carbon loss as predicted from high-temperature oxidation rates and a characteristic temperature of the thermogravimetric profile. It was suggested that burning profiles could provide a valuable, rapid laboratory method of ranking coals in terms of their burnout performance. Jayaweera et al.5 studied the effect of particle size on the percentage weight loss of a low quality bituminous coal during combustion in air by thermal analysis. It was found that the method of sieving used to prepare the samples of different particle size have a significant effect on the results. Karatepe and Ku¨ç u¨kbayrak6 used TG to determine the moisture, ash, volatile matter and fixed carbon contents of twenty-four lignite samples from different coal reserves in Turkey, and compared the results with those obtained by the ASTM standards. The results of the ASTM and TG proximate analyses were in good agreement. In particular, the differences in moisture and ash contents between the TG and ASTM were relatively low. Ko¨k et al.7 determined the effect of particle size on combustion characteristics of Ç ayırhan coal. For this purpose, nonisothermal thermogravimetry (TG/DTG) experiments were carried out on twelve different size fractions, and the thermogravimetric data were analyzed by Arrhenius type kinetic model. The results indicated that activation energies were increased as the particle size decreased. Gu¨ldogˇan et al.8 determined the pyrolysis kinetics of Tunç bilek lignite and Denizli peat blends by using a thermo balance apparatus at atmospheric pressure from 25 °C to 900 °C with a heating rate of 20 °C/min under an argon atmosphere. Their results showed an increase in both activation energies and frequency factors for the proposed reaction model with increase in the ratio of peat to lignite in blends. Durusoy et al.9 reported pyrolysis behavior of raw and microbiologically treated Mengen lignite. Their experiments were carried out in a thermo balance apparatus at atmospheric pressure from 298 to 1173 K at a heating rate of 20 K/min. The results indicated good behavior of the microbiologically treated lignite compared with raw lignite. They observed an increase in the activation energy after microbial removal of sulfur from coal. Shah et al.10 studied combustion of different sized coal samples. The results revealed that the effect of reduction (4) Morgan, P. A.; Robertson, S. D.; Unsworth, J. F. Fuel 1986, 65, 1546-1551. (5) Jayaweera, S. A. A.; Moss, J. M.; Thwaites, M. W. Thermochim. Acta 1989, 152, 215-225. (6) Karatepe, N.; Ku¨ç u¨kbayrak, S. Thermochim. Acta 1993, 213, 147-150 (7) Ko¨k, M. V.; O ¨ zbas, K. E.; Hiç yilmaz, C.; Karacan, O ¨ . Thermochim. Acta 1997, 302, 125-130. (8) Gu¨ldogˇan, Y.; Durusoy, T.; Bozdemir, T. O ¨ . Thermochim. Acta 1999, 332, 75-81. (9) Durusoy, T.; Bozdemir, T. O ¨ .; Yu¨ru¨m, Y. Fuel 1999, 78, 359363. (10) Shah, M. R.; Raza, M. Z.; Ahmed, N. Fuel Sci. Technol. Int. 1994, 1, 85-95.

Kok et al. Table 1. Dry Screen Analysis of the Samples size fraction (mm)

coal sample Tunç bilek feed

Afs¸ in-Elbistan feed

-30+18 -18+10 -10+0.5 -0.5 -30+18 -18+10 -10+0.5 -0.5

cumulative cumulative weight (%) weight (%) weight screen screen (%) oversize undersize 30.55 32.79 31.55 5.11 45.04 21.51 25.91 7.54

30.55 63.34 94.89 100.00 45.04 66.55 92.46 100.00

69.45 36.66 5.11 54.96 33.45 7.54

in particle size of coal was advantageous insofar as a reduction in particle size caused a decrease in the ignition temperature. Janikowski and Stenberg11 analyzed the 10 different coals in argon and hydrogen atmosphere; four lignite, four subbituminous, and two bituminous coals. They have distinguished two temperature regions of increased chemical reactivity, one at 75-118 °C and the second at 375-415 °C. Experimental Section Samples. Before the experiments representative samples were prepared from the coals obtained. For this purpose, a jaw crusher under close control first crushed the samples. Four different size fractions of these samples were prepared by applying closed circuit crushing and screening. These size fractions were -30+18, -18+10, -10+0.5, and -0.5 mm. The size fractions were chosen depending on the feed sizes of the thermo-power plants of Tunç bilek and the Afs¸ in-Elbistan. The results of dry screen analyses of the samples and proximateelemental analyses of the samples are given in Tables 1 , 2, and 3, respectively. Coals may be classified into two groups based on the nature of their ash constituents.12 The first one is the bituminoustype ash and the second one is the lignite-type ash. The term “lignite-type” ash is defined as an ash having more CaO plus MgO than Fe2O3. By contrast, the “bituminous-type” ash will have more Fe2O3 than CaO plus MgO. The ash compositions of the feeds were given in Table 4. According to the classification Afs¸ in-Elbistan feeds have lignite-type ashes, because it has higher CaO plus MgO content than Fe2O3 content, whereas Tunç bilek feed shows different characteristic, and its ash can be classified as bituminous-type due to its lowest CaO and MgO content than Fe2O3 content. To determine the washability characteristics of the samples used, sink-float tests were carried out for all different size fractions. Experiments were achieved by using heavy media having densities of 1.40, 1.50, 1.60, 1.70, and 1.80 g/cm3. Zinc chloride solutions were used to prepare the heavy medium for all sizes except -0.5 mm fraction. For -0.5 mm fraction the heavy medium was prepared by using carbon tetra chloride, broform, and xylol to prevent the negative effect of viscosity of zinc chloride solution. To prepare clean lignite feed for TG/ DTG experiments, it was necessary to wash the lignite of each fraction at a definite density. Since the fractions had different characteristics, the optimum separation density for each fraction was determined by using the criteria defined as degree of washability. The density, which gives the maximum degree of washability, was chosen as optimum separating density for that fraction. The degree of washability (DW) can be calculated as13

DW ) yield of clean coal (%) {[ash of raw coal (%) ash of clean coal (%)]/ash of raw coal (%)} After the determination of optimum separating densities, the final products were prepared by blending the clean coal

Combustion Characteristics of Coals

Energy & Fuels, Vol. 15, No. 6, 2001 1463 Table 2. Proximate Analysis of the Samples

coal Tunç bilek feed Afs¸ in-Elbistan feed

basis of analysis

moisture (%)

ash (%)

volatile matter (%)

fixed carbon (%)

air dried dry dry, ash free air dried dry dry, ash free

2.33

53.30 54.57

9.17

26.58 29.26

25.14 25.74 56.66 42.10 46.35 65.53

19.23 19.69 43.34 22.15 24.39 34.47

Table 3. Elemental Analysis of the Samples coal sample Tunç bilek feed Afs¸ in-Elbistan feed

basis of analysis

carbon (%)

hydrogen (%)

nitrogen (%)

sulfur (%)

oxygen (%)

air dried dry dry, ash free air dried dry dry, ash free

29.86 30.57 67.30 34.38 37.85 53.51

2.41 2.47 5.43 2.26 2.49 3.52

1.27 1.30 2.86 1.11 1.22 1.73

0.70 0.72 1.58 1.25 1.38 1.95

10.13 10.37 22.83 25.25 27.80 39.29

Table 4. Ash Composition of the Samples compound

Tunç bilek feed ash

Afs¸ in-Elbistan feed ash

SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) Na2O (%) K2O (%) SO3 (%)

53.30 21.00 11.40 2.25 6.80 0.01 0.95 2.15

8.30 6.85 2.60 54.30 1.45 0.05 0.10 16.10

products of each size fraction obtained at optimum separating densities with respect to their weight percentages. Experimental Procedure. The cleaned product and raw sample were ground and prepared for TG/DTG experiments. The PL thermogravimetry 1500 was used for the experiments. The experimental procedure of the TG/DTG includes placing 10 m. of sample, setting the heating and gas flow rates, and commencing the experiments. All the experiments were carried out at a linear heating rate of 10 °C/min. within a temperature range of 20-900 °C at an airflow rate of 5 mL/min. Prior to experiments, thermogravimetry instrument was calibrated for temperature readings, using indium as reference material. The balance was calibrated for buoyancy effect allowing the quantitative estimation of weight changes.

Results and Discussion The washability results and the degree of washability values of Tunç bilek coal sample are given in Table 5a. The optimum separation densities were 1.7 g/cm3 for -30+18 mm, -10+0.5 and -0.5 fractions, and 1.80 g/cm3 for -18+10 mm fraction. Around 92%, ash was removed by cleaning at -0.5 mm fraction while it was more than 80% at other fractions. The clean product was prepared by blending with respect to their weight percentages has the recovery of combustibles of 70.13% with respect to Tunç bilek raw coal. The properties of cleaned product of the Tunç bilek coal are given in Table 6. An increase in fixed carbon, volatile matter, and moisture contents of the cleaned products was observed. The slight increase in moisture content was due to the liquids used during the cleaning process. The carbon content of Tuncbilek feed was increased from 29.86% to 55.72% after cleaning process whereas the calorific value of the sample was increased to 4910 cal/g. Because (11) Janikowski, S. K.; Stenberg, V. I. Fuel 1989, 68, 95-99. (12) Steam: Its Generation and Use; Babcock & Wilcox Co: New York, 1985. (13) Sarkar, G. G.; Das, H. P. Fuel 1974, 53, 74-84.

Table 5. Optimum Degrees of Washability of Different Size Fractions of Tunç bilek and Afs¸ in-Elbist Samples a. Optimum Degrees of Washability of Different Size Fractions of Tunç bilek Sample size separating ash content ash content yield of fraction density of rom of clean clean degree of (mm) (g/cm3) coal (%) coal (%) coal (%) washability

-30+18

-18+10

-10+0.5

-0.5

1.40 1.50 1.60 1.70 1.80 1.40 1.50 1.60 1.70 1.80 1.40 1.50 1.60 1.70 1.80 1.40 1.50 1.60 1.70 1.80

54.47

56.82

48.16

55.48

11.20 14.91 19.07 21.40 25.03 11.35 15.46 19.40 21.88 24.30 8.21 12.51 15.55 18.24 21.04 7.05 11.76 16.20 23.32 43.94

22.76 29.31 36.17 39.58 44.24 21.74 29.18 35.60 39.30 42.60 25.75 35.93 41.45 46.24 50.96 4.77 9.69 13.60 18.79 49.99

18.08 21.29 23.51 24.03 23.91 17.40 21.24 23.45 24.17 24.38 21.36 26.60 28.07 28.73 28.70 4.16 7.64 9.63 10.89 10.40

b. Optimum Degrees of Washability ofDifferent Size Fractions of Afs¸ in-Elbistan Sample size separating ash content ash content yield of fraction density of rom of clean clean degree of (mm) (g/cm3) coal (%) coal (%) coal (%) washability

-30+18

-18+10

-10+0.5

-0.5

1.40 1.50 1.60 1.70 1.80 1.40 1.50 1.60 1.70 1.80 1.40 1.50 1.60 1.70 1.80 1.40 1.50 1.60 1.70 1.80

24.59

24.53

27.94

39.68

19.21 21.08 22.65 24.46 24.50 19.04 20.85 22.82 24.31 24.44 19.76 21.16 22.93 24.40 25.54 22.62 31.08 35.23 36.21 38.19

80.19 87.77 93.23 99.46 99.65 77.17 84.19 93.18 99.13 99.64 56.50 62.56 70.59 78.65 86.16 13.86 32.82 62.94 71.27 88.30

17.54 12.53 7.36 0.53 0.36 17.27 12.63 6.50 0.89 0.37 16.54 15.18 12.66 9.96 7.40 5.96 7.11 7.06 6.23 3.32

of the removal of some mineral matter during the cleaning process, SiO2, Al2O3, and K2O content of the cleaned coal is increased while Fe2O3, CaO, MgO, and SO3 contents are decreased slightly, while in Afs¸ inElbistan coal (Tables 5b and 6) only CaO was decreased.

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Figure 1. TG/DTG curve of Tuncbilek coal sample (-30 + 18 mm). Table 6. Proximate Analysis of Tunç bilek Cleaned Feed Sample, Elemental Analysis of Tunç bilek and Afs¸ in-Elbistan Cleaned Feed Sample, and Ash Composition of Tunç bilek and Afs¸ in-Elbistan Cleaned Feed Sample a. Proximate Analysis of Tunç bilek Cleaned Feed Sample basis of analysis

ash content (%)

volatile matter (%)

fixed carbon (%)

3.58

21.29 22.08

32.89 34.11 43.78

42.24 43.81 56.22

11.39

19.70 22.23

42.70 48.19 61.96

26.21 29.58 38.04

moisture (%)

Tunç bilek air dried dry dry, ash free Afs¸ in-Elbistan air dried dry dry, ash free

b. Elemental Analysis of Tunç bilek and Afs¸ in-Elbistan Cleaned Feed Sample basis of analysis Tunç bilek air dried dry dry, ash free Afs¸ in-Elbistan air dried dry dry, ash free

caloric carbon hydrogen nitrogen sulfur oxygen value (%) (%) (%) (%) (%) (cal/g) 55.72 57.79 74.16

3.96 4.11 5.27

2.26 2.34 3.01

1.39 1.44 1.85

11.80 12.24 15.71

4910 5090 6530

39.95 45.09 57.97

2.92 3.30 4.24

1.29 1.46 1.87

1.95 2.20 2.83

22.80 25.72 33.09

3120 3520 4530

c. Ash Composition of Tunç bilek and Afs¸ in-Elbistan Cleaned Feed Sample compound

Tunç bilek cleaned feed ash

Afs¸ in-Elbistan cleaned feed ash

SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) Na2O (%) K2O (%) SO3

55.10 23.15 9.30 1.35 4.00 0.01 1.05 -

11.80 10.70 3.75 43.10 2.15 0.01 0.15 22.50

Coal cleaning process applied to Tunç bilek and Afs¸ inElbistan raw coals improved the qualities of coal samples. However, it was shown that the rank of the coal is an important parameter. The improvements in coal quality were lower in Afs¸ in-Elbistan coal, which has the lower rank. The relative amounts of the basic and acidic constituents in the ash can are used as a means of predicting

the viscosity of the slag. The viscosity of the slug decreases as the base/acid ratio increses.10 Generally, high base-to-acid ratios are measured for coals causing problems during power generation. For the coal samples used during this research, lower base-to-acid ratios were obtained after coal cleaning. Base/acid ratios of Tuncbilek feed and cleaned feed were calculated as 0.29 and 0.20, respectively. These values were 3.86 and 2.18 for Afsin Elbistan feed and cleaned feed. A lower base/acid ratio of cleaned products indicates higher ash fusion temperature and high slag viscosity compared to raw samples. From the TG/DTG curves (Figure 1) it was observed that combustion reactions of Tuncbilek coal occur generally in three reaction regions. The first region was due to the moisture loss in coal. Release of volatile matter and burning of carbon can be called a second reaction region. The third reaction region was related to the decomposition of mineral matter in the coal. The higher amount of weight loss is observed in the second region than the other regions satisfied that the primary combustion of the samples occurred in this region. For all the cleaned size fractions of Tuncbilek sample, no temperature range was given in the third reaction region, because of the absence of any decomposition reaction. Coal cleaning which effectively removed mineral matter is the reason for this result. Thermal properties of Tuncbilek coal are given in Table 7. Slight changes of peak and burn-out temperatures of the coal were observed from TG/DTG curves. Burn-out temperatures of different size fractions of the coal are decreased as the particle size decreased. Burn-out temperatures of all the cleaned size fractions were lower than uncleaned samples indicated early completion of burning. Cleaned product had both lower peak and burn-out temperatures with respect to raw feed. It was also observed that cleaned size fractions had more combustible matter than uncleaned size fractions at peak temperature. Higher amounts of combustible matter contents of cleaned size fractions were the reason for the higher end temperatures of second reaction region. As a result, the combustion of clean coal lasts longer than uncleaned coal. Four different reaction regions occurring at different temperature ranges (Table 8) were observed from TG/ DTG curves of Afs¸ in-Elbistan coal. (Figure 2). The first


Energy & Fuels, Vol. 15, No. 6, 2001 1465

Figure 2. TG/DTG curve of Tuncbilek coal feed sample (-30 + 18 mm). Table 7. Reaction Regions and Thermogravimetric Properties of Tunç bilek Coal and Weight Percent Remained and Calculated Amount of Combustible Matter at Peak Temperature

Table 8. Reaction Regions and Thermogravimetric properties of Afs¸ in-Elbistan Coal and Weight Percent Remained and Calculated Amount of Combustible Matter at Peak Temperature

a. Reaction Regions of Tunç bilek Coal

a. Reaction Regions of Afs¸ in-Elbistan Coal

sample

region 1 (°C)

region 2 (°C)

region 3 (°C)

feed -30+18 mm (uncleaned) -30+18 mm (cleaned) -18+10 mm (uncleaned) -18+10 mm (cleaned) -10+0.5 mm (uncleaned) -10+0.5 mm (cleaned) -0.5 mm (uncleaned) -0.5 mm (cleaned) cleaned product

20-110 20-110 20-110 20-110 20-110 20-110 20-110 20-110 20-110 20-110

230-485 230-485 230-620 230-485 230-610 230-520 230-635 230-465 230-605 230-620

640-670 640-670 640-670 645-655 630-660 705-720

sample

region 1 (°C)

region 2 (°C)

region 3 (°C)

region 4 (°C)


20-110 20-110 20-110 20-110 20-110 20-110 20-110 20-110 20-110 20-110

180-420 180-415 180-425 180-410 180-415 180-415 180-440 180-315 180-330 180-445

485-545 485-555 485-565 475-545 485-550 485-555 505-555 470-520 474-525 490-550

635-735 630-730 630-700 625-725 620-690 610-730 615-660 635-780 640-755 630-705

b. Thermogravimetric Properties of Afs¸ in-Elbistan Coal

b. Thermogravimetric Properties of Tunç bilek Coal sample

peak temperature (°C)

burn-out temperature (°C)


477.03 570.78 481.67 484.66 534.42 520.63 481.85 463.83 481.05 467.84

704.56 711.14 698.08 704.53 684.68 678.42 673.52 687.21 663.44 658.76

c. Weight Percent Remained and Calculated Amount of Combustible Matter at Peak Temperature sample

weight (%)

combustible matter (%)


54.08 58.36 63.85 75.31 52.91 63.98 52.09 77.24 61.43 62.51

0.78 3.89 42.45 18.49 28.61 15.82 33.85 21.76 38.11 41.22

region was due to the loss of moisture. The magnitude of the reaction in this region was dependent on the moisture content of the sample, and to some extent, indicative of rank.2 Previous studies indicated that low

sample

peak temperature (°C)

burn-out temperature (°C)


305.80 307.49 301.59 300.04 302.04 303.21 362.77 302.51 306.76 303.89

735.70 727.25 702.26 724.72 691.77 731.18 657.78 778.78 755.90 706.83

c. Weight Percent Remained and Calculated Amount of Combustible Matter at Peak Temperature sample

weight (%)

combustible matter (%)

feed -30+18 mm (uncleaned) -30+18 mm (cleaned) -18+10 mm (uncleaned) -18+10 mm (cleaned) -10+0.5 mm (uncleaned) -10+0.5 mm (cleaned) -0.5 mm (uncleaned) -0.5 mm (cleaned)

73.24 68.51 65.29 69.92 63.02 73.48 53.83 82.10 73.06

46.66 43.92 46.08 45.39 43.98 45.54 34.07 42.42 41.98

rank coals had more tendencies to oxidation than high rank coals. TG/DTG and DTA curves of such low rank coals exhibited additional low temperature weight loss region. This low temperature weight loss region was not observed in Tuncbilek coal sample. The third and fourth regions were due to the combustion of fixed carbon and

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Kok et al.

decomposition of the mineral matter in the samples, respectively. Peak temperatures of uncleaned size fractions of Afs¸ in-Elbistan coal were close to each other. Burn-out temperatures of these fractions were also close to each other except the -0.5 mm size fraction. Due to the higher ash content of this fraction, combustion is completed at higher temperature than the other uncleaned size fractions. Burn-out temperatures of all fraction sizes decreased after coal cleaning because of the removal of some mineral matter from the samples. Burn-out temperatures of the Afs¸ in-Elbistan cleaned product were also lower than the raw feed. The amount of the samples remained at the peak temperatures showed that the weight losses of uncleaned size fractions were less than cleaned size fraction (Table 8). On the other hand, calculated amount of combustible matter of -30+18 mm size fraction was found higher than uncleaned size fraction at peak temperatures. This was again due to the low effect of coal cleaning on Afs¸ inElbistan coal. Kinetic Analysis. The calculation of the kinetic data is based on the formal kinetic equation:14

dR/dt ) kRn

(2)

where A is the Arrhenius constant, E is the activation energy, and R is the gas law constant.

(3)

k ) Ar exp(-E/RT)

(4)

Assuming first-order kinetics,

-30+18 mm (uncleaned) -30+18 mm (cleaned) -18+10 mm (uncleaned) -18+10 mm (cleaned) -10+0.5 mm (uncleaned) -10+0.5 mm (cleaned) -0.5 mm (uncleaned) -0.5 mm (cleaned) cleaned product

(5)

[(dW/dt)1/W] ) Ar exp(-E/RT)

(6)

taking the logarithm of both sides,

(7)

where dW/dt is the rate of weight change, E is the activation energy, T is the temperature, Ar is Arrhenius constant, and n is the reaction order. When log[(dW/dt)1/W] is plotted against 1/T, a straight line is obtained which will have a slope equal to E/2.303R, and from the intercept the Arrhenius constant can be estimated. The results of kinetic analysis obtained by Arrhenius kinetic model of Tuncbilek TG/DTG data are given in Table 9. Linear least-squares correlation coefficients for the identified rectilinear portions varied from 0.93 to 0.99. Different uncleaned size fractions of the coal sample showed slight changes of activation energies. Activation energies of cleaned size fractions were found lower than those of the uncleaned sizes indicating easy combustibility of cleaned size fractions. For all of the

2 3 2 3 2 3 2 3 2 3 2 3 2 3 2 3 2 3 2 3

Ea (kJ/mol)

Ar (1/min)

35.616 108.971 35.807 150.156 24.382 36.726 162.768 22.851

9.057 5.176 × 103 8.570 1.052 × 106 1.888 9.661 5.023 × 106 1.390

31.979 140.874 24.248

4.988 3.373 × 105 1.932

41.778 97.584 24.707

26.242 1.242 × 103 2.153

21.243

1.191

Table 10. Activation Energies (Ea) and Arrhenius Constants (Ar) of Afsin-Elbistan Sample sample feed -30+18 mm (uncleaned) -30+18 mm (cleaned) -18+10 mm (uncleaned) -18+10 mm (cleaned)

-10+0.5 mm (cleaned) -0.5 mm (uncleaned) -0.5 mm (cleaned)

dW/dt ) Ar exp(-E/RT)W

(14) Ko¨k, M. V. Thermochim. Acta 1993, 214, 315-324.

feed

-10+0.5 mm (uncleaned)

dW/dt ) kWn

log[(dW/dt)1/W] ) log Ar - E/2.303RT

reaction region

sample

(1)

where R is the amount of sample undergoing the reaction, n is the order of reaction, and k is the specific rate constant. The temperature dependence of k is expressed by the Arrhenius equation:

k ) A exp(-E/RT)

Table 9. Activation Energies (Ea) and Arrhenius Constants (Ar) of Tuncbilek Coal

cleaned product

reaction region

Ea (kJ/mol)

Ar (1/min.)

3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4 3 4

125.72 199.46 131.08 217.18 121.47 258.88 116.07 211.51 136.13 205.10 69.36 199.36 52.63 264.77 122.81 179.46 115.69 193.37 122.58 236.66

2.65 × 106 1.77 × 109 6.03 × 106 1.97 × 1010 1.79 × 106 5.14 × 1012 7.60 × 105 1.10 × 1010 1.87 × 107 5.87 × 109 5.71 × 102 2.27 × 109 7.62 × 107 1.85 × 1013 1.78 × 106 8.81 × 107 1.00 × 106 5.93 × 108 2.28 × 106 2.72 × 1011

cleaned size faction of Tuncbilek coal, there was no decomposition reactions observed in the third reaction region. This is the effect of coal cleaning applied to Tuncbilek coal, which was used to efficiently remove mineral matter from the coal. Table 10 represents the results of kinetic analysis of Afs¸ in-Elbistan coal. Linear least-squares correlation coefficients for the identified rectilinear portions varied from 0.95 to 0.99. Coal cleaning applied to different size fractions caused a decrease in the activation energies of primary combustion regions. However, this was not an efficient improvement, because activation energies of cleaned size fractions were still high. Afs¸ in Elbistan coal gas low degree of washability, which shows the imperfect cleaning of coal and this, may be the reason of the less decrease in the activation energy values of the samples after cleaning. Conclusions The washability and combustion characteristics of two different coals, before and after cleaning process were studied and the following conclusions were derived.


Figure 3. TG/DTG curve of Afsin-Elbistan coal sample (-30 + 18 mm).

Figure 4. TG/DTG curve of Afsin-Elbistan coal feed sample (-30 + 18 mm).

Figure 5. Arrhenius plot of Tuncbilek coal sample (-30 + 18 mm).

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Figure 6. Arrhenius plot of Afsin-Elbistan coal sample (-30 + 18 mm).

•It is possible to clean Tunç bilek coal more efficiently than Afs¸ in-Elbistan coal due to the higher rank. Coal cleaning applied to the both coal samples improved their qualities. Lower base-to-acid ratios were calculated after coal cleaning which did not cause problems during power generation due to their high ash fusion temperatures and high slag viscosities. •TG/DTG curves of Tunç bilek coal revealed three reaction regions. The first region was due to the evaporation of moisture. The second region is known as the primary reaction region, whereas the third region was related to the decomposition of mineral matter in coal. On the other hand, due to its lower rank, Afs¸ inElbistan coal presented one additional weight loss region.

•Coal cleaning reduced both peak and burn-out temperatures of Tunç bilek and Afs¸ in-Elbistan raw feeds. The TG/DTG curves and kinetic analysis have revealed that increase in reactivity and long-lasting combustion is the distinctive effect of cleaning on combustion characteristics of cleaned coal samples. Lower activation energies at the main combustion regions for all the cleaned samples. Activation energies calculated at the decomposition regions were very high compared to the activation energies of combustion regions. Afs¸ inElbistan cleaned product has a higher combustion activation energy than that of the Tunç bilek coal due to its low rank. EF0100707

Effect of Cleaning Process on the Combustion Characteristics of Two

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