Energy & Fuels - ACS Publications - American Chemical Society

Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST

Article

Combustion Behavior of Coals in Rotary Kiln and Their Interaction on Co-combustion Qiang Zhong, Jian Zhang, Yongbin Yang, Tao Jiang, Qian Li, and Bin Xu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02761 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

Energy & Fuels

Combustion Behavior of Coals in Rotary Kiln and Their Interaction on Co-combustion Qiang Zhong, Jian Zhang, Yongbin Yang, Tao Jiang, Qian Li*, and Bin Xu School of Minerals Processing and Bioengineering, Central South University, Changsha, P R China, 410083. KEYWORDS: combustion behavior, coal, thermogravimetric analysis, rotary kiln, blended coals, combustion kinetics

ABSTRACT: In order to clearly reveal coal combustion behaviors and quantitatively evaluate using coal in rotary kiln, combustion behaviors of four coals (C13, C23, C32 and C6 coals) and their blended coals were investigated. Meanwhile, their interactions and combustion kinetics were analyzed. The results showed that C13 and C23 Coals having combustion characteristics of high heating value, suitable ignition temperature, burnout time and kinetic parameters (Eα and n) conform to the requirement of rotary kiln. The partial substitution of C23 coal for C13 coal in the production is feasible, while the replacement of C32 and C6 coals for C13 coal will affect coal combustion and result production fluctuation because C32 and C6 coal have effect on C13 coal combustion. The blended coals have the similar characteristics of proximate analysis and heating value contrasting to C13 coal, but they show different combustion behaviors that can not be

ACS Paragon Plus Environment

1

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

Page 2 of 26

calculated in an additive manner. Their combustion behaviors show multiple combustion peaks and complex combustion kinetics if they compose of different combustion behavior coals.

1. INTRODUCTION Rotary kiln as a key equipment is used widely in steel industry, limestone production industry as well as other industries. The burner as an integral part of the rotary kiln determines temperature distribution, airflow field, consumption of energy, and even service life of rotary kiln. Coal is one of the most extensively used fossil fuels to meet the production demand of rotary kiln of iron ore oxide pellet.1-4 With the development of steel industry, coal demanded in rotary kiln is increasing continuously. However, available coal resource is becoming increasingly poor whilst its price is consecutively rising, and energy problem is still serious in the world. At present, some aspects of processes taking place in rotary kiln have been investigated. Mathematical modeling for coal combustion in rotary kiln has been universally applied to provide information including coal combustion, heat transfer, temperature distribution, gas composition, distribution of gas-solid flow and so on.5-7 With fuzzy multisensory data fusion and image feedback, the intelligent control models have been established to measure and predict sintering temperature in rotary kiln.8,9 Flame image recognition system has been also employed to control temperature and recognize sintering condition in rotary kiln.10 As a complex system with large delay, strong coupling, nonlinear, and time varying, the real situation of coal combustion process and temperature distribution is difficult to be effectively reflected by mathematical modeling. Thermogravimetric analysis (TGA) that covers a wide range of application in research, development and economic assessment of coal or other fuels is a simple but quite effective


2

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

Energy & Fuels

technique to observe the fuel burning profile.11,12 To date, an increasing attention has been paid to co-combustion behaviors of waste or biomass with coal by TGA, such as the change of ignition temperature and burnout temperature, and the kinetic process for various decomposition reactions.13-15 Combustion characteristics are of great importance for the estimation of combustion efficiency to establish the optimum operational conditions of coal combustion. However, combustion behavior of coal used in the rotary kiln of iron ore oxide pellet have been rarely involved. Coal combustion in rotary kiln of iron ore oxide pellet provides heat to ensure the pellet roasting at about 1523 K. Meanwhile, enough heat is transferred to kiln tail to keep the temperature of chain grate being about 1223 K and complete pellet preheating.2,3,16,17 Based on these, coal having combustion characteristics of not only high heating value but also suitable ignition temperature, burnout time and combustion intensity is required for the rotary kiln process, which is different from other coal combustion. In our previous studies, thermal behaviors of coal, coke breeze and coal tar pitch have been researched by TGA to produce metallurgical quality briquettes. Meanwhile, pyrolysis and combustion processes of a coal used in rotary kiln have been analyzed by TGA, and the effect of air flow and oxygen concentration on coal combustion behaviors has been examined with isothermal thermogravimetric analysis.18-20 Based on these works, four single coals (C13, C23, C32 and C6 coals) have been adopted and five blended coals have been designed to analyze their combustion behaviors and combustion kinetics. Furthermore, the interaction of these single coals has been analyzed to evaluate their co-combustion behaviors. Through these studies, combustion behaviors of single coals and their interaction on their co-combustion behaviors have been understood clearly. Combustion characteristics and kinetic parameters have been obtained to quantitatively evaluate using coal in rotary kiln.


3

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

Page 4 of 26

2. EXPERIMENTAL 2.1 Experiment Materials Four coals used in experiment were provided by a pellet plant of China. In order to visually show the content of volatile matter, the four coals have been respectively coded by the coding of C13, C23, C32 and C6 coals, which means the volatile matter content of the coals are 13%, 23%, 32% and 6%, respectively. In the production, the coal consumptions of C13, C23 and C32 coals are respectively 23.0 kgCe, 28.2 kgCe and 35.4 kgCe for one ton of iron ore pellet. According to the production feedback, C13 coal is the most suitable coal for rotary kiln. C23 and C32 coals can also be applied in rotary kiln, but they lead some energy and environment problems in the production, especially C32 coal. C6 coal as a comparative coal is employed in research. C6 coal is one of the highest rank of coalification coals but its using effect is not good in the iron ore pellet production. Coal sampling was performed according to Standard Practice for the Method for Preparation Coal Sampling (GB/T 474-2008). The samples were dried in an oven at 363 K for 8 h, after which four coals were crushed and sieved to make particle size under 74 µm. A comparison of the proximate analysis result and the heating value of four coals is made in Table 1. C6 coal has the highest fixed carbon content and the lowest content of volatile matter, resulting in the highest heating value (33.3 MJ/kg). The fixed carbon content and heating value of C13 coal are slightly lower than those of C6 coal. While C23 and C32 coals have low fixed carbon content, leading to lower heating value (28.5 MJ/kg and 26.0 MJ/kg). Meanwhile, they have a high content of volatile matter, especially for C32 coal.


4

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

Energy & Fuels

Table 1 Proximate analysis and heating values of four coal samples. Samples

Proximate analyses (wt.%, d)

Qnet.d

Fixed carbon

Volatile matter

Ash

(MJ/kg)

C13 coal

76.5

12.9

10.6

32.6

C23 coal

68.0

22.9

9.1

28.5

C32 coal

60.3

31.5

8.1

26.0

C6 coal

77.2

6.0

16.8

33.3

Moreover, five blended coals were designed: three two-component blended coals (80% C13 coal+20% C23 coal, 80% C13 coal+20% C32 coal, and 80% C13 coal+20% C6 coal) and two threecomponent blended coals (80% C13 coal+10% C23 coal+10% C6 coal, and 80% C13 coal+10% C32 coal+10% C6 coal). The abbreviations of five blended coals are 20C23, 20C32, 20C6, 10C2310C6 and 10C3210C6, respectively. From these coding, the component of the blends coals can be known clearly. The proximate analysis result and heating value of five blended coals are shown in Table 2. The contents of fixed carbon, volatile matter and ash, and the heating value of the five coals are similar to those of C13 coal because 80% proportion of the blended coals is C13 coal.


5

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

Page 6 of 26

Table 2 Proximate analysis and heating values of blended coal samples. Samples

Proximate analyses (wt.%, d)

Qnet.d

Fixed carbon

Volatile matter

Ash

(MJ/kg)

20C23 coal

74.4

14.8

10.8

31.6

20C32 coal

73.0

17.2

9.8

31.3

20C6 coal

76.2

11.9

11.9

32.6

10C2310C6 coal

76.1

12.4

11.5

32.4

10C3210C6 coal

74.3

15.0

10.6

32.2

2.2 Thermogravimetric Analysis Combustion tests were performed in a NETZSCH differential thermogravimetrc analyzer STA449C (precision of temperature measurement±1 K, microbalance sensitivity 0.1 µg), during which sample weight-loss value and rate as functions of temperature or time were recorded continuously under dynamic conditions. To eliminate the effect of eventual side reactions and mass and heat transfer limitations, a small amount of each material (about 8.0 mg) were thinly distributed in an Al2O3 ceramic crucible. The combustion tests were conducted at atmospheric pressure and temperatures ranging from 293 to 1273 K under artificial air atmosphere. In combustion tests of different coals, the flow rate was fixed at 100 ml/min and the heating rate was 20 K/min. The combustion parameters obtained from thermographs generally include ignition temperature (Ti), burnout temperature (Th), maximum combustion rate (DTGmax), mean combustion rate (DTGmean) and burnout time (τ). The Ti is the temperature where a sudden decrease occurs in the


6

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

Energy & Fuels

combustion profile, the Th represents the point in the combustion profile where the combustion is complete, and the τ is the time needed for coal combustion from ignition to burnout. The DTGmax is the rate of weight loss comes up to the maximum, and the DTGmean is the mean rate of weight loss.21-24 In addition, the overlapping combustion interval of a coal is the weight loss of the coal in C13 coal combustion interval divided by total weight loss of the coal in the coal combustion interval. 2.3 Kinetic Methods Thermal degradation kinetics of the single and blended coals were evaluated by the FreemanCarroll method that has been most commonly used to calculate the global kinetic parameters of polymer degradation. All kinetics studies of heterogeneous solid-state reactions can be described by Eq. (1).25-27

= (T) ( )

(1)

Where α is the conversion degree of combustible material that can be calculated using Eq. (2), t is the time and k(T) is the rate constant can be described by the Arrhenius equation (Eq. (3)). In addition, f(α) is the reaction model function, which is generally described as f(α)=(1-α)n, and n is the order of the reaction.

=

(T) = exp /

(2) (3)

Where Wo, Wf and Wt are the initial weight, the final weight and the weight at any time (t) respectively, A is the Arrhenius parameter, Eα is the apparent activation energy (kJ/mol), R is


7

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

Page 8 of 26

universal gas constant (8.314 J/mol), and T is the absolute temperature (K). Eq. (1) can be expressed as follows:

= exp / (1 − ) = (1 − ) exp /

(4)

By taking the logarithm of both sides of Eq. (4), it can be converted as the differential formation: ln

= ln + "ln(1 − ) −

(5)

If small changes of temperature result in insignificant change in the apparent activation energy, Eq. (5) can be converted as follows: △$%

& &'

△+,

= " − ) × △$%(( ) △$%(( )

(6)

This equation can be used to study the thermal degradation kinetics at a constant heating rate, which is known as Freeman-Carroll method. If the differences in ∆ln((dα/dt) and ∆ln(1-α)) are obtained at regular intervals of 1/T. By plotting ∆ln(dα/dt)/ ∆ln(1-α) against ∆T-1/∆ln(1-α), the parameters Eα and n can be obtained from the straight slope and intercept of the regression line respectively. The Arrhenius parameter (A) is then calculated from Eq. (5).28,29 3. RESULTS AND DISCUSSION 3.1 Combustion Behavior of Single Coals Combustion behaviors of different coals in the rotary kiln can be estimated from combustion characteristics that obtained by TGA. Figure 1 shows the combustion behaviors of four coals. It can be seen from Figure 1 (a and b), all of coals show only one combustion peak. Combining with the proximate analysis result, the combustion peak is the combustion of fixed carbon that makes a great contribution to the weight loss. Basic variation trends of four coals are similar, but


8

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

Energy & Fuels

there are some differences in detail. From Figure 1 (c and d), C32 coal with 673 K ignition temperature is the most easily ignited coal and its combustion process is complete at 809 K, leading to the shortest combustion time (6.8 min). Low ignition temperature and short burnout time make its combustion mainly occur in kiln head and body, resulting the heat and temperature in kiln tail and chain grate not meet the production requirement.16,17 Therefore, coal amount should be increased to provide enough heat and high temperature in kiln tail and chain grate, which leads to high C32 coal consumption (35.4 kgCe/t). While C13 and C23 coals having higher ignition temperature and longer burnout time make their combustion occur not only in kiln head but also in kiln tail, which means enough heat and high temperature can be obtained in kiln tail and chain grate with low coal consumption. The C13 and C23 coals have the similar burnout time but the DTGmax and DTGmean of C13 coal (15.7 %/min and 10.1 %/min) are faster than those of C23 coal (13.6 %/min and 10.0 %/min), meaning C13 coal has a fiercer combustion and higher heat production than those of C23 coal at the same burnout time. It leads to lower C13 coal consumption (23.0 kgCe/t) than C23 coal consumption (28.2 kgCe/t). In addition, C6 coal having fairly high ignition temperature (824 K), quite long burnout time (12.6 min) and very low DTGmax (9.6 %/min) and DTGmean (6.0 %/min) makes its combustion occur difficultly and heat release gently. The characteristics of high ignition temperature and gentle heat release may cause a lack of heating in kiln head and its temperature does not meet the production requirement. The long burnout time may also lead to insufficient coal combustion and residual coal particles mix into iron ore pellets, resulting in energy waste and pellet quality decline. Furthermore, overlapping combustion interval between C13 and C23 coal is 86.9%, meaning their combustion processes are well-matched relatively. This property contributes to create a matched heat releasing interval during their co-combustion processes. While overlapping combustion interval


9

Energy & Fuels

between C13 and C32 coal is 22.4%, and that between C13 and C6 coal is 8.7%. The low overlapping percentages mean the heat release of C32 and C6 coals hardly matched to that of C13 coal.16,17, 30, 31 Therefore, C13 and C23 coals having combustion characteristics of high heating value, suitable ignition temperature and burnout time conform to the requirement of rotary kiln. It could not only ensure high temperature in rotary kiln and chain grate, but also save energy and reduce environmental pollution. While C32 coal with the combustion characteristics of low heating value, low ignition temperature and short burnout time leads to its high consumption. High ignition temperature, long burnout time and gentle heat release of C6 coal makes it not suit to rotary kiln. 120

3

a

0

100

b

-3

DTG (%/min)

80

TG/(%)

60

40

C13 coal

-6

-9

C13 coal

-12

C23 coal

C23 coal 20

C32 coal

C32 coal

-15

C6 coal

C6 coal 0 200

400

600

800

1000

1200

-18 200

400

600

800

1000

1200

Temperature (K)

Temperature (K) 1100

d

Ti Th

1000

Temperature (K)

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

Page 10 of 26

900 8.70% 86.92% 800

12.55 min

22.44% 7.45 min 7.65 min

700

6.80 min 600

C13

C23

C32

C6

Coal

Figure 1. Combustion behaviors of different coals (a: TG profile, b: DTG profile, c: combustion characteristics, and d: burnout time and overlapping combustion interval).


10

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

Energy & Fuels

3.2. Combustion behavior of two-component blended coals Combustion behaviors of three two-component blended coals were analyzed by TGA and their results are shown in Figure 2. The 20C23 and 20C32 show one combustion peak while 20C6 has two combustion peaks. The C13 and 20C23 coals have similar combustion trends that combustion characteristics of ignition temperature, burnout time and combustion intensity are almost the same, meaning that C23 coal has no effect on the combustion behavior of C13 coal. Meanwhile, their overlapping combustion interval is up to 91.9%, indicating their combustion processes are well-matched relatively and C23 coal does not interfere with the combustion of C13 coal. Therefore, the partial substitution of C23 coal for C13 coal in the production is feasible. After added 20% C32 coal, the blended coal of 20C32 has some different characteristics contrasting to C13 coal, low ignition temperature (687 K), long burnout time (10.4 min) and gentle combustion intensity due to the effect of 20% C32 coal. Therefore, C32 coal has some effect on the combustion of C13 coal. For 20C6 coal, its combustion trend is different to that of C13 coal. Its first peak is quite well-matched with that of C13 coal, revealing that the peak is the combustion of C13 coal that creates main contribution of weight loss. The second peak is the combustion of C6 coal. The two combustion peaks can not only make its combustion become complex, but also break its combustion with the characteristics of very long burnout time (12.0 min) and weak combustion intensity,32, 33 which is disadvantageous to the coal combustion in rotary kiln. In addition, 80% C13 coal makes the overlapping combustion intervals of 20C32 and 20C6 coals are 74.4% and 86.7%, respectively. But 20% different property coals have effect on the combustion of C13 coal, which leads their blended coals have different combustion characteristics, especially for 20C6 coal.


11

Energy & Fuels

120

3

a

b

0

100

-3

DTG (%/min)

80

TG (%)

60

40

C13 coal

-6

-9

C13 coal 20C23 coal

-12

20C23 coal 20

20C32 coal

20C32 coal

20C6 coal

-15

20C6 coal 0 200

400

600

800

1000

-18 200

1200

400

Temperature (K)

600

800

1000

1200

Temperature (K)

1100

d

Ti Th

1000

Temperature (K)

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

Page 12 of 26

7.45 min

8.05 min

10.40 min

900

91.94%

86.69%

74.43%

800

12.00 min 700

600

C13

20C23

20C32

Coal

20C6

Figure 2. Combustion behaviors of two-component blended coals (a: TG profile, b: DTG profile, c: combustion characteristics, and d: burnout time and overlapping combustion interval). 3.3. Combustion behavior of three-component blended coals Combustion behaviors of two three-component blended coals were analyzed by TGA and their results are shown in Figure 3. The two blended coals both show multiple combustion peaks: two peaks in 10C2310C6 coal and three peaks in 10C3210C6 coal. It means their combustion processes are complicated contrasting to their component coals, although they have the similar characteristics of fixed carbon content, volatile matter content and heating value. The two blended coals were designed to match the C13 coal but their component coals keep the combustion behaviors of themselves. Moreover, the multiple combustion peaks make them have very long burnout time (14.3 min and 16.6 min) and weak combustion intensity, which is


12

Page 13 of 26

disadvantageous to coal combustion in rotary kiln. Furthermore, they have high overlapping combustion intervals with C13 coal (88.4% and 67.7%), but their non-overlapping combustion intervals are the combustion peaks that is different from that of C13 coals.32-34 The different combustion peaks may be cause coal combustion complication, kiln temperature fluctuation and local high temperature. 120

3

a

b

0

100

-3

DTG (%/min)

80

TG (%)

60

40

0 200

-9

-12

C13 coal 20

-6

C13 coal

10C2310C6 coal

400

600

10C2310C6 coal

-15

10C3210C6 coal

10C3210C6 coal 800

1000

1200

-18 200

400

Temperature (K)

600

800

1000

1200

Temperature (K)

1100

d

14.25 min

16.55 min

Ti Th

1000

Temperature (K)

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

Energy & Fuels

7.45 min 900 88.25%

67.74%

800

700

600

C13

10C2310C6

10C3210C6

Coal

Figure 3. Combustion behaviors of three-component blended coals (a: TG profile, b: DTG profile, c: combustion characteristics, and d: burnout time and overlapping combustion interval). 3.4. Interaction of coals If there are no interactions in the combustion of the four coals, the co-combustion behaviors of the blended coals will follow the behaviors of their component coals in an additive manner. To


13

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

Page 14 of 26

evaluate the interactions among the four coals, the experimental and calculated TGA and DTG profiles are shown in Figure 4. It can be seen that the calculated profiles are different from the experimental ones, except for 20C23 coal. The calculated profiles of 20C23 relatively overlap with the experimental ones, indicating that there are no interactions in the co-combustion of C13 and C23 coals. For the blended coals with the component of C32 and C6 coals, the calculated profiles do not overlap with the experimental ones, meaning there are some interactions in their cocombustion. It can be also known that the C13 and C23 coals have the similar combustion behaviors, while that among C32, C6 and C13 coals are dissimilar. Therefore, there are no interactions in their co-combustion when the coals have similar combustion behaviors. While the coals with dissimilar combustion behaviors create interactions in their co-combustion.


14

Page 15 of 26

3

120

a

0

100

-3

-6 60 -9 40

DTG (%/min)

TG (%)

80

-12 20

0 200

Exp. (20C23)

-15

Cal. (20C23)

-18 400

600

800

1000

1200

Temperature (K)

3

b

120

0

100

3

c

0

100

-3

-3

-6 60 -9 40

-6 60 -9 40

-12 Exp. (20C32)

20

0 200

-12

600

800

1000

1200

-15

Cal. (20C 6)

0 200

-18 400

Exp. (20C 6)

20

-15

Cal. (20C 32)

-18 400

600

Temperature (K)

800

1000

1200

Temperature (K)

120

3

d

120

3

e

0

100

0

100

-3

-3 80

-6 60 -9 40

DTG (%/min) TG (%)

80

-6 60 -9 40

-12 20

DTG (%/min)

80

DTG (%/min) TG (%)

TG (%)

80

Exp. (10C2310C6)

-15

Cal. (10C2310C6)

DTG (%/min)

120

TG (%)

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

Energy & Fuels

-12 20

-15

Exp. (10C3210C6) Cal. (10C3210C6)

0 200

-18 400

600

800

1000

1200

0 200

-18 400

Temperature (K)

600

800

1000

1200

Temperature (K)

Figure 4. Comparisons of experimental and calculated TG and DTG profiles. The combustion peak temperature of the single and blended coals is plotted in Figure 5. A comparison results of weight loss between the experimental and calculated TGA profiles are shown in Table 3. Contrasting their combustion peak temperature and weight loss with the four single coals, the peak temperature of 20C23, 20C32, 20C6 peak 1, 10C2310C6 peak 1, and 10 C3210C6 peak 2 comes near the C13 coal peak temperature, meaning these combustion peaks are


15

Energy & Fuels

mainly about the C13 coal combustion that results main contribution of weight loss. The peak temperature of 20C6 peak 2, 10C2310C6 peak 2, and 10C3210C6 peak 3 is close to that of C6 coal, indicating that these combustion peaks are mainly about the combustion of C6 coal. Meanwhile, all of these peak temperature and combustion intervals are higher than that of C6 coal. It may be because that the low temperature combustion component of C6 coal burns with the C13 coal in lower temperature peak, while the high temperature component will reserve and then burn in high temperature peak. Therefore, only the high temperature combustion component of C6 coal can burn in these peaks, which results new combustion characteristics in contrast to C6 coal. 1050

20C6

10C2310C6

Peak 2

1000

Peak 2

C6

10C3210C6 Peak 3

950

Temperature (K)

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

Page 16 of 26

900

20C6

850

800

C13 20C23 C23

Peak 1

20C32

10C2310C6

10C3210C6 Peak 2

Peak 1

750

10C3210C6

C32

Peak 1

700

Peak value

Figure 5. Combustion peak temperature of different coals.


16

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

Energy & Fuels

Table 3 A comparison results of weight loss between the experimental and calculated TGA profiles. 1st peak

2nd peak

3rd peak

Weight loss (%)

Weight loss

Weight loss (%)

Coals 20% C23

14.8

80% C13

64.7

20C23

77.7

20% C32

13.9

80% C13

68.9

20C32

86.0

20% C6

2.5

7.4

80% C13

61.6

0.3

20C6

66.6

9.3

10%C23

7.2

0.1

10% C6

1.4

4.9

80% C13

64.4

0.3

10C2310C6

69.5

6.5

10% C32

4.3

2.1

0.2

10% C6

0.1

1.1

4.8

80% C13

6.3

64.1

0.5

10C3210C6

19.4

59.1

6.3

Calculation Experiment Calculation Experiment Calculation Experiment

Calculation

Experiment

Calculation

Experiment

In addition to the first peak temperature of 10C3210C6 coal, the temperature is close to that of C32 coal but its weight loss is much higher than the calculated ones, revealing there are the


17

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

Page 18 of 26

interactions between C32 and C13 coals that make some component of C13 coal combustion at low temperature. And it can be also known that the peak temperature of 20C32 coal is lower than that of C13 coal because of the effect of C32 coal whose peak temperature is only 728 K. Therefore, there are no interactions in the co-combustion of coals whose component coals have similar combustion behaviors. While for the coals with different combustion behaviors, there are interactions in the co-combustion. 3.5. Combustion kinetics of single and blended coals Kinetic parameters of coals provide quantitative insights into the combustion behaviors of coals. The kinetic parameters of coals were obtained by the Freeman-Carroll method. The activation energy (Eα) is a quantitative measure of the energy threshold for the formation of product during combustion, and the reaction order (n) is an indication of the reaction mechanism for the combustion. The activation energy (Eα) implies a quantitative measure of the energy demanding to start combustion, and the reaction order (n) is an indication of the reaction mechanism for the coal combustion.26,35-38 Combustion kinetics of the single and blended coals was evaluated by the Freeman-Carroll method. Linear regression equations and correlation coefficients for the single and blended coals are shown in Table 4, combustion kinetic parameters of coals are plotted in Figure 6. For the single coals, C6 coal has the highest Eα and n, indicating that its combustion reaction needs greatest energy and has most complex combustion mechanism.26,35, 37,39-41 While C13, C23 and C32 coals have the similar Eα and n that are respectively about 100 kJ/mol and 1.16,42,43 Meanwhile, the results of Eα are match to their ignition temperature: The higher the Eα is, the higher ignition temperature is.


18

Page 19 of 26

Table 4 Linear regression equations and correlation coefficients by the Freeman-Carroll method for single and blended coals. Coals

1st peak

2nd peak

Equation

R2

C13

y=1.16-1.53x

0.9992

C23

y=1.10-1.21x

0.9988

C32

y=0.99-1.09x

0.9940

C6

y=2.10-2.22x

0.9800

20C23

y=1.18-1.43x

0.9995

20C32

y=1.15-1.34x

0.9958

20C6

y=1.80-1.85x

10C2310C6 10C3210C6

400

R2

0.9691

y=1.97-4.49x

0.9937

y=1.56-1.75x

0.9645

y=1.75-4.88x

0.9904

y=2.09-2.91x

0.9940

y=1.60-1.78x

0.9933

2.5

a

10C2310C6 20C6

Peak 2

Peak 2

350

10C3210C6

10C3210C6

250 200

Peak 1 20C6

bC

C13 C23

100 50

C32

10C3210C6

20C23 20C32

y=1.81-4.43x

0.9888

Peak 1 10C2310C6

2.0

20C6

Peak 2

10C3210C6 Peak 3

Peak 1 1.5

10C2310C6 C13 C23

Peak 1 150

R2

10C3210C6

20C6

6

Equation

Peak 2

Peak 3

300

C6

3rd peak

Equation

Reaction order

450

Activation energy (kJ/mol)

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

Energy & Fuels

1.0

Peak 2

20C23 20C32

Peak 1

10C3210C6 Peak 2

C32

10C2310C6 Peak 1 0.5

Coal

Coal

Figure 6. Combustion kinetics of single and blended coals.


19

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

Page 20 of 26

The combustion kinetics of blended coals is more complex than their single coals because of the interactions of coals. The Eα of 20C23, 20C32, 20C6 peak 1, 10C2310C6 peak 1 and 10C3210C6 peak 2 are near to that of C13 coal because they are mainly about the combustion of C13. While the 20C6 peak 2, 10C2310C6 peak 2 and 10C3210C6 peak 3 have the high Eα that outclass that of C6 coal. For this result, the reason is that these peaks are the combustion of the high temperature combustion component of C6 coal whose combustion needs higher energy and starts at higher temperature contrasting to C6 coal. In addition, the Eα of 10C3210C6 peak 1 is higher than that of C32 and C13 coals because there are a mass of energy demanding to complete combustion of some component of C13 coal at low temperature that is lower than ignition temperature of C13 coal. It can be also known that the n of 20C23 and 20C32 is near to 1, while that of other combustion peaks is higher than 1. The reason for these combustion peaks have a high n is the effect of C6 coal combustion that makes the combustion become complicated.26,35, 37,44,45 Furthermore, 10C3210C6 peak 1 has a high n and complex combustion mechanism because of some C13 coal combustion at low temperature. Therefore, the blended coals have the similar combustion kinetics to their component coals when their component coals have the similar combustion behaviors. While the component coals with different combustion behaviors, their blended coals have multiple combustion peaks whose Eα and n are greatly different. 4. CONCLUSIONS Combustion behaviors of four single coals (C13, C23, C32 and C6 coals) and co-combustion behaviors of their blended coals were investigated by thermogravimetric analysis. Further, their interactions and combustion kinetics were analyzed to evaluate their combustion behaviors. A


20

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

Energy & Fuels

summary of the results obtained in this study is as follows: 1) C13 and C23 coals having combustion characteristics of high heating value, suitable ignition temperature and burnout time conform to the requirement of rotary kiln. C32 coal with the combustion characteristics of low heating value, low ignition temperature and short burnout time leads to its high consumption. High ignition temperature, long burnout time and gentle heat release of C6 coal makes it not suit rotary kiln. 2) C23 coal has no effect on the C13 coal combustion and their combustion processes are wellmatched relatively. C32 coal has some effect on C13 coal combustion, while C6 coal has obvious effect on C13 coal combustion, which results their blended coals have different combustion characteristics contrasting to C13 coal. The partial substitution of C23 coal for C13 coal in the production is feasible, while the replacement of C32 and C6 coals for C13 coal will affect coal combustion and result production fluctuation. The three-component blended coals have the similar characteristics of fixed carbon content, volatile matter content and heating value contrasting to C13 coal, but they show multiple combustion peaks and complicated combustion processes. 3) The coals having similar combustion behaviors make no interactions in their co-combustion that follow the component coals behaviors in an additive manner. While there are interactions in their co-combustion when the component coals have dissimilar combustion behaviors and the cocombustion behaviors can not be calculated in an additive manner. 4) The blended coals have the similar combustion kinetics to their component coals because of their similar combustion behaviors. While the blended coals composed of different combustion behavior coals, they have more complex combustion kinetics than their component coals. The


21

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

Page 22 of 26

blended coals show multiple combustion peaks whose Eα and n are greatly different. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National Natural Science foundation of China (No. 51574284 and No. 51504293) and the Postdoctoral Science Foundation of Central South University. ABBREVIATIONS 20C23, 80% C13 coal+20% C23 coal; 20C32, 80% C13 coal+20% C32 coal; 20C6, 80% C13 coal+20% C6 coal; 10C2310C6, 80% C13 coal+10% C23 coal+10% C6 coal; 10C3210C6, 80% C13 coal+10% C32 coal+10% C6 coal. REFERENCES 1. Ljung, A. L.; Lundstrma, T. S.; Marjavaarab, B. D.; Tanob, K. Int. J. Heat Mass Transfer 2011, 54, 3882-3890.


22

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

Energy & Fuels

2. Umadevi, T.; Kumar, P.; Lobo, N. F.; Prabhu, M.; Mahapatra, P. C.; Ranjan, M. ISIJ Int. 2011, 51, 14-20. 3. Ustaoglua, A.; Alptekinb, M.; Akayd, M. E. Appl. Therm. Eng. 2017, 112, 281-295. 4. Fan, X. H; Wang, Y.; Chen, X. L. J. Cent. South Univ. 2012, 19, 1724-1727. 5. Georgallis, M.; Nowak, P.; Salcudean, M.; Gartshore, I. S. Can. J. Chem. Eng. 2005, 83, 212223. 6. Promdee, K.; Chanvidhwatanakita, J.; Satitkune, S.; Boonmee, C.; Kawichai, T.; Jarernprasert, S.; Vitidsant, T. Renew. Sust. Energ. Rev. 2017, 75, 1175-1186. 7. Yin, Q.; Chen, Q.; Du, W. J.; Ji, X. L.; Cheng, L. Int. J. Heat Mass Tran. 2016, 93, 1-8. 8. Wang, J. Z.; Zhang, Y. P.; Li, Y.; Huang, S. H. J. Energy INST. 2015, 88, 143-150. 9. Diego, M. E.; Arias, B.; Abanades, J. C. J. Clean. Prod. 2016, 117, 110-121. 10. Smart, J. P.; Riley, G. S. J. Energy INST. 2012, 85, 131-134. 11. Zhang, Y.Y.; Zhang, Z. Z.; Zhu, M. M.; Cheng, F. Q.; Zhang, D. K. Bioresour. Technol. 2016, 214, 396-403. 12. Otero, M.; Gomeza, X.; Garcia, A. I.; Moran, Chemosphere 2007, 69, 1740-1750. 13. Zhang, J. L.; Guo, J.; Wang, G. W.; Xu, T.; Chai, Y. F.; Zheng, C. L.; Xu, R. S. Int. J. Miner. Metall. Mater. 2016, 23, 1001-1010. 14. Font, R.; Fullana, A.; Conesa, J. J. Anal. Appl. Pyrolysis 2005, 74, 429-438.


23

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

Page 24 of 26

15. Li, F. X.; Li, S. Z.; Chou, Y. Z. J. Therm. Anal. Calorim. 2009, 95, 633-635. 16. Liao, Y. F.; Ma, X. Q. Appl. Energy 2010, 87, 3529-3532. 17. Dwarapudi, S.; Devi, T. U.; Rao, S. M.; Ranjan, M. ISIJ Int. 2008, 48, 768-776. 18. Zhong, Q.; Yang, Y. B.; Jiang, T.; Li, Q.; Xu, B. Fuel Process. Technol. 2016, 148, 12-18. 19. Zhong, Q.; Yang, Y. B.; Li, Q.; Xu, B.; Jiang, T. Fuel Process. Technol. 2017, 157, 12-19. 20. Zhong, Q.; Yang, Y. B.; Li, Q.; Jiang, T. Thermogravimetric Analysis of Coal Used in Rotary Kiln of Iron Ore Oxide Pellet; 7th International Symposium on High-Temperature Metallurgical Processing: Hoboken, 2016; pp 351-358. 21. Mendez, A.; Fidalgo, J. M.; Guerrero, F.; Gasco, G. J. Anal. Appl. Pyrol. 2009, 86, 66-73. 22. Vamvuka, D.; Salpigidou, N.; Kastanaki, E.; Sfakiotakis, S. Fuel 2009, 88, 637-643. 23. Liu, Z. J.; Hu, W. H.; Jiang, Z. H.; Mi, B. B.; Fei, B. H. Renew. Energ. 2016, 87, 346-352. 24. Junga, R.; Knauerb, W.; Niemieca, P.; Tańczukc, M. Renew. Energ. 2017, 111, 245-255. 25. Vyazovkin, S.; Burnham, A. K.; Criado, J. M.; Perez-Maqueda, L. A.; Popescu, C.; Sbirrazzuoli, N. Thermochim. Acta 2011, 520, 1-2. 26. Ioannidou, O.; Zabaniotou, A. Renew. Sust. Energ. Rev. 2007, 11, 1966-2005. 27. Yorulmaz, S. Y.; Atimtay, A. T. Fuel Process. Technol. 2009, 90, 939-946. 28. Paik, P.; Kar, K. K. Polym. Degrad. Stab. 2008, 93, 24-35.


24

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

Energy & Fuels

29. Vuthaluru, H. B. Fuel Process. Technol. 2004, 85, 141-155. 30. Granados, D. A.; Chejne, F.; Mejia J. M. Appl. Energ. 2015, 158, 107-117. 31. Stjernberg, J.; Ion, J. C.; Antti, M.; Nordin, L.; Lindblom, B.; Odén, M. J. Eur. Ceram. Soc. 2012, 32, 1519-1528. 32. Lu, K. M.; Lee, W. J.; Chen, W. H.; Lin, T. C. Appl. Energ. 2013, 105, 57-65. 33. Idris S. S.; Rahman, N. A.; Ismail, K.; Alias, A. B.; Rashid, A.; Aris, M. J. Bioresource Technol. 2010, 101, 4584-4592. 34. Idris S. S.; Rahman, N. A.; Rahman, N. A.; Ismail, K. Bioresource Technol. 2012, 123, 581591. 35. White, J. E.; Catallo, W. J.; Legendre, B. L. J. Anal. Appl. Pyrolysis 2011, 91, 1-33. 36. Farmahini-farahani, M.; Jafari, S. H.; Khonakdar, H. A.; Bohme, F.; Yavari, A.; Tarameshlou, M. Polym. Int. 2008, 57, 612-617. 37. Ko, K.H.; Rawal, A.; Sahajwalla, V. Energy Convers. Manage. 2014, 86, 154-164. 38. Wang, M. Y.; Liao, B.; Liu, Y. Q.; Wang, S. B.; Qing, S.; Zhang, A. M. Appl. Therm. Eng. 2016, 103, 491-500. 39. Nassar, M. M. Energ. Source Part A 1998, 20, 831-837. 40. Farmahini-Farahani, M.; Jafari, S. H.; Khonakdar, H. A.; Bohme, F.; Yavari, A.; Taameshlou, M. J. Appl. Polym. Sci. 2008, 10, 2924-2931.


25

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

Page 26 of 26

41. Lapina, N. A.; Ostrovskii, V. S.; Syskov, K.I. Carbon 1976, 14, 39–41. 42. Yang, Z. Q.; Zhang, L.; Peng, J. Energ. Source. Part A 2016, 38, 309-314. 43. Zou, C.; Zhao, J. X. J. Energy Inst. 2017, 90, 797-805. 44. Peng, W. M.; Wu, Q. Y.; Tu, P. G.; Zhao, N. M. Bioresource Technol. 2001, 80, 1-7. 45. Friedman, H. L. J. Polym. Sci. Polym. Symp. 1964, 6, 183-195.


26

Energy & Fuels - ACS Publications - American Chemical Society

Recommend Documents