Biomass Blends

Article pubs.acs.org/EF

Thermal Behaviors and Kinetics of Pingshuo Coal/Biomass Blends during Copyrolysis and Cocombustion Jian Wang,† Shou-yu Zhang,*,† Xi Guo,† Ai-xia Dong,† Chuan Chen,† Shao-wu Xiong,† Yi-tian Fang,‡ and Wei-di Yin§ †

School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai, 200093, People's Republic of China ‡ Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 03001, People's Republic of China § Department of Thermal Engineering, Tsinghua University, Beijing, 100084, People's Republic of China ABSTRACT: The copyrolysis and cocombustion behaviors of Pingshuo coal and the biomasses (sawdust and rice straw) have been investigated using a thermogravimetric analyzer. The experimental results indicate that there exist synergetic effects between the biomasses and Pingshuo coal during their coconversion process. The initial temperature of volatile emission from Pingshuo coal and the temperature corresponding to the maximum conversion rate during the copyrolysis change with the biomass mixture ratio. Moreover, it can be deduced from the comparison between the experimental and the calculated DTG curves that the copyrolysis process is not the sum of Pingshuo coal and the biomass conversion. During their cocombustion process, the larger the mixture ratio of the biomass is, the lower the ignition temperature and the burnout temperature are, and the larger the combustion characteristic index is. In addition, the maximum combustion rate and the combustion performance are the best when the mixture ratio of the biomass is 70 wt % in the research. Moreover, the activation energy and the frequency factor of the copyrolysis and the cocombustion were calculated by the Coats−Redfern method and the first-order reaction model. The results show that the activation energy and the frequency factor change with the mixture ratio of the biomass, and the regularity was consistent with the above-mentioned conclusions. Therefore, it can be deduced that the addition of the biomass can facilitate the pyrolysis and the combustion of Pingshuo coal, and improve the utilization field of Pingshuo coal.

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renewable fuels.3 Several reasons can be listed for blending biomass with coal prior to burning. The cocombustion of coal/ biomass blends will help to reduce the consumption of fossil fuels. Sometimes, biofuel is mixed with coal to achieve better control of the burning process.4 In cocombustion processes, a volatile matter yield greater than 35% is sought in order to provide a stable flame, which could be attained by using biomass.5 What’s more, the ash deposition and fouling problems on heat surfaces, which are commonly encountered during biomass combustion, can be reduced or probably eliminated by burning coal/biomass blends.6 As we all know, pyrolysis is the first reaction step during coal and biomass combustion and gasification processes. It plays a key role in clean coal utilization to study their pyrolysis characteristics and process properties. Therefore, the copyrolysis and cocombustion of Pingshuo coal/biomass are the focus of this research. However, as a typical thermal coal of China, little attention has been focused on the clean utilization of Pingshuo coal. The aim of this work was to investigate the thermal properties and kinetic behavior of Pingshuo coal, biomass, and their blends using a thermogravimetric analyzer, and the attempts were made to observe whether there are synergetic effects between the copyrolysis and the cocombustion of Pingshuo coal and biomass.

INTRODUCTION Because of the global environmental concerns over excessive fossil fuel usage, a new and clean energy has been sought to substitute for fossil resources. Compared with petroleum and natural gas, which are in deeper shortages, the amount of coal deposit is abundant. Coal, as a traditional fossil fuel, will still be a major energy source in the foreseeable future, especially for China. Pingshuo coal is a typical thermal coal in China, which characteristically has a high content of sulfur and chlorine. Because of the great amount of CO2, SOx, and NOx emission during the direct combustion of coal, it is highly important to develop the most effective technology to utilize coal as an efficient and clean source of energy. As we know, biomass is one of the most important renewable energy resources, and its application becomes more and more significant for climate protection.1 Compared with other renewable energy resources, biomass is abundant in annual production, with a geographically widespread distribution in the world. The application of biomass for energy can lead to a zero net CO2 emission in a very short life cycle period, since carbon in the form of CO2 and energy are fixed by photosynthesis during biomass growth.2 Consequently, the co-utilization of biomass in the existing coal-fired plants is very appealing nowadays. Besides, its high thermochemical reactivity and high volatile matter yield facilitate the conversion and upgrading of the fuel. To date, many technologies have been developed for the possible utilization of coal with biomass, such as cocombustion, copyrolysis, cogasification, and coliquefaction. Cocombustion is a promising short-term option for the application with the © 2012 American Chemical Society

Received: September 11, 2012 Revised: November 5, 2012 Published: November 6, 2012 7120

dx.doi.org/10.1021/ef301473k | Energy Fuels 2012, 26, 7120−7126

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Table 1. Proximate and Ultimate Analyses of the Pure Samples proximate analysis (wt %)

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ultimate analysis (wt %)

sample

Mad

Aad

Vad

FCad

Cad

Had

Oad

Nad

St,ad

Qnet,v,ad (MJ/kg)

Pingshuo coal sawdust rice straw

2.16 3.13 3.63

31.26 1.64 11.83

29.21 79.67 64.82

37.37 15.56 19.72

52.05 46.92 45.26

3.07 5.63 4.15

8.31 41.78 34.71

0.79 0.86 0.30

2.36 0.04 0.12

19.83 17.79 14.94

EXPERIMENTAL PROCEDURES

Raw Materials. Three raw materials, sawdust, rice straw, and Pingshuo coal, were chosen for the investigation. The air-dried samples were milled, sieved, and classified to obtain the fractions of uniform particle sizes of 100−200 mesh for both biomass and Pingshuo coal. To eliminate the effect of moisture content, the samples were oven-dried at 105 °C for 2h and then stored in a desiccator for the following test. Table 1 shows the proximate and ultimate analyses of the three samples. Sawdust and rice straw were blended with Pingshuo coal to prepare different binary blends with varying proportions of Pingshuo coal/ biomass. Pingshuo coal/biomass ratios in different blend compositions have been selected as 80:20, 50:50, and 30:70, respectively. Pyrolysis Experiment. Mixed samples (10 mg) were put into an Al2O3 crucible, and the crucible was placed into a pyrolysis reactor. During the startup, an inert gas was used to purge air out of the pyrolysis reactor, which had already been filled with mixed sample particles. The final pyrolysis temperature was designed as 1273 K, and the heating rate was chosen as 10 K/min. The carrier gas used was N2 with a flow rate of 70 mL/min. Combustion Experiment. The method that investigated the cocombustion of Pingshuo coal char and biomass char was used in the research. Each of the samples was pyrolyzed at 1273 K to obtain different char samples; the heating rate was chosen as 10 K/min. Char samples were pulverized to 100 mesh size. Char samples can then be used to do combustion experiments. The experimental procedures were the same as those in the pyrolysis experiment; the only difference was that the carrier gas was the N2/air (10% air) mixture atmosphere.

Figure 2. DTG curves of pyrolysis behaviors of rice straw/Pingshuo coal blends.

Table 2. Pyrolysis Characteristics of Biomass/Pingshuo Coal Blends T1 (K)

sample Pingshuo coal mixtures 1:4 (20% mixtures 1:1 (50% mixtures 7:3 (70% sawdust mixtures 1:4 (20% mixtures 1:1 (50% mixtures 7:3 (70% rice straw

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EXPERIMENTAL RESULTS AND DISCUSSION Copyrolysis. Figures 1 and 2 describe the DTG curves of the pyrolysis behaviors of biomass/Pingshuo coal blends, and the pyrolysis characteristics of biomass/Pingshuo coal blends are shown in Table 2. As shown in the figures, the pyrolysis behaviors of the biomass/Pingshuo coal blends have three weight loss areas that present three peaks. The first peak results from the dewatering phase, the second one is the biomass

sawdust) sawdust) sawdust) rice straw) rice straw) rice straw)

657.5 565.9 537.9 539.8 527.2 546.8 522.9 499.6 508.3

Tmax1 (K) 626.6 620.3 624.2 620.9 593.4 595.8 597.2 594.9

Tmax2 (K)

T2 (K)

722.3 710.5 708.8 713.6

789.2 652.3 649.6 661.2 660.1 647.8 642.8 658.4 643.3

721.5 719.2 723.1

devolatilization phase, and the third one is the Pingshuo coal devolatilization phase. The larger the mixture ratio of the biomass is, the bigger the pyrolysis rate of the biomass is, and the smaller the pyrolysis rate of Pingshuo coal is. During the copyrolysis of biomass and coal, the initial temperature of the volatile emission from the coal cannot be directly defined by the characteristics of copyrolysis curves. According to the relevant literature,7 it can be assumed that the initial temperature of the volatile emission from the coal is equal to the terminal temperature of the volatile emission from the biomasses. In this study, the terminal temperature of the volatile emission from sawdust/rice straw was considered as the initial temperature of the volatile emission from Pingshuo coal. Table 2 demonstrates the pyrolysis characteristics of biomass/Pingshuo coal blends. Among these data, T1 is the initial temperature of the volatile emission from the biomass, that is, the temperature corresponding to dx/dt = 0.1 mg/min on the DTG curve.15 T2 is the initial temperature of the volatile emission from Pingshuo coal in the mixture samples. Tmax1 and Tmax2 are the temperatures corresponding to the first and the second maximum pyrolysis rate, respectively.

Figure 1. DTG curves of pyrolysis behaviors of sawdust/Pingshuo coal blends. 7121


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considered that there are some synergetic effects during the copyrolysis of the biomass and Pingshuo coal. To further understand the interactions between the biomass and the Pingshuo coal during the copyrolysis, the theoretical TG curves of the blends were calculated as the sum of the decomposition curves of each individual component. In this research, from the angles of the pyrolysis weight loss and weight loss rate, the different temperature spots of the beginning, middle, and ending during the pyrolysis process of individual samples were chosen. The weight loss of mixed samples during the pyrolysis process is obtained as follows10

As shown in Table 2, considering the copyrolysis of the biomass and Pingshuo coal, the initial temperature of the volatile emission from Pingshuo coal and the temperature corresponding to the maximum pyrolysis rate change with the mixture ratio of the biomass. It can be found that the temperature range of the first DTG peak, which represents the devolatilization of biomass, shifts to a higher temperature range and the temperature range of the second DTG peak shifts to a lower temperature range. When the mixture ratio of the biomass is below 50 wt %, with the increase of the mixture ratio, the temperature corresponding to the maximum pyrolysis rate of Pingshuo coal is lower than that of Pingshuo coal pyrolysis, and the temperature decreases gradually. When the mixture ratio increases to 50 wt %, the corresponding temperature begins to increase, even higher than that of Pingshuo coal pyrolysis. Considering the copyrolysis of the biomass and Pingshuo coal, it can be deduced that the initial temperature of the volatile emission from Pingshuo coal is lower than that of Pingshuo coal pyrolysis. The experimental results indicate that the addition of the biomass leads to the shifts of the temperature range of Pingshuo coal pyrolysis to a lower temperature range. For example, the initial temperature of the volatile emission from Pingshuo coal becomes lower, and so is the temperature corresponding to the maximum pyrolysis rate. Therefore, the appearance of the biomass results in some synergetic effects. However, the synergetic effects disappear gradually with the increase of the mixture ratio of the biomass, and this may be owing to that the appearance of the biomass restrains Pingshuo coal pyrolysis. The appearance of the biomass leads to different effects on Pingshuo coal pyrolysis. On the one hand, biomass characteristically has a higher alkaline content, especially K- and Nacontaining matters. The alkali metal elements in biomass can have some catalysis effects on Pingshuo coal pyrolysis. Moreover, the ratio of hydrogen to carbon of the biomass is much higher than that of Pingshuo coal. The ratio of hydrogen to carbon is 0.12 for the sawdust and 0.092 for the rice straw, but only 0.059 for Pingshuo coal. Furthermore, during the copyrolysis process of the biomass and Pingshuo coal, the biomass pyrolysis produces much hydrogen-containing radicals. Therefore, the hydrogen may transfer from the biomass for the pyrolysis reactions of Pingshuo coal during their copyrolysis.7,8 As we all know, hydrogen has remarkable effects on Pingshuo coal pyrolysis, so the abundant hydrogen in the biomass could be considered as the fine supplement for the pyrolysis of Pingshuo coal. Consequently, the hydrogen within the biomass may transfer to Pingshuo coal and benefit Pingshuo pyrolysis. On the other hand, the biomass will soften during the pyrolysis process, and the large amount of biomasses will adhere to the surface of Pingshuo coal particles before the devolatilization of Pingshuo coal. Consequently, the pores of Pingshuo coal may be blocked and the devolatilization of Pingshuo coal will be restrained.9 Besides, it needs more heat during the pyrolysis process of the biomass; therefore, the initial temperature of the volatile emission from Pingshuo coal will increase. Maybe it is the reason why the initial temperature of the volatile emission from Pingshuo coal during the copyrolysis rises. On the basis of the above discussion, as the mixture ratio of the biomass is below 50%, there are some positive effects on the pyrolysis of Pingshuo coal. However, the synergetic effects disappear gradually with the increase of the mixture ratio of the biomass, and this may be owing to that the appearance of the biomass restrains the pyrolysis of Pingshuo coal. On the whole, it can be

C = Cps∗(1 − w) + C b∗w

(1)

where C is the weight loss of the different temperature spots by calculating, w is the biomass percentage of the mixed samples, b is for biomass, and ps is for Pingshuo coal. The curves of weight loss rate were obtained by taking the derivative of the experimental curves and the calculated curves. Figures 3 and 4 are the comparison diagrams between the

Figure 3. Comparison between experimental and calculated DTG curves of copyrolysis of sawdust/Pingshuo coal blends.

experimental curves and the calculated curves of the weight loss rate during the copyrolysis. It can be found that the experimental curves show some deviations from the calculated curves. Whether in the low-temperature or high-temperature region, the experimental value is greater than the calculated

Figure 4. Comparison between experimental and calculated DTG curves of copyrolysis of rice straw/Pingshuo coal blends. 7122


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value during the copyrolysis of sawdust and Pingshuo coal. This result maybe suggests that the high contents of alkali metal and hydrogen contribute to the interactions of sawdust and Pingshuo coal.11−13 During the copyrolysis of rice straw and Pingshuo coal, in the high-temperature region, the experimental value is greater than the calculated value, whereas it is just the opposite in the low-temperature region. In the main pyrolysis temperature range of rice straw, Pingshuo coal has not even started pyrolysis. Owing to that, Pingshuo coal has little effect on the pyrolysis of rice straw. These results also indicate that there are some synergetic effects during the copyrolysis of the biomass and Pingshuo coal. Cocombustion. Table 3 shows the ash fusion temperature of the mixture samples. The deformation temperature (DT), Table 3. Ash Fusion Temperature of the Mixture Samples sample Pingshuo coal sawdust rice straw 80% Pingshuo 50% Pingshuo 30% Pingshuo 80% Pingshuo 50% Pingshuo 30% Pingshuo

coal coal coal coal coal coal

+ + + + + +

20% 50% 70% 20% 50% 70%

sawdust sawdust sawdust rice straw rice straw rice straw

DT (K)

ST (K)

FT (K)

1773 1490 1390 1773 1773 1733 1733 1605 1548

>1773 1504 1461 >1773 >1773 >1773 >1773 1636 1573

>1773 1515 1476 >1773 >1773 >1773 >1773 1661 1636

Figure 6. DTG curves of combustion behaviors of Pingshuo coal/rice straw chars with different blending ratios.

of Pingshuo coal/biomass char blends are shown in Table 4. Pure sample chars were compared to the char blends with respect to their performance during combustion. As shown in the figures, the combustion behaviors of Pingshuo coal/biomass char blends only have one weight loss area caused by the char burning. According to the combustion process, some characteristic parameters can be defined. Ti is the ignition temperature; Th is the burnout temperature. Both of them can be determined by the method of TG-DTG.14 Wmean is the average burning rate, Wmax and Tmax are the maximum combustion rate and the corresponding temperature, Rw is the ignition characteristic index, and S is the combustion characteristic index.15 As seen in Table 4, the ignition temperature and the burnout temperature of the mixed char are lower than those of Pingshuo coal char. With the increase of the biomass mixture ratio, the ignition temperature and the burnout temperature of the mixed chars decrease. Thus, the ignition performance and the burnout performance of the char blends increase compared with those of Pingshuo coal char. This can be attributed to the high reactivity of the biomass char due to its loose, porous, and highly disordered carbon structure, high specific area, and high alkaline content.16,17 Furthermore, the ignition temperature and the burnout temperature of Pingshuo coal/rice straw chars are both lower than those of the Pingshuo coal/sawdust chars. This may be due to the fact that the mineral content of the rice straw, which is higher than that of the sawdust, results in a slight improvement in the ignition performance and the burnout performance.18 The maximum combustion rate of the mixed chars increases, and the corresponding temperature decreases with the increase of the biomass mixture ratio. Moreover, compared with the other chars (except biomass char) employed in the research, the combustion rate of the mixed char with the biomass blending ratio of 70 wt % is the highest. In addition, the ignition performance, the burnout performance, Rw, and S are also better than those of the other chars. However, because of the high reactivity of the biomass char, more oxygen will react with the biomass char first, and this may slow down the oxidation reaction of Pingshuo coal char. When the blending ratio of the biomass is 20 wt %, the fast burning of a small amount of the biomass char will improve the combustion performance of the mixed chars. However, when the mixture ratio of the biomass is 50 wt %, less oxygen reacts with

softening temperature (ST), and flowing temperature (FT) of Pingshuo coal are much higher than those of sawdust and rice straw. What’s more, DT, ST, and FT of the mixture samples are also much higher than those of the biomasses. SiO2, which is the main component of Pingshuo coal ash, can restrain the fouling and slagging caused by the emission of K- and Nacontaining matters to some extent. Thus, it can be deduced that the ash melting performance of the biomass can be improved in the presence of Pingshuo coal during their cocombustion. Consequently, the slagging and fouling problem may be overcome to some degree during the cocombustion of Pingshuo coal and biomass. Figures 5 and 6 show the DTG curves of combustion behaviors of Pingshuo coal/biomass chars with different mixture ratios, and the combustion characteristic parameters

Figure 5. DTG curves of combustion behaviors of Pingshuo coal/ sawdust chars with different blending ratios. 7123


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Table 4. Combustion Characteristic Parameters of Biomass/Pingshuo Coal Chars with Different Blending Ratios char Pingshuo coal 80% Pingshuo 50% Pingshuo 30% Pingshuo sawdust 80% Pingshuo 50% Pingshuo 30% Pingshuo rice straw

coal + 20% sawdust coal + 50% sawdust coal + 70% sawdust coal + 20% rice straw coal + 50% rice straw coal + 70% rice straw

Ti (K)

Th (K)

Wmean (mg·min−1)

Wmax (mg·min−1)

Tmax (K)

Rw

S × 10−10 (mg2·min−2·°C−3)

842.52 835.97 828.66 823.64 771.06 812.62 773.99 761.87 655.23

1049.15 1072.7 1024.45 972.53 909.49 1034.48 1014.82 960.81 798.73

0.2995 0.2947 0.3848 0.5255 0.7211 0.2991 0.2001 0.4136 0.3365

0.5226 0.5676 0.6054 0.8274 0.5634 0.5760 0.3462 0.7196 0.6001

883.52 885.83 879.78 870.22 830.30 872.41 852.65 797.47 699.84

2.2264 1.9652 2.0377 2.1888 2.9960 2.2796 2.3339 2.5810 3.1536

6.3207 6.6156 10.6500 20.5520 25.8030 7.7892 3.7440 18.1557 26.3845

Table 5. Pyrolysis Kinetic Parameters of Biomass/Pingshuo Coal Blends reaction 1

activation energy E (kJ·mol−1)

frequency factor A (min−1)

related coefficient R2

629−758

44.94

66.55

0.9970

665−770

30.78

9.73

0.9916

0.9955

667−743

7.81

0.09

0.9968

5015.59

0.9937

686−772

4.02

0.03

0.9729

46.61

360.99

0.9935

696−791

26.52

3.38

0.9910

529−628

56.01

5071.59

0.9970

668−795

16.01

0.55

0.9964

480−685

61.48

0.9999

686−772

11.18

0.36

0.9947



+

501−658 476−627 506−663

57.13 53.86 54.64

9211.42 6102.52 1522.78

0.9947 0.9982 0.9969

+

506−679

55.40

4054.57

+

480−685

56.29

+

554−627

+ +

char Pingshuo coal sawdust rice straw 80% Pingshuo coal 20% sawdust 50% Pingshuo coal 50% sawdust 30% Pingshuo coal 70% sawdust 80% Pingshuo coal 20% rice straw 50% Pingshuo coal 50% rice straw 30% Pingshuo coal 70% rice straw

reaction 2 range of temperature T (K)


range of temperature T (K)

32862.3

the intercept term of eq 2, ln(AR/βE). The pyrolysis kinetic parameters and the combustion reaction kinetic parameters are listed in Tables 5 and 6, respectively. The high coefficient values indicate that the corresponding reaction model satisfactorily fitted the experimental data. With regards to the kinetic parameters, the frequency factor, A, is more closely related to the material structure, whereas the reactivity of samples is determined by the activation energy, E. As seen in Table 5, during the copyrolysis of Pingshuo coal and sawdust, the first pyrolysis activation energy of the mixture is lower than that of the sawdust, and decreases with the increase of the Pingshuo coal mixture ratio. Besides, it can be seen from the DTG curves that Pingshuo coal has begun to pyrolyze during the main pyrolysis temperature region of the sawdust. Therefore, this phenomenon can be attributed to the interactions between Pingshuo coal pyrolysis and sawdust pyrolysis. However, the first pyrolysis activation energy during the copyrolysis of Pingshuo coal and sawdust shows some differences from that of Pingshuo coal and rice straw blends. The first pyrolysis activation energy during the copyrolysis of Pingshuo coal and rice straw is higher than the value of rice straw pyrolysis. The pyrolysis temperature of rice straw is relatively low, and the pyrolysis of rice straw has already ended when Pingshuo coal pyrolysis starts. In this sense, Pingshuo coal has little effect on the rice straw pyrolysis in the first pyrolysis stage. The second pyrolysis is Pingshuo coal pyrolysis. the larger the mixture ratio of the biomass is, the lower the second pyrolysis activation energy is. Moreover, the second pyrolysis activation energy of the blends is lower than the value of Pingshuo coal pyrolysis. It could be deduced that the high

Pingshuo coal char due to the high reactivity of the biomass char and the combustion rate of the mixed chars decreases. When the mixture ratio of the biomass is 70 wt %, the positive effect caused by the presence of biomass prevailed over the negative effect, and the combustion rate was the highest in the research. This result suggests that the addition of the biomass can facilitate the combustion process, and there is an optimal blending proportion during the whole combustion process. These conclusions have been confirmed in previous studies.19,20 Kinetic Analysis. The Coats−Redfern method21 and the first-order reaction model, which were often used by other researchers,10,22−26 were used in the present work for both the copyrolysis and the cocombustion processes in order to obtain the parameters of thermal events. On the basis of the Arrhenius equation and the law of mass conservation, the kinetics of the reaction is described as ln[− ln(1 − x)/T 2] = ln[AR /βE(1 − 2RT /E)] − E /RT (2) −1

where A is the frequency factor (min ), E is the activation energy (kJ/mol), R is the gas constant (8.314 J K−1 mol−1), T is the absolute temperature (K), x is the loss in mass fraction or mass conversion ratio, and β (K min−1) is a constant heating rate during the copyrolysis and the cocombustion. Because it can be demonstrated that, for the value of E and for the temperature range, the expression ln[AR/βE(1 − 2RT/ E)] in eq 2 is essentially constant; if ln[−ln(1 − x)/T2] is plotted versus 1/T, a straight line should be obtained. The activation energy, E, can be calculated from the slope of the line, −E/R; and the frequency factor A can be calculated from 7124


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Table 6. Combustion Reaction Kinetic Parameters of Biomass/Pingshuo Coal Chars with Different Blending Ratios range of temperature T (K)

char Pingshuo coal sawdust rice straw 80% Pingshuo 50% Pingshuo 30% Pingshuo 80% Pingshuo 50% Pingshuo 30% Pingshuo

coal coal coal coal coal coal

+ + + + + +

20% 50% 70% 20% 50% 70%

sawdust sawdust sawdust rice straw rice straw rice straw




199.92 137.50 110.61 161.98 158.51 80.85 179.50 236.15 134.38

1.1888 × 10 9.5479 × 107 3.389 × 107 6.4831 × 108 4.7022 × 108 11504.75 6.7426 × 109 5.2943 × 1014 1.1765 × 108

0.9965 0.9994 0.9965 0.9903 0.9947 0.9983 0.9950 0.9953 0.9580

843−950 771−878 655−763 836−922 829−942 824−920 813−915 774−893 762−816

(4) The first-order reaction model may be the main mechanism responsible for the copyrolysis and the cocombustion process. The model satisfactorily and simultaneously represents the results obtained from the conversion of the different samples. The variation in the activation energy calculated during the copyrolysis and the cocombustion is consistent with the experimental data. The addition of the biomass facilitates the copyrolysis and the cocombustion processes. (5) Because the experiments have been performed using a thermobalance apparatus, there exist some differences between the experimental conditions and the real practice. However, these problems could be solved using a drop-tube reactor with a high heating rate, and the experiment will be performed in our future studies.

ash yield and hydrogen content of the biomass is a significant factor affecting the pyrolysis behavior of Pingshuo coal. Therefore, there may be some synergetic effects occurring during the copyrolysis of the biomass and Pingshuo coal. From Table 6, obviously, it can be seen that the activation energy of both the sawdust char and the rice straw char is lower than that of Pingshuo coal char and the activation energy of sawdust char is higher than the value of rice straw char. Along with the change of the biomass mixture ratio, the activation energy and the frequency factor of the mixed char vary regularly. The activation energy of the mixed char lies between that of the biomass char and that of the Pingshuo coal char. The larger the mixture ratio of the biomass is, the lower the activation energy of the mixed char is. What’s more, these results are consistent with the conclusions drawn by the change in the maximum combustion rate of the mixed char. The synergistic effect existing between Pingshuo coal char and the biomass chars can be confirmed, and the addition of the biomass can facilitate the burning process of the mixed char.

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11

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

CONCLUSIONS

Notes

The authors declare no competing financial interest.

(1) There are three weight loss areas during the pyrolysis process of the biomass and Pingshuo coal blend. The first is the dewatering phase, the second is the biomass devolatilization phase, and the third is the Pingshuo coal devolatilization phase. During the copyrolysis process, the temperature range of the second DTG peak caused by biomass devolatilization shifts to a higher temperature range and the temperature range of the third DTG peak shifts to a lower temperature range. The initial temperature of the volatile emission from Pingshuo coal and the temperature corresponding to the maximum pyrolysis rate change with the mixture ratio of the biomass. The comparison between the experimental and the calculated curves of the weight loss rate indicates that there exists some synergetic effect during the copyrolysis process. (2) The chars from the biomass and Pingshuo coal blends are subjected to only one main combustion step. The ignition temperature and the burnout temperature of the mixed char decrease with the increase of the biomass mixture ratio, and the combustion performance of the mixtures is promoted. (3) When the biomass mixture ratio is 70 wt %, the maximum combustion rate of the mixed char is the biggest in the research. Moreover, the ignition performance, the combustion performance, and the burnout performance all increase, and the addition of the biomass can improve the combustion performance of Pingshuo coal.

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ACKNOWLEDGMENTS This work was financially supported by the National Program on Key Basic Research Project (973 Program) of China (No. 2012CB214900).

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REFERENCES

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