Thermal behavior of co-combustion of oil shale semi-coke with torrefied cornstalk

Applied Thermal Engineering 109 (2016) 413–422

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Research Paper

Thermal behavior of co-combustion of oil shale semi-coke with torrefied cornstalk Hong-peng Liu ⇑, Wen-Xue Liang, Hong Qin, Qing Wang Engineering Research Centre of Oil Shale Comprehensive Utilization, Ministry of Education, Northeast Dianli University, Jilin 132012, Jilin Province, China

h i g h l i g h t s
The co-combustion of oil shale semi-coke with torrefied cornstalks shows additive behavior.
The effects of oil shale semi-coke with torrefied cornstalks are studied by TG-DTG-DSC curves and mathematical formulas.
Vyazovkin method is successfully used to evaluate the kinetic parameters.

a r t i c l e

i n f o

Article history: Received 11 April 2016 Accepted 15 August 2016 Available online 17 August 2016 Keywords: Oil shale semi-coke Torrefied cornstalk Co-combustion Thermogravimetric analysis Kinetics

a b s t r a c t Oil shale semi-coke is a hazardous byproduct from the thermal processing of oil shale, with potential dangerous environmental impacts. Previous studies have found that adding various biomass matters to the retorting waste allows for easier combustion of the retorting waste, making for safer disposal of materials. Here, oil shale semi-coke from Huadian retorts were mixed with torrefied cornstalks of various degrees (250, 275, 300 °C) in order to assess the combustion behavior and effect of the mixture. Kinetic parameters of combustion were then calculated by using the Vyazovkin method. Based on TGDTG-DSC curves, co-combustion of oil shale semi-coke and cornstalk increased with the greater degree of heat-treated cornstalk. From activation energy distribution curve, the interaction of torrefied cornstalk in the combustion process mainly occurred in the co-firing transition phase, and the activation energy of the 275 °C cornstalk was the least of the three samples and had the most obvious influence on the process. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Oil shale is a sedimentary rock consisting of a variety of minerals and preserved organic matter. Oil shale semi-coke is formed during the thermal processing of oil shale and is a low-grade fuel. This material typically has a low volatile content and minimal calorific value along with a high proportion of ash, and is also difficult to ignite in order to burn. Discarded retorting solid waste fills valuable landfill space and contains toxic environmental hazards such as polycyclic aromatic hydrocarbons, phenolic compounds and sulfuret [1–3]. In order to reduce environmental impacts, there is currently significant interest in developing effective methods of treating retorting solid waste. Many researchers have studied the co-combustion of oil shale semi-coke. Kaljuvee et al. [4] studied the co-combustion of Estonia oil shale semi-coke and oil shale in

⇑ Corresponding author. E-mail address: [email protected] (H.-p. Liu). http://dx.doi.org/10.1016/j.applthermaleng.2016.08.084 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.

circulating fluidized bed (CFBB) and found that semi-coke with a moisture content <10% can be combusted alone in CFBB, while semi-coke with a moisture content >10% is difficult to burn when incorporated into a small amount of shale. Arro et al. [5] investigated the co-combustion of oil shale semi-coke with oil shale in a (CFBB). Wang et al. [6–10] studied the combustion behavior of oil shale semi-coke in a thermogravimetric analyzer and the results indicate that the combustion process can be divided into several low temperature sections, a transition section and a high temperature section. Sun et al. [11] found the combustion characteristics of ignition and burnout could be improved when semi-coke was mixed with bituminous coal. Liu et al. [12,13] studied the combustion characteristics and kinetic parameters of co-combustion of the Hua Dian oil shale retorting waste and cornstalks by the FWO method and found that the main reaction period was changed from the high temperature period to one of the low temperature periods with an increase of cornstalk mixing proportions. The activation energy of the volatile and frequency factor increased, while the combustion activation energy and frequency factor of the coke

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presented a downward trend. Under the heating rate of 20 °C min1, the synergy of sample combustion occurred mainly in the third and the fourth stages, while the positive effect mainly occurred in the third stage and the adverse effect mainly occurred in the second stage, which was due to the delayed combustion of fixed carbon. The combustion of mixtures of oil shale retorting solid waste and cornstalk particles in a circulating fluidized bed under different conditions was also analyzed; the results show that oil shale retorting solid waste mixed with cornstalk particles in the appropriate ratio can undergo completely stable and efficient combustion. With increasing cornstalk particle fractions, the combustion characteristics improved, such that the furnace exit temperature was elevated while the concentrations of O2 and CO decreased. Although the N content of the cornstalks is greater than that of the retorting solid waste, increases in the proportion of cornstalk material in the fuel was not associated with increased emissions of NOX [14]. Taro Sonobe et al. [15] studied the synergy of copyrolysis of lignite mixed with corn straw. The results indicate that these two showed additive behavior and had a positive influence to the formation of combustion products. The corn straw is highly volatile and released significant amount of heat while reducing the ignition temperature of coal. Therefore, the co-combustion of semi-coke with cornstalks can fully utilize the oil shale semi-coke, decreasing potential environmental hazards, thereby saving energy and protecting the environment. However, raw biomass as a potential energy source also has certain drawbacks, stemming from to its own nature. These include its heterogeneity and low energy density. In order to overcome these shortcomings, torrefaction is widely considered as a promising pre-treatment for reducing some of these deficiencies. Torrefaction is defined as a thermal treatment under inert atmosphere at temperatures between 200 and 300 °C. Torrefaction is able to improve the biomass energy density and reduce its moisture content. Torrefication at 250–300 °C of the cornstalk was selected based on the fact that torrefaction of biomass below 250 °C tends to produce products with poor grindability and above 300 °C will yield products with decomposed cellulose and lignin. Under the low temperature inert atmosphere, biomass pyrolysis can precipitate moisture and 20–75% of hemicelluloses decompose into low molecular volatile compounds, which are then heated to generate carbon. Torreficated biomass produces high energy density products. Rubiera et al. [16] studied the co-combustion of torreficated biomass and coal and found that torrefaction effectively improved the grindability and produced lower emissions of NO and SO2. In this paper, the combustion characteristics and the effect of oil shale semi-coke with torrefied cornstalks were studied. Using TGDTG-DSC curves, the combustion process was completed in stages, combining mathematical formulas with the experimental curve, and from the overall and locally analyzed effects of different final temperatures of the cornstalk with semi-coke, the activation energy distribution was then calculated using the Vyazovkin method.

2. Experimental materials and equipment 2.1. Materials Oil shale semi-coke (SC) was obtained from the Huadian oil shale retort factory. Torrefied cornstalks (TCS) were collected from Jilin City. According to the GB 474-474 and GB/T 28730-2012 standards, the particle size of the blends was <0.2 mm. Torrefaction experiments were carried out in an OTL1200 tube under nitrogen flow. The cornstalks were heated to 250 °C, 275 °C and 300 °C, respectively, at a heating rate of 10 °C min1, and held at this temperature for 22 min. Only when the tube was cooled to room temperature, N2 was stopped.

Sample nomenclature and degree of torrefication is shown in Table 1. The data on proximate and ultimate analyses of SC and TCS blends are presented in Table 2. 2.2. TG/DSC experiment Combustion characteristics of the torreficated cornstalk and oil shale semi-coke were determined by Mettler-Toledo TGA/DSC analyzer. The flow rate of air was kept at 50 ml min1. The sample was heated from 50 °C up to 950 °C using several different heating rates: 10, 20, 40, 80 °C min1. Weight loss and DSC curves were continuously recorded during the process. 3. Results and discussion 3.1. The combustion characteristic analysis of the sample Ignition temperature Ti [17] was solved by using the method of TG/DTG extrapolation. The combustion process of oil shale semicoke and cornstalks was divided into a low temperature section (LTC), transition section (TC) and high temperature section (HTC), as shown in Table 3. The TG and DTG curves obtained from thermogravimetric experiments at the heating rates of 10, 20, 40, 80 °C min1 are shown in Fig. 1. The first stage that was extended from Ti to 400 °C is attributed to the combustion of cornstalk volatiles. The DTG curve shows an obvious peak and with increase of heating rate, each peak widened and deepened while shifting toward higher temperature values. Overall, the ignition temperature increased because the spread of media and heat transfer require a certain amount of time, which has some delay, the heating rate causes the reaction to move backwards. Meanwhile the faster the heating rate at the same time, the higher burning temperature. The second stage extended from 400 to 650 °C, which was the combustion of the semi-coke volatile and the fixed carbon in the sample. The DTG curve has a peak region where there was severe weight loss and rapidly occurring processes, and it was found that with the deepening of the heat treatment the phase peak seen gradually became deeper. This is because the torrefied cornstalks have a higher energy density and form ‘‘reactive cellulose,” which then transform into fixed carbon. The third stage extended from 650 to 700 °C, which was the decomposition of the difficult pyrolytic material within the sample. 3.2. Co-combustion analyses Mixing during the combustion process will occur, whether if it is a mutual influence is not easy to determine, yet this impact is used as the basis to describe the nature of the reaction. Mixed sample combustion process, if there is a response, is difficult to assess if there is a beneficial effect, therefore co-firing is considered a reasonable method of approximation. To understand if there is an interaction between oil shale semicoke and torrefied cornstalks, theoretical DTG curves were calculated by Eq. (2.2.1). The curves represent the sum of the individual component’s behavior in the mixtures.




 dW dW dW ¼ x1 þ x2 dt cal dt 1 dt 2

ð2:2:1Þ

Table 1 Sample nomenclature and degree of torrefication. Torreficated temperature

250 °C

275 °C

300 °C

SC:TCS

A1

A2

A3

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H.-p. Liu et al. / Applied Thermal Engineering 109 (2016) 413–422 Table 2 Proximate and ultimate analysis of oil shale semi-coke and torrefied cornstalk. Sample

Proximate (%)

TCS-250 °C TCS-275 °C TCS-300 °C SC

Net heating value

Ultimate (%)

Mad

Vad

Aad

FCad

Qnet,v,ar (kJ/kg)

Cad

Had

Oad

Nad

Sad

1.91 1.50 1.60 0.89

70.10 66.82 60.12 10.44

6.98 7.82 8.94 82.62

21.02 23.86 29.35 6.09

17020.03 17852.78 19162.65 3868.29

48.06 48.17 51.87 11.29

6.55 5.96 6.23 0.35

35.68 35.71 30.54 4.21

0.66 0.70 0.68 0.11

0.16 0.14 0.14 0.53

Table 3 Divided combustion sections of each sample. Sample

A1

Heating rate (°C/min)

Ti (°C)

LTC (°C)

TC (°C)

HTC (°C)

Ti (°C)

A2 LTC (°C)

TC (°C)

HTC (°C)

Ti (°C)

LTC (°C)

TC (°C)

HTC (°C)

10 20 40 80

276 280 292 296

276–374 280–382 292–398 296–416

374–615 382–633 398–650 416–674

615–711 633–715 650–751 674–787

278 289 295 311

278–371 289–384 295–401 311–418

371–609 384–630 401–651 418–665

609–696 630–739 651–751 665–783

272 285 294 304

272–367 285–384 294–394 304–409

367–610 384–633 394–646 409–667

610–699 633–732 646–750 667–783

where x1 and x2 are the mass fractions of oil shale semi-coke and torrefied cornstalk in the blends, respectively, and (dW/dt)1 and (dW/dt)2 are the corresponding mass loss rate (%/min). Total heat losses were calculated by:

Q cal ¼ x1  Q 1 þ x2  Q 2

ð2:2:2Þ

where x1 and x2 are the mass fraction of oil shale semi-coke and torrefied cornstalk in the mixtures, respectively, and Q1 and Q2 are the corresponding burning releases heat (J). In this paper, there were two methods were compared that were used to describe the effect of a sample during the combustion process. The first was the transition DTG curves from an image using a mathematical formula and the introduction of the interaction index MR (mean of the absolute error divided by the mean of the calculated value). The second method used the various stages of combustion heat release obtained by integrating the DSC curve. Using DQ = Qe  Qc, the heat difference was used to describe the amount of effect of a sample. MR was used to analyze whether or not the interaction was beneficial. When MR > 0, the interaction was beneficial; when MR < 0, the reaction had adverse effects and was not beneficial. The MR Value can be represented by the following equation:

Pn

MR ¼

i i¼1 ðxexp  nxmean cal

xical Þ

ð2:2:3Þ

where xiexp and xical represent the experimental value and calculated value, respectively. (1) The synergies of the Huadian oil shale semi-coke and torrefied cornstalks by DTG curves. As seen from Fig. 2, the DTG curves showed additive behavior. In order to analyze if the torrefied cornstalk had an influence on combustion process from the view of nature, the MR values were obtained by Eq. (2.2.3). (2) The effects of the Huadian oil shale semi-coke and torrefied cornstalks by DSC curves. As seen from Fig. 3, the DSC curves showed additive behavior (the values increased when the heating rate was increased). In order to quantitatively analyze if the torrefied cornstalk had an influence on the combustion process, the theoretical value of the oil shale semi-coke and torrefied cornstalk were calculated by Eq. (2.2.2), and then calculated in a mixed burn scenario, the difference between the experimental values with the theoretical values. When DQ > 0, indicating that the reaction was favorable, the greater the

A3

DQ increase, the more favorable the degree of influence; on the contrary, when DQ < 0, the reaction had adverse effects. From an energy point of view, the total quantity of energy released from the mixture and the energy released from each component of the mixture can greatly affect each other, while one can still adversely affect the overall beneficial outcomes. As can be seen from Table 4, at a heating rate of 10 °C min1, the difference between the heat of sample A1 was 167.40 J, the difference between the heat of sample A2 was 1167.09 J, the difference between the heat of sample A3 was 130.88 J. At the same heating rate, 275 °C torrefied cornstalks had a larger impact than other two. Similar to changing the heating rate, samples have different impact on Q. Sample A1, at a heating rate of 40 °C/min, the difference of heat was the largest and Sample A2, A3, at a heating rate of 80 °C/min, the difference of heat was the largest. 3.2.1. Effect degree phase different samples at different heating rates It is obvious from Fig. 4 that the MR values in the second phase of the heating process was always >0, indicating that the process was beneficial in a mixed combustion process. During this second phase of the heating process, combustion of the cornstalk fixed carbon and semi-coke volatile are the main processes occurring, causing the MR > 0. The same is seen from the DQ curve: the difference of the heat during the second phase was always at a maximum for each heating rate, providing evidence that among the three stages, the second stage is not only the most affected, but also has the greatest the beneficial effects to the process. As can be seen from the diagram, each sample under different heating rates is basically identical. There are differences when the same sample underwent different heating rates; for example, the 250 °C torrefied cornstalk at a heating rate of 40 °C/min the difference between the maximum heat was 1300 J; 275 °C torrefied cornstalk at a heating rate of 80 °C/min the heat difference maximum was 2000 J; and 300 °C torrefied cornstalk at a heating rate of 80 °C/min, the difference between the maximum heat was 2250 J. As the degree of torrefied processing increased, effect and heating rate also increased. The reason was that torrefied cornstalk had an increased energy density and corn straw fixed carbon combustion moved to a higher temperature. Adverse effects mainly occurred during the third phase mixture when there were a few difficult materials undergoing decomposition, most likely due to the torrefied cornstalk fixed carbon combustion moving to a higher temperature. Therefore, at this stage the reaction material ratio decreased.

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1 2 3 4

100

10°C/min 20°C/min 40°C/min 80°C/min

DTG/(%/min)

TG(%)

90 4

80

0

1

70 60

1

-5 4

-10 -15

0

200

400

600

800

-20

1000

1 2 3 4

0

200

400

T/(°C)

1 2 3 4

10°C/min 20°C/min 40°C/min 80°C/min

4 1

80

0

70 60

1

-5 4

-10 -15

0

200

400

600

800

-20

1000

1 2 3 4

0

200

400

T/(°C)

1 2 3 4

1

70 60

0

10°C/min 20°C/min 40°C/min 80°C/min

4

80

200

400

600

1000

1

-4 4

-8

-12 0

800

(b) DTG curves of A2 at different heating rates.

DTG/(%/min)

TG(%)

90

600

10°C/min 20°C/min 40°C/min 80°C/min

T/(°C)

(a) TG curves of A2 at different heating rates. 100

1000

(b) DTG curves of A1 at different heating rates.

DTG/(%/min)

TG(%)

90

800

T/(°C)

(a) TG curves of A1 at different heating rates. 100

600

10°C/min 20°C/min 40°C/min 80°C/min

800

1000

T/(°C)

(a) TG curves of A3 at different heating rates.

1 2 3 4

0

200

400

600

800

10°C/min 20°C/min 40°C/min 80°C/min

1000

T/(°C)

(b) DTG curves of A3 at different heating rates.

Fig. 1. TG-DTG curves of the sample at different heating rate of 10, 20, 40, 80 °C min1.

3.2.2. The different stages of the degree of effect analysis of samples at different heating rates As seen from Fig. 5, under the same heating rate, the different temperature of the torrefied cornstalk had basically the same trend, rendering triangles. At different heating rate, beneficial

effects during the combustion process were mainly in the second phase of the torrefied cornstalk fixed carbon and semi-coke volatile. When the heating rate was 10 or 20 °C/min, the heat difference of the co-combustion of 275 °C torrefied cornstalk and oil shale semi-coke reached a maximum, with the largest effect. When the

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H.-p. Liu et al. / Applied Thermal Engineering 109 (2016) 413–422

0.0

0

DTG/(%/min)

DTG/(%/min)

-1

-2 A1 calculated A2 calculated A3 calculated A1 experimental A2 experimental A3 experimental

-3 0

200

400

600

800

-1.7

-3.4 A1 calculated A2 calculated A3 calculated A1 experimental A2 experimental A3 experimental

-5.1

-6.8

1000

0

200

400

T/(°C)

(a) DTG curves of experimental and calculated blends at a heating rate of 10ć/min.

800

1000

(b) DTG curves of experimental and calculated blends at a heating rate of 20ć/min.

0

0

-5

DTG/(%/min)

-4

DTG/(%/min)

600

T/(°C)

-8

A1 calculated A2 calculated A3 calculated A1 experimental A2 experimental A3 experimental

-12 0

200

400

600

800

1000

-10

A1 calculated A2 calculated A3 calculated A1 experimental A2 experimental A3 experimental

-15

-20 0

200

400

600

800

1000

T/(°C)

T/(°C)

(c) DTG curves of experimental and calculated blends at a heating rate of 40ć/min.

(d) DTG curves of experimental and calculated blends at a heating rate of 80ć/min.

Fig. 2. DTG curves of experimental and calculated blends at the different heating rate.

heating rate was 40 or 80 °C/min, the DQ curve showing obvious effect were the samples who under a processing temperature of 250 °C torrefied cornstalk. 3.3. Co-combustion kinetic analysis Based on the Vyazovkin method [18–20], the activation energy from the curves plotted as a function of conversion a for the samples are shown in Fig. 6. The Vyazovkin method is based on nonlinear solution principles, thus has higher accuracy, and is recommended by the International Confederation for Thermal Analysis and Calorimetry (ICTAC). The constancy of Ea is assumed only for small intervals of conversion, Da:

GðaÞ ¼

A b

Z

Ta

exp T a Da


 Ea A dT ¼ IðEa ; T a Þ b RT

ð2:3:1Þ

The basic assumption of this method is that the reaction model G(a) is independent of the heating rate. Under this assumption, G (a) is a constant at a given conversion:

A A A IðEa ; T a1 Þ ¼ IðEa ; T a2 Þ ¼    ¼ IðEa ; T an Þ b1 b2 bn

ð2:3:2Þ

According to the Vyazovkin expression, substituting experimental values T and b into Eq. (2.3.3) and varying E to reach a minimum gives the value of the activation energy at a given conversion:

UðEa Þ ¼ min

  n X n X I Ea ; T a;i  bj   I Ea ; T a;j  bi i¼1 i–j

ð2:3:3Þ

The algorithm based on generalized nonlinear constraint gradient method is thus derived, and is one of the most powerful nonlinear programming methods. Generally, the improvement of the conversion method is that the conversion a corresponds with different heating rates, allowing for a varying E in order to reach a minimum so as to obtain the corresponding activation energy distribution. I can be obtained after series expansion. We can write Ea under different conversions, a = (0.02, 0.04, . . . , 1). In this paper, the data was simulated at four different heating rates, b = 10, 20, 40, 80 °C min1.

H.-p. Liu et al. / Applied Thermal Engineering 109 (2016) 413–422

A1 calculated A1 experimental A2 calculated A2 experimental A3 calculated A3 experimental

20

DSC/(W/g)

15 10 5

15

DSC/(W/g)

418

A1 calculated A1 experimental A2 calculated A2 experimental A3 calculated A3 experimental

10

5

0 -5

0 200

0

400

600

800

1000

0

200

400

(a) DSC curves of samples at a heating rate of 10ć/min.

7

0

45

DSC/(w/g)

DSC/(W/g)

14

800

1000

(b) DSC curves of samples at a heating rate of 20ć/min.

A1 calculated A1 experimental A2 calculated A2 experimental A3 calculated A3 experimental

21

600

T/(°C)

T/(°C)

30

A1 calculated A1 experimental A2 calculated A2 experimental A3 calculated A3 experimental

15

0 200

0

400

600

800

1000

0

200

T/(°C)

400

600

800

1000

T/(°C)

(c) DSC curves of samples at a heating rate of 40ć/min.

(d) DSC curves of samples at a heating rate of 80ć/min.

Fig. 3. DSC curves of experimental and calculated blends at the different heating rate.

I4ðEa ; T a Þ ¼ Teu

Table 4 Total quantity of heat error of the sample at the different heating rates. Heating rate (°C/min)

A1 DQs = DQe  DQc

A2 DQs = DQe  DQc

A3 DQs = DQe  DQc

10 20 40 80

167.40 213.13 2141.58 1099.77

1167.09 936.97 1972.60 3115.80

130.88 611.67 92.02 2949.34

8 9 IðEa ; T a;1 Þb2 IðEa ; T a;1 Þb3 IðEa ; T a;1 Þb4 > > þ þ > > > IðEa ; T a;2 Þb1 IðEa ; T a;3 Þb1 IðEa ; T a;4 Þb1 > > > > > > > > > > IðEa ; T a;2 Þb1 IðEa ; T a;2 Þb3 IðEa ; T a;2 Þb4 > > > > < þ IðEa ; T Þb þ IðEa ; T Þb þ IðEa ; T Þb > = a;1 2 a;3 2 a;4 2 UðEa Þ ¼ min IðEa ; T a;3 Þb IðEa ; T a;3 Þb IðEa ; T a;3 Þb > > > > > þ I E ; T b1 þ I E ; T b2 þ I E ; T b4 > > > > ð a a;1 Þ 3 ð a a;2 Þ 3 ð a a;4 Þ 3 > > > > > > I E ;T b > > > > : þ ð a a;4 Þ 1 þ IðEa ; T a;4 Þb2 þ IðEa ; T a;4 Þb3 > ; IðEa ; T a;1 Þb4 IðEa ; T a;2 Þb4 IðEa ; T a;3 Þb4

ð2:3:4Þ

when IðEa ; T a Þ undergoes the senum – Yang [21] four approximate expression as shown in (2.3.5):




u3 þ 18u2 þ 88u þ 96 u4 þ 20u3 þ 120u2 þ 240u þ 120

 ð2:3:5Þ

The minimum value (the activation energy E) can then be obtained using MATLAB. The activation energy distribution is shown in Fig. 6. The trend of the activation energy curve of each sample is similar that activation energy values show the trend of decreasing first and then increasing. The activation energy decreased from 300–695 kJ/mol to 125–250 kJ/mol when the conversion rate was between 0 and 0.6. This is due to the combustion of a large number of fixed carbon atoms. The activation energy increased from 125–250 kJ/mol to 250–550 kJ/mol when the conversion ranged from 0.6 to 1, which showed that the sample was difficult to undergo pyrolysis, which then led to the increase in this stage. The combustion effect of the second stage was the best among the combustion processes. As can be seen from Fig. 6, the activation energy of the 275 °C torrefied cornstalk and oil shale semi-coke was lower than that of other two. Therefore, it can be expected that the effect of the 275 °C torrefied cornstalk and oil shale semi-coke had more of an impact on the combustion process than other two.

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0.08

The first stage The second stage The third stage

0.04

900

ΔQ/J

MR

The first stage The second stage The third stage

1200

0.00

600

-0.04

300

-0.08

0 15

30

45

60

75

15

Heating rate/(°C/min)

(a) The interaction index MR of A1 at different heating rates.

45

60

75

(b) The interaction index ΔQ of A1 at different heating rates.

0.07

2000

The first stage The second stage The third stage

1500

ΔQ/J

0.00

MR

30

Heating rate/(°C/min)

-0.07

1000 500

-0.14

The first stage The second stage The third stage

15

30

0 45

60

75

15

45

60

75

Heating rate/(°C/min)

Heating rate/(°C/min)

(a) The interaction index MR of A2 at different heating rates.

(b) The interaction index ΔQ of A2 at different heating rates. 2500

The first stage The second stage The third stage

0.4

2000

0.2

The first stage The second stage The third stage

1500

ΔQ/J

MR

30

0.0

1000

-0.2

500

-0.4

0 15

30

45

60

75

Heating rate/(°C/min)

(a) The interaction index MR of A3 at different heating rates.

15

30

45

60

75

Heating rate/(°C/min)

(b) The interaction index ΔQ of A3 at different heating rates.

Fig. 4. The interaction index MR and DQ at different heating rates.

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H.-p. Liu et al. / Applied Thermal Engineering 109 (2016) 413–422

0.10

800

The first stage The second stage The third stage

0.08

600 400

ΔQ/J

MR

0.06 0.04 0.02 0.00

The first stage The second stage The third stage

200 0

A1

A2

-200

A3

A1

A2

Sample

Sample

(a) The interaction index MR at a heating rate of 10ć/min.

0.08

A3

(b) The interaction index Δ Q at a heating rate of 10ć/min. 600

The first stage The second stage The third stage

The first stage The second stage The third stage

450

MR

ΔQ/J

0.04 300

0.00 150 -0.04 A1

A2

0

A3

A1

Sample

A3

Sample

(b) The interaction index Δ Q at a heating rate of 20ć/min.

(a) The interaction index MR at a heating rate of 20ć/min.

0.04

A2

The first stage The second stage The third stage

The first stage The second stage The third stage

1200

0.00

ΔQ/J

MR

900 600

-0.04 300 -0.08 A1

A2

A3

0

A1

A2

A3

Sample

Sample

(a) The interaction index MR at a heating rate of 40ć/min.

(b) The interaction index Δ Q at a heating rate of 40ć/min.

Fig. 5. The interaction index MR and DQ in different stages under heating rates of 10, 20, 40, 80 °C min1 for samples A1, A2, A3.

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2500 2000

0.0

The first stage The second stage The third stage

ΔQ/J

MR

1500 -0.2

500

The first stage The second stage The third stage

-0.4

1000

A1

A2

0

A3

A1

A2

A3

Sample

Sample

(a) The interaction index MR at a heating rate of 80ć/min.

(b) The interaction index Δ Q at a heating rate of 80ć/min. Fig. 5 (continued)

750

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E(KJ/mol)

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450

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150 0.0

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insufficient, whose depolymerization of the hemicelluloses and lignin became less; the hemicelluloses and lignin of the 300 °C torrefied cornstalk were damaged during the heating process. 3. The sample activation energy was calculated by using Vyazovkin programming. The activation energy of the sample curve was similar, in that activation energy reduced to 125 from 300 and 695 kJ/mol to 250 kJ/mol, then rose to 250–550 kJ/mol. The activation energy of the 275 °C torrefied cornstalk and oil shale semi-coke was lower than that of other two. Therefore, it can be expected that the effect of the 275 °C torrefied cornstalk and oil shale semi-coke was the largest.

1.0

α Fig. 6. Activation energy of the samples based on the Vyazovkin method.

4. Conclusions 1. As can be seen from the DTG curves, a bimodal curve region became more obvious with the increased degree of torrefied cornstalk, especially during the transitional phase of the 300 °C torrefied cornstalk. 2. The quantity of the heat released during the combustion process was calculated by integrating the DSC. Under the same heating rate, compared to other two torrefied cornstalk samples, the 275 °C torrefied cornstalk released the greatest quantity of heat with the change of heating rate: 1167.09 J, 936.97 J, 1972.60 J, 3115.80 J, and had the greatest impact on the combustion process. The same as the change of heating rate, each sample had a different impact on the amount of heat released. Sample A1, at a heating rate of 40 °C/min, the difference of heat was the largest and Sample A2, A3, at the heating rate 80 °C/min, the difference of heat was the largest. In terms of phases, comparing MR index and the difference of heat, the majority of beneficial effects mainly occurred during the second phase, when the combustion of the torrefied cornstalk fixed carbon and semi-coke refractory organics matter occurred. As the degree of torrefied cornstalk increased, the stronger the synergy effect because, as the heating rate increased. The synergy effect of the 275 °C torrefied cornstalk was the largest. Compared to that, the degree of the 250 °C torrefied cornstalk was

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