Effect of pore structure on CO2 gasification reactivity of biomass chars under high-temperature pyrolysis

Effect of pore structure on CO2 gasification reactivity of biomass chars under high-temperature pyrolysis

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Journal of the Energy Institute xxx (xxxx) xxx

Contents lists available at ScienceDirect

Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute

Effect of pore structure on CO2 gasification reactivity of biomass chars under high-temperature pyrolysis Wei Tong a, b, c, d, Qingcai Liu a, b, c, **, Chen Yang a, Zelong Cai a, Hongli Wu a, c, Shan Ren a, b, * a

College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China Chongqing Key Laboratory of Vanadium-Titanium Metallurgy and New Materials, Chongqing University, Chongqing, 400044, China College of Resources and Environmental Science, Chongqing University, Chongqing, 400044, China d Department of Chemistry, Tsinghua University, Beijing, 100084, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 June 2019 Received in revised form 25 July 2019 Accepted 20 August 2019 Available online xxx

The CO2 gasification reactivity of pine sawdust chars (PS char) obtained from the different hightemperature pyrolysis is studied based on non-isothermal thermogravimetric method. Results show that the order of gasification reactivity is PS char-1073 > PS char-1273 > PS char-1473. Under the effect of high-temperature pyrolysis, the surface structure of biomass char is gradually destroyed and the pore structure parameters of specific surface area, total pore volume and average pore diameter increase. By means of the N2 adsorption-desorption isotherms, it is seen that biomass char has more micro- and mesoporous at higher pyrolysis temperature. Besides, the PS char-1073 mostly has rich closed cylinder pores and parallel plate pores, and the PS char-1273 and PS char-1473 have plentiful open cylinder pores and parallel plate pores. An increase of pyrolysis temperature contributes to the development of porosity and improves diffusion path, which promotes the gasification reactivity. But, its effect on the decline of active site hinders the gasification reactivity. What's more, the kinetic model of distributed activation energy model (DAEM) is applied to calculate activation energy and pre-exponential factor with the integral and differential methods. The calculation results of integral method is more accurate and precise because the differential method is more sensitive than integral method for experimental noise. There is a compensation effect in the CO2 gasification process. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.

Keywords: Biomass char Gasification Pore structure DAEM High-temperature pyrolysis

1. Introduction Along with the rapid demand of society, a good number of resources have been consumed, especially the nonrenewable fossil fuels (i.e., coal, natural gas and petroleum, etc.) which have played an important role in economic development. And air pollution released from the combustion of fossil fuels, such as NOx and SO2, causes environmental damage and ecological deterioration. Therefore, the attention for sustainable biomass energy has been more noticed, which has the favorable condition of reproduction, low pollutants and widespread distribution [1,2]. However, the unfavorable factors of low energy density, low utilization efficiency and unstable supply limit its industry utilization compared with fossil fuels [3]. Hence, biomass is usually processed by means of gasification, combustion or pyrolysis. Among of these, gasification is a relatively high efficient utilization method to provide miscellaneous organic matter [4,5]. Gasification reaction of biomass is very complex in the gasifier, including biomass pyrolysis, water evaporation and char gasification [6]. Char gasification is considered as the crucial step because the reaction rate of char gasification is lower than that of water evaporation [7]. At present, the gasification agents of water steam and carbon dioxide have been widely used. And, carbon dioxide was chosen as the gasification agent in this experiment, because carbon dioxide, is defined as greenhouse gas, can be captured and utilized for char gasification [8,9], also the char gasification rate under CO2 atmosphere is lower than rate under water stream atmosphere [10e12]. Therefore, study on

* Corresponding author. College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China. ** Corresponding author. College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China. E-mail addresses: [email protected] (Q. Liu), [email protected] (S. Ren). https://doi.org/10.1016/j.joei.2019.08.007 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.

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thermochemical characteristics and kinetic parameters of biomass char CO2 gasification not only contribute to cognizing gasification mechanism, but also designing gasification reactor [13]. Some researchers used the kinetic models to study the CO2 gasification of biomass char. Wang et al. [14] employed the random pore model and modified random pore model to research gasification characteristic of biomass chars obtained from peanut shell, maize cob, wheat straw, rice lemma, pine sawdust and bamboo sawdust. Bhat et al. [15] studied the kinetics parameters of rice husk char gasification under CO2 atmosphere and the results displayed that the values of activation energies from the volume reaction model and shrinking core model were in close agreement, which were 200 kJ/mol and 180 kJ/mol, respectively. Besides, other kinetic models have been proposed for thermogravimetric analysis [16e19]. Among these models, the distributed activation energy model (DAEM) is preferable for dealing with complex reactions and describes the distribution curves of activation energy. Besides, activation energy and pre-exponential factor are overviewed and the numerical calculation of DAEM is discussed. This model assumes that a number of parallel, irreversible and first-order reactions occur concurrently with various activation energy. Therefore, it can be used to describe the CO2 gasification of biomass char. In our previous research [20], CO2 gasification reactivity of pine sawdust chars obtained from the different pyrolysis temperatures was studied. And the abnormal rule that variation tendency of specific surface area was opposite to gasification reactivity was found. The same phenomena were also reported [21,22]. These researchers only pointed out the specific surface area is not a decisive factor for gasification reactivity, but did not explain the reason and illuminate the mechanism in detail. As a result, the relationships between pore structure and gasification reactivity are further investigated in this paper. Generally, the CO2 gasification reactivity was effected by multi-factors like crystal structure, pore structure, char composition, active site and so on. And some researchers explored the effect factors of pore structure and its relationship of gasification. Cetin et al. [23] found biomass char (pinus radiata, eucalyptus maculata and sugar cane bagasse) at high heating rate underwent plastic deformation to develop structure and mainly consisted of macropores. Malekshahian et al. [24] presented that the increase of pyrolysis pressure promoted the development of surface area and the reactivity of petroleum coke char. Siauw et al. [25] studied the pore structure and reactivity of 12 coal chars derived from Canadian coal and pointed the lignite char was more porous which had high concentrations of feeder pores, active sites and Ca element. Fu et al. [26] concluded that the pore evolution during gasification included three stage development: pore creation, pore coalescence and pore collapse. Whereas, the literature about the effect of pore structure on reactivity under high-temperature pyrolysis was seldom reported. Besides, the pore structure acts as a channel connecting with biomass char and reactant gas, and it mainly effects gas diffusion. Some researched the effect of diffusion on reactivity. Barea et al. [27] researched the diffusional effects in the condition of temperature, particle size and CO2 partial pressure, the results showed that diffusional effect was obvious at high temperature and large particle size. Kajitani et al. [28] displayed that gasification rate was controlled by pore diffusion and carbon monoxide restricted gasification reaction. Mani et al. [29] studied the diffusional parameters and found that char gasification followed the chemically controlled reaction regime within the temperature range. Huo et al. [30] used the effectiveness factor to quantify the effect of pore diffusion on gasification. However, these studies focused on diffusion factors and mathematic calculation, and little work reported about the effect of realistic pore structure on diffusion and it works on gasification reactivity. Therefore, the CO2 gasification property of pine sawdust char obtained from different high-temperature pyrolysis (1073 K, 1273 K and 1473 K) was investigated using a non-isothermal thermogravimetric analysis (TGA), and kinetic parameters were calculated by distributed activation energy model (DAEM) with the integral and differential methods. Meanwhile, the surface microstructure was observed by scanning electron microscope and pore structure parameters were analyzed by nitrogen adsorption. 2. Experimental 2.1. Preparation of samples The raw materials of pine sawdust (PS) were gathered from the downtown of Chongqing City (China). Before the devolatilization experiment, the material was cut to the size of 0.5e1.0 mm. The fragmentary samples were devolatilized in muffle furnace at the heating rate of 5 K/min. The pyrolysis temperatures were automatically designed for 1073 K,1273 K and 1473 K, respectively. To ensure pyrolysis fully completed, the reaction process kept 30 min at the flowing rate of 2 L/min under nitrogen atmosphere. The obtained biomass chars were ground to fine powder and the particle size was lower than 0.074 mm. The results of proximate analysis and ultimate analysis are shown in Table 1. 2.2. Gasification tests The equipment of thermogravimetric analyzer (HCT-1/2, Hengjiu Scientific Instrument Factory, Beijing) was used to conduct experiment. The sample (15 mg) was heated in a corundum crucible with the radius of 4 mm and the height of 8 mm. The high purity of CO2 was 99.99% as gasifying agent, which was injected at the flowing rate of 100 mL/min. All the samples were heated to 1473 K under non-isothermal conditions of three different heating rates: 6 K/min, 12 K/min and 24 K/min. The schematic illustration of gasification experiment is shown in Fig. 1.

Table 1 Proximate and ultimate analysis of different biomass chars (wt. %). Char

PS char-1073 PS char-1273 PS char-1473

Proximate analysis (wt. %)

Ultimate analysis (wt. %)

Mole ratio

FCda

Ad

Vd

Cd

Hd

Oda

Nd

Sd

H/C

O/C

79.26 83.97 85.40

2.66 5.51 6.03

18.08 10.52 8.57

85.89 85.98 87.62

2.32 1.08 0.61

8.24 6.56 4.86

0.79 0.78 0.79

0.10 0.09 0.09

0.0270 0.0126 0.0070

0.0959 0.0763 0.0555

Note: FC, fixed carbon; A, ash; V, volatile matter; d, dry basis. a Calculated by difference.

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For the purpose of reducing error and improving accuracy, each test was repeated at least three times. And, the data were not considered reasonable and valid until the temperature differences were less than 3 K. The gasification conversion (x) can be calculated:



m0  mt m0  m∞

(1)

where m0 and m∞ represent the initial and final mass of sample; mt represents the mass of sample at the time t. 2.3. Characterization of char CO2 gasification In order to quantify and compare gasification reactivity of biomass chars, the description parameters are introduced, that initial gasification temperature (Ti), peak conversion rate temperature (Tm), final gasification temperature (Tf) and the time from initial mass loss to final mass loss (tg). These parameters can be used to evaluate for different biomass chars in the same reaction process [31]. The comprehensive gasification characteristic index S is determined by the following equation and this index is used as quantitative description of the overall gasification process [32]:

  S¼

dx dt

  $ dx dt

max T 2i $Tf

mean

(2)

where dx/dtmax and dx/dtmean represent the maximum and mean value of gasification rate. 2.4. Structural characterization The surface morphologies of biomass chars were observed by using scanning electron microscopy (FEI Quanta-450) at the conditions of a 10 kV voltage and amplified 500 and 2000 times. The pore structures of samples were characterized by a N2 adsorption technique at 77 K using Micromeritics 3H-2000PS1. Specific surface areas were analyzed by the Brunauer-Emmett-Teller (BET) model and the BET specific surface areas was calculated from the linear plot in the relative pressure range of 0.05e0.30. The total pore volume was given at a relative pressure of 0.99. The pore size distribution was presented by the Barrett-Johner-Halenda (BJH) model. 3. Kinetic models The distributed activation energy model (DAEM) has been widely applied for the kinetics of complex reactions during the thermal degradation of fossil fuels and biomass [33,34]. Therefore, the DAEM could be used to research CO2 gasification reactivity. The assumption of model is proposed that a number of parallel, irreversible and first-order reactions occur concurrently with various activation energy. The equation is expressed as:

1

V ¼ V∞

Z∞ 4ðE; TÞf ðEÞdE

(3)

0

where:

Fig. 1. Schematic illustration of gasification experiment.

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0 k 4ðE; TÞ ¼ exp@  0

b

ZT

1 eRT dT A E

(4)

T0

And

V ¼ m0  mt ::::V∞ ¼ m0  m∞

(5)

where f(E) is the normalized distribution curve of activation energy. The function f(E) shows the variation of activation energies corresponding to first-order irreversible reactions. Besides, for the approximation of function 4(E,T), Miura [35] postulated a step function U at an activation energy Es and the equation was given:

4ðE; TÞ ¼ UðE  ES Þ

(6)

According to derivation and approximation [36], the function 4(E,T) is chosen as a constant (4(E,T) ¼ 0.58) and the equation is given: E 0:545bE ¼ eRT k0 RT 2

(7)

This approximation that the ith reaction occurs at a specified temperature T was given mathematically by Miura and Maki [37]:

  d VV∞ dT

 Vi V∞

d y

dT

 ¼

   E Vi∞ Vi exp  i  b RT V∞ V∞

k0i

(8)

where Vi and Vi∞ are the amount of evolved volatiles and the effective volatile content for the ith reaction. Based on Eq. (7) and Eq. (8), Miura-Maki integral method could be further simplified as:

    b k R E1 þ 0:6075  ln 2 ¼ ln 0 E RT T

(9)

Through the Eq. (9), E and k0 can be calculated by the slope and intercept of fitting line that plotting ln (b/T2) versus 1/T at a certain conversion degree. Based on Friedman differential iso-conversional method [38], Miura differential method could be expressed as follows:

  dx E ¼ ln½k0 f ðxÞ  ln b dT RT

(10)

where f(x) is the reaction function. According to the Eq. (10), E can be calculated by the slope of fitting line that plotting ln (bdx/dT) versus 1/T. And k0 can be obtained by Eq. (7) at a certain heating rate.

4. Results and discussion 4.1. Gasification reactivity analysis Gasification conversion and conversion rate curves of different chars are shown in Fig. 2. All the curves of gasification conversion are like reverse S-shape that slowly decrease at the beginning, then sharp decline, level off at the end of reaction. So it is showed that gasification process could be separated into three stages: the weight of the first stage keeps flat basically; at the second stage, the curves decrease steeply because of the chemical reaction between PS char and CO2 gas; the last stage is the termination of gasification. Therefore, the study of gasification kinetics mainly focuses on the middle stage. In general, based on the concept of active site, the Boudouard reaction is considered as the explanation of char gasification [39]. The mechanism equations are given as follow.

C þ CO2 42CO k1

Cf þ CO2 /CðOÞ þ CO k2

CðOÞ þ CO/Cf þ CO2 k3

CðOÞ/CO

(11) (12) (13) (14)

where ki is the rate of the ith reaction; Cf is an active carbon site and C(O) is a carbon-oxygen complex. Gasification reactivity parameters of different biomass chars are showed in Table 2. For the identical biomass char, the values of initial temperature (Ti) and final temperature (Tf) increase with the heating rate rising, which may be attributed to heating rate influences temperature gradient and heat transfer [40]. Besides, the value of maximum rate (dx/dtmax) and mean rate (dx/dtmean) also increase because the improvement of heating rate shorten heating duration and make gasification rate quicken. Based on the gasification index S, it can be seen Please cite this article as: W. Tong et al., Effect of pore structure on CO2 gasification reactivity of biomass chars under high-temperature pyrolysis, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.007

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Fig. 2. Gasification conversion and conversion rate curves of different biomass chars.

that the order of gasification reactivity is PS char-1073 > PS char-1273 > PS char-1473 under the same heating rate. It shows that the increase of pyrolysis temperature is not beneficial to char gasification characteristics. This conclusion has been reported before. Moreover, as the heating rate rising, the values of S gradually enlarge, taking PS char-1073 as the example that the values increase from 0.23  108 to 1.64  108. It indicates that higher heating rate is good for gasification reactivity, which is attributed to vast heat floods into system and contributes to reaction. What's more, according to Eq. (2), it is found that when the reaction rate and gasification temperature concurrently rise, the value of index S will also increase, which indicates the influence of reaction rate on index S is more intense. 4.2. Pore structure analysis Fig. 3 shows the surface morphology of biomass chars under different pyrolysis temperature. It can be seen that with the pyrolysis temperature changing, surface structures of biomass char have different morphologies. For the PS char-1073, the surface of carbon material Please cite this article as: W. Tong et al., Effect of pore structure on CO2 gasification reactivity of biomass chars under high-temperature pyrolysis, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.007

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Table 2 Gasification reactivity parameters of different biomass chars at different heating rates. Sample

Heating rate (K/min)

Ti (K)

Tm (K)

Tf (K)

dx/dtmax (%/min)

dx/dtmean (%/min)

tg (min)

S (108)

PS char-1073

6 12 24 6 12 24 6 12 24

1056.0 1089.2 1101.3 1108.9 1135.5 1153.0 1173.1 1191.2 1220.1

1176.9 1255.5 1303.3 1189.4 1265.4 1336.7 1272.7 1330.7 1379.5

1224.1 1284.7 1344.8 1233.6 1297.6 1366.8 1325.1 1366.8 1429.1

5.23 7.05 12.46 5.26 7.27 13.57 4.73 7.68 12.84

0.60 1.19 2.14 0.51 1.09 2.02 0.43 0.93 1.61

29.9 16.9 10.4 28.1 15.6 9.7 26.4 15.1 9.1

0.23 0.55 1.64 0.18 0.48 1.51 0.11 0.37 0.97

PS char-1273

PS char-1473

Note: Ti, initial gasification temperature; Tm, peak conversion rate temperature; Tf, final gasification temperature; dx/dtmax, maximum value of gasification rate; dx/dtmean, mean value of gasification rate; tg, the time from initial mass loss to final mass loss; S, comprehensive gasification characteristic index.

is composed of a series of uniform gullies, attaching abundant fiber carbon structures. After magnification times, there are some cavities that connect the inside of carbonaceous sample and it can enlarge the reaction area in the process of gasification. When the pyrolysis temperature is 1273 K, the gullies are gradually broken and many shaped pore structures begin to appear. The number and size of cavity are larger in low-magnification compared with PS char-1073. The fiber carbon structures is mostly disappeared under the effect of higher

Fig. 3. Surface morphology of different biomass chars.

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pyrolysis temperature. Besides, small particles are discovered and some of particles block the cavity. In the highest pyrolysis temperature of 1473 K, the surface structure is completely destroyed and collapsed, and the undamaged gully is covered by vast debris. It is showed that the surface structure of biomass char is increasingly destroyed and the gullies gradually fade away. Combined with the rule of index S in Table 2, it can be inferred that destroyed surface structure under pyrolysis temperature is not good for gasification reactivity. For further investigation on the structure characterization of different biomass chars, the pore structure parameters of specific surface area, total pore volume and average pore diameter are introduced to quantitatively compare in the Fig. 4. In general, pyrolysis of biomass is the thermochemical processes that volatile and fixed carbon decompose syngas, tar and char, which is determined by raw material and experimental condition. From this figure, the specific surface areas gradually increase from 536.97 to 983.09 m2/g with pyrolysis temperature rising, which indicates that the release of volatile could cause the augment of inside hole at higher pyrolysis temperature. The total pore volume and average pore diameter also increase. Normally, the specific surface area is larger, which means gasification reactivity is better [41]. But this rule is not found in this paper. Therefore, in order to explore the relationship between pyrolysis temperature and pore structure and its effect on reactivity, pore structure parameters of biomass chars from others’ studies are shown in Table 3. From Table 3, with an increase of pyrolysis temperature, the specific surface area of biomass chars of whatman filter paper, rice straw, chinar leaves and pine wood, though the operation conditions of size, pyrolysis rate and pyrolysis time are different, have a augmentation tendency. It is contributed to that the release and diffusion of gas and tar cause the amplification of pore structure during pyrolysis process, and the reaction is stronger in the higher pyrolysis temperature. But the corncob chars from Wang et al. [42] have an inverse trend and other biomass chars (maize stalk, wheat straw and corn straw) display a wavy development. Authors explained that the pore or cavity are plugged by molten ash in biomass char and the part of volatile matter has no enough time to be freed, which causes the specific surface area decline. Hence, it is considered that the factors of pyrolysis time and molten ash could influence the specific surface area. Biomass is mainly composed by cellulose, hemicellulose, lignin and a small amount of inorganic matter, and various raw materials have different ratio. It builds that each biomass have a corresponding pyrolysis time that ensures pyrolysis process is complete. Besides, under the same operation condition, the woody biomass chars have higher specific surface area than agricultural biomass char, such as hardwood chips is 462 m2/g and sweet sorghum bagasse is 316 m2/g. The opinion was presented that the ash content between woody biomass and agricultural biomass is different, blocking the formation of pore structure in biomass char [50]. The variation tendency of total pore volume is basically similar to that of specific surface area when pyrolysis temperature rises, such as the whatman filter paper chars is from 0.009 to 0.226 cm3/g. And for the same pyrolysis condition, different biomass chars also have a similar rule from these references in Table 3, the biomass chars of walnut shell, almond tree, almond shell and olive stone are 0.138, 0.097, 0.023 and 0.028 cm3/g, respectively. The total pore volume increases which might be attributed to the release of more volatiles promote the formation of pore at higher pyrolysis temperature. Commonly, the larger specific surface area is better for reactivity, but this conclusion can work on the condition that the physical and chemistry properties of surface area are unbroken and it is more meaningful for the hierarchical materials like catalysts [54]. It is obviously that the too high pyrolysis temperature changes this condition. The impact factors of reaction progress are physical adsorption and chemical contact, and the surface area could only provide reaction interface and give landing point for the active site. Therefore, the factor of physical adsorption is studied in the next part. Fig. 5 shows the N2 adsorption and desorption isotherms of different biomass chars. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, all biomass chars have type IV isotherms with H4 hysteresis loop, resulting from slit-like pores. The type IV is a characteristic of mesoporous structure [55], which indicates biomass chars have rich mesoporous. From this figure, N2 adsorbed volume is quickly full at the relative pressure below 0.1, which indicates the interaction between carbon material and nitrogen is strong and proves more micropores exist. When the relative pressure rises, the hysteresis loop of biomass chars appears, especially the PS char-1273 and PS char-1473 are obvious, associating with appearance of mesoporous. The phenomenon of hysteresis loop is explained by Kelvin equation that when the relative pressure of condensation and evaporation are different, the adsorption and desorption isotherms will

Fig. 4. Pore structure parameters of different biomass chars.

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Table 3 Pore structure parameters of others’ studies. Sample

Size

Pyrolysis temperature (K)

Corncob

1.5e2 cm

Whatman filter paper

10*20 mm

1073 1273 1473 673 773 973 1273 1073 1273 1473 1073 1273 1473 573 723 873 873 973 1073 1173 1273 773 873 973 1073 773 873 973 1073 773

Rice straw

Chinar leaves

Pine wood

Maize stalk

6 mm (pellet diameter) <0.295 mm

Wheat straw

300e800 mm

Corn straw

Beech wood

Softwood chips Hardwood chips Sweet sorghum bagasse Black wattle Vineyard Sugar cane bagasse Pine cone Pine bark Pine dust Grape seed Safflower seed cake Walnut shell Almond tree Almond shell Olive stone Soybean oil Oil-palm

10.2 mm (pellet diameter) <300 mm

Pyrolysis rate (K/min)

2.5

Pyrolysis time (min)

Specific surface areas (m2/g)

Total pore volume (cm3/g)

Average pore diameter (nm)

References

25

71.66 35.97 7.48 2.60 5.24 437.63 449.06 85.5 102.6 133.9 186.1 239.3 225.3 6 23 127 12.99 9.32 48.63 81.63 78.79 157 123 138 239 160 125 137 238 637 605

0.071 0.046 0.014 0.009 0.011 0.224 0.226

3.94 5.16 7.49

[42]

480

[43]

[22]

17

60

10

5

10

30

2.6 12

180 40

1373

10

60

387 462 316

0.08 0.10 0.07

748

17

60

241 92 259

0.082 0.029 0.088

773

8.0e10.0

60

213 220 210 5 5

0.40 0.41 0.40 0.07 0.04

[50]

60

280 204 42 53 5.37 176

0.138 0.097 0.023 0.028 0.001 0.11

[51]

1e2 mm

873

2.0e2.8 mm

1073 873

5 10

60 120

[44]

0.0202 0.0179 0.0462 0.0827 0.0830 0.090 0.085 0.070 0.125 0.097 0.105 0.080 0.150

46.12 46.01 14.37 7.07 8.93

[45]

[46]

[47]

[48]

2.05 2.47 2.09

5.23

[49]

[52] [53]

separate. And the shift of relative pressure is attributed to capillary condensation. Generally, porosity material like coal and coal char can be considered to consist of rich capillaries. Besides, the amount absorbed significantly increases with pyrolysis temperature rising, it shows the pore structure becomes more developed. The pore size distribution of different biomass chars is presented in Fig. 6. Based on the IUPAC recommendations, pores are divided into three groups depending on pore width: micropores of widths less than 2 nm, mesopores of widths between 2 and 50 nm and macropores of widths greater than 50 nm. The Fig. 6(a) shows that peaks of pore diameter shift with pyrolysis temperature rising. When the pyrolysis temperature is 1073 K, the peak concentrates on less than 2 nm and a small part of pore diameters is located in 2e4 nm. It indicates the PS char-1073 contains micro- and mesoporous. At the pyrolysis temperature of 1273 K, one peak around 4 nm is obvious, which means the augment of pore diameter under higher temperature. For the PS char-1473, the peak, pore diameter is 0e2 nm and 2e8 nm, is stronger than other biomass chars. It is seen that the micro- and mesoporous are widely distribution in PS char-1473. Too high pyrolysis temperature makes the increase of pore diameter, and the amount of micro- and mesoporous increase. This change rule is the same as average pore

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Fig. 5. N2 adsorption and desorption isotherms of different biomass chars.

Fig. 6. Pore size distribution of different biomass chars.

diameter from Fig. 4. From the curves of cumulative pore volumes, cumulative pore volume rises at the beginning and continues to increase after a short platform, which elucidates the nature of bimodal micro- and mesoporous [54]. For the Fig. 6(c), with pyrolysis temperature rising, the specific surface areas of micro- and mesoporous increase. The higher the peak intensity is, the more the specific surface area increases. The specific surface areas of mesoporous for PS char-1073 and PS char-1273 are relatively smaller. It manifests higher pyrolysis temperature contributes to the development of porosity. All the biomass chars have rich micro- and mesoporous, and the pore structure is more well-developed at higher pyrolysis temperature. Meanwhile, the shapes of pore can be speculated by the capillary condensation. The shapes of pore in biomass char are complex and can be idealized for five common geometric models in Fig. 7. For the Fig. 7(a) and (b), the cylinder pore and parallel plate pore are closed at one end. Whether it is condensation or evaporation, the gas-liquid interface has the same semi-spherical. This shape makes the relative pressure is invariable when the nitrogen condense and evaporate, and the hysteresis loop disappears. If the cylinder pore and parallel plate pore are open at both ends, the hysteresis loop will present, because the gas-liquid interface is plane during condensation and interface transforms semi-spherical in the process of evaporation, like the shapes of Fig. 7(c) and (d). Besides, the closed pores shaping like ink-bottle of Fig. 7(e)

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Fig. 7. Shapes of five common geometric models.

also cause hysteresis loop owing to pore throat, but the loop has a vertical desorption isotherm [54]. Combined with adsorption and desorption isotherms in Fig. 5, it is can be inferred that the PS char-1073 mostly has rich closed cylinder pores and parallel plate pores. The PS char-1273 and PS char-1473 have plentiful open cylinder pores and parallel plate pores. Therefore, it is considered that when pyrolysis temperature rises, the close pore structures will dissolve and open, and it causes pore volume increase and gas flow is unlimited. The development and expansion of pore structure could improve the diffusion of carbon dioxide and it has a less meaning for restricting carbon dioxide to access to pores because its molecular diameter is 0.33 nm. In a word, pore structure under the effect of pyrolysis temperature contributes to diffusion path. Though gasification reactivity is influenced by multi-factors of crystal structure, pore structure, char composition, active site and so on, it is considered as that the determination of gasification are diffusion rate and contact rate, relating to pore structure and active site. In previous studies, the active site usually located at the crystallite edges, where there were carbon crystallite defects or catalytic minerals [56,57]. Besides, others’ researches indicated crystal structure and char composition are in connection with active site. Xu et al. [58] presented that the graphitization of crystalline structure made char have less active and inhibit gasification reactivity. Fu et al. [59], Lin et al. [60] and Matsumoto et al. [61] pointed the relationship between char composition and reactivity that an decrease of atomic ratios (H/C and O/C) in char showed the aromatization and graphitization of char. Therefore, referring to our previous study [20] and Li et al. [31], this conclusion was found that microcrystalline structure of biomass char was gradually graphitized with pyrolysis temperature rising. And, it is seen that the atomic ratios (H/C and O/C) successively reduce under the high-temperature pyrolysis from Table 1. Also, it can be inferred that pyrolysis temperature causes the decline of active site. Hence, pyrolysis temperature is beneficial for pore structure and reduces active site, both of factors are inverse. It shows the greater factor decides reactivity that the positive effect of pore structure is negligible compared with chemical contact. This is the reason why specific surface area, pore volume and pore diameter increase, while gasification reactivity decreases, not the pore structure is bad for it.

4.3. Kinetic analysis The parameter estimation methods can be divided by model-fitting and model-free method [62]. Based on the complex equation structure, the calculation methods of DAEM distributes into distribution-fitting and distribution-free methods. For the distribution-fitting method, the distribution curves of activation energy f(E) is assumed on account of direct search optimization methods. To further discussed without the assumption, the distribution-free methods of Miura differential method and Miura-Maki integral method were proposed [63]. Besides, the error and uncertainty of kinetic parameters from distribution-free method is relatively smaller than that from distributionfitting method [40,64]. Therefore, the kinetic parameters are calculated by DAEM with the integral method and differential method. According to Eq. (9) and Eq. (10), the plots of ln (b/T2), ln (bdx/dT) versus 1/T with the DAEM model are depicted in Figs. 8e10. The activation energy of different biomass chars are calculated with the slope of fitting straight line. And Table 4 displays the values of activation energy for the different chars by the integral method and differential method under different conversions (x ¼ 0.1e0.9). For the integral method, the average values of chars are 184.33 kJ/mol, 188.17 kJ/mol and 218.63 kJ/mol, respectively. It indicates that higher pyrolysis temperature causes an increase of activation energy and make gasification more difficult. Also, the same results are proved by the differential method that the order of activation energy are 165.63 kJ/mol, 171.35 kJ/mol and 199.23 kJ/mol. Both of operational methods show that higher pyrolysis temperature is adverse to gasification reaction. Besides, the correlation coefficients are higher than 0.97, it manifests the accuracy of model and the process of gasification could be described by a set of single first-order reactions, which are expressed by DAEM [36,65]. For the integral method, the pre-exponential factor (k0) can be obtained by the intercept of fitting straight line that plotting of ln (b/T2) versus 1/T. And for the differential method, the values of k0 can be calculated by Eq. (7) at a certain heating rate. Table 5 shows the values of pre-exponential factor of different chars with two methods. Compared with the k0 values at the same conversion, both of the methods are in close agreement, which demonstrates the calculation results are reasonable and reliable. Fig. 11 shows the relationships between activation energy and conversion with the integral method and differential method for different chars. From this figure, it is seen that the development trends of activation energy are the same for two different methods, which is a decreasing function. It is reason that higher reaction temperature accelerates molecular motion and reduces reaction barrier. Besides, the values of activation energy with the integral method are larger than those with the differential method at a certain conversion for different chars. Though the differential method and integral method are distribution-free methods for DAEM, which were proposed by Miura and Please cite this article as: W. Tong et al., Effect of pore structure on CO2 gasification reactivity of biomass chars under high-temperature pyrolysis, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.007

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Fig. 8. Arrhenius plots for PS char-1073 (a) Integral method and (b) Differential method.

Fig. 9. Arrhenius plots for PS char-1273 (a) Integral method and (b) Differential method.

Maki et al. [35,37], the differences of parameter estimation exist [64]. The integral and differential methods are best suitable for respectively analyzing integral (e.g., TGA) and differential (e.g., DSC) data [66]. Though the differential method do not employ any approximations, this method is more sensitive to experimental noise because the imprecision is introduced into data in the process of smoothing dx/dT-T data [63] and the inaccuracy is unavoidable owing to reaction heat is relative to heating rate [67]. On the other hand, the assumption of integral method is that E is independent of x. But, the E is relevant to x in fact and this assumption causes a systematic error in the value of E [68]. If this relationship is enough strong, the error will be up to 20e30% [69]. However, this error can be eliminated in the integral method by

Fig. 10. Arrhenius plots for PS char-1473 (a) Integral method and (b) Differential method.

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Table 4 Values of activation energy of different biomass chars. Sample

x

PS char-1073

Integral method

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Average 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Average 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Average

PS char-1273

PS char-1473

Differential method

E (KJ/mol)

R2

E (KJ/mol)

R2

264.50 242.64 220.76 188.00 167.60 154.08 145.25 139.28 136.83 184.33 307.64 278.58 220.78 182.73 160.26 145.94 136.47 131.79 129.33 188.17 294.69 269.59 240.14 214.04 203.31 189.69 186.55 185.44 184.17 218.63

0.9738 0.9912 1.0000 0.9919 0.9951 1.0000 0.9992 0.9971 0.9972

249.07 219.17 193.33 173.95 146.71 140.65 134.96 130.21 102.61 165.63 278.27 252.38 208.70 175.98 155.83 137.54 119.69 108.40 105.39 171.35 272.01 250.21 225.94 204.22 190.06 181.00 173.48 162.38 133.82 199.23

0.9937 1.0000 1.0000 0.9992 0.9976 1.0000 0.9916 0.9871 0.9980

0.9795 0.9955 0.9922 0.9838 0.9938 0.9991 0.9999 0.9997 0.9994 0.9717 0.9990 0.9995 0.9950 0.9942 0.9891 0.9907 0.9923 0.9897

0.9998 0.9953 0.9937 0.9879 0.9971 0.9992 0.9996 0.9959 0.9942 0.9997 0.9973 0.9999 0.9999 1.0000 0.9993 0.9993 0.9873 0.9963

Table 5 Values of pre-exponential factor of different biomass chars. Sample

PS char-1073

PS char-1273

PS char-1473

x

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Integral method

Differential method

k0

k0 (b ¼ 6)

k0 (b ¼ 12)

k0 (b ¼ 24)

Average

7.36Eþ11 2.17Eþ10 1.03Eþ09 2.01Eþ07 1.71Eþ06 3.23Eþ05 1.05Eþ05 4.67Eþ04 2.99Eþ04 2.20Eþ13 4.25Eþ11 5.80Eþ08 7.68Eþ06 5.70Eþ05 1.05Eþ05 3.32Eþ04 1.75Eþ04 1.16Eþ04 9.45Eþ11 3.20Eþ10 1.01Eþ09 5.35Eþ07 1.44Eþ07 3.04Eþ06 1.85Eþ06 1.39Eþ06 1.02Eþ06

1.28Eþ11 1.56Eþ09 4.90Eþ07 4.17Eþ06 1.69Eþ05 7.43Eþ04 3.45Eþ04 1.77Eþ04 7.53Eþ02 8.74Eþ11 2.50Eþ10 1.53Eþ08 3.56Eþ06 3.46Eþ05 4.20Eþ04 5.42Eþ03 1.41Eþ03 9.01Eþ02 8.22Eþ10 4.29Eþ09 2.35Eþ08 1.94Eþ07 3.71Eþ06 1.24Eþ06 4.92Eþ05 1.39Eþ05 6.98Eþ03

1.09Eþ11 1.56Eþ09 5.31Eþ07 4.68Eþ06 1.91Eþ05 7.86Eþ04 3.53Eþ04 1.78Eþ04 8.50Eþ02 8.43Eþ11 2.53Eþ10 1.69Eþ08 4.02Eþ06 3.73Eþ05 4.45Eþ04 5.80Eþ03 1.55Eþ03 9.92Eþ02 9.85Eþ10 4.60Eþ09 2.49Eþ08 2.11Eþ07 4.09Eþ06 1.38Eþ06 5.52Eþ05 1.59Eþ05 8.78Eþ03

1.47Eþ11 1.78Eþ09 5.74Eþ07 4.60Eþ06 1.97Eþ05 8.26Eþ04 3.76Eþ04 1.92Eþ04 1.02Eþ03 1.00Eþ12 2.83Eþ10 1.65Eþ08 3.76Eþ06 3.59Eþ05 4.51Eþ04 6.28Eþ03 1.74Eþ03 1.12Eþ03 9.23Eþ10 3.96Eþ09 2.15Eþ08 1.89Eþ07 3.77Eþ06 1.32Eþ06 5.39Eþ05 1.62Eþ05 9.82Eþ03

1.28Eþ11 1.64Eþ09 5.32Eþ07 4.49Eþ06 1.86Eþ05 7.85Eþ04 3.58Eþ04 1.82Eþ04 8.73Eþ02 9.06Eþ11 2.62Eþ10 1.62Eþ08 3.78Eþ06 3.59Eþ05 4.39Eþ04 5.83Eþ03 1.57Eþ03 1.01Eþ03 9.10Eþ10 4.28Eþ09 2.33Eþ08 1.98Eþ07 3.86Eþ06 1.31Eþ06 5.28Eþ05 1.53Eþ05 8.53Eþ03

integrating temperature. Moreover, the integral method makes use of the x-T data at different heating rates to obtain the kinetic parameters. Therefore, it has good reason to consider that the integral method is more accurate and precise than differential method. The same conclusion was also presented by Huang et al. [65].

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Fig. 11. Relationships between E and x of different biomass chars.

Fig. 12. Linear relationships between lnk0 and E of different biomass chars.

Commonly, the compensation effect between activation energy (E) and pre-exponential factor (k0) is in the process of gasification reaction. From Arrhenius law, it can be found that when the pre-exponential factor (k0) increases, the reaction rate (K) will enhance which means the improvement of gasification reactivity. On the other hand, when the activation energy (E) increases, the reaction rate (K) will reduce which implies the lower gasification reaction ability. This opposite effect can be offset by the same changing trend of E and k0. Therefore, according to the values of Tables 4 and 5, the linear relationships between lnk0 and E are drawn in Fig. 12. The values of correlation coefficients are over 0.99, which indicates the compensation effects exist in the CO2 gasification process.

5. Conclusion This paper studies the effect of pore structures on CO2 gasification reactivity of biomass char under different high-temperature pyrolysis. Results show that the order of gasification reactivity is PS char-1073 > PS char-1273 > PS char-1473. With the pyrolysis temperature rising, the surface structure of biomass char is gradually destroyed and the pore structure parameters of specific surface area, total pore volume and average pore diameter increase. At higher pyrolysis temperature, the biomass char has more micro- and mesoporous. Besides, the PS char1073 mostly has rich closed cylinder pores and parallel plate pores, and the PS char-1273 and PS char-1473 have plentiful open cylinder pores and parallel plate pores. Pyrolysis temperature promotes pore structure and reduces active site, and the greater factor decides reactivity that the positive effect of pore structure is negligible compared with chemical contact. The kinetic parameters are calculated by DAEM with the integral and differential methods. Both of the methods are close, but the integral method is more reliable because differential method is more sensitive to experimental noise. Besides, there is a compensation effect in CO2 gasification process.

Acknowledgements The authors gratefully acknowledged the National Natural Science Foundation of China (51604048 and 51874058); Fundamental Research Funds for the Central Universities (No. 2018CDYJSY0055); Graduate Research and Innovation Foundation of Chongqing, China (No. CYS18001); Fund of Chongqing Science and Technology (cstc2017shms-zdyfx0055) and Fund of Sichuan Key Research & Development Projects (2018SZ0281) for financial support. Please cite this article as: W. Tong et al., Effect of pore structure on CO2 gasification reactivity of biomass chars under high-temperature pyrolysis, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.007

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Please cite this article as: W. Tong et al., Effect of pore structure on CO2 gasification reactivity of biomass chars under high-temperature pyrolysis, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.08.007