Oxidation Reactivity of Char Produced in a Pilot-Scale Blowpipe: Effect

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Oxidation Reactivity of Char Produced in Pilot-Scale Blowpipe -Effect of Heating Rate During PyrolysisShota Akaotsu, Junichi Tanimoto, Tatsuya Soma, Yasuhiro Saito, Yohsuke Matsushita, Hideyuki Aoki, and Akinori Murao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01923 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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Oxidation Reactivity of Char Produced in PilotScale Blowpipe -Effect of Heating Rate During PyrolysisShota Akaotsua*, Junichi Tanimotoa, Tatsuya Somaa, Yasuhiro Saitoa, Yohsuke Matsushitaa, Hideyuki Aokia, Akinori Muraoa, b a

Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-

07 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan b

Steel Research Laboratory, JFE Steel Corporation, 1 Kokan-cho, Fukuyama, Hiroshima 721-

8510, Japan Keywords. Pulverized Coal Oxidation, Pyrolysis, Heating Rate, Blowpipe.

Abstract. In this study, oxidation reactivity of char produced in a pilot-scale blowpipe was investigated with a focus on the difference in heating rate during pyrolysis. In addition to the blowpipe, a thermogravimetry and Curie-point pyrolyzer were employed for preparing the char sample produced at different heating rate conditions. The changes of morphological and crystalline structure due to the heating rate were evaluated. Both the morphological and crystalline structure of the char differed according to the heating rate during pyrolysis. For the char produced in a blowpipe, the melting of the char was observed, and the specific surface area

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of the char drastically increased compared with that of the char produced in the other reactors. On the other hand, the crystalline structure of the char also developed in the rapid heating condition. According to the study results, the positive effect of the expanded specific surface area of the char was stronger than the negative effect by the ordered crystalline structure. The oxidation reactivity of the char produced in the blowpipe were much higher than those produced in lab-scale apparatuses. Therefore, the appropriate parameters obtained from the char produced in the same heating condition as the target system were necessary to predict the phenomena in the pulverized coal combustion system.

1.

Introduction Nowadays, coal has many key functions. In the field of power generation, coal-fired

thermal power plants realize the stable supply of electricity.1 New energy conversion systems, such as gasification2 and oxy-fuel combustion,3, 4 have been recently developed with the upsurge of the suppression of CO2 emissions. In addition, pulverized coal is utilized for materials in a blast furnace operation,5 while also serving as power generation fuel. In the blast furnace operation, pulverized coal injection (PCI) technology is one of the most useful techniques to support the application of coke as a reducing agent and heat source. In the PCI system, an immense number of pulverized coal particles are injected into the blast furnace from the blowpipe (BP) equipped at the bottom of the furnace. Theoretically, the amount of reducing agent decreases as the amount of pulverized coal increases. However, the accumulation of unburned coal particles in the blast furnace has adverse effects on the stable operation by obstructing gas flow.6 To solve this problem, it is necessary to understand the


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reaction process of pulverized coals.7-10 The main heterogeneous reactions of char are oxidation and gasification. Generally, the reaction order of gasification rates of char is approximately the second order, whereas gasification rates are much lower than oxidation rates. Here, the residence time of the coal particles in the BP is remarkably short at approximately a millisecond order. Therefore, many studies have focused on oxidation rates in the heterogeneous reactions of char.11-14 For the factors related to char reactivity, (i) inherent characters, such as crystalline structure and functional groups; (ii) morphological factors, such as swelling and pore development; and (iii) distribution of catalytic materials within ash were reported by Suuberg.15 In terms of gasification of char, there have been a number of reports on the effect of catalytic minerals (e.g. Ohtsuka et al.,16 Radovic et al.17 and Miura et al.18). On the other hand, the reaction order of oxidation rate is much higher than that of gasification rate. Even if there is catalytic effect, the time scale of the contribution to oxidation rates would be much smaller than that of gasification rate. Thus, the effect of catalytic materials described as (iii) would be negligible compared with the other two factors. In this study, we investigated the char reactivity in the BP by mainly focusing on the (i) inherent characters and (ii) morphological factors. It is well known that char reactivity largely depends on the heating rate during pyrolysis.1923

To determine the important factors in the change of char reactivity, the effect of the heating

rate on the crystalline structure and morphological structure of char has been investigated. Using the coal char produced by a drop tube reactor, Gale et al. investigated the oxidation reactivity of char from the specific surface area, reaction rate, and carbon aromaticity.21 As a result, the oxidation reactivity of char decreased with a higher heating rate during pyrolysis. They concluded that the negative effect for the char reactivity was caused by the ordered crystalline


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structure of carbon with a rapid heating rate condition. Their target heating rate was limited to a high range around 104 to 105 K/s. Meanwhile, Lu et al. found that the crystalline structure of the char surface was developed with a higher maximum temperature and heating rate during pyrolysis.22 Owing to the ordered crystalline structure of char, the oxidation reactivity of char produced at a rapid heating rate condition decreased compared with that of char produced at a slow heating rate condition. However, in that study of Lu et al.,22 the heating rate range was limited in a relatively slow heating rate of approximately 101 to 103 K/s. The pyrolysis condition in the BP is different from that of the lab-scale apparatus used in the previous study.21, 24 Thermogravimetry (TG) is often used for a char reactivity study because the experimental conditions, such as thermal history and holding time, can be strictly controlled. However, the possible heating rate is limited in terms of the slow heating rate. To achieve a relatively higher heating rate, a drop tube furnace is employed for preparing char samples. Although the rapid heating rate is realizable, the residence time in the furnace is relatively longer compared with that in the BP by approximately 10−2 to 10−3 s. Thus, it is impossible to realize the severe conditions in the BP with a lab-scale apparatus. The purpose of the present study is thus to identify and examine the difference of char reactivity in the industrial BP versus the lab-scale apparatus in terms of the heating rate during pyrolysis. To understand the effect of the rapid heating system in the BP on char reactivity, we used a pilot-scale BP and produced the char sample at similar conditions in the industrial PCI system. In addition to the BP, a thermogravimetry (TG) and Curie-point pyrolyzer (CPP) were employed for preparing the char sample produced at the different heating rate conditions from a BP. The range of heating rates was wider compared with that in the previous works at approximately 10−1, 103 and 105 K/s. The morphological and crystalline structure of char was


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analyzed. Furthermore, using the reaction rate parameters obtained by char oxidation tests, the combustion simulation of a single coal char was performed. Then, the char burnout time was estimated and compared with the three coal chars. 2. 2.1

Experimental Coal and char sample preparation method The sample used in the present study was JL coal classified as bituminous coal. The

proximate and ultimate analyses of JL coal are given in Table 1. The sample was milled to 78.63 µm in a volume average particle diameter. Using the three reactors, we prepared the char samples at the three different conditions of heating rates. The experimental conditions for the sample preparation are summarized in Table 2. The first reactor was a TG type (NETZSCH STA449 F1 Jupiter). An approximately 5 mg coal sample was heated at 0.33 K/s from room temperature to 378 K in an N2 environment and held for 60 min to remove moisture in the sample. Next, the sample was heated to 1173 K with a heating rate of 0.33 K/s and held for 30 min. Then, TG char was obtained. The second reactor was a CPP (Japan Analytical Industry Co., JHP-5). In this system, the sample wrapped in a ferromagnetic sheet was heated by a pulse heating method applied with induction heating. The coal sample, approximately 1 mg, was wrapped in a ferromagnetic sheet and pyrolyzed at a heating rate of approximately 5000 K/s from room temperature to 1323 K in an N2 environment. The holding time was 1 min. The third reactor was a pilot-scale BP. The details of the BP and sampling point are shown in Figure 1. In this apparatus, the coal sample was supplied to the BP with N2 and pyrolyzed in air at high temperature. Here, the heating rate in the BP was more rapid than that in the CPP. Coal chars at 40 mm in a radial direction from the center of the BP were collected with the


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sampling probe at a 300 mm downstream region from the inlet of the lance. This system enabled us to obtain the char produced at conditions similar to those of an industrial PCI system. The experimental conditions of char sampling from the pilot-scale BP are shown in Table 3. The pilot rig was heated by the inlet gas. The temperature of inlet gas was preheated to 1473 K. The outside of the wall was covered with refractory and cooled with water. Several limits existed in producing char at such a rapid heating system. The first concern was that the sample might have oxidized in air atmosphere. In the char preparation process, a huge number of coal particles were fed into the BP to reproduce the same conditions as in an industrial operation. A high concentration of coal particles could have contributed to minimizing the influence of oxidation on the char characteristics because the oxygen lean region was generated in the BP. In addition, we collected the sample near the center of the BP, in which the concentration of oxygen would have been the smallest in the BP. Accordingly, the effect of oxidation on char characteristics would have been small in the present study. In this case, Fletcher reported that the swelling behavior, which is the main morphological change during pyrolysis, does not depend on the oxygen concentration at a rapid heating condition.25 Thus, the effect of oxygen on the morphological factors would be adequately small to be neglected, even if oxygen existed around the coal particles. The next concern was that the maximum gas phase temperature in the BP was much higher than that in the two reactors. In the previous study, it was suggested that the maximum temperature during pyrolysis influenced the crystallite structure and char reactivities.22 However, it is impossible to fulfill all requirements, such as the maximum temperature and holding time, with the other two reactors. For the reasons mentioned above, the maximum temperature in CPP and TG were also adjusted as close as possible to that in the BP to minimize the differences in


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the char preparation conditions among CPP, TG, and BP. Although the particle temperature was 1700–2000 K at the exit of an industrial blowpipe, the sampling point was much closer to the lance than the exit of blowpipe. Accordingly, the particle temperature can be lower than 1700– 2000 K. To confirm the reliability of above statement, we performed combustion simulation of a single coal particle with experimental conditions in the pilot-scale blowpipe. The details of the simulation are written in section 2.3.2. Note that the residence time of particles from the lance to the sampling point is remarkably short because the gas velocity at the inlet of the blowpipe is approximately 100 m/s. As a result, the particle is heated to approximately 1200 K from the lance to the sampling point. Following the numerical result, we assumed that the effect of other factors such as maximum particle temperature on the char particles was smaller than that of the heating rate, and we identified the effect of the heating rate on the char oxidation reactivity. To discuss the heating rate effect on char reactivity during pyrolysis, the heating rate of coal particles in the BP had to be estimated. However, it is difficult to directly measure the heating rate of coal particles. Accordingly, in solving the mass conservation equation and the energy conservation equation of a single coal particle, the heating rate of the coal particle was calculated from a thermal balance of the particle. The procedure of estimating the heating rate of coal particles was the same as that of the combustion simulation in this paper. The details are summarized in Section 2.3.2. As a result, the heating rates of coal particles were 5×104 to 1×107 K/s with diameters of 10 to 150 µm. We additionally checked whether these values were appropriate by the comparison of the estimated heating rate with those in the previous work of Hashimoto et al.

26

Hashimoto et al. reported that the heating rate of pulverized coal particles in

the pulverized coal furnace was from 4.43 × 104 to 1.47 × 105.

26

Thus, the char heating rate

estimated in the present study would be valid.


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2.2

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Char characterization

2.2.1

Morphological observation with scanning electron microscopy images and gas adsorption method The surface characteristic of char was examined with scanning electron microscopy (SEM)

(Hitachi High-Technologies Co., Miniscope TM3000). In addition, the particle surface area and the pore size distributions of raw coal and char were evaluated with nitrogen adsorption isotherms measured by a pore surface and size analyzer (Quantachrome, Autosorb AS1-MP). Before the gas adsorption measurements were performed, the samples were heated to 373 K and held for 2 h under low pressure to remove moisture from the samples. The Brunauer, Emmett, and Teller method (BET) method27 was used to estimate specific surface area of the samples. The Barrett–Joyner–Halenda method (BJH) method28 and density functional theory (DFT) method29 were used to analyze the pore size distributions of each sample. The BJH method was used for pores under 30 nm in diameter, whereas the DFT method was used for pores over 30 nm in diameter. 2.2.2

Evaluation of crystalline domain with Raman spectroscopy The crystalline structure of the char surface was analyzed using laser Raman spectroscopy

(JASCO, NRS-5100). Approximately 15 to 20 particles were randomly selected, and the samples placed on slide glasses were analyzed. A laser with a wavelength of 532 nm was used as a light source for the Raman spectroscopy. The beam diameter of the laser was 1 µm. The spectra were recorded in the range of 1000 to 1950 cm−1. The exposure time and cumulative number were 30 s and three, respectively, on each measurement. The effect of inorganic matter in ash on Raman spectra is known to be small enough to be ignored.30, 31 Using curve fitting software (JASCO,


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Spectra Manager Version 2), peak fitting was performed on each measured spectrum to resolve the curve into five bands, which peaked at 1150 cm−1 (D4 band), 1350 cm−1 (D1 band), 1500 cm−1 (D3 band), 1580 cm−1 (G band), and 1620 cm−1 (D2 band)32. Figure 2 shows an image of the Raman spectra and fitting curve with a band combination for coal chars. In the present study, using the general technique with the peak intensity ratio of G bands and D1 bands, the crystallinity of the carbon on the pulverized coal particle surface was evaluated. 2.3 2.3.1

Quantitative evaluation of the effect of heating rate during pyrolysis on char reactivities Char oxidation test with thermogravimetry The oxidation reactivities of char produced under various heating rate conditions were

evaluated using thermogravimetry (NETZSCH, STA449 F1, Jupiter). The char oxidation experiments were conducted by heating thermal analysis. An approximately 0.5 mg char sample was placed in a TG pan and heated at 0.33 K/s from room temperature to 1473 K in an air environment. After the heating process, the sample was held for 60 min. The char oxidation rates were calculated with the mass loss of char samples. Here, the char samples were dispersed on alumina wool in the TG pan to suppress the effect of the mass transfer of oxidant caused by lean oxidant concentration around the char particles. Using this method, the chemical reactivities without the effect of the mass transfer of oxidant were obtained from the experiments. The conversion of char was calculated using Eq. (1): =

− , −

(1)

where and denote the mass at the elapsed time, and initial condition, respectively. is the mass of ash in the sample. For the char reaction model, the volume reaction model was


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employed. The chemical reaction rate constants were estimated using the Arrhenius equation. The reaction rate was given by d = 1 − , d

= exp −

,

(2) (3)

where , , , and are the pre-exponential factor, activation energy, chemical reaction rate

constant, and gas constant, respectively. In addition, denotes the gas temperature around the

char sample. The pre-exponential factor, , and activation energy, , were obtained by curve

fitting to the measured data. Here, the experimental data above 0.1–0.9 in the char conversion were used for the curve fitting. 2.3.2

Evaluation of differences in burnout time with the combustion simulation of a single char particle Partial oxidation and full oxidation can respectively occur during the char oxidation

process. Arthur suggested that the main chemical reactions in char oxidation depend on the gas temperature.33 According to the function of CO/CO2 proposed by Arthur, the partial oxidation reaction (R1) mainly occurs in high temperature. Since the target system in the present study was evaluated in a high temperature condition, only partial oxidation (R1) was considered for char oxidation: 1 C char + O! → CO. 2

(R1)

The particle mass was evaluated by the mass conservation equations as:


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d$ = −%, d

(4)

where % is the overall reaction rate of char that combines the chemical reaction rate and mass transfer rate of the oxidant to the particle surface. The chemical reaction rate of char according to the volume reaction model is expressed by: %& = $ '()

(5)

where , $ , '() are the reaction rate constant, char particle mass, and partial pressure of the oxidant, respectively. The pre-exponential factor and the activation energy involving reaction rate constant in Eq. (5) were obtained from the experiments. On the other hand, the mass transfer rate of oxidant was difficult to measure in the experiments. Thus, several equations were developed to estimate the mass transfer rate of oxidant to the particle.34, 35 In the present study, the analytical solution proposed by Mulcahy and Smith35 was applied. This analytical solution contains the effect of Stefan flow caused by the generation of products at the particle surface, and the validity of the equation was confirmed by Matsushita et al.36 The equation is: , -., 3 .45 ln91 − :; B , 2 @ A( $ 0.51$ : ? )

(6)

where 1$ , -C, , and ;D are the particle diameter, oxidant diffusion coefficient in the reference

@ , B$ and are the molecular weight condition, and oxidant volume fraction, respectively. ?& , ?

of carbon, mean molecular weight of the surrounding mixed gas, particle surface area, and reference temperature, respectively. The reference temperature was 1500 K in the present study. In addition, ,, A() , and : were the gas density, stoichiometric coefficient of oxygen in the partial


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oxidation, and dimensionless model parameter. The dimensionless model parameter, :, is defined by: :=

@ A. E . A. -.

(7)

For the partial oxidation of char, : ≈ −1. The particle temperature is calculated by the energy conservation equation, which includes the three heat transfer terms (i.e., convective (Eq. (9)), radiative (Eq. (10)), and reaction heat (Eq. (11))). $ H$,$

d$ = IJKLD + IMNO + INPJ , d

IJKLD = ℎJKLD B$ 3 − $ , IMNO = R$ S B$ TU − $U , INPJ = V%Δℎ.

(8) (9) (10) (11)

Here, V is the contribution ratio of the heat of the reaction to the dispersed phase. Although the

appropriate value for V has not been established, the value of 0.3–1.0 has often been employed in

previous studies.37-39 In the present study, the contribution ratio was set to 0.5. The convective heat transfer coefficient was calculated by the Nusselt number model equation as: XY =

] ] ℎJKLD 1$ = 2 + 0.6\$! '% ^ , Z

(12)

where Z is the thermal conductivity. The char particle was assumed to be fully surrounded by gas, and the Nusselt number was set to two. The governing equations were solved with the second-order Runge–Kutta method. When 99 % of the initial mass was consumed with char oxidation, we regarded the time as the burnout. Then, the burnout times of each char particle were compared. The details of the calculation conditions are summarized in Table 4.


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To validate the above assumptions and models, we compared the calculation result with the experimental data.40 Although we were not able to validate our calculation results directly because the experimental conditions were slightly different with our calculation conditions, we confirmed that the difference of our calculation results with the experimental data was not so large as shown in Figure 3. The difference is reasonably small to be accepted and would be attributed to the heat release due to the combustion of volatile matter in the previous experiment. The aim of combustion simulation is to indicate the large impact of the change of oxidation reactivity due to the different heating rate conditions. Therefore, the accuracy of our calculation results is sufficient to recognize the effect of heating rate on char reactivity. 3. 3.1

Results and discussion Effect of heating rate during pyrolysis on morphological structure Figure 3 shows the SEM images of the chars produced with different heating rates during

pyrolysis. The particle surface of TG char was roughly compared to those of the other chars produced with rapid heating rates. Pores were not clearly observed on the particle surface in TG char. On the other hand, CPP char and BP char had many pores, which would be formed by the release of volatile matter. This is because the rapid heating resulted in the increase of the amount of volatile matter and devolatilization rates. The surface of BP char was smoother than that of the other chars. This result suggested that some chars melted at rapid heating rate conditions. The morphological changes of char due to the heating rate conditions affected the char crystallization and reactivity. Table 5 shows the specific surface areas of raw coal, TG char and BP char measured with the BET method. The specific surface area of BP char was much larger than that of TG char. The


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increase in the amount of volatile matter and devolatilization rate by a rapid heating rate resulted in the elevation of pressure in the pores and thus the swelling and development of the pore structure. On the other hand, for TG char, the specific surface area slightly decreased by pyrolysis. When the heating rate is low, it is well known that the devolatilization rate becomes low compared with that at rapid heating rate.41, 42 This is because the amount of volatile matter does not increase based on the proximate analysis under relatively mild conditions. Figures 4 and 5 show the pore size distribution of raw coal and char samples measured with the BHJ and DFT methods. The pore size distribution of TG char is almost the same as that of the raw coal. This result means that the effect of swelling and development of pores on the pore structure was extremely small at slow heating conditions. On the other hand, for BP char, the volume of pores increased compared with that of the raw coal regardless of pore size. In particular, the increase of mesopores with diameters of 2 to 50 nm was remarkable. Thus, the development of pore structure was promoted by not only the growth of initial pores, but also the generation of new pores. In previous works, it was indicated that the specific surface area of micropores was larger than that of mesopores for the coal char produced at rapid heating rate conditions, i.e., approximately 104 to 105 K/s.19, 43 The heating rate of coal particle in BP, at which BP char was produced, was slightly higher than 105 K/s. Based on previous works, we could surmise that the volume of mesopores was the largest of the three pore types. Generally, in the early stage of pyrolysis, micropores are generated and expand with the melting and devolatilization. After those processes, the micropores overlap and become mesopores. Thus, the volume of mesopores remarkably increased for BP char in the present study. From the morphological changes of char


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discussed in this section, the reactivity of char oxidation seemed to be higher as the heating rate increased. 3.2

Effect of heating rate on crystalline structure during pyrolysis Figure 6 shows the intensity ratio of recorded G bands and D1 bands. Here, the intensity

ratio of BP char is plotted as the results in the case of the 105 K/s heating rate. As the heating rate of pulverized coal increased, the intensity ratio became smaller. This trend means that the carbon crystalline structure developed for the char produced at rapid heating rate conditions. In the previous study of melted coals, the mobility of hexagonal crystal lattice increased by the release of volatile matter and bridge scission between fragments composing coals.44, 45 From the comparison of the SEM image shown in Figure 3, it is also observed that the softening and melting of BP char progressed further than that of TG or CPP char. Thus, it is suggested that softening and melting of the char were promoted at rapid heating rate conditions, and the amorphous domain in the char decreased. In other words, as the heating rate increased, the carbon crystallites developed. These results are consistent with the outcomes of the work by Lu et al.22 Since the char reactivity depends on the crystal structure of carbon, the change of crystal structure contributes to the difference of the char reactivity. From the crystalline changes of char discussed in this section, the reactivity of char oxidation seemed to be lower as the heating rate increased. Combining the outcome of the increase of the specific surface area at a rapid heating rate, as mentioned earlier, the effect on the char reactivity seemed to be complex. Specifically, in the rapid heating rate condition, the char reactivity was higher from the morphological changes, whereas it was lower from the more ordered crystalline structure. 3.3

Effect of heating rate during pyrolysis on char oxidation reactivity


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In the previous sections, the change of morphological and crystalline structure by the heating rate was investigated. In this section, the change of oxidation reactivity is discussed. Figure 7 shows the histories of char oxidation rates measured with TG. First, the oxidation rate of char increased on account of the temperature elevation. When the temperature reached 900 to 1000 K, the oxidation rates of char decreased as the char conversion became higher. In the case of the oxidation rate of TG char, two peaks were observed, whereas the number of peaks was only one in the case of BP and CPP char. Accordingly, the oxidation process of TG char could be divided into two stages, whereas that of BP and CPP char was a single stage. Miura et al. suggested that char oxidation could occur in two separate stages owing to the difference in reactivities of maceral components.46 They indicated that the oxidation of vitrinite mainly occurred and that of inertinite was delayed. During pyrolysis, the volatile matter released faster from vitrinite and exinite than inertinite. Generally, the amount of volatile matter released from pulverized coal increased with the elevation of heating rate during pyrolysis. In the case of rapid heating system, the difference of components in the residual becomes smaller compared with the relatively slow heating rate. Therefore, it is suggested that the effect of the differences of the reactivity of maceral on the overall char reactivity becomes small in relatively rapid heating conditions. Although the maximum value of oxidation rate of BP char was almost the same as that of CPP char, the temperature at which the oxidation rate reached a maximum value in BP char was slightly higher than that in CPP char. This result was attributed to the difference of the crystal structures between BP and CPP char. Additionally, the maximum reaction rate of TG char was lower than that of BP char and CPP char. This is because, in the case of TG char, the pore structure, which


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directly influenced the char reactivity, did not develop during pyrolysis compared with BP and CPP char. The pre-exponential factors and activation energies obtained from the experiments are summarized in Table 6. The pre-exponential factors and activation energies increased with the elevation of heating rate. In terms of the meaning of each parameter, the pre-exponential factor depends on the morphological structure, whereas the activation energy is related to the crystalline structure of the char particle. Based on the above discussions, both morphological and crystalline structures of char drastically change by the heating rate conditions during pyrolysis. Figure 8 shows Arrhenius plots of the overall reaction rate of char. The oxidation rate of CPP char was the highest in the oxidation rates of all cases below 1000 K, whereas that of BP char was the highest above 1000 K. Figure 9 shows the burnout times of char particles with diameters of 50 and 150 µm. The burnout time was longer with the increase of particle diameter because the initial mass of particle was large. Focusing on the burnout time order, the burnout time decreased in the order of TG, CPP, and BP char, and the oxidation reactivity was higher as the heating rate increased. This result indicated that the positive effect by the expanded specific surface area of char was stronger than the negative effect by the ordered crystalline structure. Therefore, in this study, the morphological factors were stronger than crystalline factors.

4.

Conclusion In this study, the effect of heating rate of pulverized coal during pyrolysis on the reactivity

of char oxidation was investigated with a focus on the char produced in the BP. To extend the previous investigation under relatively slow heating rates, pilot-scale BP and CPP were


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employed as well as a TG. For the qualitative and quantitative evaluation of the morphological change, SEM images and a gas adsorption method were employed. From the SEM images of the char formed in the BP, softening and melting were observed. The specific surface area increased as the heating rate increased. Based on the evaluation of the crystalline structure using Raman spectroscopy, it was suggested that the carbon structure developed with the elevation of heating rate of pulverized coal particles. The reaction rate parameters for char oxidation were largely different from the heating rate condition during the pyrolysis. From the combustion simulations of a single coal particle, the burnout time also drastically changed by the heating rate condition during pyrolysis. The oxidation reactivity of char produced in the pilot-scale BP was higher than those produced in the other reactors. This result means that the positive effect by the expanded specific surface area of char was stronger than the negative effect by the ordered crystalline structure. Therefore, the appropriate parameters obtained from the char produced in the same heating condition as the target system were necessary to predict the phenomena in the pulverized coal combustion system.


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Acknowledgement The authors would like to thank Editage, a brand of Cactus Communications for the proofreading of the article.

CAPTIONS Table 1 Property of coal. Table 2 Experimental condition of char preparation. Table 3 Details of the char sampling from the pilot-scale blowpipe. Table 4 Numerical conditions. Table 5 Specific surface area of raw coal and chars measured using the BET method. Table 6 Estimated parameters of the reaction rate. Figure 1 Schematic diagram of small-scale BP and sample positions. Figure 2 Raman spectra and fitting curve with a band combination for coal chars. Figure 3 Comparison of burnout time between our calculations and the experimental data40. Figure 4 SEM images of sample char. Figure 5 Pore size distributions of raw coal and TG chars with different measurement methods. Figure 6 Pore size distributions of raw coal and BP chars with different measurement methods.


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Figure 7 Ratio of band peak intensity (ID1/IG) (error bars represent the standard derivations). Figure 8 Histories of oxidation rates of chars. Figure 9 Arrhenius plots of oxidation rates of obtained chars. Figure 10 Estimated burnout time of obtained chars under the isothermal condition.


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Tables

Table 1 Property of coal. Proximate analysis Ash

[wt%−db]

10.0

Volatile matter (VM)

[wt%− db]

14.7

Fixed carbon (FC)

[wt%− db]

75.3

Moisture

[wt%]

1.4

C

[wt%−daf]

89.11

H

[wt%−daf]

3.98

N

[wt%−daf]

2.03

S

[wt%−daf]

0.68

O (diff.)

[wt%−daf]

4.20

Higher heating value (HHV)

[MJ/kg]

31.81

Volume mean diameter

[µm]

78.63

Ultimate analysis


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Table 2 Experimental conditions of char preparation. TG char CPP char Apparatus

BP char

Thermogravimetric analyzer

Curie-point pyrolyzer

Experimental blowpipe

0.33

5150

About 105−106

Maximum temperature [K]

1173

1323

About 1200 (calc.)

Holding time

[s]

600

60

About 0.1

Pressure

[MPa] 0.1

0.1

About 0.3

Heating rate

[K/s]

Table 3 Details of the char sampling from the pilot-scale blowpipe. Flow rate of inflow gas from the blowpipe

[Nm3/h]

350

Flow rate of inflow gas from the lance

[Nm3/h]

15

Flow rate of pulverized coals

[kg/h]

60

Inflow gas temperature from the blowpipe

[K]

1473.15

Inflow gas temperature from the lance

[K]

293.15

Oxygen excess ratio

[%]

3.7


22

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Energy & Fuels

Table 4 Numerical conditions.

Raw coal

Particle diameter

[µm]

50 or 150

O2 concentration

[vol%] 21

Initial particle temperature

[K]

293

Particle emissivity

[-]

0.9

Gas temperature

[K]

1273, 1473, 1673, 1873 or 2073

Wall temperature

[K]

1200

Table 5 Specific surface area of raw coal and chars measured with the BET method. [m2/g] 1.90

TG char

[m2/g]

1.01

BP char

[m2/g]

51.3

Table 6 Estimated parameters of the reaction rate. Heating rate

Activation energy, E

Pre-exponential factor, A

[K/s]

[kJ/mol]

[1/s]

TG char

0.3

96.8

6.66×102

CPP char

5000

107.3

6.02×103

BP char

100000

133.3

1.44×105

Coal char


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Figures

Figure 1 Schematic diagram of small-scale BP and sample positions.

Figure 2 Raman spectra and fitting curve with band combination for coal chars.


24

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Figure 3 Comparison of burnout time between our calculations and the experimental data40.


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(a) TG char, ×600

(b) TG char, ×1200

(c) CPP char, ×600

(d) CPP char, ×1200

(e) BP char, ×600

(f) BP char, ×2000 Figure 4 SEM images of sample char.


26

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(a) BJH method

(b) DFT method

Figure 5 Pore size distributions of raw coal and TG chars using different measurement methods.

(a) BJH method

(b) DFT method

Figure 6 Pore size distributions of raw coal and BP chars using different measurement methods.


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Figure 7 Ratio of band peak intensity (ID1/IG) (error bars represent the standard derivations).

Figure 8 Histories of oxidation rates of chars.


28

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Figure 9 Arrhenius plots of oxidation rates of obtained chars.

(a) dp = 50 µm

(b) dp = 150 µm

Figure 10 Estimated burnout time of obtained chars under the isothermal condition.


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AUTHOR INFORMATION Corresponding Author *Shota Akaotsu TEL: +81 22 795 7251 Fax: +81 22 795 6165 E-mail address: [email protected] Present Addresses a

Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-

07 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan b

Steel Research Laboratory, JFE Steel Corporation, 1 Kokan-cho, Fukuyama, Hiroshima 721-

8510, Japan NOMENCLATURES

H$,$ -C

1$

;D,()

ℎJKLD _ℎ

pre-exponential factor

[1/s]

specific heat of particle

[J/(kg⋅K)]

diffusion coefficient

[m2/s]

particle diameter

[m]

activation energy

[kJ/mol]

volume fraction of O2

[-]

convection heat transfer coefficient

[W/(m2⋅K)]

heat of reaction

[J/kg]

rate constant

[1/s]


30

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?

molecular weight

[g/mol]

mass of char

[kg]

mass of ash

[kg]

partial pressure of oxidant

[Pa]

Prandtl number

[-]

amount of heat transfer

[W]

gas constant

[J/(mol⋅K)]

Particle Reynolds number

[-]

overall reaction rate

[kg/s]

chemical reaction rate

[kg/s]

mass transfer rate of oxidant

[kg/s]

particle surface area

[m2]

gas temperature

[K]

particle temperature

[K]

wall temperature

[K]

time

[s]

X

conversion

[-]

$

'() '% I

\$ %

%&

%* B$ 3

$

T

Greek symbols

γ

model parameter for mass transfer rate of oxidant

[-]

R$

particle emissivity

[-]

stoichiometric coefficient

[-]

thermal conductivity

[W/(m⋅K)]

`

contribution ratio of heat of reaction to the particle

[-]

ρ

density

[kg/m3]

S

Stefan–Boltzmann constant

[W/(m2⋅K4)]

A

Z

Subscripts


31

Energy & Fuels

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

conv

convection

i

chemical species

reac

Reaction

trad

thermal radiation

Page 32 of 35

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Oxidation Reactivity of Char Produced in a Pilot-Scale Blowpipe: Effect

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