Pressurized oxy-fuel combustion characteristics of single coal particle in a visualized fluidized bed combustor

Combustion and Flame 211 (2020) 218–228

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Pressurized oxy-fuel combustion characteristics of single coal particle in a visualized fluidized bed combustor Lin Li, Lunbo Duan∗, Zhihao Yang, Changsui Zhao Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China

a r t i c l e

i n f o

Article history: Received 8 May 2019 Revised 16 September 2019 Accepted 27 September 2019

Keywords: Pressurized oxy-fuel combustion Fluidized bed Ignition Char combustion Burnout time

a b s t r a c t Pressurized oxy-fuel combustion (POFC) is recognized as the second generation of oxy-fuel combustion for the reduction of CO2 emissions, which has gradually gained more and more attention recently. In this work, the influence of pressure on the combustion characteristics (i.e., ignition, volatiles combustion, char combustion) of a single lignite particle was investigated in a pressurized visualized fluidized bed combustor under O2 /N2 and O2 /CO2 atmospheres. The combustion process was recorded by a color video camera, and the temperatures of volatiles flame and char particle were measured by pre-calibrated two-color pyrometry. The results indicated that increasing the overall pressure and increasing the oxygen volume fraction to a same partial pressure of oxygen will affect the ignition delay time (td ) differently. td increased with the operating pressure at the same oxygen volume fraction, but decreased with the increase of oxygen volume fraction under atmospheric pressure. Pressurization significantly changes the coal flame shape and the volatiles flame becomes longer and thinner at a higher pressure. At the same oxygen volume fraction, higher flame and char temperatures and shorter burnout time (tb ) of coal particle were observed under a higher pressure due to the better mass transfer of oxygen. With the same oxygen partial pressure, particle temperature is lower at high overall pressure than that at high oxygen volume fraction, which is probably a potential advantage of pressurized oxy-fuel combustion to resist agglomeration. Moreover, compared with atmospheric oxygen-combustion, the gasification of char particle in pressurized oxygen-combustion is more remarkable and cannot be neglected. © 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction In the past two decades, oxy-fuel combustion technology is considered as a promising CO2 capture approach for coal-fired power plants, which has been extensively studied [1,2]. However, the high economic cost and low efficiency limit its further commercialization [3,4]. Recently, as the second generation of oxy-fuel combustion, pressurized oxy-fuel combustion (POFC) has gradually attracted the interest of academia and industry. Compared with the conventional oxy-fuel combustion, not only the energy loss can be dramatically reduced, but also many other advantages (e.g., smaller boiler size, avoiding air leakage, latent heat recovery of steam from flue gas, and enhancing heat transfer) can be achieved in POFC system [5,6]. As the most important equipment, pressurized boiler has two technical routes: pressurized pulverized coal (PPC) boiler and pressurized fluidized bed (PFB) boiler. To date, the PFB technology

∗

Corresponding author. E-mail address: [email protected] (L. Duan).

has been considered as a mature choice, which has been widely used in the energy and chemical industries [7]. Moreover, employing fluidized bed combustion technology in POFC system also can bring the advantages of flexible fuel types, stable furnace temperature control, low SOx /NOx emissions and so on [8]. Up to now, many research institutions have conducted relevant studies on simulation and experimental of POFC. The results of system simulation all showed that the net efficiency of POFC system was significantly higher than that of atmospheric oxy-combustion system [9–13]. However, there are few experimental studies on the pressurized oxy-fuel fluidized bed combustion due to the difficulty of conducting pressurized experiment, the majority of which were mainly focused on coal combustion characteristics in pressurized thermo-gravimetric analyzer (PTGA) [14–16] and pollutants (CO, NOx , SOx and PM) emission characteristics in PFB reactor [6,17–20]. To the best of our knowledge, there have been no publications on the combustion mechanism of a single coal particle in a fluidized bed under pressurized O2 /CO2 atmosphere until now, while it is very important to the design and operation of the boiler. There were a large number of studies on the combustion characteristics of the single fuel particle in fluidized bed reactor

https://doi.org/10.1016/j.combustflame.2019.09.032 0010-2180/© 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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Fig. 1. The possible influence mechanism of pressure on different combustion stages of coal particle.

[21–25], but all of them were conducted in atmospheric pressure condition. However, it is well considered that promoting the operating pressure will dramatically affect the heat and mass transfer between fuel particle and ambient environment [26], the flow characteristics of bed materials [27] and physical properties (i.e., density, diffusion rate, viscosity and specific heat capacity) of the gas. These factors will have important impact on the different combustion stages (i.e., drying, devolatilization and char combustion) of the coal particle. According to the existing research progress [28–30], the possible influence routes of operating pressure on devolatilization process are summarized in Fig. 1a. It is shown that the higher pressure brings in higher oxygen partial pressure and lower oxygen diffusion rate, and subsequently the two parameters have opposite effects on volatiles flame temperature (Tf ) and flame morphology [31]. Furthermore, increasing pressure can bring a lower total volatiles content and have effect on volatiles composition [28,32]. These may affect the temperature of volatiles flame (Tf ) and devolatilization time (tdev ). Taking all the factors together, it is difficult to give an accurate path of the influence of pressure on the volatiles release and combustion process. The influence of POFC condition on char combustion process is also complicated, the possible influence routes of which are depicted in Fig. 1b. Increasing pressure can remarkably promote the reaction rate of oxidation (C + O2 –CO2 , ࢞H= −394 kJ/mol) and gasification (C+CO2 -2CO, ࢞H= +171 kJ/mol) reactions due to the increase in the partial pressure of O2 and CO2 . Both of the reactions can contribute to the char consumption, but exert opposite effects on particle temperature (Tp ). The Tp is the most important parameter in fluidized bed combustion, which not only affects the burning characteristics of coal, but also plays a decisive role in pollutants emission and agglomeration characteristics. In addition, increasing pressure will also influence the coal swelling, fragmentation behavior and microstructure of char [28,33]. In order to provide theoretical support for the industrial application of fluidized bed pressurized oxy-fuel combustion, the pressurized oxy-fuel combustion characteristics of the single coal particle in fluidized bed need to be investigated systematically. It is well known that the temperatures of volatiles flame and char particle are the crucial parameters in the research of single coal combustion, which were difficult to monitor, especially in a high pressure. The commonly used methods in FB and PC furnaces include contact (e.g., thermocouple) and non-contact (e.g., twocolor pyrometry) measurements [21,24,31,34–37]. The thermocouple was widely used to investigate the combustion characteristics of coal particle and heat transfer [21,31,37]. However, this method not only can’t measure the temperature of volatiles flame, but also interferes with the free-motion of coal particle. Salinero et al. [38] studied the influence of thermocouples on char particle combustion behaviors in a FB combustor, and pointed out the

thermocouple should be used cautiously because the thermocouple can restrict the rotation and increase the movement resistance of particles. Two-color pyrometry can measure the temperature of flame and particle surface without affecting the free-motion of coal particle and surrounding flow field. It should be pointed out that the two-color pyrometry cannot measure the center temperature of particle. However, the temperature gradient of millimeter-scale coal particle is small [39], thus, the char particle can be considered uniform [40,41]. Overall, as a powerful temperature measurement technology, two-color pyrometry has been proven to be applicable to particle combustion diagnosis in a FB reactor [24,31,42]. The goal of this work is to investigate the combustion characteristics (ignition behavior, volatiles flame temperature, devolatilization time, char temperature, burnout time) of a single coal particle under different atmospheres (O2 /CO2 or O2 /N2 atmosphere) and pressures (1–5 bar). The experiments were carried out in a visualized pressurized fluidized bed combustor, and the whole combustion behavior of coal particle was recorded by a color video camera, the temperature of volatiles flame and char particle were estimated by the pre-calibrated two-color pyrometry. 2. Experimental 2.1. Visualized pressurized fluidized bed reactor The combustion tests of coal particles were carried out in a visualized pressurized fluidized bed reactor in present work. The schematic of pressurized fluidized bed system is shown in Fig. 2. The detailed description of the experimental system can be obtained in reference [27,31]. A two-stage feeding system was introduced to solve the feeding problem under high temperature and pressure, and a water-cooling sleeve was welded below the feed valve to prevent particle from being heated before being entered into the reactor. A color video camera, capable of up to 60 frames per second (fps) at full resolution of 1920 × 1080 pixel, was chosen to record the combustion process (from ignition to char burnout) of coal particle through the quartz window (the transmittance is greater than 0.99). The frame rate, the exposure time, and the f-number were set to be 32 fps, 1/800 s, and f/5.6, respectively. 2.2. Materials A lignite was chosen as the test coal, the chemical composition of the test samples are given in Table 1. The irregular shape of coal was first carved into spherical particle (diameter: 6 mm; weight: 170–175 mg) before testing, which can not only ensure a continuous and distinct volatiles flame, but also facilitate the potential modeling work.

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Gas mixer Water-cooling Counterbalance valve Window Water removal

Analyzer

Reactor Mass flow controller

Temperature controller Camera Pre-heater CO2

O2

N2

Gas cylinder

Fig. 2. The schematic diagram of pressurized fluidized bed system. Table 1 Proximate and ultimate analysis of the fuel. Fuel

Lignite a

Proximate analysis, (wt%) (as received)

Ultimate analysis, (wt%) (dry and ash-free basis)

M

V

FC

A

C

H

Oa

N

S

16.17

35.53

39.18

9.12

67.11

4.23

25.07

1.45

2.14

By difference.

Quartz sand with the size range of 0.3–0.35 mm was selected as bed material with an unexpanded bed height of 120 mm in each test. The minimum fluidization velocity (umf ) under different operating conditions can be calculated by the formula in reference [27], which can be applied to high temperature and high pressure environment. 2.3. Two-color pyrometry In this work, the two-color pyrometry was used to measure the temperature of volatiles flame and char particle, which has been widely used in nonintrusive in-situ observation of coal combustion behavior [24,34–36,43,44]. The detailed principle and derivation process of two-color pyrometry were shown in Supplementary material A. The specific calibration and verification tests are shown in Supplementary material B. The comparison between the standard value (black furnace temperature) and measured temperature is displayed in Fig. 3. It is indicated that the two series of data agree well, and the maximum temperature difference is less than 10 °C. By using the pre-calibration two-color pyrometry, the typical image of combustion process (volatiles and char combustion) and the corresponding temperature distribution are shown in Fig. 4. Herein the peak and the average temperatures of volatiles flame were chosen to evaluate the quality of volatiles flame, and the peak temperature of char was used to represent the char temperature [31].

Fig. 3. Comparison of standard and predicted value.

Table 2 Operating conditions used in tests. Fuel

Pressure (bar)

Lignite 1, 2, 3, 4, 5 1

Atmosphere

O2 (%)

Tb (°C)

O2 /N2 O2 /CO2 O2 /N2 O2 /CO2

10

800

10, 20, 30, 40, 50

2.4. Experimental conditions Considering the security of quartz window, all the tests were performed at a constant bed temperature (Tb ) of 800 °C. The pressure of testing system was controlled by adjusting the opening of the counterbalance valve. The fluidizing gas velocity (uf ) was set to 2.5 umf . The single coal particle was fed into the reactor after the

pressure and fluidization reached steady state. Different combinations of pressures (1 bar, 2 bar, 3 bar, 4 bar and 5 bar) and oxygen volume fractions (10%, 20%, 30%, 40% and 50%) in O2 /N2 or O2 /CO2 atmosphere were tested. The operating conditions are summarized

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Fig. 4. A typical image and the corresponding temperature distribution. (a) Volatiles combustion and (b) char combustion.

Fig. 5. (a) Typical dynamic behavior of landing, ignition, volatiles combustion and char combustion in 30%O2 /70%N2 and 30%O2 /70%CO2 atmospheres under atmospheric pressure, exposure time is 1/800 s; (b) O2 /N2 atmosphere, exposure time is 1/100 s and (c) O2 /CO2 atmosphere, exposure time is 1/60 s.

in Table 2. Each test was repeated no less than three times to guarantee the repeatability. 3. Results and discussion 3.1. Overall combustion behavior of coal particle The typical images captured during the coal particle combustion processes are displayed in Fig. 5a. The particle combustion process in sequence was observed: (1) Drying: after the particle was injected into the combustor, coal particle was heated rapidly and it needed some seconds to be dried until the end of evaporation of water. In this process, no flame can be observed. (2) Volatiles release and combustion: the dried particle was further heated and most volatiles (CO, H2 , light aliphatic, CH4 , and tar) were released and immediately burnt, and a bright volatiles flame could be observed. (3) Char combustion: the char particle was heated continuously and severe oxidation occurred on the surface of the particle. In this process, a bright char particle would be observed until its burnout. Figure 5a also showed that the volatiles flame and char surface in O2 /N2 atmosphere is slightly brighter than that in O2 /CO2 atmosphere at the same oxygen volume fraction (the luminous of volatiles flame and char surface in different pressures and atmospheres are shown in Supplementary material

C). This was mainly caused by two aspects: (1) the diffusion rate of O2 (DO2 ) in O2 /N2 atmosphere (1.96 cm2 /s, 1 bar) is 1.27 times larger than that in O2 /CO2 atmosphere (1.54 cm2 /s, 1 bar) at 800 °C; (2) the thermal capacity (cv,g ) of CO2 is 1.68 times larger than that of N2 . On one hand, a lower DO2 can reduce the oxidation reaction rate of volatiles and char, resulting in a lower flame and char temperatures; On the other hand, a higher cv,g of gas means that more heat needs to be absorbed from the flame and char [45]. The physical parameters of background gas under different operating conditions are shown in Table 3. The detailed effects of pressure and atmosphere on coal particle combustion will be quantitatively discussed in Sections 3.2–3.5, combining with the results of particle temperature. Interestingly, in Fig. 5c, there was a thin “flame layer” on the surface of particle when the char was burning in the O2 /CO2 atmosphere, while no similar phenomenon were observed in the O2 /N2 atmosphere (see Fig. 5b). This may be caused by the CO combustion near the surface of particle, which comes from the gasification reaction (C+CO2 =2CO) in O2 /CO2 atmosphere. The temperature range and thickness of the “flame layer” were 857– 973 °C (the average temperature was 921 °C) and 3.4 mm, which were measured by pre-calibration two-color pyrometry. Then we assumed that the flame layer was produced by CO combustion, the gasification rate (CO2 + C=2CO) and the corresponding CO

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Table 3 The physical parameters of background gas (at 800 °C). Pressure

1 bar

2 bar

3 bar

4 bar

5 bar

ρ N2 (kg/m3 )

0.31 1187 368.0 1.96 27.73 0.49 1260 617.4 1.54 28.44 1.68

0.62 1187 736.0 0.98 34.74 0.98 1260 1234.8 0.77 35.01 1.68

0.93 1187 1103.9 0.65 40.13 1.47 1260 1852.2 0.51 40.04 1.68

1.24 1187 1471.9 0.49 44.66 1.96 1260 2469.6 0.39 44.29 1.68

1.55 1187 1839.9 0.39 48.66 2.45 1260 3087.0 0.31 48.03 1.68

cN2 (J/kg-K) ρ N2 cN2 (J/m3 -K) DO2/N2 (cm2 /s) hconv, N2 (W/(m2 K)) ρ CO2 (kg/m3 ) cCO2 (J/kg-K) ρ CO2 cCO2 (J/m3 -K) DO2/CO2 (cm2 /s) hconv, CO2 (W/(m2 K)) ρ CO2 cCO2 /ρ N2 cN2

flame temperature were calculated in Supplementary material D. The calculated result of the CO flame temperature was 958 °C, which was close to the measured temperature. However, the gas composition of the “flame layer” cannot be in-situ quantitatively measured by our current testing method, so this is only a conjecture that needs to be proven by further research. The coal particle combustion can be divided into four stages with the particle temperature [24]: (1) Drying (Tp ≤100 °C). (2) Primary devolatilization (100 °C
more accurate when the smaller fuel particles (such as Bi<0.1, the temperature can be considered a uniform) burning in FB reactor, but the temperature of smaller particle is difficult to measure by thermocouple. Generally, the drying and devolatilization process depends on the heating rate of coal particle [31], and the char combustion process is mainly influenced by the oxidation reaction and gasification reaction (in O2 /CO2 atmosphere) [40]. In this work, the Biot number (Bi=hlc /λp=hrp /3λp, the detailed calculation process is shown in Supplementary material E) of testing coal particle are shown in Table 4 (at 800 °C), which was close to 0.1 (Bi ≤ 0.1, the heating process of particle is completely controlled by external heat transfer). So it can be considered that the lignite particle was mainly controlled by external heat transfer in the qualitative analysis of particle temperature due to the internal temperature gradient of the particles is small. Similar treatment methods were widely used in single coal particle (dp : 5–10 mm) combustion [22,40,41,46]. In order to analyze the influence mechanism of different operating conditions on particle temperature, a thermal balance equation of coal particle combustion in fluidized bed was established:

ρp V c p

dTp 4 = hpc Ap (Tb −Tp )+hgc Ap (Tb −Tp )+εσ Ap (T b −T 4p ) dt + hf Ap (Tf −Tp )+qC−O2 −qC−CO2

(1)

where ρ p , V, cp , Tp and Ap are the density, volume, specific heat capacity, temperature and superficial area of coal particle, respectively; λ, h and ɛ represent the heat transfer coefficient of thermal conduction, thermal convection and thermal radiation, respectively; σ is the Stefan–Boltzmann constant; hf is the heat transfer coefficient between the flame and particle; qC – O2 represents the heat release of char oxidation; qC – CO2 represents the heat absorption of char gasification. The first to third terms on the right side of the Eq. (1) represent the heat transferred from environment to the particle by thermal conduction (qcond ), thermal convection (qconv ) and thermal radiation (qrad ), respectively, which are closely related to the bed temperature and operating pressure. The reference values/equation of each parameter in Eq. (1) are shown in Supplementary material F. It is clear that the qC – O2 is mainly controlled by the diffusion coefficient of oxygen, while the qC – CO2 is largely dependent the temperature of the char particle. Numerous studies have shown that the gasification can not be ignored in oxy-fuel combustion under atmospheric pressure [21,25,41,47], the effect of gasification may be more pronounced under a high pressure.

3.2. Ignition Ignition delay time (td ) is an important index for judging ignition characteristics [48]. The variation of td versus pressure and atmospheres are shown in Fig. 7. At the same oxygen volume fraction under atmospheric pressure, the td of particle in O2 /CO2 atmosphere was longer than that in O2 /N2 atmosphere, and the td decreased with the increase of oxygen volume fraction. This was mainly caused by two reasons [31]: (1) the specific heat capacity (cv,g ) of CO2 (617.4 J/m3 -K, at 1 bar) is 1.68 times greater than that in N2 atmosphere (368.0 J/m3 -K, at 1 bar) and result in a difficult ignition in CO2 atmosphere. (2) High oxygen volume fraction can enhance the oxidation and exothermic heat on the surface of coal, thereby increasing the heating rate of particles and reducing td . Unexpectedly, as shown in Fig. 7, the td increased significantly with the increase of pressure at the same oxygen volume fraction, although improving the pressure can result in a higher oxygen partial pressure and higher heat transfer coefficien (see Table 4). This phenomenon might be comprehensively controlled by the cv,g of

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Fig. 6. Comparison diagram of observational method and particle temperature division method in 30%O2 /70% CO2 atmosphere under atmospheric pressure. Te is the evaporation temperature of water; Tdev is the terminal temperature of primary devolatilization process; td,2 and tdev,2 are the drying time and devolatilization time obtained from the particle temperature profile, respectively. Table 4 The h and Bi of particle in different atmosphere and pressure. Name h Bi

Unit 2

W/(m K) W/(m2 K) – –

Atmosphere

1 bar

2 bar

3 bar

4 bar

5 bar

N2 CO2 N2 CO2

160.78 174.22 0.146 0.158

191.30 207.47 0.174 0.189

212.20 230.18 0.193 0.209

228.61 247.99 0.208 0.225

242.34 262.86 0.220 0.239

increases 400%, this can bring a longer td in high pressure. This was in agreement with the previous results [24,31,36,45], which However, to the best of our knowledge, the influence mechanism of pressure on the YF,0 of coal has not been published. Existing studies [29,50,51] only show that the resistance of volatiles release increases with the pressure, and the secondary reaction of volatiles is strengthened, resulting in the decrease in volatiles yield. This could also lead to an increase of td in high pressure. Figure 7 also showed that the td in atmospheric pressure was shorter than that in high pressure at the same oxygen partial pressure. Sun and Zhang [52] investigated the ignition temperature of different type coal at different pressure in a PTGA under air condition, and got a similar results and found that the ignition temperature of coal increased as pressure at the same oxygen partial pressure. In addition, the ignition characteristics of coal particle were closely related to the coal type, particle size, heating rate and volatiles content [52]. 3.3. Volatiles combustion Fig. 7. Ignition delay time of coal particle in different pressures and oxygen volume fractions.

gas and the release rate of volatiles. On the basis of ignition theory [49], the td can be calculated by

cv,g (T 0 /Ta ) qcYF,0 Aexp(−Ta /T0 ) 2

td =

(2)

where qc is the heat release by burning per mass of volatiles; YF,0 is the initial mass-fraction of volatiles; A is the pre-exponential factor; Ta is the activation temperature which is defined on the basis of activation energy [49]; T0 is the initial temperature of oxygen and volatiles mixture. The cv,g is proportional to the pressure, when the gas pressure increases from 1 bar to 5 bar, the cv,g

3.3.1. Flame morphology and temperature The typical volatiles flame in different pressure and atmospheres are illustrated in Fig. 8, respectively. At the same oxygen volume fraction, the brightness of volatiles flame was strengthened slightly with the increase of pressure, and the flame became narrower and longer. However, in atmospheric pressure, with the increase of oxygen partial pressure, the brightness of volatiles flame was remarkably strengthened, and its size became smaller. At the same oxygen partial pressure, the flame in atmospheric pressure was brighter and smaller than that in a high pressure, the quantitative luminous intensity of volatiles flame can be found in Fig. S6a. This was mainly caused by two reasons: (1) Buoyancy. The buoyancy of volatiles under a high pressure was greater than that under atmospheric pressure due to the density of backgroud gas was proportional to the pressure (the density of background gas under different atmosphere and pressure can be found in

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Fig. 8. Typical image of volatiles flame in different atmospheres and pressure. (a) 10%O2 , 1–5 bar and (b) 10−50% O2 , 1 bar.

Table 3). Therefore, a longer volatiles flame can be observed under high pressure. (2) Diffusivity. The diffusion rate of gas is inversely proportional to the ambient pressure. On one hand, with the increase of pressure, the diffusion rate of volatiles decreases, and resulting in a narrower flame; on the other hand, under high pressure, the DO2 in the O2 /N2 or O2 /CO2 atmosphere is smaller than that in atmospheric pressure. As shown in Table 3, when the pressure from 1 bar increase to 5 bar, the DO2 of both O2 /N2 and O2 /CO2 atmospheres were reduced by 80%, so the oxidation reaction of volatiles in high pressure is weaker than that in atmospheric pressure at the same oxygen partial pressure. Figure 8 also shown that the flame in O2 /CO2 atmosphere was darker than that in O2 /N2 atmosphere under the same pressure, this is because the DO2 in O2 /N2 atmosphere (1.96 cm2 /s) is 1.27 times larger than that in O2 /CO2 atmosphere (1.54 cm2 /s), the detailed discussion can be found in the previous work [45]. Combined with the results of two-color pyrometer, the temperatures of volatile flame with different pressure and atmospheres are shown in Fig. 9. It is clear that the peak and average temperature of volatiles flame increased with the pressure and oxygen volume fraction, but the increment gradually decreased. At the same partial pressure of oxygen, the average temperature of flame in a high pressure was lower than that in atmospheric pressure, and the temperature difference increased with pressure. When the oxygen partial pressure was 0.5 bar, the average temperature differences of flame between atmospheric pressure and high pressure were 39.3 °C (O2 /N2 atmosphere) and 23.1 °C (O2 /CO2 atmosphere), respectively. Furthermore, the average temperature of flame in O2 /CO2 atmospheres was about 31– 47 °C lower than those in O2 /N2 atmosphere under atmopspheric pressure, and the temperature difference became smaller as the pressure increased

(the mean temperature of flame also shows the same change rule). These phenomena were mainly caused by two reasons: (1) The cv,CO2 /cv,N2 = 1.66 (as shown in Table 3), so the bulk gas in O2 /CO2 atmosphere would absorb more heat from flame than that in O2 /N2 atmosphere. (2) The DO2 in O2 /N2 atmosphere is 1.27 times larger than that in O2 /CO2 at the same pressure (as shown in Fig. S7), which means a higher flame temperature in O2 /N2 atmosphere. 3.3.2. Devolatilization time (tdev ) For a direct comparison, the tdev in different atmosphere and pressure are plotted in Fig. 10. It is obvious that the tdev of coal particle was significantly decreased with the increase of pressure and oxygen volume fraction. Since the flow characteristics are similar (with the same bed material and fluidization number) in all the test, the qcond and qrad in Eq. (1) can be considered unchanged, and the volatiles flame and the particle were almost completely separated (see Fig. 5) in the dnse bed, the heating effect of volatiles flame on particle could be negligible [24,31]. There are three main reasons for the decrease of tdev : (1) A higher pressure can improve the qconv by increasing the cv,g of bulk gas. (2) The increase of oxygen partial pressure can strengthen the oxidation of coal char, thus increasing qC – O2 and shortening tdev . (3) Under high pressure, the diffusion resistance of volatiles increases along the radius, resulting in a longer residence time of volatiles in particle and a greater probability of secondary reactions between char and volatiles [28]. Ultimately, the tar yield and the total yield of volatiles all decreased with increasing pressure, which is presumably another reason for reducing the tdev at high pressure. In addition, Fig. 10 also showed that the tdev of coal particle in O2 /N2 atmosphere was slightly shorter than that in O2 /CO2

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Fig. 9. The volatiles flame temperature under different conditions.

3.4. Char combustion

Fig. 10. The devolatilization time of coal particles in different pressure and atmosphere.

atmosphere under the same pressure. Similar phenomenon was also observed by Bu et al. [24] and our previous work [31,45] at atmospheric pressure. This was mainly caused by two factors: (1) The diffusion rate of oxygen. the DO2 in O2 /N2 atmosphere is larger than that in O2 /CO2 atmosphere (the detailed quantitative values of DO2 are shown in Table 3), which can improve heating rate of particle by promoting char oxidation at the surface of char, and reduce the tdev . (2) The effect of gasification reaction (C+CO2 – 2CO, ࢞H = +173 kJ/mol), which is an endothermic reaction. The gasification reaction on the char surface may reduce the particle temperature in O2 /CO2 atmosphere, and thus the devolatilization stage will be prolonged. Of course, this effect can be ignored when the particle is small. When the coal particle size is large enough (Bi>>0.1), a distinct temperature gradient will appear inside the particle [24,25], the temperature of char surface can be rapidly heated to the bed temperature by the bed material, and the oxidation and gasification (O2 /CO2 atmosphere) will occur simultaneously on the surface of the particles. In particular, as the operating pressure increases, the gasification reaction rate is remarkably enhanced, which may extend the devolatilization process in pressurized oxy-fuel combustion.

The typical images selected from the video of the char combustion in different operating conditions are shown in Fig. 11. It should be pointed out that to ensure the readability of the images in higher pressure, the images in Fig. 11a were taken under the exposure time of 1/80 s. When the oxygen volume fraction was 10%, the brightness of char particle slightly increased with the operating pressure, and it was not very obvious, see Fig. 11a. However, in atmospheric pressure, the brightness of char particle significantly increased with the oxygen volume fraction, as shown in Fig. 11b. The quantitative luminous intensity of char in different pressures and atmospheres are shown in Supplementary material C. Combined with the two-color pyrometry, the peak temperature of char (Tpeak ) in different conditions are shown in Fig. 12. It is obvious that the Tpeak increased with the oxygen partial pressure both at high pressure and atmospheric pressure. When the oxygen partial pressure increased from 0.1 bar to 0.5 bar, the Tpeak was increased by 145 °C (in O2 /N2 atmosphere, higher pressure), 114 °C (in O2 /CO2 atmosphere, higher pressure), 362 °C (in O2 /N2 atmosphere, atmospheric pressure), 311 °C (in O2 /CO2 atmosphere, atmospheric pressure), respectively. However, at the same oxygen partial pressure, the Tpeak of char in high pressure was dramatically lower than that in atmospheric pressure. As the oxygen partial pressure increases to 0.5 bar, the temperature difference of Tpeak between atmospheric pressure and high pressure condition were increased 217 °C and 197 °C in O2 /N2 atmosphere (1012 °C in 10%O2 /90%N2 atmosphere, 5 bar, 1248 °C in 50%O2 /50%N2 atmosphere, 1 bar) and O2 /CO2 atmosphere (957 °C in 10%O2 /90%CO2 atmosphere, 5 bar; 1154 °C in 50%O2 /50%CO2 atmosphere, 1 bar), respectively. These phenomena were mainly caused by different mass transfer rate under different operating conditions. In the same pressure and oxygen volume fraction, the Tpeak in O2 /N2 atmosphere was higher than that in O2 /CO2 atmosphere, this is mainly because the DO2 in CO2 atmosphere is lower than that in N2 atmosphere, the oxidation exothermic is low. There is also an endothermic reaction of gasification (C+CO2 =2CO) in O2 /CO2 atmosphere, which further reduces the surface temperature of particles, many studies have found similar conclusions under atmospheric pressure [21,22,40]. Since Tp is much higher than Tb in the char combustion stage, it can be considered that the char combustion process is controlled by external mass transfer [40,41]. Therefore, the diffusion mass flux of oxygen (m ) plays a decisive role in char conversion, which can be given by

m =hm ×(Ys,O2 −Y∞,O2 )=Sh × DO2 × (Ys,O2 −Y∞,O2 )/dp

(3)

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Fig. 11. Typical image of lignite char combustion in O2 /N2 and O2 /CO2 atmospheres with different oxygen volume fraction and pressure. (a) 10%O2 , 1–5 bar, the exposure time is 1/80 s; (b) 10−50% O2 , 1 bar, the exposure time is 1/800 s.

atmospheres were significantly lower than that in O2 /N2 atmospheres. These conclusions are consistent with the observed variation of the Tpeak of particles. Furthermore, high pressure not only promotes the oxidation of coal char, but also greatly promotes gasification (especially at high temperatures) in O2 /CO2 atmosphere, which will greatly reduce the char temperature. 3.5. Burnout time

Fig. 12. The peak temperature of char surface in various O2 volume fractions and pressures under N2 and CO2 atmospheres.

where hm is the mass transfer coefficient; Ys,O2 -Y∞,O2 represent the difference in oxygen volume fraction between particle surface and environment; Sh is the Sherwood number; dp is the diameter of char particle. However, the char particle may stay in different locations (emulsion phase, bubble phase or splash zone) during combustion process, and the m varies greatly among different locations [41]. The detailed calculation process are shown in Supplementary material G. When the oxygen volume fraction is 10%, the theoretical hm and m in various locations, pressures and atmospheres are shown in Fig. 13. As the increase of pressure, the m increased, and the hm decreased, regardless of the location and atmosphere. At the same location and pressure, the m in O2 /CO2

Burnout time (tb ) is an important parameter of coal combustion, which consists of drying time, devolatilization time and char combustion time. Thereinto, the char combustion time accounts for more than 80%, so the reaction rate of char has a crucial influence on the burnout time [39,40]. The burnout time of coal particles in various pressure and oxygen volume fraction of O2 /N2 and O2 /CO2 atmospheres is shown in Fig. 14. It is clear that the tb of particle decreased dramatically with the increase of oxygen partial pressure, and the burnout time of particles in O2 /CO2 condition were significantly longer than that in O2 /N2 conditions. When the oxygen partial pressure increased from 0.1 bar to 0.5 bar, the burnout time of coal particle decreased by 60.2% (in 10%O2 /90%N2 atmosphere, 5 bar), 54.8% (in 10%O2 /90%CO2 atmosphere, 5 bar), 79.8% (in 50%O2 /50%N2 atmosphere, 1 bar) and 78.7% (in 50%O2 /50%CO2 atmosphere, 1 bar), respectively. This means when the pressure increased by the same multiple, the decrement of coal burnout time in O2 /CO2 atmospheres was lower than that in O2 /N2 atmospheres. The mainly reasons for the above phenomena include: (1) The diffusion mass flux of oxygen (m ). As mentioned in Fig. 13b, the m in the three locations (emulsion phase, bubble phase, splash zone) of fluidized bed all increase with the pressure and oxygen volume fraction, thereby greatly reducing the burnout time of char. (2) Gasification. High pressure can significantly promote the gasification in CO2 atmosphere [53]. Although

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227

Fig. 13. The hm and m in various locations, pressure and atmospheres.

There was a “flame layer” on the surface of particle in the O2 /CO2 atmosphere, which may be caused by gasification. (2) At same oxygen volume fraction, the ignition delay time of coal particle increased with the increase of operating pressure. At the same pressure, the ignition delay time of the particle in O2 /CO2 atmosphere was longer than that in O2 /N2 atmosphere. (3) With the increase of operating pressure, the flame temperature and char temperature increased but the increment gradually decreases, and the size of flame became narrower and longer. (4) The burnout time of particle decreased dramatically as the increase of operating pressure, and the burnout time of particle in O2 /CO2 atmosphere was significantly longer than that in O2 /N2 atmosphere. Declaration of Competing Interest

Fig. 14. The burnout time of coal particles in various pressure of O2 /N2 and O2 /CO2 atmosphere.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments

the gasification contributes to the char conversion, it also reduces the temperature of particles, then reducing the rate of oxidation. Compare to the particle burning in N2 atmosphere, the lower diffusion mass flux of oxygen and lower particle temperature (caused by gasification) are the main reasons for the longer burnout time in O2 /CO2 atmosphere. The similar conclusion were also obtained under atmospheric pressure [21,39,40,45,47]. Combined with the thermal balance equation (Eq. (1)) and particle temperature (see Fig. 12), it can be inferred that the gasification reaction plays an important role in Tp and burnout characteristics of coal particle in pressurized oxy-fuel combustion, and can not be neglected. 4. Conclusion Experimental investigations of the combustion characteristics of coal particles in O2 /N2 and O2 /CO2 atmospheres under different pressure were conducted in a visualized fluidized bed combustor. The main conclusions can be drawn as follows: (1) According to the images of the combustion process, the volatiles flame in O2 /N2 atmosphere was brighter than that in O2 /CO2 atmosphere at the same oxygen volume fraction.

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