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Effect of CO2 and H2O on the combustion characteristics and ash formation of pulverized coal in oxy-fuel conditions
Applied Thermal Engineering 133 (2018) 308–315
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Effect of CO2 and H2O on the combustion characteristics and ash formation of pulverized coal in oxy-fuel conditions
T
⁎
Ming Lei , Chan Zou, Xubin Xu, Chunbo Wang School of Energy and Power Engineering, North China Electric Power University, Baoding 071000, China
H I G H L I G H T S thermal analysis with a bituminous coal. • High-heating-rate on the effects of CO and H O on coal conversion and ash mineral transformation. • Focus and H O gasifications increase the overall reaction rate at high temperatures and low O • CO • CO and H O have little effect on the mineral transformation of coal ash. 2
2
2
2
2
2
2
concentrations.
A R T I C L E I N F O
A B S T R A C T
Keywords: Oxy-fuel combustion CO2 H2O Combustion characteristics Ash formation Isothermal thermal analysis system
The combustion characteristics and ash formation of pulverized coal under different atmospheres (O2/N2, O2/ CO2 and O2/H2O/CO2) were investigated using an isothermal thermal analysis system. The burning rate in O2/ N2 was faster than that in O2/CO2 due to differences in the physical properties between N2 and CO2 at 800 °C. The effect of CO2 gasification was enhanced with increasing temperature; as a result, the overall reaction rate in O2/CO2 approached and exceeded the combustion rate in O2/N2. However, the effect of gasification was weakened by increasing O2 concentration, and the overall reaction rate in O2/CO2 decreased and eventually lagged the combustion rate in O2/N2. Because of the distinct physical properties of H2O, the burning rate in O2/CO2/ H2O was lower than that in O2/N2 but higher than that in O2/CO2 at 800 °C. At a low O2 concentration, the influence of CO2 and H2O gasification was enhanced with increasing temperature; thus, the overall reaction rate in O2/H2O/CO2 was higher than that in O2/CO2. Moreover, the burning rate in O2/H2O/CO2 increased slightly with increasing H2O concentration. However, higher O2 concentrations weakened the effect of gasification, leading to similar burning processes under different atmospheres. The XRD analysis of the ash samples showed that CO2 and H2O had no significant effect on the transformation of main minerals.
1. 1.Introduction In oxy-fuel combustion, fossil fuels are burned with the mixture of O2 and recycled flue gas instead of air, resulting in CO2 enrichment in the flue gas to levels as high as 90% by volume, which in turn leads to a relatively low cost of CO2 capture and storage (CSS) [1]. The reaction atmosphere used in oxy-fuel combustion differs substantially from that used in conventional air combustion because of the replacement of air with O2. N2 is the main component of flue gas in air combustion, whereas it changes to CO2 and H2O in oxy-fuel combustion [2]. In the oxy-fuel atmosphere, CO2 and H2O will physically and chemically affect the burning process of coal particles [3]. The physical effects are based mainly on the different physical properties of the carrier gases, which have been investigated by many scholars [4–8]. For example, the
⁎
Corresponding author. E-mail address: [email protected] (M. Lei).
https://doi.org/10.1016/j.applthermaleng.2018.01.060 Received 6 June 2017; Received in revised form 14 January 2018; Accepted 16 January 2018 Available online 20 February 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.
adiabatic flame temperature was lower in an oxy-fuel atmosphere than in air at the same O2 concentration because CO2 has a larger specific heat than N2 [9]. Additionally, char oxidation was relatively slower during oxy-fuel combustion due to the lower diffusivity of O2 in CO2 than in N2 [5]. The chemical effects mainly stem from the char gasification by CO2 (or H2O). On the one hand, this strongly endothermic gasification reaction can lower the particle temperature, leading to a decrease in char oxidation. On the other hand, CO2 (or H2O) can react with the char directly, resulting in an increase in the overall carbon consumption rate. Therefore, the effect of gasification on the combustion characteristics of pulverized coal is determined by the competition between these two aspects. By the use of high-speed camera and two-colour pyrometer, Zhang et al. [8] diagnosed the coal burning transient phenomena in situ under
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established by Marek et al. [7] to observe the single particle burning process in different atmospheres. Their results revealed that particle temperatures were higher in O2/CO2 atmospheres with H2O than in those without because of the lower specific heat of water compared with that of CO2 and the faster reaction rate for H2O gasification than for CO2gasification. In a oxy-fuel fluidized bed (Oxy-FB), Roy and Bhattacharya [18] analyzed the combustion performance of individual and large spherical single char particles in an atmosphere consisting of O2, CO2 and steam and concluded that the gasification reactions were almost negligible. On the basis of the aforementioned literature review, there are some studies on the influence of CO2 and H2O on combustion behaviors in oxy-fuel atmosphere, but a few distinct results were drawn by different authors, especially in the presence of high H2O concentrations. For example, both of Riaza et al. [13] and Yi et al. [17] found that the ignition temperature of pulverized coal increased after adding steam into the O2/CO2 mixtures; however, their conclusions about the burnout behaviors were different: Riaza et al. [13] found that the coal burnout decreased slightly, while Yi et al. [17] determined that the coal burnout was promoted after the addition of steam. Moreover, Hecht et al. [2] found that CO2 and H2O gasifications can increase the char consumption rate to some extent, but Roy and Bhattacharya [18] concluded that effect of gasification reactions on the char overall conversion were almost negligible. The reasons for the various conclusions may be due to the different experimental conditions (mainly the reaction temperature and the O2 concentration). Therefore, more effort was need to clarify the effect of CO2 and H2O on the combustion characteristics of pulverized coal at different conditions. Additionally, some studies have focused on the effect of a change in atmosphere (from air to oxy-fuel) on the behavior of ash formation, whereas the addition of H2O to the atmosphere has rarely been considered. Yu et al. [19] investigated the transformation of iron in ash under O2/N2 and O2/CO2 conditions and reported that varying the atmosphere had appreciable influences on the relative proportions of the iron combustion products. Zhang et al. [20] studied the transformation behavior of mineral matter and concluded that the evolution of included minerals in air was different from that in O2/CO2 because of the different char characteristics by replaying in air and O2/CO2. Sheng et al. [21] also found that O2/ CO2 combustion did not substantially affect the mineral phases in the ash aside from affecting their relative amounts. All these aforementioned tests were conducted in dry environments without H2O; however, according to Binner [22], steam might affect the transformation of char-bound metals by altering the char oxidation rate through its large specific heat. In this paper, the effects of CO2 and H2O on the combustion characteristics of pulverized coal in a range of temperatures (800–1200 °C) and O2 concentrations (2–21%) were investigated using a customized isothermal thermal analysis system that can achieve relatively high heating rates. Additionally, we analyzed the ash from the combustion to study the transformation of mineral phases during oxy-fuel combustion reactions.
O2/N2 and O2/CO2 atmospheres in a drop-tube furnace (DTF) and concluded the effect of CO2 quenching on the char particle temperatures can be offset by the homogeneous oxidation of char gasificationdriven CO. Naredi and Pisupati [10] investigated the influence of CO2 on the char burnout during oxy-fuel combustion in drop tube reactor (DTR) and found that the difference in char burnouts between the O2/ N2 and O2/CO2 mixtures decreased with increasing the furnace temperature, implying that the decrease in the char oxidation rate due to the lower particle temperature was compensated by the increase in the char consumption rate from char-CO2 reaction. The results indicated that CO2 gasification should be included in combustion model to accurately predict the burning behavior in oxy-fuel atmospheres. Hecht et al. [11] used the Surface Kinetics in Porous Particles (SKIPPY) code to calculate the char conversion rate during oxy-fuel combustion. They found that the increase in carbon consumption (owing to a direct reaction between CO2 and char) “exceeded” the decrease in char oxidation (due to a drop in particle temperature caused by gasification) and concluded that CO2 gasification can increase the overall carbon consumption rate for O2 concentrations up to 24%. At O2 concentrations greater than 24%, the additional carbon removal from the gasification can be offset and then surpassed by the reduced rate of char oxidation, leading to a decrease in the overall carbon consumption rate. GonzaloTirado et al. [9] used the kinetic parameters obtained from an entrained flow reactor (EFR) to modify a single-film model and then calculated the evolution of char conversion with time. The calculation results indicated that, at 4% O2, CO2 gasification enhanced the carbon consumption rate of oxy-combustion over the char oxidation in air (O2/N2), whereas the overall carbon consumption rate at 21% O2 was smaller in an oxy-fuel atmosphere than in air. In addition, subsequent analysis revealed that CO2 gasification accounted for up to 20% of the char consumption rate for lignite (at 4% O2 and 1573 K). Similarly, Kim et al.’s [5] calculations demonstrated that the carbon consumption rate increased due to the contribution of the direct gasification reaction to carbon consumption. Recently, Wang et al. [12] investigated the effect of key parameters (such as O2 concentration and bed temperature) on the char combustion characteristics in a fluidized bed (FB) system and found that the CO2 gasification can increase the char consumption rate to some extent, especially at elevated bed temperature and reduced O2 concentration. By comparing the combustion behaviors in the oxy-fuel atmospheres with and without steam addition, Riaza et al. [13] observed that the ignition temperatures increased and the coal burnout values decreased in an entrained flow reactor (EFR) as the steam was added into the oxy-fuel atmospheres. Taking into account the combined effect of CO2 and H2O, Gharebaghi et al. [14] set up a modified char burnout kinetic (CBK8) model to predict the char burnout during oxy-fuel combustion and found that the model was capable of predicting the burnout process with reasonable accuracy. Geier et al. [15] established an extended single-film reaction model that included both CO2 and H2O gasifications to determine the char particle temperature; their results showed that the predicted temperatures were in good agreement with those of tests in oxy-fuel atmospheres. A further study by Hecht et al. [2] revealed that CO2 and H2O gasifications reduced the rate of char oxidation by lowering the char temperature but also promoted char consumption by reacting with carbon directly; the final result was that the gasifications increased the char consumption rate by approximately 10% in oxy-fuel environments. Singer et al. [16] used a CFD simulation of a pilot-scale oxy-fuel test facility to evaluate the effects of the local environment and char conversion on the importance of the gasification reactions in determining the overall rate of char consumption and concluded that char gasification should not be neglected without first considering the particular char kinetics and trajectories through the furnace. Yi et al. [17] determined, through thermogravimetric analysis experiments under low heating rates, that adding H2O into an oxy-fuel atmosphere retarded the ignition but promoted the burnout of pulverized coal. A single-particle combustion stand (SPC stand) was
2. Experimental section 2.1. Materials A typical Chinese coal, Datong (DT), was chosen for this study. The proximate and ultimate analyses of the pulverized coal are shown in Table 1. The coal samples were pulverized and then screened between 75 and 96 μm. 2.2. Apparatus and procedure The isothermal thermal analysis system is presented in Fig. 1. The main equipment of the system includes a tube furnace, a data acquisition system and a steam generator, etc. More details on the system can 309
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Table 1 Properties of Datong coal.a Coal sample
Datong a b
Proximate analysis/mass %
Ultimate analysis/mass %
Moisture
Volatile
Ash
Fixed Carbon
Carbon
Hydrogen
Nitrogen
Sulfur
Oxygenb
3.27
22.71
24.25
49.77
61.22
3.42
0.77
1.13
5.94
Air-dried basis. Calculated by difference.
be found in a previous report [23]. In the tests, the gas was depressurized and then discharged into the furnace through the flowmeter; the water vapor was generated by the steam generator and carried by the gas into the furnace. All of the tests followed the same procedure: First, the furnace was heated to a given temperature. After flushing the furnace with N2 for 15 min, the gas supply was switched to the reaction gas at 1 L/min. Approximately 80 ± 0.05 mg of the coal sample was spread evenly in a quartz boat, and then the boat was inserted into the furnace to complete the burning process. In the experimental system, the quartz boat was fed into the tube furnace within a very short time because of the high speed of the guide rail. Therefore, a small amount of coal sample could be ignited at a relatively high heating rate because of the large quantities of heat provided by the furnace. The weight signals were monitored continuously during the test. The buoyancy effects were eliminated through evaluation of blank experiments. When the test was completed, the ash was removed from the quartz boat and analyzed using a Bruker D8 Advance X-ray diffractometer (XRD). To study the combustion characteristics of the pulverized coal, the conversion rate X is introduced and defined as below:
X=
m 0−mt × 100% m 0−mc
100 o
t=800 C
X/%
80
2%O2+98%CO2
60
2%O2+98%N 2
40
5%O2+95%N 2
20
10%O 2+90%N 2
5%O2+95%CO 2 10%O 2+90%CO 2 21%O 2+79%CO 2 21%O 2+79%N 2
0 0
200
400
600
800
time/s
1000
1200
processes of O2/N2 and O2/CO2. At low O2 concentrations (i.e., high CO2 concentrations), the specific heat of O2/CO2 was much higher than that of O2/N2 and the diffusion coefficient of O2 in CO2 was substantially lower than that in N2. Meanwhile, the burning rates of the samples were relatively low, which enhanced the effect of the physical properties of mixtures on the combustion characteristics. As the O2 concentration increased, the burning rate increased and the differences in the physical properties between gas mixtures decreased, which reduced the effect of the physical properties and thus gradually made the burning processes in different atmospheres more similar. Fig. 3 shows the conversion rate of DT coal in O2/N2 and O2/CO2 at 1000 °C. A comparison of Figs. 2 and 3 reveals that the elevated temperature increased the conversion rate of pulverized coal and shortened the burnout time at the same O2 concentration. Because of the differences in the physical properties of N2 and CO2, the conversion rate in O2/CO2 was still lower than that in O2/N2. However, when the temperature increased to 1000 °C, the gap between the burning processes in O2/N2 and O2/CO2 decreased at low O2 concentrations compared with the gap at 800 °C, whereas the gap between burning rates in the different atmospheres increased again at higher O2 concentrations. The gasification had a limited effect on the burning process, likely
where m0 is the initial mass of the sample, mt is the mass of the sample at time t, and mc is the remaining mass of the sample after burnout. 3. Results and discussion 3.1. Combustion characteristics 3.1.1. CO2 effect The effect of CO2 on the combustion characteristics of DT coal was investigated under different O2 concentrations at 800 °C; the results are shown in Fig. 2. As seen in the figure, with increasing O2 concentration, the conversion rates of pulverized coal in O2/CO2 and O2/N2 were both accelerated, which decreased the burnout time. Compared to the conversion rate and burnout in O2/N2, the conversion rate was slowed and the burnout was delayed in O2/CO2 because of the greater specific heat of CO2 and the lower diffusion coefficient of O2 in CO2 [4,5]. However, the increased O2 concentration reduced the gap between the burning Heater band
Tube furnace Quartz boat
Gas buffer
High-temperature resistant stent
Flowmeter Pressure reducing valve
Computer
Steam generator Temperature controller O2
1600
Fig. 2. Conversion rates of DT coal in O2/N2 and in O2/CO2 at 800 °C.
(1)
N2
1400
CO2
Temperature controller for heater band
Guide rail
Fig. 1. Isothermal thermal analysis system.
310
Data acquisition system
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100
100 o
80
t=1000 C
80
o
t=1200 C
2%O2+98%N 2
60
X /%
X/%
2%O2+98%CO 2 5%O2+95%CO 2 5%O2+95%N 2
40
2%O2+98%CO2
60
2%O2+98%N2 5%O2+95%CO2
40
5%O2+95%N2
10%O 2+90%CO 2 20
10%O 2+90%N 2
0
21%O 2+79%N 2
10%O2+90%CO2 10%O2+90%N2
20
21%O2+79%CO2
21%O 2+79%CO 2
0
200
400
600
time/s
800
1000
21%O2+79%N2
0 0
1200
100
200
300
400
time/s
500
600
700
800
Fig. 4. Conversion rate of DT coal in O2/N2 and O2/CO2 at 1200 °C.
Fig. 3. Conversion rate of DT coal in O2/N2 and O2/CO2 at 1000 °C.
was maintained at a low level of 2%, the gap between the gasification rate and the combustion rate was further reduced compared to that under similar conditions at 1000 °C and the effect of gasification was enhanced. These results indicate that the magnitude of the increase in the carbon consumption rate for the reaction between char and CO2 was greater than the magnitude of the decrease in the combustion rate (because of the lower particle temperature caused by endothermic gasification and the physical properties of CO2), which increased the overall reaction rate in O2/CO2. As a result, the burnout time of pulverized coal in O2/CO2 was lower than that in O2/N2. When the O2 concentration increased to 5%, the influence of gasification on the burning process decreased because of the increase in the combustion rate and the decrease in the gasification rate; however, the promoting effect of gasification on the carbon consumption rate was still stronger than its inhibiting effect (as a result of being endothermic and because of the physical properties of CO2) on the combustion rate. As the O2 concentration was increased further, the difference between the combustion rate and the gasification rate again increased; thus, the influence of CO2 gasification weakened, whereas the effect of the physical properties (greater specific heat and lower O2 diffusivity) was enhanced, which led to an earlier burnout in O2/N2 than in O2/CO2.
because the reaction rate of CO2 gasification was relatively low (800 °C); the differences in the combustion characteristics of O2/N2 and O2/CO2 were mainly due to the differences between the physical properties of N2 and CO2. When the temperature increased to 1000 °C, the CO2 gasification accelerated, producing two different effects: (1) strongly endothermic gasification, which can lower the particle temperature, leading to a decrease in the char oxidation, and (2) direct reaction of CO2 with the char, increasing the overall carbon consumption rate. Actually, the effect of CO2 on the overall coal consumption rate (combustion and gasification) was controlled by competition between the combustion and gasification processes and the physical properties of CO2. At low O2 concentrations, although the gasification rate was lower than the combustion rate, the difference between the reaction rates of gasification and combustion was less than that at high O2 concentrations. A comparison of Fig. 2 with Fig. 3 reveals that the gap between the burning processes of O2/N2 and O2/CO2 was reduced with increasing temperature, which indicated that CO2 gasification increased the carbon consumption rate more than it decreased the combustion rate (because of the lower particle temperature caused by endothermic gasification and the physical properties of CO2), and as a result the overall reaction rate of pulverized coal increased in O2/CO2. As the O2 concentration increased, the combustion rate accelerated and the gasification rate decreased, which meant that the gap between the gasification rate and the combustion rate increased gradually, and thus the influence of gasification on the char consumption rate diminished and the burning process was mainly controlled by combustion. Without consideration of the gasification reaction, the combustion reaction was dominated by the physical properties of reaction atmosphere. And because of the distinct physical properties of CO2 (greater specific heat and lower O2 diffusivity), the burning process of pulverized coal in O2/CO2 lagged behind that in O2/N2. Fig. 4 shows the conversion rate of DT coal in O2/N2 and O2/CO2 at 1200 °C. A comparison of Fig. 4 with Figs. 2 and 3 shows that the conversion rate of pulverized coal was further accelerated when the temperature increased to 1200 °C. However, unlike at 1000 or 800 °C, the burnout in O2/CO2 at 1200 °C substantially decreased compared to that in O2/N2 with 2% O2. When the O2 concentration increased to 5% and the gap between the burning processes in the two different atmospheres diminished, the burnout in O2/CO2 remained ahead of that in O2/N2. However, when the O2 concentration was increased further, the burnout in O2/CO2 eventually lagged that in O2/N2. When the temperature was increased from 1000 °C to 1200 °C, both the combustion rate and the gasification rate accelerated; however, the gasification rate may have increased more significantly than the combustion rate because the combustion reaction likely preceded the gasification reaction in transitioning from Zone I into Zone II. When the O2 concentration
3.1.2. CO2 and H2O effects On the basis of the analysis presented in Section 3.1.1, the combined effect of CO2 and H2O on the combustion characteristics of DT coal was investigated by adding water vapor to the reaction atmosphere. Fig. 5 shows the conversion rate of DT coal in O2/N2, O2/CO2 and O2/H2O/CO2 atmospheres at 800 °C. As shown in the figure, the conversion rate of pulverized coal with 2% O2 in O2/H2O/CO2 was slightly faster than that in O2/CO2 but still slower than that in O2/N2. As the O2 concentration was increased to 10%, the gaps among the burning processes of the different atmospheres were gradually minimized. At 800 °C, the H2O gasification likely had a limited effect on the burning process because of its relatively low reaction rate; thus, the effect of the H2O physical properties was mainly considered. The specific heat of H2O is greater than that of N2 but less than that of CO2, and the diffusion coefficient of O2 in H2O is also between those of O2 in N2 and in CO2 [17]. Therefore, the temperature of the coal particles increased and the diffusion of O2 to the particles was accelerated by replacing part of the CO2 with H2O in the gas mixture, resulting in a faster conversion rate for pulverized coal. However, the conversion rate in O2/H2O/CO2 was still slower than that in O2/N2. As the O2 concentration increased, the burning rate of pulverized coal increased and the differences in physical properties among the three atmospheres decreased, which mitigated the influence of the gases’ physical properties. Fig. 6 shows the conversion rate of DT coal in O2/N2, O2/CO2 and O2/H2O/CO2 atmospheres at 1000 °C. A comparison of Figs. 5 and 6 311
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M. Lei et al.
80
80
60
X /%
100
X/%
100
o
t=800 C 2%O2+98%CO2
o
t=1200 C
2%O2+98%CO2 2%O2+20%H2O+78%CO2
40
2%O2+20%H2O+78%CO2
40
60
2%O2+98%N2
2%O2+98%N2
20
10%O2+90%CO2
20
10%O2+90%CO2
10%O2+20%H2O+70%CO2
10%O2+20%H2O+70%CO2 10%O2+90%N2
0 0
200
400
600
800
1000
1200
10%O2+90%N2
0 1400
0
1600
100
time/s
X/%
80
o
t=1000 C 2%O 2+98%CO 2 2%O 2+20%H 2O+78%CO 2 2%O 2+98%N 2 10%O 2+90%CO 2
20
10%O 2+20%H 2O+70%CO 2 10%O 2+90%N 2
0 0
200
400
600
800
1000
400 time/s
500
600
700
800
conversion rates in the order of O2/N2, O2/H2O/CO2, O2/CO2. Fig. 7 shows the conversion rate of DT coal in O2/N2, O2/CO2, O2/ H2O/CO2 at 1200 °C. As shown in the figure, the conversion rates of pulverized coal decreased in the order of O2/H2O/CO2, O2/CO2, O2/N2 in 2% O2, whereas the gaps among the burning processes of the different atmospheres were substantially greater than those at 1000 °C. When the O2 concentration increased to 10%, the burning processes in the different atmospheres became more similar; however, the conversion rate in O2/H2O/CO2 was still the highest, followed by those in O2/ N2 and O2/CO2. As previously mentioned, the growth of the gasification rate exceeded that of the combustion rate as the temperature increased, and, because the burning rate was relatively slow at low O2 concentrations, the influence of CO2/H2O gasification was enhanced, which led to an obvious increase of the overall reaction rate of pulverized coal. As the O2 concentration increased, the influence of the gasification was weakened because of the increased combustion rate. However, the CO2/H2O gasification can still accelerate the conversion of pulverized coal to a certain extent because of its faster gasification rate compared with that of CO2 gasification. As shown in Fig. 8, three different H2O concentrations, 10%, 20% and 30%, were selected to study the effect of variable H2O concentrations on the overall reaction rate of pulverized coal during oxy-fuel combustion. It can found that the conversion rates of pulverized coal increased slightly with increasing H2O concentration in 2% O2. According to Bai et al. [24], the char gasification rate in the CO2/H2O mixture increased monotonously with increasing H2O concentration. Thus, at elevated H2O concentration, the influence of CO2/H2O gasification can be enhanced on the basis of above analysis, and thus the overall reaction rate of pulverized coal increased accordingly. However, in 10% O2, it was hardly to distinguish the effect of variable H2O concentrations on the overall reaction rate, because the effect of the gasification was weakened with increasing O2 concentration.
100
40
300
Fig. 7. Conversion of DT coal in O2/N2, O2/CO2 and O2/H2O/CO2 at 1200 °C.
Fig. 5. Conversion rate of DT coal in O2/N2, O2/CO2 and O2/H2O/CO2 atmospheres at 800 °C.
60
200
1200
time/s Fig. 6. Conversion rate of DT coal in O2/N2, O2/CO2 and O2/H2O/CO2 at 1000 °C.
reveals that the conversion rate of pulverized coal was accelerated and that the burnout time decreased with increasing temperature. In 2% O2, the conversion rate of pulverized coal in O2/H2O/CO2 was the fastest, followed by that in O2/N2 and in O2/CO2. However, at 10% O2, the conversion rate in O2/N2 was the fastest, followed by those in O2/H2O/ CO2 and O2/CO2, and the gaps among the burning processes in the three different atmospheres were reduced compared to that under low O2 concentrations. When the temperature increased to 1000 °C, the reaction rates of H2O gasification and CO2 gasification both accelerated and thus the gasification reactions had the potential to affect the burning process, especially at low O2 concentration. Bai et al. [24] investigated the gasification reactivity of coal chars using CO2/H2O mixtures and found that the char gasification rate in the CO2/H2O mixture was much higher than that in pure CO2, indicating that the synergy existed during char gasification. Therefore, in 2% O2, compared with the effect of CO2 gasification alone, the effect of CO2/H2O gasification was enhanced due to the synergistic effect [24]. As a result, the carbon consumption rate for CO2/H2O reacting with char directly increased more than the combustion rate decreased at (because of a decrease in the particle temperature caused by the endothermic gasification and by the physical properties of CO2 and H2O), which led to an increase in the overall reaction rate of pulverized coal. As the O2 concentration increased, the burning rate accelerated and the gasification rate slowed; thus, the influence of CO2/H2O gasification was weakened. The burning process of pulverized coal was mainly affected by the physical properties of CO2 and H2O, which resulted in a decrease in the
3.2. Ash formation Fig. 9 presents the XRD patterns of DT coal ashes formed under 2% O2 at 1000 °C. The major minerals in the DT ashes were quartz, hematite and anhydrite. Few differences were observed in the main minerals among the ashes generated in O2/N2, O2/CO2 and O2/CO2/H2O, which indicated that the transformation of minerals was not strongly affected by CO2 and H2O (either their physical or chemical effects) at 1000 °C. The XRD patterns of ashes formed in 2% and 21% O2 at 1200 °C are shown in Figs. 10 and 11, respectively. A comparison of Fig. 9 with Figs. 10 and 11 reveals stronger mineral peaks in the patterns of the ashes generated at 1200 °C, implying that the crystallinity of the minerals increased at higher temperatures. The anhydrite likely decomposed because of its lower decomposition temperature. Peaks 312
Applied Thermal Engineering 133 (2018) 308–315
M. Lei et al.
100
80
80
60
X /%
X /%
100
o
t=1200 C 2%O2+98%CO2
40
60 o
t=1200 C 10%O2+90%CO2
40
10%O2+10%H2O+80%CO2
2%O2+10%H2O+88%CO2 2%O2+20%H2O+78%CO2
20
10%O2+20%H2O+70%CO2
20
10%O2+30%H2O+60%CO2
2%O2+30%H2O+68%CO2
2%O2+98%N2
0 0
100
200
300
400
500
600
10%O2+90%N2
0
700
800
0
50
100
150
200
250
time/s
time/s
(a) 2% O2 concentration
(b) 10% O2 concentration
300
Fig. 8. Conversion of DT coal under different H2O concentrations at 1200 °C.
longer than that of DT coal in 10% O2; thus, more quartz likely transformed to cristobalite over this extended period of time.
attributable to the typical high-temperature mineral, mullite, was also identified in the XRD pattern. Additionally, the cristobalite in the ash disappeared as the O2 concentration increased from 2% to 21% at 1200 °C. Figs. 10 and 11 also show that the main mineral phases identified in the different ashes were approximately the same, implying that CO2 and H2O did not substantially influence the transformation of mineral phases. Sheng et al. [21] have investigated the mineral transformations of coal ash under air and O2/CO2 atmospheres and found that O2/CO2 combustion did not substantially change the mineral phases, consistent with the experimental results presented here. Pagliari et al. [25] found that the formation of cristobalite from quartz was substantially enhanced as the reaction time increased at temperatures greater than 1200 °C. As shown in Fig. 7, the reaction time of DT coal in 2% O2 was
2200
15
20
25
30
35
Q
40
45
4. Conclusions An isothermal thermal analysis system, which can be operated at high-heating-rate, were used to investigate the effects of CO2 and H2O on the combustion characteristics and ash formation of pulverized coal under oxy-fuel conditions. The results from the study demonstrated that: 1. Neglecting the influence of CO2 gasification at 800 °C, the conversion rate of pulverized coal in O2/N2 was faster than that in O2/CO2 due to the differences in the physical properties between N2 and
50
55
2%O2+20%H2O+78%CO2
65
70
A-anhydrite; H-hematite; Q-quartz;
1650
Inensity
60
1100 Q
550
A
A
H
HQ
A QQH Q
Q
Q
H
Q
Q
Q
Q
Q
Q
2200
2%O2+98%CO2
Q
Inensity
1650
1100 Q
550
A
2200
A
H
HQ
Q
A QQH Q
Q
Q
H
2%O2+98%N2
Inensity
1650 1100
550
0 15
Q A
20
25
A
30
H
HQ
35
A Q QH Q
40
Q
45
Q
50
H
55
2ș
313
60
65
70
Fig. 9. XRD patterns of DT coal ashes generated under 2% O2 at 1000 °C.
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2400
Q
Fig. 10. XRD patterns of DT coal ashes generated under 2% O2 at 1200 °C.
2%O2+20%H2O+78%CO2
C-cristobalite H-hematite; M-mullite; Q-quartz;
Inensity
1800
1200
600
Q M
C
H H H M QM Q M M M Q Q
M
Q Q
QM
M
H
QQ M
M
H
QM
M
60
65
H
Q
2400
2%O2+98%CO2 Q
Inensity
1800
1200
600
Q M
C
H H H M QM Q M M M Q Q
M
Q
Q
2400
1800
Inensity
2%O2+98%N2
Q
1200
600
Q M
0 15
20
C
H H H MQM Q M Q M M Q
M
25
30
35
40
Q
Q
45
50
55
Q
70
2ș
2600
Fig. 11. XRD patterns of DT coal ashes generated under 10% O2 at 1200 °C.
10%O2+20%H2O+70%CO2
Q
H-hematite; M-mullite; Q-quartz;
Inensity
1950 1300 Q
650
M
M
M
M H H MH QM QQM Q
Q
Q
H
QM
M
H
QM
M
H
QM
M
Q
2600
10%O2+90%CO2
Q
Inensity
1950 1300 Q
650
M
M
2600
M
M H H MH QM QQM Q
Q
Q
Q
Q
10%O2+90%N2
Inensity
1950 1300 Q
650
M
M
0 15
20
25
M
30
M H H MH QM QQM Q
35
40
Q
45
Q
50
55
2ș
314
60
65
Q
70
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CO2. As the temperature was increased to 1000 °C and then to 1200 °C, the overall reaction rate (combustion and gasification) of pulverized coal in O2/CO2 gradually approached and then exceeded the combustion rate in O2/N2 because of the growing effect of the CO2 gasification reaction. However, the effect of gasification was weakened with increasing O2 concentrations, and thus the overall reaction rate of pulverized coal in O2/CO2 eventually lagged the combustion rate in the O2/N2 due to the different physical properties of the gas mixtures. 2. Before the CO2 and H2O gasification exerted a clear effect at 800 °C, the conversion rate of pulverized coal in O2/H2O/CO2 was lower than that in O2/N2 but higher than that in O2/CO2, because the specific heat of H2O was higher than that of N2 but less than that of CO2 and because the O2 diffusivity in H2O was also between that in N2 and that in CO2. At low O2 concentrations, with increasing temperature, the overall reaction rate in O2/H2O/CO2 increased compared to that in O2/CO2 due to the enhanced effect of CO2 and H2O gasifications. However, as the O2 concentration increased, the burning process in the two atmospheres became more similar due to the reduced effect of CO2 and H2O gasification. Increasing H2O concentration in the gas mixtures can slightly increase the conversion rates of pulverized coal at low O2 concentrations due to the enhanced effect of CO2/H2O gasification. But the effect of CO2/H2O gasification was weakened at elevated O2 concentration, which made it difficult to distinguish the effect of variable H2O concentrations on the overall reaction rate. 3. According to the XRD analysis of coal ashes, the main minerals between the ashes generated in O2/N2, O2/CO2 and O2/CO2/H2O differed only slightly, which indicated that CO2 and H2O did not strongly influence the transformation of the main minerals.
[3] L. Zhang, D. Wu, L. Cai, et al., The chemical and physical effects of CO2 on the homogeneous and heterogeneous ignition of the coal particle in O2/CO2 atmospheres, Proc. Combust. Inst. 36 (2017) 2113–2121. [4] H. Tolvanen, R. Raiko, An experimental study and numerical modeling of combusting two coal chars in a drop-tube reactor: a comparison between N2/O2, CO2/ O2, and N2/CO2/O2 atmospheres, Fuel 124 (2014) 190–201. [5] D. Kim, S. Choi, C.R. Shaddix, et al., Effect of CO2 gasification reaction on char particle combustion in oxy-fuel conditions, Fuel 120 (2014) 130–140. [6] R.K. Rathnam, L.K. Elliott, T.F. Wall, et al., Differences in reactivity of pulverised coal in air (O2/N2) and oxy-fuel (O2/CO2) conditions, Fuel Process. Technol. 90 (2009) 797–802. [7] E. Marek, B. Świątkowski, Experimental studies of single particle combustion in air and different oxy-fuel atmospheres, Appl. Therm. Eng. 66 (2014) 35–42. [8] L. Zhang, E. Binner, Y. Qiao, et al., In situ diagnostics of Victorian brown coal combustion in O2/N2 and O2/CO2mixtures in drop-tube furnace, Fuel 89 (2010) 2703–2712. [9] C. Gonzalo-Tirado, S. Jiménez, J. Ballester, Gasification of a pulverized sub-bituminous coal in CO2 at atmospheric pressure in an entrained flow reactor, Combust. Flame 159 (2012) 385–395. [10] P. Naredi, S. Pisupati, Effect of CO2 during coal pyrolysis and char burnout in oxycoal combustion, Energy Fuel 25 (2011) 2452–2459. [11] E.S. Hecht, C.R. Shaddix, A. Molina, et al., Effect of CO2 gasification reaction on oxy-combustion of pulverized coal char, Proc. Combust. Inst. 33 (2010) 1699–1706. [12] W.K. Wang, C.S. Bu, A. Gómez-Barea, et al., O2/CO2 and O2/N2 combustion of bituminous char particles in a bubbling fluidized bed under simulated combustor conditions, Chem. Eng. J. 336 (2018) 74–81. [13] J. Riaza, L. Álvarez, M.V. Gil, et al., Effect of oxy-fuel combustion with steam addition on coal ignition and burnout in an entrained flow reactor, Energy 36 (2011) 5314–5319. [14] M. Gharebaghi, R.M. Ironsb, M. Pourkashaniana, et al., An investigation into a carbon burnout kinetic model for oxy-coal combustion, Fuel Process. Technol. 92 (2011) 2455–2464. [15] M. Geier, C.R. Shaddix, K.A. Davis, et al., On the use of single-film models to describe the oxy-fuel combustion of pulverized coal char, Appl. Energy 93 (2012) 675–679. [16] S. Singer, L. Chen, A.F. Ghoniem, The influence of gasification reactions on char consumption under oxy-combustion conditions: Effects of particle trajectory and conversion, Proc. Combust. Inst. 34 (2013) 3471–3478. [17] B.J. Yi, L.Q. Zhang, F. Huang, et al., Effect of H2O on the combustion characteristics of pulverized coal in O2/CO2 atmosphere, Appl. Energy 132 (2014) 349–357. [18] B. Roy, S. Bhattacharya, Combustion of single char particles from Victorian brown coal under oxy-fuel fluidized bed conditions, Fuel 165 (2016) 477–483. [19] D.X. Yu, L. Zhao, Z.Y. Zhang, et al., Iron transformation and ash fusibility during coal combustion in air and O2/CO2 medium, Energy Fuel 26 (2012) 3150–3155. [20] L. Zhang, F. Jiao, E. Binner, et al., Experimental investigation of the combustion of bituminous coal in airand O2/CO2 mixtures: 2. Variation of the transformation behaviour of mineral matter with bulk gas composition, Fuel 90 (2011) 1361–1369. [21] C.D. Sheng, Y. Li, Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures, Fuel 87 (2008) 1297–1305. [22] E. Binnera, L. Zhang, C.Z. Li, In-situ observation of the combustion of air-dried and wet Victorian brown coal, Proc. Combust. Inst. 33 (2011) 1739–1746. [23] C.B. Wang, H. Shao, M. Lei, et al., Effect of the coupling action between volatiles, char and steam on isothermal combustion of coal char, Appl. Therm. Eng. 93 (2016) 438–445. [24] Y.H. Bai, Y.L. Wang, S.H. Zhu, et al., Synergistic effect between CO2 and H2O on reactivity during coal chars gasification, Fuel 126 (2014) 1–7. [25] L. Pagliari, M. Dapiaggi, A. Pavese, et al., A kinetic study of the quartz-cristobalite phase transition, J. Eur. Ceram. Soc. 33 (2013) 3403–3410.
Acknowledgements This work was supported by the National Key R&D Program of China (2016YFB0600701), the Hebei Province Natural Science Foundation (E2016502094) and the Fundamental Research Funds for the Central Universities (2017MS120). References [1] L. Chen, S.Z. Yong, A.F. Ghoniem, Oxy-fuel combustion of pulverized coal: characterization, fundamentals, stabilization and CFD modeling, Prog. Energy Combust. 38 (2012) 156–214. [2] E.S. Hecht, C.R. Shaddix, M. Geier, et al., Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char, Combust. Flame 159 (2012) 3437–3447.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Effect of CO2 and H2O on the combustion characteristics and ash formation of pulverized coal in oxy-fuel conditions
T
⁎
Ming Lei , Chan Zou, Xubin Xu, Chunbo Wang School of Energy and Power Engineering, North China Electric Power University, Baoding 071000, China
H I G H L I G H T S thermal analysis with a bituminous coal. • High-heating-rate on the effects of CO and H O on coal conversion and ash mineral transformation. • Focus and H O gasifications increase the overall reaction rate at high temperatures and low O • CO • CO and H O have little effect on the mineral transformation of coal ash. 2
2
2
2
2
2
2
concentrations.
A R T I C L E I N F O
A B S T R A C T
Keywords: Oxy-fuel combustion CO2 H2O Combustion characteristics Ash formation Isothermal thermal analysis system
The combustion characteristics and ash formation of pulverized coal under different atmospheres (O2/N2, O2/ CO2 and O2/H2O/CO2) were investigated using an isothermal thermal analysis system. The burning rate in O2/ N2 was faster than that in O2/CO2 due to differences in the physical properties between N2 and CO2 at 800 °C. The effect of CO2 gasification was enhanced with increasing temperature; as a result, the overall reaction rate in O2/CO2 approached and exceeded the combustion rate in O2/N2. However, the effect of gasification was weakened by increasing O2 concentration, and the overall reaction rate in O2/CO2 decreased and eventually lagged the combustion rate in O2/N2. Because of the distinct physical properties of H2O, the burning rate in O2/CO2/ H2O was lower than that in O2/N2 but higher than that in O2/CO2 at 800 °C. At a low O2 concentration, the influence of CO2 and H2O gasification was enhanced with increasing temperature; thus, the overall reaction rate in O2/H2O/CO2 was higher than that in O2/CO2. Moreover, the burning rate in O2/H2O/CO2 increased slightly with increasing H2O concentration. However, higher O2 concentrations weakened the effect of gasification, leading to similar burning processes under different atmospheres. The XRD analysis of the ash samples showed that CO2 and H2O had no significant effect on the transformation of main minerals.
1. 1.Introduction In oxy-fuel combustion, fossil fuels are burned with the mixture of O2 and recycled flue gas instead of air, resulting in CO2 enrichment in the flue gas to levels as high as 90% by volume, which in turn leads to a relatively low cost of CO2 capture and storage (CSS) [1]. The reaction atmosphere used in oxy-fuel combustion differs substantially from that used in conventional air combustion because of the replacement of air with O2. N2 is the main component of flue gas in air combustion, whereas it changes to CO2 and H2O in oxy-fuel combustion [2]. In the oxy-fuel atmosphere, CO2 and H2O will physically and chemically affect the burning process of coal particles [3]. The physical effects are based mainly on the different physical properties of the carrier gases, which have been investigated by many scholars [4–8]. For example, the
⁎
Corresponding author. E-mail address: [email protected] (M. Lei).
https://doi.org/10.1016/j.applthermaleng.2018.01.060 Received 6 June 2017; Received in revised form 14 January 2018; Accepted 16 January 2018 Available online 20 February 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.
adiabatic flame temperature was lower in an oxy-fuel atmosphere than in air at the same O2 concentration because CO2 has a larger specific heat than N2 [9]. Additionally, char oxidation was relatively slower during oxy-fuel combustion due to the lower diffusivity of O2 in CO2 than in N2 [5]. The chemical effects mainly stem from the char gasification by CO2 (or H2O). On the one hand, this strongly endothermic gasification reaction can lower the particle temperature, leading to a decrease in char oxidation. On the other hand, CO2 (or H2O) can react with the char directly, resulting in an increase in the overall carbon consumption rate. Therefore, the effect of gasification on the combustion characteristics of pulverized coal is determined by the competition between these two aspects. By the use of high-speed camera and two-colour pyrometer, Zhang et al. [8] diagnosed the coal burning transient phenomena in situ under
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established by Marek et al. [7] to observe the single particle burning process in different atmospheres. Their results revealed that particle temperatures were higher in O2/CO2 atmospheres with H2O than in those without because of the lower specific heat of water compared with that of CO2 and the faster reaction rate for H2O gasification than for CO2gasification. In a oxy-fuel fluidized bed (Oxy-FB), Roy and Bhattacharya [18] analyzed the combustion performance of individual and large spherical single char particles in an atmosphere consisting of O2, CO2 and steam and concluded that the gasification reactions were almost negligible. On the basis of the aforementioned literature review, there are some studies on the influence of CO2 and H2O on combustion behaviors in oxy-fuel atmosphere, but a few distinct results were drawn by different authors, especially in the presence of high H2O concentrations. For example, both of Riaza et al. [13] and Yi et al. [17] found that the ignition temperature of pulverized coal increased after adding steam into the O2/CO2 mixtures; however, their conclusions about the burnout behaviors were different: Riaza et al. [13] found that the coal burnout decreased slightly, while Yi et al. [17] determined that the coal burnout was promoted after the addition of steam. Moreover, Hecht et al. [2] found that CO2 and H2O gasifications can increase the char consumption rate to some extent, but Roy and Bhattacharya [18] concluded that effect of gasification reactions on the char overall conversion were almost negligible. The reasons for the various conclusions may be due to the different experimental conditions (mainly the reaction temperature and the O2 concentration). Therefore, more effort was need to clarify the effect of CO2 and H2O on the combustion characteristics of pulverized coal at different conditions. Additionally, some studies have focused on the effect of a change in atmosphere (from air to oxy-fuel) on the behavior of ash formation, whereas the addition of H2O to the atmosphere has rarely been considered. Yu et al. [19] investigated the transformation of iron in ash under O2/N2 and O2/CO2 conditions and reported that varying the atmosphere had appreciable influences on the relative proportions of the iron combustion products. Zhang et al. [20] studied the transformation behavior of mineral matter and concluded that the evolution of included minerals in air was different from that in O2/CO2 because of the different char characteristics by replaying in air and O2/CO2. Sheng et al. [21] also found that O2/ CO2 combustion did not substantially affect the mineral phases in the ash aside from affecting their relative amounts. All these aforementioned tests were conducted in dry environments without H2O; however, according to Binner [22], steam might affect the transformation of char-bound metals by altering the char oxidation rate through its large specific heat. In this paper, the effects of CO2 and H2O on the combustion characteristics of pulverized coal in a range of temperatures (800–1200 °C) and O2 concentrations (2–21%) were investigated using a customized isothermal thermal analysis system that can achieve relatively high heating rates. Additionally, we analyzed the ash from the combustion to study the transformation of mineral phases during oxy-fuel combustion reactions.
O2/N2 and O2/CO2 atmospheres in a drop-tube furnace (DTF) and concluded the effect of CO2 quenching on the char particle temperatures can be offset by the homogeneous oxidation of char gasificationdriven CO. Naredi and Pisupati [10] investigated the influence of CO2 on the char burnout during oxy-fuel combustion in drop tube reactor (DTR) and found that the difference in char burnouts between the O2/ N2 and O2/CO2 mixtures decreased with increasing the furnace temperature, implying that the decrease in the char oxidation rate due to the lower particle temperature was compensated by the increase in the char consumption rate from char-CO2 reaction. The results indicated that CO2 gasification should be included in combustion model to accurately predict the burning behavior in oxy-fuel atmospheres. Hecht et al. [11] used the Surface Kinetics in Porous Particles (SKIPPY) code to calculate the char conversion rate during oxy-fuel combustion. They found that the increase in carbon consumption (owing to a direct reaction between CO2 and char) “exceeded” the decrease in char oxidation (due to a drop in particle temperature caused by gasification) and concluded that CO2 gasification can increase the overall carbon consumption rate for O2 concentrations up to 24%. At O2 concentrations greater than 24%, the additional carbon removal from the gasification can be offset and then surpassed by the reduced rate of char oxidation, leading to a decrease in the overall carbon consumption rate. GonzaloTirado et al. [9] used the kinetic parameters obtained from an entrained flow reactor (EFR) to modify a single-film model and then calculated the evolution of char conversion with time. The calculation results indicated that, at 4% O2, CO2 gasification enhanced the carbon consumption rate of oxy-combustion over the char oxidation in air (O2/N2), whereas the overall carbon consumption rate at 21% O2 was smaller in an oxy-fuel atmosphere than in air. In addition, subsequent analysis revealed that CO2 gasification accounted for up to 20% of the char consumption rate for lignite (at 4% O2 and 1573 K). Similarly, Kim et al.’s [5] calculations demonstrated that the carbon consumption rate increased due to the contribution of the direct gasification reaction to carbon consumption. Recently, Wang et al. [12] investigated the effect of key parameters (such as O2 concentration and bed temperature) on the char combustion characteristics in a fluidized bed (FB) system and found that the CO2 gasification can increase the char consumption rate to some extent, especially at elevated bed temperature and reduced O2 concentration. By comparing the combustion behaviors in the oxy-fuel atmospheres with and without steam addition, Riaza et al. [13] observed that the ignition temperatures increased and the coal burnout values decreased in an entrained flow reactor (EFR) as the steam was added into the oxy-fuel atmospheres. Taking into account the combined effect of CO2 and H2O, Gharebaghi et al. [14] set up a modified char burnout kinetic (CBK8) model to predict the char burnout during oxy-fuel combustion and found that the model was capable of predicting the burnout process with reasonable accuracy. Geier et al. [15] established an extended single-film reaction model that included both CO2 and H2O gasifications to determine the char particle temperature; their results showed that the predicted temperatures were in good agreement with those of tests in oxy-fuel atmospheres. A further study by Hecht et al. [2] revealed that CO2 and H2O gasifications reduced the rate of char oxidation by lowering the char temperature but also promoted char consumption by reacting with carbon directly; the final result was that the gasifications increased the char consumption rate by approximately 10% in oxy-fuel environments. Singer et al. [16] used a CFD simulation of a pilot-scale oxy-fuel test facility to evaluate the effects of the local environment and char conversion on the importance of the gasification reactions in determining the overall rate of char consumption and concluded that char gasification should not be neglected without first considering the particular char kinetics and trajectories through the furnace. Yi et al. [17] determined, through thermogravimetric analysis experiments under low heating rates, that adding H2O into an oxy-fuel atmosphere retarded the ignition but promoted the burnout of pulverized coal. A single-particle combustion stand (SPC stand) was
2. Experimental section 2.1. Materials A typical Chinese coal, Datong (DT), was chosen for this study. The proximate and ultimate analyses of the pulverized coal are shown in Table 1. The coal samples were pulverized and then screened between 75 and 96 μm. 2.2. Apparatus and procedure The isothermal thermal analysis system is presented in Fig. 1. The main equipment of the system includes a tube furnace, a data acquisition system and a steam generator, etc. More details on the system can 309
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Table 1 Properties of Datong coal.a Coal sample
Datong a b
Proximate analysis/mass %
Ultimate analysis/mass %
Moisture
Volatile
Ash
Fixed Carbon
Carbon
Hydrogen
Nitrogen
Sulfur
Oxygenb
3.27
22.71
24.25
49.77
61.22
3.42
0.77
1.13
5.94
Air-dried basis. Calculated by difference.
be found in a previous report [23]. In the tests, the gas was depressurized and then discharged into the furnace through the flowmeter; the water vapor was generated by the steam generator and carried by the gas into the furnace. All of the tests followed the same procedure: First, the furnace was heated to a given temperature. After flushing the furnace with N2 for 15 min, the gas supply was switched to the reaction gas at 1 L/min. Approximately 80 ± 0.05 mg of the coal sample was spread evenly in a quartz boat, and then the boat was inserted into the furnace to complete the burning process. In the experimental system, the quartz boat was fed into the tube furnace within a very short time because of the high speed of the guide rail. Therefore, a small amount of coal sample could be ignited at a relatively high heating rate because of the large quantities of heat provided by the furnace. The weight signals were monitored continuously during the test. The buoyancy effects were eliminated through evaluation of blank experiments. When the test was completed, the ash was removed from the quartz boat and analyzed using a Bruker D8 Advance X-ray diffractometer (XRD). To study the combustion characteristics of the pulverized coal, the conversion rate X is introduced and defined as below:
X=
m 0−mt × 100% m 0−mc
100 o
t=800 C
X/%
80
2%O2+98%CO2
60
2%O2+98%N 2
40
5%O2+95%N 2
20
10%O 2+90%N 2
5%O2+95%CO 2 10%O 2+90%CO 2 21%O 2+79%CO 2 21%O 2+79%N 2
0 0
200
400
600
800
time/s
1000
1200
processes of O2/N2 and O2/CO2. At low O2 concentrations (i.e., high CO2 concentrations), the specific heat of O2/CO2 was much higher than that of O2/N2 and the diffusion coefficient of O2 in CO2 was substantially lower than that in N2. Meanwhile, the burning rates of the samples were relatively low, which enhanced the effect of the physical properties of mixtures on the combustion characteristics. As the O2 concentration increased, the burning rate increased and the differences in the physical properties between gas mixtures decreased, which reduced the effect of the physical properties and thus gradually made the burning processes in different atmospheres more similar. Fig. 3 shows the conversion rate of DT coal in O2/N2 and O2/CO2 at 1000 °C. A comparison of Figs. 2 and 3 reveals that the elevated temperature increased the conversion rate of pulverized coal and shortened the burnout time at the same O2 concentration. Because of the differences in the physical properties of N2 and CO2, the conversion rate in O2/CO2 was still lower than that in O2/N2. However, when the temperature increased to 1000 °C, the gap between the burning processes in O2/N2 and O2/CO2 decreased at low O2 concentrations compared with the gap at 800 °C, whereas the gap between burning rates in the different atmospheres increased again at higher O2 concentrations. The gasification had a limited effect on the burning process, likely
where m0 is the initial mass of the sample, mt is the mass of the sample at time t, and mc is the remaining mass of the sample after burnout. 3. Results and discussion 3.1. Combustion characteristics 3.1.1. CO2 effect The effect of CO2 on the combustion characteristics of DT coal was investigated under different O2 concentrations at 800 °C; the results are shown in Fig. 2. As seen in the figure, with increasing O2 concentration, the conversion rates of pulverized coal in O2/CO2 and O2/N2 were both accelerated, which decreased the burnout time. Compared to the conversion rate and burnout in O2/N2, the conversion rate was slowed and the burnout was delayed in O2/CO2 because of the greater specific heat of CO2 and the lower diffusion coefficient of O2 in CO2 [4,5]. However, the increased O2 concentration reduced the gap between the burning Heater band
Tube furnace Quartz boat
Gas buffer
High-temperature resistant stent
Flowmeter Pressure reducing valve
Computer
Steam generator Temperature controller O2
1600
Fig. 2. Conversion rates of DT coal in O2/N2 and in O2/CO2 at 800 °C.
(1)
N2
1400
CO2
Temperature controller for heater band
Guide rail
Fig. 1. Isothermal thermal analysis system.
310
Data acquisition system
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100
100 o
80
t=1000 C
80
o
t=1200 C
2%O2+98%N 2
60
X /%
X/%
2%O2+98%CO 2 5%O2+95%CO 2 5%O2+95%N 2
40
2%O2+98%CO2
60
2%O2+98%N2 5%O2+95%CO2
40
5%O2+95%N2
10%O 2+90%CO 2 20
10%O 2+90%N 2
0
21%O 2+79%N 2
10%O2+90%CO2 10%O2+90%N2
20
21%O2+79%CO2
21%O 2+79%CO 2
0
200
400
600
time/s
800
1000
21%O2+79%N2
0 0
1200
100
200
300
400
time/s
500
600
700
800
Fig. 4. Conversion rate of DT coal in O2/N2 and O2/CO2 at 1200 °C.
Fig. 3. Conversion rate of DT coal in O2/N2 and O2/CO2 at 1000 °C.
was maintained at a low level of 2%, the gap between the gasification rate and the combustion rate was further reduced compared to that under similar conditions at 1000 °C and the effect of gasification was enhanced. These results indicate that the magnitude of the increase in the carbon consumption rate for the reaction between char and CO2 was greater than the magnitude of the decrease in the combustion rate (because of the lower particle temperature caused by endothermic gasification and the physical properties of CO2), which increased the overall reaction rate in O2/CO2. As a result, the burnout time of pulverized coal in O2/CO2 was lower than that in O2/N2. When the O2 concentration increased to 5%, the influence of gasification on the burning process decreased because of the increase in the combustion rate and the decrease in the gasification rate; however, the promoting effect of gasification on the carbon consumption rate was still stronger than its inhibiting effect (as a result of being endothermic and because of the physical properties of CO2) on the combustion rate. As the O2 concentration was increased further, the difference between the combustion rate and the gasification rate again increased; thus, the influence of CO2 gasification weakened, whereas the effect of the physical properties (greater specific heat and lower O2 diffusivity) was enhanced, which led to an earlier burnout in O2/N2 than in O2/CO2.
because the reaction rate of CO2 gasification was relatively low (800 °C); the differences in the combustion characteristics of O2/N2 and O2/CO2 were mainly due to the differences between the physical properties of N2 and CO2. When the temperature increased to 1000 °C, the CO2 gasification accelerated, producing two different effects: (1) strongly endothermic gasification, which can lower the particle temperature, leading to a decrease in the char oxidation, and (2) direct reaction of CO2 with the char, increasing the overall carbon consumption rate. Actually, the effect of CO2 on the overall coal consumption rate (combustion and gasification) was controlled by competition between the combustion and gasification processes and the physical properties of CO2. At low O2 concentrations, although the gasification rate was lower than the combustion rate, the difference between the reaction rates of gasification and combustion was less than that at high O2 concentrations. A comparison of Fig. 2 with Fig. 3 reveals that the gap between the burning processes of O2/N2 and O2/CO2 was reduced with increasing temperature, which indicated that CO2 gasification increased the carbon consumption rate more than it decreased the combustion rate (because of the lower particle temperature caused by endothermic gasification and the physical properties of CO2), and as a result the overall reaction rate of pulverized coal increased in O2/CO2. As the O2 concentration increased, the combustion rate accelerated and the gasification rate decreased, which meant that the gap between the gasification rate and the combustion rate increased gradually, and thus the influence of gasification on the char consumption rate diminished and the burning process was mainly controlled by combustion. Without consideration of the gasification reaction, the combustion reaction was dominated by the physical properties of reaction atmosphere. And because of the distinct physical properties of CO2 (greater specific heat and lower O2 diffusivity), the burning process of pulverized coal in O2/CO2 lagged behind that in O2/N2. Fig. 4 shows the conversion rate of DT coal in O2/N2 and O2/CO2 at 1200 °C. A comparison of Fig. 4 with Figs. 2 and 3 shows that the conversion rate of pulverized coal was further accelerated when the temperature increased to 1200 °C. However, unlike at 1000 or 800 °C, the burnout in O2/CO2 at 1200 °C substantially decreased compared to that in O2/N2 with 2% O2. When the O2 concentration increased to 5% and the gap between the burning processes in the two different atmospheres diminished, the burnout in O2/CO2 remained ahead of that in O2/N2. However, when the O2 concentration was increased further, the burnout in O2/CO2 eventually lagged that in O2/N2. When the temperature was increased from 1000 °C to 1200 °C, both the combustion rate and the gasification rate accelerated; however, the gasification rate may have increased more significantly than the combustion rate because the combustion reaction likely preceded the gasification reaction in transitioning from Zone I into Zone II. When the O2 concentration
3.1.2. CO2 and H2O effects On the basis of the analysis presented in Section 3.1.1, the combined effect of CO2 and H2O on the combustion characteristics of DT coal was investigated by adding water vapor to the reaction atmosphere. Fig. 5 shows the conversion rate of DT coal in O2/N2, O2/CO2 and O2/H2O/CO2 atmospheres at 800 °C. As shown in the figure, the conversion rate of pulverized coal with 2% O2 in O2/H2O/CO2 was slightly faster than that in O2/CO2 but still slower than that in O2/N2. As the O2 concentration was increased to 10%, the gaps among the burning processes of the different atmospheres were gradually minimized. At 800 °C, the H2O gasification likely had a limited effect on the burning process because of its relatively low reaction rate; thus, the effect of the H2O physical properties was mainly considered. The specific heat of H2O is greater than that of N2 but less than that of CO2, and the diffusion coefficient of O2 in H2O is also between those of O2 in N2 and in CO2 [17]. Therefore, the temperature of the coal particles increased and the diffusion of O2 to the particles was accelerated by replacing part of the CO2 with H2O in the gas mixture, resulting in a faster conversion rate for pulverized coal. However, the conversion rate in O2/H2O/CO2 was still slower than that in O2/N2. As the O2 concentration increased, the burning rate of pulverized coal increased and the differences in physical properties among the three atmospheres decreased, which mitigated the influence of the gases’ physical properties. Fig. 6 shows the conversion rate of DT coal in O2/N2, O2/CO2 and O2/H2O/CO2 atmospheres at 1000 °C. A comparison of Figs. 5 and 6 311
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conversion rates in the order of O2/N2, O2/H2O/CO2, O2/CO2. Fig. 7 shows the conversion rate of DT coal in O2/N2, O2/CO2, O2/ H2O/CO2 at 1200 °C. As shown in the figure, the conversion rates of pulverized coal decreased in the order of O2/H2O/CO2, O2/CO2, O2/N2 in 2% O2, whereas the gaps among the burning processes of the different atmospheres were substantially greater than those at 1000 °C. When the O2 concentration increased to 10%, the burning processes in the different atmospheres became more similar; however, the conversion rate in O2/H2O/CO2 was still the highest, followed by those in O2/ N2 and O2/CO2. As previously mentioned, the growth of the gasification rate exceeded that of the combustion rate as the temperature increased, and, because the burning rate was relatively slow at low O2 concentrations, the influence of CO2/H2O gasification was enhanced, which led to an obvious increase of the overall reaction rate of pulverized coal. As the O2 concentration increased, the influence of the gasification was weakened because of the increased combustion rate. However, the CO2/H2O gasification can still accelerate the conversion of pulverized coal to a certain extent because of its faster gasification rate compared with that of CO2 gasification. As shown in Fig. 8, three different H2O concentrations, 10%, 20% and 30%, were selected to study the effect of variable H2O concentrations on the overall reaction rate of pulverized coal during oxy-fuel combustion. It can found that the conversion rates of pulverized coal increased slightly with increasing H2O concentration in 2% O2. According to Bai et al. [24], the char gasification rate in the CO2/H2O mixture increased monotonously with increasing H2O concentration. Thus, at elevated H2O concentration, the influence of CO2/H2O gasification can be enhanced on the basis of above analysis, and thus the overall reaction rate of pulverized coal increased accordingly. However, in 10% O2, it was hardly to distinguish the effect of variable H2O concentrations on the overall reaction rate, because the effect of the gasification was weakened with increasing O2 concentration.
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Fig. 7. Conversion of DT coal in O2/N2, O2/CO2 and O2/H2O/CO2 at 1200 °C.
Fig. 5. Conversion rate of DT coal in O2/N2, O2/CO2 and O2/H2O/CO2 atmospheres at 800 °C.
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reveals that the conversion rate of pulverized coal was accelerated and that the burnout time decreased with increasing temperature. In 2% O2, the conversion rate of pulverized coal in O2/H2O/CO2 was the fastest, followed by that in O2/N2 and in O2/CO2. However, at 10% O2, the conversion rate in O2/N2 was the fastest, followed by those in O2/H2O/ CO2 and O2/CO2, and the gaps among the burning processes in the three different atmospheres were reduced compared to that under low O2 concentrations. When the temperature increased to 1000 °C, the reaction rates of H2O gasification and CO2 gasification both accelerated and thus the gasification reactions had the potential to affect the burning process, especially at low O2 concentration. Bai et al. [24] investigated the gasification reactivity of coal chars using CO2/H2O mixtures and found that the char gasification rate in the CO2/H2O mixture was much higher than that in pure CO2, indicating that the synergy existed during char gasification. Therefore, in 2% O2, compared with the effect of CO2 gasification alone, the effect of CO2/H2O gasification was enhanced due to the synergistic effect [24]. As a result, the carbon consumption rate for CO2/H2O reacting with char directly increased more than the combustion rate decreased at (because of a decrease in the particle temperature caused by the endothermic gasification and by the physical properties of CO2 and H2O), which led to an increase in the overall reaction rate of pulverized coal. As the O2 concentration increased, the burning rate accelerated and the gasification rate slowed; thus, the influence of CO2/H2O gasification was weakened. The burning process of pulverized coal was mainly affected by the physical properties of CO2 and H2O, which resulted in a decrease in the
3.2. Ash formation Fig. 9 presents the XRD patterns of DT coal ashes formed under 2% O2 at 1000 °C. The major minerals in the DT ashes were quartz, hematite and anhydrite. Few differences were observed in the main minerals among the ashes generated in O2/N2, O2/CO2 and O2/CO2/H2O, which indicated that the transformation of minerals was not strongly affected by CO2 and H2O (either their physical or chemical effects) at 1000 °C. The XRD patterns of ashes formed in 2% and 21% O2 at 1200 °C are shown in Figs. 10 and 11, respectively. A comparison of Fig. 9 with Figs. 10 and 11 reveals stronger mineral peaks in the patterns of the ashes generated at 1200 °C, implying that the crystallinity of the minerals increased at higher temperatures. The anhydrite likely decomposed because of its lower decomposition temperature. Peaks 312
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Fig. 8. Conversion of DT coal under different H2O concentrations at 1200 °C.
longer than that of DT coal in 10% O2; thus, more quartz likely transformed to cristobalite over this extended period of time.
attributable to the typical high-temperature mineral, mullite, was also identified in the XRD pattern. Additionally, the cristobalite in the ash disappeared as the O2 concentration increased from 2% to 21% at 1200 °C. Figs. 10 and 11 also show that the main mineral phases identified in the different ashes were approximately the same, implying that CO2 and H2O did not substantially influence the transformation of mineral phases. Sheng et al. [21] have investigated the mineral transformations of coal ash under air and O2/CO2 atmospheres and found that O2/CO2 combustion did not substantially change the mineral phases, consistent with the experimental results presented here. Pagliari et al. [25] found that the formation of cristobalite from quartz was substantially enhanced as the reaction time increased at temperatures greater than 1200 °C. As shown in Fig. 7, the reaction time of DT coal in 2% O2 was
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4. Conclusions An isothermal thermal analysis system, which can be operated at high-heating-rate, were used to investigate the effects of CO2 and H2O on the combustion characteristics and ash formation of pulverized coal under oxy-fuel conditions. The results from the study demonstrated that: 1. Neglecting the influence of CO2 gasification at 800 °C, the conversion rate of pulverized coal in O2/N2 was faster than that in O2/CO2 due to the differences in the physical properties between N2 and
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Fig. 9. XRD patterns of DT coal ashes generated under 2% O2 at 1000 °C.
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Q
Fig. 10. XRD patterns of DT coal ashes generated under 2% O2 at 1200 °C.
2%O2+20%H2O+78%CO2
C-cristobalite H-hematite; M-mullite; Q-quartz;
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Fig. 11. XRD patterns of DT coal ashes generated under 10% O2 at 1200 °C.
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CO2. As the temperature was increased to 1000 °C and then to 1200 °C, the overall reaction rate (combustion and gasification) of pulverized coal in O2/CO2 gradually approached and then exceeded the combustion rate in O2/N2 because of the growing effect of the CO2 gasification reaction. However, the effect of gasification was weakened with increasing O2 concentrations, and thus the overall reaction rate of pulverized coal in O2/CO2 eventually lagged the combustion rate in the O2/N2 due to the different physical properties of the gas mixtures. 2. Before the CO2 and H2O gasification exerted a clear effect at 800 °C, the conversion rate of pulverized coal in O2/H2O/CO2 was lower than that in O2/N2 but higher than that in O2/CO2, because the specific heat of H2O was higher than that of N2 but less than that of CO2 and because the O2 diffusivity in H2O was also between that in N2 and that in CO2. At low O2 concentrations, with increasing temperature, the overall reaction rate in O2/H2O/CO2 increased compared to that in O2/CO2 due to the enhanced effect of CO2 and H2O gasifications. However, as the O2 concentration increased, the burning process in the two atmospheres became more similar due to the reduced effect of CO2 and H2O gasification. Increasing H2O concentration in the gas mixtures can slightly increase the conversion rates of pulverized coal at low O2 concentrations due to the enhanced effect of CO2/H2O gasification. But the effect of CO2/H2O gasification was weakened at elevated O2 concentration, which made it difficult to distinguish the effect of variable H2O concentrations on the overall reaction rate. 3. According to the XRD analysis of coal ashes, the main minerals between the ashes generated in O2/N2, O2/CO2 and O2/CO2/H2O differed only slightly, which indicated that CO2 and H2O did not strongly influence the transformation of the main minerals.
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Acknowledgements This work was supported by the National Key R&D Program of China (2016YFB0600701), the Hebei Province Natural Science Foundation (E2016502094) and the Fundamental Research Funds for the Central Universities (2017MS120). References [1] L. Chen, S.Z. Yong, A.F. Ghoniem, Oxy-fuel combustion of pulverized coal: characterization, fundamentals, stabilization and CFD modeling, Prog. Energy Combust. 38 (2012) 156–214. [2] E.S. Hecht, C.R. Shaddix, M. Geier, et al., Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char, Combust. Flame 159 (2012) 3437–3447.
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