Fly Ash Deposition during Oxy-fuel Combustion in a Bench-Scale

Jul 22, 2013 - Particulate matter with aerodynamic diameters less than 10 μm (PM10) was measured by an electrical low-pressure impactor (ELPI), and t...
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Fly Ash Deposition during Oxy-fuel Combustion in a Bench-Scale Fluidized-Bed Combustor Zhimin Zheng, Hui Wang,* Shuai Guo, Yongjun Luo, Qian Du, and Shaohua Wu School of Energy Science and Engineering, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, People’s Republic of China ABSTRACT: Ash deposition on heat-exchanger surfaces in boiler systems can cause numerous problems, including slagging, fouling, and corrosion. These deleterious processes can be compounded if the boiler combustion process is changed from air to oxy-fuel. In this paper, fly ash deposition characteristics under both air and oxy-fuel combustion conditions were investigated using a bench-scale fluidized-bed combustor (FBC) based on measurements of ash deposition rates via a temperature-controlled probe. Three different combustion atmospheres were studied, and results demonstrated that, under similar combustion temperature profiles and equivalent fluidization velocities, the deposition rate increased when transitioning from combustion atmospheres consisting of 21% O2/79% CO2 to air to 30% O2/70% CO2. To determine the primary factors associated with the observed variations in deposition rates, the chemical compositions and micromorphologies of ash and fly ash deposits were analyzed by inductively coupled plasma−atomic emission spectrometry (ICP−AES) and scanning electron microscopy (SEM). Particulate matter with aerodynamic diameters less than 10 μm (PM10) was measured by an electrical low-pressure impactor (ELPI), and the particle size distributions (PSDs) and carbon contents of the collected filter ash were also ascertained. The results indicate that the higher deposition propensity associated with a 30% O2/70% CO2 atmosphere can be largely attributed to a wider PSD rather than any changes in the chemical compositions of the fly ash or deposited ash, in which there are no obvious differences between air and oxy-fuel combustion. In addition, the slightly higher concentration of fine particles produced under this atmosphere also promotes the deposition of fly ash. flue gases and gas flow field as well as a wider particle size distribution (PSD) of the ash in the case of oxy-fuel combustion. Yu et al.12 obtained similar results when studying the ash deposition behavior under the 32% O2/68% CO2 atmosphere compared to an air-fired system. Interestingly, Li et al.13 obtained completely opposite results during studies of ash deposition and reported that a lower Stokes number and slightly smaller bulk particles resulted from combustion under a 30% O2/70% CO2 atmosphere and led to reduced ash deposition rates. The authors concluded that their contrary results were largely due to the differences between a drop-tube furnace (DTF) and a one-dimensional combustor, meaning that the observed variations in experimental outcomes resulted from different experimental setups. All of the studies noted above were performed during oxyfuel combustion in PC boilers, while relatively little research has been devoted to studying ash deposition in CFB boilers during oxy-fuel combustion. There will be some differences in the ash generated by CFB and PC boilers because of the different combustion temperatures, combustion modes, and feed coal particle sizes between the two boiler types, all of which directly affect ash formation.14 Wu et al.9 investigated characteristics of the ash produced by a 100 kWth pilot-scale CFB boiler during oxy-fuel combustion incorporating a limestone sorbent but paid more attention to the desulfurization mechanism and combustion process rather than fly ash deposition. It should

1. INTRODUCTION Oxy-fuel combustion (O2/CO2) is one of several advanced carbon capture and storage (CCS) technologies and has been studied primarily with regard to its application in pulverized coal (PC) and circulating fluidized-bed (CFB) boiler systems. In comparison to oxy-fuel combustion in a PC boiler, oxy-fuel combustion in a CFB system has several advantages, with one being that the bed temperature can be controlled by adjusting the amount of circulating ash, which allows for higher O2 concentrations with no risk of drastically increasing the furnace temperature. As a result, oxy-fuel combustion can significantly reduce the amount of recycled flue gas and the volume of the boiler island, which, in turn, lowers both the operating and running costs. Because of the differences between oxy-fuel combustion and air combustion atmospheres, various groups have researched ignition and devolatilization,1 flame propagation properties,2 SO2 and NOx emissions,3−6 ash formation,7−9 and other aspects of oxy-fuel boiler processes. Few studies, however, have focused on ash deposition rates in boilers operating under oxy-fuel combustion conditions. Fryda et al.10,11 studied the ash formation and deposition behaviors of coal and coal/biomass blends during oxy-fuel combustion, using a lab-scale pulverized coal combustor or drop-tube apparatus. This work determined that combustion under a 30% O2/70% CO2 atmosphere produced increases in both the deposition rate and deposition propensity compared to combustion in an air-fired system but also found that there were no significant differences in the chemical compositions of fly ash and ash deposits between oxy-fuel and air combustion. It was concluded that the observed increase in deposits under oxy-firing was due to changes in the physical properties of the © 2013 American Chemical Society

Received: April 26, 2013 Revised: July 20, 2013 Published: July 22, 2013 4609

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Table 1. Ultimate and Proximate Analyses of the Coal Sample proximate analysis (%)

ultimate analysis (%)

heating value

sample

Mar

Var

Aar

FCar

Car

Har

Nar

Sar

Oar

Qnet,ar (kJ/kg)

Jincheng anthracite

4

6.72

30.24

59.04

59.35

2.56

0.72

1.84

1.29

20.48

Table 2. Ash Composition of the Coal Sample compound

Na2O

MgO

Al2O3

SiO2

P2O5

SO3

K2O

CaO

TiO2

Fe2O3

wt %

0.73

0.17

36.04

44.23

0.23

2.67

1.04

2.99

0.69

5.27

also be noted that excess free CaO in fly ash deposits on the heat-transfer surfaces located at the back end of boilers can react with CO2 in the flue gas and form hardened deposits. Pilot experimental work with an oxy-fuel CFB combustor has confirmed the occurrence of carbonization,15 and Mönckert et al.16 have shown that both sulfatization and carbonization occur in deposits under oxy-fuel combustion conditions. Wang et al.17 conducted a series of tests in a thermogravimetric analyser (TGA) to study fly ash carbonation under oxy-fuel combustion conditions and found that increases in both CO2 concentrations and temperature will significantly increase the carbonation conversion ratio. In addition, it is important to keep in mind that ash deposition is also associated with the formation of fine ash.18 When fine particles in the molten state deposit on heat-exchanger surfaces, these surfaces become stickier and, consequently, retain coarse ash particles more easily. Other researchers have also reported higher proportions of larger mass particles in the fine particulate matter generated under oxy-fuel combustion running at higher O2 concentrations compared to air combustion.13,19 In general, there is still a lack of knowledge concerning ash deposition on heat-exchanger surfaces in a CFB combustor operating under oxy-fuel conditions, and therefore, further investigations are necessary. The goal of our work was therefore to investigate fly ash deposition on heat-exchanger surfaces during oxy-fuel combustion and compare the results to observations from air combustion systems without calciumbased sorbents. Experiments attempting to examine the dominant factors leading to differences in ash deposition were conducted using a bench-scale fluidized-bed combustor (FBC) with a temperature-controlled ash deposit probe employed as a substitute for the heat-exchanger surface. To further examine any differences between combustion modes, the particulate matter with aerodynamic diameters less than 10 μm (PM10) contents of ash deposits were also measured by an electrical low-pressure impactor (ELPI). The combustion apparatus employed in this work was based on a bubbling fluidized bed without recycling of flue gas and ash but still allowed us to obtain suitable levels of performance and operating conditions.

Figure 1. PSD of the coal sample. 2.2. FBC Apparatus: Description and Operating Conditions. The fly ash deposition experiments were conducted in a bench-scale FBC, as shown in Figure 2. The main body of the combustor consists of a preheating section, a furnace section (containing both dense phase and dilute phase portions), and a convective section, in addition to a cyclone and a filter bag. The total height of the combustor is 3500 mm, with the preheating section being 385 mm, the dense section being 315 mm, and the dilute phase section being 2800 mm. The inner diameters of the dense section and the dilute section are 51 and 83 mm, respectively, between which there is a transition cone section with a slope of 11. A cyclone was connected to the outlet of the combustor to capture coarse ash, followed by a convective section, which is 850 mm in length and has a 56 mm inner diameter, on which there are three sampling ports to allow for measurement of flue gas, fly ash, and ash deposit, respectively. The apparatus also contains a hightemperature filter bag, which has an 80 mm outer diameter and is 150 mm in length and used to collect fly ash. The temperature of the combustor is controlled by 6, 4, 12, and 4 kW electric heaters in the four sections. The combustor is equipped with a screw feeder, which allows for a steady coal feed rate to be set between 5 and 20 g/min. The flows of O2 and CO2 into the apparatus are individually controlled by a mass flow control system, and these gases are subsequently mixed in a static mixer and then introduced into the furnace after the preheating section as fluidization gas. Switching the apparatus between air- and oxy-fuel-fired combustion modes is simple and easy to control, and during these trials, three combustion atmospheres were applied: pure air, 21% O2/79% CO2, and 30% O2/70% CO2. According to literature reports,10−13 there are many possible factors that may affect fly ash deposition, and two of the most important factors are the extent of fly ash formation and the flue gas flow field. To determine the primary causes of observed variations in fly ash deposition between different combustion conditions, some potential variables need to be held constant. For this reason, the same fluidization gas velocity was applied when working with all three combustion atmospheres, to diminish the effects of variations in flue gas velocity. Holding the fluidization gas velocity constant produces

2. EXPERIMENTAL SECTION 2.1. Fuel and Bed Materials. Samples of anthracite coal (from the Jincheng mine in China) were obtained and sieved, and the results of ultimate and proximate analyses of this coal are listed in Table 1, while the chemical composition of its ash is provided in Table 2. The particle sizes of the sieved coal ranged from 0 to 2.36 mm, with a mean diameter of 0.883 mm, and the PSD is shown in Figure 1. During experimental trials, quartz sand with a particle size range of 0.18−0.55 mm and a mean particle diameter of 0.235 mm was used as the bed material and an initial quantity of approximately 150 g was loaded as bed material prior to heating. No sorbents were used in this study. 4610

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Figure 2. Schematic of the bench-scale FBC. the same fluidization states and residence times, both important parameters of fluidized-bed combustion, in all experimental trials. In addition, the velocity of the flue gas can vary the separation efficiency of the cyclone and, thus, indirectly affect fly ash deposition. For all of these reasons, therefore, all work in this study was performed at a constant fluidization gas velocity. Table 3 presents a summary of the experimental trials and the associated system parameters. The excess oxygen coefficient shown

Table 3. List of Combustion Experimental Cases fluidizing gas

superficial gas velocity (m/s)

coal feeding rate (g/min)

gas flow rate (m3/h)

excess oxygen coefficient

air 21% O2/79% CO2 30% O2/70% CO2

0.7 0.7 0.7

10.05 10.05 15.56

5.04 5.04 5.04

1.4 1.4 1.28

Figure 3. Temperature profiles of FBC along the flue gas flow direction.

here is defined as the ratio of actual oxygen to theoretical oxygen demand. The theoretical furnace outlet O2 concentration in the flue gas was as high as 6% for all three combustion atmosphere conditions. During the start-up stage, small amounts of coal were continually fed into the furnace until the bed temperature rose to more than 800 °C and the temperature of the dense phase section reached the desired temperature of 900 °C, allowing for fluctuations of ±10 °C. The FBC temperature profiles observed along the flue gas flow direction when using each of the three combustion conditions are shown in Figure 3. When applying the 30% O2/70% CO2 atmosphere, the temperature in the upper portion of the dense section reached 950 °C, likely because of a higher O2 concentration and a greater quantity of fed coal. The temperature of the cyclone without the application of external heaters was approximately 550 °C, and the average temperature of the convective section was about 770 °C. The flue gases were analyzed by Fourier transform infrared spectroscopy (FTIR), and the concentrations of O2, CO, CO2, SO2,

NOx, and H2O were determined. Flue gas compositions obtained when applying the three combustion conditions are summarized in Table 4, from which it is evident that oxy-fuel combustion has a pronounced effect on the emission of pollutants. The higher O2 concentration and slightly elevated temperature profile under the 30% O2/70% CO2 conditions play an important role in these increased emissions. The lower emissions of SO2 and NOx in the case of the 21% O2/79% CO2 atmosphere are likely related to increased CO, resulting from the reaction of the higher CO2 concentration and char.5 2.3. Ash Sampling and Analysis. The ash deposit and fly ash resulting from each experimental trial were analyzed. Ash deposits were collected using an ash deposition probe, which had a 22 mm outer diameter and was 50 mm in length, positioned horizontally approximately 650 mm from the inlet of the convective section (see 4611

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deposition, because it is normalized to the fuel feeding rate. Figure 4 summarizes the values of DP, from which it can be

Table 4. Flue Gas Composition Tested by FTIR combustion atmospheres flue gas compositions O2 (%) CO (ppm) CO2 (ppm) SO2 (ppm) NOx (ppm) H2O (%)

air 9.52 154 10 840 417 2.7

± ± ± ± ± ±

21% O2/79% CO2 0.2 2 0.2 31 4 0.03

10.06 248 86 718 354 2.7

± ± ± ± ± ±

30% O2/70% CO2

0.1 15 0.2 23 3 0.04

11 239.7 84 1512 488 3

± ± ± ± ± ±

0.5 5 0.4 67 6 0.02

Figure 2). The surface temperature of the probe was maintained at 560 °C by an air-cooled automatic temperature control system to simulate the wall temperature of superheaters and reheaters at the tail of the boiler and could be readily adjusted to different temperatures. The sampling time for the ash deposition tests was 4 h, after which ash deposits were collected and weighed. In the case of fly ash from the convective section, two types of ash samples were obtained: size-graded ash and filter ash. Size-graded ash was collected by ELPI, as shown in Figure 2. During sampling, flue gas from the isokinetic sampling probe enters a cyclone to remove coarse particles with aerodynamic diameters greater than approximately 10 μm and then passes through a two-stage dilution system. The firststage dilution, known as isothermal dilution, has a ratio of 7.29 and uses air heated to 190 °C with flue gas as the dilution gas. The purpose of this stage is to dilute gaseous H2O and H2SO4 as well as the small amount of volatile organic compounds (VOCs) in the flue gas and ensure an unsaturated state. The second dilution stage is referred to as low-temperature dilution and has a ratio of 7.62. In this stage, the gas temperature is decreased to 40 °C to further dilute the concentration of fine particulate matter and prolong the acquisition time of the ELPI; the sample mass on the substrate at each level in the ELPI cannot exceed 1 mg, to avoid errors because of particle interactions. The ELPI contains 13 stages with 50% aerodynamic diameter cutoff values of 0.029, 0.057, 0.093, 0.154, 0.260, 0.380, 0.609, 0.943, 1.590, 2.380, 3.970, 6.650, and 9.860 μm. Polycarbonate membranes were used as stage substrates to collect particle samples for subsequent PSD measurements. The filter ash was collected at the reactor exit with a high-temperature filter bag. The collected ash deposit and filter ash were analyzed via scanning electron microscopy (SEM) and inductively coupled plasma−atomic emission spectrometry (ICP−AES) to ascertain morphology and chemical compositions, respectively. The filter ash was also examined using a Malvern Mastersizer 2000 to determine volume particle size distributions and, in addition, was baked in a muffle furnace to determine the carbon content.

Figure 4. Deposition propensities in different combustion conditions.

seen that DP was higher under air combustion compared to 21% O2/79% CO2 conditions but obviously lower than when using a 30% O2/70% CO2 atmosphere. These results show that increasing the oxygen concentration can result in a higher ash deposition propensity, while increases in CO2 have the opposite effect at the same oxygen concentration. These observations are consistent with those reported previously in the literature,10,12 although the reasons for this phenomenon are still uncertain. Experiments under all three combustion modes were performed using the same fluidization velocity and similar levels of outlet theoretical oxygen concentration (approximately 6%); therefore, the flue gas velocities near the ash deposition probe as well as the temperatures were basically equivalent between all three conditions. Neglecting any effects of the change in flue gas compositions, the effects of flue gas flow on fly ash deposition can be ignored, and therefore, differences in fly ash deposition should be attributed to changes in the characteristics of the ash deposit and fly ash, such as variations in the morphology or chemical composition of the ash deposit, as well as changes in the particle size distribution, carbon content, or chemical composition of the fly ash. These effects are discussed in the following sections. 3.1.2. Morphology of Ash Deposits. The ash deposits were observed to be gray, weakly bound, and uniformly distributed over the surface of the probe to a thickness ranging between 0 and 2 mm and, thus, would be classified as loose fouling (Figure 5). The majority of each ash deposit appeared on the windward side of the probe, with a small amount on the wing sides but only a very thin layer on the leeward side. Most of the ash particles in the deposits were less than 10 μm in size, and the particles primarily exhibited a flake or flocculated morphology (Figure 6). Closer examination showed that the sub-micrometer particles were spherical and agglomerated, while particles greater than 1 μm size appeared as flakes. Ash deposition mechanisms are known to include inertial impaction, thermophoresis, condensation, and chemical reaction,20 and the dominant deposition mechanism is thermophoresis in the case of fly ash with particle sizes of 0.5−5 μm and inertial impaction for fly ash with particle sizes above 10 μm. On the basis of the macro- and microscopic morphologies of the ash

3. RESULTS AND DISCUSSION 3.1. Ash Deposit. 3.1.1. Deposition Propensity. Two parameters were used to quantify and compare fly ash depositions under the different combustion conditions: deposition rate (DR) and deposition propensity (DP), defined as below. DR = mass of collected ash deposit (g) surface area of the probe (m 2) × deposition time (h) DP =

DR fuel feeding rate (g/h) × ash content (%)

Because of different coal feeding rates under the three combustion conditions (air, 21% O2/79% CO2, and 30% O2/ 70% CO2), the value of DR does not allow for direct comparison of the true deposition tendencies, and therefore, the value of DP was instead used to determine fly ash 4612

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Figure 5. Macromorphology of the ash deposits.

Figure 6. Micromorphology of the ash deposits.

sizes less than 1 μm contain most of the sodium compounds and can deposit on the heat-exchanger surfaces by thermophoresis. Under the 30% O2/70% CO2 atmosphere, the relatively low Na2O content in the ash deposit may be attributed to the higher fly ash deposition rate observed when applying this condition, during which there was a relatively low proportion of the sub-micrometer particles, which would contain Na2O. The data for the SO3 content show a different trend; the relationship of the SO3 content with the combustion atmosphere is 21% O2/79% CO2 < 30% O2/70% CO2 < air, which is consistent with the findings by Yu et al.12 The lower level of SO3 resulting from oxy-fuel combustion might be attributed to the reduced concentrations of alkali metals and alkaline earth metals in the ash deposit. In general, however, alkaline compounds in the ash deposits of oxy-fuel combustion are present at slightly lower concentrations than in the ash resulting from air combustion, which does not appear to facilitate fly ash deposition, and therefore, there must be other reasons for the observed higher ash deposition propensity. 3.2. Fly Ash. 3.2.1. PM10. The particle size distribution of the ash also has an effect on ash deposition, and therefore, the effects of oxy-fuel combustion on the release of fine fly ash particles was evaluated by measuring PM10 using the ELPI (Figure 8). The mass concentration of fine particles is typically reported in units of mass per volume; however, this is difficult in terms of our data because the coal feeding rate was high during the 30% O2/70% CO2 combustion trials compared to other combustion conditions. The effects of different coal feeding rates were eliminated by converting the measured concentration of fine particles from units of mass per volume to units of mass per energy, which more clearly communicates differences in the generation of PM10 under different combustion modes. The PM10 mass distributions are presented in panels a and b of Figure 8 in mass per volume and mass per energy, respectively. The PM10 emission distributions resulting from all three combustion atmospheres were similar. When mass per volume is used (as in Figure 8a), it appears that 30% O2 oxy-fuel combustion produces slightly more PM10 than conventional air combustion and significantly more than 21% O2 oxy-fuel combustion. However, when mass per energy is used instead, the differences in the amount of PM10 generated

deposits, we deduced that the dominant deposition mechanisms during these trials were inertial impaction and thermophoresis. Thermophoresis is primarily affected by temperature gradient and particle size, and therefore, because the velocity and temperature at the deposition probe were the same under all three combustion conditions, inertial impaction rather than thermophoresis was considered the cause of the observed variations in ash deposition behavior. 3.1.3. Chemical Compositions of the Ash Deposits. The chemical compositions of the ash deposits are shown in Figure 7. It should be noted that SiO2 and Al2O3 were the main

Figure 7. Chemical compositions of ash deposits.

species, while the others only accounted for relatively small proportions. The ratios of alkali and acid compounds in the ash produced under the three combustion atmospheres were 0.109, 0.092, and 0.086, respectively, which demonstrates that the combustion mode has minimal influence on ash deposit composition. The effect of the combustion mode was evident, however, in the case of Na2O and SO3, which can affect fouling of heating surfaces by promoting ash deposition and corrosion.20 With regard to the Na2O content in the ash, its relationship with the combustion atmosphere was 30% O2/70% CO2 < 21% O2/79% CO2 < air. Typically, fine particles with 4613

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deposits will also interfere with the analysis of test results. The carbon contents found in fly ash produced during combustion under air, 21% O2/79% CO2, and 30% O2/70% CO2 conditions were 5.25, 5.13, and 6.33 wt %, respectively. The carbon content in fly ash generated when using 30% O2/70% CO2 is therefore slightly higher than that produced by the other two combustion conditions, but still within the acceptable range of about 5−20 wt %.24 The carbon contents of the ash deposit samples were not measured, because macroscopic morphological observations showed few carbon particles in the ash deposits. These results indicate that there are only minor differences in the carbon contents of the fly ash, and therefore, the effects of carbon content on ash deposition can essentially be neglected. 3.2.3. PSDs. The PSDs of the filter ash resulting from air and oxy-fuel combustion are similar, although slightly larger particles are generated during the 30% O 2 /70% CO 2 combustion mode (Figure 9), such that the average particle

Figure 8. Emissions of PM10 in different combustion conditions.

between 30% O2 oxy-fuel and air combustion are largely reduced, especially with regard to fine particles with aerodynamic diameters larger than 0.2 μm. Differences in the concentrations of particles with aerodynamic diameters less than 0.2 μm are still fairly obvious between the three combustion atmospheres, which shows that increasing O2 concentrations have an effect on the formation of fine particles (those smaller than 0.2 μm), primarily because of greater vaporization and nucleation under the elevated O2 concentration associated with oxy-fuel combustion.13,19 The slightly higher temperature profile of 30% O2/70% CO2 combustion compared to those of air and 21% O2/79% CO2 combustion also contributes to this effect. These results agree with the findings of other studies.8,21,22 The lowest levels of PM10 are observed in the case of 21% O2/79% CO2 combustion, likely because the lower particle temperatures associated with that combustion atmosphere affect the vaporization and fragmentation of inorganic matter because of the higher thermal capacity of CO2 and the lower diffusivity of O2.21 In addition, it is seen that a small quantity of sub-micrometer particles are produced under each atmosphere, accounting for 2.24, 1.94, and 2.39% of the ash resulting from air, 21% O2/79% CO2, and 30% O2/70% CO2 atmospheres, respectively. From these results, under the 30% O2/70% CO2 atmosphere, a slightly higher concentration of fine particles, especially for those smaller than 0.2 μm, may improve the deposition of fly ash. 3.2.2. Carbon Content. The carbon content of fly ash can affect its deposition,23 and the residual, unburned carbon in ash

Figure 9. PSDs of filter ash in different combustion conditions.

diameters are 5.33, 4.93, and 7.85 μm. The variations in particle size may be due to the effects of a higher oxygen concentration and slightly higher bed temperature profile in the upper portion of the dense phase section during some combustion modes, both of which can enhance char fragmentation. It should be noted that the average particle diameter of the filter ash observed in this work is significantly below that of the fly ash generated in an industrial-size CFB.24 This is likely because the concentration of coarse particles (those in the range of 1−100 μm) exiting from the outlet of the cyclone under bubbling fluidized-bed combustion is lower than that resulting from CFB combustion. The difference in fly ash size may also be at least partly related to variations in the properties of the coal used as fuel in these systems, because the PSD of the coal is known to affect the PSD of the resulting fly ash. The anthracite coal used in our work has elevated carbon and ash contents, and these factors could affect its fragmentation behavior. On the basis of the above analysis, inertial impaction is the dominant deposition mechanism and is influenced by two factors: impaction efficiency and capture efficiency. Capture efficiency is related to the chemical composition of the fly ash particles as well as the surface properties of the ash deposits, which are 4614

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discussed below. Impaction efficiency is highly dependent upon the Stokes number, St, defined by the following equation: St =

combustion modes. There were, however, no marked differences in the chemical compositions of the ash deposits and fly ash among the three different combustion modes. Oxy-fuel combustion modes were shown to affect the emission of PM10 particulates. The PM10 emission distributions resulting from all three combustion atmospheres were similar, but the mass concentration of PM10 generated when applying the 30% O2/ 70% CO2 combustion mode was slightly higher, especially in the case of particles with aerodynamic diameters less than 0.2 μm, which might also be an important factor related to the increased ash deposition rates seen when working under this combustion mode. We also did find that the average particle size of fly ash generated by 30% O2/70% CO2 combustion was slightly larger than that resulting from the other two combustion modes, which could be the cause of the higher deposition rate seen when operating under a 30% O2 atmosphere, because of the higher fragmentation of coal particles in a high oxygen environment.

ρp Vpd p2 9μg Dprobe

Here, ρp is the particle density (kg/m3), dp is the particle diameter (m), Vp is the particle velocity (m/s), μg is the dynamic viscosity (Pa s), and Dprobe is the probe diameter (m). The viscosity of N2 at 900 °C is 4.83 × 10−5 Pa s, while that of CO2 is 4.70 × 10−5 Pa s. Assuming that the fly ash density is uniform, the Stokes number of the particles is dependent upon the particle diameters and, according to the relationship between ash diameter, Stokes number, and impaction efficiency,13 both the Stokes number and the impact efficiency will increase with an increasing particle diameter. As a result, a wider particle diameter distribution is also an important factor responsible for the observed higher deposition rate observed during 30% O2/70% CO2 combustion. 3.2.4. Chemical Compositions of Fly Ash. The chemical compositions of the filter ash generated during each of the three combustion modes are very nearly identical, with the exception of the levels of SO 3 (Figure 10); the SO 3

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-451-86413231. Fax: +86-451-86412528. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the National Science Foundation for Young Scientists of China (Grant 51106038).

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REFERENCES

(1) Molina, A.; Shaddix, C. R. Proc. Combust. Inst. 2007, 31, 1905− 1912. (2) Kiga, T.; Takano, S.; Kimura, N.; Omata, K.; Okawa, M.; Mori, T.; Kato, M. Energy Convers. Manage. 1997, 38, 129−134. (3) Jia, L.; Tan, Y.; Anthony, E. J. Energy Fuels 2010, 24, 910−915. (4) Jia, L.; Tan, Y.; Anthony, E. Proceedings of the 20th International Conference on Fluidized Bed Combustion; Xian, China, 2010; pp 936− 940. (5) Duan, L.; Zhao, C.; Zhou, W.; Qu, C.; Chen, X. Int. J. Greenhouse Gas Control 2011, 5, 770−776. (6) Lupiáñez, C.; Guedea, I.; Bolea, I.; Díez, L. I.; Romeo, L. M. Fuel Process. Technol. 2012, 106, 587−594. (7) Sheng, C.; Li, Y.; Liu, X.; Yao, H.; Xu, M. Fuel Process. Technol. 2007, 88, 1021−1028. (8) Sheng, C.; Li, Y. Fuel 2008, 87, 1297−1305. (9) Wu, Y.; Wang, C.; Tan, Y.; Jia, L.; Anthony, E. J. Appl. Energy 2011, 88, 2940−2948. (10) Fryda, L.; Sobrino, C.; Cieplik, M.; van de Kamp, W. L. Fuel 2010, 89, 1889−1902. (11) Fryda, L.; Sobrino, C.; Glazer, M.; Bertrand, C.; Cieplik, M. Fuel 2012, 92, 308−317. (12) Yu, D.; Morris, W. J.; Erickson, R.; Wendt, J. O. L.; Fry, A.; Senior, C. L. Int. J. Greenhouse Gas Control 2011, 5 (Supplement1), S195−S167. (13) Li, G.; Li, S.; Dong, M.; Yao, Q.; Guo, Y.; Axelbaum, R. L. Fuel 2013, 106, 544−551. (14) Arro, H.; Pihu, T.; Prikk, A.; Rootamm, R.; Konist, A. Proceedings of the 20th International Conference on Fluidized Bed Combustion; Xian, China, 2009; pp 1054−1060. (15) Nsakala, N. y.; Liljedahl, G. N.; Turek, D. G. Commercialization Development of Oxygen Fired CFB for Greenhouse Gas Control; U.S. Department of Energy: Pittsburgh, PA, 2007; PPL Report PPL-07-CT20.

Figure 10. Chemical compositions of filter ash in different combustion conditions.

concentrations in the filter ash increase in the order 30% O2/ 70% CO2 < 21% O2/79% CO2 < air. Interestingly, these results contradict the findings of prior studies12 because the retention of SO3 in fly ash resulting from the 30% O2/70% CO2 combustion mode was not observed. There are no obvious differences in the CaO and MgO levels between all three combustion atmospheres, indicating that the concentrations of these compounds have little effect on SO3 retention, and thus, the reasons for this unusual result are still unclear. In general, the data demonstrate that oxy-fuel combustion does not have a significant effect on the chemical composition of the fly ash and that the viscosity of the fly ash and capture efficiency are also basically unchanged.

4. CONCLUSION Ash deposition experiments were conducted in a bench-scale FBC under both oxy-fuel and air combustion conditions. The results showed that the propensity for fly ash deposition during 30% O2/70% CO2 combustion was significantly higher than during either the air combustion or 21% O2/79% CO2 4615

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(16) Mönckert, P.; Dhungel, B.; Kull, R.; Maier, J. Proceedings of the 3th International Energy Agency (IEA) Greenhouse Gas R&D Programme (IEA GHG) Internationl Oxy-Combustion Workshop; Yokohama, Japan, 2008. (17) Wang, C.; Jia, L.; Tan, Y.; Anthony, E. J. Fuel 2008, 87, 1108− 1114. (18) Valmari, T.; Kauppinen, E. I.; Kurkela, J.; Jokiniemi, J. K.; Sfiris, G.; Revitzer, H. J. Aerosol Sci. 1998, 29, 445−459. (19) Qu, C.; Zhao, C.; Zhou, W.; Duan, L. Proc. Combust. Inst. 2011, 33, 2829−2835. (20) Baxter, L. L. Biomass Bioenergy 1993, 4, 85−102. (21) Buhre, B. J. P.; Hinkley, J. T.; Gupta, R. P.; Nelson, P. F.; Wall, T. F. Fuel 2006, 85, 185−193. (22) Sheng, C.; Lu, Y.; Gao, X.; Yao, H. Energy Fuels 2006, 21, 435− 440. (23) Li, S.; Whitty, K. J. Energy Fuels 2009, 23, 1998−2005. (24) Junfu, L.; Wang, Q.; Li, Y.; Yue, G.; Yam, Y.; Eliasson, B.; Shen, J.; Yu, L. Tsinghua Sci. Technol. 2003, 8, 687−691.

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dx.doi.org/10.1021/ef400774b | Energy Fuels 2013, 27, 4609−4616