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

Fly Ash Deposition during Oxy-fuel Combustion in a Bench-ScaleFluidized-Bed CombustorZhimin 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 Republicof 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 tooxy-fuel. In this paper, fly ash deposition characteristics under both air and oxy-fuel combustion conditions were investigatedusing a bench-scale fluidized-bed combustor (FBC) based on measurements of ash deposition rates via a temperature-controlledprobe. Three different combustion atmospheres were studied, and results demonstrated that, under similar combustiontemperature profiles and equivalent fluidization velocities, the deposition rate increased when transitioning from combustionatmospheres consisting of 21% O2/79% CO2 to air to 30% O2/70% CO2. To determine the primary factors associated with theobserved variations in deposition rates, the chemical compositions and micromorphologies of ash and fly ash deposits wereanalyzed 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. Theresults indicate that the higher deposition propensity associated with a 30% O2/70% CO2 atmosphere can be largely attributed toa wider PSD rather than any changes in the chemical compositions of the fly ash or deposited ash, in which there are no obviousdifferences between air and oxy-fuel combustion. In addition, the slightly higher concentration of fine particles produced underthis atmosphere also promotes the deposition of fly ash.

1. INTRODUCTIONOxy-fuel combustion (O2/CO2) is one of several advancedcarbon capture and storage (CCS) technologies and has beenstudied primarily with regard to its application in pulverizedcoal (PC) and circulating fluidized-bed (CFB) boiler systems.In comparison to oxy-fuel combustion in a PC boiler, oxy-fuelcombustion in a CFB system has several advantages, with onebeing that the bed temperature can be controlled by adjustingthe amount of circulating ash, which allows for higher O2concentrations with no risk of drastically increasing the furnacetemperature. As a result, oxy-fuel combustion can significantlyreduce the amount of recycled flue gas and the volume of theboiler island, which, in turn, lowers both the operating andrunning costs. Because of the differences between oxy-fuelcombustion and air combustion atmospheres, various groupshave researched ignition and devolatilization,1 flame propaga-tion 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 depositionbehaviors of coal and coal/biomass blends during oxy-fuelcombustion, using a lab-scale pulverized coal combustor ordrop-tube apparatus. This work determined that combustionunder a 30% O2/70% CO2 atmosphere produced increases inboth the deposition rate and deposition propensity comparedto combustion in an air-fired system but also found that therewere no significant differences in the chemical compositions offly ash and ash deposits between oxy-fuel and air combustion. Itwas concluded that the observed increase in deposits underoxy-firing was due to changes in the physical properties of the

flue gases and gas flow field as well as a wider particle sizedistribution (PSD) of the ash in the case of oxy-fuelcombustion. Yu et al.12 obtained similar results when studyingthe ash deposition behavior under the 32% O2/68% CO2

atmosphere compared to an air-fired system. Interestingly, Liet al.13 obtained completely opposite results during studies ofash deposition and reported that a lower Stokes number andslightly smaller bulk particles resulted from combustion under a30% O2/70% CO2 atmosphere and led to reduced ashdeposition rates. The authors concluded that their contraryresults were largely due to the differences between a drop-tubefurnace (DTF) and a one-dimensional combustor, meaningthat the observed variations in experimental outcomes resultedfrom different experimental setups.All of the studies noted above were performed during oxy-

fuel combustion in PC boilers, while relatively little research hasbeen devoted to studying ash deposition in CFB boilers duringoxy-fuel combustion. There will be some differences in the ashgenerated by CFB and PC boilers because of the differentcombustion temperatures, combustion modes, and feed coalparticle sizes between the two boiler types, all of which directlyaffect ash formation.14 Wu et al.9 investigated characteristics ofthe ash produced by a 100 kWth pilot-scale CFB boiler duringoxy-fuel combustion incorporating a limestone sorbent but paidmore attention to the desulfurization mechanism andcombustion process rather than fly ash deposition. It should

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

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

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also be noted that excess free CaO in fly ash deposits on theheat-transfer surfaces located at the back end of boilers canreact with CO2 in the flue gas and form hardened deposits.Pilot experimental work with an oxy-fuel CFB combustor hasconfirmed the occurrence of carbonization,15 and Monckert etal.16 have shown that both sulfatization and carbonization occurin 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 combustionconditions and found that increases in both CO2 concen-trations and temperature will significantly increase thecarbonation conversion ratio. In addition, it is important tokeep in mind that ash deposition is also associated with theformation of fine ash.18 When fine particles in the molten statedeposit on heat-exchanger surfaces, these surfaces becomestickier and, consequently, retain coarse ash particles moreeasily. Other researchers have also reported higher proportionsof larger mass particles in the fine particulate matter generatedunder oxy-fuel combustion running at higher O2 concentrationscompared to air combustion.13,19

In general, there is still a lack of knowledge concerning ashdeposition on heat-exchanger surfaces in a CFB combustoroperating under oxy-fuel conditions, and therefore, furtherinvestigations are necessary. The goal of our work was thereforeto investigate fly ash deposition on heat-exchanger surfacesduring oxy-fuel combustion and compare the results toobservations from air combustion systems without calcium-based sorbents. Experiments attempting to examine thedominant factors leading to differences in ash depositionwere conducted using a bench-scale fluidized-bed combustor(FBC) with a temperature-controlled ash deposit probeemployed as a substitute for the heat-exchanger surface. Tofurther examine any differences between combustion modes,the particulate matter with aerodynamic diameters less than 10μm (PM10) contents of ash deposits were also measured by anelectrical low-pressure impactor (ELPI). The combustionapparatus employed in this work was based on a bubblingfluidized bed without recycling of flue gas and ash but stillallowed us to obtain suitable levels of performance andoperating conditions.

2. EXPERIMENTAL SECTION2.1. Fuel and Bed Materials. Samples of anthracite coal (from the

Jincheng mine in China) were obtained and sieved, and the results ofultimate and proximate analyses of this coal are listed in Table 1, whilethe chemical composition of its ash is provided in Table 2. The particlesizes of the sieved coal ranged from 0 to 2.36 mm, with a meandiameter of 0.883 mm, and the PSD is shown in Figure 1. Duringexperimental trials, quartz sand with a particle size range of 0.18−0.55mm and a mean particle diameter of 0.235 mm was used as the bedmaterial and an initial quantity of approximately 150 g was loaded asbed material prior to heating. No sorbents were used in this study.

2.2. FBC Apparatus: Description and Operating Conditions.The fly ash deposition experiments were conducted in a bench-scaleFBC, as shown in Figure 2. The main body of the combustor consistsof a preheating section, a furnace section (containing both dense phaseand dilute phase portions), and a convective section, in addition to acyclone and a filter bag. The total height of the combustor is 3500 mm,with the preheating section being 385 mm, the dense section being315 mm, and the dilute phase section being 2800 mm. The innerdiameters of the dense section and the dilute section are 51 and 83mm, respectively, between which there is a transition cone section witha slope of 11. A cyclone was connected to the outlet of the combustorto capture coarse ash, followed by a convective section, which is 850mm in length and has a 56 mm inner diameter, on which there arethree sampling ports to allow for measurement of flue gas, fly ash, andash deposit, respectively. The apparatus also contains a high-temperature filter bag, which has an 80 mm outer diameter and is150 mm in length and used to collect fly ash. The temperature of thecombustor is controlled by 6, 4, 12, and 4 kW electric heaters in thefour sections. The combustor is equipped with a screw feeder, whichallows 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 controlledby a mass flow control system, and these gases are subsequently mixedin a static mixer and then introduced into the furnace after thepreheating section as fluidization gas.

Switching the apparatus between air- and oxy-fuel-fired combustionmodes is simple and easy to control, and during these trials, threecombustion atmospheres were applied: pure air, 21% O2/79% CO2,and 30% O2/70% CO2. According to literature reports,10−13 there aremany possible factors that may affect fly ash deposition, and two of themost important factors are the extent of fly ash formation and the fluegas flow field. To determine the primary causes of observed variationsin fly ash deposition between different combustion conditions, somepotential variables need to be held constant. For this reason, the samefluidization gas velocity was applied when working with all threecombustion atmospheres, to diminish the effects of variations in fluegas velocity. Holding the fluidization gas velocity constant produces

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

Figure 1. PSD of the coal sample.

Energy & Fuels Article

dx.doi.org/10.1021/ef400774b | Energy Fuels 2013, 27, 4609−46164610

the same fluidization states and residence times, both importantparameters of fluidized-bed combustion, in all experimental trials. Inaddition, the velocity of the flue gas can vary the separation efficiencyof the cyclone and, thus, indirectly affect fly ash deposition. For all ofthese reasons, therefore, all work in this study was performed at aconstant fluidization gas velocity.Table 3 presents a summary of the experimental trials and the

associated system parameters. The excess oxygen coefficient shown

here is defined as the ratio of actual oxygen to theoretical oxygendemand. The theoretical furnace outlet O2 concentration in the fluegas was as high as 6% for all three combustion atmosphere conditions.During the start-up stage, small amounts of coal were continually fedinto the furnace until the bed temperature rose to more than 800 °Cand the temperature of the dense phase section reached the desiredtemperature of 900 °C, allowing for fluctuations of ±10 °C. The FBCtemperature profiles observed along the flue gas flow direction whenusing each of the three combustion conditions are shown in Figure 3.When applying the 30% O2/70% CO2 atmosphere, the temperature inthe upper portion of the dense section reached 950 °C, likely becauseof a higher O2 concentration and a greater quantity of fed coal. Thetemperature of the cyclone without the application of external heaterswas approximately 550 °C, and the average temperature of theconvective 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 obtainedwhen applying the three combustion conditions are summarized inTable 4, from which it is evident that oxy-fuel combustion has apronounced effect on the emission of pollutants. The higher O2concentration and slightly elevated temperature profile under the 30%O2/70% CO2 conditions play an important role in these increasedemissions. The lower emissions of SO2 and NOx in the case of the 21%O2/79% CO2 atmosphere are likely related to increased CO, resultingfrom the reaction of the higher CO2 concentration and char.5

2.3. Ash Sampling and Analysis. The ash deposit and fly ashresulting from each experimental trial were analyzed. Ash depositswere collected using an ash deposition probe, which had a 22 mmouter diameter and was 50 mm in length, positioned horizontallyapproximately 650 mm from the inlet of the convective section (see

Figure 2. Schematic of the bench-scale FBC.

Table 3. List of Combustion Experimental Cases

fluidizing gas

superficialgas velocity

(m/s)

coal feedingrate

(g/min)

gas flowrate

(m3/h)

excessoxygen

coefficient

air 0.7 10.05 5.04 1.421% O2/79% CO2 0.7 10.05 5.04 1.430% O2/70% CO2 0.7 15.56 5.04 1.28

Figure 3. Temperature profiles of FBC along the flue gas flowdirection.



Figure 2). The surface temperature of the probe was maintained at 560°C by an air-cooled automatic temperature control system to simulatethe wall temperature of superheaters and reheaters at the tail of theboiler and could be readily adjusted to different temperatures. Thesampling time for the ash deposition tests was 4 h, after which ashdeposits 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 ashwas collected by ELPI, as shown in Figure 2. During sampling, flue gasfrom the isokinetic sampling probe enters a cyclone to remove coarseparticles with aerodynamic diameters greater than approximately 10μm and then passes through a two-stage dilution system. The first-stage dilution, known as isothermal dilution, has a ratio of 7.29 anduses air heated to 190 °C with flue gas as the dilution gas. The purposeof this stage is to dilute gaseous H2O and H2SO4 as well as the smallamount of volatile organic compounds (VOCs) in the flue gas andensure an unsaturated state. The second dilution stage is referred to aslow-temperature dilution and has a ratio of 7.62. In this stage, the gastemperature is decreased to 40 °C to further dilute the concentrationof fine particulate matter and prolong the acquisition time of the ELPI;the sample mass on the substrate at each level in the ELPI cannotexceed 1 mg, to avoid errors because of particle interactions. The ELPIcontains 13 stages with 50% aerodynamic diameter cutoff values of0.029, 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 asstage substrates to collect particle samples for subsequent PSDmeasurements. The filter ash was collected at the reactor exit with ahigh-temperature filter bag.The collected ash deposit and filter ash were analyzed via scanning

electron microscopy (SEM) and inductively coupled plasma−atomicemission spectrometry (ICP−AES) to ascertain morphology andchemical compositions, respectively. The filter ash was also examinedusing a Malvern Mastersizer 2000 to determine volume particle sizedistributions and, in addition, was baked in a muffle furnace todetermine the carbon content.

3. RESULTS AND DISCUSSION3.1. Ash Deposit. 3.1.1. Deposition Propensity. Two

parameters were used to quantify and compare fly ashdepositions under the different combustion conditions:deposition rate (DR) and deposition propensity (DP), definedas below.

=

×

DRmass of collected ash deposit (g)

surface area of the probe (m ) deposition time (h)2

=×

DPDR

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

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

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

seen that DP was higher under air combustion compared to21% O2/79% CO2 conditions but obviously lower than whenusing a 30% O2/70% CO2 atmosphere. These results show thatincreasing the oxygen concentration can result in a higher ashdeposition propensity, while increases in CO2 have theopposite effect at the same oxygen concentration. Theseobservations are consistent with those reported previously inthe literature,10,12 although the reasons for this phenomenonare still uncertain. Experiments under all three combustionmodes were performed using the same fluidization velocity andsimilar levels of outlet theoretical oxygen concentration(approximately 6%); therefore, the flue gas velocities near theash deposition probe as well as the temperatures were basicallyequivalent between all three conditions. Neglecting any effectsof the change in flue gas compositions, the effects of flue gasflow on fly ash deposition can be ignored, and therefore,differences in fly ash deposition should be attributed to changesin the characteristics of the ash deposit and fly ash, such asvariations in the morphology or chemical composition of theash deposit, as well as changes in the particle size distribution,carbon content, or chemical composition of the fly ash. Theseeffects are discussed in the following sections.

3.1.2. Morphology of Ash Deposits. The ash deposits wereobserved to be gray, weakly bound, and uniformly distributedover the surface of the probe to a thickness ranging between 0and 2 mm and, thus, would be classified as loose fouling (Figure5). The majority of each ash deposit appeared on the windwardside of the probe, with a small amount on the wing sides butonly a very thin layer on the leeward side. Most of the ashparticles in the deposits were less than 10 μm in size, and theparticles primarily exhibited a flake or flocculated morphology(Figure 6). Closer examination showed that the sub-micro-meter particles were spherical and agglomerated, while particlesgreater than 1 μm size appeared as flakes. Ash depositionmechanisms are known to include inertial impaction,thermophoresis, condensation, and chemical reaction,20 andthe dominant deposition mechanism is thermophoresis in thecase of fly ash with particle sizes of 0.5−5 μm and inertialimpaction for fly ash with particle sizes above 10 μm. On thebasis of the macro- and microscopic morphologies of the ash

Table 4. Flue Gas Composition Tested by FTIR

combustion atmospheres

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

O2 (%) 9.52 ± 0.2 10.06 ± 0.1 11 ± 0.5CO (ppm) 154 ± 2 248 ± 15 239.7 ± 5CO2 (ppm) 10 ± 0.2 86 ± 0.2 84 ± 0.4SO2 (ppm) 840 ± 31 718 ± 23 1512 ± 67NOx (ppm) 417 ± 4 354 ± 3 488 ± 6H2O (%) 2.7 ± 0.03 2.7 ± 0.04 3 ± 0.02

Figure 4. Deposition propensities in different combustion conditions.



deposits, we deduced that the dominant deposition mecha-nisms during these trials were inertial impaction andthermophoresis. Thermophoresis is primarily affected bytemperature gradient and particle size, and therefore, becausethe velocity and temperature at the deposition probe were thesame under all three combustion conditions, inertial impactionrather than thermophoresis was considered the cause of theobserved variations in ash deposition behavior.3.1.3. Chemical Compositions of the Ash Deposits. The

chemical compositions of the ash deposits are shown in Figure7. It should be noted that SiO2 and Al2O3 were the main

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

sizes less than 1 μm contain most of the sodium compoundsand can deposit on the heat-exchanger surfaces by thermopho-resis. Under the 30% O2/70% CO2 atmosphere, the relativelylow Na2O content in the ash deposit may be attributed to thehigher fly ash deposition rate observed when applying thiscondition, during which there was a relatively low proportion ofthe sub-micrometer particles, which would contain Na2O. Thedata for the SO3 content show a different trend; therelationship of the SO3 content with the combustionatmosphere is 21% O2/79% CO2 < 30% O2/70% CO2 < air,which is consistent with the findings by Yu et al.12 The lowerlevel of SO3 resulting from oxy-fuel combustion might beattributed to the reduced concentrations of alkali metals andalkaline earth metals in the ash deposit. In general, however,alkaline compounds in the ash deposits of oxy-fuel combustionare present at slightly lower concentrations than in the ashresulting from air combustion, which does not appear tofacilitate fly ash deposition, and therefore, there must be otherreasons for the observed higher ash deposition propensity.

3.2. Fly Ash. 3.2.1. PM10. The particle size distribution ofthe ash also has an effect on ash deposition, and therefore, theeffects of oxy-fuel combustion on the release of fine fly ashparticles was evaluated by measuring PM10 using the ELPI(Figure 8). The mass concentration of fine particles is typicallyreported in units of mass per volume; however, this is difficultin terms of our data because the coal feeding rate was highduring the 30% O2/70% CO2 combustion trials compared toother combustion conditions. The effects of different coalfeeding rates were eliminated by converting the measuredconcentration of fine particles from units of mass per volume tounits of mass per energy, which more clearly communicatesdifferences in the generation of PM10 under differentcombustion modes. The PM10 mass distributions are presentedin panels a and b of Figure 8 in mass per volume and mass perenergy, respectively. The PM10 emission distributions resultingfrom all three combustion atmospheres were similar. Whenmass per volume is used (as in Figure 8a), it appears that 30%O2 oxy-fuel combustion produces slightly more PM10 thanconventional air combustion and significantly more than 21%O2 oxy-fuel combustion. However, when mass per energy isused instead, the differences in the amount of PM10 generated

Figure 5. Macromorphology of the ash deposits.

Figure 6. Micromorphology of the ash deposits.

Figure 7. Chemical compositions of ash deposits.



between 30% O2 oxy-fuel and air combustion are largelyreduced, especially with regard to fine particles withaerodynamic diameters larger than 0.2 μm. Differences in theconcentrations of particles with aerodynamic diameters lessthan 0.2 μm are still fairly obvious between the threecombustion atmospheres, which shows that increasing O2concentrations have an effect on the formation of fine particles(those smaller than 0.2 μm), primarily because of greatervaporization and nucleation under the elevated O2 concen-tration associated with oxy-fuel combustion.13,19 The slightlyhigher temperature profile of 30% O2/70% CO2 combustioncompared to those of air and 21% O2/79% CO2 combustionalso contributes to this effect. These results agree with thefindings of other studies.8,21,22 The lowest levels of PM10 areobserved in the case of 21% O2/79% CO2 combustion, likelybecause the lower particle temperatures associated with thatcombustion atmosphere affect the vaporization and fragmenta-tion of inorganic matter because of the higher thermal capacityof CO2 and the lower diffusivity of O2.

21 In addition, it is seenthat a small quantity of sub-micrometer particles are producedunder each atmosphere, accounting for 2.24, 1.94, and 2.39% ofthe ash resulting from air, 21% O2/79% CO2, and 30% O2/70%CO2 atmospheres, respectively. From these results, under the30% O2/70% CO2 atmosphere, a slightly higher concentrationof fine particles, especially for those smaller than 0.2 μm, mayimprove 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

deposits will also interfere with the analysis of test results. Thecarbon contents found in fly ash produced during combustionunder air, 21% O2/79% CO2, and 30% O2/70% CO2 conditionswere 5.25, 5.13, and 6.33 wt %, respectively. The carboncontent in fly ash generated when using 30% O2/70% CO2 istherefore slightly higher than that produced by the other twocombustion conditions, but still within the acceptable range ofabout 5−20 wt %.24 The carbon contents of the ash depositsamples were not measured, because macroscopic morpho-logical observations showed few carbon particles in the ashdeposits. These results indicate that there are only minordifferences in the carbon contents of the fly ash, and therefore,the effects of carbon content on ash deposition can essentiallybe neglected.

3.2.3. PSDs. The PSDs of the filter ash resulting from air andoxy-fuel combustion are similar, although slightly largerparticles are generated during the 30% O2/70% CO2combustion mode (Figure 9), such that the average particle

diameters are 5.33, 4.93, and 7.85 μm. The variations in particlesize may be due to the effects of a higher oxygen concentrationand slightly higher bed temperature profile in the upper portionof the dense phase section during some combustion modes,both of which can enhance char fragmentation. It should benoted that the average particle diameter of the filter ashobserved in this work is significantly below that of the fly ashgenerated in an industrial-size CFB.24 This is likely because theconcentration of coarse particles (those in the range of 1−100μm) exiting from the outlet of the cyclone under bubblingfluidized-bed combustion is lower than that resulting from CFBcombustion. The difference in fly ash size may also be at leastpartly related to variations in the properties of the coal used asfuel in these systems, because the PSD of the coal is known toaffect the PSD of the resulting fly ash. The anthracite coal usedin our work has elevated carbon and ash contents, and thesefactors could affect its fragmentation behavior. On the basis ofthe above analysis, inertial impaction is the dominantdeposition mechanism and is influenced by two factors:impaction efficiency and capture efficiency. Capture efficiencyis related to the chemical composition of the fly ash particles aswell as the surface properties of the ash deposits, which are

Figure 8. Emissions of PM10 in different combustion conditions.

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



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

ρ

μ=St

V d

D9p p p

2

g probe

Here, ρp is the particle density (kg/m3), dp is the particlediameter (m), Vp is the particle velocity (m/s), μg is thedynamic 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 ofCO2 is 4.70 × 10−5 Pa s. Assuming that the fly ash density isuniform, the Stokes number of the particles is dependent uponthe particle diameters and, according to the relationshipbetween ash diameter, Stokes number, and impactionefficiency,13 both the Stokes number and the impact efficiencywill increase with an increasing particle diameter. As a result, awider particle diameter distribution is also an important factorresponsible for the observed higher deposition rate observedduring 30% O2/70% CO2 combustion.3.2.4. Chemical Compositions of Fly Ash. The chemical

compositions of the filter ash generated during each of thethree combustion modes are very nearly identical, with theexception of the levels of SO3 (Figure 10); the SO3

concentrations in the filter ash increase in the order 30% O2/70% CO2 < 21% O2/79% CO2 < air. Interestingly, these resultscontradict the findings of prior studies12 because the retentionof SO3 in fly ash resulting from the 30% O2/70% CO2combustion mode was not observed. There are no obviousdifferences in the CaO and MgO levels between all threecombustion atmospheres, indicating that the concentrations ofthese 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 asignificant effect on the chemical composition of the fly ash andthat the viscosity of the fly ash and capture efficiency are alsobasically unchanged.

4. CONCLUSIONAsh deposition experiments were conducted in a bench-scaleFBC under both oxy-fuel and air combustion conditions. Theresults showed that the propensity for fly ash deposition during30% O2/70% CO2 combustion was significantly higher thanduring either the air combustion or 21% O2/79% CO2

combustion modes. There were, however, no marked differ-ences in the chemical compositions of the ash deposits and flyash among the three different combustion modes. Oxy-fuelcombustion modes were shown to affect the emission of PM10particulates. The PM10 emission distributions resulting from allthree combustion atmospheres were similar, but the massconcentration of PM10 generated when applying the 30% O2/70% CO2 combustion mode was slightly higher, especially inthe case of particles with aerodynamic diameters less than 0.2μm, which might also be an important factor related to theincreased ash deposition rates seen when working under thiscombustion mode. We also did find that the average particlesize of fly ash generated by 30% O2/70% CO2 combustion wasslightly larger than that resulting from the other twocombustion modes, which could be the cause of the higherdeposition rate seen when operating under a 30% O2atmosphere, because of the higher fragmentation of coalparticles in a high oxygen environment.

■ AUTHOR INFORMATIONCorresponding Author*Telephone: +86-451-86413231. Fax: +86-451-86412528. E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe gratefully acknowledge the financial support provided bythe National Science Foundation for Young Scientists of China(Grant 51106038).

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Figure 10. Chemical compositions of filter ash in different combustionconditions.



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

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