Ash Particulate Formation from Pulverized Coal under Oxy-FuelCombustion ConditionsYunlu Jia and JoAnn S. Lighty*

Department of Chemical Engineering, University of Utah, 50 South Central Campus Drive, Salt Lake City, Utah 84112, United States

*S Supporting Information

ABSTRACT: Aerosol particulates are generated by coalcombustion. The amount and properties of aerosol partic-ulates, specifically size distribution and composition, can beaffected by combustion conditions. Understanding theformation of these particles is important for predictingemissions and understanding potential deposition. Oxy-fuelcombustion conditions utilize an oxygen-enriched gas environ-ment with CO2. The high concentration of CO2 is a result ofrecycle flue gas which is used to maintain temperature. Ahypothesis is that high CO2 concentration reduces thevaporization of refractory oxides from combustion. A high-temperature drop-tube furnace was used under differentoxygen concentrations and CO2 versus N2 to study the effectsof furnace temperature, coal type, and gas phase conditions on particulate formation. A scanning mobility particle sizer (SMPS)and aerodynamic particle sizer (APS) were utilized for particle size distributions ranging from 14.3 nm to 20 μm. In addition,particles were collected on a Berner low pressure impactor (BLPI) for elemental analysis using scanning electron microscopy andenergy dispersive spectroscopy. Three particle size modes were seen: ultrafine (below 0.1 μm), fine (0.1 to 1.0 μm), and coarse(above 1 μm). Ultrafine mass concentrations were directly related to estimated particle temperature, increasing with increasingtemperature. For high silicon and calcium coals, Utah Skyline and PRB, there was a secondary effect due to CO2 and thehypothesized reaction. Illinois #6, a high sulfur coal, had the highest amount of ultrafine mass and most of the sulfur wasconcentrated in the ultrafine and fine modes. Fine and coarse mode mass concentrations did not show a temperature or CO2relationship. (The table of contents graphic and abstract graphic are adapted from ref 27.)

1. INTRODUCTIONCoal is the least expensive and most widely used solid fuel toproduce electricity. The U.S. Department of Energy and theEnergy Information Administration1 estimates that the world’scoal consumption will increase by 56% from 2007 to 2035. Inthe U.S., 90% of the coal is used in electricity generation.Emissions from coal combustion have been of significantconcern because of their damage to both the environment andhuman and animal health. In oxy-fuel combustion, oxygen isused versus air in the combustion zone, with recycled flue gasto keep the system cooler. The result is a flue gas with mostlywater and CO2. Ultrafine particulate is a consequence of theminerals in the coal volatilizing and recondensing or nucleatingto form submicrometer particles. Raask2 and Bryers3 haveinvestigated particulates, focusing on organic mineral matter inthe ash, and they found that formation was highly dependenton the local temperature and gas environment.The formation of submicrometer-size aerosols during coal

combustion is known to be due to mineral vaporization underlocal combustion conditions and subsequent particle formation.Quann and Sarofim,4 supported by the studies of Neville et al.,5

Senior and Flagan,6 Kaupppinen and Pakkanen,7 and Haynes etal.,8 hypothesized that the vaporization of refractory oxides was

governed by: MOn(s) + CO(g) ↔ CO2(g) + MOn−1(g). Thevapor pressure of the vaporizing oxides or metal (MOn−1) isdetermined by the equilibrium of the reaction between therefractory metal oxides MOn and CO inside the particle at hightemperatures. Therefore, increasing CO2 concentration couldeffectively reduce the vaporization of oxides, and reduce theyield of ultrafine and fine particulate.Wall et al.9 investigated oxy-fuel combustion in a pilot-scale

furnace with an average of 27% oxygen. They found nosignificant differences in chemical composition and sizedistribution of bulk fly ash between O2/CO2 and O2/N2

combustion. They did find differences in slagging and foulingin some coal tests. Ash and deposit formation from oxy-fuelcombustion, with 27% and 32% oxygen, in a pilot-scale furnacewere studied by Yu et al.10 They found no significant impactson bulk ash particle size distributions (PSDs) and compositionin O2/CO2 combustion. Apparent impacts of oxy-fuelcombustion were found for the composition of deposits,

Received: November 29, 2011Revised: March 22, 2012Accepted: April 2, 2012Published: April 2, 2012

Article

pubs.acs.org/est

© 2012 American Chemical Society 5214 dx.doi.org/10.1021/es204196s | Environ. Sci. Technol. 2012, 46, 5214−5221

especially for Ca, Fe, and S oxides. Suriyawong et al.11 studied asubbituminous coal with a drop tube furnace over a range ofoxygen conditions, 21% to 50%, and observed that the meansize and the total amount of submicrometer ash particlesformed in O2/CO2 combustion was smaller than that formed inO2/N2 combustion for same O2 concentration. Similar resultswere found in Sheng and co-worker’s drop tube studies at 20and 40% oxygen.12,13 They concluded that the combustiontemperature was the dominant factor, versus CO2 effect on ashformation. Krishnamoorthy and Veranth14 modeled the CO/CO2 ratio inside a single char particle when it combusted. Theirresults suggested that changing CO2 concentration in the bulkgas significantly changed the CO/CO2 ratio which could affectthe vaporization ratio of refractory oxides and the formation offine ash particulates.The focus of this study was to investigate the effects of

furnace temperature, coal type, and gas phase conditions,namely, CO2 versus N2 on ash particulate formation andrefractory oxides composition under controlled conditions. Thestudy utilized particle sizing equipment allowing a wide range ofparticle sizes, from submicrometer to micrometer. In addition,composition was determined for the different size fractions.

2. EXPERIMENTAL METHODS AND MATERIALSA schematic diagram of the high-temperature, drop-tubefurnace (HDTF)15 is shown in Figure 1. Synthetic flue gaswas used to simulate oxy-fuel conditions and compare theseresults with air. The furnace was heated by a pair of graphiteelectric heating elements. An alumina muffle tube (50 mminside diameter, 40 cm long) served as the central reaction zonewith an alumina honeycomb on the top to act as a preheaterand flow straightener. The main gas flow rate was maintained at0.2 L/s and coal feed rate was set at 1.5 g/h. The coal feed rateis low enough that the drop tube operates with little change ingas environment (high excess oxygen conditions).

A water-cooled collection probe was inserted along the axisthrough the bottom of the furnace. The probe position wasadjustable to allow for variable residence times. All gas andparticulate combustion products were withdrawn through acollection probe for further sampling tests. Pure N2 was addedand permeated through a porous tube to quench chemicalreactions inside the collection probe. The temperature of theflue gas after quenching was approximately 350 K.The outlet stream was further diluted by particle-free air

while passing through a dilution tunnel for sampling with theparticle sizing instruments. A stream (0.3 L/min) wasintroduced into a TSI scanning mobility particle sizer (SMPS,model 3080) and an aerodynamic particle sizer (APS, model3321) to determine the PSDs. The SMPS has a theoreticalmeasurement range of 14.3−673.2 nm, and the APS rangesfrom 532 nm to 20 μm.A Berner low pressure impactor (BLPI) was utilized for ash

size segregation and collection, as discussed in Hillamo andKauppien16 and Linak et al.17 The BLPI was connected to theoutlet of the collection probe with N2 gas dilution to maintainthe total gas flow rate at a constant value of 25.4 L/min(standard). The BLPI uses low pressure and high jet velocitiesfor particle size segregation, collecting particles on 11 stageswith particle sizes (50% aerodynamic cutoff diameterinmicrometers). Sample was collected on greased aluminum foilsfor gravimetric measurements. To collect sufficient mass ofultrafine particles for analysis, the BLPI was operated in twosteps. First, a cyclone was attached to the outlet of thecollection probe to remove particles larger than 1 μm.Operation with the cyclone lasted about 180 min to ensureenough ultrafine particles were collected on the bottom stages,without larger particles overloading the top stages (stages 11−7). The cyclone was then removed and larger particles werecollected on the upper stages. Data from the BLPI and SMPSwere compared and found to be the same except for the

Figure 1. High temperature drop tube furnace.

Environmental Science & Technology Article

dx.doi.org/10.1021/es204196s | Environ. Sci. Technol. 2012, 46, 5214−52215215

ultrafine particle range (less than 0.1 μm) where the smallamount of sample is difficult to weigh.Particulates were also collected on polycarbonate membranes

for elemental analysis. Particles were deposited on each BLPIimpactor stage for 15−20 min. Collected samples wereexamined by a FEI Nova Nano scanning electron microscope(SEM) and an energy dispersive X-ray spectrometer (EDS) asdiscussed by Vassilev and Vassileva.18 Six major refractoryoxides and sulfur were considered in the elemental analysis(reported as oxides): Na, Mg, Al, Si, Ca, and Fe.Two bituminous coals, Utah Skyline and Illinois #6, and one

subbituminous coal, Powder River Basin Black Thunder (PRB)are reported on in this paper. The properties of the coals issummarized in Table 1. All coal samples were dried at 105 °C

and sieved to a size range of 54−72 μm to ensure a stablefeeding rate. The residence time of particles passing throughthe drop-tube reaction zone was about 0.9 s which allowed forburnout of the particle; no soot was detected in the exhaust.An O2/CO2 mixture was used as the oxidant in the HTDT

experiments to simulate O2/recycled flue gas combustion, andthese results were compared with O2/N2 combustionconditions. Two oxygen concentrations, 21% and 31.5%, andfurnace wall temperatures were studied. The gas temperaturewas measured and found to be relatively constant along thelength of the reactor and under the different gas compositions.The higher gas temperature was approximately 1520 K, whereasthe lower was 1410 K.

3. SINGLE-PARTICLE MODELParticle temperature changes as a function of oxygen, withhigher temperatures at higher concentrations (Suriyawong etal.11). To elucidate temperature differences over differentcombustion conditions, a simplified, single-particle model wasdeveloped for coal char to estimate particle temperatures. As inthe studies of Badzioch et al.19 and Carslaw and Jaeger,20 aspherical, homogeneous particle was considered, reactions inthe boundary layer were ignored, and no temperature gradientwithin the particle was used. The only reaction considered atthe char particle surface was the direct oxidation of carbon toform carbon monoxide, C + 1/2O2 →CO, following theapproach of Field21 and Gavalas,22 and supported by thefindings of Tognotti et al.23 Gas-phase oxidation of CO to CO2was assumed to occur in the free stream and, therefore, did notenter into the energy and material balance equations.The following heat balance expression was used which gives

the rate of change of particle temperature with time:

γ= −εσ − − − + Δm CT

tA T T A T T H

d

d( ) h ( )p p

pp p

4W4

p p g

(1)

In eq 1 the wall temperature and gas temperature wereconsidered equal and gas temperatures were measured. Theoverall reaction rate, γ, kg/s considered oxygen diffusion fromthe bulk gas to the surface and the surface reaction rate, where,

γ = · ·k P Ao O p2 (2)

Properties were from studies of Murphy and Shaddix24 and theoxidation reaction rates of the chars were assumed to be thesame for all the coals. A diameter of 65 μm was used. Theproperties are shown in the Supporting Information.

4. RESULTS AND DISCUSSIONTemperatures were estimated from the above-mentionedmodel. Constant properties were considered comparisons aremade for each coal at the combustion conditions in questionseparately. Table 2 indicates the estimated temperatures for the

particles at the indicated gas temperatures. It is estimated thattemperatures are within ±40 K.Comparing O2/N2 and O2/CO2 combustion conditions, the

particle temperature in O2/N2 was estimated to be higher thanthat in O2/CO2 at the same gas temperature and oxygenconcentration conditions. The most likely reason is that themass diffusivity of O2 in CO2 is lower than that of O2 in N2which inhibits the oxygen at the particle surface. These resultsare consistent with data in the literature25,26 and thetemperatures are within the same range.Typical PSDs for the higher gas temperature condition, 1520

K, are shown in Figure 2 for the three coals. As seen in thisfigure, there were three modes of particles: ultrafine, less than0.1 μm; fine, 0.1 to 1.0 μm; and coarse, greater than 1 μm.Illinois #6 had the most defined segregation between modes.

Table 1. Coal Analysis Dataa

Utah Skyline PRB Illinois #6

Proximate AnalysisLOD (%) 3.2 23.7 9.7ash (%) 8.8 4.9 8.0volatile matter (%) 38.6 33.4 36.8fixed C (%) 49.4 38.0 45.6

Ultimate Analysis (%), Dry, Ash-Freecarbon 77.4 56.5 70.3hydrogen 5.9 6.8 6.1nitrogen 1.3 0.8 1.2sulfur 0.5 0.2 4.3oxygen (by difference) 14.9 35.7 18.1

Ash Composition (%)Al as Al2O3 14.52 14.78 17.66Ca as CaO 6.11 22.19 1.87Fe as Fe2O3 5.09 5.2 14.57Mg as MgO 1.39 5.17 0.98Mn as MnO 0.02 0.01 0.02P as P2O5 0.59 1.07 0.11K as K2O 0.57 0.35 2.26Si as SiO2 60.89 30.46 49.28Na as Na2O 1.41 1.94 1.51S as SO3 2.33 8.83 2.22Ti as TiO2 0.88 1.3 0.85

aNote: Loss on drying (LOD) was determined in air at 105°C for onehour and is reported on an as received sample weight basis. Note: Ashanalysis results are reported on an ashed sample weight basis.

Table 2. Char Temperatures for a Particle Diameter of 65μm (K)

Tg = 1410 K Tg = 1520 K

O2(%) 21 31.5 21 31.5O2/N2 1747 1940 1860 2044O2/CO2 1694 1843 1807 1949

Environmental Science & Technology Article

dx.doi.org/10.1021/es204196s | Environ. Sci. Technol. 2012, 46, 5214−52215216

The mean size of each particle mode was found and the SMPSand APS PSD data were integrated to obtain total mass forultrafine, fine, and coarse modes. Results were normalized to1000 mg of ash. As illustrated in Figure 2, the diameter andmass for the coarse modes (particles greater than 1 μm) did notchange for the three coals. This result is expected since particlesin this size range are usually the result of particle attrition.Changes in the ultrafine and fine modes were found for allthree coals under the different conditions. In these ranges,particles are formed from volatilization and subsequentcondensation and/or nucleation and coagulation.27 Thesesteps are more dependent upon the temperature and gasphase environment.The mass concentrations of ultrafine particle modes, under

100 nm, are shown in Figure 3 for all three coals. The massconcentrations were obtained by integrating the SMPS dataover the ranges previously discussed. This number wasnormalized by the total mass, which was obtained by summingall the fractions. The data are plotted as temperature which wasobtained from the model; the error bars show both temperature(horizontal) and concentration (vertical) errors. Utah Skyline

(Figure 3a) showed a significant increase in ultrafine particlemass concentration with temperature. The data for PRB(Figure 3b) are more scattered but also show a slight increasewith temperature. Illinois #6 (Figure 3c) had the highestultrafine mass concentration of the three coals (notice thechange in scale). This coal had a high concentration of sulfur(see Table 1) and the ultrafine mass was relatively consistentover all conditions, with the exception of one point. This islikely due to the vaporization of the sulfur which would beindependent of temperature and gas environment at theseconditions. Both Utah Skyline and PRB coals show a secondorder effect of CO2 at the higher temperatures where, for thesame temperature, the mass of ultrafine was lower for the CO2condition versus the N2 (comparing dark and light bars). Thisresult is due to the high content of silica in the Utah Skylineand calcium and silica in the PRB. These compounds would beexpected to have the hypothesized relationship with CO2 wherereaction is driven toward the solid oxide not a reducedvaporized oxide.

Figure 2. PSDs of Utah Skyline (top), PRB (middle) and Illinois #6(bottom) at higher furnace temperature with a gas temperature of1520 K. Figure 3. Mass concentration of ultrafine particles for Utah Skyline,

PRB and Illinois #6 coals.

Environmental Science & Technology Article

dx.doi.org/10.1021/es204196s | Environ. Sci. Technol. 2012, 46, 5214−52215217

Figure 4. Mean diameter ultrafine for Utah Skyline as a function of temperature.

Figure 5. Mass concentration of fine mode particles.

Figure 6. Elemental mass fraction size distribution of Utah Skyline at 1520K gas condition. The lines show the three modes: ultrafine, fine, andcoarse.

Environmental Science & Technology Article

dx.doi.org/10.1021/es204196s | Environ. Sci. Technol. 2012, 46, 5214−52215218

Figure 4 shows the mean sizes for the Utah Skyline ultrafineparticles; as seen in this figure, the diameters are constantacross the temperatures and conditions.The mass concentration of the fine mode did not vary

significantly except at the highest temperature for the UtahSkyline coal (Figure 5). The fine mode mass concentrations ofIllinois #6, not shown here, were much higher than the othertwo coals in this mode.Figure 6 shows the results from the BLPI and EDS analysis

for the Utah Skyline coal with the three modes, ultrafine (<0.1μm), fine (0.1−1.0 μm), and coarse (greater than 1 μm)indicated by the lines. The shapes are similar to the SMPS/APSPSD data of Figure 2. As previously discussed, Utah Skylinecontains a high Si content. The mass fraction of SiO2 was thehighest in the coarse mode (from 1.98 to 7.33 μm). Asexpected, the amount is relatively consistent in this size range.The highest temperature is for the 31.5% oxygen/nitrogenenvironment. As seen in the figure, the amount of ultrafine is

higher across the stages and silica is in higher concentrations forthis condition. The fine mode also shows enrichment in silicafor the highest temperature case.PRB coal has a high content of Ca and Si, and Ca is the

predominant ash compound, as Ca is more easily vaporizedthan Si (Figure 7). Again, the highest amount of calcium wasseen at the conditions of the lower, right graph. There is alsoslightly more silica in this case. The coarse fractionconcentrations are also relatively constant.The Illinois #6 data are shown in Figure 8. Illinois #6 coal

has a high content of sulfur, Si, and Fe. Large concentrations ofsulfur were found in the ultrafine and fine particles, with almostno sulfur in the coarse mode. Si and Fe showed moredependence on O2 concentration versus the combustionenvironment (N2/CO2) for all stage samples. These resultsare consistent with the fact that the sulfur was easily vaporizedat the temperatures in question.

Figure 7. Elemental mass fraction size distribution of PRB at 1520 K gas condition. The lines show the three modes: ultrafine, fine, and coarse.

Figure 8. Elemental mass fraction size distribution of Illinois #6 at 1520K gas condition. The lines show the three modes: ultrafine, fine, and coarse.

Environmental Science & Technology Article

dx.doi.org/10.1021/es204196s | Environ. Sci. Technol. 2012, 46, 5214−52215219

5. CONCLUSIONS

Based on the results, temperature was the driving mechanismfor increasing the amount of ultrafine formation with anincrease in mass at higher temperatures. For fine particles, thetemperature relationship was less pronounced. As a secondorder effect, combustion in an O2/CO2 environment yieldedsmaller amounts of ultrafine particles for Utah Skyline and PRBcoals based on the solid oxide formation for Si and Ca. EDSresults supported the SMPS/APS data and showed that thecoarse composition did not change significantly for the coals,but the ultrafine compositions were dependent upon the Si, forUtah, and Ca, for PRB. Illinois #6, a high-sulfur coal, had highconcentrations of sulfur in the fine and ultrafine ash indicatingthat the sulfur vaporized and then recondensed.

■ ASSOCIATED CONTENT

*S Supporting InformationInformation regarding the model parameters. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Phone: 801-581-5763; fax: 801-585-9291; e-mail: jlighty@utah.edu.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This material is based upon work supported by the Departmentof Energy under Award Number DE-NT0005015. The viewsand opinions of authors expressed herein do not necessarilystate or reflect those of the United States Government or anyagency thereof. The advice, support, and assistance of DanaOveracker and Dr. Matthew C. DeLong are also greatlyappreciated. The authors also acknowledge the comments andinput of Professors Jost Wendt and Adel Sarofim (deceased).

■ NOMENCLATURE

Ap: surface area of the particle, m2

Cp: specific heat of the char, J/kg·Kdp: particle diameter, mh: convective heat transfer coefficient, W/m2·KΔH: heat release at particle surface per unit mass of charcombustion, kJ/kgko: combined diffusion and reaction rate, [(1/kd) + (1/kp)]

−1, kg/(m2atm·s)mp: particle mass, kgPo2: O2 partial pressure, atmTp: particle surface temperature, KTw: wall temperature, Kε: particle emissivity, usually uses 0.8 for coal particleρ: particle density, kg/m3

σ: the Stefan−Boltzmann constant, (W/m2)·K4

■ REFERENCES(1) International Energy Outlook 2010, DOE/EIA-0484; Departmentof Energy/U.SEnergy Information Administration: Washington, DC,2010.(2) Raask, E. Mineral Impurities in Coal Combustion; HemispherePublishing Corporation: New York, 1982.

(3) Bryers, R. W. Fireside slagging, fouling, and high-temperaturecorrosion of heat−transfer surface due to impurities in stream raisingfuels. Prog. Energy Combust. Sci. 1996, 22, 29−120.(4) Quann, R. J.; Sarofim, A. F. Vaporization of refractory oxidesduring pulverized coal combustion. In Nineteenth Symposium (Interna-tional) on Combustion, 1982; Vol. 19(1) pp 1429−40.(5) Neville, M.; Quann, R. J.; Haynes, B. S.; Sarofim, A. F.Vaporization and condensation of mineral matter during pulverizedcoal combustion. In Symposium (International) on Combustion, 1981;Vol. 18(1) pp 1267−74.(6) Senior, C. L.; Flagan, R. C. Ash vaporization and condensationduring combustion of suspended coal particle. Aerosol Sci. Technol.1982, 1, 371−83.(7) Kauppinen, E. I.; Pakkanen, T. A. Coal combustion aerosols: Afield study. Environ. Sci. Technol. 1990, 24 (12), 1811−18.(8) Haynes, B. S.; Neville, M.; Quann, R. J.; Sarofim, A. F. Factorsgoverning the surface enrichment of fly ash in volatile trace species. J.Colloid Interface Sci. 1982, 87 (1), 266−78.(9) Wall, T. F. et al. Ash impacts in oxy-fuel combustion. In Impactsof Fuel Quality on Power Production; EPRI: Palo Alto, CA, 2006.(10) Yu, D. Ash and deposit formation from oxy-coal combustion ina 100 kW test furnace. Int. J. Greenhouse Gas Control 2011, 5 (1),S159−S167, DOI: 10.1016/j.ijggc.2011.04.003.(11) Suriyawong, A. Submicrometer particle formation and mercuryspeciation under O2−CO2 coal combustion. Energy Fuels 2006, 20 (6),2357−2363.(12) Sheng, C. Ash particle formation during O2/CO2 combustion ofpulverized coals. Fuel Process. Technol. 2007, 88 (11−12), 1021−28.(13) Sheng, C.; Lu, Y.; Gao, X.; Yao, H. Fine ash formation duringpulverized coal combustion. A comparison of O2/CO2 combustionversus air combustion. Energy Fuels 2007, 21 (2), 435−40.(14) Krishnamoorthy, G.; Veranth, J. M. Computational modeling ofCO/CO2 ratio inside single char particles during pulverized coalcombustion. Energy Fuels 2003, 17 (5), 1367−1371.(15) Jia, Y. Particulate formation from pulverized coal under oxy-fuelcombustion conditions. MS Thesis. University of Utah, Salt Lake CityUT, 2011.(16) Hillamo, R. E.; Kauppinen, I. On the performance of the bernerlow pressure impactor. Aerosol Sci. Technol. 1991, 14 (1), 33−47.(17) Linak, W. P.; Miller, C. A.; Wendt, J. O. L. Comparison ofparticle size distributions and elemental partitioning from thecombustion of pulverized coal and residual fuel oil. J. Air WasteManage. Assoc. 2000, 20, 1532−44.(18) Vassilev, S. V.; Vassileva, C. G. Methods for characterization ofcomposition of fly ashes from coal-fired power stations: A criticaloverview. Energy Fuels 2005, 19 (3), 1084−1098.(19) Badzioch, S.; Gregory, D. R.; Field, M. A. Investigation of thetemperature variation of the thermal conductivity and thermaldiffusivity of coal. Fuel 1964, 43, 267−80.(20) Carslaw, H. S.; Jaeger, J. C. Conduction of Heat in Solids; OxfordUniv Press: Oxford, UK, 1959.(21) Field, M. A. Rate of combustion of size-graded fractions of charfrom a low-rank coal between 1200° K and 2000° K. Combust. Flame1969, 13, 237−48.(22) Gavalas, G. R. Analysis of char combustion including the effectof pore enlargement. Combust. Sci. Technol. 1981, 24 (5−6), 197−210.(23) Tognotti, L.; Longwell, J. P.; Sarofim, A. F. The products of thehigh temperature oxidation of a single char particle in an electro-dynamic balance. In 23rd Symposiun (International) on Combustion,1991; Vol. 23, pp 1207−1213.(24) Murphy, J. J.; Shaddix, C. R. Combustion kinetics of coal charsin oxygen-enriched environments. Combust. Flame 2006, 144, 710−29.(25) Bejarano, P. A.; Levendis., Y. A. Single-coal-particle combustionin O2/N2 and O2/CO2 environments. Combust. Flame 2008, 153,270−287.(26) Khatami, R. Combustion behavior of single particles from threedifferent coal ranks and from sugar cane bagasse in O2/N2 and O2/CO2 atmospheres. Combust. Flame 2011, 159, 1253−1271.

Environmental Science & Technology Article

dx.doi.org/10.1021/es204196s | Environ. Sci. Technol. 2012, 46, 5214−52215220

(27) Lighty, J. S.; Sarofim, A. F.; Veranth, J. M. Combustion aerosols:Factors governing their size and composition and implications tohuman health. J. Air Waste Manage. Assoc. 2000, 50, 1565−1618.

Environmental Science & Technology Article

dx.doi.org/10.1021/es204196s | Environ. Sci. Technol. 2012, 46, 5214−52215221