High Temperature Interactions Between Residual Oil Ash and Dispersed Kaolinite Powders

Aerosol Science and Technology, 38:900–913, 2004Copyright c© American Association for Aerosol ResearchISSN: 0278-6826 print / 1521-7388 onlineDOI: 10.1080/027868290500805

High Temperature Interactions Between Residual Oil Ashand Dispersed Kaolinite Powders

William P. Linak,1 C. Andrew Miller,1 Joseph P. Wood,1 Takuya Shinagawa,1∗

Jong-Ik Yoo,1∗∗ Dawn A. Santoianni,2 Charles J. King,2 Jost O. L. Wendt,3

and Yong-Chil Seo4

1National Risk Management Research Laboratory, U.S. Environmental Protection Agency,Research Triangle Park, North Carolina, USA2ARCADIS Geraghty & Miller, Inc., Durham, North Carolina, USA3Department of Chemical and Environmental Engineering, University of Arizona, Tucson, Arizona, USA4Department of Environmental Engineering, Yonsei Institute of Environmental Science and Technology,Yonsei University, Wonju, Korea

The potential use of sorbents to manage ultrafine ash aerosolemissions from residual oil combustion was investigated using adownfired 82 kW laboratory-scale refractory-lined combustor. Themajor constituents were vanadium (V), nickel (Ni), iron (Fe), andzinc (Zn). The overall ash content of residual oil is very low,resulting in total ash vaporization at 1725 K with appreciablevaporization occurring at temperatures as low as 1400 K. There-fore, the possibility of interactions between ash vapor and sorbentsubstrates exists. Kaolinite powder was injected at various loca-tions in the combustor. Ash scavenging was determined from par-ticle size distributions (PSDs) measured by a Scanning MobilityParticle Sizer. Impactor samples and X-ray fluorescence (XRF)analyses supported these data. Injection of kaolinite sorbent wasable to capture up to 60% of all the ash in the residual fuel oil.

Received 10 October 2003; accepted 27 May 2004.Portions of this work were conducted under EPA Purchase Order No.

1CR183NASA with J. O. L. Wendt and EPA Contract 68-C-99-201 withARCADIS Geraghty & Miller, Inc. This work was also partially sup-ported by the KEMCO academic research fund, No. 2002CCT03P01,in Korea. The authors would like to thank EPA’s Shirley Wasson forher kind assistance with the XRF analysis. The research described inthis article has been reviewed by the Air Pollution Prevention and Con-trol Division, U.S. EPA, and approved for publication. The contents ofthis article should not be construed to represent agency policy, nor doesmention of trade names or commercial products constitute endorsementor recommendation for use.

∗On leave from JFE Engineering Corporation, Engineering Re-search Center, Kawasaki, Kanagawa 210-0855, Japan.

∗∗Postdoctoral appointment through the Oak Ridge Institute for Sci-ence and Education (ORISE).

Address correspondence to William P. Linak, National RiskManagement Research Laboratory, U.S. Environmental ProtectionAgency, Research Triangle Park, NC 27711, USA. E-mail: [email protected]

However, captures of ∼30% were more common when sorbent in-jection occurred downstream of the combustion zone, rather thanwith the combustion air into the main flame. Without sorbent ad-dition, baseline measurements of the fly ash PSD and chemicalcomposition indicate that under the practical combustion condi-tions examined here, essentially all of the metals contained in theresidual oil form ultrafine particles (∼0.1 µm diameter). Theo-retical calculations showed that coagulation between the oil ashnuclei and the kaolinite sorbent could account for, at most, 17% ofthe metal capture which was always less than that measured. Thedata suggest that kaolinite powders reactively capture a portion ofthe vapor phase metals. Mechanisms and rates still remain to bequantified.

INTRODUCTIONAirborne fine particulate matter (PM) is the subject of con-

siderable environmental interest due to the results of a numberof studies correlating short-term exposures to ambient levels offine PM with acute adverse health effects (Wilson and Spengler1996; U.S. EPA 1996; Bachmann et al. 1996; Wolff 1996). Con-sequently, ambient concentrations and source emissions of PMsmaller than 2.5 µm in aerodynamic diameter (PM2.5) face in-creased regulation (Federal Register 1997). Numerous theoriesexist to explain the mechanisms causing physiological damage.Health effects studies have identified theories related to ambientparticle composition that appear to exacerbate adverse health ef-fects, including the presence of transition metals (e.g., vanadium(V), nickel (Ni), iron (Fe), zinc (Zn), and copper (Cu); Dreheret al. 1997) and aerosol acidity. In addition to particle compo-sition, another apparent factor influencing health impacts is thepresence of ultrafine particles (<0.1 µm diameter; U.S. EPA1996). All of these characteristics—transition metals, acidity,

900

INTERACTIONS BETWEEN OIL ASH AND KAOLINITE SORBENT 901

and ultrafine size—are exhibited by the PM generated from thecombustion of residual fuel oils.

Whereas previous work by our group examined the physicaland chemical characteristics of fine PM from residual fuel oilcombustion (Miller et al. 1998) and determined effects of com-bustor design on differences in emissions (Linak et al. 2000),the research presented here focuses on the potential of an inor-ganic sorbent to reduce the emission of ultrafine metal ash nucleifrom a residual oil-fired refractory-lined combustor. Particle sizedistributions (PSDs) and size-dependent chemical compositionswere compared without and with sorbent injection. Experimen-tal results were compared to predictions of an equilibrium chem-istry model and also to an aerosol growth model. The primarymetals contained in residual oil ash are V, Ni, Fe, and Zn. Previ-ous research has indicated that injection of a sorbent can partiallydepress the mass of ultrafines emitted from residual oil combus-tion (Linak et al. 2003), although the mechanisms governing theprocess are unclear.

The first objective of the current work was to investigatethis process further by determining its response, in a practicalcombustion configuration, to various process parameters such assorbent injection temperature and flow rate. With results fromthese parametric tests, it may be possible to determine whetherthe amount of ash captured could be increased from the low lev-els measured previously (Linak et al. 2003), and consequentlywhether this approach has the potential to be a practical technol-ogy for controlling ultrafine residual fuel oil ash emissions fromcombustion sources. Another objective was to use parametrictest results to shed new light on pertinent mechanisms.

While the occurrence of ash in residual fuel oils is quite dif-ferent than that of coal, much can be learned from the results ofthe extensive studies on coal ash transformations. This researchhas been conducted by a large number of groups to predict thesize-dependent composition of the ash aerosol evolving withinpulverized coal combustion systems. The primary purpose ofthese efforts was to characterize and control the corrosion, foul-ing, slagging, and other detrimental effects ash species have oncombustor materials and heat transfer surfaces. Data on the size-classified PM emission from full-scale coal combustion unitswere first published by Davison et al. (1974), Kaakinen et al.(1975), Klein et al. (1975), Gladney et al. (1976), Ondov et al.(1978, 1979), Nettleton (1979), and Smith et al. (1979), to namebut a few. These data indicated, in general, a bimodal particlesize distribution and the conclusion that the submicron particlesare enriched in a number of trace elements. Detailed mechanismsdescribing ash transformations and aerosol formation from pul-verized coal combustion were first discussed by Sarofim et al.(1977), Flagan and Friedlander (1978), and Flagan (1979), andreviewed by Smith (1980) and Damle et al. (1982). These mod-els propose that major ash inclusions inside the coal particlesoften, melt, and coalesce during char combustion. As the car-bon matrix recedes, these micron-scale structures are released toconstitute the supermicron mode. A small fraction (∼1%) of themineral matter, however, is vaporized. Most readily vaporized

are ash species with higher vapor pressures than the commonrefractory oxides present in coal. The formation and subsequentvaporization of higher vapor pressure refractory metal suboxidesdue to the reducing environment around burning coal particlesis used to account for their relatively high concentrations in thesubmicron aerosol.

In addition to full-scale field studies, numerous smaller scalecombustion studies have made major advances in the under-standing of inorganic ash transformations and aerosol forma-tion from pulverized coal combustion. Early work by Sarofimet al. (1977), Mims et al. (1980), Neville et al. (1981), Quannet al. (1982), Haynes et al. (1982), Quann and Sarofim (1982),Neville and Sarofim (1982), and Neville et al. (1983) used well-defined drop-tube systems, while at the same time Flaganand Taylor (1981) and Taylor and Flagan (1982) used muchlarger laboratory-scale equipment. In general, these studies con-firmed mechanisms of selective vaporization of volatile tracespecies and suboxides from the burning char particles, followedby nucleation or condensation of the less-volatile oxides andtrace elements. Since then, a number of research groups havegreatly expanded the scope of this research, so that todaythere exists a reasonable understanding of the mechani-sms controlling the fate of ash species during pulverized coalcombustion.

While the behavior of inorganic ash constituents within coal-fired power plants has been extensively studied, the study ofresidual oil combustion has focused primarily on the physi-cal and chemical behavior of organic constituents, includingthe formation and combustion of large carbon cenosphere parti-cles (Williams 1976). In fact, for small residual oil-fired processheaters and package boilers, particle emissions are often charac-terized by large (>10 µm diameter) cenospheric coke particlesand high loss on ignition (LOI) values. This is in contrast tolarge utility-scale boilers where high thermal efficiencies andlow values of LOI are more typical. For these systems, particleemissions are characterized by a large submicron ash mode anda much smaller unburned coke mode (Piper and Nazimowitz1985; Walsh et al. 1991). Walsh et al. (1991) show that, in con-trast to pulverized coal, the majority of the sampled fly ash massfrom residual fuel oil-fired utility boilers lies below 1 µm indiameter. This is consistent with the understanding that oil ashis primarily organically bound, while the major fraction of coalash is contained within micron-scale mineral inclusions and ex-cluded overburden (dirt) incorporated during mining (Gluskoter1978). For coals, inherent (organically bound) ash typically con-stitutes only a small portion of the total. In general, the ash con-tents of residual oils are approximately 0.1%, while for coalsash contents are, in general, two orders of magnitude greater.For combustion typical of utility-scale residual oil boilers withlow residual carbon in the fly ash, the majority of the emitted flyash is found within particle sizes described by mechanisms ofash vaporization, nucleation, and growth (Linak et al. 2000). Atstack locations, these particles are present in a mode centered at∼0.1 µm diameter. In contrast, typically less than 1% of coal fly

902 W. P. LINAK ET AL.

Table 1Comparison of utility residual oil and coal usage and contribution of ash to ultrafine particle sizes (uncontrolled)

Residual oila Coalb Residual oil % of total

Electric utility utilization 1.5 × 1010 kg/year 7.1 × 1011 kg/year 2.2% ash in fuel 0.1 10% ash reporting to ultrafine particle mode (approximate) 100 0.5Ultrafine PM produced (uncontrolled) 1.5 × 107 kg/year 3.6 × 108 kg/year 4.3

a1997 fuel oil sales (DOE 1998).b1999 coal sales (Kilgroe et al. 2000).

ash is found in this ultrafine mode, with the vast majority beingfound in fragmentation modes, centered at ∼2 µm and ∼20 µmdiameter (Linak et al. 2002). The formation of primary ultrafineparticles from U.S. utility coal and residual oil usage can be es-timated using the information presented in Table 1 (U.S. DOE1998; Kilgroe et al. 2002). These estimates indicate that for theU.S., coal combustion generates larger quantities of ultrafine PMthan oil, but oil combustion generates larger quantities of ultra-fine PM per equivalent unit of heat produced. Note that discus-sion here is for ultrafine PM, not PM2.5, which, for coal, is dom-inated by a fine fragmentation mode (Linak et al. 2002). Clearly,as far as inorganic primary ultrafine particles are concerned coalstill dominates, with 95.7% of total potential ultrafine emissionsnationally, although the fraction of ultrafine to total coal ash islow. The potential release of oil ultrafines, however, is not neg-ligible at 4.3% and may be even more important in particularlocations. Residual oil fly ash also represents a prototype case forcombustion-generated PM in which all of the mineral matter hasbeen vaporized, without complications of possible interactionswith the usually omnipresent alumino-silicate particles charac-teristic of coal combustion. Hence, combustion of residual fueloil, with the systematic addition of alumino-silicate materialdownstream, allows interactions between organometallics anddispersed crystalline alumino-silicates to be investigated in acontrolled and parametric manner, which was also the purposeof this work.

EXPERIMENTAL APPROACHA new, vertically fired, 82 kW semi-industrial scale combus-

tor shown in Figure 1 was the central experimental device usedfor this study. This rebuilt unit was based on the design of aprevious horizontal unit that had been used for many years inthis laboratory (Linak et al. 1994). This 5.36 m tall modular,steel-shell, refractory-lined research combustor was designed tosimulate the time–temperature histories typical of utility boilers.The 0.52 m inside diameter by 1.07 m long upper burner sec-tion equipped with an International Flame Research Foundation(IFRF) moveable-block variable air swirl burner provide for re-alistic near-flame fuel-air mixing processes, so that experimen-tal results from this laboratory-scale unit might pertain directlyto expected results from practical utility boilers. Following theburner section, the 0.27 m inside diameter lower sections are

equipped with numerous access ports, permitting temperaturemeasurement, gas and particle sampling, and the injection ofsorbents. Temperatures, determined by a shielded type R ther-mocouple, ranged from a peak of ∼1750 K to 1300 K, 5 mand 2.6 s downstream. Figure 1 shows the relationship betweenmeasured temperatures, residence times, and location. This com-bustor was not designed to be an ideal reactor to study fun-damental mechanisms pertaining to single particles. However,because the movable-block burner simulates the segregated airand fuel streams and subsequent contacting in practical residualoil flames, one could, for example, explore the effects of inject-ing metered amounts of sorbent into the air stream upstream ofthe burner, and still have confidence that results would replicatewhat might be expected in large practical boiler flames. In addi-tion, as the postflame gases in this combustor were sufficientlylaminar (Re ∼ 800), one could determine mean reaction timesand temperatures for sorbent injected downstream of the flameand interpret the ensuing data in terms of fundamental mecha-nisms. Thus, the combustor was designed to yield experimentalresults that were both of immediate practical significance, as faras utility boilers are concerned, and also of use for elucidationof mechanisms.

The IFRF burner was equipped with a Spraying Systems Co.(model Air Atom 1/4-JSS) air-atomizing oil nozzle. Oil temper-ature and air pressure were maintained between 380–400 K and200–240 kPa (gauge), respectively. Residual oil was fed at a rateof 7.1 l/h. These settings produced a relatively narrow residualoil droplet size distribution with a mean diameter between 30and 40 µm. The burner was set to produce an IFRF Type 2 (SwirlNumber = 1.48) turbulent diffusion flame. Powdered kaolinitesorbent (Burgess Pigment Co., Sandersville, GA, USA; No. 40,nominally 1.4 µm diameter) was introduced into the combustorusing a K-Tron twin-screw loss-in-weight feeder and air educ-tor (model KCL 2420). Transport air introduced at the screwexit entrained the kaolinite and carried it into the combustor atone of three locations, namely into the air at the burner beforecombustion, where it would see the maximum temperature mea-sured, or at two postflame locations corresponding to tempera-tures of 1600 and 1430 K (see Figure 1). Physical properties andchemical composition of the kaolinite used, as well as the No.6 fuel oil, are shown in Table 2. Sorbent feed rates are reportedbased on sorbent equivalence ratio,�, defined as g-moles Al2O3-2SiO2 per g-moles of V + Ni + Fe + Zn. For this study, target


Figure 1. EPA downfired combustor with port temperatures and calculated residence times indicated. No. 6 residual fuel oil firedat a rate of 7.1 l/h.

values for � varied from 0 to 15. It should be noted, however,that measured sorbent concentrations at the exit of the furnacewere always considerably lower that those directly based onfeeder weight loss. The fine kaolinite powder used proved to beexceedingly difficult to handle, and was easily deposited withinprobes, transfer lines, and other surfaces. In fact, once sorbenthad been fed into the system, a small residual amount seemedto be lodged in crevices and ledges, and continued to enter theflue gas for quite some time after a sorbent experiment had beencompleted. This turns out to be a problem with sorbent injectionin larger and full-scale units as well (Biermann et al. 2003).

Scavenging of residual oil fly ash by dispersed sorbent wasquantified using a modification of the aerosol fractionationmethod used by Gale and Wendt (2002). In this modification, themeasured decrease in the ultrafine mode of the sampled aerosolyielded the fraction of the total ash that was scavenged. Thismethod avoided uncertainties derived from ambiguities presentin the absolute value of the sorbent mode in the PSD, as discussedbelow. Note that in this refractory-lined furnace, where carbonburnout approaches 100%, all of the ash aerosol is normallyfound in the ultrafine mode. This behavior has been shown pre-viously (Linak et al. 2000) for residual oil fly ash from a similar


Table 2Ultimate analysis, heating value, and concentrations of four

major metals in residual fuel oil examined

Ultimate analysis (wt%)Carbon 86.45Hydrogen 10.23Nitrogen 0.26Sulfur 2.07Oxygen 0.90Ash 0.08

Trace elements (µg/g)Vanadium 221Nickel 47Iron 51Zinc 9Higher heating value (kg cal/kg) 10,067Water (wt%) 0.7

Properties of kaolinite sorbent examinedSpecific gravity 2.63Mean particle diameter (µm) 1.4Silica (wt%) 44.8−45.3Alumina (wt%) 37.5−39.7

horizontal refractory-lined furnace but not from a fire-tubeboiler, where steep temperature profiles and high quench ratesproduce significant amounts of carbon, in the form of unburnedchar, in the emitted particles.

PSDs were determined by electrical mobility, time-of-flight,and inertial impaction techniques for sampled aerosols. Extrac-tive samples were taken using an isokinetic aerosol samplingsystem described elsewhere (Linak et al. 1994). Diluted sam-ples were directed to a TSI Inc. scanning mobility particle sizer(SMPS) configured to yield 54 channels, evenly spaced (loga-rithmically) over a 0.015 to 0.7 µm diameter range, and to a TSIInc. aerodynamic particle sizer (APS), which uses time-of-flightprinciples to measure particles with diameters of 0.5–20 µm. To-gether, these two analyzers can measure PSDs over a range fromapproximately 0.01 to 20 µm diameter.

An MSP Inc. nine-stage, 30 l/min micro-orifice uniform de-posit impactor (MOUDI) was used to size-classify and collectphysical samples for chemical analysis. MOUDI stages (andafterfilters) were examined by X-ray fluorescence (XRF) spec-troscopy to examine changes in metal PSDs as a result of sorbentinjection. Raw XRF data is given as counts for a specific element,where the proportionality constant relating counts to concentra-tion varies from element to element on a given stage, but not fromstage to stage for a given element. Therefore, the metal mass frac-tions presented are equivalent to metal XRF count fractions andare determined by individual metal counts (V, Ni, Fe, or Zn) on agiven stage divided by XRF counts for the same metal summedover all stages (and afterfilter). Absolute concentrations of µgmetal per µg of particle are not available.

EQUILIBRIAThe Chemical Equilibrium Analysis (CEA) code (McBride

et al. 1993) was used to predict the element speciation and,more importantly, the fractions of vapor-phase species as a func-tion of temperature for the four principal metals of interest (V,Ni, Fe, and Zn). References for the thermochemical propertiesand species data sets used are found in Linak et al. (2003).These equilibrium predictions suggest vapor-phase V and Fespecies above 1475 K and vapor-phase Ni species above1200 K. Vapor-phase Zn species are predicted to be presentabove 1050 K. Figure 1 shows the temperatures and furnace lo-cations at which each metal is expected to start being removedfrom the vapor (denoted as onset of condensation) and whereit finishes being removed from the vapor (denoted as end ofcondensation).

It is useful to quantify these results further as shown inFigure 2, in which the fraction of total ash predicted to be vaporat a point in the combustion zone is plotted against temperature,with that amount being divided into segments showing relativeamounts of V, Ni, Fe, and Zn in the vapor phase. Thus, when100% of all the ash is predicted to be vapor, the compositionof the vapor is the composition of the original ash in the oil.Figure 2 shows that 100% of the metal is predicted to be va-por at temperatures of 1725 K and above. Thus, at 1740 K, thehighest bulk temperature measured here, all of the oil ash met-als are predicted to be vapor. However, at 1600 K, the highestpostflame sorbent injection temperature, only 25% of the totalmetal is vapor, while at 1430 K the metal vapor fraction is lessthan 10%. If we neglect nuclei-sorbent coagulation and assumethat only metal vapor can interact via chemical or physical pro-cesses with a solid substrate, these calculations suggest that,depending on sorbent injection location, anywhere from 10 to100% of the metal might be captured in this particular system.However, this would only be the case if the interaction weresufficiently rapid and if the contact between metal and substratewere optimum. These equilibrium predictions suggest that onlypartial capture of residual fuel oil ash can be achieved if temper-atures at which metal-sorbent interactions occur fail to exceed1725 K. The equilibrium predictions are, however, sufficientlypromising to suggest that appreciable residual oil ash scaveng-ing is possible, leading to the experimental results describedbelow.

RESULTS

Particle Size DistributionsFigure 3 presents PSDs measured for the baseline case (with-

out sorbent) and also with sorbent addition. These PSDs areindicated by diamond and triangle symbols, respectively.Figure 3, therefore, shows representative SMPS/APS data onthe effects of sorbent injection. Experimental parameters, in-cluding sorbent equivalence ratio (�), sorbent injection temper-ature (Tinj), sampling temperature (Ts), and residence time (τ ),are included. Sorbent was injected through the burner, and these


Figure 2. Equilibrium predictions of the mass fraction and composition of oil ash metals in the vapor phase.

Figure 3. Measured volume PSDs using SMPS (0.015–0.7 µm) and APS (0.5–20 µm) instrumentation.


conditions represent the largest capture observed in this work.These data are in agreement with previous literature (Linak et al.2000, 2002) and clearly suggest that essentially all metals in theresidual oil did vaporize, and subsequently nucleated to form theultrafine particles with a unimodal peak centered at 0.07 µm.The small hump in the large particle region for the base case(no sorbent) is an artifact caused by carryover of residual sor-bent from crevices and ledges in the combustor from previousexperiments. It was not observed in baseline experiments beforesorbent injection experiments had been attempted. Total vapor-ization of all oil ash is consistent with the equilibrium predictionson Figure 2, where 100% vaporization of all ash is predicted attemperatures greater than 1725 K. While the largest bulk tem-perature measured was 1740 K, higher temperatures may wellbe reached locally in regions surrounding burning oil dropletsand within burning char particles. Similarly, local regions mayalso exhibit temperatures below the bulk temperature, resultingin less-effective sorbent capture potential. It should be noted thatthe ultrafine particle mode centered at 0.07 µm diameter cannotbe the result of unvaporized residual ash from the vaporizingfuel droplets. Calculated residual ash particles determined fromthe fuel oil droplet size distribution (30–40 µm mean diame-ter), oil density (0.993 g/cm3), oil ash content (0.08%), and oilash density (3.5 g/cm3) are predicted to have mean diametersbetween 2.0 and 2.3 µm.

The triangle symbols on Figure 3 depict the bimodal PSDsmeasured by the SMPS and APS when kaolinite sorbent atan equivalence ratio (�) of 15 (� defined as g-moles Al2O3-2SiO2/g-moles V + Ni + Fe + Zn). These data represent con-ditions in which the sorbent was injected through the burnerwith the combustion air, making it likely to experience the peaktemperatures of 1740 K or higher. The area under the ultra-fine particle peak, as determined from all the SMPS channels,has decreased by 52%, which was among the largest decreasesmeasured. In this work this difference is taken to reflect theamount of metal ash that was captured. Hence, in Figure 3, ap-proximately 50% of the oil ash was captured by the sorbent.Replicate experiments showed this to vary from 39 to 59%capture for a sorbent injection temperature equal to the max-imum measured (i.e., 1740 K). It should be noted that theseexperiments are conducted in a semi-industrial–scale, swirlingturbulent diffusion flame environment, and therefore some vari-ability in contacting of sorbent and metals and local tempera-ture is to be expected from experiment to experiment. This isespecially true where the sorbent is introduced at the burner,where some degradation of burner performance with time wasobserved as sorbent deposited on and partially blocked the swirlvanes.

The area under the PSD for the supermicron particle sizerange cannot be used to calculate the fraction of ash scavengedsince it changes very little due to metal sorption. Furthermore,it appears that the APS yields a PSD peak that is far too smallto represent a sorbent loading of � = 15. A portion of this

error is due to the considerable deposition of sorbent within thecombustor and transfer lines referred to above. In addition, wedo not believe the rising tail in the PSD above 10 µm reflectsa real phenomenon, as the APS model used has manufacturer-identified problems with small-particle recirculation in theoptics chamber, which results in false large-particle counts.This is further confirmed with additional data, as describedbelow.

Composition PSDs were obtained using MOUDI samples,and XRF analysis. Figures 4a (baseline) and 4b (with sorbent)represent MOUDI-XRF data for sorbent injection at 1600 K(downstream of the flame). The mass fractions of V, Ni, Fe,and Zn measured on each impactor stage divided by the stagedelta log Dp are presented. Similar mass fraction PSDs for Aland Si, found only in the kaolinite sorbent, are also shown onFigure 4b. Again, without sorbent (Figure 4a) almost all of the V,Ni, Fe, and Zn formed (condensing) nuclei in the ultrafine par-ticle size mode. This supports the data on Figure 3 and previousdata (Linak et al. 2000, 2002) showing (almost) complete vapor-ization of the oil ash, and, as noted above, is also consistent withequilibrium predictions provided the metals see temperatureshigher than 1725 K.

Figure 4b shows the bimodal metal fraction PSDs for V, Ni,Fe, and Z, indicating partial capture of each metal when sorbentis injected. Note that the Al and Si PSDs are (mostly) unimodaland can be used to track the sorbent. The small ultrafine peakfor Si suggests the interesting observation that a small portionof silicon in the sorbent may have vaporized, which one wouldnormally expect only under locally reducing conditions (Quannand Sarofim 1982). Note that the MOUDI data show no “tail”reaching upwards at 10 µm, as does the APS. This suggeststhat this aspect of the APS data is indeed an artifact related torecirculation of small particles in the optics field and need notbe considered further.

One can use the MOUDI-XRF data to calculate the per-centage of metals in the oil that were captured by the sor-bent and compare this to the SMPS/APS data. This can bedone in two ways. Method A uses only a single PSD withsorbent injection and focuses attention on the weighted frac-tions of V, Ni, Fe, and Zn collected on the top six stages ofthe MOUDI (0.5 µm < Dp < 20 µm) divided by the totalamount of those four metals in the oil. Method B uses PSDsfrom two experiments (baseline data and sorbent injection data),and focuses attention on the weighted decrease of metals on thesmall particle stages of the MOUDI (comparing Figures 4a andb). The two methods are only identical when the integral un-der the ultrafine particle mode of the baseline data is equal toone.

Specifically, the MOUDI-XRF data are analyzed as follows.For a specific experiment, let Xi

A be the number of XRF countsfor metal A, (V, Ni, Fe, or Zn) on MOUDI stage i (or afterfilter,AF), WA the concentration (µg/g) of metal A in the oil, and Ml

kthe mass fraction of all metals captured on all stages k through l


Figure 4. Measured elemental PSDs for four major residual oil metals and Al and Si present in sorbent: (a) baseline, withoutsorbent; (b) with sorbent.

of the MOUDI. Then, following Method A, the weighted fractionof all metals captured on the top six stages, M6

1 , is equal to thefraction captured (χ captured) and is given by:

χcaptured = M61

=( ∑6

i=1 XiV

/ ∑9+AF

i=1 XiV

)WV +

( ∑6i=1 Xi

Ni

/ ∑9+AF

i=1 XiNi

)WNi +

( ∑6i=1 Xi

Fe

/ ∑9+AF

i=1 XiFe

)WFe +

( ∑6i=1 Xi

Zn

/ ∑9+AF

i=1 XiZn

)WZn

WV + WNi + WFe + WZn.

Following Method B, a comparison between sorbent injectionand baseline cases of the mass of all four metals captured on thebottom three stages and afterfilter (the ultrafine mode) is used


to yield the fraction captured:

χcaptured

= 1 −

( ∑9+AFi=7 Xi

V

/ ∑9+AFi=1 Xi

V

)sorbent

WV +( ∑9 + AF

i=7 XiNi

/ ∑9+AFi=1 Xi

Ni

)sorbent

WNi +( ∑9+AF

i=7 XiFe

/ ∑9+AFi=1 Xi

Fe

)sorbent

WFe +( ∑9+AF

i=7 XiZn

/ ∑9+AFi=1 Xi

Zn

)sorbent

WZn( ∑9+AF

i=7 XiV

/ ∑9+AFi=1 Xi

V

)baseline

WV +( ∑9+AF

i=7 XiNi

/ ∑9+AFi=1 Xi

Ni

)baseline

WNi +( ∑9+AF

i=7 XiFe

/ ∑9+AFi=1 Xi

Fe

)baseline

WFe +( ∑9+AF

i=7 XiZn

/∑9+AFi=1 Xi

Zn

)baseline

WZn

.

For experimental conditions of � = 15, Tinj = 1600 K, Ts =1310 K, and τ = 1.2 s, Method A yields χ captured of 24.9 and27.9% for two replicate cases. This compares favorably withthe decrease of the SMPS areas, averaging 27.9%. Method Byields χ captured of 18.5 and 22.8% and is also within the range ofvalues of the decrease of the SMPS mode. Therefore, these threeindependent methods, namely, using baseline and sorbent SMPSdata, large-particle MOUDI-XRF data, and baseline and sorbentultrafine (only) MOUDI-XRF data, yield similar results for thefraction of ash captured. This provides assurance that ash capturecan be quantified using the experimentally convenient and rapidmethod of measuring the decrease of the SMPS ultrafine mode.

The data presented on Figure 4b also suggest that at this sor-bent injection temperature (Tinj = 1600 K), greater fractions ofNi, Fe, and Zn were captured (about 25–35%), than V (15%).This is qualitatively consistent with the equilibrium predictionsat 1600 K (see Figure 2). The MOUDI-XRF data suggest that at1600 K the solid phase contains 23% of all metals (V + Ni + Fe +Zn) originally in the oil, of which 11% is V, 5% Ni, 6% Fe, and1% Zn. The corresponding equilibrium values for metals in thevapor phase is 24% (total), of which 7% is V, 9% Ni, 5% Fe, and3% Zn. According to both theory and experiment, these elemen-tal percentages change with temperature. However, althoughthere is some qualitative agreement between theory and experi-ment on trends with temperature, there is not quantitative agree-ment at the other temperatures, as shown in the subsection below.

Effects of Sorbent Equivalence Ratio, InjectionTemperature (Location), and Available Residence Timeon Residual Oil Ash Capture

The effects of sorbent equivalence ratio (�), sorbent injectiontemperature (Tinj) and available residence time (τ ) are shown inFigures 5a–c. Data were selected so that results from changingone variable at a time (as closely as experimentally possible)are reported. Figure 5 does not report all the data collected, ornecessarily all the data referred to in other sections of this article.

Figure 5a indicates the effect of increasing sorbent equiva-lence ratio (�), and is important since it shows that capture ef-fects are seen even for small amounts of sorbent. However, theeffects are not linear with sorbent addition. For Tinj = 1600 Kand τ = 0.24 s, a sorbent equivalence ratio (�) of 8 was suf-ficient to reach the asymptotic yield of 24%, as measured bySMPS ultrafine mode decrease. This translates to 12 g sorbentper kg oil. It should be noted that values of � = 2, 4, 8, and15 relate to target sorbent concentrations of approximately 200,400, 800, and 1500 mg/m3. However, as noted above, much sor-bent is lost in the combustion system. Replicate measurements

at the bottom of the furnace indicate actual suspended sorbentconcentrations between 450 and 500 mg/m3 at � = 15.

Figure 5b shows that an important variable is the injectiontemperature of the sorbent (Tinj), with � = 15 and Ts = 1300 K.In general, the higher the sorbent injection temperature thegreater the fraction captured. A maximum capture of 59% wasachieved for one test with the sorbent injected with the burnerair. The lowest capture at the same condition was 39%, andthe average was 52%. No MOUDI samples were obtained forthis condition. Injection of this amount of sorbent through theburner led to a significant fouling problem at the burner. Thisis not surprising, as the IFRF burner is not designed for solids,and its movable block design is known to have a high pressuredrop and high impaction possibilities for particles introducedwith the air. Figure 5b shows data from both the SMPS Methodand from Method A for the MOUDI-XRF data (note diamondand triangle symbols). Agreement between both methods to de-termine capture is good for the two injection temperatures, forwhich both methods were performed. Not all data gathered arepresented in Figure 5, which focuses only in those data relat-ing to changes of one variable at a time. However, some datanot shown in Figure 5 are discussed in the text. At 1600 K thesorbent captures an amount of ash similar to amounts predictedby equilibrium to be vaporized (16–30% measured versus 24%predicted). At 1400 K the sorbent capture exceeds the amountpredicted by equilibrium to be vapor (12–35% measured ver-sus 10% predicted). For injection at the burner, with a max-imum measured temperature of 1740 K, equilibrium predicts100% of the ash to be vapor, while the data show 39–59% cap-tured. Possibly the sorbent did not contact enough of the metalat the highest temperature, resulting in less-than-optimum cap-ture. At 1600 K capture tracked the amount vaporized. However,if only vapor-phase metal reacted with sorbent, the equilibriumpredictions at 1430 K appear to underestimate the amount va-porized. These results may also indicate contributions of addi-tional mechanisms, such as sorbent deactivation at high temper-atures, and perhaps additional physical mechanisms discussedbelow.

Figure 5c shows the effect of residence time (τ ) for Tinj =1600 K and � = 15. In these experiments the sampling positionwas changed. Here, the SMPS data show almost no effect in therange of 0.3–1.2 s. The MOUDI data show somewhat scatteredagreement with the SMPS data, but overall the conclusion tobe drawn is that capture occurs within fractions of a second.Previous work (Davis and Wendt 2000; Gale and Wendt 2002)shows rapid sorbent deactivation during sorption of sodium,lead, and cadmium, and scanning electron micrographs of sam-ples here suggest that a similar process may also be occurringhere.


Figure 5. Measured mass fraction of residual oil metal captured as a function of: (a) sorbent-to-metal equivalence ratio (�);(b) sorbent injection temperature (Tinj); and (c) sorbent residence time (τ ).

DISCUSSIONIt has heretofore been assumed that the primary capture mech-

anism is the reaction between sorbent substrate and metal va-por. However, metal capture can also occur through coagulationof nuclei with sorbent particles or through condensation of themetal vapor on the sorbent particles. Only when sampling oc-curs before the onset of metal condensation must the processbe due to reaction (which may be slow or fast). In this work,sampling was never possible at temperatures above the onset ofcondensation of vanadium, the major metal in the ash.

Coagulation was modeled using the multicomponent aerosolsimulation (MAEROS) code (Gelbard and Seinfeld 1980). Input

parameters include 71 mg/m3 of oil ash (measured), an initialnuclei diameter of 2 nm (geometric standard deviation = 1.1), asorbent feed rate of 800 mg/m3, a sorbent particle mean diameterof 1.4 µm (geometric standard deviation = 1.4), and a tempera-ture profile falling from 1600 to 1290 K in 2.5 s. Results of thesecalculations are shown in Figure 6, in which the evolution of bothmodes with respect to time is depicted. The upper panel showsthe initially assumed PSDs. After 0.25 s calculations show thatapproximately 9% of the oil ash nuclei had coagulated with thesorbent (second panel), while after 2.5 s approximately 17% ofthe nuclei had coagulated with the sorbent. Therefore, coagula-tion might explain a portion of the metal–sorbent interaction, but


Figure 6. Predicted evolution of residual oil nuclei in the presence of sorbent. MAEROS2 was used to predict the amount ofmetal sorbent interaction as a result of coagulation. Input parameters include: oil ash (nuclei) concentration = 71 mg/m3, initialnuclei diameter = 2 nm (geometric standard deviation = 1.1), sorbent concentration = 800 mg/m3, initial sorbent diameter =1.4 µm (geometric standard deviation = 1.4), and a temperature profile falling from 1600 to 1290 K in 2.5 s.

it cannot explain all the capture measured, especially at the moretypical system residence times of less than 1 s. Coagulation is,surprisingly, not negligible, but neither does it seem to be thedominant mechanism to explain the partial captures observed.The coagulation calculations presented in Figure 6 assume thatnone of the vapor-phase oil ash interacts with the sorbent and thatall this ash contributes to the nuclei mode. If one assumes that35% of the vapor-phase oil ash interacts with the sorbent prior tonucleation, then model calculations predict 8 and 15% of the oilash to coagulate with the sorbent at 0.25 and 2.5 s, respectively.This also results in a slight reduction in the nuclei mean diameterat 2.5 s from ∼0.09 to ∼0.08 µm, and this qualitative behavioris noted in the measured PSDs presented in Figure 3.

Friedlander et al. (1991) examined conditions where coagu-lation between a fine aerosol and a coarse particle mode might

be an important mechanism. They concluded that the half-lifeof the fine mode depends strongly on its initial nuclei massconcentration (ρgρpvo) and on the coarse-mode diffusion in-tegral (M1) given by Flagan and Friedlander (1978). Figure 4of the Friedlander et al. (1991) paper presents a stability di-agram, determined for a system temperature of 1000 K and1 nm nuclei. Using values for ρgρpvo (71 mg/m3) and M1

(1.2 × 106 m/kg) for our system, Figure 4 from the Friedlan-der et al. (1991) paper indicates a fine mode half-life (due tocoagulation) of ∼100 s. While this time scale is far larger thanthat available in the combustor, it is sufficiently small enough toindicate that measurable coagulation between the oil ash nucleiand the sorbent is possible. This is consistent with MAEROScalculations that were allowed to continue for 100 s, which pre-dict oil ash nuclei–sorbent coagulation of 54%. As noted by


Friedlander et al. (1991), the major temperature dependence isincorporated into the gas density as well as a weak T

1/2 de-pendence for particle diffusion in the free molecular regime.MAEROS calculations performed at ambient temperatures pre-dict much smaller coagulation rates of 2 and 4% at 0.25 and 2.5 s,respectively.

The bulk of the capture, therefore, occurs either by surfacechemical reaction or surface physical condensation. If surfacereaction (or pore diffusion) is controlling, the metal composi-tion of supermicron particles should have 1/Dp dependence. Ifgas-film diffusion is controlling (condensation or very fast, ex-ternal surface, reaction) the metal composition should have 1/D2

pdependence (Friedlander 1977; Haynes et al. 1982; Linak andWendt 1993). Coagulation might also lead to 1/D2

p dependence.The MOUDI-XRF data show that particles on the four impactorstages to which the sorbent reports are in the applicable contin-uum regime. Results for all four metals for the four applicableparticle sizes are plotted on Figure 7. Results are inconclusive

Figure 7. Mass concentration of residual oil metals versus particle diameter over the continuum regime where sorbent particlesare present.

because the data are scattered, and so interpretation should beviewed as tentative at best. Logarithmic slopes vary from −2(gas film diffusion controlled), through −1 (external surface re-action, or pore diffusion controlled), to zero (interior surfacereaction controlled). In general, higher injection temperatureslead to less-sensitive particle diameter dependence, especiallyfor Fe and Zn but also for V and Ni. This trend is consistent withmechanisms in which higher injection temperatures lead to aninitial rapid formation of larger mesopores inside the kaoliniteparticles caused by greater separations between the crystallinealumino-silicate platelets in the freshly formed meta-kaolinite.This leads to greater penetration of metal vapors into the cen-ter of the particle and, consequently, to a possible mechanismtransition to that of interior surface reaction control. However,melting can later close these mesopores and deactivate the pro-cess (Gale and Wendt 2003) as it proceeds, and this was observed(through scanning electron micrographs) in many but not all re-acted particles.


CONCLUSIONSThe potential for high temperature capture of metals in resid-

ual oil fuel ash by powdered kaolinite has been investigated.Partial capture of oil ash is possible, although capture efficien-cies (determined three ways) greater than ∼30% could not beachieved when sorbent was injected downstream of the flame.This is roughly consistent with the partial ash vaporization pre-dictions of equilibrium, although captures at lower tempera-tures were greater than those predicted by equilibrium. Injectionthrough the burner caused fouling problems but led to signifi-cantly higher capture, in one case up to 59%. The through-the-burner injection process has yet to be optimized, and future workmight address how flame aerodynamics may be exploited in or-der to enable oil ash to be captured by sorbent injected with theair. Since short residence times are effective, the key seems to berelated to mixing between the sorbent and vaporized ash comingfrom the oil droplets. Sorbent deactivation by processes includ-ing sintering, melting, and pore closure of the kaolinite may belimiting metal captures at high temperatures, and these have beenidentified previously for high temperature kaolinite sorption oflead and cadmium (Davis 2000). However, at low temperaturesthe effective metal sorption may be enhanced by nuclei-sorbentcoagulation processes, causing metal capture greater than thosepredicted by metal volatility.

While coagulation between the oil ash nuclei and coarse sor-bent particles is nonnegligible, it cannot account for the amountsof metal capture observed. Particle composition data do not sup-port only film diffusion-controlled processes. Neither do theysupport only external surface-reaction–controlled processes, oronly internal surface-reaction–controlled processes. Most likelysome combination of all of these is appropriate, with an appar-ent transition to (interior) reaction control at the higher sorbentinjection temperatures.

These data do not currently provide great encouragement forthe practical utility of sorbent injection to remove more than50% of the ultrafine oil ash particles. However, the fact thatsorbent injection through the burner was able to vastly increasethe fraction of ash captured, including vanadium, may providesome hope that future research in this direction may lead tosubstantially greater captures.

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High Temperature Interactions Between Residual Oil Ash and Dispersed Kaolinite Powders