INVESTIGATION OF FLY ASH AND ACTIVATED CARBON

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INVESTIGATION OF FLY ASH AND ACTIVATEDCARBON OBTAINED FROM PULVERIZED COAL BOILERSFINAL REPORTSeptember 1, 2003 to August 31, 2006Edward K. Levy, Christopher Kiely and Zheng YaoNovember, 2006DE – FG26-03NT41796Energy Research CenterLehigh University117 ATLSS DriveBethlehem, PA 18015

DISCLAIMER“This report was prepared as an account of work sponsored by an agency of theUnited States Government. Neither the United States Government nor any agencythereof, nor any of their employees, makes any warranty, express or implied, orassumes any legal liability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or process disclosed, or representsthat its use would not infringe privately owned rights. Reference herein to any specificcommercial product, process, or service by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply its endorsement, recommendation, orfavoring by the United States Go vernment or any agency thereof. The views andopinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.”ACKNOWLEDGEMENTSThe authors of this report are grateful to the following individuals for theirassistance in carrying out the various tasks: Dr. Andrew Burrows, Electron Microscopy Specialist in Materials ScienceBrian Celeste, Graduate Student in Mechanical EngineeringDr. Alfred Miller, Research Scientist in Chemistryii

ABSTRACTOne of the techniques for Hg capture in coal-fired boilers involves injection ofactivated carbon (AC) into the boiler downstream of the air preheater. Hg is adsorbedonto the AC particles and fly ash, which are then both removed in an electrostaticprecipitator or baghouse.This project addressed the issues of Hg on activated carbon and on fly ash froma materials re-use point of view. It also addressed the possible connection betweenSCR reactors, fly ash properties and Hg capture. The project has determined thefeasibility of separating AC from fly ash in a fluidized bed and of regenerating theseparated AC by heating the AC to elevated temperatures in a fluidized bed. Thetemperatures needed to drive off the Hg from the ash in a fluidized bed have also beendetermined. Finally, samples of fly ash from power plants with SCR reactors for NOxcontrol have been analyzed in an effort to determine the effects of SCR on the ash.iii

TABLE OF CUTIVE SUMMARY3Background3Results and Recommendations3Separation of Activated Carbon and Fly Ash in a Fluidized Bed3Relation Between Mercury Concentration and Carbon Content3Removal of Hg From Activated Carbon and Fly Ash4Morphologies of Fly Ash and Activated Carbon4Detection of Hg on Individual Carbon Particles4Effect of SCR on Fly Ash Surface Chemistry5EXPERIMENTAL6Task 1: Separation of Activated Carbon a nd Fly Ash in a Fluidized Bed6Task 2: Removal of Hg from Activated Carbon and Fly Ash9Task 3: Microstructural and Chemical Analysis of Fly Ash and ActivatedCarbon10Task 4: Effect of SCR on Ash Properties12RESULTS AND DISCUSSION13Separation of Activated Carbon a nd Fly Ash in a Fluidized Bed13Removal of Hg from Activated Carbon and Fly Ash19Morphologies of Fly-Ash/Activated Carbon Mixtures23General Characteristics23Additional Studies of Activated Carbon and Unburned Carbonin Fly Ash29Fly Ash/AC Mixture Obtained by Segregation30Pure Activated Carbon30iv

TABLE OF CONTENTS (continued)PageFly Ash With High Unburned Carbon32Detection of Hg on Individual Carbon Particles35Microstructural Characterization of Fly Ash Passing Through SCRCatalysts36SUMMARY AND CONCLUSIONS40Separation of Activated Carbon and Fly Ash in a Fluidized Bed40Relation Between Mercury Concentration and Carbon Content40Removal of Hg from Activated Carbon and Fly Ash40Morphologies of Fly Ash and Activated Carbon41Detection of Hg on Individual Carbon Particles42Effect of SCR on Fly Ash Surface Chemistry42REFERENCES42v

LIST OF FIGURESFigurePage1Laboratory Batch Fluidized Bed.72Variation of Minimum Bubbling Velocity with Sound Pressure Level.73Sketch Showing Layering Technique for Analysis of CarbonStratification in Fluidized Bed.84Vertical Stratification of Carbon and Hg. V air 0.6 cm/s.145Vertical Stratification of Carbon. V air 0.7 cm/s.146Vertical Stratification of Hg and Carbon. V air 0.7 cm/s.157Vertical Stratification of Hg and Carbon. V air 0.8 cm/s.158Vertical Stratification of Carbon. V air 0.9 cm/s.169Vertical Stratification of Carbon. V air 1.1 cm/s.1610Magnitude of Carbon Stratification Versus Superficial Air Velocity.1711Minimum Fluidization Velocity From Bed Pressure Drop Test.1812Hg Content Versus Carbon Content.1813Bed Temperature Versus Time (High Carbon).2014Hg Content Versus Bed Temperature (High Carbon).2015Bed Temperature Versus Time (Medium Carbon).2116Hg Content Versus Bed Temperature (Medium Carbon).2117Bed Temperature Versus Time (Low Carbon).2218Hg Content Versus Bed Temperature (Low Carbon).2219Critical Temperature Versus Carbon.2320Low Magnification SEM Micrograph of a Fly Ash/AC Samplefrom Boiler A. Note the Diverse Range of Particle Sizes andMicrostructures That Can Co-Exist.24vi

LIST OF FIGURES (continued)FigurePage21SEM Micrograph (a) and EDS Spectrum (b) From a TypicalParticle of Unburnt Carbon.2422SEM Micrograph (a) and EDS Spectrum (b) From a TypicalSpherical Morphology Alumino-Silicate Particle.2523SEM Micrographs From Large Carbon Particles Having a HollowShell Morphology.2624SEM Micrograph (a) and Corresponding EDS Elemental Map(b) From a Typical Hollow Morphology Carbon Particle.2625TEM Micrograph of a Typical Agglomerate of Activated CarbonParticles2726TEM Micrograph of an Alumino-Silicate Sphere Decorated withActivated Carbon Particles2727An SEM Micrograph Showing the Typical Morphology of a ‘Pure’Activated Carbon Sample.2828EDS Spectrum Obtained From the Entire Area Imaged in Figure 27.2829Bright Field TEM Micrograph Showing an Irregular Carbon Particlein the Pure AC Sample Decorated with Smaller Carbon Particles.2930Higher Resolution Electron Micrograph of the Smaller DecorationParticles in Figure 29, Showing Them to be Amorphous Carbon.2931Top Layer of FA-1 Fly Ash Segregation.3132Bottom Layer of FA-1 Fly Ash Segregation.3133Optical Microscopy Image of Pure Activated Carbon Particles.3234Optical Microscopy Image of Typical Pure Activated Carbon Particle. 3335SEM Image of Fly Ash With High Unburned Carbon.3336SEM Image of Typical Large Unburned Carbon Particle.3437Optical Microscopy Image of Large Unburned Carbon Particles.34vii

LIST OF FIGURES (continued)FigurePage38Optical Microscopy of Typical Large Unburned Carbon Particles.3539Bright Field Low Magnification Micrograph of the ‘After’ SCRSamples3740Bright Field Micrograph of a Typical Spherical Fly Ash SampleShowing Surface Decoration with Carbon.3841High Resolution Bright Field Micrograph of an Agglomerate ofAmorphous Carbon Particles.3842Widescan XPS Survey Spectra of the ‘Before’ and ‘After’ SCRMaterials.3943Higher Resolution XPS Spectra From the ‘Before’ and ‘After’SCR Materials.39viii

LIST OF TABLESTable1PageFly Ash and Activated Carbon Samples Examinedix30

INTRODUCTIONBackgroundOne of the techniques for mercury (Hg) capture in coal-fired boilers involvesinjection of activated carbon (AC) into the boiler downstream of the air preheater. Hg isadsorbed onto the AC particles and onto the fly ash, which are then both removed in anelectrostatic precipitator or baghouse. While field trials with AC injection havedemonstrated the ability to remove significant fractions of the Hg at some units, thereare also problems in using AC for Hg capture. Activated carbon is relatively expensive,leading to high projected costs for Hg capture. The AC can increase opacity at unitswith electrostatic precipitators, due to increased particulate loading and the lowresistivity of AC particles. The feed rates of AC required to control Hg can also result insignificant increases in the carbon content of the ash. However, fly ash used inconcrete must have carbon contents of 4 percent or less in order that the concrete haveacceptable mechanical properties. This has raised concerns that widespread use of ACfor Hg capture will eliminate concrete as a viable market for re-use of ash, therebygreatly reducing the percentage of coal ash which can be re-used.Data reported in the literature on field tests at boilers equipped with SelectiveCatalytic Reduction (SCR) reactors show that SCR reactors promote oxidation ofelemental Hg, which then results in increased capture of flue gas Hg by fly ash particles.However, the actual mechanisms for the enhanced oxidation in the SCR reactor havenot yet been adequately explained.This project addressed the issues of Hg on activated carbon and on fly ash froma materials re-use point of view. It also addressed the possible connection betweenSCR reactors, fly ash characteristics and Hg capture. The project determined thefeasibility of separating AC from fly ash in a fluidized bed and of regenerating theseparated AC by heating the AC to elevated temperatures in a fluidized bed. Theproject also determined the temperatures needed to drive off the Hg from the ash in afluidized bed. Finally, samples of fly ash from power plants with SCR reactors for NOxcontrol were analyzed in an effort to determine the effects of SCR on the ash.1

ObjectivesThe specific objectives of this project were as follows: Determine the potential for separation of AC from fly ash in a bubblingfluidized bed Determine the temperatures needed to remove Hg from spent AC in abubbling fluidized bed, thereby creating the possibility of recyclingregenerated AC back to the boiler Determine the temperatures needed to remove Hg from fly ash in a bubblingfluidized bed Determine which components of fly ash are important for Hg capture Determine if SCR reactors affect fly ash chemistry in relation to Hg capture2

EXECUTIVE SUMMARYBackgroundOne of the techniques for mercury (Hg) capture in coal-fired boilers involvesinjection of activated carbon (AC) into the boiler downstream of the air preheater. Hg isadsorbed onto the AC particles and fly ash, which are then removed in an electrostaticprecipitator or baghouse.This project addressed the issues of Hg on activated carbon and fly ash from amaterials re-use point of view. It also addressed the possible connection betweenSelective Catalytic Reduction (SCR) reactors, fly ash properties and Hg capture. Theproject determined the feasibility of separating AC from fly ash in a bubbling fluidizedbed and of regenerating the separated AC by heating the AC to elevated temperaturesin a fluidized bed. The temperatures needed to drive the Hg from the fly ash in afluidized bed were also determined. Finally, samples of fly ash from power plants withSCR for NOx control were analyzed to determine the effects of SCR on the ash surfacechemistry.Results and RecommendationsSeparation of Activated Carbon and Fly Ash in a Fluidized Bed. Experiments wereperformed with a mixture of activated carbon and fly ash to determine to what extent theunburned carbon in the fly ash and the activated carbon can be separated from the inertportion of the fly ash in a bubbling fluidized bed. The data show that carbonsegregation is very sensitive to superficial gas velocity, with the strongest segregationoccurring at superficial velocities of 0.7 to 0.8 cm/s. At these conditions, the carboncontent at the top of the bed was approximately 27 percent and it was less than 17percent in the bottom layer. Very little or no carbon segregation occurred at fluidizationvelocities much lower than 0.7 to 0.8 cm/s or higher than 1.1 cm/s.While these experiments show it is possible to achieve carbon separation in abubbling fluidized bed, the differences in particle density between the carbon particlesand the inert fly ash particles appear not to be large enough to make this separationapproach practical for commercial applications. There are other particle separationtechniques based on triboelectric and electrostatic principles, and these might be bette rsuited for this application. It is recommended that feasibility tests be performed onAC/fly ash mixtures using the triboelectric and electrostatic approaches.Relation Between Mercury Concentration and Carbon Content. Multistageseparation experiments were performed on the AC/fly ash mixture to expand the rangeof carbon contents between the top and bottom layers of the fluidized bed. Analyses ofcarbon and bulk mercury contents of the samples obtained from these tests show astrong linear relationship between Hg and carbon content, with the bulk Hg contentapproaching zero as the carbon content of the material goes towards zero.3

Removal of Hg from Activated Carbon and Fly Ash. Elevated temperature fluidizedbed experiments were performed on the low carbon content mixture from the bottomlayers of the fluidized bed, on the high carbon content mixture from the top layers of thefluidized bed and on AC/ash mixture with average carbon content. All behavedqualitatively the same way, with a constant Hg content until a critical temperature wasreached and then with rapidly decreasing Hg content as the temperature was increasedto higher levels. The critical temperature was found to be a linear function of carboncontent, increasing from 330 C at 17 percent carbon to 370 C at 33 percent carbon.The temperature at which all of the Hg was removed is in the 450 to 500 C range.These results confirm that it is possible to remove Hg from AC and fly ashparticles by heating the material in air, but that particle temperatures as high as 500 Cwould be needed to remove all the Hg. If it were desired to use this approach toregenerate used AC, experiments would first be needed to determine if the Hgadsorption properties of the AC are changed by having been heated to thesetemperatures.The test results also show that the Hg on fly ash and AC will not be released tothe atmosphere through heating of the material, provided the temperatures do notexceed 300 C.Morphologies of Fly Ash and Activated Carbon. Scanning Electron (SEM) andTransmission Electron (TEM) Microscopes were used to study the physical andchemical characteristics of the AC/ ash mixture used in the fluidized bed separationexperiments. Five distinct morphologies were identified: large (30-100 µm) irregularlyshaped carbon particles, 0.1 to 20 µm spherical alumino -silicate particles, 50-100 µmhollow carbon particles with porous walls, fine 50-200 nm amorphous carbon particles,and large 20-50 µm angular activated carbon particles.Four additional ash and activated carbon samples were then evaluated by LightOptical and Scanning Electron Microscopy. SEM studies of the fly ash/AC mixtureobtained from the fluidized bed segregation experiments showed marked differencesbetween the materials from the top and bottom layers of the fluidized bed. The top layerwas dominated by large, irregularly shaped particles while the bottom layer had morespherical high-mineral content particles. This finding is consistent with the physicalmechanism of segregation which results in denser, smaller particles moving downwardtowards the distributor and lighter, larger particles floating at the top of the bed.Light Optical Microscopy images of large (50 to 100 microns) activated carbonparticles showed them to be irregular in shape and filled with voids. Light OpticalMicroscopy studies of a fly ash, with a high naturally-occurring carbon content, showedthe carbon in fly ash (usually referred to as unburned carbon) has an internal structurewhich is simila r in appearance to that of pure activated carbon.Detection of Hg on Individual Carbon Particles. Measurements were performed withboth Transmission Electron Microscopy (TEM) and X-Ray Photoelectron Spectroscopy(XPS) in an attempt to detect Hg on individual carbon particles. The results showed theHg concentrations were too small to be detected by either measurement method.4

Effect of SCR on Fly Ash Surface Chemistry. Samples of fly ash were obtained froma boiler with a Selective Catalytic Reduction (SCR) reactor for NOx control. Thesesamples, which were obtained from upstream and downstream of the SCR, wereanalyzed by X-ray Photoelectron Spectroscopy (XPS) to determine the effects of theSCR on the surface chemistry of the fly ash. The most significant differences were asfollows: The ‘after’ SCR material had a significant surface Cl content, while the‘before’ SCR material was essentially devoid of Cl. The surface S signal in the ‘after’ SCR sample was about half of thatobserved in the ‘before’ SCR sample. The surface Fe content shows the opposite trend to the S signal. It wasabout 50 percent larger in the ‘after’ SCR sample than in the ‘before’ SCRsample.5

EXPERIMENTALThis project was, predominately, an experimental study, involving experiments influidized beds and laboratory analyses of activated carbon and fly ash by electronmicroscopy methods and other analytical techniques.Task 1: Separation of Activated Carbon and Fly Ash in a Fluidized BedBecause of the strong solid phase mixing and gas-solids interactions which occurin a bubbling fluidized bed operated at gas velocities well above minimum fluidization,fluidized beds are in widespread use in industry for applications such as heatexchangers, combustors, gasifiers, chemical reactors, and solids dryers. However, atsuperficial gas velocities just slightly above minimum bubbling, the solids do not mixwell, and as a result, particle segregation occurs in the vertical direction, with the moredense particles settling downward to wards the distributor and the lowest densityparticles moving towards the free surface of the bed (Ref. 1 and 2).The activated carbon used in Hg capture field trials sponsored by EPRI and DOEhas approximately the same mean particle size as fly ash (15 to 20 µm mass meandiameter), but it has a lower particle density than fly ash due to the higher porosity ofactivated carbon (Ref. 3). It was expected this difference in particle density would makeit possible to separate the bulk of the spent activated carbon from the fly ash in afluidized bed, and the first group of experiments determined the bed operatingconditions needed to accomplish this and the resulting degree of separation.Previous research on the behavior of fly ash in bubbling fluidized beds showedthat, under normal circumstances, fly ash can be difficult to fluidize. The very smallparticle size leads to significant attractive (Van der Waal) forces between particleswhich can make the ash cohesive. Fly ash falls into the category of Geldart type Cpowders, which exhibit gas channeling and spouting, instead of bubbling (Ref. 4).However, the research has also shown that cohesive powders can be made to bubble ina gas fluidized bed with the assistance of high intensity sound (Ref. 5 and 6). Figure 16

shows a gas fluidized bed with a loudspeaker positioned above the free surface of thebed. The high intensity sound waves agitate the bed material, disrupt the inter-particleforces, and make it possible to obtain stable bubbling. Figure 2 shows the effect ofsound pressure level (SPL) in the bed on minimum bubbling velocity for three cohesivepowders, including fly ash (Ref. 6). The background SPL in the laboratory was 85 dB.The data show the powders could not be made to fluidize unless the SPL exceeded 120to 133 dB. At higher values of SPL, the minimum bubbling velocity decreased withincreasing SPL.SpeakerAshDistributorAirFigure 1: Laboratory Batch Fluidized Bed.0.600.50dpU mb (cm/s)0.40Poor fluidization,channeling andspouting0.300.20Fly AshTalc0.10Aluminum Nitride0.008090100110120130140150160SPL o (db)Figure 2: Variation of Minimum Bubbling Velocity with Sound Pressure Level.7

All of the fluidized bed experiments being performed in this project were carriedout with acoustic exc

Removal of Hg From Activated Carbon an d Fly Ash 4 Morphologies of Fly Ash and Activated Carbon 4 . unburned carbon in the fly ash and the activated carbon can be separated from the inert portion of the fly ash in a bubbling

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