Energy & Fuels 1993, 7, 803-813
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Thermal Treatment of Hazardous Wastes: A Comparison of Fluidized Bed and Rotary Kiln Incineration Karl K. Rink,? Fred S. Larsen, Janusz A. Kozihski, JoAnn S. Lighty,' Geoffrey D. Silcox, and David W. Pershing Departments of Chemical and Fuels Engineering and Mechanical Engineering, 3290 MEB, University of Utah, Salt Lake City, Utah 84112 Received May 21, 1993. Revised Manuscript Received September 14, 1 9 9 9
Large volumes of sludge are produced by a wide variety of industrial processes and by municipal waste water treatment. Interest in incinerating these sludges, either alone or co-fired with other fuels, is increasing. The issues surrounding sludge incineration in rotary kilns and fluidized beds were identified through a series of pilot-scale tests using two slightly different paper mill sludges. The specific issues examined include hydrocarbon emissions, NO, emissions, and bottom and fly ash properties. A 61-cm i.d. X 61-cm long, 130-kW pilot-scale rotary kiln simulator (RKS) and a 23-cm i.d., 300-kW circulating fluidized bed combustor (CFB) were maintained at a nominal temperature of 1100 K and a stoichiometric ratio of 1.5. The rotary kiln was fed in a batch mode in order to simulate the passage of solids through a kiln. The fluidized bed was fed in both batch and continuous modes. Samples were removed from the kiln (bottom ash) and transition section (fly ash). Samples of the fluidized bed materials were removed from the bed (bottom ash) and after the cyclone (fly ash). The exhaust gases were analyzed continuously for hydrocarbons, CO, 02,NO, and COz. This paper presents data on these analyses as well as NO conversion and ash properties. The production of NO in the RKS was dependent on the supply of nitrogen (in the sludge) and oxygen (in the gas phase) in the reactor. The availability of oxygen to the sludge was affected by the particle diameter of the sludge, the charge size, and whether a solids bed was present at the time of the incineration. In the CFB, the nitrogen-containing compounds were oxidized primarily downstream of the feedboard region, resulting in elevated levels of NO in the transition and cyclone regions. Carbon monoxide concentrations were high immediately above the bed which lead to the reduction of NO inside the freeboard zone. In both the CFB and the RKS tests little unburned hydrocarbons were present in the exhaust gas streams. Formation of fly ash particles was dependent on types of incinerated material (sludge; mixture of sludge and silica sand). Bottom ash material resembled randomly organized skeletons (or cenospheric skeletons), the structure of which was independent of the type of sludge or reactor. Smaller fly ash and bottom ash particles were formed during CFB incineration experiments.
Introduction According to Barton,' the U.S.Environmental Protection Agency (EPA) estimated that 7.7 million dry metric tons of sewage sludge are generated every year; no estimate is given on the amount of industrial waste water treatment sludge that is generated. Of this amount, only about 20% is incinerated. Several types of facilities are used to incinerate sludges: multiple hearth incinerators are the most common systems used with fluidized bed systems following (82% and 13%). Other alternatives to incineration include land application, land disposal, and Ocean disposal. A variety of studies have been conducted on the emissions from sludge incinerators. Specifically, Parrish et al.? Barton et al.,l Dewling et al.? Gerstle and Albrinck? and Hentz et al.6have investigated the emissions from Author to whom correspondence should be directed. Mechanical Engineering. Abstract published in Advance ACS Abstracts, October 15, 1993. (1) Barton, R.G.;Seeker,W. R.; Bostian, H. E. Trans. ZChemE. 1991, 69,Part B, 29-36. (2)Parrish, C. R.; Palazzolo, M. A.; Vancil, M. A,; Bostian, H. E.; Crumpler, E. P.TTons. IChemE. 1991,69,Part B, 20-28. (3)Dewling, R.T.;Manganelli, R. M.; Baer, G. T. J. WPCF 1980,52 (lo), 2552-2557. (4)Gentle, R. W.; Albrinck, D. N. J. Air Pollution Control Assoc. 1982,32,1119-1123. t Department of
full-scale sewage sludge incinerators, including both multiple hearths and fluidized beds. Tireyet and Dellinger et ala7have completed studies at the laboratory scale on organic emissions and have made comparisons with fullscale units. These studies indicate that a number of hydrocarbons and metals may be emitted during combustion of sludge materials. Cadmium and lead were frequently emitted, and benzene, acrylonitrile, and vinyl chloride are three of the many hydrocarbons which were identified in the exhaust of several systems. Comparisons between the different types of full-scale facilities is difficult; for example, temperature measurementa at full-scaleare hard toperform. Facilities are often equipped with different air pollution control devices, making comparisons of exit stack emissions an inaccurate means of characterizing combustion performance. For these reasons, this study was performed to investigate the incineration of two sludges in a pilot-scale rotary kiln simulator and a pilot-scale fluidized bed facility. The kiln was operated in a batch mode while the fluidized bed was (5)Hentz, L. H.;Johnson, F. B.; Baturay, A. Water Enuiron. Res. 1992,64,111-119. (6)Tirey,D. A.;Striebich,R. C.; Dellinger,B.;Bostian,H. E. Hazardous Waste and Hazardous Materials 1991,8,201-218. (7) Dellinger, B.; Mazer, S. L.; Bobbs, R. A. J. Air Waste Manage. Assoc. 1991,41, 838-843.
0887-0624/93/2507-0803$04.00/00 1993 American Chemical Society
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804 Energy & Fuels, Vol. 7, No. 6, 1993
Purged Sight Port
Exhaust
Relief Valve And Rupture Disk
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Figure 1. Overhead view of the rotary kiln simulator (RKS).
operated in both batch and continuousmodes. The smaller sizes of the facilities make them easier to control and instrument so that detailed and accurate information can be obtained. The studies were conducted at a nominal temperature of 1100 K, with multiple feed rates, and with different sludges. Total hydrocarbons, nitric oxide, and carbon monoxide were the pollutants monitored. Two industrial waste water treatment sludges were studied. Investigations of the bed and fly ash solid materials from the two systems were also conducted.
Experimental Section Rotary Kiln Simulator (RKS). The small, pilot-scale rotary kiln, shown schematicallyin Figure 1,consisted of a kiln, a flue, and an afterburner to incinerate any organic compounds that were not destroyed in the kiln. The kiln's inside dimensions were 0.61 X 0.61 m and it was heated directly by a natural gas burner. The burner had a maximum firing rate of 130 kW. The kiln was rotated (maximum rotation rate = 2 rpm) by a chain and sprocket assemblywhich was driven by a 1-HP electric motor. Solidswere introduced into the kiln with a 12 X 10-cmstainless steel cup welded on the end of a 2-m-long stainless steel bar. The cup fit through the 15-cm blast gate at the back of the kiln. Solid samples of the RKS solids bed were also taken through the blast gate with the same device. The person manipulating the sample probe was protected by asbestosgloves and a face shieldto prevent accidental burns. The 0.91-m X 15-cm i.d. (throat) flue section of the facilitywas located just down stream of the kiln. The samples for the analytical instruments used in this experiment were withdrawn from this region. The 5-m by 15-cm i.d. afterburner section was located down stream of the flue section. Two 70-kW, horizontally opposed natural gas burners were located in the afterburner section. Any hydrocarbons remaining in the kiln exhaust were destroyed by these two burners. Further details of the rotary kiln simulator are discussed by Owens et al.* Temperatures were measured at several locations in the kiln with type K, ungrounded thermocouples. For the RKS experiments, the thermocouples were placed in (1)the kiln wall, (2) the center of the kiln's freeboard (suction pyrometer), and (3) the flue. Gas samples were continuously withdrawn from the flue. CirculatingFluidized Bed Combustor (CFB). A schematic of the CFB facility is shown in Figure 2. The entire unit was 9 m high and had outside and inside diameters of 61 and 26 cm, (8)Owens, W. D.; Silcox, G. D.; Lighty, J. S.; Deng, X. X.; Pershing, D. W. Combust. Flame 1991,86,101-114.
Nozzle Plate
%,ew\ ,,
Auxiliary Auger Feed Mechanism
Figure 2. Circulating fluidized bed (CFB) facility. respectively. The maximum firing rate of the reactor was 300 kW. The design of the primary air control system allowed the facility to be operated in either the bubbling or the circulating modes, with maximum superficial velocities in excess of 7 m/s. For these studies the reactor was operated in the bubbling mode. The reactor was comprised of nine individual sections lined with two layers of castable refractory. The outer layer was 10.16 cm thick and relatively light (561 kg/m3)while the inner layer was 7.62 cm thick and dense (2130 kg/m3). The reactor could be fired to the limiting temperature of the inner refractory (1700 K) while maintaining an acceptable external wall temperature since each individual section of the reactor had two separate, coiled, 1.27-cm-diameter stainless steel cooling tubes embedded in the refractory. Each sectionfeatured a number of porta for samplingpurposes. Diametrically opposed, 6.4-cm diameter porta provided access for the insertion of large probes,such asthe solid particle sampling probe or a suction pyrometer probe. A large number of smaller ports were spaced axially and circumferentially in each section. Pressure from any number of these taps could be read individually or averaged by the data acquisition system. Alternatively, small thermocouple probes could be inserted through these porta to variable depths in the reactor. In this study, temperatures were taken at 20 locations along the wall and six locations in the fluidized bed. Gas samples were withdrawn from the CFB at five different axial locations as shown in Figure 2. The facilitywas designed to burn a wide variety of fuels. Four primary fuel injectors were located near the bottom of the reador to preheat the bed and the entire system to a desired temperature, as well as to provide heat input when incinerating low-calorific solids. Two secondaryinjectorswere located further downstream in the reactor. Fuel injector operation was monitored using ultraviolet radiation detectors and flame ionization detectors. The reactor was fueled with natural gas. T w o separate systems existed to feed solid material directly into the reactor 30.5 cm above the nozzle plate. One system used a Monyo pump to supply high water content sludgesat a nominal rate of 20-120 kg/h. For the tests described in this paper, an alternative mechanism consisting of a 10-cm-diameter,stainless steel, auxiliary auger was used to feed sludge material into the bed of the reactor. Sludge was supplied to the reactor in either a continuous-feed or batch mode. During batch operation, the
Energy & Fuels, Vol. 7, No. 6, 1993 805
Thermal Treatment of Hazardous Wastes Table I. Ultimate Analysis of Experimental Sludges (All Values Are on a Dry Basis) sludge 1 sludge 2 43.0 % carbon 42.8% 5.6% hydrogen 5.4% 0.25% 0.06 % nitrogen 0.23% sulfur 0.085% 48.2 % oxygen 33.94% 0.04 % chlorine 0.04% 17.5% 5.6% ash 17 200 kJ/kg 16 800 kJ/kg energy content 76.2% 72.8% moisture ~~~
auger was operated at high speeds to rapidly feed 0.5-1.0-kg plugs of sludge into the bed of the reactor. Conversely,the auger was operated at relatively slow speeds during continuous-feed testa so a steady feed rate of sludge was attained. Sludge was stored in a large hopper above the auger inlet pipe. In addition, this system was used to remove ash and bed material from the facility. Primary and secondary air was supplied by a single-stage, centrifugal blower. The capacity of the blower was adequate to permit circulation of solids in both hot and cold flow cases. Primary air was distributed into the reactor through a nozzle plate after passing through a laminar flow element used to accurately meter the flow rate. Air from the blower not used for fluidization flowed into the secondary branch of the air system and was used for loop seal fluidization and multiple air injection porta in the freeboard region of the reactor. Entrained solids were collected in a refractory-lined cyclone and passed through a stainless steel standpipe into a loop seal. A transition section between the reactor elbow and the cyclone prepared the flow for entrance into the cyclone through reduction in cross-sectional area and by gradually changing the internal cross-sectionalshape from circularto rectangular. The refractorylined loop seal was 91.5 cm in height with external and internal diameters of 50.8 and 44.5 cm. Solid material was withdrawn from the loop seal through a 3.81-cm-diameterdrain tube pressfitted into the center bore of the nozzle plate and extending out the bottom of the plenum chamber. Gas Analysis. Gas sampleswere withdrawn from the locations mentioned above. The samples were continuously analyzed for oxygen (Beckman755),carbon dioxide, carbon monoxide (Anarad AR 600), and nitric oxide (Thermo Electron Model 10) and were passed through a water condensate system prior to analysis. A separate wet gas sample was pulled through the THC (Ratfisch RS 55) analyzer via a heated line. The voltage signal produced by these instruments was converted to a volume fraction and recorded on a Macintosh computer using Omegabench data acquisition software. The same analytical instrumentation was used for monitoring the incineration process in both the RKS and the CFB reactors. All of the instruments were calibrated with gases purchased from the Scott or Matheson Gas Co. The oxygen and CO/COz meters were calibrated to an accuracy of hO.l%, the NO/NO, meter was calibrated to fl ppm, and the total hydrocarbon meter was calibrated to *2 ppm as methane. Sludge Material. The sludgeswere stored in 55-gdon drums. The ultimate analysis and energy content of both are shown in Table I. The ash, nitrogen, and oxygen contents are quite different for the two materials. Sludge 1has a much higher ash and nitrogen content but much less oxygen. RKS Procedure. A typical experiment consisted of (1) preparation of solids, (2) loading solids into a hot, rotating kiln, (3)taking solid samples, gas samples, and temperature data from the kiln, and (4) analyzing the data. For the kiln experiments the sludges were sieved into three size fractions: small (d, C 7 mm), medium (7 mm < d, < 13 mm), and large (d, > 13 mm). The sized sludge was placed in freezer bags and stored in a laboratory refrigerator to retard the loss of volatile5 and moisture. Immediately before a test the sludge was taken from the refrigerator, and an appropriate amount was weighed out using a Mettler PM 4000 balance. The sample was placed in the cup/
Table 11. RKS Experimental Matrix (Performed on Both Types of Sludge) particle size bed constituents wall temperature (K) 920 small sludge 1060 small sludge 1140 small sludge 1140 medium sludge 1140 large sludge sludge-silicasand 1140 small probe and covered by aluminum foil until the experiment was started, roughly 5 min later. The experiment was started by turning on all gas sample pumps, starting the kiln rotation, and turning on the data acquisition system to record the data from the gas analyzers and the thermocouples. After recording baseline gas concentrations and temperatures, the blast-gate was opened and the sludge in the cup/probe was poured in the kiln. The probe was quickly withdrawn and the blast-gate closed. During the experimenta the kiln rotated at 0.5 rpm and the natural gas burner was fired at a rate of 25 kW. Gas compositions and temperatures were recorded, as previously described, until the profiles returned to their baseline readings. At this point, any bottom ash samples were taken with the same probe that was used to place the sludge in the kiln. The experiments determined the effect of particle diameter, bed constituents, and kiln temperature on the evolution of COz, CO, 02,NO, and THC from the two types of sludge. The experimental matrix for the kiln is shown in Table 11. Additional experiments were performed with a bed of preheated silica sand already in the kiln to determine ita effect on emissions from the burning sludge. The size of the sludge charge placed in the kiln was also varied (80,120, and 160 g) to alter its combustion characteristics. CFB Procedure. The CFB facility was prepared by first removing any unwanted or contaminated bed material and filling the reactor with new bed material. Tests reported in this paper were performed with beds composed of silica sand with average particle diameters of 0.9-1.1mm. Silica sand was chosen for its fluidization characteristics and potential for a comparison with other CFB research. Similarly, the loop seal was purged of all silica sand and contaminated material remaining from previous tests. All sampling probes were then placed into position and the appropriate cooling and purge flows connected. A t this point a brief check was made to ensure proper operation of the sparkignition and ultraviolet detection system of the pilot fuel injector. Before a test, the CFB was preheated for at least 10 h at a firing rate of 65 kW and a superficial velocity of 0.4 m/s in order to achieve steady-state conditions. Gaseous emissions and solid particles were collected from multiple locations from the CFB system. Gaseous emissions were drawn through five separate, 1.27-cm external diameter, watercooled, stainless steel sampling probes. The water flow rate to each probe was varied so the gas sample was quenched to 410 K in each probe. Emissions were continually monitored for the concentrations of carbon monoxide (CO), carbon dioxide (COz), total hydrocarbons (THC), nitric oxide (NO), and oxygen (02) using the sampling train previously described. The locationsof the five gas-sampling probes in the CFB system are shown in Figure 2. Probe 1was located about 1.0 m above the expanded bed surface. There was no provision for in-bed sampling in the CFB in this study. Probes 2 and 3 were located 1.2- and 2.4-m downstream from probe 1,respectively. Probe 4 was located at the entrance to the cyclone. Finally, probe 5 was located downstream of the cyclone, but slightly upstream of the air-assisted water injectors in the exhaust duct. Each probe was positioned so that sample was drawn from the center line of the reactor. Two auxiliary fuel injectors were used for all testa described in this paper. A primary fuel injector fired deep into the bed of the reactor, while a lower flow capacity pilot flame fired into the freeboard region above the bed. The two fuel injectors were circumferentially positioned 90° from each other and affected
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806 Energy & Fuels, Vol. 7, No. 6, 1993 0.15 0.14
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Figure 3. Influence of sludge type on oxygen and CO emissions in the RKS at a wall temperature of 1140 K, sludge weight of 80 g, particle diameter less than 7 mm, and the average oxygen available in the kiln equal to 5%. the internal flow patterns in a complex manner. Secondary air, for window-purging purposes, was introduced into the reactor at various locations as indicated in Figure 2. A series of tests were performed with the CFB system while feeding sludge continually to the reactor. A baseline test, representing the fluidization of a hot silica sand bed with the addition of auxiliary natural gas fuel, preceded each test involving incineration of sludge. During the baseline tests, gaseous emissions were continually monitored and recorded. After representative emission traces were observed for the base case, the flow of sludge to the reactor commenced. Sludge flow was continued until steady, representative emission traces were again observed. Ash Sampling and Analysis. Samples of solid material were collectedfrom both the fluidized bed combustor (CFB)and rotary kiln simulator (RKS). Two different types of material were distinguished fly ash particles and bottom ash particles. The fly ash particles were sampled from the exhaust region after the cyclone in the case of CFB (samping location 5) and after the afterburner in the case of the rotary kiln (measurement point was located 60 cm above the burners). The bottom ash solids were collected from the primary chamber in the RKS case and from the bed in the CFB case after the experiment was completed. Additional samples were obtained from the transition section of the RKS (between the primary combustion chamber and the afterburner) and from three additional locations in the CFB: (1) the loop seal barrel, (2) before the cyclone (sampling location 4), and (3) the inside of the reactor (1.8 m above the bed surface level). The samples were taken during incineration of sludge 1, silica sand, or a mixture of both. A wall temperature of 1140 K was used as a reference parameter for both reactors. In the case of the RKS, solid samples were collected during incineration of the small particles (d, < 7 mm). The same sampling system was used for both facilities. The system consisted of a Thomas CE18 pump with controllablesamplingvelocity, a stainless steel suction probe, and a micro-fiberglass filter located in the internal tube through which the collected sample flowed. The filter had very little reactivity with combustion gases and a high filtration efficiency, above 99.7 % No evidence of secondary reactions of solid samples on the collection filters was observed. The distance from the filter to the inlet section of the probe was regulated. Therefore, it was possible to place the filter close to the probe entrance which prevented the deposition of solid particles on the inner probe walls. The samples were collected on the filter directly from the reactors and immediately prepared for chemical and physical microstructure analyses. A dry solid sample weighing 0.15 g (or less, in the case of the fly ash from the RKS) was mixed with epoxy resin. The samples from the CFB and RKS were then grouped together and placed in a 30-mm-diameter mold. Additional epoxy resin-hardener mixture was prepared in the same (9) Kittelson, D.B.;Moon, K.C.; Lin, B.Y.H.Sci. Total Eno. 1984, 36,153-158.
proportions and poured over the specimen. Using this preparation technique, the bias caused by different settling rates of ash particles could be minimized. Afterward, the hardened sample was polished by four different polishing plates with polishing particles ranging from 45 to 0.3 pm in diameter. The samples were cleaned in the ultrasonic generator and further conditioned by vibrating diamonds using a Syntron Vibromet I. The sample surface was then coated with spectrally clean carbon. Analyses were performed on a Cameca SX-50 electron probe microanalyzer (EPMA) equipped with four wavelength-dispersive spectrometers (WDS).The multiple layer 60-A W/Si X-ray reflector (OVONYX OV-Moa) was deposited on a single-crystal substrate of Si (100). The remaining spectrometers were equipped with TAP, PET, and LiF analyzing crystala. WDS resolution was 5 eV. This is significantly higher than for energydispersive spectrometry (EDS) where the resolution is 110 eV. The EPMA was controlled by a SUN SPARC station 330 computer. Analyses were made at 15-keV accelerating voltage and a beam current of 30 nA. The following fly ash and bottom ash parameters were determined: area and perimeter of particles, equivalent diameter defined as a diameter of a sphere whose projected area is equal to the projected area, A, of a particle: D, = (4A/r)l/2, equivalent mean arithmetic diameter defined as Zi niD,Jni, where ni denotes the number of particles corresponding to the diameter D,j (in the case of semispherical fly ash particles, their diameter was generally defined without calculating the equivalent diameter and then the mean arithmetic diameter equalled Zi n,di/ni),concentration of elements and compounds (oxides)on the surface and inside the particles, circularity shape factor CSF (CSF was determined on the basis of the measurementa of a particle's area, A, and its perimeter, P: CSF = P/ 4rA). The spatial coordinates of the analyzed particles were also stored in the computer's memory. In order to obtain the physical image of the particles' surface and interior morphologies,classical scanning electron microscopy (Cambridge Stereoscope 240 SEMI was used. Ash and sludge photomicrographs were taken at magnifications varying from 252 to 11OOO times, in order to minimize "losses" of small particles in the background. To provide a representative statistical distribution, at least 1500 individual fly ash particles and 500 bottom ash particles were examined for each data set. Ash photomicrographs were used to quantitatively establish the particle size distributions and number densities. The inaccuracy of microscopic measurements was below 4 % with the same operator.
Results and Discussion Gaseous Emissions: Rotary Kiln. T h e emissions from the two types of sludge burning in t h e rotary kiln simulator were compared at a kiln operating temperature (wall) of 1140 K. Figure 3 shows typical CO and 02 traces as a function of time for both sludges. (All gas samples from t h e RKS were taken from t h e flue sampling ports.)
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.
Almost immediately after the sludge is charged, it begins to heat up and evolve volatile hydrocarbons which ignite and reduce the available oxygen in the kiln. In both cases the initial oxygen concentration was approximately 6 5% , and within approximately 40 s the exit concentration had fallen to less than 1% due to the volatile combustion. The oxygen demand for sludge 1was only slightly higher than that for sludge 2 (1.22 vs 1.14 lb of Odlb of dry sludge) based on the constant 80-g charge size and the chemical analysis of the sludges. With sludge 1 the peak hydrocarbon evolution rate exceeded the rate at which oxygen was being supplied to the kiln resulting in temporary fuelrich conditions and, hence, the CO spike at 40 s. Kiln exit hydrocarbons emissions also increased correspondingly in this case. With sludge 2 the peak oxygen consumption rate was slightly smaller (as would have been expected based on the oxygen demand) and the CO peak was much smaller. In this case no incremental hydrocarbon emissions were measured. The corresponding effect on the NO emissions is shown in Figure 4. As nitrogen-containing volatiles are evolved early in the process, NO emissions from sludge 1begin to increase due to the oxidation of this nitrogen. However, when the locally available oxygen fraction falls below 0.01, the NO emissions begin to drop due to the lack of oxygen available to oxidize the nitrogen evolvingfrom the sludge. The emissions rise again once the hydrocarbon evolution rate slows sufficientlythat the available oxygen is not being consumed. The final decrease in NO occurs when the sludge nitrogen has been fully evolved and reacted. In contrast, sludge 2 does not appear to contribute any NO to the exhaust gases. The exit emissions remained unchanged until the oxygen dropped below 3%,and then
the NO emissions decreased. Thus, in this instance the sludge, which was essentially nitrogen free, only served to produce an NO-reducing, fuel-rich environment which reduced the baseline thermal NO concentration. Note that the NO baseline concentrations are slightly different for the experiments shown in Figure 4. This is due to the fact that the experiments took place on different days and the NO baseline (due to the auxiliary gas flame) varies between 70 and 85 ppm from day to day. Clearly the primary cause of the differences in the NO emissions from the two sludges is the 4-fold difference in nitrogencontent (0.25vs0.06%). However,the datashown in Figure 4 suggest that the nitrogen in sludge 2 is surprisingly unreactive. This could be due to the fact that it is associated with the char and burns out late in the process via heterogeneous reactions known to favor NZformation or that it is in the form of nitrates that convert to nitrogen oxides not measured in this study (e.g. NO2 or NzO). It should also be noted that there are physical differences in the two sludges that could influence the conversion process. Sludge 1particles maintained their shape during the entire experiment until an ash shell remained in the kiln (very little fly ash was created) while the sludge 2 particles tended to flake apart during the experiments, and these flakes were entrained by the gas flow through the kiln, sometimes before all the carbon had burned out of the flakes. The effect of having a silica bed in the rotary kiln when the sludge is burned is illustrated in Figure 5. The presence of the silica moderates the depletion of the oxygen in the kiln by retarding the rate of hydrocarbon evolution and, hence, combustion. This moderation effect is especially noticeable when comparing the time for the sludge char
Rink et al.
808 Energy & Fuels, Vol. 7,No. 6, 1993 I d
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Figure 6. Influence of sludge charge size on the maximum change in THC and oxygen emissions in the RKS at a wall temperature of 1140 K and the particle diameter less than 7 mm.
to completely burn out (5 vs 10 min for a silica bed based on visual observation of the bed). The diffusional resistance of the bed slows down the rate of combustion of the sludge because it slows the rate of particle heat-up and the external mass transfer of the evolved hydrocarbons. This reduces the peak evolution rate and ensures that some oxygen is available for the oxidation of evolved fuel nitrogen fragments throughout the entire evolutionperiod. The increased availabilityof oxygen for the case of a silica sand bed causesan increased conversionof the fuel nitrogen to NO. The size of the sludge charge placed in the batch kiln has a marked effect on the emissions exiting the kiln because it controls the mean oxygen concentration during the critical evolution period. This is directly analogous to the effectof feed-ratevariations in a full-scale continuous process.. Figure 6 shows the increase in unburned hydrocarbons with increased charge size for both types of sludge. This increase (as well as a corresponding increase in CO emissions) was the result of a total depletion of oxygen in the kiln during the peak evolution period. As the oxygen profiles illustrate, with the 160-g charge, the measured kiln exit oxygen concentration was essentially zero for nearly 50 s. NO emissions were also greatly reduced with the higher charge size. Finally,the particle diameter of the sludge,at a constant charge weight of 80 g, has a marked effect on the emission characteristics in the rotary kiln incineration process. Figure 7 illustrate's the impact of changing sludge particle sizes on the kiln combustion process. As expected, the smallest particles (13 mm) burn much slower and only slightlydecreasethe mean oxygen concentration. Figure 8 illustrates the impact of this behavior on NO formation. Ultimate analyses were performed on the sludges of different particle diameters to verify that there was no difference in elemental nitrogen content. Due to the homogeneity of the sludge production, it is also unlikely that there are significant differences in mineral matter
10
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composition between different particle sizes. Therefore, any difference in incineration characteristics is probably related to particle diameter, not chemical differences. The effect shown in Figure 8 is most likely due to the increased diffusional resistance as well as slower heat-up rate produced by the larger sludge particles. This increased resistance results in a slower mean hydrocarbon evolution rate, and thus the maximum depletion of the availableoxygen in the kiln is far less severeover the course of the sludge incineration. The increased oxygen availability results in an increase in NO production from the fuel nitrogen. Gaseous Emissions: Fluidized Bed. It is important to note that the operating parameters of the CFB facility are not identical in the tests of the different sludges. This was primarily due to the differences in the feeding characteristics of the different sludges. The sludges were also dried to varying degrees to help facilitate feeding through the auxiliary feed mechanism. Further, one consequence of the differencesin sludge composition was that the quantity of air introduced through the auxiliary natural gas fuel injectors was different when burning the
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Figure 9. Typical emissions profiles from the CFB for sludge 1 incineration. The CFB was fired at 190 kW (51% of energy from the sludge), a superficial velocity of 0.42 m/s, and a bed temperature of 589 K.
different sludges. Thus, the overall firing rates, including the energy contribution of the natural gas auxiliary fuel and the energy content of the sludge, were not held constant in these tests. The influence of these differences is particularly evident when examining axial profiles of CO in the reactor. Typical axial profiles of CO2, NO, and CO concentrations obtained from the CFB while incinerating sludge 1, as compared to a baseline test condition (natural gas only), are illustrated in Figure 9. In both cases an identical silica sand bed was fluidized at a superficial velocity typical of commercial systems, about 0.5 m/s, while the flow rate of auxiliary natural gas fuel was held constant. Thus the facility was operated as a conventional, bubbling fluidized bed. It is evident from Figure 9 that, when incinerating
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sludge 1,the C02 levels increase along the length of the reactor, whereas for the base case the levels initially remain constant but then begin to decrease with axial distance. The decrease in C02 concentration is due to the dilution from additional secondary air introduced into the freeboard. Figure 9 indicates that the sludge continues to burn along the length of the freeboard region. Similarly, Figure 9 showsincreasedproduction of NO along the length of the reactor as compared to the baseline. In the base case the majority of the NO is created in the freeboard region above the bed surface, while during incineration of sludge 1,elevated concentrations of NO in excess of the baseline level are evident further downstream in the reactor. Significant levels of CO are produced just above the bed in both cases as seen in Figure 9. This is not surprising since it is well-known that at the bed temperatures typical of this study, mixtures of methane and air will not burn within the bed, but combustion will occur immediately above the bed surface.1° However, in both cases, CO concentrations decrease dramaticallyalong the length of the reactor until they are negligible in the flue gas. This is to be expected since measurements using a suction pyrometer indicatetemperatures in excess of 1350 K in the freeboard region. Axial emission profiles are somewhat different when incinerating sludge 2. Figure 10 illustrates the CO2 emissions from the reactor when burning sludge 2 as compared to the base case. The same general trend of decreasing COa concentrations along the length of the reactor evident in Figure 9 is observed in Figure 10. In contrast to Figure 9, however, the C02 concentrations illustrated in Figure 10are seen to also decrease with axial location when incinerating sludge2. This result is probably due to the small amount of sludge combusted (12 % of the fuel input versus 51% for sludge 1). Another important difference observed when burning sludge 2 as opposed to sludge 1is that NO levels do not differ appreciably along the length of the reactor as compared to the base case, as seen in Figure 10. This difference in behavior can be attributed to the difference in fuel-bound nitrogen between the two sludges; recall that sludge 1contains about 4 times as much nitrogen by weight as sludge 2. Figure 10 also illustrates the change in concentration of CO with axial position in the reactor when burning sludge 2. The CO levels remain elevated further downstream in the reactor than those shown in Figure 9, and those CO levels are increased when incinerating sludge 2, as compared to the baseline. The elevated CO levels are attributed to the (10) Hayhurst, A.
N. Combust. Flame 1991,85,155-168.
810 Energy & Fuels, Vol. 7, No. 6, 1993 8 ,
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Figure 10. Typical emissions profiles from the CFB for sludge 2 incineration. The CFB was fired at 127 kW (12% of energy from the sludge), a superficial velocity of 0.91 m/s, and a bed temperature of 567 K.
fact that characteristics of sludge 2 dictated different operational parameters between the two baseline cases, as discussed in the preceding sections. One consequence of these differences was that the air flow rate to the auxiliary fuel injectors was decreased in the series tests involvingsludge 2. The mixture ratio of the auxiliary fuel supply has a strong influence on the CO concentrations in the freeboard region of the reactor. Also the superficial velocity was higher in the case of incinerating sludge 2. Although there were some operational differences when burning sludges 1 and 2, the same trend of dramatic decrease in CO with axial location in the reactor, while CO emissions are negligible in the exhaust, is clearly evident in both cases. Gas Emission Comparison: RKS vs CFB. The differences in conversion of the fuel nitrogen to NO between the RKS and the CFB are illustrated in Figure 11. A comparison was also made between the different modes of sludge feeding (batch and continuous). The fuel nitrogen that was not converted to NO was assumed to end up primarily as N:! or bound up in the ash. The overall conversion of the fuel nitrogen in sludge 1 was estimated to be approximately 14% (based on integrating the additional NO produced during the combustion of sludge 1 relative to the baseline value measured while firing the kiln solely on natural gas). As discussed previously, the time-resolved measurements for sludge 2 (Figure 4) actually showed a decrease in exhaust NO emissions during much of the combustion period; hence, the net fuelnitrogen conversionfor this low-nitrogensludge could not be evaluated. In essence, the consumption of the NO produced from the natural gas flame (by the
Sludge 1
Sludge 2
Figure 11. Comparison of NO conversions in the CFB and RKS for different feeding modes. The RKS data is from the incinerationof the small particle size sludges at a wall temperature of 1140 K, sludgeweight of 80g, and the average oxygen available in the kiln equal to 5%. The CFB data is from the incineration of the full distribution of particle sizes at a wall temperature of 1140 K and the average oxygen available equal to 10%.
hydrocarbon radicals during the sludge combustion) greatly exceeded the oxidation of the small amount of nitrogen contained in this sludge. To allow direct comparison to the RKS, limited tests were also performed with the CFB operating as a batch process. Batch tests involving sludge 1 and sludge 2 resulted in slightly lower values of conversion of fuel nitrogen to NO as compared to the continuous-feed tests. This is attributed to the fact that, while operating in the batch mode, the reactor is characterized by locally oxygendeficient regions which pass through the reactor in plug flow. In these oxygen-deficientregions, the concentration of unburned hydrocarbons and CO competing for the oxygen is relatively high, thereby reducing the amount of fuel-nitrogen that is converted to NO. Conversely,when feeding the sludge in a continuous manner, oxygen concentrationsare uniformly high throughout the reactor. This results in a greater conversion of fuel-boundnitrogen to NO. A direct comparison between the CFB and the RKS is difficult due to the inherent operating characteristics of both reactors. The CFB is a continuous-feed combustor with high gas/solid contacting times that ensure burnout of the sludge whereas the RKS is a batch reactor where the exposure of the sludge to the hot combustion gases is limited. This less than perfect gas/solid mixing in the RKS can produce greater CO and THC emissions if the rate of hydrocarbon evolution exceeds the rate of oxidant supply. However, even in this case an afterburner can be used to completely destroy these emissions. In the case of NO production, the conversion of fuel nitrogen to NO appears to be surprisingly less in the CFB vs the RKS for sludge 1. It has been shown that elevated CO levels in fluidized bed systems lead to reduced NO emissions.ll In this study, CO concentrations have been shown to be high immediately above the bed, which could lead to the reduction of NO in the freeboard region. In addition, solid carbon is known to catalyze the reduction of NO in combustion systems.12 Analysis showed significant amounts of carbon (4-16 % by mass) in the contents of the loop sealwhen burning sludge 1. Further, iron oxide impurities such as Fez03 present in silica sand beds, in (11) Chan, L.K.;Sarofim,A. F.; Beer, J. M. Combust. Flame 1983,52, 37-45. (12) Pereira, F. J.; Beer, J. M.; Gibbs, B.; Hedley, A. B. Fifteenth Symposium (International)on Combustion;The Combustion Institute: Pittsburgh, PA, 1974; 1149-1156.
Thermal Treatment of Hazardous Wastes
I .
Figure 12. Scanningelectronphotomicrographof sludgeshowing typical fiber-like structure.
conjunction with CO, are also known to catalyze the reduction of NO to nitrogen,13 The silica sand used in this study naturally contains small amounts of Fe203. Finally, the concentration of volatiles in coal and char has also been shown to possibly affect conversion of nitrogen to NO in fluidized beds.12J4 The volatilecontents of sludge 1 and sludge 2 were 70% and 82%, respectively. It is possible that the unusually high volatile content of the sludgeinfluences the production of NO from fuel nitrogen. In general, a combination of the above-mentioned effects are probably responsible for the low nitrogen to NO conversion in the CFB system. Both the CFB and RKS were operated such that little or no unburned hydrocarbons were present in the exhaust gas streams (THC 5 20 ppm). However, as the charge size to the RKS was increased from 80 to 160 g, the peak emissions of unburned hydrocarbons from the kiln itself increased from 0 to 4000 ppm in the kiln exhaust (upstream of the afterburner) becausethe peak hydrocarbon evolution rate exceeded the available oxygen. The CFB produced no significant THC emissions despite variations in the chargefeed rate. This phenomena is due to the large excess of oxygen always available and the extended gas residence time at higher temperatures allowingfor operation without an afterburner in the CFB facility. Once theexhaust from the rotary kiln passes through the afterburner, the THC emissions fall to zero for all charge sizes. Ash Identification and Size Analysis. Different types of solid particles were formed during sludge incineration in the fluidized bed and rotary kiln reactors. The sludge itself (sludge 1) was composed of randomly distributed fibers as shown in Figure 12. The major fibers (20-pm width and a few hundreds microns in length) were either directly connected one to another or connected through a cotton-like continuous phase attached to the fiber surfaces. Incineration of such a material formed three major solid structures which are illustrated in Figure 13: supermicronfly ash particles, submicron fly ash particles, and bottom ash particles. These three structures were observed in the samples collected from the fluidized bed combustor during incineration of sludgeand silicasand mixtures. The submicron particles were absent from the rotary kiln samples. Both types of fly ash particles were glassy-spheric (or ceno(13)Allen, D.;Hayhuret, A. N. FBC Technology and The Enuironmental Challenge; IOP Publishing: Briatol, England, 1991; 221-230. (14)Furusawa, T.;Ishikawa; S.; Sudo, S.; Kunii, D. J . Chem. Eng. Jpn. 1983,16 (11,7677.
Energy & Fuels, Vol. 7, No. 6,1993 811
spheric) in character as seen in Figure 14. The submicron particles were usually attached to the surface of the supermicronfly ash and were rarely present as independent structures. When only sludge was incinerated, supermicron fly ash particles were rather porous and submicron particles were not observed. This suggests that the presence of silica sand can locally provide high-temperature sites and increase solids’ residence time stimulating development of cenosphericsurfaces. Sand also influences the formation of submicron fly ash particles. A t both incinerator conditions (sludge,sludge + sand), the physical appearance of the bottom ash was the same for both incinerator types (Figure 15). The bottom ash material was composed of individual fragments which were kept together by an irregular internal and external framework. Those particles resembled randomly organized skeletons or, in a few cases, cenospheric skeletons. During sludge incineration in the rotary kiln,submicron fly ash particles were not detected. Instead of submicron particles, Fe-rich and Ca-rich plates adhered to the supermicron fly ash particles which is shown in Figure 16. Therefore, the particles’ surfaces were not as smooth as in the samples from the fluidized bed reactor. However, the Fe and Ca plates appeared to cover external openings/ orifices in the particles’ surfaces, providing a continuous external surface layer. The relationship between the type of material incinerated and particles’ character was similar to that in the CFB case, i.e., porous fly ash particles were present during sludgeincineration,and cenospheric/glassyspheric particles were present during sludge + sand incineration. In addition, the sizesof the fly and bottom ash particles were significantlyinfluenced by the presence of silica sand. The sizes of both the supermicron fly ash particles and bottom ash particles (skeletons) during the incineration of sludge and sludge-sand mixtures in the CFB and RKS are presented in Table 111. It is clear that supermicron fly ash particles were markedly smaller in the sludge-sand experiments than in the “pure” sludge experiments. In the CFB the particle sizes were decreased almost 7 times, while in the RKS the decrease was only 2 times. This behavior was connected with observed local temperature peaks created by the primary/pilot flames and the presence of the silica sand. Initially formed porous fly ash particles underwent fragmentation passing through those high-temperature regions (around 1600 K). The porous particles did not have a spherical compact shape. It is known that most structures whose shapes depart from the spherical form are liable to have any projecting parts knocked off, mainly due to collisionswith other particles (microscopicregime 1to l@ nm).15J6 Thus, collisions of the porous particles can lead to their fragmentation rather than coagulation. Locally high temperature and intensive external and internal oxidation can strongly affect the links between different parts of a particle and lead to its disintegration. The turbulent character of the CFB incineration promotes this behavior. Small disintegrated fragments formed semispherical submicron fly ash particles. Simultaneously,the particles were losing their porous character, and cenospheric and glassy particles with smooth external surface (15)Grubin, H.L.;Heae, K.; Iafrate,G. J.; and Ferry, D. K. (Eda) The Physics of Submicron Structures; Plenum Preas: New York, 19W, pp 112-1 17. (16)Krakow, W.,Ponce, F. A, Smith,D. J., Ede. High Resolution Microscopy of Materials; Materials Research Society: New York, 1989, Vol. 139, pp 103-109.
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*pcnnlcron FLYASH P.rEides
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Figure 13. Overview of the major solid structures formed during sludge incineration.
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Figure 14. Scanning electron photomicrograph of fly ash collected from the CFB (sludge + silica sand case) illustrating Si-coatedsupermicron particle with visible submicron particles attached to the surface. Particle equivalent diameter is 5.8 pm.
Figure 16. Scanning electron photomicrograph of fly ash collected from the RKS (sludge + silica sand case) s h o w an example of a supermicronfly ash particle with adhering Fe-rich and Ca-rich plates on the surface. Particle equivalentdiameter is 27.7 pm. Table 111. A Comparison of the Ash Particles Size in the CFB and RKS ExDeriments* sludge 1with silica sand sludge 1
CFB fly ash CFB bottom ash
32.1 (40-60) 19.6 (1Cb30) 46.3 (3Cb70) 86.5 (60-120)
6.2 (4-10) (submicron < 0.7) 14.4 (10-20) 24.7 (15-30) 27.9 (20-40)
RKS fly ash RKS bottom ash Particleeizerangesarelistedintheparentheees.Datarepresent equivalent mean arithmetic diameter (ccm). Q
d
. Figure 16. Scanning electron photomicrograph of bottom ash collected from the CFB (sludge only case) showing internally burned skeleton structure. Skeletonequivalentdiameter is 12.4 pm.
were developed. In the RKS, the attrition process was not as strong as in the CFB reactor since the incineration process is not as dynamic. In addition, the local high-
temperature regions in the CFB (exceeding 1600 K)wem approximately 1.0 m above the bed surface level while in the RKS they were located inside the bed (lower than 1300 K). Therefore, the most dramatic influence of the silica's presence in the RKS was the variations it caused in the bottom ash skeleton sizes. The skeletons' equivalent mean arithmetic diameter was decreased from 86.5 to 27.9 pm. In the CFB experiments no significant changes in the bottom ash sizeswere observed. This is not surprising since the in-bed characteristics were similar in both the sludge and sludge-sand tests. Generally, the bottom ash and the fly ash particles had smaller sizes during incineration in the fluidized bed
Thermal Treatment of Hazardous Wastes
reactor. For the fly ash, one reason for this might be that the fluidized bed samples were obtained downstream of ita cyclone. In addition, higher rates of in-bed mixing and particles’ attrition occur in the fluidized bed reactor. Smaller diameters may be also connected with the presence of heterogeneous vaporization and condensation processes in the CFB due to its higher peak temperatures (CFB, 1650 K us RKS, 1250 K), comparing to only softening of the external layers of the ash material in the RKS environment. These processes, together with the detailed characteristics of the chemical microstructure of fly ash particles and bottom ash skeletons, are an object of continuing study. An understanding of the environmental impact of the disposal or use of the fly ash and bottom ash is important for both types of incineration processes. The surface smoothing during incineration of sludge-silica sand mixtures is suspected of significantly influencing the possible reaction mechanisms and leaching behavior of the ash. In general, compact, glassy spheres/cenospheres formed during incineration of mixtures are expected to exhibit much lower leachability than the porous particles generated during incineration of “pure” sludge. Other researcher~~’ have clearly demonstrated that the ash sintering or compacting process reduces leachability of metals and thus reduces the potential environmental impact of ash disposal (through leachate contamination). Such ash particles also have higher compressive strength which is important for their future utilization in the construction industry.
Conclusions The production of NO from fuel-bound nitrogen in the rotary kiln simulator is significantly affected by a number of facility operating factors. NO production in the RKS was dependent on the supply of nitrogen (in the sludge) and oxygen (in the gas phase) in the reactor. The availability of oxygen to the sludge was affected by the particle diameter of the sludge,the charge size,and whether a solids bed was present at the time of the incineration. The higher surface area to volume ratio for the smaller particle size resulted in much better contacting between the oxygen and the sludge which caused a greater depletion of oxidant (faster hydrocarbon release) during the reaction which in turn led to less NO production. The larger batch size also resulted in a depletion of oxygen and a reduction in NO production. On the other hand, the presence of inert sand in the kiln produced less intimate gas/solid contacting which resulted in a slower hydrocarbon release rate (greater oxygen availability) and, therefore, a greater production of NO. Sludge 2 conversions could not be evaluated since the material did not contribute to any additional NO emissions. As indicated by comparison of baseline combustion of natural gas and sludge, it appears as though the sludge (17)Sun,C. C.; Peterson, C. H.; Newby, R. A.; Vaux, W. G.; Kearns, D. L. Disoosal o f Solid Residue from Fluidized-Bed Combustion: Engineerihg and Laboratory Studiks; Final Report for EPA,Program NO.EHE623A, 1978; pp 63-85.
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combusted downstream of the freeboard region in the CFB. Measured bed temperatures in this study were low relative to temperatures typical of coal combustion in fluidized bed@ and rarely exceeded 675 K. These bed temperatures may lead to pyrolysis of nitrogen-containing species in the bed and freeboard regions. The nitrogen-containing compounds are then oxidized further downstream in the system, resulting in elevated levels of NO in the transition and cyclone. Gas-phase analysis to determine reaction intermediates will resolve these questions, and tests are planned for the near future. Due to the different combustion conditions when burning the two sludges, it is difficult to directly compare the incineration of the two sludges,however, as expected by ita lower nitrogen content, sludge 2 demonstrated higher conversions in the CFB. The overall fuel nitrogen conversion measured for the high nitrogen sludge (no. 1)in the rotary kiln was higher than that measured for the fluidized bed in spite of the fact that the overall oxygen available in the CFB was much higher. There are several factors which might account for this behavior. Future work will more thoroughly investigate these issues. Formation of fly ash particles was found to be dependent on types of incinerated material. When only sludge was used, porous supermicron fly ash particles were formed in both reactors and submicron particles were not observed. During incineration of sludge and silica sand mixtures in the CFB, both the supermicron and submicron fly ash particles were identified. They were glassy spheres or cenospheres. The submicron spheres were usually attached to the surface of the supermicron fly ash and were rarely present as individual particles. The submicron particles were absent from the RKS samples. Bottom ash material was found to resemble randomly organized skeletons or skeletons cenospheric in character, independent of the type of sludge or reactor. Generally, smaller fly ash and bottom ash particles were formed during CFB incineration. Application of silica sand as a bed material in CFB and RKS influenced particles’ sizes and shapes. Significantly smaller particles were formed. This is attributed to fragmentation of the originally formed porous particles. Simultaneously, the particles with spherical compact shape and fairly smooth external surface were developed. Formation of such ash particles may markedly reduce their potential environmental impact since compact, glassy spheres are expected to exhibit much lower leachability than are porous particles generated during incineration of “pure” sludge.
Acknowledgment. Funding for this project was provided primarily by the National Science Foundation through the Advanced Combustion Engineering Research Center. Funding from the NSF/PYI award is also appreciated. Funds for the ACERC center are received from the National Science Foundation, the State of Utah, 26 industrial participants, and the US. Department of Energy. The authors gratefully acknowledge the extensive help provided by David A. Wagner, Xioaxue Deng, and Tim DeJulius.
