Silcox, GD
1997
Numerical Modeling of the Temperature Distribution in a Commercial Hazardous Waste Slagging Rotary Kiln
Veranth, J.M.; Silcox, G.D. and Pershing, D.W.
Environmental Science and Technology, 31:2534-539(1997). Funded by ACERC.
The gas, wall and bed temperatures in a hazardous wasted incineration kiln were studied using a commercially available, CFD-based, r4eacting flow code, which included radiation heat transfer. The model was compared to field measurements make on a co-current flow, 35 MW slagging rotary kiln. Cases were run to determine the sensitivity of the predictions to changes in the model assumptions and to simulate the normal variation in combustion inputs. The model predictions of the peak bed temperature, of the axial temperature profile, and of the gas temperature at the exit-plane were consistent with the measurement at a full-scale wasted incinerator during normal operation. The model and the field observations both indicate that the peak bed temperature occurs near the middle of the kiln and that the difference between the peak bed temperature and the exit-plane gas temperature depends on the inlet flow. The geometry of the transition between the kiln and the secondary combustion chamber and the fuel-to-air equivalence ratio have the greatest effect on the calculated temperature distribution. Modeling studies provide useful information such as the relationship between available measurements and the temperature at inaccessible locations inside a full-scale kiln.
1996
Field Investigation of the Temperature Distribution in a Commercial Hazardous Waste Slagging Rotary Kiln
Veranth, J.M.; Gao, D.-C. and Silcox, G.D.
ES&T, 30:3053-3060, 1996. Funded by ACERC and Gas Research Institute.
Gas and bed temperatures were studied in a 4.4 m by 12 m, co-current flow, slagging rotary kiln at a commercial hazardous waste incinerator. The visual observations used by the kiln operators to control the process are described. These observations were quantified using thermocouples, radiation pyrometers, and phase-change indicators. The objectives were to estimate the peak bed temperature and compare this to measurements at the kiln exit. The maximum bed temperature occurs toward the middle of this type of kiln and not at the discharge. The slag melting temperature and test pellets with known melting points indicate that the peak bed temperature can be 100-300 K higher than the kiln exit temperature reported by the permanent instruments at this facility. Both broadband radiation pyrometers and thermocouples give a qualitative temperature indication that can be used for process control, but the readings depend on the sensor locations relative to the incompletely mixed air and combustion products. Two-color radiation pyrometer measurements of surface temperature near the kiln exit are higher than the actual temperature due to reflected radiation.
1994
Fundamental Study of the Thermal Desoprtion of Toluene from Montmorillonite Clay Particles
Keyes, B.R. and Silcox, G.D.
Environmental Science Technology, 28:840, 1994. Funded by ACERC and Gas Research Institute.
Nonisothermal desorption of toluene from individual montmorillonite clay particles was measured experimentally and modeled mathematically to elucidate details of the overall thermal desorption process. A single-particle reactor was used. It consisted of a porous, 2-6mm clay pellet formed around a 0.05-mm diameter thermocouple in a 6- or 9-mm o.d. glass tube. The tube was surrounded by a supplementary heater and placed in a GC oven. Desorption rates were obtained as a function of heating rate, clay type, particle size, and purge gas flow rate. In addition, the absorption isotherms for two toluene/clay systems and one n-dodecane/clay system were measured and correlated using the Freundlich isotherm. At the conditions examined, the rate-controlling mechanism is associated with intraparticle diffusion. Isothermal desorption experiments using clay pellets of different sizes demonstrate that local desorption kinetics are not rate controlling. Toluene shows a slower desorption rate than n-dodecane at low concentrations. This is attributed to the hindered removal of toluene from interlamellar regions of the clay. Comparisons of single-particle reactor and pilot-scale rotary kiln results show that mass transport resistances associated with a bed of particles dominate the desorption process at rotary kiln conditions.
1993
The Effect of Treatment Temperature of Metal Recovery from a Porous Silica Sorbent by EPA Method 3050 and by an HF-Based Method
Gao, D. and Silcox, G.D.
JAWMA 43:1004-1005, July 1993. Funded by Gas Research Institute and ACERC.
Determining the fate of metals during hazardous waste incineration is important because of environmental concerns. Such a determination requires an analysis of the waste ash. At high temperatures, metals are frequently immobilized by reactions with silicates or by encapsulation with silicates. In order to determine the metal retention levels in waste ash, a digestion method that insures total metal recovery is necessary. This study compares two digestion techniques and uses each to look at the effect of the incinerator's temperature on the apparent fraction of metal retained in the waste ash. Four metals on a diatomaceous earth matrix were studied: Cr, Cd, Pb, and Ni. An HF-based digestion yielded fractional metal retentions of about 1.0 while an HNO3-based digestion yielded apparent retention levels that decrease from 1 to 0.2 as the reactor temperature was increased from 25 to 870ºC.
A commonly used HNO3-based digestion procedures is EPA SW-846, Method 3050. It is successful at extracting those metal compounds that are soluble at conditions that characterize natural leaching and biological processes. However, Method 3050 was not designed to digest metals immobilized by silicates.
Thermal Treatment of Hazardous Wastes: A Comparison of Fluidized Bed and Rotary Kiln Incineration
Rink, K.K.; Larsen, F.S.; Kozinski, J.A.; Lighty, J.S.; Silcox, G.D. and Pershing, D.W.
Energy & Fuels, 7 (6):803-814, 1993. Funded by ACERC.
Large volumes of sludge are produced by a wide variety of industrial processes and by municipal wastewater 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, NOx 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 continuous for hydrocarbons, CO, O2, NO, and CO2. 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 led to the reduction of NO inside the freeboard zone. In both the CFB and 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.
On-Line Monitoring of Formaldehyde in Combustion Gases Using Gas Chromatography/Mass Spectrometry
McClennen, W.H.; Sheya, S.A.N.; Arnold, N.S.; Meuzelaar, H.L.C.; Larsen, F.S. and Silcox, G.D.
Incineration of Hazardous Wastes-2; Toxic Combustion By-Products:545-555, (in press). Funded by Consortium for Fossil Fuel Liquefaction and ACERC.
This paper describes a method for on-line gas chromatography/mass spectrometry (GC/MS) of formaldehyde in combustion gases. The method uses a recently developed vapor-sampling inlet to monitor the concentration of formaldehyde and other products of incomplete combustion (PICs) from the burning of plain and phenol-formaldehyde resin treated wood chips. Other PICs that were simultaneously monitored included ketene, propylene, propyne and acetaldehyde. The direct analysis method has detection limits of less than 1 ppm for the reactive formaldehyde and excellent selectivity for determinations in the complex mixtures of combustion products. The rapid sampling technique allows monitoring of transient events of only a few minutes or less duration. Examples of the technique include the detection of sample line problems and the comparison of PIC concentrations from different points in the combustion exhaust stream.
1992
Solid Waste Incineration in Rotary Kilns
Pershing, D.W.; Lighty, J.S.; Silcox, G.D.; Heap, M.P. and Owens, W.D.
Combustion Science and Technology, 1992 (in press). (Previously presented at the First International Conference on Combustion Technologies for a Clean Environment, Vilamoura, Portugal, September 1991.) Funded by the National Science Foundation, Environmental Protection Agency, Gas Research Institute and ACERC.
Rotary kilns are used to dispose of many solid wastes and sludges and to thermally treat contaminated soils. In this communication the fates of hydrocarbon and metal species are examined with a view toward optimization of new kiln designs and maximizing existing unit throughout while minimizing pollutant emissions. Initially, process fundamentals are considered to characterize the controlling phenomena. Pilot- and large-scale data are then examined to define practical system complexities. Finally, techniques for data scale-up and performance prediction are summarized. Temperature is clearly the most important parameter with respect to the fate of both metal and hydrocarbon species; hence, heat transfer is often rate limiting. High temperatures favor hydrocarbon evolution, but can also enhance the formation of toxic metal fumes. Both the solid composition and the moisture content can significantly influence the time at temperature required for hydrocarbon destruction and metal vaporization.
Improving bed mixing helps contaminant release but can also aggravate puffing tendencies with batch charging. Full-scale performance predictions currently require a combination of small-scale data and computer modeling. Future work needs to focus on verification of large-scale predictions for complex mixtures and sludges so that expensive trial burns can be minimized.
The Desorption of Toluene from a Montmorillonite Clay Adsorbent in a Rotary Kiln Environment
Owens, W.D.; Silcox, G.D.; Lighty, J.S.; Deng, X.-X.; Pershing, D.W.; Cundy, V.A.; Leger, C.B. and Jakway, A.J.
Journal of Air Waste Management Assoc., 42:681-690, 1992. Funded by US Environmental Protection Agency, Gas Research Institute and ACERC.
The vaporization of toluene from pre-dried, 6mm montmorillonite clay particles was studied in a 130 kW pilot-scale rotary kiln with inside dimensions of 0.61 by 0.61 meters. Vaporization rates were obtained with a toluene weight fraction of 0.25 percent as a function of kiln fill fractions from 3 to 8 percent, rotation rates from 0.1 to 0.9 rpm, and kiln wall temperatures from 189 to 793ºC. Toluene desorption rates were obtained from gas-phase measurements and interpreted using a desorption model that incorporates the slumping frequency of the solids. Fill fraction of the kiln, binary gas diffusion in the bed, and particle desorption using an Arrhenius-type expression that is a function of bed temperature and average bed concentration. The model included three adjustable desorption parameters which were obtained by fitting the experimental data with a least squares technique. Solid and kiln-wall temperatures were continuously recorded and used by the model to perform toluene desorption predictions. The model was successful at predicting the effects of fill fraction and rotation rate over a range of temperatures. It was shown that an increase in kiln temperature and rotation rate increased toluene desorption rates. A decrease in kiln fill fraction showed an increase n desorption rate. Desorption predictions were performed using both predicted and measured temperature profiles. In addition, the model was used to perform sensitivity tests examine the relative importance of bed diffusion and particle desorption resistances. A methodology for predicting full-scale performance was developed. A full-scale, rotary-kiln heat-transfer model was used to estimate the bed thermal profile. This profile was then utilized by the model tp predict toluene desorption at full-scale.
Hydrocarbon and Formaldehyde Emissions from the Combustion of Pulverized Wood Waste
Larsen, F.S.; McClennen, W.H.; Deng, X.-X.; Silcox, G.D.and Allison, K.
Combustion Science and Technology, 1992 (in press). (Also presented at The Second International Congress on Toxic By-Products: Formation and Control, Salt Lake City, UT, March 1992). Funded by Weyerhaeuser Corp. and ACERC.
Hydrocarbon and formaldehyde emissions from the combustion of pulverized wood waste were measured in 100 kW, cylindrical combustion chamber measuring 0.61 by 0.61 m. The wood was pneumatically conveyed to the burner and natural gas was used as an auxiliary fuel. The wood was screened prior to feeding so that its size distribution was representative of the suspension phase of a stoker boiler. Chamber wall and gas temperatures ranged from 920 to 1200 K and oxygen concentrations ranged from 2 to 9 percent, dry. Two types of waste were studied, plain wood and wood that was impregnated with a phenol-formaldehyde resin. The latter was a by-product of particleboard production. In general, the emissions of products of incomplete combustion (PICs) from the resinated waste were higher than those produced by plain wood. This may have been due to three factors: 1) the resinated wood was slightly wetter than the plain wood (6-9 percent by weight vs. 3 percent), 2) there was a difference in particle size distribution between the two materials as received, the resinated wood being larger, and 3) the resin may have had an effect on the emissions. Ultimate analyses of the two wastes showed no significant differences, other than moisture, in composition. At temperatures above 1200 K, total hydrocarbon emissions were roughly 10 to 29 ppm and formaldehyde emissions were less than the detection limit of 1 ppm. Typical waste wood boiler temperatures are roughly 1600 K. Hence, unless there are cool, poorly mixed regions in the full-scale facility, hydrocarbon and formaldehyde emissions should not be significant. However, the emissions from burning the two different types of wood would probably be different if all operating parameters in the wood-fired boiler are held constant.
Modeling Sorbent Injection and Sulfur Capture in Pulverized Coal Combustion
Boardman, R.D.; Brewster, B.S.; Huque, Z.; Smoot, L.D. and Silcox, G.D.
Air Toxic Reduction and Combustion Modeling, 15:1-9, 1992. (Also presented at the ASME International Joint Power Generation Conference, Atlanta, GA, October 1992). Funded by Advanced Fuel Research and ACERC.
A computer model has been developed for predicting mixing and reactions of injected sorbent particles in pulverized coal combustors and gasifiers. A shrinking-core, grain model was used for sulfation. The model accounts for the effects of surface area, pore diffusion, diffusion through the product layer, chemical reaction, and reduction of the pore volume due to grain swelling. The submodel was evaluated for a fuel-lean case and for a fuel-rich case. Predictions were compared with limited experimental data (for the fuel-rich case). The results illustrate the model's capability for predicting the effectiveness of sulfur capture. The importance of sorbent particle properties was also investigated parametrically, and model limitations were identified.
1991
Hydrocarbon and Formaldehyde Emissions from the Combustion of Pulverized Wood Waste
Larsen, F.S.; McClennen, W.H.; Deng, X.-X.; Silcox, G.D. and Allison, K.
Combustion Science and Technology, 1991 (in press). Funded by Weyerhaeuser Corp. and ACERC.
Hydrocarbon and formaldehyde emissions from the combustion of pulverized wood waste were measured in 100 kW, cylindrical combustion chamber measuring 0.61 by 0.61 m. The wood was pneumatically conveyed to the burner and natural gas was used as an auxiliary fuel. The wood was screened prior to feeding so that its size distribution was representative of the suspension phase of a stoker boiler. Chamber wall and gas temperatures ranged from 920 to 1200 K and oxygen concentrations ranged from 2 to 9 percent, dry. Two types of waste were studied, plain wood and wood that was impregnated with a phenol-formaldehyde resin. The latter was a by-product of particleboard production. In general, the emissions of products of incomplete combustion (PICs) from the resinated waste were higher than those produced by plain wood. This may have been due to three factors: 1) the resinated wood was slightly wetter than the plain wood (6-9 percent by weight vs. 3 percent), 2) there was a difference in particle size distribution between the two materials as received, the resinated wood being larger, and 3) the resin may have had an effect on the emissions. Ultimate analyses of the two wastes showed no significant differences, other than moisture, in composition. At temperatures above 1200 K, total hydrocarbon emissions were roughly 10 to 29 ppm and formaldehyde emissions were less than the detection limit of 1 ppm. Typical waste wood boiler temperatures are roughly 1600 K. Hence, unless there are cool, poorly mixed regions in the full-scale facility, hydrocarbon and formaldehyde emissions should not be significant. However, the emissions from burning the two different types of wood would probably be different if all operating parameters in the wood-fired boiler are held constant.
On-Line Monitoring of Formaldehyde in Comubustion Gases Using Gas Chromatography/Mass Spectrometry
McClennen, W.H.; Sheya, S.A.N.; Arnold, N.S.; Meuzelaar, H.L.C.; Deng, X.-X.; Larsen, F.S. and Silcox, G.D.
Combustion Science and Technology, 1991 (in press). Funded by ACERC.
This paper describes a method for on-line gas chromatography/mass spectrometry (GC/MS) of formaldehyde in combustion gases. The method uses a recently developed vapor-sampling inlet to monitor the concentration of formaldehyde and other products of incomplete combustion (PICs) from the burning of plain and phenol-formaldehyde resin treated wood chips. Other PICs that were simultaneously monitored included ketene, propylene, propyne and acetaldehyde. The direct analysis method has detection limits of less than 1 ppm for the reactive formaldehyde and excellent selectivity for determinations in the complex mixtures of combustion products. The rapid sampling technique allows monitoring of transient events of only a few minutes or less duration. Examples of the technique include the detection of sample line problems and the comparison of PIC concentrations from different points in the combustion exhaust stream.
Rotary Kiln Incineration: Comparison and Scaling of Field-Scale Contaminant Evolution Rates from Sorbent Beds
Lester, T.W.; Cundy, V.A.; Sterling, A.M.; Montestruc, A.N.; Jakway, A.J.; Lu, C.; Leger, C.B.; Pershing D.W.; Lighty, J.S.; Silcox, G.D. and Owens, W.D.
Environmental Science Technology, 25:1142-1152, 1991. Funded by Environmental Protection Agency, Louisiana State University/Hazardous Waste Research Center and ACERC.
A comparison is made, for the first time, between the evolution of hydrocarbons from clay sorbent beds in a field-scale rotary kiln incinerator and in a pilot-scale rotary kiln simulator. To relate the data from the different sized units, due allowance is made for bed dynamical similitude, bed geometrical factors, and bed heat-up. To minimize the effects of disturbances caused by foreign matter in the field scale bed and differences in loading techniques, the rate of evolution is characterized by an "evolution interval" defined as the time required for the middle 80% of the ultimate containment evolution to occur. A comparison of evolution intervals with reciprocal bed temperature reveals that the data are consistent with an analysis that assumes a uniform bed temperature (at any instant of time) and desorption controlled evolution rate. Furthermore, the evolution intervals scale inversely with a modified Froude Number, which characterizes bed dynamics. The success in comparing field and simulator results indicates that pilot scale rotary kilns may be used to simulate certain features of industrial-scale units if dynamical, geometrical and thermal parameters are matched appropriately.
Thermal Analysis of Rotary Kiln Incineration: Comparison of Theory and Experiment
Owens, W.D.; Silcox, G.D.; Lighty, J.S.; Deng, X.-X.; Pershing D.W.; Cundy, V.A.; Leger, C.B. and Jakway, A.J.
Combustion and Flame, 86:101-114, 1991. Funded by Gas Research Institute, ACERC and Louisiana State University/Hazardous Waste Research Center.
A comprehensive heat-transfer model and associated simplified scaling laws are developed and verified using a pilot-scale, directly fired rotary kiln with a slumping bed of dry or wet, 6-mm clay sorbent particles. The kiln operating conditions examined include: rotation rate (0.1 to 0.9 rpm), percent fill fraction (3-8), feed moisture content (0-20 wt.%), and inner-wall temperature (190º to 790ºC). The model is used to determine the relative importance of several heat-transfer mechanisms including radiation, gas-to-solid convection, and wall-to-solid convection. Simple scaling laws are also developed for water vaporization. Generally good agreement is obtained between theory and experiment without adjusting any model parameters. Further, the simplified scaling laws provide a reasonable estimate of the pilot scale performance. The key conclusions of this study for kilns at the conditions examined are (1) water exerts a profound effect on the solids thermal profile, (2) simple geometrical scaling is not sufficient, (3) the assumption of a well mixed (radially isothermal) solids bed for the heat transfer analysis is appropriate, (4) a dimensionless group, which is a function of temperature, can be defined giving the relative importance of radiative and convective modes of heat transfer, and (5) moisture vaporization rates can be roughly approximated by assuming that the water vaporizes at the boiling point at a rate controlled by the rate of heat transfer to the bed. The implications of the scaling laws for scale-up and kiln design are also examined.
Solid Waste Incineration in Rotary Kilns
Pershing, D.W.; Lighty, J.S.; Silcox, G.D.; Heap, M.P. and Owens, W.D.
First International Conference on Combustion Technologies for a Clean Environment, Vilamoura, Portugal, September 1991. Funded by the National Science Foundation, Environmental Protection Agency, Gas Research Institute and ACERC.
Rotary kilns are used to dispose of many solid wastes and sludges and to thermally treat contaminated soils. In this communication the fates of hydrocarbon and metal species are examined with a view toward optimization of new kiln designs and maximizing existing unit throughout while minimizing pollutant emissions. Initially, process fundamentals are considered to characterize the controlling phenomena. Pilot- and large-scale data are then examined to define practical system complexities. Finally, techniques for data scale-up and performance prediction are summarized. Temperature is clearly the most important parameter with respect to the fate of both metal and hydrocarbon species; hence, heat transfer is often rate limiting. High temperatures favor hydrocarbon evolution, but can also enhance the formation of toxic metal fumes. Both the solid composition and the moisture content can significantly influence the time at temperature required for hydrocarbon destruction and metal vaporization.
Improving bed mixing helps contaminant release but can also aggravate puffing tendencies with batch charging. Full-scale performance predictions currently require a combination of small-scale data and computer modeling. Future work needs to focus on verification of large-scale predictions for complex mixtures and sludges so that expensive trial burns can be minimized.
1990
Fundamentals for the Thermal Remediation of Contaminated Soils - Particle and Bed Desorption Models
Lighty, J.S.; Silcox, G.D.; Pershing, D.W.; Cundy, V.A. and Linz, D.G.
Environmental Science & Technology, 24 (5), 750-757, 1990. Funded by the Gas Research Institute, Dave Linz, Project Manager, NSF - Presidential Young Investigator Program, ACERC (National Science Foundation and Associates and Affiliates), the State of Utah, and US Department of Energy.
A major research effort has been initiated to characterize the rate-controlling processes associated with the evolution of hazardous materials from soils. A threefold experimental approach is being used in conjunction with computer modeling to analyze thermal desorption of contaminants. Phenomena occurring both inside particles (intraparticle) and with a bed of particles (interparticle) were studied.
The results obtained suggest that the most important process variable are local thermal environment and gas-phase contaminant concentration because the adsorption equilibrium characteristics of the contaminant/soil pair control the desorption of contaminant from a particle at a given temperature. A mass-transfer/desorption model, which assumes gas/solid equilibrium at all points and time, is proposed and the model was found to predict the measured temperature dependence.
Mathematical and Physical Modeling of Rotary Kilns with Applications to Scaling and Design
Silcox, G.D.; Larsen, F.S. and Pershing, D.W.
Combustion Science and Technology, 1990 (In press). Funded by ACERC and Gas Research Institute.
The heat and mass transfer in an indirectly fired rotary kiln are examined using a combination of physical and mathematical modeling. The physical modeling is used to determine characteristic mixing times in a slumping kiln bed. These times are compared with characteristic times for diffusion of heat and mass through the bed in order to justify a lumped capacitance analysis of heat and mass transfer.
Mathematical models of heat and mass transfer are used to examine the effects of design and operating parameters on bed temperature and desorption rates. Limited comparisons with temperature measurements are presented. The design and operating parameters studied include kiln length, solid residence time, solid feed rate, and feed moisture content. The effects of moisture are particularly important to both heat and mass transfer. Scaling considerations are examined and it is shown that maintaining equivalent wall temperature profiles, fill fractions, moisture levels, and burden residence times does not necessarily result in equivalent bed thermal profiles.
The Effects of Rotary Kiln Operating Conditions and Design on Burden Heating Rates as Determined by a Mathematical Model of Rotary Kiln Heat Transfer
Silcox, G.D. and Pershing, D. W.
J. Air Waste Management Association, 40, 1990. Funded by ACERC.
A mathematical model of heat transfer in a directly-fired rotary kiln is developed and used to examine the effects of operating and design parameters on burden temperature. The model includes a mean beam length radiation model and axial zoning. Conductive and convective heat transfer are also included. Radiation between immediately adjacent zones is permitted. Calculation of heat transfer rates is facilitated by the use of an electric circuit analogue. An iterative solution procedure is adopted to solve the energy balance equations.
At the conditions examined, the model predicts that coflowing gas and solid streams result in higher average burden temperatures than do counter flowing streams. The moisture level of the feed is predicted to be a key operating parameter. The effects of kiln length, burden residence time, firing rate, and flame length are also examined.
High-Temperature, Short Time Sulfation of Calcium-Based Sorbents: I. Theoretical Sulfaction Model
Milne, C.R.; Silcox, G.D. and Pershing, D.W.
I & EC Research, 29, 2192, 1990. Funded by Environmental Protection Agency and ACERC.
A mathematical model for the sulfation of CaO is developed around the overlapping grain concept employed in the calcination and sintering models of Milne et al. (1988). The potential influence of high mass-transfer rates from simultaneous calcination of CaCO3 or Ca(OH)2 is incorporated in the mass-transfer coefficient for SO2 diffusion to the particle. A solution scheme for the nonlinear differential equation governing pore diffusion with changing particle structure is developed. The influence of grain overlap on product-layer diffusion is quantified. The model predictions show good agreement with the differential reactor data of Borgwardt and Bruce (1986) that include the influences of surface area, temperature, and SO2 partial pressure.
High-Temperature, Short Time Sulfation of Calcium-Based Sorbents: II. Experimental Data and Theoretical Model Predictions
Milne, C.R.; Silcox, G.D. and Pershing, D.W.
I & EC Research, 29, 2202, 1990. Funded by Environmental Protection Agency and ACERC.
The fundamental processes for injection of CaCO3 and Ca(OH)2 for the removal of SO2 from combustion gases of coal-fired boilers are analyzed on the basis of experimental data and a comprehensive theoretical model. Sulfation data were obtained in a 30-KW isothermal dispersed-phase reactor at conditions simulating those of upper-furnace injection. The theoretical model accounts for particle structure, calcination, sintering, sulfation, and heat and mass transfer. Pore diffusion, product-layer diffusion, and sintering appear to be the principle processes that govern the rate of SO2 capture for the hydrate particles of interest for commercial dry sorbent injection.
Calcination and Sintering Models for Application to High-Temperature, Short Time Sulfation of Calcium-Based Sorbents
Milne, C.R.; Silcox, G.D. and Pershing, D.W.
I & EC Research, 29, 139, 1990. Funded by Environmental Protection Agency and ACERC.
To simulate the staged availability of transient high surface area CaO observed in high-temperature flow-reactor data, the rate of calcination of CaCO3 of Ca(OH)2 is described by an empirical modification of the shrinking-core model. The physical model depicts particle decomposition by the shrinking-core mechanism. The subsequent time dependent decrease in CaO reactivity (surface area and porosity) due to sintering is simulated by reducing the grain-center spacing for the matrix of overlapping CaO grains. Information from SEM micrographs and from other physical property measurements of the porous particles is incorporated. This submodel simulates the time dependent availability and reactivity of CaO for a comprehensive model used to study sulfation of CaCO3 and Ca(OH)2 particles at upper-furnace injection conditions.
Rotary Kiln Incineration: Comparison of Field and Pilot Scale Measurements of Contaminant Evolution Rates from Sorbent Beds
Lester, T.W.; Cundy, V.A.; Sterling, A.M.; Montestruc, A.N.; Jakway, A.J.; Leger, C.B.; Pershing D.W.; Lighty, J.S.; Silcox, G.D. and Owens, W.D.
Environmental Science Technology, 1990. Funded by Environmental Protection Agency and Louisiana State University Hazardous Waste Research Center.
A comparison is made, for the first time, between the evolution of hydrocarbons from sorbent beds in an industrial rotary kiln incinerator and in a laboratory scale rotary kiln simulator. To relate the data from the different sized units, due allowance is made for bed dynamical similitude, bed geometrical factors, and bed heat-up. To minimize the effects of disturbances caused by foreign matter in the full scale bed and differences in loading techniques, the rate of evolution is characterized by an "evolution interval," that is defined as the time required for total hydrocarbon evolution at the maximum evolution rate. A comparison of evolution intervals with reciprocal bed temperature reveals that the data are consistent with an analysis that assumes a uniform bed temperature (at any instant of time) and desorption control of the evolution rate. Furthermore, the evolution intervals scale inversely with modified Froude Number, which characterizes bed dynamics. The success in comparing field and simulator results indicates that pilot scale rotary kilns may be used to simulate certain features of industrial scale units if appropriate care is taken in matching dynamical, geometrical and thermal parameters.
Thermal Analysis of Rotary Kiln Incineration: Comparison of Theory and Experiment
Owens, W.D.; Silcox, G.D.; Lighty, J.S.; Deng, X.-X.; Pershing D.W.; Cundy, V.A.; Leger, C.B. and Jakway, A.J.
Combustion and Flame, 1990. Funded by Gas Research Institute, ACERC and Louisiana State University Hazardous Waste Research Center.
A comprehensive heat-transfer model and associated simplified scaling laws are developed and verified using a pilot-scale, directly-fired rotary kiln with a slumping bed of dry or wet, 2 mm clay sorbent particles. The kiln operating conditions examined include: rotation rate (0.1 to 0.9 rpm), percent fill fraction (3 to 8), feed moisture content (0 to 20 wt. percent), and inner-wall temperature (190 to 790º). The model is used to determine the relative importance of several heat-transfer mechanisms including radiation, gas-to-solid convection, and wall-to-solid convection. Simple scaling laws are also developed for water vaporization. Generally good agreement is obtained between theory and experiment without adjusting any model parameters. Further, the simplified scaling laws provide a reasonable estimate of the pilot scale performance.
The key conclusions of this study for kilns at the conditions examined are: (1) water exerts a profound effect on the solids thermal profile, (2) simple geometrical scaling is not sufficient, (3) the assumption of a well mixed (radially isothermal) solids bed for the heat transfer analysis is appropriate, (4) a dimensionless group, which is a function of temperature, can be defined giving the relative importance of radiative and convective modes of heat transfer, and (5) moisture vaporization rates can be roughly approximated by assuming that the water vaporized at the boiling point at a rate controlled by the rate of heat transfer to the bed. The implications of the scaling laws for scale-up and kiln design are also examined.
1989
Fundamental Experiments on Thermal Desorption of Contaminants from Soils
Lighty, J.S.; Silcox, G.D.; Pershing, D.W.; Cundy, V.A. and Linz, D.G.
Environmental Progress, 8 (1), 1989. Funded by the Gas Research Institute, Dave Linz, Project Manager, National Science Foundation - Presidential Young Investigator Program, ACERC (National Science Foundation and Associates and Affiliates), the State of Utah, and US Department of Energy.
The goals of this research are to develop an understanding of the transport phenomena that occur during the desorption of contaminants from soil and to obtain rate information. This information can then be used to develop a model to predict the performance of full-scale thermal treatment systems for optimization and cost reduction.
The Effects of Rotary Kiln Operating Conditions and Design on Burden Heating Rates as Determined by a Mathematical Model of Rotary Kiln Heat Transfer
Silcox, G.D. and Pershing, D.W.
Accepted for publication in JAPCA, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates), the State of Utah, and US Department of Energy.
A mathematical model of heat transfer in a directly fired rotary kiln is developed and used to examine the effects of operating and design parameters on burden temperature. The model includes a mean beam length radiation model and axial zoning. Conductive and convective heat transfers are also included. Radiation between immediately adjacent zones is permitted. Calculation of heat transfer rates is facilitated by the use of an electric circuit analogue. An iterative solution procedure is adopted to solve the energy balance equations.
At the conditions examined, the model predicts that coflowing gas and solid streams result in higher average burden temperatures than do counter flowing streams. The moisture level of the feed is predicted to be a key operating parameter. The effects of kiln length, burden residence time, firing rate, and flame length are also examined.
Calcination and Sintering Models for Application to High-Temperature, Short-Time Sulfation of Calcium-Based Sorbents
Milne, C.R.; Silcox, G.D.; Pershing, D.W. and Kirchgessner, D.A.
Accepted for publication in I & EC Res., 1989. Funded by the US Environmental Protection Agency, ACERC (National Science Foundation and Associates and Affiliates), the State of Utah, and US Department of Energy.
To simulate the staged availability of transient high surface area CaO observed in high-temperature flow-reactor data, the rate of calcination of CaCO3 or Ca(OH)2 is described by an empirical modification of the shrinking-core model. The physical model depicts particle decomposition by the shrinking-core mechanism. The subsequent time dependent decrease in CaO reactivity (surface area and porosity) due to sintering is simulated by reducing the grain-center spacing for the matrix of overlapping CaO grains. Information from SEM micrographs and from other physical property measurements of the porous particles is incorporated. This submodel simulates the time dependent availability and reactivity of CaO for a comprehensive model used to study sulfation of CaCO3 or Ca(OH)2 particles at upper-furnace injection conditions.
Rotary Kiln Incineration--Combustion Chamber Dynamics
Cundy, V.A.; Lester, T.W.; Leger, C.B.; Miller, G.; Montestruc, A.N.; Acharya, S.; Sterling, A.M.; Pershing, D.W.; Lighty, J.S.; Silcox, G.D. and Owens, W.D.
Journal of Hazardous Materials, 22, 195-219,1989. Funded by US Environmental Proctection Agency and ACERC (National Science Foundation and Associates and Affiliates).
A multifaceted experimental and theoretical program aimed at understanding rotary kiln performance is underway. The overall program involves university, industry, and government participation and is broken into distinct sub-programs. This paper discusses in some detail the research effort performed to date in two of the sub-programs: Full-scale in situ sampling and kiln-simulator experimentation. Full-scale in situ measurements are obtained from the Louisiana Division rotary kiln facility of Dow Chemical USA, located in Plaquemine, Louisiana. Summary results obtained from controlled experiments that were performed during continuous processing of carbon tetrachloride and preliminary results obtained during batch mode processing of toluene-laden sorbent packs are presented. Kiln-simulator data are obtained by using the facilities of the Chemical Engineering Department at the University of Utah. Recent kiln-simulator work, conducted in support of the full-scale measurements sub-program, has aided in providing an understanding of the results that have been obtained at the full-scale. Modeling efforts, conducted at Louisiana State University and the University of Utah, have concentrated on the development of realistic, fluid-flow and heat-transfer models, near-term chlorinated kinetic models and bed mass-transfer models to be incorporated into a global three-dimensional kiln-simulator model. The paper concludes with an overview of these modeling efforts.
Experimental and Theoretical Studies on the Combustion of Waste Zirconium Sponge in a Rotary Kiln Simulator
Lemieux, P.M.; Silcox, G.D. and Pershing, D.W.
Waste Management, 9:125-137, 1989. Funded by Westinghouse Corp.
Previous experimental studies have indicated that rotary kilns may be suitable combustion systems to incinerate small quantities of off grade zirconium sponge produced during the manufacturing of zirconium for the nuclear industry. This paper describes a mathematical model of zirconium sponge combustion in a rotary kiln environment and specifically examines the use of the bed submodel to analyze detailed zirconium combustion data obtained previously in a rotary kiln simulator. The results of this analysis indicated that the experimentally observed burning rates could be predicted within 25% based on available transport correlations and current understanding of zirconium combustion. Without any adjustment of the physical parameters in the model, both the experimental results and the model predictions indicated that the primary combustion parameters are the bulk oxygen concentration within the kin and the local kiln bed temperature. Charge size was found to be less significant. A detailed analysis of the theoretical predictions indicates that the zirconium sponge oxidation rate is controlled by three sequential processes (the convection of oxygen from the bulk gas to the top of the bed, the diffusion of oxygen through the bed to the particle surface, and the diffusion of oxygen through previously formed zirconium dioxide product to the unreacted zirconium metal). Under conditions typical of commercial rotary kiln operation all three of these resistances appear to be significant. Both the experimental data and the model suggest that intrinsic chemical kinetics are fast and not controlling except during the first few minutes after the zirconium is charges. The model assumes that the zirconium oxide product layer increases until it reaches a maximum value after which it remains constant due to continuous formation and abrasion.
An In-Depth Study of Incineration - Rotary Kiln Performance Characterization
Cundy, V.A.; Lester, T.W.; Conway, L.R.; Jakway, A.J.; Leger, C.B.; Montestruc, A.N.; Acharya, S.; Sterling, A.M.; Owens, W.D.; Lighty, J.S.; Pershing, D.W. and Silcox, G.D.
Louisiana State University/Hazardous Waste Research Center SAC Review Meeting, Baton Rouge, Louisiana, 1989. Funded by Louisiana State University/Hazardous Waste Research Center (Supported by US Environmental Protection Agency).
A comprehensive study aimed at understanding rotary kiln performance is underway. The program is led by personnel from Louisiana State University. Bench and pilot-scale facilities at the University of Utah are available for use in solids desorption studies. Full-scale in situ measurements are obtained from the Louisiana Division rotary kiln facility of Dow Chemical USA, located in Plaquemine, Louisiana. This paper presents a summary of the project providing some detail of the work that has been accomplished from 1 January through 31 August 1989.
Fundamentals for the Thermal Remediation of Contaminated Soils - Particle and Bed Desorption Models
Lighty, J.S.; Silcox, G.D.; Pershing, D.W.; Cundy, V.A. and Linz, D.G.
Accepted for publication in Environ. Science Tech., 1989. Funded by the Gas Research Institute, Dave Linz, Project Manager, National Science Foundation/Presidential Young Investigators, ACERC (National Science Foundation and Associates and Affiliates), the State of Utah, and US Department of Energy.
A major research effort has been initiated to characterize the rate-controlling processes associated with the evolution of hazardous materials from soils. A threefold experimental approach is being used in conjunction with computer modeling to analyze thermal desorption of contaminants. Phenomena occurring both inside particles (intraparticle) and with a bed of particles (interparticle) were studied.
The results obtained suggest that the most important process variables are local thermal environment and gas-phase contaminant concentration because the adsorption equilibrium characteristics of the contaminant/soil pair control the desorption of contaminant from a particle at a given temperature. A mass-transfer/desorption model, which assumes gas/solid equilibrium at all points and time, is proposed and the model was found to predict the measured temperature dependence.
1988-1987
Transport Fundamentals for the Thermal Remediation of Contaminated Soils
Lighty, J.S.; Silcox, G.D.; Pershing, D.W.; Cundy, V.A. and Linz, D.G.
Submitted for Review to AIChE Journal, 1988. 23 pgs. Funded by Gas Research Institute, ACERC (National Science Foundation and Associates and Affiliates), and National Science Foundation/Presidential Young Investigators.
A major research effort has been initiated to characterize the transport phenomena associated with the evolution of hazardous materials from contaminated soils. A threefold experimental approach is being used in conjunction with computer modeling to analyze thermal desorption of contaminants from soils under a variety of experimental conditions. First, a particle-characterization reactor (PCR) is being employed to characterize intraparticle transport under conditions where the bulk concentration and temperature at the particle surface are known. Secondly, a packed-bed reactor is being used to examine interparticle transport within a bed of particles. In the third portion of the work, a 73 kW pilot-scale rotary kiln is providing time-resolved measurements of the species evolution. The results from the PCR experiments and modeling are reported in this manuscript.
The PCR experimental results obtained suggest that the most important process variable is local thermal environment and that the absorption characteristics of the contaminant/soil pair are important in the desorption process. The adsorption characteristics are a function of local temperature, thus accounting for the steep dependence of the cleanup efficiency of a soil on temperature. In addition, the adsorption characteristics are influenced by soil type and moisture; it has been found that these parameters also effect desorption of contaminants. A mass-transfer model, which assumes gas/solid equilibrium at all points and times, is proposed and the model predictions were found to exhibit temperature dependence similar to the data.
A Mathematical Model for the Flash Calcination of Dispersed CaCO3 and Ca(OH)2 Particles
Silcox, G.D.; Kramlich, J.C. and Pershing, D.W.
Submitted to I&EC/Research, 1987. 28 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).
A mathematical model for the flash calcination of Ca(OH)2 and CaCO3 is presented. The model describes the decomposition of the parent material at the reactant-product interface, the diffusion of CO2 or H2O through the growing CaO layer, and the sintering of the CaO layer. The model is qualitative, but it provides useful estimates of peak CO2 pressures in the CaO layer, relative rates of surface area development and loss for Ca(OH)2 and CaCO3, the effect of particle size, and the effects of time and temperature.
Fundamentals of Hazardous Solid Waste Incineration in a Rotary Kiln Environment
Lighty, J.S.; Silcox, G.D.; Britt, R.; Owens, W.D.; Pershing, D.W. and Cundy, V.A.
Proc. AFRC Int'l. Symposium on Waste Incineration, 1987, Palm Springs. Funded by Environmental Protection Agency.
With landfill costs increasing and regulations on landfilling becoming more stringent, alternatives to conventional hazardous waste treatment strategies are becoming more desirable. Incineration is presently a permanent, proven solution for the disposal of most organic contaminants, but also a costly one, especially in the case of solids that require some auxiliary fuel. The goal of this research is to develop an understanding of the phenomena associated with the evolution of contaminants from solids in the primary combustor of an incineration system. A threefold approach has been used. First, a bench-scale Particle Characterization Reactor was developed to study the transport phenomena on a particle basis, where the controlling processes are mainly intraparticle. Second, a Bed Characterization Reactor was built to examine the controlling transport phenomena within a bed of particles, where the processes are primarily interparticle. The results of these studies can be applied to any primary combustor. Finally a pilot-scale rotary kiln was developed to study the evolution of contaminants from solids within a realistic temperature and rotation environment.
This paper describes results obtained in a study using a commercial sorbent contaminated with toluene. The data are from the Particle Characterization Reactor and the Rotary-Kiln Simulator. The results show that the method of contamination and charge size does not have a large effect on desorption, while temperature and contaminant concentration are important parameters in the evolution of contaminants in a rotary kiln. Preliminary modeling efforts for the kiln are also discussed.
Silcox, GD
1997
Numerical Modeling of the Temperature Distribution in a Commercial Hazardous Waste Slagging Rotary Kiln
Veranth, J.M.; Silcox, G.D. and Pershing, D.W.
Environmental Science and Technology, 31:2534-539(1997). Funded by ACERC.
The gas, wall and bed temperatures in a hazardous wasted incineration kiln were studied using a commercially available, CFD-based, r4eacting flow code, which included radiation heat transfer. The model was compared to field measurements make on a co-current flow, 35 MW slagging rotary kiln. Cases were run to determine the sensitivity of the predictions to changes in the model assumptions and to simulate the normal variation in combustion inputs. The model predictions of the peak bed temperature, of the axial temperature profile, and of the gas temperature at the exit-plane were consistent with the measurement at a full-scale wasted incinerator during normal operation. The model and the field observations both indicate that the peak bed temperature occurs near the middle of the kiln and that the difference between the peak bed temperature and the exit-plane gas temperature depends on the inlet flow. The geometry of the transition between the kiln and the secondary combustion chamber and the fuel-to-air equivalence ratio have the greatest effect on the calculated temperature distribution. Modeling studies provide useful information such as the relationship between available measurements and the temperature at inaccessible locations inside a full-scale kiln.
1996
Field Investigation of the Temperature Distribution in a Commercial Hazardous Waste Slagging Rotary Kiln
Veranth, J.M.; Gao, D.-C. and Silcox, G.D.
ES&T, 30:3053-3060, 1996. Funded by ACERC and Gas Research Institute.
Gas and bed temperatures were studied in a 4.4 m by 12 m, co-current flow, slagging rotary kiln at a commercial hazardous waste incinerator. The visual observations used by the kiln operators to control the process are described. These observations were quantified using thermocouples, radiation pyrometers, and phase-change indicators. The objectives were to estimate the peak bed temperature and compare this to measurements at the kiln exit. The maximum bed temperature occurs toward the middle of this type of kiln and not at the discharge. The slag melting temperature and test pellets with known melting points indicate that the peak bed temperature can be 100-300 K higher than the kiln exit temperature reported by the permanent instruments at this facility. Both broadband radiation pyrometers and thermocouples give a qualitative temperature indication that can be used for process control, but the readings depend on the sensor locations relative to the incompletely mixed air and combustion products. Two-color radiation pyrometer measurements of surface temperature near the kiln exit are higher than the actual temperature due to reflected radiation.
1994
Fundamental Study of the Thermal Desoprtion of Toluene from Montmorillonite Clay Particles
Keyes, B.R. and Silcox, G.D.
Environmental Science Technology, 28:840, 1994. Funded by ACERC and Gas Research Institute.
Nonisothermal desorption of toluene from individual montmorillonite clay particles was measured experimentally and modeled mathematically to elucidate details of the overall thermal desorption process. A single-particle reactor was used. It consisted of a porous, 2-6mm clay pellet formed around a 0.05-mm diameter thermocouple in a 6- or 9-mm o.d. glass tube. The tube was surrounded by a supplementary heater and placed in a GC oven. Desorption rates were obtained as a function of heating rate, clay type, particle size, and purge gas flow rate. In addition, the absorption isotherms for two toluene/clay systems and one n-dodecane/clay system were measured and correlated using the Freundlich isotherm. At the conditions examined, the rate-controlling mechanism is associated with intraparticle diffusion. Isothermal desorption experiments using clay pellets of different sizes demonstrate that local desorption kinetics are not rate controlling. Toluene shows a slower desorption rate than n-dodecane at low concentrations. This is attributed to the hindered removal of toluene from interlamellar regions of the clay. Comparisons of single-particle reactor and pilot-scale rotary kiln results show that mass transport resistances associated with a bed of particles dominate the desorption process at rotary kiln conditions.
1993
The Effect of Treatment Temperature of Metal Recovery from a Porous Silica Sorbent by EPA Method 3050 and by an HF-Based Method
Gao, D. and Silcox, G.D.
JAWMA 43:1004-1005, July 1993. Funded by Gas Research Institute and ACERC.
Determining the fate of metals during hazardous waste incineration is important because of environmental concerns. Such a determination requires an analysis of the waste ash. At high temperatures, metals are frequently immobilized by reactions with silicates or by encapsulation with silicates. In order to determine the metal retention levels in waste ash, a digestion method that insures total metal recovery is necessary. This study compares two digestion techniques and uses each to look at the effect of the incinerator's temperature on the apparent fraction of metal retained in the waste ash. Four metals on a diatomaceous earth matrix were studied: Cr, Cd, Pb, and Ni. An HF-based digestion yielded fractional metal retentions of about 1.0 while an HNO3-based digestion yielded apparent retention levels that decrease from 1 to 0.2 as the reactor temperature was increased from 25 to 870ºC.
A commonly used HNO3-based digestion procedures is EPA SW-846, Method 3050. It is successful at extracting those metal compounds that are soluble at conditions that characterize natural leaching and biological processes. However, Method 3050 was not designed to digest metals immobilized by silicates.
Thermal Treatment of Hazardous Wastes: A Comparison of Fluidized Bed and Rotary Kiln Incineration
Rink, K.K.; Larsen, F.S.; Kozinski, J.A.; Lighty, J.S.; Silcox, G.D. and Pershing, D.W.
Energy & Fuels, 7 (6):803-814, 1993. Funded by ACERC.
Large volumes of sludge are produced by a wide variety of industrial processes and by municipal wastewater 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, NOx 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 continuous for hydrocarbons, CO, O2, NO, and CO2. 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 led to the reduction of NO inside the freeboard zone. In both the CFB and 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.
On-Line Monitoring of Formaldehyde in Combustion Gases Using Gas Chromatography/Mass Spectrometry
McClennen, W.H.; Sheya, S.A.N.; Arnold, N.S.; Meuzelaar, H.L.C.; Larsen, F.S. and Silcox, G.D.
Incineration of Hazardous Wastes-2; Toxic Combustion By-Products:545-555, (in press). Funded by Consortium for Fossil Fuel Liquefaction and ACERC.
This paper describes a method for on-line gas chromatography/mass spectrometry (GC/MS) of formaldehyde in combustion gases. The method uses a recently developed vapor-sampling inlet to monitor the concentration of formaldehyde and other products of incomplete combustion (PICs) from the burning of plain and phenol-formaldehyde resin treated wood chips. Other PICs that were simultaneously monitored included ketene, propylene, propyne and acetaldehyde. The direct analysis method has detection limits of less than 1 ppm for the reactive formaldehyde and excellent selectivity for determinations in the complex mixtures of combustion products. The rapid sampling technique allows monitoring of transient events of only a few minutes or less duration. Examples of the technique include the detection of sample line problems and the comparison of PIC concentrations from different points in the combustion exhaust stream.
1992
Solid Waste Incineration in Rotary Kilns
Pershing, D.W.; Lighty, J.S.; Silcox, G.D.; Heap, M.P. and Owens, W.D.
Combustion Science and Technology, 1992 (in press). (Previously presented at the First International Conference on Combustion Technologies for a Clean Environment, Vilamoura, Portugal, September 1991.) Funded by the National Science Foundation, Environmental Protection Agency, Gas Research Institute and ACERC.
Rotary kilns are used to dispose of many solid wastes and sludges and to thermally treat contaminated soils. In this communication the fates of hydrocarbon and metal species are examined with a view toward optimization of new kiln designs and maximizing existing unit throughout while minimizing pollutant emissions. Initially, process fundamentals are considered to characterize the controlling phenomena. Pilot- and large-scale data are then examined to define practical system complexities. Finally, techniques for data scale-up and performance prediction are summarized. Temperature is clearly the most important parameter with respect to the fate of both metal and hydrocarbon species; hence, heat transfer is often rate limiting. High temperatures favor hydrocarbon evolution, but can also enhance the formation of toxic metal fumes. Both the solid composition and the moisture content can significantly influence the time at temperature required for hydrocarbon destruction and metal vaporization.
Improving bed mixing helps contaminant release but can also aggravate puffing tendencies with batch charging. Full-scale performance predictions currently require a combination of small-scale data and computer modeling. Future work needs to focus on verification of large-scale predictions for complex mixtures and sludges so that expensive trial burns can be minimized.
The Desorption of Toluene from a Montmorillonite Clay Adsorbent in a Rotary Kiln Environment
Owens, W.D.; Silcox, G.D.; Lighty, J.S.; Deng, X.-X.; Pershing, D.W.; Cundy, V.A.; Leger, C.B. and Jakway, A.J.
Journal of Air Waste Management Assoc., 42:681-690, 1992. Funded by US Environmental Protection Agency, Gas Research Institute and ACERC.
The vaporization of toluene from pre-dried, 6mm montmorillonite clay particles was studied in a 130 kW pilot-scale rotary kiln with inside dimensions of 0.61 by 0.61 meters. Vaporization rates were obtained with a toluene weight fraction of 0.25 percent as a function of kiln fill fractions from 3 to 8 percent, rotation rates from 0.1 to 0.9 rpm, and kiln wall temperatures from 189 to 793ºC. Toluene desorption rates were obtained from gas-phase measurements and interpreted using a desorption model that incorporates the slumping frequency of the solids. Fill fraction of the kiln, binary gas diffusion in the bed, and particle desorption using an Arrhenius-type expression that is a function of bed temperature and average bed concentration. The model included three adjustable desorption parameters which were obtained by fitting the experimental data with a least squares technique. Solid and kiln-wall temperatures were continuously recorded and used by the model to perform toluene desorption predictions. The model was successful at predicting the effects of fill fraction and rotation rate over a range of temperatures. It was shown that an increase in kiln temperature and rotation rate increased toluene desorption rates. A decrease in kiln fill fraction showed an increase n desorption rate. Desorption predictions were performed using both predicted and measured temperature profiles. In addition, the model was used to perform sensitivity tests examine the relative importance of bed diffusion and particle desorption resistances. A methodology for predicting full-scale performance was developed. A full-scale, rotary-kiln heat-transfer model was used to estimate the bed thermal profile. This profile was then utilized by the model tp predict toluene desorption at full-scale.
Hydrocarbon and Formaldehyde Emissions from the Combustion of Pulverized Wood Waste
Larsen, F.S.; McClennen, W.H.; Deng, X.-X.; Silcox, G.D.and Allison, K.
Combustion Science and Technology, 1992 (in press). (Also presented at The Second International Congress on Toxic By-Products: Formation and Control, Salt Lake City, UT, March 1992). Funded by Weyerhaeuser Corp. and ACERC.
Hydrocarbon and formaldehyde emissions from the combustion of pulverized wood waste were measured in 100 kW, cylindrical combustion chamber measuring 0.61 by 0.61 m. The wood was pneumatically conveyed to the burner and natural gas was used as an auxiliary fuel. The wood was screened prior to feeding so that its size distribution was representative of the suspension phase of a stoker boiler. Chamber wall and gas temperatures ranged from 920 to 1200 K and oxygen concentrations ranged from 2 to 9 percent, dry. Two types of waste were studied, plain wood and wood that was impregnated with a phenol-formaldehyde resin. The latter was a by-product of particleboard production. In general, the emissions of products of incomplete combustion (PICs) from the resinated waste were higher than those produced by plain wood. This may have been due to three factors: 1) the resinated wood was slightly wetter than the plain wood (6-9 percent by weight vs. 3 percent), 2) there was a difference in particle size distribution between the two materials as received, the resinated wood being larger, and 3) the resin may have had an effect on the emissions. Ultimate analyses of the two wastes showed no significant differences, other than moisture, in composition. At temperatures above 1200 K, total hydrocarbon emissions were roughly 10 to 29 ppm and formaldehyde emissions were less than the detection limit of 1 ppm. Typical waste wood boiler temperatures are roughly 1600 K. Hence, unless there are cool, poorly mixed regions in the full-scale facility, hydrocarbon and formaldehyde emissions should not be significant. However, the emissions from burning the two different types of wood would probably be different if all operating parameters in the wood-fired boiler are held constant.
Modeling Sorbent Injection and Sulfur Capture in Pulverized Coal Combustion
Boardman, R.D.; Brewster, B.S.; Huque, Z.; Smoot, L.D. and Silcox, G.D.
Air Toxic Reduction and Combustion Modeling, 15:1-9, 1992. (Also presented at the ASME International Joint Power Generation Conference, Atlanta, GA, October 1992). Funded by Advanced Fuel Research and ACERC.
A computer model has been developed for predicting mixing and reactions of injected sorbent particles in pulverized coal combustors and gasifiers. A shrinking-core, grain model was used for sulfation. The model accounts for the effects of surface area, pore diffusion, diffusion through the product layer, chemical reaction, and reduction of the pore volume due to grain swelling. The submodel was evaluated for a fuel-lean case and for a fuel-rich case. Predictions were compared with limited experimental data (for the fuel-rich case). The results illustrate the model's capability for predicting the effectiveness of sulfur capture. The importance of sorbent particle properties was also investigated parametrically, and model limitations were identified.
1991
Hydrocarbon and Formaldehyde Emissions from the Combustion of Pulverized Wood Waste
Larsen, F.S.; McClennen, W.H.; Deng, X.-X.; Silcox, G.D. and Allison, K.
Combustion Science and Technology, 1991 (in press). Funded by Weyerhaeuser Corp. and ACERC.
Hydrocarbon and formaldehyde emissions from the combustion of pulverized wood waste were measured in 100 kW, cylindrical combustion chamber measuring 0.61 by 0.61 m. The wood was pneumatically conveyed to the burner and natural gas was used as an auxiliary fuel. The wood was screened prior to feeding so that its size distribution was representative of the suspension phase of a stoker boiler. Chamber wall and gas temperatures ranged from 920 to 1200 K and oxygen concentrations ranged from 2 to 9 percent, dry. Two types of waste were studied, plain wood and wood that was impregnated with a phenol-formaldehyde resin. The latter was a by-product of particleboard production. In general, the emissions of products of incomplete combustion (PICs) from the resinated waste were higher than those produced by plain wood. This may have been due to three factors: 1) the resinated wood was slightly wetter than the plain wood (6-9 percent by weight vs. 3 percent), 2) there was a difference in particle size distribution between the two materials as received, the resinated wood being larger, and 3) the resin may have had an effect on the emissions. Ultimate analyses of the two wastes showed no significant differences, other than moisture, in composition. At temperatures above 1200 K, total hydrocarbon emissions were roughly 10 to 29 ppm and formaldehyde emissions were less than the detection limit of 1 ppm. Typical waste wood boiler temperatures are roughly 1600 K. Hence, unless there are cool, poorly mixed regions in the full-scale facility, hydrocarbon and formaldehyde emissions should not be significant. However, the emissions from burning the two different types of wood would probably be different if all operating parameters in the wood-fired boiler are held constant.
On-Line Monitoring of Formaldehyde in Comubustion Gases Using Gas Chromatography/Mass Spectrometry
McClennen, W.H.; Sheya, S.A.N.; Arnold, N.S.; Meuzelaar, H.L.C.; Deng, X.-X.; Larsen, F.S. and Silcox, G.D.
Combustion Science and Technology, 1991 (in press). Funded by ACERC.
This paper describes a method for on-line gas chromatography/mass spectrometry (GC/MS) of formaldehyde in combustion gases. The method uses a recently developed vapor-sampling inlet to monitor the concentration of formaldehyde and other products of incomplete combustion (PICs) from the burning of plain and phenol-formaldehyde resin treated wood chips. Other PICs that were simultaneously monitored included ketene, propylene, propyne and acetaldehyde. The direct analysis method has detection limits of less than 1 ppm for the reactive formaldehyde and excellent selectivity for determinations in the complex mixtures of combustion products. The rapid sampling technique allows monitoring of transient events of only a few minutes or less duration. Examples of the technique include the detection of sample line problems and the comparison of PIC concentrations from different points in the combustion exhaust stream.
Rotary Kiln Incineration: Comparison and Scaling of Field-Scale Contaminant Evolution Rates from Sorbent Beds
Lester, T.W.; Cundy, V.A.; Sterling, A.M.; Montestruc, A.N.; Jakway, A.J.; Lu, C.; Leger, C.B.; Pershing D.W.; Lighty, J.S.; Silcox, G.D. and Owens, W.D.
Environmental Science Technology, 25:1142-1152, 1991. Funded by Environmental Protection Agency, Louisiana State University/Hazardous Waste Research Center and ACERC.
A comparison is made, for the first time, between the evolution of hydrocarbons from clay sorbent beds in a field-scale rotary kiln incinerator and in a pilot-scale rotary kiln simulator. To relate the data from the different sized units, due allowance is made for bed dynamical similitude, bed geometrical factors, and bed heat-up. To minimize the effects of disturbances caused by foreign matter in the field scale bed and differences in loading techniques, the rate of evolution is characterized by an "evolution interval" defined as the time required for the middle 80% of the ultimate containment evolution to occur. A comparison of evolution intervals with reciprocal bed temperature reveals that the data are consistent with an analysis that assumes a uniform bed temperature (at any instant of time) and desorption controlled evolution rate. Furthermore, the evolution intervals scale inversely with a modified Froude Number, which characterizes bed dynamics. The success in comparing field and simulator results indicates that pilot scale rotary kilns may be used to simulate certain features of industrial-scale units if dynamical, geometrical and thermal parameters are matched appropriately.
Thermal Analysis of Rotary Kiln Incineration: Comparison of Theory and Experiment
Owens, W.D.; Silcox, G.D.; Lighty, J.S.; Deng, X.-X.; Pershing D.W.; Cundy, V.A.; Leger, C.B. and Jakway, A.J.
Combustion and Flame, 86:101-114, 1991. Funded by Gas Research Institute, ACERC and Louisiana State University/Hazardous Waste Research Center.
A comprehensive heat-transfer model and associated simplified scaling laws are developed and verified using a pilot-scale, directly fired rotary kiln with a slumping bed of dry or wet, 6-mm clay sorbent particles. The kiln operating conditions examined include: rotation rate (0.1 to 0.9 rpm), percent fill fraction (3-8), feed moisture content (0-20 wt.%), and inner-wall temperature (190º to 790ºC). The model is used to determine the relative importance of several heat-transfer mechanisms including radiation, gas-to-solid convection, and wall-to-solid convection. Simple scaling laws are also developed for water vaporization. Generally good agreement is obtained between theory and experiment without adjusting any model parameters. Further, the simplified scaling laws provide a reasonable estimate of the pilot scale performance. The key conclusions of this study for kilns at the conditions examined are (1) water exerts a profound effect on the solids thermal profile, (2) simple geometrical scaling is not sufficient, (3) the assumption of a well mixed (radially isothermal) solids bed for the heat transfer analysis is appropriate, (4) a dimensionless group, which is a function of temperature, can be defined giving the relative importance of radiative and convective modes of heat transfer, and (5) moisture vaporization rates can be roughly approximated by assuming that the water vaporizes at the boiling point at a rate controlled by the rate of heat transfer to the bed. The implications of the scaling laws for scale-up and kiln design are also examined.
Solid Waste Incineration in Rotary Kilns
Pershing, D.W.; Lighty, J.S.; Silcox, G.D.; Heap, M.P. and Owens, W.D.
First International Conference on Combustion Technologies for a Clean Environment, Vilamoura, Portugal, September 1991. Funded by the National Science Foundation, Environmental Protection Agency, Gas Research Institute and ACERC.
Rotary kilns are used to dispose of many solid wastes and sludges and to thermally treat contaminated soils. In this communication the fates of hydrocarbon and metal species are examined with a view toward optimization of new kiln designs and maximizing existing unit throughout while minimizing pollutant emissions. Initially, process fundamentals are considered to characterize the controlling phenomena. Pilot- and large-scale data are then examined to define practical system complexities. Finally, techniques for data scale-up and performance prediction are summarized. Temperature is clearly the most important parameter with respect to the fate of both metal and hydrocarbon species; hence, heat transfer is often rate limiting. High temperatures favor hydrocarbon evolution, but can also enhance the formation of toxic metal fumes. Both the solid composition and the moisture content can significantly influence the time at temperature required for hydrocarbon destruction and metal vaporization.
Improving bed mixing helps contaminant release but can also aggravate puffing tendencies with batch charging. Full-scale performance predictions currently require a combination of small-scale data and computer modeling. Future work needs to focus on verification of large-scale predictions for complex mixtures and sludges so that expensive trial burns can be minimized.
1990
Fundamentals for the Thermal Remediation of Contaminated Soils - Particle and Bed Desorption Models
Lighty, J.S.; Silcox, G.D.; Pershing, D.W.; Cundy, V.A. and Linz, D.G.
Environmental Science & Technology, 24 (5), 750-757, 1990. Funded by the Gas Research Institute, Dave Linz, Project Manager, NSF - Presidential Young Investigator Program, ACERC (National Science Foundation and Associates and Affiliates), the State of Utah, and US Department of Energy.
A major research effort has been initiated to characterize the rate-controlling processes associated with the evolution of hazardous materials from soils. A threefold experimental approach is being used in conjunction with computer modeling to analyze thermal desorption of contaminants. Phenomena occurring both inside particles (intraparticle) and with a bed of particles (interparticle) were studied.
The results obtained suggest that the most important process variable are local thermal environment and gas-phase contaminant concentration because the adsorption equilibrium characteristics of the contaminant/soil pair control the desorption of contaminant from a particle at a given temperature. A mass-transfer/desorption model, which assumes gas/solid equilibrium at all points and time, is proposed and the model was found to predict the measured temperature dependence.
Mathematical and Physical Modeling of Rotary Kilns with Applications to Scaling and Design
Silcox, G.D.; Larsen, F.S. and Pershing, D.W.
Combustion Science and Technology, 1990 (In press). Funded by ACERC and Gas Research Institute.
The heat and mass transfer in an indirectly fired rotary kiln are examined using a combination of physical and mathematical modeling. The physical modeling is used to determine characteristic mixing times in a slumping kiln bed. These times are compared with characteristic times for diffusion of heat and mass through the bed in order to justify a lumped capacitance analysis of heat and mass transfer.
Mathematical models of heat and mass transfer are used to examine the effects of design and operating parameters on bed temperature and desorption rates. Limited comparisons with temperature measurements are presented. The design and operating parameters studied include kiln length, solid residence time, solid feed rate, and feed moisture content. The effects of moisture are particularly important to both heat and mass transfer. Scaling considerations are examined and it is shown that maintaining equivalent wall temperature profiles, fill fractions, moisture levels, and burden residence times does not necessarily result in equivalent bed thermal profiles.
The Effects of Rotary Kiln Operating Conditions and Design on Burden Heating Rates as Determined by a Mathematical Model of Rotary Kiln Heat Transfer
Silcox, G.D. and Pershing, D. W.
J. Air Waste Management Association, 40, 1990. Funded by ACERC.
A mathematical model of heat transfer in a directly-fired rotary kiln is developed and used to examine the effects of operating and design parameters on burden temperature. The model includes a mean beam length radiation model and axial zoning. Conductive and convective heat transfer are also included. Radiation between immediately adjacent zones is permitted. Calculation of heat transfer rates is facilitated by the use of an electric circuit analogue. An iterative solution procedure is adopted to solve the energy balance equations.
At the conditions examined, the model predicts that coflowing gas and solid streams result in higher average burden temperatures than do counter flowing streams. The moisture level of the feed is predicted to be a key operating parameter. The effects of kiln length, burden residence time, firing rate, and flame length are also examined.
High-Temperature, Short Time Sulfation of Calcium-Based Sorbents: I. Theoretical Sulfaction Model
Milne, C.R.; Silcox, G.D. and Pershing, D.W.
I & EC Research, 29, 2192, 1990. Funded by Environmental Protection Agency and ACERC.
A mathematical model for the sulfation of CaO is developed around the overlapping grain concept employed in the calcination and sintering models of Milne et al. (1988). The potential influence of high mass-transfer rates from simultaneous calcination of CaCO3 or Ca(OH)2 is incorporated in the mass-transfer coefficient for SO2 diffusion to the particle. A solution scheme for the nonlinear differential equation governing pore diffusion with changing particle structure is developed. The influence of grain overlap on product-layer diffusion is quantified. The model predictions show good agreement with the differential reactor data of Borgwardt and Bruce (1986) that include the influences of surface area, temperature, and SO2 partial pressure.
High-Temperature, Short Time Sulfation of Calcium-Based Sorbents: II. Experimental Data and Theoretical Model Predictions
Milne, C.R.; Silcox, G.D. and Pershing, D.W.
I & EC Research, 29, 2202, 1990. Funded by Environmental Protection Agency and ACERC.
The fundamental processes for injection of CaCO3 and Ca(OH)2 for the removal of SO2 from combustion gases of coal-fired boilers are analyzed on the basis of experimental data and a comprehensive theoretical model. Sulfation data were obtained in a 30-KW isothermal dispersed-phase reactor at conditions simulating those of upper-furnace injection. The theoretical model accounts for particle structure, calcination, sintering, sulfation, and heat and mass transfer. Pore diffusion, product-layer diffusion, and sintering appear to be the principle processes that govern the rate of SO2 capture for the hydrate particles of interest for commercial dry sorbent injection.
Calcination and Sintering Models for Application to High-Temperature, Short Time Sulfation of Calcium-Based Sorbents
Milne, C.R.; Silcox, G.D. and Pershing, D.W.
I & EC Research, 29, 139, 1990. Funded by Environmental Protection Agency and ACERC.
To simulate the staged availability of transient high surface area CaO observed in high-temperature flow-reactor data, the rate of calcination of CaCO3 of Ca(OH)2 is described by an empirical modification of the shrinking-core model. The physical model depicts particle decomposition by the shrinking-core mechanism. The subsequent time dependent decrease in CaO reactivity (surface area and porosity) due to sintering is simulated by reducing the grain-center spacing for the matrix of overlapping CaO grains. Information from SEM micrographs and from other physical property measurements of the porous particles is incorporated. This submodel simulates the time dependent availability and reactivity of CaO for a comprehensive model used to study sulfation of CaCO3 and Ca(OH)2 particles at upper-furnace injection conditions.
Rotary Kiln Incineration: Comparison of Field and Pilot Scale Measurements of Contaminant Evolution Rates from Sorbent Beds
Lester, T.W.; Cundy, V.A.; Sterling, A.M.; Montestruc, A.N.; Jakway, A.J.; Leger, C.B.; Pershing D.W.; Lighty, J.S.; Silcox, G.D. and Owens, W.D.
Environmental Science Technology, 1990. Funded by Environmental Protection Agency and Louisiana State University Hazardous Waste Research Center.
A comparison is made, for the first time, between the evolution of hydrocarbons from sorbent beds in an industrial rotary kiln incinerator and in a laboratory scale rotary kiln simulator. To relate the data from the different sized units, due allowance is made for bed dynamical similitude, bed geometrical factors, and bed heat-up. To minimize the effects of disturbances caused by foreign matter in the full scale bed and differences in loading techniques, the rate of evolution is characterized by an "evolution interval," that is defined as the time required for total hydrocarbon evolution at the maximum evolution rate. A comparison of evolution intervals with reciprocal bed temperature reveals that the data are consistent with an analysis that assumes a uniform bed temperature (at any instant of time) and desorption control of the evolution rate. Furthermore, the evolution intervals scale inversely with modified Froude Number, which characterizes bed dynamics. The success in comparing field and simulator results indicates that pilot scale rotary kilns may be used to simulate certain features of industrial scale units if appropriate care is taken in matching dynamical, geometrical and thermal parameters.
Thermal Analysis of Rotary Kiln Incineration: Comparison of Theory and Experiment
Owens, W.D.; Silcox, G.D.; Lighty, J.S.; Deng, X.-X.; Pershing D.W.; Cundy, V.A.; Leger, C.B. and Jakway, A.J.
Combustion and Flame, 1990. Funded by Gas Research Institute, ACERC and Louisiana State University Hazardous Waste Research Center.
A comprehensive heat-transfer model and associated simplified scaling laws are developed and verified using a pilot-scale, directly-fired rotary kiln with a slumping bed of dry or wet, 2 mm clay sorbent particles. The kiln operating conditions examined include: rotation rate (0.1 to 0.9 rpm), percent fill fraction (3 to 8), feed moisture content (0 to 20 wt. percent), and inner-wall temperature (190 to 790º). The model is used to determine the relative importance of several heat-transfer mechanisms including radiation, gas-to-solid convection, and wall-to-solid convection. Simple scaling laws are also developed for water vaporization. Generally good agreement is obtained between theory and experiment without adjusting any model parameters. Further, the simplified scaling laws provide a reasonable estimate of the pilot scale performance.
The key conclusions of this study for kilns at the conditions examined are: (1) water exerts a profound effect on the solids thermal profile, (2) simple geometrical scaling is not sufficient, (3) the assumption of a well mixed (radially isothermal) solids bed for the heat transfer analysis is appropriate, (4) a dimensionless group, which is a function of temperature, can be defined giving the relative importance of radiative and convective modes of heat transfer, and (5) moisture vaporization rates can be roughly approximated by assuming that the water vaporized at the boiling point at a rate controlled by the rate of heat transfer to the bed. The implications of the scaling laws for scale-up and kiln design are also examined.
1989
Fundamental Experiments on Thermal Desorption of Contaminants from Soils
Lighty, J.S.; Silcox, G.D.; Pershing, D.W.; Cundy, V.A. and Linz, D.G.
Environmental Progress, 8 (1), 1989. Funded by the Gas Research Institute, Dave Linz, Project Manager, National Science Foundation - Presidential Young Investigator Program, ACERC (National Science Foundation and Associates and Affiliates), the State of Utah, and US Department of Energy.
The goals of this research are to develop an understanding of the transport phenomena that occur during the desorption of contaminants from soil and to obtain rate information. This information can then be used to develop a model to predict the performance of full-scale thermal treatment systems for optimization and cost reduction.
The Effects of Rotary Kiln Operating Conditions and Design on Burden Heating Rates as Determined by a Mathematical Model of Rotary Kiln Heat Transfer
Silcox, G.D. and Pershing, D.W.
Accepted for publication in JAPCA, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates), the State of Utah, and US Department of Energy.
A mathematical model of heat transfer in a directly fired rotary kiln is developed and used to examine the effects of operating and design parameters on burden temperature. The model includes a mean beam length radiation model and axial zoning. Conductive and convective heat transfers are also included. Radiation between immediately adjacent zones is permitted. Calculation of heat transfer rates is facilitated by the use of an electric circuit analogue. An iterative solution procedure is adopted to solve the energy balance equations.
At the conditions examined, the model predicts that coflowing gas and solid streams result in higher average burden temperatures than do counter flowing streams. The moisture level of the feed is predicted to be a key operating parameter. The effects of kiln length, burden residence time, firing rate, and flame length are also examined.
Calcination and Sintering Models for Application to High-Temperature, Short-Time Sulfation of Calcium-Based Sorbents
Milne, C.R.; Silcox, G.D.; Pershing, D.W. and Kirchgessner, D.A.
Accepted for publication in I & EC Res., 1989. Funded by the US Environmental Protection Agency, ACERC (National Science Foundation and Associates and Affiliates), the State of Utah, and US Department of Energy.
To simulate the staged availability of transient high surface area CaO observed in high-temperature flow-reactor data, the rate of calcination of CaCO3 or Ca(OH)2 is described by an empirical modification of the shrinking-core model. The physical model depicts particle decomposition by the shrinking-core mechanism. The subsequent time dependent decrease in CaO reactivity (surface area and porosity) due to sintering is simulated by reducing the grain-center spacing for the matrix of overlapping CaO grains. Information from SEM micrographs and from other physical property measurements of the porous particles is incorporated. This submodel simulates the time dependent availability and reactivity of CaO for a comprehensive model used to study sulfation of CaCO3 or Ca(OH)2 particles at upper-furnace injection conditions.
Rotary Kiln Incineration--Combustion Chamber Dynamics
Cundy, V.A.; Lester, T.W.; Leger, C.B.; Miller, G.; Montestruc, A.N.; Acharya, S.; Sterling, A.M.; Pershing, D.W.; Lighty, J.S.; Silcox, G.D. and Owens, W.D.
Journal of Hazardous Materials, 22, 195-219,1989. Funded by US Environmental Proctection Agency and ACERC (National Science Foundation and Associates and Affiliates).
A multifaceted experimental and theoretical program aimed at understanding rotary kiln performance is underway. The overall program involves university, industry, and government participation and is broken into distinct sub-programs. This paper discusses in some detail the research effort performed to date in two of the sub-programs: Full-scale in situ sampling and kiln-simulator experimentation. Full-scale in situ measurements are obtained from the Louisiana Division rotary kiln facility of Dow Chemical USA, located in Plaquemine, Louisiana. Summary results obtained from controlled experiments that were performed during continuous processing of carbon tetrachloride and preliminary results obtained during batch mode processing of toluene-laden sorbent packs are presented. Kiln-simulator data are obtained by using the facilities of the Chemical Engineering Department at the University of Utah. Recent kiln-simulator work, conducted in support of the full-scale measurements sub-program, has aided in providing an understanding of the results that have been obtained at the full-scale. Modeling efforts, conducted at Louisiana State University and the University of Utah, have concentrated on the development of realistic, fluid-flow and heat-transfer models, near-term chlorinated kinetic models and bed mass-transfer models to be incorporated into a global three-dimensional kiln-simulator model. The paper concludes with an overview of these modeling efforts.
Experimental and Theoretical Studies on the Combustion of Waste Zirconium Sponge in a Rotary Kiln Simulator
Lemieux, P.M.; Silcox, G.D. and Pershing, D.W.
Waste Management, 9:125-137, 1989. Funded by Westinghouse Corp.
Previous experimental studies have indicated that rotary kilns may be suitable combustion systems to incinerate small quantities of off grade zirconium sponge produced during the manufacturing of zirconium for the nuclear industry. This paper describes a mathematical model of zirconium sponge combustion in a rotary kiln environment and specifically examines the use of the bed submodel to analyze detailed zirconium combustion data obtained previously in a rotary kiln simulator. The results of this analysis indicated that the experimentally observed burning rates could be predicted within 25% based on available transport correlations and current understanding of zirconium combustion. Without any adjustment of the physical parameters in the model, both the experimental results and the model predictions indicated that the primary combustion parameters are the bulk oxygen concentration within the kin and the local kiln bed temperature. Charge size was found to be less significant. A detailed analysis of the theoretical predictions indicates that the zirconium sponge oxidation rate is controlled by three sequential processes (the convection of oxygen from the bulk gas to the top of the bed, the diffusion of oxygen through the bed to the particle surface, and the diffusion of oxygen through previously formed zirconium dioxide product to the unreacted zirconium metal). Under conditions typical of commercial rotary kiln operation all three of these resistances appear to be significant. Both the experimental data and the model suggest that intrinsic chemical kinetics are fast and not controlling except during the first few minutes after the zirconium is charges. The model assumes that the zirconium oxide product layer increases until it reaches a maximum value after which it remains constant due to continuous formation and abrasion.
An In-Depth Study of Incineration - Rotary Kiln Performance Characterization
Cundy, V.A.; Lester, T.W.; Conway, L.R.; Jakway, A.J.; Leger, C.B.; Montestruc, A.N.; Acharya, S.; Sterling, A.M.; Owens, W.D.; Lighty, J.S.; Pershing, D.W. and Silcox, G.D.
Louisiana State University/Hazardous Waste Research Center SAC Review Meeting, Baton Rouge, Louisiana, 1989. Funded by Louisiana State University/Hazardous Waste Research Center (Supported by US Environmental Protection Agency).
A comprehensive study aimed at understanding rotary kiln performance is underway. The program is led by personnel from Louisiana State University. Bench and pilot-scale facilities at the University of Utah are available for use in solids desorption studies. Full-scale in situ measurements are obtained from the Louisiana Division rotary kiln facility of Dow Chemical USA, located in Plaquemine, Louisiana. This paper presents a summary of the project providing some detail of the work that has been accomplished from 1 January through 31 August 1989.
Fundamentals for the Thermal Remediation of Contaminated Soils - Particle and Bed Desorption Models
Lighty, J.S.; Silcox, G.D.; Pershing, D.W.; Cundy, V.A. and Linz, D.G.
Accepted for publication in Environ. Science Tech., 1989. Funded by the Gas Research Institute, Dave Linz, Project Manager, National Science Foundation/Presidential Young Investigators, ACERC (National Science Foundation and Associates and Affiliates), the State of Utah, and US Department of Energy.
A major research effort has been initiated to characterize the rate-controlling processes associated with the evolution of hazardous materials from soils. A threefold experimental approach is being used in conjunction with computer modeling to analyze thermal desorption of contaminants. Phenomena occurring both inside particles (intraparticle) and with a bed of particles (interparticle) were studied.
The results obtained suggest that the most important process variables are local thermal environment and gas-phase contaminant concentration because the adsorption equilibrium characteristics of the contaminant/soil pair control the desorption of contaminant from a particle at a given temperature. A mass-transfer/desorption model, which assumes gas/solid equilibrium at all points and time, is proposed and the model was found to predict the measured temperature dependence.
1988-1987
Transport Fundamentals for the Thermal Remediation of Contaminated Soils
Lighty, J.S.; Silcox, G.D.; Pershing, D.W.; Cundy, V.A. and Linz, D.G.
Submitted for Review to AIChE Journal, 1988. 23 pgs. Funded by Gas Research Institute, ACERC (National Science Foundation and Associates and Affiliates), and National Science Foundation/Presidential Young Investigators.
A major research effort has been initiated to characterize the transport phenomena associated with the evolution of hazardous materials from contaminated soils. A threefold experimental approach is being used in conjunction with computer modeling to analyze thermal desorption of contaminants from soils under a variety of experimental conditions. First, a particle-characterization reactor (PCR) is being employed to characterize intraparticle transport under conditions where the bulk concentration and temperature at the particle surface are known. Secondly, a packed-bed reactor is being used to examine interparticle transport within a bed of particles. In the third portion of the work, a 73 kW pilot-scale rotary kiln is providing time-resolved measurements of the species evolution. The results from the PCR experiments and modeling are reported in this manuscript.
The PCR experimental results obtained suggest that the most important process variable is local thermal environment and that the absorption characteristics of the contaminant/soil pair are important in the desorption process. The adsorption characteristics are a function of local temperature, thus accounting for the steep dependence of the cleanup efficiency of a soil on temperature. In addition, the adsorption characteristics are influenced by soil type and moisture; it has been found that these parameters also effect desorption of contaminants. A mass-transfer model, which assumes gas/solid equilibrium at all points and times, is proposed and the model predictions were found to exhibit temperature dependence similar to the data.
A Mathematical Model for the Flash Calcination of Dispersed CaCO3 and Ca(OH)2 Particles
Silcox, G.D.; Kramlich, J.C. and Pershing, D.W.
Submitted to I&EC/Research, 1987. 28 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).
A mathematical model for the flash calcination of Ca(OH)2 and CaCO3 is presented. The model describes the decomposition of the parent material at the reactant-product interface, the diffusion of CO2 or H2O through the growing CaO layer, and the sintering of the CaO layer. The model is qualitative, but it provides useful estimates of peak CO2 pressures in the CaO layer, relative rates of surface area development and loss for Ca(OH)2 and CaCO3, the effect of particle size, and the effects of time and temperature.
Fundamentals of Hazardous Solid Waste Incineration in a Rotary Kiln Environment
Lighty, J.S.; Silcox, G.D.; Britt, R.; Owens, W.D.; Pershing, D.W. and Cundy, V.A.
Proc. AFRC Int'l. Symposium on Waste Incineration, 1987, Palm Springs. Funded by Environmental Protection Agency.
With landfill costs increasing and regulations on landfilling becoming more stringent, alternatives to conventional hazardous waste treatment strategies are becoming more desirable. Incineration is presently a permanent, proven solution for the disposal of most organic contaminants, but also a costly one, especially in the case of solids that require some auxiliary fuel. The goal of this research is to develop an understanding of the phenomena associated with the evolution of contaminants from solids in the primary combustor of an incineration system. A threefold approach has been used. First, a bench-scale Particle Characterization Reactor was developed to study the transport phenomena on a particle basis, where the controlling processes are mainly intraparticle. Second, a Bed Characterization Reactor was built to examine the controlling transport phenomena within a bed of particles, where the processes are primarily interparticle. The results of these studies can be applied to any primary combustor. Finally a pilot-scale rotary kiln was developed to study the evolution of contaminants from solids within a realistic temperature and rotation environment.
This paper describes results obtained in a study using a commercial sorbent contaminated with toluene. The data are from the Particle Characterization Reactor and the Rotary-Kiln Simulator. The results show that the method of contamination and charge size does not have a large effect on desorption, while temperature and contaminant concentration are important parameters in the evolution of contaminants in a rotary kiln. Preliminary modeling efforts for the kiln are also discussed.