Energy & Fuels 1993, 7, 782-785
782
Pollutant Formation and Control/Hazardous Waste Incineration David W. Pershing Chemical and Fuels Engineering, University of Utah, Salt Lake City, Utah 84112 Received May 21, 1993. Revised Manuscript Received June 2,1993@
This paper summarizes the ACERC research activities being conducted as part of the Thrust Area 3 Working Group entitled Pollutant Formation and Control/Waste Incineration. The primary goals of the studies in this thrust area are to provide fundamental understanding regarding the fate of hazardous species during incineration processes, to identify new low emission control strategies for both conventional fossil fuel and waste treatment systems, and to develop computer-aided design tools which can be used to evaluate the potential of various air pollutant control options. The thrust area includes three projects in the emissions subgroup and three in the incineratiodthermaltreatment subgroup. Each project is briefly described as are the most recent significant findings.
Introduction
Table I ~~
Environmental issues are of major importance in most combustion applications because low-rank fuels and essentially all waste materials contain compounds which, when improperly oxidized, and not collected, are air pollutants. Combustion of most US.coals produces both NO, and SO, emissions which can ultimately lead to acid rain; trace components can also lead to the emission of air toxics, particularly mercury and possibly selenium. Fortunately, cost effective NO, control can be achieved by the application of combustion modification technologies (e.g., burner modifications, staged combustion, or reburning) but emission levels below 250 ppm are difficult to achieve with pulverized coal firing. SO2 emissions can be controlled with either wet or dry scrubbing systems; in retrofit applications with severe space and/or lifetime constraints, dry sorbent injection may also be a reasonable alternative. Dry scrubbers have been shown to be useful for air toxics control, particularly in conjunction with carbon injection, but the process fundamentals are not well understood. Hazardous esmissions may also be produced by the incomplete destruction of originally toxic material during hazardous waste incineration or by the formation of new toxic compounds during the incineration of municipal, industrial, or medical wastes. Increasingly stringent federal and local incinerator performance standards, and the need to handle larger quantities of different types of waste streams (due to the elimination of landfill deposal sites), are demanding the development of new, efficient thermal treatment technology, but the controlling processes are poorly understood. At present, the matching of incinerator or desorber and waste is often done by trialand-error testing in large scale systems. Objectives and Approach The overall goals of the studies in this thrust area are to (1)provide fundamental understanding regarding the
fate of hazardous species during the combustion process, (2) develop advanced waste assessment technology which e Abstract
published in Aduance ACS Abstracts, October 15, 1993.
no. project title 3C Prediction of Hazardous Waste Destruction During Thermal Incineration 3F Fluidized Bed Incineration 3G Real-Time MS Monitoring 35 NOJSO, Submodel Implementation in 3D Code 3K Low NO, Coal Combustion 3L Removal of NHs and CO from Stack Gas
investigator Geoff Silcox David Pershing JoAnn Lighty JoAnn Lighty Henk Meuzelaar Douglas Smoot Scott Hill
university
u of Utah u of Utah u of Utah u of Utah u of Utah BYU BYU
David Pershing u of Utah Geoff Silcox u of Utah Cal Bartholomew BYU William Hecker BYU
can be used to both predict and optimize the performance of thermal treatment systems, (3) identify new, lowemission control strategies, (4) develop advanced monitoring techniques, and ( 5 ) develop computer aided design tools which can be used to evaluate probable air pollution emission levels and estimate the potential of various pollutant control options. The projects are divided into two subgroups: (1)emission characterization and control and (2) thermal treatment of waste. All of the projects are summarized in Table I. The overall objective of the projects in the emissions subgroup is to assist with the creation of advanced technologies by developing sophisticated predictive tools and advanced control concepts. Emphasis in this area has increased significantly during the past two years due to the passage of the new Clean Air Act and the associated strong industrial interest in emissions control. During previous years submodels for NO, formation and SO, control via dry sorbent injection were developed and initially evaluated. The objective of Project 35 is to improve and integrate these submodels into the ACERC comprehensive codes. A new project (3K), initiated one year ago in conjunction with the DOE Combustion 2000 program, is focusing on the development of ultralow NO, burner/combustor designs. The ACERC portion of this program involves an experimental evaluation of advanced NO, control concepts for coal furnaces not constrained by current boiler design configurations. The experimental studies will be supported by 3-D combustion modeling. Another new project (3M), initiated at the beginning of 0 1993 American Chemical Society
Pollutant Formation and Control last year, is investigating the activity/selectivity properties of new Pt/Zeolite catalysts for low-temperature oxidation of NHs and CO and reduction of NO, emissions in stack gas (SCR). Studies in the incineration subgroup are designed to provide new measurement techniques and fundamental information on the rate-controlling steps in thermal treatment processes and, thereby, facilitate the development of advanced waste assessment systems and procedures to make thermal treatment more cost effective and politically acceptable. These studies have been primarily focused on solids and sludges because of the lack of information in this area and complexity of the problems. (Interest in this area is increasing because some of the high-tech nonthermal technology alternatives have now been shown to be less effective than thermal treatment.) The experimental facilities are also being used to provide test beds for industrial affiliates and associates to evaluate specific problems. Recent results from each of the projects in this thrust area are described in the Results section. The project numbers refer to Table I; the numbers are not always sequential because some projects were completed in prior years.
Anticipated Products The primary product from the pollutant emission subgroup will be an improved, three-dimensional comprehensive code containing appropriate NO, and SO, sorbent submodels. In addition, this subgroup will provide detailed experimental data on innovative new concepts for ultralow NO, coal burning. New insights into catalytic science and new catalyst technology for emissions control will be further products. The primary product of the incineration subgroup will be a model based waste assessment methodology for use in conjunction with fluidized bed and rotary kiln incinerators. Advanced measurement techniques are also being developed and evaluated. One specialty code for the rotary kiln has already been released and advanced versions of this software and companion software for fluidized bed systems will be released in later years of the effort. In addition to the specific products described above, the project is also producing fundamental understanding of the rate controlling processes associated with pollutant formation and the release of hydrocarbon and metal contaminants during incineration. These experimental results have direct, immediate applicability in other cleanup programs elsewhere. Results Project 35: NO,/SO, Submodel Implementation. The overall objective of this project is to incorporate improved pollutant submodels into the comprehensive combustion codes. During this past year the current acid rain precursor (NO,/SO,) submodel was fully integrated into the two-dimensional ACERC code (PCGC-2). Evaluation of this submodel was also conducted (see Brewster et al., this journal) and this included comparisons with available experimental data and variation of operating conditions to see if the predictions produced reasonable trends. Analysis of experimental data from the controlled profile reactor at BYU was also completed. A complete data set including radiative heat transfer, particle density distribution, and species evolution is now available which
Energy & Fuels, Vol. 7,No. 6, 1993 783
1.0-
Calculated From Equilibrium
0
10
20
30
0
0 2 % (Percentage of molecular oxygen In comburtlon products)
Figure 1. 0 radical predictions from equilibrium or elementary kinetics. Ethylene-air a t atmospheric pressure at 2 ms after reaction start. Reprinted with permission from: Smoot, L. D.; Hill, S. C.; Foli, A.; Chen, W. Seventh Annual Progress Report; Advanced Combustion Engineering Research Center, Brigham Young University, Provo, UT, 1993;p 169.
will facilitate the evaluation and development of both comprehensive codes and acid rain precursor submodels. Evaluation of fundamental NO,/SO, mechanisms was performed to study radical effects. Preliminary evaluation of the global mechanisms for fundamental kinetics was needed to allow the inclusion of more extensive kinetics for NzO and NO2 chemistry. Kinetic studies were done to seek a correlative relationship between 0 atom concentrations and molecular oxygen, since such a relationship is required for the NO, submodel. Figure 1 illustrates a near-linear relationship between predicted oxygen radicals and oxygen molecules under one set of reaction conditions which suggests it may be possibe to estimate 0 radical concentrations based on quasi-equilibrium conditions. The current acid rain precursor submodel was also integrated into the three-dimensional ACERC code (PCGC-3) and comparisons of predictions from this code with experimental data are on-going. Project 3K: Low NO, Coal Combustion. A new 100 000 Btu/h coal-fired test facility has been designed and fabricated at the Univesity of Utah as illustrated in Figure 2. This new combustor is being used to obtain the experimental data required for evaluating ultralow NO, coal burning concepts as part of the DOE Combustion 2000 program. The down-fired, ceramic-walled combustion system is 6.5 in. in inside diameter and has a total length of approximately 25 ft. The high-temperature portion of the furnace has been designed to allow approximately 1 s residence time and accommodate both the gas reburning and subsequent burnout air addition. Downstream sections include provision for heat extraction and advanced SNCR techniques with controlled thermal profiles. NO, emission levels below 200 ppm have been achieved by using long axial flames in conjunction with natural gas reburning. Current testing is focused on the impact of combustion air vitiation, fuel injector design and advanced reburning in conjunction with SNCR. Project 3L: Removal of NH3 and CO from Stack Gases. Laboratory experiments are being conducted in
784 Energy & Fuels, Vol. 7, No. 6,1993 &I L
A
23 SeCUO" 1
SeCC1Y)" 2
2
sactan 3
+.-
29 112
_qk 20 112 4
Figure 2. Multifuel U-shaped furnace.
g
5
6 .....0 ....
60
__-*---.
NO
40!
20 0 0.0
1.0
2.0
3.0
NHdNO ratio
Figure 3. NH3 + NO + CO + H20 + 0 2 reaction over Cu-ZSM-5 catalyst: effect of NH3/NO ratio at 450 O C . a joint WINCO/ACERC program to determine the performance of modified Pt/alumina and Cu/zeolite catalyst systems for final cleanup of NO,, NH3, and CO emissions. Based on the results obtained thus far, it appears that both catalysts have useful temperature windows in which simple oxidation of ammonia and CO occurs readily. Although significantly more active for either of these oxidation reactions, the Pt/alumina catalyst suffers from selectivity limitations in complex reaction mixtures and at higher reaction temperatures. The Cu/zeolite system selectivity reduces all of the reactant NO with NH3 to N2 to the temperature range of 260-450 "C as illustrated in Figure 3. The only drawback with the Cu/zeolite catalyst is that only 20-40% of the CO in the reactant mixture is oxidized to COz in the temperature range of 300-500 "C. Thus a combination of the two catalysts may be needed to achieve cleanup to low ppm levels. Project 3C: Prediction of Hazardous Waste Destruction. This study is the heart of the incineration program and it is providing generic, fundamental information on the evolution of hydrocarbons and metals from contaminatedsoils and solids (Pershing et al., 19931.' Two scales of experimental activities are currently underway. The bench scale rotary reactor (shown in Figure 4) was recently conceived, fabricated, and validated for the convenient, rapid characterization of hydrocarbon and metal desorption kinetics from solid waste under controlled thermal conditions. The advantage of this reactor is that (1) Pershing, D.W.; Lighty, J. S.; Silcox, G . D.; Heap, M. P.; Owens, W. D.Solid Waete Incineration in Rotary Kilns. Combust. Sci. Technol. 1993,93, 1-6, 245.
Pershing
only a relatively small sample of the material is required (e.g. 100 g) and the thermal profile can be completely controlled and rapidly altered using the induction heating system. By stepping the temperature profile it is possible to obtain time-resolved evolution data at multiple temperatures with one test and this is an efficient method of obtaining the calibration data required for the computer modeling described below. Experiments are also being completed in a 0.6 m diameter rotary kiln simulator to determine the influences of multicomponent hydrocarbon chemistry, moisture, and solid particle size on the rate of hydrocarbon evolution. Recent results are shown in Figure 5 for a clay soil contaminated with toluene, naphthalene, hexadecane, and water. The presence of other hydrocarbons increases the rate of vaporization of naphthalene relative to the base case. Water initially significantly enhances the evolution of naphthalene; however, once the water has been removed (and the possibility of site competition and steam stripping have been eliminated), the naphthalene evolution rate falls quite dramatically. Similar data are also being obtained for industrial sludges and these results are described in detail in the paper by Rink et al. (this journal). Because of the new interest in Air Toxics, experimental data are being obtained on the evolution of hazardous and toxic metals species from solid substrates using a small electric tube furnace. These studies also include consideration of metal-sorbent reactions and the results have indicated that significant metal retention is possible, particularly if the amount of chlorine available is limited. The resulting ash has also proved to be extremely inert with respect to subsequent metal removal by leaching, even with highly acidic environments. The fundamental experimental data have been used in conjunction with the small, pilot scale testing to create a model of rotary kiln incinerators. User-friendly software for the calculation of thermal profiles in rotary kilns has now been released in both Apple (ODEKILN) and IBM (TEMPRO) formats. The fundamental results and computer software are also being used to create a model-based treatment assessment methodology which is now being used by industrial affiliates to evaluate waste treatment alternatives. This project is cofunded by the Gas Research Institute (GRI) and the methodology commercialization is being conducted in conjunction with an industrial partner (Reaction Engineering International, Salt Lake City, UT) as required by GRI. Project 3F: Fluidized Bed Incineration. In this project, the fundamental results from Project 3C are being applied to the other major type of incineration system, the fluidized bed incinerator. A new CFBC facility was completed during the past year and it has been successfully operated in both bubbling and circulating modes. Baseline fluidization tests were conducted to determine minimum fluidization velocity and general fluidization tests were conducted to determine minimum fluidization velocity and general fluidization characteristics for a number of different bed materials. In general, the results were in good agreement with previously developed correlations from the literature. Initial testing is underway with industrial sludges and these results are described in the following paper by Rink et al. (thisjournal). A companion software package is also being developed. Project 3G: Real Time GCMS Monitor. The objective of this project is to develop the new analytical
Energy & Fuels, Vol. 7, No. 6,1993 785
Pollutant Formation and Control Indudion
Coils /
Cemmic
/ Cloth
To Sample Train
-___) Thermocouple
II I
I
Figure 4. Schematic of the bench-scale rotary reactor developed by REI and the University of Utah. 120 1W
e4
Wtx Naphthalono e4 Romalnlng In Bod 40
m 0
0
2w
400
600
800
1000
1200
Time@)
Figure 5. Naphthalene vaporization in the presence of n-hexadecane, toluene, and water.
techniques for real time, on-line emissions measurements that are desperately needed for quantification of trace pollutants from practical systems. All of the incineration studies are now utilizing the new on-line G U M S monitoring techniques developed in Project 3G (see McClellen (2) McClennen, W. H.; Amold, N. S.;Roberta, K. S.;Meuzelaar, H. L. C.; Lighty, J. 5.;Lindgren, E. R. Fast Repetitive GClMS Analysis of Thermally Desorbed Polycyclic Aromatic Hydrocarbons (PAHs) from Contaminated Soils. Combust. Sci. Technol. 1990, 74, 1-6.
et al., 1990).2 This technique has been particularly useful in support of studies in the rotary kiln simulator and the new rotary reactor as $11 as the particle characterization reactors. Work has continued on the development of online analysis by means of GC/FTIR/MS and GC/MS/MS techniques as platforms for characterizing the complex emissions from waste combustion. The AVS/GC/MS system has also been used in some preliminary experiments to examine the feasibility of monitoring metalvapor species which may evolve from waste incineration. Direct vapor mass spectra of chromyl chloride (Cr02C12)were obtained, indicating that detailed metal species information can be obtained with MS. This avenue will be further pursued during the upcoming year. In a related part of this program, on-line Fourier transform infrared spectrometry and mass spectrometry monitoring (TG/IR/MS) are being coupled with thermogravimetry in conjunction with the micro reactor technique to study the products, mechanisms, and kinetics of thermal decomposition as part of examining the suitability thermal remediation for remediation of industrial waste water treatment sludges. Future activities will focus on expanding these systems to larger pilot scale reactors and on the developing of micro scale TG systems for rapid screening.
