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Waste incineration and energy recovery
Energy Vol. 13, No. 12, pp. 845-851, 1988 Printed in Great Britain. All rights reserved
0360~5442/88 $3.00 + 0.00 Copyright @ 1988 PergamonPressplc
WASTE INCINERATION
AND ENERGY
S. S. PENNER,? D. P. Y. CHANG,$ R. GOULARD,§
RECOVERY
and T. LESTER~
t Department of Applied Mechanics and Engineering Sciences and Center for Energy and Combustion Research, University of California, San Diego, La Jolla, CA 92093, $ Department of Civil Engineering, Umversity of California, Davis, CA 95616, 5 National Science Foundation, 1800 G Street NW, Washington, DC 20550, and 1 Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 708034413, U.S.A. (Received
1 July 1988)
Abstract-We discuss the implications of waste incineration on energy production currently achieved conversion efficiencies to electricity and for realistic estimates
at of
incinerable fractions of municipal, hazardous, and biomedical wastes. We find approximate steady-state contributions in the U.S. of 6.1 x 103, (3.0-3.9) x lo3 and 0.5 x lo3 MW, for these three technologies, respectively. A combustion-research agenda for waste incinerators is summarized briefly. Its implementation may be expected to contribute to the efficacious development of waste-incineration technologies and associated energy recovery.
INTRODUCTION
Worldwide, there are today more than 1500 operating municipal solid-waste incinerator (MSWI) furnace units built by selected major manufacturers, with an average waste-burning capacity of about 220 tons per day (TPD);lY2 the total number of MSWI furnace units is perhaps as large as 3000. Thus, we are dealing here with an established, large-scale technology. Performance measurements on newer units have yielded very low emissions of chlorinated hydrocarbons.’ Although heavy-metal emissions with fly ash and possible leaching from bottom ash are currently considered to be problematic, there are indications that heavy-metal contamination in fly ash may be controlled by bag-house cleanup at acceptable levels, while stabilization and utilization procedures are being implemented for bottom ash.’ Hazardous-waste incinerators (HWIs) are fundamentally different from municipal-waste incinerators because hazardous-waste incineration (a) is a relatively small-scale industry with probably not more than a few hundred furnace units with effluent monitoring operating worldwidet and burning, on the average, a few to a few hundred tons per day, (b) the input feed is far better identified and less variable than for MSWIs, and (c) units tend to be tailor-made or at least specially modified for particular applications rather than designed to accept a wide variety of inputs, as is the case for mass-burning MSWIS.‘-~ For these reasons, a much more scientific approach may be appropriate for hazardous-waste than for municipalwaste burners. This long-range research approach may well contribute to better designed, specialized hazardous-waste incinerators during a lo-15-yr period. Biomedical (hospital) waste incineration is the third incineration technology considered in this assessment. There are very many (about 7000) installed biomedical-waste incineration units (BMWIs) in U.S. hospitals.‘*4,5 Most of these are probably operational, although incomplete evidence suggests that many have been derated to meet minimal applicable environmental controls (e.g., particulate emissions), while others are burning biomedical wastes under temporary use permits. Operational sizes range from ~100 lb/h (-1.2 TPD) for pathologicalwaste incinerators to >2000 lb/h (-24 TPD). Although the incinerators are customarily described by a variety of terms that tend to emphasize visually-striking aspects (e.g., rotary kilns, moving grates, starved- and excess-air two-stage combustors, etc.), nearly all utilize the same fundamental combustion principles, t The total number of operating hazardous-waste furnace units, including those without emission controls, may be much larger, perhaps as large as a few thousand.
S. S. PENNERet al
846
albeit with different designs. These fundamental combustion principles involve the following sequence of incineration steps: waste drying, pyrolysis, partial combustion in an air-deficient environment, completion of combustion with excess air. Although this sequence of conversion steps may not be immediately apparent in some fluidized-bed incinerators, it also occurs in these systems. Applicable regulatory constraints impose requirements on uniformity of performance that may be (temporarily) relaxed for smaller units but will nevertheless, over the long term, lead to imposition of emission controls at levels that are deemed to be environmentally acceptable, For the large MSWIs, emission-control equipment represents important but not dominant control costs rapidly capital-cost items. As the units become smaller, the environmental become major or even the dominant investments. Most BMWIs appear to have been sold with the idea that emission controls are not needed. When these units fail to meet applicable control requirements (e.g., particulate emissions), they may be retrofitted with baghouses or scrubbers to alleviate problems of this type. At the present time, measurements and controls of criteria pollutants represent paramount constraints on all incineration units. These operational requirements transcend near-term incineration needs and appear, in one form or another, as the priority implementation issue to incinerator constructors and operators alike. The incinerator scale and associated total capital cost are not only important in qualifying allowable environmental and control costs but also determine the magnitudes of the emissions problems. Generally speaking, the larger the incinerator, the longer the residence time of reactants within the combustion chamber and the more complete the destruction of input toxic compounds and of precursors that may be responsible for downstream formations of dioxins, furans and other toxic outputs. 6 These are compelling reasons for choosing incinerators of substantial scale whenever it is possible to do so, provided that localized pollutant levels do not become so large as to pose health risks in the immediate vicinities of incinerators. The FRG model, as implemented in Munich, of using a biomedical incinerator as an input stage to an MSWI is an important step in this direction.‘,’ More generally, judicious additions of both biomedical wastes (BMWs) and hazardous wastes (HWs) to MSWIs should be explored, as should also be the general utilization of HWIs and BMWIs as input stages to MSWIs.’ Pollutant diagnostics and environmental controls will then be needed only on large MSWIs and relatively inefficient small BMWIs and HWIs may become entirely acceptable stages in this type of dual-incineration technology. In view of the history and status of the industry summarized in the preceding paragraph, it may be taken as a given that industry representatives will tend to emphasize R & D issues that lead to longer plant life, reduce tipping fees to the public and produce safe residues or lead to safe disposal practices, while improving the public perception of the safety and efficacy of incinerator construction and operation.
ELECTRIC
ENERGY
PRODUCTION
FROM
WASTE
INCINERATORS
In the following discussion, we utilize the currently achieved efficiency for electricity generation in incinerators, namely, -20% conversion efficiency from thermal to electrical energy. While this efficiency may be improved somewhat in the future,’ it constitutes a reasonable estimate in view of the relatively low temperatures utilized in waste incinerators. For efficient energy utilization, these electrical generators will probably be coupled to steam and hot water cogenerators in the future. Since we are primarily interested in steady-state power generation, we shall not consider the supplementary incineration of accumulated wastes but will instead restrict this discussion to steady-state conversion of currently generated wastes. Steady-state electrical power generation from municipal solid wastes Using a 1990 forecast for municipal-waste generation of 1200 lb/yr-p with an energy content of about lo4 kJ/kg,7 we find a maximum steady-state electrical power-generation capacity for a
Waste incineration and energy recovery
847
population of 250 X lo6 people (p) of P,sw, = (1200 lb/yr-p) x (250 x lo6 p) X (lo4 kJ/kg) x (1 kg/2.2 lb) x (2.78 x 10m4kW-h/kJ) x (lyr/8.76
lo3 h) X (0.2 kW,/kW)
X
x (1 MW,/103 kW,) = 8.7 x lo3 MW,. Worldwide estimates of sorting and cornposting practices have recently ranged from a minimum of 2% in Switzerland (where 80% of the municipal waste was disposed of by incineration in 1985) to a maximum of 23% in Austria (where landfills and incineration accounted for 59 and 18%, respectively, in 1985).1’6 In view of these experiences in land-poor European countries, we consider it unlikely that more than 30% of municipal solid waste will ultimately be recycled. The remaining 70% will then be incinerated since the capacity for landfilling is rapidly being exhausted. Our estimate for steady-state electrical power generation from municipal wastes in the U.S. therefore becomes P MSWI= 6.1 x lo3 MW,.
(1)
Steady-state electrical power generation from hazardous wastes According to a 1985 EPA survey, approximately 577 x lo6 mt (metric tons) of hazardous wastes were generated in the U.S. during 1985.’ Of this total, 247 X lo6 mt were managed under RCRA and 330 X lo6 mt under other government programs.’ About 71, 22 and 7% of the total were produced, respectively, by the chemical/petroleum, metal-related and other industries; -60, -35, -5, and -1% were disposed of by underground injection, impoundment in pits or ponds or lagoons, in landfills, and disposal in waste piles or by land applications, respectively.8 Most of these hazardous wastes are in aqueous solutions and are therefore unsuitable for direct incineration without considerable energy expenditures. It has been estimated that about 25% of the non-aqueous wastes are suitable for incineration.* According to Oppelt,3 there remained 47.25 X lo6 mt of incinerable hazardous wastes after recovery and recycling in 1983. The heats of combustion of the hazardous wastes are highly variable but, on the average, may not be very different from the average value of lo4 kJ/kg used for municipal solid wastes. In order to obtain a first-order estimate for the electricity-generation capacity from hazardous-waste incinerators (HWIs), we assume that an amount equivalent to 25% of the 247 x lo6 mt managed under RCRA in 1985 will be available at the steady state for power generation, i.e., PHwI = (0.25 x 247 x lo6 mt/yr) X (lo3 kg/mt) x (lo4 kJ/kg) x
(2.78 x lob4 kW-h/kJ) x (lyr18.76 x lo3 h) x (1 MW/ld
kW)
x (0.2 MW,/MW) = 3.9 x lo3 MW,.
(2)
With Oppelt’s 1983 estimate, the value of PHw is reduced to 3.0 X lo3 MWe. Steady-state electrical power generation from biomedical wastes According to Somerlad,’ the heating values of biomedical wastes range from 1.16 x lo4 to 2.31 x lo4 kJ/kg, with an average value of 1.74 X lo4 kJ/kg; 1.4-14 kg of waste are generated per hospital bed per day with an average value of 9 kg/bed-day; there were 1.3 x lo6 beds in 6870 hospitals during 1987 in the U.S. It is likely that essentially all of the biomedical wastes produced in hospitals will ultimately be disposed of by incineration for health reasons. The corresponding steady-state electrical power capacity for 100% bed occupancy and derivable from biomedical-waste incinerators (BMWIs) is, therefore, approximately PBMWI= (1.3 X lo6 beds) x (9 kg/bed-day)
x (1.74 x lo4 kJ/kg)
x (2.73 x lop4 kW-h/kJ) x (1 day/24 h) x (1 MW/ld
kW)
x (0.2 MW,/MW) = 0.46 x lo3 MW,. This relatively small estimate is not likely to change dramatically
(3) in the near future.
S. S. PENNERet al
848
Total steady-state electrical power-generation capacity The total steady-state electrical power-generation capacity from waste incineration is seen to be approximately 10 X ld MW, and thus corresponds to about 10 large generating stations. Furthermore, since the MSWIs and BMWIs will generally be located in urban areas, their power outputs should be directly usable in urban population centers, whereas the HWIs will probably be coupled to the industries that are responsible for the production of hazardous wastes. According to a 1985 EPRI study,’ the installed generating capacity in the U.S. was distributed as follows: coal-fired, 308,430 MW,; oil- and gas-fired, 213,210 MW,; nuclear, 91,490 MW,; hydroelectric, 84,620 MW,; geothermal, 1740 MW,; other, 3070 MW,. The total installed U.S. generating capacity in 1985 was thus about 702,56OMW,. We conclude that
Table 1. Overview of priority research areas for municipal solid-waste incinerators (MSWIs). Research
and ( ) Application Areas
T
Monitorirw of Effluents fron ucineratora [Research Reccomendations (1,3)] (i)
(ii)
Reliable On-Line Monitoring of Criteria Pollutants Reliable On-Line Monitoring of NonCriteria Pollutants (e.g. Chlorinated Hydrocarbons, HCl)
Metal Deoositions on merials of Constructioq and Metals Emissions [Research Recommendations A-71 (i)
Fouling and Slagging
(if)
Metals Emissions with Fly Ash
Fundamental Research Needs Short-Term
Identification of special roblems, if any, arising ith MWIs and implementatior #f appropriate corrective leasures. Development of equipment 'or continuous emission monitoring of HCl and other Ion-criteria pollutants; mproved diagnostic tech.iques for particulate haracterizations.
Metal partitioning under .on-equilibrium conditions eween oxides and halides n multiphase flow streams.
Long-Term
Utilization of novel or new techniques (e.g., laser-based applications) for continuous emission monitoring of hazardous compounds such as chlorinated hydrocarbons at concentrations below parts per billion. Development of quantitative models on formation and destruction mechanisms involving hazardous compounds in :?IJIs.
Improved understanding of the influence of combustion conditions on metal partitioning between fly and bottom ash. Development of quantitative models for netal-compound formations and distributions.
(iii) Metals Concentrations in Bottom Ash (iv)
Ash Management and Stabilization
(y)
Recycling and Reuse of Municipal Waste Components
Effluent Clean-Uu Svstems [Research Recommendation (2)l (i)
Improved NO,, SO, and Other Effluent-Gas Removal Systems for MWIS
(ii)
Improved Systems for Fly Ash Retention and Utilization
Development of methodolo,ies for optimizing process echnologies involving sepaation and recycling.
Development of clean-up ystems that are optimized 'or the special conditions ncountered in HWI efflunts. Fundamental chemical tudies of mechanisms and ate+ involved in NO, and 0, removal.
___
Development of new catalytic and non-catalytic reaction schemes for more complete removal and control of criteria and non-criteria pollutants than is currently achieved.
Waste incineration
and energy recovery
849
waste incineration at 20% conversion efficiency and at the steady state may be expected to contribute about 5.75 times the 1985 geothermal generating capacity or about 1.4% of total 1985 electricity-generating capacity.
IMPLEMENTATION
OF
WASTE-INCINERATION
TECHNOLOGIES
The studies in Ref. 1 dealt specifically with the combustion-science underpinnings of the incineration technologies. The following are the priority research areas that should be pursued in order to facilitate incinerator construction and operation: (1) Reliable, on-line diagnostic Table
2. Overview
of
priority research areas for hazardous-waste incinerators; research need; L = long-term research need. Priority
Research and Application Areas Fates of Solids and Orzanics [Research Recommendations (4, 6. 7)1 (i) Feed Preparatio (ii) Heat-Up and Devolatilizarion of Organics from Solids (iii) Heterogeneous Reactions of Organic on and in Solids
Chemical and Phvsical Rate Processes [Research Recommendations (4, 6, 7)] (i) Efficient Furnace Designs (ii) Modeling and Scale-Up
Fates of Particulateg and Xetals [Research Recommendations 4, 5, 6, 7)l
I (Critical)
II (Important)
S: What physical and Heat transfer to chemical effects limit and mass transfer from solid particles removals below 10‘4 of in slowly mixed beds initial hazardousfor dry and slagging material concentraconditions. Volatile tions? release rates and develoument of incinerability rankings. S:
L:Xodels for unsteady 3-D mixing and for scale-up. S and L: Rates and models for chemical transformations invalving hazardous species; transport phenomena in twophase mixtures for species with greatly disparate molecular weights; !nu1tic0mp0nent droplet burning; macromixing in multiphase systems.
L: Thermochemical and thermophysical properties for organic wastes and intermediates. Overall and detailed reaction models.
S: Kinetics and ther- S: Characterization of mochemistry of parti- combustion-generated culate formations in particles. the presence of halogens; chemical transformations of metals.
(i) Service-Life Times for Materials (ii) Requirements for L: Interactions of P*st-Combustion gases and particuClean-Up lates with materials of construction. Stabilitv and Heasurements of Hazardous Comuounds [Research Recommendations 1. 2. 3, 4)]
S =short-term
S: Development of acceptable combustionL surrogates for toxic compounds.
L: Development of onMeasurements line techniques for (i) and Controls monitoring hazardous (ii) Identification compounds. of Combustion Surrogates (iii) Relation of Health-Hazards to Combustion Conditions
III (Useful) L:Application of useful research results from other combustion research areas such as coal or oil-shale devolatilizatLan, fluidized-bed combustion, etc.
S. S. PENNERet al
850
procedures at acceptable costs should be implemented and verified for criteria pollutants at all scales of incinerator operation and for all types of wastes. Measurements on incinerators involve environments that are very different from those encountered in conventional fossil-fuel technologies. For this reason, proper sampling techniques and utilization of spectroscopic facilities may represent new and challenging research areas. (2) Reliable, on-line control strategies should be implemented at acceptable costs and verified for criteria pollutants at all scales of incinerator operation. The transfer of control technologies developed for fossil-fuel plants to waste incinerators may also represent new research challenges because commerciallyavailable techniques may need to be applied outside of the justifiable composition regimes for which they were developed and have been tested. (3) Fundamental research should be pursued on selected, non-criteria pollutants (e.g., chlorinated hydrocarbons, particulate-size distributions) in order to allow monitoring and control at acceptable costs if it becomes desirable or Table 3. Overview of priority research areas for biomedical-waste Fundamental Research
pesearch and ( ) Application Areas
I
Short-Term
Inout-Stream Characteation and Combustor Desisn [Research Recommendations (1 - 7)l
Active control and sensing systems to maintain proper stoichiometry, temperature and mixing.
Potential (i) and Separation
Define relations between air/fuel-input rates and continuous vs intermittent operation on emission levels in various furnace designs.
Presorting of Inputs
(ii) Combustor Design Combustion Control
and
incinerators. Needs Long-Term
Methodology and implementation technique for desirable source separations.
(iii) Post-Combustion Clean-Up
Use of combustion enhancers (e.g., free radicals) to improve conversion efficiencies in heterogeneous systems.
On-Line wostics & pollutant ConW for m/ (Research Recommendations il. 2, 3)l
Sorption of metals on ashes. Examine the role of halogens on enrichment of Pb and Heterogeneous catalysis in Cd on fine oarticles and the destruction and formation other gas-phase-to-particle of hazardous compounds. conversion processes.
(i) Compact, Inexpensive, On-Line Monitoring Equipment (ii) Compact, Inexpensive, Clean-Up Systems
Evaluate existing information relating to aerosol science, fire research, and burning of coal and of hazardous and municipal wastes to determine if existing models on particle dynamics and relations of aerosol properties to trace-element distributions at control devices can be applied in systems in which high-volatility metallic species may be condensing in or just ahead of the control devices. Development of an inexpensive bioassay for the characterization of effluents such as dioxins and furans.
Destr ction GQrwJG&
of Pathogen_&
(i) Education Operators
of BMW1
Development of an incineration surrogate for pathogenic compounds.
Models of the dynamics, showing the effects of tracemetal distributions on particulates . Improved understanding of particulate-removal techniques . Development and utilization of biomarkers that characterize the potential health effects of combustion effluents.
Waste incineration and energy recovery
851
necessary to do so. (4) Fundamental research should be pursued on metal-compound formations (such as oxides or halides) in incineration to allow useful input separations, additions of reactants for metals uptake, and reductions of metals contents in fly ash and bottom ash to acceptable levels or else permit stabilization of ash outputs for useful commercial applications. (5) Fundamental research should be pursued to minimize fouling, slagging, erosion, and operational time losses in incineration technologies. (6) Fundamental research should be pursued on heat transfer and the development of advanced, cost-effective materials, which may be used on waste-heat boilers installed on commercial hazardous-waste incinerators and in new boilers that are designed to augment electricity generation or steam production in energy-recovery units at all scales of operation. (7) Fundamental combustion research should be pursued to enhance our knowledge base and provide scientific understanding and innovative ideas as we proceed to implement waste-destruction technologies on an increasing range of operational scales. Elaborations of the identified areas of combustion research are given in Tables l-3. Acknowledgemenr-This work was supported, in part, under NSF Contract No. NSF CBTBB-053528. Most of the information contained in this publication has been abstracted from an NSF-sponsored workshop report at which the authors served as Panel Chairmen and to which many people contributed. The NSF report is listed as Ref. 1 and may be obtained on request from the ENG/CBTE Division of NSF.
REFERENCES 1. “Research Recommendations on Incineration of Municipal, Hazardous and Biomedical Wastes,” derived from an NSF-sponsored Workshop (18-19 April 1988), submitted by S. S. Penner, and available from Publications, NSF, 1800 G St. NW, Washington, DC 20550. 2. S. S. Penner, D. F. Wiesenhahn, and C. P. Li, “Mass Burning of Municipal Wastes,” pp. 415-444, in Annual Review of Energy, Vol. 12, J. M. Hollander, H. Brooks, and D. Sternlight eds., Annual Reviews, Inc., Palo Alto, CA (1987). 3. E. T. Oppelt, “Incineration of Hazardous Waste: A Critical Review,” .I. Air Pollut. Control Ass. 37, 558 (1987). 4. “Hospital Waste Combustion Study,” prepared for the EPA by Radian Corporation, P.O. Box 13000, Research Triangle Park, NC 27711 (October 1987). 5. R. E. Sommerlad, “Hospital Waste Issues and Strategies,” Energy and Environmental Research Corporation, King Georges Post Road, Suite 104, Edison, NJ 08837 (February 1988). 6. S. S. Penner, C. P. Li, and D. F. Wiesenhahn, “A Model for Dioxin and Furan Production in Municipal-Waste Incinerators,” in Eleventh International Colloquium on Gasdynamics of Explosion and Reactive Systems, AIAA, New York, NY (1988); C. P. Li, D. F. Wiesenhahn, and S. S. Penner, Energy W, 217 (1988); D. F. Wiesenhahn, C. P. Li, and S. S. Penner, Energy W, 225 (1988). 7. R. M. Dogett, M. K. O’Farrel, and A. L. Watson, “Forecasts of the Quantity and Composition of Solid Waste,” 171 pp., EPA-600/5-80-001, EPA, Cincinnati, OH (1980). 8. K. A. Pluenneke, Mech. Engng., pp. 55-58 (April 1988). 9. “Electricity Outlook, The Foundation for EPRI R&D Planning,” PED.100.2.88, Electric Power Research Institute, P.O. Box 10412, Palo Alto, CA 94303 (1985).
0360~5442/88 $3.00 + 0.00 Copyright @ 1988 PergamonPressplc
WASTE INCINERATION
AND ENERGY
S. S. PENNER,? D. P. Y. CHANG,$ R. GOULARD,§
RECOVERY
and T. LESTER~
t Department of Applied Mechanics and Engineering Sciences and Center for Energy and Combustion Research, University of California, San Diego, La Jolla, CA 92093, $ Department of Civil Engineering, Umversity of California, Davis, CA 95616, 5 National Science Foundation, 1800 G Street NW, Washington, DC 20550, and 1 Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 708034413, U.S.A. (Received
1 July 1988)
Abstract-We discuss the implications of waste incineration on energy production currently achieved conversion efficiencies to electricity and for realistic estimates
at of
incinerable fractions of municipal, hazardous, and biomedical wastes. We find approximate steady-state contributions in the U.S. of 6.1 x 103, (3.0-3.9) x lo3 and 0.5 x lo3 MW, for these three technologies, respectively. A combustion-research agenda for waste incinerators is summarized briefly. Its implementation may be expected to contribute to the efficacious development of waste-incineration technologies and associated energy recovery.
INTRODUCTION
Worldwide, there are today more than 1500 operating municipal solid-waste incinerator (MSWI) furnace units built by selected major manufacturers, with an average waste-burning capacity of about 220 tons per day (TPD);lY2 the total number of MSWI furnace units is perhaps as large as 3000. Thus, we are dealing here with an established, large-scale technology. Performance measurements on newer units have yielded very low emissions of chlorinated hydrocarbons.’ Although heavy-metal emissions with fly ash and possible leaching from bottom ash are currently considered to be problematic, there are indications that heavy-metal contamination in fly ash may be controlled by bag-house cleanup at acceptable levels, while stabilization and utilization procedures are being implemented for bottom ash.’ Hazardous-waste incinerators (HWIs) are fundamentally different from municipal-waste incinerators because hazardous-waste incineration (a) is a relatively small-scale industry with probably not more than a few hundred furnace units with effluent monitoring operating worldwidet and burning, on the average, a few to a few hundred tons per day, (b) the input feed is far better identified and less variable than for MSWIs, and (c) units tend to be tailor-made or at least specially modified for particular applications rather than designed to accept a wide variety of inputs, as is the case for mass-burning MSWIS.‘-~ For these reasons, a much more scientific approach may be appropriate for hazardous-waste than for municipalwaste burners. This long-range research approach may well contribute to better designed, specialized hazardous-waste incinerators during a lo-15-yr period. Biomedical (hospital) waste incineration is the third incineration technology considered in this assessment. There are very many (about 7000) installed biomedical-waste incineration units (BMWIs) in U.S. hospitals.‘*4,5 Most of these are probably operational, although incomplete evidence suggests that many have been derated to meet minimal applicable environmental controls (e.g., particulate emissions), while others are burning biomedical wastes under temporary use permits. Operational sizes range from ~100 lb/h (-1.2 TPD) for pathologicalwaste incinerators to >2000 lb/h (-24 TPD). Although the incinerators are customarily described by a variety of terms that tend to emphasize visually-striking aspects (e.g., rotary kilns, moving grates, starved- and excess-air two-stage combustors, etc.), nearly all utilize the same fundamental combustion principles, t The total number of operating hazardous-waste furnace units, including those without emission controls, may be much larger, perhaps as large as a few thousand.
S. S. PENNERet al
846
albeit with different designs. These fundamental combustion principles involve the following sequence of incineration steps: waste drying, pyrolysis, partial combustion in an air-deficient environment, completion of combustion with excess air. Although this sequence of conversion steps may not be immediately apparent in some fluidized-bed incinerators, it also occurs in these systems. Applicable regulatory constraints impose requirements on uniformity of performance that may be (temporarily) relaxed for smaller units but will nevertheless, over the long term, lead to imposition of emission controls at levels that are deemed to be environmentally acceptable, For the large MSWIs, emission-control equipment represents important but not dominant control costs rapidly capital-cost items. As the units become smaller, the environmental become major or even the dominant investments. Most BMWIs appear to have been sold with the idea that emission controls are not needed. When these units fail to meet applicable control requirements (e.g., particulate emissions), they may be retrofitted with baghouses or scrubbers to alleviate problems of this type. At the present time, measurements and controls of criteria pollutants represent paramount constraints on all incineration units. These operational requirements transcend near-term incineration needs and appear, in one form or another, as the priority implementation issue to incinerator constructors and operators alike. The incinerator scale and associated total capital cost are not only important in qualifying allowable environmental and control costs but also determine the magnitudes of the emissions problems. Generally speaking, the larger the incinerator, the longer the residence time of reactants within the combustion chamber and the more complete the destruction of input toxic compounds and of precursors that may be responsible for downstream formations of dioxins, furans and other toxic outputs. 6 These are compelling reasons for choosing incinerators of substantial scale whenever it is possible to do so, provided that localized pollutant levels do not become so large as to pose health risks in the immediate vicinities of incinerators. The FRG model, as implemented in Munich, of using a biomedical incinerator as an input stage to an MSWI is an important step in this direction.‘,’ More generally, judicious additions of both biomedical wastes (BMWs) and hazardous wastes (HWs) to MSWIs should be explored, as should also be the general utilization of HWIs and BMWIs as input stages to MSWIs.’ Pollutant diagnostics and environmental controls will then be needed only on large MSWIs and relatively inefficient small BMWIs and HWIs may become entirely acceptable stages in this type of dual-incineration technology. In view of the history and status of the industry summarized in the preceding paragraph, it may be taken as a given that industry representatives will tend to emphasize R & D issues that lead to longer plant life, reduce tipping fees to the public and produce safe residues or lead to safe disposal practices, while improving the public perception of the safety and efficacy of incinerator construction and operation.
ELECTRIC
ENERGY
PRODUCTION
FROM
WASTE
INCINERATORS
In the following discussion, we utilize the currently achieved efficiency for electricity generation in incinerators, namely, -20% conversion efficiency from thermal to electrical energy. While this efficiency may be improved somewhat in the future,’ it constitutes a reasonable estimate in view of the relatively low temperatures utilized in waste incinerators. For efficient energy utilization, these electrical generators will probably be coupled to steam and hot water cogenerators in the future. Since we are primarily interested in steady-state power generation, we shall not consider the supplementary incineration of accumulated wastes but will instead restrict this discussion to steady-state conversion of currently generated wastes. Steady-state electrical power generation from municipal solid wastes Using a 1990 forecast for municipal-waste generation of 1200 lb/yr-p with an energy content of about lo4 kJ/kg,7 we find a maximum steady-state electrical power-generation capacity for a
Waste incineration and energy recovery
847
population of 250 X lo6 people (p) of P,sw, = (1200 lb/yr-p) x (250 x lo6 p) X (lo4 kJ/kg) x (1 kg/2.2 lb) x (2.78 x 10m4kW-h/kJ) x (lyr/8.76
lo3 h) X (0.2 kW,/kW)
X
x (1 MW,/103 kW,) = 8.7 x lo3 MW,. Worldwide estimates of sorting and cornposting practices have recently ranged from a minimum of 2% in Switzerland (where 80% of the municipal waste was disposed of by incineration in 1985) to a maximum of 23% in Austria (where landfills and incineration accounted for 59 and 18%, respectively, in 1985).1’6 In view of these experiences in land-poor European countries, we consider it unlikely that more than 30% of municipal solid waste will ultimately be recycled. The remaining 70% will then be incinerated since the capacity for landfilling is rapidly being exhausted. Our estimate for steady-state electrical power generation from municipal wastes in the U.S. therefore becomes P MSWI= 6.1 x lo3 MW,.
(1)
Steady-state electrical power generation from hazardous wastes According to a 1985 EPA survey, approximately 577 x lo6 mt (metric tons) of hazardous wastes were generated in the U.S. during 1985.’ Of this total, 247 X lo6 mt were managed under RCRA and 330 X lo6 mt under other government programs.’ About 71, 22 and 7% of the total were produced, respectively, by the chemical/petroleum, metal-related and other industries; -60, -35, -5, and -1% were disposed of by underground injection, impoundment in pits or ponds or lagoons, in landfills, and disposal in waste piles or by land applications, respectively.8 Most of these hazardous wastes are in aqueous solutions and are therefore unsuitable for direct incineration without considerable energy expenditures. It has been estimated that about 25% of the non-aqueous wastes are suitable for incineration.* According to Oppelt,3 there remained 47.25 X lo6 mt of incinerable hazardous wastes after recovery and recycling in 1983. The heats of combustion of the hazardous wastes are highly variable but, on the average, may not be very different from the average value of lo4 kJ/kg used for municipal solid wastes. In order to obtain a first-order estimate for the electricity-generation capacity from hazardous-waste incinerators (HWIs), we assume that an amount equivalent to 25% of the 247 x lo6 mt managed under RCRA in 1985 will be available at the steady state for power generation, i.e., PHwI = (0.25 x 247 x lo6 mt/yr) X (lo3 kg/mt) x (lo4 kJ/kg) x
(2.78 x lob4 kW-h/kJ) x (lyr18.76 x lo3 h) x (1 MW/ld
kW)
x (0.2 MW,/MW) = 3.9 x lo3 MW,.
(2)
With Oppelt’s 1983 estimate, the value of PHw is reduced to 3.0 X lo3 MWe. Steady-state electrical power generation from biomedical wastes According to Somerlad,’ the heating values of biomedical wastes range from 1.16 x lo4 to 2.31 x lo4 kJ/kg, with an average value of 1.74 X lo4 kJ/kg; 1.4-14 kg of waste are generated per hospital bed per day with an average value of 9 kg/bed-day; there were 1.3 x lo6 beds in 6870 hospitals during 1987 in the U.S. It is likely that essentially all of the biomedical wastes produced in hospitals will ultimately be disposed of by incineration for health reasons. The corresponding steady-state electrical power capacity for 100% bed occupancy and derivable from biomedical-waste incinerators (BMWIs) is, therefore, approximately PBMWI= (1.3 X lo6 beds) x (9 kg/bed-day)
x (1.74 x lo4 kJ/kg)
x (2.73 x lop4 kW-h/kJ) x (1 day/24 h) x (1 MW/ld
kW)
x (0.2 MW,/MW) = 0.46 x lo3 MW,. This relatively small estimate is not likely to change dramatically
(3) in the near future.
S. S. PENNERet al
848
Total steady-state electrical power-generation capacity The total steady-state electrical power-generation capacity from waste incineration is seen to be approximately 10 X ld MW, and thus corresponds to about 10 large generating stations. Furthermore, since the MSWIs and BMWIs will generally be located in urban areas, their power outputs should be directly usable in urban population centers, whereas the HWIs will probably be coupled to the industries that are responsible for the production of hazardous wastes. According to a 1985 EPRI study,’ the installed generating capacity in the U.S. was distributed as follows: coal-fired, 308,430 MW,; oil- and gas-fired, 213,210 MW,; nuclear, 91,490 MW,; hydroelectric, 84,620 MW,; geothermal, 1740 MW,; other, 3070 MW,. The total installed U.S. generating capacity in 1985 was thus about 702,56OMW,. We conclude that
Table 1. Overview of priority research areas for municipal solid-waste incinerators (MSWIs). Research
and ( ) Application Areas
T
Monitorirw of Effluents fron ucineratora [Research Reccomendations (1,3)] (i)
(ii)
Reliable On-Line Monitoring of Criteria Pollutants Reliable On-Line Monitoring of NonCriteria Pollutants (e.g. Chlorinated Hydrocarbons, HCl)
Metal Deoositions on merials of Constructioq and Metals Emissions [Research Recommendations A-71 (i)
Fouling and Slagging
(if)
Metals Emissions with Fly Ash
Fundamental Research Needs Short-Term
Identification of special roblems, if any, arising ith MWIs and implementatior #f appropriate corrective leasures. Development of equipment 'or continuous emission monitoring of HCl and other Ion-criteria pollutants; mproved diagnostic tech.iques for particulate haracterizations.
Metal partitioning under .on-equilibrium conditions eween oxides and halides n multiphase flow streams.
Long-Term
Utilization of novel or new techniques (e.g., laser-based applications) for continuous emission monitoring of hazardous compounds such as chlorinated hydrocarbons at concentrations below parts per billion. Development of quantitative models on formation and destruction mechanisms involving hazardous compounds in :?IJIs.
Improved understanding of the influence of combustion conditions on metal partitioning between fly and bottom ash. Development of quantitative models for netal-compound formations and distributions.
(iii) Metals Concentrations in Bottom Ash (iv)
Ash Management and Stabilization
(y)
Recycling and Reuse of Municipal Waste Components
Effluent Clean-Uu Svstems [Research Recommendation (2)l (i)
Improved NO,, SO, and Other Effluent-Gas Removal Systems for MWIS
(ii)
Improved Systems for Fly Ash Retention and Utilization
Development of methodolo,ies for optimizing process echnologies involving sepaation and recycling.
Development of clean-up ystems that are optimized 'or the special conditions ncountered in HWI efflunts. Fundamental chemical tudies of mechanisms and ate+ involved in NO, and 0, removal.
___
Development of new catalytic and non-catalytic reaction schemes for more complete removal and control of criteria and non-criteria pollutants than is currently achieved.
Waste incineration
and energy recovery
849
waste incineration at 20% conversion efficiency and at the steady state may be expected to contribute about 5.75 times the 1985 geothermal generating capacity or about 1.4% of total 1985 electricity-generating capacity.
IMPLEMENTATION
OF
WASTE-INCINERATION
TECHNOLOGIES
The studies in Ref. 1 dealt specifically with the combustion-science underpinnings of the incineration technologies. The following are the priority research areas that should be pursued in order to facilitate incinerator construction and operation: (1) Reliable, on-line diagnostic Table
2. Overview
of
priority research areas for hazardous-waste incinerators; research need; L = long-term research need. Priority
Research and Application Areas Fates of Solids and Orzanics [Research Recommendations (4, 6. 7)1 (i) Feed Preparatio (ii) Heat-Up and Devolatilizarion of Organics from Solids (iii) Heterogeneous Reactions of Organic on and in Solids
Chemical and Phvsical Rate Processes [Research Recommendations (4, 6, 7)] (i) Efficient Furnace Designs (ii) Modeling and Scale-Up
Fates of Particulateg and Xetals [Research Recommendations 4, 5, 6, 7)l
I (Critical)
II (Important)
S: What physical and Heat transfer to chemical effects limit and mass transfer from solid particles removals below 10‘4 of in slowly mixed beds initial hazardousfor dry and slagging material concentraconditions. Volatile tions? release rates and develoument of incinerability rankings. S:
L:Xodels for unsteady 3-D mixing and for scale-up. S and L: Rates and models for chemical transformations invalving hazardous species; transport phenomena in twophase mixtures for species with greatly disparate molecular weights; !nu1tic0mp0nent droplet burning; macromixing in multiphase systems.
L: Thermochemical and thermophysical properties for organic wastes and intermediates. Overall and detailed reaction models.
S: Kinetics and ther- S: Characterization of mochemistry of parti- combustion-generated culate formations in particles. the presence of halogens; chemical transformations of metals.
(i) Service-Life Times for Materials (ii) Requirements for L: Interactions of P*st-Combustion gases and particuClean-Up lates with materials of construction. Stabilitv and Heasurements of Hazardous Comuounds [Research Recommendations 1. 2. 3, 4)]
S =short-term
S: Development of acceptable combustionL surrogates for toxic compounds.
L: Development of onMeasurements line techniques for (i) and Controls monitoring hazardous (ii) Identification compounds. of Combustion Surrogates (iii) Relation of Health-Hazards to Combustion Conditions
III (Useful) L:Application of useful research results from other combustion research areas such as coal or oil-shale devolatilizatLan, fluidized-bed combustion, etc.
S. S. PENNERet al
850
procedures at acceptable costs should be implemented and verified for criteria pollutants at all scales of incinerator operation and for all types of wastes. Measurements on incinerators involve environments that are very different from those encountered in conventional fossil-fuel technologies. For this reason, proper sampling techniques and utilization of spectroscopic facilities may represent new and challenging research areas. (2) Reliable, on-line control strategies should be implemented at acceptable costs and verified for criteria pollutants at all scales of incinerator operation. The transfer of control technologies developed for fossil-fuel plants to waste incinerators may also represent new research challenges because commerciallyavailable techniques may need to be applied outside of the justifiable composition regimes for which they were developed and have been tested. (3) Fundamental research should be pursued on selected, non-criteria pollutants (e.g., chlorinated hydrocarbons, particulate-size distributions) in order to allow monitoring and control at acceptable costs if it becomes desirable or Table 3. Overview of priority research areas for biomedical-waste Fundamental Research
pesearch and ( ) Application Areas
I
Short-Term
Inout-Stream Characteation and Combustor Desisn [Research Recommendations (1 - 7)l
Active control and sensing systems to maintain proper stoichiometry, temperature and mixing.
Potential (i) and Separation
Define relations between air/fuel-input rates and continuous vs intermittent operation on emission levels in various furnace designs.
Presorting of Inputs
(ii) Combustor Design Combustion Control
and
incinerators. Needs Long-Term
Methodology and implementation technique for desirable source separations.
(iii) Post-Combustion Clean-Up
Use of combustion enhancers (e.g., free radicals) to improve conversion efficiencies in heterogeneous systems.
On-Line wostics & pollutant ConW for m/ (Research Recommendations il. 2, 3)l
Sorption of metals on ashes. Examine the role of halogens on enrichment of Pb and Heterogeneous catalysis in Cd on fine oarticles and the destruction and formation other gas-phase-to-particle of hazardous compounds. conversion processes.
(i) Compact, Inexpensive, On-Line Monitoring Equipment (ii) Compact, Inexpensive, Clean-Up Systems
Evaluate existing information relating to aerosol science, fire research, and burning of coal and of hazardous and municipal wastes to determine if existing models on particle dynamics and relations of aerosol properties to trace-element distributions at control devices can be applied in systems in which high-volatility metallic species may be condensing in or just ahead of the control devices. Development of an inexpensive bioassay for the characterization of effluents such as dioxins and furans.
Destr ction GQrwJG&
of Pathogen_&
(i) Education Operators
of BMW1
Development of an incineration surrogate for pathogenic compounds.
Models of the dynamics, showing the effects of tracemetal distributions on particulates . Improved understanding of particulate-removal techniques . Development and utilization of biomarkers that characterize the potential health effects of combustion effluents.
Waste incineration and energy recovery
851
necessary to do so. (4) Fundamental research should be pursued on metal-compound formations (such as oxides or halides) in incineration to allow useful input separations, additions of reactants for metals uptake, and reductions of metals contents in fly ash and bottom ash to acceptable levels or else permit stabilization of ash outputs for useful commercial applications. (5) Fundamental research should be pursued to minimize fouling, slagging, erosion, and operational time losses in incineration technologies. (6) Fundamental research should be pursued on heat transfer and the development of advanced, cost-effective materials, which may be used on waste-heat boilers installed on commercial hazardous-waste incinerators and in new boilers that are designed to augment electricity generation or steam production in energy-recovery units at all scales of operation. (7) Fundamental combustion research should be pursued to enhance our knowledge base and provide scientific understanding and innovative ideas as we proceed to implement waste-destruction technologies on an increasing range of operational scales. Elaborations of the identified areas of combustion research are given in Tables l-3. Acknowledgemenr-This work was supported, in part, under NSF Contract No. NSF CBTBB-053528. Most of the information contained in this publication has been abstracted from an NSF-sponsored workshop report at which the authors served as Panel Chairmen and to which many people contributed. The NSF report is listed as Ref. 1 and may be obtained on request from the ENG/CBTE Division of NSF.
REFERENCES 1. “Research Recommendations on Incineration of Municipal, Hazardous and Biomedical Wastes,” derived from an NSF-sponsored Workshop (18-19 April 1988), submitted by S. S. Penner, and available from Publications, NSF, 1800 G St. NW, Washington, DC 20550. 2. S. S. Penner, D. F. Wiesenhahn, and C. P. Li, “Mass Burning of Municipal Wastes,” pp. 415-444, in Annual Review of Energy, Vol. 12, J. M. Hollander, H. Brooks, and D. Sternlight eds., Annual Reviews, Inc., Palo Alto, CA (1987). 3. E. T. Oppelt, “Incineration of Hazardous Waste: A Critical Review,” .I. Air Pollut. Control Ass. 37, 558 (1987). 4. “Hospital Waste Combustion Study,” prepared for the EPA by Radian Corporation, P.O. Box 13000, Research Triangle Park, NC 27711 (October 1987). 5. R. E. Sommerlad, “Hospital Waste Issues and Strategies,” Energy and Environmental Research Corporation, King Georges Post Road, Suite 104, Edison, NJ 08837 (February 1988). 6. S. S. Penner, C. P. Li, and D. F. Wiesenhahn, “A Model for Dioxin and Furan Production in Municipal-Waste Incinerators,” in Eleventh International Colloquium on Gasdynamics of Explosion and Reactive Systems, AIAA, New York, NY (1988); C. P. Li, D. F. Wiesenhahn, and S. S. Penner, Energy W, 217 (1988); D. F. Wiesenhahn, C. P. Li, and S. S. Penner, Energy W, 225 (1988). 7. R. M. Dogett, M. K. O’Farrel, and A. L. Watson, “Forecasts of the Quantity and Composition of Solid Waste,” 171 pp., EPA-600/5-80-001, EPA, Cincinnati, OH (1980). 8. K. A. Pluenneke, Mech. Engng., pp. 55-58 (April 1988). 9. “Electricity Outlook, The Foundation for EPRI R&D Planning,” PED.100.2.88, Electric Power Research Institute, P.O. Box 10412, Palo Alto, CA 94303 (1985).