Waste combustion

Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 867-885

WASTE

COMBUSTION

WM. RANDALL SEEKER Energy and Environmental Research Corporation Irvine, CA

Waste materials are generated by every activity of modern industrial society. Combustion or thermal treatment can be a viable disposal technique for many of these waste streams because it can destroy hazardous organics and pathogens, and can significantly reduce mass and volume. The waste streams that are commonly burned include refuse from municipalities, medical and infectious waste from medical institutions, sludges from municipal sewage treatment, hazardous waste streams from industrial processes, and materials from the cleanup of abandoned landfill sites. The combustion systems used for these different waste streams are widely divergent because of the different physical characteristics of the materials. The state of the art in combustion systems that is discussed include liquid injection incineration systems for pumpable liquids, rotary kilns, reciprocating grate stokers, controlled air fixed hearths, and multiple hearths for unprocessed solids, and spreader stokers and fluidized beds for processed fuels from waste. In addition there is a wide range of industrial furnaces and boilers where waste streams are cofired with auxiliary fuels. The key challenges to waste combustion in the future are the optimization of the total combustion system to reliably burn wastes while minimizing emissions of all pollutants into any effluent stream (solid, liquid or air) and the development of performance assurance techniques to continually monitor the system. The ability to design, operate and monitor a waste combustion system to assure the optimized control of all emissions requires detailed knowledge of physical and chemical processes taking place within the waste combustion system and the chemistry of formation and control of these pollutant species. The current state of the knowledge regarding the formation and control of these species in waste combustion systems has resulted in significant reductions in emissions; however significant data gaps exist that preclude further advances. These data gaps must be filled with basic engineering research and vertically integrated assessment and system development studies. This paper characterizes current combustible waste streams and waste combustion technologies, reports on the current knowledge concerning formation and control of important emissions species from full scale equipment, and suggests future directions for research which could lead to the next generation of waste combustion equipment.

Introduction

which now must be recovered and permanently destroyed. Treatment of waste to eliminate hazardous characteristics is the best alternative to not generating the waste in the first place. Combustion is one of the most broadly applicable treatment techniques and has a number of advantages such as

Virtually every human activity generates waste materials. The wastes can be classified by the source from which they came such as industrial, municipal and medical. Society must dispose of these wastes because they are harmful to human health and the environment. The most prevalent disposal practice has been to discard the materials in a contained manner in an attempt to preclude the waste from reentering the human environment. The inability in the past to prevent the discarded waste from eventually reappearing such as by leaching of hazardous constituents from landfills into ground water, has led to even more difficult cleanup operations. Thus today's waste burden includes not only the waste currently being generated but also the waste that were generated and discarded in the past and

9destruction of hazardous constituents such as organics and pathogens, 9reduction of volume and mass, 9potential for energy recovery, 9rendering the waste unrecognizable from its original form (which is often a requirement for medical waste). Most important waste streams are candidates for combustion including all of the municipal waste, sewage sludge, organic fume, medical waste, and 867

868

INVITED LECTURE

significant fractions of the industrial and military waste streams. In addition, materials from the cleanup of former landfill sites are also candidate materials for combustion and thermal treatment. The function of waste combustion processes is to reliably burn the organic fraction in the waste, ideally leaving behind only an inert residue with minimum carbon content and in the process, destroy the hazardous characteristics associated with the organics. In order to be a successful waste management option, combustion must accomplish this in a cost-effective and fuel efficient manner without creating significant risks from other emissions. Because of public attention and the potential for severe adverse health impacts, there must also be a performance assurance procedure established that ensures the continuous acceptable performance of the waste combustion system. There has been significant progress made in the assessment of emissions from waste combustion systems and even some progress made on the development of a phenomenological understanding of the impacts of design and operation on emissions. However there is not a comprehensive knowledge base currently available that will allow an optimization of waste combustion practices. This paper characterizes current combustible waste streams and waste combustion technologies, reports on the cur-

rent knowledge concerning formation and control of important emissions species from full scale equipment, and suggests future directions for research which could lead to the next generation of waste combustion equipment. This paper does not address the literature on scientific studies related to waste combustion in a comprehensive manner due to the breadth of systems and equipment. Rather, the focus is placed on the what is known and needs to be known about the performance o~f practical equipment to meet future requirements.

Combustible Waste Streams

The amount of waste generated in the United States annually from different sources is shown in Table I. It is clear that almost all aspects of modern living generate waste materials. It is estimated that every person in the United States generates approximately 3.5 pounds of trash every day. This generation rate has been increasing rapidly in mode m history; for instance, it compares to 2.9 pounds in 1960 and some estimates as high as 6.5 pounds per day per capita by the year 2000.1 Other industrial countries have a similar profile of waste production scaled proportionally to their population and degree of industrialization.

TABLE I Combustible waste generated in the United States: amounts and properties

Waste type

Annual U.S. Combustile generation waste rate (million (million tons/yr) tons/yr)

Municipal solid waste

180

Sewage sludge

6.5-10.4 dry

Medical waste

Industrial hazardous waste Organic fume Materials from cleanup of superfund sites

4.8

250

3.6 1500-4200 sites

180

6.5--10.4 dry

4.8

Amount burned annually (million tons/yr)

(Btu/lb)

6.5

4500

1.2

1500 (wet)

0.72

6000

50

0.7 10-30

Fuel value

60O0

0.14

Moisture (%)

JRefuse, yard waste, food waste, paper and paperboard 50-90

Solid, semi-solid and liquid residue removed during the treatment of municipal wastewater sewage General refuse, plastics, food waste, chemicals, bedding, gauze, needles

10-40

15-350 Btu/scf 0-6000

Components/physical form

Liquids, semi-solids, tarry materials, sludges, and solids Gaseous

20

Solis, debris, liquids/ sludges, solids

WASTE COMBUSTION Much of this waste is combustible. The simplest definition of combustible waste are those materials that have an organic content that can be oxidized in the combustion process. Also shown in Table I is the amount of the various waste streams that is considered to be combustible by this definition, the amount that is currently burned, and the general chemical characteristics of individual waste steams. The make up of several of the waste streams is provided in Table II. While the sources of waste in other countries are similar, the percentage of waste burned in other countries is widely variant and is dependent on the countries environmental regulations, available land for land based disposal alternatives and the political climate of the country. For example, several countries including Japan, Germany and Denmark, have more aggressive waste combustion programs. 2 Other countries, such as Sweden and even some States in the United States have had moratoriums on the use of waste combustion systems because of concern and uncertainties about the environmental impacts of combustion. a Dehneated in Table III are the types of parameters that commercial incinerator operators examine when considering the appropriate design and operation of an incinerator for a particular waste stream. Three features of the waste generally determine the combustion characteristics and type of equipment that is suitable. These include the average physical and chemical characteristics of the waste, the special constituents in the waste streams, and the variability of the waste properties. The most important physical parameters to waste incinerator designers and operators are the waste feed characteristics. Waste materials can have the full speetum of physical forms including pumpable liquids, sludges, slurries, tarry semisolids, contaminated soils, solid refuse (paper, plastic) and bulky solids. These physical characteristics largely dictate the systems used

869

to introduce the waste into the device and the combustion chamber configuration. Another key parameter that dictates the design and operation of waste combustion systems for a particular waste is the presence of special constituents that can influence system operation or peformance. The important constituents include those that *must be destroyed to high efficiency (e.g., organics and pathogens), *can lead to pollution formation such as heavy metals and acid gas precursors (e.g., CI, N, S, and other halogens), *can be flame retardants in larger concentrations (e.g., halogens), .can form fine salt particles (e.g., K and Na), and .can cause severe corrosion (e.g. from chlorine compounds). A key issue is the impact of the design and operation of the combustion system on the fate of these trace contaminants as will be discussed later. The final critical waste property that dictates the combustion system is its variability. The important physical characteristics of these materials are widely variable among the various waste streams and can be highly variable even when the waste comes from a single source. In large commercial incinerator operations, much effort is placed on blending waste to more uniform and consistent properties. However, in smaller scale waste combustion systems such as hospital incinerators, there is little chance for blending and batch to batch variations can play a dominant role in system performance.

Waste Combustion Systems Several review articles have been written about waste combustion technologies including industrial

TABLE II Typical physical and chemical characteristics of selected waste streams Amount in waste, (percent by weight) Waste component Dry cellulosic solids Wet cellulosic solids Plastics Rubber Solvents Non-Combustibles Pathological Heating value

Medical waste

Municipal waste

Hazardous waste*

45.1 18.0 14.2 0.7

54.2 12.2 7.4

0 0 12

20.4 1.62

26.2

6000 Btu/lb

4335 Btu/lb

58

6030 Btu/lb

*For a typical commercial incinerator accepting a broad range of wastes. 1 Btu/lb = 2.324 J/g.

870

INVITED LECTURE

TABLE Ill Waste parameters required for the specification of waste combustor design Name of Waste Generation Rate

Waste Type Solid Semi-Solid (Pasty) Liquid Pumpable at 70~ F Pumpable and atomizable at 70~ F

Method of Delivery In Containers % In Barrels (steel, plastic, or cardboard) % By Tank Truck % Bulk, Bales %

Composition Water % Inorganics PCB's % S% N% Halogens: Element and % Heavy Metals: Element and % Other Noxious Matter Forming Compounds: Name and % Salts: Name and %

Special Properties Gross Heat Value (Btu/lb) Explosive (Yes/No) Toxic (Yes/No) Pyrophoric (Yes/No) pH Other Other pertinent information available.

hazardous waste, a-7 medical waste, s-9 municipal waste 1~ and sewage sludge. 11 A waste combustion facility generally consists of five major components: (1) waste preparation and feeding, (2) combustion equipment, (3) heat recovery equipment, (4) air pollution control equipment, and (5) ash/solid residue stabilization and disposal. For example, in Fig. 1 is shown a modern commercial incinerator facility that is used for the treatment of hazardous waste. This system has the capability to treat both pumpable liquid wastes via liquid injection burners and solids via the use of a rotary kiln. It consists of a waste preparation and feed system that includes both liquid blending and holding tanks as well as bulk solid hold up pits. The rotary kiln exhausts into a

secondary combustion zone wherein more liquid waste or auxiliary fuel can be fired in an afterburner mode. The exhaust gases from the secondary combustion chamber enter into a waste heat boiler consisting of water tubes for steam production. The recovery of heat from waste combustion in the form of usable steam is an option that can improve the economics of waste disposal if the steam can be used. On modern large scale municipal waste incineration systems, the boiler is often used 'to generate high quality steam for use in driving an electric generator. Almost all modern waste combustion systems are equipped with high performance air pollution control devices that control acid gases via scrubbing and particle emissions via separation. Surveys of the design of modern air pollution control devices that are suitable for waste combustion systems are available elsewhere (e.g., see Refs. 1213). The combustion process generally leaves a sold residue that may require further stabilization before it can be put into a secure landfill to prevent leaching of trace metal constituents. The selection of the combustion equipment is based upon the characteristics of the waste material to be burned and the applicable regulations. The major classes of equipment currently used for waste combustion are shown in Fig. 2 along with an indication of the type of waste material that is usually burned in the equipment. They range from simple spray flames (for pumpable waste streams with high quality fuel characteristics) to rotary kilns and movable grate combustors that can handle virtually unprocessed bulky solid waste streams. In addition, there are a range of industrial boilers and other high temperature processes that are fired on industrial waste. In the United States alone there are over 900 industrial process/boiler units burning over 3.5 million tons per year of hazardous industrial waste. These include cement kilns, other process kilns and industrial boilers cofired on industrial waste. This compares with 250 dedicated combustion units burning approximately 2.5 million tons per year of industrial hazardous waste in the United States. la Liquid injection incinerators are by far the most common type of industrial waste combustion systems even though they are restricted to pumpable liquids with moderate viscosity and low dispersed solids. In their simplest form they involve the atomization of the waste stream into a combustion chamber that can be refractory lined (Fig. 2(a)) or can be an industrial process such as boiler furnaces. A burner is typically used that mixes the droplets with air in a turbulent diffusion spray flame. A wide range of atomizers and burners are used for the variety of liquid waste of interest. The combustion processes associated with burning droplets of waste liquid compounds have recently been the subject of a number of investigations that have indicated the variety of physical and chemical phenomenon

WASTE COMBUSTION WASTE PREPARATION AND FEED

COMBUSTION SYSTEM

871

HEAT RECOVERY

POST COMBUSTION GAS CLEANUP

kVLSTEHEAT BOILER

I IC I

III-r~

/ , Z l O Bt,.V,L /IONIZI~ ~r / SCR==R

/

~---.---.---;--:IH=~to

~F~

\ SECONDARY CO~BI~E 10N CHAMBER

RADIANT SECTION

FIG9 1. Components of commercial large scale waste combustion facility. that can take place including microexplosions for multicomponent materials. 18'85,9~-z~176 It is difficult to process many waste materials because of the hazards associated with handling and limitations of size reduction equipment. For unprocessed solids, the combustion systems must be able to handle bulky materials such as drummed waste (a)

L~Jid Inleclion

and general refuse. These must be introduced into a burning environment that will dry then ignite the material and hold it at temperature until the combustion process is complete, which can take hours. The most common type of combustion device for unprocessed and drummed industrial solid waste is the rotary kiln (Fig. 2(b)). Materials are introduced Fluidized Bed

Wastes ~njec~able ThrouQh Nozz!es HoPoger .=cus Wastes 9 L,quids 9 Atomizable Pasty Organcs 9 Water with Organics

9 UnifocmCompositioin 9 Low Coraent Fus~)le Ash Compounds - - Liquids - - Size Reduced Solids

Rotary Kiln with Altefoumer Chamber

(b)

ml~

Industrial Wastes 9 Heterogene~Js or Homogeneous 9 Medium to Higtl Bulk Oeflsity 9 Fusible Compounds (Feed and Slag) - - Liquids

Multiple Hearths

- - Pasty Organics - - Solids i Wastes in Containers

/

S~udoes 9 High Moisture Sludges 9 Low Voletile C~ganics

(c) Refuse Wastes 9 9 9 9 9

Medical Instifut~ons Small Municipal Unprocessed Refuse Infectious Waste Low to Medium Bulk Density Solids

(g)

Spredder Stokers/

Traveling Grate

(a)

Processed Wastes

Grate Stoke~

9 Shredded Refuse Derived Fuels 9 Hog Fuels (Bark) 9 Unltoml Medium Density Solidlt

Unprocesse~Refuse Waste 9 Low Bu~ Density Solids 9 Sludges 9 Infectious Waste

MI

1

I

FIG. 2. Common types of waste and combustion facilities.

872

INVITED LECTURE

into a slowly turning refratory lined cylinder and move slowly down the kiln length since it is slightly tilted from horizontal. Several recent studies on the dynamics of the waste combustion in rotary kilns have been conducted that have demonstrated the unsteady nature of the combustion process. ~'92-94 For unprocessed refuse type materials, the two common types of combustion systems are the fixed hearth stoker (Fig. 2(d)) and reciprocating grate (Fig. 2(c)). In fixed hearth combustion systems the waste is introduced onto a refractory hearth through which air is injected via small ports. The waste is transferred along the grate usually with a series of transfer rams that operate intermittently. These units are often operated under controlled air conditions in which the grate region is operated substoichiometrically to minimize the entrainment of the waste particles from the combustion zone. In reciprocating grate units the waste is pushed onto a burning bed of waste that is on a grate that is tilted downward; the waste is moved down the grate by the pushing action of the multiple step grates. Some grate designs are large rollers that slowly rotate and move the waste to the next roller. The distinguishing combustion feature of these units is the burning of the waste in thick beds (up to one meter in depth on larger units) wherein mass and heat transfer are limited by, the mechanical bed mixing process. Williams et al.~ and Niessen et al.95 first examined the combustion processes that dictate combustion of waste in thick beds. Because of these transfer limitations, the bed combustion process is divided into distinct zones wherein the waste first dries at moderate temperatures, then volatilizes and burns in an active combustion zone and then the remaining fixed carbon is burned in a burnout zone. They found that the bed heats by radiative heat transfer from the hot combustion gas and the refractory lining of the lower furnace. The top of the bed ignites and the ignition wave propagates down into the fuel bed. The experimental data indicate that the underfire air flow rate and the fuel consumption rate were essentially in stoichiometric proportion but that the gas composition at the top of the fuel bed generally corresponded to the equilibration of the water-gas shift reaction. In actual waste combustion systems, the combustion processes are complicated by the heterogeneous nature of the waste and the mixing and aeration processes that disperse the ignition and burning process. In some instances the waste material has more uniform properties or can be reduced in size. For processed waste, a number of other types of combustion systems are commonly used including fluidized beds (Fig. 2(e)), multiple hearths (Fig. 2if)) and spreader stokers (Fig. 2(g)). Fluidized beds are often used on sludges and soils which have good fluidization characteristics. Multiple hearths consist

of stacked trays of fixed hearths. Sludges are introduced on the top hearth and the bed of waste is raked using air cooled rotating arms down to the next hearth. Combustion air flows upward through the device counter to the movement of the waste. Spreader stokers are used primarily for dry shredded fuels such as refuse prepared fuels and wood processing waste such as bark. The waste is thrown into a furnace and spread onto a traveling grate. The burning takes place partially in suspension as the material falls to the grate and partially in a bed on the grate. Processed wastes can also be cofired in industrial processes and boilers if the fuel properties can be improved sufficiently to be compatible with the process. Pollutant Emissions From Waste Combustion The objective of waste combustion is to burn the waste without generating pollutant emissions. Much of the public concern and regulatory pressures arise because of pollutant emissions. A wide range of pollutant emissions are potentially emitted from waste combustion systems including: 9organics constituents in the waste 9organic byproduct emissions 9toxic metals 9pathogens (especially for medical waste) 9acid gases (NO~, SO2, HC1) As a result of compliance testing, a large data base exists on the characterization of the emissions of these species both into the air and in solid residue. However, there is a dearth of information regarding controlling and limiting processes in waste combustion systems. Much of the understanding comes by analogy to other combustion systems. The present level of understanding about pollutant formation in waste combustion systems is discussed in the next sections.

Organic and Pathogen Constituents in the Waste: The primary objective of waste combustion is to destroy the organic and pathogen constituents in the waste streams. One of the regulatory requirements in the United States for hazardous waste incinerators is that each facility must demonstrate the ability to destroy or remove the principal organic hazardous constituent to an efficiency of 99.99%. This is based on the stack emissions of the constituent relative to the total feed rate of the constituent. All permitted incinerators in the United States have now been tested for compliance and with few exceptions were found to achieve this level of destruction or removal with relative ease. For example, in studies conducted by the EPA, 15-17 eight incinerators were comprehensively tested to estab-

WASTE COMBUSTION lish whether incinerators could meet the regulations under typical operating conditions, The average efficiency of destruction and removal found in these tests for a broad range of incinerators (including both solid and liquid incinerators) and organic constituents exceeded 99,99%. A detailed statistical analysis of these data was conducted to determine if any parameters could be defined which correlated with poor destruction. Only two parameters were found to impact the destruction efficiency: (1) the type of organic compound and (2) the concentration of the organic constituent in the waste stream. Other parameters expected to be determining factors for organic destruction (e.g., heat input rate, combustion chamber exit temperature, mean combustion chamber residence time, and stack 02) were very wealdy or not correlated with destruction efficiency. These parameters are certainly important for incinerators not operated near optimum conditions but do not appear to control destruction efficiency for well operated systems. The first parameter that correlated with destruction efficiency is the organic compound. Organics compounds such as benzene, naphthalene, chloroform, and methylene chloride were sometimes found to have lower destruction efficiencies. These lower efficiencies are likely because they are combustion byproducts formed during the combustion of other organics present in the waste or the auxiliary fuel. z7 Organic byproduct formation will be discussed in more detail in later sections. The other parameter found to influence the destruction of organics in normal operation of industrial hazardous incinerators, is the concentration of the organic in the waste stream. The influence of concentration on destruction or removal efficiency of volatile organics is shown in Fig. 3 from data based on field testing.l~ This same dependence on concentration has been duplicated in the laboratory using a spray flame reactor, is These destruction efficiency data correlated inversely with the organic feed concentration indicating that it is much more difficult to get high destruction efficiency at low initial concentrations. The field data in Fig. 3 indicate the total mass emissions from hazardous waste incinerators vary little over a range of waste feed concentrations approaching six orders of magnitude. This observation suggest that there is some fundamental limit on complete destruction of organics in waste combustion systems and an acceptable explanation of this phenomenon has not yet been put forth, Simple explanation such as analytical deficiencies can be eliminated because these data are well above the detection limit of the instrumentation, even at the lowest concentrations. In addition, equilibrium calculations indicate that if the organics mix with air at moderate temperatures, the thermochemical equilibrium concentration of organics will be insignificant. 19-2o

873

UI

-

N 10 "q Z 10

t tl,li,[ 1 0 -~;

i i ,i,,,,l 10 "s

t i lqtI,tl 1 0 "4

, , ,,,,lll

10 `3

i i ttptt, l 1 0 -I~

, I I,,rrl

10-~

Waste Feed Concentration (mass fraction)

FIG. 3. Impact of concentration of the organic constituent in the waste stream on the field measurement of destruction efficiency (from Reference 17), Mixing and kinetic inadequacies may likely be the limiting factors for destruction of organics in waste combustion systems. Numerous studies on the kinetics of nonflame thermal oxidation of pure and mixed organic compounds have been carried out. 2126 More recently several studies have been conducted on detailed chemical kinetics of flame zone processes of simple hazardous organic compoundsY -a~ The nonflame studies have been used to define the temperature at which a one second residence time is sufficient to produce 99.99% oxidation of the starting compound even in the absence of flame radical concentrations. These temperatures are generally below 1650~ F (900~ C) for most organics of interest as summarized in Table IV. Also provided in Table IV is the first order global Arrhenius parameters for destruction under nonflame conditions. Almost all incinerators are designed to operate at significantly higher temperatures (1800-2200~ F, 980-1200 ~ C) and thus should be conservative relative to kinetic requirements. Kinetic modeling studies by Tsang 101 using available elementary rate data suggested the importance of the reaction mixture and mixing on the destruction of chlorinated organics. Research by Lyon and Hardy31-32 and more recently by Lyon et al.~3 suggests that the extent to which a compound is destroyed by incineration may be dependent on its concentration due to a kinetic threshold of oxidation that exists at low concentrations. They suggest that the kinetic threshold arises from the fact that generating a high enough equilibrium concentration of free radicals to sustain the oxidation rate requires a minimum amount of fuel. Laboratory studies in

874

INVITED LECTURE TABLE IV Stability temperature of selected organics from nonflame laboratory testing

Compound

Empirical formula

To.~t(2) (~

Tin(2) (~

Tm 99(2) (~

Acetonitrile Tetrachloroethylene Acrylonitrile Methane Hexachlorobenzene 1,2,3,4, -Tetrachlorobenzene Pyridine Dichloromethane Carbon tetrachloride Hexachlorobutadiene 1,2,4-Trichlorobenzene 1,2-dichlorobenzene Ethane Benzene Aniline Monochlorobenzene Nitrobenzene Hexachloroethane Chloroform 1,1,1-Trichloroethane

CzHaN C2C14 CaHaN CH4 C6C16 C6H2C14 C HsN CH2CI~ CCL4 C4C16 CsHaCl3 CsH4CIz C2H8 C6H~ CsH~N CsHsCI CsHsNO2 C2C1~ CHCs C2HsCI3

760 660 650 660 650 660 620 650 600 620 640 630 500 630 620 540 570 470 410 390

900 850 830 830 820 800 770 770 750 750 750 740 735 730 730 710 670 600 590 570

-950 920 860 870 880 850 840 780 820 780 790 780 785 760 750 780 700 640 620 600

a flow reactor a4 have indicated that the dependence of destruction on concentration for benzene, chloroform and chlorobenzene is strong and can be accelerated with the addition of a co-oxidizing fuel at higher concentrations in order to generate higher concentrations of radicals as shown in Fig. 4. Thus, these data suggest that kinetic limitations can be important at low concentrations even at the high temperatures normally encountered in waste combustion systems. The other parameter that can limit organic destruction is poor mixing of the organics with air. Field measurements inside of a full scale rotary kiln burning waste have indicated striking stratification in the mixing patterns even in well operated units, a4 Kramlich is observed that the absolute emissions levels in Fig. 3 were relatively constant and independent of initial concentration. He suggested that the unmixed fuel or droplets are convected through the flame and a fraction of these are mixed with the hot vitiated combustion products forming a certain fraction of fuel-rich packets. These fuel-rich pockets approach thermochemical equilibrium in composition and below 10 percent theoretical air the equilibrium concentrations of organics of interest rises to high levels. These packets can be subsequently convected out of the flame region where they can be quenched. These quench packets contain unburned hydrocarbons that is a base level of

Ea

(kcal/mole)

A(s-~) 4.7 2.6 1.3 3.5 2.5 1.9 1.1 3.0 2.8 6.3 2.2 3.0 1.3 2.8 9.3 8.0 1.4 1.9 2.9 1.9

• • x X •

x x X

• • x X

• x X

• x • x x

107 106 10~ 109 l& 108 l0 s 1013 105 10TM l0 s l0 s 105 10s 10TM 104 lOts 107 10TM 10s

40 33 31 48 41 30 24 64 26 59 39 39 24 38 71 23 29 49 32

FIGURE 411

IO0

(, REMAINING

i 1

100.1

J 10

i

i

100

10oo

[C~HsCI]o, ppm

FIGURE 4b

100

% REMAINING ---C-- [Cd4sCI]o ffi 26.7ppm ---o-- [CsHqo = 46ppm

1011

0,1

0.01 O.OOl

i 200

i 400

i 600

i 800

i 1000

i 1200

1400

1600

[CO]0, ppm

FIG. 4. Lab scale data on the impacts of reactant concentrations on destruction efficiency by benzene and chlorobenzene (a) impaets of reactant concentration (b) impacts of CO (800 ~ C, 0.80 see, 4.8%, 02, 1.5% H20).

WASTE COMBUSTION organics that is difficult to lower without substantial improvements in mixing. Thus there appears to be some kinetic or mixing limits that exist in practical waste incinerator devices that preclude complete destruction of organics and leads to a low level of mass emissions. However, the kinetic or mixing hypothesis discussed above must be further quantified and shown to account for the observed concentration dependence of destruction efficiency of organics. Another critically important factor, particularly for soils and other contaminated solids, is the removal of the organic constituents from the solid matrix. The goal is to generate a treated residue that is free of hazardous constituents. In practice, a cleanup criteria is usually established for the level of hydrocarbon that can remain in the residue. At the same time, it is important to avoid excessive treatment temperatures in order to minimize the release of toxic metals. These tradeoffs are discussed more fully in the next section. Studies at the University of Utah have begun to examine the limits to organic removal from soils matrixes when treated in rotary kiln environments. 35 These studies have used lab and bench scale reactors to define the controlling parameters for soil matrices contaminated with organics and then related the results to full scale performance with mathematical modeling of the appropriate mass and heat transfer processes. These studies have identified the importance of soil type, hydrocarbon type, treatment temperature, and moisture content on the ultimate removal of organics. They found that for reactive, porous soils (e.g., days) the last monolayer of constituent molecules may be tightly bound to the soil necessitating temperature significantly above the boiling point to insure adequate cleanup. New insight has also been gained on the influence of moisture. Moisture not only influences the thermal requirements for heating but also was found to play a role in how organics are physically absorbed in the soil since it replaces sites within the matrix where organics can absorb and how the organics are subsequently removed from the solid via "steam distillation." These types of studies may ultimately provide a means to optimize the design and operation of soil thermal treatment equipment to ensure adequate cleanup of organics. In addition to organics, pathogens such as viruses, bacteria, mycobacteria, parasites and rickettsia can also be present in waste materials particularly those from medical institutions and municipal sources, as These pathogens have been found to be extremely fragile relative to the conditions that exist in waste combustion units. A limited number of tests have been conducted on medical and municipal waste combustion devices to define the emissions of viable pathogens. 37-41 These early studies

875

have used a variety of techniques usually involving the spiking of a sample of a target pathogen, recovering the sample, and analysis for viable pathogens. They have concluded that pathogen destruction is quantitative since no viable target pathogens were found in either the stack or the residual solids under normal operating conditions. Under excessively low operating temperatures (1100~ F, 600~ C) viable target pathogens were found in the bottom ash of one test and other pathogens have been found in the air emissions of other tests. There has been some concern about the validity of the measurement protocol used in these early studies, and therefore the U.S. Environmental protection Agency is continuing to develop and validate a pathogen emissions assessment protocol and is planning a series of tests of pathogen emissions from a series of medical waste incinerator facilities.

Trace Organic Byproduct Formation: There is great interest in the emissions of organic byproducts from waste combustion systems as indicated by the recent promulgation of new regulations both within the United States and in the European Community. They also represent one of the most prominent fears and severe criticisms of the technology by the general pubic. These concerns and regulations about byproduct organic emissions along with those on toxic and carcinogenic metals emissions will likely dictate the design, operation and monitoring of waste combustion systems in the future. Several environmental agencies have attempted to quantify the emissions of organic byproducts from a variety of waste incinerators. The Environment Canada has a National Incinerator Testing and Evaluation Program (NITEP), which has to date focused on the evaluation of the control and operation of municipal waste incinerators for trace organic control. 42-4a The U.S. E n v i r o n m e n t a l Protection Agency has programs on the evaluation of industrial waste combustion, 4AS-17,4s sewage sludge combustion, 44 municipal waste combustion45-4~ and has recently initiated a program on medical waste combustion systems. In addition, the European environmental authorities have assessment programs focused on industrial and municipal waste combustion facilities. However, to date, a comprehensive evaluation of all organic byproduct emissions from waste combustors has not been accomplished due to the limitations and costs associated with the sampling and analysis procedures required for assessment of trace levels of organics. The most comprehensive analysis conducted to date on an operating industrial waste incinerator using concentration sampling techniques and analysis by gas chromatography/mass spectrometry was only able

876

INVITED LECTURE

to account for 50-60% of the organics as measured by a total hydrocarbon monitor (flame ionization detector. 17,47 Even a smaller fraction could be quantified in terms of identifying the actual species and no attempt was made to quantify the nonvolatile organic components. The emissions of polycyclic aromatic hydrocarbons on fly ash from waste combustion systems was

first confirmed in 1976.49 Since then, the types of organic combustion byproducts that have been identified in the exhaust of waste combustion systems have dramatically increased with the development of more sensitive analysis techniques as summarized in Table V. Even the most predominant byproduct organics were found to be emitted in the range 1 to 1000 mg/min (generally less than

TABLE V Organic emissions from waste combustion facilities Emissions rates (mg/min) Compound

Range

Mean

Median

0.17-590 0.12-95 ND-140 0.04-2,800 ND-14 ND-99 ND-16

34 17 10 110 3.2 11 1.4

1.5 5.0 0.49 0.73 1.1 0.92 0.76

ND-160

12

0.27

Volatiles Benzene Tuluene Carbon tetrachloride Chloroform Methylene chloride Trichloroethylene Tetrchloroethylene 1,1, l-Trichloroethane Chlorobenzene cis-1,4-Dichloro-2-butene Bromochloromethane Bromodichloromethane Bromoform Bromomethane Methylene bromide Methyl ethyl ketone

I

Semivolatiles Bis(2-ethylhexyl) phthalate Naphthalene Phenol Diethyl Phthalate Butyl benzuyl phthalate 2, 4-Dimethylphenol o-Diehlorobenzene m-Dichlorobenzene p-Dichlorobenzene Hexachlorobenzene 2, 4,6-Trichlorophenol Fluoranthene o-Nitrophenol 1,2,4-Tirehlorobenzene o-Chlorophenol Pentachlorophenol Pyrene Dimethyl phthalate Mononitrobenzene 2,6-Toluene diisoeyanate ND = Not Detected Source: Reference 1

0.07-55

w

8.1

1.6

877

WASTE COMBUSTION 5 ppb). There is clearly a broad range of organic compounds that can be formed in the combustion process of waste combustion systems in trace amounts including volatile, semivolatile and nonvolatile species. Mechanisms that can deal with this myriad of organic compounds in a such a complex and diverse set of combustion systems have not yet been developed. The state of the art in designing and operating waste combustion systems for minimization of organic emissions has been based upon a phenomenological approach. This approach uses insights gained from smaller scale studies and engineering analysis combined with full scale field studies which have examined the impact of design and operating parameters on organic emissions. The success of this approach is indicated by the dramatic improvements made in the control of chlorinated dioxin and related species from municipal waste combustors, estimated to be a three orders of magnitude reduction in the last decade. 48 Much of the attention relative to combustion byproduct emissions from waste combustion systems has been focussed on polychlorinated dibenzo(p) dioxin and furans (PCDD/PCDF) since it was first reported to be present in the exhaust of municipal waste combustion in 1977.5~ Data from a wide range of incinerator systems (Fig. 5) indicate that many incinerators produce levels of PCDD/PCDF that are of concern to regulators (e.g., a New Source Performance Standard for large municipal waste combustion systems has been recently proposed in the United States at 10 ng/dscm of total PCDD/ PCDF). However, the emissions of PCDD/PCDF is widely variable and appears to depend on the waste type and system design and operating conditions. The phenomenological mechanisms developed for this class of species can be used to indicate the behavior of waste combustion systems relative to other types of trace organic byproduct emissions. It is now clear that several global mechanisms contribute to the emissions of PCDD/PCDF and the relative importance of each of the formation pathways depends on the specific design and operation of the combustor and the waste properties. Municipal Waste Incinerators

The mechanisms can be grouped into four categories as shown in Figure 6. The first category of mechanisms involves the lack of destruction of P C D D / P C D F that is originally in the waste stream. 51-52 Since very low levels of emissions are of interest, very small amounts of dioxin in the waste stream could account for the emission levels if they were not destroyed during the combustion process. Also, as discussed previously, at very low initial concentrations the destruction efficiency is generally low. This mechanism, however, does not likely account for most of PCDD/PCDF emissions since insufficient quantities of dioxin have been found in the waste streams. ~3-54 The second category of mechanisms involves the formation of PCDD/PCDF from vapor phase reactions within the combustion zone. Two types of reactions have been proposed: those involving unchlorinated hydrocarbon and a chlorine donor and those involving gas phase reactions of chlorinated hydrocarbons with similar structures. Bumb et al. ~5 and Crummet~ first suggested that dioxins formed due to "trace chemistry of flames" involving gas phase reactions of unchlorinated hydrocarbons and chlorine compounds. The ubiquitous nature of hydrocarbons and chlorine makes the formation of dioxin an inevitable consequence of combustion of waste materials. This global mechanism is highly controversial and supported by no direct evidence. There are data that indicate that some gas phase chlorine, likely either HCI or C12 is required to form PCDD/PCDF 57 under certain conditions. Ballschmiter et al. 5s and Benefenati et al. 59 examined emissions from fidl sale incinerators and found a close relationship between the dioxin emissions and the quantity of polychlorobenzenes and polychlorinated phenols in the exhaust. They interpreted this to indicate that dioxins are formed by reactions involving these gas phase species which were in the waste or were formed in the combustion process. Shaub and Tsang 6~ developed a kinetic model to study the characteristics of the reactions involving chlorinated hydrocarbons alone.

Medical Waste lnclnerators

12000

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]!

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~

-

%

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.

.

.

.

.

.

.

Batch

Fie. 5. Polychlorinated dibenzo(p)dioxin and furan emissions from waste incinerator systems.

2. I ~ D D / P C D F f o f m ~ ~ combtmtm inte~ate from gas preoJm-~r~ w l t b t , the ne~ zeee

3. ~ D D / T ~ D F focmcd ~ p r l t t l l y hnrr,ed p~rtkks o f waste t i m escaped ~ to o v ~ a r l ~ . g ~ s h ~ r~id~lJrr~

FIG. 6. Mechanisms of PCDD/PCDF Formation in Waste Combustion Systems.

878

INVITED LECTURE

The third category of mechanisms are heterogeneous mechanisms within the combustion zone, likely related to the particulate matter. Barton et al. 6~ first pointed out the strong correlation between PCDD emissions and the amount of particulate matter entrained from the combustion zone of a mass burn municipal waste combustion unit. This strong relationship was later confirmed to exist for a range of municipal waste combustion systems including refuse derived fuel fired speader stokers62 and starved air medical waste incinerators. 9 Recent pilot-scale municipal waste combustion studies in our laboratory have indicated that the PCDD/PCDF formed within the combustion zone is largely associated with the large, partially burned particles indicating a heterogeneous mechanism rather than a condensation mechanism. The final category of mechanisms involves processes downstream of the combustion zone. In these mechanisms, P C D D / P C D F are formed by lowtemperature, catalyzed reactions associated with the fly ash. Data supporting this category were first provided by Vogg et al.~and Stieglitz et al. ~ They found that heating fly ash from waste combustion in an oven to 250-350 ~ C resulted in the formation of PCDD/PCDF on the fly ash. This occurred despite the fact that no additional chlorine was added to the material. They were able to identify the importance of catalytic metals such as copper chloride, on the formation process. Karasek et al.~ also found that dioxins form on fly ash particles in the absence of gas phase chlorine. In addition, Karasek found that it is possible for adsorbed inorganic chlorides to chlorinate aromatic rings and promote the formation of chlorinated dioxin. The importance of the downstream mechanism has now been confirmed in a series of full scale and pilot scale tests that indicate that if fly ash particles are held in the temperature window of 250-350 ~ C such as in a hot side electrostatic precipitator, PCDD/PCDF will form. ~-68 The downstream reactions appear to be a "magnifier" of the combustion formation causing an increase of the PCDD/PCDF escaping the combustion zone by as much as an order of magnitude. The insight gained from combustion science has resulted in the establishment of good combustion practice that can significantly lower trace organic emissions.69 71 For PCDD/PCDF, different mechanisms have been identified and found to become dominant under different conditions. All of the formation mechanisms for all types of organics must be addressed if total mass emissions of trace organics are to be minimized. The basis of combustion control strategies must be to attack the temporal and spatial variations in temperatures and mixing that allow any organics to escape the combustion zone. These organics can be a toxic pollutant in their own right or can be precursors to the formation of organics that are more toxic. At the same time,

conditions must be established that minimize the entrainment and maximize the burnout of particulate matter. A key failure of waste combustion operation is the overcharging of the primary combustion zone that results in a large release of unburned materials. Finally, conditions downstream of the combustion zone must also be avoided which will hold up particles in the temperature window of 250350 ~ C. Below 200 ~ C, semi-volatiles will condense onto particulate matter that can be removed by high performance particulate control devices. Even though the organics are not destroyed they will at least not be dispersed into the air. However, it is clearly better to prevent the formation of the trace organics in the combustion zone instead of just removing the materials from the emissions. Substantial additional research is required to adequately understand the formation of other trace organics in the full range of waste combustion equipment and allow for optimum control.

Fate of Metals in Waste Combustion Systems: Waste materials can contain literally any metal species in a variety of physical and chemical forms. Many of these metals are carcinogenic or toxic to humans if they are exposed to them by inhalation or ingestion. The U.S. Environmental Protection Agency has conducted risk assessments from industrial waste and sewage sludge incinerator emissions and has concluded that exposure to metals via inhalation of air emissions poses a significant threat. For this reason, it has recently proposed new regulations on the control of metal emissions from both sources. 7z-73 The metals that are regulated are listed in Table VI. In addition, metals may be leachable from the solid residue remaining after the combustion process and the leachability is influenced by combustion conditions. Since waste combustion systems do not destroy metals, the ultimate fate of the metals is of concern. Metals that enter the combustion system with the waste exit by any of several pathways. Most of the metals in a waste pass through the incinerator unchanged and are found in the residual ash or bottom ash. Other metals will become entrained in the flue gases and exit the combustion zone as fly ash. Fly ash can be caught subsequently in heat recovery and flue gas cleaning equipment. Some metals can escape the combustion system and flue gas cleaning device and hence, be emitted into the air. The division of the metal is sometimes referred to as the partitioning of the metal since an individual metal will partition itself into the various emissions streams. An example of the partitioning that can occur in a waste combustion process for four metals is shown in Fig. 7 based upon detailed studies conducted by Brunner and Monch TM on a large municipal waste combustion facility. Three exit

WASTE COMBUSTION

879

TABLE VI Regulated metals and the volatility temperature With no chlorine Metal Chromium Nickel Beryllium Silver Barium Thallium Antimony Lead Selenium Cadmium Osmium Arsenic Mercury

With 10% chlorine in waste

Volatility temp, (~

Principal species

Volatility temp, (~

Principal species

2935 2210 1930 1660 1560 1330 1220 1160 605 417 105 90 57

CrOz/CrOa Ni(OH)2 Be(OH)2 Ag Ba(OH)z TlzO3 Sb2Oa Pb SeOz Cd

2930 1280 1930 1160 1660 280 1220 5 605 417 105 90 57

CrO~/CrO~ NiCI2 Be(OHh) AgC! BaClz TIOH Sb~Oa PbCI4

OsO 4

As20a Hg

pathways were examined and compared to the amount of each metal fed: residual bottom ash, captured fly ash and emissions. Each metal partitioned itself very differently through the combustion system. Mercury was largely emitted while copper largely stayed with the bottom ash residue. Cadmium and lead were more evenly distributed into the three emissions streams. Little detailed data are currently available on the fate of metals in waste combustion systems. However, it is clear that the combustion process can significantly influence the fate of metals. For example, in Fig. 8 is shown the enrichment of metals in the fly ash relative to the waste feed concentration after the thermal treatment of a soil from a superfund site in a pilot scale rotary kiln. At low temperatures, the metals in the fly ash are of similar concentration as in the untreated soil. After high temperature treatment, many of the metals are highly concentrated in the fly ash. Also, the organic removal from the soil takes substantially longer times at the lower treatment temperature and hence there is a trade-off between organic removal and destruction, and metal release as a function of temperature. This tradeoff suggests that an optimum temperature exists, just sufficient to remove the organics without significantly releasing the metals. Barton et al. 74 have attempted to use fundamental studies from the combustion of other fuels to explain the fate of metals in waste combustion systems and to predict the impact of the design and operation of the system of metal emissions. They adapted the figure first developed by Neville and Sarofim75 for metal particulate behavior from coal combustion to summarize the physical and chemical transformations that occur to metals in waste corn-

SeO2

Cd Os04 As~Oa Hg

bustion systems. This schematic is shown in Fig. 9. The metals can escape the combustion chamber in only two ways: either in the solid residue or in the flue gas. Metals that enter as a inorganic compound may be unaffected by the combustion process and stay as the inorganic fraction as the carbon matrix is burned off. The rest of the metal can be entrained into the combustion gases and exit with the gas. For example, ff the waste is injected as a liquid spray or if the metal is in the form of a fine powder then it can be entrained in the combustion gases. 76,77 Metals can also enter the flue gas if they are vaporized in the combustion zone. Many metals and metal species found in wastes are volatile and vaporize during the high temperature combustion process. 74'78 The metal species can also react to form more volatile species such as metal chlorides. The temperature at which the vapor pressure of each metal of interest is one millionth of an atmosphere is also provided in Table VI based upon thermochemical calculations under conditions where chlorine is and is not present. These data clearly show that several of the metals such as mercury, lead, cadmium and arsenic are expected to be volatile at normal waste burning temperatures and that chlorine species are sometimes significantly more volatile particularly for lead and silver. The impact of chlorine on metal partitioning to the fly ash has recently79been confirmed in pilot scale rotary kiln tests. These metals will subsequently condense in the cooler regions of the combustion system both homogeneously to form fine fume particless~ and heterogeneously, again preferentially on finer particles. 77 The fine metal containing particles are the most difficult to capture by normal air pollution

880

INVITED LECTURE

M E

MEASURED

As

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MERCURY

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4•R C EMI'I'i'ED

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58

COPPER

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89 FIc. 7. Example of the partitioning of four metals in a municipal waste combustion facility (adapted from Reference "/4).

control equipment and high performance devices are required to meet the more stringent standards. A metal partitioning model has been developed to account for the major physical and chemical phenomena in waste combustion systems which is described more fully in Barton et al.74 It is based upon

A L 4O E N 30 R

Cd iPb 1800 F

C 20 H 1000 F MI 10 E AsSb PbCu CrZn Cd N T

(%)

~

u II1"1"~C Z || |Zt Cr

FIG. 8. Impact of treatment temperature on the enrichment of metals in the fly ash after the thermal treatment of soils from a superfund site.

equilibrium calculations at local conditions within the combustion chamber for the chemistry and volatilization, Li's entrainment model, 76 Friedlander's condensation model, sl and Gelbard and Seinfeld's coagulation model. 82 It also includes modules for the prediction of the performance of different air pollution control devices. This computer tool provides a new procedure to evaluate the impacts of the design and operation of waste combustion systems on the fate of metals. However, it does not allow a complete assessment of waste combustion systems for the optimal management of metals. Comparisons conducted to date indicate that the equilibrium assumption of the model is reasonable under some situations but fails in others. In particular, situations have been identified where the metal is apparently held in a physical matrix that prevents volatilization. More detailed chemistry is required to evaluate the impact of reaction kinetics and to allow the specific metals species to be predicted. The chemical form of the metal is key to the definition of most appropriate disposal for the residue since different chemical forms will leach differently. Significant basic research is required in this area in order to improve the capabilities for optimum metals management. Monitoring of Waste Combustion Systems The public concern about the environmental impacts of waste combustion is due to a large extent to the fact that there are no direct real time monitoring techniques available that allow toxic emissions to be checked on a continuous basis. The regulatory and health based emissions levels for organics and metals are extremely low. For example, 99.99% destruction efficiency converts to a concentration of 70 ppb for a waste containing one percent of the organic constituent. Regulatory standards for total

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FIG. 9. Physical chemical transformation of metals in waste combustion systems. PCDD/PCDF are even as low as 10 ng/dscm ( - 6 ppt). The sampling protocolsa-s4 required to measure these low levels involves stack sampling for several hours to collect enough sample in filters, organic sorbents, and impingers to analyze. The analysis procedure usually involves analysis by gas chromatography/mass spectrometry for organics and either atomic absorption or inductively coupled plasma atomic emission spectrosopy for metals. These analysis procedures dictate specialized equipment and personnel and can require several weeks before results are available. Thus, the ability to measure the levels of pollutants of interest is available but the assessment is far from being on a real time basis. Significant progress will have to be made on the development of continuous monitoring of trace organics and metals if a direct real time monitoring procedure is to become available. In lieu of the development of direct monitoring techniques, indirect continuous monitoring techniques have been used to assess the performance of waste conbustion systems. Two indirect approaches have met with some success: (1) monitoring of combustion operating parameters related to performance and (2) the use of surrogate gas phase species that can be measured in the stack in real time and are related to the emissions of the species of interest. These indirect monitoring approaches rely on the knowledge of the relationship between the parameter being monitored and the trace species emissions. The first indirect monitoring technique for waste combustion systems involves the continuous monitoring of the key operating variables that influence the behavior of organics and metals. This is the basic approach used by permit writers in the United States for hazardous waste incinerators. Permit conditions are established to maintain the operation at the same point as was proven to be acceptable in

a compliance trial burn. However, at present, the key operating parameters have not been completely defined and additional monitoring procedures are required. Further research is required to confidently implement this technique. The other indirect monitoring technique involves the use of surrogates, Several surrogate species for organic emissions have been suggested for real time monitoring including species that are combustion intermediates such as carbon monoxide and total hydrocarbons (THC, as measured by flame ionization detectors) and species that are added to the waste stream and can be monitored continuously. The use of combustion intermediates such as CO and THC relies on the correlation of these with the emissions of ta'ace organics and is based upon the hypothesis that conditions that favor organic destruction also favor oxidation of these intermediates. Kramlich et al.84 demonstrated a strong relationship between CO and THC with organic destruction in a subscale spray flame when the destruction efficiency was relatively poor. They first introduced the concept of failure modes by proving that under flame conditions the destruction of organics is essentially quantitative and only conditions which allowed organics to bypass the flame would result in incomplete destruction. These intermediates were found to be indicators that some of these failure mode conditions existed. Staley et al. s~ found that this correlation did not extend to conditions of high destruction efficiency. Correlation~ available from field data 17 also demonstrates the same behavior i.e., CO and THC do not correlate directly with organic destruction but are generally conservative indicators of destruction efficiency. The correlation of combustion intermediates is poorer when considering the emissions of trace organic emissions such as PCDD/PCDF. Formation mechanisms for these byproducts can entail more

INVITED LECTURE

882

than just gross failures of the combustion process indicated by CO and THC alone. Microscale failure phenomenon and even processes downstream of the combustion zone can be important as was discussed above. Nonetheless, low CO and THC is an indication of good combustion conditions. Therefore CO and THC can be used as indicators of good combustion practice and indicators that the system is being operated at conditions similar to those during a compliance test. However they cannot be directly correlated with emissions and no single "safe" value has been established for all types of waste combustion systems. The current practice is to use CO and THC only as indicators of good combustion practice and to establish appropriate levels based upon technical achievability. A second type of surrogate is the use of a waste organic that is more difficult to destroy than other compounds and can be measured with real time instrumentation. In addition, nontoxic metal surrogates could be used to mimic the behavior of metals if real time analysis procedures could be developed. Sulfur hexafluoride is the most frequently suggested surrogate for organic destruction, and its application has been the subject of extensive research, a6-88 Others have suggested the use of benzene since it is generally emitted at higher levels than other trace organics and its formation is related to the formation of other aromatic compounds. While this approach holds some promise for the future, there is still significant uncertainty concerning the basic assumptions and implementation of the technique. For example, there is no agreement on the appropriate procedure for determining the most difficult to burn compounds. The ordering has been found to depend for example, on failure mode 8~ and the presence of other waste compounds, s~ The method of introduction is also in question to ensure that the surrogate experiences the same history as the organics in the waste.

Conclusions-Research Needs in Waste

Combustion The challenge to waste combustion technology of the future is to burn the ever changing waste streams in a cost-effective, fuel efficient manner without creating other emissions or operating problems. To convince the public to accept incineration will require comprehensive performance assurance techniques that continually demonstrate the safe operation of the facility. The suggested research involves vertically integrated development projects targeted towards the development of new systems, retrofit technologies and design procedures. Significant basic and engineering research will be required to allow this development to proceed. The specific high priority needs relative to waste

combustion systems are identified to include the following: 9a general assessment procedure to evaluate environmental consequences of new wastes and combustion systems, 9development of an integrated small scale incinerator for small quantity generators, 9studies on feed systems for solid and viscous liquid wastes, 9investigations of the chemistry of organic byproduct formation, 9evaluation of metal transformation during waste combustion, 9development of improved combustion zone online diagnostic techniques, 9development of improved predictive combustion zone models, 9development of surrogates to ensure equipment compliance on a real time basis, and 9development of expert system for monitoring and control of combustion equipment. With a rigorous research agenda, combustion science can be moved forward and waste combustion systems can be improved based upon a strong scientific base. Looking to the next generation of technology, these advances in combustion science will result in more powerful models capable of dynamic simulation of complex chemistry incorporating chemistry of trace organic species, nitrogen, chlorine and metals with heat and mass transfer and detailed fluid dynamics for actual combustion equipment. This will lead to the ability not only to design and build optimized waste combustion systems, but also to predict emission performance, define system limitations and failure modes. Ultimately this should lead to "smart" combustion systems which, along with new and improved on-line diagnostics, will be capable of on-line optimization of the waste combustion process. In this manner the waste combustion system of the twenty-first century can be developed that can overcome many of the problems and concerns of the present.

Acknowledgment The author is indebted to a number of colleagues within EER who have significantly contributed to this manuscript. These include Bob Barton upon whose work the dioxin and metalls sections were based, Dr. John Kramlich and Dr. Richard Lyon who contributed significantly to the organic destruction discussion, and Dr. Mike Heap who made an invaluable contribution of ideas and information relative to research needs. The author also wishes to express appreciation to Dr. C. C. Lee of the US EPA in Cincinnati who sponsored much of the engineering analysis work on waste combustion sys-

WASTE COMBUSTION tems that formed the basis of this paper, David Linz of the Gas Research Institute who supported many of the industrial studies and Blair Martin of the US Environmental Protection Agency in North Carolina who sponsored the development of the research needs in the area of waste combustion. These three individuals provided not only the resources but much of the inspiration for this endeavor. The author also wishes to acknowledge the significant contribution of Prof. Scott Samuelsen of UCI in the review and technical editing of this manuscript.

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50. OLIE, K., VERMEULEN, P. L., AND HUTZINGER, O.: Chemosphere, 6,454 (1977). 51. LUSTENHOWER, J. W. A., OLIE, K., AND HUTZlNGER, O.: Chemosphere, 9,501 (1980). 52. GRAHAM,J. L., HALL, D. L., AND DELLINGER, B.: Env. Sci. Tech,, 20,703 (1986). 53. CLEMENT, R. E., TOSINE, H. M., OSBORNE, J., OZVACIC, V., AND WONG, G.: Chlorinated dioxins and Dibenzofuans in the total Environment, S. Chaoudhary, Kieth and Rappe, eds., Ann Arbor Science, Ann Arbor, MI, Section 2, p. 489 (1985). 54. OZVACIC, F., WONG, G., TOSINE, H., CLEMENT, R. E., AND OSBORNE, J.: JAPCA, 35,849 (1985). 55. BUMB, R. R., CRUMMET, W., CUTIE, S., GLEDHILL, J. et al.: Science, June (1980). 56. CRUMMEa'r, W. B.: Chlorinated Dioxins and Related Compounds: Impact on the Environment, O. Hutzinger, et al., Eds.), Pergamon Press, NY, p. 235 (1980). 57. TIERNAN, T. O., TAYLOR, M. L., GARRETT, J., VANNESS, G., SOLCH, J., DEIS, D., AND WAGEL, D.: Chemosphere, 12, 1080 (1987). 48. BALLSCHMITER,K., ZOLLER, W., SCHL(YI'Z, C., AND NOTTRODT, A.: Chemosphere, 12,585 (1983). 59. BENEFENATI, E., GIZZI, F., REGINATO, R., FANELL1, R., LODI, M., AND TAGLIAFERRI, R.: Chemosphere, 12, 1151 (1983). 60. SHAUB, W. M., AND TSANG, W.: Env. Sei. Tech., 17,721 (1983). 61. BARTON,R. G., CLARK, W. D., LANIER, W. S. AND SEEKER, W. R.: Dioxin 89, to appear in Chemosphere, (1990). 62. SEEKER, W. R., ENGLAND, G. C., AND MALY, P.: Proc. Int. Conf. on Municipal Waste Combustion, EPA/Environment Canada, Florida, 1989. 63. VOGG, H., AND STIEGLITZ, L.: Chemosphere, 15,1373 (1986). 64. STIEGLITZ, L. AND VOGG, H.: 6th Int. Symposium on Chlorinated Dioxins and Related Compounds, 1986. 65. KARASEK, F. W., DICKSON, L. C.: Proc. Municipal Waste Incineration Conf., Montreal, Canada, Environmental Canada, 1986. 66. McGRATH, T, AND SEEKER, W. R.: Pilot Scale Studies of P C D D / P C D F Formation and Control using Natural Gas Cofiring, Gas Research Institute, Final Report, 5087-254-1574, 1990. 67. INOUE, S., YANAMOTO,T., AND INOUE Z.: Formation of P C D D / P C D F in Municipal Waste Incineration Facilities: Influence of Operating Parameters on Formation, Dioxin 89, to be published in Chemoshere, 1990. 68. U.S. ENVIRONMENTALPROTECTION AGENCY: J. D. Kilgroe, personal communication of tests results from Dayton, 1990.

WASTE COMBUSTION 69. CANADIANCOUNCIL OF RESOURCEAND ENVIRONMENT MINISTERS: O p e r a t i n g and Emissions Guidelines for Municipal Solid Waste Incinerators, Oct 1988. 70. FEDERAL REGISTER: Nov 1989, Environmental Protection Agency, Standards for Municipal Waste Combustion, Proposed Rule, 1989. 71. MOYEDA, D. K., SEEKER, W. R., ENGLAND, G. C. AND LINZ, D . G. : Dioxin 89, to appear in Chemosphere (1990). 72. FEDERAL REGISTER, Nov 7, 1986 Environmental Protection Agency, 40 CFR Parts 260, 264, 265, 266, 270 and 271. Hazardous Waste Management System: Standards for Owners and Operators of Boilers and Industrial Furnaces, 1986; U.S. Environmental Protection Agency, Guidance on Metals and Hydrogen Chloride Controls for Hazardous Waste Incinerators, EPA/SW, 1989. 73. FEDERAL REGISTER: Monday February 6, 1989, Environmental Protection Agency, 40 CFR Parts 257 and 503 Standards for the Disposal of Sewage Sludge; Proposed Rule, 1989. 74. BARTON, R. G., CLARK, W. D. AND SEEKER, W. R. : accepted for publication in Comb. Sci. Tech., Proc. First Int. Cong. on Toxic Combustion Byproducts, 1990. 65. NEVILLE, n . AND SAROFIM, A. F.: Nineteenth Symposium (International) on Combustion, The Combustion Institute, p. 1141, 1983. 76. LI, K. W.: AIChE J. 20,432 (1974). 77. LINAK, W. P. AND PETERSON, T. W.: Aerosol Sci. Tech 3,77 (1984). 78. VOGG, H., METZGER, M. AND SCHNEIDER, J.; Waste Manage. Res. 4,65 (1986). 79. WATERLAND, L. : Tests to Evaluate the Fate of Trace Metals in a Rotary Kiln Incinerator, EPA report, 1990. 80. SENIOR, C. L., AND FLAGAN, R. C.: Aerosol Sci. Tech. 1,371 (1982). 81. FRIEDLANDER, S. K.: Smoke, Dust and Haze, John Wiley and Sons. New York, 1977. 82. GELRARD, F. AND SEINFELD, J. H.: J. Colloid Int. Sci., 78,485 (1980). 83. EPA SW 846: Test Methods for Evaluating Solid Waste. 84. JOHNSON, L. D.: Incinerating Hazadous Wastes, Section 2.8, p. 65, Technomic Publishing Co., Lancaster, PA, 1988. 85. KRAMLICH,J. C., HEAP, M. P., SEEKER, W. R., AND SAMUELSEN, G, S.: Twentieth Symposium (International) on Combustion, p. 1991, The Combustion Institute, 1985. 86. STALEY, L. J., R/CHARDS, M. K., HUFFMAN, G.

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