Detecting waste combustion emissiOons Several advanced methods are usefil for sampling air contaminants from hazardous waste incinerator stacks
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Larry D. J o h n Environmental Protection Agency Research Triangle Park, N.C. 27711 The disposal of hazardous wastes, especially organic chemicals, by incineration has been the subject of rapidly increasing interest during the past several years. When such wastes are incinerated, their composition is not the only characteristic that must be determined. Other factors of at least equal importance must be ascertained. These include the varieties and concentrations of any air contaminants that may be emitted during the incineration process. The presence or absence of contaminants shows how well a unit is operating and whether it will perform well enough to meet environmental Standards.
Research on developing adequate methods of sampling and analysis of the emissions is in progress. These sampling methods are generally applicable not only to incineration but also to pmesses closely related to incineration, such as the cofiring of waste in indushial boilers and the burning of contaminated heating oil. Although this article briefly discusses methods for sampling inorganic hazardous compounds, its primary emphasis is on ways of sampling organic compounds likely to be designated as principal organic hazardous constituents (POHCs) for a trial burn. These methods employ equipment such as the modified method five train (MM5), which includes an XAD-2 sorbent This article not subpcllo IJS Copyr8ghl Pbbl shed 1986 American Chem ca#Society
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module; the source assessment sampling system (SASS); the recently developed volatile organic sampling train 0'0s"); and assorted containers such as glass bulbs and plastic bags. 'kdmical advances Although qmny measurements may be made to characterize the operation of an incinerator, almost all require that flue gas and waste feed'material be sampled in order to evaluate incinerator performance: Although incineratgr emissions have been sampled and analyzed in the past, several advances in this area have been made recently. These advances widen the range of chemical compounds that can he sampled and analyzed to evaluate a combustion process, for example, or to document compliance with air regulations. Compounds to be sampled consist mainly of POHCs; however, interest in sampling products of incomplete cornbution (PICs) is increasing. EPA's Office of Research and Development is conducting a program to provide adequate methods for characterizing the efficiency of incinerators in destroying hazardous wastes. A document for use by those condhcting incineration engineering R&D,programs,by EPA's Office of Solid Waste, and by the regulated community has been released 224 Environ. Sci. Technol., Vol. 20, No,3,1986
(I). The methods it describs represent the state of the art. Validation p r o m s now under way will greatly benefit the users of these methods by demonstrating good or poor sampler performance. Such demonstration allows better judgment in planning future sampling systems. Meanwhile, the methods desdribed below are best used by those with considerable experience. in stack testing and chemical analysis. Users should seek as much expert advice as possible when they prepre or carry out an incineration characterization project. Source sssessment sampling system Although there are numerous methods available for sampling incinerators,
four approaches are used most frequently because of their generally high performance and their applicability to a wide range of materials. One of these approaches is the SASS (Figure 1). Developed for environmental assessment programs, the SASS is the sampling train of choice. when a large amount of sample is necessary for extended chemical analysis or biological testing. It consists of cyclones for particle sizing, a glass or quartz fiber filter for collecting fine particles, a sorbent module for collecting semivolatile organic compounds, and impingers for
collecting volatile metals. The system operates at 110-140 Umin (4-5 cfm) and is usual1 operated long enough to collect 30 m of flue gas. It is possible, although inconvenient, to traverse a stack with an SASS, it is usually operated at a single point in the stack under pseudoisokinetic conditions. Pseudoisokinetic sampling is the term used to describe the following approach to stack sampling: A full velocity traverse of the duct is carried out to select a representative sampling point and to choose a site to place a probe nozzle (2). The probe is fined with a nozzle so that sampling is initially nearly isokinetic (camed out in such a manner that the velocity of the sample into the probe nozzle is matched closely to that of the gas stream being sampled), and the flow rate through the cyclones is that required for proper particle sizing. As sampling progresses, the flow is monitored but the sampling rate is not altered to match small changes in stack velocity. Major changes in stack velocity or pressure drop in the sampling train are cause for shutdown and corrective action. Under most circumstances, this mode of operation results in samples
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M o d M method five train
that are indistinguishable from those taken under true isokinetic conditions. The meaning and potential ramifications of this mode of sampling are discussed in detail by Johnson and Merrill (3). The corrosion of stainless steel in the sorbent module of the SASS has prompted the development of glass sorbent modules, which appear to perform adequately One of these modules is described by Schlickenrieder, Adams, and Thrun, who also provide detailed information concerning the construction and operation of both the SASS and the MM5 trains (4).
downward through it. The sorbent of choice for most sampling projects for both the SASS and MM5 is XAD-2 (3). Because of its more convenient sample size and ready availability, the MM5 usually is chosen over the SASS for incinerator sampling unless larger samples are needed for lower detection limits or more extensive analysis. In cases in which the SASS is necessary, those planning the test must decide whether to operate the SASS in its usual pseudoisokinetic and single-point mode, to maintain isokinetic sampling but not traverse, to traverse but not maintain isokinesis, or to perform a full Modified method five train isokinetic traverse. If any of the first The MM5 is conceptually similar to three options is selected, there is althe SASS, but it operates at a lower ways some finite chance that the collecflow rate, usually 14-28 L h i n (0.5-1 tion will be less quantitative or less repcfm). It was developed from a simple resentative than it would be with the modification of commercial sampling full isokinetic traverse. In practice, trains that conform to the requirements such collection errors generally are of EPA Method 5 for sampling flue quite small. In most cases, however, because such errors exist and their gases (3. The MM5 apparatus, shown in Fig- magnitude is unknown, their presence ure 2, does not include particle-sizing might call the defensibility of sampling cyclones, and it is usually constructed data into question (3). The SASS is of glass rather than stainless steel. Its large enough to make traversing inconsorbent module with cooling capability venient but not impossible unless it is is inserted between the filter and the precluded by physical arrangements at first impinger. The sorbent module the sampling site. The SASS has been must be positioned vertically so that the used in the full traverse and isokinetic gas and any condensed liquids flow sampling modes, but the difficulty of
this option makes it unattractive. Both SASS and MM5 allow the collection of particulate material, acid gases such as hydrochloric acid, gaseous metal compounds (if appropriate collection liquids are chosen), mediumboiling organic species (bp 100300 OC), and high-boiling organic compounds (bp > 300 "C). Organic species with boiling points between 100 OC and 120 OC require individual attention during the sampling planning stages and may require decreased sampling times to prevent volumetric breakthrough. Volumetric breakthrough is related to the migration rate of sorbed material through unsaturated sorbent beds (3). Considerable field experience has been gained with the SASS and MM5 sampling trains, although formal validation procedures have only recently been completed. Much of the confidence in these trains' ability to collect organic compounds rests on knowledge of the ability of sorbents to collect and recover organic materials (3). A particularly efficient demonstration of these trains' performance when a dioxin is injected into combustion gas is described by Cooke et al. (6).Similar research with other compounds is under way, and a validation project has begun. The American Society for MeEnviion. Sci. Technol., Vol. 20, NO. 3. 1986 225
w a s wwi particulate
chanical Engineers, in cooperation with the American Society for Testing and Materials and EPA, has developed prctocols for the sampling and analysis of polychlorinated dibenzodioxins and related c h l ~ r i ~ t ecompounds d by means of the MM5 train. These protocols should be published soon.
Volatile organic sampling train Because of the volumetric breakthrough mentioned above, the MM5 and SASS generally are not suitable as quantitative collection trains for organic compounds with boiling points
