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Air Pollution Control For znfectious Waste Incineration ~- ~~~

Medical waste incinerators have gone

from being uncontrolled to having multistage air

pollution control equipment.

by Robert P. Newman, PE

The health care industry in this coun- try is in the middle of a waste crisis regarding the safe disposal of infectious waste material. Disposal options for hospitals, clinics, laboratories, pharma- ceutical companies, health care practi- tioners and similar institutions have been greatly limited by federal, state and local regulations.

By far the most effective means of treating infectious waste material is in- cineration. Proper incineration steril- izes infectious waste, reduces waste vol- umes by more than 90 percent, and in some cases provides the economic benefits of heat recovery. On-site incin- eration also minimizes the costs of waste handling and transportation and reduces the liability associated with tak- ing waste off site.

The incineration process is not with- out controversy, however. All incinera- tors, including those treating .medical waste, have been highly scrutinized be- cause of the air pollutants emitted. In- fectious waste incinerators can produce emissions of particulate matter both respirable and non-respirable, metals, organic compounds, carbon monoxide, nitrogen oxides and sulfur oxides. Bio- logical agents including spores, viruses and associated pathogens also can be emitted.

Because of the emphasis on advanced air pollution control to reduce these emissions, it is increasingly important for the solid waste professional to un- derstand control technology options. Air pollution control required by many states resuits in capital costs greatly ex- ceeding the cost of an incineration unit alone. Although sophisticated control systems are being mandated primarily for new incineration units, there is a trend toward requiring similar control for modified and retrofitted incinera- tors, as well.

Air pollutants As with any combustion source, the

pollutants of concern from medical waste incinerations are produced from the waste feed material or are formed in the combustion process. Major pol- lutants include:

Particulate matter. Particulate emis- sions from any combustion process are determined by three factors: sus- pension of non-combustible materi- als; incomplete combustion of com- bustible materials; and condensation of vaporous emissions. Metals. Metal emissions are deter- mined by the nature of waste being incinerated. Some metals contained in waste are converted to metallic ox- ides during combustion and are emit- ted as a submicron particulate. Other metals, such as lead, volatilize during the combustion process and subse- quently condense on small particulate present in the flue gas. Organic matter. Theoretically, all or- ganic material can be completely oxi- dized to water (H20) and carbon di- oxide (CO,) during the combustion process. In any incineration process, however, complete combustion never occurs. The results are so-called prod- ucts of incomplete combustion (PIC), which can include toxic compounds such as dioxins and furans. Carbon monoxide. As with organic compounds, carbon monoxide (CO) is also the result of incomplete com- bustion. Combustion conditions that result in PIC formation also will pro- duce carbon monoxide. Formation of carbon monoxide is primarily dic- tated by the oxygen concentration, degree of mixing of fuellair, and temperature. Acid gases. For any type of medical waste incinerator, the principle acid gas of concern is hydrogen chloride (HCl). The determining factor for HCl formation is a source of chlorine. Nitrogen oxides. Nitrogen oxides are formed during the combustion proc- ess. Nitrogen can either be provided from the waste material or supplied from nitrogen in combustion air.

Air pollution control There are three ways to control air

pollutants from waste incinerators: con- trolling feed material, controlling com- bustion, and add-on air pollution con- trol systems.

Controlling feed material consists of

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eliminating certain materials either by segregating for separate disposal or for recycling. Typical materials that can be segregated include metal-bearing waste such as syringes and other non-combus- tible material.

Combustion control is paramount for any incineration system. Optimizing combustion consists of regulating tem- peratures, excess air rates, mixing, and retention time to maximize the thermal oxidization of the waste. Improving combustion can dramatically lower emissions of particulate, organics and CO. Other pollutants, however, such as HC1, metals, and sulfur oxides (SO,) are little affected by combustion controls.

Add-on pollution control can be basi- cally categorized into wet scrubbers, dry scrubbers and fabric filtration. Elec- trostatic precipitation has not been widely used to date and so will not be included here. For most installations, required levels of control dictate using a combination of technologies to attain emissions limitations.

Wet scrubbers Three basic varieties of wet scrubbing

systems are used on medical waste in- cinerators: venturi, packed-bed and spray tower scrubbers. Venturi scrub- bers are used primarily to control par- ticulate and gaseous emissions. Packed- bed units reduce acid gas emissions. Spray towers control particulate and gaseous emissions, but are unsuitable for eliminating fine particulate matter.

The venturi scrubber consists of a vessel containing a converging and di- verging cross-sectional area. See Figure 1. Liquid is sprayed upstream of the zone referred to as the throat. The throat has minimal cross-sectional ar- eas, which results in maximum gas ve- locity. The high velocity in the throat atomizes liquid droplets, which, in turn, impact particulate matter. For high collection efficiencies, venturi scrubbers require velocities in the throat in the range of 10,000 to 40,000 feet per minute.

Overall performance of a venturi scrubber is related to the size distribu- tion of particulate matter. Efficiencies are high for particles greater than 1 mi- crometer diameter, but performance drops for smaller particles. Unfortu- nately, small particle size distribution is typical for hospital incinerators due to the condensation of partially com- busted organic compounds and metallic vapors.

With venturi scrubbers, particulate

Figure 1. Performance efficiencies for venturi scrubbers drop for particles smaller than 1 micrometer.

Figure 2. A packed-bed wet scrubber uses absorption as the main method of collecting acid gases.

OCTOBER 1991 POLLUTION ENGINEERING 69

POLLUTION ENGINEERING

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Dry scrubbers use absorption to remove hydrogen chloride, sulfur dioxide and other acid gases.

matter collection efficiency increases as the static pressure drop increases. Pres- sure drop defines the total amount of energy used in the scrubber to acceler- ate the gas stream and atomize scrub- bing liquid into droplets.

Packed-bed scrubbers are used pri- marily for acid gas removal. See Figure 2. Such scrubbers are not effective as stand-alone systems for tine particulate.

Absorption is the main means of col- lecting acid gases in packed-bed scrub- bers and is affected by the extensive liquid surface contacted by the gas stream as liquid passes over packing material. Packing material comes in a variety of shapes in diameters from 0.5 inches to 3 inches.

Frequently, sodium hydroxide (NaOH) is used with water to neutralize absorbed acid gases. HCl and SO2 col- lected in the scrubber react with the NaOH to produce sodium chloride (NaCl) and sodium sulfite (Na2S07) in an aqueous solution.

One major problem with packed-bed scrubbers is the accumulation of solids. Dissolved and suspended solids must be monitored and regulated through blowdown to prevent maintenance problems.

Spray towers are relatively simple in design and consist of an empty vessel with an array of liquid spraying nozzles. See Figure 3. Such scrubbers normally incorporate counter-current flow to maximize efficiency.

Generally, the smaller the droplets formed by the spray nozzles the higher the collection efficiency for gaseous compounds and particulate matter. Gas velocity must not be too high, however. High velocities will result in transport of liquid droplets and will reduce col- lection efficiency. Exhaust velocities for spray dryers should normally be from 1 to 4 feet per second.

Spray dryers are low-energy scrub- bers compared to venturi systems, and pressure drops across such units are generally less than 1 inch of water. The collection efficiencies for spray dryers are satisfactory for particle diameters greater than 10 micrometers.

Dry scrubbers Dry scrubbing is a relatively new con-

trol technology for medical waste incin- erators. Dry scrubbers use absorption to remove hydrogen chloride, sulfur di- oxide and other acid gases. An alkaline sorbent is used to react with and, in turn, neutralize the acid gas. There are two varieties of dry scrubbing systems:

70 POLLUTION ENGINEERING OCTOBER 199 1

spray dryer absorbers and dry injection absorption systems.

In spray dryer absorbers, the alkaline reagent, usually lime, is prepared in a slurry consisting of 5 to 20 by weight solids. The slurry is atomized into a large absorber vessel with a gas resi- dence time of 6 to 20 seconds. Slurry droplets must evaporate prior to exiting the absorber with the gas stream. Con- versely, drying too rapidly can reduce the eficiency of acid gas removal. To optimize performance, spray dry ab- sorbers are normally operated with exit gas temperatures 90" to 180°F above the saturation temperature. For particu- late control, these systems are normally followed by a fabric filtration system.

Dry injection absorption systems use a dry reagent, normally calcium hy- droxide or sodium hydroxide, to neu- tralize acid gases. The sorbent is nor- mally transported to the scrubber by a pneumatic conveyor which assists in fluidizing the material. The alkaline sorbent is injected under pressure coun- tercurrent into the gas stream. Alterna- tively, the sorbent may be injected di- rectly into a reaction chamber. After neutralization occurs, the gas stream containing the entrained sorbent is di- rected to a particulate control device, usually a fabric filter, which allows for further acid gas neutralization on the filter cake.

Fabric filters Fabric filtration systems are used ex-

clusively for particulate emissions con- trol. A fabric filter, sometimes referred to as a baghouse, is a collection of bags constructed of a fabric material and hung inside a housing. See Figure 4. Exhaust gases are drawn into the hous- ing, pass through the bags, and are dis- charged to an exit stack. Particulate matter is retained on the fabric material while clean gases pass through. Particu- late matter is removed by either me- chanical movement or pneumatic means and collected for disposal in a hopper below the structure.

The design criteria for a baghouse are the air-to-cloth ratio, bag material, op- erating temperature, chemical degrada- tion problems and cost. Air-to-cloth ra- tio is a measure of the flow rate of gas through a fabric filter to the area of the bags. Typical air-to-cloth ratios for waste combusters range from 5 to 10 acfm/ft3 of bag area.

Filter material should be selected based on abrasiveness of the gas stream, operating temperature requirements,

chemical degradation problems and cost. Operating temperature for a fabric filter used on a medical waste incinera- tor is of critical importance. Exhaust gases from such incinerators contain acid gases so the entire system should be maintained at temperatures in excess of the acid dewpoint. The boiling point of HCl is 230"F, so gas temperatures should be kept sufficiently higher to prevent condensation and subsequent corrosion. Gas stream temperatures should not exceed the thermal exposure limits of filtration material.

Pressure drops in fabric filters should be monitored frequently and recorded. Pressure drops below the minimum in- dicate leaks have developed or exces- sive cleaning has removed the benefi- cial base filter cake. Both conditions re- duce performance. Excessive pressure drop is the result of the blinding of the

Figure 4. A pulse jet baghouse is a fabric filtration system used for particulate emissions control.

bags by potential moisture condensa- tion or excessive cake buildup. This situation produces reduced system air flows and potential positive pressure in the combustion chambers.

Robert P. Newman, PE, is manager of air programs for EA Engineering, Sci- ence and Technology, Hunt Valley, Md.

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