POLLUTI

Incineration Technologies for Managing Solid Waste ~~

Most solid waste management plan- ning recognizes incineration as a viable alternative, although the public percep- tion of waste incinerators as a major source of air pollution and hazardous emissions has made it a less popular alternative than the realities justify. The technologies available and the emissions standards mandated by the

Modern particularly waste-to-

a viable option for solid waste management.

technologYy can be

Clean Air Act Amendments of 1990, however, mitigate the risks to public health and make incineration an alter- native worth exploring.

Incineration can reduce solid waste to 85 percent to 90 percent of the in- coming volume, or 65 percent to 80 percent of the incoming weight. With some modification, a waste incinerator can be designed to recover energy in the form of steam, hot water or electric- ity. This latter type is known as a re- source recovery or waste-to-energy fa- cility. Most newer waste incinerators are the waste-to-energy type.

Incineration facilities can be very costly to develop, but once operational, waste disposal costs tend to remain

by Bruce Bawkon

fairly stable. Also, life-cycle costs can be significantly reduced through the re- covery and use or sale of thermal and/or electric energy. Although all incinera- tors now require pollution control de- vices, there remain issues and concerns about air quality, ash disposal and envi- ronmental impacts.

Technology Regardless of the specific technolo-

gies used, all incinerator types have common features. Each has a receiving area where waste is deposited and me- chanical processing systems remove non-combustible material and provide for a level of material recovery before combustion. Waste is then fed into a combustion chamber. The combustion gases pass through heat exchangers and pollution control equipment before be- ing discharged through a stack into the atmosphere.

The most common and proven meth- ods used to combust solid waste are mass burn, refuse derived fuel (RDF) and starved air modular combustors.

96 POLLUTION ENGINEERING SEPTEMBER I99 1

In a waste-to-energy facility, combus- tion heat is used to produce steam, hot water or electricity for use in the plant or for sale to outside energy markets. Bottom ash or residue, approximately 10 percent of the original waste vol- ume, is removed from the combustion chamber.

Mass burn combustion In this country, most large incinera-

tors are mass bum facilities. Refuse is burned in the same form as it is deliv- ered with the exception that some large metal items are removed from,the waste stream. This technology has been used s i n e the 1970s and has experienced the greatest technical and financial operat- ing success. Typical unit size is in the range of 400 to 1000 tons per day (TPD) with some facilities as large as 3000 TPD.

The air required for combustion is supplied by air ducts below the direct- combustion grate system, as well as by secondary air injectors above the grate system around the fire zone. The under- fire primary air system is usually sec- tioned so that a series of supply points and quantities of underfire air (the pri- mary combustion air) can be sectionally adjusted to aid in combustion control. The overfire air jets provide oxygen to complete the combustion of gases ex- pelled from the primary combustion area. Air jets also allow, in conjunction

with the furnace and boiler size, the proper time, temperature and turbu- lence necessary for complete combus- tion of the gas stream.

Two arrangements can be used for steam generation. In a waterwall unit, shown in figure 1, the furnace walls are lined with boiler water tubes in which water is heated to the boiling point by the combustion process. The other ar- rangement involves a dedicated boiler where steam is generated by hot com- bustion gases after leaving the furnace.

Key advantages of mass bum facili- ties relate to their well established and proven technology, demonstrated long- term reliability, good thermal efficiency and minimal refuse processing require- ments. Disadvantages relate to the long lead times required to design and build plants and their significant capital construction cost.

Starved air (modular) combustion Starved air modular incinerators are

relatively small combustion and heat recovery systems typically ranging in size from five to 100 TPD capacity. See Figure 2. Multiple units can be in- stalled when greater capacity is needed.

Starved air incineration of solid waste is achieved in either two- or three-stage systems. Partial combustion occurs in the primary chamber produc- ing a gas with a low energy content. A starved air environment (less than stoi-

chiometric gas conditions) is created in the primary chamber restricting the amount of air fed into the chamber. Combustible gases produced in the pri- mary chamber are then completely burned in the secondary chamber. A waste heat boiler typically is used to recover the energy of combustion.

Once waste enters the primary cham- ber of a typical controlled-air system, a three-stage reaction occurs:

Drying Gasification. Burnout. A two-chamber configuration sup-

ports the three-stage reaction. Combus- tion temperature typically ranges from

Two-stage controlled air combustion technology limits air pollutant emis- sions when burning solid waste. The relatively low combustion temperature in the primary chamber aids' in pollu- tion control by minimizing the vapori- zation of the metallic components of the waste, as well as slagging of the glass components. Gases generated in the primary chamber are transferred to the secondary chamber for high-tempera- ture burning to allow for more complete combustion.

1500"F.to 1800°F.

Figure 1. The furnace walls of a waterwall unit are lined with boiler water tubes.

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* I Refuse derived fuel combustion uses trash as fuel, reducing a plant’s need for other fuel sources.

Other key advantages relate to fast construction time, relatively low con- struction cost and flexibility. Disadvan- tages are limited size, lower thermal ef- ficiency, higher maintenance costs and shorter equipment life.

Refuse derived fuel combustion Refuse derived fuel (RDF) is a sys-

tem whereby refuse is used as fuel for a plant producing energy. This reduces or eliminates the plant’s need for other fuels. RDF systems are different from mass bum technologies because they are designed to separate solid waste into combustible and non-combustible factions.

The waste stream is sorted and proc- essed, with non-combustibles, such as metal and glass, removed to be recycled or landfilled. The combustible portion of the waste has a higher energy content and is a more efficient fuel.

There are two RDF technologies cur- rently available in this country:

RDF combusted in grate boilers, where RDF is burned on the surface of a grate. RDF combusted in fluidized-bed boilers, where RDF is burned in sus- pension by an upward flow of com- bustion air in a bed of sand or lime- stone.

An RDF facility is actually two facili- ties: a preprocessing or fuel plant; and a combustion and energy recovery plant. These facilities do not have to be located at the same site. A processing facility removes non-combustibles and prepares a fuel by shredding the com- bustible fraction. Additional processing of the fuel, including adding fuel amendments and pelletizing the RDF fuel, would be performed only if re- quired by the combustion facility. An RDF facility can be designed to bum RDF exclusively at a dedicated waste combustion facility or combined with other fuels to co-fire it with sludge, wood chips, peat or coal. Typi- cally, an RDF plant produces steam or electricity.

RDF with dedicated boilers involves both production and combustion of RDF fuel. This combined function re- quires the construction of a dedicated boiler to burn the RDF.

RDF with fluidized-bed combustion units burn finely processed refuse in a turbulent bed. The bed contains a flu- idizing medium of inert particles kept in a state of agitation and fluidity by a high velocity flow of combustion air in- troduced into the bottom of the com- bustor through a series of nozzles. The boiler section is located above the com-

bustion process. The bed media may be sand or limestone. Limestone beds also help decrease acid gas emissions.

There are two types of fluidized-bed combustors used with RDF. The first, bubbling-bed, suspends the RDF along with ash and inert material such as sand, with an upward flow of combus- tion air. The combustion process and bed media are contained in a combus- tion vessel usually designed to recover energy through a system of boiler tubes. The bed material, ash and other inert material remain in the combustor. Hot gases are ducted to a boiler and air pol- lution control system.

The second type, circulating flu- idized-bed, shown in Figure 3, com- busts the RDF in a medium of ash and inert material with an upward flow of combustion air that forces some of the bed material to pass out of the primary combustion into a cyclone separator. The cyclone separates the inert particles from the hot exhaust gas. The particles are returned to the combustion cham- ber to be added to the bed material. The hot gases are ducted to the boiler

Figure 2. Starved air modular incinerators involve either two- or three- stage systems.

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land air pollution control system. Fluidized-bed combustion units re-

quire a high quality RDF product with nearly all ferrous materials, glass and aluminum removed. Several units are operating suceessfully in Europe with loose and pelletized RDF, as well as other fuels.

Bubbling-bed combustion offers sev- eral advantages over conventional mass

1 burn combustion processes. A more ho- mogeneous fuel is burned and the fuel is more completely mixed in the com- bustion chamber for more efficient

!combustion. Addition of lime to the bed media can promote desulfuriza- tion. The combustion process is more stable as a tremendous amount of heat is absorbed in the bed media and can maintain a constant combustion tem- perature as variations in the fuel occur. Therefore, a more steady control of nitrogen oxide emissions can be maintained.

Combustors represent a major depar- ture from the waste-to-energy technolo- gies in that the circulating fluidized-bed allows for several advantages over the more common mass burn and bubbling- bed combustors. The circulating flu-

Figure 3. Circulating fluidized-bed incincerators combust refuse derived fuel in a medium of ash and inert material.

idized-bed can accommodate greater fluctuations in fuel heat content by varying the density of the bed. The tur- bulent mixing promotes an improved combustion efficiency and can impact reduced dioxin emissions. Boiler efi- ciency can approach 80 percent d m to low excess air requirements, which ape

greater than that typically expected from mass bum and bubbling-bed com- bustors. Turn down capabilities are sig- nificantly greater, with operating capac- ity ranging from 25 percent to 100 per- cent of capacity.

Advantages of RDF systems relate to more homogeneous fuel stock, higher

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Most modern waste incineration plants use combustion heat to produce thermal and/or electric energy.

Particulate material (PM)

Lead (Pb)

Acidic acids

Hydrogen chloride

Hydrogen fluoride

Chromium

Nickel

Lead

Organic material

Polychlorinated dibenzo-pdioxins (PCDD)

Polychlorinated dibenzofurans (PCDF)

Polynuclear aromatic hydrocarbons (PNAH)

heat content, ability to be burned as supplemental fuel and compatibility with recyclable material recovery. Key disadvantages deal with extensive and expensive equipment needs for process- ing and lack of demonstrated long-term reliability.

Disadvantages of the fluidized-bed combustors include a requirement for a refined, preprocessed fuel, with a con- trolled size for the RDF. This includes a preprocessing facility that efficiently removes glass to prevent slagging in the bed, and control and monitoring of ero- sion in the combustor walls caused by the recirculation of the sand bed mate- rial. This can be accomplished through design of the duct work and periodic inspection.

Finally, the most important disad- vantage is the limited operating experi- ence in the U.S., as compared to mass burn and RDF combustion on a grate.

Historically, facilities using fluidized- bed combustion have been more expen- sive than conventional boiler facilities.

Resource recovery and cogeneration Most modern waste incineration

plants are designed to use combustion heat to produce thermal and/or electric energy. Resource recovery facilities vary with respect to the energy products produced and the required energy gen- eration systems.

Thermal energy generated from solid waste in the form of steam, hot water or hot air, may be used for space heat- ing and cooling, process heating and for the production of power, including electricity.

The most common form of thermal energy from waste is steam. Steam tem- perature requirements generally range from 250°F to 1000°F. Steam produced from solid waste is identical to, and in- distinguishable from, steam produced by other fuels.

Electrical energy is produced at a waste-to-energy facility by passing steam produced by the incinerator's boiler system through a turbine genera- tor. Within the turbine, entering steam is expanded through nozzles, trans- forming the steam's energy to velocity. This expanded steam jet then strikes blades on a turbine wheel, forcing the wheel to revolve. This mechanical power is used to turn a generator, pro- ducing electricity. Electrical energy can be used in the facility, sold to nearby industries, sold to the local utility or transmitted to other utilities or customers.

The design of the turbine system must match the types and specifications of the energy products to be sold, in- cluding electricity and steam. Gener- ally, turbines can be classified as con- densing, non-condensing (back pres-

sure), or extraction turbines. The type of turbine selected will be dedicated by the requirements of power or thermal energy markets.

In tons of actual energy production, each ton of solid waste can produce ap- proximately 5500 pounds of exportable steam or 550 kilowatt-hours of export- able electricity. The energy equivalence of solid waste and fossil fuels is shown in Table 1.

Ash and residue disposal Incineration still leaves a residue of

ash and inert material that comprises 10 to 15 percent of incoming volume or 20 to 35 percent of incoming weight. This residue must be landfilled.

Due to concerns about content and hazard potential, incinerator ash is clas- sified as a special waste in most states and cannot be co-disposed with other solid waste at a conventional landfill. See Table 2. The safest current disposal mode is a lined monofill (only incinera- tor ash is disposed in the landfill cell, and is not mixed with other refuse) equipped with leachate collection and monitoring systems.

Impact of recycling on waste-to-energy

A popular myth is that recycling un- dermines waste-to-energy facilities. Both field tests and quantitative deriva- tions show that combustion plants benefit from the removal of recyclable materials from the waste stream prior to incineration. Recycling increases the heat content of municipal solid waste while reducing air pollution. Removal of batteries from the waste stream is particularly beneficial because it can significantly reduce heavy metal emissions.

A recent study for the University of Illinois shows that if 50 percent of the paper and all plastic, metal and glass were removed from the waste srream, its heat value would increase from 5500 Btu/lb to 6 148 Btu/lb.

Similar conclusions were reached from a series of tests run by National Recovery Technologies Inc. (NRT). In addition to increasing heat value, recy- cling increased boiler efficiency and re- duced ash quantities. Also, it resulted in the reduction of carbon monoxide, hydrocarbon, heavy metal and acid gas emissions.

Air emissions control The Clean Air Act Amendments of

1990 establish area classifications based

100 POLLUTION ENGINEERING SEPTEMBER 199 1

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*< The Clean Air Act Amendments require EPA to establish waste incinerator standards.

I Target I

on ambient air quality, and identify the performance and criteria required for threshold controls for major sources.

The Act calls for the Environmental Protection Agency (EPA) to establish by Nov. 15, 1991, waste incinerator standards that provide maximum re- ductions in air emissions, taking into account costs, health and environ- mental impacts and energy require- ments. Standards for new sources must be no less stringent than those achieved in practice by the best controlled simi- lar unit or Best Available Control Tech- nology as determined by EPA. Combus-

tion practices recommended by EPA are listed in Table 3.

Incinerator standards must include numerical limits for particulate matter, opacity, sulfur dioxide, hydrogen chlo- ride, oxides of nitrogen, carbon monox- ide, lead, cadmium, mercury, and diox- ins and dibenzofurans. For other pollut- ants, EPA may identify numerical lim- its or require monitoring of surrogate substances, parameters or residence times.

The Act requires EPA to establish monitoring and operation guidelines for incinerators. EPA must issue

i

a model state program for operator training.

Bruce Bawkon is senior environment scientist with Envirodyne Engineers Inc., Chicago, Ill.

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102 POLLUTION ENGINEERING SEPTEMBER 1991