Nicholas P. Cheremisinoff. Handbook of Solid Waste Management and Waste Minimization Technologies

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MASS INCINERATION TECHNOLOGIES FOR MSW

MSW has a broad compositional and size distribution. It is comprised of both

organic matter (combustible materials) and non-organic (non-combustible)

matter. Particle sizes range from dust to large, bulky materials like discarded

furniture and appliances. The specific composition in any one location is a major

factor in choosing from among reduction, reuse, and recycling strategies.

The average energy content of typical MSW is about 10,000 kilojoules per

kilogram (U/kg). Table 2 provides a breakdown of the heating value of MSW by

approximate composition of materials.

A typical mass incineration electricity generating plant would require about 45

tons of MSW to generate 1 megawatt (MW) of electricity of power for 24 hours.

For many large cities in North America, MSW can supply as much as 10% of the

electrical demand. Since the landfill tipping fees for many large municipalities

tend to be high, mass burning may be an economically attractive alternative. To

evaluate the merits of such an option, a full life cycle costing analysis (LCCA) is

needed, using information pertinent to the local economy and cost factors of the

region.

Table 2. Average Composition and Heating Values for MSW.

Waste Component

Paper and paper products

Plastic

Rubber and leather

Textiles

Wood

Food wastes

Yard wastes

Glass and ceramics

Metals

Miscellaneous inorganic

Weight %

37.8

4.6

2.2

3.3

3.0

14.2

14.6

9.0

8.2

3.1

Heating Value

(MJ/kg)

17.7

33.5

23.5

32.5

20.0

15.1

17.0

Mass burning is the oldest, simplest, and most widely used method of recovering

energy from MSW. Mass burn incineration typically involves waste throughputs

of between 10 and 50 tons per hour, with other types of incineration dealing with

throughputs of around 1 to 2 tons per hour.

Successful combustion of waste material relies on time, temperature and the

degree of turbulence. The specific incinerator design requirements with regards

to these parameters are as follows:

• Residence time in furnace - 2 seconds minimum for effective combustion

of any organic material

• Temperature - 85O

C average (measured via a time-temperature profile

throughout the furnace) for municipal and less offensive wastes

• Turbulence - High turbulence required in the combustion zone to avoid

dead spots or short circuiting in the furnace

The suitability of incineration as a waste management option depends upon the

properties and composition of the waste type, in particular its fuel properties. In

this respect, the proximate analysis (ash, moisture, and volatile contents) and the

ultimate analysis (of constituent elements) can be used to assess how a particular

waste stream will burn in an incinerator and the calculation of any potential

emissions. Typically, municipal solid waste can sustain combustion without the

requirement for auxiliary fuel where the moisture content falls below 60%, the

ash content is less than 25%, and the volatile fraction does not exceed 50%.

A schematic diagram of a typical mass burn incinerator with energy recovery is

illustrated in Fig. 2. The various operations and steps involved in a mass

incineration plant for MSW with energy recovery are as follows:

• Waste is deposited into a pit where an overhead crane removes oversized

items and mixes the waste to evenly distribute materials and moisture.

• The crane feeds the waste into a charging hopper from which it is fed on

to a grate, usually by means of a hydraulic ram.

• The grate agitates and transports the waste across the combustion

chamber, promoting combustion efficiency.

• Combustion air is introduced from under the grate (underfire air) and

from nozzles located in the furnace above the grate (overfire air).

Underfire air initiates combustion and cools the grate. Overfire air helps

to mix the combustion gases, ensures more complete combustion, and

reduces oxides of nitrogen (NO

• Non-combustible (i.e., inert) material and ash are discharged from the

end of the grate into a water quench tank. From there they are removed

for further treatment, and, ultimately, either recovered for use in

construction or other applications, or disposed of to landfill.

• Energy is transferred from the hot flue gases to water in the tubes of a

waterwall boiler, generating hot water and steam. The steam is used

either to turn a turbine to generate electricity or for local heating and/or

power combinations.

• The cooled flue gases pass through pollution control equipment

including scrubbers (for acid gas removal), electrostatic precipitators

(for dust removal) and/or fabric filters (for fine paniculate removal) and

sometimes activated carbon (for additional mercury and dioxin control)

before exhaustion to the atmosphere via a stack.

1 Collection vehicles 9 Steam boiler 17 Air-cooled condensers

2 Refuse bunker 10 Superheater 18 Ash discharger

3 Cranes 11 Economiser 19 Residue handling system

4 Feed hopper 12 Gas scrubber 20 Magnetic separator

5 Hydraulic ram feeders 13 Bag-house filter 21 Residue pit

6 Stoker grate 14 Induced draft fan 22 Lime storage silo

7 Forced draught fan 15 Turbine hall 23 Ash silo

8 Overfire air fan 16 Air preheater

Figure 2. Schematic of mass incineration system for energy recovery.

WATERWALL INCINERATION TECHNOLOGY

In this technology the raw MSW is incinerated directly in the furnace, usually

without any preprocessing of the waste. The primary product is steam, which can

be used directly or converted to electricity, hot water, or chilled water. A

schematic for a typical waterwall furnace for unprocessed MSW is illustrated in

Fig. 3 and further details are shown in Fig. 4.

This technology has been around for many years and is relied upon in various

industry applications. Coal-burning operations in the utility sector are prime

examples of the efficient use of this technology. The schematics shown in Fig. 3

and 4 provide a summary of the basic steps involved in the burning operation. As

with all combustion technologies, a solid waste residue or ash is left that

ultimately must be disposed of. The vast majority of this residue winds up in

landfills.

iomisi

crn iK

Figure 3. Waterwall furnace for unprocessed MSW.

Some installations perform shredding prior to burning to reduce the waste size.

This practice also facilitates material recovery. This adds equipment and

operating costs to the investment, so resource recovery is favored to offset some

of the OM&R costs and achieve an earlier payback for the investment. Sorting

practices for the raw MSW can reduce or eliminate the need for shredding, but

again these represent trade-off costs. Some of the responsibility for sorting can be

passed on to the generator by restricting the waste forms that are considered

acceptable for the mass incineration system.

Residue

Grates

Refuse Pit

Furnace

Unloading

Shed or Pit

Loading Crane

Convection

Section

Boiler

To Air

Pollution

Controls

Figure 4. Details of waterwall furnace system.

Additional resource recovery can be performed on the ash. Ferrous metal can be

separated out by magnetic separation after incineration is performed. Again, the

economics should be examined in a LCCA to determine if this is cost effective.

Waterwall incineration is not a new technology. The origins of this technology go

back to post World War II in Europe. Today's European designs favor several

Ash

1367°K

1200

Steam Drum

Generating

Tube Bank

1090

Waterwalls

Mud Drum

667°K

505-533

Economizer

Superheater

small modular furnaces operated in parallel. In the United States, the practice is

to use larger units that do not favor modular units.

SMALL-SCALE INCINERATION UNITS

Small-scale modular incinerators are capable of achieving heat recovery as steam

or hot water. They generally do not require materials recovery stages. Many

applications have focused on wastes from hospitals, schools, various institutions,

and industries where the wastes are more homogeneous than MSW.

For MSW the system designs generally involve small individual furnaces. Large-

scale incineration is accomplished by operating several identical units or

modules. MSW is usually incinerated in two stages. In the first stage, raw MSW

is combusted in a starved air environment (i.e., insufficient air is supplied so that

incomplete combustion occurs). This results in the formation of a combustible

gas and a by-product residue. The gas from the first or primary combustion stage

is then burned with an auxiliary fuel such as oil or natural gas in a secondary

combustion chamber with excess air. The hot gases from this chamber are passed

through a waste-heat recovery boiler (or a heat exchanger) in order to generate

steam, hot water, or hot air for space heating. Two-stage systems tend to be

more economical and have the advantage of reducing particulate air emissions.

FLUIDIZED BED INCINERATORS

Fluidized-bed incinerators consist of a bed of sand or similar inert material

contained in a chamber. The bed is fluidized by an upward flow of primary

combustion air. The waste is introduced to the preheated bed, where it is

efficiently dispersed and heated to the mixture's ignition temperature. There are

three main types of fluidized-bed furnaces, which can be identified according to

the flow dynamics of the two-phase system: bubbling, turbulent, or circulating

beds.

A wide variety of wastes have been treated in fluidized bed incinerators,

including MSW, sewage sludge, hazardous waste, liquid and gaseous wastes, and

wastes with difficult combustion properties. Incineration of MSW based on this

technology is best achieved with prescreening and shredding or the production of

RDF (refuse-derived fuel) pellets.

The advantages of the fluidized bed techniques include the following:

• There is a high combustion efficiency at a relatively low temperature,

with the waste feed normally being ignited spontaneously once the bed

has been preheated to around 500

• The operating temperature (circa 850

C) is ideal for in situ removal of

/SO

by dry means (limestone or dolomite addition).

• The bed cools very slowly, and a rapid startup is therefore possible after

8 to 16 hours of shutdown.

• It possesses good waste feed flexibility.

• It is capable of high conversion efficiency for power generation,

especially where the combustion is pressurized.

• It can achieve high heat transfer rates from the bed to the wall and bed

internals, leading to a strictly homogenous bed temperature.

The technology's disadvantages include the following:

• A waste feed particle size of less than 300 mm is required.

• A relatively high pressure drop is needed to fluidize a bed of granular

particles.

• The incinerator flue gas carries a high dust load.

• The regulation and control of fluidized beds is complicated.

• The possible silica incrustation of bed material limits the operating

temperature to around 850 to 950

• The wear on submerged surfaces, the occurrence of attrition upon bed

particles, the evolution of the particle size distribution, and the

composition of the bed material cannot yet be predicted because of

inadequate operating experience with fluidized bed combustors.

A fluidized-bed incineration system can effectively incinerate wet refuse, which

is usually difficult to burn without adding fuel, as well as sewage sludge and

night soil sludge. It can therefore eliminate problems such as the generation of

noxious odors from the putrefaction of refuse and sludge at landfill sites, which

attract flies and birds to cause nearby residents to complain. The sand in the

fluidized-bed incinerator (FBI) diffuses the intense heat which results when

plastics, which have extremely high calorific values, are burned. As a result,

there is no generation of clinkers that can be formed by irregular high

temperatures at various locations within the incinerator. Therefore, plastics can

be stably burned using an FBI. This means that refuse, including such plastics,

can be collected and easily incinerated. Since sand has great heat capacity, the

temperature in the incinerator does not greatly vary even after the incinerator has

been shut down. When the incinerator is restarted on the following day, its

internal temperature reaches the rated treatment capacity range almost

immediately, without the need to operate any auxiliary fuel firing equipment.

Also,

no unburnt combustibles remain after the operation is shut down, and as a

result there is no discharge of foul odors or smoke due to continuing combustion,

as is found with stoker-type incinerators. Fig. 5 illustrates the major components

of an FBI. The design illustrated is based on a classical fluid-bed system

configuration which has been around for many years.

Figure 5. Details of fluid-bed incinerator.

SAND

DISCHARGE

OUTLET

PURGE AIR

INLET

DIFFUSION

PIPE

AUXILIARY

FUEL INLET

REFRACTORY

MATERIAL,

HEAT

INSULATOR

FLUIDIZED BED ZONE

HOT AIR

INLET

STARTUP

BURNER

SAND SUPPLY INLET

FLUE GAS OUTLET

FREEBOARD

INSPECTION

HOLE

FEEDER

SLUDGE

Infilco Degremont, Inc. (see www.Equip.com and Infilco Degremont, Inc., P.O.

Box 71390, Richmond, VA 23255-1390, telephone: 804-756-7600) markets a

smokeless, odorless process to incinerate wastewater and other sludges. This

nonhazardous technology, which meets or exceeds air quality emission

regulations, is currently in use in the municipal and industrial markets. Gas

temperatures leaving the reactor are 1500

F, thus providing complete destruction

of odor-causing organics and eliminating the need for expensive, fuel-hungry

afterburners. Figure 6 shows a teardrop shape design which reduces freeboard

gas velocity and increases residence time for more enhanced combustion. There

are many industry sectors that utilize FBI or operate systems such as dryers and

various types of chemical reactors that rely on the principles of gas-solids

fluidization. It is a well-established technology with equipment scale-up

principles that are generally well understood. The most challenging issues

associated with FBI designs are the proper selection of the operating flow regime

(bubbling, churning, slugging). This is largely tied to the nature of the waste

being incinerated. Such characteristics as waste particle size, density, moisture

content, and whether or not agglomerates or so-called clinkers are formed can

greatly influence the fludization regime and the combustion efficiency.

Figure 6. Teardrop FBI configuration manufactured by Infilco Degremont, Inc.,

P.O.

Box 71390, Richmond, VA 23255-1390.

REFUSE-DERIVED FUEL SYSTEMS

Refuse-derived fuel (RDF) is a result of processing solid waste to separate the

combustible fraction from the noncombustibles, such as metals, glass, and

cinders in MSW. RDF is predominantly composed of paper, plastic, wood and

kitchen or yard wastes and has a higher energy content than untreated MSW,

typically in the range of 12,000 to 13,000 kJ/kg. This heating value will vary,

depending upon local paper and plastic recycling programs. Like MSW, RDF

can be burned to produce electricity and/or heat. RDF processing is often

combined with the recovery of metals, glass, and other recyclable materials in a

resource recovery facility, thereby improving on paybacks for investments and

offsetting OM&R costs. At the present time, RDF combustion is less common

than mass burning for MSW, but this may change in the future as recovery of

recyclable materials and environmental concerns over incinerator emissions

become more important.

Two major benefits of RDF are:

• It can be shredded into uniformly sized particles or densified into

briquets. Both of these characteristics facilitate handling, transportation,

and combustion. Easily handled, RDF can often be burned or co-fired

with another fuel such as wood or coal in an existing facility. RDF is

thus valuable as a low-cost additive, which can reduce the costs of

generating heat or electricity in a variety of applications. Mass burning

of MSW requires specially designed boilers to handle the uneven

composition of MSW.

• Fewer noncombustibles such as heavy metals are incinerated. Although

metals are inert and give off no energy when they are incinerated, the

high temperatures of a MSW furnace cause metals to partially volatize,

resulting in release of toxic fumes and fly ash. The composition of RDF

is more uniform than that of MSW; therefore fewer combustion controls

are required for RDF combustion facilities than for facilities burning

untreated MSW.

The majority of RDF combustion facilities generate electricity. On average,

capital costs per ton of capacity are higher for RDF combustion units than for

mass-burn and modular WTE units. RDFs dehydrate and the municipal solid

waste forms into small pellets or fluffy material which allows easy transportation,

storage, and combustible stability. Furthermore, efficient use of RDF as an

energy resource contributes to recycling strategies. Noncombustible materials

such as ferrous materials, glass, grit, and other materials that are not combustible

are first removed. The remaining material is then sold as RDF and used in

dedicated RDF boilers or coincinerated with coal or oil in a multifuel boiler.