GASIFICATION AND ASH-MELTING SYSTEM FOR RESIDUE MINIMIZATION

Chikashi Tame, and Keizo Taniguchi*

International Business Div., Global Marketing & Sales Group, EBARA Corp.,

1-6-27 Kohnan, Minato-ku, Tokyo Japan,

* Environmental Plant Division, Waste & Resource Plant Engineering, EBARA Corp.,

1-6-27 Kohnan, Minato-ku, Tokyo Japan

ABSTRACTThis paper covers Elements required in waste disposal and Gasification and ash-melting system to

solve or minimize problems in current waste treatment. Construction and features of Fluidized-bed

gasifier, Ash-melting furnace and other equipments is explained in detail with illustrations.

Keywords: Gasification, Ash Melting, Waste Disposal, Residue Minimization, Dioxin, Recycling

1. IntroductionReflecting the recent aspirations toward a recycling society, management of incinerated

residue for environmental conservation at final disposal sites and areas around incineration

facilities, as well as reductions of carbon dioxide favorable from the standpoint of

preventing global warming, are also becoming important elements in the waste disposal

field. These elements are important in addition to energy recovery and appropriate

management of exhaust gases.

High expectations are therefore placed on a gasification and ash melting system of a

resource recycling type that can prolong the disposal life of final disposal sites through

effective utilization of molten slag obtained in ash melting. This system renders exhaust

gases completely harmless and melts incineration ash, to reduce total emissions of the

PCDDs (polychlorinated-dibenzo-p-dioxins) that recently are regarded as most serious

toxic substances.

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2. Elements Required in Waste Disposal

At present, incineration is widespread in Japan and other countries as the most common

disposal method in waste disposal. Needless to say, the concept of the three Rs, namely,

reuse, recycle and reduce, to promote the use of waste as much as possible before

incinerating it, is important. Generally speaking, elements such as environment

conservation, prevention of global warming, suppression of carbon dioxide emissions, the

3Rs, economic efficiency and reduction of loads on final disposal sites are being given

importance.

1) Environmental Conservation

Needless to say, all elements must be considered in dealing with the environment.

Consideration of the exhaust gases and ash that are produced in incineration disposal,

which is thermal treatment, is the element carrying the largest impact. The critical point is

minimization of toxic substances contained in the waste gases and ashes. Dioxin that is

especially attracting attention throughout the world is a substance which needs most careful

attention. In Japan, the tolerable daily intake (TDI) is below 4pg-TEQ/kg.day and below

5μg/ton-waste as an area-wide total pollutant load control value when the load of dioxin on

the environment is considered. A regulatory value of below 0.1ng-TEQ/g has also been set

as the quantity that may be contained in ashes from the standpoint of preventing soil

pollution.

2) Prevention of Global Warming and Suppression of Carbon Dioxide Emissions

When considered from the standpoint of thermal treatment, the generation of carbon

dioxide is unavoidable. However, carbon dioxide emissions must be reduced to a

minimum. This means that an auxiliary fuel must not be used in thermal treatment and that

waste must be heat-treated by the heat value which the waste has itself. Needless to say,

smaller exhaust gas emissions altogether would be preferable.

3) Reuse, Recycling and Reduce

These three Rs are the basis of waste disposal. It is important to carry out the 3Rs to the

maximum before thermal treatment, so as to minimize carbon dioxide emissions. Pre-

treatment or pre-processing alone does not solve problems. What is important is how

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effectively and efficiently the approach to the 3Rs can be incorporated in thermal treatment.

Thermal recycling and material recycling must be introduced. In the past also, the basis of

thermal recycling has been to generate steam by incineration and to generate electricity

using the steam. Steam recovery and electricity generation must be performed at as high an

efficiency as possible. Recovery of metals and other materials contained in waste in the

process will reduce the labor required in pre-treatment or pre-processing.

4) Economic Efficiency

Economic efficiency is an important element for the business entities that install and

operate plants, regardless of what treatment or processing these plants employ. There are

many factors that need be considered, such as low construction costs, small installation

footprint and, needless to say, low running costs.

5) Reduction of Loads on Final Disposal Site

Incineration inevitably generates residue and ash. Generally speaking, these residue and

ash are brought to final disposal sites for landfill. The volume of residue and ash for

landfill must be reduced to prolong the disposal life of landfill sites. A shortage of landfill

sites has become a serious problem in countries like Japan which have small land areas.

3. Gasification and Ash Melting Technology

Technologies that will solve the foregoing problems or that will minimize these problems

will be needed in future waste disposal. One of the hints in this direction is the gasification

and ash melting technology for waste that is described below. Gasification and ash melting

technologies are varied in type. Basically, three types are in use at present: (1) Fluidized

bed gasification + melting furnace, (2) Kiln-type gasification + melting furnace, and (3)

Shaft furnace.

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1) Drastic reduction of PCDDs (polychlorinated-dibenzo-p-dioxins)

The installation of a melting furnace enables treatment at a high temperature. Generally

speaking, melting furnaces are operated at a temperature higher than 1300°C. This

drastically reduces generation of dioxin. Fig. 1 shows the theoretical value of the residence

time needed to achieve pyrolysis of dioxin.

Fig1 Residence Time Needed to Achieve Pyrolysis of Dioxin (Theoretical Value)

2) Molten Ash Slag

Ash contained in waste melts in the high-temperature treatment and becomes molten slag.

This molten slag is very stable. It not only prevents secondary pollution at landfill sites, but

also paves the way for the development of new uses, such as road materials. Needless to

say, ash is melted and its volume reduced, thereby contributing to prolonging the disposal

life of landfill sites.

3) Economic Efficiency

Compared with ordinary incineration, the gasification process requires only a very small

amount of air and, as a result, exhaust gas volume is small. The gasification process offers

the following features that contribute to high economic efficiency:

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● Reduces sizes of equipment for waste heat recovery and waste gas treatment

● Reduces sizes of buildings for housing equipment

● Reduces motive power of fans for inducing exhaust gases

● Increases boiler efficiency and power generation efficiency as a result

4) Effective Utilization of Residue

In general, the gasification process gasifies around a temperature of about 500°C so that

metals contained in waste can be recovered before they melt. Thus, aluminum and other

nonferrous metals can also be recovered in addition to ordinary metals. Molten slag has the

potential to be used for various applications as mentioned above.

3.1 Concept of Gasification and Ash Melting Technology

The characteristics of the various technologies are illustrated below.

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Fig. 2 Fluidized Bed Gasification

3.2 Features

As an example, the features of the gasification and ash melting furnace of the fluidized bed

type are described in detail. (See Fig. 2.)

This system combines a gasification furnace, which is based on the fluidized-bed

incineration furnace of proven reliability through many years’ municipal waste incineration

experience, and a cyclonic flow melting furnace, which has been confirmed as reliable in

melting-sewage-sludge treatment. The gasification furnace offers the following features:

(1) Stable gasification

(2) Adaptability to waste quality variations

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(3) Recovery of valuable metals in an unoxidized condition

(4) Excellent incombustible discharge performances

The cyclonic-flow melting furnace has the following features:

(1) Almost complete decomposition of PCDDs (polychlorinated-dibenzo-p-dioxins)

by combustion at a high temperature (1250 to 1450°C)

(2) High slagging ratio by high-temperature combustion and cyclonic flow

(3) Slag can be discharged continuously in small quantities.

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3.3 Gasification Furnace

Fig. 3 illustrates the features of the gasification furnace.

Fig. 3 Revolving Type Fluidized Bed

Combining the functions of fluid air, of the deflector and of the air dispersion plate, sand, a

fluid medium, forms a powerful revolving flow from all directions toward the center of the

furnace inside the cyclonic-flow type furnace without the installation of a mechanical

moving part. As a result, the following effects are achieved to accomplish stable

gasification furnace operation:

(1) Effect of destruction by a cyclonic flow allowing disposal of more bulky waste

compared with ordinary fluidized bed.

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Gasification Gasification

Deflector

Deflector

Partial Oxidation

Partial

Oxidation

Incombustibles + Valuable Metals

Incombustibles + Valuable Metals

Air Chamber

Fluid Air (High Velocity)

Fluid Air (High Velocity)

Fluid Air(Low Velocity)

(3)

Temperature in Gasification Part:

550 - 630°C

Air Dispersion Plate

(1)

(2)

Waste

Product Gas

(2) Homogenization of the load inside the furnace by revolving flow, contributing to

stable gasification inside the furnace.

(3) Achievement of a considerable effect in discharge of incombustibles contained

in waste by cyclonic flow.

(4) Selective combustion of carbon contained in combustibles, to maintain stable

combustion.

Fig. 4 illustrates the discharge mechanism of incombustibles and metals.

Fig. 4 Discharge Mechanism of Incombustibles

Incombustibles contained in waste charged into the gasification furnace move smoothly

through the cyclonic flow of sand to incombustible removal openings provided on the

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Aluminum Incombustible

Turned into molten slag in melting furnace

Iron (Unoxidized)

Magnetic Separator

Fluid Sand

Incombustible Discharge Conveyor

Vibrating Screen

Aluminum Separator

entire circumference of the furnace bottom. Incombustibles discharged by the

incombustible removal openings and chutes are separated further into sand and

incombustibles by a vibrating screen. Iron in incombustibles, which are now free of sand,

is selected by a magnetic separator. Aluminum in incombustibles is recovered by an

aluminum separator. After these separations, incombustibles are pulverized and are melted

in a melting furnace.

The furnace bed temperature of the gasification furnace is below the aluminum melting

temperature, thereby allowing recovery of iron and aluminum in the same way as normal

incombustibles. All discharged materials are clean and dry and handling, including

separation of metals, is very easily.

3.4 Melting Furnace

The temperature in the cyclonic flow melting furnace increases uniformly to a high

temperature (1250 to 1450°C) through the cyclonic effect of gas inside the melting furnace,

to produce almost perfect decomposition of generated PCDDs (polychlorinated-dibenzo-p-

dioxins). Precursors are decomposed to prevent resynthesis of PCDDs (polychlorinated-

dibenzo- p-dioxins). Through the cyclonic effect of the gas inside the melting furnace, the

primary chamber of the melting furnace acts as a cyclone so that fly ash can be collected

easily on the furnace walls. For this reason, the slagging ratio increases and less fly ash is

generated. Slag is discharged continuously in small amounts and danger through human

handling or from steam explosion can be ruled out.

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Discharge of Slag

Granulated Slag

Startup Burner

Tertiary Combustion Chamber

Secondary Combustion Chamber

Cyclonic Flow Melting Furnace

Primary Combustion Chamber

Molten Slag

Air

Temperature in Melting Part:

1250 - 1450°C

Fig. 5 Features of Cyclonic Flow Melting Furnace

3.5 Features of Process

Fig. 6 Fluidized Bed Type (Ebara)

Using the process examples of the fluidized bed system, an approach to anti-pollution

measures is described.

● Gasification furnace:

Calcium in waste reacts with hydrogen chloride and sulfur oxides on the

fluidized bed to suppress generation of these pollutants.

● Melting furnace:

Dioxin undergoes decomposition by high temperature.

● Waste heat boiler:

Waste heat boiler and air pre-heater have a construction that assures maximum

possible exhausting of gases and reduces accumulation of gases to prevent

resynthesis of dioxin.

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● Gas cooler:

Gas cooler cools the temperature of exhaust gases at the inlet of the toxic gas

treatment system to below 150°C, to prevent resynthesis of dioxin.

● Dust precipitator:

The dust precipitator is installed to collect fly ash. By installing a two-stage dust

precipitator, fly ash is collected by the first stage and is sent back to the melting

furnace, to achieve a higher slagging ratio. The second stage catches remaining

fly ash and causes it to react with hydrogen chloride and sulfur oxides to remove

such hydrogen chloride and sulfur oxides.

● Catalytic reactor:

Heats up exhaust gas to a temperature suitable for dioxin decomposition (about

200°C) by reheating and feeds the gas to the catalytic reactor, which is capable

of decomposing dioxin.

3.6 Dioxin

Fig. 7illustrates an approach to dioxin at the stage of each process. As illustrated in this

diagram, consideration regarding dioxin at each stage of the process is thorough and

minutely thought- out.

Fig. 7 Concentrations of PCDDs (Polychlorinated-dibenzo-p-dioxins) in Emissions

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3.7 Recycling and Effective Utilization

Type of Residues Valuable Metals Molten Slag Fly Ash Remarks

Standards Conformance

Requirement

Soil environmental

standard

Soil environmental

standard

Landfill standard

Elution Value of Heavy

Metals

○ ○ ○(Chemical treatment)

PCDDs (Polychlorinated-

dibenzo-p dioxins)

ND (Not detected) 0.00ng-TEQ/g or less 0.1ng-TEQ/g or less Landfill standard

3ng-TEQ/g or less

Application of Recycled

Resources

Can be sold as

valuable metals

Road pavement

materials, cement

aggregate, backfill

materials, etc.

Return to mines (Future

plan)

Recycling Effective utilization Effective utilization Effective utilization

Table 1 Residues in Gasification and Ash Melting Process

Approaches to the balance naturally vary greatly depending on the properties of waste.

Valuable metals recovered in the gasification furnace can be sold and molten slag produced

by the melting furnace can be used as a road paving material, cement aggregate and

backfilling material. Fly ash recovered from the bag filter, a dust precipitator, is expected

to be returned to mines in the future.

Slag meets the soil environmental standard value for elution of heavy metals. Sandy slag

which has undergone disintegration by the disintegrator can be effectively used in terms of

specific gravity, amount of water absorption, stability and grain size. Slag has been used as

an asphalt pavement material and as secondary concrete product.

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3.8 Economic Efficiency

The system excels in initial costs for the following reasons:

(1) Exhaust gas volume is small compared with the combustion process and exhaust

gas treatment equipment can be built compactly. For this reason, the initial cost

can be low.

(2) For the reason outlined above, the building to house the equipment can be built

small.

The system excels in terms of running costs for the following reasons:

(1) High-efficiency power generation as less exhaust gas is emitted.

(2) Auxiliary materials such as coke and limestone are not needed.

(3) Chemicals to treat dioxin are not needed.

3.9 Summary

The features of gasification and ash melting technology are summarized below:

(1) Environmental conservation

1) Minimum PCDDs (polychlorinated-dibenzo-p-dioxins) due to high temperature

melting furnace.

2) Special auxiliary materials need not be charged from the outside

Less carbon dioxide is generated.

3) Long disposal life of final disposal sites

Less fly ash generated through a high slagging ratio.

(2) Economic efficiency

1) The system is simple and compact so that initial cost can be low.

2) High-efficiency power generation as less exhaust gas is emitted.

3) Auxiliary materials such as coke and limestone are not needed

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Chemicals to treat dioxin are not needed.

4) Valuable metals can be recovered in an unoxidized condition.

(3) Operation and Safety

1) Safe system that precludes explosions or gas leakages.

2) Stable gasification even when waste quality varies.

3) Easy operation because this system is a combination of conventional

technologies.

3.10 Examples of Construction Sites

Fig. 8 shows examples of construction sites.

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■ Delivery to Sakata Area Clean AssociationPlant Scale: 196 t/day Completion: March 2002

Guaranteed Exhaust Gas Value (O2 12% Equivalent)

■ Delivery to City of Kawaguchi Plant Scale: 420t/day Completion: November 2002

Guaranteed Exhaust Gas Value (O2 12% Equivalent)

Environmental Conservation item

Stack Outlet Concentration

Nitrogen Oxide Concentration

100ppm or less

Sulfur Oxide Concentration 20ppm or less

Hydrogen Chloride Concentration

50ppm or less

Dust Concentration 0.01g/m3 (NTP) or less

Concentration of PCDDs 0.1ng-TEQ/m3 (NTP) or less

Environmental Conservation item

Stack Outlet Concentration

Nitrogen Oxide Concentration

50ppm or less

Sulfur Oxide Concentration 10ppm or less

Hydrogen Chloride Concentration

10ppm or less

Dust Concentration 0.01g/m3 (NTP) or less

Concentration of PCDDs 0.05ng-TEQ/m3 (NTP) or less

Fig. 8 Examples of Construction Sites

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3.11 Reference of the Plant

Reference of the plants are listed below. Out of ten plants, eight plants are in full operation and remaining plants are under construction.

No. Location Operating Entity Capacity U

ni

t

Power

Generation

(KW)

Plant Completion Remarks

1 Aomori Aomori RER Corp. 450 t/day 2 17,800 March, 2000 Industrial waste treatment plant

2 Niigata Joetsu Widearea Administrative

Association

15.7 t/day 1 - March, 2001 Night soil sludge

3 Toyama Nikko Mikkaichi Recycling Corp. 63 t/day 1 - June, 2002 Waste plastics, copper slag

4 Yamagata Sakata Area Clean-environment

Association

196 t/day 2 1,990 March, 2002 Municipal waste, night soil sludge

5 Saitama Kawaguchi Municipality 420 t/day 3 11,700 November, 2002 Municipal waste, waste incineration ash

6 Yamaguchi Ube Municipality 198 t/day 3 4,000 November, 2002 Municipal waste, night soil sludge

7 Gifu Chuno Widearea Administrative

Association

168 t/day 3 1,980 March, 2003 Municipal waste, landfill and incineration

residues

8 Nagano Minami-shinshu Widearea Federation 93 t/day 2 700 March, 2003 Municipal waste

9 Chiba Nagareyama Municipality 207 t/day 3 3,000 February, 2004 Municipal waste, night soil sludge

10 Selangor Malaysia Government 1500 t/day 5 28,000 December, 2006 Municipal waste

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