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
ABSTRACT
This 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.
Introduction
Reflecting 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.
1
/16
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.
minimum.
However, carbon dioxide emissions must be reduced to a
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
effectively and efficiently the approach to the 3Rs can be incorporated in thermal treatment.
2
/16
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.
ash are brought to final disposal sites for landfill.
Generally speaking, these residue and
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.
and ash melting technologies are varied in type.
Gasification
Basically, three types are in use at
present: (1) Fluidized bed gasification + melting furnace, (2) Kiln-type gasification +
melting furnace, and (3) Shaft furnace.
3
/16
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.
drastically reduces generation of dioxin.
This
Fig. 1 shows the theoretical value of the
Residence Time (sec)
residence time needed to achieve pyrolysis of dioxin.
100
10
1
0.1
0.01
0.001
600
Source: Guide for Reducing PCDDs
700
800
900
1000
1100
1200
1300
(polychlorinated-dibenzo-p-dioxins)
in Waste Disposal, Masakatsu
Exposure Temperature (°C)
Hiraoka and Shigenobu Okajima
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:
●
Reduces sizes of equipment for waste heat recovery and waste gas treatment
4
/16
4)
●
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
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.
Fig. 2
Fluidized Bed Gasification
5
/16
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
(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.
6
/16
3.3 Gasification Furnace
Fig. 3 illustrates the features of the gasification furnace.
Product Gas
Temperature in
Gasification Part:
550 - 630°C
Waste
Deflector
Deflector
(2)
Gasification
Partial
Gasification
Oxidation
Partial
Oxidation
(3)
Air Chamber
Air Dispersion
Plate
(1)
Incombustibles
+ Valuable Metals
Fluid Air
(High Velocity)
Fig. 3
Fluid Air
(Low Velocity)
Incombustibles
Fluid Air
+ Valuable Metals
(High Velocity)
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.
7
/16
(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.
Incombustible Discharge Conveyor
Vibrating Screen
Fluid Sand
Magnetic
Separator
Aluminum Separator
Incombustible
Aluminum
Turned into molten slag in
melting furnace
Fig. 4
Iron (Unoxidized)
Sold to metal dealer
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
entire circumference of the furnace bottom.
8
Incombustibles discharged by the
/16
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
(polychlorinated-dibenzo-p-dioxins).
decomposition
of
generated
PCDDs
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.
increases and less fly ash is generated.
For this reason, the slagging ratio
Slag is discharged continuously in small amounts
and danger through human handling or from steam explosion can be ruled out.
Startup Burner
Primary Combustion Chamber
Discharge of Slag
Tertiary
Combustion
Chamber
Air
Secondary Combustion Chamber
Temperature in
Melting Part:
1250 - 1450°C
Molten Slag
Granulated Slag
Cyclonic Flow Melting Furnace
Fig. 5 Features of Cyclonic Flow Melting
Furnace
9
/16
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.
10
/16
●
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
11
/16
12
/16
3.7 Recycling and Effective Utilization
Type of Residues
Valuable Metals
Molten Slag
Fly Ash
Standards Conformance
Requirement
Soil environmental
standard
Soil environmental
standard
Landfill standard
Elution Value of Heavy
Metals
○
○
Remarks
○
(Chemical treatment)
PCDDs
(Polychlorinated-dibenzo-p
dioxins)
ND (Not detected)
0.00ng-TEQ/g or less
0.1ng-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
Landfill standard
3ng-TEQ/g or less
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.
13
/16
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)
3.9
Chemicals to treat dioxin are not needed.
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
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.
14
/16
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.
■ Delivery to Sakata Area Clean Association
Plant Scale: 196 t/day
Completion: March 2002
■ Delivery to City of Kawaguchi
Plant Scale: 420t/day
Completion: November 2002
Guaranteed Exhaust Gas Value
(O2 12% Equivalent)
Guaranteed Exhaust Gas Value
(O2 12% Equivalent)
Environmental
Conservation item
Stack Outlet Concentration
Environmental
Conservation item
Stack Outlet Concentration
Nitrogen Oxide
Concentration
100ppm or less
Nitrogen Oxide
Concentration
50ppm or less
Sulfur Oxide Concentration
20ppm or less
Sulfur Oxide Concentration
10ppm or less
Hydrogen Chloride
Concentration
50ppm or less
Hydrogen Chloride
Concentration
10ppm or less
Dust Concentration
0.01g/m3 (NTP) or less
Dust Concentration
0.01g/m3 (NTP) or less
Concentration of PCDDs
0.1ng-TEQ/m3 (NTP) or less
Concentration of PCDDs
0.05ng-TEQ/m3 (NTP) or less
Fig. 8
Examples of Construction Sites
15
/16
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
Unit
Power
Plant Completion
Remarks
Generation
(KW)
1
Aomori
Aomori RER Corp.
450 t/day
2
17,800
March, 2000
Industrial waste treatment plant
2
Niigata
Joetsu Widearea Administrative
15.7 t/day
1
-
March, 2001
Night soil sludge
63 t/day
1
-
June, 2002
196 t/day
2
1,990
March, 2002
Association
3
Toyama
Nikko Mikkaichi Recycling Corp.
4
Yamagata
Sakata Area Clean-environment
Waste plastics, copper slag
Municipal waste, night soil sludge
Association
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
168 t/day
3
1,980
March, 2003
Association
8
Nagano
Minami-shinshu Widearea Federation
9
Chiba
Nagareyama Municipality
10
Selangor
Malaysia Government
Municipal waste, landfill and incineration
residues
93 t/day
2
700
March, 2003
207 t/day
3
3,000
February, 2004
1500 t/day
5
28,000
December, 2006
16
/16
Municipal waste
Municipal waste, night soil sludge
Municipal waste