C o n t r i bu t i o n o f Wa s t e t oEnergy Technology to GlobalWarming
MASAO TAKUMA* 1
YUUJI KAIHARA* 2
AKIHIRO YAMADA* 2

Based on the additional new energy measures in the “Guideline of Measures to prevent Global Warming,” a target
has been set in Japan to introduce the waste-to-energy generation amounting to 4.17 million kW by the year 2010.
This report describes the waste-to-energy technologies of Mitsubishi Heavy Industries (MHI) in correspondence withthe aforesaid measures. The report particularly gives an outline of the activities and achievements of MHI in theimprovements of power generation efficiency through high-temperature and high-pressure boiler for waste-to-energy generation, the improvements in heat recovery efficiency through oxygen-enriched combustion system of thenext-generation stoker incinerator, the prevention of nitrous oxide generation, reduction of power consumption inplasma ash melting furnace, etc. together with the contribution of MHI waste-to-energy technologies to global warming countermeasures.
in Japan for 1990 and 2001. Compared with other fields,
the waste field has the emission rate itself low, but ascompared with the emission level of 1990, the emission
rate has been increased by 40% because of increase in
the consumption rate of plastic, etc. In order to reduce
the CO 2 emission in waste field, it is essential to prevent the generation of waste itself and to reduce thevolume of incineration through promotion of recycling.
However, in spite of the efforts made for high efficiency
through energy-saving and resource-saving, it is impossible to prevent the generation of waste itself, so that it
is important to take the waste as a useful fuel and torestore it as an effective energy.

1. Introduction
Based on the "United Nations Framework Convention for Climate Change" enforced in March 1994, the"Kyoto Protocol" stipulating for legal undertakings regarding the reduction of greenhouse effect gasses suchas carbon dioxide and others, was adopted in December
1997. The Kyoto Protocol precisely sets the target levelsof reduction of the emission of greenhouse effect gasses
for the advanced countries, and calls for efforts to reduce the total emission of greenhouse gasses in order toachieve the set target(1).
Figure 1 shows the changes in CO2 emission per field

CO 2 emission rate (million ton)

400350300
250200150100500Industry

Energyconversion

Transportation

Generalbusiness

HouseholdIndustrial
process

Waste

1990

369339211

735757172001366351

260

9666

51

25

Fig. 1 Change in CO2 emission rate per field
CO2 emission rate itself for the waste field is low but shows an upward trend.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)*1 Yokohama Research & Development Center, Technical Headquarters*2 Yokohama Dockyard & Machinery Works

1Hence, introduction of waste-to-energy plant of 4.17
million kW by the year 2010 was set as one of the targets as an additional new energy countermeasure in the"Guideline of Measures to prevent Global Warming,"
concluded by the “Global Warming Prevention Headquarters” in March 2002.
This report throws light in the trends of waste-to-energy technologies and describes MHI technologies
contributing to the countermeasures against globalwarming.

MHI put the 4 MPa, 670 K class boilers in practicaluse for the exports starting from 1986, is now constructing a 6.5 MPa, 723 K class MSW incinerator in China,and has outstanding records of manufacturing top-level
MSW incineration power generation boilers over the
4 MPa, 670 K class in Japan.
MHI has been in charge of the development of elemental technologies such as high-efficiency stoker
incinerator, corrosion-resistant super heater, etc. in the
project of "Development of High-efficiency Waste-to-ene r g y Te c h n o l o g y b y N e w E n e r g y a n d I n d u s t r i a lTechnology Development Organization (NEDO)” since1991, and successfully constructed a plant equipped witha high-temperature, high-pressure boiler of 10 MPa,
770 K class (100 kg/cm 2, 500 oC class) for the first timein the world in 1998, which is currently operatedsmoothly.
Figure 3 shows the structures of 4 MPa, 670 K classboiler, the main boiler currently in use, and the highlevel 10 MPa, 770 K class boiler. The 4 MPa 670 K classboiler is equipped with radiation heating surface to ensure adequate retention for the combustion gas in passno.1 for ascending flow and pass no.2 for descending flow,
with the super heater installed at pass no.3. The 10 MPa,
770 K class boiler, on the other hand, has the tertiary
super heater installed at pass no.3, with the combustion
gas temperature around this area kept to 650 oC or under. As for the materials of super heaters, the primarysuper heater is made of carbon steel, the secondary oneof stainless steel and the tertiary one of high-alloy steel.
The photograph at the top of the first page shows theMSW incineration power generation plant, the largest
in the world, built by MHI in Singapore. The plant, composed of 720 t/day x 6 incinerators, is capable of treating4 320 t of municipal solid wastes per day and is equippedwith 3.7 MPa, 643 K boiler, with the rated power 132 600
kW generated from 2000. The plant is currently operating normally.

2. Improvement in waste-to-energy generation efficiency
In introducing the waste-to-energy, it is necessary toincrease the waste-to-energy facilities, and to improve
the waste-to-energy generation efficiency. MHI is activelyengaged not only in the construction of waste-to-energy
facilities but also in the improvement of waste-to-energy
efficiency. Figure 2 shows the changes in steam temperatures in the boiler for municipal solid wastes (MSW)
incinerators(2). In Japan 3 MPa, 570 K class (30 kg/cm2,300 o C class) boilers were mainly used until the early
1990's, which are currently replaced with 4 MPa, 670 Kclass (40 kg/cm2, 400oC class) boilers, with the boilers ofhigher pressure and higher temperature expected to comein the future.
The MSW incineration boilers had the steam conditions kept to lower levels because of the chlorine, etc.
contained in the municipal solid wastes caused the acidicgasses such as hydrogen chloride gas, etc. to generate in
the combustion flue gas, leading to corrosion of boiler
tubes, particularly the super heater tube. As a result
power generation efficiency by incineration of municipal solid wastes was confined to approximately a dozen
percentage over a long time period. However, thanks tothe prompt developments made on the anti-corrosive
steel tubes for boiler super heaters, the efficiency has
been improved to over 20%.
550

: South-east Asian countries
NEDO: Japan450o

Steam temperature ( C)500

: Germany: USA400350300
250200150'64 '66 '68 '70 '72 '74 '76 '78 '80 '82 '84 '86 '88 '90 '92 '94 '96 '98

Fig. 2 Change in steam temperatureof boiler for waste-to-energy
generation
4 MPa, 670 K class boilers are mainly
used these days, with the tendency
increased for high-temperature andhigh-pressure boilers.

Year of completion

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)

2Secondary
super heater

Super heater

Tertiary super heater

HopperHopperPrimarysuper heater

Feeder
Stoker
StokerAsh discharger(a) 4 MPa, 670 K class (40 kg/cm2, 400 C) class: Gas: Steam(b) 10 MPa, 770 K (100 kg/cm2, 500 C) classooFig. 3 Structural diagram of high-temperature, high-pressure boiler
Structures of most commonly used 4 MPa, 670 K class boiler and the world's first 10 MPa, 770 K
class boiler are shown.

(2) Recirculation of flue gas as secondary air improves
the mixing of combustion gas in the furnace to complete gaseous combustion and prevents NOx fromincreasing.
(3) Substantial decrease in the flue gas flow rate allows
compact design of equipment such as boilers and fluegas treatment systems.
(4) High-temperature combustion and reduction in fluegas flow rate improve the boiler heat recovery efficiency, contributing to the countermeasures against
global warming.
(5) Ensures high reliability and long-term stability ofoperation based on the long-stored technologies instoker incinerator combustion.

3. Contribution of the next-generation stoker
incinerator to countermeasures against global
warming
MHI has already developed the next-generation stoker
incinerator using oxygen-enriched combustion system,with commercial plants already deployed(3). The oxygenenriched combustion system as shown in Fig. 4 has thefeatures given below.
(1) Utilization of oxygen-enriched primary air promotes
intense combustion of solid waste bed on the grate
so that reduction of unburnt combustibles (CO,DXNs, etc) in the flue gas and improvement of ash
quality is achieved.

.
.

Enhanced power
generation rate
Steam turbine

Secondary air
OFA
MSW

BoilerReduction of DXN and
CO contained in flue gas
Reduction in flue gas flow rate

Gas cooler
Bag filter

Incinerator
furnace..

Reduction of DXN in fly ash
Reduction of fly ash flow rate

Recirculation of flue gas..
.

OxygenPSA

Reduction of DXN in bottom ash
Improvement of bottom ash ignition loss
Reduction of bottom ash heavy metal leaching rate

Forceddraft fanBottom ash
Reduction in combustion
air flow rate

Fig. 4 Flow of "oxygen-enriched combustion system for next-generation stoker incinerator
System flow of next-generation stoker incinerator with low air ratio and high-temperature combustion
due to oxygen enrichment, giving less environmental burden.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)

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: Conventional operation
1 016 C

1203 ppm

100
Relative value (%): Oxygen-enriched operationo0.032
33% ng-TEQ/m No877 C

0.01
ng-TEQ/g

1.03
ng-TEQ/g

800.0059

600.49401ppm

1%0.0083

200Flue gas

COemission rate

N2O

Furnace
Bottom ash DXN in
temperature ignition loss flue gas

DXN inDXN inbottom ash fly ashFig. 5 Example of verification test result of oxygen-enriched combustion system
High-temperature combustion and low environmental burden have been confirmed
through verification test result.

tom ash from a conventional waste incinerator is divided into bottom ash fine and coarse fraction, thedioxins and heavy metals get mustered to the bottom
ash fine fraction, while the bottom ash coarse fraction
is known to have excellent qualities. This characteristic of the bottom ash is applied to the bottom ash
recirculation system. The bottom ash fine fraction withinferior qualities is recirculated to the furnace to be
subjected to heat treatment under oxygen-enrichedhigh-temperature environment. This enables the bottom ash qualities to get further improved without an
ash melting furnace.
The system has been confirmed effective in improving the bottom ash qualities by constructing a small-scale
pilot stoker incinerator with treating capacity 7.2 t/day
and conducting combustion test(4).

Demonstration tests have so far been conducted using
commercial incinerators in Japan (146 t/day and 300 t/dayincinerators) and in Germany (264 t/day incinerator) toconfirm the aforesaid features as shown in Fig. 5
5. Besides,
nitrous oxide with higher greenhouse effect than carbon
dioxide or methane has also been drastically reduced, contributing to the countermeasures against global warming.
The first oxygen-enriched combustion system isscheduled to start operation in Austria (Arnoldstein:256 t/day x 1 incinerator) and in Japan in 2004 (SendaiMatsumori: 200 t/day x 3 incinerators).
At present a bottom ash recirculation system without the ash melting furnace while improving thequality of the bottom ash as shown in Fig. 6 is underdevelopment to make more effective use of the featuresof oxygen-enriched combustion system. When the bot-Wastes

Air+EGR

Oxygen-enriched air

Improvement in bottom ash qualities
through high-temperature combustion
Separationand cleaning

Bottom ash
Bottom ash
recirculation

Bottom ash
fine fraction

Bottom ashcoarse fraction

Fig. 6 Bottom ash recirculation system flow
Bottom ash fine fraction withinferior qualities is treated underhigh-temperature environment
inside the furnace to improve
the qualities.

Discharge

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)

41

Main electrodePower consumption rates (-)

Overheadinspection
window

HopperFlue gas outlet
AshInspection
window
Slag level

Screw
feeder

0.8

0.60.4

0.20Slag cooler
Bottom electrode

0.2

0.4

0.6

0.81

Volume ratio of treated ash (-)

Fig. 7 Plasma ash melting furnace
Cross-sectional view of the furnace for melting ash by means of plasma

Fig. 8 Relation between treatment volume and power consumption
The increase in ash treatment volume is confirmed to reduce
power consumption.

4. Reduction of power consumption in plasma ash
melting furnace

5. Conclusion
This report describes the activities of MHI regarding
the waste-to-energy generation with the set target levelof introducing 4.17 million kW plant by the year 2010
based on the additional energy measure in the “Guideline of Measures to prevent Global Warming.” The report
particularly summarizes the activities and achievements
for improving power generation efficiency through hightemperature and high-pressure boiler for waste
incineration plants, for improving heat recovery rate
through the next-generation stoker incinerator and oxygen-enriched combustion system, for preventing
generation of nitrous oxide, for reducing power consumption in plasma ash melting furnace, etc.
MHI will continue to make efforts in further development of waste-to-energy technologies, hoping that MHI
technologies will go a long way in contributing to thecountermeasures against global warming.

In addition to the thermal recycling of the potential
heat quantity of municipal solid wastes through power
generation using the waste incineration as mentioned
above, the ash is currently subjected to fusion and solidification for further reduction of waste in volume andto make it harmless. As a method of promoting recyclingof the molten slag into road construction material, etc.
the plasma ash melting furnace is widely used in recentyears.
The plasma ash melting furnace whose structure isshown in Fig. 7 melts ash by generating high-temperature plasma between main electrode (graphite) andbottom electrode, and makes use of the power generated
in the MSW incinerator. The reduction in power consumption would contribute to the countermeasuresagainst global warming.
MHI has so far succeeded in continuous operation ofplasma ash melting furnace for 3 months through development of effective furnace cooling, excellent slag chute andrefractory material. Further, MHI is currently engaged in
the design and construction of a plant with 100 t/day x 4
plasma ash melting furnaces, the largest in size in theworld, and has a record of building 10 plants in total.
With the plasma ash melting furnace becoming larger
in size in recent years, the cooling and heat radiation
loss rates get decreased, and the power consumption canbe reduced, as shown in Fig. 88, which in turn is considered to contribute to the countermeasures against globalwarming.

Masao Takuma

Yuuji Kaihara

References(1) Hayashi, Recent Trend of Measures to prevent Global Warming, Journal of Japan Waste Management Association, Vol.
57 N.258 (2004) p.6(2) Orita et al., Development of High-Efficiency Waste-to-Energy
Plant, Mitsubishi Heavy Industiries Technical Review, Vol.37
No.1 (2000) p.1(3) Kira et al., Development of New Stoker Incinerator for Municipal Solid Wastes Using Oxygen enrichment, Mitsubishi
Heavy Industiries Technical Review, Vol.38 No.2 (2001) p.78
(4) Mawatari et al., Development of Advanced Stoker Incinerator [Oxygen-Enriched Combustion System], 2003 Symposiun
on Enviromental Engineering, Japan Society of MechanicalEngineers, No.03-10 (2003) p.193(5) Nishikawa et al., Development of MSW Incinerator residue
Melting Furnace by Electric Arc Plasma, Mitsubishi Heavy
Industiries Technical Review, Vol.30 No.1 (1993) p.56

Akihiro Yamada

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 41 No. 4 (Aug. 2004)

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