Biomass Solid FuelP r o d u c t i o n f ro m S e w a g eSludge with Pyrolysis andCo-firing in Coal Power Plant
YOUICHI KOGA* 1

YUUKI ENDO* 1

HIROSHI OONUKI* 1

KAZUAKI KAKURATA* 1

TAKESHI AMARI* 2

KIMITOSHI OSE* 2

MHI has developed a carbonization system to recycle sewage sludge for electric power generation fuel. The technology can satisfy two needs at the same time: first, the need to recycle sludge and suppress the emission of global
warming gases in sewage sludge treatment plants; second, the need to substitute fuel with carbon-free fossil fuel inthermal power plants. The main component of the system is an indirectly heated rotary kiln of a type already inpractical use in power generation facilities with gasified wood waste. The kiln was considered optimal for the system in light of its ability to produce pyrolysis coke steadily and efficiently from sewage sludge. Bio Fuel Co., Inc.
placed an order with MHI for the construction of a sewage sludge carbonization facility, the first facility to use thesewage sludge for power generation fuel by converting it to carbonized fuel. The construction is scheduled to becompleted within 2007.
accumulated through its work to solve environment andenergy challenges. In this connection, MHI has developed a technology of a carbonization system, excellent
in stabilizing supply and properties, to recycle sewage
sludge as a fuel for power generation. This new system
can satisfy the needs for sewage sludge treatment ,forreduced emission of global warming gases, and for carbon-neutral fuel utilization in thermal power plants.
This describes the progress of our efforts to realize
the world's first "sewage sludge carbonization business"through the approach described above.

1. Introduction
The use of biomass as a new energy is being promoted
to prevent global warming by reducing greenhouse effect gases and to use waste more effectively for therealization of a recycling-oriented society. To use theenergy from biomass resources, energy producers mustfind mechanisms that meet the needs of business entities for achieving stable supply, economic efficiency, andadequate control of fluctuating fuel properties during theprocesses of biomass collection, energy conversion, andthe use of the biomass resources.
MHI has been engaged in energy conversion and thedevelopment of utilization technologies and practicalapplications relative to various types of biomasses basedon the environmental equipment technologies and energy conversion technologies the company has

2. Outline and advantage of the sewage sludge carbonization system
2.1 Outline of the system
Fig. 1 is a flow diagram of the carbonization system
and Table 1 shows the major system specifications.

Air
Dewateredsludge

Air preheater

Fuel
Dryer

Heat
exchanger

Pyrolysis gas
combustion
furnaceDry sludge
Pyrolysis
kiln

PyrolysiscokeFuelAir

Exhaust gas
treatment tower

Combustion chamber
Fig. 1 Flow diagram of sewage sludge carbonization system
Plant system configuration diagram

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)*1 Yokohama Dockyard & Machinery Works*2 Yokohama Research & Development Center, Technical Headquarters1

(SOx) and desalinized (HCl) in an exhaust gas treatment
tower, cooled and dehumidified, and treated for removalof soot and dust by an wet electric precipitator, if required. If necessary, white smoke prevention is carriedout at the outlet of the stack.
2.2 Advantages of the system(1) Stable carbonization by indirect heating
By adopting indirect heating with hot combustion
gas from outside of the rotary kiln, the water vaporization and heating temperature rise take place more
slowly than they do during direct combustion inside
the kiln. As an added advantage, the pyrolysis conditions never fluctuate rapidly, even if the water contentof the sludge does. Thus, a stable heating pyrolysis
is ensured.
The inside of the muffle for the heating gas aroundthe kiln circumference is divided into multiple divisions in a longitudinal direction, and the respective
heating gas amounts can be adjusted. With this kiln
configuration, the heating value distribution can beadjusted to suit the water content of the sludge. Thus,
pyrolysis coke of constant quality can be obtained bystable pyrolysis gasification.
(2) Hot and clean combustion and the use of pyrolysisgas
An independent pyrolysis gas combustion furnace
is installed in this system for the clean combustionof the pyrolysis gas at high temperature and the combustion and deodorization of dryer exhaust gas
containing odorous components. By adopting an independent combustion furnace, the pyrolysis gas canbe combusted at an appropriate air ratio and combustion temperature, enabling stable and cleancombustion. The released N 2 O (dinitrogen monoxide), a gas with a greenhouse effect 310 times morepotent than that of carbon dioxide, can be reduced
to levels far lower than the levels released by thefluidized bed incinerator generally adopted for sewage sludge treatment.
(3) Reduction of the fossil fuel utilities through the recovery and use of waste heat
Multiple heat exchangers provided in this system take the combustion heat of the pyrolysis gas
generated from the sewage sludge and effectively
use it as the heat source for the drying and carbonization processes. The combustion exhaust gas
generated after heating the indirectly heated rotarykiln is used as a heat source for preheating the combustion air. The combustion exhaust gas of thepyrolysis gas is used as a heat source to preheat thedryer exhaust gas before this gas is introduced into
the combustion furnace. The heat recovery from thesecombustion exhaust gases reduces the amount of fossil fuel utilities.

Table 1 Major equipment specifications for the sewage sludge
carbonization system

ItemDryer
Pyrolysis kiln
Pyrolysis gas
combustion furnace
Exhaust gas treatment

Specification
Hot air dryer with rotary kiln type agitator
Indirectly heated rotary kiln
Cylindrical furnace insulated
by a fire-resistant construction
Wet gas scrubbing tower

MHI has concentrated its technologies to realize asystem that makes the properties of the sewage sludge
carbonized fuel stable and safe, and that can performthe carbonization with a small amount of fossil fuel utilities. The system shown below is now configured.
This system consists of three processes: carbonization,
pyrolysis gas combustion and waste heat recovery, andexhaust gas treatment. In the drying process, dewateredsludge with a water content of about 76% is dried to asludge with a water content of about 25% directly with agas heating dryer with agitator. Dewatered sludge
charged into a rotating drum is dried efficiently by direct contact with hot exhaust gas of 800 to 850oC during
agitation by an agitator installed inside the drum. Oncethe sludge is dried, the dryer exhaust gas used to dry it
drops to a temperature of 200oC and is combusted anddeodorized in the subsequent process. A water dry sludge
water content of about 25% helps to reduce the consumption of the fossil fuel utilities of the system and reducethe influence of the load fluctuation of the water content during the carbonization process in the later step.
In the carbonization process, pyrolysis coke is obtained by heating the dry sludge to about 500 o C andpyrolyzing it with an indirectly heated rotary kiln in alow oxygen atmosphere. The dry sludge is indirectly
heated and pyrolyzed with hot gas at a temperature ofabout 950 to 1,100oC. After being cooled by the coolingconveyer, the pyrolysis coke produced is humidified toprevent spontaneous ignition, and then treated as carbonized fuel. In the pyrolysis gas combustion and waste
heat recovery process, the pyrolysis gas and dryer exhaust gas generated in the pyrolysis kiln are blown withair by an independent combustion furnace (pyrolysis gas
combustion furnace) and combusted at high temperature(950oC). A part of the combustion exhaust gas is used as
the heat source for the indirectly heated rotary kiln. The
waste heat is recovered by two heat exchanges: one between the combustion exhaust gas of the pyrolysis gas
and the dryer exhaust gas, and one between the combustion air and the combustion exhaust gas after theheating of the pyrolysis kiln.
In the exhaust gas treatment process, the exhaust gas
generated during the combustion process is desulfurized

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)2

1. Properties of the sludge and pyrolysis coke, and evaluation of the reduction in the environmental load
Survey of sewage sludge properties and pyrolysis coke properties
Evaluation of the environmental loading reduction effect during fuel utilization by sewage sludge carbonization

2. Carbonization test and carbonized fuel evaluation (Verification of item 1 above)
Performance verification of carbonization system- Operation stability- Environmental impact such as exhaust gas, waste water, etc.
Evaluation of carbonized fuel- Combustion properties (ignitionability, exhaust gas, combustion ash)
- Handling properties (grindability, dust dispersion, etc.)- Safety (spontaneous ignition property, dust explosion)- Harmfulness (bacteria, elution)
3. Test with an actual coal-firing thermal power generation plant(Cooperation with an electric power company that uses carbonized fuel)Fig. 2 Major verification and evaluation items aiming at the use of the carbonization system and carbonized fuel(Approach aiming at practical application)

Fig. 3 External view of the demonstration facility
An external view of the demonstration facility is shown.

an integral system. Fig.3 shows an external viewof the test facility. The system consists of a dryerand sludge carbonization facility now under operation in the Yokohama Dockyard & Machinery Worksof MHI. The combustion exhaust gas of pyrolysis
gas was used as the heat source for the dryer andpyrolysis kiln for drying and carbonizing, respectively.
According to the operation method of the dryer, thehot exhaust gas of the pyrolysis gas combustion furnace is introduced into the dryer with the fan at thedryer outlet, and the water content of dry sludge isadjusted to about 25% by adjusting the amount of the
hot exhaust gas from the pyrolysis gas combustion
furnace to a level that brings the dry exhaust gas temperature at the dryer outlet to about 200 o C. The
carbonization conditions in the pyrolysis kiln are adjusted by adjusting the amount of exhaust gas fromthe pyrolysis gas combustion furnace to a level thatbrings the carbonization temperature to about 500oC.

3. Developments aiming at practical application
MHI has completed the verification tests shown in
Fig. 2 to assess the use of the carbonization system forthe production of carbonized fuels from the sewage
sludge, and to assess the use of carbonized fuels for cofiring in a pulverized coal firing boiler (a facility
developed under the joint development projects with
Tokyo Electric Power Company, Inc. targeting the use ofsewage sludge pyrolysis coke as power generation fuel).
The verification data on operability of the carbonization
system and the evaluation of the carbonized fuel are
shown below.
(1) Verification of operability of carbonization system
We carried out a test to verify the stable operability of the system and the stability of theproperties of the fuels produced. The test system
is assumed to be an actual system consisting of adryer and pyrolysis kiln. Various conditions, suchas fluctuating amounts of inlet input, are set using

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)

3Temperature ofthe pyrolysis gas combustion furnace 950 oC

1 200
1 000

Kiln temperature 500 oC
600
400

Dryer outlet gas temperature 200 oC

200
0
10:00

14:00

18:00

22:00
02:00
06:00
Time(a) Temperatures of the respective portions of the carbonization system

100

7Feed amount (kg/h),
Water content (%)

Water content of dewatered sludge 80%

680
Amount of dewatered sludge feed 40-60 kg/h

Amount of pyrolysis
coke recovered

60

54
340

220

1

Water content of dry sludge 25%010:00

14:00

18:00

22:00

02:00

Amount of carbide recovered (kg/h)

Temperature (oC)

800

0
06:00

Time(b) Water contents and feed amounts of dewatered sludge and dry sludge,
and the amount of pyrolysis coke recovered

HHV (MJ/kg-DB)

25
20
15
10
5
10:00

14:00

18:00

22:00

02:00

06:00

Time(c) Pyrolysis coke heating value

Fig. 4 Operation of the carbonization system
Pyrolysis coke can be stably produced by controlling the dryer outlet temperature and pyrolysis
temperature to constant values.

Furthermore, a hot gas producer is installed at theinlet of the pyrolysis kiln to adjust the exhaust gas
temperature of the pyrolysis gas combustion furnace
to an arbitrary temperature and to provide a heatsource for the pyrolysis kiln.
Fig
Fig.. 4 shows the test results. The respective figures verify the stability of the system when the sludge
supply amount is varied. Even if the dewatered sludge
supply amount varies from 60 to 40 kg/h, the dry
sludge water content is stabilized at about 25 +/-10%
during operation. The carbonization temperature isalso stably controlled. The heating value of the obtained pyrolysis coke is stable, as shown in Fig. 4(c).

(2) Evaluation of carbonized fuel properties
Table 2 shows the properties of sewage sludge andcarbonized fuel, and the appearance of the carbonized fuel. The carbonized fuel has a heating value ofabout half that of coal. The fuel particles measureseveral millimeters in diameter and can be handled
in the same manner as the coal for the existing pulverized coal firing boiler. The grindability isexcellent, hence the fuel can be ground by a coal pulverizer without any problems. A combustion test has
confirmed that fuel mixing combustion of several cal% with coal leads to no problems with ignitability orflammability.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)

4

Table 2 Properties of sewage sludge and pyrolysis coke
Dewatered sludge
Water content(wt. % -WB)

79.7

-

Ash content(wt. % -DB)

17.7

55.3

Combustiblescontent(wt. % -DB)

82.3

44.7

(wt. % -DB)

44.4

38.6

(wt. % -DB)

6.5

0.8

N

(wt. % -DB)

S(wt. % -DB)

4.5
0.82

3.00.62Cl(wt. % -DB)

0.090.05(kJ/kg-DB)(kcal/kg-DB)

20 040(4 790)13 950(3 330)CH

HHV

4. Practical application of technology to convert sewage sludge to fuel by carbonization

Pyrolysis coke

MHI received an order to construct a sewage sludge
carbonization facility from Bio Fuel Co., Inc. (subsidiarycompany of Tokyo Electric Power Company, Inc.), a company engaged in the sewage sludge carbonization
business. The facility is now under construction in theTobu Sludge Plant of Tokyo Metropolitan Government.
The construction is scheduled to be complete within 2007.
The project is to annually produce 8,700 tons of carbonized fuel from 99,000 thousand tons of sludge, and touse this fuel as an alternative to coal.

5. Conclusion
This article has described the development of a carbonization system for sewage sludge, studies on the useof fuel produced for power generation, and the progress
towards the realization of what will be Japan's first sewage sludge carbonization business.
In the future we intend to practically apply and promulgate the fruits of these efforts in response to thepublic demand for the promotion of the effective use ofcarbon neutral biomass resources, and to further develop
and improve related new technologies.

Appearance ofcarbonized fuelThe sewage sludge pyrolysis coke has a spontaneous
ignitability like that of coal, but this property can besuppressed by adjusting the carbonization temperaturein the carbonization system and by humidifying the pyrolysis coke to 10-20 %.

Youichi Koga

Yuuki Endo

Hiroshi Oonuki

Kazuaki Kakurata

Takeshi Amari

Kimitoshi Ose

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 44 No. 2 (Jul. 2007)5