PYROLISIS OF MUNICIPAL SOLID WASTE

M. IGARASHI Y. HAYAFUNE R. SUGAMIYA Y. NAKAGAWA

K. MAKISHIMA Tsukishima Kikai Co., Ltd.

Chuo-ku, Tokyo, Japan

ABSTRACT

Funabashi City's Municipal Solid Waste Pyrolysis Plant is the first full-scale plant having a dual fluidized bed gasification system. The plant has the capacity of process­ing 450 TPD of mixed municipal solid waste with a very limited emission of air, water and land pollutants. The energy is recovered as high calorific value fuel gas. Since April 1983, the plant has been in continuous operation.

The purpose of this paper is to report on the system and the experience obtained during the 5 months in which it was in operation. Data on the material balance of the pyrolysis, the analysis regarding the gas produced, the flue gas composition and the equipment used are included.

BACKGROUN D

According to the data from Japan's Ministry of Public Welfare, 60.4 percent of all the municipal solid waste in Japan was incinerated in 1980 [1]. In Japan, large cities and their surrounding satellite cities incinerate the greater portion of their solid waste. However, solid waste that has various types of plastic material mixed in is causing serious problems such as the corrosion of stokers and refractories, and the hydrochloride (in the flue gas) produced in the conventional-type incinerators is causing pollution problems.

Therefore, in many of the cities, plastic waste is col­lected separately and disposed of in a different way. Another problem faCing incinerator usage is the rather high content of organic material in residue.

In Funabashi City, located in eastern Tokyo with a population of about 500,000, about 80 percent of 400

272

TPD of the general household and commercial waste was being incinerated at two incineration plants until the new plant was constructed. The other 20 percent of the waste, which is equivalent to 70 to 80 ton/day of plastic waste, metals and other inorganic materials, is being solidified and dumped at landfill sites.

The problem with the solidification of waste is that it is becoming too expensive to solidify and ship such waste to landfill sites. In addition to this, it is also becoming increasingly difficult to reserve sites for landfill operations in the future. As a result of these problems, the municipal government of Funabashi adopted the pyrolysis system and built such a plant to replace its older incinerator plant.

The reasons for the adoption of this system were as follows:

. (a) processing of plastic waste with other municipal solid waste without operational troubles;

(b) minimizing pollution due to flue gas to meet the stringent regulations;

(c) adoption of the closed-type waste water treatment system because there was no river into which plant effluent water could be dumped;

(d) obtaining high calorific value fuel gas as the reo covered energy;

(e) getting residue which contains the minimum amount of organic material.

We at Tsukishima began construction of this new system in July 1979 and carried out our first test run in January 1982. As a result of startup problems and mech­anical problems, modifications were performed during the testing period. At present (September 1983), 250 - 300 TPD of waste, which is equivalent to the 75 percent of the

city's total municipal solid waste and including about 15 percent of plastic waste, is being processed by this system. The ferrous material recovered is sold and the other inorganic materials are landfilled. The hazardous materials in the flue gas are reduced to a very low level.

At present, the fuel gas recovered is used.in the plant as auxiliary fuel for pyrolysis system and as fuel to gas-fire the boiler.

Refining and methanation test of crude pyrolysis gas is being conducted by the subsidiary of the Ministry of International Trade and Industry in an adjacent pilot plant.

A test for generation of electric power, using gas engines so as to proVide more and cheaper energy, is scheduled.

THE PYROLYSIS SYSTEM

The plant is operated 24 hr a day and has three trains for the pyrolysis reactors. A bird's eye veiw of the plant can be seen in Fig. 1 . Figure 2 shows that sectional view of the plant. The system has eight major components: the solid waste holding and size reduction unit, the pyrolysis unit, the residue treating unit, the waste water treatment unit, the flue gas treatment unit, the ash handling unit and the energy recovery unit. (Refer to Fig. 3 for the system flow diagram and Table 1 for main processing equipment.)

THE PR INCIPLE OF THE PYROLYSIS

The dual fluidized bed reactor was developed to pyrolyze organic solid waste and to obtain high calorific value fuel gas under atmospheric pressure and moderate temperatures (650 - 800°C) [2,3] . It consists of a crack­ing reactor and a regenerator (Fig. 4). These two reactors are ftlled with sand. Superheated steam is blown into the both reactors through the bottom nozzles. The sand goes up through the reactors with. steam and forms fluidized beds. From the upper end of the first fluidized bed, the sand overflows and goes down through the circulation pipe to the bottom of the second reactor.

Thus, in this way the circulation of the sand between the two reactors occurs.

Solid waste Is fed to the reactor where it is mixed with the hot sand and pyrolyzed. The cracking reaction pyrolyzes organic materials into three components: fuel gas, oil and tar, and char (carbon).

The gas produced and oil-tar vapor are taken out of the top of the reactor along with the steam. Char overflows with the sand from the reactors to the regenerator where it is mixed with air and burns. To support the pyrolysis,

the auxiliary fuel gas is fed to the regenerator. (The gas produced is used as fuel gas.)

Incinerator flue gases come out of the top of the re­generator. The circulating sand is cooled in the reactor by the drying and pyrolysis and reheated in the regenerator by the incineration of char and fuel gas.

In this way, as the pyrolysis reactor is separated from the incinerator the cracking reaction occurs under an optimum oxygen free condition and incineration flue gas is not mixed with the produced gas, allowing high calorific pyrolysis gas to be obtained.

THE SOLID WASTES HOLDING AND ,

DISINTEGRATION UNIT

Household solid waste brought in by collecting trucks and other solid waste carried in by private trucks are first weighed on truck scales and then dumped into a holding pit. Combustible bulk waste such as wooden furniture and mats are cut into sections of less than 400 mm (16 in.) by the press cutter prior to dumping into the pit.

The solid waste stored in the pit is then fed by a bucket crane into the three vertical-type hammer mills and crushed into sizes measuring less than 1 00 mm (4 in.) After this process it is stocked in the second holding pit. Normally, dumping and crushing operations are carried out only during the daytime hours.

PYROLYSIS UNIT

The pulverized refuse is picked up from the secondary holding pit by grab bucket crane which has an electric weighing device-. It is then fed into the hoppers of the feed conveyors. Bucket conveyors and feeders then take the refuse to the pyrolysis furnace.

As mentioned above, the pyrolysis unit consists of two fluidized bed furnaces connected by two pipes. One furnace is the reactor in which pyrolysis occurs and the other is a regenerator in which the combustible materials is incinerated (Fig. 4).

A feed dumper and a screw feeder are Installed to the reactor. The regenerator consists of a main burner and an auxiliary burner. At the bottom of the two furnaces, there are valves and holding tanks for blowing down the noncombustible residue with sand.

Ordinary sand, which is dugout of the ground, elutriated and dried by use of a kiln and screened into certain sizes, is used for the fluidized bed.

When the sand in the furnaces is fluidized by super­heated steam and air, it circulates between the two fur-

273

FIG.1 VIEW OF THE PLANT

naces and transfers the heat. The circulating rate is roughly controlled by the amount of steam fed in.

The pulverized solid fed by the screw feeder into the reactor is mixed with hot sand, then heated and pyrolyzed in a very short period of time.

The organic materials are decomposed into gases, oil vapor and char. As a result of drying and the endothermic reaction, the sand in the reactor is cooled down and is carried to the regenerator along with char.

In the regenerator, the sand is heated by the incinera­tion of the char and fuel gas (Refer to Pyrolysis Unit), and then is returned to the reactor.

The temperature of the fluidized bed in the regenerator can be controlled by the feed rate of fuel gas.

RESIDUE TREATING UNIT

The noncombustible materials which are drawn through the valve unit with the sand are fed into the vibrating cooler. These materials are then separated from the sand by use of a screen. The coarse inorganic residue is cooled again by use of the vibrating air cooler and fed into the magnetic separator, where ferrous metals and the other materials are separated. These two procesSed fractions are then stored in the hoppers. The fine ash is stored in the sand hopper, then the greater part is sent back to the

regenerators and the other part overflows from the hopper to be disposed.

PYROLYSIS GAS HANDLING UNIT

The pyrolysis gas produced is scrubbed and cooled at the three-stage scrubber where water, oil and particles are separated from the gas. The circulating liquid of the scrubbers is cooled by cooling water with plate-type heat exchangers. After scrubbing, the gas is passed through a wet electrostatic precipitator in which the fine particles and the mist of oil and tar are removed, then the gas is stored for use in a water-sealed gasholder.

WASTE WATER TREATMENT UNIT

The waste water coming from the scrubbers flows into the floating sedimentation tank in which oil, tar and char (carbon sludge) are separated. The sedimented sludge and floated oil-tar fraction are mixed and dewatered by the press roll filter. The filter cake is sent to the regenerator for incineration.

The sludge and oil-free waste water are condensed at the pressure-type evaporator. On the one hand, the vapor is fed to the reactor as the fluidized steam; on the other hand, the condensed liquid is drawn and fed to the second vacuum-type evaporator. The vapor of the second vacuum evaporator is condensed and treated by the active carbon adsorption tower, then used as supplimentary water for the cooling tower. The condensed liquid is mixed with fly ash and solidified for landfill. In this way, no effluent water flows out of the plant.

FLUE GAS TREATMENT UNIT

Hot flue gas from the regenerator passes the cyclone where sand and the rather large-size particles are separated. Next, it goes through the waste heat recovery boiler. After the heat recovery, it passes through the double cyclone and electrostatic precipitator. The fly ash that is separated from the flue gas is carried by a pneumatic conveyor to the ash hopper.

ASH HANDLING UNIT

274

The fly ash and the condensed process waste water are mixed at a constant ratio in the mixer. A solidifying chemical agent is added to the mixed sludge to fix the hazardous material in the kneader. The solidified output is then landfilled.

Wosi. heot retOYll'y boll ...

-

Pucker

Scoles

Hold ing pi t

D isinleo -l olion

Hold ing pi t

Feeder

Ylbrotln, cool.r RnkNt

'(iln,er

...--F,.. dump ...

Cron.

HokI,..

pi' mill

Crone

Holdin9 ."

FIG.2 THE SECTIONAL VIEW OF THE PLANT

Prirate '\ I Veicles Steam --.-.j

Bulk Waste heat culler recovery

boi ler ir= . PYrOlysi!. l�R*"1 Gos scru- GaS holder erolor I bbing ;

Evoporo - Sl udge a Gas fired tor ���rolor boi ler

Residue screen

Treoled" Press roll Ash soli-" woter f i ller d i fyinO

Moonetic seporotor Cool iog

lower

rpe�y� r� -'\ rSolidifyed--

�I ted 'sluciJe.l osh '�I';;i�fs'

FIG.3 SYSTEM FLOW DIAGRAM

275

Dust

DumP'n9 plollorm

separation

Turbine oenerotor

De Nox reoctor

-

/ , Flue gas

Stock

Component

Pulverizer Feeder

Pyrolysis

Residue handling

Pyrolysis gas treatment

Water treatment

Flue gas treatment

ENERGY RECOVERY UNIT

TABLE 1 MAIN PROCESSING EaUIPMENT

Description

105"¢, 520 KW hammer mill 30 T/Hr

1000 mmW (40") valiable speed apron conveyor

7 T/Hr dumper screw feeder

Cracking reactor and regenerator (150 T/D)

20 mesh sand screen

Residue cooler

Magnetic separator

,

1300 mm¢ x 5070 mmH gas scrubber

1800 mm¢ x 1 1,400 mmH gas cooler

2300 mm¢ x 9 100 mmH wet type electrostatic precipitator

3 10,000 m water-seal gas holder

5500 mm¢ x 5000 mmH tar & sludge separator

1500 mmW press roll filter

Three stage evaporator

1900 mm¢ x 2250 rnrnH adsorption column

Cooling tower (water 1300 m3

/H)

975 mm¢ double cyclone

Electrostatic precipitator

14,000 Nm3

/Hr

Quantity of Unit

3

3

3

3

1

1

1

3

2

1

1

1

1

1

3

1

3

3

The heat energy of the regenerator flue gas is recovered by the waste heat recovery boiler as steam. The Pyrolysis gas is used as supplementary fuel for the regenerator and as fuel for the gas-fired boiler. The steam from the boilers

is used for the waste water evaporator and the steam tur­bine generator. The catalylic deNOx reactor is installed for the gas· fired boiler exhgust gas to meet the stringent emission regulations (NO x should not exceed 100 ppm at 5 percent oxygen).

276

Solid Waste Pyrc·ly�is Gas t

Feed Dumper

Reactor

Screw Feeder t--/�1 Flui diz

bed

Steam --1--�

I

Steam'----I

Residue

Flue Gas

t

Regenerator Auxiliary Burner

Fluid ized Ma in Burner bed e:;� ""-Fuel Gas

Air

1----- Stea m

Residue

FIG.4 DUAL FLUIDIZED BED REACTOR

OPERATION

Solid waste was fed initially. to the plant in January 1982. Operational troubles developed with the gas handling unit, waste water treatment unit and some other areas, so it was necessary to make modifications to the above areas.

In the beginning period of the test run, metals were collected along with plastic materials, so large-size metal plates and pipes had to be processed. Some of the metal waste could not be reduced into sizes of less than 100 mm. Then large-size metals caused serious problems, many times bridging the outlet valves of the furnace. Therefore, the solid waste collection method was changed by the municipal government at the beginning of July 1 983, and since that time, the metal waste in the refuse has decreased considerably and there have been few occurrences of the bridging of the bottom valves.

The feed rate of the pulverized solid waste to the re­actor changed to a great extent, since the constant draw­ing out of the refuse from the feed hopper by the apron conveyor was difficult. Also, the quality of the refuse such as the content of moisture and plastic materials varied with time. Under the fluctuating feed conditions, the temperature of the reactor and the regenerator could

277

be maintained and the operation of the furnace was stable.

Approximately 37,000 tons of refuse was pyrolyzed in five months (April to August 1 983).

In general, the pyrolysis plant can be operated con­tinuously, but periodic cleaning of the heat transfer units installed in the gas handling and water treatment process is necessary (about once every six months) because of scaling.

COMPOSITION OF PYROLYZED MUNICIPAL

SOLID WASTE

The average component of the municipal solid waste after pulverization is shown in Table 2. The designed basic maximum calorific value (lower value) is 3600 Btu/lb (8374 kJ/kg), but the current value is around 2880 Btu/lb (6700 kJ/lb).

MATERIALS BALANCE OF PYROLYSIS

The temperatures of the fluidized bed of the re­generator were controlled to between 7500e and 8000e during normal operation.

Solid waste 12.5 t/H

S team 3.6 t/H

TABLE 2 MUNICIPAL SOLID WASTE COMPONENT

Paper Plastics Wood Cloth Food waste

Component

Metal and other inorganic Other organic material

Total organic material Total inorganic material

•

MOlsture content

% by Weight

46.9 15.1

2.7 3.3

17.7 material 6.3

8.0

100.0

4 1.5 13.5 45.0

100.0

Lower calorific value: 2880 Btu/lb (6700 KJ/kg)

Pyrolysis I ! I

I

\1

•

I I

Inorganic 1.3 t/H

r _ (rude gas 3060 Nm3/H uOS scru-

I bb i ng

Waste water Steam 11.3 _t!�[ • 4.6 t/H

. •

Waste heat Dust bOi er !

Carbon foec�very separation Flue gas I Sludge 1·9tl -- 31000 Nm3

Crude a s 1600 Nm3/H Ash OA t/H Steam

Air 22000 Nm3/H 10 0t/H Gas fired

Crude gas 1460 : boiler , •

Nm 3/H • 3 Air 7300 Nm I H

FIG.5 MATERIAL BALANCE OF PYROLYSIS

Flue gas

Flue go s

39500NmJ/H

Consequentially, the temperatures of the reactor could be maintained between 650°C and 700°C. The pressure in the reactor freeboard was atmospheric pres­sure. Under this reactor condition, 240 Nm3 to 250 Nm3 of pyrolysis gas was produced from one ton of refuse.

If the calorific value of the refuse is 2880 Btu/lb (8374 kJ/kg) (equal to the maximum design base), then the gas feed rate to the gas-fired boiler would ncrease from 1460 Nm3/h to 2520 Nm3/h.

The gasification ratio of the organic material was 54 to 56 percent .

The material balance of the pyrolysis is shown in Fig. 5. In this case, 300 tons of solid waste which had a calorific value of 6700 kJ/kg was pyrolized in a day using the two reactors.

278

PYROLYSIS GAS

The average component of the pyrolysis gas produced is shown in Table 3. The content of the hydrocarbons like methane, ethylene and propylene was high and the nitro­gen was low, so that crude gas has a high calorific value.

TABLE 3 PRODUCT GAS ANALYSES

.

Component

Hydrogene Oxygene Nitrogene Carbon dioxide Carbon monoxide Methane Ethane Ethylene Propane Propylene Others*

* H S 2

•

% by Volume

15.32 0.17 2.41

16.72 31. 34 17.04

2.33 9.79 0.11 3.09

1. 68

100.00

800 - 2200 p.p.m.

* Hydrochloric gas 200 - 1200 p.p.m.

Dry base calorific value: 630 Btu/ft3

(23,420 KJ/Nm3

)

TABLE 4. FLUE GAS ANALYSES

Component

Carbon dioxide Oxygene Carbon monoxide Nytrogene

SO x

NOx

HCl

* parts per million

Dust consistency:

In the plant, the crude gas was used only as the fuel for the regenerators and the gas-fired boiler, but the possible conversion of this gas into town gas seems highly feasible.

The crude gas contains a little oil and tar after the separation using the wet-type electrostatic precipitator. The tar remains inside the gas line equipment (valves and

0.02

% by Volume

10.5 8.8 o

80.7

100.0

17*

83*

92*

3 - 0.03 g/Nm

blower) and needs to be cleaned away once very six months.

FLUE GAS

The exhaust gas emitted from the stack is the mixed

279

gas of the regenerator flue gas and the gas-fired boiler flue gas. The component is shown in Table 4.

The NOx concentration of the gas-fired boiler flue gas was 150 - 200 ppm and could not meet the government regulations; therefore, it was reduced by use of a catalytic reactor in which NOx reacted·with ammonia to a concen­tration of less than 100 ppm.

The SOx and HCl concentration in the gas-fired boiler flue gas are nearly equal to the digits that re computed from the fuel gas composition. However, in case of the flue gas of the regenerator, the HCl concentration was higher than the consistency which was expected from the analyses of the fuel pyrolysis gas, and the content of SOx was lower than expected. (See the concentration of H2S and chloride in Table 3.)

It is believed that as Kubota reported [4] , the Hargreaves reaction:

occurred in the regenerator. Then, S02 is fixed as salt and chloride is decomposed into HCl gas and emitted.

UTILITIES

The principal utilities which were consumed in the plant are as shown in Table 5.

TABLE 5 UTILITIES

Per Ton Solid Waste

300 TPD 450 TPD

Electricity

- for the processing 161.85 kW'h

- for the lighting, 66.18 kW'h

air conditioning

228.03 KW·h

Water 1.2 ton

Caustic soda (as 100%) 8.1 kg

Active carbon 0.7 kg

129.31 KW'h

44.12 kW'h

173.43 kW'h

1.1 ton

8.1 kg

0.7 kg

Almost all the plant equipment was installed in a build­ing, so the power consumed for lighting, ventilation and air conditioning was comparatively high.

During processing time, the power consumed due to pulverization and the supplying of air for the regenera­tors was relatively high.

During five months operation period the calorific value of the solid waste was around 6700 kJ/kg and the re­covered energy as the generated power was small. In the event that the calorific value was 8374 kJ/kg (the designed

base), about 40 percent of the power consumption could be recovered by the power generation.

. .

So as to improve the efficiency of the energy recovery, a power generation test using a 1 SO hp gas engine utilizing the pyrolysis gas produced is scheduled to start at the beginning of 1984.

Caustic soda was used to fix chlorine in the waste water treatment process by the reaction:

NH4 Cl + NaOH = NaCl + NH3 + H20.

Ammonium chloride (NH4 Cl) is formed in the pyrolysis gas scrubbers in which ammonia and hydrochlorine in the pyrolysis gas reacts due to the equation:

and dissolved in the scrubber effluent. The activated carbon was used for the final water treat­

ment.

RESIDUE

The residue of pyrolysis and incineration was drawn out of the bottom of the reactors and the regenerators every twenty minutes through the valve and tank units.

The ignition loss of the residue was kept less than 0.7 percent throughout the operation.

CONCLUSION

The following points became clear after five months operation at the Funabashi Plant:

(a) municipal solid waste can be pyrolyzed in a stable condition using the dual fluidized bed system;

(b) plastic waste materials can be processed success­fully along with other refuse;

(c) the pyrolysis gas produced has a high calorific value;

(d) air pollutants like SOx' NOx and hydrochloric gas can be kept at a very low level;

(e) waste water reuse as supplementary cooling water is possible;

if) the residue contains very small amount of organic material and is easy to dispose of.

The following points need to be improved on for a more efficient future design of the pyrolysis plant:

(a) decreasing the energy used by the system using shallow bed reactors and a more simplified system;

280

(b) improving the efficiency of the power generation by use of a gas engine instead of the boiler turbine genera­tor system;

(c) preventing the scaling troubles of the heat ex­changer for trouble-free long-run operations.

The municipal solid waste includes various kinds of wastes like plastics, wood, many carbohydrates, etc. Therefore, the successful result of the operation of the system implies that application of this pyrolysis system to the gasification of other wastes like biomass wastes, pulp and paper wastes, petrochemical wastes and other industrial organic wastes is highly possible.

ACKNOWLE DGMENTS

The authors would like to thank the municipal govern­ment of Funabashi for running the plant and for allowing

281

this report to be published. Also, the authors are deeply indebted to the people who participated in the design, construction and operation of the plant.

REFERENCES

[1] Journal of Solid Wastes 26, Vol. 13 No.6

[2] M. Hasegawa, J. Fukuda, D. Kunii, "Research and Devel­

opment of Circulation System between Fluidized Beds for Applica­

tion of Gas-Solid Reactions," Second Pacific Chemical Engineering

Congress (pachec '77).

[3] M. Kagayama, M. Igarashi et . al. "Gasification of Solid

Waste in Dual Fluidized Bed Reactor" ,Abstract of 178th American

Chemical Society National Meeting, Washington, D.C., Sept. 9-14,

1979.

[4] H. Kubota, K. Kanaya, Journal of Solid Wastes, 19, Vol. 9 No. 12.