THE PRACTICE OF REFUSE INCINERATION IN JAPAN BURNING OF REFUSE WITH HIGH

MOISTURE CONTENT AND LOW CALORIFIC VALUE K. MATSUMOTO

R.ASUKATA T. KAWASHIMA

Mitsubishi Heavy Industries, Ltd. Yokohama, Japan

ABSTRACT

As in Europe and America, strenuous effort has recent­ly been made to improve and expand environmental sani­tation equipment and facilities in Japan.

This presentation briefly describes the history and de­velopment of the Japanese refuse incinerator facilities up to the present date. Also discussed are incinerator vari­ations to deal with the higher moisture content and lower calorific value of Japanese refuse, several Japanese incin­erator plants, and other related topics.

REFUSE INCINERATION IN JAPAN

Prior to World War II, the majority of refuse incinera­tors used in Japan were small, batch-fed, fixed-grate furnaces: no significant attempt was made to modernize refuse incinerators until after the war.

With the growing concentration of population in the large cities, the amount of refuse naturally increased, necessitating appropriate means for its disposal. This could not be accomplished by the existing batch-fed fur­naces, but called for ultimate development of large-capac­ity, mechanical-refuse incinerators.

Refuse charging and ash dispos� equipment were first mechanized in the batch-fed furnaces, and in 1955, mechanically movable grates were introduced. However, incineration was still unsatisfactory, combustion tempera­ture was low, methods of incineration had not been ap-

180

preciably improved, and performance of furnaces and auxiliary equipment was inefficient. The maximum daily processing capacity achieved for a single plant remained between 200 and 300 tons even though a combination of several 20- to 30-ton capacity furnaces were used. There were no high-capacity furnaces for processing large volumes of refuse. Figs. 1 and 2 illustrate the smaller furnaces.

To cope with ever increasing requirements for adequate environmental sanitation facilities, the Government passed emergency legislation in 1963 calling for environmental facilities. A 5-year plan for the construction of incinera­tors, based upon the new law, was contemplated. Mean­while, various technical studies were made with respect to incineration of refuse as a part of metropolitan indus­tries. These studies were conducted mainly by boiler manufacturers who had extensive experience in the de­sign and production of combustion equipment such as industrial boilers. At the same time, engineering studies were conducted on refuse incineration, and incinerators used in Europe and America were investigated. Test equipment was developed, and the design and production of continuous feed, mechanical incinerators was com­menced. In the course of the design and construction, consideration was given to public health and sanitation, to prevent public hazards and nuisances.

The first equipment of this type was installed in the Osaka Sumiyoshi Plant (150 tons/day x 3 units), as shown

FIG. 1 HOSHIKAWA, YOKOHAMA.

30 TONS/8 HOURS x 9 UNITS, 1962

in Fig. 3. The Sagamihara City Plant (90 tons/day x 2 units), shown in Fig. 4, and Tsurumi Plant, Yokohama (150 tons/day x 3 units), shown in Fig. 5, were completed at about the same time. These plant designs were based upon the different conception of combustion and incineration. Following the construction of these plants, other plants were then built using various types of incinerators.

In Japan, incineration servic'e is provided either directly or indirectly by public service organizations, and, in view of the recent public interest and growing concern about prevention of public hazards and nuis�nces, stringent measures have been taken to provide assured protection against such hazards and nuisances. For example, dust collectors have been installed, and consideration has been given to the elimination of unpleasant odors, prevention of water pollution, and noise controL Although proper refuse incineration requires a high degree of technical knowledge to cope with these conditions, only a few engineers have concerned themselves with this subject. As difficulties were encountered in the development of suitable refuse incineration plants, there was a tendency to establish standard specifications with respect to such facilities. Public interest on this subject was aroused and appeals from local public service organizations began to come in to the various technical institutions. Thus, numerous studies have been conducted on, for example, analysis of refuse, problems of public hazards and nui­sances, and theories on drying and combustion. This has

FIG. 2 OHSAKI, TOKYO, 30 TONS/8 HOURS x 1 UNIT, 1960

181

FIG. 3 SUMIYOSHI PLANT, OSAKA, 150 TONS/DAY x 3 UNITS,

1963, BY TAGUMA BOILER MFG. CO.

FIG. 4 SAGAMIHARA CITY PLANT, 90 TONS/DA Y x 2 UNITS,

1964, BY MITSUBISHI HEAVY INDUSTRIES

182

t

FIG. 5 TSURUMI PLANT, YOKOHAMA, 150 TONSIDA Y x 3 UNITS,

1965, BY MITSUBISHI HEAVY INDUSTRIES

brought about considerable discussion and joint efforts by the Ministry of Health and Welfare, university scholars, local public service organizations, manufacturers, and has resulted in the development of a Standard for Incineration Facilities which was set forth in 1966. These maintenance and control standards for refuse incineration facilities have since provided a stringent policy guidance for incinerat�r construction as well as effective guidance to manufac­turers of incinerator equipment with respect to future de­velopment of incinerating equipment and facilities.

As a result of the growing interest in this subject, new mechanical incinerator plants have sprung up in one city after another. The Government responded to the concern for improved sanitation facilities by developing a new 5-year plan that began in 1967. A brief summary of this plan is given below:

Number of cities, towns, and villages requiring refuse incineration facilities . . . . . . . . . . . . . . . . . . . ... 1,273 Refuse to be processed . . . . . . .... . ... . . . . 61,650 tons/day

183

Processing capacity at the end of 1966 .. . . . .... . . . . . . . . .... ..... . .. . . . . . . . . . . . . . 27,685 tons/day Additional amount to be processed during and after 1967 ...... . . . . . . . . . . . . . .. 33,965 tons/day

Breakdown

Amount to be processed by continuous mechanical furnaces ..... 19,530 tons/day Amount to be processed by batch-fed furnaces ..... . . . . . . . . .. . . . . . . . .... 14,435 tons/day

(Data obtained from Study on Long-Term Plan for De­velopment of Facilities for Daily Living, published by the Environmental Sanitation Division of the Ministry of Health and Welfare)

NATURE OF REFUSE

The term "refuse" as used in this presentation includes various rubbish, trash, garbage and other wastes disposed of in daily human living. The nature of this refuse varies depending upon daily living conditions, season, weather,

etc. Refuse in Japan has a far higher moisture content and lower calorific value (lower heating value: 500 to 1,300 Kcal/kg, 40 to 70 percent moisture content) than that of Europe or America (lower heating value: 1,000-2,500 Kcal/kg, 10 to 45 percent moisture content). A comparative example showing values for refuse in Tokyo and for that in New York is provided in Table 1.

TABLE 1 COMPARISON OF TOKYO AND NEW YORK REFUSE.

IN WEIGHT PERCENT

Tokyo New York

Paper 24.8 42.0

Ral!s 3.6 0.6

Wood 3.5 2.4

Garbage 34.7 12.0

Plastics 2.2 0.7

Greens, Leaves 2.2 10.5

Dirt, Ashes L9.3 14.0

Glass, Ceramics 3.3 6.0

Meta I J ics 2.8 8.0

Oils 0.9

Unclaaai fied 3.6 2.9

100.0 100.0

These data Bre components of raw refuse that were obtained in 1963.

From this table, it can be clearly seen that the refuse in Japan has a higher percentage of garbage and a lower quantity of paper. Approximately 70 to 90 percent of all garbage consists of waste from vegetables, meat, corn, and uneaten remains of foodstuff. It is readily apparent that the amount of garbage in the refuse has a pronounced affect upon the moisture content, as has been borne out from results of actual studies. (See Fig. 6)

Moreover, in examining the physical component of raw refuse and that of dried refuse, it can be clearly seen that garbage is considerably reduced as a result of de­hydration. Table 2 shows the effect on the major com­ponents.

The high content of garbage in Japanese refuse is at­tributed to the differences in food processing, the dif­ferences in methods of cooking as contrasted to those em­ployed in Europe and America, and the lack of garbage disposal units.

The outline below shows the amount of vinyl chloride resins in raw refuse which cause incinerator plant corrosion. Table 3 shows the annual change of plastic content in raw refuse.

184

)l1 10

'0

- JO c: ! c: 40 0 u � � JO 1/1

0 � to

10

I b 20 JO 40 so 60 � � Garbage

FIG. 6 RELA TIONSHIP BETWEEN GARBAGE

AND MOISTURE CONTENT IN RAW

REFUSE

TABLE 2 PHYSICAL COMPONENT OF RAW REFUSE AND

DRIED REFUSE, IN WEIGHT PERCENT

Paper

Rags

Wood

Garba/!,e

Plastics

Greens, Leaves

Glass, Ceramics

Metal1ics

Raw Refuse

25.35

1.95

2.20

32.79

4.15

6.38

4.11

1. 59

Dirt, Ashes ·the others 20.82

100.00

These data were obtained in Tokyo in 1966.

TABLE 3

Dried Refuse

27.29

2.53

3.42

17.11

7.10

6.34

8.64

3.18

23.79

100.00

ANNUAL CHANGE IN PLASTIC CONTENT OF RAW REFUSE. IN WEIGHT PERCENT

1963

1964

1965

2.7

3.4

4.2 These data were obtained in Tokyo.

The amount of vinyl chloride resins contained in raw refuse can be estimated based on the fact that approxi­mately 35 percent of all plastic products are vinyl chloride resins. Moreover, from the table it can be seen that the amount of plastic waste contained in refuse is increasing annually, making corrosion due to the presence of Hel gas an ever-increasing problem.

The nature of refuse can generally be represented by proportionate amounts of dry combustible content, mois­ture content, and ash content. On the basis of this clas­sification, refuse in Japan can be represented by the data shown in Fig. 7.

In Fig. 7, the combustible content represents the amount of refuse reduced by combustion-in other words, the amount of refuse less moisture and ash content. The amount of moisture refers to the natural moisture con­tent of the materials as well as moisture added by rain fall or in the cooking process. The ash content represents the residue, including metal, glass, ceramics, stones, which are noncombustible, as well as the burnt residue of the combustible materials. By examining the values for refuse in Europe and America in Fig. 7, it can be readily seen

60 10

that the refuse in Japan has a much higher moisture con­tent and lower calorific value.

Although refuse is a mixture of various kinds of ma­terial, the composition and calorific value of the combus­tible content in the refuse are comparatively constant. These values are shown in Table 4.

TABLE 4

CHEMICAL COMPOSITION AND CALORIFIC

VALUE OF COMBUSTIBLE CONTENT

Carbon 42 - S6 �

Hydro�en S - 7

Oxy�en 40 - 48 �

SuI fur 0.3-0.9 �

Nitro�en 0.6-1.9 �

Hi�her Heating Value 4000 - 6000 kcal/k�

The following equation for estimating lower heating value (HI Kcal/kg) of refuse, based upon combustible content (B percent) and moisture content (W percent) is

w

B

W

Range of Refus e in Japan

Range of Refuse in Europe and America

Combustible Content in %

Moisture Content in %

A Ash Content in %

Range of Refuse firing without assisting fuel

FIG. 7 COMPOSITION AND LOWER HEATING VALUE OF REFUSE

185

introduced from a statistical study conducted in Yoko­hama City:

HI = 45.91B - 6.00W

This equation can also be applied with comparatively reliable accuracy to refuse in other parts of Japan. This relationship is also shown in Fig. 7.

From experience it has been found that refuse can be burned at sustained high temperatures of 700 C or higher without adding fuel to assist in burning, where moisture content is 60 percent or less, combustible content is 25 percent or more, and lower heating value is BOO Kcal/kg or greater. Referring to Fig. 7, it can be seen that the refuse in Japan will not burn continuously at high tem­perature without using fuel to assist burning in many cases, while the refuse in Europe and America usually does not require any fuel to bum at high temperatures.

It is a well known fact that fuel combustion rate (kg/sq. m/hr) is affected by the calorific value of the fuel, temperature of air used for combustion, type of grate,

2fO

., ... a

0:: c: 200 0 .-

...

., "

..c

E 0 I�O U

100

so

Air Temperature 300°C

I I

I I I /

too 1000

/ /

I

1/

I

ISOf) 2000 kca I/kg Lower Heating Value

FIG. 8 RELATION BETWEEN REFUSE COMBUSTION RATE AND LOWER HEATING VALUE

1B6

etc. Our experience with the relationship between com­bustion rate and lower heating value is shown in Fig. B.

The relationship between combustion rate and lower heating value shown in this graph was obtained from refuse having a lower heating value of BOO to 1,300 Kca1!kg. By assuming that this basic relationship holds, and extending this curve, it can be estimated that refuse having lower heating value of approximately 2,000 Kcal/kg (such as that found in Europe and America) has a combustion rate of approximately 350 kg/sq. m/hr (71.5 Ib/sq. ft/hr). From this information, it can be readily concluded that Japanese incinerators designed for processing refuse which has a lower heating value on the order of 700 Kcal/kg must have a grate area of almost twice that of incinerators of equivalent capacity used in Europe and America. Thus, the following essentials must be carefully considered in the design and production of high-quality incinerators which will completely burn refuse with a high moisture content and low calorific value.

• For improved incinerating efficiency, means should be provided for effective drying of refuse.

• The amount of fuel consumed to assist in combus­tion should be held to a minimum.

• The combustion rate should be maximized. Efforts by Japanese incinerator manufacturers have

primarily been directed toward development of the type of incinerators that will satisfy the conditions described above.

TYPES OF JAPANESE INCINERATORS

As previously stated, refuse in Japan has a high mois­ture content and low calorific value. Therefore, to com­pletely burn this refuse at high temperature, the continu­ous-feed mechanical incinerators recently constructed in Japan must be equipped to preheat the air used for com­bustion.

Drying equipment is used to decrease moisture con­tent. This equipment features several design concepts based on the theories advanced by various manufacturers. One method employed is that in which the surface of the refuse is subjected to a high temperature by radiation and convection from hot gas flame. This has long been ac­cepted as the simplest and most effective method. It alone is sufficient to incinerate comparatively dry refuse.

For refuse which has a high moisture content, the inner layers of refuse cannot be effectively dried, therefore these layers must be turned over. For this type of process· ing, it is highly desirable to employ a simple, reliable con-

struction. However, for refuse which has an extremely high moisture content (such as that in Japan), the refuse cannot be turned over readily. Thus, to dry sufficiently the inner layers of refuse, a method incorporating both ventilation and surface drying which utilizes the porosity of the refuse layers is employed in Japan. Two kinds of ventilation drying are employed; hot-gas drying and hot­air drying. The difference between these methods is the drying equipment. In the hot-gas method there is no fear of blow-off (crater) due to partial burning of the refuse layer, and a conventional and reliable traveling grate stoker can be employed. Since the refuse partially burns in the hot-air method, a reciprocating stoker which proper­ly shakes the refuse layers must be used to prevent blow­off (crater).

There are various types of reciprocating stokers. As classified according to type of stoker, the mechanical in­cinerators in Japan can be generally classified into the following types:

Traveling grate stoker Chain grate stoker Gas drying Reciprocating stokers:

Stepped grate stokers Shaking stokers

(Von Roll type) Rocker action grat� stokers Air drying

Rotating grate stoker (VKW type) Rotary kiln stoker

r

t

.

Since the refuse is dried and burned on the stoker, the performance of this mechanism has a direct bearing upon that of the incinerator, and the reliability and durability of the stoker is a highly important factor from the stand­points of operation, maintenance and control.

In Japan there are currently approximately 28 inciner­ator manufacturers, including manufacturers of mechan­ical and batch-fed incinerators. of these manufacturers, approximately 10 are boiler makers or ship builders. Facilities of this type have not yet been completed in large cities because modern mechanical incinerators have just recently been introduced. Average capacity of in­cinerator processing is between 90 and 150 tons per day per unit. There are only a few incinerators of 200 to 300 tons per day capacity. Incinerator plants with capacities of up to 900 tons per day using these high-capacity incin­erators are under construction.

The following descriptions are of typical incinerator plants just constructed or now under construction.

Fig. 9 shows a 600-ton-per-day plant in Edogawa, Tokyo (three 200-tons-per-day units, 1966 ). This plant employs water-spray gas cooling equipment which is quite common in Japan and uses both stepped and chain grate stokers. These incinerators are equipped with a cranking mechanism which spreads the layers of refuse on the chain grate stoker. It also features a heavy oil burner to assist in burning. Since the plant is located in a residential area, an electrostatic precipitator is used for dust collection.

• " . .

•

J

FIG. 9 EOOGAWA PLANT, TOKYO, 200 TONS/OA Y x 3 UNITS,

1966, by TAGUMA BOIi:ER MFG. CO.

187

According to general Japanese regulations the unburned residue in the ash must be kept within 10 percent.

Fig. 10 shows a refuse incineration plant in !sogo, Yokohama which has a 450-tons-per-day capacity (150 tons/day x 3 units, 1968). In this plant, gas cooling is accomplished by a water-tube boiler. A portion of the steam generated is presently utilized for a night-soil processing plant and will be used in the future for con­verting sea water to fresh water. Multi-stage traveling grate stokers are used as well as special 3-stage drum feeders. This plant is located in the Keihin industrial area and electrostatic precipitators are used for dust col­lection to prevent creating a public nuisance. This plant is also equipped with smoke purification equipment to ftlter out harmful toxic or corrosive gases (S02,CI2, HCI). At the outlet of the 85-meter stack, a nozzle is provided so as to give the same effect as a 100-meter stack. These plants are equivalent to those in Europe from the stand­points of both capacity and performance.

Fig. 11 shows the Nishi-Yodo Plant at Osaka which has a capacity of 400 tons per day (200 tons/day x 2 units, 1966) and a power generating plant. The equip­ment installed in this plant is the first imported to Japan. Several plants with such power generating equipment are currently being planned for various locations. This type

_ . ' -- --

of plant has many features and design'conceptions that can be effectively employed in future incinerator plants. Much has been learned from this equipment about modern European design.

--

. --

+

In the Nishi-Yodo plant, gas cooling is performed in a water-tube boiler, and with the steam of 23 kg/ cm 2 g, 350 C generated at the rate of 17 tons per hour, 2,700 KW of electric power is generated and transmitted to the out­side. The plant is located in the heart of the Hanshin in­dustrial area and employs an electrostatic precipitator for dust collection.

Fig. 12 shows the Yao Plant in Osaka City which has a 600-tons-per-day capacity (150 tons/day x 4 units, 1965). This plant uses multi-stage, stepped-grate stokers; gas cooling is accomplished by a water-spray system. An electrostatic precipitator is also used in this plant. Electro­static precipitators are used in all Japanese plants in large cities to prevent public hazards and nuisance. There is also a trend toward increasing use of this type of precipi­tator in the industrial areas in small and medium-sized • • CInes.

Fig. 13 shows the Adachi Plant in Tokyo, which has a capacity of 600 tons per day (100 tons/day x 6 units, 1964). This plant employs multi-stage, chain-grate stokers.

Fig. 14 shows a plant in Moriguchi City which has a single unit with a 90-tons-per-day capacity (1965). This plant uses a Diisseldorf-type, rotating-grate stoker. In a small-to-medium-sized city, a multicyclone dust collector is used and the dust content of the stack gas is limited to under 0.7 g/Nm3• A small economizer is also employed so as to utilize the heat generated. The hot water pro­duced is utilized for hot water supply and heating. Gas

FIG. 10 ISOGO PLANT, YOKOHAMA, 150 TONSIDA Y x 3 UNITS, 1968, BY MITSUBISHI HEA VY INDUSTRIES

188

FIG. 11 NISHI· YODO PLANT, OSAKA, 200 TONS/DA Y x 2 UNITS,

1966, BY VON ROLL CO.

FIG. 12 YAO PLANT, OSAKA, 150 TONS/DAY x 4 UNITS,

1965, BY HITACHI, LTD.

189

I

FIG. 13 ADACHI PLANT, TOKYO, 100 TONS/DA Y x 6 UNITS, 1964

FIG. 14 MORIGUCHI CITY PLANT, 90 TONS/DA Y x 1 UNIT, 1965

190

cooling is accomplished entirely through a water-spray cooling device.

Fig. 15 shows a plant in Kamakura City which has a 150-tons-per-day capacity (75 tons/day x 2 units, 1965). This plant employs rotary kiln drying equipment and rocker-action grate incinerating stokers.

The foregoing is a description of typical Japanese in­cinerators. Descriptions of numerous other incinerators are omitted because they are either the same as the ones described or are combinations of the various types described.

INCINERATORS WITH THE HOT-GAS DRYING METHOD

This section describes the hot-gas drying method in which combustion gas is recirculated for drying, and the operation of typical incinerators employing this type of drying equipment.

There are many important reasons why refuse should be dried; ftrst, as previously stated, the refuse has a high

. moisture content, second, the refuse is not of a uniform consistency. In considering the method to be employed in incineration, the ignition temperature, combustion time, and caloriftc value are the primary factors to be considered. Ordinary fuels have a low ignition tempera­ture, high caloriftc value and are of a uniform consistency which has uniform combustion time. Thus, in planning

complete incineration, materials which are not of uniform consistency must be changed to approach the consistency of ordinary fuels. This is a highly important factor. Al­though the ignition temperature of the refuse is low, its caloriftc value varies from below average to that of paper or wood which have the same caloriftc value as ordinary fuels. Similar variations in combustion time also occur. These variations complicate stoker design. In attempting to obtain complete incineration, it would be necessary to use equipment which is excessively large and even so complete incineration is hard to attain.

In studying refuse incineration time, it can be seen that more time is required for drying and heating refuse to the ignition temperature, while the actual burning time is very short. Hence, in attempting to obtain com­plete incineration, refuse-drying time must be reduced, and combustion time made as uniforf!l as possible.

In drying refuse with high moisture content, burning of only the easily combustible materials such as dry paper and wood, lowers the calorific value of the remaining

. refuse and further reduces the burning temperature, mak­ing complete incineration and deodorization extremely difftcult. Thus, it seems to be preferable that easily­combustible materials with high caloriftc value should not be burnt until the moisture content is reduced to 40- 50 percent, and drying should be performed so that the caloriftc value is retained.

Refuse-drying tests using a model incinerator have

FIG. 15 KAMUKURA CITY PLANT, 75 TONS/DA Y x 2 UNITS, 1965

191

been performed. In these tests the temperature of the drying medium was regulated by mixing tempering air

with combustion gases from heavy oil or propane fuel gas of 19 to 20 percent 02 (almost the same as air). This mixture was supplied under pressure from below the grate, and refuse was dried by ventilation. As a result, since the drying medium was at temperatures of 200 C or higher, when the temperature of the refuse layer reached 180 to 200 C, the easiiy combustible material was par­tially ignited, and burning started quickly. It was also discovered that partial blow-off (cratering) occurred on part of the refuse layer. In a second test, the drying was performed by varying the percentage of °2, This was done by using a drying medium of heavy-oil burnt gas and regulating the temperature by water spray. It was found that ignition became increasingly difficult as the per­centage of 02 was reduced. Results of these tests are shown in Fig. 16.

-

c �

·c roo

Gl300 Q,

E Gl

I-c 0 200 .-

-.-

c C>

-

/00

10 IS

Oz % in Dry ing Gas

FIG. 16 RELA TIONSHIP BETWEEN IGNITION

TEMPERATURE AND 02 PERCENT IN GAS

20

As can be seen from this figure, in the drying of refuse without burning the combustible content within the dry­ing range, combustion gas can be used more effectively as a high temperature drying medium. Also, more drying heat can be applied.

It was found from this experiment that ventilation drying that uses the drying medium air with a high-oxygen

192

content, causes the refuse layers to ignite partially, re­sulting in blow-off. If this condition continues, the dry­ing of wet refuse, which requires the most drying, will not be accomplished. Moreover, the air which is not used for drying will be blown into the furnace through the craters, the -amount of excessive air will increase, furnace tem­perature will be lowered, and a large-capacity exhaust-gas induction system will be required. Conversely, this con­dition does not occur in hot-gas drying. Moreover, no high-capacity gas-exhaust devices such as dust collectors or fans are required because the gas is recirculated. Also, a high percentage of CO2 can be effectively maintained.

If preheated air is used for burning refuse with a high calorific value, furnace temperature will become extreme­ly high and may even exceed 1,000 C (1832 F) which may adversely affect refractories. Thus, the temperature of the air used for incineration must normally be lowered. However, as was found in the experiment described above, the ignition property of the refuse is lowered when the air temperature is lowered below 180 to 200 C, complete incineration is not obtained and combustion is highly in­efficient. By introducing recirculated gas having a tem­perature lower than that of the furnace, furnace tempera­tures are reduced. In this way, a greater amount of re­circulated gas is required to lower furnace temperature than for refuse drying.

Advantages of recirculating gas drying are: • Flue gas with temperatures higher than air tempera­

tures can be employed as the drying medium. • The CO2 content of the combustion gas can be in­

creased, and the required capacity of the gas exhaust system can be materially reduced.

• Greater drying heat can be applied at the same tem­peratures because the specific heat of flue gas with its higher moisture content is higher than that of air .

•

• Excessive furnace temperature rise can be prevented by controling the amount of recirculating gas.

A flow diagram of a recirculating-gas, drying-type in­cinerator is shown in Fig. 17. Also shown is an incinera­tor (Kobe City Plant) which was recently put into opera­tion. An outline of results of operating tests for this plant is also provided. (Fig. 18 and Fig. 19).

CORROSION

Since the construction of the Sagamihara Plant in 1964, the company has constructed eigh t plan ts (15 incinerators, having an overall capacity of 1,980 tons per day) all of which are in operation, and is in the process of construct­ing 6 plants (14 incinerators, having an overall capacity

of 1,920 tons per day). During this time, corrosion ap­parently caused by components in the flue gas, has been observed in the plants already constructed. Various types of corrosion have been noted and they can be gen­erally classified into two types: 1) Corrosion that oc­curs at high temperatures, such as corrosion of ducts leading from the furnace outlet to the gas cooling cham­ber, the air preheater inlet tube plate or the tube inlet of air preheater (made of austenitic stainless steel, AISI 304)

which are constantly exposed to flue gas at temperatures on the order of 700 to 1,000 Cj and 2) Corrosion attack­ing the ducts, dust-collector casing, and other components which are exposed to comparatively low environmental temperatures, normally 200 C or less.

n ' 2' ",.

n

This is attributed to corrosive elements in the flue gas, corrosive materials in fly ash, and environmental tem­perature. The composition and concentration of the incinerator flue gas and fly ash were measured for an in-

Incinerating Processes

Flow of Refuse

The refuse charging hopper is equipped, at its bottom, with a proper feeding device (3-drum feeder) to feed refuse into the furnace. Refuse charged into the furnace is fIrst dried fully by hot exhaust gas on the drying stoker and then forwarded to the combus­tion stoker on which the refuse burns actively with the supply of hot fresh air. Residue dropping from the end of combustion stoker is completely burnt to ashes in the residue combustion chamber of hopper-shape and thereafter delivered out of the furnace by means of the ash delivery.

Flow of Air

The air drawn from the upper part of the refuse storage pit by the forced-draft fan, passes through the air preheater where it receives heat from hot exhaust gas after incinera­tion. Then the air is heated up to about 300 C and forced into the furnace through the bottom of the combustion stoker. In addition, some portion of hot air is blown into the furnace and residue combustion chamber through the nozzles fitted on the side walls of means, which contributes greatly to the perfect com bustion of refuse.

Flow of Gas

As combustion gas drawn out of the furnace is subjected to water jet in the gas cooling chamber. After the gas temperature has been lowered in this way, the gas is forwarded to the dust collector for removal of dust and prevention of public harm. The combustion exhaust gas is branched off partially at the furnace outlet, and fed into the drying stoker after the temperature being reduced to around 350 C in the air preheater. Temperatures of burning air and drying gas are controlled by the damper (which regulates gas weight through air preheater) at the ou tlet of gas cooling cham ber.

1. Refuse • storage

pit 2. Crane 3. Charging

hopper

I 4. Drum

feeder 5. Furnace 6. Drying

5 stoker , 0 ' ,

. - ' 7. Combustion

stoker II t4< 8. Residue

combustion chamber

��

FIG. 17 FLOW DIAGRAM OF MITSUBISHI GAS DRYING REFUSE INCINERATOR

193

9. Ash deli-very conveyor

10. Burner 11. Gas cool-

. mg chamber

12. Dust collector

13. Induced draft fan

14. Stack 15. Air pre-

heater 16. Gas re-

circulat-ing fan

17. Forced draft fan

TABLE 5 cinerator under actual operating conditions. Tables 5 through 7 show the results of measurements taken at the Yokohama Tsurumi Plant.

VARIATIONS IN ANALYTICAL VALUES OF GASANO GAS TEMPERATURES AT VARIOUS PARTS

The resulting analyses attribute the corrosion primar­ily to the presence of such gases as HCI, S02' S03' and high moisture content.

OF THE INCINERATOR

Similar results were obtained from measurements taken at other plants. Yokohama Tsurumi Plant, mentioned above, is located in the Keihin Industrial area and there was a high incident of plastic waste from industrial plants (including a high vinyl chloride resin content) in the refuse. An inordinately high amount of HCI gas (more than 2,000 PPM) was detected with respect to this type of refuse. As previously stated, even ordinary household refuse has a respectably high vinyl chloride resin content. It appears, therefore, that there is a fairly high concen­tration of HCI gas for refuse from almost any location.

Alkali sulfates (Na2C04' K2S04, CaS04) and chlorides (NaCI, KCI, CaCI2), along with other oxide elements, were found in the fly ash. It is believed that

Purnace outlet

c .. cool!nc c .... b ...

inlet

Duet collect.-

or inlet

Due. collect..

or o_tl.t

Air prehe.�

er inlet

ain. •• x. a.an

aln. •• s. •• an

aln. ...... a.an

ain .. •• x. a.an

aln .. •• x.

I··en

I

HCl

p""

300 ZZOO

850

290 1600

800

260 560 .00

220 360 280

200

.00 930 I

S02 s03

p"" p""

! 10

12.0

52 6.0 28 3.7

nil trace 95 1.5 43 3.5

21 1.1 193 1.9

14 1.8

.9 1 •• 140 1 ••

60 1..

trace 0.1·1 12 0.14 1

3.3 I

0.1. !

FIG. 18 SECTIONAL DIAGRAM OF KOBE CITY PLANT,

150 TONS/DAY X 2 UNITS, 1967

194

Moi.ture CO2 02

� � �

12.0 9.9 9.2 22.0 11.5 10.0 17.0 10.6 9.6

14.0 10.1 10.1 21.0 10.8 10.2 11.0 10.5 10.1

22.0 6.3 12.5 36.0 1.9 14.6 1 28.0 7.1 13.5

18.9 6.3 12.9 25 • • 7.6 1 •• 7 ZZ.O 1.0 13.1

13.0 6.3 1 12 •• 15.0 8.1 14.5 14.0 7.1 13.5

I

C •• T_pe .. ature

°c

700 900 800

600 150 680

200 2.0 220

190 220 200

500 600 550

TABLE 6

RESULTS OF CHEMICAL ANALYSIS OF INCINERATOR

FL Y ASH IN WEIGHT PERCENT

Na

K

Si02

CLIO

Fez03

A1203

COlO

MaO

znO

504 Cl

pH at 25 °c

Fly Aah at Furnace outlet

3.73

3.75

2 2.9

trace

3.20

14.70

21. 3

2.90

trace

29.5

0.14

6.7

Fly Aah at Duat Collector

9.25

12.3

9.40

1.90

6.87

6.58

4.76

0.91

trace

42.9

5.68

4.2 • ,

pH value was measured with hot pure water in which 2 gr/lOO ml of fly ash were immersed.

TABLE 7

RESULTS OF X-RAY ANALYSIS OF FLY ASH

Fly Ash at Furnace outlet - Ca504• K2S04' Na2S04'

CaC12. HzO. KCl

Fly Ash at Dust Collector - NaCl. K2S04. KCl. Ca504

195

corrosion may result from these alkali elements in the burning refuse and ash. The ash accumulated in the dust collector, a low-temperature part, shows a comparatively strong acidic reaction property. It was deduced that this action was due to acids such as HCI and H2S04 absorbed from the gases by the ash deposited on the metal surfaces as the gases cooled. This was considered to be a dew­point corrosion.

Considerable research is now being conducted in various countries on the condition and mechanism of corrosion on various incinerator components. It is re­ported that corrosion is caused in high temperature units, by so-called sulphate chloride attack, i.e. the combination of alkali sulfates and chlorides, or by alkali iron trisulfate, K3Fe (S04)3' Na3Fe (S04h·

We are also conducting extensive research on the condition and mechanism of corrosion, and are conducting laboratory experiments based on test data and measure­ments taken under actual incinerator plant operations. In the plant, practical counter measures have been applied according to the mechanism of the corrosion on each component.

Incinerator flue gas corrosion, however, is not the only problem; air pollution prevention is equally important. Further comprehensive study and research will therefore be required.

CONCLUSION

This paper described the different practices of refuse incineration in Japan and Europe and America. We sincerely hope that this work will be of value in future studies and for development of mechanical incinerator equipment to be used for refuse having a high moisture content and low calorific value.

11 Ib 15 I� IJ 11 II

� I � t 7 b S .. J 2

07 .Ob

i: as 8.01-

Vl aJ 02

:r: )_ I - If " If � IJ � Il

II (; I(J ->l. a "

v; I

z o �

� ..<= Ql

7 " S

.4j 30 � " � � III c u c

10

t q

r r� I I : I

Qrsat (C02)

,.. ------------., � ___ J

10

I �----------� I ____ � I I I I I

II 12 1.3 IS Ib 17 "

-·-Drum Feeder - Plate Feeder (at the Res idue

Combustion Chamber) ---Ash Delivery Conveyor

- Drying Stoker ---Combustion Stoker

," 20 21 12 Time, Hours

FIG. 19-1 RESUL TS OF OPERA TlONAL TEST A T KOBE CITY PLANT

196

� '" c: c: ., Q. o ., Q. E o

o

0. <:{ /0 E 0

.5 -10

� :> � ., Q. E .,

I-

- /

/ .---,. - '" ,

, -�, /�----- -------------� , "'v'" ....". ,

r--' 1 \ , \

1 \ I \

I \ f \ r, , \r-·---:::=o�·-·---·-·J _\._

L �\ ,'- -------------", -'--i ______ --J '--_./ " /

'.J

--�--------�_r--�--.�� ____ ------

/"- r·-----------·-----�·---··-'

. _ ...... ../ t--. / ' I ' /

. \J

-- Forced Draft Fan Inlet ----Gas Recirculating Fan Inlet

-- Furnace ---- Combustion Air --- Drying Gas (Orifice %) --'- Dust Collecter Inlet

Dust Collector Outlet --- Stack Inlet

F urance Outlet Gas Combustion Air Drying Gas

---- Exhoust Gas

-�1---41�--�'--��--41----+1--�1r_--41-__+---+1---�--�'----r---+----r-f q 10 /1 12 IJ 14 /J Ii> 17 il 19 ;0 2/ 21 Time, Hours

FIG. 19-2 RESUL TS OF OPERA TlONAL TEST A T KOBE CITY PLANT

197