Disposal of municipal solid wastes by incineration in Hong Kong

Conservntion G Recycling, Printed in Great Britain

DISPOSAL

Vo1.7,No.2-4,

pp.73-82,

1984

0361-3658/84 Pergamon

OF MUNICIPAL

SOLID WASTES

TSAI-HONG Electrical

BY INCINERATION

$03.00/0 Press Ltd.

IN HONG KONG

LO

and Mechanica1 Services Hong Kong Government

Department

ABSTRACT tonnes out of a total generation of 6,500 tonnes refuse are processed Currently, about 2,500 daily at the four Government Incineration Plants and smal1 village-type incinerators. Various modifications have been or wil1 be carried out on the existing old plants to improve gas This paper gives comprehensive data emission and to increase the throughput capacities. on the successful operation of the new Kwai Chung Plant which proves that mass incineration of 'as-received' municipal refuse is a viable method of disposal by comparison with other the end-product of incineration is sterile, suitable for availabie methods. Furthermore, land reclamation and can be used as a covering material in controlled tips.

INTRODUCTION Hong Kong, comprising Victoria Island, Kowloon Peninsula and the New Territories, hals a total area of 1,036km2 with a population of 5,313,200 according to the latest statistical Lying within the tropics, the summer temperatures are in the region survey in June 1983. of 28'C to 35'C with relative humidity regularly reaching beyond 95%. The average rainfall in a year is 2,224.7mm. Typhoons may strike any time between May and November often bringing heavy tropical rain. On the other hand between November and March, sunny cool days make an invigorating contrast to the long hot and wet summer. Among tbe 6,500 tonnes of solid waste generated per day in Hong Kong, 3,600, 1,700 and 1,200 tonnes are of domestic, industrial and construction nature respectively. Refuse ccllecting vehicles, which are operated by the City Services Department and the New Territories Services tour on-street collecting points to which refuse mostly of domestic nature is Department, brought at scheduled times. Normally trade, industrial and commercial wastes are delivered by private operators for disposal at the controlled tips. Only a smal1 amount of commercial and industrial wastes, in the region of 100 tonnes per day, is accepted via a permit system to improve the overall combustion efficiency with domestic waste at the two incineration plants at Kennedy Town and Lai Chi Kok. In remote areas such as outlying islands, some 45 tonnes of solid waste are disposed of each day by the City Services Department, New Territories Service Department and Housing Department in 76 village-type incinerators. The majority of these primitive incinerators are not fitted with oil burners, blowers or secondary burning chambers. The usual problems associated with the village incinerators are smoke emission and poor burn-out. Some of these inefficient incinerators wil1 soon be discarded when vehicle accesses for bulk refuse collection are provided. The remaining old units wil1 be replaced by modern village-type modular incinerators complete with secondary burning chambers and accessories such as blowers and burners. The dust burden from the modern village incinerator is much improved and does not exceed 400 mg Nm -3 corrected to 12% Co2.

73

74

Tsai-Hong

REFUSE DISPOSAL

Lo

BY MASS INCINERATION

About 56% of the daily generation of 6,500 tonnes of solid wastes is directly disposed of at the controlled tips which are operated by the Civil Engineering Office of the Engineering Development Department. The Electrical and Mechanica1 Services Department, where the author is working, is responsible for processing 38% of the total waste by the four incineration plants and also approxiamtely 6% by the Chai Wan Composting and Pulverising Plant. In genera1 refuse incineration achieves a mass reduction ratio of 4:l and a volume reduction ratio of 17:l. The location of the refuse processing plants in the Hong Kong Territory is shown in Fig.1.

NEW TERRITORIES

__

KOWLOON

IANTAU

Fig.1. Key:

(1) (2) (3) (4) (5)

Location

of Major

Refuse

Processing

Plants

Kennedy Town Incineration Plant - normal throughput 650 tonnes/day Lai Chi Kok 'A' Incineration Plant - normal throughput 550 tonnes/day Lai Chi Kok 'B' Incineration Plant - normal throughput 450 tonnes/day Kwai Chung Incineration Plant - normal throughput 870 tonnes/day Chai Wan Composting & Pulverising Plant - normal throughput 380tonnes/day

The first incineration plant was built at Kennedy Town on Hong Kong Island and commissioned in 1966 with four Vdlund Incineration Lines each designed for 250 tonnes per day. A sectional view of an incineration line is shown in Fig.2. Hot gases from two incinerator units pass through a waste heat boiler. Saturated steam generated from the two boilers at 13,600 kg h-1 at 204'C and 16 bar is used to run a 1200kW condensing turbo-alternator for self-generation of electricity. A fifth incineration line, which has a big after-burn chamber with air infiltration in place of a waste heat boiler, was commissioned in 1975. Normally only four units are put in operation with one taken out for shut-down maintenance. The incineration process, which is similar among the four incineration plants, is described in more detail later, when reference is made to the Kwai Chung Incineration Plant.

Disposal

of Municipal

Solid Wastes

75

in Hong Kong

PUNP

Fig.2.

Section

of an Incineration

Unit of Kennedy

Town Incineration

Plant

Various modifications have been or wil1 be implemented to improve the performance of the Kennedy Town Incineration Plant. In 1975, a venturi tube was installed at the top of each chimney to improve the gas dispersion. In order to reduce the amount of dust emitted from the chimneys, two pilot high voltage electrostatic precipitators of capita1 tost of HK$8.65 million have just been installed for Units 3 and 4 to replace the less efficient mechanica1 The results are most encouraging and the dust emission can be brought dust collectors. down below the strict standard of 115 mg Nmw3 corrected to 12% CO2 with a collecting efficIt is planned to insta11 similar precipitators to the remaining three iency above 95%. A bulky waste shredder of capita1 tost around HKs8.0 million has recently incinerators. been installed to crush bulky wastes and chicken baskets to sizes suitable for charging into the incinerator hoppers. The second incineration plant located at Lai Chi Kok Plant 'A', consisting of four incineration lines each of designed capacity 250 tonnes per day, was commissioned in 1969. Each similar in structure to the Kennedy Town Incinerators, has its own waste heat incinerator, superheated steam at 295'C and 16 bar. boiler capable of producing 13,900 kg h-1 Steam from the operating boilers is used to drive a 2500kW turbo-alternator for self-generation In 1974, Plant 'B' was commissioned with four incineration lines each of of electricity. designed capacity of 250 tonnes per day. There are no waste heat boilers and the electricity supply is normally provided by the turbo-alternator at Plant 'A'. Since the commissioning of Plant 'A' tal1 housing developments have been built at the neighbouring Mei Poo Sun Chuen. A new'tall multi-flue chimney of 120111was built in 1978 with inter-connecting ducting to achieve a better dispersion Of al1 flue gases from both Plants 'A' and 'B' in place of the original twin 60m chimneys. With a view to further reducing the dust emission, a scheme is in hand to insta11 high voltage electrostatic precipitators to al1 eight units. A shredder has been installed at Plant 'A' to crush bulky wastes prior to incineration. As can be seen in Fig.3, there are big variations in the moisture content (40%-65%) in the crude refuse generated in Hong Kong. Hence the heating values vary from 1312 to 2248 kcalkg-1 which are low compared with some ether countries. Such high moisture content and big fluctuation in the heating value mainly attribute to the low output efficiency. Based on the

76

Tsai-Hong

Lo

working experience with these old incineration plants, the following areas of concern were identified and the respective remedies were subsequently incorporated in the design of the new Kwai Chung Incineration Plant.

Afff CONTENT it.6

2500

% W/W It.6

500 40

15 Fig.3.

He ating Values Moisture

45

content

% W/W

Gross calorific value Low heating value % combustible % water % ash

(i)

Pre-drying

55

50

of Crude Refuse

65

60

(as received)

(by weight) = 2248 = 2004 = 38.4 = 40.0 = 21.6

2050 1770 35 45.31 19.69

1312 916 22.4 65.6 12.6

kcal kg-' kcal kg-l % W/W % W/W % w/w

treatment

The wide and frequent variations in the moisture content have resulted in fluctuations in the time required for drying and combustion. The net effect is substandard burn-out unless the charging rate can be corrected in time to compensate for such fluctuations. For refuse with high moisture content, the effect of radiation and convection from only the flame is not adequate. Even with a reciprocating grate, turbulance and turn-over of the under-layers are difficult. When there are large quantities of moisture vapour in the combustion chamber, there is a noticeable retardation of both drying and combustion of released volatiles. Hence the Kwai Chung Plant has special features such and secondary air nozzles inside the drying chambers.

as separate

drying

chambers

Disposal

(ii)

Combustion

of Municipal

Solid Wastes

in Hong Kong

77

air

is supplied to the incinerators installed Combustion air at ambient temperature To achieve more effective drying and burning, at Kennedy Town and Lai Chi Kok. provision of pre-heating of supplying air is required. (iii)

Grate combustion

ratings

The grates and kilns of the Kennedy Town and the Lai Chi Kok Plants were sized from a combustion rating for refuse having a moisture content in the order of For effectively dealing with the local refuse with heating values ranging 50/54x. from 1312 to 2248 kcal kg-1, a fully flexible grate system is necessary to maintain This can be achieved constant throughput and an acceptable burn-out standard. by designing a nomina1 combustion rating of the effective grate area (i.e. 238 kg m-2h-lplus a rotary kiln adjustable in rotating speed. (iv)

By-pass

duet arrangement

The existing incinerator units at both Kennedy Town and Lai Chi Kok have flat For Kwai horizontal gas ductings where considerable dust settlement takes place. the occurrence of dust Chung Incineration Plant, steps were taken to minimise settlement by decreasing wherever possible areas of flat horizontal duet section Furthermore, the exit position of the by-pass duet should be and tight bends. at the rear of the drying grate where a better mixed gas temperature could be achieved and flat horizontal duet areas could be minimised. (v)

Riddlings

discharge

from the drying grates

from the drying grates consist largely of unburnt debris Riddlings discharged In the new design, which cannot be handled effectively in a conveyor system. the riddlings were routed to fa11 from the hoppers of the drying grates directly into the combustion chamber for incineration. (vi)

Gas cooling The variation in Hong Kong's refuse leads to a non-uniform furnace atmosphere. fhe incorporation of a forced water circulation counteracts the effect of transient and localised variatione in gas temperatures across a waste heat boiler.

(vii)

Gas cleaning

plant

Gas cleaning by multi-vortex cyclone grit arrestors was considered one of the effective means prevalent at the time of designing the Kennedy Town and the Lai Chi Kok Incineration Plants. Due to iomprovement in modern technology, it is possible to apply high voltage electrostatic precipitation to achieve a high standard in dust emission. A scale-down electrostatic precipitator was installed for experimental purposes to treat part of the flue gases arising from incineration of the local refuse in one Lai Chi Kok Incinerator. The objectives were to establish optimum operating conditions for maximum collection efficiency and minimum internal corrosion. Al1 such data were utilised in the design of electrostatic precipitators for Kwai Chung Incineration Plant. With the above points in mind, tenders were invited in 1973 for the supply and installation of four incinerator units, each with a designed capacity of 300 tonnes per day, at Kwai Chung. The contract was eventually awarded to Messrs. Clarke Chapman - John Thompson Ltd., United Kingdom, the same supplier for the previous contracts for installation of the Kennedy Town and the Lai Chi Kok Incineration Plants. An outline of the Kwai Chung Incineration Plant is illustrated in Fig.4. The Plant was designed to process 900 tonnes per day of refuse with three lines in operation. During the commissioning stage, the Plant was capable of meeting the designed throughput. For everyday operation, the throughput rate is set at 850-870 tonnes per day.

8

Tsai-Rong

Fig.4.

Section

of an Incineration

Unit

Lo

of Kwai Chung Incineration

Plant

Refuse is delivered to the Plant in refuse collecting vehicles which are operated by the They are first weighed City Services Department and the New Territories Services Department. Bulky waste is first stored before the contents are off-loaded into the refuse bunkers. in a separate chamber and subsequently grabbed into either of the two bulky waste shredders The composition of municipal solid waste each of shredding capacity of 10 tonnes per hour. analysed during the commissioning test is shown in Table 1.

TABLE 1. Composition of municipal solid waste analysed during the commissioning test on Kwai Chung Incineration Plant in August 1979 by the main plant contractor NE1 - ICL Ltd. UK. (fermer Clarke Chapman - John Thompson Ltd.)[21 Percentage

Type Vegetable

and putrescible

3.73 22.18

Paper and card Metals

by weight

(ferrous

and non-ferrous)

4.52 6.63

Textiles

5.96

Glass

10.19

Plastics

6.12

Wood Unclassified

+45mm siZe

19.86

Unclassified

-45mm but t12mm Size

16.12

Unclassified

-12mm

Moisture

value

4.69 45

.%

Bulk density Heating

size

(as received) (as fired)

333.3 kg m-3 1706

kcal kg

-1

Disposal

of Municipal

Solid Wastes

in Hong Kong

79

One of the two overhead electrohydraulic grabs, each fitted with a load cell, loads the The refuse passes from refuse at 3.5 tonnes each time into the incinerator charge hopper. The grate the hopper down the feed chute onto the first grate in the pre-drying chamber. assemblies consist of alternate longitudinal rows of four fixed and five moving grate blocks A hydraulic system drives al1 the moving girders of the grates inclined downwards at 15'. The refuse then falls down a vertical brick wal1 at a speed adjustable from 0 to 15 mm s-? Opposite Grates 1 and 2 are two oil burners onto the drying grate No. 2 in the same chamber. A recirculation of hot gas which are used to maintain the gaseous product temperature. at 7OO'C rises from the combustion chamber over Grate 2 to the by-pass duet fitted with Further drying is attained by drawing a portion of exhaust gas, by regulating dampers. Co01 air can be admitted means of a drying gas fan, undergrate through the grate blocks. Roth overgrate at the side walls of Grate 2 by an air fan to reduce the gas temperature. Variations of these fans can be set for automatie operation with overriding manual controls. in the calorific value of the raw refuse entering the drying chamber is sensed by a temperof the supply of undergrate dr:ying ature change which, in turn, controls the modulation The same signal in temperature change also controls the operation of the oil burners. gas. Normally the supply of drying gas and oil should only be necessary for calorific values below 1312 kcal kg-1. From the drying chamber to the combustion chamber, the refuse is discharged into a vertical drying chamber of 7116mm high where further drying takes place from contact with the reIn falling, there is a tendency for any agglomerated refuse to break circulated hot gas. The refuse proceeds from Grate 3 through a short vertical firebrick wal1 onto Grate.4. up. There is a forced draught fan for each incinerator unit capable of discharging combustion For more efficient combustion, pre-heating of the combustion air at a rate of lom-3 s-k. Normally the air is achieved by passing through a steam/air heater and a gas/air heater. air temperature is increased to 200'-230'C. The combustion air enters the combustion chamber both undergrate through the grate holes and overgrate through high velocity nozzles at the two sidewalls. Additional heat is provided, if required, by means of an oil burner in the combustion chamber, this being controlled from a temperature signal at a set point of 850~C. The maximum temperature in the combustion chamber is 95O'C-1OOO'C. The refuse is further directed into the rotary kiln where final burn-out produces a sterile The kiln, inclining at 3’ to the horizontal, can be adjusted to rotate from ash product. 0 to 12 revolutions per hour to cape with refuse of different heating values. Additional air is fed by a kiln air fan through the two end sealing boxes into the kiln so as to enable of the combustion products. combustion to take place and also to limit the temperature Control of the air supply is determined by the kiln outlet gas temperature which should be maintained at 95O'C. Flue gases from both the by-pass and the kiln then enter the after-burn chamber where any remaining volatiles are burned and mixing takes place before entry into the waste heat boiler of a forced circulation type. The boiler has a fully water-cooled enclosure from the radiant inlet chamber to the outlet from al1 the convection surface. The dust accumulated at the boiler tubes is removed frequently by retractable sootblowers. A steam and water mixture returns to the steam drum where separation takes place by a series of cyclone separators. The superhea$er in the form of pendant tubes is at the rear end. The boiler can generate 18,600 kg h superheated steam at 260' and 17 bar. kg hm1 of steam from the Only 28,000 operating boilers feeds a back-pressure turbine coupled to a brushless, salient pole alternator having an output of 2000kW. Exhaust steam at 0.3 bar is condensed in a low pressure air/steam condenser and live steam surplus to the turbine requirement is condensed in a separate high pressure air/steam condenser, the condensate being returned to the deaerator. The loss in the boiler water circuit is replenished by the output of a demineralised plant comprising mainly a cation and an anion column. The flue gas from the boiler enters a gas/air heater and the gas outlet temperature has to be maintained at 300°C irrespective of the incinerator conditions. The flue gas is subsequently directed into an electrostatic precipitator having a collecting efficiency of 95% and providing a dust outlet concentration below 0.318 g Nm-3 dry corrected to 12% CO . It is a horizontal gas flow plate type with two electrical stages operated on 35-40 kV2D.C. It contains vertically-hung collecting electrodes and discharge electrode wires. Electrically operated, rapping gear is fitted to the collecting plates and discharge wires. The flue gases yle finally discharged into a 150m high multi-flue chimney at an efflux velocity of 23 m s by means of an induced draught fan. The fan has a full load speed of 730 rpm with a maximum capacity of 49 rnm3sm1 and is coupled with a variable speed motor.

80

Tsai-Hong

Lo

Below each incinerator, the riddlings from the ccmbustion grates, the ash from the afterburn chamber, grits and dust from the boiler and precipitator are received by the submerged ash conveyor. This quenches the ash which is then conveyed to the ash bunker. Prior to dumping the ash into the bunker, the conveyor passes it over a magnetic separator to remove ferrous materials. These materials are then conveyed to the baling press which bales the metal into 3OOmm tubes. The ash delivered into the ash bunker is loaded by either of the two electrohydraulic overhead travelling cranes into ash trucks usually for landfill as a covering material but also for reclamation. Other applications for the ash have been for sub-soil drainage strata and action is being taken to evaluate the possibility of using the ash as a gravel material for road construction.

RESULTS Durinp the commissioning stage, the Kwai Chung Plant was capable of meeting the designed throughput of 900 tonnes per day with three incineration lines in operation. Except for periods of partial commissioning, plant overhaul and inadequate supply of refuse, the daily throughput of the Plant is 850-870 tonnes per day. As indicated in Tables 2, 3 and 4, the operation of Kwai Chung Plant is considered successful.

TABLE

2.

Flue gas characteristics measured at the two ends of the electrostatic precipitator of Unit 4 during the commissioning test in August 1979, Kwai Chung Incineration Plant by NEI-ICL Ltd, UK [2]

Flue gas characteristics

Average Average

Electrostatic Inlet 306.7'C

296.6OC

pressure

-67.3mm

-77.5mm

9.2

8.6

12.0

12.0

dioxide

Oxygen

% V/V

W.G.

% V/V

NO

Not measured

46.2 pg m 26.5 ug m

Sulphur

dioxide

Not measured

Sulphur

trioxide

Not measured

Chlorine

Average

chloride moisture

% V/V

Volume of gas at temperature and pressure Volume

of gas at NTP dry

Dust burden Total

Outlet

temperature

Carbon

Hydrogen

precipitator

NTP dry

dust burden

Precipitator efficiency by total dust

nil ug m

Not measured

12.0 pg m

16.47 -3 h-l m

62,143 m-3 h-l -3

1280.6 mg m

79.05 kg h-1

-3

6.9 pg me3 -3

Not measured

158,491

-3

-3

16.47 161,017

m

-3 h-3

64,198 mm3 hW1 -3 69.75 mg m -1 4.45 kg h

Disposal

TABLE

3.

of Municipal

Solid Wastes

81

in Hong Kong

Characteristics of incinerator ash found during the commissioning test in August 1979 Kwai Chung Incineration Plant by NEI-ICL Ltd. IJK [PI Ash characteristics 19%

(al

Moisture

(bl

Bulk density

(cl

Ignition

(dl

Metal

Ce)

Putrescible

Content

(dry, metal free basis)

(f)

Heavy metal

content

(dry basis)

(i) (ii) (iii)

4.

loss (dry, metal

content

of Operation

1.05%

-1

0.029 g kg 6.3

Zinc

Chromium

0.05%

2.7 g kg

Mercury

(v)

-3

8.7%

Lead

-1

g kg-1 -1

/ I

< 100 g kg

500 g kg-' -1 200 g kg -1 27.9 g kg

Nickel Arsenic

Some performance indicators for Kwai Chung Incineration Plant since first commissioning in 1979. Derived from plant records.

Amountof refuse Year

free basis)

(dry basis)

Cadmium

(vi)

1220 kg m

(wet)

(iv)

(vii)

TABLE

content

processed

Average

/

Feb-Dec 1979

120,835

414

4.6:1

3.15

During the initial commissioning period, not al1 the incinerators were available for operation. Much modification work was carried out by the Plant Contractor tc solve the teething problems. The relativ ely high oil consumption was due to the requirement of warming up the refractory with oil during the starting stage.

Jan-Dec 1980

249,660

728

4.5:1

0.46

The plant was completely shut. down for 23 days for modification work by the Contractor and preventive maintenance. When there was inadequate supply of refuse, only two incinerators were in operation.

Jan-Dec 1981

244,200

694

3.8:1

0.66

The plant was completely shut down for 13 days for modification work and preventive maintenance. The ~plant throughput was affected by inadequate supply of refuse.

Jan-Dec 1982

301,740

827

3.9:l

0.65

The plant operated

Jan-Aug 1983

194,990

848

4.0:1

1.06

Again the plant operated smoothly without major breakdowns.

smoothly.

82

Tsai-Hong

Lo

DISCUSSION In order to provide adequate disposal facilities in future years, tost, environmental and operational appraisals are being carried out on al1 potential controlled tip and incinerator sites by the Government Departments concerned, in conjunction with the Consultants. Also schemes for setting up refuse transfer stations at convenient points are being considered to achieve a tost effective transportation of solid refuse. Compacted/pulverised refuse in containers wil1 further be delivered to disposal points by barges and/or transfer vehicles from transfer stations. It is likely that a larger portion of industrial waste would be delivered from transfer stations to incineration plants. Hence future incineration plants would most likely be designed to process a mixture of domestic and industrial wastes at a higher calorific value kcal kg-1. It is estimated that steam can be generated at a rate of 2.5 kg around 2690 per kg of solid waste at 4.14 MPa and 4OO'C. Through a back pressure turbine, ap~roximately 226 kWh tonne -' can be extracted. As the in-house_;equirement is 55 kWh tonne only, the . available power for export would be 171 kWh tonne Furthermore, the estimated amount of lower steam quality of 1.03 MPa and 182"C, available for export from the incineration plant, would amount to 1870 kg of steam per tonne of waste fired. Such concepts of energy 131 wil1 certainly be put in the design of future incineration recovery and utilisation plants particularly if located in close proximity to energy-consuming Government organisations such as hospitals and laundries.

CONCLUSION making improvements to the The Hong Kong Government is conscious of continuously environmental conditions. Various pollution control ordinances have been/will be and subsequently implemented.

present enacted

A programme wil1 be worked out soon for the replacement of al1 primitive village-type inCinof the pilot electrostatic precipitators erators. Following the satisfactory performance to two incinerator units at Kennedy Town, plans are in hand to provide the same to the three remaining units at Kennedy Town and al1 eight units at Lai Chi Kok. As far as mass and volume reductions are concerned, incineration process is the best compared Effective air such as composting and baling processes. with other disposal alternatives pollution control equipment has been developed which can ensure that aerial emissions frOm waste incineration do not contribute significantly to ground leve1 concentrations of potenThe Kennedy Town and the Lai Chi Kok Incineration Plants, built between tial pollutants. 1965 and 1975 without such devices, have given the impression that combustion of wastes This is mainly due to the plume formation from the chimney stacks, is highly polluting. of fine resulting from the combination of high moisture content gases and concentrations The modern plant particulates which are highly visible during periods of high humidity. precipitators, has a far less visible at Kwai Chung, which is fitted with electrostatic flue gas analysis, indicates that combustion This fact, supported by comprehensive plume. of solid wastes need not necessarily be viewed as polluting. There is a large quantity of high energy This energy can and should be recovered.

value

solid

wastes

generated

daily

in Hong

Kong.

ACKNOWLEDGEMENTS Acknowledgement is made to the Directer of Electrical and Mechanica1 Services, Mr GJ Osborne, The author would like to thank al1 colleagues for his permission to publish this paper. who have given their encouragement and assistance in the preparation of this paper. REFERENCES 1. 2. 3.

National Incinerator Conference, Wallace, J.A., Incineration of refuse in Hong Kong. The American Society of Mechanica1 Engineers (1974). Test Report for the Internal Combustion Limited (NEI), No.4 Stream Plant Performance Refuse Incineration Plant at Kwai Chung, Hong Kong (1979). from municipal solid waste - present status and future Collier, J., Energy recovery potentiai, Hong Kong Engineer, 9, 37-48 (1980).