0957–5820/04/$30.00+0.00# 2004 Institution of Chemical Engineers

www.ingentaselect.com=titles=09575820.htm Trans IChemE, Part B, May 2004Process Safety and Environmental Protection, 82(B3): 200–207

MUNICIPAL SOLID WASTE UTILIZATION FOR INTEGRATEDCEMENT PROCESSING WITH WASTE MINIMIZATION

A Pilot Scale Proposal

K. K. H. CHOY, D. C. K. KO, W.-H. CHEUNG, J. S. C. FUNG,D. C. W. HUI, J. F. PORTER and G. MCKAY*

Department of Chemical Engineering, Hong Kong University of Science and Technology,Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China

Anovel design of an integrated process for cement production incorporating municipalsolid waste (MSW) separation and combustion has been developed. The novel designoffers significant opportunities for waste minimization. The MSW combustion system

design incorporates the use of supplementary fuel from waste marine oil. Very hightemperature, 1200�C, high turbulence and high residence time, >4 s, are achieved. This resultsin very high burnout of MSW, resulting in negligible particulate organic matter to form dioxinby de-novo synthesis. The energy produced is used for the cement process calcination oflimestone and residual heat is converted into energy to drive the cement plant. The calcinationprocess itself is used as a large scrubber to remove SOx and HCl, the latter minimizing thechance of dioxin formation further. A front end materials recovery and recycling facility,MRRF, is used to remove valuable recyclable components, chlorine-based plastics and metal-containing materials, such as batteries. The combustion of the MSW achieves a 85–90%volume reduction and the MSW ash is used as a feedstock for the production of the cementclinker.

Keywords: solid waste; integrated cement processing; waste minimization; process design;simulation.

INTRODUCTION

The opportunities for landfilling as a disposal method formunicipal solid waste (MSW) are rapidly declining withdepleting available cheap land resources and the wastefulnature of disposing of useful resources in the landfilloperation. The costs of landfilling in terms of site acquisi-tion, preparation and maintenance are extremely high. Forexample, in Hong Kong, there are three landfill sites for 6.8million people; the construction of the three sites costUS$750 million, the total area of the 270 hectares isworth US$1000 million as agricultural land and overUS$3000 million as prime real estate.

The environmental problems in terms of gaseous emis-sions and leachate containment and treatment are alsosignificant and expensive (Mackay et al., 1985). In the1970s there was a significant move towards the use ofincineration for MSW treatment. However, due to lack ofknowledge relating to emissions problems and poor incin-erator design, these early MSW incinerators developed abad environmental reputation (Law and Gordon, 1979;

Shaub and Tsang, 1983; Karasek and Dickson, 1987;Stieglitz and Vogg, 1987). The opportunities for publicsector waste reduction translate into separation, recyclingand resource recovery. Owing to the limited economicbenefits of separation and recycling (Basta et al., 1991;Donovan, 1991), and significant improvements in incin-erator design (McKay, 2002), resource recovery in theform of heat and power production has again gained favorin the past 20 years (Hasselriis, 1987; Addlink and Olie,1995; Kilgroe, 1996; Andersson et al., 1998). Mostcountries are operating at least 10 MSW incineratorunits, including the USA, Japan, Korea, the UK, Germanyand France (Shin and Chang, 1999; USEPA, 1987; WHO,1986; European Commission, 1994). The benefits ofmodern incineration plants are:

� the volume and mass of the MSW are significantlyreduced (85–90% by volume);

� the careful selection of incinerator site locations reducestransportation costs;

� the waste reduction is immediate and not dependent onlong biological reaction times;

� the energy sales from heat recovery offset the operatingcosts;

� air emissions can be controlled to achieve the strictBATNEEC incinerator guidelines.

200

*Correspondence to: Dr G. McKay, Department of Chemical Engineering,Hong Kong University of Science and Technology, Clear Water Bay,Kowloon, Hong Kong.E-mail: kemckayg@ust.hk

However, conventional MSW incineration does have itsdrawbacks:

� all the MSW is incinerated, whereas some componentsare more valuable for recycling (USEPA, 1989; Sakaiet al., 1996);

� poor incineration practices and MSW components con-taining chlorine may lead to highly toxic dioxin and furanemissions, and sulphur components lead to SOx emissions;

� controlling metal emissions is problematic; these includearsenic, cadmium, chromium, copper, lead and mercury;

� expensive primary fuels are frequently required to achievethe high combustion temperatures;

� there is 10–15% volume ash produced for landfill disposal;� limited thermal efficiencies in converting the heat energy

to electrical power.

Modern conventional MSW incineration is now acceptedworldwide; however, it does have the listed drawbacks. Thepresent study investigates the application of MSW incinera-tion as an integral component of a cement manufacturingfacility and discusses how several natural features of thecement production process combine synergistically withMSW incineration to minimize and even take advantageof its drawbacks. The integrated process, currently underconstruction at pilot plant scale, will be described, followedby a discussion on how this novel process achieves optimumwaste minimization and finally summarizes on the benefitsof a full-scale integrated cement production facility withMSW incineration.

PROCESS DESCRIPTION

The MSW obtained, after screening by the materialrecovery processes from the MSW reception and handlingarea, will undergo a series of thermal treatment operations inthe co-combustion (Co-Co) pilot-plant treatment process by,first entering the Co-Co treatment units for total hydrocar-bon destruction of combustibles into basic gaseous combus-tion products. The combustion gas is then vented throughthe precalciner for neutralization of acidic gas and chloridewith an excess amount of calcined alkaline material forscrubbing. NOx reduction is implemented at the ducting ofthe precalciner. The adsorption material carrying neutralizedproducts containing chloride will be extracted from the gasby the cyclone system and will be further cooled with thelime cooling process through an arrangement of multiplestaged high-efficiency cyclones systems. The gas is furthercooled at the heat exchanger for downstream treatment atthe baghouse filtration before venting to the atmosphere. Inaddition, the gas is polished in a carbon adsorption systemwhen necessary. The neutralized chloride material is furtherextracted downstream of the lime cooling process forcollection, sampling and analysed for further handling. Allcalcined material in excess is again recharged to the precal-ciner to exploit the neutralization effect in this closedsystem. Cleaned gas is vented to the atmosphere. Theindividual process units of the Co-Co pilot facility will bedescribed below in more detail.

MSW Reception and Handling

MSW is brought to the Co-Co pilot plant facility and firstenters an MSW reception hall. The MSW reception hall is

an enclosed building. It is divided into a bunker area and amaterial sorting area by an isolating partition wall. TheMSW is only temporarily retained in a coverable storage pitin the bunker area. Air inside the entire MSW reception hallis maintained at a negative pressure. This includes thebunker area and sorting area, which are both ventilated toprovide cooling. This draft odorous air is eventuallysupplied as primary and secondary air for the Co-Cothermal oxidation units, or, when required, it can be directedto the air cleaning device, for maintaining fresh air exchangeand odor-free exhaust. MSW drained water is collected andis injected to and treated at the secondary combustionchamber by thermal oxidation.

The MSW is picked up from the storage pit and conveyedto a bulk reduction system area, where waste materials inwrapping materials is exposed, ploughed and broken apartfor later classification before direct thermal treatment. Bulkywaste that is found to be too large is removed from theMSW. Moreover, any material that is found to contain anyhazards, like chemical, dangerous goods, pesticides, danger-ous substances, chemical and clinical waste, and otherhazardous materials, controlled or not controlled by anyspecified ordinances and subsidiary regulations, is removedusing a manually operated ‘Y’ section and handled sepa-rately from the material stream going to the next classifica-tion stage.

Screening of undersize materials is classified by a classi-fying trommel. This equipment is relatively low in capital,operating and maintenance costs compared with otherseparation systems (Savage et al., 1986). The classificationof the undersize material is to remove mainly the non-combustibles from the MSW. Most of the undersize materi-als dropping through the 50 mm trommel screen are debriscomprising mainly food and other wastes of heavier solidpieces, such as household batteries, a significant source ofheavy metal contaminants in MSW. These batteries andbatteries containing such material, like mobile phones, inthe heavier fraction will be picked up and removed from theundersize debris.

The irremovable material from the mixed undersize andoversize material stream is the final product to be treated bythe Co-Co treatment process.

Co-Co Treatment Units

The Co-Co treatment units consist of a series of energyintensive thermal oxidation units. They are the MSW rotarykiln and the secondary combustion chamber (SCC). Theremaining material passing though the equipment will betotally oxidized to basic flue gas products by the ‘3Ts’,namely, extremely high temperature, incomparable turbu-lences, and extended residence time. Those applied featuresof the ‘3Ts’ can normally be found fully exploited andinherent in the cement manufacturing process. Supplemen-tary fuel is used in addition to MSW to provide theadditional energy for the very high temperature used inthe process. This fuel is waste marine oil, termed MARPOLor marine polluting oil. All hydrocarbons in the mixedmaterial are destroyed at this stage and are broken downhere to basic combustion gas products as exiting flue gas,which exits and passes to the next stage for acidic gasremoval.

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MUNICIPAL SOLID WASTE UTILIZATION 201

All volatile matter and water content from the MSW arethermally desorbed once it is pushed from the kiln hopperinto the MSW rotary kiln and thermally oxidized graduallyalong a 2.3 m diameter by 27.6 m length, counter current hotair and material flow MSW rotary kiln. The material andvapour are brought to an extremely high temperature by thehot gas passing through a flame of temperature greater than1600�C. Unlike the conventional incinerator, the MSWrotary kiln is able to tumble any buried MSW material,enhancing the direct heat exchange. It enhances the thermaldesorption and thermal oxidation by increasing direct heatexchange between the hot air and the material. Combustiblematerial is totally destroyed and oxidized into basic combus-tion products of carbon dioxide and water exiting the MSWrotary kiln at 950�C and this hot flue gas stream now entersthe SCC. The sintered material exits the MSW rotary kilnfrom the other end at 1100�C. The 1100�C hot ‘solid’material from the MSW rotary kiln is quenched in a watertrough to room temperature. The evaporated quenchingwater is vented to the MSW rotary kiln. The residue willbe collected, sampled and analysed for further handling.

The flue gas coming out of the MSW rotary kiln, carryingthe thermally desorbed vapour, water, basic combustionproducts and other volatile matters, is ultimately cleansedin the SCC using a second stage of complete destruction bythermal oxidation. The gas is further brought to an extremetemperature of 1200�C in the SCC. The distinctive 16 mlong chamber with extra tangential air injection into theSCC is creating a vigorous swirling turbulence and main-taining a far longer residence time for the total thermaloxidation to take place. The gas mixture is kept at 1200�Cfor at least 4 s in the SCC to ensure ultimate cleansing of theoff gas. Aromatic and other organic compounds aredestroyed and dissociated down to their basic flue gasproducts. To ensure complete destruction and thermaloxidation of hydrocarbons in the gas stream from theSCC, a 6% oxygen content by weight on a wet basis ismaintained at the flue gas exit from the SCC. Owing to thebenefits of the ‘3Ts’, such high destruction and reductionefficiency (DRE) of hydrocarbons is achieved. The gas isvented to the next stage of the process to the Precalciner foracidic gases removal.

Precalciner

The precalciner consists of a cylindrical reactor with along duct, followed by stages of cyclone separators. In theprecalciner, the cement raw meal is brought into contactwith the flue gas at 1200�C from the SCC, bringing aboutde-carbonization, or calcination, of the limestone.

CaCO3 þ Heat �!CaO þ CO2

The reaction demands a high-energy consumption, whichthe hot flue gas from the SCC supplies the energy to fulfill.At the same time, the alkaline lime, CaO, absorbs the acidicgaseous pollutants present in the flue gas by dry scrubbingand neutralization reactions.

The dry scrubbing process incorporating the effective dryscrubbing and neutralization intrinsically inherent in theprecalciner and preheater process in cement manufacturinginvolves the massive overdosing of amounts of alkaline limematerial as the absorbing agent for the gaseous pollutantsincluding sulphur oxides, SOx, and chlorides evolved during

the prior combustion process. Limestone, in itself, is a majorcomponent abundantly used in cement manufacture andconstitutes up to 80% of the feed raw material.

An extended residence time of 3 s is allowed for thelimestone to be de-carbonized in the elongated precalciner,and an extended reaction duct of more than 28 m provides afavorable ‘3Ts’ condition for the scrubbing and neutraliza-tion to take place.

Hydrogen chloride (HCl) will be absorbed and scrubbedby neutralization. Chloride is a key precursor of dioxinreformation. It is extracted as the non-hazardous neutraliza-tion product, namely as calcium chloride (CaCl2). There-fore, the determining factor chloride is withdrawn from theprocess, in principle leaving no chloride for any possibledioxin reformation in the flue gas at later stages.

CaO þ 2HCl�!CaCl2 þ H2O

(reaction with hydrogen chloride)

CaO þ SO2 þ 1=2 O2 �!CaSO4 (reaction with SO2)

The neutralization product of CaCl2 is gradually extractedfrom the un-reacted CaO overdose and is collected, sampledand analysed for other possible utilization or handling.The un-reacted CaO overdose, after cooling in the limecooling system, is recharged back to the precalciner processfor re-circulation and repeat neutralization operations. Thecleaned flue gas, normally free of acidic gases, is vented tothe next stage for further cooling and polishing.

A lime cooling system is required for cooling the CaOoverdose for recharging at the precalciner. The lime coolingsystem is responsible for the cooling of this CaO overdoseand the extraction of CaCl2 from this lime material. Air fromthe bunker and sorting area of the reception hall is drawn intoan extended cooling duct for the material, and then enters aset of high efficiency cyclones for further cooling and air-solid separation to take place. The air and the CaO overdoselime material are brought to a temperature of 200�C.

The cooled CaO lime material overdose is recharged backto the precalciner. The heated cooling air is vented to therotary kiln and SCC as primary and secondary air for thethermal oxidation. At the same time, it serves to maintainfresh air exchange and de-odorizing solution for the recep-tion hall during the Co-Co operation.

De-NOx

Apart from the removal of chloride and acidic gases, theCo-Co process is also equipped with the capability tominimize NOx emission by implementing a De-NOx

system to reduce the nitrogen oxides from the thermaloxidation process. The overall reaction for NO reduction,using a urea solution injection system, is given as follows:

H2NCONH2 þ 2NO þ 1=2O2 �!2 N2 þ CO2 þ 2H2O

The De-NOx system adopted for this Co-Co pilot plant is anSNCR (selective non-catalytic reaction) type. Urea solutionwill be atomized when injected at a series of positions, oftemperature ranges from 950 to 980�C, along the ductingof the precalciner to enhance gas contact and reductionreaction.

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202 CHOY et al.

Energy Recovery System

A heat exchanger is installed to cool the flue gas from thelast stage cyclones of the precalciner system for energyrecovery optimization and subsequent flue gas polishing inthe downstream baghouse filtration unit.

The cleaned flue gas, typically free of acidic gases, isvented to the gas cooler at 950�C, and is cooled down to200�C by indirect heat exchange with cooling water. Nosteam will be generated from the gas cooler.

This heat energy recovered from the flue gas can beutilized in power generation. The design of this heatexchanger allows for the collection of thermodynamic datain establishing the specification of such a power generationunit as part of the research objectives.

The gas cooler is designed to rapidly quench the cleanedflue gas by quickly passing the flue gas through severalcooling water tube modules. The retention time for the cleanflue gas is minimized to less than 1.4 s at a temperatureregion from 450 to 200�C. Indirect heat exchange byconvection and conduction takes place at the tube surface.The cooling water carrying the heat energy away from thecleaned flue gas rises to a maximum temperature of 70�C.The cooling water is further cooled and recycled in acooling water system.

As the gas is cleaned from the preceding cyclones, only aminimum amount of particles is settled at the surface of theheat exchanger tubes and the bottom of the gas coolerhoppers. The particles collected are sampled and analysedfor further handling.

Carbon Adsorption System

The Co-Co process is intended to convert and absorbcompletely the chemical precursors of dioxins=furans intobasic flue gas products, i.e. carbon dioxide and water.Injection of activated carbon can be used for further adsorp-tion cleaning of the flue gas and forms an additionalemission control measure for absorption of dioxins andfurans, as well as heavy metals. Finely powdered activatedcarbon is used as required and injected into the flue gasstream exiting the heat exchanger. The carbon is filtered onthe baghouse filter fabrics.

Baghouse Filtration

The cleaned flue gases will be directed through fabric airfilters for removal of remaining pollutants and particulates inthe gas stream. The best available heat-resistant fabricslaminated with PTFE membrane will be adopted as thefiltration material. The cleaned flue gas is further filtered asit is vented through the baghouse. The baghouse is equippedwith a self-cleaning system to maintain its collection effi-ciency. Residue collected is sampled and analysed forfurther handling.

All air pollution control measures ensure that theexhausted stack gas meets the most exacting standards forair emission levels when it reaches the atmosphere.

Residue Handling

Residues from the thermal treatment process are sampledand analysed. The analysed results can be used to determine

subsequent predefined handling and treatment methodsapproved by the appropriate authority to adopt in handlingand treating the residue.

Residues come from the following unit operations: kilnresidue quench, extract of lime cooling system, gas coolerand baghouse. These residues are tested for contaminants ofhazardous materials by methods approved by the relevantauthority. Where appropriate, these residues are rechargedback to the process to minimize disposal handling. Residuefrom the thermal treatment process is treated by solidifica-tion as needed for disposal purposes, if an amount ofcontaminants exceeding the approved standard is foundafter the analysis.

The solidification product is sampled and analysed bymethods approved by the relevant authority. Solidification isrepeated until the local discharge standards are met by thesolidification product.

RESULTS AND DISCUSSION

Process Simulation

Material and energy balances are two of the most impor-tant elements in the process design. All equipment designsand costings are based on the result of the material andenergy balances. Flow rate and temperature are the twotypical parameters used in the material and energy balancecalculation. The basic principle for both material and energybalance is simply based on the following equation:

in � out þ generation � consumption ¼ accumulation

The overall process simulation including mass and energybalances were conducted by Microsoft Excel. The processflow diagram is presented in Figure 1.

Physical Composition of Municipal Solid Waste

Information and data on the physical composition of solidwastes are important in the selection and operation ofequipment and facilities, in assessing the feasibility ofresource and energy recovery, and in the analysis anddesign of landfill disposal facilities. For example, if thesolid wastes generated at a commercial facility consist ofonly paper products, the use of special processing equip-ment, such as shredders and balers, may be appropriate.Separate collection may also be considered if the city orcollection agency is involved in a paper-products recyclingprogram. Physical composition is the term used to describethe individual components that make up a solid waste streamand their relative distribution, usually based on percentageby weight. Typical data from the environmental protectiondepartment (EPD) on the distribution of MSW in HongKong (EPD, 2000a) are presented in Table 1. The municipalsolid waste is mainly divided into two types—domesticwaste and commercial and industrial waste. From Table 1,putrescibles, paper and plastics are the major components,constituting about 76% of the municipal solid waste, repre-senting about 33.1, 26.7 and 16.6%, respectively. Otherminor components include textiles (3.2%), metals (3.0%),glass (3.1%), bulky waste (3.5%) and wood=rattan (4.3%)and the average moisture content for municipal solid wasteis 28%.

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MUNICIPAL SOLID WASTE UTILIZATION 203

Chemical Properties of Municipal Solid Waste

Information on the chemical composition of the compo-nents that constitute MSW is important in evaluating alter-native processing and recovery options. The feasibility ofcombustion depends on the chemical composition of themunicipal solid waste. Determining the elemental composi-tion of MSW by ultimate analysis is a key factor for thedetailed design of the MSW gasification plant and helpsconfirm the accuracy of material and energy balances of theMSW gasification process. The ultimate analysis of a MSWcomponent typically involves the determination of thepercentage of carbon (C), hydrogen (H), oxygen (O), nitro-gen (N), sulfur (S) and ash. Because of the concerns over theemission of chlorinated compounds, e.g. dioxins, duringcombustion, the determination of halogens is often includedin an ultimate analysis. The results of the ultimate analysisare used to characterize the chemical composition of theorganic matter in MSW. Representative data on the ultimateanalysis for the typical MSW components given in Table 1are presented in Table 2 (Tchobanoglous et al., 1993). The

average chemical composition of municipal solid waste isestimated and is shown in Table 2. The major elements arecarbon (43.9%), oxygen (32.1%) and ash (17.1%), account-ing for around 93% of MSW in Hong Kong. Other elementsinclude hydrogen (5.6%), nitrogen (1.1%) and sulfur (0.3%).In the calculation of the material and energy balances of theMSW gasification process, the difference, 0.8% of chlorine,is assumed to be present and has been used to simulate theformation of the chlorinated compounds, such as dioxins,during co-combustion.

Energy Content of Municipal Solid Waste

After estimating the elemental composition of the MSW,the energy content of the MSW can be estimated. Typicaldata for the energy content for the components of MSW arereported in Table 3. Based on 100 kg of MSW and using thephysical components in Table 1, the total energy content ofthe MSW using the data given in Table 3 is estimated and isalso shown in Table 3, where the energy content values areon an as-discarded basis. The estimated energy contentvalue of MSW in Hong Kong is 5843.5 Btu lb�1, whichcompares well with the typical value of 5750 Btu lb�1,including 20% moisture (Tchobanoglous et al., 1993). Ifthe moisture of the MSW is 55%, the calorific value ofMSW will become 3234 Btu lb�1 (1800 kcal kg�1).

Waste Minimization Benefits

Volume reduction of MSW for the Co-Co processThe initial amount of MSW input into the simulated

Co-Co pilot plant process is 2000 kg hr�1, and the densityof the MSW varies with season and geographical location.The MSW in the Asian countries is usually wetter, and canhave a density of 500 kg m�3 (or 857 lb y or d�3) (WinrockInternational Institute for Agricultural Development, 1996).After the combustion process, the total ash output from theprocess is 375 kg hr�1. If it is assumed that the ash gener-ated from the combustion process would mostly consist ofsand and have a density of 700 kg m�3 (or 1200 lb y or d�3),which is the density of the SiO2, the total volume reduction

Figure 1. Process flow diagram.

Table 1. Physical composition of municipal solid waste in Hong Kongin 2000.

Quantity (tpd) and percentage by weight

Physicalcomponent

Domesticwaste (a)

Commercialand industrialwaste (b)

Municipalsolid waste(c)¼ (a) þ (b)

Bulky wastea 223 (3%) 106 (5.9%) 329 (3.5%)Glass 260 (3.4%) 28 (1.6%) 288 (3.1%)Metals 232 (3.1%) 52 (2.0%) 284 (3.0%)Paper 2003 (26.6%) 490 (27.3%) 2493 (26.7%)Plastics 1210 (16.0%) 334 (18.6%) 1544 (16.6%)Putrescibles 2792 (37.0%) 299 (16.7%) 3091 (33.1%)Textiles 224 (3.0%) 73 (4.0%) 297 (3.2%)Wood=rattan 152 (2.0%) 247 (13.7%) 399 (4.3%)Othersb 444 (5.9%) 166 (9.3%) 610 (6.5%)Total 7540 (100%) 1795 (100%) 9334 (100%)

aBulky waste—big furniture, household machines (e.g. refrigerator, airconditioning or washing machines); it is assumed to include 50% woodand 50% metal.bOthers—ash, pottery, dirt (e.g. office=house, sweepings).

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204 CHOY et al.

of MSW by co-combustion with MARPOL is around 87%.The ash may then be used as one of the raw materials for thecement process.

Energy savingsThe energy benefits of the MSW gasification process

options mainly come from the generation of electricity andthe production of high-temperature raw feed material, clin-ker, of cement production plant by utilizing the waste heatenergy from the flue gas while the electricity is supplied todrive the cement manufacturing process.

According to the calculated material and energy balancesin the MSW Co-Co simulation program, the total heatenergy of the flue gas coming from the SSC is29,659 MJ h�1. The hot flue gas is used to heat 4.19tonnes h�1 of cement raw meal, limestone, from 25 to950�C in order to produce 2.5 tonnes h�1 of clinker materialfor the cement manufacturing process. The limestone isbrought into contact with the flue gas at 1200�C from theSCC, bringing about de-carbonization, or calcination, of thelimestone. The total energy consumption on the precalcinersystem was 2524 MJ h�1 and 8.6% of flue gas energy can beutilized to provide all the energy required in pre-calcinationprocess to save HK$367,000 (US$47,000) annually.

It is proposed that the remaining waste heat energy fromthe flue gas, 27,135 MJ h�1, is used to generate electricityfor the Co-Co process and the cement production process.If the electricity generation efficiency of the steam turbine is11% (Steltz, 1992), and the energy content (55% moisture)of the MSW in Hong Kong is around 1800 kcal kg�1, theMSW Co-Co pilot plant will generate 14,328 kW h(597 kW) electricity per day and the annual electricitygeneration will be 4,728,240 kW h. According to the elec-tricity bills from China Light Power CLP (Hong Kong)Company, the charge for electricity is 74 cents per KWhunit. Hence, the potential annual revenue from the electricitygeneration system is HK$3,500,000 (US$450,000).

The other possible revenue from the process is the use ofthe waste oil, MARPOL, as a source of energy for theprocess. The disposal cost for MARPOL given by theEnvironmental Protection Department (EPD, 1998) isHK$450 m�3 (US$ 60 m�3). In the simulation study, itwas found that the process can utilize 328 kg h�1 ofMARPOL instead of using fuel oil as the energy forcombustion. Assuming that the density of the MARPOL is0.85 g cm�3, the annual revenue from utilizing theMARPOL is around HK$ 1.46 million (US$180,000).

In 2000, over 6 million tonnes of waste was generated inHong Kong, and the annual cost of running landfill isHK$400 million (EPD, 2000b). Therefore, the cost oflandfill is around HK$65 ton�1 (US$8 ton�1). This Co-Copilot scale process can have a capacity of 2 ton h�1, whichcould induce a further saving of HK$1 million year�1

(US$132,000 year�1) from the cost of landfill.

Emission reductionAn MSW and ash handling system with a series of air

pollutant treatment devices has been designed for this plantin order to meet the current disposal and emission require-ments. The airborne emissions from the Co-Combustionprocess are controlled and must not exceed the concentra-tion limits set by the Hong Kong Environmental ProtectionDepartment. The concentration limits are tabulated inTable 4. All air pollutant concentrations are expressed atreference conditions of 0�C temperature, 101.3 kPa pressure,dry and 11% oxygen content conditions.

In our simulation study, the concentration of the pollutantwas dependent on the removal efficiency of the equipment and

Table 2. Typical data on the ultimate analysis of the combustible materials.

Percentage by weight (dry basis)

Type of waste Carbon Hydrogen Oxygen Nitrogen Sulfur Ash

Bulky wastea 27.0 3.3 23.4 0.2 0.1 46.0Glass 0.5 0.1 0.4 <0.1 0 98.9Metals 4.5 0.6 4.3 <0.1 0 90.5Paper 43.5 6.0 44.0 0.3 0.2 6.0Plastics 60.0 7.2 22.8 0 0 10.0Putrescibles 48.0 6.4 37.6 2.6 0.4 5.0Textiles 55.0 6.6 31.2 4.5 0.2 2.5Wood=rattan 49.5 6.0 42.7 0.2 0.1 1.5Othersb 26.3 3.0 2.0 0.5 0.2 68.0MSW (average) 43.9 5.6 32.1 1.1 0.3 17.1

aBulky waste—big furniture, household machine (e.g. refrigerator, air conditioning or washing machines);it is assumed to include 50% wood and 50% metal.bOthers—ash, pottery, dirt (e.g. office=house, sweepings).Note: the estimated composition of MSW is based on the data in Tables 1 and 2.

Table 3. Energy content of municipal solid waste in Hong Kong in 2001.

100 kg MSW

Physicalcomponent

MSW(kg)

Energy,Btu lb�1a

Totalenergy,b Btu

Bulky waste 3.5 4150 31,955Glass 3.1 60 409Metals 3.0 300 1980Paper 26.7 7200 422,928Plastics 16.6 14,000 511,280Putrescibles 33.1 2000 145,640Textiles 3.2 7500 52,800Wood=rattan 4.3 8000 75,680Others 6.5 3000 42,900Total 100 1,285,572Heat value of MSW (Btu lb�1)¼ 5843.5Heat value of MSW (kcal kg�1)¼ 3248.6

aAdapted in part from Tchobanoglous et al. (1993).bAs-discarded basis.Note: 1 Btu lb�1� 2.326¼ 1 kJ kg�1.

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MUNICIPAL SOLID WASTE UTILIZATION 205

the conversion in the combustion process. It was assumed thatHCl has a 98% removal by calcium carbonate and the overallconversion of CO to CO2 is 99.98% in the combustionchamber. The removal efficiency of NO2 and SO2 in theCo-Co process is greater than 90%. The particulates wereremoved from cyclone and filter bag with a total removalefficiency of 99%. The emission concentrations of thesepollutants were lower than the concentration limits set bythe Environmental Protection Department.

Bottom ash reductionSimulated laboratory scale experiments showed that the

bottom ash discharged from the MSW rotary kiln can beincorporated into the cement process as a raw materialfeedstock. This results in a direct saving in the purchaseof silica sand.

Table 5 shows the results of a series of tests using variouscement mix compositions; samples A–C incorporate fly ashand sample D is the normal processing feed composition. Theresults are based on over 50 theoretical compositions of thecement feed composition generated by simulation in colla-boration with Green Island Cement Co. Ltd, Hong Kong. Thecompositions in Table 5 show those typical of high MSWincineration ash, thus maximizing the use of the ash. All thesimulation compositions were tested in laboratory experimentsand all produced acceptable quality cement clinker product.Based on the current full-scale operating cement plant, theproduction rate is 250 t clinker h�1. The ash produced inthe MSW rotary kiln would represent 6.5–7.0% by weight ofthe feed rate. Consequently, the potential economic savingscould be estimated based on the feed composition shown incolumn A. The main economic savings would then beUS$762,000 p.a. sand and US$244,000 p.a. limestone.

Effectiveness of combustion conditionsSince this process is not purely incineration but is a supply

of process energy, it must meet a specific temperature targetof 1200�C, significantly higher than the 850�C for the 2 snormally specified in conventional waste incinerators. There-fore, in the present process a secondary combustion chamberis provided using an additional supply of MARPOL injected

into the chamber through two burners tangentially to achieveswirling turbulence for a residence time of 4 s and heatingthe 850�C MSW incineration flue gas to a temperature of1200�C, more than satisfying the 3Ts requirement in modernwaste combustors. The ability to use the waste marine oil asa relatively cheap supplementary fuel has a major benefit inthat the process is not totally dependent on the CV of theMSW. Therefore a material recovery and recycling facilitywill be developed at the front end of the process.

MRRFThis facility is designed to take out the easily recoverable

plastics, paper, wood, metals etc., even though some have ahigh calorific value (CV). The selection is made on aneconomic basis and an environmental basis. For example,high-value recyclable plastics are recovered and also PVCunits; the latter reduces the chlorine loading to reduce dioxinformation potential. After the initial materials recovery andrecycling facility (MRRF) separation, size reduction takesplace, followed by trommelling. In another pilot study (Lau,2002), this technique was shown to be very effective for theremoval of batteries, the major source of volatile heavymetals in the emissions.

Therefore, as a direct result of the MRRF with anestimated 15% recovery, two major emission sources arealso minimized, saving US$1370 and US$73,000 p.a. fromthe decreased usage of activated carbon and limestone,respectively. An MRRF facility is generally regarded asimpractical in conventional MSW incinerators due to thelowering of the CV of the feedstock and the value of thefinal product, compared with expensive conventional supple-mentary oil energy sources.

Dioxin emission minimizationUnder the 3Ts conditions of 1200�C, >4 s residence time,

rotary kiln turbulence and tangential burner swirl turbulence,the fuel burn out is extremely high, thus minimizing theavailable organic carbon for the downstream reformationsynthesis of dioxins (McKay, 2002). The MRRF also reducesthe chlorine content of the MSW feed by the removal ofPVC. In addition, there is scrubbing of HCl in the pre-calciner, in which the scrubbing ratio is more than 10 timesgreater than that in a normal flue gas scrubber. The HClforms calcium chloride, which then participates in clinkerforming reactions. Since the temperature in the precalcinerbecomes very high, there will probably be some dissociationreactions and some chlorine may be rejected. The cementclinker forming reactions are too complex to simulateand make predictions. Consequently, we will only have

Table 4. Concentration limits for emission from incineration processes.

Air pollutant

Daily averageconcentrationlimit(mg m7 3)

Removalefficiency

Simulationresults(mg m7 3)

Particulates 30 99% 9Hydrogen chloride (HCl) 50 98% 17Hydrogen fluoride (HF) 2 — —Sulfur dioxide (SO2) 200 90% 163Nitrogen oxides (NO2) 400 90% 1Carbon monoxide (CO) 100 99.98% 50

Table 5. Various cement raw material compositions.

Sample A B C D

Limestone 76.49 78.10 78.18 80.55Fly ash 5.53 5.78 5.64 7.07Slag 2.22 2.29 2.22 2.33Sand 9.40 9.62 9.71 10.05MSW ash 6.36 4.21 4.25 —

Table 6. Potential annual savings for MSW-integrated cement process.

Waste category saving Savings US$ (per year)

Landfill costs 103,327,000Energy for process 4,700,000Power for process 15,000,000Power for export 30,000,000Limestone feed 244,000Silica sand feed 762,000Revenue from MARPOL treatment 18,000,000AC injection 457,000Lime injection 244,000Total 172,734,000

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206 CHOY et al.

quantitative data on this aspect of the project after the pilotplant tests finish in approximately 12 months. Finally, there isa carbon=lime injection system for the cooled flue gas(473 K) immediately prior to the baghouse filter.

Based on the dioxin content from conventional MSWincinerators with similar baghouse filters (McKay, 2002) inthe combustion temperature range 850–1150�C, the currentsystem, without the other dioxin minimization factors,should reduce the dioxin emission level to well below0.08 ng TEQ N�1m�3.

Waste fly ash and active carbon residuesThe major wastes from the process are flyash and

carbon=lime injection residues for the baghouse filter.In conventional incinerators, these wastes are usually stabi-lized using cement and a stabilization facility is to beinstalled in the present project. However, other opportunitiesare being reviewed. For example, the waste flyash, where thevolatile heavy metals usually deposit, will be analysed andif the composition is negligible, these will be blended withthe bottom ash as a cement raw material feed. The activecarbon=lime residues will also be tested for volatile heavymetals, VOCs and dioxins. If the loading is negligible, thematerial will be recycled for re-use.

Owing to the MRRF, trommelling and the precalcinationscrubbing, an estimated >75% heavy metals and chlorine-based plastics will be removed. There at minimum an estimated50% saving on the costs of active carbon and lime treatmentwill be achieved amounting to a total of US$7000 p.a.

Economic benefits of the Co-Co processThe savings based on the simulated pilot plant study

reported in the previous sections for the 2 t MSW day�1

pilot plant can be scaled up to the full-scale 250 t cementclinker per day plant. The results are summarized in Table 6.The values obtained are likely to be minimum value scenar-ios but until pilot scale tests are completed the moreoptimistic values cannot be confirmed. The data in thetable reflect the direct difference between the MSWcement integrated process and conventional incineration.In terms of assessing the rate of return, a conventionalcement plant will incur additional capital and operatingcosts for the MRRF, the MSW incineration system and theemission plant and treatment chemicals.

CONCLUSION

The use of MSW as a source of raw materials and energy inthe production of cement has been studied. A novel integratedco-combustion design of a cement production facility has beendeveloped and a pilot-scale plant is under construction. Wasteminimization will enable significant economic savings to bemade compared to the conventional MSW incinerators, in thefollowing categories: (i) landfill costs; (ii) energy for process;(iii) power for process; (iv) power for export; (v) limestonefeed; (vi) silica sand feed; (vii) revenue from MARPOLtreatment; (viii) AC injection; and (ix) lime injection.

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ACKNOWLEDGEMENTS

The authors are grateful to ITF, Hong Kong SAR for the provision offinancial support during this research programme, and thanks to BarrieCook, Gary Yu, Henry Law, Peter Leung, Aung Khine, Raymond Cheung,Thomas Tao, Michael Wong, Sunny Kwong and Vivian Kwok for theassistances and advices throughout this research project.

The manuscript was received 24 June 2003 and accepted for publicationafter revision 18 February 2004.

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