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Energy recovery from municipal solid waste incineration—A review
Heat Recovery Systems dE CHP Vol. 9, No. 2, pp. 115--126, 1989
0890-4332/89 S3.00+ .00 Pergamon Preu plc
Printed in Great Britain
ENERGY
RECOVERY FROM MUNICIPAL SOLID INCINERATION--A REVIEW
WASTE
V. H. MORCOS* School of Technical Education, University of Technology, PO Box 35010, Baghdad, Iraq (Received 21 March 1988; in revised form 16 June 1988)
Alam'act--Heat recovery from the incineration of municipal solid waste (MSW) can make a useful contribution to the nation's energy needs. This paper reviews the literature on the field of energy recovery from MSW incineration. This review includes (1) An historical background on the MSW methods of disposal and the recent trends. (2) The potential role of waste as a fuel from the knowledge of its analysis, beating value and quantity. (3) The alternative methods of waste disposal including the thermal (mass incineration, refuse-derived fuel and pyrolysis), the mechanical and the biochemical processes. (4) Some economic considerations regarding MSW disposal costs. (5) The combustion characteristics of MSW and waste combustion plants. (6) Heat recovery from waste burning including the main types of waste heat boilers; the heat recovery rates; heat exchange boundary conditions; fouling, erosion and corrosion of waste heat boilers; control of waste incineration paniculate emissions and finally the control of nitrogen oxides emissions. The intensive data given here is essential to the proper design and operation of any MSW-to-energy plant. The design needs to be undertaken with careful consideration of the experience that already exists in order to achieve success.
1. H I S T O R I C A L Municipal solid waste generated by residences and commercial facilities is primarily non-toxic and historically has been disposed of by incineration a n d / o r burial in landfills. Solid waste was incinerated in the U.S. as early as 1885. Primarily, incineration is used for reducing the volume of waste prior to disposal in a landfill. Waste incinerators producing electricity or supplying steam for heating purposes were in use in the U.K. as long ago as 1895. These relatively inefficient stations were closed down primarily due to the availability of cheap fuel. Although the availability of low-cost conventional fuels (until 1973) made almost all thermal waste-to-energy systems uneconomical in the U.S., waste-to-energy systems have been in use for many years in Europe and Japan where energy costs have always been higher. The energy crisis of the early 1970s and increasing costs for energy, coupled with the difficulties and restrictions experienced in disposing of waste, have led to renewed interest in refuse as a fuel, starting with more accurate knowledge of its calorific value. With current landfills reaching capacity and the decreasing availability of new landfills, most of the developed countries are facing a phenomenal waste disposal problem. According to some estimates, the U.S.A. will run out of landfill capacity by the year 2000 [1]. Some major metropolitan areas are already experiencing problems. In an effort to solve the problem many municipalities are building waste-to-energy plants for burning their waste. Electricity and steam generated by the plants are sold to raise revenue, thus helping to stabilize or reduce the costs of refuse disposal. Most industrial analysts involved with the industry are projecting a market of $10 billion over the next 10 yr for construction of waste-to-energy plants using both the mass burning and refuse-derived fuels technologies. The performance of mass incineration waste-to-energy plants has been as reliable as other power producing plants and the system can be a viable proposition and an attractive alternative to the traditional method of landfill of refuse. Since the early 1970s the Greater London Council in England has burned an average of 1140 tpd of MSW in its Edmonton solid waste incineration plant [2]. The Edmonton station was the first of the new generation plants in the U.K. and it has overshadowed earlier plants in scale, complexity *On leave from the Faculty of Engineering, Assiut University, Assiut, Egypt. !15
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and reliability. The objective of the Edmonton plant is to reduce the maximum amount of refuse to an hygienic residue at a minimum cost. The income received from the sale of electricity is directly related to the throughput and efficiency of the plant. On a somewhat smaller scale, refuse is incinerated and energy recovered at a few other plants in the British Isles. The Flingern refuse power plant in Dusseldorf, West Germany serves a population of approximately 650,000 people and commenced operation in 1966. The annual throughput for the plant is approximately 280,000 t; approximately 600,000 t of steam are supplied to the local electricity power station and for district heating. Another plant was commissioned in Zurich in 1978. In 1980 the plant incinerated 270,000 t of refuse producing combined heat and power (CHP) [3]. Denmark is in the forefront with CHP and refuse is used as a cheap low grade fuel. Whilst European countries have predominated in the field of mass incineration, many plants are to be found in Japan and in Scandinavian countries. In Saugus, Massachusetts, U.S.A., between 1973 and 1975 a waste-to-energy facility was constructed. The plant has now processed more than 3 million t of waste which is converted into useful heat and electricity. A plant in Peekshill, New York was commissioned in 1985 to burn 2250 t of refuse per day and to generate 60 MW electricity [3]. 2. T H E
POTENTIAL
ROLE
OF
WASTE
AS A F U E L
Approximately I kg of refuse is produced per person per day and approximately 5 t of refuse has the same energy potential as 2 t of coal or I t of oil.The potential for augmenting the world's diminishing fossilfuel stocks is obvious, even though the efficiencyof conversion to electricityfor a small refuse-firedplant is less than half that of a modern coal- or oil-firedplant. Refuse collected can be a heterogeneous mixture from paper, plastics,textiles,glass, metals, vegetable matter, dust and cinder and other unclassifiedmaterials. A waste-to-energy facilitymust cater for all refuse delivered from the collection vehicles. A typical material analysis for a city municipal waste could be as shown in Table I; Table 2 shows a typical ultimate analysis [I--4]. Municipal solid waste chemical composition can be found in the range shown in Table 3 [5]. Some 30--40% of the waste fed to mass incinerator is non-combustible and leaves the furnace as residual material for disposal. The major ash component in waste fuels is from the clay added during the manufacture of paper. Incineration does however reduce the volume of material to be disposed of as residuals. A 10-I volume reduction is an important factor in subsequent residual disposal costs [2]. Municipal refuse has a moisture content of between 15 and 60%. The moisture content of the constituents varies widely, from littleor none in commercial paper to 7 0 % or more in food and yard wastes [1,6]. A large proportion of municipal wastes are readily combustible (25-70%) and possess a calorific value high enough to give good potential for incineration and subsequent recovery of the heat evolved by the combustion process [7]. A mean H.H.V. for raw municipal waste is around 8400 kJ kg-*. Allowing for moisture content (typically 20-30% by weight) good quality raw refuse has an average H.H.V. of 10500 kJ kg -* [I, 2]. Annually, in the U.K., more than 29 million t of municipal refuse (that which is gathered from homes, shops, offices etc. but not from industry) is generated and about 10% of this is incinerated. Table 1. Material analysis of sample of refuse by weight
Material Dust and cinder Vegetable matter Paper and cardboard Metals Textiles Glass Plastics Unclassified
Weight (%) 14.8 19.7 33.8 6.4 4.2 9.3 5.3 6.5
Table 2. Ultimate analysis by weight
Chemical Weight composition ........... (o/==) Carbon 25 Hydrogen 3 Oxygen 18.4 Nitrogen 0.4 Sulphur 0.2 Water 26 Ash and inerts 27
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Table 3. The range of ultimate analysis by weight for MSW Chemical composition Moisture Carbon Oxygen Hydrogen Nitrogen Sulphur Non-combustibles Higher heating value (H.H.V.)
Weight (%) 15-35 15-30 12-24 2-5 0.20-1.0 0.02-0, I0 15-25 7000-14000 Id kg-'
Even less is converted into useful energy, but nevertheless even this small fraction makes a worthwhile contribution to the U.K. economy [4]. Domestic waste is the major component of municipal refuse. Municipal waste generated within the U.S. has increased 57% over what it was 25 years ago, with 225 million t expected per year by 1990 and increases continuing thereafter [1]. According to the estimates of Cairo Cleaning Authority (February, 1988), the capital of Egypt produces about 4000-5000 t of municipal waste daily. The majority is disposed of by incineration without heat recovery and landfill. Small amounts of this refuse are processed in two factories for producing organic fertilizer, which is used in land reclammation. Refuse incineration could make a valuable contribution to large-scale CHP schemes or could offer the prospect of commercially viable smaller scale district heating development. 3. ALTERNATIVE METHODS OF WASTE DISPOSAL There are a number of alternative methods of refuse disposal varying from the traditional landfill concept to the now more accepted use of refuse as a fuel. Even within this latter category there are alternatives. 3.1. Thermal processes (a) Mass incineration (burning). In mass burning systems the refuse is burned in an "as received" condition. Generally, in mass burning systems all of the waste entering the facility is dumped into a large storage pit, with bulky items like stoves, refrigerators and similar items being removed prior to entering the combustion chamber. (b) Refuse-derived fuel (RDF). Refuse-derived fuels are processed So that all non-combustible materials are removed prior to burning. In many instances the waste remaining after processing is shredded into confetti-like particles. (c) Pyrolysis. Pyrolysis is thermal decomposition in the absence of oxygen. When the heat for pyrolysis is provided by combustion of part of the waste in air or oxygen, the term "gasification" is more appropriate. The chemistry of thermal degradation is extremely complex and only partially understood. By manipulating the environmental conditions within the reactor, the yield of any desired product (gas of low calorific value, liquid oil and carbonaceous char) may be optimized [6, 81. 3.2. Mechanical processes Before materials can be recovered in a systematic manner, or usually before energy can be extracted systematically from the refuse, it has to be sorted into its different fractions (combustibles, metals, glass and unclassifieds). Often the first stage in the treatment of refuse is the removal of moisture, but this can be a highly energy expensive process. However, moisture, which is not chemically bound to materials is readily amenable to mechanical dewatering. Usually it is far cheaper to remove water by mechanical separation (e.g. by squashing the wet item) rather than by heating. If the metal and glass fractions have remained separate, they can be recycled at much reduced energy costs. Similarly, it is usually worthwhile recycling the coarser sized items of paper and plastics. The residual combustible fractions (including rags and wood, as well as the smaller size items of dirty paper and plastics) can then undergo combustion, pyrolysis, pelletization or briquetting to produce heat or various forms of fuels [4]. HRS 9!2--§
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3.3. Biochemical processes During the decomposition of refuse in a tip, the temperature rises and may reach as high as 65~'C but starts to fall after I-2 months. However the process of fermentation goes on for a long time and a number of gases are produced, including small amounts of carbon monoxide and hydrogen sulphide. Under anaerobic conditions methane is produced and there have been cases of small explosions or fires where methane has entered a building. In relatively shallow tips the methane is likely to escape but if the tip is deep, particularly if it is in a steep sided valley so that sealing of the top is relatively easy, it may be possible to collect the gas. The gas is said to be 52% methane and 46% carbon dioxide. The gas from a tip in the Sun Valley near Los Angeles is being collected as a demonstration project. The method seems applicable only to large metropolitan areas. It is said that the Los Angeles Department of water and power has proposals to extract gas from up to about 20 tips and that the total power potential is 80-100 MW [8]. Mass incineration without pretreatment of the refuse with electricity generation is regarded as the most reliable and economical option due to the following reasons. (i) The majority of wastes will burn without giving rise to noxious products of combustion (HCI, HF, SO2 and NOx) in significant quantities. (ii) The volume and mass occupied by the waste is vastly reduced. A small volume of incombustible residue is left. The heat of combustion is recovered in a waste heat boiler for steam generation. (iii) Waste in its initial form may be objectionable-in nature containing decaying organic matter and such like. The incineration process produces an effectively sterile ash residue. As with all forms of waste disposal there are the very necessary standards to be met by incineration plants in order to protect the environment. These are based on the principal that the best practicable means or all possible means shall be made to minimize the emission of pollutants to atmosphere. Legislation applies also to lanfill tips where combustion residues or ash are disposed of. 4. ECONOMIC C O N S I D E R A T I O N S It has been demonstrated that incineration with a heat recovery option performed better than the landfill option [!, 3]. In the U.K., the average cost per t for the waste-to-energy incinerator fell to £9.6 in 1983/4 whilst the most modern transfer plant to remote tips was £15.2 t -I more costly [2]. The total cost, with the eventual disposal at the tip, may lie anywhere between £20 to £40 per t [9]. Sanitary landfill costs will escalate at a faster rate than the general rate of inflation. Some of the main reasons for this prediction are higher land costs, increasing costs for transporting waste outside the urban areas and the higher operating costs resulting from more stringent landfill regulations [1]. The capital cost of a waste-to-energy plant, which has been built already, is fixed. Electricity and steam generated by this plant are sold to raise revenue, thus helping to stabilize or reduce the costs of refuse disposal. Therfore, waste-to-energy plants can play a part in steadying or decreasing costs. In addition, such plants generate revenue from tipping fees, sale of dectrieity and steam, and in some cases, sale of recycled metals and glass. A waste-to-energy facility can promote other local developments (e.g. by providing less expensive power for small businesses and industrial parks) as well as addressing the community's health, economic and waste disposal problems. 5. COMBUSTION OF WASTES 5.1. Combustion characteristics o f waste
The fixed carbon and volatile matter content from proximate analysis of a fuel indicate to a combustion engineer the mode of combustion which is favourable. A typical refuse mix contains around 20% fixed carbon and 80% volatile matter. The major proportion of the heat release then will be from volatiles evolving from the solid matter. The mode of combustion best suited to refuse
Energy recovery from municipal solid waste incineration
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is less certain, neither grate, suspension burning or fluidized bed combustion being likely to be as controllable as with coal. Moreover, quite different supplies of primary and secondary combustion air are required, as is a much large combustion volume. The proximate analysis and physical dimensions of waste on the other hand do favour an alternative mode of combustion namely air gasification (starved-air or semi-pyrolitic incineration) [10]. To maintain combustion without the use of a secondary fuel the incoming refuse must have a moisture content less than 50% by weight, an ash content less than 60% by weight and a combustible content more than 25% by weight. Air for combustion requires to be 60-80% in excess of stochiometric conditions [2, 7]. 5.2. Waste combustion plant Ideally a waste combustion plant should be able to meet environmental requirements with: (i) The minimum of gas cleaning plant. (ii) No pretreatment of waste. (iii) Minimum and closely controlled use of auxiliary fuel. It is very rare for industrial incinerators to be larger than 2 t h- i. Generally speaking incineration with energy recovery is not commercially viable below 180-270kgh -I [11]. It is interesting to review equipment available and assess the benefits and pitfalls each particular type have to offer. This will be limited specifically to the incineration plant although it is acknowledged that several designs of waste burning boilers have proved successful on certain specific applications. Many early, and ind~d, recent forms of incinerators have been of the refuse-burning grate form [11] shown in Fig. 1. In these devices copious amounts of air were passed through the firebed from the grate in an attempt to give efficient combustion. The result of this highly turbulent introduction of air was the carryover from the firebed of a large amount of dust and fly ash. This level of particulate emission necessitates some form of gas cleaning plant. On larger plants scrubbers and latterly electrostatic precipitators were used. Smaller plants used reversal chambers, refractory cyclones or filters. When burning more than 5-10% plastic many of the cruder types of incinerator produce dense black smoke. A further problem when the waste calorific value varies is that very rapid and uncontrolled rises in temperature can occur which can damage the plant by overheating. On smaller plants the auxiliary fuel usage on afterburning chambers can be excessively high. Generally speaking these incinerators were capable of accepting untreated waste loaded either manually or by mechanical means. Far larger versions of the moving grate incinerator with water wall boilers were built throughout the world as large municipal incinerators [2]. The success or failure of the combustion process rests to a large extent on the selection, operation and maintenance of a suitable grate. The refuse-burning grate must be robust enough to withstand the physical and heat load of the fuel; it must be capable of responding to sudden load changes brought about by the heterogeneous nature of the fuel; it must be able to cope with both thick and thin fuel beds to achieve an optimum residue burn-out both with regard to carbon in ash and biodegradable material. Ease and simplicity of maintenance are essential. Smoke and flyash in stack gases
Waste ignition burner
Grate
lili0 l Fig. 1. Early form of solid waste incinerator.
Reversal chamber
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V.H. MoRcos
A relatively recent marketable product is the fluidized bed incinerator [11-14] shown in Fig. 2. Fluidizing air is forced up through a bed of inert material (sand) or limestone at a high enough velocity (2-6 m s-~) so that the bed behaves very similarly to a boiling liquid. Pretreated waste is metered into the bottom of the bed operating at a temperature of about 800-900°C. Ignition starts by heating the incoming combustion/fluidizing air by fitting a burner in the air plenum system or by firing an ignition burner directly above the bed. There are two principal types of bed in use, bubbling beds and circulating beds, distinguished by the extent to which solids are entrained from the bed. The majority of the combustion reaction actually takes place above the bed. The fuel gasifies in the bed releasing its volatile components which pass out of the bed into a secondary burning zone (free board). The majority of the carbon residue burns away in the bed itself.
Hoouo n!c
cornbustorto boiler
/l
• ,7.
////1. Meteredfeedof
Fluidisedbed
Air dl=ributor
"~t_; ~
"~ ")~
~
j Air
I
I
Fig. 2. Basic fluidizedbed incinerator.
There has been much interest in the application of fluidized bed incinerators to the burning of waste. This interest is centred on the heat transfer which takes place within the bed proper without cooling the bed to such an extent that unsatisfactory combustion results. Fluid beds, because of the nature of their operation, require a finely controlled feed, generally using a shredded form of waste. A further problem is the carry-over of paniculate matter which has to be separated out from the stack gases before venting to atmosphere, by means of a multicyclone or other more efficient particulate-removal system. A further type of incinerator, the starved-air or semi-pyrotitic incinerator, has emerged in regent years, which represents the state of the art in waste management technology [6, 10, 11, 12, 15, 16]. There are now several thousand of these units in operation ranging from as small as 25 kg h- i units operating for 6-8 h per day to massive municipal waste plants operating continuously. The basic principles of operation are shown in Fig. 3. The incinerator comprises a refractory-lined primary combustion chamber, into which waste is loaded and to which a small flow of air is admitted. The chamber has no grate and the waste is reduced to sterile ash by a process involving pyrolysis, volatilisation and gasification, with a small amount of burning. The waste is initially heated by small auxiliary burners and undergoes essentially a pyrolysis process. Normally the reaction is autothermic once an operating temperature of 700-800°C has been achieved. The burners in this primary chamber can be either gas or oil and only operate until conditions are autothermic, which usually means they are off except for the first 30-60 rain in a day. The temperature in the primary chamber is maintained by a waterspray which operates on an "on/off" principle, being activated through the main panel according to the registered temperature.
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Gal CO(
particles
ste destroyed Waste introduced
Fig. 3. Basic principles of starved-air incinerator.
The advantage of this method or primary burning, as opposed to the more usual technique of inducing active combustion by introduction of large volumes of forced air, is that the waste decomposes under quiescent conditions. Consequently, carry over of particulate matter, which would subsequently contribute to stack emissions, is minimized. The partial combustion products, mainly carbon monoxide rich smoke from the substoichiometric combustion, pass directly into an afterburner chamber, mounted immediately above the main combustion chamber, at extremely low velocities under 0.30 m s- I. The gases are admixed with additional air and elevated to a temperature of 1000-1200°C to ensure successful burn out of smoke. In average plants the burner in the oxidation chamber operates for only 50% of the time, as satisfactory conditions persist in the remaining period. The exhaust gases are then cooled by an air attemperator mounted after and on top of the second chamber. The residue in the batch plants is usually removed as dictated by the rate of build up, commonly on a daily basis. The sterile ash is of such a composition that it can be used for road infill. The final result is an incinerator based on a new technology which, because of automation, operates at the touch of the button. The advantages of the starved-air incinerator plants are: (1) The extremely low emission levels attained without the need for any external or inbuilt gas cleaning system. (2) No requirement for the over-engineered material handling of conventional incineration. The waste fed to the incinerator is completely untreated, no shredding or preseparation is carried out. (3) A rugged machine with no grate which meant lower maintenance. (4) The system is modular, which means it can be installed at factories with lower capital cost. (5) The system offers the maximum protection to the operator from fume emission, blowbacks and exploding items such as aerosol cans. (6) The optimisation of auxillary fuel usage. The fuel rate is automatically adjusted to suit the loading and burning of the incinerator. (7) This modular incinerator is specifically designed for small quantities of waste, and can thus be contemplated by communities producing between 10-100 t of waste per day, for which the only other economic option is probably landfill. It is possible to consider a waste heat recovery unit using starved-air incinerator exhaust gases without having to consider gas cleaning. Waste beat recovery from the conventional incinerator, although long recognised as desirable, has not been too often practised because any recovery boilers fitted are susceptible to clogging and corrosion, The starved-air incinerator waste heat recovery system can produce 3.4 t of steam for every tonne of waste destroyed. By positioning the system adjacent to an industrial plant, office, building, factory or housing facility, significant revenue can be earned by selling the steam. As the plants are composed of modules, they can respond immediately to seasonal demands; a defect in any one unit means only a fraction of the disposal requirement is affected. They can be
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removed and resited, require little labour and have the same spares on one or on many sites. If a maintenance engineer understands one unit then he will understand them all.
6. HEAT RECOVERY FROM WASTE B U R N I N G With proper waste management and the installation of appropriate energy recovery equipment, waste can supply a high percentage of the total energy demand of any country. The cost of waste arising on site and used as fuel can be low. Recovering energy from it on site saves it's disposal cost.
By the very nature of the material they use as fuel, waste fired energy recovery systems must be robust. Technology has progressed beyond the large, box-type incinerator usually fitted with a moving grate and incorporating a heat exchanger. Instead, modern systems favour the modular approach, several small furnaces feed hot, complete products of combustion into a separate waste heat recovery boiler. Many furnaces are employed due to the straight economic fact that a modular furnace which is entirely factory made and transportable by a standard commercial vehicle is of least cost. Moreover, a small furnace does not require special foundations and the combustion capacity is virtually guaranteed with many furnaces, it being unlikely that more than one furnace would require repair at any one time. As a result, the use of fewer and larger furnaces tends to lead to a more expensive installation. Several factors govern whether an incinerator with heat recovery is a viable economic proposition. Some of the significant ones are as follows: (i) Gas cleaning plants to reduce particulate and chemical emission levels to acceptable levels are expensive to purchase and maintain. (ii) The need to pretreat waste using a shredder or separator adds to capital and running costs. (iii) Auxiliary fuel used on afterburners increases the running cost of the plant. (a) Waste-heat boiler A fire-tube, horizontal boiler has gained universal acceptance, however, many plants have tended to use extended surface water tube boilers. An induced draught fan is fitted to overcome flow losses in the waste heat boiler on associated ductwork, this also serves to maintain the incinerator under slight negative pressure (50-150 Pa). Most waste heat boilers need a pressure drop across the boiler of 0.50-1.0 KPa [11, 15]. (b) Heat recovery rates The heat recovery rates are a function of several variables. These variables are [1 l]: (1) The loading rate of the waste into the incinerator. (2) The calorific value of the material charged. These two effectively fix the thermal input. (3) The boiler or heat exchanger efficiency and the heat loss from the plant to its surroundings by convection and radiation from hot surfaces on the plant. (4) Boiler or heat exchanger surface fouling. These last two factors are the obvious heat losses from the plant which cannot be recovered for economic reasons, for instance the diminishing financial returns as insulation thickness is increased in order to avoid too low a gas outlet temperature from the boiler to avoid corrosion. (S) Incinerators which operate for 8-10 h per day seldom burn at a rate equal to their loading rate. Over the period of operation a firebed of considerable size builds up, which burns on for a number of hours after loading has ceased. As a result the heat fibcration rate is often far less than expected. It must be said that although the heat liberation rate is lower, the heat liberated during burn down will make up for this to some extent. Additionally, by using longer residence times achieved by using large combustion chambers, the burning rate is equal to the loading rate. Extra heat losses occur due to the large surface area of the plant and the heat lost by quenching the hot ash. It must be understood that the mass of ash is often as high as 30--40% of the charging rate, so the loss is significant.
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(6) The thermal inertia of the plant. On a plant starting up once a day a considerable amount of heat is absorbed by the refractory linings in the chamber and ductwork of the plant. This heat is not recovered; it is lost when the plant is shut down at the end of the burndown period. On continuous plants operating 24 h per day the thermal inertia losses are, of course, only encountered on start-up and are of less significance. (c) Heat exchange boundary conditions Most waste materials contain inorganic salts, such as NaCI, Na, SO4, CuSO4 and FeSO4, which accelerate wear on the refractory in the combustion zone. Upon evaporation and combustion the salts form particulates of NaCI, Na:SO4, Cu20 and Fe~O3. Molten salts and oxides solidify on the waste-heat boiler tubing when cooled, causing reduced efficiency and possible plugging in the waste-heat boilers. This problem can be alleviated by cooling the incinerator exhaust gas below the salt melting points before it enters the waste-heat boiler, either by quenching with water, ambient air, or recycled gas from the waste-heat boiler exhaust. Melting points for salts and oxides found in incinerators are in the range of 750-1000°C. A gas-side boiler inlet temperature not exceeding 950°C normally ensures that any gas-side deposits remain friable and easily cleaned, and a working temperature of 800°C is chosen frequently so as to permit reasonable fluctuation [15, 17, 18]. At the outlet of the boiler, minimum gas temperatures are dictated by the probability of condensation of unknown or unexpected gaseous components and by the heat losses between the boiler exit and the top of the chimney. Boiler outlet temperatures are not allowed to fall below, typically, 230°C at full-load and 200°C at part-load conditions. The safe chimney top temperature is 170-175°C [2, 15, 17, 18]. The percentage of recoverable heat (Ra) based on boiler inlet gas temperature is calculated from
RB- - C. Tt- C.0 To x Cp, T~
IO0
where Cp, = m e a n specific heat of flue gases between entry temperature and 0°C (kJ[kg K]- I), Cp0 = mean specific heat of flue gases between outlet temperature and 0°C (kJ[kg K]-I), T~= boiler inlet gas temperature (K), TO= boiler outlet gas temperature (K). Overall efficiencies of heat recovery are lower than the above, due to sensible heat losses from the combustor (about 4%) and heat exchanger (about 3%) as well as losses due to unburnt material (about 7%) [6, 11)]. The highest practical overall thermal efficiency of heat recovery (boiler efficiency) does not exceed 65-70% of the gross thermal input [6, 9, 11,15]. (d) Fouling, erosion and corrosion of waste-heat boilers Fouling is a recurring availability problem which can be met by regular, gas-side cleaning (750-1000h intervals are convenient). So long as the gas temperatures are maintained between 900°C (maximum inlet) and 230°C (minimum outlet), any panicles adhering to tubes, etc. will be dry and friable; thus light brushing and vacuum-cleaning will suffice. Erosion problems can be minimized to the point where they do not affect the life of heat exchanger merely by keeping tube gas velocities reasonable ( < 25 m s-I) [15]. Also, by reducing gas velocities in the superheater section and arranging tubes parallel to the gas flow, erosion can be controlled. Corrosion mechanisms have been well-documented in respect of municipal waste burning. Sulphur, chlorine and phosphorous are the main elements in waste which are responsible for high and low temperature corrosion and deposits affecting heat exchange surfaces and other components downstream of a boiler. PVC plastics (45.4% CI) and common salt are among the several chlorides commonly found in waste. Low-temperature corrosion can occur at the back end of the boiler and subsequent plant if the metal temperatures fall below acid dew point. The presence of even a small amount of sulphur in waste can lead to SO: and SO3. The possibility of condensation of sulphuric and sulphurous acids
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(H2 SO4; H2 SO3), from the combination of SOj and SO2 with water vapour, on these cooled metal surfaces must be considered because of their corrosive effects. Chlorinated organic compounds produce HCI and some CI 2 as combustion products. Upon cooling in the waste-heat boiler there is a possibility of condensation of HCI with water vapor. Hydrochloric acid (HCI) and chlorine (Ci2) are also corrosive. By maintaining high back-end gas temperatures (200-250°C) at the expense of some boiler efficiency and a fairly high feedwater temperature (about 150°C), which will exceed the flue gas acid dew point at all times, low-temperature corrosion has been avoided in economizers, precipitators, induced draught fans and ducting [2, 17, 18]. High-temperature corrosion occurs when metal temperatures exceed about 300°C with the corrosion effect worsening at higher temperatures. Huck [19] has described this corrosion as the reaction between the tube metal and chlorine released during combustion of the waste forming Fe2 C16 which permeates the ash adhering to tube surfaces. At a sufficiently high metal temperature the Fe2CI6 decomposes to form Fe 3C14, HCI, CI and 02 which, because it is trapped in the ash, reacts inwards to attack the tube wall and forms more Fe2CI6. The reaction is thus perpetuated. The phenomenon is also described by Dunn and Temple-Pediani [10] who acknowledge the lack of comprehensive data on chloride corrosion and draw attention to two corrosive reactions which occur on the steel tube surfaces (Fe + C12 = FeCI2; Fe + 2HC1 = FeCI2 + H2) resulting in the formation of ferrous chloride scale. The ameliorating effect of a dual system---coal plus refuse--has also been noted in certain European plants. Krause et al. [20] have shown that the damaging reiietions increase significantly in rate with metal temperatures above 315°C and recommendations are made that boiler durability is obtainable only when all metal temperatures are below 260°C. Allowing say a 30 K temperature drop across the heat exchange surfaces yields 230°C as the maximum saturation or superheated steam temperature which can be generated with safety to the boiler. But when waste is intended to generate high temperature steam, over 230°C, treatment of the flue gases by scrubbing before ducting to the boiler must be recommended. The boiler can then be designed to operate safely and efficiently without reference to either high or low temperature corrosion and deposits. Krause et al. [2 I] have demonstrated that the addition of low-chloride sludge to municipal refuse is recommended as a corrosion prevention measure and a waste disposal technique. (e) Control of waste incinerator particulate emissions
One of the critical factors involved in the use of municipal solid waste as a source of heat energy is the requirement to remove particulate matter from the hot flue gases. In the incineration process, gases will leave the combustion chamber/furnace area at temperatures around 1000°C, containing solid particulate matter at a concentration ranging from 1-20 g m -3 at STP (0°C and 101.3 kPa), Emissions legislation in various countries throughout the world demands that these emissions be reduced to levels ranging from 25 to 200 mg m -3. This defines the range of efficiency for the gas cleaning plant as being from 80.000 to 99.875%. This can be accomplished using special design features of electrostatic precipitators [22, 23]. The starved-air principle of incineration achieves the extremely low emission levels without the requirement of any gas cleaning plant. Tests have shown that the units will meet all the particulate emission standards [11]. (f) Control of nitrogen oxides emissions Temperature, types and amounts of nitrogen-containing compounds in the waste and the percent theoretical air are factors which will affect the degree of conversion of waste nitrogen to NO, [17,23]. Thermal NOx formation occurs primarily in the range 1540-1930°C. Very little thermal NOx formation occurs below 1540°C. At lower waste nitrogen levels more than 60% of the elemental N is converted to NOx. The fraction conversion is about 45% at 0.4°/. waste nitrogen and decreases further to about 25% at 2% nitrogen.
Energy recovery from municipal solid waste incineration
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NOx concentration peaks at around 90% the0i-eti~l ai~: a~ording to one study. Other studies indicate from 83-105% as the amount of theoretical air at which NO~ concentration reaches a maximum. CONCLUSIONS From this review of the energy recovery from MSW incineration, the following main conclusions can be obtained. (1) The performance of mass incineration waste-to-energy plants has been as reliable as other power producing plants and the system can be a viable proposition and an attractive alternative to the traditional method of landfill of refuse. (2) Refuse incineration could make a valuable contribution to large-scale CHP schemes or could offer a prospect for commercially viable smaller scale district heating development. (3) Mass incineration without pretreatment of refuse with electricity generation is regarded as the most reliable and economical option for waste disposal. (4) Waste-to-energy plants can play a part in steadying or decreasing costs. (5) The starved-air incinerator represents the state of the art in waste management technology. The starved-air incinerator waste heat recovery system can produce 3.4 t of steam for every tonne of waste destroyed. (6) By the very nature of the material they use as fuel, xvaste-fired energy recovery systems must be robust. (a) A fire-tube, horizontal boiler has gained universal acceptance in waste-to-energy plants, however many plants have tended to use extended surface water tube boilers. (b) The heat recovery rates are a function of the loading rate and the calorific value of waste; the efficiency and fouling of the waste-heat boiler and the thermal inertia of the plant. (c) A gas-side boiler inlet temperature not exceeding 950°C normally ensures that any gas-side deposits remain friable and easily cleaned and a working temperature of 800°C is chosen frequently so as to permit reasonable fluctuation. The gas temperature at the boiler outlets is not allowed to fall below 230°C to avoid the probability of condensation of any gaseous components. The highest practical overall thermal efficiency of heat recovery does not exceed 65-70% of the gross thermal input. (d) Fouling of waste-heat boilers is avoided by regular gas-side brushing and vacuum cleaning. Erosion can be minimized by keeping tube gas velocities less than 25 m s-~ and by arranging tubes parallel to the gas flow. Low-temperature corrosion can be avoided by maintaining high back end gas temperatures (200-250°C). (e) Particulate emissions must be reduced to levels ranging from 25 to 200 mg m- 3. This can be achieved by using special design features of electrostatic precipitators. The starved-air principle of incineration achieves the extremely low emission levels without the requirement of any gas cleaning plant. (f) Very little thermal NOx formation occurs below 1540°C. Nox concentration reaches a maximum at around 83-105% theoretical air. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
D. J. Smith, Waste fuels, refuse-derived fuels and cogeneration, Power Engng 19, 40--46 (1986). J. H. S. Smart, The Greater London Council's refuse-fired power Station, Proc. Inst. Mech. Engrs 2@8,255-266 0986). N. Barnes, Operational performance of waste-to-energy plants, Proc. Inst. Mech. Engrs 199, 117-122 0985). S. D. Probert and B. Samuel, Domestic, municipal and industrial refuse in the U.K., Appi. Energy 22, 85-106 0986). J. L. Pavoni, J. E. Heer and D. J. Hagerty, Handbook of Solid Waste Disposal--Materials and Energy Recovery, pp. 57-167. Van Nostrand Reinhold, New York (1975). D. C. Wilson, Waste Management, Planning, Evaluation, Technology, pp. 6-17; 253-264. Clarendon Press, Oxford (1981). C. R. Veizy and C. O. Velzy, Incineration, in Mark's Standard Handbook for Mechanical Engineers, 8th edition, pp. 7.53-7.60. McGraw-Hill, New York (1978). J. M. Burnett, Recent developments in the use of municipal refuse as a fuel, Proc. Inst. Mech. Engrs 191, 89-98 (1977). R. W. Temple-Pediani, Energy recovery from combustible industrial and domestic refuse, CME 28, 43-46 (1981). A. Dunn and R. W. Temple-Pediani, Combined coal and waste firing of boilers, CME 30, 57-60 0983).
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11. J. A. J. Clark, Recovery of heat from industrial waste materials, 1ChemE Symposium Series 72, AI-A17 (1982). 12. H. M. Freeman, Hazardous waste destruction process, Environ. Prog. 2, 205-210 (1983). 13. J. C. Gwozdz and W. C. Walke, Fluidized bed combustion moves into utility applications, Power Engng 19, 42--46 (1986). 14. V. H. Morcos, Refuse-fired, pressurized, fluidized-bed combustion combined cycle analysis, Heat Recovery Systems 7, 465-472 (1987). 15. C. T. Chamberlain, Waste heat recovery from combustible wastes, IChemE Symposium Series 72, HI-H15 (1982). 16. G. H. Smith, Using waste to raise steam, CME 28, 44--46 (1981). 17. J. E. Ward and A. P. Ting, Waste incineration and heat recovery, Environ. Prog. 1, 30-38 (1982). 18. R. K. Tanner, Incineration of industrial waste. Prog. Energy combust. Sci. 5, 245-251 (1979). 19. R. Huck, Hydrogen chloride corrosion in refuse incineration plants, Brennst-Warme und Kraft (1966). 20. H. H. Krause, D. A. Vaughan and P. D. Miller, Corrosion and deposits from combustion of solid waste, ASME J. Engng Power 95, 45-52 (1973). 21. H. H. Krause, P. W. Cover, W. E. Berry and R. A. Olexscy, Corrosion and deposits from combustion of solid waste, part VII--Coincineration of refuse and sewage sludge, ASME J. Engng Power 102, 698-705 (1980). 22. K. R. Parker and A. Russell-Jones, Control of refuse incinerator particulate emissions, Proc. Inst. Mech. Engrs 200, 41--49 (1986). 23. J. Bromley, Monitoring of emissions from hazardous waste incineration, Proc. Inst. Mech. Engrs 201, 21-27 (1987).
0890-4332/89 S3.00+ .00 Pergamon Preu plc
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ENERGY
RECOVERY FROM MUNICIPAL SOLID INCINERATION--A REVIEW
WASTE
V. H. MORCOS* School of Technical Education, University of Technology, PO Box 35010, Baghdad, Iraq (Received 21 March 1988; in revised form 16 June 1988)
Alam'act--Heat recovery from the incineration of municipal solid waste (MSW) can make a useful contribution to the nation's energy needs. This paper reviews the literature on the field of energy recovery from MSW incineration. This review includes (1) An historical background on the MSW methods of disposal and the recent trends. (2) The potential role of waste as a fuel from the knowledge of its analysis, beating value and quantity. (3) The alternative methods of waste disposal including the thermal (mass incineration, refuse-derived fuel and pyrolysis), the mechanical and the biochemical processes. (4) Some economic considerations regarding MSW disposal costs. (5) The combustion characteristics of MSW and waste combustion plants. (6) Heat recovery from waste burning including the main types of waste heat boilers; the heat recovery rates; heat exchange boundary conditions; fouling, erosion and corrosion of waste heat boilers; control of waste incineration paniculate emissions and finally the control of nitrogen oxides emissions. The intensive data given here is essential to the proper design and operation of any MSW-to-energy plant. The design needs to be undertaken with careful consideration of the experience that already exists in order to achieve success.
1. H I S T O R I C A L Municipal solid waste generated by residences and commercial facilities is primarily non-toxic and historically has been disposed of by incineration a n d / o r burial in landfills. Solid waste was incinerated in the U.S. as early as 1885. Primarily, incineration is used for reducing the volume of waste prior to disposal in a landfill. Waste incinerators producing electricity or supplying steam for heating purposes were in use in the U.K. as long ago as 1895. These relatively inefficient stations were closed down primarily due to the availability of cheap fuel. Although the availability of low-cost conventional fuels (until 1973) made almost all thermal waste-to-energy systems uneconomical in the U.S., waste-to-energy systems have been in use for many years in Europe and Japan where energy costs have always been higher. The energy crisis of the early 1970s and increasing costs for energy, coupled with the difficulties and restrictions experienced in disposing of waste, have led to renewed interest in refuse as a fuel, starting with more accurate knowledge of its calorific value. With current landfills reaching capacity and the decreasing availability of new landfills, most of the developed countries are facing a phenomenal waste disposal problem. According to some estimates, the U.S.A. will run out of landfill capacity by the year 2000 [1]. Some major metropolitan areas are already experiencing problems. In an effort to solve the problem many municipalities are building waste-to-energy plants for burning their waste. Electricity and steam generated by the plants are sold to raise revenue, thus helping to stabilize or reduce the costs of refuse disposal. Most industrial analysts involved with the industry are projecting a market of $10 billion over the next 10 yr for construction of waste-to-energy plants using both the mass burning and refuse-derived fuels technologies. The performance of mass incineration waste-to-energy plants has been as reliable as other power producing plants and the system can be a viable proposition and an attractive alternative to the traditional method of landfill of refuse. Since the early 1970s the Greater London Council in England has burned an average of 1140 tpd of MSW in its Edmonton solid waste incineration plant [2]. The Edmonton station was the first of the new generation plants in the U.K. and it has overshadowed earlier plants in scale, complexity *On leave from the Faculty of Engineering, Assiut University, Assiut, Egypt. !15
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and reliability. The objective of the Edmonton plant is to reduce the maximum amount of refuse to an hygienic residue at a minimum cost. The income received from the sale of electricity is directly related to the throughput and efficiency of the plant. On a somewhat smaller scale, refuse is incinerated and energy recovered at a few other plants in the British Isles. The Flingern refuse power plant in Dusseldorf, West Germany serves a population of approximately 650,000 people and commenced operation in 1966. The annual throughput for the plant is approximately 280,000 t; approximately 600,000 t of steam are supplied to the local electricity power station and for district heating. Another plant was commissioned in Zurich in 1978. In 1980 the plant incinerated 270,000 t of refuse producing combined heat and power (CHP) [3]. Denmark is in the forefront with CHP and refuse is used as a cheap low grade fuel. Whilst European countries have predominated in the field of mass incineration, many plants are to be found in Japan and in Scandinavian countries. In Saugus, Massachusetts, U.S.A., between 1973 and 1975 a waste-to-energy facility was constructed. The plant has now processed more than 3 million t of waste which is converted into useful heat and electricity. A plant in Peekshill, New York was commissioned in 1985 to burn 2250 t of refuse per day and to generate 60 MW electricity [3]. 2. T H E
POTENTIAL
ROLE
OF
WASTE
AS A F U E L
Approximately I kg of refuse is produced per person per day and approximately 5 t of refuse has the same energy potential as 2 t of coal or I t of oil.The potential for augmenting the world's diminishing fossilfuel stocks is obvious, even though the efficiencyof conversion to electricityfor a small refuse-firedplant is less than half that of a modern coal- or oil-firedplant. Refuse collected can be a heterogeneous mixture from paper, plastics,textiles,glass, metals, vegetable matter, dust and cinder and other unclassifiedmaterials. A waste-to-energy facilitymust cater for all refuse delivered from the collection vehicles. A typical material analysis for a city municipal waste could be as shown in Table I; Table 2 shows a typical ultimate analysis [I--4]. Municipal solid waste chemical composition can be found in the range shown in Table 3 [5]. Some 30--40% of the waste fed to mass incinerator is non-combustible and leaves the furnace as residual material for disposal. The major ash component in waste fuels is from the clay added during the manufacture of paper. Incineration does however reduce the volume of material to be disposed of as residuals. A 10-I volume reduction is an important factor in subsequent residual disposal costs [2]. Municipal refuse has a moisture content of between 15 and 60%. The moisture content of the constituents varies widely, from littleor none in commercial paper to 7 0 % or more in food and yard wastes [1,6]. A large proportion of municipal wastes are readily combustible (25-70%) and possess a calorific value high enough to give good potential for incineration and subsequent recovery of the heat evolved by the combustion process [7]. A mean H.H.V. for raw municipal waste is around 8400 kJ kg-*. Allowing for moisture content (typically 20-30% by weight) good quality raw refuse has an average H.H.V. of 10500 kJ kg -* [I, 2]. Annually, in the U.K., more than 29 million t of municipal refuse (that which is gathered from homes, shops, offices etc. but not from industry) is generated and about 10% of this is incinerated. Table 1. Material analysis of sample of refuse by weight
Material Dust and cinder Vegetable matter Paper and cardboard Metals Textiles Glass Plastics Unclassified
Weight (%) 14.8 19.7 33.8 6.4 4.2 9.3 5.3 6.5
Table 2. Ultimate analysis by weight
Chemical Weight composition ........... (o/==) Carbon 25 Hydrogen 3 Oxygen 18.4 Nitrogen 0.4 Sulphur 0.2 Water 26 Ash and inerts 27
Energy recovery from municipal solid waste incineration
II 7
Table 3. The range of ultimate analysis by weight for MSW Chemical composition Moisture Carbon Oxygen Hydrogen Nitrogen Sulphur Non-combustibles Higher heating value (H.H.V.)
Weight (%) 15-35 15-30 12-24 2-5 0.20-1.0 0.02-0, I0 15-25 7000-14000 Id kg-'
Even less is converted into useful energy, but nevertheless even this small fraction makes a worthwhile contribution to the U.K. economy [4]. Domestic waste is the major component of municipal refuse. Municipal waste generated within the U.S. has increased 57% over what it was 25 years ago, with 225 million t expected per year by 1990 and increases continuing thereafter [1]. According to the estimates of Cairo Cleaning Authority (February, 1988), the capital of Egypt produces about 4000-5000 t of municipal waste daily. The majority is disposed of by incineration without heat recovery and landfill. Small amounts of this refuse are processed in two factories for producing organic fertilizer, which is used in land reclammation. Refuse incineration could make a valuable contribution to large-scale CHP schemes or could offer the prospect of commercially viable smaller scale district heating development. 3. ALTERNATIVE METHODS OF WASTE DISPOSAL There are a number of alternative methods of refuse disposal varying from the traditional landfill concept to the now more accepted use of refuse as a fuel. Even within this latter category there are alternatives. 3.1. Thermal processes (a) Mass incineration (burning). In mass burning systems the refuse is burned in an "as received" condition. Generally, in mass burning systems all of the waste entering the facility is dumped into a large storage pit, with bulky items like stoves, refrigerators and similar items being removed prior to entering the combustion chamber. (b) Refuse-derived fuel (RDF). Refuse-derived fuels are processed So that all non-combustible materials are removed prior to burning. In many instances the waste remaining after processing is shredded into confetti-like particles. (c) Pyrolysis. Pyrolysis is thermal decomposition in the absence of oxygen. When the heat for pyrolysis is provided by combustion of part of the waste in air or oxygen, the term "gasification" is more appropriate. The chemistry of thermal degradation is extremely complex and only partially understood. By manipulating the environmental conditions within the reactor, the yield of any desired product (gas of low calorific value, liquid oil and carbonaceous char) may be optimized [6, 81. 3.2. Mechanical processes Before materials can be recovered in a systematic manner, or usually before energy can be extracted systematically from the refuse, it has to be sorted into its different fractions (combustibles, metals, glass and unclassifieds). Often the first stage in the treatment of refuse is the removal of moisture, but this can be a highly energy expensive process. However, moisture, which is not chemically bound to materials is readily amenable to mechanical dewatering. Usually it is far cheaper to remove water by mechanical separation (e.g. by squashing the wet item) rather than by heating. If the metal and glass fractions have remained separate, they can be recycled at much reduced energy costs. Similarly, it is usually worthwhile recycling the coarser sized items of paper and plastics. The residual combustible fractions (including rags and wood, as well as the smaller size items of dirty paper and plastics) can then undergo combustion, pyrolysis, pelletization or briquetting to produce heat or various forms of fuels [4]. HRS 9!2--§
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3.3. Biochemical processes During the decomposition of refuse in a tip, the temperature rises and may reach as high as 65~'C but starts to fall after I-2 months. However the process of fermentation goes on for a long time and a number of gases are produced, including small amounts of carbon monoxide and hydrogen sulphide. Under anaerobic conditions methane is produced and there have been cases of small explosions or fires where methane has entered a building. In relatively shallow tips the methane is likely to escape but if the tip is deep, particularly if it is in a steep sided valley so that sealing of the top is relatively easy, it may be possible to collect the gas. The gas is said to be 52% methane and 46% carbon dioxide. The gas from a tip in the Sun Valley near Los Angeles is being collected as a demonstration project. The method seems applicable only to large metropolitan areas. It is said that the Los Angeles Department of water and power has proposals to extract gas from up to about 20 tips and that the total power potential is 80-100 MW [8]. Mass incineration without pretreatment of the refuse with electricity generation is regarded as the most reliable and economical option due to the following reasons. (i) The majority of wastes will burn without giving rise to noxious products of combustion (HCI, HF, SO2 and NOx) in significant quantities. (ii) The volume and mass occupied by the waste is vastly reduced. A small volume of incombustible residue is left. The heat of combustion is recovered in a waste heat boiler for steam generation. (iii) Waste in its initial form may be objectionable-in nature containing decaying organic matter and such like. The incineration process produces an effectively sterile ash residue. As with all forms of waste disposal there are the very necessary standards to be met by incineration plants in order to protect the environment. These are based on the principal that the best practicable means or all possible means shall be made to minimize the emission of pollutants to atmosphere. Legislation applies also to lanfill tips where combustion residues or ash are disposed of. 4. ECONOMIC C O N S I D E R A T I O N S It has been demonstrated that incineration with a heat recovery option performed better than the landfill option [!, 3]. In the U.K., the average cost per t for the waste-to-energy incinerator fell to £9.6 in 1983/4 whilst the most modern transfer plant to remote tips was £15.2 t -I more costly [2]. The total cost, with the eventual disposal at the tip, may lie anywhere between £20 to £40 per t [9]. Sanitary landfill costs will escalate at a faster rate than the general rate of inflation. Some of the main reasons for this prediction are higher land costs, increasing costs for transporting waste outside the urban areas and the higher operating costs resulting from more stringent landfill regulations [1]. The capital cost of a waste-to-energy plant, which has been built already, is fixed. Electricity and steam generated by this plant are sold to raise revenue, thus helping to stabilize or reduce the costs of refuse disposal. Therfore, waste-to-energy plants can play a part in steadying or decreasing costs. In addition, such plants generate revenue from tipping fees, sale of dectrieity and steam, and in some cases, sale of recycled metals and glass. A waste-to-energy facility can promote other local developments (e.g. by providing less expensive power for small businesses and industrial parks) as well as addressing the community's health, economic and waste disposal problems. 5. COMBUSTION OF WASTES 5.1. Combustion characteristics o f waste
The fixed carbon and volatile matter content from proximate analysis of a fuel indicate to a combustion engineer the mode of combustion which is favourable. A typical refuse mix contains around 20% fixed carbon and 80% volatile matter. The major proportion of the heat release then will be from volatiles evolving from the solid matter. The mode of combustion best suited to refuse
Energy recovery from municipal solid waste incineration
i 19
is less certain, neither grate, suspension burning or fluidized bed combustion being likely to be as controllable as with coal. Moreover, quite different supplies of primary and secondary combustion air are required, as is a much large combustion volume. The proximate analysis and physical dimensions of waste on the other hand do favour an alternative mode of combustion namely air gasification (starved-air or semi-pyrolitic incineration) [10]. To maintain combustion without the use of a secondary fuel the incoming refuse must have a moisture content less than 50% by weight, an ash content less than 60% by weight and a combustible content more than 25% by weight. Air for combustion requires to be 60-80% in excess of stochiometric conditions [2, 7]. 5.2. Waste combustion plant Ideally a waste combustion plant should be able to meet environmental requirements with: (i) The minimum of gas cleaning plant. (ii) No pretreatment of waste. (iii) Minimum and closely controlled use of auxiliary fuel. It is very rare for industrial incinerators to be larger than 2 t h- i. Generally speaking incineration with energy recovery is not commercially viable below 180-270kgh -I [11]. It is interesting to review equipment available and assess the benefits and pitfalls each particular type have to offer. This will be limited specifically to the incineration plant although it is acknowledged that several designs of waste burning boilers have proved successful on certain specific applications. Many early, and ind~d, recent forms of incinerators have been of the refuse-burning grate form [11] shown in Fig. 1. In these devices copious amounts of air were passed through the firebed from the grate in an attempt to give efficient combustion. The result of this highly turbulent introduction of air was the carryover from the firebed of a large amount of dust and fly ash. This level of particulate emission necessitates some form of gas cleaning plant. On larger plants scrubbers and latterly electrostatic precipitators were used. Smaller plants used reversal chambers, refractory cyclones or filters. When burning more than 5-10% plastic many of the cruder types of incinerator produce dense black smoke. A further problem when the waste calorific value varies is that very rapid and uncontrolled rises in temperature can occur which can damage the plant by overheating. On smaller plants the auxiliary fuel usage on afterburning chambers can be excessively high. Generally speaking these incinerators were capable of accepting untreated waste loaded either manually or by mechanical means. Far larger versions of the moving grate incinerator with water wall boilers were built throughout the world as large municipal incinerators [2]. The success or failure of the combustion process rests to a large extent on the selection, operation and maintenance of a suitable grate. The refuse-burning grate must be robust enough to withstand the physical and heat load of the fuel; it must be capable of responding to sudden load changes brought about by the heterogeneous nature of the fuel; it must be able to cope with both thick and thin fuel beds to achieve an optimum residue burn-out both with regard to carbon in ash and biodegradable material. Ease and simplicity of maintenance are essential. Smoke and flyash in stack gases
Waste ignition burner
Grate
lili0 l Fig. 1. Early form of solid waste incinerator.
Reversal chamber
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V.H. MoRcos
A relatively recent marketable product is the fluidized bed incinerator [11-14] shown in Fig. 2. Fluidizing air is forced up through a bed of inert material (sand) or limestone at a high enough velocity (2-6 m s-~) so that the bed behaves very similarly to a boiling liquid. Pretreated waste is metered into the bottom of the bed operating at a temperature of about 800-900°C. Ignition starts by heating the incoming combustion/fluidizing air by fitting a burner in the air plenum system or by firing an ignition burner directly above the bed. There are two principal types of bed in use, bubbling beds and circulating beds, distinguished by the extent to which solids are entrained from the bed. The majority of the combustion reaction actually takes place above the bed. The fuel gasifies in the bed releasing its volatile components which pass out of the bed into a secondary burning zone (free board). The majority of the carbon residue burns away in the bed itself.
Hoouo n!c
cornbustorto boiler
/l
• ,7.
////1. Meteredfeedof
Fluidisedbed
Air dl=ributor
"~t_; ~
"~ ")~
~
j Air
I
I
Fig. 2. Basic fluidizedbed incinerator.
There has been much interest in the application of fluidized bed incinerators to the burning of waste. This interest is centred on the heat transfer which takes place within the bed proper without cooling the bed to such an extent that unsatisfactory combustion results. Fluid beds, because of the nature of their operation, require a finely controlled feed, generally using a shredded form of waste. A further problem is the carry-over of paniculate matter which has to be separated out from the stack gases before venting to atmosphere, by means of a multicyclone or other more efficient particulate-removal system. A further type of incinerator, the starved-air or semi-pyrotitic incinerator, has emerged in regent years, which represents the state of the art in waste management technology [6, 10, 11, 12, 15, 16]. There are now several thousand of these units in operation ranging from as small as 25 kg h- i units operating for 6-8 h per day to massive municipal waste plants operating continuously. The basic principles of operation are shown in Fig. 3. The incinerator comprises a refractory-lined primary combustion chamber, into which waste is loaded and to which a small flow of air is admitted. The chamber has no grate and the waste is reduced to sterile ash by a process involving pyrolysis, volatilisation and gasification, with a small amount of burning. The waste is initially heated by small auxiliary burners and undergoes essentially a pyrolysis process. Normally the reaction is autothermic once an operating temperature of 700-800°C has been achieved. The burners in this primary chamber can be either gas or oil and only operate until conditions are autothermic, which usually means they are off except for the first 30-60 rain in a day. The temperature in the primary chamber is maintained by a waterspray which operates on an "on/off" principle, being activated through the main panel according to the registered temperature.
Energy recovery from municipal solid waste incineration
121
Gal CO(
particles
ste destroyed Waste introduced
Fig. 3. Basic principles of starved-air incinerator.
The advantage of this method or primary burning, as opposed to the more usual technique of inducing active combustion by introduction of large volumes of forced air, is that the waste decomposes under quiescent conditions. Consequently, carry over of particulate matter, which would subsequently contribute to stack emissions, is minimized. The partial combustion products, mainly carbon monoxide rich smoke from the substoichiometric combustion, pass directly into an afterburner chamber, mounted immediately above the main combustion chamber, at extremely low velocities under 0.30 m s- I. The gases are admixed with additional air and elevated to a temperature of 1000-1200°C to ensure successful burn out of smoke. In average plants the burner in the oxidation chamber operates for only 50% of the time, as satisfactory conditions persist in the remaining period. The exhaust gases are then cooled by an air attemperator mounted after and on top of the second chamber. The residue in the batch plants is usually removed as dictated by the rate of build up, commonly on a daily basis. The sterile ash is of such a composition that it can be used for road infill. The final result is an incinerator based on a new technology which, because of automation, operates at the touch of the button. The advantages of the starved-air incinerator plants are: (1) The extremely low emission levels attained without the need for any external or inbuilt gas cleaning system. (2) No requirement for the over-engineered material handling of conventional incineration. The waste fed to the incinerator is completely untreated, no shredding or preseparation is carried out. (3) A rugged machine with no grate which meant lower maintenance. (4) The system is modular, which means it can be installed at factories with lower capital cost. (5) The system offers the maximum protection to the operator from fume emission, blowbacks and exploding items such as aerosol cans. (6) The optimisation of auxillary fuel usage. The fuel rate is automatically adjusted to suit the loading and burning of the incinerator. (7) This modular incinerator is specifically designed for small quantities of waste, and can thus be contemplated by communities producing between 10-100 t of waste per day, for which the only other economic option is probably landfill. It is possible to consider a waste heat recovery unit using starved-air incinerator exhaust gases without having to consider gas cleaning. Waste beat recovery from the conventional incinerator, although long recognised as desirable, has not been too often practised because any recovery boilers fitted are susceptible to clogging and corrosion, The starved-air incinerator waste heat recovery system can produce 3.4 t of steam for every tonne of waste destroyed. By positioning the system adjacent to an industrial plant, office, building, factory or housing facility, significant revenue can be earned by selling the steam. As the plants are composed of modules, they can respond immediately to seasonal demands; a defect in any one unit means only a fraction of the disposal requirement is affected. They can be
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V . H . Mogcos
removed and resited, require little labour and have the same spares on one or on many sites. If a maintenance engineer understands one unit then he will understand them all.
6. HEAT RECOVERY FROM WASTE B U R N I N G With proper waste management and the installation of appropriate energy recovery equipment, waste can supply a high percentage of the total energy demand of any country. The cost of waste arising on site and used as fuel can be low. Recovering energy from it on site saves it's disposal cost.
By the very nature of the material they use as fuel, waste fired energy recovery systems must be robust. Technology has progressed beyond the large, box-type incinerator usually fitted with a moving grate and incorporating a heat exchanger. Instead, modern systems favour the modular approach, several small furnaces feed hot, complete products of combustion into a separate waste heat recovery boiler. Many furnaces are employed due to the straight economic fact that a modular furnace which is entirely factory made and transportable by a standard commercial vehicle is of least cost. Moreover, a small furnace does not require special foundations and the combustion capacity is virtually guaranteed with many furnaces, it being unlikely that more than one furnace would require repair at any one time. As a result, the use of fewer and larger furnaces tends to lead to a more expensive installation. Several factors govern whether an incinerator with heat recovery is a viable economic proposition. Some of the significant ones are as follows: (i) Gas cleaning plants to reduce particulate and chemical emission levels to acceptable levels are expensive to purchase and maintain. (ii) The need to pretreat waste using a shredder or separator adds to capital and running costs. (iii) Auxiliary fuel used on afterburners increases the running cost of the plant. (a) Waste-heat boiler A fire-tube, horizontal boiler has gained universal acceptance, however, many plants have tended to use extended surface water tube boilers. An induced draught fan is fitted to overcome flow losses in the waste heat boiler on associated ductwork, this also serves to maintain the incinerator under slight negative pressure (50-150 Pa). Most waste heat boilers need a pressure drop across the boiler of 0.50-1.0 KPa [11, 15]. (b) Heat recovery rates The heat recovery rates are a function of several variables. These variables are [1 l]: (1) The loading rate of the waste into the incinerator. (2) The calorific value of the material charged. These two effectively fix the thermal input. (3) The boiler or heat exchanger efficiency and the heat loss from the plant to its surroundings by convection and radiation from hot surfaces on the plant. (4) Boiler or heat exchanger surface fouling. These last two factors are the obvious heat losses from the plant which cannot be recovered for economic reasons, for instance the diminishing financial returns as insulation thickness is increased in order to avoid too low a gas outlet temperature from the boiler to avoid corrosion. (S) Incinerators which operate for 8-10 h per day seldom burn at a rate equal to their loading rate. Over the period of operation a firebed of considerable size builds up, which burns on for a number of hours after loading has ceased. As a result the heat fibcration rate is often far less than expected. It must be said that although the heat liberation rate is lower, the heat liberated during burn down will make up for this to some extent. Additionally, by using longer residence times achieved by using large combustion chambers, the burning rate is equal to the loading rate. Extra heat losses occur due to the large surface area of the plant and the heat lost by quenching the hot ash. It must be understood that the mass of ash is often as high as 30--40% of the charging rate, so the loss is significant.
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(6) The thermal inertia of the plant. On a plant starting up once a day a considerable amount of heat is absorbed by the refractory linings in the chamber and ductwork of the plant. This heat is not recovered; it is lost when the plant is shut down at the end of the burndown period. On continuous plants operating 24 h per day the thermal inertia losses are, of course, only encountered on start-up and are of less significance. (c) Heat exchange boundary conditions Most waste materials contain inorganic salts, such as NaCI, Na, SO4, CuSO4 and FeSO4, which accelerate wear on the refractory in the combustion zone. Upon evaporation and combustion the salts form particulates of NaCI, Na:SO4, Cu20 and Fe~O3. Molten salts and oxides solidify on the waste-heat boiler tubing when cooled, causing reduced efficiency and possible plugging in the waste-heat boilers. This problem can be alleviated by cooling the incinerator exhaust gas below the salt melting points before it enters the waste-heat boiler, either by quenching with water, ambient air, or recycled gas from the waste-heat boiler exhaust. Melting points for salts and oxides found in incinerators are in the range of 750-1000°C. A gas-side boiler inlet temperature not exceeding 950°C normally ensures that any gas-side deposits remain friable and easily cleaned, and a working temperature of 800°C is chosen frequently so as to permit reasonable fluctuation [15, 17, 18]. At the outlet of the boiler, minimum gas temperatures are dictated by the probability of condensation of unknown or unexpected gaseous components and by the heat losses between the boiler exit and the top of the chimney. Boiler outlet temperatures are not allowed to fall below, typically, 230°C at full-load and 200°C at part-load conditions. The safe chimney top temperature is 170-175°C [2, 15, 17, 18]. The percentage of recoverable heat (Ra) based on boiler inlet gas temperature is calculated from
RB- - C. Tt- C.0 To x Cp, T~
IO0
where Cp, = m e a n specific heat of flue gases between entry temperature and 0°C (kJ[kg K]- I), Cp0 = mean specific heat of flue gases between outlet temperature and 0°C (kJ[kg K]-I), T~= boiler inlet gas temperature (K), TO= boiler outlet gas temperature (K). Overall efficiencies of heat recovery are lower than the above, due to sensible heat losses from the combustor (about 4%) and heat exchanger (about 3%) as well as losses due to unburnt material (about 7%) [6, 11)]. The highest practical overall thermal efficiency of heat recovery (boiler efficiency) does not exceed 65-70% of the gross thermal input [6, 9, 11,15]. (d) Fouling, erosion and corrosion of waste-heat boilers Fouling is a recurring availability problem which can be met by regular, gas-side cleaning (750-1000h intervals are convenient). So long as the gas temperatures are maintained between 900°C (maximum inlet) and 230°C (minimum outlet), any panicles adhering to tubes, etc. will be dry and friable; thus light brushing and vacuum-cleaning will suffice. Erosion problems can be minimized to the point where they do not affect the life of heat exchanger merely by keeping tube gas velocities reasonable ( < 25 m s-I) [15]. Also, by reducing gas velocities in the superheater section and arranging tubes parallel to the gas flow, erosion can be controlled. Corrosion mechanisms have been well-documented in respect of municipal waste burning. Sulphur, chlorine and phosphorous are the main elements in waste which are responsible for high and low temperature corrosion and deposits affecting heat exchange surfaces and other components downstream of a boiler. PVC plastics (45.4% CI) and common salt are among the several chlorides commonly found in waste. Low-temperature corrosion can occur at the back end of the boiler and subsequent plant if the metal temperatures fall below acid dew point. The presence of even a small amount of sulphur in waste can lead to SO: and SO3. The possibility of condensation of sulphuric and sulphurous acids
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(H2 SO4; H2 SO3), from the combination of SOj and SO2 with water vapour, on these cooled metal surfaces must be considered because of their corrosive effects. Chlorinated organic compounds produce HCI and some CI 2 as combustion products. Upon cooling in the waste-heat boiler there is a possibility of condensation of HCI with water vapor. Hydrochloric acid (HCI) and chlorine (Ci2) are also corrosive. By maintaining high back-end gas temperatures (200-250°C) at the expense of some boiler efficiency and a fairly high feedwater temperature (about 150°C), which will exceed the flue gas acid dew point at all times, low-temperature corrosion has been avoided in economizers, precipitators, induced draught fans and ducting [2, 17, 18]. High-temperature corrosion occurs when metal temperatures exceed about 300°C with the corrosion effect worsening at higher temperatures. Huck [19] has described this corrosion as the reaction between the tube metal and chlorine released during combustion of the waste forming Fe2 C16 which permeates the ash adhering to tube surfaces. At a sufficiently high metal temperature the Fe2CI6 decomposes to form Fe 3C14, HCI, CI and 02 which, because it is trapped in the ash, reacts inwards to attack the tube wall and forms more Fe2CI6. The reaction is thus perpetuated. The phenomenon is also described by Dunn and Temple-Pediani [10] who acknowledge the lack of comprehensive data on chloride corrosion and draw attention to two corrosive reactions which occur on the steel tube surfaces (Fe + C12 = FeCI2; Fe + 2HC1 = FeCI2 + H2) resulting in the formation of ferrous chloride scale. The ameliorating effect of a dual system---coal plus refuse--has also been noted in certain European plants. Krause et al. [20] have shown that the damaging reiietions increase significantly in rate with metal temperatures above 315°C and recommendations are made that boiler durability is obtainable only when all metal temperatures are below 260°C. Allowing say a 30 K temperature drop across the heat exchange surfaces yields 230°C as the maximum saturation or superheated steam temperature which can be generated with safety to the boiler. But when waste is intended to generate high temperature steam, over 230°C, treatment of the flue gases by scrubbing before ducting to the boiler must be recommended. The boiler can then be designed to operate safely and efficiently without reference to either high or low temperature corrosion and deposits. Krause et al. [2 I] have demonstrated that the addition of low-chloride sludge to municipal refuse is recommended as a corrosion prevention measure and a waste disposal technique. (e) Control of waste incinerator particulate emissions
One of the critical factors involved in the use of municipal solid waste as a source of heat energy is the requirement to remove particulate matter from the hot flue gases. In the incineration process, gases will leave the combustion chamber/furnace area at temperatures around 1000°C, containing solid particulate matter at a concentration ranging from 1-20 g m -3 at STP (0°C and 101.3 kPa), Emissions legislation in various countries throughout the world demands that these emissions be reduced to levels ranging from 25 to 200 mg m -3. This defines the range of efficiency for the gas cleaning plant as being from 80.000 to 99.875%. This can be accomplished using special design features of electrostatic precipitators [22, 23]. The starved-air principle of incineration achieves the extremely low emission levels without the requirement of any gas cleaning plant. Tests have shown that the units will meet all the particulate emission standards [11]. (f) Control of nitrogen oxides emissions Temperature, types and amounts of nitrogen-containing compounds in the waste and the percent theoretical air are factors which will affect the degree of conversion of waste nitrogen to NO, [17,23]. Thermal NOx formation occurs primarily in the range 1540-1930°C. Very little thermal NOx formation occurs below 1540°C. At lower waste nitrogen levels more than 60% of the elemental N is converted to NOx. The fraction conversion is about 45% at 0.4°/. waste nitrogen and decreases further to about 25% at 2% nitrogen.
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125
NOx concentration peaks at around 90% the0i-eti~l ai~: a~ording to one study. Other studies indicate from 83-105% as the amount of theoretical air at which NO~ concentration reaches a maximum. CONCLUSIONS From this review of the energy recovery from MSW incineration, the following main conclusions can be obtained. (1) The performance of mass incineration waste-to-energy plants has been as reliable as other power producing plants and the system can be a viable proposition and an attractive alternative to the traditional method of landfill of refuse. (2) Refuse incineration could make a valuable contribution to large-scale CHP schemes or could offer a prospect for commercially viable smaller scale district heating development. (3) Mass incineration without pretreatment of refuse with electricity generation is regarded as the most reliable and economical option for waste disposal. (4) Waste-to-energy plants can play a part in steadying or decreasing costs. (5) The starved-air incinerator represents the state of the art in waste management technology. The starved-air incinerator waste heat recovery system can produce 3.4 t of steam for every tonne of waste destroyed. (6) By the very nature of the material they use as fuel, xvaste-fired energy recovery systems must be robust. (a) A fire-tube, horizontal boiler has gained universal acceptance in waste-to-energy plants, however many plants have tended to use extended surface water tube boilers. (b) The heat recovery rates are a function of the loading rate and the calorific value of waste; the efficiency and fouling of the waste-heat boiler and the thermal inertia of the plant. (c) A gas-side boiler inlet temperature not exceeding 950°C normally ensures that any gas-side deposits remain friable and easily cleaned and a working temperature of 800°C is chosen frequently so as to permit reasonable fluctuation. The gas temperature at the boiler outlets is not allowed to fall below 230°C to avoid the probability of condensation of any gaseous components. The highest practical overall thermal efficiency of heat recovery does not exceed 65-70% of the gross thermal input. (d) Fouling of waste-heat boilers is avoided by regular gas-side brushing and vacuum cleaning. Erosion can be minimized by keeping tube gas velocities less than 25 m s-~ and by arranging tubes parallel to the gas flow. Low-temperature corrosion can be avoided by maintaining high back end gas temperatures (200-250°C). (e) Particulate emissions must be reduced to levels ranging from 25 to 200 mg m- 3. This can be achieved by using special design features of electrostatic precipitators. The starved-air principle of incineration achieves the extremely low emission levels without the requirement of any gas cleaning plant. (f) Very little thermal NOx formation occurs below 1540°C. Nox concentration reaches a maximum at around 83-105% theoretical air. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
D. J. Smith, Waste fuels, refuse-derived fuels and cogeneration, Power Engng 19, 40--46 (1986). J. H. S. Smart, The Greater London Council's refuse-fired power Station, Proc. Inst. Mech. Engrs 2@8,255-266 0986). N. Barnes, Operational performance of waste-to-energy plants, Proc. Inst. Mech. Engrs 199, 117-122 0985). S. D. Probert and B. Samuel, Domestic, municipal and industrial refuse in the U.K., Appi. Energy 22, 85-106 0986). J. L. Pavoni, J. E. Heer and D. J. Hagerty, Handbook of Solid Waste Disposal--Materials and Energy Recovery, pp. 57-167. Van Nostrand Reinhold, New York (1975). D. C. Wilson, Waste Management, Planning, Evaluation, Technology, pp. 6-17; 253-264. Clarendon Press, Oxford (1981). C. R. Veizy and C. O. Velzy, Incineration, in Mark's Standard Handbook for Mechanical Engineers, 8th edition, pp. 7.53-7.60. McGraw-Hill, New York (1978). J. M. Burnett, Recent developments in the use of municipal refuse as a fuel, Proc. Inst. Mech. Engrs 191, 89-98 (1977). R. W. Temple-Pediani, Energy recovery from combustible industrial and domestic refuse, CME 28, 43-46 (1981). A. Dunn and R. W. Temple-Pediani, Combined coal and waste firing of boilers, CME 30, 57-60 0983).
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11. J. A. J. Clark, Recovery of heat from industrial waste materials, 1ChemE Symposium Series 72, AI-A17 (1982). 12. H. M. Freeman, Hazardous waste destruction process, Environ. Prog. 2, 205-210 (1983). 13. J. C. Gwozdz and W. C. Walke, Fluidized bed combustion moves into utility applications, Power Engng 19, 42--46 (1986). 14. V. H. Morcos, Refuse-fired, pressurized, fluidized-bed combustion combined cycle analysis, Heat Recovery Systems 7, 465-472 (1987). 15. C. T. Chamberlain, Waste heat recovery from combustible wastes, IChemE Symposium Series 72, HI-H15 (1982). 16. G. H. Smith, Using waste to raise steam, CME 28, 44--46 (1981). 17. J. E. Ward and A. P. Ting, Waste incineration and heat recovery, Environ. Prog. 1, 30-38 (1982). 18. R. K. Tanner, Incineration of industrial waste. Prog. Energy combust. Sci. 5, 245-251 (1979). 19. R. Huck, Hydrogen chloride corrosion in refuse incineration plants, Brennst-Warme und Kraft (1966). 20. H. H. Krause, D. A. Vaughan and P. D. Miller, Corrosion and deposits from combustion of solid waste, ASME J. Engng Power 95, 45-52 (1973). 21. H. H. Krause, P. W. Cover, W. E. Berry and R. A. Olexscy, Corrosion and deposits from combustion of solid waste, part VII--Coincineration of refuse and sewage sludge, ASME J. Engng Power 102, 698-705 (1980). 22. K. R. Parker and A. Russell-Jones, Control of refuse incinerator particulate emissions, Proc. Inst. Mech. Engrs 200, 41--49 (1986). 23. J. Bromley, Monitoring of emissions from hazardous waste incineration, Proc. Inst. Mech. Engrs 201, 21-27 (1987).