Municipal Solid Waste Conversion DONALD K. WALTER
1. INTRODUCTION
The purpose of this chapter is to present data me productive use of municipal solid waste (MSW). The unique characteristics of MSW at both encourage and hinder attempts to use it further before the ultimate disposal of 1conomic components will be discussed.
The terms ‘waste’ and ‘refuse’ will be used interchangeably to signify anything that is normally thrown away as having no value yet en passed through an energy- or materials very system, it can be put to use economically. Energy-related uses will be emphasized, although other uses will also be considered.
No material is likely to be recycled unless s either required in a specific process or is the economical to use than virgin materials. : example of the former are certain steel production processes that require scrap, the latter is exemplified by the recycling of aluminum cans, thus conserving 95% of the energy required to make the cans from raw material.
As the term is used in this chapter, MSW is limited to the waste normally discarded in a sanitary landfill without special controls. This definition excludes: hazardous wastes; wastes are normally recycled at home; and prompt or intermediate industrial scrap.
1.1 History
The City of Chicago operated rendering plants in the 1850s and used the resulting grease and oil to lubricate the municipal cable- car system. By 1885 the first incinerator had been constructed, and by 1905 the logical step of adding a boiler to an incinerator to produce steam had been undertaken in New York City and in Hamburg, Germany. During the two World Wars, source separation and recycling of strategic materials for war efforts were commonly practiced. Following the Second World War combustion technology continued to develop in Europe, where energy costs were
high, land was scarce, and where environmental concerns emerged son, what earlier than in the USA. Higher population densities and municipal district-heating systems provided an excellent market for the steam generated. The tendency was to construct refractory- walled incinerators with waste heat boilers to simplify pollution control and recover energy.
By the 1950s, the European industry began to develop the more efficient integrated water-walled boiler. The United States, with cheaper energy and more available land, relied primarily on the use of landfills. In the more populous regions on the east coast, the tendency was to construct incinerators as a means of reducing the volume of waste requiring disposal. Frequently, these incinerators were constructed without proper pollution control devices and were overloaded. As result they frequently belched black smoke, smelled bad, and were marked by litter. Recycling was limited to the use of garbage as animal feed or as compost.
The environmental legislation of the 1960s, with its requirements for air pollution control, led to the closing of most US incinerators. Solid waste legislation in the 1970s led to experimentation with pyrolysis and use of refuse-derived fuel (RDF) in the US. The European and Japanese experience tended towards the boiler system, with somewhat greater emphasis on the efficiency of energy recovery. Where the plants of the l96OswouId have an extra combustion unit and be designed to have 60% of capacity in operation, the plant of the late 1970s would be designed to have 75% to 80% of capacity in operation. By the 1980s, the United States seems to nave rediscovered the combustion system; meanwhile, in Europe and Japan the trend now seems to be towards the development of RDF and prolysis systems.
2 THE RESOURCE 2. 1 Composition
Discarded from residences, commercial establishments, institutions, and industries, MSW is a most heterogeneous substance; with the quantity of any particular constituent varying widely with location. During 1978, the US Department
of Energy (DOE) sponsored a number of feasibility studies of waste-to energy facilities. Those studies produced a total of 13 different estimates of the constituents of MSW. Some of the significant results are that estimates of paper content varied from 30% to 55.6% of the waste; aluminum, from 0.4% to 3.4% (paper and aluminum are the largest materials components, respectively)
Constituent Weight
t,%) Heating value (MJ/kg Paper and paper products
Plastic
Rubber and leather Textiles
Wood Food wastes Yard wastes
Glass and ceramics Metals Miscellaneous 37.8 4.6 2.2 3.3 3.0 14.2 14.6 9.0 8.2 3.1 100.0 The moisture content averages about 25%. The Inorganics average about 20%
The ash in the organics average about 5%.
Compiled from a series of Department of Energy reports published between !975 and 1982 by the author.
and moisture (albeit not consistently reported), from 17.4% to 47.4%.
Table 1 shows the average constituents compiled from a series of Department f Energy reports. Table 2 shows the classification of wastes adopted by the Incinerator
Institute of America (now the Solid W Processing Division, American Socie1 Mechanical Engineers (SWPD)).
2.2 Solid waste management
In addition to physical characteristics, institutional matters are of great significance i productive use of MSW. Waste is man through a solid waste management which may be thought of in terms of t components - collection, transportation. disposal.
2.2. 1 Collection
Collection systems are not considered in d in this chapter. Waste collection constitutes necessary public function, whatever the mate disposal mechanism. As a result, no accrues to the energy- or materials- recovery system (shortened to ‘energy system’ herearter)
Contents composition moisture (MJ/kg) Highly combustible
waste (paper, wood, plastic, etc.): commercial and industrial sources Combustible waste (paper, rags, wood, floor sweepings, etc.): domestic, commercial, and industrial Sources Rubbish and garbage: residential sources
Animal and vegetable matter: restaurant, hotel, market, dub, and other sources
and organic a wastes
organic wastes: hospital. laboratory, .abattoir, and other sources
Gaseous, liquid, or semi liquid
Industrial process wastes
Semi-solid or solid
Combustibles requiring hearth retort or grate burning equipment
unless the collection system has to be 6adified for that system. As an example, if a system requires residents to separate specific elements for separate collection (source separation), then any added collection costs should be charged in an economic evaluation the source separation system. The appropriate formula would include the additional of vehicle amortization and operating, including both collection and delivery, pressing costs, etc., and subtract savings reduction in other collection costs, reduced vehicle maintenance, etc.). One of many publications that describes collection systems is that of Diaz era!. (1982).
2.2.2 Transportation
The second component, transportation, also not considered here in any detail, because is a necessary service function of local government agencies. However, transportation is likely to affect the overall cost of solid waste management, so it should be considered in overall refuse system economics. Frequently, the site of the MSW refuse plant is not at an existing landfill, but close to the energy user: the change in costs, between moving the collection or transfer vehicle from the centre of the collection site to the plant rather than to the existing landfill should be included in system economics. Because the energy user is generally closer to the municipality than is the landfill, this cost is normally negative.
A second transportation cost to be considered is that of vehicle maintenance. An MSW plant always includes a hardstand tipping floor, so vehicle maintenance is significantly reduced when compared with that incurred as a result of operations in the harsh environment of the landfill. This maintenance
The last component, disposal, is of great significance for an energy from municipal waste (EMW) system. Municipalities presently pay a disposal or tipping fee to discard their waste in a landfill. An alternative system that uses the waste to recoup a product(s) for sale should earn an identical disposal fee. Thus, MSW is the only energy source that is collected at a zero or negative cost and delivered to the site of use at a zero or negative cost. (And then the plant is paid to accept it!) In the northeastern section of the United States in 1984, the average tipping fee was $25 per ton.
2.3 Public versus private perspectives
Along with the positive economic aspects of using MSW as an energy or materials source, one must note a number of liabilities. These liabilities are associated with institutional issues concerning the political, economic, and business aspects involved in the operation of an MSW facility. Such issues are often considered from the viewpoint of either the service or the business aspects of EMW facilities. The service aspect refers to the munkipality’s view. The local agency wants all of the waste to disappear all of the time, with the least political liability and the least perceived risk, using as few facilities as possible. In contrast, the business aspect is shaped primarily by the desire that profit be maximized (i.e. all the product is sold at the maximum price, with the minimum operating and construction costs). Not surprisingly, there two viewpoints can occasionally be in striking conflict. The siting of facilities is one’ major battleground. From the service agency’s point of view, the ideal system site is out of sight responding to the public’s NIMBY (not in my back yard) stance; frequently, the existing landfill is chosen. In contrast, the ideal site from the business point of view is at the desired market.
The current optimum market is a steam operating on oil, and the optimum business is at a factory.
2.4 Financing
Another major institutional issue concerns economics and financing of EMW system the United States, there are methods used to finance EMW facilities
(1) General obligation (GO) bonds. (2) Revenue bonds (1DB).
(3) A combination of public and financing. (4) Private financing.
GO bonds offer the least expensive r of financing. They are based on the L strength of the municipality issuing and are a pledge of the tax income Li municipality. They are tax-exempt: obligations. Generally the interest that during construction is paid from funds and is not accounted for in the economics of the system. The disadvantage’ of GO bonds is that the total amount that may be issued is limited by law; the, the municipality must be willing to hi other uses of GO-bond funds. Despite‘ disadvantages, the GO bond is the most popular investment mechanism for the small scale system.
The 1DB offers tax-exempt obligation are repaid from project revenues. bonds required that debt-reserve and working capital funds he developed and that during construction be amortized. Theo the size of the bond issue may be twice large as the construction cost of the project.
Private financing has the highest interest rates; however, the system can be cons with less back-up and can be matched energy market. Few systems have been using private financing alone, because return on investment is marginal.
The combination of public and private financing is the most popular form of financing for a large plant. A business invests approximately 25% of the required capital and assumes much of the risk. In return, it acquires ownership and the full tax benefits. The net present US law, the net present value the tax benefits can be as
much as 40% of thee capital cost of the plant. The municipality sponsors tax-exempt IDBs for the rest of the ital. In return, it avoids much of the risk; addition, the capital requirements are reduced, and the initial tipping fee is lower. further details can be found in Anon (1981) Argonne National Laboratory (1983).
TECHNOLOGIES FOR RECOVERY %D CONVERSION OF RDF
Municipal solid waste contains recoverable it-al, glass, plastic, and paper, useful for the manufacture of new metal, glass, paper, or products. In addition, the remaining and organic materials make a better fuel than the original MSW, because some of i-combustible components have been d and the material has a more uniform size. The fuel portion is called refuse-derived RDF) in the United States and waste derived fuel (WDF) in the United Kingdom.
A variety of technologies for the productive use MSW and RDF are now or soon will be available, and can be classified into three broad categories:
1. Mechanical processing. These technologies include source separation and the mechanical manipulation of as received MSW to produce a solid fuel called refuse-derived fuel (RDF) and materials for recycling. Loose (fluff), densified (dRDF), and powdered pRDF) forms of RDF are produced. They may be used as a solid fuel or as a feedstock for other conversion processes. All source separation systems to date -have recovered materials for recycling which is a form of energy conservation, since the value of the system products is almost entirely determined by the cost of the energy required to produce the same items from virgin materials.
2. Thermochemical conversion. These technologies convert MSW or RDF to a more useful product by the application of heat derived from MSW and RDF. The end product may be steam, electricity, or another fuel. Discounting source separation, the oldest of the EMW technologies is combustion. However, pyrolysis and gasification systems are emerging for these biomass materials (see this volume Chapter 10 and 11).
3. Biochemical conversion. These technologies use organisms or their by-product enzymes to convert MSW to liquid or gaseous fuels or more stable solids. Anaerobic digestion converts the natural organic matter in MSW to a mixture of methane and carbon dioxide (see Chapter 13). in addition, certain organisms selectively convert cellulose to glucose, which may then be fermented to produce alcohol. The organic fraction of MSW can be stabilized by either aerobic or anaerobic bacteria to produce compost. Compost is useful as a soil conditioner and a weak fertilizer, and it has also been used as a base for making other fertilizers.
3.1 Source separation
In the context of source separation, mechanical processing is arguably the oldest of the systems for recycling of wastes. Source separation has occurred in times of emergency, to recover food wastes as feed for domestic livestock, and recycling of scrap has occurred in industry, but the source separation of recyclable material from MSW for materials began in the late 1960s. The prime recyclable materials are aluminum, paper, iron and steel, and glass. Other potentially recyclable materials include plastics, food wastes, tyres, and other metals (Martin, 1982).
The value of a recoverable material is, in part, related to the energy conserved in its remanufacture to new materials. Table 3 lists certain recoverable materials for which recovery and energy conserv9ton estimates have been made.
Perhaps the best US example of a source separation programme with home collection is in Madison, Wisconsin. Special racks have been added to Madison’s trash collection vehicles. Bundled. newsprint separated and placed at the curbside by residents is placed in the special rack by the collector at the same time as other (mixed) waste is being collected. At the landfill, before the vehicle is emptied of its normal load, mechanical trips on the racks facilitate dumping the newspaper. Thus, collection occurs at virtually no cost to the citizens or their local government.
Source separation is practiced in other countries, as well as in the United States. In certain parts of Europe where the local brewery is a fixture, beer is delivered to the door, and empty bottles are collected and returned to the brewery to be used again.
A frequent argument is the compatibility of source separation and energy recovery. In fact the argument can he reduced to the compatibility of paper and plastic recycling, versus their use as a fuel. In the US, the argument stems from the 1970s when plant size was based upon an average available waste rather than the weight of the waste available for the recovery plant. As a result several plants were constructed that were grossly oversized. For example, a 1000 ton per day plant was structured in an area that controlled only tons per day. In fact, removal of as much the recyclable paper and plastic as feasible will only remove 5% of the energy content the waste, while removal of the inorganics (particularly the aluminum with its lowing point) will improve the efficiency reduce maintenance costs of the system. Tm there is no conflict so long as the en system is not oversized and the combustible components should be allowed to seek the most economic market.
A further discussion of source separation and recycling is available in Vesilind Rimer (1981).
3.2 Mechanical separation systems
Mechanical systems, designed to separate waste into specific components to be revered and used again, represent a young technology; yet when development started in early 1970s, combustion technologies already mature. So why was there a push something new? The most likely reasons a twofold. First, technology development initiated in the US, where various environmental acts had resulted in the closing of a large number of incinerators because of the uncontrolled stack emissions. Undoubtedly, this wave of closings created a basic concern about any direct combustion technology.
Second, the principal concern - then as now - was that MSW should disappear. A system that would produce a solid fuel that could be burned in existing boilers as a partial replacement for coal should be less expensive, because the cost of the combustion chamber and boiler would be eliminated and utility boilers existed in or near every large city.
Mechanical systems perform two basic functions - homogenization (or size reduction) and separation. Homogenization is known by a variety of names, including shredding, hammer milling, and grinding. Diaz et a!. (1982) suggest that the most descriptive term is shredding, because no than 25% of the waste stream is brittle. Shredding results in smaller and more uniform particles for subsequent separation operations, and since the principal value of the separated components is their fuel value, the small particles enhance combustion characteristics. Figure 1 shows typical size distributions aw (as-received) and shredded MSW components.
Anything that is not wanted may be found MSW. Therefore, used paint cans, spray cans, old gasoline cans, old or unwanted ammunition, powdered resins, solvents, and other potentially flammable or explosive materials will be found on occasion. These constituents can explode in a hot, confined, and spark-filled space - such as a shredder (ERDA, 1975).
After the raw waste has been shredded to a more easily handled size, it may be separated a number of components. If the prime interest is in the production of fuel, then a major goal is to remove the non-combustible matter. If the aim is to recover specific components for recycling, then recycled purity of covered product becomes very important. The nature of the separation equipment depends on the physical and chemical proper of the material to he recovered. A binary device divides the material stream into two products, and a ternary device into three or more streams (Alter, 1983).
Sometimes separation can be effected by means of the electrical properties of the recovered materials. Magnetic separators for ferrous metals, eddy current
deflectors for non- magnetic metals such as aluminum, and possibly electrostatic separators, for dielectrics such as paper and plastics (Vesilind and Rimer 1981).
Screens separate waste based on its physical dimensions. There are many types of screens. The desired characteristics - beyond the size of the opening - include the ability to agitate the shredded waste to expose fresh material to the screen and the ability to clear material that blinds screen openings. Three types of screens are in use at MSW mechanical processing plants - flat, trommel or rotary, and disc screens. The most frequently used screen is the rotary screen which can simultaneously be used to open bags, smash glass and remove undersized material before it reaches a primary shredder.
Density separation by means of air classifiers and flotation is also being utilized. Air classification can be used in a ternary system separating light ‘combustibles’ from aluminum and heavy materials through the use of baffles and air ‘knives’.
The principal product of the mechanical processing system is RDF. In its original shredded and separated form, it is fluff RDF having a low bulk density and relatively high moisture content, so it is difficult to store and transport. There are cases on record where as much as 2000 tons of shredded waste stored for a week solidified into a papier mâché-like mass. To improve the characteristics of fluff RDF, research was undertaken to adapt the various pelletizers, cubetters, and briquetters that had been developed principally for the agriculture and charcoal industries. Their operation depends upon either forcing shredded and separated waste through a die (pelletizer and cubetter) or squeezing it between two wheels. All function reasonably well, although in the US, the devices do not reach their rated capacities. The UK has been very successful in producing dRDF for industrial boilers. (The UK produces 70% of its steam for industry in coal fired boilers with grates.) Another effort to improve the characteristics of RDF involves treating the natural organic with acid. The acid embrittles the organic material, which is then ground in a d ball mill. The resultant pRDF is very uniform and quite dry. Unfortunately, the full-scale plant
encountered a large number of difficulties. Before these could be resolved and the plant made commercially operational, the developer went bankrupt. On a positive note, however, the fuel produced by this plant was burned successfully in a cyclone utility furnace at up to 50% pRDF.
The systems that have been built to date have been designed for three purposes: the recovery of specific materials for recycling; the recovery of RDF to be used in an existing boiler: and the production of fuel for a special boiler designed for the RDF. In the US, the original intent of mechanical processing was to provide fuel to large suspension, coal fired boilers. Of the 22 processing plants built in the US, only two have been designed by the same engineer for the same purpose (i.e. as integrated, RDF-dedicated boiler plants). Even these two are not identical; one is twice the size of the other and is designed accept coal as an alternative feedstock. they were also designed at the same time so that neither could gain experience from the other. The remaining 20 plants are of separate designs and were essentially designed at the same time; thus, their designers were unable to draw upon each other’s experience. This provides some explanation for the relatively poor experience with these systems in the US.
Of the 22 US plants, two were designed to recover materials, and both are closed, neither was able to develop acceptable markets for the product and both had technical problems that were not fully resolved. Eleven plants have been constructed to recover RDF and only secondarily to recover materials: four are operational and sell RDF to existing utility boilers. The plants which are closed had a combination of technical and institutional problems. Nine plants have been built to provide fuel to a dedicated boiler. Six are operational while the boiler for a seventh is in construction.
In the UK a slower more orderly research program has been developed; it is considered that the collection of sufficient waste in one place to make a significant impact on the fuel consumption of a large utility boiler would not be economic in the situation. Seventy percent of industrial steam generation is by means of small coal fired boilers with grates and MSW and RDF systems have been developed to fit.
Early work developed fluff RDF as feed for these boilers plus cement kilns. After success with these efforts, the UK began developing pelletizers to extend the utility and enhance the fuel value of RDF. Currently, there are two RDF plants in operation and two dRDF pilot plants gathering data.
Research and full scale or test scale facilities are also underway in the rest of Europe and Japan. These units and tests are meeting with varying degrees of success. Most are reporting some difficulty.
3.3 Thermochemical conversion system
Thermochemical systems are designed to change MSW - either as-received MSW or RDF - into such energy forms as hot water, steam, or electricity or into a more valuable fuel form. More than 500 systems worldwide are burning waste to produce steam, but there are few commercial-scale systems producing gaseous fuel from waste.
The distinguishing characteristic among thermal systems is the amount of oxygen made available. If sufficient oxygen is available to complete the oxidation of the components, then the system is an incinerator. As the oxygen supply is restricted, the oxidation is less complete; the resulting products comprise a range of combustible gases, liquids, and solids.
3.3.1 Combustion
If the heat produced by an incinerator is used for a productive purpose, then the system is better defined as a combustor or boiler. All MSW combustion systems have the following components in common :
(1) receiving area; (2) storage area; (3) preparation system;
(4) feed system (to feed to combustor); (5) combustion chamber(s);
(6) boiler;
(7) pollution control equipment; and (8) ash removal equipment.
Some of these individual components may be combined.
The feeding system depends on the design of the combustor. Most large mass burning systems use a crane with clam-shell or orange-peel bucket to transfer the charge of waste into a feed hopper. The RDF dedicated boiler usually is fed by a series of conveyers and an air-swept spreader-stoker. The metering device can be variable speed screw or belt conveyers, simple flappers, or some other device; alternatively, the metering may not be controlled at this point. First and foremost, the combustion chamber - the heart of the thermal system - provides a surface upon which the waste can burn. The typical small combustor-has a heat- resistant surface and a series of hydraulic rams to move the smoldering waste through the furnace, while the mass burning furnace generally has a proprietary grate that supports, transports and admits air underneath (under-fire air) the burning refuse. (Department of Commerce, 1978 and Turner, 1983 include schematic drawings of different grate systems.) The dedicated boiler usually has a traveling grate to perform the same functions as the grate of the mass burner.
The second function of the combustion chamber is to provide combustion air to the burning waste. In addition to the underline air, the combustion chamber admits over fire air to mix the combustion products and ensure their complete burnout. The typical small
often mechanically conveyed to the water pit be combined with the bottom ash and tings for ultimate disposal: Some dry ash systems are being constructed especially when ash disposal site is distant from the plant. More than 400 field-erected mass burning plants worldwide are currently operating and recovering energy. Of these, 16 are operating n the US. In addition, seven RDF-dedicated boiler plants are operating (six in the US and one in Canada), and there are over 50 factory-crected small modular combustors (30 of them the US).
Figure 2 shows a typical mass burning waste-to-energy plant.
It shows the Saugus, Massachusetts, plant which produces superheated steam (4.7 MPa, 4669 °C) for sale to a nearby industry. A steam turbine generator set is being installed to broaden the market for energy from the plant. The plant’s technical details include a pit for storage of the waste, a crane-and-bucket feed system, a proprietary European grate system, and n ESP. Not shown are the over fire air, injected at the throat on the water wall section, and the hydraulic ram, which pushes waste into the combustion chamber. The typical mass burning MSW combustion system controls steam production by cooling the speed of the feed ram and grates and the amount and location of the over fire air. Control within 5% of-a et point is common.
The principal operating problems are erosion and corrosion of the tube surfaces These problems are thought to be functions of the temperature of the tube surface and to be caused by a complex iron-sulphur-chlorine reaction, A classic curve of the corrosion relation in a municipal waste combustor shows a sharp increase over a 55 °C range centered at about 170 °C. The corrosion rate then becomes low up to about 470 °C, at which point the corrosion proceeds rapidly. Figure 3 is a graph of this corrosion mechanism. The corrosion appears to be worse in the vicinity of steam soot blowers, because soot blowers tend to dislodge any protective coating on the tubes. The solutions lie in boiler design, control of steam temperatures, tube metallurgy, and mechanical rapping systems. Corrosion and erosion also occur in the vicinity of changing oxidizing and reducing atmospheres. Proper design of over fire
air systems and coating of refractory surfaces in exposed zones can overcome the problem in such areas. In addition to corrosion and erosion problems, the walls and tubes can become fouled with slag if temperatures are not controlled and provisions are not made for cleaning of the walls. In one new plant, slag build-up was sufficient essentially to prevent the walls from removing beat from the combustion gas. The temperatures were to drop 170 °C but only dropped 15 °C. As a result the temperature was too high at the super heater tubes and some tube failure was experienced.
A typical dedicated RDF combustion system is shown schematically in Figure 4. Not shown is the RDF preparation plant, which delivers RDF pneumatically from a storage device to the hopper (on the upper left). The RDF falls by gravity to the air swept spreader-stoker above and to the right of the person shown in Figure 4. The RDF is blown into the boiler, where half of it burns in suspension and half falls on the rear of traveling grate. As the grate moves forward (towards the figure) the RDF complete burn out and ash falls into an ash pit.
The principal operating problems include the corrosion and erosion problems described for the mass burning system. In addition, if the temperature on the grate is not carefully controlled, aluminum and glass may melt and resolidify on moving parts; the correction of this problem does not seem to be well in hand although limiting grate temperature will help. Dedicated RDF units are also susceptible to slagging and development of deposits on the rear wall of the boiler; this problem can be corrected by controlling the air-swept feeder and by adding coatings and soot blowers on the rear wall.
A variation on the dedicated RDF boiler burns RDF normally as a partial replacement for coal in an existing spreader-stoker boiler. Test burns at up to 100% RDF by energy content have been successful; however, the boiler must be derated up to 15% of output in order to accommodate the higher moisture and ash contents of RDF. Densified RDF has been burned successfully with coal. Problems include the somewhat higher particulate matter rating on the pollution control equipment (offset
by lower sulphur emissions), the boiler derating and where the percentage of RDF is high, overheating on the grate surfaces.
The suspension fired boiler, designed to burn coal, constitutes a third major variety of boiler for MSW-based fuels. Typically, utility boilers have been used because of their availability; a good grade of RDF fuel must be produced, and the ratio of PDF to coal is limited to 10% by energy content. The basic design includes an RDF plant located at the utility boiler site. At the utility plant, some type of storage and reclamation facility (supplied by truck or conveyer) is provided. The RDF is reclaimed by a metering device, pneumatically transported, and blown into the boiler (usually through coal nozzles that have been repiped and converted for RDF). Generally, only half the RDF burns in suspension; the remainder falls to the bottom of the furnace. It is common practice for dump grates to be retrofitted in the furnace to hold the RDF until it completes combustion. The dump grates also tend to retain coal clinkers, which cool and are no longer brittle after they pass through the ash pit. This adversely affects the operation of the clinker grinders.
Some success has been achieved, although the majority of the plants that have been closed in the US have been designed to provide RDF to existing boilers. These systems have experienced several problems. The dump grate has permitted completion of burn out; however, RDF ash has formed clinkers on the grate that required manual rodding for removal, and as noted above, the coal ash has changed its characteristics in a way that adversely affects the ash grinders.
Another problem is strictly institutional. Most utilities operate on the principle of economic dispatch (i.e. they obtain electricity from the cheapest source even if they must buy rather than internally produce the electricity). The plants converted to burn RDF are typically the older units. These units are operated for peaking power, so they are frequently off line. Meanwhile, MSW is made to RDF every day and is not easily stored.
The last type of commercially operational MSW combustion unit is the modular combustor. Modular combustors have many names and are made by a large
number of manufacturers. They are characterized by assembly line construct on in a factory and by possession of one or more secondary chambers to allow for complete burnout of combustion gases. The typical unit (with energy recovery) has a capacity range of 25 - 105 t/d.
The receiving area is usually a flat floor. Small skid-steer tractors mix and push the MSW into a small, single charge point. A door closes off the pit and a hydraulic ram pushes the waste through an interlocked door into the prime combustion chamber operates under start air conditions. The gases produced trave through a throat into a secondary combustion chamber, where excess air is added to complete combustion. The secondary chamber has an auxiliary gas or oil burner for start and has a pilot light to ensure combustion. Normally, this burner operates only during start-up. The waste is transported through primary chamber by a series of hydraulic ram operating on a stepped hearth.
The original units (many of which operation as incinerators) were allowed to burn out each evening. In the morning, the back of chamber was opened and the ash manual removed before the day’s processing begin. The modern units have automatic ash remove systems that permit continuous operation Figure 5 is a schematic of a modular combustor.
In principle the low velocity in the prime chamber and the fire in the second chamber should combine to decrease oxidize the particulate matters so that pollutant control equipment would be unnecessary practice, the units are on the borderline meeting US particulate control standards, at many small plants include pollution control. Corrosion is not a problem in the present modular combustors, because steam conditions have been limited to saturation conditions. As the steam temperature is allowed to rise, some corrosion will occur (as in larger combustion units). Fouling can be problem in these modular units, particularly the boilers have obstruction. Other problems include refractory wear and combustion control.
Another thermal system nearly commercial status is the fluidized bed combustion (FBC) system. It has a bed of sand or other fine material that is kept fluid
by a risk current of air. Because of the large mass of bed material and the inherent mixing of the air, RDF burns uniformly and rapidly. Bed temperature can be controlled precisely if this temperature is limited to 815oC agglomeration of the bed is
no problem because the glass does not soften. The amount of air necessary for combustion is controlled as much by the need to fluidize the bed as by the need to provide sufficient combustion air. Excess air can be limited to 45% to 50%, in comparison with 100% in conventional burners. In addition, the bed material can be varied and used to capture chlorine and sulphur compounds (Trezak, 1983).
The first large-scale fluidized bed combustor built at Duluth, Michigan has encountered fuel preparation and fuel feeding problems. The fuel preparation and fluid bed configuration has been modified and the system is in operation again.
Table 4 lists some operating facilities in the US and Canada. This list is not meant to be complete and the inclusion or omission of any specific plant is not to be construed meaningful.
3.3.2 Pyrolysers and gasifiers
A number of commercial scale MSW gasification systems have been constructed (Kuester, 1983). Six ANDCO-Torrax systems were built: four in Europe and one each in Japan arid the US. Of these six systems, two are operating as of the preparation of this document (July 1985), three are operational, and one has been dismantled. The ANDCO-Torrax system uses as-received MSW stored in a pit. A crane loads a lock hopper which in turn drops the MSW into a cylindrical chamber. As the waste falls, it is successfully dried and gasified. At the bottom of the chamber the ungasified carbon char is burned with high temperature air to provide a rising current of hot gas. The combustion zone temperatures are hot enough to slag the metals, glass, and ash which flow from the unit through a slag tap. The hot gas produced, which has an energy content of 5.6 MJ/m3 without including the sensible
heat, is immediately burned in a secondary combustion chamber. Part of the hot gas is exhausted through regenerative towers, which heat the combustion air to 980 °C. The
remainder of the gas then passes through a boiler and joins the gas from the regenerative tower before being exhausted through a pollution control device.
Nipon Steel has constructed a direct heat system that uses preheated air enriched heat oxygen. The facility, with a capacity of 400 t/d has been constructed at Ibaragi City, Japan. It uses RDF-2, coke, and limestone. The end product, a fuel gas, probably has an energy content between that of the ANDCO-Torrrax system and a Purox system.
The Purox system was developed by Union Carbide Corporation. A unit is operating at Cichibu City, Japan. This system uses uses essentially pure oxygen to gasify RDF-2. The design is similar to the ANDCO-Torrax unit. RDF-2 is introduced through twin ram feeders. As the waste settles it is successively dried and gasified. At the bottom the char is combusted with pure oxygen. The slag from the bottom of the reactor is an obsidian like glass that is stable and hard.
A number of indirect heat systems have also reached reasonable large scale. At Redwood City, California, a 45 t/d unit produces a gaseous product from automobile shredder organic materials. The system uses a look hopper feeding a vibrating conveyer in a long radiant heated horizontal chamber. Part of the gaseous product is used for the radiant heaters; the rest is burned to produce steam in a conventional boiler.
The Occidental Research system at El Cajon. California, was designed to use an RDF-4 fuel in an entrained reactor to produce a liquid. The char was separated in a cyclone and burned to provide hot gas for the process. The liquid product was condensed from the gas stream from the reactor. The remaining gas stream (after the liquid product was condensed) was also used for system energy. The product liquid was highly oxygenated and viscous, and it deteriorated in storage. This system was shut down until some research questions were answered.
In Japan, two companies are developing indirectly heated fluidized bed gasifiers. RDF is fed to a bed of hot sand, which is fluidized with product gas; the RDF gasifies. The bed velocity is high to elutriate the char, which is removed in a cyclone
and burned in second fluidized bed to provide hot sand for the first bed. A similar system, using dual-media beds, is under development in the US use with wood.
UNICIPAL SOLID WASTE CONVERSION Table 4. Selected MSW thermal facilities Facility type and location Year
operational Design capacity (t/d) Product Field-erected mass burning plants Westchester County, NY Saugus, Mass Montreal, Quebec Hampton, Va Gallatin, Tenn 1984 1975 1970 1980 1981 2250 1500 1200 200 200 Electricity
Steam for industry District heat
Steam for government Steam for industry RDF-dedicated boilers
Dade County, Fla Niagara Falls, NY Akron, Ohio Albany, NY Hamilton, Ontario 1982 1981 1978 1982 1972 3000 2000 1000 750 500 .
Electricity and materials Cogeneration District heat District heat Electric (1982) Modular combustors Pittsfield, Mass Auburn, ME Pascagoula, Miss Susanville, Calif Red Wing, Minn
1981 1981 1984 1985 1982 360 200 150 96 72
Steam for industry Steam for industry Steam for industry Cogeneration District heat
Osceola, Ark Groveton, NH 1980 1972 50 24
Steam for industry Steam for industry RDF for existing boilers
Baltimore County, MD Ames, Iowa Madison, Wis 1978 1975 1979 1200 400 400
Cyclone utility boiler Suspension utility boiler Industrial boiler
3.4. Biochemical systems
Biochemical systems use organisms or the products of organisms to change MSW into fuel. In controlled reactors, typically the MSW must be processed into a RDF, while uncontrolled reactors typically use the MSW in its as-received form. An example of the former is a reactor converting MSW to sugars for fermentation to ethanol and of the latter a sanitary landfill naturally degrading organics to methane and carbon dioxide.
There are a number of distinguishing characteristics of biochemical systems, One is the availability of oxygen to the system. Organisms are either aerobic, anaerobic, or facultative (living with or without oxygen). Another major distinguishing characteristic is whether the organism performs the conversion function as a part of its life cycle or produces an enzyme that catalyses the conversion reaction. When compared to thermo-chemical systems, biochemical systems are slow (reaction times of hours or days) but operate at mild conditions (near atmospheric pressure and below 100°C and produce specific products.
There are three major biochemical systems at an advanced research state; anaerobic digestion to produce methane-rich gas, fermentation of cellulose to glucose for further conversion to ethanol, and composting. Further there are two subsets of anaerobic digestion; controlled and landfill.
A large fraction of MSW is biodegradable (see Table 1). However, certain organic molecules resist breakdown. These include the lignins that form 25% of wood, and the majority of the man-made plastics and fibres. An appropriate feedstock preparation system must remove as much of this material as possible along with the in organics which are not biodegradable. The measure of the organic material in a substance is volatile solid (VS). VS is determined by heating a sample to a controlled temperature for a controlled period of time. The loss in weight of the sample is the VS. Normally, the sample is then heated until the weight is stable. The remainder is ash. Thus the non-biodegradable portions of the sample contribute to the volatile solids. Other tests are in use which subject a sample to a controlled set of biological conditions. The loss of weight is the biodegradable solids.
The most modern type reactor being developed uses some form of a solid bed on which the organisms can be immobilized. Bed designs vary. A facility operating on distillery waste in Puerto Rico has plastic egg crate packing with the substrate flowing upwards through it. A 190 m3 sludge digester (ANFLOW) tested by the US
Department of Energy had plastic rings with the substrate flowing upwards. Other schemes in development use horizontal or zigzag flow with plastic baffles and expanded or fluidized beds of sand. These units all have the same characteristics, very short hydraulic retention time (HRT) (8 hours of less) and very long retention time (SRT) (a year or more). The short HRT adversely affects the energy recovery since as much as half of the methane can remain dissolved in the effluent but the long SRT decreases the sludge disposal problems common with waste water treatment plants.
There are no commercial MSW anaerobic digestion systems in operation in the world; although there are numerous digesters operating on sewage sludge primarily for stabilization. In addition, there are a large number of digesters operating on manures particularly in China and India. There are research; programs on MSW digestion in the US, Italy and Spain.
The initial US research (Pfeffer, 1985) and economic studies established the viability of the technology. St Louis, Missouri, RDF was used in laboratory digesters;
0.37 m3/kz of volatile solids were fed at thermophilic temperatures up to 60 °C. Table
S presents the results as related to volatile solids destruction.
Based on studies by Dynatech Corp (Wise et al., 1974), the Department of Energy issued a competitive solicitation of construct a proof-of-concept scale MSW anaerobic digestion system. Waste Management Inc was selected to construct and operate the system. Construction of the 50 to 100 ton per day facility was completed in 1978 and has produced gas since. Initial operation at mesophilic and subsequent operation at thermophilic temperatures confirmed the laboratory research results. Walter (1985) contains a complete list of research data collected since 1981. The earlier data are flawed since the digester could not be fed consistently and the means of collection was not operating properly. Table contains some data from the April 1985 period.
3.4.2. Landfill gas
A second form of anaerobic digestion is much further advanced. It is the uncontrolled digestion that naturally occurs in existing landfills.
The origin of this’ emerging industry is the requirement to control emissions for safety reasons. The sanitary landfill that required daily cover resulted in gas migration. On occasion, the gas was trapped, concentrated, and with a spark would burn or explode. Thus biogas was collected on the perimeter of landfill for migration control. In 1975 NRG Inc, a predecessor of Getty Synthetic Fuels, recognized the value of the flared gas, developed a molecular sieve based on natural zeolite formations and began the purification of biogas from the Palos Verdes, California landfill to a synthetic natural gas (essentially pure methane) which is sold to a southern Californian Edison Corp, gas distribution main. Subsequently the Azusa Land Reclamation Company tapped its landfill at Azusa, California and sold biogas directly to Reich-hold Chemical Company as a boiler fuel. The final market development was the installation of engine generator sets to generate electricity.
The technology of the recovery of landfill gas is relatively simple. A well is drilled vertically in an existing landfill or is created horizontally while the landfill is under construction. Typically, slotted plastic pipe with slip joints is inserted in the centre of the well and packed with gravel. Near the surface a concrete or clay cap seals the well against infiltration. The wells are then manifolded together and a slight vacuum applied to draw the biogas to a treatment station. If the end use requires medium energy gas (15 MJ/m3) then the treatment system generally removes the
condensate. Where high energy gas (essentially pure methane) is desired after condensate control, the carbon dioxide is removed by a number of processes including molecular sieve, scrubbing with water or a variety of proprietary compounds, and membrane filtration (OTA, 1982, Ashare et al., 1982). The electric generation option generally uses condensate control in front of a reciprocating or gas turbine engine generator set in a few instances the gas is sold to an existing utility boiler as a supplemental fuel, For the purposes of this discussion, that is akin to the sale of a boiler fuel.
The preferred landfill gas system in the US is the sale of a medium energy gas for use a boiler fuel since capital costs are low and the oil or gas displaced is a high value fuel. However, the co-location of a landfill with an appropriate boiler is rare. The choice between producing high energy gas for the pipeline and electricity is a matter of economics although the potential for a nearby electric distribution system is higher than that for a natural gas pipeline, and US law requires that electric utilities purchase third party generated electricity.
A landfill s expected to produce between 250 and 900 m3/(t.y) for at least 20
years. For at least 10 years the gas will be produced in economic quantities, rough formulae for the costs of landfill gas recovery in the US are given by Pfeffer et al. (1983). There are existing research efforts that are designed to understand the generation and recovery processes. For instance, some computer models related to the design of wells and their sphere of influence have been developed. Existing and planned work is designed to understand the boundary conditions of the landfill and
the microbial populations responsible for biogas production. Table 7 is a partial list of landfill gas recovery facilities in the US.
3.4.3 Ethanol
A second biochemical process converts MSW to ethanol. From an energy standpoint the ethanol is used as a fuel or automotive fuel additive although there is a wide variety of chemical uses of the industrial sector. The use of ethanol and other alcohols as fuels is not new. Recently, the use of alcohol as a fuel has become prevalent and some countries such as Brazil (see this volume Chapter 16) have large-scale use or vehicles fuelled with pure ethanol. Until recently all industrial grade ethanol was produced by thermal processes from natural gas. With the increase in oil prices and the availability of excess grain in the United States, the interest in the fermentation of ethanol from sugar and starch for fuel purposes was rekindled.
Conversion of both wood-band MSW celluloses is in experimental development. Some research has been conducted and proposals advanced for pilot and commercial facilities. The majority of the research has been based on wood and agricultural wastes. This work is expected to be applicable to MSW.
The MSW conversion system has five distinct steps: the concentration and pretreatment of the cellulosic and carbohydrate components; the conversion of those components to sugars; the fermentation of sugars to ethanol; and the distillation of the ethanol/water mixture.
In essence, the concentration step has described in the feedstock preparation systems above. A potential complicating factor is the removal of plastics. A source separation system concentrating the food components might be particularly advantageous in a MSW to alcohol system since these tend to be higher in starch and sugars.
The conversion of the cellulosic components may be accomplished by two techniques, acid and enzymatic hydrolysis. The former was described in patents in 1880 and used in the First World War to hydrolyze cellulose to sugar mostly for
animal fodder. It is relatively quick with reaction times in the range of seconds to minutes. Unfortunately, the acids then hydrolyze the sugars to undesireable products from an energy standpoint with reaction times of the same order of magnitude. Neither of these techniques is used commercially, although research and development is being conducted on both.
Much of the current acid hydrolysis research is being accomplished in Canada and Japan on wood and agricultural feed-stocks. The principal research on MSW was done by Dr Rugg at the New York University, under the sponsorship of the US Environmental protection Administration. The basic process is an extrusion system based upon twin screw machines such as those used in industrial plastic extruders. Rugg’s work was based upon the hydrolysis of newsprint. Subsequently working on wood as a feedstock, he found typical reaction times were 6.6 seconds 240 °C while the acid concentrations varied from 1% to 4% (Rugg, 1982). At the higher concentrations the product was almost free of unreacted cellulose, although the overall glucose yield dropped from 54% to 36%. The concentrations of other sugars such as xylose and mannose were high. At this time neither at these five-carbon sugars is commercially converted to ethanol although research has identified some yeasts which do ferment xylose to alcohol. A relatively short increase a reaction time resulted in a further conversion of the sugars to undesirable energy products such as furfural. Direct fermentation of the hydrolysates was carried out with 99% of the glucose being consumed in 28 hours.
The enzymatic hydrolysis process was initially researched by the US Army, principally at its Natick laboratories as part of a materials/preservation program. Subsequently, the research turned to use of the process first as food source and subsequently as a fuel source (Reese, 1976). Subsequently a number of researches began efforts to develop further enzymatic hydrolysis of MSW feedstock. This work continues. One example is the work by Dr Emert which began in 1971 for the Gulf Oil Chemicals Corp, and subsequently transferred to the University of Arkansas Foundation where he conducted research that included a one ton per day prepilot
plant at Pittsburg, Kansas. MSW, pulp mill, agricultural and saw mill wastes were all tested as substrates for the production of cellulose enzymes and the subsequent conversion of those substrates to ethanol. The overall system used the enzyme in a simultaneous saccharification fermentation step. The development of the process continues with a 50 ton per day facility reported as the next logical scale-up step. One of the major requirements to advance enzymatic hydrolysis of MSW is the development of a viable pretreatment process that will improve the digestibility of the cellulose fibre.
3.4.4 Composting
A unique MSW biochemical system is composting. Composting stabilizes the putrescible fraction of MSW and sewage solids (SS) to humus. It is an ancient natural process that man has to learn to control. Scientific examination of the process began in the middle of this century under the auspices of governmental agencies and universities. The state of the art has advanced to the point that commercial facilities are offered today.
The basic process uses micro-organisms to oxidatively break down the organics to stable compounds, carbon dioxide, and heat. Corn- posting can be either aerobic or anaerobic and may operate at mesophilic or thermophilic temperatures to sterilize the final product. The resulting compost is currently used as a soil conditioner, low grade fertilizer or carrier for chemical fertilizers. Since the heat generated drives off much of the moisture, particularly from the normal wet feedstocks such as SS and manures, the compost may also be a viable solid fuel although it is not in commercial use for this purpose at this time.
Most composting operations have four steps : (1) preparation (and mixing) of the feed-stock(s); (2) decomposition;
(3) curing;
For MSW the preparation step is some form of a mechanical system designed to concentrate the organics and to remove the glass and metals that are undesirable in the finished product. The preparation step for wood and other bulking agents provides appropriate size materials. For sewage sludge and manures the preparation involves mechanical dewatering. If there are more than two feeds-tocks they are then mixed in commercial equipment such as a pug mill.
The decomposition step generally is mechanically assisted. The simplest form is a flat floor with provision for forcing air through the pile and including special mobile or other equipment to mix the composting material on a regular basis. More complex systems use structures to initiate and control the composting process. For instance, the Dano system uses a long rotating compartmented kiln with a residence time of about three days. As the material proceeds through the kiln, it is mixed, air and temperature are controlled the decomposition is initiated. An installation in Sao Paulo, Brazil, uses MSW. It has a receiving bin and hand picking of recyclable materials preceding the Dano reactor. The compost, after three days in the reactor is cured in open windrows for 30 days before screening into various grades for sale. The curing stage allows time for the stabilization of the compost. Generally, it takes place outside (with possible protection from the weather) on a gravel or concrete slab. After completion of the curing process, the finished compost is screened to provide a uniform product size, to recover bulking agent particularly if wood chips are used, and/or to grade the end product. Another system beginning operation at May, New Jersey uses the ABV/Purac system. The unit is a vertical plug flow digester operating with sewage sludge and sawdust. The mixture (optimum moisture content from 30% to 35%) is spread on top of a BIO reactor where it remains for 14 days. This material is discharged from the bottom and moved to the top a cure reactor with a residence time of 14 days. The finished compost is removed from the bottom of the cure reactor. Air is introduced into the bottom of the reactor and exhausted through a scrubber for odour and pollution control (Cathcart, 1985).
Most of the commercial compost systems operating are in Europe with some facilities in Brazil and other South American countries. The European success, in large measure, is based upon careful matching of product to the available market, and the availability of vineyards where the use of compost as a mulch, soil conditioner and low grade slow release fertilizer is particularly advantageous. The experience in the US has been limited although the number of installations is expanding, especially for the stabilization of SS since alternative disposal means are energy intensive. The principal deterrent to compost processes is the lack of large markets for the product. 4. ECONOMICS
An energy from municipal waste (EMW) ant has two sources of income. The sale energy and/or materials produced by the project and the disposal or tipping fee. The disposal fee should not exceed the fee for the current or least cost system to dispose of waste. Generally the competing system of disposal is a landfill. From the city perspective e tipping fee increases annually with the increase in the cost of living. In an EMW system the tipping fee tends to stabilize or decrease since the debt service is not subject to inflation. Figure 6 graphically shows this effect across a time period.
The economics of any waste to energy system is very site specific. However, some generalizations are possible. Any well designed and operated system though profitable over its life cycle is not economic in first cost, that is, the system will have a negative cash flow in its first few years (breakeven seems to occur between years three and seven). This presents a serious barrier to implementation particularly in the US since municipal funding decisions tend to be made on a first cost basis. Generally, source separation systems are not economic unless some of the costs are transferred to others. For instance, volunteer labor or the transfer of collection costs to individuals can result in economic systems from the viewpoint of municipal cash flow.
One means to reduce the initial cash shortfall of an EMW system is through the financing mechanism. In the US if GO bonds are used to finance the project, then
typically the interest on the bond while the plant is under construction is paid from general revenue funds and reserves for debt service and startup are not created. One of the most economically successful US plants (Hampton, Virginia) used GO bonds and a 25% federal grant to realize a zero tipping fee in its full year of operation. In this case the federal government is the energy customer and the grant is being repaid through reduced energy costs. On very large plants where GO bonds are not practical, industry is providing 25% of the capital as an equity contribution. That equity is then recovered during the first live years of plant operation through favorable tax laws. This serves to reduce the early year cash flow. Other city-related means come negative cash flows including disposal fees or acquisition of waste from other areas. Disposal fees are the municipality, thus the discussion above takes the viewpoint of the city.
The disposal fee in the EMW plant actually has two elements the fee that would otherwise be paid to a landfill and any modification that may occur in the solid waste management system such as a change in travel distance or maintenance costs. Frequently these costs are not well defined. A study in the mid-1970s to define the reasons for European success in waste-to-energy plants quickly showed that many city financial records were not sufficiently detailed to define what specific city functions cost. The EMW, district heat, solid waste management functions, and electric supply system were intermingled and the actual costs of each could not be separated from the other. A similar standard of record keeping is also common in the US.
Energy and materials sales must be acceptable to the marketplace. In the case of the sale of steam to industry the sales price is usually tied to another fuel (i.e. a shadow price, commonly oil) and generally discounted from that fuel price. An exception is the Hampton plant mentioned above. There the sale of steam is discounted from the cost for a local military base to produce steam internally. In the US, in 1985 a discount from fuel oil price equates to a sale price of energy of $4.00 to $4.50/GJ while discount from steam price equates to $8.00 to $8.50. As regards the
sale of electricity, US law (PURPA requires that utilities purchase electricity from any qualifying source (EMW, industrial cogeneration, windmill, etc) at the full avoided cost to the utility of generating that same amount of electricity. Full avoided cost in this context includes the fuel, operating and capital costs associated with generation practice the avoided cost is subject to local interpretation and definition. Energy and materials sales have also been used to modi5 the early year cash shortfall. Energy purchasers have provided a higher initial energy purchase price in return for a decrease future energy costs. A typical contract would reduce the future escalation of the price energy sold from the plant. Energy sales the concern of the energy purchaser, a private party who operates the plant and the municipality to the extent that energy price affects the tipping fee.
As noted any EMW system is very sensitive to the local development conditions. Therefore specific examples of economics are difficult develop. Figure 6 is a projection of the tipping fee versus time of the Pinellas County, Florida facility. The facility was financed with a $160 million revenue bond sold in 1980 with varying interest rates that depend upon the purpose of the funds. The funds raised by the bond issue were applied as follows: construction plus escalation, $86 million; site work and transfer station, $11 million; fees, utilities, taxes, contingency, etc $13 million; capitalized interest during construction, $41 million; debt fund reserve, $17 million and bond discount, insurance, etc $5 million. The $109 million construction fund was invested during construction and $13 million to provide the balance of construction fund requirement. The project include funds to amortize the entire $160 million debt against project revenues. The projections are based on the simplified data from the official statement for the bond issue. A number of items have been lumped under energy sales and operating costs and uniform inflation factors (7.8% on energy sales and operating costs, 6.0% on materials sales) have been applied. The inflation factors are based on the official statement. The project was completed ahead of schedule and the start-up period was six months shorter than expected. These changes not reflected in the sketch.
