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waste heat recovered from incinerators and composting. Heat recovery from incinerators has been practiced in Europe and Japan for some time. Recently, heat recovery incinerators of European design have been introduced into the U.S. and Canada. Although heat recovery from incinerators has been practiced for some time, there are still some significant technical problems with these systems such as erosion and corrosion of the boilers and reliable deliverability of the product. The technology of composting is well established. There are several composting techniques, the most successful being the Fairfield-Hardy and the Varro systems. Poor marketability of the fiinshed product has been a factor in a rather unimpressive history of composting in the U.S.A.

There has been a marked increase in the development of new technology for resource recovery from municipal waste during the last few years. Included in this emerging technology are: (1) energy recovery processes, (2) materials recovery processes, (3) pyrolysis processes, and (4) chemical conversion processes.

The emerging energy recovery technology includes fuel recovery processes, steam generation processes, and electrical power generation processes. Energy recovery is applicable only to the organic fraction of wastes, but many of the energy recovery processes also recover some of the inorganics (metals and glass). Two of the promising fuel recovery systems are the Horner-Shifrin and the A. M. Kinney processes. The Horner-Shifrin process involves dry shredding of the refuse and using it as a supplementary fuel in existing power plant furnaces. A. M. Kinney has a design to wet pulp waste organics for use as a supplementary industrial or power plant fuel.

Two new steam generation systems, designed by the American Thermogen Company and Torrax Systems, Inc., involve the recovery of heat from the combustion of refuse in special furnaces. The novel aspect of these systems is the use of high-temperature furnaces which require no preseparation or preparation of the waste, and which melts all of the residue to a lava-like frit.

Another new energy recovery system, called the CPU-400, is designed to burn shredded municipal waste in a high pressure fluid-bed combuster and uses the hot gases to drive a gas turbine-electric generator. This system is presently in the pilot plant development stage.

The materials recovery processes are designed to remove paper, ferrous and nonferrous metals, and glass from the refuse. In most processes all four materials are recovered. Both wet and dry processes have been devised to separate the paper from mixed waste. Techniques to remove the metals both from the mixed waste and from incinerator residues are being developed. Most of the ferrous metal separation techniques are based upon magnetic separation-a well-developed technology. The glass is separated by air classifiers (separation by density) and color sorting using optical devices or by flotation techniques. The materials recovered in these systems are generally of a quality that subsequent refinement or additional upgrading may be necessary to obtain fully marketable products. The most developed materials recovery systems are the Black-Clawson Fibre-claim system, and an incinerator residue recovery system developed by the U.S. Bureau of Mines.

A number of organizations are in the process of developing pyrolysis processes that recover synthetic fuel oil, gas or other potentially valuable materials from municipal wastes. These pyrolysis systems involve the thermal degradation of the waste in a controlled amount of oxygen. Some of the products that have been obtained from municipal waste by pyrolysis systems are oils, gas, tar, acetone, and char. Pyrolysis is an attractive method for waste resource recovery because of the basic flexibility of the technique; changes in operating conditions can be made to vary the nature of the recovered products.

The Garrett Research and Development Company has developed a pyrolysis process that recovers synthetic fuel oil from refuse (glass and ferrous metal are also recovered). The Garrett system appears attractive because of the reported high yield of low sulfur oil and substitutability for low-grade fuel oil. However, it has not yet been determined whether the recovered oil will be readily usable as a substitute for commercial fuel oils. Union Carbide has a high-temperature pyrolysis process from which the combustible off-gases can be cleaned for use as a fuel gas for utility furnaces. The adaptability of the synthetic gas to commercial furnace fuel systems has not been fully determined yet. Monsanto has a pyrolysis system that has been tested to a much more greater extent than any of the other pyrolysis systems. Furthermore, their pyrolysis unit is based upon extensive rotary kiln design experience. Both facets speak well for probable success of the Monsanto pyrolysis system. The primary pyrolysis unit (fluid-bed type) proposed by the Hercules Company is feasible, but unproven; their back-up unit is a well-developed furnace for producing wood charcoal. Battelle Northwest and West Virginia University have also been working on the development of pyrolysis processes for mixed municipal wastes.

There are a variety of chemical conversion processes (anaerobic digestion, acid hydrolysis, wet oxidation, hydrogenation, and photodegradation) which have been conceived for mixed municipal waste, resulting in such products as proteins, methane, glucose sugar, oils, alcohol, yeasts, and other organic chemicals. Since most of these processes utilize only the cellulose portion of the waste, separation and pretreatment of the waste is necessary. Most of these processes are in early stages of development.

Economic Summary: The most obvious finding of our economic analysis is that resource recovery systems are not self-sustaining economic operations under the conditions of the analysis used. They do not recover revenue sufficient to offset total costs; all systems analyzed show a net cost of operation. However, where incineration, remote landfill, or other high-cost waste disposal is necessary, resource recovery offers an economically viable alternative. Most resource recovery systems show lower costs than conventional incineration (without resource recovery); several have net costs (for large capacity plants) low enough to compete with landfill, if the recovered products can be sold at or above the assumed prices.

Under the conditions used in the generalized economic analysis, the process ranking by lowest net cost is: (1) fuel recovery, (2) materials recovery, (3) pyrolysis, (4) composting, (5) steam generation with incinerator residue recovery, (6) steam recovery, (7) incinerator residue recovery, and (8) electrical energy generation. The net opera

tional costs (based on a 1,000 TPD plant) range from about $3.00/ton for fuel recovery systems to about $9.00/ton for electrical energy. generation.

Most of the emerging systems for resource recovery utilize new technology or at least unique combinations of existing industrial technology. Political jurisdictional units are often hesitant to experiment with new or unproven technology since this represents a radical departure from traditional waste management practices and introduces "high risk" of taxpayer funds. This is true even though a system developer may guarantee performance of a specific system. However, in order to introduce technically and economically viable disposal/ resource recovery systems waste management jurisdictions will be required to adopt relatively sophisticated technology and competitive marketing skills.

Most of the resource recovery systems examined are capital intensive, i.e., a large capital investment is required for each system. Therefore, the fixed costs of operation are quite high in relation to total costs. These systems should be operated at or near capacity to minimize unit costs and maximize salable product output. In addition, the systems show economies of scale, so that the larger the system, the more attractive the unit cost of operation.

Perhaps the most critical economic factor is marketability of the output products. All of the resource recovery techniques produce products that must compete with established commodities directly or indirectly in the marketplace. The variables of most importance are unit price (or value), throughput quantity and the percent of input (or output) that is salable. In turn, these variables are dependent upon the quality of the recovered product and its applications or demand in the specific situation in which it occurs.

In summary, waste processing for resource recovery requires sophisticated industrial technology and a large capital investment, and must be operated within competitive industrial market conditions. Nonetheless, resource recovery is a viable alternative to traditional waste disposal practices and should be carefully assessed by any municipality or jurisdictional unit faced with a waste disposal investment decision and/or high-cost waste disposal.

2. John Edgerton, "Transforming Trash in Nashville," The Progressive (February 1974), pp. 25–26:

Americans throw away about 200 million tons of stuff a year— trash, garbage, junk, so-called "disposable solid waste." By the most conservative estimates, it costs more than $20 a ton to collect it and dispose of it in an incinerator or a sanitary land-fill-the latter being a euphemism for the malodorous junk graveyards that gobble up about 60,000 acres of land each year.

Trash, then, is a four-billion-dollar-a-year liability, not to mention. a visual blight and a health hazard. That being so, it is mind-boggling to discover that one American city is about to reduce heating and cooling costs, cut fossil fuel consumption, lessen air pollution, and lower the cost of collecting and disposing of solid waste-and do all of that by converting its refuse from an energy-consuming eyesore to an energy-producing fuel.

Nashville, Tennessee, is not the first city to burn solid waste, nor the first to operate a centralized heating and cooling plant, nor even the first to recover heat from trash incineration. But in April, when the contents of local trash cans are used for the first time to stoke the boilers of the city's new thermal transfer plant, Nashville will have the world's first large-scale plant producing both steam and chilled water from solid waste.

The facility, operated by a nonprofit corporation, will heat and cool more than thirty city, state, and nongovernmental buildings in the downtown area under thirty-year, non-cancelable contracts. The city, which has about 1,400 tons of trash a day to dispose of, has agreed to give the new corporation as much of it as it wants, free of charge, for the next thirty years. Initially, the plant will consume about half of the available solid waste to produce almost 400,000 pounds of steam per hour and about 14,000 tons of air conditioning. Eventually, it will use all of the city's available refuse, and its heating and cooling capacity will be increased accordingly.

There is no new technology involved in the Nashville facility. No fewer than 150 European cities create steam from the burning of solid waste, and while only four U.S. cities (Chicago, Miami, Norfolk, and Harrisburg) do that, almost twenty produce some centralized heating and cooling by using conventional fossil fuels. The Nashville plant simply combines those proven ideas, and the happy result is beginning to look like one of those rare bonanzas that helps everybody. When it is in full operation next June, the plant will:

Replace the individual heating and cooling systems of its customers, at a savings of some twenty million kilowatt hours of electricity the first year;

Reduce the heating and cooling costs of the customers by at least twenty-five per cent;

Reduce the amount of electricity needed in a typical building by as much as fifty per cent;

Save the city government more than $1.25 million a year through reductions in the cost of garbage collection and sanitary land-fill operations;

Virtually eliminate the city's solid waste disposal problem by reducing its volume by ninety-five per cent and leaving a five per cent residue of metal that can be recycled and ash that can be resold for a variety of uses;

Require less water than the heating and cooling plants it replaces;

Bring about a substantial reduction in air pollution.

The thermal transfer plant is located on the site of an old railway terminal beside the Cumberland River in downtown Nashville. A network of underground pipes will distribute steam and chilled water to and from the plant and the buildings it serves. Garbage and trash collection trucks that used to dump their cargo in sanitary land-fills will deliver it instead to strategically located transfer stations, where fully enclosed semi-trailer trucks will pick it up and take it to a refuse pit at the plant site. There, a crane will pick up the waste matter in one-ton bites and feed it into the incinerator-boilers. The process of incineration is such that potential odor problems will be eliminated, and the

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emission of nitrogen oxides, sulphur dioxides, and smoke will be at least seventy-five per cent less than the pre-existing heating and cooling systems created.

The plant will consume virtually every kind of solid waste; only large, bulky items-old refrigerators and the like-will be culled.

Carl Avers, the thirty-five-year-old general manager and chief engineer of the Nashville Thermal Transfer Corporation, says the plant will produce even more economies as it grows: "On the strength of the contracts we signed with our customers, we raised $16.5 million from revenue bonds to finance construction. Our rate scale is based on the amount of money needed to retire the bonds and operate the plant. With free fuel, and with a more efficient and economical heating and cooling system, we can offer our customers a real bargain. And the more customers we add, the greater the savings."

The corporation developing the plant is a private legal entity with no government subsidy. Its board of directors is made up of its major customers-state and local government officials. As a nonprofit corporation, it is exempt from supervision by the Tennessee Public Utilities Commission.

Avers points out that the new plant is replacing more than thirty heating and cooling systems that had no environmental controls with a single system that more than meets the requirements of the Environmental Protection Agency. "By any measure," he says, "we are replacing old methods of operation with new and improved ones. With this one system, we are attacking the energy problem, the waste-disposal problem, the pollution problem, and the inflation problem. When you can conserve energy, save money, get rid of trash, and reduce pollution all at once, that's a real accomplishment."

The Nashville plant eventually will be enlarged to consume all of the city's disposal solid waste, and since it can also be operated on conventional fossil fuels, it could be made even larger. Other cities have sent representatives to Nashville to investigate the facility, and many of them are likely to build their own thermal transfer plants in the future. Presumably, the idea could be applied not just to downtown buildings but to large institutions, shopping centers, industrial parks, and even concentrated housing projects.

Of all the advantages thermal transfer plants afford, perhaps none is more intriguing than the elimination of solid waste. To think of trash and garbage as a valuable resource instead of a stinking nuisance will require some getting used to. The day may even come when you will be able to sell the stuff instead of paying somebody to haul it away, and therein may lie the only potential disadvantage to using it as fuel: If Americans produce 200 million tons of waste now, how much more would we be inspired to create if we could sell it at a profit? 3. "Garbage, the Cinderella Fuel," New York Times, February 24, 1974:

Garbage is today "a vital natural resource."

That comment came not from a speaker at a sanitationmen's convention but from a steel-industry executive addressing a recent beverage packaging conference conducted by the American Management Association.

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