Research Directions in Solid Waste Management Edward J. Farkas Department of Man-Environment Studies, University of Waterloo, Waterloo, Ontario, Canada
The major alternatives for dealing with the combustible portion of residential and commercial solid waste are materials recovery and energy recovery. Energy recovery appears optimal because it can lead to reduced use of fossil fuel, a raw material more valuable than waste paper and similar materials found in solid waste. There are several choices for energy recovery from solid waste-incineration with heat recovery, pyrolysis, digestion, hydrogasification. Incineration is more energy-efficient and releases fossil fuel for higher uses. Research in the field of recovery of energy from residential and commercial solid waste should be directed at improvement of the incineration process rather than at less efficient processes such as pyrolysis.
Introduction About a dozen years ago, Professor T. K. Sherwood (1964) commented on the importance of study, placing in context, correlation, and re-presentation of already-published results in a form that encourages their practical use. He saw effort of this type as even more important to the advancement of engineering than the effort which resulted in the original publication. In the present paper, the object is to report some of the results of a study, the overall goals of which are in line with Professor Sherwood’s prescription. These goals include: (1) collection and correlation of information on technology currently being used in North America and in Europe for solid waste management, and drawing conclusions as to appropriate choices among technological alternatives; (2) presentation of this information to appropriate municipal and other authorities to assist them in making rational solid waste management decisions; (3) drawing of conclusions as to appropriate directions for further research in solid waste management, and presentation of these conclusions to the research fraternity. In the solid waste management field, as well as in a number of other pollution control and energy problem areas, there has been a marked proliferation of alternative solutions. In a number of instances, actions which will require substantial lead times before results are seen are being delayed by protracted consideration of alternatives of roughly similar viability. An example from the solid waste management field of this phenomenon, which was termed “disruptive”, has been reported (Pyle, 1975). There are many recent publications with goals similar to items 1and 2 above (Franklin et al., 1973; Huang and Dalton, 1975; MacDonald, 1974; Schwieger, 1975; International Research and Technology Corp., 1973). In the present study, the additional objective was to arrive a t a substantial narrowing of the field of alternatives. A more rapid approach toward solution of certain solid waste management problems would follow from a more focussed research and decision-making process. The Basic Alternatives “Solid waste” in this study means primarily material put out a t the curb by households and commercial establishments. The most frequent means of disposal of this material in North America and Europe is landfilling. With land no longer as readily available, and with increased awareness of the undesirable ecological and environmental implications of landfilling, there has in recent years been widespread consideration of alternatives to landfill. The most desirable alternative is source reduction, i.e., reduction in the amount of waste generated. There are re40
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quirements for research here, to find ways of making products more durable, and to find ways of packaging that are less wasteful, while a t the same time providing employment opportunities. On a long-term basis, rising energy and material costs will force source reduction, even without overt government action. In the meantime, how is the solid waste currently being generated to be handled, if not by landfill? Solid waste on a moisture-free basis can be as much as 50% paper by weight. So the first set of alternatives from which a choice must be made is: (A) reclamation of the paper fiber for re-use in manufacturing of paper products; or (B) reclamation of energy from the combustible portion of the solid waste. If we select alternative (B),then presumably somewhere in the industrial society less coal, oil, or natural gas will be burned. Since coal, oil, and natural gas are more valuable industrial raw materials than is waste paper, I believe that alternative (B) is clearly better. There has been some excellent research and development work done on processes under alternative (A), e.g., the Black-Clawson process (Franklin et al., 1973). However, the oil and gas supply situation will, I believe, increasingly force us toward alternative (B). This conclusion certainly applies to mixed solid waste, the mixture produced by a typical household where source separation is not practiced. Even where, for example, bundles of newspaper are set out separately, there are indications that it is possible to over-emphasize alternative (A). In a t least one study (Fooks and Langford, 1974,1975) it was found that more energy was needed to produce paper using recycled material than to produce paper from new material. If so, we are burning irreplaceable coal, oil, and natural gas to preserve trees, which are to some extent replaceable. Energy Recovery Alternatives Once it is decided to follow alternative (B),Le., to attempt to reclaim energy rather than materials from the combustible fraction of solid waste, then a decision must be made between two further alternatives: (a) incineration (burning) with recovery and utilization of heat, and (b) pyrolytic, chemical, or biological treatment to produce synthetic gaseous or liquid fuels. Our society uses great quantities of steam to drive turbines for the production of electricity, for process purposes in industry, and for district heating schemes for university campuses, hospital complexes, and portions of cities. Alternative (a) is readily adapted for steam production and therefore appears as a natural choice. Proponents of alternative (b) have pointed to the difficulty of transporting or storing energy in the form of steam. There is also of course the seasonal aspect of use of steam for district heating purposes.
In my opinion a very major consideration in deciding among the energy reclamation alternatives is energy efficiency. Regardless of which alternative is put into practice, the initial state is the solid waste and the final state is carbon dioxide and water. These initial and final states define the maximum amount of energy that can be reclaimed. Efficient incineration as in alternative (a)can produce amounts of energy very close to the theoretical maximum. For example, Sheng and Alter (1975) calculated that separation of metals and preparation of the combustible portion of the solid waste for incineration would require about 3% of the energy content of the waste. On the other hand, Kispert e t al. (1975) studied biological production of methane from waste and concluded that “operating energy requirements of the process consume. . .37.5% of the gas produced”. Similarly, it has been estimated that the Union Carbide Purox pyrolysis process would require for its own operation over 30% of the energy value of the as-received solid waste (Fisher et al., 1975; Kuester and Lutes, 1976).Various other authors have also commented on the energy inefficiency of pyrolysis (Dair and Schwegler, 1974; Smith, 1975). In addition, gas, and to some extent oil, produced in alternative (b) may have relatively low values and may be difficult to market (Maugh, 1972). The gas may be so impure as to make it economically unattractive to transport without expensive and energy-consuming upgrading. Symptomatic of this problem is that the Monsanto “Landgard” pyrolysis plant installed a t Baltimore will sell its energy in the form of steam, produced by burning its own pyrolysis gases. At the same time initial plans called for the plant to consume “2.2 million gallons of fuel oil annually to maintain the required 1800 O F temperature in the kiln” (Linaweaver and Crooks, 1974). Parenthetically, it may be noted that Heer and Hagerty (1974) take the position that the Monsanto Landgard system and another system, the Andco-Torrax system, are not in fact pyrolysis systems, although the latter description is used by the proponents of these systems. Heer and Hagerty view these systems as “in fact more closely related to high-temperature incineration”. The major benefit conferred by these systems may be a reduced volume of flue gas to be sent to air pollution control equipment. This saving must be balanced against the increased complexity of the process itself. Proponents of these high-temperature incineration systems also point to the absence of moving parts in the form of grates (Davidson, 1975). Again this saving must be balanced against other complexities in the process. Furthermore, the moving grate, exemplified by the Martin and von Roll systems, has reached a very satisfactory level of development, as evidenced by repeated installation in Europe (e.g., Rasch, 1975) and increasing use in North America. In summary, in the case of residential and commercial solid waste, energy efficiency considerations in my opinion point very clearly to alternative (a) as most desirable. Again some very excellent research and development has been done on processes coming under alternative (b). An example is carbon monoxide treating (Chem. Eng., 1971; Appell e t al., 1975). These processes may be better for other types of waste, such as animal wastes in agriculture. But in the case of residential and commercial solid waste, these alternative (b) processes may well represent a scattering and inefficient use of research resources. Although a number of variations of alternative (a) are widely practiced and represent proven, workable technology, there are still areas that could benefit from research. I recommend these areas to those who wish to carry out research on reclamation of energy from residential and commercial solid waste. In the remainder of this paper I will provide details of some of the various alternatives in an attempt to provide support for the conclusions presented.
Discussion of E n e r g y Alternatives Some of the enthusiasm for pyrolysis during the early 1970’s was based on favorable estimates of costs, as compared to estimates for incineration. When large-scale pyrolysis units are in routine operation, more realistic cost data will be available. Without prejudging what these costs will be, it is safe to say that the apparently inevitable continuing increases in the cost of energy will work to the disadvantage of any process that is less energy efficient than ordinary incineration with heat recovery. In particular, processes requiring routine use of purchased supplemental fuel will be a t a disadvantage. The possibility of producing a storable and transportable synthetic fuel is attractive, as compared to production of steam. But steam produced by burning solid waste can be used in electricity generation, thus releasing for higher uses some of the oil and natural gas currently used by base-load electricity generating stations. The net effect of this sequence is the “production” of “real” oil and gas from solid waste. However, this sequence is more efficient than attempting to produce synthetic oil and gas directly from solid waste by pyrolysis or similar processes. In addition, the possibilities of storage problems and in particular of polymerization during storage of synthetic oil have been raised (Chem. Wceh, 1974). Concerning the seasonal demand problem, the economic situation of a given facility is of course greatly improved if there are industrial customers as well as space heating customers. Also, steam can be used in the summer in absorption air conditioning. But, in general, fortunately or unfortunately, energy from solid waste will never provide more than a few percent of the total amount of energy that we use. Therefore energy-from-waste schemes do not have to solve all the load factor and distribution problems that are met in the overall energy system. Another very constructive year-round use that can be made of steam is in drying of digested sludge from sewage treatment plants. In general, digested sewage sludge poses an increasingly serious disposal problem, and steam produced from solid waste could displace the fossil fuel currently being used to deal with sludge. There is ample opportunity for research on methods of integrating sewage sludge and solid waste handling, including burning the dried sludge along with the solid waste. T h e Incineration Alternatives The opinion presented here that the incineration or “burning” alternative is better than alternatives which produce synthetic oil or gas will be strongly debated, undoubtedly. But, if this opinion can be accepted for the moment or a t least if it can be agreed that incineration is a n attractive alternative if not the attractive alternative, then it will be of interest to look a t the various choices open within the alternative. Here the choices are not clearcut and there is ample room for both engineering research and economic analysis. An important question is whether the solid waste should be burned alone or in combination with fossil fuels. If the objective is to provide steam for district heating, there of course would be provision for shouldering of the full load by fossil fuel in emergencies in either case. The real question concerns day to day, routine operation. There are large-scale operations which burn only solid waste and there are large scale operations which have found it advantageous to use solid waste as a supplemental fuel to provide about 10% of the heating value of the fuel. As will be shown below, there are also systems which combine the steam produced by two boiler plants, one consuming solid waste only and one consuming fossil fuel. In some smaller systems, it has not proven workable Ind. Eng. Chem., Fundam., Vol. 16, No. 1, 1977
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to rely on solid waste alone due to variation in supply and properties of the waste. Some of these systems have actually been closed (Solid Wastes Mgt., 1973 b,c). Another question is whether or not the solid waste should be shredded before burning. Mass firing, in which the solid waste is burned in an “as received” condition, is a widely practiced, apparently workable technology. Its proponents state that shredding creates fire and explosion hazards as well as air pollution problems. They point to the substantial energy and maintenance requirements of the shredding operation. Proponents of shredding the solid waste before burning point out that this step facilitates efficient recovery of metals or other materials, and also leads to more efficient burning. There are fewer areas in the furnace where reducing atmospheres are created. Reducing atmospheres are a major cause of boiler tube corrosion. Shredded solid waste can be suspension fired, carrying with it all the advantages of control of feed rate and combustion conditions found in suspension firing of powdered coal. Another area requiring study is the best use of the heat produced in the burning step. If district heating is envisaged, is it best to produce steam or to produce hot water? There is at least one large-scale application using hot water distribution, and hot water has been found to be more attractive than steam distribution in a t least one study (Turton et al., 1974). Hot water can be economically transported over larger distances. Steam, on the other hand, can be transported at higher temperatures, thus reducing the size of the equipment needed to transport and utilize the energy. But what about production of electricity? Much of the evolution of the energy production system has been in the direction of larger and larger scale. But higher energy costs and better equipment may again make it attractive to generate electricity in smaller plants, using solid waste as the fuel. Again, all these aspects require study and research. An example is the Combustion Power Company CPU-400 scheme. Solid waste is burned in a fluidized bed. The hot combustion gases are subjected to particulate removal and then are sent directly to turbines for electricity generation. The concept is hampered by the extremely rigorous cleaning of high-temperature gas required to avoid damage to the turbine. A solution may be use of recently announced (Chem. Eng., 1976) ceramic topping cycle equipment, which can accept hot gases containing some particulate matter. Existing Incineration Operations Before presenting in additional detail some of the areas where research is needed in incineration technology, a few randomly chosen examples of applications of this technology will be described. The object is to provide support for the conclusion that this technology is workable and practical and therefore more worthy of attention from researchers and municipal authorities alike than are other technologies. The Nashville Thermal Transfer Corporation has been set up to accept solid waste from Nashville, Tenn., and surrounding areas. The solid waste is incinerated in an as-received condition. Steam is produced and sold to 30 downtown government and commercial buildings for heating. Chilled water is produced and sold to the same customers for cooling purposes. The plant was started up in February, 1974. There were initial difficulties with inadequate air pollution control equipment and boiler tube wastage due to periodic creation of reducing atmospheres. T o solve these problems, electrostatic precipitators are being installed and air supply arrangements are being improved (Resource Recovery, Energy Reu., 1976; Buckman, 1976). In England, the Corporation of the City of Nottingham is developing a large scheme which by 1980 will have a heating load equivalent to 1 7 000 homes. The two plants used to 42
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produce steam were started up in 1973. One plant burns locally mined coal and the other burns as-received’solid waste. Steam from both plants is used to produce hot water by direct contact. The hot water is piped to shopping centers, public buildings, educational institutions, and homes. Part of the steam is used to drive equipment which produces the electricity used by the system. Exhaust steam is then sold to industries (Energy Dig.,1973).The rate at which solid waste can be accepted has been somewhat reduced by periodic downtime for removal of a hard encrustation on the lower few inches of the heat exchanger tubes (Haslam, 1976). A large-scale demonstration project underway since April 1972 in St. Louis, Mo., has taken a different approach. Solid waste is shredded to a -1 in. size. Various separation steps are then applied to remove ferrous metals and other noncombustibles. The light combustible fraction is then burned in suspension with pulverized coal, in existing boilers operated by Union Electric Co. The solid waste displaces between 10 and 20% of the coal formerly required (Sutterfield, 1975) and thus there is a saving in environmental and energy costs associated with mining and transportation of coal. The success of this project has led to the creation of similar projects in other cities, e.g., Ames, Iowa (Chantland, 1974). Also, Union Electric Co. has decided to undertake this type of operation on a permanent and much larger scale (Sutterfield, 1975). This approach is attractive because of the ability to use existing boilers, thus greatly reducing capital costs (Fernandes and Sherk, 1974). Oil-fired boilers which formerly burned coal, and which still have ash handling facilities in place, are also amenable to this type of operation. Another benefit is that consumption of low-sulfur coal and oil can be somewhat reduced. Solid waste is low in sulfur, and many plants are regulated by pollution control agencies on the basis of the average sulfur content of the fuels used. Expected to begin operation soon is a facility a t Saugus, Mass., which will burn solid waste primarily in an as-received condition. The plant will use von Roll grates and steam will be sold to nearby industry. An interesting aspect of this project is that it will be owned and operated by private enterprise (Enuiron. Sci. Technol., 1974; Metcalf and Eddy, 1972). There are numerous installations in continental Europe which recover energy from solid waste, some which have been in operation for a substantial period of time. Frankfurt and Amsterdam generate respectively 7 and 6% of their electrical energy from solid waste. There are energy recovery plants in Vienna, Geneva, and Gothenburg (Sweden) (Enuiron. Sei. Technol., 1974). The Issy-les-Moulineaux plant in Paris has been in operation for over ten years. Steam is produced and sent into the municipal distribution system. Electricity is also produced and the surplus above the needs of the plant is sold to the national electricity system. The proportions of steam and electricity produced are varied according to the demand. The Munich North facility has been in operation for nearly ten years. The first phase of the project involved two boilers, one operating on solid waste and one on coal. The second phase of the project is a single boiler that burns 20% solid waste and 80% coal. The major product is electricity. A plant in Zurich began operation in 1969. There are two boilers. The solid waste is burned in an as-received condition and the main product is again electricity (Franklin et al., 1973). What is probably the earliest plant of this type was built in 1954 in Berne, Switzerland. This plant is still in continuous operation and since then over 50 incineration with heat recovery plants have been built in various parts of the world (MacAdam, 1975). Finally, a large installation in Rotterdam was completed in 1973. This facility, the largest in continental Europe, in-
tegrates the disposal of residential and commercial solid waste with various industrial wastes including sludges and oils. Products will be steam, electricity, and distilled water (Solid Wastes Mgt., 1973a).
Research Needs The experience gained with the numerous plants built over the past twenty-odd years has led to solutions of many problems (MacAdam, 1975). There are still further improvements that can be made, and research is needed to achieve these improvements. A general concern remains corrosion of and other types of damage to the waterwall membranes and other surfaces exposed to the hot flue gases. A major contribution here would be reduction of use of packaging materials which contain halogens (Mencher, 1974). Polyvinyl chloride plastic, for example, can be as high as 50% chlorine. A reduction in use of this type of material could be achieved through development of synthetic materials which do not contain halogens and through development of new types of packaging systems based entirely on paper. There is also need for additional research on mechanism of attack on furnace walls by halogens (Schwieger, 1975). With better information of this type available, more effective preventive measures could be instituted. Corrosion resulting from the presence of reducing conditions could also he profitably studied. Reducing conditions can occur due to inadequate mixing even when, overall, adequate excess air is available. Another problem requiring constant attention is deposition of solids on heat transfer surfaces. Current practice involves controlling operating conditions so that ash and slag have solidified before they reach furnace walls. Also, mechanical rappers are increasingly being used (Enuiron. Sci. Technol., 1974). A general problem still requiring research attention is removal of small solid particles from gas streams (Azarniouch et al., 1975). This problem is met in incineration of solid wastes with heat recovery and of course in many other applications also. Several large solid waste disposal facilities going into operation recently in North America have had initial problems with excessive release of particulate material into the atmosphere and have had to install better equipment, usually electrostatic precipitators. There are research opportunities here in characterizing the size distribution and other properties of particulates produced by thermal processing of solid waste, and of course in developing better collection equipment. Summary and Conclusion The best solid waste management alternative on the basis of energy efficiency is burning with heat recovery. The waste material can be burned in an as-received condition or after shredding and removal of noncombustibles. In the latter case the waste material can he burned in combination with coal or oil. Any of these burning options allows release of valuable fossil fuels for higher uses, thus in effect "producing" fossil fuels from solid waste.
In an era where the most efficient use of energy is mandatory, research efforts on solid waste management should be concentrated on those techniques which are energy efficient for each type of waste. For household and commercial wastes, the optimal technique appears to be some variation of burning with heat recovery, rather than what might be termed the more sophisticated techniques of pyrolysis, hydrogasification, etc. However, for agricultural wastes and specifically for manure, these more sophisticated techniques may he optimal.
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Received for revietc J u n e 28,1976 Accepted October 11,1976
Financial support was received from the Office of t h e President, University of Waterloo.
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