FEATURE
CosVbenefits of solid waste reuse
Helmut W. Schulz Columbia University New York, N. Y . 10027
The economics of recovering energy, glass and metals from municipal solid waste reaffirms that this refuse is actually an “urban ore”
Recent studies indicate that the U.S.generates municipal solid waste at the daily rate of 3.4 Ib of residential and commercial refuse per person, or 130 million tons per year. This concomitant of our way of life represents a present waste and potential source of 12 million tons of metals, 12 million tons of glass, and energy equivalent to 150 million barrels of petroleum per year. Although the nation’s ecological conscience has been aroused by environmentalists, this mass of refuse is currently still disposed of according to the following pollution-prone pattern: 0 open dumps, 69%; 0 sanitary landfill, 22%; 0 incineration, 8%; and 0 resource recovery, 1%.
costly disposal means; many metropolitan centers are rapidly running out of available landfill sites; and, above all, the tripling in price of petroleum and natural gas is providing a market and substantial credits for refusederived fuels. In assessing the technology and economics of resource recovery, the following generalizations may be made: 0 The time i s propitious for municipalities and regional authorities to implement the transition from landfill disposal and incineration to resource recovery with emphasis on energy utilization. 0 Most of the energy recovery processes make the inert components of MSW available in comminuted and segregated form, so that the mechanical retrieval of se-
Figure 1.
Oxygen refuse converter (PUROX)
Off
gas
Refuse feed
These figures suggest that municipal solid waste (MSW) is indeed an ”urban ore.” At present prices for imported crude oil, the energy equivalent alone is worth $1.5 billion. The secondary metals and glass that can now be reclaimed by established procedures could contribute an additional $1 billion. Combined with a present disposal cost of $1 billion, this constitutes a $3.5 billion market, which has stimulated many innovative developments for turning garbage and trash from a pollution problem into a local resource. Until recently, however, most resource recovery technologies, though ecologically intriguing, have not been able to compete economically with sanitary landfill or airpolluting incineration. Now three things have conspired to change this picture: the Clean Air Act has rendered incineration with adequate air pollution controls a very
lected materials also becomes economically viable. 0 The costs associated with refuse preparation (conveying, shredding, air classification) are so substantial that it is necessary to distribute them over a full spectrum of secondary materials and fuel in order to cover the capital and operating costs without public subsidy. 0 Each system must be designed and optimized for a given municipality or waste disposal region with emphasis on markets, transportation, energy users and, above all, possible savings in collection costs, since collection represents 80-90% of the total waste disposal bill. 0 An important value increment is potentially available through the recovery of refined metals from the now generally undifferentiated nonferrous metals fraction (NFM). To implement this, the establishment of a number of geographically dispersed Metals Recovery Centers is proVolume 9,Number 5, May 1975
423
TABLE 1
Economics of energy recovery from municipal solid waste Co-combustion vs pyrolysis: 1000 tpd System
Co-Combustion of RDF
Proponent
Horner-Shifrin/ R. Parsons Development status 650 tpd demonstration plant Materials recovered Iron Full scrap spectrumo
Slagging Pyrolysis wlth On Union Carbide (PUROX)
200 tpd pilot plant
Full
FusQd k i t
Wtrum
1,200
3,700
Investment $/daily
posed, which would accept, refine, and market the crude NFM fractions reclaimed by local resource recovery plants. 0 The progressive increase in fuel prices, the growing scarcity of virgin metals (which consume energy in mining, smelting, and refining), and the pace of technological advance in reclamation and recycling combine to ensure a continuing transition from landfill disposal to resource recovery. Municipal decision-makers are confronted by a wealth of advanced technology options for the recovery of values from MSW. Processes have been developed and demonstrated for converting the organic components into yeast protein for animal feed, into ethyl alcohol by hydrolysis and fermentation, and into methane by anaerobic digestion. The technologies to recover and recycle ferrous scrap, aluminum, and glass are known. There are rival processes for energy recovery based on incineration with production of steam, co-combustion of refuse-derived fuel in coal-burning utility boilers, and pyrolysis of organic components to char, oil, or fuel gas. However, the environmental impact and the economics of technically feasible alternatives can differ widely. Energy recovery A comprehensive catalog of resource recovery processes was prepared by the Midwest Research Institute. More recently the National Center for Resource Recovery published an analysis of the economics of resource reclamation. A study conducted at Columbia University under sponsorship of the National Science Foundation has made a critical assessment of the most promising technologies in terms of such economic determinants as capital investment, operating costs, and realistic by-product credits. Based on the Columbia study, the two most cost-effective, environmentally acceptable alternatives are the high temperature, oxygen-fed pyrolysis process developed by Union Carbide (the PUROX system), and the co-combustion of refuse-derived fuel (RDF) in coal-burning utility boilers, now being demonstrated by the Union Electric Co. in St. Louis, Mo. (the Homer-Shifrin approach). Flow diagrams for these processes are presented in Figures 1 and 2, respectively. The PUROX pyrolysis process, which uses oxygen in place of air, possesses a number of advantages over other combustion and pyrolysis processes. First, the process is compatible with any degree of front-end separation that may be imposed for the recovery of secondary materials, but it accepts all rejected inorganic components and reduces them to a fused metallovitreous frit that has utility as a road-building aggregate. Second, it produces no stack emissions or other noxious effluents that may cause air, water, or land pollution: the outputs are saleable products. Third, it produces energy in the form of a clean fuel gas that is scrubbed free of particulates and acidic air pollutants. This gas can be burned in existing utility boilers without contributing to corrosion of the boiler tubes or undesirable stack emissions. It can be 424
Environmental Science & Technology
ton Front end separation Energy recovery Self-support power Total
9,500
11,000
0 0
0 0
18,000 4,500
18,000 4,500
9,500
11,000
23,700
26,200
Capital costb $/ton
2.86
3.30
7.09
8.30
Operating costs $/ton Direct tabor Maintenance Powerc Other utilities Administration Residue disposal Total
2.79 .78 2.20 .07 .26 .90 7.00
3.89 .91 3.00 .21 .30 .36 8.67
3.34 1.64
4.65 1.92
0
0
5.51
7.27
9.86
11.97
12.60
15.57
5.6
5.4
10.08
9.72
Amortized operating cost
Od
Od
.21 .32
.34 .35
Energy credits Units for export (MM Btu/ton) RDF at $0.80/ MMBtw $/ton Purox gas at $1.80/MM Btu6 $/ton
7.5
7.5
6.00
6.00
Materials credits $/ton
1.87
4.76
.40
4.88
Total credits $/ton
7.87
10.76
10.48
14.60
Net disposal cost $/ton
1.99
1.21
2.12
0.97
Assumptions Magnetic Metals Aluminum, Glassand Nonferrous Metals(NFM) b Fifteen year amohzation 7% municipal interest rate = A t 4 cents/kWh d Self-generated: costsare included e Reflects utility's willingness fo pay(Con Ed 0f.N.Y.) Lower va!ue,for RDF partly due to utility's additional investment in boiler modification for burning RDF
upgraded to pipeline quality or utilized as a chemical synthesis gas, but these alternatives require added processing. It may also be converted to electric energy by onsite operation of a gas turbine generator if it is not practical to locate the pyrolysis unit reasonably close to a fuel consumer. Fourth, at current price levels for fuel oil, the energy credit generated substantially offsets the amortized operating cost, including operation and amortization of the oxygen plant (Table 1 ) . The PUROX system is currently being tested in a 200 tpd pilot plant in Charleston, W.Va., and cannot be regarded as commercially available until protracted continuous operation at full rating has been demonstrated. The RDF co-combustion process affords the following advantages: 0 It utilizes an existing facility for combustion and heat recovery that requires little modification if the unit was designed and equipped for the burning of powdered coal. For this reason it has the lowest capital requirement of all leading alternatives. 0 In order to implement this approach it becomes necessary to build and operate a front-end separation fa-
TABLE 2
Front end separation processes incremental capital and operating costs basis: 1000 tpd plant Unit operation
Investmenta $/daily ton
Primary shredding Air classification Secondary shredding Magnetic metal separation Rising current/heavy media separation Roll crushing & electrostatic separation Color sorting
Capital costb %/ton MSW'
Amortized Operating operating cost cost, $/ton $/ton MSW' MSW'
1250 950 625 75
0.38 0.28 0.19 0.02
2.52 1.36 1.60 0.39
2.90 1.64 1.79 0.41
260
0.08
0.41
0.49
280
0.08
0.44
0.52
425
0.13
0.40
0.53
Does Vot include land or buildings; b Assumes 15 year amortization, 7% municipal interest rates. a
cility based on shredding and air classification to recover the light, combustible organic fraction reasonably free of inerts. This imposed separation step serves as an incentive to the further processing of the noncombustible, inorganic fraction for the recovery of such values as magnetic metals, aluminum, glass cullet, and the other nonferrous metals. Since the cost of shredding and air classification may be charged against fuel recovery, the secondary materials need not bear this processing charge, and their reclamation is thus more easily justified. 0 As compared with coal, prepared refuse generally contains less w l f u r , thus reducing the concentration of sulfur oxides in the stack gases. However, refuse-derived fuel may require higher levels of excess air, which tends to increase the concentration of nitrogen oxides and particulates in the stack emissions. 0 Although the fuel credits are necessarily limited by the price of coal, the energy and by-product credits generated by this process are sufficient to reduce the net amortized operating cost to a level more favorable than current landfill disposal costs (Table 1). The economics of these two processes are compared in Table 1. The RDF process shows the investment for the front-end recovery operation on two bases. The fuel credit represents the maximum that Consolidated Edison Co. of New York would be willing to pay when assuming the capital expenditures associated with the adaptation of its oil-burning boilers for the co-combustion of this solid fuel. The cost projections for the PUROX system are made on two bases: one includes shredding and the recovery of fused frit only; the other provides a comprehensive front-end recovery system. The energy credits are those Consolidated Edison is currently willing to pay for clean fuel gas of PUROX quality. The materials credits were derived as shown in Table 3.
Materials recovery The recovery of materials from municipal solid waste is no recent innovation. In 1903 an entrepreneur paid the City of New York an annual fee of $47,000 for the privilege of hand-picking the refuse stream. Today, the secondary materials industry reclaims $8 billion of scrap per year. A typical front-end separation scheme for the recovery of magnetic metals, aluminum, glass, and an undifferentiated NFM fraction is that proposed by the National Center for Resource Recovery. The Urban Technology Center of Columbia University made a cost analysis of the incremental unit operations comprising this type of front-end recovery process based on representative equipment cost quotations, energy re-
quirements, and labor rates prevailing in the fourth quarter of 1974. These unit costs per ton of MSW are presented in Table 2. The credits available from the marketing of reclaimed materials will vary with the state of the economy, the presence of deleterious contaminants, the concentration of a given component in the MSW stream, and proximity to markets (as reflected in the difference between f.0.b. and delivered prices). I t is noteworthy that the price for No. 2 bundle steel scrap fluctuated between $15 and $55 per ton in the span of one year. Based on MSW compositions typical of the New YorK City area and the scrap prices prevailing in the fourth quarter of 1974, the credits available for materials recovered by conventional frontend separation are shown in Table 3. Thus, the net revenue from the sale of recovered materials (other than RDF) amounts to $4.76 per ton of MSW. The remainder of the front-end processing cost (approximately $3.50 per ton of MSW) represents the production cost of the quantity of RDF derived from one ton of MSW. Although front-end separation processes of this type have been proposed for the City of New Orleans and for Bridgeport, Conn., there is, as yet, no large-scale experience to document the economics of actual operation, or the quality of the reclaimed materials. At this writing, it is doubtful that any municipality (with the possible exception of New York City) can economically justify a comprehensive front-end materials recovery operation. This is due in part to the wide fluctuation in scrap prices and, in part, to the fact that the relatively small scale of local resource recovery operations does not justify the process refinements that are necessary to produce specification-grade recyclable materials. An alternative is proposed that would place emphasis on the recovery of premium products, while simplifying the materials processing performed at municipal centers Volume 9, Number 5, May 1975
425
A. Local resource recovery plant
B. Metals recovery center e derived I (RDD
I
inorganics
+
I
in extracting energy from the MSW stream. The alternative approach calls for the establishment of a relatively small number of geographically dispersed Metals Recovery Centers (MRC) that would process two crude metal fractions received from a large number of local resource recovery plants. One fraction consists of the magnetically separated ferrous scrap, while the other comprises all nonferrous metals, including aluminum. The local plants and the regional Metals Recovery Centers would each employ new technologies that have thus far been demonstrated only on a pilot plant scale. The local resource recovery plant would place emphasis on the production of RDF and the two crude metal fractions described above. Crushed glass would not be refined, but either sold for use as a road building aggregate or consigned to a slagging pyrolysis unit for conversion to fused frit. The modified front-end recovery operation performed locally would (see Figure 3a): 0 Handpick and screen to remove appliances, rubber tires, and containers of flammables. These would be sheared and returned to the main stream. Such pretreatment would reduce the energy and maintenance requirements in the shredding operation. 0 Shred in flail or hammer mills. 0 Air classify to recover RDF. 0 Process the underflow from the air classifier for the
426
Environmental Science & Technology
Crude
magnetic metals
-
Aluminum
magnetic separation of ferrous metals. 0 Process the remaining inerts in a linear motor separator that would selectively remove all nonferrous metals from the substrate of crushed glass and minerals. Eddy current separators are being developed by Garrett R&D and Combustion Power; a novel Falling Stream Linear Motor separator (or NFM magnet) will be evaluated at Columbia University. 0 Recover heavy organics from the remaining inerts in a rising current separator. The organic fraction would be dried and incorporated in the RDF product. The residual glass fraction could be further refined by froth flotation, although this procedure is currently not economically justified except for locations close to a glass plant. Similarly, the extensive processing associated with the color sorting of glass cullet is, to date, an unwarranted complication. The magnetic metals fraction and the NFM fraction recover would then be shipped to the Metals Recovery Center for further separation, refining and marketing. Metals recovery centers
The proposed Metals Recovery Centers would afford a strong incentive for the practice of materials recovery by local MSW processing plants. The MRC would provide an assured outlet for the crude magnetic metals and nonferrous metals, which have a potential value of $300 million
and $700 million, respectively. They would relieve the local plants of the difficult tasks of producing specification-grade materials. and of finding markets for relatively small and variable product streams. The MRC plants can afford to use the advanced technology and processing sophistication that is needed to recover refined metal fractions of reproducible purity. i t is iikeiy that the establishment of such Metals Recovery Centers will require the initiative of the U.S. EPA, or of a large metals processor, such as American Metals Climax (AMAX). The Metals Recovery Centers would (see Figure 3b) s process the magnetic metals fraction to spec tion-grade ferrous scrap and market this product: s recover aluminum from the crude nonferrous metals fraction by processing this small but valuable fraction through a ferrofluid levitation cell operated with an apparent specific gravity of 4.0: s recover zinc and zinc alloys from the "sinks" of the previous step by a second pass through a ferrofluid levitation cell operated with an apparent specific gravity of 7.2: s recover electrolytic copper from the "sinks" of the previous step by casting them into anodes for electrolytic refining: s recover lead, tin, chromium and nickel salts from the electrolyte of the copper refining operation, if warranted by the production scale. (It is suggested that the MRC plants be designed to process a minimum of 200 tonsof NFM per day.) Ferrofluid levitation is an intriguing concept now being developed by Avco Corporation that makes it possible to "dial" the density of a.suspension medium (finely ground magnetite slurried in kerosene or water) by changing the strength of an imposed magnetic field. This technique is applied to the separate aluminum and zinc and zinc alloys from the remainder of the NFM fraction by two successive passes through a pair of continuous ferrofluid levitation cells. The amortized processing cost is estimated to be 3-5@/ib of metals processed, depending on the number of shifts per day. The "sinks" from the ferrofluid separator are then melted and cast into anodes with appropriate fume control. These are anodically dissolved in an electrolytic Cell by using a copper sulfate-sulfuric acid electrolyte. Copper is simultaneously plated out on copper sheet cathodes. Such ingredients of the anode alloy as iron, zinc, nickel, chromium, and tin remain in solution, while lead sulfate is precipitated with the anode slime. The cell is operated continuously by bleeding and processing the electrolyte to maintain the copper ion concentration at a level Of 30 g/i to ensure a bright deposit. Copper hydroxide recovered from the spent electrolyte is recycled to the Cell with makeup sulfuric acid. The processing of the electrolyte for the recovery Of copper, lead, zinc, tin, chromium and nickel by differential precipitation requires a sequence of eight steps. A description of the operating conditions, yields, and purities pertaining to the recovery of each component is beyond the scope of this article. Copper is thus recovered as relatively pure metal, while the other metal compounds are marketed as concentrates to established refiners of these metals. A rough economic appraisal for the electrolytic recovery scheme developed in the Electrochemical Engineering Laboratory of Columbia University projects an amortized operating cost of 206/lb of cathode copper when processing 200 tpd of the NFM fraction. This estimate provides for the nonpolluting disposal of iron salts and other unrecovered ingredients of the processed electrolyte. No credits have been assigned for the separated tin, lead, nickel, and chromium compounds. It is assumed that these metal concentrates will be further refined only if the credits more than offset the incremental processing costs.
Private enterprise The brightening economics for resource recovery offer the opportunity to municipalities and regional authorities to shift the burden of solid waste disposal to private industry at considerable savings to the taxpayer. Given the now available energy and materials credits, private firms have an incentive to build and operate resource recovery plants against the payment of a moderate tipping fee. A relatively long-term contract (say, 15 years) would be required, and should be so drawn that the tipping fee varies in inverse ratio with with the price of crude oil, and directly with the cost of labor. Provision can ais0 be made that the municipality or county share in the profits from the sale of secondary materials at a fixed ratio. One disadvantage to private ownership is the higher cost of private as compared with municipal borrowing. An analysis of this factor by Midwest Research Institute showed that the amortized operating cost is increased by approximately $2.00-$3.00/ton if deprived of the benefit of municipal interest rates. Since this dfferential must ultimately be reflected in the disposal cost to the community, the preferred approach is to couple municipal funding of the resource recovery facility with operation and management by responsible private enterpreneurs. I n most states, the municipal and county authorities are constrained by law from negotiating contracts that extend beyond the term of the incumbent administration. Private enterprise cannot assume the risk of investing its own capital unless afforded the protection of a sufficiently long contract to recover its investment and earn a profit. The resolution of this dilemma will require special enabling legislation by the states. Connecticut has recently pioneered in this field through the establishment of its Resources Recovery Authority. which is expressly empowered to negotiate long-term contracts for the entrepreneurial management of publicly owned resource recovery facilities. Other states are currently examining the Connecticut model in order to free city and county public officials from an increasingly burdensome pollution problem. and to exDedite the commitment of caDital that is needed for the transition from landfill disposa'l and incineration to resource recovery and recycling. Additional reading Midwest Research Institute. Resource Recovery: The State of Technology. February 1973, and Resource Recovery Processes tor Mixed Municipal Solid Waste. Part il-Catalogue of Processes. December 18,1972. Abert, J. G., Alter, H. and Bernheisel. J. F. "The Economics of Resource Recovery from Municipal Solid Waste." Science 183, 1052-1058 (1974). Schulz, H. W.. et al. A series of interim reports prepared under the National Science Foundation project entitled "A Pollution-. Free System for the Economic Utilization of Municipal Solid Waste lor the City of New York:' School of Engineering & Applied Science, Columbia University, New York. Marks, D. H.. et al. "Evaluation of Policy-Related Research in the Field of Municipal Solid Waste Management." September 1 s174. School of Engineering, Massachusetts Institute of Techn1)logy. Cambridge, Mass. inunicipal Solid Waste: its Volume, Composition and Value." "I National Center for Resource Recovery Bulletin. VoI. I II. No. 2. Helmut W. Schulz is adjunct professor of Chemical Engineering and director of the Urban Technology Center in the School of Engineering & Applied Science, Columbia University. Dr. Schulz is directing a National Science Foundation RANN project concerned with the application of advanced technology to energy and materials recovery from solid waste. Coordinated by LRE Volume 9. Number 5, May 1975
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