FEATURE
Fuels from recvclincl systems Three systems, operating at sufficient scale, produce fuels that may be alternatives to oil and gas David A. Tillman Materials Associates, Inc. Washington, D. C. 20037
Recycling to produce fuels, usable either in the generation of electricity or as process heat, has gained considerable attention. The general public, policy makers, and energy consumers consider this energy form as an alternative to oil and gas. Engjneers and scientists, confronted with this attitude, are charged with designing systems to produce and use such fuels. Although numerous systems have been designed, and many are under construction, only three have been built and operated at a sufficient scale (150 tons per day or better) over a sufficient period of time to permit careful analysis. These three recycling systems are: 0 Black Clawson Fiberclaim, Franklin, Ohio; 0 Union Carbide, South Charleston, W.Va.; and 0 Union Electric, St. Louis, Mo. The three systems produce a wet fuel, a pyrolytic gas and a dry fuel, respectively.
Process trains The Black Clawson system prepares the solid waste by processing it through a hydropulper. This paper industry machine achieves size reduction by pounding the organics and inorganics underwater. Large ferrous objects are removed by a "junker." All fuel fraction materials plus most glass and small-sized metals are reduced to less than one inch. They are then passed through the bottom of the machine in a slurry of 3% solids, 97% water. A liquid cyclone separates the organics from the inorganics. Eighty percent of the material flows from the top of this
cyclone as organic matter while 20% flows from the bottom as heavy organics, glass and metals. The light (top) fraction may proceed through fiber refiners where reusable paper fibers can be removed. The fuel is then dewatered on inclined dewatering screws to a 20% solids, 80% liquids level. At that point, sewage sludge ( 5 % solids) is added, and the entire mat is mea consistency of chanically dewatered to 50% solids-to woodbark. The fuel is ready for burning. The Union Carbide Linde Division PUROX system accepts solid waste, rough grinds it in a vertical Eidal Mill, and magnetically removes the metal. The nonmagnetic material remaining is moved to the top of a 40-ft high shaft furnace. According to Richard S. Paul, manager of Union Carbide's Solid Waste Division, Environmental Systems Department, three zones operate in the pyrolysis chamber: a drying zone, a reaction zone, and a firing zone. This furnace is based on the U.S. Bureau of Mines pyrolysis experiments. The solid waste first enters the drying zone where rising heat prepares the material. In the reaction zone, organics (principally cellulosic materials) are decomposed to a combustible gaseous material and a fuel char. The gas is removed, passed through a Venturi scrubber and delivered to the user. The char and inorganics fall from the reaction zone, where temperatures range from 900"F-1700"F, to the firing zone. Here pure oxygen reacts with the char at a combustion temperature of 3000°F. The inorganics and ash melt into a slag, PROCESS TRAIN
Fuel type
Shredder
Boiler
Classifier
Air classified in Lights moved Shreds in vertical to boiler hortizontal mill straight drop device Metals magPyrolysis Union Gasb Rough Carbide grinds in reactor netically separated from vertical nonmagnetic mill material
Union Electric
Dryfi
Black Wetc Shreds with Clawson water
In liquid
cyclone; separates by differences in specific gravity
Dewatered to 20% solids, 80% liquids
11 Fuel consists of light paper a n d lastics fraction (heterogeneous): b Pyrolysis chamber produces gas (homogeneous); sistency cf woodbark (homogeneous?
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Cleanup system
Burn in exist- Electrostatic ing boilers precipitator designed for coal Venturi Burn clean scrubber gas in existing boilers designed for natural gas Sewage Dewatered Burn in a bark- Currently uses type boiler, baghouses sludge to 50% (5% solids) solids if 100% of but may use added feed Venturi scrubber or electrostatic precipitator c
Fuel has t h e Con-
which is quenched as a frit; the carbon monoxide resulting from char combustion passes upward to the reaction zone, and enriches the fuel gas. The Union Electric system simulates the Bureau of Mines raw refuse experiments. Waste material enters the system and is shredded in a horizontal Grundler mill. After shredding to a nominal size (1.5 in.), particulates are air classified in a vertical straight drop device. Negative pressures fluidize the light paper and plastics while the heavy organics and inorganics drop through the air classifier for further processing. The light paper and plastics are then shipped to the Merremac Power Plant as fuel, according to project engineers William Hackel and David Klumb.
Fuel value According to the U.S. Bureau of Mines, after removal of inorganic materials, the energy potential of solid waste is 6,000 Btu/lb. The paper and light plastics alone contain 7,800 Btu/lb. This compares to underground mined c'oal that has a value of 11,800 Btu/lb and crude oil that has a value of 19,900 Btu/lb. Each of the three systems, then, receives about 9.6 million Btu per incoming ton of refuse. Each seeks to achieve maximum energy recovery in the most usable form. Black Clawson, because it produces a wet fuel, offers an energy source with between 4,000 and 4,360 Btu/lb. This fuel is homogeneous, with known chemical composition and stoichiometric air requirements. "We sacrificed Btu's for a homogeneous fuel," states Paul Marsh, technical director of Black Clawson Fiberclaim. Union Carbide produces a gas with 286 to 300 Btu/scf; when converted to output pounds, equivalent to the measure used at Black Clawson, this fuel contains 5,317 Btu/lb. Energy used in the conversion process represents the loss from raw refuse. The PUROX fuel is also homogeneous and provides control over stoichiometric air requirements. Union Electric retains most of the 6,000 Btu/lb fuel value by not processing the fuel beyond separation. I t loses some heavy organics. This system does not provide homogeneity within the fuel, creating a practical upper limit to its usage. Black Clawson and Union Carbide can offer a substitute fuel; their products can be 100% of the feed to a boiler. Union Electric must use at least 80% coal with its refuse; it normally uses a 10-207'0 solid waste/90-80% coal mixture. Both the Union Carbide and Union Electric fuels can Volume 9, Number 5 , May 1975
419
Liquid cyclone. This Black Clawson unit separates 80% of the material out the top as organics, and 20% out the bottom as inorganics
be burned in existing boilers designed for natural gas or coal, respectively. The Black Clawson fuel, as an alternative, must be burned in a bark-type boiler capable of accepting the added moisture. Boiler efficiencies for the solid dry fuel and gaseous fuel are in the 80-90% range; bark burner boiler efficiency, with a 50% moisture content fuel, is about 65%, but reaches 75% when the fuel is thermally dried to 80% solids. System efficiencies The analysis of the capability for turning incoming Btu's into useful energy forms considers energy consumed to produce the fuel by the mechanical recycling apparatus, energy consumed in any chemical reactions used to produce fuel, and boiler efficiencies or energy cost in fuel consumption. This analysis does not consider inefficiencies of converting steam to electricity: it also ignores transporting the fuel to and from the process train. Those considerations are external to the system efficiency analysis. The Union Carbide PUROX system consumes 8% of its own energy to run mechanical feeders and provide process steam and building heating. Chemical conversion of solid waste to gas-heating the solid waste in the drying zone, breaking the long cellulosic chains into simple hydrocarbon, carbon, and hydrogen compounds-results in another 17% loss. Of the energy available for combustion under a boiler, one-sixth is required for oxygen generation; the gas boiler itself has a 90% efficiency rating.
Comparison of the chemical composition of fuels
H1O Moisture Cz Carbon H1 Hydrogen CO Carbon monoxide COSCarbon dioxide CHI Methane CzH, Hydrocarbon NzNitrogen C I Chlorine OpOxygen Ash Sa Sulfur
8.96 63.31 4.75
26.04 27.23 3.85
1.02 0.12 9.98 11.28 3.38
0.28
-
-
0.20 21.49 20.63 0.26
*Trace elements
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Environmental Science & Technology
50 23.26 3.3 0.33 0.72 17.26 5.6 0.09
0
33 47 14 4 1
1 0 0 0
*
Overall. Union Carbide has a useful energy conversion rate of about 65%. Black Clawson's wet fuel, the other homogenized fuel. suffers similar losses. Some 19% of the incoming energy is required to operate the mechanical systems that produce the fuel. There are no losses in chemical conversion: there is no chemical conversion. But boiler efficiencies, assuming a 50% moisture content in the fuel, are about 65%. The overall system has a net useful energy release of 52.6%. The Union Electric system suffers mechanical system losses, air classification losses of heavy organics, and boiler efficiency losses caused by increased moisture content in the fuel. Mechanical system losses are about 5%; air classification may remove another 15%; the 30% moisture can decrease boiler efficiency by at least 10% over normal operating efficiencies. These losses result in a net efficiency of 65%; this has been confirmed by General Electric. In their report, "A Proposed Plan of Solid Waste Management for Connecticut: Summary," General Electric states, "A review of pyrolysis systems shows a greater spread in yields. The highest energy yield is obtained from the fuel gas process. Energy yield is of the same order of magnitude as that of the solid fuel preparation processes." Both the Union Carbide and Union Electric systems approximate 65% efficiencies. Each fuel product can be upgraded. Extensive drying of solid waste prior to pyrolysis will produce more gas with higher Btu (energy) values. According to Bureau of Mines' researchers, three chemical reactions take place simultaneously in pyrolysis: 0 H20 GO s GO2 HZ (water-gas shift) 0 H20 C s CO H2 (steam-carbon reaction) 0 H20 CH4 s GO 3H2 (steam-hydrocarbon reaction) Drying shifts the reactions to the left, increasing the carbon monoxide and methane values, Btu content and gas volume. The energy required in drying, however, may not yield an appreciable increase i n net fuel efficiency at the commercial scale of operation. Similarly, Black Clawson can improve the value of its fuel by drying. Wet fuel, dried to 20% moisture, increases boiler efficiency to 75%. The overall impact may be negligible, however, again because of increased energy costs in drying. Union Electric faces similar trade-offs.
+ + +
+
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The Perfect Fuel.
Combustion Ecrubment Associates introduces ECO-FUEL 11.'" A
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Manufactured from crarbacre. Low in sulphur,domestic,versaBle.Nin-depleting.
OUR ANSWER TO MUNICIPALWASTE DISPOSALPROBLEMS AND INDUSTRY'S FUEL PROBLEMS. because it provides a new reliable CEA has developed the ,
Resource F:ecovery technology which converts organic solid waste into ECO-FUEL 11, a premium quality fossil fuel substitute. ECO-FUEL 11, can be shipped, stored and combusted in existing power systems like any other fuel. It will be available to industry in either briquette or powder for:m,or suspended as a mixture in oil. ECO-FUEL I1 can also be a monetary benefit to a municipality because when your town delivers its waste to 'a CEA Resource Recovery plant, we'll return some of the revenue we receive from selling it to industry. Which is why ECO-FUEL I1 is perfect for everybody. To the municipality because it's an ecologically and monetarily better alternative to landfilling, dumping, or incineration. . . and to industry
source of fuel. For more information on how CEA's Resource Recovery systems can help your town and environment, write : Combustion Equipment Associates, 555 Madison Avenue, New York, N.Y. 10022.
Tomorrows fuel today COMBUSTION EQUIPMENTASSOCIATES
a
We design, build and operate systems to abate pollution, recycle wastes, and produce energy.
ECO T J E L
IS '1 registeyed
traderam of Cornbastior Eouiprne~tAssociates
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Additional feedstocks Municipal solid waste is the primary feedstock, but other environmentally unsound materials may be used as well. For example, wood waste can be handled by all systems if it can be shredded to acceptable sizes. In metropolitan areas, sewage sludge remains the other principal waste disposal problem in addition to solid waste. Systems that are able to handle sewage sludge offer a powerful bonus. The Union Electric system does not handle sludge; it is not set up to do this. However, if the sludge is dried to 20% solids, it can be pyrolyzed in the Union Carbide furnace. No additional gas will be produced; the sludge will sustain its own reactions, and not deteriorate the output derived from the solid waste. Black Clawson. by dewatering the sludge to 50% solids against the mat or wet solid waste, gains mechanical drying at a low energy cost. This system, by achieving such a high dewatering level, also turns the sludge into additionally recoverable Btu's that can be used in a productive manner. Thus, this system has the highest level of acceptability in terms of handling the sludge problem.
Pollution potential The chemical composition of a fuel determines both its heating value and its emission control problems. Because the Union Carbide system produces a gas, the composition of the final product is expressed in compounds: solid fuels can be expressed in more elemental terms. The pollution problems of ash, sulfur, nitrogen, and chlorine exist for each system. Across the spectrum of fuels, coal contains 68% fuel elements and pollutant potentials of 24.76%; solid waste contains 31 .OS% fuel elements and potential pollutants of 21.37%; wet fuel offers 27.55% fuel elements and 5.71% potential pollutants: and pyrolytic gas consists of 85% fuel elements plus 1% potential pollutants. The two processed fuels demonstrate that homogenization can reduce the pollution potential. Wet fuel dilutes the potential harm from nitrogen, sulfur, chlorine and ash; pyrolysis eliminates the problems in fuel generation. A second source of pollution is the volume of air used in combustion of any fuel; the nitrogen in the air combines with unconsumed oxygen to produce NO, emissions. By pyrolyzing trash with pure oxygen, the Union Carbide system reduces the production of nitrogen oxides. Producing a fuel requiring only one-seventh the volume of air needed in incineration holds NO, emissions to levels well below EPA regulations. Sulfur dioxide and nitrogen oxides present no particular problems with any of the solid waste fuels created to date. Ash appears to be the only potential hazard. The ash content in Union Carbide's fuel is "scrubbed" prior to using the gas as an energy source. Thus, combustion of PUROX gas results in 0.008 grains of dry particulate per cubic foot (adjusted to 12% GO2). This is below the EPA standard by a factor of ten. Ash content in the Black Clawson fuel is higher, and combustion of the fuel requires scrubbers or electrostatic precipitators. Dilution of the solid waste controls this problem, however. Since the ash content in dry solid waste is twice that in coal, but the Btu content i s halved, one would assume that generating the same heat from 100% dry solid refuse as from 100% coal would produce a quadrupling of the load on the electrostatic precipitator. Initial tests by Union Electric indicate maximum inlet dust loadings of 2.2904 gr/scf when firing with solid waste. These loadings are well within EPA regulations for existing power sources, but above the new source standards of 0.1 Ib/mm Btu. (With solid waste, firing at 140 MW, Union 422
Environmental Science B Technology
Electric has 5.4 Ib/mm Btu: 100% coal offers uncontrolled particulate of 4.9 Ib/mm Btu under the same conditions.) Because solid waste has a higher ash content, precipitator efficiency decreases as trash burning increases. The differential can be as high as 3-4%. For example, precipitators are 97% effective when the Merremac station is generating 140 MW on 100% coal: firing coal and 30% refuse reduces this efficiency to 93.5%, according to initial Union Electric tests. Union Electric is metering its precipitators to optimize their efficiencies; the company is retesting air pollution emissions to provide more definitive data. Other fuels Other forms of solid waste fuels will become available for testing and evaluation within the next few years. These include products from the Landgard system of the Monsanto Chemical plant in Baltimore, Md., and the pyrolytic oil of the Garrett Research and Development plant in San Diego. Calif. The Landgard system provides steam for heating downtown buildings. This steam is generated from a waste heat boiler connected to a recently constructed 1000 ton per day pyrolysis plant. The Garrett system pyrolyzes municipal solid waste to generate heavy oil compatible with No. 6 residual oil. This pyrolysis operation produces one barrel of oil containing 4.8 million Btu for every incoming ton of trash (bearing 9.6 million Btu's). The fuel contains 0.3% sulfur and is low in pollution potential. It, however, contains 75% of the energy value of comparable virgin industrial oil. SCA Associates, the Americology Division of American Can Go., and Raytheon Service Co., all offer variations on the dry solid waste fuel theme. Garrett Research, a division of Occidental Petroleum, also offers this option. Devco Engineering markets a gas pyrolysis unit. The variety of systems and fuels now coming into the marketplace is most broad: vendors are trying to match overall capabilities with local markets. Summing up No company offers the single best fuel for all conditions. The trade-offs are sufficiently varied to produce significantly different but good fuels. Dry shredded fuel has the highest Btu content per output ton, a net energy efficiency equal to pyrolysis, and few retrofit problems. But it can only be used as a supplemental fuel. It also has the highest pollution potential. Pyrolytic fuels have mod.erate Btu content pkr output ton and one of the highest net efficiencies. They have the lowest pollution potential and can be used as an alternative fuel with few retrofit problems. Wet homogenized fuels have a high Btu recovery per incoming ton of trash, relatively low pollution potential, can be used as an alternate fuel, and c a n w e sewage sludge in a productive manner. Retrofit problems are more significant: and overall net efficiency of the fuel is lower than either dry solid waste or pyrolytic fuels. Any community selecting a recycling process to produce fuels must weigh the specific energy fraction characteristics against the requirements of the potential enduser David A. Tillman is vice-president of Materials Associates, Inc., Washington, D.C., and contributing editor to Area Development magazine. Prior to his present position, he was editor of Vermont Business World. Mr. Tillman is writing a book on industrial and municipal waste recycling for McGraw-Hill Book Co. Coordinated by LRE
