Pollutantsfrom waste-to-energy conversion Wskrns d l -
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As more cities turn to these systems to supplement their energy supplies and reduce their solid waste management costs, the associated potential pollution problems need to be assessed
Harry Freeman
US.Environmental Protection Agency Cincinnati, Ohio 45268 Only a few years ago municipalities around the country were closing their solid waste incinerators because they could not meet the more restrictive emerging air pollution standards. Sanitary landfills were being touted as the ultimate answer to solid waste management problems. Also, the relatively low cost of fuels repressed the economic appeal of recovering energy from wastes. Today, with fewer convenient landfill sites available, and the costs of solid waste management and energy increasing, cities and counties are again considering combustion systems as an economically attractive option. However, the systems now being considered incorporate energy recovery in addition to volume reduction. 1252
Environmental Science & Technology
The new generation of combustion systems is an improvement over its smoky predecessors. Nevertheless, these new units still produce atmospheric, aquatic, and solid pollutants that must be controlled. Since solid wastes, by their very nature, cannot be easily characterized, it is difficult to generalize about the quantities and types of emissions that can be expected from the conversion and use of wastes as fuel. The situation is further complicated because each system is very much a unique plant.
Solid waste as fuels Waste-to-energy conversion plants are now being planned for many US. cities and, therefore, the question of environmental impact must be addressed. The question of marginal contribution of waste processing systems is especially important for cities that are not attaining National Ambient Air Quality Standards. What can we expect from these new systems?
Although the actual make-up of municipal solid waste can vary widely depending upon geographical, seasonal, and weather-related factors, it has generally been found that from 60-75% of the waste stream is combustible. The combustible fraction consists of paper, plastics, yard wastes, and wood. The estimated input for the largest components in a “typical” waste stream for the new solid waste processing plant in Chicago is shown in Table 1. For the purpose of an environmental assessment, it is much more useful to have the input feed analyzed according to its chemical constituents rather than its combustible constituents. Table 2 shows the results of a typical, ultimate analysis of mixed municipal solid wastes. Although many analyses such as the one shown in Table 2 have been prepared, relatively few trace-metal analyses of wastes have been made. As shown in Table 3, published work
This article not subject to U.S. Copyright. Published 1978 American Chemical Society
( E S & T. May 1976, p 436) has indicated that the constituents of urban refuse include trace elements. The existence of these trace elements in the municipal refuse should not be construed as necessarily indicating that the metals will be transferred to the atmosphere during conversion. Such emissions could occur, but much more information concerning the form of the elements must be known before any projections can be made. For example, chlorine commonly appears in the waste stream either as a chlorinated synthetic or as sodium chloride. Should it be i n the form of the chlorinated synthetic it would probably be freed and would exit the conibustion chamber as a waste gas, HCI. Should it, instead, be in the form of sodium chloride it would more probably end up as bottom ash. So, although the elements listed in Table 3 should be considered as potential environmental pollutants, the quantities should be viewed as worst case amounts. Conbersion technology
The various thermal processes for extracting energy from solid wastes can be categorized into three major arcas: Incineration with heat recovery. T h e uaste is fired, usually in an unprocessed form, in a boiler lined with water-filled tubes that transform the collected heat into steam. Combined firing systems. T h e uaste is first processed to remove nonconibustibles and to reduce its particles to common sizes, and is then fired as a supplemental fuel with coal, oil. or natural gas in ;I modified conventional boiler. T h e processed refuse is sometimes referred to as Refuse Derived Fuel ( R D F ) . Pyrolysis processes. The solid IV a s t e is converted t 'h r o ug h the r mochemical conversion in an oxygenstarved environment into a liquid. gxeous. or solid fuel product. This storable rind transportable product can be utilized as either a primary or suppl c ine nt a I fuel. Of the three categlsries of thermal processes. only incineration with heat recovery (waterwall incineration) and combined firing systems are presently being operated on a commercial scale. Some pyrolysis processes have been demonstrated and are being considered for commercial use: none has yet been adopted. For discussions of the available technologies, the reader is referred to ES& T , May 1976, p 430; Etigitireritig and Economic Analysis oJ' Wrrstes to Etiergj. Systems, EPA Contract Report N o . 68-02-2101;
TABLE 1
Mass balance for largest fractions of 1000 tpd plant
a
A. rec'd,
Ftaw rduse
rrt%
oomporwmt
HHVa Btullb
24.77 12.73 4.09 3.74 3.67 3.16 2.23 1.22
Misc. paper Newspaper Plastics Magazines/books Cormgatedboxboard Teuctibs Wood Rubber i3 leather
5193 5980 14 230 5810 5600 6670 7060 8450
Fuel fractlon Tld %
235 125 38 35 35 30 18 10
Reject traction Tld
33.0 17.6 5.3 5.0 4.9 4.4 2.5 1.4
12.7 2.3 2.9 2.4 1.7 1.6 4.3 2.2
HHV = High heating value.
Socace:€PA Report NO. 88-02-2101.
TABLE 2
Analysis, heating value for typical mixed municipal wastes compomn(
AnaJycls (as recolved) % bY W . l g M
Moistwe Carbon Hydrogen Oxygen Nitrogen Chlorine (organic 0.16) (inorganic 0.14) Sulfur Metal Glass,ceramics Ash Total High heating value, HHV 4400 Btu/lb
Analysk (dry bases)
%by-
25.1 25.2 3.2 18.8 0.4
0.0 33.5 4.3 25.2 0.5
0.3 0.1 8.7 12.2 6.0 100.0
0.4 0.1 11.6 16.3 8.1 100.0 5600 Btu/lb
Source: News of EnvironmentalResearch, ind. Env. Res. Lab.,July 1977.
T A M 3
Trace elements in urban refuse M a p elements Averconhnt (loO0-100 OW ppm)
Aluminum Calcium Chlorine lrOn
Magnesium Phosphorus Potassium Silidbn Sodium sutka Titanium Zinc
Mlnw elements Amcontenl (1-099 ppm)
Antimony Arsenic Barium Beryllium Bismuth Boron Cadmium Cesium Chromium Cobalt Copper Germanium Gold Lead Lithium
Manganese Mercury Molybdenum Nickel Niobium Platinum Rubidium Selenium Silver Strontium Tantalum Tin Tungsten Vanadium Zirconium
SOWGO: EPA 600/7-77-091, AWUt 1977
Volume 12, Number 12, November 1978
1253
1978; Fourth Report to Congress: Resource Recouery and Wastes R e duction (EPA Report No. SW-600).
Air emissions In waste-to-energy conversion systems, air emissions come from two main sources: the receiving area and processing equipment (grinders, classifiers, etc.); and the stack from the combustion chamber. The emissions from the first area are typically dusts, and from the second area are small particles and gases. Some controversy regarding waste processing plants arose in 1975 when an environmental assessment of the EPA R D F demonstration plant in St. Louis, Mo., indicated high counts of bacteria and virus in the ducts. Given the nature of the material being handled, the presence of bacteria in the process lines was to be expected. EPA researchers and others were, however, concerned about the levels of bacteria and viruses that might be present in the ambient air surrounding processing plants. Another study was undertaken to determine bacteria counts outside the ducts, and in the ambient air around the plant, and to compare these counts at the processing plant with counts at the handling facilities for other types of waste streams. The study found that although airborne bacterial levels are generally higher both in-plant and at the property line for plants with uncontrolled dust emissions than at other wastehandling facilities, these emissions can be controlled by using fabric filters applied to the primary source of emissions. The efficiency of a fabric filter as a dust and bacteria-control device was also documented by another EPA research program (EPA Contract No. 68-02-1871) at an R D F processing facility in Houston, Tex. In that study it was found that efficiencies of removal of up to 99.6% were realized for dust a r 1 associated bacteria. Waste-to-energy conversion system emissions include particulate matter, sulfur oxides, nitrogen oxides, hydrogen chloride, hydrocarbons, carbon monoxide, and trace elements. Of these pollutants, particulate matter is the most significant in terms of present environmental regulations. Particulate matter is any solid or liquid material in the gas stream, except uncombined water. It consists of fly ash, dus L, aerosols (microscopic-size particles) and mists. Emission rates for particulate matter vary widely depending on moisture and ash content of the fuel, unit design, and combustion parameters. However, uncontrolled rates of from 15-24 Ib of par1254
Environmental Science & Technology
ticulate matter per ton of refuse fired are generally produced by waterwall incinerators. National pollution control standards [New Stationary Source Performance Standards (NSSPS)] are presently in effect for particulate emissions from waste-to-energy conversion systems built since 197 1. For incineration, the standard is 0.08 gr/dscf corrected to 12% C 0 2 , which corresponds to approximately 1.9 lb of particulate per ton of refuse fired. Several operating units have complied with this standard (Figure 1). Since EPA has decided that modification of a coal-fired utility boiler built before 1971 to enable it to burn R D F does not constitute a new source of air emissions, no NSSPS compliance tests for utility boilers firing R D F as a supplement have been done. However, as part of an R D F demonstration project co-sponsored by the EPA and the Union Electric Company in St. Louis, Mo., it was determined that the Missouri state pollution control standard for particulate matter from a coal-fired boiler could be met while firing 20% RDF. The EPA is presently revising its NSSPS standard for fossil-fuel-fired boilers. The new standard will include within its purview the firing of supplemental solid fuels.
Gaseous emissions, trace elements Gaseous emissions from waste-toenergy conversion are not presently viewed as a significant environmental problem. Refuse is a low-sulfur fuel, averaging only about 0.3% sulfur (as compared to 1-3% for coal). Consequently, SO2 production is minimal. Significant nitrogen oxides formation usually occurs at temperatures of above 2000 OF. Most waterwall incinerators operate at a much lower temperature. Although combustion temperatures are higher in R D F cofiring situations, in utility boilers the marginal contribution of the R D F to NO, production is minimal.
Unburned hydrocarbons (HCs) and carbon monoxide (CO) are usually present in significant amounts only if proper combustion is not taking place. Proper flame turbulence, ample combustion times, and sufficient temperatures will reduce the quantities of C O and HCs to negligible levels. Hydrogen chlorides have been detected in the stacks from waste combustion systems and contribute to increased equipment corrosion. It is not presently believed, however, that these emissions represent a hazard to the environment significant enough to require control. No NSSPS standards have been developed for gaseous emissions from incinerators, and although SO, and NO, standards are in effect for utility boilers that can be modified to fire solid wastes, the firing of the wastes is not considered to contribute significantly to SO, and NO, emissions. In fact, demonstrations at St. Louis and Ames, Iowa, have indicated that SO2 and NO, emissions a r e reduced through the cofiring of solid waste with the coal. Although data are extremely limited concerning trace-element emissions from waste-to-energy conversion systems, several studies have indicated that refuse combustion can produce airborne pollutants not produced by conventional fuel combustion. The EPA evaluation of the St. Louis R D F plant indicated that such potentially hazardous substances as beryllium, cadmium, mercury, copper, and lead were present in higher concentrations in the emission streams from coal plus R D F combustion than in emissions from coal-only combustion. A National Science Foundation study found that in two metropolitan areas, refuse incineration could account for major portions of zinc, cadmium, and antimony observed on airborne particles. The study also suggested that refuse incineration was a large source of another toxic element, vapor-phase mercury.
FIGURE 1
Emissions from burning solid waste Saugus, Mass.
Norfolk,
Va.
I
I
1
0.02
0.04
I
0.06
Particulate emissions (gridscf)
0.08
0.1
Resource recovery mixed waste facilities as of March 1978 Location
Type a
Capacity (TPD)
Productslmarkets
Start-up date
In operation: Altoona, Pa. Ames, Iowa Baltimore, Md. (D)
Compost RDF Pyrolysis
Baltimore County, Md. (D) Blytheville, Ark. Braintree, Mass. E. Bridgewater, Mass. (D) Franklin, Ohio (D) Groveton, N.H. Milwaukee, Wis.
RDF MCU
Nashville, Tenn. Norfolk, Va. Oceanside, N.Y. Palos Verdes, Calif. Saugus, Mass. Siloam Springs, Ark. South Charleston, W. Va. (D)
wwc RDF Wet pulp MCU RDF
wwc wwc
200 400 700
Humus RDF-utility, Fe, AI Steam heating & cooling, Fe
1963 1975 1975
550 50 240 160 150 30 1000
RDF, Fe, AI, glass Steam process Steam process RDF-utility Fiber, Fe, glass, AI Steam process RDF-utility, paper Fe, AI
1976 1975 1971 1974 1971 1975 1977
720 360 750
1974 1967 1965174 1975 1976 1975 1974
MCU Pyrolysis
1200 20 200
Steam heating & cooling Steam (Navy Base) Steam Gas-utility & Fe Steam process Steam process Gas, Fe
Akron, Ohio Bridgeport, Conn.
RDFIWWC RDF
1000 1800
Steam heating & cooling RDF utility, Fe, AI, glass
1978 1978
Chicago, 111. Hempstead, N.Y.
RDF Wet pulplWWC
1000 2000
RDF-utility, Fe Electricity, Fe, AI, glass
1976 1978
Lane County, Ore. Monroe County, N.Y.
RDFIWWC RDF
750 2000
RDF-institution, Fe RDF-utility, Fe, AI, glass
1978 1978
Mountain View, Calif. (D) New Orleans, La. (D)
Methane recovery Materials
Gas-ut iIity Nonferrous, Fe, glass, paper
1977 1978
Niagara Falls, N.Y. North Little Rock, Ark. Portsmouth, Va. San Diego, Calif. (D)
RDFIWWC MCU
Steam industry, Fe Steam process Steam loop Liquid fuel-utility, Fe, AI, glass
1977 1976 1977
RWIIWWC Methane recovery
wwc
Under construction; startup:
wwc
Pyrolysis
650
2200 100 160 200
-
a RDF = Refuse-derived fuel: WWC = Waterwall combustion; RWI = Refractory wall incinerator with waste heat boiler; MCU = Modular combustion unit; RDFIWWC = Waterwall combustion using processed waste; D = Pilot or demonstration facility.
Source: EPA. SW13004, 1978.
Baltimore, Md.
. .
Chicago, Ill. 9
North Little Rock. Ark.
Saugus, Mass.
Volun?e 12, Number 12, November 1978
1255
To date no standards for trace element emissions from these systems have been developed. None of the EPA standards for hazardous pollutants apply to either incinerators or fossilfuel-fired boilers. With increasing attention being given by the EPA to toxic substances, more consideration may be given in the future to trace element regulation.
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Water pollution, solid residuals Since waste-to-energy systems usually use water only for ash quenching and facility clean-up, they are not typically seen as a major source of water pollution. Two exceptions to this general statement are RDF/coal cofiring installations in which water is used to sluice the ash, and pyrolysis systems in which water is used as a scrubbing medium for the product gas. In the cofiring case, sluice water from the St. Louis demonstration unit was found to exceed state standards for BOD, dissolved oxygen and total dissolved solids and to contain higher concentrations of twelve other pollution parameters than a sluice stream from a coal-only boiler. I n the pyrolysis case, the untreated wastewater stream from these units is expected to have very high concentrations of BOD, COD, phenols, and other organics and will certainly require treatment. Such treatment is included as part of the commercialscale units presently being developed. I f a waste-to-energy system is discharging into a municipal sewer system, which is the case for most units in operation today, usually only minimal treatment such as settling and pH adjustment is required. I f the effluents were discharged into public waterways, more extensive treatment would be necessary. Characterization of the solid residuals from waste-to-energy conversion systems has been minimal to date. These residuals are produced as bottom ash and processing rejects, recovered fly ash and, for high temperature slagging incineration and pyrolysis units, an inert slag. Trace elements such as beryllium, mercury, cadmium, and lead have been reported to be present in the fly ash from coal plus refuse systems. The extent to which trace elements are enriched in the fly ash is not fully understood. In August 1976, EPA's Cincinnati-based Industrial Environmental Research Laboratory initiated a research program to characterize emissions from waste-to-energy systems.
WLKS Foxboro Analytical
@ Registered Trademarkof The FoxboroCompany
CIRCLE 7 ON READER SERVICE CARD
1256
Environmental Science 8 Technology
As part of this program the laboratory contracted with Midwest Research Institute (Kansas City, Mo.) to carry out environmental assessments of selected systems throughout the country. These assessments are being done on units that are either converting solid wastes to more desirable forms of fuel, or actually combusting wastes to produce energy. To date, the project has assessed the Union Carbide Purox process at South Charleston, W . Va., the mass-fired incinerator at Braintree, Mass., a wood/coal cofiring unit at the University of Missouri at Rolla, and a wood-tired utility boiler in Burlington, Vt. Reports of these projects will be available from the author.
A last word Since waste-to-energy technology is still relatively young in this country, a large amount of environmental data is not yet available. Programs of the EPA and others are now being implemented to remedy this deficiency. These programs are intended to develop the data base needed to ensure that waste-toenergy systems do indeed fulfill their potential as positive contributions to improving solid waste management practices in the country.
Additional reading EPA, Environmental Assessment of Waste to Energy Processes: Source Assessment Document, EPA 60017-77-091, August 1977. Environmental Quality-I 977, Eighth Annual Report of the Council on Environmental Quality. EPA, Evaluation of the Ames Solid Waste Recovery System, EPA-600/2-77-205. Gorman, P. G., Shannon, L. J., Schrag, M. P., Fiscus, D.. St. Louis Demonstration Project Final Report: Power Plant Equipment Facilities and Encironmental Ecaluation, EPA Contract No. 68-02I87 I , July 1976. Greenberg, R. R., Zoller, W. H., Gordon,
G.E. Composition and Size Distributions
of Particles Released in Refuse Incineration, Enciron. Sci. Technol.; 12, 566
(1978).
Harry Freeman is research program manager for the Waste-to-Energy Systems Encironniental Assessment Program at EPA 's Industrial Encironmental Research Laboratory in Cincinnati, Ohio.
Coordinated by LRE
