(24) Brownawell, B. J.; Farrington. J. W. Geochem. Cormochim. Acta 1986. 50. 157-69. (25) Quensen. J. F.; Tiedje. I. M.; Boyd, S. A. Science 1988,242. 752-54. (26) Oliver. B. G.; Charltan. M. N.; Durham, R. W. Environ. Sci. Technol. 1989, 23. 200-208. (27) Alford-Stevens. A. L.: Budde. W. L.; Bellar. T. A. Anal. Chem. 1985. 57. 2452-Sl. (28) Alford-Stevens. A. L.; Eichelberger. 1. W.; Budde. W. L. Environ. So'. Technnl . I.N..,57. . . 304-12. .. (29) Intergovernmental Oceanographic Commission: IOC Tech.Series 1984; UNESCO. New York, 1986. (30) Marlin. J. M.; Whitfield. M. In Tram Mer& in Sea Wore,: Wone. C. S. et al.. Fds.: Plenum: New York. y983; pp. 26596.
Acknowledgment The authors thank Dr. Robert Magnien for helping with archival data.
kid
Chistapher E D'Elia is a professor at ChesapeakeBiological Laboratory and Director of rhe Mapland Sea Grant College Program. He received his Ph.D. in zoology from the Universiry of Georgia in 1974. His inreresis include the nutrient dynamics of aquatic ecosystems and sciencepolicy relationships. 1
Glenn A. Norton Ames Loboratory
Iowa State University Ames, IA 50011
Audrey D. Levine Iowa State University Ames. IA 50011 James G. Sanders (I)is Director and Associate Curator of the Academy of Notum1 Sciences' Benedicr Estuarine Research Laboratory on rhe Chesapeake Bay His research inrerests include trace element biogeochemistry in coastal ecosystems and organism response to chronic loading of toxic substances. He obtained his B.S. from Duke Universiry and his Ph.D. from the University of North Carolina at Chapel Hill. (r) a professor at Chesapeake Biological Laboratory. received his Ph.D. from the University af Miami in oceanography in 1978. His research deals with microbial ecology, biogeochemistry, and microbe-pollutant interactions.
Douglas C. -ne,
l74 Envimn. Sci. Technol.. Vol. 23. No. 7. 1989
It is estimated that the annual generation of municipal solid waste (MSW)in the United States will reach about 200 million tons by 1990 (I). Alternatives to disposal of solid waste in landfillsare needed because of increasingly stringent environmental regulations. One alternative is to process MSW into refusederived fuel (RDF) that can be used as a fuel supplement for combustion of coal in dedicated or retrofitted boilers for the production of electricity. The energy content of 200 million tons of MSW is equivalent to about 75
million tons of coal or about 300 million barrels of oil and thus represents a significant fuel source (2). In addition, by producing RDF from MSW,the volume of waste to be disposed of is decreased; therefore the useful life of a landfill can be extended by as much as 400% ( I ) . The processed MSW can be fired as 'fluff RDF or densified RDF (dRDF). Fluff RDF is typically 3141 in. in size and is generally prepared by processing the MSW through primary and secondary shreddiag, magnetic separation, and air classification (3, 4). The fluff RDF can be densified to form dRDF by pelletizing, briquetting, or extruding techniques ( I , 4). (In this paper, "RDF" refers to both the fluff and densified forms unless otherwise noted.) The first major experiment in which RDF was cofired with coal was performed in St. Louis at the Union Elec-
001?-936W89/09230774.$014501.5010
CI 1989 American Chemical Socieh,
tric Meramec Station power plant in the 1970s (4, 5). Since that time, RDF has been fired with coal in boilers at a number of facilities. Emission rates of environmental pollutants are controlled by the composition of RDF and coal and by the combustion conditions. In addition, possible aberrations resulting from cocombustion may influence the performance of the boilers and emission control devices. Emissions of sulfur oxides (SO,), nitrogen oxides (NOJ, and particulate matter are limited by state and federal regulations. Therefore, it is important to evaluate the influence of cocombustion of RDF and coal on emission rates of those pollutants. The primary objective of this paper is to evaluate the effects of combusting RDF with coal on selected emissions using published data from a variety of studies. We will summarize emissions of SO,, NO,, and particulate matter from coal-RDF combustion in conventional coal-fired boilers and discuss some studies in which laboratory-scale boilers were used. The purpose of this paper is not to critique the individual studies, but to review general trends and typical operating experiences with regard to SOx, NO,, and particulate emissions from firing coal-RDF mixtures relative to firing coal alone. The objectives of the studies reviewed in this paper varied widely and, in some cases, minimal environmental emission data were collected. When possible, a preliminary data screening was conducted to select those data that best reflected trends typical of a given facility, In multiyear studies at facilities in which continuous improvements were made in RDF quality and coalRDF cofiring technology, the more recent data were selected as being more representative of the typical operating experience. Some publications were reviewed but not included in this paper because of apparent duplication of data presented in other reports. In such cases, the most recent or comprehensive publications were selected for inclusion.
Site descriptions Descriptions of the coal-RDF combustion facilities for which pertinent environmental emission data have been published are given in Tables 1 and 2. The studies are grouped according to the type (fluff or densified) of RDF combusted; facilities where fluff RDF was fired are shown in Table 1 and facilities where dRDF was fired are presented in Table 2. Within each of these tables, the sites are listed in alphabetical order according to the name of the city or area where the study was performed.
TABLE 1
Site descriptionsfor coalhefuse-derivedfuel (RDF) combustion studies in which fluff RDF was fired' Site (reference no.)
Ames, IA City Power Plant (6, 7) Ames, IA Citv Power Plant 18) ,, Baltimore, MD Ballimore Gas & Electric's Crane Stalion 19. , 101 , Columbus. OH City Power Plant ( I I ) Dickerson, MD Polomac Elecfric Plant (12) twine. CA KVB, lncorDorated (13) Madison, WI Oscar Mayer and Company (14) Madison, WI Madison Gas & Electric's Blount Streef Slafion (15) Milwaukee, WI Wisconsin Electric's Oak Creek Plant (16) Rochester, NY Rochesler Gas and Electric's Russel Stalion (171 . . SI. Louis, MO Union Etecfric Meramec Plant llb' . . 191, Stevenage. England Warren Spring Laboratory (20)
Unit(s) used
Bailer information Steam flow (1000 Iblh)' Type of firlng
Psrtlcle
5,6
95 125
spreader-stoker
multicyclone
langential
ESP
collector
7
360
2
1360
cyclone furnace
ESPs
6
150
spreader-stoker
cyclone
3
1300
tangential
ESPs
NRC
NRr (0 8 MWJ suspension-fired
baghouse
5
125
spreader-stoker
multicyclone
9
425
front-fired. pulverized coal
mech collector, ESP
tangential
ESP
7.8
2000 each
3
465
tangential
ESP
1
925
tangenfiat
ESP
C
NRC(27 MW) stoker-fired
NRE
'Both fluff and powdered RDF were fired at Ihine. 5Al nominal capacity. =Not reported.
In general, particulate emissions were collected and determined in accordance with procedures described in EPA Method 5. Exceptions include the tests performed by Union Electric at St. Louis, where American Society of Mechanical Engineers (ASME) Performance Test Code 27 procedures (35) were used, and by Battelle Labs in Columbus, where a Source Assessment Sampling System (SASS) was used. The SASS is similar to a Method 5 sampling system, but isokinetic samSampling and analysis pling is usually only approximated In most of the studies cited here, the (36). Minor modifications in the samSO, emissions were sampled and mea- pling systems, such as replacing a wasured in accordance with procedures ter impinger with a hydrogen peroxide described in EPA Method 6 (34), al- impinger in order to collect SO2 emisthough in many cases continuous in- sions, were sometimes made in order to strumental analyzers were used. Other meet specific sampling objectives. In accordance with standard EPA methods used in several of the studies include EPA Method 8 and modified sampling protocol, sampling of particuMethod 5 sampling systems (34). EPA late effluents should be performed at Method 8 is essentially the same as least eight stack or duct diameters EPA Method 6, except that sulhric acid downstream and at least two stack or duct diameters upstream from a flow mist is determined in addition to SO,. The NOx emissions were sampred disturbance (34). Such ideal locations and quantified using EPA Method 7 were not available in many of the studprocedures (34) or continuous instru- ies, particularly for efflnents collected mental analyzers. Both of these tech- upstream of the cyclone or ESP,and the niques were used to quantify the NO, number of sampling points used in such cases was not always in accordance emissions at many of the facilities.
In these studies (6331,the fluff RDF was fired in both stoker- and suspension-fired boilers. The dRDF, on the other hand, was always burned in stoker-fired boilers, since dRDF is not appropriate for firing in suspension. Particulate emissions were generally controlled with cyclones or electrostatic precipitators (ESPs). It is also worth noting that the boiler size varied substantially among the sites, as indicated by the rated steam flows.
Environ. Sci. Technol.. Vol. 23.No. 7, 1989 775
with recommended procedures for collecting representative samples.
Composition of fuels The MSW used to produce RDF for the studies discussed in this paper was processed typically by shredding, air classification, and ferrous metal r e moval. However, the complexity of the various processing stages varied among the studies. At many of the sites, including Ames (6, s),Milwaukee (16, 2 3 , Dickerson (12), and BatteUe Labs at Columbus (Z),nonferrous metal removal was included in MSW processing. The differences in the composition of the RDFs reflect variations in p m essing the MSW as well as the diversity in the composition of the MSW used to produce the RDI? The most notable exception to the typical processing scheme used for most nf the studies was for the tests conducted on Unit 1 at Wrigh-Patterson Air Force Base. In that study, a hydro-pulper was used to slued the MSW while mixing it with water to form a slurry that passed through a liquid cyclone to remove inorganic materials. The resulting RDF was dewatered and dried to about 20% moisture prior to peuetiziig (30). On an "as-received" basis, heating valnes were typically 1 0 , ~ 1 4 , 0 0 0 Btuilb for the coals and 5000-8000 Btuilb for the RDF. Average sulfur, nitrogen, ash, and moisture concentrations in the coal and RDF used in the Studies discussed in this paper are presented in Table 3. If comparative data between the composition of the coal and RDF for a given site were not provided, no entry for that site was made in the table. Also,data on the fuels used in the laboratory-scale boilers and at Erie are excluded, since emission data for those studies are not included in the subsequent tables. In some of the studies, only typical concentrations were provided, and these were used in lieu of average values. However, it is important to note that the compition of the coal and RDF can vary substantially, and in some studies the concentration ranges of a given constituent for different fuels overlapped considerably. (An expanded Table 3 is available; see Table of Contents for information on ordering supplemental material). These data are intended to show only typical relative concentrations for a given constituent between the coal and RDF at a given facility. Therefore, the data have been rounded to a maximum of two significant figures. The sulfur data xemed to be the most variable for both fuels, having coefficients of variation (CVs) of 4060%in some cases. Overall, CVs for the fuel analyses were typically less than 20%. 776 Envimn. Sci. Technol., MI. 23, NO. 7. I989
, nitrogen, and ash concentrations in coal and RDF’
Baltimore. MD (8, 14)
1.7 4.3 1.3 2.0 3.3
NR
0.38
0.30 NR 0.27 0.21 0.3 0.22
NR 1.2 1.4
NR NR 0.46
NR
NR
2.2 2.3 1.5
0.18
1.5
0.58
0.7
0.12
NR
NR
0.70
0.13
1.5
0.35
0.31
The data presented in Table 3 are reported on an as-received or an “asfired” basis, although the reporting basis was not specified in several of the studies. In cases where the data were reported on both a dry and an as-received basis, the as-received values were used to reflect the actual fuel composition at the time of firing. In most cases, relative concentrations of a given constituent in the coal and RDF for a given study would remain the same, despite occasional ambiguity in reporting. However, because of this ambiguity, caution should be used when comparing values among studies for a given constituent. At Arlington 91, 22), the sulfur, nitrogen, and ash concentrations were reported only on a dry basis. Average compositions of RDF and coal are compared in Figure 1. Coals contained about 1 4 % sulfur, whereas the sulfur content of the RDF was about 0.1-0.4%. Nitrogen contents were 12% for the coals and 0.6% or less for the RDF. The ash contents of both coal and RDF were quite variable. The RDF had substantially higher moisture concentrations than that of the coal. However, it should be noted that the moisture content of the RDF can vary both seasonally and geographically. The inhence of cocombustion on emissions of SO,, NOx, and ash depends on the sulfur, nitrogen, ash, and moisture contents of the coal and RDF and the relative amounts of each fuel in the mixture. For the typical operating ranges used in cocombustion facilities, the effect of mixing coal and RDF on the composition of the fuel admix is shown in Figure 2. The major changes in the fuel composition are a decrease in sulfur content and an increase in ash and moisture content. The nitrogen content would also decrease, but to a lesser degree than sulfur.
Emission results Specific operating parameters and the type and efficiency of particulate and gaseous emission control devices regulate the composition of emissions. However, for design and operation of pollution control systems for cocombustion facilities, it is important to evaluate the effect of the altered fuel compositions on emissions. The SO, emissions are directly related to the sulfur content of the fuels, whereas NO, emission levels are dependent primarily on the combustion temperature 99). The particulate emission levels from the stack are related largely to the ash content of the fuels as well as the collection efficiencies of the particle control devices. Based on fuel analyses, a decrease in sulfur emissions and an increase in particulate emissions would Environ. Sci. Technol., Val. 23,NO. 7. 1989 777
be expected to result from coal-RDF combustion. Because nitrogen emissions are more dependent on combustion temperatures, it is more difficult to predict cocombustion effects on NO,. For the facilities listed in Tables 1 and 2, emission data for SO, and NO, generated during the combustion of coal and coal-RDF mixtures are summarized in Table 4, and particulate emission results are reported in Table 5. The SO, and NO, emissions were usually reported as S@ and NO,, respectively, although in some studies the data were given as SO, or NOI without reporting a specific chemical form. In cases where NO, emissions were reported as NO, the values were converted to NO2 before entering them into the table. If a report contained no numerical emission data for a given site, the site was not included. (Expanded Tables 4 and 5 are available; see Table of Contents for ordering information). The data are often reported in parts per million (ppm) or pounds/million Btu (Ib/lo6Btu) for gaseous emissions and in Ib/lbBtu or graiddry standard cubic foot (grldscf) for particulate emissions. For some sets of data, slightly different trends in emissions were observed, depending on methods of expressing results. For the particulate matter, the emission levels are partially dependent on boiler efficiency when the emissions are expressed in terms of Ib/l@Btu (14). When emissions are expressed in gr/ dscf, factors such as the amount of excess air come into play. In order to minimize the effects of some of the uncontrolled variables, the data in the reports used for this paper were often "corrected" to 50% excess air, 12% C q , or some other reference gas composition when emissions were expressed in ppm or gr/dscf. Before discussing the emission data, it is important to mention some of the variables and problems that contributed to the data scatter observed in the individual studies. Emission results had CVs that were sometimes as high as 40%or more, but were typically on the order of 10-30%. The magnitude of this data scatter often precluded drawing definitive conclusions about the effects of cofiring on emission levels of the pollutants in question because the range of emission levels reported for firing coal only and cod-RDF mixtures at a given facility overlapped considerably. Some variables that were evident in the studies are the boiler size and design, firing method, RDF type (fluffor densified) and composition, coal composition, type of emission control device@), boiler load, and percentage RDF fired. Many of the studies were 778 Envlron. Sci. Technol., Vol. 23.No. 7,1989
conducted with minimal control of boiler operation parameters, especially boiler load and fuel blends. In many cases, boiler loads were dictated largely by on-site needs (e.g., electricity demand) of the facility at the time of firing. The number of tests performed (often 3 or fewer) under a given set of conditions was often inadequate for evaluating trends in view of the uncontrolled variables involved. In many of the studies, the entire sampling program was brief and the sampling matrix was not always well defined. For example, sampling of particulate emissions upstream of the cyclone or ESP was not always coordinated with stack or ESP outlet sampling. Furthermore, the effect of stabilization time on emission results has not been firmly established. In one study, the efficiencyof the ESP continued to degrade with increasing time of ccfiring (16). In another study, differences in stabilization time of the ESP were believed to be responsible for discrep ancies in particulate emission levels resulting from different sets of tests conducted under similar experimental conditions (19). Because of limited sampling statistics, mechanical problems unrelated to cofiring (such as abnormal performance of the boiler and particle control device), site-specific variations, and numerous uncontrolled variables, comparison of actual emission levels among various sites is not warranted. However, interstudy comparisons are valuable for identifying trends. Despite the factors mentioned above, some general conclusions can be drawn from the
emission results. These results are discussed below for each of the pollutants individually. The relatively large variations in ash and sulfur concentrations among the coals used at Erie, combined with uncertainties regarding the data accuracy, precluded drawing any realistic conclusions about the relative emission levels between coal and coal-RDF combustion for that study. Thus, no emission data from that study are reported in Tables 4 or 5. Similarly, since the SO, data were reported to be suspect and because no comparative (coal vs. coalRDF combustion) data were given for the NO, emissions at Madison (14), no entry for that facility was made in Table 4. In addition, data on emissions from the laboratory-scale boilers are excluded from Tables 4 and 5. Only average or typical changes in emission levels for a given set of conditions are presented, and these data do not necessarily reflect trends observed under all operating conditions. SO, emissions. The SO, emission levels decreased substantially during coal-RDF combustion relative to combusting coal only. This trend correlates well with the sulfur content reported in the fuel analyses. When emission results were expressed in ppm in the laboratory-scale study at Battelle Labs at Columbus, SOx reductions were evident for both types of RDF used, but were more prominent when cofiring the Americology RDF (from American Can Company) than when cofiring E c d I RDF (from Combustion Equipment Associates). A possible explanation may be that the Americology RDF had a lower sulfur content (23). How-
TABLE 4
Effect of cofiring on SO, and NO. emlssions Sae (relermce no.) Ames, IA (6, 7) (Units 5 & 6) Ames, IA (8) (Unit 7) Arlington, VA(27, 22) Baititnore. MD(W Columbus. OH (17) Haprstown, M D (26) Unit 11 Hage&own. MD (26) (Unit 2) Milwaukee Co.,WI (27) Rmhesler, NY (77) st.Louis, MO(16, 19) Stevenage, England (N) Wnghl-Panenw, Air Force Base, OH (28-30)
Boiler losd (%)
Refme-
derhred tuel(4cy
% Chsnsd
NO.
SO.
80
20
-3410-42
-1llo-52
80-100
20
-3210-42
-15
20-65 100 50-80
20-40 5
-26 -34
-39
20-30
-28
-a
34-55
20
-44 -49 t18 - 24
-7 -21
0
t11
35
30-55
20
35 M75 80-100 60-95 90-1
w
IO 8-14 10-20 50(wt)
- 17 t5 -33
23 -44 60 - 52 100 30 Unit 3) p i , 33) 40(wl) -9 75-95 Unit 4) (37, 32) *Basedon Blu mpul, unless mhewsa noted %crease ( + ) or decrease (-) as B resull of firing RDF with mal
NR"
t56 +12 -10 -4 NRO
-41
- 32 - 25
ever, when those emissions were expressed in terms of Ib/l@Btu, no decrease in SOx emissions was observed when the Eo-11 RDF was fired with
TABLE 5
Effect of cofiring refuse-derived fuel (RDF) with coal on particulate emissions
coal.
A possible reason for this discrepancy may be difficulties in determining the heat input rates for the laboratoryscale boiler. Other exceptions were the studies at St. Louis (18, 19), Milwaukee County 0,and some of the tests at Hagerstown OS),where little or no decrease in SO, emissions was observed during cofiring, even though the RDF had lower sulfur concentrations than the coal. The reasons for these anomalies are not clear The decrease in SO, concentrations resulting from cofiring RDF with coal is attributable largely to the lower sulfur content of the RDE In addition, the fly ash resulting from the addition of RDF to the coal has higher levels of alkaline components that may react with SO,, thereby decreasing SO,.-emissions (iij. NO. emissions. The NO. emissions tendd to decrease duringcofiring at most of the facilities. However, the magnitude of these decreases was often small, and no significant trends beyond the data scatter were observed in most cases. The NO, emissions also tended to decrease as a result of cofiring RDF with coal at Irvine (13) and Milwaukee (IS), even though no numerical data were reported. The slight average decrease in NO, emissions during cofiring is believed to be a result of a lower temperature in the combustion chamber, which may be related to the increased monnts of excess air used during cofiring and the higher moisture levels in the fuel mixture (7, 16). When firing with about 40% RDF in Unit 1 at Wright-Patterson AFB, the operators had trouble controlling the furnace temperamre, fuel distribution, and fuel-to-air ratios, resulting in erratic NO, emissions (28-30). These factors could contributeto the relatively high average NO, concentrations observed in those tests. Particulate emissions. Ash emissions are strongly influenced by sampling location, type of particle control device, efficiency of particle collectors, and boiler operation parameters. The effects of cocombustion on particulate emissions upstream and downstream of the particle control device, as well as on the efficiencies of particle collectors, are discussed below. Upstream of control device. Although relatively few data on particulate emission levels prior to the particle control device are available, these emissions generally tended to increase at Ames (6-8) and Baltimore (10) and appeared to decrease slightly or remain
-
Boiler load SI& (rete"
1%)
no.)
Ames, IA (6, 7) (Unit 5) Ames. IA (6,7) (Unit 6) Ames. IA (8) (Unit 7) Arlington, VA (21,22J Baltimore. MD (10) Columbus, OH (1 1) Dickerson. MD 1121 Hagerstown. M b & (Units 1 a 2) Madison. WI (14) Madison. WI (15) Milwaukee Co., WI (27) Rochester, NY (17) St. Louis, MO (18, 19) Slevenase. Enqland 1201 Wnght-ianerGn Air Force Base. OH (Unit 1) (28-30) (Unit 3)131. 33) (Unit 4) (31, 32)
H Change' U Slream SlscWESP % R D P - oPco11ec1w outlet
80
20
+ 19
110
60
20
+ 87
t162
80 100 20-65 100 50-80 NR
20 20 20-40 5 20-30 10-20 20
126 -5 NR
+ 76 + 26
30-55
60-90 NR 65-75
60 100 60-95 90-100 60
60 30 75-95
~
10-50 10-15 10 8-14 8-14 10-20 50 (wl)
23
+ 30
- 49
+TI
-10 NR NR
+ 46
NR NR
+ 17
- 32 NR NR
-66 - 21 -5
- 30 t25
-3
-5
t16
NR +7
+376
37 -3 -1 100 40 (dry wl.) - 10
-6 +to -i7
- 10
*Based on Blu input, unless OlherWiSe noted. %crease [ + 1 or decrease ( - ) as a result of firing ROF With mal. CSamplingwas performed after the mechanical ~ o l l e ~and t ~ before i the ESP if the bcilar had both tmes of emission ~ontioldevices.
essentially constant at St. Lonis (18, 19). Columbus ( I I ) , and Unit 4 at Wright-Patterson AFB (28-33). At Milwaukee County, the apparent decrease in particulate emissions at the ESP inlet was believed to be partially a result of the lower boiler loads used for the cofiring tests (27). The ash emission results for the various sites did not always correlate with the ash contents of the fuels. Downstream of control device. The effect of coal-RDF combustion on particulate emission levels at the stack (or outlet of the cyclone or ESP) varied. Increased emissions were noted at Ames ( 6 4 ,Stevenage PO),Columbus (11), St. Louis (18, 19), Madison (14). and Baltimore (IO), although the magnitude of the apparent increases varied among the sites and is probably not satistically significant in some cases. Although no numerical data were reported for the Milwaukee study, it was noted that stack particulate levels increased during cofiring (16). In some studies, the specific RDF used influenced particulate emission rates. At Batelle Labs at Columbus, particulate emissions appeared to increase when firing the densified Americology RDF with coal, but not when the densified Eco-I1 RDF was used, even though the two RDFs had about the same ash content (23). Those results may be related to the fact that,
prior to pelletizing, the Americology RDF was in fluff form whereas the Eo-11RDF was a coarse powder. Particulate emissions appeared to decrease at Dickerson (12), Hagerstown (26),Arlington (21, 22), Milwaukee County (23,and the tests using Units 3 and 4 at Wright-Patterson AFB (31, 32), whereas emission levels were either mixed or did not appear to be significantly affected by the RDF in the fuel admix for most of the other sites. In some studies, including the one performed at Hagerstown, the observed trends were not believed to be statistically significant at a 90% confidence interval. At Dickerson, air leaks were fixed and the ESP rapper was optimized between the baseline tests and the cofiring tests (12). This operational change would explain the substantially lower particulate emission levels observed during cofiring, even though the RDF had a higher ash content than the coal. Particulate emission levels in the Baltimore study were unusually high because of a faulty ESP (10). The emission of trace elements associated with the ash component of the effluents may increase in conjunction with increases in particulate emission rates resulting from cofiring RDF and coal. In addition, trace e1emer.t emission rates may increase during cocombustion due to the fact that the fly ash from coal-RDF combustion is enriched Envimn. Sci. Technol., Vol. 23, No. 7, 1989 779
in some trace elements relative to fly ash from burning coal only (14,17,27, 29, 30, 37). However, a detailed review of trace element emissions is beyond the scope of this paper. Collector efficiency. A change in the particulate emission levels from the stack could be related to a change in the efficiency of the particle control device rather than to changes in emission rates from the boiler. Collector efficiencies tended to decrease as a result of cofiring RDF with coal at Milwaukee (16) and in Boiler No. 7 at Ames (8). Either no significant changes in collector efficiency or inconclusive results were generally observed at Wright-Patterson AFB (28-33),Milwaukee County (27), Erie (24), St. Louis (19), and Units 5 and 6 at Ames (6). For an ESP, fly ash resistivity is one of the most important fly ash characteristics which affects collection efficiency; it is influenced largely by the fuel composition, combustion technique, and gas temperature in the collection zone (I 1). An apparent increase in fly ash resistivity during cofiring was noted for Unit 3 at Wright-Patterson AFB (31), Hagerstown (26), and Erie (24);fly ash resistivity remained essentially constant or increased only slightly at St. Louis (18),Columbus (II), and Unit 4 at Wright-Patterson AFB (32). No decrease in resistivity was noted in any of the studies. Depending on the resistivity of the coal fly ash, an increase in resistivity from cofiring RDF with coal could either increase or decrease ESP efficiency (24). Collector efficiencies can also be affected by the flue gas volumetric flow, grain loading, and particle size (6). At Ames, an increase in the air flow rate through the boiler was noted for both the tangentially fired and stoker-fired units (6, 8). This increased air flow was believed to contribute to the increase in particulate levels prior to the ESP for the tests with Unit 7 at an 80% boiler load (8). Based on all of the studies as a whole, the effect of cocombustion on fly ash particle size is not clear due to limited, inconclusive, or conflicting data. At Ames, differing results were sometimes obtained with different samplers (8),indicating difficulty in obtaining representative samples or the need for improved sampling systems or procedures. This is complicated by additional difficulties in interpreting the sizing data. For example, a decrease in the size of fly ash from a cyclone may indicate either that cocombustion resulted in smaller particle sizes or merely that smaller particles were collected more efficiently because of changes in flue gas volumetric flows or physical properties of the ash. 780 Environ. Sci. Technol., Vol. 23,No. 7,1989
summing up When RDF was used as a fuel supplement, SO, emissions tended to decrease relative to combustion of coal alone. This reduction is primarily a result of the relatively low sulfur content of the RDF as compared to most of the coals. By combusting RDF with coal, some facilities may be able to burn coals that are relatively high in sulfur without exceeding emission regulations. Altered combustion conditions apparently tended to decrease average NO, emission levels slightly, although the magnitude of the decrease was usually not significant in view of the data scatter observed when firing coal and coal-RDF mixtures. Thus, cocombustion does not appear to have any significant implications with respect to NO, emission regulations. The effect of coal-RDF combustion on particulate emissions is difficult to quantify in view of the mixed results. However, in studies where the control of boiler load and percentage RDF was relatively stringent, the ash emissions appeared to increase during cocombustion. In order to more accurately assess the effects of coal-RDF combustion on particulate emissions, additional studies are needed in which some of the major variables, including boiler load, percentage RDF fired, and coal composition, are more stringently controlled. The emissions discussed in this paper constitute only a fraction of the pollutants of interest. Emissions of other pollutants, both orgsulic and inorganic, must be examined before a thorough assessment of the environmental effects of burning RDF with coal can be made.
References (1) “Technology Review: Producing and Burning d-RDF”; NCRR Bulletin, National Center for Resource Recovery, Inc.: Washington, DC, December 1980, 10(4), pp. 86-90. (2) McGowin, C. R. Chem. Eng. Prog. 1985, 81 57. (3) Stillman, G. In Proceedings: Municipal Solid Waste as a Utility Fuel; EPRI WS79-225; Electric Power Research Institute: Palo Alto, CA, October 1980; pp. 15-1-15-44. (4) “Refuse-Derived Fuel”; NCRR Briefs; National Center for Resource Recovery, Inc.: Washington, DC, May 1980. ( 5 ) O’Toole, J. J.; et al. “Health and Environmental Effects of Refuse Derived Fuel (RDF) Production and RDF/Coal Co-Firing Technologies”; Ames Laboratory: Ames, IA, October 1982. (6) Hall, J. L. et al. “Co-Firing of Solid Wastes and Coal at Ames: Stoker Boilers”: U.S. Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, September 1985; EPAl600/2-85/114. (7) Hall, J . L. et al. “Evaluation of the Ames Solid Waste Recovery System, Part 111: Environmental Emissions of the Stoker Fired Steam Generators”; U.S. Environmental Protection Agency. U.S. Govern-
ment Printing Office: Washington, DC, October 1979; EPA-60017-79-222. Hall, J. L. et al. In Proceedings of the 9th National Waste Processing Conference; American Society of Mechanical Engineers, New York, 1980, pp. 497-521. Bourquin, R. H., Jr. In Resource Recovery from Solid Wastes; Sengupta, S.; Wong, K.-F. V., Eds.; Pergamon: Elmsford, NY, 1982; pp. 359-66. ‘To-Combustion of Refuse Derived Fuel and Coal in a Cyclone Furnace at the Baltimore Gas and Electric Company, C. P. Crane Station”; Prepared for the U.S. Department of Energy under Contract No. DE-FG01-80CS24320, March 1982; DOE/CSI24320-1. Vaughan, D. A. et al. “Environmental Effects of Utilizing Solid Waste as a Supplementary Power Plant Fuel”; U.S. Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, November 1984; EPA-600/284- 178. Ulman, R. R. In Proceedings: Municipal Solid Waste as a Utility Fuel; Electric Power Research Institute: Palo Alto, CA, November 1986; EPRI CS-4900-SR, pp. 11-1-11-17. Arand, J. K.; Muzio, L. J.; Barbour, R. L. “Emission Assessment of Refuse-Derived Fuel Combustion: Suspension Firing”; U.S. Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, September 1985; EPA-600/2-85/ 117. Vetter, R. J. et al. “Test Firing RefuseDerived Fuel in an Industrial Coal-Fired Boiler”; U.S. Environmental Protection Agency. U S . Government Printing Office: Washington, DC, September 1985; EPA/600/2-85/113. Barlow, K. M. In Proceedings: Municipal Solid Waste as a Utility Fuel; Electric Power Research Institute: Palo Alto, CA, November 1982; EPRI-(3-2723, pp. 4- 1-4- 14. Petersdorf, R. J.; Sansone, S . In Proceedings: Municipal Solid Waste as a Utility Fuel; Electric Power Research Institute: Palo Alto, CA, October 1980; EPRI WS-79-225, pp. 6-1-6-22. Stine, G.; Burton, R. M., Jr. In Proc. 77th APCA Annu. Meet. Paper no. 8437.4; Air Pollution Control Association, Pittsburgh, PA, 1984. Gorman, l? G. et al. “St. Louis Demonstration Final Report: Power Plant Equipment, Facilities, and Environmental Evaluations”; U S . Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, December 1977; EPA-600/2-77-155b. (19) Shannon, L. J. et al. “St. Louis/Union Electric Refuse Firing Demonstration Air Pollution Test Report”; U.S. Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, August 1974; EPA-65012-74-073. (20) Davis, B. J.; Clayton, €? “Combustion of Waste Derived Fuel”; Warren Spring Laboratory: Stevenage, England, 1984; LR-491(AP)-M. (21) Campbell, J. “Final Test Report: Demonstration D-RDF Burn GSA Pentagon Power Plant”; Prepared for the U.S. Department of Energy under Contract No. DE-AC01-76CS20167, October 1981; DOE/CS/20167-7. (22) Golembiewski, M. A. “Environmental Assessment of A Waste-To-Energy Process: Boiler Co-Fired with Coal and Densified Refuse-Derived Fuel”; Draft final report to the U.S. Environmental Protection Agency under Contract No. 68-022166; Midwest Research Institute: Kansas City, MO, August 1979. (231I Rising, B. W.; Allen, J. M. “Emissions Assessment for Refuse-Derived Fuel Combustion”; U.S.Environmental Protection Agency. U.S. Government Print-
ing Ofice: Washington, DC. September 1985; EPA-60012-85-II6. (24) Kleinhenz. N. 1. "Coal:dRDF (Densified Refuse Derived Fuel) Demonstration Test in an Industrial Spreader Stoker Boiler. Use of Coal:dRDF Blends in StokerFired Boilers. Volume I"; US. Environmental Protection Aeencv. U S . Government Printing Ofi;; Washington. DC. September 1981. F.PA-MW)IZ-81-l83 (25) Klcmhenz. N . 1. 'Coal JRDF (Densified Refuse Derived Fucl, D m " s t r a u u n Test an I~.IUWJI spreader Stoker ~odrr. Use of Coal JRDF Bleno\ m StokerFired Boilcrr. Appendices A. R. C . and D . Volume 11". U S Environmental Prnteelion Agency. U.S. Gmcrnment Printing ONm Washington. DC. Srplemhcr 19x1. I'PA-6OOl2.X 1. IR4. (26) D&r. G H , R i p . H G.: Riley. B T.. J r "A Field Test Crmg CoaI.dRDF Blends tn SpreoJer Stoker-Fired Rode,~": U.S. Environmental Proleition Agency. U.S. Gwernment Printing Office: Washington. DC. Augur1 1980: EPA.6001240-095. (27) Hecklinger. R S , Rrhm. F. R. "CoallD. RDF I D e n ~ t i e dRefuv Derived Furl) Cc,-Firmg Project. Milwaukee Count). W~sconsin". U S Enrironmental Proteclion Agene). U S Gorernment Printing OfFlcc. Washington. DC. November 1985: EPAI600 2-851136. (28) Jockson. J W. "A Biocnrironmcntal Study of Emissions from Refuse Derived Fuel": Environmcnlal Health Labora. tor). McClcllan Air Force Rs,e. CA. lanuary 1976. Report No. 76M-2 (29) Jackson. 1 . W : Ledbetter. 1. 0. I . Environ. E t . Heolrh. Part A. 1977, A12l9). 465. (30) Rolsten, R. E; Glaspell. L.; Waltz. J. r! Nuel. Teehnol. 1977.36, 314. (31) Joensen. A. W. et al. I n Proc. 11th Nor/. Worte Process. ConJ; American Society of Mechanical Engineers: New York.
604.
Acknowledgment Ames Laboratory i s operated for the US. Department of Energy b y I o w a Slate University under Contract W-7405-Eng-82. T h i s w o r k was supported, in part, b y the Assistant Secretary for Fossil Energy through the Morgantown Energy Technology Center and, in part. b y the I o w a State Water Resources Research Institute. This article has been reviewed for suitability as an ES&T feature b y Rodney R. Ruch. Illinois State Geological Survey, Urbana, IL 61081: and by D. R. Taylor, A c u r e x C o r p . , M o u n t a i n V i e w , CA
94039.
Glenn A. Norton is an associate chemist wirh Ames Laboratory at Iowa Store Universiry where he has been working for the past 10 years. He has been agilioted with the Fossil Energy Program for rhe lastfive years and was previously ajiliated with rhe Environmental Program. He is also a candidate for an M.S. degree in Water Resources in the Department of Civil En&+ neering at Iowa State Universiry. His research interests include the environmental effects of burning refuse-derived fuel with coal, fate of trace elements during preparation and combustion of fossilfuels. and chemical and mineralogical characterization of coal andpy ash. Audrey D. Lcvine is an assistant professor in the Department of Civil Engineering at Iowa State. She holds a B.A. from Bates College in Lewiston. Maine; an M.S. from Wane Universiry in New Orleans. Louisim a ; and a Ph.D. in Civil Engineering from the Universiry of California at Davis. Her research interests include characrerizatian of waste materials; recovery of energy, materials, and byproducts from waste materials; and particle and molecular size effects on treatment process effectiveness.
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