Chemical Characterization of Ash Produced during Combustion of Refuse-Derived Fuel with Coal Glenn A. Norton," Kenneth L. Malaby, and Edward L. DeKalb Ames Laboratory, I o w a State University, Ames, I o w a 5001 1
Samples of bottom ash, electrostatic precipitator (ESP) fly ash, and suspended stack particulate matter were collected when coal alone and mixtures of coal and refuse-derived fuel (RDF) were being combusted in a 35-MW boiler at a municipal power plant. Analytical data obtained by X-ray fluorescence, neutron activation, and atomic absorption analyses on these samples indicated that levels of Br, Cr, Ga, Na, Ni, Pb, Sb, Sn, Ti, and Zn in the bottom ash were 2-5 times higher when RDF was fired with coal. Coal/RDF combustion appeared to increase average levels of As, Br, Cr, Ga, Ge, Pb, Sn, and Zn in the ESP fly ash, although increases of a factor of 2 or more generally were not observed. In the stack fly ash, the most prominent increases in trace element concentrations resulting from cocombustion were for Br, Cd, Pb, Sb, Se, Sn, and Zn. The magnitude of the increases in trace element levels in the ash streams does not appear to be sufficient to warrant any greater environmental concern for ash from firing coal/RDF mixtures than for burning coal only. Introduction
In recent years, mounting interest in environmentally and economically acceptable alternatives to landfill disposal of municipal solid waste (MSW) has become evident. The energy content of the MSW produced annually in the United States is equivalent to about 50 million tons of coal (I). Thus, MSW is a prevalent resource with a significant potential for energy recovery. One approach to recovering energy from MSW is to process it into a refuse-derived fuel (RDF) for use as a supplement to coal in coal-fired boilers. Combustion of coal/RDF mixtures has been tested in boilers at various facilities (2-10),and reviews of some of those studies are available in the literature (1,11-13).In view of numerous variations between studies, including type of RDF fired, boiler size and design, firing parameters, and fuel blends, additional studies on coal/RDF combustion are warranted. Two major ash streams, bottom ash and fly ash, are produced from coal combustion in a conventional coal-fired boiler. The fly ash fraction is entrained in the flue gas and carried in suspension from the boiler. Although the majority of the fly ash is retained by a particIe control device, the smaller and more respirable particles pass through the collector and are emitted to the environment through the stack system. This latter portion will be referred to as stack fly ash. One of the areas of potential environmental concern associated with firing RDF with coal is the trace element content of the ash streams produced. In view of the po0013-936X/88/0922-1279$01.50/0
tential environmental and technological impacts of many of these elements (I4-I6),the trace element content of ash streams from combusting coal only has been addressed extensively in the literature. Many potentially toxic elements, such as As, Cd, Cr, Cu, Ga, Ni, Pb, Sb, Se, Sn, V, and Zn, have been found to be concentrated in stack fly ash from coal combustion (17-21).Because many of these elements are concentrated preferentially on the particle surfaces (229, surface concentrations can be a factor of 10 or more higher than the bulk concentrations (23).Thus, the potential environmental effects of the ash can be substantially underestimated when based on conventional bulk analyses. The trace element content of bottom ash and collector fly ash is important because of the leaching potential of these ash streams in a landfill or settling pond (24-28). This is particularly important since several trace elements in fly ash have been reported to be more readily leached with water when the ash was derived from cocombustion (7). Since RDF is known to be enriched in elements such as Cd, Cr, Cu, Hg, Mn, Pb, and Zn relative to coal (2-5), the toxicity of the ash formed when combusting coal/RDF mixtures may be enhanced relative to ash derived from combusting coal only. However, few studies dealing with the trace element content of the bottom ash and fly ash streams from coal/RDF combustion have been performed. In this study, X-ray fluorescence (XRF), flame atomic absorption (FAA),and neutron activation analysis (NAA) were used to characterize bottom ash, electrostatic precipitator (ESP) fly ash, and stack fly ash produced during coal and coal/RDF combustion. Results of similar analyses on sized stack fly ash samples were reported previously (29). For this paper, weighted averages were calculated for the elemental concentrations in those sized stack fly ash samples to obtain concentrations in the stack fly ash as a whole. These values were compared to elemental concentrations in the bottom ash and ESP fly ash. The primary purpose of this study was to observe the effects of adding RDF to coal on the trace element content of the ash streams produced during combustion. Although the emphasis was placed on trace elements, several major and minor constituents were also determined. These data were used to assess more fully the potential toxicity of ash from coal/RDF combustion relative to that produced from firing coal only. Experimental Section
@ 1988 American Chemical Society
Power Plant Description. At Ames, IA, a nominal 150 Environ. Sci. Technol., Vol. 22, No. 11, 1988
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Table I. Selected Test Parameters during Collection of the Six Sample Sets
boiler load (MW) heat input, lo8 Btu/ h coal blend,
%CO / %IA RDF content
1
2
32 400
30 376
test no. 3 4
5
6
33 414
29 362
35 438
28 352
70130 75/25 55/45 45/55 45/55 45/55
0 of fuel, % stack temperature, 168
0
0
20
20
20
166
160
182
185
182
O C
ton/day processing plant produces a refuse-derived fuel (RDF) through ferrous and non-ferrous metal recovery, shredding, and air density separation of raw municipal solid waste. The RDF is then transferred pneumatically to firing ports in the boilers at the municipal power plant. The unit used for this study is a retrofitted 35-MW tangentially fired boiler with a cold-side ESP for particulate emission control. The rated heat input at the nominal boiler capacity is about 435 X lo6 Btu/h. The unit normally burns 15-20% RDF, based on the percent of the total Btus in the fuel admix supplied by the RDF. When firing 20% RDF a t 35 MW, RDF feed rates are about 7 ton/h and coal feed rates are about 18 ton/h. Sample Collection and Handling. .In this study, prominent rather than subtle differences in elemental concentrations between ash from coal and coal/RDF combustion were sought. Consequently, boiler loads, RDF feed rates, and coal blend ratios were not rigidly controlled, and sampling was not designed for performing elemental mass balances. A total of six sample sets were collected, three while burning coal only and three while burning coal/RDF mixtures. Each set of samples included bottom ash, sluice water, ESP fly ash, and stack fly ash. A summary of various boiler, fuel, and sampling parameters for each of the six tests is shown in Table I. During a previous year of testing, coal and RDF samples were also collected. Although those samples were not collected in conjunction with the ash samples discussed in this paper, analytical data for those fuel samples are included here since the general elemental enrichments in the RDF relative to coal are indicative of the RDF and coal fired at the City of Ames Power Plant as a whole. Samples were collected at 30-min intervals over a testing period of several hours and were later combined to form a composite sample of each fuel component for each test. A total of 18 composite samples of the coal and RDF were collected. Coal samples were collected from a feed conveyor in the power plant and were prepared for analysis by grinding in a hammer mill. Moisture levels were determined in these samples by ASTM D-3173 (30),and trace element data on the coals were then converted to a dry basis with these moisture values. The RDF samples were collected in a storage bin where RDF from a drag conveyer dropped into the pneumatic transport system. The RDF samples were subsequently dried in an oven at 110 "C and shredded in a Wiley Mill prior to analysis. Bottom ash was sluiced to a settling pond with untreated well water. To help ensure sample validity, bottom ash was sluiced from the boiler just prior to each test after the desired firing parameters and fuel blends had been achieved. Because there was no satisfactory location for collecting dry bottom ash, samples of the ash/water slurry were collected by placing a 1-gallon plastic bucket in the slurry as it discharged from the sluicing pipe. Each 11280
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gallon bucket of sample was emptied into a plastic-lined can, and sampling continued until about 15 gallons of slurry were collected. This procedure yielded about 100 g (dry) of ash for each of the six tests. To minimize possible leaching of the ash, the composite slurry sample was pressure-filtered in the field by using compressed nitrogen gas and a Nuclepore 420800 pressure filter holder. Millipore type AP prefilters were used in conjunction with Millipore type HA 0.45-pm filters. For each test, portions of the filtrate were acidified to a pH of
