Environ. Sci. Technol. 2000, 34, 2501-2506
Factors Determining the Element Behavior in Municipal Solid Waste Incinerators. 1. Field Studies HASAN BELEVI* AND HERMANN MOENCH Swiss Federal Institute for Environmental Science and Technology, EAWAG, 8600 Dubendorf, Switzerland
This paper investigates the factors determining element behavior in municipal solid waste incinerators. The method is based primarily on field measurements in full-scale incinerators. Material flow analyses are carried out in an incinerator. The main focus is placed on the processes in the furnace. The following master variables determine element transfer behavior to the raw gas in the furnace: (i) occurrence and distribution of the elements in the input waste; (ii) temperature, redox conditions, and content of chlorine and of reaction partners other than oxygen and chlorine in the furnace bed; and (iii) residence time and mixing conditions in the furnace bed. Twenty-nine elements are divided into two groups with respect to their transfer behavior to the raw gas and to the bottom ash in the furnace. The results indicate that the elements Si, Fe, Co, Cr, Mn, Ni, P, Al, Ca, Mg, Na, Ba, Li, Ti, and K are transferred mainly to the raw gas by entrainment. Occurrence and distribution of these elements in the input waste determine primarily their transfer coefficients. Evaporation is the main transfer process for the remaining investigated elements to the raw gas. In addition to the occurrence and distribution of the elements in the input waste, physical and chemical conditions as well as kinetics are the main factors responsible for the different transfer behaviors between the elements F, Cu, Mo, Pb, Sn, Zn, Br, Sb, C, S, Cl, As, Cd, and Hg.
Introduction Worldwide, less than 15% of the generated municipal solid waste (MSW) is currently incinerated. Although about 30% of the MSW is incinerated in industrialized nations, this percent is increasing rapidly as a result of the recently implemented strategies and guidelines. Most incinerators meet the prescribed exhaust gas quality standards, as the technology has consistently improved over the past 30 years. However, the solid residues produced by incinerators do not fully meet the stipulated requirements. Although bottom ash can be defined as a rocklike material as far as the major elements are considered, most of the minor and trace elements are highly enriched (1). Since bottom ash is also a reactive material, several processes in monofills could lead to leachates that are environmentally not compatible in the long term (2, 3). Furthermore, although gas cleaning residues contain elements such as Zn and Pb in similar concentrations as ores, an improved separation of potentially harmful elements from bottom ash and their concentration in gas cleaning residues would make their recovery more attractive. * Corresponding author phone: +41-1-8235514; fax: +41-18235399; e-mail:
[email protected]. 10.1021/es991078m CCC: $19.00 Published on Web 05/12/2000
2000 American Chemical Society
Consequently, new plants have started to be designed in order to achieve “tailor-made” product qualities. This is only possible, if the factors determining the element behavior in incinerators are understood. The influence of varying input qualities and of physical and chemical conditions in incinerators on the quality of the incinerator products is not yet well-known. The first investigations on element behavior in MSW incinerators were conducted in the late 1970s and the beginning of the 1980s (4, 5). Element transfer to the incinerator products has been determined, and the processes determining gaseous emissions have also been dealt with in numerous publications (6, 7). In recent years, additional efforts have been made to investigate the behavior of heavy metals in incinerators (8, 9). These thermodynamic studies are theoretical but provide very valuable information on the stability of species under incineration conditions. Barton et al. investigated the fate of metals in a pilot scale rotary kiln incinerator (10). This paper presents a method to investigate the factors determining element behavior in incinerators. It characterizes the waste input and provides an insight into several processes in incinerators. The following specific questions are formulated: (i) How do major, minor, and trace elements behave in a municipal sold waste incinerator? (ii) What are the factors determining element behavior? The hypotheses based on field experiments in a full-scale system (MSWI) are presented in this paper. The hypotheses based on additional laboratory experiments are presented in a companion paper, also published in this issue (11).
Experimental Section Incinerator. The field experiments were conducted in the MSW incinerator of St. Gallen, Switzerland. Its grate-type furnace is designed to incinerate 5.2 tons of waste per h, and the furnace room-temperature averages between 820 °C and 880 °C. Solid residues from the combustion process, i.e., bottom ash, fall into a water bath where they are cooled before being conveyed to the ash bunker. The hot gases (raw gas) from combustion are conveyed to the boiler. The boiler converts the energy of the hot gases into steam. The boiler ash is mixed with gas cleaning residues. However, during the field experiments described hereafter, the boiler ash is collected separately. The gases leaving the boiler pass through a spray dryer/ absorber where the wet scrubber liquor, neutralized with lime, is injected into the gas stream. Part of the dry powder is removed at this stage. An electrostatic precipitator removes most of the remaining particulate. The residues from the electrostatic precipitator are mixed with the residues from the spray dryer. The gas leaving the electrostatic precipitator is conveyed to the wet scrubber and subsequently released through the stack into the atmosphere. Full-Scale Experiments, Sampling, and Sample Preparation. The incinerator in St. Gallen was fed with two different types of waste: on the first day with waste of residential origin (household waste) and on the second day with waste of commercial and residential origin (mixed waste). The material flow analyses were conducted for both waste types. Input and output quantities were measured for a period of 8-10 h. The waste input was not analyzed but merely weighed by a calibrated crane balance. Sampling of bottom ash took 10 h and was carried out in 1-h sampling periods (totally 10 samples). All the bottom ash was collected for an hour, loaded onto a truck, and weighed. The bottom ash was then unloaded onto a screen. The residues over 100 mm were defined as VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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“coarse goods” and collected in a skip over the entire sampling period and weighed at the end of the sampling period. They make up about 4% of the total bulk of bottom ash. Assessment of element concentrations in the coarse goods is explained elsewhere (12). The ash passing the screen was mixed thoroughly with shovels, and a 2-4 kg sample was taken. Boiler ash samples were taken before they were mixed with gas cleaning residues. A device was installed into the pipe. The entire boiler ash was collected in 200-L drums at hourly intervals for a period of 10 h. After weighing the ash, it was thoroughly mixed, and an approximately 2-kg sample was taken. The gas cleaning residues were collected in large plastic bags which were weighed continuously. The mass of the material collected was registered every 30 min, and a 2-kg sample was taken. Eighteen samples were taken over a period of 9 h. Lime slurry and water consumption were registered regularly for 10 h. Since caustic soda consumption could not be registered, its annual mean consumption was used for mass balance calculations. Lime slurry and water samples were also taken. The mass flow and analysis of exhaust gas were carried out in accordance with the guidelines of the Swiss Agency for the Environment, Forests and Landscape (SAEFL) (13). All solid samples were dried at 105 °C for 24 h. About 500 mL of each aqueous sample was acidified with HNO3 to a pH of about 2. The residue and all other samples were maintained at 4 °C. Bottom ash samples were ground for 4 h in a steel ball mill and sieved through a 0.5 mm screen. The amount of oversize material (sieving residues) totaled between 4 wt % and 11 wt %. The entire oversize material of eight bottom ash samples was chosen randomly to assess the concentrations in sieving residues. The size of sieving residues does not allow to digest them by conventional methods. A reasonably priced method of acceptable precision and accuracy was developed to determine element concentrations (a detailed description is given in ref 12). The material passing the screen was used as fine ground laboratory samples for element analyses. The same procedure was used for boiler ash samples. Gas cleaning residue samples were ground in a hammer mill. All the resulting powdered boiler ash and gas cleaning dust completely passed the 0.5 mm screen and were used for element analysis. Analytical Techniques. Depending on the element to be analyzed, different methods were used for sample dissolution: digestion in aqua regia, digestion in a mixture of hydrofluoric, hydrochloric, and nitric acid, digestion according to Wurzschmitt (14), digestion in hot sodium hydroxide, and extraction with deionized water. Several metal concentrations were measured by atomic absorption spectrometry (Varian AA 10/20 or Perkin-Elmer 306/307). Depending on the element to be determined, flame or graphite furnace or cold vapor or hydride generation atomic absorption spectrometry was used. Inductively coupled plasma atomic emission spectrometry (Spectroflame) was applied for multielement analysis. Several anions were measured by ion chromatography with Sykam. Fluoride and bromide concentrations were determined potentiometrically by ion selective electrodes. Phosphorus concentration was determined by flow injection analysis using the automated molybdenum blue colorimetric method. The molybdenum concentration was determined by polarography according to the method suggested by Wunderli (15). The polarographic measurements were conducted with the polarograph Metrohm VA-Processor 693/VA-Stand 694. Na, Mg, Al, P, Si, K, Ca, Ti, and Mn concentrations were determined by X-ray fluorescence spectrometry. For this purpose, cast glass specimens are produced. The measurements were performed with the X-ray fluorescence spec2502
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trometer ARL 8410. Spiked samples and international certified reference materials were used for the calibration. Total carbon, hydrogen, nitrogen, and sulfur were determined simultaneously with the CNS-Analyzer of Carlo Erba CNS 1500. The sample was acidified, and the produced carbon dioxide was analyzed by coulometry (Coulometrics Inc.) to determine the carbonate carbon content (total inorganic carbon TIC). Calculations of Mass and Element Flows as well as of Transfer Coefficients. To avoid misunderstandings, the term “mass flow” is herewith used for mass flow of goods, and the term “element flow” is used to designate the mass flow of elements irrespective of their speciation. Element flows are calculated by simply multiplying mass flows with element concentrations. Element concentrations in the waste input were, however, not measured. They were calculated by performing mass balances for each element and assuming steady-state conditions. Transfer coefficients are defined by the following equation
ki,j )
Ai,j Ai,w
where ki,j corresponds to the transfer coefficient of the total mass or of the element i to the output j, Ai,j corresponds to the total mass or element flow of the element i through the output j, and Ai,W corresponds to the total mass or element flow of the element i through the waste input. For bottom ash j ≡ BMA, for boiler ash j ≡ BRA, for gas cleaning residues j ≡ GCR, for the raw gas after the boiler (gas cleaning devices) j ≡ CD, and for the exhaust gas j ≡ GAS. Transfer coefficients describe the distribution of the input mass among the various products. A high transfer coefficient of an element to the bottom ash indicates for example that this element is mainly transferred to the bottom ash. A high transfer coefficient of an element to the gas cleaning residues indicates that this element is volatilized in the furnace and condensed in the gas cleaning devices.
Results and Discussion Daily Mean Concentrations in the Municipal Solid Waste. Figure 1 illustrates element concentrations in the household and in the mixed waste. Besides water, organics show the highest concentrations. The mixed waste reveals higher hydrogen and carbon concentrations than household waste. The carbon to phosphorus ratio is about 800:1 for household waste and about 1400:1 for mixed waste. Since phosphorus is an adequate indicator element for kitchen and garden waste, the household waste appears to contain more kitchen and garden waste than the mixed waste. Furthermore, the carbon to oxygen and hydrogen-to-oxygen ratios are higher in the mixed waste than in the household waste. This indicates that the plastic content in the mixed waste is higher than in the household waste. The water content of the mixed waste is also lower. The lower heating values Hu are assessed through the energy production data. They total 10.5 ( 1.5 MJ/kg for household waste and 12.5 ( 1.5 MJ/kg for mixed waste. The higher carbon and lower water contents of the mixed waste account for its higher energy content. The sum of contents of metals and metalloids Al, Ca, Fe, K, Mg, Na, and Si is about 11-12 wt %. The nonmetals P, S, Br, Cl, and F make up less than 1 wt %, and minor and trace elements make up less than 1 wt %. As, Co, Hg, Li, and Mo show the lowest concentrations among the measured elements (e10 mg/kg). Daily Mean Element Concentrations in the Incineration Products. Table 1 contains the daily mean element concentrations in the solid residues from the incineration of household and mixed waste. Al, Ca, Fe, K, Mg, Na, and Si are
FIGURE 1. Daily mean concentrations in the household waste and mixed waste; % corresponds to wt %. the major elements whose concentrations are equal or higher than 1% in the bottom ash. The carbon concentration is also higher than 1%. Chlorine shows the highest concentration among the halogens. The concentration range of the other elements ranges from less than 0.0002 g/kg (Hg) to 7 g/kg (Ti). As mentioned in the Experimental Section, element concentrations in the various fractions of bottom ash are determined separately. The mean concentrations of all elements except Fe, Co, Cr, Cu, Mn, Mo, Ni, Pb, Sn, and Zn in the sieving residues (ci,SR) are lower than or equal to those in the fine ground laboratory sample (ci,FG). About 15% Pb, about 20% Co and Mn, and about 28% Zn exist in the sieving residues and coarse goods. About 27-32% Cr, 24-34% Sn, 30-36% Mo, 43-47% Fe, 47% Cu, and 60-67% Ni come from the sieving residues and coarse goods. Element concentrations in the fine ground samples, sieving residues, and coarse goods are differentiated elsewhere (12). In addition to Al, Ca, Fe,K, Mg, Na, and Si, the element concentrations of C, S, Cl, Pb, Ti, and Zn in boiler ash are equal or higher than 1%. The concentration of the remaining elements ranges from less than 0.0015 g/kg (Hg) to 6 g/kg (P). In the gas cleaning residues, Al, Ca, Fe, K, Mg, Na, Si, C, S, Cl, Pb, and Zn concentrations are equal or higher than 1%. The concentration of the remaining elements ranges from 0.01 g/kg (Co) to 7 g/kg (Ti). Daily Mean Element Concentrations in the Exhaust Gas. The daily mean carbon concentrations in the exhaust gas ranges from 8.0 vol % to 8.4 vol %, the oxygen concentration from 10.9 vol % to 11.4 vol %, the water content of the wet gas from 19.2 vol % to 19.3 vol %, the sulfur concentration from 19 ( 6 to 30 ( 11 mg/Nm3 dry, and the mercury concentration from 0.14 ( 0.13 to 0.16 ( 0.15 mg/Nm3 dry for incineration of household and mixed waste, respectively. Dust concentrations are very low due to the very efficient gas cleaning system (7.4 ( 2.4 and 4.8 ( 3.2 mg/Nm3 dry). Therefore, the concentrations of the remaining elements, which are mostly particle-bound, are not measured but estimated on the basis of the measured dust concentrations and literature values.
FIGURE 2. Transfer coefficients during incineration of household waste and mixed waste. All values are expressed in percent: sw, solid waste; bma, bottom ash; bra, boiler ash; gcr, gas cleaning residues; and gas, exhaust gas. Transfer Coefficients. Figure 2 illustrates mass and element transfer coefficients in percent for the incineration of household and mixed waste. About 78 wt % household waste mass and about 79 wt % mixed waste mass are transferred to the exhaust gas. Each ton of household waste produces 200 kg bottom ash dry matter, 4 kg boiler ash, and VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Daily Mean Element Concentrations in the Incinerator Residuesa concn [g/kg dry matter] bottom ash element TC TIC TOC N P S Br Cl F
As Ba Cd Co Cr Cu Hg Li Mn Mo Ni Pb Sb Sn Ti Zn TC: total carbon
mixed waste
boiler ash household waste
mixed waste
gas cleaning residues household waste
mixed waste
20 ( 2 4.3 ( 0.8 15.6 ( 1.8 0.50 ( 0.14 4.3 ( 0.4 1.7 ( 0.5
19 ( 1 7.0 ( 0.8 12.0 ( 1.3 0.38 ( 0.09 3.2 ( 0.3 1.3 ( 0.4
Nonmetals 45 ( 9 5.3 ( 0.7 40 ( 10 0.59 ( 0.15 6.1 ( 0.7 32 ( 8
28 ( 8 4.2 ( 1.0 24 ( 9 0.36 ( 0.09 5.1 ( 0.6 62 ( 15
17.9 ( 1.6 1.7 ( 0.2 16.2 ( 1.4 0.44 ( 0.09 3.6 ( 0.4 30 ( 3
0.09 ( 0.02 3.4 ( 0.5 0.61 ( 0.27
0.08 ( 0.02 3.4 ( 0.5 0.33 ( 0.11
Halogens 0.31 ( 0.08 23 ( 3 1.4 ( 0.5
0.39 ( 0.12 26 ( 4 2.2 ( 0.8
3.6 ( 0.8 259 ( 20 3.3 ( 1.0
5.5 ( 2.7 282 ( 18 5.1 ( 1.5
48 ( 6 184 ( 19 7.6 ( 0.6 29 ( 3 8.2 ( 0.5 40 ( 4 53 ( 7
33 ( 4 175 ( 18 9.6 ( 0.8 29 ( 3 7.1 ( 0.4 38 ( 4 40 ( 6
58 ( 7 107 ( 17 158 ( 19 8.7 ( 1.2 15 ( 1 22 ( 4 184 ( 15
Al Ca Fe K Mg Na Si
a
household waste
0.010 ( 0.002 1.7 ( 0.3 0.006 ( 0.003 0.017 ( 0.003 1.5 ( 0.6 4.1 ( 1.4 2000 3.3 ( 0.2
>1300 3.8 ( 0.3
C P
a
ki,CD/ki,BRA [-]
household
9 ( 44 67 ( 7
S
Halogens F
household
mixed
7.2 ( 2.0
3.6 ( 0.9
15 ( 3
14 ( 3
Al Ca Fe K
3.0 ( 0.2 3.6 ( 0.3 3.0 ( 0.3 7.8 ( 0.8
Metals and Metalloids 2.9 ( 0.4 Mg 2.7 ( 0.4 Na 2.1 ( 0.2 Si 5.7 ( 0.5
3.4 ( 0.1 4.6 ( 1.2 2.5 ( 0.3
As Ba Cd Co Cr Cu Hg Li
10.6 ( 1.4 6.1 ( 1.5 18 ( 4 5.1 ( 0.8 3.9 ( 0.3 6.4 ( 0.7 >180 3.7 ( 0.4
Minor and Trace Elements 9.3 ( 1.0 Mn 4.2 ( 0.7 Mo 12 ( 2 Ni 4.5 ( 1.1 Pb 3.1 ( 0.3 Sb 6.8 ( 1.3 Sn >200 Ti 4.0 ( 0.7 Zn
4.0 ( 0.3 6.4 ( 0.8 3.1 ( 0.5 4.9 ( 1.3 9.6 ( 1.8 33 ( 20 3.2 ( 0.3 9.4 ( 1.5
2.9 ( 0.2 2.8 ( 0.77 2.2 ( 0.3 2.9 ( 0.2 5.5 ( 1.3 1.7 ( 0.9 4.1 ( 0.9 12.5 ( 2.8 37 ( 8 2.4 ( 0.4 8.5 ( 0.8
ki,j, transfer coefficient; ki,CD ) ki,GCR + ki,GAS; BRA, boiler ash; GCR, gas cleaning residues; GAS, exhaust gas; CD, gas cleaning devices.
TABLE 3. Main Indicators and Their Indicationsa main indicators
element
ki,BMA
Group 1a >0.90b Si, Fe, Co, Cr, Mn, Ni
t-test ki,BMA
c
ki,CD/ ki,BRA
t-test ki,CD/ ki,BRAc
no sign. ci,FGf
ENTR
PART
BULK
cannot be shown
ALL
cannot be shown
Group 1b P, Al, Ca, Mg, Na, Ba, Li, Ti
0.82-0.90 no sign. 4.5 difference
ci,SR < ci,FG
ENTR and EVAP?
FINE/GAS?h NON-BULK
cannot be shown
Group 2a F, Cu, Mo, Pb, Sn, Zn
0.27-0.98 low
>4.5e no sign. ci,SR > ci,FGf difference
EVAP
FINE/GAS
BULK
SENS
Group 2b Br, Sb
0.07-0.33 low
>4.5
no sign. ci,SR e ci,FG difference
EVAP
FINE/GAS
NON-BULK
SENS
4.5 difference
no sign. ci,SR e ci,FG difference
EVAPg
FINE/GAS
NON-BULK
cannot be shown
Group 2c C, S, Cl, As, Cd, Hg
low
a ENTR or EVAP, transfer to the raw gas occurs mainly by entrainment or by evaporation; PART or FINE/GAS, transport in the raw gas occurs either by entrained particle matrix or by fine particles and as gaseous species; BULK or NON-BULK or ALL, occurrence in bulky or in easy entrainable inorganic and organic goods or in both; SENS, sensitive to process conditions in the furnace within the investigated parameter range; SR, sieving residues of bottom ash sample; FG, fine ground-bottom ash sample. For other abbreviations see the text. b kCo,BMA ) 0.85-0.93. c “low” means that the mixed waste incineration produces lower values than the household waste incineration. d kCo,CD/kCo,BRA ) 3.4-5.9, kNa,CD/kNa,BRA ) 2.0-5.8, kBa,CD/kBa,BRA ) 3.5-7.6, kMo,CD/kMo,BRA ) 4.2-7.2, kPb,CD/kPb,BRA ) 3.5-6.2. e Al and Li are exceptions. f Si and F are exceptions. g S is transferred to the raw gas by evaporation as well as by entrained particle matrix (12). h Acid solubility indicates that a high part of K in the raw gas is not bound in the silicate matrix (12).
in the furnace bed, such as residence time and mixing conditions. Information on processes responsible for element transfer is obtained by focusing on the combustion chamber and on the boiler. For this purpose, a distinction is made between the three kinds of physical forms in the raw gas. (i) Species in entrained particle matrix: These species have not undergone evaporation processes in the furnace. (ii) Species in fine particles: These species have evaporated in the furnace and have subsequently condensed homogeneously or heterogeneously. (iii) Gaseous species: These species have evaporated in the furnace and occur in gaseous form.
Most species transported in “entrained particle matrix” can be removed from the raw gas by simple dust separation processes. The boiler can capture micron and submicron particles less effectively than coarser particles. Species transported with “fine particles” and as “gaseous species”, therefore, need more efficient dust separation systems such as electrostatic precipitators and scrubbers. This property is used here to obtain information on transport in the raw gas and, thus, on volatilization processes in the furnace bed. The ki,CD/ki,BRA ratios (transfer coefficient to gas cleaning devices divided by transfer coefficient to boiler ash) are calculated. A higher value indicates that transfer of elements VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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is greater to the gas cleaning residues and/or to the exhaust gas than to the boiler ash. Since coarse particles are more easily separated in the boiler ash, elements with high ki,CD/ ki,BRA values are transported by fine particles and/or as gaseous species and mostly condensed in the gas cleaning devices or leave the incinerator through the exhaust gas. Furthermore, if the ki,CD/ki,BRA value of an element is sensitive to input changes (t-tests), it is very likely that the element is transported in the raw gas by coarse particles (>10 µm). At lower values, which are also sensitive to input changes, it is quite likely that the particular element is transferred to the raw gas mainly by entrainment. Consequently, ki,CD/ki,BRA is a useful indicator of volatilization processes in the furnace bed. However, one has to bear in mind that the indicators are not always definite, as evaporated particles can condense also on coarser particles or entrained particles can also contain fine particles. Table 2 contains the mean ki,CD/ki,BRA values and confidence intervals for the incineration of household waste and mixed waste. ki,CDR/ki,BRA amounts to less than 4.5 for phosphorus and silicon. Since they are expected to be transported “as species in entrained particle matrix” to the raw gas, at least a significant part of an element with a ki,CD/ ki,BRA ratio significantly higher than 4.5 is postulated to be transported with fine particles and/or as gaseous species. Consequently, the elements C, S, Br, Cl, F, K, As, Cd, Cu, Hg, Mo, S, Sb, Sn, and Zn may be transferred mainly to the raw gas “by fine particles” or “as gaseous species”. In light of these results, it is postulated that their transfer occur mainly by evaporation in the furnace bed. Compared to the incineration of mixed waste, household waste incineration produces higher ki,CD/ki,BRA values of the elements P, S, Ca, Fe, K, Mg, Na, Cr, Mn, Ni, and Ti at 95% confidence level and Si and Ba at 80% confidence level. In other words, their separation in the boiler is sensitive to input changes. This speaks in favor of the hypothesis that these elements are transported by particles in the raw gas. Household waste incineration seems to produce a different concentration vs particle size distribution in the raw gas than incineration of mixed waste. This may be due to the higher kitchen and garden waste content of household waste, exhibiting a higher surface-to-volume ratio than bulky goods. Table 3 contains the main indicators, the hypotheses pertaining to the elements transfer to the raw gas, transport in the raw gas, occurrence and distribution in the waste input, and sensitivity to process conditions in the furnace bed. The hypotheses are preliminary assessments and do not claim to be complete. The elements are divided in two groups. The elements in Group 1 are transferred to the raw gas by entrainment and transported mainly by entrained particle matrix (ki,CD/ki,BRA < 4.5). Occurrence and distribution is the main master variable. This explains the differences in transfer coefficients to the raw gas within Group 1. While the elements in Group 1a are mostly present in bulky goods (ci,SR > ci,FG), the elements in Group 1b also occur in easily entrainable inorganic or in organic goods (ci,SR e ci,FG). The low transfer to the raw gas (ki,RAW < 0.18) can be explained by the low dust generation in the furnace (kmass,CD/kmass,BMA < 0.15). Entrainment and evaporation may be responsible for the transfer of potassium to the raw gas. Potassium is transported in the raw gas by particles (incineration of mixed waste produces different kK,CD/kK,BRA values than household waste). However, the particles must be fine due to high kK,CD/kK,BRA values. Its occurrence in organic MSW goods (cK;SR < cK,FG) probably favors the high transfer to the raw gas. The behavior of potassium is explained in detail elsewhere (12). Incineration of different types of waste could not significantly indicate whether transfer to the raw gas of the elements in Group 1 is sensitive to input variations. 2506
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The elements in Group 2 are transferred to the raw gas by evaporation and transported in the raw gas by fine particles and as gaseous species (ki,CD/ki,BRA is higher than 4.5 and not significantly different for both incineration experiments). The wide range of transfer coefficients in Group 2a can be explained to a certain extent by the occurrence of these elements in bulky goods in MSW (ci,SR > ci,FG). The evaporation rate is decreased by the low surface-to-volume ratio of the bulky goods. Therefore, these elements are also very sensitive to input variations (incineration of mixed waste produces different ki,BMA values than incineration of household waste). Bromine and antimony (Group 2b) occur mainly in easy entrainable inorganic or in organic goods (ci,SR e ci,FG) and are also sensitive to input variations. The elements of the Group 2c also occur mainly in easy entrainable inorganic or in organic goods. However, they are not highly sensitive to input variations (t-test ki,BMA). The results reveal that the elements in Group 1 are mainly transferred to the raw gas by entrainment. Their occurrence and distribution in the input waste primarily explain the differences between their transfer coefficients. The parameter variations influencing entrainment in the furnace, i.e., furnace construction, primary air feed rate, mixing on the grate, may have an influence on the transfer coefficients of these elements. Evaporation is the main process responsible for the transfer of the elements in Group 2. Apart from occurrence and distribution, physical and chemical conditions as well as kinetics in the furnace have an influence on the transfer to the raw gas. Additional laboratory experiments, presented in a companion paper, also published in this issue, provide further information on the transfer mechanisms of these elements (11).
Acknowledgments We are grateful to Prof. P. Baccini and many members of his Resource and Waste Management team at EAWAG for their field and laboratory assistance. We also thank the staff of the St. Gallen incinerator for their help in carrying out the material flow analyses.
Literature Cited (1) Baccini, P.; Belevi, H.; Lichtensteiger, Th. GAIA 1992, 1, 34-49. (2) Belevi, H.; Sta¨mpfli, D. M.; Baccini, P. Waste Management Res. 1992, 10, 153-167. (3) Johnson, A.; Brandenberger, S.; Baccini, P. Environ. Sci. Technol. 1995, 28, 142-147. (4) Greenberg, R. R.; Zoller, W. H.; Gordon, G. E. Environ. Sci. Technol. 1978, 12, 566-573. (5) Brunner, P. H.; Moench, H. Waste Management Res. 1986, 4, 105-119. (6) Hagenmaier, H.; Kraft, M.; Brunner, H.; Haag, R. Environ. Sci. Technol. 1987, 21, 1080-1084. (7) Vogg, H.; Braun, H.; Metzger, M.; Schneider, J. Waste Management Res. 1986, 4, 105-119. (8) Fernandez, M. A.; Martinez, L.; Segarra, M.; Garci, J. C.; Espiell, F. Environ. Sci. Technol. 1992, 26, 1040-1047. (9) Verhulst, D.; Buekens, A.; Spencer, P. J.; Eriksson, G. Environ. Sci. Technol. 1996, 30, 50-56. (10) Barton, R. G.; Clark, W. D.; Seeker, W. R. Combust. Sci. Technol. 1990, 74, 327-342. (11) Belevi, H; Langmeier, M. Environ. Sci. Technol. 2000, 34, 25072512. (12) Belevi, H. Environmental Engineering of Municipal Solid Waste Incineration; vdf University Press: Zu ¨ rich, 1998. (13) BUWAL Suggestions for emission measurements of air pollutants at stationary plants; Swiss Agency for the Environment, Forests and Landscape: Bern, 1987. (14) Wurzschmitt, B. Microchim. Acta 1951 36/37, 369. (15) Wunderli, S. Ph.D. Thesis, No. 8523, ETH, Zu ¨ rich, 1988.
Received for review September 20, 1999. Revised manuscript received March 20, 2000. Accepted March 20, 2000. ES991078M
