Environ. Sci. Technol. 2000, 34, 2507-2512
Factors Determining the Element Behavior in Municipal Solid Waste Incinerators. 2. Laboratory Experiments HASAN BELEVI* AND MADELEINE LANGMEIER Swiss Federal Institute for Environmental Science and Technology, EAWAG, 8600 Dubendorf, Switzerland
Laboratory experiments with synthetic samples are conducted to obtain information on the evaporation behavior of metals in incinerators. In combining the results obtained through field measurements presented in a companion paper in this issue, hypotheses are postulated on the influence of physical and chemical conditions as well as of kinetics on the evaporation behavior of the chosen elements in the furnace bed. These are validated by thermal treatment of bottom ash samples in the laboratory. A temperature increase of from 500 °C to 900 °C causes higher transfers of most metals into the gaseous phase. Mo and Sb are the exceptions. Chlorine availability generally favors evaporation. Increasing oxidative conditions cause lower Sn transfers. Transfer of the elements Cd, Mo, Sb, Sn, and Zn is negatively affected by their reaction with reaction partners other than oxygen and chlorine occurring in municipal solid waste. Oxidation of organic carbon to CO and CO2 as well as HCl formation determine the distribution of temperature, redox conditions, and chlorine availability in the furnace bed. First indications are obtained on the residence times of goods under these different local conditions. The knowledge acquired can be used to design new incinerators with “tailor-made” products that can either be recycled or landfilled without an adverse environmental impact for the long term. For this purpose, mechanical processing techniques, thermal treatments at temperatures between 500 °C and 1100 °C, and hightemperature treatments (>1100 °C) including melting processes will have to be combined.
Introduction Until the 1980s, solid waste disposal focused mainly on health and safety problems. In recent years, industrialized nations began to develop solid waste management strategies and to issue corresponding guidelines. These guidelines for sustainable resource management require that municipal solid waste incinerators (MSWI) produce well-designed products. These “tailor-made” products will either be used as secondary raw materials (to save resources) or landfilled for future generations to find a healthy aquatic and terrestrial ecosystem (to protect resources). Therefore, incinerators should separate the environmentally harmful compounds from MSW and concentrate them in the gas cleaning residues. Thus, bottom ash could be used in construction material or landfilled on * Corresponding author phone: 41-1-823-55-14; fax: 41-1-8235399; e-mail:
[email protected]. 10.1021/es991079e CCC: $19.00 Published on Web 05/12/2000
2000 American Chemical Society
a long-term basis without causing environmentally damaging effects, and gas cleaning residues could become attractive for resource recovery processes. However, “tailor-made” product qualities can only be achieved if the factors controlling element transfers from MSW to incinerator products are understood. A companion paper, also published in this issue (1), presents field measurements in a MSW incinerator. Based on the results of these measurements, the transfer behavior of 29 elements in incinerators is determined. The following master variables explain the element transfer behavior to the raw gas: (i) occurrence and distribution of the elements in the input waste; (ii) temperature, redox conditions, and content of chlorine and reaction partners other than oxygen and chlorine in the furnace bed; and (iii) residence time and mixing conditions in the furnace bed. The results indicate that the elements Al, Ba, Ca, Co, Cr, Fe, K, Li, Mg, Mn, Na, Ni, P, Si, and Ti are mainly transferred to the raw gas by entrainment. Their occurrence and distribution in the input waste primarily explain the differences in their transfer coefficients. Evaporation is the main process in the transfer of the remaining investigated elements As, Br, C, Cd, Cl, Cu, F, Hg, Mo, Pb, S, Sb, Sn, and Zn. The behavior of C is presented elsewhere (2). Several research groups (3, 4) have investigated the behavior of Hg. The behavior of As is well-studied in cement kilns (5). Therefore, additional experiments are not necessary for C, Hg, and As. Laboratory experiments with synthetic samples are presented in this paper to obtain information on the evaporation behavior of the elements Cd, Cu, Mo, Pb, Sb, Sn, and Zn. Additionally Co, Mn, Ni, and Ti are chosen for control purposes, since these elements are not expected to evaporate in laboratory experiments. The evaporation behavior of the remaining elements Br, Cl, F, and S is not investigated. Special experiments with these elements will have to be developed in future studies. In combining the field and laboratory results obtained, hypotheses are postulated on the factors controlling element transfer behavior. These are validated by thermal treatment of bottom ash samples in the laboratory. The following specific questions are raised: (ii) What are the factors determining the evaporation behavior of the elements As, C, Cd, Cu, Hg, Mo, Pb, Sb, Sn, and Zn in MSW incinerators? (ii) Is it possible to use the knowledge obtained by the field and laboratory experiments to attain “tailormade” product qualities by optimizing existing incinerators or by designing new plants?
Experimental Section Figure 1 contains the evaporation equipment used in the laboratory experiments. It comprises a furnace with a gas collection device, a condensation flask in an ice water bath, three gas washing bottles, a rotameter, and a suction pump. The first gas washing bottle contains deionized water, and the others contain an aqueous solution of 5 wt % nitric acid and 3 wt % hydrogen peroxide. After setting the air feeding rate, the furnace is heated to the desired temperature. Ten grams of the sample is put in a 7-cm diameter ceramic crucible and placed in the furnace. After a selected residence time, the crucible is taken out and cooled. The sample is pressed into a pellet after cooling. Determination of the concentration is performed by X-ray fluorescence spectrometer ARL 8410 using the UNIQUANT program. The experiments are conducted with synthetic and bottom ash samples. In addition to quartz, the synthetic samples contain the elements given in Table 1. The element combinations are selected in such a way as to minimize element VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Apparatus for the evaporation experiments in the laboratory.
TABLE 1. Concentrations in the Synthetic Samples element
element concn [g/kg] [mmol/kg]
element as oxide [%]
element as chloride [%]
Ti Co Zn Sn
2.4 4.4 4.8 9.5
Synthetic Sample 1 50 100 75 50 74 50 80 50
0 50 50 50
Cu Mo Sb
4.6 6.8 9.0
Synthetic Sample 2 73 52 71 49 74 47
48 51 53
Mn Ni Cd Pb
4.1 4.8 8.8 9.1
Synthetic Sample 3 75 46 82 49 79 49 44 51
54 51 51 49
interactions by determining the concentration with an X-ray fluorescence spectrometer. The master variables temperature, chlorine content, redox conditions, and existence of reaction partners other than chlorine and oxygen, performed by bottom ash addition, are varied. For bottom ash addition, the same amount of quartz is replaced by bottom ash previously dried and ground to 0.5 mm diameter. Polyethylene is added as carbon source. Ammonium chloride is used as the chlorine source as it is more appropriate than many other inorganic salts, such as sodium or potassium chloride. The amount of carbon and chlorine added is based on the amount of samples without added species. An air feeding rate of 15 Nm3 kg-1 h-1 was selected for all the experiments. This value is lower than the specific air feeding rate per mass bottom ash in an incinerator. The concentrations of chlorine and carbon are lower and those of the investigated metals higher in the synthetic sample than in MSW. However, due to different kinetics, these conditions should not be compared with the conditions in an incinerator. Consequently, the results are described qualitatively. These experiments do not aim at simulating the incinerator conditions.
Results and Discussion Effect of Physical and Chemical Conditions on Evaporation. Figure 2 shows the element transfer into the gaseous phase. The negative transfer at low values arises due to chemical analytical errors. The concentrations in untreated samples are highly accurate as they are determined on the basis of synthetic sample composition. After thermal treatment, the concentrations are measured, however, by XRF with an error of 20%. During the experiment with Sample 1, about 70% Zn is transferred to the gaseous phase at 700 °C, and about 80% is transferred at 800 °C and 900 °C. This suggests that 202508
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30% ZnO is transferred to ZnCl2 by consuming chlorine of the other species and evaporated as ZnCl2 (boiling point 732 °C). If chlorine is added, up to 96% can be transferred at 900 °C. Even at 700 °C the transfer is about 94%. If part of the quartz is replaced by bottom ash, almost no zinc is transferred to the gaseous phase. Zinc chloride probably reacts to some nonvolatile species, such as zinc silicates or aluminates. However, if chlorine is also added besides bottom ash, almost 90% is volatilized at 900 °C. A decrease in oxidative conditions (carbon addition) reduces transfer, and an addition of carbon and chlorine increases transfer. In light of the results obtained, it can be postulated with regards to zinc transfer to the raw gas in an incinerator furnace, that the most important parameter is the availability of chlorine at temperatures between 700 °C and 900 °C. No significant amount of Sn is evaporated during thermal treatment of Sample 1. Addition of bottom ash has no effect either. About 75% transfer can be reached between 500 °C and 900 °C if chlorine is added (114 °C is the boiling point of SnCl4). If bottom ash is also added, transfer is lower. If only carbon is added, about 50% Sn can volatilize, and if both carbon and chlorine are added about 96%. These results suggest that Sn transfer is influenced by redox conditions and by chlorine concentrations. Furthermore, the bottom ash matrix reduces transfer. Consequently, tin oxides are postulated to be easily produced in oxidizing conditions. If the conditions are not highly oxidative and chlorine is in excess, tin chlorides can predominate and are subsequently evaporated. At high temperatures, Sn reaction with partners other than chlorine and oxygen is also significant and reduces evaporation. Copper transfer to the gaseous phase is negligible if Sample 2 is thermally treated. Experiments with the addition of carbon, including carbon and chlorine, reveal no copper transfer. If part of the quartz is replaced by bottom ash, about 40% copper is transferred at 900 °C. If chlorine is added transfer also occurs at 500 °C. More copper is transferred to the gaseous phase if both chlorine and bottom ash are added. Addition of chlorine and replacement of quartz with bottom ash can lead to a 55% transfer. In general, addition of carbon prevents transfer. These results suggest that copper transfer is influenced by redox conditions, by temperature, and by chlorine concentrations. Copper chlorides are probably only stable under highly oxidative conditions. If Sample 2 is thermally treated, 83% Sb is transferred to the gaseous phase at 500 °C, and about 65% at 700 °C and 25% at 900 °C are transferred. This suggests that up to 70% Sb2O3 is transferred to chlorides by consuming chlorine of the other species and evaporated as chlorides (283 °C is the boiling point of SbCl3). If chlorine is added, up to 100% transfer can be reached. However, if part of the quartz is replaced by bottom ash, almost no antimony is transferred to the gaseous phase. Antimony chloride must react to some nonvolatile species. If chlorine is added besides bottom ash, over 95% is volatilized. Higher temperatures and a decrease in oxidative conditions (carbon addition) cause lower transfer; however, addition of chlorine accelerates transfer. In light of these results it is postulated that availability of chlorine is the most important parameter. Conditions should not be reductive and temperature closer to 500 °C than to 900 °C for a high transfer to the gaseous phase. During treatment of Sample 3, about 67% Cd is volatilized at 700 °C, and about 86% is volatilized at 900 °C. This suggests that 30-70% CdO is transferred to CdCl2 or to Cd and subsequently evaporated (765 °C and 960 °C, respectively, are the boiling points of Cd and CdCl2). Addition of chlorine slightly changes the transfer ratios. If part of the quartz is replaced by bottom ash, less Cd is evaporated at lower temperatures (e700). Cadmium chloride must react to some nonvolatile species. However, over 95% Cd is volatilized at
FIGURE 2. Transfer to gaseous phase in the laboratory experiments: 9, sample; 2, bottom ash addition; +, chlorine addition; [, carbon addition; ×, chlorine and carbon addition; O, chlorine and bottom ash addition; residence time, 30 min; air feed rate, 15 Nm3 kg-1 h-1; chlorine addition, 2.9 mol/kg; carbon addition, 10.7 mol/kg; and bottom ash addition, replacement of 500 g/kg quartz with bottom ash. 900 °C if both bottom ash and chlorine are added. Decreasing oxidative conditions do not significantly affect the evaporation process. Temperature seems to be the most important master variable. Addition of chlorine causes higher transfers if bottom ash is also present. In Sample 3, about 53% Pb is transferred to the gaseous phase at 500 °C, and about 73% is transferred at 900 °C. This suggests that a part of PbO is transferred to PbCl2 and subsequently evaporated (950 °C is the boiling point of PbCl2). Transfer is slightly higher if chlorine is added. The effects of carbon and bottom ash additions are also not very significant. In light of these results it is postulated that temperature is the most important parameter with regards to the transfer of lead to the raw gas in a furnace. As anticipated, no significant amounts of Co, Mn, Ni, and Ti are transferred to the gaseous phase. In incinerators, these elements are mainly transferred to the raw gas by entrainment (1). Mo is only transferred to the gaseous phase if excess chlorine is present (not shown here). There is a high transfer (90%) at 500 °C if chlorine is added. Transfer decreases with increasing temperatures. Molybdenum chlorides are volatile (258 °C is the boiling point of MoCl5) and probably not stable at higher temperatures (6). Figure 4 is an attempt to simplify the effects of temperature, redox conditions, and content of chlorine and reaction partners other than oxygen and chlorine on the element
transfer to the raw gas in a furnace. The transfer of the elements Al, Ba, Ca, Co, Cr, Fe, K, Li, Mg, Mn, Na, Ni, P, Si, and Ti is not significantly affected by physical and chemical conditions (1). Their transfer to the raw gas occurs primarily by entrainment. However, mercury is almost completely transferred to the raw gas as gaseous species. High temperatures and high oxidative conditions favor the transfer of C, Cu, and Zn to the raw gas. Transfer of Cd and Pb is higher if the temperature is increased. However, redox conditions do not play a major role. More Sn is transferred to the raw gas by decreasing oxidative conditions and increasing temperatures. Mo and Sb do not require temperatures over 500 °C to transfer to the raw gas. A decrease in oxidative conditions causes lower transfers. Arsenic can react with calcium oxide to form calcium arsenate which is transferred mainly to the bottom ash (5). In cement kilns, formation mechanisms are linked with three conditions: (i) excess calcium oxide, (ii) oxidizing atmosphere, and (iii) elevated temperatures. Consequently, the arsenic transfer to the raw gas may be adversely affected by an increase in oxidative conditions and temperature. An increase in chlorine content favors all the aforementioned metals. Reaction partners other than oxygen and chlorine negatively affect the transfer of As, Cd, Mo, Sb, Sn, and Zn. However, the effect of the other reaction partners on the transfer to the raw gas greatly depends on excess VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Effect of physical and chemical conditions on element transfer to raw gas in the furnace bed. A positive effect means a higher element transfer occurs by condition change within the parameter range existing in incinerators: O, an increase of chlorine content favors a higher transfer to the raw gas; dotted circle, an increase of chlorine content favors a higher transfer to the raw gas in a secondary importance; and italic, reaction partners other than oxygen and chlorine negatively affect the transfer to a significant extent. chlorine and temperature. If excess chlorine is present for example, this effect is very low for Sb. If this is not the case, transfer of antimony to the gaseous phase is prevented by the reaction with the other waste components. Effect of Kinetics on Evaporation. Figure 4 illustrates the effect of residence time on the element transfer to the gaseous phase in laboratory experiments. This effect is investigated with samples containing additional chlorine and bottom ash. An increase in residence time from 10 to 120 min increases Zn transfer only at temperatures between 500 °C and 700 °C. Residence times over 10 min do not increase the transfer of Sn and Sb. Cu transfer increases significantly (from 18% to 84%) at 900 °C, and the effect of residence time is low between 500 °C and 700 °C. Higher residence times increase the transfer of Cd and Pb at all temperatures. However, after 30 min, transfer of Cd and after 10 min, transfer of Pb reach almost maximum values at 900 °C. According to the results obtained, the expected residence times of solid goods in the furnace bed (minutes to hours) may strongly influence the transfer of elements to the raw gas. However, the fact that residence time may also be dependent on physical and chemical conditions should also be taken into consideration. Further experiments have indicated for example that a few minutes of residence time in the furnace bed under lower oxidative conditions are likely to cause a higher Sn transfer than high residence times (hours) under oxidative conditions (6). With regards to the oxidation of organic substances, the combustion processes in a furnace are generally divided into three zones: drying, gasification/pyrolysis, and combustion. An additional “burnout” zone in the furnace bed is also important for element transfer to the raw gas as well as for further mineralization of organic substances, even though most of the initial carbon is transferred to the raw gas in the gasification/pyrolysis and combustion zones. Depending on the physical and chemical conditions in these zones, several elements have different evaporation rates. Behavior of the elements Cd, Cu, Pb, and Sn provides information on the kinetics in the furnace bed. Thermodynamic calculations suggest a copper transfer coefficient of over 0.9 to the raw gas (7, 8). The measured values are, 2510
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however, lower than 0.05. As aforementioned, high temperatures and excess chlorine under oxidative conditions are necessary for copper transfer (Figure 3). Although these conditions generally prevail on an average basis in the furnace bed, they should not be present in the same spot and at the same time. Residence times of solid goods at high temperatures and under oxidative conditions with excess chlorine must be very low (seconds to minutes). A decrease in oxidative conditions and an increase in chlorine availability favor tin transfer (Figure 3). Since tin occurs mostly in bulky goods, the evaporation rate is decreased due to mixing controlled kinetics. Nevertheless, relatively high transfer coefficients to the raw gas are determined in incinerators (0.39-0.73) (1). This suggests that high chlorine availability should be present along with reduced oxidative conditions in the furnace bed. Temperature is the key factor with regard to lead and cadmium transfer (Figure 3). High cadmium transfer coefficients of 0.85-0.93 (1) suggest that residence times up to hours at temperatures above 700 °C occur in the furnace bed. Higher transfer coefficients are probably prevented by the reaction of cadmium with reaction partners other than chlorine and oxygen to nonvolatile species. Lower lead transfer coefficients (0.34-0.63) are probably caused by its occurrence in bulky goods. Distribution of the physical and chemical conditions in the furnace bed are postulated on the basis of these indications (Figure 5). In the drying zone, temperature is low (700 °C), and the conditions are oxidative. Total residence time of solid goods in the combustion and burnout zones reach up to hours, since high quantities of lead and cadmium are transferred to the raw gas. Thermal Treatment of Bottom Ash in the Laboratory. To validate the aforementioned hypotheses, evaporation experiments are conducted in the laboratory with two bottom ash samples. Sample A is a finely ground-bottom ash sample taken during the field measurements (1). Sample B is taken in such a way as to exclude coarse material (>2 cm). It can be assumed that Sample B represents a bottom ash treated mechanically by grinding and separation processes. Both bottom ash samples are subjected to the same thermal treatment as the synthetic samples. Extreme conditions are chosen, but the experiments are not optimized. Temperature ranges between 800 °C and 1000 °C and residence time amounts to 2.5 h. The air feeding rate totals 15 Nm3 kg-1 h-1, and 14.5 mol chlorine is added per kg bottom ash (chlorine input to the incinerator is about 35 mol/kg bottom ash). Table 2 lists the concentrations of 10 elements in these samples before and after thermal treatment. It also compares the element concentrations in inputs and/or outputs at two high-temperature plants, including the concentrations in Swiss soil-forming rock types and in the earth’s crust. Over 90% Pb and over 85% Cu are transferred to the gaseous phase. This confirms that Pb and Cu transfer is highly favored at high temperatures and excess chlorine. As anticipated, about 50% Mo and 20-30% Sn are transferred to the gaseous phase. Mo evaporation is not high at high temperatures (800-1000
FIGURE 4. Effect of residence time in the laboratory experiments: air feed rate, 15 Nm3 kg-1 h-1; chlorine addition, 2.9 mol/kg; and bottom ash addition, replacement of 500 g/kg quartz with bottom ash.
FIGURE 5. Distribution of physical and chemical conditions in the furnace bed; (chemical element): transfer in a secondary importance. °C), and the oxidizing atmosphere does not favor Sn volatilization. About 80% Zn is transferred. Excess chlorine and high temperatures favor Zn transfer; however, the effect of the reaction partners other than oxygen and chlorine is
also significant. Zn has probably already reacted in the incinerator or reacts during these experiments to Zn silicates or to zinc spinel. Cd behavior also confirms the hypothesis that part of some elements reacts with reaction partners other than oxygen and chlorine, thereby, preventing their volatilization. Since only a small amount (60%) could be removed by grinding and sieving (1). This explains the difference in element concentrations in bottom ash samples prior to thermal treatment. The nickel concentrations are higher than in rocks. However, high-temperature plants can attain the same nickel concentrations as those found in rocks. VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Element Concentrations in Bottom Ash Samples before and after the Thermal Treatment and a Comparison with the Concentrations in Inputs and/or Outputs of Two High Temperature Plants as Well as with the Concentrations in Soil Forming Rock Types of Switzerland and the Earth’s Crusta concn [mg/kg] Sample A
bottom ash in HSR-processb
Sample B
element
before thermal treatm
after thermal treatm
before thermal treatm
after thermal treatm
before thermal treatm
after thermal treatm
thermo-select molten slagc
rocksd
Cd Co Cr Cu Mn Mo Ni Pb Sn Zn
4.5 26 1900 3000 1400 33 340 1900 230 4500
2.9 26 1900 470 1400 16 340 65 160 1000
3.5 28 1700 1400 900 11.0 200 1100 50 2600
1.8 28 1700 90 900 4.9 200 55 40 430
13 nde 2100 8500 nde nde 1000 1900 50 4100
0.5 nde 2800 400 1600 nde 50 75 20 400
0.1 50 10000 650 1500 nde 60 200 30 580
