Volatilization Behaviors of Low-boiling-point Elements in Municipal Solid Waste Gasification and Melting Processes

Transcription

1 Volatilization Behaviors of Low-boiling-point Elements in Municipal Solid Waste Gasification and Melting Processes Shohichi Osada 1, Morihiro Osada 2 1. Nishinihon Branch Office, Nippon Steel Engineering Co., Ltd, Osaka, Japan 2. Environmental Solution Division, Tokyo, Japan Abstract: In order to reduce the contents of heavy metals such as Pb and Zn in slag which is utilized in civil and construction fields, it is of great importance to investigate the volatilization behaviors of these heavy metals from molten slag under various conditions. In the present study, the melting furnace reaction model which represents distribution behaviors between the gas and slag phases of low-boiling-point elements in the incineration-residue melting process was built by using calculation software for chemical thermodynamics. The results derived from the calculation were compared with those obtained from actual operations undertaken in a test plant with a coke-bed type incineration-residue melting furnace. As a result, it was found that melting conditions such as melting temperature and oxygen partial pressure had great effect on the volatilization behavior of low-boiling-point elements. The volatilization of Pb became higher and the content of Pb in the molten slag became lower, as the melting temperature was raised and the oxygen partial pressure was decreased. These results were in good agreement with the data obtained from actual operations. Based on the validity of the present calculation results, this computational method was applied to the prediction of heavy metals volatilization in a shaft-type gasification and melting furnace. In shaft-type gasification and melting furnace, a decomposition zone is located in the upper part of the melting zone. In this study, the volatilization behaviors of heavy metals in the decomposition and the melting zones were compared. Then, the characteristics of heavy metals volatilization in these two zones were investigated, comparing the heavy metal species volatilized and the amount of heavy metals volatilization. From a comparison of the calculation results with the data of actual operation, it was found that the melting temperature and oxygen partial pressure affected the volatilization of low-boiling-point elements in a shaft-type gasification and melting furnace, which was similar to the results obtained from incineration-residue melting process. In particular, it was shown from the calculation results that Na, K, Pb and Zn were volatilized as metal chlorides in the decomposition zone and that they were volatilized as metal elements in the melting zone. Keywords: municipal solid waste, gasification and melting, slag, thermodynamics, heavy metals, low-boiling-point elements 1. INTRODUCTION The amount of municipal solid waste generated in Japan is estimated to be about 50 million metric tons per year, and melting treatment is widely applied for its intermediate treatment. In contrast to conventional incineration treatment, since solid waste is melted at high temperature, it can be detoxified and the resulting volume is reduced. The melting treatment is largely categorized into two types: a process using an ash-melting furnace, which melts the combustion ash resulting from the incineration of waste; and another process using a gasification and melting furnace, which directly melts the waste. In either melting treatment, slag and fly ash are generated. In recent melting treatment processes for municipal solid waste, an increasing number of gasification furnaces are put into operation [1]. In this system, in contrast to the process that melts the incineration residue, municipal solid waste is directly put into the melting furnace where the organic components in the waste is thermally decomposed, and the residual ash is melted and finally turned into slag. This system features a single-step process to turn municipal solid waste into melt slag rather than going through two steps: the first step to incinerate the Corresponding author: Shohichi Osada, osada.shohichi@eng.nsc.co.jp 807

2 waste and produce the incineration residue, and the second step of melting for detoxifying and reducing the volume. In Japan, the amount of slag generated from melting treatment is 0.5 million metric tons per year. Utilization of the slag will extend the service life of the final disposal site and eliminate the need for disposal sites of general waste. While the use for civil construction materials is one promising application of the slag, it is critical to ensure that the amount of toxic elements contained in the slag is environmentally safe in line with regulations on toxic elements. In this respect, it is necessary to study the relationship between the operating conditions of the furnace and the migration of toxic metals into slag and gas (fly ash) from loaded municipal solid waste. The behavior of low-boiling-point elements contained in incineration residue distributed into gas and slag has been reported[2,3]. These studies focused on the relationship between the Pb content in slag produced in an actual furnace and the operating conditions of melting treatment facilities, and investigated the influence of melting treatment conditions on the Pb content, which is an important factor in environmental safety when utilizing the slag produced by melting incineration residue. These studies have shown that a reducing atmosphere (low-oxygen atmosphere) in the furnace is effective for suppressing the Pb content in the slag produced. Another study from the viewpoint of chemical equilibrium theory has examined the relationship between the volatilization behavior of Pb and the Pb content in slag using computational thermodynamics software. It has been shown that computational thermodynamics is an effective tool for the study of multi-component solid waste[4]. However, thermodynamic study on the volatilization behaviors of low-boiling-point elements is yet to be conducted for gasification and melting treatment, the mainstream melting treatment, of municipal solid waste. Unlike incineration residue, municipal solid waste is composed of combustible, water and ash components. Therefore, evaporation of the water and thermal decomposition and gasification of the combustible components take place during the melting treatment process. The melting treatment of municipal solid waste is thus characterized by two stages: the thermal decomposition stage of organic components and the melting stage, in contrast to the system for melting combustion residue where the entire furnace works almost only in the melting stage [5]. In this study, we performed computational thermodynamic analysis [6] on the volatilization dynamic analysis [6] on the volatilization behaviors of low-boiling-point elements, Na, K, Pb, and Zn, during the thermal decomposition and melting processes of municipal solid waste. We have also constructed a thermodynamic model for the reaction in the shaft furnace system, one of the gasification and melting systems, to clarify the influence of volatilization behavior on the volatilization ratio at various temperatures of municipal solid waste. 2. METHODOLOGY 2.1 Composition of municipal solid waste To ensure the representation of waste samples taken for analysis, sample dividing is performed prior to waste sample preparation. Using several samples taken, the kinds and chemical composition of the solid waste are analyzed. When analyzing municipal solid waste, three-component analysis is mainly performed on the combustible, water, and ash components. Element analyses are mostly performed for C, H, O, N, Cl, and S derived from the combustible components[7,8], and only a limited number of results are available on the analysis of other composing elements. In addition, due to the non-uniform characteristics of municipal solid waste, the representation of analyzed samples is not highly reliable. We have therefore calculated the chemical composition of municipal solid waste from the material balance between the amounts of elements coming into and those going out of the melting furnace. In this method, the verification test furnace is operated for a long time under steady operating conditions by feeding municipal solid waste, coke, and lime stone. The chemical compositions and amounts of combustible (C, H, O) components and water and ash components in municipal solid waste are reversecalculated from the chemical composition and amount of substances loaded into the furnace and the molten substance and ash discharged out of the furnace. With this method, the chemical composition of municipal solid waste is thus estimated by reverse calculation of the material balance during the melting treatment. Table 1 shows reverse-calculated typical characteristics of municipal solid waste. The tabulated data are average values that are reverse calculated from 10 test charges of municipal solid waste treated in the steady melting operation for 137 hours. The combustible components and water account for 45% and 44% of the waste, respectively, and the rest of the 11% is ash components. The contents of the low-boiling-point elements are 0.33% Na, 0.15% K, 0.02% Pb, and 0.07% Zn. In addition, the contents of Cl and S are 808

3 0.48% and 0.09%, respectively. The reverse calculation process for the solid waste characteristics also provides the distribution ratio of each element between the gas and solution phase. Table 1 Components of municipal solid waste Combustible Cl 0.48 S 0.09 Total Water Ash CaO 1.25 SiO Al 2 O MgO 0.25 Pb 0.02 Zn 0.07 Na 0.33 K 0.15 Cu 0.10 Fe 2.35 Total Table 2 Volatilization ratios in actual operations of the shaft-type gasification and melting furnace Element volatilization ratios Na K Pb Zn The calculation is based on the discharge ratios of the elements migrating into the fly ash being discharged into the gas phase, return ash, or molten substance. The result is shown in Table 2, revealing that the elements discharged into the gas phase account for 45% for Na, 75% for K, and nearly 100% for Pb and Zn Volatilization behaviors of low-boiling-point elements in municipal solid waste at a Melting Temperature Among low-boiling-point elements contained in municipal solid waste, we have examined the behaviors of typical alkali metals, Na and K, and low-boiling-point heavy metals, Pb and Zn. Pb is specified as one of the eight toxic elements listed in the Soil Contamination Countermeasures Law, and the contents of these elements are controlled. Regulation of Pb content in slag is also specified in the JIS standards for road construction and slag concrete aggregate. From the standpoint of environmental safety, Pb is thus a critical metal element when we consider the utilization of melt slag as a civil construction material. At various temperatures and gas atmospheres, we analyzed the volatilization behaviors of the low-boiling-point elements in municipal solid waste using computational thermodynamics software FACTSAGE [9] Version 5.5. The temperatures for the analysis were selected within the general melting temperature range as 1573K and 1823K, and a partial pressure of oxygen po 2 was used as the indicator for the gas atmosphere. The distribution ratios of Na, K, Pb, and Zn between the gas and molten phases were obtained by chemical equilibrium calculation. When municipal solid waste is kept in a molten temperature condition, the po 2 becomes 10-9 atm at 1573K and 10-7 atm at 1823K, creating a reducing atmosphere (low-oxygen atmosphere). Oxygen is eventually added to change the po 2 level. According to the calculation, C, H, O, and water in the combustible components of municipal solid waste are gasified under the above described conditions, and CaO, SiO 2, and Al 2 O 3 migrate into the molten substance. Under the same melting conditions, Na, K, Pb, and Zn are distributed between the gas and molten substance. Some of the Na and K migrate into the gas phase in chloride form, NaCl and KCl, while the rest exist within the molten phase in the form of Na 2 O and K 2 O. Pb exists in the gas phase as Pb in a reducing atmosphere and as PbO in an oxidation atmosphere. Pb also exists as PbO in the molten phase in an oxidation atmosphere. Gasification of Zn advances in a reducing atmosphere, while in an oxidation atmosphere, it exists in the molten phase in the form of ZnO. As the temperature rises, these four elements are increasingly diffuse into the gas phase and the volatilization ratio increases. Meanwhile, the behavior of Na and K has little dependence on the po 2 level, whereas the volatilizations of Pb and Zn are highly dependent on the po 2 level. That is to say, in a reducing atmosphere, the volatilization is accelerated, 809

4 while in an oxidation atmosphere, these elements migrate into the molten phase. 2.3 Volatilization behavior of low-boiling-point elements in shaft-type gasification and melting furnaces Structure of shaft-type gasification and melting furnaces Gasification and melting furnaces are used for the melting treatment of municipal solid waste. There are largely three types of gasification and melting furnaces, and some furnaces have a unified thermal decomposition and melting zone, whereas the others have separate zones [10]. Figure 1 illustrates the structure of shaft-type gasification and melting furnaces with a unified zone. The shaft-type furnace is loaded with municipal solid waste, coke as supplemental heat source and reducing material, and lime stone for adjusting the slag basicity, all of which are fed from the top of the furnace. Air is blown in through the tuyere at the bottom of the furnace to burn the carbon content and create a high-temperature zone inside the furnace. While the gases generated in the melting zone are flowing upward, they give the heat to the solid waste moving downward. The solid waste is affected by the gases generated in the furnace and the reactions of water drying, thermal decomposition, and melting progress in this order. In the thermal decomposition zone, combustible components in the solid waste are thermally decomposed and generate thermally decomposed gases and combustion residue. In the melting zone, solid carbon resulted from the thermal decomposition and coke carbon are burned with the oxygen in the air blown in through the tuyere, and the residue of thermal decomposition is turned into molten substance and decomposition gases Input data and calculation procedure of the thermodynamic reaction model Table 3 shows the list of input data used for the calculation: the amount of elements and compounds contained in municipal solid waste, coke and lime stone loaded into the shaft-type gasification and melting furnace, as well as in the air blown in through the tuyere at the bottom of the furnace. For the calculation, the feeding rate and discharge rate are expressed in Kmol/h. Fig. 1 Shaft-type gasification and melting furnace Table 3 Input data for calculation Elements and Compounds Kmol/h Municipal C 1.74E+01 solid waste O 9.43E+00 H 2.89E+01 Cl 1.17E-01 S 2.55E-02 H 2 O 2.15E+01 CaO 1.93E-01 SiO E-01 Al 2 O E-01 MgO 5.48E-02 Pb 7.53E-04 Zn 9.29E-03 Na 1.25E-01 K 3.38E-02 Cu 1.38E-02 Fe 3.66E-01 Coke C 4.12E+00 SiO E-01 Lime stone CaCO E-01 Air N E E+00 O 2 While it is difficult to estimate an accurate melting temperature in the shaft-type furnace, the temperature of molten substance ranges from 1773 to 1823K when it is periodically discharged from the bottom of the furnace, and the temperature becomes higher when the red-hot coke reacts with the air near the tuyere. The melting temperature is consequently set at two points: 1823K and 2023K. Partial pressure of oxygen po 2 in the furnace is determined by the ratio between the amount of carbon loaded into the furnace and the amount of 810

5 loaded into the furnace and the amount of oxygen blown in through the tuyere, that is to say: C(s) + O 2 (g) = CO 2 (g) (1) 2CO(g) + O 2 (g) = 2CO 2 (g) (2) po 2 = 1/Kp 1 (pco 2 /pco) 2 (3) Kp 1 : a function of temperature Equation (1) describes the incineration reaction of carbon, and equation (2) expresses the chemical equilibrium condition between CO, O 2, and CO 2 gases in the gas phase. Consequently, po 2 varies according to the change in temperature[11]. po 2 can thus be controlled by changing the ratio between carbon and oxygen in the furnace. However, due to the effect of combustion characteristics, the operating condition of melting treatment cannot be varied in such a wide po 2 range as in the case of the calculation. Each type of gasification and melting furnace is operated within a certain limited po 2 range. For the shaft-type gasification and melting furnace, calculation was carried out assuming standard amounts of carbon and oxygen that are estimated from the operation data obtained from the actual operations. FACTSAGE calculates the kinds and ratios of elements and compounds in a system so that the Gibbs free energy of the subject system is minimized, and the thermodynamic computing engine includes the latest data of thermodynamics. Output of the calculation indicates the kind and amount of each element or compound in the gas, liquid, or solid phase Calculation results with the thermodynamic reaction model Table 4 shows the calculation results of the kinds and product ratios of the elements and compounds generated in the gas phase at various temperatures. Volatilization ratios are also obtained from the calculated product ratios of the elements as shown in Table 5. Na volatilizes either in the form of NaCl or Na itself, while at a higher temperature, the ratio of Na in metallic form increases. While K shows a similar tendency to Na, the bonding force to chlorine is stronger and thus volatilization in the form of KCl is dominant. Pb and Zn volatilize in metallic form. The calculation results show that the volatilization ratio of Na is 41% at 1823K and 62% at 2023K; and for K, 92% at 1823K and 95% at 2023K. For both of Pb and Zn, the volatilization ratio is 100% at either temperature. These results are compared with the volatilization ratios of the elements as shown in Table 2, which have been obtained from the material balance on the actual operations. Both results tend to show good agreement. The calculated volatilization ratio of K shows a higher value than that of the actual operations, which indicates that K creates chloride compounds more easily than Na and hence volatilizes more easily. The above-described results suggest that the volatilization mode of each element changes, either in the form of chloride or metal itself, depending on the temperature. Table 4 Product ratios of Na, K, Pb and Zn compounds in melting zone derived by calculations. Temperature (K) Na K Pb Zn Product ratio Species ( % ) Species Na NaCl Na NaCl (NaCl) (NaCl) Product ratio ( % ) NaOH 7.0 NaOH 11.7 K KCl K KCl (KCl) 2 KOH (KCl) 2 KOH Pb 91.5 Pb 93.3 PbCl PbCl PbCl 0.9 PbCl 0.6 PbS PbO PbS PbO Zn ZnCl Zn ZnCl Table 5 Volatilization ratios of Na, K, Pb and Zn compounds in melting zone derived by calculations Temperature (K) Na K Pb Zn Relationship between the volatilization behavior and the temperature of shaft-type gasification and melting furnaces Assumptions for calculation By calculating equilibrium at each temperature step after municipal solid waste is loaded from the top of the furnace until it reaches the melting temperature at the bottom of the furnace, the volatilization characteristics are examined at each temperature step. When calculating equilibrium at each temperature step, the following conditions are assumed on the evaporation of the water contained in the municipal solid waste, combustible C, H, and O components, 811

6 and the substance condensed from the generated gas phase: 1) Water in municipal solid waste evaporates at 573K. Unlike CO or CO 2, however, H 2 O and H 2 are gasified in the drying zone or thermal decomposition zone in the upper or middle part of the furnace and discharged out of the furnace, having no influence on the atmosphere in the melting zone. 2) The bonding of hydrocarbons composed of C, H, and O is disconnected and these combustible elements are in the atomic state. 3) Substances created in the condensation process of the gases generated at various temperature steps are discharged out of the furnace in the form of dust Calculation procedure Figure 2 illustrates the calculation model for chemical equilibrium in the shaft-type gasification and melting process. The carbon contained in the municipal solid waste and coke reacts with the air at the bottom of the furnace, and CO, CO 2, and N 2 gases are generated. These gases that govern the reactive atmosphere in the furnace move upward from the bottom of the furnace and are discharged from the top of the furnace. The temperature of the gases lowers while they are moving upward in the furnace, Municipal Solid waste Carbon in MSW and Coke Exhaust gas Exhaust gas Temperature Oxygen partial pressure Gas phase T 1 Po 2 1 T 2 Po 2 2 Solid phase Equibrium reaction Air through tuyere T n Po 2 n Molten slag Thermal decomposition matter Fig.2 Step-up reaction model of shaft type gasification and melting furnace 812

7 Fig.3 Amounts of Na, K, Pb and Zn elements and compounds in gas phase and the po 2 consequently changes. Since the temperature changes in a wide range, the calculation is performed for the equilibrium between three phases: the gas, liquid, and solid phases. The equilibrium calculation is carried out from 573K to 1573K with a temperature step of 200K to obtain the kinds and amounts of elements and compounds in the gas and solid phases. Calculation results of the elements and Table 6 Product ratios of Na, K, Pb and Zn elements and compounds in gas phase Temperature (K) NaCl Na (NaCl) Na KCl K (KCl) K PbCl Pb PbCl Pb PbS PbO Zn ZnCl Zn compounds existing in the solid and liquid phases are used as the input data for the next-step calculation at a temperature of +200K, where those migrating into the gas phase are excluded. This procedure is repeated to a final melting temperature of 2023K Calculation results and discussion Figure 3 shows the amounts of Na, K, Pb, and Zn compounds in the gas phase at various temperatures. The volatilization ratio of each element reaches its maximum at a different temperature. The volatilization ratio of Na reaches a maximum at around 1173K, and then decreases afterward. The maximum volatilization temperature of K is 1173K. These alkali metals volatilize predominantly in the form of chloride at around 1173K. Pb already volatilizes at 973K mainly in the form of PbCl 2, whereas the amount of volatilization is very small above 1173K. Up to 1173K, Zn volatilizes in the form of chloride, whereas in metallic form, beyond that temperature. Table 6 shows the product ratios by element at various temperatures. Up to 1373K, Na and K volatilize predominantly in the form of chloride, NaCl and KCl, whereas they begin to volatilize predominantly in metallic form at 1573KºC. Up to 1173K, Pb predominantly volatilizes in chloride form, PbCl and PbCl 2, whereas in metallic form, above 1373K. Up to 973K, Zn volatilizes in chloride form, ZnCl 2, whereas in metallic form, above 1373K. In summary, Na, K, and Zn volatilize predominantly in chloride form up to 1173K, whereas in metallic form, above 1573K. Compared to Na, K, and Zn, Pb begins to volatilize at a lower temperature, and up to 813

8 1173K, it volatilizes in chloride form. As the temperature further rises, the amount of volatilization decreases and it begins to volatilize in metallic form. Since the Pb content is low in comparison to the other three elements, the transition of volatilization from chloride to metallic form does not clearly appear. In this study, the calculation is based on the typical composition of municipal solid waste. It is thought that the result will be slightly different in the cases of higher Pb content or different chlorine concentration. The temperature dependence of the saturated vapor pressures of Na, K, Pb, and Zn chlorides indicates that ZnCl 2 has the highest vapor pressures at the same temperature, followed by PbCl 2, KCl and NaCl. The melting points of Zn and Pb chlorides are 599K and 771K, respectively. Above the melting point temperature, the gas and liquid phases exist in equilibrium and the pressure in the gas phase follows the saturated vapor pressure. Na and K chlorides become liquid above 1073K, and they consequently begin to volatilize at a higher temperature than those of Pb or Zn. Under the existence of chlorine, Pb volatilizes at a relatively lower temperature around 973K. Since the bonding force of K or Na to chlorine is stronger than that of Pb or Zn, creation of KCl and NaCl starting at around 1173K suppresses the reaction to create Pb or Zn chlorides, resulting in a drastic change at 1373K in the volatilization mode from chloride compounds to metallic form. In addition, the amount of volatilization of Na and K increases as the temperature rises above 1573K, in contrast to the decrease in the amount of volatilization of Pb and Zn above 1373K. In a reducing atmosphere, Pb and Zn do not diffuse into the molten slag, but are mostly gasified as the temperature rises. Meanwhile, regardless the partial pressure level of oxygen, Na and K are taken into the molten slag in the form of Na 2 O and K 2 O within the silicate structure, as shown in Fig. 1. Consequently, as the temperature rises, these Na 2 O and K 2 O are gasified and released from the slag. 3. Conclusion Using computational thermodynamics software FACTSAGE version 5.5, we have examined the volatilization behaviors in the melting process of the low-boiling-point elements, Na, K, Pb, and Zn, contained in municipal solid waste. 1) As the temperature rises, the volatilization ratios of Na, K, Pb, and Zn increase. The behavior of Na and K has a little dependence on the po 2 level, whereas the volatilization of Pb and Zn is accelerated in a reducing atmosphere and suppressed in an oxidation atmosphere. 2) We have calculated the volatilization ratios of Na, K, Pb, and Zn contained in municipal solid waste loaded into the shaft-type gasification and melting furnace. The calculation results show good agreement with the volatilization ratios obtained from the actual operations. 3) The calculation results indicate that the melting point temperature affects the mode of volatilization of Na, K, Pb, and Zn, that is to say, the volatilization occurs in the form of metal chlorides up to 1373K, whereas in metallic form, above 1573K. REFERENCES 1. Japan society of industrial machinery manufacturers Eco-slag utilization promotion center annual report 2005, p13,(2005) (in Japanese) 2. F.Kanbayashi, S.Abe, M.Kokado, 8 th Conference of the Japan society of waste management experts, pp ,(1997) (in Japanese) 3. M.Kokado, 8 th Conference of the Japan society of waste management experts, pp ,(1997) (in Japanese) 4. S.Osada, M.Osada, M.Kokado, M.Tokuda, Thermodynamic study of the Behavior of Elements with Low Boiling Points in the Melting Process Journal of Japan society of waste management experts, Vol.15, No.5, pp ,(2004) (in Japanese) 5. Japan waste management association, Design / construction point of waste disposal plant maintenance, pp ,(1999) (in Japanese) 6. T.Matsumiya, A.Kiyose, tetsu-to-hagane, Vol.88, No.2, pp51-58,(2002)(in Japanese) 7. N.Watanabe, Proximate Analysis, heat Measurement and elemental analysis of waste, Journal of Japan society of waste management experts, Vol.11, No.6, pp , (2000) (in Japanese) 8. Japan waste management association, Design / construction point of waste disposal plant maintenance, pp ,(1999) (in Japanese) 9.Bale,C.W.,Chartrand,P.,Degterov,S.A.,Eriksson,G.,Hack,K.,BenMahfoud,R.melancon,J.,Pelton,A.D.,and Petersen,S. FACTSAGE thermochemical software and databases. Calphad,Vol.26,No.2 pp ,(2002). 10. Japan waste management association, Design / construction point of waste disposal plant maintenance, Appendix5 Gasification and Melting Concepts of Each Company, pp ,(1999) (in Japanese) 11. M. Otani, Iron metallurgical thermodynamics, Nikkan-Kogyo-Shinbunsya, pp106,(1971) (in Japanese) 814