thermodynamic and dynamic analysis of sludge
THERMODYNAMIC AND DYNAMIC ANALYSIS OF SLUDGE INCINERATION FLY ASH MELTING PROCESS WITH DIFFERENT PARTICLE SIZE FENG WANG, YANLONG LI, TIANHUA YANG, RUNDONG LI* College of Energy and Environment, Shenyang Aerospace University, Key Laboratory of Clean Energy, Liaoning Province, Shenyang 110136, P.R. China
SUMMARY: In this paper, the sludge incineration fly ash collected by the small circulating fluidized bed was separated by the different particle sizes: 50μm. The differential thermal and thermo-gravimetric analysis of fly ash from different sizes were carried out by using TG-DSC integrated thermal analyzer. The fly ash samples were calcined at different temperatures (900°C, 1000°C, 1100°C) in a tube furnace, and the change of mineral composition of fly ash is discussed. The results shows that: the fly ash melting process is an endothermic reaction. The overall endothermic response of fly ash melting comprises three reactions: drying dehydration, polycrystalline transformation and melt phase transformation. The endothermic peak at temperature range of 420-500°C should be the polycrystalline transition process. The melting process of fly ash occurs in the temperature range of 900~1200°C, and as the fly ash particle size increases, the starting temperature of the melting process increases: 902°C, 916°C, 968°C, 1006°C and 1013°C, respectively. In the process of fly ash melting, the components of fly ash SO3 began to volatilize from 900°C, and the content of SO3 decreases from the original 6.3% to 0.6%, this is mainly in the form of anhydrite decomposition of volatile. With the increase in the particle size of fly ash, the temperature required to achieve the maximum weight loss rate of fly ash is significantly increased as follows: 1041°C, 1061°C, 1094°C, 1144°C and 1148°C. With the heating rate increase from 10°C/min to 30°C/min, the initial temperature of the melting process increase. The reason may be that the increase of the heating rate increases the inhomogeneity of the internal temperature distribution of the fly ash, which leads to overheating in the outer part of the fly ash. Based on the thermodynamic analysis, a zero-order kinetic model of fly ash melting with different particle size was established by using the Arrhenius temperature integral. The melting reaction of fly ash belongs to non-isothermal and heterogeneous reaction kinetics. Finally, the apparent activation energy of fly ash with different particle size melting transition was obtained. Keyword: sludge incineration fly ash; different particle sizes; TG-DSC; kinetic model
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
1. INTRODUCTION The rapid development of the economy and the process of urbanization have brought a considerable volume of sewage sludge which comprise some harmful and toxic substances, such as viruses, bacteria, dioxins, non-biodegradable organic compounds, heavy metals, and so on(Le-huan Yu et al., 2016; Smith K.M. et al., 2009; Syed Shatir A.S. et al., 2017). In 2016, China produced 30-40 million tons of water content of about 80% of the sewage sludge, and the processing rate is less than 30%, the historical stacking of sewage sludge is huge and the production has dramatically increased at an annual rate of 4% in recent years (Wen-yi Deng et al., 2009). Many conventional methods such as farmland application and sewage sludge disposal in landfills and oceans can cause serious pollution accidents or need high treatment costs, and are not environmentally friendly or cost-effective during the treatment of sewage sludge (Xue-bin Wang et al., 2016). Sludge incineration is a means of heat treatment of sludge with high temperature, which the organic waste in the sludge and the sufficient air in the incinerator oxidation decomposition reaction. Incineration has been considered as the most thorough, quick and economical way of sludge disposal due to its advantages on stabilization, volume reduction and resource recovery (Khiari B. et al., 2006; Werther J. and Ogada T., 1999; Wen-yi Deng et al., 2009). It is expected that sewage sludge incineration will increase rapidly in the next decade in China (Wen-yi Deng et al., 2009). Nevertheless even the incineration has advantages due to the reduction of sewage sludge volume and lower disposal costs, it is not a complete method since almost 30% of the solids remain in incineration residues as fly ash and air pollution control (APC) residues (Monika K. et al., 2016). The incineration fly ash that contains higher concentrations of toxic heavy metals (Okada T. and Hiroki T., 2013; T Okada et al., 2007) and dioxin (He-fa Cheng et al., 2010) is recognized as hazard waste. In China, fly ash was included in the "National List of Hazardous Wastes" in 2008. As the sewage sludge composition is more complex, containing heavy metal pollutants in the process of incineration and transformation. If the fly ash directly landfill or improper handling, due to acid rain and other factors, the heavy metal will gradually leach out in the acidic environment, polluted of underground water and endanger humans. Therefore, it is urgent to take effective measures to address incineration fly ash in China. The appropriate treatments for APC residues can be grouped into three classes: (i) separation processes; (ii) solidification/stabilization (S/S); (iii) thermal methods (Margarida J. Q. et al., 2008; Hong-yun Hu et al., 2012). Sintering, melting and other high-temperature treatment methods has the advantage of good treating effect and reducing the amount of fly ash and it becomes a tendency of fly ash treating in the world. The fly ash melting process is newly developed since 1990s, and the use of fly ash melting technology is on the rise in Japan (Hiroki H. et al., 2005). After the melting process, the mass and volume of the residues is greatly reduced, producing a high-density melted product. By melting the residues at such a high temperature and with the change in physical and chemical state, it is possible to produce a melted slag with high stability. The results show that the increase of the density of the fly ash can be reduced by about 70%, and if the slag is taken into account, the slump capacity can be up to 1/20 (Chang-Hwan Jung and Masahiro O., 2007). The melting technology makes incinerator residues, bottom ash and fly ash, stable and non-toxic. Moreover, this type of treatment allows the melted slag to be used as a resource again (Sakai S. and Hiraoka M., 2000). For this reason, the melting of fly ash in many countries to achieve rapid development, many scholars on the melting of incineration fly ash carried out a lot of research. Run-dong Li has studied the mechanism of melting MSWI fly ash: When more than one crystal state of the same substance can be found, classic thermodynamic considerations show that the polymorph that has the lowest free enthalpy is most stable. The crystal becomes mechanically unstable or
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
unrealizable when temperature is increased, which leads to a polymorphic transformation and equilibrium change from one phase to another (Run-dong Li et al., 2007). Takashi Okada finds that Pb and Zn are volatilizing while leaving behind Fe and Cu in the ash-melting processes (Okada T. and Hiroki T., 2013). In order to get a better understanding of the process of fly ash melting, much work had been conducted using thermo-gravimetry/derivative thermo-gravimetry (TG/DTG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) techniques in recent decades. Thermo-gravimetric analysis (TGA) was used to characterize the thermal decomposition of coal and, more recently, to characterize biomass fuels (Saldarriaga J.F. et al., 2015; Sanchez M.E. et al., 2009) and their co-combustion with coal (Otero M. et al., 2008; Otero M. et al., 2011), and also the pyrolysis process oily sludge with plant biomass (Shuang-hui Deng et al., 2016; Mustafa V.K. 2012; Wan-fen Pu et al., 2015). The advantage of TGA is that provides a rapid assessment of the fuel value, the temperatures at which combustion starts and ends and other characteristics such as maximum reactivity temperature or total combustion time (Coimbra R.N. et al., 2015). Massimo Tettamanti studies the results of fly-ash characterization by differential scanning calorimetry (DSC) and finds that: the endothermic processes are thermodynamically controlled and include desorption of organic compounds along with a phase transition, instead, exothermic processes are kinetically controlled and include two distinct combustion reactions (Tettamanti M. et al., 1998). Therefore, this paper focused on the thermal behavior of fly ash and the kinetic of the fly ash melting process. In this paper, the incineration fly ash was separated into five different sizes ( 50μm) by a high-precision air classification equipment. The differential thermal and thermo-gravimetric analysis of fly ash from different sizes were carried out by using TG-DSC integrated thermal analyzer. Based on the thermodynamic analysis, a zero-order kinetic model of fly ash melting with different particle size was established by using the Arrhenius temperature integral. The research on the melt kinetics of fly ash with different particle size will contribute to the classification of fly ash and play an important role in the development of new fly ash treatment technology and then find a potential solution for fly ash management.
2. THERMAL ANALYSIS EXPERIMENT 2.1 Sample preparation The fly ash used in the experiments was collected form a sewage sludge incineration power plant in Zhejiang Province, China. The different sizes of fly ash was separated by a highprecision air classification equipment which can form an overall flow with almost no vortex or only a very small local vortex. The fine particles of different particle size is no longer mixed, which can flow out soon witch the air flow. In order to obtain the fineness and precision of the grading treatment, the rotation speed of the induced draft fan was adjusted to regulate the size of the internal negative pressure traction of the equipment. Meanwhile, it is also necessary to control the feeding speed to maintain the stability of the equipment’s internal gas powder concentration. Compared with the sizing screen, the high-precision air classification equipment has the minimum damage to the original particles. So the final classification’s accuracy was improved significantly. The fly ash was separated by the different particle sizes: 50μm. The particle size distribution is analyzed by a Mastersizer 2000, which uses a wet measurement, as shown in Fig.1. The chemical composition of the samples were analyzed by an X-ray fluorescence spectrum analyzer (ZSX100e), as shown in Table 1. X-ray diffraction
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
(PRO/MPD) was used to analyze the mineral phase of the different particle sizes, as shown in Fig.2. Simultaneously, the microstructure of the samples were accessed by means of the scanning electron microscopy, as shown in Fig.3: A, B, C, D and E. with the magnification of 20,000 times. The image A1 is a particle size
SUMMARY: In this paper, the sludge incineration fly ash collected by the small circulating fluidized bed was separated by the different particle sizes: 50μm. The differential thermal and thermo-gravimetric analysis of fly ash from different sizes were carried out by using TG-DSC integrated thermal analyzer. The fly ash samples were calcined at different temperatures (900°C, 1000°C, 1100°C) in a tube furnace, and the change of mineral composition of fly ash is discussed. The results shows that: the fly ash melting process is an endothermic reaction. The overall endothermic response of fly ash melting comprises three reactions: drying dehydration, polycrystalline transformation and melt phase transformation. The endothermic peak at temperature range of 420-500°C should be the polycrystalline transition process. The melting process of fly ash occurs in the temperature range of 900~1200°C, and as the fly ash particle size increases, the starting temperature of the melting process increases: 902°C, 916°C, 968°C, 1006°C and 1013°C, respectively. In the process of fly ash melting, the components of fly ash SO3 began to volatilize from 900°C, and the content of SO3 decreases from the original 6.3% to 0.6%, this is mainly in the form of anhydrite decomposition of volatile. With the increase in the particle size of fly ash, the temperature required to achieve the maximum weight loss rate of fly ash is significantly increased as follows: 1041°C, 1061°C, 1094°C, 1144°C and 1148°C. With the heating rate increase from 10°C/min to 30°C/min, the initial temperature of the melting process increase. The reason may be that the increase of the heating rate increases the inhomogeneity of the internal temperature distribution of the fly ash, which leads to overheating in the outer part of the fly ash. Based on the thermodynamic analysis, a zero-order kinetic model of fly ash melting with different particle size was established by using the Arrhenius temperature integral. The melting reaction of fly ash belongs to non-isothermal and heterogeneous reaction kinetics. Finally, the apparent activation energy of fly ash with different particle size melting transition was obtained. Keyword: sludge incineration fly ash; different particle sizes; TG-DSC; kinetic model
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
1. INTRODUCTION The rapid development of the economy and the process of urbanization have brought a considerable volume of sewage sludge which comprise some harmful and toxic substances, such as viruses, bacteria, dioxins, non-biodegradable organic compounds, heavy metals, and so on(Le-huan Yu et al., 2016; Smith K.M. et al., 2009; Syed Shatir A.S. et al., 2017). In 2016, China produced 30-40 million tons of water content of about 80% of the sewage sludge, and the processing rate is less than 30%, the historical stacking of sewage sludge is huge and the production has dramatically increased at an annual rate of 4% in recent years (Wen-yi Deng et al., 2009). Many conventional methods such as farmland application and sewage sludge disposal in landfills and oceans can cause serious pollution accidents or need high treatment costs, and are not environmentally friendly or cost-effective during the treatment of sewage sludge (Xue-bin Wang et al., 2016). Sludge incineration is a means of heat treatment of sludge with high temperature, which the organic waste in the sludge and the sufficient air in the incinerator oxidation decomposition reaction. Incineration has been considered as the most thorough, quick and economical way of sludge disposal due to its advantages on stabilization, volume reduction and resource recovery (Khiari B. et al., 2006; Werther J. and Ogada T., 1999; Wen-yi Deng et al., 2009). It is expected that sewage sludge incineration will increase rapidly in the next decade in China (Wen-yi Deng et al., 2009). Nevertheless even the incineration has advantages due to the reduction of sewage sludge volume and lower disposal costs, it is not a complete method since almost 30% of the solids remain in incineration residues as fly ash and air pollution control (APC) residues (Monika K. et al., 2016). The incineration fly ash that contains higher concentrations of toxic heavy metals (Okada T. and Hiroki T., 2013; T Okada et al., 2007) and dioxin (He-fa Cheng et al., 2010) is recognized as hazard waste. In China, fly ash was included in the "National List of Hazardous Wastes" in 2008. As the sewage sludge composition is more complex, containing heavy metal pollutants in the process of incineration and transformation. If the fly ash directly landfill or improper handling, due to acid rain and other factors, the heavy metal will gradually leach out in the acidic environment, polluted of underground water and endanger humans. Therefore, it is urgent to take effective measures to address incineration fly ash in China. The appropriate treatments for APC residues can be grouped into three classes: (i) separation processes; (ii) solidification/stabilization (S/S); (iii) thermal methods (Margarida J. Q. et al., 2008; Hong-yun Hu et al., 2012). Sintering, melting and other high-temperature treatment methods has the advantage of good treating effect and reducing the amount of fly ash and it becomes a tendency of fly ash treating in the world. The fly ash melting process is newly developed since 1990s, and the use of fly ash melting technology is on the rise in Japan (Hiroki H. et al., 2005). After the melting process, the mass and volume of the residues is greatly reduced, producing a high-density melted product. By melting the residues at such a high temperature and with the change in physical and chemical state, it is possible to produce a melted slag with high stability. The results show that the increase of the density of the fly ash can be reduced by about 70%, and if the slag is taken into account, the slump capacity can be up to 1/20 (Chang-Hwan Jung and Masahiro O., 2007). The melting technology makes incinerator residues, bottom ash and fly ash, stable and non-toxic. Moreover, this type of treatment allows the melted slag to be used as a resource again (Sakai S. and Hiraoka M., 2000). For this reason, the melting of fly ash in many countries to achieve rapid development, many scholars on the melting of incineration fly ash carried out a lot of research. Run-dong Li has studied the mechanism of melting MSWI fly ash: When more than one crystal state of the same substance can be found, classic thermodynamic considerations show that the polymorph that has the lowest free enthalpy is most stable. The crystal becomes mechanically unstable or
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
unrealizable when temperature is increased, which leads to a polymorphic transformation and equilibrium change from one phase to another (Run-dong Li et al., 2007). Takashi Okada finds that Pb and Zn are volatilizing while leaving behind Fe and Cu in the ash-melting processes (Okada T. and Hiroki T., 2013). In order to get a better understanding of the process of fly ash melting, much work had been conducted using thermo-gravimetry/derivative thermo-gravimetry (TG/DTG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) techniques in recent decades. Thermo-gravimetric analysis (TGA) was used to characterize the thermal decomposition of coal and, more recently, to characterize biomass fuels (Saldarriaga J.F. et al., 2015; Sanchez M.E. et al., 2009) and their co-combustion with coal (Otero M. et al., 2008; Otero M. et al., 2011), and also the pyrolysis process oily sludge with plant biomass (Shuang-hui Deng et al., 2016; Mustafa V.K. 2012; Wan-fen Pu et al., 2015). The advantage of TGA is that provides a rapid assessment of the fuel value, the temperatures at which combustion starts and ends and other characteristics such as maximum reactivity temperature or total combustion time (Coimbra R.N. et al., 2015). Massimo Tettamanti studies the results of fly-ash characterization by differential scanning calorimetry (DSC) and finds that: the endothermic processes are thermodynamically controlled and include desorption of organic compounds along with a phase transition, instead, exothermic processes are kinetically controlled and include two distinct combustion reactions (Tettamanti M. et al., 1998). Therefore, this paper focused on the thermal behavior of fly ash and the kinetic of the fly ash melting process. In this paper, the incineration fly ash was separated into five different sizes ( 50μm) by a high-precision air classification equipment. The differential thermal and thermo-gravimetric analysis of fly ash from different sizes were carried out by using TG-DSC integrated thermal analyzer. Based on the thermodynamic analysis, a zero-order kinetic model of fly ash melting with different particle size was established by using the Arrhenius temperature integral. The research on the melt kinetics of fly ash with different particle size will contribute to the classification of fly ash and play an important role in the development of new fly ash treatment technology and then find a potential solution for fly ash management.
2. THERMAL ANALYSIS EXPERIMENT 2.1 Sample preparation The fly ash used in the experiments was collected form a sewage sludge incineration power plant in Zhejiang Province, China. The different sizes of fly ash was separated by a highprecision air classification equipment which can form an overall flow with almost no vortex or only a very small local vortex. The fine particles of different particle size is no longer mixed, which can flow out soon witch the air flow. In order to obtain the fineness and precision of the grading treatment, the rotation speed of the induced draft fan was adjusted to regulate the size of the internal negative pressure traction of the equipment. Meanwhile, it is also necessary to control the feeding speed to maintain the stability of the equipment’s internal gas powder concentration. Compared with the sizing screen, the high-precision air classification equipment has the minimum damage to the original particles. So the final classification’s accuracy was improved significantly. The fly ash was separated by the different particle sizes: 50μm. The particle size distribution is analyzed by a Mastersizer 2000, which uses a wet measurement, as shown in Fig.1. The chemical composition of the samples were analyzed by an X-ray fluorescence spectrum analyzer (ZSX100e), as shown in Table 1. X-ray diffraction
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
(PRO/MPD) was used to analyze the mineral phase of the different particle sizes, as shown in Fig.2. Simultaneously, the microstructure of the samples were accessed by means of the scanning electron microscopy, as shown in Fig.3: A, B, C, D and E. with the magnification of 20,000 times. The image A1 is a particle size
