- Email: [email protected]
Thermogravimetric study and kinetic analysis of dried industrial sludge pyrolysis
Waste Management 41 (2015) 128–133
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Thermogravimetric study and kinetic analysis of dried industrial sludge pyrolysis Guangrui Liu a, Huijuan Song a,b, Jinhu Wu a,⇑ a b
Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 20 November 2014 Accepted 31 March 2015 Available online 16 April 2015 Keywords: Sludge Pyrolysis DAEM TGA Kinetic study
a b s t r a c t Thermogravimetric experiments of two different industrial sludge samples were carried out with nonisothermal temperature programs. The results indicated that the pyrolysis process contains three obvious stages and the main decomposition reaction occurred in the range of 200–600 °C. The distributed activation energy model (DAEM) was also proposed describing equally well the pyrolysis behavior of the samples. The calculated activation energy was ranged from 170 to 593 kJ/mol and 125 to 756 kJ/mol for SLYG (sludge sample from chemical fiber factory) and SQD (sludge sample from woody industry), respectively. The reliability of this model not only provided good fit for all experiments, but also allowed accurate extrapolations to relative higher heating rates. Besides, the FTIR measurement was also used to further understand the relationship between pyrolysis behavior and chemical structures for industrial sludge. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The sludge is one of the most significant hazardous solid wastes generated from the waste water treatment processes. It is a complex emulsion of various organic chemical compounds and solid particles, as well as high concentration of water and heavy metals (Werle and Wilk, 2010). The improper disposal or insufficient treatment of sludge could lead serious environmental destruction and human health impacts. Thus, a variety of effective sludge treatment technologies have been developed to eliminate the hazardous constituent and mitigate its negative impacts, due to its increasing production quantity around the world (Kelessidis and Stasinakis, 2012). The most common sludge treatment methods are containing landfill, incineration and pyrolysis (Tian et al., 2011). However, considering the farm land limitations and more restrictive environmental regulations, the landfill and incineration technologies are becoming more and more difficult to operate (Montusiewicz and Lebiocka, 2011; Smidt and Parravicini, 2009; Donatello and Cheeseman, 2013). Thus, the energetic valorization method of thermal decomposition was regarded as a promising alterative technology and fast developed due to its more economical and environmental friendly.
⇑ Corresponding author. Tel.: +86 532 80662763. E-mail addresses: [email protected] (G. Liu), [email protected] (J. Wu). http://dx.doi.org/10.1016/j.wasman.2015.03.042 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
In order to transform the organic sludge to the fuels or valuable chemical sources, the pyrolysis method was employed around the world (Manara and Zabaniotou, 2012; Chang et al., 2013; Velghe et al., 2013). Pyrolysis is the thermal decomposition of organic compounds at elevated temperatures in an inert environment. The main products are including non-condensable gases, lower molecular weight liquid bio-oils as well as solid char (Fonts et al., 2012). Besides, the merits of this method are focusing on its lower emissions and capacity that enables heavy metals and most of other hazardous trace elements to be concentrated or retained partially in the final char (Hu et al., 2013). Zhang et al. (2014) studied the pyrolysis of sewage sludge in a free-fall reactor. They indicated that the high temperature was in favor of syngas generation in gas product and the volatile matter could be completely released to gaseous product at 1300 °C. Besides, the energy balance results also showed that the gaseous product contains the most heating value of sludge. Magdziarz and Werle (2014) studied the combustion and pyrolysis characteristics of three types of sewage sludge from waste water treatment plants. They found the main devolatilization pyrolysis reaction took place between 200 and 540 °C, associated to the degradation of protein and soluble polysaccharide. They also obtained the detailed identification of gaseous evolved during thermal reaction. Analyses showed the main gaseous components are mainly containing H2, CO, CO2 and CH4. In order to obtain a better understanding of the physical and chemical characteristics of this thermal process as well as
129
G. Liu et al. / Waste Management 41 (2015) 128–133
reasonable devolatilization mechanisms, many researchers have done lots of contributions on the kinetic study of sewage sludge pyrolysis (Gao et al., 2014). Meantime, the distribution activation energy model (DAEM) was succeeding employed to investigate the kinetic behaviors during the pyrolysis of dried sewage sludge, due to its well prediction capability during the whole devolatilization process for all kinds of complicated reaction systems such as coal, charcoal, shale, oil asphalt and biomass (Tang et al., 2014). However, few studies focused on the kinetic analysis of pyrolysis for dried industrial sludge. Considering the fast development of society and great concern on the environment, large quantity of waste sludge were generated from different industrial plants. The sludge exhibited various constitutes and structures because they were used to deal with different waste water sources. Furthermore, the effects of functional groups structures for sludge pyrolysis have often been neglected in previously studies. In present study, two different chemical sludge samples were used to investigate the thermal degradation characteristics and pyrolysis kinetics by TGA method. The sludge was pyrolyzed in a non-isothermal environment, and thermogravimetric analysis by DAEM method was carried out. In order to further explore the relationship between the chemical structure and pyrolysis, the variation of functional groups was also detected by FTIR method.
dispersed in KBr. The spectral range was ranged from 4000 to 400 cm1. Ten scans were performed with a resolution of 1 cm1. 2.3. Thermogravimetric analysis (TGA) The pyrolysis characteristics of the samples were examined using a thermogravimetric analyzer (TG, PerkinElmer Diamond TG/DTA). The sludge was loaded in a crucible and placed inside the TG where the weight of the sample was constantly measured. For each experimental run, around 5 mg of sample with particle size of approximate 200 meshes was used. The heating temperature was ranged from 25 to 1000 °C at four different heating rates (5, 10, 20 and 40 °C/min). In order to provide an inert environment without oxygen for the sludge pyrolysis, the argon gas was used as the carrier gas, whose flow rate was fixed at 40 mL/min. Each test was repeated four times obtaining differences lower than 5% to guarantee repeatability. 2.4. Pyrolysis kinetics The non-isothermal kinetics for dried sludge decomposition can be written as follows:
dX ¼ kf ðXÞ dt
2. Materials and methodology
ð1Þ
where X is sample conversion, and is given by
2.1. Materials Two sludge samples (SQD and SLYG) were obtained from different chemical industries in China. These samples were heated for 24 h at the temperature of 110 °C to remove the free water before the experiments. In order to avoid the effect of heat and mass transfer during the thermal decomposition, all the dried sludge was sieved into 200 meshes particle. The ultimate analysis was carried out in a TruSpec CHN and TruSpec S analyzer (Leco, USA) while the proximate analysis was run in a TGA Q500 TA instrument (TA, USA). The moisture, volatiles and ashes were measured according to the ASTM standard test method No. E871, E872 and D1102, respectively. The proximate and ultimate analysis of sludge samples were shown in Table 1. The samples exhibited the usual properties of sludge materials, such as relative high oxygen content and lower hydrogen content. Their nitrogen content is also remarkably high, which is due to proteins and alkaloids in their structural compositions.
X¼
Vi V Vi Vf
ð2Þ
For this equation, V, Vi and Vf represent the instantaneous, initial and final weights of the sludge sample. The function f(X) can be written as
f ðXÞ ¼ ð1 XÞn
ð3Þ
where n is assuming reaction order. For the DAEM method, it is assumed that many irreversible first-order parallel reactions with different rate parameters occur simultaneously. In other words, all frequency factors differ only in activation energy and the number of independent reactions is larger enough to permit continuous Gaussian distribution of the activation energy, which is to be expressed as a function of f(E) and
Z
1
f ðEÞdE ¼ 1
ð4Þ
0
2.2. Fourier transform infrared spectroscopy analysis (FTIR)
Then the total amount of the volatile material released up to time t is given by
Surface functional groups of different chemical sludge samples were analyzed using a VECTOR22 FTIR spectrometer (Bruker, Germany). The tablet was made with 1 wt.% of the sludge sample
Ultimate analysis (dry-ash-free basis) C (wt.%) H (wt.%) O (wt.%) N (wt.%) S (wt.%)
SQD
SLYG
6.82 50.39 5.94 36.85
8.70 59.50 9.57 22.23
51.70 6.83 30.76 9.82 0.89
Z
1
0
Z exp k0 0
t
E dt f ðEÞdE exp RT
ð5Þ
For the process with a constant heating rate, b, temperature, T, against time, t, follows
Table 1 The proximate and ultimate analysis of sludge samples.
Proximate analysis (dry basis) Moisture (wt.%) Volatiles (wt.%) Fixed carbon (wt.%) Ash (wt.%)
1X ¼
49.49 6.76 32.51 10.21 1.03
T ¼ T 0 þ bt
ð6Þ
The integral method based on DAEM equation is used in previous work (Cai et al., 2007), and the approximate integration of Eq. (5) gives
ln
b T
2
¼ ln
k0 R E þ 0:6075 E RT
ð7Þ
The Arrhenius plot of ln(b/T2) versus 1/T becomes a linear line for each rate of devolatilization. Thus, the activation energy (E) and frequency factor (k0) can be determined from the slope and intercept of the regression line, respectively.
130
G. Liu et al. / Waste Management 41 (2015) 128–133
and ACAO function group, respectively. Moreover, the C@O group is mainly from the acids and aldehydes. Compared with other research, the similar absorption peaks of ACONHA were also detected at 1549 cm1 (amid II band), 1656 cm1 (amid I band) and 3420 cm1 (NAH stretching), which might be the characteristic of certain amounts of proteins in sludge samples (Ma et al., 2013; Zhao et al., 2013). In the range of 670–860 cm1, the absorption peaks are characterized by aromatic rings. Besides, the absorption peak at 1549, 1410 and 470–640 cm1 is characteristic of oxygen-functional group with nitrogen, sulfur and phosphorus, respectively. 3.2. TG and DTG results Fig. 1. The FTIR spectra of different chemical sludge samples.
3. Results and discussion 3.1. FTIR spectra of sludge samples In order to further understand the relationship between the pyrolysis behaviors and chemical structures of industrial sludge, the distributions of various functional groups were detected by FTIR analysis. FTIR spectra of different chemical sludge samples were shown in Fig. 1. The two sludge samples had similar spectrum because their composition are both varies extremophile bacteria, whose structure could be decomposed into small organic acids, amine and proteins. The spectrum for all sludge has a very broad and strong absorption band between 3250 and 3750 cm1, which was caused by OAH stretching vibration (Silva et al., 2012; Verma et al., 2012). Another wide band ranged from 2850 to 2960 cm1 was induced by the symmetric and asymmetric stretching vibration of CAH, which indicates the existence of aliphatic chain. There are also two obvious absorption peaks at 1655 and 1030 cm1, which are induced by stretching vibration of C@O
Sludge pyrolysis was a thermal process that takes place in an inert atmosphere, and where the organic matter undergoes a series of complex reactions that generate volatile gas and liquid, as well as solid char products. The TG and DTG curves of different heating rates were illustrated in Figs. 2 and 3, where it is seen that remaining mass fraction curves was shifted up the temperature scale by an increase in the heating rate. Compared with SQD sludge sample, the SLYG sample would generate much more volatiles under the same pyrolysis conditions. The similar tendency was also reflected in Table 1 while the SLYG sample has less ash amount. The reason might focus on that the SQD sludge would contain large amount of complicated matters, which were derived from the wood treatment. SLYG sample would contain more organic compounds while dealing with chemical fiber wastewater. However, a large amount of large-molecular organic matters as well as complicated inorganic compounds would accumulate in SQD sludge sample because of special material, solvents and additives used for treatment of wood products. The devolatilization process of sludge samples occurred in a wide range of temperatures between 180 and 800 °C. In order to obtain a high conversion rate, the temperature must be as high as 1000 °C, which is in good agreement to that obtained by other researchers (Scott et al., 2006; Soria-Verdugo et al., 2013).
Fig. 2. The TG profiles of sludge pyrolysis at different heating rates.
131
G. Liu et al. / Waste Management 41 (2015) 128–133
Fig. 3. The DTG profiles of sludge pyrolysis at different heating rates.
Fig. 4. The Arrhenius plot of ln(b/T2) vs. 1/T at selected conversion rates b = 5, 10, 20 °C/min.
During the whole sludge pyrolysis range, three main intervals were considered: (a) in the first interval while the temperature was below 200 °C, the total weight loss was accounting for about 10 wt.% with the reason of most moisture released from the sludge. However, this weight loss percentage was larger than the moisture content which was detected by proximate analysis (6.82 wt.% for SQD, 8.70 wt.% for SLYG). Thus, the volatilization for a certain amount of light reactive components would occur in this stage (Folgueras et al., 2013). (b) The second interval ranged from 200 to 600 °C, the pyrolysis of most biodegradable organic matters occurred together with the partial pyrolysis of other less reactive bacterial matters such as protein and carboxyl groups such as sugar components and some saturated aliphatic chains with relatively large molecular weight. This interval was the main stage for the cleavage of CAC bond (2850–2960 cm1), CAO bond (1655 and 1030 cm1) as well as aromatic rings (670–860 cm1) and oxygen-functional group with nitrogen (1549 cm1) for the generation of volatiles. During this pyrolysis stages the DTG curves also exhibited the largest peaks, which represented the maximum rate of mass loss. The total weight loss percentage of sludge was approximately up to 40 wt.%. This result was useful for directing the development of
low-temperature pyrolysis technology of sludge treatment. (c) During the third interval, a small peak would be observed at temperatures higher than 600 °C. This was mainly due to the secondary decomposition such as light char devolatilization as well as the decomposition of non-biodegradable components such as CaCO3 and other inorganic matters. However, the DTG curves for SLYG did not show this peak obviously. The reason might be focusing on that the sludge sample from chemical fiber factory contained much organic matters instead of inorganic matters. Table 2 The calculated E values for selected conversion rates. Conversion rate
ELYG (kJ/mol)
R2
EQD (kJ/mol)
R2
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
170 191 220 303 428 205 593 304 295
0.999 0.948 0.941 0.999 0.976 0.957 0.996 0.916 0.912
548 663 509 756 250 125 134 486 682
0.983 0.999 0.965 0.976 0.990 0.914 0.983 0.972 0.971
132
G. Liu et al. / Waste Management 41 (2015) 128–133
Fig. 5. Experimental and calculated T vs. X curves at b = 10, 40 °C/min for sludge pyrolysis.
3.3. Kinetic results by DAEM method For the each heating rate, a ln(b/T2) versus 1/T plot under selected values of devolatilization rate was established and presented in Fig. 4. All curves were represented by Eq. (7). Therefore, the activation energy and the frequency factor can be calculated from the slope and intercept in the Arrhenius plots at each rate of devolatilization. The selected conversion rates were also exhibited in Table 2. The results indicated that the activation energy was ranged from 170 to 593 kJ/mol and 125 to 756 kJ/mol for SLYG and SQD, respectively. The high correlation coefficients indicated that this model fits the experimental data very well. Fig. 5 showed the sludge devolatilization profiles (with the heating rate of b = 10 °C/min) which were predicted by Eq. (5) without any mathematical fitting techniques using the results of E and k0 obtained from DAEM. The results indicated that both the experimental curves and the calculated curves for every sample at existed lower heating rates well correspond each other. In order to calculate E and k0 with high accuracy, the fitting coefficient of Arrhenius plot from Eq. (7) R2 > 0.91 will be necessary. Besides, Fig. 5 also showed the comparison of devolatilization curves between the predicted and experimental results for relatively higher heating rate of b = 40 °C/min. The calculated temperatures for each conversion ratio were agreed with experiment results with minor errors. All the above findings indicate that the DAEM method is suitable to describe the pyrolysis and predict the kinetic behavior of chemical sludge even at relative higher heating rate. However, Fig. 5 also exhibited that the calculated temperature (T) was slight higher than experimental results. The reason might be focusing on that the large amount of volatiles would generate at higher heating rate. Thus, the predictions of sludge pyrolysis behaviors under much higher heating rates still are the key points during our further research. 4. Conclusions In present study, the pyrolysis of two different industrial sludge samples was analyzed by means of non-isothermal thermogravimetric experiments. The FTIR analysis was also used to determine
the decomposition behaviors by the chemical structural compositions. The results showed that three main intervals were occurred during the whole pyrolysis process. The main weight loss (about up to 40 wt.%) of industrial sludge was ranging from 200 to 600 °C, which might be accompanying with the cleavage of CAC bond, CAO bond as well as aromatic rings and oxygen-functional group with nitrogen. The distribution activation energy model was applied to reproduce the weight losses curve in order to obtain the kinetic parameters of the process and it provided good fit for all experiments. The results indicated that the activation energy was ranged from 170 to 593 kJ/mol and 125 to 756 kJ/mol for SLYG and SQD, respectively. Besides, this model could also supply accurate extrapolations to relatively higher heating rates. Acknowledgements Financial support from the Shandong Provincial Natural Science Foundation, China (Grant ZR2014EEQ013) and Applied Basic Research Programs of Qingdao (Grant 14-2-4-53-jch) is greatly acknowledged. Besides, the paper was also supported by the Qingdao Institute of Bioenergy and Bioprocess Technology Director Innovation Foundation for Young Scientists (Grant CASKLB201502). References Cai, J.M., He, F., Yao, F.S., 2007. Nonisothermal nth-order DAEM equation and its parametric study – use in the kinetic analysis of biomass pyrolysis. J. Math. Chem. 42 (4), 949–956. Chang, F.C., Ko, C.H., Wu, J.Y., Wang, H.P., Chen, W.S., 2013. Resource recovery of organic sludge as refuse derived fuel by fry-drying process. Bioresour. Technol. 141, 240–244. Donatello, S., Cheeseman, C.R., 2013. Recycling and recovery routes for incinerated sewage sludge ash (ISSA): a review. Waste Manage. 33 (11), 2328–2340. Folgueras, M.B., Alonso, M., Diaz, R.M., 2013. Influence of sewage sludge treatment on pyrolysis and combustion of dry sludge. Energy 55, 426–435. Fonts, I., Gea, G., Azuara, M., Abrego, J., Arauzo, J., 2012. Sewage sludge pyrolysis for liquid production: a review. Renew. Sustain. Energy Rev. 16 (5), 2781–2805. Gao, N.B., Li, J.J., Qi, B.Y., Li, A.M., Duan, Y., Wang, Z., 2014. Thermal analysis and products distribution of dried sewage sludge pyrolysis. J. Anal. Appl. Pyrol. 105, 43–48. Hu, G.J., Li, J.B., Zeng, G.M., 2013. Recent development in the treatment of oily sludge from petroleum industry: a review. J. Hazard. Mater. 261, 470–490.
G. Liu et al. / Waste Management 41 (2015) 128–133 Kelessidis, A., Stasinakis, A.S., 2012. Comparative study of the methods used for treatment and final disposal of sewage sludge in European countries. Waste Manage. 32 (6), 1186–1195. Ma, X.Y., Duan, Y.F., Liu, M., 2013. Effects of petrochemical sludge on the slurryability of coke water slurry. Exp. Thermal Fluid Sci. 48, 238–244. Magdziarz, A., Werle, S., 2014. Analysis of the combustion and pyrolysis of dried sewage sludge by TGA and MS. Waste Manage. 34 (1), 174–179. Manara, P., Zabaniotou, A., 2012. Towards sewage sludge based biofuels via thermochemical conversion – a review. Renew. Sustain. Energy Rev. 16 (5), 2566–2582. Montusiewicz, A., Lebiocka, M., 2011. Co-digestion of intermediate landfill leachate and sewage sludge as a method of leachate utilization. Bioresour. Technol. 102 (3), 2563–2571. Scott, S.A., Dennis, J.S., Davidson, J.F., Hayhurst, A.N., 2006. Thermogravimetric measurements of the kinetics of pyrolysis of dried sewage sludge. Fuel 85 (9), 1248–1253. Silva, J.D., Rodrigues, G., Meireles, C.D., Ribeiro, S.D., Vieira, J.G., da Silva, C.V., Cerqueira, D.A., 2012. Thermal analysis and FTIR studies of sewage sludge produced in treatment plants. The case of sludge in the city of Uberlandia-MG, Brazil. Thermochim. Acta 528, 72–75. Smidt, E., Parravicini, V., 2009. Effect of sewage sludge treatment and additional aerobic post-stabilization revealed by infrared spectroscopy and multivariate data analysis. Bioresour. Technol. 100 (5), 1775–1780.
133
Soria-Verdugo, A., Garcia-Hernando, N., Garcia-Gutierrez, L.M., Ruiz-Rivas, U., 2013. Analysis of biomass and sewage sludge devolatilization using the distributed activation energy model. Energy Convers. Manage. 65, 239–244. Tang, Q., Zheng, Y.Y., Qang, T.F., Wang, J.F., 2014. Study on the kinetics of plant oil asphalt pyrolysis using thermogravimetry and the distributed activation energy model. Energy Fuels 28, 2035–2040. Tian, Y., Zuo, W., Ren, Z.Y., Chen, D.D., 2011. Estimation of a novel method to produce bio-oil from sewage sludge by microwave pyrolysis with the consideration of efficiency and safety. Bioresour. Technol. 102 (2), 2053–2061. Velghe, I., Carleer, R., Yperman, J., Schreurs, S., 2013. Study of the pyrolysis of sludge and sludge/disposal filter cake mix for the production of value added products. Bioresour. Technol. 134, 1–9. Verma, S., Prasad, B., Mishra, I.M., 2012. Physicochemical and thermal characteristics of the sludge produced after thermochemical treatment of petrochemical wastewater. Environ. Technol. 33 (15), 1789–1801. Werle, S., Wilk, R.K., 2010. A review of methods for the thermal utilization of sewage sludge: The Polish perspective. Renewable Energy 35 (9), 1914–1919. Zhang, L.G., Xiao, B., Hu, Z.Q., Liu, S.M., Cheng, G., He, P.W., Sun, L., 2014. Tar-free fuel gas production from high temperature pyrolysis of sewage sludge. Waste Manage. 34 (1), 180–184. Zhao, P.T., Chen, H.F., Ge, S.F., Yoshikawa, K., 2013. Effect of the hydrothermal pretreatment for the reduction of NO emission from sewage sludge combustion. Appl. Energy 111, 199–205.
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Thermogravimetric study and kinetic analysis of dried industrial sludge pyrolysis Guangrui Liu a, Huijuan Song a,b, Jinhu Wu a,⇑ a b
Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 20 November 2014 Accepted 31 March 2015 Available online 16 April 2015 Keywords: Sludge Pyrolysis DAEM TGA Kinetic study
a b s t r a c t Thermogravimetric experiments of two different industrial sludge samples were carried out with nonisothermal temperature programs. The results indicated that the pyrolysis process contains three obvious stages and the main decomposition reaction occurred in the range of 200–600 °C. The distributed activation energy model (DAEM) was also proposed describing equally well the pyrolysis behavior of the samples. The calculated activation energy was ranged from 170 to 593 kJ/mol and 125 to 756 kJ/mol for SLYG (sludge sample from chemical fiber factory) and SQD (sludge sample from woody industry), respectively. The reliability of this model not only provided good fit for all experiments, but also allowed accurate extrapolations to relative higher heating rates. Besides, the FTIR measurement was also used to further understand the relationship between pyrolysis behavior and chemical structures for industrial sludge. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The sludge is one of the most significant hazardous solid wastes generated from the waste water treatment processes. It is a complex emulsion of various organic chemical compounds and solid particles, as well as high concentration of water and heavy metals (Werle and Wilk, 2010). The improper disposal or insufficient treatment of sludge could lead serious environmental destruction and human health impacts. Thus, a variety of effective sludge treatment technologies have been developed to eliminate the hazardous constituent and mitigate its negative impacts, due to its increasing production quantity around the world (Kelessidis and Stasinakis, 2012). The most common sludge treatment methods are containing landfill, incineration and pyrolysis (Tian et al., 2011). However, considering the farm land limitations and more restrictive environmental regulations, the landfill and incineration technologies are becoming more and more difficult to operate (Montusiewicz and Lebiocka, 2011; Smidt and Parravicini, 2009; Donatello and Cheeseman, 2013). Thus, the energetic valorization method of thermal decomposition was regarded as a promising alterative technology and fast developed due to its more economical and environmental friendly.
⇑ Corresponding author. Tel.: +86 532 80662763. E-mail addresses: [email protected] (G. Liu), [email protected] (J. Wu). http://dx.doi.org/10.1016/j.wasman.2015.03.042 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
In order to transform the organic sludge to the fuels or valuable chemical sources, the pyrolysis method was employed around the world (Manara and Zabaniotou, 2012; Chang et al., 2013; Velghe et al., 2013). Pyrolysis is the thermal decomposition of organic compounds at elevated temperatures in an inert environment. The main products are including non-condensable gases, lower molecular weight liquid bio-oils as well as solid char (Fonts et al., 2012). Besides, the merits of this method are focusing on its lower emissions and capacity that enables heavy metals and most of other hazardous trace elements to be concentrated or retained partially in the final char (Hu et al., 2013). Zhang et al. (2014) studied the pyrolysis of sewage sludge in a free-fall reactor. They indicated that the high temperature was in favor of syngas generation in gas product and the volatile matter could be completely released to gaseous product at 1300 °C. Besides, the energy balance results also showed that the gaseous product contains the most heating value of sludge. Magdziarz and Werle (2014) studied the combustion and pyrolysis characteristics of three types of sewage sludge from waste water treatment plants. They found the main devolatilization pyrolysis reaction took place between 200 and 540 °C, associated to the degradation of protein and soluble polysaccharide. They also obtained the detailed identification of gaseous evolved during thermal reaction. Analyses showed the main gaseous components are mainly containing H2, CO, CO2 and CH4. In order to obtain a better understanding of the physical and chemical characteristics of this thermal process as well as
129
G. Liu et al. / Waste Management 41 (2015) 128–133
reasonable devolatilization mechanisms, many researchers have done lots of contributions on the kinetic study of sewage sludge pyrolysis (Gao et al., 2014). Meantime, the distribution activation energy model (DAEM) was succeeding employed to investigate the kinetic behaviors during the pyrolysis of dried sewage sludge, due to its well prediction capability during the whole devolatilization process for all kinds of complicated reaction systems such as coal, charcoal, shale, oil asphalt and biomass (Tang et al., 2014). However, few studies focused on the kinetic analysis of pyrolysis for dried industrial sludge. Considering the fast development of society and great concern on the environment, large quantity of waste sludge were generated from different industrial plants. The sludge exhibited various constitutes and structures because they were used to deal with different waste water sources. Furthermore, the effects of functional groups structures for sludge pyrolysis have often been neglected in previously studies. In present study, two different chemical sludge samples were used to investigate the thermal degradation characteristics and pyrolysis kinetics by TGA method. The sludge was pyrolyzed in a non-isothermal environment, and thermogravimetric analysis by DAEM method was carried out. In order to further explore the relationship between the chemical structure and pyrolysis, the variation of functional groups was also detected by FTIR method.
dispersed in KBr. The spectral range was ranged from 4000 to 400 cm1. Ten scans were performed with a resolution of 1 cm1. 2.3. Thermogravimetric analysis (TGA) The pyrolysis characteristics of the samples were examined using a thermogravimetric analyzer (TG, PerkinElmer Diamond TG/DTA). The sludge was loaded in a crucible and placed inside the TG where the weight of the sample was constantly measured. For each experimental run, around 5 mg of sample with particle size of approximate 200 meshes was used. The heating temperature was ranged from 25 to 1000 °C at four different heating rates (5, 10, 20 and 40 °C/min). In order to provide an inert environment without oxygen for the sludge pyrolysis, the argon gas was used as the carrier gas, whose flow rate was fixed at 40 mL/min. Each test was repeated four times obtaining differences lower than 5% to guarantee repeatability. 2.4. Pyrolysis kinetics The non-isothermal kinetics for dried sludge decomposition can be written as follows:
dX ¼ kf ðXÞ dt
2. Materials and methodology
ð1Þ
where X is sample conversion, and is given by
2.1. Materials Two sludge samples (SQD and SLYG) were obtained from different chemical industries in China. These samples were heated for 24 h at the temperature of 110 °C to remove the free water before the experiments. In order to avoid the effect of heat and mass transfer during the thermal decomposition, all the dried sludge was sieved into 200 meshes particle. The ultimate analysis was carried out in a TruSpec CHN and TruSpec S analyzer (Leco, USA) while the proximate analysis was run in a TGA Q500 TA instrument (TA, USA). The moisture, volatiles and ashes were measured according to the ASTM standard test method No. E871, E872 and D1102, respectively. The proximate and ultimate analysis of sludge samples were shown in Table 1. The samples exhibited the usual properties of sludge materials, such as relative high oxygen content and lower hydrogen content. Their nitrogen content is also remarkably high, which is due to proteins and alkaloids in their structural compositions.
X¼
Vi V Vi Vf
ð2Þ
For this equation, V, Vi and Vf represent the instantaneous, initial and final weights of the sludge sample. The function f(X) can be written as
f ðXÞ ¼ ð1 XÞn
ð3Þ
where n is assuming reaction order. For the DAEM method, it is assumed that many irreversible first-order parallel reactions with different rate parameters occur simultaneously. In other words, all frequency factors differ only in activation energy and the number of independent reactions is larger enough to permit continuous Gaussian distribution of the activation energy, which is to be expressed as a function of f(E) and
Z
1
f ðEÞdE ¼ 1
ð4Þ
0
2.2. Fourier transform infrared spectroscopy analysis (FTIR)
Then the total amount of the volatile material released up to time t is given by
Surface functional groups of different chemical sludge samples were analyzed using a VECTOR22 FTIR spectrometer (Bruker, Germany). The tablet was made with 1 wt.% of the sludge sample
Ultimate analysis (dry-ash-free basis) C (wt.%) H (wt.%) O (wt.%) N (wt.%) S (wt.%)
SQD
SLYG
6.82 50.39 5.94 36.85
8.70 59.50 9.57 22.23
51.70 6.83 30.76 9.82 0.89
Z
1
0
Z exp k0 0
t
E dt f ðEÞdE exp RT
ð5Þ
For the process with a constant heating rate, b, temperature, T, against time, t, follows
Table 1 The proximate and ultimate analysis of sludge samples.
Proximate analysis (dry basis) Moisture (wt.%) Volatiles (wt.%) Fixed carbon (wt.%) Ash (wt.%)
1X ¼
49.49 6.76 32.51 10.21 1.03
T ¼ T 0 þ bt
ð6Þ
The integral method based on DAEM equation is used in previous work (Cai et al., 2007), and the approximate integration of Eq. (5) gives
ln
b T
2
¼ ln
k0 R E þ 0:6075 E RT
ð7Þ
The Arrhenius plot of ln(b/T2) versus 1/T becomes a linear line for each rate of devolatilization. Thus, the activation energy (E) and frequency factor (k0) can be determined from the slope and intercept of the regression line, respectively.
130
G. Liu et al. / Waste Management 41 (2015) 128–133
and ACAO function group, respectively. Moreover, the C@O group is mainly from the acids and aldehydes. Compared with other research, the similar absorption peaks of ACONHA were also detected at 1549 cm1 (amid II band), 1656 cm1 (amid I band) and 3420 cm1 (NAH stretching), which might be the characteristic of certain amounts of proteins in sludge samples (Ma et al., 2013; Zhao et al., 2013). In the range of 670–860 cm1, the absorption peaks are characterized by aromatic rings. Besides, the absorption peak at 1549, 1410 and 470–640 cm1 is characteristic of oxygen-functional group with nitrogen, sulfur and phosphorus, respectively. 3.2. TG and DTG results Fig. 1. The FTIR spectra of different chemical sludge samples.
3. Results and discussion 3.1. FTIR spectra of sludge samples In order to further understand the relationship between the pyrolysis behaviors and chemical structures of industrial sludge, the distributions of various functional groups were detected by FTIR analysis. FTIR spectra of different chemical sludge samples were shown in Fig. 1. The two sludge samples had similar spectrum because their composition are both varies extremophile bacteria, whose structure could be decomposed into small organic acids, amine and proteins. The spectrum for all sludge has a very broad and strong absorption band between 3250 and 3750 cm1, which was caused by OAH stretching vibration (Silva et al., 2012; Verma et al., 2012). Another wide band ranged from 2850 to 2960 cm1 was induced by the symmetric and asymmetric stretching vibration of CAH, which indicates the existence of aliphatic chain. There are also two obvious absorption peaks at 1655 and 1030 cm1, which are induced by stretching vibration of C@O
Sludge pyrolysis was a thermal process that takes place in an inert atmosphere, and where the organic matter undergoes a series of complex reactions that generate volatile gas and liquid, as well as solid char products. The TG and DTG curves of different heating rates were illustrated in Figs. 2 and 3, where it is seen that remaining mass fraction curves was shifted up the temperature scale by an increase in the heating rate. Compared with SQD sludge sample, the SLYG sample would generate much more volatiles under the same pyrolysis conditions. The similar tendency was also reflected in Table 1 while the SLYG sample has less ash amount. The reason might focus on that the SQD sludge would contain large amount of complicated matters, which were derived from the wood treatment. SLYG sample would contain more organic compounds while dealing with chemical fiber wastewater. However, a large amount of large-molecular organic matters as well as complicated inorganic compounds would accumulate in SQD sludge sample because of special material, solvents and additives used for treatment of wood products. The devolatilization process of sludge samples occurred in a wide range of temperatures between 180 and 800 °C. In order to obtain a high conversion rate, the temperature must be as high as 1000 °C, which is in good agreement to that obtained by other researchers (Scott et al., 2006; Soria-Verdugo et al., 2013).
Fig. 2. The TG profiles of sludge pyrolysis at different heating rates.
131
G. Liu et al. / Waste Management 41 (2015) 128–133
Fig. 3. The DTG profiles of sludge pyrolysis at different heating rates.
Fig. 4. The Arrhenius plot of ln(b/T2) vs. 1/T at selected conversion rates b = 5, 10, 20 °C/min.
During the whole sludge pyrolysis range, three main intervals were considered: (a) in the first interval while the temperature was below 200 °C, the total weight loss was accounting for about 10 wt.% with the reason of most moisture released from the sludge. However, this weight loss percentage was larger than the moisture content which was detected by proximate analysis (6.82 wt.% for SQD, 8.70 wt.% for SLYG). Thus, the volatilization for a certain amount of light reactive components would occur in this stage (Folgueras et al., 2013). (b) The second interval ranged from 200 to 600 °C, the pyrolysis of most biodegradable organic matters occurred together with the partial pyrolysis of other less reactive bacterial matters such as protein and carboxyl groups such as sugar components and some saturated aliphatic chains with relatively large molecular weight. This interval was the main stage for the cleavage of CAC bond (2850–2960 cm1), CAO bond (1655 and 1030 cm1) as well as aromatic rings (670–860 cm1) and oxygen-functional group with nitrogen (1549 cm1) for the generation of volatiles. During this pyrolysis stages the DTG curves also exhibited the largest peaks, which represented the maximum rate of mass loss. The total weight loss percentage of sludge was approximately up to 40 wt.%. This result was useful for directing the development of
low-temperature pyrolysis technology of sludge treatment. (c) During the third interval, a small peak would be observed at temperatures higher than 600 °C. This was mainly due to the secondary decomposition such as light char devolatilization as well as the decomposition of non-biodegradable components such as CaCO3 and other inorganic matters. However, the DTG curves for SLYG did not show this peak obviously. The reason might be focusing on that the sludge sample from chemical fiber factory contained much organic matters instead of inorganic matters. Table 2 The calculated E values for selected conversion rates. Conversion rate
ELYG (kJ/mol)
R2
EQD (kJ/mol)
R2
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
170 191 220 303 428 205 593 304 295
0.999 0.948 0.941 0.999 0.976 0.957 0.996 0.916 0.912
548 663 509 756 250 125 134 486 682
0.983 0.999 0.965 0.976 0.990 0.914 0.983 0.972 0.971
132
G. Liu et al. / Waste Management 41 (2015) 128–133
Fig. 5. Experimental and calculated T vs. X curves at b = 10, 40 °C/min for sludge pyrolysis.
3.3. Kinetic results by DAEM method For the each heating rate, a ln(b/T2) versus 1/T plot under selected values of devolatilization rate was established and presented in Fig. 4. All curves were represented by Eq. (7). Therefore, the activation energy and the frequency factor can be calculated from the slope and intercept in the Arrhenius plots at each rate of devolatilization. The selected conversion rates were also exhibited in Table 2. The results indicated that the activation energy was ranged from 170 to 593 kJ/mol and 125 to 756 kJ/mol for SLYG and SQD, respectively. The high correlation coefficients indicated that this model fits the experimental data very well. Fig. 5 showed the sludge devolatilization profiles (with the heating rate of b = 10 °C/min) which were predicted by Eq. (5) without any mathematical fitting techniques using the results of E and k0 obtained from DAEM. The results indicated that both the experimental curves and the calculated curves for every sample at existed lower heating rates well correspond each other. In order to calculate E and k0 with high accuracy, the fitting coefficient of Arrhenius plot from Eq. (7) R2 > 0.91 will be necessary. Besides, Fig. 5 also showed the comparison of devolatilization curves between the predicted and experimental results for relatively higher heating rate of b = 40 °C/min. The calculated temperatures for each conversion ratio were agreed with experiment results with minor errors. All the above findings indicate that the DAEM method is suitable to describe the pyrolysis and predict the kinetic behavior of chemical sludge even at relative higher heating rate. However, Fig. 5 also exhibited that the calculated temperature (T) was slight higher than experimental results. The reason might be focusing on that the large amount of volatiles would generate at higher heating rate. Thus, the predictions of sludge pyrolysis behaviors under much higher heating rates still are the key points during our further research. 4. Conclusions In present study, the pyrolysis of two different industrial sludge samples was analyzed by means of non-isothermal thermogravimetric experiments. The FTIR analysis was also used to determine
the decomposition behaviors by the chemical structural compositions. The results showed that three main intervals were occurred during the whole pyrolysis process. The main weight loss (about up to 40 wt.%) of industrial sludge was ranging from 200 to 600 °C, which might be accompanying with the cleavage of CAC bond, CAO bond as well as aromatic rings and oxygen-functional group with nitrogen. The distribution activation energy model was applied to reproduce the weight losses curve in order to obtain the kinetic parameters of the process and it provided good fit for all experiments. The results indicated that the activation energy was ranged from 170 to 593 kJ/mol and 125 to 756 kJ/mol for SLYG and SQD, respectively. Besides, this model could also supply accurate extrapolations to relatively higher heating rates. Acknowledgements Financial support from the Shandong Provincial Natural Science Foundation, China (Grant ZR2014EEQ013) and Applied Basic Research Programs of Qingdao (Grant 14-2-4-53-jch) is greatly acknowledged. Besides, the paper was also supported by the Qingdao Institute of Bioenergy and Bioprocess Technology Director Innovation Foundation for Young Scientists (Grant CASKLB201502). References Cai, J.M., He, F., Yao, F.S., 2007. Nonisothermal nth-order DAEM equation and its parametric study – use in the kinetic analysis of biomass pyrolysis. J. Math. Chem. 42 (4), 949–956. Chang, F.C., Ko, C.H., Wu, J.Y., Wang, H.P., Chen, W.S., 2013. Resource recovery of organic sludge as refuse derived fuel by fry-drying process. Bioresour. Technol. 141, 240–244. Donatello, S., Cheeseman, C.R., 2013. Recycling and recovery routes for incinerated sewage sludge ash (ISSA): a review. Waste Manage. 33 (11), 2328–2340. Folgueras, M.B., Alonso, M., Diaz, R.M., 2013. Influence of sewage sludge treatment on pyrolysis and combustion of dry sludge. Energy 55, 426–435. Fonts, I., Gea, G., Azuara, M., Abrego, J., Arauzo, J., 2012. Sewage sludge pyrolysis for liquid production: a review. Renew. Sustain. Energy Rev. 16 (5), 2781–2805. Gao, N.B., Li, J.J., Qi, B.Y., Li, A.M., Duan, Y., Wang, Z., 2014. Thermal analysis and products distribution of dried sewage sludge pyrolysis. J. Anal. Appl. Pyrol. 105, 43–48. Hu, G.J., Li, J.B., Zeng, G.M., 2013. Recent development in the treatment of oily sludge from petroleum industry: a review. J. Hazard. Mater. 261, 470–490.
G. Liu et al. / Waste Management 41 (2015) 128–133 Kelessidis, A., Stasinakis, A.S., 2012. Comparative study of the methods used for treatment and final disposal of sewage sludge in European countries. Waste Manage. 32 (6), 1186–1195. Ma, X.Y., Duan, Y.F., Liu, M., 2013. Effects of petrochemical sludge on the slurryability of coke water slurry. Exp. Thermal Fluid Sci. 48, 238–244. Magdziarz, A., Werle, S., 2014. Analysis of the combustion and pyrolysis of dried sewage sludge by TGA and MS. Waste Manage. 34 (1), 174–179. Manara, P., Zabaniotou, A., 2012. Towards sewage sludge based biofuels via thermochemical conversion – a review. Renew. Sustain. Energy Rev. 16 (5), 2566–2582. Montusiewicz, A., Lebiocka, M., 2011. Co-digestion of intermediate landfill leachate and sewage sludge as a method of leachate utilization. Bioresour. Technol. 102 (3), 2563–2571. Scott, S.A., Dennis, J.S., Davidson, J.F., Hayhurst, A.N., 2006. Thermogravimetric measurements of the kinetics of pyrolysis of dried sewage sludge. Fuel 85 (9), 1248–1253. Silva, J.D., Rodrigues, G., Meireles, C.D., Ribeiro, S.D., Vieira, J.G., da Silva, C.V., Cerqueira, D.A., 2012. Thermal analysis and FTIR studies of sewage sludge produced in treatment plants. The case of sludge in the city of Uberlandia-MG, Brazil. Thermochim. Acta 528, 72–75. Smidt, E., Parravicini, V., 2009. Effect of sewage sludge treatment and additional aerobic post-stabilization revealed by infrared spectroscopy and multivariate data analysis. Bioresour. Technol. 100 (5), 1775–1780.
133
Soria-Verdugo, A., Garcia-Hernando, N., Garcia-Gutierrez, L.M., Ruiz-Rivas, U., 2013. Analysis of biomass and sewage sludge devolatilization using the distributed activation energy model. Energy Convers. Manage. 65, 239–244. Tang, Q., Zheng, Y.Y., Qang, T.F., Wang, J.F., 2014. Study on the kinetics of plant oil asphalt pyrolysis using thermogravimetry and the distributed activation energy model. Energy Fuels 28, 2035–2040. Tian, Y., Zuo, W., Ren, Z.Y., Chen, D.D., 2011. Estimation of a novel method to produce bio-oil from sewage sludge by microwave pyrolysis with the consideration of efficiency and safety. Bioresour. Technol. 102 (2), 2053–2061. Velghe, I., Carleer, R., Yperman, J., Schreurs, S., 2013. Study of the pyrolysis of sludge and sludge/disposal filter cake mix for the production of value added products. Bioresour. Technol. 134, 1–9. Verma, S., Prasad, B., Mishra, I.M., 2012. Physicochemical and thermal characteristics of the sludge produced after thermochemical treatment of petrochemical wastewater. Environ. Technol. 33 (15), 1789–1801. Werle, S., Wilk, R.K., 2010. A review of methods for the thermal utilization of sewage sludge: The Polish perspective. Renewable Energy 35 (9), 1914–1919. Zhang, L.G., Xiao, B., Hu, Z.Q., Liu, S.M., Cheng, G., He, P.W., Sun, L., 2014. Tar-free fuel gas production from high temperature pyrolysis of sewage sludge. Waste Manage. 34 (1), 180–184. Zhao, P.T., Chen, H.F., Ge, S.F., Yoshikawa, K., 2013. Effect of the hydrothermal pretreatment for the reduction of NO emission from sewage sludge combustion. Appl. Energy 111, 199–205.