Catalytic effect of metal oxides on pyrolysis of sewage sludge
Introduction
Sewage sludge or bio-solid is the major by-product generated from wastewater treatment plant. During the past decades, the production of sludge annually in the USA increased dramatically from 4.9 million dry tons in 1972 to 7.6 million dry tons in 2005, and will probably increase to about 8.2 million dry tons in 2010 due to more stringent regulations [1], [2]. Sewage sludge is harmful to the environment, due to the high concentrations of toxic metals, organic pollutants and pathogens [2]. The costs of disposal of sewage sludge may account for up to 50% of the total wastewater treatment cost [3].
The current methods of treatment of sewage sludge are land filling, agriculture application, and incineration, none of which are exempt from drawbacks [4]. It is important and urgent to develop a technology for the appropriate treatment of such huge amount of sewage sludge to reduce environmental problems and costs.
Increasing attention has been paid on pyrolysis since 1980 [5], [6], [7], [8]. It is considered as a promising alternative method to sewage sludge treatment and to convert this kind of waste to generate clean energy and more valuable chemicals [4]. In addition, pyrolysis is more favorable, as the process conditions can be optimized to maximize the production of gases, oils or chars depending on the specific purpose [9], [10], [11].
Sewage sludge is considered as a very heterogeneous material and comprising a mixture of various compounds [12]. Rulkens [13] summarized that sewage sludge consists of six groups of components: (1) nontoxic organic carbon compounds, for a large part from biological origin, (2) nitrogen- and phosphorous-containing components, (3) toxic inorganic and organic pollutants, (4) pathogens and other microbiological pollutants, (5) inorganic compounds, such as silicates, aluminates, and calcium- and magnesium-containing compounds, and (6) water. However, taking into account of thermal behavior, it is necessary to know the sludge fractions according to temperature response instead of the chemical composition [14]. In this case, the pyrolysis behavior of the main components of the sewage sludge was comparable to those reported for lignocellulosic biomass comprising cellulose, hemicellulose, and lignin [12], [14].
Sewage sludge is also known as high ash material on dry basis. The ash content in sludge may range from 20 to 50% [4]. The main inherent ash-forming elements are Al, Ca, Fe, K, Mg, Na, P, S, and Si, together with trace amounts of Cl, Cu, Ti, and Zn. They could exist as oxides, silicates, carbonates, sulfates, chlorides, and phosphates [15]. Usually, it is assumed that elements in ash exist as oxides phase. Previous studies [16], [17], [18], [19], [20], [21] have revealed that mineral matters may have positive or negative effects on the pyrolysis of solid fuels. In those studies, selective demineralization method (water washing or acid treatment) has been used to investigate the effect of individual elements. Yang et al. [18] found that most of the mineral additives (KCl, Na2CO3, CaMg(CO3)2, Fe2O3, and Al2O3) demonstrated negligible effects on the pyrolysis of palm oil wastes, while K2CO3 was found to inhibit the pyrolysis of hemicellulose but promote the degradation of cellulose. Shie et al. [19] reported that the additives might increase the conversion of oil sludge by the following order Fe2O3 > AlCl3 > Fe2SO4·7H2O > Al2O3 > FeCl3 > Al > Fe > no additives. Liu et al. [20] showed that CaO, K2CO3, and Al2O3 had a catalytic effect on the reactivity of coal pyrolysis. Li et al. [21] demonstrated that metal oxides (CuO, Fe2O3, and ZnO) might decrease the ignition temperature of coal by 8–50 K, and increase the combustion rate and burnout of the fixed carbon. The previous studies provided some useful results of the catalytic pyrolysis of different feedstocks. However, there is not much information available about the influence of metal oxides on the pyrolysis behavior of sewage sludge. Moreover, little has been reported on the relationship between metal oxides and the kinetics of sludge pyrolysis.
In this work, the catalytic degradation of sewage sludge in the presence of metal oxides was studied. The sludge sample was demineralized and then impregnated with five kinds of metal oxides (including Al2O3, CaO, Fe2O3, TiO2, and ZnO), respectively. The catalytic pyrolysis was carried out using a thermogravimetric analyzer at the heating rate of 10 K/min in nitrogen atmosphere from room temperature to 1100 K. The kinetic parameters due to the catalytic effect were also determined according to the Coats–Redfern method.
Section snippets
Material
Sewage sludge (SS) sample was obtained from a wastewater treatment plant. To minimize the changes of sludge property, it was dried at 378 ± 3 K for 24 h to a constant weight to remove moisture prior to characterization. The dried sludge sample was then ground and sieved. The fraction 106–125 μm was used for demineralization, impregnation and analysis. The proximate analysis of dried sludge sample was performed according to ASTM D3172. It showed that the sludge includes about 75.3 wt.% of volatile
Effect of pretreatment method on pyrolysis of sludge
Differential thermogravimetric (DTG) results comparing the effect of acid pretreatment method on the pyrolysis of sewage sludge are shown in Fig. 1. Two main peaks were found from pyrolysis of raw sewage sludge. The first peak with a slight shoulder is located between 400 and 640 K, centered at 596 K, while the second peak is between 640 and 790 K, with the peak temperature of 713 K. Pyrolysis of the main components of sludge sludge was found to be similar to that of hemicellulose, cellulose, and
Conclusions
The effects of metal oxides on the pyrolysis of sewage sludge were investigated in a thermogravimetric analyzer at the heating rate of 10 K/min. The chemical structure might be changed due to the decrease of mineral matters during demineralization process by acid treatment. The reduction of inorganic mineral matters promotes the decomposition of organic matter in the pyrolysis of sewage sludge. The catalytic effects of metal oxides on the pyrolysis of demineralized sewage sludge samples were
Acknowledgement
This research is supported by the National Natural Science Foundation of China (Grant No. 50676037 and No.50721005) and ETRP fund for Project 0901 140 of Singapore, National Environmental Agency (NEA). This work is also part of the project “Co-control of Pollutants during Solid Fuel Utilization”, supported by the Programme of Introducing Talents of Discipline to Universities (project B06019), China.
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