Characterization and analysis of sludge char prepared from bench-scale fluidized bed pyrolysis of sewage sludge
Graphical abstract
Introduction
Sewage sludge (SS) is a by-product from the municipal sewage treatment plant, which is a semi-solid with high water content. SS contains nitrogen and phosphorus salts, various organics and inorganics. Pathogenic microorganisms, parasitic and saprophytic organisms, and heavy metals are also found in SS. These toxic substances can cause serious environmental problems if they are not treated and disposed properly. Owing to the rapid population growth and urbanization, the production of SS increased significantly in recent years and is expected to reach 60–90 million tons (80% moisture content) by 2020 in China [1]. The traditional methods for the treatment of SS are mainly landfill, incineration, composting and land use. SS can also be used as construction materials (e.g. mold brick and lightweight concrete bone grains) [2]. Pyrolysis is an important thermochemical technology, heating SS to a certain temperature under inert atmosphere. SS is then transformed into pyrolysis oil, non-condensable gas and char [3]. There has been growing interests in pyrolysis of complex materials in recent years, which can provide an environmentally and economically viable energy solution. Pyrolysis helps to improve safety of global energy supply, and meet environmental and climatic regulations on sludge treatment with reduced carbon dioxide emissions [4].
There have been many researches on SS pyrolysis, aiming at technical feasibility of different technologies and process fundamentals. Lehmann and Joseph employed pyrolysis techniques to effectively convert SS into syngas, bio-oil and bio-char at relatively high temperature (300–1000 °C) [5]. Fonts et al. [6] investigated the effects of solid feed rate, temperature and nitrogen flow rate on fast pyrolysis of digested sludge. It was found that the highest oil yield of 33 wt% was achieved at 540 °C under nitrogen flow rate of 4.5 NL/min. W.D. Chanaka Udayanga et al. [7] studied the effects of organic and inorganic constituents on the product properties during SS pyrolysis in a vertical fixed-bed pyrolysis unit. It was found that the additives significantly affected the yields and compositions of non-condensable gas, tar and char. Yunxue Xia et al. [8] conducted SS pyrolysis in a horizontal furnace equipped with a quartz tube to investigate the enhanced phosphorus availability and heavy metal removal by chlorination, proposing a novel method to use the sludge biochar as potential P-fertilizer. Eilhann E. Kwon et al. [9] demonstrated that CaCO3 reduce the content of polycyclic aromatic hydrocarbons (PAHs) in the pyrolysis products from SS. Salman Raza Naqvi et al. [10] investigated the thermo-kinetics of high-ash SS using thermogravimetric analysis, the results indicated that the SS pyrolysis may be divided into three stages.
Current SS pyrolysis research is focused on fixed bed and thermogravimetric analyzer, the main research scopes are yields and properties of pyrolysis products, concentrations and risks of heavy metals, and pyrolysis kinetics. Furthermore, there are also some studies on fluidized bed pyrolysis (FBP) of SS. FBP is an efficient method for converting biomass into gas, oil and solid products due to excellent gas-solid contact and fast reaction rate in the fluidized bed [11]. Alejandro Jaramillo-Arangoa [12] investigated the effect of temperature on pyrolysis of anaerobically digested SS by means of chemical characterization of the gases, liquids and solids produced in a fluidized bed reactor at temperature between 300 and 800 °C. The pyrolysis gas, in mass fractions, was mainly composed of CO and CO2, while the liquid product was mostly pyrolytic water. Antonio Soria-Verdugo [13] studied different operating conditions to analyze the sample mass evolution with time during pyrolysis, including bed temperature and velocity of nitrogen used as inert gas. An increase of nitrogen velocity (e.g. from 0.8Umf to 2.5Umf) reduced pyrolysis time and accelerated pyrolysis remarkably. A circulating fluidized bed was used for SS pyrolysis to produce liquids rich in nitrogenated heterocyclic compounds [14]. A low fluidized gas velocity (e.g. 1.13 m/s), high sludge feed rate (e.g. 10.78 kg/h) and large particle size (e.g. 1–2 mm) increased the yield of heterocyclic nitrogenated compounds. Effects of temperature, SS particle size and vapor residence time on bio-oil yield and properties, such as high heating value (HHV) and moisture content, were evaluated by Arazo et al. [15] in a fast pyrolysis fluidized bed reactor for SS disposal. At optimum conditions, the unit produced a bio-oil product with HHV nearly twice as much as lignocellulosic-derived bio-oil, and its properties were comparable to heavy fuel oil. This FBP study focused on effects of operating conditions (e.g. temperature and gas velocity) on the yield of pyrolysis products and properties of liquid products.
In addition, there were some studies on the catalytic pyrolysis of SS and CaO was known to benefit pyrolysis, for instance, CaO was found to adsorb CO2 from non-condensable gases [16] and facilitate the retention of sulfur [17] in the derived char during pyrolysis of CaO conditioned sludge. However, there were limited studies regarding the influence of aluminosilicate compounds on sludge pyrolysis [18]. Aluminosilicate clay would act as solid acid catalyst in the cracking reactions of the organic component of sludge and improve the yield of volatile products. Ischia et al. [19] reported that the release of crystalline water from clay could result in an increase in CH4 content. Fixation of heavy metals into the derived char by addition of clay in the sludge was also demonstrated [20]. However, the impacts of aluminosilicate compounds (e.g. kaolin and Ca-bentonite) on the pyrolysis product distribution and properties have not been clearly determined. As natural ores, kaolin and Ca-bentonite had some merits, such as good catalytic performance, fast response and low price.
Hence, we investigated FBP of SS and the influences of pyrolysis temperature and additives on product yields, sludge char (SC) characteristics, and migration, existing forms and potential ecological risks of heavy metals in SC. Based on these scenarios, technical feasibility of FBP and immobilization of heavy metals in SC were assessed, which is expected to be meaningful for solid wastes treatment and disposal. We also studied the transformation and balance of carbon element during SS pyrolysis, which were rarely studied in previous literatures. In addition, the energy balance and energy yield of the FBP system were analyzed and the economy of the FBP technique was also discussed.
Section snippets
Materials
Raw SS was obtained from a sewage treatment plant situated in Hebei province. The SS was dried at 105 °C for 12 h, then crushed and sieved into 0.105–1.0 mm particles for the FBP experiment. Required chemicals (analytical reagent) such as ethanol, dichloromethane, hydrochloric acid, nitric acid and catalysts were purchased from Beijing Chemical Works.
Experimental device
Fig. 1 shows the schematics of FBP of semi-continuous operation. For more detailed descriptions, please refer to previous literatures [21]. About
Effects of temperature on yields of pyrolysis products
Product yields from SS pyrolysis are depicted in Fig. 2. The SC yield declined steadily from 74.33 wt% to 58.66 wt% with increasing pyrolysis temperature mainly due to continuous release of volatile materials (Fig. 2a).
It was noticed that char yield from SS was considerably higher than that from lignocellulosic biomass [22], due mainly to higher ash content in SS. Meanwhile, the yield of non-condensable gas increased from 4.08 wt% to 11.75 wt% as the temperature increased from 450 °C to 950 °C,
Conclusions
The effects of temperature and catalysts on the FBP of SS were systematically investigated. With addition of kaolin, the minimum yield of SC (i.e. 54.64 wt%) was found at 850 °C. Catalysts significantly decreased the content of carbon, hydrogen and nitrogen in SC. Carbon in the SS and SC surface layer was mainly present in the forms of C–C, C–O, C–Cl, CO, COOR and MCO3. Increase in temperature promoted the thermal decomposition of aliphatic alcohol, alkane and proteinaceous compounds. Kaolin
CRediT authorship contribution statement
Yang Liu: Conceptualization, Methodology, Writing - original draft. Chunmei Ran: Methodology, Investigation. Azka R. Siddiqui: Methodology, Writing - review & editing. Asif Ali Siyal: Writing - review & editing. Yongmeng Song: Methodology, Resources, Investigation. Jianjun Dai: Supervision, Writing - review & editing, Funding acquisition. Polina Chtaeva: Resources, Investigation, Methodology. Jie Fu: Data curation. Wenya Ao: Investigation. Zeyu Deng: Investigation. Zhihui Jiang: Formal
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Funding: This work was supported by the Ministry of Science and Technology of the People’s Republic of China [grant number 2017YFE0124800].
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