Effect of temperature on the sulfur fate during hydrothermal carbonization of sewage sludge☆
Graphical abstract
Introduction
Recently, sewage sludge (SS) as a by-product from sewage treatment plants has aroused extensive public concern. SS is composed of wastewater, mineral particles, organic debris, and bacteria (Zhai et al., 2016). Approximately 300 million tons per year of dry sludge is produced in China (Zhang et al., 2017), and only a small amount of this sludge can be reutilized. Thermal conversion technologies, such as pyrolysis, gasification, and combustion (Samolada and Zabaniotou, 2014; Yaman, 2004; Zhao et al., 2013), have been applied to SS. However, potential risks from high moisture content, heavy metals, and hazardous and toxic organic components limit the further utilization of SS. Therefore, effective treatment technology is critical.
Hydrothermal carbonization (HTC) is a promising thermochemical technology that has received attention because this process can directly utilize the moisture of raw materials as a reaction medium (Chiang et al., 2001; Yoshikawa, 2012). This process generally occurs at a low temperature range (180–250 °C) in high-pressure vessels (Wang et al., 2018b). Moreover, the hydrothermal carbonization process can be affected by reaction conditions, such as temperature, residence time, solid-to-liquid ratio, and raw material selection (Danso-Boateng et al., 2015; Parshetti et al., 2013), although temperature is the most important of these factors.
With increasing applications of hydrochar, concerns are also increasing regarding the environment owing to the release of sulfur-containing gases when biochar is combusted (Meng et al., 2016). The sulfur content in sludge varies from 0.2 to 9.5% (Anna et al., 2007; Chen et al., 2015; He et al., 2013; Ros et al., 2006). Therefore, a thorough investigation of the sulfur species in SS is essential. Generally, the sulfur species in SS are divided into two major categories: organic and inorganic sulfur. Organic sulfur species include sulfide, sulfone, sulfoxide, thiophene and mercaptan; inorganic sulfur species are typically sulfate and sulfite (Zhong et al., 2018). Until now, only a handful of studies had been conducted on sulfur species transformation of SS during pyrolysis process. The transformation of sulfur species in sludge had been previously investigated for both microwave pyrolysis and pyrolysis at several temperatures (Zhang et al., 2017). Their results indicated that aliphatic sulfur compounds (e.g., mercaptan) were likely to degrade below 500 °C when pyrolysis broke the C–S bond in these compounds. However, stable aromatic sulfur required a higher temperature in the range of 500–800 °C before degradation (Zhang et al., 2017). Furthermore, the yield of H2S tended to increase with increasing (microwave) pyrolysis temperature. Chiang and coworkers claimed that the sulfur concentration in petrochemical sludge increased with increasing pyrolysis temperature (Chiang et al., 2001). Because only a few researchers have estimated the sulfur evolution and distribution in SS during HTC, investigating the effect of HTC temperature on sulfur evolution is essential and meaningful for the further application of SS.
This is the first study to discuss the effect of HTC temperature on sulfur species in SS. The main objectives of this study are: (1) to investigate the forms of sulfur in different phases; (2) to understand the effect of HTC temperature on the transformation of sulfur species.
Section snippets
Materials
The sludge used in this study was obtained from the secondary clarifier of a petrochemical wastewater treatment plant (Henan, China). Sludge was black and sticky, and its composition was summarized in Table 1. Prepared sludge was stored in plastic bags at 4 °C. Before every experiment, sludge was naturally dried for two weeks.
HTC experiments
SS hydrochar was collected from the HTC experimental apparatus (a 500 mL 316 stainless steel enclosed reactor). 66.66 g SS particles was added to 100 mL deionized water
Properties of SS and hydrochar
The properties of SS and hydrochar are presented in Table 1. As shown in Table 1, hydrochar had lower O/C ratios than those of SS, which resulted from the loss of oxygen-containing functional groups and irreversible dewatering (Mursito et al., 2010). Similar results can be found in previous research. Previous studies claimed that the H/C and O/C ratios decreased continuously when the HTC temperature was raised from 150 °C to 380 °C (Mursito et al., 2010; Wu et al., 2015a). The carbon contents
Conclusions
In this paper, upgraded SS was produced, and the evolution route of sulfur species was summarized. The decrease of the O/C ratio was caused by the reduction of oxygen-containing function groups (-OH). The amount of sulfur species in the liquid fraction (mainly sulfate ions) continuously decreased until a minimum value was achieved at 300 °C. Moreover, sulfoxide and sulfate were generated above 180 °C. The amounts of stable thiophene and sulfate increased with increasing temperature. The organic
CRediT authorship contribution statement
Zhexian Wang: Writing - original draft, Methodology, Writing - review & editing. Yunbo Zhai: Supervision, Validation. Tengfei Wang: Writing - review & editing, Supervision. Chuan Peng: Conceptualization, Investigation. Shanhong Li: Data curation, Investigation. Bei Wang: Visualization, Investigation. Xiangmin Liu: Visualization, Investigation. Caiting Li: Software, Supervision.
Declaration of competing interest
The authors declare that they have no conflict of interests.
Acknowledgments
This research was financially supported by the State Key Laboratory of pollution control and treatment of petroleum and petrochemical industry (PPC2017003), a project of the National Natural Science Foundation of China (No. 51679083), a project of the National a key research and development project of Hunan Province (2018WK2011).
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This paper has been recommended for acceptance by Yong Sik Ok.
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a,b,c contributed equally to this work and should be considered co-first Affiliation.