Elsevier

Waste Management

Volume 120, 1 February 2021, Pages 513-521
Waste Management

Comparative analysis for pyrolysis of sewage sludge in tube reactor heated by electromagnetic induction and electrical resistance furnace

https://doi.org/10.1016/j.wasman.2020.10.015Get rights and content

Highlights

  • Propose a comparative study on EMI and ERF pyrolysis.

  • Different yield, distribution and properties of products are obtained.

  • EMI shows high yield of energy recovery and low energy consumption.

  • EMI pyrolysis is beneficial to improve liquid and gaseous products.

Abstract

A comparative study was conducted on the pyrolysis of sewage sludge in tube reactors heated by electromagnetic induction (EMI) and conventional electrical resistance furnace (ERF). A minimal effect of pyrolysis temperature and initial moisture content on the distribution of pyrolytic products was obtained. Compared with the counterpart from ERF pyrolysis, the bio-char from EMI pyrolysis exhibited less ash content (46.38 wt%) and higher organic matter content (53.62 wt%). SEM and FTIR test showed similar microstructure characterizations in the two bio-chars. The specific area of bio-char from EMI pyrolysis was 8.6 m2/g. EMI pyrolysis increased the total content of aliphatic/aromatics in the bio-oil from 10.8 wt% to 15.6 wt% and the hydrogen/carbon monoxide in the bio-gas from 33.8 vol% to 41.1 vol% because of possible cracking and reforming reactions. Increased sulfur content in the bio-oil and decreased hazard gas content (such as hydrogen sulfide and sulfur dioxide) in the bio-gas were obtained during EMI pyrolysis. The actual energy consumption for EMI and ERF pyrolysis were 4.62 MJ/kg and 6.65 MJ/kg. Increasing the feedstock content would reduce the energy consumption unit energy consumption. Less system energy loss during EMI pyrolysis might explain the higher energy recovery from EMI pyrolysis than that from ERF. Despite some disadvantages, EMI pyrolysis shows potential in real-plant applications.

Introduction

Sewage sludge (SS) is usually generated by municipal wastewater treatment plants. Substantial untreated SS is causing severe environmental impacts as well as health issues to humans because of its complicated mixture of microorganisms, undigested organic matters, and heavy metals (Deng et al., 2009, Ma et al., 2016, Morgano et al., 2016). Landfill, composting and incineration are the most common methods for SS treatment and disposal. Landfills are gradually constricted given the limitation of land sites. Conversely, incineration seems less acceptable because of the legislative regulations for environmental protection (Huang et al., 2014, Lin and Ma, 2012). For SS composting, emissions of odor gas and bioaerosol along with leachability of heavy metals are serious concerns (Fang and Wong, 1999).

Pyrolysis, a technology for thermochemical conversion, has been widely used in industry. Given its fast reaction, small footprint, reduction of emissions, immobilization of heavy metals, and convenience of control and scale-up, pyrolysis is recently gaining greater popularity for SS treatment compared with other treatment technologies (Cao et al., 2010, Domínguez et al., 2006, Han et al., 2015, Jin et al., 2017). Moreover, pyrolysis is believed to convert the organic waste of SS into valuable gaseous, liquid, and solid by-products, a feature which makes their storage, transportation, and usage easier (Cao and Pawłowski, 2012, Chen et al., 2014, Kim and Parker, 2008, Zhang et al., 2017a). Previous research identified ablative, fluid bed, circulating fluid bed, and vacuum pyrolysis as the main types of pyrolysis technologies (Bridgwater et al., 1999). These technologies typically employ an electric furnace and regenerative gas heater as heating sources (Agrafioti et al., 2013, Jin et al., 2017, Ruiz-Gómez et al., 2017). However, those methods of external heat transfer could lower pyrolysis efficiency and generate high energy loss. Over the last decade, microwave-assisted pyrolysis has become predominant and a potential technology for SS pyrolysis (Huang et al., 2015, Ma et al., 2017, Zhang et al., 2017b). In comparison with the familiar heating ways, non-contact and well-established microwave heating method facilitates the reduction of energy consumption and pyrolysis reaction time (Foong et al., 2020, Huang et al., 2015, Lam et al., 2019). However, safety measurements might require greater attention when using microwaves with 2.45 GHz or higher frequencies during pyrolysis to protect human health and the environment from radiation. To date, heating temperature and rate play key roles in SS pyrolysis, as demonstrated by many studies (Shen and Zhang, 2003, Shie et al., 2015, Wang et al., 2007). During pyrolysis, additional free radicals are generated because of the high heating rates, an outcome which is consequently beneficial for the yield of gaseous and liquid products. By contrast, a slow heating rate might lead to the generation of solid products due to the long vapor residence time (Iyoti, 2006). Although pyrolysis technologies and the parameters of the reaction processes have been intensively studied, no research has been reported on pyrolysis products through electromagnetic induction (EMI) pyrolysis of SS.

EMI has been extensively used as heating method in many fields, such as the thermal treatment of metals, plastics and textile (Bayerl et al., 2014, Bera and Babadagli, 2015, Norambuena-Contreras and Garcia, 2016, Sadeghi et al., 2017). Given its non-contact, energy saving, and pollution free nature and its effective thermal conversion, EMI heating is very effective and environment-friendly through its fast distribution of thermal energy into induction objects. Electrical conductivity dependent on the heating method such as EMI used in industry usually shows low frequency (20–40 kHz) corresponding to a lower radiation risk. Specifically, by cutting the magnetic lines of force in an alternating magnetic field, alternating current produces and heats the non-insulated conductance magnet induction media, thereby simultaneously heating the target by the effect of thermal conduction. Regarding the thermal treatment of SS, Xue et al. have considered and investigated using EMI for drying SS. They concluded that the drying characteristics of SS was significantly influenced by the working voltage of the EMI device. During EMI drying, a quick weight reduction related to the release of volatile organic matters as well as an obvious carbonation phenomena of SS can be observed when the working voltage exceeds 350 V (Xue et al., 2018b). A further investigation on SS pyrolysis by EMI is promising. Bio-char can be prepared through EMI pyrolysis and shows similar general properties bio-char as well as its better adsorption capacity on heavy metals compared with bio-char derived from conventional pyrolysis (Xue et al., 2019). Experimental results indicate that EMI pyrolysis temperature was determined by the relationship between the working voltage of the EMI device and the induction media. Bio-char yield rate ranged from 89.7 wt% to 51.2 wt% at temperatures from 300 ℃. to 600 ℃. Elements, surface characterizations and micromorphology, thermogravimetric properties, and gas evolution changed with the increasing pyrolysis temperature. Hydroxyl groups were decomposed during EMI pyrolysis (Xue et al., 2019). EMI has also been conducted on food processing SS and exhibited a potential for recycling biomass energy (Wentien et al., 2009). The risk of polycyclic aromatic hydrocarbons in bio-oil pyrolyzed by induction-heating has been evaluated (Tsai et al., 2009). These findings indicate that EMI pyrolysis can be used for the production of liquid and gaseous products. Although several properties of bio-char from EMI pyrolysis have been improved, the distribution and composition of liquid and gaseous products which is great of importance for energy conversion during SS pyrolysis remains unclear. The energy consumption of EMI pyrolysis must also be evaluated in relation to that of conventional pyrolysis, such as an electrical resistance furnace (ERF) with a tube reactor.

To understand SS pyrolysis by EMI, this work presents an improved pyrolysis reactor and method designed on the basis of our previous research. The temperature and initial moisture content of SS affecting the yield of pyrolysis products were discussed. A comparative analysis for the product distributions and characterization in solid, liquid, and gas fractions between the EMI and ERF pyrolysis of SS were investigated. The mass and energy balance was evaluated. This work helps to understand the difference of pyrolytic product distribution and energy consumption between EMI and ERF pyrolysis by providing a comparative analysis.

Section snippets

Materials

SS used in this work involved mixed sludge obtained from the Luobuzui wastewater treatment plant in Wuhan, China. Raw SS was stored at 4 ℃ in a refrigerator. The main properties of SS are listed in Tables S1 and 1. The original moisture content of this kind of raw SS is 95 wt%. Raw SS samples were placed in an oven at 105 ℃ for 2 h to obtain the dried SS. To evaluate the effect of different moisture content on pyrolysis, moisture contents corresponding to 30, 50, and 70 wt% of SS were obtained

Effect of temperature

The effect of pyrolysis temperature on the yield of products is given in Fig. 2 (a). By increasing the pyrolysis temperature, the bio-char yield was dominated and significantly decreased from 65.3 wt% to 44.0 wt%. The higher yield of bio-char depended on SS composition corresponding to the higher ash content. The yield of bio-oil which consisted of water and pyrolyzed oil increased from 20.3 wt% at 300 ℃ to 23.5 wt% at 400℃, and then decreased to 15.8 wt% at 600 ℃ with increasing temperature.

Conclusions

A comparative study was conducted on pyrolysis of SS in tube reactors heated by EMI and a conventional ERF. The effect of the main pyrolysis parameters on the distribution of pyrolytic products was similar and consistent with those of previous research.

The bio-char from EMI pyrolysis exhibited lower H/C ratio corresponding to carbonization and aromaticity and indicated a better application of soil remediation. No significant difference in morphology and no new functional group was found through

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.

Acknowledgments

Financial and technical supports from National Science Foundation of China (51878526) are gratefully acknowledged.

References (50)

  • A. Domínguez et al.

    Production of bio-fuels by high temperature pyrolysis of sewage sludge using conventional and microwave heating

    Bioresour. Technol.

    (2006)
  • M. Fakhroleslam et al.

    Thermal/catalytic cracking of hydrocarbons for the production of olefins; a state-of-the-art review III: Process modeling and simulation

    Fuel

    (2019)
  • M. Fang et al.

    Effects of lime amendment on availability of heavy metals and maturation in sewage sludge composting

    Environ. Pollut.

    (1999)
  • S.Y. Foong et al.

    Valorization of biomass waste to engineered activated biochar by microwave pyrolysis: Progress, challenges, and future directions

    Chem. Eng. J.

    (2020)
  • J. Han et al.

    Modeling downdraft biomass gasification process by restricting chemical reaction equilibrium with Aspen Plus

    Energy Convers. Manage.

    (2017)
  • R. Han et al.

    Thermal characterization and syngas production from the pyrolysis of biophysical dried and traditional thermal dried sewage sludge

    Bioresour. Technol.

    (2015)
  • H.-J. Huang et al.

    Co-pyrolysis of sewage sludge and sawdust/rice straw for the production of biochar

    J. Anal. Appl. Pyrol.

    (2017)
  • X. Huang et al.

    Influences of pyrolysis conditions in the production and chemical composition of the bio-oils from fast pyrolysis of sewage sludge

    J. Anal. Appl. Pyrol.

    (2014)
  • Y.F. Huang et al.

    Microwave co-pyrolysis of sewage sludge and rice straw

    Energy

    (2015)
  • Z. Huang et al.

    The effects of Fe2O3 catalyst on the conversion of organic matter and bio-fuel production during pyrolysis of sewage sludge

    J. Energy Inst.

    (2019)
  • Y. Kim et al.

    A technical and economic evaluation of the pyrolysis of sewage sludge for the production of bio-oil

    Bioresour. Technol.

    (2008)
  • S.S. Lam et al.

    Microwave pyrolysis valorization of used baby diaper

    Chemosphere

    (2019)
  • L. Leng et al.

    Biochar stability assessment methods: A review

    Sci. Total Environ.

    (2019)
  • H. Lin et al.

    Simulation of co-incineration of sewage sludge with municipal solid waste in a grate furnace incinerator

    Waste Manage.

    (2012)
  • H. Liu et al.

    Influence of residual moisture on deep dewatered sludge pyrolysis

    Int. J. Hydrogen Energy

    (2014)
  • Cited by (16)

    • Plasma-electrolytic biorefinery of sewage sludge for clean oil and bio-derived metal oxide production

      2023, Sustainable Materials and Technologies
      Citation Excerpt :

      PEL is based on the solvent-catalyzed liquefaction which is a thermal conversion process. The addition of solvent and catalyst could decrease the temperature and realize biomass conversion in atmospheric pressure compared with hydrothermal liquefaction with a temperature of 200–400 °C and pressure of 2–20 MPa and pyrolysis at over 400 °C and with a protective gas [25,31,32]. So far, this method has been used to liquefy lignocellulosic biomass, microalgae, and, most recently, human feces [6,33–37].

    • Cornstalk pyrolysis for syngas in a two-stage electromagnetic induction reactor

      2023, Fuel
      Citation Excerpt :

      Therefore, it may be a viable alternative to heat the biomass pyrolysis and more importantly to convert heavy oil into lighter products as indicated by a few studies in the literature. Xue et al. [19] studied the pyrolysis of sludge by placing it on an iron sheet in an induction field of 150 kHz, and reported oil and gas yields of 15.8 wt% and 40.3 wt%, respectively, in 30 min at 600 °C. The gas products included H2, CH4, CO and CO2, among which H2 accounted for 14.8 % hydrogen in the sludge.

    • Experimental investigation of a new ammonium bicarbonate reactor induced with an electromagnetic heater

      2023, International Journal of Hydrogen Energy
      Citation Excerpt :

      Their results showed significant enhancements in making ammonium bicarbonate from carbon dioxide and aqueous ammonia, but the chosen catalyst is extremely expensive compared to typically used catalysts in the chemical industry. In addition, electromagnetic induction heaters have been studied for various applications to improve the chemical production of different chemicals [20–22]. For example, Belkhiria et al. [23] introduced an electromagnetic induction heater to a metallically catalyzed hydrogen reactor.

    View all citing articles on Scopus
    View full text