Characterization of copper (II) chemical forms and heavy metal distribution in chemical looping gasification of municipal solid waste

doi:10.1016/j.joei.2021.03.005

Journal of the Energy Institute

Volume 96, June 2021, Pages 140-147

https://doi.org/10.1016/j.joei.2021.03.005 Get rights and content

Highlights

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Formation of copper (II) in chemical looping gasification of municipal solid waste were studied.
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Heavy metals in chemical looping gasification of municipal solid waste were investigated.
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High hydrogen concentration promotes the formation of HCl (g) instead of CuCl₂ (g).
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Copper (II) can be reduced to solid-copper with the presence at H₂O/C of 2 and CaO/C of 2.
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Heavy metals of Pb, Cr, and Cu can be stabilized in solid compounds but except for Cd.

Abstract

Calcium‒based chemical looping gasification (calcium‒based CLG) shows promise for as a solution for waste problems. There has, however, been little discussion about dioxin emissions and heavy metal contamination in calcium‒based CLG. Herein, a thermodynamic equilibrium analysis was investigated to determine the characterization of copper (II) chemical forms (a main catalytic metal for the formation of dioxins) and heavy metal distribution in calcium‒based CLG of municipal solid waste (MSW). Results demonstrate that the high hydrogen concentration in calcium‒based CLG could promote the formation of HCl (g) instead of CuCl₂ (g); therefore, it was beneficial in suppressing formation of dioxins. Furthermore, during the operating temperature of gasifier in calcium‒based CLG (600–750 °C), some heavy metals (Pb, Cr, Cu) could be stabilized as solid compounds except for Pb. The valences of heavy metals decreased in the calcium‒based CLG. If sulfur was present, some heavy metals, such as Pb, could convert into solid sulfides; sulfur, therefore, has a positive effect on the stabilization of heavy metals. Consequently, this study may provide information useful in restricting dioxin emissions and heavy metal contamination in calcium‒based CLG.

Graphical abstract

Characterization of copper (II) chemical forms and heavy metal distribution in chemical looping gasification of municipal solid waste were comprehensive studied.

Introduction

Waste symbolizes the misallocation of resources and is represent the inefficiency of society [1]. Approximately 55% of humanity (4.0 billion) currently lives in urban centers. By 2100, the urban population will increase to 85% of the global human population [2]. Urban humanity produces approximately 70% of global waste [1,3]. Production of any waste will create a series of problems such as exhaustion of natural resources, environmental pollution, and the additional economic cost for waste disposal. Waste‒to‒energy (WtE) is one of the most visionary strategies for solving waste problems [4,5].

At present, incineration is one of the most mature WtE technologies, with approximately 1400 incineration facilities having been built around the world [6,7]. However, waste incineration is among the primary contributors to the total environmental levels of extremely toxic polychlorinated dibenzo‒p‒dioxins and dibenzofurans (PCDD/Fs) [8,9]. To meet the local legal levels of dioxin emission, incinerators must be equipped with advanced air pollution control devices (APCDs) and consume high volumes of activated carbon [10,11]. In comparison combustion, gasification has been considered as a promising WtE technology because PCDD/Fs formation can be inhibited in reduction at high temperature with the oxygen‒deficient atmosphere [7,12]. Calcium‒based chemical looping gasification (Calcium‒based CLG) with the addition of steam and calcium‒based sorbent as a potential inhibitor of PCDD/Fs formation [13,14] can produce hydrogen‒enriched syngas [[15], [16], [17]]; therefore, calcium‒based CLG has the a potential to further minimizing PCDD/Fs formation.

Some studies have indicated that the formation of dioxins could be affected by carbon‒Cl containing hydrocarbons [18]. The inhibition of carbon‒Cl bond formation is an efficient way to inhibit the formation of dioxins; theoretically, the hydrogen‒enriched syngas could transfer the carbon‒Cl bond of active chlorine species into hydrogen‒Cl bond of HCl molecules [19,20]. Chen et al. have suggested that catalytic metals, such as copper (Cu), iron (Fe), chromium (Cr), and zinc (Zn), are more significant in the synthesis of dioxin compared to the compositions of chlorine and residual carbon [21]. The catalytic capacities of metals ranked as Cu > Fe > Cr > Zn [22,23]. Fujimori et al. proved that the chlorinating force of copper (II) chloride is stronger than that of copper (I) chloride during at approximately 300 °C [24]. Therefore, the control of copper (II) chloride concentration as highly volatile and mobile is important in reducing the synthesis of dioxins in the thermal processing of waste [25].

Heavy metal contamination is another predominant type of pollution occurring during the thermal disposal of waste. The replacement of nitrogen by carbon dioxide could reduce the volatilization rate of Cd and Cr and increase that of Zn in waste combustion [26]. The transformation and migration of heavy metals varies in the surrounding atmosphere due to evaporation‒condensation. Chen et al. have suggested that Cd was distributed mainly in fly ash because of the generation of the reducing gas during CLG based on copper and iron‒based oxygen carriers; however, Cd is primarily distributed in bed materials during chemical‒looping combustion (CLC) [27]. The addition of calcium‒based sorbents reduced the volatilization of Cr, Cu, and Zn, while enhanced that of Pb [28]. Although the number publications about heavy metals control during CLG is increasing [27], research into the migration behavior of heavy metals during calcium‒based CLG are scarce.

Consequently, this study aims to consider two predominant types of pollution, namely dioxin emissions and heavy metal contamination, in the calcium‒based CLG of waste. Because copper (II) chloride is the predominant catalytic metal in synthesis of dioxins, this study investigates the potential pathway for synthesizing copper (II) chloride. In addition, the effects of chlorine concentration and hydrogen concentration on the synthesis of copper (II) chloride are evaluated thermodynamically with HSC Chemistry 6.0. The transformation and migration of heavy metals, such as Pb, Cd, Zn, Cr, and Cu, in calcium‒based CLG are also systematically explored through a comparison using equivalence ratio (ER) of 1.2, the mass ratio of steam to carbon (S/C) of 2, and the mass ratio of calcium to carbon (Ca/C) of 2.

Section snippets

Waste material characterization

In this study, the waste used in thermodynamic equilibrium calculations was the municipal solid waste (MSW) mixture which was collected, from the Guilin MSW incineration. The ultimate and proximate analysis of MSW is shown in Table 1. The heavy metal contents of MSW were analyzed using an inductively coupled plasma optical emission spectrometer (ICP‒OES, Agilent 730 series), made by Agilent Technologies of the United States. During characterization, approximately 0.3 g of the MSW sample (after

The potential reactions for synthesizing copper (II) chloride

According to the compounds of MSW, 13 potential reactions for synthesizing copper (II) chloride were selected in this study (Table 4). The potential reactions of R1, R2, R3, R4, R7, R11, R12, and R13 could directly synthesize copper (II) chloride; while other reactions could synthesize reactant compounds. These potential reactions were just an overview and some other reactions could also occur. In the real system, chemical equilibrium is not necessarily present [29]. Some other governing

Conclusions

This study has investigated the characterization of copper (II) chemical forms and heavy metal distribution in calcium‒based CLG of MSW. The concentrations of CuCl₂ (g), Cl₂ (g), and CuCl₂ (s) decreased with the increase of hydrogen concentration in 0–1400 °C; this trend was opposite to the concentrations of HCl (g), Cu (s), and CO (g). Therefore, calcium‒based CLG was beneficial to suppress the formation of dioxins. In ER of 1.2, the elements of Pb and Cd belong to the high volatile metals,

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

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China [52006096], the Natural Science Foundation of Guangxi Province, China [2020GXNSFBA297075, 2018GXNSFAA138071], Foundation for Science and Technology Base and Talents of Guangxi Provence, China [GUIKE AD20297010], and the Scientific Research and Technology Development Plan of Guilin, China [20180107‒2].

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Characterization of copper (II) chemical forms and heavy metal distribution in chemical looping gasification of municipal solid waste

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Waste material characterization

The potential reactions for synthesizing copper (II) chloride

Conclusions

Declaration of competing interest

Acknowledgements

J. Clean. Prod.

Appl. Energy

Renew. Sustain. Energy Rev.

Appl. Energy

Waste Manag.

Energy Convers. Manag.

Waste Manag.

Chemosphere

Environ. Pollut.

Process Saf. Environ. Protect.

J. Clean. Prod.

Process Saf. Environ. Protect.

Waste Manag.

J. Hazard Mater.

Chemosphere

Bioresour. Technol.

Chem. Eng. J.

Ecotoxicol. Environ. Saf.

Fuel Process. Technol.

Fuel

Chem. Eng. Process: Process Intensification

Renew. Sustain. Energy Rev.

Fuel

Renew. Energy