Carbonation of municipal solid waste gasification fly ash: Effects of pre-washing and treatment period on carbon capture and heavy metal immobilization☆
Graphical abstract
Introduction
The emission of greenhouse gases has become an important climate problem faced by human society. Due to its high emission, CO2 contributes more than 60% to global warming among greenhouse gases (Adebayo et al., 2021; Begum et al., 2015). According to Blunden and Boyer (2021), the atmospheric CO2 concentration reached 412.5 ppm in 2020, increased by 2.5 ppm compared with 2019. With the increasing atmospheric CO2 concentration, the global surface temperature in 2020 was one of the three warmest since the 1800s with records (Blunden and Boyer, 2021). The rising temperature may lead to a series of problems including rising sea level, loss of land, and following societal problems (Bamber et al., 2019), and it is necessary to decrease the emission of CO2. Apart from decreasing carbon emission activities, CO2 capture and storage is also a pathway to decrease the emission of CO2 (Li et al., 2013; Pan et al., 2020; Vassilev and Vassileva, 2020). Alkaline materials can react with CO2 in the presence of water and could be used for CO2 capture (Pan et al., 2012; Ji et al., 2018; Pan et al., 2020; Chen et al., 2021a). Pan et al. (2020) reported that 12.5% of global CO2 emission is potential to be reduced by CO2 mineralization and utilization with alkali waste. Incineration fly ash (IFA) is a solid residue from the municipal solid waste (MSW) incineration process and could be carbonated due to its alkali content. However, due to the high temperature during the incineration process, heavy metals such as Pb, Zn, and Cd volatilized and concentrated in the IFA. The high contents of heavy metals and other pollutants such as chlorides and dioxins make IFA a hazardous waste (Quina et al., 2018; Chen et al., 2021b; Zhang et al., 2021; Wang et al., 2022). It has been proved that after carbonation some heavy metals in IFA could be effectively immobilized, especially for Pb (Ecke, 2003; Jianguo et al., 2009; Ni et al., 2017; Ren et al., 2022; Chen et al., 2022). Through carbonation of IFA, the CO2 capture and the immobilization of heavy metals in IFA could be achieved simultaneously. Ren et al. (2022) used the carbonated IFA in cement paste and reported a replacement ratio of 30% could be achieved without the reduction of the strength of the paste. However, there are also some limitations of gas carbonation. With exceeding carbonation, the pH of IFA will reduce to even below 7, increasing the leachabilities of Pb, Zn, Cd, and Cu (Ecke, 2003; Wang et al., 2010; Chen et al., 2022). Current studies also revealed that conventional gas carbonation is not effective to stabilize or remove chlorides in IFA (Wang et al., 2010; De Boom et al., 2014).
Slagging-gasification is a novel method to treat municipal solid waste (MSW). Slagging-gasification produces valuable syngas and optimizes the compositions of solid residues, i.e., recyclable slag and metal alloy, without the generation of incineration bottom ash (IBA) in conventional MSW incineration (Arena, 2012; Tanigaki et al., 2012). The only solid residue that could not be reused currently is gasification fly ash (GFA). Compared with conventional IFA, heavy metals such as Pb and Zn are even more concentrated in the GFA (Tanigaki et al., 2012; Viet et al., 2020) because of the higher temperature in the slagging-gasification furnace (Jung et al., 2005), which increases the difficulty to treat GFA. However, there are limited studies about the carbonation of GFA. Viet et al. (2020) injected CO2 into a water-GFA mixture to carbonate GFA and used the carbonated GFA as a supplementary cementitious material (SCM). However, Viet et al. (2020) controlled the carbonation by monitoring the pH to around 8.3 for all samples and applied a very high liquid to solid ratio of 10/1. The optimal condition for carbonating GFA was not studied, and the high liquid content may hinder the diffusion of CO2 and affect the carbonation (Bertos et al., 2004). To obtain the optimal condition of treatment, more studies are required to analyze the carbonation of GFA. Water washing is a method that is usually applied to treat conventional IFA (Chen et al., 2012; Mao et al., 2020; Liu et al., 2021) for the removal of soluble chlorides (NaCl and KCl), which benefits its application as a cementitious material. Furthermore, some scholars have proved the effectiveness of water washing as a pre-treatment method before other treatments. For example, Ji et al. (2022) emphasized the importance of pre-washing before the sintering process because it benefits the product quality and could limit the generation of dioxins. Wang et al. (2009) claimed that pre-washing improved the efficiency to remove heavy metals in the following bioleaching. However, the effect of pre-washing on the carbonation of IFA or GFA has not been studied.
In order to investigate the effects of the treatment period and pre-washing on the carbonation of GFA, and to assess the properties of GFA and washed GFA (WGFA) as materials for CO2 capture, GFA and washed GFA (WGFA) were carbonated in sealed containers with CO2 with different treatment periods in this study. The changes in sample weight, pH value, and mineral composition of treated samples were determined and recorded with treatment period. To evaluate the effect of carbonation on immobilizing heavy metals in GFA and obtain the optimal treatment condition, the relationship between leached heavy metal concentrations from carbonated GFAs and WGFAs, the treatment period, and the pH value were analyzed. SEM-EDX test was done for the micromorphological analysis of carbonated GFAs and carbonated WGFAs.
Section snippets
Materials
GFA in this study was collected from the Waste to Energy Research Facility (WTERF) in Singapore, a slagging-gasification plant with a capacity to treat 11.5 t/d MSW. Water washing with a liquid to solid ratio of 10:1 was conducted to prepare WGFA. GFA and distilled water were mixed in a beaker and the mixture was then shaken at an orbital shaker. During the washing of GFA, heavy metals such as Pb and Zn dissolved first and then continually re-precipitated, and the washing time of 24 h was
Carbonation ratio and pH values
Fig. 1 presents the carbonation ratio of GFAs and WGFAs with different initial water contents. A proper water content is important for the carbonation treatment of alkaline solid wastes, since CO2 needs to dissolve into water prior to carbonation reactions (Ecke, 2003), while excessive water can hinder the diffusion of CO2 (Bertos et al., 2004; Li and Yi, 2019; Li et al., 2021; Xu and Yi, 2022). Fig. 1 shows that when the water content of GFA is between 5 and 25%, the final carbonation ratios
Conclusions
In this study, the properties of GFA and WGFA as the material for CO2 capture were evaluated. The effects of the treatment period and pre-washing on the properties of carbonated GFA were studied. The carbonation reactions of GFA were rapid, the weight increment and the change in mineral compositions mainly occurred in the first hour. The pH of GFA and WGFA continuously decreased during the carbonation process. The effect of carbonation on immobilizing heavy metals was closely related to the
Credit author statement
Junde Qin: Methodology, Validation, Formal analysis, Investigation, Writing – original draft.; Yunhui Zhang: Formal analysis, Writing – review & editing.; Yaolin Yi: Conceptualization, Formal analysis, Writing – review & editing, Supervision, Project administration, Funding acquisition.; Mingliang Fang: Writing – review & editing.
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.
Acknowledgement
This research is supported by the Ministry of Education, Singapore, under its Academic Research Fund Tier 1 grant (RG139/20). The authors appreciate Nanyang Environment and Water Research Institute (NEWRI) for providing gasification fly ash used in this study.
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This paper has been recommended for acceptance by Dr Hefa Cheng.