The role of CaO and its influence on chlorine during the thermochemical conversion of shredder residue
Graphical abstract
Introduction
Proper management of urban and industrial solid waste is one of the major challenges faced by society. In this context, the steel production, if not controlled, can cause significant environmental impacts, since different wastes are generated, including the shredder residue (SR). The SR is generated in the ferrous scrap preparation, which is used in the feeding of electric arc furnaces for the production of different types of steel.
The pyrolysis process is among the possible routes for the treatment of SR. Pyrolysis is a thermochemical process conducted at moderate temperatures and in the absence of oxygen, in which the volatile organic matter is decomposed into three fractions (solid, liquid and gas). The pyrolysis process is an attractive option because it reduces corrosion and emissions of sulfur, chlorine, and Polychlorinated Dioxins and Furans (PCDD/F). Meanwhile, pyrolysis lowers the rate of thermal NOX formation, due to its lower reaction temperatures (Guo et al., 2012).
Studies on the pyrolysis of automobile shredder residues (ASR) have been reported in the literature (Anzano et al., 2017; Day et al., 1996; de Marco et al., 2007; Edo et al., 2013; Galvagno et al., 2001; Haydary et al., 2016; Notarnicola et al., 2017; Rausa and Pollesel, 1997; Roy and Chaala, 2001; Shen et al., 1995). In this context, Table 1 shows the articles published in recent years that mention the thermochemical conversion of ASR. Nevertheless, this residue presents different properties and characteristics from the residue generated by the Brazilian steel industry. The residue generated by Brazilian shredders differs from other countries due to its feeding. Commonly, in other countries the shredders are fed with automobiles, whereas in Brazil the materials fed into the shredder come from a variety of sources (mixed scrap), making the composition of the residue particularly heterogeneous. The studies developed until this moment on the pyrolysis of ASR did not include any statistical analyses. Considering the SR heterogeneity, it is very important to use statistical tools to obtain reliable results. Therefore, a 2k factorial design was used in this study in order to evaluate the pyrolysis process of SR generated by a Brazilian steel industry.
The SR has all the characteristics of a complex waste, which makes its treatment and reuse difficult processes. Although SR is generated in large quantities, and is available to provide a safe feed in a process (Ambrose et al., 2002; Börjeson et al., 2000; Galvagno et al., 2001; Johnson and Wang, 2002; Robson and Goodhead, 2003), it contains significant amounts of sulfur, chlorine and heavy metals.
In general, the main source of chlorine in the SR is the polyvinyl chloride (PVC). The chlorine elimination is considered a major concern in the development of polymer heat treatment processes. Pyrolysis is mentioned as a possibility of dehalogenation. It is divided into three groups: pyrolysis in stages, catalytic pyrolysis and co-pyrolysis with adsorbents. During the pyrolysis in stages, a step at a low temperature is conducted in order to remove chlorine from the original samples as HCl; afterwards the pyrolysis is conducted in a conventional manner (Bockhorn et al., 1999a, 1999b). In catalytic pyrolysis, some metal catalysts have double potential, acting as catalysts and reducers of HCl emission. Co-pyrolysis with adsorbents can reduce HCl emission, since HCl is retained by physicochemical adsorption and maintained in the solid fraction. In this case, some renewable materials such as biomasses (Kuramochi et al., 2008), and alkaline adsorbents (CaCO3, CaO, Ca(OH)2, Na2CO3, NaHCO3) (López et al., 2011), have been used as HCl adsorbents.
Therefore, the calcium oxide (CaO) presence in the SR pyrolysis process can reduce the acid gases release (especially HCl). In addition, CaO has been used in pyrolysis processes for the in situ carbon dioxide (CO2) capture in the non-condensable gas.
The objective of this study was to evaluate the role of CaO and its influence on chlorine during the thermochemical conversion of shredder residue under different operating parameters (temperature, heating rate, inert gas flow, CaO addition) using a factorial design.
Section snippets
Materials
SR samples used during the development of this work come from the Brazilian steel industry. These samples were generated in a shredder equipment, which has the capacity to process approximately 45,000 tons of scrap/month. An equipment image is available in the supplementary material (S1). Residues generated in the shredding process (> 100 mm) were sampled at the beginning and at the end of the process. Afterwards, they were placed in bags with a capacity of 600 kg max. After the quartering
Thermogravimetric analysis
Fig. 2 shows the thermogravimetric curve of SR and the main materials constituting the SR: polystyrene (28.69%wt.), high density and low density polyethylene (17.85%wt.), polyamide 6 (5.73%wt.), polypropylene (4.94%wt.), polyamide 6.6 (1.77%wt.), wood (3.29%wt.) and polyamide 6.12 (2.21%wt.).
According to Fig. 2, the SR sample has a residual mass (sample mass remaining at 800 °C) of approximately 17%. The polymers present in the sample have negligible residual mass at the end of the experiment.
Conclusions
The chlorine retention in the char was higher in the experiments conducted in the presence of CaO. The reduction of chlorine retention in char by increasing the temperature for the experiments conducted in the presence of CaO was attributed to the volatilization of metal chlorides.
The retention of metals in char was influenced by the presence of CaO. A decrease in the retention of some metals (Co, Cu, Cr, Fe, Ni and Zn) in char in the presence of CaO, as well as an increase in Al and Mn
Acknowledgments
The authors would like to acknowledge the National Council for Scientific and Technological Development (CNPq) for providing the scholarship, the Coordination for the Improvement of Higher Education Personnel (CAPES) and are grateful to Gerdau Riograndense (Rio Grande do Sul State- Brazil) for providing the SR samples.
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