**1. Introduction**

With the rapid development of the automobile industry, the disposal of increasing nonbiodegradable waste tires has brought a great burden on the natural environment. After the removal of steel, carcass and textiles, the tire material generally consists of synthetic and natural rubber, carbon black, and inorganic components such as ZnO and SiO2 [1–4]. The carbon content of waste tires exceeds 80% and the ash content is comparable to that of coal [4,5]. The moisture content of waste tires is relatively low in comparison with other alternative energy sources such as municipal solid waste (MSW) or biomass [4,5]. In addition, the waste tires possess a high calorific value of 30–40 MJ/kg, which is larger than those of coal and other solid fuels [1–6]. Considering these characteristics, combustion, gasification and pyrolysis are proposed as the potential approaches to utilizing waste tires [7–27]. The pyrolysis of waste wires in an inert atmosphere can produce 10–30% of gas, 38–55% of pyrolytic oil and 33–38% of char, all of which are valuable products [17–21]. The pyrolysis gas is composed of methane, ethane, butadiene, hydrogen and other hydrocarbons with a high calorific value (37–42 MJ/kg), which can be used as a source of energy for the pyrolysis process itself and other required processes [19–23]. The tire pyrolysis oil (TPO) is a complex of C5-C20 hydrocarbons containing paraffins, olefins and aromatic compounds with a high calorific value of 41–44 MJ/kg, which can be employed as a substitute for diesel fuel to reduce fossil fuel consumption and as a high value-added chemical source for producing benzene, toluene, xylenes, isoprene and limonene [20–23]. The produced char has a calorific value of 30–40 MJ/kg and can be used as a fuel [23]. Additionally, the char can be reused as a low-quality carbon black in the tire industry because it contains high contents of ash (12–16%), S (1.8–4%) and Zn (3–5%) and as high-quality activated carbon

**Citation:** Lan, Y.; Jin, S.; Wang, J.; Wang, X.; Zhang, R.; Ling, L.; Jin, M. Effects of S and Mineral Elements (Ca, Al, Si and Fe) on Thermochemical Behaviors of Zn during Co-Pyrolysis of Coal and Waste Tire: A Combined Experimental and Thermodynamic Simulation Study. *Processes* **2022**, *10*, 1635. https://doi.org/10.3390/ pr10081635

Academic Editors: Elio Santacesaria, Riccardo Tesser and Vincenzo Russo

Received: 26 July 2022 Accepted: 15 August 2022 Published: 18 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

via activation using steam or carbon dioxide as activation agents for adsorption, catalysis and electrochemical applications [24–27].

Apart from classic pyrolysis, the co-pyrolysis of waste tires with coal for metallurgical coke production is considered an economical route for recycling waste products in the cokemaking process, decreasing coal consumption and reducing the cost of waste disposal without causing an apparent deterioration in coke quality under suitable conditions [28–32]. W.R. Leeder studied the effect of particle size of waste tires on the quality of coke and found that 5 wt.% of finely pulverized tires can be incorporated into coal blends without weakening the coke quality, but the coke yield is reduced because of the higher volatile matter of tires [28]. C. Barriocanal et al. compared the yields and characteristics of pyrolysis products from blends of coal and tire wastes in a fixed bed (FB) and a rotary oven (RO) [30]. It was found that the char yields obtained in the two configurations were similar and the RO promoted the production of gas while the FB produced a higher amount of oil. The pore characteristics of chars obtained from the two ovens were similar and the chars generated from the waste tires are mainly mesoporous whereas that from the coal contained a larger amount of macro- and micropores [31–33]. The oil produced in the RO was more aromatic and contained a smaller number of oxygen functional groups due to their higher residence time in the hot zone of the reactor. A.M. Fernández et al. found that the decrease in the fluidity of industrial coal blends after the addition of tire wastes and the ash composition of tires contributed to the deterioration in coke quality [34,35]. They found that the presence of Zn-bearing phases in the tire wastes increased the coke reactivity and proposed that a small number of waste tires (2 wt.%) should be incorporated into the industrial coal blends to guarantee coke quality and good blast furnace performance.

ZnO, as an activator for sulfur vulcanization, is widely used in tire rubber manufacturing with a weight content of 1–2% [36,37]. Zn is also known as a deleterious element for the production and life of the blast furnace [35–40], which is present in the blast furnace as a component of sinter charge or the coke in the form of an oxide (ZnO) and a sulphide (ZnS) and can be reduced by CO or carbon to metallic Zn [38,39]. The melting and boiling points of metallic Zn are 420 ◦C and 910 ◦C, respectively. Therefore, the metallic Zn is easily vaporized in the high-temperature regions and then condenses in the low-temperature zones or is re-oxidized to ZnO by CO2 and water vapor [40]. Consequently, reduction, vaporization, condensation, oxidation and circulation of Zn could occur in the blast furnace [38,41]. The harmful effects of Zn on blast furnaces have been investigated by several studies [42,43]. The Zn vapor can flow into the air holes of the refractory lining, then the deposition and/or the oxidation of Zn can give rise to internal stress, volume expansion and material damage [42]. The coke acts as a fuel, a reductant, structural support and a carburizer in the blast furnace [43]. The penetration and deposition of Zn in the coke pores can weaken the coke strength and accelerate the pulverization of coke. The Zn vapor can also react with the primary minerals near the coke pores to form new Zn-bearing compounds, resulting in the accumulation of Zn in the coke [43]. Meanwhile, Zn can catalyze the gasification reaction of coke, thus the high content of Zn will increase the coke reactivity index (CRI) and decrease the coke strength after reaction (CSR) [43].

As mentioned above, the presence of Zn in the coke is detrimental to the quality of coke and the production of a blast furnace. Actually, the volatilization and condensation of Zn vapor may also be harmful to the refractory materials of the coke oven during the co-pyrolysis of coal with waste tires. Therefore, it is essential to study the transformation behaviors of Zn during co-pyrolysis of waste tire and coal. The main mineral components in coal are SiO2, Al2O3, Fe2O3 and CaO [44]. Although the reactions between ZnO, carbon and S have been studied during the pyrolysis of waste tires alone [45–50], the impacts of inorganic components (Ca, Al, Si, Fe) originating from the coal on the Zn conversion and Zn distribution in the pyrolytic products during co-pyrolysis remain poorly understood. Considering these problems, the co-pyrolysis of coal and waste tires was carried out in a fixed bed reactor, and the Zn contents in the pyrolytic products (coke, tar and gas) with the different addition ratios of waste tires (or ZnO) at typical temperatures were studied. The mineral compositions of cokes were analyzed by XRD measurement. Meanwhile, the thermodynamic equilibrium calculations were conducted using FactSage 8.0 to simulate the thermochemical conversion behaviors and phase distributions of Zn and other mineral elements (Ca, Al, Si and Fe) under different co-pyrolysis conditions. Additionally, the consistency between the experimental results and the thermodynamic equilibrium analysis was verified [51–53].

## **2. Materials and Methods**

#### *2.1. Materials*

An industrial coal blend provided by an iron-making plant in China was used as the base coal, and three kinds of waste rubber powders (WT-1, WT-2 and WT-3) gained from the tire recycling industry were employed as additives, which were obtained by the grinding of tread rubbers from truck (WT-1) and car (WT-2) tires and sidewall rubber (WT-3), respectively. Table 1 provides the proximate and elemental analyses of base coal and tire wastes. The moisture content of sample was obtained by drying at 105 ◦C to constant mass. The ash content of the sample was obtained by calcining in air at 800 ◦C to constant mass. The sample was heated at 900 ◦C for 7 min in air to measure the volatile matter content. The contents of C, H, O, N and S elements were determined on an elemental analyzer (Elementar-vario EL cube). The contents of Zn, Al, Ca, Fe, Mg, Si and Ti elements in coal and WT-2 were measured using inductively coupled plasma atomic emission spectrometry (ICP-AES) on Agilent 720 spectrometer, as listed in Table 2. It is shown that the chemical compositions of three tire wastes are similar, and their H, O, S and Zn contents are higher than the base coal. WT-2 was selected as the additive to study the Zn distributions in the pyrolytic products by fixed-bed pyrolysis experiments and the transformation behaviors of Zn via thermodynamic simulations using FactSage 8.0.



<sup>a</sup> Dry basis. <sup>b</sup> Volatile matter. <sup>c</sup> Dry ash-free basis.

**Table 2.** Mineral compositions of coal and WT-2.

