*2.4. Thermodynamic Equilibrium Simulation*

In order to better understand the conversion behaviors of Zn during co-pyrolysis process, the thermodynamic equilibrium simulations were carried out using FactSage 8.0 based on the principle of Gibbs free energy minimization. The contents of C, H, O, N, S and mineral elements (Ca, Al, Si and Fe) of coal were used as the inputs. The amounts of S, Ca, Al, Si and Fe are 0.24, 0.07, 0.46, 0.53 and 0.06 mol per kg of coal, respectively. Additionally, the amount of Zn is 0.0019, 0.0052, 0.0117, 0.0182, 0.12 and 0.62 mol in per kg of coal when the Zn content is 0.01, 0.02, 0.06, 0.1, 0.8 and 4 wt.%, respectively. Equilibrium calculations were performed at the temperature range of 100–1500 ◦C with an interval of 100 ◦C in the nitrogen atmosphere under a pressure of 1 atm.

#### **3. Results and Discussion**

#### *3.1. Phase Compositions of Coal and Tire Wastes*

The phase compositions of coal and three tire wastes were analyzed by XRD, as shown in Figure 2a. The main compounds in coal are kaolinite Al2Si2O5(OH)4 (JCPDS 29-1488), quartz SiO2 (JCPDS 99-0088), goethite FeO(OH) (JCPDS 99-0055) and gypsum CaSO4(H2O)2 (JCPDS 06-0047). Calcite containing magnesium (Ca, Mg)CO3 (JCPDS 43-0697) is found in WT-2 and limestone CaCO3 (JCPDS 86-0174) exists in WT-3. CaSO4 (JCPDS 74-2421) and ZnO (JCPDS 99-0111) are detected in all three tire wastes. CaSO4 is commonly used in rubber modification, which can effectively improve the mechanical performance and

thermal stability of rubber [54]. The XRD patterns of ashes obtained by the calcination of coal and tire wastes at 800 ◦C for 1 h are displayed in Figure 2b. SiO2, CaSO4 and Fe2O3 (JCPDS 85-0599) are detected in the ash of coal. The ashes of three tire wastes are composed of SiO2, CaSO4, ZnO and Zn2SiO4 (JCPDS 37-1485). Zn2SiO4 is formed by the reaction of ZnO and SiO2 at high temperatures [55].

**Figure 2.** XRD patterns of base coal and three tire wastes (**a**) and their ashes (**b**).

## *3.2. TG-DSC Analysis*

The influences of waste tires and ZnO additives on the thermal stability of coal were examined by TG-DSC measurements. The TG-DTG and DSC curves of coal, WT-2 and coal containing 5 wt.% of WT-2 or 5 wt.% of ZnO were displayed in Figure 3. The TG curve of coal can be divided into three stages, including dehydration, primary pyrolysis and secondary pyrolysis processes, giving rise to a total weight loss of 29.2% from room temperature to 1100 ◦C. The first stage from room temperature to 200 ◦C is attributed to the removal of physically adsorbed water, corresponding to a peak at 74 ◦C in the DTG curve and a small endothermic peak at around 82 ◦C in the DSC curve. The second weight loss takes place at 400–600 ◦C; meanwhile, a peak at 479 ◦C in the DTG curve and a broad endothermic peak at 482 ◦C in the DSC curve are observed, which is related to the primary pyrolysis of coal with the breaking and recombining of organic functional groups accompanied by the evolution of gas products such as CO2, CO, light aliphatics, CH4, H2O and so on [56–58]. The third stage occurs above 700 ◦C, which is assigned to the secondary pyrolysis of condensed carbon matrix with the release of CO and H2, corresponding to a broad endothermic peak at 600–1000 ◦C. The decomposition of waste tire WT-2 starts at 200 ◦C and a sharp DTG peak at 382 ◦C is observed to exhibit a weight loss of 68.9% up to 1100 ◦C. The low thermal stability of WT-2 should be due to the high volatile content and easily breakable molecular bonds of rubber [35,59]. The co-pyrolysis of coal with 5 wt.% of WT-2 displays an additional peak at 384 ◦C in the DTG curve and the weight loss increases by 1.7% up to 1100 ◦C compared with that of coal, indicating that more volatile matter is released after the addition of waste tire [60]. In comparison with coal, the DTG curve for a mixture of coal and 5 wt.% of ZnO shows an extra broad peak around 800 ◦C, and an exothermic peak at 830 ◦C in the DSC curve is detected, which is attributed to the carbothermal reduction of ZnO.

**Figure 3.** TG-DTG (**a**) and DSC (**b**) curves of coal, WT-2, coal containing 5 wt.% of WT-2 or 5 wt.% of ZnO.

## *3.3. Phase Compositions of Cokes*

The XRD patterns of cokes generated by the separate pyrolysis of coal and WT-2 are displayed in Figure 4. As shown in Figure 4a, SiO2 is detected in the coke obtained by the pyrolysis of coal at 700, 900 and 1050 ◦C. Additionally, CaS (JCPDS 08-0464) is found in the coke generated at 900 and 1050 ◦C. When the WT-2 pyrolyzes alone at 700 ◦C, the sphalerite ZnS (JCPDS 77-2100), ZnO, CaS and SiO2 are detected in the resultant coke. With the temperature rising, the diffraction peaks of ZnO disappear in the coke obtained at 900 ◦C and those of ZnS cannot be detected in the coke formed at 1050 ◦C, indicating that the ZnS is more stable than ZnO in the coke, but ZnS can also be reduced by carbon to metallic Zn vapor at high temperature.

**Figure 4.** XRD patterns of cokes obtained by the separate pyrolysis of coal (**a**) and WT-2 (**b**) at different temperatures.

Figure 5 shows the XRD patterns of coke produced by pyrolysis of coal containing 1, 3 and 5 wt.% of WT-2, 1 and 5 wt.% of ZnO, corresponding to 0.02, 0.06, 0.1, 0.8 and 4 wt.% of Zn content in the blends, respectively. When the Zn content is low (0.06 and 0.1 wt.%), ZnS is only detected in the cokes obtained at 700 ◦C, and as the Zn content increases to 0.8 and 4 wt.%, ZnS exists in the cokes formed at 700–1050 ◦C. ZnS cannot be detected in the coke obtained by the pyrolysis of coal containing 0.02 wt.% of Zn, which should be due to the ultra-low content of Zn and the resultant ultra-fine crystallite size of ZnS. As the

Zn content increases to 4 wt.%, ZnO and ZnS coexist in the coke produced at 700 ◦C. The diffraction peaks of ZnO vanish and those of ZnAl2O4 (JCPDS 73-1961) appear at 900 ◦C, whereas the ZnAl2O4 phase disappears and the diffraction peaks of ZnS become weak at 1050 ◦C. The above results show that Zn preferentially combines with S to generate ZnS, and when the Zn content exceeds the molar of S (4 wt.% of Zn), excess ZnO can react with Al2O3 to form ZnAl2O4, which is also not stable at 1050 ◦C and can be further reduced to metallic Zn vapor by carbon. Moreover, the CaS phase can be detected in the cokes formed at 700, 900 and 1050 ◦C when the Zn content is low (0.02, 0.06 and 0.1 wt.%), which can only be observed in the cokes obtained at 900 and 1050 ◦C as the Zn content increases to 0.8 and 4 wt.%. This suggests that the S preferentially binds with Zn rather than Ca because CaS can only be formed once some ZnS decomposes.

39

**Figure 5.** *Cont.*

**Figure 5.** XRD patterns of coke generated by pyrolysis of coal containing 1 wt.% (**a**), 3 wt.% (**b**) and 5 wt.% (**c**) of WT-2, 1 wt.% (**d**) and 5 wt.% (**e**) of ZnO.

#### *3.4. Migration of Zn during Pyrolysis*

The mass distributions of Zn in the pyrolytic products (coke, tar and gas) under different pyrolysis conditions are listed in Table S1, and the corresponding relative percentages (%) and contents (g/g coke) of Zn are displayed in Figure 6. It should be emphasized that the results for separate pyrolysis of coal and co-pyrolysis of coal with 1 wt.% of WT-2 are not included due to the extremely low content of Zn in the pyrolytic products and the resultant large detection error. As shown in Figure 6a, c and e, the relative percentages of Zn in the coke decrease with the temperature increasing. The Zn residual rate in the coke produced at 700 ◦C is 97.4%, 97.3% and 96.8% as the Zn content is 0.06, 0.1 and 0.8 wt.% in the coal mixture, which declines to 90.3% when the Zn content is 4 wt.%. When the co-pyrolysis temperature rises to 900 and 1050 ◦C, most of the Zn escapes from the coke and enters into the tar. Moreover, the residual rate of Zn in the coke obtained at 900 ◦C when the Zn content is 0.8 and 4 wt.% is higher than that of coke produced from the coal containing 0.06 and 0.1 wt.% of Zn. Additionally, the residual rate of Zn in the coke obtained at 1050 ◦C from the coal owning 4 wt.% of Zn is slightly lower than that of coke produced from the coal containing 0.06, 0.1 and 0.8 wt.% of Zn. The Zn contents in the pyrolytic products were also calculated based on the per gram coke, as shown in Figure 6b,d,f. It is shown that the Zn content of obtained coke increases with the increase of Zn content in the coal at the same pyrolysis temperature and decreases with the increase of pyrolysis temperature under the same Zn content of coal. The Zn content of coke formed at 700 ◦C is highest as the coal containing 4 wt.% of Zn, reaching 0.047 g/g coke and the largest amount of Zn (0.051 g/g coke) migrates into the tar produced at 1050 ◦C from the coal owning 4 wt.% of Zn. When the WT-2 pyrolyzes alone, the Zn residual rate in the coke obtained at 700 ◦C is 88.3% and then decreases to 40.2% and 0.05% at 900 and 1050 ◦C, respectively (Figure 6a). Compared with the co-pyrolysis of coal and WT-2, the coke yield by separate pyrolysis of WT-2 is almost reduced by half (Table S1), which results in the enhancement of Zn content calculated based on the per gram coke in the pyrolytic products (Figure 6b,d,f).

**Figure 6.** The relative percentages (%) (**a**,**c**,**e**) and contents (g/g coke) (**b**,**d**,**f**) of Zn in the coke, tar and gas under different pyrolysis conditions.

To sum up, when the coal containing 0.06, 0.1 and 0.8 wt.% of Zn (the molar ratio of Zn to S < 1) pyrolyzes at 700 ◦C, Zn is fixed in coke in the form of ZnS, resulting in a 97% Zn residual rate in coke. When the molar of Zn exceeds that of S (4 wt.% of Zn), ZnO and ZnS coexist in the coke produced at 700 ◦C and partial ZnO is reduced to gaseous Zn by carbon, causing a slight decrease in Zn residual rate in the coke (90.3%). It is noted that the Zn residual rate in the coke obtained by separate pyrolysis of WT-2 at 700 ◦C is 88.3%, which is lower than that of co-pyrolysis of coal with WT-2, although the molar ratio of Zn to S in the WT-2 is 0.64. This should be attributed to the large crystallite size of ZnO in the WT-2, which cannot be completely transformed into ZnS at 700 ◦C, as shown in Figure 4b, resulting in the coexistence of ZnO and ZnS in the coke obtained at 700 ◦C. As the pyrolysis temperature increases, ZnS is reduced to metallic Zn vapor by carbon, so the residual rate of Zn in coke decreases and the percentage of Zn in tar increases. In addition, the residual rate of Zn in coke is related to the Zn content of coal. As the Zn content of coal is 0.8 wt.%, a large amount of ZnO is reduced slowly by carbon with the temperature rising and leads to a higher Zn residual rate in the cokes obtained at 900 and 1050 ◦C. When the Zn content of coal is 4 wt.%, excess ZnO reacts with Al2O3 to generate ZnAl2O4 at 900 ◦C to enhance the Zn residual rate in coke to 50%. Additionally, ZnAl2O4 can also be reduced by carbon to generate gaseous Zn at 1050 ◦C, giving rise to a reduction of Zn residual rate in coke of 10%. Based on the above experimental results, the relative percentage distribution of Zn in the pyrolytic products is closely related to the content of S and mineral elements in the coal. FactSage 8.0 is further used to calculate the thermochemical conversion behaviors and phase distributions of Zn at different temperatures in detail.
