**3. Results**

The results of the numerical calculations contain information about, among others, temperature field in the gas and solid domains. Exemplary visualization of the obtained temperature results for case 7 is shown in Figures 6 and 7.

**Figure 6.** Temperature contours after 2 h of the heat treatment (**a**) 0.3 m and (**b**) 0.5 m above the furnace bogie level—case 7.

**Figure 7.** Temperature of the load surface after 2 h of the heat treatment—case 7.

In this study the focus was on the mold temperature levels, especially after 2 h of the treatment. Acquisition of the data took place at every time step, therefore at least every 15 s of the simulated process time. The results for the two analyzed cases (1—the reference one, and 7—the SG co-firing scenario with the results most similar to case 1) are presented in Figure 8.

**Figure 8.** Volume average (avg), minimum (min), and maximum (max) temperatures of the load during the first 2 h of the heat treatment: (**a**) case 1, (**b**) case 7.

The applicability and efficiency of each powering mode was assessed on the basis of load temperature difference (LTD) and volume average load temperature (ALT), respectively. Value of LTD is defined as the temperature difference between the hottest (Tmax) and the coldest (Tmin) point of the load—lower LTD means better thermal uniformity. ALT is an indicator of thermal efficiency, i.e., the ratio of the energy fed in fuel to the energy received by the load—higher values of this measure denote improved usage of heat.

Substituting 40% of NG by low-calorific SG with a constant amount of air supplied to the NG burners has a negative or no effect on thermal uniformity of the load, depending on whether the power among the furnace corners has not or has been equated, respectively (cases 8, 9; Figure 9a). Introducing syngas, while maintaining λNG at 2.0, increases the maximum load temperature difference up to the level noted in the reference case (cases 5, 7; Figure 9a), for which λNG is 1.42. Adjusting the natural gas burners' power in cases where λNG is equal to 2.37 or 3.0 is noticeably beneficial from the LTD perspective (cases 4, 6, 8, and 9).

**Figure 9.** Load temperature difference (**a**) and average load temperature (**b**) values for different ways of supplying power (number of kW represents the power of NG burners).

Use of different NG burners power outputs has no impact on average load temperature, regardless of λNG applied (cases 4, 6 and 5, 7 and 8, 9; Figure 9b). Reducing the air–fuel equivalence ratio is correlated with elevation of the heating efficiency, especially in case of no syngas addition. Implementation of co-firing significantly raises the amount of heat absorbed by the load—for λNG equal 3.0 by 28.1% ± 0.7%, and for λNG equal 2.0 by 17.3% ± 0.1%, reaching 98.5% ± 0.2% (cases 5 and 7) of the reference case heating efficiency.

Within 1 h the furnace powered conventionally (cases 1–3) consumes ca. 103.6 kg of natural gas and emits 285.5 kg of CO2 (under the assumption of complete fuel combustion). When 40% of NG is replaced by SG (cases 4–9), the amount of fossil carbon dioxide added to the atmosphere drops by 40% as well—in that case the emission is 171.3 kg CO2/h.
