*2.2. Operation Modes*

To evaluate the influence of NG-SG co-firing on the quality of the heat treatment, nine different furnace powering scenarios were developed and implemented in the furnace model—the test matrix showing the case numeration and respective substitution rates, NG burner power, and air supply levels is shown in Figure 3. For each case the furnace power is equal to the initial one, i.e., 1440 kW. Three main factors that can affect the process course were identified and tested:


**Figure 3.** Parameters of the considered furnace operation modes.

The air–fuel equivalence ratio for SG burners λSG is fixed to 1.15, because syngas as a low calorific fuel is considered more prone to incomplete combustion or flame blowout [19], and as such needs conducive conditions to avoid it.

The first 2 h of the process were simulated—during that time the burners are turned on continuously, and the tracked indicators, i.e., average temperature and maximum temperature difference of the load are expected to increase. At the beginning of the heat treatment (t = 0 s), the load and the air inside the furnace are 20 ◦C (293 K). Case 1 (no syngas, 1980 m3 <sup>N</sup>/h of air) serves as the reference case.
