2.1.1. Furnace

The numerical model is based on the existing preheating furnace BOSIO 1 located in Store, Slovenia. For this study a medium-size preheating furnace was selected. This particular furnace was chosen because it is a representative example of thousands of similar, simple operation units installed around the world. The characteristic batch loading (using bogie hearth) enables high flexibility for thermal treatment of loads of different sizes and shapes. The obtained results should be extrapolated with ease to furnaces of similar type.

This gas-fired unit is equipped with four burners combusting natural gas (NG) able to operate with power up to 400 kW each, four independent recuperators preheating the air from room temperature to 60 ◦C (333 K), and a doubled chimney with the natural draft. During the heat treatment a vortex-like flow structure inside the furnace is created, and direct contact between streams of flue gas and the load is avoided (Figure A1). For co-firing purposes, an addition of two burners dedicated to combust syngas (SG) was proposed—their operating power is 360 kW each, and they are located in the vicinity of NG burners to ensure complete combustion of syngas. The geometry of the furnace model is presented in Figure 1. The load is a 4.1 m long steel cylindrical mold with an outer diameter of 1.8 m. It was assumed that its material density is 7700 kg/m3, specific heat is 502.5 J/(kg·K), and thermal conductivity is 50 W/(m·K).

**Figure 1.** Geometry of the furnace model (NG—natural gas, SG—synthesis gas).

Numerical domain of the furnace was discretized with a hybrid mesh—structured elements form the load, chimneys, and part of the syngas burners interior, and the rest of the geometry was filled with polygonal elements (total number of cells: 875,119).

Figure 2 shows the NG and SG burners' primary (17% of comburent air, marked blue) and secondary (83% of burner air, marked red) preheated air inlets separated by the inlet of the respective fuel (green for syngas, yellow for natural gas). The secondary air swirl angle is 30◦. The air consists of 77.45% nitrogen, 20.59% oxygen, and 1.96% water vapor by volume, what is an exemplary value corresponding to air at 85% relative humidity at 20 ◦C. The heat leaves the furnace to ambient at 20 ◦C (293 K) through convection: heat transfer coefficient at the outer walls of the furnace was specified as 25 W/m2·K, and with exhaust gas through the chimney outlets. Thermal insulance of the walls was estimated as 0.1 m2·K/W (0.2 m2·K/W for the walls surrounding NG burner no. 1 (the closest to the viewer in Figure 1) due to their reduced thickness of 0.1 m). Emissivity was 0.9, regardless of the temperature level and the material type.

**Figure 2.** Discretization of surfaces near syngas (left) and natural gas (right) burners—colors mark respective air and fuel inlets (blue—core air, yellow—NG, green—SG, red—secondary air).

Total power of the considered furnace is limited to 1440 kW. The same power was fed in fuel in the numerical model. The influence of the recuperators extracting a fraction of the exhaust gases from the inside of the furnace on the gas flow was represented by assigning fixed velocity (1.33 m/s) to the recuperator outlets. In cases where no syngas is fed into the system, inlets of the SG burners were treated as adiabatic walls.
