*2.1. Investigated Duct Location in the Boiler*

The numerical model includes the flue gas flow through the external duct located downstream of the main boiler flue to the SCR installation, including introducing hot flue gases from the bypass duct into the main flue gas stream. A representative geometry of an OP-650 class coal-fired power boiler was chosen for the study. The analyzed boiler unit schematic with the key components highlighted is shown in Figure 1: the boiler outline, the SCR installation and the section of pipeline analyzed in this paper placed directly upstream of the SCR installation. In Figure 1, the flue gas flow path is also marked with blue arrows. The analyzed duct section is symmetrically divided on two sides of the boiler, which is well illustrated by the axonometric view located in the upper left corner of Figure 1.

**Figure 1.** The analyzed boiler unit schematic with the key components: 1—boiler outline, 2—SCR installation, 3—analyzed duct section.

For further analyses, a section of the flue gas duct leading from the main boiler building to the SCR installation, marked in Figure 1 as number 3, was extracted from the presented geometry. As the analyzed duct section upstream of the SCR installation consists of two symmetrical parts, one of them was simulated in the numerical calculations using symmetry conditions.
