*2.3. Calculation Method*

CFD tools are widely used to simulate and optimize the processes of heat transfer [20] and energy release from various fuels [21]. A transient 3D model was prepared and used to determine the changing conditions inside the furnace over time. The model based on URANS approach covers turbulent gas flow by solving three-dimensional Reynolds-Averaged Navier-Stokes equations with realizable k-ε turbulence model applied to solve the Reynolds stresses. Gravitational forces were included. The solver is pressure-based and because adaptive time step sizes (0.1 s for the first 1 s of the process, 1 s for the period between 1 s and 15 s of the process, and 15 s for the rest of the process) were used in the simulations, the coupled algorithm for calculating pressure was chosen.

The gas is a multicomponent single-phase mixture composed of CO2, CO, CH4, C2H6, H2, O2, H2O, and N2, whose local mass fractions are predicted through solving the conservation equations for each species (except for nitrogen, which is the balancing species), and both density (following the incompressible ideal gas law) and specific heat of each compound are temperature dependent. Species can participate in the volumetric chemical reactions Equations (1)–(4), which are the source of thermal energy in the system.

$$\text{C}\_2\text{H}\_6 + 2.5\text{ O}\_2 \to 2\text{ CO} + 3\text{ H}\_2\text{O},\tag{1}$$

$$\text{CH}\_4 + 1.5\,\text{O}\_2 \to \text{CO} + 2\,\text{H}\_2\text{O},\tag{2}$$

$$\text{CO} + 0.5\,\text{O}\_2 \to \text{CO}\_2\tag{3}$$

$$\text{H}\_2 + 0.5\text{ O}\_2 \rightarrow \text{H}\_2\text{O} \tag{4}$$

The rate of these chemical reactions is controlled by turbulent mixing, that is an acknowledged approach used for modelling non-premixed combustion (eddy-dissipation model) [22].

The numerical model allows the heat to be transferred by convection, conduction (within the load's volume), and radiation mechanisms. Radiative heat transfer is calculated using discrete ordinates method with the weighted-sum-of-grey-gases model included. Temperature is obtained through solving the energy equation. The CFD simulations were performed in Ansys Fluent 19.0.
