# Simulation of Biomass Combustion with Modified Flue Gas Tract
## 2 Mathematical Simulation  
### The Description of Model  
The model used in the simulation is shown in Figure 1. This model is a modified wood stove, composed of two parts. The first part shows a combustion chamber with a retort burner and holes for the air supply. This part represents a prism with a length of 400 mm, a width of 420 mm, and a thickness of 400 mm. The second part shows a flue gas tract with a deflector and three baffles. It has a length of 500 mm, a width of 420 mm, and a thickness of 400 mm. These components are designed to capture solid pollutants such as particulate matter. Solid pollutants are particles consisting of carbon, ammonium, metals, organic materials, nitrates, and sulfates.  
The position of the baffles placed in the flue gas tract was also observed and the best variant was used in the complete model. The proposed variants are shown in Figure 2. The baffles were placed perpendicular to the vertical plane in variant 1. In variant 2, baffles were also perpendicular, but the last third of the baffle was turned down by an angle of 30\({}^{\circ}\). Variant 3 had perpendicular baffles, and the last third of the baffle pointed vertically downward and then turned by an angle of 65\({}^{\circ}\) from the vertical plane. In variant 4, they were parallel to each other and rotated at an angle of \(\pm\)20\({}^{\circ}\) from the horizontal plane.  
The used material in the complete model was glass on the front wall of the model and then fireday panels on the other walls. The air supply was realized by using three types of inlets. The supply of primary air was realized through the retort burner for primary combustion. The second type of air supply was provided through holes on the rear wall of the model for secondary combustion. The supply of the last tertiary air was used to blow the glass.  
The created mesh of the complete model had nearly 575,500 number of elements, whose shapes were mostly tetragonal, then hexagonal. The quality of the mesh was assessed based on the orthogonal quality with the average value of 0.85, the skewness with the average value of 0.16, and the aspect ratio with the average value of 1.55. Based on the evaluation criteria, the quality of the mesh was recognized as sufficient. The mesh of the  
Figure 1: The model of modified wood stove: (I) A: Outlet; (II) B, I and J: Burner; (III) C: Secondary inlet; (IV) D: Tertiary inlet; (V) E, F and G: Baffles; (VI) H: Deflector.  
Figure 2: The proposed variants of the flue gas tract: (I) Variant 1; (II) Variant 2; (III) Variant 3; (IV) Variant 4; (V) A, B and C: Baffles.  
complete model is shown in Figure 3. Wall boundary conditions were set up to the baffles, deflector, and retort burner. The burner zone was also used. The exit from the chimney was called the outlet. Inlets were realized by using a retort burner, holes for the secondary inlet as well as a hole for the tertiary inlet.  
The grid sensitivity test was realized only on the model of flue gas tract with baffles (variant 4). The denser mesh was created, where the number of elements was 6.7 times higher compared to the base model. The evaluated results showed differences of 2.4% based on the particle number and 2.3% based on the mass flow in the particle outlet stream. As differences in the results were insignificant, less dense mesh was used in all analyzed cases. The final calculation mesh was constructed based on similar element sizing.  
The CFD model of the wood stove was developed in Ansys software, where the CFD solver was Ansys Fluent.