Syngas Fuel Combustion in Gas Turbine Can Combustor: CFD simulation

Description

ANSYS Fluent CFD simulation of Schwarze Pumpe syngas in a Gas Turbine Can Combustor (GTCC)

In this project, a Gas Turbine Can Combustor (GTCC) has been simulated using computational fluid dynamics. This system consists of 6 fuel injectors, a primary air inlet, and 6 secondary air inlets around the combustion chamber. The system geometry along with dimensions is shown in Figure 1. The fuel inlet mass flow rate is 0.001 kg/s, and the air velocity at the primary and secondary air inlets is 10 and 6 m/s, respectively. The temperature for the species in all inlets is assumed to be 300 K. Thus, a non-premixed combustion simulation has been performed.

Figure 1: The geometry of the gas turbine can combustor

The volume fraction of the selected syngas, which is Schwarze Pumpe syngas, is presented in Table 1. In this project, the combustion and emission characteristics of this syngas have been simulated and analyzed. In other words, syngas compositions are controlled by many parameters, which GTCCs can use to perform CFD simulations under different conditions and at very low cost to achieve the best composition. Thus, the combination presented in Table 1 allows the user to provide comprehensive training, despite the different types, so that she/he can simulate these systems in an excellent manner.

Figure 2 also shows the mesh used in this project for GTCC. This mesh was generated by Ansys Meshing 2024 R2 software. Also, due to the complexity of the system, an unstructured and high-quality mesh was used in the simulation. Some of the characteristics of the generated mesh are as follows:

• Maximum aspect rate: 50
• Maximum skewness: 0.886 and its average: 0.24
• Average mesh quality: 0.71

For a turbulent flow simulation due to syngas combustion, the standard k-ε turbulence model has been used. Also, the radiation due to energy release in GTCC has been simulated using the P1 model. On the other hand, the non-premixed combustion model has been used for species transport. Thus, the state relation has been considered as chemical equilibrium, and the energy treatment has been considered as non-adiabatic. Finally, the PDF/mixture fraction has been used to simulate the combustion of non-premixed mixture. In this approach, species transport equations are not solved separately, and all equations are solved with together. The PDF table based on the mean temperature, mean mixture fraction and scaled variance, for the Schwarze pumpe syngas is shown in Figure 3.

Figure 2: The computational mesh

Figure 3: PDF table

Pressure and velocity coupling is performed by the SIMPLE algorithm, and to increase the accuracy of the simulation, all equations are discretized to the second order. In such problems where the number of species in transition is more than 3, the adjustment of Under Relaxation Factors (URFs) has a great impact on the convergence process. In this project, these parameters have been adjusted in such a way that the convergence has been done ideally. Figure 4 shows the residual diagram of the solution confirming this issue. The URFs are also set in such a way that at the beginning of the solution, some of them are used up to 0.7, and after 1200 iterations, they are increased to the values ​​provided in the case file.

Figure 4: Solution residuals diagram

 Results

In this section, various contours from the simulation are presented. These graphical figures, by presenting the physical concept of the problem, show that the simulation has been carried out with high accuracy. The figure 5 shows the pressure on the outer wall of the GTCC. As can be seen, the primary inlet air loses pressure as it passes through the turbine, and in other words, accelerates. This allows for better mixing of the fuel and air.

Figure 5: Pressure contour on the outer wall of GTCC

Figure 6 shows the temperature on the outer walls. As can be seen in this figure, the maximum temperature on these areas has increased to about 935 K. This maximum temperature is achieved by the completion of combustion at the outlet of the system. However, in the inner regions, i.e. the main part of the flame, the maximum temperature has increased to about 1900 K.

Figure 6: Temperature contour on the outer wall of GTCC

The velocity contour shown in Figure 7 also represents the different velocities of the species in the system. The way the flow becomes circular can be clearly seen in this figure.

Figure 7: The velocity contour on the different section of GTCC

Figures 8 to 13 also show the volume fraction of different species of CH₄, H₂, O₂, CO, CO₂, and H₂O. The mixing and transport of species in these contours indicates an accurate simulation of the system.

Figure 8: CH4 on the different section of GTCC

Figure 9: H2 on the different section of GTCC

Figure 10: O2 on the different section of GTCC

Figure 11: CO on the different section of GTCC

Figure 12: CO2 on the different section of GTCC

Figure 13: H2O on the different section of GTCC

The pollution phenomenon caused by syngas combustion is shown in Figure 14. By considering NOx in simulations, the efficiency of the system can be greatly increased, based on the reduction of pollution in the systems in which combustion takes place.

Figure 14: The pollutant NO on the different section of GTCC

Figure 15 shows the streamlines from the fuel inlet and air inlets. As can be seen, the air passing through the turbine causes the flow to rotate, and this factor has had a significant impact on better mixing of fuel and oxidant.

Figure 15: 3D streamlines from different and all inlets

Conclusion

In this project, a GTCC has been simulated using Ansys Fluent 2024 R2. Models such as the standard k-ε turbulence model, the P1 radiation model, and the non-premixed combustion model have been used to accurately simulate the system. In the combustion model, the PDF table is used to solve for the simultaneous transfer of Schwarze Pumpe syngas species. In this way, the physical and chemical phenomena of the system have been modeled with very high accuracy. To increase the efficiency of modeling, as well as to increase the level of training in simulating such systems, the pollution phenomenon (NOx) has also been considered. The results show that the presence of turbine blades to circulate the flow has a very large effect on the mixture mixing, and combustion has been better. Thus, the volume fraction contours of the fuel species clearly show that, at the outlet of the system, the fuel has been completely burned, and only combustion products have been present. In other words, the combustion efficiency in this system was very high with the boundary conditions used.

Reference:

[1] Yilmaz, Harun, Omer Cam, and Ilker Yilmaz. “A comparison study on combustion and emission characteristics of actual synthetic gas mixtures.” Fuel 263 (2020): 116712.