The hottest gas turbine combustor burns natural ga

Comparison between burning natural gas and burning medium and low calorific value gas in gas turbine combustor

preface

coal cogeneration system can improve the comprehensive benefits of coal production, reduce air pollution, and support the transformation of coal enterprises from traditional industries to high-tech industries. The key problem is to provide a new type of gas turbine combustor with low NOx emissions suitable for a variety of fuels (including medium and low calorific value gas, natural gas, etc.). Changing the fuel of the combustion chamber from natural gas to medium and low calorific value gas will face the following problems:

· increase of fuel flow: with the power of the combustion chamber unchanged, due to the decrease of the calorific value of the medium and low calorific value gas, the fuel flow will increase by 3-10 times. Accordingly, it is necessary to provide greater air flow to the combustion area, change the combustion cooling mixing air ratio, and the combustion flame will be longer. Therefore, it is necessary to change the overall size and structure of the fuel system, combustion zone and mixing zone

· combustion stability: the combustion of medium and low calorific value gas is characterized by low average temperature rise and high local temperature rise. In addition, the increase of fuel flow leads to the increase of gas injection speed, and the lower limit of ignition of the main substance CO in the gas is high. When the unit works under low load conditions, it is prone to flameout

· control the contradiction between NOx emission and CO emission: the combustion temperature rise of medium and low calorific value gas is higher, while the flame propagation speed is lower, which is easy to produce CO emission exceeding the standard. CO emission can be greatly reduced by increasing the temperature of the combustion zone. However, this is contradictory to reducing NOx content by reducing the temperature of combustion zone. Now the more advanced DLN combustion chamber reduces the combustion temperature through premixed combustion, which has reduced the NOx content. However, the stability of premixed combustion is low, and the combustion stability of medium and low calorific value gas itself is low, so it is difficult to transplant to medium and low calorific value gas combustion chamber

· cooling problem: the gas contains H2, and the local combustion temperature is high, which is easy to burn out the nozzle. As mentioned earlier, due to the increase of fuel flow, it is necessary to provide greater air flow to the combustion area, which will lead to the reduction of the amount of air used for cooling

the solution of these problems depends on the close combination of experiment and calculation: the combustion problem itself is relatively complex, and the composition of medium and low calorific value gas is more than that of natural gas, and the reflection process is correspondingly complex. The previous work was mainly based on experiments, and numerical simulation was more difficult. With the development of CFD technology, we now have tools to solve such problems, which can conduct joint simulation of turbulence, multicomponent diffusion and chemical reaction. In this paper, fluent software is used to calculate the temperature distribution, flame structure, combustion efficiency and NOx distribution of the same combustion chamber using the natural gas and the gas with medium and low calorific value that the pendulum impact tester has failed, according to the insiders

introduction to algorithm

this paper uses UNS algorithm in fluent software. The simple method is used to solve the Reynolds averaged NS equation, and the standard k is used for the turbulence model- ε Model, "the wall function adopts the standard wall function. The combustion model adopts the non adiabatic PDF model

· the core algorithm of UNS: this algorithm is derived from the classic simple algorithm. Its application range is incompressible flow and medium compressible flow (Mach number less than 1). This algorithm does not solve the Navier Stokes equation simultaneously, but modifies the momentum equation by pressure. This algorithm is a very mature algorithm, which has been widely verified in application

· PDF combustion model: this model does not solve the transport equation of single component, but the transport equation of mixed component distribution. The concentration of each component is obtained from the distribution of mixed components. PDF model is especially suitable for the simulation of turbulent diffusion flames and similar reaction processes. In this model, the probability density function PDF is used to consider the turbulence effect. This model does not require the user to explicitly define the reaction mechanism, but is handled by the flame surface method (i.e. mixed combustion model) or chemical equilibrium calculation, so it has more advantages than the finite rate model. The model is suitable for non premixed combustion (turbulent diffusion flame), and can be used to calculate combustion problems in gas turbine combustors and complex combustion problems in liquid/solid rocket motors

· NOx simulation: Fluent software provides three NOx formation models: thermal NOx, prompt NOx and fuel NOx formation models. In this paper, thermal NOx and prompt NOx

· radiation models are selected: Fluent software provides four radiation models, and P therefore -1 model is selected in this paper. P-1 model is a simplification of p-n model, which is suitable for large-scale radiation calculation. Compared with dtrm model, it has the advantage of less computation and includes scattering effect. When the size of combustion calculation domain is relatively large, P-1 model is very effective. In addition, P-1 model can be applied in more complex computing domain

analysis of calculation examples

the composition of medium and low calorific value gas used in calculation is

it has been mentioned in the preface that one of the main problems of medium and low calorific value gas combustion is combustion stability. For medium and low calorific value gas combustion chamber, there are two structures to choose from, the first is to adopt cylindrical combustion chamber, and the second is to adopt annular or annular tube combustion chamber. The cylindrical structure is easier to solve the problem of medium and low calorific value gas combustion [3]. Therefore, the combustion chamber calculated in this paper is the cylindrical combustion chamber [1] of an experimental model of Shanghai Jiaotong University. The burner used in the experiment has no primary air jet hole and mixing hole. The nozzle is of direct type, with 6 fuel injection holes with a diameter of 1.2mm in the circumferential direction, and the twist angle of the hydrocyclone blade is 50O. Then the combustion of natural gas and the combustion of medium and low calorific value gas in the combustion chamber of a Ge gas turbine are simulated. Figures 1 and 2 show the structure diagram and grid diagram of the cylindrical combustion chamber. The grid is qualified with 150000 mixture. Figures 3 and 4 show the structure diagram and grid diagram of Ge combustion chamber. There are 18 nozzles in the whole combustion chamber. In this paper, periodic boundary conditions are used to simulate a single nozzle, with a total of 208000 grids

1. Simulation of natural gas combustion in cylindrical combustion chamber

Figure 5 is the temperature contour map. It can be seen from the figure that the high temperature area is large, and two high temperature vortex areas are formed at the upper and lower ends, but they are relatively small. From the jet trace diagram (Figure 6), it can be clearly seen that the natural gas injected by the fuel nozzle rotates into the combustion section under the action of swirling air, and is divided into two paths at the outlet of the combustion section. One path is sucked back to the combustion section to form a high-temperature vortex Zone, and the other path is until it reaches the outlet

2. Simulation of gas with medium and low calorific value in cylindrical combustion chamber

in the temperature contour map of longitudinal section (Fig. 7), it can be seen that the high-temperature area of flame is significantly smaller than that of burning natural gas. Due to the rapid injection speed of gas, the reflux area curls upward in the trace map (Fig. 8), and the flow state is completely different from that of burning natural gas

3. Simulation of natural gas combustion in Ge combustor

Figure 9 shows the temperature distribution in the longitudinal section. It can be seen that the combustion is mainly concentrated in the primary air inlet area. The first flame retardant hole on the upper cylinder wall successfully blocks the flame, and the flame retardant hole on the lower cylinder wall and the downstream mixing hole also play a role. Combined with figure 10 and Figure 11, the three-dimensional structure of the whole high temperature zone is analyzed. Figure 10 shows the cross-sectional temperature distribution. In the main combustion zone, the distribution of high temperature zone is very regular, and the flame has low central temperature and high peripheral temperature; When it comes to the flame-retardant area, it is affected by the flame-retardant jet, and the high-temperature area is squeezed. This can be seen more clearly from Figure 11. Figure 11 is the temperature contour map of 1400k. Due to the influence of the central jet of the upper cylinder wall, the flame is blocked and the high-temperature gas deforms to both sides. Figure 12 shows the velocity vector diagram of the longitudinal section. Each jet hole has a great impact on the mainstream, and each step cooling joint also plays a role of film cooling. Figure 13 shows the distribution of CH4 components at the outlet. It can be seen that the combustion is very sufficient, and the existence of CH4 can no longer be seen. Figure 14 shows the NOx component distribution at the outlet of the combustion chamber, which is not evaluated here, but compared with other working conditions (burning medium and low calorific value gas, improving fuel nozzle). Figure 12 shows the velocity vector diagram of the longitudinal section, and each jet hole has a great impact on the mainstream

4. Simulation of low calorific value gas in Ge combustion chamber

due to the low calorific value of gas, the fuel flow required to obtain the same power needs to be increased by 5 times. Using the same geometric structure and the same air flow as burning natural gas, the flow structure in the combustion chamber has changed greatly. Figure 15 shows the temperature distribution in the longitudinal section. It can be seen that the flame retardant holes do not work. It can be seen from Figure 16 that the high-temperature area is obviously smaller, and the effect of the jet in the step gap is not obvious. The high-temperature gas directly rushes to the lower cylinder wall near the outlet. The temperature distribution at the outlet is also unreasonable. It can be seen from the vector diagram in Figure 17 that the above phenomenon is caused by the increase of fuel injection flow, the unchanged nozzle size, the increase of injection speed and the enhancement of fuel jet stiffness. There is a little residual CO in the outlet section (Fig. 18), but the NOx content (Fig. 19) is lower than that when burning natural gas. This is mainly because the combustion temperature is lower than that when burning natural gas

5. Simulation of medium and low calorific value gas combustion after improving the nozzle

from the previous paragraph, we can see that the key to the problem is that the flow rate of fuel is too fast, so we will expand the nozzle to reduce the fuel speed - increasing the nozzle area by 4 times. Figure 20 shows the temperature distribution in the longitudinal section. Compared with Figure 15, the flow pattern structure has been improved to a certain extent. The flame-retardant jet on the upper cylinder wall plays a certain role, and the film cooling jet in the step gap also lifts the high-temperature zone away from the lower cylinder wall. The effects of flame retardant holes and jet holes can be seen more clearly in the cross-sectional temperature distribution and 1400k temperature contour map in figures 21 and 22. The drawback is that the NOx component in the outlet section has increased

conclusion

the combustion of natural gas and medium and low calorific value gas in cylindrical combustion chamber and annular combustion chamber is numerically simulated. There is a certain difference in the combustion flow structure of the two fuels in the cylindrical combustion chamber, but the difference is smaller than that in the annular combustion chamber. The original combustion chamber burning natural gas is directly changed to burning medium and low calorific value gas, which will cause the problem of too fast fuel jet speed, make the flame retardant hole invalid, the flame is too long, and the high-temperature gas will directly impact the outlet of the lower cylinder wall. Only increasing the diameter of the fuel nozzle can improve the flow structure in the combustion chamber to a certain extent, so that the flame retardant holes and mixing holes play a certain role, and the film cooling jet also lifts the high temperature zone away from the lower cylinder wall. However, the overall optimization of flame retardant jet, mixing jet and film cooling air flow is still needed

references

[1] Hu zongjun, et al., high humidity combustion calculation model and the influence of steam injection on combustion flow field characteristics of combustion chamber, thermal power engineering, Volume 15, 2000.3

[2] Zhao Shihang, gas turbine cycle and off design performance, Tsinghua University Press, 1993.7

[3] Jiao Shujian, integrated gasification gas steam combined cycle (IGCC), China Electric Power Press, 1996.12

[4] Fluent Inc. Product Documentation-User's Guide(end)