Study on combustion performance of microgas turbine combustor with different fuels

A low‐pollution single‐tube microgas turbine combustor using different gas fuels was studied. The combustion performance of two kinds of landfill gas and natural gas was tested in a single‐head combustor. The effects of fuel nozzle position, and the fuel distribution ratio between pilot and main combustion stage on pollutant emission were investigated under rated load condition. The flow field, temperature field, and pollutant generation characteristics with different fuel nozzle diameters and nozzle positions were analyzed by numerical simulation. The results show that: the combustor can form an obvious central recirculation zone, which is convenient for ignition and stable flame propagation; in the case of limited premixed length, the influence of fuel nozzle position on fuel–air mixing uniformity is less obvious than that of fuel nozzle diameter. The fuel–air mixing uniformity plays a decisive role in the temperature field and the generation of pollutants in the combustor. The combustor has good pollutant emission characteristics, and the NOx emission can be reduced to about 10 ppmv (15% O2 concentration) under the appropriate combination of fuel nozzle diameter and position, which provides a reference for the further optimization of low‐emission combustor.


| INTRODUCTION
Dry low emissions (DLEs) combustion technology is the mainstream technology for reducing pollutant emissions from gas turbines. The basic principle is lean homogeneous premixed combustion, the key is the uniform mixing of fuel and air to avoid local high temperature in the combustor. 1 The mixing characteristics of fuel and air are related to the physical boundary and geometric boundary of the combustor. Generally, the physical boundary conditions include the classification mode of fuel and air, the air distribution in the main combustion zone, the fuel distribution (FD) mode, and so forth, while the geometric boundary conditions include the fuel jet structure, premixed section structure, swirl intensity, and so forth. 2 The premixing process has been carefully designed for the existing advanced dry low-emission combustors in the world, such as DLN2.6 combustor of GE and HR3 combustor of Siemens to ensure better mixing uniformity of fuel and air. 3,4 The common feature is that the fuel is preinjected into the air through many small holes on the air mixing mechanism, the incoming air has considerable swirl and turbulence intensity, and has a long enough premixed distance. 5 The microgas turbine combustor is compact, and the space and time for sufficient premixing of fuel and air are limited. Fully premixing fuel and air in a compact space and effectively reducing pollutant emissions has become the key to the research on low-emission combustion technology for microgas turbines. Fric proposed the concept of premixed inhomogeneity in 1993 and concluded that the fuel mixing characteristics have a great influence on the combustion characteristics. 6 After that, researchers studied the factors that affect the mixing characteristics of fuel and air [7][8][9][10] and the effect of mixing characteristics on combustion characteristics [11][12][13] and pollutant emission characteristics. [14][15][16][17][18][19][20] Li et al. studied the pollutant generation mechanism and the main design parameters influencing rules of natural gas (NG) and biomass gas combustion, and proposed the optimization design method of premixed and graded low-emission combustor suitable for multifuel. 21,22 Xing et al. studied a miniature hybrid nozzle using 40 external jet misflows with conical circular outlets positioned by a distribution arrangement to improve the temperature distribution uniformity of the field flow. The optimal relationship between the axial liner length and fuel hole diameter is proposed. 23, 24 Liu et al. analyzed the effects of the two staging techniques on combustion characteristics and emissions, fuel staging has a more significant effect on reducing NO emission than air staging. 25 Microgas turbine has the characteristics of compact structure, strong fuel versatility, and flexible use, but it also faces the problem of reducing pollutant emission. Due to size and space constraints, the formation of full premixed fuel and air in the combustor of microgas turbine becomes the key to the research of low-emission combustion technology of microgas turbine. This paper introduces a low-emission single-tube combustor designed for microgas turbine; the effects of the FD ratio and fuel supply mode on pollutant emissions are investigated experimentally; the effects of key structural parameters such as fuel nozzle position and diameter on mixing performance, temperature field, and pollutant emission characteristics are studied by mathematical simulation. The conclusions can provide reference for the design and development of microgas turbine combustor.

| Combustor
The structure of the combustor is shown in Figure 1, which is a counterflow combustor. Lean premixed combustion technology is used to reduce pollutant emissions in the combustor, and central classification technology is used to meet the combustion stability. Two axial swirlers are designed for the DLE combustor. The swirler for pilot burner has greater swirl number, greater than 0.6 for a typical gas turbine combustor, to create a vortex breakdown reverse flow feature along the axis of the combustor. This flow feature is called as internal reverse flow zone or central recirculation zone. In this DLE concept, the reverse flow zone remains attached to the center of the combustor as shown in Figure 2, thereby establishing a firm aerodynamic base for flame stabilization. In the wake of the sudden expansion, an external reverse flow zone is established. The flame is stabilized in the aerodynamically generated shear layers around the internal and external reverse flow regions.
The fuel enters the combustor from the fuel inlet, in which the fuel inlet1 and fuel inlet2 cooperate with the swirler1 to form pilot burner, which is used to ensure the combustion stability. The fuel inlet1 adopts the diffusion combustion method. The main combustion stage of the combustor adopts premixed combustion to reduce pollutant emission. Fuel inlet3 and fuel inlet4 are main-stage fuel nozzles located in different positions as shown in F I G U R E 1 Schematic diagram of the combustor.
F I G U R E 2 Flow feature in the combustor. CRZ, central recirculation zone; ERZ, external reverse flow zone. Figure 1, which are used to analyze the influence of mainstage nozzle position and diameter on combustion characteristics. Table 1 lists the eight main-stage nozzle combinations to be studied. The front, middle, and back position diagrams are shown in Figure 1. Among them, "front (F)" refers to the inlet position of the swirler2, "middle (M)" refers to the middle position of the swirler2, and "back (B)" refers to the outlet position of the swirler2. The nozzle diameters were selected as 1.5 and 2 mm, respectively, and the fuel mass flow under different nozzle diameters was the same.

| Experimental test system
The structure of the experimental test system for the performance study of the combustor is shown in Figure 3. The test system mainly includes air supply system, fuel supply system, ignition system, test system, and related pipelines. The air supply system includes fan, storage tank, heater, and flow controller. The air flow rate can be measured by an orifice flowmeter at the inlet of the combustor, and the measurement error is ±1.0%. The gas fuel is obtained by mixing CH 4 , CO 2 , and N 2 in the mixing device. The flow rate of each component is measured by a mass flowmeter with a measurement error of ±1.0%. The pressure sensor is arranged at the inlet and outlet of the combustor. The measurement error of the pressure sensor is ±0.5%. K-type thermocouple is used for the temperature of import and export, and the measurement error is ±1.0%. The exhaust gas at the outlet of the combustor is measured by Testo 350 flue gas analyzer. The gas composition at the outlet of the combustor is sampled through the sampling ring. The burned gas enters the sampling ring through 12 small holes in the front of the sampling ring. The gas flow flows through the thin pipe with cooling to achieve quenching and freeze the combustion chemical reaction. After the sampling ring gas comes out of the sampling outlet, it enters the gas composition analysis through a 150°C thermostatic pipe for composition analysis. The measurement error of each component is as follows: the measurement error of O 2 concentration is ±0.8% of full range, the measurement error of CO concentration is ±5% of the measured value; the measurement error of NO concentration is ±5% of the measured value; the measurement error of NO 2 concentration is ±5% of the measured value.

| Experimental study
The combustible composition and heat value will fluctuate for different gaseous fuels, even for the same gaseous fuel, the concentration of combustible components will fluctuate at different times. For the microgas turbines using landfill gas (LFG), the composition can be fluctuated, even when the amount of LFG is not enough, NG needs to be used as supplementary fuel. Therefore, for the microgas turbine using LFG as fuel, the combustor needs to have good fuel adaptability. When the fuel composition fluctuates, it can still maintain good combustion characteristics. To analyze the influence of fuel composition change on combustion characteristics, two representative LFG and NG were selected which are from two different landfills for a long time operation. The composition and heat value are shown in Table 2.
To reduce the influence of fuel composition change on the operation characteristics of microgas turbine, the fuel flow rate is adjusted through the test of fuel composition to ensure that the outlet temperature of the combustor remains unchanged. When the fuel composition changes, the fuel flow will change to ensure the same outlet temperature, that is, the fuel flow will increase with the decrease of fuel heat value. On the premise that the geometric parameters of the fuel nozzle remain unchanged, the change of fuel flow rate will lead to the change of momentum and jet depth when the fuel enters the combustor, and then lead to the change of combustion characteristics.
The central classification technology is used in the combustor, the function of the pilot stage is to stabilize combustion, and the function of the main combustion stage is to reduce pollutant emissions. The pilot stage and the main combustion stage will form two different combustion zones, the pilot combustion zone and the main combustion zone, respectively, in the combustion zone, and the two combustion zones will also affect each other. The FD ratio between the pilot stage fuel and the main combustion stage fuel is one of the main control parameters affecting the combustion characteristics. To analyze the influence of the FD ratio, the combustion characteristics of the FD ratio in the range of 0.2-0.5 were tested under the premise of keeping the inlet and outlet parameters of the combustor unchanged.
Therefore, using the fuel inlet scheme of case7 and case8 in Table 1, using the three fuel components in Table 2, and the FD ratio in the range of 0.2-0.5 to study the generation characteristics of pollutants in the combustor. To ensure the reliability of the test, three repeated tests were conducted for each test point.

| Numerical simulation
To obtain the mixing characteristics of fuel and air, the temperature distribution characteristics and the pollutant generation characteristics inside the combustor when the main combustion stage nozzle is located at different locations and with different diameters, numerical simulation method is adopted to carry out numerical calculation of case1-case6 shown in Table 1. The fuel used is LFG1 as shown in Table 2 with the same FD ratio for all the cases. The inlet and outlet conditions of the combustor are the same under different working conditions. The established mesh model for numerical calculation is shown in Figure 4. Because the fuel orifice is not only small in diameter but also large in number, the area method and unstructured grid are used to divide the orifice. The basic unit of the grid is tetrahedral and hexahedral grid, and the positions of the fuel orifice and the main combustion zone are partially encrypted. The T A B L E 2 Experimental gas fuel. | 2645 grid independence of the DLE combustor model is also verified. Figure 5 shows the temperature distribution on the central axis of the combustor when the fuel flow ratio of the pilot stage to the main stage is 0.24. As can be seen from the figure, the calculation results of 2.86 million grids and 3.59 million grids have little difference, so it can be considered that 2.86 million grids have met the requirements of grid independence. Therefore, the final grid number of the combustor fluid domain is 2.86 million.

Contents
For numerical calculation, the realizable k-ε model was used for the turbulence model because the realizable k-ε model can be used to deal with the swirl problem, standard wall functions were used for near-wall surface treatment, the Finite-rate/Eddy Dissipation model was selected according to the structural characteristics of the central hierarchical combustion chamber to simulate component transport and chemical reaction process, and the SIMPLE algorithm was adopted for pressure and velocity coupling. The inlet boundary conditions of air and fuel are the mass inlet, and the outlet of the combustor is the pressure outlet boundary condition.

| Effect of fuel composition and distribution ratio
The effects of the FD ratio on combustion performance for three different fuels were tested. During the experiment, the inlet and outlet conditions of the combustor were kept unchanged at the design point. Therefore, for a specified fuel, the total fuel flow remains unchanged in the test, only changing the FD ratio between the pilot stage and the main stage. The FD ratio is defined as the fuel flow ratio of the pilot stage to the main stage. After the combustor reaches a stable state, the fuel flow of the main combustion stage is gradually reduced, while the fuel flow of the pilot stage is added. The fuel regulation is stopped at the investigated FD ratio. After stability, the composition of the combustor exit is recorded and the pollutant emission performance is calculated. The changes of NO x and CO emissions for the three different fuels are shown in Figure 6.
The pollutant emission is usually converted to 15% oxygen content of dry base gas, which is conducive to the comparison of the combustion pollutant components of different gas turbines. The calculation formula is as shown in Equation (1) where the unit of gaseous pollutants with 15% oxygen concentration on dry basis is ppmv (dry), and the oxygen concentration is the measured percentage. It can be seen from the test results that the fuel composition will have a significant impact on the formation of pollutants. With the decrease of fuel calorific value, the generation of NO x decreases and the generation of CO increases. The result is consistent with other studies. The influence of fuel composition on pollutant generation is mainly due to the fact that the fuel nozzle is not replaced in the test. When the calorific value of the used fuel is low, it is necessary to increase the fuel pressure to increase the fuel flow. This leads to an increase in the momentum of the fuel, which can improve the mixing uniformity of fuel and air. At the same time, LFG fuel contains N 2 and CO 2 , which is also F I G U R E 5 Grid independence verification.
F I G U R E 6 Comparison of NO x and CO emissions for different fuels. LFG, landfill gas; NG, natural gas. beneficial to reduce the fuel concentration in local areas in the air. The higher fuel concentration in the local area in the air will lead to local hot spots in the area, which is favorable to the generation of NO x .
From the change trend, with the increase of fuel flow at pilot stage, the NO x production of NG changed greatly, increasing from 20 to 30 ppmv, while the NO x change of the two LFGs with lower heat value was not obvious, and the NO x change range of LFG2 was 3.7-10.1 ppmv, LFG1 is 7.2-12.3 ppmv. This combustor shows good NO x emission characteristics. Different FDs have effect on the CO. The change trend of CO is the same for the three fuels, which decreases first, reaches the lowest value and then increases. In the studied fuel, the LFG2 has the highest CO emission from 49.3 to 25.8 ppmv. The FD ratio has different influences on the generation of NO x and CO, but a better distribution proportion can be found to achieve a lowest pollutant emission value. This is mainly due to the different heat values of the fuel and the different jet velocities when the fuel passes through the fuel nozzle at different distribution ratios, which leads to the different mixing characteristics of the fuel and air.

| Effect of main fuel hole position
From the influence of the FD ratio on pollutants emission, the FD ratio has an effect on the pollutant emission. So two certain FD methods (FD1 and FD2) have been selected for the study.
The test results of pollutant generation characteristics for different main fuel hole positions are shown in Table 3. It can be seen from the test results that the location of the main fuel nozzle will have an obvious impact on the formation of pollutants. Under the two FD methods, the pollutant generation of case8 is higher than case7. The difference for case7 and case8 is the location of the main combustion stage nozzle. For case7, the main combustion stage nozzle is located in the front of the swirler2, while it is situated in the middle of the swirler2 for case8. It can be preliminarily judged from the position of the main combustion stage nozzle, the case7 equivalent to lengthen the premixed channel compared with the case8. The longer premixed channel is beneficial to improve mixing uniformity. Better mixing uniformity is conducive to the reduction of pollutants. This conclusion is inferred from the experimental results. To further obtain the influence of fuel nozzle position on mixing uniformity and pollutant generation characteristics, further research is carried out by numerical simulation in the following.

| Verification of mathematical model
To verify the established mathematical calculation model and confirm the accuracy of the model, the numerical calculation results of case7 were compared with the tested results. Table 4 shows the comparison of the average exit temperature under different FD ratios. According to the data in the table, the error between the experimental results and the calculated results can be kept below 5%, and the temperature change trend is consistent, indicating that the two results are basically consistent. At the same time, according to the two curves of 2.86 million grids and 3.59 million grids in Figure 5, the temperature on the central axis of the combustor gradually decreases after the main combustion zone, and there is no sudden change in temperature, which is in line with the real combustion situation in the combustor. Figure 7 shows the NO x emission curves of the exit section of the combustor obtained by calculation and experiment. It can be seen the calculated results are generally lower than the experimental results, and the relative error is less than 15%. This is caused by several factors. On the one hand, the numerical simulation only calculates thermal NO x , but in actual combustion, fuel NO x and instantaneous NO x are produced due to the N 2 and CO 2 contained in the fuel, so the calculation results are lower. On the other hand, the NO x distribution is uneven for the combustor exit, while the calculation results are for the whole combustor exit average which can result in deviation. Therefore, the overall comparison results show that the NO x emission of the experiment is slightly higher than that of the calculation, but the overall change trend is consistent. The comparison between temperature variation and NO x generation shows that the calculation method is reasonable and reliable, and can be used to analyze combustion characteristics in the combustor. Figure 8 shows the fuel-air mixing characteristics of the six cases in Table 1 at a distance of 40 mm from the outlet of the swirler2. It can be seen from the calculation results that different fuel nozzle diameters and positions have a great influence on the mixing uniformity of fuel and air. From the influence of nozzle diameter on mixing uniformity, the mixing uniformity with a small nozzle diameter (1.5 mm for case1, case3, and case5) is significantly better than that with a large nozzle diameter (2 mm for case2, case4, and case6). At the same time, it can be seen from the comparison of case1, case3, and case5 that the position of the fuel nozzle has a certain influence on the fuel-air mixing uniformity, but the influence is not obvious. The definition of mixing uniformity is shown in Equation (2). 4 The mixing uniformity of the six cases was calculated at the distance of 40 mm from the outlet of the swirler2. The results are shown in Table 5. When the mixing uniformity is 1, it means no mixing, and when the mixing uniformity is 0, it means full mixing.

| Fuel-air mixing characteristics
where δ 2 is the variance, and f is the mean value. It can be seen from the calculation results that the uniformity of case1, case3, and case5 is one order of magnitude lower than that of the case2, case4, and case6, and the difference between the maximum and minimum fuel volume concentration values of case2, case4, and case6 is also significantly higher than that of case1, case3, and case5. This result corresponds to the qualitative calculation result as shown in Figure 8, indicating that the nozzle diameter is more important than the position of the fuel nozzle in the influence on the fuel-air mixing uniformity. This conclusion is related to the structural characteristics of the combustor studied. Due to the limitation of the space size of the combustor, the mixing distance between fuel and air is relatively short, and the change in the position of the fuel nozzle has no obvious effect on the mixing characteristics. This conclusion is especially applicable to microgas turbine combustor. In the design of microgas turbine combustor, more consideration should be given to the influence of the ratio of fuel to air jet momentum on the mixing characteristics.

| Characteristics of temperature field
The diameter of the fuel nozzle will affect the jet velocity and jet depth of the fuel. Under the premise of constant flow characteristics of the inlet air, the momentum ratio of the fuel and air will be affected, and then the mixing uniformity of the fuel and air will be affected. The influence of nozzle diameter can be clearly seen from the calculated results of the temperature field as shown in Figures 9-11. When the nozzle positions are the same, as in case1 and case2, it can be seen that the increase of nozzle diameter leads to the rise of temperature in the main combustion zone, and the high-temperature zone is mainly concentrated at the nozzle exit. Compared with nozzle diameter, the influence of fuel nozzle location on the temperature field is not obvious. The difference of case1, case3, and case5 (case2, case4, and case6) is that the positions of fuel nozzles 3 and 4 are different, and there is only a slight difference from the temperature field. The farther forward the fuel nozzle is, the longer the mixing distance between fuel and air is, which is conducive to the uniform mixing of fuel and air. As a result, the more uniform the temperature field is, the smaller the local high temperature is. Figure 10 shows the cross section of the temperature field at a distance of 40 mm from the swirler2 outlet, which is the same with the fuel-air mixing uniformity position. It can be seen from the temperature distribution of the cross section that the uniformity of fuel and air T A B L E 5 Fuel-air mixing uniformity for six cases. mixing is corresponding to the temperature distribution. The more uniform the mixture is, the lower the temperature difference in the radial distribution of temperature and the fewer the high-temperature points. From the comparison of the six cases, case1 has the most uniform temperature distribution, while case6 has the highest temperature points. Figure 11 shows the radial temperature field section at the exit of the combustor. Compared with Figure 10, the temperature field distribution at the exit of the combustor is more uniform due to mixing. As can be seen from the temperature distribution cloud map, the outlet temperature is in line with the low temperature at both ends and high temperature in the middle.  Hot spot index outlet temperature distribution factor (OTDF) is a parameter used to measure the temperature distribution at the outlet. The definition is as shown in Equation (3).

Case
where T 4local is the maximum temperature of gas in the exit temperature field, T 4av is the average outlet temperature, T 3av is the inlet average temperature. Equation (3) is used to calculate the outlet temperature fields of the six cases, and the OTDF for each case has been shown in Figure 11. For industrial gas turbine, the OTDF should be about 0.2, and for low-pressure ratio burning NG, the OTDF should be about 0.15. According to the comparison of OTDF, it can be seen that the nozzle diameter has a more obvious influence on outlet temperature distribution than the nozzle position. The OTDF of case1, case3, and case5 meets the requirements, which is beneficial to improving the turbine guide's life. The OTDF of case4 and case6 is too high, and the service life of turbine guide will be affected under the premise of the same average outlet temperature of the combustor.
It can be seen from the comparison of nozzle diameter and location on the distribution of temperature field inside the combustor and outlet hot spot index that small fuel nozzle diameter and increased fuel-air mixing distance (i.e., the fuel nozzle is closer to the front) are conducive to the full mixing of fuel and air, and are beneficial to the elimination of hot spots at the combustor and outlet of the combustor

| Emission characteristics
The main pollutants of microgas turbine are NO x and CO. At present, most international gas turbine manufacturers can accept NO x < 25 ppmv and CO < 50 ppmv as pollutant emission standards. In the microgas turbine field, according to publicly reported information, Capstone's microgas turbine NO x emissions can be controlled below 10 ppmv. The established mathematical model is used to numerically calculate the pollutant emission characteristics of the combustor at the design point for the six cases. The NO x generation characteristics in the combustor are shown in Figure 12. In the calculation of the combustor, the generation of NO x adopts the thermodynamic NO x generation mechanism, so the distribution characteristics of NO x in the combustor are consistent with the temperature distribution characteristics. The more uniform the fuel-air mixture is, the more uniform the temperature distribution in the combustor is, and the local high temperature is reduced, which is conducive to reducing the generation of NO x . Table 6 shows the calculated results of NO x and CO emission characteristics at the combustor outlet for the six cases, which are the values converted to dry base 15% O 2 . It can be seen from the calculation results that the NO x emission of case1 is the lowest and has reached the single digit, while the NO x emission of case6 is more than 30 ppmv at the highest. The NO x emission of case1, case3, and case5 all met the requirement of less than 15 ppmv, but the CO emission of case1 was close to the maximum value required. The higher NO x emission of case2, case4, and case6 is due to the increase of fuel nozzle diameter, which leads to the decrease of momentum and velocity of fuel jet and the deterioration of fuel-air mixing uniformity. Mixing uniformity decline will lead to the existence of local high, and the formation of NO x is index with temperature, so lead to this several kinds of cases of NO x emission is higher, and also proved that the fuel-air mixing uniformity has a decisive influence on NO x emission. Factors affecting the mixing uniformity will also affect the formation of NO x . The F I G U R E 12 NO x generation characteristics in the combustor. LIU ET AL.
| 2651 mixing uniformity also has a certain influence on the generation of CO, but it is not obvious.

| CONCLUSION
In this paper, a microgas turbine low-emission combustor has been studied using the experimental study and numerical calculation. The combustor is designed according to the center-staged premixed lean combustion technology. The effects of fuel composition, main combustion nozzle position, and diameter on the combustion characteristic have been studied, the main conclusions are as follows: 1. The change of the FD ratio between pilot stage and main combustion stage has obvious influence on combustion characteristics of the combustor. Optimal FD ratios exist for different fuels to achieve the best combustion performance. 2. The more uniform the fuel-air mixture, the more uniform the temperature distribution in the combustion chamber, the less local high temperature, and the less NO x is generated. 3. In the microgas turbine combustor with limited space size, the influence of fuel nozzle diameter on mixing uniformity is greater than that of fuel nozzle position. 4. The pollutant emission characteristics of the designed combustor have reached the emission level of advanced low-emission microgas turbine combustor, especially the NO x emission can be controlled within 10 ppm.