Investigations of the working processes in a gas turbine low-emission combustor operating on the synthesis gas, in which the principle of RQL (Rich-Burn, Quick-Mix, and Lean-Burn) combustion technology is realized, have been performed. Selected concept of a gas turbine combustor can provide higher performance and lower emission of nitrogen oxides and demonstrates satisfactory major key parameters. Obtained results and recommendations can be used for the gas turbine combustor operation modes modeling, geometry optimization, and prospective power generation units design and engineering.
Gas turbine engines (GTE) are designed primarily to be fueled with natural gas, which mainly consists of methane. Recent increases in natural gas prices and concerns about its availability, as well as significant advances in synthesis gas producing and cleanup, have made opportunities for the synthesis gas as a primary GTE fuel [
Plasma-assisted gasification can be efficiently used to convert carbon-containing materials to synthesis gas that can be used to generate power [
Rather serious increase of toxic substances emission is possible when using synthetic fuels in GTE combustors in comparison to engines which operate on traditional fuel. Thus, there is necessity to provide design solutions for hazardous elements emission mitigation.
Almost all the known methods of
RQL combustion scheme includes initial combustion of rich air-fuel mixture and quick mixing and burning of lean air-fuel mixture [
Selection of rational correlations of geometry parameters of this two-zone combustor which operates on synthesis gas and defining the most effective ways of oxidant (air) supply to the zone of quick mixing is a rather important practical issue. Its solution through experimental means requires high expenses of material and human resources. So using the computational models will provide sharp reduction of time and resources for designing a RQL-combustor working on the synthesis gas.
An offered CFD mathematical model of a combustor working on the synthesis gas is based on the following equations [
For the modeling of synthesis gas combustion processes the Eddy Dissipation Concept (EDC) model is used. It is an expanded model of turbulent eddy dissipation which includes low-level modeling of chemical processes of fuel oxidation under the conditions of flame with turbulent flow excitations [
The selection of a hydrodynamic turbulence model significantly influences the estimated properties of a mathematical model of a low-emission gas turbine combustor. The RNG modification of the
Combustion of synthesis gas requires attention to key operation processes factors, including emission formation. The range of compositions found in synthesis gas varies more substantially than similar properties of natural gas. Synthesis gas behaves differently in many ways including specific heat, diffusivity, flammability limits, and flame speed [
The study of modern gas turbine burning system operation shows that the diffusion-type burners show more stable results and have a more flexibility options in comparison with combustors even with partial preliminary mixing of different fuels with an oxidant [
If fuel has such components as СО, Н2, and СН4, then in terms of numerical modeling of combustion processes there should be grounded selection of the most proper kinetic schemes for defining the main flame parameters at reasonable computational efforts.
The detailed expanded chemical mechanisms which describe combustion of hydrocarbon fuel are developed properly for combustion of СО/Н2 mixtures. There are also additional simplified or so-called global mechanisms for CFD modeling which are mainly used for calculations of oxidation of hydrocarbon fuels and synthesis gases. For example, Bohni and others developed a six-stage kinetic mechanism by means of systematic decrease of the detailed kinetic mechanism and Andrea de Pascale and others offered a two-stage global mechanism for СО/Н2 mixtures [
Two mechanisms of chemical kinetics were used in this investigation to provide the detailed analysis of operating processes: (
Reactions of reduced mechanism.
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Reactions of Yetter mechanism.
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The main chemical components of reduced mechanism are CH4; O2; CO2; CO; H2O; H; OH; O; H2; HO2; CH3; HCO; CH3O; CH2O; H2O2; N2.
The main chemical components of Yetter mechanism are H2; H; O2; O; OH; H2O; HO2; H2O2; CO; CO2; HCO; N2; AR.
To verify a mathematical model of a combustor, which operates on synthesis gas, an ejection type afterburner of synthesis gas [
For designing an afterburner the following initial data were accepted: synthesis gas consumption 25 g/s; temperature ≤ 700 K; air flow via an afterburner 400 g/s; air temperature 350 K. As the pressure of the obtained synthesis gas is close to the atmospheric one, an original ejection system is developed for providing synthesis gas supply to the afterburner.
The data acquisition scheme is shown in Figure
Scheme of the measurements.
Air from the compressor 3 is fed into the afterburner full-size model 1 through the Coriolis flowmeter 2 with capacity of 0 to 0.5 kg/s. Air from a compressor is introduced through the inlet duct and enters the receiver where it divides into five streams. The first stream enters an injector through three channels. Fuel gas enters through a pipe, injects through the injector, partially mixes with air in a mixer, and enters a flame tube. The second air flow enters a swirler, as a swirling flow enters the flame tube, forming a vortex type combustion zone. The third airflow enters the flame tube through a number of holes. The fourth airflow enters a gap for combustor wall cooling and after passing through an external vortex generator mixes with the combustion products to reach the necessary exit afterburner temperature. The fifth stream enters channel, intended for air feeding into primary combustion zone at the region of the afterburner axis [
The operating processes in the afterburner were studied at atmospheric pressure (101,325 Pa). The mass air consumption varied from 0.05 to 0.4 kg/s. Propane-butane is used as a pilot gas.
The obtained data were verified by comparison of the main afterburner parameters (consumption of the ejected air and the amount of toxic components on the exhaust) obtained by computational (using the reduced chemical mechanism) and experimental methods.
The comparisons of the calculated and experimental values of the afterburner parameters in the modes of cool (without fuel feeding and combustion) and hot (with fuel feeding and combustion) blow are shown in Figures
Experimental and calculated dependence of air consumption via the ejector on air consumption via the combustor: (a) cool blow mode; (b) hot blow mode.
Experimental and calculated dependence of nitrogen oxides emission on air consumption via the afterburner.
The analysis of the obtained results shows the acceptable level of coincidences of experimental and calculated values which proves the adequacy of the offered mathematical and physical model and possibility of their use for calculating the gas turbine combustors which operate on synthesis gas.
To study the influence of the synthesis gas content on the gas turbine combustor parameters using a computer-aided design system, a parameter digital model of
Geometry model of the combustor: (a) swirler; (b) combustion liner; (c) longitudinal section of the combustor.
The calculations were performed for three variants of fuel gas content. Variant 1 provided methane supply to the combustor, the results of its calculations were compared to the results of experimental studies of a traditional diffusion-type combustor and used for verification of a kinetic scheme of hydrocarbons oxidation. Variants 2 and 3 suppose supply of synthesis gas of various content and various calorific values to the combustor (Table
Gas turbine combustor calculation initial data.
Variant 1 | Variant 2 | Variant 3 | |
---|---|---|---|
Lower calorific value, kJ/kg | 48,800 | 21,791 | 12,448 |
Gas content, % (vol.) | |||
СO2 | 0 | 12.59 | 2.66 |
Н2O | 0 | 0 | 4.88 |
СO | 0 | 15.09 | 59.39 |
Н2 | 0 | 50.63 | 31.76 |
СН4 | 100 | 19.08 | 0 |
N2 | 0 | 2.61 | 1.31 |
O2 | 0 | 0 | 0 |
Stoichiometric air amount, kg/kg | 16.72 | 6.70 | 3.21 |
Air flow via combustor, kg/s | 6.91 | 6.91 | 6.91 |
Air temperature on combustor inlet, K | 650.0 | 650.0 | 650.0 |
Air pressure on combustor inlet, MPa | 11.91 | 11.91 | 11.91 |
Fuel flow via combustor, kg/s | 0.097 | 0.217 | 0.375 |
Fuel temperature, K | 303 | 303 | 303 |
Total air excess coefficient | 4.26 | 4.74 | 5.74 |
Average fuel velocity in outflow holes, m/s | 200 | 218 | 200 |
To provide the same heat power of a burning device which corresponds to the nominal mode of a GTE, the synthesis gas flows were increased in comparison to the methane flow proportionally to reduction of the lower calorific value. The areas of the flow sections of a burning device were increased which provided the required velocity of synthesis gas flow from the fuel holes without flame blow out. At this stage, apart from the change of the flow sections of a burning device no design upgrade of the combustor was made.
As a result of calculations with the use of the EDC combustion model and a simplified reduced kinetic mechanism [
Figure
Distribution of the velocity in the longitudinal section of the combustor, m/s: (a) variant 1; (b) variant 2; (c) variant 3.
For variant 3 the flow velocity in the area of the output section of the swirler reaches 120 m/s when for the combustor operating on methane it is on the rational level which is about 80 m/s. This feature shows the necessity of changing open flow areas in the swirler to provide the calculated velocities in the combustion zone during combustor modification.
It is possible to analyze the details of the changing character of the working medium velocities using Figure
Distribution of the axial velocity component in the longitudinal section, m/s: (a) variant 1; (b) variant 2; (c) variant 3.
Figures
Change of average mass temperature (
Temperature contours in the longitudinal section of the combustor, K: (a) variant 1; (b) variant 2; (c) variant 3.
There is no decrease of the maximum temperature in the combustor volume in connection with decrease of the fuel gas calorific value as the diffusion burning principle is implemented in the considered gas turbine combustor.
Distribution of carbon oxide CO in the combustion liner is shown in Figure
Distribution of CO mass fractions in the longitudinal section of the combustor: (a) variant 1; (b) variant 2; (c) variant 3.
Nevertheless, considering the obtained data, it is possible to stipulate that the growth of the combustion volume is positive for the level of CO burning off and the combustion efficiency.
Figure
Distribution of NO volume fractions in the longitudinal sections of the combustor.
It should be mentioned that air redistribution along the combustion liner at further combustor modification will allow avoiding the local values of the air excess coefficient at which nitrogen oxides emission is maximum and reducing the volume of the zones of possible nitrogen oxides formation.
The results of the performed three-dimension CFD calculations are shown in Table
Results of combustor calculations.
Variant 1 | Variant 2 | Variant 3 | |
---|---|---|---|
Low calorific value, kJ/kg | 48,800 | 21,791 | 12,448 |
Temperature on combustor outlet, K | 1,224 | 1,167 | 1,199 |
Velocity on combustor outlet, m/s | 74.18 | 74.56 | 81.35 |
Mole fraction of СН4 | 0.00042 | 0.00022 | 0 |
Mole fraction of O2 | 0.1544 | 0.1604 | 0.1631 |
Mole fraction of СO2 | 0.0235 | 0.0261 | 0.0508 |
Mole fraction of Н2O | 0.0501 | 0.051 | 0.0304 |
Content of N2O, ppm | 0.363 | 0.260 | 0.0393 |
Content of NO, ppm | 65.7 | 94.0 | 200.3 |
Total pressure losses, % | 4.51 | 4.31 | 4.03 |
At calculating a gas turbine unit cycle it was determined that the average temperature on the turbine outlet of the GTE with the power of 2.5 MW operating on natural gas is 740 K (that corresponds to the experimental data 733 K [
Unlike natural gas which has been used over many years and for which much combustion data is collected, synthesis gas fuel mixtures and their kinetics have not been widely investigated [
Therefore the investigation of the influence of kinetic schemes on parameter distribution in the volume of the gas turbine combustor operating on synthesis gas with minimum low calorific value (variant 3) was performed. Two detailed mechanisms of chemical kinetics were used for comparative calculations: mechanism 1 simplified 35-reaction reduced mechanism [
The results of modeling are shown in Table
Calculation results for combustor using various chemical kinetics mechanisms.
Mechanism 1 | Mechanism 2 | |
---|---|---|
Low calorific value, kJ/kg | 12,448 | 12,448 |
Temperature on combustor outlet, K | 1,199 | 1,150 |
Velocity on combustor outlet, m/s | 81.35 | 79.98 |
Mole fraction of СН4 | 0 | 0 |
Mole fraction of O2 | 0.1631 | 0.1665 |
Mole fraction of СO2 | 0.0508 | 0.0457 |
Mole fraction of Н2O | 0.0304 | 0.0276 |
Distribution of the velocities in the longitudinal section of the combustor, m/s: (a) mechanism 1; (b) mechanism 2.
Temperature field in the longitudinal section of the combustor, K: (a) mechanism 1; (b) mechanism 2.
The results of the calculations show that there is no significant difference in contours of the velocities at using both mechanisms (Figure
Thus, we can recommend both considered kinetic mechanisms for defining the main parameters of the combustor operating on synthesis gas.
It should be mentioned that emission parameters of the series production combustor of the GTE with the power of 2.5 MW [
Numerical study of the 2.5 MW gas turbine combustor showed the necessity to provide significant changes to the scheme of the combustor working process organization in order to increase stability and efficiency of its operation especially when operating on low calorific synthesis gas. Simple replacement of natural gas (methane) with synthesis gas for a series produced gas turbine combustor leads to increase of toxic components emission, particularly nitrogen oxides.
To improve the series produced UGT2500 combustor [
Initially, the following changes were made in the series produced combustor design: The amount of air supplied to the primary zone of combustion was reduced due to shifting from a barrier cooling system to a convective one and formation of an RQL scheme of synthetic fuel combustion. The air swirler was moved beyond the combustion liner and the form of the primary combustion zone was changed to the conic one. Special plugs (shells) were added to the primary air holes to increase the depth of entering of air flows and improve the quality of components mixing on the output.
To reduce the possibility of formation of local areas in the combustor with the stoichiometric value of the air excess coefficient and reduction of air suctions to the primary zone (Rich-Burn) it is offered to pinch the section of the combustion liner before the holes of primary air supply.
For modified designs 1 and 2 the diameter of the corresponding shell ring of the combustion liner was reduced by 10% (Figure
Structure designs of combustors: (a) modified design 1; (b) modified design 2.
The graph dependence shows the minimum air suction to the side of the swirler in design 2 (Figure
Lines of the air tracks of the quick mixing (Quick-Mix) zone: (a) modified design 1; (b) modified design 2.
Temperature field in the longitudinal section of the combustor, K: (a) modified design 1; (b) modified design 2.
Despite this, in the output section of the combustion liner there is no significant increase of carbon oxide emission (Figure
Distribution of CO mass fractions in the longitudinal sections of the combustor: (a) modified design 1; (b) modified design 2.
The obvious effect of lack of primary air suction to the rich air-fuel mixture combustion zone is sharp decrease of the calculated concentrations of nitrogen oxides in the outlet section to 29 ppm for modified design 2 (Figure
Results of modeling of modified combustor designs operating on synthesis gas.
Design 1 | Design 2 | |
---|---|---|
Temperature on the combustor outlet, K | 1,192 | 1,190 |
Velocity on combustor outlet, m/s | 82.02 | 75.00 |
Mole fraction of СН4 | 0.000001 | 0.00001 |
Mole fraction of O2 | 0.1582 | 0.1577 |
Mole fraction of СO2 | 0.0277 | 0.0276 |
Mole fraction of Н2O | 0.0537 | 0.0544 |
Content of N2O, ppm | 0.173 | 0.225 |
Content of NO, ppm | 109.1 | 28.9 |
Distribution of NO mass fractions in the longitudinal sections of the combustor: (a) modified design 1; (b) modified design 2.
Figure
Emission of nitrogen oxides NO in the combustor operating on synthesis gas.
As at operation of the gas turbine combustor on synthetic fuel (to provide its constant heat capacity) the synthesis gas flow grows to a high rate in comparison to the natural gas flow, there is practical interest for the interval of the values of the synthesis gas calorific value which allows implementing its stable and effective burning off without additional supply of natural gas.
The given variant calculations supposed supply of synthesis gas of various content and calorific value obtained by plasma treatment of low grade coal [
The data of three-dimension CFD calculations (Table
Calculation results of the combustor which operates on various-content synthesis gas.
Case |
Case |
Case |
|
---|---|---|---|
Low calorific value, kJ/kg | 33,089 | 21,791 | 12,448 |
Temperature on the combustor outlet, K | 1,182 | 1,191 | 1,300 |
NO content on the outlet, ppm | 31.4 | 28.9 | 116.4 |
Total pressure losses, % | 4.72 | 4.71 | 3.87 |
Combustor characteristics: (a) change of the nitrogen oxides emission; (b) change of average temperature along the combustor which operates on various content synthesis gas.
Increase of the values of the maximum temperature in the combustion zone (Figure
Theoretical investigations of the gas turbine combustor operating on the synthesis gas are performed. Numerical study showed the necessity to provide significant changes to the scheme of the combustor working process organization in order to increase stability and efficiency of its operation especially when operating on low calorific synthesis gas. To improve the characteristics of the GTE combustor operating on synthesis gas use of promising RQL combustion technology is proposed. The three-dimensional mathematical model and different kinetic mechanisms of a synthesis gas combustion processing have been used for enhancement of the 2.5 MW gas turbine ecological parameters. The application of the practical recommendations can reduce the nitrogen oxides emission up to 29 ppm in the exit section of the gas turbine combustors working on synthesis gas which are developed.
The authors have no conflicts of interest related to the conduct and reporting of this research.