Emission level calculation for water injection in gas turbines.

Popescu, Jeni ; Vilag, Valeriu ; Barbu, Ene 等

1. INTRODUCTION

The technical-scientific development the last decades led to modernization of terrestrial and aero-transportation constituting sources of noxious emissions raising acute ecological problems and forcing the research of fast and efficient limitative solutions.

The gas turbine chemical pollution consists in production of complete combustion resultant (C[O.sub.2], water), incomplete combustion resultants (smoke, CO, hydrocarbons), nitrogen oxides (N[O.sub.2], [N.sub.2]O and particularly NO) and ash particles. Some important achievements have been reached concerning the reduction of the visible smoke and the total emission level through advanced injection techniques, therefore the problem of the emissions produced by gas turbines can be restricted to reducing the N[O.sub.x] level, mainly caused by the spectacular increase in thermodynamic parameters, such as compression ratio, admission temperature in the combustion chamber and power turbine (Carlanescu, 1998).

The data presented in the paper are the result of the researches made at INCDT COMOTI Bucharest in the framework of the contracts 21-056/2007 and 22-108/2008 ("Partnership in prior fields" Programme)

2. INFLUENCES AND REDUCTION METHODS

The problem of emission level reduction is particularly complex due to the fact that the efficient solutions for decreasing some pollutants prove to be stimulant for producing others. The concentration levels of numerous pollutant agents coming from gas turbines are directly connected to temperature distribution, concentration and residence time in the combustion chamber, distributions modifying form a chamber to another and from a working regime to another. The incomplete combustion products having maximum values on low regimes (low pressures, temperatures, poor mixture). In these conditions, the N[O.sub.x] production is also low, to increase fast with the regime, caused by the fact that improving the combustion conditions leads to nitrogen oxides production, proportionally to the temperature increase (Khartchenko, 1997).

The ratio and atomization of the fuel-air mixture influences directly the combustion efficiency, therefore the quantity of the products. The N[O.sub.x] emission is not affected by the atomization, but tends to increase with the fuel quantity in the mixture. The compression ratio influences the air pressure at the combustion chamber inlet, its increase leading to the decrease of incomplete combustion products and the increase in N[O.sub.x] production. From the point of view of the velocity, a quick passing of the air, respectively of the burning gases, through the combustion chamber involves an increase in the incomplete combustion products level, while reducing the N[O.sub.x] generation which needs high temperatures and pressures, as well as relatively long time.

Improving combustion conditions, by using pre-mixing rooms and low pressure water injection, along with additives based on manganese, barium and calcium, and depending on the fuel type (Giampaolo, 2006), pulls in step the decrease of smoke and soot emission. Another solution for theoretical high efficiency and emission control consists in variable geometry combustion chambers, characterized by using or closing a set of additional orifices, based on the working conditions. This solution for optimizing the ratio and gases velocity constitutes the main technological conception for future gas turbines, particularly turbojets. The most utilized solutions for reducing the emission level in gas turbines, by internal control, are catalytic combustion chambers and water or steam injection.

The water injection is a method of cooling the combustion chambers by adding water or water-methanol to the fuel-air mixture. The initial water injection cools significantly the fuel-air mixture increasing the density and the mass of the fluid, therefore the thrust for turbojets and the power for turboprops, with the subsequent effect, during combustion, of absorbing the energy and reducing the temperature peak and the N[O.sub.x] formation (Carlanescu, 1998).

The importance of the data in the present paper comes from establishing the basics in applying a method of N[O.sub.x] reduction by completely modifying the fuel system of an existing aviation gas turbine, as part of a complex project dedicated to decrease the emission level for gas turbines by involving both a water injection system and a supplementary firing system.

3. THEORETICAL CALCULATION METHOD

The combustion analysis and the theoretical calculation of emissions were made with the help of the CEA program developed by NASA Lewis Research Centre. The program allows obtaining the chemical equilibrium for thermodynamic states imposed by the user through defining the parameters, completed with included data bases for substances composition and common thermodynamic data (McBride, 1996).

A theoretical analysis was made for kerosene and methane combustion, with seven different injected water concentrations, in a known combustion chamber of an aviation turbo-shaft, in the conditions of pressure and temperature parameters recommended for the nominal regime and experimental for the idle regime of the gas turbine. The combustion reactions of kerosene and methane, with water injection, are given by equations (1) and (2), where the coefficient "a" of the water was considered as a function of the fuel coefficient.

[C.sub.12][H.sub.23] + 17.75([O.sub.2]+3.76[N.sub.2]) + a[H.sub.2]O [right arrow] 11.5[H.sub.2]O + C[O.sub.2] + 66.74[N.sub.2] (1)

C[H.sub.4] + 2([O.sub.2]+3.76[N.sub.2]) + a[H.sub.2]O [right arrow] 2[H.sub.2]O + C[O.sub.2] + 7.52[N.sub.2] (2)

The maximum temperature of the original gas turbine, of 1063 [K], was tracked in order to establish the necessary air/oxidant excess for the two regimes, along with the concentrations of the combustion resultants, particularly N[O.sub.x]. Knowing the recommended kerosene mass flow rate, the theoretical air quantity necessary for the combustion process and the air/oxidant excess, the air/oxidant mass flow rate and the fuel mass flow rate can be calculated for each case, in order to be further used as input data in numerical simulations of the combustion process.

4. NUMERICAL APPROACH

For the most relevant reduction of emissions, the methane combustion at nominal working regime of the gas turbine, another method allowing the calculation of maximum temperatures and N[O.sub.x] level is applied, consisting in simulating the combustion process in CFD environment.

The combustion chamber of the aviation gas turbine, along with the injection system, initially designed for kerosene, were geometrically adjusted, the annular combustion chamber, well known, was next divided in eight identical slices, corresponding to the injectors number, for reducing the necessary time and calculating resources. The meshing of the used geometry was made in three variants, based on fineness, the evolution being required by the necessity of obtaining most relevant results.

The most important numerical approaches are characterized by the definition of the fluid material of the domain, as a reactive mixture of perfect gases, where the reactions are controlled by the methane-air combustion, with the tracking of the NO level from the CFD code internal library (Ansys, 2007). The pressure and temperature conditions, as well as the mass flow rates and the mass fractions of the reactants are dictated by the known regimes of the gas turbine.

The results are presented in the following figures as temperature plots on a radial plan defined in the centre of the domain, Fig. 1 and 3, and on the outlet of the combustion chamber, Fig. 2 and 4, for the methane combustion at nominal regime without water injection, respectively with water injection. For the second approach, the purpose was to restore the maximum temperature of the gases on the outlet of the combustion chamber. The figures show the shape of the flame and the maximum temperature inside the combustion chamber. The post-processing of the results also includes the calculation of the thermodynamic parameters for regions of interest, particularly the outlet of the combustion chamber.

Table 1 presents the comparative results of the theoretical and numerical calculation from the point of view of the obtained maximum temperature and N[O.sub.x] emission level.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

5. CONCLUSIONS

Comparing the theoretical and numerical results of the methane combustion, with and without water injection, the temperature is obtained at nearly the same value, while the N[O.sub.x] emission level has different values, however on the same order of magnitude. This is caused by several hypotheses used for the two methods. The theoretical calculation involves NO, NO2 and NO3 emissions, while the numerical case only calculates the NO. The injection area in the combustion chamber is not defined, both methods considering only the mass fractions of the reactants, slightly different following a recalculation of the air excess. Another important factor is that, even for stable processes, the experiments and measurements made on international level proved an unstable NO concentration.

The paper establishes two methods for calculating the N[O.sub.x] emission level from gas turbines, a theoretical one by using the CEA NASA program and a numerical one by using a commercial CFD code, with comparable results. Future work will consist in defining a water injection system suitable for the studied gas turbine, testing the modified gas turbine on experimentation bench and comparing the experimental results with the already obtained ones.

6. REFERENCES

Carlanescu, C. (1998). Turbomotoare. Fenomenologia Producerii si Controlul Noxelor (Gas Turbines. Phenomonology of Production and Emission Control), Ed. Academiei Tehnice Militare, Bucharest, Romania

Khartchenko, N.V. (1997). Advanced Energy Systems, First Edition, Taylor & Francis, ISBN 978-1560326113, Bristol, UK

Giampaolo, T. (2006). Gas Turbine Handbook: Principles and Practices, Third Edition, The Fairmont Press, Inc., ISBN 0-88173-515-9, London, UK

McBride, B.J. & Gordon, S. (1992). NASA RP-1271, Computer Program for Calculating and Fitting Thermodynamic Functions, NASA Lewis Research Center, NSN 7540-01280-5500, Cleveland, Ohio, USA

*** (2007) Release 11.0 Documentation for Ansys, SAS, IP

Tab. 1. Comparative results for methane combustion

 Without water injection With water injection

Results Theoretic Numeric Theoretic Numeric

Temp [K] 1063 1069 1063 1059
N[O.sub.x] [ppm] 53.38 29.03 25 11.02