Aero-derivative gas turbines fuelled by biogas.

Popescu, Jeni ; Vilag, Valeriu ; Petcu, Romulus 等

1. INTRODUCTION

Environmental laws adopted at Brussels require a stabilized level of waste production until 2012. Moreover, the member states confronts to new limitations in the field of waste management. Studies are made and solutions are demanded in many countries. The use of waste materials is not only excellent suitability for alternative fuel production--biogas, it also creates some additional benefits by reducing large polluting biomass quantities. The biogas is produced by means of anaerobic digestion its main components being C[H.sub.4] and C[O.sub.2] in different proportions (Nikolic, 2005).

2. BIOGAS COMBUSTION IN GAS TURBINES

2.1 Background

The use of biogas in gas turbines has gained interest in recent years because of its CO2 neutral aspect. However, only two ground applications, cogenerative plants, are known in this direction: first developed in 1992 in U.K. by SITA Packington, using a Rolls-Royce TM 501-KB5, and the second one in Sweden, by TURBEC, using a T100 micro gas turbine. Both gas turbines were primarily designed for gaseous fuels and industrial use.

2.2 Theoretical considerations on combustion process

Considering known the chemical composition of a system, the calculation of its thermodynamic properties are permitted, proprieties that can be used in a large variety of applications in chemistry and chemical engineering. The combustion process was analyzed through the Gordon program (Gordon, 2002) allowing the obtaining of the chemical equilibrium of the composition for imposed thermodynamic states by defining the parameters. The considered gas turbine is an aviation turbo-shaft of 882 KW, with known performances and design, originally equipping a transport helicopter.

[FIGURE 1 OMITTED]

Knowing the admission temperature in the compressor the compression ratio for a considered turbo-shaft, of 6.6, and the adiabatic coefficient for air, 1.4, the admission temperature in the combustion chamber can be calculated as input data for the calculus program.

It is of interest for us to compare the combustion process parameters for the classic jet fuel, kerosene, and the bio-gas.

The maximum temperature for kerosene is calculated from the general combustion reaction, taking into consideration an air excess of 1--6, is:

[C.sub.12][H.sub.23] + a x ([O.sub.2] + 3.76[N.sub.2]) [right arrow] b x [H.sub.2]O + c x C[O.sub.2] + d x [N.sub.2] + e x [O.sub.2] (1)

The bio-gas composition is 50% methane and 50% carbon dioxide. The general combustion reaction is applied in the same manner as for kerosene and helps determining the variation of the maximum temperature (Fig.1).

(C[H.sub.4] + C[O.sub.2]) + a x ([O.sub.2] + 3.76 x [N.sub.2]) [right arrow] b x [H.sub.2]O + c x C[O.sub.2] + d x [N.sub.2] + e x [O.sub.2] (2)

From the obtained diagrams, the air excess results for the maximum temperature reached in the gas turbine, 1063 K. Knowing (KLIMOV CORPORATION, 1973) the fuel mass flow rate, the quantity of air necessary for the combustion process and the air excess, we can calculate the air mass flow rate and the fuel mass flow rate (Pimsner, 1964).

2.3 Numerical applications for biogas combustion

Computational Fluid Dynamics (CFD) is a computer-based tool for simulating the behaviour of systems involving fluid flow, heat transfer and other related physical processes. It works by solving the equations of a fluid flow over a region of interest, with specified conditions on the boundary of that region.

Different calculus techniques for the thermodynamic phenomenon inside the combustion chamber of the gas turbines were mostly based on experiments (Carlanescu, 1997). The high cost of experimentation determined its reduction by using virtual experiments with the help of numerical simulations.

The geometry of the combustion chamber was considered well known. The fuel to be burned with the air inside this chamber is a biogas in equal volumetric proportions of methane and carbon dioxide. The other contained species are neglected, considered as impurities. The combustion chamber geometry was modelled in CAD software and processed in a CFD environment.

The first numerical study consists in the determination of the geometry of the injector such as to obtain a good injection jet. The injector is considered as in Fig. 2, the simulation consisting in the variation of x, but mainly the angle a.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

For the numerical simulation some hypotheses have been proposed in order to reduce the calculation domain and, consequently, the necessary time to obtain the solution. Due to the fact that the original combustion chamber has eight identical injectors positioned in the same plane at the same radius from the longitudinal axis of the gas turbine, the main simplifying hypothesis is that the calculation domain can be reduced to a single sector from the eight identical ones formed, Fig. 3. This hypothesis implies that the air mass flow rate is equally divided among the eight "pies" and the fuel mass flow rate is equally divided among the eight injectors.

The second hypothesis is that the walls of the combustion chamber adiabatically isolate the internal flow, meaning that are no loses of heat produced during the combustion.

The third important hypothesis, which reduces the number of equations to be solved, is that the thermal conductivity of the wall is neglected, their only purpose being to direct the flow. This is close to reality since our purpose is to study steady states only.

The boundary conditions used for the simulations have been calculated from theory or have been taken from the given data from the constructor of the engine. The main goal of the CFD application is to define the injector geometrical configuration to transform the combustor chamber for bio gas so that the new gas turbine engine functions in safety conditions.

The air pumped into the combustion chamber is formed by oxygen [O.sub.2] and nitrogen [N.sub.2] in 1:3.76 molar ratios. The mass flow is 6.75 kg/s = 8 x 0.84375 kg/s and the working pressure is 6.4 bars. The mass flows of the fuels were determined from thermodynamics after selecting the working regime from the comparison shown before between the original kerosene fuel and the biogas.

2.4 Numerical results

Considering the previously described hypotheses, the injection angle and the temperatures in the combustion chamber were determined. From the temperature and its variation with the air excess for biogas it was determined the necessary biogas mass flow to be injected. The value was used to define the geometry of the injector, length x and angle a. The results for different angles are presented in Fig. 4.

[FIGURE 5 OMITTED]

The most important numerical result is the temperature in of the combustor chamber and at the exit of the combustion chamber, in front of the turbine inlet, and it was used like a constriction for configuration and dimensioning of the burner. Actually, by its help and by the given mass fuel flux it was verified that a certain working regime was obtained. The obtained average temperatures at the exit of the chamber are close enough to the ones given by the constructors.

3. CONCLUSIONS

The bio-gas was analyzed from the combustion point of view compared to kerosene, for an aviation gas turbine and diagrams were realized tracking the variation of the maximum temperature with the air excess.

CFD simulations are used for design purposes in many activity fields from the engineering domain. For the current work CFD was used for redesign the combustion chamber of an aviation gas-turbine in order to be able to work on bio-gas instead of the original fuel, kerosene. First it was studied the geometry of the injector by varying two of its parameters, second the necessary injection velocity and third the combustion itself for the desired working regime. All the simulations were conducted with respect to reliability and safety since the gas-turbine shall be use for industrial plants.

4. REFERENCES

Carlanescu, C. (1997), Turbomotoare de aviafie. Aplicafii industrial (Aviation Gas-Turbines. Industrial Applications), Editura Didactica si Pedagogica, Bucuresti

Gordon, S. (2002), NASA Glenn Coefficients for Calculating Thermodynamic Properties of Individual Species, TP-2002-211556

KLIMOV CORPORATION (1973), Motorul de aviatie turbopropulsor TV2-117-A si reductorul VR-8 (TV2-117-A Aviation Turboprop and VR-8 Gear Box),

Nikolic, V. (2005), Producerea si utilizarea biogazului (Producing and Using the Biogas), Chiminform Data, Bucuresti

Pimsner, V. (1964), Energetica turbomotoarelor cu ardere interna (Energetics of Internal Combustion Gas-Turbines), Editura Academiei Republicii Populare Romane

POPESCU, J[eni]; VILAG, V[aleriu]; PETCU, R[omulus]; SILIVESTRU, V[alentin]; STANCIU, V[irgil]*

* Supervisor, Mentor

Table 1. Air quantity, air excess and mass
flow rate for the two studied fuels

Fuel minL [lamdba] [m.sub.fuel]
 [kg/s]

Kerosene 14.6 4.6 0.0983
Biogas 4.57 4.3 0.3355