As shown in Figure 1, the simplified NGCC (Natural Gas Combined Cycle) power plant is divided into seven different control volumes: Compressor (K), Combustor (CC), Combustion Turbine Generator (CTG), Steam Turbine Generator (STG), Heat Recovery Steam Generator (HRSG), Condenser (C) and Pump (P).

Following the process flow diagram: 1) Fresh air enters the compressor. 2) It is then compressed to a higher pressure. Upon leaving the compressor, air enters

the combustion system, where natural gas (NG) is injected and combustion occurs. The combustion process occurs essentially at constant pressure. 3) The exhaust gas leaves the combustor and enters the Combustion Turbine Generator (CTG). In the turbine section, the flue gas is expanded to produce electricity. 4) The flue gas leaves the CTG at high temperature. This first part of the power plant is called Open Cycle Gas turbine (OCGT). In the second part of the NGCC, the hot stream from the gas turbine will generate steam in a Heat Recovery Steam Generator (HRSG).The steam cycle consists of a HRSG, Steam Turbine Generator (STG), condenser, and pump to form a Rankine cycle for electricity production. Water enters the HRSG at high pressure and the resulting steam produce electricity in the steam turbine generator (STG). The saturated steam leaving the steam turbine is first condensed and then its pressure increased before returning to the HRSG. The de-aerator of the steam cycle is not included in this investigation.

The objective of the preliminary calculations is to determine 1) the amount of natural gas, 2) excess air and 3) the theoretical flue gas recirculation ratio in a natural gas combined cycle (NGCC) power plant in order to produce about 600 MW electricity.

For an effective absorption by amine solutions, the molar percentage of CO₂ in the flue gas should be higher than 10%. In order to reduce technical problems linked to oxidative degradation of amine in the CO₂ capture plant, the percentage of O₂ in the flue gas should also be lower than 5%.The positive effects of Flue Gas Recirculation (FGR) on the composition the flue gas have also been investigated [2] [5] [12] [14] Akram et al. [5] studied the effects of recirculating part of flue gas in a 100 KW (plus 150 KW hot water) CHP gas turbine TurbecT100. Their preliminary results indicate that a recirculation ratio of 0.45 increased the molar percentage of CO₂ from 1.5% to 3.0% and the molar percentage of O₂ decreased from 18.2% to 16.0%. Their results show also that recycling part of the flue gas decreases NOx emissions by decreasing the flame temperature. Using a natural gas-fired power plant of 700 MW, the results presented by Bolland and Saether [2] show that, using a recirculation ratio of 0.4, the molar percentage of CO₂ increased from 3.3% to 5.5% and the molar percentage of O₂ decreased from 13.8% to 9.8%. To maintain high cycle efficiency, flue gas was cooled before reinjection in the combustor. Abu Zahra [14] used Aspen plus and the Peng-Robinson equation of state, to simulate the effect of flue gas recirculation in an IGCC power plant using a low-pressure gasifier. With all the other parameters of the gas turbine constant, the results indicated that the concentration of CO₂ increased from 7.3 (% mol.) to 14.0 (% mol.) with the increase of the flue gas recirculation ratio from 0% to 45%. Moreover, the concentration of O₂ decreased from 14.2 (% mol.) to 8.0 (% mol.) with the same increase of flue gas recirculation. This part of this investigation is to simulate the impact of flue gas recirculation ratio on its composition for a gas turbine with an excess air (EA) of 100%. First, the simulation results of the molar percentage of CO₂in the flue gas are shown in Figure 4.

As shown in Figure 4, The Flue gas recirculation increases the molar percentage of carbon dioxide in the flue gas entering the absorption unit from 5.8 (FGR = 0.1) to 11.0 (FGR = 0.53). However, from a value of FGR ratio equal to 0.5, the

molar percentage of carbon dioxide stays constant. This observation could indicate that there is not enough oxygen for the combustion process and carbon monoxide is being formed instead of carbon dioxide. This could also be explained by the fact that the simulated mass flow rate of fresh air (361.1 kg/s) is lower than the minimum needed stoichiometric value of 384.53 kg/s. The optimum value of the flue gas recirculation ratio should therefore be lower than 0.50. Secondly, the effects of the flue gas recirculation (FGR) ratio on the concentration of O₂ of the flue gas entering the absorption unit are shown in Figure 5.

As expected, the flue gas recirculation decreases the molar percentage of oxygen in the stream entering the absorption unit from 9.4 (FGR = 0.1) to 0.0 (FGR = 0.53). However, the slope of curve for the values of FGR ratio above 0.42 is larger than the slope of the part of the curve for value of FGR ratio lower than 0.42. This value is selected in this study as the optimum FGR ratio. The corresponding excess air in the primary air is 17% and the molar composition of oxygen in the mix natural gas-primary air is 0.19. This molar composition of oxygen in the gas ?air mixture is higher than the value of 0.17 given in the literature as the minimum concentration for the stability of the flame [13] . In practice, good efficiency of the combustion could be obtained by controlling oxygen and carbon monoxide content in the exhaust gas. For example, the measured CO concentration can be used for combustion control by two methods: controlling O₂ when the O₂ concentration exceeds a prescribed value and overriding to CO control when the O₂ concentration falls below the value, or giving a CO concentration bias (compensation) to the O₂ concentration (Figure 6).

As shown in Figure 7, a 600 MW-natural gas combined cycle (NGCC) with a flue gas recirculation ratio of 0.42 is simulated in this study. 23.81 kg/s of natural gas (CH₄ = 93% mol) is supposed to be available in a battery limit of the plant at 3.1 MPa and 25˚C and 449.2 kg/s of fresh air at design condition is compressed up to 3.1 MPa in a three stages compressor with intercooling. NG mixes with primary air in the combustor and it is assumed that combustion is complete and all the carbons in NG convert to CO₂. The temperature of the combustion gases is 2100˚C. Secondary air is mixed with recirculated flue gas to cool the combustion

gases until 1300˚C before entering the turbine. The exhaust gases leave the gas turbine at atmospheric pressure and 618˚C. The flue gas enters to HRSG in order to produce 55 kg/s of steam at three levels of pressure: high pressure (HP) steam (173 Bars, 600˚C), an intermediate pressure (IP) steam (65 bars, 565˚C), and a low pressure (LP) steam (2 bars, 350˚C) with double reheat. The medium pressure steam (IP) is heated from 438˚C to 565˚C and the low pressure steam (LP) is heated from 206˚C to 350˚C. After leaving the Heat Recovery Steam Generator (HRSG), the flue gas is cooled from 67˚C to 40˚C in order to remove water in the separator V-100 and 42% of the flue gas is recycled. The pressure of the recycled gas will increase from 110 kPa to 3.1 MPa using three stages compressor with intercooling at 40˚C and mixed with secondary ambient air.

The impact of the recirculation of flue gas on the power input or output of the different equipment of the power plant are investigated. The objective is to compare the performance of each equipment of the plant with flue gas recirculation ratio of 0.42 to its performance without flue gas recirculation.

Natural Gas Combined Cycle (NGCC) power generation is an energy conversion system of generating electric power that combines gas turbine power generation (CTG) with steam turbine power generation (STG). The thermodynamic power cycles can be categorized as gas cycles and steam cycles. Gas turbines operate using the principles of the Brayton cycle. The isentropic efficiency (η_iso) of the Brayton cycle can be expressed in terms of the temperatures and pressures entering (P₁ & T₁) and leaving (P₂, T₂) the compressor:

η i s o = 1 − T 1 T 2 = 1 − ( P 1 P 2 ) k − 1 k (6)

where k is the specific heat ratio. Based on Equation (6), increasing the temperature of the stream leaving the compressor or increasing the pressure ratio will enhance the efficiency of the gas turbine. In this investigation, fresh air enters the compressor at design conditions of 15˚C and the temperature of the exhaust gas is limited to 1300˚C. Based on these operating conditions, the isentropic efficiency of the gas turbine is 0.82. This value was selected as the adiabatic efficiency of the gas turbine for the simulation of the natural gas combined cycle. The corresponding power output of the gas turbine is 708.5 MW. The compressors of fresh air and recirculation of flue gas consume respectively 174.21 MW and 125.89 MW. The net output of the gas turbine is equal to 408.4 MW.

The overall efficiency of the gas turbine could be estimated by the following equation:

η = W netoutput m N G × LHV (7)

where W_{net output} is the difference between the energy produced and the energy consumed by the different equipment of the combustion turbine generator (CTG), m_NG is the mass flowrate of natural gas and LHV is the low heating value of natural gas. Using the Low heating value (LHV) and the mass flow rate of the natural gas (46.7 MJ/kg and 23.81 kg/s), the efficiency of the gas turbine is equal to 36.1%. This value corresponds to the efficiency of the Open Cycle Gas Turbine (OCGT). The hot stream (exhaust gas) with a temperature of 617˚C will be used to produce more electricity by generating steam in the Heat Recovery Steam Generator (HRSG). Steam turbine in a combined cycle powerplant operates using the principles of the Rankine cycle. Like the Brayton cycle, the ideal Rankine cycle operates on four internally reversible processes. The theoretical maximum efficiency of a Rankine cycle is the Carnot efficiency given in Equation (8).

η Carnot = 1 − T Cold T Hot (8)

In this investigation, the highest temperature is the temperature of the steam entering the HP turbine at 585˚C and the cold temperature is the temperature of water leaving the condenser at 30˚C. Based on these data, the maximum efficiency of the steam cycle is 64.7%. The thermal efficiency of a Rankine cycle is lower than that of a Carnot cycle operating between the same temperature levels. This is primarily because of the fact that the energy transfer as heat in the boiler does not take place at constant temperature in the Rankine cycle. The purpose of a reheating cycle is to remove the moisture carried by the steam at the final stages of the expansion process and improves the efficiency of the cycle, because more of the heat flow into the cycle occurs at higher temperature.

Based on Figure 14, the thermal efficiency of the Rankine cycle with reheat is given by Equation (9):

η = ( h 1 − h 2 ) + ( h 3 − h 4 ) + ( h 6 − h 5 ) ( h 1 − h 6 ) + ( h 3 − h 2 ) = W net-out Q in (9)

The power produced by the three turbines is equal to 32.74 MW (HP), 114.3 MW (IP) and 66.8 MW (LP). The pump consumes 3.15 MW. Based on the low heating value of the natural gas, the efficiency of the steam turbine is equal to 18.9%.

Because of the complementary temperature ranges of the Brayton GT cycle (1300˚C - 618˚C) and the Rankine steam cycle (618˚C - 67˚C), natural gas combined cycle (NGCC) can produce significantly improved thermodynamic cycle efficiency (Figure 15).

The net output of the NGCC power plant without flue gas recirculation and with a FGR ratio equal to 0.42 are respectively 610.89 and 618.57 MW. Based on the Low heating value (LHV) of the natural gas, the overall efficiency of the power plant increased by 1.1% from 57.5% from to 58.2%.