Process Simulation of a 620 Mw-Natural Gas Combined Cycle Power Plant with Optimum Flue Gas Recirculation

Abstract

The main objective of this investigation is to obtain an optimum value for the flue gas recirculation ratio in a 620 MW-Natural Gas Combined Cycle (NGCC) power plant with a 100% excess air in order to have a composition of the exhaust gas suitable for an effective absorption by amine solutions. To reach this goal, the recirculated flue gas is added to the secondary air (dilution air) used for cooling the turbine. The originality of this work is that the optimum value of a Flue Gas Recirculation (FGR) ratio of 0.42 is obtained from the change of the slope related to the effects of flue gas recirculation ratio on the molar percentage of oxygen in the exhaust gas. Compared to the NGCC power plant without flue gas recirculation, the molar percentage of carbon dioxide in the flue gas increases from 5% to 9.2% and the molar percentage of oxygen decreases from 10.9% to 3.5%. Since energy efficiency is the key parameter of energy conversion systems, the impact of the flue gas recirculation on the different energy inputs and outputs and the overall efficiency of the power plant are also investigated. It is found the positive effects of the flue gas recirculation on the electricity produced by the steam turbine generator (STG) are more important than its cooling effects on the power output of the combustion turbine generator (CTG). The flue gas recirculation has no effects on the water pump of the steam cycle and the increase of energy consumed by the compressor of flue gas is compensated by the decrease of energy consumed by the compressor of fresh air. Based on the Low heating value (LHV) of the natural gas, the flue gas recirculation increases the overall efficiency of the power plant by 1.1% from 57.5% from to 58.2%.

Exergy Efficiency and Environmental Impact of Electricity of a 620 MW-Natural Gas Combined Cycle

“…In the combustor (CC), combustion takes Figure 1. Schematic representation of natural gas combined cycle [14].…”

“…As shown in Figure 3, a 620 MW-natural gas combined cycle (NGCC) with a flue gas recirculation ratio of 0.42 was simulated in the first part of this study [14]. A mass flowrate of 23.81 kg/s of natural gas, which consists of 93 mol% methane, is meant to be available in a battery limit of the plant at 3.1 MPa and 25˚C.…”

“…In the combustion chamber, natural gas mixes with primary air and it is assumed complete combustion where all the carbon element of the natural gas is turned into carbon dioxide. Based on the first part of this investigation [14], the temperature of the combustion gases is 2100˚C. Secondary air is mixed with recirculated flue gas to reduce its temperature to 1300˚C before entering the turbine [14].…”

“…Based on the first part of this investigation [14], the temperature of the combustion gases is 2100˚C. Secondary air is mixed with recirculated flue gas to reduce its temperature to 1300˚C before entering the turbine [14].…”

“…A 620 MW-Natural Gas Combined Cycle (NGCC) power generation plant using 100% excess air was simulated in the first part of this investigation [14]. In Journal of Power and Energy Engineering order to have a composition of the exhaust gas suitable for an effective absorption by amine solutions, an optimum value of a Flue Gas Recirculation (FGR) ratio of 0.42 was calculated.…”