Energy, Exergy & Environmental Analysis of a Combined Cycle with Pre-combustion CO2 Capture and N2 Injected into the Compressor of the Gas Turbine

Energy, Exergy & Environmental Analysis of a Combined Cycle with Pre-combustion CO₂ Capture and N₂ Injected into the Compressor of the Gas Turbine

H. Betelmal¹, Abdurrauf. M. Naas², Mohamed A. Naas³

Affiliation: Department of Mechanical Engineering Tripoli University,

Faculty of Engineering Tripoli-Libya

DOI: https://doi.org/10.51244/IJRSI.2024.1102032

 Received: 02 February 2024; Revised: 14 February 2024; Accepted: 20 February 2024; Published: 19 March 2024

ABSTRACT 

Power plants are a major source of CO₂ emissions in the environment, which cause global warming; due to this issue, CO₂ capture from hydrocarbon fuels is one of the key technology options to reduce greenhouse gases. Pre-combustion capture of CO₂ in the combined gas and steam turbine cycle has been investigated in this paper. The first step in the pre-combustion method is to react the fuel with oxygen, which comes from the air separation unit and produces a mixture of hydrogen and carbon monoxide. The CO is converted to CO₂ in a water-shift reactor, and a physical absorbent removes the CO₂; a hydrogen-rich fuel is produced, which can be burnt in a gas turbine with minimal CO₂ emissions.

This paper presents a thermodynamic cycle analysis, where pre-combustion CO₂ capture is applied in a combined cycle where the main goal is to reduce the CO₂ emission into the atmosphere. The main parameters are varied to examine the influence on cycle performance. The cycle performance results for the combined cycle with CH₄ as fuel are presented and compared to the combined cycle with precombustion CO₂ capture, both with and without Nitrogen injection into the compressor. An exergy analysis is carried out to determine which case has more exergy destruction.  The results indicate that at 45 ^oC the combined cycle efficiency is raised by around 3 % when the pre-combustion CO₂ capture without Nitrogen injection into the compressor is used, and the power output has been increased by 29 %. The performance of combined cycle with CO₂ capture can be further enhanced by the N₂ acquired from the air separation process and injected into the compressor’s middle stage. The results demonstrate a good improvement in the performance; the power output increases by 48 % and efficiency by 4.2 %. However, exergy destruction is increased when the CO₂ capture, both with and without N₂ injection into the compressor is used. Nevertheless, pre-combustion capture requires large monetary investment for a new-build plant where also the CO₂ emission is reduced by 100 %.

Keywords: CO₂ Capture, gas turbine, combined cycle, N2 injected, energy, and exergy analyses

Nomenclature

Symbol

Cp                          specific heat (kJ/kg. K)

CO2                       Carbon dioxide

N2                         Nitrogen

ɳ                            Efficiency

h                            Enthalpy (kJ/kg)

I                             Irreversibility (kW)

m                            mass flow rate (kg/s)

P_G           is the power of the gas turbine.

Q_in          inlet heat (kW)

R_g         universal gas constant

s             Entropy (kJ/kg. K)

T₀            Atmospheric temperature (298 K)

φ             exergy (kW)

Subscripts

a             air

ex           exit

in            inlet

f              fuel

g             gas

gen          generation

INTRODUCTION

Gas turbine technology is widely used, the simple gas turbine cycle has low thermal efficiency, especially in hot climatic conditions, but innovation constantly drives new improvements in performance. There are a few different methods that are used to improve a gas turbine’s efficiency and power output, the most popular way is the gas and steam turbine combined cycle, in addition, the methods employing steam injected into the gas turbine cycle components show even more success in improving the gas turbine cycle performance [1]. In its most basic configuration, the air in the simple cycle gas turbine is compressed and mixed with the fuel (usually natural gas) and burned in a combustor. The resulting exhaust gases expand through the turbine that drives the compressor and an electric generator. In a combined cycle gas turbine plant, the hot exhaust gases leaving the turbine pass through a heat recovery steam generator, producing steam to generate more electricity with no additional fuel as shown in figure 1.

Figure 1 Gas and Steam Combined Cycle

On the other hand, the levels of atmospheric carbon dioxide, and other greenhouse gases are on the rise and are causing global warming.  Fossil-fuelled power generation and other man-made greenhouse gas sources mostly emit CO₂, with the power generation sector being the largest emitter of carbon dioxide (International Energy Agency, 2006) [2]. Thus, CO₂ capture in fossil fuel power plants can be used to control and limit the emission of greenhouse gases and thereby reduce the rise in atmospheric temperature.

One of the methods for the capture of CO₂ from fossil fuel power generation is Pre-combustion capture, which is removing CO₂ from the fuel before combustion is completed, this method is an alternative to combusting fuel directly in a combustor. Initially, synthesis gas (syngas), which is a mixture of mainly H₂ and CO with a trace of CO₂, is produced from a fossil fuel, it can be done by adding oxygen that is separated from the air in an air separation plant to the fossil fuel [3]. This process is called partial oxidation and the reaction for this process is given below,

C_x H_y + x/2 O₂ → xCO+(y/2) H₂

This synthesis gas, or syngas, can then undergo the water-gas shift reaction to convert CO and H₂O to H₂ and CO₂ [4].

Water gas shift reaction: CO+ H₂O ⇋ CO₂+H₂.

The CO₂ can then be captured, separated, transported, and ultimately sequestered, and pure H₂ is further used as a fuel for the combined cycle to produce electricity [5].

This study focuses on the pre-combustion approach to control the gas emission by removing the greenhouse gases such as CO₂ to the environment, we first convert the fuel into H₂ and CO₂, subsequently separating CO₂ from the fuel gases (CH₄) which can be stored permanently or reutilized industrially while the hydrogen is used as a fuel. Furthermore, this approach examines how Nitrogen (N₂), after separating it from air and injecting it into the compressor, improves the cycle’s performance, it is used to compensate for the fuel mass shortage when the combined cycle runs at full load. The injection of Nitrogen into the compressor has a significant role in improving the combined cycle’s performance, increasing efficiency and power output. These benefits can lead to the ability to produce more power in peak time and baseload operation. The IPSEpro program has been used to model and examine the combined thermodynamic cycles to determine the optimum mode of operation and configuration [6].

SYSTEM ANALYSIS

Another option currently being developed is to utilize hydrogen as a fuel for the power plant using a pre-combustion method. The studies show that the combined cycles with natural gas as a fuel using a pre-combustion method to capture the carbon dioxide improves greenhouse gas control. The main disadvantage of pre-combustion capture is that it requires a chemical plant in front of the turbine. Complicated chemical processes normally cause extra shutdowns of the plant, resulting in lower power output [7]. In this study after using pre combustion capture, H₂ has been obtained and used as a fuel for the combined cycle. A simplified schematic of the combined cycle integrated with pre-combustion capture is shown in figure 2.

Figure 2 Pre-Combustion Combined Cycle

The system shown in figure 3 represents the combined cycle and the injection of N₂ into the compressor to enhance the cycle’s performance. The purpose of the compressor is to compress the inlet air (raise its pressure). After the air- N₂ mixture is discharged from the compressor at high-pressure, it is sent to the combustor, mixed with H₂, and combustion occurs. The hot pressurized gas exits the combustors at a very high temperature up to 1117 °C. This gas then passes through a gas turbine, giving the turbine energy to spin the compressor and the power for a generator to produce electricity and the hot exhaust gases leaving the turbine pass through a heat recovery steam generator, producing steam that is used to generate more electricity with no additional fuel. Based on the first and second laws of thermodynamics, this work examines the combined cycle’s performance, using the energy and the exergy analysis in the combined cycle operation.

Figure 3 Pre-Combustion Combined Cycle with N₂ Injection

THERMODYNAMIC ANALYSIS

The present study introduces the energy and exergy analysis for the combined cycle with pre-combustion CO₂ capture and N₂ injected into the compressor. The analysis investigated the cycle performance due to changes in the ambient temperatures.

Assumptions

The following assumptions are made to simplify the calculations; however, they can be refined to reach more real solutions. The main assumptions are:

Hydrogen (H₂) gas enters a steady-flow adiabatic combustion chamber at 25°C and 20 bar.

Mass of air m_air = 393kg/s

Design Parameters
Ambient temperature Ta	15oC
Inlet pressure Pair Air Humidity	1.002 bar 60 %
The input data for the compressor
Compressor isentropic efficiency ηs	0.87
Compressor mechanical efficiency ηm	0.99
Pressure ratio P2/P1	11
Pressure drops in the combustion chamber (∆P)	0.1 bar
The input data for turbine:
Turbine mechanical efficiency ηm	0.99
Turbine isentropic efficiency ηs	0.9
The characteristics of the heat exchanger are:
The pressure drops of the hot temperature side	0.1 bar
The pressure drops of the low temperature side	0.1 bar

The temperature increase of the air during the compression is:

T_in: Compressor inlet Temperature [K]

T_ex: Compressor outlet Temperature [K]

dT_ex-in: Temperature different between inlet and exit

P_in: Compressor inlet pressure

P_ex: Compressor outlet pressure

Compressor Injected with Nitrogen Analysis

The pressure ratio P_ex/P_in can be obtained directly from the combustion chamber pressure loss.

The overall efficiency of the combined cycle (  is calculated using the following equation.

EXERGY DESTRUCTION IN THE COMPONENTS OF THE COMBINED CYCLE

The complex thermodynamic analysis of the combined cycle is based on the second law of thermodynamics because the conventional first law analysis of any thermodynamic system has the capability to determine the energy distribution across the system boundaries but does not explain the means of energy degradation that cause decreases in the power output of the system. The exergy analyses for the gas turbine components have been conducted to determine the energy loss caused by irreversible processes. Exergy analysis usually predicts an energy system’s thermodynamic performance and the system components efficiency by quantifying the entropy generation of the components. This analysis provides a tool for the optimal design and operation of complex thermal systems. The exergy and irreversibility equations for each component are written as follows [8],

Compressor: The exergy and irreversibility in the compressor is given by

Combustion Chamber: As an approximation, the virtual power is used to calculate the combustor component’s exergy. The exergy balance of this component and exergy loss due to irreversibility in the combustion chamber is given by:

And = average (T_ex, T_in)

Turbine

The exergy loss due to irreversibility in a gas turbine is given by:

DISCUSSION OF THE RESULTS

Based on the methodology developed and the thermodynamic equations shown in this study, the effect of ambient conditions on the performance and the destruction of exergy due to irreversibility in the gas turbine components is displayed graphically, with and without nitrogen injection into the compressor. The exergy destruction is obtained from the exergy analysis, in addition, the impact of CO₂ capture is shown in the results. The following figures show the change in the combined cycle performance with changing ambient temperature [15-45 ⁰C].

To improve the power output of the combined cycle, N₂ injection into the compressor is employed. The higher value of LHV “lower heating value” of H₂ means less mass flow rate in the fuel stream which leads to increased efficiency and power output.

Figure 4: The power output of the combined cycle with injection of different N₂ mass flow rate as a function of ambient temperature

Figure 5: The efficiency of the combined cycle with injection of different N₂ mass flow rate as a function of ambient temperature

Detailed pre combustion combined cycle efficiency and power output analyses are performed on four different N₂ mass flowrate injections (30 kg/s – 45 kg/s), also the effect of ambient temperature for each N₂ mass flowrates is performed. Figures 4 & 5 shows that total power output and the combined cycle efficiency are increases as the mass flowrate of N₂ injected increases, and that indicates that the pre-combustion combined cycle with N₂ injection improves the cycle performance. However, the power output and efficiency are decreases as the ambient temperature increases in all examined N2 flowrates. So that the mass flowrate of ₍45 kg/s) N2 injected into the cycle _is recommended for farther analysis.

The variation of the power output and efficiency against ambient temperature for the three different cases, (normal combined cycle, pre combustion combined cycle without N₂ injection and pre combustion combined cycle with ₍45 kg/s) N₂ injection) are shown in Figures 6 and 7 respectively.

Figure 6: The efficiency of the 3 cases of the combined cycle as a function of ambient temperature

Figure 7: The power output of the 3 cases pf the combined cycle as a function of ambient temperature

Figures 6 and 7, demonstrates the effect of ambient temperature on the efficiency and the power output of the combined cycle for the different 3 cases. The cycle efficiency and the power output does not show much change with ambient temperature between 15 ºC to 45 ºC. The bottom curve in figure 6, represents the efficiency of conventional cycle that uses CH₄ as fuel, where the efficiency is lower compared to the pre-combustion cycles represented by the top 2 curves, the top curve demonstrating that when N₂ has been injected into the pre-combustion cycle, the cycle’s performance has further improved, with an increase in efficiency by 4.2 % compared to the normal combined cycle. Figure 7 shows that the combined cycle with CO₂ capture without N₂ injection increases the power output by about 29 % compared to the normal cycle, moreover, the N₂ injected into the compressor achieves the highest power output. And the result shows an improvement in the power output is increased by about 48 % in the combined cycle with N₂ injection and CO₂ capture when compared with the normal combined cycle.

Exergy analysis identifies the causes and locations of thermodynamic losses more clearly than energy analysis. Consequently, exergy analysis can help in improving and optimizing designs. The exergy of the combustion chamber is affected by modifications made to the normal cycle as shown in figure 8. The exergy losses in the combined cycle components in three different cases are presented. The results show that when CO₂ capture is implemented, the exergy loss will be higher than the normal cycle exergy loss, however, as the N₂ is injected into the compressor, the exergy loss will be maximized. Exergy loss in the combustion chamber is the largest of all component losses in the gas turbine cycle. The exergy destruction during the combustion process was different for the different fuels. The exergy loss when using H₂ as a fuel increased by 18 % compared to CH₄. The combustion chamber has the biggest exergy loss in the combined cycle, a loss of around 130 MW, while the air compressor has a loss of roughly 7.5 MW.

Figure 8:  Exergy destruction of the combined cycle components at 15 ⁰C for the 3 cases

The effect of the environment temperature variation (between 15°C, and 45°C) on the component exergy destruction rate when the pressure ratio stays at 11 bar is shown in figure 9.  Clearly, the combustion chamber’s exergy destruction decreases as the ambient temperature increases in the case of N₂ injection into the compressor with CO₂ capture, however, the exergy loss for all other components does not change much when the ambient temperature increases. In the combustion chamber, at high ambient temperature, the exergy loss will be lower than the exergy loss at the lower ambient temperature. The exergy loss in the combustion chamber has various causes, mainly due to the chemical reaction and heat transfer occurring inside the combustion chamber.

Figure 9:  Exergy destruction of the CO₂ capture + (45 kg/s) N₂ injection as a function of ambient temperature

The exhaust gas temperature from the gas turbine in the pre-combustion combined cycle with and without N₂ injection has a favourably high heat content as shown in figure 10, leading to a high exhaust temperature that is used to operate the steam turbine cycle, for farther increase in ambient temperature the exhaust gas temperature starts to decrease. On the other hand, upon examining the amount of fuel consumption, it is discovered that the fuel consumption drastically decreases when pure H₂ is used as a fuel because of a very high lower heating value compared with the CH₄ fuel as shown in figure 11.

Figure 10: The gas turbine exhaust gas temperature as a function of ambient temperature for the 3 cases

Figure 11:  Fuel Consumption for two cases

There was a significant reduction in CO₂ emission when CO₂ capture is applied in the combined cycle as demonstrated in this study as shown in figure 12, where the CO₂ emission is reduced by 100 % also the exhaust gas analysis dose not recorded any amount of NO_x emission, thereby positively reducing climate change.

Figure 12:  CO₂ Emission in Three Cases

CONCLUSION

In this work the effect of injected N₂ into the compressor on the energy and exergy of the combined cycle was analysed. The simulation program IPSEpro has been applied successfully to the gas and steam turbine combined cycle using conventional thermodynamic analysis applying the first law and second law of thermodynamics. The analysis investigated the effects of different ambient temperatures and different masses of N₂ used in injection on the cycle performance. The air separation cycle is used to separate the Oxygen O₂ from the N₂ so we can use O₂ for CO₂ capture and the N₂ for injection into the compressor of the gas turbine. The system performance heavily depends on the type of fuel, with H₂ being the best, the present study introduces a comparative energy and exergy analysis for the CO₂ capture combined cycle with N₂ injection, this causes an increase in the power output of a gas turbine and an increase in cycle efficiency, both being higher than that in the common combined cycle, whilst the CO₂ emission is reduced by 100 %. Finally, when this system is employed, exergy destruction is increased, the increase in the non-injection case is small, but the largest increase is when N₂ is injected. Further studies should evaluate investment costs associated with the cycle, in addition to the cost of exergy destruction within each system component.

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