Energy, Exergy and Economic Analysis of Al-Qayyarah Gas Power Plant

The globe faces several difficulties, chief among them the scarcity of energy and the rising demand for it brought on by the rise of the global economy and population growth. It forced nations to consider carefully how they would supply this energy. One of the factors behind the phenomena of global warming is energy. Energy is essential for human usage and plays a significant role in social and economic advancement. Studies have indicated a correlation between the expansion of the economy and the usage of electricity. This link suggests that increased electrical energy usage has an impact on economic expansion [1]. In contrast to conventional energy sources like nuclear power, hydropower, solar energy, wind energy, and others, renewable energy is clean energy that doesn't harm the environment. However, because of its limited supply and damage to the ozone layer, which impacts both human health and the planet's climate, it can't meet demand. Like other nations, Iraq experiences a deficit of electrical energy as a result of the recent rise in demand for it. This has led to a need for electrical energy sources, which has impeded Iraq's progress in the majority of fields related to modern development [2, 3]. The development of electrical energy consumption and satisfying the growing demand for it depends on boosting production efficiency and streamlining usage. This is considered an excellent way to lessen the influence on the environment as it lowers emissions and the amount of fuel used in the thermal station, which tends to be replaced by clean energy to cut down on the usage of fossil fuels and lower prices. However, they can transform thermal energy into electrical energy in gas turbine units, as shown in Figure 1, fossil fuels are regarded as one of the most significant sources of electrical energy generation [4]. The thermal energy in the hot gases produced by burning is transformed into mechanical energy by gas turbines. The stored thermal energy is transformed into energy by the gas turbine (T3), which raises air pressure and directs compressed air to the combustion chamber, where fuel and air are combined. Hot gasses are produced when they are ignited. The compressor and turbine shaft are run by the energy contained in the gases. The turbine's rotating action causes the air compressor's main shaft to revolve independently at the same time, utilizing mechanical energy [5].

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Figure 1. Gas turbine unit [6]

The capacity of gas power plants to generate vast amounts of energy surpasses that of other forms of power plants that concentrate solely on producing electrical energy, which is just one of their numerous advantages. This is a result of how much fuel they can efficiently turn into electricity. Gas turbines are distinguished from other types of turbines by their quick start-up times and high energy distribution, which frequently exceeds 60% or more. Gas turbines can effectively satisfy the necessary requirements in a short amount of time, which makes them highly sought after. Gas turbines' reliance on carbon as a fuel source also restricts the fields in which they may be used to produce electricity [6]. Because gas power plants are so efficient at producing energy, they are vital to the economy, especially when it comes to the creation of electricity. Gas turbines are devices that convert the thermal energy released during the combustion of natural gas into mechanical energy, as shown in Figure 2. This mechanical energy is converted into electrical energy by electrical generators. Gas-fired power plants have the capacity to generate electricity at a high rate during times of demand, which makes them a potentially viable option for meeting significant energy needs. Additionally, these power plants significantly contribute to the stability of the electrical system as a whole [7]. The goal of engineering is to increase the gas power plants' part-load operational efficiency. The goal of gas turbine technological advancements is to improve efficiency and operational capabilities. Control and switching system functionality improved, enabling plants to respond more quickly to variations in energy demand and preserve grid stability. Many tactics have been developed to lower power plant gas emissions. This entails employing cutting-edge combustion technology and putting in place gas processing systems to manage combustion byproducts. Water usage is to be decreased while plant operating efficiency increases through the development of cooling systems [7].

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Figure 2. Gas turbine

This study was conducted at the Qayyarah gas power station located in the Qayyarah district of Nineveh Governorate/Iraq. The plant shown in Figure 3 was designed by General Electric (GE) and implemented by the Turkish company Gailk Energy. The system has six 9th generation generating units (frame 9E), single shaft turbine specifications, and engine speed (3000rpm). The unit is specifically designed to operate with a load of (125MW) as the intended power capacity and typically operates with a load of approximately (100MW) as the actual power capacity. The gas unit has been specifically developed for simple cycle operation and is capable of using three different forms of fuel (crude oil, natural gas, and light fuel).

Usin software techniques to simulate a power plant from data taken from the operating department for the year 2023, this research aims to theoretically analyze the energy and available energy and study the effect of relative humidity, pressure ratio, heat load and external ambient temperature on the equipment of the Qayyarah gas station.

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Figure 3. Qayyarah gas power station [7]

Prior to being acquired by Aspen Tech and rebranded as Aspen HYSYS, Hyprotech developed the HYSYS software. Oil, natural gas, petrochemical, gas, and thermal facilities are all simulated using the HYSYS program as it includes all the industrial components required for most businesses. A thorough database with the majority of the materials used in the aforementioned areas is also provided by the program, and users have the option to add more materials or compounds. Reactors, distillation towers, absorption columns, heat exchangers, and a plethora of other industrial equipment are among the numerous that use it in their design [10]. Major design companies such as Enppi and others utilize the HYSYS software to build a complete design for gas plants, petrochemical plants, and petroleum refining facilities. This program is used in design, along with other auxiliary programs, whether in the Arab world or elsewhere in the globe. Additionally, the application computes the required values based on the values of the different parameters. Additionally, the algorithm selects the optimal heat amount, pressure, temperature, and production rate settings that will maximise facility profits while minimising expenses [11]. With the HYSAS program, DCS control units may also be emulated. The software is also used to investigate changing operating circumstances. Our approach to production rate differs in that we use the HYSAS software to simulate the unit whose operating parameters we desire to modify first, then we progressively adjust the settings and assess the impact of this modification on production rates. Temperature, pressure, heat, and production pace that will enable the facility to make the most significant money with the least expenses [12]. The core ideas of integration and innovation serve as the foundation for the adaptable and dependable process simulation provided by the HYSAS application. The most recent chemical process technologies, unified functions in a single software environment, seamless connection to the chemical engineering computing environment with tool links like MS Excel and Word and interfaces like (COM, DCOM), and HYSAS combining a state-of-the-art graphical user interface are just a few of the many major benefits it offers HYSAS users. Additionally, special thermodynamics, unit operations, computations, and reporting may be supported by customising the program. Systems in both dynamic and stable states are simulated [13]. A simple model for simulating gas turbines is created, as shown in Figure 4. Three fundamental parts make up the model: a turbine, a combustion chamber, and a compressor. Throughout the year, the conditions under which air enters the compressor vary.

For natural gas-fired Unit 6, simulations were performed using HYSYS software to ascertain variations in gas turbine performance and efficiency as well as predict production range and performance under different operating situations. The following assumptions guided the simulation implementation: Assumption that each cycle process remains constant throughout; The air entering the compressor and the air leaving the exhaust has the same standard pressure (1.01325 bar), which is atmospheric pressure. The inlet air temperature is equal to the ambient air temperature. Introduction of fuel, air, combustion products, and ideal gases According to all ideal gas laws, the gases leaving the combustion chamber can be calculated, and they are equal to (0.97*P2). Assuming that each part of the cycle - the compressor, the combustion chamber, and other parts -leads to nitrogen entering and leaving the compressor at a molar rate of (79%) and oxygen at a molar rate of (21%) only when entering the compressor, the mechanical efficiency of the turbine and compressor reaches (95%) [14, 15].

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Figure 4. An Aspen HYSYS gas turbine model's flowchart

3.1 Energy analysis

To study the energy for the gas turbine unit, a series of assumptions is used to simplify the resultant mathematical model:

1. The gas turbine is operating under steady-state conditions.
2. KE and PE are negligible this is because its value is very small and does not affect the conduct of theoretical calculations, so it is neglected.
3. The efficiency of the electric generator and the mechanical efficiency of the turbine are equal to approximately 97% and 98%, respectively, taking into account the friction losses resulting from the vibration of the drive shaft and the work consumed in the secondary devices [16].

The gas turbine power for each component is evaluated based on Figure 5. which illustrates its actual cycle on a T-S diagram:

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Figure 5. Actual gas turbine unit cycle [17]

A. Compressor Work $\left(W_{A C}\right)$

The calculation for the compressor work is as follows [18]:

$\dot{W}_{A C}=\dot{m}_a \times \bar{C} p_i \times\left(T_2-T_1\right)$     (1)

where,

$\dot{m}_a=\dot{m}_i=\dot{m}_e$     (2)

And $\bar{C} p_i$ represented the combustion chamber's products and is computed as [18]:

$\bar{C} p_i=a_i+b_i T+c_i T^2+d_i T^3$     (3)

The heat added to the combustion chamber during the burning of fuel and air may be computed as follows [19]:

$\dot{Q}_{a d d}=\dot{m}_f \times L H V$     (4)

B. Turbine Work $\left(W_{G T}\right)$

Can indicate the work performed by the turbine as a consequence of the expansion of the combustion products via its blades [20]:

$\dot{W}_{G T}=\dot{m}_g \times C p_g \times\left(T_3-T_4\right)$     (5)

where,

$\dot{m}_g=\dot{m}_a+\dot{m}_f$     (6)

C. Thermal Efficiency $(\eta I)$

The ratio of the network created by the turbine to the heat energy produced by the combustion process in the combustion chamber is known as the thermodynamic cycle's thermal efficiency [19, 21]:

$\eta \mathrm{I}=\frac{\dot{W}_{\text {net }}}{\dot{m}_f \times(L H V)}$     (7)

D. Brake Work Ratio (B.W.R)

The ratio of heat energy to turbine work is known as the brake work ratio [22]:

$B.W.R=\frac{\dot{W}_{A C}}{\dot{W}_{G T}} \times 100 \%$     (8)

E. Specific Fuel Consumption (SFC)

When examining the energy and exergy for gas power plants, one of the most important factors is specific fuel consumption (SFC), which is a measurement of the fuel used by a gas turbine to produce energy for a certain amount of time [23]. Its definition is the ratio of an hour's fuel mass consumption ($\dot{m} f$) to a network ($\dot{W}_{\text {net }}$).The low values of (SFC) show that the turbine is operating effectively [24], can be calculated [25, 26]:

$S F C=\frac{3600 \times \dot{m}_f}{\dot{W}_{\text {net }}}$     (9)

3.2 Exergy analysis

When the effects of surface tension, magnetic, electrical, and nuclear reactions are disregarded, the exergy of the gas turbine power plant may be classified into four primary categories. The overall exergy is represented as follows [26-28]:

$\dot{E}_X=\dot{E} x_{C H}+\dot{E} x_{P H}+\dot{E} x_{K N}+\dot{E} x_{P T}$     (10)

The open cycle (JBC) calculates the quantity of energy to each component of the system as follows [29]:

$e x=\left(h_i-h_o\right)-T_a \times\left(s_i-s_o\right)+\frac{V^2}{2}+g z+e x_{C H}$     (11)

The following rule is used to calculate the available power rate [30]:

$\dot{E}_X=\dot{m} \times e x$     (12)

The following formula may be used to define $\dot{E x}_{P H}$, which is the consequence of temperature changes for air, fuel, and combustion gases entering and leaving a system in relation to the outside environment [18]:

$e x_{P H}=\left(h_i-h_o\right)-T_a \times\left(s_i-s_o\right)$     (13)

To compute the amount of ∆S between two states entry and exit in each process while applying the specific entropy of an ideal gas as follows [31]:

$\Delta S=S_{i+1}-S_o$     (14)

$\Delta S=C_{P, i} \times \ln \left(\frac{T_{i+1}}{T_o}\right)-R \times \ln \left(\frac{P_{i+1}}{P_o}\right)$     (15)

The hII for each component may be written as [32]:

For compressor $\eta \mathrm{II}$ AC:

$\eta$ II. AC. $=\frac{E x_2-E x_1}{w} \times 100 \%$     (16)

For combustion chamber $\eta \mathrm{II}$ C.C:

$\eta$ II C. C. $=\frac{E x_{\text {fule gas }}}{E x_{\text {fule }}+E x_{\text {air }}} \times 100 \%$     (17)

For gas turbine $\eta \mathrm{II}$ GT:

$\eta \mathrm{II} \mathrm{GT}=\left(1-\frac{\dot{E} x_{d e s, G T}}{E x_3-E x_4}\right) \times 100 \%$     (18)

$\eta \mathrm{II}$ is calculated from [33]:

$\eta$ II cycle $=\frac{W_{\text {net }}}{E x_f} \times 100 \%$     (19)

And $\dot{W}_{\text {net,load }}$ is calculated from [34, 35]:

$\dot{W}_{\text {net,load }}=\left(\dot{W}_{G T}-\dot{W}_{A C}\right) \times\left(\eta_{\text {generator }} \times \eta_{\text {mechanical }}\right)$     (20)

And $\dot{E} x_{\text {des }}$ is determined from [36]:

$\dot{E} x_{d e s}=T_a \times \sigma_{C V}$     (21)

The study showed the effect of external variables (Load, Ta, RH, rp) on the system performance and the rate of specific fuel consumption, in addition to analyzing the energy and exergy of the power plant (Qayyarah gas station) to ascertain the losses resulting from it in actual working conditions. The following conclusions were drawn from the results of the system operation:

1. The temperature of the external environment influences the efficiency of the system (gas station). Hence, the efficiency of the gas unit decreases with rising external temperature.
2. When the compression ratio (rp) improves, the efficiency of the system increases
3. The combustion chamber achieved maximum exergy distortion as a result of irreversible processes that were followed by the gas turbine and compressor, respectively.
4. The price of producing energy is impacted economically by fuel consumption, which rises in response to changes in the outside temperature.