Enhancing Processes and Performance of Gas Turbine Power Plants (GTPPs) Using Six Sigma DMAIC Method in Hot and Dry Climate

2.1 Methodology

It is the first time in this study to use the Six Sigma DMAIC method with Aspen HYSYS software to optimize the inlet cooling system and enhance the performance of GTPP by using a fogging system. The research employed the five phases of the Six-Sigma DMAIC methodology: Define, measure, analyze, improve, and control with their tools to increase the efficiency of GTPP and power output. Considering that these tools might vary from case to case, each phase consists of many processes that employ pertinent engineering tools.

2.2 Material

This study, which is actually a case study, was conducted within the AL Taji Power Plant company, which generates and distributes electricity in the Iraq -Baghdad, Iraq. The Gas-Turbine Unit (GTU) that is the subject of this case study is (PG6111FA), which was manufactured by General Electric ("GE Energy"), is described in this part, as shown in Figure 4 [22]. A description of the key characteristics of the GT operation is provided in Table 1 [23]. An 18-speed axial compressor (stages) compresses the ambient air after it has been filtered by the input mixing air chamber assembly during GTU operation. The compressor's pressurized air enters the annular zone, which surrounds the six combustion chambers. The air passes through the metering holes in the lining of each combustion chamber and then reaches the combustion zone through the gap between the inner lining and the outer casing. Each of the six combustion chambers receives fuel from fuel injectors, which then mix it with inflammation and ignition. Prior to accessing the three-stage turbine section, the hot gases expand from the combustion chambers and go through six separate transition compartments that are attached to the inner lining of the combustion chambers' discharge side. The pressure drops and the jet's kinetic energy increases with each extra row of nozzles. The kinetic energy of the jet increases with each extra row of nozzles, but the pressure simultaneously decreases. The turbine rotor uses some of the jet's kinetic energy to drive the blades of the following row of spinning devices. The third stage's blades pass through, and the exhaust gases are then collected by a diffuser and exhaust casing. The gases are then sent to the waste-heat recovery boiler for processing. Following its passage through the reduction gear, the shaft revolves, driving the generator rotor, which generates power [23].

4.png

Figure 4. Typical GT cross section [22]

Table 1. The standard references parameters for GT GTU (PG6111FA) [23]

2.3 Applications of the Six Sigma DMAIC approach with rising ambient temperatures

When the air temperature rises during the hot summer causes decreases in mass flow rate in GTPP, as result un sufficient in power output is happing and decreases in supplying electric energy to the customers, so it is necessary to discover this issues and analyses the data with find the suitable solve with improving the processes of GTPP by application Six Sigma DMAIC method.

2.3.1 Define phase

The Six Sigma and DMAIC methodologies begin with the "define" step. Establishing the project's boundaries and scope is the goal of this phase [24]. Finding the objectives of the project and the voice of the customer (VOC), or the needs of the client or the business, is important (Table 2) [7, 8]. However, the Six Sigma team must be established before these project components are defined. The team for this improvement project consisted of three individuals: the improvement project leader, an experienced operator from the energy product from GTPP, and a production energy engineer. The next step in the DMAIC "define" stage was to state the project's scope. Jirasukprasert et al. [24] recommend that a Six Sigma project be chosen in light of business problems pertaining to failing to meet consumer expectations. A major and favorable influence on clients should be the main goal of the initiatives that are selected. Given these suggestions, the project's selected task was to reduce or eliminate the increase in ambient temperature, especially during hot days, which reduces the mass flow rate of air, power output, and efficiency of GTPP.

Table 2. A project charter [8]

The next stage to identify is the SIPOC diagram (Supply Input Processes Output Customer) in order to have a better understanding of the issue with the GTPP process for providing clients with energy power. The SIPOC diagram for GTPP processes is shown in Figure 5 [7].

Figure 5. SIPOC Diagram of GTPP processes [7]

The basic idea of a GT is that a compressor compresses a working gas, such as air, and then uses the fuel's combustion energy to heat it. Both the temperature and the pressure are elevated in the working gas. The turbine changes the energy of the flow of gas into the rotating energy to drive the shaft, taking advantage of the contact between the gas and the blades. Before being released into the atmosphere, this generates mechanical power. GT uses some of its mechanical energy to spin or drive the shaft of the compressor and the rest to create electricity [16].

2.3.2 Measure phase

Establishing trustworthy metrics to aid in tracking progress toward the goal or goals is part of the "measure" phase of this methodology [25]. The goal of this paper was to lessen the high ambient temperature, which caused a decrease in the capacity of GT, Air mass flow rate, and GTPP efficiency. Specifically, the "measure" step in this study indicated the process of defining and choosing useful measurements to identify the main flaws that needed to be fixed. The quantity with less power output on hot days was one of the measures that was established. To compare the "before and after" states, two additional metrics were employed: GT operational parameters and GT (ISO 3977).

Data collection and parameters. The actual and genuine ISO condition data utilized in this calculation came from the manufacturer's operating volume, which was given to the client. and the logbook, which documents the GT unit's daily analog and digital readings, also includes actual temperature data that was used in other computations. In our calculations, the ambient temperature was at typical standard values for GT (ISO 3977), despite the ambient pressure being 101.3 kPa (14.7 psia) and T1 = 15℃ + 273 = (288k) (Table 3).

Table 3. Standard reference conditions for GTPP (ISO 3977)

The temperature increases of more than 15℃ reduce the GTPP's capacity (power production) because the compressor will not have adequate air mass. In the area of Iraq, especially in the hot summer, air temperatures can reach 45 to 50℃ and sometimes much higher. Furthermore, Table 4 presents the data gathered at different temperatures, which were used in a Pareto chart (Figure 6) [25].

Table 4. Different air temperatures and parameters GTPP

Figure 6. Pareto chart for different ambient temperatures (T₁ °C the drop of mass flow rate of air with rising of ambient temperature [25]

2.3.3 Analysis phase

The system is analyzed during this phase of the DMAIC improvement model to find strategies for closing the performance gap between the intended target or objectives and the present performance [8]. In order to do this, the data is analyzed in this phase, and then the underlying cause of the issue is identified and comprehended through an inquiry and understand the root cause of the problem (Figure 7) [26]. The next step is to identify and rank the opportunities for improvement. The tasks conducted during the analysis stage can be carried out by utilizing particular methodologies and strategies that are often used in this stage of DMAIC [8]. In the analysis stage, methods and strategies, including process mapping, brainstorming, cause-and-effect diagrams, DOE, hypothesis testing, statistical process control (SPC) charts, and simulation, are typically employed [27]. The choice of the most successful strategies will often depend on the project's type and methodology [8].

The performance of GTPPs is influenced by the ambient conditions [28]. These conditions (Temperature, Pressure Density, and Mass flow rate, etc.) vary with time and place. Both air density, air mass flow, and power output are greatly reduced on hot days due to high temperature and low air mass flow [29]. The relationship between mass flow rate and density with ambient temperature is as follows [30].

$\begin{gathered}m=P_1 V_1 / R T_1 \\ \rho=P / \mathrm{RT} \text { and } \rho=m / \mathrm{v} \therefore P / R T \alpha ~ m / v\end{gathered}$

m: mass flow rate, $\boldsymbol{\rho}$: density, $\boldsymbol{P}$: air pressure, $\mathbf{v}$: volume, $\boldsymbol{T}$: ambient temperature, $\boldsymbol{R}$: constant of the air.

These changes significantly affect GTPP performance [27]. Therefore, the power output decreases, and SFC increases when the ambient temperature increases. Changes in the flow rate of the air mass entering a GT affect the performance of the whole system, including changes in the air pressure, humidity, and temperature, all of which affect the air density (Figure 7) [26, 27].

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Figure 7. Cause-and-effect diagram related to the lack in power output of GTPP [26, 27]

The equations of flow energy in a steady state and its derivatives. Ibrahim et al. [29] stated that every component of the GT engine functions stably together. The following equations were obtained by applying the equations of flow energy in steady state for an ideal turbine cycle to a steady flow process and adjusting it to assume that the compressor and turbine processes that zero heat addition is zero (q follows the first law of thermodynamics for an open system as the following [30]:

$\begin{gathered}h_1+\frac{v_1^2}{2}+g z_1+w=h_2+\frac{v_2^2}{2}+g z_2+q \\ \text { since, } w=q+\left(h_2-h_1\right)+\frac{\left(v_2-v_1\right)^2}{2}+\left(z_2 g-\right. \left.z_1 g\right)\end{gathered}$          (1)

$\boldsymbol{w}=\boldsymbol{h}_{\mathbf{2}}-\boldsymbol{h}_{\mathbf{1}}=\operatorname{Cpa}\left(\boldsymbol{T}_{\mathbf{2}}\right)-\operatorname{Cpa}\left(\boldsymbol{T}_{\mathbf{1}}\right)=\boldsymbol{C}_{\boldsymbol{p}}\left(\boldsymbol{T}_{\mathbf{2}}-\boldsymbol{T}_{\mathbf{1}})\right.$          (2)

Considering that all work and heat energy units will be in kJ/kg, the mass flow is constant, and the system's total energy input equals its total energy output.

The GT work net equations [30]:

Work of the compressor:

$w c=\acute{h}_2-h_1=\operatorname{Cpa}\left(\acute{T}_2-T_1\right)$          (3)

Work of the turbine:

$w t=\acute{h}_3-\acute{h}_4={Cpg}\left(\acute{T}_3-\acute{T}_4\right)$          (4)

The work net:

$\begin{gathered}{ Wnet }=w t-w c={Cpg}\left(\acute{T}_3-\acute{T}_4\right)-{Cpa}\left(\acute{T}_2-T_1\right)\end{gathered}$          (5)

The GT heat addition and efficiency equations [30]:

The additional heat:

${\text { Qin }=\acute{h}_3-{\acute{\acute{h}}_2}}=heat\ \acute{Comb.}-heat\ Com ={Cpg}\left(\acute{T}_3-\acute{T}_2\right)$          (6)

Thermal efficiency:

$\eta t h=\frac{W n e t}{Q i n}=\frac{{Cpg}\left(\acute{T}_3-\acute{T}_4\right)-{Cpa}\left(\acute{T}_2-T_1\right)}{{Cpg}\left(\acute{T}_3-\acute{T}_2\right)}$          (7)

The power output equation:

The units that follow from ignoring the flow mass in the S.S.F.E.E. equations and their derived formulas will be in kJ/kg since the flow is constant. Taking into account the mass flow, multiply it by the network to determine the power output.

$p_{{output }}=W_{{net }} \times \mathrm{m}$          (8)

The isentropic efficiency equations [30]:

One metric used to gauge the extent of energy loss in steady-flow devices is isentropic efficiency. It entails contrasting a device's real performance with the performance that would be attained for the identical inlet and exit state under idealized conditions.

$\begin{gathered}\eta C=\frac{ { Ideal\ Com.work }}{{ Actual\ Com.work }} \\ \eta C=\frac{{ma\ cpa}\left(T_2-T_1\right)}{{ma\ cpa}\left(\acute{T}_2-T_1\right)}\end{gathered}$

Compressor isentropic efficiency:

$\eta C=\frac{\left(T_2-T_1\right)}{\left(\acute{T}_2-T_1\right)}$          (9)

$\begin{aligned} \eta T & =\frac{{ Actual\ Tur.work }}{{ Ideal\ Tur.work }} \\ \eta T & =\frac{m g {cpg}\left(\acute{T}_3-\acute{T}_4\right)}{m g {cpg}\left((\acute{T}_3-T_4\right)}\end{aligned}$

Turbine isentropic efficiency:

$\eta T=\frac{\left(\acute{T}_3-(\acute{T}_4\right)}{\left(\acute{T}_3-T_4\right)}$          (10)

The calculations for actual compressor temperature ($\acute{T}_2​$) [29]:

From given Table 5:

$T_1=25^{\circ} \mathrm{C}=25+273=298 k^0$ and pressure ratio $\left(\frac{\boldsymbol{P}_{\mathbf{2}}}{\boldsymbol{P}_{\mathbf{1}}}\right) =15.8, T_2$ can be calculated is entropically: $\acute{T}_2$

$T_2=T_1\left(\frac{\boldsymbol{P}_{\mathbf{2}}}{\boldsymbol{P}_{\mathbf{1}}}\right)^{\frac{\gamma-\mathbf{1}}{\gamma}} T_2=298 \times(15.8)^{o \cdot 28}=645 k^0$

The calculation $\acute{T}_2$ using: Eq. (9),

$\begin{gathered}\left.\acute{T}_2=\left(T_2-T_1\right)+\left(\eta C \times T_1\right) / \eta C=(645-298)+0.88 \times 298\right) / 0.88 =692 . k^0\end{gathered}$

2.3.4 The calculations actual turbine outlet temperature ($\acute{T}_4$) [30]

It is necessary to assume $P_2$ = P3 and P1 = P4 since the process is isentropic (there is no loss in the pressure isentropically, the pressure entering the compressor is equal to the pressure leaving the turbine).

From given Table 5:

$T_3=\acute{T}_3=1335^{\circ} \mathrm{C}$ and pressure ratio $\left(\frac{P_3}{P_4}\right)=\left(\frac{P_2}{P_1}\right)=15.8$

$\frac{T_3}{T_4}=\left(\frac{P_2}{P_1}\right)^{\frac{\gamma-1}{\gamma}} \frac{P_3}{P_4}=\frac{P_2}{P_1}, T_3=\acute{T}_3$

$T_4$ can be calculated isentropically: $T_4=\acute{T}_3/\left(\frac{\boldsymbol{P}_{\mathbf{2}}}{\boldsymbol{P}_{\mathbf{1}}}\right)^{\frac{\boldsymbol{\gamma}-\mathbf{1}}{\boldsymbol{\gamma}}}=T_4 =(1335+273) k^0 /(15.8)^{0.23}=855 k^0$. The calculation of actual turbine outlet temperature($\acute{T}_4$) using Eq. (10):

$\begin{gathered}\acute{T}_4=\acute{T}_3-\eta T x\left(\acute{T}_3-T_4\right) \\ \acute{T}_4=1608-(0.90) \times(1608-855)=930 k^0 \\ \text { Since, } T_1=298 k^0, \acute{T}_2=692 k^0, \acute{T}_3=(1335+273) =1608 k^0, \acute{T}_4=930 k^0 \\ W c=\acute{h2}-h 1=C p a\left(\acute{T}_2-T_1\right)=1.005 \times(692-298) =396 \mathrm{~kJ} / \mathrm{kg} \\ W t=\acute{h3}-\acute{h4}=C p g\left(\acute{T}_3-\acute{T}_4\right)=1.15 \times(1608-930) =779 \mathrm{~kJ} / \mathrm{kg} \\ W n e t=w t-w c =C p g\left(\acute{T}_3-\acute{T}_4\right)-C p a\left(\acute{T}_2-T_1\right)\end{gathered}$          (11)

$W \text { net }=W t-W C=C p g\left(\acute{T}_3-\acute{T}_4\right)-C p a\left(\left(\acute{T}_2-T_1\right)=779-396=\right.\mathbf{3 8 3 ~ k J} / \mathrm{kg}$

Power output $=Wnet × m=383 \times 190=72.7 \mathrm{MW}$.

Real specific W net $=$ Power output $/$ total mass flow rate.

Real specific W net = 72.7 / 398 = 182 kJ/kg.

2.3.5 The equations of thermal efficiency above (5.6,7) [29]

At $T_1$ = 25°C thermal efficiency of the turbine.

$\begin{aligned} & \eta t h=W \text { net } / Q i=C p g\left(\acute{T}_3-\acute{T}_4\right)-C p a\left(\left(\acute{T}_2-T_1\right) / C p g\left(\acute{T}_3-\right.\right.  \left.\acute{T}_2\right) \\ & \eta t h=383 / 1.15(1608-692)=362 / 1048=36.4 \% \\ & \text { Cp air }=1.005, \mathrm{Cp} \text { gas }=1.15 \mathrm{kJ} / \mathrm{kg}\ \mathrm{k} \\ & \text { SFC }=3600 \times \mathrm{mf} / \text { power output }=3600 \times 7.75 / 72.7=0.385 \\ & \mathrm{~kg} / \mathrm{kW} . \mathrm{h} \\ & \text { HR }=\text { SFC } \times \mathrm{LHV}=0.385 \times 28.580=11.003 \mathrm{~kJ} / \mathrm{kW} . \mathrm{h}\end{aligned}$

The values of SFC and HR are shown in the Figure 8 and Figure 9.

where, SFC: Specific fuel consumption HR: Heat rate.

Emission fuel = fuel consumption × emission factor of CO₂ (11) [31].

Emission fuel = 0.385 kg/KWh × 34.260 kWh = 13.190 kg/hour

Figure 8. SFC increases with rising ambient temperatures T₁ (℃)

Figure 9. HR rises with the increase of ambient temperature T₁ (℃)

Table 5. Parameters and working results for GTPP

2.3.6 Improve phase

The enhanced step of the DMAIC seeks to explore ways to lessen and address the root cause or causes after they have been identified [26]. According to experience and common sense dictated the exists a correlation between (ambient temperature and mass flow rate of GTPP). To do this and in order to analyze the experiment’s results, a two-way analysis of variance (ANOVA) was used. ANOVA is a statistical model for comparing differences among means of more than two cases [26]. However, if there are two sources of data, the variables that need to be evaluated in this case are the mass flow rate and the ambient temperature, necessitating the use of two-way ANOVA, a statistical method for analyzing the effects of two factors [30]. Four different GTPP temperature parameters—15, 25, 35, and 50℃—were used to analyze the two aforementioned components. The goal of improving the performance of GTPP is especially in the hot session. The members of the improvement team, the production engineer and production operator in particular, had process expertise and experience that was used to determine these characteristics. Two elements (the ambient temperature and the mass flow rate) were then used in the experiment, each of which had four levels. However, by employing turbine inlet air conditioning (TIAC) to lower the surrounding temperature and raise the mass flow rate and power output, the enhancing of GTPP's efficiency and performance are enhanced while boosting electric power during the hot season [31]. The Fogging system, a proven method for increasing GT production and efficiency, particularly in hot and dry climates, will be the main focus of GT intake air-cooling (TIAC) (Figure 10) [32].

Figure 10. Schematic diagram of the fogging unit incorporates to GT [31]

By implementing a fogging system in the GTPP at an ambient air temperature of 50℃ (typical of hot summer conditions), and using Aspen HYSYS simulation software [33], enhancements in air mass flow rate, power output, and overall efficiency were demonstrated, as shown in Table 6.

Fogging system. The selection of fogging system technology is critical due to its ability to generate extremely fine water droplets at high pressure, which are introduced at the compressor’s inlet. As illustrated in Table 7, the fogging system is modeled using a combination of mixer and separator units. When hot ambient air enters the mixer, demineralized water is injected through high-pressure nozzles by a high-pressure pump, producing a fine mist or fog. The heated air mixes with the fog, increasing its humidity and achieving near-saturation before exiting the mixer. The saturated air then flows into the separator, where any unevaporated water is removed. As shown in Figure 11, residual water that fails to evaporate is discharged from the bottom of the separator [32, 33]. This process increases both the mass flow rate and density of the inlet air, resulting in higher turbine output power and reduced compressor power consumption.

Table 6. The variation of parameters of GTPP without and with the fogging system

Table 7. Operation of fogging system parameters [32]

Additionally, despite the use of fogging, the relative humidity of the air exiting the cooler is maintained below 60%. The performance of the fogging system was simulated using Aspen HYSYS software, based on real operational data collected during peak summer conditions, and the results were thoroughly evaluated.

Figure 11. Schematic fogging system [31]

The current study was conducted under conditions and limitations of steady state flow energy, ignoring the mechanical losses of GT, using methane gas only as a fuel, and the humidity was less than 60% etc. Future studies will focus on enhancing the performance of GT under different conditions, in coastal areas, and with different types of fuel by using advanced technology systems.

2.3.7 Control phase

The objective of the control phase is to manage ongoing operations and institutionalize process or product innovations in order to preserve and maintain the advantages of process improvement [34]. The processes may then be monitored using design controls to make sure the enhanced processes have stayed under control. In the case of this improvement project, the organization studied the institutionalized improvements made by including the optimum parameters for the GTPP (decreasing ambient temperature by turbine inlet air colling TIAC [35], or fogging inlet cooling, and increasing mass flow rate, power output, and thermal efficiency, especially on hot days). Figure 12 and Figure 13 are used to control and sustain the enhancement of the project for customer satisfaction. This has enabled the company to maintain the gains made [36].

Figure 12. GT thermal efficiency function of the ambient temperature

Figure 13. GT power output as a function of the ambient temperature

The improving of the performance of GTPP especially in the hot session is to reduce the problems resulting from the stoppage or decline the production of the energy which causes great material losses in industry and requirements of the citizen, on the contrary the use of the technological and economic cooling systems to enhance the capacity of GTPP is the object of current study also saved economic and decline CO₂ emissions.

Production capacity and GTPP efficiency are significantly reduced in regions that get high air temperatures during hot seasons. Increases in ambient temperature cause the air mass's flow rate to drop, which lowers the energy generated by GTs. In order to identify the issue and increase the efficiency and power output of a GTPP during hot seasons, as well as to save time, environment, and cost, the Six Sigma DMAIC methodology, which consists of five phases (Define, Measure, Analysis, Improve, and Control), was applied in this case study. The fogging cooling system was chosen to enhance GTPP's operating parameters, efficiency, and power production since it is easier to use and less expensive to maintain. Aspen HYSYS software was utilized to model and evaluate actual data that was gathered from the plant throughout the hot summer. The simulation's outcomes were determined for a GTPP both without and with air-cooling equipment. The data indicate that the fogging cooling system's incoming air temperature decreased. The findings demonstrated that the mass flow rate and turbine capacity decrease with increasing air temperature, resulting in a shortage of electric power from GTPP to the consumers. Furthermore, a rise in air temperature causes thermal efficiency to fall, as well as a rise in SFC, HR, and emissions of CO₂. Choosing the best method or approach and resources after comparing them with other methods to help and preserve the products (product electricity). Eliminate faults, cut costs, and shorten cycle times in GTPP. As a consequence, this study proved that Six Sigma with the DMAIC approach and tools is highly successful, so many firms have used it to gain significant benefits. One of the six sigma common approaches is DMAIC to problem-solving.