Elsevier

Energy

Volume 118, 1 January 2017, Pages 399-413
Energy

The optimal design and 4E analysis of double pressure HRSG utilizing steam injection for Damavand power plant

https://doi.org/10.1016/j.energy.2016.12.064Get rights and content

Highlights

  • 4E analysis of CCPP utilizing steam injection.

  • HRSG design for steam injection condition.

  • HRSG optimization for power loss compensation in CCPP in steam injection condition.

  • Sensitivity analysis of combined cycle by steam injection flow rate.

  • Investigation of the environmental effects of steam injection into the CC.

Abstract

Energy, Exergy, Exergoeconomic and Environmental (4E) analysis of combined cycle power plant utilizing the method of steam injection into the combustion chamber is performed in the current study. Steam is supplied from low pressure (LP) side of the heat recovery steam generator (HRSG). According to genetic algorithm's results, the steam injection elevates the total combined cycle power by 2 MW while lowers the design costs considerably at optimum state. Some parameters restrict the steam injection usage within combined cycles that could be noted as Gas turbine power decline; Gas turbine loss increase due to pressure loss; HRSG's manufacturing costs growth; opposition between high and low pressure flow rate variations; NOX and CO emission costs dependency on optimum conditions. This paper discusses the above limitations for implementation of such technology in combined cycles. At the optimum point, Exergy and thermal efficiencies values increased from 42% and 47.6% to 47.28% and 48.94% respectively. Optimum pinch and approach temperatures for HP and LP sides were computed to be 35, 20, 17 and 37.5, respectively. Also, the optimum value of X parameter is 23.28%; X is defined as the ratio of steam to the inlet air flow rate divided by the other fraction which is the ratio of fuel to input air flow rate while there is no steam injection into the combustion chamber.

Introduction

Most researches about steam injection into the combustion chamber are experimental and numerical and it could be claimed that there are few researches on 4E ((1) Energy, (2) Exergy, (3) Exergo-economic and (4) Environmental) analysis of combined cycles which utilize steam injection. 4E assessment of non-steam injection combined cycle was performed by Ahmadi et al. [1] and is currently being developed. Quality improvement of steam turbine output vapor is recently analyzed by Ganjehkaviri et al. [2]. The optimization process in this work is not related to a particular type of steam turbine. Some parameters such as compressor pressure ratio, compressor, and turbine isentropic efficiencies and gas turbine inlet temperature are included in their calculation. Their triple-objective optimization led to the determination of 15 parameters of the combined cycle operation.

Normally, HRGS is designed in non-steam injection condition for combined cycles. There is no investigation on the influences of HRSG new configuration and design on the combined cycle operation in the case the combustion chamber is steam-injected. The required amendments in HRSG to have the best performance for combined cycle in this situation are also uncharted. In this area, many articles could be exemplified which have paid attention to HRSG design in non-steam injection condition. Casarosa and Franco [3] are from HRSG design pioneers. From other researchers who have contributed in HRSG design and optimization, Pasha and Sanjeev [4], Esmaieli et al. [5], Behbahaninia et al. [6] and Ganjehkaviri et al. [7] could be mentioned. To determine the optimum fuel flow rate in duct burners for Damavand combined cycle power plant, Mokhtari et al. [8] modeled HRSG in order to decrease gas turbine losses in various temperatures. In 2015, Mokhtari et al. [9] designed HRSG as a part of their investigations on solar water desalination to select the appropriate working fluid in power generation section of the cycle.

There are several configurations of Humidified Gas Turbines (HGT) cycles including the Steam-Injected Gas Turbine (SIGT), Humid Air Turbine (HAT) and Evaporative Gas Turbine (EvGT) [10]. In an SIGT system, steam is generated using a HRSG, injected into the gas turbine combustion chamber and utilized as working fluid with air [11]. Recently, several studies have examined injected-steam gas turbines [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Paepe and Dick [14] analyzed the water recovery in steam injected gas turbines in terms of the technology and economy. Nishida et al. [11] analyzed the performance characteristics of two types of regenerative steam injection gas turbines and compared their performance with characteristics associated with simple, regenerative, water injection and steam injected gas turbine cycles. They presented that the steam injection configuration can be applied in a flexible heat-and-power cogeneration system. Wang and Lior [10] investigated the performance of an SIGT-based combined system with thermal desalination systems. Their analysis improved authors' understanding of the combined SIGT power and water desalination process and opened their ways to improve and optimize it.

Hassan Kayhan et al. [21], investigated the steam injection into the combustion chamber of a gas turbine cycle. In their study the amount of steam injection, thermal efficiency and environmental impacts (production of NOx and CO) were investigated. There is no consideration of the pressure drop, HRSG design and economic analysis in their research. From other valuable works, Tufalo and et al. [22] study could be mentioned. They dedicated their researches to obtain mathematical equations in the case in which steam is injected into the combustion chamber. The installed control system results in their investigated gas turbine (LM2500-PE0) led to a semi-experimental relation for emission analysis. The system adjusted its coefficients with experimental data in order to predict CO and NOx amounts.

One of the problems of combined cycles and power generation cycles which operate with gas turbine is their unexpected power decrease. This decline is caused by site environmental conditions which can also disrupt the preparation of other units. For a power plant, two important points are required to be considered:

  • 1.

    Supply the network-base load by maximum nominal power.

  • 2.

    Power generation standby regarding the network load variations.

Governmental power plants in Iran profit from two responsibilities taken by ministry of energy:

  • 1.

    Energy conversion

  • 2.

    Circuit existence standby

In this paper, circuit existence standby of the combined cycle is remarked. The only controlling system in the combined cycle is combustion chamber fuel rate. The usage of duct burner increases the fuel consumption which causes separation of the upstream cycle (top cycle) from the downstream (bottom cycle) one. The fin tips temperature in heat exchangers especially superheaters should not exceed 600 °C. As a result, this system can be used in limited circumstances even though this approach reduces the cycle efficiency. As noted previously, several investigations considered this method, which [1], [8] and [23], [24], [25] can be referred.

Damavand power plant is located near Tehran, at the elevation of 1300 m above sea level with the annual mean temperature and humidity of 16.6 °C and 52%, respectively. It comprises 12 gas turbines (V94.2) with 160 MW nominal power for each one. The modeling data is presented in Table 1.

In this study, only one block of this power plant including a gas turbine, a steam turbine and a HRSG is studied (Fig. 1). Average annual temperature and ISO-temperature (15 °C) don not differ sensibly. Compressor inlet volumetric flow rate is assumed to be fixed while mass flow rate varies with temperature and pressure. Therefore, the decrease in the pressure and also air mass flow rates are the reason for power decline.

The plant is assumed to operate in two states of having and lacking duct burners. Owing to the fact that the power is better to be improved without additional fuel consumption, the process is modeled without utilizing duct burners.

Table 1, indicates three main air properties deflection from the ISO mode. For example, moisture content change represents the variation from 60% to 52% while temperature and pressure of ISO state remain constant. It well helps to trace the related effects of each parameter variations on gas turbine cycle parameters.

As can be observed, cycle working pressure change is a factor that heavily influences the cycle operational parameters. As a result, one method is required to be chosen to increase the power generation in the downstream cycle.

One way is cooling compressor inlet air which is reasonable due to the high temperature difference in ISO mode. However, when compared to Table 1, it seems as an inappropriate method because of the low difference between yearly mean temperature and ISO mode temperature.

One of the constraints which lead to power production dissipations in gas turbine is its inlet temperature which should not exceed defined boundaries [1], [26], [27]. Since cycle operation increases CO2 and environmental negative effects, steam injection into combustion chamber is proposed for power generation growth and environmental negative effects diminution. Current research, assesses the cycle from 4E points of view.

In this study, it can be realized that the steam injection into the combustion chamber increases the gas side pressure drop within HRSG. Hence, the purpose of this paper is to optimally design HRSG configuration regarding the application of steam injection.

Fig. 1 illustrates schematically how HRSG and gas turbine parameters affect the gas turbine power generation. Due to the sensitive dependence of pressure drop to HRSG's configuration, HRSG should be designed how that the minimum power loss occurs in the gas turbine. Thus, it cannot be definitely reported that whether the increase in steam injection leads to net power decrease or not. This phenomenon depends on HRSG design which is going to be addressed in this study.

The equation which is introduced in Fig. 2, demonstrates the power loss relation. Supplementary descriptions about it can be met in the HRSG modeling section; but the dependence of power dissipations to steam injection should be noted here. There is a direct relationship between gas turbine exhaust mass flow rate (m˙g) and power loss. The more exhaust mass flow rate due to steam injection, the more pressure drop and power loss. It seems to be an unavoidable process. Hence, to decrease the power loss, some improvements should be executed for the systems in which power dissipation parameters may play an important role. According to Fig. 2, HRSG and gas turbine are influential parts of the cycle. Thus, HRSG design and configuration and associated power dissipations during steam injection process are analyzed in this article.

The scenario here attends these objectives: increasing the upstream generated power by changing pinch and approach temperatures (steam production increase) [5], [8], [28]; gas turbine power increment; reduction of the environmental costs and greenhouse gases emission; reduction of HRSG construction, exploitation and exergy destruction costs.

Gas turbine power increase is performed by: (1) Reduction of the gas side pressure drop in HRSG and thus the power dissipation [6]; (2) Increasing the gas turbine inlet flow rate compared to the flow rate of the case in which there is no steam injection at the same temperature.

In this study, the following assumptions and limitations are considered:

  • 1.

    Air and combustion gaseous products properties are considered temperature-variant.

  • 2.

    The moister of the site is 52% and it is supposed to be 60% at ISO state [29].

  • 3.

    Steam turbine outlet quality is 88% [9], [29].

  • 4.

    Gas turbine inlet temperature is assumed to be constant and equal to 1060 °C [30].

  • 5.

    In order to prevent burning, fin density and height in superheaters are assumed to be lower than ones of other heat exchangers [31].

  • 6.

    Tube arrangements are supposed to be triangular (based on Damavand power plant information)

  • 7.

    The calculation of the captured energy in each tube row is performed differently according to ref. [32].

  • 8.

    In order to calculate the fin tip temperatures, the fins are assumed longitudinal and after computing the base temperature, the fin tip temperatures could be obtained by using references [32], [33], [34].

  • 9.

    Steam pressure and temperature in HP and LP sides of HRSG is respectively 90 bar, 520 °C and 8.5 bar, 233 °C. Pinch and approach temperatures and condenser pressure are acquired from Ref. [39]. For gas side, some parameters like gas mass flow rate, HRSG inlet temperature and flue gas combination should be known.

  • 10.

    The amount of blow down (BD) in HRSG is assumed to be 5% [29], [32].

  • 11.

    Because of the presence of hydrogen sulphide gas (6.5 ppm) in the fuel, the minimum HRSG outlet gas temperature is assumed to be 110 °C.

The following items are outstanding novelties in the current study:

  • Exergy, exergoeconomic and environmental analysis of combined cycle considering steam injection

  • HRSG design considering steam injection

  • HRSG optimization toward lowering power dissipation in combined cycle using steam injection

  • Investigation of the variation of the related environmental effects by steam injection into the combustion chamber

  • HRSG optimization and Pareto curve plotting for the sake of net power increase as well as costs reduction

  • Sensitivity analysis of injected steam flow rate

  • Thermal and exergy efficiency study by importing them as weight coefficients in objective functions

Section snippets

Energy analysis (1E)

Atmospheric pressure is expressed considering elevation from sea level (H) according to the following relation:Patm=(760×(1-226×107×H)5.25)735where H is in meter and Patm is in bar.

Exergy and exergo-economic analysis (2nd E & 3rd E)

In this paper, exergy at each point is a combination of physical and chemical exergy which is calculated according to the following equation [39], [40], [41]:E˙x=E˙xPH+E˙xCH

Exergy destruction rate (E˙xD) for each component is calculated according to the following equation (‘fuel–product–loss’ (F–P–L)) [35], [42]:E˙xF=E˙xP+E˙xD+E˙xL

By defining the following parameter, the ratio of exergy destruction to the total exergy destruction could be obtained [35]:yD=E˙xDE˙xD,Total

Exergy efficiency of the

NOx and CO emissions

Internal combustion engines are characterized by a remarkable development during past decades; but they constitute a very important source of polluting gas.

The nitrogen oxides (NOX) have negative effects on human and environment health. Many computerized simulations have been developed to elaborate a calculation model close to reality because of the complexity of this pollutants formation process. NOX is formed at high temperature by the dissociation of O2 molecules and the reaction of the

Objective functions selection

HRSG parameters play an important role in power dissipations and steam generation. Those parameters along with X are considered as decision parameters in Table 2. Optimization is done utilizing genetic algorithm (GA). More description of GA application method is referred to [1], [5], [6], [8], [9], [52].

The followings are the main targets in the optimization:

  • Combined cycle power increase without any extra fuel consumption.

W˙net(max)=W˙GT,realW˙ACW˙BFP+W˙ST,Where:W˙GT,real=W˙GTEq.(18)ΔW˙Eq.(42

Validation process

The validation details for HRSG design at ISO condition by the obtained information from Damavand combined cycle power plant are presented in Table 3.

References [8], [52], which are prepared by the authors of this study, could be referred for validation. The same codes and validations are utilized in current study. Mokhtari et al. [8], [52] have designed and validated a double-pressure heat regenerative boiler in a combined cycle. In Ref. [8], a precise modeling for gas turbine and its

Environmental conditions and steam injection

Table 4 exhibits the modeling results in steam injection and non-injection conditions for site and ISO modes. The HRSG design parameters which are known from Table 2, are presumed to be fixed and equivalent to Damavand power plant ones. Based on these assumptions, HRSG is modeled. So, the required thermal surfaces of each heat exchanger and thus the entire costs of HRSG are determined.

As it can be seen, compared to ISO mode values, the steam turbine power generation decreases dramatically at

Conclusions

  • A)

    Steam injection reduces the amount of exergy destruction within Damavand combined cycle such that it declines from 254.52 MW to 222.12 MW.

  • B)

    Greenhouse emission decreases by utilizing steam injection into combustion chamber. CO2 emission costs falls from 0.1215 (cent/kWh) to 0.1178 (cent/kWh) in optimized mode.

  • C)

    Optimized design of HRSG leads to the gas turbine power dissipations decrement and hence the net combined cycle power increment. The pressure drop which is a consequence of steam injection

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