Elsevier

Energy

Volume 36, Issue 2, February 2011, Pages 1196-1205
Energy

Effect of various inlet air cooling methods on gas turbine performance

https://doi.org/10.1016/j.energy.2010.11.027Get rights and content

Abstract

Turbine air inlet cooling is one of many available commercial methods to improve the efficiency of an existing gas turbine. The method has various configurations which could be utilized for almost all installed gas turbines. This paper presents a comparison between two commons and one novel inlet air cooling method using turbo-expanders to improve performance of a gas turbine located at the Khangiran refinery in Iran. These methods have been applied to one of the refinery gas turbines located at the Khangiran refinery in Iran. Two common air cooling methods use evaporative media or a mechanical chiller. The idea behind the novel method is to utilize the potential cooling and power capacity of the refinery natural gas pressure drop station by replacing throttling valves with a turbo-expander. The study is part of a comprehensive program with the goal of enhancing gas turbine performance at the Khangiran gas refinery. Based on the results, it is found that using turbo-expanders is the most economically feasible option and so is recommended to be utilized for improving gas turbine performance at the Khangiran refinery.

Research highlights

► 3 inlet air cooling methods to improve performance of a gas turbine are studied. ► Evaporative media, EM, mechanical chiller, MC, and turbo-expander, TE, are the methods. ► The idea of TE is to utilize cooling and power capacity of the natural gas pressure drop station by a TE. ► The efficiency enhancement in MC, EM and TE systems are 5%, 3% and 4% respectively. ► The payback period for TE method is lower than the other methods.

Introduction

Gas turbines are widely used for producing electricity, operating airplanes and for various industrial applications such as in refineries and petrochemical plants. The basic gas turbine cycle has low thermal efficiency so it is important to look for improved gas turbine based cycles.

It is well known that ambient temperature, humidity and pressure are important factors in gas turbine performance. Thermodynamic analyses exposed that thermal efficiency and specific output decrease with an increase of humidity and ambient temperature as shown by Tsujikawa and Sawada [1]. Bird and Grabe [2] have studied the effects of humidity on gas turbine performance and formulated correlations for expressing relations between humidity and gas turbine performance. El-Hadik [3] carried out a parametric study on the effects of ambient temperature, pressure, humidity and turbine inlet temperature on power and thermal efficiency. He concluded that the ambient temperature has the greatest effect on gas turbine performance, which increases with the turbine inlet temperature and pressure ratio. Reductions of power and efficiency due to a 1 K temperature growth were found to be around 0.6 and 0.18% respectively.

Jonsson and Yan [4] comprehensively reviewed the literature on research and development on humidified gas turbines and identified the cycles with the largest potential for the future. Remaining development work required for implementing the various humidified gas turbine cycles was also discussed. They concluded that the interest in inlet air cooling systems for gas turbines has increased in recent years due to the increasing need for power to a low specific investment cost, especially during the summer when the ambient temperature is high. They classified the available inlet air cooling systems into following groups:

  • 1.

    Evaporative coolers. Water is distributed over media (such as wood wool fibers, or corrugated paper impregnated with anti-rot salts) through which the inlet air passes to be cooled and humidified.

  • 2.

    Spray inlet coolers or fogging systems. Water is injected into the air through nozzles and creates a fog of small water droplets, 5–20 μm in diameter. The systems are divided into two subgroups as, a) Saturated systems, where the air is saturated before the compressor and, b) Overspray systems, where more water than is needed for saturation is injected. Water droplets enter the compressor, evaporate and cool the air.

  • 3.

    Mechanical vapor compression or absorption chillers, where a heat exchanger cools the inlet air. Chillers can increase the gas turbine power output by 15–20% and the efficiency by 1–2% (i.e. if gas turbine exhaust gas energy is recovered). A chiller can cool the inlet air regardless of the ambient conditions; however, the specific investment cost is much higher than for media and spray coolers.

Johnke and Mast [5] reported that a media cooler can increase the relative humidity of the inlet air to about 90%, thus increasing the power output by 5–10% and the efficiency by 1.5–2.5% and decreasing the NOx emissions by 10% for a conventional diffusion combustor. Saturated spray systems provide similar performance improvements as media coolers, while overspray systems can increase the power output by 10–20%, the efficiency by 1.5–3% and decrease the NOx emissions by 20–40% for a conventional diffusion combustor.

Ameri and Hejazi [6] have carried out feasibility of installing an absorption chiller system to cool the inlet air to the Chabahar power plant gas turbines (Six frame-5 gas turbines with the rated power of 16.60 MW). It has been shown that the average power output can be increased by as much as 11.3%. The maximum power augmentation is around 2.4 MW. The electricity production is increased by 14,000 MW h per year. The economical analysis has shown that the internal rate of return (ROR) is 23.4% and the payback period is 4.2 years.

Kakaras et al. [7] have presented a computer simulation of the integration of an innovative absorption chiller technology for reducing the intake air temperature in gas turbine plants. Following a description of the air cooling system, the simulation results for two test cases were presented: a simple cycle gas turbine and a combined cycle plant. They concluded that the effect of ambient air temperature variation results in a large penalty in the plant’s performance for high ambient temperatures. The results from the integration of an evaporative cooler and of the air cooling system under consideration showed the gain in power output and efficiency. The absorption chiller cooling system demonstrated a higher gain in power output and efficiency than evaporative cooling for a simple cycle gas turbine, independent of ambient air temperature. The results for the combined cycle case also demonstrated that the absorption chiller can considerably increase the power output, although there is an efficiency reduction.

De Lucia et al. [8] reported that evaporative inlet cooling is economical and simple, but suitable for only dry hot climates. He concluded that evaporative inlet cooling could enhance power by 2–4% depending on the weather. Bassily [9] presented the effects of the turbine’s inlet temperature, ambient temperature, and relative humidity on the performance of the recuperated gas turbine cycle with evaporative inlet cooling and an intercooler reheat regenerative gas turbine cycle with indirect evaporative inlet cooling. His results showed that evaporative cooling of the inlet air could boost the efficiency by up to 3.2% and that evaporative after cooling could increase the power by up to about 110% and cycle efficiency by up to 16%.

Ait-Ali [10] presented the concept of inlet air refrigeration to boost the power output from the gas turbine. The absorption chiller works on the principle of vapor absorption refrigeration cycle. The main advantage of this chiller lies in the fact that the inlet air can be cooled down to a specific temperature for a wide range of ambient air temperatures and, therefore the power output of a gas turbine remains more or less constant, independent of ambient air conditions. The low grade exhaust energy can be used to drive the chiller. The chilled water (≈5 °C), produced by the absorption system, is passed through the inlet air cooler, which is an indirect type air to water heat exchanger.

Gareta et al. [11] has proposed a methodology for the economic evaluation of the cooling systems and to analyse and compare the different alternatives, including the time-dependent variables involved in the technical and economical system behaviour. These variables include: ambient temperature, relative humidity, hourly electricity prices and natural gas tariff. The proposed methodology takes into account the performance simulation at the combined cycle, the air cooling system, ambient conditions, equipment maintenance and investment, and electricity and natural gas prices, in order to obtain cash flows and other relevant economical variables that maximise the profit of the integrated system. They concluded that the method offers more straightforward information for sizing and selecting the cooling equipment technology.

Boonnasa et al. [12] studied how to improve the capacity of the combined cycle power plant by lowering intake air temperature to around 15 °C and 100% RH before entering the air compressor of a gas turbine in Bangkok, Thailand. They proposed a steam absorption chiller to cool intake air to the desired temperature level. They concluded that, it could increase the power output of a gas turbine by about 10.6% and the combined cycle power plant by around 6.24% annually. In economic analysis, the payback period was calculated to be about 3.81 years and internal rate of return 40%.

Zadpoor and Golshan [13] have focused on power augmentation of a typical gas turbine cycle by using a desiccant-based evaporative cooling system. This technique requires a desiccant-based dehumidifying process be used to direct the air through an evaporative cooler, which could be either media-based or spray type. They compared performance improvement achieved by this technique and those of other evaporative cooling systems in different climatic conditions and concluded that the proposed technique, at least for hot and humid climates, is more effective than other evaporative cooling techniques.

There are various places in the natural gas industry where high pressure gas has to be dropped to a lower value. During this pressure reduction, valuable pressure exergy of the gas is wasted. Farzaneh-Gord and Magrebi [14] studied exergy destruction in Iran’s natural gas fields. They concluded that one could generate 4200 MW of electricity from this pressure exergy. Farzaneh-Gord et al. [15] studied methods of using the pressure exergy of natural gas in the Bandar Abbas (Iran) refinery pressure drop station. They investigated the effect of gas pre-heating on the amount of electricity generation. They also proposed some thermodynamic systems to produce and use refrigeration. Farzaneh-Gord et al. [16] investigated the amount of energy destruction in Iran’s pipeline network. Based on a comprehensive program, they concluded that one can generate 2.27 W for each cubic meter/day of natural gas flow through the network. They also concluded the total amount of obtainable energy is around 762 MW.

There is a pressure reduction point in most Iranian refineries and power plants, including the Khangiran gas refinery, which is under investigation in this study. The natural gas which is supplied to the refinery comes from a high pressure natural gas transmission pipeline and passes through a metering and reducing station. Farzaneh-Gord and Deymi [17] have proposed a system to capture this energy. The proposed system consist of preheat section and a turbo-expander, as the natural gas passes through the turbo-expander, its temperature drops and a considerable cooling capacity is created. Then, Farzaneh-Gord and Deymi [18] have proposed a system to benefit from the cooling capacity aiming to improve the gas turbine performance.

Farzaneh-Gord et al. [19] proposed another system just to utilize the available cooling capacity in the Khangiran refinery pressure drop station in order to cool inlet air and to improve performance of one of the Khangiran refinery gas turbines. They concluded that the gas turbine inlet air temperature was reduced by 4–25 °C and the performance could be improved in a range of 1%–3.5% in one gas turbine for almost 9 months.

In this study, the proposed method of Farzaneh-Gord et al. [18] for improving the gas turbine performance has been compared with 2 other standard inlet air cooling methods technically and financially. The comparison enables selection of the most beneficial system for inlet air cooling and gas turbine enhancement.

Section snippets

Chemical composition of natural gas

Natural gas composition (mixture) varies with location, climate and other factors. The gas is refined before flowing into the pipelines. Table 1 [20] details the chemical composition of the refined gas, as presented in the Khangiran refinery official site. It can be seen that the most components comprise a very low fraction of the gas. Natural gas is about 99% methane. Thus, for the sake of simplicity, the natural gas consumed in the Khangiran gas refinery can be modelled as pure methane.

The studied inlet air cooling methods

As discussed in the Introduction, heat should be removed by using a heat exchanger and available coolant stream or by evaporating water droplets to reduce the compressor inlet air temperature. Three methods are studied and compared with the novel inlet cooling method which was discussed in Farzaneh-Gord et al. [18].

Calculation procedure

The calculation procedure in this study was based on the actual recorded site values in the Khangiran refinery. In the refinery, temperature and pressure have been recorded every 2 h and natural gas mass flow rate through the pressure drop station recoded on daily bases. Fig. 3, Fig. 4 and Table 2 show average daily recorded values on 21/6/2007.

In the refinery, m˙air is not recorded on a daily basis, however it was measured during an energy audit project in 2004. To be able to calculate m˙air,

Results and discussions

It would be instructive to investigate the effect of the ambient temperature on the Khangiran gas turbines thermal efficiency. Fig. 6 shows the effect the ambient temperature on the thermal efficiency the Khangiran gas turbine. It is seen that the cycle thermal efficiency varies from as low as 7%–13.5% when the temperature drops from 46 °C to 3 °C. It could be concluded that the thermal efficiency is generally low.

Fig. 7 shows the daily ambient relative humidity. Note from this figure, the

Cost analysis

To evaluate the cost effectiveness of the studied cooling methods in order to see whether the benefits outweigh the costs, a cost analysis has been carried out in this section.

A cost benefit analysis is carried out for the mechanical chiller, evaporative media, and turbo-expander systems. In this method, additional revenues are calculated as a result of added electricity production in MW h per year. The revenue calculation is based on the current electricity price in Iran which is 6 Cents/kW h.

Conclusion

There are many places in the natural gas industry where high pressure gas has to be reduced to a lower value including pressure reduction points in most Iranian refineries and power plants. The Khangiran gas refinery which is under investigation in this study receives high pressure natural gas from several transmission pipelines. The gas passes through a metering and reducing station, sometimes referred to as the pressure drop station. During this pressure reduction, a valuable of pressure

Acknowledgments

This study is part of a comprehensive program aimed to enhance gas turbines performance of the Khangiran refinery and has been supported by the refinery.

Nomenclature

cp
constant pressure specific heat (kJ/kg K)
E
sensation coefficient
h
specific enthalpy (kJ/kg)
m˙
mass flow rate (kg/s)
P
pressure (MPa or kPa)
Pe
generator output power(MW)
Q˙in
heat transfer rate (kW)
Q˙mech
cooling capacity to the mechanical chiller (kW)
qactual
actual heat transfer in the heat exchanger (kJ)
qmax
maximum possible heat transfer in the heat exchanger (kJ)
RH
relative humidity
s
specific entropy (kJ/kg K)
T
temperature (K or °C)
W
actual work (kJ/kg)
W˙
actual work rate (kW or MW)
ω
humidity ratio
C6+
all

References (26)

  • J. Bird et al.

    Humidity effects on gas turbine performance

    ASME Paper

    (1991)
  • EI-Hadik

    The impact of atmospheric conditions on gas turbine performance

    Journal of Engineering for Gas Turbines and Power

    (1993)
  • T. Johnke et al.

    Power boosters—technologies to enhance gas turbine power output on demand

    Siemens Power Journal Online

    (May 2002)
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