A review: Techniques and Processes for Performance Improvement of Gas and Steam Turbine Power Plants

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008 Certified Journal, Volume 4, Issue 1, January 2014)

85

A review: Techniques and Processes for Performance

Improvement of Gas and Steam Turbine Power Plants

Vinod Kumar Singoria

1

, Samsher

2

1

Research Scholar, 2Professor, Department of Mechanical Engineering , Delhi Technological University (DTU), (Formerly Delhi College of Engineering)Bawana Road, Delhi –India-110042

Abstract— In this paper the review of techniques and processes for performance improvement of power plants (using Gas and Steam turbine) are presented along with the of results of computational studies for finding Local Loss Coefficients for pitch wise position for the turbines and compressors cascades using Computational Fluid Dynamics (CFD) commercial softwares, Gambit 2.4.6® and FLUENT 6.2.16® for creating geometry and solving the governing equations respectively. The processes for performance improvement of steam turbine power plants include regeneration & reheating separately & combining both. The net output of work is more with reheat compared to Ideal cycle {without Reheat}. Both regeneration & reheating are also combined in simple cycle plant to

increase thermal efficiency (



th_{) and output respectively.}

The other processes which may be useful for the performance improvement of all type of power are increasing turbine inlet temperature, using better quality of fuel & new material (which can withstand high temperature). In order to improve performance of gas turbine plants intercooling between various stages of compressor are also made functional. The provision of combining steam and gas turbine power plant is made so

that



th_{of simple cycle plant of gas turbine plant may}

increase. Such arrangement is termed as heat recovery steam generation (HRSG). Last but not the least, power plant engineers reduce the losses by increasing aerodynamic efficiency of the power plant in order to get higher overall efficiency increased. Efficiency loss occurs when erosion, deposition or corrosion results in increased surface roughness of blades of compressor or turbine blades. The Authors found pitch wise loss coefficients to evaluate integrated mass averaged loss coefficient for various span position from bottom end wall till complete blade height for each of cascade of turbines and compressor. Based upon pitch wise Local Loss Coefficients, Mass averaged total loss coefficient for every span position is calculated. In order to compute effect on total, profile and secondary losses for particular magnitude of roughness over entire blade height for the given cascades, the flow area of the section of the cascade and representing height for each of Mass averaged total loss coefficient over span wise direction are taken into consideration. Thus, loss coefficient over pitch & span wise direction are obtained by doing double integration. It is observed that the total loss increases as roughness increases on the same surfaces for both cascades. The total losses as well, profile loss are minimum for smooth cascade. These losses increased with the increase in roughness both for turbine & compressor Cascades.

Keywords-- Cascade, Energy Loss, Secondary Flow Loss,

Profile Loss, Roughness.

I. INTRODUCTION

In the power generation industry gas turbines and steam turbines are widely used for generating power. The efficiency of a turbine is largely dependent on its aerodynamic performance. Hence, the design of blade profiles for nozzles and rotors are continuously improved over the decades to achieve better overall efficiency for the turbine.

Among the various energy Converting devices, gas turbines are extensively used as prime movers as these use fossil fuels. Gas turbines are also suited for Jet propulsion

Schematic diagram of Gas Turbine Power Plant is shown in Figure 1.

1.1 Classification of Gas Turbines: On the basis of the path of working substance, gas turbines are classified as

I.Open Cycle Gas Turbines II.Closed Cycle Gas Turbines III.Semi Closed Cycle Gas Turbines

1.2 Cycle: A series of thermodynamic processes during which the working fluid can be made to undergo changes involving energy transition and is subsequently returned to its original state.

The Joule or Brayton cycle is the most idealized Cycle for the Simple gas turbine Power plant represented by Open or Closed Cycle Gas Turbines.

1.3 Joule or Brayton cycle: Figure 2 & 3 show various processes in Joule or Brayton cycle on P -V & T-S diagram respectively Various processes used in Joule‟s cycle are given below :

1-2 Work In put by compressor 2-3 Heat addition at constant pressure

3- 4 Isentropic Expansion producing useful work, in turn generator Shaft coupled with turbine shaft is rotated and electricity is generated.

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International Journal of Emerging Technology and Advanced Engineering

86 Applying the fist law of thermodynamics i.e.

q

w

  



Net heat interaction in the cycle = heat supplied – heat rejected

=

C

P



T

3



T

2





T



4



T

1



















3 4 P 3 4

2 1

P 2 1

P 3 4 P 2 1

The turbine work h

h

C

T

The Compressor work h

h

C

T

Net work done in ideal Joule‟s cycle is given by :

Net Work

Turbine work

Compressor work

C

T

C

T





















Efficiency of Joule or Brayton cycle

Efficiency (



th_{)= Net Work / Heat supplied}

th















P 3 4 P 2 1

P 3 2

C

T

C

T

C

T













43 21



T T

1 T

T







assuming P2/ p1 = r p

) 1 (

r

1

 







_

p th

(1)

Equation 1 clearly depicts that



th_{increases as r}

p

increases. The



th _{as shown by Eqn .1 is theoretical and}

for actual expansion in gas turbine the relation needs to be modified.

1.4 Means for improving the efficiency and the Specific output: The improvement in efficiency and the Specific output of simple cycle plant may be done by various means.

Both regeneration & reheating are also combined in Simple cycle plant to increase



th_{and output}

respectively.

1.4.1Regeneration

th



regeneration is more than



th_{of simple cycle}

plant as compressed air reaches to Combustion chamber at comparatively higher temperature utilizing heat of exhaust from last Stage of turbine. Thus



th_{of Simple}

steam turbine Power plant with regeneration is more than

th



of simple cycle plant

1.4.2Reheating:

For steam turbine power plant, reheating is applied to increase output. In reheating the temperature which has lowered due to expansion in brought back to such a limit so that expansion in next Stage can lead to more output.

The net work is more with reheat compared to Ideal cycle {without Reheat}.

Methods which are useful for all types of power plant include increasing turbine inlet temperature, using better quality of fuel & new material (which can withstand high temperature)

1.4.3Reducing compressor work

Inter cooling: Inter cooling the air between Compresses Stages Reduces compressor input.

Reducing compressor Inlet temperature: It is not desirable because by reducing compressor Inlet temperature r p decreases thus reduces compressor input.

1.4.3Combined steam and gas turbine power plant:

A heat recovery steam generation Plant is combined with simple cycle plant to increase



th

. These plants have low operating and maintenance costs. They also have the advantage of long-term fuel price stability, fuel flexibility and low emissions. These plants can be located close to the power-user reducing transmission costs and increasing reliability. Studies have identified combined cycles to be the most economic of available power generating methods. A shake-up in the electricity market is forecasted and the competitive edge of combined cycle plants provides them with a promising future.

1.5 Factors affecting Efficiency of gas turbines

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International Journal of Emerging Technology and Advanced Engineering

87

1.5.1 Compressor

Compressor fouling is unavoidable in more or less all conditions and ingested air would cause compressor fouling. Its intensity is subject to ambient conditions for the particular area. In general, fouling due to ingesting of contaminated air may cause the deposition of contaminants which may cause blade profile deterioration; and also blade erosion may occur if large air borne particles above 5 microns in size are allowed through the air filtration system. The most obvious sign of compressor fouling is a loss in the compressor discharge pressure. Among the most common types of contaminants are dirt or soil, sand, sea shells, insects, salt and oil.

1.5.2 Turbine

In the turbine, the source of deposits on hot gas path components come from the additive and ash content in fuel. The low grade liquid fuels such as the heavy blended distillates and residual fuel oil have significant ash bearing components. The contaminants can be sodium, potassium, calcium, vanadium and traces of other metals. Due to the non-availability of natural gas and distillate, user usually has no choice but to base its power generation using the residual/ crude oil.

The first problem which has to be addressed to when burning crude oil is that of the hot corrosion of turbine super-alloys due to the entrainment of certain trace metals, particularly Vanadium. The elimination of hot corrosion manifests itself in proper fuel treatment which involves the removal of these contaminants if possible, or chemical inhibition with additives.

Nonetheless vanadium exists as an oil soluble contaminant and cannot be removed by washing. The corrosive effects of vanadium are neutralized by the addition of a magnesium base compound as an inhibitor in the fuel.

1.5.3 Corrosion

Corrosion is the loss of material caused by chemical reaction between machine components and contaminants which can enter the gas turbine through the gas stream, fuel system or water/steam injection system.

1.5.4 Erosion

Erosion is the abrasive removal of material by hard particles suspended in the gas stream. Particles causing erosion are normally 10 microns or larger in diameter. Particles with diameters between 5 and 10 microns fall in a transition zone between fouling and erosion. Erosion damage increases with increasing particle diameter and density, flow turning and gas velocity, and with decreasing blade size.

Turbine and compressor manufacturers minimize erosion by increasing trailing edge thickness, installing field replaceable shields and using improved alloys. Nevertheless, they all recommend fine inlet filtration to prevent hard particles from entering the turbines.

1.5.5 Fouling

Fouling is the adherence of particles and droplets to the surface of the turbo machine blading. This degrades flow capacity and reduces efficiency in a short period of time. Fouling can normally be reversed by cleaning, but it often requires downtime. The deposition trajectories can be predicted for some turbine blades, the actual fouling is very much dependent on inlet gas cleanliness which varies unless it is controlled.

1.5.6 Mechanism of Blade Fouling

The primary mechanisms of particle delivery in turbines are inertial impaction, turbulent eddy diffusion and Brownian motion.

For inertial impaction, particle inertia causes its trajectory to deviate from flow streamlines in the area where flow changes direction. This causes particles to impact on blade surfaces. Mass flux (arrival date) due to this mechanism decreases with decreasing particle diameters and inertia becomes very small for particles smaller than micron.

For turbulent eddy diffusion, particles become entrained in eddies of turbulent boundary layers and are swept toward the blade and vane surfaces. Small particles (0.1-1.0µm ) are slowed and trapped by the viscous drag forces of boundary layers.

In Brownian motion, very small particles (0.1µm and finer) are randomly transported to the turbine surfaces. Decreasing particle (aerosol) size and density increase the deposition rate due to Brownian motion.

There is another type of very small particles (0.01-0.1µm) deposition on the turbine blades caused by temperature gradient (Thermophoresis) which is usually negligible in low temperature applications. This is typical in oil and gas industry operations. Because of the broad distribution of particles and droplets in gas streams, (0.1-10µm) almost all of the above mechanisms govern the fouling of turbine blades. The formation of low melting point eutectics is an important fouling mechanism in power recovery hot gas expanders used in fluid catalytic cracking units.

1.5.7 Turbine Power Degradation

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International Journal of Emerging Technology and Advanced Engineering

88 In attempting to compensate for the loss in output, turbines consume additional fuel which increase firing temperature and generate undesirable increases in heat rate.

It is customary to shutdown the turbine at the point of 10% power loss for cleaning.

1.5.8 Inlet Gas Filtration

With the current advances in filtering solid particles and coalescing aerosols from gases, the turbine inlet gas can economically be filtered to contain no more than 0.01 ppm with a particle size cut-off at 0.3µm. That absolutely eliminates erosion. Fouling should virtually be controlled to limit 10% power loss in no less than 20,000 operating hours which is beyond normal plant shutdown.

Regarding corrosion control, particle filters and liquid gas coalesces are mechanical septa and do not separate corrosive vapor and gases from fuel gas. However, most of corrosive salts are dissolved and carried by liquid aerosols. Most aerosols are between 0.1 - 0.6µm and are quite removable by fine liquid/gas coalesces. That should reduce, not eliminate, corrosion as a bonus of fine filtration.

II. LITERATURE REVIEW

The viscous diffusion in the flow through turbine cascade results in the decrease in integrated flux of total pressure through the cascade. Since this decrease in total pressure flux is related to the amount of kinetic and potential energy loss in the cascade, hence this pressure flux is termed as „total pressure loss‟ or simply „loss‟. Minimizing losses by getting best aerodynamic performance is a very important step in this direction. Optimizing blades conditions i.e. Length, Aerodynamic Section, Shape, Aerofoil Thickness and last but not least the surface smoothness would be inevitable for a better aerodynamic performance.

The two major losses encountered in the cascade are termed as „profile loss‟ and „end loss‟ or secondary loss.

Denton, (1993) said, “End wall loss is the most difficult loss component to understand & predict as virtually all prediction methods are still based on correlations of empirical data, often with very little underlying physics”.

III. METHODOLOGY

In order to compute effect of roughness on total, profile and secondary losses for particular magnitude of roughness Computational studies separately for turbine and compressor are conducted.

The study is carried out using the Computational Fluid Dynamics (CFD) commercial softwares, Gambit 2.4.6® and FLUENT 6.2.16® for creating geometry and solving the governing equations respectively. In view to measure effects of blade deterioration on performance of turbines and compressors “Three Dimensional Rectilinear Turbine Cascade” and “low-speed axial flow rectilinear Compressor Cascade” are modeled respectively. Air with an inlet velocity of 102 m/s and 30 m/s is passed through the cascade of turbine and compressor respectively. The turbine and compressor cascades are open to atmosphere at the exit. First of all results for turbine cascade are obtained. Intially both surfaces of the blades of the cascade are kept as smooth and losses are analyzed for turbine cascade. effect of roughness on total, profile and secondary losses for particular magnitude of roughness is found alternatively by employing different combinations of profiles and roughnesses on surfaces of blades of cascades

The local energy loss coefficient

The value of local energy loss coefficient (y) is measured along the blade span both for turbine and compressor cascade choosing the blades of cascade to be smooth. The total loss coefficient at a given location is a measure of total losses. The total loss coefficient is a fraction which shows as to how much amount the energy of the incoming air has been lost during the course of flow through turbine or compressor cascades.

Local energy loss coefficient is calculated using the following relation proposed by Dejc and Trojanovskij (1973) as shown in equation 2.

y



=         1 01 2 2 01 02 01 1 01 2 1 01 2 2 01 02 01 1 01 2

1

   























































































































P

s s s s s s (2) Where, P2s is static pressure at outlet of the cascade,

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International Journal of Emerging Technology and Advanced Engineering

89 IV. RESULTS AND DISCUSSIONS

To calculate a single value of energy loss coefficient, the mass average value of loss coefficient was calculated. It is observed for turbine cascade with both surfaces smooth the total loss is 10.53%. Whereas this loss gets almost doubled and becomes 19.72% when a roughness of 1000 µm is applied on both the surfaces. When roughness of 1000 µm is applied separately on the suction and pressure surfaces of all the blades, the total loss is 17.96% and 12.33% respectively. Total loss for the smooth cascade was found to be minimum. There after both surfaces of the blades of the cascade are kept as smooth and secondary loss is analyzed for compressor cascade. The total loss for the smooth cascade is found to be 24.48%. These losses are maximum when roughness of 750 µm was applied on both the surfaces of blades of Compressor Cascade.

V. CONCLUSIONS

Performance improvement of gas turbine power plants is a key area for modern day researchers of power plant area. They hanker for maximum use of available energy. The turbine engineers therefore need to select the processes based on available cap of capital investment. They also need to minimize losses by getting best aerodynamic performance for their plant. They also need to optimize blades conditions i.e. Length, Aerodynamic Section, Shape, Aerofoil Thickness & Surface finish for a better aerodynamic performance.

VI. NOMENCLATURE



_Density

ui Velocity vector

Sm Momentum Source Term P Static Pressure

i

g



Gravitational Body Force Fi External Body Force

ij



Stress Tensor

Keff Effective Thermal Conductivity Jj‟ Diffusion Flux

Sh Source term includes heat of chemical reaction T Temperature

E Energy term h Enthalpy

P2s Static pressure at outlet Po1 Total pressure at inlet Po2 Total pressure at outlet



_{Ratio of specific heats for air}

y Local energy loss coefficient REFERENCES

[1 ] Baig, M. S., 2000 -2001, “Causes & Extent of Formation of Contamination / Fouling” Institution of Engineers Pakistan, Saudi Arabian Center

[2 ] Dejc, M. E., and Trojanovskij, B. M., 1973, Untersuchung und berechnung axialer turbinenstufen (Investigations and computations of the axial turbines stages), Veb Verlag Technik Berlin (in German).

[3 ] Denton, J.D., 1993 “Loss Mechanisms in Turbomachines” ASME Journal of Turbomachinery, 115, pp. 621-656.

[4 ] Stamatis, A., Mathioudakis, K., and Papailiou, K., 1999, Assessing the effect of deposits on turbine blading in a twin shaft gas turbine, Proc. International Gas Turbine & Aero-engine Conference and Exhibition, Indianapolis, Indiana.

[5 ] Vinod Kumar Singoria, Deepika Sharma, Samsher, 2012 “Effect of Roughness on Secondary Flow in a Rectilinear Turbine” proceedings of conference organized by YMCA University of Science & Technology, Faridabad, Haryana, India, 132-141. [6 ] Vinod Kumar Singoria, Samsher, 2013 “The Study of End Losses

in a Three Dimensional Rectilinear Turbine Cascade” International Journal of Emerging Technology and Advanced Engineering (IJETAE), 3, 782-797

[7 ] Vinod Kumar Singoria, Samsher, 2013, “Mechanism, Characterization, Pattern and Effect of Roughness over Turbine Blade: A Review” International Journal of Engineering and Innovative Technology (IJEIT), 2, 191-200.

[8 ] Vinod Kumar Singoria, Samsher, 2013, The Study of End Losses in a Three Dimensional Rectilinear low-speed axial flow Compressor Cascade, International Journal of Emerging Technology and Advanced Engineering (IJETAE), 3, 330-340 [9 ] Yadav,R., 2003, Steam & gas turbine, Central Publishing House

Allahbad

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International Journal of Emerging Technology and Advanced Engineering

[image:6.595.73.539.91.483.2]

90 VII. FIGURES AND GRAPHS

Figure 1: Schematic diagrams of Gas Turbine Power Plant

[image:6.595.63.265.141.266.2]

Figure 2 : P -V diagram of Joule’ s

Figure 3: T-S diagram of Joule cycle

[image:6.595.93.233.297.452.2]