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Exergy analysis of power plants

Chemical Engineering Department

Omar Mohamed Alhosani & Abdulla Ali Alhosani

Advisor: Zin Eddine Dadach

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Founded in 1988 by Federal Decree, the Higher Colleges of Technology (HCT) is

a community of approximately 20,000 students and 2,000 staff based on 17

campuses throughout the United Arab Emirates (UAE) – the largest higher

education institution in the UAE.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Inductive, cooperative and problem based learning are the main active

learning strategies : lab experiments, industrial projects, team activities

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Senior students have a work placement in local industries, they learn about

processes and collect data of a specific technical problem. After collecting data

from the plant, students and their advisor investigate the technical problem

using learned theories and search for solutions from literature.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Why this study?

What is exergy ?

Issue with Energy balance

Introduction of the power plant

Exergy analysis of the plant: Evaluating exergy destructions

Exergy analysis of the plant by simulation

a) Effects of temperature of air on the exergy destructions

b) Effects of T & humidity of air on the exergy destruction

I.

II.

III.

IV.

V.

VI.

VII. Analysis of the results

VIII. Recommendations

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

 Largest contribution to global warming

Power generation plants release about 40% of CO2

emissions in the UAE

 Lower performance due to atmospheric conditions

There is a difference between the design conditions

(T=288K, RH=60%) of the power plants and the summer

conditions (T=316K, RH=50%) in Abu Dhabi

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Energy has a Quantity and a Quality (Exergy).

Exergy is a measure of usefulness of the energy.

Work has higher exergy than heat because 1 kJ of

electricity is fully useful but not 1 kJ of heat.

Energy is conserved (1st law) but not exergy (2nd

law). Exergy destruction caused by irreversibilities

in the process).

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

 The real plant inefficiencies

are not related to heat loss

(estimated by energy

balance) but to

irreversibilities in the

process (estimated by

exergy destructions).

 Energy balance is not useful

for energy conversion

processes because the

irreversibilities in the system

are invisible.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

 Open Cycle Gas Turbine (OCGT) with a production of 160 MW

 The compressor of the power plant is an axial compressor with 21

stages and the compression pressure ratio is 13.5.

 The combustion chamber is equipped in ring with 72 environmental

burners.

 The expansion section is composed of a 5-stage gas turbine and a

generator.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

1) Fuel composition, flow rate and (P,T)

2) Air composition, flow rate and (P, T).

3) Flow, P,T of air leaving compressor

4) Power input to the compressor

5) Power generated by the turbine

6) P,T, composition, flow of flue gas before and

after turbine

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

First Objective of this exergy analysis is to

determine the exergy destruction (irreversibilities)

of each equipment of the power plant.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

 for a component of a stream:

(ex)t= (ex)tm+ (ex)ch

tm: thermo-mechanical part of exergy : Difference in (P,T)

between the stream and the dead state

ch: Chemical part of the exergy: Difference in composition

between the stream and he dead state

(ex)t= (h-h0) –T0(s-s0)- ΣRT0ln (x)0

Where : h0,T0, s0, x0are the values for the dead state

 Exergy of streams 1, 2,3,4 is evaluated with respect to a

reference state called “dead state”

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

In our study, the atmospheric pressure,

average temperature and relative humidity in

Abu Dhabi during summer (P= 100.8 kPa, T=

316 K and RH= 50% ) are selected as the

parameters of the “dead state”

Therefore: the exergy of the ambient air

(stream 1) is equal to zero.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

 Compressor:

(ex)D,K= WK- ma .(ex2-ex1)

 Combustor:

(ex)D,CC= ma.ex2+mf.(ex)f– mfg. ex3

 Gas turbine:

(ex)D,GT = mfg. (ex4-ex3) - WGT

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Exergy destruction

(kW)

% Exergy

destruction

Equipment

Exergy efficiency

Compressor

17445.31

12.4

0.916

Combustion

chamber

98155.6

70.2

0.851

Turbine

24417.2

17.4

0.938

Power plant

140018.1

100

0.331

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

 Soave-Redlich-Kwong (SRK) equation of state was utilized.

This is commonly used EOS for simulation of gas turbine.

 To fit the real exit temperature of the compressor, the

isentropic efficiency of the compressor was selected as

equal to 0.8.

 To fit the real exit temperature of the turbine, the turbine

isentropic efficiency is 0.88.

 Assumptions: mechanical efficiency of the turbine and

compressor equal to 0.98 and the generator efficiency

equal to 0.99.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

The net output of the power plant under

design conditions (T=288K, RH=60%) is 160

MW ( Source: Company).

Our simulation results indicate that the net

power produced at design conditions is 163

MW.

 The relative shift is 1.9%. This result presents

a validation of the simulation.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Figure 1: Temperature increases the exergy destruction in the

compressor and combustion chamber but decreases the

exergy destruction in the turbine.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Figure 2: Temperature decreases the net power output of the

plant.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Figure 3: (1) Increasing temperature and absolute humidity of air increases the exergy

destruction in the combustion chamber. (2) Exergy destruction in the compressor

increases slightly with temperature and absolute humidity. (3) exergy destruction in

the turbine decreases slightly with temperature and absolute humidity.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

When comparing the slopes (109.49 and 151.12), both

temperature of ambient air and its absolute humidity increase

the exergy destruction in the compressor. However,

temperature effects seem to be more important.

 Based on:(??)?= ??− ????(???2− ???1)

These results could be explained by the fact that the

compressor requires more work to compress air of a higher

temperature because air density becomes smaller. Moreover,

due to molecular weight, air absolute humidity decreases the

density of air, by consequence, the work needed to compress

the humid air.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Comparing the slopes (1126 and 829.09) related to the exergy destruction

in the combustion chamber, temperature of ambient air increases the

exergy destruction of the combustion but humidity decreases it with

smaller effects. Based on:

(ED)cc = ????? ???2+ ?????? ??????− ???.???3

The exergy destruction in the combustor increases with the exergy of the

compressed air. Both temperature of air and its absolute humidity

increases the exergy of the compressed air but humidity could have a

cooling effect too. So it could therefore decrease the exergy destruction in

the combustion process.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Because the slope (-3053.5) of the effects of ambient air on the

exergy destruction in the turbine is much higher than the slope (-

267.36) of the corresponding curve for both temperature and

humidity , it could be concluded that the exergy destruction in the

turbine decreases with temperature but increases with air absolute

humidity. However, the effect of temperature could be more

important. Based on:

(??)?= ???? ???3− ???4 − ??

The findings could be related to the fact that the work output (WT) of the

turbine increases with the compressor inlet temperature and decreases

with air humidity.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

I. OCGT: The temperature of the flue gas leaving

the turbine is very high: 700 K

II. Furnace has the largest exergy destruction

III. Power and Exergy loss of the power plant

compared to design conditions.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

This is an Open Cycle Gas turbine (OCGT) and the

temperature of the exhaust gas is around 700 K. A heat

recovery steam generator (HRSG) should be added to increase

the net power output of the plant.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

The combustion process is the main source of exergy

destruction (70.2 %) .

This could be explained by the fact that the fuel supply

may not meet the gas turbine combustion requirements.

Requirements regarding fuel gas composition in original

equipment manufacturer (OE) specs are driven by a

number of factors including auto-ignition, flame

temperature, emissions and stability.

It is critical to successfully obtaining a fuel stream that

meets the original equipment manufacturer (OEM)

specifications.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Actual Process Control strategy : Ratio controller ( Actual AFR

= 50.4 (kg/kg).

This is a Feedforward control which does not take into

account all the disturbances (FG composition change, change

in Temperature of air,..)

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

 Replacing the feedforward control (Ratio control) by a feedback

control with Continuous measurement of both O2 and CO leaving

the combustor could provides the needed information for effective

combustion for significant energy savings by minimizing excess air.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

Compared to design conditions, the power plant lost 4.66 % of its net

power output.

Results show that temperature has more negative effects than the

absolute humidity on the power plant.

A thermo-economic optimization study is needed in order to select

between the different cooling systems in order to decrease the negative

effects of the high temperatures of the summer in Abu Dhabi (UAE) :

Example : Fogging cooling

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

 Exergy Analysis of a Power Plant in Abu Dhabi (UAE)

 International Journal of Energy Engineering

 Jun. 2015, Vol. 5 Iss. 3, PP. 43-56

 16 References

 Main Ref.: Book by Bejan, Tsatsaronis & Moran

 Title: “Thermal Design and Optimization”;

 Edition: J. Wiley & Sons (1996).

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

In exergy costing, a certain cost is

determined for each of the exergy-flows.

The goal of conducting exergo-economic

analysis of the power plant is to compare

the cost of exergy to the capital and

operations cost for each equipment.

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


Exergy analysis of power plants

2nd International Conference and Exhibition on Industrial Engineering

November 16-18, 2015 Dubai (UAE)


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