Zin Eddine Dadach

Higher Colleges of Technology,UAE

Introduction

Energy = Quantity + Quality

Issues about energy balance

Advantages of exergy analysis

Comparing energy and exergy flows in power

plants

Equations for exergy destructions in OCGT

Recommendations

Exergy analysis is a useful

concept for ecology and

sustainability because it can

used as a common measure of

resource quality along with

quantity.

Quantity (1st law: Energy is conserved)

Quality (2nd law: Exergy is destroyed).

The exergy of an energy form or a

substance is a measure of its

usefulness or work potential

Work has higher energy quality than heat. 1 kJ of

electricity is fully useful but not 1 kJ of heat.

Plant efficiency management in the first instance is

concerned with thermal efficiency. Ultimately a plant

wants to produce as much output for the least

expenditure of fuel.

When plant efficiency deteriorates, the challenge is first

to locate the cause and then cost the value of repairs.

Energy balance is not useful because the real plant

inefficiencies are not related to energy loss but to exergy

destruction.

Determining the exergy destruction of each

equipment of the plant.

By analyzing the exergy destroyed by each

component in a process, we can see where

we should be focusing our efforts to improve

system efficiency.

Exergy of an energy stream can be lost to the

surroundings (flue gas, cooling water and heat

loss),

However, the primary contributors to exergy

destruction are irreversibilities associated with

chemical reaction, heat transfer, mixing, and

friction

No exergy destruction during a reversible process.

Reversible work is then the maximum amount of

useful work output

The initial and final energy and exergy are similar

Large amount of heat but small amount of exergy leave the process . Exergy destruction is due mainly to irreversibilities in the process ( exergy flow very narrow).

Furnace losses represent small percentage of the total energy conversion. From the exergy diagram that something drastic happens (Irreversibilities due to combustion).

Energy flows, the losses are heaviest in the condenser. However, very low temperature has a very low quality

Energy and exergy loss similar in the turbine. Exergy destruction due to energy loss through the boundaries

Irreversibilities (chemical reaction, heat transfer,

mixing, and friction ) are invisible in energy

balance.

Irreversibilities destroy the quality and then the

economic value of energy flows. Therefore

increasing their environmental impact.

Exergy analysis distinguish between the exergy

destroyed by irreversibilities and those lost

through the boundaries of the system.

Furnace has the highest exergy destruction .

It is critical to use a fuel that meets the

original specifications for optimum

combustion.

Continuous measurement of both O2 and CO

leaving the combustor provides also

information needed for effective combustion

for significant benefits in energy savings

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

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

3) Air to fuel ratio or excess air

4) Flow, P,T of air leaving compressor

5) Power input to the compressor

6) Power generated by the turbine

7) P,T, composition, flow of flue gas before and after

turbine

Exergy destruction and exergy efficiency of

the power plant and its components are

found by solving the three balance

equations applied at any control volume, in

other words, at any component of the plant: Mass balance: Σmin =Σmout

Energy balance: Σmout.hout −Σmin .hin =Qnet,in −Wnet,out

Exergy balance: Σmin.exin −Σmout .exout - W +Σ Q (1- T0/T) = (EX)destroyed

Exergy of streams is evaluated with respect

to a reference environment (i.e. dead state).

for a component of a stream at rest: ek=ep=0

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

= (h-h0 ) –T0 (s-s0)- RT0 ln (x)0

In our study, the atmospheric pressure, annual average

temperature and relative humidity in Abu Dhabi will be

selected as the parameters of the “dead state”

Compressor:

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

Combustor:

(ex)D,CC= ma.ex2 +mf.(ex)f – mfg. ex3

Gas turbine:

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

Work from a gas turbine can be defined as the product of

mass flow, heat energy in the combusted gas (Cp), and

temperature differential across the turbine

WT= mfg. Cp,fg. (T3-T4)

The mass flow of flue gas is the sum of compressor

airflow and fuel flow.

The heat energy of the flue gas is a function of the

elements in the fuel and the products of combustion.

Energy output and efficiency of gas turbine vary according to operating conditions.

The main parameters to be considered are:

Ambient temperature,

Compressor pressure ratios

Fuel temperature

Turbine inlet temperature

Air to fuel ratio

.

With the exergy analysis of power plants , we can detect causes to irreversibilities:

Recoverable loss which can be rectified by water washing or, more thoroughly, by mechanically cleaning the compressor blades and vanes after opening the unit.

Non-recoverable loss. Because this loss is caused by reduction in component efficiencies, replacement of affected parts are recommended during inspection intervals.

In Abu Dhabi, Ambient temperature is much

higher than design value of 150C.

Plant energetic and exergetic efficiencies could

increase by 20% and 12% by decreasing air

temperature by 10oC.

Wet air compression technology increases power,

reduces NOx emissions, and improves heat rate

and is not ambient temperature dependent.

If the fuel consists only of hydrocarbons with no inert gases and no oxygen atoms, work output increases as LHV increases

Heating fuel will result in higher turbine efficiency due to the reduced fuel flow required to raise the total gas temperature to firing temperature.

But reducing fuel flow will have slight decrease in turbine output.

Energy and Exergy efficiencies increase to maximum value as compressor pressure ratio (rp) increases.

However, increasing the compressor pressure ratio has less improvement when it is over 12.5.

Maximum Compressor pressure ratio should be estimated.

Higher temperature at the exit of the combustor increases the output of the power plant.

The constraint is the metallurgical thermal limitations of the turbine.

Minimize the use of excess air by appropriate

process control system but continuous

measurement of both O2 and CO leaving the

combustor provides also information needed for

effective combustion for significant benefits in

energy savings.

Results indicate that the thermal efficiency of a

gas turbine will increase by about 2.4% when the

air to fuel ratio decreases from 50 to 30.