the laws of thermodynamics & co generation cycle

7/25/2019 The Laws of Thermodynamics & Co Generation Cycle

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Principles of Thermodynamics:

The laws of thermodynamics, in principle, describe the specifics for the transport of heat and

work inthermodynamic processes.Since their inception, however, theselaws have become someof the most important in all of physics and other types of science associated with

thermodynamics.It is wise to distinguish classicalthermodynamics,which is focused on systems

in thermodynamic equilibrium, from non-equilibrium thermodynamics. The present article isfocused on classical or thermodynamic equilibrium thermodynamics.

There are generally considered to be four principles (referred to as "laws"):

1. Thezeroth law of thermodynamics,which underlies the definition of temperature.

2. Thefirst law of thermodynamics,which mandatesconservation of energy,and states inparticular thatheat is a form of energy.

3.

Thesecond law of thermodynamics,which states that theentropy of an isolatedmacroscopic system never decreases, or (equivalently) thatperpetual motion machinesare impossible.

4. Thethird law of thermodynamics,which concerns the entropy of a perfect crystal at

absolute zero temperature, and implies that it is impossible to cool a system all the way to

exactly absolute zero.

Zeroth law of thermodynamics

If two thermodynamic systems are each in thermal equilibrium with a third, then they are inthermal equilibrium with each other.

When two systems are put in contact with each other, there will be a net exchange ofenergy

between them unless or until they are inthermal equilibrium,that is, they are at the sametemperature. While this is a fundamental concept of thermodynamics, the need to state it

explicitly was not perceived until the first third of the 20th century, long after the first three

principles were already widely in use, hence the zero numbering. The Zeroth Law asserts that

thermal equilibrium, viewed as abinary relation,is atransitive relation (and since any systemis always in equilibrium with itself, it is furthermore anequivalence relation).

First law of thermodynamics

Energy can neither be created nor destroyed. It can only change forms.

In any process in an isolated system, the total energy remains the same.

For athermodynamic cycle the netheat supplied to the system equals the net work done by the system.

The First Law states that energy cannot be created or destroyed; rather, the amount of energylost in a steady state process cannot be greater than the amount of energy gained. This is the
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statement ofconservation of energy for athermodynamic system.It refers to the two ways that

aclosed system transfers energy to and from its surroundings by the process of heating (or

cooling) and the process of mechanical work. The rate of gain or loss in the stored energy of asystem is determined by the rates of these two processes. In open systems, the flow of matter is

another energy transfer mechanism, and extra terms must be included in the expression of the

first law.

The First Law clarifies the nature of energy. It is a stored quantity which is independent of any

particular process path, i.e., it is independent of the system history. If a system undergoes athermodynamic cycle,whether it becomes warmer, cooler, larger, or smaller, then it will have

the same amount of energy each time it returns to a particular state. Mathematically speaking,

energy is astate function and infinitesimal changes in the energy areexact differentials.

All laws of thermodynamics but the First are statistical and simply describe the tendencies of

macroscopic systems. For microscopic systems with few particles, the variations in the

parameters become larger than the parameters themselves, and the assumptions of

thermodynamics become meaningless. The First Law, i.e. the law of conservation, has becomethe most secure of all basic principles of science. At present, it is unquestioned (although it is

said to be criticized by people who do not accept the idea that the potential to gain energy is aform of actual energy).

Fundamental Thermodynamic Relation:

The first law can be expressed as theFundamental Thermodynamic Relation:

Heat supplied= internal energy+ work done

Internal energy= Heat supplied- work done

Here, E isinternal energy,T istemperature,S isentropy,p ispressure,and V isvolume.This is

a statement of conservation of energy: The net change in internal energy (dE) equals the heat

energy that flows in (TdS), minus the energy that flows out via the system performing work(pdV).

Second law of thermodynamics

Theentropy of anisolated system consisting of two regions of space, isolated from one another, each in

thermodynamic equilibrium in itself, but not inequilibrium with each other, will, when the isolation thatseparates the two regions is broken, so that the two regions become able to exchange matter or energy,

tend to increase over time, approaching a maximum value when the jointly communicating systemreachesthermodynamic equilibrium.

In a simple manner, the second law states "energy systems have a tendency to increase their

entropy rather than decrease it." This can also be stated as "heat can spontaneously flow from a
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higher-temperature region to a lower-temperature region, but not the other way around." (Heat

can flow from cold to hot, but not spontaneously- for example, when a refrigerator expends

electrical power.)

A way of thinking about the second law for non-scientists is to consider entropy as a measure of

ignorance of the microscopic details of the system. So, for example, one has less knowledgeabout the separate fragments of a broken cup than about an intact one, because when the

fragments are separated, one does not know exactly whether they will fit together again, or

whether perhaps there is a missing shard. Solid crystals, the most regularly structured form ofmatter, have very low entropy values; andgases,which are very disorganized, have high entropy

values. This is because the positions of the crystal atoms are more predictable than are those of

the gas atoms.

Theentropy of an isolated macroscopic system never decreases. However, a microscopic system

may exhibit fluctuations of entropy opposite to that stated by the Second Law (see Maxwell's

demon andFluctuation Theorem).

Third law of thermodynamics

As temperature approachesabsolute zero,theentropy of a system approaches a constant minimum.

Briefly, this postulates that entropy is temperature dependent and results in the formulation of the

idea ofabsolute zero.

Topping cycle:

In a topping cycle, the fuel supplied is used to first produce power and then thermal energy,

which is the by-product of the cycle and is used to satisfy process heat or other thermalrequirements. Topping cycle cogeneration is widely used and is the most popular method ofcogeneration.

The four types of topping cycle cogeneration systems are briefly explained in Table

1.A gas turbine or dieselengine producing electrical or

mechanical power followed

by a heat recovery boiler tocreate steam to drive a

secondary steam turbine. This

is called a combined-cycletopping system.
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2.The second type of system

burns fuel (any type) toproduce high-pressure steam

that then passes through a

steam turbine to produce

power with the exhaustprovides low-pressure process

steam. This is a steam-turbine

topping system.

3.A third type employs heatrecovery from an engine

exhaust and/or jacket cooling

system flowing to a heatrecovery boiler, where it is

converted to process steam /

hot water for further use.

4.The fourth type is a gas-

turbine topping system. A

natural gas turbine drives agenerator. The exhaust gas

goes to a heat recovery boiler

that makes process steam and

process heat

Bottoming cycle:

In a bottoming cycle, the primary fuel produces high temperature thermal energy and the heat

rejected from the process is used to generate power through a recovery boiler and a turbinegenerator. Bottoming cycles are suitable for manufacturing processes that require heat at high

temperature in furnaces and kilns, and reject heat at significantly high temperatures. Typical

areas of application include cement, steel, ceramic, gas and petrochemical industries. Bottoming

cycle plants are much less common than topping cycle plants. The Figure 7.6 illustrates thebottoming cycle where fuel is burnt in a furnace to produce synthetic rutile. The waste gases

coming out of the furnace is utilized in a boiler to generate steam, which drives the turbine to

produce electricity.


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Bottoming cycle:

ORGANIC RANKINE CYCLE:

The Organic Rankine cycle(ORC) is named for its use of anorganic, high molecular mass fluidwith a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the

water-steam phase change . The fluid allows Rankine cycle heat recovery from lower

temperature sources such as industrial waste heat, geothermal heat, solar ponds, etc. The lowtemperature heat is converted into useful work, that can itself be converted into electricity. A

prototype was first developed and exhibited in 1961 byIsraeli solar engineersHarry Zvi TaborandLucien Bronicki.

Working principle of the ORC:

The working principle of the organic Rankine cycle is the same as that of the Rankine cycle : the

working fluid is pumped to a boiler where it is evaporated, passes through a turbine and is finally

re-condensed.

In the ideal cycle, the expansion isisentropic and the evaporation and condensation processes are

isobaric.

In the real cycle, the presence ofirreversibilities lowers the cycleefficiency.Those

irreversibilities mainly occur :

During the expansion : Only a part of the energy recoverable from the pressure difference

is transformed into useful work. The other part is converted into heat and is lost. Theefficiency of the expander is defined by comparison with an isentropic expansion.
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In the heat exchangers : The working fluid takes a long and sinuous path which ensures

good heat exchange but causespressure drops that lower the amount of power

recoverable from the cycle.

Improvement of the organic Rankine cycle

ORC with Regenerator

In the case of a "dry fluid", the cycle can be improved by the use of a regenerator : Since the

fluid has not reached the two-phase state at the end of the expansion, its temperature at this pointis higher than the condensing temperature. This higher temperature fluid can be used to preheat

the liquid before it enters the evaporator.

A counter-flow heat exchanger is thus installed between the expander outlet and the evaporator

inlet. The power required from the heat source is therefore reduced and the efficiency is

increased.

Applications for the ORC:

The organic Rankine cycle technology has many possible applications. Among them, the most

widespread and promising fields are the following:

Waste heat recovery

Waste heat recovery is the most important development field for the ORC. It can be applied toheat andpower plants (for example a small scalecogenerationplant on a domestic water heater),

or to industrial and farming processes such as organic products fermentation, hot exhausts from

ovens or furnaces, flue gas condensation, exhaust gases from vehicles, intercooling of acompressor, condenser of a power cycle, etc.

[1]

Biomasspower plant

Biomass is available all over the world and can be used for the production of electricity on smallto medium size scaled power plants. The problem of high specific investment costs for

machinery such as steam boilers are overcome due to the low working pressures in ORC power

plants. The ORC process also helps to overcome the relatively small amount of input fuel
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available in many regions because an efficient ORC power plant is possible for smaller sized

plants.[2]

Geothermal plants

Geothermic heat sources vary in temperature from 50 to 350C. The ORC is therefore perfectlyadapted for this kind of application. However, it is important to keep in mind that for low-

temperature geothermal sources (typically less than 100C), the efficiency is very low and

depends strongly on heat sink temperature (defined by the ambient temperature).

Solar thermal power

The organic Rankine cycle can be used in the solarparabolic trough technology in place of the

usual steam Rankine cycle. The ORC allows a lower collector temperature, a better collecting

efficiency (reduced ambient losses) and hence the possibility of reducing the size of the solarfield.

[3][4][5]

Choice of the working fluid:

The selection of the working fluid is of key importance in low temperature Rankine Cycles.Because of the low temperature, heat transfer inefficiencies are highly prejudicial. These

inefficiencies depend very strongly on the thermodynamic characteristics of the fluid and on the

operating conditions.

In order to recover low-grade heat, the fluid generally has a lower boiling temperature thanwater. Refrigerants and hydrocarbons are the two commonly used components.

Optimal characteristics of the working fluid :

Isentropicsaturation vapor curve :

Since the purpose of the ORC focuses on the recovery of low grade heat power, a superheated

approach like the traditional Rankine cycle is not appropriate. Therefore, a small superheating at

the exhaust of the evaporator will always be preferred, which disadvantages "wet" fluids (that are

in two-phase state at the end of the expansion). In the case of dry fluids, a regenerator should beused.

Low freezing point, high stability temperature :

Unlike water, organic fluids usually suffer chemical deteriorations and decomposition at hightemperatures. The maximum hot source temperature is thus limited by the chemical stability of

the working fluid. The freezing point should be lower than the lowest temperature in the cycle.
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High heat of vaporisation and density :

A fluid with a high latent heat and density will absorb more energy from the source in theevaporator and thus reduce the required flow rate, the size of the facility, and the pump

consumption.

Low environmental impact

The main parameters taken into account are theOzone depletion potential (ODP) and theglobal

warming potential (GWP).

Safety

The fluid should to be non-corrosive, non-flammable, and non-toxic. The ASHRAE safetyclassification of refrigerants can be used as an indicator of the fluid dangerousness level.

Good availability and low cost Acceptable pressures

WASTE HEAT RECOVERY:

Introduction:

Waste heat is heat, which is generated in a process by way of fuel combustion or chemical

reaction, and then dumped into the environment even though it could still be reused for someuseful and economic purpose. The essential quality of heat is not the amount but rather its

value. The strategy of how to recover this heat depends in part on the temperature of the waste

heat gases and the economics involved.

Large quantity of hot flue gases is generated from Boilers, Kilns, Ovens and Furnaces. If some of

this waste heat could be recovered, a considerable amount of primary fuel could be saved. The

energy lost in waste gases cannot be fully recovered. However, much of the heat could be

recovered and loss minimized by adopting following measures as outlined in this chapter.

Heat LossesQuality

Depending upon the type of process, waste heat can be rejected at virtually any temperature from

that of chilled cooling water to high temperature waste gases from an industrial furnace or kiln.

Usually higher the temperature, higher the quality and more cost effective is the heat recovery. Inany study of waste heat recovery, it is absolutely necessary that there should be some use for the

recovered heat. Typical examples of use would be preheating of combustion air, space heating,or pre-heating boiler feed water or process water. With high temperature heat recovery, a cascade

system of waste heat recovery may be practiced to ensure that the maximum amount of heat is

recovered at the highest potential. An example of this technique of waste heat recovery would be

where the high temperature stage was used for air pre-heating and the low temperature stageused for process feed water heating or steam raising.
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Heat LossesQuantity

In any heat recovery situation it is essential to know the amount of heat recoverable and also howit can be used. An example of the availability of waste heat is given below:

Heat recovery from heat treatment furnace

In a heat treatment furnace, the exhaust gases are leaving the furnace at 900o

C at the rate

of 2100 m3

/hour. The total heat recoverable at 180o

C final exhaust can be calculated as

Q = V x x Cp

x T

Q is the heat content in kCal

V is the flowrate of the substance in m3

/hr

is density of the flue gas in kg/m3

Cp

is the specific heat of the substance in kCal/kgo

C

T is the temperature difference ino

C

Cp (Specific heat of flue gas) = 0.24 kCal/kg/o

C

Heat available(Q) = 2100 x 1.19 x 0.24 x (900-180) = 4,31,827 kCal/hr

By installing a recuperator, this heat can be recovered to pre-heat the combustion air. The fuel

savings would be 33% (@ 1% fuel reduction for every 22o

C reduction in temperature of flue gas.

Waste heat sources ant quality:

WASTE SOURCE AND QUALITY

S.No. Source Quality

1. Heat in flue gases. The higher the temperature, the greater the

potential value for heat recovery

2. Heat in vapour streams. As above but when condensed, latent heat also

recoverable.

3 Convective and radiant heat lost

from exterior of equipment

Low grade if collected may be used for

space heating or air preheats.


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4. Heat losses in cooling water. Low gradeuseful gains if heat is exchanged

with incoming fresh water.

5. Heat losses in providing chilled

water or in the disposal of

chilled water.

a) High grade if it can be utilized to reduce

demand for refrigeration.

b) Low grade if refrigeration unit used as a

form of heat pump.

6. Heat stored in products leaving

the process

Quality depends upon temperature.

7. Heat in gaseous and liquid

effluents leaving process.

Poor if heavily contaminated and thus

requiring alloy heat exchanger.

Classification

High Temperature Heat RecoveryThe following Table 8.2 gives temperatures of waste gases from industrial process equipment in

the high temperature range. All of these results from direct fuel fired processes.

TYPICAL WASTE HEAT TEMPERATURE AT HIGH TEMPERATURE RANGE

FROM VARIOUS SOURCES

Types of device TemperatureNickel refining furnace 1370-1650

Aluminium refining furnace 650-760

Zinc refining furnace 760-1100

Copper refining furnace 760-815

Steel heating furnaces 925-1050

Copper reverberatory furnace 900-1100

Open hearth furnace 650-700

Cement Kiln (Dry process) 620-730

Glass melting furnace 1000-1550

Hydrogen plants 650-1000

Solid waste incinerators 650-1000

Fume incinerators 650-1450


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Medium Temperature Heat Recovery

The following Table 8.3 gives the temperatures of waste gases from process equipment in the

medium temperature range. Most of the waste heat in this temperature range comes from the

exhaust of directly fired process units.

TYPICAL WASTE HEAT TEMPERATURE AT MEDIUM TEMPERATURE

RANGE FROM VARIOUS SOURCES

Types of device Temperature

Steam boiler exhaust 230-480

Gas turbine exhausts 370-540

Reciprocating engine exhausts 315-600

Reciprocating engine exhausts(turbo charged) 230-370

Heat treating furnaces 425-650

Drying and baking ovens 230-600Catalytic crackers 425-650

Annealing furnace cooling systems 425-650

Low Temperature Heat Recovery

The following Table 8.4 lists some heat sources in the low temperature range. In this range it is

usually not practical to extract work from the source, though steam production may not be

completely excluded if there is a need for low-pressure steam. Low temperature waste heat maybe useful in a supplementary way for preheating purposes.

TYPICAL WASTE HEAT TEMPERATURE AT LOW TEMPERATURE RANGE

FROM VARIOUS SOURCES

Source Temperature

Process steam condensate 55-88

Cooling water from:

Furnace doors 32-55

Bearings 32-88Welding machines 32-88

Injection molding machine 32-88

Annealing furnaces 66-230

Forming dies 27-88

Air compressors 27-50

Pumps 27-88

Internal combustion engines 66-120


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Air conditioning and refrigeration

condensers

32-43

Liquid still condensers 32-88

Drying,baking and curing ovens 93-230

Hot processed liquids 32-232

Hot processed solids 93-232

Benefits of Waste Heat Recovery :

Benefits of waste heat recovery can be broadly classified in two categories:

Direct Benefits:

Recovery of waste heat has a direct effect on the efficiency of the process. This is reflected by

reduction in the utility consumption & costs, and process cost.

Indirect Benefits:

a) Reduction in pollution: A number of toxic combustible wastes such as carbon monoxide

gas, sour gas, carbon black off gases, oil sludge, Acrylonitrile and other plastic chemicals

etc, releasing to atmosphere if/when burnt in the incinerators serves dual purpose i.e.recovers heat and reduces the environmental pollution levels.

b) Reduction in equipment sizes: Waste heat recovery reduces the fuel consumption, which

leads to reduction in the flue gas produced. This results in reduction in equipment sizes of

all flue gas handling equipments such as fans, stacks, ducts, burners, etc.

c) Reduction in auxiliary energy consumption: Reduction in equipment sizes gives

additional benefits in the form of reduction in auxiliary energy consumption like

electricity for fans, pumps etc..

the laws of thermodynamics & co generation cycle

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