111Equation Chapter 1 Section

1Performance Evaluation of an Industrial Boiler

By

Yasir Nadeem

A Thesis Submitted in Partial Fulfillment of the

Requirements for the Masters’ Degree

In

Chemical Engineering

Department of Chemical Engineering

University of Engineering and Technology, Lahore

Research Supervisor:

Dr.-Ing Naveed Ramazan

1

©Yasir Nadeem 2012

Performance Evaluation of an Industrial Boiler

This project is submitted to the Department of Chemical Engineering, University of

Engineering and Technology, Lahore for the partial fulfillment of the requirement for the

Masters degree in Chemical Engineering.

Approved on: ________________

Examining Committee:

---------------------------------Internal ExaminerDr.-Ing Naveed Ramzan

---------------------------------External Examiner

---------------------------------ChairmanProf. Dr. Nadeem Feroze

---------------------------------DeanProf. Dr. Shahid Naveed

Department of Chemical Engineering

2

University of Engineering and Technology, Lahore

And He has subjected to you, as from Him, all that is in the heavens and on earth: Behold! in that are signs indeed for those who reflect.

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(Al-Jathiya:13)

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A B S T R A C T

Pakistani Industrial Sector is facing vital energy crisis. There is a need of optimizing

energy consumptions at demand side. Industrial Boilers are considered as highly energy

intensive equipment. In present work, the opportunities to improve efficiency of a three

pass boiler are defined with energy audit of the boiler. Primary objective of the study is to

determine the causes of low efficiencies in industrial boilers. Several measures have been

taken such as, combustion efficiency, optimum excess air, stack gas temperature and

blow down calculations. Overall efficiency of the system has been also determined as

well, as a result of all up mentioned measurements. From calculations, it is found that

boiler combustion and overall efficiencies are 75.7% and 70% respectively. Stack gas

leaves with a lot of energy in it (temp. 319 oC). Excess oxygen present in stack is also on

higher side (3.5% instead of 2.2%). And also, blow down come up with high potential of

saving opportunities.

While proposing guidelines and recommendations for efficient boiler operation in this

system, saving measures resulted in annual fuel saving of about 8.4 Million PKRs.

Key words: Combustion efficiency, Boiler, Overall efficiency, Blow down, Excess oxygen, Stack gas, thermal effectiveness

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C H A P T E R 1

INTRODUCTION

1.1. World Energy Scenario:

Energy is the most important prerequisite for the economic development of a country.

Besides industrial and agricultural energy demands, domestic sector applies a great

pressure for energy availability. Also, growing worldwide population and changing life

styles give rise to energy requirement and generation of increasing quantities of waste

which has become another threat to our already degraded environment. Fossil fuels such

as oil, coal and gas, for energy production have become primary energy sources and more

than 80% of total energy supply in the world is obtained by burning fossil fuels (EIA,

2012).

Figure 1.1: Worldwide energy consumption

On the other hand currently proven energy sources will deplete within next 40-50 years

and drive the world to an expected energy crises if alternate energy technologies are not

introduced in the market (Saidur et al., 2011). Moreover burning fossil fuel has damaged

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the environment more than anything else by emitting an intense amount of green house

gases. However in recent years, zero-waste and energy recovery from waste (waste-to-

energy) technologies have been developed to produce clean energy which are equipped

with the high efficient pollution control equipment to produce least emissions.

A major fraction of fossil fuel burned in the industry is used for steam production which

in turn is used for either heating purposes in various sections of process industry or for

power generation. Typically 57% of total fossil fuel is used for steam generation in food

processing, 81% in pulp and paper, 42% in chemicals, 23% in petroleum refining and

10% in primary metals (Saidur et al. 2010).

Table 1.1: World energy consumption (quadrillion BTU)

Year Liquid Natural gas Coal Nuclear Others Total2005 170.8 105.1 122.3 27.5 45.4 471.12010 173.2 116.7 149.4 27.6 55.2 522.12015 187.2 127.3 157.3 33.1 68.5 573.42020 195.8 138.5 164.6 38.9 82.2 620.1

The limited reserves of combustible fuels and the damage to the environment by low

efficiency of combustion reaction require more efficient systems that convert the fuel into

useful energy with minimum carbon emission. Carbon dioxide (CO2) is major greenhouse

gas (GHG) produced from the combustion of fossil fuels in the boiler. It has most

significant impact on global warming as compared to other green house gases (Ayres &

Walter, 1991). Other GHG emissions from boilers may include water vapors’, Nitrous

oxide (N2O) and methane (CH4). These GHG emissions results severe consequences of

rising temperature, floods, droughts and earth’s climate change.

Table 1.2: Emission level at various era (CDIAC, 2012)

Gas Preindustrial level Current level Increase since 1750Carbon dioxide 280 ppm  388 ppm 108 ppmMethane 700 ppb 1745 ppb 1045 ppbNitrous oxide 270 ppb  314 ppb  44 ppb

Both increasing energy cost and greenhouse gas emissions exert pressure on industrial

bottom line and on global climate as well. The two factors causing pressure on user

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economic growth are directly affected by the boiler efficiency and its ability of steam

generation. An efficient boiler is the one that produce greater steam with minimum fuel

consumption and reduced GHG emissions. Combustion air supplied to the boiler also

effects boiler efficiency.

If combustion air flow rate is too low, concentration of carbon monoxide builds up in flue

gases and in extreme cases smoke (unburned carbon particles) is produced due to

incomplete combustion. If combustion air flow rate is too high (excess air), it results

unneeded air, which carry lot of heat in exhaust and reduces heat available for steam

generation (West, 2002). Recommended excess air for various fuels is shown in table 1.3.

Table 1.3: Excess air used for different fuels

Fuels Excess Air (%)Coal (Pulverized) 15-30Coke 20-40Wood 25-50Bagasse 25-45Oil 3-15Natural Gas 5-10Refinery Gas 8-15Blast furnace gas 15-25Coke-Oven gas 5-10

In order to cater with Increasing energy demand and environmental constraints, it is

required to enhance boiler performance by reducing loses that severely impacts boiler

efficiency. Besides controlling air supply, burner performance is another factor effecting

boiler efficiency. Proper mixing of fuel and air is a key performance factor for fuel

combustion, which in turn affects energy available for steam production. Though burner

performance is not an operating parameter, yet fixing fuel and air proportion within

flammability limits and provision of steady state and continuous combustion by burner

directly affects boiler efficiency.

Modern boilers are capable of maintaining recommended excess air through turndown

ratio of the burner. Continuous operation of burner may cause wear on cams, linkages

and pins that results variation in fuel/air ration and reduces boiler efficiency. A poor

burner causes uncontrolled fuel/air ratio which is responsible for unsmooth and

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inefficient fuel combustion and results wastage of fuel. For safe and economic operation

of the boiler, control of excess air supply and stack temperature must be maintained at

optimum level which is established through proper tune-up procedure (Ozdemir, 2004).

Pakistani Industrial Sector is facing vital energy crisis. There is a need of optimizing

energy consumptions at demand side. Steam is an integrated part of process industry.

There is an opportunity of about 10 to 30% of thermal energy saving from more efficient

usage of boilers.

1.2. Fuel considerations for Boiler operation:

Most common fuels used in steam boilers include: Coal, oil and gas. Domestic and

industrial wastes are also incinerated for electricity generation and solid waste

management. Choice of fuel for combustion in the boiler depends on the design of

combustor. Most existing boilers have either single fuel firing facility or very limited

flexibility to fire alternative fuels. In latest designs of boiler combustors, application of

alternative fuels is being considered so that boiler can be switched to a different fuel for

future applications.

Natural Gas is the most simple, attractive and traditional fuel which is readily available.

Natural gas is readily mixes with air providing best fuel/air mixture for combustion

purposes. Factors that make natural gas a proffered fuel for combustion purposes include:

1. Low cost for same energy value as compared to oil and coal.

2. Very limited types of equipment are required for monitoring and handling,

typically flow meters, pipelines, knock out drums and control instrumentations.

3. High radiant flame characteristics and high velocities results enhanced heat

transfer and low heat transfer surface area which in turns requires small boiler

size and reduces purchase cost of the boiler.

4. Natural gas is more atmospheric friendly fuel. So cost associated with flue gas

treatment from natural gas fired combustor is less as compared to other fuels.

Liquid Fuels are either distillate fine products or residue from straight run distillation.

They are classified on the basis of physical characteristics and more often graded as No.

1, 2, 3, 4, 5 and No. 6 fuel oil. Grade 4, 5 are 6 are residual fuels. No. 2 is suitable for

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industrial and domestic heating purposes. Distillate oils are preferred on residual oils

because it is easier to handle and requires no heating to transport and no temperature

control for viscosity adjustment for atomization and combustion.

For better atomization, fuel/air mixing, ease of handling and efficient boiler operation;

heating value, viscosity, flash point, pour point, sulfur and ash contents are carefully

controlled. Fluctuating market prices of crude oil affects economic trend of steam

generation. On the other side natural gas rate are more stable and cheaper than fuel oil.

Coal is widely used for heating, steam and electricity generation purposes in commercial

and industrial sector. Coal is a solid fuel and has varying quality grades based on non-

carbon contaminations. Though Coal is widely used solid fuel, yet following factors have

negative impacts on its selection as a fuel;

1. Combustion of coal results in the emission of NOx and SOx along with CO2

which adversely affect the atmosphere if not controlled before emission.

2. Growing environmental limitations necessitate the installation of emission control

equipment that is highly expensive.

3. Coal receiving, storage, transport, preparation and processing requires heavy

machinery that that requires significantly high capital investment.

4. Coal requires road transportation, so political scenarios may affect its

uninterrupted supply.

Though safe and environmental friendly operation of coal combustion demands high

capital cost yet substantial operating cost saving of coal as compared to oil and gas

market prices, coal is likely to escalate in coming days as it significantly justifies a major

portion of investment made on coal feed preparation and post combustion emission

control (Turner & Doty, 2007).

1.3. Boiler Performance Evaluation:

Boilers are highly energy intensive thermal units. A significant amount of energy is

consumed in the boiler for steam generation. More than two-third of fossil fuel is burned

in the boilers, furnaces and other fired heaters. Typically 57% of total fossil fuel, burned

in industry, is used for steam generation in food processing, 81% in pulp and paper, 42%

in chemicals, 23% in petroleum refining and 10% in primary metals (Saidur et al., 2010).

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Steam generated in the boiler is either used as direct heating purposes or used to rotate

steam turbines for electricity generation. Unlike other electric systems (hydroelectric,

nuclear, geothermal etc.) boilers are considered as inherently energy inefficient systems.

Perfect burning is the first requirement for a system to be efficient that involve energy

production from fossil fuel. Combustion reactions are inherently irreversible processes

and always cause an increase in entropy which is useless for of energy. An ideal system

that can result 100 % combustion efficiency, all of its energy will not be available as

useful energy because of intrinsic nature of combustion process. Energy analysis is a

tradition methodology carried out for performance evaluation of energy conversion

systems (Guoqiang, 2011).

An actual amount of thermal energy can only be achieved by quantitative and qualitative

analysis of energy conversion, transport and distribution (Paredo et al., 2002). Many

studies have been done on the importance of energy analysis for performance evaluation

of thermal systems. They also reveal the misleading concept of energy analysis and

energy efficiency because of its lake of true identification and quantification of potential

losses (Rosen, 2002).

The basic objective of thermoeconomic evaluation is the determination of actual product

cost, provide basis to control expenditures caused by system inefficiencies optimize the

design and operation of systems (Gaggioli, 1983). It is powerful tool to analyze

(Tsatsaronis & Winhold, 1985; Lozano et al. 1993b) diagnose (Lozano, 1994; Arena &

Borchiellini, 1999) and optimize (Lozano, 1996; Spakovsky, 1990) the performance of

energy conversion systems.

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C H A P T E R 2

LITERATURE REVIEW

First and second law analysis are widely used in industry to check the performance of

energy systems. Especially the introduction of second law analysis is becoming a prime

toll for the identification and quantification of thermal losses at each stage of energy

system. In this way the thermodynamic analysis has changed the perception about

performance evaluation of energy systems.

Various research groups made a great contribution in thermoeconomic analysis of

thermal systems e.g. power plant, energy storage system and other unit operations.

Flavio et al. (2000) presented thermoeconomic evaluation of gas turbine

cogeneration system. They evaluated power plant on the basis of first and second

law in order to determine the production cost of steam and electricity. He

concluded higher cost of electrical power and lower cost of steam and the product

on exergetic basis due to increase in ratio of product to fixed cost.

Zhang et al. (2007) presented thermoeconomic analysis of coal fired power plant

to diagnose the operational faults to prevent anomaly progression and reduce

economic loss. They concluded that irreversibilities in many components are not

due to their inefficiencies but they are caused by dysfunctions by other

components.

Eric Conklin (2010) applied second law analysis on steam boiler and combined

heat and power (CHP) plant in order to determine true losses in the system and

propose different ways to reduce that losses and increase second law efficiency of

the system.

Dong et al. (2009) studied thermal efficiency of 300MW CFB boiler. They are

studied the effect of boiler load, excess air, air-to-fuel ration, combustion air

temperature and feed water and boiler temperature on first and second law

efficiency. According to their analysis energy efficiency increases with increase

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in combustion air temperature, feed water and superheated steam temperature

while decreases with increase in excess air.

Bakhshesh & Amir (2012) performed parametric study of boiler in order to

decrease irreversibilities in various sections of the boiler. They found energy

efficiency of boiler to be 89.21% and 45.48% respectively. Combustion chamber

and steam generator were major source of irreversibilities in the boiler. They also

studied the effect of excess air and stack temperature on energy efficiency.

Combined pinch and energy analysis (CPEA) has also been applied by Abtin &

ChangKyoo (2010) for efficiency optimization of 325MW steam power plant

using Cycle Tempo. They plotted energy composite curves for minimum

approach temperature to enhance process to process energy transfer and reduce

fuel and utility requirements. They concluded that applying CPEA fuel

consumption reduces while the thermal cycle performance of power plant

increases.

Chen et al. (2012) carried out extensive study in order to recover thermal energy

from flue gas using condensing boilers. He concluded that using polypropylene as

corrosion resistant material flue gases can be cooled down to 30 oC recovering

major portion of thermal energy and almost all of its energy at the expense of

increase in heat transfer area.

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C H A P T E R 3

PROCESS DESCRIPTION AND METHODOLOGY

3.1. Boiler Schematic Diagram:

Schematic of process is shown in figure 3.1. Major components of boiler operation

include 1) combustor 2) steam generator 3) steam super-heater and 4) stack to exit flue

gases to the environment.

Figure 3.1: Schematic diagram of boiler

Combustion air (stream-1) and fuel (stream-2) enters the combustion section where

combustion reaction takes place and huge amount of heat is released. This heat of

reaction is utilized to convert boiler feed water (stream-4) into superheated steam

(stream-7) in two stages. At first stage of three pass fire tube boiler, combustion products

(stream-3) pass through tube side of steam generator and produce saturated steam

(stream-6) in the shell side of packaged boiler. In the second stage saturated steam from

the shell side of boiler is superheated in shell and tube type super-heater to almost 10

degree Celsius above the saturation point. After recovering major fraction of heat from

combustion products, the flue gas (stream-8) leaves through stack to the environment.

Natural gas enters at 1.045 bar abs pressure and temperature of 15 oC while combustion

air enters at 1.013 bar abs and 35 oC respectively. Boiler feed water circulates through

other section of process plant and available at boiler head at a temperature of 140 oC.

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Table 3.1: Technical data of the boiler

No. Parameter Unit Natural Gas Boiler1 Design Capacity tons/hr 82 Operating Capacity tons/hr Variable (1-8)

3 Boiler Type ….Packaged typeThree Pass, Fire Tube

4 Heating area m2 2055 Draft Type ….. FD6 Design pressure bar abs 307 Operating pressure bar abs 25-308 Furner type ….. Dual (Natural gas & Diesel)

Steam requirement varies with the operation of plant. At shutdown conditions boiler is

operated at 1 ton steam/hr. Other operating conditions at variable capacity are given in

table 3.2.

Table 3.2: Stream data for boiler operation

Capacity ṁ1 ṁ2 ṁ3 ṁ4 P4 T6 T7 T8 λ

tons/hr Kg/sec Kg/sec Kg/sec Kg/sec bar abs oC oC oC

0.5 0.154 0.009 0.163 0.139 26.5 228 239 256 1.181 0.295 0.018 0.313 0.278 28.8 233 241 259 1.162 0.609 0.037 0.646 0.556 26.2 229 243 265 1.173 0.922 0.056 0.978 0.833 27.3 230 245 266 1.174 1.214 0.075 1.289 1.111 29.5 235 253 270 1.155 1.527 0.094 1.621 1.389 27.8 231 249 268 1.166 1.848 0.111 1.959 1.667 28.7 232 252 273 1.197 2.103 0.130 2.233 1.944 28.5 232 255 272 1.158 2.478 0.150 2.628 2.222 29.3 234 256 275 1.18

3.2. Assumptions:1. Boiler and all of its components are considered as working on steady state.

2. Dry ambient air is considered to be composed of 79 vol% Nitrogen and 21 vol%

Oxygen and combustion air is considered as entering the boiler at 35 oC having

relative humidity of 65%.

3. Reference environment is taken as 25 oC temperature and 1.013 bar pressure and

composition of air is considered as 75.67% N2, 20.30% O2, 3.12% H2O, 0.9% Ar

and 0.03% CO2 on mol basis (Gaggioli & Petit, 1977).

4. Combustion efficiency of fuel is considered as 100%.

5. Carbon monoxide emission is negligible and no NOx emissions take place.

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6. Combustion products enter the steam generator and leave the stack at ambient

pressure.

7. Natural gas contains 90% CH4, 7% N2 and 3% CO2 on mole basis.

8. Kinetic and Potential energy effects are negligible as compared to thermal effects.

9. Transmission pipes are fully insulated and no heat loss takes place in pipes.

3.3. General Balance Equation for boiler:

Boiler is subdivided into combustor, steam generator and steam superheater.

Thermoeconomic evaluation of boiler involves general mass balance, energy and exergy

balance and efficiency equation. These equations help in the determination of energy and

exergy efficiency, energy loss, exergy destruction and efficiency of individual

components and overall boiler as well. Necessary data of boiler operation at different

capacities is given in table 3.2 and thermodynamic properties for first and second law

analysis were taken from H. Perry (1997 & 2008). Chemical reaction occurring in

combustion chamber is given as;

Dincer et al. (2007) and Cengel & Boles (2006) presented first and second law analysis

in detail. General balance for a quantity in a system is written as

(1)

(2)

(3)

For steady state process there is no accumulation of conserved quantity. So,

General mass balance equation:

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(4)

General energy balance equation:

(5)

Neglecting potential and kinetic energy effects, equation can be written as:

(6)

General exergy balance equation:

(7)

For flowing streams exergy flow rate can be written as:

(8)

Where, specific thermo-chemical and chemical exergies are expressed as:

(9)

(10)

Overall energy efficiency of the boiler can be written as:

(11)

Overall energy efficiency of the boiler can be written as:

(12)

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Specific enthalpy, entropy and chemical exergy of various substances used in the analysis

are expressed as:

Table 3.3: Reference thermodynamic properties of selected components (Perry, 2008; Younglove, 1982 & 1987)

Component ho (kJ/mol) so (kJ/mol-K) exochem (kJ/mol)

CO2 (g) 35.861 0.214 19.89N2 (g) 8.652 0.171 0.721O2 (g) 8.416 0.185 3.970H2O (l) 1.890 0.007 0.884

Other boiler performance parameters thermal effectiveness, boiler yield and quality

preservation index can be calculated as;

Thermal effectiveness as defined by Bird et al. (1960);

(13)

(14)

Another basic parameter used for boiler evaluation is termed as quality preservation

index. It is the measure of exergy/energy carry over ratio. It simply divides product

quality delivered to the source quality.

(15)

Boiler is subdivided into combustor, steam generator and superheater. Each subsystem

can be analyzed separately using mass, energy and exergy balance equation to find out

energy and exergy efficiencies and losses occurring in each component.

3.3.1. Energy and exergy analysis of combustor:

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Both fuel and combustion air enters the combustion chamber where combustion reaction

takes place and organic fuel is converted to combustion products (CO2 and H2O). In the

presence of excess air no carbon monoxide produces and also NOx emissions are also

negligible. No work and heat transfer takes place, so at steady state material, energy and

exergy expressions can be written as described by (Aljundi, 2009a; Aljundi, 2009b);

Mass balance:

(17)

Energy balance:

(18)

First law efficiency can be written as;

(19)

For adiabatic combustor energy efficiency always yields 100%.

Exergy balance:

(20)

Second law efficiency can be written as;

(21)

At ambient conditions specific fuel exergy reduces to chemical exergy (Saidur et al., 2007)

(22)

Where, γ is fuel exergy grade function defined as specific fuel exergy to its heating value.

Table shows exf, γ and hf of some typical hydrocarbon fuels.

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Table 3.4: Properties of selected fuel (Saidur et al., 2007; Reistad, 1975)

FuelHeating value,

hf (kJ/kg)Chemical Exergy,

exf (kJ/kg)Exergy grade

function, γGasoline 47,849 47,349 0.99Natural Gas 55,448 51,702 0.93Fuel Oil 47,405 47,101 0.99Kerosene 46,117 45,897 0.99

3.3.2. Energy and exergy analysis of steam generator:

Steam generator is first heat recovery heat transfer unit after combustor. Its objective is to

recover major fraction of heat from combustion products and convert boiler feed water

into saturated steam. Steam generator contains three passes for flue gases to transfer heat

to boiler feed water in the shell side. As it is only a heat transfer unit so no work involves

in its operation. Although it is well insulated to prevent heat from dissipation, still there is

heat loss that reduces its efficiency. Steam generated don not require mass balance,

continuous blow down is too small that it can be ignored, so first and second law analysis

can be written as;

Energy balance:

(23)

(24)

First law efficiency of steam generator can be written as:

(25)

Exergy balance:

(26)

(27)

Second law efficiency of steam generated can be written similar to first law efficiency:

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(28)

3.3.3. Energy and exergy analysis of steam superheater:

Objective of superheater is to increase the temperature of steam as much as possible by

using waste heat of flue gas. It increases the degree of superheat of saturated steam

obtained from steam generator. For steady state operation energy and exergy balance can

be applied similar to steam generator.

Energy balance:

(29)

As, & , so

(30)

First law efficiency of superheater can be written as;

(31)

Exergy balance:

(32)

As, & , so

(33)

Second law efficiency of superheater can be determined as;

(34)

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Energy and exergy analysis of stack:

As no energy transfer takes place in stack, so all of energy at stack is lost to the

environment. Energy loss and exergy destruction in stack can be found out as;

(35)

And exergy destruction;

(36)

Energy and exergy analysis of overall boiler:

Hence overall first law efficiency of the boiler is:

(37)

And energy lost in the boiler:

(38)

Overall second law efficiency of the boiler is:

(39)

And exergy destruction in the boiler:

(40)

3.3.4. Economic evaluation of energy saving potential:

Various energy saving measures are suggested in various studies e.g. energy recovery

from flue gases, use of variable speed drive (VSD) in boiler fan and pump, use of

turbulator in combustion chamber, minimization of radiation loss from boiler geometry,

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energy recovery from blow down etc. Economic associated to energy saving can be

formulated as;

(41)

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C H A P T E R 4

RESULTS AND DISCUSSIONS

This chapter covers major findings as a result of thermoeconomic evaluation of three pass

fire tube boiler. To perform thermoeconomic evaluation various operating parameters

against different capacities are selected given in table 3.1. Then various investigations

were performed in order to analyze energy and exergy of boiler and then at various

sections of the boiler to identify the location and the potential energy loss and exergy

destruction.

Table 4.1: Total energy flow for each stream

Capacity

E1 E2 E3 E4 E5 E6 E7 E8

tons/hr kW kW kW kW kW kW kW kW

0.5 1.89 428.19 426.77 67.52 84.44 375.42 380.35 79.13

1 3.61 834.42 832.97 135.08 161.08 751.47 758.89 153.21

2 7.45 1701.77 1703.01 270.07 345.4 1504.31 1528.98 319.17

3 11.29 2591.09 2582.14 404.68 522.17 2251.72 2286.37 484.63

4 14.85 3447.46 3426.35 538.89 709.17 3005.96 3069.92 640.48

5 18.69 4314.81 4290.56 674.83 888.72 3754.05 3833.15 804.56

6 22.61 5105.31 5080.65 809.98 1096.03 4501.37 4607.78 981.08

7 25.73 5972.67 5936.11 944.56 1263.69 5251.9 5392.37 1112.85

8 30.32 6883.94 6845.54 1079.78 1488.69 6006.98 6161.68 1321.58

Table 4.2: Total exergy flow for each stream

Capacity

Ex1 Ex2 Ex3 Ex4 Ex5 Ex6 Ex7 Ex8

tons/hr kW kW kW kW kW kW kW kW

0.5 1.27 398.22 251.69 10.76 21.26 132.03 134.06 19.04

1 2.43 776.01 504.03 21.57 40.34 267.07 270.15 37.09

2 5.01 1582.651019.3

143.01 89.4 528.57 538.64 78.13

3 7.57 2409.711522.8

364.53

134.91

794.59 808.97 118.84

4 9.97 3206.142055.3

886.31

188.55

1071.62 1098.7 158.54

5 12.54 4012.772561.1

4107.66 235.1 1328.31

1361.44

198.30

24

6 15.18 4747.943087.7

1129.36

295.12

1598.841643.6

1244.27

7 17.27 5554.583619.8

7150.81

343.88

1863.941923.2

8277.12

8 20.35 6402.064137.9

8172.55

405.14

2139.432205.5

5331.56

Economic loss as a result of inefficiency of the boiler was also explored. Various energy

saving strategies, their economic perspectives and payback period to each energy

conservation measure was also calculated. At first step energy and exergy value of each

stream was calculated then various parameters were evaluated.

4.1. First and Second law analysis:

Energy and exergy value for each stream are presented in table 4.1 & 4.2. Boiler inputs

are energy and exergy of fuel and air where air imparts negligible contribution as

compared to the fuel values while outputs of the boiler are energy and exergy values

carried by superheated steam. Various boiler performance parameters were calculated.

Thermal effectiveness of the boiler is nearly constant at various steam production

capacity value and relatively low due to excessive thermal losses in the boiler (figure

4.1). For an efficient boiler thermal effectiveness should be greater than 0.90. Quality

preservation index refers to the ratio of product quality (exergy/energy of steam) to the

source quality (exergy/energy of fuel). Source quality is equal to fuel exergy grade

function given in table 3.4 while delivered product quality is exergy value of steam to

energy supplied by fuel. Low quality preservation index is due to very low exergy/energy

ratio of steam which is seldom greater than 0.4.

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Figure 4.1: Thermal effectiveness and quality preservation Index of boiler

It is observed that at very low capacity boiler yield is 16.67 tons steam/ton fuel burned

that decreases to a magnitude of 15 at 37.5% capacity then remains nearly constant at

higher capacity (figure 4.2). At very low capacity more fuel energy is available for steam

production and less energy is lost in steam generator and stack. It is observed that at

higher capacity energy loss increases majorly due to increase in stack temperature. Table

3.2 shows that for each 50% decrease in boiler capacity stack temperature lowers by

almost 5 oC which results more fuel energy available for steam production.

Figure 4.2: Boiler yield vs capacity

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Figure 4.3 shows variation in energy and exergy efficiency of boiler along the capacity.

Energy and exergy efficiency of the boiler are only 73.75% and 31.41% which vary

between 73.06-74.76% and 30.8-31.91% respectively. 26.25% fuel energy and 68.59%

fuel exergy are lost due to inefficiency of the boiler. An efficient boiler must have overall

efficiency more than 90% which is far less than the target value. As in this work

combustion efficiency is considered as 100% in the presence of excess air, so 26.25% of

thermal inefficiency is very high for a boiler running on natural gas.

Figure 4.4 shows energy lost in kW in various sections of boiler. As capacity of boiler

increases total energy lost increases that provides more energy saving potential at higher

capacity. Stack results maximum energy lost followed by steam generator, combustor and

superheater.

Figure 4.3: Energy and exergy efficiency vs capacity

Negligible amount of energy is lost in superheater due to its low duty. Ideally there is no

energy lost in combustion chamber as it is considered to be operating adiabatically and all

of energy of combustion reaction is available to raise the temperature of combustion

products.

27

Figure 4.4: Energy lost in boiler components vs capacity

Figure 4.5 shows exergy destruction in kW in various sections of boiler. Unlike energy

loss in the boiler, exergy destruction have different trend along proceeding stages of the

boiler. Maximum exergy destruction has been observed in combustion chamber due to

intrinsic nature of combustion reaction. Steam generator, stack and superheater contribute

next major exergy destroying components. Magnitude of exergy destruction in boiler

increases with increase in capacity.

28

Figure 4.5: Exergy destruction in boiler components vs capacity

Figure 4.6 shows energy loss and exergy destruction distribution in various sections of

boiler. It is observed that stack results maximum energy loss (72.5 %) because exhaust

gases leaves at very high temperature carrying large amount of energy which is lost in

environment (Barreras at al. 2004). From table 3.2 it is observed that exhaust temperature

increases with capacity. For each 50% decrease in capacity exhaust temperature lowers to

almost 5 oC. Exhaust gases temperature can be reduced to minimize energy losses. Steam

generator is the second major unit that causes energy loss. Upto 27% energy is lost due to

radiation loss and inefficiency of steam generator. Maximum exergy is destroyed in the

combustion chamber (52.37%) where combustion reaction takes place. Chemical reaction

and heat transfer are responsible for irreversibilities in combustor (Moran & Boehm,

1997; Durmayaz & Yavuz, 2001). In most of physical processes internal thermal energy

exchange plays a major role in irreversibilities (Soma & Datta, 2008). By nature

combustion reaction is an irreversible process and exergy loss associated to this intrinsic

irreversibility cannot be avoided. Steam generator and stack contribute to exergy

destruction as 40.3% and 7.2% respectively. Stream to stream heat transfer in steam

generator and exhaust of flue gases at relatively high temperature are major causes of

exergy destruction in steam generator and stack respectively.

29

Figure 4.6: Energy loss and Exergy destruction distribution in boiler components

Energy and exergy flow diagram (figure 4.7) represents a breakdown of total fuel energy

lost and fuel exergy destruction in individual component of the boiler.

Figure 4.7: Energy loss and exergy destruction flow diagram

Stack is the major contributor in fuel energy loss followed by steam generator while

combustor is major contributor in fuel exergy destruction followed by steam generator. In

combustor negligible fuel energy is lost due to adiabatic conditions while 36.03% fuel

exergy is lost due to inherent irreversibilities, excess air and low combustion air

temperature. Steam generator shares 7% and 27.73%, superheater 0.12% and 0.09%,

30

stack shares 18.73% and 4.96% and overall 26.68% and 68.81% fuel energy loss and

exergy destruction occurs respectively.

Figure 4.8 shows energy and exergy efficiency of individual component and overall

boiler. It is observed that combustor is 99.17% energy and 64.08% exergy efficient,

steam generator is 91.15% and 52.58% and superheater is 93.2% and 90.23% energy and

exergy efficient respectively while overall system is 73.75% energy and 31.41% exergy

efficient.

Figure 4.8: Comparison of energy and exergy efficiency

4.2. Exergy saving potential:

Exergy analysis identified the potential loss in various sections of the boiler and provides

basis for exergy savings. Combustor, steam generator and stack are major contributors of

exergy loss and magnitude of loss increases with increase in boiler capacity. At higher

capacity boiler have more potential of exergy and hence economic saving. Exergy saving

measures can be applied on individual component.

4.2.1. Exergy saving potential in combustor:

Combustor results 36.03% fuel exergy destruction and shares 52.37% of total exergy

destruction in the boiler. Intrinsic irreversibility, low combustion air temperature and

31

fuel/air ratio are major sources of exergy destruction. Irreversibilities due to combustion

reaction are unavoidable.

Yet the magnitude of irreversibility can be minimized by suitably controlling combustion

air temperature and the excess air. Excess air is analyzed by measuring the concentration

of oxygen in flue gases. A minimum percentage of excess air is unavoidable for complete

combustion yet excess air over that causes both energy and exergy loss by carrying

energy through flue gases. Combustion reaction gives maximum adiabatic flame

temperature at stoichiometric amount of air. As the percentage of excess air increases,

adiabatic flames temperature decreases, overall volume of flue gas increases and energy

quality also decreases. Rate of exergetic destruction can be reduced by oxygen

enrichment of air. Also preheating combustion air increases the energy availability for

steam production. Exergy efficiency can be increased by reducing heat conduction due to

internal mixing which can be achieved by controlling temperature gradient in combustion

chamber e.g. air preheating, fuel-air staging and controlling jet velocity. All these

strategies reduce mass flow and the temperature of flue gases, hence increasing energy

and exergy efficiency of the system. Soma et al. (2008) also presented the effect of fuel

inlet pressure on combustion and exergetic efficiency of various light hydrocarbon fuels.

He concluded that with increase in inlet pressure the combustion efficiency decreases

while exergetic efficiency increases.

Figure 4.10 shows effect of combustion air temperature on exergy efficiency of

combustor. In this work combustion air was supplied at 35 oC which requires a part of

energy available to raise its temperature to reaction temperature. It was observed that by

raising air temperature from 25 oC to 150 oC exergy efficiency increased by 4.26%. Also

an increase in air temperature increases available energy in combustion products by

increasing the extent of reaction. Waste heat from exhaust gases can be utilized to preheat

combustion air.

4.2.2. Exergy saving potential in steam generator:

Steam generator results 7% and 27.73% fuel energy loss and exergy destruction and

shares 27.01% and 40.3% in total respectively. Heat transfer irreversibilities are

32

responsible for energy and exergy inefficiency. In steam generator heat transfer

Irreversibilities reduces as approach temperature (minimum temperature difference

between heat transfer streams) decreases (Saidur, 2010). It is observed that average

approach temperature at steam generator outlet is 76 oC that leaves a wider room for

energy saving.

Figure 4.9: Effect of combustion air temperature on combustor exergy efficiency

Also from the experience it is observed that due to burning of fuel oil in the boiler, an

unburned carbon layer has been deposited inside boiler tubes. It acts as heat insulation

and reduces heat transfer flux. Proper cleaning of boiler tubes will increase heat transfer

rate and will increase boiler yield decreasing steam cost and energy and exergy lost.

Increased energy and exergy efficiency in steam generator and superheater can be

achieved by reducing radiation losses and using high thermal conductivity material for

heat transfer.

Figure 4.11 shows the effect of superheated steam temperature on boiler efficiency. It is

observed that by increasing steam temperature from 235 oC to 260 oC energy and exergy

efficiency increases by 2.55% and 1.17% respectively. Also by increasing pressure from

25 bar to 30 bar there is negligible effect on exergy efficiency and energy efficiency

decreases due to reduction in steam quality.

33

Figure 4.10: Effect of superheated steam temperature on exergy efficiency

Figure 4.12 shows increase in exergy efficiency of steam generator as a function of

recovery in thermal losses. It is observed that at full capacity 430MW thermal energy is

lost in steam generator. Radiation losses and heat lost due to carbon deposits inside boiler

tubes are major sources of heat loss. If all heat losses in the steam generator are recovered

by proper insulation and applying maintenance then its exergy efficiency can be

increased by 2.95%.

Table 4.11: Thermal loss recovered in steam generator vs exergy efficiency

4.2.3. Exergy saving potential in stack:

Typical flue gases contain 15-40% of fuel heat contents depending upon stack

temperature (Chen et al., 2010). In this study stack results 18.73% fuel energy and 4.96%

34

fuel exergy loss while 72.5% overall energy loss and 7.2% overall exergy loss due to

very high temperature of exhaust gases leaving in atmosphere. It is observed that stack

temperature ranges between 256 oC to 275 oC. Energy from stack can be utilized to

preheat combustion air that in turn increases combustor and overall energy and exergy

efficiency.

Flue gas dew point is a major obstruction in waste heat recovery. Stack temperature can

be reduced to the dew point of exhaust gases with simple heat exchange and even below

the dew point (upto 30 oC) by using condensing boiler (Riffat et al., 2006, Chen et al.,

2010). Below the flue gas dew point sulphuric acid formation takes place that cause cold

end corrosion in boiler tube and stack lining (Barreras & Barroso, 2004). With natural gas

boiler, thermal efficiency can be increased from 75% to 90% and considerable reduction

in CO2 emissions using condensing boilers (Weber, 2002). As the temperature of flue

gases decreases overall energy available for air preheating increases yet the quality of

energy decreases with decrease in exhaust temperature. Figure 4.13 shows energy and

exergy recovery potential in stack. It is observed that decreasing stack temperature

energy and exergy saving potential increases. By lowering stack temperature from 275 oC

to 145 oC 76.88% exergy destruction and 39.85% energy losses in stack can be avoided.

Boiler energy efficiency increases by 6.56% and exergy efficiency increases by 4.02% as

shown in Figure 4.14.

4.3. Financial saving associated with exergy recovery:

Exergy analysis provides true economic saving potential as a result of exergy recovery in

various sections of the boiler. Overall exergy destroyed and the exergy based fuel saving

has been economically evaluated. For economic analysis following assumptions has been

made:

35

Table 4.3: Assumptions for economic analysis

Parameter Unit ValueNatural gas teriff PKR/MMBTU 507Unit energy cost PKR/MJ 0.480Unit exergy cost PKR/MJ 0.516Annual operating hours hr 7900

Figure 4.12: Energy and exergy recovery potential in stack

Figure 4.13: Effect of stack temperature on overall exergy efficiency

36

Methodology described in section 3.4 is used for economic evaluation. Unit exergy cost

is calculated by considering total fuel exergy consumed (fuel heating value*exergy grade

function) and total fuel cost associated to that exergy consumption. Cost analysis of

boiler at 100% load is determined and results are summarized in table:

Table 4.4: Annual cost balance sheet

Parameter Unit ValueFuel consumption MNm3 4.953Fuel cost MPKR 94.008Steam production tons 63200Steam worth MPKR 29.853Financial lossCombustor MPKR 33.545Steam generator MPKR 25.931Superheater MPKR 0.110Stack MPKR 4.569

4.3.1. Cost saving potential in Combustor:

Exergy based fuel cost saving potential in combustor has been determined. Exergy saving

potential as a function of combustion air temperature is given in figure 4.14. It is

observed that by raising combustion air temperature upto 150 oC, 11.924% to financial

loss in combustor due to exergy destruction can be recovered. Annual fuel saving

potential in combustion chamber is found to be 3.99 Million PKR.

Figure 4.14: Annual fuel saving potential in combustor vs combustion air temperature

37

4.3.2. Cost saving potential in Steam generator:

Cost saving potential in steam generator as function of thermal loss recovered is

evaluated. It is found that annually 25.931 Million PKRs are lost due to thermal losses.

Fuel cost saving potential as a function of thermal loss recovered is determined and given

in figure 4.15. It is found that by recovering 80% thermal losses, 2.157 Million PKRs can

be saved.

Figure 4.15: Annual fuel saving potential in steam generator vs thermal loss recovered

4.3.3. Cost saving potential in stack:

Cost saving potential in stack as a function of reduction in exhaust gases temperature is

determined and results are shown in figure 4.16. It is found that by lowering stack

temperature upto 145 oC, 3.742 Million PKRs can be saved.

So total fuel saving as a result of exergy saving measures is given in table 4.5:

Table 4.5: Annual fuel saving

Component Unit Saving

Combustor MPKRs 3.99

Steam generator MPKRs 2.17

Stack MPKRs 3.74

Total MPKRs 9.90

38

Figure 4.16: Annual fuel saving potential in stack vs stack temperature

39

C H A P T E R 5

CONCLUSIONS & RECOMMENDATIONS

After detailed first and second law analysis of fire tube industrial boiler, following

conclusions has been prepared:

Second law (exergy) analysis provides true basis for thermal and economic

evaluation of industrial boiler and power plants. Sometime first law (energy)

analysis misleads in performance evaluation, also it does not have a capability to

identify and quantify the potential losses at various sections of the boiler. Exergy

analysis gives quantitative and qualitative insight of energy storage and

conversion systems. Also it helps in the design, diagnose and optimize energy

conversion systems.

Thermoeconomic evaluation is a good tool for the determination of economic

perspectives of different operating variables in the boiler. Effect of combustion air

temperature, steam generator temperature & pressure, heat recovery from steam

generator and superheater, flue gases exhaust temperature excess air on annual

cost saving and their optimization can be better analyzed using thermoeconomic

methodology.

First law analysis of the boiler revealed that boiler is only 73.75% thermally

efficient. Whereas steam generator and superheater are 91.15% and 93.2%

efficient respectively. Rest of the heat is lost to the environment from different

sections of the boiler. It is observed that out of 854.94kW of fuel energy

221.08kW s lost per ton steam production. Stack is major source of energy loss

that results 18.73% fuel energy and 72.5% of total energy lost. While steam

generator causes 7% of fuel energy and 27.01% of total energy lost. Superheater

and combustor result negligible heat loss due to very low heat duty in superheater

and adiabatic combustion in combustion chamber.

40

Second law analysis revealed very low exergy efficiency. Exergy efficiency of the

boiler is 31.41% while combustor, steam generator and superheater are 64.08%,

52.58% and 90.23% efficient respectively. In the boiler out of 795.15kW of fuel

exergy 547.11kW exergy is destroyed per ton steam production. Combustor is a

major source of exergy destruction that results 36.03% of fuel exergy and 52.37%

of total exergy destruction. Steam generator and stack results 27.73% and 4.96%

of fuel exergy whereas 40.3% and 7.2% of total exergy destruction respectively.

Superheater results negligible exergy destruction due to very low exergy duty.

Economic analysis of the boiler shows that per ton steam costs 1478 PKRs on

exergy basis. Actual fuel cost per ton steam production is just 461 PKRs while

1017PKRs are lost due to irreversibilities of the boiler. Combustor, steam

generator and superheater shares individual loss of 533, 410 and 73 PKRs. A part

of this economic loss can be recovered by applying conservation measure, while a

part is unavoidable due to some intrinsic irreversibilities e.g. combustion reaction.

Stack causes 18.73 % fuel energy loss and 4.96% fuel exergy destruction.

Analysis of results provided that if flue gas exhaust temperature is lowered to 145 oC then 39.84% energy loss and 76.88% exergy destruction in the boiler can be

recovered. Also this recovery improves boiler energy efficiency from 73.75% to

80.31% by 6.56% and boiler exergy efficiency from 31.41% to 35.43% by a

factor of 4.02%. Exhaust can be lowered to further extent by using condensing

coils but unit exergy recovery below 145 oC is not so cheaper.

Steam generator as another major source of exergy destruction. 220.48 kW of

exergy is destroyed in steam generator per ton steam production. It causes 7% of

fuel energy loss and 27.73% of fuel exergy destruction. Thermal losses in steam

generator are a result of radiation losses and carbon deposit inside the boiler tubes

due which produces due to the burning of heavy fuel oil as a replacement of

natural gas. If thermal losses are recovered by proper insulation and internal

cleaning of boiler tubes then exergy efficiency of steam generator is increased

from 52.58% to 55.53% by a factor of 2.95%.

Combustion chamber is the biggest source of exergy destruction. 286.52 kW of

fuel exergy is destroyed per ton steam production due to combustion and transport

41

irreversibilities. Temperature gradient between feed streams and combustion

product is an avoidable while combustion is an unavoidable source of

irreversibilities. If combustion air is preheated to 150 oC then its exergy efficiency

can be increased from 64.08% to 68.34% by a factor of 4.28%. Combustion air

can be preheated by utilizing waste energy from flue gases.

In order to save exergy from stack it is required to pinch flue gas heat with

combustion air. It will reduce the irreversibilities both in stack and combustor as

well.

Periodic cleaning and maintenance of fireside of boiler tubes will increase heat

transfer between combustion products and boiler feed water and reduces steam

generator irreversibilities.

It is recommended that sootblowers be installed in order to remove the deposits

formed inside firetubes due to burning heavy fuel.

More studies are required to distinguish the irreversibilities caused by combustion

reaction, excess air and air temperature. It will help in more exergy saving in

combustor.

42

NOMENCLATURE:

Destroyed irreversibility (kW)

Energy flow (kW)

Exergy flow (kW)

First law efficiency

Heat transfer (kW)

Mass flow rate (kg/s)

Second law efficiency

Specific exergy (kJ/kg)

Work done (kW)

Subscripts & Superscripts:

a air

B boiler

--chem chemical

comb combustor

f fuel

g gas

h Specific enthalpy (kJ/kg)

i inlet

o outlet

s steam

sg steam generator

sh superheater

st stack

T Temperature (oC)

--th thermal

w water

43

APPENDIX:

Table A1: Boiler performance Evaluation

Capacity (%)

Thermal Effectiveness

Quality preservation index

Boiler yield

Energy efficiency (%)

Exergy efficiency (%)

6.3 0.73 0.42 16.67 73.06 30.8612.5 0.75 0.43 16.67 74.76 31.9325.0 0.74 0.42 15.38 73.98 31.2237.5 0.73 0.43 15.01 72.62 30.8050.0 0.73 0.43 14.81 73.42 31.4862.5 0.73 0.43 14.71 73.21 31.1575.0 0.74 0.43 15.06 74.39 31.7987.5 0.74 0.43 14.89 74.47 31.81100 0.74 0.43 14.81 73.82 31.65

Table A2: Fuel energy loss and exergy destruction in boiler components

Component Fuel energy loss (%) Fuel exergy destruction (%)

Combustor 0.83 36.03Steam Generator 7.01 27.73Superheater 0.12 0.09Stack 18.73 4.96Overall 26.68 68.81

Table A3: Energy and exergy efficiency in boiler components

Component Energy efficiency (%) Exergy efficiency (%)Combustor 99.17 64.08Steam Generator 91.15 52.58Superheater 93.20 90.23Overall 73.75 31.41

44

Table A4: Energy and exergy efficiency in boiler components

Capacity ηcomb ηsg ηsh ηB ψcomb ψsg ψsh ψB

% % % % % % % % %

6.3 99.23 89.94 92.84 73.06 63.02 52.63 91.44 30.8612.5 99.40 91.74 94.28 74.76 64.75 52.94 94.77 31.9325.0 99.64 90.91 94.05 73.98 64.20 52.22 89.35 31.2237.5 99.22 89.66 92.30 72.62 63.01 52.61 89.48 30.8150.0 98.96 90.80 93.11 73.42 63.91 52.78 90.24 31.4862.5 99.01 90.52 93.99 73.20 63.63 52.48 90.03 31.1575.0 99.08 92.64 92.57 74.39 64.83 52.62 88.04 31.7987.5 98.96 92.19 93.13 74.47 64.97 52.29 88.89 31.81100 99.01 91.98 92.57 73.82 64.43 52.69 89.86 31.65

Table A5: Exergy destruction causes and saving measure

Component Causes Mitigation

Combustor Temperature gradient Preheat combustion air

Excess air Use of VSD

Reaction irreversibilities Unavoidable

Steam generator Radiation losses Insulation

Carbon deposits Cleaning

Stack High Temperature Energy pinch

45