Fyp Report Final.pdf

OPTMIZATION OF WASTE HEAT RECOVERY BOILER

Final Year Project Report

Group: 15

Batch: 2013-2014

Mohsin Ali Khan

ME-13211

Muhammad Daniyal

ME-13214

Syed Wajahat Hussain

ME-13216

Syed Ahmed Hasan

ME-13218

Internal Advisor:

Mohammad Ehteshamul Haque Assistant Professor Department of Mechanical Engineering

Reference#: 15/2017 DEPARTMENT OF MECHANICAL ENGINEERING NED UNIVERSITY OF ENGINEERING AND TECHNOLOGY

CERTIFICATE It is to certify that the following students have completed their project “Optimization of Waste Heat Recovery Boiler” satisfactorily.

Group: 15

Batch: 2013-2014

Name

Seat No.

Mohsin Ali Khan

ME-13211

Muhammad Daniyal

ME-13214

Syed Wajahat Hussain

ME-13216

Syed Ahmed Hasan

ME-13218

Internal Advisor Mohammad Ehteshamul Haque Assistant Professor Department of Mechanical Engineering NED University of Engg. & Tech.

Projects’ Coordinator Mr. Masood Ahmed Khan Department of Mechanical Engineering NED University of Engg. & Tech.

DEPARTMENT OF MECHANICAL ENGINEERING NED UNIVERSITY OF ENGINEERING AND TECHNOLOGY

ACKNOWLEDGEMENTS We would like to take this opportunity to thank Almighty Allah to whom we owe everything. It is by the will of Almighty that we are finally able to complete this project.

Extensive credit goes to Mr. Mohammad Ehteshamul Haque, (project internal advisor) for providing the encouragement and helpful discussion throughout the course of our project. His presence has been a source of knowledge, spirit, encouragement and enthusiasm. His efforts steered us in fulfilling the arduous task of meeting our project.

We would like to express our gratitude, appreciation and special thanks to Mr Masood Ahmed Khan (project coordinator) for his time to time co-operation.

We would also like to acknowledge with much appreciation the crucial role of the staff of Mechanical Engineering Department’s computer programming laboratory and the Digital Library for their extended support, who gave the permission to use all required software’s and the necessary material to complete the project.

II

ABSTRACT Waste heat recovery is relatively better technology to improve the process in the plant by extracting waste energy and utilizing it in the processing and power generation. Waste heat boilers are the equipment that extract these energies and used for steam generation which is used in various processes. In cement plant the waste heat is recovered from Air quenched cooler (AQC) and Suspension Preheater (SP). The primary of objective is to increase the efficiency of the Waste heat recovery boilers which includes Boiler analysis, optimization of parameters, custom and design of boiler. Boiler analysis includes the energy balance for the all the equipment of the boiler at actual data and heat transfer across the tubes. Optimization of parameters includes the effect of Inlet temperature and flow rate of exhaust gases on the boiler efficiency and life. Custom design of boiler includes the changing tube materials, and tube area. Estimation of boiler tube life is also analyzed and optimized through erosion theory because the erosion is the most important parameter in defining the boiler tube life

III

TABLE OF CONTENTS 1.

INTRODUCTION........................................................................................ 9 1.1

WASTE HEAT RECOVERY: .................................................................. 9

1.2

CYCLES FOLLOWED IN WASTE HEAT RECOVERY SYSTEMS .... 9

1.2.1

Simple Rankine cycle: ......................................................................... 10

1.2.2

Organic Rankine cycle: ........................................................................ 10

1.2.3

Kalina cycle: ........................................................................................ 11

1.3

CONCERNED INDUSTRY .................................................................... 11

1.4

BASIC OPERATIONS OF CEMENT PLANT: ..................................... 12

1.4.1 1.5

Sources of waste heat in a cement plant: ............................................. 13 PROJECT & BACKGROUND: .............................................................. 14

1.5.1

Pre-requisites: ...................................................................................... 14

1.6

OBJECTIVES OF PROJECT: ................................................................. 15

1.7

SCOPE OF PROJECT ............................................................................. 15

2.

PROPERTIES OF FLUE GASES ............................................................ 16 2.1

CONSTITUENTS OF FLUE GASES: .................................................... 17

2.2

CALCULATIONS: .................................................................................. 17

2.2.1

Viscosity: ............................................................................................. 17

2.2.2

Specific heat: ........................................................................................ 19

2.2.3

Thermal conductivity: .......................................................................... 21

2.2.4

Prandtl number: .................................................................................... 22

3.

DESIGN OF WASTE HEAT RECOVERY BOILER: .......................... 24 3.1

ECONOMIZER DESIGN:....................................................................... 24

3.1.1

Design considerations: ......................................................................... 25

3.1.2

Governing parameters: ......................................................................... 25

3.1.3

Heat balance: ........................................................................................ 25

3.1.4

Considerations and assumptions: ......................................................... 26

3.1.5

Economizer constraints: ....................................................................... 26

3.2

ECONOMIZER DESIGN CALCULATIONS: ....................................... 26

4

3.2.1 3.3

Estimating the overall heat transfer co-efficient: ................................. 32 EVAPORATOR DESIGN ....................................................................... 36

3.3.1

Design and operational criterion: ......................................................... 36

3.4

EVAPORATOR DESIGN CALCULATIONS: ...................................... 37

3.5

SUPER HEATER DESIGN..................................................................... 44

3.5.1

Design consideration:........................................................................... 44

3.5.2

Design aspects:..................................................................................... 44

3.5.3

Design assumptions: ............................................................................ 45

3.6

SUPERHEATER DESIGN CALCULATIONS: ..................................... 45

3.6.1

OVERALL HEAT TRANSFER COEFFICIENT OF PLAIN TUBES 50

3.6.2

TOTAL SURFACE AREA FOR HEAT TRANSFER ........................ 50

3.6.3

NUMBER OF TUBES WIDE ............................................................. 51

3.6.4

TOTAL AREA FOR MASS FLOW.................................................... 51

3.6.5

TRANSVERSE PITCH ....................................................................... 51

3.6.6

WIDTH OF SUPERHEATER ............................................................. 51

3.7 4.

INDIRECT EFFICIENCY BOILER OLD CONDITION ....................... 52 OPTIMIZATION OF BOILER GAS SIDE PARAMETERS: .............. 54

4.1

Inlet gas temperature: ............................................................................... 54

4.2

Inlet gas flow rate: ................................................................................... 57

4.3

Constant Inlet energy: .............................................................................. 60

4.4

Conclusion: .............................................................................................. 61

5.

MATERIAL OPTIMIZATION: .............................................................. 62 5.1

GENERAL CRITERIA FOR MATERIALS SELECTION: ................... 62

5.1.1

Properties of different Steel Grades ..................................................... 63

5.1.2

TREND OF THERMAL CONDUCTIVITY ON ‘U’ VALUE:.......... 67

5.1.3

Overall heat transfer coefficient: ......................................................... 67

5.1.4

Evaluating U-value for mild steel: ....................................................... 67

5.1.5

CONCLUSIONS: ................................................................................ 72

5.1.6

EFFECT ON FINAL TEMPERATURES OF COLD FLUID: ........... 73

5.2 5.2.1

ECONOMIC ANALYSIS OF BOILER TUBE MATERIAL ................. 75 VOLUME OF MATERIAL USED IN HEAT EXCHANGERS

PIPING: ............................................................................................................. 76 5

5.2.2

TOTAL VOLUME .............................................................................. 77

5.2.3

MASS REQUIRED BY DIFFERENT GRADES ............................... 77

5.2.4

COST FOR EACH STEEL GRADE ................................................... 78

6.

MATERIAL INSPECTION: .................................................................... 79 6.1 6.1.1 6.2

CORROSION CALCULATIONS ........................................................... 79 JUSTIFICATION OF USING AISI-1010 ........................................... 80 EROSION IN BOILER TUBES .............................................................. 83

6.2.1

THEORY ............................................................................................. 83

6.2.2

LITERATURE REVIEW: ................................................................... 84

6.2.3

CALCULATION ................................................................................. 87

6.2.4

WEAR RATE AND VOLUME REMOVED PER HOUR AT

DIFFERENT VELOCITIES ............................................................................. 90 

Conclusion ………………………………………………………………..

90



References ……………………………………………………………….

91

6

LIST OF FIGURES Figure 1-2: T-s diagram of Organo Cycle ................................................................... 10 Figure 1-1: T-s diagram of Rankine Cycle .................................................................. 10 Figure 1-3: T-s diagram of Kalina Cycle .................................................................... 11 Figure 1-4: Schematic of Cement Plant....................................................................... 13 Figure 3-1: Rough Schematic of Boiler ....................................................................... 24 Figure 3-2: Standard dimensions for steel tubes ......................................................... 28 Figure 3-3: Different tube arrangement ...................................................................... 31 Figure 3-4: Thermal circuit of a fouled heat transfer surface..................................... 32 Figure 3-5: Resistance to the flow of heat through pipe walls .................................... 32 Figure 3-6: Fouling factor for different fluids ............................................................. 34 Figure 3-7: Constants for Nusselt correlation............................................................. 40 Figure 3-8: Heat flow across In-line Arrangement (Heat and Mass Transfer by Incopera ) ..................................................................................................................... 42 Figure 3-9: Table of constant for Nusselt correlation ................................................. 49 Figure 3-10: Indirect Method for Efficiency evaluation.............................................. 52 Figure 4-1: Temperature vs Overall Heat Transfer Coefficient for Economizer, Evaporator and Superheater........................................................................................ 54 Figure 4-2: Effectiveness Vs Inlet Gas Temperature for Economizer, Evaporator and Superheater .................................................................................................................. 55 Figure 4-3: Mass flow rate of gas Vs Overall Heat Transfer coefficient for Economizer, Evaporator and Superheater .................................................................. 58 Figure 4-4: Mass flow rate of gas Vs Effectiveness for Economizer, Evaporator and Superheater .................................................................................................................. 59 Figure 4-5: Mass flow rate Vs Effectiveness & Inlet Temperature Vs Effectiveness for Economizer .................................................................................................................. 60 Figure 4-6: Mass flow rate Vs Effectiveness & Inlet Gas Temperature Vs Effectiveness for Evaporator........................................................................................ 60 Figure 4-7: Mass flow rate Vs Effectiveness & Inlet Temperature Vs Effectiveness .. 61 Figure 6-1: T-s diagram of Rankine Cycle .................................................................. 81

7

LIST OF TABLES Table 1-1: High Temperature Source for Waste Heat Recovery ................................... 9 Table 2-1: Weight Fraction and Molecular Weight of Flue gas constituents ............. 17 Table 2-2: Viscosity of Flue gas constituents at different temperatures ..................... 17 Table 3-1: Design specifications for Economizer ........................................................ 26 Table 3-0-3: Standard dimension for steel tubes ......................................................... 28 Table 3-4: Maximum Velocity Ranges ......................................................................... 30 Table 3-5: Tube Specifications for Economizer........................................................... 30 Table 3-7: Design specifications for Evaporator ........................................................ 37 Table 3-10: Design specifications for Super-heater .................................................... 46 Table 4-3: Heat Loss across the Outside Wall Vs Inlet Gas Temperature .................. 57 Table 4-4: Efficiency VS Temperature ......................................................................... 57 Table 5-1: Chemical composition of ASTM A36 ......................................................... 63 Table 5-2: Mechanical properties of ASTM A36 ......................................................... 63 Table 5-3: Chemical composition of AISI 1040 ........................................................... 64 Table 5-4: Mechanical properties of AISI 1040 .......................................................... 64 Table 5-5: Chemical composition of AISI 1010 ........................................................... 64 Table 5-6: Mechanical properties of AISI 1010 .......................................................... 65 Table 5-7: Chemical composition of Stainless Steel Grade 410.................................. 65 Table 5-8: Mechanical properties of Stainless Steel Grade 410 ................................. 65 Table 5-9: Chemical composition of Stainless Steel 316 ............................................. 66 Table 5-10: Mechanical properties of Stainless Steel 316 .......................................... 66 Table 5-11: Thermal conductivities of different steel grades ...................................... 67 Table 5-12: Overall Heat Transfer coefficient and Surface Area of economizer for different materials ........................................................................................................ 69 Table 5-13: Overall Heat Transfer coefficient and Surface Area of evaporator for different material ......................................................................................................... 70 Table 5-14: Overall Heat Transfer coefficient and Surfaces Area of Super-heater for different material ......................................................................................................... 72 Table 5-15: Ranges of Price per Ton for different Steel Grades ................................. 75 Table 5-16: Mass required by boiler tubes for different material ............................... 78 Table 5-17: Cost of each steel grade ........................................................................... 78 8

CHAPTER #1 1. INTRODUCTION 1.1 WASTE HEAT RECOVERY: Waste Heat as the name defines “The Unused Potential”. Process industries using thermal energy may have this recovery option. Particular research shows that exhaust gases above 250°F (=122°C) have potential to produce energy. In this way fuel is economically utilized by not dumping the exhaust gases directly into the atmosphere. Large, quantity, of, hot flue gases, is generated from boilers, kilns, ovens and furnaces. The, essential, quality, of heat, is not its quantity but its quality. If some of this waste heat, could, be recovered, a, considerable, amount, of, primary, fuel could be saved. Although we cannot recover full energy from waste gases because there are certain limitations. The term Waste Heat Recovery (WHR) means reuse of the wasted potential other than the process; therefore it must not affect the production capacity of the plant anyhow. Following are some high temperature sources for the waste heat recovery: TYPES, OF, DEVICES

TEMPERATURE, (°C)

Nickel, refining, furnace

1400-1650

Steel, heating, furnaces

925-1050

Cement, kiln, (dry process)

620-730

Hydrogen, plants

650-1000

Fume, incinerators

600-1450

Table 1-1: High Temperature Source for Waste Heat Recovery

1.2 CYCLES FOLLOWED IN WASTE HEAT RECOVERY SYSTEMS There are three basic cycles followed: 1. 2.

Simple Rankine, Cycle Organic, Rankine, Cycle

3. Kalina, Cycle 9

1.2.1

Simple Rankine cycle:

There are four components:

Simple process is that, superheated steams

flow

through

the

turbine

converting its energy (thermal) to shaft power coupled

by

expansion with

process,

generator

shaft

producing

electricity. The lower energy steam then condenses in a condenser, and then this water is pumped to boiler again for

Figure 1-1: T-s diagram of Rankine

steam production.

Cycle

1.2.2

Organic Rankine cycle:

In simple Rankine cycle there is a condition for the steam to be superheated otherwise during expansion there could be high moisture content eroding the blades of the turbine. Thus organic fluids are used i.e. below 400°C do not have to be overheated resulting in an efficient cycle (fuel requirement of overheating discarded). The working fluid superheats as pressure is reduced, as also they have low freezing point.

Figure 1-2: T-s diagram of Organo Cycle

10

1.2.3

Kalina cycle:

A recent development in power generation technology is the Kalina cycle, which basically follows the Rankine cycle concept except that the working fluid is 70% ammonia–water mixture. It has the potential to be 10–15% more efficient than the Rankine cycle and uses conventional materials of construction, making the technology viable.

Figure 1-3: T-s diagram of Kalina Cycle

1.3 CONCERNED INDUSTRY Generally, a waste heat boiler costs too much If waste heat is of low quality it can be difficult to efficiently utilize the low quality contained by the waste heat medium 

WHRS may out-weight the benefit.



Additional equipments required



Complexity of the system increase



Additional maintenance costs

So the waste heat recovery boilers (WHRB) are efficient but not useful for every industry. Industries like: 

Chemical industry



Cement industry



Sulphuric acid industry



Coke dry quenching industry etc.

11

We have chosen Cement Industry as our primary focus, reasons are: 

Among energy-intensive industries, cement is the one where waste heat recovery (WHR) has been most developed.



Eligible for thorough learning.

 Ease of approach to learn. 1.4 BASIC OPERATIONS OF CEMENT PLANT: 1. The raw material required to manufacture cement are limestones and clay. Rocks extracted from the quarry are routed to cement plant nearby. 2. The minerals from the quarry are routed to grinding plant where they undergo initial milling before being reduced to fine powder. The raw material (80% limestone and 20% clay) stored in pre homogenization pile. This mixture is called the “raw meal”. 3. The raw mix is fed into pre-heating tower at 800 °C before returning to inclined rotary kiln where it is heated to about 1450 °C. Combustion process causes decarbonization of limestone. The fired material take the form of hard granules called “clinkers”. 4. Following the cooling, the clinker is stored in the silos then transformed into cement according to the production requirement. At final stage, for increasing the settling time gypsum is added to the clinker in proportion of 3-5% and mixture is finely ground. 5. The cement is stored in silos before being delivered in bulk using trucks or packed into for the shipment. Generally, the processes can be summarized as:        

Blasting Crushing Grinding Pre-heating (suspension pre-heater) Combustion Cooling (Air Quenched Cooler) Fine grinding Packaging

12

Figure 1-4: Schematic of Cement Plant

1.4.1

Sources of waste heat in a cement plant:

Considering the kiln; generally twelve feet or more in diameter and length of around 500 feet made of steel lined with fire bricks, gradually slanted with intake end higher than output end, mounted on roller bearings. In kiln burning of the blended material take place. The burning zone can be heated up to 3000°F such that material turns its color. The combustion of the fuel takes place only in the kiln. From here the exhaust gases enters in the suspension pre-heater through k-line where the pre-heating of the blended material is done. Afterwards, the clinker is send to grate cooler, where clinker is spread on perforated moving bed where air is blown from the bottom to cool the clinker. This process is air quenching, also called Air Quenched Cooling. Therefore, the exhaust gases leaving the suspension pre heater and the hot air by the quenching are the two major sources of heat recovery.

13

1.5 PROJECT & BACKGROUND: The energy crisis is the concern that the world & demand on the limited natural resources that are used to power the industrial sector as resources are limited and demand is continuously increasing. Natural resources may take hundred/ thousands of years to emerge out again to replenish the stores.     

Causes of energy crisis are; Poor infrastructure of equipment. Over – population. Delay in commissioning of power plants. Wastage of energy.

Population is somewhat we can’t control. Infrastructure and recovery of waste energy are the major ways to decelerate the increasing demand shortages. However,

WHRvfsremainsehuneconomicvfincnumerousrcountries,bespeciallygcin

areasdwhereflowcelectricitydpricesedoenotcmakevthevconversioneofwwastenheatm intowelectricityzofqinterest.qWherevelectricityvsupplywiswazproblemr(becauseecof uncertaintiesxregardingwelectricityvvoltage),cevenbifstheqpricenofeelectricitynisslow ,qWHRqcanqbeqadvantageousqasqitqallowsqthe plant to be more self-sufficient.

1.5.1 Pre-requisites: To accomplish the project in a right manner, there are several things to be fulfilled.     

Parametric analysis: EXCEL. General knowledge about the design of boiler. Fluid mechanics. Thermodynamics. Chemical stoichiometry.

14

1.6 OBJECTIVES OF PROJECT: Performance evaluation and optimization of the waste heat recovery boiler by varying different parameters.

1.7 SCOPE OF PROJECT The scope of this report includes:  Basic understanding of the operations of a Waste Heat Recovery System in a cement plant.  Boiler Analysis.  Optimization of Boiler includes the effect of Inlet Temperature and flow rate of exhaust gases on the boiler efficiency and life.  Custom design of boiler includes the changing in the tube materials, and tube area.

 Efficiency enhancement of the accessories attached within the Waste Heat Recovery System.

15

CHAPTER # 2 2. PROPERTIES OF FLUE GASES In the design of waster heat recovery boiler properties of flue gases are required to perform energy – balance calculation and heat transfer and pressure drop studies. While applying energy – balance to calculate Heat content

We will need

:

(Specific Heat).

It can also be seen in the design calculations that we are interested in finding the heating surface area of the tubes. Using formula:

In the above expression, for finding ‘U’, we have:

In order to find ‘ ’ (Convective Heat Transfer Coefficient), we must know the following dimensionless numbers:  

Reynold Number Nusselt Number

And the formula for both Reynold

and Nusselt

And, . Finally to calculate :

As we can see, in order to: 16

we will be using are:

-

Find

we need

-

Find

, we need

-

Find , we need

(viscosity) (Prandtl Number) (Thermal Conductivity)

So we have to calculate these parameters first at average temperatures. Here, we will be calculating all the flue gas parameters at average temperatures i.e. 333.45

and 280 .

2.1 CONSTITUENTS OF FLUE GASES:

CONSTITUENTS

WEIGHT FRACTION

MOLECULAR WEIGHT

CO2

0.22

44

H2 O

0.06

18

SO2

0.015

64

O2

0.02

32

NO2

0.67

46

Table 2-1: Weight Fraction and Molecular Weight of Flue gas constituents

2.2 CALCULATIONS: 2.2.1 Viscosity:

Temperature (℃) 380 300 245.5 219 0.1079 0.0953 0.0883 0.848 CO2 0.087 0.074 0.067 0.0641 H2 O 0.1002 0.0868 0.0796 0.076 SO2 0.133 0.1207 0.1135 0.11 O2 0.1264 0.1138 0.0891 0.1 NO2 Table 2-2: Viscosity of Flue gas constituents at different temperatures μ of

17

-

-

FOR TEMPERATURE = 380 ˚C

.

.

.

Gases

(weight frac. each)

(each)

CO2

0.22

0.1079

6.633249

0.15746

1.45931478

H 2O

0.06

0.087

4.24264

0.02215

0.2545584

SO2

0.015

0.1002

8

0.01202

0.12

O2

0.03

0.133

5.65685

0.02257

0.1697055

NO2

0.67

0.1264

6.78233

0.57438

4.5441611

Viscosity @ 380

(kg/m/hr) =

0.120435

Viscosity @ 380

(kg/m/sec) =

3.3454 E – 05

FOR TEMPERATURE = 300 ˚C

Gases

.

.

.

(weight frac. each)

(each)

CO2

0.22

0.0953

6.633249

0.13907

1.45931478

H 2O

0.06

0.074

4.24264

0.01884

0.2545584

SO2

0.015

0.0868

8

0.01042

0.12

O2

0.03

0.1207

5.65685

0.02048

0.1697055

NO2

0.67

0.1138

6.78233

0.51713

4.5441611

Viscosity @ 300

(kg/m/hr) =

0.10781354

Viscosity @ 300

(kg/m/sec) =

2.9948 E – 05

18

-

FOR TEMPERATURE = 245.5 ˚C

.

Gases

CO2 H 2O SO2 O2 NO2

-

(weight frac. each) 0.22 0.06 0.015 0.02 0.67

.

.

(each) 0.0883 0.067 0.0796 0.1135 0.0891

6.633249 4.24264 8 5.65685 6.78233

0.12886 0.01706 0.00955 0.01284 0.40488

Viscosity @ 245.5

(kg/m/hr) =

0.08852081

Viscosity @ 245.5

(kg/m/sec) =

2.45289 E – 05

1.45931478 0.2545584 0.12 0.113137 4.5441611

FOR TEMPERATURE = 219 ˚C

Gases (weight frac. each) 0.22 CO2 0.06 H 2O 0.015 SO2 0.03 O2 0.67 NO2

.

.

.

(each) 0.0848 0.0641 0.076 0.11 0.1

6.633249 4.24264 8 5.65685 6.78233

0.12375 0.01632 0.00912 0.01867 0.45442

Viscosity @ 219

(kg/m/hr) =

0.09503597

Viscosity @ 219

(kg/m/sec) =

2.6399E – 05

2.2.2 Specific heat: Formula: Cp = Cp=

19

1.45931478 0.2545584 0.12 0.1697055 4.5441611

FOR TEMP = 380 ˚C GASES

Cp@380 kcal/kg ˚C

weight fraction

CO2 H2O SO2 O2 NO2

0.2347 0.4652 0.1689 0.2301 0.236

0.22 0.06 0.02 0.03 0.67

GASES

Cp@280 kcal/kg˚C

weight fraction

CO2 H2O SO2 O2 NO2

0.2266 0.4585 0.1641 0.2267 0.2335

0.22 0.06 0.02 0.03 0.67

GASES

Cp@250 kcal/kg˚C

weight fraction

CO2 H2O SO2 O2 NO2

0.222 0.4554 0.1612 0.2251 0.2331

0.22 0.06 0.02 0.03 0.67

Cp * weight fraction 0.051634 0.027912 0.003378 0.006903 0.15812

FOR TEMP = 280 ˚C Cp * weight fraction 0.049852 0.02751 0.003282 0.006801 0.156445

FOR TEMP = 250 ˚C

20

Cp * weight fraction 0.04884 0.027324 0.003224 0.006753 0.156177

FOR TEMP = 219 ˚C GASES

Cp@219 kcal/kg˚C

weight fraction

CO2 H2O SO2 O2 NO2

0.22 0.4545 0.1599 0.224 0.2312

0.22 0.06 0.02 0.03 0.67

Cp * weight fraction 0.0484 0.02727 0.003198 0.00672 0.154904

2.2.3 Thermal conductivity: Formula:

FOR TEMP = 380 ˚C Gases CO2 H2O SO2 O2 NO2

K@380 k.cal/m.hr.˚C 0.0348 0.0454 0.0257 0.0434 0.0447

K= FOR TEMP = 300 ˚C Gases CO2 H2O SO2 O2 NO2

k@300 k.cal/m.hr.˚C 0.032 0.0372 0.0231 0.0336 0.039

K= 21

FOR TEMP = 245.5 ˚C Gases CO2 H2O SO2 O2 NO2

[email protected] k.cal/m.hr.˚C 0.0287 0.0333 0.0198 0.0337 0.0321

K= FOR TEMP = 219 ˚C Gases CO2 H2O SO2 O2 NO2

k@219 k.cal/m.hr.˚C 0.028 0.0315 0.085 0.0339 0.035

K=

2.2.4 Prandtl number: PRANDTL NUMBER

=

FOR TEMP = 380 ˚C (Sp.Heat Cap.) 1041.0000

(Thermal

(Viscosity)

Cond.) 0.049

3.345E-05

22

(Prandtl Number) 0.711

FOR TEMP = 300 ˚C (Sp.Heat Cap.) 1024.0000

(Thermal

(Viscosity)

Cond.) 0.043

2.994 E – 05

(Prandtl Number) 0.713

FOR TEMP = 245.5 ˚C (Sp.Heat Cap.) 1040.5800

(Thermal

(Viscosity)

Cond.) 0.0364

2.457 E – 05

(Prandtl Number) 0.706

FOR TEMP = 219 ˚C (Sp.Heat Cap.) 1010.0000

(Thermal

(Viscosity)

Cond.) 0.04

2.63 E – 05

23

(Prandtl Number) 0.662

CHAPTER # 3 3. DESIGN OF WASTE HEAT RECOVERY BOILER:  The design method of waste heat recovery boiler starts from data collection from plant such as operating data, properties of flue gas, existing cement plant and available space, water resources, etc.  Identification of potential energy available from flue gas and usages.  The main equipment in this system is a WHRG boiler that consist three heat exchangers i.e. Economizer, Evaporator and Super-heater. Traditionally, the heat exchanger performance analysis and simulation are performed using steady-state energy balance across the heat exchanger. The energy balance on the hot and cold fluids together with the heat-transfer equation constitutes the model of heat exchangers. A simplified model generally uses an average driving force such as log mean temperature difference (LMTD) and assumes uniform properties of the fluids along the length of the heat exchanger to determine the overall heat-transfer coefficient. Under the assumption that there is no heat loss to the surroundings, the heat lost by the hot fluid stream shall be equal to the heat gained by the cold fluid stream. Waste heat recovered in cement plant is gases with high dust content, which can be as high as 150 g/m3. Dust on the pre-heater side is sticky, and dust at the clinker cooler exhaust is abrasive. This aspect has an impact on the design of heat exchangers used for waste heat recovery. In addition, sulfur contained in those gases can condensate, which limits the amount of waste heat that can be recovered.

3.1 ECONOMIZER DESIGN:

Figure 3-1: Rough Schematic of Boiler

24

1. Its function is to preheat the feed water before it enters in the evaporator. Only sensible heating take place, no phase change process otherwise steaming can cause blockage of tubes and may disturbs the circulation. 2. Therefore required heat transfer in economizer is lower than the saturation temperature of water at given pressure. 3. But the decision is also dependent on mass flow rate of water. Large quantity of water can cause steaming too; therefore we kept a margin of 15 to 20 degrees below saturation temperature.

3.1.1 Design considerations: 1. The cooler the water the more effective will be the heat transfer. 2. If approach point is kept low then the economizer can steam up at partial loads. Or prevention is making the flow of water from bottom to top against the gravity. 3. Economizers are usually finned due to the fact that there is low temperature difference between flue gases and water thus required larger surface area. 4. Due to finned tubes there is a large pressure drop of exhaust gases across economizer, and cleaning also becomes difficult. Therefore using un-finned design of economizer. 5. As economizer is the last stage of quantifying lost heat of flue gases, we must consider the dew point of flue gases. If flue gases contain SO3, SO2 and other chlorine compounds and water is below the dew point temperature, then there is a great danger for corrosion due to formation possibility of hydrochloric or sulfuric acid. 6. The design pressure of economizer should within a few inches of water column gauge. The higher pressure drop results in the decrease in efficiency of the process.

3.1.2 Governing parameters: 1. 2. 3. 4. 5. 6.

The following parameters are to be determined while designing the boiler. The heat content absorbed by the water or given away by gases. The area of heat exchange The number of tubes required. The arrangement of tubes. Pressure drop across the tubes.

3.1.3 Heat balance: 1. For any heat exchanger heat balance is a useful mean to extract the basic parameters like inlet and outlet temperatures of either fluid. 2. Applying heat balance would give:

25

3.1.4 Considerations and assumptions: 1. The operating pressure of the boiler and the degree of superheat of steam should be fixed. 2. Normally the pressure of steam is high for power generation. 3. The operation pressure should be around 13 bars. 4. The pinch and approach point should be selected; these greatly affect the area for heat exchange and stability of operation. 5. Suitable temperature of feed water is set at the inlet of economizer. 6. The log-mean temperature difference should be evaluated. 7. The gas-mass velocity should be fixed (depending on the nature of gas i-e clean or dirty). 8. Dew point should be estimated. 9. Transport properties should be evaluated. 10. The area for the flow or the number of tubes required for a fixed size tube and velocity can be determined by the equation ṁ= . Here A is the water side flow area. 11. The gas side area is determined by the equation:

3.1.5 Economizer constraints: 1. The dew point temperature should be 130˚C, so the outlet temperature will be 15˚C above it. 2. The incoming temperature of the gas is 221˚C. 3. The operating pressure of the boiler is taken to be 13 bar and corresponding saturation temperature to be 191.6˚C.

3.2 ECONOMIZER DESIGN CALCULATIONS: Specifications on which we are designing the economizer are as follows:

Inlet gas temperature Outlet gas temperature Gas mass flow rate Specific heat Inlet water temperature

321.94°C ? 83.5 kg/sec 1.04 kJ/kg °C 40°C

Outlet water temperature Inlet enthalpy

175°C 167.57 kJ/kg

Exit enthalpy Mass flow of water ṁ Table 3-1: Design specifications for Economizer 26

741.5 kJ/kg 2.22 kg/sec

The outlet temperature of water is taken short of the saturation temperature at operating pressure of 13 bars, in order to prevent steaming. The values of enthalpies are taken from book of “Heat Transfer” by Yunus A. Cengel. Now Applying Energy Balance; Energy gain by feed water = Energy lost by the gases Energy gain by feed water = ṁ ( ) = 2.222(741.5 – 167.57) = 1274.124 kW Therefore energy gain by water is equal energy loss by the gases = 1274.124 kW = ṁ

∆T

1274.124 = 83.5 × 1.04 × (

–

)

= 306.83 °C

Temperatures for transport property: In order to get the properties of fluids, average temperatures are likely to be found. Average temperature of gas =

= 314.385°C

Average temperature of water =

= 107.5°C

Average film temperature of gas Properties of fluid such as density viscosity Prandtl number etc are calculated at the film temperature (temperature of the fluid at solid fluid interface). Since temperature is changing continuously along the flow direction, therefore finding the mean of mean temperature of water and flue gases. = Log-mean temperature-difference (LMTD) Since the purpose of economizer is to preheat the fluid, therefore counter flow configuration is used. Otherwise disturbance would be created through steaming, especially when the flow is downward. Log mean temperature difference of parallel flow is less than that of counter flow.

27

∆T LMTD = Where ∆ Ta =

–

∆T b =

321.94°C 306.83°C 40°C 175°C

∆T LMTD =

∆T LMTD = 200.96 °C

Assuming a suitable tube size Assuming the diameter of tubes and thickness in accordance with the mass flow rate. Outer diameter OD = 25.4 mm = 0.0254m (1 inch)

Table 3-3-2: Standard dimension for steel tubes

Figure 3-2: Standard dimensions for steel tubes Source: TEMA (Tubular Exchanger Manufacturer Association) According to the table above thickness with respect to outer diameter is given, selecting higher value in order to

28

compensate fouling and corrosion and also reduces pressure drop. Thickness: t =3.175 mm =0.003175 m (

)

Now internal diameter would be; OD -2(t) = ID Di =ID = 25.4 – (3.175x2) Di = ID = 19.05 mm = 0.01905 m ( inches)

Cross-sectionalqareaqforqoneqtube: Theqcrossqsectionalqareaqofqtheqtube would be used in the formula of mass flow rate to find out the number of tubes width and hence width of the boiler. A=

=

A = 2.85×

(0.01905)2 m2

Number of tubes Continuity equation tells us that in how many tubes does a mass flow rate of water (ṁ) of 2.22 kg/sec would be divided. Nw= Where,

For density taking average temperature of water in economizer =

= 107.5°C

Since, =

= 937 kg/m3

= ṁ=2.22 kg/sec A = 2.85×

m2

For velocity of the fluid iterations are required within the suitable range as given in the table below 29

Table 3-3: Maximum Velocity Ranges V=0.5m/sec (Assumed) in good assumption with the above table. Nw= = 16.626 ≈ 17 tubes Thus 7 tubes are quite reasonable for lesser frontal area. This would curtail the length of the tubes as given by the following relation = Tubes specs for economizer: outer diameter

25.4 mm

inner diameter

19.05 mm

thickness

3.175 mm

number of tubes

17

area Table 3-4: Tube Specifications for Economizer

m2

Fixing the gas mass velocity: Gas mass velocity is basically the mass flux (amount of exhaust gas passing through per unit area). The range for the gas flux typically ranges from 6-16 kg/m2sec, depending upon the type of gas which ascends from dirtiest to clean. ṁ = ρAV = ρV =G Setting up =G=9kg/m2sec for cement industry exhaust gas; G=

30

Where

is the frontal area, re-arranging the equation and putting values we get = = 9.278 m2

Now length of economizer tubes can be calculated.

Pitch of tubes

Figure 3-3: Different tube arrangement

Transverse pitch of the tube can be evaluated by the formula Aff = L× (ST-do) × Nw

Setting up length up to 3.8 meters because of evaporator length found to be 3.8m. Since length of the tubes is fixed therefore only variable is the transverse pitch in the equation. ST =

+ do

Then; ST =

+ 0.0254

ST = 0.169 m = 6.65 in.

Width of tubes The numbers of tubes are known and we have to find the pitch. Now setting the width for bank of tubes: Width = ST × Nw W = 6.65×17 W= 113.05 in = 2.87 m

31

3.2.1 Estimating the overall heat transfer co-efficient: Overall-heat-transfer coefficient is useful for finding the surfaceqareaqforqheat transfer. Since convective heat-transfer-coefficient of gas side has much lower value than water side, therefore gas side coefficient is the governing parameter. Even though they don’t have much impact, but for higher accuracy we have considered it. Also we have to consider the fouling factor on gas side and corrosion factor on water side. Therefore the equation for overall coefficient becomes

Figure 3-4: Thermal circuit of a fouled heat transfer surface Where

Figure 3-5: Resistance to the flow of heat through pipe walls

32

For water side: = 0.0254m (

) H₂O = 107.5°C = 225.5°F

ṁ = 2.22kg/sec = 7.992 tones/hr. At average temperature of water 107.5°C properties are: These properties are extracted from table of properties of saturated water from INCROPERA “fundamentals of heat and mass transfer” µf = 324×10-6 Ns/m2 Pr = 2.02 kf = 674×10-3 W/m.K 3 f = 937 kg/m

Dynamic viscosity Prandtl number Thermal conductivity Density Reynolds number:

Re =

=

Re = Re = 27546.06 Nusselt number: As we know that critical Reynold number for internal flows is Re 2300, here Reynold number is greater than this, hence the flow is turbulent. Now using Dittus-Boelter correlation for finding Nusselt number (Book: Incropera) NU = 0.023 NU = 0.023 ( NU

0.8

(2.02)0.4

= 108.6215

hi = h = 3.843 kW/m2K

33

For Gas side: Properties of gases evaluated at the film temperature =

314.385

= 1.010 kJ/kg µ = 2.630×10-5 kg/m sec k = 4.013×10-5 kJ/m sec Reynold number: Using gas mass velocity for finding Reynold number

Where

is the dynamic viscosity for the flue gas

Figure 3-6: Fouling factor for

Prandtl number:

different fluids Nusselt number: Since Reynold number for flow over tube bank is less than the critical Reynold numbers. For outside heat transfer coefficient using correlation Nu

= 0.33

.

This relation is known as GRIMSON MODEL and is used for flow over tube banks when Re

4

Pr and m are constants. For values of these, two tables are given below

34

Using tube material to be low Carbon steel thermal conductivity k = 50 W/m.K Now;

Substituting all values, +

+

In

+0.001+0.0002

U = 90.36635 W/m2K Flow area required: Now calculating the surface area:

A = 70.1605 m2 Number of tubes high required: = =

× ×

= 13.61

13

Total number of tubes N = Nh × Nw N = 170

35

3.3 EVAPORATOR DESIGN 3.3.1 Design and operational criterion: During the calculation of design parameters we have to take certain considerations for better utilization and safer working conditions. In the following lines some general points will be mentioned which are thought to be vital for the boiler. 1. To avoid thermal shock it is suggested to introduce makeup water or feed water in a way that there should be no sudden temperature change occurs. 2. For a high temperature and pressure boiler, material should be, strong enough against creep, enough structural stability against crystal changes, enough surface stability against corrosion, erosion and oxidation, alloys that can be easily welded or machined. 3. Design and shape of boiler depends on, conditions and heat content of hot gases, maximum utilization of all three forms of heat. 4. The best possible heat transfer occur in condition of nucleate boiling so try to avoid film boiling. Moreover stagnation film of either gas or water causes reduction in heat transfer. 5. Scaling is one of the fatal factors for tube causes reduction in surface conductance that ultimately brings temperature high enough to be damaged. 6. Although forced convection increases rate of heat transfer but it is limited by heat transfer area and most important cost of surface area. 7. In water tube boiler shell and tube are not exposed to radiant heat fire, therefore not subjected to overheating. 8. Corrosion is another threat to boiler. 9. Generally speaking all parts of boiler are accessible for cleaning, inspection and repair. 10. According to temperature, pressure and gas conditions different types of materials can be used like carbon steel with different grades, alloys steel with composition with chromium, nickel, molybdenum etc. 11. Tubes failures are due to reasons, scaling, corrosion and stress concentration, high concentration of heat, poor circulation. 12. Bent tube design offers certain benefits like, boiler can be made wide and low or narrow high, more surface available for radiant heat of flame, and provide greater flexibility in tube design. 13. It is necessary to keep in mind that, as temperature difference becomes smaller and smaller heat transfer area requirement increases. 14. For heavy dust flue gases water tube are appropriate for one more reason that baffles can be used not only for gas flow help but also to remove dust particle that are deposited. 15. Usually tube banks are concerned with convection form of heat transfer.

36

16. In case of clean surface, gas film resistance is important and considerable as compared to steam or water film resistance or tube wall conductance.

3.4 EVAPORATOR DESIGN CALCULATIONS:

Specifications on which we are designing the evaporator are as follows: Pressure in Evaporator

P

13 bar 373.4

Flue Gas Inlet Temperature Flue Gas Outlet Temperature

? 175

Inlet Water Temperature Outlet Water Temperature = Saturation Temperature @ given pressure

191.6

Massqflowqrateqofqflueqgas

83.5 kg/s

Mass flow rate of steam

2.22 kg/s

Enthalpy @ 175

743.2 kJ/ kg

Enthalpy @ 191.6

2786.4 kJ/ kg 0.99679692 kJ/kg ˚C

SpecificqHeatqofqFlueqgas Viscosity of Flue gas Prandtl Number

Pr

Table 3-5: Design specifications for Evaporator

37

Amount of heat to be generated: This heat will be equal to the heat gained by water: = = (2.22

2022)

= 4488 kW Applying Energy Balance: Energy gained by the water = Energy lost by flue gas

4488

= (83.5) (1.024) (373.4 = 321 C

For estimation of surface area:

Where, =

After Rearranging,

In order to find Area, we need to find out: 1. 2. Overall Heat transfer coefficient (U)

-

FIRST WE FIND LMTD:

38

)

Where,

And we know that, 373.4 321 C 176.64 191.64

After calculating, we get,

-

NOW WE WILL FIND ‘U’:

Here,

For

and

:

We will follow the following steps, first we will calculate Nusselt Number, then Reynolds and then Convective Heat Transfer Coefficient.

39

For gas side: : From Heat and Mass Transfer by Incropera,

Here, ……. [Gas mass velocity is nothing but the mass flux

] ----------- (1)

It depend upon the gas It ranges from 6-16 which ascends from dirtiest to clean = 9 kg/m2.s

Setting up =

[Reason: acoustic cleaning system]

Assuming suitable tube dimension:

After solving, we get:

Hence the flow is turbulent. Using correlation from Incropera, Nu

= 1.13

.

Value of ‘C’ and ‘m’ can be taken from the table

Nu = 1.13

.

According to the calculated Reynold Number

Figure

– C = 0.229 and m = 0.632

correlation

Hence, Nu = 102.3 Now,

40

3-7:

Constants

for

Nusselt

For water side: :

Here,

Solving, we get, Nu= 0.023 Nu= 1529.0108 Now,

Finally, putting in,

We get,

Putting back in equation (A) to get Surface Area,

41

For gas side:

Where

is the frontal area ,

Also, Comparing both, we get Figure 3-8: Heat flow across In-line Arrangement (Heat and Mass Transfer by Incopera ) Here,

……… (B) Also we have, A = Total required surface area taking all the tubes. A= Putting

Iterating for

----------------- (C)

from eq (B):

and

for different ,

If taking 25 according to ease, then equation (C) becomes

If taking

25 then length will be

It is sufficient length and can be easily managed and accommodated.

42

For water side: Now finding the velocity of fluid (water) in evaporator Again using equation (1) = Where,

After rearranging, -------------------- (D) Where, = Velocity of fluid in evaporator --------------- (E) Here,

Also, C. R = 4,

Substituting, we get = 244.59 kg/ = 0.0312 Putting in equation (D)

43

3.5 SUPER HEATER DESIGN 1. 2. 3. 4. 5.

The production of steam at higherqtemperatureqthanqtheqsaturationqtemperature is called superheating. The temperature added to the saturationqtemperatureqisqcalledqtheqdegreeqof superheat. A superheater is employed in the steam boiler to add additional energy into steam and raise its temperature. Superheater is in demanding condition, and their failure could mean shut down for few days resulting in large financial losses. The function of superheater is to increase the capacity of the steam boiler, eliminates erosion of the steam turbine, and reduces steam consumption of the steam turbine.

3.5.1 Design consideration: There are basically three types of superheaters: 1. Radiant superheater 2. Convection superheater 3. Semi-radiant superheater But the cement plant exhaust will not be concerned with radiant or semi-radiant superheater. Our concern is convective superheater as we are not taking heat from any furnace. The final sections of superheater are placed in the highest gas temperatures, which calls for adopting the most appropriate high-temperature alloy for the tubing from considerations of metal temperatures, fouling due to ash compounds and corrosion due to salts in ash.`` Strict control of the chemical composition of the ash and strict control of the operational variables are necessary in order to avoid severe material loss in the superheaters.

3.5.2 Design aspects: Some of the most important aspects of the design of superheaters are: 1. Uniform distribution of steam and gas across all the sections to minimize imbalance of flows. 2. Provision of thermal expansion of headers, tubes, spacers and supports. 3. Accessibility of cleaning, examination and removal of elements 4. Optimally high steam velocity in all the tubes to keep the metal temperatures as low as possible 5. Minimum steam pressure losses.

44

3.5.3 Design assumptions: Velocity: Superheaters transfer heat from flue gas to steam. Heat transfer between two gases is not very effective compared to heat transfer from gas to fluid. For that reason, steam must flow fast enough (10-20 m/s) in order to give the superheater tubes enough cooling. Lower steam pressure weakens the heat transfer rate, so with lower pressures, steam must have a greater velocity (15-40 m/s). Spacing: Superheater of boiler consists of banks of tubes. A system of tubes is located in the path of the furnace gases in the top of furnace. A superheater must be built so that it superheats approximately the same amount of steam from low to high loads. Changing tube lengths between passes can control temperature differences. The outermost tube that receives the most radiative flux should be shorter than the rest of the tubes.

Tubes: Tubes in superheaters can be arranged according to inline or staggered arrangement. Inlineqtubeqarrangementqis preferred forqfoulingqboilers, andqrecovery. Staggered arrangementqisqpreferredqforqoil, gasqandqheatqrecoveryqsteam generator. As free space with staggered arrangement is much smaller than with inline arrangement the reason for decreased fouling with inline is evident.

3.6 SUPERHEATER DESIGN CALCULATIONS: Specifications on which we are designing the superheater are as follows: Pressure in superheater

P

13 bar

,

380°C

Flue gas inlet temperature Flue gas exit temperature

,

?

Flue gas mass flow rate

83.5 kg/sec

Specific heat of flue gas Inlet steam temperature

g

Tsi

45

1.041 kJ/kg °C 205.7°C

Tso

294.28°C

Enthalpy at inlet

h @205.7°C

2824 kJ/kg

Enthalpy at exit

h @294.28°C

3030 kJ/kg

Outlet steam temperature

Mass flow rate of steam

2.22 kg/sec ρg

Density of flue gas

0.5 kg/m3 3.345x10-5 N.s

Viscosity of gas Thermalqconductivity

K

0.0364qW/m.K

Outer diameter of tube

do

0.0508 m (2 in)

Inner diameter of tube

di

0.043 m (1.7 in)

Length of tube L Table 3-6: Design specifications for Super-heater Energy gain by steam = Energy lost by the gases E gain =

×

E gain = 457.32 kW

Temperature for Transport property: Average temperature of steam:

Outlet temperature of flue gas: Applying energy balance: E= Where 0.95 is for fouling effects 0.98 is for radiation losses 374.34

46

3.8 m

Average temperature of flue gas:

377.17 Log mean temperature difference:

Where, ∆

=

out,–

∆

=

, – s,

sin,

And we know that,

,

380

,

373.4°C

s,

205.7°C

s,

294.28°C

After calculating, we get, = 122.159 ℃

For gas side Reynold Number: Assuming diameter of exhaust gases pipe = 4 m Area = Area = 12.568 m2 Assuming volumetric flow rate of flue gases =

47

= 166.3 m/s

Finding the velocity of flue gases: Vf = Vf = 13.23 m/s Reyonld number = At ρ = 0.5 kg/m3 V = Vf = 13.23 m/s D = Do = 0.0508 m Re = 10000 Finding Reynold number at particular gas mass velocity At G = 12 kg/m2 sec

Re = 12 x 0.0508 /3.345 x 10-5 Re = 18224.215 Nusselt number of flue gas: Zukauskas has proposed a correlation of the form of Nusselt number (Eq 7-58 Heat Transfer By Incropera) Nu = C RemD,max Pr0.36 (Pr/Prs) 1/4 Where, C & m selected on basis of Reynold number. C (at Re ) = 0.27 m (at Re) = 0.63

48

Figure 3-9: Table of constant for Nusselt correlation C2

selected

on

the

basis

of

number

of

tubes C2

(8

rows)

=

0.96

Pr at inlet of the air = Pr = 0.713 Prs at the mean temp at inlet & exit = Prs = 0.706 Nu = (0.27) x (18224.215)0.63 x (0.713)0.36 x (0.713/0.706)1/4 Nu = 115.8126419 Heat transfer co-efficient: h0 = (Nu) (k)/do h0 = 115.8126419 x 0.0364/0.0508 h0 = 82.98 W/m2K

For water side Following data is obtained from steam table Density Viscosity

Thermal conductivity

5.62 kg/m3

ρs

5.5

k

49

43.5

N.s

W/m.K

Specific heat of steam

3.37 kJ/kg K

s

Prandtl number

Pr

1.295

Mass flow rate of steam

ms

2.22kg/s

Heat transfer coefficient: The value of internal heat transfer co-efficient is taken from Appendix B from Applied Heat Transfer by V Ganapathy:

= 1300 W/m.K

3.6.1 OVERALL HEAT TRANSFER COEFFICIENT OF PLAIN TUBES

Here, = = =

(steam

) = .0002 m2.oC/W

=

(

) = .001 m2.oC/W

When, ho = 82.98 W/m.K = 1300 W/m.K The overall-heat-transfer coefficient is found to be: = 69.94 W/m2K

3.6.2 TOTAL

SURFACE

AREA

50

FOR

HEAT

TRANSFER

Q = Energy absorbed by steam = 457.32 kW LMTD = 122.159 ℃ A = 53.520 m2

3.6.3 NUMBER OF TUBES WIDE At tube length = 3.8 m, if we select the number of tube high as 4, then the number of tubes wide are given by the following relation: A = Nh × Nw × Assuming Nh

× do × L 4

53.520 = (4) Nw ( ) (0.0508) (3.8) Nw = 22.06

3.6.4

TOTAL AREA FOR MASS FLOW (di)2

A = Nw

A = 0.032 m2 From the definition of continuity equation, velocity of steam is given by

= V= 12.327 m/s

3.6.5

TRANSVERSE PITCH

Then transverse pitch calculated is ST = 0.1338 m

3.6.6

WIDTH OF SUPERHEATER

W = 2.95 m 51

3.7 INDIRECT EFFICIENCY BOILER OLD CONDITION

Figure 3-10: Indirect Method for Efficiency evaluation

In order to find the current condition of boiler, we use indirect method of efficiency calculation. The efficiency can be measured easily by measuring all the losses occurring in the boilers using the principles to be described. The disadvantages of the direct method can be overcome by this method, which calculates the various heat losses associated with boiler. The efficiency can be arrived at, by subtracting the heat loss fractions from 100.An important advantage of this method is that the errors in measurement do not make significant change in efficiency.

For calculation of efficiency, following losses are considered 1. Dry flue gas loss 2. Radiation loss 3. Unaccountable losses 4. Manufacturer margin

52

Dry flue gas loss:

Radiation losses 2% Unaccountable loses 2% Manufacturer’s margin

Therefore

53

CHAPTER # 4 4. OPTIMIZATION OF BOILER GAS SIDE PARAMETERS: There are two ways to increase the efficiency of boiler which include either to change the design of the boiler or its auxiliaries and second one is to optimize the boiler parameters to obtain the best efficiency of the boiler. There are various parameters of the boiler which can be controlled. Following are the variables that may change without altering the design of plant and boiler to obtain the optimization. 1. Inlet gas temperature 2. Inlet mass flow rate of gas 3. Feed water flow

4.1 Inlet gas temperature: Inlet gas temperature can increase the effectiveness of the boiler which will led to change in properties of boiler and increase the overall convective-heat-transfer coefficient of the boiler and can easily be seen by graphs which are obtain by the calculations with different temperatures.

Figure 4-1: Temperature vs Overall Heat

Transfer

Economizer, Superheater

54

Coefficient Evaporator

for and

And the effectiveness of each component of the boiler is shown below the following graph showing the increase in effectiveness of the boiler as a function of inlet gas temperature.

Figure 4-2: Effectiveness Vs Inlet Gas Temperature for Economizer, Evaporator and Superheater 55

But due to high temperature of gases there will be an overall increase in efficiency of the boiler as the heat transfer enhance more than the losses. The graph between efficiency and the temperature clearly showing the efficiency so make the temperature about 385oC with constant mass flow of the gases. So, these are the advantages of the increase in inlet gas temperature. Further if we increase the temperature of the inlet gas the feed water flow is adjusted accordingly and it will increase the overall power output of the turbine but there are several disadvantages of temperature increment on the gas side which includes 1. Reduction of the tubes life due to increase in velocity of flue gas which will enhance the erosion rate discuss in the next chapters. 2. Heat transfer across the outside wall will also increase significantly led to increment of the Heat loss from 1.2% to 1.3%. Following graph shows the loss of the energy as the inlet gas temperature increases. 3. Increase in creep which is another factor of life reduction. 4. Increase in boiler temperature also increases the irreversibility’s of the cycle and boiler by increasing the pinch point of the system and decrease the efficiency of the boiler. Pinch and approach points are important parameters which determine directly the steam generation rate, the temperature profiles, and the heat transfer surface area required in the WHRB. PINCH POINT: The pinch point is defined as the minimum temperature difference between two streams, in this case the exhaust gas and the water or steam. In practice, the pinch point is usually the difference between saturation temperature and temperature of the exhaust gas at the evaporator outlet in waste heat recovery steam boilers. After the exhaust gases have transmitted a certain amount of heat and cooled to a certain temperature, there is a point below where the further cooling of the gases requires disproportionately large heat transfer surface area resulting in expensive WHRB design. As the pinch point decreases, the increase of the heat transfer surface area of the evaporator is exponential, whereas the increase of recovered heat is only linear. In other words, the smaller the pinch point, more efficient the WHRB, but also more expensive the design. The commonly used pinch point in WHRB designs is usually between 8 to 15 °C.

56

Table 4-1: Heat Loss across the Outside Wall Vs Inlet Gas Temperature

Table 4-2: Efficiency VS Temperature

4.2 Inlet gas flow rate: Increase in the flow rate of source fluid flowing over the tubes will lead to increase in the effectiveness of the boiler by increasing the Reynold number and velocity. This leads to the heat transfer enhancement across the tube and ultimately leads to increase in efficiency of the boiler.

57

Figure 4-3: Mass flow rate of gas Vs Overall Heat Transfer coefficient for Economizer, Evaporator and Superheater All the graphs drawn above showsqthatqtheqheatqtransferqincreasesqwith increase in flow rate and the feed water flow is adjusted accordingly with flow rate which will also increase and there are certain advantages of increase in mass flow rate over 58

increase in temperature which includes No heat transfer increment across the wall of the boiler and No growth in creeping phenomenon of the tubes. But the only disadvantage of flow rate increment is the increase in velocity of the gases and gases has significant amount of kiln dust content which will enhance the erosion of the wall exponentialy .The complete relation of the velocity and erosion and discuss in the chapter of erosion. So it should be 45 to 55 kg/s when operating at about 350oC to 380 oC. Itqisqimportantqtoqnoteqthatqincreaseqin mass flow will increase the pressure losses across the tubes which will lead to the requirement of larger Induced Draft (ID) fans operating at its maximum power for obtaining the maximum efficiency of the boiler.

Figure 4-4: Mass flow rate of gas Vs Effectiveness

for

Evaporator and Superheater

59

Economizer,

4.3 Constant Inlet energy: If the inlet energy is constant and the inlet temperature of the gases is increased then their will be reduction of mass-flow-rate of the gas.The following graph is showing the detail of the decrease in effectiveness with increase in inlet temperature of the gases at constant energy because of the mass flow rate reduction which will reduce the effectiveness and so the efficiency. From the graph it is clear that the effectiveness is reduced and it is recommended that one should increase the mass flow rate and reduce the temperature if constant amount of energy is transferred to the boiler. However the erosion rate will also increase with the increase in mass flow significantly and reduce the boiler life.

Figure 4-5: Mass flow rate Vs Effectiveness & Inlet Temperature Vs Effectiveness for Economizer

Figure 4-6: Mass flow rate Vs Effectiveness & Inlet Gas Temperature Vs Effectiveness for Evaporator

60

Figure 4-7: Mass flow rate Vs Effectiveness & Inlet Temperature Vs Effectiveness

From the graph it is clear that the effectiveness is reduced and it is recommended that one should increase the mass flow rate and reduce the temperature if constant amount of energy is transferred to the boiler. However the erosion rate will also increase with the increase in mass flow significantly and reduce the boiler life.

4.4 Conclusion: So from the results obtain in the form of the graphs showing above it can be concluded that: 1. Temperature should be greater than 370oC and less than 450oC provided that the mass flow rate in constant to obtain the maximum efficiency. 2. Flow rate increase the efficiency and effectiveness of boiler greater than the inlet temperature of the gases. 3. All the efficiency increment is at the cost of boiler life.

61

CHAPTER # 5 5. MATERIAL OPTIMIZATION: 5.1 GENERAL CRITERIA FOR MATERIALS SELECTION: The engineer making the materials selection must know all the aspects involved in the construction, operation and maintenance of the heat exchanger tubing. The importance of this is illustrated with the following examples: an operator may isolate a heat exchanger with raw water for sufficient time to initiate a pitting corrosion; partial blockage of tubes, specially of small diameter, would result in stagnant conditions that may cause pitting in alloys that are so prone; fouling may result in operating the heat exchangers in throttled/part load condition.

A general procedure that could be used for identifying the most appropriate material for a specific heat exchanger application would consist of the following steps: 1. 2. 3. 4. 5.

Defineqtheqheatqexchangerqrequirements. Establishqaqstrategyqforqevaluatingqcandidateqmaterials. Identifyqcandidateqmaterials. Evaluateqmaterialsqinqdepth. Selectqtheqoptimumqmaterial.

In identifying candidate materials, it is desirable to narrow the field to a comparatively small number of materials for more extensive evaluation. There is no hard and fast rule as to how many candidate materials should be selected for detailed study. The initial identification and selection procedure, if done properly, will eliminate those materials which are unsuitable and those which are excessively expensive. Special considerations which affect materials selection include:

Physical properties: 1. High heat transfer coefficient (requiring high thermal conductivity for tube material). 2. Thermal expansion coefficient to be low and as compatible as possible if tubes are welded at the header section.

62

Mechanical properties: 1. Good tensile and creep properties (High creep rupture strength at the highest temperature of operation and adequate creep ductility to accommodate localized strain at notches are important). 2. Good fatigue, corrosion fatigue and creep-fatigue behavior. 3. High fracture toughness and impact strength to avoid fast fracture.

Corrosion resistance: 1. Low corrosion rate to minimize the corrosion allowance. 2. Resistance to corrosion from off normal chemistry resulting from leak in upstream heat exchanger or failure in the chemistry control 3. Tolerance to chemistry resulting from mix up of shell and tube fluids.

5.1.1 Properties of different Steel Grades

ASTM A36: ASTMqA36qisqtheqmostqcommonlyqusedqmildqandqhotqrolledqsteel.qItqhas excellentqweldingqpropertiesqandqisqsuitableqforqgrinding,qpunching,qandqtapping, drilling and machining processes. Chemical composition is given below. Carbon Copper Iron Manganese Phosphorus Silicon Sulfur Table 5-1: Chemical composition of ASTM A36

0.25-0.29% 0.20% 98.0% 1.03% 0.04% 0.28% 0.05%

Mechanical properties: Ultimate tensileqstrength Yield strength Elasticqmodulus Bulkqmodulus Shearqmodulus Poissonqratio Table 5-2: Mechanical properties of ASTM A36

400-550qMPa 250qMPa 200qGPa 140qGPa 79.3qGPa 0.26

Machining: The machinabilityqrateqofqASTMqA36qisqestimatedqtoqbeq72%, andqtheqaverage surfaceqcuttingqfeedqofqASTMqA36qisq120 ft. /min. Machining of ASTM A36 steel is not as easy as that of AISI 1018 steel.

63

Welding: ASTM A36 steel is easy to weld using any type of welding methods, and the welds and joints so formed are of excellent quality.

AISI-1040 AISIq1040qcarbonqsteelqhasqhighqcarbonqcontentqandqcanqbeqhardenedqbyqheat treatment followed by quenching and tempering to achieve 150 to 250 ksi tensile strength. Chemical composition is given below. Carbon Iron Manganese Phosphorus Sulfur Table 5-3: Chemical composition of AISI 1040

0.37-0.44% 98.6-99.0% 0.60-0.90% ≤ 0.04% ≤ 0.05%

Mechanical properties: Ultimateqtensileqstrength Yieldqstrength Elasticqmodulus Bulkqmodulus Shearqmodulus Poissonqratio Table 5-4: Mechanical properties of AISI 1040

620qMPa 415qMPa 190-210qGPa 140qGPa 80qGPa 0.27-0.30

Machining: The machinability rating of AISI 1040 carbon steel is 60. Weldability: AISI 1040 carbon steel can be welded using all welding techniques. It can be preheated at 149 to 260°C (300 to 500°F) and post heated at 594 to 649°C (1100 to 1200°F) due to its high carbon content.

AISI-1010 AISI 1010qcarbonqsteelqisqaqplainqcarbonqsteelqwithq0.10%qcarbonqcontent. This steel has relatively low strength but it can be quenched and tempered to increase strength. Chemical composition is given as: Carbon Manganese Phosphorus Sulfur Iron Table 5-5: Chemical composition of AISI 1010 64

0.080% 0.30% 0.05% 0.04% 99.18%

Mechanical properties: Tensile strength Yield strength Elastic modulus Table 5-6: Mechanical properties of AISI 1010

365 MPa 190 MPa 305 GPa

Machinability: TheqmachinabilityqofqAISIq1010qcarbonqsteel, cold worked state, is considered as fairly good.

especiallyqinqtheqcoldqdrawnqor

Welding: AISI 1010 carbon steel can be welded using all the conventional welding techniques.

STAINLESS STEEL GRADE-410: Gradeq410qstainlessqsteelsqareqgeneralqpurposeqmartensiticqstainlessqsteels containingq11.5%qchromium,qwhichqprovideqgoodqcorrosionqresistanceqproperties . However, the corrosion resistance of grade 410 steels can be further enhanced by a series of processes such as hardening, tempering and polishing. Quenching and tempering can harden grade 410 steels. Chemical composition is given as: Carbon 0.15% Manganese 1.00% Silicon 1.00% Phosphorus 0.04% Sulfur 0.03% Chromium 13.00% Nickel 0.75% Iron 84.03% Table 5-7: Chemical composition of Stainless Steel Grade 410 Mechanical properties: Tensile strength 985 MPa Yield strength 730 MPa Brinell hardness 321 Table 5-8: Mechanical properties of Stainless Steel Grade 410 Welding: Gradeq410qsteelsqcanqbeqweldedqusingqallqconventionalqweldingqtechniques,qbut theqmaterialsqshouldqpreheatedqat 150 to 260°C followedqbyqpostweldqannealing treatment,qtoqmitigateqcracking. According to AS 1554.6 standards, grade 309 electrodes or rods are preferred for welding 410 steels. 65

Machining: Grade 410 steels can be easily machined in highly tempered or annealed conditions. However, it is hard to machine grade 410 steels if they are hardened above 30HRC. Free machining grade 416 is the best alternative.

STAINLESS STEEL 316: Grade 316 is the standard molybdenum bearing grade, second in importance to 304 amongst the austenitic stainless steels. The molybdenum gives 316 better overall corrosion resistant properties than Grade 304, particularly higher resistance to pitting and crevice corrosion in chloride environments. Chemical composition is: Carbon 0.08% Manganese 2.00% Silicon 0.75% Phosphorus 0.045% Sulfur 0.03% Chromium 18.0% molybdenum 3.0% Nickel 14.0% Nitrogen 0.10% Iron 62.0% Table 5-9: Chemical composition of Stainless Steel 316 Mechanical properties: Tensile strength 515 MPa Yield strength 205 MPa Elastic modulus 193 GPa Table 5-10: Mechanical properties of Stainless Steel 316 Welding: Excellent weldability by all standard fusion methods, both with and without filler metals. AS 1554.6 prequalifies welding of 316 with Grade 316 and 316L with Grade 316L rods or electrodes (or their high silicon equivalents). Heavy welded sections in Grade 316 require postweld annealing for maximum corrosion resistance. This is not required for 316L. Grade 316Ti may also be used as an alternative to 316 for heavy section welding. Machinability: A “Ugima” improved machinability version of grade 316 is available in round and hollow bar products. This machine is significantly better than standard 316 or 316L, giving higher machining rates and lower tool wear in many operations. 66

5.1.2 TREND OF THERMAL CONDUCTIVITY ON ‘U’ VALUE: Different grades of AISI and ASTM standards are considered whose composition and properties are mentioned in the above section. Now thermal conductivities of each grade are considered. The table below shows thermal conductivities: THERMAL CONDUCTIVITIES (W/m.K) Mild Steel 42 AISI 1010 63.9 AISI 1018 51.9 AISI 1020 51.9 AISI 1040 51.9 ASTM A36 50 StainlessqSteelq405 30 StainlessqSteelq304 16.2 StainlessqSteel 410 16.2 Stainless Steel 316 16.2 Stainless Steel 440A 30 Table 5-11: Thermal conductivities of different steel grades GRADES

5.1.3 Overall heat transfer coefficient:

Where,

5.1.4 Evaluating U-value for mild steel: 5.1.4.1 ECONOMIZER: In order to calculate the overall-heat-transfer-coefficient as the function of thermal conductivities, all other values are needed to be fixed. From the design section the values are given below:

67

K/W K/W W/ W/

.K

.K

Thermal conductivity of mild steel is 42 W/m.K at elevated temperatures. Now, +

1.16×

+

In

9.52×

+0.001+0.0002

)

)

0.001

4.64×

1.135

U

89.07

Surface area required: Now calculating the surface area:

= 71.16 U-values of other grades: In a similar manner as above, the values of surface area and overall heat transfer coefficient is calculated. This work is done directly on Microsoft Excel sheet. The values are as follows: Material

Thermal

U

Cond (k)

68

A

Economizer

Mild 42 0.0109441 91.373163 69.374039 AISI 1010 63.9 0.0108115 92.4943 68.533146 AISI 1018 51.9 0.0108152 92.462069 68.557036 AISI 1020 51.9 0.0108152 92.462069 68.557036 AISI 1040 51.9 0.0108152 92.462069 68.557036 ASTM A36 50 0.010816 92.45555 68.56187 Stainless steel grade 405 30 0.0108299 92.336991 68.649902 Stainless steel grade 304 16.2 0.0108595 92.085514 68.837378 Stainless steel grade 410 16.2 0.0108595 92.085514 68.837378 Stainless steel grade 316 16.2 0.0108595 92.085514 68.837378 Stainless steel grade 440A 30 0.0108299 92.336991 68.649902 Table 5-12: Overall Heat Transfer coefficient and Surface Area of economizer for different materials 5.1.4.2 EVAPORATOR: In order to calculate the overall-heat-transfer-coefficient as the function of thermal conductivities, all other values are needed to be fixed. From the design section the values are given below:

K/W K/W W/ W/

.K

.K

Thermal conductivity of mild steel is 42 W/m.K at elevated temperatures. Now,

+

6.12×

+

In

1.16×

+0.001+0.0002

)

2.36×

69

)

0.001

1.2983

U

75.0236

Surface area required: Now calculating the surface area:

= 369.55 U-values of other grades: In a similar manner as above, the values of surface area and overall heat transfer coefficient is calculated. This work is done directly on Microsoft Excel sheet. The values are as follows: Material

Thermal cond. (k)

U

A

Evaporator

Mild AISI 1010 AISI 1018

42 63.9 51.9

0.0129965 0.0128763 0.01288

76.943707 77.661975 77.639717

82.383936 81.621995 81.645395

AISI 1020 AISI 1040 ASTM A36 Stainless steel grade 405 Stainless steel grade 304 Stainless steel grade 410 Stainless steel grade 316

51.9 51.9 50 30 16.2 16.2 16.2

0.01288 0.01288 0.0128808 0.0128944 0.0129233 0.0129233 0.0129233

77.639717 77.639717 77.635214 77.553313 77.379468 77.379468 77.379468

81. 645395 81. 645395 81.650131 81.736358 81.91991 81.91991 81.91991

Stainless steel grade 440A 30 0.0128944 77.553313 81. 736358 Table 5-13: Overall Heat Transfer coefficient and Surface Area of evaporator for different material

70

5.1.4.3 SUPERHEATER: In order to calculate the overall-heat-transfer-coefficient as the function of thermal conductivities, all other values are needed to be fixed. From the design section the values are given below:

K/W K/W W/ W/

.K .K

Thermal conductivity of mild steel is 42 W/m.K at elevated temperatures. Now,

+

8.87×

+

In

+0.001+0.0002

1.29×

)

2.31× 1.511 U

66.6

Surface area required: Now calculating the surface area:

= 64.57

71

)

0.001

U-values of other grades: In a similar manner as above, the values of surface area and overall heat transfer coefficient is calculated. This work is done directly on Microsoft Excel sheet. The values are as follows: Material

Thermal

U

cond. (k)

A

Superheater

Mild 42 0.0151072 AISI 1010 63.9 0.0149052 AISI 1018 51.9 0.0149088 AISI 1020 51.9 0.0149088 AISI 1040 51.9 0.0149088 ASTM A36 50 0.0149096 Stainless steel grade 405 30 0.0149232 Stainless steel grade 304 16.2 0.014952 Stainless steel grade 410 16.2 0.014952 Stainless steel grade 316 16.2 0.014952 Stainless steel grade 440A 30 0.0149232 Table 5-14: Overall Heat Transfer coefficient and

66.193796 95.763135 67.09083 94.482739 67.074269 94.506067 67.074269 94. 506067 67.074269 94. 506067 67.009972 94.510787 67.009972 94.596747 66.880546 94.779809 66.880546 94.779809 66.880546 94.779809 67.009972 94. 596747 Surfaces Area of Super-

heater for different material

5.1.5 CONCLUSIONS: From the above table you can observe the trend that thermal conductivity has on the overall-heat-transfer-coefficient and surface area required for heat transfer. As thermal conductivity increases, overall-heat-transfer-coefficient increases, therefore lesser area will be required for same amount of energy transfer.

Conversely we can also say that if material of greater thermal conductivity is used, then for same surface area amount of energy transfer would be maximized.

We will be showing this result through calculations. For this purpose we have to redesign the boiler.

72

5.1.6 EFFECT ON FINAL TEMPERATURES OF COLD FLUID: For the same surface area and new overall-heat-transfer-coefficient the amount of power ( ) would also be maximized. This will give us a rise in the outlet temperatures of water at every section. After material shifted from mild steel to high thermal conductivity material AISI 1010, then U values of every section becomes = 92.92 W/

.K

= 77.61 W/

.K

= 77.81 W/

.K

Now, = = 1290.87 = 4525.02 = 469.50 ECONOMIZER For the changed amount of heat rate the outlet temperature in economizer is given as = 1290.87

= 2.22

4.203

138.35 For inlet temperature of 40

the final temperature would become

= 178.35 Previously this temperature was 175 . An obvious increase of 3.35

occurs after material change.

73

EVAPORATOR For the previous temperature difference and phase change, evaporator requires 4488 KW of heat rate. Now here are two changes: 1. Inlet temperature of evaporator changes. 2. Material change will further increase the heat rate and outlet temperature. Now At 178.35

=C

At 191.5

= 440

= 784.74

= 2792

Therefore for this temperature difference heat rate required is = = 4456.12 After material changed, the heat rate available has been calculated and is found to be greater than the previous one. = 4525.02 Therefore extra amount of energy will be utilized in increasing the outlet temperature of saturated steam. =

68.9

This extra energy will increase the outlet temperature of saturated steam coming out of evaporator section. =

= 31.04 =

= 2824

Now from steam table at pressure 13 bars superheated section across enthalpy, final outlet temperature is found through interpolation. = 205.7 Previously this temperature was 191.5

.

74

SUPERHEATER For the changed amount of heat rate the outlet temperature in economizer is given as: = 469.5

= 2.22 246.38 =

= 3071

Now from steam table at pressure 13 bars superheated section across enthalpy, final outlet temperature is found through interpolation. = 313

5.2 ECONOMIC ANALYSIS OF BOILER TUBE MATERIAL After taking the advantage of thermal conductivity for heat exchanger piping material, here we are justifying on the basis of cost. In case we have higher cost of the new material selected, then we will also have to compare the results of advantage with the price i.e. degree of superheat increased will produce how much extra power. That power in terms of cost will have to be compared with the newly compatible material extra cost. Also we have the chance to do corrosion and erosion analysis of old and new material so as to fully justify our material. MATERIAL

DENSITY

PRICE PER TON (ranges)

Mild steel AISI 1010 AISI 1040

7850 7870 7845

350 -- 400 300 -- 550 580 -- 830

ASTM A36 7850 400 -- 700 SS 316 8000 1500 -- 6000 SS 410 7800 1000 -- 1500 Table 5-15: Ranges of Price per Ton for different Steel Grades The prices of material are taken from the site (www.alibaba.com) . Billets of standard sizes are considered for the cost analysis.

75

5.2.1 VOLUME OF MATERIAL USED IN HEAT EXCHANGERS PIPING: 5.2.1.1 ECONOMIZER Tube specification of economizer are:

Volume of material required for one tube:

For number of tubes

226

5.2.1.2 EVAPORATOR Tube specification of evaporator are:

Volume of material required for one tube:

For number of tubes

625

76

5.2.1.3 SUPERHEATER Tube specification of super-heater are:

Volume of material required for one tube:

For number of tubes

92

5.2.2 TOTAL VOLUME Total volume required would be the sum of all volumes of three sections. Therefore,

5.2.3 MASS REQUIRED BY DIFFERENT GRADES According to theqdefinitionqofqdensity:

Therefore mass can be calculated as:

Also

Since each steel grade has different density, therefore the mass required for each grade will be different. This is a good way to truly justify our material.

77

MATERIAL

DENSITY

TOTAL

TOTAL MASS

VOLUME / MILD STEEL

7850

1.8041

14141

AISI 1010

7870

1.8041

14177

AISI 1040

7845

1.8041

14132

ASTM A36

7850

1.8041

14141

SS 316

8000

1.8041

14411.2

SS 410

7800

1.8041

14051

Table 5-16: Mass required by boiler tubes for different material

5.2.4 COST FOR EACH STEEL GRADE MATERIAL

TOTAL MASS

PRICE PER TON

PRICE $

MILD STEEL

14141

350

5455.8

AISI 1010

14177

300

4688.1

AISI 1040

14132

580

9034.6

ASTM A36

14141

400

6235.2

SS 316

14411.2

1500

23828

SS 410

14051

1000

15488

Table 5-17: Cost of each steel grade

78

CHAPTER # 6 6. MATERIAL INSPECTION: 6.1 CORROSION CALCULATIONS Comparing different materials require the satisfactory amount of justification. From the material point of view corrosion calculation is one of the important parameter. by previous calculations related to material, we find out that AISI 1010 material is best fitted in terms of enhancing the overall heat transfer i.e. ring the degree of superheat up-to 20 degrees more. Here now we are only considering AISI 1010 and mild steel for corrosion purpose. Since our flue gases contain sulfur dioxide

in considerable amount, therefore

production of sulfuric acid may be probable. Thus testing our materials in acidic medium of sulfuric acid will give us right magnitudes of corrosion. 0.5 molar solution of sulfuric acid without corrosion inhibitor conditions were taken from research articles for mild steel and AISI 1010.

FOR MILD STEEL For mild steel directly measured corrosion rate is given:

FOR AISI 1010 We have to calculate it. Formula for corrosion rate in terms of weight loss is given by:

Where,

For weight loss specimen dimensions are Length

2.5 cm

Diameter of rod

79

0.6 cm

Therefore surface area is

Weight lost per unit area is found to be Therefore total weight lost is:

Density of AISI 1010 is 7.87 And exposed area is 2 The specimen is tested for 3 days dipped in the solution, therefore time in hours would be 72 hours Now,

It is found that the corrosion rate of AISI 1010 is approximately 1.5 times the corrosion rate of mild steel and comparing the tubes on same dimensions (thickness) AISI 1010 needs to be changed earlier than mild steel. On the other hand by using AISI 1010 sufficient increase in the degree of superheat is observed i.e. 20 degrees increment. So you cannot let it go by seeing only one minor difference of corrosion estimation.

6.1.1

JUSTIFICATION OF USING AISI-1010

Using simple Rankine cycle to calculate the difference of superheat had by using AISI 1010. Consider the power plant operating between two pressure curves on T-s diagram as:

80

Figure 6-1: T-s diagram of Rankine Cycle

While our system utilizing mild steel as tubing material the degree of superheat was

When we upgrade our material from mild steel to AISI 1010, our degree of superheat is found to be:

Enthalpies of all four points are

Just because we neglected the pump work, enthalpy at point 1 and point 2 are same. For mild steel @ 295

For AISI 1010 @ 313

Now,

81

WORK OUTPUT The difference in superheat in terms of work output is given as: FOR MILD STEEL

FOR AISI 1010

Fractional change in turbine work output is given by:

This means that upgrading the material is improving our work output up-to 13.46%. Therefore changing the material is better.

82

6.2 EROSION IN BOILER TUBES 6.2.1 THEORY EROSION: Erosion-is-a-form-of-wear. Wear can be defined as progressive loss of a surface material due to mechanical action involving impingement of abrasive particles. Unlike corrosion, which is a chemical or electrochemical action, wear is purely mechanical. Abrasion and erosion are two types of wear. 1.

Abrasionqcanqbeqlikenedqtoqsand-paperingqinqwhichqsolidqparticlesqmoveqin

contactqwithqaqparallelqsurface.

Abrasionqaffectsqtheqhighqspotsqofqtheqsurface

withoutqmuchqeffectqonqtheqmainqbody. The resulting loss of material is smaller in comparison with erosion. Abrasion resistance can be built by a boundary layer of high and preferably hard spots. 2. Erosion is the impingement of hard particles at an inclination, and it has more energy and destructive power than abrasion. The impinging particles cut through the boundary layer and destroy the main matrix. Hence, abrasion-resistant material cannot withstand erosion. Erosionqofqhotqpartsqdueqtoqash,qparticularlyqtheqtubes,qisqaqseriousqproblem affecting the life of boiler tubes. The hard constituents moving with flue gas at high velocities impinge on tubes, refractory, and other parts in the gas path, causing erosion. Erosion is influenced by the following:

Erosion is an inseparable. It can be minimized but not eliminated altogether. The aim is to minimize and predict the erosion rate so that the intervals between the downtimes 83

can be extended to coincide with the planned outages, thus improving the unit availability. The erosion prevention and protection measures essentially fall into two categories: 1. Design stage measures 2. Provision of sacrificial protective material

Figure 6-2: Erosion Mechanism

6.2.2

LITERATURE REVIEW:

METAL CUTTING THEORY: Finne( 1960) developed a model for surface erosion. Chinese Boiler Thermal Standards developed an equation by following the model using very large amount of experimental data. The maximum corrosion rate in convective section is written as (Basu.el/1999):

,

are the coefficient accounts forqtheqnon-uniformityqofqtheqflyqashqdensity

and gas velocity field, temperature, respectively = the time of erosion of tubes (h) =the density of fly ash =ratio of the gas velocity to average running load = percentage of fly gas smaller than 90

84

= Gas velocity in the narrowest section of tubes = erosion coefficient in gas .Depends on type of coal = least square errors = factor taking account of strike efficiency = strike efficiency EROSION RATE (BEP,1992):

K is constant dependent on tubes = erosion rate = ash abrasiveness factor = the flue gas temperature, K = Gas velocity between tubes = ash to carbon ratio in fuel EROSION BY PLASTIC CONTACT:

= =

85

According to BeckmannqandqGotzmann, the ratio

isqaquniversalqparameterqused

toqdetermineqtheqwearqresistanceqofqmetals. On the basis statistical data, a graph between pure metals, carbon and alloy steel and White cast iron is plotted:

Figure: 6-2

Source: Solid Particle Erosion: Occurrence, Prediction & Control by Ilmar Kleis, Priit Kulu = Wear rate in volume of material removed per kg of particle (

)

Stands for the share of wear caused by tangential component of velocity Stands for the share of wear caused by normal component of velocity = depth of indentation by the corroding material R = radius of abrasive particle in flue gas =is the dimension-less quantity, where numeratorqmeans the shearingqstress of theqtargetqmaterial and denominator means specific shear energy density = density of target material = Density of particle in flue gas

86

= Modulus of Elasticity of Target material = Modulus of Elasticity of Particle = Hardness of target material (HV GPA) = Hardness of corroding particle (HV GPA)

6.2.3

CALCULATION

EROSION RATE AT DIFFERENT IMPINGMENT ANGLE 6.2.3.1 OBJECTIVE: Analysis of boiler tube wearing rate at different angle of impingement FORMULA USED: Erosion by plastic contact:

The reason of using this formula is that it is more general and does not depend on the type of fuel used, while other approaches are restricted to the erosion caused by fly ash of coal using as a fuel in fired tube boiler. In our case, the boiler is waste heat boiler no firing happens only the cause of dust content is the Cement kiln dust (CKD). Assumptions: 

The particles are spherical



Particlesqcausingqerosion is homogeneousqandqelasticallyqdeformable



The velocity of particle before immediate impact is constant



The particles have no rotational energy



Homogenous dust content distribution



Flow rate and dust content are taken at nominal operating point. Changes due to temperature is not incorporated

6.2.3.2

ANGLE OF IMPINGEMENT:

DATA: Density of particle =

= 2799.92

87

Hardness of target material (AISI 1010) = 60 (Rockwell B) Hardness of the Particle (CKD) = 113 (Rockwell B) ModulusqofqElasticityqof Target material = 190GPa ModulusqofqElasticityqof Particle = 20GPa Poisson’s ratio of target material = Poisson’s ratioqofqparticle =

= 0.27

= 0.15 (approx.)

Vickers Hardness of Target material = HV= 108 = 108*0.0009807= 1.06GPa Impingement Velocity =

= 20 m/s (Current working parameter)

= 0.07 (from the graph presented by to Beckmann and Gotzmann given in literature review heading) 6.2.3.3 CALCULATION OF RADIUS OF PARTICLE: Particle size distribution is given by: AVERAGE DIAMETER(

PARTICLE

WEIGHT PERCENTAGE

),

1

03

5

20

10

15

20

20

30

16

40

10

50

06

60

03

>60

07

Weighted average Diameter of particle:

88

= 18.7096 R = 9.3548 Reduced modulus of elasticity is given by:

= 18.6031028GPa

= 2.47963e-07 m = = 5.97204e-4 = = =0

= 3.5644e-09

89

6.2.3.4 WEAR RATE (

AT DIFFERENT ANGLE OF IMPINGEMENT:

The velocity is kept constant as well as the hardness, density and viscosity the resultant effect will be in following figure:

Figure 6-3: Impingement Angle Vs Wear Rate 6.2.3.5 RESULT: 

It is observed the wear rate increases sharply up to 50 degrees



Shows almost constant wear rate between 60-90 degrees



Maximum wear rate at 50-70 degrees

6.2.4 WEAR RATE AND VOLUME REMOVED PER HOUR AT DIFFERENT VELOCITIES Angle of impingement = Density of particle =

= 2799.92

Hardness of target material (AISI 1010) = 60 (Rockwell B) Hardness of the Particle (CKD) = 113 (Rockwell B) Modulus of Elasticity of Target material = 190GPa Modulus of Elasticity of Particle = 20GPa Poisson’s ratio of target material =

= 0.27 90

Poisson’s ratio of particle =

= 0.15 (approx.)

Vickers Hardness of Target material = HV= 108 = 108*0.0009807= 1.06GPa

6.2.4.1.1 WEAR RATE: The effect of wear rate at different velocity at 90 degree angle is:

Figure 6-4: Velocity Vs Wear rate

6.2.4.1.2 MATERIAL VOLUME REMOVED PER HOUR: It is given the nominal flow rate and dust inlet content. To find the volume of material removed from the boiler tubes we have just find the dust content mass flow rate and then simply by multiplying the mass flow rate by the wear rate The angle of impingement is kept constant i.e. 90 degrees Angle of impingement = Current dust content = 15 Flow rate = 588700 m3/hr Mass flow rate of Dust content = (588700 * 15)/1000 = 8830.5 kg/hr. Volume removed at different wear rate will be given as: = Volume removed (

) = mass flow rate of dust content (kg/s) *

91

(

)

At different wear rate calculated using above formulae at different velocities the graph between volumes removed per hour with velocity will be:

Figure 6-5: Velocity Vs Volume removed per hrs

92

CONCLUSION We have a boiler operating at degraded performance. Through heat transfer analysis boiler was defined in terms of number of tubes, heat transfer area and overall heat transfer coefficient. Our work was on the material of tubes in boiler and optimizing boiler gas side parameters. In our boiler material was mild steel that has a thermal conductivity of 42 W/m.K through this the boiler was giving superheated steam at 295 C. while optimizing gas sides parameters we considered: 1. Inlet gas temperature effect on overall heat transfer coefficient and effectiveness. 2. Mass flow rate of gas effect on overall heat transfer coefficient. 3. Mass flow rate of gas effect on effectiveness of heat exchanger. 4. Inlet gas temperature effect when constant energy input. 5. Mass flow rate of gas effect when constant energy input. 

For inlet gas temperature effects at constant mass flow rate of gas, it is recommended to use temperature of 380 to 400 degrees not greater than this because this will lead to lesser tube life due to creep loading. At temperature 385 degrees overall efficiency found to be 85.11%.



For mass flow rate of gas effects at constant inlet temperature, it is recommended to use 80 to 90 kg/sec otherwise inlet dust content and increase in velocity will increase, thus increase erosion rate of tubes.



For constant energy inlet, increasing mass flow rate increases the effectiveness, while increasing the inlet temperature reduces it. The optimum point is achieved by increasing the mass flow rate and decreasing the inlet temperature. Keeping erosion rate in mind.

Also by material upgrading to AISI 1010 that has high thermal conductivity, it is found that degree of superheat enhanced and work output increased by approximately 13%. Overall optimization can be concluded by considering the life of boiler. Factors like corrosion, erosion can reduce lifespan and thus increase frequency of overhauling. Final conclusion is increase the mass flow rate up-to 90 kg/sec for fixed hea0t transfer ra0te, th0us reducing the high inlet temperature requirement. This will increase the tubes life by reducing creep load and also increase the overall efficiency of boiler. 93

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APPENDIX (section for appendix material)