Boilers and Thermic Fluid Heaters (2)

7/27/2019 Boilers and Thermic Fluid Heaters (2)

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ACADs (08-006) Covered

Keywords

Boiler, efficiency, blowdown, assessment, boiler feedwater.

Description

Supporting Material

1.3.4.1 5.2.1.9.c 5.2.2.2 5.2.2.2


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Training Session on Energy

Equipment

Boilers & ThermicFluid Heaters


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1. Describe the theory, construction,

and applications of boilers

2. Describe the common type of

boilers3. Explain how to assess the

performance and efficiency of a

boiler

4. Describe methods to improve

boiler efficiency

5.List energy efficiency opportunities

Objectives


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Introduction Purpose?

Type of boilers

Boiler Assessment

Energy efficiency opportunities

Objectives


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What is a Boiler?

Introduction

Vessel that heats water to become

hot water or steam

At atmospheric pressure water

volume increases 1,600 times

Hot water or steam used to transfer

heat to a process


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Introduction

BURNERWATER

SOURCE

BRINE

SOFTENERS

CHEMICAL FEED

FUELBLOW DOWN

SEPARATOR

VENT

VENTEXHAUST GASSTEAM TOPROCESS

STACK DEAERATOR

PUMPS

Figure: Schematic overview of a boiler room

BOILER

ECO-

NOMI-

ZER


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Introduction

Type of boilers

Assessment of a boiler



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Types of Boilers

1. Fire Tube Boiler

2. Water Tube Boiler

3. Packaged Boiler

4. Fluidized Bed (FBC) Boiler

5. Stoker Fired Boiler

6. Pulverized Fuel Boiler

7. Waste Heat Boiler

What Type of Boilers Are There?


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Type of Boilers

(Light Rail Transit Association)

1. Fire Tube Boiler

Relatively small steamcapacities (12,000

kg/hour)

Low to medium steam

pressures (18 kg/cm2)

Operates with oil, gas

or solid fuels


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Type of Boilers

2. Water Tube Boiler

(Your Dictionary.com)

Used for high steam

demand and pressure

requirements

Capacity range of 4,500

120,000 kg/hour

Combustion efficiency

enhanced by induced

draft provisions

Lower tolerance for

water quality and needs

water treatment plant


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Type of Boilers

(BIB Cochran, 2003)

3. Packaged Boiler

Oil

Burner

To

Chimney

Comes in complete

package

Features

High heat transfer Faster evaporation

Good convective

heat transfer

Good combustion

efficiency High thermal

efficiency

Classified based on

number of passes


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Type of Boilers

4. Fluidized Bed Combustion(FBC) Boiler Particles (e.g. sand) are

suspended in high velocity air

stream: bubbling fluidized bed

Combustion at 840950

C

Fuels: coal, washery rejects,

rice husk, bagasse andagricultural wastes

Benefits: compactness, fuel

flexibility, higher combustion

efficiency, reduced SOx & NOx


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Type of Boilers

5. Stoke Fired Boilers

a) Spreader stokers

Coal is first burnt in suspension then incoal bed

Flexibility to meet load fluctuations

Favored in many industrial applications


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Type of Boilers

5. Stoke Fired Boilers

b) Chain-grate or traveling-grate stoker

(University of Missouri, 2004)

Coal is burnt on moving

steel grate

Coal gate controls coal

feeding rate

Uniform coal size for

complete combustion


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Type of Boilers

Tangential firing

6. Pulverized Fuel Boiler

Pulverized coal powder blown with combustion

air into boiler through burner nozzles Combustion

temperature at 1300 -

1700 C

Benefits: varying coalquality coal, quick

response to load

changes and high pre-

heat air temperatures


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Independence Steam Electric Station

- Newark, AR

Operation: Unit 1 - January 1983/Unit 2 - December 1984

Fuel:Low-sulfur coal mined near Gillette, Wyoming

Capability:1,678 megawatts


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Type of Boilers

Agriculture and Agri-FoodCanada, 2001

7. Waste Heat Boiler

Used when waste heat

available at medium/high

temp

Auxiliary fuel burners

used if steam demand is

more than the waste heat

can generate

Used in heat recovery

from exhaust gases from

gas turbines and diesel

engines


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Introduction

Type of boilers

Boiler Assesment


Training Agenda: Boiler


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1. Boiler

2. Boiler blow down

3. Boiler feed water treatment

Boiler Assessment


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Assessment of a Boiler

1. Boiler performance

Causes of poor boiler performance-Poor combustion

-Heat transfer surface fouling

-Poor operation and maintenance

-Deteriorating fuel and water quality

Heat balance: identify heat losses

Boiler efficiency: determine

deviation from best efficiency


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Heat Balance

An energy flow diagram describes geographically

how energy is transformed from fuel into useful

energy, heat and lossesStochiometric

Excess Air

Un burnt

FUEL INPUT STEAMOUTPUT

Stack Gas

Ash and Un-burnt parts

of Fuel in Ash

Blow

Down

Convection &

Radiation


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Heat Balance

Balancing total energy entering a boiler against

the energy that leaves the boiler in different forms

Heat in Steam

BOILER

Heat loss due to dry flue gas

Heat loss due to steam in fuel gas

Heat loss due to moisture in fuel

Heat loss due to unburnts in residue

Heat loss due to moisture in air

Heat loss due to radiation & other

unaccounted loss

12.7 %

8.1 %

1.7 %

0.3 %

2.4 %

1.0 %

73.8 %

100.0 %

Fuel

73.8 %


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Heat Balance

Goal: improve energy efficiency by reducing

avoidable lossesAvoidable losses include:

- Stack gas losses (excess air, stack gas

temperature)

- Losses by unburnt fuel- Blow down losses

- Condensate losses

- Convection and radiation


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1. Boiler Efficiency

Thermal efficiency: % of (heat) energy input that is

effectively useful in the generated steam


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Controls total dissolved solids (TDS) in the

water that is boiled

Blows off water and replaces it with feed water

Conductivity measured as indication of TDS

levels

Calculation of quantity blow down required:

2. Boiler Blow Down

Blow down (%) =Feed water TDS x % Make up water

Maximum Permissible TDS in Boiler water


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Two types of blow down

Intermittent Manually operated valve reduces TDS

Large short-term increases in feed water

Substantial heat loss

Continuous Ensures constant TDS and steam purity

Heat lost can be recovered

Common in high-pressure boilers

Boiler Blow Down


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Benefits

Lower pretreatment costs

Less make-up water consumption

Reduced maintenance downtime

Increased boiler life

Lower consumption of treatment

chemicals

Boiler Blow Down


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Quality of steam depend on water

treatment to controlSteam purity

Deposits

Corrosion

Efficient heat transfer only if boiler

water is free from deposit-forming

solids

3. Boiler Feed Water Treatment


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Deposit control

To avoid efficiency losses and

reduced heat transfer

Hardness salts of calcium and

magnesium

Alkaline hardness: removed by boiling

Non-alkaline: difficult to remove

Silica forms hard silica scales

Boiler Feed Water Treatment


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Internal water treatment

Chemicals added to boiler to prevent scale Different chemicals for different water types

Conditions:

Feed water is low in hardness salts

Low pressure, high TDS content is tolerated

Small water quantities treated

Internal treatment alone not recommended



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External water treatment:

Removal of suspended/dissolved solids and

dissolved gases

Pre-treatment: sedimentation and settling

First treatment stage: removal of salts

Processesa) Ion exchange

b) Demineralization

c) De-aeration

d) Reverse osmoses



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1. Stack temperature control

2. Feed water preheating using economizers

3. Combustion air pre-heating

4. Incomplete combustion minimization

5. Excess air control

6. Avoid radiation and convection heat loss7. Automatic blow down control

8. Reduction of scaling and soot losses

9. Reduction of boiler steam pressure

10. Variable speed control

11. Controlling boiler loading

12. Proper boiler scheduling13. Boiler replacement

Energy Efficiency Opportunities


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1. Stack Temperature Control

Keep as low as possible

If >200C then recover waste heat


2. Feed Water Preheating

Economizers

Potential to recover heat from 200 300 oC flue

gases leaving a modern 3-pass shell boiler

3. Combustion Air Preheating

If combustion air raised by 20C= 1% improvethermal efficiency


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4. Minimize Incomplete Combustion

Symptoms:

Smoke, high CO levels in exit flue gas

Causes:

Air shortage, fuel surplus, poor fuel distribution

Poor mixing of fuel and air

Oil-fired boiler: Improper viscosity, worn tops, cabonization on

dips, deterioration of diffusers or spinner plates

Coal-fired boiler:non-uniform coal size



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5. Excess Air Control

Excess air required for complete combustion

Optimum excess air levels varies

1% excess air reduction = 0.6% efficiency rise

Portable or continuous oxygen analyzers

Fuel Kg air req./kg fuel %CO2in flue gas in practice

Solid Fuels

Bagasse

Coal (bituminous)

Lignite

Paddy Husk

Wood

3.3

10.7

8.5

4.5

5.7

10-12

10-13

9 -13

14-15

11.13

Liquid Fuels

Furnace OilLSHS

13.814.1

9-149-14


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7. Automatic Blow Down Control

6. Radiation and Convection Heat

Loss Minimization

Fixed heat loss from boiler shell, regardless ofboiler output

Repairing insulation can reduce loss

Sense and respond to boiler water conductivity

and pH


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9. Reduced Boiler Steam Pressure

8. Scaling and Soot Loss Reduction

Every 22oC increase in stack temperature= 1%

efficiency loss

3 mm of soot = 2.5% fuel increase

Lower steam pressure

= lower saturated steam temperature

= lower flue gas temperature

Steam generation pressure dictated by process


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11. Control Boiler Loading

10. Variable Speed Control for Fans,

Blowers and Pumps

Suited for fans, blowers, pumps Should be considered if boiler loads are

variable

Maximum boiler efficiency: 65-85% of rated load

Significant efficiency loss: < 25% of rated load


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13. Boiler Replacement

12. Proper Boiler Scheduling Optimum efficiency: 65-85% of full load

Few boilers at high loads is more efficient than

large number at low loads

Financially attractive if existing boiler is

Old and inefficient

Not capable of firing cheaper substitution fuel

Over or under-sized for present requirements

Not designed for ideal loading conditions


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Training Session on Energy

Equipment

Boilers & Thermic

Fluid Heaters

THANK YOU

FOR YOUR ATTENTION

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Boilers and Thermic Fluid Heaters (2)