Boilers & Thermic Fluid Heaters

ACADs (08-006) Covered

KeywordsBoiler, 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 & Thermic Fluid Heaters

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1. Describe the theory, construction, and applications of boilers

2. Describe the common type of boilers

3. 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

SOFTENERSCHEMICAL FEED

FUELBLOW DOWN SEPARATOR

VENT

VENTEXHAUST GASSTEAM TO PROCESS

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

Energy efficiency opportunities

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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 steam capacities (12,000 kg/hour)

• Low to medium steam pressures (18 kg/cm2)

• Operates with oil, gas or solid fuels

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11

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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 840° – 950° C

• Fuels: coal, washery rejects, rice husk, bagasse and agricultural 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 in

coal 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 coal quality 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 1984Fuel: Low-sulfur coal mined near Gillette, Wyoming Capability: 1,678 megawatts

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Type of Boilers

Agriculture and Agri-Food Canada, 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

Energy efficiency opportunities

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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Assessment of a Boiler

Heat BalanceAn energy flow diagram describes geographically how energy is transformed from fuel into useful energy, heat and losses

StochiometricExcess AirUn burnt

FUEL INPUT STEAM OUTPUT

Stack Gas

Ash and Un-burnt parts of Fuel in Ash

Blow Down

Convection & Radiation

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Assessment of a Boiler

Heat BalanceBalancing 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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Assessment of a Boiler

Heat BalanceGoal: 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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Assessment of a Boiler

1. Boiler EfficiencyThermal efficiency: % of (heat) energy input that is effectively useful in the generated steam

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Assessment of a Boiler

• 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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Assessment of a Boiler

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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Assessment of a Boiler

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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Assessment of a Boiler

• Quality of steam depend on water treatment to control• Steam 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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Assessment of a Boiler

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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Assessment of a Boiler

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

Boiler Feed Water Treatment

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Assessment of a Boiler

External water treatment:• Removal of suspended/dissolved solids and

dissolved gases

• Pre-treatment: sedimentation and settling

• First treatment stage: removal of salts

• Processes

a) Ion exchange

b) Demineralization

c) De-aeration

d) Reverse osmoses

Boiler Feed Water Treatment

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1. Stack temperature control2. Feed water preheating using economizers3. Combustion air pre-heating4. Incomplete combustion minimization5. Excess air control6. Avoid radiation and convection heat loss7. Automatic blow down control8. Reduction of scaling and soot losses9. Reduction of boiler steam pressure10. Variable speed control11. Controlling boiler loading12. Proper boiler scheduling13. Boiler replacement

Energy Efficiency Opportunities

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1. Stack Temperature Control• Keep as low as possible• If >200°C then recover waste heat

Energy Efficiency Opportunities

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 20°C = 1% improve

thermal 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

Energy Efficiency Opportunities

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Energy Efficiency Opportunities

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 %CO2 in 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 Oil LSHS

13.8 14.1

9-14 9-14

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Energy Efficiency Opportunities

7. Automatic Blow Down Control

6. Radiation and Convection Heat Loss Minimization• Fixed heat loss from boiler shell, regardless of

boiler output

• Repairing insulation can reduce loss

• Sense and respond to boiler water conductivity and pH

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Energy Efficiency Opportunities

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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Energy Efficiency Opportunities

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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Energy Efficiency Opportunities

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 YOUFOR YOUR ATTENTION

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