Boilers And Thermic Fluid Heaters
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Presentation on Energy Equipment For Karachi University Students Thermal Equipment/ Boilers
Boilers & Thermic Fluid Heaters Presentation from the “Energy Efficiency Guide for Industry in Pakistan”
1
Training Agenda: Boiler
Thermal Equipment/ Boilers
Introduction Type of boilers Assessment of a boiler Energy efficiency opportunities
2
Introduction
Thermal Equipment/ Boilers
What is a Boiler? • 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 3
Introduction STEAM TO PROCESS
EXHAUST GAS
DEAERATOR
PUMPS ECONOMIZER
Thermal Equipment/ Boilers
STACK
VENT
VENT
BOILER BLOW DOWN SEPARATOR
BURNER
WATER SOURCE
FUEL BRINE CHEMICAL FEED
Figure: Schematic overview of a boiler room
SOFTENERS
4
Training Agenda: Boiler
Thermal Equipment/ Boilers
Introduction Type of boilers Assessment of a boiler Energy efficiency opportunities
5
Types of Boilers
Thermal Equipment/ Boilers
What Type of Boilers Are There? 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 8. Thermic Fluid Heater (not a boiler!) 6
Type of Boilers
Thermal Equipment/ Boilers
1. Fire Tube Boiler
(Light Rail Transit Association)
•
Relatively small steam capacities (12,000 kg/hour)
•
Low to medium steam pressures (18 kg/cm2)
•
Operates with oil, gas or solid fuels
7
Type of Boilers
Thermal Equipment/ 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 8
Type of Boilers
Thermal Equipment/ Boilers
3. Packaged Boiler To Chimney
Oil Burner
(BIB Cochran, 2003)
• 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
9
Type of Boilers
Thermal Equipment/ 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
•
Capacity range 0,5 T/hr to 100 T/hr
•
Fuels: coal, washery rejects, rice husk, bagasse and agricultural wastes
•
Benefits: compactness, fuel flexibility, higher combustion efficiency, reduced SOx & NOx 10
Type of Boilers
Thermal Equipment/ Boilers
4a. Atmospheric Fluidized Bed Combustion (AFBC) Boiler • Most common FBC boiler that uses preheated atmospheric air as fluidization and combustion air
4b. Pressurized Fluidized Bed Combustion (PFBC) Boiler • Compressor supplies the forced draft and combustor is a pressure vessel • Used for cogeneration or combined cycle power generation 11
Type of Boilers
Thermal Equipment/ Boilers
4c. Atmospheric Circulating Fluidized Bed Combustion (CFBC) Boiler •
Solids lifted from bed, rise, return to bed
•
Steam generation in convection section
•
Benefits: more economical, better space utilization and efficient combustion
(Thermax Babcock & Wilcox Ltd, 2001)
12
Type of Boilers
Thermal Equipment/ 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
13
Type of Boilers
Thermal Equipment/ Boilers
5. Stoke Fired Boilers b) Chain-grate or traveling-grate stoker • Coal is burnt on moving steel grate • Coal gate controls coal feeding rate • Uniform coal size for complete combustion
(University of Missouri, 2004)
14
Type of Boilers
Thermal Equipment/ Boilers
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 preheat air temperatures
Tangential firing 15
Type of Boilers
Thermal Equipment/ Boilers
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
Agriculture and Agri-Food Canada, 2001
• Used in heat recovery from exhaust gases from gas turbines and diesel engines
16
Type of Boilers
Thermal Equipment/ Boilers
8. Thermic Fluid Heater •
Wide application for indirect process heating
•
Thermic fluid (petroleum-based) is heat transfer medium
•
Benefits: • Closed cycle = minimal losses • Non-pressurized system operation at 250 °C • Automatic controls = operational flexibility • Good thermal efficiencies 17
Type of Boilers 8. Thermic Fluid Heater Thermal Equipment/ Boilers
User equipment
2. Circulated to user equipment
1. Thermic fluid heated in the heater
3. Heat transfer through heat exchanged
4. Fluid returned to heater Control panel Insulated outer wall
Blower motor unit Fuel oil filter
Exhaust
(Energy Machine India) 18
Training Agenda: Boiler
Thermal Equipment/ Boilers
Introduction Type of boilers Assessment of a boiler Energy efficiency opportunities
19
Assessment of a boiler
Thermal Equipment/ Boilers
1. Boiler 2. Boiler blow down 3. Boiler feed water treatment
20
Assessment of a Boiler
Thermal Equipment/ Boilers
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 21
Assessment of a Boiler
Thermal Equipment/ Boilers
Heat Balance An energy flow diagram describes geographically how energy is transformed from fuel into useful energy, heat and losses Stochiometric Excess Air Un burnt
Stack Gas
FUEL INPUT
STEAM OUTPUT
Convection & Radiation
Blow Down
Ash and Un-burnt parts of Fuel in Ash
22
Assessment of a Boiler
Thermal Equipment/ Boilers
Heat Balance Balancing total energy entering a boiler against the energy that leaves the boiler in different forms 12.7 % 8.1 %
100.0 % Fuel
1.7 %
BOILER
0.3 % 2.4 % 1.0 %
73.8 %
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 moisture in air Heat loss due to unburnts in residue Heat loss due to radiation & other unaccounted loss
Heat in Steam 23
Assessment of a Boiler
Thermal Equipment/ Boilers
Heat Balance Goal: improve energy efficiency by reducing avoidable losses Avoidable losses include: - Stack gas losses (excess air, stack gas temperature) - Losses by unburnt fuel - Blow down losses - Condensate losses - Convection and radiation
24
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Efficiency Thermal efficiency: % of (heat) energy input that is effectively useful in the generated steam
BOILER EFFICENCY CALCULATION
1)
DIRECT METHOD:
2) INDIRECT METHOD:
The energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel.
The efficiency is the different between losses and energy input
25
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Efficiency: Direct Method Boiler efficiency (η ) =
Heat Input x 100 Q x (hg – hf) x 100 = Heat Output Q x GCV
hg -the enthalpy of saturated steam in kcal/kg of steam hf -the enthalpy of feed water in kcal/kg of water Parameters to be monitored: - Quantity of steam generated per hour (Q) in kg/hr - Quantity of fuel used per hour (q) in kg/hr - The working pressure (in kg/cm2(g)) and superheat temperature (oC), if any - The temperature of feed water (oC) - Type of fuel and gross calorific value of the fuel (GCV) in 26 kcal/kg of fuel
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Efficiency: Direct Method Advantages • • • •
Quick evaluation Few parameters for computation Few monitoring instruments Easy to compare evaporation ratios with benchmark figures
Disadvantages • •
No explanation of low efficiency Various losses not calculated 27
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Efficiency: Indirect Method Efficiency of boiler (η ) = 100 – (i+ii+iii+iv+v+vi+vii)
Principle losses: i) Dry flue gas ii) Evaporation of water formed due to H2 in fuel iii) Evaporation of moisture in fuel iv) Moisture present in combustion air v) Unburnt fuel in fly ash vi) Unburnt fuel in bottom ash vii) Radiation and other unaccounted losses
28
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Efficiency: Indirect Method Required calculation data • Ultimate analysis of fuel (H2, O2, S, C, moisture content, ash content) • % oxygen or CO2 in the flue gas • Fuel gas temperature in ◦C (Tf) • Ambient temperature in ◦C (Ta) and humidity of air in kg/kg of dry air • GCV of fuel in kcal/kg • % combustible in ash (in case of solid fuels) • GCV of ash in kcal/kg (in case of solid fuels)
29
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Efficiency: Indirect Method Advantages • •
Complete mass and energy balance for each individual stream Makes it easier to identify options to improve boiler efficiency
Disadvantages • •
Time consuming Requires lab facilities for analysis 30
Assessment of a Boiler 2. Boiler Blow Down Thermal Equipment/ Boilers
•
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:
Blow down (%) =
Feed water TDS x % Make up water Maximum Permissible TDS in Boiler water 31
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Blow Down 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 32
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Blow Down Benefits • Lower pretreatment costs • Less make-up water consumption • Reduced maintenance downtime • Increased boiler life • Lower consumption of treatment chemicals
33
Assessment of a Boiler
Thermal Equipment/ Boilers
3. Boiler Feed Water Treatment • 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 34
Assessment of a Boiler Boiler Feed Water Treatment Thermal Equipment/ Boilers
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 35
Assessment of a Boiler Boiler Feed Water Treatment Thermal Equipment/ Boilers
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 36
Assessment of a Boiler Boiler Feed Water Treatment Thermal Equipment/ Boilers
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
37
Assessment of a Boiler External Water Treatment Thermal Equipment/ Boilers
a) Ion-exchange process (softener plant) • Water passes through bed of natural zeolite of synthetic resin to remove hardness • Base exchange: calcium (Ca) and magnesium (Mg) replaced with sodium (Na) ions • Does not reduce TDS, blow down quantity and alkalinity
b) Demineralization • Complete removal of salts • Cations in raw water replaced with hydrogen ions 38
Assessment of a Boiler
Thermal Equipment/ Boilers
External Water Treatment c) De-aeration • Dissolved corrosive gases (O2, CO2) expelled by preheating the feed water • Two types: • Mechanical de-aeration: used prior to addition of chemical oxygen scavangers • Chemical de-aeration: removes trace oxygen 39
Assessment of a Boiler External Water Treatment Thermal Equipment/ Boilers
Mechanical de-aeration
Vent Spray Nozzles
Boiler Feed Water
Stea m
Scrubber Section (Trays)
• O2 and CO2 removed by heating feed water • Economical treatment process • Vacuum type can reduce O2 to 0.02 mg/l
Storage Section De-aerated Boiler Feed Water
( National Productivity Council)
• Pressure type can reduce O2 to 0.005 mg/l 40
Assessment of a Boiler External Water Treatment Thermal Equipment/ Boilers
Chemical de-aeration • Removal of trace oxygen with scavenger • Sodium sulphite: • Reacts with oxygen: sodium sulphate • Increases TDS: increased blow down • Hydrazine • Reacts with oxygen: nitrogen + water • Does not increase TDS: used in high pressure boilers
41
Assessment of a Boiler
Thermal Equipment/ Boilers
External Water Treatment d) Reverse osmosis • Osmosis • Solutions of differing concentrations • Separated by a semi-permeable membrane • Water moves to the higher concentration
• Reversed osmosis • Higher concentrated liquid pressurized • Water moves in reversed direction
42
Assessment of a Boiler
Thermal Equipment/ Boilers
External water treatment d) Reverse osmosis Pressure
Fresh Water
Feed Water More Concentrated Solution Concentrate Flow
Water Flow
Semi Permeable Membrane
43
Training Agenda: Boiler
Thermal Equipment/ Boilers
Introduction Type of boilers Assessment of a boiler Energy efficiency opportunities
44
Energy Efficiency Opportunities
Thermal Equipment/ Boilers
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Stack temperature control Feed water preheating using economizers Combustion air pre-heating Incomplete combustion minimization Excess air control Avoid radiation and convection heat loss Automatic blow down control Reduction of scaling and soot losses Reduction of boiler steam pressure Variable speed control
45
Energy Efficiency Opportunities 1. Stack Temperature Control Thermal Equipment/ Boilers
• Keep as low as possible • If >200°C 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 20°C = 1% improve thermal efficiency
46
Energy Efficiency Opportunities 4. Minimize Incomplete Combustion Thermal Equipment/ Boilers
• 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
47
Energy Efficiency Opportunities 5. Excess Air Control Thermal Equipment/ Boilers
• 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
48
Energy Efficiency Opportunities
Thermal Equipment/ Boilers
6. Radiation and Convection Heat Loss Minimization •
Fixed heat loss from boiler shell, regardless of boiler output
•
Repairing insulation can reduce loss
7. Automatic Blow Down Control •
Sense and respond to boiler water conductivity and pH 49
Energy Efficiency Opportunities 8. Scaling and Soot Loss Reduction Thermal Equipment/ Boilers
•
Every 22oC increase in stack temperature = 1% efficiency loss
•
3 mm of soot = 2.5% fuel increase
9. Reduced Boiler Steam Pressure •
Lower steam pressure = lower saturated steam temperature = lower flue gas temperature
•
Steam generation pressure dictated by process
50
Energy Efficiency Opportunities
Thermal Equipment/ Boilers
10. Variable Speed Control for Fans, Blowers and Pumps •
Suited for fans, blowers, pumps
•
Should be considered if boiler loads are variable
11. Control Boiler Loading •
Maximum boiler efficiency: 65-85% of rated load
•
Significant efficiency loss: < 25% of rated load 51
Energy Efficiency Opportunities 12. Proper Boiler Scheduling Thermal Equipment/ Boilers
•
Optimum efficiency: 65-85% of full load
•
Few boilers at high loads is more efficient than large number at low loads
13. Boiler Replacement 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
52
Training Session on Energy Equipment Thermal Equipment/ Boilers
Boilers & Thermic Fluid Heaters THANK YOU FOR YOUR ATTENTION
53
Boilers & Thermic Fluid Heaters Presentation from the “Energy Efficiency Guide for Industry in Pakistan”
1
Training Agenda: Boiler
Thermal Equipment/ Boilers
Introduction Type of boilers Assessment of a boiler Energy efficiency opportunities
2
Introduction
Thermal Equipment/ Boilers
What is a Boiler? • 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 3
Introduction STEAM TO PROCESS
EXHAUST GAS
DEAERATOR
PUMPS ECONOMIZER
Thermal Equipment/ Boilers
STACK
VENT
VENT
BOILER BLOW DOWN SEPARATOR
BURNER
WATER SOURCE
FUEL BRINE CHEMICAL FEED
Figure: Schematic overview of a boiler room
SOFTENERS
4
Training Agenda: Boiler
Thermal Equipment/ Boilers
Introduction Type of boilers Assessment of a boiler Energy efficiency opportunities
5
Types of Boilers
Thermal Equipment/ Boilers
What Type of Boilers Are There? 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 8. Thermic Fluid Heater (not a boiler!) 6
Type of Boilers
Thermal Equipment/ Boilers
1. Fire Tube Boiler
(Light Rail Transit Association)
•
Relatively small steam capacities (12,000 kg/hour)
•
Low to medium steam pressures (18 kg/cm2)
•
Operates with oil, gas or solid fuels
7
Type of Boilers
Thermal Equipment/ 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 8
Type of Boilers
Thermal Equipment/ Boilers
3. Packaged Boiler To Chimney
Oil Burner
(BIB Cochran, 2003)
• 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
9
Type of Boilers
Thermal Equipment/ 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
•
Capacity range 0,5 T/hr to 100 T/hr
•
Fuels: coal, washery rejects, rice husk, bagasse and agricultural wastes
•
Benefits: compactness, fuel flexibility, higher combustion efficiency, reduced SOx & NOx 10
Type of Boilers
Thermal Equipment/ Boilers
4a. Atmospheric Fluidized Bed Combustion (AFBC) Boiler • Most common FBC boiler that uses preheated atmospheric air as fluidization and combustion air
4b. Pressurized Fluidized Bed Combustion (PFBC) Boiler • Compressor supplies the forced draft and combustor is a pressure vessel • Used for cogeneration or combined cycle power generation 11
Type of Boilers
Thermal Equipment/ Boilers
4c. Atmospheric Circulating Fluidized Bed Combustion (CFBC) Boiler •
Solids lifted from bed, rise, return to bed
•
Steam generation in convection section
•
Benefits: more economical, better space utilization and efficient combustion
(Thermax Babcock & Wilcox Ltd, 2001)
12
Type of Boilers
Thermal Equipment/ 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
13
Type of Boilers
Thermal Equipment/ Boilers
5. Stoke Fired Boilers b) Chain-grate or traveling-grate stoker • Coal is burnt on moving steel grate • Coal gate controls coal feeding rate • Uniform coal size for complete combustion
(University of Missouri, 2004)
14
Type of Boilers
Thermal Equipment/ Boilers
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 preheat air temperatures
Tangential firing 15
Type of Boilers
Thermal Equipment/ Boilers
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
Agriculture and Agri-Food Canada, 2001
• Used in heat recovery from exhaust gases from gas turbines and diesel engines
16
Type of Boilers
Thermal Equipment/ Boilers
8. Thermic Fluid Heater •
Wide application for indirect process heating
•
Thermic fluid (petroleum-based) is heat transfer medium
•
Benefits: • Closed cycle = minimal losses • Non-pressurized system operation at 250 °C • Automatic controls = operational flexibility • Good thermal efficiencies 17
Type of Boilers 8. Thermic Fluid Heater Thermal Equipment/ Boilers
User equipment
2. Circulated to user equipment
1. Thermic fluid heated in the heater
3. Heat transfer through heat exchanged
4. Fluid returned to heater Control panel Insulated outer wall
Blower motor unit Fuel oil filter
Exhaust
(Energy Machine India) 18
Training Agenda: Boiler
Thermal Equipment/ Boilers
Introduction Type of boilers Assessment of a boiler Energy efficiency opportunities
19
Assessment of a boiler
Thermal Equipment/ Boilers
1. Boiler 2. Boiler blow down 3. Boiler feed water treatment
20
Assessment of a Boiler
Thermal Equipment/ Boilers
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 21
Assessment of a Boiler
Thermal Equipment/ Boilers
Heat Balance An energy flow diagram describes geographically how energy is transformed from fuel into useful energy, heat and losses Stochiometric Excess Air Un burnt
Stack Gas
FUEL INPUT
STEAM OUTPUT
Convection & Radiation
Blow Down
Ash and Un-burnt parts of Fuel in Ash
22
Assessment of a Boiler
Thermal Equipment/ Boilers
Heat Balance Balancing total energy entering a boiler against the energy that leaves the boiler in different forms 12.7 % 8.1 %
100.0 % Fuel
1.7 %
BOILER
0.3 % 2.4 % 1.0 %
73.8 %
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 moisture in air Heat loss due to unburnts in residue Heat loss due to radiation & other unaccounted loss
Heat in Steam 23
Assessment of a Boiler
Thermal Equipment/ Boilers
Heat Balance Goal: improve energy efficiency by reducing avoidable losses Avoidable losses include: - Stack gas losses (excess air, stack gas temperature) - Losses by unburnt fuel - Blow down losses - Condensate losses - Convection and radiation
24
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Efficiency Thermal efficiency: % of (heat) energy input that is effectively useful in the generated steam
BOILER EFFICENCY CALCULATION
1)
DIRECT METHOD:
2) INDIRECT METHOD:
The energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel.
The efficiency is the different between losses and energy input
25
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Efficiency: Direct Method Boiler efficiency (η ) =
Heat Input x 100 Q x (hg – hf) x 100 = Heat Output Q x GCV
hg -the enthalpy of saturated steam in kcal/kg of steam hf -the enthalpy of feed water in kcal/kg of water Parameters to be monitored: - Quantity of steam generated per hour (Q) in kg/hr - Quantity of fuel used per hour (q) in kg/hr - The working pressure (in kg/cm2(g)) and superheat temperature (oC), if any - The temperature of feed water (oC) - Type of fuel and gross calorific value of the fuel (GCV) in 26 kcal/kg of fuel
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Efficiency: Direct Method Advantages • • • •
Quick evaluation Few parameters for computation Few monitoring instruments Easy to compare evaporation ratios with benchmark figures
Disadvantages • •
No explanation of low efficiency Various losses not calculated 27
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Efficiency: Indirect Method Efficiency of boiler (η ) = 100 – (i+ii+iii+iv+v+vi+vii)
Principle losses: i) Dry flue gas ii) Evaporation of water formed due to H2 in fuel iii) Evaporation of moisture in fuel iv) Moisture present in combustion air v) Unburnt fuel in fly ash vi) Unburnt fuel in bottom ash vii) Radiation and other unaccounted losses
28
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Efficiency: Indirect Method Required calculation data • Ultimate analysis of fuel (H2, O2, S, C, moisture content, ash content) • % oxygen or CO2 in the flue gas • Fuel gas temperature in ◦C (Tf) • Ambient temperature in ◦C (Ta) and humidity of air in kg/kg of dry air • GCV of fuel in kcal/kg • % combustible in ash (in case of solid fuels) • GCV of ash in kcal/kg (in case of solid fuels)
29
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Efficiency: Indirect Method Advantages • •
Complete mass and energy balance for each individual stream Makes it easier to identify options to improve boiler efficiency
Disadvantages • •
Time consuming Requires lab facilities for analysis 30
Assessment of a Boiler 2. Boiler Blow Down Thermal Equipment/ Boilers
•
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:
Blow down (%) =
Feed water TDS x % Make up water Maximum Permissible TDS in Boiler water 31
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Blow Down 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 32
Assessment of a Boiler
Thermal Equipment/ Boilers
Boiler Blow Down Benefits • Lower pretreatment costs • Less make-up water consumption • Reduced maintenance downtime • Increased boiler life • Lower consumption of treatment chemicals
33
Assessment of a Boiler
Thermal Equipment/ Boilers
3. Boiler Feed Water Treatment • 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 34
Assessment of a Boiler Boiler Feed Water Treatment Thermal Equipment/ Boilers
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 35
Assessment of a Boiler Boiler Feed Water Treatment Thermal Equipment/ Boilers
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 36
Assessment of a Boiler Boiler Feed Water Treatment Thermal Equipment/ Boilers
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
37
Assessment of a Boiler External Water Treatment Thermal Equipment/ Boilers
a) Ion-exchange process (softener plant) • Water passes through bed of natural zeolite of synthetic resin to remove hardness • Base exchange: calcium (Ca) and magnesium (Mg) replaced with sodium (Na) ions • Does not reduce TDS, blow down quantity and alkalinity
b) Demineralization • Complete removal of salts • Cations in raw water replaced with hydrogen ions 38
Assessment of a Boiler
Thermal Equipment/ Boilers
External Water Treatment c) De-aeration • Dissolved corrosive gases (O2, CO2) expelled by preheating the feed water • Two types: • Mechanical de-aeration: used prior to addition of chemical oxygen scavangers • Chemical de-aeration: removes trace oxygen 39
Assessment of a Boiler External Water Treatment Thermal Equipment/ Boilers
Mechanical de-aeration
Vent Spray Nozzles
Boiler Feed Water
Stea m
Scrubber Section (Trays)
• O2 and CO2 removed by heating feed water • Economical treatment process • Vacuum type can reduce O2 to 0.02 mg/l
Storage Section De-aerated Boiler Feed Water
( National Productivity Council)
• Pressure type can reduce O2 to 0.005 mg/l 40
Assessment of a Boiler External Water Treatment Thermal Equipment/ Boilers
Chemical de-aeration • Removal of trace oxygen with scavenger • Sodium sulphite: • Reacts with oxygen: sodium sulphate • Increases TDS: increased blow down • Hydrazine • Reacts with oxygen: nitrogen + water • Does not increase TDS: used in high pressure boilers
41
Assessment of a Boiler
Thermal Equipment/ Boilers
External Water Treatment d) Reverse osmosis • Osmosis • Solutions of differing concentrations • Separated by a semi-permeable membrane • Water moves to the higher concentration
• Reversed osmosis • Higher concentrated liquid pressurized • Water moves in reversed direction
42
Assessment of a Boiler
Thermal Equipment/ Boilers
External water treatment d) Reverse osmosis Pressure
Fresh Water
Feed Water More Concentrated Solution Concentrate Flow
Water Flow
Semi Permeable Membrane
43
Training Agenda: Boiler
Thermal Equipment/ Boilers
Introduction Type of boilers Assessment of a boiler Energy efficiency opportunities
44
Energy Efficiency Opportunities
Thermal Equipment/ Boilers
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Stack temperature control Feed water preheating using economizers Combustion air pre-heating Incomplete combustion minimization Excess air control Avoid radiation and convection heat loss Automatic blow down control Reduction of scaling and soot losses Reduction of boiler steam pressure Variable speed control
45
Energy Efficiency Opportunities 1. Stack Temperature Control Thermal Equipment/ Boilers
• Keep as low as possible • If >200°C 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 20°C = 1% improve thermal efficiency
46
Energy Efficiency Opportunities 4. Minimize Incomplete Combustion Thermal Equipment/ Boilers
• 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
47
Energy Efficiency Opportunities 5. Excess Air Control Thermal Equipment/ Boilers
• 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
48
Energy Efficiency Opportunities
Thermal Equipment/ Boilers
6. Radiation and Convection Heat Loss Minimization •
Fixed heat loss from boiler shell, regardless of boiler output
•
Repairing insulation can reduce loss
7. Automatic Blow Down Control •
Sense and respond to boiler water conductivity and pH 49
Energy Efficiency Opportunities 8. Scaling and Soot Loss Reduction Thermal Equipment/ Boilers
•
Every 22oC increase in stack temperature = 1% efficiency loss
•
3 mm of soot = 2.5% fuel increase
9. Reduced Boiler Steam Pressure •
Lower steam pressure = lower saturated steam temperature = lower flue gas temperature
•
Steam generation pressure dictated by process
50
Energy Efficiency Opportunities
Thermal Equipment/ Boilers
10. Variable Speed Control for Fans, Blowers and Pumps •
Suited for fans, blowers, pumps
•
Should be considered if boiler loads are variable
11. Control Boiler Loading •
Maximum boiler efficiency: 65-85% of rated load
•
Significant efficiency loss: < 25% of rated load 51
Energy Efficiency Opportunities 12. Proper Boiler Scheduling Thermal Equipment/ Boilers
•
Optimum efficiency: 65-85% of full load
•
Few boilers at high loads is more efficient than large number at low loads
13. Boiler Replacement 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
52
Training Session on Energy Equipment Thermal Equipment/ Boilers
Boilers & Thermic Fluid Heaters THANK YOU FOR YOUR ATTENTION
53
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