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Imbaba Aviation Institute Mechanical Power Department, 4th YEAR Fall Semester 2010/2011
- Introduction to Combustion systems - Definition of combustion efficiency and factors affecting it. - Methods of energy conservation in combustion systems. - Control systems in combustion. - Waste heat recovery. - Performance control of various systems.
1
Energy Management
What Is Energy Management? The use of Engineering and Economic principles to CONTROL the cost of energy to provide needed services in buildings and industries.
2
Energy Management NEED FOR ENERGY MANAGEMENT IMPORTANT REASONS: 1. ENVIRONMENTAL QUALITY 2. ECONOMIC COMPETITIVENESS
3. REDUCE COSTS AND CREATE JOBS 4. ENERGY SECURITY 5. CORPORATE REQUIREMENT
3
DEFINITIONS
ENERGY: the capacity of doing work Thermal, Electromagnetic, Nuclear, Mechanical, Chemical, etc. ENERGY CONSERVATION LAW
Energy is transformed from one form to another and the total amount of energy
remains the same. 4
DEFINITIONS • EFFICIENCY – is the ratio of the output of a system in relation with its input.
• MOTORS – a device that converts electrical energy into mechanical energy. • GENERATOR – converts mechanical energy into electrical energy. • TRANSFORMER - Is a device that converts AC electric energy at one voltage level to an AC electric energy at another voltage level. They are classified as “step-up” or “step-down” transformers depending of the function they are being used for.
5
DEFINITIONS • POWER FACTOR : is the ratio of the total power produced between the power used. • PF = COS θ
KVA KVAR θ
KW 6
DEFINITIONS
• COGENERATION – is the sequential production of thermal and electric energy from a single fuel source. Heat, that would normally be lost, is recovered in the production of one form of energy. The heat is then used to generate the second form of energy.
7
HOW& WHY ENERGY CONSERVATION
• HOW? – Energy Audits
– Fuel Switching – Electric Rate Structures – Electrical System Utilization • PF Correction – Lighting Improvements – Motors And Applications – Insulation – HVAC Improvements – Waste Heat Recovery; Cogeneration, ETC. 8
ENERGY AUDITS
ENERGY AUDITS • An Energy Audit (or Energy Survey) is a study of how energy is used in a facility and an analysis of what alternatives could be used to reduce energy costs. • This process starts by collecting information of the facility’s operation and about its past record of utility bills. This data is then analyzed to get a picture of how the facility uses ( and possibly wastes) energy, and identify • ECO’s (Energy Conservation Opportunities).
9
COMBUSTION EFFICIENCY • In any closed combustion system such as a boiler, we can measure precisely what occurred at the burner by carefully measuring the exhaust. • The goal is to be able to carefully control the fuel and airflow to ensure the complete and efficient combustion. • We will see why excess air is important and why too much excess air is expensive. 10
SAVINGS
% SAVINGS IN FUEL = (New Eff. – Old Eff.)/New Eff. Savings = (% Savings)(Fuel consumption)
11
SAVINGS Example 1 • Last year a 20 x 106 BTU/HR boiler consumed 19000 MCF of natural gas at $4.00/MCF. The
boiler operates at 6% O2 and 700 ºF STR. What is the saving to correcting that to 3% O2 ?
12
SAVINGS As can be seen later
Eff 1. = 74.5% Eff. 2 = 77% % Savings = (77 – 74.5)/77 = 3.2 % $ Savings = (3.2%) [ 19,000 MCF][$ 4.00] = $ 2,500 / YR
13
Introduction to Furnaces
Introduction Type of furnaces and refractory materials Assessment of furnaces Energy efficiency opportunities
14
MAIN COMPONENTS OF COMBUSTION SYSTEM
• There are six components that may be important in industrial combustion processes load itself, a combustor, heat recovery device flow control system air pollution control system.
Schematic of an industrial combustion process. 15
16
FURNACES
17
COMBUSTION PRINCIPLES
Combustion chemistry In practice, since combustion conditions are never ideal. The actual quantity depends on many factors, such as fuel type and composition, furnace design, firing rate, and the design and adjustment of the burners stoichiometric requirement industrial processes.
18
The “other species” depends on what oxidizer is used and what is the ratio of the fuel to oxidizer is air nearly 79% N2 by volume. If the combustion is fuel rich, If the combustion is fuel lean.
Figure 18. Stoichiometric Air Requirements for Combustion 19
Unburned hydrocarbons Fuel was not fully combusted Fuel properties: Heating value of the fuel either the higher heating value (HHV) lower heating value (LHV) excludes the heat of vaporization.
The stoichiometry or mixture ratio in industry is as follows:
The mixture ratio :
20
Where SP is the stoichiometric ratio for theoretically perfect combustion. fuel-rich combustion of CH4, S2 < 9.52. For the fuellean combustion of CH4, S2 > 9.52. Using the above definition for the mixture ratio, 1.0 <λ for fuel-rich flames and 1.0 > λ for fuel-lean flames.
21
Combustion properties
Combustion products The oxidizer composition, mixture ratio, air and fuel preheat temperatures, and fuel composition.
22
An adiabatic process means that no heat is lost during the reaction, or that the reaction occurs in a perfectly insulated chamber.
An equilibrium process means that there is an infinite amount of time for the chemical reactions to take place.
23
FLAME TEMPERATURE
• The actual flame temperature is lower than the adiabatic equilibrium flame temperature due to imperfect combustion and radiation from the flame. • A highly luminous flame generally has a lower flame temperature than a highly non-luminous flame. The actual flame temperature will also be lower when the load and the walls are more radiatively absorptive. • The flame temperature is a critical variable in determining the heat transfer from the flame to the load.
24
Oxidizer and fuel composition
The flame temperature increases significantly when air is replaced with oxygen
25
26
Nearly all industrial combustion applications are run at fuel-lean conditions to ensure that the CO emissions are low. NOx emissions are also maximized since NOx increases approximately exponentially with gas temperature.
27
STACK GAS COMPOSITION
1.
Point A: 2C + O22
2.
→
CO + heat
Point B: 2CO + O22 → CO2 + heat
3.
Point C CO to have reached a low level. A small amount of oxygen
4.
To achieve .complete. combustion, a small amount of air must be added over. Point D. At this point, the CO 2 level reaches a peak (typically around 15- 16 percent for oil fuels, and 11-12 percent for natural gas).
5.
Point E, oxygen level builds towards 20.9 percent. 28
BURNER TESTING: Operating parameters, pollutant emissions, flame dimensions, heat flux profile, safety limitations, and noise data heat release range of the burner. Turndown is defined as the ratio of maximum heat release to minimum heat release:
29
• An operator also needs to know the point at which a burner will become unstable if fired below the minimum heat release absolute minimum the combustion air settings can be determined through testing to ensure the efficient operation. •
The emissions of pollutants such as NOx ,CO, and unburned hydrocarbons (UHC). When firing burners on a wide variety of fuels, flame dimensions can change, depending on the fuel fired.
• Another valuable piece of data that can be collected is noise. • API 535 gives some good guidelines for specifications and data required for burners used in fired
30
Introduction to Furnaces
What are Refractories: Materials that • Withstand high temperatures and sudden changes • Withstand action of molten slag, glass, hot gases etc • Withstand load at service conditions • Withstand abrasive forces • Conserve heat • Have low coefficient of thermal expansion • Will not contaminate the load 31
Introduction to Furnaces
Refractories Refractory lining of a furnace arc
Refractory walls of a furnace interior with burner blocks (BEE India, 2005) 32
Introduction to Furnaces
Properties of Refractories • Melting point • Temperature at which a ‘test pyramid’ (cone) fails to its own weight
• Size • Affects stability of furnace structure
• Bulk density • Amount of refractory material within a volume (kg/m3) • High bulk density = high volume stability, heat capacity and resistance 33
Introduction to Furnaces
Properties of Refractories • Porosity • Volume of open pores as % of total refractory volume • Low porosity = less penetration of molten material
• Cold crushing strength • Resistance of refractory to crushing
• Creep at high temperature • Deformation of refractory material under stress at given time and temperature 34
Introduction to Furnaces
Properties of Refractories • Pyrometric cones • Used in ceramic industries to test ‘refractoriness’ of refractory bricks • Each cone is mix of oxides that melt at specific temperatures (BEE India, 2004)
• Pyrometric Cone Equivalent (PCE) • Temperature at which the refractory brick and the cone bend • Refractory cannot be used above this temp 35
Introduction to Furnaces
Properties of Refractories • Volume stability, expansion & shrinkage • Permanent changes during refractory service life • Occurs at high temperatures
• Reversible thermal expansion • Phase transformations during heating and cooling
36
Introduction to Furnaces
Properties of Refractories • Thermal conductivity • Depends on composition and silica content • Increases with rising temperature
• High thermal conductivity: • Heat transfer through brickwork required • E.g. recuperators, regenerators
• Low thermal conductivity: • Heat conservation required (insulating refractories) • E.g. heat treatment furnaces 37
Type of Furnaces and Refractories
Classification of Refractories Classification method
Examples
Chemical composition ACID, which readily combines with bases
Silica, Semisilica, Aluminosilicate
BASIC, which consists mainly of metallic oxides that resist the action of bases
Magnesite, Chrome-magnesite, Magnesitechromite, Dolomite
NEUTRAL, which does not combine with acids nor bases
Fireclay bricks, Chrome, Pure Alumina
Special
Carbon, Silicon Carbide, Zirconia
End use
Blast furnace casting pit
Method of manufacture
Dry press process, fused cast, hand moulded, formed normal, fired or chemically bonded, unformed (monolithics, plastics, ramming mass, gunning castable, spraying) 38
Type of Furnaces and Refractories
Fireclay Refractories • Common in industry: materials available and inexpensive • Consist of aluminium silicates • Decreasing melting point (PCE) with increasing impurity and decreasing AL2O3
High Alumina Refractories • 45 - 100% alumina • High alumina % = high refractoriness
• Applications: hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks 39
Type of Furnaces and Refractories
Silica Brick • >93% SiO2 made from quality rocks • Iron & steel, glass industry
• Advantages: no softening until fusion point is reached; high refractoriness; high resistance to spalling, flux and slag, volume stability
Magnesite • Chemically basic: >85% magnesium oxide • Properties depend on silicate bond concentration • High slag resistance, especially lime and iron 40
Type of Furnaces and Refractories
Chromite Refractories • Chrome-magnesite • • • •
15-35% Cr2O3 and 42-50% MgO Used for critical parts of high temp furnaces Withstand corrosive slags High refractories
• Magnesite-chromite • • • •
>60% MgO and 8-18% Cr2O3 High temp resistance Basic slags in steel melting Better spalling resistance 41
Type of Furnaces and Refractories
Zirconia Refractories • Zirconium dioxide ZrO2 • Stabilized with calcium, magnesium, etc.
• High strength, low thermal conductivity, not reactive, low thermal loss • Used in glass furnaces, insulating refractory
Oxide Refractories (Alumina) • Aluminium oxide + alumina impurities • Chemically stable, strong, insoluble, high resistance in oxidizing and reducing atmosphere • Used in heat processing industry, crucible shaping
42
Type of Furnaces and Refractories
Monolithics • Single piece casts in equipment shape • Replacing conventional refractories • Advantages • • • • • • •
Elimination of ts Faster application Heat savings Better spalling resistance Volume stability Easy to transport, handle, install Reduced downtime for repairs 43
Type of Furnaces and Refractories
Insulating Materials Classification • Material with low heat conductivity: keeps furnace surface temperature low • Classification into five groups • • • • •
Insulating bricks Insulating castables and concrete Ceramic fiber Calcium silicate Ceramic coatings (high emissivity coatings) 44
Type of Furnaces and Refractories
Castables and Concretes • Consist of • Insulation materials used for making piece refractories • Concretes contain Portland or high-alumina cement
• Application • Monolithic linings of furnace sections • Bases of tunnel kiln cars in ceramics industry
45
Type of Furnaces and Refractories
Ceramic Fibers • Thermal mass insulation materials • Manufactured by blending alumina and silica • Bulk wool to make insulation products • Blankets, strips, paper, ropes, wet felt etc
• Produced in two temperature grades 46
Type of Furnaces and Refractories
Ceramic Fibers Remarkable properties and benefits • • • • • • • • • •
Low thermal conductivity Light weight Lower heat storage Thermal shock resistant Chemical resistance Mechanical resilience Low installation costs Ease of maintenance Ease of handling Thermal efficiency
• Lightweight furnace • Simple steel fabrication work • Low down time • Increased productivity • Additional capacity • Low maintenance costs • Longer service life • High thermal efficiency • Faster response 47
Type of Furnaces and Refractories
High Emissivity Coatings • Emissivity: ability to absorb and radiate heat • Coatings applied to interior furnace surface: • • • •
emissivity stays constant Increase emissivity from 0.3 to 0.8 Uniform heating and extended refractory life Fuel reduction by up to 25-45%
48
Type of Furnaces and Refractories
High Emissivity Coatings
49
Assessment of Furnaces
Introduction Type of furnaces and refractory materials
Assessment of furnaces Energy efficiency opportunities
50
Assessment of Furnaces Heat Losses Affecting Furnace Performance
Heat input
FURNACE Heat in stock
Other losses
Furnace surface/skin
Openings in furnace
Hydrogen in fuel
Moisture in fuel
Flue gas
51
Assessment of Furnaces Instruments to Assess Furnace Performance Parameters to be measured
Location of measurement
Instrument required
Required Value
Furnace soaking zone temperature (reheating furnaces)
Soaking zone and side wall
Pt/Pt-Rh thermocouple with indicator and recorder
1200-1300oC
Flue gas temperature
In duct near the discharge end, and entry to recuperator
Chromel Alummel Thermocouple with indicator
700oC max.
Flue gas temperature
After recuperator
Hg in steel thermometer
300oC (max)
Furnace hearth pressure in the heating zone
Near charging end and side wall over the hearth
Low pressure ring gauge
+0.1 mm of Wc
Oxygen in flue gas
In duct near the discharge end
Fuel efficiency monitor for oxygen and temperature
5% O2
Billet temperature
Portable
Infrared pyrometer or optical pyrometer
-
52
Assessment of Furnaces Calculating Furnace Performance Direct Method • Thermal efficiency of furnace = Heat in the stock / Heat in fuel consumed for heating the stock • Heat in the stock Q: Q = m x (t1 – t2) Q = Quantity of heat of stock in kCal m = Weight of the stock in kg = Mean specific heat of stock in kCal/kg oC t1 = Final temperature of stock in oC t2 = Initial temperature of the stock before it enters the furnace in oC 53
Assessment of Furnaces
Calculating Furnace Performance Direct Method - example • Heat in the stock Q = • m x (t1 – t2) • 6000 kg X 0.12 X (1340 – 40) • 936000 kCal • Efficiency = • (heat input / heat output) x 100 • [936000 / (368 x 10000) x 100 = 25.43%
m = Weight of the stock = 6000 kg = Mean specific heat of stock = 0.12 kCal/kg oC t1 = Final temperature of stock = 1340 oC t2 = Initial temperature of the stock = 40 oC Calorific value of oil = 10000 kCal/kg Fuel consumption = 368 kg/hr
• Heat loss = 100% - 25% = 75% 54
Assessment of Furnaces
Calculating Furnace Performance Indirect Method Heat losses a) Flue gas loss
= 57.29 %
b) Loss due to moisture in fuel
= 1.36 %
c) Loss due to H2 in fuel
= 9.13 %
d) Loss due to openings in furnace
= 5.56 %
e) Loss through furnace skin
= 2.64 %
Total losses
= 75.98 %
Furnace efficiency = • Heat supply minus total heat loss •
100% – 76% = 24% 55
Assessment of Furnaces
Calculating Furnace Performance Typical efficiencies for industrial furnaces Furnace type
Thermal efficiencies (%)
1) Low Temperature furnaces a. 540 – 980 oC (Batch type)
20-30
b. 540 – 980 oC (Continous type)
15-25
c. Coil Anneal (Bell) radiant type
5-7
d. Strip Anneal Muffle
7-12
2) High temperature furnaces a. Pusher, Rotary
7-15
b. Batch forge
5-10
3) Continuous Kiln a. Hoffman
25-90
b. Tunnel
20-80
4) Ovens a. Indirect fired ovens (20 oC –370 oC)
35-40
b. Direct fired ovens (20 oC –370 oC)
35-40
56
Energy Efficiency Opportunities Introduction
Type of furnaces and refractory materials Assessment of furnaces
Energy efficiency opportunities
57
Energy Efficiency Opportunities 1. Complete combustion with minimum excess air 2. Proper heat distribution 3. Operation at the optimum furnace temperature 4. Reducing heat losses from furnace openings
5. Maintaining correct amount of furnace draft 6. Optimum capacity utilization 7. Waste heat recovery from the flue gases
8. Minimize furnace skin losses 9. Use of ceramic coatings 10.Selecting the right refractories
58
Energy Efficiency Opportunities
1. Complete Combustion with Minimum Excess Air • Importance of excess air • Too much: reduced flame temp, furnace temp, heating rate • Too little: unburnt in flue gases, scale losses
• Indication of excess air: actual air / theoretical combustion air • Optimizing excess air • • • •
Control air infiltration Maintain pressure of combustion air Ensure high fuel quality Monitor excess air
59
Energy Efficiency Opportunities
2. Proper Heat Distribution When using burners • Flame should not touch or be obstructed
• No intersecting flames from different burners • Burner in small furnace should face upwards but not hit roof
• More burners with less capacity (not one big burner) in large furnaces • Burner with long flame to improve uniform heating in small furnace 60
Energy Efficiency Opportunities
3. Operate at Optimum Furnace Temperature • Operating at too high temperature: heat loss, oxidation, decarbonization, refractory stress • Automatic controls eliminate human error Slab Reheating furnaces
1200oC
Rolling Mill furnaces
1200oC
Bar furnace for Sheet Mill
800oC
Bogie type annealing furnaces
650oC –750oC 61
Energy Efficiency Opportunities
4. Reduce Heat Loss from Furnace Openings • Heat loss through openings • Direct radiation through openings • Combustion gases leaking through the openings • Biggest loss: air infiltration into the furnace
• Energy saving measures • Keep opening small • Seal openings • Open furnace doors less frequent and shorter 62
Energy Efficiency Opportunities
5. Correct Amount of Furnace Draft • Negative pressure in furnace: air infiltration
• Maintain slight positive pressure • Not too high pressure difference: air ex-filtration
Heat loss only about 1% if furnace pressure is controlled properly! 63
Energy Efficiency Opportunities
6. Optimum Capacity Utilization • Optimum load • Underloading: lower efficiency • Overloading: load not heated to right temp
• Optimum load arrangement • Load receives maximum radiation • Hot gases are efficiently circulated • Stock not placed in burner path, blocking flue system, close to openings
• Optimum residence time • Coordination between personnel
• Planning at design and installation stage
64
Energy Efficiency Opportunities
7. Waste Heat Recovery from Flue Gases • Charge/Load pre-heating • Reduced fuel needed to heat them in furnace
• Pre-heating of combustion air • Applied to compact industrial furnaces • Equipment used: recuperator, selfrecuperative burner • Up to 30% energy savings
• Heat source for other processes • Install waste heat boiler to produce steam • Heating in other equipment (with care!) 65
Energy Efficiency Opportunities
8. Minimum Furnace Skin Loss • Choosing appropriate refractories • Increasing wall thickness • Installing insulation bricks (= lower conductivity) • Planning furnace operating times • 24 hrs in 3 days: 100% heat in refractories lost • 8 hrs/day for 3 days: 55% heat lost 66
Energy Efficiency Opportunities
9. Use of Ceramic Coatings • High emissivity coatings • Long life at temp up to 1350 oC • Most important benefits • Rapid efficient heat transfer • Uniform heating and extended refractory life • Emissivity stays constant
• Energy savings: 8 – 20% 67
Energy Efficiency Opportunities
10. Selecting the Right Refractory Selection criteria • Type of furnace
• Type of metal charge • Presence of slag • Area of application • Working temperatures • Extent of abrasion and impact
• Structural load of furnace
• Stress due to temp gradient & fluctuations • Chemical compatibility • Heat transfer & fuel conservation • Costs 68
- Introduction to Combustion systems - Definition of combustion efficiency and factors affecting it. - Methods of energy conservation in combustion systems. - Control systems in combustion. - Waste heat recovery. - Performance control of various systems.
1
Energy Management
What Is Energy Management? The use of Engineering and Economic principles to CONTROL the cost of energy to provide needed services in buildings and industries.
2
Energy Management NEED FOR ENERGY MANAGEMENT IMPORTANT REASONS: 1. ENVIRONMENTAL QUALITY 2. ECONOMIC COMPETITIVENESS
3. REDUCE COSTS AND CREATE JOBS 4. ENERGY SECURITY 5. CORPORATE REQUIREMENT
3
DEFINITIONS
ENERGY: the capacity of doing work Thermal, Electromagnetic, Nuclear, Mechanical, Chemical, etc. ENERGY CONSERVATION LAW
Energy is transformed from one form to another and the total amount of energy
remains the same. 4
DEFINITIONS • EFFICIENCY – is the ratio of the output of a system in relation with its input.
• MOTORS – a device that converts electrical energy into mechanical energy. • GENERATOR – converts mechanical energy into electrical energy. • TRANSFORMER - Is a device that converts AC electric energy at one voltage level to an AC electric energy at another voltage level. They are classified as “step-up” or “step-down” transformers depending of the function they are being used for.
5
DEFINITIONS • POWER FACTOR : is the ratio of the total power produced between the power used. • PF = COS θ
KVA KVAR θ
KW 6
DEFINITIONS
• COGENERATION – is the sequential production of thermal and electric energy from a single fuel source. Heat, that would normally be lost, is recovered in the production of one form of energy. The heat is then used to generate the second form of energy.
7
HOW& WHY ENERGY CONSERVATION
• HOW? – Energy Audits
– Fuel Switching – Electric Rate Structures – Electrical System Utilization • PF Correction – Lighting Improvements – Motors And Applications – Insulation – HVAC Improvements – Waste Heat Recovery; Cogeneration, ETC. 8
ENERGY AUDITS
ENERGY AUDITS • An Energy Audit (or Energy Survey) is a study of how energy is used in a facility and an analysis of what alternatives could be used to reduce energy costs. • This process starts by collecting information of the facility’s operation and about its past record of utility bills. This data is then analyzed to get a picture of how the facility uses ( and possibly wastes) energy, and identify • ECO’s (Energy Conservation Opportunities).
9
COMBUSTION EFFICIENCY • In any closed combustion system such as a boiler, we can measure precisely what occurred at the burner by carefully measuring the exhaust. • The goal is to be able to carefully control the fuel and airflow to ensure the complete and efficient combustion. • We will see why excess air is important and why too much excess air is expensive. 10
SAVINGS
% SAVINGS IN FUEL = (New Eff. – Old Eff.)/New Eff. Savings = (% Savings)(Fuel consumption)
11
SAVINGS Example 1 • Last year a 20 x 106 BTU/HR boiler consumed 19000 MCF of natural gas at $4.00/MCF. The
boiler operates at 6% O2 and 700 ºF STR. What is the saving to correcting that to 3% O2 ?
12
SAVINGS As can be seen later
Eff 1. = 74.5% Eff. 2 = 77% % Savings = (77 – 74.5)/77 = 3.2 % $ Savings = (3.2%) [ 19,000 MCF][$ 4.00] = $ 2,500 / YR
13
Introduction to Furnaces
Introduction Type of furnaces and refractory materials Assessment of furnaces Energy efficiency opportunities
14
MAIN COMPONENTS OF COMBUSTION SYSTEM
• There are six components that may be important in industrial combustion processes load itself, a combustor, heat recovery device flow control system air pollution control system.
Schematic of an industrial combustion process. 15
16
FURNACES
17
COMBUSTION PRINCIPLES
Combustion chemistry In practice, since combustion conditions are never ideal. The actual quantity depends on many factors, such as fuel type and composition, furnace design, firing rate, and the design and adjustment of the burners stoichiometric requirement industrial processes.
18
The “other species” depends on what oxidizer is used and what is the ratio of the fuel to oxidizer is air nearly 79% N2 by volume. If the combustion is fuel rich, If the combustion is fuel lean.
Figure 18. Stoichiometric Air Requirements for Combustion 19
Unburned hydrocarbons Fuel was not fully combusted Fuel properties: Heating value of the fuel either the higher heating value (HHV) lower heating value (LHV) excludes the heat of vaporization.
The stoichiometry or mixture ratio in industry is as follows:
The mixture ratio :
20
Where SP is the stoichiometric ratio for theoretically perfect combustion. fuel-rich combustion of CH4, S2 < 9.52. For the fuellean combustion of CH4, S2 > 9.52. Using the above definition for the mixture ratio, 1.0 <λ for fuel-rich flames and 1.0 > λ for fuel-lean flames.
21
Combustion properties
Combustion products The oxidizer composition, mixture ratio, air and fuel preheat temperatures, and fuel composition.
22
An adiabatic process means that no heat is lost during the reaction, or that the reaction occurs in a perfectly insulated chamber.
An equilibrium process means that there is an infinite amount of time for the chemical reactions to take place.
23
FLAME TEMPERATURE
• The actual flame temperature is lower than the adiabatic equilibrium flame temperature due to imperfect combustion and radiation from the flame. • A highly luminous flame generally has a lower flame temperature than a highly non-luminous flame. The actual flame temperature will also be lower when the load and the walls are more radiatively absorptive. • The flame temperature is a critical variable in determining the heat transfer from the flame to the load.
24
Oxidizer and fuel composition
The flame temperature increases significantly when air is replaced with oxygen
25
26
Nearly all industrial combustion applications are run at fuel-lean conditions to ensure that the CO emissions are low. NOx emissions are also maximized since NOx increases approximately exponentially with gas temperature.
27
STACK GAS COMPOSITION
1.
Point A: 2C + O22
2.
→
CO + heat
Point B: 2CO + O22 → CO2 + heat
3.
Point C CO to have reached a low level. A small amount of oxygen
4.
To achieve .complete. combustion, a small amount of air must be added over. Point D. At this point, the CO 2 level reaches a peak (typically around 15- 16 percent for oil fuels, and 11-12 percent for natural gas).
5.
Point E, oxygen level builds towards 20.9 percent. 28
BURNER TESTING: Operating parameters, pollutant emissions, flame dimensions, heat flux profile, safety limitations, and noise data heat release range of the burner. Turndown is defined as the ratio of maximum heat release to minimum heat release:
29
• An operator also needs to know the point at which a burner will become unstable if fired below the minimum heat release absolute minimum the combustion air settings can be determined through testing to ensure the efficient operation. •
The emissions of pollutants such as NOx ,CO, and unburned hydrocarbons (UHC). When firing burners on a wide variety of fuels, flame dimensions can change, depending on the fuel fired.
• Another valuable piece of data that can be collected is noise. • API 535 gives some good guidelines for specifications and data required for burners used in fired
30
Introduction to Furnaces
What are Refractories: Materials that • Withstand high temperatures and sudden changes • Withstand action of molten slag, glass, hot gases etc • Withstand load at service conditions • Withstand abrasive forces • Conserve heat • Have low coefficient of thermal expansion • Will not contaminate the load 31
Introduction to Furnaces
Refractories Refractory lining of a furnace arc
Refractory walls of a furnace interior with burner blocks (BEE India, 2005) 32
Introduction to Furnaces
Properties of Refractories • Melting point • Temperature at which a ‘test pyramid’ (cone) fails to its own weight
• Size • Affects stability of furnace structure
• Bulk density • Amount of refractory material within a volume (kg/m3) • High bulk density = high volume stability, heat capacity and resistance 33
Introduction to Furnaces
Properties of Refractories • Porosity • Volume of open pores as % of total refractory volume • Low porosity = less penetration of molten material
• Cold crushing strength • Resistance of refractory to crushing
• Creep at high temperature • Deformation of refractory material under stress at given time and temperature 34
Introduction to Furnaces
Properties of Refractories • Pyrometric cones • Used in ceramic industries to test ‘refractoriness’ of refractory bricks • Each cone is mix of oxides that melt at specific temperatures (BEE India, 2004)
• Pyrometric Cone Equivalent (PCE) • Temperature at which the refractory brick and the cone bend • Refractory cannot be used above this temp 35
Introduction to Furnaces
Properties of Refractories • Volume stability, expansion & shrinkage • Permanent changes during refractory service life • Occurs at high temperatures
• Reversible thermal expansion • Phase transformations during heating and cooling
36
Introduction to Furnaces
Properties of Refractories • Thermal conductivity • Depends on composition and silica content • Increases with rising temperature
• High thermal conductivity: • Heat transfer through brickwork required • E.g. recuperators, regenerators
• Low thermal conductivity: • Heat conservation required (insulating refractories) • E.g. heat treatment furnaces 37
Type of Furnaces and Refractories
Classification of Refractories Classification method
Examples
Chemical composition ACID, which readily combines with bases
Silica, Semisilica, Aluminosilicate
BASIC, which consists mainly of metallic oxides that resist the action of bases
Magnesite, Chrome-magnesite, Magnesitechromite, Dolomite
NEUTRAL, which does not combine with acids nor bases
Fireclay bricks, Chrome, Pure Alumina
Special
Carbon, Silicon Carbide, Zirconia
End use
Blast furnace casting pit
Method of manufacture
Dry press process, fused cast, hand moulded, formed normal, fired or chemically bonded, unformed (monolithics, plastics, ramming mass, gunning castable, spraying) 38
Type of Furnaces and Refractories
Fireclay Refractories • Common in industry: materials available and inexpensive • Consist of aluminium silicates • Decreasing melting point (PCE) with increasing impurity and decreasing AL2O3
High Alumina Refractories • 45 - 100% alumina • High alumina % = high refractoriness
• Applications: hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks 39
Type of Furnaces and Refractories
Silica Brick • >93% SiO2 made from quality rocks • Iron & steel, glass industry
• Advantages: no softening until fusion point is reached; high refractoriness; high resistance to spalling, flux and slag, volume stability
Magnesite • Chemically basic: >85% magnesium oxide • Properties depend on silicate bond concentration • High slag resistance, especially lime and iron 40
Type of Furnaces and Refractories
Chromite Refractories • Chrome-magnesite • • • •
15-35% Cr2O3 and 42-50% MgO Used for critical parts of high temp furnaces Withstand corrosive slags High refractories
• Magnesite-chromite • • • •
>60% MgO and 8-18% Cr2O3 High temp resistance Basic slags in steel melting Better spalling resistance 41
Type of Furnaces and Refractories
Zirconia Refractories • Zirconium dioxide ZrO2 • Stabilized with calcium, magnesium, etc.
• High strength, low thermal conductivity, not reactive, low thermal loss • Used in glass furnaces, insulating refractory
Oxide Refractories (Alumina) • Aluminium oxide + alumina impurities • Chemically stable, strong, insoluble, high resistance in oxidizing and reducing atmosphere • Used in heat processing industry, crucible shaping
42
Type of Furnaces and Refractories
Monolithics • Single piece casts in equipment shape • Replacing conventional refractories • Advantages • • • • • • •
Elimination of ts Faster application Heat savings Better spalling resistance Volume stability Easy to transport, handle, install Reduced downtime for repairs 43
Type of Furnaces and Refractories
Insulating Materials Classification • Material with low heat conductivity: keeps furnace surface temperature low • Classification into five groups • • • • •
Insulating bricks Insulating castables and concrete Ceramic fiber Calcium silicate Ceramic coatings (high emissivity coatings) 44
Type of Furnaces and Refractories
Castables and Concretes • Consist of • Insulation materials used for making piece refractories • Concretes contain Portland or high-alumina cement
• Application • Monolithic linings of furnace sections • Bases of tunnel kiln cars in ceramics industry
45
Type of Furnaces and Refractories
Ceramic Fibers • Thermal mass insulation materials • Manufactured by blending alumina and silica • Bulk wool to make insulation products • Blankets, strips, paper, ropes, wet felt etc
• Produced in two temperature grades 46
Type of Furnaces and Refractories
Ceramic Fibers Remarkable properties and benefits • • • • • • • • • •
Low thermal conductivity Light weight Lower heat storage Thermal shock resistant Chemical resistance Mechanical resilience Low installation costs Ease of maintenance Ease of handling Thermal efficiency
• Lightweight furnace • Simple steel fabrication work • Low down time • Increased productivity • Additional capacity • Low maintenance costs • Longer service life • High thermal efficiency • Faster response 47
Type of Furnaces and Refractories
High Emissivity Coatings • Emissivity: ability to absorb and radiate heat • Coatings applied to interior furnace surface: • • • •
emissivity stays constant Increase emissivity from 0.3 to 0.8 Uniform heating and extended refractory life Fuel reduction by up to 25-45%
48
Type of Furnaces and Refractories
High Emissivity Coatings
49
Assessment of Furnaces
Introduction Type of furnaces and refractory materials
Assessment of furnaces Energy efficiency opportunities
50
Assessment of Furnaces Heat Losses Affecting Furnace Performance
Heat input
FURNACE Heat in stock
Other losses
Furnace surface/skin
Openings in furnace
Hydrogen in fuel
Moisture in fuel
Flue gas
51
Assessment of Furnaces Instruments to Assess Furnace Performance Parameters to be measured
Location of measurement
Instrument required
Required Value
Furnace soaking zone temperature (reheating furnaces)
Soaking zone and side wall
Pt/Pt-Rh thermocouple with indicator and recorder
1200-1300oC
Flue gas temperature
In duct near the discharge end, and entry to recuperator
Chromel Alummel Thermocouple with indicator
700oC max.
Flue gas temperature
After recuperator
Hg in steel thermometer
300oC (max)
Furnace hearth pressure in the heating zone
Near charging end and side wall over the hearth
Low pressure ring gauge
+0.1 mm of Wc
Oxygen in flue gas
In duct near the discharge end
Fuel efficiency monitor for oxygen and temperature
5% O2
Billet temperature
Portable
Infrared pyrometer or optical pyrometer
-
52
Assessment of Furnaces Calculating Furnace Performance Direct Method • Thermal efficiency of furnace = Heat in the stock / Heat in fuel consumed for heating the stock • Heat in the stock Q: Q = m x (t1 – t2) Q = Quantity of heat of stock in kCal m = Weight of the stock in kg = Mean specific heat of stock in kCal/kg oC t1 = Final temperature of stock in oC t2 = Initial temperature of the stock before it enters the furnace in oC 53
Assessment of Furnaces
Calculating Furnace Performance Direct Method - example • Heat in the stock Q = • m x (t1 – t2) • 6000 kg X 0.12 X (1340 – 40) • 936000 kCal • Efficiency = • (heat input / heat output) x 100 • [936000 / (368 x 10000) x 100 = 25.43%
m = Weight of the stock = 6000 kg = Mean specific heat of stock = 0.12 kCal/kg oC t1 = Final temperature of stock = 1340 oC t2 = Initial temperature of the stock = 40 oC Calorific value of oil = 10000 kCal/kg Fuel consumption = 368 kg/hr
• Heat loss = 100% - 25% = 75% 54
Assessment of Furnaces
Calculating Furnace Performance Indirect Method Heat losses a) Flue gas loss
= 57.29 %
b) Loss due to moisture in fuel
= 1.36 %
c) Loss due to H2 in fuel
= 9.13 %
d) Loss due to openings in furnace
= 5.56 %
e) Loss through furnace skin
= 2.64 %
Total losses
= 75.98 %
Furnace efficiency = • Heat supply minus total heat loss •
100% – 76% = 24% 55
Assessment of Furnaces
Calculating Furnace Performance Typical efficiencies for industrial furnaces Furnace type
Thermal efficiencies (%)
1) Low Temperature furnaces a. 540 – 980 oC (Batch type)
20-30
b. 540 – 980 oC (Continous type)
15-25
c. Coil Anneal (Bell) radiant type
5-7
d. Strip Anneal Muffle
7-12
2) High temperature furnaces a. Pusher, Rotary
7-15
b. Batch forge
5-10
3) Continuous Kiln a. Hoffman
25-90
b. Tunnel
20-80
4) Ovens a. Indirect fired ovens (20 oC –370 oC)
35-40
b. Direct fired ovens (20 oC –370 oC)
35-40
56
Energy Efficiency Opportunities Introduction
Type of furnaces and refractory materials Assessment of furnaces
Energy efficiency opportunities
57
Energy Efficiency Opportunities 1. Complete combustion with minimum excess air 2. Proper heat distribution 3. Operation at the optimum furnace temperature 4. Reducing heat losses from furnace openings
5. Maintaining correct amount of furnace draft 6. Optimum capacity utilization 7. Waste heat recovery from the flue gases
8. Minimize furnace skin losses 9. Use of ceramic coatings 10.Selecting the right refractories
58
Energy Efficiency Opportunities
1. Complete Combustion with Minimum Excess Air • Importance of excess air • Too much: reduced flame temp, furnace temp, heating rate • Too little: unburnt in flue gases, scale losses
• Indication of excess air: actual air / theoretical combustion air • Optimizing excess air • • • •
Control air infiltration Maintain pressure of combustion air Ensure high fuel quality Monitor excess air
59
Energy Efficiency Opportunities
2. Proper Heat Distribution When using burners • Flame should not touch or be obstructed
• No intersecting flames from different burners • Burner in small furnace should face upwards but not hit roof
• More burners with less capacity (not one big burner) in large furnaces • Burner with long flame to improve uniform heating in small furnace 60
Energy Efficiency Opportunities
3. Operate at Optimum Furnace Temperature • Operating at too high temperature: heat loss, oxidation, decarbonization, refractory stress • Automatic controls eliminate human error Slab Reheating furnaces
1200oC
Rolling Mill furnaces
1200oC
Bar furnace for Sheet Mill
800oC
Bogie type annealing furnaces
650oC –750oC 61
Energy Efficiency Opportunities
4. Reduce Heat Loss from Furnace Openings • Heat loss through openings • Direct radiation through openings • Combustion gases leaking through the openings • Biggest loss: air infiltration into the furnace
• Energy saving measures • Keep opening small • Seal openings • Open furnace doors less frequent and shorter 62
Energy Efficiency Opportunities
5. Correct Amount of Furnace Draft • Negative pressure in furnace: air infiltration
• Maintain slight positive pressure • Not too high pressure difference: air ex-filtration
Heat loss only about 1% if furnace pressure is controlled properly! 63
Energy Efficiency Opportunities
6. Optimum Capacity Utilization • Optimum load • Underloading: lower efficiency • Overloading: load not heated to right temp
• Optimum load arrangement • Load receives maximum radiation • Hot gases are efficiently circulated • Stock not placed in burner path, blocking flue system, close to openings
• Optimum residence time • Coordination between personnel
• Planning at design and installation stage
64
Energy Efficiency Opportunities
7. Waste Heat Recovery from Flue Gases • Charge/Load pre-heating • Reduced fuel needed to heat them in furnace
• Pre-heating of combustion air • Applied to compact industrial furnaces • Equipment used: recuperator, selfrecuperative burner • Up to 30% energy savings
• Heat source for other processes • Install waste heat boiler to produce steam • Heating in other equipment (with care!) 65
Energy Efficiency Opportunities
8. Minimum Furnace Skin Loss • Choosing appropriate refractories • Increasing wall thickness • Installing insulation bricks (= lower conductivity) • Planning furnace operating times • 24 hrs in 3 days: 100% heat in refractories lost • 8 hrs/day for 3 days: 55% heat lost 66
Energy Efficiency Opportunities
9. Use of Ceramic Coatings • High emissivity coatings • Long life at temp up to 1350 oC • Most important benefits • Rapid efficient heat transfer • Uniform heating and extended refractory life • Emissivity stays constant
• Energy savings: 8 – 20% 67
Energy Efficiency Opportunities
10. Selecting the Right Refractory Selection criteria • Type of furnace
• Type of metal charge • Presence of slag • Area of application • Working temperatures • Extent of abrasion and impact
• Structural load of furnace
• Stress due to temp gradient & fluctuations • Chemical compatibility • Heat transfer & fuel conservation • Costs 68
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