COMBUSTION OPTIMISATION ASPECTS
THE EFFECT OF COMBUSTION ON THE EFFICIENCY OF THE HEATING APPLIANCE
INTRODUCTION
The combustion efficiency is affected by the
manner in which the combustion occurs
That is, the
air:fuel ratio air:fuel ratio
degree of atomising (liquid fuels)
fuel-air mixing
flame temperature
flame shape
fuel residence time in the combustion zone
And the amount of heat lost out of the system
AIR:FUEL RATIO
The theoretical air:fuel ratio for complete combustion is known as the STOICHIOMETRIC ratio
In practice this ratio does not achieve complete combustion as the degree of mixing is never sufficient to allow every oxygen molecule to come into contact with a fuel moleculemolecule to come into contact with a fuel molecule
Thus a certain amount of excess oxygen (air) is required to achieve full combustion
The range of excess oxygen required to achieve complete combustion in practical applications is between 1% and 5% depending on the combustion appliance
This implies that an excess air requirement of 5% - 25% is necessary, as there is only ~21% oxygen in air.
AIR:FUEL RATIO
FLUE GAS ANALYSIS
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13
14
15
16
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PERCENT OF FLUE GAS BY VOLUME
CARBON DIOXIDE
CARBON
MONOXIDE
0
1
2
3
4
5
6
7
8
9
10
11
-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70
% EXCESS AIR
PERCENT OF FLUE GAS BY VOLUME
OXYGEN
AIR:FUEL RATIO
An amount of excess air is necessary for complete combustion
Too much excess air is undesirable as it reduces efficiency by absorbing and
Too much excess air is undesirable as it reduces efficiency by absorbing and carrying away heat
Typically the energy loss due to excess air is in the order of 1,2% for every 10% of excess air by volume
Heating oil in liquid form must be turned into vapor and mixed with air before it can burn. When the oil from the storage tank reaches the burner's nozzle, it's broken into small droplets. This process is called atomizing.These droplets are mixed with air and then ignited by the burner.
The efficiency of the oil-air mix achievedby a burner depends on its design. The biggest difference between old burners and modern ones is the air handling step of the process.
Oil Burner
Atomization exposes more surface area per unit mass of fuel oil.
Twin-fluid burners; using high pressure steam (or air) to break oil drops into fine droplets.
Steam (or air) atomized oil burner
Combustion of Gas
Combustion of gas is easy and clean.
No atomization required.
1 m3 of natural gas requires roughly 20 m3 of air.air.
ATOMISING
Applies to liquid fuels only
Is required to generate an even spray of droplets
sufficiently small to allow good mixing with the
oxygen to achieve complete combustion (usually oxygen to achieve complete combustion (usually
What the Nozzle Does
Atomizing speeds up the vaporization process
One litre of oil becomes 15 billion droplets at 7kg/cm2 with size 0.0002 inch 0.010 inch7kg/cm2 with size 0.0002 inch 0.010 inch
Metering deliver a fixed amount of atomized fuel to the combustion chamber
Patterning uniform spray
pattern and spray angle
ATOMISING
Primary causes of poor atomisation are:
Worn nozzles
Insufficient fuel-oil pressure
Excessive fuel-oil viscosity
Insufficient atomising air or steam pressure
Incorrect nozzle size excessive turndown
Poor nozzle design
Excessive fuel viscosity (>20 cSt)
Spray at 10 psi pressureSpray at 100-psi pressure
Spray at 10 psi pressureSpray at 100-psi pressure
Spray at 300-psi pressure
FUEL:AIR MIXING
The effectiveness of the burner in achieveing adequate mixing of the fuel and air is crucial to efficient combustion
The burner must provide a stable spray The burner must provide a stable spray of atomised fuel particles expanding into the combustion air in a manner that will sustain good combustion
The quarl helps sustain the shape of the flame necessary for good combustion
FUEL:AIR MIXING
Causes of poor mixing:
Imbalanced air:fuel pressures
Incorrectly set up burners Incorrectly set up burners
Worn burner parts
Misaligned burners
Damaged or badly made burner tile (quarl)
Dirty or blocked swirl plates
STACK LOSSES
The heat load in the combustion gases is a loss of useful energy
Therefore the stack temperature should be kept as low as possiblekept as low as possible
The volume of gas should be minimised (excess air)
Stack temperature in a boiler application goes up when the heat transfer surfaces become dirty
Pulverized Coal Firing System
First commercial application in 1920.
become almost universal in central utility stations using coal as fuel.utility stations using coal as fuel.
First ground to dustlike size.
Then, powdered coal is carried by air to the burners.
Pulverized Coal Burner
Conditions for Pulverized Firing
Large quantities of very fine particle of coal.
Pass 200 mesh (0.074 mm opening) sieve
Small size => large surface-to-volume ratio Small size => large surface-to-volume ratio
Minimum quantity of coarser particles.
Higher surface area per unit mass of coal allows faster combustion reactions.
More carbon exposed to heat and oxygen.
Reduce excess air needed to complete combustion.
Advantages of Pulverized Coal Firing
Low excess air requirement Less fan power Ability to use highly preheated air reducing exhaust losses
Higher boiler efficiency Higher boiler efficiency Ability to burn a wide variety of coals Fast response to load changes Ease of burning alternately with, or in combination with gas and oil
Capacity up to 2,000 t/h steam Less pressure losses and draught need.
MEASUREMENT
It is virtually impossible to set a burners air:fuel ratio by eye to ensure complete combustion (minimum CO) and minimum excess air.air.
The only reliable way is to measure the Oxygen (O2) and Carbon Monoxide (CO< 10 ppm) content in the stack
The burner should be set for minimum O2 in the stack gas without producing more than 10ppm of CO over a range of turn-down
CONTROLS
The only effective way is to install combustion analysers and control the fuel:air mixture automaticallyfuel:air mixture automatically
There is a range of such instruments and systems on the market
EE Issues in Boilers EE Issues in Boilers
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What is a Boiler?
Vessel that heats water to become hot water or steam
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hot water or steam
At atmospheric pressure water volume increases 1,600 times
Hot water or steam used to transfer heat to a process
IntroductionIntroduction
VENTVENTEXHAUST GASEXHAUST GASSTEAM TO STEAM TO PROCESSPROCESS
STACKSTACK DEAERATORDEAERATOR
PUMPSPUMPS
ECOECO--NOMINOMI--
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BURNERBURNERWATER WATER
SOURCESOURCE
BRINEBRINE
SOFTENERSSOFTENERSCHEMICAL FEEDCHEMICAL FEED
FUELFUELBLOW DOWN BLOW DOWN
SEPARATORSEPARATOR
VENTVENT
Figure: Schematic overview of a boiler room
BOILERBOILER
NOMINOMI--ZERZER
Introduction
Type of boilers
Assessment of a boiler
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Assessment of a boiler
Energy efficiency opportunities
Types of BoilersTypes of Boilers
1. Fire Tube Boiler
2. Water Tube Boiler
What Type of Boilers Are There?
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2. Water Tube Boiler
3. Packaged Boiler
4. Fluidized Bed (FBC) Boiler
5. Stoker Fired Boiler
6. Pulverized Fuel Boiler
7. Waste Heat Boiler
Fluidised Bed Combustion
Assessment of a BoilerAssessment of a Boiler
1. Boiler performance
Causes of poor boiler performance-Poor combustion
-Heat transfer surface fouling
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-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
Assessment of a BoilerAssessment of a Boiler
Heat Balance
An energy flow diagram describes graphically how energy is transformed from fuel into useful energy, heat and losses
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StochiometricExcess AirUn burnt
FUEL INPUT STEAM OUTPUT
Stack Gas
Ash and Un-burnt parts of Fuel in Ash
Blow Down
Convection & Radiation
Assessment of a BoilerAssessment of a Boiler
Heat Balance
Balancing total energy entering a boiler against the energy that leaves the boiler in different forms
Heat loss due to dry flue gas 12.7 %
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 %
Assessment of a BoilerAssessment of a Boiler
Heat Balance
Goal: improve energy efficiency by reducing avoidable lossesAvoidable losses include:
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Avoidable losses include:
- Stack gas losses (excess air, stack gas temperature)
- Losses by unburnt fuel
- Blow down losses
- Condensate losses
- Convection and radiation
Assessment of a BoilerAssessment 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 BoilerAssessment of a Boiler
Boiler Efficiency: Direct Method
Boiler efficiency () = hf/hg
Heat Input * 100%
Heat Output
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Assessment of a BoilerAssessment of a Boiler
Controls total dissolved solids (TDS) in the water that is boiled
Blows off water and replaces it with feed water
2. Boiler Blow Down
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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
Assessment of a BoilerAssessment of a Boiler
Quality of steam depend on water treatment to control
3. Boiler Feed Water Treatment
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Steam purity
Deposits
Corrosion
Efficient heat transfer only if boiler water is free from deposit-forming solids
1. Stack Temperature Control
Keep as low as possible
If >200C then recover waste heat
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
2. Feed Water Preheating
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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% improve thermal efficiency
4. Minimize Incomplete Combustion
Symptoms:
Smoke, high CO levels in exit flue gas
Causes:
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
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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 OpportunitiesEnergy 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
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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
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
6. Radiation and Convection Heat Loss Minimization Fixed heat loss from boiler shell, regardless of
boiler output
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7. Automatic Blow Down Control
boiler output
Repairing insulation can reduce loss
Sense and respond to boiler water conductivity and pH
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
8. Scaling and Soot Loss Reduction
Every 22oC increase in stack temperature = 1% efficiency loss
3 mm of soot = 2.5% fuel increase
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9. Reduced Boiler Steam Pressure
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
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
10. Variable Speed Control for Fans, Blowers and Pumps Suited for fans, blowers, pumps
Should be considered if boiler loads are
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11. Control Boiler Loading
Should be considered if boiler loads are variable
Maximum boiler efficiency: 65-85% of rated load
Significant efficiency loss: < 25% of rated load
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
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
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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
STEAM SYSTEM
Introduction
Why steam is popular mode of heating?
Highest specific heat and latent heat Highest specific heat and latent heat
Highest heat transfer coefficient
Easy to control and distribute
Cheap and inert
Properties of Steam374.15 C ,221.2 bar (a)
Steam tablesPressure
(kg/cm2)
Temperature oC
Enthalpy in kCal/kg Specific Volume
(m3/kg)
Water
(hf ) Evaporation (hfg) Steam (hg)
1 100 100.09 539.06 639.15 1.673
2 120 119.92 526.26 646.18 0.901
3 133 133.42 517.15 650.57 0.616
4 143 143.70 509.96 653.66 0.470
5 151 152.13 503.90 656.03 0.381
6 158 159.33 498.59 657.92 0.321
7 164 165.67 493.82 659.49 0.277
8 170 171.35 489.46 660.81 0.244
Typical Steam Distribution
1. Monitoring Steam Traps Condensate discharge
Inverted bucket and thermodynamic disc traps should have intermittent condensate discharge.
Float and thermostatic traps should have a continuous condensate discharge.
Energy Saving Opportunities
Thermostatic traps can have either continuous or intermittent discharge depending upon the load.
If inverted bucket traps are used for extremely small load, it will have a continuous condensate discharge
Flash steam Users get confused between a flash steam and leaking steam.
Flash steam and the leaking steam can be approx.ly identified as follows
If steam blows out continuously in a blue stream, it is a leaking steam.
If a steam floats out intermittently in a whitish cloud, it is a flash steam
2. Continuous steam blow and no flow indicate, there is a problem in the trap
Whenever a trap fails to operate and the reasons are not readily apparent, the discharge from the trap should be observed.
A step-by-step analysis has to be carried out A step-by-step analysis has to be carried out mainly with reference to lack of discharge from the trap, steam loss, continuous flow, sluggish heating, to find out whether it is a system problem or the mechanical problem in the steam trap
3. Avoiding Steam Leakages
ExampleExamplePlume Length = 700 mmSteam loss = 10 kg/h
4. Providing Dry Steam for Process
The best steam for industrial process heating is the dry saturated steam.
Wet steam reduces total heat in the steam. Also water forms a wet film on heat transfer Also water forms a wet film on heat transfer and overloads traps and condensate equipment.
Super heated steam is not desirable for process heating because it gives up heat at a rate slower than the condensation heat transfer of saturated steam
5. Utilising Steam at the Lowest Acceptable Pressure for the Process
the latent heat in steam reduces as the steam pressure increases
but lower the steam pressure, the lower but lower the steam pressure, the lower will be its temperature
Therefore, there is a limit to the reduction of steam pressure
7. Minimising Heat Transfer Barriers
8. Proper Air Venting
9. Condensate Recovery
For every 60C rise in the feed water temperature, there will be approximately 1% saving of fuel in the boilerboiler
Financial reasons
Water charges
Effluent restrictions
Maximising boiler output
Boiler feedwater quality
12. Reducing the Work to be done by Steam
Reduction in operating hours Reduction in steam quantity required per hour Use of more efficient technology Minimizing wastage.