Energy Efficiency in Combustion

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


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


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 %


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


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

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Boiler Efficiency: Direct Method

Boiler efficiency () = hf/hg

Heat Input * 100%

Heat Output

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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


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:


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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


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


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


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


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


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.

Energy Efficiency in Combustion

Documents

fuel ratio air

excess oxygen air

complete combustion

combustion efficiency

theoretical air

excess air requirement

atomized fuel

combustion zone