BOILER OPERATORS TRAINING COURSE

THEORY OF COMBUSTION AND BOILER FUELS

By

COSMAS CHIGORA
(C & C INDUSTRIAL CONSUMABLES)

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 COURSE CONTENT
• 1.0 WHAT IS A BOILER & ITS USES
• 2.O HOW IS STEAM PRODUCED
• 3.0 TYPES OF BOILERS & CLASSIFICATIONS
• 4.0 TRENDS IN DESIGN OF BOILERS
• 5.0 TYPES OF STEAM GENERATED
• 6.0 WHAT IS COMBUSTION, THEORY
• 7.0 BOILER FUELS DISCUSSED, ADV & DISADV.

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8.0 BOILER EFFICIENCIES
9.0 EMISSIONS &ENVIRONMENTAL
STANDARDS
10.0 BOILER OPERATIONAL
WATCHPOINTS

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 WHAT IS A BOILER AND ITS USES?

Definition:
– an implement that applies heat energy to water
to produce steam for industrial applications and
power generation.

Where boilers are used:

– in chemicals, paper, food, refining, metals,
clothing industries, power stations etc.

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 PROCESS OF PRODUCING STEAM

– heat energy  pipes  water  steam

– heat energy comes from fuels: wood, coal, oil,

natural gas, electricity, biomass, solar, nuclear
e.g In a firetube boiler, hot combustion
gases flow inside tubes and heat
water flowing outside

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 TYPES OF BOILERS AND CLASSIFICATIONS

General classifications:
Water-tube and/or Firetube or by Fuel used.
-Water-tube: water is heated by combustion
gases flowing outside tubes.
– Firetube: hot combustion gases flow inside
tubes and heat water flowing
outside
– Fuel used: wood, oil ,coal, gas, biomass,
electricity.
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 TRENDS IN BOILER DESIGNS AND SELECTION

Design focus is on efficiency & low cost design

– At design stage -consider output steam mass
flow rate, pressure & temperature.
– Efficiency is determined by type of fuel used.

Selection is based on desired temperature,

pressure and steam flow rate 7
 OTHER DESIGN CRITERIA
- give attention to emission of air pollutants –
standards exist from EMA
- Pollutants: CO, hydrogen chloride, mercury,
selected metals, greenhouse gases (CO2,
SO2, NOx)

- boiler based on renewable energy(biomass,

solar energy) becoming popular.
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Selection:
- Firetube: developed 18th century, more
fuel efficient, lower initial cost, easier to
operate, limited to 25t/hr at pressure17.5
bar.
- water-tube : preferred for high pressure
steam, less fracture, higher efficiency at
any steam capacity & pressure.
- Electrode/diesel boiler: low pressure and
low steam capacity, no emissions

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 TYPES OF STEAM GENERATED

- Saturated steam: steam at boiling point for a given

pressure.
- Superheated steam: steam temp. 100⁰C @ 1 bar.

- Supercritical steam: steam at pressure and temp

above water critical point 374.15⁰C @ 218.3 atm
where boiling no longer occurs (used in power

generation)
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 ENERGY SOURCES FOR BOILER
COMBUSTION

- Wood - Electricity
- Coal - Biomass
- Oil - Solar
- Natural gas - Nuclear

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 WHAT IS COMBUSTION AND ITS
CAUSES?

Definition:
- simply a process of burning substance +
oxygen

Causes:
- material + oxygen = heat = energy

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 FACTORS INFLUENCING COMBUSTION PROCESS
- Ignition temperature of fuel used
- Turbulence rate when fuel and oxygen mix
- Time taken for complete combustion- enough?

If factors not correctly set lead to

- incomplete combustion, carbon soot, black smoke &
reduces boiler efficiency.

QUALITY OF COMBUSTION
i) Too little or excess air – black smoke/dense white smoke.
Good mix gives a faint, light brown haze.

General rule of thump: 1 kg of fuel oil requires 14.1kg of air.

Correct air results in 21% CO2 and 79% NO.
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ii) Causes of incomplete combustion:
- shortage of air
- surplus fuel & distribution
- poor mixing of fuel and air
-poor viscosity
- non uniform fuel size (fines etc..)

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Combustion on a Chaingrate Stoker

• Combustion on a chaingrate stoker takes place

in 3 stages:
Stage 1: volatiles or tarry fraction is distilled off
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Combustion on a chain grate stoker
• Combustion on a chain grate stoker takes place in 3 stages:
Stage 1: volatiles or tarry fraction is distilled off and coal is
ignited
Stage 2 : coke formed in stage 1 is burned leaving a layer of ash
on the grate
Stage 3: the ash is transported to ashport through a high
temperature oxidising atmosphere to burn off most of remaining
carbon.

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• For efficient combustion to take place:
Air provided by FD fan must be evenly
distributed across the fuel bed:-
fixed inlet guide vanes distribute air evenly
across its width; controllable undergrate
dampers regulate the amount of air which is
allowed to flow to the tail end of stoker ; fixed
vanes located at end of stoker distribute air
through the carbon lean ash layer.
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Coal characteristics that affect performance:

- Rate at which coal can be ignited- volatiles content, washed

- Particle size distribution- affects resistance to airflow through

fuel bed.

- Fusion temperature- low means coal cannot be burnt at high

combustion intensities resulting in formation of slag on grate.

Therefore a balanced draft is important to avoid tubes burning

if there is scale on waterside as temp rises inside.
Avoid clinker by spreading coal thinly(less than 150mm) or use
‘slag-softening’ chemical into the combustion.
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TYPES OF FUELS, ADVANTAGES & DISADVANTAGES

1.Coal fuel: a fossil fuel most popular in

Zimbabwe & the world over.
- Coal composition: carbon, hydrogen, sulphur, oxygen,
nitrogen etc
-Coal types: peat, lignite, sub-bituminous,
bituminous, anthracite, graphite.
Bituminous is the one used in steam-electric power
generation.
- Burn pulverized coal or mixed with biomass at times.
- Produces residual ash and removal mechanisms required.
- Energy value of coal(heating value)- 24MJ/ kg
- 1 kg of coal produces approx 8 kg of steam 19
 Coal as a fuel cont…..
Advantages:
-when pulverized, burns almost as efficiently as gas;
- readily accessible & plentiful;
- costs less;
- simple & easily replaceable components;
- long term investment 30-40 years;
- lowest energy density of common fuels.

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 Coal as a fuel cont….
Disadvantages:
- being a fossil fuel, it releases CO, CO2, Hg (bad for
environment –pollution& ozone depletion);
- non-renewable-finite source;
- uses a lot of water;
- high transport costs & handling by operator;
- much storage space;
- heat output not easily controlled;
- generally high installation costs.
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 Coal as a fuel cont….
Environmental effects
- dust nuisance
- waste products disposal
- acid rain from high sulphur content(1.5-3% by wt)
(use of gas scrubbers)
- lung cancer
- emits mercury, asenic, selenium all harmful to health
- releases CO2, a greenhouse gas(climate change &
global warming)
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 Coal as a fuel cont….
Technological measures to mitigate disadv
CO2 emission factor comparison
Coal - 0.963kg/kWh
Oil - 0.881kg/kWh
Natural gas - 0.569kg/kWh
- CCT (Clean coal technology)
coal is chemically washed of minerals & impurities, or
gasified then burned and resulting flue gases treated with
steam to remove SO2 & re-burned to make CO2 in flue
gas economically recoverable.
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 TYPES OF FUELS, ADVANTAGES &
DISADVANTAGES (cont.)
2. oil fuel: a fossil fuel mainly used in the food
industry in Zimbabwe
Advantages:
- does not produce toxic gases & only traces of ash, so cleaner to
operate;
- burns more efficiently than coal, wood & thus produces high energy.
Shorter response between demand and generation of steam
- little storage space required;
- virtually no handling from operator;
- easy to adjust heat output electronically;
- low installation cost and mechanical stoker eliminated;
- 1 kg of oil= 15kg steam or I litre oil= 14kg steam

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 Oil as a fuel cont…

Disadvantages:
- used for low steam output;
- high cost of purchase.
- production of acid rain & harmful
gases(particulates, CO2, Nox, SOx
water vapour)
- releases greenhouse gases
- non-renewable & fast depleting 25
 TYPES OF FUELS, ADVANTAGES &
DISADVANTAGES (cont.)
3. Electricity: electrode boilers are used mainly
in clothing and textile industry.
- Elements heat the water directly
- Requires protection against ground faults, over
current & loss of phase;
- Unit must be shut down periodically so that
insulators can be washed off to prevent arcing;
- Water treatment a must or solids will accumulate
slowly on isolators that support the electrodes.

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 TYPES OF FUELS, ADVANTAGES &
DISADVANTAGES (cont.)
4. Wood fuel: a fossil fuel, generally popular in
lumber processing operations
Disadvantages:
- don’t know exactly how it will burn: too many
variables
(i.e species of tree, where grown, bark, sapwood or
hardwood, high % of resins & pitches; moisture
content; wood particle sizes; storage and can be
explosive.)
- ash & soot build-up in firetubes(need soot
blowers) 27
 Wood as a fuel cont…
Note that build up of soot leads to:
- excessive fuel usage
- loss of energy & boiler efficiency
- loss of production time while cleaning tubes

Energy produced:
0.75kg wood will give about 8500 BTU energy

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 Wood as a fuel cont…
Environmental effects
- Produces fine particle matter which causes:
- eye, nose, throat & lung irritations
- coughing, sneezing, runny nose & shortness
of breath.
- may lead to chronic bronchitis, lung cancer,

heart disease.

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 5. Natural Gas: cleanest, quietest &
easiest to handle.

- Produces CO2 and water vapour and only traces of sulphur

- Combustion efficiency is 2.3% lower than oil

- Difficult to store compared with oil but can be piped.

1 Therm unit of gas produces 42kg of steam for a 10bar boiler

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 LOSS OF FUEL & EFFICIENCY DUE TO
SOOT(mixture of solid carbon, ash & molten ash-sticks)
Thk of soot % loss of eff % average
in firetube in firetube fuel loss
0.8mm 9.5% 2.9%
1.6mm 26.2% 7.8%
2.4mm 35.7% 10.7%
3.16mm 45.3% 12.6%
4.8mm 69% 20.7%
• How do you know that boiler has accumulated soot in tubes?

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Boiler flue gas temp rise of 5% from normal reduces
boiler efficiency by 1%. Investigate cause.

Causes: high excess air, fouled firesides, secondary

combustion, broken baffles.

Effect of soot on corrosion: water + SO2 + SO3 = acid

which condenses at 120-175 degC and if it drops to
55 degC acid dilutes and causes rapid corrosion.

Remedy for soot:

i) Brush tube boiler when temp rises by 10 degC

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ii) Maintain flue gas exit temp above 200 degC
& acid will not form in chimney & flues

iii) Use smoke density meter to measure

efficiency of combustion

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 EFFICIENCIES OF BOILERS
Efficiencies of boilers (with feedwater economizers, air pre-
heating & 3% O2 in flue gas)

Fuel type combustion eff %

Natural gas 85.7

Distillate oil 88.7-89.6
Coal 90.3

Efficiencies of boilers (typical)

Coal 85%
Oil 80%
Natural gas 75%
Biomass 70%
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Why stress efficiency?
- Major cost in boiler operation is fuel at
about 96%, with investment & maintenance
accounting for 4%.

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 ENERGY REQUIREMENTS FOR STEAM PRODUCTION

• Energy(kJ) required to produce 1 kg saturated

steam
Feedwater Temperature ⁰C

Pressure 10 deg C 38 deg C 66 deg C 93 deg C 121deg C

(kPa)

1034 kPa 2740 kJ 2624 kJ 2507 kJ 2391 kJ 2272 kJ

3103 kPa 2761 kJ 2645 kJ 2528 kJ 2421 kJ 2293 kJ

4137 kPa 2754 kJ 2638 kJ 2521 kJ 2405 kJ 2289 kJ

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Question: Fuel consumption is increasing but
steam output same?

Solution:
- Flue gas analysis to check combustion efficiency

- Review rate of blowdown & chemical treatment to

remove silica, TDS & conductivity.

- Check for steam and boiler water leaks i.e. safety

valves, blowdown pipes, gauge glasses

- Check temp of incoming feedwater & recovery of

condensate
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Specific Heat Capacities & Relative
densities
• Relative density specific heat capacity
kJ/kg degC
------------------------- ----------------------------
a. Coal 0,64 – 0.93 1.08 – 1.54
b. Wood 1.32 1.38
c. Fuel oil 0.96 1.67
0.91 1.84
d. Gasoline - 2.21

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 METHODS OF MEASURING HEAT TRANSFER EFFICIENCIES

Measure combustion efficiency using a

combustion gas analyzer.

Efficiency = Total heat released in combustion

less heat lost in stack divided by total heat
released.

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 METHODS OF INCREASING HEAT
TRANSFER EFFICIENCIES
- Equip boiler with systems that prevent or
recover energy losses
- Feed water preheating using an economizer to
extract heat from boiler exhaust(5% increase)
- Combustion air pre-heating using boiler
exhaust recuperation(1-2% increase)
- Heat recovery from blowdown waste & preheat
feedwater(1% increase). Blowdown waste can
also be used in a flash tank for de-aeration or
low pressure steam processes.
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 METHODS OF INCREASING HEAT
TRANSFER EFFICIENCIES (cont.)

- Insulation of pipes & valves

- Use of turbulence promoters inserted in tubing
to increase heat transfer for firetube
boilers(1% increase)
- O2 trim controls to ensure optimum fuel/O2
mixture in burner(1% increase)
- Condensate return systems
- Select boilers with more passes
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 FUEL EMMISSIONS, STATUTORY
REGULATIONS (EMA)
1. EMA regulations
- EMA Act Chapter 20:27
- effluent and solid waste disposal SI 6 of 2007
- waste management plans and targets
- design & construction of boiler blowdown sump
2. World Standard limits on emissions:
STD 2001/80/EC

Natural gas - 50.29 kg/TJ CO2 emissions

Distillate oil - 69.33 kg/TJ CO2 emissions
Fuel oil - 74.69 kg/TJ CO2 emissions
Coal - 89.08 kg/TJ CO2 emissions

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BOILER OPERATIONAL WATCH-POINTS
FOR COMBUSTION EFFICIENCIES

- Reduce time required to start-up(fire banking etc)

- Check steam quality(dry or wet)
- Blow-down schedules to be adhered to(rate, length)
- Water treatment to avoid scale build up(RO)
RO water results in:
i) TDS levels down to 100ppm saves 20% coal usage
ii) saves 60% on chemicals
iii) reduces scaling & corrosion by up to 90%
iv) prevents boiler foaming & carryover of steam
- Measure & record stack gas temperatures
- Use of controls to control pressure, heat recovery systems, steam
delivery & distribution, condensate drainage, air release, quality of coal
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Blowdowns
- Encourage 4-second blowdowns every 8
hours.
Steam measurement
- Why measure steam usage & cost
• Able to calculate plant efficiency-
Show steam flowrate from switch off to full
load
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• Able to work on energy efficiency schemes
• Helps to control process
• Costing & custody-measure & cost at
individual points

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Show deterioration of plant over time-
clean/replace
Track steam demand & changing trends
Establish peak periods
Identify sectors or items of plant who are
major users
Change production methods

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Process control
• Control quantity of steam being supplied to
process- is it at correct temp & pressure
• Monitor rate of increase of flow at start-up,
slow warm up control value

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 CONCLUSION

Considering that
fuel accounts for 96% of the cost of
running a boiler ,
SURELY
fuel should be an important point
of focus for any Operator.

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