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Boiler Combustion(Simplified)

Coal Fired

MODEL LOCOMOTIVES

2

HEAT INTO POWER

• Fuel and Air combustion :- Chemical Reaction

• Mechanical work :- Useful Work

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Solid Fuel (Coal)

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Anthracite

• Almost consist of pure carbon

• Smokeless

• Ignites with difficulty

• Burns slowly

• Requires a thin fire

• Requires strong draft

• The larger the grate-area the more efficient the combustion

• Effective radiant heat properties

• Generally contains low moister content (average 1½%)

5

Dry Welsh steam coal

• Semi Anthracite

• Requires a reasonable forced draft

• Contains high % of Carbon

• Burns off % of volatile gases

• Hydro-carbons produce a luminous flame

• Excellent radiant heat transmitting properties

• High heat value ratio compared with non-luminous gases

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Bituminous

• Low in carbon content

• High in volatile matter (gases)

• Generally high in moister content (average 7%)

• Burns freely produces a long flame

• Emits smoke if not carefully handled ie:-correct drafting techniques

• Cakes becomes very pasty

• Produces volatile hydro-carbons

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Effects of impurities in Coal

• Ash None combustible material in certain temperature conditions forms a semi- molten liquid (silicate of iron) Clinker

• Sulphur Introduced to excessive heat is converted to Sulphide Dioxide (SO2)

When absorbed by water it forms Sulphurous Acid (H2SO3)

In certain circumstances can oxidise further to become Sulphuric Acid (H2SO4)

Chemically corrosive !!

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COMPOSITION of AIR

23 parts Oxygen :- 77 parts Nitrogen in every 100 parts of air by weight.

21 parts Oxygen and 79 parts Nitrogen by volume.

9Primary Air

Secondary Air

Primary Air

Fire Box

Air intake in fire box

Blast nozzle blower ring

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Three stages of combustion

1. Distillation of the coal gasses (hydrocarbons)

2. For complete combustion to burn gasses:- Sufficient supply oxygen Sufficient high temperatures

3. Solid fuel remains behind as (carbon)

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Units of heat

1 lb Carbon effectively consumed produces

carbon dioxide 14,600 B.Th.U.s

1 lb Carbon incompletely consumed produces

carbon monoxide 4,400 B.Th.U.s

1 lb Hydrogen burned to water vapour produces

62,000 B.Th.U.s

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Primary Air Below the fire bed

• Combustion of the volatiles hydrogen (H) combines with the oxygen (O2) and forms water vapour steam (H2O)

• The oxygen combines with the carbon (C) burns off forming a colourless gas known as carbon dioxide (CO2)

• Passing through the fire bed looses part of its oxygen unites with the upper layers of carbon (C) and forms carbon monoxide (CO)

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Secondary Air Above the fire bed

• Insufficient oxygen incomplete combustion

• Heat losses 70% Example :- carbon consumed to (CO2)

14,600 B.Th.U.s per 1lb

carbon unconsumed to (CO)

4,400 B.Th.U.s per 1lb

• Carbon Monoxide (CO) vapours re-ignite at 1200ºF

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Hydrocarbons 1200º

Carbon Dioxide Carbon Monoxide Damper

Lever

Front damper

Rear damper

Ash

Air Oxygen and Nitrogen

At 60º F

Carbon Dioxide Water Vapour

Nitrogen 2000ºF

Ash pan

Air Oxygen and Nitrogen

700ºF

Blast Nozzle Blower Ring

Coal - Carbon - Hydrogen + Ash

Complete combustion of the coal at high temperatures upwards of 2000ºF

Gasses cool down on passage through tubes EXOTHERMIC REACTION

Fire hole door

partially openIncandescent fire bed

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Incomplete combustion showing miss management of the fire

Air- Oxygen and Nitrogen at 60ºF

Fire hole door fully

open

Holes in the fire bed

Drop in fire box temperature

Uneven thick fire bed

Heat losses up to 70%

Clinker grate area reduced

impeding draft

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Deflector plate

Stainless steel arch

Promotes combustion absorbs the heat from the volatile gasses before being

discharged reduces temperature drop

Directs the path

of air

Other uses' of absorbing heat

Volatile gasses

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Combustion chamber

Shortened tube length improved heat transfer

Volatile gases

Combustion chamber

Larger heating service