Fuel Technology Solid Fuels

FUEL TECHNOLOGY-SOLID FUELS

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UNIT III

FUEL

COMBUSTION OF FUEL

CLASSIFICATION OF FUEL

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CALORIFIC VALUE

CHARACTERISTICS OF GOOD FUEL

FUEL

The combustible substances which

on burning in air produces large

amount of heat that can be used

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amount of heat that can be used

economically for domestic and

industrial purposes are called fuels.

Eg. Wood , Coal etc

COMBUSTION OF FUEL

The term combustion refers to the

exothermal oxidation of a fuel, by air or

oxygen occurring at a sufficiently rapid

rate to produce a high temperature,

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rate to produce a high temperature,

usually with the appearance of a flame.

As most of the fuels contain carbon or

carbon and hydrogen, the combustion

involves the oxidation of carbon to carbon

dioxide and hydrogen to water. Sulphur, if

present, is oxidised to sulphur dioxide while

the mineral matter forms the ash.

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the mineral matter forms the ash.

Complex fuels like coal undergo thermal

decomposition during combustion to give

simpler products which are then oxidised to

carbon dioxide, water etc.

e.g.: Coke on combustion gives carbon

dioxide

Coal Coke + Coal gas

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C (coke) + O2 CO2


FUEL

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OCCURENCE PHYSICAL STATE

On the basis of occurrence

FUEL

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PRIMARY OR

NATURAL FUELSECONDARY OR

ARTIFICIAL FUEL


Fuels are classified as

Primary fuels Fuels which occur naturally such as coal, crude petroleumand natural gas. Coal and crude petroleum, formed from organic matter many millions of years ago, are referred

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many millions of years ago, are referred to as fossil fuels.

Secondary fuels Fuels which are derived from naturally occurring ones by a treatment process such as coke, gasoline, coal gas etc.

On basis of physical state

FUEL

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SOLID LIQUID GAS

FUEL

Primary Fuels Secondary fuels

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Solid

Eg. Wood,peat Liquid

Eg.crude oil

Gas

Eg.Natural gas

Solid

EgCoke,charcoal

Liquid

Eg. Petrol ,LPG

Gas

Eg.coal gas ,water gas

CHARACTERISTICS OF

GOOD FUEL

1.HIGH CALORIFIC VALUE:

A good fuel should have high

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A good fuel should have high

calorific value i.e. it should

produce large amount of heat on

burning.

CALORIFIC VALUE

The calorific value of a fuel is defined as

the quantity of heat (expressed in

calories or kilo calories) liberated by the

complete combustion of unit weight

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complete combustion of unit weight

(1gm or 1kg) of the fuel in air or oxygen,

with subsequent cooling of the

products of combustion to the initial

temperature of the fuel.

contd

The calorific value of a fuel depends

upon the nature of the fuel and the

relative proportions of the elements

present, increasing with increasing

amounts of hydrogen. Moisture if

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amounts of hydrogen. Moisture if

present, considerably reduces the

calorific value of a fuel. The calorific

value may be theoretically

calculated from the chemical

composition of the fuel.

contd

If both hydrogen and oxygen are

present, it may be assumed that all the

oxygen are already combined with 1/8

of its weight of hydrogen to form water.

This fraction is then deducted from the

hydrogen content of the fuel in the

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hydrogen content of the fuel in the

calculation. Thus for a fuel containing

carbon, hydrogen, oxygen and sulphur,

the calorific value of the fuel is given

by DULONG FORMULA

Determination of calorific value

from Dulong formula

Calorific value =

1/100[8080 C + 34500 {H O/8 } +2240 S] kcal/kg

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where C, H, O, S refer to % of carbon, hydrogen, oxygen and sulphur respectively.

GROSS AND NET CALORIFIC VALUE

With fuels containing hydrogen, two calorific values are distinguished, the gross and the net calorific value.

GROSS CALORIFIC VALUE

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The gross calorific value refers to the heat evolved when the water produced by combustion is condensed as a liquid. The net value gives the heat liberated when water is in the form of steam or water vapour.

contd

Thus the gross calorific value (or the

higher heating value) is the quantity of

heat liberated by the complete

combustion of unit weight of the fuel with

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combustion of unit weight of the fuel with

subsequent cooling of the products of

combustion to the initial temperature of

the fuel.

NET CALORIFIC VALUE

Under normal working conditions,

water vapours produced during

combustion are not condensed

and escape as such along with the

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and escape as such along with the

hot gases.Hence lesser amount of

heat is available, which is called

Lower or net calorific value.

Contd.

Net calorific value is the heat

produced when unit mass of fuel

is burnt completely and products

of combustion are allowed to

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of combustion are allowed to

escape.

contd

The net calorific value (or the

lower heating value) is defined as

the gross calorific value minus

the latent heat of condensation of

water (at the initial temperature of

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water (at the initial temperature of

the fuel), formed by the

combustion of hydrogen in the

fuel.The latent heat of steam at

ordinary temperature may be

taken as 587cal/g

contd

Net calorific value=Gross calorific

value-Latent heat of water vapours

NCV=GCV-weight of hydrogen x 9 x

Latent heat of water vapours

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Latent heat of water vapours

Latent heat of water vapours is 587

kcal/kg

Calculation of Net calorific valueHydrogen in the fuel reacts with oxygen to

give water

H2 + 1/2 O2 H2O

2H = 1/2O = H O

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2H = 1/2O2 = H2O

2parts = 16parts = 18parts

1parts = 8parts = 9parts

Contd

Let H is the percentage of hydrogen in

the fuel

Amount of water produced by burning

unit mass of fuel=9H/100 g

Latent heat of steam=587cal/g

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Latent heat of steam=587cal/g

Amount of heat produced by

condensation of steam=9H/100 x587 cal

NCV=[GCV-9H/100 x 587]

=[GCV-0.09 x 587] cal/g

Energy Units

Need to distinguish between energy and

power

Common energy units:

Btu (British Thermal Unit)- energy required to Btu (British Thermal Unit)- energy required to

heat one lbm of water one degree Fahrenheit

1 Btu = 778.16 ft-lbf = 1.055 kJ = 0.252 Cal

Commonly used measure of fuel energy, heating and

cooling quantities

1 MBtu = 1000 BTU; 1 MMBtu = 106 Btu

1 Quad = 1015 Btu (billion 109, trillion 1012, quadrillion)

1 Q = 1018 Btu

Energy Units

kJ- standard SI-mks energy unit

1 kJ = 1000 J = 1000 N-m

kJ used to measure fuel energy and heating and

cooling quantities

kWh- used to measure electrical energy kWh- used to measure electrical energy

1 kWh = 3412 Btu = 3600 kJ = 860 Cal

Calorie- used to measure food energy,

technically should be called a kilocalorie.

Chemists calorie (lower case c) is the energy

needed to raise 1 g of water 1 degree C.

1 Cal = 1000 cal = 4.186 kJ = 3.968 Btu

Power Units

Horsepower- used to measure rate of

mechanical work

1 hp = 2545 Btu/hr = 0.746 kW

kW- SI power unit used for both work and kW- SI power unit used for both work and

heat transfer. Sometimes see kWth for

thermal kW.

1 kW = 3412 Btu/hr = 1.34 hp = 0.2843 Tons

Ton- American unit of cooling rate commonly

employed to measure air conditioning

capacity

1 Ton = 12,000 Btu/hr = 3.517 kWth

2. MODERATE IGNITION

TEMPERATURE:

Ignition temperature: the lowest

temperature to which fuel must be

preheated so that it starts burning

smoothly. If ignition temp. is low, the

fuel catches fire easily. Low ignition

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fuel catches fire easily. Low ignition

temperature is dangerous for storage

and transportation of fuel. High

temperature causes difficulty in

kindling. So ,a good fuel should have

moderate ignition temperature.

3.LOW MOISTURE CONTENT:Agood fuel should have low moisture

content as moisture content reduces

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content as moisture content reduces

the calorific value.

4.LOW NON-COMBUSTIBLE

MATTER CONTENT

A good fuel should have low

contents of non-combustible

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contents of non-combustible

material as non-combustible matter

is left in form of ash which

decreases the calorific value of fuel

5.MODERATE RATE OF

COMBUSTION:

The temperature of combustion of

fuel depends upon the rate of

combustion . If the rate of

combustion is low ,then required

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combustion is low ,then required

high temperature may not be

reached soon. On the other hand

,too high combustion rate causes

high temperature very quickly.

6.MINIMUM SMOKE AND NON-

POISONOUS GASES

On burning, Fuel should not give

out objectionable and poisonous

gases. In other words, gaseous

products should not pollute the

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products should not pollute the

atmosphere. Gases like

CO,SO2,H2S etc. are some of

harmful gases.

7.CHEAP: A good fuel should be cheap and readily available.

8.EASY TRANSPORTATION :

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8.EASY TRANSPORTATION : A good fuel should be easy to

handle and transport at low cost

9.CONTROLLABLE

COMBUSTION:

Combustion of fuel should be easy

to start or stop when required.

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10.NON SPONTANEOUS

COMBUSTION: Combustion of fuel

should be non-spontaneous

otherwise it can cause fire hazards.

Contd.

11.LOW STORAGE COST:A good fuel should be easily stored at

low cost.

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The substance to be burned is massed into the

bomb, which is fitted with a device that can

deliver a spark and with a tube that can deliver

oxygen under pressure.

The bomb is then sealed and immersed in a

well-insulated vat of water.

Oxygen is let into the bomb, the sparkOxygen is let into the bomb, the spark

generated, the reaction occurs, and no products

escape as the heat is generated.

The heat warms the bomb and thus

the water surrounding it.

The stuff absorbing the heat is not

only the water, but also anything

immersed in it.

It includes the thermometer and the It includes the thermometer and the

stirrer which ensures that any heat is

uniformly distributed before the final

temperature is read.

Bomb Calorimetry

To obtain precise heat measurements, you

must know or find out the heat capacity of

the bomb calorimeter

Heat capacity takes into account all the Heat capacity takes into account all the

parts of the calorimeter that can lose or

gain.

ctotal= cwater + cthermometer + cstirrer + ccontainer

Bomb Calorimetry

Since mass of the other parts are

constant, there is no need for the mass

units in the heat capacity value.

Manufacturers include the heat capacity Manufacturers include the heat capacity

(C) of a calorimeter when it is purchased.

Therefore, qcal= CT

C= heat capacity of the calorimeter

Coal in truth stands not beside but

entirely above all other commodities. It is

the material energy of the country- the

universal aid, the factor in everything weuniversal aid, the factor in everything we

do with coal, almost any feat is possible;

without it we are thrown back into the

laborious poverty of early times

(DiCiccio, 1996).

What is Coal?

Coal:

A sedimentary rock that burns

Mineralized vegetative material deposited over a

long period of time (although miniscule long period of time (although miniscule

geologically)

altered chemical composition

Formed by increased T and P

Partial decay resulting from restricted access to

oxygen

Coal Composition

Carbon > 50%

Impurities Volatile Matter

Sulphur

Chlorine Chlorine

Phosphorus

Nitrogen

Trace amounts Dirt

Other elements

Classification of coal

Classification means classifying or categorizing

objects as per their characteristics or property.

Objective is to place like things together and

separate things that differs.

Coal is a naturally available heterogeneous organic

mass. So very difficult to classify.

Hence for last 150 years many attempts have been

made according to different classification basis.

Classification by visual characters

Category Attributes Flame

Brown coal/lignite Brown colour, woody

structure

----------------

Bituminous coal Black and banded Smoky yellow flame

Anthracite Black and lustrous Burns without flame

Changes in the average composition from wood to anthracite

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Where Does The Carbon

Come From? Coke: pure carbon obtained from heating wood at high temperatures. This heating evaporates volatile organic compounds and leaves essentially pure carbon.

Coke was the originally used source of carbon in Coke was the originally used source of carbon in iron smelting. However, population growth and rapid industrial development caused an increase in price and resulted in a declining source of supply (trees) created need for a cheaper substitute for the charcoal.

Welcome to

Coke-Land

Coke = charcoal made from coal

Heating value 25million BTUs/ton Heating value 25million BTUs/ton

Process of coke-making discovered in Sixteenth Century England:. Originally called (charking).

Obtained by heating coal at high temperatures (900-1150 C) in the absence of oxygen; much the same way as charcoal was made from wood.

Beehive Coke Ovens First Beehive coke oven was made in Connellsville,

Fayette County, PA during the 1830s.

Widespread use of these ovens was delayed until the

1850s.

These ovens proved much more efficient, producing These ovens proved much more efficient, producing

coke with carbon contents of up to 67%.

Beehive oven: A fire brick chamber shaped like a dome is

used. It is 4 m wide and 2.5 m high.

The roof has a hole for charging the coal from the top. The

discharging hole is provided in the circumference of the

lower part of the wall. Number of ovens is built in a row with

common walls between neighboring ovens.

Coal is introduced from the top and produces an even layer

about 60-90 cm deep. Air is supplied initially to ignite the

coal. Carbonization starts and volatile matter burns insidecoal. Carbonization starts and volatile matter burns inside

the partially closed side door. Carbonization proceeds from

top to bottom and is completed in 2-3 days.

Heat is supplied by burning of volatile matter and hence no

by-products recovered. The exhaust gases are allowed to

escape to the atmosphere. The hot coke is quenched with

water and discharged, manually through the side door. The

walls and roof retain enough heat to initiate, carbonization

of the next charge.

The yield of coke is about 60-80%.

Beehive Coke Ovens

Demerits:

No recovery of by-products

Lower coke yield due to partial combustion

Lack of flexibility in operation

Otto Hoffmann oven or by product oven

Construction:

A number of narrow rectangular chambers made of silica

bricks 12-14 m in length, 4-5 m in height, 0.5 m width ire

used.

It is tightly closed so that no air can enter.

Each chamber at the top has three holes for charging coal,

Otto Hoffmann oven or by product oven

Each chamber at the top has three holes for charging coal,

a gas take off and refractory lined cast iron door for

discharging the coke.

The carbonization chambers are erected side by side with

vertical flues or interspaced for combustion in between

them. 10-100 ovens are set together.

One oven is capable of holding 16-24 tones of coal.

Recovery of byproducts

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Fuel Technology Solid Fuels