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THERMODYNAMICS

Lecture 12: Combustion

Thermodynamics

Thermodynamics

IntroductionIntroduction

• Solar energy is converted to chemical energy through photo-synthesis in plants

• Energy produced by burning wood or fossil fuels

• Fossil fuels: coal, oil and natural gas

The Formation of Fuels

Thermodynamics

Type of FuelsType of Fuels

Solid FuelsCoal classification• Anthracite: hard and geologically the

oldest

• Bituminous

• Lignite: soft coal and the youngest

• Further classification: semi- anthracite, semi-bituminous, and sub-bituminous

Thermodynamics


Solid Fuels

Physical properties• Heating or calorific value (GCV)

• Moisture content

• Volatile matter

• Ash

Chemical properties• Chemical constituents: carbon, hydrogen,

oxygen, sulphur

Thermodynamics


Solid Fuels (Physical properties)

Heating or calorific value• The typical GVCs for various coals are:

Parameter Lignite(Dry Basis)

Indian Coal

Indonesian Coal South African Coal

GCV (kJ/kg) 18,850 16,700 23,000 25,100

Thermodynamics


Solid Fuels (Physical properties)Moisture content• % of moisture in fuel (0.5 – 10%)

• Reduces heating value of fuel

• Weight loss from heated and then cooled powdered raw coal

Volatile matter• Methane, hydrocarbons, hydrogen, CO, other

• Typically 25-35%

• Easy ignition with high volatile matter

• Weight loss from heated then cooled crushed coal

Thermodynamics


Solid Fuels (Physical properties)Ash• Impurity that will not burn (5-40%)

• Important for design of furnace

• Ash = residue after combustion

Fixed carbon• Fixed carbon = 100 – (moisture + volatile matter + ash)

• Carbon + hydrogen, oxygen, sulphur, nitrogen residues

• Heat generator during combustion

Thermodynamics


Solid Fuels (Physical properties)Proximate analysis of coal• Determines only fixed carbon, volatile matter,

moisture and ash

• Useful to find out heating value (GCV)

• Simple analysis equipment

Ultimate analysis of coal• Determines all coal component elements: carbon,

hydrogen, oxygen, sulphur, other

• Useful for furnace design (e.g flame temperature, flue duct design)

• Laboratory analysis

Thermodynamics


Solid Fuels (Physical properties)Proximate analysis

Typical proximate analysis of various coals (%)

Indian Coal Indonesian Coal South African Coal

Moisture 5.98 9.43 8.5

Ash 38.63 13.99 17

Volatile matter 20.70 29.79 23.28

Fixed Carbon 34.69 46.79 51.22

Thermodynamics


Solid Fuels (Chemical Properties)Ultimate analysis

Typical ultimate analysis of coal (%)

Parameter Indian Coal, % Indonesian Coal, % Moisture 5.98 9.43 Mineral Matter (1.1 x Ash) 38.63 13.99 Carbon 41.11 58.96 Hydrogen 2.76 4.16 Nitrogen 1.22 1.02 Sulphur 0.41 0.56 Oxygen 9.89 11.88 GCV (kJ/kg) 16 700 23 000

Thermodynamics


Solid Fuels (Chemical Properties)Storage, Handling & Preparation• Storage to minimize carpet loss and loss due

to spontaneous combustion

• Reduce carpet loss: a) a hard surface b) standard concrete/brick storage bays

• Coal preparation before use is important for good combustion

Thermodynamics


Liquid FuelsUsage• Used extensively in industrial applications

Examples• Furnace oil

• Light diesel oil

• Petrol

• Kerosine

• Ethanol

• LSHS (low sulphur heavy stock)

Thermodynamics


Liquid FuelsDensity• Ratio of the fuel’s mass to its volume at 15 oC,

• kg/m3

• Useful for determining fuel quantity and quality

Thermodynamics


Liquid FuelsSpecific gravity• Ratio of weight of oil volume to weight of same water volume at a given temperature

• Specific gravity of water is 1

• Hydrometer used to measure

Fuel oil type LDO(Light Diesel Oil)

Furnace oil LSHS (Low SulphurHeavy Stock)

Specific Gravity

0.85-0.87 0.89-0.95 0.88-0.98

Table 1. Specific gravity of various fuel oils (adapted from Thermax India Ltd.)

Thermodynamics


Liquid FuelsViscosity• Measure of fuel’s internal resistance to flow

• Most important characteristic for storage and use

• Decreases as temperature increases

Flash point• Lowest temperature at which a fuel can be heated so that the vapour gives off flashes when an open flame is passes over it

• Flash point of furnace oil: 66oC

Thermodynamics


Liquid Fuels

Pour point• Lowest temperature at which fuel will flow

• Indication of temperature at which fuel can be pumped

Specific heat• kCal needed to raise temperature of 1 kg oil by

1oC (kcal/kgoC)

• Indicates how much steam/electricity it takes to heat oil to a desired temperature

Thermodynamics


Liquid FuelsCalorific value• Heat or energy produced

• Gross calorific value (GCV): vapour is fully condensed

• Net calorific value (NCV): water is not fully condensed

Fuel Oil Gross Calorific Value (kCal/kg)Kerosene 11,100Diesel Oil 10,800L.D.O 10,700Furnace Oil 10,500LSHS 10,600

Thermodynamics


Liquid FuelsSulphur content• Depends on source of crude oil and less on the refining process

• Furnace oil: 2-4 % sulphur

• Sulphuric acid causes corrosion

Ash content• Inorganic material in fuel

• Typically 0.03 - 0.07%

• Corrosion of burner tips and damage to materials /equipments at high temperatures

Thermodynamics


Liquid FuelsCarbon residue• Tendency of oil to deposit a carbonaceous solid residue on a hot surface

• Residual oil: >1% carbon residue

Water content• Normally low in furnace oil supplied (<1% at

refinery)

• Free or emulsified form

• Can damage furnace surface and impact flame

Thermodynamics


Liquid FuelsStorage of fuels• Store in cylindrical tanks above or below

the ground

• Recommended storage: >10 days of normal consumption

• Cleaning at regular intervals

Thermodynamics


Liquid Fuels

Fuel OilsPropertiesFurnace Oil L.S.H.S L.D.O

Density (Approx. g/cc at 150C)

0.89-0.95 0.88-0.98 0.85-0.87

Flash Point (0C) 66 93 66

Pour Point (0C) 20 72 18

G.C.V. (Kcal/kg) 10500 10600 10700

Sediment, % Wt. Max. 0.25 0.25 0.1

Sulphur Total, % Wt. Max. < 4.0 < 0.5 < 1.8

Water Content, % Vol. Max. 1.0 1.0 0.25

Ash % Wt. Max. 0.1 0.1 0.02

Typical specifications of fuel oils(adapted from Thermax India Ltd.)

Thermodynamics


Gaseous FuelsAdvantages of gaseous fuels• Least amount of handling

• Simplest burners systems

• Burner systems require least maintenance

• Environmental benefits: lowest GHG and other emissions

Thermodynamics


Gaseous FuelsClassification of gaseous fuels

(A) Fuels naturally found in nature-Natural gas-Methane from coal mines(B) Fuel gases made from solid fuel-Gases derived from coal-Gases derived from waste and biomass-From other industrial processes (C) Gases made from petroleum-Liquefied Petroleum gas (LPG)-Refinery gases-Gases from oil gasification(D) Gases from some fermentation

Thermodynamics


Gaseous FuelsCalorific value• Fuel should be compared based on the net

calorific value (NCV), especially natural gas

Typical physical and chemical properties of various gaseous fuelsFuel Gas Relative

Density Higher Heating Value kCal/Nm3

Air/Fuel ratio m3/m3

Flame Temp oC

Flame speed m/s

Natural Gas 0.6 9350 10 1954 0.290

Propane 1.52 22200 25 1967 0.460

Butane 1.96 28500 32 1973 0.870

Thermodynamics


Gaseous FuelsLiquefied Petroleum Gas (LPG)• Propane, butane and unsaturates, lighter C2 and heavier C5 fractions

• Hydrocarbons are gaseous at atmospheric pressure but can be condensed to liquid state

• LPG vapour is denser than air: leaking gases can flow long distances from the source

Thermodynamics


Gaseous FuelsNatural gas• Methane: 95%

• Remaing 5%: ethane, propane, butane, pentane, nitrogen, carbon dioxide, other gases

• High calorific value fuel

• Does not require storage facilities

• No sulphur

• Mixes readily with air without producing smoke or soot

Thermodynamics


Comparing FuelsFuel Oil Coal Natural Gas

Carbon 84 41.11 74

Hydrogen 12 2.76 25

Sulphur 3 0.41 -

Oxygen 1 9.89 Trace

Nitrogen Trace 1.22 0.75

Ash Trace 38.63 -

Water Trace 5.98 -

Thermodynamics

Performance EvaluationPerformance Evaluation

• Combustion: rapid oxidation of a fuel

• Complete combustion: total oxidation of fuel (adequate supply of oxygen needed)

• Air: 20.9% oxygen, 79% nitrogen and other

• Nitrogen: (a) reduces the combustion efficiency (b) forms NOx at high temperatures

• Carbon forms (a) CO2 (b) CO resulting in less heat production

Principles of Combustion

Thermodynamics


• Control the 3 Ts to optimize combustion:

• Water vapor is a by-product of burning fuel that contains hydrogen and this robs heat from the flue gases

Principles of Combustion

1T) Temperature

2T) Turbulence

3T) Time

Thermodynamics


Oxygen is the key to combustion

Principle of Combustion

Bureau of Energy Efficiency, India, 2004

Thermodynamics


Stochiometric calculation of air required

Stochiometric air needed for combustion of furnace oil

Theoretical CO2 content in the flue gases

Actual CO2 content and % excess air

Constituents of flue gas with excess air

Theoretical CO2 and O2 in dry flue gas by volume

Thermodynamics


• Measure CO2 in flue gases to estimate excess air level and stack losses

Concept of Excess Air

Carbon dioxide (%)

Exce

ss a

ir (%

)

Source: Bureau of Energy Efficiency, India, 2004

Thermodynamics


Concept of Excess Air

Residual oxygen (%)

Exce

ss a

ir (%

)

Bureau of Energy Efficiency, India, 2004

• Measure O2 in flue gases to estimate excess air level and stack losses

Thermodynamics


To exhaust combustion products to atmosphere

Natural draft:• Caused by weight difference between the hot gases

inside the chimney and outside air

• No fans or blowers are used

Mechanical draft:• Artificially produced by fans

• Three types a) balanced draft, b) induced draft and c) forced draft

Draft System

Thermodynamics

Energy Efficiency OpportunitiesEnergy Efficiency Opportunities

Preheating of combustion oil

Temperature control of combustion oil

Preparation of solid fuels

Combustion controls

Four main areas

Thermodynamics


Purpose: to make furnace oil easier to pump

Two methods:• Preheating the entire tank

• Preheating through an outflow heater as the oil flows out

Preheating of Combustion Oil

Thermodynamics


To prevent overheating• With reduced or stopped oil flow

• Especially electric heaters

Using thermostats

Temperature Control of Combustion Oil

Thermodynamics


Sizing and screening of coal• Important for efficient combustion

• Size reduction through crushing and pulverizing (< 4 - 6 mm)

• Screen to separate fines and small particles

• Magnetic separator for iron pieces in coal

Preparation of Solid Fuels

Thermodynamics


Conditioning of coal:• Coal fines cause combustion problems

• Segregation can be reduced by conditioning coal with water

• Decrease % unburnt carbon

• Decrease excess air level required


Thermodynamics


Blending of coal• Used with excessive coal fines

• Blending of lumped coal with coal containing fines

• Limits fines in coal being fired to <25%

• Ensures more uniform coal supply


Thermodynamics


• Assist burner to achieve optimum boiler efficiency through the regulation of fuel supply, air supply, and removal of combustion gases

• Three controls:• On/Off control: burner is firing at full rate or it is

turned off

• High/Low/Off control: burners with two firing rates

• Modulating control: matches steam pressure demand by altering the firing rate

Combustion Controls

Thermodynamics

Introductory definitions

Combustion is a quick egsothermal process of reduction(oxidation) of a fuel during which heat is released.

Fuels are the compounds of carbon, hydrogen, sulphur andoxygen which can be found in abundance in nature.

Fuels are solid, liquid or gaseous (natural – coal, crude oil, natural gas and artificial – coke, pellets, petrol, town gas).

Fuels consists of a combustible part and a balast. In solid (liquid) fuels the balast is understood as ash (naturalsubstance) and moisture, whereas in gaseous fuels the balast is the nitrogen, carbon dioxide and vapour of water.

Thermodynamics

Phases of combustion processPhase I – for solid fuels: heating and drying of fuels (dehumidification).

Phase II – transformation of combustible substance into simplecompounds ready for oxidation (hydrogen, carbon in form of soot orcoke, carbon monoxide.

a) In solid fuels – generation of volatile substances (carbohydrates) and pyrolisis (thermal decomposition of carbohydrates).

b) In liquid fuels – dehumidification and pyrolisis,

c) In gaseous fuels – pyrolisis of carbohydrates (CxHy decomposesinto hydrogen, carbon and carbon monoxide.

Phase III – appropriate reduction (oxidation) process (usually smallcontent of sulphur but leading to corrosion following condensation ofexhausts:

22

222

22

21

SOOS

HOOH

COOC

→+

→+

→+

Thermodynamics

Schematic of combustion processSubstances supplied to the combustion chamber (air and fuel) arereactants, whereas the products of combustion consist of gaseousexhausts and solid substances.

Combustion is complete when all the carbon present in the fuel is burnedto carbon dioxide, all the hydrogen is burned to water, all sulfur is burnedto sulfur dioxide and all other combustible elements are fully oxidised. Otherwise combustion in incomplete. In such case the flue gases maycontain Co, H2, CH4 and other CxHy, soot, unburned coal or coke in ash.

In our analysis we will assumethat reactants will be denotedwith ‘ whereas products with ‘’.

Thermodynamics

Combustion of solid and liquid fuelsChemical composition of solid and liquid fuels is determined by the mass fraction denoted with small letters: c- carbon, h – hydrogen, s – sulfur, o – oxygen, n – nitrogen, w – water, p – ash.

Composition of 1 kg of fuel:

c + h+s+n+o+w+a=1 and gc=c [kg C/kg fuel]

Stoichiometry of combustion

reactants → products

fuel + oxidiser → products

22

22

222

222

22

22

111

11121

111

SOkmolOkmolSkmolSOOS

OHkmolOkmolHkmol

OHOH

COkmolOkmolCkmolCOOC

→+→+

→+

→+

→+→+

Thermodynamics

Combustion of solid and liquid fuelsBearing in mind that m=nM, we are going to express the amount ofsubstance not in kg/kg, but in kmol/kg.

The number of particular reactants in a unit of fuel is expressed as:

fuelkgCkmol

fuelkmolCkg

fuelkgCkg

cMmn

c

c

c=

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

==12

fuelkgOHkmolwn

fuelkgNkmolnn

fuelkgOkmolon

fuelkgHkmolhn

fuelkgSkmolsn

fuelkgCkmolcn

OHNO

Hsc

2'2'2'

2'''

182832

23212

222

2

===

===

Thermodynamics

Air-fuel ratioTheoretical amount of air is a minimum amount of air for completecombustion of a unit of fuel.

In case of gaseous oxidiser we are interested in volume V[m3]; 1kmol=22.41 nm3

fuelkgOkmoloshcnnnnn

OHscO2

323241221 '

2

'

2

''

min2−++=−++=

fuelkgOkmol

nVOO

2min2min2

41.22=fuelkg

airkmoln

nO

air 21.0

min2

min=

fuelkgairnmnnV

Oairair

3

minminmin 221.041.2241.22 ==

Real air demand for assurance of complete combustion is greater

fuelkgairnmVAFVV

OOair

3

min2

min2

== λ

Thermodynamics

Air-fuel ratio

Thermodynamics

Thermodynamics

Flue gases from combustion of solid or liquid

22

222

22

21

SOOS

OHOH

COOC

→+

→+

→+

434214342144 344 21irexcessfuelfromcombustionofproducts

ONwNOHSOCOfluennnnnnnn **

222222++++++=

**

2222 ONwNHSCfluennnnnnnn ++++++=

( )( )fuelkg

gasesfluekmolnnwnshcnairairflue min

121.079.0182832212

−++++++= λ

( )fuelkg

OkmolnnnnairairairO

2

minmin

*

2121.021.021.0 −=−= λ

fuelkgNkmolnnn

airairN2

min

*

279.079.0 λ==

( )fuelkgfluenmVVnV

flueflueflueflue

3

141.22min

−+== λ

Thermodynamics

Flue gases from combustion of solid or liquid

( )fuelkgfluenmVVnV

flueflueflueflue

3

141.22min

−+== λ

OHfluedryflueVVV

2

−=

Thermodynamics

Composition of flue gases

22

2

%100 COnn

rdryflue

COCO

≡= ⎟⎠⎞

⎜⎝⎛ +−=

182whnn

fluedryflue

( )2

min2

2

%100121.0

%100 On

n

nn

rdryflue

air

dryflue

OO

≡−

==λ

2

min2

2

%10079.0

28%100 Nn

nn

nn

rdryflue

air

dryflue

NN

≡+

==λ

22

2

%10032%100 SOn

s

nn

rdryfluedryflue

SOSO

≡==

OHn

wh

nn

rdryfluedryflue

OHOH 2

2

2

%100182%100 ≡+

==

Thermodynamics

Combustion of gaseous fuelsComposition of fuels is given by means of volumetric (r) or molar (z) shares.

222

3

HH

ii

zHrfuelkg

kmolzfuelkg

nmr

==

=

1'2

'2

'2

''22

'4

''2

=+++++++ ∑ 44 344 2143421balastlittle

yx CONOHCHCCHCOH

OHyxCOOyxHC

OHCOOHCOHCOOHC

OHCOOCH

COOCO

OHOH

yx 222

22262

22222

2224

22

222

24

325.3225.2

222121

+→⎟⎠⎞

⎜⎝⎛ ++

+→+

+→+

+→+

→+

→+

Thermodynamics

Demand for oxygen and airComposition of fuels is given by means of volumetric (r) or molar (z) shares.

( )fuelkmol

Okmolnnyxn

nnn

OCxHyCHCOH

O24/2

2'

242

min2

−+++= ∑+

fuelkmol

Okmol

fuelnm

OnmnV

OO22

3

3

min2

min2

==

( )fuelkmolairkmolO

yH

xCyxCH

COHn

nO

air ⎥⎦

⎤⎢⎣

⎡∑ −+++

+== '

2'4/2

221.01

21.0'4

''2

min2

min

minairairVV λ=

Thermodynamics

Amount and composition of combustion products

4342143421

excessair

ONfuelfrom

balastreagentsflueVVVVV

22

+= ++

⎥⎦

⎤⎢⎣

⎡∑= ⎟

⎠⎞

⎜⎝⎛

+++

fuelnmairnm

fuelkmolairkmol

yH

xCxCHCOCOV

CO 3

3'4

''2

''2

'

⎥⎦

⎤⎢⎣

⎡∑+= ⎟

⎠⎞

⎜⎝⎛

+

fuelnmairnm

fuelkmolairkmol

yH

xCyCHHV

OH 3

3'4

'2

''2

'2/2

⎥⎦

⎤⎢⎣

⎡+=+=

fuelnmairnm

fuelkmolairkmolVNVNV

airairN 3

3

min

'2

'2

''2

79.079.0 λ

( ) ( ) ⎥⎦

⎤⎢⎣

⎡−== −

fuelnmairnm

fuelkmolairkmolVVVV

airairairO 3

3

minmin

''2

121.021.0 λ

''2

''2

''2

''2 ONOHCOflue

VVVVV +++= ''2OHfluedryflue

VVV −=

Thermodynamics

Control of combustion processesIf in combustion products we find CO and H2 (detected with analysers) then extra air must be supplied to the process or temperature ofcombustion is too small.

If the excess air is too high the cooling of the combustion bed takes place and thermal energy is removed to the chimney.

combustion process.If we observe the soot (black smoke) then that points at incorrect

lecture 12: combustion - wydział mechaniczny · pdf filelecture 12: combustion. ... (gcv)...

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