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Pierwsza strona
THERMODYNAMICS
Lecture 12: Combustion
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Thermodynamics
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Thermodynamics
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Thermodynamics
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Thermodynamics
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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
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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
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Thermodynamics
Type of FuelsType of Fuels
Solid Fuels
Physical properties• Heating or calorific value (GCV)
• Moisture content
• Volatile matter
• Ash
Chemical properties• Chemical constituents: carbon, hydrogen,
oxygen, sulphur
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
Liquid FuelsUsage• Used extensively in industrial applications
Examples• Furnace oil
• Light diesel oil
• Petrol
• Kerosine
• Ethanol
• LSHS (low sulphur heavy stock)
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Thermodynamics
Type of FuelsType of Fuels
Liquid FuelsDensity• Ratio of the fuel’s mass to its volume at 15 oC,
• kg/m3
• Useful for determining fuel quantity and quality
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Thermodynamics
Type of FuelsType of Fuels
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.)
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
Liquid FuelsStorage of fuels• Store in cylindrical tanks above or below
the ground
• Recommended storage: >10 days of normal consumption
• Cleaning at regular intervals
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Thermodynamics
Type of FuelsType of Fuels
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.)
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Thermodynamics
Type of FuelsType of Fuels
Gaseous FuelsAdvantages of gaseous fuels• Least amount of handling
• Simplest burners systems
• Burner systems require least maintenance
• Environmental benefits: lowest GHG and other emissions
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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
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Thermodynamics
Type of FuelsType of Fuels
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 -
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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
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Thermodynamics
Performance EvaluationPerformance Evaluation
• 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
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Thermodynamics
Performance EvaluationPerformance Evaluation
Oxygen is the key to combustion
Principle of Combustion
Bureau of Energy Efficiency, India, 2004
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Thermodynamics
Performance EvaluationPerformance Evaluation
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
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Thermodynamics
Performance EvaluationPerformance Evaluation
• 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
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Thermodynamics
Performance EvaluationPerformance Evaluation
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
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Thermodynamics
Performance EvaluationPerformance Evaluation
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
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Thermodynamics
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
Preheating of combustion oil
Temperature control of combustion oil
Preparation of solid fuels
Combustion controls
Four main areas
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Thermodynamics
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
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
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Thermodynamics
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
To prevent overheating• With reduced or stopped oil flow
• Especially electric heaters
Using thermostats
Temperature Control of Combustion Oil
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Thermodynamics
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
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
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Thermodynamics
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
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
Preparation of Solid Fuels
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Thermodynamics
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
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
Preparation of Solid Fuels
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Thermodynamics
Energy Efficiency OpportunitiesEnergy Efficiency Opportunities
• 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
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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.
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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
→+
→+
→+
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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 ‘’.
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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
→+→+
→+
→+
→+→+
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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
===
===
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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
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Thermodynamics
Air-fuel ratio
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Thermodynamics
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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
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Thermodynamics
Flue gases from combustion of solid or liquid
( )fuelkgfluenmVVnV
flueflueflueflue
3
141.22min
−+== λ
OHfluedryflueVVV
2
−=
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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 ≡+
==
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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
+→⎟⎠⎞
⎜⎝⎛ ++
+→+
+→+
+→+
→+
→+
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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=
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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 −=
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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