THERMOCHEMISTRY

OF COMBUSTION

COMBUSTION AND FUELS

 IST RULE OF THERMODYNAMICS
Q – heat
H – inner enthalpy
dQ = dH – Vdp V – volume
P - pressure

For isobaric processes: dp = 0

dQ = dH
dH = cpdT → dQ = cpdT
Q = ∫ c p dT

COMBUSTION AND FUELS

 CALORIC EFFECTS OF COMBUSTION

1. Every compound has an inner energy in which an

important part is chemical energy resulting from its
chemical structure.
2. Every change of chemical structure of the matter is
connected with variation of its inner energy.
Caloric effect of combustion results from the oxidation
reactions of a fuel due to changes of its chemical
structure in the combustion process.

COMBUSTION AND FUELS

 CALORIC EFFECTS OF CHEMICAL COMPOUNDS
FORMATION
Every substance has its individual chemical energy, which
could be called as heat of its synthesis. It can be denoted:

enthalpy of formation, ∆hf [J/mol]

For convenience of comparison enthalpies of formation for different
compounds standard enthalpy (∆hf,st) of formation is used.
The standard enthalpy is determined at standard conditions:

p = 101.3 kPa , T = 298 K

Values of standard enthalpies of formation ∆hf,st are given in tables

COMBUSTION AND FUELS

 HEAT OF CHEMICAL REACTION
Knowledge of heat (enthalpy) formation of compounds makes
itpossible to calculate heat of chemical reaction ∆hrst:

ΣiνiAi = 0
(νi – stoichiometric coefficients)

as a sum of enthalpies of formation of reagents ∆hif,st:

∆hrst = Σiνi ∆hif,st

where νi assumes νi < 0 for subtracts and νi > 0 for products, and subscript st
denotes standard conditions.

COMBUSTION AND FUELS

CALORIC EFFECT OF CHEMICAL REACTIONS:
types of chemical reactions
Types of chemical reaction:
•exothermic,
• endothermic.
Example (oxidation of coal):
C + O2 = CO2 – 393.5 kJ/mol exothermic
C + 0.5O2= CO – 110.5 kJ/mol exothermic
C + CO2 = 2CO +172.5 kJ/mol endothermic

COMBUSTION AND FUELS

 CALORIC EFFECT OF CHEMICAL REACTION:
Hess law
I The caloric effect of a chemical reaction
I
i
depends only on the initial and final state of the
i reacting system and is separated from the route
of conversion.
I

II

III

COMBUSTION AND FUELS

 CALORIC EFFECT OF COMBUSTION
– incomplete combustion
Consider methane oxidation:
I. CH4 + 3/2O2 → CO + 2H2O + Q1
II. CH4 + 2O2 → CO2 + 2H2O + Q2
Q2 > Q1
I. incomplete combustion,
II. complete combustion.
CO + 1/2O2 → CO2 + Q3
Q1 + Q3 = Q2
COMBUSTION AND FUELS
 LOWER CALORIC VALUE LCV
UPPER CALORIC VALUE HCV
Every fuel is evaluated by two caloric
parameters:

Upper caloric value: HCV (Qs) [J/kg]

Lower caloric value: LCV (Qi) [J/kg]
Both caloric values are strictly defined and possible to
be calculated on a basis of chemical structure or
determined experimentally. The lower and higher
caloric values are given for the almost every fuel in the
tables.
COMBUSTION AND FUELS
 STANDARD UPPER CALORIC VALUE

UCVst = ∆hf,st(af) – ∆hf,st(fg) Qs,st

where:
af – air and fuel, T = 298 K
fg – flue gas p = 101.3 kPa

In practice index „st” is omitted and it is assumed that

the caloric value is determined in the standard
conditions.
COMBUSTION AND FUELS
 STANDARD LOWER CALORIC VALUE

∗(Wa+8.94Ha)
LCVast = UCVast – r∗ Qai,st

r – heat of evaporation of water at the temperature of 25 °

C corresponding to 1% of water in fuel [J/g],
r = 24.42 [J/g]
Wa = moisture content in analytical sample of fuel, %
Ha – hydrogen content in analytical sample of fuel, %
(8.94 – coefficient for water)

COMBUSTION AND FUELS

 LOWER AND UPPER CALORIC VALUE
The difference between the upper and lower caloric
value results form the fact that water in flue gas is in
liquid form during determination of UCV and in gaseous
form (steam) when LCV is determined. Hence:

UCV > LCV Qs > Qi

Upper caloric value is higher than lower caloric value.

COMBUSTION AND FUELS

ENTHALPIES OF FORMATION OF SOME REACTANTS
The zero value was assumed for enthalpies of formation for
molecules, like: oxygen O2, hydrogen H2, methane CH4
Radicals:
Atom of hydrogen H: 217.99 [kJ/mol]
Atom of oxygen O: 249.19 [kJ/mol]
Water and CO
Water (liquid) H2O: - 285.83 [kJ/mol]
Water (steam) H2O: - 241.81 [kJ/mol]
Carbon monoxide CO: -110.52 [kJ/mol]

COMBUSTION AND FUELS

 APPROXIMATE FORMULAS OF
LOWER CALORIC VALUES

LCV = 4.187[81C + 246H +26(O – S) – 6W] kJ/kg

C, H, O, S, W – weight contents of elements in a fuel (%).

COMBUSTION AND FUELS

LOWER CALORIC VALUES OF SELECTED SOLID FUELS

Fuel type Name LCV

MJ/kg

Solid Wood 14.3

(analytical Peat 14.5
state, air-dry)
Lignite 17-23.2
Bituminous coal 29.4
Anthracite 31.1
Wood char 28.6
Coke 30.1

COMBUSTION AND FUELS

COMPARISON OF CALORIC VALUES FOR COALS

Type of Name LCV UCV

fuel MJ/kg MJ/kg

Solid Bituminous 31.55 32.71

(analytical coal
state, air-dry) (KWK Śląsk) 20.73 21.94
Lignite
(KWB Konin)

COMBUSTION AND FUELS

COMPARISON OF CALORIC VALUES FOR LIQUID FUELS

TYPE OF NAME LCV UCV

FUEL MJ/kg MJ/kg

Liquid Ethylene 26.8 29.7

Gasoline 42.0 45.2
Diesel oil 41.8 44.7
Heating oil 42.1 44.8

COMBUSTION AND FUELS

COMPARISON OF CALORIC VALUES FOR GASEOS FUELS

TYPE OF NAME LCV UCV

FUEL MJ/kg MJ/kg

GAS Acetylene 48.6 50.4

Buthane 45.7 49.6
Methane 50.0 55.6
CO 10.1 10.1
Hydrogen 119.6 142.0

COMBUSTION AND FUELS

 Exchangeability of gases and Wobbe No
Gases are exchangeable if they can be burnt properly in the same
burners not requiring adaptation.
More precisely: if an apparatus is supplied be two different gases
under the same overpressure (∆ ∆p), and it has the same power N
(N1 = N2), then these gases are called exchangeable.
The condition of exchangeability is determined by the Wobbe
no:
Wobbe no Wb

Wb = Qs/dν0,5
Density ratio dν

dν = ρgas/ρair

COMBUSTION AND FUELS