Combustion Reactions

S.Gunabalan Associate Professor Mechanical Engineering Department Bharathiyar College of Engineering & Technology Karaikal - 609 609. e-Mail : gunabalans@yahoo.com

Part - 1

Combustion Reactions

The entropy of substances at the absolute zero of temperature, called absolute entropy (third law of thermodynamics).

A combustion reaction is a chemical reaction in which products are formed from reactants with the release or absorption of energy as heat is transferred to as from the surroundings

The numerical coefficients in the equation, which precede the chemical symbols to give equal amounts of each chemical element on both sides of the equation, are called stoichiometric coefficients.

the total mass of reactants must equal the total mass of products Since 1 kmol of H2 equals 2 kg, 1 2 kmol of O2 equals 16 kg, and 1 kmol of H2O equals 18 kg,

The air–fuel ratio is simply the ratio of the amount of air in a reaction to the amount of fuel. The ratio can be written on a molar basis (moles of air divided by moles of fuel) or on a mass basis (mass of air divided by mass of fuel).

Theoretical Air The minimum amount of air that supplies sufficient oxygen for the complete combustion of all the carbon, hydrogen, and sulfur present in the fuel is called the theoretical amount of air

Gas Molar Weight ( M)Kg/Kmol

Air 28.97

Nitrogen 28.01

Oxygen 32

Hydrogen 2.016

Helium 4.004

Carbon dioxide 44.01

Steam 18.02

Stoichiometric (or chemically correct) mixture of air and fuel is one that contains just sufficient oxygen for complete combustion of the fuel. A weak mixture is one which has an excess of air. A rich mixture is one which has a deficiency of air

The ratios are expressed as follows : For gaseous fuels :By volume For solid and liquid fuels : By mass

AIR-FUEL RATIO

• air-fuel ratio can be calculated by the following methods :

1. Fuel composition known (i) Carbon balance method (ii) Hydrogen balance method (iii) Carbon-hydrogen balance method. 2. Fuel composition unknown (i) Carbon-hydrogen balance method.

Carbon balance method : combustion takes place with excess air and when free (solid) carbon is not present in the combustion products. Hydrogen balance method is used when solid carbon is present in the combustion product. Carbon-hydrogen balance method: This method may be employed when there is some uncertainty about the nitrogen percentage reported by the Orsat analysis.

ANALYSIS OF EXHAUST AND FLUE GAS

Wet analysis : An analysis which includes the steam in the exhaust. Orsat apparatus : is used for the analysis of combustion products.

ENTHALPY OF FORMATION

The enthalpy of formation (∆Hf) is the increase in enthalpy when a compound is formed from its constituent elements in their natural form and in a standard state. The standard state is 25°C, and 1 atm.

∆퐻0푅푒푎푐푡푖표푛 = ∆퐻0풇표푟푚푎푡푖표푛풑풓풐풅풖풄풕풔 − ∆퐻0풇푹풆풂풄풕풂풏풕풔

∆퐻0푅푒푎푐푡푖표푛 = 푛.∆퐻0

풇 풑풓풐풅풖풄풕풔 − 푛.∆퐻0

풇 푹풆풂풄풕풂풏풕풔

Where n – number of moles Also Enthalpy of formation of elements in its elemental form is zero. For example H2,O2 ext

Ref: http://www.youtube.com/watch?v=O36WGkHGXLI

http://en.wikipedia.org/wiki/Standard_enthalpy_of_formation

Reference • Moran, M. J. 2011. Fundamentals of engineering thermodynamics. Wiley,

[Hoboken, N.J.?]. • Rajput, R. K. 2010. Engineering thermodynamics. Jones and Bartlett

Publishers, Sudbury, Mass. • Nag, P. K. 2002. Basic and applied thermodynamics. Tata McGraw-Hill, New

Delhi.