How oxygen & air amounts for combustion are explained to starters?

 by  Maryam Hussain

A chemical reaction, named “Combustion” has played major roles in human civilization. Since man discovered how to create fire, we have relied on combustion to perform a variety of tasks. The fire was used for heating and cooking, and later to manufacture tools and weapons. It was not until the onset of the Industrial Revolution in the nineteenth century that man started to obtain power from combustion. Rapid progress in the application of combustion systems since then, and many industries have come into existence as a direct result of this achievement.

Combustion is one of the most complex subjects that involve primarily such disciplines as physics, chemistry, thermodynamics, and fluid mechanics. Thanks to the branch of engineering & science, the thermodynamics, which enables us to calculate the energies of system changes in composition. Combustion has a wide variety of uses. Combustion is used for energy production in power plants, gas turbines, and engines.

Combustion is a process of heat release in exothermic reactions, which is accompanied by mass and heat transfer. Combustion involves a chemical transformation between a substance or substances called fuels and other chemicals called oxidizers, both of these being reactants.

In the process of combustion, that is a type of chemical reactions, the nuclei of the reactants are not altered but the bonds of the compounds are altered which involved the electrons of the molecules and atoms. In the case of combustion, it leads to the release of heat, while there are chemical reactions where heat is required to form the bonds. The amount of heat, that releases in the process of chemical bonding change, is the most interesting since this energy can be harvested and usefully exploited. Combustion may involve all phases of matter; the phases being solid, liquid, and gas; for example, liquid and gaseous combustion in SI engines.

Most internal combustion engines, spark ignition, and compression ignition obtain their energy from the combustion of a hydrocarbon fuel with oxygen, that converts the chemical energy of the fuel to internal energy in the gases within the engine.

The oxygen required for a given combustion reaction comes from the air; on a volume basis, air contains about 78% Nitrogen gas in the form of diatomic N₂ molecule, about 1 % of rare & noble gases such as helium, argon, 21 % Oxygen gas in the form of a diatomic O₂ molecule.

Combustion is the rapid reaction of a burnable substance with oxygen. It is worth mentioning that oxidation reactions of elements release significant quantities of energy as heat;

CARBON to carbon monoxide, carbon dioxide, CO and CO₂ respectively,

HYDROGEN to water, H₂O, and

NITROGEN to nitric oxide and nitrogen dioxide, NO and NO₂ respectively,

The most important of the combustion reactions are those in which hydrocarbon fossil fuels, such as coal, natural gas, etc. are burned to release energy that ultimately shows up as electricity, commercial and domestic heating, and automobile motion. There are thousands of different hydrocarbon fuel components, which consist of hydrogen and carbon. Still there are few hydrocarbon fuels that also contain oxygen, e.g. alcohols.

The major products of complete combustion from practically all fuels are CO₂ and water. A combustion reaction in which CO is formed is referred to as partial combustion or incomplete combustion, even though CO₂ may also be produced.

Examples:

C3H8    +    3.5O2    ⇀    3CO    +    4H2O    Partial combustion of propane

C    +    O2    ⇀    CO2    Complete combustion of carbon

C3H8    +    5O2    ⇀    3CO2    +    4H2O    Complete combustion of propane

The most important property of a fuel is its heating value, which is numerically equal to its standard heat of combustion. The heating value of a fuel is usually determined directly by experimental measurements. The maximum amount of chemical energy that can be released, in the form of heat energy from the fuel, is when fuel reacts (combusts) with a stoichiometric amount of oxygen. The stoichiometric amount of oxygen (also called theoretical oxygen) is just enough to convert all carbon in the fuel to CO₂ and all hydrogen of the fuel to H₂O, with no oxygen left over in the exhaust.

To simplify calculations without causing any large error, the amounts of argon and other trace gases in the air can be assumed to be a part of nitrogen gas, as all of these gases remain neutral during the combustion reactions, and atmospheric air can be modeled as being made up of 21% oxygen and 79% nitrogen (volume or molal basis). Therefore, for every mole of air injected into the combustion reaction, there are 0.21 moles of oxygen and 0.79 moles of nitrogen. For every one mole of oxygen, there are 0.79/0.21 = 3.76 moles of nitrogen in the air intake of the combustion chamber. So, for every mole of oxygen needed for combustion, 4.76 moles of air must be supplied: one mole of oxygen plus 3.76 moles of nitrogen.

The stoichiometric combustion of methane with air is given by the following equation:

CH4    +    2O2    +    2(3.76)N2    ⇀    CO2    +    2H2O    +    2(3.76)N2

The stoichiometric combustion of isooctane with air is given as:

C8H18    +    12.5O2    +    12.5(3.76)N2    ⇀    8CO2    +    9H2O    +    12.5(3.76)N2

The terminologies used for the amount of air or oxygen used in combustion are:

1). 85% stoichiometric air = 85% theoretical air = 85% air = 15% deficiency of air

2). 120 % stoichiometric oxygen = 120 % theoretical oxygen= 120 % oxygen = 20 % excess oxygen

The concept of excess air means the introduction of air in addition to that required for combustion. However, if one does not provide enough air, combustion may still continue but it will generate large quantities of CO and combustibles. This is referred to as sub-stoichiometric combustion. Process heaters, furnaces, and boilers should NEVER be operated in this mode.

For actual combustion in an engine, we introduce a useful term “equivalence ratio”. The equivalence ratio is a measure of the actual fuel-air mixture relative to the stoichiometric conditions of the fuel-air mixture. It is defined as:

ɸ  =   (FA)act /  (FA) stoich   =    (AF)stoich  /   (AF)act

where:

FA    =         mf  / ma = fuel-air ratio

AF    =         ma  /mf   = air-fuel ratio

ma    =         mass of air or mass flowrate of air

mf    =         mass of fuel or mass flowrate of fuel

Precautions for units of quantities:

During calculations, when using the mass flow rate of air, essentially use the mass flow rate of fuel.

During calculations, when using the mass of air, essentially use the mass of fuel.

when:

ɸ < 1 then the mass of air is more than that in stoichiometric amounts is needed, the FAmix will be lean in fuel & oxygen will be in exhaust gases.

ɸ > 1 then the mass of air is less than that in stoichiometric amounts is needed, the FAmix will be rich in fuel, there will be no oxygen in exhaust gases, but there will be CO and fuel in exhaust gases.

ɸ = 1 then FAmix will be in stoichiometric amounts, maximum energy will be released from fuel, and there will be neither oxygen nor CO in exhaust gases.

Spark ignition engines usually operate with an equivalence ratio in the range of 0.9 to 1.2, depending on the type of operation.