Combustion

The flames caused as a result of a fuel undergoing combustion (burning)

Combustion or burning is a chemical process, an exothermic reaction between a substance (the fuel) and a gas (the oxidizer), usually O₂, to release heat. In a complete combustion reaction, a compound reacts with an oxidizing element, and the products are compounds of each element in the fuel with the oxidizing element. For example:

CH_4 + 2O_2 \rightarrow \; CO_2 + 2H_2O + heat
CH_2S + 6F_2 \rightarrow \; CF_4 + 2HF + SF_6 + heat

In most cases, combustion uses oxygen (O₂) obtained from the ambient air, which can be taken as 21 mole percent oxygen and 79 mole percent nitrogen (N₂). Thus, when methane (CH₄) is combusted using air as the oxygen source, the first example equation above becomes:

CH_4 + 2O_2 + 7.52N_2 \rightarrow \; CO_2 + 2H_2O + 7.52 N_2 + heat

As can be seen, when air is the source of the oxygen, nitrogen is by far the largest part of the products of combustion.

Rapid combustion

Rapid combustion is a form of combustion in which large amounts of heat and light energy are released. This often occurs as a fire. This is used in a form of machinery, such as internal combustion engines, and in thermobaric weapons.

Slower combustion

Slow combustion is a form of combustion which takes place at low temperatures. Respiration is an example of slow combustion.

Complete combustion

In complete combustion, the reactant will burn in oxygen, producing a limited number of products. When a hydrocarbon burns in oxygen, the reaction will only yield carbon dioxide and water. When a hydrocarbon or any fuel burns in air, the combustion products will also include nitrogen. When elements such as carbon, nitrogen, sulfur, and iron are burned, they will yield the most common oxides. Carbon will yield carbon dioxide. Nitrogen will yield nitrogen dioxide. Sulfur will yield sulfur dioxide. Iron will yield iron(III) oxide. Complete combustion is generally impossible to achieve unless the reaction occurs where conditions are carefully controlled (e.g. in a laboratory environment).

Turbulent combustion

Turbulent combustion is a combustion characterized by turbulent flows. It is the most used for industrial application (e.g. gas turbines, diesel engines, etc.) because the turbulence helps the mixing process between the fuel and oxidizer.

Incomplete combustion

Incomplete combustion happens when there is an inadequate supply of oxygen for combustion to occur completely. The reactant will burn in oxygen, but will produce numerous products. When a hydrocarbon burns in air, the reaction will yield carbon dioxide, water, carbon monoxide, and various other compounds such as nitrogen oxides. Incomplete combustion is much more common and will produce large amounts of byproducts, and in the case of burning fuel in automobiles, these byproducts can be quite unhealthy and damaging to the environment.

Smouldering combustion

Smouldering combustion is a flameless form of combustion, deriving its heat from heterogeneous reactions occurring on the surface of a solid fuel when heated in an oxidizing environment. The fundamental difference between smouldering and flaming combustion is that in smouldering, the oxidation of the reactant species occurs on the surface of the solid rather than in the gas phase. The characteristic temperature and heat released during smouldering are low compared to those in the flaming combustion of a solid. Typical values in smouldering are around 600 °C for the peak temperature and 5 kJ/g-O2 for the heat released; typical values during flaming are around 1500 °C and 13 kJ/g-O2 respectively. These characteristics cause smoulder to propagate at low velocities, typically around 0.1 mm/s, which is about two orders of magnitude lower than the velocity of flame spread over a solid. In spite of its weak combustion characteristics, smouldering is a significant fire hazard.

Chemical equation

Generally, the chemical equation for burning a hydrocarbon (such as octane) in oxygen is as follows:

C_xH_y + (\frac{x + y}{4})O_2 \rightarrow \; xCO_2 + (\frac{y}{2})H_2O

For example, the burning of propane is:

C_3H_8 + 5O_2 \rightarrow \; 3CO_2 + 4H_2O

The simple word equation for the combustion of a hydrocarbon in oxygen is:

Fuel + Oxygen \rightarrow \; Heat + Water + Carbon\ dioxide

If the combustion takes place using air as the oxygen source, the corresponding equations are:

C_xH_y + (\frac{x + y}{4})O_2 + 3.76(\frac{x + y}{4})N_2 \rightarrow \; xCO_2 + (\frac{y}{2})H_2O + 3.76(\frac{x + y}{4})N_2

For example, the burning of propane is:

C_3H_8 + 5O_2 + 18.8N_2 \rightarrow \; 3CO_2 + 4H_2O + 18.8N_2

The simple word equation for the combustion of a hydrocarbon in air is:

Fuel + Oxygen + Nitrogen \rightarrow \; Heat + Water + Carbon\ dioxide + Nitrogen

Combustion of liquid fuels

Combustion of a liquid fuel in an oxidizing atmosphere actually happens in the gas phase. It is the vapour that burns, not the liquid. Therefore, a liquid will normally catch fire only above a certain temperature, its flash point. The flash point of a liquid fuel is the lowest temperature at which it can form an ignitable mix with air. It is also the minimum temperature at which there is enough evaporated fuel in the air to start combustion.

Combustion of solid fuels

The act of combustion consists of three relatively distinct but overlapping phases:

Preheating phase, when the unburned fuel is heated up to its flash point and then fire point. Flammable gases start being evolved in a process similar to dry distillation.
Distillation phase or gaseous phase, when the mix of evolved flammable gases with oxygen is ignited. Energy is produced in the form of heat and light, flame is often visible.
Charcoal phase or solid phase, when the output of flammable gases from the material is too low for persistent presence of flame and the charred fuel does not burn rapidly anymore but just glows and later only smoulders.

Combustion temperatures

Assuming perfect combustion conditions, such as an adiabatic (no heat loss and no heat gain) and complete combustion, the adiabatic combustion temperature can be determined. The formula that yields this temperature is based on the first law of thermodynamics and takes note of the fact that the heat of combustion is used entirely for heating the fuel, the combustion air or oxygen, and the combustion product gases (commonly referred to as the flue gas).

In the case of fossil fuels burnt in air, the combustion temperature depends on

the heating value
the stoichiometric air to fuel ratio Lambda
the heat capacity of fuel and air
the air and fuel inlet temperatures

The adiabatic combustion temperature (also known as the adiabatic flame temperature) increases for higher heating values and inlet air and fuel temperatures and for stoichiometric air ratios approaching one.

Typically, the adiabatic combustion temperatures for coals are around 1500 °C (for inlet air and fuel at ambient temperatures and for Lambda = 1.0), around 2000 °C for oil and 2200 °C for natural gas.

In industrial fired heaters, power plant steam generators, and large gas-fired turbines, the more common way of expressing the usage of more than the stoichiometric combustion air is percent excess combustion air. For example, excess combustion air of 15 percent means that 15 percent more than the required stoichiometric air is being used.

Combustion analysis

Combustion analysis is a process used to determine the composition of organic compounds.

Combustion instabilities

Combustion instabilities are typically violent pressure oscillations in a combustion chamber. In rockets, such as the F1 used in the Saturn V program, instabilities led to massive damage of the combustion chamber and surrounding components. This problem was solved by re-designing the fuel injector. In liquid jet engines the droplet size and distribution can be used to attenuate the instabilities. Combustion instabilities are a major concern in ground-based gas turbine engines because of NOx emissions. The tendency is to run lean, an equivalence ratio less than 1, to reduce the combustion temperature and thus reduce the NOx emissions; however, running the combustor lean makes it very susceptible to combustion instabilities, especially in premixed combustors. The Rayleigh Criteron is the basis for analysis of combustion instabilities. It states that the pressure oscillations will be amplified as long as the heat release oscillations are in phase with them. These pressure oscillations can be as high as 180dB, and long term exposure to these cyclic pressure and thermal loads reduces the life of engine components.

External links

Hydrocarbon combustion Simple applet that illustrates the Chemical equation.
Fuel efficiency (stoichiometric air fuel mixture) vs. decreased emissions in combustion engines
Combustion Analysis The principles of exhaust fume analysis for assessing combustion quality in boilers.