# Basics (Combustion)

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

Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame. Fuels of interest often include organic compounds (especially hydrocarbons) in the gas, liquid or solid phase.

In a complete combustion reaction, a compound reacts with an oxidizing element, such as oxygen or fluorine, and the products are compounds of each element in the fuel with the oxidizing element. For example:

CH4 + 2O2 → CO2 + 2H2O + energy
CH2S + 6F2 → CF4 + 2HF + SF6

A simple example can be seen in the combustion of hydrogen and oxygen, which is a commonly used reaction in rocket engines:

2H2 + O2 → 2H2O(g) + heat

The result is water vapor.

Complete combustion is almost impossible to achieve. In reality, as actual combustion reactions come to equilibrium, a wide variety of major and minor species will be present such as carbon monoxide and pure carbon (soot or ash). Additionally, any combustion in air, which is 78% nitrogen, will also create several forms of nitrogen oxides.

## Types

### Complete vs. Incomplete Combustion

In complete combustion, the reactant burns in oxygen, producing a limited number of products. When a hydrocarbon burns in oxygen, the reaction will only yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides. Carbon will yield carbon dioxide, nitrogen will yield nitrogen dioxide, sulfur will yield sulfur dioxide, and iron will yield iron(III) oxide.

In most industrial applications and in fires, air is the source of oxygen (O2). In air, each kg (lbm) of oxygen is mixed with approximately 3.76 kg (lbm) of nitrogen. Nitrogen does not take part in combustion, but at high temperatures, some nitrogen will be converted to NOx, usually between 1% and 0.002% (2ppm).(NortheastCHP) A more complete air combustion reaction is therefore:

2CH4 + xO2 + N2 → CO2 + 4H2O + CO + 2NOx + heat

### Incomplete

Incomplete combustion occurs when there isn't enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. It also happens when the combustion is quenched by a heat sink such as a solid surface or flame trap.

The quality of combustion can be improved by design of combustion devices, such as burners and internal combustion engines. Further improvements are achievable by catalytic after-burning devices (such as catalytic converters) or by the simple partial return of the exhaust gases into the combustion process. Such devices are required by environmental legislation for cars in most countries, and may be necessary in large combustion devices, such as thermal power plants, to reach legal emission standards.

The degree of combustion can be measured and analyzed, with test equipment. HVAC contractors, firemen and engineers use combustion analyzers to test the efficiency of a burner during the combustion process. In addition, the efficiency of an internal combustion engine can be measured in this way, and some states and local municipalities are using combustion analysis to define and rate the efficiency of vehicles on the road today.

### Smoldering

Smoldering is the slow, low-temperature, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel. It is a typically incomplete combustion reaction. Solid materials that can sustain a smoldering reaction include coal, cellulose, wood, cotton, tobacco, peat, duff, humus, synthetic foams, charring polymers including polyurethane foam, and dust. Common examples of smoldering phenomena are the initiation of residential fires on upholstered furniture by weak heat sources (e.g., a cigarette, a short-circuited wire), and the persistent combustion of biomass behind the flaming front of wildfires

### Rapid

Rapid combustion is a form of combustion, otherwise known as a fire, in which large amounts of heat and light energy are released, which often results in a flame. This is used in a form of machinery such as internal combustion engines and in thermobaric weapons. Sometimes, a large volume of gas is liberated in combustion besides the production of heat and light. The sudden evolution of large quantities of gas creates excessive pressure that produces a loud noise. Such a combustion is known as an explosion. Combustion need not involve oxygen; e.g., hydrogen burns in chlorine to form hydrogen chloride with the liberation of heat and light characteristic of combustion.

## Chemical Equation

Generally, the chemical equation for stoichiometric burning of hydrocarbon in oxygen is

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

For example, the burning of propane is

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

Generally, the chemical equation for stoichiometric incomplete combustion of hydrocarbon in oxygen is as follows:

$\left( z \right) C_xH_y + \left( z \left(\frac{x}{2} + \frac{y}{4} \right) \right) O_2 \rightarrow \; z \cdot xCO + \left( \frac{z \cdot y}{2} \right) H_2O$

For example, the incomplete combustion of propane is:

$2C_3H_8 + 7O_2 \rightarrow \; 2C + 2CO + 8H_2O + 2CO_2$

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

$\textrm{Fuel} + \textrm{Oxygen} \rightarrow \; \textrm{Heat} + \textrm{Water} + \textrm{Carbon\ dioxide}$

If the combustion takes place using air as the oxygen source, the nitrogen can be added to the equation,as and although it does not react, to show the composition of the flue gas:

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

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:

$\textrm{Fuel} + \textrm{Air} \rightarrow \; \textrm{Heat} + \textrm{Water} + \textrm{Carbon\ dioxide} + \textrm{Nitrogen}$

Nitrogen may also oxidize when there is an excess of oxygen. The reaction is thermodynamically favored only at high temperatures. Diesel engines are run with an excess of oxygen to combust small particles that tend to form with only a stoichiometric amount of oxygen, necessarily producing nitrogen oxide emissions. Both the United States and European Union are planning to impose limits to nitrogen oxide emissions, which necessitate the use of a special catalytic converter or treatment of the exhaust with urea.

## Fuels

### 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.

### 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. Flames are often visible. Heat transfer from the combustion to the solid maintains the evolution of flammable vapours.
• 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.
A general scheme of polymer combustion

## Reaction Mechanism

Combustion in oxygen is a radical chain reaction where many distinct radical intermediates participate.

The high energy required for initiation is explained by the unusual structure of the dioxygen molecule. The lowest-energy configuration of the dioxygen molecule is a stable, relatively unreactive diradical in a triplet spin state. Bonding can be described with three bonding electron pairs and two antibonding electrons, whose spins are aligned, such that the molecule has nonzero total angular momentum. Most fuels, on the other hand, are in a singlet state, with paired spins and zero total angular momentum. Interaction between the two is quantum mechanically a "forbidden transition", i.e. possible with a very low probability. To initiate combustion, energy is required to force dioxygen into a spin-paired state, or singlet oxygen. This intermediate is extremely reactive. The energy is supplied as heat. The reaction produces heat, which keeps it going.

Combustion of hydrocarbons is thought to be initiated by hydrogen atom abstraction (not proton abstraction) from the fuel to oxygen, to give a hydroperoxide radical (HOO). This reacts further to give hydroperoxides, which break up to give hydroxyl radicals. There are a great variety of these processes that produce fuel radicals and oxidizing radicals. Oxidizing species include singlet oxygen, hydroxyl, monatomic oxygen, and hydroperoxyl. Such intermediates are short-lived and cannot be isolated. However, non-radical intermediates are stable and are produced in incomplete combustion. An example is acetaldehyde produced in the combustion of ethanol. An intermediate in the combustion of carbon and hydrocarbons, carbon monoxide, is of special importance because it is a poisonous gas, but also economically useful for the production of syngas.

Solid fuels also undergo a great number of pyrolysis reactions that give more easily oxidized, gaseous fuels. These reactions are endothermic and require constant energy input from the combustion reactions. A lack of oxygen or other poorly designed conditions result in these noxious and carcinogenic pyrolysis products being emitted as thick, black smoke.

## Temperature

Assuming perfect combustion conditions, such as complete combustion under adiabatic conditions (i.e., no heat loss or gain), 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 all of the following:

• the heating value;
• the stoichiometric air to fuel ratio λ;
• the specific 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.

Most commonly, the adiabatic combustion temperatures for coals are around 2,200 °C (3,992 °F) (for inlet air and fuel at ambient temperatures and for λ = 1.0), around 2,150 °C (3,902 °F) for oil and 2,000 °C (3,632 °F) 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.

## References

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