Combustion

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Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames.

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:

$CH_4 + 2O_2 \rightarrow \; CO_2 + 2H_2O + \textrm{heat}$

$CH_2S + 6F_2 \rightarrow \; CF_4 + 2HF + SF_6 + \textrm{heat}$

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

$2H_2 + O_2 \rightarrow \; 2H_2O + \textrm{heat}$

The result is simply water vapor.

In the large majority of the real world uses of combustion, the oxygen (O₂) oxidant is obtained from the ambient air and the resultant flue gas from the combustion will contain nitrogen:

$CH_4 + 2O_2 + 7.52N_2 \rightarrow \; CO_2 + 2H_2O + 7.52 N_2 + \textrm{heat}$

As can be seen, when air is the source of the oxygen, nitrogen is by far the largest part of the resultant flue gas.

In reality, combustion processes are never perfect or complete. In flue gases from combustion of carbon (as in coal combustion) or carbon compounds (as in combustion of hydrocarbons, wood etc.) both unburned carbon (as soot) and carbon compounds (CO and others) will be present. Also, when air is the oxidant, some nitrogen will be oxidized to various, mostly harmful, nitrogen oxides (NO_x).

[edit] Types

[edit] Rapid

Rapid combustion is a form of combustion in which large amounts of heat and light energy are released, which often results in a fire. This is used in a form of machinery, such as internal combustion engines, and in thermobaric weapons. Combustion is double replacement, on the other hand a chemical reaction is single replacement.

[edit] Slow

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

[edit] Complete

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. It should be noted that complete combustion is impossible to achieve. In reality, as actual combustion reactions come to equilibrium, a wide variety of major and minor species will be present. For example, the combustion of methane in air will yield, in addition to the major products of carbon dioxide and water, the minor products which include carbon monoxide, hydroxyl, nitrogen oxides, monatomic hydrogen, and monatomic oxygen.

[edit] Turbulent

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.

[edit] Incomplete

Incomplete combustion occurs when there isn't enough oxygen to allow the fuel (usually a hydrocarbon) to react completely with the oxygen to produce carbon dioxide and water. 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, pure carbon (soot or ash) 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.

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). 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.

[edit] Smouldering

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.

[edit] Chemical equation

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

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

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:

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

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

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

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:

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

[edit] Fuels

[edit] 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.

[edit] 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.
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.

[edit] Temperature

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 $λ$
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.

Most commonly, the adiabatic combustion temperatures for coals are around 2200 °C (for inlet air and fuel at ambient temperatures and for $λ = 1.0$ ), around 2150 °C for oil and 2000 °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.

[edit] Analysis

This section provides a combustion analysis for a few typical fuel cases (carbon, hydrogen, sulfur, coal, oil and gas) when the fuel reacts with air at stoichiometric conditions.

In the presented combustion analysis, both fuel and air are at standard inlet combustion conditions of 298 K and 1 atm of absolute pressure. Furthermore, combustion is complete and with no heat loss.

Figure 1: Reactants and combustion products enthalpy vs temperature

During the combustion, a large amount of reactants' chemical energy gets released in the form of thermal energy.

Enthalpy of combustion (HHV) is the difference between the reactants enthalpy value minus the combustion products enthalpy value at the standard reference temperature, which is 298 K.

When the reactants enthalpy value is equal to the combustion products enthalpy value, one can calculate the combustion products flame temperature (adiabatic temperature).

The plot in Figure 1 depicts the reactants and combustion products enthalpy value change with an increase in the temperature.

Physical properties for both reactants and combustion products are very important and need to be known in order to carry out successful combustion calculations.

The plot in Figure 2 depicts how the reactants and combustion products species enthalpy values change with the temperature. The physical properties provided in this plot come from the JANAF Thermochemical Data - Tables, 1970.

Figure 2: Reactants and combustion products species enthalpy vs temperature

It is interesting to note that the enthalpy value for basic combustion elements such as carbon (C), hydrogen (H), sulfur (S), oxygen (O) and nitrogen (N) is equal to zero at the standard combustion conditions of 298 K and 1 atm.

Also, it should be mentioned that for ideal gas species, the enthalpy value is only dependent on the temperature.

In addition to knowing the reactants and combustion products physical properties, for any kind of combustion analysis and calculations, it is important to know both fuel and oxidant compositions.

For solid and liquid type fuels, the fuel compositions is given on the weight basis for a unit mass amount. For the gas type fuels, the fuel compositions is provided on the mole/volume basis for a unit volume amount. In this analysis, CH₄ is the only gas fuel considered. In order to keep the combustion analysis simple and straightforward, the CH₄ composition is provided on the weight basis. Oxidant composition is usually given on the mole/volume basis.

Table 1 provides the fuel composition.

Table 1: Fuel Composition (proportion by mass)
Fuel	C	H	S	N	O	H₂O	CH₄
Carbon	1.000	0.000	0.000	0.000	0.000	0.000	-
Hydrogen	0.000	1.000	0.000	0.000	0.000	0.000	-
Sulfur	0.000	0.000	1.000	0.000	0.000	0.000	-
Coal	0.780	0.050	0.030	0.040	0.080	0.020	-
Oil	0.860	0.140	0.000	0.000	0.000	0.000	-
Gas	-	-	-	-	-	-	1.000

Table 2 provides the standard air composition.

Table 2: Oxidant/Air Composition
Oxidant	N kg/kg	O kg/kg	N₂ mol/mol	O₂ mol/mol
Air	0.767	0.233	0.790	0.210

Again, in this combustion analysis, only the stoichiometric combustion is analyzed. Results of such analysis are provided, including a combustion products composition on weight and mole/volume basis, a flame temperature, a stoichiometric ratio and a higher heating value (HHV).

Table 3 provides the combustion products composition on the weight basis.

Table 3: Combustion Products on Weight Basis (proportion by mass)
Fuel	CO₂	H₂O	SO₂	N₂
Carbon	0.295	0.000	0.000	0.705
Hydrogen	0.000	0.255	0.000	0.745
Sulfur	0.000	0.000	0.378	0.622
Coal	0.249	0.041	0.005	0.705
Oil	0.203	0.079	0.000	0.718
Gas	0.151	0.124	0.000	0.725

Table 4 provides the combustion products composition on the volume basis.

Table 4: Combustion Products on Mole Basis (proportion by moles)
Fuel	CO₂	H₂O	SO₂	N₂
Carbon	0.210	0.000	0.000	0.790
Hydrogen	0.000	0.347	0.000	0.653
Sulfur	0.000	0.000	0.210	0.789
Coal	0.170	0.068	0.002	0.759
Oil	0.133	0.127	0.000	0.740
Gas	0.095	0.190	0.000	0.715

When considering coal, oil and gas as the fuel, coal has the largest amount of CO₂ in the combustion products on both weight and mole basis.

Table 5 provides the combustion products flame temperature, stoichiometric ratio and the fuel higher heating value.

Table 5: Other Fuel Characteristics
Fuel	Flame Temperature (K)	Stoichiometric Ratio (see note below)	HHV (kJ/kg)
Carbon	2,460	11.444	32,779.8
Hydrogen	2,525	34.333	141,866.8
Sulfur	1,972	4.292	9,261.3
Coal	2,484	10.487	32,937.9
Oil	2,484	14.580	47,630.0
Gas	2,327	17.167	50,151.2
Note: Stoichiometric ratio is the mass of air required for complete combustion of a unit mass of fuel. Thus, 1 kg of carbon fuel requires 11.444 kg of air for complete, ideal combustion.

Today, global warming is becoming more evident and it is being said that it is primarily caused by CO₂ emissions. A detailed combustion analysis, as it is provided here, can be very useful in determining different fuel and technology scenarios that would result in the reduction of current CO₂ emissions.

[edit] Instabilities

Combustion instabilities are typically violent pressure oscillations in a combustion chamber. 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. 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.

The Rayleigh Criterion is the basis for analysis of thermoacoustic combustion instabilities and is evaluated using the Rayleigh Index over one cycle of instability.

$G(x)=\frac{1}{T}\int_{T}q'(x,t)p'(x,t)dt$

When the heat release oscillations are in phase with the pressure oscillations the Rayleigh Index is positive and the magnitude of the thermoacoustic instability increases. Consecutively if the Rayleigh Index is negative then thermoacoustic damping occurs. The Rayleigh Criterion implies that a thermoacoustic instability can be optimally controlled by having heat release oscillations 180 degrees out of phase with pressure oscillations at the same frequency. This minimizes the Rayleigh Index.

[edit] See also

[edit] Related concepts

Air-fuel ratio
Deflagration
Detonation
Fire
Flue gas emissions from fossil fuel combustion
Flue gas stacks
Gas stoichiometry
Heat of combustion
Phlogiston theory (historical)
Pyrolysis
Pyrophoric
Smouldering
Spontaneous combustion
Stoichiometry

[edit] Machines and equipment

[edit] Measurement techniques

[edit] Social applications and issues

[edit] 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.
Simulation of gas combustion

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Categories: Combustion | Chemical processes | Chemical engineering | Chemical engineering topics