In the study of
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...
, the adiabatic flame temperature is the temperature reached by a flame under ideal conditions. It is an upper bound of the temperature that is reached in actual processes.
There are two types adiabatic flame temperature: ''constant volume'' and ''constant pressure'', depending on how the process is completed. The
constant volume
In thermodynamics, an isochoric process, also called a constant-volume process, an isovolumetric process, or an isometric process, is a thermodynamic process during which the volume of the closed system undergoing such a process remains constant. ...
adiabatic flame temperature is the temperature that results from a complete combustion process that occurs without any
work
Work may refer to:
* Work (human activity), intentional activity people perform to support themselves, others, or the community
** Manual labour, physical work done by humans
** House work, housework, or homemaking
** Working animal, an animal t ...
,
heat transfer
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
or changes in
kinetic or
potential energy. Its temperature is higher than in the ''constant pressure'' process because no energy is utilized to change the volume of the system (i.e., generate work).
Common flames
In daily life, the vast majority of flames one encounters are those caused by rapid
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
of
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
s in materials such as
wood
Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...
,
wax
Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to giv ...
,
fat
In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food.
The term often refers specifically to triglycerides (triple est ...
,
plastic
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
s,
propane, and
gasoline
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organi ...
. The constant-pressure adiabatic flame temperature of such substances in air is in a relatively narrow range around 1950 °C. This is mostly because the
heat of combustion of these compounds is roughly proportional to the amount of oxygen consumed, which proportionally increases the amount of air that has to be heated, so the effect of a larger heat of combustion on the flame temperature is offset. Incomplete reaction at higher temperature further curtails the effect of a larger heat of combustion.
Because most combustion processes that happen naturally occur in the open air, there is nothing that confines the gas to a particular volume like the cylinder in an engine. As a result, these substances will burn at a constant pressure, which allows the gas to expand during the process.
Common flame temperatures
Assuming initial atmospheric conditions (1bar and 20 °C), the following table
[See under "Tables" in the external references below.] lists the flame temperature for various fuels under constant pressure conditions. The temperatures mentioned here are for a
stoichiometric
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...
fuel-oxidizer mixture (i.e.
equivalence ratio ''φ'' = 1).
Note that these are theoretical, not actual, flame temperatures produced by a flame that loses no heat. The closest will be the hottest part of a flame, where the combustion reaction is most efficient. This also assumes complete combustion (e.g. perfectly balanced, non-smoky, usually bluish flame). Several values in the table significantly disagree with the literature
or predictions by online calculators.
Thermodynamics
From the
first law of thermodynamics
The first law of thermodynamics is a formulation of the law of conservation of energy, adapted for thermodynamic processes. It distinguishes in principle two forms of energy transfer, heat and thermodynamic work for a system of a constant amou ...
for a closed reacting system we have
:
where,
and
are the heat and work transferred from the system to the surroundings during the process, respectively, and
and
are the internal energy of the reactants and products, respectively.
In the constant volume adiabatic flame temperature case, the volume of the system is held constant and hence there is no work occurring:
:
There is also no heat transfer because the process is defined to be adiabatic:
. As a result, the internal energy of the products is equal to the internal energy of the reactants:
.
Because this is a closed system, the mass of the products and reactants is constant and the first law can be written on a mass basis,
:
.
In the case of the constant pressure adiabatic flame temperature, the pressure of the system is held constant, which results in the following equation for the work:
:
Again there is no heat transfer occurring because the process is defined to be adiabatic:
. From the first law, we find that,
:
Recalling the definition of enthalpy we obtain
. Because this is a closed system, the mass of the products and reactants is the same and the first law can be written on a mass basis:
:
.
We see that the adiabatic flame temperature of the constant pressure process is lower than that of the constant volume process. This is because some of the energy released during combustion goes, as work, into changing the volume of the control system.
If we make the assumption that combustion goes to completion (i.e. forming only and ), we can calculate the adiabatic flame temperature by hand either at
stoichiometric
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...
conditions or lean of stoichiometry (excess air). This is because there are enough variables and molar equations to balance the left and right hand sides,
:
Rich of stoichiometry there are not enough variables because combustion cannot go to completion with at least and needed for the molar balance (these are the most common products of incomplete combustion),
:
However, if we include the
water gas shift reaction
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a s ...
,
:
and use the equilibrium constant for this reaction, we will have enough variables to complete the calculation.
Different fuels with different levels of energy and molar constituents will have different adiabatic flame temperatures.
We can see by the following figure why
nitromethane
Nitromethane, sometimes shortened to simply "nitro", is an organic compound with the chemical formula . It is the simplest organic nitro compound. It is a polar liquid commonly used as a solvent in a variety of industrial applications such as in ...
(CH
3NO
2) is often used as a power boost for cars. Since each molecule of nitromethane contains an oxidant with relatively high-energy bonds between nitrogen and oxygen, it can burn much hotter than hydrocarbons or oxygen-containing methanol. This is analogous to adding pure oxygen, which also raises the adiabatic flame temperature. This in turn allows it to build up more pressure during a constant volume process. The higher the pressure, the more force upon the piston creating more work and more power in the engine. It stays relatively hot rich of stoichiometry because it contains its own oxidant. However, continual running of an engine on nitromethane will eventually melt the piston and/or cylinder because of this higher temperature.
In real world applications, complete combustion does not typically occur. Chemistry dictates that
dissociation and
kinetics will change the composition of the products. There are a number of programs available that can calculate the adiabatic flame temperature taking into account dissociation through equilibrium constants (Stanjan, NASA CEA, AFTP). The following figure illustrates that the effects of dissociation tend to lower the adiabatic flame temperature. This result can be explained through
Le Chatelier's principle.
See also
*
Flame speed The flame speed is the measured rate of expansion of the flame front in a combustion reaction. Whereas ''flame velocity'' is generally used for a fuel, a related term is explosive velocity, which is the same relationship measured for an explosive. ...
References
External links
General information
*
Computation of adiabatic flame temperature
Tables
* adiabatic flame temperature of hydrogen, methane, propane and octane with oxygen or air as oxidizers
* {{Cite web
, title = Flame Temperatures for some Common Gases
, work = The Engineering Toolbox
, accessdate = 2008-01-27
, url = http://www.engineeringtoolbox.com/flame-temperatures-gases-d_422.html
, archiveurl= https://web.archive.org/web/20080107164751/http://www.engineeringtoolbox.com/flame-temperatures-gases-d_422.html, archivedate= 7 January 2008 , url-status= live
Temperature of a blue flame and common materials
Calculators
Online adiabatic flame temperature calculatorusing
Cantera
Cantera, literally meaning "quarry" in Spanish, is a term used in Spain to refer to youth academies and farm teams organized by sports clubs. It is also used to refer to the geographical area that clubs recruit players from. The term is widely u ...
Adiabatic flame temperature programGaseq program for performing chemical equilibrium calculations.
Flame Temperature Calculator- Constant pressure bipropellant adiabatic combustion
Combustion
Temperature
Threshold temperatures