Adiabatic flame temperature

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There are two types of adiabatic flame temperature depending on how the process is completed: constant volume and constant pressure. The constant volume adiabatic flame temperature is the temperature that results from a complete combustion process that occurs without any work, heat transfer or changes in kinetic or potential energy. This is the maximum temperature that can be achieved for given reactants because any heat transfer from the reacting substances and/or any incomplete combustion would tend to lower the temperature of the products. The constant pressure adiabatic flame temperature is the temperature that results from a complete combustion process that occurs without any heat transfer or changes in kinetic or potential energy. Its temperature is lower than the constant volume process because some of the energy is utilized to change the volume of the system (i.e. generate work).

From the first law of thermodynamics for a closed reacting system we have:

[_RQ_P - _RW_P = U_P - U_R ].

where [_RQ_P] and [_RW_P] are the heat and work transferred during the process respectively, while [U_R] and [ U_P ] are the internal energy of the reactants and products respectively. This process is illustrated by the figure below.

In the constant volume adiabatic flame temperature case, the volume of the system is held constant hence there is no work occurring: [ _RW_P = \int\limits_R^P = 0].

There is no heat transfer because the process is defined to be adiabatic: [ _RQ_P = 0 ].

As a result, the internal energy of the products is equal to the internal energy of the reactants: [ U_P = U_R ].

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: [ U_P = U_R \Rightarrow m_P u_P = m_R u_R \Rightarrow u_P = u_R ].

An internal energy versus temperature diagram illustrates this concept using a sample calculation for isooctane starting at a pressure of 1 bar and a temperature of 400 K:

In the constant pressure adiabatic flame temperature case, the pressure of the system is held constant which results in the following equation for the work: [ _RW_P = \int\limits_R^P = p\left( \right) ].

Again there is no heat transfer occurring because the process is defined to be adiabatic: [ _RQ_P = 0 ].

From the first law, we find that [ - p\left( \right) = U_P - U_R \Rightarrow U_P + pV_P = U_R + pV_R ].

Recalling the definition of enthalpy we recover: [ H_P = H_R ].

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: [ H_P = H_R \Rightarrow m_P h_P = m_R h_R \Rightarrow h_P = h_R ].

Using the same initial conditions as the previous example, we plot the enthalpy versus temperature:

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 into changing the volume of the control system. One analogy that is commonly made between the two processes is through combustion in an internal combustion engine. For the constant volume adiabatic process, combustion is thought to occur instantaneously when the piston reaches the top of its apex (Otto cycle or constant volume cycle). For the constant pressure adiabatic process, while combustion is occurring the piston is moving in order to keep the pressure constant (Diesel cycle or constant pressure cycle).

If we make the assumption that combustion goes to completion (i.e. [CO_2] and [H_2O]), we can calculate the adiabatic flame temperature by hand either at stoichiometric 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:

[}_\alpha }_\beta }_\gamma }_\delta + \left( }_} + b}_} } \right) \to \nu _1 }_} + \nu _2 }_} } + \nu _3 }_} + \nu _4 }_} ]

Rich of stoichiometry there are not enough variables because combustion cannot go to completion with at least [CO] and [H_2] needed for the molar balance (these are the most common incomplete products of combustion):

[ }_\alpha }_\beta }_\gamma }_\delta + \left( }_} + b}_} } \right) \to \nu _1 }_} + \nu _2 }_} } + \nu _3 }_} + \nu _5 } + \nu _6 }_} ]

However, if we include the Water Gas Shift reaction: [ }_} + H_2 \Leftrightarrow } + }_} } ] and use the equilibrium constant for this reaction, we will have enough variables to complete the calculation.

The following figure illustrates the adiabatic flame temperatures and pressures of both the constant volume and constant pressure processes as a function of stoichiometry (relative ratio of fuel and air).

Different fuels with different levels of energy and molar constituents will have different adiabatic flame temperatures.

Due to the stoichiometry of organic compounds, from wood to propane to gasoline, the constant pressure adiabatic flame temperature of most common flammable substances in air is in a relatively-narrow range around 1950°C. Since most combustion processes that happen naturally occur in the open air, there is not anything 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 allowing the gas to expand during the process

We can see by the following figure why nitromethane [(CH_3NO_2)] is often used as a power boost for cars. Since it contains two moles of oxygen in its molecular makeup, it can burn much hotter because it provides its own oxidant along with fuel. 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 is interesting to note that 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 relative constituents 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.

External links

[Adiabatic Flame Temperature Program]

Adiabatic flame temperature

External links

See also