Introduction to combustion phenomena
Gaetano Continillo
Università del Sannio, Benevento, Italy
Orléans - Wednesday, 23 September 2011
Rome Rome
Naples Naples
Benevento Benevento Venice
Venice Florence
Florence
Milan
Milan
Introduction
Combustion and its control are essential to our life on Earth.
For example 85% of the energy used in the USA come from combustion sources [US DOE 1996 Annual Energy Review]
Other fonts
Combustion
Transportation is mostly based on combustion (cars, trucks,
aircrafts, ships, except railways)
Industrial processes rely heavily on combustion:
• Iron, Steel, Aluminum and other metal refining industries
employ furnaces
• Heat treatments are conducted in ovens
Other industrial combustion devices include boilers,
refinery and drying ovens, organic fume incinerators
Cement (beton) manufacturing is based on combustion
(Rotary kilns to produce clinker)
End-of-lifecycle use of combustion includes waste
incineration
Combustion processes call for improvement in terms of:
• Thermal efficiency
• Pollution control
• Development of combustion technologies for renewable fuel sources
•
Thermal efficiency contributes to save energy
• Pollution control is necessary for global and local environmental protection
• Renewable fuels are necessary in view of fossil
fuel shortage and to avoid greenhouse effect due
to increase of CO
2concentration in the atmosphere.
A definition of Combustion
Rapid oxidation generating heat, or both light and heat; also, slow oxidation accompanied by relatively
little heat and no light.
This definition emphasizes the intrinsic importance of chemical reactions to combustion.
It also emphasizes why combustion is so important:
combustion transforms energy stored in chemical
bonds to heat that can be utilized in a variety of
ways.
Combustion modes and flame types
Combustion can occur in flame and non-flame mode.
The difference between flame and non-flame mode can be explained for example with the knocking phenomena in spark ignition internal combustion engines.
A flame is a thin zone of intense chemical
reaction.
As the flame moves across the combustion space, temperature and pressure rise in the unburned gas.
Unburned fuel-air mixture Burnt fuel-air mixture
Propagating flame Spark
location
Under certain conditions, rapid oxidation reactions occur at many locations
Autoigniting fuel-air mixture
There are premixed flames and non-premixed (diffusion) flames
In a premixed flame fuel and oxidizer are mixed at the molecular level prior to the occurence of any significant reaction. Spark ignition engines are a good example.
In a diffusion flame, the reactants are initially separated, and reaction occurs only at the interface between the fuel and the oxidizer. An example af a diffusion flame is a
simple candle.
Oxidizer diffusing from outside
Flame sheet
Liquid fuel climbing for capillarity
Solid fuel
Vapor fuel diffusing from inside
Important topics in the study of combustion are:
• Thermochemistry
• Chemical kinetics
• Molecular transport of mass and heat
• Fluid mechanics
Thermochemistry provides the link between chemical composition of the system, temperature and pressure.
Chemical bonds break and form during chemical reactions.
Chemical potential energy is destroyed (exothermic
reactions) and accumulated (endothermic reactions), to the benefit or expense of kinetic energy of the molecules. The kinetic energy of the molecules in the system is related to temperature.
Under proper circumstances, all systems reach equilibrium.
Equilibrium is achieved via the available path according to
physical constraints (for example contant volume, constant
pressure). Equilibrium is met when a conveniently defined
state function (e.g. Gibbs free energy) reaches its minimum.
Chemical reactions proceed towards equilibrium with a finite rate. This rate depends on composition and
temperature according to laws studied and assessed in chemical kinetic studies.
Thus, thermochemistry sets the target and chemical
kinetics dictates the rate.
To have a chemical transformation
event involving one or more molecules, some energy barrier must be won
(activation energy).
Strong exothermic reactions can
deliver large amounts of kinetic energy, if the reactant molecules are supplied some energy to overcome this barrier.
Chemically reactive gaseous (or gaseous/condensed two–
phase) systems exist when molecules move and hit each other.
Such energy is often provided by surrounding molecules, which hit reactant molecules and increase their
mechanical energy.
Example: first–order gas–phase bi–molecular reactions.
Chemical transformation events in a gaseous system occur in large amount if:
r kc c A B
0exp A /
k k T k E RT
•
Both reactant molecules are present in large number (the more there are, the more probably they hit each other)
•
Kinetic energy of molecules (temperature) is high (each
hit is more probable to break/form chemical bonds)
Real systems are non-uniform in space. This produces gradients in species concentrations and temperature.
Flames are present when strong gradients exist. Gradients are the driving forces for molecular transport of mass and heat.
Molecular transport phenomena govern both premixed and
diffusion flames.
Most combustion processes take place in fluids in motion. Thus, when composition and temperature
gradients are present in a flow field, convection can be an important, if not dominant, mechanism of transport.
Moreover, most combustion processes are designed to
take place in a turbulent flow. Turbulent flows are the
most complex phenomena in fluid mechanics.
Finally, observe that all of the above phenomena are physically coupled to each other. Temperature
increases due to exothermic chemical reactions, thus density must decrease according to the constitutive
equation (for example the ideal gas law). Density decrease implies expansion in the fluid flow. This creates motion that influences the spatial distribution of species and
temperature, and so on. Moreover, molecular transport
coefficients, including viscosity, all depend on temperature.
Simulation of many gas–phase combustion processes is nowadays feasible by means of computer codes solving initial and boundary value problems on Navier–Stokes equations written for reactive systems.
Multi–phase combustion can also be described by
coupling gas–phase and condensed–phase equations, via boundary conditions when a separation interface is
present, or via source terms for dispersed condensed–
phase.
However, most systems are just too complex to be fully
described in their detail. Therefore one has to resort to
some modelling.
There are distributed–parameter models, in which state variabes are function of position and time, and lumped–
parameter models, in which variables may be only function of time or of a time-like variable.
Lumped parameter models, in contrast with distributed parameter models, are built by making assumptions on spatial uniformity of the state variables.
Models
Classic models for ideal chemical reactors, developed in chemical reaction engineering:
• Batch Reactor
• Continuous Stirred Tank Reactor (CSTR)
• Plug-Flow (tubular) Reactor
