6 marzo 2006

Introduzione alla combustione

Gaetano ContinilloUniversita del Sannio, Benevento, Italy

6 marzo 2006

Introduzione

La combustione ed il suo controllo sono essenziali alla vita sulla Terra.

Per esempio l’85% dell’energia usata negli USA proviene dalla combustione [US DOE 1996 Annual Energy Review]

Altre fonti

Combustione

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Il trasporto è in massima parte alimentato dalla combustione (automobili, autocarri, aeromobili, natanti – eccetto le ferrovie)

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Numerosi processi industriali si basano estesamente sulla combustione:

• Le industrie del ferro, dell’acciaio, dell’alluminio ed altre industrie metallurgiche impiegano fornaci

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• Trattamenti termici sono realizzati in forni

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La combustione è alla base di molti altri dispositivi e processi industriali quali ad esempio i generatori di vapore, in generale le raffinerie, i forni di essiccazione, gli inceneritori di materie organiche.

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La fabbricazione dei cementi si basa su un forno rotativo in cui si produce il clinker)

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L’uso della combustione al termine del ciclo di vita (end-of-lifecycle) di un manufatto include l’incenerimento

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I processi di combustione richiedono sviluppo in termini:

• 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 CO2 concentration in the atmosphere.

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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.

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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.

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As the flame moves across the combustion space, temperature and pressure rise in the unburned gas.

Unburned fuel-air mixture Burned fuel-air mixture

Propagating flame Spark location

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Under certain conditions, rapid oxidation reactions occur at many locations

Autoigniting fuel-air mixture

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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.

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Oxidizer diffusing from outside

Flame sheet

Liquid fuel climbing for capillarity

Solid fuel

Vapor fuel diffusing from inside

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Important topics in the study of combustion are:

• Thermochemistry

• Chemical kinetics

• Molecular transport of mass and heat

• Fluid mechanics

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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.

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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.

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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.

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Example: first–order gas–phase bi–molecular reactions.

Chemical transformation events in a gaseous system occur in large amount if:

A Br kc c

0 exp /Ak 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)

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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.

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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.

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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.

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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.

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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

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Classic models for ideal chemical reactors, developed in chemical reaction engineering:

• Batch Reactor

• Continuous Stirred Tank Reactor (CSTR)

• Plug-Flow (tubular) Reactor