Fundamentals of Combustion Processes

Fuel and oxidizer are the two essential ingredients of a combustion process. Fuels can be classified as substances that liberate heat when reacted chemically with an oxidizer. Practical application of a fuel requires that it be abundant and inexpensive, and its use must comply with environmental regulations. Most fuels currently used in combustion systems are derived from non-renewable fossil sources. Use of these “fossil fuels” contributes to global warming effects because of the net-positive amount of carbon dioxide emissions inherent to their utilization. Fuels derived from biomass or from other renewable means represent potentially attractive alternatives to fossil fuels and are currently the subject of intensive research and development. Topics covered in this chapter include: (1) different types of fuels (fossil fuels, biofuels, and hydrogen), (2) fuel usage in the United States, (3) basic considerations when choosing a fuel, (4) identification of fuels by their molecular structure, and (5) properties of liquid fuels.

Although combustion processes often involve chemical reactions that may be far from equilibrium, the equilibrium state provides a useful guide on the ultimate combustion state if sufficient time is given. Chemical compositions of the combustion products at equilibrium, heating value of a fuel, and flame temperature can be determined from thermodynamics. In comparison to the thermodynamics of a pure substance, the thermodynamics of combustion systems are complicated by the change of components during combustion. That is, the components in the final state are different from those in the initial state. With the introduction of enthalpy of formation, the general approach normally used to solve thermodynamic problems of a pure substance can be extended to combustion systems. The following topics will be discussed in this chapter: (1) properties of mixtures, (2) combustion stoichiometry, (3) heating values and enthalpy of formation, (4) adiabatic flame temperatures, and (5) equilibrium state (Cantera Program).

While thermodynamics provides steady-state information of the combustion process, chemical kinetics describes the transient states of the system during the combustion process. Particularly important is information related to the rate at which species are consumed and produced, and the rate at which the heat is released. Combustion chemistry has two important characteristics not commonly observed in other chemical systems. First, combustion reaction rates are highly sensitive to temperature. Second, a large amount of heat is released during a chemical reaction. The rate at which fuel and oxidizer are consumed is of great importance to combustion engineering, as one needs to ensure sufficient time for chemical reactions when designing a combustion system. Chemical kinetics is the science of chemical reaction rates. When chemical kinetics is coupled with fluid dynamics and heat transfer, a combustion system can be characterized. The following topics will be discussed in this chapter: (1) the nature of combustion chemistry including global and elementary reactions, (2) global and elementary reaction rates and equilibrium, and (3) simplified models of combustion chemistry including the partial equilibrium and quasi-steady state assumptions.

The transport of heat and species generated by the chemical reactions is an essential aspect of most combustion processes. These transport processes can be described by the conservation equations commonly used in fluid and heat transfer analysis of engineering problems. Additional terms in the mass, momentum, and energy conservation equations account for the effects of the chemical reactions. This chapter briefly discusses heat and mass transfer, the equations governing combustion systems (conservation of mass, species, momentum, and energy), the normalization of the conservation equations, and simple expressions for the variation of viscosity, conductivity and diffusivity.

Ignition is the mechanism leading to the onset of a vigorous combustion reaction and is characterized by a rapid increase of temperature. An understanding of ignition is important in a wide range of combustion processes, from designing practical combustion devices to preventing unwanted fires. Ignition of a combustible material is often classified in two ways: spontaneous ignition, also known as autoignition, occurs through the self heating of the reactants, whereas piloted ignition occurs with the assistance of an ignition source. Topics included in this chapter are: (1) the thermal theory of spontaneous ignition of a gas phase mixture, (2) piloted ignition of a gas phase mixture, and (3) ignition of condensed fuels.

Premixed flames refer to the combustion mode that takes place when a fuel and oxidizer have been mixed prior to their combustion. Premixed flames are present in many practical combustion devices. Two such applications are a home heating furnace and a spark ignited internal combustion engine. In premixed flame combustors, the fuel and oxidizer are mixed thoroughly before being introduced into the combustor. Combustion is initiated either by ignition from a spark or by a pilot flame, creating a ‘flame’ that propagates into the unburned mixture. It is important to understand the characteristics of such a propagating flame in order to design a proper combustor. Some relevant engineering questions arise, such as: How fast will the flame consume the unburned mixture? How will flame propagation change with operating conditions such as equivalence ratio, temperature, and pressure? From a fire protection viewpoint, how can flame propagation be stopped? Topics covered in this chapter include: (1) the physical processes in a premixed flames, (2) flame speed and flame thickness, (3) flammability limits, (4) flame quenching, (5) minimum energy for sustained ignition and subsequent flame propagation, and (6) turbulent premixed flames.

In many combustion processes, the fuel and oxidizer are separated before entering the reaction zone where they mix and burn. The combustion reactions in such cases are called “non-premixed flames,” or traditionally, “diffusion flames” because the transport of fuel and oxidizer into the reaction zone occurs primarily by diffusion. Many combustors operate in the non-premixed burning mode, often for safety reasons. Since the fuel and oxidizer are not premixed, the risk of sudden combustion (explosion) is eliminated. Chemical reactions between fuel and oxidizer occur only at the molecular level, so “mixing” between fuel and oxidizer must take place before combustion. In non-premixed combustion the fuel and oxidizer are transported independently to the reaction zone, by convection and diffusion, where mixing of the fuel and oxidizer occurs prior to their reaction. Often the chemical reactions are fast, hence the burning rate is limited by the transport and mixing process rather than by the chemical kinetics. Consequently, greater flame stability can be maintained. This stable characteristic makes diffusion flames attractive for many applications, notably aircraft gas-turbine engines. Topics covered in this chapter include: (1) a detailed description of a candle flame, (2) the structure of non-premixed laminar jet flames, (3) theoretical and empirical expressions for laminar jet flame height, (4) Burke-Schumann jet diffusion flames, (5) turbulent jet flames including liftoff height and blowout limit, and (6) a short discussion of condensed fuel fires.

Liquid fuels are widely used in various combustion systems for their ease of transport and storage. Due to their high energy content, liquid fuels are the most common fuels in transportation applications. Before combustion can take place, liquid fuel must be vaporized and mixed with the oxidizer. To achieve fast vaporization, liquid fuel is injected into the oxidizer (normally air) at high speeds. Soon after injection, the liquid fuel breaks up into droplets, forming a spray. Droplets then collide and coalesce, producing droplets of different sizes. Due to the high density of liquid fuel, the momentum of the liquid spray has a profound impact on local flow fields, creating turbulence and gas entrainment. In piston engines, the complexities of droplet combustion are further complicated by the occurrence of successive multiple transient events including gasification, ignition, flame propagation, and, ultimately, burn-out. As such, droplets can be considered the building block for providing fuel vapor in combustion systems. Understanding of single-droplet evaporation and combustion processes therefore provides important guidance in design of practical burners. Topics covered in this chapter include (1) droplet evaporation in both quiescent and convective environments, (2) droplet combustion, (3) initial droplet heating, and (4) characterization of droplet distributions.

Emissions from combustion of fossil fuels are of great concern due to their impact on the environment and public health. The primary combustion products, carbon dioxide (CO₂) and water (H₂O), affect the environment through greenhouse effects and potential localized fog. Both products are inherent to the combustion of fossil fuels and their emission can only be reduced through modifications in the fuel or by exhaust treatment. The other major pollutants from combustion are secondary products and include carbon monoxide (CO), unburned hydrocarbons (HC), soot, nitric oxides (NO_x), sulfur oxides (SO_x), and oxides of metals. Pollutants cause health problems in humans and animals and can contribute to acid rain. Topics included in this chapter are: (1) the negative effects of combustion products, (2) parameters affecting the formation of pollutants, (3) pathways of pollutant formation and methods of control, and (4) methods of emission quantification.

Internal combustion (IC) engines have been moving the industrial world for over three centuries. A type of IC engine is the spark ignition (SI) engine, where the fuel and oxidizer are premixed prior to entering the engine. Because of the high power density, low cost of production, and the vast infrastructure for gasoline, SI engines are ideal power platforms for passenger cars, small trucks, motorcycles, lawn mowers, and small electrical power generators. SI engines are robust and capable of producing high levels of power at wide speed ranges. Current opportunities for internal combustion engine development include efficiency improvement, novel fuel implementation, and pollution reduction. Topics in this chapter include: (1) principles of SI engines, (2) thermodynamic analysis, (3) discussion of the octane number, (4) fuel preparation, (5) ignition timing, (6) flame propagation, (7) modeling of SI engine combustion, (8) emissions and their control including catalytic converters, and (9) gasoline direct injection engines.

Another type of IC engine is the compression ignition (CI) or diesel engine, where the fuel is injected into the engine cylinder after compression of the air. The advantage of a CI engine is the possibility of attaining high compression ratios and consequently high efficiencies. Modern compression-ignition engines (diesel engines) have evolved from the 3:1 compression ratio engine that Rudolph Diesel built in 1890 to compression ratios up to 20:1 with high-pressure fuel injection systems, outputting up to 10,000 hp. CI engines are generally found on heavy-duty trucks, construction vehicles/equipment, stationary power generators, trains, and large ships because of the higher power output required. Topics included in this chapter are: (1) overall comparison to SI engines including advantages and disadvantages, (2) thermodynamic analysis, (3) diesel spray and combustion, (4) discussion of the cetane number, (5) emissions and their control, and (6) homogeneous charge compression ignition engines (HCCI).