introduction to combustion

introduction to combustion

MAJOR AREAS OF COMBUSTION APPLICATION

It is fair to say that the ability to use fire is an important factor in ushering the dawn of civilization. Today our dependence on the service of fire is almost total, from heating and lighting our homes to powering the various modes of transportation vehicles.

Useful as it is, fire can also be menacing and sometimes deadly.Wildland and urban fires cause tremendous loss of property and lives every year; the noxious pollutants from automotive and industrial power plants poison the very environment in which we live; and the use of chemical weapons continues to be an agent of destruction with ever greater efficiency. Combustion is certainly one branch of science that affects almost every aspect of human activities.

Practical combustion problems can be roughly divided into the following five major categories, in each of which we cite some examples of current interest.

1- Energy and Combustion Devices: Despite the large variety of alternate energy sources available, such as nuclear, solar, wind, hydroelectric, geothermal, and OTEC(ocean thermal energy conversion), chemical energy derived from burning fossil fuels supplies a disproportionately large fraction of the total world energy needs—around 85 percent at present. This trend will continue in the foreseeable future because of its convenience, high-energy density, and the economics.

Combustion energy is mainly used to generate heat and power. Examples of this application are domestic heating, firing of industrial furnaces, and the operation of automotive engines and gas turbines. Hence the design and operation of heat and power devices and engines is closely related to the issue of efficient energy utilization.

Because of the importance of transportation vehicles as a major consumer of petroleum fuels and contributor of air pollution

there has been extensive development since the early 1970s for more efficient and cleaner burning internal combustion engines for automobiles. For example, the diesel engine offers substantial advantage over the more widely used gasoline engines, for several reasons. First, even though

its combustion cycle efficiency is less than that of the gasoline engine for the same compression ratio, it is more efficient overall because it operates at higher compression ratios. Furthermore, unlike the gasoline engine, which requires highly refined fuels with narrow specifications, the diesel engine is very fuel tolerant. Thus diesel fuel requires less refining than gasoline and, consequently, results in a net saving in processing energy at the refinery stage. 

This property of fuel tolerance also implies that the diesel engine is a good candidate for the use of unconventional or low-grade fuels. The diesel engine, however, does have the potential disadvantages of being relatively noisier and a heavy emitter of soot and oxides of nitrogen (NOx); both problems have their origin in its operational principle and therefore require fundamental combustion research. It is nevertheless gratifying to note that much progress has been made recently in alleviating these problems.

An important concept in engine development is that of stratified charge combustion. The basic idea is that the combustion of lean mixtures has the potential of simultaneously increasing the combustion efficiency and reducing the formation of most pollutants. Lean mixtures, however, are hard to ignite. Therefore, the concept of stratified charge combustion is to stratify an overall fuel lean mixture from relatively rich to ultra lean. Since the relatively rich portion can be ignited easier, the hot combustion products so generated can in turn ignite the ultra lean portion of the charge. 

Thus by combining the merits of high-pressure combustion, direct fuel injection for uniform cylinder-to-cylinder charge distribution and controlled fuel vaporization, spark ignition for controlled ignition event, and stratified charge combustion, there has been considerable development on high-compression-ratio, direct-injection, spark-assisted, stratified charge engines.

In contrast to stratified charge engines, there is also

there is also considerable interest in the development of HCCI (homogeneous charge compression ignition) engines.

Conceptually, by having reaction taking place homogeneously within the entire engine cylinder, instead of being confined to localized, high-temperature regions constituting the flames, the formation of soot and NOx can be substantially reduced.

Furthermore, higher compression ratios and hence higher efficiency can be attained with compression ignition.

The fact that improvements in the engine performance can be pursued through the opposite concepts of stratified and homogeneous charges not only demonstrates the complexity of the combustion phenomena underlying such technological processes, but it also highlights the richness of the possible avenues that can be explored for optimization.

2- Fuels: Combustion needs fuel. Furthermore, the satisfactory operation of different heat and power engines usually depends critically on the compatibility of the fuel used. Examples are the unsuitability of diesel fuel for use in gasoline engines because it is relatively less volatile, and the narrow compositional specifications of gases which can be used in domestic gas stoves in order to maintain flame stabilization by avoiding blowoff and flashback.

The importance of fuel in combustion has been receiving increased interest because of the concern over the shortage and reliability of petroleum supply. Thus “energy crisis” is simply a “fuel crises.” Since the world’s petroleum supply is projected to be severely depleted within this century, the long term solution for the next few centuries in terms of fossil fuels appears to largely depend on the burning of coal, either through direct utilization or as coal-derived fuels. Two approaches for direct coal utilization are being actively pursued. The first is fluidized-bed combustion, in which air is introduced through the bottom of a bed of coal particles at a sufficiently fast rate such that the particles are levitated, that is, fluidized. This approach has the advantages that the coal particles are in direct contact with the oxidizing air such that their burning rates are maximized, that neutralization of oxides of sulfur (SOx) can be facilitated by mixing limestone with the coal particles, and that the production of NOx can be minimized by controlling the fluidization rate. The second approach for direct coal utilization is the burning of coal–water slurries. Here, finely crushed coal particles of sizes ranging between 40–70 μm are mixed in water and sprayed directly into the combustion chamber of industrial furnaces. The advantages are that the physical processes of coal crushing and mixing are less energy expensive than the chemical process of coal liquefaction, and that the slurries can be transported through pipelines and subsequently directly burned in conventional oil-fired combustors.

This requires minimum hardware modification, and thereby capital outlay and combustor downtime. Slurries up to 70 percent coal content have been successfully

burned.

Oil can also be derived from coal. These coal-derived oils have higher boiling points, wider boiling point ranges, and higher contents of aromatics and nitrogencontainingcompounds. Consequently, they tend to produce more soot and NOx.

Various alternate and hybrid fuels have also been formulated. Prominent among these are methanol, ethanol, and mixtures of ethanol with oil. Methanol can be derived from natural gas and coal, while both methanol and ethanol can be produced from biomass. Alcohols have smaller heats of combustion because of the extra oxygen atom in the molecule. 

However, they have higher knock ratings in gasoline engines and produce less NOx and soot. 

Blends of ethanol and gasoline, and methanol and gasoline, have been successfully marketed.

Coal, of course, can also be gasified in the presence of air, with or without steam, to produce a combustible gaseous fuel that consists of hydrogen and carbon monoxide.

Coal gasification becomes progressively more attractive as a source of clean fuel with the dwindling supply of natural gas.

3- Pollution and Health: The major pollutants from combustion are soot, SOx, NOx, unburned hydrocarbons (UHC), and carbon monoxide (CO). As just mentioned, soot is expected to be a serious problem with the burning of coal-derived fuels and the large-scale deployment of high-compression engines such as the diesel. Soot not only is unsightly but can also be carcinogenic due to the condensation and thereby presence of carcinogenic liquid combustion products on the particle surface.

The main source of SOx is from burning coal. When combined with water in the atmosphere, the emitted SOx forms sulfuric acid and precipitates as acid rain, with devastating effects on aquatic life and soil erosion.

NOx can be formed from either the N2 in the atmosphere or the nitrogen atoms in the fuel molecules, with the former produced under high-temperature, intense combustion situations because of the need to dissociate the nominally inert N2 in theair. 

Fuel-bound NOx is less temperature sensitive and could be a major contributor of NOx emission from burning coal or coal-derived oils. When it reacts with UHC and ozone in the presence of sunlight, NOx forms smog that is detrimental to the respiratory system.

A problem of potential concern is indoor pollution.With houses being better insulated to conserve energy, the trace pollutants (CO, NOx, UHC), from such domestic heating devices as the gas stove, furnace, and kerosene heater, may exist at sufficiently high levels as to be injurious to health.

There is also interest in applying combustion technology in the management of municipal, munition, and chemical hazardous wastes through incineration. The problems with burning these wastes are the uncertainty of the toxicity of the combustion intermediates and products and the fact that some of the chemicals are halogenated compounds, which can be resistant to efficient burning because of the scavenging of the crucial hydrogen atom by the halogen radicals in the oxidation process.

A serious, and potentially catastrophic, environmental problem is global warming caused by the increased amount of anthropogenic CO2 in the atmosphere. Since CO2 is a by-product of hydrocarbon combustion, suggestions have been made to use hydrogen as the primary fuel source. In the event that hydrogen is derived through the conversion of hydrocarbons, CO2 is still produced during conversion and needs to be sequestered properly in order to prevent its release into the atmosphere.

A discussion on the adverse effects of combustion on health would not be complete without mentioning the well-established cancer-causing consequence of cigarette smoking, which is simply the slow combustion of tobacco leaves.

 The knowledge of combustion science has not been sufficiently brought to bear on this problem of

immense importance

4- Safety: This topic can be divided into three categories, namely fires, explosions, and materials. Fires, both structural and wildland, are costly in terms of human suffering as well as financial loss. Problems of interest include improving fire detection technology and understanding the dynamics of fire propagation in confined spaces such as buildings and aircraft cabins.

Explosions are of concern to safety in mine galleries and grain elevators, as a consequence of LNG (liquefied natural gas) spills or rupturing of pressurized hydrogen storage tanks in urban areas, and in nuclear reactor accidents. In the last example, hydrogen gas is generated and could accumulate in sufficient quantity to cause an explosion. This would in turn rupture the reactor containment structure, causing the release of radioactive gases into the environment.

Since the inhalation of smoke and the toxic products of combustion is a cause of fatality in fires, the choice of materials for structure and decoration is also an

important consideration in the overall strategy for fire control.

A strategy toward the prevention of fires and explosions in aircraft and combat vehicles, such as tanks, is the development of fire-safe fuels which, while burning well within the engine, will not catch fire upon spillage. For example, diesel oil emulsified with a small amount of water has been found to be fire resistant.

5- Defense and Space: The various defense establishments are interested in the formulation of high-energy munitions and propellants; the suppression of combustion instability within jet engines, rockets and guns; signature and detection vulnerability from the exhausts of jet engines and rockets; and measures at preventing explosion of fuel tanks when being penetrated by projectiles. The development of chemical lasers as an intense power source and of hypersonic aircraft up to Mach 25 are also of interest to the national defense.

Since combustion experiments conducted on earth are frequently complicated by the presence of buoyant flows, there has been much interest to conduct these experiments in the weightless environment of a space shuttle or station. The intrusion of buoyancy is particularly problematic when the burning is slow as in the propagation of a flame in a weak mixture, or for long-duration phenomena such as smoldering.

The presence of buoyancy can also distort the flame configuration from an otherwise symmetrical one, and hence significantly complicates data reduction as well as theoretical analysis or computational simulation of the phenomenon of interest.

Fire safety is of paramount interest in space exploration. For example, while earthbound smoke detectors of incipient fires are placed at the ceiling of a room in order to capture the buoyancy-driven, upwardly rising smoke, they are clearly inoperative in the weightless space environment. Furthermore, flammability standards

SCIENTIFIC DISCIPLINES COMPRISING COMBUSTION

Combustion is the study of chemically reacting flows with rapid, highly exothermic reactions. It is interdisciplinary in nature. 

1- comprising thermodynamics

2- chemical kinetics

3-  fluid mechanics

4-  transport phenomena 

CLASSIFICATIONS OF FUNDAMENTAL COMBUSTION PHENOMENA

we introduce the various classifications of fundamental combustion phenomena and the terminology usually associated with them.

1- Premixed versus Nonpremixed Combustion.

2- Laminar versus Turbulent Combustion.

3- Subsonic versus Supersonic Combustion.

4- Homogeneous versus Heterogeneous Combustion.

FUNDAMENTALS OF COMBUSTION

Air–Fuel Ratios

Combustion is rapid oxidation, usually for the purpose of changing chemical energy into thermal energy—heat. This energy usually comes from oxidation of carbon, hydrogen, sulfur, or compounds containing C, H, and/or S. The oxidant is usually O2—molecular oxygen from the air.

To ensure complete combustion of the fuel used combustion chambers are supplied with excess air. Excess air increase the amount of oxygen to the combustion and the combustion of fuel.

when fuel and oxygen from the air are in perfect balance - the combustion is said to be stoichiometric

The combustion efficiency increases with increased excess air - until the heat loss in the excess air is larger than the heat provided by more efficient combustion.

Fuels

Fuels used in practical industrial combustion processes have such a major effect on the combustion that they must be studied simultaneously with combustion.

1- Gaseous fuels

 are generally easier to burn, handle, and control than liquid or solid fuels. Molecular mixing of a gaseous fuel with oxygen need not wait for vaporization or mass transport within a solid. Burning rates are limited only by mixing rates and the kinetics of the combustion reactions; therefore, combustion can be compact and intense. Reaction times

as short as 0.001 sec and combustion volumes from 10⁴ to 10⁷ Btu/hr⋅ft³ are possible at atmospheric pressure. Gases of low calorific value may be so dilute after mixing with air that their combustion rates will be limited by the mixing time.

2- Liquid fuels

 are usually not as easily burned, handled, or controlled as gaseous fuels. Mixing with oxygen can occur only after the liquid fuel is evaporated; therefore, burning rates are limited by vaporization rates. In practice, combustion intensities are usually less with liquid fuels than with high calorific gaseous fuels such as natural gas

Because vaporization is such an integral part of most liquid fuel burning processes,

much of the emphasis in evaluating liquid fuel properties is on factors that relate to vaporization.

One of the most critical properties is viscosity, which hinders good atomization, since atomization or the creation of small droplets is the primary method for enhancing vaporization. Much concern is also devoted to properties that affect storage and handling because, unlike gaseous fuels that usually come through public utility main pipelines, liquid  fuels must be stored and distributed by the user.

3- Solid fuels 

are frequently more difficult to burn, handle, and control than liquid or gaseous fuels. After initial devolatilization or release of volatile matter, the combustion reaction rate depends on diffusion of oxygen into the remaining char particle, and the diffusion of carbon monoxide back to its surface, where it burns as a gas. Reaction rates are usuallylow and required combustion volumes high, even with pulverized solid fuels burned in suspension.

Some cyclone combustors have been reported to reach the intensities of gas and oil flames.

4- Waste or by-product fuels and gasified solids

 are being used more as fuel costs rise.

Operations that produce such materials should attempt to consume them as energy sources or to sell them as a fuel to others. Problems with handling the lack of a steady supply, and pollution problems often complicate such fuel usage.

For the precise temperature control and uniformity required in many industrial heating processes, the burning of solids, especially the variable quality solids found in wastes, presents a critical problem. Such fuels are often left to very large combustion chambers, particularly boilers and cement kilns. When solids and wastes must be used as heat sources in small and accurate heating processes, a better approach is to gasify them to produce a synthesis gas, which can be cleaned and then controlled more precisely.