Flames

Type of physical science: Flames, Fire, Thermodynamics, Chemistry

Field of study: Chemical reactions

Flames are regions in space that emit light and high temperatures through combustion of fuels. They serve purposes such as heating homes and producing electricity; however, uncontrolled, they may do damage to people and property.

Overview

A flame is a region of space that emits light and heat at a high temperature. It is the result of a combustion process in which one or more oxidation reactions occur between several substances. These substances are most often gases, but they may also be solids or liquids that can be vaporized. When a flame is produced on a large scale for a purpose such as to heat a home, the substances that are used most are hydrocarbon mixtures called fossil fuels. Fossil fuels are most often natural gas, coal, gasoline, or the fuel oils derived from petroleum. However, a huge number of substances composed of elements other than hydrogen and carbon are capable of combustion to produce flames. The heat and the light that are released by any flame are caused by the inherent, very exothermic (heat-releasing) chemical reactions that are involved in its production. The more exothermic the chemical reactions involved, the hotter and brighter are the flames.

Heat is released by a flame because of the breakage of chemical bonds that occurs in the combustion process. For example, when a candle burns, combustion turns the hydrocarbons it uses as fuel into carbon dioxide and water. Before the start of the combustion process, the hydrocarbon fuel is a mixture of chemical compounds composed of hydrogen and carbon atoms. These atoms are held together by chemical bonds, electrostatic forces that occur between atoms and hold them in complex shapes. When the combustion occurs, all the hydrogen and carbon atoms in each fuel molecule react with oxygen to form molecules of carbon dioxide and water. The breaking apart of the hydrogen and carbon atoms and their reforming into simpler molecules (carbon dioxide and water) releases the binding energy that held them together. Only part of this binding energy is needed to form water and carbon dioxide, so all the excess energy released produces heat (thermal energy) that raises the temperature of the flame and creates light.

The combustion process that results in a flame--at least with hydrocarbon fuels--is a complex sequence of events that produce a wide variety of substances intermediate in form between the fuel burned (one or more hydrocarbons) and the reaction products (water and carbon dioxide). These other substances, which are often free radicals, are unstable and propagate the combustion process by interacting with one another, with unburned fuel, and with the oxidizing agent needed for combustion. The oxidizing agent is usually the oxygen present in air. The very great variety of fuel substances and the myriad intermediate substances that form cause flames to have many different colors. For example, hydrocarbon flames usually range from light blue to yellowish. However, if a little calcium salt is added to the mixture, the flame turns red. Fireworks, which are flames that only last for a short time, are colored by adding various substances to their fuel.

Flames are usually classified as either premixed or diffusion flames. In a premixed flame, the fuel and the oxidizing agent are mixed together at room temperature and transferred to the area where the flame will occur. Once ignited, they burn quite rapidly. Whenever the ratio of fuel to oxygen is suitable in a premixed fuel sample, the mixture burns completely. One example of a premixed flame is a Bunsen burner in which both the fuel (mostly methane gas) and oxygen enter the bottom of the burner and mix before reaching the top of the burner where the flame forms.

The flame from a Bunsen burner has three distinct areas. An innermost dark region consists of a cool, unburned gas-air mixture. Just outside of this layer is a very thin region where the gas and the oxidant (oxygen from the air) begin to react, producing hydrogen, carbon monoxide, and many other substances. The outermost region of the flame is the brightest. In this part of the flame, all of the intermediate reaction products are converted to carbon dioxide and water and most of the heat is released. Premixed flames are either laminar (smooth flowing) or turbulent. Turbulence is produced when the gas flow into the flame is very rapid. If this gas flow becomes too rapid, the turbulence becomes so great that the flame blows out.

In a diffusion flame, the fuel is not premixed with the oxidant (usually oxygen from the air), which is diffused into the flame. A very good example of a diffusion flame is a candle. After the candle is lit, its solid fuel (a waxy mixture of hydrocarbons) melts, runs up the wick, and then vaporizes to maintain the flame. As the wax liquefies and vaporizes, air diffuses toward it and provides the oxidant. Such flames tend to be very bright and are suitable for lighting rooms. Their brightness is partly caused by the incomplete burning of their fuel and the resultant incandescent soot present in the flame. These smoky flames broadcast cooled soot into the air around them. Usually, diffusion flames are cooler than premixed flames using the same fuel because they burn the fuel less efficiently. They release less excess bonding energy to become heat.

Flames move (are propagated) through unburned mixtures of fuel and air via the transfer of heat and free radicals. The heat raises the temperature of the unburned mixture enough to enable combustion to begin. The free radicals produced trigger many combustion reactions in the heated areas of the mixtures. For laminar hydrocarbon flames, the maximum rate of propagation is about one hundred feet per minute. This propagation rate can be increased by making the flames turbulent.

To ignite a flame, it is necessary to provide enough energy to break the chemical bonds in the fuel and oxidant molecules. When a spark or pilot light is used, the ignition is termed forced ignition and the fuel-oxidant mix is called flammable. In the absence of a forced ignition source, ignition is caused by heating a mixture until the fuel becomes hot enough to ignite spontaneously. The temperature that must be reached to ignite the flame is called the flash point of the mixture. Typically hydrocarbons--usually in the form of liquids--reach their flash points at temperatures that cause about 1 percent of the volume of the fuel to be found in the vapor phase, where it can mix with air and burn. The flash points of different liquid fuels vary greatly. The higher the volatility (relative ability to evaporate) of a fuel, the lower its flash point. Larger and more complex hydrocarbons, which have lower volatilities, have higher flash points that make them less likely to accidentally ignite and safer to store and handle.

The temperature a flame reaches is important because many hydrocarbons are used as fuels for heating, and a fuel with a high flame temperature is better at heating than one with a lower flame temperature. Flame temperature depends on a number of factors, including the amount of heat the fuel releases, which is not necessarily easy to estimate. The heat released depends greatly on the nature of the gases produced by combustion and other factors such as the milieu in which the flame is produced. Flame temperature is determined in part by the specific heats (amounts of heat that will raise the temperature of a gram of a substance 1 degree Celsius) of the substances in the flame's surroundings. Because these factors vary greatly, combustion temperature values do not always match the calorific values of fuels. For example, the optimized flame temperatures of methane and the acetylene used in welding torches are 189 and 232 degrees Celsius, respectively, although methane has a much higher calorific value than acetylene. Its higher flame temperature makes acetylene preferable to methane for welding.

Applications

The use of diffusion flames--burning candles, lit kerosene lamps--for lighting was once a very common application. Though this use is pretty much relegated to a campsite or other remote location, diffusion flames are used in the heating of people's homes and businesses. Typically, fuel in a furnace or boiler is ignited by a pilot light. Air is drawn through a pipe from outside the building into the furnace where the oxygen in the air diffuses toward the flame and keeps it burning.

When the heat produced by diffusion flames is drawn to the walls of a furnace or boiler and used to heat air or water inside a second compartment, the flame used should be very bright. This maximizes the energy available to heat the external gas or water effectively. Use of a second compartment in such systems is necessary so that toxic chemicals such as carbon monoxide that are generated by the hot gas are not distributed throughout the home or business along with the heat. Power plants also use diffusion flames in gas turbines that generate steam to produce electricity.

Diffusion flames are also produced during unplanned fires such as house or forest fires, which can cause a great deal of damage to individuals and to society. Although people perceive these fires as burning solids, in reality most of the combustion derives from liquefaction and then vaporization of all the volatile components in the wood or other flammable material involved. The vapor forms and burns, and much of the charred material left behind actually provides relatively little fuel for the fire. Usually combustion that involves the nonvolatile components of a solid is flameless.

Understanding the concepts associated with spontaneous ignition and flash points of materials and the rate of movement of flames in, through, and away from accidental fires enables architects and builders to choose the safest possible building materials. It also helps fire-fighting agencies put out blazes. Shipping companies, whether they use railroads, trucks, or airplanes, can use a knowledge of diffusion flames to design or choose shipping vehicles and methods that will protect the cities through which their vehicles pass and help them fight any fires that occur as a result of transportation accidents.

Common premixed flames include Bunsen burners, furnace pilot lights, and the internal combustion engines in personal and industrial motor vehicles and airplanes. Devices that use premixed flames often require that the combustion chamber be separated from the user. This ensures that the potentially toxic gases produced leave the flame without hurting the user. For example, Bunsen burners are used either under fume hoods or in large, well-ventilated laboratories so that any toxic gases produced by the flame will be carried away and not present a danger for anyone. In motor vehicles and airplanes, the engine compartment is not connected to the cab or cabin in which users ride and is vented to the outside by an exhaust system that safely carries toxic gases away from anyone in the vehicles or airplanes.

Scientists use flames to detect and identify materials in flame photometry, a type of spectrochemical analysis. Flame photometry is an analytic technique that examines the nature of and measures the intensity of light emitted by substances whose atoms are excited by interaction with the thermal energy of a flame. This technique is especially valuable in detecting the presence of metals in a sample and determining how much of the metal is present. A solution containing a metal salt or a mixture of metal salts is sprayed into the flame that makes up one component of the photometer. The liquid in which the salt was dissolved evaporates and deposits the finely powdered metal salt in the flame. The salt is quickly vaporized and broken into atoms by the high temperature of the flame. The metal atoms are excited, that is, changed into higher energy forms by the heat. When the excited atoms lose their energy, they emit it as colored light. For example, potassium, barium, strontium, and calcium respectively turn the flame violet, green, and two different shades of red.

In flame photometry the exact color of the flame is determined and used to identify the metal in the sample. The intensity of the color of the flame reveals the amount of the metal present in the sample. The photometer uses flow meters to control carrier gas injection, an atomizer, a burner, and an optical system associated with photosensitive light detectors to make an exact identification of the metal. It also records the detector output. The amount of the metal present is determined by comparing the results with those produced by a sample that contains a known concentration of the salt. Flame photometry is used in many ways, ranging from identifying the exact composition of alloys so they can perform optimally in industrial processes to identifying substances in forensic science applications.

Context

No one knows who first discovered flames and put them to use. One theory is that primitive humans somehow recognized that hot embers could be removed from fires started after thunderstorms or by natural spontaneous combustion and used to produce flames where and when they needed them. Scientists believe flames were used to keep wild animals away, to cook food, for warmth in cold weather, and later to shape metals to yield better tools.

As humankind evolved, people learned to start fires using flints and other simple methods. Flames were put to use by shamans who cast metal salts into fires and interpreted the colors as bearing messages from the gods. As civilization progressed, people began to harness the energy produced by flames to power machines in factories and then vehicles. For example, James Watt used hot flames to produce steam that powered a very usable steam engine. Other people used the energy of flames to develop automobile engines, to power aircraft and submarines, and to run the machines that made mass production possible in factories.

As the use of flames spread through civilization, so did accidental fires. People began to look for ways to prevent fires by attempting to understand the properties of flammable materials such as flame velocity and flash point. This understanding was also put to use in developing ways to increase the usefulness of flames and resulted in improvements in industrial materials production and ways of moving surface and airborne vehicles. Hotter and hotter flames were produced as they were needed to melt and shape new materials.

Much of the research into flame use was conducted by physicists, engineers, and materials scientists. However, chemistry also played a huge role in flame research. Chemistry was used to identify the basis of the combustion process that led to flames and the toxic substances that were produced by flames and to help industry avoid the negative effects of both desired and undesired flames through the development of better fuels and safer combustion methods. In addition, the modern chemist harnessed the magic of ancient shamans, using flame colors to identify the content and makeup of metal alloys and other substances via flame photometry and several other techniques. Even physicians and biological scientists have explored flames. Their goals were to develop treatments for toxic gases and substances produced by flames.

In the future, more and more applications are likely to be developed for flames, and scientists will continue studying and solving the problems associated with flame use and misuse. Some of the applications that will arise probably will lead, as did the steam engine, to new categories of professionals who work with the applications or in industries or pursuits made possible by them.

Principal terms

COMBUSTION: Burning, a chemical change that is accompanied by the production of heat and light

DIFFUSION FLAME: A flame in which the fuel is not premixed with oxidant and whose burning depends on oxidant diffusion into the flame

EXOTHERMIC: A chemical process that occurs spontaneously and produces energy that is usually heat and/or light

FLASH POINT: The temperature at which a fuel-oxidant mixture becomes hot enough to ignite spontaneously

FORCED IGNITION: An instance where a flame is started by use of a spark or pilot light

FOSSIL FUEL: A common fuel obtained from natural sources (for example, heating oil) and used to produce flames for heating and for many other societal uses; they are hydrocarbons or hydrocarbon mixtures

FREE RADICAL: A reactive chemical substance that contains an unpaired electron

HYDROCARBON: A substance made up entirely of the elements hydrogen and carbon

LAMINAR: A premixed flame that is both smooth flowing and nonturbulent

PREMIXED FLAME: A flame produced in a mixture of fuel and oxidizing agent prepared at room temperature and before ignition

SPECIFIC HEAT: The amount of heat needed to raise the temperature of a gram of a substance 1 degree Celsius

VAPORIZATION: The process that turns a substance into a gas, its vapor form

Bibliography

Bradey, James E., and John R. Holum. Chemistry: The Study of Matter and Its Changes. New York: John Wiley & Sons, 1993. This college chemistry textbook covers many interesting aspects of the chemistry of combustion and flames and contains a discussion of flame photometry. It also provides many definitions of associated terms that will be useful to the reader.

Bradley, John N. Flame and Combustion Phenomena. New York: Barnes & Noble, 1969. This easy-to-read book briefly covers most important aspects of flames and combustion in a simple, interesting fashion. Contains helpful illustrations.

Fenimore, Charles P. Chemistry in Premixed Flames. New York: Macmillan, 1964. Many issues related to the chemistry of combustion occurring in premixed flames are described in detail. An extensive bibliography is included.

Fristom, R. M. Flame Structure and Processes. New York: Oxford University Press, 1994. This interesting book touches all bases. Examples deal with flame types, the basis of flame formation, and the properties of flames. Contains numerous bibliographic references and useful diagrams.

Gaydon, Alfred G., and Hans G. Wolfhard. Flames, Their Structure, Radiation, and Temperature. London: Chapman and Hall, 1970. This solid, well-illustrated book explores many useful topics related to flames and combustion. It is well worth reading by those wishing depth of coverage. Contains numerous bibliographic references.

Griffiths, J. F., and J. A. Barnard. Flame and Combustion. London: Chapman and Hall, 1995. This well-crafted book discusses many relevant topics associated with flames in an interesting and clear fashion. Included are numerous bibliographic references and useful illustrations.

Hess, Frederic O. Flame of Man. Philadelphia: Franklin Institute, 1969. This brief book simply describes the history of understanding and dealing with flames and combustion.