Flames and Burning

Experiment 1

FLAMES AND BURNING

Part of the discussion below on flames and fuels is taken from “The Chemistry of Flames”, by William C. Gardiner, Jr., Scientific American, February, 1982, p. 110.

If one burns wood in air the amount of material left as ashes is less than the amount of wood originally; it appears that part of the wood has disappeared. In 1774, Lavoisier realized that the apparent disappearance of matter (such as wood) when it is burned is an illusion. He showed that an invisible component of air (which he later named oxygen) reacts chemically with matter at high temperatures to give heat and a variety of chemical products. Some of these products are gases and carry some of the matter away into the air, giving the illusion that the matter disappears. These concepts today define a fuel: a substance that can participate in an exothermic (heat evolving) chemical reaction with oxygen.

The reaction of matter with oxygen is called combustion if it occurs rapidly and gives off heat. Rapid reaction generally takes place at high temperatures. At low temperatures a slow reaction with oxygen usually occurs and when the reaction involves a metal it is called corrosion (rusting if the metal is iron).

The study of the combustion of fuels has taken on added significance over the past several decades owing to increased awareness of the finite supply of comparatively inexpensive fossil fuels (oil and coal) and of the injurious effects of some of the combustion products on the environment and human health. The practical objective of the study of combustion and fuels has been to learn how to burn the cheapest available fuels as efficiently, intensely and cleanly as possible.

The earliest fuels were undoubtedly wood and camel (or other animal) dung. For the past 100 years or so, there has been a steady increase in the use of petroleum products, natural gas and coal (and things derived from them) as fuels. There are some things that satisfy the strict definition of a fuel such as aluminum and ammonia but they require more energy to produce than they liberate when they burn so they are not considered practical.

Most practical fuels in use today consist of compounds that are composed of carbon and hydrogen (hydrocarbons) or are composed mostly of carbon and hydrogen and some oxygen also. There are thousands of different hydrocarbons. Some examples of hydrocarbons used as fuels are

CH4 methane C3H8 propane C4H10 butane C2H2 acetylene

Methane is the major component of natural gas. Propane is sold in small metal cylinders for heating purposes such as soldering copper tubing (called “sweating”) in plumbing. Butane and/or propane are used as fuels to heat homes where natural gas is not available. Acetylene is used to cut and weld steel in shipyards, etc.

Gasoline is a mixture of hydrocarbon compounds whose molecules have from 5 or 6 carbon atoms to 12 or 13 carbon atoms. Gasoline also has some other additives to make it burn smoothly in the automobile engine and also to burn more efficiently so as to reduce air pollution.

Using methane as an example, the reaction of a hydrocarbon fuel with oxygen can be written as shown in Table 1. The g, l and s in parentheses behind the formulas indicate whether the compound is a gas, liquid or solid in the reaction which is important in regard to the amount of heat evolved. The information in Table 1 indicates the following:

1. Complete combustion requires the largest ratio of oxygen to fuel. The ratios given in Table 1 are the mole ratios of oxygen and air to methane for each reaction. Avogadro’s principle says that the same number of molecules of any gas are contained in the same volume (T and P fixed) so that the ratios in Table 1 are also the volume ratios needed (at fixed pressure). Since air is 20% oxygen, one must use 5 times as much air as oxygen to obtain the oxygen to methane ratio of 2 to 1 needed for complete combustion. In fact a larger amount of oxygen than the amounts shown in Table 1 would be useful to insure complete combustion.

2. Complete combustion gives the most heat per gram of fuel.

3. Complete combustion gives the (relatively) harmless products CO2 and water. Incomplete combustion (which occurs if insufficient oxygen is available) gives poisonous carbon monoxide and sooty elemental carbon.

Table 1. The Reaction of Methane with Oxygen

In the field of combustion science, the term fire refers to either an accidental or deliberate combustion event in the “real” world (outside the laboratory) such as a forest fire or a furnace fire. Flames are subunits of a fire. Flames are generally classified into two categories:

1. Premixed flames - the fuel and oxygen are mixed prior to ignition. Gasoline and air are mixed in the carburetor and intake manifold of an internal combustion engine (automobile engine) and after compression are ignited in the cylinder of the engine. A laboratory burner (see below) burning with a blue flame has the air and fuel gas mixed in the barrel and ignited at the top of the barrel. Premixed flames are more likely to give complete combustion although this does not have to be the case.

2. Diffusion flames - no pre-mixing occurs; the fuel and oxygen come together and burn in the combustion zone. Vaporized wax and oxygen from air meet above a candle wick to burn. A laboratory burner with its air intake opening closed has pure fuel gas going up the barrel to meet with air at the top of the barrel and burn. The burner flame under these circumstances is yellow due to glowing carbon produced because of the incomplete combustion of the fuel. A candle flame is similarly yellow.

Diffusion is the movement of the molecules of two gases into the spaces between molecules of each other. The fuel gas molecules and oxygen molecules diffuse into each other in order to burn in a diffusion flame.

In the case of pure solid carbon it is not possible to have pre-mixing without powdering the carbon prior to burning. This is done in power plants that use coal (which is primarily carbon) to generate electricity. The coal is pulverized into a fine powder that is fed into the combustion zone with the air. This gives more complete combustion. The reaction for complete combustion of carbon is C + O2 ----> CO2 + H2O

However, when one burns briquettes (which are primarily carbon) in a barbeque grill, the carbon is not in small particles and complete combustion does not occur. Some CO is produced in addition to the CO2. Briquettes should not be burned in a closed space or the buildup of CO may become toxic.

Given in Table 2 are some chemical equations representing the overall reaction that occurs in some flames. Also given is the temperature the flame produces and the energy (as heat) that is released per gram of reactant mixture (includes the weight of the oxygen and nitrogen (if air) also).

Table 2. Characteristics of Various Fuels

Reaction of hydrocarbons with oxygen is an example of a type of chemical reaction that is classified as an oxidation reaction. The oxygen is said to oxidize the compound it reacts with and the oxygen is called an oxidizer. Other compounds that react similarly with hydrocarbons and other fuels (not necessarily to produce CO2 and/or CO and water) are also called oxidizers in this context. The N2O4 in the methyl hydrazine-dinitrogen tetroxide reaction in Table 2 is classified as an oxidizer in that reaction.

Hydrogen is the only fuel in Table 2 that is not a hydrocarbon or a compound that contains appreciable carbon. Fuels do not have to be carbon compounds but most are. In principle one of the reactants does not have to be oxygen in order for the reaction to be used to produce heat as is shown by the methyl hydrazine - dinitrogen tetroxide reaction. Any chemical reaction that is exothermic will serve. However, in practice, reasonable fuels are those that react with oxygen since that is the most readily available oxidizer.

In most cases the mixture of fuel and oxidizer must be ignited using a match or a spark after which the combustion sustains itself. The methyl hydrazine - dinitrogen tetroxide mixture, however, ignites spontaneously on contact. This reaction has been used in the small rocket engines used to rotate and maneuver the space shuttle after it has reached orbit.

Hydrogen/oxygen and cyanogen/oxygen flames are used in the laboratory for high temperature spectroscopic studies. Producer gas is a mixture of CO and H2 and is a by product of some industrial processes or can be made from coal and water.

II. LABORATORY BURNERS

A laboratory burner is a device which burns a fuel in air to produce heat. The fuel is often natural gas (methane). Burners of the type shown in Figure 1 are often generically called Bunsen burners because the first burner of this type was invented by Robert Bunsen in 1855. The smaller burner used in this laboratory is a Terrill burner. The larger burner is a Meker or Fisher burner.

If the air inlets are completely closed (by different methods on different burners), a yellow luminous flame results. No air is entering the barrel at the bottom and the gas must mix with air by diffusion at the top leading to incomplete combustion. This gives carbon (also known as soot in this context) which is heated by the flame until it glows due to incandescence, causing the yellow color and the luminosity of the flame. The air inlets cannot be completely closed on the Meker burner so it does not exhibit the yellow flame.

As the air inlets are opened, the flame goes from yellow to blue. In this mode of operation, air enters the bottom of the barrel of the burner and premixes with the gas before burning at the top of the barrel. Complete combustion burns the gas to carbon dioxide and water. In the process of going from methane to carbon dioxide, some carbon monoxide is formed and converting this carbon monoxide to carbon dioxide by reaction with oxygen (actually an OH radical) produces the blue color of the flame.

The yellow flame should not be used for heating purposes. If less heat is needed, reduce the gas and air flow but keep the blue flame.

If too much air is admitted, the flame may “strike back”, i.e., burn back down inside the barrel. This can make the burner very hot and can even melt the rubber hose connecting the burner to the gas valve which may lead to a fire in the laboratory. The heavy grid on the top of the Meker burner is designed to conduct heat away from the base of the flame and prevent the ignition of gas inside the barrel.

If one places a wire in the burner flame, an indication of the temperature can be obtained by observing the color of the wire. As the wire is heated the atoms in the solid wire vibrate with greater frequency and emit light whose wavelength is dependent on the temperature of the wire. Table 3 gives an approximate relationship between the temperature and the color of the wire.

Table 3. Temperature and Color of a Glowing Wire

Temperature, oC Color

600 - 900 dull red

900 - 1000 cherry red

1000 - 1200 orange to yellow

1200 - 1500 white to brilliant white

above 1500 dazzling white