Fire Characteristics: Gaseous Combustibles

 

 

 

 

MINISTRY OF EDUCATION AND SCIENCE

Federal State Educational Establishment higher education

"Zabaikalsky State University"

(FGBOU VPO "ZabGU")

 

 

 

 

 

 

 

 

Fire Characteristics: Gaseous Combustibles

 

 

Third Edition

Principles of Fire Protection Chemistry and Physics

Raymond Friedman

National Fire Protection Association, wpi Quincy, Massachusetts

 

 

 

 

Prepared: Sosnovchik Y.F.

 

Checked: Master of Sciences,

 

Kozykin V. А.

 

 

 

 

 

 

 

 

 

 

 

 

 

CHITA – 2013

 

 

Сontent.

 

Preface………...…………………………………………………………………3

1. Chemical Elements and Compounds: Atoms and Molecules…………..….4

1.1.  What is an atom………………………………………………...…………...4

1.2. How stable is an atom……………………………………………….…….…6

1.3. What are the mass and size of an atom………………………………….….. 7

 

 

2. Physical and Chemical Change………………………………………….…..7

2.1. States of matter…………………………………………………………..…..7

2.2. Properties of Gases…………………………………………………….…….8

2.3. Properties of Liquids………………………………………………………...9

2.4. Properties of Solids…………………………………………………….….9

 

3. Fire Characteristics: Gaseous Combustibles……………………………….11

3.1. Types of gaseous flames…………………………………………………….11

3.2. Premixed Versus Diffusion Flames……………………………………..…..11

3.3. Laminar Versus Turbulent Flames……………………………………….…12

3.4. Stationary Versus Propagating Flames………………………………...……13

3.5. Ignition of gases………………………………………………………....…..13

Conclusion…………………………….………………………………………....15

Literature……………………………….……………………………………….16

Vocabulary…..………………………….……………………………………….17

 

 

 

 

 

 

 

 

Preface.

 

Fire is basically a chemical reaction between combustibles and the oxygen of the air. The product of fire (smoke) consists of a complex mixture of chemicals that interact with humans trying to fight a fire or escape from a fire. Extinguishing agents and fire retardants are also chemicals. Clearly, an understanding of chemistry is a prerequisite to a thorough understanding of fire protection. Certain branches of physics are also important in the understanding of fire. The rate of air mixing into flames, the buoyant rise of fire gases to the ceiling and the subsequent motion under the ceiling, and the escape of smoke from the fire compartment into connecting compartments are all key elements of the behavior of a fire. The rate at which heat is transferred from the flames to not yet ignited material or to humans trying to escape the fire is also clearly an important aspect. Finally, any attempt to understand the computer models now available for predicting fire behavior requires a knowledge of the underlying physics as well as the underlying chemistry. Part I of this book is an elementary review of selected fundamentals of chemistry and physics that are most relevant to fire. Some readers who are already well trained in chemistry and physics will want to skip this part and proceed directly to Part II. Other readers might appreciate the opportunity to refresh their memories on basic aspects of chemistry and physics. Many terms in the text are defined in the glossary at the back of the book. For those readers who have never been exposed to chemistry or physics instruction, it will be difficult to obtain complete mastery of all concepts in this book. Such readers would benefit from studying an introductory textbook on chemistry or physics, such as Introduction to Chemistry, 7th ed., by T. R. Dickson (J. Wiley, New York, 1995,589 pp.), or Fundamentals of Physics, 5th ed., by D. Halliday, R. Resnick, and J. Walker (J. Wiley, New York, 1997, 984 pp.).

 

 

 

 

1. Chemical Elements and Compounds: Atoms and Molecules

1.1.  What is an atom?

There are 109 known chemical elements. Some of the more familiar elements are carbon, oxygen, hydrogen, helium, chlorine, iron, copper, lead, silver, and gold. Elements consist of atoms, which are tiny particles characteristic of the various elements.

Each atom consists of a relatively heavy nucleus having a positive electric charge, and a surrounding cloud of orbiting electrons, which are negatively charged and relatively light. More than 99.9 percent of an atom's mass is concentrated in the nucleus.

Each element consists of atoms unique to that element and different from the atoms of all other elements. Each element has been assigned an atomic number. The atomic number of hydrogen is 1; the hydrogen atom consists of a nucleus with an electric charge designated as +1, about which orbits an electron with a charge of -1. Helium is the second element, with an atomic number of 2; its atoms each have a nucleus with an electric charge of +2 (i.e., twice that of hydrogen). Two electrons orbit around each helium nucleus. The atoms of the third element, lithium, have a nuclear charge of +3 and three orbiting electrons per atom ... and so forth, through the list of elements.

Earlier in this century this model would have been presented as a probable theory of how the elements differ from one another. Today, however, sophisticated scientific measurements confirm the correctness of the model.

Table 2.1 presents a partial list of the chemical elements, including most elements mentioned in this book. The properties of all 109 elements can be found in the Handbook of Chemistry and Physics [1], along with historical information on the discovery of each element.

One might infer, from the description presented so far, that a helium atom is two times as heavy as a hydrogen atom, a lithium atom three times as heavy as a hydrogen atom, and so on. Examination of the atomic weights shown in Table 2.1 makes it clear that this is not the case; for example, a lithium atom is about seven times as heavy as a hydrogen atom even though the electrical charge within its nucleus is only three times as large. This concept was difficult to explain until the neutron was discovered in 1930. The neutron is a particle with almost exactly the same mass as the nucleus of a hydrogen atom (which is called a proton), but the neutron is uncharged (electrically neutral) while the proton has a positive unit charge. The relative masses of the various kinds of atoms become understandable if each nucleus is considered to consist of a combination of protons and neutrons, which are bound together very tightly.

For example, if a helium nucleus consisted of two protons and two neutrons, it would have twice the electric charge and four times the mass of the hydrogen nucleus (a single proton). This is indeed the case. Likewise, a lithium nucleus consists of three protons and four neutrons and has seven times the mass of a hydrogen nucleus.

The atoms of the heaviest naturally occurring element, uranium, with atomic number 92, have 92 protons and about 146 neutrons in each nucleus and a mass 238 times that of a hydrogen nucleus. Notice the words "about 146 neutrons" in the previous sentence. Actually, a small percentage of uranium atoms have 92 protons and 143 neutrons in the nucleus. These atoms, which are slightly different forms of the same element, are called isotopes. The two isotopes of uranium are called U-238 and U-235, respectively.

The element chlorine is a mixture of two isotopes, Cl-35 and Cl-37. Each of these two isotopic forms has an electric charge of +17. The two isotopes contain 18 and 20 neutrons, respectively. The chlorine found on earth contains a larger proportion of Cl-35 than Cl-37, and the average atomic weight of chlorine is 35.45.

Actually, each element comprises several isotopes. However, a single isotope predominates for most elements, so that 99 percent or more of the atoms of that element belong to the dominant form. The atomic weights for each element shown in Table 2.1 represent the average of isotopes for that element as it occurs in nature relative to the C-12 isotope of carbon, which, by definition, is 12. We must know atomic weights in order to calculate the combining proportions of reactants when chemical reactions, such as combustion, take place. It is important to memorize approximate atomic weights of certain commonly encountered elements.

 

1.2. How stable is an atom?

If an atom participates in an ordinary physical or chemical process, it can experience one of the following changes:

1. Lose a few of its electrons
2. Share a few of its electrons with a neighboring atom to form a chemical 
   bond
3. Gain one or two extra electrons

Its nucleus, however, is completely unaffected. For example, if an atom is passed through a flame, an electric arc, or a laser beam, its nucleus will be unchanged. Atoms cannot be changed by fire, which produces temperatures of only a few thousand degrees Celsius at most.

However, if an atom is heated to a temperature of hundreds of millions of degrees, such as in a thermonuclear explosion or in the interior of the sun, then the nucleus can be changed. The original atom of element A can become an atom of element B, or perhaps it can split into two atoms of elements C and D. Another way to change a nucleus is to strike it with another nucleus that has been accelerated to nearly the velocity of light (3 ■ 10 m/s) by the application of millions of volts of electricity. This book does not address the chemistry of nuclear transformations. Finally, certain atoms, such as those of radium, are radioactive, which means that their nuclei are unstable. These atoms can spontaneously emit charged particles and become atoms of a different element.

1.3. What are the mass and size of an atom?

Atoms are not infinitely small; they have finite, measurable size and mass.

Scientists who use refined equipment, such as mass spectrometers and X-ray diffraction cameras, are able to measure the mass and size of individual atoms. For example, the hydrogen atom has a mass of 3.3 • 10 g and a diameter of 1.06 • 10""⁸ cm. In proportion to their atomic weights, other types of atoms are more massive than hydrogen atoms.

 

2. Physical and Chemical Change.

2.1. States of matter.

Three basic classifications of matter are found in the material world:

1. Gas (or vapor)
2. Liquid
3. Solid

These terms are part of the basic vocabulary of technology and science and usually can be identified by sight, smell, or touch. Steam and air are examples of gases. Water and mercury are liquids. Ice and iron are solid substances. Steam is a gaseous vapor of water, and air is the colorless, odorless gas in which we live and breathe. Vapors usually cannot be seen except in rare cases, such as the brown vapors of the element bromine or the greenish color of chlorine gas. However, the effects that are caused by gases when they blow dust and smoke around, or when they make rubber balloons expand, can be observed.

 

2.2. Properties of Gases.

Gases consist of individual atoms or molecules moving at high velocities (approximately at the speed of sound, which is about 335 m/s in air). At atmospheric pressure and room temperature, the atoms or molecules them

selves occupy only about 0.1 percent of the space, and the remaining 99.9 percent of the space is empty. Each atom or molecule is colliding with others and changing direction about 10 times per second.

If the temperature of a gas is increased at constant pressure, the average velocity of the atoms or molecules increases and the gas expands. If the pressure on the gas is increased by compression at constant temperature, then the molecular velocities do not change, but the density increases.

The density p of a gas is directly proportional to its pressure P, and inversely proportional to its absolute temperature T. The density also is proportional to the atomic or molecular weight M of the gas at a given temperature and pressure. These interrelationships are described by the perfect gas law, which is valid except at pressures far above normal atmospheric pressure:

 

 

 

2.3. Properties of Liquids.

If the temperature of a gas is reduced, which reduces the molecular speed, or if the pressure is increased, which forces the molecules closer together, a point can be reached at which the gas condenses into a liquid. This is the point at which the attractive forces between the molecules overcome their tendency to separate after a collision.

In a liquid, the molecules are in contact with one another but also move relative to one another. For example, if a drop of ink is put into a glass of water, the ink will diffuse slowly through the water because of the molecular motion.

Every liquid has a vapor pressure, which increases with increasing temperature. The vapor pressure of a liquid is the pressure of the vapor over the liquid at which the rate of evaporation is equal to the rate of condensation; therefore, no net transfer occurs across the interface.

Vapor pressure can be expressed in millimeters of mercury (mm Hg) because a mercury column often is used to measure vapor pressure (760 mm Hg equals 1 atm, which equals 101.3 kPa). Consider liquid water, which has a vapor pressure of 17.5 mm Hg at 20°C. If water vapor at a pressure greater than 17.5 mm Hg exists above the liquid water, then the water vapor will condense. However, if water vapor is present at a pressure less than 17.5 mm Hg, or if no water vapor is present, then liquid water at 20°C will evaporate until the pressure of the water vapor reaches 17.5 mm Hg, or until the temperature of the liquid water drops below 20°C because of evaporative cooling.

If dry air exists over liquid water at 20°C, evaporation into the dry air will occur. If the dry air is at a pressure above 17.5 mm Hg, evaporation will occur slowly. However, if the pressure of the dry air is below 17.5 mm Hg, the water will boil, with rapid evaporation. If the water temperature is 100°C, boiling will occur unless the pressure of the dry air is greater than 1 atm.

 

2.4. Properties of Solids.

When the temperature of a liquid is reduced, generally a freezing point will be reached at which the liquid changes into a crystalline solid. In a crystal, the atoms, ions, or molecules are fixed in regular geometric positions and cannot move through the solid. However, they can vibrate — move back and forth on either side of their equilibrium positions in the crystalline lattice. When crystals such as ice or sodium chloride are heated sufficiently, they melt. The melting point is the same temperature as the freezing point.

Some liquids do not have a sharp freezing point and do not form crystals upon cooling. Instead, these liquids become progressively more viscous as they are cooled, so that the molecules are less and less free to move about, and finally the substance entirely loses its capability of flowing and it becomes a solid. The liquids capable of such transitions generally consist of relatively large molecules. When they solidify, the molecules are trapped in a random arrangement, as contrasted with the orderly arrangement found in a crystal. Such solids are called amorphous substances or glasses; ordinary window glass is a common example. When window glass is heated, it gradually softens over a range of hundreds of degrees, rather than having a sharp melting point, as do crystals of ice and sodium chloride. Other examples of glassy solids include tar, asphalt, cold molasses, waxes, and many synthetic polymers such as polyvinyl chloride (PVC) and polymethyl methacrylate (Plexiglas™).

Further, the molecules might not be oriented completely randomly in some glassy substances, but might be aligned with each other to some degree, depending on the process by which the melt was solidified. In such cases, the degree of crystallinity of the solid can be determined by X-ray diffraction tests.

Metals are a special kind of solid. Metals generally consist of positively charged atomic ions in a geometrically defined crystal lattice, with electrons free to move through the lattice. The fact that metals conduct heat and electricity far better than other solids is due to the high mobility of the electrons through the crystal lattice.

A metal can consist of a single pure element such as copper, aluminum, iron, or 24-carat gold, or it can be an alloy of two or more elements. For example, brass is an alloy of copper and zinc, steel is an alloy of iron and carbon, and 18-carat gold is an alloy of gold and silver.

 

3. Fire Characteristics: Gaseous Combustibles.

3.1. Types of gaseous flames.

Flames can be categorized as premixed flames or diffusion flames, that is, fuel gas mixed with oxygen before or during combustion, respectively. In addition, they can be categorized as laminar or turbulent flames, as well as stationary or propagating flames. Any combination is possible, such as a turbulent propagating premixed flame or a laminar stationary diffusion flame. Each of these categories, as well as the possibility that a flame could be a detonation, is discussed.

3.2. Premixed Versus Diffusion Flames.

Imagine a compartment containing 9.5 percent methane gas (CH4) by volume and 90.5 percent air, thoroughly mixed. Because air contains 21 percent oxygen by volume, and because 21 percent of 90.5 is 19, the compartment must contain 19 percent oxygen by volume. Recall from Chapter 3 that x volume percent of a gas is the same as x mole percent. Therefore, the ratio of the moles of oxygen in the compartment to the moles of methane is 19/9.5, or 2. This is a stoichiometric mixture, according to the equation

If an ignition source, such as a spark, is provided in the center of the compartment, then a small blue spherical flame will form around the spark and spread radially outward at about 3 m/s (10 ft/s).

Similar behavior would result if the methane percentage in air is somewhat lower or somewhat higher than 9.5 percent, except that the flame would propagate more slowly. If less than 5 percent or more than 15 percent of methane is present, the mixture would be too far from stoichiometric, and no ignition would occur. For combustible mixtures with greater than 9.5 percent methane ("rich" mixtures), there would be insufficient oxygen to completely oxidize the CH₄ to CO₂ and H₂O, and the products would include some CO, some H₂, and, for very "rich" mixtures, some solid carbon (soot).

This type of flame, whether stoichiometric or not, is a premixed flame. Pre-mixed flames can be either moving through space (propagating), as described above, or stationary, as will be described later.

The contrasting type of flame is a diffusion flame. Assume that there is a cloud of methane, resulting from a sudden release of gas from a tank, surrounded by air, but mixing has not occurred yet except in a thin zone at the interface between methane and air. If an ignition source is provided at this interface, then combustion will spread rapidly over the surface of the cloud. Subsequently, the bulk of the methane within the cloud will burn more slowly, as air and methane interdiffuse. Meanwhile, the hot burning ball of gas will rise. The flame will be yellow.

Diffusion flames can be stationary. A familiar example is a candle flame, in which molten wax evaporates from the wick and the wax vapor interdif-fuses with oxygen from the surrounding air. Most fires burn in this way, except on a larger scale.

In summary, premixed flames burn more rapidly than diffusion flames, and the chemistry is different (i.e., blue flames versus yellow flames. (See Chemical Mechanisms of Combustion later in this chapter for more details.)

 

3.3. Laminar Versus Turbulent Flames.

Large flames or flames burning in high-velocity flows generally are turbulent. That is, the velocity and temperature fluctuate at any selected point in the flame, and the track of any particle moving through the flame (a streamline) is erratic, with many changes of direction, rather than a straight or gently curving line.

On the other hand, small flames such as candle flames or the flame cones on a domestic gas burner generally are laminar; that is, the streamlines are smooth and the fluctuations are absent or negligibly small.

The presence of turbulence in a flame enhances heat transfer and mixing and can even affect the chemistry. Accordingly, rates of combustion are considerably higher in turbulent than in laminar flames. This phenomenon makes it difficult to predict large-scale fire behavior from small-scale (bench-top) fire tests.

As a general rule, a diffusion flame taller than 0.3 m (1 ft) will be turbulent, and a diffusion flame shorter than 0.1 m (4 in.) will be laminar unless a high-velocity jet is involved.

 

3.4. Stationary Versus Propagating Flames.

A premixed flame can propagate through a combustible gas mixture at 1 m/s (3 ft/s) or faster. However, there are several techniques for stabilizing a premixed flame so that it burns in a fixed position. Figure 8.1 shows three burner arrangements for studying premixed flames.

Such burners permit combustion scientists to measure flame properties accurately. For example, the gas-flow rate at which the flame will stabilize is a good way to measure the speed of flame propagation. The distribution of temperature and chemical species within the flame can be measured. These measurements lead to a detailed understanding of the combustion process.

Figure 8.2 shows four arrangements for stabilizing a diffusion flame. Once again, measurements of the stabilized flame structure lead to understanding of this type of flame, which more nearly resembles a fire.