Chemistry and behavior of fire

CHEMISTRY AND BEHAVIOR OF FIRE

DEFINITION OF TERMS• Fire (Various definitions):

1. Rapid oxidation of matter accompanied by heat or flame;

2. A rapid chemical reaction that gives off energy and products of combustion that are very different in composition from the fuel and oxygen that combine to produce them;

3. A chemical reaction that occurs, when fuel, air and a source of ignition are brought

together at the same time and in proper proportions.

• Oxidation – A complex chemical reaction of organic materials with oxygen in air or other oxidizing agents resulting in the formation of a more stable compound.

• Organic materials – Substances containing carbon, such as plant and animal materials. More stable compounds are simply those with less bound up chemical energy.

• Combustion – A self sustaining process of rapid oxidation.

• Fuel – Any form of matter capable of burning.

• Heat – A form of energy that raises temperature.

• British thermal unit (BTU) – The amount of heat needed to raise the temperature of one pound of water to one degree Fahrenheit.

• Calorie – The amount of heat needed to raise the temperature of one gram of water to one degree Celsius.

• Fahrenheit – English unit of temperature with 32⁰ as freezing point of water and 212⁰ as boiling point.

• Celsius – Metric unit of temperature with 0⁰ as freezing point of water and 100⁰ as boiling point.

• Vapor pressure – A measure of the tendency of a substance to evaporate.

• Boiling point – When vapor pressure exceeds atmospheric pressure. Also when rate of evaporation exceeds rate of condensation.

• Flash point – The minimum temperature at which a liquid fuel gives off vapors to form an ignitable mixture with oxygen in air.

• Fire point – The temperature at which a liquid fuel will produce vapors sufficient to support continuous combustion when heated.

• Vaporization – Process by which gases (vapors) are evolved from liquid when heated.

• Pyrolysis – Process by which gases (vapors) are evolved from solid when heated.

• Flammable limits (range) – The limits within the percentage of a substance in vapor state in air will burn once it is ignited. This is where the fuel vapor in air mixture is in proper proportions to support combustion.

• Specific gravity – Density of a liquid in relation to water. This is the weight of the liquid compared to the weight of an equal volume of water.

• Vapor density – Density of gas or vapor in relation to air. This is the weight of the gas or vapor compared to the weight of an equal volume of air.

THREE (3) ELEMENTS OF FIRE

• Heat

• Fuel

• Oxygen

For a fire to occur, three (3) things must be present at the same time and in proper proportions; a fuel, a source of ignition (heat), and a source of oxygen (air or any oxidizing agent).

FUEL

• Fuel is the material or substance being oxidized or burned in the combustion process. In scientific terms, the fuel in a combustion reaction is known as reducing agent. Most common fuels contain carbon along with combinations of hydrogen and oxygen. These fuels can broken down further into hydrocarbon-based fuels (such as gasoline, oil, and plastics) and cellulose-based materials (such as wood an paper).

• There are also other based fuels that are less complex in their chemical makeup, including hydrogen gas and combustible metals such as magnesium and sodium. In the combustion process there are two (2) fuel-related factors:

1. the physical state of the fuel; and

2. its distribution.

• These factors are discussed as follows:

• A fuel may be found in any of the three states of matter: solid, liquid or gas.

• Only gases burn; to burn fuels must normally be in its gaseous state.

• For solids and liquids, energy must be expended to cause these physical changes.

• The initiation of combustion of a liquid or solid fuel require their conversion into gaseous state by heating.

SOLID FUELS

• Fuel gases are evolved from solid fuels by pyrolysis. Pyrolysis is the chemical decomposition of a substance through the action of heat.

• Simply stated, as solid fuels are heated, combustible materials are driven from the substance. If there is sufficient fuel and heat, the process of pyrolysis generates sufficient quantities of burnable gases to ignite.

• Because of their nature, solid fuels have a definite shape and size. This property significantly affects the ignition of the fuel.

• Of primary importance is the surface-to-massratio of the fuel. The surface-to-mass ratio is the surface area of the fuel in relation to the mass.

• Wood is one of the best examples of the surface-to-mass ratio. To produce usable materials, a tree must cut into a log.

• The mass of the log is very high, but the surface area is relatively low, thus the surface-to-mass ratio is low.

• The log is then milled into boards. The result of this process is to reduce the mass of the individual boards as compared to the log, but

the resulting surface area is increased, thus increasing the surface to mass ratio.

• As the surface area increases, more of the material is exposed to heat and thus generates more burnable gases by pyrolysis.

• The physical position of the solid fuel is also of great importance.

• If the solid fuel is in a vertical position, fire will spread will be more rapid than if it is in horizontal position.

• The rapidity of fire spread is due to the increased heat transfer through the three (3) ways of heat transfer, which will be explained later.

LIQUID FUELS

• Fuels gases are evolved from liquid fuels through a process known as vaporization.

• Vaporization, in scientific terms, is the transformation of a liquid to its vapor or gaseous state. There must be some energy input in order to cause this transformation.

• In most cases, this energy is provided in the form of heat.

• A liquid assumes the shape of its container.

• The surface-to-volume ratio of liquids is an important factor.

• When contained in a container, the specific volume of a liquid has relatively low surface-to-volume ratio.

• When it is released, this ratio increases significantly as does the amount of the fuel vaporized from the surface.

• The specific gravity fuel liquid is an important factor in the degree of hazard of a liquid.

• A liquid fuel with a specific gravity of less than one (1) is more hazardous that a fuel liquid with a specific gravity of more that one (1).

• Solubility of a fuel liquid is also an important factor.

• Polar Solvents, such as alcohol, dissolves or mix with water.

• Hydrocarbons, such as oils or gasoline, do not mix with water

HEAT

• Heat is the energy element of fire. It is a form of energy which may be described as a condition caused by “molecules in motion”

• When heat, through a source of ignition, comes in contact with a fuel, the heat (as a form of energy) supports the combustion process.

SOURCES OF HEAT

• Heat can be derived from other forms of energies that results in the ignition of a fuel.

• The five (5) general categories of heat energy are:

1. Chemical Heat Energy

2. Electrical Heat Energy

3. Mechanical Heat Energy

4. Nuclear Heat Energy

5. Solar Heat Energy

CHEMICAL HEAT ENERGY

• Common types of heat generated as a result of chemical reaction are:

1. Heat of Combustion – Heat generated by the process of oxidation of matter. Ex: Flame of a candle.

2. Spontaneous heating – Heating of an organic substance without the addition of an external heat. Ex: Oil soaked rag.

• Heat of decomposition – Release of heat from decomposing compounds, usually due to bacterial action. Ex: Compost pile

• Heat of solution – Heat released by the solution of matter in a liquid. Some acids when dissolved in water, can produce violent reactions, spewing heat with explosive force.

ELECTRICAL HEAT ENERGY

• Electrical heat energy has the ability to generate high temperature that are capable of igniting any combustible material near the heated area.

• Heat generated by electricity can occur in a variety of ways, such as:

• Resistance heating – Generated by an electrical current passing through a conductor with a small resistance: Ex: Overloaded circuits.

• Dielectric heating – Action of pulsating a DC or AC at high frequency on a non conductive material. Ex: Defective micro-oven.

• Leakage current heating – Generated by current leaks to surrounding combustible materials. Ex: Wires which are not well insulated.

• Static electricity – Build up of a positive charge on one surface and a negative charge on the another surface. Ex: Lightning.

MECHANICAL HEAT ENERGY

• Mechanical Heat Energy is generated in two (2) ways; by friction and by compression.

1. Heat by friction is created by the movement of two surfaces against its other.

2. Heat by compression is generated when a gas is compressed. Diesel engines ignite fuel vapor without a spark plug by the use of this principle.

NUCLEAR HEAT ENERGY

• Nuclear heat energy is generated when atoms are either split apart or combined.

1. Fission is the splitting of atoms.

2. Fusion is the combining of atoms.

SOLAR HEAT ENERGY

• Solar heat energy is heat transmitted from the sun in the from of electromagnetic radiation.

• Typically, solar energy is transmitted fairly and evenly over the surface of the earth and in itself is not capable of starting a fire. However, when it is concentrated on a particular point, as through the use of a lens, it may ignite combustible materials.

TRANSMISSION OF HEAT

• The transfer of heat from one point or object to another is a basic concept in the study of fire. Heat is the energy transferred from one body to another when the temperature of the two bodies are different. Temperature is an indicator of heat and is the measure of the warmth or coldness of an object based on an standard.

• The transfer of heat from the initial fuel to other fuels in and beyond the area of the fire origin controls the growth of the fire. Investigators use this knowledge of heat transfer as they analyze the fire scene and determine the method of heat transfer from the fuels involved in the ignition to other fuels in the area of origin.

• Heat moves from warmer objects to those that are cooler.

• The rate that heat is transferred is related to the temperature differential of the two bodies. The greater the temperature difference is between the bodies, the greater is the transfer rate.

• Heat can be transferred from one body to another in three (3) ways; conduction, convection, and radiation.

CONDUCTION

• Conduction is the transfer of heat from one body to another by direct contact or by an intervening heat conducting medium.

• It is the point-to-point transmission of heat energy.

• Conduction occurs when a body is heated as a result of direct contact with a heat source.

CONVECTION

• Convection is the transfer of heat by the movement of heated liquids or gases.

• When heat is transferred by convection, there is a movement or circulation of a fluid (any substance, liquid or gas, that will flow) from one place to another.

• As with all heat transfer, the flow of heat is from the warmer area to the cooler area.

RADIATION

• Radiation is the transfer of heat through the medium of space or atmosphere.

• It is the transformation of energy as an electromagnetic wave, without an intervening medium.

• The best example of heat transfer by radiation is the sun’s heat. The energy travels from the sun through space and warms the earth surface.

HEAT MEASUREMENTS

• The temperature of a material is the measurement of heat condition of a material which indicates whether it can take up heat from its surroundings or give it out to its surroundings.

• It is a purely relative measurement and has been standardized in terms of the freezing point and the boiling point of pure water

• Fahrenheit denote the freezing point of water as 32⁰ and the boiling point as 212⁰. The difference between these points is divided equally into 180 divisions (or degrees).

• Celsius denote the freezing point of water as 0⁰ and the boiling point as 100⁰. The difference between these points are divided equally into 100 divisions.

• To convert Fahrenheit degrees to Celsius, subtract 32 from ⁰F and divide the result by 1.8.

• To convert Celsius degrees to Fahrenheit, multiply the ⁰C by 1.8 and add 32.

• Remember the above additions and subtractions must be done algebraically.

• It is also convenient to note that -40⁰C is equal to -40⁰F.

HEAT CAPACITY

• The heat capacity of a substance, or its thermal capacity, is that property of a material for absorbing heat with a consequent temperature rise per unit weight.

• It is measured in terms of heat units necessary to raise the temperature of the substance to one degree; i.e. BTU or Calorie.

SPECIFIC HEAT

• The specific heat a substance is the ratio of the heat capacity of the substance to the heat capacity of water.

• This value is 1.00 Btu per pound or 1.00 calorie per gram.

• It can be seen that specific heat, heat capacity, and thermal capacity are synonymous terms.

• The specific heat of various substances vary over a considerable range.

• This can be seen by the following table:

Substance Specific Heat Substance Specific Heat

Water 1.00 Zinc 0.093

Paraffin 0.694 Iron 0.109

Copper 0.093 Mercury 0.033

• The above table shows that if it takes 1000 Btu of heat from a fire to raise the temperature of 1000 lbs. of water to 1⁰F, it only requires a heat input of 109 Btu from a fire to raise the temperature of 1000 ponds of iron to 1⁰F.

• Conversely, the same relationship holds in lowering substances 1⁰F by removing heat or subtracting Btu’s in order to cool them to safe temperatures.

OXIDIZING AGENTS

• The oxygen in air around us is considered as the primary oxidizing agent.

• Normally, air consists of about 21% oxygen.

• Aside from air there are other oxidizing agents. These are those materials that yield oxygen or other oxidizing gases during the course of a chemical reaction.

• Oxidizers themselves are not combustible, but they support combustion when combined with fuel.

COMMON OXIDIZERS

• While oxygen is the most common oxidizer, there are other substances that fall into the category, such as the following:

1. Bromates 7. Nitrates

2. Bromine 8. Nitric Acid

3. Chlorates 9. Nitrites

4. Chlorine 10. Perchlorates

5. Fluorine 11. Permanganates

6. Iodine 12. Peroxides

HOW A FIRE WILL START

• For a fire to start, all the three elements, fuel, heat, and oxygen must come together at the same time and in proper proportions.

• When a source of ignition (heat) comes in contact with fuel in air, the heat energy supports the combustion reaction through pyrolysis or vaporization of solid or liquid fuels.

• The heat energy from the source of ignition is also necessary for ignition to occur and for the continuous production and ignition of fuel vapors or gases.

• The time it takes for a combustion reaction to occur is the determining factor in the type of reaction that is observed.

• If the fuel is in liquid state, the temperature of the heat absorbed from the source of ignition must reach flash point the fuel, at which vapors will start to be evolved from the fuel but not sufficient enough to support combustion.

• If the liquid fuel continue to absorb more heat energy from the source of ignition, temperature increases until it reaches fire point at which vapors produced are already sufficient enough to support continuous combustion.

• However, for combustion to occur after a fuel has been converted into vapor or gaseous state, it must be mixed with air (oxidizer) in proper ratio.

• The range of concentration of the fuel vapor and air is called flammable limits (range).

FLASH POINTS AND IGNITION TEMPERATURES OF SELECTED FUEL LIQUIDS

Substance Flash Point Ignition Temp.

Asphalt 400⁰F 905⁰F

Fuel Oil 105⁰F 490⁰F

Turpentine 95⁰F 464⁰F

Alcohol 55⁰F 700⁰F

Gasoline -45⁰F 495⁰F

IGNITION TEMPERATURES OF SELECTED FUELS

Substance Ignition Temp.

Hydrogen 1085⁰F

Carbon 925⁰F

Sulphur 450⁰F

Coal 750⁰F

Wood 450⁰F

Kerosene 490⁰F

FLAMMABLE LIMITS (RANGES)FOR SELECTED MATERIALS

Material Lower Flammable Upper Flammable

Limit (LFL) Limit (UFL)

Acetylene 2.5% 100%

Carbon Monoxide 12.5% 74.0%

Ethyl Alcohol 3.3% 19.0%

Fuel Oil 0.7% 5.0%

Gasoline 1.4% 7.6%

Methane 5.0% 15.0%

Propane 2.1% 9.5%

THE FIRE TRIANGLE ANDTHE FIRE TETRAHEDORN

• The fire triangle is a graphical representation of fire in the smoldering mode and incipient free burning mode.

• The fire tetrahedron is a graphical representation of fire in the flaming mode.

• For many years the fire triangle was used to teach the elements of fire. While this simple example is useful, it is not technically complete.

• When a fire becomes free burning, combustion occurs and the fire is in flaming mode.

• Once the fire is in flaming mode, it can continue when enough heat energy produced causes the continuous development of the fire.

• In the flaming mode, there are four (4) components of fire necessary to continuous combustion process. These components are:

1. Oxygen (oxidizing agent)

2. Fuel (reducing agent)

3. Heat

4. Self-sustained chemical reaction.

FIRE DEVELOPMENT

• When the four components of the fire tetrahedron come together, combustion occurs.

• For a fire to grow beyond the first material ignited, heat must be transmitted beyond the first material to additional fuel available.

• In the early development of the fire, heat rises and forms a plume of hot gas.

• If the fire is in the open (outside or in large building), the fire plume rises unobstructed and air is drawn into it as it rises. The spread of the fire in an open area is primarily due to heat energy that is transmitted from the plume to nearby fuels.

• Fire spread in outside fires can be increased by wind and sloping terrain that allow exposed fuels to be preheated.

• The development of a fire in a compartment is more complex than a fire in the open.

• A compartment is an enclosed room or space within the building. The term compartment fire is defined as a fire that occurs within such space.

• The growth and development of a compartment fire is usually controlled by the availability of fuel and oxygen.

• When the amount of fuel to burn is limited, the fire is said to be fuel controlled.

• When the amount of available oxygen is limited, the condition is called ventilation controlled.

• Recently, researchers have attempted to describe compartment fires in terms of stages or phases that occur as fire develops.

STAGES OF COMPARTMENT FIRES

• The stages of compartment fires are:

1. Ignition

2. Growth

3. Flashover

4. Fully Developed

5. Decay

IGNITION STAGE

• Ignition stage describes the period when the four components of fire come together and combustion begins.

• The physical act of ignition can be piloted (caused by spark or flame) or non-piloted (caused when a material reaches its ignition temperature as a result of self heating) such as spontaneous ignition.

• At this point, the fire is small and generally confined in the material (fuel) first ignited.

GROWTH STAGE

• Shortly after ignition, a fire plume begins to form above the burning fuel. As the plume develops, it begins to draw air from the surrounding space.

• Unlike an unconfined fire, the plume in a compartment is rapidly affected by the ceiling and walls of the space. The first impact is the amount of air that is drawn into the plume.

• Because the air is cooler than the hot gases generated by the fire, the air has a cooling effect on the plume temperatures. The location of the fuel package in relation to the compartment walls determines the amount of air that is drawn and thus the amount of cooling that takes place.

• Fuel packages that are located near walls will draw less air and will have higher plume temperature.

• Fuel packages located in corners draw even less air and will have the highest plume temperature.

• This factor significantly impacts the temperature that are developed in the hot gas layer above the fire. As the hot gas rise, they begin to spread outward when they hit the ceiling. This spread continues until the walls that make up the compartment are reached.

• The depth of the gas layer then begins to increase.

• The temperatures in the compartment during this period are dependent on the amount of heat conducted into the compartment ceiling and walls as the gases flow over them, the location of the initial fuel package, and the resulting air drawn into the plume.

• Research shows that the gas temperatures decrease as the distance from the centerline of the plume increases.

• The growth stage continues if enough fuel and oxygen are available.

• Compartment fires in the growth stage are generally fuel controlled.

• As the fire grows, the overall temperature in the compartment increases as does the temperature of the gas layer at the at the ceiling level.

FLASHOVER STAGE

• Flashover is the transition between the growth and fully developed fire stages and is not a specific event such as ignition.

• During flashover, conditions in the compartment change very rapidly as the fire changes from one that is dominated by the burning of the materials first ignited to one that involves all of the combustibles within the compartment.

• The hot gas layer that develops at the ceiling level during the growth stage causes radiant heating of combustible materials remote from the origin of the fire.

• Typically, this radiant heating causes pyrolysisto take place in the combustible materials in the compartment.

• The gases generated during this time are heated to their ignition temperatures by the radiant heat energy from the gas layer at the ceiling.

• Just prior to flashover, several things are happening within the burning compartment. The temperatures are rapidly increasing, additional fuel packages are becoming involved, and the fuel packages in the compartment are giving off combustible gases as a result of pyrolysis.

• As flashover occurs, the combustible materials in the compartment and the pyrolysis gases ignite. The result is a full-room involvement.

FULLY DEVELOPED STAGE

• The fully developed stage occurs when all combustible materials in the compartment are involved in the fire.

• During this period of time, the burning fuels in the compartment are releasing maximum amount of heat possible and producing large volumes of fire gases.

• The heat released and the volume of fire gases produced depend on the number and size of the ventilation openings in the compartment.

• The fire frequently becomes ventilation controlled; and therefore, large volumes of unburned gases are produced.

• During this stage, hot unburned fire gases are likely to begin flowing from the compartment of origin into adjacent spaces or compartments. These hot gases ignite as they enter a space where air is more abundant.

DECAY STAGE

• As the available fuel in the compartment is consumed by the fire, the rate of heat release begins to decline. Once again the fire becomes fuel controlled, the amount of fire diminishes, and the temperatures within the compartment begin to decline.

• The remaining mass of glowing embers can, however, result in moderately high temperatures in the compartment for some time.

FACTORS THAT IMPACT FIRE DEVELOPMENT

• As the compartment fire progresses from ignition to decay, several factors impact the behavior of a fire and its development within a compartment. These factors are:

1. Size, number, and arrangement of ventilation openings;

2. Volume of the compartment;

3. Thermal properties of the compartment enclosures;

4. Ceiling height of the compartment;

5. Size, composition, and location of the fuel package that is first ignited; and

6. Availability and locations of additional fuel packages.

• For a fire to develop, there must be sufficient air available to support burning beyond ignition stage.

• The size and number of ventilation openings in a compartment determine how the fire develops within the space.

• The compartment size and shape and the ceiling height determine whether a significant hot gas will form or not.

• The location of the initial fuel package is also very important in the development of the hot gas layer.

• The plumes of burning fuel packages in the center of a compartment draw more air and are cooler than those against the walls or in the corners of the compartment.

SPECIAL CONSIDERATIONS

• Several conditions or situations occur during the course of a fire’s development that should be discussed. These conditions can occur as a fire proceeds through the stages of growth and development. These conditions or situations are:

1. Flameover/Rollover

2. Thermal Layering of Gases

3. Backdraft

FLAMEOVER/ROLLOVER

• The terms flameover and rollover are used to describe a condition where flames move through or across the unburned gases during a fire’s progression. Flameover is distinguished from flashover involvement of only the fire gases and not the surfaces of other fuel packages within a compartment. This condition may occur during the growth stage as the hot gas layer forms at the ceiling of the compartment.

• Flames may be observed in the layer when the combustible gases reach their ignition temperature. While the flames add to the total heat generated in the compartment, this condition is not flashover. Flameover may also be observed when unburned fire gases vent from a compartment during the growth and fully-developed stages. As these hot gases vent from the burning compartment into the adjacent space, they mix with oxygen; if they are at their ignition temperature, flames often become visible in the layer.

• A flameover will not result in the ignition of target fuels within the compartment but may result in burn patterns on walls and other combustible building elements with which the gases are in direct contact

THERMAL LAYERING OF GASES• The thermal layering of gases is the tendency

of gases to form into layers according to temperature. Other terms used to describe this layering of gases by heat are heat stratification and thermal balance. The hottest gases tend to be in the top layer, while the cooler gases form the lower layers. Smoke, a heated mixture of air, gases, and particles, rises. If a hole is made in a roof, smoke will rise from the building or room to the outside.

• Thermal layering is critical in fire fighting activities. As long as the hottest air and gases are allowed to rise, the lower levels will be safer for firefighters. This normal layering of the hottest gases to the top and out the ventilation opening can be disrupted if water is applied directly into the layer.

• When water is applied to the upper level of the layer, where the temperature is hottest, the rapid conversion to steam can cause the gases to mix rapidly. This swirling mixture of smoke and steam disrupts normal thermal layering, and hot gases mix throughout the compartment. This process is sometimes referred to as disrupting the thermal balance or creating a thermal imbalance.

• Many firefighters have been burned when thermal layering was disrupted. Should this condition occur during fire fighting operations, the investigator could observe uncharacteristic patterns in the compartment during scene examination.

BACKDRAFT

• As the fire grows in a compartment, large volume of hot, unburned fire gases can collect in unventilated spaces. These gases may be at or above their ignition temperatures but have insufficient oxygen available for actual ignition to take place. Any action during fire fighting operations that allows air to mix with these hot gases can result in an explosive ignition called backdraft.

• Many firefighters have been killed or injured as a result of a backdraft. The potential for backdraft can be reduced with proper vertical ventilation (opening at highest point). Because the unburned gases rise, the building or space should be opened at the highest possible point to allow them to escape before entry is made.

• The following conditions may indicate the potential for a backdraft to occur:

1. Pressured smoke exiting small openings.

2. Black smoke becoming dense gray-yellow.

3. Confined and excessive heat.

4. Little or no visible flame.

5. Smoke leaving the building in puffs or at intervals.

6. Smoke-stained windows.

• Should a backdraft occur, the fire investigator will observe blastlike damage in the building. Homes have been known to be moved from their foundations, and walls of multiple-storey buildings have been blown off.

PRODUCTS OF COMBUSTION• Heat

1. Responsible for the spread of fire.

2. Causes burn, dehydration, exhaustion and/or injury to respiratory tract.

• Flame

1. The visible luminous body of burning gas.

2. When burning gas is mixed with proper amounts of oxygen, becomes hotter and less luminous.

• Smoke

1. Unburned, finely divided particles of soot.

2. Contents vary depending of the exact material that is burning.

3. Causes suffocation.

• Fire gases

1. Evolved from fuel during process of combustion.

2.Most gases evolved are toxic to humans.