Introduction to Combustion and Fire - Example

Name : Subject: Prof : Date : Introduction to Combustion and Fire “Fire is a fundamental force in nature” (Cote and Bugbee 1988). To appreciate fire, we must have scientific description of fire unfailing with our perceptions. We must recognize the role fire has played in history, its benefits, and it cost to society in terms of people and property destruction. Controlled fire, combustion, or the burning of substances in the air (Findlay 2007:61), for useful power is studied in combination with the market forces that drive our economy. The study of uncontrolled fire seems to be stimulated by clear risks to society and by societies having the means to invest in such study. The development of the science of fire has accelerated over the last 150 years. It is a multifaceted area linking various fields, and it is comparatively archaic compared to other technological fields. ”Before there was life, there was fire” (Quinteire 1998:1). It has left its mark on history in countless ways. In scientific terms, fire or combustion is a chemical reaction involving fuel and oxidizer. This us normally the oxygen (O2) in the air we breath. Rusting and yellowing of old newsprint do fit this definition. However, those processes are neither combustion nor fire. In scientific terms, combustion and fire are synonymous and the flammability and fire behaviour of materials are used synonymously when describing the ignition, combustion, and fire propagation behaviours of materials separately or in combination (Andrady 2003:404). In conventional terms, we generally treat fire as distinct from combustion, in that fire is combustion that is not intended to be controlled. Fire fighters attempt to control it by adding water or other agents, but the process of fire is not “designed” combustion, as in furnace or an engine. Combustion experts who study such systems may know very little about fire, and those who deal with fire may know very little about combustion (Quinteire 1998:1). Fire is a chemical reaction that involves the evolution of light and energy in sufficient amounts to be perceptible. A chemical reaction concerning solids, liquids, and gases that ignite and go through a quick, self-contained oxidation process, go together with the progression of heat and light of varying strength (NUREG 2005:4). Oxygen is requires to combine with the fuel for combustion to occur. However, an oxidizing agent can be used instead of oxygen itself in the combustion process (IAFC-NFPA 2004:127). Therefore, oxidation is the chemical reaction between the burning material and oxygen. The vigour of combustion can be increased by increasing the rate at which oxygen is supplied to the burning materials similar to the reaction when a man blows a fire with a bellows (Findlay 2007:66). For instance, the burning hydrogen (H2) with air or oxygen generates only water vapour from its chemical reaction. While considerable energy is produce, we would not see flame. However, in most other fire classifications we would see the combustion process and a fire or a flame would be brisk enough to be sensed, predominantly with ample energy to harm our skins. It may not be very immense, but its energy discharge rate per unit volume of the chemical reaction zone would be enough to give us local burn injury. The rate of reaction is the quantitative measure of the number of moles of the product produced or reactant consumed per unit time per unit volume which is : rate = change in moles of species divided by time increment multiplied volume (Kanury 1975:9). A fire is the consequence of striking a match, the blaze of the charcoal briquette, the flames of the forest, and the impulsiveness of a burning haystack (Quinteire 1998:1). The difference between a flame and fire according to Baukal and Schwartz (2001) is that a flame is controlled and desirable, while a fire is uncontrolled and undesirable (p.331). However, the chemical and physical reactions that occur during a fire are exceptionally multifaceted and a lot difficult to explain absolutely (NUREG 2005:4). One earth, chemical reactions like fire characteristically involves hydrocarbon-based fuels, those composed of atoms of carbon (C), hydrogen (H), and conceivably some oxygen, and nitrogen. Synthetic substances have been added to this assortment of material (molecules), such as chlorine (CI), bromine (Br), fluorine (F) and other atoms. For instance, wood molecules consist of atoms involving H, C, and O; polyvinyl chloride (plastic) contains H, C, and O. These additions to the H-C-O base obscure the nature of combustion products and their impending threat to the surroundings. All chemical reactions conserve mass, which means all the atoms survive, on the contrary to a nuclear reaction in which some atoms are converted into new atoms with some substance changed to energy. In a chemical reaction, however, molecules are not conserved. Their obliteration is the essence of a chemical reaction in which they are converted to new molecules. For combustion or a fire, the creation of new molecules from the fuel and oxygen molecules gives off a net amount of energy. This energy comes from discharge the binding forces that hold the molecules simultaneously (Quinteire 1998:1). The most widespread fires start because of the ignition of solid or liquid fuels or combustible materials. Solid and liquid fuels normally become volatile and serve as suppliers of gaseous fuel to support combustion. In the physical model, the process of fire development begins when the fuel surface starts to heat up because of heat transfer from the adjacent surroundings. Consequently, as the temperature of the fuel surface climb in response to his heat input, the fuel surface begins to emit fuel vapours. The fuel vapours mix by convection and diffusion with oxygen in the adjacent boundary layer, ignite through a chemical reaction, and release additional heat. Some this liberated heat energy may further increase the surface temperature of the fuel and thereby accelerate the fire growth process (NUREG 2005:4). Moreover, after the fire has been initiated, energy is still required to sustain the flame (Baukal and Schwartz 2001:331). According to Findlay (2007), the velocity or strength of every chemical change is increased by raising temperature. At the normal temperature, the oxidation of a material takes place so leisurely that no change is apparent even over long periods. However, when the temperature of a combustible material is increased, the rate at which it reacts with the oxygen of the rapidly increases, and as a result the production of heat which accompanies the reaction also rapidly intensifies, the temperature rises more and more (p.67). Natural phenomena that can cause fire because of their high temperature are lightning and molten rock from volcanic activity. These phenomena date back to the formation of the earth. As organic matter developed, we could expect to have fire. Various materials react with oxygen to some extent but these materials differ in the respective rates of reaction. The difference between slow and rapid oxidation reactions is that the latter occur so rapidly that heat is generated faster than it is dissipated, causing the material being oxidized to reach its ignition temperature. As a result, once a material reaches its ignition temperature, either it ignites and continues to burn until the fuel or the oxygen consumed (NUREG 2005:4). Even today, these natural phenomena are a leading cause of accidental fire. The workers keeping watch over our forest record lighting strikes. These strikes can cause smouldering underbrush on the forest floor, and a day or two after a storm, flames can erupt. Of more concern to modern society is the effect of an earthquake, rather than direct volcanic activity. An earthquake can play havoc with fire and fuel sources used for heating and cooking. More damage was done by the conflagrations resulting from the San Francisco earthquake of 1906 and the Great Kanto earthquake of 1923 in Tokyo and Yokohama than from the actual quakes (Quinteire 1998:1). We can categorize fire into four distinct phenomena; diffusion flames, smouldering, spontaneous combustion, and premixed flames. Diffusion flames represent the predominant category. It is the building fire, the forest fire, the lit match. Smouldering can be the birth or follow the death of a diffusion flame. It is the glowing embers of the tragic house fire, the result of a lighting strike to the forest bed, or the glow of a blown out match flame. Spontaneous combustion is the incubation of a chemical reaction that leads to smouldering or diffusion flaming. It can occur in oily cotton rags, a haystack, or a pile of wood chips. Premixed flames represent controlled combustion processes such as in the gasoline internal combustion engine with spark ignition or the diesel engine with auto-ignition. Auto-ignition is the process through which a flammable liquid’s vapours are capable of extracting enough energy form the environment to self-ignite, without the presence of a spark or flame (Baukal and Schwartz 2001:329). Premixed flames also represent the incipient flame in the ignition of solids and liquids before a diffusion flame emerges (Quinteire 1998:1). On the other hand, Cote and Bugbee (1988) classifies four major products of combustion; fire gases, flame, heat, and visible smoke. All of these are produced in varying degrees by fire (p.55). The heat released during “combustion is usually accompanied by a visible flame” (NUREG: 4). A diffusion flame is a combustion process in which the fuel gas and oxygen are transported into the reaction zone due to concentration differences. This transport process is called diffusion and is governed by Fick’s Law, which says that a given species in connection with fire, oxygen, fuel, CO2 will move from a high to low concentration in the mixture. A drop of blue ink in a glass of water will eventually diffuse into the water to give a blue tinge. Oxygen in air will move to the flame where it has a concentration of zero as it is consumed in the reaction. Fuel is transported into the opposite side of the flame by the same process. The combustion products diffuse away from the flame in both directions. Most natural flaming natural flaming fires are diffusion flames. A common example is the flame of a match or a candle. In a candle, the flame melts the wax, which is transported up the wick by capillary action. The flame then vaporizes the wax, and the gaseous fuel diffuses in the flame where it meets oxygen (Quinteire 1998:1). An ideal diffusion flames consist of an infinitesimally thin exothermic reaction zone, which separates the fuel stream from the oxidant stream. Diffusion flames are either homogenous or heterogeneous. In homogenous diffusion flames, the fuels as well as the oxidant are in gaseous state such as in the case of combustion of a jet of natural gas issuing into air. Normally, heterogeneous diffusion flames occur when a solid or liquid fuel burns in an oxidizing gaseous atmosphere (Kanury 1975:143). According to Tse et. al. (1989), the combustion process is observed as taking place in either of two modes: the flaming mode or the glowing mode. The flaming mode is an attribute suggestive of the free burning of either gases or vapours that are derived implicitly from liquid or solid fuels or straight from flammable gases. The second necessitate no preparation to go into the burning process. Flammable liquids, by itself, do not go into the burning process directly since evaporation is necessary to form the flammable vapours. The heat energy to realize this is derived from the heat (p.1) released by the fire itself in the form of radiative feedback. Heat energy is required to ignite a fire and once the fire is ignited, the combustion process produces more heat (IAFC-NFPA 2004:128). Through flaming solid fuels, vapours are derived from the pyrolytic disintegration through the outcome of radiative feedback from the fuel itself. Radiative feedback also induces other stage of the combustion process such as the “cracking” of compound vapours, consequently forming free radicals as well as simpler vapours. Correspondingly, radiative feedback stimulates the interplay chain reactions, in addition to the combustion of fire-generated gases. The strength of any combustion process is at all times articulated as the time rate of heat energy freed to the immediate environment. In expressions, that we are accustomed with, we would state this rate as British thermal units per minute (BTU/min) (Tse et. al. 1989:2). Liquid fuels in the form of steam or mist, and solid fuels in the form of powder or crushed materials; flame in a manner is comparable to liquid fuels. Volatility and large particle surface area’s mass ratio are accountable for the resemblance, as all combustion processes transpires at the crossing point between the fuel and the air. After ignition, the speed of burning hastens exponentially. Combustion is a display of the conversion of chemical energy, confined within the fuel molecules, to heat energy. In the early stage of the fire, the chemical reaction are energy absorbing (endothermic), and these are trailed by energy-emitting (exothermic) reactions. Combustion is a chemical reaction which is exothermic when considered as a whole, and which accelerates after a slow start to become rapid and even violent (Borghi and Destriau 1998:31). The second reactions produce higher rates of heat energy discharge than that absorbed by the endothermic reactions. This attribute gives the fire its self-sufficient character, provided none of the factors is interfered with. These factors according to IAFC-NFPA (2004) are fuel, oxygen, and heat. When these factors are present, under most circumstances, combustion will occur (p.128). Because of the abovementioned, the escalating rate of heat discharge will cause the fire to spread out until it reaches the threshold of its environment, it die down for lack of fuel and air, or it ends because of extinguishment (Tse et. al. 1989:2) The glowing mode is exhibited in the form of surface combustion in which the burning is restricted to the interfacial surface between the fuel and the air. Pure examples of fuels that burn according to the glowing mode consist of several metallic and non-metallic chemicals elements, from which gases and vapours are not distilled and no flaming exists. Instance of these elements are magnesium aluminium, lithium, potassium, boron etc. (Tse et. al. 1989:3). Once all the volatiles of the solid are released, the carbonaceous solid residue is attacked by oxygen to yield glowing combustion (Kanury 1975:196). In primary and secondary combustion, as we mentioned earlier, the flames of the diffusion type are the product of aggressive, unrestrained free burning that, because of the fire’s exothermic character, will grow in any direction it can, growing ever larger. This fire is the concern of fire services within its innumerable forms. A characteristic of this type of combustion is that it is always under ventilated whether it is fully out in the open or partly covered. One of the reasons for this condition is the noticeable existence of two phases in the overall combustion process. For simplicity, we can call the first combustion as the primary phase to differentiate it from the secondary phase (Tse et. al. 1989:86). In the primary phase, no new fuel of any kind has been added but instead, the gaseous effluent from the primary phase passes through a momentary transformation becoming alternately combustible and oxidizes. The process experienced an alternately endothermic and exothermic series of sub phases. These subphases are referred to as the secondary combustion phase. This evident redundancy of the overall burning process can only happen when free glowing carbon particles are present in the flame, as substantiated by its characteristic yellow orange hue consequential from spectral emission of heated carbon. By means of the simple hydrocarbons, methane and ethane, the flames do not emit smoke as the glowing carbon is practically oxidized. Heavier fuels become increasingly ruddy and release even thick clouds of black smoke. Fuels like simple alcohols, ketones, ether, and the like, that contains oxygen within their molecular structure burn with smokeless and colourless flame. The participants in this secondary phase are hydrogen, carbon, carbon dioxide, carbon monoxide, and carbon, which are perceptibly slowest, and this accounts for the long duration of glowing carbon within the discernible flame (Tse et. al. 1989:86). Combustion itself does not immediately appear to be easy to understand. However, it is the very chemistry of the system, which governs the combustion process occurring in the heart of the flame. The system can be described as being a set of chemical reaction whose principal reactants are oxygen in the air, and gaseous vapours evaporating off the wick. On the other hand, the physical aspect of heat transfer is clearly apparent since it is partly produced by the yellow radiation, which the most visible feature of a fire. References: Andrady Anthony, 2003, Plastics and the Environment, Published 2003 Wiley-IEEE, ISBN 0471095206 Baukal Charles and Schwartz Robert, 2001, The John Zink Combustion Handbook, Published 2001 CRC Press, ISBN 0849323371 Borghi Roland and Destriau Michel, 1998, Combustion And Flames: Chemical and Physical Principles, Published 1998 Editions TECHNIP, ISBN 2710807408 Cote Arthur and Bugbee Percy, 1988, Principles of Fire Protection, Published 1988 Jones & Bartlett Publishers, ISBN 0877653453 Findlay Alexander, 2007, Chemistry in the Service of Man, Published 2007 READ BOOKS, ISBN 1406758159 IAFC-NFPA, 2004, Fundamentals of Fire Fighter Skills, International Association of Fire Chiefs, National Fire Protection Association Published 2004 Jones & Bartlett Publishers, ISBN 0763722332 Kanury Murty, 1975, Introduction to Combustion Phenomena: For Fire, Incineration, Pollution, and Energy Application, Published 1975 Taylor & Francis, ISBN 0677026900 NUREG, 2005, NUREG-Series Report, United States Nuclear Regulatory Commission, United States of America Quintiere James G., 1998, Principles of Fire Behavior, Published 1998 Thomson Delmar Learning, ISBN 0827377320 Tse Francis, Haessler Walter, and Morse Ivan E., 1989, Fire: Fundamentals and Control, Published 1989 CRC Press, ISBN 0824780248 Read More