An Introduction to Combustion - Assignment Example

Introduction to combustion xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Name xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Course xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Instructor xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Date submitted xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Q1.) The three states of matter are solids, liquids and gases. They differ in their microscopic structure or rather molecular activity. Molecules in solid form a repetitive structure known as the lattice structure. They are closely packed, have very minimal vibrations and are usually fixed on one position. Liquids on the other hand, are less structured and composed of loose chains of molecules. The molecules move a faster speed than solid molecules and are less tightly packed. Gases have random and move in the vessel at a very high speed compared to liquids and solids. Molecules are independent of each other with no bonds between them. One state can change to another by the effect of heat (Cracolice 2011). Q2.) Free radicals are atoms or molecules that consist of an unpaired electron but are neither negatively nor positively charged. They are highly unstable and reactive. During instances of fire, smoke accelerates formation of free radicals in the body. For example cigarette smoke is highly loaded with free radical which is believed to be the main cause of lung cancer. This phenomenon has been closely linked to death of individuals who have been previously subjected to large fumes from burning buildings (Cracolice 2011). Q3.) The formula used for temperature conversion of degrees in Degree Celsius to Kelvin’s Is: K= (C +273.15), Where k represents Kelvin while C represents degrees centigrade. For 146.5°C Answer= (146.5+273.15) = 419.65 Kelvin. For 374°F the formula changes to K= (F32) /2.25 Answer= (37432)/2.25=136.8 Kelvin. For 765°C Answer= (146.5+765) =911.15 Kelvin. For 112°F Answer= (112-32) /2.25 = 31.1111 Kelvin. Q4.) 3Fe + 3H2SO4 =>Fe3(SO4)3 + 3H2 SnO2 + 2H2 => Sn + 2H2O  4(NH4)3PO4 +3Pb(NO3)4 => Pb3(PO4)4 + 12NH4NO3 SeCl6 + O2 + C => SeO2 + 3Cl2 Q5.) The first determinant of the upward movement of a flame or gases is earth’s gravitational force. Gravity ensures support of the flame buoyancy. Flames also move upwards to make use of atmospheric oxygen which is fundamental in combustion. Gas density determines its upward movement. If the gas is less dense than air, it will move upwards but if it is denser it will move in the direction of wind or terrain. Qn.6) Heat needed =thermal conductivity of gypsum  mass in kg temperature change in Kelvin. 220+ 273= 493K. Answer=0.482493= 473.28 Joules Q7.) When gases enclosed in vessel they are in constant motion with each other. When volume of the gases is reduced, their pressure increases which implies an increase in kinetic energy, that is, the energy with which the molecules are interacting with each other. As a result, molecules will bombard each other at a higher rate hence increasing the rate of reaction (Cracolice 2011). Q8.) Heat is the kinetic energy (thermal energy) contained in a system or that which is transferred from one system to another. Heat is measured in terms of metric units known as Joules (J). Transfer of heat is either through radiation, convention or conduction. On the other hand, temperature is the measure of the average kinetic energy or average molecular motion of particles in a system. The SI units of temperature are Kelvin’s (K), Celsius (C) and Fahrenheit (F). Another difference is that energy can be transferred from one system to another without temperature of the substance (Smith 2008). Q9.) Benzene the most common aromatic compound has a molecular formula composed of six carbon atoms bonded by each other by double bonds. Each carbon is also bonded to one hydrogen atom. Benzene forms a hexagonal ring with each carbon representing a corner of the ring. Within the ring, the double bonds are separated by single bonds which introduce the aspect of conjugation of bonds (Smith 2008). C6H6 = Qn.10) Temperature difference = 275°C 32°C=243C. Conversion to Kelvin: 243273= 516 K Heat conduction Q/ Time = (Thermal conductivity) x (Area) x (Thot - Tcold)/Thickness Answer= (0.480.64516/ 0.032)/3600=1.376 Watts/m2 K. Q11.) There are various kinds of fire extinguisher and they are designed depending on the type of fire involved. There are six basic categories of fire as listed below: I. Class A: These fires are caused by ordinary combustible solid materials such as wood, papers, plastics and cardboards. II. Class B: Fires caused by combustible or flammable liquids such as kerosene, grease, oil, and gasoline. III. Class C: These are caused by electrical equipment such as circuit breakers, wiring and electronic appliances. IV. Class D: Are those that break in chemical laboratories especially due to combustible metals such as sodium, titanium, potassium and magnesium. V. Class F: Those that result from flammable gases such as propane methane and butane. VI. Class K: these are fires that occur in restaurants kitchens due to spoilage of cooking oil or fat on electrical cooking appliances. The main fire extinguishers that have been developed as a result of these types of fires are; Water extinguishers: They contain water pressurized with air and used on Class A fires. They should never be used electrical fires since they accelerate them. Dry chemical extinguishers: They are filled with powder or foam that has been pressurized with nitrogen. They are appropriate for Class A, B and C fires. The kind of foam contained in them may differ. For instance, some are filled with potassium bicarbonate or sodium bicarbonate which is mildly corrosive on materials and should be wiped off immediately. Carbon Dioxide (CO2) extinguishers: They contain highly pressurized carbon dioxide, a no-flammable gas. They are most appropriate for all kinds of fires including electrical fires and do not leave a harmful residue. Q12.) In a premixed flame the oxidizer and fuel are mixed before reaching the flame front while in diffusion flame mixing and combustion takes it place at the same place as the fuel and oxidizer are supplied to the burner. Diffusion flames burn slowly as compared to premixed flames as the oxidizer might be insufficient to complete the combustion reaction. Generally, diffusion flames will move further downstream while premixed flames will remain upstream until all the components are consumed. In addition, diffusion flames are sooty due to insufficiency of oxidizers while premixed are less sooty. It is easier to determine the flame temperature of premixed flames by simply altering the fuel to oxidizer ratio which is rather difficult in diffusion flames. Diffusion flames are less localized while those of premixed flames are roughly conical. Temperature profile at the turbine inlet of premixed flame is 850ᵒ (Warnatz, Maas and Dibble 2006). Q13.) Smoldering combustion is one where the all the elements of combustion; fuel, air, oxygen and heat are present but in unbalanced proportions. It occurs where the reaction is self sustaining. Smoldering combustion occurs in any inorganic material that is subjected to limited heat temperature and ventilation. Such materials include wood, cellulose, vegetable fibers, some leather, and viscose rayon among others. A typical example of smoldering combustion is the burning of cigarette (Warnatz, Maas and Dibble 2006). . Q14.) 1. Conduction, convection that occurs due to collapse of a wall between the source and the fire free region. 1. Convection, radiation occurring due to provision of an opening by the existing barrier between the fir source and the fire free region. 2. Conduction, convection where a conductor connects region between the fire free region and the fire source, thus leading to a spread. 3. Direct pyrolysis where the fire spreads from the source under restrictions through available air like fire spreads from the source into roofs then spreads to other building parts. Q15.) Turbulence of a flame is increased by addition of excess air to the flame. Significance of turbulence is that it improves combustion of components in the burner. A flame with high turbulence undergoes complete combustion. Q16.) I. Thermal radiation enables calculation of heating of structures and the flame spread in a building. II. The concept also makes it easy to predict fire growth of charring wall linings that re combustible. III. Once heat is fed back to the structure in form of thermal radiation, safe-operational time for fire rescuers and safe-escape-time for structure occupants can be estimated. Qn.17) Denotation is defined as a process in which fuels auto-ignite or pre-ignite in an engine combustion chamber. It occurs when a flame containing shock waves and at a supersonic velocity, travels along a pipe. Normally, it occurs as a result of change in acceleration of the flame caused by roughness in the pipe due to either valves or bends. Shock waves are caused by change in pressure and air density which subsequently changes subsonic velocity of the flame to supersonic velocity. In other words, wave propagation of the combustion reaction is faster than the speed of sound. Deflagration differs from detonation by the fact that wave propagation of combustion in deflagration is less than the velocity of sound. Qn.18) Boyles Law is a law that relates pressure and volume of a gas. The law states that for a fixed gas mass at a constant temperature, the gas volume is inversely proportional to the applied pressure. PV =P1V1 or PV =Constant. The Ideal Gas Law state that, relate gases their volumes, temperature and pressure. It state that fixed gas volumes contain the e same gas molecules. PV=n R T where P=Gas pressure in the atmosphere V= Gas volume in litres N=gas amount in moles. T= Gas temperature in Kelvins. R =ideal gas constant that stands at 22.4L Calculations T= (PV)/ (nR) thus the answer is T= (5.6 12) / (4  12) = 1.4104K N=RV/RT thus answer is n= (1.231)/(36022.4)=0.0046 moles. The combined gas laws relate the temperature, volume and pressure. It’s derived from the combination of Boyle’s law, Gay-Lussac’s law and Boyle’s law. P1V1/T1=P2V2/T. (Moore 2011) Q19.) 1. Size and shape of the compartments that determines the oxygen reservoir needed during combustion. 2. Amount of fuel available. 3. The number and point of placement of the openings in the compartments (Quintiere 2009). Q20.) When a fire starts in one room, it has the probability of extinguished or spread. The second step is that the object gains potential of spreading to other objects if it survives the latent or incubation period. The fire is at the post-flashover stage and it may either trigger the alarm or not It the alarm is triggered, occupants will safely leave the building but if not, they might be trapped in there. The spread continues to other houses in the compartment leading to flashovers and fully developed fires (Yung 2008). Q21.) Q22.) High amount of moisture in the air imply that the air is saturated with water molecules. This makes it difficult to filter out oxygen atoms for use in the combustion process. Thus, such kind of combustion is incomplete. Q23.) According to the law of conversion of matter, matter cannot be gained or lost but can be converted from one form to another. Thus, the number of atoms and molecules of reactants remains the same as those of the products. However, moles of reactants differ from those of the products (Smith 2008). . Q24.) Air-fuel ratio is the ratio of mass of air and fuel used in a combustion chamber. It provides a measure of the amount of air that is required to burn the fuel completely. It is measure using an Air-fuel monitor. Q25.) Air-fuel ratio is normally expressed on the basis of mass but it is now possible to express it on mole basis. It is interpreted as the ratio of number of moles of air to the number of moles of fuel. Q26.) Incomplete combustion causes inefficiency of a combustion burner. Main cause of incomplete combustion is inadequate supply of oxygen which limits the reaction from reaching end point to form the usual end products; carbon dioxide and water. Q27.) CO is more likely to be found after incomplete combustion of a hydrocarbon fuel. The reaction takes place under limited oxygen whereby the hydrocarbon undergoes partial combustion to form carbon monoxide and water. Q28.) Theoretical air is the amount of air that is theoretically enough to react with carbon in the fuel that has been supplied. 100% theoretical air represents the level where concentration of air is increased to the extent where CO rapidly decreases and oxygen atoms increase such that Co2 is formed. Q29.) Enthalpy of combustion is the amount of heat emitted during the steady-flow of a combustion process when 1 kmol is completely burned at a specific pressure and temperature. It differs from enthalpy of reaction which is the enthalpy involved in the transition state of reactants to products. The enthalpy of formation is the enthalpy of a substance at a specified state as a result of its chemical composition. Whereas the determining factor of enthalpy of formation chemical composition, variables in enthalpy of combustion are temperature and pressure. Heat of combustion is defined as the amount heat released when one mole of a substance undergoes complete combustion using oxygen at constant pressure (Smith 2008). . Q30.) 1. Diffusion, turbulent flame that consists of open fire inclusive of forest fires. 2. Laminar, diffusion flame is one with its fuel source as waxes vapour a good example being the candle flame. 3. Turbulent, premixed flame where the fuel and air are premixed. They occur in engineered combustion systems like furnaces. 4. Laminar, premixed flame occur where there is stream lined flow of the fuel and without significant bouncing around and the fuel and the oxidiser are before their introductions into the combustion zone. A good example is one from a Bunsen burner (Babrauskas 2006). References Babrauskas, V, 2005, Ignition Handbook, A Division of Science and Technology. Isaquah.WA: Fire Science Publishers Bettellhelm, F, 2006, Introduction to general, Organic and Biochemistry, California: Belmont Publishers. Cracolice, M, 2011, Introductory chemistry: an active learning approach. Belmont, CA : Thomson Brooks/Cole. Moore, J, 2011, Chemistry for Dummies, Hoboken : Wiley Publisher. Quintiere, G., 2006, Fundamentals of Fire Phenomena, West Sussex, London: John Wiley and Sons, Ltd. Smith, E, 2008, Asic Chemical Thermodynamics. London: Imperial College Press. Warnatz, J, Maas, U and Dibble, R, 2006, Combustion: physical and chemical fundamentals, modeling and simulation. Berlin: Springer-Vertlag Yung, D, 2008, Principles of fire risk assessment in buildings. Chichester: Wiley. Read More