Combustion and Fire Issues - Assignment Example

Name: Instructor: Course: Date: Introduction to Fire and Combustion 1. Matter is any substance that has mass and occupies space, with West arguing that there are three main states of matter: “solids, liquids, and gases” (20). The molecules in each of these states of matter move and behave in particular ways, depending on forces involved. According to the behaviors, the states of matter can be differentiated using two main characteristics: shape and volume. A solid has a definite, measurable shape that cannot be easily changed unlike a liquid or gas, which has no dimensions and no shape that can be measured without being in a container. West states that solids and liquids have defined and measurable volumes, while gases do not (20). Although, the molecules in gases exhibit constant motion, they are in a defined amount of space. 2. Free radicals are atoms or molecules, which poses unpaired electrons. They can be anions, cations or neutral e.g. hydrogen bromide and oxygen atoms (Punchard and Kelly 70). They are formed when compounds especially hydrocarbons are burned. The formed radicals combine with other radicals (e.g. oxygen radicals (O)) which sustain fire and finally produce products of combustion. 3. Heat of combustion is the energy produced in form of heat when compounds (especially hydrocarbons) undergo complete combustion with oxygen under standard conditions producing water and carbon dioxide (Stoessel 35). 4. i. 46.5°C converted to Kelvin K=oC+273 (Dunn 26) K: Kelvin oC: degrees Celsius 46.5oC= (46.5+273) K=319.5K ii. 174°F converted to Kelvin, oC= (oF-32)*5/9 174oF= ((174-32)*5/9)oC=78.89oC 78.89oC= (78.89+273) K= 351.89K iii. 705°C converted to Kelvin, 705oC = (705+273) K= 978K iv. 212°F in kelvins,212oF= ((212-32)*5/9)oC= 100oC 5. a. H3PO4 + KOH => K3PO4 + H2O step 1: balance hydrogen atoms i.e. H3PO4 + KOH => K3PO4 + 2H2O step 2: balance oxygen atoms i.e. H3PO4 + 6KOH => 2K3PO4 + 2H2O step 3: balance phosphorous atoms i.e. 2H3PO4 + 6KOH => 2K3PO4 + 2H2O However, the equation is not yet balanced step 4: balance the hydrogen and oxygen atoms - 2H3PO4 + 6KOH => 2K3PO4 + 6H2O Thus the balanced equation is given as: 2H3PO4+6KOH=>2K3PO4+6H2O b. H3PO4 + Mg(OH)2 => Mg3(PO4)2 + H2O. Following the above steps using the atoms involved in the reaction, the balanced equation will be; 2H3PO4 + 3Mg(OH)2 =>Mg3(PO4)2 + 6H2O Totals: 12H,14O,2P and 3Mg(on both sides of the equation) c. C2H6 + O2 => CO2 + H2O The balanced equation will be: 2C2H6 +7O2=>4 CO2 +6H2O Totals: 12H,14O,and 4C (on both sides of the equation) d. Ca3(PO4)2 + SiO2 + C => CaSiO3 + CO + P The balanced equation is given as: Ca3(PO4)2 + 3SiO2 + 5C => 3CaSiO3 +5 CO +2 P Totals: 3Ca,2P,14O,3Si and 5C (on either side of the equation) (West 67) 6. The upward movement of flames and gases can be affected and altered by a number of factors, including convection of the flames, the atmospheric pressure, density of gas, concentration of oxygen, and gravity of the area (Mannan and Lees 17-46). 7. Heat energy(Q)=mass(m)*specific heat capacity(c)*change in temperature (Dunn 42), Q= 1kg*1.09kJkg-1*(180-30) =163.5kJ Thus heat required to raise the temperature of 1kg of gypsum plaster from 30°C to 180°C is 163.5kJ*1000=163500J 8. A mixture of gas reacts faster when the volume is reduced because the gas molecules have decreased spaces between them and the pressure on them is increased, and as such, they collide faster during reactions (West 38). 9. Temperature is how hot or cold a state of matter is and makes the states of matter to change their forms. Heat is form of energy and makes molecules or/and atoms in matter to move or vibrate (Dunn 4). 10. Let the molecular formula be (ClCH2) n [relative atomic masses: Cl=35.5, C=12, H=1] The relative molecular mass will be, n (35.5+12+1(2)) = 49.5n (West 17). However, it is given that the molecular mass is 98.96 Therefore,49.5n=98.96 n=98.96/49.5 => n=2 Thus the molecular formula of the compound is (ClCH2)2 ,which is Cl2C2H4 11. Figure: Semenov diagram of fire explosions (Stoessel 50) Thermal explosion occurs as result of pressure differences between the cooling and the heat production systems, i.e. as cooling continues higher temperatures in a higher reaction rate causes further increase in heat product rate (Stoessel 51). Exponential increase in heat production makes the cooling capacity of the reactor to increase linearly (by Newtonian cooling equation of heat removal) with the temperature becoming insufficient. From the graph above, we have exothermal reaction where heat release rate, varies as an exponential function of temperature, and heat removal (cooling system), varies linearly with temperatures. Heat balances are in equilibrium, i.e. occurring at the intersection points of the exponential heat release curve with the linear line of heat removal curve. However, the stable equilibrium point is at the intersection (S) which is at a lower temperature. Any rise in temperature above S, means heat removal increases reducing temperatures until its production equals the removal, and vice versa. The intersection at (I) shows that the system is instable and a small temperature rise causes excess heat production resulting into thermal explosion. In other words, the intersection of cooling line with temperature axis represents the temperature cooling system, For higher cooling system, the straight line shifts downwards (the dash lines). When the points of dash lines merge, the corresponding temperature of the cooling system is known as the critical temperature. This shows that heat balance equation cannot be solved hence explosion will occur. 12. Where (Stoessel 43), Q = heat loss or gain (watts) L = thickness of insulation in meters (m) = 0.042M K =Thermal Conductivity of the insulation material in watts/meter C = 0.48W/mK A = surface area of outside of container (m2) = 0.32M2 h = Heat Transfer Coefficient of the surface material in watts/meter2 C = 0 TO = Outside temperature in C = 6750C TI = Inside temperature in C = 320C 13. What are the different types of fire extinguisher and on what can they be used? There are various fire extinguishers classified according to type of fire application. Common types include (Silberstein 62-63), a) Class A Fire Extinguishers - Used on fires resulting from burning of ordinary combustible materials such as wood, paper etc. They contain water and compressed gas, and labeled with a picture of burning combustibles, or a green triangle with a clear visible letter A. b) Class B Fire Extinguishers – They contain dry chemicals, halon, or carbon dioxide and are used on fires involving flammable liquids e.g. gasoline or oil. They are labeled with a picture of burning fuel or letter B inside a red square. c) Class C Fire Extinguishers – They are applied on electrically energized fires and contain non-conductive extinguishing agent to prevent fire from spreading. They are labeled with a picture of a burning outlet or letter C inside a blue circle. d) Class D Fire Extinguishers – They are used on flammable metals and they are designed for specific metals and materials, and are labeled with letter D inside a yellow star. 14. Differences between the Premixed and the Diffusion flame Premixed flame Diffusion flame 1. Temperatures and concentration of products increase uniformly in the combustion zone Not uniformly distributed in the combustion zone 2. Fuel and oxidizer are initially mixed Fuel and oxidizer are initially separated 3. The fuel- oxidizer ratio is constant everywhere in the flame The fuel-oxidizer ratio varies throughout the flame 4. Partly, it has a blue flame It has a yellow flame 5. Combustion is more rapid resulting to higher temperatures Combustion is less rapid resulting to low temperatures 6. The rate of burning is limited by kinetic reactions The rate is limited by the diffusion mechanism Their temperature profiles cause large flow acceleration at the flame front. Hence, the premixed flame temperatures rise up to 40000F, and that of diffusion flames range from 19000 to 20000F (Mannan and Lees 16). 15. Materials that can undergo smoldering are the ones that produce carbon monoxide and carbon dioxide to catalyze oxidation process (Mannan and Lees 16-58). Therefore, material that forms significant char during thermal decomposition are mostly preferred. 16. Factors that spearhead the spreading of fire involve conduction through sharp corners where materials meet. A convectional and radiation factor of the burning material still leads to quick spreading of fire (Mannan and Lees 16-13). In addition, fire can spread through direct pyrolysis within roofing materials. Wind also plays a role in spreading fires from affected to non-affected regions of the fire compartment. 17. Turbulence is important in combustion because it assists in increasing the mixing process during all stages of combustion (Stoessel 195). 18. Burning velocity of a gas mixture is the velocity with which a homogeneous fuel-air mixture must be fed to maintain the combustion process. The flame can spread with faster velocity than that of burning because of expansion of the combustible reactants, volatility, and disorderly buckle of the flame (Mannan and Lees 17-37). 19. Thermal radiation from a flame plays significant part in fire safety because it controls heat transfer from reacting gases and the combusting products. It also controls combustion velocities and oxygen concentration due to its heat transfer mechanisms (Silberstein 13). Through its heat emission, a heating expansion reduces creation of an environment for cooling. 20. Detonation is a supersonic condition caused by shock compression where an erratic form of combustion occurs due to fuel pre-ignition causing head gasket failure as well as damaging the engine. The difference between detonation and deflagration is that shock compression causes fuel pre-ignition resulting into detonation, while subsonic spread of combustion resulting from thermal conductivity causes deflagration (Mannan and Lees 17-5). 21. Fires can generally be classified in to three general categories, which include (Mannan and Lees 16-13) a) Class A fires – Include fires fueled by materials such as paper and wood that leave ash residue when burnt. b) Class B fires - Fires involving flammable liquids and gasses, for example gasoline c) Class D fires – Fires involving exotic metals, such as magnesium, sodium, among others. 22. The above fires produce toxic emissions, including (Dunn 36), a) Class A fires – They produce green house gases, for example carbon dioxide, smog, among others b) Class D fires – They produce poisonous aqueous species c) Class B fires – They can produce toxic salts which poison both land and water bodies 23. Factors affecting depletion of oxygen in a fire compartment include (Dunn 247), a) High levels of carbon monoxide b) Limited air supply through the vents c) Presence of materials that use oxygen in combustion 24. When fire catches the compartment, occupants start experiencing high heat levels, oxygen supply reduce leading to suffocation, and the compartment may collapse (Mannan and Lees 16-174). 25. Figure: standard fire curve (Rockwool International A/S) Works Cited Dunn, Andrew. Heat. London: Thomson Learning, 1993. Print. Mannan and Frank Lees. Lee's Loss Prevention in the Process Industries; Hazard Identification, Assessment and Control, Volume 1. (3rd ed.). Amsterdam: Elsevier, 2005. Print. Punchard, Neville A., and Frank J. Kelly: Free Radicals: A Practical Approach. Oxford: Oxford University Press, 1996. Print Rockwool International A/S. Fire Curve. Rockwool Firesafe Insulator, n.d. Web. 23 Feb. 2011. Silberstein, Eugene. Residential Construction Academy: HVAC. Florence, KY: Cengage Learning, 2004. Print. Stoessel, Francis. Thermal Safety of Chemical Processes: Risk Assessment and Process Design. Hoboken, NJ: Wiley-VCH, 2008. Print. West, Krista. States of Matter: Gases. Liquids and Solids. New York, NY: Infobase Publishing, 2008. Print. . Read More