Fire Chapter 2

Information about Fire Chapter 2

Published on July 28, 2014

Author: pthomp

Source: authorstream.com

Content

PowerPoint Presentation: Fire, Explosions and Process Safety CHEMISTRY OF FIRES Dr Pauline Thompson PowerPoint Presentation: General chemical reaction Simple chemical reaction for fires may be written molecular concentration of D be C D , of G be C G rate of disappearance of D ( r D ) PowerPoint Presentation: General chemical reaction Law of mass action: where the d and g are from the proportions in the reaction k is the rate constant for the reaction from Arrhenius equation (k not constant) A = Arrhenius factor, E a = activation energy, R = gas constant = 8.314 JK 1mol 1, T = absolute temperature (K) PowerPoint Presentation: General chemical reaction For example: Reaction rate is thus PowerPoint Presentation: Combustion and Energy Release Exothermic oxidative reaction = produces heat and consumes oxygen Heat of combustion ( H C ) is the heat released when a unit quantity of fuel (25°C and atm pressure) is oxidised completely E.g. C 3 H 8 + 5O 2 → 3CO 2 + 4H 2 O H Cliq (C 3 H 8 ) = 2220 kJmol -1 for water in its liquid state H Cvap (C 3 H 8 ) = 2044 kJmol -1 for water in its vapour state Latent heat of vaporisation of water H C vap - H C liq (C 3 H 8 ) = 2044 - 2220 = -176 kJmol -1 In flames water is vapour so use H C vap (= H C from now on) Either energy per mol or gram Mol weight of C 3 H 8 = 44 so 2044 kJmol -1 = 46.45 kJg -1 PowerPoint Presentation: Combustion and Energy Release Tricky – estimate oxygen instead of fuel consumed Typical organic liquids/gases H COx = 12.72 (± 3%) kJgram -1 O 2 Most polymers H COx = 13.02 (± 4%) kJgram -1 of O 2 As an overall estimate, H COx = 13 kJgram -1 may be taken. Avagadro’s Law - equal volumes of gases contain equal numbers of molecules at the same T and P Initial proportion of oxygen in air can be taken to be 0.21 Heat release rate: ( kJs -1 or kW) Can also express in terms of air – as 21% H C air = 3 kJgram -1 = 3000 kJkg -1 PowerPoint Presentation: Stoichiometric burning Complete combustion of a fuel All the carbon becomes carbon dioxide All the hydrogen oxides to water N 2 actually goes straight through 1 mole of PMMA requires 6 moles of O 2 and 22.56 moles of N 2 = a total of 28.56 moles of air Molecular weight of PMMA ~ 100, air ~28.95 1 gram of PMMA requires (28.56 x 28.95)/100 = 8.27 g of air PowerPoint Presentation: Stoichiometric burning General equation of conservation of mass 1 g fuel + Sr g air → (1 + Sr ) g products where Sr is called the stoichiometric air/fuel mass ratio For PMMA the stoichiometric ratio is thus 8.27. Sr may also be used to estimate H C for a fuel. E.g. H Cair = 3 kJg -1 , then H C(PMMA) = 3 × 8.27 = 24.8 kJg -1 PowerPoint Presentation: Flammability limits More later… Combustion is a gas phase reaction Liquids and solids must produce vapours above the surface in order to burn Solids the escape of gas from the surface is known as pyrolysis Enhanced at higher temperatures Air/fuel gas mixture must lie between flammability limits E.g. methane = only if concentration in air = 5-15% PowerPoint Presentation: Summary Chemical reactions to calculate the rate of loss of fuels. The rate of reaction increases exponentially with temperature. Combustion is an exothermic process. The heat released per unit mass consumed is called the heat of combustion H C . Heat released complicated when there are many products. Simplification = use heat of combustion for oxygen H COx . Remarkably constant value =13 kJgram -1 for common fuels. Equivalent value for air is approximately H Cair = 3 kJgram -1 . Heat released calculated by measuring the oxygen consumed in combustion. Avogadro’s Law ( kW) Complete combustion of a fuel occurs when the proportion of air to fuel corresponds to the stoichiometric ratio. PowerPoint Presentation: Discussion Questions Now we recommend: Read through these sections in the notes Tackle the discussion the questions at the end Work with your fellow students on the discussion boards PowerPoint Presentation: Any Questions ? If so – post them in the discussion boards

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