Chapter41-FireBASICCONCEPTSDougalDrysdaleTheChemistryandPhysicsofFireFireisamanifestationofuncontrolledcombustion.Itinvolvescombustiblematerialswhicharefoundaroundusinthebuildingsinwhichwelive,workandplay,aswellasawiderangeofgases,liquidsandsolidswhichareencounteredinindustryandcommerce.Theyarecommonlycarbon-based,andmaybereferredtocollectivelyasfuelsinthecontextofthisdiscussion.Despitethewidevarietyofthesefuelsinboththeirchemicalandphysicalstates,infiretheysharefeaturesthatarecommontothemall.Differencesareencounteredintheeasewithwhichfirecanbeinitiated(ignition),theratewithwhichfirecandevelop(flamespread),andthepowerthatcanbegenerated(rateofheatrelease),butasourunderstandingofthescienceoffireimproves,webecomebetterabletoquantifyandpredictfirebehaviourandapplyourknowledgetofiresafetyingeneral.Thepurposeofthissectionistoreviewsomeoftheunderlyingprinciplesandprovideguidancetoanunderstandingoffireprocesses.BasicConceptsCombustiblematerialsareallaroundus.Giventheappropriatecircumstances,theycanbemadetoburnbysubjectingthemtoanignitionsourcewhichiscapableofinitiatingaself-sustainingreaction.Inthisprocess,the“fuel”reactswithoxygenfromtheairtoreleaseenergy(heat),whilebeingconvertedtoproductsofcombustion,someofwhichmaybeharmful.Themechanismsofignitionandburningneedtobeclearlyunderstood.Mosteverydayfiresinvolvesolidmaterials(e.g.,wood,woodproductsandsyntheticpolymers),althoughgaseousandliquidfuelsarenotuncommon.Abriefreviewofthecombustionofgasesandliquidsisdesirablebeforesomeofthebasicconceptsarediscussed.DiffusionandpremixedflamesAflammablegas(e.g.,propane,C3H8)canbeburnedintwoways:astreamorjetofgasfromapipe(cf.thesimpleBunsenburnerwiththeairinletclosed)canbeignitedandwillburnasadiffusionflameinwhichburningoccursinthoseregionswheregaseousfuelandairmixbydiffusiveprocesses.Suchaflamehasacharacteristicyellowluminosity,indicatingthepresenceofminutesootparticlesformedasaresultofincompletecombustion.Someofthesewillburnintheflame,butotherswillemergefromtheflametiptoformsmoke.Ifthegasandairareintimatelymixedbeforeignition,thenpremixedcombustionwilloccur,providedthatthegas/airmixturelieswithinarangeofconcentrationsboundedbythelowerandupperflammabilitylimits(seetable41.1).Outsidetheselimits,themixtureisnon-flammable.(NotethatapremixedflameisstabilizedatthemouthofaBunsenburnerwhentheairinletisopen.)Ifamixtureisflammable,thenitcanbeignitedbyasmallignitionsource,suchasanelectricalspark.Thestoichiometricmixtureisthemostreadilyignited,inwhichtheamountofoxygenpresentisinthecorrectproportiontoburnallthefueltocarbondioxideandwater(seeaccompanyingequation,below,inwhichnitrogencanbeseentobepresentinthesameproportionasinairbutdoesnottakepartinthereaction).Propane(C3H8)isthecombustiblematerialinthisreaction:C3H8+5O2+18.8N2=3CO2+4H2O+18.8N2Table41.1LowerandupperflammabilitylimitsinairLowerflammabilitylimit(%byvolume)Upperflammabilitylimit(%byvolume)Carbonmonoxide12.574Methane5.015Propane2.19.5n-Hexane1.27.4n-Decane0.755.6Methanol6.736Ethanol3.319Acetone2.613Benzene1.37.9Anelectricaldischargeassmallas0.3mJissufficienttoigniteastoichiometricpropane/airmixtureinthereactionillustrated.Thisrepresentsabarelyperceptiblestaticspark,asexperiencedbysomeonewhohaswalkedacrossasyntheticcarpetandtouchedagroundedobject.Evensmalleramountsofenergyarerequiredforcertainreactivegasessuchashydrogen,ethyleneandethyne.Inp...
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