Chapter 18 - Fuels (Courtesy of Biscuit Notes)

@biscuitnotes on carousell 1

@biscuitnotes on carousell Chapter18-FuelsandCrudeOil Sourcesoffuel:- Fossil fuelsintheformofcrudeoil,natural gasandcoal (carbon)- Biofuels Fossil fuelsareburnedinoxygentoformCO2 andH2 O.Thisexothermicreactioniscombustionandthethermal energygeneratedfromthecombustionoffossil fuelscanbeconvertedtootherformsofenergy Combustion:Fuel burnsinthepresenceofexcess/sufficientoxygentoproduceCO2 andH2 OIncompletecombustion:FuelsburninthepresenceofinsufficientoxygentoproduceCOand/orC(s)andH2 O CharacteristicsofNatural gas1. Itiscolourless2. Itisamixtureoflighthydrocarbons,mainlyconsistingofmethane3. Itcanbeeasilyliquefied(duetolowboilingpoint)andstoredintanks.Itcanflow4. Itproducesalmostnopollutant5. Combustion: - CompletecombustionCH4 burnsinexcessoxygentoformCO2 andH2 Otoproduceenergy𝐶𝐻4 (𝑔) + 2𝑂2 (𝑔)→𝐶𝑂2 (𝑔) + 2𝐻2 𝑂(𝑙) - IncompletecombustionCH4 burnsininsufficientoxygentoformCOand/orC(s)andH2 Otoproduceenergy𝐶𝐻4 (𝑔) +𝑂2 (𝑔)→𝐶𝑂(𝑔) + 𝐶(𝑠) +𝐻2 𝑂(𝑙) - Comparingcompleteandincompletecombustion,for1moleofCH4 ,2molesofO2 isrequiredforcompletecombustionwhereasonly1moleofO2 isrequiredforincompletecombustion.Hence,whenthereisinsufficientoxygensupply,completecombustionismorelikelytooccuraslessmoleofO2 isrequiredforthesamemoleofCH4 - However,formethane,mostofthetimes,completecombustionoccurs99%ofthetime,asinmostcases,therewill besufficientoxygensuppliedforcompletecombustiontotheitslowpercentagemassofcarbon(ithasonly1carbonatomineachmolecule)- Lowpercentagemassofcarbon: -[1]𝐶𝐻4 (𝑔) + 2𝑂2 (𝑔)→𝐶𝑂2 (𝑔) + 2𝐻2 𝑂(𝑙)- Higherpercentageofcarbon: -[2]𝐶3 𝐻8 (𝑔) + 5𝑂2 (𝑔)→3𝐶𝑂2 (𝑔) + 4𝐻2 𝑂(𝑙) Whichequation[1]or[2]will giveoutsootierproducts/havemoreincompletecombustion? 71

@biscuitnotes on carousell - ForthesamemoleofCH4 andC3 H8 ,C3 H8 requiresmoreoxygenthanCH4 forcompletecombustion.Hence,thismeansthatC3 H8 will undergomoreincompletecombustionascomparedto ,henceproducingmoresoot.𝐶𝐻4 - Ruleofthumb:Forthesamemoleoffuel,thefuel thatrequiresthegreaternumberofmolesofoxygenforcompletecombustionwill undergomoreincompletecombustionandhencemoresoot/COwill beproduced Characteristicsofcrudeoil1. Itisadarkbrownliquid2. Itisamixtureofhydrocarbons(compoundswithhydrogenandcarbononly)3. Itcanbestoredandtransportedeasilybytankers.Itcanflowthroughapipeeasily.4. Itismoderatelypollutingassomesootisproducedduetoincompletecombustion(duetoslightlyhigherpercentagemassofcarboncomparedtonatural gas)5. Italsoundergoescombustionintheformsofcompleteandincompletecombustion. Characteristicsofcoal:1. Itisablacksolid2. Itismainlyjustcarbon3. Itisnoteasilystoredandtransported.4. Itisextremelypollutingaslargeamountofsootisproducedduetoincompletecombustion(dueto100%percentagemassofcarbo)5. Itundergoescombustionintheformsofcompleteandincompletecombustion. Completecombustion:- Coal burnsinexcessoxygentoformCO2 toproduce- 𝐶(𝑠) +𝑂2 (𝑔)→𝐶𝑂2 (𝑔) Incompletecombustion:- Coal burnsininsufficientoxygentoformCOtoproduceenergy- 2𝐶(𝑠) +𝑂2 (𝑔)→2𝐶𝑂(𝑔) 72

@biscuitnotes on carousell Separationofcrudeoil:Fractional Distillation- Petroleum/crudeoil isheatedinthefurnaceandvaporizeintothefractionatingcolumn- Asthevapourrisesupthefractionatingcolumn,itcoolsandcondenseatdifferentboilingpoints.- Fractionwithlowerboilingpointcondenseatthetopofthecolumn(smallerhydrocerbons)andisdistilledoverfirst- Fractionwithhigherboilingpointcondenseatthebottomofthecolumn(largerhydrocarbons) Fractional distillationmakesuseofthefactthat- Hydrocarbonswithmorecarbonatomshaveahigherboilingpointthanonewithfewercarbonatoms- Thisisbecausehydrocarbonswithmorecarbonatomshavealargermolecularsizethanhydrocarbonswithfewercarbonatoms,andhence,therewill bemoreintermolecularforcesofattractionbetweenhydrocarbonswithmorecarbonatoms.- Hence,moreenergyisrequiredtoovercometheintermolecularforcesofattractionbetweenhydrocarbonswithmorecarbonatoms,thusresultinginahigherboilingpoint- Hence,hydrocarbonswithfewercarbonatomswill distill overfirstatthetopofthefractionatingcolumn,leavingthehydrocarbonswithmorecarbonatomstodistill overatthebottom 73

@biscuitnotes on carousell Hydrogenasapotentialfuel- Itisarenewableenergyresource- Itisobtainedfrom:1.Reactionofnatural gaswithsteam+ CO+3 , +CO +𝐶𝐻4 𝐻2 𝑂(𝑔)→ 𝐻2 𝐻2 𝑂 → 𝐶𝑂2 𝐻2 2.Electrolysisofwater+2𝐻2 𝑂(𝑙)→𝑂2 (𝑔) 2𝐻2 (𝑔)3.Crackingofhydrocarbons+𝐶2 𝐻6 (𝑔)→𝐶2 𝐻4 (𝑔) 𝐻2 (𝑔) Advantagesofusinghydrogen:- Hydrogenisrenewable- Hydrogendoesnotcausepollutionasonlywaterisformedwhen reactswith𝐻2 𝑂2 (fossil fuelsform )𝐶𝑂2 - Whenhydrogenisburnt,itreleasesmorethantwicetheamountofenergycomparedtomanyotherfuels(efficientpowersource) Disadvantagesofusinghydrogen:- Expensive.Obtaining fromelectrolysisisexpensive/crackingofhydrocarbons𝐻2 - Hydrogenisextremelyflammableandexplosive- Hydrogenisagasatrtpandrequireslargestoragevolumescomparedtootherfuelswhichareliquidsatrtp(liquidsrequirelessstoragevolume) Fuelcell(similartoasimplecell):Afuel cell isachemical cell inwhichreactants(fuelandoxygen)arecontinuouslysuppliedtoproduceelectricitydirectly(chemical energy→electrical energy) AdvantagesofFuel Cell:- Therearenopollutantsformedashydrogenreactswithoxygentoproducewater- Itismoreefficientascomparedtocoal/fossil fuelsasitconvertschemical energydirectlytoelectrical energy DisadvantagesofFuel Cell:- Itisexpensivetosourceforhydrogengasthroughelectrolysisofwaterorcrackingofhydrocarbons Howiselectricityproduced?- Electronsareproducedattheanodewhenhydrogengasreactswiththeelectrolyte( ions)𝑂𝐻 − - Theelectronsproducedattheanodizedwill betransferredtothecathodewherereductionoccurs- Theflowofelectronsfromanodetocathodegenerateselectricity 74