CHE Combustion Reactions Fuels –Gaseous –Liquid –Solid Oxidants Elementary combustion reactions Theoretical and excess air

Gaseous Fuels Natural gas –Comprised mainly of methane (CH 4 ) –Found in underground gas reservoirs –Often contaminated by H 2 S, removed in “sweetening” processes –Either compressed or liquified (LNG) at low temperature (< 190 K) –Odorless (“odorizers” added for detection) Liquefied petroleum gas (LPG) –Mainly propane (C 3 H 8 ) and butane (C 4 H 10 ) –Derived from petroleum refining –Gaseous at room temperature and 1 atm –Liquid under pressure at room temperature

Gaseous Fuels Cont’d Water gas –Mixture of carbon monoxide (CO) and hydrogen (H 2 ) –Obtained by reacting water with carbon containing materials –Stored as a gas (e.g. “gasometers”) –Also called “blue gas” because of blue flame –Highly toxic and odorless –Odorizers added for detection (e.g. mercaptans) Examples: C + H 2 O  CO + H 2 CH 4 + H 2 O  CO + 3 H 2

Liquid Fuels Gasoline –Complex mixture of hydrocarbons including aromatics (e.g. toluene) and branched paraffins (e.g. isoctane) –Approximate overall formula: C 7 H 14 –Obtained from petroleum refining: Topping (distillation) Cracking Reforming, alkylation –Composition designed to reduce “knocking” in internal combustion engines - branched paraffins improve anti-knocking properties Octane number = percentage of isooctane in an octane/isooctane mixture that performs like the actual gasoline in a standard engine –Many other properties including vapor pressure and viscosity critical and adjusted seasonally

Liquid Fuels Cont’d Fuel oil –Complex mixture of hydrocarbons including mainly paraffins (e.g. dodecane, C 12 H 26 ) heavier than those found in gasoline –Obtained from petroleum refining as the portion that cannot be economically converted to gasoline: Topping (distillation) –Viscous liquid at room temperature Special burners required for efficient combustion –May contain sulfur (normally < 1%), which can produce SO 2

Solid Fuels Coal –Complex molecule comprising predominately fused anthracene rings –Sulfur normally present (up to 5%), which produces SO 2 upon combustion –Empirical formula (typical): C 70 H 41 O 6 N –Generates most CO 2 because of high carbon content

Renewable/Alternative Fuels Biodiesel –Obtained from transesterification of fats + NaOH + CH 3 OH + 3

Renewable/Alternative Fuels Ethanol –Obtained from fermentation –Current main feedstock is sugar from corn in US and sugar from sugar cane in Brazil –Lower energy content than gasoline –Water soluble and hygroscopic –Must be nearly anhydrous in order to mix with gasoline –Difficult to remove water by distillation because of azeotrope

Renewable/Alternative Fuels Hydrogen –Obtained by steam reforming hydrocarbons –Difficult to store/transport High pressure cylinders Cylinders filled with hydrogen adsorbent –Explosive and highly flammable –Not a practical combustion fuel –Makes sense for use in fuel cells Example: CH 4 + H 2 O  CO H 2

Fuel Cells Proton Exchange Membrane (PEM) Cell H2H2 2H + 2e - ½O 2 H2OH2O Electron current Proton current Anode (oxidation of H 2 ) Cathode (reaction with O 2 ) Overall reaction H 2 + ½O 2  H 2 O

Oxidant Air –Typical dry air composition –Practical: 21 mol% O 2, 79 mol% N 2 –Actual 21 mol% O 2 78 mol% N 2 1 mol% Ar 0.03 mol% CO 2 He, Ne, Kr, Xe ~ 0.01 mol% Humid air contains significant amounts of water vapor: e.g. at 70 F, 50% RH ~ 1.3 mol% (normally ignored in combustion calculations)

Elementary Combustion Reactions Complete combustion of Carbon Incomplete combustion of Carbon Other elements C + O 2  CO 2 C + ½ O 2  CO H + ¼ O 2  ½ H 2 O S + O 2  SO 2

Theoretical and Excess Air Theoretical air = air required for complete and stoichiometric combustion of all the fuel fed % Excess air = Actual air = (Theoretical air) x (1+%Excess/100) (Actual air - Theoretical air) (Theoretical air) x100