Thermodynamics

782 | ThermodynamicsHybrid power systems (HPS) that combine high-temperature fuel cells andgas turbines have the potential for very high efficiency in converting naturalgas (or even coal) to electricity. Also, some car manufacturers are planningto introduce cars powered by fuel-cell engines, thus more than doubling theefficiency from less than 30 percent for the gasoline engines to up to 60 percentfor fuel cells. In 1999, DaimlerChrysler unveiled its hydrogen fuel-cellpowered car called NECAR IV that has a refueling range of 280 miles andcan carry 4 passengers at 90 mph. Some research programs to develop suchhybrid systems with an efficiency of at least 70 percent by 2010 are underway.SUMMARYAny material that can be burned to release energy is called afuel, and a chemical reaction during which a fuel is oxidizedand a large quantity of energy is released is called combustion.The oxidizer most often used in combustion processes isair. The dry air can be approximated as 21 percent oxygenand 79 percent nitrogen by mole numbers. Therefore,1 kmol O 2 3.76 kmol N 2 4.76 kmol airDuring a combustion process, the components that existbefore the reaction are called reactants and the componentsthat exist after the reaction are called products. Chemicalequations are balanced on the basis of the conservation ofmass principle, which states that the total mass of each elementis conserved during a chemical reaction. The ratio ofthe mass of air to the mass of fuel during a combustionprocess is called the air–fuel ratio AF:AF m airm fuelwhere m air (NM) air and m fuel (N i M i ) fuel .A combustion process is complete if all the carbon in thefuel burns to CO 2 , all the hydrogen burns to H 2 O, and allthe sulfur (if any) burns to SO 2 . The minimum amount of airneeded for the complete combustion of a fuel is calledthe stoichiometric or theoretical air. The theoretical air isalso referred to as the chemically correct amount of air or100 percent theoretical air. The ideal combustion process duringwhich a fuel is burned completely with theoretical air iscalled the stoichiometric or theoretical combustion of thatfuel. The air in excess of the stoichiometric amount is calledthe excess air. The amount of excess air is usually expressedin terms of the stoichiometric air as percent excess air or percenttheoretical air.During a chemical reaction, some chemical bonds are brokenand others are formed. Therefore, a process that involveschemical reactions involves changes in chemical energies.Because of the changed composition, it is necessary to have astandard reference state for all substances, which is chosen tobe 25°C (77°F) and 1 atm.The difference between the enthalpy of the products at aspecified state and the enthalpy of the reactants at thesame state for a complete reaction is called the enthalpy ofreaction h R . For combustion processes, the enthalpy of reactionis usually referred to as the enthalpy of combustion h C ,which represents the amount of heat released during a steadyflowcombustion process when 1 kmol (or 1 kg) of fuel isburned completely at a specified temperature and pressure.The enthalpy of a substance at a specified state due to itschemical composition is called the enthalpy of formation h f .The enthalpy of formation of all stable elements is assigned avalue of zero at the standard reference state of 25°C and 1atm. The heating value of a fuel is defined as the amount ofheat released when a fuel is burned completely in a steadyflowprocess and the products are returned to the state of thereactants. The heating value of a fuel is equal to the absolutevalue of the enthalpy of combustion of the fuel,Heating value 0h C 01kJ>kg fuel2Taking heat transfer to the system and work done by thesystem to be positive quantities, the conservation of energyrelation for chemically reacting steady-flow systems can beexpressed per unit mole of fuel asQ W a N p 1h° f h h°2 p a N r 1h° f h h°2 rwhere the superscript ° represents properties at the standard referencestate of 25°C and 1 atm. For a closed system, it becomesQ W a N p 1h° f h h° Pv 2 p a N r 1h° f h h° Pv 2 rThe Pv terms are negligible for solids and liquids and can bereplaced by R u T for gases that behave as ideal gases.