Yunus A. Çengel,
University of Nevada, Reno
Michael A. Boles,
North Carolina State University
| Absolute entropy | is the entropy value relative to the absolute base established by the third law of thermodynamics.
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| Adiabatic flame or adiabatic combustion temperature | is the maximum temperature the products of combustion will reach in the limiting case of no heat loss to the surroundings during the combustion process. The adiabatic flame temperature attains its maximum value when complete combustion occurs with the theoretical amount of air.
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| Air-fuel ratio AF | is a frequently used quantity in the analysis of combustion processes to quantify the amounts of fuel and air. It is usually expressed on a mass basis and is defined as the ratio of the mass of air to the mass of fuel for a combustion process.
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| Chemically correct amount of air | is the stoichiometric or theoretical air, or 100 percent theoretical air.
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| Combustion | is a chemical reaction during which a fuel is oxidized and a large quantity of energy is released.
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| Combustion air | is dry air which can be approximated as 21 percent oxygen and 79 percent nitrogen by mole numbers. Therefore, each mole of oxygen entering a combustion chamber will be accompanied by 0.79/0.21 = 3.76 mol of nitrogen. To supply one mole of oxygen to a combustion process, 4.76 mol of combustion air are required.
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| Complete combustion | is a combustion process in which all the carbon in the fuel burns to CO2, all the hydrogen burns to H2O, and all the sulfur (if any) burns to SO2. That is, all the combustible components of a fuel are burned to completion during a complete combustion process.
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| Conservation of mass principle (or the mass balance) | is the principle used to balance chemical reaction equations. It can be stated as follows: The total mass of each element is conserved during a chemical reaction. The total mass of each element on the right-hand side of the reaction equation (the products) must be equal to the total mass of that element on the left-hand side (the reactants) even though the elements exist in different chemical compounds in the reactants and products. Even though the mass must be conserved, the total number of moles is not conserved during a chemical reaction.
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| Deficiency of air | results when the amounts of air are less than the stoichiometric amount.
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| Enthalpy of a chemical component | at a specified state is the sum of the enthalpy of formation of the component at 25°C, 1 atm, and the sensible enthalpy of the component relative to 25°C, 1 atm, which is the difference between the sensible enthalpy at the specified state ad the sensible enthalpy at the standard reference state of 25°C and 1 atm. This definition enables us to use enthalpy values from tables regardless of the reference state used in their construction.
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| Enthalpy of combustion | hC is the enthalpy of reaction during a steady-flow combustion process when 1 kmol (or 1 kg) of fuel is burned completely at a specified temperature and pressure and represents the amount of heat released.
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| Enthalpy of formation | is the enthalpy of a substance at a specified state due to its chemical composition. The enthalpy of formation of all stable elements (such as O2, N2, H2, and C) has a value of zero at the standard reference state of 25°C and 1 atm.
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| Enthalpy of reaction | hR is defined as the difference between the enthalpy of the products at a specified state and the enthalpy of the reactants at the same state for a complete reaction.
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| Equivalence ratio | is the ratio of the actual fuel-air ratio to the stoichiometric fuel-air ratio.
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| Excess air | is the amount of air in excess of the stoichiometric amount.
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| Exothermic reaction | is a reaction during which chemical energy is released in the form of heat.
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| Fuel | is any material that can be burned to release energy.
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| Fuel-air ratio | is the reciprocal of air-fuel ratio.
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| Fuel cells | operate on the principle of electrolytic cells in which the chemical energy of the fuel is directly converted to electric energy, and electrons are exchanged through conductor wires connected to a load. Fuel cells are not heat engines, and thus their efficiencies are not limited by the Carnot efficiency. They convert chemical energy to electric energy essentially in an isothermal manner.
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| Heating value | of a fuel is defined as the amount of heat released when a fuel is burned completely in a steady-flow process and the products are returned to the state of the reactants. In other words, the heating value of a fuel is equal to the absolute value of the enthalpy of combustion of the fuel.
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| Higher heating value | (HHV) is the heating value when the H2O in the products is in the liquid form.
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| Hydrocarbon fuels | are the most familiar fuels and consist primarily of hydrogen and carbon. They are denoted by the general formula CnHm. Hydrocarbon fuels exist in all phases, some examples being coal, gasoline, and natural gas.
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| Ignition temperature | is the minimum temperature to which a fuel must be brought to start the combustion.
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| Incomplete combustion | is a combustion process in which the combustion products contain any unburned fuel or components such as C, H2, CO, or OH.
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| Liquefied petroleum gas | (LPG) is a byproduct of natural gas processing or crude oil refining. It consists mainly of propane (over 90 percent), and thus LPG is usually referred to as propane. However, it also contains varying amounts of butane, propylene, and butylenes.
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| Lower heating value | (LHV) is the heating value when the H2O in the products is in the vapor form.
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| Natural gas | is produced from gas wells or oil wells rich in natural gas. It is composed mainly of methane, but it also contains small amounts of ethane, propane, hydrogen, helium, carbon dioxide, nitrogen, hydrogen sulfate, and water vapor. It is stored either in the gas phase at pressures of 150 to 250 atm as CNG (compressed natural gas) or in the liquid phase at 162°
C as LNG (liquefied natural gas).
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| Orsat gas analyzer | is a commonly used device to analyze the composition of combustion gases. The amounts of carbon dioxide, carbon monoxide, and oxygen are measured on a percent by volume and are based on a dry analysis.
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| Percent deficiency of air | is the deficiency of air expressed as a percent of stoichiometric air. For example, 90 percent theoretical air is equivalent to 10 percent deficiency of air.
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| Percent excess air | or percent theoretical air is the amount of excess air usually expressed in terms of the stoichiometric air. For example, 50 percent excess air is equivalent to 150 percent theoretical air.
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| Products | are the components that exist after the reaction in a combustion process.
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| Reactants | are the components that exist before the reaction in a combustion process.
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| Stable form of an element | is the chemically stable form of that element at 25°
C and 1 atm. Nitrogen, for example, exists in diatomic form (N2 ) at 25°
C and 1 atm. Therefore, the stable form of nitrogen at the standard reference state is diatomic nitrogen N2 , not monatomic nitrogen N.
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| Standard reference state | for the properties of chemical components is chosen as 25°C (77°F) and 1 atm. Property values at the standard reference state are indicated by a superscript (°) (such as h°and u°).
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| Stoichiometric air or theoretical air | is the minimum amount of air needed for the complete combustion of a fuel. When a fuel is completely burned with theoretical air, no uncombined oxygen will be present in the product gases.
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| Stoichiometric combustion or theoretical combustion | is the ideal combustion process during which a fuel is burned completely with theoretical air.
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| Third law of thermodynamics | is stated as the entropy of a pure crystalline substance at absolute zero temperature is zero.
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2002 McGraw-Hill Higher Education