Fundamentals of Combustion Lec 3: Chemical Thermodynamics Dr. Zayed Al-Hamamre Content
Process Heat Transfer 1-3 Process Heat Transfer 1-4
Process Heat Transfer 1-5 Theoretical and Excess Air Combustion reactions are usually run with more air than is needed to supply oxygen in stoichiometric proportion to the fuel. This has the effect of increasing the conversion of the valuable reactant at the expense of the cost of the excess reactant and additional pumping costs. Theoretical Oxygen: The moles (batch) or molar flow rate (continuous) of O 2 needed for complete combustion of all the fuel fed to the reactor, assuming that all carbon in the fuel is oxidized to CO 2 and all the hydrogen is oxidized to H 2 O Theoretical Air: The quantity of air that contains the theoretical oxygen.
Excess Air: The amount by which the air fed to the reactor exceeds the theoretical air If 50% excess air is supplied Example One hundred mol/h of butane (C 4 H 10 ) and 5000 mol/h of air are fed into a combustion reactor. Calculate the percent excess air. The stoichiometric equation for complete combustion of butane: The theoretical air from the feed rate of fuel and the stoichiometric equation
Example Ethane is burned with 50% excess air. The percentage conversion of the ethane is 90%; of the ethane burned. 25% reacts to form CO and the balance reacts to form CO 2. Calculate the molar composition of the stack gas on a dry basis and the mole ratio of water to dry stack gas. Basis: 100 mol C 2 H 6 Fed Assumption: nitrogen is inert-that is, Thus, neglect the trace amounts of No x that might form
Degree-of-Freedom Analysis
Another approach: 25% Conversion to CO 75% Conversion to CO 2 0.75 CO 2 CO 2 Atomic Hydrogen Balance: Atomic Oxygen Balance:
The analysis of the stack gas is now complete Quiz: Find the composition in wet base The mole ratio of water to dry stack gas is Example A hydrocarbon gas is burned with air. The dry-basis product gas composition is 1.5 mole% CO, 6.0% CO 2, 8.2% O 2, and 84.3% N 2. There is no atomic oxygen in the fuel. Calculate the ratio of hydrogen to carbon in the fuel gas and speculate on what the fuel might be. Then calculate the percent excess air fed to the reactor. Basis: 100 mol Product Gas
Degree-of-Freedom Analysis
The fuel composition described by the formula To find the Percent Excess Air Process Heat Transfer 1-20
Process Heat Transfer 1-21 Process Heat Transfer 1-22
Process Heat Transfer 1-23 Process Heat Transfer 1-24
Process Heat Transfer 1-25 Process Heat Transfer 1-26
Process Heat Transfer 1-27 Process Heat Transfer 1-28
Process Heat Transfer 1-29 Process Heat Transfer 1-30
Process Heat Transfer 1-31 Process Heat Transfer 1-32
Summary and Examples 25 o C and 1 atm Example Example Cont.
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Heat of reaction If there are no gaseous reactants or products, then Example Formation reaction and heat of formation The standard heat of formation of an elemental species (e.g., O 2 ) is zero. If ν A is the stoichiometric coefficient of the i th species participating in a reaction (+ for products, - for reactants) and is the standard heat of formation of this species, then the standard heat of the reaction is Example
Heat of combustion The standard heat of combustion of a substance,, is the heat of the combustion of that substance with oxygen to yield specified products [e.g., CO 2 (g) and H 2 O(l)], with both reactants and products at 25 o C and 1 atm (the arbitrary but conventional reference state). standard heats of combustion for a substances are tabulated. The given values are based on the following assumptions: (a) All carbon in the fuel forms CO 2 (g), (b) All hydrogen forms H 2 O(l), (c) All sulfur forms SO 2 (g), and (d) All nitrogen forms N 2 (g). Standard heats of reactions that involve only combustible substances and combustion products can be calculated from tabulated standard heats of combustion. For CO 2,H 2 OandSO 2, the standard heats of combustion is equal to zero. Heat of combustion For combustion reaction, Example
Energy balance on reactive processes Example Example Cont. 1. Use the heat of reaction method for the energy balance. 2. Choosing as references the reactant and product species in the states for which the heat of reaction is given
Example Cont. Or one step or heat the liquid water from 25 C to l00 C, vaporize it, heat the vapor from l00 C to 300 C, Example Cont.
Example Cont. Thus, 19,700 kw of heat must be transferred from the reactor to maintain the product temperature at 300 C. If less heat were transferred, more of the heat of reaction would go into the reaction mixture and the outlet temperature would increase. Example
Example Cont. Two or multiple reactions system or Single reaction with unknown ΔΗ r material balance is not required Example Cont.
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Example Cont. to calculate then ΔΗ Processes with Unknown Outlet Conditions: Adiabatic Reactors Example Quiz: Find the composition of the output stream
Example Cont. Use either the Heat of Reaction Method or Heat of Formation Method As references for enthalpy calculations Example Cont.
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Example cont. Quiz Example cont.
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Fuels and Their Properties The heating value The heating value of a combustible material is the negative of the standard heat of combustion. The higher heating value (or total heating value or gross heating value) is With H 2 O(l) as a combustion product,. The lower heating value (or net heating value) is the value based on H 2 O (v) as a product. Since is always negative, the heating value is positive. To calculate a lower heating value of a fuel from a higher heating value or vice versa, you must determine the moles of water produced when one mole of the fuel is burned. o If this quantity is designated n, then Where the heat of vaporization of water at 25 o C is If a fuel contains a mixture of combustible substances, its heating value (lower or higher) is Fuels and Their Properties x i is the mass fractions of the fuel components if the heating values are expressed in units of (energy)/(mass), or x i mole fractions if the dimensions of the heating values are (energy)/(mole).
Example Cont. Example Example Cont.
Adiabatic Flame Temperature When a fuel is burned, o A considerable amount of energy is released. o Some of this energy is transferred as heat through the reactor walls, o The remainder raises the temperature of the reaction products; o The less heat transferred, the higher the product temperature. o The highest achievable temperature is reached if the reactor is adiabatic and all of the energy released by the combustion goes to raise the temperature of the combustion products (adiabatic flame temperature, T ad ). In adiabatic reactor, Adiabatic flame temperature depends on air/fuel ratio T ad : Increases for preheated air, or for O 2 instead of air. T ad : decreases, if cooled exhaust gas is mixed with reactants (recycling) Process Heat Transfer 1-68
Adiabatic Flame Temperature Example Example Cont.
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