Problems
Combustion Analysis
One kmol of ethane (C2H6) is burned with an unknown amount of air. An analysis of the combustion products reveals that the combustion is complete, and there are 3 kmol of free O2 in the products. Determine the air-fuel ratio and the percentage of theoretical air.
Combustion Analysis
Octane (C8H18) is burned with dry air. The mole fractions of the products on a dry basis are 9.21 percent CO2, 0.61 percent CO, 7.06 percent O2, and 83.12 percent N2.
Determine the air–fuel ratio and the percentage of theoretical air used.
Dewpoint Temperature
In a closed system, 3 kg of C4H10 (butane) is burned with 75 kg of saturated moist air at 30 ◦C, 90 kPa. Determine the air-fuel ratio and the dewpoint temperature of the combustion product when the products are at 0.5 bar.
Dewpoint Temperature
A mass flow of 5 kg of C12H26 (dodecane) is burned with a mass flow of 150 kg of moist air at 30 ◦C, 90 kPa with relative humidity of 80%. Determine the relative amount of excess air, and the dewpoint temperature of the combustion product when the products are at 0.7 bar.
Combustion of Liquid Fuel Mixture
A liquid mixture of 90 mol octane (C8H18) and 10 mol ethyl alcohol (C2H5OH) at 25 ◦C, 1 atm is burned isobarically at 150% theoretical air with dry air at 25 ◦C. Heat is transferred to the surroundings and the final product temperature is 25 ◦C.
1. Determine the mole numbers of the combustion products.
2. Determine the amount of liquid water in the product.
3. Determine the heat transferred to the surroundings.
Incomplete Combustion
When hydrocarbon fuels are burnt with less than theoretical air, the products may contain carbon monoxide, carbon (as soot) and hydrogen. When there is only little deficiency of oxygen, all hydrogen in the fuel will form water, but some carbon monoxide will exist. This is due to reaction kinetics: water and carbon monoxide are formed earlier in the combustion process, while carbon dioxide is formed later from the reaction between CO and O2.
Benzene gas, C6H6 at 25 ◦C is burned in a steady flow process with 95% of theoretical air that enters at 25 ◦C as well. The products leave at 1000 K.
Determine the mole fraction of CO in the product and the heat transfer from the combustion chamber.
Soot Formation
2 mole of propylene gas (C3H6) react with 6 moles of oxygen gas (O2) to form a mixture of water (H2O), carbon dioxide (CO2), carbon monoxide (CO) and soot (i.e., pure carbon, C). Determine the mole numbers of the products.
Non-adiabatic Flame Temperature of Acetylene
Acetylene gas (C2H2) at 25 ◦C is burned with 30% excess air at 27 ◦C, combustion is complete. The combustion chamber loses 75 kJ of heat per mole of fuel. Determine the temperature of the combustion products.
Adiabatic Combustion of Methanol
Liquid methanol (CH3OH) at 25 ◦C is burned adiabatically with excess air that enters the combustion chamber at a temperature of 47 ◦C, combustion is complete. The temperature of the combustion products is 1500 K. Determine the relative amount of excess air.
Combustion
Liquid Ethanol (C2H5OH) at 25 ◦C, 1 atm is burned isobarically with 50% excess air (dry air) at 25 ◦C. Heat is transferred to the surroundings and the final product temperature is 600 K.
1. Determine the mole fractions of the combustion products.
2. Determine the heat transferred to the surroundings for a mass flow of ethanol of 15 kg/ h.
Combustion of Dodecane
Dodecane (C12H26) is burned adiabatically with 150% excess air.
Determine the balanced reaction equation and compute the upper heating value.
Combustion of Ethane
Consider the combustion of ethane (C2H6) with 100% of excess air at a pressure of 1.74 bar. Consider the air as dry air.
1. Set up the chemical equation.
2. Compute the higher and the lower heating value.
3. Compute the dewpoint of the combustion products.
4. Compute the heat of reaction at the dewpoint.
Combustion of Diesel Fuel
Combustion: Heat and Entropy Generation In a technical process, a mass flow of 2 kg of liquid ethyl alcohol C2H5OH is burned with 50% excess air; both incoming flows are at standard conditions
(1 bar, 25 ◦C). The exhaust leaves at 500 K. Determine the heat provided by this process, and the entropy generation rate, assuming that the heat is received at 500 K.
Isochoric Combustion
Consider the adiabatic combustion of methyl alcohol vapor CH3OH with the stoichiometric amount of air in an 0.8 litre combustion chamber. Initially, the mixture is at 25 ◦C and 98 kPa.
Determine the maximum pressure that can occur in the combustion cham- ber if the combustion takes place at constant volume.
Combustion Analysis
An equimolar mixture of carbon monoxide (CO) and methane (CH4) is burned with 200% theoretical air (dry air). The mole flow of fuel is 2 kmol . Fuel stream and air enter the combustor at 25 ◦C, 1 atm, and the reactants leave at 127 ◦C. Determine:
1. The air fuel ratio on per mass basis.
2. The dew point of the products.
3. The heat transfer out of the combustion chamber.
4. Entropy generation and work loss.
5. The work to isothermally separate the CO2 from the exhaust gas.
Combustion: Ammonia as a Fuel
By means of catalysts, gaseous ammonia (NH3) reacts with oxygen (O2) to water (H2O) and molecular nitrogen (N2). Consider the oxidation of ammonia with with 200% excess air (dry) at a pressure of 3 bar.
1. Set up the combustion equation.
2. Determine the dewpoint of the combustion products.
3. Determine the heat of reaction per mole of NH3 at 25 ◦C for the cases that all product water is liquid, or all water is vapor, respectively.
Combustion Plant: Heat and Entropy Generation
In a technical process, a mass flow of 20 g of liquid ethyl alcohol C2H5OH is burned with 100% excess air; both incoming flows are at standard conditions (1 bar, 25 ◦C). The process produces a power of 300 kW, and the exhaust
leaves the plant at standard conditions.
1. Determine the heat rejection rate of the plant.
2. Determine the entropy generation rate, assuming that the plant rejects waste heat into the environment at 25 ◦C.
Hint: start with a good sketch.
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