1320 Fundamentals of Engineering Thermodynamics 8th edition

13.20. (Fundamentals of Engineering Thermodynamics 8th edition). A gaseous fuel mixture with a specified molar analysis burns completely with moist air to form gaseus products as shown in the figure. Determine the dew point temperature of the products in degrees Celsius.

uc 70% CH4, 10% H 3%2, 5% CO2, 12% N2 Moist air = 0.01 kg H2O/kg dry air Gaseous combustion products CO2. H,O, N2 at I a CO2. H20, N2 at 1 atm Fig. P13.20

Solution

Let us assume that 1 kg of dry air is intaken.

The combustion reaction is as follows:

CH₄ + 2O₂ + 2(4) N₂ = CO₂ + 2H₂O + 8N₂

[for 1mole of O₂, 12/3=4 mole of N₂ will be used ]

Here in the combustion reaction, 2 mole of H₂O is formed.

Also, 0.01 kg of H₂O per kg of dry air is introduced.

Now, 18 kg H₂O = 1 mole H₂O

So, 0.01 kg H₂O = (1/18)*0.01 mole H₂O = 0.0006 mole H₂O

In the flue gas, the total amount of vapour content = 2 + 0.0006 = 2.0006 mole H₂O

In the flue gas, CO₂ = 1 mole

H₂O = 2.0006 mole

N₂ = 8 mole

So, the % content of H₂O in the flue gas = 2.0006/11.0006 = 0.182

Now from the relation, = {P_v /(P_a - P_v)} *0.622

So, 0.182 = {P_v /(1.013- P_v)}*0.622 [1 atmospheric pressure = 1.013 bar]

P_v = 0.23 bar

The corresponding value of saturation temperature is 63.14°C [from steam table]

Hence, the dew point temperature of the product is 63.14°C.