Weight of dry exhaust gas formed per unit of fuel

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Reference no: EM13943528

COMBUSTION PROBLEM NO 29-

Ocatne is burnt with 10% excess air. Calculate-

A - air/fuel ratio by weight

b- air /fuel ratio by volume

c- weight of dry exhaust gas formed per unit of fuel.

d- mole of oxygen in the exhaust gas per unit weight of fuel.

e- mole of water vapour in exhaust gas per unit weight of fuel.

F - volume of exhaust gas at 1 atmosphere and 260 oc per unit weight of fuel.

The specific gravity of octane may be taken as 0.7.

SOLUTION -

Basis -1 kg mole of octane burnt-

Reaction - C8H18 +25/2 O2 = 8CO2 +9H2O

PART a- Theoretical oxygen demand = 12.5 kg mole

Oxygen supplied by 10% excess air = 12.5*110/100 = 13.75 kg- moles =13.75 *32 =440 kg

Nitrogen supplied by air = 13.75*79/21 = 51.73 kg - mole =51.73 *28 = 1448.4 kg

Amount of air supplied =13.75 +51.73 = 65.48 kg - mole =65.48 *29 kg

Molecular wt of air =29 kg/kg mole

Molecular wt of nitrogen = 28 kg/kg-mole

Molecular wt of oxygen = 32 kg/kg-mole

Molecular wt of octane ( fuel) = C8H18 = 8*12 +1*18 =96 +18 = 114 kg/kg mole

Weight of air/weight of fuel ( octane ) = 1888.4/114 = 16.56

Part - b - specific gravity of octane =0.7

Density of octane =0.7 g /cc = 0.7*1000 =700 kg /m³

Volume of fuel = 114/700 =0.163 m³

Assuming ideal gas law , volume of air at N.T.P

= 65.48 *22.4 = 1466.75 NM³

Volume of air/volume of fuel = 1466.75/0.163 = 8998.5

Part -c

Excess O₂ = supplied O₂ -- used O2 = 13.75 -12.50 =1.25 kg mole

Dry flue gas analysis-

Constituents amount kg mole molecular wt amout ,kg

CO₂ 8.0 44 44*8 =352

O₂ 1.25 32 1.25*32 =40

N₂ 51.73 28 28*51.73=1448.4

TOTAL 60.98 1840.40

WEIGHT OF DRY EXHAUST GAS/WEIGHT OF FUEL =1840.4/114 = 16.4

Part - d -

Mole of O₂ in the exhaust gas/weight of fuel = 1.25/114 = 0.011

Part - e -

Mole of water vapour in the exhaust gas/ weight of fuel = 9.0/114 = 0.079

Part - f -

Moles of exhaust gas ( wet) = 60.98 +9.0 = 69.98

By ideal gas law-

Volume at 260^oc and 1 atmosphere

V = n.RT/P = 69.98 *0.082 *(260 +273)/1.0 = 3060.8 M₃

VOLUME OF EXHAUST GAS ( WET)/WEIGHT OF FUEL = 3060.8/114 = 26.85

Reference no: EM13943528

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