Question

A coal gasifier in a power plant produces a gas mixture with the following volumetric percent composition:

CH4:0.3, H2:29.6, CO: 41.0, CO2:10.0, N2:0.8, H2O:17.0, H2S:1.1, NH3:0.2

This gas is cooled to 40°C, 3 MPa, and the H2S and NH3 are removed in water scrubbers. Assuming that the resulting mixture, which is sent to the combustor, is saturated with water, determine the mixture composition and the theoretical air-fuel ratio in the combustors.

Answer 1

Cool to 40°C PG = 7.384,

P = 3000 kPa

yH2O MAX= 7.384 /3000=nV/ nV+81.7

nV = 0.2016

CH4 H2 CO CO2 N2 H2O(v)

0.3 kmol 29.6 41.0 10.0 0.8 0.2016

81.9016 kmol (from 100 kmol of the original gas mixture)

0.3 CH4 + 0.6 O2 → 0.3 CO2 + 0.6 H2O

29.6 H2 + 14.8 O2 → 29.6 H2O

41 CO + 20.5 O2 → 41 CO2

⇒ Number of moles of O2 = 0.6 + 14.8 + 20.5 = 35.9

Number of moles of air = 35.9 + 3.76 × 35.9 (N2)

A/F = 28.97(35.9 + 3.76(35.9)) /0.3(16) + 29.6(2) + 41(28) + 10(44) + 0.8(28) + 0.2016(18)

= 2.95 kg air/kg fuel