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ENGR5003 / Week 1
Lecture 3 Example 2 - physical and chemical exergy of hydrogenTutorialCurrent specThird8 min

ENGR5003 Lecture 3 Example 2 (slide 22)

Figure 1 follows hydrogen from storage to exhaust: H2\mathrm{H_2} at 1 MPa, 298 K is throttled to 0.1 MPa, mixed with air, burnt (products at 2 MPa, 1500 K), cooled to 0.1 MPa, 298 K and finally exhausted to the atmosphere (ambient T0T_0, p0p_0; mole fractions N2\mathrm{N_2} 75.6%, O2\mathrm{O_2} 20.3%, H2O\mathrm{H_2O} 3%, CO2\mathrm{CO_2} 0.03%, Ar 0.91%). Each bar shows the exergy of the stream split into reaction (chemical), diffusion (concentration), mechanical (pressure) and thermal parts; the white part of a bar is exergy that has been lost.

Figure 1: exergy of the hydrogen stream at each stage, split into reaction, diffusion, mechanical and thermal exergy (Lecture 3, slide 22).
Figure 1: exergy of the hydrogen stream at each stage, split into reaction, diffusion, mechanical and thermal exergy (Lecture 3, slide 22).
Formulas you may need
  • Exergy: the maximum useful work obtainable as a system comes into equilibrium with its surroundings (learn this, Lecture 3)
  • Exergy destroyed =T0Sg= T_0 S_g, with Sg≥0S_g \ge 0 for any real process (learn this; ΔSiso=Sg≥0\Delta S_{iso} = S_g \ge 0 is on the formula sheet)
  1. (a)
    For each step (pressure drop, mixing, combustion, cooling, exhaust), state which kind of exergy is lost and why.
  2. (b)
    Which step destroys the most exergy, and why does the stream still have exergy after cooling to 0.1 MPa and 298 K?
  3. (c)
    Suggest how the losses in the first three steps could be avoided or reduced.