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Exergy efficiency improvement in hydrogen production process by recovery of chemical energy versus thermal energy

Abstract

Exergy analysis is recently being employed as one of the preferred methods to improve the design performance of a system and to achieve overall sustainability. Exergy is mainly composed of physical or thermo-mechanical and chemical components and a single stream can possess one or more forms of exergy. Where there is exergy lost in unused chemical streams or wasted energy, the recovery of exergy would reduce losses and increase the second law efficiency of the process. In many chemical process plants such as hydrogen (H2), ammonia, nitric acid, etc., there is a potential to recover waste or excess heat by process heat exchange or by generating utilities. For a process like steam–methane (CH4) reforming (SMR), exergy efficiency can be improved by recovering the available excess heat partially or fully in the form of chemical energy or thermal energy. This paper presents the generalised system analysis to show that the recovery of exergy in the form of chemical energy is better than in thermal energy form due to fewer losses and higher efficiency. The concept is illustrated with the example of a simple combustion system with excess heat in which saving fuel proves to be more exergy efficient than generating utility. The approach is applied to an industrial case study of H2-producing SMR plant with two modified cases of steam generation and recycling portion of unconverted CH4 as feed. In the case study, heat exchanger network is treated as a separate process component and a simple methodology is proposed to calculate the exergy losses for the same. The results of the case study prove that the recovery of chemical energy is more efficient than that of thermal energy from an exergy perspective.

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Abbreviations

b :

Exergy of stream (kW)

b a :

Exergy of air (kJ/mol)

b f :

Chemical exergy of fuel (kJ/mol)

C vf :

Constant volume specific heat capacity of fuel (kJ/mol K)

ExC :

Chemical exergy (kW)

Exch :

Recoverable chemical exergy (kW)

E x in :

Total input exergy (kW)

ExP :

Physical exergy (kW)

E x prod :

Exergy of desired products (kW)

\({\text{Ex}}_{\text{th}}^{\hbox{max} }\) :

Maximum thermal exergy (kW)

ε :

Exergy factor

\(\overline{h}\) :

Generalised enthalpy (kJ/mol)

int:

Interval

m :

Mass/molar flowrate (g/s, mol/s)

m fr :

Reduced mass/molar flowrate of fuel (g/s, mol/s)

Δm a :

Change in mass/mole of air (g/s, mol/s)

η Ex :

Exergy efficiency (%)

P 0 :

Reference pressure (atm)

Q in :

Heat reservoir at temperature T in (kW)

Q r :

Heat recovered (kW)

Q 0 :

Heat interaction with surroundings (kW)

S :

Entropy (kJ/K, kW/K)

S gench :

Entropy generation (chemical exergy recovery case, kW/K)

S genth :

Entropy generation (thermal exergy recovery case, kW/K)

T C :

Temperature of a cold system (°C)

T flame :

Flame temperature of fuel (°C)

T flamelm :

Logarithmic mean flame temperature of fuel (°C)

T H :

Temperature of a hot system (°C)

T r :

Recovery temperature (°C)

T rlm :

Logarithmic mean recovery temperature (°C)

T 0 :

Reference temperature (°C)

W :

Work transfer (kW)

φ :

Ratio of chemical exergy–LHV of fuel

ψ :

Ratio of chemical exergy–HHV of fuel

GCC:

Grand composite curve

HEN:

Heat exchanger network

HHV:

Higher heating value

ΔH :

Heat of reaction (kJ/mol)

LHV:

Lower heating value

MPTA:

Modified problem table algorithm

SMR:

Steam–methane reforming

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Acknowledgments

Authors would like to acknowledge the financial support obtained from Orica Mining Services, Newcastle, Australia for this study.

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Correspondence to Santanu Bandyopadhyay.

Appendix

Appendix

See Tables 2, 3, 4, 5, 6 and 7.

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Thengane, S.K., Hoadley, A., Bhattacharya, S. et al. Exergy efficiency improvement in hydrogen production process by recovery of chemical energy versus thermal energy. Clean Techn Environ Policy 18, 1391–1404 (2016). https://doi.org/10.1007/s10098-016-1094-2

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Keywords

  • Exergy recovery
  • Thermal exergy
  • Chemical exergy
  • Combustion
  • Steam–methane reforming