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Thermodynamic, exergy and exergo-environmental analysis of waste feedstock for the H2 production: A simulation study

Authors: Bilal Kazmi,Syed Ali Ammar Taqvi,Usama Ahmed,Libor Štěpanec,Salman Raza Naqvi
Journal: Energy Conversion and Management: X
Publisher: Elsevier BV
Publish date: 2025-10
ISSN: 2590-1745 DOI: 10.1016/j.ecmx.2025.101211
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In Table 8 (page 8), the authors report a H₂ yield of 99.96% for Case VI (10% OSW + 90% PW) at 600°C with a steam-to-biomass ratio of 0.89 and CaO-to-biomass ratio of 2.22.

However, this value is thermodynamically and chemically unlikely for the following reasons:

-Thermodynamic Limitation:
Steam gasification of carbonaceous materials (even with catalysts like CaO) cannot achieve nearly 100% hydrogen yield due to:

The water-gas shift reaction equilibrium:

CO+H2O↔CO2+H2

​This reaction has a limited equilibrium constant that prevents complete conversion to H₂.
The presence of other elements (C, O, N, S) in the feedstock necessarily leads to the formation of CO, CO₂, CH₄, and other gases.

– Empirical Evidence:
The authors themselves cite numerous studies in Table 9 (page 11) where H₂ yields from co-gasification of various biomass and waste blends typically range between 25% and 93%. A value of 99.96% is unprecedented and contradicts both the literature and fundamental thermodynamics.

– Internal Inconsistency:
In Figure 4 (page 9), the mole fraction of H₂ for Case VI is shown as ~0.99, which is already suspicious.
However, in Table 8, the value is reported as 99.96%, which implies near-pure hydrogen with almost no other gases; a physical impossibility in a gasification process that inherently produces syngas (a mixture of H₂, CO, CO₂, CH₄, etc.).

-Modeling Artifact:
The authors used the RGibbs reactor in Aspen Plus, which minimizes Gibbs free energy. While this is a valid approach, it is highly sensitive to:

      + Incorrectly defined feedstock properties (ultimate/proximate analysis)
      + Unrealistic assumptions (e.g., perfect mixing, no tar formation, ideal kinetics)
      + Over-simplified reaction sets
   

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