Oxidative steam reforming of glycerol for hydrogen production: Thermodynamic analysis including different carbon deposits representation and CO_2 adsorption

摘要

Glycerol production is associated with growing biodiesel industry, and because of its poor fuel skills it has been signed as a candidate for hydrogen production by Steam Reforming (SR). In various reforming reaction systems, many different types of Carbon deposits have been reported, but this variety has not been incorporated in thermodynamic studies as previous studies have represented carbon deposits only as Graphite. This work proposes a new representation including Graphite, Carbon Nanotubes, Amorphous, and Polymeric carbon for Glycerol Reforming systems. The study also includes an analysis of CO_2 sorption effects, comparing Hydrotalcites and CaO as sorbents, with their respective variation of sorption capacity with temperature. All thermodynamic analysis is performed by Gibbs free energy minimization, following an algorithm for discrete nonlinear minimization. The extended representation of carbon deposits reveals the existence of two regions: below 450 ℃ the most favorable carbonaceous solid type is graphite; and above, carbon nanotubes. The use of CO_2 sorbents in Glycerol Reforming systems shifts the equilibrium to products, increasing H_2 yield. In those systems where Hydrotalcites were included as CO_2 sorbent, H_2 yield is maximized between 350 ℃ and 450 ℃ and S/G ratio above the stoi chiometric ratio, while for CaO sorbent and no sorbent systems the maximization of H_2 yield is given at 600 ℃ and S/G = 10. From the thermodynamic analysis, once the O/G ratio has been chosen according to energetic consideration, it is advisable to carry out the Glycerol Reforming reaction with at least a stoichiometric S/G ratio, the addition of enough mass of Hydrotalcites for stoi-chiometric CO_2 sorption and a temperature between 375 ℃ and 450 ℃. Those conditions maximize the H_2 yield with no other product gases or carbonaceous solids.