Hydrogen production from reforming of glycerol in supercritical water

Abstract

This dissertation investigates different glycerol reforming technologies for hydrogen production. The studies are divided into two parts including i) thermodynamic analysis under thermal neutral gas phase reforming and supercritical water reforming conditions, and ii) experimental studies under supercritical water reforming condition in an Inconel 625 reactor with and without catalysts. Thermodynamic analysis by Gibbs minimization indicates that the thermal neutral condition can be achieved when water to glycerol ratio (WGR), oxygen to glycerol ratio (OGR), and operating temperature are carefully selected. The reformer level shows that only a small amount of OGR is required in the operation. Much higher OGR is required to provide sufficient energy especially for the feed preheating in the system level. Considering two modes of air feeding in the System level, the Single-feed mode is a superior mode in term of suppressing carbon formation _ no carbon formation is observed when operating at temperature above 900 K. The Split-feed mode offers higher H2 mole fraction in the product gas because N2 from air and part of CO2 are not present in the gas product in the Split-feed mode unlike in the Single-feed mode. Use of pure O2 in the Single-feed mode can increase the H2 mole fraction in the product and H2 yield. In addition, using the afterburner products stream to supply heat is beneficial to increase the H2 yield in the Split-feed mode as it decreases the fuel requirement (glycerol and air). Thermodynamic calculations of the supercritical water reforming indicates that higher operating temperature, lower pressure, and lower glycerol feed concentration promote hydrogen production. CO is almost inhibited at high pressure, achieving less than 0.01 of mole fraction. Above 1048 K, only slight change of components is found by increasing of operating temperature. The experimental results in the empty Inconel 625 reactor show that conversion of glycerol and hydrogen yield increases when increasing the operating temperature. However, carbon formation is found as a serious problem for low and medium feed rates at high operating temperatures. Lower feed glycerol concentration supports the hydrogen production without carbon formation. For the catalytic supercritical water reforming, cobalt and nickel metals with the La2O3, ɤ-Al2O3, α-Al2O3, ZrO2, and YSZ supports are chosen. Co/YSZ and Ni/La2O3 catalysts are found to be suitable catalysts in terms of glycerol conversion, hydrogen yield, and carbon formation at 773 and 798 K, respectively. The metal loading is an important parameter to enhance the hydrogen production. The optimum cobalt and nickel loadings are 10 and 15 wt.%, respectively.