Bio-compounds as reducing agents of reforming catalyst and their subsequent steam reforming performance

At present, H2 is mainly produced through catalytic steam reforming of natural gas. Sustainable H2 production from renewable resources is of great significance to achieve a ‘hydrogen economy’ in the future. Aiming at exploring the potential of bio-derived fuel (e.g. bio-oil) for H2 production via chemical looping reforming (CLR), this study investigated the direct reduction of a reforming catalyst (18 wt% NiO/Al2O3) with five bio-compounds (acetic acid, ethanol, acetone, furfural and glucose) and subsequent steam reforming (SR), which represented one half of a cycle in CLR.
First, thermodynamic analysis was conducted. Results indicated that for a system consisting of bio-compound, steam and NiO above 200 °C, the bio-compounds would preferably reduce NiO rather than react with steam or decompose. The reduction was hardly affected by temperature, pressure, or the presence of steam. The formation of carbon during reduction depended on temperature and the availability of NiO. Moreover, the dependence of SR performance (equilibrium yields, and carbon formation) on temperature, molar steam to carbon ratio (S/C) and the type of bio-compound was studied. Equilibrium yields of H2, CO, CO2 and CH4 were successfully fitted into linear functions of the O/C and H/C ratios in bio-compound molecules. The wide suitability of these fitted equations made it possible to predict the potential of various feedstocks in H2 production without doing repeated simulation work.
Moreover, the integrated catalyst reduction and SR process was experimentally investigated in a packed bed reactor over the temperature range of 500-750 °C and S/C range of 4.5-9 for glucose and 0-5 for the other bio-compounds at atmospheric pressure. The effects of temperature and S/C on reduction kinetics as well as the subsequent SR were systematically investigated. Kinetic modelling was performed within NiO conversion of 0-50% and two-dimensional nuclei growth model (A2) was found to fit very well except for glucose. For all the bio-compounds, the apparent activation energy of NiO reduction was between 30 and 40 kJ/mol. Their pre-exponential factors decreased in this order: CH4>ethanol≈acetone>acetic acid>furfural> glucose, probably due to the different activities of reducing species they produced. Optimal S/C values for reduction kinetics were found between 1 and 2. The main barrier for each bio-compound in SR process was summarised.
In addition, temperature programmed reduction (TPR) of the NiO catalyst with solid bio-compounds (glucose and citric acid) under N2 was investigated by TGA-FTIR technique. It was found that the coke formed by bio-compound pyrolysis acted as the actual reductant for NiO reduction with CO2 as main reduction product. The reduction extent depended on the initial loading of bio-compounds and their charring characteristics. The reduction kinetics was studied by the Kissinger method. A two-step reduction mechanism (initially solid-solid reduction, and then gaseous reduction with CO) was proposed to explain the multiple reduction phases observed.