(538c) Steam Reforming of Ethanol-Phenol Mixture As a Model for Biomass Tars: Effects of Ni Loading, Catalyst Conditioning and Boron and Magnesium Addition On Activity and Sulphur Poisoning of Ni-Al2O3 Catalysts

Introduction

The production of renewable hydrogen can be performed through biomass gasification, followed by treatment of the resulting syngas. This process is hampered by the presence of tars (a complex mixture of heavier and lighter oxygenated organic compounds) in the syngas, which causes several draw-backs.

The most promising industrial strategy for tar abatement after biomass gasification seems to be catalytic steam reforming. Ni-Al₂O₃ based catalysts are active in this reaction but they appear to be deactivated in particular by sulphur contaminants. For this reason they are reported to be not applicable below a reaction temperature limit. Sulphur is usually present in variable amounts in syngases produced by gasification in the range 20-200 ppm v_H2S/v. On the other hand, reaction temperature for the tar abatement  clean-up step may depend from the overall gasification plant configuration.

In this communication we will present our results concerning a study of steam reforming of ethanol-phenol mixture as a model for biomass tar, over silica-stabilized Ni-Al₂O₃ catalysts. The effect of Ni loading, catalysts  conditioning, boron and magnesium addition on the activity and sulphur deactivation behaviour.

Experimental

Catalysts preparation

Different families of catalysts have been used, prepared by conventional wet impregnation of Siralox 5/170 support (Alumina with 5% SiO₂ from Sasol, 170 m²/g) using Ni hexahydrate nitrate water solution. After impregnation, drying at 363K for 8 hours and calcination at 973K for 5 hours have been performed. Four different samples were used to investigate the effect of Ni loading: pure support (hereinafter denoted as NiØ) and Ni-Al₂O₃ containing 5%, 16% and 39% of Ni metal w_Ni/w_Al2O3 (hereinafter denoted as Ni5, Ni16 and Ni39 respectively). The addition of boron (1% w_H3BO3/w_cat) was performed by wet impregnation of Ni16% catalyst with boric acid solution, followed by a second drying and calcination step (B1Ni16 catalyst). The magnesium containing catalyst (Ni16Mg20, with 20% of MgO w_MgO/w_Al2O3) was prepared by sequential wet impregnation of magnesium nitrate and nickel nitrate, both steps followed by drying and calcination.

Catalytic reaction conditions

The total flowrate fed to the tubular quartz flow reactor was 40 Nml/min, with the following composition: 4.1% (v/v) of ethanol, 2% (v/v) of phenol, 54.6% (v/v) of water and 39.3% (v/v) of helium as carrier gas. The reactor was filled up for every test with 440 mg of sieved quartz (60-70 mesh) and 44.1 mg of catalyst. Products analysis was performed with a gas-chromatograph Agilent 4890 equipped with a Varian capillary column “Molsieve 5A/Porabond Q Tandem” and TCD and FID detectors in series. Between them a Nickel Catalyst Tube was employed to reduce CO to CH₄. The sampling of the gases was made by injection, using a gas-tight with a nominal volume of 0.25*10^-6 m³.  It is also available a sampling injection point at the end of the preheating zone to analyse the reagents and determine the possible decomposition or change in the feed. Products analysis was also performed on GC/MS (ThermoScientific), in order to have a precise identification of the compounds.

In order to reveal conditioning effect catalytic experiments were performed both rising and reducing reaction temperature  (773K, 873 K, 973 K, 1023 K and reverse).

To investigate sulphur poisoning  tetrahydrotiophene (C₄H₈S, THT) was used as a contaminant of the feed. In one series of experiments performed at 973 K 200 ppm of THT were continuously fed with the reactants, by increasing time on stream up to six hours. In another series of experiments, also performed at 973 K, the effects of two sequential pulses of THT (first 0.011 mol_S/mol_Ni and after 0.033 mol_S/mol_Ni) and of a further stay on the sulphur-free stream on the catalytic activity of the different catalysts were studied.

Results

The experiments performed with sulphur free feed show that pure alumina catalyses the conversion of ethanol to ethylene with traces of hydrogen exchange reaction to give ethane and acetaldehyde. Alkylation of phenol mainly to o-ethylphenol is observed. Experiments using increasing reaction temperature show that Ni containing catalysts catalyse alkylation of phenol at low temperature more than pure alumina while complete steam reforming of both reactants to CO and CO2 with traces of methane is obtained at 973 K for Ni5 and Ni16 while for Ni39 steam reforming is almost complete at already 873 K. Experiments by decreasing reaction temperature reveal catalysts conditioning during high temperature operation, resulting in higher catalytic activity to steam reforming at lower temperature.

Experiments performed with continuous feed of THT reveal that Ni39, which is the most active catalyst in the steam reforming of the mixture, is the more sensitive to sulphur being almost fully deactivated for steam reforming after 290 min on stream. When steam reforming is inhibited by sulphur, phenol conversion falls down but ethanol dehydration to ethylene as well as dehydrogenation to acetaldehyde reappear.

Experiment performed with THT pulses confirm that Ni39 is the most prone to deactivation by sulphur. On the other hand, the contact of the sulphur-deactivated catalyst with the sulphur-free feed reactivated all catalysts for steam reforming. However Ni39 is the slowest catalyst to reactivate.