Hydrogen production by steam reforming of biodiesel

An experimental set up was designed and commissioned for the steam reforming of biodiesel. The performance of the reactor was evaluated by varying molar steam/carbon ratio in the feed (S/C) , temperature, residence time, catalyst and biodiesel characteristics, at constant input of 1.50 10-5 mol of C/s. The effect of temperature was examined in the range of 600-800°C with S/C from 2 to 3. The weight hourly space velocity (a measure of inverse residence time in the reformer) was varied from 3.52 to 2.85 h-1. The experimental results were compared with their chemical equilibrium counterparts in order to determine the efficiency of the processes. The process outputs were measured using a micro-gas chromatograph and material balances were performed to determine the parameters such as H2 yield, biodiesel and steam conversions, and selectivity to carbon gases such as CO, CO2, CH4, C2H4, C2H6, C3H6 and C3H8. An elemental carbon balance was performed taking into account the carbon converted to C-containing gases, C deposited on the catalyst and C present in the condensate. Commercial Ni on alumina supported catalysts and in house prepared Ni catalysts were used for the steam reforming evaluation. Nickel catalysts supported on two different kinds Ce/Zr supports (Zr rich and Ce rich) were formulated in house. The Ce rich catalyst supports were prepared by sol gel synthesis employing tamarind seed polysaccharide as a gelling agent. The Zr support was provided by a commercial manufacturer. The Ni was loaded on the supports by impregnation using nitrate salts. Three kinds of biodiesels i.e. commercial, palm and in house prepared biodiesel were used for hydrogen production. The in house made biodiesel was prepared by acid-base transesterification of waste cooking oil obtained from a local fish and chip shop. It was found that among the three types of biodiesels tested, commercial biodiesel exhibited higher biodiesel and water conversion resulting in higher H2 yield. The highest hydrogen yield and biodiesel conversion was found in the temperature range of 650-700°C with S/C of 3. The optimum residence time to achieve high biodiesel and steam conversion was 3.18 h-1. The selectivity to the main carbon containing gases (CO, CO2 and CH4) and to H2 was very close to its chemical equilibrium value for all the catalysts. The commercial Ni/Al2O3 and Zr-rich Ce/Zr supported Ni catalyst were effective for steam reforming and should lower carbon formation as compared to the Ni supported on Ce rich Ce/Zr support. The highest YH2 of 27.8 wt% of biodiesel, i.e. 96% of the chemical equilibrium value, was measured for the Ni/Ce-Zr catalyst using commercial biodiesel. Biodiesel and steam conversions of 96% and 41.3% were responsible for providing such high H2 yield. The catalyst also exhibited very high H2 selectivity (99%). Among the carbon CO2 selectivity was highest 63.6% followed by CO which 33% and finally CH4 which was a mere 2.1%. Using the catalyst Ni/Ce-Zr, a C balance closure within 2.5% was obtained which corresponded to a carbon deposition of 1.25% of the carbon feed. H2 production using autothermal reforming was attempted, i.e. where a source of oxygen, here in the form of air, is introduced in the biodiesel/steam feed to encourage exothermic partial oxidation reactions intended to neutralise the heat demand in the reformer. The experimental set up designed indicated that further optimisation would be required for this process due to large amount of carbon forming at the reactor inlet, in addition to the carbon deposited on the catalyst surface, significantly affecting the process efficiency. Despite this, conditions 12% close to autothermality, as reflected in the ratio of enthalpy balance on the isothermal reactor at 650 °C to the total enthalpy output, were obtained for the condition S/C of 2 for O2/C of 0.38 and WHSV of 3.23 h-1 on the Ni/Ca-Al catalyst, this corresponded to a YH2 that was 69.2 % of the chemical equilibrium value.