Hydrogen production via Catalytic Glycerol Reforming

Abstract

The present study focuses on hydrogen production from steam reforming of glycerol using nickel based catalysts. Renewable energy sources are highly desirable in this era of dwindling petroleum reserves and increasing environmental concerns. Several alternatives of renewable fuels, such as ethanol and biodiesel, are currently been exploited in order to promote a more sustainable development. Among the various renewable feedstock sources, glycerol is one alternative because it has relatively high hydrogen content, it is nontoxic, and its storage and handling is safe. Glycerol is produced as a byproduct during biodiesel production from trans-esterification of vegetable oils. With increase in production of biodiesel, a glut of glycerol is expected in the world market and therefore, it is essential to find useful applications of glycerol. About 1 kg of glycerol is formed as a byproduct for every 10 kg of biodiesel. The increasing abundance and attractive pricing of glycerol make this product an appealing source of chemical to derive value added commercial compound hydrogen. Hydrogen is expected to play an important role in future energy systems. Production of hydrogen from glycerol is environmentally friendly because it adds value to byproduct generated from biodiesel plants. Hydrogen production from glycerol by steam reforming process has received considerable attention during recent years. Hydrogen produced by steam reforming of glycerol can be used for fuel cell applications, automobile applications and electricity production. Hydrogen is also used as an important material in chemical synthesis and refinery. Steam reforming is the most energy efficient technology available, and it is the most cost effective. It is strongly endothermic carried at high temperatures, low pressure and high steam to glycerin ratio to achieve higher conversion. Commercially, nickel is used in steam reforming of glycerol due to its inherent availability, high stability and lower cost compared to noble metals. γ-Al2O3 support has high surface area, good dispersion of active phase, good mechanical strength, high thermal stability, and high porosity. In the present investigation an attempt has been made to develop multicomponent nickel catalysts (Ni/Al2O3, Ni/CeO2/Al2O3, Ni/ZrO2/Al2O3, Ni/CeO2/ZrO2/Al2O3) for the steam reforming of glycerol (SRG) for high glycerol conversion, high hydrogen selectivity, suppression of CO formation and reduce catalyst deactivation. The catalysts were prepared by wet impregnation and co-precipitation methods. All these catalysts were characterized for their surface area, pore volume and pore size distribution, x-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed reduction (TPR), temperature programmed oxidation (TPO), temperature programmed desorption (CO2-TPD) and thermogravimetric/differential thermal analysis (TG/DTA) techniques were used for the characterization of fresh and used catalysts to correlate the activity and properties of catalysts for subsequent improvement in the catalyst performance. The performance of catalysts was evaluated in atmospheric gas solid fixed bed catalytic reactor over a wide range of operating conditions. The reactor temperature was controlled using PID controllers. Prior to runs all the catalysts were reduced in-situ in the stream of 15% H2/N2 mixture. A mixture of glycerol and water vapor generated in an evaporator was passed through a preheating zone along with nitrogen prior entering the reactor. 1 gram of catalyst was placed in the reactor plugged with glass wool. The composition of gas products that contained mainly H2, N2, CO, CH4, and CO2 at the outlet of the vapor-liquid separator was analyzed with a gas chromatograph (GC2010 Shimadzu, TCD detector). The process variable used for the screening of catalyst were contact time (W/F)= 7-15 kgcat s mole-1, temperature 700-800 0C, pressure 1 atm, and steam to glycerol (S/G) molar ratio 9:1. The conversion, yield and selectivity were calculated. The mass balance with respect to carbon was found to be 95%. Results suggested that Ni/CeO2/ZrO2/Al2O3 catalyst enhanced the catalysts activity significantly. The promoters like ceria, zirconia were incorporated in the nickel catalysts to enhance the activity and stability of catalyst. Ni/CeO2/ZrO2/Al2O3 (Ni/Ce/Zr/Al: 10/3/2/85) catalyst prepared by wet impregnation method gave maximum glycerol conversion, hydrogen production, CO minimum formation and CH4 almost constant with good stability. The enhanced activity of ceria based catalysts is due to higher dispersion and surface area of nickel with easy reducibility of NiO to Ni. For the ceria based catalyst CO formation was also low due to oxygen storage- release capacity of ceria. Doping of ZrO2 over Ni/CeO2/Al2O3 catalyst also improved the catalyst performance however CO formation was high. The effect of run time on the catalyst activity showed that Ni/CeO2/ZrO2/Al2O3 is more stable than Ni/CeO2/Al2O3. The results of the various characterization techniques were used to relate the observed catalytic activity and stability to the catalyst property. Kinetic study was carried out for the steam reforming of glycerol for catalyst (Ni/Ce/Zr/Al:10/3/2/85) prepared by wet impregnation method at following operating conditions: temperature 700-800 0C, contact time (W/F) 7-15 kgcat s mole-1, pressure 1 atm with steam to glycerol (S/G) molar ratio 9:1. The preliminary runs at various conditions were carried out in order to obtain the kinetic data in chemical reaction controlled regime by eliminating film diffusion and pore diffusion. All the kinetic data were collected at total flow rate greater than 300 ml/min where external mass transfer limitations were not prevailed. The inert SiO2 (0.5 – 1.5 mm) granules to catalyst weight ratio was maintained at 4 in order to reduce the specific pressure drop across the reactor and to avoid any local temperature gradients in the catalyst bed. The catalyst temperature was maintained across the catalyst bed within ± 2 K difference using PID controller. The plug flow conditions were maintained in the reactor by providing catalyst bed height to catalyst particle size ratio L/DP ≥ 50. Glycerol conversion and hydrogen production rate increased with the contact-time and temperature. The hydrogen selectivity decreased whereas selectivity of carbon monoxide increased with increase in temperature and contact time. Investigations suggested that the carbon monoxide formation takes place as a secondary product via reverse water gas shift reaction. Kinetic study was carried out for the steam reforming of glycerol over Ni/CeO2/ZrO2/Al2O3 using power law model. The activation energy was found to be 17.67 kcal/mole. Detailed deactivation study was also done for steam reforming of glycerol over Nickle based catalysts at different temperatures ranging from 700-800 0C, contact time (W/F) 15 kgcat s mole-1, steam to glycerol (S/G) molar ratio 9:1 at atmospheric pressure for 14 hours of run time. The amount of carbon deposited on to the catalysts was estimated using TG/DTA. The results of deactivation study revealed that the coke formation was the major cause for the catalyst deactivation and contribution of sintering. The excess steam used in the present study also inhibit the coke formation.