Hydrogen Production by Supercritical Water Reforming of Bio-Oil Components
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The concept of a hydrogen economy is gaining traction with an ever increasing body of work focusing on all aspects of the overall process which consists of converting a fuel source to hydrogen followed by storage and distribution. Among the several fossil fuel and non-fossil fuel based resources that are being considered for hydrogen production, biomass is considered to be an abundant, CO2 neutral and ultimately viable alternative. Biomass which comprises a complex chemical structure needs to be processed to convert it into the simple molecule that is H2. One technique is to treat the biomass hydrothermally with rapid heating to produce a liquid called Bio-oil. There are several issues in directly using the bio-oil as a fuel; the high oxygen content implies lower energy intensity and furthermore, it is not stable over time and degrades into a viscous mixture that would make its application as a fuel difficult. One solution to circumvent this issue is to convert the variety of oxygenated compounds present to hydrogen by the reforming reaction. Carrying out the reforming reaction in the supercritical water phase allows for complete miscibility of gases and provides a homogenous reaction medium to carry out the reaction with the additional advantages of enhanced mass and heat transfer. The sheer number of compounds (typically over 300) present in bio-oil significantly increases the complexity of the reforming process. Acetic acid is thought of as a model oxygenate of bio-oil. Therefore, supercritical water reforming of acetic acid is examined as a first step in the overall goal of converting crude bio-oil to H¬2. AspenPlus© software was used to carry out a Gibb’s free energy minimization to determine the effect of different thermodynamic conditions on the equilibrium composition of product gas obtained in the process. The parametric effect of reaction temperature (400 – 900oC) and feed concentration (Steam to carbon mole ratio from 1:1 to 9:1) on the selectivity and yield of different gases was examined. It was found that temperatures greater than 700 oC and steam to carbon ratio in excess of 6:1 gave high hydrogen yield. It was also found that no graphitic coke formation was observed at temperatures > 500 oC when the steam to carbon ratio was 6 or higher. Thermal decomposition of acetic acid (given by a feed of pure acetic acid with no water entering the reactor) resulted in formation of graphitic carbon in the reactor even at temperatures as high as 900oC. Following the acetic acid study, thermodynamic analysis of the supercritical water reforming of a synthetic bio-oil mixture to simulate the aqueous phase of bio-oil compound was performed. Since the aqueous phase typically consists of organic acids, ketones, aldehydes and alcohols this study focused on model compound mixture containing methanol, acetic acid, acetaldehyde and acetol. Here again, the non-stoichiometric Gibb’s free energy minimization method using the Peng-Robinson equation of state with Boston-Mathias mixing rule was used to determine the effect of temperature (500- 900 oC) and steam to carbon ratio (1:1 to 9:1) on product gas composition. In-situ CO2 removal leads to an increase in equilibrium H2 yields during reforming operation which can be explained using the Le-Chatelier principle. One technique of CO¬2 removal is the use of solid sorbents such as minerals of calcium and magnesium. Carbonation of magnesium silicates, called mineral carbonation, is a useful method for CO2 sequestration due to the stable carbonate formation and the abundance of Mg-based minerals. The kinetics of direct carbonation of such minerals is very poor with reaction rates in the order of 1000’s of years at standard temperature and pressures. Therefore the magnesium silicate must be activated either physically by the action of steam, by increasing the temperature or by chemically treating with strong acids to form Mg(OH)2 which has a higher reactivity to CO2 compared to other Mg-based compounds. In this work, the effect of supercritical water on the pore structure and morphology of talc is investigated. BET surface area and pore volume were obtained following activation under supercritical water at various temperatures and reaction times. The supercritical water reforming of methanol in a fixed bed packed with only sorbent and no catalyst was studied. Plugging of the reactor due to the fine powdery nature of Mg(OH)2 was observed. This was overcome by loading the reactor with a mixture of Mg(OH)2 and CaCO3. It was found that the loading of sorbent was too low to detect the effect of in-situ CO2 removal. The catalytic conversion of glycerol to hydrogen was studied for two different catalyst support systems, 5% Ru/ZrO2 and 10% Ni/TiO¬2 was studied experimentally. The effect of reaction temperature and residence time was investigated and it was found that lower residence time gave higher hydrogen yield and reduced methane concentration in the product gas. Additionally, 10% Ni/TiO2 ¬catalyst was able to gasify close to 36 wt% of glycerol at temperatures of 600 oC and 650 oC with no reactor plugging.