| Cellulose ethanol reforming fro hydrogen production is an important way for the energetic utilization wood biomass, which is suitable for scattered hydrogen refueling station of small cars. In the reforming process for hydrogen production, there coexist many reactions and the process characteristics can be studied with systematic analysis which is also helpful for catalyst screening, process optimization and reactor designation.etc. In the present study, the thermodynamic trends of biomass-based small molecules (polyols, alkenes and alcohols) for hydrogen production are analyzed with thermal equilibrium constants method. And that polyols are more suitable for aqueous-phase reforming for hydrogen production while ethanol and alkenes tend to vapor-phase reforming. Afterwards, the thermodynamic equilibrium compositions are calculated under temperatures from700to1400K, steam-to-ethanol molar ratio between0-10and oxygen-to-ethanol molar ratio between0-3at atmosphere pressure and the results are fitted in polynomials. Finally, the thermal effects and energy efficiencies for the three technologies are computed and analyzed, on the base of which, the difference between the latest experimental results and thermodynamic compositions are compared and analyzed, which could be a reference for experiment study on ethanol to hydrogen. Some major achievements are listed as follows:It is showed by the calculation that the conversion for ethanol reforming can achieve100%even at low temperatures from380-500K, with the equilibrium productions mainly carbon, water and methane. In the process of ethanol SR, high temperatures and S/E ratios contribute the formation of hydrogen. More than5.4moles of hydrogen yield can be obtained above700K with S/E>10. High temperature and low S/E ratio leads to large amount of CO formation and1.8moles of CO can be generated above1200K with S/E≈1.2. Mean time, low S/E ratio is the main cause for coke formation. It is suggested that the thermodynamic favorable conditions for ethanol steam reforming should be above900K with S/E ratio higher than6.In the process of ethanol POX, the theoretical maximum hydrogen production per mole ethanol is3moles and high temperature and low O/E ratio promote the hydrogen yield and more than2.9moles of hydrogen are produced above1300K with O/E=0. Mean while, high temperature and low O/E also promote the CO yield and more than1.9moles of CO can be achieved above1200K with O/E=0.5. Coke is easy to generate at low temperatures and O/E and with O/E<0.5, coke is always formed.In the process of ethanol ATR, high temperature, S/E ratio and low O/E ratio favors the formation of hydrogen with the maximum yield close to5.1moles. And the yield of CO increases gradually with the increase of temperature while with no apparent changes with S/E ratio. The amount of coke is the smallest among the three technologies and when S/E ratio is above1, the coke formation can be negligible.The process of ethanol SR is highly endothermic the adsorbed energy increases with the increase of S/E ratio and temperature which is equivalent to the burning heat by some0.2mole of ethanol above1200K. Also, the system hydrogen energy efficiencies increase with temperature which can be up to88%and the remaining12%is shared by CH4, CO and C. The process of ethanol POX is highly exothermic and the heat generated increases obviously with the increasing of O/E ratio and when O/E ratio reaches3, ethanol is totally combusted. Generally, the energy efficiencies of ethanol POX are relevantly low and when O/E ratio equals to1.5, the energy efficiency is about50%and then decrease sharply to zero with O/E=3. The process of ethanol ATR is slightly exothermic or endothermic which can be seen as thermal neutral. At a given O/E ratio, with the increasing of S/E ratio, the T-N points move to lower temperatures; with S/E ratio fixed, with the increasing of O/E ratio, the T-N points tend to higher temperatures. Furthermore, the system energy efficiency achieves89%at100K with S/E=10and O/E=0.25.It is found by comparing the latest experiment researches and thermodynamic equilibrium compositions that the reaction rate is very slow with no catalysts, which is controlled by dynamic factors. The comparison also reveals that with appropriate catalyst, the reaction conversion of ethanol reaches100%with the hydrogen yield close to the equilibrium composition and in this case, the process is thermodynamic controlled. The analysis shows that when Gibbs free energy changes1-2%, the equilibrium production compositions change a lot and sub-stable thermodynamic equilibrium state may exist, under which the hydrogen yield is higher or lower than that under thermodynamic equilibrium state... |
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