| Fossil fuels are the main energy sources in the world. However, fossil fuels will be in large shortage in the near future. Moreover, the utilization of fossil fuels makes great challenge in environment protection and aggravates the global warming. As a result, it is rather significant to use renewable energy and develop advanced energy utilization technologies, which are clean, highly efficient and with low carbon emission. The CaO sorption enhanced biomass anaerobic gasification is a novel technology to produce hydrogen, which can not only produce syngas with high purity hydrogen using renewable biomass resource, but also be capable to obtain flue gases with high CO2concentration. The development of this technology will helps to construct high efficiency energy utilization system and be beneficical for CO2transport and storage. This thesis focuses on both mechanism and experimental studies relating to CaO sorption enhanced biomass anaerobic gasification.The study on biomass pyrolysis mechanism in the presence of abundant CaO is important to analyze reactant distributions prior to biomass anaerobic gasification and examine the influences of CaO additives on the evolution of CO2and biomass tar species. Using thermogravimetric Fourier transform infrared (TG-FTIR) analysis, wheat-straw pyrolysis experiments were conducted under different CaO addition amounts and heating rates. Results show that wheat-straw pyrolysis exhibits two stages in the presence of CaO. In the first and also the main stage, the addition of CaO apparently reduces the yields of H2O、CO、CO2、CH4and biomass tar species such as toluene, p-xylene, phenol and formic acid, and the total mass loss in this stage correspondingly decreases. CaO plays dual roles of both CO2sorbent and tar reduction catalyst in this stage. Kinetic calculation reveals that the addition of CaO can also lower the activation energy of biomass pyrolysis. The second stage is caused by the decomposition of CaCO3at high temperatures and should be avoided during realistic gasification operations.Pressurized operation implies great benefit for CaO carbonation and plays a key role to enhance hydrogen production reactions. The kinetic reaction properties of CaO carbonation under high pressure were examined using a pressurized thermogravimetric analyzer. Experiments were performed at different total pressures and different CO2concentrations. It is found that the increase in total pressure and CO2concentration both accelerate the initial reaction rate of CaO carbonation and increase the final conversion of CaO sorbents. The shrinking core model is adopted for the isothermal kinetic analysis. Results of kinetic calculation show that the activation energy of CaO carbonation decreases with increasing pressures. An empirical reaction rate equation for CaO carbonation is proposed coupling both reaction temperature and CO2partial pressure.The cyclic carbonation reactivity of CaO sorbents is critical for the economic of the whole system. Using a pressurized thermogravimetric analyzer and Scanning Electron Microscopy (SEM) technology, the effect of calcination pressure on CaO cyclic carbonation reactivity was examined. Meanwhile, the reactivation of CaO sorbents during cyclic calcination-carbonation (CC) reactions was also surveyed using distilled water hydration at atmospheric pressure and saturated steam hydration at high pressure. It is found that CaO sorbents calcined under lower CO2partial pressure exhibit higher porosity and lose reactivity more slowly with increasing cycle number than those calcined at high pressure. Both two hydration methods efficiently improve the CaO reactivity during cyclic CC reactions as a result of the promoted surface area and porosity. The mean values of reactivity increase for water hydration and steam hydration after6cycles were～22%and～27%respectively.The change of gasification operating variables under realistic fluidized bed conditions will have large influences on the investigated technology. The effects of CaO to carbon mole ratio (CaO/C), H2O to carbon mole ratio (H2O/C) and gasification temperature (T) on hydrogen production from sawdust anaerobic gasification were examined at atmospheric pressure, using a self-design bubbling fluidized bed reactor. Results show that, over the ranges examined in this study (CaO/C:0-2; H2O/C:1.2-2.18, T:489-740℃), the increase of CaO/C, H2O/C and T are all favorable for promoting H2production. A maximum H2output with a concentration of62%and a yield of72g/kg-biomass is achieved at CaO/C=1, H2O/C=2.18and T=740℃. The comparison with previous studies on fluidized bed biomass gasification reveals that this method has the advantage of being capable to produce a syngas with high H2concentration and low CO2concentration.In order to investigate the effects of reaction pressure on CaO sorption enhanced biomass anaerobic gasification, a pressurized dual circulating fluidized beds reactor was designed and constructed, which can be operated at pressures up to1.0MPa. After the cold state tests, hydrogen production experiments were performed in this facility using sawdust anaerobic gasification within the following experimental ranges: pressure (P)1-4bar, T530-760℃, CaO/C0-1.2and H2O/C0.72-0.89. Results show that pressurized operations are beneficial to increase both H2concentration and H2yield. Compared with the atmospheric runs, pressurized gasification further decreases the CO2concentration in syngas, reduces the consumption of steam and promotes the biomass carbon conversion and cold gas efficiency. H2output with a concentration of67.7%and a yield of68g/kg-sawdust was achieved at P=4bar, CaO/C=1.2, H2O/C=0.89and T=680℃.To get systematic operation properties of the whole H2production system and provide theoretical guide for the realistic dual fluidized bed runs in next step, a kinetic model for CaO sorption enhanced biomass anaerobic gasification was established. This model involves in five key reaction processes, i.e., biomass pyrolysis, gasification of the pyrolysis products, CaO carbonation, biomass char combustion and CaCO3calcination. Using this model, effects of reaction pressure, temperature and reaction atmosphere on system operation properties were systematically predicted. According to the calculation results, the suitable operation conditions are:gasification and calcination pressure1MPa, gasification temperature973K, calcination temperature1173K. At a typical calculation condition, the whole system achieves reasonable operation properties and H2output. |
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