| CaO sorption enhanced biomass steam gasification is a novel one-step technology for H2production in recent years, which can not only convert low grade biomass into high grade H2of high purity, but also be able to achieve CO2capture and emission reduction. However, the current studies of this technology mainly focus on high temperature and high pressure gasification to achieve high carbon conversion rate and low CO2concentration, which is difficult for technical implementation and application in actual processes. Under this background, from the point of view of industrial application, a new idea of "in-situ CO2sorption enhanced H2production" was put forward by State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, which concerned with the properties of biomass fuels and the characteristics of biomass thermochemical conversion. The goal is to produce H2-rich gas at lower temperature (600-700℃) and atmospheric or slightly pressurized conditions, and a CaO sorption enhanced biomass steam gasification system was also built up. This thesis mainly focuses on mechanism and experimental studies relating to CaO sorption enhanced biomass steam gasification.The experiments of enhanced pyrolysis and steam gasification of corn stalk under high amount of CaO addition were conducted using Thermal Gravimetric Analyzer (TGA), and the results showed that enhanced pyrolysis of biomass and secondary reaction of volatile matter arc the main processes during the CaO sorption enhanced biomass steam gasification, while coke gasification has no obvious effect on the gasification process. The corn stalk pyrolysis exhibits two mass loss stages in the presence of CaO. In the first stage, CaO in situ absorption CO2from the pyrolysis gas could significantly reduce the mass loss of this stage, and the activation energy of pyrolysis was also decreased. The second stage was mainly caused by the calcination of CaCO3formed in the first stage, and it should be avoided during the actual H2production process.Pyrolysis is the initial stage of gasification, and the composition and distribution of pyrolysis volatile will significantly influence the gasification process. Thus the experiments of enhanced pyrolysis of corn stalk under high amount of CaO addition were conducted on tube furnace, and the results showed that as temperature rang form550to650℃, CaO showed good performance of CO2absorption, and it also catalyzed the pyrolysis process, which resulted in the yield of H2and CH4in the pyrolysis gas increasing and the content of polycyclic aromatic hydrocarbons (PAHs) in the pyrolysis oil decreasing, and it was beneficial for the following steam reforming process of pyrolysis volatile. As temperature rang from700to850℃, CaO showed almost no absorption effect of CO2, and CaO was just presented as a catalyst during the pyrolysis process.Enhanced reforming of carbonaceous gas is the key process to obtain high concentration of H2in CaO sorption enhanced steam gasification of biomass. Quartz tube experiment and thermodynamic simulation were both carried out to reveal the enhanced effect and mechanism of CaO addition, and the results showed that700℃was also the thermodynamic dividing point for the enhanced reforming process. Before700℃, the enhanced effect of CaO decreased gradually as temperature increasing, and after700℃, it decreased to0. CaO addition had significant enhanced effect on the actual Water Gas Shift (WGS) reaction, and the increasing of the molar ratio of H2O to CO was beneficial for the WGS reaction, while the increasing of the space velocity was opposite. As the molar ratio of H2O to CO of3, the space velocity of1500h-1and the reaction time of about20min, the H2concentration could get a maximum value of71.71%, meanwhile, the CO2absorption efficiency could reach to91.44%, and the enhanced ratio of CaO to WGS reaction was81.12%. While in the whole range of test conditions, CaO addition had no obviously enhanced effect on Steam Methane Reforming (SMR) reaction.The reactivity of CaO carbonation directly affects the enhanced degree of the H2production process. Using tube furnace, the performance of CaO carbonation was studied, and the effect of inorganic composition of biomass was emphatically analyzed. The results showed that as calcination temperature of900℃and calcination time of15min, the CaO showed the best reactivity of CaO carbonation corresponding to the best characteristics of pore distribution. Along with the carbonation temperature increasing, the conversion rate of CaO first increased and then decreased, and it got a maximum value of73.80%at675℃, while the effect of CO2concentration on the performance of CaO carbonation was relatively small. The addition of Na salts. K2CO3and KiSiO3were shown to inhibit the carbonation reaction, whereas the addition of KC1, K2SO4, MgO and SiO2could promote the carbonation reaction, and there were also significant interactions between different inorganic components. The addition of corn stalk ash, rice straw ash and wheat straw ash were beneficial for the carbonation reaction, while the addition of peanut shell ash and sawdust ash showed obvious inhibition to the carbonation reaction, and these were closely related to the ash composition.To study the influence of CaO addition at different fluidized bed conditions is significant for the actual application of enhanced H2production of biomass steam gasification. The experiment was carried out at atmospheric pressure on a bench scale fluidized bed reactor, and the results showed that the increasing of the addition amount of CaO and the mass ratio of steam to biomass (S/B) were both beneficial for the enhanced H2production of corn stalk. As the temperature of650℃, the molar ratio of CaO to C of1and the S/B of1, the H2concentration and yield could reach to61.23%and51.21g/(kg daf biomass), respectively. Temperature had obvious impact on the enhanced effect of CaO, at550-650℃, the enhanced effect of CaO was very significant, and the content of CO and CO2in the product gas decreased largely; while at700-800℃, the CaO addition was dominated by the catalytic effect, and the CO2absorption of CaO can be neglected. It could be also observed that different biomass showed different characteristics of enhanced H2production, and the order of the H2yield (as received basis) of5tested biomass was as follow:sawdust> peanut shell> corn stalk> wheat straw> rice straw, which was in accordance with the content of the volatile matter (as received basis).Using phenol as the model compound of volatile matter (or biomass), the comparative thermodynamic analysis between CaO sorption enhanced steam gasification of biomass and coal based enhanced H2production showed that the performance of H2production from biomass is better than that from coal. Thus, the technology of CaO sorption enhanced biomass steam gasification of biomass is proved to be feasible, and it is also expected to obtain a higher H2production efficiency using biomass rather than coal at a lower temperature. Using ASPEN Plus software based on Gibbs free energy minimization, the process of enhanced H2production process from biomass was comprehensively simulated, and the results showed that CaO addition had obviously enhanced effect on the H2 production process, and the desired H2concentration and yield could be obtained just at atmospheric pressure or slight pressurization (<0.5MPa) and relatively lower temperature (600-700℃). As the temperature of650℃and the S/B of1.2, the theoretical concentration of H2could reach to94.24%at atmospheric pressure. After the temperature was higher than750℃, CaO addition showed no enhanced effect on the H2production process. |
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