Lithium-Based Co₂Acceptors Applied For Hydrogen Production In Sorption-Enhanced Steam-Methane Reforming Process

Hydrogen is an important chemical raw material as well as a kind of clean secondary energy. With improvement of oil refining processing technology and increasingly stringent regulations for environmental protection purpose, the requirement for hydrogen with high purity has become increasingly urgent. At the present main methods for hydrogen production in the industry are still based on natural gas (methane) steam reforming technology, in which two main reactions, i.e., methane-steam reforming (MSR), and water-gas shift (WGS), are included. However, this technology has drawbacks such as high energy consumption and process complexity. With development of technology and increasing requirements for lowering energy consumption, recently a research topic has been conducted on the in-situ removal of the produced CO2from MSR and WGS reactions by a sorbent mixed with a reforming and shift reaction catalyst to enhance the yield and purity of hydrogen, which is so-called " Sorption-Enhanced Reaction Process"(SERP). This can lead to MSR and WGS reactions taking place at relatively low temperatures (500-600℃), which makes a low energy consumption as well as to the simplification of the process.In this thesis, hydrogen production via MSR and WGS in combination with SERP has been studied systematically, in which conventional catalysts are unsuitable and lithium-based CO2sorbents, such as Li2ZrO3and Li4SiO4with improved CO2capture properties should be developed. Based on the previous research in our group, a study from the following two aspects was carried out.1) A series of lithium-based CO2sorbents were prepared by a soft-chemistry method. The CO2sorption analysis showed that Li4SiO4presented the higher sorption rate than that for 3. The results also revealed that Fe-doped Li4SiO4produced a substantially increased CO2sorption rate, compared with undoped Li4SiO4. In addition, after a number of capture cycles, the lithium-based acceptors displayed the same capacity and capture/regeneration kinetics. The differences in the CO2sorption kinetics between Li4SiO4and Li2ZrO3can be correlated to the mobility of ions into their crystalline structures. In the case of Li4SiO4, lithium ions move very quickly by hopping from site tosite, since there are many sites for lithium-ion hopping. However, the structure of Li2Zr03turns out to be more packed, in which lithium movement is more limited. The higher CO2sorption rate for Fe-doped Li4SiO4was due to the improved ion diffusion resulting from defects in crystalline Li4SiO4.2) To realize the benefits of SERP, the catalyst for MSR and WGS is expected to have a high activity at sorption temperatures for CO2capture. In a sorbent/catalyst mixture test, conversional catalysts such as Ni/Al2O3, Ni/SiO2, and Ni/MgO could be poinsoned by alkali metal ions released by the sorbent. MgSiO3having a relatively weak interaction with alkali ions should be effective as a catalyst support to reduce alkali poisoning. For this purpose, the catalyst20wt.%Ni/MgSiO3was prepared, and it exhibited a very high activity in and the presence of the developed sorbents.A mixture of20wt.%Ni/MgSiO3and Fe-doped Li4SiO4was used in a fixed-bed reactor at550℃, a H2O/CH4molar ratio of4, and a total pressure of atmospheric pressure. Hydrogen concentrations above90mol.%on dry base could be obtained in the enhancement period. Under the same conditions, the maximum concentration of hydrogen in the conventional MSR and WGS (without CO2sorbent) was only72%. The agreement between experimental observation and theoretical analysis confirmed that the developed high-temperature CO2sorbent applied for hydrogen production in SERP would be feasible. In addition, it must be noted that no enhancement of hydrogen concentration was observed using K-doped Li2ZrO3as sorbent at low reaction pressures, because the uptake of CO2in K-doped Li2ZrO3was too low to be coupled with MSR and WGS reactions.