| High purity hydrogen, as clean, effective, and environmental benign energy and an important raw material, plays a more and more significant role in the contemporary era which is facing with an increasing shortage of energy and severely environmental pollution. The development of new energy sources is growing, resulting in the enhancement of hydrogen demand, which is driven by hydrogen-based economies because of its efficient and zero pollution characteristics.At present main methods for hydrogen production in the industry still use three major fossil fuels as raw materials,about 80% of the world's hydrogen production through natural gas (methane) steam reforming, in which three units, i.e. methane-steam reforming (MSR), water-gas shift (WGS), and pressure swing adsorption (PSA), are included. However, methane-steam reforming at high temperatures implies a huge energy consumption and the separation of CO2 by means of PSA makes the process complicated. In order to reduce the energy consumption and simplify the process, recently a research topic has been conducted on the in-situ removal of the produced CO2 from 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 methane-steam reforming and water-gas shift reaction in combination with the SERP has been studied systematically, for which conventional catalysts are unsuitable and high-temperature CO2 sorbents should be developed. Therefore, a new catalyst is developed, which can be applied in both reactions at the same temperature. By using the home-made Li2ZrO3 nanoparticles as sorbent to in-situ remove, the produced CO2 from the reactions, a concept of the SERP applied into MSR and WGF reactions for hydrogen production has been demonstrated and inherently the processes have been optimized.The Ni/Al2O3 catalysts prepared by an impregnation method have been used in the MSR reaction, from which it is found that 19.6 wt.%Ni/γ-Al2O3 catalyst is the best. The process conditions for the MSR over 19.6 wt.%Ni/γ-Al2O3 has been optimized, in combination with thermodynamic data analysis, as follows:pressure of 0.1 MPa, space velocity of 2000 h-1, temperature of 550℃, and H2O:CH4=4:1, under which CH4 conversion is about 60%. Over the same catalyst, the optimized reaction conditions for the WGS reaction are:pressure of 0.1 MPa, space velocity of 2000 h-1, temperature of 550℃, H2O:CO=4:1, under which CO conversion is 90% with a by-product CH4 concentration of about 2%in the reactor outlet.The physical mixture of the 19.6 wt.%Ni/γ-Al2O3 catalyst and the Li2ZrO3 sorbent was loaded into the reactor to investigate the effects of the SERP on either water-gas shift or methane-steam reforming. Under the optimized conditions, mass ratio of catalyst to sorbent of 1:12, pressure of 0.1 MPa, space velocity of 2000 h-1, temperature of 550℃, and H2O:CO=4:1, CO conversion in the water-gas shift reaction is increased by 10%, compared with that without addition of CO2 sorbent, the duration for effective H2 production time is about 27.5-35 min, and H2 purity is above 95%. When the SERP is applied into methane steam reforming, under the optimized conditions, mass ratio of catalyst to sorbent of 2:3, pressure of 0.1 MPa, space velocity of 2000 h-1, temperature of 550℃, and H2O:CO=4:1, CH4 conversion is increased by 25%, compared with that without addition of CO2 sorbent, the duration for effective H2 production time is about 20-27.5 min, and H2 purity is above 90%.This work presented in this thesis demonstrates the applicability of the concept SERP in both methane-steam reforming and water-gas shift reactions for hydrogen production, which has not only an academic significance but also provides some reliable technological parameters for industrial H2 production with high purity.
|
PDF Full Text Request