Basic Research On HI Catalytic Decomposition,Membrane Separation And Purification In The Sulfur-iodine Cycle For Hydrogen Production

Hydrogen is considered as a clean, efficiency and promising energy source, and it will play an important role in the future energy systems. The large-scale, high-efficient and low-cost hydrogen production technology is the prerequisite foundation for hydrogen utilization. Hydrogen can be produced by the decomposition of water through a series of related thermal reactions and all other reactants can be recovered and reused. Among these cycles, sulfur-iodine (SI or IS) cycle has been proposed as one of the most promising routes for continuous, efficient, large-scale and environmentally sound hydrogen production without CO2emission. The SI cycle is consisted of the following three reactions:Bunsen reaction:SO2+I2+2H2Oâ†'29～390K2HI+H2SO4H2SO4decomposition:H2SO4â†'970～1270H2O+SO2+1/2O2HI decomposition:2HIâ†'570～770k I2+H2The HI decomposition is the key step for hydrogen production in SI cycle, and the HI conversion determines, to a great extent, the total thermal efficiency and hydrogen production rate. The HI catalytic decomposition, membrane separation and purification for HIx and H2SO4phase in Hi decomposition section are studied in the thesis.Different carbon materials were studied as support for nickel catalysts for HI decomposition. The Ni/AC shows the best catalytic activity, and activated carbon (AC) had the best catalytic performance during the carbon supports. The pore structure of AC was well developed, and the interficial nickel crystallites had stronger interaction with AC support. Different nickel salts were studied as precursor for Ni/AC catalysts for HI decomposition. The anion portions of the nickel salts influenced the adsorption and disperse of nickel crystallites. The nickel nitrate had stronger interaction with AC support, and the nickel crystallite was the smallest and well dispersed. Different nickel content in Ni/AC catalyst was studied for HI decomposition. The nickel crystallite was small and well dispersed in Ni/AC when the nickel content was below12%. The nickel crystallite tended to agglomerate, forming larger block of irregular particles when the nickel content is above12%. The12%Ni/AC showed the best catalytic activity. The edge plane sites, structural defects (e.g., structural carbon vacancies and nonaromatic rings) and nickel crystallite were considered as the main active sites for HI decomposition. A hypothetic mechanism for HI decomposition over Ni/AC catalysts was proposed.The loss of AC with water during reaction was studied. No loss was detected during the Experiments at temperature below700℃. The loss of AC was started at800℃. The loss was aggravated by the temperature and humidity since800℃. The activity and stability test for modified AC catalyst was conducted at500℃for24h. The HI conversion for modified AC at500℃was ca.22%. It was remarkable that the AC showed a stable catalytic activity after24h of operating lifetime. The pore structure and surface functional groups changed little, and some I2was attracted on the surface of AC. The activity and stability test for12%Ni/AC catalyst was conducted at500℃for12h. The HI conversion for12%Ni/AC at500℃was ca.22.4%. The pore structure of12%Ni/AC catalyst was destroyed slightly, and the agglomeration of nickel crystallites was detected after12h of operating lifetime. Owing to the low cost, abundant resource, various source and regeneration, the AC basted catalysts were promising for HI decomposition. The water content in HI solution, which mainly influence the residual time, had a slight impact on HI conversion. But a lot of energy was wasted for heating water in HI solution. The I2content in HI solution restrained the HI decomposition. Thus, the most water should be extracted in HIx rectification, and the I2content to HI should be below0.1:1.The stability of Pd membrane in HI-H2O phase was tested at500℃. The non-used Pd membrane was continuous and compact. The defects were detected after1h of operating tests. The surface of Pd membrane was strongly destroyed by after2h of tests. The single-component permeation of H2and Ar through carbon membrane was tested. The H2/Ar separation factor was50-106in the experiments. The H2permeation was dominated by the activated diffusion. The permeance of H2, H2O and HI through carbon membrane in H2-H2O-HI gaseous mixture was tested at300～500℃. Hydrogen permeance was nearly the same as single-component permeance. Most of HI but not H2O was stopped by carbon membrane. The separation factor was over310for H2-HI. The hydrogen performance was slightly increased after6h of test, which indicated the slight damage on the surface of carbon membrane. The single-component permeation of H2and N2through silica membrane was tested. The hydrogen performance of silica membrane was increased, maintaining high H2/N2separation, when the ceramic support was modified by γ-Al2O3. The hydrogen performance was5.96×10-8mo·m-2·s-1·Pa-1, and the H2/N2separation factor was8.5at500℃. The hydrogen performance through silica membrane was dominated by activated diffusion. The permeance of H2, H2O and HI through silica membrane in H2-H2O-HI gaseous mixture was tested at300～500℃. Hydrogen permeance were nearly the same as single-component permeance. The separation factor was over630for H2-HI.The purification of HIx and H2SO4phases by the application of reverse Bunsen reaction was studied. The temperature significantly influenced the purification. Higher temperature obviously benefited the purification. The lower feed flow rate will benefit the residual time, thus improved the purification efficiency. The higher sweeping gas rate accelerated the spill of produced SO2, thus improved the purification efficiency. Oxygen will oxidize I" in the acid. Oxygen can be implied as sweeping gas in H2SO4phase purification. The application of oxygen will result in the loss of HI in HIx phase.