Integrated hot and cold synthesis gas clean-up

It is generally accepted that gasification of coal is the most promising route for clean utilization of coal for different applications. The gasification process converts coal and other feedstocks into synthetic raw gas suitable for different utilization areas such as production of electricity, value-added chemicals and liquid fuels. However, coal-derived synthesis gas contains harmful contaminants which need to be removed. The sulfur removal is a must prior to utilization of the gasification products either as a fuel for gas turbines, gas engines or fuel cells or as a synthesis gas for Fischer-Tropsch syntesis, ammonia or methanol productions. Hydrogen sulfide and other sulfur compounds can cause corrosion in pipelines thus limit the plant lifetime. Hydrogen sulfide is also a well-known catalyst poison. For these reasons, it is necessary to reduce the H₂S concentration around one ppmv level. A semi-pilot scale bubbling bed gasification system equipped with hot and cold synthesis gas (syngas) clean-up units has been installed at Energy Institute of TUBITAK MAM. This system has been used for coal derived syngas cleanup purposes. A pilot scale pressurized bubbling fluidized bed gasification system is also current being developed by TUBITAK MAM. By using a combination of various techniques, we have already been able to we reduced the H₂S content of syngas below 1 ppmv level. The ongoing research efforts will be described in this paper with a particular focus on sulfur compounds removal. High temperature desulfurization performance of dolomite for coal derived syngas was investigated in this study. The results indicated that dolomite can be active in sulfur removal at the temperatures higher than 750°C. However, due to the thermodynamic equilibrium conditions, H₂S clean-up was limited to around 150-200 ppmv in presence of water vapor. The presence of water vapor in the dolomite bed seemed to inhibit sulfidation reactions. On the other hand, the presence of CO₂ in the gas stream did not have a suppressing effect on sulfidation reactions, whereas, it tended to depress the calcination of CaCO₃ to CaO.