(72e) Conversion of Non-Transportable Brown Coals to Hydrogen By Steam Gasification Process

Australian brown coals are non-transportable lignite because of their high content of water. On drying, brown coals show self-ignition characteristics making them hazardous substances to transport. Their use is only limited to local thermal power generation plants. Recently, there have been advances to develop technologies to transport these coals to the point of use such as converting to char at place of production. However, in addition to environmental reasons, from economy development point of view of brown coal producing country and the energy requirement point of view of energy resources deficient country, converting brown coal to hydrogen/liquid fuels is the best suited approach. Efficiently converting brown coal to hydrogen/synthesis gas will be the main driver for this business model to be economically successful. Therefore, development of efficient conversion technologies is necessary. Currently, conventional oxygen blown gasification process is a primary commercial process to gasify coals to synthesis gas or hydrogen. However, low temperature catalytic coal gasification is more efficient and economical way to convert coal into hydrogen/synthesis gas. AIST is developing a catalytic gasification technology to efficiently convert non-transportable fossil fuels like brown coals, biomass and bio-process wastes to hydrogen/liquid fuels via synthesis gas-FT combined process. Gas composition of synthesis gas is primarily determined by temperature (thermodynamic equilibrium) and gasifying agent composition. Gasification in steam at low temperature leads to hydrogen rich gas production but chemical kinetics becomes the limiting factor for process development. This disadvantage can be overcome by using alkali catalyst. Use of catalyst for coal gasification has not been preferred because of loss of catalyst by irreversible reaction with mineral matters present in coals which means large amount of make-up catalyst will be needed. However, brown coals have less than 1 wt% of mineral matter making the make-up catalyst almost equal to operating loss of catalyst. Therefore, conversion of brown coal by catalytic steam gasification is most economical and efficient way. This study presents a brown coal catalytic gasification process to produce hydrogen rich gas or synthesis gas by simply adjusting the gasifying agent composition. The highlight of the process is the production of the hydrogen/synthesis gas at 650~700°C using steam/CO₂ as gasifying agent. Synthesis gas suitable for FT-synthesis process was produced by changing the steam/CO₂ ratio of the gasifying agent. When gasifying agent is pure steam, hydrogen and CO₂ are the main component of the gas from which hydrogen can be separated. Hydrogen can be transported by hydrogen-carrier process which involves hydrogenating the hydrogen carrier at the place of production, transporting it to consumption site and then dehydrogenating to recover the hydrogen or as liquefied H₂. FT-process suitable synthesis gas can be produced by using steam/CO₂ mixture as gasifying agent. The process does not use oxygen, thus saving significant energy needed to run ASU, and leading to higher thermal conversion efficiencies than the conventional oxygen blown gasification process.