Substitute Natural Gas (SNG) from Coal and Lignite - Methanation of synthesis gas from allothermal gasification

摘要

This paper deals with results from gasification tests, which serve as a starting point for the investigation of the methanation, as well as results from methanation tests themselves. In order to get representative gas compositions, lignite was gasified by means of water steam in a 500 kW indirectly heated fluidized bed gasifier, the so called Heatpipe-Reformer. The obtained gas compositions with an H_2/CO ratio of about 3 provide ideal conditions for the synthesis of methane. The methanation process itself takes place in fixed bed reactors with different, commercially available, nickel catalysts. In the first step it was operated with synthetic synthesis gases. The goal was to find ideal operating parameters for SNG production. For reducing the plant complexity and increasing the efficiency a simplification of the gas cleaning is proposed. Fine particles and alkalis are removed by means of a sinter metal filter, impurities like sulfur compounds and chlorines by means of different adsorptive materials. In contrast to state-of-the-art cold/wet gas cleaning techniques, where all impurities are removed, higher hydrocarbons are not considered in this hot gas cleaning. For this reason, the methanation catalysts must be able to deal with these components. Due to the tubular design and polytropic operating condition of the methanation reactor a hot reaction zone (500 - 550 °C) originates at its inlet. This reaction zone provides enough exothermic heat for the reformation of higher hydrocarbons on the methanation catalyst. The results of long-term methanation tests with synthetic as well as real synthesis gas from a lab-scale gasifier provide the basis for the optimization of the process to increase the catalyst life.