Thermo-economic assessment of CO2 separation technologies in the framework of synthetic natural gas (SNG) production

Synthetic Natural Gas (SNG) is one of the alternative fuels that can be produced from biomass. Its potential advantages are the possibility of mixing with fossil Natural Gas in the existing distribution infrastructure, and a production process based on proven technologies. SNG production is a highly integrated energy conversion process. It is based on gasification of biomass and the SNG produced has to be upgraded to meet the quality standards of Natural Gas. The gas upgrading process leads to a considerable energy penalty for the system, mainly due to the high energy demand for the separation of carbon dioxide from the methane. This thesis is part of a broader project that aims to identify synergies between sub-processes in SNG production, in order to achieve a fully optimized process and a realistic SNG production cost. The object of this thesis was to compare three different configurations for the gas upgrade section based on different CO2 separation technologies. They were integrated within the framework of a SNG production where the syngas at the inlet of the gas upgrade section contained around 46 % of CH4 and 46 % of CO2. The technologies investigated are: Membrane separation, vacuum pressure swing adsorption (VPSA) and absorption with mono-ethanolamine (MEA). For each technology a model of the gas upgrade section has been developed, using process simulation software (Aspen Plus). In the SNG production process, captured CO2 can be considered as a product, and in the future revenue resulting from CO2 capture can be significant for the economic performance of the plant. The VPSA configuration was judged not to be relevant for CO2 capture, given the major energy penalty associated with compression of the separated CO2 stream. The results of the simulations were combined with data from an existing model of SNG production, and pinch analysis was used as a tool to estimate the potential electricity production resulting from harnessing heat flows within the plant. The studied configurations were even compared for their upgrade performance, their energy consumption and their economic performance calculated as variations of the costs and the revenues of the plant. Four different possible future energy market scenarios were used. The configurations achieve a CH4 recovery between 80% for MEA absorption and 91% for membrane separation, with a power consumption of 2,7 MW and 5,6 MW, respectively. The economic results show that the revenues from CO2 recovery are extremely relevant for the revenues of the plant and, depending on the scenario they can vary from 1 M€/y to 15 M€/y. The membrane configuration results in the best difference between revenues and costs, but it involves the highest investment cost, while the MEA results in the possibility to produce SNG with a system independent from the electricity-market