RECO2

This project deals with CO2 recycling and valorisation in a lignocellulosic biomass steam gasification process performed in a fluidised bed, with the aim of producing gaseous fuel (bioSNG: Synthetic Natural Gas) or liquid fuel (with Fischer-Tropsch synthesis). In this kind of process, the gas directly coming from biomass thermochemical conversion contains CO2. Moreover, in order to produce fuel, it is generally necessary to perform a water-gas shift step in order to adjust the H2/CO ratio, which leads to an additional CO2 production. This project aims at studying recycling of a part of this CO2 by reintroducing it into the gasifier, either instead of neutral gas with biomass feeding, or to replace a part of the steam in the gasifier itself, CO2 then acting as a gasifying agent. Some experimental studies have shown that biomass pyrolysis and gasification take place differently in a mixture of H2O and CO2 and in a mixture of H2O and N2. The literature review shows that data are still missing and that mechanisms are not well understood. So, the present project aims first at studying and understanding mechanisms which govern biomass pyrolysis and gasification in a mixture H2O+CO2, and secondly at evaluating the possibilities and advantages of recycling the CO2 produced in a double fluidised bed process, from both technico-economical and environmental points of view. Firstly, analytical studies will be led at the particle scale to isolate the pyrolysis and gasification mechanisms and so to better understand the phenomena involved. Parameters will be temperature and composition of the gaseous atmosphere around the particle. Several complementary installations implemented at the CEA and RAPSODEE will be used. In a second stage, the same mechanisms will be studied in a fluidised bed continuously fed with biomass. In this installation, phenomena are not isolated and can thus interfere. All the results obtained at the particle scale and at the small pilot gasifier scale will be synthesized, so as to show the progress made in reaction mechanisms and key parameters comprehension. After this important experimental step, these results will be analyzed and transposed so as to be used in technico-economical simulation tools for industrial processes evaluation. Biomass gasification in a double fluidised bed process for the production of SNG and of Fischer-Tropsch diesel will be simulated. For each of both applications, the reference case without CO2 recycling will be compared to several options of CO2 recycling. The last step of the project consists in an environmental evaluation of the process and of its alternatives according to both studied applications. This approach will again be focused on environmental advantages and drawbacks of the different options of CO2 recycling. It will end up with a life cycle analysis of the studied processes.