Promoteur : Prof. Guy De Weireld

Résumé :

This PhD thesis focuses on the reduction of the CO2 anthropogenic emissions coming from the cement sector by investigating its possible conversion into methanol. To this extent, a preliminary study has been performed to introduce the cement manufacturing process and define the flue gas exiting the cement kilns that will be considered as raw material of the CO2 purification process. In this perspective, the conventional purification technologies (such as dust, SOx and NOx removal techniques) and the CO2 capture and purification technologies that are applied to industrial flue gases have been summarised.
The wide panel of Carbon Capture and Reuse or Storage (CCUS) technologies have been presented and methanol has been selected as one of the most promising CCU technologies. As a result, the thermodynamic study of the hydrogenation of CO2 into methanol has been presented through several sensitivity analyses because this reaction is highly thermodynamically limited. This thermodynamic study was completed by a wide literature review regarding the catalysts to be used and their related kinetic mechanisms, models and deactivation phenomena.
A kinetic study is also proposed on a commercial CuO/ZnO/Al2O3 catalyst and on an innovative homemade CuO/ZnO/ZrO2 one specially designed for the conversion of pure CO2 into methanol. The experiments were conducted on a homemade-designed semi-pilot installation, allowing the determination of the kinetic constants of two power-law and one LHHW models related to both these catalysts.
The entire CO2 conversion chain from its capture to its conversion into methanol was also investigated. In this perspective, the CO2 conversion process has been optimized with regards to the operating conditions and considering internal and external integrations. Finally, the integrated and optimised process is able to treat 2475 tonnes per day CO2 and convert 90% of them into 1546 tonnes methanol per day. The heat integration inside the conversion unit reduces the reboiler duty required for the distillation of the water-methanol mixture by 100%. The heat integration between the CO2 capture and conversion units also reduces the reboiler duty required for the regeneration of the solvent by 32.7%. The water integration from the coproduct of the methanol synthesis completely fulfils the water demand of the CO2 capture unit for water make-up.
The LCA also emphasises the high potential of this alternative route compared to the conventional production of methanol from steam methane reforming as its contribution to climate change could be drastically reduced from 819 to -494 kgCO2eq per tonne methanol for the conventional methanol production from syngas and our optimised process respectively.
Finally, the proposed CO2-based conversion process can be considered as a very good substitution alternative for methanol production and a solution for climate change mitigation. However, its economic breakthrough is strongly related to the costs of the electricity required for the production of hydrogen.