Soutenance publique de thèse de Doctorat

Promoteur : Prof. Diane Thomas

Résumé :

For the cement industry, reduction of carbon dioxide emissions has become one of the main objectives as carbon dioxide emitted in this case represents 30% of the total annual CO2 released by the industrial sector. This work was carried out in the framework of the ECRA academic Chair at UMONS and aimed to reduce these CO2 emissions considering Carbon Capture and Storage/Utilization (CCS/U) applied to flue gases deriving from the cement industry. For this purpose, in this study, two treatment technologies were investigated for the cement industry case.

The first step of the CO2 purification Unit (CPU), called “Sour-Compression Unit”, was simulated in Aspen PlusTM by means of a de-SOx and de-NOx process corresponding to a water scrubbing in pressurized flue gas systems. This process was applied to flue gases deriving from a combustion conducted with pure oxygen, such process being called “full oxy-fuel combustion”. A previous air separation unit is therefore needed and this type of combustion leads to a high CO2 concentration in the flue gas, namely between 70 and 90%. The first key step was the implementation of a comprehensive, realistic and innovative chemical mechanism representing the reactive absorption which depends on two key elements, namely the pH of the solution and the SOx/NOx interaction reactions, the global mechanism being composed of 27 reactions. This mechanism provided satisfactory results with a two-column configuration of the SCU, which is the base case considered (namely 97% of abatement ratio for NOx species and 100% of abatement ratio for SOx species). A parametric study of the process was also conducted in a single-column configuration and the main driven conclusions are that the recycle ratio of the absorber, the pressure and the total liquid flow rate induce the most influent effects. It has to be noted that the NOx components are the limiting species. In fact, 100% NOx removal could not be reached even when working with 100% fresh water as scrubbing solution. The main purpose of the SCU being to reach a specific purity target on SOx and NOx components, the feature of the investigated model was to optimize the way to reach the purity target in order to decrease installation costs (CAPEX) and energy requirements (OPEX). CAPEX and OPEX studies showed that the compression step is the most cost demanding operation with over 88% of the total equipment costs and over 74% of the total energy requirements.

The second CO2 treatment technology studied here is an alternative solution for capturing CO2 called post-combustion capture in amine-based solvents, applied to cement kilns working under O2-enriched air combustion, namely “partial oxy-fuel combustion”. This combustion type leads to a more CO2-concentrated cement flue gas (between 30 and 70%) compared to the conventional one. An experimental screening of simple and blended amine-based solvents was conducted at lab and micro-pilot scales for the best solvents screened. Both scales showed that MMEA (methylmonoethanolamine, a secondary amine) is the simple solvent presenting the best absorption performances. Among the activators, PZ (piperazine, a cyclical diamine) allows enhancing the kinetics of the reaction better than TETRA (tetramine) but the latter showed interesting absorption capacities even when loaded in CO2. AMP (2-amino-2-methyl-1-propanol, a sterically hindered amine) and DEA (diethanolamine, a secondary amine) activated with PZ also presented interesting characteristics, the best solvents allowing to reach a compromise between high absorption ratios and low energies of regeneration.

Simulations of the micro-pilot unit showed that the model fits with the obtained experimental results. Regarding the industrial scale, simulations of the industrial CASTOR/CESAR pilot unit revealed realistic values of the regeneration energy of 3.4 GJ/tCO2 for a CO2 content of 20% in the gas to treat. An important conclusion for this treatment technology was the beneficial decrease of the solvents regeneration energies when the CO2 content increases (i.e. 2.5 GJ/tCO2 at 40% of CO2 in the gas to treat). The total OPEX was estimated between 33 and 45 €/tcaptured CO2 with the heat duty for the solvent regeneration being the most significant utility.

This PhD thesis studied two innovative CO2 treatment technologies applied to the cement industry. On the one hand, the implementation of the global CPU and, on the other hand, an alternative configuration of the absorption-regeneration process using the best solvents screened are the main prospects of this work. These prospects will include the respective economic evaluations for the application to the cement industry.