Post-Combustion Carbon Dioxide (CO2) Capture with Biomass Derived Activated Carbon

Abstract

The carbon dioxide (CO2) capture and storage are indispensable for reducing greenhouse gas emissions. Post-combustion capture is one of the most promising technologies to capture CO2 because it can be retrofitted to any facility with an acceptable footprint. Adsorption-based technologies are appealing for CO2 capture mainly due to the ease of the regeneration and the benign character of solid sorbents. To date, the greatest research efforts have aimed at improving efficient adsorbents with higher working capacity for CO2, higher selectivity, and better impurity tolerance. Among the alternative methodologies in adsorbent production, valorization of agricultural residues is an efficient way in terms of a sustainable circular economy. Besides, valorizing the agricultural residues in porous carbon materials contributes to the reduction of the overall cost of carbon capture since they are ubiquitous and inherently of low-cost. It may also provide a further benefit for more cost-effective waste management. In addition, in order to scale-up the adsorption based CO2 capture technology, cyclic adsorption processes are being designed and tried to be optimized. Especially, the temperature swing adsorption (TSA) processes are gaining more and more attention because they only require thermal energy, offering an additional advantage over pressure/vacuum swing adsorption (PSA/VSA) processes.

In post-combustion CO2 capture literature, there are many studies that address the use of waste materials as precursors of adsorbents; the thesis study puts for the first time on evaluating the performance of hazelnut shells. The first aim of this dissertation is to develop activated carbon from hazelnut shells with suitable textural development in terms of microporosity and surface area to enhance the adsorption capacity, and to investigate its potential use for CO2 capture under post-combustion capture conditions with particular emphasis on the thermal energy requirements for regeneration. The second aim is to evaluate the performance of the hazelnut shell based activated carbon under dynamic conditions in a fixed-bed reactor over consecutive adsorption−desorption cycles. For that, the maximum CO2 capture capacities were determined from breakthrough curves in CO2/N2 binary mixture at different temperature and partial pressure conditions (14% and 30% CO2 at 30°C and 50°C) which close to the real ones encountered in an industrial process. The last aim is to design different TSA processes providing higher product (CO2) purity, recovery, productivity and lower specific energy consumption and to compare these processes performances with the ones obtained in VSA processes, which were also tested in the current study.

The results obtained in this dissertation revealed that it is possible to develop highly microporous carbonaceous adsorbent using hazelnut-shells and the higher separation factor of CO2 over N2 observed at low CO2 partial pressure makes the adsorbent a good candidate for CO2 capture from post-combustion flue gases. According to evaluation of adsorption capacity under static conditions, the net theoretical CO2 uptake of the hazelnut-shells derived activated carbon (HS-AC) would be up to 0.064 kg CO2/kg adsorbent. Secondly, the dynamic evaluation of CO2 capture capacity of HS-AC with breakthrough experiments in a fixed-bed showed that HS-AC has a fast adsorption and desorption kinetics, which is very crucial in rapid swing adsorption processes. Lastly, novel TSA configurations, which were not experimented in literature before, were proposed and they resulted in CO2 purities of 93% and 89% with recoveries of 71% and 87%, respectively (with feed condition of 30% CO2 at 30°C, and regeneration at 120°C). The thermal energy requirements of TSA configurations were calculated between 1.06 – 2.35 MJ/kg- CO2 (46.6 – 103.4 kJ/mol-CO2) depending on the feed condition and cycle configuration, which are lower than the energy requirement of amine absorption, which is the mature technology of CO2 post-combustion capture. The CO2 recovery and purity of optimal VSA configuration were found 100% and 92%, respectively pressure at 0.05 kPa for 7 min evacuation and 10 mL/min N2 purging (with feed condition of 30% CO2 at 30°C). The comparison of the best results among the two methods, TSA and VSA, illustrated that the cyclic performance parameters are slightly lower when TSA is used; however, considering the possibility of using available waste heat sources in the real applications, the utilization of TSA process is a promising method for capturing CO2 from post-combustion flue gases.