Membrane-based technologies for CO2 capture and mineralization from energy-intensive industries
Session chaired by Dr. Ruben Van de Vijver
Mitigation of the adverse effects of climate change requires a transition to a CO2 economy with recycling of CO2 to (carbon-neutral) fuels and (carbon-negative) chemicals and minerals using renewable sources. Mineralization (Carbonation) technology is based on reacting carbon dioxide with calcium (Ca) or magnesium (Mg) oxides or silicates to form a solid carbonate mineral structure. These materials can be found either in natural form or in waste streams. The mineralization of CO2 is an alternative to conventional geological storage and results in permanent storage as a solid, with no need for long term monitoring. The carbonation reaction can be accelerated by using high CO2 concentrations and optimized reaction conditions. The reaction is exothermic (releases energy as heat) and does not require any significant input of renewable energy. CO2 mineralization is an attractive option for the cement industry, one of the most energy- and carbon-intensive manufacturing processes where CO2 releases are inherent in their fundamental chemistry. Novel technologies for CO2 capture and mineralization involve the use of gas-liquid membrane contactors for post-combustion capture. Hollow fiber membrane contactors are well established in the field of gas separation/bubbling/extraction applications since very high and well defined surface areas can be obtained in hyper compact membrane modules. The membrane contactor process has several advantages compared to conventional bubbling columns since the gas-liquid interface lies at the pores edge with no dispersion of the gaseous phase into the liquid solvent. Membrane contactors can be used for direct CO2 capture from the flue gases and simultaneous conversion to useful chemical compounds, depending on the appropriate solvent selection. In this study the process of simultaneous CO2 capture from flue gases and direct mineralization to nano-CaCO3 is evaluated. A CO2/N2 (19/81% v/v) binary gas mixture was fed in the lumen side of the membrane module (1x5.5 Liqui-Cel™ MiniModule), while a CaCl2/NH3 aqueous solution was used as liquid feed (shell side). Gas and liquid streams were co-currently fed to the membrane module in a once-through contactor mode. The Ca2+ concentration, gas (Qg) and liquid (Ql) flowrates, as well as the Qg/Ql ratio were identified as the main process parameters affecting the performance of the process (mass transfer coefficient, CO2 removal efficiency), as well as the morphological properties of CaCO3 particles produced. Benchmark tests were also performed with a 100% CO2 stream with continuous recirculation of the CaCl2/NH3 aqueous phase, both in contactor and bubbling mode. Rapid pH reduction was found in both modes of operation (once-through, recirculation), and the CO2 recovery was monitored at 50-90%, in the whole Qg/Ql ratio, examined. CaCO3 particles were precipitated in any mode of operation. In bubbling mode, CaCO3 demonstrated all its crystalline structures, while in the contactor mode only calcite was detected. Average particle size was estimated to approximately 1μm, while nanoparticle aggregates were also observed. The average size of CaCO3 particles was slightly smaller in the case of bubbling mode operation compared to those from the contactor mode. Membrane precipitator operated in the bubbling mode had higher productivity than in the contactor mode. The experimental results revealed that membrane-based precipitation of carbonates offers an ideal route for mineralization with controllable morphological and structural properties of the precipitated particles. A very promising potential application could be calcium carbonate nanoparticles production for partially substituting cement in high-performance concrete.
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