Carbon Dioxide Sequestration By Mineralisation Of Coal Fly Ash

In the present study,various fly ashes were selected for carbonation experiments.Carbonation experiments were conducted in a batch reactor within a wide range of temperature and at the same initial CO₂ pressure,solid/liquid ratio,stirring rate,and reaction time to investigate the carbonation characteristics of different Mg/Ca-bearing phases and the effect of temperature on carbonation reaction.X-ray fluorescence spectroscopy?XRF?,X-ray diffraction?XRD?,and scanning electron microscope?SEM?equipped with X-ray spectroscopy?EDS?were used to determine chemical compositions of crystalline and amorphous phases of three fly ashes qualitatively and quantitatively.BJ fly ash displayed the best carbonation performance.We then deeply studied the carbonation efficiency and reaction mechanism of BJ fly ash.The effects of temperature,solid to liquid ratio and gas flow rate on the carbonation efficiency of the fly ash were systematically investigated in the direct mineralisation process.Apart from parameters optimization,the promotion of additives like Na₂CO₃,NaHCO₃ and NaCl to carbonation efficiency was also investigated.After the investigation of the effect of parameter on carbonation efficiency,a typical Victorian brown coal fly ash from Hazelwood power plant which has very high Ca fraction was selected for carbonation experiments to investigate its CO2 sequestration potential and the kinetic mechanisms involved.Experiments were conducted in a vessel reactor at various temperatures?40,50,60,and 70?C?,stirring rates?900,1050,1200 and 1350 rpm?and CO₂ pressures?3,4,5,6 and 7 bar?to investigate the reaction kinetics and identify the rate-limiting steps of carbonation.The results indicated that calcium in lime and portlandite had a higher reactivity towards CO₂ than that in other calcium bearing phases either crystalline or amorphous.Both parameter optimization and introduction of additives could improve carbonation efficiency and enhance the carbonation reactions between fly ash and CO₂,but the kinetic was still not fast enough for large-scale application of CO₂ mineralisation by fly ash.To deal with problem,we developed a novel process?integrated CO₂ absorption-mineralisation?IAM??that integrates amine scrubbing,CO₂ mineralisation and amine regeneration in a single process.The regeneration performance of five commonly used amines by calcium oxide?CaO?and their associated mechanisms were investigated in detail.These amine absorbents included monoethanolamine?MEA?,diethanolamine?DEA?,piperazine?PZ?,N-methyldiethanolamine?MDEA?and 2-amino-2-methy-1-propanol?AMP?.We also investigated the technical feasibility of using fly ash as a feedstock for absorbent regeneration.Then the generation performance of the best amine?PZ?was further investigated in multicycle experiments using calcium oxide solid as well as the industrial waste?fly ash?as feedstock.The benchmark solvent MEA was then selected to deeply investigate the reaction mechnisms of IAM.The results show that amine solutions can be regenerated after a carbonation reaction with both calcium oxide and fly ash at 40?C,and that the CO₂ absorbed by amine solution can be precipitated as calcium carbonate.Compared with the traditional thermal regeneration-based CO₂capture,IAM has great advantages in energy reduction and capital saving due to larger cyclic CO₂ capacity,less energy requirement of amine regeneration and no need of CO₂ compression and pipeline transport.This technology has great potential for industrial application,particularly with CaO-containing wastes,such as fly ash and carbide slag.However,a more detailed study is required to investigate potential problems.For example,various amines should be analysed to find a more suitable absorbent for IAM.The technology is also likely to require a new amine-CO₂ contactor instead of a packing column,because the carbonation of the calcium ions in lean solutions may block the traditional packing column.In addition,the leaching behaviours of the metals from fly ash and their effect on amine degradation should be studied in detail.