Scrubbing CO2 from a Simulated Flue Gas via the Dissolution of Heat-Activated Serpentinite

Our research concerns the aqueous scrubbing of CO₂ from a simulated three component flue gas via the dissolution of magnesium (Mg) rich ground heat-activated (HA) serpentinite. HA serpentinite is an alkaline solid transformed from the raw serpentine mineral into a largely reactive (and amorphous) form through heat treatment. The research involves conducting dissolution experiments with HA serpentinite (sub 75 µm size fraction) in batch configuration using a column-based scrubbing unit operated at atmospheric pressure and temperatures between of 30-60°C. The simulated flue gas being used in experimentation is comprised of 10-15% CO₂, an oxygen (O₂) component of less than 10%, with the balance nitrogen (N₂). This composition was chosen as it is comparable with that of the major components of a typical flue gas from a coal-fired generator.

Major factors affecting the rate and extent of scrubbing of CO₂ from the synthetic flue gas are examined, as well as the dependency of scrubbing efficiency on these factors. The primary variables considered in experimentation include; gas composition, gas flow rate, bubble size and solids loading. We report results from a range of experiments using a primary flue gas composition of 12% CO₂ in N₂ in the absence of O₂ as a basis for comparison.

The extent of Mg extraction is measured and the scrubbing efficiency is determined by processing progressive liquid samples taken from the reactor and analysing these samples to determine elemental concentrations of Mg, as well as other elements, using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The Mg concentration is then compared to the amount of carbon captured, based on the difference between the concentration of CO₂ in the input and exhaust streams. The composition and flow rate of both streams are recorded by a gas analyser at discrete time intervals. In order to consider possible influence of bubble characterisation on mass transfer, scrubbing performance and Mg extraction, bubble classification is facilitated by photographic methods whereby an off-flow stream is monitored to establish Sauter mean diameter and an empirical estimation of bubble size distribution.

Thermodynamic calculation using PHREEQC thermodynamic software is used to determine whether the extent of dissolution of the HA serpentinite, using measured Mg extraction as the metric, represents what would be predicted should the CO₂ gaseous and liquid phases be in equilibrium. In addition to elemental Mg concentrations in progressively collected liquid samples, the rate of Mg extraction is further outlined by an increase in pH over time and increase of solution conductivity.

Analysis of the solids, HA serpentinite prior to reaction and leached solid collected post reaction, is undertaken using Thermogravimetric Analysis (TGA) to identify any evolution of CO₂ due to the presence of precipitated carbonate during dissolution. Additionally, Quantitative X-ray Diffraction (QXRD) is being used to determine the change in amount of amorphous reactive phase in the unreacted and leached solids.