Feasibility of Carbonation of Alkaline Waste Oil Shale Ash in the Rocking-Type Autoclave Reactor

The increasing carbon dioxide emissions from fossil fuel combustion, has led to concerns about climate change. Mineral carbonation, which includes the reaction of Ca or Mg containing minerals with CO₂, has been proposed as one of the options for reducing carbon dioxide emissions, especially in case the suitable waste materials and industrial by-products are available.

In Estonia, local low-grade carbonaceous fuel, kukersite oil shale, is used as a primary energy source. The mineral matter of oil shale is dominated by calcite and dolomite, which also contribute to its high specific carbon emission factor (27.85 tC/TJ). Estonia is one of the highest per-capita CO₂ emitters (13.4 Mt in 2013) among EU Member States; its power sector is the main CO₂ emitter and also a producer of large amounts of calcium rich (up to 30% of free CaO and ~30% Ca-Mg silicates) waste ash (~7 Mt/year). The oil shale based power sector can benefit from mineral carbonation through the stabilization of waste ashes as well as CO₂ capture and storage.

In this study, the feasibility of an innovative carbonation approach for CO₂ mineralization on the basis of Estonian oil shale waste ashes was studied in a rocking-type autoclave reactor. The influence of process parameters (temperature, CO₂partial pressure, time, ash type, addition of additives) on the ash carbonation conversion was investigated, seeking the optimal conditions for maximizing the utilization of ash as a carbon sink.

A characterization of the initial samples and the treated ones as well as the liquid phase was made using various analytical techniques: TGA and total carbon analysis for estimation of the carbonate content, XRD-analysis for determination of phase composition, SEM-analysis for surface morphology characterization, LDA-analysis for determination of particle size and ICP for determination of metals content in the liquid phase.

The results of carbonation (without the additives) undertaken with different types of oil shale ashes at P=100 bar, T=180C, residence time 60 min, showed that the main compounds in the treated samples were calcite, aragonite and anhydrite. The presence of nesquehonite (Mg(HCO₃)(OH)*2H₂O) and hydromagnesite (Mg₄(CO₃)₃(OH)₂*3H₂O) was also confirmed. According to XRD, TGA and TC analysis, the carbonated samples contained 37-59% of CaCO₃, whereas the content of residual lime and portlandite was 0% at the applied conditions. The liquid phase of the carbonated suspension contained mainly Mg (0,5-1,2 g/L), Ca (0,12-0,54 g/L) ja Si (0,07-0,14 g/L) ions. Also, SEM micrograms as well as the particle size distribution curves of the samples before and after treatment were obtained for comparative investigations. A distinguish morphology of aragonite in the carbonated samples is observed from SEM images. The mean particle size of the initial ash samples was in the range of 11-45 µm and depending on the carbonation conditions increased to 19-50 µm.

It can be concluded, that the potential of the free lime in oil shale ash was utilized in full amount under studied conditions. The overall carbonation efficiency calculated on the bases of Ca+Mg content in ash varied in the range of 50-83% dependent on the applied conditions and the type of ash. So, this method allows to involve into the CO₂ mineralization process in addition to lime also Ca-Mg silicates. It should be also pointed out, that the additives possibly used for intensification can be recovered, so they are not consumed in the carbonation process.