“CO2 Energy Reactor” – Integrated Mineral Carbonation: Results and Possible Application

Mineral CO₂ sequestration is hindered by both process and economic challenges. Both of these items could be compensated by an inventive reactor technology and more focus towards utilization of the generated products.

The main process bottleneck (with regards to abundantly available magnesium silicates such as olivine) is the slow reaction rate. As a result, the process requires excessive intensity. Typical residence time is in the order of hours, while a CO₂ pressure above 40 bar and temperatures of 180°C are required. These conditions are difficult (or expensive) to reach in a conventional reactor, but can be more easily handled in a Gravity Pressure Vessel (GPV).

The main economic drawback is the combination of the processing costs and the low value of sequestered CO₂. However, if more focus is given to the application of the reaction products and their replacement of more costly conventional materials, the economic situation could change. The challenge is that this requires the manufacturing of a tailored product, meeting commercial specifications.

A Gravity Pressure Vessel is an elegant way to handle rather extreme process conditions. This device consists of a reactor with a length (more precisely, a depth) of about 1200 meter. The required pressure is obtained hydrostatically, the residence time by the volume, and the temperature by the integrated heat-exchanger. As this type of equipment cannot be “scaled down”, in order to test the mineralization process, a ‘rocking autoclave’ was built as a representation of this foreseen full-scale process.

The rocking autoclave consists of a 1.8 litre tube, which can be filled with the material, fluids and additives and then sealed. The tube is rotated 180° in order to simulate the slurry flow in the GPV and to promote the mixing (the solids descend in the liquid and the gas moves up). During the test the tube can be heated and cooled, so a preferred temperature/time profile can be applied. While rocking and heating, CO₂ is added to carry out the carbonation.

The following materials have been tested in this rocking autoclave: Olivine; Wollastonite; Asbestos; Steel slags; Oil shale residue.  In this presentation, the various results are explained. As olivine was the most studied material, it is discussed in more detail.

The olivine dissolution was increased by:

-       Higher pressure: As expected

-       Residence time: For bigger particles approximately a linear increase between the conversion and the residence time for batches between ½ hour to 6 hours. This was not noted for smaller particles.

-       Specific Surface Area. The conversion degree is dependent on the geometrical surface area.

-       Additives: the additives could be recovered and re-used, proving that they are not consumed in the reaction, but are rather working as pseudo-catalysts.

In contrast we found no effect of the following parameters (although they are usually mentioned in the literature):

-       Added NaCl had no marked effect.

-       CO₂ partial pressure: only a slight dependency on the CO₂ concentration was observed; this may be related to the presence of added NaHCO₃, which promotes the MgCO₃ precipitation.

-       Amount of added material; in previous experiments (with a CSTR) it was noted that higher solids loadings resulted in higher conversions (up to a maximum). This relation was not observed in the Rocking Autoclave, as the conversion was independent of the amount of added material. This can be explained by the absence of a passivating layer and the limited particle-particle interaction in the rocking autoclave. SEM images of the reaction products confirm that the reaction products are not depositing on the olivine itself, but forming individual particles, with their own properties (and applications).

The second major drawback of mineral CO₂ sequestration is the economics of the process. The current CO₂ prices do not sufficiently encourage mineral carbonation solely for the purposes of a carbon sink. However, if valuable end-products can be produced (including recovery of the exothermic heat of reaction), this could contribute to an economic “business case” and, as CO₂ is the feedstock, also a sustainable process. As the focus is converting CO₂ from a waste to a product, the specification demands of the intended application will determine the process requirements. For example, looking at the carbonation reaction of Olivine, according to:

Mg₂SiO₄ + 2 CO₂ = 2 MgCO₃ + SiO₂

It is evident that more attention needs to be given to the right-hand side of the equation: the formed products and their applications. When the formula is converted to a mass balance and to “an economic balance”, this becomes even clearer:

	Input		Process	Output
	Mg2SiO4	2 CO2		2 MgCO3	SiO2
Mass	1.6 t	1 t		1.9 t	0.7 t
Unit price	- $ 30/t	$ 5/t		$ 40/t	$ 200/t
Economic	- $ 48	$ 5	- $ 50	$ 77	$ 136
Balance						$ 120

As can be seen, although the process sequesters CO₂, the main driver is shifted to the product value and their applications. The reaction products from Olivine carbonation are characterized as:

-       Crystalline Magnesium Carbonate, in the form of Magnesite (about 2/3 of the total products),

-       Amorphous Silica (about 1/3), and

-       Iron, Chromium and Nickel compounds.

Notably, all reaction products are smaller than 15 micrometer, and SEM imaging indicates that the Magnesite and Silica are present as distinct particles. The physical separation of these two main products was also studied.

The following applications have been investigated:

-       Uses as additive in concrete. Both the composition (amorphous silica) and the size of the particles make this an interesting potential market. Amorphous silica is known to have a pozzolanic effect, so it can replace the costly and CO₂-emitting cement, or the very finely grinded materials that are sometimes added to concrete in order to improve its physical properties (e.g. blocking pores, which results in less freeze/thaw attack, chloride corrosion, etc.).

-       Use as an inert filler in papers, polymers and ceramics. Both Precipitated Carbonates and Amorphous Silica are used for these applications, so this is a potential market.

-       If the exothermic heat can be recovered this could be a valuable “product”, as it could be used to overcome the efficiency loss of a typical Carbon Capture step.

-       The Olivine contains Iron, Chromium and Nickel, so the recovery of these metals is of interest to mineral processors.

In summary, the reaction mechanisms in the gently mixed Rocking Autoclave autoclave are different from the mechanisms in a typical CSTR. A dissolution-type process is indicated, unhindered by passivating layers. Further, in order to get an economically interesting case for mineral CO₂ sequestration, most of the value must originate from the reaction products. Even with the foreseen higher CO₂ prices, these highly engineered high pressure, high temperature processes are only feasible when a sufficiently high revenue of the products is obtained. For this purpose, concrete is an attractive market. Also, as the used product contains CO₂ and could partially replace cement, this could be a very efficient way to lower the carbon footprint of this sector. In conclusion, the approach of using a Gravity Pressure Vessel and focusing on the valuable reaction products could make CO₂ mineralization a truly sustainable process.