Single and Multi-Step Processes for Flue Gas CO2 Mineralization Using Thermally Activated Serpentine

Traditionally, aqueous CO₂ mineralization was studied at high
temperature with the acidifying help of high CO₂ partial pressures
to extract alkaline earth metals from a solid feed, which then form carbonates
by precipitation. Recent developments in mineralization research have seen
process concepts that accept dilute CO₂ streams as input, thus
avoiding the cost associated to a preliminary capture step. Usually, these
concepts employ pH-tuning agents, which require full recovery, and/or a rather
complex multi-step design to deal with the low reactivity of the solid feed at
flue gas conditions. More simply, the CO₂ absorption, feed
dissolution, and carbonate precipitation could take place simultaneously in a
single reactor without additives. This, however, requires special feed
materials, such as alkaline industrial residues (fly ashes, cement kiln dust,
etc.) or the magnesium (Mg) silicate serpentine in its activated form. Alkaline
residues are highly reactive, but suffer from limited availability. Natural
serpentine is worldwide abundant, but needs to be thermally pre-treated to
maximize its reactivity [1].

The feasibility of single-step flue
gas CO₂ mineralization was investigated in a first series of
experiments. Ground (sub 125 μm) and heat-treated (75% dehydroxylated)
serpentine was mixed with ultrapure water, and exposed to CO₂ at
ambient pressure, in a 100mL Teflon reactor equipped with a gas dip tube,
reflux condenser, sampling port, and either a pH or Raman probe. Experiments
were carried out at T = 30, 50, 60,
and 90°C, and slurry densities S/L
(solids to liquid ratio) of 5, 10, 15, and 20% by weight. Slurry samples were
taken at regular intervals of t =
0.5, 1, 1.5, 2, 3, 4, 6, 8 h and filtered immediately. The filtrate was diluted
and analyzed for solute concentration of Mg²⁺, silica and iron. The
solids were dried under vacuum overnight and analyzed using TGA, XRD, and SEM
image analysis.

As expected from literature [2, 3],
the hydrated Mg-carbonate nesquehonite precipitated during low-T experiments (30-50°C), while a basic
Mg-carbonate (dypingite or hydromagnesite) was formed at 90°C. Transformation
of the former into the latter was observed at 60°C. The onset of precipitation,
t_onset,
decreased with slurry density S/L and
with temperature T. However, the
conversion of the Mg in the feed to carbonates, R_Mg, did not exceed 25% at 30 and 50°C, and even lower values
were measured at 60 and 90°C (R_Mg
≤ 15%). A complete analysis of the system composition suggested that low
carbonation (R_Mg) extents
were the results of low dissolution efficiency of activated serpentine at the
operated conditions [4].

In order to tackle the equilibrium
inhibitions obtained in the single step experiments, a double step strategy was
proposed [4]. Here, dissolution of activated serpentine and precipitation of
carbonates occur in two different reactors that are operated at different reaction
temperatures (see Fig.1). A temperature swing in combination with a pCO₂ swing was used that exploits
the fact that the solubility of the relevant Mg-carbonates decreases with
increasing T and decreasing pCO₂. For this purpose, two
Teflon reactors were connected via a double-head peristaltic pump, which circulates
aqueous Mg²⁺ between the two reactors. Sensitivity analyses were
performed for the flow sheet at steady state operating conditions, and suitable
operating conditions were identified based on thermodynamics. A few
proof-of-concept experiments were performed that aimed to validate our proposed
double-step strategy. Our preliminary results are promising, where we obtained
a higher R_Mg than from
any of the single-step experiments. A further improvement in the process scheme
was proposed by having multiple dissolution steps that leads to a multi-step
mineralization process. In this regard, a linear temperature ramp between 30
and 90°C was introduced experimentally in the dissolution reactor. This once
again performed better than our double step-process. An optimal dissolution
strategy for activated serpentine is currently being devised that would enable
us to obtain the maximum carbonation extent, R_Mg.

In summary, thermally activated serpentine
can be carbonated under very lean operating conditions of less than 100°C and
CO₂ pressures not higher than 1 bar, which serves the purpose of
direct flue gas CO₂ mineralization. By exploiting basic
thermodynamics, it is possible to separate and optimize the dissolution and the
precipitation step without utilizing additional chemicals for pH control. The
presentation will describe our experimental results from the single and
double-step strategies, and show simulations for the proposed multi-step strategy.

Fig1. Schematic flow sheet for a double-step CO₂
mineralization process. Dissolution of activated serpentine is done at
low temperatures (like 30°C) and moderate pCO₂
(like 1.0 bar), while carbonate precipitation is done at moderate temperatures
(90°C) and low pCO₂ (0.1
bar). The liquid stream is transported between the reactors and transports the
Mg²⁺ solute for carbonate precipitation.

References

[1] McKelvy et al., ?Exploration of the role of heat activation
in enhancing serpentine carbon sequestration reactions?, Environ. Sci. Technol. (2004), 38, 6897-903.

[2] Hänchen et al., ?Precipitation in the
Mg-carbonate system - effects of temperature and CO₂ pressure? Chem. Eng. Sci. (2008), 63,
1012-28.

[3] Prigiobbe and
Mazzotti, ?Precipitation of Mg-carbonates
at elevated temperature and partial pressure of CO₂?, Chem. Eng.
J. (2013), 223, 755-63.

[4] Werner et al., ?Flue
gas CO₂ mineralization using thermally activated serpentine: From
single - to double-step carbonation?, Phys.
Chem. Chem. Phys (2014), Article in press