Effect of H2O on Mg(OH)2 carbonation pathways for combined CO2 capture and storage
Highlights
► The extent of gas-solid Mg(OH)2 carbonation is limited in dry conditions. ► The presence of H2O can enhance Mg(OH)2 carbonation in gas-solid and slurry modes. ► Mg(OH)2 carbonation often involves the formation of hydrated intermediate phases. ► Anhydrous carbonate formation is favored at high temperature and H2O loading. ► Mg(OH)2 has shown potential to be used as a combined CO2 capture and storage media.
Introduction
The rapid increase in carbon dioxide (CO2) emissions from industrial sources is considered one of the main causes for the Earth's changing climate (IPCC, 2007). Reduction of CO2 emissions can be achieved by improving energy efficiency, implementing renewable carbon-free energy sources, and developing carbon capture, utilization and storage (CCUS) technologies. Worldwide energy use will continue increasing (IEA, 2010), and thus, CCUS could provide an immediate solution to the global carbon imbalance while renewable energy technologies develop. By sequestering CO2, the atmospheric CO2 concentration can be stabilized or reduced. Most focus in the CCUS field has been placed on amine-based CO2 capture combined with geological storage. While these technologies have already been demonstrated in large scales (Rochelle, 2009), amine-based CO2 capture process and the geological storage of CO2 still face challenges, such as high parasitic energy consumption during solvent regeneration and the permanence and accountability issues for long term CO2 storage. Furthermore, these schemes would not allow direct integration of carbon capture and storage with high temperature energy conversion systems.
A few high temperature carbon capture schemes exist that utilize a metal oxide as a carbon capture medium such as Zero Emission Coal Alliance (ZECA) process and calcium looping technologies (Feng et al., 2007). Numerous studies have shown that Ca-based sorbents, often in the form of Ca(OH)2 or CaO derived from CaCO3, provide substantial carbonation conversion and kinetics (Feng et al., 2007). Ca-based sorbents are attractive because they can be prepared using inexpensive resources such as limestone; however, since they are derived from carbonate mineral, Ca-based sorbents cannot be used as direct carbon storage. The spent sorbents need to be regenerated, which requires significant energy and cost, especially when accounting for sorbent degradation (Dasgupta et al., 2008, Senthoorselvan et al., 2009).
On the other hand, carbon mineralization technology that converts Mg-bearing minerals into mineral carbonates is a CCUS scheme that could combine CO2 capture and storage technologies (IPCC, 2005). Research has shown that the abundance of suitable minerals, particularly those containing high magnesium fractions (e.g., olivine and serpentine), far exceeds the total CO2 that could be produced from fossil fuel reserves (Lackner et al., 1995). Mineralized carbon is significantly more thermodynamically stable than gaseous carbon, and carbonation reactions are exothermic. Thus, carbon mineralization is the most secure and permanent solution for carbon storage that does not require long-term monitoring (Lackner et al., 1995). Unfortunately, mineral weathering naturally occurs on geological timescales; therefore, feasible carbon mineralization processes must provide significant enhancement to mineral dissolution and carbonation rates. As a result, most of the research in this area has been focused on the enhancement of silicate mineral dissolution (1), CO2 hydration (2), (3), and carbonation (4) (Park and Fan, 2004):Mg3Si2O5(OH)4(s)+6H+=3Mg2++2Si(OH)4+H2OCO2(g)=CO2(aq)CO2(aq)+H2O=H2CO3(aq)=HCO3−+H+=CO32−+H+Mg2++CO32−=MgCO3(s)
These reactions have generally been performed in the aqueous phase, which limits their application to relatively low reaction temperatures. By raising the pH without introducing CO2 and producing Mg(OH)2 instead of MgCO3, a solid Mg(OH)2 sorbent can be formed to capture CO2 via high temperature gas–solid reactions:Mg(OH)2(s)=MgO(s)+H2O(g)MgO(s)+CO2(g)=MgCO3(s)
The overall reaction becomes:Mg(OH)2(s)+CO2(g)=MgCO3(s)+H2O(g)
Carbonation of Mg-based sorbents extracted from silicate minerals has seen less research interest, mainly due to its slower kinetics, though optimized reaction conditions and sorbent characteristics, such as surface area, can improve sorbent reactivity (Béarat et al., 2002, Butt et al., 1996, Fagerlund et al., 2010, Fagerlund and Zevenhoven, 2011, Goff and Lackner, 1998, Lin et al., 2008, Zevenhoven et al., 2008). Much of the complexity of the Mg(OH)2 carbonation system arises from the simultaneous dehydroxylation and carbonation reactions (reactions (5), (6)), which occur in similar temperature ranges (Butt et al., 1996). MgO carbonation has been shown to be considerably slower than Mg(OH)2 carbonation. In fact, MgO is effectively unreacitve at low partial pressures of CO2 (Béarat et al., 2002, Zevenhoven et al., 2008). Though Mg(OH)2 is more reactive, the carbonation reaction can produce a diffusion limited carbonate shell which restricts the overall carbonation conversion (Butt et al., 1996). Some argue that the effect of water on Mg(OH)2 carbonation was to prevent dehydroxylation (Fagerlund et al., 2011). Other literature available on carbonation of Mg and Ca bearing oxides, hydroxides, and raw minerals supports a water enhanced carbonation theory under a wide range of reaction conditions (Beruto and Botter, 2000, Kwak et al., 2010, Kwak et al., 2011, Kwon et al., 2011, Larachi et al., 2012, Loring et al., 2012, Schaef et al., 2011, Shih et al., 1999, Torres-Rodríguez and Pfeiffer, 2011). Considering Mg(OH)2 carbonation in slurry phase is relatively rapid (Botha and Strydom, 2001, Park et al., 2003), the reaction mechanism likely proceeds through a different pathway when H2O is involved. Highly hydrated environments may even eliminate the occurence of the heterogeneous carbonation reaction (Zhao et al., 2010). Thus, this study aimed to investigate the effect of H2O on the reaction pathways of Mg(OH)2 carbonation in high pressure gas–solid experiments and a slurry phase experiment through systematic solid product analyses.
Section snippets
Sample preparation
Reagent-grade Mg(OH)2 (Acros Organics) was used throughout the carbonation experiments. The particle size distribution was obtained through the laser diffraction measurement (LSTM 13 320 MW, Beckman Coulter, Inc.). All Mg(OH)2 particles were under 150 μm with the majority under 50 μm. Mg(OH)2 particles had a surface area of 6.93 m2 g−1, and the majority of pores were under 5 nm in diameter (NOVA-win 2002 BET analyzer, Quantachrome Corporation). A thin layer of Mg(OH)2 was coated on glass slides to
Two-step vs. one-step carbonation of Mg(OH)2
As mentioned earlier, some literature has reported that MgO solid is effectively unreactive with gaseous CO2 (Béarat et al., 2002, Zevenhoven et al., 2008). Thus, in order to verify this claim with our Mg-bearing sorbent and investigate the reaction mechanism and kinetics of each step involved (reactions (5), (6)), the carbonation of Mg(OH)2 was performed in a two-step mode: dehydroxylation of Mg(OH)2 and carbonation of MgO. Using a TGA setup, Mg(OH)2was first calcined in a He environment to
Conclusions
After exploring and verifying the limitations observed in gas–solid carbonation of Mg(OH)2, the current study investigated the enhancement of carbonation with increasing temperature and H2O pressure through systematic solid product analyses. Low pressure Mg(OH)2 carbonation experienced limitations over a large temperature range, while elevated CO2 pressures only marginally increased reactivity. In the presence of steam however, Mg(OH)2 carbonation was enhanced, likely due to a shift in the
Notation
- λ
Wavelength
- BET
Surface area analysis technique (Brunauer, Emmett, and Teller)
- CCUS
Carbon, capture, utilization, and storage
- dTG
Differential thermogravimetry
- MgCO3
Magnesium carbonate, magnesite
- MgCO3·3H2O
Nesquehonite
- Mg5(CO3)4(OH)2·4H2O
Hydromasgnesite
- MgCO3·Mg(OH)2·3H2O
Artinite
- MgO
Magnesium oxide
- Mg(OH)2
Magnesium hydroxide, brucite
- Mg3Si2O5(OH)4
Serpentine (Magnesium iron silicate hydroxide)
- Px
Partial pressure of species X
- Po,x
Intial partial pressure of species X
- RH
Relative humidity
- T
Temperature
- TGA
Acknowledgments
This publication was based on work supported by the National Science Foundation CAREER Award number 0846846. We are also grateful for the experimental assistance of Mr. David Dogon and the critical review of the manuscript by Dr. Camille Petit.
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