Effect of Steam on the Reactivity of MgO-Based Sorbents in Precombustion CO₂ Capture Processes

Abstract

Low-cost MgO-based sorbents were prepared through modification of natural dolomite and were found to be capable of capturing CO 2 from coal gas at elevated temperatures and pressures (i.e., 330°< T < 450 °C and P > 20 bar). The effect of steam on the reactivity of the MgO sorbents was investigated experimentally. The results indicate that the presence of steam significantly improves the overall rate of the carbonation reaction through two different mechanisms. The presence of steam improves the intrinsic rate of the carbonation reaction by altering the carbonation reaction pathway through the formation of more reactive transient MgO•H 2 O • /Mg(OH) 2 • intermediate compounds. The formation of the transient intermediate compounds with larger molar volume (compared with MgO) results in the expansion of the inner pores, which lowers diffusion resistance to the reactant gas (i.e., CO 2 ) during carbonation.

CO2 Uptake and Cyclic Stability of MgO-Based CO2 Sorbents Promoted with Alkali Metal Nitrates and Their Eutectic Mixtures

“…Therefore, the primary focus of previous research efforts has been the development of approaches to reduce the limitations due to product layer formation. For example, Zarghami et al 16 found that increasing the system pressure to 20 bar and adding 30 vol. % water vapor improved the apparent rate of carbonation, leading in turn to an almost complete conversion of the solid sorbent.…”

Structural and kinetic analysis of CO2 sorption on NaNO2-promoted MgO at moderate temperatures

“…Increasing the pressure may increase the reaction rate, but this is problematic for practical applications [21]. Alternatively, the use of steam can improve the CO 2 sorption rates of MgO due to the formation of Mg(OH) 2 , [22]. Nevertheless, the practical temperature range for this sorbent is limited to 200-315 °C.…”

CO2 capture using solid sorbents: Fundamental aspects, mechanistic insights and recent advances

“…That the carbonation reaction of MgO is limited to its surface has been substantiated experimentally by the strong correlation between CO2 uptake and surface area [140]. It is possible to improve the CO2 uptake of MgO by fabricating materials with a high surface area of up to 1000 m 2 g -1 , achieving capacities of 20 wt% at temperatures between 25 -400 °C [59], [127]- [129], [140]- [142]. In Section 5.1, we describe how infrared spectroscopy has been used to gain further insight to how CO2 is adsorbed on the surface of pure MgO.…”

“…In pre-combustion CO2 capture, e.g. for integrated gasification combined cycle (IGCC) power plants, partial pressures of CO2 from coal gas can range from 0.5 to 10 bar [59]- [61]. , respectively.…”

“…Several experimental studies have shown that the reactivity of MgO-based sorbents increases in the presence of steam, introduced either via a pretreatment step or during carbonation [59], [64], [160], [254]- [258]. For example, Zarghami et al [59] observed a 38 % higher uptake after 100 seconds of carbonation when 10 % steam was added during carbonation and a 30 % higher uptake after 5 min of carbonation when a MgO sorbent was pretreated in 30 vol.% H2O/N2 at 330 °C (Figure 3a and Figure 3b, respectively). Most of these studies have shown that it is necessary to use a steam concentration above 5 vol.% during carbonation in order to observe a beneficial effect [64], while the use of a lower concentration of 1 vol.% steam had no measurable impact [63], in contrast to CaO described above.…”