Regenerable sodium-based lithium silicate sorbents with a new mechanism for CO2 capture at high temperature
Introduction
The increase in greenhouse gases, especially carbon dioxide (CO2) generated by the combustion of fossil fuels (oil, natural gas, and coal), is widely known as the main cause of global warming and climate change. Coal-fired power plants emit significant amounts of CO2 in all industrial processes [1,2]. With growing global awareness of the negative effects of CO2 emission, regulations and technologies are currently being developed for its reduction.
For large-scale CO2 emissions sites such as coal-fired power plants, a major concern for capturing CO2 is reducing the of regeneration energy required for CO2 desorption. Park et al. proposed a multi-stage solid sorbent CO2 capture process in order to reduce this regeneration energy requirement for CO2 desorption. This multi-stage CO2 capture process consisted of three-stages of different sorption and desorption temperatures: low temperature (40–150 °C, amine-based sorbent), medium temperature (300–450 °C, alkali-promoted MgO), and high-temperature (600–750 °C, Li4SiO4) [3,4]. This previous report focused on the development of a novel CO2 capture process that can reduce 30% of the operating costs of CO2 capture for application in coal-fired power plants [3]. For the success of energy exchange-type three-stage CO2 capture processes, it is essential to develop a solid sorbent with optimal performance (i.e., CO2 capture capacity, sorption rate, and long-term stability) at each stage.
In this study, we focused on developing solid sorbents with excellent performance for CO2 capture at high temperature. CaO [[5], [6], [7]] and lithium orthosilicate (Li4SiO4) solid sorbents [[8], [9], [10]] are able to capture CO2 at high temperature and both are able to sorb CO2 continuously during reversible sorption and desorption processes. In the case of the CaO solid sorbent, the reverse calcination reaction is performed in the high temperature range of 850–950 °C. The CaO solid sorbent is very cheap and has a high CO2 capture capacity for CO2 sorption at high temperatures. However, it has the disadvantage of requiring a large amount of energy for complete regeneration (>900 °C) following long-term operation over several cycles [10]. By comparison, the Li4SiO4 solid sorbent is considered one of the most promising CO2 acceptor materials at high temperatures, because it can be used at temperatures (450–800 °C) lower than those required with the CaO solid sorbent [10,11]. The Li4SiO4 solid sorbent can theoretically sorb CO2 in amounts up to 0.36 g CO2/g sorbent. It has been widely reported that the Li4SiO4 solid sorbent reacts reversibly with CO2 leading to the formation of Li2CO3 and Li2SiO3 as follows: Li4SiO4 + CO2 ↔ Li2SiO3 + Li2CO3 [[12], [13], [14], [15], [16]]. However, deactivation caused by sintering during cycling must be overcome to facilitate the regeneration of Li4SiO4-based solid sorbents [17,18]. These problems have been significantly improved by many techniques including the use of additives [15,19,20], nanoparticles [17,21,22], the preparation of various precursors [[23], [24], [25], [26], [27], [28]], and alkali metal doping [[29], [30], [31], [32]].
Recently, however, the lithium ion battery [33,34] industry has been attracting attention as the most important technology of the ‘Smartization era’ due to the fourth industrial revolution. Lithium ion batteries are widely deployed as a source of power or energy 76 for everything from portable electronics to electric vehicles. As a result, the consumption of lithium is increasing rapidly, which is directly related to an increase of its price [35]. With this trend, worldwide lithium-ion battery demand is expected to nearly double in 2018 compared with 2017, and is expected to increase 6–7 fold by 2026 [36]. This is clearly not economical as such consumption would increase the cost of synthesizing Li4SiO4–based solid sorbents.
In this study, a sodium-based lithium silicate sorbent is developed that uses less lithium than that consumed in the synthesis of Li4SiO4-based solid sorbents. Despite this difference, the developed sorbent exhibited almost similar performance to Li4SiO4 solid sorbents. The CO2 capture capacities of these solid sorbents were investigated during multiple cycles in the presence of 10 vol% CO2 and 10 vol% H2O in a fixed-bed reactor. Additionally, the performances of the developed sodium-based lithium silicate solid sorbents were characterized by studying their CO2 capture capacity, sorption rate, cyclic stability, and CO2 sorption/desorption through powder X-ray diffraction (XRD) and temperature-programmed desorption (TPD) analyses.
Section snippets
Preparation of sorbents
First, lithium silicate-based solid sorbents (Li4SiO4 and/or Li2SiO3) were synthesized using the solid-state method. Lithium carbonate (Li2CO3, Aldrich, 99.9%) and silica oxide (SiO2, Aldrich, 99.9%) were mixed in 1:1 and 2:1 M ratios. The obtained mixture was then placed in an alumina crucible and calcined in an electric muffle furnace under air for 5 h at 700 °C. The temperature ramping rate was maintained at 5 °C/min. These samples are denoted as LSx, where L and S represent Li2CO3 and SiO2,
Characterization of novel sodium-based lithium silicate solid sorbents
Fig. 2 shows the XRD patterns of the LS1, LS2, LONS1, LONS2, and LONS3 solid sorbents before CO2 capture. The lithium silicate-based LS1 and LS2 solid sorbents were prepared by mixing 1 and 2 mol of Li2CO3 with SiO2 powder, respectively. Likewise, the LONS1, LONS2, and LONS3 solid sorbents were prepared by mixing 1, 2, and 3 mol of LiOH as the Li precursor with a sodium silicate solution (instead of SiO2 powder), respectively. In the case of the LS1 solid sorbent, its corresponding XRD pattern
Conclusion
The CO2 capture capacity of the regenerable sodium-based lithium silicate solid sorbents (LONS2) developed in this study was maintained at approximately 253–223 mg CO2/g sorbent during multiple sorption/regeneration tests in the high temperature range between 550 and 700 °C. The CO2 sorption rate of the LONS2 solid sorbent was much faster than the Li4SiO4 (LS2) solid sorbent. Notably, we found that the LONS2 solid sorbent contained Li4SiO4 as well as Li3NaSiO4 phases. The new Li3NaSiO4 phase
Acknowledgment
This work was supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology [grant number 2009-0093819]. Support from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea [grant number 20173010050110] is also acknowledged.
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Yong Mok Kwon and Soo Chool Lee have contributed equally to this work.