Article pubs.acs.org/est
Fabrication of CaO-Based Sorbents for CO2 Capture by a Mixing Method Changlei Qin, Wenqiang Liu, Hui An, Junjun Yin, and Bo Feng* School of Mechanical and Mining Engineering, The University of Queensland, St Lucia, Queensland 4072, Australia S Supporting Information *
ABSTRACT: Three types of sorbent were fabricated using various calcium and support precursors via a simple mixing method, in order to develop highly effective, durable, and cheap CaO-based sorbents suitable for CO2 capture. The sorption performance and morphology of the sorbents were measured in a thermogravimetric analyzer and a scanning electron microscopy, respectively. The experimental results indicate that cement is a promising low-cost support precursor for contributing to the enhancement of cyclic CO2 sorption capacity, especially when organometallic calcium precursors were used. A sorbent (with 75% CaO content) made from calcium L-lactate hydrate and cement showed the highest CO2 sorption capacity of 0.36 g of CO2/g of sorbent and its capacity decreased only slightly after 70 cycles of carbonation and calcination.
1. INTRODUCTION CO2 capture and sequestration (CCS) has been identified to be one of the most effective ways of mitigating the global warming problem.1 In CCS, the process of CO2 capture is the major barrier, because it accounts for about 75% of the total cost, while CO2 transportation and storage account for the rest.2,3 Various methods have been proposed to capture CO2; however, only few of them are proved to have acceptable technical and economical performance. Currently, amine scrubbing is the only commercially available technology, but it has a number of intrinsic undesirable problems: the corrosive nature of the solvents, the relatively high utility cost, and severe efficiency penalty.4−6 Comparatively, the chemical looping of CaO-based sorbents exhibits great advantages for its abundant cheap sorbent precursors and the potential much higher CO2 carrying capacity.7 Calcium looping cycle is based on the reversible chemical reactions between CaO and CaCO3, which typically repeatedly operate at atmospheric pressure between 650 and 900 °C. Carbonation is characterized with an initial fast rate stage, followed by a transition to a slow diffusion-controlled process.8 Then, the produced CaCO3 is calcined, which usually completes rapidly in minutes over a wide range of conditions,4,9 and recycled back ready for the next cycle. Though the carbonation/calcination cycle is a simple chemical process, the CaO-based sorbents suffer from a well-know problem of lossin-capacity, i.e., the sorption capacity of sorbents usually reduces dramatically to 8−10% of its initial value after a number of cycles.4,10 Loss-in-capacity is the most challenging problem that is imperative to solve before CaO-based sorbents can be © 2012 American Chemical Society
commercially applied. Till now, great efforts have been made to solve the problem by either reducing the rate of decay in reactivity or improving the long-term performance of sorbents, and various degrees of success have been achieved. These mainly include pretreatment of sorbents10−12 before they are subjected to cyclic sorption/desorption and dispersion of CaO into inert materials such as Ca12Al14O33,5,10,13−15 CaTiO3,16,17 MgO,11,18−21 SBA-15,22 and rice husk ash23 to prevent sintering and pore structure change of the sorbents. Among these efforts, a simple wet mixing method has been developed in our group to synthesize CaO-based sorbents.24 Briefly, water or a solvent is used to dissolve calcium and support precursors in order to uniformly mix CaO and support at the molecular level. Sorbents could be obtained in the form of powder after processes of solution mixing, drying, and calcination. Remarkable CO2 capture performance was demonstrated of all the sorbents derived from a wide range of calcium and magnesium precursors. And almost 90% of the theoretical maximum capacity was sustained over 24 cycles of carbonation/calcination under the following conditions: 650 °C, 15% CO2, 30 min for carbonation, 900 °C, 100% N2, and 10 min for calcination. The superior performance of these sorbents is believed to be owing to the fine distribution of CaO on the support. Although the wet mixing method has been proved to be effective, calcium and support precursors are required to have Received: Revised: Accepted: Published: 1932
October 25, 2011 December 30, 2011 January 3, 2012 January 3, 2012 dx.doi.org/10.1021/es203525y | Environ. Sci. Technol. 2012, 46, 1932−1939
Environmental Science & Technology
Article
Table 1. Composition of the Cement component percentage (wt %)
CaO ≤37.0
Al2O3 ≥39.8
SiO2 ≤6.0
Fe2O3 ≤18.5
2. EXPERIMENTAL SECTION 2.1. Raw Materials. Six calcium precursors were used, among which calcium carbonate (CC, 99%, Ajax Finechem) and calcium hydroxide (CH, 96%, Ajax Finechem) are insoluble. The four soluble precursors are as follows: calcium acetate hydrate (CA, 99%, Sigma-Aldrich), calcium D-gluconate monohydrate (CG, 98%, Sigma), calcium formate (CF, 99%, Aldrich), and calcium L-lactate hydrate (CL, 98%, Aldrich). Calcium aluminate cement (CE) produced by Kerneos Aluminate Technologies and clay (LY) and fly ash (AH) from Cement Australia were used as insoluble support precursors. The composition of cement is as shown in Table 1. Three soluble support precursors are magnesium acetate (MA, 99.5%, Ajax Finechem), magnesium D-gluconate hydrate (MG, 98%, Sigma), and magnesium L-lactate monohydrate (ML, 95%, Aldrich). 2.2. Sorbents Fabrication. Table 2 summarizes the total 19 sorbents prepared using mixing methods, which were Table 2. Summary of the Synthetic Sorbents insoluble−soluble
soluble−insoluble
CC-CE-75 CH-CE-75 CC-LY-75 CH-LY-75 CC-AH-75 CH-AH-75
CH-MA-75 CC-MA-75 CH-MG-75 CH-ML-75 CH-MG-50
CA-CE-75 CG-CE-75 CF-CE-75 CL-CE-75 CA-LY-75 CA-AH-75 CL-CE-50 CG-CE-50
MgO
