Article pubs.acs.org/IECR
Carbon Nanofiber-Supported K2CO3 as an Efficient Low-Temperature Regenerable CO2 Sorbent for Post-Combustion Capture Niels N.A.H. Meis, Anne Mette Frey, Johannes H. Bitter, and Krijn P. de Jong* Inorganic Chemistry and Catalysis, Department of Chemistry, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80 083, 3508 TB Utrecht, The Netherlands S Supporting Information *
ABSTRACT: This study focuses on regenerable sorbents for post-combustion CO2 capture at low temperature (373 K). K2CO3 loaded on three different supports, carbon nanofibers (CNF), alumina (γ-Al2O3), and activated carbon (AC), was investigated. K2CO3−CNF revealed excellent properties as CO2 sorbent, displaying capacities of 1.2−1.6 mmol g−1 and fast desorption kinetics at low temperatures (423 K). This temperature was too low to completely regenerate K2CO3−Al2O3 and K2CO3−AC, and consequently, these sorbents lost 8% and 50%, respectively, of their capacity after the first absorption−desorption cycle. K2CO3−CNF could be regenerated to restore 80% of its capacity with a low energy input, estimated at 2−3 GJ/ton CO2, which is competitive to currently used amines.
■
INTRODUCTION The increasing use of fossil energy brings about the emission of large amounts of carbon dioxide (CO2) into the atmosphere and carbon capture and storage (CCS) is one of the options to mitigate these emissions.1 CO2 capture can be divided in two classes. One method is pre-combustion capture that involves CO2 trapped at high temperatures (523−773 K) accompanied by the production of hydrogen, for example, via the water−gas shift reaction (CO + H2O ↔ CO2 + H2). Removal of CO2 from flue gas streams, for example, from power plants, at low temperatures (373−423 K) is referred to as post-combustion capture. For the latter, amine-based scrubbers are currently the commercial option.2,3 The use of amines, however, requires the use of special reactor materials due to their corrosive nature.2 Other drawbacks of amines are their limited stability during operation, toxicity, and need of a solvent (water) to prevent foaming and to keep the viscosity low.3 The use of water as solvent makes the regeneration (desorption of CO2) energy intensive because a considerable amount of energy is used for evaporating the water to release the entrapped CO2. Reported alternatives for amine scrubbers are solid regenerable adsorbents such as amines supported on SBA-15,4−6 MCM41,5,6 MOFs,7−10 ZIFs,11,12 and modified carbons.13,14 Though promising, these materials are prone to fouling and deterioration. Alternatively, alkali carbonate supported on carbon and refractory supports has been reported to be promising.14−21 However, most of the reported K2CO3-based materials have low CO2 capacity17,20 or limited stability (e.g., K2CO3/MgO17,18). In addition, regeneration is an important factor to consider, and most of the currently known CO2 storage materials require high temperatures to be regenerated.17,18,22,23 Alternative materials that are regenerable at low temperatures are thus needed.24 The process for CO2 capture with K2CO3 consists of absorption (carbonation) 1 and regeneration 2, which can be expressed as © 2013 American Chemical Society
K 2CO3 + H 2O + CO2 → 2HKCO3 ΔH 0 = −103 kJ mol−1 (absorption)
(1)
2HKCO3 → K 2CO3 + H 2O + CO2 ΔH 0 = +103 kJ mol−1 (regeneration)
(2)
20,25,26
According to the literature, however, the system is slightly more complex and can better be described by K 2CO3 × 1.5H 2O + CO2 → 2HKCO3 + 0.5H 2O ΔH 0 = −38 kJ mol−1 (absorption)
(3)
2HKCO3 + 0.5H 2O → K 2CO3 × 1.5H 2O + CO2 ΔH 0 = +38 kJ mol−1 (regeneration)
(4)
Because of the low enthalpy of CO2 absorption for reaction 3, the regeneration of K2CO3 sorbents requires less energy input that apparent from reaction 1 and 2, making it an attractive sorbent system to develop. Here, we report the performance of three supported K2CO3 sorbents for CO2 capture. Carbon nanofibers (CNF) was used as a support and compared to activated carbon (AC) and to a more traditional support, γ-alumina. CNF supports have previously been used to study nanostructured NaAlH427 for hydrogen storage and have shown to be an excellent support for solid base catalysts such as hydrotalcites,28 lanthanum oxide,29 and alkaline earth metal oxides.30 We investigated the role of the support on absorption and regeneration properties on supported K2CO3. We show that the energy consumption of K2CO3−CNF is competitive to the commercially used amines. Because this new material simply consists of potassium Received: Revised: Accepted: Published: 12812
May 30, 2013 August 13, 2013 August 14, 2013 August 14, 2013 dx.doi.org/10.1021/ie4017072 | Ind. Eng. Chem. Res. 2013, 52, 12812−12818
Industrial & Engineering Chemistry Research
Article
Table 1. Experimental Conditions for Sorption Measurements at 373 K with 5 g Activated Supported Sample flow (mL min−1)
composition
temp. (K)
30 30 30
100% N2 83% N2/12% H2O/5% CO2 88% N2/12% H2O
323 → 773 (5 K min−1, 1 h) 373 (30−300 min)s 373 → 423/573 (5 K min−1, 1 h)
activation absorption desorption
Table 2. Physio-Chemical Properties of Support Materials and Various K2CO3 Sorbents
a
support
SBET (m2 g−1)
Vtot (cm3 g−1)
CNF
150
0.37
Al2O3
160
0.80
AC
816
0.39
sorbent
SBET (m2 g−1)
Vtot (cm3 g−1)
Vmicro (cm3 g−1)
K2CO3 K16−CNF K29−CNF K16−Al2O3 K29−Al2O3 K16−293−AC
