Article pubs.acs.org/IECR
Highly Active CaO-Based Sorbents for CO2 Capture Using the Precipitation Method: Preparation and Characterization of the Sorbent Powder Davood Karami and Nader Mahinpey* Department of Chemical and Petroleum Engineering, Schulich School of Engineering, The University of Calgary, Calgary, Alberta, Canada T2N 1N4 ABSTRACT: Calcium oxide is a known adsorbent for the capture of carbon dioxide. In this study, CaO-based sorbents were prepared using the precipitation of solutions containing different anion precursors, including nitrate NO3− and chloride Cl−, by different alkaline precipitants. The sorbents prepared from the precipitation of salt solutions by alkaline solutions under specific precipitation conditions resulted in the excellent uptake capacity for CO2. These sorbents formed as a fine powder with a BET surface area (16.5 m2/g) and pore volume (0.35 cm3/g) showed almost 100% carbonation, at temperatures between 650 and 750 °C. Moreover, the carbonation proceeded predominantly during an initial short period. Under numerous carbonation/calcination cycles, these sorbents demonstrated a good reversibility. During a 17-cycle operation, the sorbents maintained a fairly high conversion of 70% at 700 °C. As the carbonation/calcination cycles progress, sorbent particles conglomerate to a loosely integrated lump resulting in a greater mass transfer resistance for CO2 molecules to reach the unreacted core of calcium oxide. It is observed that grinding of the formed chunk to fine particles could recover the activity of sorbent completely.
1. INTRODUCTION Concern over the impact of CO2 emissions on the global climate has drawn remarkable interest in carbon capture and sequestration (CCS) technologies. The promising processes involve the production of a nearly pure stream of CO2 suitable for geological sequestration from a flue gas stream of lower CO2 concentration (10−20%).1,2 Consequently, efforts to reduce CO2 emissions into the atmosphere have intensified, including research on materials to capture CO2 efficiently from mixtures. Currently, the technology for flue gas separation of CO2 is absorption by monoethanolamine (MEA) solvent. The major drawbacks of solvent absorption processes are the highenergy requirements and environmental issues from the loss of solvent due to its high volatility. Separation processes based on solid sorbents have the potential for environmental and energy benefits. However, a major challenge in the development of solid state CO2 capture is to design and fabricate sorbents with a high adsorption capacity for CO2 at low partial pressures and long-term performance. Recently, some studies have been published looking at calcium oxide for this task due to its low cost and wide availability in natural minerals.3 The large adsorption capacity makes calcium oxide a potential CO2 sorbent at high temperatures. CaO is carbonated into CaCO3, followed by regeneration into CaO with the calcination occurring at a higher temperature. The exothermic carbonation reaction and endothermic calcination reaction of calcium oxide with CO2 form a cyclic process. The major challenge of this cycle is a sharp decrease in total capacity with the number of carbonation/calcination cycles.4,5 It has been reported that the adsorption capacity for CaO-based sorbents drops as a function of the number of cycles.6−8 Recently, many methods have been proposed for improving the uptake capacity and the life cycle performance of a CaO-based sorbent, such as (1) the incorporation of high surface inert materials, (2) the © 2012 American Chemical Society
modification of the pore structure in order to increase surface area, (3) the synthesis of the mix of CaO and calcium titanate, (4) the precipitation of calcium carbonate from calcium hydrate in order to achieve high CaO sorbent performance, (5) the enhancement of the reactivity and durability of CaO sorbents by an intermediate hydration treatment during cyclic operations, and (6) the promotion effect of doping alkali metals on CaO sorbents.9−12 The objective of this research was to develop highperformance sorbents for CO2 uptake at high temperatures using the conventional precipitation of calcium hydroxide and carbonate from solutions. The reversibility of the sorbents was tested at different reaction conditions. The effect of the various precipitating conditions on sorbent performance was also studied.
2. EXPERIMENTAL SECTION 2.1. Sorbent Preparation. The chemicals were used without any further purification, direct from the supplier. The following reactants were used: sodium hydroxide pellets (99%, Sigma-Aldrich), sodium carbonate decahydrate (>99%, SigmaAldrich), calcium chloride dehydrate (ACS reagent >99%, Sigma-Aldrich), calcium nitrate tetrahydrate (>99%, SigmaAldrich), and ammonia solution (ACS reagent, 28−30%, Sigma-Aldrich). CaO sorbents were synthesized by precipitation. Calcium nitrate solution was made by dissolving Ca(NO3)2·4H2O, and calcium chloride solution was made by dissolving CaCl2·2H2O in deionized water. The precipitants were sodium hydroxide, Received: Revised: Accepted: Published: 4567
October 22, 2011 March 7, 2012 March 8, 2012 March 8, 2012 dx.doi.org/10.1021/ie2024257 | Ind. Eng. Chem. Res. 2012, 51, 4567−4572
Industrial & Engineering Chemistry Research
Article
kept short to prevent possible sintering of the sorbents. During the entire process, the sorbent weight and the temperature were continuously recorded.
sodium carbonate decahydrate, and ammonia solutions. Two stock solutions of calcium salt and alkaline precipitant with specific concentrations (molar concentrations ranging between 0.5 and 2 mol) were added to heal water (water with a specific pH was poured into the reaction vessel) dropwise to keep the pH constant and at constant temperature in the range of room temperature and 50 °C. After precipitation completed, the resulting slurry was filtered and washed twice and finally dried at 120 °C in an oven. During the entire precipitation progress, the slurry was stirred vigorously in order to achieve homogeneity. In this work, the CaO sorbents with various surface areas and particle sizes were synthesized based on the stock solution concentrations and precipitation conditions. 2.2. Characterization. BET surface area and pore size distribution measurements were performed using nitrogen adsorption and desorption isotherms at −196 °C on a Micromeritics Tristar volumetric adsorption analyzer. The CaO sorbents were degassed at 150 °C for at least 3 h in the degassing port of the apparatus before the actual measurements. The adsorption and desorption isotherms of nitrogen for BET measurements were collected at −196 °C using the values of pressure ranging from about 1 to 760 mmHg. The pore size distribution measurements were obtained using the Barrett−Joyner−Halenda (BJH) pore size and volume analysis method. 2.3. Carbonation/Calcination. Carbonation/calcination experiments were conducted with a Perkin-Elmer PyrisSTA6000 thermogravimetric analyzer (TGA), a Perkin-Elmer thermal analysis gas station (TFGS), and Pyris instrument managing v10.1 software from Perkin-Elmer. The microbalance of the Pyris-STA6000 TGA operates as a high-gain electromechanical servo system that permits detection of changes in weight as small as 0.1 μg as a function of time. To maintain the TGA balance accuracy, an ultrapure nitrogen flow of 20 mL/ min was used as the balance purge gas to flow over the sample. The TFGS has four gas channels that automatically switch on to introduce gas over the sample according to the reaction program. The shift between CO2 and nitrogen and their flow were accurately maintained by the TFGS and the reaction program. All steps of the carbonation and calcination experiments, including heating/cooling the sample and shifting gases between CO2 and nitrogen were programmable and operated batchwise. About 15 mg of sorbent was placed in an alumina sample pan and heated to the carbonation temperature at a ramp rate of 40 °C/min under nitrogen atmosphere. Once the sample had reached the carbonation temperature, the program was automatically switched to the carbonation process. A 50 mL/min flow of the reactant gas CO2 (35% carbon dioxide with balanced nitrogen) was automatically switched into the system to replace the 20 mL/min flow of pure nitrogen. When the carbonation process completed, the temperature increased or decreased at the same ramp rate of 40 °C/min to the programmed calcination temperature by switching reactive gas to inert gas. After the initial carbonation/calcination cycle progress completed, the 20 mL/min flow of CO2 was replaced by a 20 mL/min flow of helium. A new carbonation/calcination cycle began with the same programmed course. In this work, the typical carbonation time was set at 60 min to achieve a relatively high uptake capacity of CO2, and the calcination time was set at less than 10 min to allow the sorbent to release the carbonate completely. The calcination time was intentionally
3. RESULTS AND DISCUSSION The results of the 1 h carbonation reactions over these sorbents at 700 °C are shown in Figure 1. A discrepancy of 3% over
Figure 1. Uptake of CO2 over CaO sorbents prepared with various precursors: (a) nitrate salt, (b) chloride salt, and (c) nitrate salt (precipitant, sodium carbonate). Conditions: carbonation temperature, 700 °C; CO2 concentration, 35 vol %; balance nitrogen.
three repetitions of the experiment was considered sufficient to validate the results and ensure reproducibility. The carbonation and calcination processes are described by the following reactions:
carbonation/calcination: CaO + CO2 ⇄ CaCO3
(1)
In order to compare the performances of all CaO sorbents, carbonation experiments were performed at 700 °C under a CO2 partial pressure of 140 kPa. For all sorbents, a monotonic increase in sorbent uptake was observed. Conversion is calculated from the following equation: conversion (%) = (56/44)(M2 /M1 − 1)·100
(2)
In eq 2, 56 and 44 are the molecular weights of calcium oxide and carbon dioxide. M2 and M1 are sample weights from the end of carbonation and calcination processes recorded by TGA, respectively. M2 − M1 indicates a weight gain due to CO2 uptake by sorbent. Thus, from the stoichiometry in reaction 1, the CaO converted is (56/44)(M2 − M1). M1 is the assumed weight of the active CaO exposed to gas. The division of two weights gives the conversion. All sorbents showed relatively high carbonation rates during the initial stage followed by an abrupt shift to a lower rate. During the high carbonation rate, the sample temperature increased slightly over the programmed temperature due to the exothermic reaction of the carbonation. During the 1 h carbonation process at 700 °C, the sodium hydroxide Ca(OH)2 sorbents were carbonated. There was an almost complete conversion of CO2 under the same carbonation conditions, and each one was precipitated by a sodium hydroxide solution. The sodium carbonate Ca(OH)2 sorbent precipitated by the sodium carbonate exhibited nearly 100% conversion depending on the 4568
dx.doi.org/10.1021/ie2024257 | Ind. Eng. Chem. Res. 2012, 51, 4567−4572
Industrial & Engineering Chemistry Research
Article
calcination weights (M1), due to the fineness of the prepared calcium carbonate. When M1 was less than 5 mg, in the first cycles, conversion reached completeness. When the sodium hydroxide Ca(OH) sorbents possess some coarse particles, the M1 can be raised to less than 15 mg. Dependence of sorbent capacities on M1 is presented in Figure 2. The reason for this
Figure 4. BJH desorption pore size distributions of CaO sorbents prepared from chloride salt.
PSD trend regardless of their surface area.13 Table 1 summarizes BET surface areas and pore volumes of a few
Figure 2. Conversion dependence on calcination sample weight (M1).
Table 1. BET Surface Area and Pore Volume for the Various CaO Sorbents
dependency may be attributed to the sintering and conglomeration of fine particles in the calcination stage to an integrated chunk. This makes it difficult for CO2 molecules to access the unreacted core solid. The same sorbents prepared by direct calcination of calcium salts exhibited a low uptake capacity of CO2, and the carbonation reaction rate was extremely low under these experimental conditions. The reason for this difference is that these sorbents derived directly from calcium salts do not contain many pores, and they have a significantly lower BET surface area than the precipitated sorbents.8 Therefore, their capacity was limited to a low value. Figures 3
sample name
surf. area (m2/g)
pore vol (cm3/g)
ref
CaO−CaCl2 CaO−Ca(NO3)2 CaO−SH CaO−SC CaO−NH3
