ARTICLE pubs.acs.org/EF
High-Temperature Pressure Swing Adsorption Process for CO2 Separation Junjun Yin, Changlei Qin, Hui An, Wenqiang Liu, and Bo Feng* School of Mechanical and Mining Engineering, The University of Queensland, St Lucia, Queensland 4072, Australia
bS Supporting Information ABSTRACT: This paper presents a novel pressure swing adsorption process and the development of specifically designed sorbents for the process. It is operated at high temperature (650800 °C) using the reversible reaction of calcium oxide with CO2, i.e., CaO + CO2 a CaCO3. The new process directly stores the reaction heat released from the forward reaction in the sorbent and then releases it for sorbent regeneration under reduced CO2 partial pressure, so that the need of pure oxygen for oxy-fuel combustion is avoided. Two potential problems of the new process, namely, loss in capacity and slow and unmatched reaction rates of chemicalcontrolled carbonation and calcination, were discussed in detail. Three specifically designed calcium-based sorbents showed stable performance during 92 isothermal carbonationcalcination cycles at either 680 or 750 °C. The calcination rate was significantly enhanced by increasing the reaction temperature and the introduction of steam to match the reaction rate of chemical-controlled carbonation. This pressure swing adsorption process could be used for low-cost CO2 separation using specifically designed sorbents under carefully selected operating conditions.
1. INTRODUCTION One of the emerging CO2 capture technologies is the use of calcium-based sorbents, which undergo carbonation and calcination reactions via CaOðsÞ þ CO2 ðgÞ a CaCO3 ðsÞ
Another problem in calcium looping is loss in capacity of sorbent after cycles that also increases the cost of system operation because of the need of makeup flow, which is of the order of the mass flow of fuel entering a plant.7 The loss-incapacity problem is an old issue for a calcium-based sorbent, which is attributed to the dramatic decrease of the surface area and pore volume as a result of sintering under high calcination temperatures and closure of pores during the carbonation process.8,9 To date, several methods have been proposed to solve this problem, including hydration by steam,10 thermal pretreatment,11 and synthesis with inert support.12 The thermal pretreatment and synthesis are applied for sorbent preparation, while hydration is mainly used during the operation of the calcium looping process. These technologies have achieved various degrees of improvement on the reactivity of sorbent in the laboratory experiment, but no clear solutions for commercial use have yet been found. Aiming at reducing the cost of calcium looping by eliminating both the need of oxygen production and the problem of loss in capacity, this paper presents a novel pressure swing adsorption (PSA) process that operates at high temperature (650800 °C) using a specifically designed calcium-based sorbent. The objective of this paper is to demonstrate the feasibility of the new process and performance of the specifically designed sorbents suitable for the process. Some potential operational challenges and a cost estimation of specifically designed sorbent are also outlined in this work.
ΔH25 °C ¼ (178 kJ=mol
A well-accepted approach is to conduct carbonation and calcination in two fluidized-bed reactors connected by solid transportation lines.1 The solid sorbent is continuously circulated between the reactors, absorbing CO2 in a carbonator (600700 °C) and then desorbing CO2 in a calciner (900 950 °C). The adsorption reaction releases a large amount of energy that is recovered by driving an additional steam cycle, while the desorption process requires the same amount of energy for sorbent regeneration that is typically provided by oxy-fuel combustion.2 In a typical calcium-looping process, nearly 1/3 of the oxygen demand in an oxy-fuel power plant is required for oxy-fuel combustion, which represents significant capital, operation, and maintenance costs.3 Either avoiding or minimizing the need of pure oxygen in the calciner operation could further reduce the cost of calcium looping. A possible method is to combine metal oxide chemical looping (e.g., Fe/FeO chemical looping) with calcium looping, in which the oxidation reaction of metal with air provides energy necessary for sorbent regeneration.4,5 However, the generated CO2 in the regeneration step could be highly diluted by N2 in air. Abanades et al.6 recently improved this method by combining calcium looping with Cu/CuO chemical looping. In their method, the decomposition of CaCO3 occurs simultaneously with the highly exothermic CuO reduction reaction in the presence of syngas to obtain CO2 stream with high purity. r 2011 American Chemical Society
Special Issue: 2011 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: July 31, 2011 Revised: October 25, 2011 Published: October 26, 2011 169
dx.doi.org/10.1021/ef201142w | Energy Fuels 2012, 26, 169–175
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has to be restricted not to exceed Teq equilibrium temperature corresponding to P1; otherwise, the calcination of CaCO3 will be dominant. This allows the temperature increase at atmospheric pressure to be between 50 and 100 °C). Afterward, the exiting CO2-free flue gas can be sent back to the boiler tail flue for further heat exchange. 2.1.2. Calcination Mode. After about 1 min of carbonation (because the chemical-controlled stage of carbonation normally finishes in 1 min), the reactor is switched to calcination mode by disconnecting the flue gas and introducing steam as sweep gas to dilute CO2 at the reaction front. Then, CaCO3 starts to decompose with required energy from the sorbent itself under low CO2 partial pressure, which is expected to complete in 1 min to achieve a 1 min switching time. The ratio of steam/CO2 is important for the reaction front movement and calcination rate. Then, the regenerated sorbent is recycled to capture CO2, while the gas mixture containing released CO2 and steam exits the reactor for further condensation, compression, and storage. Therefore, the process is cycled by swinging the CO2 partial pressure around the equilibrium curve at atmospheric pressure, which can be seen as a hot pressure swing adsorption (HotPSA) process. Another uniqueness is that the solid flow is stagnant in the reactor rather than circulating between a carbonator and a calciner. Also, it is believed that the calcination temperature of the HotPSA process is 100150 °C lower than that in conventional calcium looping because of internal energy use and PSA operation. Therefore, it is envisaged that the new process has the following advantages in contrast with conventional calcium looping: (1) much lower cost mainly because fuel, pure oxygen, and external energy for regeneration are unnecessary or minimized, (2) lower attrition rate of sorbent because of the mitigated contact between sorbent particles and between the particle and reactor wall as a result of stagnant solid in the reactor, and (3) milder sintering rate caused by a lower calcination temperature. The last two merits can contribute to less sorbent makeup flow that further reduces the system cost. 2.2. Potential Operational Challenges. With regard to the feasibility of the new process, however, it is possible that the process would suffer from the following potential problems of reaction heat loss, loss in capacity, and slow and unmatched carbonationcalcination reaction rates. 2.2.1. Reaction Heat Loss. The reaction heat might be stored in the sorbent or be carried away by feeding gas, and the latter results in reaction heat loss. To minimize the amount of heat being carried away by feeding gas, the heat conduction rate inside the sorbent should be much faster than that of the heat convection between the gas and sorbent, so that the reaction heat can be used to heat the sorbent mostly. Meanwhile, the heat release rate should not be limited by CO2 diffusion in the sorbent. In other words, the Biot number and pseudo-Lewis number for the sorbent reaction with CO2 should both be much less than 1. Rough computation was deduced to conclude that the particle size should be less than 0.050.3 m (assuming that the thermal conductivity of cement and the heat-transfer coefficient of flue gas at atmospheric pressure are 1.74 W m1 K1 and 535 W m2 K1, respectively). It is possible to control the particle size and properties, so that the two numbers are much less than 1 and the heat loss is at a minimum. 2.2.2. Loss-in-Capacity Problem of the Calcium-Based Sorbent. The loss-in-capacity problem causes a large makeup flow (of the order of the mass flow of fuel entering a plant in the case of natural minerals) to maintain the sorbent activity.13 It has been
Figure 1. Operation modes of the new HotPSA process.
Figure 2. Equilibrium of pure CaCO3 above CO2 partial pressure as a function of the temperature.
2. HIGH-TEMPERATURE PSA PROCESS 2.1. Fundamentals of the Proposed Process. The new system conducts the reversible reaction of CaO with CO2 for CO2 capture by swinging CO2 partial pressure. The innovation of the process is in the storage of the reaction heat released from the carbonation process in the specifically designed calciumbased sorbent and subsequent reuse of it for CaCO3 decomposition. The new process operates in the following two modes (see Figure 1). 2.1.1. Carbonation Mode. The reactor (including sorbent) is initially heated to a desired operating temperature (650800 °C) suitable for the reaction between CaO and CO2. Then, the flue gas (with a similar temperature as the carbonation temperature possibly coming from the high-temperature section in the boiler tail flue) enters the reactor to remove CO2. Simultaneously, the released energy from carbonation is stored in the sorbent, which will result in an increase by a small margin of the temperature (as shown in Figure 2, the temperature raises from T1 to T2, and T2 170
dx.doi.org/10.1021/ef201142w |Energy Fuels 2012, 26, 169–175
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the CaO content in the final sorbents should be around 5%, assuming 100% conversion and 50 °C temperature rise. When calcium aluminate cements were employed as an inert support, the CaO content was set as 15% because of the potential reaction between CaO and Al2O3 over certain temperatures.18 3.1. Experimental Section. 3.1.1. Sorbent Preparation. Simplified solgel, physical mixing, and wet mixing were applied for the preparation of different sorbents. Detailed preparation procedures are as follows: 3.1.1.1. Simplified SolGel. This was a simplified solgel method to achieve 15% CaO loading in the final sorbents. It was called simplified solgel because only colloidal sol formed by dissolving 1.99 g of Ca(OH)2 (96%, Ajax Finechem) into distilled water. Then, 8.63 g of calcium aluminate cement (Kerneos Aluminum Technology) was added into the sol while maintaining stirring for 12 h. The final sorbents were obtained by drying the sol at 110 °C for 1 h and subsequently calcining in air at 900 °C for 4 h. The final sorbent made by solgel with cement as an inert support is denoted as CaCSG. 3.1.1.2. Physical Mixing. To load 15% CaO in the final sorbent, 1.99 g of Ca(OH)2 (96%, Ajax Finechem) and 8.63 g of calcium aluminate cement (Kerneos Aluminum Technology) were manually mixed in a mortar for 3 min. The mixture was dried at 110 °C for 1 h and calcined in air at 900 °C for 4 h. The final sorbent made by physical mixing with cement as an inert support is denoted as CaCPM. 3.1.1.3. Wet Mixing. Wet mixing was chosen to synthesize the inert support of Ca12Al14O33 and to load 5% CaO in the final sorbent.12 A total of 20.74 g of calcium D-gluconate monohydrate (Fluka) and 18 g of aluminum nitrate nonahyrate (98%, Sigma-Aldrich) were fully dissolved in the distilled water separately. The added amount of water was calculated according to the solubility of chemicals. Then, one of the transparent solutions was poured into another while maintaining stirring for 1 h. Finally, the mixed solution was dried overnight at 110 °C to form a dry solid powder that was subsequently calcined for 2 h in air at 900 °C. The final sorbent made by wet mixing with mayenite as an inert support is denoted as CaAlWM. 3.1.2. Sorbent Performance Tests. The reversibility test was conducted in a Cahn thermogravimetric analyzer (TGA) under the following conditions. Because the PSA is an approximate isothermal process, it is necessary to run the reversibility tests under isothermal conditions to simulate the realistic reaction condition. Initially, the furnace temperature was heated to 900 °C at a rate of 20 °C/min to achieve complete decomposition of CaCO3 to CaO. Then, the temperature was decreased to the isothermal temperature of 680 or 750 °C. Once the isothermal temperature was obtained, a reactant gas flow with 15% CO2 (N2 balance) was introduced into the furnace. After 30 min of carbonation, CO2 flow was disconnected and followed by 30 min of calcination in 85 mL/min N2 atmosphere. The calcium-based sorbents were subjected to multiple cycles. 3.1.3. X-ray Diffraction (XRD) Patterns. The diffraction patterns of sorbents were analyzed in a Bruker AXS D8 Advance X-ray diffractometer equipped with a scintillation counter, graphite monochromators, and copper target. The XRD analysis was conducted at room temperature (25 °C) with a 2 s step time, 10° start test angle, and 70° end angle. 3.1.4. Scanning Electron Microscope (SEM) Mapping. The morphologies of the sample were observed by a JEOL JSM-6610 SEM. Before the SEM analysis, samples were dispersed on a conductive adhesive tab placed on a SEM mount and coated with platinum using an EIKO IB-5 sputter coater for 5 min to form a sample layer with approximately 15 nm thickness. Then, SEM images were obtained from secondary electrons with 20 kV of accelerating voltage. 3.1.5. Steam Treatment. The steam was generated by heating an aqueous glass bottle on a hot plate at 110 °C. The glass bottle has two Teflon tubes: one tube connecting with the nitrogen cylinder was immersed under the water surface, while the other tube connecting
Table 1. Chemical Composition of Calcium Aluminate Cements chemical constituents
minor constituents
Al2O3
CaO
SiO2
Fe2O3
TiO2
MgO
SO3
K2O + Na2O
g37.0
e39.8
e6.0
e18.5
