Article pubs.acs.org/IECR
Analysis of Hydroxide Sorbents for CO2 Capture from Warm Syngas David J. Couling, Ujjal Das,† and William H. Green* Massachusetts Institute of Technology, Chemical Engineering, 77 Massachusetts Avenue, 66-352 Cambridge, Massachusetts 02139, United States S Supporting Information *
ABSTRACT: Integrated gasification combined cycle (IGCC) with CO2 capture and sequestration (CCS) is a promising technology to efficiently mitigate the emission of CO2. Warm CO2 removal has been predicted to make the CO2 capture process more efficient. Here, we investigate the efficiency penalties associated with CO2 removal via a pressure swing adsorption (PSA) process using metal hydroxide sorbents at elevated temperature. We use numerical models constructed in MATLAB and integrate these with Aspen Plus process simulations. We apply these models to both general metal hydroxides of variable enthalpy of adsorption and real metal hydroxides identified using density functional theory (DFT) calculations. We show that having an enthalpy of adsorption between 15 and 20 kJ/mol results in a PSA process that gives an overall IGCC−CCS efficiency that is competitive with the conventional IGCC−CCS process using (cold) Selexol. An enthalpy of adsorption of 20 kJ/mol is predicted to be the most favorable because it yielded a promising combination of HHV efficiency and higher working capacity. In addition, we identify Fe(OH)2, Co(OH)2, Ni(OH)2, and Zn(OH)2 as potentially favorable real materials, with IGCC−CCS efficiencies predicted to be within 1% HHV of that of Selexol.
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INTRODUCTION Due to its high abundance and relatively low cost, coal is predicted to continue to be a major part of the world’s energy portfolio, especially in the developing world.1,2 However, conventional coal combustion releases large amounts of the greenhouse gas CO2 into the atmosphere. Coal is also wellknown to contain a large number of pollutants whose removal is necessary to avoid environmental contamination. Integrated gasification combined cycle (IGCC) with CO2 capture and sequestration (CCS) presents a method to achieve tight emissions on criteria air pollutants such as NOx, SOx, and particulate matter, while at the same time mitigating greenhouse gas emissions. Currently, the technologies that are commercially available for IGCC with CCS involve cooling the gas stream to low temperatures and using physical or chemical solvents to remove sulfur and CO2. The U.S. Department of Energy (DOE) and National Energy Technology Laboratory (NETL) have shown that using these technologies can lead to a decrease in overall higher heating value (HHV) efficiency of the IGCC plant of 6− 9% (i.e., a 15−23% reduction in kWh per ton of coal).3 It would be very desirable to avoid this efficiency loss by identifying novel CO2 separation techniques with lower efficiency penalties. Previous researchers have predicted that performing the gas cleanup at elevated temperatures can lead to an efficiency gain of as much as 3% HHV (that is, a decrease of only 3−6% from the IGCC without CO2 capture case).4,5 Previously, we have investigated this phenomenon with CO2 capture via H2-permeable Pd-alloy membranes, CO2- and H2permeable polymer membranes, and metal oxide adsorbents for CO2 capture using IGCC process simulations within Aspen Plus.6 Our results showed that H2-permeable Pd-alloy membranes and metal oxide sorbents, given the right material properties, both allow for small gains in efficiency (up to 1% HHV) over a cold solvent process such as Selexol. Here, we © 2012 American Chemical Society
extend our previous analysis to include metal hydroxide sorbents for CO2 capture. We investigate favorable properties, such as enthalpy of adsorption, of a hypothetical metal hydroxide sorbent material. In addition, we perform process simulations of five “real” materials identified using density functional theory computations as potential candidates for CO2 capture.
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METAL HYDROXIDES AS CO2 SORBENTS There is currently a significant amount of effort to develop hydroxide-based solid sorbents, such as sodium hydroxide or magnesium hydroxide7,8 or layered double hydroxides9,10 (also known as hydrotalcite-like compounds). A simplified form of the CO2 adsorption reaction for a metal hydroxide sorbent is shown in eq 1. CO2 (g) + M(OH)2 (s) ⇔ H 2O(g) + MCO3(s)
(1)
We do not consider the formation of bicarbonate compounds in this study because, in general, the formation of carbonates is favored with respect to the enthalpy change of reaction. In addition, because gas-phase H2O is produced in the formation of a carbonate (as shown in eq 1), carbonates are favored with respect to the entropy change as well. With this small entropy of adsorption, the enthalpy of adsorption does not need to be as large in magnitude for the adsorption process to occur at elevated temperatures. As a result, it is possible that large temperature fluctuations within the adsorption bed can be avoided, which is a significant benefit, since temperature changes decrease both the working capacity of the sorbent and the overall efficiency of the IGCC process.6 Received: Revised: Accepted: Published: 13473
January 20, 2012 August 13, 2012 September 4, 2012 September 4, 2012 dx.doi.org/10.1021/ie300189a | Ind. Eng. Chem. Res. 2012, 51, 13473−13481
Industrial & Engineering Chemistry Research
Article
Figure 1. IGCC with CCS and warm syngas cleanup.
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METHODOLOGY A base case simulation of IGCC with CO2 capture at low temperature was developed in Aspen Plus by Field and Brasington.11 We modified this simulation to include warm CO2 capture, with the general flowsheet being shown in Figure 1. The sulfur is removed via the ZnO desulfurization process and recovered using the direct sulfur recovery process (DSRP), both of which were developed by RTI and Eastman Chemical.12,13 For full details of the IGCC with warm CCS flowsheet, please see Couling et al.6 Metal Hydroxide Adsorption Model. Assuming the adsorption behavior follows the reaction shown in eq 1, the total number of moles of gas adsorbed on the bed at any one time is constanteach adsorption site is filled with either a molecule of CO2 or a molecule of H2O. Mathematically, this means that the sum of the adsorption rates of all species, ∑i(dqi/dt), is equal to zero. This in turn yields a simplification to the total and component mass balances, shown below in eqs 2 and 3, respectively. As was the case in our earlier work, eq 3 resulted from a linear combination of the component and total mass balances, following the method of Simo et al.14 ε
ε
∂(uC) ∂C = −ε ∂z ∂t
∂yi ∂t
= εu
∂yi ∂z
−
The equilibrium mole fraction of CO2 is related to the CO2 loading on the surface (q) and the equilibrium mole fraction of H2O using the Langmuir isotherm and assuming the partial pressures are high enough that all sites are occupied by either CO2 or H2O, as shown in eq 5. qCO = 2
(3)
The rate of adsorption itself is shown in eq 4. A linear driving force (LDF) approximation is used, following the method of Seader and Henley,15 in which the driving force is treated as a difference between the bulk gas-phase concentration and the equilibrium concentration just above the surface of the sorbent (C*). dqCO
2
dt
ε ρs (1 − ε) εp * ) − yCO 2 RTρ (1 − ε) s
dqCO
2
dt
2
2
(5)
⎞ ⎛ qCO (yCO + yH O ) 2 2 2 ⎟ = kLDF⎜⎜yCO − sat ⎟ 2 q K ( q q ) + − eq CO2 ⎝ CO2 CO2 ⎠ εp × RTρs (1 − ε)
(6)
The rate of adsorption of H2O is then just the negative of the above expression. This LDF expression is different from that used in our previous work6 (since corrected16) and those commonly used elsewhere17 in that the LDF dependence is on the difference in the gas phase concentration, C − C*, rather
* ) = kLDF(CCO2 − CCO 2 = kLDF(yCO
2
Equation 5 assumes that no bicarbonates or metal oxides are formed so that essentially all surface sites are hydroxides (containing H2O) or carbonates (containing CO2). The equilibrium constant Keq is then the equilibrium constant between the CO2, H2O, hydroxide, and carbonate species. We relate the equilibrium partial pressures (pi*) to the bulk partial pressures by assuming the total pressure of the bulk CO2 and H2O is equal to the total pressure of the CO2 and H2O at equilibrium near the surface of the adsorbent. We note that this assumes that the partial pressures of the nonadsorbing species, such as H2, are equal in the bulk gas stream and near the adsorbent surface. We expect this assumption to hold because the surface is assumed to be completely occupied by either CO2 or H2O, meaning that the movement of a CO2 molecule adsorbing will be balanced by that of an H2O molecule desorbing, and vice versa. This allows the nonadsorbing species to have partial pressures that are relatively unchanged. Rearranging our expression in terms of the equilibrium partial pressure of CO2, we obtain the final LDF expression shown in eq 6.
(1 − ε)ρs RT dqi dt
2
* yH* O + KeqyCO 2
(2)
p
sat * qCO KeqyCO
(4) 13474
dx.doi.org/10.1021/ie300189a | Ind. Eng. Chem. Res. 2012, 51, 13473−13481
Industrial & Engineering Chemistry Research
Article
Table 1. 5-Stage PSA System DescriptionAdsorption on Metal Hydroxidea step
a
boundary conditions z=0
description
pressurization (
