Sorption-Enhanced Hydrogen Production: A Review - Industrial

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Ind. Eng. Chem. Res. 2008, 47, 6486–6501

Sorption-Enhanced Hydrogen Production: A Review Douglas P. Harrison† Cain Department of Chemical Engineering, Louisiana State UniVersity, Baton Rouge, Louisiana 70803

In the sorption-enhanced hydrogen production process, hydrocarbon reforming, water gas shift, and CO2 separation reactions occur simultaneously in a single reaction step over a reforming catalyst mixed with a CO2 sorbent. Transferring CO2 as it is formed from the gas to the solid phase shifts the normal equilibrium restrictions and allows both the reforming and water gas shift reactions to approach completion. Depending on reaction conditions, the product (dry basis) may contain as much as 98% H2 and only ppmv levels of CO and CO2, thereby minimizing the final H2 purification step or even eliminating it for some applications. A number of CO2 sorbents have been studied including calcium-based oxides, K-promoted hydrotalcite, and mixed metal oxides of lithium and sodium. The sorbent is consumed during H2 production so that the process is intrinsically unsteady state. Process economics requires that the sorbent be regenerable and used in many reaction-regeneration cycles. Regeneration may occur via temperature swing, pressure swing, or a combination. Much of the current research is devoted to testing and improving sorbent multicycle durability. Both circulating fluid-bed reactors and dual fixed-bed reactors with alternating reaction-regeneration functions have been proposed to provide overall steady state H2 production. Introduction

enhanced H2 production, the CO2 is removed using an appropriate solid sorbent that for the time being we represent by the symbol Σ. The CO2 capture reaction may be written as

Hydrogen has long been an important raw material for the manufacture of commodity chemicals such as ammonia and methanol. Increased quantities of H2 are now being used in petroleum refining as sour, heavy crude increasingly replaces sweet, light crude. Very large increases in H2 demand will result as the world shifts to a hydrogen-based energy economy. About 95% of the H2 currently produced in the US uses the steammethane reforming process.1 However, the expected increased demand coupled with anticipated limits on CO2 emissions provide an opportunity for new processes that may be more economical, energy efficient, and environmentally friendly. The sorption-enhanced hydrogen process may fill this opportunity. This Review does not attempt to cover all of the recent research on sorption-enhanced H2 production. Indeed, that would be impractical as the overall effort in this area has increased dramatically in recent years. The papers selected have proven to be of benefit to the author and provide a reasonably complete overview of research activities. Other studies may be easily found by examining the references listed in the papers that are cited here. The primary reactions involved in H2 production from natural gas are steam-methane reforming and water gas shift

As will be seen, the sorbent may truly react with CO2 to form a solid carbonate or may be physically or chemically adsorbed on the surface of the sorbent. Calcium-based sorbents and potassium promoted hydrotalcite (K-HTC) sorbents have received the bulk of research attention and these materials are discussed in detail below. Other sorbents based on mixed metal oxides of lithium and sodium have been studied less extensively and are covered briefly at the end of this review. Because the sorbent is effectively consumed in reaction 3, sorption-enhanced processes are inherently dynamic in operation. A sorbent regeneration step is necessary, and the sorbent must maintain activity through many cycles for the process to be economically viable.

CH4(g) + H2O(g) T 3H2(g) + CO(g)

(1)

Potential advantages of sorption-enhanced H2 production as listed by Mayorga et al.2 include the following:

CO(g) + H2O(g) T H2(g) + CO2(g)

(2)

Reforming is highly endothermic while the shift reaction is moderately exothermic. Both reactions are equilibrium limited, and it is impossible to achieve complete conversion of the CH4 and CO in a single reactor under normal reaction conditions. However, if the CO2 can be removed from the gas phase as it is formed, the normal equilibrium limits are displaced and complete conversion can be closely approached. In sorption†

E-mail: [email protected]. Tel: 225 578 3066. Fax: 225 578 1476.

CO2(g) + Σ (s) f Σ · CO2(s)

(3)

The overall reaction thus becomes CH4(g) + 2H2O(g) + Σ (s) f 4H2(g) + Σ · CO2(s)

(4)

• replacement of the high-temperature, high-alloy steels required in the reforming reactor with less expensive materials of construction; • simplification (or in some cases elimination) of the hydrogen purification section due to higher H2 purity and lower concentrations of CO and CO2; • elimination of the shift reactor(s); • reduction or possible elimination of carbon deposition in the reforming reactor;

10.1021/ie800298z CCC: $40.75  2008 American Chemical Society Published on Web 07/31/2008

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Figure 1. Schematic of the steam-methane reforming process showing two product purification options.

• decreased size of heat exchange equipment associated with reduction in sensible heat loss. The Standard Steam-Methane Reforming Process Hydrogen production using the standard steam-methane process represents mature technology and descriptions may be found in a number of references.1–5 For this review, it is instructive to discuss briefly the standard process and identify some of the limitations that sorption-enhanced hydrogen may eliminate or at least mitigate. A schematic flow diagram of the standard process showing two product purification options is shown in Figure 1. Primary process streams leading to the purification step are shown by solid lines. Streams leading to and from the two purification options are indicated by dashed lines, and primary supplemental energy streams are designated by dotted lines. Sulfur-free natural gas, mixed with steam at a steam-to-carbon molar ratio (S/C) of 2.5 to 5, is fed to the reformer where both H2 and CO are formed according to reaction 1. Reforming is highly endothermic and substantial amounts of supplemental energy are required to overcome the heat of reaction and achieve the operating temperature of 800-900 °C. High-pressure operation, in the range of 15-20 bar, has been found to be economic in spite of the adverse effect of pressure on CH4 conversion. The reaction occurs on a nickel-based catalyst inside tubes within a furnace where the supplemental energy is supplied by the combustion of off-gas from product purification and/or supplemental natural gas. The water gas shift reaction also occurs to some extent so that the reformer product contains substantial amounts of H2, CO2, CO, H2O, and unreacted CH4. Supplemental steam may then be added (if desired) and additional H2 produced via the water gas shift reaction. Two reactors, a high-temperature shift operating at 350-400 °C followed by a low-temperature shift at about 200 °C, have traditionally been used to take advantage of the improved equilibrium conversion of CO associated with this exothermic reaction at low temperature. A chromium iron oxide catalyst is typically used in the high-temperature reactor and a copper-zinc catalyst is used at low temperature. The typical gas composition (dry basis) leaving the second shift reactor is about 76% H2, 17% CO2, 4% unreacted CH4, and 3% CO2. Optional purification methods exist depending on product specifications. In one option, CO2 is removed by wet scrubbing using a solution such as monoethanolamine (MEA). The MEA-CO2 complex formed in the scrubber is decomposed by steam stripping to produce CO2-rich off-gas and regenerated MEA for recycle to the scrubber. Very stringent specifications

on CO concentration exist if the product H2 is to be used in downstream processes involving a catalyst that is sensitive to CO poisoning. The CO may be reduced to acceptable levels by methanation (the reverse of the reforming reaction) or by preferential oxidation (PROX). PROX is a catalytic process in which a small amount of O2 is added to selectively oxidize CO in preference to H2. The final product from this purification option typically consists of 95+% H2 with only trace concentrations of CO. Pressure swing adsorption (PSA) has become an important option for producing extremely high purity hydrogen. An excellent description of the PSA process for H2 purification is provided by Sto¨cker et al.6 Multiple packed beds containing molecular sieves, silica gel, or activated carbon are used to provide continuous operation with constant product and offgas compositions. Impurities are removed by the adsorbent while hydrogen passes through the bed essentially unchanged. Adsorption occurs at high pressure and near ambient temperature while regeneration also occurs near ambient temperature and at significantly lower pressure. H2 concentrations of greater than 99.9% may be produced and modern units can provide up to 90% H2 recovery. The off-gas, containing the feed stream impurities and unrecovered H2, is fed to the reformer furnace to provide a portion of the energy required by that unit. It is common practice, when using PSA purification, to omit the lowtemperature shift reactor shown in Figure 1. The high temperature and highly endothermic nature of the reforming reaction result in high supplemental fuel requirements. The exothermic shift reaction does not provide efficient energy integration since it occurs at considerably lower temperature. In addition, the overall process is reasonably complex and consists of a number of units in series. When two stages of shift reaction are used, three different sulfur-sensitive catalysts are required. Finally, the purification steps are not without problems. H2 losses are invariably associated with both methanation and PROX. The cost of MEA scrubbing is significant, primarily because of the stripper steam requirements. PSA units are relatively complex and also result in loss of 10% or more of the H2 fed. Calcium-Based Sorbents The concept of sorption-enhanced H2 production in the presence of a Ca-based sorbent is not new. Rostrop-Nielsen7 reports that the first description of the conversion of hydrocarbons in the presence of steam and calcium-based sorbent was published in 1868. Williams8 was issued a patent in 1933 for a process in which steam and CH4 react in the presence of lime

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Figure 2. Equilibrium H2 content as a function of temperature with and without CaO sorbent. Reprinted with permission from ref 12. Copyright 1999, Elsevier.

and catalyst to produce H2. Gorin and Retallick9 patented a fluidized-bed process using reforming catalyst and calciumbased sorbent. Brun-Tsekhovoi et al.10 showed that the combined reaction equilibrium could be closely approached at 625 °C, 2 MPa, and space velocities as large as 12 000 h-1. The carbon capture reaction 3 for the specific case of CaO sorbent is CO2(g) + CaO(s) f CaCO3(s)

(5)

This reaction is exothermic and the combined reactions 1, 2, and 5 are almost thermally neutral so that little or no supplemental energy is required by the reformer. Supplemental energy is required to regenerate the CaCO3 and permit the sorbent to function through many carbonation-regeneration cycles. Brun-Tsekhovoi et al.10 and Lopez and Harrison11 have, however, reported overall potential energy savings of 20% to 25% compared to the standard steam-methane process. Thermodynamic Analysis. The potential advantage of adding CaO may be understood from a thermodynamic analysis of the combined reactions 1, 2, and 5. Figure 2 from Balasubramanian et al.12 compares the equilibrium mole percent H2 (dry basis) in the products from both the standard SMR process and sorption-enhanced SMR using CaO as a function of temperature at a pressure of 15 atm and (S/C) ) 4. In the standard endothermic SMR process the H2 content increases with temperature and reaches a maximum of 76% (dry basis) at 900 °C. Two branches of the equilibrium line are shown when CaO is present. The lower branch allows for the formation of both CaCO3 and calcium hydroxide, Ca(OH)2, while, in the upper branch only CaCO3 was permitted in the equilibrium calculations. Ca(OH)2 begins to decompose at about 600 °C and the two branches merge at 630 °C. The H2 content reaches a maximum of about 96% at 650 °C and is equal to or greater than 95% at all temperatures below 750 °C (based on the Ca(OH)2-free branch). The almost total lack of temperature dependence is a result of the energy required by the endothermic reforming reaction being almost exactly balanced by the exothermic shift and carbonation reactions. Above 850 °C, CaCO3 can no longer be formed and the equilibrium product composition is the same with or without CaO. Increased equilibrium H2 purity is possible if the reactions are carried out at lower pressure because of the increase in gas moles associated with the reforming reaction. However, the maximum temper-

Figure 3. Equilibrium CO2 pressure as a function of temperature. Reprinted from ref 11. Copyright 2001, American Chemical Society.

atures for the formation of both Ca(OH)2 and CaCO3 also decrease at lower reactor pressure. At the lower temperatures of Figure 2, essentially all of the CO and CO2 can be removed and the primary impurity at equilibrium is unreacted CH4. At the higher temperatures, more CH4 reacts and the primary impurities become carbon oxides. From Figure 2 we see that with CaO at 650 °C, 15 atm, and (S/C) ) 4, 96% H2 (dry basis) can be produced at equilibrium. Without CaO, the H2 content is limited to 64%. These equilibrium concentrations correspond to a potential yield (calculated by material balance) of 3.46 moles H2 per mole CH4 fed with CaO and to only 1.86 moles H2 per mole CH4 without CaO. The maximum yield of 4.0 moles H2 per mole would correspond to complete conversion of CH4. In a commercial process the sorbent must be used in many carbonation-regeneration cycles. Figure 3 shows the conditions needed for CaCO3 regeneration in a plot of equilibrium CO2 pressure versus temperature (Lopez and Harrison11). Temperatures and pressures above and to the left of the equilibrium line favor CaCO3 formation while carbonate decomposition is favored at conditions below and to the right of the line. The possible production of high-purity CO2 suitable for use or sequestration is an important consideration in the regeneration step. If high-purity CO2 is to be produced the regeneration atmosphere should consist of pure CO2, pure steam, or perhaps a CO2-steam mixture where the steam could be separated by condensation. At 1 bar of CO2 pressure, a temperature of at least 900 °C is required for CaCO3 decomposition while a temperature in excess of 1100 °C is required at a CO2 pressure of 15 bar. If CO2 capture is not a consideration, regeneration may be carried out in a low-CO2 atmosphere such as flue gas and the required regeneration temperature would be considerably lower. For example, at 0.1 bar of CO2 the CaCO3 decomposition temperature is reduced to about 760 °C. The high regeneration temperature is one of the major problems associated with the calcium-based sorption-enhanced process as the sorbent tends to sinter and lose activity in multicycle operation. This problem is discussed in greater detail later in this review. Experimental Studies on Hydrogen Production. The ability of the combined reactions to closely approach equilibrium product composition has been proven in a number of experimental studies over a wide range of reaction conditions. Many of the experimental studies have used small fixed-bed reactors, with the bed containing varying proportions of reforming


Figure 6. Experimental capacities of dolomite and limestone sorbent precursors in five cycle tests. Reprinted with permission from ref 14. Copyright 1996, Taylor and Francis.

Figure 4. Typical response from a fixed-bed reactor test using reagent grade CaCO3 sorbent precursor. Reprinted with permission from ref 12. Copyright 1999, Elsevier.

Figure 5. Hydrogen concentration as a function of temperature using reagent grade CaCO3 sorbent precursor. Reprinted with permission from ref 12. Copyright 1999, Elsevier.

catalyst and Ca-based sorbent. Typical results from a fixed-bed reactor test using reagent grade CaCO3 sorbent precursor at 650 °C, 15 bar, and (S/C) ) 4 are shown in Figure 4 (Balasubramanian et al.12). Concentrations of H2, CH4, CO, and CO2 (dry basis) in the product gas are shown as a function of time. Following an unsteady state start-up period, the concentrations of all species are effectively constant during a prebreakthrough period and closely approach the equilibrium composition (indicated by the solid horizontal lines) at these reaction conditions. During this period the product gas contained 94.7% H2, 5.2% CH4, and approximately 400 and 600 ppmv of CO2 and CO, respectively. The beginning of breakthrough corresponds to the leading edge of the carbonation reaction front reaching the exit of the packed bed. The CO2 removal efficiency decreases, which, in turn, reduces the extents of the reforming and shift reactions. The H2 concentration decreases, whereas the CH4, CO, and CO2 concentrations increase. The second steady-state or postbreakthrough period begins when the sorbent is exhausted and the carbonation reaction rate approaches zero. From this point the product gas composition approaches the equilibrium associated with the standard CH4 reforming reaction. Product gas equilibrium composition, again indicated by the solid lines, can be closely approached at 15 bar over a wide temperature range as shown in Figure 5 (Balasubramanian et al.12), where H2 concentration (dry basis) during both the prebreakthrough and postbreakthrough periods is shown between 450 and 750 °C. The prebreakthrough

concentrations were significantly below equilibrium only at temperatures below 550 °C while the postbreakthrough concentrations were near equilibrium over the entire temperature range. It should be pointed out that the feed gas in this series of tests contained 70% N2 diluent so that the equilibrium prebreakthrough H2 concentration was only about 24%. The diluent was added for experimental simplicity and does not alter the important conclusion that equilibrium composition was closely approached. The decrease in experimental H2 concentration relative to the equilibrium value at the lower temperatures was attributed to one or a combination of two factorssdecreased reaction rates and/or formation of Ca(OH)2. The excellent reproducibility possible in such tests is shown by the results of duplicate runs at 650 °C. The thermodynamic characteristics of reactions 1 and 2 suggest that somewhat larger H2 and significantly lower CO concentrations can be achieved at lower pressure, although lower reaction temperature is also required. For example, Yi and Harrison13 reported results of a series of tests between 400 and 460 °C at 1 bar using (S/C) ) 3. Equilibrium concentrations at these conditions varied only between 97.7% and 97.8% H2, and from 1 ppmv to 17 ppmv CO. At 460 °C the experimental concentrations were 96.4% H2 and 49 ppmv CO while at 400 °C the experimental concentrations were 87.3% H2 and 12 ppmv CO. Sorbent Durability. Interest in the use of Ca-based sorbents for CO2 capture from flue gas has produced a number of multicycle studies involving only sorbent carbonation–regeneration. The experimental approach is considerably simplified and the durability problem is equally applicable to H2 production as well as flue gas CO2 capture applications since the loss of activity is generally attributed to structural property changes associated with sintering during the high-temperature regeneration step. The experimental results described in Figures 4 and 5 used reagent grade CaCO3 as the sorbent precursor in order to eliminate possible adverse effects associated with impurities. However, there are strong economic incentives in a commercial process to use a naturally occurring, inexpensive precursor such as limestone or dolomite. Limestone has the advantage of wider availability and a higher theoretical capacity of 0.79 g CO2/g CaO compared to the theoretical capacity of calcined dolomite (CaO · MgO) of 0.46 g CO2/g CaO · MgO. Dolomite, however, has been found to give improved performance in multicycle tests. This is illustrated in Figure 6 (Silaban et al.14) where the achievable capacity of CaO and CaO · MgO from limestone and dolomite precursors is plotted versus cycle number. These tests used a TGA reactor, and the reaction time was 20 min for both carbonation and calcination. Calcination was carried out at 750 °C and 1 atm in 100% N2 while carbonation occurred at 750


Figure 7. Influence of inert MgO on normalized multicycle CO2 sorption capacity. Reprinted with permission from ref 15.

°C and 1 atm in 15% CO2/N2, conditions where MgCO3 cannot be formed. Thus, after the initial decomposition of CaCO3 · MgCO3, the dolomite precursor cycled between the fully calcined CaO · MgO and half-calcined CaCO3 · MgO forms. In the first three cycles the experimental capacity of CaO was somewhat larger, but the positions were reversed by the fourth cycle. The first cycle capacities of 0.61 and 0.42 g CO2/g sorbent for CaO and CaO · MgO, respectively, correspond to fractional Ca conversions of 0.77 for CaO and 0.91 for CaO · MgO. By the fifth cycle the experimental capacity of CaO had decreased to 0.35 while the corresponding value for CaO · MgO was still 0.40. The larger fractional calcium conversion associated with the dolomite precursor was attributed to the “excess” pore volume created by the initial MgCO3 decomposition and the improved multicycle performance to structural property stabilization due to the presence of MgO. In the absence of structural property changes associated with sintering, the pore volume created during CaCO3 decomposition should just be sufficient to permit complete recarbonation. However, carbonation tends to occur near the exterior of the individual particles so that surface porosity approaches zero leaving inaccessible CaO near the particle interior. Bandi et al.15 compared the normalized CO2 capacities of calcite, dolomite, and huntite precursors through 47 carbonation-calcination cycles with results shown in Figure 7. Huntite is a naturally occurring mineral represented by the formula CaCO3 · 3MgCO3. Even greater levels of excess pore volume are created by the calcination of huntite since the molar ratio of Mg-to-Ca is 3 times that of dolomite. After 47 cycles huntite retained about 84% of its initial capacity compared to 55% for dolomite and 38% for calcite. While the limited availability of huntite and its reduced theoretical CO2 capacity (0.25 g CO2/g CaO · 3MgO) probably preclude its use in a commercial process, these results do support the explanations regarding the importance of pore volume and the stabilization effects of Mg. Abanades16 compared multicycle results from a number of studies and found that, up to 20 cycles, the rate of activity decrease was largely independent of regeneration conditions. The CO2 capacity decreased by about a factor of 4 by the end of 20 cycles. Grasa and Abanades17 have recently reported results from 500 carbonation/calcination cycles using limestone as the sorbent precursor. The CO2 capacity decreased rapidly during the first 10 cycles and relatively slowly thereafter. The residual capacity after 500 cycles was about 10% of the initial capacity. Even more recently, Sun et al.18 extended the number of cycles to greater than 1000. Limestone was used as the sorbent precursor and the study was conducted in a TGA reactor operated isothermally at 850 °C. Carbonation occurred in 100%

Figure 8. Comparison of CO2 capture in the 20th cycle with and without steam reactivation. Reprinted from ref 20. Copyright 2007, American Chemical Society.

CO2 and calcination in 100% N2. A nonzero asymptote was reached after about 150 cycles with the sorbent retaining as much as 17% of original capacity under the most favorable conditions. Results were interpreted using a mechanistic model based on changing pore size distribution. Hughes et al.19 showed that the formation and subsequent decomposition of Ca(OH)2 during regeneration was effective in restoring at least a portion of the sorbent reactivity. Manovic and Anthony20 found in studies using limestone sorbent precursor that the extent of carbonation following steam reactivation was significantly improved as shown in Figure 8. After 20 calcination-carbonation cycles without steam activation the degree of carbonation slowly increased to about 35%, and there was only a moderate effect of sorbent particle size. After 20 calcination-carbonation cycles without reactivation, the sorbent that was reactivated in steam and the degree of carbonation quickly increased to about 60% followed by slower carbonation to about 75%. Calcination conditions were 850 °C in N2 for 30 min with carbonation at 650 °C in 20% CO2/N2. Reactivation was carried out in a Parr bomb at 200 °C under saturated steam for 30 min. The authors postulate that Ca(OH)2 decomposition is effective in maintaining the inventory of small pores (e200 µm) needed for CO2 capture. Fennell et al.21 have reported that exposing the sorbent to moist air was an effective method of reactivation. While, to the authors knowledge, no similar studies have been performed using dolomite, there is no reason to believe that similar improvements would not occur in that system as well. Other researchers have prepared and tested synthetic Ca-based sorbents in an effort to improve sorbent durability. Li et al.22 supported CaO (75 wt %) on a mixed metal oxide of composition Ca12Al14O33 (25 wt%). While the sorbent experienced some deactivation in a multicycle TGA test, the durability over 50 cycles was reported to be superior to that of natural dolomite at the same test conditions. This synthetic sorbent mixed with a standard Ni-based reforming catalyst was used in a continuous 400 min sorption-enhanced H2 production test involving two parallel fixed-bed reactors operated in a cyclic manner (Li et al.23). As shown in Figure 9, the product gas contained about 95% H2 and 2-4% CO2 throughout the test. Reaction conditions were 630 °C, 1 atm, and (S/C) ) 5 for H2 production and 850 °C, 1 atm, in pure argon for regeneration. Switching times are indicated by the dashed vertical lines. Satrio et al.24 described the preparation and testing of a novel core-in-shell combined catalyst and sorbent. A reactive sorbent, calcined limestone or dolomite, formed the core while the shell was formed from strong, porous alumina loaded with nickel reforming catalyst. While this structure is said to be very durable, only limited multicycle test results have been reported to date.


Figure 9. H2 and CO2 product concentrations during a continuous 400 min test using two parallel fixed-bed reactors operated in a cyclic manner. Reprinted from ref 23. Copyright 2006, American Chemical Society.

Researchers at ChevronTexaco Technology Ventures (Stevens et al.25) carried out extensive tests using a number of synthetic Ca-based sorbents prepared by Cabot Superior MicroPowders using spray-based technology and containing various concentrations of Al2O3 and/or MgO. As many as 500 carbonation/ calcination cycles were carried out in TGA tests. The 500-cycle CO2 capacity data were curve fit and then extrapolated to predict CO2 capacity after one year of operation (>4000 cycles). As an example of their results, one sorbent containing 90% CaO/ 10% MgO whose experimental first cycle capacity was about 0.42 g CO2/g sorbent had a one year predicted capacity of 0.19 g CO2/g sorbent. In spite of reasonable success in extending the lifetime of calcium sorbents, it is almost certain that continuous or periodic addition of fresh sorbent and purge of spent sorbent will be necessary to maintain desired product composition. While the number of cycles that the sorbent must undergo to result in an economically feasible process is not known, Abanades et al.26 concluded that, because of the low cost and wide availability of limestone, the cost of spent sorbent replacement should not be a limiting factor in the use of limestone in flue gas CO2 capture applications. It is reasonable, therefore, to reach the same conclusion with respect to sorption-enhanced H2 production, assuming that inexpensive natural limestone or dolomite is used as the sorbent precursor. The sensitivity of the reforming catalyst to sulfur poisoning must also be considered. It is assumed that the natural gas feed will have undergone desulfurization, for example, using a zinc oxide guard bed prior to being fed to the system. However, most naturally occurring limestones and dolomites contain small amounts of sulfur, presumably in the form of CaSO4 and MgSO4. In the reducing atmosphere of the reforming reactor sufficient sulfur can be transferred from the sorbent to the gas phase to quickly poison the nickel reforming catalyst. Lopez and Harrison11 showed that sorbent pretreatment, first in a H2-N2 mixture and then in H2O(steam)-N2 at high temperature, was sufficient to remove the sulfur and permit the combined reforming, shift, and carbonation reactions to proceed normally. Figure 10 compares the fixed-bed reactor breakthrough curves using Rockwell dolomite (≈0.01 wt % S), with and without, sorbent pretreatment. Reaction conditions are shown on the figure. The feed for this test contained 70% N2 so that equilibrium H2 concentration was only 24.5% (dry basis). Without pretreatment, the H2 concentration peaked at about 23%, significantly below the equilibrium level, and decreased thereafter. The final value was considerably below the 17% equilibrium H2 concentration in the absence of sorbent, thus proving that the activity of the catalyst had been affected. Following pretreatment, both the prebreakthrough and postbreakthrough concentrations were effectively equal to the respective equilibrium levels. Several naturally occurring limestones and dolomites have been tested at Louisiana State University (LSU),

Figure 10. Comparison of hydrogen production with and without dolomite pretreatment for sulfur removal. Reprinted from ref 11. Copyright 2001, American Chemical Society.

Figure 11. Schematic of a possible sorption-enhanced hydrogen production process using dual circulating fluidized bed reactors with sorbent hydration for activity maintenance.

and only the Arctic dolomite used by Yi and Harrison13 and also by Johnsen et al.27 in fluid-bed tests had sufficiently low sulfur concentration that pretreatment was not required. Process Considerations. Circulating fluidized bed (or transport) reactors provide an ideal system for transporting regenerated and spent sorbent between the H2 production and sorbent regeneration reactors, to add fresh and remove spent sorbent, and also to divert a portion of the sorbent to a hydration reactor for reactivation. A commercial process might, therefore, look something like that shown in Figure 11. Multiple fixed-bed reactors with periodic switching between H2 production and sorbent regeneration functions have also been suggested. It is likely, however, that a number of problems including operating large valves at high temperature with periodic switching and sorbent/catalyst separation would limit fixed-bed systems to small-scale operations. Johnsen et al.27 have proven that gas-solid contact in a laboratory-scale bubbling fluidized bed reactor is sufficient to achieve close approximation to equilibrium conditions. Product hydrogen concentrations as large as 98% (dry basis) were obtained during the steady-state prebreakthrough period (see Figure 12). Reaction conditions were ambient pressure, 600 °C, and S/C ) 3. The fluid bed contained commercial Haldor-Topsoe Ni-based reforming catalyst mixed with Franzefoss A/S, Arctic dolomite SHB in a mass ratio of 2.5 g of catalyst/g of calcined dolomite. The single reactor was operated in a batchwise manner and calcination was carried out at ambient pressure and 850 °C using an N2 purge. Both the catalyst and sorbent were exposed to the regeneration conditions, and H2 was added after each


Figure 12. Outlet composition (dry basis) as a function of time in multicycle fluid-bed reactor tests. Reprinted with permission from ref 27. Copyright 2006, Elsevier.

regeneration to ensure that the catalyst was reduced before CH4 and H2O were added. The maximum H2 concentration was effectively equal in each of the four cycles, although the duration of the prebreakthrough period was reduced somewhat as the number of cycles increased. Johnsen et al.28 also published a mathematical model describing the reforming and regeneration steps in coupled fluidized bubbling bed reactors. Similar experimental studies using a fluidized-bed reactor have been reported by Hildenbrand et al.29 who examined the effect of possible Ca(OH)2 formation on the product gas composition. The required regeneration temperature will depend on the CO2 pressure. If carbon capture is desired, it will be necessary that the regenerator off-gas be essentially pure CO2 or perhaps a steam-CO2 mixture where the steam can be condensed to produce pure CO2 product. Regeneration energy may be supplied either by direct contact with the spent sorbent or indirectly through heat transfer surface. The actual operating mode depends on the intended use of the H2 and on the importance of carbon capture to the overall process. Two of the proposed processes discussed below utilize the H2 product to generate electricity using solid oxide fuel cells (SOFC). In those two cases the exhaust gas from the SOFC will be used for regeneration. Supplemental fuel must be used if SOFC exhaust energy is not available or is not sufficient. If a steam-CO2 regenerator carrier gas is used, regeneration may be accomplished without additional carbon emissions by using direct contact combustion of natural gas in O2. If the regeneration carrier gas is pure CO2, in order to avoid carbon emissions it will be necessary to supply the energy by burning a slip stream of product H2 in O2, using either direct or indirect contact. Two recently published process simulations indicate that sorption-enhanced H2 production compares favorably to standard technology in terms of process efficiency. Ochoa-Fernandez et al.30 performed Hysis simulations that compared both thermal and net efficiencies of standard steam methane reforming and sorption-enhanced steam methane reforming using both CaO and Li2ZrO3 sorbents (the properties of Li2ZrO3 sorbent are discussed near the end of this review). Thermal efficiency was defined as the ratio of the LHV of the product to that of the feed. Net efficiency was reduced by considering the energy penalty associated with, for example, addition of the MEA unit for CO2 capture in the standard reforming process. All simulations were based on the achievement of equilibrium conditions. Different reaction temperatures, pressures, and S/C ratios were chosen to maximize CH4 conversion for both the CaO and Li2ZrO3 sorption-enhanced processes. Fixed-bed reactors were used so that the H2 production and regeneration reactors could

operate at different pressures. Hydrogen product was compressed to 25 bar before final purification in a PSA unit. 99.9% H2 was produced in each case with 90% H2 recovery in the PSA units. Off-gas from the PSA was burned in pure O2 from an air separation unit for sorbent regeneration in the sorption-enhanced cases and to supply a portion of the reformer energy in the standard process. The air separation unit was not modeled but an energy charge of 1.0 MJ/kg O2 was included in the calculations. An MEA unit was included to capture CO2 in the standard process. The calculated thermal efficiency for the standard process of 86% in the absence of CO2 capture compared favorably to 88% thermal efficiency reported in the literature. Addition of CO2 capture reduced the calculated net efficiency to 71% for the standard process. The calculated net efficiency of the CaO-based sorption-enhanced process with CO2 capture was 79%, 8 points higher than the standard process, while that of the sorption-enhanced process using Li2ZrO3 sorbent was 69%, 2 points lower than the standard process. Reijers et al.31 performed an Aspen Plus simulation to compare the efficiencies of CaO-based sorption-enhanced H2 production coupled with electricity generation in a combined gas-steam cycle to that of a standard natural gas combined cycle based on Siemens V94.3A technology with and without carbon capture. The CaO-based H2-production cycle operated with S/C ) 3 at 600 °C and 17 bar. Regeneration was accomplished at 1000 °C and 17 bar using steam purge at a steam-to-CO2 ratio of 1.8 with regeneration energy supplied by burning a portion of the H2 product supplemented with natural gas. Steam was separated by condensation and CO2 was compressed to 110 bar suitable for transport and sequestration. The CaO process resulted in 93% CH4 conversion, 85% carbon capture, and operated with an overall efficiency of 52.6%. This compared to the standard natural gas combined cycle efficiency of 57.1% without carbon capture, and 48% after addition of postcombustion CO2 capture using MEA scrubbing that resulted in 86.2% CO2 capture. Several organizations are in various stages of process development in which Ca-based sorption-enhanced H2 production plays a role. Two processes use natural gas as the hydrocarbon feed and are conceptually similar to the laboratoryscale research described above. Two additional processes use coal as the raw material and, although sorption-enhanced hydrogen production is involved, the overall processes are significantly different. One process that utilizes natural gas is designated Zero Emission Gas Power Project (ZEG), and is being led by the Institute of Gas Technology in cooperation with Christian Michelsen Research AS and Prototech AS in Norway. A brief discussion of the process may be found on the Internet,32 and an update on the status of the project was recently presented by Johnsen.33 H2 is produced by the sorption-enhanced reforming of natural gas. A number of candidate sorbents have been considered with Arctic dolomite, which does not require pretreatment for sulfur removal,13,27 receiving the most attention. H2 is to be used to produce electricity in a high-temperature solid oxide fuel cell with the exhaust heat used for sorbent regeneration. Electrical efficiencies of from 50% to 80% based on the net power output (LHV) of four process configurations having varying degrees of heat integration are reported. Pratt and Whitney Rocketdyne (PWR) is now in the pilot stages of a sorption-enhanced H2 production process development based on natural gas. While few details have been released, the company claims a 90% size reduction, 30-40% reduction in capital costs, 5-20% higher hydrogen yield, and reduced


product purification requirements that will lead to a smaller PSA system. The comparisons are relative to a standard steam methane reforming process with PSA purification. Upon completion of the current pilot tests, PWR plans to construct a 5 MMscf/d commercial demonstration plant (Stewart34). The Zero Emission Carbon (ZEC) process developed at Los Alamos National Laboratory is designed to produce carbonfree energy in the form of electricity and hydrogen from coal (Ziock35). Coal is first hydrogasified to produce CH4, which is then reformed to H2 using the calcium-based sorption-enhanced process. Roughly one-half of the H2 produced is used in the hydrogasification step with the remainder used to generate electricity using a solid oxide fuel cell. Like the Norwegian ZEG process, the high-temperature off-gas from the fuel cell is used to regenerate the spent sorbent. Recovered CO2 may be used for enhanced oil recovery, enhanced coal bed methane production, or sequestered in saline aquifers or as a mineral carbonate. A systems analysis performed by Nexant Corp. (Nawaz and Ruby36) estimated coal to electricity conversion efficiency on the order of 70%. Research on this concept is continuing in a joint study at Cambridge University and Imperial College in the UK (Paterson37). The HyPr-RING (hydrogen production by reaction-integrated novel gasification) process is under development in Japan (Lin et al.38). Coal, steam, and Ca-based sorbent are fed to the reactor (gasifier). The steam and coal react to form H2, CO, and CO2 with the CO2 simultaneously removed from the gas by reaction with the sorbent which permits additional H2 to be formed via the water gas shift reaction. The reaction products, after being cooled in a heat exchanger, are separated into gas (H2 product), liquid (condensed unreacted steam plus soluble salts), and solid (CaCO3, unreacted carbon and coal ash) streams. The condensate is recycled to the reactor after removal of the salts. The solids are sent to the regenerator where O2 is added, and the exothermic oxidation of the unreacted carbon supplies the energy needed for sorbent regeneration. CO2 exits from the top of the regenerator with coal ash and regenerated sorbent exiting from the bottom. Most of the regenerated sorbent is in the hydrated form, Ca(OH)2, for improved activity maintenance. Coal ash and a portion of the spent sorbent are purged from the system. Conditions in the reactor of 873-973 K and 3 MPa are reported to result in slightly over 50% carbon conversion with about 90% H2 in the product gas. The remainder of the product gas is predominantly CH4 with less than 0.4% (CO + CO2). The regenerator operates at 1073 K and 0.1 MPa. Hydrotalcite (HTC) Sorbent Hydrotalcite is a member of the family of double-layered hydroxides having the general formula [MII1-xMIIIx(OH)2][An-]x/n · zH2O, where MII and MIII are divalent and trivalent cations, respectively, and An- is an interlayer anion. The structure consists of Mg(OH)2-like layers separated by the interlayer anions. In the natural mineral hydrotalcite, MII ) Mg, MIII ) Al, and An-) CO3 with the formula [Mg6Al2(OH)16][CO3] · 4H2O. The HTC structure is shown in Figure 13.39 Upon heating, the HTC begins to lose water of hydration near 250 °C, with thermal decomposition beginning near the same temperature. The product is a high surface area material that has found use in catalysis, and, more recently, as a hightemperature sorbent for CO2. Extensive deposits of HTC exist throughout the world, often as a secondary mineral with serpentine. HTC may also be synthesized and synthetic materials have been used in several of the studies discussed below. CO2 Adsorption Equilibrium and Kinetics. Mayorga et al.,2 among the first to explore the use of HTC for CO2 capture,

Figure 13. Hydrotalcite structure. Reprinted from ref 39. Copyright 2006, American Chemical Society.

Figure 14. CO2 Chemisorption isotherms for K-HTC (ref 2).

used synthetic HTC materials having Mg/Al ratios somewhat different from the natural mineral - an Alcoa product having a Mg/Al ratio of 3.4-3.6 and a LaRoche material with Mg/Al ratio of 3.7. The HTCs were promoted with K2CO3 solution (K-HTC) and activated at 500 °C at which temperature the HTC decomposed into high surface area, poorly crystalline MgO and Al2O3. These authors described CO2 capture in terms of a combination of reversible and irreversible chemisorption, while other authors (described subsequently) have referred to a combination of reversible physisorption and irreversible chemisorption. Adsorption isotherms of the Langmuir type over the temperature range of 300-500 °C in a high steam atmosphere are shown in Figure 14. The capacity decreased with temperature, increased with CO2 pressure, and reached a maximum of about 1 mmol/g sorbent at 1.5 atm CO2 and 300 °C. Importantly, the adsorption capacity was not adversely affected by the presence of steam. Multicycle tests showed a capacity decrease through the first 10 cycles to a constant value thereafter. For example, as shown in Figure 14, the initial capacity of 0.8 mmol/g at 400 °C and PCO2 ) 1.5 atm decreased to about 0.45 mmol/g after 10 cycles. The loss in capacity was ascribed to irreversible chemisorption. Ding and Alpay40 reported qualitatively similar adsorption capacities using a commercial (unspecified source) K-HTC. The isotherms were of the Langmuir type and initial CO2 capacities were 0.65 and 0.58 mol/kg sorbent at PCO2 ≈ 0.6 bar and 673 and 733 K, respectively. The capacities decreased by ≈30-40% in early cycles, which was attributed to irreversible chemisorp-


tion, before reaching a steady value. Operation in a steam atmosphere enhanced the process and resulted in about a 10% increase in capacity and improved multicycle performance. There was substantial retention of H2O on the sorbent during regeneration, consistent with partial rehydration of the HTC. Adsorption kinetics was quite rapid and could be described by a linear driving force model with mass transfer control when the nonlinearity of the adsorption isotherm was accounted for. Desorption tests found that the amount of CO2 removed by depressurization was very small and that the presence of H2O in the purge gas (H2O/N2) considerably enhanced CO2 desorption. Ebner et al.41 studied the dynamics of CO2 adsorption on K-HTC and reported capacities at 980 torr of CO2 of 2.25 and 1.02 mol/kg at 250 and 500 °C, respectively, with steady-state working capacities, which were achieved after 5-12 cycles, varying from 0.11 mol/kg at 250 °C to 0.46 mol/kg at 500 °C, with a maximum of working capacity of 0.55 mol/kg at 450 °C. The rate of approach to equilibrium was a strong function of temperature and was extremely slow at 250 °C. The results were interpreted in terms of three temperature-dependent, coupled, reversible, and equilibrium-driven reactions. The first, adsorption of CO2 on HTC, exhibited very rapid kinetics and relatively small capacity. Full incorporation of CO2 into the HTC occurred in two reaction steps, the first showing moderate kinetics and intermediate capacity, and the final step occurring with very slow kinetics but having very high capacity. Detailed studies of the effect of temperature on the structural properties of HTC (non-K promoted) and subsequent chemisorption and physisorption of CO2 were reported by Hutson et al.42 At 200 °C the material maintained its layered structure, although the interlayer spacing decreased due to the loss of loosely held water. When heated to 400 °C, the interlayer CO32decomposed and the HTC was completely dehydrated and partially dehydroxylated. Upon further heating to 800 °C, the material was completely decomposed to a solid solution of MgO and Al2O3. A very large increase in BET surface area, from roughly 60 to 170 m2/g, and pore volume, from 0.14 to 0.30 cm3/g, occurred between 200 and 300 °C. The mesopore size distribution peaked at about 61 Å and was not affected by temperature. However, the average size of micropores decreased from about 6.3 to 5.1 Å between 120 and 400 °C. Adsorption isotherms, shown in Figure 15, for CO2 pressures between 0 and 120 kPa were measured using HTC samples that had been pretreated at both 200 and 400 °C to determine the effect of the large increase in surface area on CO2 adsorption. Total adsorption (physisorption plus chemisorption) was measured first (Figure 15, upper). The sample heated to 200 °C exhibited larger capacity at low CO2 pressures, but only a moderate increase in capacity as the CO2 pressure increased. In contrast, the sample heated to 400 °C showed relatively low capacity at low CO2 pressure, but a much stronger response as the CO2 pressure increased. The sample was then evacuated to remove the weakly held physisorbed fraction (Figure 15, middle). The proportion of the total CO2 removed at this stage was much larger with the 400 °C sample. The difference between the total CO2 and that removed by evacuation represented irreversibly chemisorbed CO2 (Figure 15, lower). The decrease in chemisorption between 200 and 400 °C was attributed to the decreased availability of Mg2+ cation while the increase in physisorption was attributed to the increased surface area. CO2 adsorption isotherms shown in Figure 16 (Lee et al.43) are substantially different from those previously discussed in that the data do not conform to the Langmuir model. A new model, shown by the solid and dashed lines in Figure 16, which

Figure 15. Isotherms showing total adsorption (upper), physisorption (middle), and chemisorption (lower) for CO2 on HTC treated at 200 and 400 °C. Reprinted from ref 42. Copyright 2004, American Chemical Society.

accounts for simultaneous chemisorption of CO2 on the surface of the K-HTC and additional reaction between gaseous CO2 and the chemisorbed molecules was developed to describe the experimental data. The model has the same form as the Langmuir model in the low-pressure region and asymptotically approaches a limit as PCO2 increases. The asymptotic limit is reasonably consistent with the earlier isotherm results. This model was used to estimate the heat of CO2 chemisorption of 21.0 kJ/mol and the heat of the surface chemical reaction of 42.2 kJ/mol. Experimental CO2 breakthrough data shown in Figure 17 from a packed bed column at 400 °C (a) and 520 °C (b) from the same paper (Lee et al.43) confirm that the kinetics of CO2 adsorption is rapid at both temperatures to values of y/y° ≈ 0.7. y/y° is the concentration of the column effluent normalized to the feed concentration and t/t* is the normalized time. t* is the stoichiometric breakthrough time based on the isotherm data shown in Figure 16. The decrease in adsorption rate for values


Ding and Alpay45 made similar fixed-bed reactor studies and reported the results in terms of a CH4 enhancement factor, E(t), defined by E(t) ) {[(XCH4)ad/(XCH4)nad - 1]} × 100

Figure 16. Equilibrium chemisorption isotherms of CO2 on K-HTC. Reprinted from ref 43. Copyright 2007, American Chemical Society.

of y/y° > 0.7 was attributed to column nonisothermality. The isothermal portions of the breakthrough data were well simulated by a “CSTR in series” reactor model coupled with a linear driving force model for the chemisorption kinetics. The importance of K2CO3 loading in increasing the CO2 capacity is shown in Figure 18 (Reijers et al.39). Commercial HTC (Pural MG70) was activated by heating overnight to 400 °C, then impregnated with varying quantities of K2CO3 and dried overnight. CO2 adsorption (Figure 18a) and desorption (Figure 18b) concentrations as a function of time for the 20th cycle of a multicycle test using a laboratory-scale fixed-bed reactor are shown. Both the adsorption and desorption steps were carried out at 400 °C with the adsorption gas containing 5% CO2/29% H2O/66% N2 and the desorption gas containing 29% H2O and 71% N2. Addition of K2CO3 caused the CO2 capacity to increase by a factor of as much as 3 compared to nonimpregnated HTC, but the capacity was relatively insensitive to K2CO3 loading. The CO2 capacity was effectively constant at 400 and 450 °C and decreased only a small amount at 500 °C. Like the previous studies, the presence of H2O was reported to enhance both the adsorption and desorption steps. Hydrogen Production. Results of a fixed-bed reactor test for sorption-enhanced H2 production using K-HTC are shown in Figure 19 (Mayorga et al.2). The test was conducted using a feed containing 11% CH4 and 89% H2O at 450 °C and 55 psig, with the reactor packed with a 3:1 mixture of K-HTC to reformer catalyst. The abscissa represents cumulative H2 production, which is proportional to time as long as the reactor product is almost pure H2 (dry basis). The shaded portion to the left of the figure represents H2 used to initially pressurize the reactor. During the early stages of the test the product contained about 98% H2 and 2% CH4. CO2 and CO were not detected until the latter stages of the test (detection limits were not stated). 98% H2 corresponds to about 92% CH4 conversion, and both numbers are much higher than the equilibrium CH4 conversion of 34% and 57% H2 content at these conditions in the absence of the CO2 sorbent. Note, however, that (S/C) ) 8 is considerably larger than values typically used in steam-methane reforming. Similar results at the same reaction temperature and pressure using a larger reactor, a one-to-one ratio of sorbent to catalyst, and lower steam content, (S/C) ) 6 were subsequently published by Hufton et al.44 At these conditions the reactor product contained >95% H2, 98% purity) is produced in the reactor effluent. This step continues until the


Figure 17. CO2 breakthrough data at (a) 400 °C and (b) 520 °C. Reprinted from 43. Copyright 2007, American Chemical Society.

Figure 19. Reactor effluent composition for sorption-enhanced hydrogen test: 1/3 sorbent to catalyst, 450 °C, 55 psig, (S/C) ) 8 (ref 2).

Figure 18. Adsorption and desorption profiles of K-HTC with different K2CO3 loadings: 20th cycle. Reprinted from ref 39. Copyright 2006, American Chemical Society.

product purity decreases to some preset level, when the feed is diverted to a second identical reactor. (2) Depressurization. Countercurrent depressurization is used and the effluent gas, containing CO2, H2, and H2O, is either recycled as feed to another reactor or used as fuel. Depressurization and subsequent regeneration steps occur at the temperature of the adsorption-reaction step and at a pressure between 0.2 and 1.1 atm. (3) Purge. Countercurrent purge with a weakly adsorbing gas such as CH4 is used to regenerate the sorbent. The effluent contains CH4, CO2, H2, and H2O that may be separated for CH4 recycle or used as fuel. (4) Product Purge. The CH4 purge gas is displaced from the reactor using countercurrent flow of product H2. The displaced CH4 may be recycled or used as fuel. It is necessary to use product H2 in this and the following step if high-purity H2 is to be produced during the reaction-adsorption step. (5) Product Pressurization. Countercurrent pressurization with H2 prepares the reactor for the next reaction-sorption cycle.

Figure 20. Effect of (S/C) on the CH4 conversion enhancement factor. Reprinted with permission from ref 45. Copyright 2000, Elsevier.

In the process described by Reijers et al.,39 regeneration would be accomplished in a single step using steam purge. Steam is specified because, compared to other possible inert gas purges, steam can be easily separated from H2 by condensation. The adsorption and regeneration steps both operate at the same temperature and total pressure, 400 °C and 1 atm. Minimizing the amount of steam required is important for the process to be economical since, as the authors point out, production of steam is energy intensive. The amount of purge steam required should be in the same range as the (S/C) ratio (3 to 6) in the reactor feed gas in order to obtain acceptable system efficiency. Experimental results (Figure 23) using a purge gas containing 29% H2O/71% N2 show that the quantities of steam required


Figure 21. Results of 100-cycle stability test using K-HTC. Reprinted with permission from ref 31.

Figure 22. Schematic illustration of the PSA H2-SER process steps (ref 2).

Figure 23. K-HTC regeneration using steam purge. Reprinted from ref 39. Copyright 2006, American Chemical Society.

are far in excess of the desired amount. The amount of purge gas required is a strong function of the purge gas flow rate, with the total amount decreasing by a factor of about 5 when the purge gas rate is reduced from 100 to 10 mL/min. The total purge time, however, at the lower purge rate would be about a factor of 2 longer than at the high rate. In none of the cases

shown in the figure does the required purge gas rate approach the desired level. The authors attribute the CO2 desorption percentages exceeding 100 to the release of CO2 from the sorbent structure during regeneration, and the large required purge gas rate to CO2 transport limitations from the particle interior. Lee et al.43 have proposed a combination pressure and temperature swing process with H2 production occurring at about 490 °C and 1.5-2.0 bar, and using steam to desorb CO2 from the K-HTC sorbent at a temperature of 590 °C and pressure near atmospheric. Although the authors refer to this concept as thermal swing sorption-enhanced reaction (TSSER), they specify changes in both the temperature and pressure between the adsorption and regeneration steps. This concept avoids the expensive subatmospheric pressures normally associated with straight PSA and the higher regeneration temperature reduces the quantity of steam needed. In addition, the higher temperature of the regenerated K-HTC provides the energy needed to drive the reforming reaction. Production of 99.99+% H2 containing approximately 10 ppmv CO, 13 ppmv CO2, and 60 ppmv CH4 is claimed to be possible at these conditions and the concept is said to be particularly applicable to small, efficient, fuel-cell grade H2 production for residential or industrial use. A unit capable of producing 24 slpm H2 at 1.5 bar containing ≈10 ppmv CO for use in a residential 2 KW PEM fuel cell is described. Two shell-and-tube exchanger type reactors (see following paragraph) each containing 80 tubes, 1.73 cm diameter by 200 cm length, operating on a 20 min cycle time will be required. The diameter of each exchanger will be ≈17 cm. A novel shell-and-tube reactor suitable for carrying out K-HTC sorption-enhanced H2 production in a fixed-bed was described by Mayorga et al.2 and is shown in Figure 24. The K-HTC-catalyst mixture is contained in the tubes with heat transfer fluid on the shell side. During H2 production a reaction mass transfer zone travels along the bed from the inlet to product end. Energy must be supplied in this mass transfer zone to maintain isothermal conditions to compensate for the net endothermic reforming, shift and carbon capture reactions. However, the energy flux should be near zero above and below the reaction zone to avoid overheating the solids in these sections. Heat must also be supplied during the endothermic desorption step. A recycled heat transfer fluid that condenses


Figure 25. Durability of Li4SiO4-LiZrO3 sorbent in 50-cycle tests. Reprinted with permission from ref 49. Copyright 2004, The Ceramic Society of Japan.

Figure 24. Novel reactor concept for H2 production using K-HTC (ref 2).

at the desired reaction temperature flows through the shell so that heat is transferred only to that section of the reactor tubes where the endothermic reactions occur. Reijers et al.31 analyzed a K-HTC enhanced H2 production process in their Aspen Plus simulation studies referred to earlier in the section of CaO enhanced H2 production. The sorber-reactor operates at 17 bar and 400 °C with a feed (S/C) ) 3. The regenerator operates at 2.8 bar and 400 °C using steam purge gas at a steam to CO2 ratio of 1.8. Product H2 was used to generate electricity in a combined gas-steam cycle. Consistent with the CaO analysis, CH4 conversion was fixed at 93% to result in 85% CO2 capture. The calculated overall efficiency of 51.6% was one point lower than the CaO-based system but significantly larger than the 48% efficiency of the standard natural gas combined cycle process retrofitted for CO2 capture using MEA scrubbing. Other Potential Sorbents Research is underway on alternate sorbents that will, on the one hand, have larger CO2 capacity than HTC, and, on the other hand, be regenerable at temperatures lower than required for Ca-based sorbents. Larger capacity will reduce the quantity of sorbent required and/or increase the duration of the reactionsorption cycle. Lower regeneration temperature should reduce sintering, increase sorbent durability, and possibly reduce the energy required for sorbent regeneration. Researchers at Toshiba Corp. were among the earliest entries into this area with their K2CO3-promoted Li2ZrO346,47 sorbent. The original application proposed was not for H2 production but for the high-temperature capture of CO2 in association with IGCC applications. The reversible chemical reaction is Li2ZrO3(s) + CO2(g) T Li2CO3(s) + ZrO2(s)

(6)

Little CO2 was adsorbed using pure Li2ZrO3, and both the rate and quantity adsorbed increased with increasing K2CO3 pro-

moter content. The authors concluded that these improvements were due to the formation near the exterior of the particle of a eutectic carbonate containing both potassium and lithium that melted about 500 °C. ZrO2 was limited to the core of the particle. Thus the reaction was actually between CO2 and Li2O in the liquid phase to form Li2CO3. The increased rate was due to faster diffusion through the liquid eutectic than through the solid phase. Depending on CO2 pressure, capture occurred between 450 and 590 °C with the reverse reaction proceeding at temperatures above 590 °C. Essentially complete CO2 removal was reported from a bench-scale test at 560 °C and 5 atm with the feed gas containing 20% CO2 in N2. No multicycle results were reported in these early studies. In a later paper Kato et al.48 compared the CO2 adsorption properties of several Li-containing mixed oxides: Li2ZrO3, LiFeO2, LiNiO2, Li2TiO2, Li2SiO3, and Li4SiO4. Among this group, lithium orthosilicate, Li4SiO4, was found to have the highest reactivity with CO2. In addition, the Si-based sorbent is lighter in weight and lower in cost that the Zr-based material. The stoichiometry for the Li4SiO4 reaction is Li4SiO4(s) + CO2(g) T Li2SiO3(s) + Li2CO3(s)

(7)

Five cycle tests at atmospheric pressure involving CO2 capture at 700 °C and regeneration at 850 °C, both in pure CO2, were reported with no significant deterioration in performance. It is also interesting to note that the Li4SiO4 sorbent was found to be effective in capturing CO2 at room temperature. After 450 h exposure to ambient air containing 500 ppmv CO2 the Li4SiO4 showed an almost 30% weight gain compared to only about 1% weight gain for Li2ZrO3. K2CO3-doped Li4SiO4 sorbent durability was studied by Kato et al.49 who found that the addition of small amounts of Li2ZrO3 resulted in significant improvement in 50-cycle tests. In these tests CO2 capture occurred at 1 atm and 600 °C in an atmosphere of 20% CO2 in N2 with regeneration at 1 atm and 800 °C in N2. The duration of both the capture and regeneration cycles was 1 h. Results, shown in Figure 25, were presented in terms of retention ratio versus cycle number, where retention ratio was defined as the CO2 adsorption rate after 1 h in cycle n normalized to the adsorption rate after 1 h in cycle 1. Tests using pure Li4SiO4 and with 2% and 5% (mass) Li2ZrO3 are shown. Both the pure Li4SiO4 and the sample containing 2% Li2ZrO3 experienced a quite rapid initial decrease in retention ratio before leveling off in the 70-80% range after 50 cycles. In contrast, the initial loss in retention ratio was much smaller


with the sample containing 5% Li2ZrO3 and the 50-cycle value was well above 90%. The benefit associated with Li2ZrO3 addition was attributed to stabilization of the structure against sintering. The pure Li4SiO4 experienced an almost 75% loss in surface area after 50 cycles, whereas the surface area of the sorbent containing 2% Li2ZrO3 decreased by about 50%, and the sorbent containing 5% Li2ZrO3 experienced almost no loss in surface area. A number of studies have examined the effect of preparation methods on both the kinetics and durability of lithium-based sorbents. Xiong et al.50 used the solid-state reaction method to prepare several K2CO3-doped Li2ZrO3 sorbents having similar crystallite but different aggregate sizes. They concluded that that the aggregate size, not the crystallite size, determines the CO2 adsorption rate, with the rate increasing with decreasing aggregate size. Escobedo Bretado et al.51 compared the performance of Li4SiO4 sorbents prepared using the solid-state reaction and impregnated suspension methods. The impregnated suspension method was said to provide improved control of the morphology and microstructure of the product, and resulted in increased conversion of Li4SiO4 in TGA reactor tests. Gauer and Heschel52 compared the effects of K2CO3 doping of Li4SiO4 sorbents to the effect of heteroatom doping by introducing Al and Fe atoms to produce defects in the Li4SiO4 crystalline structure, thus improving ion mobility. At moderate temperatures, all doped sorbents produced substantially increased rates of CO2 update compared to undoped Li4SiO4 with the maximum rate at 500 °C associated with Li3.7Fe0.1SiO4. However, the overall maximum rate of CO2 uptake occurred with undoped Li4SiO4 at a substantially higher temperature of 685 °C. The Fe-doped sorbent was also found to be superior to both Al-doped and K-doped sorbents in releasing CO2 during sorbent regeneration. In addition, the presence of Fe was said to contribute catalytic activity for the water gas shift reaction. Kimura et al.53 recognized the problem that might be caused by the formation of the low melting eutectic associated with the K2CO3 doping in terms of particle adhesion, particularly in fluid-bed reactors. This group produced fluid-bed sized particles in which a K2CO3-doped Li4SiO4 core was coated with a porous alumina shell to prevent the liquid from wetting the particle surface. Although some deterioration in CO2 capture was observed in multicycle tests the authors reported no deterioration in fluidization performance and at least 50% sorbent conversion through 60 cycles. Sodium-based sorbents based on a Na2CO3-NaHCO3 cycle have been suggested for the capture of CO2 from flue gases at low temperatures of about 70 °C (Liang et al.54). Lopez et al.55 reported the feasibility of high-temperature CO2 capture using the mixed oxide Na2ZrO3. The sorbent was prepared by solid state reaction and the sorption/desorption properties of CO2 were studied in a TGA. The CO2 sorption rate at 600 °C was higher than with either Li2ZrO3 or Li4SiO4 prepared in a similar manner, but the regeneration performance was not as good as the Li-based sorbents. Zhao et al.56 prepared nanosized Na2ZrO3 having well-defined crystal structure by dissolving sodium citrate and zirconoxy nitrate in deionized water, mixing the solutions, and evaporating the mixture with continuous stirring to produce an amorphous zirconium complex. A number of calcination procedures were tested. Heating to 1073 at 10 K/min under argon, followed by calcination in air for 3 h, produced a highly active and relatively pure monoclinic Na2ZrO3. Calcination in air throughout the cycle, or calcination in argon to 1173 K followed by air, produced predominantly hexagonal Na2ZrO3, which was less active for CO2 capture. A highly exothermic

Figure 26. Sorption-enhanced H2 production using Na2ZrO3 sorbent. Reprinted with permission from ref 57.

Figure 27. Equilibrium CO2 pressure as a function of temperature for CaO, Li2ZrO3, Na2ZrO3, and Li4SiO4 sorbents. Reprinted with permission from ref 30. Copyright 2007, The Royal Society of Chemistry.

reaction between nitrate and citrate occurred accompanied by coke deposition during calcination in Ar. The carbon served as a dispersant of the oxide and subsequent carbon burnoff in air promoted the formation of nanocrystalline Na2ZrO3 having an open pore structure. Little experimental data on the use of Li- and Na-based sorbents for hydrogen production has been published. One exception is shown in Figure 26 (Yi et al.57). The product gas contained 96.8% H2 (dry basis) during the prebreakthrough period of a fixed-bed test using Na2ZrO3 sorbent prepared using a liquid-liquid procedure. Reaction conditions were 600 °C, 1 bar, with (S/C) ) 4. The paper did not discuss sorbent regeneration or present multicycle results. The thermodynamic properties of Li- and Na-based sorbents are not as favorable as those of Ca-based sorbents for sorptionenhanced H2 production. This is shown by the plot of equilibrium CO2 pressure as a function of temperature in Figure 27 (Ochoa-Fernandez et al.30). For example, at 850 K the equilibrium CO2 pressure is less than 0.002 bar with CaO, about 0.025 bar with Na2ZrO4, 0.045 bar with Li2ZrO3, and 0.1 bar with Li4SiO4. These equilibrium pressures translate to the possibility, at 10 bar, 850 K and (S/C) ) 5, of producing a product containing about 98.3% H2 (dry basis) using CaO, 90% with Na2ZrO3, 89% with Li2ZrO3, and only about 82% with Li2SiO4. In addition, the authors found that the kinetics of CO2 sorption was substantially faster with CaO compared to the other sorbents. However, each of the lithium- and sodium-based


sorbents showed better durability in multicycle experiments consisting of eight sorption and regeneration cycles. Conclusions Sorption-enhanced H2 production combines hydrocarbon reforming, water gas shift, and CO2 separation reactions to produce H2 in a single reaction step. The simultaneous reactions occur over a mixture of reforming catalyst and CO2 sorbent. The process possesses many potential advantages in comparison to the standard steam-CH4 reforming process including improved energy efficiency and capital cost reduction through process simplification. Only two reactors are required: one for H2 production and the other for sorbent regeneration. The water gas shift reactor is not needed. Depending on reaction conditions, the H2 content of the product may be in the 95-98% range (dry basis) with the concentration of carbon oxides in the low ppmv level. Product purification may be eliminated for some applications or simplified for others. The high-alloy steels required to withstand the high temperature and heat flux in the reformer furnace in the standard process may be replaced with less expensive materials of construction. Finally, in some process options, a high-purity stream of CO2, suitable for use or for compression and sequestration, may be produced during sorbent regeneration. A number of candidate CO2 sorbents have been studied including Ca-based oxides, K-HTC, and mixed oxides of Li and Na. Ca-based oxides from naturally occurring precursors such as limestone and dolomite have the advantage of being widely available and inexpensive. They possess high CO2 capacity and react rapidly over a wide range of temperatures and pressures. Their primary disadvantage is associated with the high temperature required for sorbent regeneration. The relatively large swing between reaction and regeneration temperatures increases the energy required for regeneration, and the high temperature promotes sorbent sintering that leads to rapid deactivation. Much of the current research on Ca-based sorbents is aimed at improving multicycle durability, either by altering process conditions or by synthesizing sorbents that have intrinsically better durability. HTCs are members of the family of double-layered hydroxides that, when doped with K2CO3, can serve as hightemperature CO2 sorbents. They react rapidly but have much lower CO2 capacity than Ca-based sorbents and are also considerably more expensive. Their advantage is that regeneration is possible with less energy input and at lower temperature so that sorbent durability should be improved. The HTC-CO2 complex may be regenerated by pressure swing, temperature swing, or a combination of the two. Recent interest in mixed metal oxide sorbents of Li and Na such as Li2ZrO3, Li4SiO4, and Na2ZrO3 was spawned, on the one hand, by the desire to find a replacement for Ca-based sorbents that could be regenerated at lower temperature, and, on the other hand, would have considerably higher CO2 capacity than HTC. To date, these sorbents have largely been evaluated only for their multicycle ability to capture CO2 and relatively little information is available on their use in the overall sorptionenhanced H2 production process. Because of less favorable thermodynamic properties associated with these sorbents, the equilibrium CO2 pressures are higher and product H2 concentrations must be lower than can be obtained using Ca-based sorbents at equivalent reaction conditions.

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ReceiVed for reView February 20, 2008 ReVised manuscript receiVed May 8, 2008 Accepted May 16, 2008 IE800298Z

Sorption-Enhanced Hydrogen Production: A Review - Industrial

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