Hydrotalcite as CO 2 sorbent for sorptionenhanced steam reforming of methane

Transcription

1 ECN-RX Hydrotalcite as CO 2 sorbent for sorptionenhanced steam reforming of methane H.Th.J. Reijers, S.E.A. Valster-Schiermeier, P.D. Cobden and R.W. van den Brink to be published in the CO 2 Capture special issue of Industrial & Engineering Chemistry Research edited by name: H.Th.J. Reijers function: Project leader d.d.: verified by name: S.C.A. Kluiters function: Project leader d.d.: verified by name: J.W. Erisman function: Unit manager d.d.: modified by name: H.Th.J. Reijers function: Project leader d.d.: authorized by name: R.W. van den Brink function: Group manager d.d.: JULY 2005

2 Acknowledgement/Preface This research has been carried out by ECN in the CATO-project (Carbon Capture, Transport and Storage), which is financed by the BSIK subsidy program of the Dutch Government agency SenterNovem. It is also part of ECN's DECAFF-program (Decarbonisation of Fossil Fuels), which is sponsored by the Dutch ministry of Economic Affairs. Abstract Sorption-enhanced reforming of methane is an attractive option for the combined production of electricity and capture of CO 2. In this process, the steam reforming catalyst is mixed with a CO 2 sorbent. During the reaction, CO 2 is adsorbed implying an increase of the hydrogen production rate. Once the sorbent is saturated, it must be regenerated using purge gas, usually steam. The amount of steam needed for CO 2 removal from the saturated sorbent determines the system efficiency of the process to a large extent. In this paper, several potassium-promoted hydrotalcite samples, both commercially obtained and in-house prepared, are tested for their suitability as CO 2 sorbent for sorption-enhanced reforming of methane. In particular, the purge gas to adsorbed CO 2 ratio is determined at various operation conditions. It is shown that this ratio is still too large at the investigated conditions. Mixed with steam reforming catalyst the hydrotalcite sorbent can adsorb sufficient CO 2 to enhance the CH 4 conversion to almost 100%. Keywords: adsorption, desorption, sorption-enhanced, CO 2 capture, hydrotalcite, steam reforming 2 ECN-RX

3 Contents List of tables 4 List of figures 4 Summary 5 1. Introduction 7 2. Sorbent selection 8 3. Hydrotalcites 9 4. Experimental Data analysis Results and discussion Conclusions 13 List of symbols 13 References 13 ECN-RX

4 List of tables Table 1 Characteristics of CO 2 sorbent materials (*** = good, ** = fair, * = poor,?=unknown) Table 2 Specifications of HTCs according to the supplier Table 3 Standard experimental conditions Table 4 CO 2 adsorption capacities at breakthrough and during the whole adsorption step at various conditions List of figures Figure 1 Schematic representation of a SERP system for electricity production Figure 2 Structure of hydrotalcite Figure 3 Schematic diagram of the lab-scale experimental apparatus Figure 4 Approximation of peak area given by the solid line (dashed line: curve fit, symbols:experimental data points) Figure 5 Adsorption and desorption profiles of various potassium- promoted HTC samples (the desorption profiles of ECN-HTC/22 wt% K 2 CO 3 coincides with that of PURAL MG70/22 wt% K 2 CO 3 ) Figure 6 Adsorption and desorption profiles of PURAL MG70 with different K 2 CO 3 loadings 24 Figure 7 Desorption characteristics determined for different desorption flows (the dashed curve is a repetition of 100 ml/min at the end of the experiment) Figure 8 Desorption characteristics determined for different desorption times (t ads = 10 min, F des = 30 ml/min) Figure 9 Effluent gas composition during reaction and regeneration using a mixture of steam reforming catalyst and HTC impregnated with 22 wt% K 2 CO Figure 10 CO 2 flow during regeneration in 17.4% and 29% H 2 O containing N 2 gas ECN-RX

5 Summary Sorption-enhanced reforming of methane is an attractive option for the combined production of electricity and capture of CO 2. In this process, the steam reforming catalyst is mixed with a CO 2 sorbent. During the reaction, CO 2 is adsorbed implying an increase of the hydrogen production rate. Once the sorbent is saturated, it must be regenerated using purge gas, usually steam. Compared with other purge gases (air, nitrogen, methane), steam can be easily separated, thus enabling the supply of a concentrated CO 2 stream suitable for sequestration. The amount of steam needed for CO 2 removal from the saturated sorbent determines the system efficiency of the process to a large extent. In this paper, several potassium-promoted Mg-Al based hydrotalcite samples, both commercially obtained and in-house prepared, are tested for their suitability as CO 2 sorbent for sorption-enhanced reforming of methane. The effects of promoter loading, temperature, duration of the desorption step and purge flow on the adsorption and desorption properties are studied. In particular, the ratio of the amount of purge gas needed to remove all adsorbed CO 2 and the amount of adsorbed CO 2 ratio is used to find the optimum sorbent properties and experimental conditions. It is shown that this ratio is still too large at the investigated conditions. As a proof-of-principle experiment, it is shown that mixed with steam reforming catalyst, the hydrotalcite sorbent can adsorb sufficient CO 2 to enhance the CH 4 conversion to almost 100%. ECN-RX

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7 1. Introduction There is a growing awareness that energy must be produced at lower greenhouse gas emissions. Fossil fuels, though, will remain the most important energy source for the first half of this century. This has led to new technologies to reduce the emission of the CO 2 produced from the burning of fossil fuels. This CO 2 can be captured with end-of-pipe technologies from the flue gas of the energy production unit using a CO 2 scrubber, e.g. an amine solution. Other possibilities include the application of nitrogen-free combustion of fossil fuels which facilitate CO 2 capture, but require a costly nitrogen-oxygen separation step 1. A third possibility is to use precombustion decarbonization. Here, the CO 2 produced is captured prior to combustion, while transferring the energy content of the fuel to hydrogen. Various pre-combustion routes for electricity production have been investigated, using a membrane reactor 2,3 or the sorption-enhanced reaction process (= SERP) 4,5. The relevant reaction(s) are: CO + H +, ( 1 ) 2O CO2 H 2 for water gas shift, and in addition CH + +. ( 2 ) 4 H 2O CO 3H 2 if steam reforming is included. Conventional steam reforming of methane is performed at high temperatures, typically between 850 and 950 ºC, and 80% to 90% of the methane is converted to hydrogen. When the steam reforming catalyst is mixed with a sorbent, as in the sorptionenhanced reaction process, the CO 2 produced in the above reactions is simultaneously adsorbed, so that the hydrogen production rate is enhanced. Thus, to obtain the same CH 4 conversion, the steam reforming process may be performed at lower temperatures compared with conventional reforming, typically between 450 and 550 ºC. Sorption-enhanced reforming is a batch process necessitating the regeneration of the sorbent which is saturated with CO 2. The preferred regeneration or purge gas is steam if the captured CO 2 is to be sequestrated. Compared with other purge gases (air, nitrogen or methane), steam can be easily separated from the purge stream, leaving a pure CO 2 flow. An example of a sorption-enhanced reforming system for electricity production is shown in Figure 1. Both SERP reactors are filled with a mixture of steam reforming catalyst and CO 2 sorbent. A CH 4 /H 2 O mixture is fed to reactor 1, purge gas to reactor 2 for regeneration. To improve the desorption of CO 2, the temperature may be increased or the pressure may be decreased with respect to the temperature and pressure during adsorption. After the sorbent of reactor 1 is saturated with CO 2, the gas flows to both reactors are interchanged and CO 2 is desorbed from the bed of reactor 1, whereas steam reforming is performed in reactor 2. Part of the produced H 2 is fed to a gas turbine. The other part is used as fuel for the burner, which in turn drives the endothermic steam reforming reaction and the regeneration process. Production of 1 mole of steam is energy-intensive requiring about 20% of the reaction enthalpy of the steam reforming of the same amount of methane. Thus, its consumption for regeneration must be limited in order to obtain an acceptable system efficiency 6. None of the mentioned groups who investigated sorption-enhanced steam reforming of methane, has optimized the sorbent properties and the system configuration such that the lowest cost of CO 2 removal is obtained. The purpose of this paper is to investigate the relation between the purge gas to adsorbed CO 2 ratio and the operation conditions in more detail. The amount of purge steam needed for complete desorption per mol CO 2 adsorbed by the bed, will be used as the parameter to characterize the efficiency of the desorption step. This ratio depends only on the material properties of the sorbent, not on the system configuration. It should be in the same range as the steam/methane ratio of the feed gas (i.e. 3-6) to obtain an acceptable system efficiency. Note, ECN-RX

8 that the amount of purge steam is expressed with respect to the amount of adsorbed CO 2, since it is proportional with the amount of adsorbed CO 2 to first approximation. Clearly, this is a simplifying assumption since a real system is more complex. A smaller adsorption capacity implies a higher switch frequency between adsorption and desorption steps, and higher pressure drop losses due to the larger adsorbent bed. Though these will affect the efficiency as well, such factors will not be taken into account here. The purge steam/adsorbed CO 2 ratio needed to desorb a certain amount of CO 2, hereafter S/CO 2, will be experimentally determined. The plot of the desorbed fraction of CO 2 versus S/CO 2, is called the desorption characteristic. The steeper the characteristic, the more easily the CO 2 is desorbed. The relation between the purge steam/adsorbed CO 2 ratio for complete desorption, hereafter (S/CO 2 ) c and the properties of the chosen sorbent, promoted hydrotalcite, and operation conditions for sorption-enhanced steam reforming of methane is investigated in this paper. 2. Sorbent selection For use in a steam reforming reactor, the sorbent material should fulfill the following requirements: 1. It should be able to remove sufficient CO 2 in the temperature range ºC and in the pressure range 1-40 bar. 2. It should be able to withstand the high p steam /p CO2 ratios at the steam reforming conditions in a SERP reactor (> 20), where p steam and p CO2 are the partial pressures of respectively steam and CO 2 gas. 3. It should be mechanically and chemically stable for extended periods at these conditions. 4. The kinetics of adsorption and desorption should be sufficiently fast. From a literature search follows that the materials which are of interest for SERP applied to a fuel-processing step can be divided in the following 5 groups: 1. metal oxides 7,8,9 2. hydrotalcites 5,10,11,12 3. double salts lithium metal oxides 14,15, and 5. supported sorbents. Metal oxides, like CaO and MgO, react chemically with CO 2. Hydrotalcites are discussed in detail below. Potassium double salts were identified as the active phase in the decomposition products of K 2 CO 3 -impregnated hydrotalcite using high-temperature X-ray diffraction. Synthesizing double salts directly, rather than preparing them as a side product by decomposition of hydrotalcite, resulted in sorbents having higher adsorption capacity and improved kinetics compared with promoted hydrotalcites. Lithium metal oxides were discovered by Toshiba in their molten carbonate research when they added ZrO 2 to the molten electrolyte. The product, Li 2 ZrO 3, was found to be a good CO 2 sorbent, as were other lithium metal oxides. Supported sorbents are combinations of a sorbent of group 1 4 and a suitable support material. An example is CaO supported by Cabot Superior Micropowder 16. The properties of the above 5 groups of sorbents are summarised in Table 1. Those sorbent materials which are used for CO 2 removal at other conditions are excluded. Materials like zeolites, amines, carbonates and active carbon are used at relatively low temperatures (mostly lower than 100 ºC) and are thus not taken into consideration here. The stability of double salts has not been investigated very well since a good recipe for a binder to produce pellets from the precipitates has not been found yet. For that reason, they were not used for our research. Trying to select a sorbent from the remaining 4 ones on the basis of this table proves hard since the overall performance of the various sorbent groups is rather similar. Therefore, system aspects need to be taken into account as well. Note that of these 5 groups, the materials of the 1 st and 4 th group are preferably regenerated by temperature swing since they react with CO 2 in a strongly exothermic reaction (ΔH = 170 kj/mol for CaO at 800 ºC, ΔH = 170 kj/mol for Li 4 SiO 4 at 700 ºC). Those of 8 ECN-RX

9 the 2 nd and 3 rd groups can be regenerated by pressure swing since the heat of adsorption is relatively low (17 kj/mol 17 ). Those of the last group may be regenerated using either temperature or pressure swing depending on the sorbent material. Clearly, the chosen regeneration method (temperature swing or pressure swing) has important consequences for the system configuration and efficiency. Without knowing the details of the system, we decided that pressure swing is to be preferred to temperature swing. In the case of temperature swing, the reaction step and regeneration take place in two different, fluidized bed reactors at different temperatures, between which the sorbent particles are continuously circulated 18. This makes the system more complex and thus less attractive. Also, heat-resistant alloys must be used for vessel construction due to the higher temperatures involved (> 700 ºC). For that reason, the CaO and lithium metal oxide based adsorbents were dropped from our experimental work and attention was focused on hydrotalcites which could be obtained commercially as well as prepared relatively easily in our laboratory. It should be kept in mind that in the case of pressure swing operation, the parameter (S/CO 2 ) c provides only a relative indication of the ease of regeneration of the CO 2 sorbent. It is not suitable for process design purposes since the bed is never fully regenerated. Instead, a lower purge gas amount would be used with an associated penalty in the CO 2 adsorption capacity. An optimization of energy and capital cost would determine which purge gas amount is most desirable. 3. Hydrotalcites HTCs (= Hydrotalcites), also called layered double hydroxides or Feitknecht compounds, belong to the family of anionic clays. Their general formula is n M 1 M x ( OH ) 2 ( A ) x / n. mh O x 2, where M 2+ and M 3+ are divalent and trivalent metal ions respectively, and A n- is an anion. The value of x should be in the range The metal ions and anions appear in different layers (Figure 2). The metal ion host layer has the brucite structure of Mg(OH) 2, in which the metal ions are octahedrally coordinated by OH - ions. Part of the divalent metal ions is replaced by trivalent ions, leaving the brucite structure intact. Consequently, this layer has a net positive charge which is compensated by the charge of the anion layer. The empty sites of the anion layer are filled with water molecules. The most common HTC is ( Mg) ( Al) ( OH ) ( CO ) mh O, 1 x x 2 3 x / 2. 2 occurring in nature as Mg 0.75 Al 0.25 (OH) 2 (CO 3 ) H 2 O (the mineral hydrotalcite ) with which the above class of anionic clays is isostructural. HTCs can be easily synthesized by coprecipitation from a solution of soluble salts containing the metal ions, usually at slightly elevated temperature and constant ph 19. By slow addition of a carbonate salt, a precipitate is formed. The precipitate is separated by filtration and dried. Thanks to the structure of HTC, the metals are well mixed and dispersed and thus, HTCs are ideal candidate catalyst precursors. When the HTC is calcined in air or N 2, first it loses its interlayer water up to approximately 200 ºC. In the range ºC, dehydroxylation and decarbonisation take place and the specific surface area increases strongly. The suitability of HTC as high temperature CO 2 sorbent stems from the plurality of strong basic sites at the surface this structure offers, which favours the adsorption of the acidic CO 2 according to the Lewis acidbase theory 20. When no further heating occurs, the thus obtained mixture of metal oxides can be transformed back into the original HTC structure by contacting it with a carbonate solution. This is the so-called memory-effect of HTCs. If the metal oxide mixture is heated to temperatures above 500 ºC, the structure changes irreversibly and amongst other phases a spinel phase is formed. ECN-RX

10 4. Experimental For our work, four commercially available HTCs (PURAL MG70, PURAL MG61 HT, PURAL MG50 and PURAL MG30) were obtained from SASOL. They are aluminum magnesium hydroxide carbonates of general formula (Mg) 1-x (Al) x (OH) 2 (CO 3 ) x/2.mh 2 O. Their specifications are given in Table 2. We also prepared several HTC samples in-house, called ECN-HTC. To a well-stirred NaHCO 3 solution of 65 ºC, a solution containing the metal nitrates in the ratio Mg/Al = 3 was added dropwise resulting in precipitation of the HTC precursor. The ph was kept at a constant value of 8.00 by adding a NaOH solution when needed. After that, the solution was allowed to cool down to room temperature overnight under vigorous stirring. Next day, the precipitate was separated from the suspension by filtration and dried for 24 hours at 120 ºC. The HTC samples, both commercial and in-house prepared, were activated by heating them in air to 400 ºC and by keeping them at this temperature for 4 hours. After that, the obtained powders were loaded with 22 wt% K 2 CO 3 using dry impregnation and dried overnight 21. The impregnated powders were compacted at a pressure of 275 atm and at room temperature. A sieve fraction of these particles was obtained with sizes between and mm. The experiments were performed using a glass reactor tube of 16 mm inner diameter and 150 mm length. The reactor was filled with 3 g of particles. The sample was heated under N 2 to the temperature of the experiment, usually 400 ºC. Humidified N 2 (29% H 2 O) was passed along the sample for 75 minutes to remove any calcination products, the so-called pre-desorption step. After that the sample was subjected to a series of adsorption/desorption cycles. The conditions of the adsorption and desorption steps of a standard experiment are given in Table 3. The duration of both steps is 75 minutes and 60 gas samples per step from the dry effluent are analysed by a MicroGC (Hewlett Packard M200H), using a sample interval of 75 s. A permapure is used for drying the effluent. At the end of the experiment, the sample is cooled down to room temperature under dry N 2. The water is evaporated and mixed with the feed gas flow in a CEM (Controlled Evaporation Mixer) unit. The mass flow controllers, CEM unit and valves are computercontrolled. A schematic diagram is shown in Figure 3. To determine the amount of CO 2 adsorbed and desorbed by the HTC samples only during respectively the adsorption and desorption steps, a so-called blank experiment was performed to correct for instrumental effects. For the blank experiment, a non-adsorbing material (SiC) of the same particle size range and bed volume as the adsorbing samples was used. The experimental conditions were identical to those applied in the adsorption and desorption steps of the experiments using an adsorbing sample. The following parameters were varied: K 2 CO 3 loading, H 2 O fraction of the feed gas, promoter, desorption flow and desorption time. Finally, the HTC sorbent was mixed with a low-temperature steam reforming catalyst and a CH 4 /H 2 O/N 2 gas mixture was fed to the reactor to investigate sorption-enhanced reforming of methane. 5. Data analysis The measured CO 2 concentrations were converted into CO 2 flows. Curve fitting was applied to the resulting adsorption and desorption profiles using ( 1 exp(( t a1) / a2 )) + b0 ( 1 exp(( t b1 ) / b2 ) c0 a + 0 ) and a 0 exp( ( t a1) / a2 ) + b0 exp( ( t b1 ) / b2 ) + c0 + c1t, for respectively the adsorption and desorption profiles. The parameters a 0, a 1, a 2, b 0, b 1, b 2, c 0 and c 1 are determined by a least-squares fit. The R-square value was better than and 0.99 for respectively the adsorption and desorption profiles, showing that the quality of the fitted 10 ECN-RX

11 curves was good. The area under the curves was subsequently calculated by integrating the above equations up to the appropriate time 't'. The same procedure is applied to both the adsorbing sample and the blank. The amounts of CO 2 adsorbed and desorbed at the time 't' elapsed since the start of the step, respectively V ads and V des, follow from: V ads ( t) = Area under ' blank' curve Area under ' CO2 adsorbent' curve and V des ( t) = Area under ' CO2 adsorbent' curve Area under ' blank' curve, where blank and CO 2 adsorbent curve refer to the adsorption or desorption profiles of the experiments using respectively the blank and the CO 2 adsorbent. The height of the peak, which arises shortly after switching from adsorption to desorption conditions, cannot be accurately described due to lack of data points. Therefore, for the calculation of the curve area this peak is approximated by a straight line going from the vertical axis to the second data point, see Figure Results and discussion Figure 5 shows the adsorption and desorption profiles corresponding with the 20 th cycle of the experiments up to t = 40 min using the various PURAL samples and ECN-HTC, all loaded with 22 wt% K 2 CO 3. The CO 2 adsorption decreases during the first 20 cycles for all samples. After the 20 th cycle, it becomes approximately constant. The results of the blank experiments are indicated by dashed lines. The adsorption profiles of PURAL MG70, MG61 HT and ECN-HTC are similar (breakthrough times respectively 7.5 min, 8.8 min, 9.0 min), that of PURAL MG50 starts to break through at the same time as the before-mentioned ones (7.9 min) though it rises less strongly, whereas breakthrough of PURAL MG30 clearly occurs at a later time (12.5 min). For all samples we find that after breakthrough, the profile does not show a sharp rise towards the inlet CO 2 flow (1.5 ml/min), but rather slowly creeps to this value. Table 4 shows the calculated amount of CO 2 adsorbed up to the breakthrough time t bt and during the whole adsorption step (here 75 min), respectively q ads (t bt ) and q ads (t=75min). About two thirds of all CO 2 is adsorbed before breakthrough. The desorption profiles show a rapid initial CO 2 release, which considerably slows down during deeper desorption and never becomes zero. At the end of the desorption step, the CO 2 flow is in the range ml/min. Apart from the desorption peak, which is highest for PURAL MG30, the profiles of the various samples are quite similar. The desorbed amount exceeds the adsorbed amount of CO 2. During impregnation of calcined HTC with K 2 CO 3, the original HTC structure is partly restored due to the memory effect 19. When the impregnated HTC is heated again, calcination of the restored structure occurs which is apparently not complete after the pre-desorption step. We believe that the mismatch between adsorbed and desorbed CO 2 is an inherent property of the HTC, not the result of experimental errors in the MicroGC data or the analysis procedure. First, because the mismatch gets smaller when more adsorption/desorption cycles are performed (up to 40). Second, because in only one case (the experiment performed at 500 ºC, see below), the mismatch (almost) vanished, although it did not even here disappear completely. Third, because we have checked the MicroGC by using a CO 2 analyzer allowing a CO 2 concentration measurement per 15 seconds which is a factor 5 more than the GC allows. This did not change the mismatch between adsorbed and desorbed CO 2. Using an activation temperature of 500 ºC as a measure to reduce the mismatch, is undesirable because of the decrease of the CO 2 adsorption capacity. Other measures like increasing the duration of the activation step or pre-desorption step, while keeping the temperature at 400 ºC, could not reduce the mismatch, though. ECN-RX

12 Figure 6 shows the adsorption and desorption profiles corresponding with the 20 th cycle of experiments using PURAL MG70 with different K 2 CO 3 loadings. For comparison, also the unloaded sample is included. It is seen that the CO 2 adsorption is enhanced by a factor 3 when loading the HTC with K 2 CO 3, implying that the original HTC structure is not simply restored upon contacting the calcined product with K 2 CO 3. The CO 2 adsorption is not very sensitive to the loading percentage (Table 4). The desorption profiles of the various samples are quite similar. The shift of the peak position is due to the extent of sorbent loading, not to differences in desorption properties of the various samples. The presence of water during adsorption is important. Without water, the CO 2 adsorption capacity is smaller than in the presence of water. At H 2 O fractions in the feed gas of 15% or higher, the CO 2 adsorption does not increase (Table 4). The temperature of the experiment, which refers to both the temperature of activation and the temperature at which the series of adsorption and desorption cycles is performed, does not have a strong effect on the CO 2 adsorption capacity. From 400 to 450 ºC, it hardly varies, while it has slightly decreased at 500 ºC (Table 4). The effect of water on CO 2 desorption is discussed below in the sorption-enhanced steam reforming experiment. As pointed out in the introduction, S/CO 2, the purge gas to adsorbed CO 2 ratio, is an important parameter. The desorption characteristic yields the variation of the fraction of desorbed CO 2 with S/CO 2. Figures 7 and 8 show the desorption characteristics of PURAL MG70 loaded with 22 wt% K 2 CO 3, determined at different desorption flows (t ads = t des = 75 min, F ads = 30 ml/min, F des = 10, 20, 30, 60, 100 ml/min) and desorption periods (t ads = 10 min, t des = 10, 25, 50, 75 min, F ads = F des = 30 ml/min). All characteristics rise above values of 100% implying that more CO 2 is desorbed than adsorbed. As pointed out above, this is a consequence of the release of CO 2 from the sorbent structure occurring during the desorption steps. The value of S/CO 2 for complete desorption, (S/CO 2 ) c, is in the range , which is clearly larger than the desired range 3-6. Decreasing the desorption flow results in a smaller (S/CO 2 ) c, though the CO 2 adsorption capacity is also reduced such that (S/CO 2 ) c shows a net decrease. When the desorption flow is decreased from 100 to 10 ml/min, (S/CO 2 ) c drops by approximately a factor 5. Apparently, mass transfer limitation occurs during CO 2 transport from the particle interior to the purge gas, because the amount of purge gas can be reduced by a factor 5 to remove the same amount of CO 2. The desorption characteristics shown in Figure 8 are getting steeper and (S/CO 2 ) c decreases by a about factor 3 when t des is reduced from 75 to 10 minutes. This indicates that the smaller part of the bed which is used, the more easily the adsorbed CO 2 can be removed from the bed. Finally, to deliver a proof-of-the-principle test for sorption-enhanced steam reforming of methane, an experiment has been performed using a mixture of 1.5 g low-temperature steam reforming catalyst and 3.0 g PURAL MG70 impregnated with 22 wt% K 2 CO 3. As before, the sample was subjected to a series of reaction/regeneration cycles at 400 ºC. Feed gas containing 2.9 % CH 4, 17.4% H 2 O and balance N 2 at a flow rate of 25 ml/min was fed to the sample during the reaction step, while the standard feed gas was used for regeneration (Table 3). The results of the last (100 th ) cycle are shown in Figure 9. More H 2 is produced during reaction than predicted by thermodynamic equilibrium without sorbent. An average CH 4 conversion of 95% is obtained (thermodynamic equilibrium conversion of CH 4 without sorbent: 53%), demonstrating the suitability of HTC for sorption-enhanced reforming of methane. In the same experiment, the effect of water on CO 2 desorption was investigated. An extra cycle was added with the same reaction conditions but with regeneration performed in 17.4% H 2 O containing N 2 gas. After the sample had been completely desorbed at these conditions (114 min), the H 2 O concentration was increased to 29%. A clear increase of the CO 2 flow is observed. 12 ECN-RX

13 7. Conclusions The selected high temperature CO 2 sorbent, Mg-Al HTC, is able to adsorb CO 2 at temperatures between 400 and 500 ºC. The adsorption is considerably enhanced by impregnating this HTC with K 2 CO 3. Loadings between 11 and 44 wt% have been tested, but the adsorption proved not to be very sensitive to the loading percentage. The adsorption, at least the part after breakthrough, and desorption profiles of the potassium-promoted HTC consist roughly of two parts: a part characterized by fast adsorption (desorption) and a part characterized by slow adsorption (desorption). In the performed experiments, desorption was never complete. The end of the desorption profiles did not go to zero and the desorbed amounts of CO 2 exceeded the adsorbed amounts of CO 2. This is most probably due to CO 2 released from the HTC structure. Water assists the CO 2 adsorption and desorption process. In the investigated temperature range, ºC, the CO 2 loading capacity hardly varies. Lowering the desorption flow or the duration of the desorption step, results in smaller values of (S/CO 2 ) c. A proof-of-the-principle test has shown the workability of SERP for the steam reforming of methane. List of symbols F ads = molar gas flow during the adsorption step (ml/min) F des = molar gas flow during the desorption step (ml/min) q ads (t) = specific adsorbed amount of CO 2 (mmol/g) S/CO 2 = purge steam to adsorbed CO 2 ratio (mol purge gas/mol adsorbed CO 2 ) (S/CO 2 ) c = purge steam to adsorbed CO 2 ratio for complete CO 2 removal (mol purge gas/mol adsorbed CO 2 ) t = time (min) t ads = duration of the adsorption step (min) t bt = breakthrough time (min) t des = duration of the desorption step (min) T ads = temperature during the adsorption step (ºC) T des = temperature during the desorption step (ºC) V ads = amount of adsorbed CO 2 (ml) V des = amount of desorbed CO 2 (ml) References (1) Matisson, T.;Zafar, Q.;Lyngfelt, A.; Gevert, B. Integrated hydrogen and power production from natural gas with CO 2 capture, in Proceedings of the 15 th International Hydrogen Conference, Yokomhama, 27 June- 2 July (2) Bracht, M. Water gas shift membrane reactor for CO 2 control in IGCC systems: technoeconomic feasibility study. Energy Conv. Management, 1997, 38 (supplement), (3) Dijkstra, J.W.; Jansen, D.; Leendertse, G.P.; Pex, P.P.A.C.; Hydrogen membrane reactor as key technology for sustainable use of fossil fuels, in Proceedings of the 1st European Hydrogen Energy Conference, Grenoble, 2-5 September, (4) Hufton, J.R.; Allam, R.J.; Chiang, R.; Middleton, P.; Weist, E.L.; White, V. Development of a process for CO 2 capture from gas turbines using a sorption enhanced water gas shift reactor system, in Proceedings of the 7th International Conference on Greenhouse Gas Technologies, Vancouver, September (5) Hufton, J.R.; Mayorga, S.; Sircar, S. Sorption-enhanced reaction process for hydrogen production. AIChE Journal, 1999, 45, ECN-RX

14 (6) Reijers, H.Th.J.; Roskam-Bakker, D.F.; Dijkstra, J.W.; Smit, R.; Groot, A. de; Brink, R.W. van den. Hydrogen production through sorption-enhanced reforming, in Proceedings of the 1st European Hydrogen Energy Conference, Grenoble, 2-5 September, (7) Balasubramanian, B.; Ortiz, A.L.; Kaytakoglu, S.; Harrison., D.P. Hydrogen from methane in a single-step process. Chem. Eng. Science, 1999, 54, (8) Harrison, D.P.; Peng, Z. Low-carbon monoxide hydrogen by sorption-enhanced reaction. Int. Journal Chem. Reactor Eng. 2003, 1, 1-9. (9) Meyer, J.; Eriksen, D.O.; Glöckner, R.; Ørjaster, B.E. Hydrogen production by integrated reforming and CO 2 capture, in Proceedings of the 1 st European Hydrogen Energy Conference, Grenoble, 2-5 September, (10) Waldron, W.F.; Hufton, J.R.; Sircar, S. Production of Hydrogen by Cyclic Sorption Enhanced Reaction Process. AIChE Journal, 2001, 47, (11) Ding,Y.; Alpay, E. Adsorption-enhanced steam-methane-reforming. Chem. Eng. Science, 2000, 55, (12) Ding, Y.; Alpay, E. Equilibria and kinetics of CO 2 adsorption on hydrotalcite adsorbent. Chem. Eng. Science, 2000, 55, (13) Mayorga S.G. Carbon dioxide adsorbents containing magnesium oxide suitable for use at high temperatures. Air Products and Chemicals Inc. Allentown (PA). US Patent no B1, (14) Kato, M.; Yoshikawa, S.; Nakagawa, K. Carbon dioxide absorption by lithium orthosilicate in a wide range of temperature and carbon dioxide concentrations. J. Mat. Sci. Letters. 2002, 21, (15) Nakagawa, K.; Kato, M.; Yoshikawa, S.; Essaki, K. A novel CO 2 absorbent using lithiumcontaining oxides, in Proceedings of the 2nd Annual Conference on Carbon Sequestration, Alexandria VA, 5-8 May (16) Stevens, J.F.; Atannassova, P.; Shen, J.P. Development of a 50 kw fuel processor for stationary fuel cell applications using revolutionary materials for absorption-enhanced natural gas reforming, in DOE Hydrogen Program FY2004 Progress Report, (17) Ding, Y.; Alpay, E. Equilibria and kinetics of high temperature CO 2 adsorption on hydrotalcite adsorbent, Chem. Eng. Science, 2000, 55, (18) Meyer, J.; Aaberg, R.J.; Andresen, B. Chapter 12 Generation of hydrogen fuels for the thermal power plant with integrated CO 2 capture using a CaO-CaCO 3 cycle, in Carbon dioxide capture for storage in deep geologic formations, volume 1. Elsevier: Oxford, (19) Cavani, F.; Trifirò, F.; Vaccari, A. Hydrotalcite-type anionic clays: preparation, properties and applications. Cat. Today, 1991, 11, (20) Di Cosimo J.I.; Diez, V.K.; Xu, M.; Iglesia, E.; Apesteguia, C.R. Structure and surface and catalytic properties of Mg-Al based basic oxides. J. Cat. 1998, 178, (21) Nataraj, S.; Carvill, B.T.; Hufton, J.R.; Mayorga, S.G.; Gaffney, T.R.; Brzozowski, J.R. Materials selectively adsorbing CO 2 from CO 2 containing streams. Air Products and Chemicals Inc., EP Patent no A1, ECN-RX

15 Table 1 Characteristics of CO 2 sorbent materials (*** = good, ** = fair, * = poor,?=unknown) group representative member ads.cap. stability kinetics metal oxides CaO *** * *** hydrotalcites Mg 6 Al 2 (OH) 16 [CO 3 ].4H 2 O/K 2 CO 3 * *** * double salts (K 2 CO 3 )(2KHCO 3 )(MgCO 3 )(MgO).xH 2 O **? ** Li metal oxides Li 4 SiO 4 ** ** *** supported sorbents CaO on Cabot Superior Micropowder 16 ** *** *** ECN-RX

16 Table 2 Specifications of HTCs according to the supplier spec. surface (m²/g) pore volume (g/cm³) bulk density (g/cm³) MgO:Al 2 O 3 (wt%:wt%) PURAL MG70 > 180 1) > 0.2 1) :30 PURAL MG61 HT 16 not specified :39 PURAL MG50 > 200 1) > 0.2 1) :50 PURAL MG30 2) > 250 1) > 0.5 1) :70 1) After 3 hours of activation at 550 ºC 2) PURAL MG30 contains a significant amount of boehmite 16 ECN-RX

17 Table 3 Standard experimental conditions adsorption desorption flow (ml/min) composition 5% CO 2 /29% H 2 O/66% N 2 29% H 2 O/71% N 2 T (ºC) duration (min) ECN-RX

18 Table 4 CO 2 adsorption capacities at breakthrough and during the whole adsorption step at various conditions t bt (min) q ads (t bt ) (mmol/g) q ads (t=75 min) (mmol/g) HTC sample PURAL MG PURAL MG61 HT PURAL MG PURAL MG ECN-HTC K 2 CO 3 loading (wt%) H 2 O content of feed gas (%) temperature (ºC) ECN-RX

19 1 H 2 + steam gas turbine SERP generator steam in adsorption natural gas air exhaust 2 steam SERP in regeneration water knock out CO 2 Figure 1 Schematic representation of a SERP system for electricity production ECN-RX

20 Al(OH) 6 - octahedron Mg(OH) 6 - octahedron H 2 O CO 2-3 Figure 2 Structure of hydrotalcite 20 ECN-RX

21 He liquid water through reactor valve reactor CH 4 water flow controller CO 2 N 2 CEM bypass MicroGC vent Mass flow controller permapure 7 vent flow meter Figure 3 Schematic diagram of the lab-scale experimental apparatus ECN-RX

22 5 4 CO 2 flow (ml/min) Elapsed time (min) Figure 4 Approximation of peak area given by the solid line (dashed line: curve fit, symbols:experimental data points) 22 ECN-RX

23 1.6 blank 1.2 PURAL MG70/22 wt% K 2 CO 3 PURAL MG61 HT/22 wt% K 2 CO PURAL MG50/22 wt% K 2 CO 3 ECN-HTC/22 wt% K 2 CO PURAL MG30/22 wt% K 2 CO 3 adsorption PURAL MG30/22 wt% K 2 CO 3 desorption 3 CO 2 flow (ml/min) 2 PURAL MG50/22 wt% K 2 CO 3 1 PURAL MG70/22 wt% K 2 CO 3 PURAL MG61 HT/22 wt% K 2 CO 3 blank Elapsed time (min) Figure 5 Adsorption and desorption profiles of various potassium- promoted HTC samples (the desorption profiles of ECN-HTC/22 wt% K 2 CO 3 coincides with that of PURAL MG70/22 wt% K 2 CO 3 ) ECN-RX

24 1.2 adsorption non-impregnated 44% % 11% % non-impregnated % desorption CO 2 flow (ml/min) % 11% 22% Elapsed time (min) Figure 6 Adsorption and desorption profiles of PURAL MG70 with different K 2 CO 3 loadings 24 ECN-RX

25 140 Desorption percentage (%) ml/min 20 ml/min 30 ml/min 60 ml/min 100 ml/min, beginning 100 ml/min, end Purge gas (mol gas/mol adsorbed CO 2 ) Figure 7 Desorption characteristics determined for different desorption flows (the dashed curve is a repetition of 100 ml/min at the end of the experiment) ECN-RX

26 ml/min 20 ml/min 30 ml/min 60 ml/min Desorptiepercentage (%) ml/min, beginning 100 ml/min, end Purge gas (mol gas/mol adsorbed CO 2 ) Figure 8 Desorption characteristics determined for different desorption times (t ads = 10 min, F des = 30 ml/min) 26 ECN-RX

27 1.6 reaction regeneration 16 CH 4,CO 2 concentration (%) CO 2 H 2 CH 4 CH 4 H 2 CO 2 solid line: experiment dashed line: calculated from thermodynamics without adsorbent present Elapsed time (min) Figure 9 Effluent gas composition during reaction and regeneration using a mixture of steam reforming catalyst and HTC impregnated with 22 wt% K 2 CO H 2 concentration (%) ECN-RX

28 % H 2 O in N 2 29% H 2 O in N 2 CO 2 flow (ml/min) Elapsed time (min) Figure 10 CO 2 flow during regeneration in 17.4% and 29% H 2 O containing N 2 gas 28 ECN-RX