Experimental investigation of improved calcium-based CO2 sorbent and Co3O4/SiO2 oxygen carrier for clean production of hydrogen in sorption-enhanced chemical looping reforming

In this study, highly pure hydrogen is produced in sorption enhanced chemical looping steam methane reforming (SE-CLSMR) using cobalt-based oxygen carrier (OC) and cerium promoted CaO-based sorbent. In addition, the CO₂ removal from a gas stream at high temperatures is investigated via calcium looping process prior to SE-CLSMR process. The prepared samples are characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) and energy dispersive X-ray spectroscopy (EDX) techniques. The effect of Ca/Ce molar ratio (100/0.00–0.91/0.09), sorption temperature (550–650 °C) and sorbent lifetime are studied to find the optimal sorbent. The characterization results show the uniform and orderly CeO₂ dispersed sorbent nanoparticles that notably improved the sorbent morphology compared with blank CaO. The sorption results revealed the negative effect of temperature on CO₂ uptake of all the samples. In addition, the CO₂ sorption evaluations indicate that the molar ratio of cerium to calcium plays a significant role in the stability of sorbent and improved the CO₂ sorption capacity significantly. The high CO₂ removal efficiency in the cerium modified sorbents could be due to decrease in diffusion resistance of CO₂ through the sorbent structure during the carbonation reaction. Furthermore, results show that the addition of cerium to the sorbent structure, effectively improves the thermal resistance of synthesis sorbents. The SE-CLSMR results showed that the H₂ purity could be increased up to about 95% considering Co₃O₄/SiO₂ oxygen carrier and cerium promoted calcium-based sorbent at relatively low temperature of 550 °C, which is comparable with 84% in CLR process.     

Sorption-enhanced ethanol steam reforming on Ce-Ni/MCM-41 with simultaneous CO2 adsorption over Na- and Zr- promoted CaO based sorbent

In this study, sorption-enhanced ethanol steam reforming (SEESR) is investigated using a Ce-Ni/MCM-41 as a catalyst in the presence of Na or/and Zr promoted CaO-based adsorbents. Ce-Ni/MCM-41 and promoted sorbents were synthesized by wet impregnation method. The catalyst was characterized by XRD, FTIR, TGA, EFSEM, TEM, H₂-TPR and N₂ adsorption/desorption and promoted sorbents were studied by XRD, EFSEM, BET, TEM and TGA analysis. Sorption experiments were performed to verify sorbent activity for CO₂ removing. The results indicated that with doping different promoter on CaO sorbent and also with increasing Na loading, there was an increase in BET surface area, the reduction in particle size and thereupon an enhancement in CO₂ sorption capacity. Higher BET surface area, smaller particle size, and superior CO₂ sorption capacity were obtained on Na-Zr-CaO sorbent. Sorption-enhanced steam reforming process of ethanol on synthesized catalyst and sorbents were performed at 600 °C and water to ethanol molar ratio of 6. The effect of sorbent to catalyst ratio and the arrangement of sorbent and catalyst (like two separated layers and the mixture of sorbent and catalyst in a single layer) were also studied. The best results were demonstrated on Na-Zr-CaO sorbent and with the separated array. Hydrogen production via a SEESR process with Na-Zr-CaO as sorbent was ∼94% that is 24% more than that of conventional ethanol steam reforming (ESR) reaction.     

A novel catalyst–sorbent system for an efficient H2 production with in-situ CO2 capture

This paper presents an experimental investigation for an improved process of sorption-enhanced steam reforming of methane in an admixture fixed bed reactor. A highly active Rh/Ce_αZr_1−αO₂ catalyst and K₂CO₃-promoted hydrotalcite are utilized as novel catalyst/sorbent materials for an efficient H₂ production with in situ CO₂ capture at low temperature (450–500 °C). The process performance is demonstrated in response to temperature (400–500 °C), pressure (1.5–6.0 bar), and steam/carbon ratio (3–6). Thus, direct production of high H₂ purity and fuel conversion >99% is achieved with low level of carbon oxides impurities (<100 ppm). A maximum enhancement of 162% in CH₄ conversion is obtained at a temperature of 450 °C and a pressure of 6 bar using a steam/carbon molar ratio of 4. The high catalyst activity of Rh yields an enhanced CH₄ conversion using much lower catalyst/sorbent bed composition and much smaller reactor size than Ni-based sorption enhanced processes at low temperature. The cyclic stability of the process is demonstrated over a series of 30 sorption/desorption cycles. The sorbent exhibited a stable performance in terms of the CO₂ working sorption capacity and the corresponding CH₄ conversion obtained in the sorption enhanced process. The process showed a good thermal stability in the temperature range of 400–500 °C. The effects of the sorbent regeneration time and the purge stream humidity on the achieved CH₄ conversion are also studied. Using steam purge is beneficial for high degree of CO₂ recovery from the sorbent.     

Effect of Ni precursor salts on Ni-mayenite catalysts for steam methane reforming and on Ni-CaO-mayenite materials for sorption enhanced steam methane reforming

In view of climate change containment, sorption enhanced steam methane reforming (SESMR) appears as an interesting production route for H₂ with the additional advantage of CO₂ capture application performed by high-temperature solid sorbents. CaO is largely employed as CO₂ sorbent because of its low-cost mineralized forms (limestone and dolomite), of its high sorption capacity in the high temperature range compatible with steam methane reforming (SMR). Many recent studies have proposed purposely synthesized Ni-based reforming catalysts, used with high-temperature CO₂ solid sorbents, or combined sorbent-catalyst materials (CSCM). For this last purpose, we studied the effect of Ni salt precursor (Ni nitrate hexahydrate or Ni acetate tetrahydrate) on properties and reactivity of Ni-mayenite catalysts or Ni-CaO-mayenite CSCM, synthesized by an already validated sequence of wet mixing (for sorbents synthesis) and wet impregnation (for catalysts and CSCM synthesis) methods. Although Ni acetate tetrahydrate was often reported as the best choice to improve textural properties, our study identified Ni nitrate hexahydrate as a definitely more suitable precursor than Ni acetate tetrahydrate in the purpose of developing efficient materials for SESMR. The dissimilar behaviors observed in reforming reactivity are related and explained by the differences in textural properties, Ni species dispersion, and reducibility.     

Sorption enhanced steam hydrogasification of coal for synthesis gas production with in-situ CO2 removal and self-sustained hydrogen supply

The in-situ removal of CO₂ and the increase of the energetic gas yield, including hydrogen and methane, by sorption enhanced steam hydrogasification (SE-SHR) process were investigated. Lignite was used in this study as the feedstock to the steam hydrogasification reaction (SHR) with the addition of calcined dolomite as a sorbent. CO₂ was reduced dramatically with the introduction of the sorbent into the reactor. The production of hydrogen and methane was increased simultaneously. The hydrogen yield was increased by 60% when the calcium oxide to carbon molar ratio was increased to 0.86 as compared to the results without the sorbent. The hydrogen in the product gas was sufficient to maintain a self-sustained supply back to the SHR when the calcium oxide to carbon molar ratio was over 0.29. The performance of the SE-SHR was determined at different temperatures ranging from 650 °C to 800 °C and at different steam to carbon molar ratios. Additionally, the char conversion was also enhanced in all cases with the sorbent introduction. The synthesis gas production using SE-SHR coupled with steam methane reforming was also modeled by Aspen Plus. The simulation results showed that the H₂/CO ratio of the synthesis gas generated based on SE-SHR process was 6 with higher overall energy efficiency of 74.5%. Summarily, the main findings of this study were that the overall performance of the SE-SHR was substantially improved compared to the conventional operation of the SHR and the quality of synthesis gas produced based on SE-SHR process was more flexible for the downstream processing.     

Development of a Fe/Mg-bearing metallurgical waste stabilized-CaO/NiO hybrid sorbent-catalyst for high purity H2 production through sorption-enhanced glycerol steam reforming

In this work, a Fe/Mg-bearing metallurgical waste (upgraded slag oxide, UGSO) was, for the first time, investigated as a stabilizer for increasing the cyclic stability of CaO-based sorbents. The sorbents were prepared through the wet mixing of the ball-milled UGSO particles with the limestone-derived calcium citrate under sonication. The sorption capacity of samples containing different waste loadings (5, 10, 15, and 25 wt%) was studied for 18 carbonation/regeneration cycles under conditions similar to the sorption-enhanced glycerol steam reforming process. A significant improvement of the cyclic stability was observed for all doped sorbents; however, the sample with 10 wt% UGSO showed the highest sorption capacity among all tested samples. This optimum sorbent was further used to synthesize a UGSO stabilized CaO–NiO hybrid sorbent-catalyst material (20 wt% NiO loading), whose performance was tested in sorption-enhanced steam reforming of glycerol. A H₂ purity of around 95% was obtained in the pre-breakthrough period that lasted for about 30 min. In summary, the results showed a better stability of UGSO stabilized sorbents compared to pure CaO and a good performance of the CaO-UGSO10/NiO sorbent-catalyst hybrid material in the sorption-enhanced reforming process.     

High capacity potassium-promoted hydrotalcite for CO2 capture in H2 production

This paper presents an experimental study for a newly modified K₂CO₃-promoted hydrotalcite material as a novel high capacity sorbent for in-situ CO₂ capture. The sorbent is employed in the sorption enhanced steam reforming process for an efficient H₂ production at low temperature (400–500 ^°C). A new set of adsorption data is reported for CO₂ adsorption over K-hydrotalcite at 400 °C. The equilibrium sorption data obtained from a column apparatus can be adequately described by a Freundlich isotherm. The sorbent shows fast adsorption rates and attains a relatively high sorption capacity of 0.95 mol/kg on the fresh sorbent. CO₂ desorption experiments are conducted to examine the effect of humidity content in the gas purge and the regeneration time on CO₂ desorption rates. A large portion of CO₂ is easily recovered in the first few minutes of a desorption cycle due to a fast desorption step, which is associated with a physi/chemisorption step on the monolayer surface of the fresh sorbent. The complete recovery of CO₂ was then achieved in a slower desorption step associated with a reversible chemisorption in a multi-layer surface of the sorbent. The sorbent shows a loss of 8% of its fresh capacity due to an irreversible chemisorption, however, it preserves a stable working capacity of about 0.89 mol/kg, suggesting a reversible chemisorption process. The sorbent also presents a good cyclic thermal stability in the temperature range of 400–500 °C.     

Hydrogen production via steam reforming of coke oven gas enhanced by steel slag-derived CaO

Steel slag, a waste from steelmaking plant, has been proven to be good candidate resources for low-cost calcium-based CO₂ sorbent derivation. In this work, a cheap and sintering-resistance CaO-based sorbent (CaO (SS)) was prepared from low cost waste steel slag and was applied to enhance catalytic steam reforming of coke oven gas for production of high-purity hydrogen. This steel slag-derived CaO possessed a high and stable CO₂ capture capacity of about 0.48 g CO₂/g sorbent after 35 adsorption/desorption cycles, which was mainly ascribed to the mesoporous structure and the presence of MgO and Fe₂O₃. Product gas containing 95.8 vol% H₂ and 1.4 vol% CO, with a CH₄ conversion of 91.3% was achieved at 600 °C by steam reforming of COG enhanced by CaO (SS). Although high temperature was beneficial for methane conversion, CH₄ conversion was remarkably increased at lower operation temperatures with the promotion effects from CaO (SS), and CO selectivity has been also greatly decreased. Reducing WHSV could increase methane conversion and reduce CO selectivity due to longer reactants residence time. Reducing C/A could increase methane conversion and hydrogen recovery factor, and also decrease CO selectivity. When being mixed with catalyst during SE-SRCOG, CaO (SS) with a uniform size distribution favored methane conversion due to the high utilization efficiency of catalyst. Promising stability of CaO (SS) in cyclic reforming/calcination tests was evidenced with a hydrogen recovery factor >2.1 and CH₄ conversion of 82.5% at 600 °C after 10 cycles using CaO (SS) as sorbent.     

Thermally moderated hollow fiber sorbent modules in rapidly cycled pressure swing adsorption mode for hydrogen purification

We describe thermally moderated multi-layered pseudo-monolithic hollow fiber sorbents entities, which can be packed into compact modules to provide small-footprint, efficient H₂ purification/CO₂ removal systems for use in on-site steam methane reformer product gas separations. Dual-layer hollow fibers are created via dry-jet, wet-quench spinning with an inner “active” core of cellulose acetate (porous binder) and zeolite NaY (69 wt% zeolite NaY) and an external sheath layer of pure cellulose acetate. The co-spun sheath layer reduces the surface porosity of the fiber and was used as a smooth coating surface for a poly(vinyl-alcohol) post-treatment, which reduced the gas permeance through the fiber sorbent by at least 7 orders of magnitude, essentially creating an impermeable sheath layer. The interstitial volume between the individual fibers was filled with a thermally-moderating paraffin wax. CO₂ breakthrough experiments on the hollow fiber sorbent modules with and without paraffin wax revealed that the “passively” cooled paraffin wax module had 12.5% longer breakthrough times than the “non-isothermal” module. The latent heat of fusion/melting of the wax offsets the released latent heat of sorption/desorption of the zeolites. One-hundred rapidly cycled pressure swing adsorption cycles were performed on the “passively” cooled hollow fiber sorbents using 25 vol% CO₂/75 vol% He (H₂ surrogate) at 60 °C and 113 psia, resulting in a product purity of 99.2% and a product recovery of 88.1% thus achieving process conditions and product quality comparable to conventional pellet processes. Isothermal and non-isothermal dynamic modeling of the hollow fiber sorbent module and a traditional packed bed using gPROMS^® indicated that the fiber sorbents have sharper fronts (232% sharper) and longer adsorbate breakthrough times (66% longer), further confirming the applicability of the new fiber sorbent approach for H₂ purification.     

Transient reaction phenomena of sorption-enhanced steam methane reforming in a fixed-bed reactor

The transient chemical reaction phenomena of the sorption-enhanced steam methane reforming (SE-SMR) by using Ni/Al₂O₃ catalyst and CaO sorbent in a tubular fixed-bed reactor were numerically investigated by an experimentally verified unsteady 2D model. Four chemical reactions are involved in SE-SMR including steam reforming (SR), water gas shift (WGS), global steam reforming (GSR), and CO₂ sorption. The reaction process in time is divided into period 1, transient period, and period 2. The high-purity H₂ is produced in period 1 which is defined as the outlet molar fractions of H₂ ≥ 90% and CO ≤ 1% (dry basis) in this work. In the first half of period 1, the endothermic reaction rates of SR and GSR are dominant in the entrance region of catalyst/sorbent bed. The WGS and CO₂ sorption reactions are triggered by SR and GSR reactions. The heat transfer from the wall plays an important role. Higher CaO conversion, temperature, and reaction rates appear first near the wall region, then they gradually expand to the central region.In the second half of period 1, a sharp wave-shaped curve of strong CO₂ sorption reaction occurred in downstream becomes dominant and it moves to downstream almost at a constant speed, as time progresses. The peak value of the CO₂ sorption reaction is more than twice larger than that of SR or WGS. The SR and WGS reaction rates are significantly enhanced by CO₂ sorption reaction. The great sorption, WGS, and SR reactions result in a high-purity H₂ production with the outlet molar fractions of 95.8% H₂, 0.998% CO, and 0.73% CO₂ at the end of period 1, based on the parameters used in this work such as reactor temperature of 600 °C. The maximum CaO conversion is about 76% in end of period 1 and the average CaO conversion in the reactor is 51%. The 2D distributions of CaO conversion, temperature, and reaction rates are also presented and discussed.     

Production of high purity hydrogen by sorption enhanced steam reforming of crude glycerol

Here we report effective production of pure hydrogen from crude glycerol by the one-stage sorption enhanced steam reforming (SESR) process. This process yielded H₂ up to 88% with a very high purity (99.7 vol%) at atmospheric pressure and at 550–600 °C with a steam/C = 3 in a fixed-bed reactor over a mixture of Ni/Co catalyst derived from hydrotalcite-like material (HT) and dolomite as CO₂ sorbent. The concentration of methane is lowest at 575 °C, while the CO concentration increases concurrently with increasing temperature from 525 to 600 °C. The high coking potential of glycerol and fatty acid methyl esters (C₁₇–C₁₉) resulted in the increased formation of coke, thus lower hydrogen yield. The reaction rates of methane reforming and water–gas shift reactions are much higher than the steam reforming of crude glycerol on Co–Ni catalysts. The high purity of hydrogen can be obtained even at low spatial times with low crude glycerol conversions. Our work reveals a great potential to directly convert biomass derived complex mixtures to the most clean energy carrier of hydrogen with high yield and purity.     

Sorption enhanced steam reforming of ethanol for hydrogen production,over Mg/Al hydrotalcites modified with K

Sorption-enhanced ethanol steam reforming is an interesting alternative, to produce high purity H₂. In this study, potassium promoted hydrotalcites are compared for sorption-enhanced ethanol steam reforming reaction under cyclic operation, performing sorbent regeneration at reaction temperature which is a great advantage to reduce process energy requirements. It is found that potassium promoted hydrotalcites have higher CO₂ sorption capacity compared to unpromoted ones, due to the higher concentration of intermediate and strong basic sites. The hydrotalcite modified with 15 wt% potassium shows the best performance on multicyclic CO₂ sorption-desorption (sorption capacity = 0.167 mol_CO2/kgsorbent). Therefore, there is an optimum loading of potassium, for which the opposite effects of reduction in surface area and enhanced basicity are balanced. Finally, potassium promoted hydrotalcites are tested under cyclical ethanol reforming process with simultaneous adsorption of CO₂ followed by regeneration in N₂ at reaction temperature (500 °C). At short reaction times (<5 min), H₂ purities higher than 95% are achieved, with CO₂ purities near 0%.     

Sorption-enhanced reaction process for glycerol-to-hydrogen conversion over cobalt catalyst supported on promoted hydrotalcites

New bi-functional materials comprising the reforming catalyst, cobalt, and the CO₂-sorbent, hydrotalcite were used to produce pure hydrogen (H₂) from sorption-enhanced steam glycerol reforming (SESGR). Three promoters, calcium, copper and zinc, were used for modifying the properties of hydrotalcites. All materials were characterized using X-ray diffraction, nitrogen physisorption and electron microscopy techniques. They were found to be very proficient for glycerol-to-H₂ conversion in a fixed-bed reactor, even at low temperature (623–823 K). Copper-promoted materials were especially promising, due to longest duration of the pre-breakthrough stage (40 min) and highest H₂ content of the reformed gas (93.1%) at T = 823 K. Besides, their sorption capacity was the highest (1.1 mol CO₂/kg sorbent) at T = 823 K. The effects of temperature, steam-to-carbon ratio in feed (S/C ratio) and gas hourly space velocity (GHSV) on the SESGR process were investigated. Durability tests over 20 cycles of adsorption and regeneration showed that materials promoted with calcium, copper and zinc were stable up to 8 (at 773 K), 11 and 5 cycles (at 823 K) correspondingly. The role of cobalt metal and cationic hydrotalcite promoters in the reforming pathway was elucidated. This insightful study will assist in improved H₂ production from renewably producible glycerol.     

Thermodynamic performance assessment and comparison of IGCC with solid cycling process for CO2 capture at high and medium temperatures

Solid sorbents can be used to capture CO₂ from pre-combustion sources at various temperatures. MgO and CaO are typical medium- and high-temperature CO₂ sorbents. However, pure MgO is not active toward CO₂. The addition of Na₂CO₃ increases the operating temperature and significantly increases the reactivity of sorbents to capture CO₂. Na₂CO₃-promoted MgO is a promising medium-temperature CO₂ sorbent. In this study, the thermodynamic performance of integrated gasification combined cycle (IGCC) systems with Na₂CO₃–MgO-based warm gas decarbonation (WGDC) and CaO-based hot gas decarbonation (HGDC) is evaluated and compared with that of an IGCC system with methyldiethanolamine (MDEA)-based cold gas decarbonation (CGDC). Assuming that the average CO₂ capture capacities of solid sorbents are one-third of their theoretical maxima, we reveal that the IGCC system undergoes approximately 2.8% and 3.6% improvement on net efficiency when switching from CGDC to WGDC and to HGDC, respectively. The net efficiency of the system is increased by improving the CO₂ capture capacity of the sorbent. The IGCC with Na₂CO₃–MgO experiences more significant increase in efficiency than that with CaO along with the improvement of sorbent average CO₂ capture capacity. The efficiency of the IGCC systems reaches the same value when the average CO₂ capture capacities of both sorbents are 53% of their theoretical levels. The effects of gas turbine combustor fuel gas inlet temperature on IGCC system performance are analyzed. Results show that the efficiency of the IGCC systems with HGDC and WGDC increases by 0.74% and 0.53% respectively as the fuel gas inlet temperature increases from 250 °C to 650 °C.     

Enhanced hydrogen-rich gas production from steam gasification of coal in a pressurized fluidized bed with CaO as a CO2 sorbent

Steam gasification of a typical Chinese bituminous coal for hydrogen production in a lab-scale pressurized bubbling fluidized bed with CaO as CO₂ sorbent was performed over a pressure range of ambient pressure to 4 bar. The compositions of the product gases were analyzed and correlated to the gasification operating variables that affecting H₂ production, such as pressure (P), mole ratio of steam to carbon ([H₂O]/[C]), mole ratio of CaO to carbon ([CaO]/[C]) and temperature (T). The experimental results indicated that the H₂ concentration was enhanced by raising the temperature, pressure and [H₂O]/[C] under the circumstances we observed. With the presence of CaO sorbent, CO₂ in the production gas was absorbed and converted to solid CaCO₃, thus shifting the steam reforming of hydrocarbons and water gas shift reaction beyond the equilibrium restrictions and enhancing the H₂ concentration. H₂ concentration was up to 78 vol% (dry basis) under a condition of 750 °C, 4 bar, [Ca]/[C] = 1 and [H₂O]/[C] = 2, while CO₂ (2.7 vol%) was almost in-situ captured by the CaO sorbent. This study demonstrated that CaO could be used as a substantially excellent CO₂ sorbent for the pressurized steam gasification of bituminous coal. For the gasification process with the presence of CaO, H₂-rich syngas was yielded at far lower temperatures and pressures in comparison to the commercialized coal gasification technologies. SEM/EDX and gas sorption analyses of solid residues sampled after the gasification showed that the pore structure of the sorbent was recovered after the steam gasification process, which was attributed to the formation of Ca(OH)₂. Additionally, a coal-CaO–H₂O system was simulated with using Aspen Plus software. Calculation results showed that higher temperatures and pressures favor the H₂ production within a certain range.     

Effects of impregnating variables on dynamic sorption characteristics and storage properties of composite sorbent for solar heat storage

A family of composite sorbents was prepared by impregnating silica gel in the solution of the hygroscopic salt CaCl₂ for solar heat storage. The characteristics of water adsorbed on the composite sorbents prepared under different impregnating conditions were measured by a micromeritics gas adsorption analyzer, a Calvet-type microcalorimeter and an open-type gravimetric method. From the results of these dynamic sorption measurements, the effects of impregnating variables on the characteristics of water adsorbed on the composite sorbents were evaluated. The composite sorbents prepared under different impregnating conditions were also tested on an open-type sorption storage system. The composite sorbent prepared by impregnating in the CaCl₂ solution of 30% showed a high and stable storage capacity of 1020 J g⁻¹ at the charging temperature of about 90 °C. This study demonstrates a great potential in controlling the sorption characteristics as well as the storage properties of the composite sorbents by optimizing the impregnating variables to meet the specific demands of solar heat storage.     

High hydrogen yield and purity from palm empty fruit bunch and pine pyrolysis oils

The benefits of CO₂ sorption enhanced steam reforming using calcined dolomite were demonstrated for the production of hydrogen from highly oxygenated pyrolysis oils of the agricultural waste palm empty fruit bunches (PEFB) and pine wood. At 1 atm in a down-flow packed bed reactor at 600 °C, the best molar steam to carbon ratios were between 2 and 3 using a Ni catalyst. After incorporating steam-activated calcined dolomite as the CO₂ sorbent in the reactor bed, the H₂ yield from the moisture free PEFB oil increased from 9.5 to 10.4 wt.% while that of the pine oil increased from 9.9 to 13.9 wt.%. The hydrogen purity also rose from 68 to 96% and from 54 to 87% for the PEFB and pine oils respectively, demonstrating very substantial sorption enhancement effects.     

Continuous high-purity hydrogen production by sorption enhanced steam reforming of ethanol over modified lithium silicate

Sorption-enhanced steam reforming of ethanol (SE-SRE) with in-situ CO₂ removal is an environmentally friendly and sustainable approach for hydrogen production. Researches on continuous production of high-purity H₂ by SE-SRE over the modified Li₄SiO₄ sorbent were conducted using two parallel reactor in this work. The low cost Li₄SiO₄ derived from rice husk ash (RHA) is a promising high-temperature CO₂ sorbent. However, the poor adsorption kinetics of RHA-Li₄SiO₄ sorbent at low CO₂ concentration is the major challenge. The metallic elements (K, Ca, Al, Mg) were employed to modify the RHA-Li₄SiO₄ for efficient CO₂ capture. The developed sorbents were characterized and tested to study the role of dopants on the crystal, textural, microstructure and CO₂ adsorption kinetics and cyclic stability. Results indicated that K doping effectively inhibited the growth of crystal aggregation and resulted in a fluffy morphology with abundant pores and higher specific surface area, while the addition of Ca, Al and Mg formed a nubby structure with larger particle size. K-doped RHA-Li₄SiO₄ exhibited the best CO₂ uptake properties and the optimal K doping molar content was 0.02 with the maximum capture capacity of 34.16 wt%, which is higher than 27.1 wt% of pure RHA-Li₄SiO₄. Then, the effect of operating conditions on the enhancement behaviors was considered in the SE-SRE system. High-purity H₂ (above 96%) was achieved by coupling K(0.02)/RHA-Li₄SiO₄ sorbent with Ni-based catalyst under the optimum condition (T: 525 °C, liquid hourly space velocity: 0.9 mL/(g·h), sorbent/catalyst: 4 and steam/carbon: 8.0). The adsorption activity of K(0.02)/RHA-Li₄SiO₄ maintained at a high level in ten SE-SRE/regeneration cycles. Finally, a scheme including two parallel fixed-bed reactors was designed and operated periodically for continuous production of high-purity H₂. The reaction switching time was shown to depend strongly on the pre-breakthrough time and operating conditions. As the reaction switching time was 40 min, the products were always only H₂ and CH₄ (no CO and CO₂ appear) and the H₂ purity remained above 90% during 400 min, confirming high purity hydrogen stream can be obtained continuously.     

The effect of non-uniform temperature on the sorption-enhanced steam methane reforming in a tubular fixed-bed reactor

The effect of non-uniform temperature on the sorption-enhanced steam methane reforming (SE-SMR) in a tubular fixed-bed reactor with a constant wall temperature of 600 °C is investigated numerically by an experimentally verified unsteady two-dimensional model. The reactor uses Ni/Al₂O₃ as the reforming catalyst and CaO as the sorbent. The reaction of SMR is enhanced by removing the CO₂ through the reaction of CaO + CO₂ → CaCO₃ based on the Le Chatelier's principle. A non-uniform temperature distribution instead of a uniform temperature in the reactor appears due to the rapid endothermic reaction of SMR followed by an exothermic reaction of CO₂ sorption. For a small weight hourly space velocity (WHSV) of 0.67 h^?1 before the CO₂ breakthrough, both a low and a high temperature regions exist simultaneously in the catalyst/sorbent bed, and their sizes are enlarged and the temperature distribution is more non-uniform for a larger tube diameter (D). Both the CH₄ conversion and the H₂ molar fraction are slightly increased with the increase of D. Based on the parameters adopted in this work, the CH₄ conversion, the H₂ and CO molar fractions at D = 60 mm are 84.6%, 94.4%, and 0.63%, respectively. After CO₂ breakthrough, the reaction of SMR dominates, and the reactor performance is remarkably reduced due to low reactor temperature.For a higher value of WHSV (4.03 h^?1) before CO₂ breakthrough, both the reaction times for SMR and CO₂ sorption become much shorter. The size of low temperature region becomes larger, and the high temperature region inside the catalyst/sorbent bed doesn't exist for D ≥ 30 mm. The maximum temperature difference inside the catalyst/sorbent bed is greater than 67 °C. Both the CH₄ conversion and H₂ molar fraction are slightly decreased with the increase of D. However, this phenomenon is qualitatively opposite to that for small WHSV of 0.67 h^?1. The CH₄ conversion and H₂ molar fraction at D = 60 mm are 52.6% and 78.7%, respectively, which are much lower than those for WHSV = 0.67 h^?1.     

Comparative exergy analysis of sorption enhanced and conventional methane steam reforming

Exergy efficiency analysis tool is used to evaluate sorption enhanced steam reforming in comparison with the industrial hydrogen production route, steam reforming. The study focuses on hydrogen production for use in high pressure processes. Thermodynamic sensitivity analysis (effect of reforming temperature on hydrogen yield and reforming enthalpy) was performed to indicate the optimum temperature (650 °C) for the sorption enhanced reforming. The pressure was selected to be, for both cases, 25 bar, a typical pressure used in the industrial (conventional) process. Atmospheric pressure, 1000 °C and CO₂ as inert gas were specified as the optimum operating parameters for the regeneration of the sorbent after performing exergy efficiency analysis of three realistic case scenarios. Aspen Plus simulation process schemes were built for conventional and sorption enhanced steam reforming processes to attain the mass and energy balances required to assess comparatively exergy analysis. Simulation results showed that sorption enhanced reforming can lead to a hydrogen purity increase by 17.3%, along with the recovery of pure and sequestration-ready carbon dioxide. The exergy benefit of sorption enhanced reforming was calculated equal to 3.2%. Analysis was extended by adding a CO₂ separation stage in conventional reforming to reach the hydrogen purity of sorption enhanced reforming and enable a more effective exergy efficiency comparison. Following that analysis, sorption enhanced reforming gained 10.8% in exergy efficiency.