CaO Sorbent Research Articles

Facile fabrication of mesoporous CaO sorbents using simple salt as a pore template in a template-assisted and spray-drying synthesis method

A combined templating and spray-drying synthesis technique has been proposed to fabricate a mesoporous CaO sorbent for CO2 capture applications. A simple inorganic salt, NaCl, was used as an alternative pore template to conventional, expensive organic templates. The effects of different NaCl-to-calcium precursor ratios (w/w) on porosity characteristics and CO2 capture performance of the synthesized sorbents were investigated. Cyclic CO2 capture performances of the sorbents were studied under mild and severe calcination conditions and compared with CaO derived from commercial CaCO3 (99%, Sigma–Aldrich) as a benchmark. Subsequent to 20 cycles of carbonation and severe calcination, CaO sorbent synthesized using a NaCl-to-CG weight ratio of 0.3 (denoted as CaO-0.3) exhibited a capture performance of 0.225±0.005g CO2/g sorbent whereas the benchmark sorbent exhibited a CO2 uptake of 0.082±0.002g CO2/g sorbent. This corresponds to maximum improvement of 165% in CO2 uptake under severe conditions for CaO-0.3 sorbent. Furthermore, the highest pore volume and pore area were also observed for CaO-0.3 sorbent at 17.6nm and 12.5nm, respectively. A BET surface area of 23±1m2/g and a total pore volume of 0.149±0.008cc/g were measured for this sorbent, which compares favourably with that of commercial CaCO3.

A shrinking core model for steam hydration of CaO-based sorbents cycled for CO2 capture

Calcium looping is a developing CO2 capture technology. It is based on the reversible carbonation of CaO sorbent, which becomes less reactive upon cycling. One method of increasing the reactivity of unreactive sorbent is by hydration in the calcined (CaO) form. Here, sorbent has been subjected to repeated cycles of carbonation and calcination within a small fluidised bed reactor. Cycle numbers of 0 (i.e., one calcination), 2, 6 and 13 have been studied to generate sorbents that have been deactivated to different extents. Subsequently, the sorbent generated was subjected to steam hydration tests within a thermogravimetric analyser, using hydration temperatures of 473, 573 and 673K. Sorbents that had been cycled less prior to hydration hydrated rapidly. However, the more cycled sorbents exhibited behaviour where the hydration conversion tended towards an asymptotic value, which is likely to be associated with pore blockage. This asymptotic value tended to be lower at higher hydration temperatures; however, the maximum rate of hydration was found to increase with increasing hydration temperature. A shrinking core model has been developed and applied to the data. It fits data from experiments that did not exhibit extensive pore blockage well, but fits data from experiments that exhibited pore blockage less well.

Fluidized-bed gasification combined continuous sorption-enhanced steam reforming system to continuous hydrogen production from waste plastic

This paper proposes a novel system for continuous hydrogen production from waste plastic (WP) by fluidized-bed gasification (FBG) combined sorption-enhanced steam reforming process (SERP). The system was operated in successive processes of several sections: (a) gasifying a WP into a raw gas comprising CO, H2, CH4, total hydrocarbons (THC), and small amount of HCl contaminant, etc.; (b) passing the raw gas to hydrogen production through two moving-bed reactors by continuous SERP process over Ni-based catalyst mixed CaO sorbent for in-situ CO2 capture and HCl removal; (c) simultaneously regenerating CaCO3 formed and catalyst with carbons deposited in other moving-bed reactor at the regeneration condition selected; and (d) carrying the particles of catalyst and sorbent to continuous steam reforming and their regeneration between two moving-bed reactors by riser. Gradually expanding chamber design of FBG reactor suitable for different particles flow to prolong the residence times of gas and solid phases makes high carbon conversion and the maximum value is up to 83.6% at 880 °C during FBG stage. The combination of FBG and SERP has produced a stream of high-purity hydrogen at some certain conditions, and about 88.4 vol % of hydrogen (H2O- and N2-free basis) was obtained at 818 °C of FBG temperature with 706–583 °C of SERP temperature. Reduced Ni-based catalyst efficiently converted raw gas from FBG and steam to H2, and CaO sorbent in the moving-bed reactor are capable of reducing the HCl and CO2 to low levels at all the temperatures tested.

Improved CO2 adsorption capacity and cyclic stability of CaO sorbents incorporated with MgO

The CO2 adsorption capacity of CaO sorbents with different MgO wt% (calcination temperature 800 °C, carbonation at 650 °C with 100% CO2, and de-carbonation at 800 °C in 100% N2).

Natural dolomite modified with carbon coating for cyclic high-temperature CO2 capture

An efficient MgO-stabilized CaO sorbent via the citric acid treated dolomite coupled with carbonization process (denoted as carbon coating) was developed for CO2 capture at high temperature. The citric acid was used as a carbon source to produce the coated carbon, which was expected to effectively prevent the crystallites (CaO and MgO) from sintering. The original dolomite and citric-acid-treated dolomite with subsequent calcination in air were also prepared for comparison. Different characterizations (thermal decomposition, phase composition, morphology, and nitrogen adsorption) were performed. The cyclic-CO2-capture performance was tested in a fix-bed reactor. During the carbonization in N2 atmosphere, the acidified dolomite transformed into carbon-coated, Mg-doped calcite, which could control thermal sintering and prevent the de-mixing of CaO and MgO in the secondary calcination step in air. Thus, the sorbent prepared using carbon coating achieved smaller grains, larger specific surface area and pore volume and more uniform distribution of CaO and MgO, which led to its superior activity and stability, compared to two reference sorbents.

Performance and economic assessments of a solid oxide fuel cell system with a two-step ethanol-steam-reforming process using CaO sorbent

The hydrogen production process is known to be important to a fuel cell system. In this study, a carbon-free hydrogen production process is proposed by using a two-step ethanol-steam-reforming procedure, which consists of ethanol dehydrogenation and steam reforming, as a fuel processor in the solid oxide fuel cell (SOFC) system. An addition of CaO in the reformer for CO2 capture is also considered to enhance the hydrogen production. The performance of the SOFC system is analyzed under thermally self-sufficient conditions in terms of the technical and economic aspects. The simulation results show that the two-step reforming process can be run in the operating window without carbon formation. The addition of CaO in the steam reformer, which runs at a steam-to-ethanol ratio of 5, temperature of 900 K and atmospheric pressure, minimizes the presence of CO2; 93% CO2 is removed from the steam-reforming environment. This factor causes an increase in the SOFC power density of 6.62%. Although the economic analysis shows that the proposed fuel processor provides a higher capital cost, it offers a reducing active area of the SOFC stack and the most favorable process economics in term of net cost saving.

Reactivity enhancement of calcium based sorbents by doped with metal oxides through the sol–gel process

The calcium-based sorbents are considered to be promising candidates for capturing CO2 from fossil fuel power plants and how to improve the CO2 capture and sintering-resistant performance of the sorbents during the cycling is a challenge for researchers. To improve the CO2 capture capacity and sintering resistant of the sorbents, CaO sorbents were synthesized by the sol–gel process with different precursors. Test matrices considering the important factors, such as dopant, doping amount, HNO3 addition and water addition in solvent were designed to investigate effects of synthesis conditions on reactivity of sorbents and to obtain an appropriate process for synthesizing sorbents for CO2 capture. The structural properties of the resulting sorbents were characterized by N2 physisorption and X-ray diffraction (XRD) techniques, showing that the sorbents prepared by the sol–gel process possessed wonderful microstructure as well as uniform distribution of CaO and MgO, MnO2 and TiO2. These features result in excellent reactivity enhancement and strong sintering resistant of sorbents compared with pure CaO, which give rise to high carbonation conversion, especially for CaO sorbents doped with MgO and MnO2, of 0.79 and 0.76 after 50 cycles under severe calcination condition, respectively. It demonstrates the excellent performance of sorbents synthesized by the sol–gel process.

Various Methods to Prepare High-Efficiency CaO Sorbents for Improved SO2 Capture Capacity

ABSTRACTThe reaction of calcium oxide with sulfur dioxide is one of the most important method for the flue gas desulfurization. However, the SO2-capturing capacity of calcium oxide is low because of rapid pore blockage by solid products. In the present study, various methods for producing more porous solid lime structures were investigated. These methods consisted of microwave heating, freezing/thawing, adding some decomposable materials, and acetic acid washing of lime. The SO2-capturing capacity of the produced sorbents were compared with the blank samples in a thermogravimetric analyzer at 850°C and 1000 ppm. The best performance was observed for the acid-washed lime. Finally, the experimental conversion-time profiles of natural and acid-washed limes were successfully predicted by modified grain model.

Performance of Coal Fly Ash Stabilized, CaO-based Sorbents under Different Carbonation–Calcination Conditions

The rapid deactivation of CaO sorbents due to sintering currently presents a major barrier to calcium looping for CO2 capture. In this work, we report an easy method for synthesizing highly stable CaO-based sorbents through mechanical mixing of a calcium precursor and coal fly ash (solid waste from coal-fired plants). To investigate the stable performance of as-synthesized sorbents, the effects of calcium precursors and carbonation–calcination conditions were studied and discussed. The synthetic sorbent derived from calcium oxalate (90%CaC2O4-FA-2h) showed the best performance and demonstrated a CO2 uptake of 0.38 g(CO2) g(CaO)−1 after 30 cycles. Even under the most severe calcination condition (at 920 °C in pure CO2), this sorbent maintained a stable capture capacity, with a final CO2 uptake of 0.27 g(CO2) g(CaO)−1 and an average decay rate of only 0.29% per cycle. Characterization of the sorbents confirmed that the formation of dispersed inert phase (gehlenite, Ca2Al2SiO7) was responsible for this high ...

Effect of Chemical and Physical Treatments on the Properties of a Dolomite Used in Ca Looping

Four diverse microstructured MgO-stabilized CaO sorbents with varying mixing characteristics of Ca and Mg were obtained from untreated, hydrated, precipitated, and milled dolomite. Different morphological characterizations (thermal decomposition, phase composition, morphology, and nitrogen adsorption) were performed, followed by an analysis of 30 carbonation/calcination cycles in a fixed-bed reactor. The mixed metal oxide (CaO–MgO) in the fresh calcined dolomite transformed into separate crystals of CaO and MgO in the cycled sorbent and resulted in a relative decrease in the cyclic CO2 capture capacity. Favorable structures (decreased crystallinity, increased porosity, and surface area) were generated by water hydration treatment, which was expected to enhance the recyclability, as suggested by some authors. However, this sorbent produced separate Mg and Ca as the major components, leading to a decrease in the CO2 capture compared to fresh calcined dolomite. Complete segregation between Ca and Mg was obse...

Sorption-enhanced reforming of bioethanol in dual fixed bed reactor for continuous hydrogen production

The sorption enhanced reforming (SER) of ethanol over an admixture of CaO sorbent and Ni reforming catalyst has been experimentally studied in a periodically operated dual fixed bed reactor. The optimal parameters for SER process were calculated from thermodynamic data. The experiment was carried out in cyclic mode; temperature swing adsorption and pressure swing adsorption (PSA) modes were used for periodic sorbent regeneration. It was shown that the continuous hydrogen production and the purity of hydrogen during the sorption-catalytic ethanol reforming maintained at the constant level and were 50 L/min and 97–99 vol.% respectively. The dual fixed bed SER reactor worked fine during 2 months on stream and more than 250 cycles. Both the sorbent and the catalyst maintained the sufficient activity during experiment. The obtained results suggest that sorption enhanced reforming may be a perspective approach for hydrogen production in fuel cell powered cogeneration systems.

Application of CaO Sorbent for the Implementation and Characterization of an in Situ Desulfurization Steam-Blown Bubbling Fluidized-Bed Test Rig for Biomass Gasification

In this work, a test rig is presented that is suitable for investigations on the equilibrium of high-temperature in situ desulfurization sorbents under real process conditions of a fluidized-bed gasifier. Gasification was performed using wood pellets as fuel with addition of a defined amount of CS2 to raise the content of sulfur without significantly changing the composition of permanent gases. The conversion of CS2 to H2S under gasification conditions ensures a high sulfur content in the produced gas in the range of 1200 ppmv, representing a synthesis gas derived from gasification of low-grade residual biomass. This type of gas is suitable for investigating the equilibrium of the desulfurization reaction under real process conditions, as the initial sulfur content is high enough to observe a desulfurization effect upon addition of potentially suitable sorbents. Furthermore, the low ash content of wood pellets ensures long-term stable gasification conditions. Investigations using lime as a sulfur sorbent ...

Biomass/coal steam co-gasification integrated with in-situ CO2 capture

Addressing recent environmental regulations on fossil fuel power systems and both biomass fuel supply and coal greenhouse gas issues, biomass/coal co-gasification could provide a feasible transition solution for power plants. In the quest for an even more sustainable process, steam co-gasification of switchgrass and coal was integrated with in-situ CO2 capture, with limestone as the bed material and sorbent. Five gasification/carbonation (at <700 °C) and calcination (at >850 °C) cycles were performed in an atmospheric pilot scale bubbling fluidized bed reactor. Hydrogen production was enhanced significantly (∼22%) due to partial adsorption of CO2 by the CaO sorbent, shifting the gasification reactions forward, consistent with Le Châtelier's principle. Tar yield measurements showed that reducing the gasification temperature could be achieved without experiencing higher tar yield, indicating that the lime has a catalytic effect. The sorbent particles decayed and lost their calcium utilization efficiency in the course of cycling due to sintering. The co-existence of three types of solids (biomass, coal, lime) with different particle properties led to bed segregation. An equilibrium model was found to be useful in design of lime-enhanced gasification systems.

Preparation of a Nano CaO Sorbent for Improvement the Capacity for CO2 Capture Reaction

In this work, carbonation reaction of a nano CaO sorbent derived from synthesized CaCO3 was investigated. Calcium carbonate was synthesized by the reaction of calcium nitrate and sodium bicarbonate. An ultrasonic device was used to produce fine grains of CaCO3. The sorbent was characterized by SEM, XRD, and nitrogen adsorption. The reaction of various CaO sorbents with CO2 was investigated in a thermogravimeter. The nano synthesized CaO sorbent showed an improved initial reaction rate for capturing CO2 with respect to other usual sorbents.

Thermodynamic analysis of hydrogen production via chemical looping steam methane reforming coupled with in situ CO2 capture

In this study, a detailed thermodynamic analysis of the sorption enhanced chemical looping reforming of methane (SE-CL-SMR), using CaO and NiO as CO2 sorbent and oxygen transfer material respectively, was conducted. The effect of different parameters, such as reactor temperature, pressure, H2O/CH4 ratio, CaO/CH4 ratio and CaO/NiO ratio was investigated. Moreover, the use of different sweep gases and oxidants for the re-oxidation/calcination cycle, like pure oxygen, air, steam and CO2, was specifically addressed. Conventional steam reforming (SMR) and sorption enhanced steam reforming (SE-SMR) were also investigated for comparison reasons.The results of thermodynamic analysis show that there are significant advantages of both sorption enhanced processes compared to conventional steam reforming. Presence of CaO sorbent in the reformer leads to higher methane conversion, hydrogen purity and yield at low temperatures (∼650°C). Addition of the oxygen carrier, in the chemical looping reforming concept, minimizes thermal requirements of the process, and results in superior performance compared to SE-SMR and SMR processes. A negative effect from NiO addition is reduction in hydrogen production (due to the reaction of part of methane with NiO to form CO/CO2). Hydrogen yield is up to 11% lower compared to SE-SMR for a NiO/CaO ratio of 0.7. It was found that only pure O2 can be used for re-oxidation/regeneration in order to reduce the energy requirements of the SE-CL-SMR process up to 26% compared to SE-SMR and up to 55% compared to conventional SMR.

Study of Al2O3 addition to synthetic Ca‐based sorbents for CO2 sorption capacity and stability in cyclic operations

AbstractSynthetic CaO sorbents were prepared using alumina as a sintering inhibitor via a simple precipitation method. The effects of three mixing procedures on the physical properties and CO2 capture performance of the sorbents were examined. The cyclic CO2 capture performance of the sorbent derived from the precipitation of calcium salts over colloidal alumina (highly dispersed alumina gel) showed the best performance of the three mixing methods. It was found that variation of alumina‐to‐CaO ratios did not significantly change the sintering influence on the sorbent capacity in cyclic operations. CaO particles homogeneously mingled with alumina at higher ratios. Sintering prevention, however, was not observed. This important observation indicates that alumina appeared to merely act as a binder for the fabrication of mechanically enhanced strength particles that are suitable for large‐scale operations. It was determined that CO2 uptake was not dependent on either the mixing technique or the type of synthetic materials incorporated into a sorbent in cyclic operation. The sorbent derived from the precipitation of calcium salts over colloidal alumina with an alumina‐to‐CaO ratio of 20:80 achieved the highest CO2 uptake of 13.1 moles/kg sorbent for half an hour of carbonation in the first cycle and retained a sorption capacity of 6.5 moles/kg sorbent after 17 successive cycles (50 % activity loss), which is in agreement with the reported results. It was demonstrated that the quantity of CO2 uptake increased moderately with decreasing sorbent particle sizes. The effect of pressure on sorbent CO2 uptake was insignificant.

A highly efficient CaO-based CO2 sorbent prepared by a citrate-assisted sol–gel technique

Abstract This work describes the development of an efficient Al-stabilized CaO sorbent for CO 2 capture. The sorbent (92.5 wt% CaO/7.5 wt% Al 2 O 3 ) was prepared using a citrate-assisted sol–gel technique followed by calcination in two steps, under inert and air atmospheres. The material was tested over 31 cycles of carbonation–calcination under mild and severe operating conditions. Under mild calcination condition (800–900 °C. N 2 flow), the working adsorption capacity was 0.57 g-CO 2 /g-ads over 31 cycles without any deactivation. The high stability was attributed to the favorable structural properties and high dispersion of Ca 9 Al 6 O 18 binder throughout the CaO matrix. Moreover, a number of parametric studies were performed to investigate the influence of carbonation and calcination temperatures on the activity of the material. Application of severe calcination condition (930 °C in the presence of 100% CO 2 ) yielded an uptake of 0.33 g-CO 2 /g-ads at the end of operation, indicating the superior activity and stability of the current material in comparison to previously reported Al-stabilized CaO adsorbents operated under similar conditions.

Porous spherical CaO-based sorbents via PSS-assisted fast precipitation for CO2 capture.

In this paper, we report the development of synthetic CaO-based sorbents via a fast precipitation method with the assistance of sodium poly(styrenesulfonate) (PSS). The effect of PSS on physical properties of the CaO sorbents and their CO2 capture performance were investigated. The presence of PSS dispersed the CaO particles effectively as well as increased their specific surface area and pore volume remarkably. The obtained porous spherical structure facilitated CO2 to diffuse and react with inner CaO effectively, resulting in a significant improvement in initial CO2 carbonation capacity. A proper amount of Mg(2+) precursor solution was doped during a fast precipitation process to gain CaO-based sorbents with a high anti-sintering property, which maintained the porous spherical structure with the high specific surface area. CaO-based sorbents derived from a MgxCa1-xCO3 precursor existed in the form of CaO and MgO. The homogeneous distribution of MgO in the CaO-based sorbents effectively prevented the CaO crystallite from growing and sintering, further resulting in the favorable long-term durability with carbonation capacity of about 52.0% after 30 carbonation/calcination cycles.

Thermodynamic analysis of H2production from CaO sorption-enhanced methane steam reforming thermally coupled with chemical looping combustion as a novel technology

In this novel paper, a technique for hydrogen production route of CaO sorption-enhanced methane steam reforming (SEMSR) thermally coupled with chemical looping combustion (CLC) was presented (CLC-SEMSR), which perceived as an improvement of previous methane steam reforming (MSR) thermally coupled with CLC technology (CLC-MSR). The application of CLC instead of furnace achieves the inherent separation of CO2 from flue gas without extra energy required. The required heat for the reformer is provided by thermally coupling CLC. The addition of CaO sorbents can capture CO2 as it is formed from the reformer gas to the solid phase, displacing the normal MSR equilibrium restrictions and obtaining higher purity of H2. The Aspen Plus was used to simulate this novel process on the basis of thermodynamics. The performances of this system examined included the composition of reformer gas, yield of reformer gas (YRg), methane conversion (αM), the overall energy efficiency (η), and exergy efficiency (φ) of this process. The effects of the molar ratio of CaO to methane for reforming (Ca/M) in the range of 0–1.2, the molar ratio of methane for combustion to methane for reforming (M(fuel)/M) in the range of 0.1–0.3, and the molar ratio of NiO to methane for reforming (Ni/M) in the range of 0.4–1.2 were investigated. It has been found to be favored by operating under the conditions of Ca/M = 1, M(fuel)/M = 0.2, and Ni/M = 0.8. The most excellent advantage of CLC-SEMSR was that it could obtain higher purity of H2 (95%) at lower operating temperature (655 °C), as against H2 purity of 77.1% at higher temperature (900 °C) in previous CLC-MSR. In addition, the energy efficiency of this process could reach 83.3% at the optimal conditions. Copyright © 2014 John Wiley & Sons, Ltd.

Performance of Hydration Reactivated Ca Looping Sorbents in a Pilot-Scale, Oxy-fired Dual Fluid Bed Unit

The progressive deactivation of CaO based sorbents is one of the major limitations of the Ca Looping cycle for postcombustion CO2 capture. Techniques using the hydration of the spent CaO sorbent have been identified as a promising route for the reactivation of CaO sorbents but have so far not been tested at pilot scale using realistic calcination and carbonation conditions. In this work, the performance of sorbents reactivated by two different reactivation techniques (hydration–dehydration and superheating) was assessed in an oxy-fired, pilot-scale dual fluid bed unit. Compared to a spent sorbent, reactivated materials exhibited a ≈60% increase in CO2 carrying capacity over 3 h of circulation as well as an increase in the sorbent attrition rate of 25% (superheating) and 50% (hydration–dehydration). In both cases, however, increased attrition did not lead to serious disruptions of system operation. A comparison of sorbent performance at laboratory and pilot scale suggested that high velocity impacts in the...