CaO-based Sorbents Research Articles

Research on the preparation of CO2 renewable sorbent from calcium-based waste: Towards enhanced biomass gasification for H2 production

The potential of industrial waste—carbide slag (CS) as a high-temperature CaO-based CO2 sorbent and its anti-sintering modification strategies are explored by examining the changes in maximum carbonation conversion (Xn,max) during multiple carbonation/calcination cycles. Although the pristine CS performs satisfactorily during initial carbonation, its Xn,max steadily declines with increasing cycle numbers due to sintering. To develop CO2 sorbent with robust, stable structures, various refractory dopants, including Al2O3, MgO, SiO2, Y2O3, and two industrial by-products rich in Al2O3 and SiO2—coal gangue (CG) and fly ash (FA)—are utilised to modify the CS. The SiO2-doped CS sorbent exhibits superior CO2 cycling adsorption capacity under identical testing conditions, attributed to the formation of Ca2SiO4 with a high Tammann temperature (TT) during high-temperature calcination. Notably, CSCG5 (CS/CG mass ratio of 95:5) presents a CO2 capture capacity comparable to CSSi2 (CS/SiO2 mass ratio of 98:2), providing a potential solution for low-cost and anti-sintering modification of CS sorbents. Relative to pure CaO or CS, the CSSi2 and CSCG5 samples exhibit superior stability in terms of H2 concentration and yield throughout the biomass cyclic hydrogen production tests. Adsorption kinetic analysis results suggest that the carbonation of CS sorbent during rapid reaction predominantly adheres to the Pseudo-first-order, Elovich, and Intra-particle diffusion models. The carbonation conversion (Xt) is excellently fitted (R2 > 0.99) by a Logistic equation considering CO2 concentration (xCO2) and carbonation temperature (Tcar). Moreover, comprehensive data analysis yields an innovative semi-empirical equation suitable for predicting the Xn,max during long-cycle carbonation/calcination processes. We also discussed the sintering and modification mechanisms of CS sorbent in depth.

CO2 capture performance of CaO-based sorbent modified with torrefaction condensate during calcium looping cycles

In this work, the organic acid-rich torrefaction condensate (TC) from biomass torrefaction process was attempted to replace commercial acetic acid (AC) for acidification treatment of CaO sorbent to improve its CO2 capture performance during calcium looping cycles. The results showed that TC-modified sorbents exhibited a much slower carbonation reactivity decay than the AC-modified one over the cycles. According to the results of characterizations (thermal decomposition, morphology, phase composition, and N2 adsorption), the enhanced cyclic stability was probably due to the presence of carbon deposition from the decomposition of heavy tar in TC, which might act as a spacer and protective overlay to slow down the sintering of CaO particles. In addition, more carbon deposition could be dispersed in-situ into the TC-modified sorbents by increasing torrefaction temperature and TC addition amount, resulting in more thorough carbon deposition overlay and better sintering resistance. However, excessive carbon deposition (>50 wt%) leaded to slightly lower carbonation conversions and rates in the first two or three cycles. But this deficiency would disappear in subsequent cycles, as some carbon deposition was consumed by the carbon dioxide released during calcination. And the carbonation conversion of TC-modified sorbent with a carbon deposition content of 69.1 wt% was still as high as 81.5% after 10 cycles.

A dual functional sorbent/catalyst material for in-situ CO2 capture and conversion to ethylene production

The coupling technology (CaLODHE) of calcium looping (CaL) and oxidative dehydrogenation of ethane (ODHE) has recently attracted numerous attentions. However, the optimization of the CaO-based sorbent in CaLODHE has not been extensively reported to date. In this work, a comparative study of the CO2 capture performances of the CaO-based sorbents by different synthesis methods (i.e., dry-mixing, wet-mixing, sol–gel, and coprecipitation) tested in CaL and CaLODHE conditions was conducted. The results show that the sorbent by the sol–gel method in CaLODHE condition has the highest cyclic capture performances of around 0.6 g/g during 20 cycles, attributed to the porous structure and homogeneously dispersed CeO2. In addition, the dual functional material (DFM) with CaO-based sorbent and MgGaAlO4 catalyst was first synthesized and the performances of DFM with different mixing ratios of sorbent and catalyst were investigated. The results show that the DFM with the sorbent and catalyst of the same quality has a balanced performance, and the CO2 uptake capacity and conversion are 0.27 g/g and 57%, and the ethane conversion and ethylene selectivity are 56.9% and 74.2%, respectively. In addition, the reaction mechanism of CaLODHE was also proposed in depth.

Thermodynamic modelling of integrated carbon capture and utilisation process with CaO-based sorbents in a fixed-bed reactor

The global emission of CO2 through fossil fuel combustion is still increasing, which is a major challenge for the international community. An integrated carbon capture and utilisation (ICCU) process with a CaO-based sorbent is a promising alternative to effectively reduce emissions. In this work, a comparative thermodynamic analysis of two CaO-based sorbents (commercial and sol-gel CaO) was performed for one cycle of ICCU. In addition, the influence of temperature was investigated from 600 to 750 °C in terms of the degree of CO2 conversion. Thermodynamic calculations were based on the actual gas composition and developed model, where heat consumption and entropy generation were calculated. The results indicate that the degree of CO2 conversion decreased from 84.6 to 41.2% and from 84.1 to 62.4% for the sol-gel and commercial material, respectively, as the temperatures increased. Furthermore, the total heat consumption during one cycle decreased with higher temperatures. The total amount of consumed heat decreased from 19.1 to 5.9 kJ/g and from 24.7 to 5.4 kJ/g for sol-gel and commercial CaO, respectively. Although commercial CaO always requires more heat during one cycle. Moreover, for both materials, the lowest generation of entropy was calculated at 650 °C with values of 9.5 and 10.1 J/g·K for the sol-gel and the commercial CaO, respectively. At all temperatures, the commercial CaO generated a greater entropy.

Strengthening performance of Al-stabilized, CaO-based CO2 sorbent pellets by the combination of impregnated layer solution combustion and graphite-moulding

Calcium looping (CaL) process is recognized as one of the most promising CO2 capture technologies. However, the CaO-based sorbents commonly encounter serious sintering and elutriation problem during CO2 capture cycles. In this work, Al-stabilized, CaO-based sorbents were synthesized by the impregnated layer solution combustion method, and cigarette butt was selected as the porous reactive skeleton. The Al-stabilized, CaO-based sorbent containing 15 wt% of Al2O3 exhibits the highly stable CO2 capture performance, whose CO2 capture capacity after 20 cycles is almost the same as its initial capacity, ∼0.354 g/g. It is mainly attributed to the synergistic effect of thermal pretreatment (cigarette butt-assisted combustion) and incorporation of Al-based stabilizer (i.e., Ca12Al14O33) provide a stable inert framework for the synthetic sorbents, ensuring their highly stable capability for CO2 sorption during cycles. Further, graphite-moulding is a facile and energy-saving approach to make Al-stabilized, CaO-based sorbent powders into pellets to enhance its resistance to elutriation. The produced sorbent pellets (85 wt% CaO/15 wt% Al2O3) exhibit excellent long-term CO2 capture capacity (∼0.248 g/g after 49 cycles) and high compression strength of 18.47 ± 3.47 MPa. Therefore, the combined method involved of impregnated layer solution combustion and graphite-moulding is promising to produce highly efficient CaO-based CO2 sorbent pellets used in CaL.

Ca-looping process using wastes of marble powders and limestones for CO2 capture from real flue gas in the cement industry

The potential of CaO-based sorbents using waste and natural geological materials (two wastes of marble powders and four limestones) for CO2 capture through the Calcium-looping process was assessed using two distinct carbonation gas atmospheres: a real industrial flue gas from a cement plant, and a synthetic gas mixture. The sorbents’ CO2 capture capacity was evaluated on a Fluidized Bed Reactor laboratory scale unit, through ten carbonation-calcination cycles (calcination: 930 °C, 70 % CO2 in air; carbonation: 700 °C, 15 % CO2 in air/real industrial flue gas). The sorbents carbonated with the real industrial flue gas showed a better performance and stability for CO2 capture than with the synthetic gas, which can be explained by presence of moisture in the former. The textural and mineralogical properties of spent sorbents, were characterized by N2 sorption and XRD techniques. The results show that the moisture (below 0.3 %) present in the real flue gas hinders the growth of the CaO crystallite size and leads to an increase of the surface area and total pores volume after 10 cycles. Also, better CO2 capture capacity results were associated with fresh CaO compositions above 52.0 wt. % while for values bellow 46.0 wt. % the performance decreased significantly. The sorbents with high CaO content have an interesting potential to be used in the Ca-looping process under a real carbonation atmosphere, namely the wastes of marble, that appear as a possible economically attractive option, which can contribute to minimize the adverse environmental impacts of the high volume of Portuguese industrial wastes.

Sorption-enhanced steam gasification of biomass for H2-rich gas production and in-situ CO2 capture by CaO-based sorbents: A critical review

The sorption-enhanced steam gasification of biomass (SEBSG) is considered a prospective thermo-chemical technology for high-purity H2 production with in-situ CO2 capture. Fundamental concepts and operating conditions of SEBSG technology were summarized in this review. Considerable industrial demonstration units have been conducted on pilot scales for large-scale availability of the SEBSG process. The influence of process parameters such as reaction temperature, Steam/Biomass (S/B) ratio, feedstock characteristics, cyclic CO2 capture capacity of CaO-based sorbents, and catalysis, were critically reviewed to provide theoretical recommendations for industrial operation. Bifunctional materials that have high catalytic activity and CO2 capture activity are crucial for ensuring high H2 production in the SEBSG. The application of density functional theory (DFT) and reactive force field molecular dynamic (ReaxFF MD) simulations on microcosmic reaction mechanisms in the SEBSG process, such as pyrolysis, WGS and reforming reactions, and CO2 capture of CaO-based materials are comprehensively overviewed. Several research gaps, like the exploitation of more efficient and low-cost bifunctional material, integrated process economics and revelation of well-rounded mechanisms, need to be filled for the following large-scale industrial applications.

Attrition characteristics of limestone in gas-solid fluidized beds

Calcium looping is a potential energy-efficient post-combustion CO2 capture process that is competitive with existing commercially available technology like oxy-combustion. The attrition of CaO-based sorbents during alternating calcination/carbonation cycling in calcium looping (CaL) has the potential to escalate operation cost and contribute to environmental emisssions. Here, we measure jet mill attrition rates of four commercial limestones from Ghana and Canada and compare them with fluid catalytic cracking catalyst (FCC) and commercial vanadyl pyrophosphate (VPP) in a jet mill. We assessed the effect of orifice velocity, attrition time and calcination on particle attrition resistance. The attrition rates of the limestones increased exponentially from as low as 2.4 mg h−1, which is an acceptable rate for industrial applications, up to 38 mg h−1, which is prohibitively high as the orifice velocity increased from 180 m s−1 to 230 m s−1. The attrition rates of the FCC and VPP catalyst were somewhat higher at the low orifice velocity (5 5 mg h−1 and 8 mg h−1, respectively) but lower at the high orifice velocity (21 mg h−1−1 and 26 mg h−1, respectively). Within 3 h, Bui and Nauli limestones reached a steady attrition rate of 19 mg h−1 and 21 mg h−1, respectively whereas Oterpklu limestone reached a steady rate of 17 mg h−1 after 4 h. The calcined limestones attrition rate was twice as high as their carbonated counterpart. The limestones attrited predominantly throgh fragmentation.

Cigarette butt-assisted combustion synthesis of dolomite-derived sorbents with enhanced cyclic CO2 capturing capability for direct solar-driven calcium looping

Calcium looping (CaL) is a promising technology for CO2 capture owing to the widespread applicability and low-cost CaO-based sorbents. However, the SO2 and ash from coal combustion in O2/CO2 atmosphere for CaO regeneration is prone to cause rapid degradation for CaO-based sorbent. Hereby, a direct solar-driven CaL system is proposed to eliminate the adverse influence of coal combustion on CaO-based sorbent. Different unitary Mn-doped and binary-doped (Mn/Fe-, Mn/Cu- and Mn/Co-doped) dolomite-derived sorbents were synthesized by cigarette butt-assisted combustion method. The higher Mn doping ratio, the higher solar absorptance for unitary Mn-doped sorbent due to the gradually deepened blackness of the sample. Comprehensively considering the solar absorptance and CO2 capture performance, the unitary Mn-doped sorbent (Ca&Mg: Mn=100:10) is an optimal candidate, which has a solar absorptance of 63.6 % and a carbonation conversion of 64.3 % in the 20th cycle. Compared with the unitary Mn-doped sorbent (Ca&Mg: Mn=100:10), the binary-doped sorbent (Ca&Mg: Mn: Fe=100:5:5) exhibits a comparable CO2 capture capacity and a higher solar absorptance of 76.7 %. More importantly, the binary-doped sorbent (Ca&Mg: Mn: Fe=100:5:5) possesses the markedly faster CO2 capture rate in the early carbonation stage due to the increased lattice defect of CaO.

MOF-derived nano CaO for highly efficient CO2 fast adsorption

Calcium looping is considered to be one of the best approaches for industrial CO2 high-temperature capture, whereas current CaO-based sorbent suffers poor cyclic performance, especially the relations between precursor structure and CO2 fast adsorption capacity still need to be further revealed. In this work, three calcium-based metal frameworks (Ca-MOFs) with specific structures were employed as precursors to obtain nano CaO through a two-step thermal transition process. It was found that LAC MOF-derived CaO achieved a fast adsorption capacity of 11.8 mmol g−1 (78 % of total adsorption) after 4 cycles. This was because the fiber bundle-like original LAC MOF structure changed gradually to nanosheets after 600 °C pyrolysis and finally formed regular CaO spheres with an average size of 100 nm after an 800 °C -calcination, thereby preventing further aggregation of CaO particles during the thermal transition process. In addition, the adsorption performance did not rely on pore structure, and thus pore-blocking showed little influence on the performance of CO2 capture. Hence, high stability can be achieved simply by avoiding particle sintering. The systematic study marks the significance of precise tailoring of nano-CaO for achieving the desired performance of CO2 fast adsorption.

Facile, time-saving cigarette butt-assisted combustion synthesis of modified CaO-based sorbents for high-temperature CO2 capture

Highly efficient and stable Zr-incorporated CaO-based sorbents were synthesized using a facile, cigarette butt-assisted combustion synthesis (CAC) method. Comparative investigation of CAC method with two common synthesis methods (i.e., wet-mixing, sol–gel) on producing CaO-based sorbents was conducted. The sorbents synthesized by the CAC and sol–gel methods exhibit superior cyclic CO2 capture capability compared to wet-mixing. More importantly, the CAC belongs to impregnated layer solution combustion synthesis and takes markedly shorter time compared to sol–gel, possessing the potential of scaled-up preparation. The optimization of combustion temperature and incorporation ratio of Zr-based support on synthesis of CaO-based sorbents by CAC method was further conducted. The proper combustion temperature selected to synthesize Zr-incorporated CaO-based sorbents using CAC method is 750 °C. Too high or low combustion temperature may result in unsatisfactory pore structure of synthetic CaO-based sorbents because of excessive sintering or generating in-situ CaCO3, respectively. The synthetic CaO-based sorbents with 25 wt% of ZrO2 exhibit the outstanding CO2 capture performance (CaO carbonation conversion of 63.8% after 17 cycles) due to the generation of extensive CaZrO3 grains acting as the inert skeleton. Additionally, the combination of cigarette butt-assisted combustion and graphite-moulding can prepare Zr-incorporated CaO-based pellets with desirable practicality.

Fabrication of robust CaO-based sorbent via entire utilization of MSW incineration bottom ash for CO2 capture

CO2 capture using CaO-based sorbent is a promising and effective CO2 mitigation strategy due to its high theoretical capacity of CO2 adsorption and potential use in large scale. The preparation of waste-derived CaO-based CO2 sorbents has received extensive interests due to its economic feasibility via waste recycling. In this work, the development of a novel robust CaO-based sorbent via the entire utilization of municipal solid waste incineration (MSWI) bottom ash (BA) was reported. Specifically, residues from the extraction of CaO from BA were added back to the extracted CaO after thermal passivation. The results showed that the addition of BA residue-derived stabilizer (BAS) promoted the homogeneous dispersion of CaO and increased the specific surface area (17.66 m2/g) of the BAS/CaO sorbents and pores in the range of 20–100 nm. Meanwhile, the small amounts of Al2O3 and SiO2 in the Ca-Si-Al-O species of BAS reacted with CaO to form Ca12Al14O33 and Ca2SiO4 during cyclic carbonation/calcination, which was beneficial to alleviate the aggregation and sintering between CaO particles and maintain porous structure of sorbents. In addition, the CO2 uptake exhibited a volcano-type variation with the increase of BAS additive amount and carbonation temperature, where the optimum carbonation temperature for BAS(20 wt%)/CaO sorbents was 700 °C. Among the various sorbents, the BAS(20 wt%)/CaO sorbent (BAS/CaO = 2/8) exhibited the most superior CO2 uptake of 0.14 gCO2/gsorbent after 20 cycles. This study provides a promising method to achieve a cost-effective CO2 capture candidate with enhanced capacity by completely reusing solid wastes, which eliminates the disposal of secondary residues that was rarely considered in previous studies.

Investigation of K2CO3-modified CaO sorbents for CO2 capture using in-situ X-ray diffraction

Alkali metal salt promoters, such as K2CO3 investigated in this work, are known to affect the cyclic performance of CaO-based sorbents – sometimes positively, sometimes negatively. Thus far, the influence of K2CO3 (and other salt-based promoters) is poorly understood, and detailed investigations on the interaction of K2CO3 with CaO are missing. We find that low amounts of K2CO3 in the sorbents improve the cyclic performance of CaO significantly (up to a factor of three), both under mild and harsh CO2 release conditions, while large amounts of K2CO3 (>1 mol.%) reduce the positive effect on the CO2 uptake performance. Aided by in-situ X-ray diffraction measurements, we observe that the gas environment has an important influence on the stability of K-containing phases in the sorbent. At typical carbonation conditions (650 °C, 15 vol% CO2/N2), CaO and K2CO3 readily form K-Ca double carbonates (K2Ca(CO3)2, which reacts further to form K2Ca2(CO3)3). When the calcination reaction is carried out in pure N2, the K-Ca double carbonates decompose above ∼730 °C, forming K2CO3 that becomes partially volatile near its melting point of ∼900 °C, and leads to a reduction of the K content in the samples with cycling. When the calcination reaction is carried out in a CO2-rich environment, the cyclic loss of K2CO3 is reduced substantially, but the sintering of the sorbent is increased. There are indications that very low quantities of K are incorporated into the CaO structure, and the formation of K-Ca double carbonates appears to influence the structural evolution of the CaCO3 phase formed during carbonation. With very low amounts of K2CO3 present in the sorbent, we observe an increase in pore volume for pore diameters between 20 and 80 nm, which ultimately may explain the improved CO2 uptake performance compared to the CaO sorbent without K2CO3.

One-pot preparation of H2-mixed CH4 fuel and CaO-based CO2 sorbent by the hydrogenation of waste clamshell/eggshell at room temperature.

The preparation of clean fuel or CO2 adsorbents using industrial and domestic garbage is an alternative way of meeting global energy needs and alleviating environmental problems. Herein, H2-mixed CH4 fuel and CaO-based CO2 sorbent were first prepared in one pot by the mechanochemical reaction of pretreated clamshell or eggshell wastes (carbon and calcium source) with calcium hydride (hydrogen source) at room temperature. In the above reactions, CH4 was the sole hydrocarbon product, and its yield reached 78.23%. The H2/CH4 ratio of the produced H2-mixed CH4 fuel was tunable according to the need by changing the reaction conditions. It is inspiring that the simultaneously formed solid CaO/carbon products were efficient CaO-based sorbents, which possessed a higher CO2 adsorption capacity (49.81-58.74wt.%) at 650°C and could maintain good adsorption stability in 30 carbonation/calcination cycles (average activity loss per cycle of only 1.6%). The three achievements of the idea are that it can simultaneously eliminate clamshell or eggshell wastes, obtain valuable clean fuel, and acquire efficient CaO-based sorbents.

One-step fabricated Zr-supported, CaO-based pellets via graphite-moulding method for regenerable CO2 capture

Calcium looping (CaL) belongs to a promising high-temperature CO2 capture technology because of adopting cheap and extensive source CaO-based sorbents. However, CaO-based sorbents are prone to occur the issues of sintering and elutriation in the fluidized-bed reactors. To further enhance the practicability, the Zr-supported, CaO-based sorbent pellets were produced using the graphite-moulding method. Different synthesis modes (i.e., sol-gel, hydration-mixing, and wet-mixing) were compared for preparing CaO-based composite slurry before pelletization. Sol-gel is the promising synthesis mode to prepare Zr-supported, CaO-based pellets with outstanding CO2 capture performance due to achieving a more uniform distribution of inert CaZrO3 spacer. Moreover, the Zr-based stabilizer content within sorbent pellets produced by the combined method of sol-gel and graphite-moulding was further studied. The higher content of Zr-based stabilizer promotes the enhancement of cyclic stability and mechanical strength of Zr-supported, CaO-based pellets. After 17 cycles, the sorbent pellets containing 20 wt% of Zr-based stabilizer display a high CaO carbonation conversion of 74.1 %. Moreover, CaO-based pellets with 20 wt% of Zr-based stabilizer possess a higher compression strength of 4.84 ± 1.08 MPa, which is as high as 1.8 times that of the pellets with 5 wt% of Zr-based stabilizer.

Evaluation of MgO- and CaZrO3-promoted CaO-based pellets produced via solution combustion synthesis

Synthetic CaO-based sorbents stabilized with CaZrO3 and MgO were produced using a facile one-step fabrication procedure. Powdered sorbents were pelletized using two types of binders (calcium aluminate cement and boehmite) to shape the solids into forms suitable for reactor testing and evaluation under industrially relevant conditions. The CO2 uptake capacity and stability of pelletized sorbents were initially screened in a thermogravimetric analyzer, and subsequently assessed in a fixed and fluidized bed reactor. Sorbents showed a residual uptake capacity of 0.24 g/g after 100 cycles of carbonation and calcination in TGA, indicating a near threefold improvement over the residual CO2 uptake of natural Cadomin limestone (0.08 g/g). Similar improvements were observed when the sorbents were tested in a lab-scale reactor system, where the synthetic sorbents showed 0.4–0.45 g/g uptake after 5 cycles, whereas the uptake of Cadomin dropped to 0.15 g/g. Characterization of fresh and spent sorbents indicated an increase in the BET surface area of sorbents after cycling. Pore size distribution profiles exhibit increase of pore volume in the 20–100 nm region, whereas the pore volume in the < 6 nm region reduced. SEM images indicated formation of cracks after cycling, resulting from the expansion and contraction of sorbent over carbonation and calcination cycles.

Stability of CaO-based Sorbents under Realistic Calcination Conditions: Effect of Metal Oxide Supports

Solid metal oxides with high Tammann temperatures have been investigated as stabilizers to remedy the sintering of CaO sorbents during the calcium looping process. A methodical comparison between different metal stabilizers must take into account two factors: (1) equal molar ratio of metal stabilizer to active sorbent and (2) homogenous dispersion of the stabilizer in the sorbent. The negligence of one of these two factors has caused a discrepancy among the current work in the literature with respect to the optimum metal stabilizer for CaO sorbents. In this work, seven metal promoters (Mg, Y, Ce, Nd, Zr, La, and Al) were studied to assess their stabilization effect on CaO sorbents used in cyclic carbonation and calcination experiments. Scanning electron microscopy/energy dispersive X-ray analysis elemental mapping was performed on all sorbents to ensure uniform dispersion of stabilizers in the sorbents. Results indicate that among the metal supports tested in this study, Mg-stabilized sorbents are most efficient in maintaining a high uptake capacity through cycles, under both mild and harsh calcination conditions. This is partly attributed to the high Tammann temperature of MgO stabilizer in comparison to other promoters, resulting in efficient separation of CaO grains and inhibition of sintering and agglomeration. Mg-stabilized sorbents (Mg/Ca = 1:14) showed a first-cycle uptake capacity of 0.68 and 0.63 g/g under mild and harsh calcination conditions, respectively. Sorbents with higher stabilizer ratio (as high as Mg/Ca = 1:2) were tested in 100 cycles and showed a residual uptake capacity of ∼0.15 g/g. In contrast to previously reported results in the literature, the stabilization effect of Mg promoter was observed even at Mg/Ca molar ratios as low as 1:14.

Multiple activation mechanisms of CaO-based sorbents promoted by pre-sintering-hydration

Highly reactive CaO sorbents based on natural limestone with enhanced stability and re-usability were obtained via a pre-sintering-hydration method. It was found that several active mechanisms including generation of oxygen vacancies, promotion of ion replacement and improvement in pore structure were involved during the process of pre-sintering-hydration. The limestone-based sorbents obtained at a pre-sintering temperature of 1100 °C showed the highest CO2 capture capacity of 0.71 g-CO2 g−1 in the first cycle with a specific surface area of 28.65 m2 g−1. Electron spin-resonance analysis and high-resolution transmission electron microscopy demonstrated the existence of stress-induced oxygen vacancies and the promoting effect of hydration on ion replacement. The CO2 capture capacity of sample Ca-1100Mg-1:7 can still maintain at 0.47 g-CO2 g−1 at the 30th cycle, which was 2.8 times that of Ca-lime. After every 30 cycles, the recovered CO2 capture capacity of sample Ca-1100Mg-1:7 can achieve about 96.9 % of its initial valve. The improved stability and re-usability of the prepared sorbents can be attributed to the combined action of multiple active mechanisms introduced by pre-sintering-hydration treatment.

Sorption-enhanced gasification of municipal solid waste for hydrogen production: a comparative techno-economic analysis using limestone, dolomite and doped limestone

Sorption-enhanced gasification has been shown as a viable low-carbon alternative to conventional gasification, as it enables simultaneous gasification with in-situ CO2 capture to enhance the production of H2. CaO-based sorbents have been a preferred choice due to their low cost and wide availability. This work assessed the technical and economic viability of sorption-enhanced gasification using natural limestone, doped limestone with seawater and dolomite. The techno-economic performance of the sorption-enhanced gasification using different sorbents was compared with that of conventional gasification. Regarding the thermodynamic performance, dolomite presented the worst performance (46.0% of H2 production efficiency), whereas doped limestone presented the highest H2 production efficiency (50.0%). The use of dolomite also resulted in the highest levelised cost of hydrogen (5.4 €/kg against 5.0 €/kg when limestone is used as sorbent), which translates into a CO2 avoided cost ranging between 114.9 €/tCO2 (natural limestone) and 130.4 €/tCO2 (dolomite). Although doped limestone has shown a CO2 avoided cost of 117.7 €/tCO2, this can be reduced if the production cost of doped limestone is lower than 42.6 €/t. The production costs of new sorbents for CO2 capture and H2 production need to be similar to that of natural limestone to become an attractive alternative to natural limestone.

Optimization of sol-gel combustion synthesis for calcium looping CO2 sorbents, part Ⅰ: Effects of sol-gel preparation and combustion conditions

The sol-gel combustion synthesis is a superior method to prepare high-performance CaO-based sorbent which is applicable for flue gas decarbonization and thermochemical energy storage. To optimize this synthesis, this study investigated different preparation variables in the sol-gel formation process and combustion process. The effects of these variables on the cyclic CO2 sorption performance, micromorphology, and physical parameters of the prepared pure CaO sorbents were studied. First, worse cyclic performance was shown when the H2O/Ca2+ molar ratio was over 100:1, and the optimal ratio was 80:1. Second, the citric acid/calcium nitrate ratio influenced the formation of the complex as well as the gas production and adiabatic flame temperature in the combustion process, and the optimal molar ratio was 1:1. Third, the NH4NO3 and Ca-based precipitate would be produced when the pH value of the precursor solution was over 3. The optimal pH value was 2, and the carbonation conversion of the prepared sorbent reached 99% in the 2nd cycle within the 10 min carbonation at 650 °C under 15% volume CO2. After 20 cycles, the conversion was 70.4%. Fourth, a higher heating rate facilitated better cyclic performance and the optimal heating rate was 10 °C/min. Finally, a higher initial ignition temperature accelerated grain growth and particles agglomeration of the combustion product, and the optimal temperature was 300 °C.