CaO-based Materials Research Articles

A critical assessment of the testing conditions of CaO-based CO2 sorbents

Calcium (or carbonate) looping (CaL) is a high-temperature process to capture CO2 from industrial installation using CaO-based materials as the sorbent. A large number of natural and synthetic CaO-based sorbents for CO2 capture have been developed and investigated in the laboratory by cycling the material between a low-temperature carbonation stage and a high-temperature calcination stage. We demonstrate the importance of the exact experimental protocol chosen to transition from the carbonation to the calcination step by varying the CO2 concentration and the heating rate while keeping the conditions during the actual carbonation (650 °C, 15 vol.% CO2) and calcination (950 °C, 80 vol.% CO2) stages constant. The experiments were performed in a thermogravimetric analyser, the equipment most frequently used in investigations of the cyclic CO2 uptake, using a natural limestone as the sorbent. Our results show that the reaction conditions under which the CaCO3 is decomposed to CaO determines the cyclic performance of the sorbent, whereas the effect of the CO2 concentration during the set calcination stage (which usually begins after most of the CaCO3 has been converted to CaO) appears to be insignificant. Higher heating rates facilitate a higher cyclic CO2 uptake because the carbonate phase, known to be prone to sintering, is exposed to higher temperatures for a shorter time. The reaction conditions during the transition period crucially affect the CO2 uptake determined in laboratory tests and hence need to be reported in full detail, yet this is currently hardly the case. Otherwise it is unclear whether differences in the CO2 uptake of different CO2 sorbents are due to modifications of the material or variations in the testing protocol.

Multi-cyclic evaluation of composite CaO-based structured bodies for thermochemical heat storage via the CaO/Ca(OH)2 reaction scheme

Experimental efforts have shown that thermo-chemical heat storage through cyclic hydration/dehydration of the CaO/Ca(OH)2 couple requires efficient CaO-based particles, in terms of both hydration capacity and structural robustness. Acknowledging the challenge caused by high fragmentation of pure CaO particles during multi-cyclic operation in bed reactors, the development of CaO-based materials with enhanced mechanical properties is essential. Promising results have been obtained for nearly spherical structured formulations using kaolinite as binder. The combination of natural limestone with 25wt% kaolinite rendered mechanically strong materials with a hydration capacity of up to 50% compared to the maximum hydration capacity of pure CaO. These formulations remained intact and showed stable reactivity in the course of 10 hydration/dehydration cycles. In order to obtain valid conclusions for the suitability of these materials for the here suggested reaction scheme, examination upon multiple hydration/dehydration cycles was necessary. The current work is related to the assessment of CaO-kaolinite composite materials under three different evaluation protocols, as well as to the examination of their mechanical properties through crushing strength measurements and attrition tests. For most of the materials examined, more satisfactory results were obtained for hydration and dehydration reactions in vapor-rich atmosphere at 450 and 550°C, respectively. Multi-cyclic evaluation (50–200cycles) confirmed the initial findings of the 10-cycle tests. The final selected composition constituted a promising material for the suggested reaction scheme and was qualified as in-principle suitable for long-term operation in pilot scale units.

Use of Green Mussel Shell as a Desulfurizer in the Blending of Low Rank Coal-Biomass Briquette Combustion

Calcium oxide-based material is available abundantly and naturally. A potential resource of that material comes from marine mollusk shell such as clams, scallops, mussels, oysters, winkles and nerites. The CaO-based material has exhibited a good performance as the desulfurizer or adsorbent in coal combustion in order to reduce SO2 emission. In this study, pulverized green mussel shell, without calcination, was utilized as the desulfurizer in the briquette produced from a mixture of low rank coal and palm kernel shell (PKS), also known as bio-briquette. The ratio of coal to PKS in the briquette was 90:10 (wt/wt). The influence of green mussel shell contents and combustion temperature were examined to prove the possible use of that material as a desulfurizer. The ratio of Ca to S (Ca = calcium content in desulfurizer; S = sulfur content in briquette) were fixed at 1:1, 1.25:1, 1.5:1, 1.75:1, and 2:1 (mole/mole). The burning (or desulfurization) temperature range was 300-500 °C; the reaction time was 720 seconds and the air flow rate was 1.2 L/min. The results showed that green mussel shell can be introduced as a desulfurizer in coal briquette or bio-briquette combustions. The desulfurization process using that desulfurizer exhibited the first order reaction and the highest average efficiency of 84.5%.

Flexible sorption enhanced gasification (SEG) of biomass for the production of synthetic natural gas (SNG) and liquid biofuels: Process assessment of stand-alone and power-to-gas plant schemes for SNG production

A flexible sorption enhanced gasification (SEG) process is assessed in this work, where CaO-based material circulating between gasifier and combustor reactors is adjusted for fulfilling the syngas composition requirements according to the downstream fuel synthesis process. A case study of a synthetic natural gas (SNG) production plant based on this SEG process is presented, which has been analysed under different conditions of gasification temperature or solid circulation. A possible integration of this plant with an electrolysis system for power-to-gas application for balancing the electric grid is also proposed and assessed.SNG production efficiencies as high as 62% (LHV-based) have been found for the production of SNG with final CH4 content of 98%. Excess energy recovered from the process streams can be used for producing electricity in a steam turbine, covering the electric demand in the plant. If credits associated to electricity production are considered, equivalent SNG production efficiencies higher than 70% have been calculated. Efficiencies reported in this work are in the upper limit of the range found in the literature for non-SEG concepts, which require an intermediate conditioning step of WGS and CO2 removal. When coupled with an electrolyser, power-to-gas efficiencies of about 60% have been calculated, in line with stand-alone power to gas methanation systems.

CO2 abatement from the iron and steel industry using a combined Ca–Fe chemical loop

We report an integrated CO2 capture process to manufacture iron and steel with low CO2 emissions from steel-making plants over a CaO-based, Fe-functionalized CO2 sorbent. This new process relies on a combined Ca–Fe chemical loop, where the exothermic oxidation of FeO provides the heat (or fractions of it) required to drive the endothermic decomposition of CaCO3. A key advantage of coupling a FeO/Fe2O3 redox cycle and a CaO/CaCO3 CO2 capture cycle within the as-prepared materials is that the heat could be transferred from FeO to CaCO3 directly at a molecular level. All materials synthesized require an appreciably reduced net heat, i.e. a decrease of 26–43%, for CaCO3 decomposition during the combined Ca–Fe looping when compared with conventional calcium looping. Importantly, materials CaFe-CA1EG1-pH1, CaFe-CA1EG1-pH2, and CaFe-CA1EG1-pH3 exhibit a fairly stable cyclic CO2 uptake of ∼0.16gCO2gsorbent−1 throughout 10 realistic combined Ca–Fe looping cycles. The medium capacity but excellent stability for CO2 capture of the CO2 sorbents makes them a promising alternative of naturally-derived, CaO-based materials, especially for the combined Ca–Fe chemical loop proposed. Using steel slag as feedstock, the material developed is very promising for CO2 abatement from the iron and steel industry accompanied by the recycling of CaO and Fe2O3 components in spent sorbents and recovery of energy from process gases.

Structural evolution in synthetic, Ca-based sorbents for carbon capture

The carbonation of CaO-based materials at high temperatures (e.g. >600°C) is a promising method of capturing CO2 emitted from, e.g. the combustion of carbonaceous fuels. The resulting CaCO3 can be regenerated by calcination at a temperature at which the equilibrium partial pressure exceeds that of the local partial pressure of CO2 (e.g. 950°C). A process involving repeated cycles of carbonation and calcination of a calcareous material is called calcium looping. The capacity of a CaO-based sorbent to accept and reject CO2 over many cycles is governed by a number of factors, such as chemical composition, surface morphology and pore structure, all of which often evolve with cycling. The present paper investigates the underlying mechanisms controlling the evolution of the micro-structures of a series of synthetic sorbents consisting of CaO mixed with the inert supports Ca12Al14O33 and MgO. These sorbents were subjected to cycles of calcination and carbonation and were characterised using a variety of in situ and ex situ techniques. It was found that the balance between the degree of surface cracking during calcination and the extent of sintering during carbonation was responsible for changes in uptake during cycling, giving an increase in uptake for the supported CaO and a decrease for the unsupported CaO.

Improving the stability of synthetic CaO-based CO2 sorbents by structural promoters

CaO-based materials are promising sorbents for CO2 capture via carbonate looping for high temperature applications, which however suffer from decreasing sorption capacity over multiple sorption/desorption cycles. In this study, we report the development of mixed CaO-based CO2 sorbents with inert structural promoters prepared via sol–gel auto-combustion that exhibit significant stability increase. Different synthesis parameters such as the organic material used as combustion agent, the inert material used as stabilizer and the CaO concentration in the sorbent were investigated. Citric acid was identified as the most suitable combustion agent, resulting in the formation of calcium oxide with the highest initial sorption capacity. Among the four different promoters used to increase the resistance of CaO toward sintering, Al2O3 and ZrO2 resulted in the most stable sorbents due to formation of the mixed phases Ca3Al2O6 and CaZrO3. The two sorbents exhibited a very stable performance with a sorption capacity higher than 9mol of CO2/kg of sorbent after 100 cycles under mild operating conditions (calcination at 850°C under pure N2 flow). On the other hand Ca–Mg and Ca–La demonstrated a significant decay of sorption capacity with more than 30% deactivation after 100 and 70 cycles respectively. Under severe but more realistic conditions (calcination at 950°C under pure CO2 flow), the mixed CaO–Al2O3 sorbent still retained ∼42% of its initial sorption capacity, corresponding to 4.8mol of CO2/kg of sorbent, while CaO–ZrO2 exhibited a remarkable stability, maintaining sorption capacity of 8.5mol of CO2/kg of sorbent after 100 cycles.

Acidic Treatment of CaO-Based Materials for Medium-High Temperature CO<sub>2 </sub>Capture

In this work, a series of CaO-based materials for medium-high temperature (500-800 °C) CO2 capture were synthesized from limestone by acidic treatment. In order to enhance its thermal stability, CaO-based materials were modified with Al(NO3)3. With various stirring time of 24, 48 and 72 hours, three types of CO2 capture sorbent were synthesized, named LA24, LA48 and LA72, respectively. TGA results showed the CaO-based materials were activated in the CO2 capture reaction of temperature range of 500-800 °C. LA24 exhibited the best CO2 capture capacity of 53.8 wt % as well as stability of 93.2 % after 10 cycles test. Furthermore, all of the modified CaO-based materials showed good thermal resistance upon 900 °C. It was consistent with expectation by the contributions of alumina coexistence with calcium oxide. XRD results indicated that CaO dominated in the diffraction spectrum before capturing CO2, and a side diffraction peak of Al2O3 was also observed. The characterization of CaO-based materials was also investigated by BET and SEM, respectively.

Multifunctional Pd/Ni-Co catalyst for hydrogen production by chemical looping coupled with steam reforming of acetic acid.

High yield of high-purity H2 from acetic acid, a model compound of bio-oil obtained from the fast pyrolysis of biomass, was produced by sorption-enhanced steam reforming (SESR). An oxygen carrier was introduced into a chemical loop (CL) coupled to the cyclical SESR process to supply heat in situ for the endothermic sorbent regeneration to increase the energy efficiency of the process. A new multifunctional 1 %Pd/20 %Ni-20 %Co catalyst was developed for use both as oxygen carrier in the CL and as reforming catalyst in the SESR whereas a CaO-based material was used as CO2 sorbent. In the sorbent-air regeneration step, the Ni-Co atoms in the catalyst undergo strong exothermic oxidation reactions that provide heat for the CaO decarbonation. The addition of Pd to the Ni-Co catalyst makes the catalyst active throughout the whole SESR-CL cycle. Pd significantly promotes the reduction of Ni-Co oxides to metallic Ni-Co during the reforming stage, which avoids the need for a reduction step after regeneration. H2 yield above 90 % and H2 purity above 99.2 vol % were obtained.

H2 production by steam reforming with in situ CO2 capture of biomass-derived bio-oil

Abstract High-purity H 2 was produced by the sorption enhanced steam reforming (SESR) of acetic acid, a model compound of bio-oil obtained from biomass fast pyrolysis. A fixed bed reactor comprising two sections with different temperature was used to favor the reforming reaction in the high-temperature section, and the water gas shift (WGS) and the CO 2 capture reactions along the low-temperature segment. A Pd/Ni-Co hydrotalcite-like material (HT) was used as catalyst and a CaO-based material as CO 2 sorbent. High H 2 yield (84.4%), with H 2 purity of 99.8%, was obtained at atmospheric pressure and 600 °C in the above section of the bed followed by a bed temperature of 500 °C at the bottom part. A null CO concentration was achieved, while the CH4 and CO 2 contents showed very low values (0.15% and 0.02%, respectively).

Oxidative Desulfurization of Crude Oil Incorporating Sulfone Decomposition by Alkaline Earth Metal Oxides

This work explores a novel approach to desulfurization of whole crude oil where thiophenic sulfur compounds are oxidized to sulfones followed by sulfone decomposition into sulfur-free hydrocarbons. The objective of this work is to study the effect of alkaline earth metal oxides on decomposition of sulfones in oxidized crude oil. About 73% of the sulfones in a preoxidized, medium sour crude oil was decomposed over CaO at 360 °C. SO2 was detected by TPD–MS and also measured by a quantitative method when the spent CaO sorbent was thermally treated in an inert atmosphere with sweep gas. No prominent decomposition of sulfur compounds was observed when untreated crude oil (not oxidized) was treated over CaO at 360 °C. This study demonstrates the feasibility of decomposing sulfones in oxidized crude oil over CaO-based materials.

The morphology of limestone-based pellets prepared with kaolin-based binders

Modifications and pelletization of limestone were investigated in order to improve the utilization of CaO-based materials for different catalytic reactions and environmental applications. Attempts to purify the limestone by ion-exchange with CaCl2 solution did not result in significant removal of impurities. On the other hand, acetification with 10 vol.% acetic acid enhanced pore surface area and pore volume of the sorbent by 42% and 3-fold, respectively. The acetification was found to widen small pores, and thus create a beneficial pore size distribution with more pores in the range of 25–100 nm. In order to utilize such powdered materials in fluidized beds, pelletization is the next step. Unfortunately, pelletization results suggested that natural kaolin is an unsuitable binder for preparing CaO-based pellets due to its negative impact on pellet morphology. By contrast, Al(OH)3 binder obtained from kaolin leaching had a strong positive effect on the porous texture of the pellets, demonstrated by pore surface area and volume of 22.48 m2 g−1 and 0.051 cm3 g−1 for 1 mm pellets with CaO/binder ratio of 5.5, compared to 10.92 m2 g−1 and 0.039 cm3 g−1 for natural materials. The enhancement in pellet morphology is mainly attributed to transformation of Al(OH)3 to the highly porous Al2O3 at high temperatures. Pellets synthesized from limestone modified with 10 vol.% acetic acid with Al(OH)3 binder (ratio of 5.5) exhibited high pore surface area and volume, represented by 1.3-fold and 44% increase over those for natural limestone. It was concluded that the combination of acetified limestone with Al(OH)3 binder is a promising approach for synthesis of CaO-based pellets with enhanced morphology.

Influence of morphology, porosity and crystal structure of CaCO3 precursors on the CO2 capture performance of CaO-derived sorbents

In this study, several different CaCO3 samples both natural and synthetic, showing diverse physico-chemical characteristics (particle morphology, crystal structure and porosity) were used as precursors of CaO-based sorbents for the capture of CO2. In order to identify the most favorable CaCO3 properties for a high stability and optimal CO2 uptake by CaO-derived sorbents, both series of materials were characterized using the following analytical techniques: X-ray diffraction, N2 adsorption and scanning electron microscopy. Moreover, the main differences in chemical composition, particle morphology and sorbent porosity of the CaO-based materials were correlated to the maximum CO2 uptake and loss-in-capacity after several carbonation/calcination cycles. The results indicate that the crystal structure of CaCO3parent samples strongly affects the physical properties of the calcium oxide formed during the calcination step, significantly influencing the maximum CO2 uptake. The agglomeration of CaO particles after the calcination of the CaCO3 precursors controlled to a certain extent the carbonation rate. However, this factor could not be correlated with the loss of activity of the CaO-derived sorbents in tests carried out over consecutive carbonation/calcination cycles.

Glycerol Etherification over Highly Active CaO‐Based Materials: New Mechanistic Aspects and Related Colloidal Particle Formation

Glycerol is an attractive renewable building block for the synthesis of di- and triglycerols, which have numerous applications in the cosmetic and pharmaceutical industries. In this work, the selective etherification of glycerol to di- and triglycerol was studied in the presence of alkaline earth metal oxides and the data are compared with those obtained with Na2CO3 as a homogeneous catalyst. It was found that glycerol conversion increased with increasing catalyst basicity; that is, the conversion increases in the order: MgO<CaO<SrO<BaO. The best selectivity values for (di- + tri-) glycerol (>90 % at 60 % conversion) are obtained over CaO, SrO, and BaO. For these catalysts no substantial acrolein formation was observed. Furthermore, at the start of the reaction mainly linear diglycerol was produced, whereas at higher conversion degrees branched diglycerol started to form. In another series of experiments different types of CaO materials were prepared. It was found that these CaO-based materials not only differed in their surface area and number of basic sites, but also in their Lewis acid strength. Within this series the CaO material possessing the strongest Lewis acid sites had the highest catalytic activity, comparable to that of BaO, pointing towards the important role of Lewis acidity for this etherification reaction. Based on these observations a plausible alternative reaction scheme for glycerol etherification is presented, which considers the facilitation of the hydroxyl leaving process. Finally, the stability of the catalytic solids under study was investigated and it was found that colloidal CaO particles of about 50-100 nm can be spontaneously generated during reaction. Catalytic testing of these CaO colloids, after isolation from the reaction medium, revealed a very high etherification activity. Understanding the nature of these Ca-based colloids opens new opportunities for investigating supported colloidal particle catalysts to take advantage of both their hetero- and homogeneous nature.

Microstructure and Properties of Self‐Reinforced Silicon Nitride

Problems associated with manufacturing Si3N4/SiC‐whisker composites have been overcome by developing selfreinforced Si3N4 with elongated β‐Si3N4 grains formed in situ from oxynitride glass. This Si3N4–Y2O3–MgO–SiO2–CaO‐based material has a flexure strength &gt;1000 MPa and fracture toughness &gt;8 MPa·m½. The optimum combination of mechanical properties has been obtained with Y2O3:MgO ratios ranging from 3:1 to 1:2, CaO contents ranging from 0.1 to 0.5 wt%, and Si3N4 contents between 90 and 96 wt%.