Calcination and sintering characteristics of limestone under O2/CO2 combustion atmosphere

The O₂/CO₂ coal combustion technology is an innovative combustion technology that can control CO₂, SO₂ and NO_x emissions simultaneously. Calcination and sintering characteristics of limestone under O₂/CO₂ atmosphere were investigated in this paper. The pore size, the specific pore volume and the specific surface area of CaO calcined were measured by N₂ adsorption method. The grain size of CaO calcined was determined by XRD analysis. The specific pore volume and the specific surface area of CaO calcined in O₂/CO₂ atmosphere are less than that of CaO calcined in air at the same temperature. And the pore diameter of CaO calcined in O₂/CO₂ atmosphere is larger than that in air. The specific pore volume and the specific surface area of CaO calcined in O₂/CO₂ atmosphere increase initially with temperature, and then decline as temperature exceeds 1000 °C. The peaks of the specific pore volume and the specific surface area appear at 1000 °C. The specific surface area decreases with increase in the grain size of CaO calcined. The correlations of the grain size with the specific surface area and the specific pore volume can be expressed as L = 744.67 + 464.64 lg(1 / S) and L = − 608.5 + 1342.42 lg(1 / ε), respectively. Sintering has influence on the pore structure of CaO calcined by means of influencing the grain size of CaO.     

Modeling char conversion under suspension fired conditions in O2/N2 and O2/CO2 atmospheres

The aim of this investigation has been to model combustion under suspension fired conditions in O₂/N₂ and O₂/CO₂ mixtures. Experiments used for model validation have been carried out in an electrically heated Entrained Flow Reactor (EFR) at temperatures between 1173 K and 1673 K with inlet O₂ concentrations between 5 and 28 vol.%. The COal COmbustion MOdel, COCOMO, includes the three char morphologies: cenospheric char, network char and dense char each divided between six discrete particle sizes. Both combustion and gasification with CO₂ are accounted for and reaction rates include thermal char deactivation, which was found to be important for combustion at high reactor temperatures and high O₂ concentrations. COCOMO show in general good agreement with experimental char conversion profiles at conditions covering zone I-III. From the experimental profiles no effect of CO₂ gasification on char conversion has been found. COCOMO does however suggest that CO₂ gasification in oxy-fuel combustion at low O₂ concentrations can account for as much as 70% of the overall char consumption rate during combustion in zone III.     

Differences in reactivity of pulverised coal in air (O2/N2) and oxy-fuel (O2/CO2) conditions

The reactivity of four pulverised Australian coals were measured under simulated air (O₂/N₂) and oxy-fuel (O₂/CO₂) environments using a drop tube furnace (DTF) maintained at 1673 K and a thermogravimetric analyser (TGA) run under non-isothermal (heating) conditions at temperatures up to 1473 K. The oxygen concentration, covering a wide and practical range, was varied in mixtures of O₂/N₂ and O₂/CO₂ in the range of 3 to 21 vol.% and 5 to 30 vol.%, respectively. The apparent volatile yield measured in CO₂ in the DTF was greater than in N₂ for all the coals studied. Pyrolysis experiments in the TGA also revealed an additional mass loss in a CO₂ atmosphere, not observed in a N₂ atmosphere, at relatively high temperatures. The coal burnout measured in the DTF at several O₂ concentrations revealed significantly higher burnouts for two coals and similar burnouts for the other two coals in oxy-fuel conditions. TGA experiments with char also revealed higher reactivity at high temperatures and low O₂ concentration. The results are consistent with a char–CO₂ reaction during the volatile yield experiments, but additional experiments are necessary to resolve the mechanisms determining the differences in coal burnout.     

The formation and thermal stability of spurrite,Ca5(SiO4)2CO3

The reversible equilibrium: 2Ca₂SiO₄ + CaO + CO₂ ? Ca₅(SiO₄)₂CO₃ has been studied using F^? and C?^? ions as mineralizers. A pressure-temperature curve is given for the reaction in the range of CO₂ pressures between 0.08 and 1 atmosphere. At these pressures, the decomposition temperatures of spurrite are 790 ± 5°C and 912 ± 5°C respectively. At a given CO₂ pressure the thermal stability of spurrite is greater than that of CaCO₃.     

Conditions and mechanism of interconvertibility of compounds 12CaO.7Al2O3 and 5CaO.3Al2O3

Based on a critical analysis of the data reported in the literature and our own experimental studies relating to the melt and glass formation from compositions corresponding to 12CaO.7Al₂O₃ under various atmospheric conditions, the liquid structure of the melt phase and conditions for stabilization of 12CaO.7Al₂O₃ and 5CaO.3Al₂O₃ in crystalline state have been highlighted. The role of “stabilizers” like O^2?, F^?, Cl^?, S^2? for the crystalline structure of 12CaO.7Al₂O₃, effect of temperature and atmosphere on melt structure and properties, and conditions favouring the appearance of 12CaO.7Al₂O₃ and 5CaO.Al₂O₃ crystalline phases from the given melt have been specifically dealt with.     

Effects of water vapor, CO2 and SO2 on the NO reduction by NH3 over sulfated CaO

Gas effects on NO reduction by NH₃ over sulfated CaO have been investigated in the presence of O₂ at 700–850 °C. CO₂ and SO₂ have reversible negative effects on the catalytic activity of sulfated CaO. Although H₂O alone has no obvious effect, it can depress the negative effects of CO₂ and SO₂. In the flue gas with CO₂, SO₂ and H₂O co-existing, the sulfated CaO still catalyzed the NO reduction by NH₃. The in situ DRTFTS of H₂O adsorption over sulfated CaO indicated that H₂O generated Br?nsted acid sites at high temperature, suggesting that CO₂ and SO₂ competed for only the molecularly adsorbed NH₃ over Lewis acid sites with NO, without influencing the ammonia ions adsorbed over Br?nsted acid sites. Lewis acid sites shifting to Br?nsted acid sites by H₂O adsorption at high temperature may explain the depression of the negative effect on NO reduction by CO₂ and SO₂.     

Calcium oxides for CO2 capture obtained from the thermal decomposition of CaCO3 particles coprecipitated with Al ions

Calcium-carbonate powders were coprecipitated with Al³⁺ and then decomposed in air and/or under a CO₂ flux between 590 °C and 1150 °C. The data were analysed using a consecutive-decomposition-dilatometer method and the kinetic results were discussed according to the microstructure analysis done by N₂ adsorption isotherms (78 K), SEM and FT-IR measurements. Below 1000 °C, CaCO₃ particle thermal-decomposition was pseudomorphic, resulting in the formation of a CaO grain porous network. When the CaO grains were formed, the Al³⁺ diffused among them, producing AlO₄ groups that promoted the CaO grain coarsening and reduced O²⁻ surface sites available to CO₂ adsorbed molecules to form CO₃²⁻. In pure CaO, CO₃²⁻ diffused through the grain boundary, enhancing Ca²⁺ and O²⁻ mobility; AlO₄ groups reduced CO₃²⁻ penetration and CaO sintering rate. Above 1000 °C, the sintering rate of the doped samples exceeded that of the undoped, likely because of Al³⁺ diffusion in CaO and viscous flow.     

三元复合钙基材料CaO-Ca3Al2O6-MgO的合成及其CO2吸附性能

采用Al₂O₃和MgO同时掺杂改性的方法制备了CaO-Ca₃Al₂O₆-MgO复合钙基高温吸附CO₂材料。复合钙基材料孔隙发达,活性物相为CaO,惰性骨架物相为Ca₃Al₂O₆和MgO。Ca₃Al₂O₆/MgO质量比偏小的材料,表面微粒粒径较小。在10%（体积分数,下同）CO₂和90% N₂的混合气气氛下,采用热重分析仪测量了复合钙基材料吸附CO₂容量、碳化反应速率以及循环碳化（670℃）/煅烧（900℃）过程的稳定性。结果发现,复合钙基材料CaO-Ca₃Al₂O₆-MgO具有较好的吸附CO₂性能,提高Ca₃Al₂O₆/MgO质量比,合成材料的循环稳定性较好;降低Ca₃Al₂O₆/MgO质量比,合成材料的碳化反应速率加快,CaO转化率提高。最后,通过对不同循环次数下复合钙材料的比表面积、孔径分布、微观形貌、表面元素分布,晶相、晶粒大小进行研究分析,对合成材料的失活以及掺杂物质对烧结的抑制机理进行了讨论。     

Coal devolatilization and char conversion under suspension fired conditions in O2/N2 and O2/CO2 atmospheres

The aim of the present investigation is to examine differences between O₂/N₂ and O₂/CO₂ atmospheres during devolatilization and char conversion of a bituminous coal at conditions covering temperatures between 1173 K and 1673 K and inlet oxygen concentrations between 5 and 28 vol.%. The experiments have been carried out in an electrically heated entrained flow reactor that is designed to simulate the conditions in a suspension fired boiler. Coal devolatilized in N₂ and CO₂ atmospheres provided similar results regarding char morphology, char N₂-BET surface area and volatile yield. This strongly indicates that a shift from air to oxy-fuel combustion does not influence the devolatilization process significantly. Char combustion experiments yielded similar char conversion profiles when N₂ was replaced with CO₂ under conditions where combustion was primarily controlled by chemical kinetics. When char was burned at 1573 K and 1673 K a faster conversion was found in N₂ suggesting that the lower molecular diffusion coefficient of O₂ in CO₂ lowers the char conversion rate when external mass transfer influences combustion. The reaction of char with CO₂ was not observed to have an influence on char conversion rates at the applied experimental conditions.     

CO2 gas-activated sintering of carbonated hydroxyapatites

Sintering compacts of carbonated hydroxyapatite (CHA) nanoparticles (3.4 wt% CO₃²⁻) in a CO₂ flow (4 mL/min) proceeded at a temperature which was more than 200 °C lower than that for hydroxyapatite in air (1150 °C). During heating from RT to 1200 °C (5 K/min) the rate of shrinkage of the CHA compacts showed a maximum thrice as high as that in air at about 929 °C. The shrinkage correlates with a mass loss caused by the release of CO₂ due to the thermal decomposition of CO₃²⁻ ions that substitute PO₄³⁻ ions in the CHA lattice. Firing the compacts in the CO₂ flow at 800 and 900 °C for 2 h resulted in an additional carbonatation on the B-sites and a further decrease in the sintering temperature to 890 °C. The compacts fired in the 900-1000 °C range became almost complete ceramics with high densities and mechanical properties close to those of medical implants. Firing at temperatures above 1000 °C resulted in an additional carbonatation on the A-sites. However, this led to a material with low densities and poor mechanical properties. A supposition has been proposed that the effect of CO₂ gas-activated sintering is a result of the intensification of the diffusion in the nanoparticles caused by CO₂ molecules entering the bulk from the CO₂ atmosphere and (or) releasing from the bulk due to the decomposition of carbonates on the B-sites in the lattice.     

羟基磷灰石负载钯催化剂用于H2和O2直接合成H2O2的动力学

H₂和O₂直接合成H₂O₂过程绿色环保,反应具有原子经济性,是最有潜力的H₂O₂合成新方法之一。采用等量浸渍法,将Pd负载于羟基磷灰石（HAp）载体上,得到了高分散的Pd/HAp纳米催化剂,Pd平均粒径2.5 nm。运用幂指数模型,研究该催化剂在H₂O₂加氢及H₂和O₂直接合成反应中的动力学,计算得到H₂O₂加氢、H₂O₂和H₂O的生成反应的表观活化能及O₂、H₂表观反应级数。结果表明低温及高O₂分压有利于H₂O₂的生成,而高H₂分压则有利于H₂O的生成。     

Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures

The present paper was addressed to mineral transformations and ash formation during O₂/CO₂ combustion of pulverized coal. Four Chinese thermal coals were burned in a drop tube furnace to generate ashes under various combustion conditions. The ash samples were characterized with XRD analysis and ⁵⁷Fe Mössbauer spectroscopy. The impacts of O₂/CO₂ combustion on mineral transformation and ash formation were explored through comparisons between O₂/CO₂ combustion and O₂/N₂ combustion. It was found that, O₂/CO₂ combustion did not significantly change the mineral phases formed in the residue ashes, but did affect the relative amounts of the mineral phases. The differences observed in the ashes formed in two atmospheres were attributed to the impact of the gas atmosphere on the combustion temperatures of coal char particles, which consequently influenced the ash formation behaviors of included minerals.     

Modeling and simulation for the methane steam reforming enhanced by in situ CO2 removal utilizing the CaO carbonation for H2 production

The transient behavior of catalytic methane steam reforming (MSR) coupled with simultaneous carbon dioxide removal by carbonation of CaO pellets in a packed bed reactor for hydrogen production has been analyzed through a mathematical model with reaction experiments for model verification. A dynamic model has been developed to describe both the MSR reaction and the CaO carbonation-enhanced MSR reaction at non-isothermal, non-adiabatic, and non-isobaric operating conditions assuming that the rate of the CaO carbonation in a local zone of the packed bed is governed by kinetic limitation or by mass transfer limitation of the reactant CO₂. Apparent carbonation kinetics of the CaO pellet prepared has been determined using the TGA carbonation experiments at various temperatures, and incorporated into the model. The resulting model is shown to successfully depict the transient behavior of the in situ CaO carbonation-enhanced MSR reaction. The effects of major operating parameters on the transient behavior of the CaO carbonation-enhanced MSR have been investigated using the model. The bed temperature is the most important parameter for determining the amount of CO₂ removed by carbonation of CaO, and at temperatures of 600°C, 650°C, 700°C and 750°C, the CO₂ uptake is 1.43, 2.29, 3.5 and -CO₂/kg-CaO, respectively. Simultaneously with the increase in CO₂ uptake with increasing temperature, the corresponding amounts of hydrogen produced are 1.56, 2.54, 3.91 and -H₂/kg-CaO, at the same temperatures as above. Operation at high pressure, high steam to methane feed ratio, and the decreased feed rate at a given temperature are favorable for increasing the degree of the overall utilization of CaO pellets in the reactor bed, and for lowering the CO concentration in the product.     

Similarity and differences in the oxidative dehydrogenation of C2–C4 alkanes over nano-sized VOx species using N2O and O2

The effect of O₂ and N₂O on alkane reactivity and olefin selectivity in the oxidative dehydrogenation of ethane, propane, n-butane, and iso-butane over highly dispersed VO_x species (0.79 V/nm²) supported on MCM-41 has been systematically investigated. For all the reactions studied, olefin selectivity was significantly improved upon replacing O₂ with N₂O. This is due to suppressing CO_x formation in the presence of N₂O. The most significant improving effect of N₂O was observed for iso-butane dehydrogenation: S(iso-butene) was ca. 67% at X(iso-butane) of 25%.Possible origins of the superior performance of N₂O were derived from transient experiments using ¹⁸O₂ traces. ¹⁸O¹⁶O species were detected in ¹⁸O₂ and ¹⁸O₂–C₃H₈ transient experiments indicating reversible oxygen chemisorption. In the presence of alkanes, the isotopic heteroexchange of O₂ strongly increased. Based on the distribution of labeled oxygen in CO_x and in O₂ as well as on the increased CO_x formation in sequential O₂–C₃H₈ experiments, it is suggested that non-lattice oxygen species (possibly of a bi-atomic nature) originating from O₂ are non-selective ones and responsible for CO_x formation. These species are not formed from N₂O.     

The role of sulphates in cement clinkering reactions: phase formation in the system CaOAℓ2O3Fe2O3SiO2CaSO4K2SO4

The CaO-rich portions of the systems CaOA?₂O₃SiO₂K₂SO₄, CaOA?₂O₃K₂SO₄, CaOA?₂O₃SiO₂K₂SO₄CaSO₄ and CaOA?₂O₃Fe₂O₃CaSO₄K₂SO₄ have been studied experimentally. Schemes are presented showing phase assemblages compatible at subsolidus temperatures. Melting commences at about 825°C in assemblages containing K₂SO₄. Silicate, aluminate and ferrite phases are comparatively insoluble in molten alkali-rich sulphates. At clinkering temperatures, two immiscible liquids form; one is rich in K₂O and sulphate, the other is a silicate liquid. Some features of vapour-liquid-solid equilibria relevant to the S cycle in cement kilns are discussed.     

Superplastic deformation of hydroxyapatite ceramics with B2O3 or Na2O addition fabricated by pulse current pressure sintering

High-density and fine-grained transparent hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: HAp) ceramics with B₂O₃ and Na₂O addition were fabricated using pressureless sintering and pulse-current pressure sintering between 1000 and 1100 °C; the superplastic deformation of these HAp specimens was evaluated. The relative density of pure HAp compacts pulse-current pressure sintered at 1000 °C for 10 min under a pressure of 50 MPa attained 99.9% and exhibited translucency. The tensile elongation of the pure HAp specimen, which was measured at 1000 °C under a strain rate of 1.48 × 10⁻⁴ s⁻¹, was as high as 364%. The relative density of HAp compacts with 3.0 mol.% B₂O₃ addition pulse-current pressure sintered under the same conditions as those of pure HAp compacts was 98.9%, whereas the grain size was as low as 0.24 μm. The elongation of HAp specimens, measured at a test temperature of 1000 °C under a strain rate of 1.48 × 10⁻⁴ s⁻¹, was as high as 578%.     

In situ diagnostics of Victorian brown coal combustion in O2/N2 and O2/CO2 mixtures in drop-tube furnace

Experimental investigation of the combustion of an air-dried Victorian brown coal in O₂/N₂ and O₂/CO₂ mixtures was conducted in a lab-scale drop-tube furnace (DTF). In situ diagnostics of coal burning transient phenomena were carried out with the use of high-speed camera and two-colour pyrometer for photographic observation and particle temperature measurement, respectively. The results indicate that the use of CO₂ in place of N₂ affected brown coal combustion behaviour through both its physical influence and chemical interaction with char. Distinct changes in coal pyrolysis behaviour, ignition extent, and the temperatures of volatile flame and burning char particles were observed. The large specific heat capacity of CO₂ relative to N₂ is the principal factor affecting brown coal combustion, which greatly quenched the ignition of individual coal particles. As a result, a high O₂ fraction of at least 30% in CO₂ is required to match air. Moreover, due to the accumulation of unburnt volatiles in the coal particle vicinity, coal ignition in O₂/CO₂ occurred as a form of volatile cloud rather than individual particles that occurred in air. The temperatures of volatile flame and char particles were reduced by CO₂ quenching throughout coal oxidation. Nevertheless, this negative factor was greatly offset by char-CO₂ gasification reaction which even occurred rapidly during coal pyrolysis. Up to 25% of the nascent char may undergo gasification to yield extra CO to improve the reactivity of local fuel/O₂ mixture. The subsequent homogeneous oxidation of CO released extra heat for the oxidation of both volatiles and char. As a result, the optical intensity of volatile flame in ∼27% O₂ in CO₂ was raised to a level twice that in air at the furnace temperature of 1273 K. Similar temperatures were achieved for burning char particles in 27% O₂/73% CO₂ and air. As this O₂/CO₂ ratio is lower than that for bituminous coal, 30-35%, a low consumption of O₂ is desirable for the oxy-firing of Victorian brown coal. Nevertheless, the distinct emission of volatile cloud and formation of strong reducing gas environment on char surface may affect radiative heat transfer and ash formation, which should be cautioned during the oxy-fuel combustion of Victorian brown coal.     

New acrylic monolithic carbon molecular sieves for O2/N2 and CO2/CH4 separations

The modification of activated carbon fibres prepared from a commercial textile acrylic fibre into materials with monolithic shape using phenolic resin as binder was studied. The molecular sieving properties for the gas separations CO₂/CH₄ and O₂/N₂ were evaluated from the gas uptake volume and selectivity at 100 s contact time taken from the kinetic adsorption curves of the individual gases. The pseudo-first order rate constant was also determined by the application of the LDF model. The samples produced show high CO₂ and O₂ rates of adsorption, in the range 3-35 × 10⁻³ s⁻¹, and in most cases null or very low adsorption of CH₄ and N₂ which make them very promising samples to use in PSA systems, or similar. Although the selectivity was very high, the adsorption capacity was low in certain cases. However, the gas uptake in two samples reached 23 cm³ g⁻¹ for CO₂ and 5 cm³ g⁻¹ for O₂, which can be considered very good. The materials were heat-treated using a microwave furnace, which is a novel and more economic method, when compared with conventional furnaces, to improve the molecular sieves properties.     

Reaction between CO2 and CaO under dry grinding

This study was carried out to investigate the reaction between CO₂ and materials that contain CaO under dry grinding. Chemical reagent CaO was used in this experiment, and waste concrete was also tested to examine the feasibility of CO₂ sorption into it. Samples were ground in a CO₂ atmosphere by a centrifugal ball mill. The reaction was measured with the constant volume method. The effects of amount of sample, the number and diameter of balls, the concentration of CO₂ in the mixture of CO₂ and air and the rotational speed on the CO₂ sorption were examined. The amount of the CO₂ sorption under grinding was larger than that without grinding. The grinding enhanced the reaction between CaO and CO₂. The CO₂ sorption steeply increased with time in the early stage of grinding. After that, it increased gradually. The CO₂ reacted with the CaO at the surface layer of the newly exposed surface of the CaO particles during the grinding. The initial sorption rate of CO₂ was related with the shear force. In the latter stage of grinding, the grinding process caused the CaO particles to agglomerate. As a result, the sorption of CO₂ became slow. It was found that the waste concrete had high potential for sorption of CO₂ by means of dry grinding.     

Preparation of a highly microporous carbon from a carpet material and its application as CO2 sorbent

In order to increase the use of carpet wastes (pre- and/or post-consumer wastes), this work studies for the first time the preparation and characterisation of a microporous material from a commercial carpet (pile fiber content: 80% wool/20% nylon; primary and secondary backings: woven polypropylene; binder: polyethylene) and its application for CO₂ capture. The porous material was prepared from an entire carpet material using a standard chemical activation with KOH and then, characterised in terms of their porous structure and surface functional groups. Adsorption of CO₂ was studied using a thermogravimetric analyser at several temperatures (25-100 °C) and under different CO₂ partial pressures (i.e. pure CO₂ flow and a ternary mixture of 15% CO₂, 5% O₂ and 80% N₂). In order to examine the adsorbent regenerability, multiple CO₂ adsorption/desorption cycles were also carried out. The surface area and micropore volume of the porous adsorbent were found to be 1910.17 m² g^− 1 and 0.85 cm³ g^− 1, respectively. The CO₂ adsorption profiles illustrate that the maximum CO₂ capture on the sample was reached in less than 10 min. CO₂ adsorption capacities up to 8.41 wt.% and 3.37 wt.% were achieved at 25 and 70 °C, respectively. Thermal swing regeneration studies showed that the prepared adsorbent has good cyclic regeneration capacities.