Conversion Of Limestone Research Articles

Multiscale modeling of an entrained flow solar calciner for thermochemical energy storage via calcium looping

Calcium looping has been proposed as an alternative route addressing the intermittent nature of solar energy in terms of thermochemical energy storage, with the added benefit of CO2 capture. In this work, a multiscale approach aims to predict and optimize the performance of a pilot-scale solar calciner operating at realistic conditions, incorporating a diffusion–reaction model with multiphase flow considerations in a downer reactor. The model has been successfully validated with experimental data from the literature studying the effect of particle size on the observed rate, and with limestone decomposition experiments carried out in a lab -scale downer reactor. It was revealed that particles larger than 100 μm exhibit internal mass transport resistances, due to CO2 accumulation within the particle, whereas heat transport limitations are not expected in the operating conditions of interest. A reference scenario demonstrated 92 % limestone conversion at realistic operating conditions, feasible in concentrated solar power plants. The solar flux density on the reactor wall influences the efficiency of the system critically since heat loss to the environment leads to greater solar input. To tackle the low efficiency, a partially insulated, indirectly irradiated reactor was investigated: the efficiency increased from 8 % to 26 %, while integration in a Calcium-Looping system can further boost the efficiency to 34 %. This approach constitutes a basis for future scale-up considerations.

A mathematical model of a twin-shaft parallel flow regenerative lime kiln

ABSTRACT Lime is an important flux used in the iron and steel-making processes. Tata Steel Jamshedpur uses a twin-shaft Parallel Flow Regenerative type kiln to calcine limestone. Previously, the kiln was operated based on the temperature measurement at only three locations. This led to suboptimal kiln performance and difficulty in maintaining lime quality. A thermal digital twin was developed using first principles to address these issues. A set of unsteady-state partial-differential equations representing the thermophysical phenomena is solved numerically using the finite volume method. The model was validated with the measured flue gas temperature and lime temperatures. The model is implemented online with the real-time operating parameters to estimate the temperature profile of solid and gas and the degree of conversion of limestone. This real-time visibility enables the operators to optimize the process and achieve lime conversion up to 94%, resulting in energy savings of 31.8 GJ/day and stable lime quality.

Transesterification of Waste Cooking Oil using CaO Catalyst Derived from Madura Limestone for Biodiesel Production and Its Application in Diesel Engine

In this study, we report biodiesel production from waste cooking oil using CaO catalyst derived from Madura limestone through a transesterification reaction. Many limestone quarries in Madura can be used as heterogeneous catalysts because they are cheap, easy to separate, and have high basicity. Conversion of limestone into CaO catalyst through calcination at 900°C for 3 hours. The CaO catalyst formed was characterized using X-Ray Diffraction (XRD), Fourier Transform Infra-Red (FTIR), and Scanning Electron Microscopy-Energy Dispersive X-Ray (SEM-EDX) instruments. Biodiesel formed through the transesterification reaction was analyzed using GC-MS. Furthermore, biodiesel blends from waste cooking oil and pure diesel were prepared in volume percentages (B-10, B-20, B-30, B-40, and B-100) for testing on diesel engine performance. The results of testing the highest torque and brake horsepower (BHP) were obtained on pure diesel fuel (S-100) at 2.49 Nm and 381.12 watts, respectively. The lowest fuel consumption at 1500 rpm is produced on the B-20 at 0.186 kg/h. Overall, the emission characteristics of carbon monoxide (CO), nitrogen oxides (NOx), and nitrogen monoxide (NO) with the lowest concentration resulted from biodiesel blends rather than pure diesel.

Ultrasonic-assisted conversion of limestone into needle-like hydroxyapatite nanoparticles

Needle-like hydroxyapatite nanoparticles were successfully synthesized via a reaction between calcium oxide (CaO) that was obtained from calcined limestone and orthophosphoric acid (H3PO4) under ultrasonic irradiation at 25 °C. The reaction systems were exposed to ultrasonic waves of 20 kHz for various times ranging from 0 to 4 h. The initial and final pH values of the mixtures of CaO and H3PO4 solution were continuously observed (pH < 4.0) after ultrasonic irradiation. The powder was then dried at 60 °C and calcined at 300 °C for 3 h (3 °C/min). The products were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The results showed that the formation of needle-like hydroxyapatite (HAp) nanoparticles was substantially accelerated compared with the reaction without ultrasonic irradiation. The HAp phase was increasingly visible with longer ultrasonic irradiation time compared with the monetite phase (CaHPO4). This suggests that ultrasonic waved induced a phase transition from the monetite to HAp phase. A smaller needle-like structure of HAp (diameter ∼ 7.4 nm) with a lower contamination of monetite phase was obtained following sonication for 3 h. This study shows that Thai limestone can used as a starting material for synthesizing needle-like HAp nanoparticles with the aid of ultrasonic methods.

Trace Na2CO3 Addition to Limestone Inducing High-Capacity SO2 Capture

Although the literature has reported enhanced indirect sulfation of limestone by adding Na2CO3, the amount of Na2CO3 additive required to achieve high CaO conversion is typically high (∼4.0 mol %), which commonly results in adverse effects in fluidized-bed combustion boiler systems and increased cost of sorbents. In this work, we demonstrate for the first time that trace Na2CO3 (0.1 mol %) can significantly enhance the sulfate conversion of limestone. This enhanced sulfation is attributed to the increased surface area and optimized pore size distribution. The trace Na2CO3 additive splits the pores of the original sorbents peaking at ∼70 nm into pores peaking at ∼4 nm and ∼140 nm due to the slight promotion of sintering. This well-developed pore structure results in a relatively high reactivity for sulfation. Thus, the Na2CO3 additive influences the sorbent reactivity in two ways: (1) at less than 0.5 mol %, tuning its pore structure; (2) at more than 0.5 mol %, promoting the product layer diffusion. We also find that trace amount of other metal salts, such as CaCl2 and NaCl, clearly enhance the sulfation of limestone. The strategy of enhancing limestone sulfation by the addition of trace amount of metal salts offers evident engineering and economic advantage.

Decomposition of Limestone in Hot Metal at 1300 °C

Decomposition and dissolution of cylindrical limestone samples (ø16 mm × 50 mm approximately) in hot metal at 1300 °C are experimentally studied. The cross section of burned limestone is determined by using image processing and then the lime layer thickness is determined. The three layers of the cross section of burned sample are analyzed by using SEM, and the change of the Fe content along the radial direction is also analyzed. A kinetic model is established to characterize the calcination of limestone in hot metal. The results show that there are three distinct layers on cross section of burned limestone: a reacted layer in the outside, a burned lime layer in the middle, and an unburned limestone core in the inside. Heat transfer is the control step for limestone calcination in hot metal. Fe can be oxidized by carbon dioxide from limestone calcination, and ferrous oxide can permeate into burned lime layer along the radial direction of sample. The kinetic model for limestone calcination can be used to predict limestone conversion rate in hot metal.

High and stable [formula omitted] capture capacity of natural limestone at Ca-looping conditions by heat pretreatment and recarbonation synergy

The Ca-looping (CaL) process, based on the multicyclic carbonation/calcination of limestone derived CaO, has emerged recently as a potentially economically advantageous technology to achieve sustainable postcombustion and precombustion CO2 capture efficiencies. Yet, a drawback that hinders the efficiency of the CaL process is the drastic drop of limestone capture capacity as the number of carbonation/calcination cycles is increased. Precalcination of limestone at high temperatures for a prolonged period of time has been proposed as a potential technique to reactivate the sorbent, which is however precluded by regeneration temperatures above 850°C and low CO2 concentrations in the carbonator to be found in the practical situation. Under these conditions, heat pretreatment leads to a stable yet very small CaO conversion. On the other hand, the introduction of a recarbonation stage between the ordinary carbonation and calcination stages has been shown to decelerate the rate of sorbent activity decay even though this favorable effect is not noticeable up to a number of above 10–15 cycles. The present manuscript demonstrates that the synergetic action of heat pretreatment and recarbonation yields a high and stable value for the multicyclic conversion of limestone derived CaO. It is foreseen that recarbonation of heat pretreated limestone would lead to a reduction of process costs especially in the case of precombustion applications. Even though sorbent purging will always be needed because of ash accumulation and sulphation in postcombustion CO2 capture applications, the stable and high multicyclic CaO conversion achieved by the combination of these techniques would make it necessary to a lesser extent.

Multicyclic conversion of limestone at Ca-looping conditions: The role of solid-sate diffusion controlled carbonation

Limestone derived CaO conversion when subjected to multiple carbonation/calcination cycles is a subject of interest currently fueled by several industrial applications of the so-called Ca-looping (CaL) technology. The multicyclic CaO conversion at Ca-looping conditions exhibits two main features as demonstrated by thermogravimetric analysis (TGA). On one hand, carbonation occurs by two well differentiated phases: a first kinetically-driven fast phase and a subsequent much slower solid-state diffusion controlled phase. On the other, carbonation in the fast phase usually shows a drastic decay with the cycle number along the first carbonation/calcination cycles. This trend can be reversed by means of heat pretreatment, which induces a marked loss of fast conversion in the first carbonation but enhances diffusion of CO2 in the solid. Upon decarbonation the regenerated CaO skeleton displays an increased conversion in the fast carbonation phase of the next cycle, a phenomenon which has been referred to as reactivation. Nonetheless, sorbent reactivation is hampered by looping carbonation/calcination conditions as those to be likely found in practice such as carbonation stages characterized by low CO2 concentrations and short duration and calcination stages at high temperatures in a CO2 enriched atmosphere, which causes a sintering and loss of activity of the regenerated CaO skeleton. We analyze in this work sorbent reactivation as affected by heat pretreatment and carbonation/calcination conditions. Aimed at shedding light on the role played by these conditions on reactivation we look separately at the multicyclic evolution of conversion in the kinetic and diffusive phases. Generally, the evolution of multicyclic conversion after the first cycle can be described by a balance between the surface area gain due to diffusive carbonation and the surface area loss as caused by sintering in the calcination stage. A significant gain of relative surface area after the first cycle, which is favored by harshening the heat pretreatment conditions, leads however to a marked decay of it during subsequent cycles, which precludes reactivation for an extended interval of cycles. On the other hand, sorbent grinding, if performed before heat pretreatment, leads to a less marked but more sustainable reactivation along the cycles. A novel observation reported in our work is that pretreatment of limestone in a CO2 atmosphere leads upon a subsequent quick decarbonation to a CaO skeleton with extraordinarily enhanced reactivity in the kinetically-driven carbonation phase and with a high resistance to solid-state diffusion, which can be attributed to annealing of the crystal structure as reported by independent studies.

Microstructure Improvement of Modified Calcium-Based Sorbents during Cyclic CO&lt;sub&gt;2&lt;/sub&gt; Capture

Limestone is modified with acetic acid solution as a CO2sorbent in order to solve the problem that the carbonation conversion of limestone decays sharply with the number of cycles. The results show that the carbonation conversion of modified limestone is much higher than original one, and it still achieves as high as 0.5 after 20 cycles. The mechanism of the carbonation conversion improvement for modified limestone was investigated by means of XRD, SEM and nitrogen adsorption instrument. It indicates that the grain size of CaO from modified limestone is smaller than original one. The acetification restrains the increase of grain size for CaO as the number of cycles increase. Sequentially, the extent of sintering for modified limestone is reduced. The fractal dimension of calcined modified limestone is higher than calcined original one. The inner surface of calcined modified limestone is much rougher and more complicated. The acetification makes limestone produce abundant pores. The surface area and pore volume of calcined modified limestone are far greater than those of calcined original one after the same cycles.

Agglomeration of Sorbent Particles during Sulfation of Lime in the Presence of Steam

Previous investigations of FBC systems utilizing limestone addition for SO2 capture have shown that sintering due to chemical reaction (sulfation) and related solid-state diffusion are important causes for agglomeration/fouling in FBC boiler systems firing high-sulfur fuels. When the original work was done it was widely believed that steam did not affect indirect sulfation and, therefore, its effects on agglomeration were not considered. However, recent research clearly shows that steam at levels produced by burning hydrocarbon fuels can significantly improve sulfation by enhancing solid state diffusion and limestone conversion. The present study revisits the earlier studies on agglomeration but in the presence of steam. The agglomeration tendency of two limestones, Kelly Rock (Canada) and Katowice (Poland, Upper Silesia) was tested, using two particle sizes (75−115 and 250−425 μm) in pelletized and nonpelletized forms. The samples were sulfated for up to seven days in a tube furnace (TF) and thermogravimetric analyzer (TGA) by a gas mixture containing 15% of H2O(g). The results were compared with those obtained with no steam present during reaction. The residues after sulfation were examined by scanning electron microscopy (SEM) and their surfaces and profiles were analyzed by energy dispersive X-ray (EDX) spectroscopy. The results showed that steam greatly affects sulfation and solid state diffusion and this caused enhanced agglomeration. Similar agglomeration effects noticed after 100 days of sulfation time in past studies were shown to occur in just one day in the presence of steam. SEM analysis showed that sulfated particles agglomerate, and connections between particles due to sulfation are often stronger than the original strength of the particles themselves. The effect of agglomeration is more pronounced in the case of smaller particles and/or pelletized samples. These results suggest that steam may play a significant role in the fouling of industrial boilers firing high-sulfur fuels.

Book Reviews

Book Reviews

A coupled fluid dynamic-discrete element simulation of heat and mass transfer in a lime shaft kiln

In the present study heat and mass transfer related to the chemical conversion of limestone to quicklime in a shaft kiln are investigated by means of a coupled numerical scheme for gas and solid phase transport. The three-dimensional transport of mass, momentum and energy in the gas phase is modelled by computational fluid dynamics (CFD), while a discrete element method (DEM) is employed for the mechanical movement and the conversion reactions of the solid material. The DEM simulation readily describes the mechanical and thermal particle-to-particle interactions of a large number of differently sized particles. Novel aspects addressed in this work are the simultaneous effects of inner particle heat-conduction and pore-diffusion of the gaseous product of the calcination reaction (CO 2) modelled by a shrinking core approach. Simulations of laboratory scale experiments of single reacting spheres show good agreement with the measured conversion rates. Simulations of an idealised vertical shaft kiln including pressure drop calculations demonstrate the suitability of the proposed approach for the modelling of industrial scale systems.

Experimental study on CO 2 capture conditions of a fluidized bed limestone decomposition reactor

In this study, the decomposition conditions of limestone particles (0.25–0.50 mm) for CO 2 capture in a steam dilution atmosphere (20–100% steam in CO 2) were investigated by using a continuously operating fluidized bed reactor. The results show that the decomposition conversion of limestone increased with the steam dilution percentage in the CO 2 supply gas. At a bed temperature of 920 °C, the conversions were 72% without steam dilution and 98% with 60% steam dilution. The conversion was 99% with 100% steam dilution at 850 °C of the bed temperature. Steam dilution can decrease not only the decomposition temperature of limestone, but also the residence time required for nearly complete decomposition of CaCO 3. The hydration and carbonation reactivities of the CaO produced were also tested and the results show that both the reactivities increased with the steam dilution percentage for decomposing limestone.

Limestone Attrition under Simulated Oxyfiring Fluidized‐Bed Combustion Conditions

AbstractLimestone attrition by surface wear was studied during the flue gas desulfurization under simulated fluidized‐bed (FB) oxyfiring conditions and hindered calcination. Bench‐scale experimental tests were carried out using well‐established techniques previously developed for the characterization of sulfation and attrition of sorbents in air‐blown atmospheric FB combustors. The experimental limestone conversion and attrition results were compared with those previously obtained with the same limestone under simulated air‐blown combustion conditions. The differences in the conversion and attrition extents and patterns associated with oxyfiring as compared to air‐blown atmospheric combustion were highlighted and related to the different particle morphologies and thicknesses of the sulfate layer. It was noted that attrition could play an important role in practical circulating FB combustor operation, by effectively enhancing particle sulfation under both oxyfiring and air‐blown combustion conditions.

Formation of replacement dolomite in the Latemar carbonate buildup, Dolomites, northern Italy: Part 2. Origin of the dolomitizing fluid and the amount and duration of fluid flow

Replacement dolomite in the Latemar carbonate buildup developed when limestone was infiltrated by reactive fluid. Minor-element, trace-element, and oxygen and carbon isotope compositions of dolomite and precursor limestone constrain the origin of the fluid and fundamental aspects of the flow. Inferred salinity (similar to seawater); temperature (45°–85°C); ⁸⁷Sr/⁸⁶Sr (0.7076–0.7079); Ca/Mg (&lt;1.4); and Fe, Mn, and Zn content of the dolomitizing fluid are consistent with a fluid similar to modern diffuse effluent. Modern diffuse effluent itself is approximately a mixture of seawater with up to ∼25 percent high-temperature mid-ocean ridge hydrothermal vent fluid. Time-integrated fluid flux was in the range (2–4) · 10⁶ mol fluid/cm² rock or (4–7) · 10⁷ cm³ fluid/cm²rock. Estimation of time-integrated flux leads to an internally consistent framework for the appropriate interpretation of the oxygen, strontium, and carbon isotope compositions of replacement dolomite. The oxygen and strontium isotope compositions reflect equilibration with dolomitizing fluid and provide a chemical fingerprint of the fluid. The carbon isotope composition of dolomite, however, was simply inherited directly from the precursor limestone in nearly all cases. A quantitative evaluation of the minor- and trace-element budget of dolomitization verifies that a fluid like modern diffuse effluent, but not unmodified seawater, could supply sufficient Fe, Mn, and Zn to enrich dolomite in these elements compared to limestone. If the flux of dolomitizing fluid was similar to that of modern diffuse effluent, ∼0.02 cm³/cm² · s, the duration of fluid flow and mineral-fluid reaction was short, ∼100 years. The total duration of dolomitization, however, could have been much longer if fluid flow was episodic, as in modern seafloor hydrothermal systems, depending on the time elapsed between episodes of flow. Conversion of limestone to dolomite likely occurred by a mechanism intermediate between the end-member cases of replacement at constant oxygen and carbon and replacement at constant volume.

Limestone Calcination with CO 2 Capture (II): Decomposition in CO 2/Steam and CO 2/N 2 Atmospheres

Decomposition of limestone particles (0.25−0.5 mm) in a steam dilution atmosphere (20−100% steam in CO 2) was investigated by using a continuously operating fluidized bed reactor for CO 2 capture. The decomposition conversion of limestone increased as the steam dilution percentage in the CO 2 supply gas increased. At a bed temperature of 1193 K, the conversions were 72% without dilution (100% CO 2) and 98% with 60% steam dilution. The decomposition conversions obtained with steam dilution and N 2 dilution differed significantly, and this result is explained in terms of the difference between the heat transfer to particle in steam and N 2 dilution atmosphere. The reactivities of the CaO produced from limestone decomposition with steam dilution and without dilution (100% CO 2) were tested by means of hydration and carbonation reactions. In the hydration test, the time required for complete conversion [CaO → Ca(OH) 2] of the CaO produced by steam dilution was approximately half that required for the CaO prod...

Attrition-Enhanced Sulfur Capture by Limestone Particles in Fluidized Beds

Sulfur capture by limestone particles in fluidized beds is a well-established technology. The underlying chemical and physical phenomena of the process have been extensively studied and modeled. However, most of the studies have been focused on the relatively brief initial stage of the process, which extends from a few minutes to hours, yet the residence time of the particles in the boiler is much longer. Following the initial stage, a dense product layer will be formed on the particle surface, which decreases the rate of sulfur capture and the degree of utilization of the sorbent. Attrition can enhance sulfur capture by removing this layer. A particle model for sulfur capture has been incorporated with an attrition model. After the initial stage, the rate of sulfur capture stabilizes, so that attrition removes the surface at the same rate as diffusion and chemical reaction produces new product in a thin surface layer of a particle. An analytical solution for the conversion of particles for this regime is presented. The solution includes the effects of the attrition rate, diffusion, chemical kinetics, pressure, and SO{sub 2} concentration, relative to conversion-dependent diffusivity and the rate of chemical reaction. The particle model results in models thatmore » describe the conversion of limestone in both fly ash and bottom ash. These are incorporated with the residence time (or reactor) models to calculate the average conversion of the limestone in fly ash and bottom ash, as well as the efficiency of sulfur capture. Data from a large-scale pressurized fluidized bed are compared with the model results.« less

Mechanism of Highly Efficient In-Furnace Desulfurization by Limestone under O2/CO2 Coal Combustion Atmosphere

The desulfurization characteristics of limestone under O2/CO2 atmosphere were studied by a drop-tube furnace test rig. The experimental results show that high CO2 concentration makes the calcination reaction of limestone slow and that the pore structure of calcined limestone is more suitable for high-temperature desulfurization under O2/CO2 atmosphere. A desulfurization mechanism for limestone under O2/CO2 coal combustion atmosphere is proposed based on the experiments. The system keeps on producing nascent CaO continuously. At the same time, the diffusion resistance through CaSO4 product layer is reduced because CO2 is formed at the CaO/CaCO3 interface before complete limestone decomposition into CaO. The above two factors make CaO keep on a high sulfation rate for a long time under O2/CO2 atmosphere. So the high Ca conversion of limestone can be reached under O2/CO2 atmosphere. There is an optimal desulfurization temperature for limestone in O2/CO2 atmosphere as in air. And the optimal desulfurization temperature and corresponding desulfurization efficiency increase with the CO2 concentration in O2/CO2 atmosphere. It is implied that the O2/CO2 atmosphere is beneficial to limestone desulfurization at higher temperature.

Modeling of in-line low-NO x calciners—a parametric study

Simulations with a heterogeneous model of an in-line low-NO x calciner, based on non-isothermal diffusion-reaction models for char combustion and limestone calcination combined with a kinetic model for NO formation and reduction, are reported. The analysis shows that the most important hydrodynamic parameter is the mixing rate of preheated combustion air into the sub-stoichiometric suspension leaving the reducing zone and the most important combustion parameter is the char reactivity. Also, the calcination rate modifies very much the temperature in the calciner, char and limestone conversion and NO emission. Carbon monoxide is a key component for the reduction of NO and reliable data for the kinetics of NO reduction by CO over CaO are very important for the prediction of the NO emission. The internal surface area of char and limestone particles influences the combustion and calcination rates and thereby the char and limestone conversion and the NO emission.

Influence of Periodically Changing CO2 Partial Pressure on Sulfur Capture and Free Lime Content of Residues in PFBC

PFBC technology is still under development. Further research into the capture of SO2 by limestone during PFBC is needed. The present work investigates the influence of periodically changing partial pressure of CO2 on the sulfation of limestone and also the free lime content present in residual products. Experiments were conducted in a fixed-bed reactor under conditions typical of PFBC, that is, limestone particles were alternately exposed to the conditions of calcination and recarbonation as the reaction of sulfation proceeded. It was found that the degree of sulfation achieved was considerably higher with periodically changing partial pressure of CO2 than with a steady high partial pressure of CO2. This was true even when the average value of the pressure during its periodical change exceeded the calcination equilibrium pressure. These findings may partly explain the high conversion of limestone obtained in PFBC units. Furthermore, the free lime content proved remarkably different for various cycle times...