Effects of steam on the kinetics of calcium carbonate calcination

Study on Pyrolysis Characteristics of Phosphate Tailings under H2O Atmosphere

The pyrolysis separation of calcium and magnesium from phosphate tailings is an important process due to its high-value resource utilization. In this paper, aiming to address the problems of high energy consumption, a slow decomposition rate and the low activity of decomposition products in the high-temperature pyrolysis of phosphate tailings, the medium-temperature pyrolysis of phosphate tailings under a H2O atmosphere was carried out, and the phase reconstruction and activation of pyrolysis process were discussed. The results showed that compared with N2, air and CO2 atmospheres, the pyrolysis process of phosphate tailings in a H2O atmosphere was changed from two stages to one stage, the starting decomposition temperature was reduced to 500 °C and the decomposition time was shortened to 30 min. The order of the influence of each factor on the pyrolysis of phosphate tailings was temperature > H2O pressure > holding time. Under the optimized pyrolysis conditions, the yield of CaMg(CO3)2 decomposition of phosphate tailings into MgO and CaO was 97.3% and 98.1%, respectively, and the reactivity of MgO was 31.6%. The distribution of Ca and Mg elements in the phosphate tailings after pyrolysis showed a negative correlation, and both of them no longer formed associated compounds; Ca mainly existed in the form of Ca(OH)2, Ca5(PO4)3F, CaSiO3 and CaF2, and Mg mainly existed in the form of MgO, MgF2 and Mg(OH)2.

Effect of Steam on Carbonation of CaO in Ca-Looping

Ca-looping is an effective way to capture CO₂ from coal-fired power plants. However, there are still issues that require further study. One of these issues is the effect of steam on the Ca-looping process. In this paper, a self-madethermogravimetric analyzer that can achieve rapid heating and cooling is used to measure the change of sample weight under constant temperature conditions. The parameters of the Ca-looping are studied in detail, including the addition of water vapor alone in the calcination or carbonation stage and the calcination/carbonation reaction temperatures for both calcination and carbonation stages with water vapor. Steam has a positive overall effect on CO₂ capture in the Ca-looping process. When steam is present in both calcination and carbonation processes, it increases the decomposition rate of CaCO₃ and enhances the subsequent carbonation conversion of CaO. However, when steam was present only in the calcination process, there was lower CaO carbonation conversion in the following carbonation process. In contrast, when steam was present in the carbonation stage, CO₂ capture was improved. Sample characterizations after the reaction showed that although water vapor had a negative effect on the pore structure, adding water vapor increased the diffusion coefficient of CO₂ and the carbonation conversion rate of CaO.

Scale-up disaggregation of palygorskite crystal bundles via ultrasonic process for using as potential drilling fluid

High-efficient disaggregation of palygorskite (PAL) crystal aggregates into individual nanorods is the key to exploiting its nanometer properties, which remains a challenge at present. The sonochemical cavitation effects have been successfully employed for the intensification of physical and chemical processing applications, but it still lacks the relevant study on the scale-up disaggregation of PAL crystal bundles. Here, the energy-efficient, scale-up ultrasonic process was developed to disaggregate PAL aggregates in batches, and the effects of ultrasonic treatment time, temperature, and power on physicochemical features of PAL were systematically investigated. The results showed that the single dispersed PAL nanorods could be continuously produced by sonicating 15 wt% of PAL suspension at 20 kHz, 2000 W and 30 °C for 5 min retaining the original nanorod length and layered-chain structure. It also greatly improved the dispersion of nanorod crystal, specific surface area and suspension stability of PAL. The ultrasonically disaggregated PAL has a higher pulping rate in water (14.96 m³/t) and saturated NaCl system (14.45 m³/t), which is significantly better than that of natural PAL in water (14.72 m³/t) and saturated NaCl solution (12.37 m³/t). It suggests that the disaggregated PAL exhibits excellent potential and adaptability as a viscosity enhancer for drilling fluid. Therefore, this work provides a feasible and efficient ultrasonic process for large-scale industrialized disaggregation of PAL crystal bundles, laying a foundation for the high-value utilization of natural PAL as one-dimensional nanomaterials.

Effect of Steam Injection during Carbonation on the Multicyclic Performance of Limestone (CaCO3) under Different Calcium Looping Conditions: A Comparative Study

This study explores the effect of steam addition during carbonation on the multicyclic performance of limestone under calcium looping conditions compatible with (i) CO₂ capture from postcombustion gases (CCS) and with (ii) thermochemical energy storage (TCES). Steam injection has been proposed to improve the CO₂ uptake capacity of CaO-based sorbents when the calcination and carbonation loops are carried out in CCS conditions: at moderate carbonation temperatures (∼650 °C) under low CO₂ concentration (typically ∼15% at atmospheric pressure). However, the recent proposal of calcium-looping as a TCES system for integration into concentrated solar power (CSP) plants has aroused interest in higher carbonation temperatures (∼800-850 °C) in pure CO₂. Here, we show that steam benefits the multicyclic behavior in the milder conditions required for CCS. However, at the more aggressive conditions required in TCES, steam essentially has a neutral net effect as the CO₂ uptake promoted by the reduced CO₂ partial pressure but also is offset by the substantial steam-promoted mineralization in the high temperature range. Finally, we also demonstrate that the carbonation rate depends exclusively on the partial pressure of CO₂, regardless of the diluting gas employed.