Ca-looping Technology Research Articles

Mechanism analysis of carbide slag capture of CO2via a gas-liquid-solid three-phase fluidization system

In the previous work, the industrial residue-carbide slag was used to capture CO2via Ca-looping technology with high-temperature (above 823 K). Few studies are dealing with the capture of CO2 capture at low or even ambient temperature with carbide slag. In this work, based on an environment-friendly and cost-effective approach, carbide slag has been utilized to capture CO2 at low or even ambient temperature. It was confirmed that Ca can be transformed into CaCO3 completely via a three-phase fluidization system, at ambient temperature and pressure through theoretical and experimental investigations. In this process, an absorption value of 18.13 wt% can be achieved, which is higher than the values obtained in the case of some other Ca-based absorbents. After analysis the reaction mechanism, it was found that the model of carbide slag capture of CO2via gas-liquid-solid three-phase fluidization system can be expressed based on the two-film theory and Fick’s law at the gas-liquid and liquid-solid interface. This study provides an investigation of a clean and energy-saving method to capture CO2, and an excellent pathway to reuse the industrial waste-carbide slag.

Thermodynamics and kinetics analysis of Ca-looping for CO2 capture: Application of carbide slag

The calcium looping process, which is based on the calcination/carbonation of CaCO3, has emerged over the years as a recommended low cost technology for post-combustion CO2 capture. In this work, a novel Ca-based material (carbide slag) is proposed for CO2 capture through the Ca-looping method. Carbide slag is a by-product generated in the process of acetylene production, and its main components are Ca(OH)2 and CaCO3. In this work, CO2 was chemisorbed by CaO solid particles derived from precalcination of inexpensive and widely available carbide slag. The reaction mechanism was investigated by the thermodynamics software-FactSage and by Thermogravimetric Analysis-Differential Thermal Analysis (TG-DTA). The study found that the temperature of CO2 capture should be more than 650 °C and lower than 900 °C. Moreover, the carbide slag showed good stability for over 20 cycles of calcination/carbonation. Further, the kinetics of calcination/carbonation were investigated by the Kissinger method, Flynn-Wall-Ozawa (FWO) method and Coats-Redfern method. The results of this work indicate that carbide slag is a suitable material for CO2 capture by Ca-looping technology, which would be very helpful for carbide slag resource utilization.

A new integration model of the calcium looping technology into coal fired power plants for CO2 capture

The Ca-Looping (CaL) process is at the root of a promising 2nd generation technology for post-combustion CO2 capture at coal fired power plants. The process is based on the reversible and quick carbonation/calcination reaction of CaO/CaCO​3 at high temperatures and allows using low cost, widely available and non toxic CaO precursors such as natural limestone. In this work, the efficiency penalty caused by the integration of the Ca-looping technology into a coal fired power plant is analyzed. The results of the simulations based on the proposed integration model show that efficiency penalty varies between 4% and 7% points, which yields lower energy costs than other more mature post-combustion CO2 capture technologies such as the currently commercial amine scrubbing technology. A principal feature of the CaL process at CO2 capture conditions is that it produces a large amount of energy and therefore an optimized integration of the systems energy flows is essential for the feasibility of the integration at the commercial level. As a main novel contribution, CO2 capture efficiency is calculated in our work by considering the important role of the solid-state diffusion controlled carbonation phase, which becomes relevant when CaO regeneration is carried out under high CO2 partial pressure as is the case with the CaL process for CO2 capture. The results obtained based on the new model suggest that integration energy efficiency would be significantly improved as the solids residence time in the carbonator reactor is increased.

Energy Intensity Reduction of Ca-Looping CO2 Capture by Applying Mixing Loop Seals and Cyclonic Systems

Abstract This work faces the challenge of cutting the specific energy demand in the CO2 capture process based on Ca-looping technology. The use of high-temperature sorbents allows an efficient integration of the excess heat flows. Up to now, several investigations studied the Ca-looping integration with external systems such as a steam cycle. In this research, a further step is done by comparing technological solutions for the internal heat integration with the aim of reducing the energy needs. Particles preheating before entering the regeneration reactor appears as an opportunity for energy saving since solids have to be heated up around 250–300°C from one reactor to another. Two different internal heat integration possibilities making use of a particle separation device and a mixing valve are presented and compared. The former consists of the inclusion of a cyclonic preheater. This configuration presents the a priori advantage of a more developed technology since it is widely used in the cement industry but the drawback of a worse gas–solid heat exchange. Although there is a lack of practical experience regarding the use of a single seal valve to feed two reactors, this configuration presents a promising prospective related to the excellent heat exchange features of the solid flows. The aim is to obtain comparative results by means of implementing advanced thermochemical models, in order to make progress on the development of less energy-intensive configurations of the calcium looping.

On the negative activation energy for limestone calcination at high temperatures nearby equilibrium

The study of limestone calcination under high CO2 pressure and high temperature has gained an extraordinary practical importance due to the recently emerged Ca-looping technology for post-combustion CO2 capture, which uses natural limestone (CaCO3) as precursor of the CO2 solid sorbent (CaO). A critical issue of this promising process is the necessity of producing CaO by calcination of limestone under high CO2 partial pressure. Experimental measurements on the kinetics of limestone calcination usually show that the rate of the endothermic calcination reaction is increased with temperature according to an Arrhenius law with an activation energy similar to the reaction enthalpy change. In situ microscopic observations have demonstrated that the reaction involves a crystallographic structural transformation of CaO. Calcination is started by chemical decomposition leading to the development of metastable CaO⁎ nanocrystals after which CO2 is desorbed while stable CaO cubic crystals grow. In calcination environments with low CO2 partial pressure P (as compared to the equilibrium pressure: P/Peq≪1), desorption of CO2 and the exothermic structural transformation of CaO⁎ to its stable CaO form occur extremely fast and do not play a role on the reaction kinetics, which is just determined by chemical decomposition. However, at high values of P/Peq the reaction rate may be significantly influenced by the structural transformation above a critical temperature. As a main consequence, the reaction mechanism proposed in the present paper shows that, above a critical temperature, limestone calcination is characterized by a negative activation energy in agreement with the experimental results shown.

Ca-looping for postcombustion CO2 capture: A comparative analysis on the performances of dolomite and limestone

The low cost and wide availability of natural limestone (CaCO3) is at the basis of the industrial competitiveness of the Ca-looping (CaL) technology for postcombustion CO2 capture as already demonstrated by ∼1Mwt scale pilot projects. A major focus of studies oriented towards further improving the efficiency of the CaL technology is how to prevent the gradual loss of capture capacity of limestone derived CaO as the number of carbonation/calcination cycles is increased. Natural dolomite (MgCa(CO3)2) has been proposed as an alternative sorbent precursor to limestone. Yet, carbonation of MgO is not thermodynamically favorable at CaL conditions, which may hinder the capture performance of dolomite. In the work described in this paper we carried out a thermogravimetric analysis on the multicyclic capture performance of natural dolomite under realistic regeneration conditions necessarily implying high calcination temperature, high CO2 concentration and fast transitions between the carbonation and calcination stages. Our study demonstrates that the sorbent derived from dolomite has a greater capture capacity as compared to limestone. SEM analysis shows that MgO grains in the decomposed dolomite are resistant to sintering under severe calcination conditions and segregate from CaO acting as a thermally stable support which mitigates the multicyclic loss of CaO conversion. Moreover, full decomposition of dolomite is achieved at significantly lower calcination temperatures as compared to limestone, which would help improving further the industrial competitiveness of the technology.

Role of precalcination and regeneration conditions on postcombustion CO2 capture in the Ca-looping technology

The Ca-looping (CaL) technology is already recognized as a potentially viable method to capture CO2 from postcombustion gas in coal fired power plants. In this process, CO2 is chemisorbed by CaO solid particles derived from precalcination of cheap and widely available natural limestone. The partially carbonated solids are regenerated by calcination under high CO2 concentration. Novel CaL concepts are proposed to further improve the efficiency of the technology such as the introduction of a recarbonation reactor in between the carbonation and calcination stages to mitigate the progressive deactivation of the regenerated CaO. Process simulations aimed at retrieving optimum design parameters and operating conditions to scale-up the technology yield results critically dependent on the multicyclic sorbent performance. Nevertheless, technical limitations usually preclude lab-scale tests from mimicking realistic CaL conditions necessarily involving high CO2 concentration for sorbent regeneration and quick transitions between carbonation and calcination. In this work, a lab-scale experimental analysis is reported on the CaO multicyclic conversion at CaL conditions closely resembling those to be expected in practice. The results presented evidence a relevant role of precalcination conditions. Precalcination in air leads to a strongly adverse effect on the activity of the sorbent regenerated under high CO2 concentration, which is further hindered if a recarbonation stage is introduced. On the other hand, sorbent deactivation is mitigated if precalcination is carried out at conditions similar to those used for sorbent regeneration. In this case, recarbonation helps lessening the loss of multicyclic conversion, which is further enhanced by the synergistic combination with heat pretreatment. Moreover, the present study shows that the kinetics of carbonation is strongly dependent on precalcination and regeneration conditions. The diffusion controlled carbonation phase and recarbonation are intensified if the sorbent is precalcined and regenerated under high CO2 concentration whereas the reaction controlled carbonation phase is notably hampered.

Relevant Influence of Limestone Crystallinity on CO2 Capture in The Ca-Looping Technology at Realistic Calcination Conditions

We analyze the role of limestone crystallinity on its CO2 capture performance when subjected to carbonation/calcination cycles at conditions mimicking the Ca-looping (CaL) technology for postcombustion CO2 capture. The behavior of raw and pretreated limestones (milled and thermally annealed) is investigated by means of thermogravimetric analysis (TGA) tests under realistic sorbent regeneration conditions, which necessarily involve high CO2 partial pressure in the calciner and quick heating rates. The pretreatments applied lead to contrasting effects on the solid crystal structure and, therefore, on its resistance to solid-state diffusion. Our results show that decarbonation at high CO2 partial pressure is notably promoted by decreasing solid crystallinity. CaO regeneration is fully achieved under high CO2 partial pressure at 900 °C in short residence times for the milled limestone whereas complete regeneration for raw limestone requires a minimum calcination temperature of about 950 °C. Such a reduction of the calcination temperature and the consequent mitigation of multicyclic capture capacity decay would serve to enhance the efficiency of the CaL technology. On the other hand, the results of our study suggest that the use of highly crystalline limestones would be detrimental since excessively high calcination temperatures should be required to attain full decarbonation at realistic conditions.

Effect of Heat Pretreatment/Recarbonation in the Ca-Looping Process at Realistic Calcination Conditions

Even though an increasing number of pilot-scale plants are demonstrating the potential efficiency of the Ca-looping technology to capture CO2 at a commercial level, a still standing matter of concern is the loss of carbonation reactivity of the regenerated CaO by calcination, which is expected to be particularly marked at realistic conditions necessarily implying a high CO2 partial pressure in the calciner. In this work, we address the effect of previously reported strategies for sorbent reactivation, namely heat pretreatment and the introduction of a recarbonation stage before regeneration. Both techniques, either combined or separately, are shown to favor the carbonation reactivity, albeit CaO regeneration is usually carried out at low CO2 partial pressure in lab-scale tests. Novel results reported in this paper show the opposite when the sorbent is regenerated by calcination at high CO2 concentration, which is arguably due to the diverse mechanisms that rule decarbonation depending on the CO2 concentra...

Enhancement of CO2 capture at Ca-looping conditions by high-intensity acoustic fields

The Ca-Looping (CaL) technology, based on a dual gas-fluidized bed system of CaO/CaCO3 particles operated at high temperature, is a viable technological process for highly efficient pre-combustion and post-combustion CO2 capture. In this paper we show a lab-scale experimental study on the carbonation/decarbonation of a fluidized bed of CaO particles at CaL conditions as affected by the application of a high-intensity acoustic field. The results obtained demonstrate that both carbonation and decarbonation are remarkably enhanced for sound intensity levels above 140dB and frequencies of about 100Hz. Fine particles (of size smaller than dp∼100μm) are entrained in the oscillating gas flow induced by an acoustic field of such low frequency, which yields a strong agitation of the bed and improves the gas–solid contact efficiency. On the other hand, an intense convection of gas flow (acoustic streaming) is generated on the surface of larger particles unmovable by the sound wave, which promotes the heat/mass transfer at the gas–solid boundary in this case. Either of these mechanisms, whose relative importance will depend on the average particle size and sound frequency, will contribute to increase the carbonation and decarbonation rates of CaO fluidized beds in the CaL technology.

The Calcium Looping Process for Low CO2 Emission Cement and Power

Calcium looping appears as one of the most promising technologies for CO2 capture in short-medium term plants featuring the combustion of fossil fuels. Ca-looping (CaL) is a regenerative process which takes advantage of the capacity of Calcium Oxide-based sorbents in capturing the CO2 from combustion gases by means of sequential carbonation-calcination cycles. CaL technology appears very promising for CO2 capture from cement plants, since the CaO-rich purge stream which must be extracted from the process can be a valuable raw material for clinker production.The aim of this study is to investigate from the technical and economic side the benefits arising from the integration between a coal-fired power plant with CaL process for CO2 capture and a cement plant using the CaL purge to substitute part of the raw meal. The main parameters affecting the CaL process are varied and the effects on both the plant performance and the final cost of clinker and electricityare discussed.

Ca looping technology: current status, developments and future directions

AbstractCalcium looping technology is a promising new technique for high‐temperature scrubbing of CO2 from flue gas and syngases. Current economic projections suggest it might be able to capture CO2 at costs of ∼$20/ton of avoided CO2. Nonetheless there are questions about the long‐term behavior of natural sorbents in such systems, and there is substantial R&D being done on this technology worldwide to answer questions about whether the performance of natural sorbents can be improved, or whether it would be better to use synthetic ones. The current period is particularly interesting as the first pilot plants and demonstration units capable of operating continuously are now coming on stream, and if successful these will lead to large‐scale industrial demonstrations of the technology in the next 10 to 15 years. © 2011 Society of Chemical Industry and John Wiley & Sons, Ltd