Calcium oxide has been proved to be a suitable sorbent for high temperature CO 2 capture processes based on the cyclic carbonation-calcination reaction. It is important to have reaction rate models that are able to describe the behavior of CaO particles with respect to the carbonation reaction. Fresh calcined lime is known to be a reactive solid toward carbonation, but the average sorbent particle in a CaO-based CO 2 capture system experiences many carbonation-calcination cycles and the reactivity changes with the number of cycles. This study applies the random pore model (RPM) to estimate the intrinsic rate parameters for the carbonation reaction and develops a simple model to calculate particle conversion with time as a function of the number of cycles, partial pressure of CO 2 , and temperature. This version of the RPM model integrates knowledge obtained in earlier works on intrinsic carbonation rates, critical product layer thickness, and pore structure evolution in highly cycled particles.
This paper presents the basic economics of an emerging concept for CO2 capture from flue gases in power plants. The complete system includes three key cost components: a full combustion power plant, a second power plant working as an oxy-fired fluidized bed calciner, and a fluidized bed carbonator interconnected with the calciner and capturing CO2 from the combustion power plant. The simplicity in the economic analysis is possible because the key cost data for the two major first components are well established in the open literature. It is shown that there is clear scope for a breakthrough in capture cost to around 15 $/t of CO2 avoided with this system. This is mainly because the capture system is generating additional power (from the additional coal fed to the calciner) and because the avoided CO2 comes from the capture of the CO2 generated by the coal fed to the calciner and the CO2 captured (as CaCO3) from the flue gases of the existing power plant, that is also released in the calciner.
a b s t r a c tThis work presents a conceptual design of a novel method to obtain hydrogen and/or electricity from natural gas and a concentrated stream of CO 2 suitable for permanent geological storage. The method is based on the well known Sorption Enhanced Reforming (SER) principles for H 2 production using a CaO/CaCO 3 chemical loop. A second chemical loop of Cu/CuO is employed to solve the problem of endothermic CaCO 3 calcination in order to regenerate the sorbent and release the concentrated CO 2 . The reduction reaction of CuO with natural gas, CO or H 2 is shown to be feasible for providing the necessary heat for calcination. A preliminary design of the process has been carried out based on the principles of fixed bed operation and high temperature PSA, making use of the information offered by the literature to define the operating best conditions for the key gas-solid reaction steps and assuming ideal plug flow behaviour in all the reactors during the chemical reactions and gas-solid heat transfer. This makes it possible to define the precise operating windows for the process, so that the reactors can operate close to neutrally thermal conditions. Special material properties (particularly the Ca/inert and Cu/inert ratios) are required, but these are shown to be within the limits of what have been reported in the literature for other gas/solid reaction processes using the same reactions. The conclusion is that there is a great potential for achieving a high degree of energy efficiency with the proposed process by means of a sequence of reactions under the conditions described in this work.
In recent years several processes incorporating a carbonation-calcination loop in an interconnected fluidized bed reactor have been proposed as a way to capture CO 2 from flue gases. This paper is a first approximation to the modelling of a fluidized bed carbonator reactor. In this reactor the flue gas comes into contact with an active bed composed of particles with very different activities, depending on their residence time in the bed and in the carbonation-calcination loop. The model combines the residence time distribution functions with existing knowledge about sorbent deactivation rates and sorbent reactivity. The fluid dynamics of the solids (CSTR) and gases (PF) in the carbonator are based on simple assumptions. The carbonation rates are modelled defining a characteristic time for the transition between a fast reaction regime to a regime with a zero reaction rate. On the basis of these assumptions the model is able to predict the CO 2 capture efficiency for the flue gas depending on the operating and
This paper presents a new solids looping process for capturing CO2 while generating hydrogen and/or electricity from natural gas. The process is based on the sorption enhanced reforming of CH4, employing CaO as a high temperature CO2 sorbent, combined with a second chemical loop of CuO/Cu. The exothermic reduction of CuO with CH4 is used to obtain the heat necessary for the decomposition of the CaCO3 formed in the reforming step. The main part of the process is completed by the oxidation of Cu to CuO, which is carried out with air diluted with a product gas recycle of this reactor at sufficiently low temperatures and high pressures to avoid the decomposition of a substantial fraction of CaCO3.
The influence of thermal pretreatment on the performance of a high-purity limestone (La Blanca) during CO 2 capture cycles is investigated in this paper. This limestone was chosen for more detailed investigation because, in earlier research, it failed to show any favorable effect as a result of thermal pretreatment. Here, the original sample, with a particle size of 0.4-0.6 mm, and ground samples were thermally pretreated at 1000-1200°C , for 6-24 h, and then subjected to several carbonation/calcination cycles in a thermogravimetric analyzer (TGA). This work shows that thermal pretreatment failed to produce a significant self-reactivation effect during CO 2 cycles, despite the use of a wide range of conditions during pretreatment (grinding, temperature, and pretreatment duration) as well as during cycling (CO 2 concentration and duration of the carbonation stage).Additional doping experiments showed that both high Na content and lack of Al in La Blanca limestone cause poor self-reactivation performance after thermal pretreatment. Scanning electron microscope-energy-dispersive X-ray (SEM-EDX) analyses also confirmed more pronounced sintering and loss of activity, which we believe are caused by the relatively high Na content. However, stabilization of sorbent particle morphology by Al can allow this limestone to show self-reactivation performance and higher conversions over a longer series of CO 2 cycles.
Several systems for CO 2 capture using CaO as regenerable sorbent are under development. In addition to a carbonation step, they all need a regeneration step (calcination of CaCO 3) to produce a concentrated stream of CO 2. Different options for calcination may be possible, but they all share common operating windows that appear when the mass and heat balances in the system are solved incorporating equilibrium data, sorbent performance information, and fuel composition (sulphur and ash content). These relatively narrow operating windows are calculated and discussed in this work. Due to sorbent performance limitations, low carbonation levels of the sorbent in the carbonator are expected and the heat demand in the calciner is dominated by the heating of inert solids flowing in the carbonation chemical loop. High make up flows of fresh limestone reduce this effect by increasing the average reactivity of the sorbent, but they also increase the heat demand in the calciner to calcine the fresh feed of limestone. Hence, an optimum level of sorbent activity appears under different operating conditions, processes and fuel characteristic, and these are discussed in this work.
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