Reactivity of calcined cement raw meals for carbonation (original) (raw)
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Recent attempts to improve the attrition resistance and CO 2 uptake of Ca-based sorbent by making pellets with aluminate cement have succeeded in enhancing their effectiveness. The effects of parameters on sorbent attrition were investigated. Batch experiments were also conducted in a fluidized bed at optimal reaction conditions from previous studies (Carbonation: 0.5 MPa and 700°C in 15% CO 2 /air balance for 15 min; Calcination: 0.1 MPa and 950°C in 100% CO 2 for 10 min). The pore structure characteristics (BET, BJH) were measured as a supplement to the attrition and reaction studies. Results showed that the mechanical property of the pellets with the particle size of 1.0-1.43 mm were greatly enhanced, especially for the pellet CaO-0.5% CLS-A. A slow decay in CO 2 capture capacity of the sorbents was observed after making pellets during multiple cycles. It was attributed to the attrition of sorbents and the exposure of inner core of the CaO sorbents, which are in favor of CO 2 capture. The pore structure showed that the BET surface area and BJH pore volume did not change much, which benefits CO 2 uptake of the sorbents during the cycling.
Behavior of Different Calcium-Based Sorbents in a Calcination/Carbonation Cycle for CO 2 Capture
Energy & Fuels, 2007
The aim of this work is to identify the characteristics of natural carbonates which upon calcination generate an optimum material for use as a CO 2 -capturing sorbent in large-scale industrial CO 2 -producing sources. Nine different naturally occurring Ca/Mg carbonates were selected for this study. The carbonates were fully characterized by a variety of analytical techniques including atomic absorption and redox volumetry, for the chemical characterization of the carbonates, and optical and scanning electron microscopy (SEM), X-ray diffraction, and Fourier transform infrared spectroscopy, to determine their crystallinity, morphology, and the presence of impurities. They were then subjected to successive (up to 100) calcination/recarbonation cycles, and their conversion decay curves were interpreted on the basis of the physical and chemical characteristics of the parent carbonates. The textural development of the sorbents during cycling was studied by Hg porosimetry and SEM. Hardness tests were also conducted on selected samples. It was concluded that both carbonate purity and crystallinity are important parameters in determining the performance of the sorbents. The activity of all the sorbents tested turned out to be highly dependent on the pore structure of the calcines and their variation during cycling. In turn, the natural tendency of the sorbents to develop low surface areas (poor efficiencies) during cycling seems to be enhanced by the presence of moderate amounts of Mg.
Integrating Calcium Looping CO2 Capture with the Manufacture of Cement
This paper investigates the trace element content of calcium oxide sorbent after repeated cycles of calcination and carbonation in the presence of fuel combustion during the calcination step. The trace element content of the sorbent was measured using ICP-OES after a wet acid digestion procedure. The weight % of alite, the cement phase responsible for short-term strength of the cement, has been measured using XRD for cement prepared in the laboratory from single cycled and repeatedly cycled sorbent. The results indicate that repeated cycling does lead to an increase in the concentration of some trace elements entering the sorbent from the fuel. However the increase in these concentrations was not enough to impact upon the weight % of alite in the resulting clinkers produced from the sorbent.
Industrial & Engineering Chemistry Research, 2009
A series of carbonation/calcination tests consisting of 1000 cycles was performed with CaO-based pellets prepared using hydrated lime and calcium aluminate cement. The change in CO 2 carrying capacity of the sorbent was investigated in a thermogravimetric analyzer (TGA) apparatus and the morphology of residues after those cycles in the TGA was examined by scanning electron microscopy (SEM). Larger quantities of sorbent pellets underwent 300 carbonation/calcination cycles in a tube furnace (TF), and their properties were examined by nitrogen physisorption tests (BET and BJH). The crushing strength of the pellets before and after the CO 2 cycles was determined by means of a custom-made strength testing apparatus. The results showed high CO 2 carrying capacity in long series of cycles with an extremely high residual activity of the order of 28%. This superior performance is a result of favorable morphology due to the existence of large numbers of nanosized pores suitable for carbonation. This morphology is relatively stable during cycles due to the presence of mayenite (Ca 12 Al 14 O 33 ) in the CaO structure. However, the crushing tests showed that pellets lost strength after 300 carbonation/calcination cycles, and this appears to be due to the cracks formed in the pellets. This effect was not observed in smaller particles suitable for use in fluidized bed (FBC) systems.
Ind Eng Chem Res, 2009
A series of carbonation/calcination tests consisting of 1000 cycles was performed with CaO-based pellets prepared using hydrated lime and calcium aluminate cement. The change in CO 2 carrying capacity of the sorbent was investigated in a thermogravimetric analyzer (TGA) apparatus and the morphology of residues after those cycles in the TGA was examined by scanning electron microscopy (SEM). Larger quantities of sorbent pellets underwent 300 carbonation/calcination cycles in a tube furnace (TF), and their properties were examined by nitrogen physisorption tests (BET and BJH). The crushing strength of the pellets before and after the CO 2 cycles was determined by means of a custom-made strength testing apparatus. The results showed high CO 2 carrying capacity in long series of cycles with an extremely high residual activity of the order of 28%. This superior performance is a result of favorable morphology due to the existence of large numbers of nanosized pores suitable for carbonation. This morphology is relatively stable during cycles due to the presence of mayenite (Ca 12 Al 14 O 33 ) in the CaO structure. However, the crushing tests showed that pellets lost strength after 300 carbonation/calcination cycles, and this appears to be due to the cracks formed in the pellets. This effect was not observed in smaller particles suitable for use in fluidized bed (FBC) systems.
An investigation of the kinetics of CO2 uptake by a synthetic calcium based sorbent
Chemical Engineering Science, 2012
This study examines the kinetics of carbonation by CO 2 at temperatures of ca. 750 1C of a synthetic sorbent composed of 15 wt% mayenite (Ca 12 Al 14 O 33 ) and CaO, designated HA-85-850, and draws comparisons with the carbonation of a calcined limestone. In-situ XRD has verified the inertness of mayenite, which neither interacts with the active CaO nor does it significantly alter the CaO carbonation-calcination equilibrium. An overlapping grain model was developed to predict the rate and extent of carbonation of HA-85-850 and limestone. In the model, the initial microstructure of the sorbent was defined by a discretised grain size distribution, assuming spherical grains. The initial input to the model -the size distribution of grains -was a fitted parameter, which was in good agreement with measurements made with mercury porosimetry and by the analysis of SEM images of sectioned particles. It was found that the randomly overlapping spherical grain assumption offered great simplicity to the model, despite its approximation to the actual porous structure within a particle. The model was able to predict the performance of the materials well and, particularly, was able to account for changes in rate and extent of reaction as the structure evolved after various numbers of cycles of calcination and carbonation.
Carbonation of CaO-Based Sorbents Enhanced by Steam Addition
Industrial & Engineering Chemistry Research, 2010
The carbonation reaction has recently been intensively investigated as a means of CO 2 capture from gas mixtures such as flue gas produced during fossil fuel combustion. Unfortunately, this gas-solid reaction is limited due to formation of the solid product (CaCO 3 ) at the reacting surface and sintering, all of which reduce the carrying capacity of the sorbent. In this work the enhancement of carbonation conversion by means of steam addition to the carbonating gas was studied. Seven limestones of different origin and composition as well as one synthetic sorbent (calcium aluminate pellets) were tested. A thermogravimetric analyzer (TGA) was employed for the carbonation tests at different temperatures (350-800°C) in a gas mixture containing typically 20% CO 2 and 10 or 20% H 2 O (g) . The samples tested were calcined under an N 2 (800°C) or CO 2 (950°C) atmosphere to explore the influence of different levels of sample sintering, and the results obtained were compared with those seen for carbonation in dry (no steam) gas mixtures. The morphology of samples after carbonation under different conditions was examined by a scanning electron microscope (SEM). It was found that carbonation is enhanced by steam, but this is more pronounced at lower temperatures and for more sintered samples. With increasing temperature and carbonation time, the enhancement of carbonation becomes negligible because the conversion reaches a "maximum" value (∼75-80% for samples calcined in N 2 ) even without steam. Carbonation of samples calcined in CO 2 is enhanced at different levels depending on the sorbent tested. The shape of carbonation profiles and morphology of carbonated samples show that steam enhances solid state diffusion and, consequently, conversion during carbonation. Figure 7. SEM images of carbonated Havelock (see Figure 4c) calcined in CO 2 at 950°C and carbonated (a) without steam and (b) in the presence of steam.
Sorbents for CO2 capture from biogenesis calcium wastes
Chemical Engineering Journal, 2013
Egg shells, shellfish shells and cuttlefish bones, high calcium content alimentary wastes, were used to prepare CaO sorbents for CO 2 capture. The materials were prepared by a simple procedure including two steps: crushing and calcination at 900ºC. Fine powders displaying chalky-white to pale grey-green shades were obtained depending on the starting material. All the prepared sorbents were microcrystalline limes containing various trace elements.
Industrial & Engineering Chemistry Research, 2010
Sintering and a resulting loss of activity during calcination/carbonation can introduce substantial economic penalties for a CO 2 looping cycle using CaO-based sorbents. In a real system, sorbent regeneration must be done at a high temperature to produce an almost pure CO 2 stream, and this will increase both sintering and loss of sorbent activity. The influence of severe calcination conditions on the CO 2 carrying behavior of calcium aluminate pellets is investigated here. Up to 30 calcination/carbonation cycles were performed using a thermogravimetric analyzer apparatus. The maximum temperature during the calcination stage in pure CO 2 was 950°C, using different heating/cooling rates between two carbonation stages (700°C, 20% CO 2 ). For comparison, cycles were also done using N 2 during the calcination stages. In addition, the original Cadomin limestone, used for pelletization, was also examined in its original form and the results obtained were compared with those for the aluminate pellets. As expected, high temperature during calcination strongly reduced CO 2 carrying capacities of both sorbents. However, aluminate pellets showed better resistance to these severe conditions. The conversion profiles obtained are significantly different to those obtained under milder conditions, with significant increased activity during the slower, diffusion-controlled, carbonation stage. Moreover, scanning electron microscopy analysis of samples after prolonged carbonation showed that pore filling occurred at the sorbent particle surfaces preventing diffusion of CO 2 toward the particle interior.
Influence of calcination conditions on carrying capacity of CaO-based sorbent in CO 2 looping cycles
Fuel, 2009
This study examines the loss of sorbent activity caused by sintering under realistic CO 2 capture cycle conditions. The samples tested here included two limestones: Havelock limestone from Canada (New Brunswick) and a Polish (Upper Silesia) limestone (Katowice). Samples were prepared both in a thermogravimetric analyzer (TGA) and a tube furnace (TF). Two calcination conditions were employed: in N 2 at lower temperature; and in CO 2 at high temperature. The samples obtained were observed with a scanning electron microscope (SEM) and surface compositions of the resulting materials were analyzed by the energy dispersive X-ray (EDX) method. The quantitative influence of calcination conditions was examined by nitrogen adsorption/desorption tests, gas displacement pycnometry and powder displacement pycnometry; BET surface areas, BJH pore volume distributions, skeletal densities and envelope densities were determined. The SEM images showed noticeably larger CaO sub-grains were produced by calcination in CO 2 during numerous cycles than those seen with calcination in nitrogen. The EDX elemental analyses showed a strong influence of impurities on local melting at the sorbent particle surface, which became more pronounced at higher temperature. Results of BET/BJH testing clearly support these findings on the effect of calcination/cycling conditions on sorbent morphology. Envelope density measurements showed that particles displayed densification upon cycling and that particles calcined under CO 2 showed greater densification than those calcined under N 2 . Interestingly, the Katowice limestone calcined/cycled at higher temperature in CO 2 showed an increase of activity for cycles involving calcination under N 2 in the TGA. These results clearly demonstrate that, in future development of CaO-based CO 2 looping cycle technology, more attention should be paid to loss of sorbent activity caused by realistic calcination conditions and the presence of impurities originating from fuel ash and/or limestone.