CO2 Looping Cycles Research Articles

Reactivation mode investigation of spent CaO-based sorbent subjected to CO2 looping cycles or sulfation

Rapid loss-in-capacity of naturally occurring limestone for CO2 capture is the most prominent issue in the calcium looping process. The synthesis of highly efficient sintering-resisting sorbents is a common solution to mitigate capacity decay. However, the long-term CO2 looping cycles, particularly in the presence of SO2, still cause the generation of large amounts of spent CaO-based sorbents. Comparatively, the reactivation of spent sorbent is more economical than the replenishing of fresh sorbent. In this work, three types of reactivation modes (i.e., hydration, simultaneous hydration/ impregnation and acidification) were applied to recover the CO2 capture capacities of the spent, synthetic CaO-based sorbents (CaO/MgO = 75 wt%/25 wt%). It is found that hydration is effective to recover the cyclic CO2 capture performance of the spent sorbent due to the formation of more cracks and channels that extend to the interior of sorbent particles. Therefore, the sorbent reactivated via sole hydration exhibits a high capacity of 0.390 g/g after 40 cycles. Moreover, NaCl impregnation combined with hydration produce lager amounts of macropores within the reactivated sorbents after initial sever sintering that are responsible for their desirable CO2 capture stability. The introduction of filtration during acidification reactivation process can effectively enhance the initial CO2 capture capacity (an improvement percentage of ~14.7%) owing to the removal of the irreversible CaSO4.

Development of synthetic CaO sorbents via CTAB-assisted sol–gel method for CO2 capture at high temperature

The use of calcium oxide (CaO) for the CO2 looping cycle has attracted increased attention due to several potential advantages. The main drawback of this cycle in practical applications is a sharp decay subsequent to a few cycles of carbonation–calcination. In this work, we report the development of synthetic CaO sorbents via a sol–gel method. Effect of Ca2+/CTAB molar ratios on physical properties of the CaO sorbents as well as their CO2 capture performance were investigated. The presence of CTAB was found to effectively prevent an agglomeration of CaO particles, and to greatly increase BET surface area and total pore volume of the resulting CaO sorbents. Volcano-shaped trends in the BET surface area and total pore volume with respect to the increase of CTAB concentration signal an optimum Ca2+/CTAB ratio. The CaO sorbent prepared with the Ca2+/CTAB molar ratio of 10:3 achieves the highest carbonation conversion of 76.55% in the first cycle, and retains an excellent carbonation conversion of 63.28% subsequent to 20 consecutive test cycles. Furthermore, the analysis results suggest the presence of two linear relationships between: carbonation conversion at reaction stage and BET surface area, and carbonation conversion at diffusion stage and CaO crystallite size.

Modeling of Limestone Capture Performance During CO2 Looping Cycles Based on BP Neuron Network

Modeling of Limestone Capture Performance During CO2 Looping Cycles Based on BP Neuron Network

CO2 Carrying Behavior of Calcium Aluminate Pellets under High-Temperature/High-CO2 Concentration Calcination Conditions

Sintering and a resulting loss of activity during calcination/carbonation can introduce substantial economic penalties for a CO2 looping cycle using CaO-based sorbents. In a real system, sorbent regeneration must be done at a high temperature to produce an almost pure CO2 stream, and this will increase both sintering and loss of sorbent activity. The influence of severe calcination conditions on the CO2 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 CO2 was 950 °C, using different heating/cooling rates between two carbonation stages (700 °C, 20% CO2). For comparison, cycles were also done using N2 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 CO2 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 CO2 toward the particle interior.

Integration of Carbonate CO2Capture Cycle and Coal-Fired Power Plants. A Comparative Study for Different Sorbents

Lately, an outstanding research interest for CO2 capture sorption/desorption looping systems is the improvement of sorbents reactivity and durability. In particular, in calcium-looping cycles the control of sintering processes in the sorbent by thermal pretreatments, doped limestone, or dolomite have deserved excellent works and have shown good experimental results. Also, synthetic sorbents have been tested and demonstrate a lasting capture capacity. Nevertheless, in most cases this long-term conversion enhancement increases the cost of the sorbent and, thus, the system operation and CO2 capture cost. Therefore, any comparison among sorbents will be accurate if both chemical and economical considerations are taken into account in the assessment. In this work, a common basis for sorbent comparison is presented. The integration of the sorbent cost and its chemical and mechanical performance have been studied for different options. The energetic and economical characteristics of several high temperature sorbents have been checked in a CO2 looping cycle applied to an existing coal-fired electrical generation power plant. The aim is to compare the cost of avoided CO2 as a function of average conversion of solid population and cost for different sorbents. Despite excellent conversion results, the unit cost of the sorbent is crucial to maintain the CO2 looping concept as economically attractive. High cost sorbents, even if their residual activity remains at a high level, will be preferred to operate in systems fed with inert-free fuels, that is, natural gas, instead of applications operating with coal. Both their low cost and long-term performance make thermally pretreated limestones competitive sorbents for carbonation/calcination cycles.

Screening of Binders for Pelletization of CaO-Based Sorbents for CO2Capture†

CaO-based CO2 looping cycle technology is a promising method for separation of CO2 from flue gas and syngas at high temperatures. The process of CO2 capture is expected to take place in fluidized-bed combustion (FBC) systems, which implies significant attrition and elutriation of the solid sorbent. Hence, both reactivation of spent sorbent and preparation of modified CaO-based sorbent may be required to maximize the performance of the sorbent. One of the more promising methods to achieve reactivation, namely, hydration, seems to produce very fragile particles, which are unlikely to be suitable for FBC applications. Thus, it is expected that pelletization of the obtained powder may be required. In this paper, we present initial results on the screening of suitable binders for pelletization. Two types of bentonite (Na- and Ca-bentonite) and four types of commercial calcium aluminate cements (CA-14, CA-25, Secar 51, and Secar 80) were investigated here, with a primary focus of maintaining a high CO2-capture capacity over 30−35 cycles. The tests were carried out using a thermogravimetric analyzer (TGA), and the results showed that the presence of bentonites led to faster decay in activity as a result of the formation of calcium-silica compounds with low melting points, which leads to enhanced sintering. This is confirmed by scanning electron microscopy (SEM) and also X-ray diffraction (XRD), which showed the presence of spurrite [Ca5(SiO4)2CO3] as the dominant compound in the pellet after this series of cycles. Better results were obtained with no binder, i.e., by hydration of lime, where Ca(OH)2 plays the role of the binder. Promising results were obtained also with calcium aluminate cements, where no effect of sintering because of the presence of these binders was noticed. Thus, on the basis of this study, the use of calcium aluminate cements for pelletization of CaO-based sorbent is recommended.

SO2 Retention by CaO-Based Sorbent Spent in CO2 Looping Cycles

CaO-based looping cycles are promising processes for CO2 capture from both syngas and flue gas. The technology is based on cyclical carbonation of CaO and regeneration of CaCO3 in a dual fluidized-bed reactor to produce a pure CO2 stream suitable for sequestration. The main limitation of natural sorbents is the loss of carrying capacity with increasing number of reaction cycles, resulting in the need for extra sorbent, and subsequent spent sorbent waste. Use of spent sorbent from CO2 looping cycles for SO2 capture is investigated in this study. Three limestones were investigated: Kelly Rock (Canada), La Blanca (Spain), and Katowice (Poland, Upper Silesia). Carbonation/calcination cycles were performed in a tube furnace with both the original limestones and samples thermally pretreated for different times (i.e., sintered). The spent sorbent samples were sulfated in a thermogravimetric analyzer (TGA). The changes in the resulting sorbent pore structure were then investigated using mercury porosimetry. It has been shown that the sulfation rates of both thermally pretreated and spent sorbent samples are lower in comparison with those of the original samples. However, final conversions of both spent and pretreated sorbents after longer sulfation time were comparable or higher than those observed for the original sorbents under comparable conditions. Maximum sulfation levels strongly depend on sorbent porosity and pore surface area. The shrinkage of sorbent particles during calcination/carbonation cycling resulted in a loss of sorbent porosity on the order of ≤48%, which corresponds to maximum sulfation levels of ∼55% for spent Kelly Rock and Katowice. This is ∼10% higher than that seen with the original samples after 15 h of sulfation. By contrast, La Blanca limestone had more pronounced particle shrinkage during pretreatment and cycling, leading to porosities lower than 35%, which resulted in sulfation conversion of spent samples <30%, which is significantly lower than that for the original sample (45%). These results showed that spent sorbent samples from CO2 looping cycles can be used as sorbents for SO2 retention in cases where significant porosity loss does not occur during CO2 reaction cycles. The higher conversions of spent samples are explained by a shift in pore size distribution toward larger pores that reduce the reaction rate and pore plugging near the particle’s outer surface, with formation of either unreacted core or unreacted network patterns. In the case of spent Kelly Rock and Katowice samples, sorbent particles are practically uniformly sulfated, achieving final conversions that are determined by the total pore volume available for the bulky CaSO4 product.

Sintering and Reactivity of CaCO3-Based Sorbents for In Situ CO2 Capture in Fluidized Beds under Realistic Calcination Conditions

Sintering during calcination/carbonation may introduce substantial economic penalties for a CO2 looping cycle using limestone/dolomite-derived sorbents. Here, cyclic carbonation and calcination reactions were investigated for CO2 capture under fluidized bed combustion FBC conditions. The cyclic carbonation characteristics of CaCO3-derived sorbents were compared at various calcination temperatures 700-925°C and different gas stream compositions: pure N2 and a realistic calciner environment where high concentrations of CO280-90% and the presence of SO2 are expected. The conditions during carbonation employed here were 700°C and 15% CO2 in N2 and 0.18% or 0.50% SO2 in selected tests, i.e., typically expected for a carbonator. Up to 20 calcination/carbonation cycles were conducted using a thermogravimetric analyzer TGA apparatus. Three Canadian limestones were tested: Kelly Rock, Havelock, and Cadomin, using a prescreened particle size range of 400-650 m. In addition, calcined Kelly Rock and Cadomin samples were hydrated by steam and examined. Sorbent reactivity was reduced whenever SO2 was introduced to either the calcining or carbonation streams. The multicyclic capture capacity of CaO for CO2 was substantially reduced at high concentrations of CO2 during the sorbent regeneration process and carbonation conversion of the Kelly Rock sample obtained after 20 cycles was only 10.5%. Hydrated sorbents performed better for CO2 capture, but also showed significant deterioration following calcination in high CO2 gas streams. This indicates that high CO2 and SO2 levels in the gas stream lead to lower CaO conversion because of enhanced sintering and irreversible formation of CaSO4. Such effects can be reduced by separating sulfation and carbonation and by introducing steam to avoid extremely high CO2 atmospheres, albeit at a higher cost and/or increased engineering complexity.

Improvement of CaO-based sorbent performance for CO2 looping cycles

This paper presents research on CO2 capture by lime-based looping cycles. This is a new and promising technology that may help in mitigation of global warming and climate change caused primarily by the use of fossil fuels. The intensity of the anticipated changes urgently requires solutions such as the developing technologies for CO2 capture, especially those based on CaO looping cycles. This technology is at the pilot plant demonstration stage and there are still significant challenges that require solutions. The technology is based on a dual fluidized bed reactor which contains a carbonator - a unit for CO2 capture, and a calciner - a unit for CaO regeneration. The major technology components are well known from other technologies and easily applicable. However, even though CaO is a very good candidate as a solid CO2 carrier, its performance in a practical system still has significant limitations. Thus, research on CaO performance is critical and this paper discusses some of the more important problems and potential solutions that are being examined at CETC-O.

Solid Looping Cycles: A New Technology for Coal Conversion

This article examines both oxygen looping cycles (otherwise known as chemical looping combustion), and lime-based CO2 looping cycles, where calcined limestone is used for in situ CO2 capture. There...

06/01718 Particle-metal interactions during combustion of pulp and paper biomass in a fluidized bed combustor: Eldabbagh, F. et al. Combustion and Flame, 2005, 142, (3), 249–257

Sulphation and carbonation have been performed on hydrated spent residues from a 75 kWth dual fluidized bed combustion (FBC) pilot plant operating as a CO2 looping cycle unit. The sulphation and carbonation tests were done in an atmospheric pressure thermogravimetric analyzer (TGA), with the sulphation performed using synthetic flue gas (0.45% SO2, 3% O2, 15% CO2 and N2 balance). Additional tests were carried out in a tube furnace (TF) with a higher SO2 concentration (1%) and conversions were determined by quantitative X-ray diffraction (QXRD) analyses. The morphology of the sulphated samples from the TF was examined by scanning electron microscopy (SEM). Sulphation tests were performed at 850 °C for 150 min and carbonation tests at 750 °C, 10 cycles for 15 min (7.5 min calcination + 7.5 min carbonation). Sulphation conversions obtained for the hydrated samples depended on sample type: in the TGA, they were ∼75–85% (higher values were obtained for samples from the carbonator); and in the TF, values around 90% and 70% for sample from carbonator and calciner, respectively, were achieved, in comparison to the 40% conversion seen with the original sample. The SEM analyses showed significant residual porosity that can increase total conversion with longer sulphation time. The carbonation tests showed a smaller influence of the sample type and typical conversions after 10 cycles were 50% – about 10% higher than that for the original sample. The influence of hydration duration, in the range of 15–60 min, is not apparent, indicating that samples are ready for use for either SO2 retention, or further CO2 capture after at most 15 min using saturated steam. The present results show that, upon hydration, spent residues from FBC CO2 capture cycles are good sorbents for both SO2 retention and additional CO2 capture.