Fluidized Bed Calcium Looping Research Articles

Investigations on biomass gasification of compact-fast dual fluidized bed calcium looping

In this study, a 10 kWth hot rig of dual fluidized bed is developed to investigate the performance of the calcium looping process for biomass hydrogen-rich gasification and carbon dioxide capture. The system consists of a compact fluidized bed and a fast fluidized bed riser. The compact fluidized bed, which is comprised of a lower bubbling fluidized bed and an upper riser, is used as the gasifier, and the fast fluidized bed riser is used as the regenerator. The solid from the regenerator goes upward from the bottom of the upper riser gasifier, and then downward to the lower bubble fluidized bed gasifier, bringing heat from the generator to the gasifier, and is finally circulated to the regenerator form the bottom of the bubble fluidized bed, taking carbon dioxide from the gasifier to the regenerator. Dolomite and quartz sand are used as bed materials. The effects of variables such as temperature, bed material, and solid circulating flux on product gas compositions, yield, calorific value, tar content, and cold gas efficiency were investigated. The dolomite bed material has a more obvious improvement in hydrogen production as compared with quartz sand as bed material. The maximum H2 production is 0.46 Nm3/kgFuel, the purity reaches 70.88%, the cold gas efficiency is 45.93%, and the low calorific value is 13.26 MJ/Nm3. The high temperature of the upper riser favors tar conversion. When the riser temperature was 800 °C, the tar content decreased by 4.46 g/Nm3, and the total gas production increased by 0.03 Nm3/kgFuel. In addition, the increase of solid circulating flux can improve the concentration and the yield of hydrogen, and decrease the content of tar.

Investigations at a 10 kWth calcium looping dual fluidized bed facility: Limestone calcination and CO2 capture under high CO2 and water vapor atmosphere

Calcium looping post-combustion CO2 capture process takes advantage of the reversible carbonation of CaO, during which CO2 contained in flue gas is absorbed by CaO forming CaCO3 and then is released when the reverse step of the reaction, named calcination, takes place. In the scope of the present paper, the performance of the process will be presented as recorded from experiments performed at University of Stuttgart, in the 10kWth Dual Fluidized Bed calcium looping facility under conditions closer to those expected industrially: wet flue gas in the carbonator reactor and atmospheres rich in CO2 and H2O in the regenerator reactor. The decomposition of limestone was realized under CO2 presence up to 75vol.% d.b., water vapor presence up to 35vol.%, balanced with N2 while the carbonation of CaO was carried out under 10–16vol.% CO2 and up to 10vol.% water vapor. Various temperatures were tested, i.e. between 620 and 680°C for the carbonator and 870 up to 930°C for the regenerator. Results showed that the demands in Ca to be circulated between the reactors is an increasing function of the CO2 vol.% in the regenerator and decreasing function of the presence of water during both sorbent carbonation and calcination. The carbonator CO2 capture efficiency decreases with increasing carbonator temperature and decreasing regenerator temperature. Efficiencies for both carbonator and regenerator of more than 90% were recorded for looping ratios (molCa/molCO2) less than 10 under oxyfired conditions when water vapor was present. With regard to chemical properties, the sorbent exhibited a residual activity (also referred to as average maximum carbonation conversion) almost double (∼20%) in the case where water vapor was present in the carbonator and regenerator, as opposed to when water vapor was absent. However, sorbent friability was enhanced resulting in material loss up to 0.095molCa/molCO2 to be captured or 4.75wt.%/h based on the total system inventory.

Characterization of the oxy-fired regenerator at a 10 kWth dual fluidized bed calcium looping facility

Calcium looping is a CO2 capture technology, developed in the last years with great potential to contribute to the reduction of greenhouse gases emissions. It consists of a carbonator where CO2 contained in power plant flue gas, is absorbed by CaO producing CaCO3 and a regenerator where the CaCO3 is calcined to CaO releasing a highly concentrated CO2 stream. A key aspect of the process is the oxy-combustion of the fuel in the regenerator to provide the heat for the endothermic calcination reaction. This implies high CO2 concentration in the off-gas and requires a flue gas recycle. Aim of this work is the characterization of the regenerator operation under this environment and thus experiments were performed at the 10 kWth IFK Dual Fluidized Bed facility. Regenerator efficiency and sorbent performance in terms of chemical activity and mechanical stability are the two basic parameters investigated, while the active space time is used for interpretation of the results. It is found that high calcination conversions can be achieved with temperatures less than 920 °C. Sorbent achieves a residual CO2 carrying capacity similar to the one referred to the literature. The sorbent proved to be mechanically strong thus optimistic percentage of material loss was recorded to be 0.8 wt.%/h or 0.024 mol Ca/mol CO2.

Hydration-induced reactivation of spent sorbents for fluidized bed calcium looping (double looping)

The aim of this work was to study the hydration-induced reactivation of spent sorbents from a calcium looping process. The changes of the sorbent properties induced by hydration, the regeneration of the CO2 capture capacity and the attrition tendency of the material once reused in the looping cycle were investigated. To this end, a reference limestone was subjected to multiple calcination/carbonation cycles in a lab-scale fluidized bed reactor and then reactivated by water hydration at room temperature for different times. Results suggested that the sorbent under investigation should be hydrated for times as short as possible compatible with complete chemical hydration. In fact, long hydration times bring about cramming (chemical sintering) phenomena, lower enhancement in the active porosity, increased attrition tendency and reduced reactivation. Attrition/fragmentation data referring to as-received and reactivated sorbents were compared. Relationships among CO2 capture capacity, surface properties and attrition tendency were presented and critically discussed, taking into account also the results obtained for a different sorbent.

Reactivation by water hydration of the CO2 capture capacity of a calcium looping sorbent

The potential of water hydration as a mean to reactivate a spent reactive limestone-based sorbent used in a fluidized bed calcium looping process for CO2 capture is demonstrated. The study has been specifically targeted at investigating the changes of sorbent properties induced by hydration, and the influence that this treatment had on the CO2 capture capacity and on the attrition tendency of the reactivated material. To this end, the spent sorbent obtained from calcium looping tests in a lab-scale fluidized bed reactor (calcination at 940°C in 70% CO2, carbonation at 650°C in 15% CO2) was hydrated at 25°C for times ranging from 10 to 60min, and characterized by TG, SEM and porosimetric analyses. The hydrated materials were further subjected to calcium looping cycles under the same operating conditions, and the CO2 capture capacities, the elutriation rates and the particle size distributions after each stage were measured and compared with those obtained in the tests before reactivation. It is demonstrated that hydration is effective in reactivating the spent sorbent: the CO2 capture capacity increased from 0.04gg−1 (last carbonation before hydration) to 0.32–0.37gg−1 (first carbonation after hydration). The capacity, however, rapidly decayed along with the cycles. Results suggested that the CO2 capture capacity, the sorbent properties and the attrition tendency are linked one to the other. The sorbent hydrated for the longest time (60min) not only developed an enhanced active porosity, but was also characterized by a limited attrition tendency (with respect to the material hydrated for 10min). Notably, results obtained in the present study differed from those obtained in a previous study on a different limestone, highlighting the importance of sorbent reactivity for an optimally tailored sorbent regeneration process.

Fluidized bed calcium looping cycles for CO2 capture under oxy-firing calcination conditions: Part 2. Assessment of dolomite vs. limestone

The CO2 capture performance and the attrition behavior of a dolomite were assessed during calcium looping cycles in a lab-scale fluidized bed. Results were compared to those obtained with limestone in similar tests. The experiments were carried out under alternating calcination–carbonation conditions representative of a process with calcination in an oxy-firing environment (T=940°C, 70% CO2), without or with the presence of SO2 at two concentration levels. Results showed that the CO2 capture capacity of the dolomite was larger than that of limestone, in spite of the lower calcium content of this sorbent. In fact, the large magnesium fraction in the dolomite reduced particle sintering and preserved the Ca reactivity. However, the presence of SO2 at both concentration levels significantly depressed the sorbent CO2 capture capacity. Contrary to limestone, the dolomite was subject to extensive attrition and fragmentation.

Fluidized bed calcium looping cycles for CO2 capture under oxy-firing calcination conditions: Part 1. Assessment of six limestones

Six limestones were tested for CO2 capture during calcium looping cycles in a lab-scale fluidized bed apparatus. Batch tests were carried out under alternating calcination–carbonation conditions representative of a process with calcination in an oxy-firing environment (T=940°C, 70% CO2). The effect of the presence of SO2 was also studied at two concentration levels: 1500ppm SO2 during both carbonation and calcination, simulating CO2 capture from uncontrolled flue gas and regeneration in an oxy-fired calciner burning high-sulfur coal; 75ppm SO2 during carbonation and 750ppm SO2 during calcination, simulating CO2 capture from already desulfurized flue gas and regeneration in an oxy-fired calciner burning medium-sulfur coal. Limestone attrition processes were characterized during the tests by measuring the changes of the sorbent particle size distribution and the fines elutriation rate along conversion over repeated cycles.Results showed that for all the limestones the CO2 capture capacity decreased with the number of cycles reaching a very low asymptotic value. The combination of high bed temperature and high CO2 concentration during the calcination stage significantly enhanced particle sintering. Moderate attrition rates were experienced by the sorbent particles mostly during the first cycle. In the subsequent cycles the attrition rate progressively declined, due to the concurrent chemical–thermal treatment making the sorbent surface increasingly hard. The presence of a high SO2 concentration significantly depressed the sorbent CO2 capture capacity, because of the buildup of a compact CaSO4 layer on the particle surface. Under a lower SO2 concentration level, the effect on CO2 capture capacity was less important. In both conditions, limestone attrition was only influenced to a limited extent by SO2. The slight variations of the sorbent particle size distribution indicated that very low particle fragmentation occurred over the repeated cycles.

Fluidized bed calcium looping: The effect of SO2 on sorbent attrition and CO2 capture capacity

The CO2 capture capacity and the extent of particle attrition during simulated calcium looping cycles in a fluidized bed were studied, with a focus on the influence of SO2 in the flue gas. Five calcination/carbonation cycles (calcination at 850°C and 20% CO2, carbonation at 700°C and 16% CO2) were carried out with a reference limestone using different SO2 concentrations during carbonation (110 and 1800ppm). Attrition phenomena were characterized by following the modifications of bed sorbent particle size distribution and the elutriation rates of fines throughout conversion over repeated cycles.Results showed that the CO2 capture capacity decreased with the cycles, as expected. The presence of SO2 had a detrimental effect, but the SO2 concentration affected the capacity less than anticipated. In-bed particle fragmentation was always limited, while the attrition rate decreased with the number of cycles, and was only moderately affected by the presence of SO2.

Attrition of Limestone During Fluidized Bed Calcium Looping Cycles for CO2 Capture

Attrition of a limestone during calcium looping cycles for CO2 capture was studied in a lab-scale fluidized bed apparatus. Batch experiments under alternating calcination–carbonation conditions were carried out to investigate the effect of chemical reactions and temperature changes on the attrition propensity of the sorbent particles. Attrition processes were characterized by following the modifications of bed sorbent particle size distribution and the elutriation rates of fines throughout conversion over repeated cycles. Different bed temperatures and CO2 inlet concentrations during the calcination stage were tested in the experiments. Results show that relatively large attrition rates were experienced by the sorbent particles only during the first cycle. From the second cycle on the attrition rate progressively declines, also during the calcination stage where the softer CaO is produced. It is inferred that the combined chemical-thermal treatment affects the particle structure making it increasingly hard. At the same time, the CO2 capture capacity decays toward an asymptotic level, possibly related to the very same structural modifications. The bed temperature and CO2 concentration both appear to influence the sorbent behavior in the tests.

Analysis and Comparison of Reactivity and CO2 Capture Capacity of Fresh Calcium-Based Sorbents and Samples From a Lab-Scale Dual Fluidized Bed Calcium Looping Facility

Naturally occurring limestone and samples from a lab-scale dual fluidized bed (DFB) calcium looping test facility were analyzed in a thermogravimetric analyzer. The reactivity of the samples evaluated at typical carbonation conditions prevailed in the carbonator was compared with raw samples. The rate of carbonation and carbonation capacity of the samples were compared with respect to the following three categories: number of calcination-carbonation cycles, carbonation temperature, and CO2 concentration. It is suspected that the much lower activity of the DFB sample is attributed to the differences in experimental conditions, i.e., partial carbonation of the DFB particles, fast heating rate in the calciner and thus a rapid calcination reaction, and particle attrition in the circulating fluidized bed calciner riser. These harsh conditions lead to sintering and thus a loss of surface area and reactivity. Sintered DFB samples showed low (nearly one-third of the raw samples) but stable conversions with increasing number of cycles. Hydration was used as an attempt to regenerate the lost capture capacity of partially carbonated and sintered DFB sample. Hydration of the DFB sample was successful in increasing the maximum capture capacity in the fast reaction regime to values almost as high as that of a fresh sample in its first carbonation cycle. Although more investigation is required to investigate the effect of hydration on the CaO particle morphology, a process modification to enhance the CO2 capture efficiency of the carbonator via particle hydration was proposed.