Performance of Hydration Reactivated Ca Looping Sorbents in a Pilot

Article pubs.acs.org/EF

Performance of Hydration Reactivated Ca Looping Sorbents in a Pilot-Scale, Oxy-fired Dual Fluid Bed Unit Vlatko Materić,*,† Robert Symonds,‡ Dennis Lu,‡ Robert Holt,† and Vasilije Manović§ †

Callaghan Innovation, Lower Hutt 5010, New Zealand CanmetENERGY, Ottawa K1A 1M1, Canada § Cranfield University, Beford MK43 0AL, United Kingdom ‡

ABSTRACT: The progressive deactivation of CaO based sorbents is one of the major limitations of the Ca Looping cycle for postcombustion CO2 capture. Techniques using the hydration of the spent CaO sorbent have been identified as a promising route for the reactivation of CaO sorbents but have so far not been tested at pilot scale using realistic calcination and carbonation conditions. In this work, the performance of sorbents reactivated by two different reactivation techniques (hydration− dehydration and superheating) was assessed in an oxy-fired, pilot-scale dual fluid bed unit. Compared to a spent sorbent, reactivated materials exhibited a ≈60% increase in CO2 carrying capacity over 3 h of circulation as well as an increase in the sorbent attrition rate of 25% (superheating) and 50% (hydration−dehydration). In both cases, however, increased attrition did not lead to serious disruptions of system operation. A comparison of sorbent performance at laboratory and pilot scale suggested that high velocity impacts in the transfer lines were the main cause of attrition.

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INTRODUCTION

pilot plants in the 1−2 MW range being currently established.9,10 One of the major limitations of Ca Looping systems is the progressive decay in the CO2 carrying capacity of CaO based sorbents with increasing cycle number.11−13 Indeed, the CO2 carrying capacity of natural limestone is rapidly reduced during the first 10−20 cycles, ultimately stabilizing at about 7−15% of the theoretical capacity.14 This reduction is due to sintering occurring during every calcination step, and capacity decay is accelerated with increasing intensity of sintering i.e. with the increase of temperature and CO2 concentration in the calciner.15 This progressive decay in sorbent carrying capacity leads to the operation of the Ca Looping system at a sorbent capacity much lower than the theoretical limit. In turn, the circulation of a large fraction of inactive CaO leads to an increase in carbonator and calciner sizes16 and increased oxygen consumption from the air separation unit (ASU),17 both of which negatively impact the economics of the process. Although a number of methods have been proposed to remediate this issue18,19 this work focuses on hydration based reactivation methods, as they have shown promise in restoring the activity of spent sorbents.18,19 Note that after reactivation, the progressive decay in sorbent activity is resumed,19−21 so that it is envisaged that reactivation by hydration would be performed periodically (or continuously on a slip stream of CaO) so that the frequency of reactivation would control the average CO2 carrying capacity of the sorbent.21 In hydration based reactivation methods, the first step always consists in hydrating the cycled (spent) CaO to form Ca(OH)2. Hydration was found to be effective using a wide range of

Carbon capture and sequestration (CCS) technologies are expected to play a major role in achieving the reductions in greenhouse gas emissions required to mitigate the increase of atmospheric CO2 concentration.1 The aim of CCS is to separate the CO2 emitted by large, stationary plants and sequester it out of the atmosphere. CCS technologies are composed of two distinct steps: the capture step, in which CO2 is separated from the flue gas, and the sequestration step, where the concentrated CO2 is compressed and transported to sequestration sites, in suitable geological formations. The CO2 capture step is the costly part of CCS technologies, representing up to 80% of the total cost of CCS.1 Calcium looping (Ca Looping) is an emerging CO2 capture technology based on the reversible calcination-carbonation reaction of CaCO3 generating calcium oxide and CO2.2 Although any CaCO3 containing solid can be used, natural limestone is typically used due to its availability and low cost.3 Although a number of processes can be designed upon this simple reaction,4 both in pre- and postcombustion CO2 capture,1,2 most research and development efforts are currently focused on the postcombustion process initially proposed by Shimizu et al.5 The particularity of this process is that the heat required for calcination is supplied via oxy-combustion of coal or other fuels, which allows generating concentrated CO2 from the calciner. Although this entails significant costs associated with producing pure O2, the appeal of this approach lies in the ready availability of all its components, i.e., the fluid bed reactors (carbonator/calciner) and the air separation unit.6 A number of economic analyses have shown that the cost of CO2 avoided in Ca Looping systems with oxy-fuel calciners could be considerably lower than that of solvent based systems or oxy-combustion alone.6−8 These early analyses have spurred the development and scale-up of this process, with a number of © 2014 American Chemical Society

Received: May 27, 2014 Revised: July 27, 2014 Published: July 29, 2014 5363

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conditions, e.g., with liquid water,22,23 with humid air,20 and with steam at a variety of temperatures (100−380 °C) and steam pressures.24−28 Water is able to penetrate the sintered sorbent in a relatively short time and hydration causes a substantial expansion of the sorbent particles as the molar volume of Ca(OH)2 (33 mL mol−1) is larger than that of CaO (17 mL mol−1). If such a hydrated sorbent is then heated to above ≈400 °C and dehydrated, the resulting CaO is found to be considerably more porous than the spent CaO was prior to hydration; it was also observed to exhibit a considerably higher CO2 carrying capacity than the spent CaO.29,30 The hydration−dehydration technique was successfully used to restore the activity of sorbents after 10−20 Ca Looping cycles,20,31−33 and in some cases, the sorbent performed better after reactivation when compared with the parent limestone.28,29 Furthermore, it was shown that this technique can be used repeatedly to maintain sorbent activity at a high level over a large number of cycles.25 Fennell et al. maintained an average activity of ≈35% over 85 cycles while only hydrating-dehydrating the sorbent three times (after cycles 30, 50 and 70) in a TGA apparatus.34 Materic et al. maintained an average sorbent activity of 33% over 50 cycles while periodically hydrating the sorbent every three cycles in a fluidized bed.25 However, it was observed that the hydration−dehydration technique also induced a decrease in particle density29 and significantly weakened the reactivated sorbent particles so that increased rates of attrition were observed during the CO2 capture cycles following reactivation.25,32 Blamey et al.27 reported a 4-fold increase in sorbent breakage after using the hydration−dehydration technique in a fluid bed whereas Materic et al. reported a trebling in the amount of fines generated after 49 cycles in a bubbling bed25 and a significantly increased tendency for fragmentation at low impact velocities.35 Coppola et al. correlated this increased fragmentation with an increase in macroporosity of the sorbent occurring during the hydration based reactivation.36 An alternative hydration based reactivation technique was proposed to minimize the friability of the reactivated sorbent.25 Immediately after hydration, the hydrated sorbent is heated in the presence of CO2 to a temperature over 450 °C (typically 520 °C) and annealed at this temperature for a period of time. This method is referred to as the superheating technique, because it was observed that the presence of CO2 in the gas phase can lead to the inhibition of the dehydration of Ca(OH)2 with some materials.21,25,37 The superheated reactivation technique was reported to lead to mechanically stronger reactivated sorbents as measured in bubbling fluid beds,25 high velocity impact testing35 and compression tests.38 In all cases, the friability of the superheated sorbent was similar to that of the unreactivated sorbent. In addition, the superheated technique was also reported to generate more active sorbents than the hydration−dehydration technique, as measured in a TGA apparatus21 and bubbling fluid bed.25 However, no tests of the performance of reactivated performance have yet been carried out in pilot-scale dual fluid bed (DFB) systems under realistic conditions. On one hand, it is unclear whether the mechanical strength of the reactivated sorbents would be sufficient to remain usable in the highly attriting environment of a DFB unit. On the other hand, it is unclear what effect the harsh calcining conditions expected

in the oxy-fuel calciner would have on the CO2 carrying capacity improvement induced by hydration based reactivation. Therefore, the objective of this work was to assess the performance of sorbents reactivated with both hydration based reactivation methods in a pilot-scale, oxy-fired DFB system. Batches of sorbents reactivated by two different hydration based methods (hydration−dehydration and superheating) were prepared from 30 kg of limestone each, using an electrically heated bubbling bed apparatus at Callaghan Innovation in New Zealand. Sorbent performance, in terms of CO2 carrying capacity and sorbent attrition, was then tested in an oxy-fired, pilot-scale DFB unit at CanmetENERGY in Canada.

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EXPERIMENTAL SECTION

Sorbent Sample Preparation. All sorbent samples were prepared from a high purity (96%) calcitic natural limestone with a nominal particle size between 0.4 and 0.85 mm obtained from Holcim, Te Kuiti. X-ray fluorescence (XRF) analysis showed that the raw limestone contained small amounts of SiO2 (1.2%), Al2O3 (0.39%) and Fe2O3 (0.27%). Sorbent batches were prepared using a bubbling fluid bed unit at Callaghan Innovation, New Zealand, which is capable of processing 2.5 kg batches of limestone. The apparatus used had a reaction zone of 0.083 m diameter and 1 m length with a disengagement zone of 0.21 m diameter and 0.24 m length. The experimental setup is further described elsewhere.25 This unit was used to produce three different types of sorbent: an unreactivated sorbent, a hydrated−dehydrated sorbent and a superheated sorbent. The preparation method is illustrated in Figure 1, and described in more detail below, whereas Table 1 summarizes the conditions used in each step of the preparation process.

Figure 1. Sorbent sample preparation procedure: steps and sequence.

Table 1. Conditions Used during the Preparation of Sorbent Samples in the Bubbling Bed Unit at Callaghan Innovation bubbling bed conversions calcination

carbonation

hydration

superheating

temperature (°C) gas (vol %)

870−900

650−700

250−340

300−520

air 80

20% H2O/ N2 60

100% CO2

time (min)

25% CO2/ air 20

45

Unreactivated Sorbent. A 2.5 kg batch of raw limestone was charged into the reactor and subjected to 6 calcination/carbonation cycles using the conditions given in Table 1. Upon completion of the 6 calcination/carbonation cycles, the sorbent was discharged and was found to contain 15% CaCO3. On average, 2.1 wt % of the bed charge was elutriated out of the reactor during the preparation of this sorbent. Hydrated/Dehydrated Sorbent. A 2.5 kg batch of raw limestone was charged into the reactor and subjected to 6 calcination/ carbonation cycles with the insertion of a hydration−dehydration step before the final carbonation, see Figure 1. The hydration− dehydration step consisted in cooling the calcined bed in N2 to 220 °C, and hydrating the sorbent in 20% steam in N2 for 60 min. After 5364

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Figure 2. Schematic of the dual fluidized bed pilot-plant at CanmetENERGY, Ottawa. hydration, the bed was heated to carbonation temperature (650 °C) in air. The progress of the dehydration reaction was monitored using a humidity probe in the outlet gas, and this reaction was completed as the bed temperature reached 430 °C. Finally, carbonation was performed at 650 °C, and the discharged sorbent was found to contain 58% CaCO3. On average, 4.2 wt % of the bed charge was elutriated out of the reactor during preparation of this sorbent. Superheated Sorbent. A 2.5 kg batch of raw limestone was charged into the reactor and subjected to 6 calcination/carbonation cycles with the final carbonation replaced by a superheating step. During the superheating step, the sorbent was first hydrated as described above and then heated to 520 °C in 100% CO2 and held at that temperature to anneal for 25 min. After the annealing hold, CO2 was replaced by air, which triggered the dehydration of the remaining Ca(OH)2.21,25 Finally, the bed material was discharged and found to contain 65% CaCO3. On average, 3.8 wt % of the bed charge was elutriated out of the reactor during preparation of this sorbent. In all reaction steps, the gas flow supplied was sufficient to ensure fluidization of sorbent particles. To account for temperature variations during the production process, the fluidization velocity was maintained between 0.4 and 0.7 m s−1, i.e., approximately 1.5−2.5 times the measured minimum fluidization velocity (Umf). Given that 30 kg batches of limestone equivalent were required for testing in the DFB unit each preparation procedure was repeated 12 times, and the resulting materials were mixed to form a single batch of sorbent for testing in the DFB unit. Prior to pilot plant tests, samples

of each sorbent batch were taken and tested for CO2 carrying capacity and friability using lab-scale apparatus. Note that the conditions used for sample preparation were not strictly representative of those expected in industrial units, i.e., oxyfired, dual fluidized beds. The conditions employed here were chosen as a compromise in order to respect the limitations of the electrically heated, batch operated, BFB reactor and to allow a practicable production method of large sorbent amounts (30 kg total for each sample). In particular, air was used in the calcination step to reduce the calcination temperature and achieve an acceptably short calcination time, here 80 min, see Table 1. On the basis of the conditions presented in Table 1, in particular for calcination, it can be expected that the sorbent prepared in this way would be less sintered and therefore less deactivated than equivalent materials prepared in an oxy-fired unit. CO2 carrying capacity was measured using a Stanton Redcroft 1500 TGA apparatus. Sorbent samples were subjected to 9 Ca Looping cycles, with calcinations performed for 5 min at 850 °C in N2 at 35 mL min−1 in and carbonations performed for 20 min at 620 °C in CO2 at 26 mL min−1. The conditions used in these tests favor high sorbent activity and thus this method is likely to overestimate the sorbent’s CO2 carrying capacity compared to what could be expected in an oxy-fired, industrial unit, such as the pilot system used here. CO2 carrying capacities obtained with this laboratory method were compared to those measured in the DFB unit for the reactivated sorbents. 5365

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Friability Testing. The friability testing was performed in a custom built attrition testing unit described elsewhere.39 Using an internally circulating fluidized bed (ICFB), sample particles were repeatedly impacted against a wall at a controlled velocity during 10 min periods. Sorbent friability at a given circulation velocity was assessed by measuring the weight of elutriated fines. DFB Pilot Unit. The sorbent samples were tested using a 100 kWth dual fluidized bed pilot-plant designed and constructed at CanmetENERGY, Ottawa. A detailed schematic of the system is shown in Figure 2. The system is designed to handle a variety of solid feedstock for sorbent calcination, e.g., coal, coke and biomass, whereas carbonation can be achieved with either simulated or real flue gas produced from natural gas combustion. The calciner/combustor is designed to operate in full oxy-fuel mode with flue gas recycle (either wet or dry), allowing for the production of a flue gas stream with a concentration of CO2 greater than 90%. The entire vertical length (∼5 m) of the calciner (0.1 m ID) is outfitted with electric heaters to provide the necessary heat during start-up and to ensure uniform temperatures along the riser, avoiding the possibility of recarbonation. The calciner can be operated in either bubbling or circulating mode, which allows for flexibility in sorbent particle size, and is equipped with both an overflow discharge and loop-seal (in the return-leg below the cyclone) utilized for sorbent transfer to the carbonator. Similar to the calciner, the carbonator (0.1 m ID) uses electric heaters to maintain the reactor temperature. The carbonator is designed to be operated in bubbling mode, and the sorbent is transferred back to the calciner via bed overflow discharge. Other key features of the system include an electric boiler used for steam addition to the carbonator/calciner, online gas analyzers (O2, CO, CO2, SO2, and NOx) connected to both reactor effluent lines, and a transfer auger/eductor to control the transfer rate of sorbent from the carbonator to the calciner, and vice versa. System Start-up and Operation. During start-up of the DFB system, 5 kg of olivine sand (150−850 μm) was added to both the calciner and carbonator before preheating both reactors to 700 and 650 °C, respectively. Once the required temperature was achieved, the addition of fuel (low ash wood pellets) to the calciner was initiated through the water cooled feed screw. To establish the required oxyfuel conditions, the primary fluidizing gas was switched over to recycle flue gas while simultaneously introducing O2 (>99.5% in purity) into the system to replace the compressed air used during start-up. During the transition period, the back-end pressure was tightly controlled in order to minimize air ingress into the system, thus maximizing the CO2 concentration in the calciner. The next step was to initiate the circulation of olivine sand between the calciner and the carbonator. This was achieved by switching the diverter valve position from the return-leg (to the calciner) to the eductor, allowing the solids to be pneumatically conveyed to a high efficiency cyclone, and finally into the carbonator. At the same time, the carbonator overflow valve was opened and the solids transfer auger was started, which allowed material to flow from the carbonator to the calciner. The bed inventory in each reactor was then fixed by adjusting the transfer auger rotational speed. The nominal transfer speed for the auger was set at 30 kg solid h−1. Once sand circulation was stabilized, a known amount of sorbent was progressively added to the system through the eductor. Note that the addition of sorbent proceeded slowly and was done in several batches in order not to disrupt the solids circulation. The use of an inert sand material in this manner allowed avoiding difficulties in the stabilization of system operation and normalizing the loading and initiation procedures for all sorbents tested. Furthermore, this approach allowed minimizing the thermal sintering that inevitably occurs during the loading and initiation phase, thus providing a clearer comparison between sorbents. The mass of sorbent added was normalized to provide a 48 wt % proportion of CaO in sand once fully calcined; this value was chosen as it was found to allow >90% CO2 capture efficiency in the carbonator when using the superheated sorbent. As a result, the Ca/CO2 ratio in the carbonator was constant during all three experiments and was calculated to be 6.5.

Once all the sorbent was added, the DFB system was operated for 4−5 h while maintaining the conditions shown in Table 2 and monitoring a range of parameters as discussed in the following section.

Table 2. Experimental Conditions for Both Reactors Used during DFB Tests at CANMET temperature (°C)

gas composition

gas velocity (m s−1)

sorbent residence time (min)

carbonator

650

0.6

12

calciner

910

18% CO2/ air 50% O2/ flue Gas

1.25

7

Measurements. CO2 capture efficiency was calculated from the difference between the CO2 concentration at the inlet and outlet of the carbonator, see eq 1

%ECO2 = 1 −

CCO2out CCCO2in

(1)

where %ECO2 is the CO2 capture efficiency and CCO2out and CCO2in are the measured concentration of CO2 out and in of the carbonator, respectively. Because the CO2 concentration at the inlet was fixed for each test, the capture efficiency was simply a function of the outlet CO2 concentration, which was recorded continuously over the experimental test period using an online gas analyzer. In addition, the CO2 concentration in the gas stream leaving the calciner was monitored during the experiments. This concentration was in the 77−90% range with higher values typically observed during the sorbent loading period, i.e., during the first calcination of the samples. Solid Samples and Mass balance. Between 3 and 5 samples of each ∼0.15 kg were collected from the carbonator to calciner transfer line (before the solids transfer auger) during the tests, allowing for the determination of the sorbent CO2 carrying capacity over time. At the end of each experiment, all material remaining in the DFB unit was collected and weighed. Samples were taken from the materials collected in different parts of the system (calciner, carbonator, loop seal, solids transfer auger, transfer lines, cyclones and baghouse). These samples were subsequently characterized to determine carbonation conversion using TGA and particle size distribution (PSD) using sieve screening. Repeated carbonation conversion measurements were performed on selected samples to assess the experimental error using this method. The formation of silicates species through the high temperature interaction of the sand and sorbent materials was not observed in this work.40 Finally, mass balances were closed between 93 and 105% in the three different experiments.

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RESULTS AND DISCUSSION Sorbent Characterization at Callaghan Innovation. The CO2 carrying capacity of the reactivated sorbents was measured using a TGA apparatus, and Figure 3 plots its evolution over nine Ca Looping cycles. It can be seen that the carrying capacity of the unreactivated sorbent remained essentially constant, while that of reactivated sorbents considerably reduced with cycling. As shown in Figure 3, reactivation resulted in doubling the amount of CO2 captured per unit mass of CaO over the nine cycles tested here. On the basis of these results, it can be expected that in the DFB unit, the CO2 carrying capacity increase due to reactivation will last for a limited period of time. However, the duration and the extent of this increase are expected to be smaller in the DFB unit than those measured using the TGA apparatus because more intense sintering calcination conditions prevail in the DFB unit’s oxy-fuel calciner.15 5366

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difference was more pronounced at velocities above the threshold. Reactivated sorbents exhibited considerably different friability profiles depending on the reactivation technique used. The hydrated−dehydrated sorbent sample was very friable, even at the lowest velocities tested, and did not exhibit a treshold as sorbent friability increased almost linearly with circulation velocity. In contrast, the superheated sorbent exhibited a behavior very similar to that of the parent limestone, and was found to be slightly less friable than its unreactivated counterpart at velocities above the threshold value. A more detailed analysis and interpretation of these friability profiles can be found elsewhere.35 These results suggest that regardless of the intensity of mechanical stresses experienced in the DFB unit, the hydrated−dehydrated sorbent can be expected to attrite the most followed by the superheated sorbent and finally, the unreactivated sorbent. The actual attrition level reached will however depend on the actual impact velocities and impact frequency that the particles experience in the DFB unit. Higher impact velocity and frequency are likely to increase final attrition levels and diminish the difference observed between the different sorbents as can be seen from Figure 4. Sorbent Performance in the Dual Fluid Bed System. Unreactivated Sorbent. Figure 5 plots the evolution of CO2 capture efficiency and carbonator temperature during the experiment with the unreactivated sorbent. As seen in this figure, CO2 capture efficiency increased with each addition of sorbent, then briefly stabilized at 60% which was followed by a period of decline until the end of the experiment, ultimately reaching ≈35%. Given that the CO2 carrying capacity of the unreactivated sorbent can be expected to remain essentially constant during this experiment, see Figure 3, the explanation for this decline is likely due to changes in the flow, inventory, or residence times during the experiment. Inventory, Flow and Residence Times. Figure 6 plots the evolution of the differential pressure across the two reactors during the experiment, which is an indication of the evolution of reactor inventory with time. It can be seen that the inventory of the calciner remained relatively stable during the experiment after sorbent addition was completed, while the carbonator inventory varied considerably during the run; after an initial

Figure 3. Evolution of the CO2 carrying capacity of the reactivated sorbents with cycling number as measured in the TGA. Note that cycle 0 refers to the sorbent immediately after reactivation.

Figure 4. Friability profiles of different sorbents produced in this work, as measured in an ICFB attrition testing apparatus. This data is analyzed and the apparatus is described in more detail elsewhere.35,39

Friability. The mechanical strength of the reactivated sorbents was tested using a custom-built internally circulating fluid bed apparatus described elsewhere.39 Figure 4 plots the sorbent friability (as mass of elutriated material in 10 min of circulation) as a function of impact velocity for the 500−600 μm fraction of the different sorbent samples prepared for this work. The rapid increase in sorbent friability observed above the threshold value of 8 m s−1 was previously attributed to the activation of the fragmentation mode of attrition.35,39 The unreactivated sorbent sample was found to be more friable than its raw limestone parent at all velocities; the

Figure 5. CO2 capture efficiency and carbonator temperature measured during the experiment with the unreactivated sorbent. 5367

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Figure 6. Evolution of differential pressure in the DFB reactors during the experiment with the unreactivated sorbent.

Table 3. CO2 Capture Efficiency, CO2 Content and Calculated System Parameters at the Time of Solid Sampling for the Experiment with the Unreactivated Sorbenta sample

total run time (h)

CO2 content (wt %)

% CaO converted (σ = 1.5%)

CO2 capture efficiency (%)

solid flow (kg h−1)

solid inventory (kg)

residence time (min)

1 2 3 4 final

1.75 2.75 3.75 4.75 5

2.86 2.92 2.88 3.35 2.96

9.0 9.2 9.0 10.6 9.3

51 48 43 38

31 29 26 20

6.0 4.4 3.7 3.7

12 9 9 11

a

Standard deviation (σ) was evaluated by repeating the measurement for selected samples.

leaving the carbonator was reducing throughout the experiment. Because the inventory of the carbonator was also reduced during the experiment, the same can be said for the flow of solids into the carbonator, and therefore, the Ca/CO2 ratio. Thus, the reduction in the CO2 capture efficiency can be attributed to the progressive reduction in the Ca/CO2 ratio during the test. However, despite variations in the inventory and solid flow, the residence time of the sorbent was found to remain stable because both solid flow and inventory reduced at a similar rate (Table 3). Most importantly, the residence time in the carbonator during the experiment was sufficient to achieve complete carbonation in the fast regime so that the conversions achieved during the experiment can be said to be representative of the sorbent’s carrying capacity. Its value was found to be at ≈9%, which was comparable to the value obtained during TGA testing shown in Figure 3, despite the much harsher sintering conditions experience in the oxy-fired DFB unit. The observed system instability is here attributed to the substantial change in particle size distribution of the sorbent, induced by attrition phenomena, which are discussed in the following section. Attrition and Elutriation. Sorbent friability, i.e., tendency to attrite, was assessed by comparing the particle size distribution of the sand/sorbent mixture before and after the test, as discussed in the Experimental Section and shown in Figure 7. Note that preliminary experiments have shown that the olivine sand used here has a negligible tendency to attrite in this DFB unit so that the measured fine creation is assumed to be essentially the result of the attrition of limestone particles.

increase due to sorbent addition, the carbonator inventory decreased by a factor of 1.7 during the course of the experiment, see Figure 6. Because this reduction is not accompanied by an increase in the calciner inventory, it can be deduced that some sorbent is likely accumulating in other parts of the system, e.g., return leg or transfer lines. The flow of solid material leaving the carbonator can be estimated by considering the mass balance of CO2 across the carbonator. Namely, because the CO2 captured from the gas phase leaves the carbonator with the solids then the solid flow leaving the carbonator at a given moment can be calculated knowing the amount of CO2 captured and the CO2 content of the solid leaving the carbonator. All the data required to calculate the solid flow is available at the moment that a solid sample was taken; both the CO2 content of the solid sample and the outlet gas are known at each sampling point. Assuming a steady state in the carbonator during the sampling period, the solid flow out of the carbonator can be calculated using the CO2 mass balance as follows: Fsolid,out =

%ECO2 × FCO2in %CO2 − solid

(2)

Where Fsolid,out is the mass flow of solids out of the carbonator, %ECO2 the CO2 capture efficiency, FCO2in the mass flow of CO2 in the carbonator and %CO2−solid the mass of CO2 contained in the sorbent-sand mixture, as experimentally determined. The results of this calculation for the moments where solid samples were taken and their CO2 content was measured are shown in Table 3. These results suggest that flow of solids 5368

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Such large changes in the sorbent PSD are likely to induce changes in the hydrodynamic performance of the system and thus are a plausible explanation for the observed variations in sorbent flow and carbonator inventory. Reactivated Sorbents. Inventory, Flow and Residence Times. Similarly to the experiment with the unreactivated sorbent described above, the carbonator inventory reduced with time while the calciner inventory remained stable, as shown in Figure 8. To address this issue, the speed of the transfer auger was slightly increased several times during the experiment with the hydrated−dehydrated sorbent, which effectively stabilized the carbonator inventory (Figure 8, left). The transfer rate was increased just once during the experiment with the superheated sorbent (Figure 8, right) with little effect as to the stability of the carbonator inventory. Figure 9 plots the evolution of CO2 capture efficiency during the experiments, whereas Table 4 shows the CO2 content and the CaO conversion of solid samples taken. Using the method described above, the CO2 mass balance allows calculating the flow of solids leaving the carbonator at the moment when the samples were taken. The results of these calculations, shown in Table 4, suggest that the increase in the solid transfer rate allowed maintaining a steady solid flow with the hydrated−dehydrated sorbent. In contrast, the solid flow was found to steadily decline with the superheated sorbent after ≈3 h of operation. As a result, the average residence time increased considerably during the experiment with the superheated sorbent whereas it remained stable with the hydrated−dehydrated sorbent. It is worth noting at this point that during the first 3 h of operation, the system operated at the nominal flow rates and residence times with all three sorbents. This observation will be used in the Results and Discussion section to allow for a comparison of the CO2 carrying capacity of the different sorbents. The CaO conversion of the first solid sample for both reactivated sorbents, Table 4, confirms the positive effect of reactivation on the CO2 carrying capacity of the sorbent. Namely, at the moment this sample was taken, the average particle had experienced ≈5−6 cycles and the CaO conversion measured for reactivated sorbents (13%; Table 4) is higher than that measured for the unreactivated sorbent (9%; Table 3). This improvement might appear low compared to what could have been expected after 5−6 cycles from the TGA experiments shown in Figure 3. The difference between these results can be

Figure 7. Particle size distribution of the unreactivated sorbent/sand mixture before and after the experiment in the DFB unit. Note that the data includes materials elutriated during the experiment.

Although the potential effect of the olivine sand on the quantitative attrition data obtained in this unit is difficult to assess it is not expected it would affect the validity of the comparison between the different sorbents tested in this way. Considerable attrition occurred during the experiment, as evidenced by the change in the PSD of the sand/sorbent mixture plotted in Figure 7. The proportion of particles larger than 600 μm reduced from ≈24 to 3% while the proportion of fines (600 μm) during cycling suggests that attrition occurred via the fragmentation mode,35,41 which usually occurs at velocities above the threshold value (≈8 m s−1, Figure 4). Furthermore, impacts at velocities above the fragmentation threshold can be expected to lead to elevated fine creation rates, in line with what was experimentally observed. In addition, some attrition could be occurring in the transfer auger. Considering that the nominal gas velocities in the reactors, 0.6 and 1.25 m s−1 for the carbonator and calciner, respectively, were significantly lower than the fragmentation threshold values at 6 and 8 m s−1; it appears that attrition most likely occurred in the cyclone and transfer lines where the particles were accelerated to much higher velocities.

Figure 8. Evolution of the differential pressure across the carbonator during experiments with reactivated sorbents: hydrated−dehydrated (left), superheated sorbent (right). 5369

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Figure 9. Evolution of the CO2 capture efficiency and carbonator temperature as well as sample collection time during DFB experiments with the hydrated−dehydrated sorbent (left) and superheated sorbent (right).

Table 4. CO2 Capture Efficiency, CO2 Content and Calculated System Parameters at the Time of Solid Sampling for the Experiment with the Hydrated−Dehydrated and Superheated Sorbentsa sample

total run time (h)

CO2 content (wt %)

% CaO converted (σ = 1.5%)

1 2 3 4 5 final

1.75 2.5 3.5 4.5 5.5 5.75

4.47 3.73 3.69 3.48 3.24 3.69

13 11 11 10 9 11

1 2 3 final

2.75 3.75 4.75 5

4.62 5.75 7.66 9.63

13 17 23 29

a

CO2 capture efficiency

solid flow (kg h−1)

hydrated−dehydrated sorbent 79 31 77 36 66 31 59 30 53 29 superheated sorbent 82 72 62

31 13 8

solid inventory (kg)

residence time (min)

5.8 5.1 3.9 5.1 4.6

11 8 8 10 10

5.8 4.6 4.0

11 22 30

Final refers to the sorbent remaining in the carbonator at the end of the experiment.

explained by the increased intensity of sintering occurring in the oxy-fuel calciner in the DFB unit. The final conversion of the hydrated−dehydrated sorbent appears only marginally higher than that observed for the unreactivated sorbent. This result suggests that in realistic cycling conditions such as those used in the DFB unit in this work, the benefit of a single reactivation step would have a limited duration. In contrast, the CO2 content of the solid leaving the carbonator was found to increase during the experiment with the superheated sorbent. The origin of this behavior remains unclear but it could be explained by the increased residence time that the superheated sorbent experienced in the carbonator. Namely, it has been shown that sorbents hydrated at some point in their history exhibit an increased carbonation rate in the diffusion regime31,33 when compared with that of unreactivated sorbents. Therefore, the prolonged carbonation times the superheated sorbent was exposed to (Table 4) can be expected to increase the CaO conversion42 reached. Elutriation and Attrition. Reactivated sorbents exhibited increased levels of elutriation; the elutriation rates of the hydrated−dehydrated and superheated sorbent were 0.53 and 0.74% h−1, respectively, which was higher than that of unreactivated sorbent (0.40% h−1). Over 95% of all elutriated particles were smaller than 150 μm in all cases. Reactivated sorbents experienced significant attrition during the experiments in the DFB unit. Figure 10 plots the particle size distribution of the sorbent/sand mixture before and after the experiment with the reactivated sorbents in the DFB unit.

Figure 10. Particle size distribution of the reactivated sorbents/sand mixture before and after the experiment in the DFB unit. Note that this data include the material elutriated during the experiment.

With both reactivated sorbents, particles larger than 600 μm were fragmented before the end of the experiment. Furthermore, the proportion of fines generated during the experiment increased by 12% and 15% points for the superheated and hydrated−dehydrated sorbent, respectively (Figure 10). Overall, the superheated sorbent was found to be less prone to attrition than the hydrated−dehydrated sorbent with an average fine creation rate of 2.28% h−1 vs 2.63% h−1, respectively. The intensity of elutriation of the superheated sorbent compared to that of the unreactivated sorbent is surprisingly high in light of the results presented in Figure 4, 5370

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other limestones and in other fluid bed systems in order to confirm the wider validity of this statement. In terms of sorbent CO2 carrying capacity and on the basis of literature reports and TGA results (Figure 3), it was expected that reactivated sorbents would exhibit a significant but temporary increase in CO2 carrying capacity during the period immediately following sorbent loading. One of the objectives of this work was to assess the duration and the extent of that increase when the reactivated sorbents are subjected to more intensely sintering calcination conditions like those of the oxyfired DFB system. Evaluating the increase in CO2 carrying capacity induced by reactivation is not straightforward because sorbents were added to the DFB reactor over a period of 1−1.5 h, see Figures 5 and 9. As a result, not all particles were cycled for the same duration. To estimate the increase in CO2 carrying capacity induced by reactivation, the average mass of CO2 captured per kg CaO per hour in circulation during the first 3 h of each experiment was calculated. This was achieved by dividing the total mass of CO2 captured during this period (calculated from CO2 capture efficiency data) by the average particle circulation time (eq 3).

and it cannot be easily explained by the results obtained in this study. The absence of the large particle fraction at the end of the experiment with reactivated sorbents suggests that significant attrition via the fragmentation mode had occurred, and therefore, that sorbent particles were subjected to high velocity impacts in the DFB unit. However, the reason for the increased elutriation observed with the superheated sorbent remains unclear, because the bulk densities of both reactivated sorbents were found to be comparable prior to testing in the DFB unit.

■

DISCUSSION As mentioned in the Introduction, the objective and principal novelty of this work was to assess the performance of hydration reactivated sorbents, in terms of attrition and CO2 carrying capacity, when used in an oxy-fired dual fluid bed system. Despite the variations of the solid flow and carbonator inventory during the experiments, it is possible to draw a range of useful conclusions in this respect. In regards to sorbent attrition, based on literature data25,27,29,32,35 and laboratory scale results, shown in Figure 4, it could have been expected that the hydrated−dehydrated sorbent would exhibit such high attrition levels as to be unusable in the DFB system. Also, superheated sorbents were expected to exhibit lower attrition levels but it was unknown whether they would be sufficiently strong to be usable in a realistic DFB system. Both reactivated sorbents were found to be sufficiently strong to remain usable in the DFB system unit for 5 h despite the intense mechanical stresses they were subjected to, as evidenced by the significant fragmentation observed even when circulating unreactivated sorbent particles (Figure 7). The variations in solid flow and inventory observed during the experiments were attributed to this “baseline” attrition. However, both reactivated sorbents experienced increased attrition rates compared to that of the unreactivated sorbent. As could be expected, the attrition levels experienced by the superheated sorbent were in between those of the hydrated− dehydrated sorbent and the unreactivated sorbent (Table 5).

3h

A CO2,3h =

fine creation rate (wt % h−1) elutriation rate (wt % h−1) max. CO2 capture efficiency (%) CO2 captured over 3 h (kgCO2 kgCaO−1 h−1)

hydrated− dehydrated

superheated

1.84 0.39 62

2.63 0.52 85

2.29 0.74 90

5.5

8.6

9.2

mCaO*tav.circulation

(3)

Where ACO2,3h is the average mass of CO2 captured per kg of CaO during the first 3 h of operation, FCO2inthe mass flow of CO2 in the carbonator, %ECO2 the CO2 capture efficiency, mCaO the total mass of CaO in the system, and tav.circulation the average time the sorbent spent circulating in the system during the first 3 h of operation. Using this calculation method corrects for the delayed addition of sorbents to the system, whereas using only the results obtained during the first 3 h of operation focuses on the period where solid flow and inventory were relatively stable and comparable across the three experiments. The results of this calculation are shown in Table 5 and indicate that hydration based reactivation significantly increased the CO2 carrying capacity of the sorbents during this period. In line with the TGA results, the superheated sorbent had a higher CO2 carrying capacity than the hydrated−dehydrated sorbent over the test period. However, the increase in CO2 capture capacity induced by reactivation was smaller than what could have been expected from TGA results (Figure 3); the difference can be attributed to the more intensely sintering conditions prevailing in the oxy-fuel calciner of the DFB unit. The results presented in Tables 3 and 4 suggest that the increase in sorbent activity lasted approximately 3 h with the hydrated dehydrated sorbents, which nominally corresponds to approximately 9 cycles. The duration of the improvement is similar to that observed in the TGA where it had largely diminished after 5−6 cycles, as shown in Figure 3. In the case of the superheated sorbent, the carrying capacity was found to increase after 3 h, which is most likely due to an increase in the carbonator residence time rather than a property inherent to the superheated sorbent. This observation, if confirmed, suggests that hydration based reactivation might be particularly useful in Ca Looping systems with longer carbonation residence times. This is in particular the case with systems using bubbling or fixed bed carbonator reactors. It is worth noting that the work presented here only assessed the performance of a sorbent prepared under relatively gentle

Table 5. Summary of Sorbent Performance in the DFB Unit unreactivated

∫0 FCO2in × %ECO2 ·dt

Note that the quantitative data obtained here should be extrapolated with care because each fluid bed system and each limestone source is likely to have a range of specific and unique elements that can profoundly modify these values, e.g., specific reactor configurations or the use of an inert sand material in these experiments. However, it is expected that the qualitative and relative conclusions obtained in this work should remain valid in other DFB systems. These observations then suggest that hydration reactivated sorbents derived from Te Kuiti limestone are likely to be usable in most DFB systems, particularly so if these were designed to minimize attrition. Nevertheless, it would be necessary to investigate the behavior of reactivated sorbents derived from 5371

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calcination conditions, i.e., in air at 900 °C, and this sorbent can be expected to be less deactivated than an equivalent sorbent obtained in an oxy-fired DFB unit. Furthermore, only the effect of a single reactivation step was tested whereas in practice it is intended that reactivation be periodically performed. Thus, the logical next step in the development of hydration based reactivation methods would be to test the performance of sorbents spent in the oxy-fired unit itself and periodically reactivated.

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CONCLUSION In this work, the performance of sorbents reactivated by hydration−dehydration and superheating was tested for the first time in an oxy-fired dual fluid bed system and compared to the predictions based on laboratory scale results. It was shown that both types of reactivated sorbent tested here could be used in DFB systems without serious disruptions, although their attrition levels were found to be 25−50% higher than those of the unreactivated sorbent. As expected, sorbents reactivated using the superheated method experienced less attrition than those reactivated with hydration−dehydration. Finally, hydration based reactivation increased the carrying capacity of Ca Looping sorbent by ≈60%, although the improvement was lower than that measured in a laboratory setting, which was attributed to the intense sintering conditions employed during calcination in this work.

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AUTHOR INFORMATION

Corresponding Author

*V. Materić. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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