WO2014012170A1 - System and method for reactivating and pelletizing sorbents for gas capturing - Google Patents

Abstract

The present invention uses spray of water to reactivate spent sorbent by hydration and simultaneously produce the reactivated sorbent in pelletized form. The use of spray water combined with movement of the sorbent can be controlled to produce reactivated sorbent of desirable size. The system and method eliminate the need for separate pelletization after reactivation of spent sorbent, and allow both to complete in one process and in one container. The system and method enable addition of calcium aluminate cements as coating to form pellets with core-in-shell structure thereby enhancing pellet strength and sorbent capacity for absorbing gas.

Description

SYSTEM AND METHOD FOR REACTIVATING AND PELLETIZING

SORBENTS FOR GAS CAPTURING

Field of the Invention

The present invention relates generally to sorbent reactivation systems and methods, and particularly to systems and methods for reactivating and pelletizing CaO-based sorbents used in gas capturing. Background of the Invention

Various gas capturing processes using CaO-based sorbents have been known. For example, the possibility of using CaO-based sorbent for the removal of carbon dioxide (C0₂) was considered at least at early as the 1920s. The process is based on the reversible carbonation-calcination reaction. C0₂ scrubbing is achieved in a cyclic process which involves C0₂ capture from a flue gas in a carbonator (reactor where carbonation takes place) and sorbent regeneration in a calciner (a reactor where calcinations takes place), releasing a concentrated stream of C0₂. The reactivated sorbent is ready to be used in further cycles, i.e., the same sorbent can be used in a looping process for C0₂ capture.

For example, calcium looping (CaL) cycles can be used in the process of capturing C0₂. More specifically, CaL can be used to produce a concentrated C0₂ stream from the utilization of fossil fuels and biomass, which can allow the sequestration of C0₂ from large stationary thermal power sources such as fossil-fuel-fired power plants, thereby reducing emission of C0₂ to the atmosphere.

CaL processes are based on the following reversible carbonation reaction: CaO(s) + C0₂₍g) = CaC0₃(_S) AH_r° = -178 kJ/mol

(1)

This reaction is employed to capture C0₂ from gases during pre-combustion processes such as sorption enhanced reforming or from flue gases produced during combustion in a post-combustion C0₂ capture scenario.

The reverse reaction, calcination, enables sorbent regeneration and produces a concentrated C0₂ stream. However, calcination usually takes place at higher temperatures, typically > 900 °C, which in the presence of an almost pure C0₂ atmosphere enhance sintering, that in turn significantly reduces sorbent C0₂ capture activity during cycles. Although natural CaO-based sorbent maintains a residual reactivity even over a very long series of carbonation/calcination cycles, the deactivation phenomenon is even worse in the presence of S0₂. Here the sorbent is irreversibly sulphated, but more importantly, the CaS0₄ product covers pore surfaces preventing further reaction between CaO and C0₂. This means that sorbent becomes deactivated (spent sorbent), and an increased make up of fresh sorbent is required in order to maintain the desired C0₂ capture efficiency. Unfortunately, this increases the cost of C0₂ capture, and may pose additional environmental problems and costs related to the disposal of the waste material, if it is not to be used in cement manufacture.

In the context of S0₂ capture, fluidized bed combustion (FBC) is an established combustion technology for solid fuels (including high-sulphur coals and petroleum coke). Calcium-based sorbents (calcitic limestone and dolomite) are typically used for S0₂ capture in FBC systems. The sorbent and fuel are fed into the boiler where S0₂ is captured, usually at 850-900 °C. The overall reaction for sulphur capture in an atmospheric FBC boiler involves two steps: calcination (2) and sulphation (3):

CaC0₃→ CaO + C0₂ (2) CaO + S0₂ +l/20₂→ CaS0₄ (3)

Limestone is a low-porosity material; however, upon calcinations, which typically takes many minutes at atmospheric FBC conditions (-850 °C), it becomes very porous. However, the pores are eventually blocked by the formation of the CaS0₄ product layer due to the fact that CaS0 has a larger molar volume than CaC0₃ and CaO (i.e., 46 vs. 37 and 17 cm³/mol, respectively). This leaves a significant amount of unreacted CaO in the core of the particle, leading to a relatively low utilization of CaO (typically 30-40%), which is one of the major limitations of the technology. As a result, excess limestone sorbent is required to achieve an acceptable S0₂ capture efficiency, and a typical Ca/S molar ratio of 2-3 is normal in industrial FBC boilers for >90% S0₂ removal.

One drawback of excess sorbent use is that it adversely affects the economics of the FBC technology, as the cost for both limestone use and ash disposal for the spent sorbent increases. Moreover, the C0₂ emissions are somewhat increased because extra limestone needs to be calcined in the combustor.

Another critical issue associated with the use of excessive sorbent quantities is a high level of unreacted calcium oxide (20-30% CaO) contained in the FBC solid residues (e.g., bed ash and fly ash) especially when firing high-sulphur fuels. Hence, FBC ash can experience highly exothermic reactions when in contact with water, creating potential problems for handling and disposal. Furthermore, the ash produces high-pH leachate from the landfill which must also be treated. In addition, the presence of significant quantities of CaO in the landfill ashes can also cause expansion due to the formation of ettringite, which further increases the cost for ash disposal.

To maintain sorbent carrying capacity, numerous methods have been tested, including sorbent pre-treatments by heating at high temperature, or by chemical agents, or sorbent reactivation, produced by means of hydration. Hydration is in fact one of the most well-investigated methods for spent sorbent reactivation. For example, the basic reaction during hydration of partially sulphated sorbent occurs as follows:

CaO + H₂0→ Ca(OH)₂ (4)

Water (or steam) reacts with CaO in the core and cracks the sulphated shell, because Ca(OH)₂ has a larger molar volume than that of CaO. The unreacted core is exposed so that the capacity for additional sulphur removal can be substantially restored for the hydrated sorbent once it is reintroduced into the combustor.

Unfortunately, the use of liquid water for hydration does not appear to be practical because it creates additional energy penalties for drying the wet hydrated lime or other hydrated sorbents.

Steam hydration has been proposed as an alternative sorbent treatment, and spent sorbent has been shown to be very reactive with steam, and even moisture from air can regenerate sorbents at room temperature.

Past steam hydration studies have demonstrated successful reactivation for sorbent cycled and sintered under different conditions and reactor scales such as thermogravimetric analyzer (TGA), fixed bed, or even industrial-scale fluidized bed facilities. However, these studies have also shown that large cracks were formed in the particles resulting in poor mechanical strength of the particles which easily becomes a powdered material during handling, which means that reactivated material cannot be effectively used directly in fluidized bed combustion (FBC) reactors which are currently the preferred technology for any CaL cycle processes. As a result, the reactivated sorbents are required to be subsequently processed into a form that is suitable for use in FBC. For example, a separate pelletization process implemented with a separate reactor or pelletizer is applied to the reactivated sorbents. Another possible method of mitigating such shortcomings could be calcination of hydrated sorbent in an atmosphere of C0₂. Thus, reactivated sorbent calcined at temperatures which are much higher than the decomposition temperature for Ca(OH)₂ is mechanically stronger than if the sorbent is simply calcined at a temperature sufficient to decompose Ca(OH)₂.

Therefore, although hydration can be effective for the bed material, there are two remaining issues to be addressed.

First, hydrated material tends to be fragile since the hydrated particles show severe cracks throughout their structures, which means that such materials will have a reduced lifetime in a FBC boiler due to fragmentation or attrition.

Second, it appears that fly ash typically cannot be reactivated by hydration. This in itself may not be important given that such material is already very reactive; however, fly ash has an inadequate residence time in the primary reaction loop of a circulating FBC (CFB) to achieve adequate sulphation, mainly because such particles are too small (d₅₀ = 30-40 μηι). This small size also means that fly ash does not have a core-shell structure, for effective reactivation with water or steam, so the main challenge in improving its sulphation characteristics is to increase its residence time in the combustor. However, prior art system and method have not been able to improve the performance of fly ash for sulphation, which accounts for the major ash waste formed in FBC.

In addition to a decay of sorbent activity, loss of CaO-based sorbents from FBC CaL cycle reactors due to attrition and elutriation also increases the demand for fresh sorbent makeup. Due to sintering, sorbent carrying capacity reduces, and due to attrition, produced fines are elutriated from FBC. Therefore, sorbent make-up is required, which may result in increased costs for C0₂ capturing and environmental problems because spent sorbent is a waste material and should be disposed.

Recent studies on pelletization of CaO-based sorbents showed that the mechanical strength of such materials can be enhanced by the addition of binders. It has been shown that pelletization can be employed for both reactivated spent sorbent and elutriated material. Therefore, pelletization can be considered as a process which decreases demand for fresh sorbent make up. This can be valuable when the binder contributes to enhanced sorbent C0₂ capture activity such as is the case with calcium aluminate cements. However, it requires that the sorbent be calcined and/or hydrated before pelletization. As such, pelletization increases the complexity of the reactivation of sorbent by requiring additional processing time and equipment for implementation.

Therefore, there remains a need for an improved system and method for reactivating deactivated CaO-based sorbents used in gas capturing.

Summary of the Invention

The present invention overcomes the shortcomings of the prior art by providing improvements to the sorbent reactivation system and method. The present invention provides sorbent reactivation system and method that enable substantially simultaneous reactivation and pelletization of deactivated sorbents used in absorbing a gas.

In accordance with one aspect of the present invention, there is provided a method of reactivating sorbents for absorbing a gas, said method comprising the steps of: (a) receiving a batch of ingredients into a container, said ingredients comprising at least one of sorbents deactivated during use in absorbing said gas; (b) spraying water droplets against said ingredients; and (c) moving said ingredients relative to said container; wherein said spraying and said moving cooperate to reactivate at least a portion of said ingredients by hydration and substantially simultaneously produce reactivated sorbents in pelletized form.

Preferably, said ingredients further comprise at least one of fresh sorbents.

Preferably, said ingredients comprise CaO-based sorbents for absorbing carbon dioxide.

Preferably, said ingredients comprise CaO-based sorbents for absorbing sulfur dioxide.

Preferably, wherein said ingredients comprise at least one of residues from reactors and CaO-based sorbents.

Preferably, said residues are fluidized bed reactor residues. Preferably, said residues are fixed bed reactor residues. Preferably, ingredients further comprise at least one additive.

Preferably, said at least one additive comprises at least one of CaO, calcium aluminate cement, bauxite, and aluminum hydroxide.

Preferably, the method further comprising the step of coating said reactivated pellets with at least one additive in said container thereby producing reactivated sorbent pellets in core- in-shell form.

In accordance with another aspect of the present invention, there is provided a system for reactivating sorbents for absorbing a gas, comprising: a container for receiving a batch of ingredients comprising at least one of sorbents deactivated during use in absorbing said gas; means for spraying water droplets against said ingredients; and an actuator for moving said ingredients relative to the system; wherein said spraying means and said actuator cooperate to reactivate at least a portion of said ingredients by hydration and substantially simultaneously produce reactivated sorbents in pelletized form. Preferably, the batch of ingredients further comprises at least one of fresh sorbents.

In accordance with another aspect of the present invention, there is provided a method of reusing fly ash, comprising the steps of: (a) receiving a mixture of fly ash and at least one additive in a container, said fly ash being originated from waste of a fluidized bed combustion for absorbing a gas, said at least one additive comprising CaO; (b) spraying water droplets against said mixture; and (c) moving said mixture relative to said container; wherein said spraying and said moving cooperate to hydrate at least a portion of said mixture and simultaneously granulate said mixture to produce pelletized sorbents for absorbing said gas.

Preferably, said process for absorbing a gas comprises a fluidized bed combustion process.

Other features and advantages of the present invention will become apparent from the following detailed description and the accompanying drawings, which illustrate, by way of example, the principles of the invention.

Brief Description of the Drawings

By way of examples only, preferred embodiments of the present invention are described hereinafter with reference to the accompanying drawings, wherein:

Figure 1 is a schematic view of an illustrative embodiment of a reactivation system in accordance with the present disclosure; Figure 2 illustrates SEM images of tested CaO-based sorbents: a) sectioned spent sorbent particle, b) reactivated/pelletized sorbent pellets, c) morphology of reactivated and calcined sorbent with 10 % cement, and d) morphology of reactivated sorbent (10% cement) after 30 carbonation/calcination cycles;

Figure 3a) illustrates pore surface area distribution of tested lime-based sorbents;

Figure 3b) illustrates pore volume distribution of tested lime-based sorbents; Figure 4 illustrates C0₂ capture performances of tested sorbents: a) C0₂ capture capacity during 30 cycles, and b) conversion profiles during the first 3 cycles. Conditions: carbonation in 50% C0₂ (N₂ balance) for 30 min, calcination in 100% N for 10 min, isothermally at 800 °C; Figure 5 illustrates results of attrition tests in bubbling bed reactor: a) reactivated sorbent with no cement, and b) reactivated sorbent with 10% calcium aluminate cement;

Figure 6 illustrates sulphation of calcined KR, BA and FA in TGA in synthetic flue gas (15% C0₂, 3% 0₂, 0.45% S0₂ and N₂ balance) at 850 °C;

Figure 7 illustrates TGA analysis of BA and FA (reactive gas used is 100% N₂);

Figure 8 illustrates sulphation of pellets in TGA in synthetic flue gas (15% C0₂, 3% 0₂, 0.45% S0₂ and N₂ balance) at 850 °C;

Figure 9a) illustrates pore surface area of original and pelletized samples; and Figure 9b) illustrates distribution of original and pelletized samples. Detailed Description of the Invention

The present invention will now be described with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown.

Referring to Figure 1, there is shown an illustrative embodiment of a system in accordance with the present disclosure, for reactivating sorbents for absorbing a gas. System 10 comprises a container 12 for receiving a batch of ingredients 14 comprising sorbents 16 deactivated (or spent) during use in absorbing the gas, spraying means 18 for spraying water droplets 20 against the ingredients 14, and an actuator 22 for moving the ingredients 14 relative to the system 10. System 10 is configurable so that the spraying means 18 and the actuator 22 cooperate to reactivate at least a portion of the ingredients 14 by hydration and substantially simultaneously produce reactivated sorbents in pelletized form. In operation, a batch of ingredients 14 comprising at least one of the deactivated sorbents and may include fresh sorbent is introduced into a container 12. The ingredients may include spent sorbent 16, i.e. sorbent that has been deactivated during use in capturing or absorbing a gas such as C0₂, S0₂ or any gas that is subject to capturing. In some example embodiments, the ingredients 14 comprise one or more types of fresh sorbents. In some example embodiments, the ingredients 14 comprise one or more types of deactivated sorbents 16, and may comprise additives (not explicitly shown) of any kind including binders and/or fresh sorbents such as fresh lime. The ingredients 14 may comprise deactivated sorbents 16 of any form, size, shape or configuration. For example, the deactivated sorbents 16 may be in the form of particles, pellets, powders, or any combination thereof. The deactivated sorbents 16 is CaO-based, but are not limited to any particular composition. To reactivate the spent sorbent 16, the system 10 is configurable to spray water droplets 20 against the ingredients 14, and move the ingredients 14 relative to the system 10 or a portion thereof. Preferably, the spray of water 20 is supplied by a spraying means 18 such as a nozzle. Preferably, the spraying means 18 is configurable and/or controllable with respect to its position, direction, and operating parameters that affect the amount of water, the velocity, size of water droplets and all aspects of the manner in which water 20 is supplied in relation to the ingredients 14. Water droplets 20 are applied in a manner that would allow at least a portion of the ingredients 14 or deactivated sorbents 16 to be reactivated by hydration. The hydration effectively causes some cracks in the ingredients 14 or sorbents 16 so that they become easily reducible into some particles or powders or into a form that is small relative to the original size of the sorbents. Preferably, the movement of the ingredients 14 relative to the system 10 or a portion thereof is provided using an actuating means or an actuator 22 of any suitable kind. In some example embodiments, the actuator 22 is configurable to be independently operable, and may comprise at least one movable member for facilitating movement of the ingredients 14. For example, the actuator 22 may comprise at least one of an agitator and a chopper, as shown in Figure 1. However, a person skilled in the art would appreciate that any other similar structures can be used in place of the devices 22 shown in Figure 1. In some embodiments, the actuator 22 may cause the ingredients 14 to move relative to the system 10 without directly contacting the ingredients 14. For example, the actuator 22 may comprise an air blower that directs a supply of airflow against the ingredients 14 to effect a movement thereof.

Preferably, each of the actuating means 22 and the spraying means 18 is independently operable. Preferably, the operation of the actuating means 22 is configurable and controllable to cooperate with the operation of the spraying means 18, and vice versa. Preferably, the cooperation between the actuating means 22 and the spraying means 18 is implemented automatically by way of a programmable logic unit (not shown), or manually by mechanical coordination conducted by an operator of the system 10.

Accordingly to some example embodiments of the present disclosure, the cooperation between the actuator 22 and the spraying means 18 is in such a manner that at least a portion of the ingredients 14 or the deactivated sorbents 16 are reactivated through hydration and substantially simultaneously formed into pellets. In particular, the present invention takes advantage of the fact that hydrated and reactivated sorbents are fragile or easily powdered, which in the prior art has been considered as a significant disadvantage of steam hydration process, and combines hydration and pelletization into one process. This means that with the system and method described herein, grinding of the reactivated sorbent is not required before pelletization. Furthermore, excessively large sorbent particles, which are unavoidable in prior art pelletization operation, can be powdered during the next run of the disclosed method to produce particles in the desirable size range so as to minimize oversized particles wasted. The use of particles from previous runs, therefore, reduces the amount of waste material and the process effectively can be operated continuously or semi-continuously.

Advantageously, the combination of water droplets 20 and the movement of the ingredients 14 provides improved hydration/reactivation of sorbents while simultaneously removing the need for a separate pelletization process as required by the prior art method. In some example embodiments, the spray of water 20 optimizes both the step of hydration/reactivation of sorbent and the step of pelletization of sorbent. The supply of water is controlled so that it is sufficient to reactivate sorbent into powders or otherwise achieve complete hydration, and that any further addition of water or variation in the manner of water supply facilitates granulation of the reactivated sorbent particles or powders into pellets.

In some example embodiments, the movement of ingredients 14 is controlled in a way so as to optimize both the step of hydration/reactivation of sorbents and the step of pelletization of the sorbents. The actuating means 22 is operated so that in the hydration step it facilitates reducing the size of the particles of the ingredients 14, while in the pelletization step it facilitates the formation of sorbent pellets. Still referring to Figure 1 , in some embodiments, the spray of water 20 and the movement of the ingredients 14 are provided such that at least a portion of the deactivated sorbents 16 are reactivated by hydration into powders and substantially simultaneously the powders are granulated to form reactivated sorbent pellets. In some embodiments, the spray of water 20 and the movement of the ingredients 14 are provided such that at least a portion of the deactivated sorbents 16 are reactivated by hydration into at least one of particles and powders, which are simultaneously granulated to form reactivated sorbent pellets. In some embodiments, the spray of water 20 and the movement of the ingredients 14 are controlled so as to produce reactivated sorbent pellets of a predetermined shape, size, and/or dryness. In some embodiments, the above characteristics of the pellets are controllable by way of adjusting at least one of the total amount of water supply, the size of the water droplets, the position, orientation of the actuator 22 and the direction, speed and manner of the ingredients movement.

Accordingly, in some example embodiments, the system and method disclosed herein combine granulation, hydration and pelletization of deactivated sorbents into one process that can be implemented in the same reactor or container.

In accordance with some example embodiments of the system and method disclosed herein, reactivated sorbent pellets with addition of calcium aluminate cements can be produced to enhance both pellet strength and gas sorption capacity of the sorbents. The use of spray water 20 enables production of pellets with a high resistance to attrition, which is a significant advantage when reactivated sorbent is used in FBC reactors. In addition, the system and method disclosed herein allow production of pellets with core- in-shell structure. The pellets can be produced using fresh sorbent or pure CaO-based material at the beginning of pelletization, and after that cement can be added to form a calcium aluminate shell. Apart from the enhanced strength of obtained core-in-shell pellets when compared with that of pellets in which cement is uniformly distributed throughout the volume, the unwanted sintering at the surface of sorbent particles is also reduced because the surface is enriched by A1₂0₃. The described method of simultaneous hydration by spray water and pelletization is also suitable for pelletization of ingredients 14 that comprise fresh sorbents.

In an example process of producing core-in-shell pellets, spent sorbent is first hydrated and granulated and the pellets obtained were then sieved, and particles with a size fraction of a predetermined range were returned to the pelletization container together with cement addition. In some embodiments, a cement-containing layer would be formed externally to the original pellets, i.e., around the reactivated sorbent core. To avoid damage to already formed "core" particles, the movement of the ingredients 14 can be reduced to a suitable level during the coating step.

After pelletization, the coated pellets were again sieved and dried in open air for several days before storage. It should be noted that for the preparation of the core-in-shell pellets, the cores obtained after the first stage were not dried, which results in stronger adhesion between core and shell. After each run the smaller and larger particles that fall outside of the predetermined size range were collected and returned to the pelletization container to be remade and pelletized with a new batch of spent sorbent during the next run. CaO from calcined spent sorbent adsorbs water from particles made in the previous run and releases heat during hydration, which caused powdering of particles from previous runs. This effect is also enhanced by the higher degree of movement of the ingredients 14.

The fact that the sorbent particles during hydration become fragile and easily powdered is employed here in two ways. First, spent sorbent is easily powdered during reactivation which means that grinding is not required before pelletization. Second, excessively large particles, which are unavoidable in prior art pelletization operation, can be powered during the next run to produce particles in the desirable size range so that no oversized particles are wasted. The use of particles from previous runs, therefore, reduces the amount of waste material and in effect the process becomes continuous or semi-continuous. Various tests have been carried out for the system and method described herein. The following experiments illustrate the principles of the present invention by way of examples and without loss of generality.

Experimental

Reactivation of Sorbents from CaL Cycles for CO₂ Capture

General A spent Cadomin limestone derived sorbent sample obtained from a pilot-scale fluidized bed (FBC) CaL cycle reactor is used for reactivation. The calcined spent sorbent is sprayed by water in a pelletization container. Initially, the material is hydrated forming fragile particles which are simultaneously powdered due to large amount of heat released during hydration and the mechanical work of the rotor blades attached to the reactivation/pelletization container, i.e. agitator and chopper. When a high level of hydration is achieved, further amounts of water induces granulation of hydrated sorbent. The amount of water and the speed of the rotor blades control the size of obtained pellets. The fragility of hydrated sorbent particles, which has been a shortcoming of prior art steam hydration methods, is employed here to enhance hydration by means of enhanced material powdering.

The reactivation method disclosed herein produces pellets ready to be used in FBC reactors. Moreover, this procedure enables the addition of calcium aluminate cement to additionally enhance sorbent strength. Cement can be mixed with the original material at the beginning of the process or added when pellets are already formed, which results in the formation of a core-in-shell pattern of reactivated sorbent.

The characterisation of reactivated material by nitrogen physisorption (BET, BJH) and scanning electron microscopy confirmed the enhanced morphology of sorbent particles for reaction with C0₂. Improved C0₂ carrying capacity was demonstrated in calcination/carbonation tests performed in a thermogravimetric analyzer (TGA). Finally, the resulting pellets showed a high resistance to attrition during fluidization in a bubbling bed.

Materials

Spent Cadomin limestone sample obtained after four calcination/carbonation cycles in pilot-plant dual fluidized bed (FBC) reactor was used in this study. The FBC facility is known in the art and its detailed description can be found in an article by Lu, D. Y.; Hughes, R. W.; Anthony, E. J., "Ca-based Sorbent Looping Combustion for C0₂ Capture in Pilot-Scale Dual Fluidized Beds", Fuel Process. Technol. 2008, 89, 1386-1395. The calcination occurred at 905-915 °C under oxy-fuel combustion conditions in the presence of 2000 ppm S0₂ and 15% steam present, while the carbonation process took place at 650 °C in 8% C0₂ and 15% steam balanced with air. The residue obtained after the fourth carbonations was collected and used for reactivation/pelletization tests reported here. The XRF elemental analysis of original limestone and that of the sorbent after four calcination/carbonation cycles is presented in Table 1. A commercial calcium aluminate cement, CA-14, (71% A1₂0₃ and 28% CaO), produced by Almatis Inc., was used as a binder for pelletization and was supplied as a very fine powder with >80% of the particles <45 μηι.

Reactivation/Pelletization

The reactivation tests with simultaneous pelletization were done with a mechanical pelletizer (Glatt GmbH) operated in batch mode. The calcined spent sorbent and calcium aluminate cement (total 300 g) was mixed in the reactivation/pelletization container (1 dm ). The schematic representation of the reactivation/pelletization container is illustrated in Figure 1. A certain amount of water was sprayed intermittently during hydration and pelletization with a nozzle, which can produce the micron-sized water droplets (<300 μηι) required for the reactivation and pelletization process. Moreover, when hydration started some water evaporated due to the high temperature produced, and a fog like atmosphere was formed from the spray water. The amount of spray water varied depending on the starting material used, but the water droplet size and the total amount of water required are the most important factors affecting the pellet size. The pellet size was also controlled by the speed of a pair of rotor blades attached to the container, i.e., one agitator (operated at 50 - 1000 rpm) located on the bottom and one chopper (operated at 100 - 3000 rpm) on the side. Typically one batch of pellets can be produced in 20-30 min.

The pellets were also prepared with no addition of cement. In addition, pellets with a core- in-shell structure were prepared in a two-step process. For this process, spent sorbent is first hydrated and granulated and the pellets obtained were then sieved, and particles with a size fraction 0.425-1.0 mm were returned to the pelletization container together with cement addition only. It is assumed that a cement-containing layer would be formed externally to the original pellets, i.e., around the reactivated sorbent core. To avoid damage to already formed "core" particles the rotation speed of the chopper was reduced to 300 rpm (agitator speed reduced to 100-200 rpm) during coating step.

After pelletization, the pellets were again sieved and dried in the open air for several days before storage. It should be noted that for the preparation of the core-in-shell pellets, the cores obtained after the first stage were not dried, resulting in stronger adhesion between core and shell. After each run the smaller (<0.425 mm) and larger (>1.4 mm) particles were collected and returned to the pelletization container to be remade and pelletized with new batch of spent sorbent during the next run.

CaO from calcined spent sorbent adsorbs water from particles made in the previous run and releases heat during hydration, which caused powdering of particles from previous runs. This effect is also enhanced by the high speed of the chopper. The fact that the sorbent particles during hydration become fragile and easily powdered is employed here in two ways. First, spent sorbent is easily powdered during reactivation which means that grinding is not required before pelletization. Second, excessively large particles, which are unavoidable in real pelletization operation, can be powered during the next run to produce particles in the desirable size range so that no oversized particles are wasted. The use of particles from previous runs, therefore, reduces the amount of waste material and effectively the process becomes semi-continuous.

Sorbent Characterization

CO2 carrying capacities of pellets were determined by a Perkin Elmer TGA-7 apparatus using ~30 mg samples suspended in a quartz tube (i.d. 20 mm) on a platinum pan (i.d. 5 mm). The gas flow rate was 0.04 dm /min and the temperature and gas used were controlled by Pyris software. Data on sample mass during the experiments were monitored and conversions were calculated on the basis of mass change, assuming that mass change occurs only due to formation/decomposition of CaC0₃. The experiments were done isothermally at 800 °C, with carbonation in 50% C0₂ (N₂ balance) for 30 min, and calcination in 100% N₂ for 10 min.

The attrition tests were performed in a bubbling fluidized bed (BFB), with an internal diameter 50 mm. The pellets (0.425-1.4 mm) were pre-calcined in a muffle furnace in air for 2 h at 850 °C and their particle size distribution was determined by sieving before the attrition tests. The pre-calcined sample was weighed (-80-100 g in calcined form) before loading to the reactor. The minimum fluidization velocity, U_mf, was determined experimentally for each batch of sample by pressure drop (ΔΡ) vs. superficial gas velocity (U) correlations. The sample was heated in 100% N₂ from room temperature to 850 °C at a heating rate of 30 °C/min prior to each experiment. Minimum fluidization velocities at 850 °C of pre-calcined pellet samples were found to be in the range of U_mf = 0.16-0.17 m/s. Three carbonation/calcination cycles were conducted alternating temperature and gas composition: 15% C0₂ (N₂ balance) at 700 °C for 30 min at a superficial gas velocity of 1.0 m/s for carbonation, and 100% N₂ at 850 °C for 15 min at a superficial gas velocity of 0.35 m/s during calcination. When the attrition test was finished, the bed inventory was removed from the bed by vacuum with a 10 mm internal diameter stainless steel tube. The collected bed material was then weighed and the particle size distribution was determined again.

The original limestone, spent sorbent and reactivated pellets were all submitted to N₂ adsorption/desorption analyses of pore surface area. Pore surface areas (BET) and pore size and pore volume distributions (BJH) were determined using a Micromeritics TriStar II 3020 VI .02 N₂ sorption analyzer. The sample morphologies were observed with a Hitachi S3400 Scanning Electron Microscope (SEM) with 20 kV of accelerating voltage under high vacuum. The samples were coated with gold/palladium before SEM examination and images obtained by secondary electrons are presented here. Reactivation of FBC spent Sorbents for SO₂ Capture

General Reactivation and pelletization of the spent sorbent were achieved simultaneously in a mechanical pelletizer with the addition of spray water. Four types of pellets were prepared with various proportions of bed ash and fly ash. Quick lime (CaO) powders were also tested as a useful additive for the pelletization process. The effectiveness of the reactivation technique was tested by nitrogen physisorption which confirmed that a more suitable pore surface area and pore volume distribution for sulphation were developed. The S0₂ capture capability of the pellets was also examined in a thermogravimetric analyzer (TGA). The reactivated pelletized sorbents showed an improved sulphation rate in comparison to both the original sorbent and to the spent sorbent, particularly during the diffusion-controlled reaction stage.

Materials

The spent sorbent (partially sulphated bed ash (BA) and fly ash (FA)) was from a 165 MWe circulating FBC boiler at Point Aconi, Nova Scotia, Canada. The "as-received" fly ash is in the form of very fine powders (<40 μιη), while the original bed ash had been previously sieved to eliminate oversized particles (>1 mm). Kelly Rock calcitic limestone (KR) is the commercial sorbent used for S0₂ retention in this boiler. Therefore, the original KR limestone (300-425 μηι) was also used in the sulphation tests to compare its S0₂ capture activity with that of reactivated sorbents. Commercial quick lime powder (<30 μηι), purchased from Graymont Limestone Inc., was selected as an optional additive for the pelletization process. Reactivation and Pelletization

The simultaneous reactivation and pelletization tests were done with a mechanical pelletizer (Glatt GmbH) operated in batch mode. Bed ash and fly ash were calcined at 850 °C in air for 2 h before pelletization. This pre-treatment is required because both CaC0₃ and Ca(OH)₂ were formed in these samples during storage. In the case of the fly ash sample, the pre-calcination in air also eliminates char carbon, which is normally found in substantially larger amounts in fly ash than in bed ash. However, in practical application, such calcination will not be required because fresh material coming directly from the FBC is already calcined and does not contain significant Ca(OH)₂/CaC0₃ content (providing it has not been stored and exposed to the atmosphere for extended periods). Therefore, char carbon becomes a constituent of the pellets, which provides the additional benefit of giving it a second chance for combustion when the pellets are fed into the FBC. This may also assist in pellet sulphation because combustion of the char carbon will provide additional porosity for S0₂ diffusion and accommodation of the bulky CaS0₄ product.

Four different pellet types were produced and their composition is given in the table below.

Components in pellets (wt. %)

These pellets are classified into two types: one set of materials with CaO addition, and another set without CaO addition. Approximately 300 g of calcined spent sorbent and CaO powder (when used), was mixed in the reactivation/pelletization container (1 dm ) in the desired proportions. The schematic representation of the reactivation/pelletization container is shown in Figure 1. A given amount of water was sprayed intermittently during hydration and pelletization with a nozzle, which can produce the micron-sized water droplets (<300 μηι) required for the reactivation/pelletization process. Moreover, when hydration started some water evaporated due to the high temperature produced, and a fog like atmosphere was formed from the spray water. The amount of spray water varied depending on the starting material used, water droplet size and the total amount of water required are important factors affecting the pellet size. The pellet size was also controlled by the speed of a pair of rotor blades attached to the container, i.e., one agitator (operated at 50 - 500 rpm) located on the bottom and one chopper (operated at 100 - 3000 rpm) on the side. Typically a batch of pellets can be produced in 20-30 min in this lab-scale equipment.

After pelletization, the collected pellets were sieved and air-dried before storage. Given its particular suitability for fluidized bed systems, the size fraction of 0.425-1.4 mm was used in the subsequent sulphation tests.

During pellet manufacture, after each run the smaller (<0.425 mm) and larger (>1.4 mm) particles were collected and returned to the pelletization container to be re-pelletized with fresh sample (in designated proportion) in the next run. It was previously noted that fresh CaO absorbed considerable amounts of water and released considerable heat during hydration (more water was necessarily sprayed as well), which caused the "fragmentation" of any excessively large particles from the previous run into a fine powder. Furthermore, this effect was also enhanced by the high speed of the chopper under these conditions. The use of particles from previous runs to markedly minimize the amount of waste material (too small and too large particles) and effectively made the process semi-continuous and, by reducing the amount of waste materials, would improve the economics of any commercial process based on this technique. Pore surface area and pore volume distribution for original ashes and reactivated and pelletized samples were obtained from nitrogen adsorption/desorption isotherm data using a Micromeritics ASAP 3000 apparatus for nitrogen physisorption at 77 K. Before these analyses, samples were calcined in air at 850 °C for 2 h and degassed at 150 °C under vacuum for 12 h.

Sulphation tests

A Perkin-Elmer TGA-7 thermogravimetric analyzer was used for the sulphation experiments. The sample (~30 mg) was suspended in a quartz tube (i.d. 20 mm) on a platinum pan (i.d. 5 mm). The temperature and gas used were controlled by Pyris software. The temperature program included heating to 850 °C for calcination of the samples with a heating rate of 45 °C/min and then holding the temperature at that level for sulphation. The program started with the introduction of N₂, which was replaced with synthetic flue gas (15% C0₂, 3% 0₂, 0.45% S0₂ and N₂ balance) 10 min after the temperature reached 850 °C. Sulphation was performed isothermally for 240 min. The gas flow rate during runs was controlled with a flowmeter at 40 cm /min. Data on sample mass during the experiments were monitored and conversions were calculated on the basis of mass change, assuming that mass change occurred only due to formation of CaS0₄.

Results and Discussions

The sorbent used for reactivation was obtained under realistic CaL cycle conditions including the presence of steam and S0₂ during cycles, resulted in 13.6% sulphation conversion (Table 1).

Original limestone Spent sorbent

Si0₂ 1.03 wt% 1.40 wt%

A1₂0₃ 0.33 wt% 0.34 wt%

Fe₂0₃ 0.09 wt% 0.33 wt%

P2O5 <0.03 wt% <0.03 wt%

CaO 54.91 wt% 70.65 wt%

MgO 0.20 wt% 1.21 wt%

so₃ <0.10 wt% 13.71 wt%

K₂0 0.06 wt% 0.15 wt%

Na₂0 <0.20 wt% O.20 wt%

Loss on Fusion 43.09 wt% 12.10 wt%

Sum 99.93 wt% 99.96 wt%

Table 1. Elemental composition of Cadomin limestone and Ca looping cycle residue after the fourth carbonation

Moreover, it can be seen in Figure 2a that a sulphate shell is formed at the sorbent particle surface which together with sorbent sintering hinders carbonation. Therefore, to prolong sorbent cycle life a reactivation step is required, and a hydration technique appears as the most appropriate method. However, as noted, hydration typically causes particle swelling resulting in large cracks which lead to destruction of the particles on further handling, something especially pronounced in a FBC system, in effect, an Achilles heel of any hydration reactivation technique. However, with the pelletization approach employed in the present invention, such resulting sorbent fragility becomes a benefit and any remaining need to reduce the particles size is achieved by means of the mechanical work of two blades in hydration/pelletization container (Figure 1). Furthermore, the hydration process, and release of heat causes evaporation which means that hydration readily occurs in a mixture of spray water and steam. During reactivation sorbent particles quickly becomes a very fine powder which easily achieve complete hydration and any further addition of water results in granulation of powder, and relatively spherical pellet particles are obtained which is presented in Figure 2b. Therefore, the method disclosed herein practically involves three processes in the same reactor: granulation, hydration, and pelletization of CaO based sorbent.

However, the purpose and main goal of proposed technique is reactivation of spent sorbent from CaL cycles. The morphology of obtained reactivated pellets after calcination is shown in Figure 2c. It can be seen that morphology in terms of porosity is significantly enhanced when compare it with that of the original spent sorbent presented in Figure 2a. In this case the sulphate shell is destroyed during reactivation and smaller grains and a more developed porous structure is formed. Due to presence of calcium aluminates the sorbent texture remains relatively stable during carbonation/calcination cycles conducted in TGA (Figure 2d). The results of nitrogen physisorption tests presented in Table 2 show that BET pore surface area of the spent sorbent was only 0.73 m7g.

Table 2. BET pore surface area and BJH cumulative pore volume of calcined limestone, spent sorbent, and reactivated pellets. However, after reactivation pore surface area is significantly higher (3.96-4.23 m²/g) and in range of that for fresh limestone after calcination (4.19 m²/g). Previous studies on hydration by Manovic, V.; Anthony, E. J., "Steam Reactivation of Spent CaO-Based Sorbent for Multiple C0₂ Capture Cycles", Environ. Sci. Technol. 2007, 41, 1420-1425, showed that BET surface area of reactivated sorbent can be even higher than it is for the original sorbent. However, in this case sorbent before hydration was already significantly sulphated (13.6%) which also resulted in the lower pore surface area of reactivated sorbent. The addition of calcium aluminate cement slightly increased pore surface area due to presence of aluminum compounds.

Pore surface area distribution presented in Figure 3a shows that spent sorbent is deficient in small pores which are mainly responsible for "maximum" carbonation conversion. It is interesting that the spent sorbent after reactivation showed similar pore surface area distribution to that for the original limestone. In both cases, the peak for pore surface area distribution occurs at ~5 nm pore diameter, consistent with the data for pore volume distribution. Two peaks are present. The first peak corresponds to ~5 nm pores and the second one is slightly below 100 nm. The observed enhancement of pore surface area and pore volume of pores smaller than 150 nm is crucial for carbonation conversion. Thus, an article by Fennell et al. "The Effects of Repeated Cycles of Calcination and Carbonation on a Variety of Different Limestones, as Measured in a Hot Fluidized Bed of Sand", Energy Fuels 2007, 21, 2072-2081 found a correlation between C0₂ capture capacity and available volume for accommodation of formed product (CaC0₃) in pores of size 150 nm. Similarly, Alvarez and Abanades explained in "Determination of the Critical Product Layer Thickness in the Reaction of CaO with C0₂", Ind. Eng. Chem. Res. 2005, 44, 5608- 5615 their maximum carbonation conversions during C0₂ capture cycles by the formation of a critical product layer (49 nm), in other words, the enhancement of sorbent texture should benefit in a higher C0₂ capture activity. The C0₂ capture activity of spent sorbent and that seen after reactivation, as well as conversion of original limestone in a series of 30 calcination/carbonation cycles in the TGA is presented in Figure 4a. As can be seen, the activity of spent sorbent is poor; with only about 1 1 g C0₂ is captured by 100 g sorbent, which corresponds to only 20% conversion of available CaO (as CaO from CaS0₄ is unavailable for carbonation). However, the C0₂ uptake is almost three times higher after reactivation (~30 g/lOOg), which demonstrates the beneficial effect of reactivation. While it should be noted that C0₂ uptake is still lower than that for original sorbent, due to the fact that spent/reactivated sorbent is partially sulphated. Finally, a small beneficial effect of cement addition on conversions after 30 cycles can be seen, which is attributed here to mayenite (Ca^Al^C^) formation, which was also noted and discussed in earlier studies on calcium aluminate pellets. However, the effect of cement addition is less pronounced in this study due to sulphates present in the sorbent.

Figure 4b shows that the beneficial effect of reactivation can be even more important when sorbents are used under more realistic conditions in FBC reactors. Namely, the conversion profiles show that only few percents of C0₂ capture is achieved for spent sorbent during the first few minutes which is an expected residence time of sorbent particles in the CaL FBC systems. In this case practically there is no fast initial reaction stage, i.e. the conversion progresses slowly from the beginning, and after a relatively long reaction time (30 min) the C0₂ uptake is only 11 g/100 g. By contrast, a fast initial reaction stage is clearly present after reactivation. However, reactivated sorbent also shows different conversion profiles when compared with that of the original sorbent. The shift between kinetically and diffusion controlled stage is not sharp as it is in the case of natural limestone. It can be seen that carbonation of reactivated pellets continues noticeably even during diffusion controlled stage. These differences should be also taken into account when simulating the C0₂ capture activity of a modified sorbent, since these "modified" sorbents can be considered synthetic sorbents, given that they have experienced a realistic calcination/carbonation cycles in an atmosphere containing steam and S0₂. This means that semi-empirical formulas/models developed for natural sorbent cycled under atmospheres containing only N₂ and C0₂, which correlates "maximum" conversions during capture cycles, are almost certainly not appropriate for modified sorbents or sorbents cycled under more realistic conditions.

It is also well established that the first calcination and a few first calcination/carbonation cycles in FBC reactors are associated with the most significant attrition, which may result in the loss of 40% of sorbent due to elutriation from FBC reactors. The causes for attrition are related mechanical and thermal stresses, and attrition extend greatly depends on sorbent type and sphericity the particles. The pelletization procedure applied here results in highly spherical particles (Figure 2b) which naturally resistant attrition. Moreover, given that such "agglomerated particles" already experience some "attrition" during circulation in pelletization container any material or structures which are less mechanically strong, are reshaped during pelletization. The superior resistance to attrition of obtained pellets is also confirmed by the tests in BFB, as shown in Figure 5. After three carbonation/calcination cycles performed only a few percent of small particle size fractions are formed. Moreover, as expected, sorbent with addition of cement performed better. Therefore, reactivation pelletization can be done even with no addition of cement especially if the produced material would be used in the processes which require shorter residence time in an FBC reactor. For example, if reactivated sorbent is used in a guard bed to remove S0₂ before CaL cycles. Otherwise, if reactivated material is to be used in longer series of C0 capture cycles, the addition of cement is desirable due to both mechanical strength and reduced sintering. In the case of multiple reactivations during a series of CaL cycles, the addition of cement is required only for the first reactivation/pelletization, and after that, such pellets can be reactivated and reshaped with no additional cement added, which is an important advantage.

These results show that the method disclosed herein for simultaneous reactivation and pelletization has certain advantages over the existing art. Deactivated or extremely un- reactive sorbents can be reactivated, and pellets suitable for FBC applications can be obtained. Furthermore, reactivated material can be used for additional C0₂ capture cycles with a reasonable activity even if that had been partially sulphated before reactivation. The attrition resistance and C0₂ capture activity is improved with cement addition. The method disclosed herein therefore encompasses several advantages when compared with prior art steam reactivation methods. Namely, in the present invention, spray water is used, which is less energy intensive than to produce steam. Moreover, obtained pellets are ready to be used directly in FBC reactors, which is not the case with steam hydrated materials which require further processing before its utilization in FBC reactors.

For the reactivation of spent, partially sulphated, FBC sorbent for additional S0₂ capture. In the present invention, simultaneous hydration and pelletization were integrated in one step. It was worth noting that for tests with bed ash, it was easy to produce pellets without addition of quick lime. However, the same procedure was not applicable to the fly ash, because it contains significant amounts of silicates and aluminosilicates (12.23% Si0₂, 3.67% A1₂0₃, Table 3), and every attempt to pelletize it without addition of quick lime resulted in the production of a mud-like material remaining in the pelletization container. While reactivation and pelletization with no addition of fresh lime is a less expensive scenario, this addition eliminates difficulties in pelletization of the fly ash used here. Nonetheless, given that the feasibility of pelletization depends on the particular composition and performance of the material to be pelletized, the present invention has shown that such difficulties can be successfully dealt with by the addition of fresh lime, or by mixing bed ash and fly ash directly.

MgO 0.47 0.55 0.78

S0₃ 0.30 31.44 21.78

K₂0 0.21 0.18 0.79

Na₂0 <0.20 <0.20 <0.20

Loss on Fusion 42.05 9.03 10.80

Sum 99.69 100.00 99.91

Tab e 3. Major elements composition of KR limestone and FBC residues (wt. %)

Chemical analysis indicated that KR limestone is a typical calcitic limestone with 93% CaC0₃ (Table 3) and limited MgO content.

The sulphation pattern of the calcined KR sample is shown in Figure 6. In the first 45 min sulphation was controlled by chemical reaction and then followed by a diffusion-controlled stage, which was characterized by a relatively abrupt transition from a rapid to a very slow conversion rate. As expected, sulphation of natural sorbent is substantially incomplete since the reaction product (CaS0₄) is formed and further reaction is, therefore, impeded by this outer shell formation which hinders contact of reactants. A 4-h sulphation of calcined KR shows only 24% CaO conversion.

This conversion was lower than that calculated from the BA (43%) and FA (32%) analysis results presented in Table 3, however, it is typical for this limestone in previous TGA sulphation tests. Higher conversions achieved in the samples from full-scale FBC operation are presumably due to the differences between the operation of a large commercial boiler which includes longer residence times and vigorous fluidization conditions.

Figure 6 also shows sulphation patterns for the FA and BA samples. To allow a better comparison, sulphation results hereafter are presented in terms of mg S0₂ absorbed per g of sample (in calcined form), which allows one to compare the sulphation performance directly between KR, spent sorbent samples and their reactivated and pelletized forms, to which as noted above additional fresh CaO was introduced in some cases. Both BA and FA performed better than limestone, with a higher overall S0₂ absorption capability, and an extended, kinetic-controlled, fast reaction stage from 45-50 min for KR to 90-100 min for BA and FA. FA shows the best S0₂ capture ability (375 mg S0₂/g sorbent) partly because the FA had a relatively lower original sulphation level compared to the BA. It was also found that the original BA and FA samples were partially hydrated and carbonated during storage, as shown in the TGA curves in Figure 7, which undoubtedly increased their reactivity with S0₂. The conversion of CaO to Ca(OH)₂ and CaC0₃ due to reactions between ash samples and atmospheric moisture/C0₂ was 59.9% for BA and 52.7% for FA. In addition, the TGA curve reveals that there is a significant weight loss for the FA sample after the temperature reaches 850 °C due to the fact that CaS0₄ decomposed to a major extent with char carbon (normally existing in a significant amount in FA) as a reducing agent in the N₂ atmosphere.

The results of 4-hour sulphation of the reactivated and pelletized sorbents can be seen in Figure 8. When compared with the sulphation curves for the original sorbent, there are significant qualitative differences between sulphation of the original (Figure 6) and reactivated/pelletized samples (Figure 8). Sulphation curves for BA and FA have a typical shape as does that for the limestone sample, as shown in Figure 6, which is characterized by an obvious inflection point indicating sulphation is limited by diffusion, with the sulphation rate becoming very slow thereafter.

The shapes of sulphation curves for the reactivated/pelletized samples (Figure 8) show that only initial sulphation (15-20 min) is similar to that of the original samples. After the initial period, although the reaction rate was modestly reduced for the reactivated/pelletized samples when rate control shifted to diffusion-controlled mode, the shift to the slower stage is not as abrupt as that for the original samples. Furthermore, the sulphation rate is not reduced sharply in the diffusion-controlled stage. Among the four pellets produced and tested, BA33FA33Ca33 demonstrated the best S0₂ absorption capability with 325 mg S0₂/g after 190-min sulphation. Because the sulphation conversion continued to increase at a significant rate, it is reasonable to estimate the final value of 4-h sulphation as potentially reaching 370 mg S0₂/g, equivalent to the result of FA. The other two samples, BA100 and FA50Ca50, showed lower S0₂ capture capacity of 260 and 290 mg S0₂/g, which are still better than the result of KR (240 mg/g). Sample BA50Ca50 had the lowest sulphation capability among the four pellets, of 185 mg/g, while it also maintained a relatively faster reaction rate than the original samples at the end of the 4-h sulphation (see Figure 6).

The specific pore surface area and pore volume distribution of sorbents were determined by the BET and BJH methods using N₂ adsorption/desorption isotherm data. The results are presented in Table 4 and Figure 9.

Table 4. N₂ adsorption/desorption pore analysis results (calcined samples)

For the original BA and FA samples (after calcination), both pore surface area and pore volume are very low with relatively larger pores, as shown in Table 4. One of the most important observations is that the pore surface area and pore volume were increased 2-4 times after reactivation/pelletization, while the pore diameter was moderately reduced. BA50Ca50 shows the smallest pore diameter and the highest pore surface area, resulting in its low sulphation capacity since the pores were prone to be filled or blocked by the bulky sulphation product (CaS0₄). The pore surface area displays a unimodal distribution, as shown in Figure 9a. The main contribution to pore surface area is from small pores (2-3 nm) for reactivated/pelletized samples, and the contribution decreases quickly with increasing pore diameter.

As indicated in Figure 9b, pore volume distribution for the parent samples shows one single peak at ~90 nm, while pore volume distribution for reactivated/pelletized samples shows a bimodal pattern with one smaller peak showing at 2-3 nm, which corresponds to the pore surface area distribution. The other peak, at ~90 nm, is much more noticeable, indicating pore volume increases more for larger pores. When compared with the parent samples, pore volume after reactivation/pelletization is much higher for larger pores. Sulphation is primarily limited by pore filling with sulphation product that leads to pore closure or blockage; therefore, increase of pore volume is favourable for enhancement of the performance of the sorbent. The improved sulphation activity for reactivated/pelletized samples is clearly demonstrated by the resulting TGA profiles.

These experiments have demonstrated a new method of reactivation of FBC spent sorbent for S0₂ capture. A pelletized sorbent, originating from FBC spent sorbent, was produced with a mechanical pelletizer. Reactivation and pelletization of the spent sorbent were achieved simultaneously in the pelletizer with addition of spray water. BA, FA (with addition of fresh CaO) and their mixtures can be pelletized in this process. The pore surface area and pore volume increased significantly after the process, which is favourable for sulphation. The sulphation tests in a TGA showed that the sulphation rate of the four types of pellet sorbents produced was enhanced in the diffusion-controlled stage, when compared to the original spent sorbent and to the parent limestone. Among them, BA33FA33Ca33 with 33.3% BA, 33.3% FA and 33.3% CaO demonstrated an excellent S0₂ capture capacity of 325 mg S0₂/g after 190-min sulphation, with continued S0₂ capture and potential for further capture capacity available. This result is closely comparable to the S0₂ absorption capability of the original FA sample, which is potentially the most reactive spent sorbent in the FBC system. However, FA cannot be directly reused for S0₂ capture in FBCs because of its short residence time, which is important given that fly ash represents the bulk of the waste solids from a FBC boiler. Therefore, the integration of reactivation and pelletization to produce pellet sorbents provides added benefits of reuse of the previously unusable FA, in addition to the reactivation of BA in the same process.

Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments and modifications are possible. Therefore, the scope of the appended claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

Claims

WE CLAIM:

1. A method of reactivating sorbents for absorbing a gas, said method comprising the steps of:

(a) receiving a batch of ingredients into a container, said ingredients comprising at least one of sorbents deactivated during use in absorbing said gas;

(b) spraying water droplets against said ingredients; and

(c) moving said ingredients relative to said container;

wherein said spraying and said moving cooperate to reactivate at least a portion of said ingredients by hydration and substantially simultaneously produce reactivated sorbents in pelletized form.

2. The method of claim 1 , wherein said ingredients further comprise at least one of fresh sorbents.

3. The method of claim 1, wherein said ingredients comprise CaO-based sorbents for absorbing carbon dioxide.

4. The method of claim 1 , wherein said ingredients comprise CaO-based sorbents for absorbing sulfur dioxide.

5. The method of claim 1 , wherein said ingredients comprise at least one of residues from reactors and CaO-based sorbents.

6. The method of claim 5, wherein said residues are fluidized bed reactor residues.

7. The method of claim 5, wherein said residues are fixed bed reactor residues.

8. The method of any one of claims 1 to 7, wherein said ingredients further comprise at least one additive.

9. The method of claim 8, wherein said at least one additive comprises at least one of CaO, calcium aluminate cement, bauxite, and aluminum hydroxide.

10. The method of any one of claims 1 to 9, further comprising the step of coating said reactivated pellets with at least one additive in said container thereby producing reactivated sorbent pellets in core-in-shell form.

1 1. A system for reactivating sorbents for absorbing a gas, comprising:

a container for receiving a batch of ingredients comprising at least one of sorbents deactivated during use in absorbing said gas;

means for spraying water droplets against said ingredients; and

an actuator for moving said ingredients relative to the system;

wherein said spraying means and said actuator cooperate to reactivate at least a portion of said ingredients by hydration and substantially simultaneously produce reactivated sorbents in pelletized form.

12. A system according to claim 1 1, wherein the batch of ingredients further comprises at least one of fresh sorbents.

13. A method of reusing fly ash, comprising the steps of:

(a) receiving a mixture of fly ash and at least one additive in a container, said fly ash being originated from waste of a process for absorbing a gas, said at least one additive comprising CaO;

(b) spraying water droplets against said mixture; and

(c) moving said mixture relative to said container; wherein said spraying and said moving cooperate to hydrate at least a portion of said mixture and simultaneously granulate said mixture to produce pelletized sorbents for absorbing said gas.

The method of claim 13, wherein said process for absorbing a gas comprises a fluidized bed combustion process.