WO1997006889A1

WO1997006889A1 - A method for reactivating sorbent to enable the reuse thereof

Info

Publication number: WO1997006889A1
Application number: PCT/IB1996/000930
Authority: WO
Inventors: John H. Chiu
Original assignee: Abb Research Ltd.
Priority date: 1995-08-21
Filing date: 1996-07-29
Publication date: 1997-02-27
Also published as: PL325057A1; CA2230019C; KR100312360B1; CN1199353A; TW397706B; FI980394A0; IN188644B; CA2230019A1; KR19990044099A; FI980394A; CZ51098A3; SE9800396D0; SE9800396L; CN1104946C

Abstract

A method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO that is embodied in spent ash, which is generated as a consequence of the combustion in a fossil fuel-fired combustor (14) of fossil fuel in the presence of sorbent containing Ca. The sorbent containing unreacted CaO is subjected to steam having a predetermined temperature and a predetermined partial pressure operative to effect the conversion of the unreacted CaO to Ca(OH)2 whereby a fracturing of the sorbent occurs such that the Ca(OH)2 becomes exposed. Thereafter, the sorbent now containing Ca(OH)2 is reinjected into the fossil fuel-fired combustor (14) at a temperature within the fossil fuel-fired combustor (14) operative to effect the reconversion of the Ca(OH)2 to CaO.

Description

A METHOD FOR REACTIVATING SORBENT TO ENABLE THE REUSE THEREOF

BACKGROUND OF THE INVENTION This invention relates to SO₂ emission reduction and, more specifically, to a method for reactivating for purposes of the reuse thereof the sorbent, which is injected into a fossil fuel-fired combustor in order to effectuate the capture therewith of the SO₂ that is released during the combustion of the fossil fuel within the fossil fuel-fired combustor. It has long been known in the prior art to employ a sorbent embodying CaO to effect therewith capture of the S0₂ that is released during the combustion of the fossil fuel within fossil fuel-fired combustors. Moreover, such sorbent has proven to be particularly effective in capturing the S0₂ that is released within a circulating fluidized bed combustor during the combustion therewithin of fossil fuels that are solid in nature. However, notwithstanding the fact that sorbent has proven to be particularly effective when utilized to accomplish the capture of SO₂ in circulating fluidized bed combustors, it has nevertheless been found that sorbent consumption and the need to effect the disposal of the ash, in which sorbent is contained, that is produced as a consequence of the combustion, which occurs within the circulating fluidized bed combustor, represent major operating costs to the operator of the circulating fluidized bed combustor. Furthermore, it is expected that these costs will continue to escalate as environmental constraints become even more stringent in the future. Thus, in summary the increased calcium-to-sulfur mole ratio necessary to meet very high levels of sulfur removal will seriously impact the cost effectiveness of circulating fluidized bed technology unless the consumption of sorbent can be reduced. in accordance with the process employed in circulating fluidized bed technology, fossil fuel, which may take any one of a number of different forms, e.g., coal, etc., is burned in the combustor of a circulating fluidized bed system in the presence of a sorbent embodying CaO, such sorbent being injected into the combustor in order that while the fossil fuel is being burned in the combustor such sorbent undergoes calcination and sulfation reactions. To this end, such sorbent, while the fossil fuel is being burned in the combustor, is operative to effect the capture of S0₂. In doing so though, not all of the CaO in the sorbent is fully utilized for sulfur capture. This is attributable to the fact that in capturing the sulfur the CaO in the sorbent combines with the sulfur to form CaSO₄. As such, because the specific volume of the CaSO₄ is greater than the specific volume of the CaO, the pore structure of the sorbent becomes plugged with CaSO₄. That is, the surface of the CaO essentially becomes covered with an outer layer of CaS0₄, which functions to effectively shield the unreacted free CaO in the interior of the particles of sorbent from further reaction with SO₂, i.e., inhibits further capture of sulfur by the sorbent.

It has been known heretofore to reinject ash into the combustor of the circulating fluidized bed system in an effort to effect the reuse of the sorbent contained in the reinjected ash. This reinjection of ash containing sorbent eventually reaches a point of diminishing retums. Namely, the particles of sorbent eventually become sulfated and the internal pores of the sulfur become blocked by CaS0₄. Accordingly, further sulfur capture by the sorbent can only occur if the CaO in the interior of the sorbent particles becomes exposed. Attempts, to which reference will be had more fully hereinafter, have been made to effect such exposure of the CaO.

Thus, by way of summary, a substantial effort has been put forth heretodate in the prior art in an attempt to develop new methods for reducing sorbent consumption, diminishing solids waste disposal and improving solids waste utilization. In this regard, by way of exemplification and not limitation, reference is had to U.S. Patent No. 5,342,594 entitled "Fluidized Bed Process For SO_x Removal," which issued on August 30, 1994. As taught in U.S. Patent No. 5,342,594, hot, unclean gases having gaseous sulphur oxide-containing impurities are mixed within a mixing chamber with a solid process material comprising CaCO₃ such that the gaseous sulphur oxide-containing impurities react with the CaC0₃ to form a solid impurity reaction product thereof. Thereafter, the solid process material and the solid impurity reaction product are separated from the unclean gases to provide purified gases. Whereupon, the separated solid impurity reaction product and solid process material are retumed to the mixing chamber to be once again mixed with hot, unclean gases.

Also, further in this regard, reference is had, by way of exemplification and not limitation, to U.S. Patent No. 5,345,883 entitled "Reactivation Of Sorbent In A Fluid Bed Boiler," which issued on

September 13, 1994. In accordance with the teachings of U.S. Patent No. 5,345,883, a jet of fracturing medium of liquid water or steam is injected at a sufficiently high pressure and in being so injected is directed so as to impinge upon sorbent particles containing unreacted sorbent material inside, whereby these sorbent particles are mechanically fractured to expose the unreacted sorbent contained therewithin. To this end, as taught in U.S. Patent No. 5,345,883, the fracturing medium when being so injected is at a temperature lower than the temperature of the sorbent particles that the fracturing medium is made to impinge upon such that the fracturing of the sorbent particles is the result of thermal shock. Or, as taught in U.S. Patent No. 5,345,883, the fracturing medium when being so injected is directed at the sorbent particles such that the sorbent particles are caused to be mechanically broken apart by striking a target surface or other particles.

In addition, reference is had further in this regard, by way of exemplification and not limitation, to U.S. Patent No. 5,344,632 entitled "Method For Reducing Sulfur Oxides Emissions In A Combustion Process," which issued on September 6, 1994. As taught in U.S. Patent No. 5,344,632, the mixture of flue gases and entrained fine particles, a portion of which are particles of limestone, which are both unsulfated and have undergone chemical conversion to calcined limestone, are made to enter a humidifying reactor at a reduced temperature, wherein the reduced temperature results from heat having been previously extracted from the mixture. Thereafter, water is dispersed in the form of a plurality of fine water particles that evaporate and humidify the mixture of flue gases and entrained fine particles, which in combination with the reduced temperature of the mixture, is said to be highly conducive to the formation of a thin film of alkali solution of calcium hydroxide on the surface of the particles of limestone. This alkali solution is then effective in absorbing sulfur oxides present in the mixture of flue gases and entrained fine particles to form calcium sulfate and calcium sulfide precipitation.

Lastly, reference is had in this regard, by way of exemplification and not limitation, to U.S. Patent No. 5,341 ,753 entitled "Circulating Fluidized Bed Power Plant With Improved Mixing Of Sorbents With Combustion Gases," which issued on August 30, 1994. In accordance with the teachings of U.S. Patent No. 5,341 ,743, recognition is had therein of the fact that the calcium sulfur ratio required for a desired amount of sulfur removal is a function of how much excess particle density in the gas stream is required in order to ensure that a sufficient number of sulfur dioxide molecules come in contact with the calcium dioxide particles. Thus, it is said to be desirable to improve the contact between the calcium and the sulfur dioxide particles. To this end, high velocity steam is injected into the circulating fluidized bed boiler to improve the mixing therewithin of the recirculating solids with the combustion gases whereby the circulating fluidized bed boiler for burning sulfur containing fuels is allegedly made to utilize limestone more efficiently. From the teachings of the aforementioned U.S. patents, it is thus apparent that the utilization of spent ash from a circulating fluidized bed system can be enhanced by hydration or even simple size reduction followed by the reinjection of the spent ash into the combustor of the circulating fluidized bed system in order to thereby achieve additional sulfation of the reinjected spent ash. Moreover, it is apparent from the teachings of the foregoing U.S. patents that it is possible to realize even additional sulfation through the proper selection of the location whereat the spent ash is reinjected into the circulating fluidized bed system. In addition to those which form the subject matter of the aforementioned U.S. patents, there are also known to exist in the prior art other ash activation/injection processes. Considered collectively, all of these ash activation/injection processes, for ease of reference, may be categorized as follows: direct spent ash reinjection with no treatment of the spent ash; mechanical grinding of the spent ash in order to expose unreacted CaO prior to the spent ash being reinjected; humidification of the flue gases without any spent ash recycle; injection of the sorbent into flue gases without any spent ash recycle; reinjection in dry form of hydrated spent ash; reinjection in moist form of hydrated spent ash; and reinjection in slurry form of hydrated spent ash. Although generally speaking these ash activation/injection processes have been demonstrated to be operative for their intended purpose, there has nevertheless been evidenced in the prior art a need for such ash activation/injection processes to be further improved. Namely, there has been evidenced in the prior art a need for a new and improved method for enhancing the capture of the SO₂ that is released during the combustion of fossil fuel within a fossil fuel-fired combustor, and, in particuiar, a new and improved method for reactivating for purposes of the reuse thereof the sorbent, which is injected into the combustor of a circulating fluidized bed system in order to effectuate the capture therewith of the S0₂ that is released during the combustion of fossil fuel within the combustor of the circulating fluidized bed system.

To this end, there has been evidenced in the prior art a need for such a new and improved method for reactivating sorbent for purposes of the reuse thereof that is characterized in a number of respects. One such characteristic is that such a new and improved method for reactivating sorbent for purposes of the reuse thereof would enable a substantial reduction to be realized in the amount of limestone that would otherwise be required to be utilized to achieve the same amount of SO₂ capture, i.e., enables a lower Ca/S ratio to be utilized than that which has heretofore been required to achieve the same amount of S0₂ capture. Another such characteristic is that such a new and improved method for reactivating sorbent for purposes of the reuse thereof would enable a substantial reduction to be realized in the amount of spent ash that otherwise would be required to be disposed of. A third such characteristic is that such a new and improved method for reactivating sorbent for purposes of the reuse thereof would enable a substantial reduction to be realized in the amount of Ca present in the spent ash that eventually must be disposed of thereby enabling the spent ash to be more readily disposed of. A fourth such characteristic is that such a new and improved method for reactivating sorbent for purposes of the reuse thereof would be capable of implementation without requiring any process modifications to be made in the combustion process by which fossil fuel is burned in fossil fuel-fired combustors and from whence is released the SO₂, the capture of which is effected with sorbent. A fifth such characteristic is that such a new and improved method for reactivating sorbent for purposes of the reuse thereof would be capable of implementation without requiring any significant equipment modifications to be made in the fossil fuel-fired combustors in which the fossil fuel is burned and from whence is released the S0₂, the capture of which is effected with sorbent. A sixth such characteristic is that such a new and improved method for reactivating sorbent for purposes of the reuse thereof would be capable of utilization with virtually any form of fossil fuel-fired combustor in which fossil fuel is burned and from whence is released the S0₂, the capture of which is effected with sorbent. A seventh such characteristic is that such a new and improved method for reactivating sorbent for purposes of the reuse thereof would be suitable for application in new fossil fuel-fired combustors. An eighth such characteristic is that such a new and improved method for reactivating sorbent for purposes of the reuse thereof would be suitable to be retrofitted for application in existing fossil fuel-fired combustors.

It is, therefore, an object of the present invention to provide a new and improved method for reactivating for purposes of the reuse thereof the sorbent containing unreacted CaO, which is injected into a fossil fuel-fired combustor in order to effectuate the capture therewith of the SO₂ that is released during the combustion of the fossil fuel within the fossil fuel-fired combustor. It is another object of the present invention to provide a new and improved method that is particularly suited to being utilized for reactivating for purposes of the reuse thereof the sorbent containing unreacted CaO, which is injected into the combustor of a circulating fluidized bed system in order to effectuate the capture therewith of the SO₂ that is released during the combustion of fossil fuel within the combustor of the circulating fluidized bed system.

It is still another object of the present invention to provide a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized in that through the use thereof a substantial reduction is enabled to be realized in the amount of sorbent that would otherwise be required to be utilized to achieve the same amount of SO₂ capture.

Another object of the present invention is to provide such a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized in that through the use thereof a substantial reduction is enabled to be realized in the amount of spent ash that otherwise would be required to be disposed of.

A still another object of the present invention is to provide such a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized in that through the use thereof a substantial reduction is enabled to be realized in the amount of Ca present in the spent ash that eventually must be disposed of thereby enabling the spent ash to be more readily disposed of.

A further object of the present invention is to provide such a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized in that the implementation thereof does not require any process modifications to be made in the combustion process by which fossil fuel is burned in fossil fuel-fired combustors and from whence is released the SO₂, the capture of which is effected with sorbent.

A still further object of the present invention is to provide such a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized in that the implementation thereof does not require any significant equipment modifications to be made in the fossil fuel-fired combustor in which the fossil fuel is burned and from whence is released the S0₂, the capture of which is effected with sorbent. Yet an object of the present invention is to provide such a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized in that the utilization thereof may be with virtually any form of fossil fuel-fired combustor in which fossil fuel is burned and from whence is released the SO₂, the capture of which is effected with sorbent. 5 Yet a further object of the present invention is to provide such a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized by its suitability for application in new fossil fuel-fired combustors.

Yet another object of the present invention is to provide such io a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized by its suitability to be retrofitted for application in existing fossil fuel-fired combustors.

15 SUMMARY OF THE PRESENT INVENTION

In accordance with the present invention there is provided a method for reactivating for purposes of the reuse thereof the sorbent containing unreacted CaO, which is injected into a fossil fuel-fired combustor in order to effectuate the capture therewith of the SO₂ that is

20 released during the combustion of the fossil fuel within the fossil fuel-fired combustor. More specifically, in accord with the subject method of the present invention for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, such reactivation of the sorbent containing unreacted CaO is effected through steam reactivation. To this end, spent

O S ash, which is derived from the combustion of fossil fuel in a fossil fuel-fired combustor and which has present therein particles of sorbent containing unreacted CaO, is subjected to steam, which is at a predetermined temperature and a predetermined partial pressure. As a consequence of this subjection of the spent ash to steam, the unreacted CaO contained in the sorbent particles present in the spent ash is converted to Ca(OH)₂ by virtue of the reaction of the CaO contained in the sorbent particles with the steam. Moreover, because the volume of Ca(OH)₂ is greater than the volume of CaO, this increase in volume results in a fracturing of the sorbent particles, which had previously contained unreacted CaO but which now contain Ca(OH)₂, such that by virtue of the fracturing of the sorbent particles the Ca(OH)₂ thereof becomes exposed. After having been so exposed to steam, in accord with the subject method of the instant invention the spent ash now having present therein sorbent particles containing Ca(OH)₂ rather than unreacted CaO is then reinjected into the fossil fuel-fired combustor whence the spent ash was derived. To this end, the spent ash now having present therein sorbent particles containing Ca(OH)₂ is reinjected into the fossil fuel-fired combustor at a location thereof whereat the temperature is sufficiently high, i.e., in excess of 580 degrees C, so as to cause the Ca(OH)₂ contained in the sorbent particles present in the reinjected spent ash to covert once again to CaO such that by virtue of this reactivation thereof the sorbent present in the reinjected spent ash is rendered once again operative to effectuate the capture therewith of SO₂ that is released during the combustion of fossil fuel within the fossil fuel-fired combustor.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a side elevational view of one form of fossil fuel- fired combustor, i.e., a circulating fluidized bed system, with which the method of the present invention may be employed;

Figure 2 is a graphical depiction of the disassociation pressure of Ca(OH)₂ as a function of temperature based on thermodynamic considerations; and

Figure 3 is a graphical depiction of the conversion of hydrated fly ash with SO₂. DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, and more particularly to Figure 1 thereof, there is depicted therein a form of fossil fuel-fired system generator with which the method of the present invention is capable of being utilized. More specifically, in accord with the best mode embodiment of the invention the fossil fuel-fired combustor illustrated in Figure 1 of the drawing comprises a circulating fluidized bed system, generally denoted in Figure 1 by the reference numeral 10. Fossil fuel, which is most frequently in the form of coal, along with sorbent, which is most frequently in the form of limestone, are fed, as indicated by the arrow denoted by the reference numeral 12 in Figure 1 , to the combustor, denoted generally in Figure 1 by the reference numeral 14, of the circulating fluidized bed system 10. As indicated by the arrow denoted by the reference numeral 16 in Figure 1 , primary fluidizing air, which has been preheated, is fed in known fashion to the air plenum chamber, denoted in Figure 1 by the reference numeral 18, which is located in the bottom of the combustor 14 such as to be positioned below the air distribution plate, denoted in Figure 1 by the reference numeral 20.

Continuing with the description of the circulating fluidized bed system 10 depicted in Figure 1 of the drawing, combustion supporting air, as indicated by the arrow denoted by the reference numerals 22 and 24, is fed into the combustor 14. Ash, which is generated as a consequence of the combustion in the combustor 14 of the fossil fuel introduced at 16 thereinto is removed from the combustor 14 through the pipe, denoted in Figure 1 by the reference numeral 26, and through the ash cooler, denoted in Figure 1 by the reference numeral 28.

The bottom portion, which comprises the primary combustion zone, of the combustor 14 in accordance with conventional practice is normally refractory lined in order to thereby eliminate therefrom high heat losses. On the other hand, in accordance with conventional practice the upper portion of the combustor 14 normally contains evaporative waterwall tubes in which steam is generated.

The flue gas generated as a consequence of the combustion within the combustor 14 of the fossil fuel introduced at 16 thereinto along with any solids, which may be entrained in the flue gas, are made to flow from the combustor 14 through the duct, denoted in Figure 1 by the reference numeral 30, to the cyclone separator, denoted in Figure 1 by the reference numeral 32. In the cyclone separator 32, the entrained solids are separated from the flue gas. After being separated in the cyclone separator 32 from the flue gas, the solids drop to the bottom of the cyclone separator 32 whereas the flue gas now minus the previously entrained solids flows out of the top, the latter being denoted in Figure 1 of the drawing by the reference numeral 34, of the cyclone separator 32. From the top 34 of the cyclone separator 32, the flue gas now minus the previously entrained solids then flows through the tangential duct, denoted in Figure 1 by the reference numeral 36, to the convective pass, denoted in Figure 1 by the reference numeral 38, of the circulating fluidized bed system 10, wherein in accordance with conventional practice heat exchange surfaces are typically to be found. Referring again to Figure 1 of the drawing, the circulating fluidized bed system 10 as illustrated therein is further provided on the bottom of the cyclone separator 32 with a J-leg or seal pot, denoted generally by the reference numeral 40 in Figure 1. The function of the J- leg or seal pot 40 is to effect the recirculation of the solids, which are collected in the cyclone separator 32 after the separation thereof from the flue gas, back to the combustor 14 against the pressure present in the combustor 14. Namely, these solids flow down on the inlet side, i.e., on the side, denoted in Figure 1 by the reference numeral 40a, of the J-leg or seal pot 40, up the outlet side, i.e., the side, denoted in Figure 1 by the reference numeral 40b, of the J-leg or seal pot 40, and then back to the combustor 14 through the duct, denoted in Figure 1 by the reference numeral 42. In accordance with conventional practice, the bottom portion, denoted in Figure 1 by the reference numeral 40c, of the J-leg or seal pot 40 is normally fluidized, by virtue of the injection thereinto of air, to permit the solids in the J-leg or seal pot 40 to flow therethrough. As is known to those skilled in this art, the difference in solids level between the inlet side 40a thereof and the outlet side 40b thereof corresponds to the pressure differential across the J-leg or seal pot 40. To this end, solids entering the inlet side 40a of the J-leg or seal pot 40 displace the solids flowing into the duct 42 out of the outlet side 40b of the J-leg or seal pot 40. Continuing, the circulating fluidized bed system 10, in accordance with the illustration thereof in Figure 1 of the drawing, further embodies a solids withdrawal pipe, denoted in Figure 1 by the reference numeral 44, and a solids flow control valve, represented by the arrow identified in Figure 1 by the reference numeral 46. As seen with reference to Figure 1 of the drawing, the solids withdrawal pipe 44 and the solids flow control valve 46 are suitably supported in the lower portion 40c of the J-leg or seal pot 40. The function of the solids withdrawal pipe 44 is to feed the desired portion, as established through operation of the solids flow control valve 46, of the hot recirculating solids from the J-leg or seal pot 40 to the external fluidized bed heat exchanger, denoted in Figure 1 by the reference numeral 48. In known fashion, the external fluidized bed heat exchanger 48 typically consists of one or more compartments, schematically depicted at 48a in Figure 1, with most compartments 48a containing immersed tube bundles (not shown in the interest of maintaining clarity of illustration in the drawing), which are designed to be operative as evaporative and/or reheat and/or superheat and/or economized heat transfer surface. However, some of the compartments 48a may not be provided with immersed tube bundles. The solids, which enter the external fluidized bed heat exchanger 48, are fluidized for purposes of effecting the flow thereof therethrough. During the course of their passage through the external fluidized bed heat exchanger 48 the soiids, as they gradually pass from one to another of the compartments 48a, transfer heat to the heat transfer surfaces housed therewithin. Thereafter, the solids flow out of the extemal fluidized bed heat exchanger 48 through the outlet pipe, denoted in Figure 1 by the reference numeral 50, and back to the combustor 14.

The solids, which are circulating around the circulating fluidized bed system 10, i.e., through the combustor 14, the cyclone separator 32 and the extemal fluidized bed heat exchanger 48, consist of a mixture of unreactive fossil fuel ash and particles of sorbent, which are only partially reacted. To this end, the core of these particles of sorbent comprises unreacted CaO, whereas the shell or outer layer of these particles of sorbent consist of CaSO₄.

A discussion will next be had herein of S0₂ capture by sorbent in the combustor 14 of the circulating fluidized bed system 10. In this regard, overall SO₂ capture by sorbent consisting of limestone, which is added to a circulating fluidized bed system, such as the circulating fluidized bed system 10 illustrated in Figure 1 of the drawing, is traditionally described by two consecutive reactions; namely, those of endothermic calcination and exothermic sulfation. Exothermic sulfation is a net reaction and is not to be interpreted as describing reaction mechanisms on a molecular level. The calcination reaction, which is significant above 700 degrees C, creates very porous CaO particles. As the sulfation reaction proceeds, the pore structure of CaO particles changes, since the molar volume of CaSO₄ is larger than that of CaO and this tends to block the pores near the outer surface of the particle, preventing further diffusion of SO₂ into the interior of the sorbent particles. Although there are cases of quite uniform sulfation across the particles found in small particles of fly ash from circulating fluidized bed systems, most of this type of heterogeneous reaction is commonly modeled with an unreacted-core model, which makes allowance for the following: diffusion of gaseous SO₂ to the surface of the solid particle, penetration and diffusion of SO₂ through the CaSO₄ layer to the surface of the unreacted core, and reaction of SO₂ with sorbent within the core.

Low calcium utilization, typically only 25 to 45%, leads to a relatively large quantity of waste products for disposal. Thus, if the overall utilization of fresh sorbent can be increased, a significant improvement of the desulfurization process, in terms of both economics and pollution control, could be accomplished. As noted herein previously, among the methods that have been found to have the potential to increase the utilization of sorbent significantly is hydration of spent sorbent/ash by steam reactivation at low temperature combined with recirculation of the reactivated sorbent/ash back to the combustor. Enhanced utilization of sorbent by such hydration is attributable to the difference in molar volume of CaO and Ca(OH)₂. To this end, when spent sorbent/ash is exposed to steam, the strong affinity between water and unreacted CaO leads to absorption of water in the pores of the particles so that CaO is converted to Ca(OH)₂.

In Figure 2 of the drawing the partial pressure of water in equilibrium with a mixture of CaO and Ca(OH)₂ is depicted by means of the curve denoted therein by the reference numeral 52. Thus, the curve 52 effectively represents the minimum partial pressure of water vapor in an environment to effect rehydration of CaO as a function of temperature. The curve 52 was computed using free energies of formation and heat capacities of the pure solids and gases. From a reference to Figure 2 of the drawing, it can be seen that hydration is possible at high temperatures, i.e., at temperatures within the range of 250 degrees C. to 700 degrees C. and high H₂O partial pressures up to 1.0 and beyond. Moreover, it can be seen from a reference to Figure 2 of the drawing that Ca(OH)₂ is stable below 450 degrees C. with a volume percent moisture of 60%. As such, given the smaller size of water vapor molecules compared with SO₂, penetration of H₂O to previously unutilized CaO sites is therefore feasible. Furthermore, under appropriate conditions in-situ rehydration of CaO to Ca(OH)₂ can be had with a corresponding volume change and concomitantly exposure of unreacted CaO surface area, which would then allow further SO₂ capture when the sorbent particles that have been subjected to such in-situ rehydration of CaO are reinjected into a circulating fluidized bed system, e.g., into the combustor 14 of the circulating fluidized bed system 10 illustrated in Figure 1 of the drawing.

Heretofore, the effect of temperature on hydration of partially sulfated limestone has been the subject of several studies. Such studies have found that the conversion of CaO to Ca(OH)₂ decreased when the temperature was increased from 100 degrees C. to 300 degrees C. at a constant moisture content. Further, it was observed in such studies that the rate of hydration increased as the temperature was decreased. Moreover, it was suggested that this was due to the mechanism by which steam reaches active CaO sites and proposed that the diffusional resistance toward steam is partially caused by the increase in molar volume resulting from the formation of Ca(OH)₂. To this end, it could be that the decrease in both reaction rate and ultimate conversion is caused by pore plugging, similar to that occurring during sulfation, with an increasing temperature causing the pores of the sorbent particles to plug faster.

With further reference to the aforementioned studies, it has been found therefrom that the rate of hydration is much slower with steam than with water. Such studies assumed that this was caused by the transport mechanism of water through the sorbent particles rather than because of pore plugging. To this end, it was suggested in such studies that water molecules are quickly absorbed because of the surface tension of water, which draws water into the pores of the sorbent particles, where the water reacts with the CaO.

All the work done in the course of such studies has been carried out at temperatures below 300 degrees C. In these studies, there does not appear to have been any systematic consideration given to reactivation at temperatures above 300 degrees C, perhaps partially because decomposition of Ca(OH)₂ becomes significant at 300 degrees C, and thermodynamic as well as kinetic factors become important. In particular, a minimum partial pressure of steam, above the decomposition pressure of Ca(OH)₂, is needed to effect reactivation.

Reference will next be had herein to Figure 3 of the drawing wherein a series of curves, denoted therein by the reference numerals 54, 56 and 58, respectively, are depicted. The trends, which are shown by the curves 54, 56 and 58, can be explained by a combination of thermodynamic and kinetic factors. Thermodynamic factors enter into this discussion in two ways. First, there is the determination of whether at a given steam partial pressure reactivation can occur at a specified temperature in accord with curve 52 of Figure 2 of the drawing. Secondly, thermodynamic factors have an influence on the reaction rate. It appears that the effect of temperature and steam partial pressure as shown by the curves 54, 56 and 58 in Figure 3 of the drawing is explainable where the rate of reactivation is limited by diffusion of steam through pores in a surface layer of spent ash. To this end, the rate of diffusion is proportional to two temperature dependent factors, i.e., the diffusion coefficient divided by the temperature, and the driving force. Furthermore, the driving force is equal to the difference in steam partial pressure in the bulk gas and in the center of the sorbent particle (the decomposition pressure). As regards the diffusion coefficient, the diffusion coefficient increases with temperature to a power varying between approximately 1.5 and 0.5 depending upon whether the diffusion is bulk diffusion or Knudsen diffusion. However, for a given partial pressure of steam, the driving force for diffusion decreases with temperature because of the increase in the steam partial pressure in the center of the sorbent particle. The result of these two counteracting trends is a maximum in the reactivation rate, provided that the diffusion coefficient varies with temperature to a power greater than unity. The overall trend, as illustrated by the curves 54, 56 and 58 in Figure 3 of the drawing, shows that the reaction rate increases with temperature to a maximum, because the diffusion coefficient increases. Moreover, at a temperature higher than this maximum point the reaction rate fails off as the driving force falls, reaching zero when the bulk diffusion is equal to the decomposition pressure.

The spent ash, which is to be subjected to steam reactivation in accordance with the method of the present invention may be obtained from several locations in the circulating fluidized bed system 10. Namely, the spent ash, which is to be subjected to steam reactivation in accordance with the present invention may be obtained from the ash cooler 28, or from the J-leg or seal pot 40, or from the extemal fluidized bed heat exchanger 48. In addition, it is also contemplated in accordance with the method of the present invention that steam reactivation of the spent ash may take place in-situ. That is, steam may be employed to perform a dual function, i.e., to effect the fluidization of the solids being recirculated from the cyclone separator 32 to the combustor 14 while at the same time this same steam is operative to effect the steam reactivation in-situ of the recirculated solids. To this end, as depicted by the arrows denoted by the reference numeral 60 in Figure 1 of the drawing, the steam employed for this purpose may be introduced into the outlet pipe 50 of the circulating fluidized bed system 10. Other than when the steam reactivation of the spent ash takes place in-situ, the spent ash after being subjected to steam reactivation in accordance with the present invention preferably is reintroduced into the combustor 14 at 12 along with the fossil fuel that is to be burned therewithin.

Thus, in accordance with the present invention there has been provided a new and improved method for reactivating for purposes of the reuse thereof the sorbent containing unreacted CaO, which is injected into a fossil fuel-fired combustor in order to effectuate the capture therewith of the S0₂ that is released during the combustion of the fossil fuel within the fossil fuel-fired combustor. Besides, there has been provided in accord with the present invention a new and improved that is particularly suited to being utilized for reactivating for purposes of the reuse thereof the sorbent containing unreacted CaO, which is injected into the combustor of a circulating fluidized bed system in order to effectuate the capture therewith of the SO₂ that is released during the combustion of fossil fuel within the combustor of the circulating fluidized bed system. As well, in accordance with the present invention there has been provided a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized in that through the use thereof a substantial reduction is enabled to be realized in the amount of sorbent that would otherwise be required to be utilized to achieve the same amount of SO₂ capture. Moreover, there has been provided in accord with the present invention a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized in that through the use thereof a substantial reduction is enabled to be realized in the amount of spent ash that otherwise would be required to be disposed of. Also, in accordance with the present invention there has been provided a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized in that through the use thereof a substantial reduction is enabled to be realized in the amount of Ca present in the spent ash that eventually must be disposed of thereby enabling the spent ash to be more readily disposed of. Further, there has been provided in accord with the present invention a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized in that the implementation thereof does not require any process modifications to be made in the combustion process by which fossil fuel is burned in fossil fuel-fired combustors and from whence is released the SO₂, the capture of which is effected with sorbent. In addition, in accordance with the present invention there has been provided a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized in that the implementation thereof does not require any significant equipment modifications to be made in the fossil fuel-fired combustor in which the fossil fuel is burned and from whence is released the SO₂, the capture of which is effected with sorbent. Furthermore, there has been provided in accord with the present invention a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized in that the utilization thereof may be with virtually any form of fossil fuel-fired combustor in which fossil fuel is burned and from whence is released the SO₂, the capture of which is effected with sorbent. Penultimately, in accordance with the present invention there has been provided a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized by its suitability for application in new fossil fuel-fired combustors. Finally, there has been provided in accord with the present invention a new and improved method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO, which is characterized by its suitability to be retrofitted for application in existing fossil fuel-fired combustors.

Claims

What is claimed is:

1. In a fossil fuel-fired combustor wherein sorbent containing Ca is injected for purposes of effecting the capture therewith of SO₂ released during the combustion of fossil fuel in the fossil fuel-fired combustor, the improvement of a method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO comprising the steps of: a. providing a supply of spent ash embodying sorbent containing unreacted CaO, the spent ash having been generated as a consequence of the combustion in the fossil fuel-fired combustor of the fossil fuel in the presence of the sorbent containing Ca; b. subjecting the supply of spent ash embodying sorbent containing unreacted CaO to steam having a predetermined temperature and a predetermined partial pressure operative to effect the conversion to Ca(OH)₂ of the unreacted CaO contained in the sorbent embodied in the spent ash such that as a consequence of Ca(OH)₂ having a greater volume than CaO a fracturing of the sorbent occurs thereby exposing the Ca(OH)₂; c. reinjecting into the fossil fuel-fired combustor the spent ash embodying the sorbent containing the Ca(OH)₂ at a temperature within the fossil fuel-fired combustor operative to effect the conversion to CaO of the Ca(OH)₂ contained in the sorbent embodied in the reinjected spent ash; and d. effecting with the sorbent containing the reconverted CaO the capture of SO₂ released during the combustion of fossil fuel in the fossil fuel-fired combustor.

2. In a fossil fuel-fired combustor, the method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 1 wherein the supply of spent ash embodying sorbent containing unreacted CaO is subjected to steam having a temperature in the range of 250 degrees C. to 700 degrees C.

3. In a fossil fuel-fired combustor, the method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 2 wherein the supply of spent ash embodying sorbent containing unreacted CaO is subjected to steam having a partial pressure in the range of 0.0 to 1.2.

4. In a fossil fuel-fired combustor, the method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 1 wherein the spent ash embodying the sorbent containing the Ca(OH)₂ is reinjected into the fossil fuel-fired combustor at a temperature within the fossil fuel-fired combustor in excess of 580 degrees C.

5. In a fossil fuel-fired combustor, the method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 1 wherein the supply of spent ash embodying sorbent containing unreacted CaO is subjected to steam externally of the fossil fuel-fired combustor.

6. In a fossil fuel-fired combustor, the method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 1 wherein the supply of spent ash embodying sorbent containing unreacted CaO is subjected to steam in-situ.

7. In a circulating fluidized bed system including a combustor, a cyclone separator and a heat exchanger wherein sorbent containing Ca is injected into the combustor of the circulating fluidized bed system for purposes of effecting the capture therewith of S0₂ released during the combustion of fossil fuel in the combustor of the circulating fluidized bed system, the improvement of a method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO comprising the steps of: a. providing a supply of spent ash embodying sorbent containing unreacted CaO, the spent ash having been generated as a consequence of the combustion in the combustor of the circulating fluidized bed system of fossil fuel in the presence of sorbent containing Ca; b. subjecting the supply of spent ash embodying sorbent containing unreacted CaO to steam having a predetermined temperature and a predetermined partial pressure operative to effect the conversion to Ca(OH)₂ of the unreacted CaO contained in the sorbent embodied in the spent ash such that as a consequence of Ca(OH)₂ having a greater volume than CaO a fracturing of the sorbent occurs thereby exposing the Ca(OH)₂; c. reinjecting into the combustor of the circulating fluidized bed system the spent ash embodying the sorbent containing the

Ca(OH)₂ at a temperature within the combustor of the circulating fluidized bed system operative to effect the conversion to CaO of the Ca(OH)₂ contained in the sorbent embodied in the spent ash; and d. effecting with the sorbent containing the reconverted CaO the capture of SO₂ released during the combustion of fossil fuel in the combustor of the circulating fluidized bed system.

8. In a circulating fluidized bed system, the method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 7 wherein the supply of spent ash embodying sorbent containing unreacted CaO is subjected to steam having a temperature in the range of 250 degrees C. to 700 degrees C.

9. In a circulating fluidized bed system, the method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 8 wherein the supply of spent ash embodying sorbent containing unreacted CaO is subjected to steam having a partial pressure in the range of 0.0 to 1.2.

10. In a circulating fluidized bed system, the method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 7 wherein the spent ash embodying the sorbent containing the Ca(OH)₂ is reinjected into the combustor of the circulating fluidized bed system at a temperature within the combustor of the circulating fluidized bed system in excess of 580 degrees C.

11. In a circulating fluidized bed system, the method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 7 wherein the supply of spent ash embodying sorbent containing unreacted CaO is obtained from the combustor of the circulating fluidized bed system.

12. In a circulating fluidized bed system, the method for reactivating for puφoses of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 7 wherein the supply of spent ash embodying sorbent containing unreacted CaO is obtained from the cyclone separator of the circulating fluidized bed system.

13. In a circulating fluidized bed system, the method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 7 wherein the supply of spent ash embodying sorbent containing unreacted CaO is obtained from the heat exchanger of the circulating fluidized bed system.

14. In a circulating fluidized bed system, the method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 7 wherein the supply of spent ash embodying sorbent containing unreacted CaO is subjected to steam externally of the circulating fluidized bed system.

15. In a circulating fluidized bed system, the method for reactivating for purposes of the reuse thereof sorbent containing unreacted CaO as set forth in Claim 7 wherein the supply of spent ash embodying sorbent containing unreacted CaO is subjected to steam in-situ.