WO1994007591A1

WO1994007591A1 - Process of producing calcium hydroxide for absorption

Info

Publication number: WO1994007591A1
Application number: PCT/US1992/006279
Authority: WO
Inventors: Preston L. Veltman; Karsten Felsvang
Original assignee: Niro A/S
Priority date: 1992-09-25
Filing date: 1992-09-25
Publication date: 1994-04-14

Abstract

In a preferred process for treating a first gas medium containing an acidic material, the process comprising comminuting calcium oxide in a second flowing gaseous medium containing water to form particulates of calcium hydrate sorbent suspended in the second flowing gaseous medium. The particulates in the second gaseous medium are conveyed to a spray dryer absorber and the particulates are contacted in the spray dryer absorber with the first gaseous medium in admixture with an aqueous material for a time sufficient to allow at least some of the acidic material to react with said particulates to form solid products. At least some of the solid products are separated from the first gaseous medium.

Description

PROCESS OF PRODUCING CALCIUM HYDROXIDE FOR ABSORPTION

The present invention relates generally to processes for removing undesirable components such as sulfur dioxide from a flue gas by sorbents, and more particularly, to processes wherein a flue gas is treated in a spray dryer absorber for absorption of a component or components by a nascent, finely divided particulate sorbent such as calcium hydroxide created and utilized while suspended in a gaseous medium in admixture with an atomized aqueous material, preferably a slurry containing recycled calcium hydroxide.

While the subject invention will be discussed primarily hereinafter with reference to removal of sulfur containing materials such as sulfur dioxide from a flue gas, it is to be understood that the use and the application of the subject invention is not thereby so limited. For example, the invention may be useful in the removal of other undesirable components from flue gases or for the removal of components in other industrial or like processes where a gas or gases are to be treated to remove and/or recover components contained in the gases.

In the operation of power stations, incineration plants, heating stations and the like, hot waste gases such as flue gases are produced in significant quantities. It has been the general practice to remove potentially harmful and undesirable components such as sulfur oxides, hydrogen chloride and nitrogen oxides from such waste gases so as to avoid environmental damage which may occur when the waste gases are released directly into the atmosphere.

A number of processes are known in the art to remove the potential harmful components of waste gases as noted above. For example, processes are well known in which sulfur dioxide and other acidic gases are fixed in a dry powder such as that which results when a sulfur dioxide containing flue gas is contacted with an atomized solution of a strong base or a suspension of calcium hydroxide in a spray dryer type absorber. Such a process is described in U.S. Patent No. 4,279,873 to Felsvang and other patents issued since that date. Notably, calcium hydroxide has evolved as the sorbent of choice as spray dryer technology has been developed over the years since calcium hydroxide is relatively inexpensive and readily available. In addition, the reaction product of calcium hydroxide and sulfur compounds such as sulfur dioxide which contains sulfites and sulfates is in the form of a powder which is relatively easy to handle and discard.

When calcium oxide reacts with water, the reaction proceeds on a molecule to molecule basis. A great deal of heat is produced during the reaction and if the particles of calcium oxide are small and the mixing reactants efficient, the reaction proceeds with almost explosive force. Initially individual molecules of calcium hydroxide are produced. However, the individual molecules are highly reactive and tend to quickly associate one with another to form larger particles of lesser reactivity. Classical studies by W. Whitman and G. Davis "The Hydration of Lime", I.E. Chem 18, 118 (Feb 1926) demonstrated that the smallest hydrated lime particles are produced by rapid hydration. Thus, hydrate particles produced rapidly and used immediately are given little opportunity to agglomerate.

In flue gas desulfurization processes using calcium hydroxide as the sorbent, it is known that in the process, one molecule of calcium hydroxide solvent reacts with one molecule of sulfur oxide. Thus, in order for a process to operate economically, the calcium hydroxide and sulfur dioxide reactants must be able to physically contact one another. Solid sorbent particles of calcium hydroxide must be molecularly porous so that gaseous and acidic materials such as sulfur dioxide which are in solution can achieve physical contact by diffusion. In addition, the particles must be relatively small so as to have the extensive gross surface area necessary for a timely reaction to occur. Reaction kinetics are facilitated by the presence of absorbed water. Further, it is known as was indicated above that existing small particles of calcium hydroxide tend to agglomerate with time forming larger particles. Those molecules located within a calcium hydroxide agglomerate are not readily available for reaction with sulfur oxides.

The economic attractiveness of a particular process in the flue gas desulfurization marketplace depends critically on the ability of the process to capture a required amount of sulfur oxides from a given flue gas stream with the minimum amount of sorbent, capital invention and operating costs. The particular calcium to sulfur stoiche ical mol ration obtainable in desulfurization processes is generally a direct measure of sorbent utilization economics. That is, the closer the ratio is to unity and thus ideal conditions, the more economical the process will be. Thus, for the most part, process improvements are directed towards providing sorbents capable of obtaining near 100% utilization in minimum amount of time.

In flue gas desulfurization processes using a spray dryer absorber, the temperature of the flue gases entering the spray dryer absorber is generally a fixed value determined primarily by nature of the source of flue gas such as furnace design and by economics. As a consequence, the amount of water that can be evaporated in a given flue gas desulfurization process is a fixed value. In turn, the amount of water that can be vaporized thereby limits the amount of water than can be used to form a pumpable slurry as feed to the atomizer in a spray dryer absorber process. Thus, for any given spray dryer absorber process, there is a maximum amount of solids that can be contained in the atomized slurry.

SUMMARY OF THE INVENTION

It is therefore a feature of this invention is to provide a spray dryer absorber process for desulfurization of flue gases having a sulfur oxide capability which is not limited by the heat content of the flue gases.

It is another feature of the process to use a spray dryer absorber chamber and related co-axial gas and solid distributors and atomizer means for introducing aqueous constituents of the process which are designed to obtain efficient particle/gas contacting in a manner not obtainable in a non-symmetrical duct system.

It is yet another feature of the subject invention to provide a process which utilizes nascent calcium hydroxide produced, suspended and conveyed in a gaseous medium for absorbing an acid producing substance or acidic material from a gas stream in a spray dryer type absorber.

It is another feature of the present invention to provide a process for producing nascent calcium hydroxide or hydrates where calcium oxide is comminuted by means including impact and abrasion while being suspended and conveyed in a gaseous medium containing water which then can be utilized immediately for absorption and the like.

It is a further feature of the subject invention to provide a process for the absorption of sulfur dioxide from a gaseous medium such as flue gas which requires significantly less calcium hydrate as compared to conventional absorption procedures.

It is yet another feature of the subject invention to provide a process for the absorption of sulfur dioxide from a gaseous medium such as flue gas which can be conducted at significantly less cost as compared to conventional absorption procedures.

Briefly, the present invention comprehends in its broader aspects a process for treating a first gas medium containing an acidic material, the steps comprising comminuting calcium oxide in a second flowing gaseous medium containing water to form particulates of calcium hydrate sorbent suspended in the flowing second gaseous medium and contacting the second gaseous medium with the first gaseous medium in admixture with an atomized aqueous material, preferably a slurry containing recycled calcium hydroxide.

The present invention further comprehends a process for treating a first gas medium containing an acidic material, the process comprising comminuting calcium oxide in a second flowing gaseous medium containing water to form particulates of calcium hydrate sorbent suspended in the second flowing gaseous medium, conveying said particulates in said second gaseous medium to a spray dryer absorber, contacting said particulates in said spray dryer absorber with the first gaseous medium in admixture with an atomized aqueous material, preferably a slurry containing recycled calcium hydroxide, for a time sufficient to allow at least some of the acidic material to react with said particulates to form a solid product, and separating at least some of said solid product from said first gaseous medium.

Further features, objects and advantages of the present invention will become more fully apparent from a detailed consideration of the arrangement and construction of the constituent portions of the process as set forth in the following description when taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

Figure l is a simplified process flow diagram for a known spray dryer absorber type process which utilizes conventional atomized calcium hydroxide as the sorbent, Figure 2 is a simplified flow diagram illustrating one embodiment of a process according to the present invention, -

Figure 3 is a graphical representation of Ca/S mol ratio versus sulfur dioxide concentration in the flue gas to be treated showing the mol ratio necessary to obtain a capture or recovery of 95% of the sulfur dioxide contained in the flue gas using a conventional process as depicted in Figure 1, plot A being for a single pass operation and plot B being for a recycle system, and

Figure 4 is a graphical representation of Ca/S mol ratio requirements of one embodiment of a process according to the present invention versus sulfur dioxide concentration in the flue gas to be treated at a S0₂ capture or recovery level of about 90%, plot A being for a single pass operation and plot B being for a recycle system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to Figure l, shown is a simplified process flow diagram of conventional spray dryer absorber type system 10 for flue gas desulfurization. The system is shown for purposes of illustration only to exemplify the system of the present invention. In conventional system 10, an aqueous slurry of slaked lime is carried by line 12 to atomizer feed tank 14 having agitation means. Line 16 transports the lime slurry to atomizer device 18 mounted in chamber 20 of spray dryer absorber 22.

Flue gas from a source such as a boiler (not shown) is fed by line 24 into the gas distribution chamber located above atomizer device 18 and flows from the distribution chamber around the atomizer wheel of the device for efficient contact with the atomized slurry in chamber 20 of absorber 22. Gases and entrained solids exit absorber chamber 20 by line 26 and are fed into solid-gas separation device 28. Gases exit separation device 28 via line 30 and are sent to the atmosphere by stack 32. Solids separated in separation device 28 are withdrawn through lines 34 and are sent via line 36 to disposal and via line 38 to recycle tank 40 having agitation means. In tank 40, water is supplied by line 42 and the resultant slurry is recycled to atomizer feed tank 14 by line 44. System 10 may be operated as once-through or so-called single pass or sorbent basis by closing lines 38 and 42.

Turning now to Figure 2, shown is a simplified flow diagram of a spray dryer absorber type system for desulfurization of a flue gas according to the present invention. System 50 has similarities with the system 10 shown in Figure 1 but a primary distinction is in the manner in which calcium hydroxide particulates are produced and utilized as the sorbent for undesirable components in the flue gas. As is possible using system 10, the sorbent in system 50 may be only used for a single pass and then discarded or can optionally be recycled to the absorber for multiple passes.

More specifically, system 50 includes gas swept mill 52 capable of grinding pebble lime by impact and abrasion to a fineness such that 99 to 99.9 percent will pass through a No. 200 mesh sieve. A presently preferred mill of this type is a ring-roller mill conventionally used in the beet-sugar industry to produce quicklime for the production of milk of lime. Such mills are often referred to as Raymond mills. Further details as to suitable ring- roller mills may be found in Perry's Chemical Engineers' HandBook. Fourth Edition, by Perry et al, McGraw Hill 1963, with particular attention directed to chapter 8, pages 51 and 52.

Fed into mill 52 are calcium oxide, water and steam by lines 56, 58 and 60 respectively as well as dry sorbent recycled via line 62. Flue gas to be treated by system 50 enters via line 64 from a source such as a boiler or the like (now shown) . A portion of the entering flue gas is drawn form line 64 by blower 66 into line 68 and fed into mill 52. In the mill 52, the flue gas acts as a suspending and conveying medium for the calcium hydroxide reaction product produced by the reaction between the finely divided calcium oxide and water in the mill. The flue gas conveys the reaction product from mill 52 via line 54 into a coaxial solid distributor device 70 extending into chamber 72 of spray dryer absorber 74.

The remainder of the flue gas in line 64 is fed into gas distribution chamber 76 of spray dryer absorber 74, the gas distribution chamber being located around the upper portion of solids distributor device 70. The flue gas flows from the distribution chamber 76 around the atomizer wheel of the distributor device 70 to provide for efficient contact between the reactants while in chamber 72.

Treated flue gas with entrained particulate sorbent reaction products and any unreacted sorbent exits absorber 74 by line 78 and is fed to gas-solids separator 80 where the majority of the particulates are removed and the treated gas is vented by stack 82 by line 84. Separator 80 may be a bag house, electrostatic precipitator or the like. Separated solids including sorbent reaction products are removed from separator 80 by lines 86 and part is sent to disposal by line 88, part sent to recycle back to mill 52 by line 90 along with at least a portion of the solids collected in chamber 72, and sent by line 92 to recycle tank 94. In tank 94 having agitator mans, water or a lime slurry from a source (not shown) is added by line 96 and the resultant slurry is pumped through line 98 to the atomizer wheel of distributor device 70 of spray dryer absorber 74.

Of particular significance to the subject invention as described above is the provision of grinding the lime in a mill while the lime is suspended in a gaseous medium with the simultaneous introduction of water and steam as required so as to produce finely divided nascent calcium hydroxide particulates emanating from the mill. Significantly, if the gaseous medium used in the mill is the gas to be treated, reaction of the calcium hydroxide produced can occur essentially immediately. Also of significance to the subject invention is that the nascent calcium hydroxide particulates are conveyed while suspended in the gaseous medium to the point at which the particulates are immediately utilized such as in a spray dryer absorber. Further of particular significance is that the nascent calcium hydroxide particulates may be admixed in the spray dryer absorber with atomized slurry comprising recycled sorbent containing any unreacted calcium hydroxide.

Thus the process of the subject invention specifically recognizes and uses to advantage, among other things, the nature of the reaction between lime and water to produce calcium hydroxide as discussed previously. More specifically, by recognizing that the calcium hydroxide particulates produced from the reaction of calcium oxide and water are highly reactive and molecules of the same tend to quickly associate with one another, the subject process utilizes nascent calcium hydroxide essentially immediately by producing, suspending and conveying the nascent calcium hydroxide in a gaseous medium and then absorbing an acid producing substance or acidic material such as sulfur dioxide in a spray dryer type absorber wherein the acidic gases also contact an atomized aqueous slurry reactive to the acidic gases.

More specifically, in the subject process utilizing a spray dryer type absorber, the elapsed time between the formation of calcium hydroxide in a gaseous medium and contact with flue gases is minimized such that the hydrated lime, at the time of contact with flue gases, exists in what may be termed a "nascent state". Those calcium hydroxide containing particulates issuing from the atomizer are enveloped in water which evaporates on contact with the hot flue gases thereby providing cooling of the gases by evaporation. The water and water vapor tend to promote the reaction between acidic gases with both the calcium hydrate provided as dry calcium hydroxide particulates and that introduced in the aqueous slurry. Thus an important feature of the subject process is that the flue gas is contacted with an admixture of simultaneously produced particulates of an atomized aqueous calcium hydroxide comprising slurry and particulates of dry calcium hydroxide introduced in a gaseous medium.

As is apparent from the above, the process of the invention is particularly suitable for removing sulfur dioxide from a gaseous stream such as a waste or flue gas stream. In addition, the process may be useful for the removal of other undesirable components of gas stream which are capable of producing acidic substances such as for example, combustion products from various fuels or waste or process gases from various industrial processes. For convenience, these compounds including sulfur dioxide are referred to herein as acidic materials.

While the process described above with reference to Figure 2 uses a portion of the entering flue gas as the gaseous medium for entraining the particulate sorbent, other gaseous media may be used in the process of the invention either alone or conjunction with the flue gas. For example, the gaseous medium could be ambient or heated air, another waste gas or the like such as the treated gas exiting the system via stack 82 shown in Figure 2. Preferably if another gaseous medium is used other than the flue gas, the gas is preferably at an elevated temperature for the considerations mentioned below. Generally when the flue gas to be treated is used as the gaseous medium for conveying the particulate sorbent, a major portion of the flue gas enters the absorber directly and a minor portion of the flue gas is used for conveying the sorbent.

During the grinding operation to produce the finely divided particulate sorbent, the combined amount of moisture introduced by the water and steam lines is preferably regulated so as to provide at least that water necessary to convert all the lime to calcium hydroxide and no more than that which produces an essentially dry product to be conveyed to the absorber. For most processes, this amount of water will be the stoichiometric amount or slightly in excess, for example, of about 1.0 to about 1.2 moles water per mole calcium oxide.

At the same time the water and steam are added to the lime being ground n the mill, it may be advantageous to also add other materials to facilitate and/or optimize the subject process. For example, it may be beneficial to incorporate such adjuvants as one or more glycols, amines, lignosulfonates and reactive silicious materials into the gas stream flowing through the mill and/or into the lime, water and/or steam directly.

In the spray dryer absorber, water is also introduced, the primary purpose being to control the temperature within the absorber. As a general matter, the amount of water introduced into the absorber is a function of the inlet temperature of the flue gases and the optimum or desired temperature for conducting the absorption reaction within the reaction zone of the absorber.

Conditions for operating the above described process may vary considerably depending upon, among other things, the particular flue gas being treated in terms of type of material to be removed and its concentration as well as its input temperature and gas flow rate, the degree to which the material is to be removed from the flue gas and the particular composition of the sorbent being utilized. Generally, the pressure and temperature utilized may vary significantly although it is preferred that the temperature of all gases, circulating solids, and equipment surfaces be maintained at a temperature above the adiabatic saturation temperature of the gases being treated so as to avoid condensation throughout the system which might cause operational problems such as corrosion and the like. Preferably, the temperature is at least 10"C above the adiabatic saturation temperature of the gases being treated.

Other operating conditions such as absorber residence time, recycle ratio and the like may also vary considerably. Generally an overall recycle ration of recycled sorbent to newly added nascent sorbet to of about 25:1 to about 10:1 may be utilized. In addition, a ratio of about 4:1 to about 1:4 for recycle sorbent to the mill relative to recycle sorbent to the absorber may be used. Recycle sorbent to the mill notably provides surface area on which the nascent calcium hydroxide particulates may be usefully separated one from another for reaction purposes. Alternatively, it may be preferably under certain situations to conduct the subject process with no recycle of sorbent whatsoever.

The improved results achieved by the spray dryer absorber type process of the present invention for flue gas desulfurization are illustrated by the data shown in Figures 3 and 4 which enable a comparison of the subject process as illustrated by the system shown in Figure 2 with a conventional process as is illustrated by the system of Figure l. More specifically, Figure 3 shows plots of sulfur dioxide concentration versus the calcium/sulfur mole ration necessary to obtain a capture or recovery of 95% of the sulfur dioxide contained in a flue gas in a conventional process. Plot or line A in Figure 3 is for a single pass system and plot or line B is for a recycle type system. In a similar fashion, Figure 4 shows plots of sulfur dioxide concentration versus the calcium/sulfur mole ratio necessary to obtain a capture or recovery of 95% of the sulfur dioxide contained in a flue gas by a process according to the present invention. Again, plot or line A is for a single pass system and plot or line B is for a recycle type system. Both the recycle systems of Figure 3 and 4 use recycle of separated sorbent in a slurry through the atomizer for temperature control and recovery of sorbent values. For example, a review of line B of Figure 3 indicates that at 2000 ppm sulfur dioxide concentration in the inlet gas, a Ca/S ration of about 1.38 is required to obtain a 95% capture of sulfur dioxide when a conventional process using calcium hydroxide slurry is used. In contrast, reference to line B of Figure 4 indicates that when the nascent calcium hydroxide is used as the primary sorbent according to the process of the subject invention, a sulfur dioxide capture of about 95% can be achieved with a Ca/S mol ratio of 1.18. As is thus apparent, the process of the subject invention can achieve the same level of removal of sulfur dioxide with significantly less sorbent with the attendant reduction in costs for materials, capital expenditures, energy requirements and other operating expenses.

A specific process according to the invention is presented in the following example. It should be understood that the example is given herein for the purposes of illustration and does not limit the invention as has been heretofore described to this particular example.

EXAMPLE

A test is conducted in a facility following the process depicted in Figure 4, the test being conducted using 2000 ppm S0₂ in a simulated flue gas. The temperature in the absorber is maintained at about 15°C above the adiabatic saturation temperature of the flue gas. The Ca/S ratio required to obtain about 95% S0₂ capture is shown in Figure 4 to be about 1.18. Thus about 1.18 mols of calcium oxide is introduced into the system per unit of time for every mol of sulfur dioxide entering the system.

The facility is brought on stream by first introducing a hot flue gas into line 64 and through line 68 and allowing the various process components to heat up until the entire system is heated to a temperature above 100°C. Spent sorbent is introduced into mill 52 by line 90 from a suitable source and is conveyed to absorber chamber 72 and thence to separator 80 by line 78. Sorbent is withdrawn from separator 80 via line 92 to recycle tank 94 where the solids are admixed with water to form a recycle slurry fed to atomizer 70 by line 98. A recirculating flow of sorbent is thus established in conjunction with spent sorbent passing through mill 52 via line 90.

Feed of calcium oxide through line 56 to mill 52 is then regulated such that recycle sorbent entering through line 90 is progressively reduced to a desired level and the calcium oxide as well as water and steam through lines 58 and 60 respectively are progressively adjusted as required. As a consequence, finely divided nascent calcium hydroxide sorbent is produced and pneumatically conveyed to spray dryer absorber 74 for contact with sulfur dioxide containing flue gas. As operations proceed, the withdrawal of spend sorbent via line 88 is commenced and is adjusted until steady state conditions are achieved.

During operation, the atomized slurry of spent sorbent introduced by atomizer 70 in spray dryer absorber 74 serves to control the temperature in the absorber. In addition, the introduction of the slurry also enables the economic use of unreacted calcium hydrate sorbent values contained in the recycled solids.

While there has been shown and described what are considered to be preferred embodiments of the present invention, it will be apparent to those skilled in the art to which the invention pertains that various changes and modifications may be made therein without departing from the invention as defined in the appended claims.

Claims

IT IS CLAIMED:

1. A process for treating a first gas medium containing an acidic material, the process comprising comminuting calcium oxide in a second flowing gaseous medium containing water to form particulates of calcium hydrate sorbent suspended in the second flowing gaseous medium, conveying said particulates in said second gaseous medium to a spray dryer absorber, contacting said particulates in said spray dryer absorber with the first gaseous medium in admixture with atomized aqueous material for a time sufficient to allow at least some of the acidic material to react with said particulates to form a solid product, and separating at least some of said solid product from said first gaseous medium.

2. A process according to claim 1 further including the step of recycling at least some of the solid product into the second gaseous medium.

3. A process according to claim 2 further including the step of recycling at least some of the solid product in the form of a slurry of the spray dryer absorber.

4. A process according to claim l further including the step of recycling at least some of the solid product in the form of a slurry of the spray dryer absorber.

5. A process according to claim 1 wherein the acidic material of the first gaseous medium includes sulfur dioxide.

6. A process according to claim 1 wherein the second gaseous medium contains sufficient water to form calcium hydrate from essentially all the calcium oxide.

7. A process according to claim 1 wherein the first gaseous medium and the second gaseous medium are of the same composition.

8. A process according to claim 1 wherein water is provided in the second gaseous medium by the introduction of steam and liquid water.

9. A process according to claim 1 wherein the calcium oxide is comminuted by abrasion and impact such that the size of the calcium oxide particulates is such that about 99% passes through a No. 200 mesh screen.

10. A process according to claim 1 the reaction in the spray dryer absorber is conducted at a temperature of at least about 10°C. above the adiabatic saturation temperature of the gases contained therein.

11. A process according to claim 1 wherein comminuting of the calcium oxide is conducted in the presence of at least one adjuvant selected from the group consisting of glycols, amines, lignosulfonates, and silicious materials.

12. In a process for treating a first gas medium containing an acidic material, the steps comprising comminuting calcium oxide in a second flowing gaseous medium containing water to form particulates of calcium hydrate sorbent suspended in the flowing second gaseous medium and contacting the second gaseous medium with the first gaseous medium.

13. In a process according to claim 12, further including conveying said particulates in said second gaseous medium prior to contacting with the first gaseous medium, and contacting said particulates with the first gaseous medium for a time sufficient to allow at least some of the acidic material to react with said particulates to form solid products, and separating at least some of the solid product from the first gaseous medium.

14. In process according to claim 13 further including the step of recycling at least some of the solid products into the second gaseous medium.

15. A process according to claim 12 wherein the acid material of the first gaseous medium includes sulfur dioxide.

16. A process according to claim 13 wherein the second gaseous medium contains sufficient water to form calcium hydrate from essentially all the calcium oxide.

17. A process according to claim 12 wherein the first gaseous medium and the second gaseous medium are of the same composition.

18. A process according to claim 12 wherein water is provided in the second gaseous medium by the introduction of steam and liquid water.

19. A process according to claim 12 wherein the calcium oxide is comminuted by abrasion and impact so that the size of the calcium oxide particulates is such that about 99% passes through a No. 200 mesh screen.

20. A process according to claim 12 wherein comminuting of the calcium oxide is conducted in the presence of at least one adjuvant selected form the group consisting of glycols, amines, lignosulfonates, and silicious materials.

21. A process according to claim 1 wherein the second gaseous medium is a gas essentially free of solids.