CA2357280A1 - Procedure and also apparatus for the cleaning of flue gases containing sulfur dioxide - Google Patents
Procedure and also apparatus for the cleaning of flue gases containing sulfur dioxide Download PDFInfo
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- CA2357280A1 CA2357280A1 CA002357280A CA2357280A CA2357280A1 CA 2357280 A1 CA2357280 A1 CA 2357280A1 CA 002357280 A CA002357280 A CA 002357280A CA 2357280 A CA2357280 A CA 2357280A CA 2357280 A1 CA2357280 A1 CA 2357280A1
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- mixing unit
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/46—Removing components of defined structure
- B01D53/48—Sulfur compounds
- B01D53/50—Sulfur oxides
- B01D53/508—Sulfur oxides by treating the gases with solids
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/46—Removing components of defined structure
- B01D53/48—Sulfur compounds
- B01D53/50—Sulfur oxides
- B01D53/501—Sulfur oxides by treating the gases with a solution or a suspension of an alkali or earth-alkali or ammonium compound
Abstract
Procedure for the cleaning of flue gases containing sulfur dioxide that come from a circulating fluidized-bed firing system, wherein within the combustion chamber of the fluidized-bed firing system sulfur-containing coal is burned by the addition of air at a temperature of 700 to 950 °C in a fluidized bed, and therein flue gas containing sulfur dioxide is formed and a particulate SO2 sorbent is delivered to the combustion chamber, a portion of the ash produced in the combustion and a portion of the reaction product produced in the reaction of the SO2 sorbent with the sulfur dioxide as well as a portion of the unreacted SO2 sorbent is carried off from the combustion chamber and is fed to a mixing unit, in the mixing unit, water or an aqueous sodium-containing solution is fed to the ash, the reaction product, and the unreacted SO2 sorbent and is mixed together with this, and therein the unreacted SO2 sorbent is converted into a hydration product at a reaction temperature of 60° to 100° and at atmospheric pressure, the ash together with the reaction product and the hydration product from the mixing unit is returned into the combustion chamber of the fluidized-bed firing system, and the hydration product is reactivated to an SO2 sorbent at a combustion-chamber temperature of 700° to 950° C, as well as an apparatus for the execution of this procedure (Figure 1).
Description
Specification Procedure and also apparatus for the cleaning of flue gases containing sulfur dioxide This invention relates to a procedure as well as an apparatus for the cleaning of flue gases containing sulfur dioxide that come from circulating fluidized-bed firing systems.
Circulating fluidized-bed firing systems are used in particular for the low-emission combustion of fossil fuels, e,g. coal, peat, wood, and so forth. In burning sulfur-containing coal, for example, the oxidation of the sulfur produces sulfur dioxide, which gets into the atmosphere via the flue gas. These em,ission5, which are harmful to the earth's atmosphere, are returned to the earth as acid rain by Way of the weather cycle.
Various procedures have been developed for reducing these harmful emissions to the greatest possible extent.
An overview of the retaining of sulfur dioxide in filuidized-bed firing systems was presented by E. J. Anthony during the "Mediterranean Combustion Symposium" in Antalya, Turkey in June 1999.
For example, a familiar procedure is the addition of a fine-grained alkaline SOa sorbent, generally limestone {CaC03), burnt lime (Ca0) or also dolomite, into the combustion chamber of the fluidized-bed firing system. Here, first of all the roasting (calcining process) of the limestone to burnt lime (Ca0) takes place, and subsequently a reaction occurs between the roasted limestone and the sulfur dioxide of the flue gas.
If in this connection the limestone is exposed to the temperatures of 700 °C to 950°C
that are present in a circulating fluidized-bed bring system, namely if carbon dioxide is driven off from the limestone, then what remains is burnt lime, which because of the driving off of the C02 has a high degree of porosity and thus a high specific surtace area.
Circulating fluidized-bed firing systems are used in particular for the low-emission combustion of fossil fuels, e,g. coal, peat, wood, and so forth. In burning sulfur-containing coal, for example, the oxidation of the sulfur produces sulfur dioxide, which gets into the atmosphere via the flue gas. These em,ission5, which are harmful to the earth's atmosphere, are returned to the earth as acid rain by Way of the weather cycle.
Various procedures have been developed for reducing these harmful emissions to the greatest possible extent.
An overview of the retaining of sulfur dioxide in filuidized-bed firing systems was presented by E. J. Anthony during the "Mediterranean Combustion Symposium" in Antalya, Turkey in June 1999.
For example, a familiar procedure is the addition of a fine-grained alkaline SOa sorbent, generally limestone {CaC03), burnt lime (Ca0) or also dolomite, into the combustion chamber of the fluidized-bed firing system. Here, first of all the roasting (calcining process) of the limestone to burnt lime (Ca0) takes place, and subsequently a reaction occurs between the roasted limestone and the sulfur dioxide of the flue gas.
If in this connection the limestone is exposed to the temperatures of 700 °C to 950°C
that are present in a circulating fluidized-bed bring system, namely if carbon dioxide is driven off from the limestone, then what remains is burnt lime, which because of the driving off of the C02 has a high degree of porosity and thus a high specific surtace area.
The subsequent gas-solid reaction of the burnt lime (sorbent) with sulfur dioxide and oxygen is a surface reaction, and this is why the creation of a high specific surface area is a fundamental prerequisite for this reaction. Remaining behind as a solid reaction product is calcium sulfate or gypsum (CaSOa). which stays in the pores or on the surface of the sorbent or the burnt lime.
Depending on the grain size of the limestone or S02 sorbent used and on its abrasion properties, either the aggregate of the sorbent-reaction product (Ifme-gypsum aggregate) remains long enough in the combustion chamber for it to be drawn off via to the components of the combustion-chamber ash removal system, or in the case of small particles the sorbent-reaction product aggregate leaves the combustion chamber together with the flue-gas stream and is subsequently separated out in the following flue-gas filter.
The mixture composed of fuel ash, reaction product, and free, unreacted sorbent that is drawn off via the combustion-chamber ash removal system is generally referred to as bottom ash or coarse ash. The particle size of this coarse ash is for the most part larger than 100 Nm. The maximum grain diameter can amount to several mm.
20 The ash carried off with the flue gas that is subsequently separated out in the filter is generally called filter ash. Depending on the quality of the cyclone /
separator, the grain size of this ash encompasses the small grain fractions up to about 200 Nm in diameter.
From knowledge gained by constructing fluidized-bed firing systems, it is evident that for the degree of desulfurization required in industrial use, namely a reduction in sulfur dioxide of 'From 70% to 99°~, the desulfurization reaction requires a high excess of sorbent. This requirement is all the higher the greater is the demand placed on the degree of desutfurization.
30 If one uses the Ca/S ratio as a measure for the added limestone or another sorbent, namely the molar quotient of externally supplied Ca arid the total sulfur of the fuel that is present, then typical Ca/S values for fluidized-bed firing systems lie between 2 and 4 for a degree of desulfurization of 95%.
Depending on the grain size of the limestone or S02 sorbent used and on its abrasion properties, either the aggregate of the sorbent-reaction product (Ifme-gypsum aggregate) remains long enough in the combustion chamber for it to be drawn off via to the components of the combustion-chamber ash removal system, or in the case of small particles the sorbent-reaction product aggregate leaves the combustion chamber together with the flue-gas stream and is subsequently separated out in the following flue-gas filter.
The mixture composed of fuel ash, reaction product, and free, unreacted sorbent that is drawn off via the combustion-chamber ash removal system is generally referred to as bottom ash or coarse ash. The particle size of this coarse ash is for the most part larger than 100 Nm. The maximum grain diameter can amount to several mm.
20 The ash carried off with the flue gas that is subsequently separated out in the filter is generally called filter ash. Depending on the quality of the cyclone /
separator, the grain size of this ash encompasses the small grain fractions up to about 200 Nm in diameter.
From knowledge gained by constructing fluidized-bed firing systems, it is evident that for the degree of desulfurization required in industrial use, namely a reduction in sulfur dioxide of 'From 70% to 99°~, the desulfurization reaction requires a high excess of sorbent. This requirement is all the higher the greater is the demand placed on the degree of desutfurization.
30 If one uses the Ca/S ratio as a measure for the added limestone or another sorbent, namely the molar quotient of externally supplied Ca arid the total sulfur of the fuel that is present, then typical Ca/S values for fluidized-bed firing systems lie between 2 and 4 for a degree of desulfurization of 95%.
This requirement has economic disadvantages, in particular because in general this raises not only the operating costs for the procurement of limestone or of another sorbent, but also the waste-disposal costs for the resulting ash due to the fraction of unreacted sorbent.
In connection w~h the above-named desulfurization procedure for the flue gas in a circulating fluidized-bed firing system, it has proved to be a shortcoming that the limestone or the SOz sorbent does not react completely with the sulfur dioxide, since frequently a blanket of gypsum that is almost gas-impermeable forms around the lime aggregate or sorbent aggregate, and also the pores of the lime or sorbent are clogged up by gypsum as the reaction product. The physical/chemical basis for this is the larger molar volume of SOz diffusing into the lime aggregate or sorbent aggregate compared to the expelled C02.
Especially in the interior of the grains of the sorb~nt-reaction product (lime-gypsum grains), a core of unreacted sorbent remains that is no longer available for the reaction, since the reaction partners of sulfur dioxide and oxygen can no longer penetrate down into this core.
The concentrations of unreacted free sorbent in the ash mixture can be as much as 40%, both in the case of filter ash and also with coarse ash, relative to the total ash mixture that is to be carried off. Also, within the framework of the further use of the ash mixture in the cement industry or in roadbuilding, it is desirable to have a lower concentration of free, namely unreacted, sorbent or limestone, to below 3 to 5%.
In current fluidized-bed firing systems, various techniques are being used at present in order to increase the degree of utilization of sorbents or limestone for the purposes of reducing the sulfur dioxide.
3o Thus, for example, in circulating fluidized-bed firing systems the ash accumulating in the flue-gas filter, which may still contain high proportions of unreacted sorbent, is returned again directly into the combustion chamber.
-t~,-One drawback of this ash recycling is that the utilization of the still free sorbent in the ash may be of only limited benefit, because the additional dwell time in the fluidized-bed combustion chamber is small and because the reactivity of this ash or of the free sorbent contained in the ash is considerably reduced compared to the original sorbent.
Moreover, a recycling of the bed ash drawn off from the combustion chamber is also customary. To this end, the bed ash is subjected in part to a treatment (sifting of the good grain fraction or grinding up of the bed ash) that is aimed at Increasing its degree of reactivity. But this method as well has the drawback that its effect in reducing the to requisite consumption of sorbent is very limited, since it does not eliminate the cause of the incomplete reaction, the above-mentioned gypsum blanket around the sorbent or lime aggregate.
By way of the document US Patent 4,312,280, Shearer et al., a system has furthermore been disclosed in which ash from stationary fluidized~bed firing systems is brought into contact with water or steam and is returned to the combustion chamber of the fluidized-bed system. The mixing of the ash with steam and water takes place in a complex ffuidized-bed reactor at relatively high temperatures, No reference is made to more extensive process-technology details about operating temperatures of this fluidized-bed 20 reactor or to what water admixtures are used for doing this work. This disclosed system has on the whole a high technological complexity and therefore has not gained much acceptance on the market, partly also because the potential market for stationary fluidized-bed firing systems is limited to small system sizes, and compared to circulating fluidized-bed firing systems they have the disadvantage of having smaller particle dwell times as well as an inhomogeneous temperature distribution.
The object of the invention, then, is to devise a procedure as well as an apparatus for the cleaning of flue gases containing sulfur dioxide that come from circulating fluidized-bed firing systems, in which the above-mentioned disadvantages are avoided.
3o Furthermore the efficiency or the degree of utilization of the sorbent used is to be increased and thereby the quantity of sorbent needed for the sorption of the sulfur dioxide is to be reduced.
The object specified above is achieved through a~ procedure having the features of Patent Claim 1 and through an apparatus having the features of Claim 26.
ether advantageous embodiments of the invention can be found in the dependent claims.
Through the achievement in accordance with the invention, a procedure as well as a mechanism are provided that have the following advantages.
1o By the mixing together of the ash mixture coming from the combustion chamber of the circulating fluidized-bed firing system (ash, reaction product, and unreacted SOz sorbent) with water or with an aqueous, sodium-containing solution in a mechanical mixing unit at a reaction temperature of 60° to 100 °C and at atmospheric pressure, and by recycling this into the combustion chamber of the circulating fluidized-bed firing system, the degree of utilization of the sorbent is considerably increased compared to a simple recycling of filter ash or bottom ash.
This effect arises from the fact that due to the mixing together of the ash with water or with an aqueous, sodium.containing solution in a manner corresponding the procedure 2o in accordance with the invention, the still unreaGted sorbent is first caused to react at a reaction temperature of 60 to 100 ° C with water or with an aqueous, sodium-containing solution to form a hydration product. This reaction is exothermic. The elevation in temperature as well as the reaction rate depend on the concentration of the unreacted sorbent in the ash, the temperature of the supplied ash, and the temperature of the supplied water as well as on the parallel reaction with the Si~2 and AI203 contained in the ash mixture. Since the hydration product has a lower density than the sorbent, this reaction causes the sorbent grain to "swell,u so that the adsorbate blanket (or gypsum blanket) around the sorbent aggregate gets broken up.
30 In this connection it has turned out that the degree of conversion of the S02 sorbent into hydration product as well as the reaction rate increase considerably with increasing ~6-l0 temperature, and within the range of about 60°C to about 100°C
and at atmospheric pressure they reach a value that is optimally desirable in such an operation.
If the temperature is too low, then the formation of hydration product proceeds only very slowly and is not complete within the dwell time that is available in the mixing unit. If the temperature is too high, then a "boiling" of the ash mixture takes place, with the consequence that due to the excess reaction enthalpy, additional water is evaporated, which is then no longer available for the hydration reaction. The consequences of reaction temperatures that are too high are an increased water consumption and problems associated with the additional vapor formation.
The reaction temperature in the mixing unit is regulated in an expedient manner in order to achieve an optimal conversion of unreacted SOz sorbent into a hydration product.
This enables the dosed-out quantity of water or of sodium solution to be adjusted in accordance with the concentration of the unreacted SOz sorbent in the ash.
Thus via a thermodynamic balance the optimal quantity of water to be added can be determined.
In another advantageous embodiment of the invention, the desired reaction temperature in the mixing unit is achieved by adding preheated water into the mixing unit, with the water temperature being regulated by a preheater located in front of the mixing unit. By 20 the preheating of the water, the reaction temperature in the mixing unit can be set within the temperature range that is favorable to the course of the reaction, independently of the concentration of the non-reacted sorbent in the ash. Instead of water, an aqueous, sodium-containing solution can also be supplied.
Through the regulation of the quantity of water or of aqueous sodium-containing solution that is fed to the mixing unit, the mixture product can be carried off from the mixing unit in an advantageous manner as a function of the residual moisture. In this way, the product that is to be carried off can be produced in a desirable fashion.
30 In one advantageous embodiment of the invention, the dwell time of the product introduced into the mixing unit is regulated as a function of the degree of hydration of the product to be carried off. It is especially advantageous when the minimum dwell time _7_ in the mixing unit and/or the subsequent delivery lines amounts to one minute, in order to ensure that the hydration of the introduced product occurs in the desired manner.
It is expedient for the product drawn off from the mixing unit to exist in the form of a solid and to have a residual moisture less than 10 %. This keeps the product drawn off from the mixing unit from being sludgy and thus difficult to transport due to a too-high excess of water.
It may be advantageous to construct the mixing unit in two stages, where in the first stage a portion of the water or of aqueous, sodium~containing solution required for the mixing is admixed with the ash, the reaction product, and the unreacted sorbent, and in the second stage the remaining portion of water or aqueous, sodium-containing solution is admixed in a regulated way as a function of the residual moisture of the product to be carried off from the mixing unit. In this way, in the first mixer attention can be directed toward the mixing process and in the second mixer it can be directed toward the requisite dwell time as well as to the temperature requirements.
By feeding the product carried off from the mixing unit into a drier, this product can be stored in an advantageous way after it has been dried. Thereby, in another advantageous embodiment of the invention the possibility is provided of putting this product into intermediate storage and feeding it to the combustion chamber after a certain interval of time.
It is further advantageous to return at least a portion of the product carried off from the mixing unit back into the mixing unit. With this measure, the dwell time for the reaction within the mixing unit can likewise be affected.
By feeding the ash mixture into a siftinglsizing unit before introducing it into the mixing unit, grain sizes of undesirable magnitude, for example larger than 300 microns, can be sifted out. This makes it possible to largely avoid erosions within the mixing unit as well as in the delivery lines. The same effect can be achieved by directing relatively large and undesirable grain sizes into a grinding unit and subsequently passing these on to the mixing unit.
-$_ In one advantageous embodiment ofi the invention, the ash, the reaction product, and the unreacted S02 sorbent drawn off from the circulating fluidized-bed firing system and fed to the mixing unit is supplied to the mixing unit from the flue-gas filter as filter ash and from the combustion chamber as bottom ash over separate supply fines in each case, in which connection either filter ash or bottom ash or an adjustable mixture of the two can be fed to the mixing unit. This makes it possible to respond to any operational situation of the fluidized-bed firing system.
Furthermore the apportionment of the mixture of filter ash and bottom ash can be set by 1o adjusting the grain sizes of added fuel and SOx sofient.
tt is advantageous to introduce the water or an aqueous, sodium-containing solution Into the mixing unit by means of at least one nozzle. As the situation demands, the requisite nozzles can be installed within the mixing unit at any points desired.
It is advantageous to use limestone as the S02 sorbent. This is relatively inexpensive and has worked well as an S02 sorbent. Furthermore it may also be advantageous to use dolomite as the SOa sorbent.
20 In another advantageous embodiment of the invention, 50% to 500% of solid mixture, relative to the solid mixture normally leaving the combustion chamber of the fluidized-bed firing system, is fed to the mixing unit for the hydration process and subsequently fed again to the combustion chamber. Thereby an optimal conservation of the needed S02 sorbent is achieved.
In one special case of the procedure in accordance With the invention, a portion of the S02 sorbent is delivered directly to the mixing unit. In this way, regardless of the amount of ash in circulation as well as unreacted SO2 sorbent, the requisite amount of SOz sorbent can be influenced from the outside, In another special case of the procedure in accordance with the invention, a hydration product or Ca(OH)2 can be delivered into the line between the mixing unit and combustion chamber, with it being advantageous for this product to be delivered upstream of an intermediate storage, as seen in the direction of flow of the product. In this way, this quantity of product can likewise be influenced from the outside.
If an aqueous, sodium-containing solution is used, it is advantageous for the solution to contain sodium in the form of ions on an order of magnitude of up to 3 % by weight, relative to the unreacted SOZ sorbent.
Furthermore it may be advantageous for the mixing unit to include an additional feed-in of water or of an aqueous, sodium-containing solution, by means of which water or an aqueous, sodium-containing solution is added to the ash, the reaction product, and the unreacted SOa sorbent upstream of the mixing unit. Thereby the reaction in the mixing unit can be influenced beforehand if appropriate.
Through the apparatus in accordance with the invention, the procedure in accordance with the invention can be carried out in an efficient, inexpensive, and resource-sparing manner.
Below, exemplary embodiments of the invention are explained in more detail on the basis of the drawings and the spec~cation.
These show:
Figure 1 a schematically represented apparatus for the execution of the procedure in accordance with the invention.
Figure 2 an embodiment that is alternative to Figure 1.
Figure 1 shows in a schematic way the apparatus in accordance with the invention for the cleaning of flue gases containing sulfur dioxide that come from fluidized-bed firing systems. By means of the apparatus shown, the procedure in accordance with the invention can also be carried out.
-10~
The circulating fluidized-bed firing system 1 in accordance with the invention has a fluidized-bed combustion chamber 2, in which in a familial manner coal or other particulate and carbon-containing fuels are burned by the addition of air - as a fluidizing medium via the line 5 as well as secondary air ~- at 700 to 954 °C in the fluidized bed.
For the desulfurization of the sulfur-containing fuel, in~ addition to the fuel (supply line 3) also an SOZ sorbent is delivered to the combustion chamber 2 via the line 4.
As a rule, limestone in the form of particles or fine grains, namely CaCOs, is delivered to the combustion chamber 2 as the SO2 sorbent. In place of Ilmestone, also dolomite as well as burnt lime (Ca0) or Ca(OH)2 can be used as an S02 sorbent.
When limestone is used as the S02 sorbent, first of all the process of roasting the limestone into burnt Ilme (Ca0) takes place in the combustion chamber 2 and subsequently a reaction occurs between the roasted limestone and the sulfur dioxide of the flue gas that has arisen in the combustion of the sulfur-containing fuel.
In the temperature that prevails in combustion chamber 2, carbon dioxide is expelled from the limestone and burnt lime remains behind, which because of the expulsion of the C02 is highly porous and thus has a high specffic surface area. In the subsequent gas-solid reaction of the burnt lime as an S02 sorbent with sulfur dioxide and oxygen, the latter are adsorbed on the surface and in the pores of the sorbent, and gypsum (CaS04) remains behind as a reaction product. Here the above-mentioned high speck surface area of the SOz sorbent is extremely important to its having a high absorbing capacity or reactivity.
Depending on the grain size of the limestone or SOZ sorbent used and its abrasion properties, either the sorbent-reaction product aggregate (lime~ypsum aggregate) remains in the combustion chamber until it is drawn off via the components or media 11 of the combustion-chamber ash removal system, or in the case of small particles the sorbent:reaction product aggregate leaves the combustion chamber 2 together with the flue-gas stream through the line 8, and after passing through a separator 8 (particles 3o separated out in the separator 6 are recycled to the combustion chamber 2 through the line 22) and through a steam generator or heat-recovery adjuncts 7, to which a large portion of the heat contained in the flue gas is released, it is subsequently separated out in the following flue-gas ~Iter 9.
Here the mixture of ash, reaction product or gypsum, and free, non-reacted limestone, which is drawn off via the combustion-chamber ash removal system, is usually designated as coarse or also as bottom ash, and the mixture that is separated out in filter 9 is designated as filter ash. In what follows, these designations are used for the above-mentioned mixtures.
The bottom ash and the fiilter ash thus arrive via two separate circuits - the bottom ash via the line 11 and the filter ash separated out from the flue gas in the flue-gas filter 9 via the line 8 (jointly with the flue gas) and Ifne 10 - from the combustion chamber 2 into 1o a mechanical mixing unit 12. In this mixing unit 12, wpter is fed to the above-mentioned mixture through a supply line 14 and via at least one feed nozzle 23 for their mixing together. In case it is necessary, also a number of feed nozzles 23 can be positioned in the mixing unit 12 via the product-passage segment. The mixing unit 12 can furthermore include upstream of the mixing unit 12, in the line 10 and/or 11. another feed nozzle 23.
By means of this nozzle, the mixing process can be influenced.
In accordance with the invention, the addition of water into the mixing unit 12 takes place at a reaction temperature of fi0 °C to 100 °C and at atmospheric pressure (about 1 bar). In such a case the still free, namely unreacted burnt lime reacts within the 20 framework of an exothermic reaction to form hydrated lime (hydration product), with the reaction rate and the temperature elevation depending on the free-lime concentration of the ash, the temperature of the added ash, as well as the temperature of the supplied water. Because of the lower density of hydrated lime compared to lime, the reaction causes the lime aggregate to swell, so that the gypsum blanket around the lime aggregate is broken up and the gypsum blanket becomes permeable, and thereby the porous surface of the lime aggregate regains at least partially its S02 sorbabilifiy.
Instead of water, also an aqueous, sodium..containing solution can be fed to the product mixture in the mixing unit 12. Due to the sodium contained in the solution, the surface 30 ~,nperature of the reaction product is lowered and on its surface a liquid phase forms, which improves the SOz sorbabllity of the reaction product in connection with the reaction of sodium with S02 (Na2S04).
The requisite reaction temperature in the mixing unit 12 can be regulated, with it being advantageous for the water or the aqueous, sodium-Containing solution fed to the mixing unit 12 through line 14 to be preheated in accordance with the required reaction temperature. This happens in a preheater 15 positioned in the line 14, which depending on the required reaction temperature in the mixing unit 12 is regulated by means of a heat regulating mechanism 16. It is furthermore possible to regulate the amount of water or aqueous, sodium-containing solution fed to the mixing unit 12 as a function of the residual moisture of the product to be carried ofF from the mixing unit 12. It is advantageous for this to take place by means of an appropriate driving of the regulating 1o valves 24 that lie upstream, on the side of the flow media. of the nozzle or nozzles 23 positioned at the mixing unit 12.
With respect to the product to be carried off from the mixing unit 12, in order to achieve the degree of hydration for this that is within a requisite range, the dwell time of the product introduced into the mixing unit 12 can also be regulated. This can be done, for example, by regulating the speed of transport in the mixing unit 12. It is expedient for the minimum dwell time to amount to one minute for the product that is to be mixed together and is to undergo a reaction within the mixing unit 12 and possibly also within the subsequent delivery lines.
A return line 19 makes it possible for a portion of the product carried off from the mixing unit 12 to be delivered again to the mixing unit 12 for a further or a renewed reaction and thus this makes it possible to influence the dwell time as well.
In addition to the reaction in the mixing unit 12 that preferably proceeds under atmospheric pressure and at a reaction temperature of 60 °C to 100 °C, the reaction within the mixing unit 12 can also be carried out under a higher pressure. For example, the reaction can be brought about under a pressure of 5 bars and a reaction temperature equal to or less than 151 °C, namely just below the boiling temperature associated with this pressure. The mixing unit 12 is Constructed to be pressure-tight to the extent adequate to the chosen pressure.
In one useful embodiment of the invention, plowshare or paddle mixers are used as the mixing unit 12 for mixing the product mixture together with the water or aqueous, sodium-containing solution. However, agitators can al$o be used, if appropriate.
In one embodiment of the invention, by feeding the coarse or bottom ash and the filter ash in two separate lines or circuits the proportion of the bottom ash or filter ash that is supplied each time can be varied. For example, eve quantitative shares of ground ash and one quantitative share of filter ash can be fed to the mixing unit 12.
This makes it possible to have an operating mode for the mixing unit 12 that can be coordinated wrth the quantities of bottom ash or filter ash arising from the combustion chamber 2.
Through the use of the procedure in accordance with the invention in a circulating fluidized-bed firing system its advantages become clear, since here we have a close regulation of the combustion-chamber temperature, a long dwell time for the solid particles that are to be burned and that are introduced as SOz sorbent, a high degree of gaslsolid particle intermixing, and as a result of this a low CaIS ratio.
Figure 2 shows a two~stage mixing unit 12, where in fihe first stage 12' a portion of the water or of an aqueous, sodium-containing solution required for the mixing is admixed with the ash, the gypsum aggregate, and the unreacted burnt lime, and in the second stage 12" subsequent to the first stage 12' the remaining portion of the water or of the aqueous sodium-containing solution is admixed in a regulated fashion as a function of the residual moisture of the product to be can-led off from the mixing unit 12.
It is advantageous for the ash, the gypsum, and the ~ hydration product carried off from the mixing unit 12 to exist in the form of a solid and to have a residual moisture of less than 10 %. Furthermore the product carried off from the mixing unit 12 can be fed to a drier 20, in which the product can be dried, for example, into a product capable of being stored and if appropriate can be stored in an intermediate store 21 and can be fed to the fluidized-bed combustion chamber 2 after a certain interval of time.
By means of the line 13 the ash, the gypsum (reaction product) and the hydrated lime (hydration product) is fed from the mixing unit 12 to the combustion chamber 2, in which there occurs. because of the temperatures prevailing there, a counter-reaction and a splitting off of water vapor from the hydrated lime. Due to the thermal stress, additional cracking develops in the lime aggregate (reaction product), whereby the specific surface area is increased further. Due to the breaking up of the gypsum blanket as well as through the reaction surface additionally created by the swelling of the lime core, the lime that has not yet undergone a reaction in the interior of the previously blanketed or partially blanketed lime grains, namely the SOz sorb~nt, becomes available again for a more extensive desulfurization reaction, The procedure in accordance with the invention is used especially efficiently when the concentration of non-reacted S02 sorbent in the combustion chamber 2 amounts to 5 to 40 percent by weight of the solid mixture carried off from the combustion chamber 2.
Before conducting the ash, the reaction product gypsum, and the unreacted lime into the mixing unit 12, this product can be sifted/sized and/or ground in units 17, 18 suitable for this purpose, in order to obtain the best possible conditions for the mixing process. In this connection it is expedient to use a sifter with a mesh width such that that particles larger than 300 microns are sifted out and are separated out of the circuit.
This has the advantage of preventing major erosions from occurring In the mixing unit 12 as well as the hydrating of stony materials.
The apparatus in accordance with the invention and the procedure in accordance with the invention allows either combustion-chamber bottom ash or fiilter ash or a mixture of the two to be fed to the mixing unit 12. In this connection, it is advantageous for the apportionment of the mixture of filter ash and bottom ash to be set by adjusting the grain sizes of added fuel and limestone.
In the procedure in accordance with the invention, there is a savings of 20 %
to 40 % of S42 sorbent, for example limestone, as compared to previously familiar procedures.
When compared to a customary sulfur retention of 95 % and a customary Ca/S
ratio (the ratio of SOZ sorbent to sulfur that is to be retained) of 2, in the procedure in accordance with the invention and with a sulfur retention of 95 % the result is a CaIS
ratio of 1.2. Thus this value already lies very close to the ideal stoichiometry of 1Ø With that, through the procedure in accordance with the invention considerably less sorbent is needed for the retention of the sulfur present in the flue gas.
This has a substantial effect on the operating costs of the system, especially when fuel having a high sulfur content is used.
The transporting of the product in the lines 10, 11, 13, or 19 can be done, for example, by either pneumatic or mechanical means.
The supply line 2S makes it possible to deliver more SOZ sorbent to the mixing unit 12.
Thereby, regardless of the concentration of unreacted SOz sorbent in the mixture delivered to the mixing unit 12, this mixture can be enriched by means of an externally supplied S02 sorbent. Likewise, by means of the line 26 the hydration product Ca(OM)z can be supplied to the line 13, which leads from the mixing unit 12 to the combustion chamber 2. By means of this measure, it is possible to influence the quantity of hydration medium, namely the medium that is reactivated again into an SOz sorbent in the combustion chamber 2.
In one advantageous embodiment of the invention, 50 to 500 % by weight of the solid mixture, relative to the solid mixture that customarily leaves the combustion chamber 2 of the fluidized-bed firing system 1, is supplied to the mixing unit 2 for the hydration process and is subsequently supplied again to the combustion chamber 2.
In the procedure according to the invention, usually a continuous process is involved, but in special applications it is also possible to have a discontinuous mixing together and enrichment of the ash mixture with water in the mixing unit. In this case the hydration product is either discontinuously supplied to the combustion chamber 2 or the hydration product stored in an intermediate store 21 is continuously [sic]
drawn off from this intermediate storage 21 and supplied to the combustion chamber 2 List of reference symbols 1 Fluidized-bed firing system 2 Fluidized-bed combustion chamber 3 Feeding of coal into the tluidized-bed combustion chamber 4 Feeding of S02 sorbent into fluidized-bed combustion chamber Feeding of fluidizing agent or air into fluidized-bed combustion chamber 8 Separatar or cyclone 7 Steam generator or heat recovery adjuncts 8 Line between fluidized-bed combustion chamber and flue-gas filter 9 Flue-gas filter 10 Line between flue-gas filter and mixing unit 11 Bottom-ash line between iluidized-bed combustion chamber and mixing unit 12 Mixing unit 13 Line between mixing unit and fluidized-bed combustion chamber 14 Supply line of water or an aqueous sodium-containing solution to the mixing unit Preheater of the water or of the aqueous sodium-containing solution 16 Heat regulating mechanism of the preheater 17 Sifter or sizer 18 Grinding unit 19 Return line to the mixing unit 20 Drying unit 21 Intermediate store 22 Return line between separator and fluidized-bed combustion chamber 23 Nozzle 24 Regulating valve Supply line for sorbent (external) 26 Supply line for hydration product Ca(OH)2 (external)
In connection w~h the above-named desulfurization procedure for the flue gas in a circulating fluidized-bed firing system, it has proved to be a shortcoming that the limestone or the SOz sorbent does not react completely with the sulfur dioxide, since frequently a blanket of gypsum that is almost gas-impermeable forms around the lime aggregate or sorbent aggregate, and also the pores of the lime or sorbent are clogged up by gypsum as the reaction product. The physical/chemical basis for this is the larger molar volume of SOz diffusing into the lime aggregate or sorbent aggregate compared to the expelled C02.
Especially in the interior of the grains of the sorb~nt-reaction product (lime-gypsum grains), a core of unreacted sorbent remains that is no longer available for the reaction, since the reaction partners of sulfur dioxide and oxygen can no longer penetrate down into this core.
The concentrations of unreacted free sorbent in the ash mixture can be as much as 40%, both in the case of filter ash and also with coarse ash, relative to the total ash mixture that is to be carried off. Also, within the framework of the further use of the ash mixture in the cement industry or in roadbuilding, it is desirable to have a lower concentration of free, namely unreacted, sorbent or limestone, to below 3 to 5%.
In current fluidized-bed firing systems, various techniques are being used at present in order to increase the degree of utilization of sorbents or limestone for the purposes of reducing the sulfur dioxide.
3o Thus, for example, in circulating fluidized-bed firing systems the ash accumulating in the flue-gas filter, which may still contain high proportions of unreacted sorbent, is returned again directly into the combustion chamber.
-t~,-One drawback of this ash recycling is that the utilization of the still free sorbent in the ash may be of only limited benefit, because the additional dwell time in the fluidized-bed combustion chamber is small and because the reactivity of this ash or of the free sorbent contained in the ash is considerably reduced compared to the original sorbent.
Moreover, a recycling of the bed ash drawn off from the combustion chamber is also customary. To this end, the bed ash is subjected in part to a treatment (sifting of the good grain fraction or grinding up of the bed ash) that is aimed at Increasing its degree of reactivity. But this method as well has the drawback that its effect in reducing the to requisite consumption of sorbent is very limited, since it does not eliminate the cause of the incomplete reaction, the above-mentioned gypsum blanket around the sorbent or lime aggregate.
By way of the document US Patent 4,312,280, Shearer et al., a system has furthermore been disclosed in which ash from stationary fluidized~bed firing systems is brought into contact with water or steam and is returned to the combustion chamber of the fluidized-bed system. The mixing of the ash with steam and water takes place in a complex ffuidized-bed reactor at relatively high temperatures, No reference is made to more extensive process-technology details about operating temperatures of this fluidized-bed 20 reactor or to what water admixtures are used for doing this work. This disclosed system has on the whole a high technological complexity and therefore has not gained much acceptance on the market, partly also because the potential market for stationary fluidized-bed firing systems is limited to small system sizes, and compared to circulating fluidized-bed firing systems they have the disadvantage of having smaller particle dwell times as well as an inhomogeneous temperature distribution.
The object of the invention, then, is to devise a procedure as well as an apparatus for the cleaning of flue gases containing sulfur dioxide that come from circulating fluidized-bed firing systems, in which the above-mentioned disadvantages are avoided.
3o Furthermore the efficiency or the degree of utilization of the sorbent used is to be increased and thereby the quantity of sorbent needed for the sorption of the sulfur dioxide is to be reduced.
The object specified above is achieved through a~ procedure having the features of Patent Claim 1 and through an apparatus having the features of Claim 26.
ether advantageous embodiments of the invention can be found in the dependent claims.
Through the achievement in accordance with the invention, a procedure as well as a mechanism are provided that have the following advantages.
1o By the mixing together of the ash mixture coming from the combustion chamber of the circulating fluidized-bed firing system (ash, reaction product, and unreacted SOz sorbent) with water or with an aqueous, sodium-containing solution in a mechanical mixing unit at a reaction temperature of 60° to 100 °C and at atmospheric pressure, and by recycling this into the combustion chamber of the circulating fluidized-bed firing system, the degree of utilization of the sorbent is considerably increased compared to a simple recycling of filter ash or bottom ash.
This effect arises from the fact that due to the mixing together of the ash with water or with an aqueous, sodium.containing solution in a manner corresponding the procedure 2o in accordance with the invention, the still unreaGted sorbent is first caused to react at a reaction temperature of 60 to 100 ° C with water or with an aqueous, sodium-containing solution to form a hydration product. This reaction is exothermic. The elevation in temperature as well as the reaction rate depend on the concentration of the unreacted sorbent in the ash, the temperature of the supplied ash, and the temperature of the supplied water as well as on the parallel reaction with the Si~2 and AI203 contained in the ash mixture. Since the hydration product has a lower density than the sorbent, this reaction causes the sorbent grain to "swell,u so that the adsorbate blanket (or gypsum blanket) around the sorbent aggregate gets broken up.
30 In this connection it has turned out that the degree of conversion of the S02 sorbent into hydration product as well as the reaction rate increase considerably with increasing ~6-l0 temperature, and within the range of about 60°C to about 100°C
and at atmospheric pressure they reach a value that is optimally desirable in such an operation.
If the temperature is too low, then the formation of hydration product proceeds only very slowly and is not complete within the dwell time that is available in the mixing unit. If the temperature is too high, then a "boiling" of the ash mixture takes place, with the consequence that due to the excess reaction enthalpy, additional water is evaporated, which is then no longer available for the hydration reaction. The consequences of reaction temperatures that are too high are an increased water consumption and problems associated with the additional vapor formation.
The reaction temperature in the mixing unit is regulated in an expedient manner in order to achieve an optimal conversion of unreacted SOz sorbent into a hydration product.
This enables the dosed-out quantity of water or of sodium solution to be adjusted in accordance with the concentration of the unreacted SOz sorbent in the ash.
Thus via a thermodynamic balance the optimal quantity of water to be added can be determined.
In another advantageous embodiment of the invention, the desired reaction temperature in the mixing unit is achieved by adding preheated water into the mixing unit, with the water temperature being regulated by a preheater located in front of the mixing unit. By 20 the preheating of the water, the reaction temperature in the mixing unit can be set within the temperature range that is favorable to the course of the reaction, independently of the concentration of the non-reacted sorbent in the ash. Instead of water, an aqueous, sodium-containing solution can also be supplied.
Through the regulation of the quantity of water or of aqueous sodium-containing solution that is fed to the mixing unit, the mixture product can be carried off from the mixing unit in an advantageous manner as a function of the residual moisture. In this way, the product that is to be carried off can be produced in a desirable fashion.
30 In one advantageous embodiment of the invention, the dwell time of the product introduced into the mixing unit is regulated as a function of the degree of hydration of the product to be carried off. It is especially advantageous when the minimum dwell time _7_ in the mixing unit and/or the subsequent delivery lines amounts to one minute, in order to ensure that the hydration of the introduced product occurs in the desired manner.
It is expedient for the product drawn off from the mixing unit to exist in the form of a solid and to have a residual moisture less than 10 %. This keeps the product drawn off from the mixing unit from being sludgy and thus difficult to transport due to a too-high excess of water.
It may be advantageous to construct the mixing unit in two stages, where in the first stage a portion of the water or of aqueous, sodium~containing solution required for the mixing is admixed with the ash, the reaction product, and the unreacted sorbent, and in the second stage the remaining portion of water or aqueous, sodium-containing solution is admixed in a regulated way as a function of the residual moisture of the product to be carried off from the mixing unit. In this way, in the first mixer attention can be directed toward the mixing process and in the second mixer it can be directed toward the requisite dwell time as well as to the temperature requirements.
By feeding the product carried off from the mixing unit into a drier, this product can be stored in an advantageous way after it has been dried. Thereby, in another advantageous embodiment of the invention the possibility is provided of putting this product into intermediate storage and feeding it to the combustion chamber after a certain interval of time.
It is further advantageous to return at least a portion of the product carried off from the mixing unit back into the mixing unit. With this measure, the dwell time for the reaction within the mixing unit can likewise be affected.
By feeding the ash mixture into a siftinglsizing unit before introducing it into the mixing unit, grain sizes of undesirable magnitude, for example larger than 300 microns, can be sifted out. This makes it possible to largely avoid erosions within the mixing unit as well as in the delivery lines. The same effect can be achieved by directing relatively large and undesirable grain sizes into a grinding unit and subsequently passing these on to the mixing unit.
-$_ In one advantageous embodiment ofi the invention, the ash, the reaction product, and the unreacted S02 sorbent drawn off from the circulating fluidized-bed firing system and fed to the mixing unit is supplied to the mixing unit from the flue-gas filter as filter ash and from the combustion chamber as bottom ash over separate supply fines in each case, in which connection either filter ash or bottom ash or an adjustable mixture of the two can be fed to the mixing unit. This makes it possible to respond to any operational situation of the fluidized-bed firing system.
Furthermore the apportionment of the mixture of filter ash and bottom ash can be set by 1o adjusting the grain sizes of added fuel and SOx sofient.
tt is advantageous to introduce the water or an aqueous, sodium-containing solution Into the mixing unit by means of at least one nozzle. As the situation demands, the requisite nozzles can be installed within the mixing unit at any points desired.
It is advantageous to use limestone as the S02 sorbent. This is relatively inexpensive and has worked well as an S02 sorbent. Furthermore it may also be advantageous to use dolomite as the SOa sorbent.
20 In another advantageous embodiment of the invention, 50% to 500% of solid mixture, relative to the solid mixture normally leaving the combustion chamber of the fluidized-bed firing system, is fed to the mixing unit for the hydration process and subsequently fed again to the combustion chamber. Thereby an optimal conservation of the needed S02 sorbent is achieved.
In one special case of the procedure in accordance With the invention, a portion of the S02 sorbent is delivered directly to the mixing unit. In this way, regardless of the amount of ash in circulation as well as unreacted SO2 sorbent, the requisite amount of SOz sorbent can be influenced from the outside, In another special case of the procedure in accordance with the invention, a hydration product or Ca(OH)2 can be delivered into the line between the mixing unit and combustion chamber, with it being advantageous for this product to be delivered upstream of an intermediate storage, as seen in the direction of flow of the product. In this way, this quantity of product can likewise be influenced from the outside.
If an aqueous, sodium-containing solution is used, it is advantageous for the solution to contain sodium in the form of ions on an order of magnitude of up to 3 % by weight, relative to the unreacted SOZ sorbent.
Furthermore it may be advantageous for the mixing unit to include an additional feed-in of water or of an aqueous, sodium-containing solution, by means of which water or an aqueous, sodium-containing solution is added to the ash, the reaction product, and the unreacted SOa sorbent upstream of the mixing unit. Thereby the reaction in the mixing unit can be influenced beforehand if appropriate.
Through the apparatus in accordance with the invention, the procedure in accordance with the invention can be carried out in an efficient, inexpensive, and resource-sparing manner.
Below, exemplary embodiments of the invention are explained in more detail on the basis of the drawings and the spec~cation.
These show:
Figure 1 a schematically represented apparatus for the execution of the procedure in accordance with the invention.
Figure 2 an embodiment that is alternative to Figure 1.
Figure 1 shows in a schematic way the apparatus in accordance with the invention for the cleaning of flue gases containing sulfur dioxide that come from fluidized-bed firing systems. By means of the apparatus shown, the procedure in accordance with the invention can also be carried out.
-10~
The circulating fluidized-bed firing system 1 in accordance with the invention has a fluidized-bed combustion chamber 2, in which in a familial manner coal or other particulate and carbon-containing fuels are burned by the addition of air - as a fluidizing medium via the line 5 as well as secondary air ~- at 700 to 954 °C in the fluidized bed.
For the desulfurization of the sulfur-containing fuel, in~ addition to the fuel (supply line 3) also an SOZ sorbent is delivered to the combustion chamber 2 via the line 4.
As a rule, limestone in the form of particles or fine grains, namely CaCOs, is delivered to the combustion chamber 2 as the SO2 sorbent. In place of Ilmestone, also dolomite as well as burnt lime (Ca0) or Ca(OH)2 can be used as an S02 sorbent.
When limestone is used as the S02 sorbent, first of all the process of roasting the limestone into burnt Ilme (Ca0) takes place in the combustion chamber 2 and subsequently a reaction occurs between the roasted limestone and the sulfur dioxide of the flue gas that has arisen in the combustion of the sulfur-containing fuel.
In the temperature that prevails in combustion chamber 2, carbon dioxide is expelled from the limestone and burnt lime remains behind, which because of the expulsion of the C02 is highly porous and thus has a high specffic surface area. In the subsequent gas-solid reaction of the burnt lime as an S02 sorbent with sulfur dioxide and oxygen, the latter are adsorbed on the surface and in the pores of the sorbent, and gypsum (CaS04) remains behind as a reaction product. Here the above-mentioned high speck surface area of the SOz sorbent is extremely important to its having a high absorbing capacity or reactivity.
Depending on the grain size of the limestone or SOZ sorbent used and its abrasion properties, either the sorbent-reaction product aggregate (lime~ypsum aggregate) remains in the combustion chamber until it is drawn off via the components or media 11 of the combustion-chamber ash removal system, or in the case of small particles the sorbent:reaction product aggregate leaves the combustion chamber 2 together with the flue-gas stream through the line 8, and after passing through a separator 8 (particles 3o separated out in the separator 6 are recycled to the combustion chamber 2 through the line 22) and through a steam generator or heat-recovery adjuncts 7, to which a large portion of the heat contained in the flue gas is released, it is subsequently separated out in the following flue-gas ~Iter 9.
Here the mixture of ash, reaction product or gypsum, and free, non-reacted limestone, which is drawn off via the combustion-chamber ash removal system, is usually designated as coarse or also as bottom ash, and the mixture that is separated out in filter 9 is designated as filter ash. In what follows, these designations are used for the above-mentioned mixtures.
The bottom ash and the fiilter ash thus arrive via two separate circuits - the bottom ash via the line 11 and the filter ash separated out from the flue gas in the flue-gas filter 9 via the line 8 (jointly with the flue gas) and Ifne 10 - from the combustion chamber 2 into 1o a mechanical mixing unit 12. In this mixing unit 12, wpter is fed to the above-mentioned mixture through a supply line 14 and via at least one feed nozzle 23 for their mixing together. In case it is necessary, also a number of feed nozzles 23 can be positioned in the mixing unit 12 via the product-passage segment. The mixing unit 12 can furthermore include upstream of the mixing unit 12, in the line 10 and/or 11. another feed nozzle 23.
By means of this nozzle, the mixing process can be influenced.
In accordance with the invention, the addition of water into the mixing unit 12 takes place at a reaction temperature of fi0 °C to 100 °C and at atmospheric pressure (about 1 bar). In such a case the still free, namely unreacted burnt lime reacts within the 20 framework of an exothermic reaction to form hydrated lime (hydration product), with the reaction rate and the temperature elevation depending on the free-lime concentration of the ash, the temperature of the added ash, as well as the temperature of the supplied water. Because of the lower density of hydrated lime compared to lime, the reaction causes the lime aggregate to swell, so that the gypsum blanket around the lime aggregate is broken up and the gypsum blanket becomes permeable, and thereby the porous surface of the lime aggregate regains at least partially its S02 sorbabilifiy.
Instead of water, also an aqueous, sodium..containing solution can be fed to the product mixture in the mixing unit 12. Due to the sodium contained in the solution, the surface 30 ~,nperature of the reaction product is lowered and on its surface a liquid phase forms, which improves the SOz sorbabllity of the reaction product in connection with the reaction of sodium with S02 (Na2S04).
The requisite reaction temperature in the mixing unit 12 can be regulated, with it being advantageous for the water or the aqueous, sodium-Containing solution fed to the mixing unit 12 through line 14 to be preheated in accordance with the required reaction temperature. This happens in a preheater 15 positioned in the line 14, which depending on the required reaction temperature in the mixing unit 12 is regulated by means of a heat regulating mechanism 16. It is furthermore possible to regulate the amount of water or aqueous, sodium-containing solution fed to the mixing unit 12 as a function of the residual moisture of the product to be carried ofF from the mixing unit 12. It is advantageous for this to take place by means of an appropriate driving of the regulating 1o valves 24 that lie upstream, on the side of the flow media. of the nozzle or nozzles 23 positioned at the mixing unit 12.
With respect to the product to be carried off from the mixing unit 12, in order to achieve the degree of hydration for this that is within a requisite range, the dwell time of the product introduced into the mixing unit 12 can also be regulated. This can be done, for example, by regulating the speed of transport in the mixing unit 12. It is expedient for the minimum dwell time to amount to one minute for the product that is to be mixed together and is to undergo a reaction within the mixing unit 12 and possibly also within the subsequent delivery lines.
A return line 19 makes it possible for a portion of the product carried off from the mixing unit 12 to be delivered again to the mixing unit 12 for a further or a renewed reaction and thus this makes it possible to influence the dwell time as well.
In addition to the reaction in the mixing unit 12 that preferably proceeds under atmospheric pressure and at a reaction temperature of 60 °C to 100 °C, the reaction within the mixing unit 12 can also be carried out under a higher pressure. For example, the reaction can be brought about under a pressure of 5 bars and a reaction temperature equal to or less than 151 °C, namely just below the boiling temperature associated with this pressure. The mixing unit 12 is Constructed to be pressure-tight to the extent adequate to the chosen pressure.
In one useful embodiment of the invention, plowshare or paddle mixers are used as the mixing unit 12 for mixing the product mixture together with the water or aqueous, sodium-containing solution. However, agitators can al$o be used, if appropriate.
In one embodiment of the invention, by feeding the coarse or bottom ash and the filter ash in two separate lines or circuits the proportion of the bottom ash or filter ash that is supplied each time can be varied. For example, eve quantitative shares of ground ash and one quantitative share of filter ash can be fed to the mixing unit 12.
This makes it possible to have an operating mode for the mixing unit 12 that can be coordinated wrth the quantities of bottom ash or filter ash arising from the combustion chamber 2.
Through the use of the procedure in accordance with the invention in a circulating fluidized-bed firing system its advantages become clear, since here we have a close regulation of the combustion-chamber temperature, a long dwell time for the solid particles that are to be burned and that are introduced as SOz sorbent, a high degree of gaslsolid particle intermixing, and as a result of this a low CaIS ratio.
Figure 2 shows a two~stage mixing unit 12, where in fihe first stage 12' a portion of the water or of an aqueous, sodium-containing solution required for the mixing is admixed with the ash, the gypsum aggregate, and the unreacted burnt lime, and in the second stage 12" subsequent to the first stage 12' the remaining portion of the water or of the aqueous sodium-containing solution is admixed in a regulated fashion as a function of the residual moisture of the product to be can-led off from the mixing unit 12.
It is advantageous for the ash, the gypsum, and the ~ hydration product carried off from the mixing unit 12 to exist in the form of a solid and to have a residual moisture of less than 10 %. Furthermore the product carried off from the mixing unit 12 can be fed to a drier 20, in which the product can be dried, for example, into a product capable of being stored and if appropriate can be stored in an intermediate store 21 and can be fed to the fluidized-bed combustion chamber 2 after a certain interval of time.
By means of the line 13 the ash, the gypsum (reaction product) and the hydrated lime (hydration product) is fed from the mixing unit 12 to the combustion chamber 2, in which there occurs. because of the temperatures prevailing there, a counter-reaction and a splitting off of water vapor from the hydrated lime. Due to the thermal stress, additional cracking develops in the lime aggregate (reaction product), whereby the specific surface area is increased further. Due to the breaking up of the gypsum blanket as well as through the reaction surface additionally created by the swelling of the lime core, the lime that has not yet undergone a reaction in the interior of the previously blanketed or partially blanketed lime grains, namely the SOz sorb~nt, becomes available again for a more extensive desulfurization reaction, The procedure in accordance with the invention is used especially efficiently when the concentration of non-reacted S02 sorbent in the combustion chamber 2 amounts to 5 to 40 percent by weight of the solid mixture carried off from the combustion chamber 2.
Before conducting the ash, the reaction product gypsum, and the unreacted lime into the mixing unit 12, this product can be sifted/sized and/or ground in units 17, 18 suitable for this purpose, in order to obtain the best possible conditions for the mixing process. In this connection it is expedient to use a sifter with a mesh width such that that particles larger than 300 microns are sifted out and are separated out of the circuit.
This has the advantage of preventing major erosions from occurring In the mixing unit 12 as well as the hydrating of stony materials.
The apparatus in accordance with the invention and the procedure in accordance with the invention allows either combustion-chamber bottom ash or fiilter ash or a mixture of the two to be fed to the mixing unit 12. In this connection, it is advantageous for the apportionment of the mixture of filter ash and bottom ash to be set by adjusting the grain sizes of added fuel and limestone.
In the procedure in accordance with the invention, there is a savings of 20 %
to 40 % of S42 sorbent, for example limestone, as compared to previously familiar procedures.
When compared to a customary sulfur retention of 95 % and a customary Ca/S
ratio (the ratio of SOZ sorbent to sulfur that is to be retained) of 2, in the procedure in accordance with the invention and with a sulfur retention of 95 % the result is a CaIS
ratio of 1.2. Thus this value already lies very close to the ideal stoichiometry of 1Ø With that, through the procedure in accordance with the invention considerably less sorbent is needed for the retention of the sulfur present in the flue gas.
This has a substantial effect on the operating costs of the system, especially when fuel having a high sulfur content is used.
The transporting of the product in the lines 10, 11, 13, or 19 can be done, for example, by either pneumatic or mechanical means.
The supply line 2S makes it possible to deliver more SOZ sorbent to the mixing unit 12.
Thereby, regardless of the concentration of unreacted SOz sorbent in the mixture delivered to the mixing unit 12, this mixture can be enriched by means of an externally supplied S02 sorbent. Likewise, by means of the line 26 the hydration product Ca(OM)z can be supplied to the line 13, which leads from the mixing unit 12 to the combustion chamber 2. By means of this measure, it is possible to influence the quantity of hydration medium, namely the medium that is reactivated again into an SOz sorbent in the combustion chamber 2.
In one advantageous embodiment of the invention, 50 to 500 % by weight of the solid mixture, relative to the solid mixture that customarily leaves the combustion chamber 2 of the fluidized-bed firing system 1, is supplied to the mixing unit 2 for the hydration process and is subsequently supplied again to the combustion chamber 2.
In the procedure according to the invention, usually a continuous process is involved, but in special applications it is also possible to have a discontinuous mixing together and enrichment of the ash mixture with water in the mixing unit. In this case the hydration product is either discontinuously supplied to the combustion chamber 2 or the hydration product stored in an intermediate store 21 is continuously [sic]
drawn off from this intermediate storage 21 and supplied to the combustion chamber 2 List of reference symbols 1 Fluidized-bed firing system 2 Fluidized-bed combustion chamber 3 Feeding of coal into the tluidized-bed combustion chamber 4 Feeding of S02 sorbent into fluidized-bed combustion chamber Feeding of fluidizing agent or air into fluidized-bed combustion chamber 8 Separatar or cyclone 7 Steam generator or heat recovery adjuncts 8 Line between fluidized-bed combustion chamber and flue-gas filter 9 Flue-gas filter 10 Line between flue-gas filter and mixing unit 11 Bottom-ash line between iluidized-bed combustion chamber and mixing unit 12 Mixing unit 13 Line between mixing unit and fluidized-bed combustion chamber 14 Supply line of water or an aqueous sodium-containing solution to the mixing unit Preheater of the water or of the aqueous sodium-containing solution 16 Heat regulating mechanism of the preheater 17 Sifter or sizer 18 Grinding unit 19 Return line to the mixing unit 20 Drying unit 21 Intermediate store 22 Return line between separator and fluidized-bed combustion chamber 23 Nozzle 24 Regulating valve Supply line for sorbent (external) 26 Supply line for hydration product Ca(OH)2 (external)
Claims (25)
1. Procedure for the cleaning of flue gases containing sulfur dioxide that come from a circulating fluidized-bed firing system, wherein within the combustion chamber of the fluidized-bed firing system sulfur-containing coal is burned by the addition of air at a temperature of 700 to 950 °C in a fluidized bed, and therein flue gas containing sulfur dioxide is formed and a particulate SO2 sorbent is delivered to the combustion chamber, a portion of the ash produced in the combustion and a portion of the reaction product produced in the reaction of the SO2 sorbent with the sulfur dioxide as well as a portion of the unreacted SO2 sorbent is carried off from the combustion chamber and is fed to a mixing unit, in the mixing unit, water or an aqueous sodium-containing solution is fed to the ash, the reaction product, and the unreacted SO2 sorbent and is mixed together with this, and therein the unreacted SO2 sorbent is converted into a hydration product at a reaction temperature of 60°C to 100° and at atmospheric pressure, the ash together with the reaction product and hydration product from the mixing unit is returned into the combustion chamber of the fluidized-bed firing system, and the hydration product is reactivated into an SO2 sorbent at a combustion-chamber temperature of 700° to 950° C.
2. Procedure based on Claim 1, characterized such that the mixing unit includes an additional supply of water or an aqueous sodium-containing solution, by means of which upstream of the mixing unit water or an aqueous sodium-containing solution is fed to the ash, the reaction product, and the unreacted SO2 sorbent.
3. Procedure based on Claim 1 or 2, characterized such that the reaction temperature of the mixing unit is regulated.
4. Procedure based on Claim 3, characterized such that for the regulation of the reaction temperature the water or the aqueous sodium-containing solution fed to the mixing unit is preheated so as to be adequate to the required reaction temperature.
5. Procedure based on one of the preceding claims, characterized such that the water or aqueous sodium-containing solution fed to the mixing unit is regulated as a function of the residual moisture of the product to be carried off from the mixing unit.
6. Procedure based on one of the preceding claims, characterized such that the dwell time of the products introduced into the mixing unit is regulated as a function of the degree of hydration of the product to be carried off.
7. Procedure based on one of the preceding claims, characterized such that the minimum dwell time of the products introduced into the mixing unit amounts to minute in the mixing unit and/or the subsequent delivery lines.
8. Procedure based on one of the preceding claims, characterized such that the mixing unit is constructed in two stages, wherein in the first stage a portion of the water or aqueous sodium-containing solution required for the mixing is admixed with the ash, the reaction product, and the unreacted SO2 sorbent, and in the second stage the remaining portion of the water or aqueous sodium-containing solution is admixed in a regulated fashion as a function of the residual moisture of the product to be carried off from the mixing unit.
9. Procedure based on one of the preceding claims, characterized such that the ash, the reaction product, and the hydration product carried off from the mixing unit exist in a solid form and have a residual moisture of less than 10 %.
10. Procedure based on one of the preceding claims. characterized such that the ash, the reaction product, and the hydration product carried off from the mixing unit are fed to a drier and are dried into a product capable of being stored.
11. Procedure based on one of the preceding claims, characterized such that the ash, the reaction product, and the hydration product carried off from the mixing unit are stored in an intermediate store and are fed to the fluidized-bed combustion chamber after a certain time interval.
12. Procedure based on one of the preceding claims, characterized such that at least a portion of the ash, the reaction product, and the hydration product carried off from the mixing unit is fed again to the mixing unit.
13. Procedure based on one of the preceding claims, characterized such that the ash, the reaction product, and the unreacted SO2 sorbent is sifted and/or sized before feeding into the mixing unit.
14. Procedure based on one of the preceding claims, characterized such that the ash, the reaction product, and the unreacted SO2 sorbent is ground before feeding into the mixing unit.
15. Procedure based on one of the preceding claims, characterized such that the mixture of solids consisting of ash, reaction product, and unreacted SO2 sorbent drawn off from the circulating fluidized-bed firing system and fed to the mixture unit is fed to such mixing unit from the flue-gas filter as filter ash and from the combustion chamber as bottom or coarse ash via separate supply lines each time, wherein either the filter ash or the bottom ash or an adjustable mixture of the two is fed to the mixing unit.
16. Procedure based on one of the preceding claims, characterized such that the apportionment of the mixture of filter ash and bottom ash is set by adjusting the grain sizes of added fuel and SO2 sorbent.
17. Procedure based on one of the preceding claims, characterized such that the water or an aqueous sodium-containing solution is introduced into the mixing unit by way of at least one nozzle.
18. Procedure based on one of the preceding claims, characterized such that the concentration of non-reacted SO2 sorbent in the combustion chamber amounts to to 40 % by weight of the mixture of solids carried off from the combustion chamber.
19. Procedure based on one of the preceding claims, characterized such that limestone is used as the SO2 sorbent.
20. Procedure based on one of the claims 1 to 18, characterized such that dolomite is used as the SO2 sorbent.
21. Procedure based on one of the preceding claims, characterized such that the aqueous sodium-containing solution contains sodium in the form of ions on an order of magnitude of up to 3 % by weight, relative to the non-reacted SO2 sorbent.
22. Procedure based on one of the preceding claims, characterized such that at least a portion of the SO2 sorbent is delivered directly to the mixing unit.
23. Procedure based on one of the preceding claims, characterized such that Ca(OH)2 is delivered into the line between the mixing unit and combustion chamber.
24. Procedure based on Claim 23, characterized such that Ca(OH)2 is delivered upstream of an intermediate store, seen in the direction of flow of the product flowing in the line between the mixing unit and the combustion chamber.
25. Procedure based on one of the preceding claims, characterized such that 50 to 500 % by weight of the mixture of solids, relative to the mixture of solids normally leaving the combustion chamber of the fluidized-bed firing system, is fed to the mixing unit for hydration and subsequently fed again to the combustion chamber, 28. Apparatus for the cleaning of flue gases containing sulfur dioxide that come from a circulating fluidized-bed firing system, including a combustion chamber (2), in which sulfur-containing coal is burned by the addition of air at a temperature of 700 to 950 °C in a fluidized bed, and therein flue gas containing sulfur dioxide is produced and a particulate SO2 sorbent is delivered to the combustion chamber (2), a means for carrying off a portion of the ash produced in the combustion and a portion of the reaction product produced in the reaction of the SO2 sorbent with the sulfur dioxide as well as a portion of the unreacted SO2 sorbent from the combustion chamber (2), and a feeding of this into a mechanical mixing unit (12), a mechanical mixing unit (12) for the mixing of fed water or an aqueous sodium-containing solution together with the supplied ash, the reaction product, and the unreacted SO2 sorbent at a reaction temperature of 60° to 100°
and at atmospheric pressure and for the conversion of the unreacted SO2 sorbent into a hydration product, a means for carrying the ash together with the reaction product and the hydration product off from the mixing unit (12) and for returning this into the combustion chamber (2), wherein in the combustion chamber (2) the hydration product can be reactivated at a combustion temperature of 700° to 950° C into an SO2 sorbent.
27. Apparatus based on Claim 28, characterized such that the mixing unit (12) includes a supply line (14) for water or for an aqueous sodium-containing solution, by means of which upstream of the mixing unit (12) water or an aqueous sodium-containing solution can be fed to the ash, the reaction product, and the unreacted SO2 sorbent.
28. Apparatus based on Claim 26 or 27, characterized such that the reaction temperature of the mixing unit (12) can be regulated.
29, Apparatus based on one of the claims 28 to 28, characterized such that the mixing unit (12) is constructed in two stages, wherein in the first stage (12') a portion of the water or an aqueous sodium-containing solution required for the intermixing can be admixed with the ash, the reaction product, and the unreacted SO2 sorbent, and in the second stage (12") the remaining portion of the water or an aqueous sodium-containing solution can be admixed in a regulated way as a function of the residual moisture of the product that is to be carried off from the mixing unit (12', 12").
30. Apparatus based on one of the claims 26 to 29, characterized such that the dwell time of the product to be mixed and subjected to a reaction in the mixing unit (12) can be regulated as a function of the degree of hydration of this product.
31. Apparatus based on one of the claims 26 to 30, characterized such that upstream of the mixing unit (12), seen in the direction of flow or of delivery of the unreacted SO2 sorbent, a sifting or sizing unit (17) is provided for the sifting or sizing of the ash, the reaction product, and the unreacted SO2 sorbent.
32. Apparatus based on one of the claims 26 to 31, characterized such that upstream of the mixing unit (12), seen in the direction of flow or of delivery of the unreacted SO2 sorbent, a grinding unit (18) is provided for grinding the ash, the reaction product, and the unreacted SO2 sorbent.
33. Apparatus based on one of the claims 26 to 32, characterized such that downstream of the mixing unit (12), seen in the direction of flow or of delivery of the hydration product, a drying unit (20) is provided for the drying of the ash, the reaction product, and the hydration product.
34. Apparatus based on one of the claims 26 to 33, characterized such that between the mixing unit (12) and the combustion chamber (2), seen in the direction of flow or of delivery of the hydration product, an intermediate store (21) is positioned for the storage of the ash, the reaction product, and the hydration product.
35. Apparatus based on one of the claims 26 to 34, characterized such that the mixing unit (12) is constructed with at least one nozzle (23) for the supplying of water or an aqueous sodium-containing solution.
36. Apparatus based on one of the claims 26 to 35, characterized such that the mixing unit (12) is constructed as a plowshare or paddle mixer or an agitator.
and at atmospheric pressure and for the conversion of the unreacted SO2 sorbent into a hydration product, a means for carrying the ash together with the reaction product and the hydration product off from the mixing unit (12) and for returning this into the combustion chamber (2), wherein in the combustion chamber (2) the hydration product can be reactivated at a combustion temperature of 700° to 950° C into an SO2 sorbent.
27. Apparatus based on Claim 28, characterized such that the mixing unit (12) includes a supply line (14) for water or for an aqueous sodium-containing solution, by means of which upstream of the mixing unit (12) water or an aqueous sodium-containing solution can be fed to the ash, the reaction product, and the unreacted SO2 sorbent.
28. Apparatus based on Claim 26 or 27, characterized such that the reaction temperature of the mixing unit (12) can be regulated.
29, Apparatus based on one of the claims 28 to 28, characterized such that the mixing unit (12) is constructed in two stages, wherein in the first stage (12') a portion of the water or an aqueous sodium-containing solution required for the intermixing can be admixed with the ash, the reaction product, and the unreacted SO2 sorbent, and in the second stage (12") the remaining portion of the water or an aqueous sodium-containing solution can be admixed in a regulated way as a function of the residual moisture of the product that is to be carried off from the mixing unit (12', 12").
30. Apparatus based on one of the claims 26 to 29, characterized such that the dwell time of the product to be mixed and subjected to a reaction in the mixing unit (12) can be regulated as a function of the degree of hydration of this product.
31. Apparatus based on one of the claims 26 to 30, characterized such that upstream of the mixing unit (12), seen in the direction of flow or of delivery of the unreacted SO2 sorbent, a sifting or sizing unit (17) is provided for the sifting or sizing of the ash, the reaction product, and the unreacted SO2 sorbent.
32. Apparatus based on one of the claims 26 to 31, characterized such that upstream of the mixing unit (12), seen in the direction of flow or of delivery of the unreacted SO2 sorbent, a grinding unit (18) is provided for grinding the ash, the reaction product, and the unreacted SO2 sorbent.
33. Apparatus based on one of the claims 26 to 32, characterized such that downstream of the mixing unit (12), seen in the direction of flow or of delivery of the hydration product, a drying unit (20) is provided for the drying of the ash, the reaction product, and the hydration product.
34. Apparatus based on one of the claims 26 to 33, characterized such that between the mixing unit (12) and the combustion chamber (2), seen in the direction of flow or of delivery of the hydration product, an intermediate store (21) is positioned for the storage of the ash, the reaction product, and the hydration product.
35. Apparatus based on one of the claims 26 to 34, characterized such that the mixing unit (12) is constructed with at least one nozzle (23) for the supplying of water or an aqueous sodium-containing solution.
36. Apparatus based on one of the claims 26 to 35, characterized such that the mixing unit (12) is constructed as a plowshare or paddle mixer or an agitator.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE10045586A DE10045586C2 (en) | 2000-09-15 | 2000-09-15 | Process and device for cleaning smoke gases containing sulfur dioxide |
| DE10045586.7-24 | 2000-09-15 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2357280A1 true CA2357280A1 (en) | 2002-03-15 |
Family
ID=7656256
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002357280A Abandoned CA2357280A1 (en) | 2000-09-15 | 2001-09-12 | Procedure and also apparatus for the cleaning of flue gases containing sulfur dioxide |
Country Status (8)
| Country | Link |
|---|---|
| US (2) | US6685902B2 (en) |
| EP (1) | EP1188472B1 (en) |
| CN (1) | CN1178734C (en) |
| AT (1) | ATE346674T1 (en) |
| CA (1) | CA2357280A1 (en) |
| DE (2) | DE10045586C2 (en) |
| MX (1) | MXPA01009057A (en) |
| PL (1) | PL200482B1 (en) |
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-
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-
2001
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- 2001-09-04 DE DE50111526T patent/DE50111526D1/en not_active Expired - Lifetime
- 2001-09-04 EP EP01120696A patent/EP1188472B1/en not_active Expired - Lifetime
- 2001-09-07 MX MXPA01009057A patent/MXPA01009057A/en active IP Right Grant
- 2001-09-12 CA CA002357280A patent/CA2357280A1/en not_active Abandoned
- 2001-09-12 PL PL349621A patent/PL200482B1/en unknown
- 2001-09-14 CN CNB01133102XA patent/CN1178734C/en not_active Expired - Fee Related
- 2001-09-14 US US09/952,749 patent/US6685902B2/en not_active Expired - Lifetime
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2003
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103212282A (en) * | 2013-04-11 | 2013-07-24 | 陕西科技大学 | Method for reducing pH value of fly ash-coproduced calcium silicate filler for papermaking and purifying flue gas simultaneously |
| CN103212282B (en) * | 2013-04-11 | 2015-06-03 | 陕西科技大学 | Method for reducing pH value of fly ash-coproduced calcium silicate filler for papermaking and purifying flue gas simultaneously |
Also Published As
| Publication number | Publication date |
|---|---|
| DE10045586A1 (en) | 2002-04-04 |
| US6685902B2 (en) | 2004-02-03 |
| CN1347750A (en) | 2002-05-08 |
| EP1188472A3 (en) | 2002-04-03 |
| US6912962B2 (en) | 2005-07-05 |
| DE50111526D1 (en) | 2007-01-11 |
| PL349621A1 (en) | 2002-03-25 |
| MXPA01009057A (en) | 2004-11-10 |
| EP1188472A2 (en) | 2002-03-20 |
| PL200482B1 (en) | 2009-01-30 |
| ATE346674T1 (en) | 2006-12-15 |
| US20020085961A1 (en) | 2002-07-04 |
| CN1178734C (en) | 2004-12-08 |
| US20040103830A1 (en) | 2004-06-03 |
| DE10045586C2 (en) | 2002-07-18 |
| EP1188472B1 (en) | 2006-11-29 |
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