The present invention relates to the purification of exhaust gases comprising acidifying compounds such as SO2 and HC1.

Exhaust gases comprising such acidifying compounds often arise in connection to industrial combustion of organic compounds, such as in fossil fuel fired power plants and waste incineration plants. They also arise in for instance chemical process industry where for instance SO2 or HC1 constitute residual products.

Conventionally, alkaline compounds, such as Mg(OH) ₂ , lime in the form of for instance CaO, Ca(0H) ₂ or CaCC>3, etc., are used to, in a scrubber or the similar, separate the acidifying compounds. It is also known to use alkaline fly ash for this purpose. Often, the alkaline compounds are also recirculated, so that the exposure between exhaust gases and alkaline compounds is increased.

However, such exhaust purification systems are expensive to operate, since large amounts of alkaline compounds are used in order to minimize the discharge of acidifying and thereby environmentally harmful compounds.

The present invention solves the above described problems.

Hence, the invention relates to a method for purifying exhaust gases from acidifying compounds, which exhaust gases are exposed to an alkaline compound such as Ca(OH) ₂ which is reacted with and therefore neutralizes the acidifying compounds, and is characterized in that ash comprising CaO in a first step is mixed, in a container, with water, but not with the exhaust gases, and is heated so that CaO is transformed into Ca(0H) ₂ , and in that the hence formed Ca(OH) 2 in a second step is reacted with, and therefore neutralizes, the acidifying compounds in the exhaust gases. The invention also relates to a device.

The invention will now be described in detail, with reference to exemplifying embodiments of the invention and to the enclosed drawings, wherein:

Figure 1 is an outline diagram of a device for purifying exhaust gases according to the present invention;

Figure 2 is a side view of an activation device according to a first preferred embodiment of the invention;

Figure 3 is a side view of an activation device according to a second preferred embodiment of the invention; and

Figure 4 is a side view of an activation device according to a third preferred embodiment of the invention. Figure 1 is an outline diagram illustrating a device for purifying exhaust gases comprising acidifying compounds, preferably gaseous, such as S0 ₂ , S0 ₃ , HF och HC1. An industrial process 1, such as a combustion process or another chemical process, hence emits gases, such as flue gases, compris- ing said acidifying compounds. In the following, such gases are denoted "exhaust gases". The exhaust gases are brought, using a pipe work 2, to a dust separation step 3, in which solid substances such as fly ash or other solid alkaline compounds dispersed in the exhaust gases are separated from the exhaust gases. The separated alkaline substances are brought onwards to an activation device 4, in which at least a share of the CaO contained in the alkaline substances is transformed to Ca(0H) ₂ according to CaO + H ₂ 0 → Ca (OH) ₂ -

Finally, at least a share of the activated substances are returned to the exhaust gases by redispersion before the dust 5 separating step 3, so that a cycle of operation arises for the solid substances, according to the arrows of figure 1.

The activated alkaline substances, then comprising Ca(OH)2, are hence exposed to the exhaust gases, and thereby neutrality izes their acidifying contents. One example of such a neutralizing reaction, whereby SO2 is neutralized so that CaS0 ₃ is formed, is:

Ca(0H) ₂ + S0 ₂ → CaS0 ₃ + H ₂ 0

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Other alkaline compounds, such as Mg(0H) ₂ and CaC0 ₃ , can also be used to neutralize various acidifying compounds in the exhaust gases, such as SO2 and HC1, in corresponding ways which are known as such. In the case where HC1 is neutralized 20 with Ca(OH) ₂ , for instance CaCl ₂ will be formed.

The exhaust purification device can of course comprise also other units that are conventional as such, such as dry and/or moist purification steps, such as a scrubber. What is essen- 25 tial for the present invention is that the alkaline substances are activated in the activation device 4, in a way which is described in closer detail below, before they are dispersed in the exhaust gases.

30 For cost reasons it is preferred that the alkaline substances at least partly, preferably completely, are constituted by alkaline fly ash which has been separated from the exhaust gases. However, it is both possible and in certain applications desirable to also supply additional alkaline compounds in order to be able to complete the neutralizing reaction. Such additional externally supplied alkaline compounds may for instance comprise fly ash from other processes and various forms of lime.

Figure 2 illustrates, in cross-section and from the opposite side, an activation device 100 for activating alkaline compounds according to a first preferred embodiment. The device 100 comprises an elongated container 101, in which alkaline compounds that are to be activated are fed in through an inlet 102 in a first end of the container 101, and out through an outlet 103 in the opposite end of the container 101. In the interior of the container 101, there is an elongated mixing space 104, through which a feeding means 105 is arranged to feed the alkaline compounds in the main direction 106 of extension of the container 101.

Water, in the form of steam or hot, liquid water, is supplied via a conduit system 107, a valve 108 and at least one nozzle 109. In case the water is in the form of steam, it is preferred that the steam holds a temperature of at least 110°C, if it is liquid at least 30°C. It is preferred that the majority of the nozzles 109 are arranged to distribute the supplied water evenly over the alkaline compounds comprised in the container 101, from above and along at least a majority of the length of the container 101.

According to the invention, the alkaline compounds comprise ash comprising CaO, and are preferably completely comprised by ash, preferably from the industrial process that has also given rise to the exhaust gases as such.

Hence, the ash is mixed with water in the container 101 without therein coming into contact with the exhaust gases. More- over, heat is supplied to the contents of the container 101, which in combination with mechanical mixing as well as the exposure to the supplied water, results in rapid transformation of CaO into Ca(OH) ₂ , but also in that the ash parti- cles crack open and become more porous. This increases the yield in a second step according to the invention, in which the alkaline compounds, including the Ca(OH) ₂ formed in the container 101, are exposed to the activated alkaline compounds so that the acidifying compounds in these are neutral- ized and therefore are separated from the exhaust gases.

The present method is also efficient for transforming other oxides to hydroxides, such as transformation of MgO to Mg (OH) 2 .

According to a preferred embodiment, the heat supply to the container takes places only through the heat in the supplied water. However, it is also possible to use a separate heating device 114. Irrespective of which, it is preferred that the container 101 is isolated in a suitable manner to avoid thermal losses.

According to a preferred embodiment, the amount of added water is continuously regulated, using a control device 110, so that the resulting mixture of water and alkaline compounds is so moist so that the above described transformation to hydroxide can take place for the majority of the CaO comprised in the ash, but at the same time sufficiently dry in order to both be transported through the container 101 and thereafter to be able to be injected into the exhaust stream and therein be able to react with the acidifying compounds. It is preferred that the resulting mixture, at the outlet 103, holds a moisture ratio of 2-5%. It has turned out that good results both concerning transport of the mixture of water and alkaline compounds through the container 101, as well as mechanic mixing of the mixture, can be achieved when the feeding means 105 is in the form of a feeding screw which is conventional as such, which hence both feeds and mixes the mixture in the container 101.

According to a particularly preferred embodiment, compressed air 111 is furthermore added, via an inlet 112, at the bottom of the container 101. In this case, sufficient amounts of compressed air per time unit are added for the mixture of alkaline compounds and water to form a fluidized bed in the container 101. This results in an additionally improved mixing in the container 101, and therefore higher yield. Accord- ing to a preferred embodiment, there is arranged a fluidiza- tion cloth 113, which is conventional as such, so that a bottom of the container 101 is constituted by a fluidizing part with an outer mantle and an inner fluidization cloth 113.

According to a preferred embodiment, the alkaline compounds that have been activated using water and heat in the container 101 are returned, after they have reacted with the exhaust gases, once more to the container 101 for additional treat- ment with water and heat, after which the reactivated alkaline compounds again are reacted with the exhaust gases. This circulation can then be repeated in order to maximally exploit the alkali which is present in the ash. Moreover, it is preferred that the ash which has been treated in the container with water and heat is used for supplementing an externally supplied alkaline compound for purifying the exhaust gases from the acidifying compounds. Since ash most often is relatively cheap, such a supplementation will be profitable.

As stated above, the device 100 illustrated in figure 2 is provided with a substantially horizontally extending container 101, through and along which the mixture of alkaline compounds and water is transported using the feeding means 105, and to which water is supplied through the nozzles 109, that are arranged above the surface of the mixture in the contain- er 101.

Figure 3 illustrates an alternative embodiment, in which a device 200, in a way corresponding to that of figure 2, comprises a container 201 with an inlet 202 and an outlet 203, between which the mixture of alkaline compounds and water is transported using a feeding means 205 in the form of a feeding screw. The container 201 is not horizontally arranged, but the mixture is transported substantially vertically, in the direction 206, first by the use of the feeding means 205 on to a first shaft 204a, thereafter down to the bottom of the shaft 204a and across to a second shaft 204b, located on the other side of a vertical partition wall 204c, and further up towards the outlet 203. The bottom of the container 201 preferably slants in a direction towards the outlet 203, in order to facilitate the transport of the mixture. Also, it is preferred that the transport of the mixture is also facilitated by the supply of compressed air 211, via an inlet 212, at the bottom of the container 201, so that a fluidized bed, preferably with a fluidization cloth 213, is formed as described above. Hence, the compressed air will bring with it the mixture up through the shaft 204b and towards the outlet 203. In this case, it is preferred that the water supply, which is similar to the one described above, takes place via a control device 110, a conduit system 207, a valve 208 and nozzles 209, where the nozzles 209 are arranged in the ceiling of the second shaft 204b, so that the water is distributed into the mixture from above in the second shaft 204b right before the outlet 203. The heat is supplied via the water and/or using a separate heating device 214.

Hence, the transport of the mixture in this embodiment takes place firstly, in the first shaft 204a, using energy supplied via the feeding means 205 and gravity, and thereafter, in the second shaft 204b, primarily using energy supplied via the compressed air 211.

Figure 4 illustrates an additionally alternative embodiment, in which a device 300 comprises a container 301 which, similarly to the container 201 and using a vertical partition wall 304c, is partitioned in a first shaft 304a and a second shaft 304b, whereby the mixture is transported, from an inlet 302 to an outlet 303 and in the direction 306, using a vertically arranged feeding means 305 in the second shaft 304b, in combination with compressed air 311 which is supplied via an inlet 312 below a fluidization cloth 313 at the bottom of the container 301.

In this case, the transport in the container 301 thus takes place mainly using the feeding means 305, supplemented by the compressed air 311.

Water is supplied, using a control device 310, a valve 308 and a conduit system 307, via nozzles 309 arranged in the vicinity of the bottom of the container 301 and directed upwards in the second shaft 304b. Thereby, the compressed air 311 will improve the mixing of the alkaline compounds and the supplied water in the second shaft 304b. Heat is supplied via the water and/or using a separate heating device 314. By the use of a method and a device according to the present invention, existing residual products from the process from which exhaust gases comprising acidifying compounds originate can thus be more efficiently used in order to separate the acidifying compounds. This results in a cheaper purification process for the achievement of the same environmental goals as a corresponding conventional process.

Above, preferred embodiments have been described. However, it is apparent to the skilled person that many modifications can be made to the described embodiments without departing from the basic idea of the invention.

For instance, the respective second shaft 204b, 304b in devices 200 and 300 needs not be vertical, but may slant up- wards to the outlet 203, 303. This facilitates additionally transport of the mixture, and can advantageously be used in certain embodiments in which sufficient mixture can be achieved also without the vertical transport illustrated in figures 3 and 4. In this case, the feeding means 205, 305 may constitute a screw pump of the Archimedean type, which efficiently transports the mixture up towards the outlet 203, 303 under constant mixing.

Furthermore, the hot water or steam needs not be supplied via nozzles 109, 209, 309. Instead, water can for example be added to the ash already before it enters into the container 101, 201, 301. Hence, the invention is not to be limited to the describe embodiments, but may be varied within the scope of the en closed claims.