Patent Application
20080236458
The invention relates to a method and a device for reducing polyhalogenated compounds in incineration plants, provided with at least one combustion chamber as defined in patent claim one and patent claim four. The method and the device are furthermore suitable for lowering boiler corrosion by reducing the Cl content of the ash deposits produced during the plant operation.
Polyhalogenated compounds are, for example, poly-chlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) that form during combustion processes, among other things also during the waste-material incineration, and which are released along with the waste gas. As a result of their toxicity, the legislature for the Federal Republic of Germany in the 17th Federal Emissions Safety Regulation (17th BImSchV) has established a limit value for emissions of these compounds from waste incineration plants of 0.1 ng TEQ/Nm3 (TEQ—toxicity equivalent). Based on the present level of knowledge, this limit value for PCDD/F emissions from incineration plants cannot be met by simply optimizing the conditions for incineration. Insofar, state of the art teaches the use of an additional, secondary flue gas cleaning to lower the PCDD/F concentration in the incineration waste gas below the prescribed value. Thus, incineration plants essentially consist of a combustion chamber, if applicable with a boiler, at least one subsequent dust collector, as well as at least one wet washer.
It is generally known from references [1] and [2] that the PCDD and the PCDF component in a waste gas can be reduced considerably by simply using an excess amount of sulfur for the combustion. In particular the ratio of sulfur dioxide to hydrochloric acid is important in that case, wherein the PCDD and PCDF component in the waste gas is reduced considerably with an increase in the ratio of sulfur dioxide to hydrochloric acid.
A method for reducing the corrosion in incineration plants is additionally disclosed in reference [3]. With this method, a partial stream of the flue gas is guided back or re-circulated into the combustion chamber, following the separating out of most of the hydrochloric acid in a first, acid-gas washing stage, by mean of a nozzle which serves to feed in a secondary gas. Owing to the low hydrochloric acid content and because the sulfur dioxide content has not yet been reduced in a second washing stage, the flue gas has a ratio of sulfur dioxide to hydrochloric acid which exceeds 1. The share of hydrochloric acid in the untreated waste gas inside the combustion chamber is consequently reduced solely by diluting it with the re-circulated waste gas.
However, extremely large amounts of re-circulated waste gas are required to achieve a noticeable change in the S/Cl ratio, which results in a considerable increase in the waste-gas volume flow in the area of the boiler and the dust collector. If no further measures are taken, this leads to a reduction in the combustion temperatures and thus to efficiency losses, wherein these could be mitigated through an additional heating of the gases fed into the combustion chamber or by adding chemicals or auxiliary agents.
Starting with the above, it is the object of the present invention to propose a device and a method for reducing polyhalogenated compounds in incineration plants with at least one combustion chamber, for which the aforementioned disadvantages or restrictions do not occur or only to a negligible degree.
This object is solved with a method having the features as defined in claim 1 and a device having the features as defined in claim 4. The dependent claims contain advantageous embodiments.
An essential and fundamental idea upon which the invention is based is that SO2 is selectively separated out of the flue gas in at least one washer and is re-circulated back into the combustion chamber as SO2 or sulfuric acid. As a result of the selective separating out of SO2 in a washer, highly concentrated SO2 or sulfuric acid and, in particular, not a diluted waste gas mixture with low SO2 concentrations is advantageously available for re-circulation, which considerably reduces the aforementioned disadvantages according to prior art resulting from the extreme diluting.
The invention is explained in further detail in the following with the aid of an exemplary embodiment and the following FIGURE, wherein:
Within the framework of the processing flows, a combustible material is initially burnt inside the combustion chamber 1 with a feed-in of primary gas 6. This is followed by an afterburning of any components not completely burnt, along with a feed-in of a secondary gas 16. The resulting untreated waste gas 7, which has a temperature in the range of 1000° C., is guided toward a boiler wall where it releases a certain amount of heat through heat transfer to the boiler and/or the medium inside the boiler. In the process, the untreated waste gas is cooled to temperatures ranging from 200 to 300° C. The untreated waste gas subsequently flows through the first cleaning stage and then the dust collector 3, which the waste gas for the exemplary embodiment leaves at the same temperature level and in the direction of the first washer 4.
Dioxins form primarily in the waste-gas line at a temperature level above 200° C., meaning at precisely the aforementioned temperature level for the boiler wall 2 and the dust collector 3. However, as described before, they can be reduced effectively by feeding in sulfur dioxide or even sulfuric acid, for example together with the secondary gas.
In the first acid-gas washer, hydrochloric acid is selectively separated out through absorption in water at pH values of less than 1, without this resulting in a separating out of sulfur dioxide. In addition to the water feed line 17, this washer is furthermore provided with a first line 8 for discharging hydrochloric acid in the direction of the reactor 12, as well as with a line to the second washer 5.
In contrast, the second and neutrally operated washer is used for the selective separation of sulfur dioxide from the waste gas. In addition to a feed line 9 for water-dissolved sodium hydroxide or calcium hydroxide as neutralizing agent, it also comprises a second line 10 for discharging a watery solution of sodium sulfate or sodium sulfite and/or calcium sulfate and calcium sulfite to the reactor 12, as well as a line for venting the cleaned waste gas 11.
The aforementioned sulfites are formed through reaction of sulfur dioxide with the respective hydroxides, as shown below:
2NaOH+SO2→Na2SO3+H2O (1) and/or
Ca(HO)2+SO2→CaSO3+H2O (2),
wherein at pH values below 7 the sulfites can in part oxidize further with the oxygen in the waste gas to form sulfates, as shown below:
Na2SO3+½O2→Na2SO4 (3) and/or
CaSO3+½O2→CaSO4 (4).
While the aforementioned sulfates are chemically stable and are therefore not available for generating sulfur dioxide in the reactor 12, the above-mentioned sulfites can enter in the reactor 12 into a reaction with the hydrochloric acids, supplied by the first washer 4, forming water and chlorides in the process, and can ultimately react to form sulfur dioxide:
Na2SO3+2HCl→2NaCl+SO2(g)+H2O (5) and/or
CaSO3+2HCl→CaCl2+SO2(g)+H2O (6).
In the same way as the aforementioned sulfates, the chlorides are no longer needed for the further cleaning process and are vented to the outside of the reactor 12 via the discharge line 13.
It is therefore critically important to adjust the oxidation rate and thus also the pH value in the second washer. The goal basically is to achieve a selective separation of sulfur compounds with high sulfite content and low sulfate component. If the pH value is noticeably below 7, the sulfites are further oxidized to form the undesirable sulfates [1]. On the other hand, pH values rising above 7 lead to an undesirable separation of carbon dioxide from the waste gas, resulting in the forming of carbonates in the washing solution:
2NaOH+CO2→Na2CO3+H2O (7) and/or
Ca(OH)2+CO2→CaCO3+H2O (8).
In turn, this requires the additional use of neutralizing agent. Furthermore, during the mixing with HCl in the reactor, the yield of SO2 would drop drastically, which is undesirable, since the HCl required for generating SO2 would be used up by the competing reaction:
Na2CO3+2HCl→2NaCl+CO2(g)+H2O (9) and/or
CaCO3+2HCl→CaCl2+CO2(g)+H2O (10).
For the exemplary embodiment, a sulfur dioxide yield could be achieved in the second washer for pH values between 4 and 9, wherein a maximum yield could be observed for a pH value adjustment around 7.
Two previously mentioned substance flows leave the reactor. The stable compounds sodium chloride and sodium sulfate and/or calcium chloride or calcium sulfate, which cannot be utilized with the method according to the invention, are discharged via the discharge line 13 for a further disposal, for example to a disposal site. On the other hand, the sulfur dioxide formed in the reactor during the reactions (5) and/or (6) is re-circulated via the discharge line 15 to the combustion chamber, together with the carrier gas that is fed into the reactor via the feed line 14. It makes sense in this connection to combine the line 15 with the secondary gas feed line 16, wherein combining the line 15 and the secondary gas feed-in 16 can be configured such that a certain excess or low pressure can be generated in one of the two lines by one of the two substance flows, respectively in the other line, in order to aid the transport and/or mixing.
For one practical application, the salt residues from the two washers 4 and 5 are combined inside a small stirrer vessel, functioning as reactor 12, and are mixed together to form a solution, wherein nitrogen dispersed via the feed line 14 into the solution served as non-reactive carrier gas. The sulfur dioxide gas, released during the reaction according to equations (5) and (6), is separated out in the reactor together with the carrier-gas flow and is fed to the secondary gas feed line 16. Starting with the combustion chamber 1, the concentration of sulfur dioxide in the waste gas, increased in this way, again passes through all components until it reaches the second washer 5, where the sulfur dioxide is once more separated out selectively.
The aforementioned cyclic process leads to a step-by-step increase in the sulfur dioxide concentration in the untreated waste gas, precisely in the previously mentioned areas of dioxin formation. Under ideal conditions, the ratio of sulfur dioxide to hydrochloric acid in the untreated waste gas emitted by household waste incineration plants can be raised without auxiliary agents from a value of 0.1 to values of around 0.6, wherein an increased sulfur dioxide concentration causes a reduction of the chlorine (Cl2) that is primarily responsible for the PCDD/F formation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10338752.8 | Aug 2003 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2004/009146 | 8/14/2004 | WO | 00 | 6/6/2008 |
