DE202020104616U1 - Device for the treatment of exhaust gases from incineration plants operated with heavy fuel oil - Google Patents

Abstract

Device for exhaust gas treatment of incineration systems operated with heavy fuel oil with a boiler (11) in which the fuel oil is heavily burned and in which an SNCR system (15) and in the further course of the exhaust gas at a temperature range between 850 ° C and 1,050 ° C A flue gas desulfurization system (22) is present in the exhaust gas duct in front of the chimney (26), characterized in that the SNCR system (15) is designed so that the nitrogen oxides by 55% to 65% with an NH ₃ slip that is greater than a predetermined limit value, and that in the flue gas flow direction behind the SNCR system (15) and upstream of the flue gas desulfurization system (22) there is an SCR system (27) whose catalyst (28) is smaller than the catalyst that is used for the Reduction of the nitrogen oxides produced during combustion without an SNCR system is necessary.

Description

The invention relates to a device for the treatment of exhaust gas from combustion systems operated with heavy fuel oil, with a boiler in which the heavy fuel oil (fuel oil S) is burned and in which an SNCR system and furthermore at a temperature range between 850 ° C and 1,050 ° C of the flue gas There is a flue gas desulphurisation system in front of the chimney. In particular, the invention relates to a device for treating exhaust gas from combustion systems in the power range of 5 to 50 MW, which are used in medium-sized industry.

When fossil fuels and waste are burned, nitrogen oxides (NO _x ) are produced in the smoke or exhaust gases, which pollute the environment. Because of the increasingly strict exhaust gas regulations, it is therefore necessary to further optimize and refine the already known methods for denitrification.

• 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

A proven denitrification process is generally known as the SNCR process (selective non-catalytic reduction process) and from the EP 0 426 978 B1 known. Ammonia in aqueous solution or urea in aqueous solution are used as reducing agents, which convert the nitrogen oxides contained in the exhaust gas into water vapor and nitrogen and carbon dioxide (when using urea) through thermolysis:

• 4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O

When using urea, ammonia is initially formed: NH ₂ CONH ₂ + H ₂ O → 2NH ₃ + CO ₂

The ammonia (NH ₃ ) then reacts with the nitrogen oxides according to the first-mentioned reaction. The use of urea as a reducing agent is considered more favorable than the use of ammonia or an aqueous ammonia solution because of the problem-free handling and storage of urea.

In fact, this reduction is based on several partial reactions, which among other things depend on the reaction temperature, i.e. the flue gas temperature. The temperature window which is favorable for this reduction is approximately 850 ° C. to 1,050 ° C. and in particular 900 ° C. to 1,000 ° C. However, it cannot be avoided that a certain ammonia slip always occurs, which is undesirable. Complete denitrification cannot be achieved either. At higher temperatures, the NH ₃ slip and the conversion of nitrogen oxides decrease. At lower temperatures, the NH ₃ slip and the conversion of nitrogen oxides increase. Since the new exhaust gas regulations also stipulate a limit value of NH ₃ , the use of the SNCR process is only possible with additional control effort and restrictions in the normal load of the boiler.

Another method for reducing nitrogen oxides is selective catalytic reduction (SCR, selective catalytic reduction). Here, in the presence of a catalyst and ammonia, the nitrogen oxides NO and NO _{2 are converted} into water and nitrogen: 4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (T> 250 ° C) NO + 2NH ₃ + NO ₂ → 2N ₂ + 3H ₂ O (170 ° C <T <300 ° C) 8NH ₃ + 6NO ₂ → 7N ₂ + 12H ₂ O

With the SCR process, the new limit values according to the 44th BImSchV (Federal Immission Control Ordinance) of 2019 can basically be achieved. In addition, the SCR system cannot be operated with fuel containing sulfur, such as heating oil S, since the sulfur dioxide (SO ₂ ) in the catalytic converter is oxidized to sulfur trioxide (SO ₃ ), from which sulfuric acid is produced.

Another problem with the combustion of fossil fuels is the emission of environmentally harmful sulfur dioxide (SO ₂ ) in the exhaust gas due to the sulfur content in the fuel. Here, too, the 44th BImSchV specifies lower limit values in the exhaust gas of 350 mg / m ³ . This limit value basically requires a heating oil with a sulfur content of <0.2%. However, heating oil S usually has a sulfur content of> 1.0%.

While it was previously possible to burn heating oil S with a sulfur content of> 1.0%, which is easily and inexpensively available regionally as refinery residue oil, now heating oil EL (extra light heating oil) must be used. However, this is not a residual oil, but a middle distillate with the highest water hazard class. This means that for a large number of furnaces with storage tanks for the previously used heating oil S with a lower water hazard class, the operating license for the storage tanks would expire. The consequence would be a cessation of operations.

The heating oil S, however, occurs in large quantities in a refinery. Before the 44th BImSchV there were corresponding customers that would no longer apply. Shipping is no longer a buyer of heating oil S either, as marine diesel is only allowed to have a sulfur content of <0.5%. The heating oil S must therefore be exported to other countries with different emission regulations. However, this measure does not help globally, since only the The place of origin of the environmentally harmful sulfur dioxide is relocated.

From an economic point of view, further use of heating oil S as an energy source would therefore be desirable in Germany. On the one hand, this fuel is available at low cost. On the other hand, it is also available regionally in Germany. Complex transport routes are avoided. In addition, there is no costly export. After all, old systems can continue to be operated with conventional storage tanks.

The use of SCR systems to reduce nitrogen oxides, however, leads to increased SO ₃ formation during the combustion of heating oil S due to the sulfur content, so that sulfuric acid is formed, which destroys the subsequent installations. It is known in principle to add sulfur-free components to heating oil S. However, such a blend has an inhomogeneous boiling curve and stoichiometric combustion is difficult to set.

The invention is therefore based on the object of designing an exhaust gas treatment system for combustion systems in such a way that the use of heating oil S with a sulfur content of> 1.0% is possible in compliance with the 44th BImSchV.

The object is achieved according to the invention in that the SNCR system is designed so that the nitrogen oxides are reduced by 55% to 65% with an NH ₃ slip that is greater than a predetermined limit value, and that in the flue gas flow direction behind the SNCR system and upstream of the flue gas desulphurisation system there is an SCR system with a catalyst that is smaller than the catalyst that is required for reducing nitrogen oxides without an SNCR system. The size that is required to reduce the nitrogen oxides in the exhaust gas that ultimately emerges to the same extent when the SCR system is operated without an SNCR system is to be understood as a comparative measure of the size of the catalytic converter.

The invention uses the properties of the two combined denitrification measures in order to be able to comply with the limit values of the 44th BImSchV with a relatively low expenditure on equipment, even if heating oil S is used as fuel. A large part of the nitrogen oxides produced is reduced by using the SNCR system. The SNCR system can be set so that there is an NH ₃ slip that is well above the permissible limit value of 30 mg / m ³ according to the 44th BImSchV.

This NH ₃ slip is largely converted in the subsequent SCR system in the presence of the catalyst with the non-reduced nitrogen oxides to form the desired exhaust gas products according to the reaction equations listed above. There is a further reduction in nitrogen oxides to below the permissible limit value of 200 mg / m ³ . However, since a large part of the nitrogen oxides originally formed during combustion have already been converted, a relatively small catalyst is sufficient to bring about the desired reduction in the exhaust gases emerging from the chimney.

This in turn means that despite the relatively high content of SO ₂ in the exhaust gas, only a little SO _{3 is produced} in the SCR system due to the use of heating oil S. The formation of the harmful sulfuric acid is thus suppressed or kept to a manageable extent, so that there is no risk of damage to the subsequent installations or the subsequent flue gas desulfurization system.

The investment costs for converting an existing incineration plant are therefore relatively low. Even before the 44th BImSchV came into force, the use of an SNCR system was necessary in order to be able to comply with the limit values for nitrogen oxides.

It is therefore possible according to the invention for the size of the catalyst to be between 5% and 50% and in particular between 10% and 25% of the size of the conventional catalyst. The usual size corresponds to the size of the catalytic converter in an SCR system, which is required in the exhaust gas flow without an upstream SNCR system. The smaller size of the catalytic converter is sufficient to further reduce the remaining nitrogen oxides with the NH ₃ slip that inevitably occurs in the SNCR system, so that ultimately a reduction of more than 80% of the nitrogen oxides originally produced during combustion can be achieved.

The SCR system does not require any additional ammonia metering for the desired reactions. Nevertheless, it can be useful to provide a metering device for the same reducing agent upstream of the SCR system in the exhaust duct if the NH ₃ slip would not be sufficient to reduce the remaining nitrogen oxides in the SCR system. However, there is no need to provide an additional storage container or a further metering device for another reducing agent.

In principle, it does not matter what type of boiler is used. The boiler can, for example, be designed as a shell boiler or as a water tube boiler. In the power range between 5.0 and 50.0 MW, a shell boiler or three-pass boiler is predominantly used.

In the case of a three-pass boiler, the invention provides that the SNCR system is arranged in the first train and the SCR system is arranged behind the second train. Such an arrangement is advisable because the temperatures required for the SNCR reactions are present in the first pass and the temperatures required for the SCR reactions are present after the second pass. There is therefore no need for complex modifications to the boiler system. In particular, the SCR system can be installed in the turning chamber between the second pass and the third pass of the three-pass boiler with little effort.

Overall, with these measures, it is possible to continue to operate existing combustion systems or boilers with the inexpensive S heating oil. Nonetheless, the limit values of the 44th BImSchV are complied with, and there is no export of heating oil S to other countries. Rather, heating oil S, which is easily available in the region, can be used and thus consumed profitably.

The invention is explained in more detail below with reference to the schematic drawing, the single figure of which shows a combustion system with a device according to the invention.

The shell boiler shown in the drawing 11 is from a burner 12 heated with heating oil S from a storage tank 13 is operated. The shell boiler 11 is designed as a three-pass boiler and has a first train 14th at the end of an injection device 15th is arranged for a reducing agent for the selective non-catalytic reduction of nitrogen oxides in the exhaust gas. The exhaust gas flows in the direction of the arrows 33 through the exhaust system. The reducing agent is, for example, urea and is taken from a storage container 16 with a dosing pump 17th and a metering device 18th at the favorable exhaust gas temperature between 850 ° C and 1,050 ° C. The SNCR process is known and therefore does not need any further explanation.

The second move too 19th and the third move 20th runs within the shell boiler 11 before the exhaust gas flow through a heat exchanger 21st to the flue gas desulphurization system 22nd arrives in which the SO ₂ with the addition of calcium hydroxide Ca (OH) ₂ or its precursors from a storage container 23 is converted into plaster: 2SO ₂ + 2Ca (OH) ₂ + O ₂ + 2H ₂ O → 2 [CaSO ₄ · 2H ₂ O]

The gypsum produced in this way is stored in a silo 24 caught and discharged. The flue gas desulfurization system is known and therefore does not require any further explanation.

From the flue gas desulfurization system 22nd The exhaust gas, which has been freed of nitrogen oxides and sulfur dioxide, arrives via an induced draft fan, for example 25th and a fireplace 26th in the nearby areas. Other pollutants such as dust, soot, nickel, vanadium and other metals are also retained in the flue gas desulphurisation system.

For compliance with the limit values according to 44 . BImSchV is intended that after the second move 19th the exhaust gas, which has already been largely freed of nitrogen oxides, is passed through an SCR system 27 is directed to selective catalytic reduction. The catalytic converter internals arranged therein 28 For example, titanium dioxide, tungsten dioxide and tungsten dioxide can be made much smaller than that of an SCR system that is operated without a previous SNCR system. Only a small part of the amount of nitrogen oxide originally produced during combustion needs to be reduced, so that a smaller catalyst is sufficient for the conversion of this remaining part of the nitrogen oxides. The outlay on equipment is correspondingly lower.

Ammonia is required for the reactions in the SCR system. This ammonia is from the NH ₃ slip of the SNCR plant 15th made available, which is adjusted so that the SNCR process is carried out with a significant excess of NH ₃ . The downstream SCR system 27 thus also serves as an NH ₃ brake and the nitrogen oxides are further reduced.

Due to the lower required conversion rate in the SCR system 27 Only relatively little sulfur trioxide (SO ₃ ) is produced from the sulfur dioxide still present in this section and therefore only a little sulfuric acid in the flue gas desulphurisation, which can be controlled in these quantities, for example by a primary additive to the fuel. It can, however, be provided that the reducing agent comes from the SNCR system 15th also for the SCR system 27 is used. To this end, the metering device 18th an appropriately controllable branch 29 on, via which the reducing agent, for example urea, in front of the SCR system via a metering device 30th is introduced into the exhaust gas stream.

Furthermore, primary additives, for example to improve burnout or to neutralize SO ₃ , can be taken from a storage container 31 and a dosing pump 32 be fed to the fuel before combustion.

Due to the downstream SCR system, it is possible to control the nitrogen oxide content and the NH ₃ slip in the chimney 26th Exhaust gas escaping into the environment is below the relevant limit values of the 44 . Bring BImSchV. At the same time will the sulfur dioxide still present in this section of the exhaust gas flow is only slightly converted into harmful sulfur trioxide due to the relatively small size of the catalyst, which can be neutralized by appropriate and long-known additives. The use of fuels with a higher sulfur content is therefore also possible with a flue gas desulfurization system. The cost-effective heating oil S can therefore be used again sustainably as fuel.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• EP 0426978 B1 [0003]

Claims (4)

Device for exhaust gas treatment of incineration systems operated with heavy fuel oil with a boiler (11) in which the fuel oil is heavily burned and in which an SNCR system (15) and in the further course of the exhaust gas at a temperature range between 850 ° C and 1,050 ° C A flue gas desulfurization system (22) is present in the exhaust gas duct in front of the chimney (26), characterized in that the SNCR system (15) is designed so that the nitrogen oxides by 55% to 65% with an NH ₃ slip that is greater than a predetermined limit value, and that in the flue gas flow direction behind the SNCR system (15) and upstream of the flue gas desulfurization system (22) there is an SCR system (27) whose catalyst (28) is smaller than the catalyst that is used for the Reduction of the nitrogen oxides produced during combustion without an SNCR system is necessary. Device according to Claim 1 , characterized in that the size of the catalytic converter (28) is between 5% and 50% and in particular between 10% and 25% of the size of the catalytic converter which is required for reducing the nitrogen oxides formed during combustion without an SNCR system. Device according to Claim 1 or 2 , characterized in that the boiler is designed as a shell boiler (11). Device according to one of the Claims 1 to 3 , characterized in that the heating boiler (11) is a three-pass boiler, and that the SNCR system is arranged in the first pass (14) and the SCR system (27) is arranged behind the second pass (19).

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