FI130561B - A method for removing nitrogen oxides from flue gas - Google Patents
A method for removing nitrogen oxides from flue gas Download PDFInfo
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- FI130561B FI130561B FI20215932A FI20215932A FI130561B FI 130561 B FI130561 B FI 130561B FI 20215932 A FI20215932 A FI 20215932A FI 20215932 A FI20215932 A FI 20215932A FI 130561 B FI130561 B FI 130561B
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- flue gas
- conduit
- ozone
- side stream
- nitrogen oxides
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- MWUXSHHQAYIFBG-UHFFFAOYSA-N nitrogen oxide Inorganic materials O=[N] MWUXSHHQAYIFBG-UHFFFAOYSA-N 0.000 title claims abstract description 198
- 239000003546 flue gas Substances 0.000 title claims abstract description 143
- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 title claims abstract description 142
- 238000000034 method Methods 0.000 title claims abstract description 51
- CBENFWSGALASAD-UHFFFAOYSA-N Ozone Chemical compound [O-][O+]=O CBENFWSGALASAD-UHFFFAOYSA-N 0.000 claims abstract description 93
- 238000005201 scrubbing Methods 0.000 claims abstract description 43
- 239000007788 liquid Substances 0.000 claims abstract description 29
- 230000001590 oxidative effect Effects 0.000 claims abstract description 11
- 238000007254 oxidation reaction Methods 0.000 claims description 19
- 230000003647 oxidation Effects 0.000 claims description 17
- 238000001816 cooling Methods 0.000 claims description 12
- 238000006243 chemical reaction Methods 0.000 claims description 11
- 229910052757 nitrogen Inorganic materials 0.000 claims description 7
- 235000019391 nitrogen oxide Nutrition 0.000 description 45
- 229910002089 NOx Inorganic materials 0.000 description 28
- 229960003753 nitric oxide Drugs 0.000 description 25
- 239000007789 gas Substances 0.000 description 18
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 15
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 12
- MGWGWNFMUOTEHG-UHFFFAOYSA-N 4-(3,5-dimethylphenyl)-1,3-thiazol-2-amine Chemical compound CC1=CC(C)=CC(C=2N=C(N)SC=2)=C1 MGWGWNFMUOTEHG-UHFFFAOYSA-N 0.000 description 7
- JCXJVPUVTGWSNB-UHFFFAOYSA-N nitrogen dioxide Inorganic materials O=[N]=O JCXJVPUVTGWSNB-UHFFFAOYSA-N 0.000 description 7
- 230000005611 electricity Effects 0.000 description 6
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 5
- 239000000356 contaminant Substances 0.000 description 5
- 239000001301 oxygen Substances 0.000 description 5
- 229910052760 oxygen Inorganic materials 0.000 description 5
- 239000000243 solution Substances 0.000 description 5
- 239000000126 substance Substances 0.000 description 5
- 238000002485 combustion reaction Methods 0.000 description 4
- 239000000463 material Substances 0.000 description 4
- 239000010935 stainless steel Substances 0.000 description 4
- 229910001220 stainless steel Inorganic materials 0.000 description 4
- 239000013505 freshwater Substances 0.000 description 3
- 239000000203 mixture Substances 0.000 description 3
- 239000007800 oxidant agent Substances 0.000 description 3
- 238000012856 packing Methods 0.000 description 3
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- 229910000975 Carbon steel Inorganic materials 0.000 description 2
- MYMOFIZGZYHOMD-UHFFFAOYSA-N Dioxygen Chemical compound O=O MYMOFIZGZYHOMD-UHFFFAOYSA-N 0.000 description 2
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 description 2
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 2
- 238000003916 acid precipitation Methods 0.000 description 2
- 239000010962 carbon steel Substances 0.000 description 2
- 230000008021 deposition Effects 0.000 description 2
- 229910001882 dioxygen Inorganic materials 0.000 description 2
- OUUQCZGPVNCOIJ-UHFFFAOYSA-N hydroperoxyl Chemical compound O[O] OUUQCZGPVNCOIJ-UHFFFAOYSA-N 0.000 description 2
- 238000002347 injection Methods 0.000 description 2
- 239000007924 injection Substances 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 229910017604 nitric acid Inorganic materials 0.000 description 2
- 239000008399 tap water Substances 0.000 description 2
- 235000020679 tap water Nutrition 0.000 description 2
- 208000000059 Dyspnea Diseases 0.000 description 1
- 206010013975 Dyspnoeas Diseases 0.000 description 1
- 206010019233 Headaches Diseases 0.000 description 1
- 206010028813 Nausea Diseases 0.000 description 1
- 238000003915 air pollution Methods 0.000 description 1
- 239000000926 atmospheric chemistry Substances 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 239000001569 carbon dioxide Substances 0.000 description 1
- 229910002092 carbon dioxide Inorganic materials 0.000 description 1
- 229910002091 carbon monoxide Inorganic materials 0.000 description 1
- 239000003245 coal Substances 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 230000003292 diminished effect Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 239000003344 environmental pollutant Substances 0.000 description 1
- 239000002803 fossil fuel Substances 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 231100000869 headache Toxicity 0.000 description 1
- TUJKJAMUKRIRHC-UHFFFAOYSA-N hydroxyl Chemical compound [OH] TUJKJAMUKRIRHC-UHFFFAOYSA-N 0.000 description 1
- 230000036737 immune function Effects 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- 239000012535 impurity Substances 0.000 description 1
- 230000007774 longterm Effects 0.000 description 1
- 230000008693 nausea Effects 0.000 description 1
- 239000013618 particulate matter Substances 0.000 description 1
- 238000005192 partition Methods 0.000 description 1
- 231100000719 pollutant Toxicity 0.000 description 1
- 239000002243 precursor Substances 0.000 description 1
- 238000010791 quenching Methods 0.000 description 1
- 150000003254 radicals Chemical class 0.000 description 1
- 230000004202 respiratory function Effects 0.000 description 1
- 239000007921 spray Substances 0.000 description 1
- 238000005507 spraying Methods 0.000 description 1
- XTQHKBHJIVJGKJ-UHFFFAOYSA-N sulfur monoxide Chemical class S=O XTQHKBHJIVJGKJ-UHFFFAOYSA-N 0.000 description 1
- 208000024891 symptom Diseases 0.000 description 1
- 238000005406 washing Methods 0.000 description 1
Classifications
-
- 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
- B01D53/54—Nitrogen compounds
- B01D53/56—Nitrogen oxides
-
- 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
- B01D53/30—Controlling by gas-analysis apparatus
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Analytical Chemistry (AREA)
- General Chemical & Material Sciences (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Health & Medical Sciences (AREA)
- Biomedical Technology (AREA)
- Environmental & Geological Engineering (AREA)
- Treating Waste Gases (AREA)
Abstract
This specification provides an improved method of removing nitrogen oxides from flue gas. The method comprises providing a flow of the flue gas comprising nitrogen oxides in a first conduit (101, 201, 301, 401, 501), directing a portion of the flue gas from the first conduit into a side stream conduit (105, 205, 305, 505) and providing ozone into the side stream conduit. The method further comprises oxidizing the portion of the flue gas in the side stream conduit (105, 205, 305, 505) by the ozone in order to provide oxidized portion of the flue gas, conducting at least the oxidized portion of the flue gas into a scrubber (102, 202, 302, 402, 502), and scrubbing the flue gas comprising nitrogen oxides in the scrubber (102, 202, 302, 402, 502) with a scrubbing liquid in order to remove at least some of the nitrogen oxides. Further, a system for removing nitrogen oxides from flue gas is provided.
Description
A method for removing nitrogen oxides from flue gas
This specification relates to a method of removing nitrogen oxides from flue gas. The specification also relates to a system for removing nitrogen oxides from flue gas.
It is known to use ozone as an oxidant to oxidize nitrogen monoxide of flue gas into nitrogen oxides that are soluble in water and thus can be subsequently removed from the flue gas by scrubbing.
In so-called deep oxidation nitrogen monoxide is oxidized by ozone to N2Os, which is more water soluble when compared to NO». In order to ensure the deep oxidation to take place, long enough residence time for the oxidation reaction, low enough reaction temperature and high enough O3/NO molar ratio are required. However, when the ozone feed is to be dimensioned to the entire flue gas stream, a big ozone generator as well as great electrical power for ozone generation are required. Further, investment costs related to the ozone generator can be substantial.
Document EP 3012011 A1 discloses a method and a system for removing
N contaminants, such as nitrogen oxides from gas streams. Oxidation of nitrogen
N oxides by the addition of ozone to a separated portion of a process gas stream 2 is disclosed. The total process gas stream is divided into at least two or more & streams and the portion of the total process gas stream that is to be treated
I 30 with ozone is determined based on the extent of nitrogen oxide removal a desired. The oxidized contaminants in the ozone treated portion of the stream x are removed by contacting with a scrubbing medium in a wet or semi-dry or = dry scrubber. &
Document WO 2017/112178 A1 discloses a method for the partial removal of contaminants such as nitrogen oxides from a process gas stream. The process gas stream is separated into at least two process gas streams by means of a partition, baffle, damper or other device. Ozone is fed into contact with at least one of the separated process gas streams to oxidize the contaminants therein and the at least one of the process gas streams contacted by ozone is fed to a scrubber for removal of the oxidized contaminants from the gas streams.
An improved method and a system that utilize ozone oxidation for nitrogen — oxide removal from flue gases is provided. With the solution disclosed herein the NOx emission limits are reachable with minimal consumption of ozone and thus minimal amount of the liquid oxygen needed for its production as well as with minimal consumption of electrical power by the ozone generator. Further, use of a smaller ozone generator and a smaller reaction chamber may be enabled, thus reducing investment costs.
According to an embodiment, a method of removing nitrogen oxides from flue gas is provided. The method comprises - providing a flow of the flue gas comprising nitrogen oxides in a first conduit, - directing a portion of the flue gas from the first conduit into a side stream conduit, - providing ozone into the side stream conduit, - oxidizing the portion of the flue gas in the side stream conduit by the ozone in order to provide oxidized portion of the flue gas,
N - conducting at least the oxidized portion of the flue gas into a
N scrubber, and 2 - scrubbing the flue gas comprising nitrogen oxides in the scrubber & with a scrubbing liquid in order to remove at least some of the x 30 nitrogen oxides.
N
N
N Fig. 1 illustrates, by way of an example, a system configured to remove nitrogen oxides from flue gas by implementing the method disclosed herein,
Fig. 2 illustrates, by way of an example, another system configured to remove nitrogen oxides from flue gas by implementing the method disclosed herein,
Fig. 3 illustrates, by way of an example, yet another system configured to remove nitrogen oxides from flue gas by implementing the method disclosed herein,
Fig. 4 illustrates, by way of an example, a scrubber for the system disclosed herein, and
Fig. 5 illustrates, by way of an example, still another system configured to remove nitrogen oxides from flue gas by implementing the method disclosed herein.
The figures are schematic. The figures are not in any particular scale.
The solution is described in the following in more detail with reference to some embodiments, which shall not be regarded as limiting.
In this description and claims, term “comprising” is used as an open term, but it also comprises the closed term “consisting of”. Unit of temperature
N expressed as degrees C corresponds to °C. The following reference numbers 5 and denotations are used in this application:
N 101, 201, 301, 401, 501 first conduit
Ek 30 102, 202, 302, 402, 502 scrubber a 103, 203, 303, 403, 503 line x 104, 204, 304, 504 directing means = 105, 205, 305, 405, 505 side stream conduit
N 106, 206, 306, 506 cooling means 107, 207, 307, 507 means for providing ozone 108, 208, 308, 408, 508 outlet
109, 209, 309, 409, 509 outlet 201a, 401a part of the first conduit 403a line 408a line 410 contact area 411 nozzle 412 pump 413 line 414 connection 520 sensors 521 processor
In atmospheric chemistry NOx is a generic term for the nitrogen oxides that are most relevant for air pollution, namely nitric oxide (NO, also called nitrogen — oxide or nitrogen monoxide) and nitrogen dioxide (NO2). Within context of this specification, nitrogen oxides (abbreviated as NOx) refers to binary compounds of oxygen and nitrogen and to the mixtures thereof. Thus, nitrogen oxides referred to herein also comprise for example oxidized form N20O5 in addition to NO and NO».
Nitrogen monoxide is a colourless gas that forms in combustion systems.
Nitrogen monoxide is a free radical that has an unpaired electron.
Environmental effects of nitrogen monoxide in flue gases relate for example to acid rain deposition, ozone depletion and to its behaviour as NO? precursor.
Nitrogen monoxide reacts with the hydroperoxyl radical to form nitrogen
N dioxide, which then can react with a hydroxyl radical to produce nitric acid.
N Nitric acid, along with sulfuric acid, contributes to acid rain deposition. Nitrogen 2 monoxide also participates in ozone layer depletion by reacting with & stratospheric ozone to form Oo and nitrogen dioxide. Nitrogen monoxide can
I 30 transform into nitrogen dioxide with the hydroperoxyl radical or with diatomic a oxygen. Symptoms of short-term nitrogen dioxide exposure of humans include x nausea, dyspnea and headache. Long-term effects include for example = impaired Immune and respiratory function.
O
N
Flue gas refers to a combustion exhaust gas produced in a furnace, for example a furnace of a power plant. Composition of the flue gas depends on what is being burned. Typically, the flue gas comprises nitrogen, carbon dioxide, water vapor and oxygen. It further comprises a number of pollutants, such as particulate matter, carbon monoxide, nitrogen oxides and sulphur oxides. The flue gas refers to a combustion exhaust gas that is produced 5 during combustion of a mixture of air and fuel. Especially fossil fuels, particularly coal, produces nitrogen oxides when being combusted.
In order to reduce and/or avoid the effects caused by NOx emissions, flue gas
NOx emission limits have been introduced. There is variation in the emission limits, but flue gas nitrogen oxide limit may be for example 150 mg/Nm3.
As already mentioned, use of ozone as an oxidant to oxidize nitrogen monoxide of flue gas into nitrogen oxides that are soluble in water and thus subsequently removable from the flue gas by scrubbing, is known. Nitrogen monoxide is oxidized by ozone to nitrogen dioxide, that has a moderate solubility in water. In order to improve the solubility of NO», certain reducing chemicals have to be utilized. In certain conditions it is possible to oxidize NO? further to N2Os, that is more soluble to water than NO? and does not require any further chemicals to be used for improving its solubility.
Oxidation of nitrogen monoxide by ozone to N2Os is called deep oxidation. In order to ensure the deep oxidation to take place, long enough residence time for the oxidation reaction, low enough reaction temperature and high enough
O3/NO molar ratio are required. For effective deep oxidation O3/NO molar ratio of higher than 1.6 is preferred. Further, the reaction temperature should be at
N most 130 degrees C, preferably below 120 degrees C. Residence time of at
N least 1 second, for example from 1 to 3 seconds or from 2 to 3 seconds is 2 preferred.
N
E 30 Ozone (or trioxygen) is an inorganic molecule with the chemical formula a O3.0zone is a powerful oxidant. Ozone cannot be stored and transported like x other industrial gases because of its quick decay into diatomic oxygen. = Therefore, ozone must be produced on site. Ozone generator may be used to
N produce ozone from oxygen. Ozone generator may function for example by using electric discharge to produce ozone by breaking apart oxygen molecules into single atoms, which then attach to diatomic oxygen to form ozone.
Presently known solutions utilize a system wherein ozone gas is fed to a reaction chamber into the flue gas stream. In such systems the ozone feed has to be dimensioned to the entire flue gas stream, and a big ozone generator as well as great electrical power for ozone generation are required. Further, large/long enough reaction chamber or equivalent is required in order to provide long enough residence time.
In optimal conditions wherein the deep oxidation takes place completely the treated flue gas may at its best have N2Os content of about 85 vol. % and NO: content of about 15 vol.%. In that case a nitrogen oxide reduction of over 90 % is obtainable. In case only 50 % reduction in NOx is required, it is possible to reduce the amount of ozone fed to process and thus have savings in its consumption. However, this leads to a lower O3/NO molar ratio and thus less —N205 with respect to NO? is formed, and NOz2 will be the main oxidation component. When no reduction chemicals are used, only about 20 % of the
NO: can be removed by scrubbing.
After scrubbing, the residual flue gas obtained from the scrubber mainly comprises NOx in the form of nitrogen dioxide. The flue gas having a NO? content as low as 40-100 mg/Nm? may appear as yellow or brown. Although such low contents may meet the emission levels, the yellow or brown(ish) colour of the flue gas is undesired because of aesthetic reasons.
An exemplary calculation for a power plant producing a flue gas flow of 60
N Nm3/s and the flue gas flow containing 300 mg/Nm3 of NOx under conditions
N wherein the NOx emission limit is 150 mg/Nm3 is shown in Table 1. In this 2 example ozone is fed into the entire flue gas flow and the entire ozone treated & flue gas flow is subsequently directed into a conventional NOx scrubber.
Ek 30 a
N Table 1. 3 Substance | Amount| Unit] 5 Fluegasfow | eo] Nmisdry &
rr
OJNOmolarratio = | dA7) rr rr
As illustrated in Table 1, 50 % NOx reduction requires 79 kg/h ozone and still the residual flue gas contains 143 mg/Nm3 NO», which provides the residual flue gas with undesired yellow or brown colour. It has been calculated that in order to avoid the yellow or brown colour of the residual flue gas, i.e. in order to reach low enough NOz2 content in the outcoming flue gas in the conventional system according to this example, ozone amount of 110 kg/h is required. This results into a content of 41 mg/Nm? of total NOx out (36 mg/Nm3 of NO»).
This specification aims to provide an improved method and a system that e utilize ozone oxidation for nitrogen oxide removal from flue gases. With the
S solution disclosed herein the NOx emission limits are reachable with minimal
S consumption of ozone and thus minimal amount of the liquid oxygen needed a for its production as well as with minimal consumption of electrical power by
N 15 the ozone generator. =
N A method of removing nitrogen oxides from flue gas is provided. The method 3 comprises providing a flow of the flue gas comprising nitrogen oxides in a first
N conduit and directing a portion of the flue gas from the first conduit into a side
N 20 stream conduit. The method further comprises providing ozone into the side stream conduit and oxidizing the portion of the flue gas in the side stream conduit by the ozone, thus producing oxidized portion of the flue gas. The method further comprises conducting at least the oxidized portion of the flue gas into a scrubber and scrubbing the flue gas comprising nitrogen oxides in the scrubber with a scrubbing liquid in order to remove at least some of the nitrogen oxides.
In case the flue gas provided in the first conduit is already at a temperature suitable for deep oxidation, i.e. at most 130 degrees C, no separate cooling step is needed. This may be the case if, for example, a quench or a another scrubber precedes the system described herein. In case the flue gas provided in the first conduit has not been cooled and/or has a temperature higher than 130 degrees C, the method further comprises cooling the portion of the flue gas in the side stream conduit.
By the side stream conduit optimal conditions for NOx oxidation may be provided. The side stream conduit may be dimensioned according to the prevailing needs. The side stream conduit provides a way to oxidize only the amount of the flue gas NOx that is necessary for reaching the prevailing NOx emission limits.
As only a portion of the flue gas flow is oxidized by ozone treatment before scrubbing, savings via lower consumption of ozone and electricity are achieved. Thus, a method with lower operating costs is provided. In an exemplary case, wherein the NOx emission limit to be met is over 50 mg/Nm? and the need for reducing the NOx emissions is from 25 to 75 %, the method
N disclosed herein may provide even from 15 to 25 % lower electricity
N consumption and ozone consumption, when compared to a situation wherein 2 the entire flue gas flow is to be treated by the ozone. This also has the benefit & that a smaller ozone generator and a smaller reaction chamber are needed, x 30 which means less investment costs.
N
SB According to an embodiment, the method further comprises determining a = nitrogen oxide content of the flue gas prior to directing the portion of the flue
N gas into the side stream conduit. The method further comprises determining a total amount of the flue gas flow (for example in Nm?/s). The portion of the flue gas directed to the side stream conduit may be adjusted by the determined nitrogen oxide content and the total amount of the flue gas flow.
The amount of the flue gas to be directed to the side stream conduit may be calculated by subtracting the NOx emission limit from the determined NOx content of the flue gas. The amount of ozone provided to the side stream conduit may be adjusted by the determined nitrogen oxide content. The ozone may be provided into the side stream conduit in such an amount that an O3/NO molar ratio is higher than 1.6, for example 1.8. Oxidizing the portion of the flue gas in the side stream conduit by the ozone may be performed at a temperature of at most 130 degrees C, preferably below 100 degrees C or even more preferably below 90 degrees C. The residence time for oxidation of the portion of the flue gas may be at least 1 second, for example from 1 to 3 seconds or from 2 to 3 seconds.
According to an embodiment, the ozone is provided by means for providing ozone into the side stream conduit. For example, the ozone may be provided by an ozone generator into a reaction chamber comprised by the side stream conduit.
The method comprises directing the ozone treated flue gas from the side stream conduit into the scrubber for scrubbing. The ozone treated flue gas mainly comprises N>Os that is highly soluble in water and may be readily scrubbed from the gas by aqueous scrubbing liquid.
N The residual flue gas wherefrom the portion to the side stream conduit has
N been separated may also be directed into the scrubber for scrubbing. The 2 treated flue gas and the residual flue gas may be directed to the scrubber via & separate inlets.
Ek 30 a In case the flue gas is such that only NOx is to be removed, said residual flue x gas may be directed to pass the scrubber. In that case, only the ozone treated = flue gas is scrubbed in the scrubber.
O
N
In the scrubber, the flue gas containing nitrogen oxides is scrubbed with the scrubbing liquid by contacting the flue gas with the scrubbing liquid. When contacted with the scrubbing liquid, at least some of the nitrogen oxides contained by the flue gas are transferred into the scrubbing liquid. Thus, scrubber bleed, i.e. the used scrubbing liquid is formed. The flue gas from which at least some of the nitrogen oxides are removed by the scrubbing liquid, is directed out of the scrubber.
The scrubbing liquid is aqueous scrubbing liquid. Thus, the scrubbing liquid comprises or consists of water. In case at least some of the used scrubbing liquid is circulated back into the scrubber for reuse (see Fig. 4), the scrubbing liquid comprises water and dissolved impurities removed from the flue gas, such as nitrogen oxides. The fresh, i.e. the un-used scrubbing liquid may be substantially pure water, for example fresh water. The water may for example comprise or consist of water obtained from a fresh water source. The water obtained from the fresh water source may be purified, for example filtered.
Alternatively, the fresh, i.e. the un-used scrubbing liquid may comprise or consist of for example tap water or make-up water of the boiler. The tap water and the make-up water of the boiler may also be in some way purified before use.
According to an embodiment, a system for removing nitrogen oxides from flue gas is provided. An exemplary illustration of the system is shown in Fig. 1. The method as described herein is implementable by the system. The system is connected to a source configured to produce flue gas containing nitrogen oxides. The system comprises a first conduit 101, a scrubber 102, a side stream conduit 105, directing means 104 for directing a portion of the flue gas
N from the first conduit into the side stream conduit, and means for providing
N ozone 107. © & The first conduit 101 is configured to conduct flue gas comprising nitrogen
I 30 oxides. The first conduit 101 may be connected to a source configured to a produce flue gas containing nitrogen oxides. The first conduit 101 may be x further connected to the scrubber 102. The means for directing the portion of = the flue gas (directing means 104) from the first conduit 101 into the side
N stream conduit 105 may be arranged as part of the first conduit 101. The — directing means 104 may be for example a single leaf damper. The first conduit 101 is preferably composed of carbon steel. The first conduit 101 may be provided with sensors configured to determine a nitrogen oxide content of the flue gas and total amount of the flue gas within the first conduit 101. The sensors are located prior to the directing means 104 configured to direct a portion of the flue gas from the first conduit 101 into the side stream conduit 105.
The side stream conduit 105 is arranged to diverge from the first conduit. The system may comprise cooling means 106 arranged as part of the side stream conduit. In order to illustrate the optionality of the cooling means 106, the cooling means 106, 206, 306, 506 is drawn with dashed lines in Figs. 1-3 and 5. The cooling means 106 may comprise for example a heat exchanger. Prior to the cooling means 106, the side stream conduit 105 may be composed of carbon steel. From the cooling means 106 onwards, the side stream conduit 105 may be composed of stainless steel in order to tolerate the cooled flue gas conditions. As cooling is performed for only part of the total flue gas flow, a stainless steel conduit of a smaller area is needed when compared to a situation wherein the entire flue gas flow were to be cooled for the oxidation.
Therefore, the present solution brings savings in materials costs, as the need of expensive stainless steel material is diminished. Further, less efficient or smaller cooling means may be utilized, as there is no need to cool the entire flue gas flow, but only a portion of it.
The side stream conduit 105 is configured to receive ozone via means for providing ozone 107. The side stream conduit 105 may comprise an ozone inlet. The side stream conduit 105 may comprise a reaction chamber which
N comprises the ozone inlet. The ozone inlet may be connected to an ozone
N generator or other source of ozone. The means for providing ozone 107 may 2 comprise an ozone inlet and an ozone generator or other source of ozone. The & side stream conduit 105 may be further connected to the scrubber 102. The
I 30 side stream conduit 105 may be directly connected to the scrubber 102 or the a side stream conduit 205 may be connected to the scrubber 202 via the part of 3 the first conduit 201a, as illustrated in Fig. 2.
N
N The scrubber 102 comprises at least one inlet for the flue gas. According to an embodiment, the scrubber 102 comprises two inlets for the flue gas. In that case first inlet may be in connection with the first conduit 101 and the second inlet may be in connection with the side stream conduit 105. The scrubber 102 comprises a line 103 for conveying scrubbing liquid into the scrubber 102.
Fig. 2 illustrates an example, wherein the scrubber 202 comprises only one inlet for the flue gas. The side stream conduit 205 is connected to the scrubber 202 via the first conduit 201. The part of the first conduit 201 that is arranged to receive the contents of the side stream conduit 205 and to convey the contents to the scrubber 202 is denoted with reference number 201a. In that case the part of the first conduit 201a that is arranged to receive the flue gas of the side stream conduit 205 may be composed of stainless steel.
Fig. 3 illustrates another example, wherein the scrubber 302 comprises only one inlet for the flue gas. Only the side stream conduit 305 is connected to the scrubber. Such system is suitable in case the flue gas is such that only NOx is to be removed and only the ozone treated flue gas is to be scrubbed in the scrubber. The residual flue gas from the first conduit 301, that is not directed to the side stream conduit 305, is directed to pass the scrubber. For example, the residual flue gas from the first conduit 301, that is not directed to the side stream conduit 305, is directed directly to a stack.
As illustrated in Fig. 4, the scrubber 402 may comprise a connection 414 for providing the flue gas of the first conduit 401 and/or the second conduit 405 or the part of the first conduit 401a. the scrubber 402 may comprise a contact area 410 configured to improve contact between the flue gas and the scrubbing liquid sprayed onto the flue gas. The contact area 410 may also be called a
N washing stage. The contact area may comprise for example a packed bed
N column. The packed bed column is filled with a packing material. The packed 2 bed column may be randomly filled with the packing material or it may & comprise a structured packing. Alternatively, the flue gas may be contacted
I 30 with the scrubbing liquid without a packed bed column, for example in a bath a of a scrubbing liquid, through which the flue gas runs, or by spraying the x scrubbing liquid onto a flow of the flue gas. The scrubber may comprise = nozzles 411 configured to spray a scrubbing liguid onto the flue gas. When
N contacted with the scrubbing liguid, at least some of the nitrogen oxides contained by the flue gas are transferred into the scrubbing liquid. The scrubber 402 may comprise an outlet 408 for letting out the scrubber bleed,
i.e. the used scrubbing liquid. The scrubber 402 also comprises an outlet 409 configured to direct the scrubbed flue gas out of the scrubber.
The scrubber 402 may be provided with means for circulating the used scrubbing liquid back into the scrubber for further use. This is illustrated in Fig. 4 with a dashed line to represent line 413 configured to circulate at least some of the used scrubbing liquid from the outlet 408 via a pump 412 to be combined with the fresh scrubbing liquid of 403 and to be directed via line 403a into the scrubber 402. The pump 412 may be arranged to pump the used scrubbing liquid or the remaining portion of it, if some of it is recirculated, via line 408a for disposal or further use.
Fig. 5 illustrates an exemplary embodiment, wherein the first conduit 501 is provided with sensors 520 configured to determine a nitrogen oxide content of the flue gas and total amount of the flue gas within the first conduit 501. The sensors are located prior to the directing means 504 configured to direct a portion of the flue gas from the first conduit 501 into the side stream conduit 505. The portion of the flue gas directed to the side stream conduit 505 may be adjusted by the determined nitrogen oxide content and the total amount of the flue gas flow. Fig. 5 shows a processor 521 that may be configured to control the directing means 504 by taking into account the data measured by the sensors 520. Additionally or alternatively, the processor 521 may be configured to control the means for providing ozone 507. The processor 521 may control the amount of ozone provided to the side stream conduit by taking into account the nitrogen oxide content determined by the sensor 520. The
N processor 521 may control the ozone feed such that the O3/NO molar ratio of 5 the side stream conduit 505 is higher than 1.6, for example 1.8. & Even if not shown in Figs. 1-3, sensors 520 and processor 521 may be used
I 30 also in those embodiments. Further, the exemplary scrubber 402 of the Fig. 4 a is useable in all discussed embodiments. Moreover, the system disclosed x herein may further comprise all necessary lines, pumps, valves etc. for = implementing the method disclosed herein.
O
N
Table 2 shows an exemplary calculation for the method and system disclosed above. The flue gas flow (60 Nm?/s) and the NOx contents (300 mg/Nm3) as well as the NOx emission limit (150 mg/Nm3) correspond to those of example illustrated in Table 1. However, in this example 55 % of the flue gas flow has been directed to the side stream conduit for oxidizing and O3/NO molar ratio of 1.8 has been utilized.
Table 2.
Total Oz injection | No Oz (55 %) injection (45 %)
Fluegasfow| 60] 33] 27] Nm¥sdry
NO>
NO-% of 100 100 100 % incoming
NOx
NO
.
Tl 11. ratio
NO oxidation | 542) 986] 00] % n reduction
S | |. & o
N
E | |. 3 Osneeded | 651, | | kyl] 2 Electricity | esos | | ww]
S
When compared to the situation illustrated in Table 1, 18 % reduction in the consumption of electricity is achieved (650,8 kW vs. 791,1 kW) when directing only 55 % of the flue gas to the side stream conduit for oxidizing. Further, consumption of ozone is also 18 % lower (65,1 kg/h vs. 79,1 kg/h). The residual flue gas, i.e. the outcoming flue gas, has NO? content of only 12,3 mg/Nm3, which is substantially lower than that of the example shown in Table 1 (143,3 mg/Nm?3). Thus, the unwanted yellow or brown(ish) colour of the flue gas is avoided by using less ozone, when compared to the situation wherein the ozone is fed into the entire flue gas flow and the entire ozone treated flue gas flow is subsequently treated in a conventional NOx scrubber.
As a conclusion, savings of costs related to consumption of electricity and ozone are achieved by the method and system disclosed herein. As already mentioned, the method and the system disclosed herein may provide even from 15 to 25 % lower electricity consumption and ozone consumption, especially in cases wherein the NOx emission limit to be met is over 50 mg/Nm? and the need for reducing the NOx emissions is from 25 to 75 %. As the consumption of resources is lower, also a greener and more sustainable approach is provided. The above disclosed method also enables use of a smaller ozone generator and a smaller reaction chamber, thus reducing the investment costs. e]
N
O
N
O
©
N
I a a
N
0)
O
LO
N
O
N
Claims (6)
1. A method of removing nitrogen oxides from flue gas, the method comprising - providing a flow of the flue gas comprising nitrogen oxides in a first conduit (101, 201, 301, 401, 501), - directing a portion of the flue gas from the first conduit into a side stream conduit (105, 205, 305, 405, 505), - cooling the portion of the flue gas in the side stream conduit (105, 205, 305, 405, 505), - providing ozone into the side stream conduit (105, 205, 305, 405, 505), - oxidizing the portion of the flue gas in the side stream conduit (105, 205, 305, 405, 505) by the ozone in order to provide oxidized portion of the flue gas, - conducting at least the oxidized portion of the flue gas into a scrubber (102, 202, 302, 402, 502), and - scrubbing the flue gas comprising nitrogen oxides in the scrubber (102, 202, 302, 402, 502) with a scrubbing liquid in order to remove at least some of the nitrogen oxides, wherein the ozone is provided into the side stream conduit (105, 205, 305, 405, 505) in such an amount that an O3/NO molar ratio is higher than 1.6, and wherein a residence time for oxidation of the portion of the flue gas is at least 1 second.
N
2. The method according to claim 1, the method further comprising N - determining a nitrogen oxide content of the flue gas and total amount 2 of the flue gas flow within the first conduit (101, 201, 301, 401, 501) & prior to directing the portion of the flue gas into the side stream x 30 conduit (105, 205, 305, 405, 505). N SB
3. The method according to claim 2, the method further comprising = - adjusting the portion of the flue gas directed to the side stream N conduit (105, 205, 305, 405, 505) by the determined nitrogen oxide content and the total amount of the flue gas flow.
4. The method according to claim 2 or 3, the method further comprising - adjusting an amount of ozone provided to the side stream conduit (105, 205, 305, 405, 505) by the determined nitrogen oxide content.
5. The method according to any of the preceding claims, wherein oxidizing the portion of the flue gas in the side stream conduit (105, 205, 305, 405, 505) by the ozone is performed at a temperature of at most 130 degrees C.
6. The method according to any of the preceding claims, wherein the ozone is provided by an ozone generator into a reaction chamber comprised by the side stream conduit (105, 205, 305, 405, 505). e] N O N O © N I = N 0) O LO N O N
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