WO2010016786A1 - Method and system for the reduction of the content of ammonia in flue gases from pulp mills - Google Patents

Method and system for the reduction of the content of ammonia in flue gases from pulp mills Download PDF

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Publication number
WO2010016786A1
WO2010016786A1 PCT/SE2008/050910 SE2008050910W WO2010016786A1 WO 2010016786 A1 WO2010016786 A1 WO 2010016786A1 SE 2008050910 W SE2008050910 W SE 2008050910W WO 2010016786 A1 WO2010016786 A1 WO 2010016786A1
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WIPO (PCT)
Prior art keywords
liquid
gas
trap
liquid trap
ammonia
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Application number
PCT/SE2008/050910
Other languages
French (fr)
Inventor
Sofia BLOMSTRÖM
Robert Lundahl
Original Assignee
Metso Power Ab
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Application filed by Metso Power Ab filed Critical Metso Power Ab
Priority to PCT/SE2008/050910 priority Critical patent/WO2010016786A1/en
Publication of WO2010016786A1 publication Critical patent/WO2010016786A1/en

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23G—CREMATION FURNACES; CONSUMING WASTE PRODUCTS BY COMBUSTION
    • F23G7/00—Incinerators or other apparatus for consuming industrial waste, e.g. chemicals
    • F23G7/04—Incinerators or other apparatus for consuming industrial waste, e.g. chemicals of waste liquors, e.g. sulfite liquors
    • 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
    • B01D53/14—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 by absorption
    • 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
    • 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
    • B01D53/58—Ammonia
    • 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
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/75—Multi-step processes
    • 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
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/77—Liquid phase processes
    • B01D53/78—Liquid phase processes with gas-liquid contact
    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C11/00—Regeneration of pulp liquors or effluent waste waters
    • D21C11/06—Treatment of pulp gases; Recovery of the heat content of the gases; Treatment of gases arising from various sources in pulp and paper mills; Regeneration of gaseous SO2, e.g. arising from liquors containing sulfur compounds
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23G—CREMATION FURNACES; CONSUMING WASTE PRODUCTS BY COMBUSTION
    • F23G5/00—Incineration of waste; Incinerator constructions; Details, accessories or control therefor
    • F23G5/02—Incineration of waste; Incinerator constructions; Details, accessories or control therefor with pretreatment
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23G—CREMATION FURNACES; CONSUMING WASTE PRODUCTS BY COMBUSTION
    • F23G7/00—Incinerators or other apparatus for consuming industrial waste, e.g. chemicals
    • F23G7/06—Incinerators or other apparatus for consuming industrial waste, e.g. chemicals of waste gases or noxious gases, e.g. exhaust gases
    • F23G7/061—Incinerators or other apparatus for consuming industrial waste, e.g. chemicals of waste gases or noxious gases, e.g. exhaust gases with supplementary heating
    • F23G7/065—Incinerators or other apparatus for consuming industrial waste, e.g. chemicals of waste gases or noxious gases, e.g. exhaust gases with supplementary heating using gaseous or liquid fuel
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2251/00—Reactants
    • B01D2251/10—Oxidants
    • B01D2251/11—Air
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/30—Sulfur compounds
    • B01D2257/302—Sulfur oxides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/40—Nitrogen compounds
    • B01D2257/406—Ammonia
    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C11/00—Regeneration of pulp liquors or effluent waste waters
    • D21C11/06—Treatment of pulp gases; Recovery of the heat content of the gases; Treatment of gases arising from various sources in pulp and paper mills; Regeneration of gaseous SO2, e.g. arising from liquors containing sulfur compounds
    • D21C11/08—Deodorisation ; Elimination of malodorous compounds, e.g. sulfur compounds such as hydrogen sulfide or mercaptans, from gas streams
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
    • Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Definitions

  • the present invention concerns a method and a system for the reduction of ammonia in strong gases from pulp mills containing sulphur compounds according to the preambles to claims 1 and 4, respectively.
  • the problem that is discussed here is that the content of ammonia in the strong gases creates nitrogen oxides (NO ⁇ ) during combustion with excess air.
  • the strong gases are also known as LVHC-gases (an abbreviation for Low Volume High Concentration) and constitute a concentrated gas, the concentration of which exceeds the explosive range.
  • Weak gases, or dilute gas, also known as HVLC-gases (an abbreviation for High Volume Low Concentration) constitute a gas whose concentration lies under the explosive range. Strong gases and weak gases are collected from different sources in a pulp mill.
  • US 4,431 ,617 reveals a system for the scrubber of strong gases in a gas scrubber in which it is intended that the TRS contents be removed.
  • a solution of sodium sulphide and sodium carbonate is used as the gas-scrubber liquid in this case.
  • WO 99/47746 reveals a system for reducing the content of ammonia in strong gases before they are combusted.
  • the ammonia can be separated out in a molecular sieve, although it is mentioned also here that ammonia can be washed from the strong gases in a scrubber.
  • the strong gas systems at pulp mills contain conventional pumps, preferably in the form of steam ejectors for the transport of gas, liquid traps and gas scrubbers in various sequences before the strong gases are led to the combustion process.
  • a heavy emphasis is placed on controlling the systems such that the rate of flow in the combustion systems, in particular in the gas nozzles, exceeds by a considerable amount the speed of travel of the flame front in the strong gases.
  • the strong gases are maintained at a concentration that exceeds the explosive concentration, but a disturbance in the systems may lead to the concentration temporarily falling, and the risk then arises for an explosion, as does the risk that the strong gases begin to combust backwards from the burner into the system.
  • Simple liquid traps are therefore installed in the strong gas systems in order to stop any blow-back combustion: these liquid traps are monitored only to ensure that the liquid level is maintained, something that takes place through the continuous supply of a small flow of liquid.
  • a further function of the liquid trap is that of collecting gases from different sources and keeping these separate from each other.
  • a first purpose of the invention is to reduce the content of ammonia in the strong gases in a simpler manner, whereby the strong gases can be led to combustion with excess air for the oxidation of primarily the foul-smelling sulphur compounds and where it is possible to reduce the formation of nitrogen oxides, NO ⁇ , due to a lower content of ammonia.
  • a second purpose is that of integrating the separation of ammonia with the liquid traps that are required in the strong gas systems, whereby an improved separation of the ammonia takes place in one and the same process unit.
  • the complexity of the system is considerably reduced in this case relative to that of a system that has an independent liquid trap and an independent gas scrubber.
  • Figure 1 shows a system according to the invention in which the gas scrubber is integrated into the liquid trap of the strong gas system.
  • Figure 1 shows a system for the reduction of ammonia in strong gases from pulp mills containing sulphur compounds.
  • the strong gases are collected from a number of different positions 2a/2b/2c in the pulp mill through separate pipes 20a/20b/20c to the liquid trap and onwards in a system of pipes 22-23 to be led to a burner C 2 , in which combustion takes place with excess air in order to bring about the oxidation of the sulphur compounds.
  • the strong gases may come from, for example: • the cooking process, where the liquor steam, for example, from the pressure-release tanks of the black liquor contains TRS gases, • the evaporation process, in which TRS gases are bled from the handling of foul condensate and from the heavy liquor cistern,
  • the pipe system 20a/20b/20c - 22-23 comprises at least one liquid trap 1a through which the strong gases are caused to pass.
  • the liquid trap may maintain the different gas sources 2a, 2b, 2c, separated and it may function as a non-return valve that prevents blow-back combustion in the pipe system.
  • the liquid trap 1a is integrated with a gas scrubber 40 into one single unit. Liquid Liqi is supplied to the gas scrubber through the line 46 and the regulator valve 47, and it maintains the liquid trap in a filled condition while the liquid is circulated by a pump 43 from the liquid trap 1a to the gas scrubber 40 and back again to the liquid trap 1a.
  • FCT Foul Condensate Tank
  • new liquid Liqi is added to the liquid circulation through a clean liquid connection, in order to maintain the liquid level in the liquid trap.
  • FCT Foul Condensate Tank
  • This addition takes place in the drawing through the valve controlled by a level sensor LC.
  • the withdrawal from the liquid trap can be designed as a fixed or variable throttle 44 on the pressurised side of the pump 43.
  • the drawing shows how the valve can be controlled by a sensor S1 , which sensor may be, preferably, a pH sensor. The amount of ammonia that has been dissolved in the liquid is proportional to the pH value.
  • the gas scrubber 40 comprises a bed of packing material where the strong gases are led into the bed through an inlet in the bottom of the bed, while the liquid is distributed by a spreader 41 across the upper part of the bed.
  • a drop separator 42 is preferably arranged at the top of the gas scrubber, which drop separator may be designed in a conventional manner as a wire mesh separator or as a cassette with guide blades.
  • the bed in the gas scrubber is what is known as a "packed bed” with packing material that maintains a nominal size of 25-75 mm and that gives an inner area of contact in the bed of at least 50-100 m 2 /m 3 , preferably 300-600 m 2 /m 3 .
  • the total volume of the bed is to be greater than 0.5 m 3
  • the total surface area is to be greater than 150 m 2 .
  • the amount of scrubber liquid that is sprayed across the bed is to be sufficient to maintain the surface of the bed in a liquid- coated state, preferably of the order of magnitude such that a film of liquid of theoretical thickness 0.1-1 mm is formed across the complete bed.
  • a structured bed with a corresponding area of contact, may be used.
  • the bed of the gas scrubber 40 is arranged in the drawing above the liquid level in the liquid trap, and the liquid that is led over the packing material runs directly downwards into the liquid trap.
  • the strong gases are led into the liquid trap 1a through several inlet pipes, 20a, 20b, 20c, that open out under the level of liquid, each such inlet pipe having a dedicated gas source 2a, 2b, 2c. It is advantageous for the scrubber function that the strong gases are distributed into lesser sub-flows in the liquid trap, as is shown in the drawing with several inlet pipes 20a-20c, which function as injection pipes into the liquid trap.
  • the distribution of the gas in the liquid trap may, of course, take place using other gas distribution arrangements than the ones shown in the drawing.
  • the strong gases will by this design be led down into the liquid trap as is shown schematically in the drawing with flow arrows, and the gases will then emerge from the surface of the liquid trap before the gas flow 21 is connected up towards the gas scrubber 40.
  • the strong gases will in this way be exposed to a first removal of ammonia in the liquid trap, even if the duration of contact between liquid and gas is relatively short.
  • the liquid in the liquid trap 1a is held in an enclosed tank 60 with a first chamber 1a with an overflow barrier 61 to at least one second chamber 1b, and that the circulation pump 43 is connected to the first, in the drawing to the second, chamber in the enclosed tank.
  • the overflow barrier is simply a dividing wall with a height that ensures that the inlet pipes 20a protrude down into the volume of liquid in the first chamber 1a.
  • the bed 40 is arranged in an outlet housing 63 in the upper part of the tank.
  • the liquid Liqi that is added to the liquid flow has a pH that lies under 8, and that the liquid that is recirculated is maintained at a pH that lies under 8.
  • the liquid Liqi that is added to the liquid flow may be constituted to a major part by water, possibly with the addition of acidifiers Ac through a connection 62.
  • the addition of acidifiers Ac may be controlled by the current value of pH in the flow, or a fixed amount may be added that is proportional to the flow of clean scrubber liquid LJq 1 that is added. If clean water at room temperature is used, up to 48 g of ammonia can be dissolved in 100 g water under ideal conditions, giving a concentration of ammonia of 32% by weight.
  • the pH increases as the amount of ammonia in the water rises, and at a concentration of as low as 1% ammonia in water the pH will be approximately 11.7, while it will be approximately 12.4 at 10% and approximately 13.5 at 30%. It is also an advantage in order to increase the solubility of ammonia in the liquid that it is cold. It has proved to be the case that the solubility of ammonia in water falls as the temperature increases as shown below:
  • At least one cooler 64 is situated in the liquid flow in order to reduce the temperature of the liquid, here placed between the pump 43 and the spreader 41.
  • the withdrawal of ammonia from the circulation either through the liquid or directly through separation can be empirically determined as a fixed volume per unit time, or a fixed volume per unit volume of gas that passes the liquid trap.
  • the withdrawal of ammonia can take place in a manner that involves feedback: it may, for example, depend on the pH value detected in the liquid that circulates.
  • Liquid should be bled from the system when the rate at which ammonia dissolves in the liquid begins to fall. Since it is difficult to dissolve more than 30-32% of ammonia by weight in water at room temperature, and since the rate at which it dissolves falls as this maximum is approached, an optimal limiting value at which the liquid should be bled lies in the interval 15-20%.
  • the subfraction of the recirculated liquid that is withdrawn be sent to the FCT (the foul condensate tank), which is a buffer tank for a subsequent scrubber procedure using a stripper ST and a methanol column MC, in series with a cooler. It is normal that these process units FCT/ST/MC are available at the pulp mill for the handling of foul condensate 52 from the evaporation process.
  • FCT the foul condensate tank
  • the strong gases after passing the liquid trap 1a and the gas scrubber 40, 41 , 42 are forwarded by a pump 50 followed by a drop separator 31 and by flame protection 32, onwards in the system to final combustion.
  • a water ring pump 50 is shown in this position in the drawing, but a steam ejector may alternatively be used for gas transport. It is appropriate that the condensate from the drop separator 31 be led to the liquid circulation of the water ring pump, as shown in the drawing.
  • the liquid ring pump 50 in the pipe system for transport onwards of the strong gases has a recirculation of liquid, preferably water, to its liquid ring from a liquid tank 51. It is possible also to bleed out a fraction of this liquid to the foul condensate tank FCT through the line 53, while clean replacement liquid Uq 2 is added to the tank 51.
  • liquid preferably water
  • a dot-dash line is used in the drawing to indicate an alternative in which water for the binding of ammonia is led in countercurrent flow through the system such that the clean liquid Uq 2 is added to the circulation system of the liquid ring pump, after which it is bled out through the dash-dot line to the circulation of the water trap, whereby the addition of liquid Liqi is now no longer necessary. Ammonia is finally bled out to the foul condensate tank FCT through the valve 44.
  • the combustion of the strong gases takes place in the embodiment shown in a second stage C 2 in a two-stage burner, in which a combustion is initiated in the first stage Ci while the combustion of methanol takes place with a deficiency of air (substoichiometric combustion), and final combustion takes place in a second stage C 2 with excess air (superstoichiometric combustion).
  • This type of burner is marketed by Metso Power under the name DecNOjTM. This type of burner allows ammonia that contains methanol to be combusted in the first stage Ci with minimal formation of nitrogen oxides, NO ⁇ . Since the strong gases with their content of sulphur-containing TRS are not added until the second stage C 2 , these gases can be oxidised in the manner intended.
  • That portion of the liquid that is bled out from the circulation in the liquid trap can be withdrawn directly after the liquid has flown through the bed 40 and before the liquid runs down into the liquid trap, by, for example, arranging a low collection vessel under the bed but above the surface of the liquid trap.
  • the main principle according to the invention is that the liquid trap is used to bind the ammonia in the strong gases, and that the liquid in the liquid trap is bled out and has scrubber liquid with a lower level of dissolved ammonia added to it such that the liquid trap obtains a level of dissolved ammonia that lies under the saturation limit by a certain amount.
  • the liquid trap and the subsequent gas scrubber may, in one advantageous embodiment, be integrated into a single unit, and that the liquid that circulates in the gas scrubber and in the liquid trap is the same liquid, and that the liquid is used to dissolve the ammonia in the strong gases.
  • the liquid trap prevents any blow-back combustion in the system, and the possibility is available to collect strong gases from several different sources in the liquid trap.

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  • Environmental & Geological Engineering (AREA)
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Abstract

The invention concerns a system for the reduction of ammonia in strong gases from pulp mills containing sulphur compounds (TRS). It is intended that the strong gases be destroyed in an oxidising environment without the formation of harmful nitrogen oxides. A liquid trap (1 a, 1 b) is inserted into the pipe system (20, 22, 23) for strong gases. Liquid is added to the gas scrubber and it maintains the liquid trap in a filled condition while the liquid circulates from the liquid trap to the gas scrubber and back again to the liquid trap, at the same time as a subfraction of the recirculated liquid is withdrawn (44) and new liquid (Liqi) is added to the liquid circulation in order to maintain the liquid level in the liquid trap. The liquid trap may also comprise an integrated gas scrubber (40) in one single unit. An obstacle to blow-back combustion is in this way formed, while an effective reduction in the level of ammonia in the strong gases is achieved at the same time. The amount of nitrogen oxides in the flue gases from a subsequent combustion plant for strong gases is in this way reduced.

Description

METHOD AND SYSTEM FOR THE REDUCTION OF THE CONTENT OF AMONIA IN FLUE GASES FROM PULP MILLS
Technical area
The present invention concerns a method and a system for the reduction of ammonia in strong gases from pulp mills containing sulphur compounds according to the preambles to claims 1 and 4, respectively.
The prior art
The management of foul-smelling strong gases in pulp mills is complicated, since these strong gases have compositions that include sulphur compounds, terpentine, methanol and ammonia. It is principally desired to deal with sulphur compounds, often denoted "TRS", (which is an abbreviation for "total reduced sulphur") in the form of hydrogen sulphide (H2S), methyl mercaptan (CH3SH), dimethyl sulphide (CH3SCH3) and dimethyl disulphide (CH3SSCH3). This is normally achieved through combustion with excess air such that these foul- smelling sulphur compounds can be oxidised.
The problem that is discussed here is that the content of ammonia in the strong gases creates nitrogen oxides (NOχ) during combustion with excess air. The strong gases are also known as LVHC-gases (an abbreviation for Low Volume High Concentration) and constitute a concentrated gas, the concentration of which exceeds the explosive range. Weak gases, or dilute gas, also known as HVLC-gases (an abbreviation for High Volume Low Concentration) constitute a gas whose concentration lies under the explosive range. Strong gases and weak gases are collected from different sources in a pulp mill.
Several different gas-scrubber systems are known.
US 4,001,374, for example, reveals a gas-scrubber system in which a cooled organic acid, ammonium bisulphite, is used as scrubber liquid in order to remove the ammonia from the gases emitted by a coke oven. Bisulphite is used as scrubber liquid also in WO 96/23566.
US 4,431 ,617 reveals a system for the scrubber of strong gases in a gas scrubber in which it is intended that the TRS contents be removed. A solution of sodium sulphide and sodium carbonate is used as the gas-scrubber liquid in this case.
WO 99/47746 reveals a system for reducing the content of ammonia in strong gases before they are combusted. The ammonia can be separated out in a molecular sieve, although it is mentioned also here that ammonia can be washed from the strong gases in a scrubber.
The strong gas systems at pulp mills contain conventional pumps, preferably in the form of steam ejectors for the transport of gas, liquid traps and gas scrubbers in various sequences before the strong gases are led to the combustion process. A heavy emphasis is placed on controlling the systems such that the rate of flow in the combustion systems, in particular in the gas nozzles, exceeds by a considerable amount the speed of travel of the flame front in the strong gases. The strong gases are maintained at a concentration that exceeds the explosive concentration, but a disturbance in the systems may lead to the concentration temporarily falling, and the risk then arises for an explosion, as does the risk that the strong gases begin to combust backwards from the burner into the system.
Simple liquid traps are therefore installed in the strong gas systems in order to stop any blow-back combustion: these liquid traps are monitored only to ensure that the liquid level is maintained, something that takes place through the continuous supply of a small flow of liquid. A further function of the liquid trap is that of collecting gases from different sources and keeping these separate from each other.
Since the focus has been placed on managing the TRS content of the strong gases, these liquid traps have not been regarded as potential locations for a scrubber function for the strong gases. Special gas scrubbers have been instead installed, similar to those in the previously mentioned US 4,431 ,617. The gas scrubber process in these typical gas scrubbers conventionally takes place during countercurrent flow, in which the scrubber liquid in finely divided form is led against the flow of gas, either as a finely divided spray against the flow of gas or flowing through a bed of packing material, in order to increase the area of contact between the gas and the scrubber liquid.
The purpose of the invention
A first purpose of the invention is to reduce the content of ammonia in the strong gases in a simpler manner, whereby the strong gases can be led to combustion with excess air for the oxidation of primarily the foul-smelling sulphur compounds and where it is possible to reduce the formation of nitrogen oxides, NOχ, due to a lower content of ammonia.
A second purpose is that of integrating the separation of ammonia with the liquid traps that are required in the strong gas systems, whereby an improved separation of the ammonia takes place in one and the same process unit. The complexity of the system is considerably reduced in this case relative to that of a system that has an independent liquid trap and an independent gas scrubber.
The purposes described above are achieved with a method according to the characterising part of claim 1 and a system according to the characterising part of claim 4.
Description of drawings Figure 1 shows a system according to the invention in which the gas scrubber is integrated into the liquid trap of the strong gas system.
Detailed description of the invention
Figure 1 shows a system for the reduction of ammonia in strong gases from pulp mills containing sulphur compounds. The strong gases are collected from a number of different positions 2a/2b/2c in the pulp mill through separate pipes 20a/20b/20c to the liquid trap and onwards in a system of pipes 22-23 to be led to a burner C2, in which combustion takes place with excess air in order to bring about the oxidation of the sulphur compounds. The strong gases may come from, for example: • the cooking process, where the liquor steam, for example, from the pressure-release tanks of the black liquor contains TRS gases, • the evaporation process, in which TRS gases are bled from the handling of foul condensate and from the heavy liquor cistern,
• the methanol system.
The pipe system 20a/20b/20c - 22-23 comprises at least one liquid trap 1a through which the strong gases are caused to pass. The liquid trap may maintain the different gas sources 2a, 2b, 2c, separated and it may function as a non-return valve that prevents blow-back combustion in the pipe system. According to one preferred embodiment of the invention, the liquid trap 1a is integrated with a gas scrubber 40 into one single unit. Liquid Liqi is supplied to the gas scrubber through the line 46 and the regulator valve 47, and it maintains the liquid trap in a filled condition while the liquid is circulated by a pump 43 from the liquid trap 1a to the gas scrubber 40 and back again to the liquid trap 1a. At the same time, a subfraction of the recirculated liquid is withdrawn through a control valve 44 to the Foul Condensate Tank (abbreviated as "FCT") and new liquid Liqi is added to the liquid circulation through a clean liquid connection, in order to maintain the liquid level in the liquid trap. This addition takes place in the drawing through the valve controlled by a level sensor LC. The withdrawal from the liquid trap can be designed as a fixed or variable throttle 44 on the pressurised side of the pump 43. The drawing shows how the valve can be controlled by a sensor S1 , which sensor may be, preferably, a pH sensor. The amount of ammonia that has been dissolved in the liquid is proportional to the pH value.
The gas scrubber 40 comprises a bed of packing material where the strong gases are led into the bed through an inlet in the bottom of the bed, while the liquid is distributed by a spreader 41 across the upper part of the bed. A drop separator 42 is preferably arranged at the top of the gas scrubber, which drop separator may be designed in a conventional manner as a wire mesh separator or as a cassette with guide blades.
The bed in the gas scrubber is what is known as a "packed bed" with packing material that maintains a nominal size of 25-75 mm and that gives an inner area of contact in the bed of at least 50-100 m2/m3, preferably 300-600 m2/m3. The total volume of the bed is to be greater than 0.5 m3, and the total surface area is to be greater than 150 m2. The amount of scrubber liquid that is sprayed across the bed is to be sufficient to maintain the surface of the bed in a liquid- coated state, preferably of the order of magnitude such that a film of liquid of theoretical thickness 0.1-1 mm is formed across the complete bed. Alternatively, what is known as a "structured bed", with a corresponding area of contact, may be used.
The bed of the gas scrubber 40 is arranged in the drawing above the liquid level in the liquid trap, and the liquid that is led over the packing material runs directly downwards into the liquid trap. The strong gases are led into the liquid trap 1a through several inlet pipes, 20a, 20b, 20c, that open out under the level of liquid, each such inlet pipe having a dedicated gas source 2a, 2b, 2c. It is advantageous for the scrubber function that the strong gases are distributed into lesser sub-flows in the liquid trap, as is shown in the drawing with several inlet pipes 20a-20c, which function as injection pipes into the liquid trap. The distribution of the gas in the liquid trap may, of course, take place using other gas distribution arrangements than the ones shown in the drawing. The strong gases will by this design be led down into the liquid trap as is shown schematically in the drawing with flow arrows, and the gases will then emerge from the surface of the liquid trap before the gas flow 21 is connected up towards the gas scrubber 40. The strong gases will in this way be exposed to a first removal of ammonia in the liquid trap, even if the duration of contact between liquid and gas is relatively short.
The liquid in the liquid trap 1a is held in an enclosed tank 60 with a first chamber 1a with an overflow barrier 61 to at least one second chamber 1b, and that the circulation pump 43 is connected to the first, in the drawing to the second, chamber in the enclosed tank. It is appropriate that the overflow barrier is simply a dividing wall with a height that ensures that the inlet pipes 20a protrude down into the volume of liquid in the first chamber 1a. The bed 40 is arranged in an outlet housing 63 in the upper part of the tank. In order to dissolve as much as possible of the ammonia from the strong gases in the liquid, it is advantageous if the liquid Liqi that is added to the liquid flow has a pH that lies under 8, and that the liquid that is recirculated is maintained at a pH that lies under 8. The liquid Liqi that is added to the liquid flow may be constituted to a major part by water, possibly with the addition of acidifiers Ac through a connection 62. The addition of acidifiers Ac may be controlled by the current value of pH in the flow, or a fixed amount may be added that is proportional to the flow of clean scrubber liquid LJq1 that is added. If clean water at room temperature is used, up to 48 g of ammonia can be dissolved in 100 g water under ideal conditions, giving a concentration of ammonia of 32% by weight.
The pH increases as the amount of ammonia in the water rises, and at a concentration of as low as 1% ammonia in water the pH will be approximately 11.7, while it will be approximately 12.4 at 10% and approximately 13.5 at 30%. It is also an advantage in order to increase the solubility of ammonia in the liquid that it is cold. It has proved to be the case that the solubility of ammonia in water falls as the temperature increases as shown below:
Figure imgf000007_0001
At least one cooler 64 is situated in the liquid flow in order to reduce the temperature of the liquid, here placed between the pump 43 and the spreader 41.
The withdrawal of ammonia from the circulation either through the liquid or directly through separation, can be empirically determined as a fixed volume per unit time, or a fixed volume per unit volume of gas that passes the liquid trap. Alternatively, the withdrawal of ammonia can take place in a manner that involves feedback: it may, for example, depend on the pH value detected in the liquid that circulates. Liquid should be bled from the system when the rate at which ammonia dissolves in the liquid begins to fall. Since it is difficult to dissolve more than 30-32% of ammonia by weight in water at room temperature, and since the rate at which it dissolves falls as this maximum is approached, an optimal limiting value at which the liquid should be bled lies in the interval 15-20%.
It is possible through this system for liquid with a first, higher, level of dissolved ammonia to be bled and replaced by new liquid having a second, lower, level of dissolved ammonia such that the ability of the liquid to dissolve ammonia does not reach the saturation limit.
It is appropriate that the subfraction of the recirculated liquid that is withdrawn be sent to the FCT (the foul condensate tank), which is a buffer tank for a subsequent scrubber procedure using a stripper ST and a methanol column MC, in series with a cooler. It is normal that these process units FCT/ST/MC are available at the pulp mill for the handling of foul condensate 52 from the evaporation process.
The strong gases after passing the liquid trap 1a and the gas scrubber 40, 41 , 42 are forwarded by a pump 50 followed by a drop separator 31 and by flame protection 32, onwards in the system to final combustion. A water ring pump 50 is shown in this position in the drawing, but a steam ejector may alternatively be used for gas transport. It is appropriate that the condensate from the drop separator 31 be led to the liquid circulation of the water ring pump, as shown in the drawing.
The liquid ring pump 50 in the pipe system for transport onwards of the strong gases has a recirculation of liquid, preferably water, to its liquid ring from a liquid tank 51. It is possible also to bleed out a fraction of this liquid to the foul condensate tank FCT through the line 53, while clean replacement liquid Uq2 is added to the tank 51.
There may be cooling and the addition of acidifiers in a manner equivalent to that shown for the flow through the liquid trap, in order to optimise the solubility of ammonia in the liquid that flows through the liquid ring pump.
A dot-dash line is used in the drawing to indicate an alternative in which water for the binding of ammonia is led in countercurrent flow through the system such that the clean liquid Uq2 is added to the circulation system of the liquid ring pump, after which it is bled out through the dash-dot line to the circulation of the water trap, whereby the addition of liquid Liqi is now no longer necessary. Ammonia is finally bled out to the foul condensate tank FCT through the valve 44.
The combustion of the strong gases takes place in the embodiment shown in a second stage C2 in a two-stage burner, in which a combustion is initiated in the first stage Ci while the combustion of methanol takes place with a deficiency of air (substoichiometric combustion), and final combustion takes place in a second stage C2 with excess air (superstoichiometric combustion). This type of burner is marketed by Metso Power under the name DecNOj™. This type of burner allows ammonia that contains methanol to be combusted in the first stage Ci with minimal formation of nitrogen oxides, NOχ. Since the strong gases with their content of sulphur-containing TRS are not added until the second stage C2, these gases can be oxidised in the manner intended.
The invention can be modified in a number of ways within the scope of the attached patent claims.
That portion of the liquid that is bled out from the circulation in the liquid trap, for example, can be withdrawn directly after the liquid has flown through the bed 40 and before the liquid runs down into the liquid trap, by, for example, arranging a low collection vessel under the bed but above the surface of the liquid trap.
The main principle according to the invention is that the liquid trap is used to bind the ammonia in the strong gases, and that the liquid in the liquid trap is bled out and has scrubber liquid with a lower level of dissolved ammonia added to it such that the liquid trap obtains a level of dissolved ammonia that lies under the saturation limit by a certain amount.
The liquid trap and the subsequent gas scrubber may, in one advantageous embodiment, be integrated into a single unit, and that the liquid that circulates in the gas scrubber and in the liquid trap is the same liquid, and that the liquid is used to dissolve the ammonia in the strong gases.
By bleeding out a part of the circulating liquid continuously, an effective reduction of the ammonia contents of the strong gases can be obtained, which reduces the emissions of NOχ when the strong gases are combusted. Furthermore, the liquid trap prevents any blow-back combustion in the system, and the possibility is available to collect strong gases from several different sources in the liquid trap.

Claims

1. A method for the reduction of ammonia in strong gases from pulp mills containing sulphur compounds, where the strong gases from several sources of gas (2a, 2b, 2c) are collected in a pipe system (20, 22, 23) and led to a burner (C2) in which combustion takes place with excess air for the oxidation of the sulphur compounds, and where the pipe system comprises pumps (50) for the forwarding of the strong gases and at least one liquid trap (1a) through which the strong gases from all sources of gas are caused to pass through a gas line (20a, 20b, 20c) that is dedicated to one source of gas and that opens out in the liquid trap in order to prevent blow-back combustion in the pipe system and in order to maintain the gases separated, characterised in that liquid in the liquid trap (1a) that has a first, higher, level of dissolved ammonia in the liquid is bled out and replaced by new liquid with a second, lower, level of dissolved ammonia such that the ability of the liquid to dissolve ammonia does not reach the saturation limit.
2. The method according to claim 1 , characterised in that the strong gases from each source of gas after passing the liquid trap are caused to pass also a gas scrubber (40), in which liquid is led in countercurrent flow against the flow of gas and where the liquid from the gas scrubber is subsequently led to the liquid trap.
3. The method according to claim 2, characterised in that the liquid is maintained in circulation from the liquid trap (1a) to the gas scrubber (40) and then back to the liquid trap (1a).
4. A system for the reduction of ammonia in strong gases from pulp mills containing sulphur compounds, where the strong gases from several sources of gas (2a, 2b, 2c) are collected in a pipe system (20, 22, 23) and led to a burner (C2) in which combustion takes place with excess air for the oxidation of the sulphur compounds, and where the pipe system comprises pumps (50) for the forwarding of the strong gases and at least one liquid trap (1 a) through which the strong gases from all sources of gas are caused to pass through a gas line (20a, 20b, 20c) that is dedicated to one source of gas and that opens out in the liquid trap in order to prevent blow-back combustion in the pipe system and in order to maintain the gases separated, characterised in that the liquid trap has a withdrawal outlet (45) from which liquid that has a first, higher, level of dissolved ammonia in the liquid is withdrawn from the liquid trap and a filler line (46) through which the liquid trap is filled with liquid with a second, lower, level of dissolved ammonia and a regulatory system (S1, 44, 46, 47) in order to regulate the withdrawal and the filling of liquid into the liquid trap such that the liquid in the liquid trap always lies under the limit for the amount of ammonia that can be dissolved in the liquid.
5. The system according to claim 4, characterised in that the regulatory system has feedback regulation from a parameter that is representative for the amount of ammonia dissolved in the liquid, preferably a sensor (S1) that detects pH, which parameter controls a valve (44) in the withdrawal outlet (45).
6. The system according to claim 5, characterised in that the liquid trap (1a, 1 b) is integrated into a single unit with a gas scrubber (40) arranged in series with the liquid trap, where liquid is added to the gas scrubber and maintains the liquid trap (1a, 1b) in a filled condition while the liquid is circulated from the liquid trap (1a, 1 b) to the gas scrubber (40) and back again to the liquid trap (1a) by circulation means (43, 41), while at the same time a subfraction of the recycled liquid is led through the withdrawal outlet
(45) controlled by a regulator valve (44) to the foul condensate tank (FCT) and new liquid (LJq1) is added to the liquid circulation through a filler line (46) controlled by a regulator valve (47, LC) in order to maintain the level of liquid in the liquid trap (1a, 1 b).
7. The system according to claim 6, characterised in that the liquid that is circulated passes through a cooler (64).
8. The system according to claim 6, characterised in that the gas scrubber comprises a bed (40) of packing material where the strong gases are led into the bed through an inlet in the bottom of the bed, while the liquid is distributed by a spreader (41) across the upper part of the bed.
9. The system according to claim 8, characterised in that the packing material of the bed (40) maintains a size of 25-75 mm, and that it gives an inner area of contact in the bed of at least 50-100 m2/m3, preferably 300-600 m2/m3.
10. The system according to claim 9, characterised in that the total volume of the bed (40) is to be greater than 0.5 m3, and the total surface area is to be greater than 150 m2.
11. The system according to claim 10, characterised in that the bed (40) of the gas scrubber is arranged above the liquid level in the liquid trap (1a, 1b).
12. The system according to claim 11 , characterised in that the strong gases are led into the liquid trap through at least one inlet pipe (20a, 20b, 20c) that opens out under the level of liquid in the liquid trap (1a, 1b).
13. The system according to claim 12, characterised in that one gas source (2a, 2b, 2c) is connected to at least one inlet pipe that is dedicated to the gas source.
14. The system according to claim 13, characterised in that the liquid in the liquid trap is kept in an enclosed tank (60) with a first chamber (1a) with an overflow barrier (61) to at least one second chamber (1b), and in that the circulation pump (43) is connected to the second chamber (1 b) in the enclosed tank, where the tank (60) comprises the gas scrubber (40) in an outlet housing (63) in the upper part of the tank.
15. The system according to claim 14, characterised in that the liquid that is added to the liquid circulation has a pH that lies under 8.
16. The system according to claim 15, characterised in that the liquid that is added to the liquid flow is constituted to a major part by water, possibly with the addition of acidifiers (Ac) through a connection (62).
17. The system according to claim 16, characterised in that the foul condensate tank (FCT) is a buffer tank in which the liquid that has been bled from the liquid trap (1a, 1b) is held in preparation for a subsequent scrubber procedure for liquids that contain methanol and ammonia, with a stripper (ST) and a methanol column (MC) in series in this scrubber procedure.
PCT/SE2008/050910 2008-08-08 2008-08-08 Method and system for the reduction of the content of ammonia in flue gases from pulp mills WO2010016786A1 (en)

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CN103438466A (en) * 2013-09-09 2013-12-11 北京航天动力研究所 Method and device for producing sodium pyrophosphate by forging glyphosate mother liquor at constant temperature
CN103712198A (en) * 2014-01-02 2014-04-09 潍坊亿佳节能控制科技有限公司 Multi-step steam pressure digestion absorber and application thereof
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KR20190102193A (en) * 2016-12-27 2019-09-03 스탈크랩 Equipment for the production and processing of gas streams through liquid volumes
WO2021239651A1 (en) * 2020-05-27 2021-12-02 Valmet Technologies Oy A method for removing ammonia from non-condensable gases of a pulp mill
WO2024058049A1 (en) * 2022-09-15 2024-03-21 三菱造船株式会社 Scrubber and gas treatment method in scrubber

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JP2001334126A (en) * 2000-05-29 2001-12-04 Toshiba Corp Device for treating discharge from reduction furnace
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CN103438466B (en) * 2013-09-09 2015-11-25 北京航天动力研究所 A kind of method and device utilizing glyphosate mother solution calcining at constant temperature production sodium pyrophosphate
CN103712198A (en) * 2014-01-02 2014-04-09 潍坊亿佳节能控制科技有限公司 Multi-step steam pressure digestion absorber and application thereof
CN103712198B (en) * 2014-01-02 2015-03-04 潍坊亿佳节能控制科技有限公司 Multi-step steam pressure digestion absorber and application thereof
KR20190102193A (en) * 2016-12-27 2019-09-03 스탈크랩 Equipment for the production and processing of gas streams through liquid volumes
KR102511423B1 (en) 2016-12-27 2023-03-17 스탈크랩 Equipment for the production and processing of gaseous streams through liquid volumes
FR3062799A1 (en) * 2017-02-10 2018-08-17 Starklab DEVICE FOR PRODUCING AND PROCESSING GAS FLOWS THROUGH AUTOMATICALLY CONTROLLED LIQUID VOLUME
WO2018146123A1 (en) * 2017-02-10 2018-08-16 Starklab Device for producing and treating a gas stream through an automatically controlled volume of liquid
KR20190111960A (en) * 2017-02-10 2019-10-02 스탈크랩 Apparatus for the production and processing of gas streams through automatically controlled liquid volumes
RU2751198C2 (en) * 2017-02-10 2021-07-12 Старклаб Device for creating and processing a gas stream by passing it through an automatically adjustable volume of liquid
KR102478818B1 (en) 2017-02-10 2022-12-20 스탈크랩 Apparatus for production and processing of gaseous streams with automatically controlled liquid volume
US11547966B2 (en) 2017-02-10 2023-01-10 Starklab Device for producing and treating a gas stream through an automatically controlled volume of liquid
WO2021239651A1 (en) * 2020-05-27 2021-12-02 Valmet Technologies Oy A method for removing ammonia from non-condensable gases of a pulp mill
WO2024058049A1 (en) * 2022-09-15 2024-03-21 三菱造船株式会社 Scrubber and gas treatment method in scrubber

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