Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D1/00—Evaporating
  • B01D1/0011—Heating features
  • B01D1/0058—Use of waste energy from other processes or sources, e.g. combustion gas
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D1/00—Evaporating
  • B01D1/26—Multiple-effect evaporating
• • 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
• • 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
  • D21C11/10—Concentrating spent liquor by evaporation
• • 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
  • F23G5/44—Details; Accessories
  • F23G5/46—Recuperation of heat
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES 
  • F23J15/00—Arrangements of devices for treating smoke or fumes
  • F23J15/06—Arrangements of devices for treating smoke or fumes of coolers
• • 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
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E20/00—Combustion technologies with mitigation potential
  • Y02E20/12—Heat utilisation in combustion or incineration of waste
• • 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
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E20/00—Combustion technologies with mitigation potential
  • Y02E20/30—Technologies for a more efficient combustion or heat usage
• • 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
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E50/00—Technologies for the production of fuel of non-fossil origin
  • Y02E50/10—Biofuels, e.g. bio-diesel
• • 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
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/10—Greenhouse gas [GHG] capture, material saving, heat recovery or other energy efficient measures, e.g. motor control, characterised by manufacturing processes, e.g. for rolling metal or metal working

Definitions

• the present invention relates to a method and system for recovering heat from flue gases generated in power and heat producing combustion devices.
• the flue gases are generated in the combustion devices of a power boiler, biomass mill or pulp mill, such as a recovery boiler .
• black liquor which typically has a dry solids content of over 80% (eighty percent) and combustion air are fed into a furnace of a chemical recovery boiler for burning the black liquor and recovering chemicals therefrom.
• the flue gases generated in the combustion are led into an economizer of the recovery boiler.
• the economizer heats feed water for the boiler. After flowing through the economizer, the flue gases are cleaned.
• the feed water is led from the economizer to a steam-generating bank of the boiler and into a superheater for producing steam, which may have a pressure of more than 80 bar.
• the steam flows from the recovery boiler into a steam turbine for producing electricity.
• the steam discharged from the turbine is utilized for preheating the feed water flowing into the economi zer .
• the final temperature of the flue gases may remain high when the gases are discharged from the economizer. The heat in the discharged flue gases is released to the atmosphere and the energy in the heat is lost.
• New recovery boiler plants are also beginning to apply a technique used at power plants for recovering heat from flue gas.
• the recovery is performed for generating hot water, but the heat can be utilized also for preheating combustion air.
• the flue gas coolers are located in flue gas ducts downstream of the electrostatic precipitator and upstream of the fans.
• WO 02097243 discloses an arrangement in which the temperature of the water being led into the economizer of the recovery boiler is regulated by means of bleed steam of the turbine so that the flue gases exit the economizer at a temperature of over 250°C.
• the flue gases are cleaned in at least a hot electrostatic precipitator.
• the cleaned flue gases are cooled in a preheater for combustion air or in a preheater for feed water.
• pulp mills In addition to flue gases generated by a recovery boiler, pulp mills have other combustion arrangements, such as a bark boiler and a lime kiln, which generate flue gases. Energy is produced in a pulp mill primarily from the combustion of black liquor in a recovery boiler, bark and wood wastes in an auxiliary boiler, and oil or gas in a lime sludge reburning kiln. The energy released by burning the bark of raw wood material and organic matter in the black liquor is usually sufficient to satisfy the entire energy requirement of a pulp mill. There are also pulp mills in which wood or bark is used as fuel for the lime re journeyning kiln, either as such after drying or after drying and gasifying.
• Patent application PCT/FI2011/050828 describes a process for recovering heat from flue gas generated at a biomass-based plant, such as a pulp mill or a stand ⁇ alone power boiler.
• flue gas is directed from a combustion device through a flue gas cooler and a circulation liquid is heated in the flue gas cooler with heat from the flue gas.
• the heated circulation liquid is transported to a waste heat accumulator, where the liquid is flashed. Flash vapor is extracted from the waste heat accumulator, and it can be used as a process steam.
• the flash-cooled circulation liquid is returned from the waste heat accumulator into the flue gas cooler.
• combustion devices there is a desire to improve heat recovery from flue gases generated in combustion devices at a biomass- based plant, such as a pulp mill or a power boiler plant. Flue gases are generated in combustion devices, such as in a recovery boiler, power boiler or lime kiln at the pulp mill or in a stand-alone power boiler. Combustion devices are often located at a mill or plant where different kind of liquids are concentrated by evaporation in several evaporation stages.
• effluent flows are all industrial liquid flows that bring effluent load to the mill and its environment
• such industrial liquid flows can be all flows going to the effluent water treatment plant or other flows going directly to the surrounding environment, e.g. bio-based process flows, spent liquors of the pulp and paper industry, such as black liquor, APMP effluent, CTMP effluent, or residual flows when producing bio-fuels (bio-ethanol etc.), or water-based residual flows when producing energy from wet biomass etc.
• bio-based process flows spent liquors of the pulp and paper industry, such as black liquor, APMP effluent, CTMP effluent, or residual flows when producing bio-fuels (bio-ethanol etc.), or water-based residual flows when producing energy from wet biomass etc.
• the concentration of liquids is low, typically 2000-20000 mg-effluent /dm 3 -liquid, where mg-effluent can be a COD (Chemical oxygen demand) or BOD (biological oxygen demand) or any other salts, metals or non-process elements or toxics which are beneficial to be recovered even in smaller amounts like 100-2000 mg-effluent /dm 3 - liquid.
• mg-effluent can be a COD (Chemical oxygen demand) or BOD (biological oxygen demand) or any other salts, metals or non-process elements or toxics which are beneficial to be recovered even in smaller amounts like 100-2000 mg-effluent /dm 3 - liquid.
• the system according to present invention can also advantageously be used as an evaporator or pre- evaporator when the liquid concentration is higher than20000 mg-effluent /dm 3 -liquid, hence the integrated multi-effect evaporator is also usable for liquors of higher concentrations in the range of 20000 to 500000 mg effluent /dm 3 -liquid, such as black liquor.
• the overall evaporation capacity is then reduced by a lower number of possible evaporator stages in the evaporator train because of the higher boiling point rises in each stage.
• Use as pre-evaporator is mainly dependent on the amount to be evaporated and final concentration and number of effects then needed.
• vapor can be transferred to an existing evaporator as shown in patent application PCT/FI2011/050828.
• the method according to the invention is characterized by what is presented in the characterizing part of the independent claims 1 or 2.
• the system according to the invention is characterized by what is presented in the characterizing part of the independent claims 24 or 25.
• the flue gas is used either inside the lamellas or outside the lamellas (or outside or inside the tubes), for creating a vapor from the effluent flow to be used in the succeeding multi-effect evaporator train.
• the effluent liquid is fed into heat exchangers on the side of the wall opposite to the flue gas so that there is no direct contact between flue gas and effluent.
• the heated liquid is flashed in a flash tank.
• the heat exchangers are coupled either in counter-current, cross-current or co-current form with one or more successive flash reactor/s connected to the above mentioned heat exchangers.
• one evaporation unit or more evaporation units are connected in series (without a flash reactor) where the flue gas heat exchanger works as a normal evaporator, preferably as a falling film evaporator and where the flue gas is used instead of steam or vapor in the evaporator body outside the tubes or lamellas, or inside the tubes or lamellas, as the effluent liquid is fed at the side of the opposite wall as a falling film. Liquor having a higher concentration is produced.
• the above described is particularly beneficial because of the low specific heat transfer duty (i.e. low heat flow, kW/m2), in the gas cooler due to the small temperature differences and simultaneous low heat exchanger coefficient, which is due to a low pressure gas-liquid heat exchange process.
• the low specific heat duty does not initiate fouling when heating difficult effluents, i.e. a low specific heat transfer duty, due to low heat transfer coefficients and a low temperature difference, is used as an advantage instead of a disadvantage in the above flue gas pre-concentrat ing stage .
• the flue gas heat produces clean steam or vapor that is used in the successive evaporator train in one or more stages.
• Flue gas is directed from a combustion device into an indirect contact with a liquid to be concentrated in the multiple-effect evaporation plant.
• the liquid is an effluent flow described above.
• the liquid is heated with heat from the flue gas in a flue gas cooler having at least one indirect heat exchange stage.
• the heated liquid is discharged from the flue gas cooler and flashed in at least one flashing stage so as to produce flash-cooled liquid with a higher concentration and flash vapor.
• the flashing is performed in a tank in which the liquid is cooled to a predetermined process vapor temperature such as 110-150°C for first effect vapor and 90-110°C for front end effects and 50-90°C for backend train evaporator effects.
• the flash vapor from a first flashing stage is used as a heating medium in a first effect stage of the multiple-effect evaporation plant (effect numbering according to the flow direction of the vapor) and a part of the flash-cooled liquid is directed to the multiple effect evaporation plant while the circulated liquid in the flue gas cooler is 5-1000 times the mass of the flashed vapor.
• novel method and system are very applicable and suitable for pre-concentrating different effluents, such as black liquor of a pulp mill.
• the at least one heat exchange stage of the flue gas cooler is formed of tubes, inside which the liquid flows. When there are two or more heat exchange stages, they are placed preferably inside the same housing.
• the flue gas and the liquid may flow counter-currently, co-currently or cross-currently in the flue gas cooler.
• the flue gas and the liquid flow counter-currently in the flue gas cooler having a first and a second heat exchange stage in a flow direction of the flue gas.
• the hot flue gas is cooled by the liquid circulated from the first flashing stage and in the second heat exchange stage the flue gas is further cooled by the liquid from another step of the evaporation plant.
• a part of the flash-cooled liquid from the first flashing stage is directed to the multiple effect evaporation plant, and in the second heat exchange stage the flue gas is typically cooled by the liquid from a subsequent flashing stage.
• the point or place where the feed liquid is introduced into the system depends on the properties of the liquid, such as temperature and composition.
• the feed liquid is preferably introduced into the liquid flow between the first heat exchange stage and the inlet to the first flashing stage.
• the hot feed liquid is first cooled by flashing, and thus the temperature of the liquid is adequately low to cool the hot flue gas.
• the temperature of the feed liquid of the evaporation plant is between 50 and 110 degrees of Celsius and the flue gas and liquid flow counter-currently. Then a part or all of the feed liquid is fed into the second heat exchange inlet and/or into the discharge of the second heat exchange or into the liquid between the first flashing stage and the inlet of the first heat exchange stage, depending on the temperature of the feed liquid.
• the temperature of the feed liquid of the evaporation plant is low, i.e. below 50 degrees Celsius.
• the flue gas and liquid flow counter-currently, in the second heat exchange stage the flue gas is cooled by the feed liquid, which is further heated in the first heat exchange stage, after which the heated liquid is partly circulated to the first flashing stage and partly to a subsequent flashing stage of the evaporation plant.
• the feed liquid Prior to the flue gas cooling the feed liquid is preheated by secondary heat from a last stage/effect or last stages/effects of the evaporation plant in the flow direction of vapor.
• the flue gas cooling and liquid flashing take place in different devices. According to one alternative these steps may be carried out in one or more evaporator apparatuses, where the heat exchange surface is formed of tubes or lamellas.
• the flue gas flow is not in contact with the liquid, and flows either inside or outside the lamella or inside or outside the tube surface.
• the liquid is flowing on the wall side opposite to the flue gas side as a falling film.
• flue gas is used as a heating medium instead of steam or vapor.
• vapor is evaporated off from the liquid, while the flue gas is cooled.
• This evaporated vapor is used as a heating medium in a first evaporation stage in a direction of the vapor flow.
• the vapor from successive flue gas -driven evaporator apparatuses is fed to the backend of the evaporator train.
• the flue gas is generated in a recovery boiler by burning black liquor from a pulping process, and black liquor is pre-concentrated by the heat from the flue gas.
• the flue gas is generated in a power boiler.
• the flue gas heat according to this invention is used for producing vapor for a multi- stage flash evaporator.
• Particles such as crystals or lignin or non- process elements or heavy metal agglomerates, can be separated from the liquid at least in one point between the evaporation stages of the evaporation plant. This may be carried out in a separating apparatus, which may typically comprise one or more cyclones and /or decanter centrifuges. Other suitable separator devices can also be used.
• a separating apparatus which may typically comprise one or more cyclones and /or decanter centrifuges. Other suitable separator devices can also be used.
• the temperature of the flue gas being discharged from the economizer of the power or recovery boiler may be less than 200 °C.
• the invention will however work also at this temperature or higher.
• the temperature of the flue gas is decreased in the new flue gas cooling, such as from 180°C to 125°C or even lower to 70°C particularly when using more than one heat exchanger stage.
• the new flue gas cooling such as from 180°C to 125°C or even lower to 70°C particularly when using more than one heat exchanger stage.
• the low heat flow, kW/m 2 , in the flue gas cooler is advantageous for avoiding fouling when heating an effluent liquid, i.e. a low heat transfer rate, due to low heat transfer coefficients and a low temperature difference, is used as an advantage instead of a disadvantage in a pre-concentrating stage.
• Dedicated segregation and integrated stripping can be connected to the multiple-effect evaporation plant using vapor from the flue gas heat recovery as a heating medium. It can be used to remove such components as: residual sulfur, ions (metals and other NPE's); lignin, hemicellulose, furfurals, oxalate, methanol, ethanol, formiate, formic acids, acetate, acetic acids, fatty acids and other acids or carbon-based components.
• Dedicated evaporation or pre-evaporation of effluents makes it possible to capture volatiles, like MeOH and VOCs, into secondary condensate for further cleaning and burning.
• the flue gas cooler and flash tank can be controlled to efficiently decrease energy consumption.
• the pressure in the liquid circulation is controlled to prevent boiling in the system and minimize the power consumption in the flue gas cooler recirculation.
• the pressure in the flash tank is regulated by the flash vapor amount and the back flow to the flue gas coolers.
• the flow is controlled with a circulation pump using inverters or valves.
• An embodiment of the invention provides a possibility to build new equipment parallel with an old or a new flue gas duct.
• the flue gas cooler and flue gas duct are connected in parallel so that the total flue gas flow or part thereof may be led directly through the duct into the chimney.
• FIGURE la, lb or lc is a schematic illustration of an exemplary arrangement for recovering heat from flue gas according to the present invention.
• FIGURE 2 is a schematic illustration of the basic components of one exemplary system in connection with which the present invention can be utilized.
• FIGURE 3 a schematic illustration of the basic components of another exemplary system in connection with which the present invention can be utilized.
• FIGURE 4 is a schematic illustration of an exemplary arrangement for recovering heat from flue gas according to the present invention.
• FIGURE 5 is a schematic illustration of an exemplary arrangement for recovering heat from flue gas according to the present invention.
• FIGURE 6 is a schematic illustration of the basic components of one exemplary system in connection with which the present invention can be utilized. DETAILED DESCRIPTION OF THE INVENTION
• FIGURE la, lb and lc show arrangements of the present invention.
• the temperature of the feed liquid to be pre-concentrated or concentrated varies in these examples.
• the temperature of the feed liquid (effluent) is above 110°C, in Figure lb between 50°C and 110°C and in Figure lc below 50°C.
• Liquid such as weak black liquor
• the hot flue gas can be introduced through line 14 in Fig. la.
• the hot liquid is introduced through line 15 and flash-cooled in a flash stage, which is in the form of a flash tank 16 or vessel, and is then circulated into the flue gas cooler 12.
• the flue gas cooler 12 has an outer housing 18 that contains a heat exchanger surface, such as heat pipes 20 or lamellas.
• the hot flue gas enters the lower part of the flue gas cooler through line 14 from a flue gas duct which receives the flue gas from the combustion device (not shown) , such a recovery boiler or power boiler.
• the flue gas indirectly exchanges its heat with liquid travelling counter-currently through the heat pipes 20 in the flue gas cooler.
• the cooled flue gas is discharged from the top of the housing 18 and led through line 22 into the flue gas chimney.
• the heated liquid exits the flue gas cooler and is led through line 24 to the flash tank 16.
• the upper end of the flash tank has a vapor outlet line 26, and the lower end has a liquid outlet 28 for the flash-cooled liquid. Flash vapor is directed through line 26 to an evaporator of the evaporation plant.
• a portion of the cooled liquid is circulated into the flue gas cooler through line 29 by means of pump 30.
• the flash tank has also an inlet 31 for heated liquid coming from the flue gas cooler.
• the point where the feed liquid is introduced into system depends on the properties of the liquid, such as temperature and composition.
• a very hot liquid is to be concentrated.
• the temperature of the feed liquid of the evaporation plant is 110 degrees of Celsius or more.
• the hot feed liquid from line 15 is first cooled by flashing, and thus the temperature of the liquid is adequately low to cool the hot flue gas and to be fed to the evaporation plant.
• the feed liquid is introduced through line 15 into the flash tank 16 together with the liquid flow in line 24 from the first heat exchange stage of the flue gas cooler. A large portion of the flash-cooled liquid is circulated into the flue gas cooler, and a portion thereof is directed to the evaporation plant through line 34.
• the temperature of the feed liquid of the evaporation plant is moderate, typically between 50 and 110 degrees of Celsius.
• This arrangement also has a two-stage flue gas cooler and the flue gas and liquid flow counter- currently.
• the arrangement corresponds to that of Fig. la, but the point (s) of introducing feed liquid are different.
• the feed liquid can be led through line 35 to the liquid flow in line 24 between the first flashing stage 16 and the first heat exchange stage 32, in which case the liquid is first heated and then flashed in the flash tank 16.
• a portion of the feed liquid can be led through line 36 to the second heat exchange stage 33 together with the liquid flow in line 23. If the temperature of the liquid is closer to 110°C than 50°C, the portion of the feed liquor can be led into the liquid flow discharged from the second heat exchange stage and directed to the evaporation plant through line 23.
• the temperature of the feed liquid of the evaporation plant is low, i.e. below 50 degrees Celsius.
• This arrangement also has a two-stage flue gas cooler and the flue gas and liquid flow counter-currently. The arrangement corresponds to those of Fig. la and Fig. lb, but the point of introducing feed liquid is different.
• the flue gas and liquid flow counter-currently, in the second heat exchange stage 33 the flue gas is cooled by the feed liquid fed through line 37.
• the liquid discharged from the second heat exchange stage is directed through line 38 and 29 to the first heat exchange stage 32.
• Fig. 2 shows an exemplary multi-effect evaporation plant in connection with which the present invention may be applied.
• the black liquor flow is indicated by bold lines in FIGURE 2.
• the fed liquor such as weak black liquor (or other cellulose pulp waste liquor) is led through lines 36 and 35 to the heat recovery system (flue gas cooling and flashing) , in which it is heated and flashed.
• the flashed liquor from flash tank 16 is transferred through line 34 to a subsequent flashing in a tank 40.
• the liquor is passed via line 41 into evaporation effect 5, where the liquor flashes.
• the liquor is passed via line 42 to effect 6, where it is further flashed.
• From effect 6, the liquor is passed via line 43 to effect 7, where it is further flashed, then via line 44 to effect 8 for evaporation.
• the liquor is further evaporated in effects 7, 6, 5, 4, 3, 2 and 1, as indicated by line 45.
• the product liquor in line 46 is withdrawn from effect 1, which is driven by flash vapor in line 26 from the flue gas heat recovery .
• the above-described embodiment relates to an eight-effect evaporation plant.
• the invention can, in a similar manner, naturally also be applied to other kinds of evaporation plants having, for example, five, six or seven evaporation effects, or even more than eight effects.
• a portion of the liquid to be fed into the last effect 8 is used to cool the flue gas in the flue gas cooler 12.
• the liquid portion is transferred via line 21 to the second stage 33 of the flue gas cooler according to Fig. lb.
• the heated liquid is returned to the evaporation plant via line 23 and combined with the liquor flow in line 34 from the flash tank 16.
• the combined flow is further flashed in tank 40, and fed via line 41 into effect 5 as described above.
• Warm water can be produced in a heat exchanger 47 with flash vapor in line 47' from flash tank 40.
• Evaporated water vapor contains also some methanol, volatile organic sulfur and other volatile compounds.
• the evaporation plant has a known treatment system for condensate and non-condensable gases. Different volatile components may be separated as a result of this treatment.
• the system comprises segregating evaporation elements (lamellas), flash tanks, a surface condenser and a steam stripper.
• the vapors and condensate streams are fractioned and treated so that foul condensate (FC) and secondary condensate (SC) are formed.
• FC foul condensate
• SC secondary condensate
• the foul condensate is cleaned in a steam stripper 50.
• the secondary condensate is cleaned, if needed.
• Fig. 3 shows an arrangement similar to that of
• Fig. 2 but the temperature of the feed liquid is lower, and it is led to the arrangement in a different way.
• the temperature is low, typically less than 50 °C.
• the heat recovery is shown in Fig. lc and described above .
• the cold liquid to be concentrated is first fed though line 57 into a surface condenser 49, in which secondary steam from the last evaporation effect 8 is condensed. Thus the liquid replaces cooling water normally needed in the condenser. Then a portion of the liquid is led via line 52 and further heated in heat exchangers 51 which are heated by secondary steam from evaporation effects 5-7, i.e. back-end evaporation effects.
• the preheated liquid is transferred via line 53 and 37 (Fig. lc) into a flue gas cooler 12, where heat is recovered from the flue gas as described in Fig. lc.
• the liquid heated with flue gas is passed via flash tank 40 into effect 5 and treated as described in connection of Fig. 2 above.
• the evaporation plant is provided with a separator 55, in which particles, such as crystals or lignin or Non-process elements, silica, calcium, or heavy metal agglomerates, can be separated from the liquid at least in one point between the evaporation stages of the evaporation plant. This may be carried out in a separating apparatus, which may typically comprise one or more cyclones and/or decanter centrifuges.
• the separated solids are conveyed in line 56 into the product liquid 46 to be discharged from the plant for burning or a corresponding treatment.
• the separated solids can also be directly discharged for direct disposal.
• the separation can also advantageously be performed in the front end of the evaporator train that is when the liquid flow is clearly less than the feed, e.g. 1/4-1/20 part of the feed, and then a higher concentration of contaminants is reached helping to form crystals or agglomerations of these, which are then easily separated, since small liquid flows require small separation equipment, such as for example 1/4 to 1/20 sized residence tanks, centrifuges, presses, filters etc.
• Fig. 4 shows another arrangement of an embodiment of the present invention, in which the flue gas cooler and the flue gas duct/chimney are connected in parallel. It also shows some controls, which may be used to control the system.
• the feeding of the feed liquid, the cooling of the flue gas and the liquid flow are arranged as in Fig. lc, but the arrangement of Fig. 4 can be used also in other embodiments. Whenever possible, the same reference numerals are used as in Figs .1-3.
• a control system such as a computer processor with non-transitory memory storing an executable program having control algorithms for controlling the flue gas heat recovery.
• the control algorithms may be implemented manually by technicians monitoring the pressure (PIC) , temperature and other sensors monitoring the flue gas heat recovery system.
• the executable algorithms may perform the following procedures :
• the controlling of the liquid flow includes regulating a pressure of the flash vapor 26 by adjusting the pressure set point (PIC-1) to achieve a predetermined minimum temperature of the flashed circulation liquid 61 returning into the flue gas cooler .
• the circulating pump 63 is adjusted by controlling an inverter for the pump.
• Fig. 4 the flue gas cooler and the flue gas duct/chimney of the combustion device are connected in parallel so that the total flue gas flow or part thereof may be led directly through the duct into the flue gas chimney. No flue gas or a lower flow thereof is fed into the cooler. This may be necessary if the cooler is not able to receive flue gas for some reason, e.g. for overhaul.
• the parallel system can be installed without any change to the existing flue gas fan.
• the flue gas duct 60 supplies flue gas from the combustion device, such as a recovery boiler.
• a fan 62 or like feeds the flue gas into the flue gas cooler 12.
• the flue gas duct 60 is connected to the flue gas chimney 64, so that the total flue gas flow or part thereof can be led by a fan 66 directly into the chimney bypassing the cooler 12. This may be necessary because of a process malfunction or maintenance of the flue gas cooler.
• Fig. 4 shows that the flue gas or a part of the flue gas is pumped with a fan 62 before or after the heat exchangers of the flue gas cooler 12 to compensate the pressure drop in the flue gas cooling heat exchangers when flowing in the channel 67 into the chimney 64.
• flue gas it is also possible to separate the flue gas or a part of the flue gas into a parallel flue gas cooling channel 65 and returned into main flue gas channel 60 or chimney 64 after passing through the flue gas cooler 12.
• the flue gas can be pumped with a fan 62 that is placed before or after the heat exchangers to compensate the pressure drop in the flue gas cooling heat exchangers.
• the cooled flue gas is returned into main flue gas channel 60 or chimney 64.
• the flue gas channel 60 and/or 65 may be provided with a dust separation device.
• Fig. 5 shows an embodiment, in which no separate flash tank is needed, but the flue gas cooling and vapor generation are carried out in one or more evaporator apparatuses.
• Fig. 5 shows a two-stage flue gas cooler 70, in which heat exchangers I and II are located in the same housing 77 and connected in series with respect to flue gas.
• the heat exchangers are formed of several lamellas 71 arranged a distance apart from each other, but only one element is shown.
• Each lamella is formed of two substantially parallel plates which are connected at their edges to form a closed space.
• the heat exchange surface can be formed of tubes.
• Hot flue gas is pumped by a fan 72 from a flue gas channel 73.
• the flue gas flows through the interior of the lamellas of the heat exchange stages I and II, and the cooled flue gas is discharged via line 74 into the flue gas channel 73.
• a distributor tray 76 is disposed above the lamellas and receives a liquor to be concentrated.
• the feed liquor is led via line 75 into the second heat exchange stage.
• the liquid flows through perforations of the tray and as a thin film along the exterior surface of lamellas.
• the liquid is evaporated with the heat from the flue gas, whereby vapor is generated.
• the evaporated liquid is collected at the bottom 78 of the housing.
• a portion of the liquid collected at the bottom is circulated through lines 84 to the distribution tray 76 in each stage.
• Liquid from the second stage II is led into the first heat exchange stage I, where it is heated with the hot flue gas. It is further evaporated.
• the liquid is discharged from the flue gas cooler 70 and led via line 79 to the multiple-effect evaporator plant, where it can be treated as shown in Fig. 3.
• the vapor produced in the first heat exchange stage is led through line 80 and preferably used as a heating medium in a first evaporation stage 82 of the multiple-effect evaporation plant.
• the vapor from succeeding flue gas -driven evaporator stage II is led through line 81 to the backend of the evaporator plant, for instance into the evaporation effect 4 in Fig. 3.
• the above can also be implemented so that the liquid is fed into the heat exchange stage I instead of II.
• Other equipment embodiments of the above are possible as long as not violating the main principle of no direct contact between flue gas and evaporated liquid and evaporated vapor.
• the principle of no direct contact between the flue gas and vapor is crucial to secure maximum vacuum or pressure in the vapor side of heat exchangers I and II and also in the flash stages. Already a small gas leak can greatly reduce the efficiency. Gas leaking from the flue gas side to the vapor side is therefore not recommended and nor acceptable since it reduces the heat exchanger efficiencies.
• Fig. 6 shows an exemplary multi-effect evaporation plant in connection with which the present invention may be applied.
• the arrangement is similar to that of Fig. 3, but the flue gas cooler is the cooler shown in Fig.5.
• the temperature of the feed liquid 75 is for example below 50 °C.
• the effluent feed point will change depending on flow properties. It can be feed to the back end of the MEE when it is beneficial to start evaporating effluents with a cold segregation phase before reaching the hot front end. If the feed flow is really cold (20-30 degrees of Celsius), it can be used as a sink in the cold end of the evaporation plant, i.e. as a cooling flow in the surface condenser and then it can be fed into the flue gas heat exchanger or partially warmed up by the back end vapor before feeding into the flue gas heat exchanger.
• the optional heat exchanger connections are numerous.
• a flue gas cooling system having a flue gas cooler and a flash tank (Figs, la-lc) and a flue gas system having an evaporator (Fig. 5) are connected in series so that the flue gas is first cooled in one system and then in the other system.
• the systems can be connected in any order.
• the pressure of the vapor discharged from the flue gas heat exchangers can be increased by a thermo-compressor , or any other form of compressing device, particularly when it is cost-efficient to raise the vapor pressure of any evaporator stage downstream of the flue gas heat exchanger stages.
• the new solution could also be used as a pre- evaporator for black liquor or as a hybrid that evaporates e.g. black liquor in the front end of the evaporation plant and another liquid in the back end.
• a hybrid evaporator may be chosen if the liquid to be treated is a small effluent flow or a flow that is hea- t sensitive and contains e.g. some special enzymes or polymers that need to be concentrated below a certain maximum temperature.
• the new integrated multiple-effect evaporation plant is suitable for all effluent flows but is most advantageous when recovering clean condensate from blow out flows or effluent flows that contain COD of 2000 mg/kg or higher concentrations, NPE ' s (non-process elements), small amounts of contaminants, heavy metals etc .
• the new method and system are particularly advantageous for solutions having a low boiling point rise (BPR) .
• BPR low boiling point rise
• a separator e.g. 1 or more cyclones or/and decanter centrifuges
• the flow of particles is then either transferred forward or backwards in the evaporation plant depending on the need and purpose.
• Non-process elements can be separated from the main recovery cycle.
• the separated concentrates are to be burned in a separate power boiler e.g. reducing significantly the risk of NPE disturbances in pulp bleaching or scaling in the evaporation plant and in other parts of the process.

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• 2013
  • 2013-03-22 WO PCT/FI2013/050325 patent/WO2013144438A1/fr not_active Ceased

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