BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method and system of recovering heat from flue gases generated in combustion devices at a biomass-based plant, such as a pulp mill. Flue gases are generated in combustion devices, such as a recovery boiler or power boiler at the pulp mill or a power boiler at a power boiler plant.

[0002] In a pulp mill, black liquor which typically has a dry solids content of over 80% (eighty percent) and combustion air are fed to a furnace of a chemical recovery boiler to burn the black liquor and recover chemicals therefrom. The flue gases generated in the combustion are led to 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 to a superheater for producing steam, which may have a pressure of more than 80 bar. The steam flows from the recovery boiler to a steam turbine to produce electricity. The steam discharged from the turbine is utilized for preheating the feed water flowing to the economizer.

[0003] If the temperature of the feed water flowing to the economizer is high (such as when efficient electricity production is desired and feed water preheating is used) , the final temperature of the flue gases may remain high at 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. [0004] New recovery boiler plants are also beginning to apply a technique used at power plants for recovering heat from flue gas. The recovery is done 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 more than 250 °C. After being discharged from the economizer, 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.

[0005] 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 reburning kiln, either as such after drying or after drying and gasifying. BRIEF DESCRIPTION OF THE INVENTION

[0006] 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 a stand-alone power boiler.

[0007] A method and system have been developed in which the heat recovery in a flue gas cooler is adjusted depending on the purpose for which the recovered heat is to be utilized at the mill, while simultaneously efficiently recovering the thermal energy in the flue gas .

[0008] The method according to the invention is characterized by what is presented in the characterizing part of claim 1. The system according to the invention is characterized by what is presented in the characterizing part of claim 13.

[0009] Flue gas is led from a flue gas duct into a flue gas cooler. Liquid is recirculated in the flue gas cooler to recover heat from the flue gas by transferring the heat to a liquid flowing through the flue gas cooler. The cooled flue gas is led from the flue gas cooler back into the flue gas duct and into the chimney.

[0010] The hot liquid discharged from the flue gas cooler is transported to a tank, which is referred to as a waste heat accumulator. The waste heat accumulator works as a flash tank and cools the liquid to a predetermined process vapor temperature. The cooled liquid is then returned to the flue gas cooler. The liquid is sufficiently clean to avoid fouling heat transfer surfaces of the flue gas cooler.

[0011] The liquid re-circulates in the system and make-up liquid for flashed vapor is added to the waste heat accumulator. Suitably the make-up liquid can be boiler make-up water or clean condensate from the evaporation plant of the pulp mill (either stripped condensate or clean secondary condensate, normally having 50-100 mgMeOH/l) .

[0012] The system generates waste vapor to be used at various locations at the biomass-based plant, such a pulp mill. Further, the system preferably comprises controls to minimize electrical consumption needed for flue gas cooler circulation.

[0013] The flash vapor from waste heat accumulator can be used at a suitable process stage or stages at the pulp mill. Typically flash vapor may be used at the black liquor evaporation plant of the mill as a heating medium. A suitable stage for using the extra flash vapor is to stabilize the feed temperature of hot weak black liquor that is fed to the evaporation plant. In the pulp mill there can be other evaporation processes, such as ash recrystallization for chloride removal. The ash which is separated in an electro-static precipitator of the recovery boiler is dissolved in liquid. The liquid is concentrated by evaporation using the waste flash vapor as the heating medium.

[0014] The amount of the circulation stream may be varied according to the consumption of the flash vapor at further process stages in the mill. [0015] Blow down of a portion of the recirculated liquid may be used to remove non-high volatile compounds accumulating in the liquid. The portion of the recirculated liquid that is to be blown down may be in a range of five percent (5%) to fifteen percent (15%), 8% to 12% and may be set to 10%. The control of blow down and portion of fed make-up condensate or boiler water for the blow down flow is based on available condensates and the measurement of conductivity of the make-up liquid.

[0016] In the system, the flue gas cooler and waste heat accumulator can be controlled to efficiently decrease energy consumption. In addition, 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 waste heat accumulator 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.

[0017] A minimum return temperature to the flue gas cooler is arranged by limiting the pressure of the flow of the flash vapor to the place (s) where the vapor is used . [0018] The temperature of the flue gas being discharged from the economizer of the recovery boiler may be less than 200 °C. In particular, the temperature of the flue gas is decreased, such as from 180°C to 125°C. The temperature of the return water must not be too low, the minimum return water temperature to the flue gas cooler is typically 105 °C. [0019] 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. This may be necessary if the cooler is not able to receive flue gas for some reason, e.g. for overhaul. The parallel installation allows complete redundancy and provides the following advantages:

- cleaning of the equipment is possible independent on and without any effect on the main process, whereby the cleaning is more efficient and of better quality;

- the flue gas flow is adjustable up to 100%, which allows constructing remarkably cheaper equipment (e.g. remarkably lower maximum design pressure);

- parallel application provides degrees of freedom for mechanical implementation (for example, no need to change or rebuild an existing flue gas fan; cheaper fan solution, heat surface construction, cleaning system etc . ) ;

ensures complete redundancy and thus improves the overall process safety and availability.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The process that has been developed is described in more detail with reference to the drawings, in which:

[0021] FIGURE 1 is a schematic illustration of the basic components of one exemplary system in connection with which the present invention can be utilized, and [0022] FIGURE 2 illustrates an exemplary arrangement according to the invention.

[0023] FIGURE 3 illustrates another exemplary arrangement according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The exemplary system illustrated in FIGURE 1 includes a cooking plant 2 which typically comprises a digester into which hard wood or soft wood chips, or other comminuted cellulosic material, is fed through line 1. A line is a pipe or other conduit for transporting a fluid or slurry. In the digester the wood chips are acted upon by the cooking chemicals at temperature and pressure conditions so as to produce chemical cellulose pulp, such as kraft pulp. Then the pulp may be subjected to oxygen delignification in stage 3. After oxygen delignification, the pulp proceeds to the bleach plant 4 where it is subjected to bleaching in a plurality of different bleaching stages. The pulp is passed to a further treatment via line 5.

[0025] Weak black liquor from the cooking plant 2 is passed in line 21 to evaporators (see evaporator effects 25, 22) where it is evaporated to a concentrated black liquor in line 18 to be fired in the recovery boiler. The dry solids concentration of the weak black liquor is typically 12% to 17%, and the firing liquor concentration may be at least 75%, and in a range of 80% to 85%. The evaporators may be multiple effect evaporators 25, 22 with water evaporation of 6 to 12 ton/ADT. Primary steam 19 is introduced into the first evaporator effect where part of the water in the black liquor is vaporized. The vapor is then used as heating steam in the second effect, which is operated at lower pressure and temperature than the first effect. Similarly the vapors are introduced into the subsequent effects and finally the vapor from the last effect 22 is condensed in a surface condenser (not shown) or the vapor in line 23 is used as heating steam for bleach plant effluent evaporator 9. Multiple effect evaporators have typically 5 to 8 effects and the primary steam consumption is respectively 2,2 to 0,8 ton/ADT. [0026] Evaporated water vapor contains also some methanol and volatile organic sulfur compounds but practically no inorganic compounds. The vapors can be fractionated and stripped to clean secondary condensate 24 which can be used as process water. Cooking chemicals and dissolved organic and inorganic solids from wood (e.g. chlorine, heavy metals like cadmium and lead) remain in the concentrated black liquor in line 18.

[0027] Chlorine-containing effluent 8 from the acidic bleaching stage 4 is concentrated such as in a multiple effect evaporator 9. The effluent is evaporated to concentrations of 5% to 20% or even to higher concentrations. The concentrate 10 in line 11 is fired in the recovery boiler 17. Depending on the required evaporation capacity the effluent evaporator 9 can utilize secondary vapors 23 from the black liquor evaporator back end stages 22 or primary steam 19.

[0028] The concentrated spent liquor from pulping in line 18 is fed into the furnace 43 via liquor spraying devices 16. The liquor stream in line 18 may be divided and introduced at several levels 15 into the recovery boiler furnace 43. In a kraft recovery boiler the combustion air is fed into the boiler via several air ports at several levels, 44, 45, 46. The main part of the inorganics in spent liquor, typically cooking chemicals, chemicals for the fiber line, or chemicals for energy or special chemicals production, are discharged from the lower furnace, as smelt in line 14, or recovered from flue gases 38 in a separation device such as electrostatic precipitator 36 into stream 35 to be further processed into crystals in line 26. In kraft pulping a chemical smelt 47 is formed on the bottom 48 of the furnace of the recovery boiler. The smelt flow 14 enters a dissolving tank 13 for further recovery and preparation of cooking chemicals at 12.

[0029] Flue gases which pass heat transfer surfaces 41, 39 and are discharged from the recovery boiler furnace 43 contain inorganic dry solids particles, which are separated in an electrostatic precipitator (ESP) 36. Chloride and potassium are enriched in ESP ash and therefore chloride and potassium are favorably removed from the ash. The ash 35 is dissolved in hot water or condensate 34, in a mixing tank 33, and then recrystallized in an evaporator crystallizer 27. Valuable sodium sulfate and carbonate are first crystallized and separated from the mother liquor and after the separation the crystals 26 are fed back through line 20 to black liquor evaporator 25. The mother liquor in line 28 rich in chloride and potassium is purged to sewer or may be further utilized in processes developed for that purpose. While dissolved ash solution in the mixing tank 33, is alkaline, the metal ions in the ash are insoluble forming fine metal hydroxide particles in the solution. The particles are separated from the solution 32 in the filter or in separation equipment, 30, and the filter cake is led to further treatment, 31. The filtered solution, 29, is led further to the ash recrystallizer, 27.

[0030] FIGURE 2 shows an arrangement of an embodiment of the present invention.

[0031] Water, such as secondary condensate is recirculated in a flue gas cooler 52 and heated therein. The flue gas can be introduced through line 37 in Fig. 1. The hot water is flash cooled in the waste heat accumulator 50, and is then returned to the flue gas cooler 52.

[0032] The flue gas cooler 52 has an outer housing 51 that contains a heat exchanger surface, such as heat pipes 53 or lamellas. The flue gas enters the upper part of the flue gas cooler through line 69 from a flue gas duct which receives the flue gas from the combustion device, such a recovery boiler in Fig. 1. The flue gas indirectly exchanges its heat with water travelling through the heat pipes 53 in the flue gas cooler. The cooled flue gas is discharged from the bottom of the housing 51 and led through line 69' into the flue gas chimney. The heated water exits the flue gas cooler and is led through line 54 to the waste heat accumulator 50, which is in the form of a flash tank 55 or vessel. The upper end of the accumulator has a vapor outlet 57, and the lower end has a liquid outlet 58 for the flash- cooled water. The cooled water is recirculated to the flue gas cooler through line 59 by means of pumps 67 and 68 or by means one of them. The accumulator has also an inlet 60 for make-up water and an inlet 61 for heated water coming from the flue gas cooler. [0033] Flash vapor has many benefits. It is clean, and so it has less problems with fouling (e.g. calcium carbonate deposit, chlorine corrosion, biological corrosion) . The flash vapor produced in the waste heat acccumulator may be utilized in the evaporator crystallizer 27 in Fig. 1 for ash recrystallization (ARC) . The vapor is led through line 64. Earlier it has been used as vapor from effect 2 of the evaporation plant 25.

[0034] The flash vapor can also be utilized to stabilize the hot weak liquor flashing in the evaporation. In this embodiment the flash vapor is led through lines 62 and 63 to a first and a second weak black liquor at the evaporation plant 25, 22 in Fig. 1 to reduce or eliminate heat load variations of the evaporation plant.

[0035] Secondary condensate is typically used as make ^¬ up water and fed through line 65 to the waste heat accumulator 50 which makes flash vapor for other process units.

[0036] A certain amount of blow down through line 66, such as 10%, is applied to remove detrimental substances, such as non-high volatile compounds accumulating in the system (like salts etc.) . This way detrimental substances causing fouling and corrosion can be removed from the system.

[0037] Several controls may be used to control the system and particularly the flue gas heat recovery. A control system 56, such as a computer processor with non-transitory memory storing an executable program having control algorithms for controlling the flue gas heat recovery. Alternatively, the control algorithms may be implemented manually by technicians monitoring the pressure (PIC), temperature (TI) and other sensors monitoring the flue gas heat recovery system. The executable algorithms may effect the following procedures :

[0038] The pressure (PIC-2) in the circulation is controlled to be high enough to prevent boiling in the pipelines or flue gas coolers and minimize the power consumption in flue gas cooler recirculation. The pressure set point is adjusted based on the water temperature (TI-1) from the flue gas cooler.

[0039] By adjusting the pressure set point (PIC-2) based on the temperature TI-1, the electrical consumption for the circulation pumps 67 and 68 can be minimized. Two circulation pumps are shown in Fig. 2, but one pump may be sufficient in most cases.

[0040] The pressure in the waste heat accumulator

(PIC-3) is regulated by flow to the flue gas coolers. This flow is controlled with circulation pump inverters SIC-5 and SIC-6, which are adjusted based on the pressure data obtained from the waste heat accumulator pressure sensor (PIC-3) .

[0041] To enhance efficiency, the system may set a minimum return water temperature (such as 105 degrees Celsius) to flue gas coolers. The minimum return water temperature may be arranged by limiting the opening of the valves (FIC-9, PIC-10, PIC-11) in the lines through which the vapor is led to a further use. If the pressure falls below 0.2 bar(g), which corresponds to the saturation pressure at 105 degrees C, the opening of the valves may be limited, e.g., reduced.

[0042] Medium pressure (MP) steam in line 69'' may be fed to the flue gas cooler when soot blowing of the fouled heat transfer surfaces is needed.

[0043] FIGURE 3 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. Wherever possible, the same reference numbers are used as in Fig. 2.

[0044] Water, such as secondary condensate is recirculated in a flue gas cooler 72 and heated therein. The flue gas can be introduced through line 37 in Fig. 1. The hot water is flash-cooled in the waste heat accumulator 70, and is then returned into the flue gas cooler 72.

[0045] The flue gas cooler 72 has an outer housing 71 that contains a heat exchanger surface, such as heat pipes 73 or lamellas. The flue gas enters one end of the flue gas cooler from a flue gas duct 89 and indirectly exchanges its heat with water travelling through the heat pipe 73. The cooled flue gas is discharged from the other end of the housing 71. The heated water exits the flue gas cooler and is led through line 74 to the waste heat accumulator 70, which is in the form of a flash tank 75 or vessel. One end of the accumulator has a vapor outlet 77, and the other end has a liquid outlet 78 for the flash-cooled water. The cooled water is recirculated to the flue gas cooler through line 79 by means of pump 87. The accumulator has also an inlet 80 for make-up water and an inlet 81 for heated water coming from the flue gas cooler.

[0046] The flash vapor produced in the waste heat acccumulator may be utilized in the evaporator crystallizer 27 in Fig. 1 for ash recrystallization (ARC) . The vapor is led through line 84.

[0047] The flash vapor in lines 93, 94 can also be utilized in other places at the mill, where heat is needed .

[0048] Secondary condensate is typically used as make ^¬ up water and fed through line 85 to the waste heat accumulator 70 which makes flash vapor for other process units .

[0049] A certain amount of blow down through line 86, such as 10%, is applied to remove non-high volatile compounds accumulating in the system (like salts etc.) .

[0050] Several controls may be used to control the system and particularly the flue gas heat recovery. A control system 76, such as a computer processor with non-transitory memory storing an executable program having control algorithms for controlling the flue gas heat recovery. Alternatively, the control algorithms may be implemented manually by technicians monitoring the pressure (PIC), temperature (TI) and other sensors monitoring the flue gas heat recovery system. The executable algorithms may influence the procedures in the same way as described in connection with Fig. 2.

[0051] In Fig. 3 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 to the flue gas chimney. No flue gas or a lower flow thereof is fed to 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.

[0052] The flue gas duct 90 supplies flue gas from the combustion device, such as the recovery boiler in Fig. 1. A fan 82 or like feeds the flue gas to the flue gas cooler 70. The flue gas duct 90 is connected to the flue gas chimney 92, so that the total flue gas flow or part thereof can be led directly into the chimney bypassing the cooler. This may be necessary because of a process malfunction or maintenance of the flue gas cooler.

[0053] Gas or liquid may be fed to the flue gas cooler through line 83 when cleaning of the fouled heat transfer surfaces is needed. Dust or sludge formed is purged through line 91.

[0054] The parallel installation allows complete redundancy and provides several advantages mentioned earlier.

[0055] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.