Imported: 10 Mar '17 | Published: 27 Nov '08
USPTO - Utility Patents
A method for burning chlorine-containing liquors in a chemical recovery boiler at a pulp mill, wherein the recovery boiler includes spent liquor sprayers for feeding spent liquor and a number of combustion air levels including: increasing a combustion temperature in the recovery boiler in a burning zone where a chlorine-containing liquor or a chlorine-containing effluent is burned; while burning the liquor or effluent, volatilizing the chlorine in the liquor or effluent to produce chloride-containing salts in flue gases in the boiler, and removing the chloride-containing salts from the flue gases.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/913,322, filed Apr. 23, 2007, the entirety of which is incorporated by reference.
This invention relates to recovery processes for processing natural cellulosic or other fibrous material and, particularly, to the removal of chlorine from such processes.
Chlorine (Cl) is present in wood, in make-up chemicals and pulp bleaching filtrates, especially when using chlorine-containing bleaching chemicals. Chlorides entering with the wood and input chemicals tend to build up in pulping liquors. This may be a particular problem for coastal mills where logs are transported in sea water and become saturated in chloride. Chemicals used in the process may also contain considerable amounts of chlorine. The concentration of chlorine in black liquor varies greatly from one process to another. The chlorine content in black liquor can be as low as 0.1 to 0.8% of the liquor dry solids, but in some cases the chlorine content of the liquor dry solids can be as high as about 5%, and in closed cycle processes it may rise even higher.
One proposed technique for decreasing the environmental impact of chlorine-containing chemicals is the closing of the liquid circulations of bleaching plants, and modern bleaching plants have reached the level of 10-15 m3/adt without any negative impact on the quality of the pulp. However, even when decreasing the amount of effluent from the level of 15 m3 to the level of 10 m3, an increase in chemical consumption becomes visible, which thus leads to an ever-increasing amount of organic chlorine compounds from bleaching. Thus, a conclusion can be made that closing the water circulations of bleaching does not as such have a direct effect on the amount of organic chlorine compounds, but on the other hand, a decreased amount and higher concentration of effluents allows for an easier and more economical purification thereof.
Thus, the dominance of chlorine dioxide as bleaching chemical has gained even more foothold in the recent years, and even the most up-to-date researches and industrial experiences have been unable to undermine its prominence, but as a rule the pulping industry, with only a few exceptions, has accepted the use of chlorine dioxide as the key chemical in bleaching. Therefore, if a mill is to further decrease the amount of organic chlorine compounds, the aim in the mill will, above all, be their elimination and their treatment inside the mill, rather than a decrease in the use of chlorine dioxide.
Modern ECF-bleaching used for bleaching pulp typically comprises at least three bleaching stages and three washing apparatuses. In special cases, only two washing apparatuses may be used, but there are only a few such applications in the world. All bleaching sequences using at least one chlorine dioxide stage and not using elementary chlorine in any bleaching stage, are regarded as ECF-bleaching. Further, a bleaching sequence comprises one alkaline stage, which at present typically uses either oxygen, peroxide, or both as auxiliary chemical. Additionally, ozone, various types of acid stages and a chelate stage may be used in modern bleaching for removing heavy metals.
When bleaching is referred to as ECF-bleaching, the amount of chlorine dioxide used therein is over 5 kg/adt, and even then the applications are referred to as so-called Light ECF applications. Typically, light-ECF applications make versatile use of the removal of hexenuronic acids, i.e. the A-stage (as described in U.S. Pat. No. 6,776,876); peroxide is used in one or two stages, and in some cases also an ozone stage. The total amount of chlorine dioxide varies from the mentioned 5 kg/adt up to a level of about 25 kg/adt. If chlorine dioxide is used in one bleaching stage, the charges are most typically between 5-15 kg/adt and if the mill is provided with two chlorine dioxide stages, charges less than 10 kg/adt are rarely used.
If the use of peroxide in bleaching is limited to charges below 6 kg and if chlorine dioxide is the main bleaching chemical, the chlorine dioxide charge in the bleaching increases from a level of 25 kg/adt depending on the bleaching properties of the pulp in question and on the level of decrease in the kappa number before starting the bleaching with chlorine-containing chemicals. Therefore, bleaching technique can, in view of the process, be fairly freely adjusted to various chlorine dioxide consumption levels in such a way that the amount of chlorine-containing chemicals exiting the bleaching corresponds to the capability of the chemical circulation to receive chlorides.
When chemical liquor cycle in kraft mills will be more closed, chloride, potassium, metals and other Non Process Element (NPE) concentrations in the liquor cycle are increased.
In chemical pulp mills, the chemicals of a pulping process are recovered from spent liquor, e.g., black liquor in kraft pulping, by firing the liquor in a recovery boiler either alone or together with other waste streams. The firing process is exothermic and the released energy is recovered as pressurized superheated steam. The steam energy is recovered in a steam turbine in the form of electric power and steam of different pressures for process needs. In the recovery boiler, chlorine and potassium are enriched into the fly ash and increase the corrosiveness of the flue gas especially in the superheater.
Improved methods of handling chlorine-containing liquors and effluents at pulp mills so that corrosion problems and other adverse effects caused by chlorine can be minimized.
CA 2041536 describes a treatment of a DC-stage effluent in a special evaporator and incinerator without recovering valuable chemicals from the incineration ash. U.S. Pat. No. 5,374,333 relates to a process in which all liquid effluents from a bleach plant are evaporated and incinerated independent of the recovery boiler to produce a residue containing sodium, sulfate and carbonate, which residue is leached to produce a leachate. At least a substantial portion of the leachate is fed to the chemical recovery loop associated with the recovery boiler.
EP 719359 describes a process in which liquid effluents from a bleach plant are concentrated and incinerated in a recovery boiler to produce flue gases including ash containing salts including sodium, potassium, and chloride-containing salts, and sulfur compounds. Potassium and chloride are removed from the ash while returning the sulfur containing compounds of the ash to the recovery loop, so as to balance the sulfur, chloride and potassium levels in the mill.
U.S. Pat. No. 5,989,387 discloses a method for reducing the chlorine concentration in a sulfate cellulose process, wherein part of the chlorine content in the chemical cycle is separated from the cycle and removed. In this process sulphurous odour gases are introduced into the soda recovery boiler at least in such an amount that the concentration of sulphur oxides in the soda recovery boiler is such that at least part of chlorine separating in gaseous form from the bed is in the form of hydrogen chloride in the upper part of the soda recovery boiler. The hydrogen chloride is separated from the flue gases by scrubbing the flue gases.
The above described conventional methods do not address burning chlorine-containing effluents in a recovery boiler such that chlorine could be removed from the chemical recovery loop efficiently.
A process has been developed and is disclosed herein for treating spent liquors and filtrates or effluents from bleaching using chlorine dioxide at a pulp mill and for removing chlorine (Cl) from the process. This allows high water reuse and effective production of power and heat from spent liquor, such as black liquor, and other energy-containing streams available at the mill, or brought to the mill. The process disclosed herein can also be used for balancing and stabilizing the Cl concentrations in the material circulations of the mill, specially the concentration level in the spent liquor, when chlorine enters the mill in raw materials and chemicals streams. The process disclosed herein preferably relates to sulfate or Kraft pulp mills.
A process has been developed in which the burning of chlorine-containing liquor and effluents can be controlled in such a way that the operation of the recovery boiler itself is efficient, whereby a high-temperature steam can be produced for power and heat production. The developed process may allow for chlorine to be separated efficiently and for the chlorine level of the pulp mill can be balanced without adversely affecting the pulp quality and the operation of the recovery boiler. Especially corrosion problems in the machinery can be avoided or minimized.
A method is disclosed herein for burning chlorine-containing liquors in a chemical recovery boiler at a pulp mill, wherein the recovery boiler comprises spent liquor sprayers for feeding spent liquor and a number of combustion air levels. A feature of the disclosed method is that the combustion temperature in the recovery boiler is increased in a zone, where a chlorine-containing liquor or effluent is burned, for improving the volatilization of chlorine from the liquor or effluent into flue gases to produce chloride-containing salts, and that the flue gases are treated to remove the chloride-containing salts. The chlorine-containing stream to be burned is typically a spent liquor, such as black liquor, from pulp production or a chlorine-containing effluent from a bleaching plant of the pulp mill. Also other chlorine-containing streams from the pulp mill can be treated according to the process disclosed herein.
According to an embodiment of the method disclosed herein, at least 30% calculated from the as fired liquor chlorine concentration is volatilized into the flue gases. Preferably over 40% chlorine delivery from the as fired stream chlorine concentration into flue gases is obtained by adjusting the combustion zone temperature high enough.
According to an embodiment of the method disclosed herein, the oxygen concentration in the recovery boiler is increased in the burning zone of the chlorine-containing stream for raising the temperature of the zone.
According to an embodiment of the method disclosed herein, the pulp mill has a bleach plant using chlorine dioxide, and the bleach plant has at least one chlorine dioxide stage, and chlorine-containing effluent flow from the bleach plant is concentrated and burned in the recovery boiler.
According to an embodiment of the method disclosed herein, oxygen enrichment takes place at the primary and/or secondary air levels of the combustion air. Preferably oxygen enrichment takes place at secondary air level or levels.
According to an embodiment of the method disclosed herein, the recovery boiler is provided with an integrated separate combustion chamber, where the chlorine-containing liquor is burned. Typically the chlorine-containing stream that is burned in the separate combustion chamber is a bleaching effluent.
According to an embodiment of the method disclosed herein, oxygen-enriched air is added to the zone where the stream having the highest chlorine concentration is burned.
According to an embodiment of the method disclosed herein, oxygen-enriched air is added to the integrated combustion chamber.
According to an embodiment of the method disclosed herein, the oxygen content is increased by raising the oxygen content of the combustion air supplied to the burning zone.
According to an embodiment of the method disclosed herein, the oxygen content in the boiler is increased by supplying oxygen directly to the burning zone.
According to the methods disclosed herein, the temperature in a combustion zone where a chlorine-containing liquor or effluent is burned is increased so that the delivery of chlorine from the liquor into flue gases formed in the burning is maximized. Thus, the chlorine volatilization and pyrolysis take place in the zone where the liquor is burned. The combustion zone temperature is over 800 C., typically over 850, preferably over 950 C., most preferably over 1150 C.
FIG. 3 shows a chart of the operation of a kraft recovery boiler in which the proportion (r) of Cl, calculated based on the Cl amount in as fired black liquor, found in flue gases, as the function of furnace loading (MW/m2 bottom area of the furnace). The upper line represents an operation model having a higher temperature in the combustion zone, the lower line represents an operation model having a lower temperature in the combustion zone. This shows that raising the combustion zone temperature can increase the chlorine volatilization from the as fired stream into flue gases. This result is utilized in the present invention. Chlorine concentration into flue gases is maximized via increasing the combustion zone temperature. The proportion (r) can be increased with high dry solids of the spent liquor (80-90%), with firing intensity, with proper air distribution, and/or with high air temperature and/or with addition of oxygen to the furnace, preferably close to the point where the stream having the highest chlorine (Cl) concentration is fed to the furnace. Thus one suitable way to increase the combustion zone temperature is to have stoichiometric conditions or close (the air factor is 0.85-1.0, preferably 0.9-1.0). in the combustion zone. This can typically be achieved by proper combustion air distribution or addition of oxygen. In the process disclosed herein the combustion zone temperature for a chlorine-containing liquor or effluent is raised intentionally so as to increase or maximize chlorine volatilization from as fired stream into flue gases. Chlorine compounds can then be removed from the flue gases by a suitable process. The removal of chlorine may be practiced according to many conventional or known techniques, such as evaporation/crystallization
By increasing the combustion temperature e.g. by optimizing combustion intensity/m2 it is possible to volatilize, over 30%, preferably over 40%, calculated from the as fired spent liquor Cl concentration, into flue gases, typically as sodium chloride (NaCl) or potassium chloride (KCl). Even more than 50% Cl from the as fired streams can be delivered into flue gasesin theory 100%, but not in practice. In principle chlorine could also be in form of hydrogen chloride (HCl), but HCl is favoured by low furnace temperature, which results into low delivery of Cl into flue gases. HCl could be washed out from flue gases, as is known.
Further increases in chlorine delivery into flue gases from the chlorine-containing stream can be achieved with the use of a separate combustion chamber integrated with a recovery boiler. The combustion temperature for the chlorine-containing stream, typically bleaching effluent, in the chamber can be increased with the use of oxygen or oxygen enriched air. Further the burning in the chamber can be improved by a high flame temperature producing combustion agent such as fuel oil, natural gas, methane, ethanol, methanol, other biofuels and chemicals, which are included in the mill processes. The chamber may have thermal insulation or brickwork to increase the combustion temperature in the chamber.
When the processes disclosed herein is used for balancing chlorine level in spent liquor, the chlorine concentration entering the boiler furnace may be so high that under a traditional arrangement high live steam temperature, or live steam and reheated steam temperatures cannot be achieved without corrosionunder reasonable costs. In that case a process can be applied in which the recovery boiler is provided with a separate combustion cavity or chamber having a heat exchanger for final superheating of the steam produced in the superheater section of the recovery boiler, whereby the heat exchanger is connected to the superheaters of the boiler. The cavity is heated by burning fuel in such a manner that non-corrosive conditions in the combustion chamber are guaranteed. The fuel used in the combustion chamber can be gas produced from biomass, liquefied biomass, methanol, other biofuels, natural gas, LPG, etc. The criterion for the fuel is the non-corrosive nature under the combustion chamber conditions.
Thus the recovery boiler used in connection with the processes disclosed herein can be provided with a separate combustion chamber for burning a chlorine-containing stream or for final superheating of steam from the superheater section of the boiler, or for both purposes. In the last mentioned alternative the recovery boiler has at least two separate combustion chambers, one for burning a chlorine-containing stream and one for final superheating of steam from the superheater section of the boiler.
Chlorides and potassium are enriched in the recovery boiler ash. Cl and K can be removed from the ash by methods known per se, such as leaching, evaporation/crystallization, freeze crystallization. One preferable process for ash handling is described in connection with FIG. 1.
The exemplary system illustrated in FIG. 1 includes a cooking plant 2 which typically comprises a digester, such as a continuous digester, to which hard wood or soft wood chips, or other comminuted cellulosic material, is fed through line 1. 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. The pulp is further typically treated in brown stock washing and in a screening room. Then the pulp is preferably subjected to oxygen delignification in stage 3. After oxygen delignification, the pulp proceeds to the bleach plant where it is subjected to bleaching in a plurality of different bleaching stages. The particular bleaching stages that are utilized can be varied, and are also dependent upon the particular cellulose material being treated, but in at least one bleaching stage chlorine dioxide is used as a bleaching chemical. Typical sequences are A/D-EOP-D-P and D-EOP-D-P. In FIG. 1 a D stage is after oxygen delignification 3, but there can be other stages before D-stage 4 including washing which is shown as an example only. Chlorine dioxide in line 6 is added to stage 4, and after that washing liquid, such as water through line 7. The pulp is passed to a further treatment via line 5.
Weak black liquor from the cooking plant 2 is passed in line 21 to evaporator 25, 22 where it is evaporated to a concentrated black liquor in line 18 to be fired in the recovery boiler. Dry solids concentration of the weak black liquor is typically 12-17%, and the firing liquor concentration respectively 75%, preferably 80-85%. The evaporator is most often a multiple effect evaporator with water evaporation of 6-12 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 5-8 effects and the primary steam consumption is respectively 2.2-0.8 ton/ADT.
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 in fiber line processes, such as at 3. 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.
Chlorine-containing effluent 8 from the acidic bleaching stage 4 is concentrated e.g. in a multiple effect evaporator 9. The effluent flow is typically 3-5 m3/ADT having 0.2-1% dissolved dry solids (e.g. chloride and heavy metal ions). The effluent is evaporated to concentrations of 5-20% or even to higher concentrations. The concentrate 10 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, also mechanical vapor recompression type of evaporator can be used.
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. These different locations are situated on a front wall, rear wall and sidewalls. The spent liquor burns in the furnace, as combustion air is available from several air feed points. One typical spent liquor is called black liquor, from kraft pulping, which is burned and the chemicals recovered in a so called kraft recovery boiler. In a kraft recovery boiler the combustion air is fed into the boiler via several air ports at several levels, which are primary air, at the lowest air port level(s) 46 at the lower part of the furnace, secondary air level or levels, 46, above the primary air level but below the liquor nozzles, and tertiary air level or levels, 44, above the liquor nozzles to ensure complete combustion. Sometimes the highest tertiary air level is called a quaternary air level. Combustion airs may contain weak odorous gases from the pulp mill, and/or from the recovery boiler. Oxygen or oxygen enriched air in line 45 is fed into the furnace. In EP Patent 953080 a method is described in which oxygen enriched air is fed to the lower furnace of a recovery boiler so that the air factor is lowered, which contributes to e.g. increase in the firing capacity of the boiler.
The spent liquor 18 contains typically at least some chlorine (Cl), for instance 0.05-2% based on the dry solids analysis. The concentrated bleach plant effluent flow 10, which contains typically a higher Cl concentration, based on dry solids, than flow 18, is also fed into the recovery boiler furnace 43. The feeding place or places 11 may be located in the same zones and at the same levels where the spraying devices 16 are located.
Alternatively, the flow 10 may be fed with spraying, or through a burner or burners via line 50 into a separate combustion chamber 49, which is integrated into the furnace 43 of the recovery boiler, and from which chamber flue gases enter the furnace 43.
In principle the arrangement is similar to that shown in FIG. 2 and disclosed in U.S. Patent Applications Nos. 2006-236696 or 2005-252458.
The additional combustion chamber 49 of the recovery boiler is located prior to superheaters 41, and prior to reheaters (not shown), when following the flue gas path 17 from the recovery boiler furnace 43. The chamber 49 may have thermal insulation or brickwork to increase the combustion temperature in the chamber. The flue gases from the chamber may enter the furnace 43 flowing down or flowing up.
The flows 10 and 18 can be fed separately or mixed prior to the recovery boiler 43, or inside the evaporation plant 25, and the mixed flow can be fed into the furnace via devices 16. 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 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 dissolving tank 13 for further recovery and preparation of cooking chemicals. Prior art describes various processes for the green liquor handling and caustizing 12, including removal of undesired components, such as heavy metals.
A solution has been developed for effective chlorine removal from recovered streams, i.e. chemical melt 14 formed in the recovery boiler, and stream 26 including sodium sulfate and sodium carbonate from ash handling, comprising the following:
Cl concentration into flue gases 17 is maximized via increasing the combustion zone temperature where Cl containing streams 10 and 18 are burned. The proportion of Cl, calculated based on the Cl amount in as fired black liquor, found in flue gases can be increased with high dry solids of the spent liquor, with firing intensity, with proper air distribution, and/or with high air temperature and/or with addition of oxygen to the furnace, preferably close to the point where the stream having the highest chlorine (Cl) concentration is fed to the furnace.
By optimizing e.g. combustion intensity/m2 it is possible to volatilize, at least over 30%, calculated from the as fired spent liquor Cl concentration, into flue gases, typically as sodium chloride (NaCl) or potassium chloride (KCl). The delivery of Cl into flue gases is increased via the use of oxygen or oxygen enriched air 45. If the furnace temperature is high enough, more than 40%, or even more than 50% Cl delivery from the as fired streams can be delivered into flue gasesin theory 100%, but not in practice.
Further increase in Cl delivery into flue gases from the concentrated bleaching effluent stream 10 in line 50 can be achieved by using the integrated combustion chamber described above, in which the burning is intensified with the use of oxygen enriched air via line 51. Further the burning in the chamber can be improved by a high flame temperature producing combustion agent as fuel oil, natural gas, methane, ethanol, methanol, other biofuels, chemicals, which are included in the mill processes.
Adequately high sodium (Na) and potassium (K) volatilization from the spent liquor combustion is required for binding Cl into NaCl and KCl. Also for Na and K a higher combustion temperature increases delivery into flue gases 17. In the furnace NaCl and KCl are formed, and they turn into fine particles, ash, which deposit onto heat transfer surfaces 41 and 39. The main part is captured as fly ash in the precipitator 36 to be processed, stream 35. The main part of the ash is, however, formed of useful SO4 and CO3 salts. If the chamber described above is located in the upper part of the furnace 43, part of Cl may enter precipitator 36 as gas, HCl, which can be removed from flue gases 37 exiting the precipitator by using known technology, such as scrubbing.
Flue gases from the recovery boiler 43 contain inorganic dry solids particles, which are separated in electrostatic precipitator 35. The main components in the ash are sodium sulfate and sodium carbonate. The ash contains also potassium salts, chlorides and several metals, such as e.g. cadmium and lead, which are easily vaporized in the recovery boiler 43, The ash amount is typically 6-12% of the dry solids fired in the recovery boiler, equal to about 80-200 kg/ADT. The ash is returned back to the evaporator or to the firing liquor to recover valuable chemicals.
Chloride and potassium are enriched in ESP ash and therefore chloride and potassium are favorably removed from the ash. The ash is dissolved in hot water or condensate 34, in mixing tank 33, and then recrystallized in 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 mixing tank 33, is alkaline, pH typically 10-11, 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 other 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.
When the process is used for balancing Cl level in spent liquor, the Cl stream entering the boiler burning streams, spent liquor in line 105 (FIG. 2) and optionally bleaching effluent in line 101 mixed with the spent liquor, may be so high that under traditional arrangement high live steam temperature, or live steam and reheated steam temperatures cannot be achieved without corrosionunder reasonable costs. In that case a system can be applied in which a combustion cavity or chamber is provided in connection with a recovery boiler for the final superheating of steam produced in the superheater section of the recovery boiler, as shown in FIG. 2 or described for example in US 2005-252458. The system allows heating the steam in the conventional heat transfer sections (i.e. economizers, boiler bank, and superheaters) of the recovery boiler into such a degree that high temperature corrosion does not substantially take place, i.e. below 520 C., optimally 480-500 C., and after that the steam is final superheated to 500-600 C., optimally to 520-560 C. in the combustion cavity, which serves as a final superheater. Thus the chamber can also be used for final increase of the temperatures of live steam and reheat steam, if the flue gases 126 generated in the recovery boiler furnace are too corrosive for final superheating and reheating. The corrosiveness of Cl and K increase with temperature. The corrosiveness of Cl and K impose an upper temperature limit on the steam generated in the recovery boiler. This limit for the superheated steam temperature is typically 400 C. to 490 C., depending on the chlorine and potassium content. However, the target upper temperatures for the steam are typically up to 520-560 C. or higher, as mentioned above. The fuel for superheating of steam is preferably the noncorrosive nature under the conditions of the combustion chamber. In this case the fuel used in the chamber is preferably a biofuel. The fuel can be a gas produced by gasifying biomass. Instead of the gas produced from biomass other fuels can be used, e.g. liquefied biomass, methanol, ethanol, natural gas, LPG etc.
In FIG. 2, the cavity 102 may comprise a single chamber or a plurality of cavities that are arranged in parallel and/or serial. The cavity may share a wall with the furnace 103 and the walls of the cavity may be water-cooled. Combustion gases generated in the cavity 102 flow into the furnace as additional flue gases 127. The cavity may include a superheater 113. Superheated steam flows via steam conduit 123 from the conventional superheaters 108 in the boiler to the superheater(s) 113 in the cavity 102 (or cavities). The cavity 102 may include one or more burners 125. Flue gases 127 formed in the cavity enter the furnace and combine with the flue gases 126 in the furnace of the recovery boiler. Combustion air 128 is injected into the cavity 102 to promote combustion in the burners 125. The burners 125 generally burn gas fuel generated in a gasifier 129 and that flows via gas supply conduit 130. The gas generated by the gasifier 129 may be distributed via line 131 for other purposes in addition to providing fuel for the cavity burners 25. The gas from the gasifier may be cleaned or otherwise treated in a gas treatment device 132 before flowing to the burners.
In connection with the disclosed process, combustion chambers integrated into the recovery boiler can be used for burning concentrated bleaching effluents and/or for final superheating of the steam from the recovery boiler.
The process may increase the investment costs of chemical circulation, therefore it is reasonable to set such guidelines for the bleaching that the investment costs thereof can be controlled. It is therefore reasonable to select bleaching sequence A/D-EOP-D-P with four bleaching stages as a reference sequence and to exclude ozone. For softwood, the corresponding sequence is D-EOP-D-P. In this case, the quality of the pulp can be considered to correspond to the properties of ECF-pulp and the yield remains reasonable. This way, the chlorine dioxide charge for softwood is between 25-35 kg/adt and for hardwood 20-30 kg/adt. These parameters can be regarded as rating values, and thus no new techniques need to be invented for bleaching.
When the amount of active chlorine is calculated as the amount of chlorides in the way described above, it is noted that for softwood, a bleaching line produces, in order to obtain good bleaching results, about 10 kg of chlorides per one ton of cellulose, and a hardwood bleaching line even less. If the mill is closed in such a way that less and less fresh water is introduced into the bleaching, as much as 50% larger chlorine dioxide charges may be expected, and on the other hand the amount of chlorides in bleaching effluents will increase up to a level of 15 kg. Levels higher than this cannot be considered economically reasonable, but the main idea of the bleaching corresponds to these basic solutions. By means of the disclosed process and arrangement, the chlorine/chloride concentrations in different parts of the pulping and recovery processes can be controlled.
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.