U.S. patent application number 12/107877 was filed with the patent office on 2008-11-27 for recovery process and system for a pulp mill.
This patent application is currently assigned to ANDRITZ OY. Invention is credited to Heikki Jaakkola, Kari Saviharju, Janne Vehmaa.
Application Number | 20080289782 12/107877 |
Document ID | / |
Family ID | 39875131 |
Filed Date | 2008-11-27 |
United States Patent
Application |
20080289782 |
Kind Code |
A1 |
Saviharju; Kari ; et
al. |
November 27, 2008 |
RECOVERY PROCESS AND SYSTEM FOR A PULP MILL
Abstract
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.
Inventors: |
Saviharju; Kari; (Espoo,
FI) ; Jaakkola; Heikki; (Helsinki, FI) ;
Vehmaa; Janne; (Siltakyla, FI) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
ANDRITZ OY
Helsinki
FI
|
Family ID: |
39875131 |
Appl. No.: |
12/107877 |
Filed: |
April 23, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60913322 |
Apr 23, 2007 |
|
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|
Current U.S.
Class: |
162/30.1 |
Current CPC
Class: |
D21C 11/12 20130101 |
Class at
Publication: |
162/30.1 |
International
Class: |
D21C 11/12 20060101
D21C011/12 |
Claims
1. A method for burning chlorine-containing liquors in a chemical
recovery boiler at a pulp mill comprising: increasing a combustion
temperature in a burning zone of the recovery boiler, 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.
2. The method of claim 1 wherein over 30 percent, as calculated
from as fired liquor chlorine concentration, is volatilized into
the flue gases.
3. The method of claim 1 wherein over 40 percent of chlorine from
as fired stream chlorine concentration is volatilized into the flue
gases.
4. The method of claim 1 wherein the pulp mill includes 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.
5. The method of claim 1 wherein the chlorine-containing liquor is
a spent liquor.
6. The method of claim 5 further comprising mixing a bleaching
effluent with the spent liquor before supplying the
chlorine-containing liquor to the recovery boiler furnace.
7. The method of claim 5 further comprising superheating steam from
a superheater of the recovery boiler in a separate combustion
chamber of the boiler.
8. The method of claim 5 wherein the recovery boiler is provided
with at least two separate combustion chambers, said method further
comprises burning a chlorine-containing stream in one of the two
separate combustion chamber, and superheating steam from a
superheater of the recovery boiler in another of the two separate
combustion chambers.
9. The method of claim 1 wherein the recovery boiler is provided
with an integrated combustion chamber, and the chlorine-containing
liquor is burned in the integrated combustion chamber.
10. The method of claim 1 further comprising increasing an oxygen
concentration in the burning zone to increase the combustion
temperature.
11. The method of claim 10 wherein the oxygen concentration is
increased by adding oxygen-enriched air to a location adjacent to
where the liquor or effluent is fed to the boiler.
12. The method of claim 10 wherein the oxygen concentration is
increased by raising an oxygen content of combustion air supplied
to the burning zone.
13. The method of claim 10 wherein the oxygen concentration is
increased by supplying oxygen directly to the burning zone.
14. The method of claim 10 wherein oxygen-enriched air is added to
a combustion chamber of the boiler.
15. The method of claim 10 wherein the oxygen-enriched air is added
through at least one secondary air level of the recovery boiler
furnace.
16. The method of claim 1 wherein a high dry-solids content of the
liquor or effluent contributes to the increase the combustion
temperature.
17. The method of claim 1 wherein the combustion temperature is
increased by setting a combustion firing intensity in the burning
zone.
18. The method of claim 1 wherein the combustion temperature is
increased by adjusting a distribution of combustion air in the
recovery boiler.
Description
RELATED APPLICATION
[0001] 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.
BACKGROUND OF INVENTION
[0002] This invention relates to recovery processes for processing
natural cellulosic or other fibrous material and, particularly, to
the removal of chlorine from such processes.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
SUMMARY OF THE INVENTION
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] According to an embodiment of the method disclosed herein,
oxygen-enriched air is added to the integrated combustion
chamber.
[0026] 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.
[0027] 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.
[0028] 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.degree. C., typically over 850, preferably over 950.degree. C.,
most preferably over 1150.degree. C.
[0029] 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/m.sup.2 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
[0030] By increasing the combustion temperature e.g. by optimizing
combustion intensity/m.sup.2 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 gases--in 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.
[0031] 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.
[0032] 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 corrosion--under
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.
[0033] 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.
[0034] 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.
SUMMARY OF THE DRAWINGS
[0035] The process that has been developed will be described in
more detail with reference to the attached drawings, in which:
[0036] FIG. 1 illustrates a schematic illustration of the basic
components of one exemplary system that incorporates the developed
process and arrangement;
[0037] FIG. 2 is a schematic view of a recovery boiler system with
an integrated combustion chamber; for high electricity
production.
[0038] FIG. 3 depicts, in graphic form, a proportion of chlorine
volatilized from as fired spent liquor into flue gases in Kraft
recovery boilers.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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:
[0051] 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.
[0052] By optimizing e.g. combustion intensity/m.sup.2 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 gases--in theory 100%, but not
in practice.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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
corrosion--under 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.degree. C.,
optimally 480-500.degree. C., and after that the steam is final
superheated to 500-600.degree. C., optimally to 520-560.degree. 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.degree. C. to 490.degree. C., depending on the chlorine and
potassium content. However, the target upper temperatures for the
steam are typically up to 520-560.degree. 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
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