Note: Descriptions are shown in the official language in which they were submitted.
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Desulphurization of odorous gases of a pulp mill
The present invention relates to a method for the desulphurization of odorous
sulphur-
containing gas produced in a sulphate pulp mill.
According to a method of this kind, concentrated odorous gases of the process
streams in
sulphate processes, which gases contain sulphur compounds, are collected and
these
odorous gases are combusted in order to oxidize the sulphur compounds.
The invention further relates to the use of the Claus process for the
desulphurization of
odorous sulphur-containing gas produced in a sulphate pulp mill.
The invention also relates to an arrangement suitable for desulphurization of
odorous
sulphur-containing gas produced in a sulphate pulp mill.
The circulations in modern pulp mills are closed so that, although non-
sulphurous
chemicals are substituted for sulphur-containing fresh chemicals, the
circulation must be
desulphurized to prevent uncontrollable sulphur emissions. All modern sulphate
pulp mills
burn concentrated odorous gases either in soda recovery boilers, lime sludge
returning
kilns or in separate odorous gas boilers or reserve flames.
When the concentrated odorous gases are burnt in the soda recovery boiler,
sulphur is
removed either by flue gas emissions (sulphur dioxide) or in the form of
pulverized fuel
ash. Generally, pulverized fuel ash is discharged from the system by releasing
it into the
river and lake system. Sulphur dioxide-containing flue gas emissions cause
acidification of
soil, damages to the trees and to the surface materials of buildings, and
other environmental
damages. Ash, in turn, contains abundantly wood-based heavy metals; therefore,
it is
possible that the release of pulverized fuel ash into the water system will in
the future be
prohibited on environmental grounds. A further considerable disadvantage
related to the
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removal of pulverized fuel ash is that, along with sulphur, also sodium in a
molar ratio of
Na/S 2/1 contained in the ash exits, and must then be added to the system.
When concentrated odorous gases are combusted in the lime sludge reburning
kiln, the
sulphur either exits as flue gas emission (sulphur dioxide and TRS = total
reduced sulphur,
i.e., so-called smelly sulphur compounds) or in waste lime/waste lime sludge.
In addition
to the above disadvantages, sulphur also causes running failures (cycles,
among others) in
the lime sludge reburning kiln.
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When concentrated odorous gases are burnt in the separate odorous gas boilers,
either
sodium bisulphite or a dilute sulphur dioxide solution is obtained. In sodium
bisulphite,
sodium in a ratio of Na/S = I/ 1 exits the system along with the sulphur.
When burning concentrated odorous gases in the reserve flame, all sulphur
escape as air
emission in the form of sulphur dioxide.
All desulphurization methods currently used have one or more of the following
disadvantages: emissions into the air increase, losses of sodium increase,
whereby the
operating expenses grow because of the substitution chemical needed, or the
end product
obtained in the desulphurization is useless to the mill.
It is an object of the present invention to eliminate problems related to
known technology
and to provide a completely new solution for recovering sulphur from the
process streams
of a sulphate process.
The present invention is based on the idea that the sulphur contained in
concentrated
odorous gases is recovered as elemental sulphur which can be returned to the
process or
20. which can be employed in other practical applications.
Selective oxidation of sulphur compounds into elemental sulphur is previously
known in
the art. Accordingly, US 4,522,746 discloses the treating of sulphur-
containing gas flow by
converting essentially all sulphur compounds in the gas flow first to hydrogen
sulphide and
oxidation of hydrogen sulphide into elemental sulphur at low temperature (160-
3200C)
in the presence of titanium dioxide catalyst. FI Patent Specification No.
102250 discloses
a catalyst for the selective oxidation of sulphur compounds into elemental
sulphur, a
method for preparing the catalyst, and a method for the selective oxidation of
sulphur
compounds into elemental sulphur. Published FI Patent Application No. 903673,
in turn,
suggests monolithic catalysts for the treatment of sulphur compounds-
containing gases in
the industry. Published FI Patent Application No. 933481 discloses a method
and an
arrangement for the treatment of black liquor originating from a sulphate
process for the
recovery of energy and chemicals. Hydrogen sulphide can be produced from
exhaust gas
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containing sulphur compounds when heating the black liquor. Hydrogen sulphide
can be
used for different purposes, for example it can be oxidized into elemental
sulphur.
The published GB Patent Application No. 2117749 describes a method and an
apparatus
for the combustion of gases containing a hydrogen sulphide to form elemental
sulphur.
None of the above solutions suggests the treatment of concentrated odorous
gases; neither
do they contain any suggestions for the recycling of the sulphur thus
obtained.
According to the invention, the odorous gases collected from the process
streams of the
sulphate pulp mill are combusted at a low air index so that at least an
essential part of the
sulphur compounds is oxidized into elemental sulphur, which is removed from
the system.
This solution can be used to remove sulphur from the waste gases of the pulp
mill in a
controllable way. The method of oxidation of sulphur compounds used in the
invention is
based on the so-called Claus process.
The invention also provides an arrangement for the removal of sulphur from the
circulation
of chemicals of a sulphate pulp mill, comprising a combustion unit for the
odorous gases,
which is provided with an inlet, which is connected to the source of
concentrated odorous
gases, and with an outlet, through which the oxidized sulphur compounds of the
concentrated odorous gases can be removed from the device as elemental
sulphur. The
combustion unit according to the invention comprises at least one combustion
device of the
Claus system.
More specifically, in a first embodiment, the invention comprises a method for
desulphurization of sulphur-containing, concentrated odorous gases produced in
a sulphate
pulp mill, comprising: a) collecting the concentrated odorous gases that
contain sulphur
compounds; and b) combusting them to oxidize the sulphur compounds,
characterized in
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that: i) the concentrated odorous gases are combusted at an air index below
one in a first
combustion unit, using the Claus process so that 50 to 90 molar % of the
sulphur
compounds are oxidized into elemental sulphur, and at least 10 molar % of the
sulphur
compounds from the first combustion unit are conducted to a second combustion
unit and
combusted, and ii) the elemental sulphur is recovered in liquid or solid form.
In a second embodiment, the invention comprises use of the Claus process for
desulphurization of sulphur-containing, concentrated odorous gases produced in
a sulphate
pulp mill wherein the concentrated odorous gases are combusted with
substoichiometric
amounts of air into elemental sulphur in one or more combustion units of the
Claus system
and at least 10 molar % of the sulphur compounds which pass through the Claus
system are
conducted to a combustion device of at least one second combustion unit.
In a third embodiment, the invention comprises an arrangement for
desulphurization of
sulphur-containing, concentrated odorous gases produced in a sulphate pulp
mill,
comprising at least one odorous gas combustion unit into which a flow of the
concentrated
odorous gases from a source of concentrated odorous gases is collected and
wherein
reduced sulphur contained in the concentrated odorous gases is combusted,
characterized in
that: the arrangement comprises a first combustion unit, which has at least
one Claus
system combustion device, which is provided with a feed inlet, which is
connected to the
source of concentrated odorous gases, and with an exhaust outlet, through
which the
oxidized and condensed sulphur compounds of the concentrated odorous gases are
removed from the first combustion unit as elemental sulphur in a liquid or
solid form, and a
second combustion unit, whereby the first unit has an exhaust nozzle for
gaseous,
uncondensed sulphur compounds, which is connected to the second combustion
unit.
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The invention provides considerable advantages. Accordingly, the method can be
used to
desulphurize the waste gases of the sulphate pulp mill in particular. The
sulphur of the
odorous gases is recovered in a form that can be utilized in the same or in
other processes.
The methods according to known technology provide no solutions, which,
according to the
invention, would integrate the method of desulphurization into the exhaust
system of
odorous gases of a sulphate cellulose mill.
To optimize the discharge of elemental sulphur in various temperature ranges,
the
combustion of odorous gases can be carried out in one or more steps. This can
be carried
out so that one or more combustion units of the Claus system are arranged
sequentially.
Various running conditions can be used therein.
On the other hand, the invention can be used for converting sulphur, for
example, so that it
can be exploited by combusting sulphur to sulphur dioxide which can be used in
bleaching,
and to make sulphuric acid by means of processing either for the manufacture
of bleaching
chemicals or for peeling pine oil. At the same time, the sodium losses of the
mill can be
minimized.
In a process configuration, the solution can be placed in connection with the
existing
combustion of concentrated odorous gases, whereby so-called tail gases can be
combusted
in the existing equipment, such as in a soda recovery boiler, an odorous gas
boiler, a lime
sludge reburning kiln or a flame.
In the method according to the invention, it is preferable to use for the
condensation of
sulphur, e.g., the condenser water or the boiler water of the pulp mill.
In the arrangement according to the invention, the burning device of the Claus
system, i.e.
the reactor, and a corresponding steam boiler is connected at its inlet to the
odorous gas
line of the cooking department or the evaporation plant of the sulphate
process or a
corresponding source of odorous gas, and the concentrated odorous gases
obtained
therefrom can be combusted, in accordance with the principles of the Claus
process, with
substoichiometric amount of air to oxidize the sulphur compounds. To
condensate the
oxidation products obtained from the combustion device, the arrangement
further
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comprises a condenser, which can be used to condensate the products, the
elemental
sulphur in particular, into a liquid or solid form. In order to discharge
these liquid or solid
substances, the combustion device is equipped with a discharge assembly that
is connected
to the condenser. In addition, the unit further comprises an exhaust assembly
for gaseous
5 (uncondensed) bodies, which can be used to remove any light oxidation
products, as well
as inert and not reacted compounds (TRS and SO2). As indicated in detail in
the following,
at least part of the sulphur compounds of the concentrated odorous gases
coming from the
Claus combustion are conducted to conventional odorous gas combustion which
takes
place in the soda recovery boiler, odorous gas boiler, lime sludge reburning
kiln or flame,
or in more than one of these devices. Therefore, the outlet of the gaseous
compounds of the
arrangement is preferably connected by means of pipe lines to the inlet of the
soda
recovery boiler or the lime sludge reburning kiln, or to a similar post-
processing device.
In a process configuration, the solution can be placed in connection with the
existing
combustion process of concentrated odorous gases, whereby the so-called tail
gases can be
combusted in the existing equipment, such as in a soda recovery boiler or lime
sludge
reburning kiln or similar equipment, and the required condenser of sulphur on
the cooling
water side can be integrated into the water circulations/recovery of heat of
the mill. In that
case, the equipment solution is reduced into a simple and economic form. The
solution
according to the invention can be applied both to existing and new mills.
As a simple addition, the solution is particularly well suited for mill
concepts, wherein the
soda recovery boiler is the primary place of combustion of concentrated
odorous gases.
In the following, the invention is examined more closely by means of a
detailed description
with reference to the appended drawings.
Fig. 1 shows a flow chart illustrating the collection of odorous gases.
Fig. 2 shows, again with the aid of a flow chart, the processing method of
odorous gases
according to prior art.
Fig. 3 shows the recovery of sulphur by means of the Claus process, according
to the
invention.
Fig. 4 shows the flow chart of an alternative embodiment of the invention,
wherein several
burning devices of the Claus system are connected in parallel.
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Fig. 5 shows the recovery of sulphur by means of the Claus process and the
flow chart of
the post processing of the gases from the Claus system.
One special feature of the invention is that the so-called Claus process is
used to binding
sulphur from concentrated odorous gases. The following reactions take place in
the Claus
process:
H2S + 3/2 02 <-> S02 + H2O 1 /3 (I)
2 H2S + S02 <-> 3 S + 2 H2O 2/3 (II)
The concentrated odorous gases are combusted with a substoichiometric quantity
of air or
oxygen, whereby part of the sulphur of the concentrated odorous gases remains
elemental
sulphur and part is combusted to sulphur dioxide. The amount of elemental
sulphur
remaining is 50 to 90 molecular percent, typically 70 to 85 molecular percent.
The concentrated odorous gases herein refer to the smelly gases of the pulp
mill, the
concentration of which exceeds the so-called upper explosion limit. These
gases consist of
hydrogen sulphide, methyl mercaptan, and dimethyl sulphide and dimethyl
disulphide,
among others.
The reduced sulphur in the concentrated odorous gases is enough concentrated,
is also
sodium-free and has a sufficiently rich thermal capacity. By burning at a
suitable air index,
the main part of the sulphur can be oxidized into elemental sulphur, as is the
case in the
above conventional Claus process. The sulphur can be condensed and discharged
separately as liquid sulphur, which is an inert chemical element with an
inherent market
value. The "air index" herein refers to the molecular proportion of oxygen
(02) in the air to
the sulphur (S) in the substance being treated. The air index is one, when the
amount of
oxygen in the air is in a stoichiometric proportion to the amount of sulphur
of the gas to be
treated to change all the sulphur into sulphur dioxide. "Substoichiometric
amounts of air",
in turn, means that the air index is smaller than one.
In the present invention, the odorous gases are combusted with under air or
short air,
typically, at an air index of 0.4 - 0.9, whereby the adiabatic combustion
temperature is
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about 1400 - 1800 C. For one gas mixture, the combustion was carried out at an
air index
of about 0.6, whereby the adiabatic combustion temperature was about 1600 C.
According to one preferred embodiment of the invention the combustion of
concentrated
odorous gases is carried out in adiabatic conditions at the temperature of
1400 - 1800 C.
According to the invention, part of the collected odorous gases are conducted
to Claus
combustion and part of the sulphur compounds from the Claus combustion are
conducted
to conventional odorous gas combustion, or optionally part of the collected
odorous gases
are conducted directly to conventional odorous gas combustion.
More specifically, according to the method of the invention, the flow of
odorous gases
collected from the source of concentrated odorous gases is conducted to
combustion in a
first combustion unit, using the Claus process, through appropriate pipe
lines, and the flow
of gases from the first combustion unit through appropriate pipe lines to a
second
combustion unit, such as the odorous gas boiler, soda recovery boiler, lime
sludge
reburning kiln or flame, or to more than one of these devices. Alternatively,
part of the
flow of concentrated odorous gases can be conducted directly past the first
combustion unit
and into the second combustion unit.
According to the invention, at least 10 molar percent of the collected odorous
gases are
conducted to combustion in the first combustion unit using the Claus process.
30 - 90
molar %, typically 50 - 90 molar %, are preferably conducted through the Claus
process.
The rest of the collected odorous gases, 10 - 90 molar %, can be conducted
directly to
combustion in the existing apparatuses.
The concentrated odorous gases are combusted in the first combustion unit with
substoichimetric amounts of air. The sulphur that has oxidized into elemental
sulphur is
condensed with the condenser and discharged in a molten form through the
bottom of the
odorous gas combustion unit. The elemental sulphur in liquid form is conducted
into the
collector and taken further to a means of transport. When cooling, liquid
sulphur solidifies;
however, short storage for reuse in particular should be implemented in liquid
form.
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When necessary, the exhaust gases or tail gases are conducted into the second
combustion
unit of the Claus system to maximize the desulphurization. If required, the
running
conditions used in the latter combustion device may differ from those in the
first
combustion device.
The exhaust or tail gases, such as sulphur dioxide and TRS, which come from
the Claus
system, are lead into at least one combustion device of the second combustion
unit, such as
the odorous gas boiler, soda recovery boiler, lime sludge reburning kiln or
flame, to be
disposed of. In this method, the Claus system is not intended for a complete
recovery of
sulphur as elemental sulphur; on the contrary, at least 10 molecular percent
of the sulphur
compounds taken through the Claus system are taken to be combusted in the
combustion
device of the second combustion unit. Typically, at least 10 molecular
percent, about 50
molar % at a maximum, preferably about 15 to 30 molar % of the sulphur
compounds of
the concentrated odorous gases coming from the Claus combustion are lead to
combustion
in conventional combustion systems, such as soda recovery boiler or lime
sludge reburning
kiln.
The method according to the invention functions without catalysts. When
necessary, the
Claus system uses technology, such as catalysts, which was developed in
connection with
the Claus combustion of hydrogen sulphide. Examples of suitable catalysts are
cited in the
published FI Patent Application No. 904949 or FI Patent Specification No.
102250.
The elemental sulphur obtained according to the method can be stored in a
liquid or solid
form and further transported in a liquid or solid form, for example, to a
sulphuric-acid
plant or another pulp mill. It can also be recycled into the process, e.g., by
suitably adding
to white liquor (the manufacture of a polysulphide solution) or, when
necessary, by feeding
upstream into the soda recovery boiler by means of a combination burner or in
some other
way to adjust the sulphidity. As it is easy to store and transport, the
recovery of sulphur in
a liquid or solid form is advantageous.
According to an alternative solution, the odorous gases can be combusted in a
Claus
reactor with the aid of oxygen instead of air.
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According to a second alternative solution, the elemental sulphur can further
be combusted
to sulphur dioxide or processed to form sulphuric acid. Also in that case,
however, the
elemental sulphur can first be recovered by means of the combustion unit of
the Claus
process.
According to a third alternative solution of the invention, the amount of
concentrated
odorous gases is increased, for example, by means of thermal treatment of
black liquor.
In the following, the method according to the invention is examined with
reference to the
appended figures.
Fig. 1 shows a flow diagram of the collection of concentrated odorous gases
into the
collectors of a cooking department 1 or an evaporator plant 2 from known
sources of
concentrated odorous gases either separately or in combinations thereof, using
known
methods.
Fig. 2 shows a flow diagram of a prior art processing method of odorous gases,
wherein the
concentrated odorous gases are lead into an odorous gas boiler 3 (containing a
condenser
7), soda recovery boiler 4, lime sludge reburning kiln 5 or flame 6, or into
more than one
of them.
Fig. 3 shows the arrangement according to the invention suitable for the
combustion of
odorous gases. The arrangement according to the invention comprises at least
one odorous
gas combustion unit 10 (a first combustion unit), which a flow of concentrated
odorous
gases can be collected in and lead into from a source of concentrated odorous
gases, and
wherein the reduced sulphur contained in the concentrated odorous gases can be
combusted. The combustion unit comprises at least one combustion device 10 of
the Claus
system, and a condenser 11 connected thereto, and an air feeding assembly 18.
The
combustion unit is provided with a feeding assembly 14, which is connected to
the source
of concentrated odorous gases, and an exhaust assembly 15, through which the
oxidized
and condensed sulphur compounds of the concentrated odorous gases can be
discharged
from the device as elemental sulphur in a liquid or solid form. The elemental
sulphur can
be lead into a collector 12 and further to a transporter 13. The combustion
unit also
comprises an exhaust assembly 16, through which the gaseous, uncondensed
sulphur
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compounds coming from the first combustion unit can be lead into the second
combustion
unit to be combusted. Furthermore, the feeding assembly 14 of the first
combustion unit
can be connected to a distribution piece 17 so that part of the concentrated
odorous gases
can be lead through the distribution piece and past the first combustion unit
directly into
5 the second combustion unit to be combusted.
The second combustion unit comprises at least one combustion device, such as
an odorous
gas boiler, soda recovery boiler, lime sludge reburning kiln and/or flame.
10 Fig. 4 shows an arrangement, wherein the first combustion unit comprises at
least two
combustion devices 20, 21 of the Claus system, connected in parallel, and
condensers 22,
23 connected thereto, and air feeding nozzles 33, 34. The combustion unit is
provided with
a feed nozzle 26, which is connected to the source of concentrated odorous
gases, and with
an exhaust nozzle 27, through which the oxidized and condensed sulphur
compounds of
the concentrated odorous gases can be discharged from the device as elemental
sulphur in a
liquid or solid form. The elemental sulphur can be lead into a collector 24
and further to a
transporter 25. The combustion unit also comprises an exhaust assembly 28,
through which
the gaseous, uncondensed sulphur compounds coming from the first combustion
unit can
be lead into the second combustion device 21 of the Claus system through a
feeding
assembly 29. The latter combustion device 21 is provided with an exhaust
assembly 32,
through which the oxidized and condensed sulphur compounds can be discharged
from the
device as elemental sulphur in a liquid or solid form. The elemental sulphur
can be lead
into the collector 24 and further to the transporter 25. The second combustion
device of the
Claus system is further provided with an exhaust assembly 30, through which
the gaseous,
uncondensed sulphur compounds coming from the combustion device of the Claus
system
can be lead into the second combustion unit to be combusted.
The feed nozzle 26 of the first combustion unit can be connected to the
distribution piece
31 so that part of the concentrated odorous gases can be lead through the
distribution piece
and past the first combustion unit directly into the second combustion unit to
be
combusted. Alternatively, the exhaust nozzle 28 of the first Claus combustion
device of the
Claus system can also be connected to the distribution piece, through which
part of the gas
stream from the first Claus combustion device can be lead directly into the
second
combustion unit to be combusted.
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Fig. 5 shows the recovery of sulphur by means of the Claus process, wherein
several Claus
combustion devices are connected sequentially, and the flow diagram of the
post
processing of the gases from the Claus system. The arrangement comprises a
first
combustion unit having at least two combustion devices 40, 41 of the Claus
system
connected in parallel, and condensers 42, 43 connected thereto, and air
feeding assemblies
58, 59. The first combustion device of the Claus system is provided with a
feed nozzle 46,
which is connected to the source of concentrated odorous gases, and with an
exhaust
nozzle 47, through which the oxidized and condensed sulphur compounds of the
concentrated odorous gases can be removed from the device as elemental sulphur
in a
liquid or solid form. The elemental sulphur can be lead into a collector 44
and further to a
transporter 45. The combustion device also comprises an exhaust nozzle 48,
through which
the gaseous, uncondensed sulphur compounds from the first combustion device of
the
Claus system can be lead into the second combustion device 41 of the Claus
system
through a feeding nozzle 49. The latter combustion device 41 is provided with
an exhaust
nozzle 57, through which the oxidized and condensed sulphur compounds can be
removed
from the device as elemental sulphur in a liquid or solid form. The elemental
sulphur can
be lead into the collector 44 and further to the transporter 45. The second
combustion unit
of the Claus system is further provided with an exhaust nozzle 50, through
which the
gaseous, uncondensed sulphur compounds coming the combustion device of the
Claus
system can be lead into the second combustion unit to be combusted.
As described in connection with Fig. 4, the feed nozzle assembly 46 of the
first combustion
unit can be connected to the distribution piece 51 so that part of the
concentrated odorous
gases can be lead through the distribution piece and past the first combustion
unit directly
into the second combustion unit to be combusted. Alternatively, the exhaust
nozzle 48 of
the first Claus combustion device of the Claus unit can be connected to the
distribution
piece, through which part of the gas stream coming from the first Claus
combustion device
can be lead directly into the second combustion unit to be combusted.
The exhaust or tail gases, such as sulphur dioxide and TRS, coming from the
combustion
device 41 of the Claus system of the first combustion unit, are lead to be
disposed of in at
least one combustion device of the second combustion unit, such as the odorous
gas boiler
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53 (which is provided with a condenser 52), soda recovery boiler 54, lime
sludge reburning
kiln 55 and/or flame 56.
