CA3133373A1 - Arrangement for collection of hot gas from an electrolysis process, and a method for such gas collection - Google Patents

Abstract

A dual gas collection system for collection of hot gas from an electrolysis process producing aluminium in a cell of Hall-Héroult type comprising PTS (Pot Tending Suction) channels (20, 30) with chimneys (25, 35) having openings (25', 35') for collecting gas from the interior of a gas hooding of the cell. The PTS channels (20, 30) being arranged in the superstructure of the cell, outside the said gas hooding. Inside the gas hooding, that can be thermally insulated, there is arranged a (DPS) Distributed Pot Suction channel (10) that runs along the extension of the hooding, where the channel (10) is provided with at least one the gas collection cap (11). The invention also relates to a method for dual collection of hot gas from an electrolysis process producing aluminium in a cell of Hall-Héroult type where the gas is collected via plural gas collecting caps (11, 12, 13, 14, 15, 16) arranged in a common DPS channel (10), wherein the channel is modified such that the suction rate is substantially equal at each cap along the channel. According to the invention one can extract a more CO2-concentrated flue gas from a cell than is standard procedure in the aluminium industry today, by means of distributed pot suction (DPS) devices. In one embodiment the DPS cap can be integrated with a breaker bar (4) and a feeder for feeding raw material to the cell.

Description

Arrangement for collection of hot gas from an electrolysis process, and a method for such gas collection The present invention relates to a method and an arrangement for collection of off gases in an electrolysis cell, in particular a cell of Hall-Heroult type for aluminium production. In particular the invention relates to a dual channel gas collection system and a method for operating such system.
In modern electrolysis cells (sometimes named pots) for aluminium production by prebaked anodes, the superstructure above the cell has several individual point feeders connected to the cell superstructure. The Pot Suction System (PSS) or gas collection system has several suction points or elongate slits distributed along one or more process gas ducts, located in the top of the superstructure, but as a separate system adjacent to the alumina feeding system.
Commonly, there are two such ducts arranged in parallel. Since at least one anode in the cell has to be replaced by a new anode on a merely daily basis, modern prebake cells have a superstructure with many lids covering the area between the cathode and the gas skirt located just below the anode beam to prevent the flue gases entering the potroom. In order to prevent this pollution, one normally needs negative pressure (sub atmospheric) inside the cell superstructure and large quantities of air is sucked through said openings during the anode change and into the gas suction system together with the flue gases for further treatment, today fluoride recovery and in some cases sulphur removal (purification).
The air entering the inside the superstructure, also provides air cooling of the upper part of the cell with its installed equipment (pneumatical-, electrical- and electronical-equipment). During anode replacements one needs to remove some of the lids. To prevent the flue gases from entering the potroom, and thereby protect the operators from exposure, efficient flue gas collection can be obtained during this operation by increasing the suction volume significantly by setting the cell into Pot Tending Suction (PTS) mode, for instance via an additional fan in a separate suction string. By flipping a valve, the gas suction can change from normal PS (Pot Suction) to PTS (Pot Tending Suction), and the increased suction volume enables handling the anode replacements with several lids removed from the cell without flue gases entering the potroom, i.e maintaining the negative pressure inside the cell super structure.
Feeding alumina to an electrolysis cell was performed more than a century ago by manually breaking the top crust of alumina and feeding alumina powder to the cell. The crust breakage was later done by a crust breaking wheel, then a crust breaker beam and finally an

2 electronically controlled point crust breaker, which is being installed at basically all new smelters being built. Hence, point feeding is therefore considered as state of the art.
The production of aluminium also gives effluents, mainly CO2 with traces of CO, but also significant amounts of HF and SO2. Such effluents leave the electrolytic process through the layer of solidified crust above the electrolyte, through feeding holes but also through the crust itself. Modern smelters remove most of the HF and SO2 before the effluents are released to the atmosphere, but not 002. In order to remove all the effluents being released from the cell and to cool the cell properly, the standard suction design involves several suction points along .. the main gas ducts situated approximately one meter from the top crust.
These suction points suck in a lot of false air from gaps and joints in the cell's superstructure keeping a negative pressure inside the top lids to ensure capture of all the effluents released from the cell. The gas collected is comfortably cold for the superstructure (100-1500C), and the off-gases are strongly diluted by the false air.
Up until today there has not been so much focus on CO2 scrubbing since it is a part of a natural circle, but the recent focus on how CO2 impacts the climate has changed the focus. The design limitation of modern electrolysis cells for CO2 capturing and sequestration is the low concentration of CO2 in the process gas, which typically is less than 1%. To remove low-concentration CO2 is both challenging and expensive. The cost of CO2 sequestration generally decreases with increasing CO2 concentration in the flue gas.
The present invention generally relates to a dual system for gas collection from the cells, one system that is operated during pot tending such as anode change, i.e. a Pot Tending Suction System (PTS) and a system that operates during normal mode and that collects the process gases close to the crust of the cell, by a Distributed Pot Suction (DPS). The DPS can be constructed as an integrated part inside the cell's superstructure and can have alumina feeders integrated. The invention relates to a method of collection of concentrated, process gas for further treatment. In addition, this DPS enables collection of process gas with enough elevated temperatures suitable for heat-recovery, such as flue gas that has a temperature of more than 100 C, preferably more than 150 C.
In WO 2006/009459 there is described a method and equipment for heat recovery from exhaust raw gas from an electrolysis plant for the production of aluminium.
This type of technology can advantageously be combined with the present invention.

3 Various industrial processes produce process gases that can be contaminated by particles, dust and other species that can cause fouling in energy recovery equipment.
Such fouling will imply reduced efficiency and may require extensive maintenance such as cleaning of the surfaces exposed to the gas flow. Process gas may, before it is cleaned, contain dust and/or particles that will form deposits on the heat recovery equipment and thus reduce the efficiency of the heat recovery to an undesired low level. Thus, energy recovery units are normally placed downstream a gas cleaning plant, after the gas has been cleaned.
With respect to optimising the energy recovery, it is of interest to arrange the recovery units as close to the industrial process as possible, where the energy content in the process gas is at its maximum. This implies that the energy recovery units have to be arranged upstream the gas cleaning plant, because such plants are localised relatively distant to the industrial process. For example, process gas from an aluminium electrolysis reduction cell contains large amounts of energy at a relatively low temperature level. This energy is currently utilized only to a small extent, but it can be used for heating purposes, process purposes and power production if technically and economically acceptable solutions for heat recovery are established. The temperature level achieved in the heated fluid is decisive to the value and usefulness of the recovered thermal energy. The heat should therefore be extracted from the process gas at as high a process gas temperature as possible.
Cooling the process gas will contribute to reduced gas flow rate and pressure drop, with reduced fan power as a consequence. The largest reduction in pressure drop is achieved by cooling the process gas as close to the aluminium cells as possible.
The energy content of the process gas can be recovered in a heat exchanger (heat recovery systems) in which the process gas gives off heat (is cooled) to another fluid suitable for the application in question. In principle, the heat recovery system can be located:
- upstream of the cleaning process - where the heat recovery system must operate with a gas containing particles - downstream of the cleaning process - where polluted components and particles in the gas have been removed, - In the electrolysis cell itself.
As the process gas treatment centres (GTC) available today operate most optimal at a low temperature level, energy recovery is, in practice, relevant only for the alternative where the

4 heat recovery system is located upstream of the cleaning process. This means, in practice, that the heat recovery system must be able to operate with hot gas containing particles.
Cooling of the raw gas upstream the fans in combination with heat recovery is a solution that will reduce both the process gas volume flow rate and the pressure drop in channel system and gas cleaning plant. The suction can thereby be increased without the need of changing the dimensions of channels and gas cleaning plant.
The heat recovered from the process gas is available as process heat for various heating and processing purposes, like CO2 sequestration.
The applicant's EP2337879B1 discloses a suction device for gas collection that is able to obtain an efficient collection of the flue gases produced in the cell with a limited amount of alumina or anode cover material (ACM) entering the suction device. In the combination with a point feeder this gives a compact design.
US Patent 4,770,752 from 1988 describes a system where the gas collection cap is placed in contact with the crust in correspondence of a hole provided in the crust. The purpose of this invention is to collect the flue gases from the cell for purification of fluoride components by bringing the components in contact with alumina particles and thereafter return the alumina and the fluorides to the cell again by a separate alumina feeder. CO2 scrubbing and heat recovery are not mentioned except from preheating the alumina. This invention has a limitation with respect to maintenance and possible damages occurring during anode replacements since the cap is situated so close to the anodes and the crust. There is no indication of any plant which utilised this invention, supporting the said drawbacks.
JP Patent 57174483 from 1981 describes a method and device for continuous measurement of current efficiency of an aluminium electrolysis cell. The purpose is to measure current efficiency quickly and continuously and to control supplying of raw materials by collecting the gases produced from the cell continuously, measuring the concentrations of CO2 and CO
successively, converting these to electrical signals and inputting the signals to a controller. The collection device is not fully described but seems to be situated in contact with the crust with the drawbacks just described.
US Patent 4,770,752 from 1988 describes a system where a cap is placed in contact with the crust in correspondence of a hole provided in the crust. The purpose of this invention is to collect the flue gases from the cell for purification of fluorine components by alumina situated

5 close to the cell and thereafter feed the alumina and said components directly back into the same cell from which they have been emitted.
US Patent 5,968,334 describes removal of at least one of the gases CF4 and 02F6 from the flue gases from an electrolysis cell using a membrane.
The present invention relates an integrated equipment with plural Distributed Pot Suction (DPS) zones where one can combine feeding the raw material alumina to the cell and at the same time extract a more 002-concentrated flue gas from several holes in the top crust in the cell than what is standard procedure in the aluminium industry today. However, the suction can also be arranged at other places above the crust in the cell, if appropriate.
Some net effects one can be obtained by the invention:
1. Less total volume of gas removed from the cell with the potential to reduce the overall Fume Treatment Plants / Fume Treatment Centre (FTP/FTC) or Gas Treatment Centre (GTC).
2. As a consequence of point 1, the process gas collected will be increased in temperature more than before and therefore more suitable for heat recovery.
3. Suction of less "false air" into the gas collection chambers during normal operation increases the concentration of CO2 in the off-gas significantly, enabling CO2 capture and sequestration with standard technologies used for CO2 capture from power plants.
4. Improvements of the gas flow inside the superstructure.
5. Increased suction during pot tending operations by a separate Pot Tending Suction.

6. Less heat removed from the cell can reduce the overall energy consumption of the cell.
The present invention relates to a dual channel gas collection system for collection of hot gas from an electrolysis process producing aluminium in a cell of Hall-Heroult type. The cell comprising PTS (Pot Tending Suction) channels with openings for collecting gas, said channels being arranged in the superstructure of the cell outside a gas hooding (GH) of the cell, and wherein inside the gas hooding (GH) there is arranged a DPS
(Distributed Pot Suction) system. The DPS (Distributed Pot Suction) system comprises a designed gas channel that runs along the extension of the hooding, where the channel is provided with gas collection balanced suction cap(s).
The method involves that the suction from the caps are individually tuned and optimized so that the suction rate is substantially equal at each cap along the channel.
During normal operation of the cell, all process gas or substantially all gas is collected via gas collecting cap(s) arranged in the common DPS channel which allows that the energy for gas suction and the amount of gas removed can be optimized at a sufficiently but low level.
These and further advantages can be achieved by the invention as defined in the attached patent claims.
In the following, the invention shall be further explained by examples and Figures where:
Fig. la discloses one embodiment of a dual system comprising (DPS) and (PTS) in accordance with the invention, seen from one side, where the DPS is arranged inside a cell's superstructure, Fig. lb discloses the embodiment shown in Fig. 1, seen in perspective, Fig. 1 c discloses the DPS channel of Fig. la and Fig. 1 b, seen from above, Fig. 2 discloses in a cross-sectional view the dual system of DPS and PTS of Fig.
la, seen from the right short end of the cell, the cross-section taken at one suction cap 16, Fig. 3 discloses details of a DPS collector opening together with a point breaker installed therein, above a hole in a crust, Fig. 4 discloses the embodiment of the dual system comprising (DPS) and (PTS) in accordance with Fig. 1, where arrows indicates how the gas flows in each separate system.
To obtain maximum gas collection from the dual gas suction arrangement, it can be designed in many ways.
This extra suction of the DPS creates an artificial "air wall", which gives a more efficient flue gas collection from the hole "H" in the crust "C" and decreases disturbance from cross flows.
One can also equip the DPS with pressurised air, and blow air through this joint, with the penalty of using more of the compressed air in the pot room for this application. In the Figs, the reference numeral 4 indicates the crust breaker bar.

7 Detailed description of the preferred embodiments A functional description of DPS (Distributed Pot Suction) combined with a point feeder follows:
In Figure la there is shown a DPS suction arrangement adapted to a cell. The arrangement comprises a gas channel 10 that runs along the longitudinal direction of the cell. The gas channel 10 has integrated gas collection caps 11, 12, 13, 14, 15, 16 and an outlet OT. This channel 10 is seen from above in Fig. 1 c, which discloses the gas collection caps correspondingly. Further, it can be seen that the cross-section area of the channel 10 increases .. in the direction of the suction. The parts between the suction caps are to avoid dead rooms and further to achieve the desired velocity in the duct.
Further, as a part of the dual system there is shown in Fig. lb a PTS (Pot Tending Suction) with two gas channels 20, 30, (only channel 20 shown in Fig. la for sake of clarity), where both are running in the longitudinal direction of the cell. These channels are arranged outside the gas hooding GH of the cell (see Fig. 2) and the channels have chimneys that communicates with the interior of the gas hooding GH. It can be seen in Fig. la that for instance channel 20 is provided with chimneys 21, 22, 23, 24, 25, 26.
In Figure 2 a pneumatic cylinder of a crust breaker is indicated at reference sign 1, the breaker is attached to the gas channel of the DPS. In Figure 2 there is shown a gas collection cap 16, a chisel of a breaker bar 4, and a gas suction duct 10.
Fig. 3 shows these elements in a cross-sectional view and an enlarged scale where the collection cap is indicated at 16, the chisel of the breaker bar at 4. The chisel is guided by a guiding tube 5. The guiding tube 5 is arranged inside a shielding tube 6, leaving an annular space 7 between them for supply of a cooling medium, such as air. The cooling medium can be supplied from an upper location of the arrangement and leaving the annular space 7 at positions 7', 7". This gas flow may also be applied for flushing the inner cap with fresh air to protect components/materials from aggressive process gas.
An alumina feeding tube may be arranged inside the collection cap 16 (not shown). Preferably the feeding tube is arranged in a space between an inner wall of the cap 16 and the outer wall of the shielding tube 6.

8 The chisel of the breaker bar 4 is operated periodically to secure that a hole H in the crust C
is open for feeding materials through the hole and into the electrolyte (not shown) below the crust.
During normal operation the gas flow out of the pot's hooding is collected through the DPS, where preferably a feeding point is placed at each of the gas collection caps 11, 12, 13, 14, 15, 16 of the pot. The alumina may be feed from a fluidised feeder but also via mechanical feeders.
When the gas is drawn through the gas collection caps 11, 12, 13, 14, 15, 16, see Fig. 4, it will be collected into the duct 10 inside the hooding of the pot, conveying gas from all feed points.
The gas is from this transition points transported to the fume treatment systems (i.e. Fluoride recovery, and SO2 removal) and introduced from there to any commercial CO2 scrubbing system able to handle the actual concentrations of CO2 or as input to combustion systems such as gas turbines, coal power plants or biomass combustion plant.
When the pot is to be serviced the PTS is activated, and gas is collected from the inside of the gas hooding GH of the cell via the two gas channels 20, 30 and chimneys that communicates with the interior of the gas hooding GH. See also Fig. lb where chimneys are indicated at reference signs 21 to 26 and 31 to 36. In Fig 2, PTS gas channels are indicated at 20, 30, while two corresponding chimneys are indicated at 25, 35 respectively and having openings 25', 35' communicating with the interior of the gas hooding (GH). Preferably, the gas collection via the DPS is stopped during this operation.
Fig.4 illustrates for simplifying reasons in the same figure how the gas is extracted via chimneys 21, 22, 23, 24, 25, 26 and flows into channel 20 together and how the gas flows via the gas collection caps 11, 12, 13, 14, 15, 16 and into the channel 10.
During tending operations, the main ducts in the pot superstructure is activated to support pot tending suction (PTS) from the pot (i.e. increasing the pot suction volume 2-4 times higher than normal.
The up-concentrated process gas is hotter than normal which makes it suitable for heat recovery. On the other hand, the warmer gas may damage the superstructure and electronics placed there. One way to solve this new challenge is to arrange all major hot gas components of the DPS inside the cell's hooding and thermally insulate relevant part of the gas hooding towards the parts of the superstructure comprising vulnerable parts of breakers and feeding

9 apparatus etc. It may also be possible to reduce the heat flux form the DPS
channel 10 by appropriate thermal insulation.
Process gases from several cells can be connected to the same heat recovery unit. The .. process gas is then sent for classical fume treatment, removing dust, HF
and SO2. Depending on whether one connect the flue gas to another process as combustion air or directly to a CO2 scrubber unit, the flue gas might have to be purified sufficiently not to damage these process steps.
The main features of one embodiment of the present invention consist in the integration of the point suction system with the alumina point feeder having a crust breaker. The step forward caused by the DPS is changed composition and increased temperature of the collected process gas. The gas collected by the DPS will contain much less "false air"
and consequently have higher concentration of hazardous gases (Fluoride, S0x, and 002). This will ease the fluoride recovery and SOx removal. The aim is to increase the concentration of CO2 to such a level that commercially available CO2 scrubbing technologies can be utilised to remove it. Also, because of the smaller amount of air and installation straight above the feeding points the collected off-gas has increased temperature compared to the regular process gas, which increases the potential for heat exchange.
It should be clear for any person skilled in the art that the process gas collection cap could be customised for any type of point feeder, and also be arranged in the vicinity of such feeder without being an integrated part of it.
For instance, the walls of the suction cap 11 can be of an oval cross section and slanting upwards in an outward direction. The point feeder can preferably be installed in a manner where it will be as less as possible affected by the heat inside the cells' hooding. The tip of the breaker can be actively cooled by a flow of cooling gas, such as pressurised air flowing from a lance (not shown).
The gas collection caps 11, 12, 13, 14, 15, 16 are preferably placed at a distance from the crust allowing operating space for the anodes during anode change, see for instance the arrangement of gas collection cap 11 and the anodes A', A" in fig. 2.
Advantageously, the cap is placed at a minimum distance to the crust depending on the suction rate.
Preferably the distance is in order 10 to 1000 mm.

10 The designed distance has to take into account the pickup velocity for alumina/anode cover material (ACM) which is in order of 7 metres per second, hence the said distance between the cap and the top of the crust should ensure that this level of velocities at the surface of the crust is not reached.
This embodiment of DPS is designed to separate by physical measures the hot gas to be sucked off and the technical parts of the crust breaker as much as possible, to induce as little thermal stress as possible to vital parts of the crust breaker.
By this means it is possible to withdraw most of the process gases evolved in the cell. In addition, by extracting this rather huge volume of gases just above the crust where the caps are located, the global flow pattern inside the superstructure of the cell will be influenced in a positive manner.
In one embodiment the dual collection of hot gas from an electrolysis process producing aluminium in a cell of Hall-Heroult type can comprise a pair of PTS (Pot Tending Suction) channels connected with openings (25', 35') for collecting gas from the cell during pot tending operations.
During normal operation the gas is collected in close vicinity of a feeding hole(s) in the crust in the cell by gas collecting cap(s) arranged in a common DPS (Distributed Pot Suction) channel, wherein the suction along the channel is tuned in such that the suction rate is substantially equal at each cap along the channel. All process gas or substantially all gas is collected via at least two gas collecting cap(s) arranged in the common DPS channel.
The CO2 capture and storage in accordance with the present invention can in one embodiment be performed in the following steps:
1) Cell CO2 production 2) Collecting flue gas with high CO2 content 3) Heat recovery of said gas 4) Pre-scrubbing 5) Gas to other processes and/or feeding gas to CO2 scrubber 6) Purified gas led out of scrubber, CO2 led to a pressurizing station 7) Liquified CO2 transported to storage.

11 The gas concentration may be up to 4% 002, and the temperature of the gas between 150 QC
up to 200 C. The system is a maintenance friendly modular system having chimneys and caps. The point feeders can be changed from the top, one by one.
The breakers can be provided with a cooling arrangement. Suction caps can be provided with individual collection flaps.
The channel has an increasing dimension in the direction of the gas flow.
DPS channel has a brownfield option, i.e. there is an option to install it during the lifecycle of a cell, for instance when it is taken out of production due to relining.
Combination of DPS/PTS in flushing mode. Deposit alumina can be removed by lost suction and flush dusting. Hatches can be cleaned from side and end.

Claims (11)

Claims

1. A dual channel gas collection system for collection of hot gas from an electrolysis process producing aluminium in a cell of Hall-Héroult type comprising PTS (Pot Tending Suction) channels (20, 30) with openings for collecting gas, said channels being arranged in the superstructure of the cell outside a gas hooding (GH) of the cell, and wherein inside the gas hooding (GH) there is arranged a DPS
(Distributed Pot Suction) system, characterised in that the DPS (Distributed Pot Suction) system comprises a designed gas channel (10) that runs along the extension of the hooding, where the channel is provided with gas collection balanced suction cap(s) (11).

2. A dual gas collection system in accordance to claim 1, characterised in that a point feeder is integrated with said gas collection cap(s) (11).

3. A dual gas collection system in accordance to claim 1, characterised in that the DPS channel (10) is provided with several gas collection caps (11, 12, 13, 14, 15, 16).

4. A dual gas collection system in accordance to claim 1, characterised in that the PTS channels (20, 30) are thermally insulated from the inner space of the gas hooding of the cell.

5. A dual gas collection system in accordance to claim 1.
characterised in that the PTS channels (20, 30) are provided with chimneys (25, 35) that communicates with the inner space of the cell's hooding via openings (25', 35').

6. A method for dual collection of hot gas from an electrolysis process producing aluminium in a cell of Hall-Héroult type comprising PTS (Pot Tending Suction) channels (20, 30) connected with openings (25', 35') for collecting gas from the cell during pot tending operations, wherein said PTS channels (20, 30) being arranged in the superstructure of the cell outside a gas hooding (GH) of the cell, and wherein during normal operation gas is collected in close vicinity of a feeding hole(s) in the crust in the cell, the gas is collected via gas collecting cap(s) (11, 12, 13, 14, 15, 16) characterised in that the gas collecting cap(s) are arranged in a common DPS (Distributed Pot Suction) channel (10), wherein the suction along the channel is tuned in such that the suction rate is substantially equal at each cap along the channel.

7. A method in accordance with claim 6, characterised in that the gas is collected in close vicinity of a feeding hole (H) in the crust (C).

8. A method in accordance with claim 6, characterised in that the gas has a temperature of more than 100 C, preferably more than 150 C.

9. A method in accordance with claim 6, characterised in that heat is extracted from the gas by appropriate heat exchange means, such as an exhaust gas heat exchanger.

10. A method in accordance with claim 6, characterised in that the gas is separated downstream into a CO2 enriched component.

11. A method in accordance with claim 6, characterised in that during normal operation of the cell, all process gas or substantially all gas is collected via the least two gas collecting cap(s) (11, 12, 13, 14, 15, 16) arranged in the common DPS channel (10).