Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B3/00—General features in the manufacture of pig-iron
  • C21B3/04—Recovery of by-products, e.g. slag
  • C21B3/06—Treatment of liquid slag
  • C21B3/08—Cooling slag
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B7/00—Working up raw materials other than ores, e.g. scrap, to produce non-ferrous metals and compounds thereof; Methods of a general interest or applied to the winning of more than two metals
  • C22B7/04—Working-up slag
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B2400/00—Treatment of slags originating from iron or steel processes
  • C21B2400/02—Physical or chemical treatment of slags
  • C21B2400/022—Methods of cooling or quenching molten slag
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B2400/00—Treatment of slags originating from iron or steel processes
  • C21B2400/05—Apparatus features
  • C21B2400/066—Receptacle features where the slag is treated
  • C21B2400/07—Receptacle features where the slag is treated open to atmosphere
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B2400/00—Treatment of slags originating from iron or steel processes
  • C21B2400/08—Treatment of slags originating from iron or steel processes with energy recovery

Definitions

• the present invention relates to the field of industrial waste heat recovery from steel slag, in particular with the purpose of producing saturated steam. Background, Prior Art and technical Problem to be solved
• CO 2 emissions are significant. Considering the urgent need to reduce CO 2 emissions in order to limit climate change, solutions must be found for this purpose. CO 2 capture is one of them. [0003] It is known that the carbon capture (CO 2 ) process needs a lot of energy, and sometimes in the form of saturated steam.
• Waste heat recovery means recovering heat already available on site, for example from hot process exhaust gases, radiative masses, etc. In this way no fuel, and very little electricity coming from the network, are consumed, and operational expenditures are hugely lowered.
• One of the best heat sources available on industrial site is a steel slag pit. Steel slag arrives into the pit at high temperature, around 1000°C, and stays in the pit all the time it is cooling in the ambient environment.
• FIG. 1 shows a solution known from Hui Zhang et al, A review of waste heat recovery technologies towards molten slag in steel industry, Applied Energy 112, Elsevier (2013) 956-966.
• This solution developed in Japan in the early 1980s, relates to the single rotating drum process, a technology for dry granulation based on mechanical impact or rolling of slag film by rotating drum.
• the molten slag is poured onto a rotating drum and is broken up due to direct impact. After that, the disintegrated slag is thrown into a trap under the centrifugal force and then exchanges heat with air in the cooling chamber.
• the air can be heated to a temperature of 500°C and the heat recovery rate achieves 60%.
• Document EP 162 182 A1 discloses a method comprising a rolling of the liquid slag between at least two cooling rolls of metal, preferably steel, the temperature of and the distance between the rolls being controlled such that a cohesive slag slab is obtained having a solidified surface layer and a melted central layer, the slab still being sufficiently plastic to be shapable, a shaping in conjunction with the rolling or after the same, of the slab into briquettes and a recovery of heat at least from the shaped briquettes, preferably after these have been separated from each other, via any suitable cooling means or medium.
• the corresponding apparatus comprises at least two cooling rolls arranged to roll out the liquid slag into a cohesive, shapable slab, means for briquetting the slab and means for recovering heat from the shaped briquettes.
• Document JP5560871 B2 provides a method of efficiently recovering heat energy of steel slag as a gas of high temperature from coagulated slag of high temperature obtained by cooling molten slag.
• a heat exchanger including a hopper, a belt conveyor for conveying the coagulated slag S charged from the hopper approximately in the horizontal direction, a belt conveyor for conveying the coagulated slag S conveyed by the belt conveyor approximately in the horizontal direction, a gas blowing section for blowing a gas exchanging heat with the coagulated slag S above from a lower part of the belt conveyor, and a gas heating section for heating the gas passing through the belt conveyor by the coagulated slag S falling from the belt conveyor onto the belt conveyor, is used as a heat exchanger exchanging heat between the coagulated slag S of high temperature and the gas, to recover the heat energy from the coagulated slag S as the gas of high temperature.
• Document EP 2 660 338 B1 relates to an apparatus for assembling molten slag and recovering sensible heat.
• the apparatus includes: a rotary circular plate which rotates while being cooled by cooling water, and which cools the molten slag dropping onto the top surface thereof so as to convert the molten slag into particle slag and scatter the particle slag; a rotating drum part which rotates while being cooled by the cooling water, which is spaced apart from the side surface of the rotary circular plate, and which collides with the particle slag scattered by the rotary circular part so as to cool the colliding particle slag, thereby moving the cooled particle slag; an inclination-inducing part disposed at a downward incline below the rotation drum part, the inclination-inducing part inducing the colliding particle slag to drop downward; and a sensible-heat recovery casing part connected to the lower portion of the inclination-inducing part to enable the exchange of heat between a cooling medium and the dropping particle
• Document CN 103981307 A discloses a mobile hot smoldering slag treatment line comprising a plurality of hot smoldering slag tanks on a working station of a factory building, wherein each hot smoldering slag tank is communicated with a steam diffusing chimney by virtue of a steam pipeline.
• Two steel rails are arranged on two sides of the plurality of hot smoldering slag tanks.
• a hot smoldering slag cover mobile trolley with a transmission device is arranged on each steel rail.
• a car frame of the hot smoldering slag cover mobile trolley is connected with a hot smoldering slag cover by virtue of a lifting device.
• the hot smoldering slag cover mobile trolley is used as a carrier of the hot smoldering slag cover so as to achieve rapid and flexible switching of the hot smoldering slag cover among a plurality of hot smoldering treatment stations and waiting stations and meet the requirements of production processes.
• the present invention aims to provide a cost-efficient technical solution to recover industrial waste heat from molten slag pits while avoiding the above-mentioned drawbacks of prior art.
• the present invention relates to an industrial installation for recovering waste radiative heat from steel slag, said installation comprising :
• the installation further comprises a lifting system using jacks, so that the heat exchanger under the form of tube-cooled walls can be moved vertically from an upper standby position to a lower working position and vice versa, wherein the tube-cooled walls of the heat exchanger are made of a metal allowing the heat exchanger to work in an environment presenting sharp temperature gradients and corrosion.
• the installation is further limited by at least one of the following characteristics or by a suitable combination thereof :
• said metal allowing the heat exchanger to work in an environment presenting sharp temperature gradients and corrosion is selected from the group consisting of duplex stainless steels, Monel ® alloys, Incoloy ® alloys and Inconel ® alloys ;
• said metal allowing the heat exchanger to work in an environment presenting sharp temperature gradients and corrosion is selected from the group consisting of duplex stainless steel SA 789 S31803, superalloys Incoloy ® S31277, N08028, N08367, N08825 and Inconel ® N06696, N 06625, N 06617, N06230, N06022, N10276 ;
• the steel structure comprises columns or pillars anchored in the ground, said columns or pillars being protected from heat by a concrete shield layer ;
• the mobile heat exchanger of the evaporating device is connected to a fixed part of the evaporating device thanks to a plurality of flexible hoses, the fixed part comprising a steam drum and pumps, the flexible hoses being connected close to the inlet and the outlet of the evaporating device ;
• the heat exchanger is arranged with top panel and side panels only, having the form of a cap or an upside down basket, the inlet and outlet pipes of the heat exchanger being connected to the top thereof, allowing to optimize pipes protection against temperature and corrosion, said pipes being thereby isolated from molten steel slag and further radiation.
• a second aspect of the present invention relates to the use of the industrial installation according to the above, in which the lifting system using jacks is operated to adapt the distance between a bottom of the heat exchanger and the ground so as to allow a slag conveying machine to have access to the pit area, once the bottom of the heat exchanger is in the upper standby position, either for discharging molten slag into the pit or to remove solidified slag from the pit, without any perturbation of the steel slag treatment process or collision risk with the structure.
• FIG. 1 is representing an example of waste heat recovery solution from slag according to prior art.
• FIG. 2 is a perspective view of an installation comprising a heat exchanger for recovering radiative heat above a steel slag pit, according to an embodiment of the present invention.
• FIG. 3 is a cross-sectional view of the installation according to
• FIG. 4 is a detailed perspective view representing an embodiment of the connection device of the fixed part to the moving part of the evaporating device depicted in FIG. 2.
• FIG. 5 is a cross-sectional view representing another embodiment for the connection device of the fixed part to the moving part of the heat exchanger depicted in FIG. 2.
• FIG. 6 is a detailed view of the lifting system of the evaporating device depicted in FIG. 2.
• FIG. 7 is a perspective view of the tube-cooled walls heat exchanger according to the above-mentioned embodiment.
• FIG. 8 is a list of symbols used in the following P&IDs.
• FIG. 9 is representing a general P&ID of the installation according to the present invention corresponding to the steam/water side.
• FIG. 10 is representing the P&ID specifically corresponding to the two-passes heat exchangers.
• FIG. 11 is representing the P&ID specifically corresponding to the blowdown tank and water removal lines. Description of a Preferred Embodiments of the Invention
• the present disclosure has the following structural characteristics :
• the heat exchanger 6 is placed just above a steel slag pit in order to allow radiative heat recovery in an efficient way. As stated before the heat exchanger 6 must be as close as possible to the steel slag pit radiating surface 2 for allowing the heat exchange, but it must also be kept away to let the truck providing liquid steel slag to discharge its content (not shown). No contact is then allowed between molten slag drop and heat exchanger. Accordingly it requires the ability of the heat exchanger 6 to move up and down. Moreover, very little space is available on the ground around the steel slag pits. Therefore the installation footprint must be as reduced as possible.
• the size of the structure is subject to damages in case of seism, to be also considered ;
• the heat exchanger needs to be moved up and down to allow the best radiative heat recovery while protecting the heat exchanger against the machines and the molten steel slag discharge.
• the solution provided by the present invention for solving this challenge is the use of flexible hoses, as illustrated on FIG. 4 and FIG. 5.
• the water flow is preferably divided in several hoses with smaller diameter instead of being carried by only one big hose with large diameter. Indeed small diameters allow the hoses to keep enough flexibility for this application while being sufficiently robust against pressure and temperature conditions of water.
• connection between the fixed part and the moving part is advantageously provided at the supply and at the outlet of the heat exchanger.
• the heat exchanger 6 is made of tube-cooled walls 10, 11 to recover radiative heat. These panels 10, 11 are arranged so that the exchanger forms a type of cap, looking also like a reversed basket (see FIG. 7). This cap makes a shielding protecting the tubes and flexible hoses located above the heat exchanger, avoiding that they are directly exposed to sharp temperature changes and to the highly corrosive fumes of the slag.
• This structure forms an enclosure allowing the recovery of heat rays with a better angle than if the heat exchanger were only a plane rectangle. Nevertheless the four vertical sides of the heat exchanger must be short enough in order not to elevate the steel structure too much, what would increase its wind exposure surface area.
• the walls are supplied with water at their top and a biphasic mixture exits the walls at their top as well.
• the bottom of the heat exchanger must be as clean as possible to optimize the height of the heat exchanger and futher the height of the steel structure.
• it makes the routing of the piping simpler, and keeping pipes above the heat exchanger provides a protection against temperature fluctuation and corrosion.
• the heat exchanger 6 is surrounded by a shell made up of steel plates.
• the function of this shell is multiple :
• the heat exchanger must also be protected against the highly constraining environment (temperature, corrosion) as for the steel structure.
• the solution found for this issue is to make tube-cooled walls in the following metals : « Duplex » (or austeno-ferritic) stainless steel, SA 789 S31803 or superalloys based mainly on nickel or nickel/ chrome such as Monel ® , Incoloy ® and Inconel ® alloys, and in particular Incoloy ® S31277 (or 27-7MO), N08028, N08367, N08825 and Inconel ® N06696, N06625, N 06617, N 06230, N06022 (or HASTELLOY C-22), N10276 (or C-276) (alloy designation according to UNS or Unified Numbering System).
• Such materials offer a good protection against corrosion and their behavior against temperature variations is also well suitable for the kind
• the installation described in this document aims to recover radiative heat emitted by steel slag by a heat exchanger made of tube-cooled walls.
• the heat absorbed by the heat exchanger is advantageously used for saturated steam production at low pressure.
• the heat exchanger is actually an evaporator in which saturated water gets in and is partially evaporated thanks to the recovered heat.
• - Safety position as soon as any danger for the heat exchanger integrity is detected, the heat exchanger will be raised to reach its top position, i.e. o when the steel slag is going to be discharged into the pit under the heat exchanger ; o when the pit is closed and water is injected on the hot slag (generating “dirty” steam, quite corrosive) ; o during slag removal by bulldozers ; o in case of trip of the heat recovery system : no more water mass flow exists inside the heat exchanger (dry operation), therefore the farther of the heat source, the better for the heat exchanger integrity.
• - Exchange position when the slag has just been discharged, the heat exchanger has to be lowered in order to be as close as possible to the heat source and then optimize the radiative heat recovery.
• the cold water will fill in the heat recovery system by going through the feedwater control valve 20.
• This valve will control the water level inside the steam drum depending on the steam production. This control will be performed thanks to flow elements (FE) upstream and downstream the steam drum that measure water and steam mass flows. To assist this control some water level measurements 21 are foreseen on the drum (LT for Level Transmitter, LI for Level Indicator).
• the control valve can be isolated thanks to isolation valves for maintenance purpose.
• the steam leaving the drum 9 will reach the steam network by passing through another control valve 22 set on the steam piping.
• This control valve will be adjusted to maintain a constant pressure (25 barA in the studied application) inside the heat recovery system, measured by Pressure Transmitter (PT).
• PT Pressure Transmitter
• the second goal of this valve is to generate an isenthalpic expansion of steam so that its temperature becomes slightly higher than the saturation temperature corresponding to the steam network pressure. This margin against the saturated state is useful to compensate the heat losses through the steam network. It will be developed in the section about steam network.
• the steam piping going to the steam network is equipped with a drain line to evacuate accumulated condensed water if condensation occurred. This phenomenon will typically happen during the boiler start-up phase when hot steam meets cold pipes. Moreover the closeness of the steam with saturated state may lead to some possible condensation.
• the steam drum is equipped with blowdown lines 23 : one intermittent blowdown line and one continuous blowdown line.
• the intermittent blowdown line is opened in case of water level increase, in order to help its regulation. This line is typically necessary for system start-up phase during which some water level fluctuations are expected.
• the steam drum has been designed to limit the water level fluctuations, nevertheless this line is a supplementary security.
• the continuous blowdown line is continuously draining some water from the steam drum. This allows to evacuate impurities accumulating at the bottom of the drum.
• the drained water mass flow is typically 1% of the total steam production. To compensate this loss of water, some make-up water must be foreseen, it shall be provided at the deaerator feedwater tank. This tank is set out of steel slag treatment site.
• the steam drum is equipped with a safety valve 24, to protect the whole heat recovery system against overpressure. If the pressure measured with PT reaches the design limit the safety valve will open and release the produced steam to stop the pressure increase.
• the heat exchanger is made of tube- cooled walls receiving radiative heat, and carrying evaporating water. It is hanged to a steel structure and is able to move vertically thanks to jacks. In order to allow the relative displacement between the fixed part (the steam drum and the pumps on the steel structure), and the heat exchanger that is moving, flexible hoses are foreseen.
• the tube-cooled walls are two-passes exchangers 30 : water enters at the top of the boiler, is flowing through the first half of the panel, going from first pass to second pass through the lower header, and flowing upwards through the second half of the wall to leave the heat exchanger at the top.
• Circulation pumps - FIG. 9 The evaporation circulation is achieved thanks to circulation pumps 40, as mentioned previously. Two pumps able to operate at full load are provided, one operating and one for back-up.
• Each of the pumps is equipped upstream with filters to avoid soiling. These filters are monitored with pressure difference measurement (DRT). [0084] There is also minimum flow lines for each pump, which ensure that the pump will never work below its acceptable range. In case of system shut off the pump can still work with its minimum flow to allow a quick re-startup.
• pumps can be isolated from the upstream circuit and from the downstream circuit, and are drainable.
• Water chemistry is a key topic for boiler operation. Indeed steel is subject to corrosion in contact with water or steam.
• One of the most important parameter involved in the steel corrosion process is water pH. pH regulation is carried out by injection of alkalyzing agents, this is the chemical dosing of the boiler.
• the water quality monitoring is performed through water sampling. At strategic locations of the boilers some water extractions are foreseen. These extractions will generate water samples intended to be analysed.
• Boiler conservation - FIG. 9 [0089]
• a nitrogen injection 50 is foreseen on the steam drum, in case of boiler shutdown. This injection of inert gas is for conservation purpose, to protect the boiler against water ingress. Stagnant water is a cause of steel corrosion.
• Blowdown tank and water removal - FIG. 11 This last section is related to the evacuation of dirty water to sewer.
• This tank is equipped with cooling water in order to decrease the water temperature to an acceptable level for the sewer.

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• 2022
  • 2022-06-17 WO PCT/EP2022/066563 patent/WO2023274754A1/en active Application Filing
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