HEAT RECOVERY ON STEEL SLAG Field of the Invention

[0001] 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

[0002] In heavy industry, for instance steel industry, 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.

[0004] Energy being an important cost in the process of carbon capture, for example from blast furnace exhaust gas, a solution developed to reduce steam cost is producing steam through waste heat recovery. 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. [0005] 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.

[0006] There is a need to develop a technical solution for recovering heat thereof that is :

- technically feasible ;

- implementable in an existing site ;

- with no impact on the process of the steel slag ;

- simple to operate ;

- with an efficient heat recovery potential ; and

- at a reasonable cost.

[0007] Before conceiving a new solution, a review of prior art is needed in order to firstly find suitable and already existing solutions.

[0008] Some of these already designed solutions are presented hereinafter.

[0009] 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%.

[0010] 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.

[0011] 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.

[0012] 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 slag, the sensible-heat recovery casing part discharging the particle slag and the cooling medium having undergone heat exchange to the outside. [0013] In the system of SJ Pickering et al, New process for dry granulation and heat recovery from molten blast-furnace slag, 2005 http://masters.donntu.orq/2005/fizmet/konchenko/library/arti cle11 .him, the liquid slag stream is fed into a rotating cup atomizer where centrifugal force creates droplets which are ejected at high speed and solidified in an air stream before falling into a fluidized bed or other exchange medium.

[0014] All these technical solutions use the same principle of heat recovery : slag granulation and utilization of air as heat transfer fluid. These solutions allow the exploitation of the latent heat of the slag, what constitutes a huge amount of energy.

[0015] Nevertheless, a main characteristic and drawback of these technical solutions is the high footprint and space occupied by the installation. On site however, available space is very limited, around the pits and further in the steel slag process.

[0016] Moreover, the above-mentioned installations seem to have a high level of complexity. These technologies are not industrially implemented and no working application exists. The maturity of these technologies is clearly not demonstrated, and this level of complexity poses the challenge of the maintenance and the availability level of the installation. A lot of elements, especially moving elements, are subject to failure and then decrease the potential lifetime and availability level, especially in extreme environment. And in this kind of industrial site, any interruption of the process can lead to loss of money. Talking about money, these complex installations certainly represent a huge investment cost, in addition to maintenance costs.

[0017] Finally these solutions imply a transformation of the slag, so that the process would be impacted and this should be avoided.

[0018] In conclusion it clearly appears that the prior art solutions, some of them being already patented, are not suitable for the application of interest here and the inventors decided to study different ways to recover heat from molten steel slag. [0019] According to the chosen principle, heat is recovered from steel slag as inspired from a waste heat recovery solution already designed : recovering the radiative heat emitted by a hot mass by surrounding it by tube- cooled walls inside which evaporating water is flowing. This way to recover heat is very classically found in fired boilers, where furnaces are made up of tube- cooled walls.

[0020] Even if this way to recover radiative heat has proven its efficiency in boilers, some important challenges remained to be solved in the frame of the present invention :

- there is very few space left available around pits ;

- some heavy machines are moving around the pits, with the risk of destroying any installation on their way ;

- there is a highly corrosive environment as steel slag is made of plenty of different elements that are sometimes highly corrosive ;

- temperature is highly fluctuating, inducing a high temperature gradient ;

- an environment producing soot ;

- one should be able to recover heat as close of the emitting surface as possible without interfering with the steel slag discharge ;

- the process is highly irregular, therefore steam production is also very irregular.

[0021] 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. An explosion-discharging device and a water spraying device are arranged on the hot smoldering slag cover. 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.

Aims of the Invention [0022] 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.

Summary of the Invention [0023] The present invention relates to an industrial installation for recovering waste radiative heat from steel slag, said installation comprising :

- a pit in which molten steel slag is discharged and from which solidified steel slag is removed, using a slag conveying machine or vehicle ; - an evaporating device for producing hot water and steam and auxiliary equipment, said evaporating device comprising a heat exchanger under the form of tube-cooled walls ;

- a steel structure supporting said evaporating device ; characterized in that 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. [0024] According to preferred embodiments, 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.

[0025] 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.

[0026] In document CN 103981307 A, the lifting system using jacks is not used to move a heat exchanger vertically from an upper standby position to a lower working position and vice versa. Brief Description of the Drawings

[0027] FIG. 1 is representing an example of waste heat recovery solution from slag according to prior art.

[0028] 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.

[0029] FIG. 3 is a cross-sectional view of the installation according to

FIG. 2.

[0030] 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.

[0031] 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. [0032] FIG. 6 is a detailed view of the lifting system of the evaporating device depicted in FIG. 2.

[0033] FIG. 7 is a perspective view of the tube-cooled walls heat exchanger according to the above-mentioned embodiment.

[0034] FIG. 8 is a list of symbols used in the following P&IDs. [0035] FIG. 9 is representing a general P&ID of the installation according to the present invention corresponding to the steam/water side.

[0036] FIG. 10 is representing the P&ID specifically corresponding to the two-passes heat exchangers.

[0037] 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

STRUCTURAL DESCRIPTION

[0038] According to some embodiments, the present disclosure has the following structural characteristics :

A. A steel structure

[0039] The proposed technical solution is shown in FIG. 2 and FIG. 3.

[0040] 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.

[0041] The solution provided to this requirement is :

- a steel structure 14 for a heat exchanger support above the pit 2 and also supporting the auxiliary equipments such as drum 9, pumps 40, etc. with sufficient height ;

- a lifting system using jacks 5 to allow vertical movement of the heat exchanger 6, fixed on the heat exchanger enclosure (and not on the heat exchanger itself).

[0042] The steel structure faces several constraints that are typical of this industrial site :

- the steel slag pits are wide (15m typically) which requires a steel structure with an important span ;

- the pit ground is not favourable for structure stability : civil engineering and solid foundations are requested ; - steel slag conveying machines are several meters high : the steel structure must therefore be more than about 10m high to allow the heat exchanger to be out of the way of the machine. This generates a significant wind exposure surface area ;

- the size of the structure is subject to damages in case of seism, to be also considered ;

- the environment is highly corrosive and with high temperature fluctuation ;

- the installation cannot have any impact on steel slag logistics and process.

[0043] The steel structure presented in FIG. 2 and FIG. 3 has then been designed according to the items listed hereabove. Stiffeners 14 are only set on both sides of the structure. In front of the structure, enough space has been foreseen for the molten slag conveying machine. And on the back of the structure enough space has also been foreseen for machines removing cold slag from the pit.

[0044] To solve the constraint relative to the environment (corrosion, temperature), steel columns shall be advantageously covered by a concrete layer. This solution has been proven for high temperature variation resistance, and it is expected to offer a good resistance to corrosion as well. Some maintenance is nevertheless to be expected, for example replacement of the concrete layer after a number of years.

[0045] It has to be noted that only the bottom part below the heat exchanger (when it is in top position) is to be protected against this aggressive environment. Indeed the presence of the heat exchanger constitutes a « shield » against corrosion and temperature issues for the top part of the structure, for the exchanger lifting system, and also for the equipment placed atop the structure.

[0046] Lastly the structure height offers the significant advantage of allowing the heat exchanger to work in dry conditions, i.e. without being cooled by a cooler fluid. In case the boiler faces problems and must stop working, steel slag logistics cannot often be adapted consequently. What means that steel slag discharge into the pit where exchanger is not cooled anymore cannot often be avoided. B. The movement of the moving heat exchanger vs. the fixed part of the installation

[0047] As mentioned hereabove, 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.

[0048] In the section about the steel structure, a heat exchanger support with jacks is mentioned, allowing the heat exchanger to move up and down.

[0049] Nevertheless the connection between piping connected to fixed part of the installation (such as steam drum, pumps) and piping connected to moving part (the heat exchanger) must also follow this relative movement.

[0050] 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.

[0051] The constraints to be solved for these flexibles hoses 8 are the following :

- resistance to pressure (25 barA) and temperature (225°C), therefore meet the requirements of the PED ;

- being still flexible enough to be elongated of more or less 7m ;

- resistance to regular movements ; - keep acceptable velocities of water or biphasic mixture inside.

[0052] To meet all these constraints in the same time, 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.

[0053] In order to minimize the importance of the moving part and also keeping the steam drum and the pumps in a static position, the connection between the fixed part and the moving part is advantageously provided at the supply and at the outlet of the heat exchanger. C. The architecture of the heat exchanger

[0054] As mentioned hereabove, 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.

[0055] 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.

[0056] The walls are supplied with water at their top and a biphasic mixture exits the walls at their top as well. Indeed 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. Moreover, it makes the routing of the piping simpler, and keeping pipes above the heat exchanger provides a protection against temperature fluctuation and corrosion.

[0057] Piping carrying inlet and outlet water fluxes are illustrated in

FIG. 4.

[0058] The heat exchanger 6 is surrounded by a shell made up of steel plates. The function of this shell is multiple :

- it bears the walls carrying evaporating water ;

- it allows to maintain a layer of thermal insulating material to limit heat losses ;

- it makes the link between the steel structure and the heat exchanger : the cables of the jacks 5 are attached to this shell, as it can be seen in FIG. 4 and FIG. 6 ;

- it participates to the « shielding » properties of the heat exchanger ;

- heat exchanger bending issues are avoided while minimizing the number of lifting devices. [0059] The heat exchanger must also be protected against the highly constraining environment (temperature, corrosion) as for the steel structure. According to preferred embodiments, 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 of application of the present disclosure.

[0060] More details about the operation of this heat recovery system and the architecture of the water/steam circuit are given in the functional description of the present invention below.

FUNCTIONAL DESCRIPTION

[0061] This section will describe how the heat recovery system operates. The control system has a classical boiler control architecture, with some specificities for the present application.

1. Purpose of the system and operating conditions

[0062] 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.

[0063] The heat exchanger is actually an evaporator in which saturated water gets in and is partially evaporated thanks to the recovered heat.

[0064] In the studied application where this solution has been developed, the goal is to produce saturated steam at 25 barA from feedwater received at 170°C (50°C below saturation). The steam will enter the steam network. This network carries steam to the receiving process (for example carbon capture process). [0065] Heat is available up to 1000°C, and is highly cyclical. Steel slag is discharged in a pit every 20 min (in full load conditions) so that the emitting surface temperature is cyclically fluctuating. The height of the emitting surface is slightly increasing with time since more and more steel slag layers are added. Today, between two unloadings, steel slag in pit is losing heat to the ambient air so that its temperature decreases quite fastly. The implementation of the technical solution disclosed here allows to recover this heat.

2. Details about the heat recovery system control

[0066] In this section all the main regulation systems will be described, referencing to the P&IDs (for Piping and Instrumentation Diagram) provided in FIG. 9 to 11 .

STEEUSLAG SIDE

[0067] No P&ID is available for this part yet, nevertheless the principles of control are already defined. The important parameter to be controlled is the position of the heat exchanger, which can be lowered to reach a bottom position, or raised to reach a top position.

[0068] Two cases are foreseen when the exchanger is in operation: the “exchange” position and the “safety” position :

- 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.

[0069] Two signals shall be used to control the heat exchanger height, in normal operation :

- Detection of the position of the truck conveying liquid steel slag : o when leaving the steel mill for the concerned pit, raising of the heat exchanger, because some steel slag is going to be discharged ; o when leaving the steel slag pit just after having discharged some steel slag, lowering of the heat exchanger ;

- Detection of pit closing : when a pit is considered full, it is then “closed”. In the control center managing the state of the pits a signal shall then have to be generated to give the heat exchanger the instruction to raise. The heat exchanger will then stay in the high position until the first truck leaves the steel mill to come to this pit for a new cycle.

STEAM/WATER SIDE

[0070] All the P&IDs available for this heat recovery are describing the water/steam lines and also process lines such as sampling or chemical dosing.

Feedwater line - FIG. 9

[0071] 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. Drum outlet steam - FIG. 9

[0072] 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).

[0073] 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.

[0074] 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.

Steam drum - FIG. 9

[0075] The steam drum is equipped with blowdown lines 23 : one intermittent blowdown line and one continuous blowdown line.

[0076] 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.

[0077] 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. [0078] Finally 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. Exchangers - FIG. 10

[0079] As explained above 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.

[0080] 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.

[0081] This very specific circuit requires circulation pumps (see next section). Indeed a natural circulation is very complicated to maintain at all boiler loads, so that the evaporative loop shall be an assisted circulation.

Circulation pumps - FIG. 9 [0082] 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.

[0083] 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.

[0085] Finally, pumps can be isolated from the upstream circuit and from the downstream circuit, and are drainable. Water chemistry - FIG. 9

[0086] Some other process equipments are connected to the heat recovery system, especially for water chemistry control. 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.

[0087] Another important parameter to be considered for boiler integrity is the oxygen content of water. This will be solved by a deaerating system set on the feedwater tank.

[0088] 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 [0090] This last section is related to the evacuation of dirty water to sewer. The intermittent and continuous blowdown, and also the other drains, are falling to the blowdown tank 60. This tank is equipped with cooling water in order to decrease the water temperature to an acceptable level for the sewer.

[0091] In case of water level increase, an overflow line is foreseen in order to evacuate this overflow. Reference symbols