SYSTEM AND METHOD FOR WASTE HEAT RECOVERY IN STEEL PRODUCTION FACILITIES

Abstract

A system for recovery of thermal energy from a first closed cooling loop for cooling skid pipes is provided. The first closed cooling loop comprising a circulation fluid receiving thermal energy from said skid pipes, and a cooling source. The system being capable of measuring the temperature in said first closed cooling loop converting thermal energy into electricity. The system further including a flow control system arranged to control input of thermal energy into a power conversion module, wherein said flow control system is arranged to cut off said cooling source from said first closed cooling loop when the measured temperature is below a first predetermined threshold temperature (TsTART), such that said circulation fluid is directed to a hot side of said power conversion module only, to provide a thermal energy input into said power conversion module.

No new matter is added.

Claims

1. A system for recovery of thermal energy from a first closed cooling loop for cooling skid pipes in a steel production facility, said first closed cooling loop comprising a circulation fluid which receives thermal energy from said cooling skid pipes, and a cooling source, wherein said system comprises: a temperature sensor configured to measure a temperature (T) in said first closed cooling loop, and a power conversion module configured to work in a closed loop thermodynamic cycle and convert thermal energy into electricity, wherein said power conversion module is arranged in the first closed cooling loop in fluid communication with the cooling source; wherein the power conversion module further comprises a flow control system arranged to control input of thermal energy into said power conversion module, wherein said flow control system is arranged to cut off said cooling source from said first closed cooling loop when a measured temperature (T) in said first closed cooling loop is below a first predetermined threshold temperature (TsTART), such that said circulation fluid is directed to a hot side of said power conversion module only, to provide a thermal energy input into said power conversion module.

2. The system according to claim 1, wherein said flow control system is further arranged to direct fluid flow between said cooling source and a cold side of said power conversion module in a second closed cooling loop, fluidly separate from said first closed cooling loop, when the measured temperature (T) in said first closed cooling loop is above said first predetermined threshold temperature (TsTART).

3. The system according to claim 1, wherein said flow control system is further arranged to cut off said cooling source and said power conversion module from said first closed cooling loop, such that said circulation fluid is directed to flow in a second shorter closed cooling loop, to provide a thermal energy input from said cooling skid pipes into said second shorter closed cooling loop.

4. The system according to claim 1, wherein said flow control system is further arranged to cut off said power conversion module from said first closed cooling loop and said cooling source when the measured temperature (T) in said first closed cooling loop is above a second predetermined threshold temperature (TsToP), such that said circulation fluid is directed to said cooling source only.

5. The system according to claim 4, wherein said second predetermined threshold temperature (TsToP) is in the range 110-140 C.

6. The system according to claim 1, wherein said circulation fluid is water.

7. The system according to claim 1, wherein said first predetermined threshold temperature (TsTART) is in the range 65-95 C.

8. The system according to claim 1, wherein the flow control system comprises conduits for connecting the power conversion module to the first closed cooling loop and the cooling source, and valves for selectively closing and opening the conduits to direct flow of said circulation fluid therein.

9. A steel production facility comprising skid pipes for supporting feedstock to be heated, a first closed cooling loop comprising a circulation fluid which receives thermal energy from said skid pipes, a cooling source, and a system for recovery of thermal energy according to claim 1.

10. The steel production facility according to claim 9, further comprising a skid pipe heat transferring loop comprising a circulation fluid running through said skid pipes and a heat exchanger arranged to transfer thermal energy between said skid pipe heat transferring loop and said first closed cooling loop.

11. A method for controlling input of thermal energy into a power conversion module configured to work in a closed loop thermodynamic cycle and convert thermal energy into electricity, wherein said method comprises: arranging said power conversion module in fluid communication with a first closed cooling loop comprising a circulation fluid which receives thermal energy from skid pipes in a steel production facility, and a cooling source arranged in said first closed cooling loop; measuring a temperature (T) in said first closed cooling loop; and when said measured temperature (T) is below a first predetermined threshold temperature (TsTART), cutting off said cooling source from said first closed cooling loop, such that said circulation fluid flows into a hot side of said power conversion module only, to provide a thermal energy input into said power conversion module.

12. The method according to claim 11, further comprising the step of: when said measured temperature (T) is above said first predetermined threshold temperature (TsTART), directing a fluid flow from said cooling source to a cold side of said power conversion module in a second closed cooling loop, fluidly separated from said first closed cooling loop.

13. The method according to claim 11, further comprising: when said measured temperature (T) is above a second predetermined threshold temperature (TsToP), cutting off said power conversion module from said first closed cooling loop and said cooling source, such that said circulation fluid is directed to said cooling source only.

14. The system according to claim 4, wherein said second predetermined threshold temperature (TsToP) is approximately 120 C.

15. The system according to claim 4, wherein said first predetermined threshold temperature (TsTART) is approximately 70 C.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0027] The invention is now described, by way of example, with reference to the accompanying drawings, in which:

[0028] FIGS. 1a and 1b show schematic views of conventional cooling arrangements in a steel production facility;

[0029] FIGS. 2a and 2b show schematic views of cooling arrangements in a steel production facility comprising a system for recovery of thermal energy according to the present invention in an initial warmup phase;

[0030] FIGS. 2c and 2d show schematic views of additional embodiments of cooling arrangements in a steel production facility comprising a system for recovery of thermal energy according to the present invention in an initial warmup phase

[0031] FIG. 3 shows a schematic view of a cooling arrangement in a steel production facility comprising a system for recovery of thermal energy according to the present invention in an operating phase;

[0032] FIG. 4 shows a schematic view of a cooling arrangement in a steel production facility comprising a system for recovery of thermal energy according to the present invention in a stopped phase; and

[0033] FIG. 5 shows a schematic view of a thermodynamic closed-loop cycle illustrating the working principle of an exemplary power generation module which may be to employed in the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0034] In the following, a detailed description of an improved system and method of recovering waste heat from steel production facilities, to generate electrical power in accordance with the present invention is provided.

[0035] FIGS. 1a and 1b illustrate a steel production facility 1, for instance, a reheating furnace of the walking beam or pusher type used in steel rolling, including a cooling arrangement. Such reheating furnaces are used extensively in steel mills to bring the feedstock up to temperature before mechanically forming it to a specified profile. The temperature in the furnace is usually maintained at 1100-1300 C. depending on process requirements. The elements, also called blooms, are transported through the furnace on skids 2 which mechanically advance the element through the furnace. The skids 2 are covered with refractory material and cooled internally with fluid (water) in order to keep the material temperature at acceptable levels. The skid pipes 2 are interconnected in a loop to transfer the heat away from the skids 2. In the cooling arrangement shown in FIG. 1a, the skid pipes 2 are connected in a first closed cooling loop 5 and the fluid is circulated by means of a pump 7 between the skid pipes 2 and a cooling source 6 for cooling, e.g. a cooling tower.

[0036] FIG. 1b illustrates another exemplary cooling arrangement, wherein the skid pipes 2 are connected in a skid pipe heat transferring loop 3, fluidly separated from the first closed cooling loop 5. In this case, thermal energy is transferred indirectly from the skid pipes 2 by means of a heat exchanger 4, arranged between the skid pipe heat transferring loop 3 and the first closed cooling loop 5 comprising the cooling source 6. As mentioned above, the temperature of the circulation fluid (water) in the first closed cooling loop 5 is generally in the range 10-40 C. The heat transferred from the skid pipe heat transferring loop 3 typically raises the temperature of the water in the first closed cooling loop 5 by about 10 C. to keep the temperature of the skids 2 in the furnace at an acceptable level.

[0037] Each of the skid pipe heat transferring loop 3 and the first closed cooling loop 5 comprises a pump 7 and 8, respectively, to circulate the respective fluids therein. The direction of flow may either be parallel-flow, counter-flow, or cross-flow. In the present example, the flow paths are countercurrent as indicated by the arrows. The first closed cooling loop 5 may further comprise an expansion chamber 9 to allow for expansion of the water with increased temperatures.

[0038] Turning now to FIGS. 2-4, a system for recovery of thermal energy in a steel production facility 1 according to the present invention is illustrated. The system comprises a power conversion module 10 configured to work in a closed loop thermodynamic cycle and convert thermal energy extracted from said first closed cooling loop 5 into electricity. One example of such a power conversion module 10 is described further below in connection with FIG. 5. The power conversion module 10 is arranged in the first closed cooling loop 5 in fluid communication with both the heat exchanger 4 and the cooling source 6, and comprises a flow control system including valves 11, 12, 13a, 13b, 14 and additional conduits for connecting respective hot and cold sides of the power conversion module 10 with the first closed cooling loop 5 and the cooling source 6, respectively. In addition, the power conversion module 10 comprises means (not shown) for measuring the temperature T in the first closed cooling loop 5.

[0039] More specifically, the inlet 10a to the hot side of the power conversion module 10 is connected to the first closed cooling loop 5 downstream of the heat exchanger 4 and upstream of the cooling source 6, whereas the outlet 10b of the hot side of the power conversion module 10 is connected to the first closed cooling loop 5 downstream of the cooling source 6 and upstream of the heat exchanger 4. Similarly, the inlet 10c to the cold side of the power conversion module 10 is connected to the cooling source 6 upstream of the heat exchanger 4 and the outlet 10d of the cold side of the power conversion module 10 is connected to the cooling source 6 downstream of the heat exchanger 4. In addition, valves 11, 12, 13a, 13b, 14 to control and direct the flow between the power conversion module 10 and the first closed cooling loop 5 and the cooling source 6 are provided, such as e.g. three-way valves, return valves etc. as known in the art. Depending on the operation mode, the flow of fluid (water) in the first closed cooling loop 5 may be controlled, such that the different components may be cut off/bypassed, separated, or connected as will be explained below.

[0040] In order to be able to convert thermal energy into electricity, the power conversion module 10 requires a minimum threshold temperature of about 70 C., preferably the temperature should be in the range 80-120 C. Hence, in an initial start-up or warm-up phase, the temperature of the fluid in the first closed cooling loop 5 needs to be raised from the typical 10-40 C. This mode of operation is illustrated in FIGS. 2a and 2b in relation to the two cooling arrangements shown in FIGS. 1aand 1b, respectively. When the temperature T in the first closed cooling loop 5 is below a first predetermined threshold temperature T.sub.START, the flow control system redirects the flow to cut off the cooling source 6 from the first closed cooling loop 5. The three-way valve 11 is set (indicated in white) to direct the flow of heated water from the heat exchanger 4 to the inlet 10a into the hot side of the power conversion module 10, and the valve 12 is closed (indicated in black), such that the water from the outlet 10b from the hot side of the power conversion module 10 returns back to the heat exchanger 4 (flow path indicated by the arrows) without passing through the cooling source 6 (indicated by the hatched lines). At the same time, the conduits between the power conversion module 10 and the cooling source 6 remain closed (valves 13a, 13b indicated in black). With each passage through the heat exchanger 4, and in the absence of cooling, the temperature T of the water is raised until it reaches the first predetermined threshold temperature T.sub.START.

[0041] Since water vapour has a lower specific heat capacity than water, i.e. lower cooling effect, it is important that the circulation fluid in the first closed cooling loop 5 is kept in liquid form. In order to prevent vaporisation of the circulation 4 fluid, the pressure in the first closed cooling loop 5 should be maintained at about 2 bar for the temperature range 80-120 C. Of course, at higher pressure, the temperature range could be increased without risking vaporisation. However, a pressure of about 2 bar is conventionally employed in cooling arrangements in steel production facilities and is considered optimal for the system according to the present invention.

[0042] In another embodiment of the system, as disclosed in FIGS. 2c and 2d, an additional piping 18 and an additional three-way valve 19 is arranged to cut off the cooling source 6 and the power conversion module 10 from first closed cooling loop 5. Hereby the circulation fluid is directed to flow in a second shorter closed cooling loop 5a, wherein the thermal energy input from said skid pipes 2 is more quickly heating up the circulation fluid in said second closed cooling loop 5a. Thus, the warm-up phase of the circulation fluid may be shortened. If for example, the temperature of the circulation fluid is less than for example 10 C. this quick initial warm-up of the circulation fluid may be necessary. The quick warm-up may of course also be required for other reasons than extra low temperature of the circulation fluid.

[0043] When the temperature T in the first closed cooling loop 5 is above or equal to the first predetermined threshold temperature T.sub.START, the power conversion module 10 switches to an operating mode wherein thermal energy is extracted from the heated water in the first closed cooling loop 5 and converted to electricity. This operating mode is illustrated in FIG. 3. Here, the flow control system maintains the setting of valves 11 and 12, such that the heated water continues to circulate between the heat exchanger 4 and the hot side of the power conversion module 10. At the same time, the conduits between the cold side of the power conversion module 10 and the cooling source 6 are opened (valves 13a, 13b indicated in white), such that a second closed cooling loop 16 is created therebetween, separate from the first closed cooling loop 5 between the heat exchanger 4 and the hot side of the power conversion module 10. Circulation in this second closed cooling loop 16, as shown by the arrows, may be supplemented by an additional pump 15 (indicated in white). In this operating mode, the heated water from the heat exchanger 4 is cooled indirectly in the power conversion module 10. The extracted thermal energy is used to heat a working medium in the power conversion module 10 to generate electricity, whereas the working medium is cooled by the cooling source 6 connected to the cold side of the power conversion module 10.

[0044] In order to ensure optimal working conditions and prevent vaporisation, the flow control system may also be configured to bypass the power conversion module 10 if the temperature T in the first closed cooling loop 5 increases above a second predetermined threshold temperature TsTop, e.g. beyond the operating range 80-120 C. (at pressure 2 bar) of the power conversion module 10 as defined above. This operating mode of the system is illustrated in FIG. 4 and resembles the flow path in the conventional cooling arrangement shown in FIG. 1. Here, the three-way valve 11 is set (indicated in white) to direct the heated water from the heat exchanger 4 to the cooling source 6 without flowing through the hot side of the power conversion module 10. The conduits between the cold side of the power conversion module 10 and the cooling source 6 are similarly closed (valves 13a, 13b indicated in black), such that the power conversion module 10 is completely cut off from the first closed cooling loop 5. Instead, the flow path from the cooling source 6 returns directly to the heat exchanger 4 through the open valves 12, 14 (indicated in white).

[0045] By controlling the flow paths of the system as explained above, the waste heat from the reheating furnace may be extracted and used to generate electricity. It is estimated that about 10% of the thermal energy may be converted to electrical energy without affecting the manufacturing process. The difference in temperature between the furnace atmosphere (about 1300 C.) and the cooling water is so substantial that changing the cooling temperature from 20 C. to 100 C. produces the same cooling effect without any impact on the manufacturing process. This is due to the specific heat capacity of water in liquid form only changing marginally between 20 C. and 100 C. In other words, the cooling capacity of the water in liquid form is substantially the same, regardless of the temperature.

[0046] As mentioned above, a heat conversion system utilising a thermodynamic closed-loop system to convert heat into energy is disclosed in WO 2012/128715. A similar heat conversion system comprising a power generation module is visualized in FIG. 5.

[0047] FIG. 5 shows a schematic view illustrating the working principle of an exemplary power generation module. Said power generation module is arranged to convert low-temperature heat into electricity by utilizing the phase change energy of a working medium produced in a thermodynamic cycle. The thermodynamic closed loop cycle may be a Rankine cycle, Organic Rankine Cycle, Kalina cycle or any other known thermodynamic closed loop power generating processes converting heat into power. The power generation module comprises a turbine 300, a generator 400, a hot source (HS) heat exchanger 200, a condensation device 100, and a pump 600, and a working medium is circulated through the module. The main pump in the power generation module is located directly below the condenser vessel. The working fluid is heated in the hot source heat exchanger 200, also called evaporator, to vaporisation by an incoming hot source, e.g. the heated water in the first closed cooling loop 5. The hot gaseous working fluid is then passed to the turbine 300 which drives the generator 400 for production of electrical energy. The expanded hot working fluid, still in gaseous form, is then fed into the condensation device 100 to be converted back to liquid form before being recirculated to the HS heat exchanger 200 to complete the closed-loop cycle 100, as shown on the left-hand side of FIG. 5.

[0048] The condensation device may in one embodiment be a vessel 100 or container 100 which has a generally cylindrical shape. The liquid composition of the working medium is evacuated through a bottom section of the condensation device 100 by means of a centrifugal pump 600. The stream of liquid medium Q may be separated into two streams Q1, Q2, wherein a first, bigger stream Q1 is recirculated through a first conduit to the HS heat exchanger 200 and a second, smaller stream Q2 is fed through a second conduit to a cold source (CS) heat exchanger 500 to be cooled before being led back to a top section condensation device 100. The cold source is the cooling source 6 connected to the power conversion module 10 in the second closed cooling loop 16, and the heat energy given off by the second stream Q2 may be used in different applications.

[0049] In another embodiment, the condensation of the working medium takes place directly in the CS heat exchanger 500 which then can be said to be the condensation device. In this embodiment, the entire stream of liquid medium Q may be guided directly back to the HS heat exchanger 200.