Abstract:
A process for recycling blast furnace gas is provided. At least one portion of the gases resulting from the blast furnace undergo a CO 2  purification step to create a CO-rich gas which is reinjected at a first top injection point at a temperature between 700° C. and 1000° C. through a top injection line, and at a second bottom injection point at a temperature between 1000° C. and 1300° C. through a bottom injection line. The gases from the bottom and top injection lines are heated at a temperature between 1000° C. and 1300° C. A portion of the CO-rich gas exiting the purification step is directly introduced into the top injection line via a cold gas injection line to obtain a temperature between 700° C. and 1000° C. at the first top injection point. The gas that flows through the bottom and top injection points controlled upstream of the system of the heaters. A device is also provided.

Description:
The invention relates to a process for recycling blast furnace gas in which at least one portion of the gases resulting from the blast furnace undergo a CO 2  purification step so as to create a CO-rich gas which is reinjected into the blast furnace. The invention also relates to a device that implements this process. 
     BACKGROUND 
     The blast furnace is a gas-liquid-solid counter-current chemical reactor, the main purpose of which is the production of cast iron, subsequently converted to steel by reducing its carbon content. 
     The blast furnace is fed with solid materials, mainly with sinter, pellets, iron ore and coke, in its upper part. The liquids consisting of cast iron and slag are discharged at the hearth in its lower part. 
     The conversion of the iron-containing charge (sinter, pellets and iron ore) to cast iron is conventionally carried out by reduction of the iron oxides by a reducing gas (in particular containing CO, H 2  and N 2 ), which is formed by combustion of coke at the tuyeres located in the bottom part of the blast furnace where air preheated to a temperature between 1000° C. and 1300° C., called hot blast, is injected. 
     In order to increase the productivity and reduce the costs, auxiliary fuels are also injected at the tuyeres, such as coal in pulverized form, fuel oil, natural gas or other fuels, combined with oxygen which enriches the hot blast. 
     The gases recovered in the upper part of the blast furnace, called throat gases, mainly consist of CO, CO 2 , H 2  and N 2  in respective proportions of around 22%, 22%, 3% and 53%. These gases are generally used in other parts of the plant as fuel. Blast furnaces are therefore significant producers of CO 2 . 
     In view of the considerable increase in the concentration of CO 2  in the atmosphere since the beginning of the last century, it is essential to reduce emissions of CO 2  where it is produced in a large quantity, and therefore in particular at blast furnaces. 
     For this purpose, during the last 50 years, the consumption of reducing agents in the blast furnace has been reduced by half so that, at present, in blast furnaces of conventional configuration, the consumption of carbon has reached a low limit linked to the laws of thermodynamics. 
     One known way of additionally reducing CO 2  emissions is to reintroduce throat gases that are purified of CO 2  and that are rich in CO into the blast furnace. The use of CO-rich gas as a reducing agent thus makes it possible to reduce the coke consumption and therefore the CO 2  emissions. 
     In one preferred configuration, CO is reintroduced at two levels, on the one hand level with the tuyeres at a temperature of around 1200° C., more largely between 1000° C. and 1300° C. and, on the other hand, level with the waist, in the vicinity of the waist-stack angle of the blast furnace, at a temperature of around 900° C., more largely between 1000° C. and 1300° C. This known system is described with reference to  FIG. 1 . 
     The blast furnace  1  is fed with coke, with sinter, with pellets, and with iron ore  2  via the line  3  at point  4 . The cast iron and the slag  5  are recovered at point  6  level with the hearth via the line  7 . Oxygen and coal and/or other auxiliary reducing agents  8  are introduced at point  9  level with the tuyeres via the line  10 . 
     The throat gases are recovered at point  11  of the upper part of the blast furnace by means of the line  12 . One portion  13  of these throat gases is exported via the pipe  14  into another device of the site. The other portion of the throat gases is recycled into the blast furnace by means of the pipe  15 . 
     This portion of the throat gases intended to be recycled is purified of most of its CO 2  by means of a CO 2  purifier. This purifier  16  may, for example, consist of a physicochemical absorption process using a solution of amines, or a pressure swing adsorption (PSA) process or a vacuum pressure swing adsorption (VPSA) process, these processes possibly or possibly not being combined with a supplementary cryogenic step intended to produce pure CO 2    17  ready to be stored in subsoils (this then refers to geological storage) or to be used in specific applications such as the food industry or the enhanced recovery of hydrocarbons from deposits in the final stage of extraction. 
     The CO-rich gas  18  is then heated in heat exchangers  24 , commonly referred to as ‘cowpers’, then introduced into the blast furnace  1  at a temperature between 700° C. and 1000° C. at point  20  from a top injection line  21 , and at a temperature between 1000° C. and 1300° C. at point  22  from a bottom injection line  23 . 
     The specific flow of CO-rich gas required for the top injection line  21  is between 300 and 600 Nm 3  per tonne of cast iron and for the bottom injection line  23 , it is between 200 and 500 Nm 3  per tonne of cast iron. 
     The difficulty of this configuration is in controlling these flows. Indeed, the CO-rich gas that circulates in the bottom injection line  23  and top injection line  21  is at a temperature above 700° C. for the top injection line and above 1000° C. for the bottom injection line, and it is therefore not possible to use conventional control valves since the latter do not withstand such temperatures, in particular in lines for circulating reducing gas. 
     SUMMARY OF THE INVENTION 
     The invention makes it possible to overcome this problem by proposing a process and an associated device that make it possible to inject CO-rich gas into the blast furnace at the bottom and top injection lines at the required temperatures and flows, while ensuring the safety of the device especially if one of the heat exchangers is out of service. 
     For this purpose, the present invention provides a process for recycling blast furnace gas in which at least one portion of the gases resulting from the blast furnace undergo a CO 2  purification step so as to create a CO-rich gas which is reinjected at a first top injection point located above the base of the blast furnace at a temperature between 700° C. and 1000° C. through a top injection line, and at a second bottom injection point at the base of the blast furnace at a temperature between 1000° C. and 1300° C. through a bottom injection line, in which the gases from the bottom and top injection lines are heated by means of heaters from which the gases emerge at a temperature between 1000° C. and 1300° C. This process is characterized in that a portion of the CO-rich gas exiting the purification step is directly introduced into the top injection line via a cold gas injection line in order to obtain a temperature between 700° C. and 1000° C. at the first top injection point, and in that the gas flows through the bottom and top injection points are controlled upstream of the system of heaters. 
     The process of the invention may also comprise the following optional features taken separately or in combination:
         the temperature at the top injection line is measured and the flow of cold gas to be introduced into this top injection line is adjusted according to the temperature measured previously;   the gases of the top and bottom injection lines are heated by two independent heater systems, the gas flows are measured, making it possible to evaluate the gas flows at the respective bottom and top injection points, and the gas flows to be introduced respectively into the first and second heater systems are adjusted according to the gas flows evaluated previously;   the gas flow circulating in the main line for transporting the CO 2 -purified gas before the cold gas injection line and the gas flow at the inlet to the heater system of the bottom injection line are measured, and the gas flows to be introduced respectively into the first and second heater systems are adjusted in order to obtain the targeted gas flows at the respective bottom and top injection points;   the gas flows to be introduced into the first and second heater systems are controlled, on the one hand, by adjusting the gas flow in the main line for transporting the CO 2 -purified gas before the cold gas injection line by acting either directly on the gas flow of this line, or on a compressor located upstream of the CO 2  purifier  16  into which the gas passes or else on an expansion turbine optionally integrated into the CO 2  purifier, and, on the other hand, by adjusting the gas flow at the inlet to the heater system of the bottom injection line;   there is a switch from a configuration in which the gases of the top and bottom injection lines are heated by the two independent heater systems, to a configuration in which the gases of the top and bottom injection lines are heated by a single heater system;   in the configuration with one heater system, the gas flow circulating in the main line for transporting the CO 2 -purified gas before the cold gas injection line is measured and the gas flow to be introduced into the single heater system is adjusted;   in this configuration, the gas flow that circulates in one or the other or both of the bottom and top injection lines is measured so as to evaluate the gas flow through the bottom and/or top injection points, and the gases from one or the other or both of the bottom and top injection lines pass into a system of singular pressure drops so as to act, substantially even, on the gas flows of the bottom and/or top injection points;   the temperature of the gases of the bottom injection line at the injection point in the bottom part of the blast furnace is around 1200° C., and the temperature of the gases of the top injection line at the injection point in the vicinity of the waist-stack angle of the blast furnace is around 900° C.       

     The invention also provides a device for recycling blast furnace gas comprising:
         a CO 2  purifier into which at least one portion of the gases resulting from the blast furnace flows so as to create a CO-rich gas,   a top injection line via which CO-rich gas resulting from the purifier is injected at a first top injection point above the base of the blast furnace at a temperature between 700° C. and 1000° C.,   a bottom injection line via which CO-rich gas resulting from the purifier is injected at a second bottom injection point into the bottom part of the blast furnace at a temperature between 1000° C. and 1300° C.,   two heater systems that make it possible to heat, respectively, the gas of the top and bottom injection lines,   a line for supplying cold gas to the top injection line through which a portion of the CO-rich gas exiting the purifier is introduced into the top injection line at a cold gas injection point in order to obtain a temperature between 700° C. and 1000° C. at the top injection point of the blast furnace, and   a system for controlling the gas flows at the respective bottom and top injection points which is located upstream of the heater systems.       

     The device of the invention may also comprise the following optional features taken separately or in combination:
         the device comprises:   at least one system for measuring the temperature of the gas in the top injection line, and   at least one system that makes it possible to adjust the flow of cold gas to be injected into the top injection line depending on the temperature of the gas of this top injection line;   the system for controlling the gas flows at the respective bottom and top injection points comprises:   at least one gas flow measurement system that makes it possible to evaluate the gas flows of the bottom and top injection lines at the bottom and top injection points,   a system that makes it possible to adjust the gas flows at the inlet to each of the heater systems according to the evaluated gas flows of the bottom and top injection lines;   the device comprises:   a system for measuring the gas flow in the main line for transporting the CO 2 -purified gas before the cold gas injection line,   a system for measuring the gas flow at the inlet to the heater system of the bottom injection line,   a system that makes it possible to adjust the gas flow in the main line for transporting the CO 2 -purified gas before the cold gas injection line, and   a system that makes it possible to adjust the gas flow at the inlet to the heater system of the bottom injection line;   the system that makes it possible to adjust the gas flow in the main line for transporting the CO 2 -purified gas before the cold gas injection line is either a control valve, or a compressor located upstream of the CO 2  purifier in which the gas flows or else an expansion turbine optionally integrated into the CO 2  purifier;   the heater systems are heat exchangers that each comprise a gas heating system and a heat storage system, it being possible for each of these exchangers to switch from the heat storage function to the gas heating function and vice versa, so as to keep the temperature of the gas emerging from the heater system relatively stable at a temperature between 1000° C. and 1300° C.;   the device comprises switching elements that make it possible to change from the configuration in which the gases of the top and bottom injection lines are heated by the two independent heater systems to a configuration in which the gases of the top and bottom injection lines are heated by a single heater system;   the switching elements comprise a first valve capable of connecting the two pipes for introducing gas into the first and second heater systems and a second valve capable of connecting the top and bottom injection lines before the point for injecting cold gas into the top injection line;   the device comprises, over one or the other or both of the bottom and top injection lines, singular pressure drops that make it possible to act on the gas flows of the bottom and/or top injection points.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on reading the following description, given with reference to the appended figures in which: 
         FIG. 1 , already described, represents a known device for recycling throat gas; 
         FIG. 2  represents a heater system formed of three cowpers; 
         FIG. 3  schematically represents the process and the associated device of the invention, for the configuration in which the gases of the top and bottom injection lines are heated by two independent heater systems; and 
         FIG. 4  schematically represents the process and the device used for the configuration in which the gases of the top and bottom injection lines are heated by a single heater system, for example in the event of a problem with one of the heat exchangers. 
     
    
    
     DETAILED DESCRIPTION 
     The common elements of the devices from  FIGS. 1 ,  3  and  4  bear the same references. For the sake of simplicity, the injection of the auxiliary fuels and the recovery of the cast iron and slag are not represented in  FIGS. 3 and 4  but are obviously steps that are present in the processes illustrated in these figures. 
     With reference to  FIG. 3 , the blast furnace  1  is fed with coke, with iron ore, with pellets and with sinter  2  via the line  3  at point  4 . 
     The throat gases are recovered at point  11  of the upper part of the blast furnace by means of the line  12 . One portion  13  of these throat gases is exported via the pipe  14  into another device of the site. The other portion is recycled into the blast furnace by means of the pipe  15 . 
     This portion of the throat gases intended to be recycled is passed into a compressor  19  and is purified of CO 2  by means of a CO 2  purifier  16  such as an amine absorption unit, a VPSA, a PSA or by one of these devices combined with a supplementary cryogenic step. In the example from  FIG. 3 , the CO 2  purifier is a VPSA  16 . Furthermore, the compressor  19  may be integrated into the VPSA  16 ; it is represented separately from the VPSA for the sake of clarity. The CO 2    17  is stored in the subsoil after having undergone appropriate treatments where necessary. 
     A portion of the CO-rich gas resulting from the VPSA  16  flows into a main transport line  18  and is sent via a pipe  31  to a first heater system  30  in which it is heated to a temperature of around 1200° C. The hot CO-rich gas resulting from this first heater system  30  is injected at point  22  into the bottom part of the blast furnace level with the tuyeres by means of a bottom injection line  23 . 
     Another portion of the CO-rich gas resulting from the VPSA  16  is sent via a pipe  32  to a second heater system  33  in which it is also heated to a temperature of around 1200° C. The hot CO-rich gas resulting from this second heater system is mixed at the injection point  34  with CO-rich gas resulting from the VPSA which is at a temperature close to ambient temperature by means of a cold gas feed line  35 . 
     The mixing of these two gases according to one operating mode that will be explained later on makes it possible to obtain a CO-rich gas at a temperature of around 900° C., which is injected at point  20  in the vicinity of the waist-stack angle of the blast furnace by means of a top injection line  21 . 
     A first control valve  36  is located in the main line for transporting the CO 2 -purified gas  18  before the cold gas injection line  35 . This valve  36  is connected to a gas flow measurement system  38  also located in the main line for transporting the CO 2 -purified gas  18  before the cold gas injection line  35 . As a variant, the possibility of controlling the flow of the main line of the CO 2 -purified gas  18  by the compressor  19  of the VPSA  16 , as represented by dotted lines in  FIG. 3 , in place of the control valve  36 , will be noted. 
     A second control valve  37  is located in the pipe  31  at the inlet to the first heater system  30 . This valve  37  is connected to a gas flow measurement system  39  also located in this pipe  31 . 
     The device of the invention also comprises a control valve  40  located in the cold gas feed line  35  that makes it possible to adjust the cold gas flow to be injected into the top injection line  21  depending on the temperature of the gases of this line, which is measured by a suitable system  41 , so that the CO-rich gas that is injected in the vicinity of the waist-stack angle of the blast furnace remains at a relatively stable temperature of around 900° C. 
     Measurements of the gas flows at the outlet of the VPSA  16  and at the inlet to the first heater system  30  make it possible to adjust the gas flows at the bottom injection point  22  and top injection point  20  into the blast furnace by means of the valves  36  and  37 . 
     The control of the CO-rich gas flows upstream of the heater systems and the control of the cold gas flow to be injected at the top injection line  21  make it possible to obtain a system in which the temperatures and the flows of the CO-rich gases injected into the blast furnace are relatively stable and controlled without it being necessary to use control valves in the top injection line  21  and bottom injection line  23 , in which gases at temperatures respectively above 700° C. and above 1000° C. circulate. 
     The first heater system  30  and second heater system  33  of the embodiment from  FIG. 3  are of the type of that represented in  FIG. 2 . The operation of these heater systems is explained with reference to this  FIG. 2 . 
     The heater system  50  from  FIG. 2  comprises three heat exchangers  51 , 52 , 53 , two of which are represented in heat storage mode  52 , 53  and one in gas heating mode  51 . Each of these exchangers is commonly referred to as a cowper. 
     Each cowper  51 , 52 , 53  will alternately fulfill the role of heating gas that feeds the injection line in question and the heat storage role. 
     In each of the two cowpers in heat storage mode  52 , 53 , a mixture of throat gas and of steelworks gas and/or coking plant gas and/or of any other gas suitable for the process of heating these systems  55 , and also air  56 , are introduced into a burner  58  located in the lower part of the combustion shafts  59 . 
     The combustion of this gas mixture heats the refractory checker bricks constituting the walls of the checkerwork shaft  60  and flue gases  61  are discharged at the bottom of the checkerwork shaft  60  toward the stacks. 
     In the cowper in gas heating mode  51 , the checker bricks of the checkerwork shaft  60  which have been preheated according to the principle that has just been described, will heat the CO-rich gas  18   a  that originates from the VPSA  16  ( FIGS. 3 and 4 ) and is introduced via the pipes  31  and/or  32  at the base of the checkerwork shaft  60 . The bricks will cool down with the circulation of this CO-rich gas, which is discharged from the cowper  51  at a temperature of around 1200° C. from the combustion shaft  59  to the zone of the blast furnace at which it is injected. 
     When the bricks are no longer hot enough and when the temperature of the outlet gas is below 1200° C., the cowper  51  switches to its heat storage role whilst, concomitantly, one of the two cowpers  52 , 53 , for example the cowper  52 , switches to a cycle of heating the CO-rich gas. Similarly, when this cowper  52  no longer produces enough heat, it is the last cowper  53  that will provide the heating of the gas whilst the cowper  52  will again enter into a heat storage cycle. 
     In this configuration with three cowpers, should one of the cowpers be shut down, the system can continue to operate with two cowpers, the first cowper heating the gas, the second being in a heating cycle. 
     On the other hand, in the first embodiment presented in  FIG. 3 , provision is made for each of the two heater systems  30 , 33  to comprise only two cowpers, respectively  51 , 52  and  53 , 54 . 
     This configuration is advantageous within the context of the invention. Indeed, a plant comprising two heater systems with three cowpers each would be very expensive. However, operating with two independent heater systems is necessary in order to be able to control the gas flows at the bottom injection point  22  and top injection point  20 . 
     This is why a system has been chosen that has two heater systems each comprising two cowpers. 
     However, according to this system, if one of the cowpers is out of service, the entire system must be interrupted since the heater system whose cowper is faulty cannot operate with only one cowper. 
     This is why the invention provides means that make it possible to switch the system from operating with two heater systems having two cowpers each,  FIG. 3 , to a system with a single heater system having three cowpers, represented in  FIG. 4 , thus avoiding the complete shutdown of the recycling blast furnace. 
     To switch from the configuration with four cowpers, to the configuration with three cowpers, a valve  70  connects the pipes  32  and  31 , in which the CO-rich gas  18  resulting from the CO 2  purifier  16  circulates, to the first heater system  30  and second heater system  33 . This valve  70  is kept in the closed position while the system with four cowpers from  FIG. 3  is operating. It is ordered open if the system has to switch to operating with three cowpers as represented in  FIG. 4 , forming a single pipe  43  ( FIG. 4 ), in which the CO-rich gas  18  resulting from the VPSA  16  circulates, to the heater system  45 . 
     Furthermore, a valve  71  connects the top injection line  21  and bottom injection line  23  at a level located before the mixing point  34  via which the cold gas  18  is injected into the top injection line  21 . By opening this valve  71  and by immobilizing the inlet and outlet of the cowper that is no longer operating, the gases circulating in the top injection line  21  and bottom injection line  23  are all heated by a single heater system  45  having three cowpers according to the configuration represented in  FIG. 4 . 
     In the example represented in the figure, it is the cowper  52  which is shut down; the heater system  45  thus operates with the three other cowpers  51 ,  53  and  54 . 
     The operation of the system with one heater system  45  comprising three cowpers  51 , 53 , 54  is described with reference to  FIG. 4 . 
     At the outlet of this heater system  45 , the gases are at a temperature of around 1200° C. A portion of this gas is injected at point  22  into the bottom part of the blast furnace level with the tuyeres by means of a bottom injection line  23 . The other portion is mixed at point  34  with CO-rich gas resulting from the VPSA  16 , which is at a temperature close to ambient temperature, by means of the cold gas feed line  35 . The cold gas flow is controlled as for the embodiment from  FIG. 3  with the same control valve  40  and the system  41  for measuring the temperature of the gas in the top injection line  21 . 
     The inlet flow into the heater system  45  is controlled by the control valve  36  combined with its flow measurement system  38 . The control valve  37  represented in  FIG. 3  no longer performs a controlling role, this is why it is not represented in  FIG. 4 . 
     In this configuration from  FIG. 4 , it is not possible to independently control the gas flow of the top feed line  21  and the gas flow of the bottom feed line  23 . 
     To overcome this inability, it is possible to use systems of singular pressure drops  80 , 81  positioned respectively in the top injection line after the mixing point  34 , and in the bottom injection line  23  making it possible, via their design geometry, to modify the gas flows at the respective bottom injection point  22  and top injection point  20  into the blast furnace. 
     Furthermore, in the system from  FIG. 4 , it may be necessary to have to measure the gas flow circulating in one or the other or both of the bottom injection line  23  and top injection line  21  for reasons independent of the subject of the invention. A flow measurement of this type carried out by an associated device  82 , for example in the top injection line  21 , may also help in the control of the flow of the two injection lines  21 , 23  in particular by making it possible to size the singular pressure drops  80 , 81  implanted in the respective top injection line  21  and bottom injection line  23 . It is possible to provide a single device for measuring the hot gas flow  82  located in one or the other of the bottom injection line  23  and top injection line  21  but also a device for measuring the flow located in each of these two lines  21 , 23 . 
     It will be noted that the systems of singular pressure drops  80 , 81  and the device for measuring the hot gas flow  82  were already present in the device represented in  FIG. 3 , although they were not represented, it being understood that these systems  80 , 81 , 82  do not operate within the context of the invention in the configuration with two heater systems. 
     In any case, the process and the device of the invention make it possible, on the one hand, to control the flow and the temperature of a gas circulating in two injection levels at temperatures above 700° C. using two independent heater systems  30 , 33  and, on the other hand, to ensure the safety of the system by providing a switchover to a configuration in which admittedly the gas flows of the two injection lines are no longer controlled independently, but in which the blast furnace can nevertheless continue to operate. 
     One highly advantageous aspect of the invention lies in the simplicity of switching between the configuration with four cowpers ( FIG. 3 ) and the configuration with three cowpers ( FIG. 4 ). 
     Indeed, the control valves  40  and  36  are used in both systems, without them having to be reconfigured. 
     As a variant, provision may be made for the valve  36  and its associated flow measurement  38  to be in the pipe  32  at the inlet to the second heater system  33 . In this case, to ensure the flows of the top injection line  21  and bottom injection line  23  are controlled, a supplementary flow measurement in the cold gas line  35  will be provided. In this configuration, the switchover to the configuration with a single heater system  45  is more complicated as it involves a reconfiguration of the valve  36  or  37  which will both be used in parallel, upstream of the heater system  45 . 
     As a variant, provision could also be made for the valve  37  and its associated flow measurement  39  to be installed in the pipe  32  at the inlet to the second heater system  33 . In this case, to ensure the flows of the top injection line  21  and bottom injection line  23  are controlled, a supplementary flow measurement in the cold gas line  35  will be provided. 
     As a variant, it is possible to allow for the presence of a single system of singular pressure drops positioned either in the top injection line  21 , or in the bottom injection line  23 . 
     It will also be noted that the injection temperature of 900° C. for injecting gas halfway up the blast furnace is an optimum in a range from 700° C. to 1000° C. and that the temperature of the gas of the bottom injection line presented at 1200° C. may be between 1000° C. and 1300° C., maximum temperature of the gas at the outlet of the heat exchangers. 
     Furthermore, the temperature of the gas at the top injection point  21  is understood as being strictly below 1000° C. and the temperature of the gas at the bottom injection point is understood as being strictly above 1000° C.