Abstract:
A blast furnace system is used wherein the coke rate is decreased by recycling upgraded top gas from the furnace back into its shaft section (which upgraded top gas is heated in a tubular heater prior to being recycled). The top gas, comprising CO, CO 2  and H 2 , is withdrawn from the upper part of the blast furnace; cooled and cleaned of dust, water, and CO 2  for increasing its reduction potential and is heated to a temperature above 850° C. before being recycled thus defining a first gas flow path used during normal operation of the blast furnace. Uniquely, a second gas flow path for continued circulation of top gas selectively through the heater and a cooler during operation interruptions of the blast furnace allows time for gradual controlled cool down of the heater in a manner to avoid heat-shock damage to the tubular heater.

Description:
This application is a National Stage Entry under 35 U.S.C. §371 of PCT/IB2013/001974 filed on Jul. 3, 2013, published on Jan. 9, 2014 under publication number WO 2014/006511, which claims the priority benefit of U.S Provisional Application No. 61/667,896 filed Jul. 3, 2012, the disclosures of which are incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of the iron and steel industry, more particularly to the operation of blast furnaces comprising upgraded top gas recycle through a direct fired gas heater for increasing the efficiency and productivity of the blast furnace while protecting said heater from thermal shocks when the blast furnace system starts its operation or when the top gas recycle to the blast furnace has to be interrupted. 
     BACKGROUND OF THE INVENTION 
     In a blast furnace producing pig iron, iron ore is charged together with coke and fluxes through the upper part of the furnace. A hot air blast is injected through tuyeres at the bottom of the furnace, thereby generating heat by the combustion of the carbon of coke which melts down the charge. The controlled combustion of coke also generates hydrogen and carbon monoxide which chemically reduce the iron oxides in the furnace. Periodically liquid iron and slag are tapped from the furnace. The combustion gases flow up through the furnace and reduce the iron oxides, and exit the furnace as a stream of dust-laden hot gas. Heat is recovered for preheating the air blast, and this furnace top gas, once cooled down, is then normally used as fuel in other areas of the steel plant. 
     Metallurgical coke is needed in the charge of a blast furnace because this material (produced by pyrolysis, e.g. indirect heating without oxygen presence, of coal in coke ovens) provides the structural support of the charge of the furnace above the so-called “dead man” zone where the metallic iron starts melting and falling down to the bottom part of the furnace where molten iron and slag are collected. 
     Coke also provides the heat for melting the iron charge by its combustion with an oxygen-containing gas, typically preheated air, the combustion gases, mainly composed of CO and CO 2  with some H 2  and water, flow upwardly through the shaft portion of the furnace and reduce the iron oxides to wustite (FeO). 
     Several proposals for recycling top gas in a blast furnace with the aim of reducing the coke rate, are found in the prior art, addressed to recycling top gas to the furnace and in this way decrease the coke consumption to a minimum. If the top gas is heated in a direct fired tubular heater, the tubes of the heater made of high-grade alloys to withstand the high temperatures necessary to raise the temperature of the top gas above, 850° C., preferably in the range of 900° C. to 1100° C., need to follow a specific scheduled temperature profile during start-up and shut-down periods, to avoid thermal shock to the tubes, which requires a continuous flow of the gas through the tubes of the heater during such start-up and shut-down. 
     Applicants have found several patents and patent applications concerning top gas recycle to a blast furnace for reducing coke consumption, teaching that the recycled gas is to be heated to temperatures suitable for direct reduction of iron oxides, for example: U.S. Pat. Nos. 3,784,370; 4,844,737; 4,917,727; 4,363,654; 5,234,490; U.S. Patent application No. 2010/0212457 A1; British Patent No. GB1,218,912; and Japanese Patent Publication No. JP55113814. 
     None of the above patents or patent applications teach or suggest providing an alternative path comprising cooling means for the hot gas effluent from the gas heater for protecting said gas heater components from thermal shocks due to unforeseen operation interruptions of the blast furnace. Solutions for this practical problem of the heaters for improving the operation and availability of blast furnaces with upgraded top gas recycle are not envisioned in the prior art. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to provide a method and apparatus for improving the operation of a blast furnace by upgrading and recycling top gas. 
     It is another object of the invention to provide a method and apparatus for improving the operation of a blast furnace by providing a hot gas alternative circuit including gas cooling means allowing for applying a start-up and shut-down normal operation of the heater, even if the rest of the BF plant operation stops. 
     It is a further object of the invention to provide a method and apparatus for improving the availability of a blast furnace plant by providing a gas circuit and cooling means allowing for an independent operation procedure. 
     Other objects of the invention will be evident for those skilled in the art or will be pointed out in the description of the invention. 
     SUMMARY OF THE INVENTION 
     The objects of the invention are generally achieved by providing a method of producing iron in a blast furnace wherein the coke rate is decreased by recycling upgraded top gas which is heated in a tubular heater prior to its recycling to the blast furnace and wherein said tubular heater is protected from damages that may be caused by unforeseen operation interruptions of the blast furnace system, and wherein a top gas stream comprising CO, CO 2  and H 2 , is withdrawn from the upper part of the blast furnace, is cleaned of dust and water and CO 2  are removed from said cooled top gas stream for increasing its reduction potential and is heated to a temperature above 850° C., preferably between 900° C. and 1100° C., before being recycled to the blast furnace through a first gas flow path used during normal operation of the blast furnace system. A second gas flow path for circulation of the top gas through said heater is provided so that in case of operation interruptions of the blast furnace system, the top gas continues circulating through said second gas flow path thus avoiding damage to said tubular heater by sudden thermal changes that might be caused by said operation interruptions of the blast furnace system. 
     The objects of the invention are also achieved by providing a blast furnace system for producing molten iron in a blast furnace to which iron ore, metallurgical coke and fluxes are charged at its upper part and molten iron and slag are tapped from its lower part, comprising means for cleaning the top gas stream of dust connected to said outlet means; first cooling means for washing and cooling said top gas stream and removing water therefrom; pump means for increasing the pressure of the cooled top gas stream to enable recycling of said top gas to the blast furnace; means for removing CO 2  from at least a portion of said cooled top gas stream forming a CO 2 -lean reducing gas stream, a tubular gas heater for heating said CO 2 -lean reducing gas stream to a temperature above 850° C., and first piping means connecting the components of said blast furnace system defining a first gas circulation gas path through said gas heater to recycle said hot reducing gas stream to said blast furnace during normal operation of the blast furnace system; characterized by further comprising: second cooling means for cooling hot gas effluent from said heater; second piping means connecting said second cooling means with said gas heater and said pump means defining a second path for gas circulation through said gas heater; first valve means for selectively diverting the flow of gas effluent from said heater to said second cooling means and to flow through said second gas path; and second valve means for blocking the flow of gas effluent from said heater to said blast furnace when the gas effluent from said reactor is flowing through said second gas path; whereby in case the operation of the blast furnace system is interrupted, the gas circulating through the heater is diverted through said second gas path to prevent damage of said heater from sudden thermal changes caused by said blast furnace system operation interruption. 
     The objects of the invention are also achieved in its broader aspects by providing a method of producing molten iron in a blast furnace wherein iron ore, metallurgical coke and fluxes are charged at its upper part and molten iron and slag are tapped from its lower part, said blast furnace having a plurality of tuyeres in its lower part through which an oxygen-containing gas is introduced for generating heat and reducing gases by combustion of the coke within said furnace further comprising forming during normal operation a first gas circulation path by withdrawing a stream of hot top gas from said blast furnace; cooling and washing the hot top gas stream of dust in first cooling means; increasing the pressure of the resulting cooled top gas stream by pump means to enable recycling of said top gas stream back to the blast furnace; removing CO2 from at least a portion of said cooled top gas stream forming a CO2-lean reducing gas stream; heating in a tubular gas heater said CO2-lean reducing gas stream to a temperature above 850° C.; completing the first gas circulation path by causing the resulting now-hot CO2-lean reducing gas stream to be injected back into the upper part of said blast furnace; further characterized by protecting the heater during interruption of operation of the blast furnace, with the consequent interruption in the normal flow of top gas therefrom, by forming a second gas circulation path to assure continuous gas flow of CO2-lean reducing gas through the heater during controlled heater cool-down by diverting the flow of hot gas effluent from said heater away from said blast furnace and through a cooling means; passing the resulting cooled gas effluent on through said pump means to pressurize and maintain gas flow through said second gas path; and 
     completing the second gas circulation path by re-circulating the cooled and pressurized gas effluent back into said heater. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic process diagram showing a preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , numeral  10  generally designates a blast furnace having in its lower part a crucible section  12  where molten iron and slag are collected and above that a blast section  14  where the oxygen containing gases are introduced for carrying out the combustion of coke, and in its upper part a shaft section  16  where iron ore particles in the form of sinter, pellets or lumps and mixtures thereof are charged along with coke, limestone and other fluxes  18 , and next the iron oxides are reduced to wustite and finally to metallic iron as is known in the art. Molten iron  19  and slag  21  are periodically tapped from the bottom zone  12  of blast furnace  10 . 
     Oxygen from a source  26  of industrial purity, instead of air, is fed to mixing device  24  where a temperature moderating agent is fed from a source  28  for preventing the flame temperatures from reaching excessively high levels and therefore from damaging the blast nozzles in tuyeres  27 . The temperature moderating agents  28  may be for example, steam, carbon dioxide, oil, pulverized coal, coke fines or other hydrocarbon that will undergo an endothermic reaction with the oxygen and lower the temperatures to levels of about 2000° C. to 2600° C. Also a portion of the top gas from pipe  30  after treatment can be recycled to the tuyeres for moderating the high combustion temperature of oxygen with coke. Oxygen blast  26  combined with the moderating agent  28  are fed to header  23  and then through feeding pipes  25  to tuyeres  27 . 
     The top gas composition varies in a wide range depending on the characteristics of the materials charged to the blast furnace. A typical composition on a dry basis is 25% CO, 12% CO 2 ; 5% H 2  and 56% N 2  and traces of other gases. The top gas effluent from the top of the blast furnace  10  exits through pipe  30  and is fed to a de-dusting device  32 , where dusts from the charge and soot or other solid materials  34  are separated. 
     In an optional preferred embodiment of the invention, the resulting cleaned gas flows through pipe  36  to shift reactor  38  where the composition of the cleaned and cooled gas is adjusted to increase the hydrogen content so as to obtain a H 2 /CO ratio of 1.5 to 4, preferably between 2 and 3 (measured by % volume). Steam  40  is supplied as the reactant for the shift reaction through pipe  42 . The CO reacts with H 2 O to form H 2  according to the reaction:
 
CO+H 2 O→H 2 +CO 2  
 
The temperature to carry out the above reaction is above about 300° C. The top gas stream may be heated by means known in the art as a heat exchanger (not shown) before being fed to shift reactor  38 . The shifted gas is then passed through pipe  46  and valve  104  to cooling water injection device  110 , where the temperature of the gas is lowered by contact with water  112 , and then is passed on to a first cooler/scrubber  48  (using cooling water  50 ) where the water content of the gas is condensed and combined waters are extracted as water stream  52 . In the illustrated preferred embodiment, the device  110  and cooler/scrubber  48  may also together be considered to function as the first cooler means.
 
     The de-watered gas then flows through pipe  49  from where a minor portion of the cleaned and dewatered gas  54  is purged from the recycle circuit through pipe having a pressure control valve  56  (for pressure control of, and for maintaining a N 2  concentration below 13% by volume, in the recycle circuit). A majority of the gas stream flows through pipe  58  to be recycled to the blast furnace  10 . The purged gas  54  may be advantageously utilized as fuel in burners  88  for the gas heater  70  and optionally if needed may also be supplemented with other fuel as for example coke oven gas or natural gas  86 . 
     The cleaned and dewatered reducing effluent gas is then transferred to compressor  60  through pipe  58  wherein its pressure is raised by this pump means  60  to a level suitable for further treatment prior to its ultimate recycling to blast furnace  10 . In order to upgrade the reducing potential of the recycled reducing gas, the pressurized effluent gas flows through pipe  62  to a CO 2  separation unit  64  where CO 2    66  is removed, leaving a reducing gas mainly composed of CO and H 2 . 
     Removal of CO 2  from the cooled gas stream being recycled may be carried out by absorption using an amines solution or carbonates solution or by physical adsorption in a pressure swing adsorption (PSA) or vacuum pressure swing adsorption (VPSA) unit. 
     The CO 2  lean gas is led through pipe  68  to heater  70  where its temperature is raised above 800° C. The resulting hot reducing gas is led through pipe  71  to header  72 , and this recycled reducing gas is introduced into the shaft part  16  of the blast furnace through peripheral pipes  74  and nozzles  76 . Oxygen from source  78  may be added to the hot reducing gas for further increasing the temperature of the reducing gas to between 1000° and 1100° C. A suitable fuel  86 , for example natural gas or coke oven gas, is used in burners  88  of heater  70 . 
     Upgraded recycled top gas with an improved H2/CO ratio and high reduction potential (measured as the ratio of H 2 +CO/CO 2 +H 2 O and having a value above 2), is heated in coils  80  of the heater  70  to a temperature above 800° C. for reducing the iron oxides in the shaft section  16 . A mixture of steam  82  and air/oxygen  84  may be periodically injected to the tubes  80  of the heater  70  for decoking, e.g. eliminating carbon deposits that may accumulate in the tubes  80 , depending on the composition of the top gas. 
     Tubular heaters typically have a series of heating tubes  80  made of special alloys to withstand the high wall temperatures and require specific and detailed start-up and shut-down procedures in order to avoid thermal shocks to said tubes  80 , which may be damaged if sudden temperature changes happen. The damages caused by sudden temperature changes produce cracks in the outer part of the tubes walls due to the different temperatures that the tubes walls have on the outer and inner surfaces and alloy fatigue. 
     The normal procedure for shutting down a gas heater is to start cooling the furnace from the normal working temperature which is about 900° C. in a programmed cooling rate for example of about 30° C. per hour. This gradual cooling is done while maintaining the normal flow of gas through the tubes. 
     When the temperature of the gas at the outlet of the heater is between about 600° C. and 700° C., the fuel gas flow to the burners  88  is gradually lowered and the tubes  80  are then allowed to cool down slowly. In this way the useful life of the tubes is longer than if the tubes suffer thermal fatigue for unexpected interruptions of the gas flow through said tubes. 
     In order to avoid damages to the heater tubes, the top gas flow effluent from heater  70  is diverted to pipe  90  by closing valve  116  to pipe  90  and then to water injection device  118  where the hot top gas is quenched by contact with water from source  120  through pipe  122 . The quenched top gas flows through pipe  91  to a second cooler/scrubber  92  where the top gas is washed and cooled down by direct contact with water from source  94 , which exits through pipe  96 . The cool and clean top gas passes through pipe  98  and valves  100  and  108  and is fed to compressor  60  thus maintaining a flow of top gas through the tubes  80  of heater  70  even if the operation of blast furnace  10  is interrupted. 
     In another embodiment of the invention, cooler/scrubber  92  may be dispensed with and instead the quenched top gas from pipe  90 / 98  is washed and cooled down in cooler/scrubber  48  by closing valve  108  and causing it to flow through pipe  102  (shown in dotted line to indicate an optional embodiment of the invention) to washer/scrubber  48 , cooled and clean top gas effluent from cooler/scrubber  48  then flows through pipe  58  by opening valve  59  and closing valve  104 . 
     Preferably also, the heater can be further protected, if there is an operating problem at the CO 2  removal unit, by providing a by-pass of the CO 2  removal unit using pipe  124  and isolation valves  126  and valve  128 . 
     A further preferred embodiment includes the option of having both first and second coolers  92  &amp;  48 . For example, in an embodiment, not including the shifter, the top gas will come out of the de-dusting device  32  at around 100-120° C., therefore the first cooler  48  will be designed for a low cooling capacity, while if a shifter is included, then the top gas will be at about 300° C. and then said first cooler  48  will be designed for a higher cooling capacity. In any case, the second cooler  92  will be designed for cooling top gas at much higher temperatures because the top gas will be exiting the heater at about 850 to 1000° C. and therefore, second cooler  92  is more important than the first cooler. 
     The present invention may be applied to either new or existing furnaces wherein top gas is recycled and when said recycled top gas is heated in a direct fired tubular heater. 
     It is of course to be understood that in this specification only some preferred embodiments of the invention have been described for illustration purposes and that the scope of the invention is not limited by such described embodiments but only by the scope of the appended claims.