Direct Reduction of Iron Using Coke Oven Gas

The direct reduction of iron oxide, in such forms as agglomerated pellets or lump ore, to metallic iron in the solid state has in recent years become a commercial reality in many direct reduction plants throughout the world. The combined annular capacity of these plants currently in operation or under construction is in excess of 15 million metric tons of direct reduced iron product, which is used primarily as feedstock for electric arc steelmaking furnaces. The world demand for additional direct reduced iron is projected to increase at a substantial rate for many years to satisfy a growing world need for such feedstock, as additional electric arc furnace steelmaking plants are constructed.

The majority of the commercial plants producing direct reduced iron utilize natural gas as the source of reductant. The natural gas is reformed to produce the reductants CO and H.sub.2. The most energy efficient and most productive of the commercial natural gas based direct reduction plants are the Midrex plants which include continuous catalytic reforming of natural gas using as reforming oxidants the CO.sub.2 and residual water vapor in cooled, recycled, spent reducing gas from the reduction furnace, as taught in U.S. Pat. No. 3,748,120.

It is well recognized in the art of catalytic reforming of natural gas that it is essential to maintain a very low level of sulfur in the gas mixture being reformed, in order to avoid sulfur poisoning of the catalyst. The maximum sulfur level which can be tolerated in the reforming, without poisoning the catalyst, is approximately 2 to 3 parts per million by volume (ppmv) in the gas mixture being reformed. To achieve this very low level of sulfur often requires complicated and expensive desulfurization of the gas before it can be utilized as process fuel.

Coke oven gas is available as a fuel in many of the industrial countries of the world. However, coke oven gas includes certain sulfur containing components such as COS and thiophene.

The present invention is an improvement to the direct reduction process taught in U.S. Pat. No. 4,270,739 in which process fuel is desulfurized in the reduction furnace by reacting the sulfur in the process fuel with hot direct reduced iron before the process fuel is admitted to the reformer. Sulfur in the process fuel is transferred to the iron during the reduction process, permitting sulfur levels as high as 400 ppmv in the process fuel to be tolerated without adding an undesirable amount of sulfur to the direct reduced iron product. This in-situ desulfurization of the process fuel makes practical, in the direct reduction of iron, the use of process fuels which are very difficult to desulfurize externally, such as coke oven gas or natural gas which contains organic sulfur compounds.

U.S. Pat. No. 4,270,739 teaches preheating of process gas in a process gas heater and reforming of spent top gas to reducing gas in a reformer. In the present invention, I desulfurize spent top gas by passing it through the cooling zone as cooling gas, and preheat process gas by indirect heat exchange with reformer flue gas. This results in a more fuel-efficient process.

It is the primary object of this invention to provide a highly efficient apparatus for the direct reduction of iron utilizing reforming of hydrocarbon-containing gaseous process fuel to produce reducing gas, in which the process fuel is desulfurized in the reduction process prior to being reformed.

It is also an object of the invention to provide apparatus for direct reduction of iron which is particularly well adapted for the use of gaseous process fuels which contain organic sulfur.

FIG. 1 is a diagrammatic illustration of the preferred embodiment of the invention.

FIG. 2 is a diagrammatic illustration of an alternative embodiment of the invention.

Referring now to the drawing, a refractory-lined counterflow shaft type reduction furnace is indicated generally at 10. Iron oxide feed material 12, in the form of oxide pellets, natural lump ore, or a mixture of pellets and lump ore having a nominal particle size in the range of 5 to 30 mm, is introduced to a feed hopper 14 and into the furnace through a feed pipe 16 to establish a burden 17 within the furnace. Particulate direct reduced iron product is withdrawn from the lower region of the furnace through a furnace discharge pipe 18 by a discharge conveyor 20, the speed of which controls the rate of descent of the burden 17 through furnace 10.

The middle region of furnace 10 is provided with a hot reducing gas inlet pipe 22 leading to a plurality of gas inlet ports 24 arranged in the furnace refractory wall. A hot pre-reducing gas inlet pipe 26 is provided leading to a plurality of gas inlet ports 28 arranged in the furnace refractory wall at an elevation above ports 24. Hot pre-reducing gas, which consists of a mixture of hot reformed reducing gas and hot process fuel gas, introduced through ports 28 flows inwardly, then upwardly in counterflow relationship to the descending burden. The hot reducing gas from ports 24, in its upward flow, initially fills the entire cross-section of the burden and then is forced to converge toward the center region of the burden at about the elevation ports 28 due to the flow of hot pre-reducing gas through ports 28. In the upper region of the furnace the two gas streams merge and fill the entire cross-section of the burden then exit the burden at stockline 30 and leave the furnace through top gas outlet pipe 32. This top gas is a mixture of spent reducing gas and process fuel gas.

The lower region of furnace 10 is provided with a cooling gas circuit for cooling the direct reduced iron prior to discharge.

Top gas leaving furnace 10 through outlet pipe 32 is cooled and scrubbed of dust in a cooler-scrubber 34 and withdrawn into pipe 36. The gas is divided into two streams. The first stream of cooled top gas flows into the cooling gas circuit via pipe 38. This cooling circuit includes a cooling gas blower 40 in pipe 38, a cooling gas inlet 42 leading to a cooling gas distributing member 44 within furnace 10, a cooling gas collecting member 46 positioned above the distributing member within the furnace, a cooling gas outlet 48, and an external gas cooler-scrubber 50. Cooled cleaned gas exits cooler-scrubber 50 through pipe 51, then is admitted to a plurality of heat resisting alloy reformer tubes 52, one of which is shown in the drawing. Each reformer tube 52 is filled with refractory lump at the tube entry region and the remaining major portion is filled with nickel or cobalt reforming catalyst indicated generally as 54. The reformer tubes are enclosed in a refractory lined reformer furnace 56 having a plurality of burners 58, only one of which is shown, and a flue pipe 60 for venting the spent burner combustion gases from the reformer furnace. A portion of the cooled top gas from pipe 36, together with fuel from an external source 62 is admitted to each burner through pipe 64. Flue pipe 60 communicates with two heat exchangers 66 and 68, in series. Pipe 70, which passes through heat exchanger 66, connects a source of combustion air 72 to burner 58. Process fuel gas from source 75 passes through heat exchanger 68. Heated process fuel gas is admitted to reduction furnace 10 via pipes 82 and 84 and pre-reducing gas inlet 26.

The first, usually the major, portion of the hot gas leaving reformer tubes 52 is delivered to the hot reducing gas inlet 22, as the hot reformed reducing gas, via pipes 87 and 90. A second, usually minor, portion of the hot gas leaving reformer tubes 52 passes through pipe 92 and valve 94, then is mixed with heated process fuel gas in pipe 84 and this mixture becomes the hot pre-reducing gas admitted to furnace 10 at inlet 26.

The temperature of the process fuel gas must be at least about 600.degree. C. The process fuel gas must be heated to a sufficiently high temperature that a mixture of process fuel gas and hot reformed reducing gas will have a sufficiently high temperature to carry out the direct reduction of iron oxide.

In its broadest concept the invented process includes the complete mixing of the sulfur-containing process fuel gas, such as coke oven gas, natural gas or blast furnace gas, with the hot reformed reducing gas to form a reducing gas mixture. This mixture is then introduced to the reducing zone of the furnace through a single bustle and tuyere system. This process can be performed with the apparatus shown in the drawing merely by the closing of valve 100 in pipe 84.

In the art of desulfurization of fuel gases such as natural gas, blast furnace gas or coke oven gas, there are numerous well-established commercial processes for removing H.sub.2 S (hydrogen sulfide) from such gases in a single desulfurization step. However, the removal of COS (carbonyl sulfide) and organic sulfur compounds such as thiophene (C.sub.4 H.sub.4 S) require the use of complicated and expensive multi-stage desulfurization processes to hydrogenate and convert the sulfur compounds to H.sub.2 S before these forms of sulfur can be removed.

It has recently been discovered that COS and organic sulfur compounds can be removed from gases by reaction with hot direct reduced iron pellets in the presence of hydrogen. The direct reduced iron is not effective at low temperatures for removal of these sulfur compounds, but is effective at temperatures of about 700.degree. C. and higher. The exact mechanism of this sulfur removal is not known, but it is believed that the hot direct reduced iron becomes an effective catalyst for conversion of these sulfur compounds in the presence of hydrogen, to H.sub.2 S, which then chemically reacts with the iron. In any event, the sulfur is transferred from the gas to the direct reduced iron.

Therefore, the gas mixture introduced to inlet 26 must be above about 700.degree. C. for sulfur removal. Higher temperatures are usually preferred, however, as at least about 800.degree. C. is needed for direct reduction of the iron oxide. Some pellets cluster at 800.degree. C., so they must be reduced at lower temperatures. A practical lower limit for the temperature of the gas to inlet 22 is thus 750.degree. C.

The following is a specific example of the present invention, utilizing sulfur-containing coke oven gas as the process fuel gas. The sulfur level selected for the coke oven gas in this example is 200 ppmv which is a sulfur level commonly achieved by a simple single step desulfurization process. Gas with this level of sulfur, although unuseable as a process fuel for reforming, is very acceptable as a burner fuel.

In the specific example of this invention, and referring to the drawings, hot reducing gas from the reformer tubes 52 is admitted to the reduction furnace at inlet 22 at a temperature of about 900.degree. C. Hot pre-reducing gas, which is a mixture of 900.degree. C. gas from the reformer tubes and 750.degree. C. coke oven gas from heat exchanger 68, is admitted to the reduction furnace at inlet 26 at a temperature of about 800.degree. C. The reduction furnace design provides for a furnace burden residence time of about 4 hours from the stockline 30 to ports 28, and 6 hours from stockline 30 to ports 24, which insures that a high degree of direct reduction of the iron oxide to metallic iron is achieved in the pre-reduction zone above ports 28, with the final degree of direct reduction being achieved in the reduction zone between ports 24 and ports 28.

In the pre-reduction zone, the reductants CO and H.sub.2 in the hot pre-reducing gas and in the hot reducing gas flowing up from the final reduction zone reduce the iron oxide feed material to a degree of metallization of about 94 percent. Based upon both laboratory tests and commercial experience, the methane present in the pre-reduction gas from the coke oven gas does not crack to my significant degree in its passage through the pre-reduction zone at 800.degree. C. because hydrogen is already present in the gas. Thus, the spent reducing gas or top gas exiting from the furnace burden at the stockline and from the furnace gas outlet pipe 32 contains unreacted reductants CO and H.sub.2, oxidants CO.sub.2 and H.sub.2 O vapor formed in the reduction process, and methane. In the top gas cooler-scrubber 34, a major portion of the H.sub.2 O vapor is condensed out of the top gas. The majority of the dewatered and scrubbed top gas exiting scrubber 34 is passed via line 38 to compressor 40 and into the cooling zone via the cooling gas inlet ports 44. The cool gas passes upwards in counterflow heat exchange with the descending burden of metallic iron and cools the iron to essentially ambient temperature before exiting the top of the cooling zone at 48. The descending burden of metallic iron reacts with any H.sub.2 S which is contained in the cooling gas either as residual H.sub.2 S from the process fuel gas or H.sub.2 S liberated during the early stages of reduction of a sulfur-bearing iron oxide and effectively desulfurizes the gas. The desulfurized gas is cooled and scrubbed in scrubber 50 and is a gas mixture suitable for reforming in reformer tubes 52 to produce hot fresh reducing gas. In the reformer tubes, the CO.sub.2 and residual water vapor in the cooled and scrubbed top gas serve as the reforming oxidants for the methane, as is set forth in U.S. Pat. No. 3,748,120.

The following tables show the results of a comprehensive process analysis of the invented process and are keyed to the drawing. These data are to be understood as being merely illustrative and in no way limiting. All of the tabulations are based on one metric ton of direct reduced iron product, having a degree of metallization of 92 percent and a carbon content of 1.5 percent. These are widely accepted commercial standards for direct reduced iron produced in natural gas based direct reduction plants.

Table I shows the fuel input required for the process. Coke oven gas has a higher heating value of 4618 kCal/Nm.sup.3.

Table II shows the gas flows in the process in normal cubic meters per hour at the indicated locations on the drawing.

Table III shows the gas analyses in percent at the locations indicated.

Approximately 0.018 percent sulfur is added to the metallized iron product by the sulfur transfer from the process fuel gas. This is below the acceptable limit of 0.03 percent for use of direct reduced iron in electric arc furnace steelmaking.

In the example shown, approximately 98.3% of the fuel gas consumed is required to carry out the reduction process; the remaining 1.7% is used for heating. Additional heat exchangers may be added to recover more residual heat from the reformer flue gas to preheat the process gas feed to the reformer. This will further decrease the fuel required for heating to the extent that gaseous fuel of about 2200 kCal/Nm.sup.3 heating value may be exported for use elsewhere. For example, in the present invention, when natural gas rather than coke oven gas is utilized as the process fuel gas, the required volumetric quantity of the natural gas will be approximately one-half that of coke oven gas due to the almost twice calorific value of the natural gas. This will enable the natural gas to contain approximately 400 ppmv of sulfur without adding an excessive amount of sulfur to the iron product.

Process fuel gases such as coke oven gas and naphtha vapor contain unsaturated hydrocarbons which can present carbon deposition problems in catalytic reforming. The present process, in addition to desulfurizing the process fuel gas, also serves to convert such unsaturated hydrocarbons into methane or other saturated hydrocarbon in the reduction furnace prior to the reforming and thus avoids carbon deposition problems during reforming.

As shown in FIG. 2, heat exchangers 66 and 68 can be arranged in parallel rather than in series. Flue pipe 60 feeds both spent gas combustion pipes 60-A and 60-B which are connected to heat exchangers 66 and 68 respectively.

As a second alternative, the heat exchangers of FIG. 1 can be reversed, heating the combustion air first, then heating the process fuel gas, but the FIG. 1 arrangement is preferred.

It can readily be seen from the foregoing that I have invented an improved and highly efficient process for the direct reduction of iron which will allow the direct use of sulfur-containing process fuel gases.