A method of producing liquid crude iron and high-grade top gas

Abstract

 A blast furnace is fed by a charge of conventional composition and a blast is used that contains about 50% oxygen and 20% water vapour, balance nitrogen. Additional fuel in the form of coal dust and/or oil is supplied to the raceways. Normal temperatures are maintained in the hearth and top zones. As compared with conventional operation a very high specific crude iron production per m² of hearth area and 24 hours will result. The molecular ratio CO+H₂/N₂ of the top gas will be higher than 2.8 which makes the top gas suitable for ammonia production.

Description

[0001] The present invention relates to a method of producing liquid crude iron and high-grade top gas with a molecular ratio of (CO+H₂)/N₂ > 2.8 in a blast furnace in which normal temperatures are maintained in the hearth and top zones, and which is fed by a charge of conventional composition, at the same time as additional fuel and preheated blast, obtained by mixing air, oxygen and water, are supplied to the raceways. The invention also relates to a way of making steel and ammonia by producing liquid crude iron and high-­grade top gas in the way mentioned above and processing the top gas to ammonia.

[0002] For the sake of simplicity, the following symbols and terms of units will be used throughout the specification:

[0003] Accordingly, the specific flow of crude iron, for instance, will be designated by THM/m².24h (= tons of crude iron per m² of hearth area and 24 hours).

[0004] "Raceways" refers to the hot zone ranging from the tuyeres and usually penetrating 1,5-2 m towards the blast furnace centre.

[0005] A great number of patents, magazine articles etc. discuss the way of optimizing the blast furnace operation, especially by minimizing the costs of fuel/THM. The steps suggested to obtain optimum working conditions include (1) increased blast temperature, (2) charge preparation, (3) gas injection into the stack, (4) pre-reducing the charge, (5) blast additives, (6) high top pressure, and so on. When operating modern conventional optimized furnaces, one or more of the steps (1), (2), (5) and (6) are usually applied.

[0006] Consequently, the basic idea behind the development of blast furnace optimization has implied steps that reduce the costs of fuel consumption/THM, especially by measures reducing the coke content of the charge, but also by replacing the coke with a cheaper fuel, for instance coal dust, injected into the raceways. The optimization has been very successful, and existing blast furnaces, operated in a conventional manner, usually have a fuel consumption less than 370 kgs of coke + 100 kgs of coal dust/THM. Then the specific crude iron production usually amounts to 55 - 65 THM/m².24h. Such a blast furnace may be operated in a range of ±20% of the optimum production, but the prime cost of crude iron will then be higher on both sides of the optimum conditions.

[0007] In order to give further particulars of the concept of "conventional optimized operated blast furnace", which is important to get a clear idea of the present invention, a detailed description of such a blast furnace will be given below as an example.

[0008] The furnace hearth diameter is 8.5 m and its optimum crude iron capacity, that is the capacity having the lowest costs of production/THM, amounts to an average of 130 THM/h at a degree of utilization of 96%. The optimum capacity corresponds to a specific crude iron capacity of 57 THM/m².24h. Required charging of iron raw materials, slag formers, fuel and air blast (preheated to 1.430°K) are shown in Table No 1 that also shows analyses in percentage by weight of the charge components, additional fuel, slags and crude iron together with vol% in the air blast.

[0009] The specific slag flow in the hearth area amounts to 8.5 tons of slag/m².24h. The adiabatic flame temperature in the raceways is calculated to 2460°K, and the top gas has a temperature of 430°K. The hearth and top gas analyses in vol% etc will be seen in Table 2.

[0010] The low fuel consumption/THM of a conventional optimized blast furnace, results in a lean low energy top gas with high contents of CO₂ as well as N₂, as shown in Table 2. This means that the gas is unsuitable as a raw material for further chemical processing, and its principal application has become as fuel, for instance for preheating the air-blast.

[0011] One way of improving the overall economy of a blast furnace is to change the operating conditions so as to obtain a high-grade top gas, which may be used for purposes economically more interesting than that of fuel. Several patents and magazine articles present efforts in this direction, and the following will be analysed:

(1) Okamoto et al. Trans IISJ 14, 1974, pp 122-132.

(2) Jordan, US-A 4.529.440.

(3) Jordan, US-A 4.013.454.

(4) Bradley, US-A 2.593.257

[0012] In the first three cases, the quality improvement of the top gas is obtained by replacing the normal air blast with oxygen enriched air and recirculated top gas, in case (2) combined with fuel injection into the raceways.

[0013] The first mentioned article is most interesting in relation to the present invention, since it deals with industrial ammonia production on basis of processed top gas from a blast furnace with modified operating conditions.

[0014] Owing to the CO₂-content of the recirculated top gas, the fuel consumption and the CO-content of the top gas will increase. Accordingly, see (1), by blowing through separate channels of the tuyeres, 31% of the top gas flow and oxygen enriched air with 55.1 vol% O₂ corresponding to 35.1 vol% O₂ in a total air blast a top gas was obtained that had the following analysis in vol%: 59.5% CO, 13.4% CO₂, 3.3% H₂, 23.6% N₂. The molecular ratio (CO+H₂)/N₂ then amounted to 2.66.

[0015] By shifting CO to H₂ in a conventional way by means of water vapour and addition of H₂ produced by electrolysis of water, a synthesis gas was obtained, having a molecular ratio of H₂/N₂ = 3.0 suitable for ammonia synthesis.

[0016] The same article (1) also outlines operation without recirculation of top gas - instead 80 kgs of heavy-oil/THM was injected into the raceway and the production capacity of crude iron was increased by about 10%; the molecular ratio (CO+H₂)/N₂, however, was reduced to 0.67, and therefore the suitability of the top gas for further processing to synthesis gas for ammonia production was eliminated.

[0017] US-A 4.529.440 also describes an operating cycle for blast furnaces in which the air blast is replaced with air enriched to > 65 vol% O₂ together with recirculated top gas containing carbonaceous material. Flow conditions are not stated. The top gas is said to contain a vol% of about 50 vol% CO, and gas discharged from the stack in the upper part of the hearth area is stated to contain about 75 vol% CO. The claims refer to a method of increasing the gas production without increasing the top gas temperature, by removing heat from the stack, for instance by discharging hot gas from the stack, increasing the slag production and the crude iron temperature, that is, by increasing the thermal losses on purpose.

[0018] US-A 4.013.454 outlines a way of operation of a blast furnace in which the air blast is replaced by carbon dioxide and oxygen, or oxygen enriched air in case ammonia is the end-product of the top gas processing. Also in this case it is a kind of recirculation, since the carbon dioxide will be extracted from the top gas or from the same after water-gas shifting.

[0019] US-A 2.593.957 describes a method of nitrogen-free operation of a blast furnace. The blast contains solely oxygen and water vapour. The specific crude iron production obtained with this method could have been high as compared with the normal specific production in 1948, but, with the knowledge of the present invention, it can, ex poste, be supposed that a specific crude iron production could not have been reached that would be considered high today.

[0020] According to the invention, a very high specific production of liquid crude iron and high grade top gas with a molecular ratio of (CO+H₂)N₂ > 2.8 is possible in a blast furnace together with an energy consumption/THM even lower than that which is obtained in a conventional optimized blast furnace of identical dimensions. The energy consumption/Nm₃ of the synthesis gas obtained from the top gas produced according to the invention is also surprisingly considerably lower than that required by industrial methods of production of such synthesis gas existing today. This is made possible fundamentally by the fact, that the charge flow, the additional fuel flow and the blast flow are controlled so that the specific crude iron production is > 75 THM/m².24h. The oxygen content in the blast is controlled within the interval 40-60 vol%, the vapour content within the interval 15-30 vol% and the ratio O₂/H₂O is maintained > 2.0. The produced top gas is not utilized for recirculation but entirely or almost entirely it can be processed into synthesis gas especially suitable for ammonia production. The content of water vapour should preferably exceed 18 vol%. It can for example be 18-22 vol% and then, a specific production > 82 THM/m².24h can be obtained.

[0021] The combination of high water vapour and oxygen contents in the air blast and high fuel flow, as compared with conventional optimized operation, results in a normal temperature-level in the raceways.When using oil, for instance, the fuel flow will amount to > 60 kgs/THM beyond the flow of the conventional operation according to Table 1. The high H₂- and CO-contents of the produced hearth gas, resulting in a high reduction potential and a low molecular weight in combination with correctly controlled hearth gas flow, will give acceptable flow conditions in the raceways and high specific crude iron production.

[0022] The above mentioned surprising results of the operation of a blast furnace according to the invention will be illustrated closer in two examples, elucidating the difference between said operating conditions and the operating conditions of a conventionally optimized operation. Both examples refer to blast furnaces dimensionally identical with the conventional blast furnace described above.

EXAMPLE NO 1

[0023] While normal temperatures in the raceways as well as in the top gas are maintained , a charge flow of conventional composition is supplied to the furnace top, the flow of which, however, is 54% higher than same of conventional optimized operation. 555 Nm³/THM of blast, (total blast) preheated to 1.430°, obtained by mixing air, oxygen and water vapour in volume ratios of 36.3/43.0/20.7 (giving a content of 51 vol% oxygen in the total blast), and 74 kgs of heavy-oil/THM are simultaneuously supplied to the raceways in addition to the 100 kgs of coal dust/THM supplied to the raceways in the conventional operation.

[0024] The above measures result in a theoretical adiabatic flame temperature in the raceway of 2500°K, a top gas temperature of 410° K, and a specific crude iron production of 88.2 THM/m².24h. The analysis of the top and hearth gas contents in vol% etc are found in Table No 3.

[0025] The operating conditions obtained by above mentioned the steps together with a minor adjustment of the opening diameter of the tuyeres, also imply that "penetration factor" as well as "raceway factor" (dimensionless figures for the characterization of the conditions in the raceways, (reference Hatano et al in Trans IISJ 17, pp 108 to 110)) will be the same as for the conventional optimized operation according to Table No 1. Therefore, there is no risk of channelling or irregular gas distribution, and operation according to the invention will result in just as steady and regular operation as with the conventional optimized operation.

[0026] As stated above, operation according to the invention results in much higher specific crude iron flow and slag flow than does conventional optimized operation. At the same time, however, the specific mass flow of the hearth gas is the same and volume flow is 24% higher than those of conventional optimized operation. The high Fe-content (66.6%) of the pellets with subsequent low slag flow (144 kgs of slag/THM), together with the above-mentioned hearth gas conditions and the low molecular weight as well as viscosity of the top gas, will bring about a considerable and safe distance between the optimum conditions according to the invention and those conditions, at which flooding may occur. In order to be safe in this respect, it is advisable to provide such a charge composition that results in a slag production < 250 kgs/THM.

[0027] The high molecular ratio (CO+H₂)/N₂ of the top gas according to Example No 1 enables optimal economical exploitation of the exhaust gas from the oxygen fining of the crude iron by intermixing the exhaust gas with the top gas. The gas mixture obtained in this way may be further processed to a composition suitable for ammonia synthesis as will be described below in connection with Example No. 1. According to this example crude iron is obtained which, when fined in an LD converter, gives an exhaust gas which, when mixed with the top gas, results in a gas having the analysis shown in Table No 4 (dry gas).

[0028] After dust purification, elimination of a part of the carbon dioxide content of the gas, shifting by water vapour and elimination of the water vapour and carbon dioxide content - all in a well-known manner - a raw gas with analysis in vol% according to Table No 5 is obtained.

[0029] As is clear from Table No 5, a minor adjustment of the raw gas N₂-content is necessary to obtain a high-grade synthesis gas with a molecular ratio of H₂/N₂ = 3.0, suitable for ammonia production. By supplying a minor flow of N₂ (on average 25Nm³/THM), a synthesis gas of required composition is obtained. With a yield of 96% in the ammonia production, the synthesis gas will give 300 kgs of NH₃/THM or about 65 tons of NH₃/h, according to the table.

[0030] In exemple No 1, it would be possible to directly obtain raw gas with a molecular ratio of 3.0 by increasing the quantity of air in the blast. Since oxygen in the form of air is cheaper than pure oxygen, such a process would give the lowest production cost. In practice, however, this is not possible since the analysis of the LD-gas varies within relatively wide limits, which has to be compensated for in order always to obtain the required molecular ratio of 3.0. The simplest way to arrive at a correct analysis of the synthesis gas is to have somewhat lower air content in the blast and to correct the analysis by a controlled minor flow of pure N₂, preferably from the oxygen plant that supplies O₂ for the blast since the oxygen plant has a very high surplus of N₂.

EXAMPLE NO 2

[0031] If utilization of exhaust LD-gas is of no current interest, the operating conditions are changed, as compared to Example No 1, in so far as the charge flow is controlled to be 51% higher than by conventional optimized operation, and the blast composition of air, oxygen and water vapour is changed to the proportions of 40.6/ 40.2/ 19.2, corresponding to 49 vol% oxygen. The blast flow will be controlled to 575 Nm³/THM, and the oil supply to the raceways to 74 kgs/THM in addition to the 100 kgs of coal dust/THM of the conventional operation.

[0032] The steps mentioned above will result in a theoretical adiabatic flame temperature of 2500°K, a top gas temperature of 410°K, a specific crude iron production of 86.2 THM/m².24 h and a specific slag production of 12.4 tons of slag/m².24h. The top gas and hearth gas analyses in vol% are clear from Table No 6.

[0033] After dust purification, shifting by water vapour and elimination of the water vapour and carbon dioxide content from the shifted gas - all in a well-known manner - a raw gas with an analysis in vol% etc according to Table No 7 is obtained.

[0034] As is clear from Table No 7, a very small adjusting gas flow of pure N₂ will be necessary in this case to obtain a high-grade synthesis gas with a molecular ratio of H₂/N₂ = 3.0. With a yield of 96% in the ammonia production, the synthesis gas will give 274 kgs of NH₃/THM or 58 tons of NH₃/h.

[0035] To sum up, running a blast furnace as described in examples 1 and 2, results in a very big increase of productivity as to crude iron (>50%) together with a production of top gas with a high molecular ratio (CO+H₂)/N₂. It is also possible to obtain so high a molecular ratio as to enable exploitation of exhaust gas from oxygen fining of the crude iron in production of ammonia; in that case, a yield of ammonia amounting to 30% of the crude iron yield can be expected.

[0036] The total economy for a blast furnace operated according to the invention, combined for instance with an LD-plant and a plant for ammonia synthesis, will be far superior to the economy of two separate plants based upon the same raw materials and producing steel and ammonia respectively, for instance a blast furnace + an LD-converter and a Texaco-gasifier + an NH₃-synthesis unit.

[0037] In Table No 8, this superiority is clearly demonstrated. In the table are shown the ratios between productivities, net energy consumptions, investment costs/ton per year and man hours/ton of product for the crude iron production in a conventional optimized blast furnace as well as for the ammonia production in a Texaco-­gasification/NH₃-synthesis, compared with crude iron production in an identical blast furnace operated according to the invention + NH₃-synthesis based upon processed top gas. It is clear from these key figures, that the prime cost of crude iron as well as of ammonia will be drastically reduced when a production method according to the invention is applied.

Claims

1. A method of producing liquid crude iron and high-grade top gas with a molecular ratio of (CO+H₂)/N₂ > 2.8 in a blast furnace in which normal temperatures are maintained in the hearth and top zones, and which furnace is fed by a charge of conventional composition, at the same time as additional fuel and, preferably, preheated blast, obtained by mixing air, oxygen and water, are supplied to the raceways
characterized in that the charge flow, the additional fuel flow and the blast flow are controlled so as to get a specific crude iron production of > 75 THM/m².24h, and the blast oxygen content is maintained in the interval 40 - 60 vol%, water vapour in the interval 15-30 vol% and the molar ratio O₂/H₂O > 2.0.

2. Method according to claim 1
characterized in that the oxygen content in the blast is 45-55 vol%.

3. Method according to claim 1 or 2 characterized in that the water vapour content of the blast is in the interval 18-22 vol%.

4. Method according to claim 3
characterized in that the water vapour content of the blast is 19-21 vol%.

5. Method according to claim 3 or 4 characterized in that the oxygen and water vapour contents of the blast, the flow of additional fuel and the flow of the blast are controlled so as to obtain > 82 THM/m².24h.

6. A method of producing steel and ammonia by producing crude iron in a blast furnace and processing top gas from the blast furnace operation into ammonia and oxygen fining the crude iron,
characterized in that the blast furnace is operated in accordance with any one of the preceeding claims and the top gas from the blast furnace and the waste gas from the oxygen fining are mixed and the mixture is processed to ammonia.