JP7787405B2 - Blast furnace operation method - Google Patents

Description

The present invention relates to a method for operating a blast furnace.

In the steel industry, the blast furnace process is the mainstream method for producing pig iron. In this process, ferrous raw materials (raw materials containing iron oxide, primarily sintered ore; hereafter simply referred to as "ferrous raw materials") and coke are alternately and in layers charged into the blast furnace from the top of the furnace, while hot air is blown into the furnace from tuyere openings at the bottom. The hot air reacts with the pulverized coal blown in and the coke in the blast furnace, generating high-temperature reducing gas (mainly CO gas). In other words, the hot air gasifies the coke and pulverized coal in the blast furnace. The reducing gas rises within the blast furnace, heating and reducing the ferrous raw materials. As the ferrous raw materials descend within the blast furnace, they are heated and reduced by the reducing gas. The ferrous raw materials then melt and drip down the blast furnace, where they are further reduced by the coke. The iron-based raw materials are ultimately stored in the hearth as molten pig iron (pig iron) containing just under 5% carbon by mass. The molten pig iron in the hearth is removed from the taphole and used in the subsequent steelmaking process. Therefore, in the blast furnace process, carbonaceous materials such as coke and pulverized coal are used as reducing agents.

In recent years, global warming has become a social issue, and as a countermeasure, there have been calls for reducing emissions of carbon dioxide ( _CO2 gas), one of the greenhouse gases. As mentioned above, the blast furnace process uses carbonaceous material as a reducing agent to produce large amounts of pig iron, generating large amounts of _CO2 . Therefore, the steel industry is one of the major industries in terms of _CO2 gas emissions, and must respond to this social demand. Specifically, there is an urgent need to further reduce the reducing agent ratio (the amount of reducing agent used per ton of molten pig iron) in blast furnace operation. Note that the reducing agent ratio specifically refers to the total mass of coke and pulverized coal required to produce one ton of molten pig iron.

The reducing agent serves two purposes: to generate heat in the furnace, thereby raising the temperature of the charge material, and to reduce the iron-based raw materials in the furnace. Therefore, in order to reduce the reducing agent ratio, it is necessary to increase the reduction efficiency in the furnace. Reduction reactions in the furnace can be expressed by various reaction formulas. Among these reduction reactions, the direct reduction reaction using coke (reaction formula: FeO + C ⇒ Fe + CO) is an endothermic reaction accompanied by a large heat absorption. Therefore, minimizing the occurrence of direct reduction reactions is important in reducing the reducing agent ratio. Preventing the occurrence of direct reduction reactions reduces the amount of coke required for the direct reduction reaction and the amount of reducing agent used as a heat source. Because this direct reduction reaction occurs in the lower part of the blast furnace, if the iron-based raw materials can be sufficiently reduced with reducing gases such as CO and _H2 before reaching the lower part of the furnace, the amount of iron-based raw materials subject to the direct reduction reaction can be reduced.

A known conventional technique for solving the above problems is to increase the reducing gas potential inside the furnace by blowing a carbon-containing reducing gas (COG, LPG, methane gas, etc.) into the tuyere along with hot air. With this technique, the carbon in the reducing gas blown into the blast furnace from the tuyere becomes CO gas inside the furnace, reducing the iron-based raw materials. This makes it possible to reduce the amount of iron-based raw materials that are directly subject to reduction reactions.

Furthermore, in recent years, technologies have become known that use liquid ammonia as a raw material for reducing gas, as disclosed in Patent Documents 1 and 2. In the technologies disclosed in Patent Documents 1 and 2, liquid ammonia is vaporized to produce ammonia gas, and the resulting ammonia gas is blown into the tuyere along with hot air. The ammonia gas is thermally decomposed in the blast furnace to produce a mixed gas of nitrogen gas and hydrogen gas, and the hydrogen gas reduces the iron-based raw materials in the blast furnace. By converting ammonia gas into a liquid, the volume can be significantly reduced. Therefore, ammonia gas can be easily transported in large quantities in liquid form to the blast furnace. Furthermore, because ammonia gas can be mass-produced industrially, it is thought to be relatively easy to apply to blast furnace operations that require large amounts of gas.

Patent No. 5565150 Patent No. 5593883

The inventors focused on the properties of ammonia gas and investigated injecting it into a blast furnace. As a result, they found that if ammonia gas is introduced directly into a blast furnace, a large amount of heat is required for decomposition at the tuyere. In other words, the thermal decomposition reaction of ammonia gas is an endothermic reaction. This results in stricter operational design conditions at the tuyere (for example, a significant increase in the oxygen enrichment rate), making it difficult to achieve a drastic reduction in carbon content.

The present invention was made in consideration of the above problems, and its purpose is to provide a method for operating a blast furnace that can further enhance the carbon reduction effect of ammonia gas.

In order to solve the above problems, one aspect of the present invention provides a method for operating a blast furnace, which comprises transporting liquid ammonia to a blast furnace, vaporizing the liquid ammonia to form ammonia gas, thermally decomposing the ammonia gas to generate a mixed gas containing nitrogen gas and hydrogen gas, and injecting the mixed gas into the tuyere of the blast furnace.

Here, the ammonia gas may be thermally decomposed by heating it to 400°C or higher.

Furthermore, when the amount of mixed gas injected is less than 300 Nm ³ /t and is 100 Nm ₃ /t or less, the thermal decomposition rate of ammonia gas is 0 to 100%, when the amount of mixed gas injected is more than 100 Nm ³ /t and 200 Nm ³ /t or less, the thermal decomposition rate of ammonia gas may be 80 to 100%, and when the amount of mixed gas injected is more than 200 Nm ³ /t and less than 300 Nm ³ /t, the thermal decomposition rate of ammonia gas may be 90 to 100%.

In addition, when the amount of mixed gas injected is 300 Nm ³ /t or more and the temperature at which the mixed gas is injected into the tuyere is 400°C or more and less than 600°C, the thermal decomposition rate of ammonia gas may be 90 to 100%, and when the temperature at which the mixed gas is injected into the tuyere is 600°C or more, the thermal decomposition rate of ammonia gas may be 80 to 100%.

The above aspects of the present invention can further enhance the carbon reduction effect of ammonia gas.

1 is a graph showing the correlation between the total amount of hydrogen input to a blast furnace and the carbon reduction effect in the blast furnace for each reducing gas. 1 is a graph showing the correlation between the thermal decomposition rate of ammonia gas and the carbon reduction effect in a blast furnace for each amount of mixed gas injected. 3 is a graph showing the correlation between the injection temperature of a mixed gas produced by the thermal decomposition of ammonia gas and the carbon reduction effect in a blast furnace for each thermal decomposition rate of ammonia gas. In Fig. 3, the injection rate of the mixed gas is 300 Nm ³ /t.

Preferred embodiments of the present invention are described in detail below. In the following embodiments, numerical ranges expressed using "to" refer to a range that includes the numerical values before and after "to" as the lower and upper limits. Numerical values indicated as "greater than" or "less than" are not included in the numerical range. The unit "/t" indicates a value per ton of molten iron.

<1. Investigation by the present inventors>
(1-1. Total hydrogen input and carbon reduction effect in blast furnaces)
First, the inventors' investigations will be described. The inventors first investigated the influence of hydrogen-based reducing gas (reducing gas containing hydrogen atoms) injection on the carbon reduction effect in a blast furnace. The inventors conducted the following investigations using a blast furnace operation simulation. For the blast furnace operation simulation, a so-called "blast furnace mathematical model" can be used, for example, as shown in Kouji TAKATANI, Takanobu INADA, and Yutaka UJISAWA, "Three-dimensional Dynamic Simulator for Blast Furnace," ISIJ International, Vol. 39 (1999), No. 1, pp. 15-22. This blast furnace mathematical model roughly divides the internal region of the blast furnace in the vertical, radial, and circumferential directions to define multiple meshes (small regions), and simulates the behavior of each mesh.

The calculation conditions are shown in Tables 1 and 2. Table 1 shows the calculation conditions used to obtain the data in Figure 1, and Table 2 shows the calculation conditions used to obtain the data in Figures 2 and 3. All iron-based raw materials were sintered ore. The composition of the sintered ore was T-Fe: 58.5%, FeO: 7.5%, C/S: 1.9%, _{and Al2O3} : 1.7%. It was assumed that the coke used was C: 87.2% and Ash: 12.6% (all percentages represent mass%). The air flow rate and oxygen enrichment rate in the hot blast were adjusted so that the temperature in front of the tuyere and the molten iron temperature were as constant as possible during reducing gas injection. Here, the oxygen enrichment rate is roughly the volume ratio of oxygen in the hot air to the total volume of the hot air, and is expressed as follows: oxygen enrichment rate (%) = {(air blow rate [Nm ³ /min] × 0.21 + oxygen enrichment amount [Nm ³ /min]) / (air blow rate [Nm ³ /min] + oxygen enrichment amount [Nm ³ /min])} × 100-21.

Figure 1 shows the results of the study. Specifically, the horizontal axis represents the total hydrogen input amount ( ^Nm3 /t), and the vertical axis represents the carbon reduction effect (%) in the blast furnace. The total hydrogen input amount is the amount of hydrogen gas input into the blast furnace per ton of molten iron. When a reducing gas other than hydrogen gas is input into the blast furnace, this is the value obtained by converting the input amount of the reducing gas into the input amount of hydrogen gas. The vertical axis can be obtained by the following calculation. First, Input C shown in the following formula (1) is calculated.

Input C (kg/t) = Coke ratio (kg/t) × Carbon ratio in coke (mass%) + Pulverized coal ratio (kg/t) × Carbon ratio in pulverized coal (mass%) + Reducing gas consumption rate (Nm ³ /t) × Carbon ratio in reducing gas (kg/Nm ³ ) (1)

Next, if Input C for base operation (operation without using reducing gas) is A, and Input C when using various reducing gases is B, the value on the vertical axis (Input △C) is defined by the following equation (2).

Input △C=(AB)/A×100(%) (2)

Point P1 in Figure 1 shows the correlation between the total hydrogen input amount during base operation and the carbon reduction effect in the blast furnace. Point P2 shows the correlation between the total hydrogen input amount and the carbon reduction effect in the blast furnace when coke oven gas (COG) is used as the reducing gas. The total hydrogen input amount at point P1 is a positive value because the hot blast contains pulverized coal (containing hydrogen). Point P3 shows the correlation between the total hydrogen input amount and the carbon reduction effect in the blast furnace when natural gas (NG) is used as the reducing gas. Point P4 shows the correlation between the total hydrogen input amount and the carbon reduction effect in the blast furnace when pure hydrogen gas is used as the reducing gas. Point P5 shows the correlation between the total hydrogen input amount and the carbon reduction effect in the blast furnace when ammonia gas is used as the reducing gas. By organizing the results on this horizontal axis, it is possible to determine which gas type is effective for carbon reduction. It can be seen that pure hydrogen gas is the hydrogen-based reducing gas with the greatest carbon reduction effect, while ammonia gas has little carbon reduction effect.

The reasons why ammonia gas injection is not very effective in reducing carbon emissions are thought to be: (1) the heat of decomposition of ammonia gas is very high, making it necessary to burn carbon at the tuyere tip to compensate for the heat, and (2) because the heat of decomposition is high and the tuyere tip temperature drops, it becomes necessary to design specifications that increase the oxygen enrichment rate. As a result, the heat flow ratio increases (in other words, the amount of hot air injected into the blast furnace decreases), reducing the amount of sensible heat injected into the blast furnace.

The inventors therefore considered that, from the perspective of tuyere design, it would be more desirable to decompose ammonia gas outside the furnace and then inject the resulting mixture of nitrogen and hydrogen gas into the blast furnace, rather than decomposing ammonia gas inside the furnace. They therefore investigated the carbon reduction effect of decomposing ammonia gas, as shown in Figure 2.

(1-2. Correlation between the thermal decomposition rate of ammonia gas and the carbon reduction effect in blast furnaces)
FIG. 2 shows the effect of the thermal decomposition rate of ammonia gas on the carbon reduction effect. In the example of FIG. 2, ammonia gas is thermally decomposed according to the thermal decomposition rate and then injected into a blast furnace. The temperature of the mixed gas injected into the blast furnace was 400°C. The horizontal axis of FIG. 2 represents the thermal decomposition rate (%) of ammonia gas, and the vertical axis represents the carbon reduction effect (%) in the blast furnace. The thermal decomposition rate of ammonia gas is obtained by dividing the volume of the ammonia gas pyrolyzed by the total volume of ammonia gas before pyrolysis. Specifically, the thermal decomposition rate is measured using a gas analysis device such as a gas chromatograph or mass spectrometer. The thermal decomposition rate can be adjusted by the heating temperature of the ammonia gas and the catalyst used during pyrolysis. Therefore, a thermal decomposition rate of 0% means that pure ammonia gas is injected into the blast furnace, and a thermal decomposition rate of 100% means that all ammonia gas is thermally decomposed into a mixed gas of nitrogen gas and hydrogen gas, and this mixed gas is injected into the blast furnace. When the thermal decomposition rate is more than 0% and less than 100%, a mixed gas of ammonia gas and nitrogen gas and hydrogen gas produced by the thermal decomposition of the ammonia gas is injected into the blast furnace. Point P6 shows the correlation between the thermal decomposition rate of ammonia gas and the carbon reduction effect in the blast furnace when 100 ^Nm3 /t of the mixed gas is injected into the blast furnace. Point P7 shows the correlation between the thermal decomposition rate of ammonia gas and the carbon reduction effect in the blast furnace when 200 ^Nm3 /t of the mixed gas is injected into the blast furnace. The amount of mixed gas injected can be controlled by adjusting the gas flow rate after thermal decomposition using a valve or the like.

As shown in Figure 2, it can be seen that as the amount of mixed gas injected increases, the thermal decomposition rate increases and the carbon reduction effect in the blast furnace also increases. A more detailed examination reveals that when the amount of mixed gas injected is relatively small, at 100 ^Nm3 /t, the carbon reduction effect remains roughly flat regardless of the thermal decomposition rate of ammonia gas. Therefore, it is estimated that when the amount of mixed gas injected is 100 _Nm3 /t or less, a high carbon reduction effect can be obtained when the thermal decomposition rate of ammonia gas is 0 to 100%.

On the other hand, when the injection rate of the mixed gas is relatively high, such as 200 Nm ³ /t, the higher the thermal decomposition rate of ammonia gas, the higher the carbon reduction effect, but when the thermal decomposition rate of ammonia gas is low, the carbon reduction effect is significantly reduced. The reason for this phenomenon is thought to be the influence of the heat of decomposition of ammonia gas itself and the resulting increase in the heat flow ratio, as described above. In the example of Figure 2, a good carbon reduction effect is obtained when the thermal decomposition rate is 80 to 100%. Therefore, when the injection rate of the mixed gas is more than 100 Nm ³ /t and not more than 200 Nm ³ /t, it is estimated that a high carbon reduction effect is obtained when the thermal decomposition rate of ammonia gas is 80 to 100%. Although not illustrated in Figure 2, when the amount of ammonia gas used is more than 200 Nm ³ /t and less than 300 Nm ³ /t, it is estimated that a good carbon reduction effect is obtained when the thermal decomposition rate is 90 to 100%.

The results in Figure 2 suggest that when using large amounts of ammonia gas, the heat generated by decomposition at the tip of the tuyere can become a bottleneck in operation. To solve this issue, we investigated heating ammonia gas and ammonia-derived mixed gas (i.e., a mixed gas produced by the thermal decomposition of ammonia gas) and injecting it into the tuyere. An example of this investigation is shown in Figure 3.

(1-3. Correlation between mixed gas injection temperature and carbon reduction effect in blast furnace)
Figure 3 shows the influence of the injection temperature of the mixed gas (as described above, the mixed gas is produced by the thermal decomposition of ammonia gas. When the thermal decomposition rate of ammonia gas is less than 100%, the mixed gas contains undecomposed ammonia gas) and the thermal decomposition rate of ammonia gas on the carbon reduction effect in a blast furnace. In the example of Figure 3, the mixed gas is injected into the blast furnace in a heated state (or at room temperature). The horizontal axis of Figure 3 shows the injection temperature (°C) of the mixed gas. Note that the mixed gas is injected into the blast furnace from the tuyere together with hot air, and the injection temperature here indicates the temperature before being mixed with the hot air. The vertical axis shows the carbon reduction effect (%) in the blast furnace. Point P8 shows the correlation between the injection temperature of the mixed gas and the carbon reduction effect in the blast furnace when the thermal decomposition rate is 80%. Point P9 shows the correlation between the injection temperature of the mixed gas and the carbon reduction effect in the blast furnace when the thermal decomposition rate is 90%. Point P10 shows the correlation between the mixed gas injection temperature and the carbon reduction effect in the blast furnace when the thermal decomposition rate is 100%. The mixed gas injection rate was standardized to 300 Nm ³ /t.

The results in Figure 3 show that when the mixed gas injection temperature is low (up to 300°C), the carbon reduction effect decreases when the thermal decomposition rate of ammonia gas is low. On the other hand, as the gas injection temperature increases, a higher carbon reduction effect is obtained under conditions with a lower ammonia decomposition rate. This is thought to be due to the following reason. For example, a mixed gas with a thermal decomposition rate of 100% is a mixed gas of 25 vol% _N2 + 75 vol% _H2 . If the mixed gas injection rate is 100 ^Nm3 /t, the amount of _H2 injected into the blast furnace is 100 x 75 vol% = ⁷⁵ Nm3/t. On the other hand, a gas with a thermal decomposition rate of 80% is a mixed gas of 20 vol% _NH3 + 20 vol% _N2 + 60 vol% _H2 . If the mixed gas injection rate is 100 Nm ³ /t, the amount of H ₂ injected into the blast furnace is 100 × (20 vol% × 3/2 + 60 vol%) = 90 Nm ³ /t. Therefore, under conditions of low decomposition rate, although NH ₃ , which requires decomposition heat at the tuyere tip, is contained in the mixed gas, the condition is one in which the amount of H ₂ input is large. Therefore, if the mixed gas injection temperature is increased and sufficient sensible heat is compensated, the adverse effect of the decomposition heat of NH ₃ is outweighed by the effect of the large H ₂ input, and it is thought that the carbon reduction effect is improved. From the above, it was found that a higher carbon reduction effect can be obtained by controlling the mixed gas injection temperature and controlling the thermal decomposition rate of ammonia gas according to that temperature. For example, when the mixed gas injection temperature into the tuyere is 400°C or higher but less than 600°C (the thermal decomposition temperature of ammonia gas), a high carbon reduction effect can be obtained when the thermal decomposition rate of ammonia gas is 90 to 100%. Furthermore, when the temperature of the mixed gas injected into the tuyere is 600°C or higher, a high carbon reduction effect can be obtained when the thermal decomposition rate of ammonia gas is 80 to 100%. This tendency was also observed when the mixed gas injection rate was 400 Nm ³ /t. Specifically, when the injection temperature was 400°C, the carbon reduction effect at an 80% decomposition rate was 10.4%, at a 90% decomposition rate was 12.2%, and at a 100% decomposition rate was 12.9%. Furthermore, when the injection temperature was 1200°C, the carbon reduction effect at an 80% decomposition rate was 23.1%, at a 90% decomposition rate was 22.2%, and at a 100% decomposition rate was 21.1%. Therefore, when the amount of mixed gas injected is 300 Nm ³ /t or more, when the temperature of the mixed gas injected into the tuyere is 400°C or more (thermal decomposition temperature of ammonia gas) but less than 600°C, and when the thermal decomposition rate of ammonia gas is 90 to 100%, a high carbon reduction effect can be obtained. Also, when the temperature of the mixed gas injected into the tuyere is 600°C or more, when the thermal decomposition rate of ammonia gas is 80 to 100%, a high carbon reduction effect can be obtained.

<2. Blast furnace operation method>
Next, a method for operating a blast furnace based on the above-mentioned study results will be described. In the method for operating a blast furnace according to this embodiment, liquid ammonia is transported to the blast furnace, vaporized to form ammonia gas, and the ammonia gas is thermally decomposed outside the blast furnace to form a mixed gas of nitrogen gas and hydrogen gas, which is then injected into the tuyere of the blast furnace. As a result, a high carbon reduction effect can be achieved, as described above.

Here, ammonia gas is thermally decomposed by heating it to 400°C or higher using an appropriate catalyst. Examples of ammonia gas heating means include supplying heat using an external heater, and examples of catalysts include platinum-based precious metals. Preferably, different control is performed depending on the amount of mixed gas injected. Specifically, when the amount of mixed gas injected is 100 Nm ₃ /t or less, the thermal decomposition rate of ammonia gas is set to 0 to 100%; when the amount of mixed gas injected is more than 100 Nm ³ /t and less than 200 Nm ³ /t, the thermal decomposition rate of ammonia gas is set to 80 to 100%; and when the amount of mixed gas injected is more than 200 Nm ³ /t and less than 300 Nm ³ /t, the thermal decomposition rate of ammonia gas is set to 90 to 100%.

Furthermore, when the amount of mixed gas injected is 300 Nm ³ /t or more, it is preferable to carry out the following control. That is, when the temperature at which the mixed gas is injected into the tuyere (heating temperature of ammonia gas) is 400°C or higher but lower than 600°C, the thermal decomposition rate of ammonia gas is set to 90 to 100%, and when the temperature at which the mixed gas is injected into the tuyere is 600°C or higher, the thermal decomposition rate of ammonia gas is set to 80 to 100%. The operating conditions other than those described above may be the same as those of the conventional method. Here, the injection temperature may be measured, for example, with a thermocouple or the like, and may be controlled by adjusting the output of an external heater or the like used during ammonia gas decomposition.
(Ammonia gas heating temperature)

The above describes in detail preferred embodiments of the present invention with reference to the accompanying drawings, but the present invention is not limited to such examples. It is clear that a person with ordinary skill in the technical field to which the present invention pertains can conceive of various modifications or alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

Claims (2)

transporting liquid ammonia to a blast furnace, vaporizing the liquid ammonia to form ammonia gas, thermally decomposing the ammonia gas to generate a mixed gas containing nitrogen gas and hydrogen gas, and injecting the mixed gas into a tuyere of the blast furnace;
The ammonia gas is heated to 400°C or higher to thermally decompose the ammonia gas,
The amount of the mixed gas injected is less than 300 Nm ³ /t;
When the amount of the mixed gas injected is 100 Nm ³ /t or less, the thermal decomposition rate of the ammonia gas is 0 to 100%;
When the injection rate of the mixed gas is more than 100 Nm ³ /t and not more than 200 Nm ³ /t, the thermal decomposition rate of the ammonia gas is 80 to 100%;
A method for operating a blast furnace, characterized in that when the injection rate of the mixed gas is more than 200 Nm ³ /t and less than 300 Nm ³ /t, the thermal decomposition rate of the ammonia gas is 90 to 100% . transporting liquid ammonia to a blast furnace, vaporizing the liquid ammonia to form ammonia gas, thermally decomposing the ammonia gas to generate a mixed gas containing nitrogen gas and hydrogen gas, and injecting the mixed gas into a tuyere of the blast furnace;
The ammonia gas is heated to 400°C or higher to thermally decompose the ammonia gas,
The amount of the mixed gas injected was 300 Nm ³ /t or more,
When the temperature of the mixed gas injected into the tuyere is 400°C or higher and lower than 600°C, the thermal decomposition rate of the ammonia gas is 90 to 100%,
A method for operating a blast furnace, characterized in that when the temperature of the mixed gas injected into the tuyere is 600°C or higher, the thermal decomposition rate of the ammonia gas is 80 to 100%.