JPS6256438B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a heating method prior to melting scrap, and provides a method for melting with high Fe yields and low NOx emissions with reduced energy consumption. In the production of special steel, which is mainly carried out in an arc furnace, using electricity to melt the scrap material, which is the preparatory stage, is disadvantageous from a cost standpoint, and at least part of the power consumption is used by other sources. It is desirable to switch to a heat source. Preheating charging materials using heavy oil burners was once popular, but this was mainly aimed at shortening cycle time and improving productivity, and since oil prices have skyrocketed, it has become less common. Not yet. At present, instead of this, preheating of scrap using the heat of the rapidly rising exhaust gas from the arc furnace is widely practiced. Measures to improve the power unit in the steelmaking process include:
There is a ``carbon ink injection method'' developed and already disclosed by the applicant (Japanese Patent Application Laid-Open No. 1983-1999).
No. 89414). This technology generates CO by injecting carbonaceous material (hereinafter referred to as "C") powder and oxygen into molten steel, and utilizes the heat generated by oxidation. It also bubbles slag and creates an arc within it. By embedding it in the tank, it has a kind of heat retention effect and increases thermal efficiency. . In more advanced operations, oxygen is first injected into the molten steel to raise the temperature by utilizing the large amount of heat generated by the oxidation of Fe, and then C is injected into the slag to cause a reduction reaction of FeO+C→Fe+CO↑ to remove Fe. Along with collecting
It has the form of obtaining the above-mentioned effect of CO generation (Japanese Patent Application Laid-open No. 87616/1983). The above-mentioned injection of C and O <sub>2</sub> into molten steel is extended to the scrap melting process, which is the "reactor iron making method" developed by the applicant (Japanese Patent Application Laid-open No. 198206/1983), which is 2 This developed into the ``twin reactor iron manufacturing method,'' in which separate vessels were used alternately for scrap preheating and melting (patent application 1983-
14443) (Japanese Patent Application Laid-Open No. 133309, 1983). We have also attempted to apply twin reactor technology to a combination of arc furnace melting and carbon injection (Japanese Patent Application No. 111196, 1982) (Japanese Patent Application No. 1983-1983).
1982), this led to the scrap melting method that used a separate sulfurizing furnace to generate CO to improve operational control, which was also already disclosed (Japanese Patent Application No. 58-24369). 59―150006
issue). The use of C in this way was made by focusing on the advantages of its relatively low price and easy availability. We attempted to burn FeO directly in a burner and use it as a heat source for scrap melting, and by adjusting the flame temperature and controlling the amount of FeO produced, we achieved an effect that exceeded expectations, so we also proposed this (Patent Application No. 88287-1988) ) (Japanese Patent Publication No. 59-215427
issue). The scrap melting method uses a pulverized coal burner to heat the scrap prior to arc melting.
At this time, air is used to burn the scrap while the temperature is low, and when the temperature rises, oxygen or oxygen-enriched air is used instead of air to burn the scrap, thereby adjusting the flame temperature and increasing heating efficiency. This is achieved by controlling the amount of Fe oxidation. As a result of subsequent research, we have established the most efficient method for heating scrap using this pulverized coal burner, which we will disclose here. The scrap heating method of the present invention heats scrap charged in a scrap heating container with a pulverized coal burner that burns it with oxygen and/or air, by supplying less oxygen than is necessary for combustion. , the composition of the exhaust gas leaving the heating container is
The exhaust gas is rich in CO, air is introduced into this exhaust gas to burn the CO, and the exhaust gas whose temperature has been raised thereby is led to a scrap preheating container to preheat the charged scrap. A preferred example will be described with reference to the drawings.
As shown in the figure, the heating vessel is made up of a steel shell 11 with a refractory lining 12, and a pulverized coal burner is attached to the heating vessel, which is an arc furnace body 1 with a structure that allows steel to be tapped from a tapping gutter 13 by tilting. A furnace lid 3 equipped with 2 is placed, and the scrap 8 charged inside is heated with a burner. At this time, according to the proposed technique, first air, then oxygen-enriched air, and then oxygen are supplied to the burner to increase the burner oxygen ratio and increase the flame temperature. Here, the "burner oxygen ratio" is a number that indicates how much of the total oxygen required for combustion of pulverized coal is supplied as pure oxygen; 0 means only air, and 1 means pure oxygen. and oxygen-enriched air is a number between 0 and 1, depending on its composition. Apart from changing the burner oxygen ratio, as mentioned above, the amount of oxygen supplied to the heating furnace is set to be slightly less than the amount theoretically required for the combustion of combustible materials in the furnace, so that the exhaust gas from the heating container is Ensure a CO-rich composition. The amount of oxygen deficiency shall be within 10%. A preferred range is a deficit of 3-5%, or 97-95% of the theoretical amount of oxygen. Maintaining the inside of the heating container in a weakly reducing atmosphere in this manner is effective in suppressing oxidation of scrap to a low level. The degree of scrap oxidation as the scrap temperature increases depends on the temperature of the flame and the atmosphere within the heating container, and the relationship is as shown in FIG. FeO generated by oxidation is
To some extent, due to the later carbon injection
Although it can be reduced to Fe, it is disadvantageous to generate excessive amounts of FeO. Next, making the exhaust gas composition rich in CO means that
It is also significant in reducing NOx generation. The NOx produced in the burner flame is CO
is reduced back to _N2 . When the scrap is heated to a high temperature, Fe acts as a catalyst for the above-mentioned reduction reaction, and furthermore, a reduction reaction occurs in which Fe itself removes O from NOx. The relationship between the amount of CO in the exhaust gas and the amount of NOx generated is as shown in Figure 3.The higher the burner oxygen ratio, the higher the NOx concentration, but this figure shows that overall conditions that include CO are preferable. can be understood immediately. Considering the origin of the generated NOx, one is the so-called fuel NOx due to the N content (contains about 1 to 2%) in pulverized coal, and this is difficult to avoid. Another problem is so-called thermal NOx caused by N ₂ in the combustion air, which can be reduced by selecting the combustion conditions as described above. acceptable in exhaust gas from the point of view of subsequent processing
Set the NOx concentration limit to 200ppm and
When set at two levels of 500ppm, from the graph in Figure 3, the burner oxygen ratio and the CO in the atmosphere
% and the combination with O ₂ %, the permissible range is determined. This is shown in Figure 4,
All you have to do is choose the conditions in the lower left area of each curve. The pulverized coal burner used for scrap heating is desirably one that has a short frame length, does not raise the flame temperature excessively, can continue to burn stably even under oxygen-deficient combustion conditions, and does not generate soot. A pulverized coal burner developed by the present inventors and proposed separately is suitable for this purpose. However,
Even if some soot is generated due to incomplete combustion,
There is no problem because when air is blown in and the CO is burned later before entering the preheating container, it burns at the same time. Returning to FIG. 1 and continuing the explanation, a gas sampler 51 installed in the flow path 5 of the exhaust gas exiting the heating container
A typical example of the composition of the gas sampled is as follows. CO 3% NOx 50ppm CO ₂ 15% Temperature 450°C O ₂ 0% This exhaust gas is heated by burning the CO gas contained in it with the air introduced from the air introduction means 6, and is heated to a preheating container such as a clam shell. Scrap 9 introduced into 4 and charged therein
heat up. At this time, it is recommended to install an afterburner 7 that introduces and burns auxiliary fuel, such as propane gas, in order to complete the combustion of the CO gas and further increase the temperature of the exhaust gas, which is advantageous for preheating the scrap. be done. Needless to say, the exhaust gas from the scrap preheating container is sent to a dust collector (not shown) to remove dust, undergoes necessary purification treatment, and then is released. Once the scrap has been heated to a predetermined temperature as described above, the pulverized coal burner 2 is stopped, the furnace cover 3 is moved, an electrode is inserted, electricity is applied, and the process proceeds to arc melting and refining. Starting from cold material, it is heated using the scrap heating method of the present invention and then arc melted to a temperature of 157℃.
The operation pattern for a typical operation example from obtaining molten steel to tapping is shown in Figure 5A. The conventional operation pattern is to proceed from arc heating to arc melting without preheating the scrap.
As shown in FIG. 5B. Typical values for heat balance and energy consumption per charge ton in each case are shown below.

[Table] The scrap heating method of the present invention can be applied to a variety of existing scrap melting techniques, including those proposed by the applicant. As a representative example, a case where the present invention is applied to the above-mentioned Japanese Patent Application No. 111196/1988 "Arc Furnace Melting Method", that is, the twin shell type arc furnace melting technology, is as shown in FIGS. 6 and 7. That is, arc furnace bodies 1A and 1B of the same shape
First, as shown in FIG. 6, a furnace lid 3' equipped with an electrode 2' is placed over the furnace body 1A, and the heated scrap inside is arc-melted. On the other hand, the furnace body 1B is covered with a furnace lid 3 equipped with a burner 2 to heat the preheated scrap inside. As shown in FIG. 7, these exhaust gases are initially at a low temperature, so they are not used for preheating, but are discharged after dust removal and purification treatment. When the melting of the scrap progresses at 1A and the temperature of the scrap increases at 1B, the exhaust gases are introduced into the preheating container 4 to preheat the loaded scrap. When the initial scrap is melted in 1A, the preheated scrap in the preheating container 4 is additionally loaded into the furnace body 1A. Once the additional charge is melted, carbon injection is performed on the furnace body 1A. The exhaust gas may be led to the preheating container together with the exhaust gas from the furnace body 1B which continues to be heated by the burner, or it may be used for heating through the furnace body 1B and then used for preheating. In some cases, the burner is stopped and the arc furnace furnace body 1 is undergoing carbon injection.
It is also possible to heat the scrap in the furnace body 1B using only the exhaust gas from A, and further to perform preheating using the exhaust gas. Once the steel can be melted and tapped in the furnace body 1A, the scrap in the preheating vessel is charged, the furnace cover equipped with a fine powder burner is covered, and the process moves to burner heating. On the other hand, the furnace body 1B containing the scrap that has been heated to a high temperature is covered with a furnace lid equipped with electrodes instead of the furnace lid equipped with a burner, and the process proceeds to arc melting. In this way, the pair of furnace bodies 1A and 1B are alternately used as an arc melting furnace and a scrap heating container, and the scrap preheating container is also used, thereby realizing an energy-efficient scrap preheating-heating-melting cycle. That's why.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a preferred example of an apparatus used to carry out the scrap heating method of the present invention. FIG. 2 is a graph showing changes in the amount of scrap oxidation as the scrap temperature rises at various burner oxygen ratios when scrap is heated with a pulverized coal burner in a scrap heating vessel. FIG. 3 is a graph showing the exhaust gas NOx concentration depending on the amount of oxygen supplied at various burner oxygen ratios when scrap is heated with a pulverized coal burner in the scrap heating vessel.
Figure 4 shows the exhaust gas from the scrap heating vessel.
It is a graph showing the permissible range of the combination of burner oxygen ratio and oxygen supply amount when the permissible amount of NOx concentration is 500 ppm and 200 ppm. FIG. 5 shows an example of an operation for melting scrap and refining steel using the scrap heating method of the present invention.
The operation pattern is compared to the conventional method of melting scrap and refining steel without preheating, where A represents heating and B represents conventional technology. In the figure, time passes from left to right. 6 and 7 are for explaining the case where the scrap heating method of the present invention is applied to twin shell type arc furnace melting technology, FIG. 6 shows the configuration of the device, and FIG. 7 shows the device It is a diagram conceptually showing the work of each part and the exhaust gas distribution route. 1... Heating container (arc furnace furnace body), 2... Pulverized coal burner, 2'... Electrode, 3... Furnace cover (for burner),
3'...furnace lid (for electrode), 4...preheating container (clam shell), 5...exhaust gas flow path, 6...air introduction means, 7...
afterburner, 8...scrap to be heated;
9...Scrap to be preheated.

Claims (1)

[Claims] 1 Scrap charged in a scrap heating container is heated with a pulverized coal burner that burns it with oxygen and/or air, and heating is done by supplying less oxygen than is necessary for combustion. The composition of the exhaust gas leaving the container is CO-rich, air is introduced into this exhaust gas to burn the CO, and the heated exhaust gas is guided into the scrap preheating container to preheat the scrap charged. Characteristic scrap heating method. 2 The amount of oxygen to be supplied is the theoretical amount required for combustion.
The heating method according to claim 1, wherein the shortage is within 10%, preferably 3 to 5%.