JPS6411688B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improvements in air ratio control methods in continuous heating furnaces. The air ratio control method in conventional continuous heating furnaces is called the so-called low O ₂ combustion method, and is a method of keeping the air ratio at least 1.0 and as close to 1.0 as possible. However, this method cannot be said to be the optimal method in terms of heating efficiency. The reason for this is that the air ratio that yields the maximum flame temperature is not necessarily
This is not 1.0, but a value lower than this, and the conventional control method with an air ratio of 1.0 or more would involve heating excess air. This fact was previously proposed by the present inventor in Japanese Patent Application No. 52578/1983. In addition, in the conventional method, the air ratio in the continuous heating furnace is uniformly set to a constant value of 1.0 or more even though the conditions of each zone are different, which also leads to poor heating efficiency. A special example of the conventional method is a method of performing multi-stage combustion in a furnace to reduce NO _x . This method lowers the air ratio in the first stage, but the air ratio is
In order to reduce the amount of NO _x produced, it is necessary to lower the temperature considerably, and in this state the optimum heating effect cannot be obtained. Also, overall, the air ratio is 1.0 or more,
Heating efficiency is low. Based on this, the present inventor proposed a method of increasing heating efficiency by first setting the air ratio for each zone of the continuous heating furnace and setting the range to a predetermined range of less than 1.0 (Japanese Patent Application No. 1987- No. 52577). However, this method does not take into account the influence of combustion gas flowing in from the previous zone, so it is difficult to set each zone to a desired set air ratio. The present invention was made in view of the above circumstances, and
The purpose of this is to control the air ratio of a continuous heating furnace to maximize the heating efficiency of each zone by controlling the air ratio of each zone while taking into account the combustion gas flowing in from other zones. I am trying to find a method. That is, the present invention is characterized in that the set air ratio m _i of each zone of the continuous heating furnace is controlled based on the following formula (1), and the air ratio M _i of each zone is set within the range of the following formula (2). This is an air ratio control method for a continuous heating furnace. m _i =1/F _i {M _i・_i 〓 ^l=1 F _l − _i-1 〓 ^l=1 m _l・F _l } _(i=1～k) ...(1) where m _i : i-th Set air ratio for the band F _i : Fuel input amount for the i-th band (Nm ³ /H) M _i : Actual air ratio for the i-th band defined by equation (2) 2φ(T _i )− 1≦M _i <1.0 ... (2) where φ (T _i ): air that maximizes the heating efficiency in the i-th zone (temperature T _i ) In the embodiment of the present invention, in equation (2), i The air ratio M _i {3φ(T _i )−1}/2≦M _i ≦{φ(T _i )+1}/2 in the second band is set in the range. The present invention will be explained below with reference to the drawings. FIG. 1 is an explanatory diagram showing an example of a continuous heating furnace. This continuous heating furnace is provided with a preheating zone 1, a first heating zone 2, a second heating zone 3, a third heating zone 4, and a soaking zone 5, and the steel material 6 is moved from the preheating zone 1 to the soaking zone 5 direction. The combustion gas flows from the soaking zone side to the preheating zone side and finally flows into the flue 7. Therefore, except for the soaking zone 5, combustion gas flows into each zone from the previous zone. For example, in the third heating zone 4, combustion gas corresponding to the fuel input in the zone itself and combustion gas flowing from the soaking zone 5 are present. Therefore, in order to set the air ratio of each zone so that the heating efficiency is maximized, it is necessary to consider the influence of combustion gas flowing from other zones. The present invention sets the air ratio in consideration of this influence. A continuous heating furnace having N bands will be described below as an example. However, the upper band and the lower band are considered to be one band, and the number of divisions in the longitudinal direction is N. For convenience, the soaking zone is numbered first and the preheating zones are numbered in order of flow of combustion gas. In the method of the present invention, the actual air ratio in each zone is as follows (3)
The formula preferably falls within the range of formula (4) below. 2φ(T _i )−1≦M _i <1.0 …(3) 3φ(T _i )−1/2≦M _i ≦φ(T _i )+1/2 …(4) However, M _i ; Actual air ratio in the zone T _i ; Average temperature of the combustion gas present in the i-th zone (℃) φ (T _i ); Optimal air ratio that maximizes heating efficiency when the combustion gas temperature is T _i Here φ(T _i ) is determined by calculating in advance the relationship between the combustion gas temperature T and φ(T) for the fuel used by equilibrium calculation. For example, in the case of M gas (net calorific value=2640 kcal/Nm ³ ) which is a mixture of coke oven gas, blast furnace gas, etc., the curve M is shown in FIG. Note that straight line A indicates the theoretical air ratio. The air ratio M _i is set within the range of equation (3) above because it is based on the optimum air ratio that can obtain the maximum heating efficiency, and the closer M _i is to φ(T _i ), the better. . In order to obtain the above air ratio M _i , the set air ratio for each zone is determined based on the following equation (5). m _i =1/F _i {M _i・_i 〓 ^l=1 F _l − _i-1 〓 ^l=1 m _l・F _l } _(i=1〜k) ……(5) However, m _i ;i Set air ratio F _i in the ith band; fuel input amount in the ith band (Nm ³ /H) This equation (5) is obtained as follows. In other words, if we assume that among the N bands, up to the kth band (k≦N) are the bands in which combustion is occurring, then i=
M _i of bands from 1 to k is expressed by equation (6). Therefore, from equation (4), the set air ratio m _i of the i-th band is
It is shown by equation (5). Insert a value that satisfies equation (3) or (4) into M _i in equation (5), and set the air ratio for each zone as i=
Decide in order starting from 1. Incidentally, when i=1 (soak zone), M ₁ =m _i ·F ₂ /F ₂ =m ₁ . This is because when i=1, no gas flows in from other zones. As for the soaking zone, if the surface properties of the steel to be heated, that is, the peelability of scale, is a problem, m ₁ >1.0 may be set, ignoring the reduction in heating capacity. In this case, the control method according to the present invention is performed for the other bands. Also, the k-th and subsequent bands, or the N-th band (k=
N), or a predetermined amount of air Q in the area where the gas temperature in the flue is above the spontaneous ignition temperature of unburned gas (approximately 600°C or higher).
may be supplied so that no unburned matter remains in the combustion gas that is finally exhausted. However, A ₀ ; Air input amount (Nm ³ /H) Next, examples of the present invention will be described. Using the continuous heating furnace shown in FIG. 1, the air ratio was controlled under the conditions shown in Table 1. For comparison, the conventional air ratio control method is also listed in Table 1.
In addition, in this example, the set air ratio m ₁ of the soaking zone
The removability of the scale was taken into account by setting the value to 1.1. In addition, the air ratio of the final combustion gas was set to 1.1, the same as in the conventional method, so that comparisons could be made clearly. Table 2 shows the test results under such control conditions and the differences from the conventional method.

[Table] * Air ratio of the sintered gas finally discharged

[Table] From Table 2, according to this example, compared to the conventional method, the heating efficiency was improved by about 3.3%, and the fuel consumption rate was reduced.
A reduction of 13000kcal/ton was confirmed. As described above, according to the present invention, since the actual air ratio of each zone is set in consideration of the combustion gas flowing in from other zones, it is possible to reliably increase the heating efficiency and reduce the amount of fuel used.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram showing an example of a continuous heating furnace;
The figure is a diagram showing the relationship between φ(T) and T in M gas. 1... Preheating zone, 2... First heating zone, 3... Second heating zone, 4... Third heating zone, 5... Soaking zone, 6... Steel material, 7
...flue.

Claims (1)

[Claims] 1. The set air ratio m _i of each zone of the continuous heating furnace is as follows (1)
An air ratio control method for a continuous heating furnace, characterized in that the air ratio M _i of each zone is controlled based on the following equation (2). m _i =1/F _i {M _i・_i 〓 ^l=1 F _l − _i-1 〓 ^l=1 m _l F _l } _(i=1～k) ...(1) where m _i : i-th Setting air ratio of the band F _i : Fuel input amount in the i-th band (Nm ³ /H) M _i : Actual air ratio of the i-th band specified by equation (2) 2φ(T _i )−1 ≦M _i <1.0 ...(2) where φ(T _i ): Air ratio 2 at which the heating efficiency in the i-th zone (temperature T _i ) is maximum In equation (2), the air ratio M in the i-th zone The air ratio control method for a continuous heating furnace according to claim 1, wherein _i is set in the range of {3φ(T _i )−1}/2≦M _i ≦{φ(T _i )+1}/2.