JPH09194918A - Reducing gas heating method and device therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the reduction in thickness caused by carbon deposition on the inner surface of a gas heating pipe and carburization-embrittlement of the heating pipe by rapidly heating at the temp. raising velocity having a prescribed value or higher in the dangerous temp. range easily depositing the carbon from a CO-containing reducing gas. SOLUTION: At the time of heating the reducing gas containing CO, the carbon is deposited by reaction 2CO←→CO2 +C. This deposited carbon is stuck onto the heating pipe and reacts with iron, and carburizing phenomenon is caused and the inner surface is embrittled and worn. This reaction is exothermic reaction, and according to the chemical equilibrium theory, the reaction is progressed to the right side as the temp. becomes lower, the carbon is easily deposited but on the other hand, since the reaction velocity is not high in the low temp., the dangerous temp. zone easily depositing the carbon exists. This range depends on the gas composition (particularly CO and CO2 concns.) and the operation pressure, but, this temp. is generally not over about 350 deg.C, at the time of heating this gas at >=1000 deg.C/sec temp. raising velocity, since the staying time of the gas is extremely short, such as <=0.35sec, the deposition of the carbon is effectively restrained.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for heating a carbon monoxide-containing reducing gas mainly used in a direct reduction iron-making process.

[0002]

In the direct reduction ironmaking process, 800-1
Metallic iron is produced by reducing iron ore (iron oxide) using a reducing gas heated to 000 ° C. The reducing gas used here generally contains hydrogen, carbon monoxide, carbon dioxide and water vapor, and often contains methane, nitrogen and the like in addition to this. Such reducing gas is 800 ~
To heat to 1000 ° C., a heating furnace in which a gas heating pipe is arranged is used. The heating furnace generally comprises a combustion chamber (radiation heat transfer section) equipped with a burner and a convection heat transfer section for passing combustion gas. The reducing gas first passes through a gas heating pipe arranged in the convection heat transfer section and is heated for about 30 minutes. It is preheated to 400 to 450 ° C. and then passed through a gas heating pipe arranged in the combustion chamber to be heated to a desired temperature.

The gas heating pipe provided in the combustion chamber has an outer diameter of 1
It is called a heat transfer coil because a tube of about 50 to 190 mm has a folded shape as shown in FIG. 1, and a plurality of such heat transfer coils are generally connected in parallel. The heat transfer coil is heated by receiving radiant heat from the flame of a radiant wall burner provided on both wall surfaces of the heat transfer coil, and thereby the reducing gas passing through the heat transfer coil is heated to a predetermined temperature. The heating rate of the gas at this time is about 200 to 300 ° C./sec.

[0004]

However, it has been found that when the heating furnace is continuously used, a phenomenon (metal dusting) occurs in which a part of the gas heating pipe is consumed and thinned. There is a disadvantage in that the gas heating tube in the portion whose consumption has been reduced must be replaced. This phenomenon is often seen especially in a part of the gas heating pipe arranged in the combustion chamber (first half, see FIG. 1). This is probably due to carburizing and embrittlement of the inner surface of the gas heating pipe. That is, it is considered that when a reducing gas containing carbon monoxide is heated, carbon is deposited by the following disproportionation reaction (Budard reaction). 2CO ⇔ CO ₂ + C ↓ The carbon thus deposited adheres to the inner surface of the gas heating pipe,
It is considered that the carburization phenomenon occurs by reacting with iron or the like, which is a material (main component) of the high-temperature gas heating tube, and the inner surface of the gas heating tube is embrittled and consumed.

As a measure to prevent such carburizing embrittlement, it is considered to improve the material of the gas heating pipe (in particular, its inner surface) so that carburizing embrittlement does not occur even if carbon is deposited. However, considering the usage conditions and cost of the heating furnace, it is not always possible to deal with such material improvement alone in all cases, or even if it is not possible, it is not a realistic solution. There are often times when it cannot be said.

The present invention has been made in view of the above demands, and as a measure for preventing the consumption wall thinning due to carburizing and embrittlement of the gas heating pipe from a viewpoint different from the material improvement,
It is intended to prevent the deposition of carbon itself on the inner surface of a gas heating tube of a heating furnace that heats a reducing gas containing carbon monoxide.

[0007]

The present invention provides a method of heating a carbon monoxide-containing reducing gas through a heat transfer surface. The method defines a dangerous temperature zone in which carbon easily precipitates from the gas, and rapidly heats the gas so that the temperature rising rate in the dangerous temperature zone is 1000 ° C./sec or more.

Alternatively, the method defines a dangerous temperature zone in which carbon is likely to precipitate from the gas, and rapidly heats the gas so that the residence time in the dangerous temperature zone is 0.35 or less.

The invention also provides an apparatus which is particularly suitable for carrying out the method described above. Such a device has a heating part in which a burner and a gas heating pipe are arranged in a combustion chamber, and the gas heating pipe is heated from the outside by a flame of the burner, while a carbon monoxide-containing reducing gas is supplied to the gas. A gas heating furnace that raises the temperature of the carbon monoxide-containing reducing gas by flowing through the heating tube, and has a multi-pass structure in which the gas heating tube is a straight tube having an outer diameter of 30 to 80 mm.

[0010]

As the carbon monoxide-containing reducing gas to be heated in the present invention, the reducing furnace tail gas for direct reduction ironmaking is first of all mentioned. To give an example of a typical composition of this gas,

[Table 1] Reduction furnace tail gas ( after CO ₂ removal ) H ₂ 70.7% (volume%, the same applies below) CO 16.7% CO ₂ 1.0% CH ₄ 3.5% N ₂ 6.8% H ₂ O 1.3% (saturated amount).

Of course, the gas to be heated in the present invention is not limited to the reduction furnace tail gas described above,
Reformed gas from a reformer as shown in the typical composition, coke oven gas, blast furnace gas, converter gas, etc.
Various carbon monoxide-containing reducing gases are targeted.

[Table 2] Example of composition of reformed gas from reformer H ₂ 58.2% (volume%, the same applies hereinafter) CO 12.9% CO ₂ 5.7% CH ₄ 0.3% H ₂ O 22 .9%

[Table 3] Example of composition of coke oven gas H ₂ 50 to 57% (volume%, same below) CO 5 to 7% CO _{2 2 to} 5% CH ₄ 22 to 30% C ₂ ⁺ 2 to 5% N ₂ 2 to 6% H ₂ O saturation amount

Table 4 example H ₂ 3 to 4% of the composition of the blast furnace gas (vol%, the same applies _{hereinafter) CO 21~24% CO 2 20~23%} N 2 52~56% H 2 O saturation amount

[Table 5] Example of composition of converter gas H ₂ 1.5 to 3.3% (volume%, the same applies hereinafter) CO 65 to 67% CO ₂ 12 to 14% CH ₄ 0.2 to 0.4% N ₂ 16 to 19% H ₂ O saturation amount Since a gas having a carbon monoxide content of less than 4% has a very small tendency of carbon precipitation, consumption of the gas heating pipe can be reduced without using the means of the present invention. Substantially never occurs.

The Baudard reaction for depositing carbon from a reducing gas containing carbon monoxide is an exothermic reaction, and in terms of chemical equilibrium, it can be said that carbon tends to deposit because the reaction proceeds to the right at lower temperatures. That is, the chemical equilibrium constant of the Boudart reaction Kp (eq) = P _CO2 ^* / (P _CO ^* ) ² (P _CO2 ^* ... …… Equilibrium carbon dioxide partial pressure, unit: kg / cm ² ) (P _CO ^* ……… Equilibrium The carbon monoxide partial pressure (unit: kg / cm ² ) is thermodynamically expressed as a function that monotonically decreases with temperature under a given pressure condition. For example, pressure 3.5Kg / cm ²
Assuming heating from room temperature to 900 ° C under G,
If the value of Kp (eq) calculated thermodynamically is plotted against the temperature, a curve as shown in FIG. 2 is obtained. Separately, from the composition of the reduction furnace tail gas shown above, the total pressure is 3.5 kg / c.
The actual carbon monoxide partial pressure P _CO and carbon dioxide partial pressure P _CO2 under m ² G were calculated, and P _CO2 / (P _CO ) ² = Kp (obs) [(obs) is the observed value) Is shown. ] Is also shown in FIG. Kp (eq) ＞
The temperature region of Kp (obs) is a region where carbon may be deposited, and the higher the ratio of Kp (eq) / Kp (obs), the more likely carbon is to deposit. Kp (obs)>
A gas having a concentration of 2.0 (kg / cm ² ) ^-1 has a very small tendency of carbon precipitation, so that even if the means of the present invention is not used, consumption of the gas heating pipe will not be substantially reduced.

On the other hand, since the reaction rate of the Boudard reaction is not high at low temperature, it can be said that high temperature is more likely to deposit in a region where carbon may be deposited.
Therefore, the critical temperature zone in which carbon is likely to be deposited is defined as a temperature range in which the potential for carbon deposition is thermodynamically high and the reaction rate is relatively large. Generally, K
When p (eq)> 2.5 Kp (obs), it can be said that the carbon deposition potential is thermodynamically high, and at 450 ° C. or lower, the reaction rate is small and the carbon deposition tendency is small. For example, in the example shown in FIG. 2, the dangerous temperature zone in which carbon easily precipitates is generally set to 500 ° C. to 775 ° C.

As described above, the dangerous temperature zone in which carbon is likely to be deposited varies depending on the composition of the gas to be heated (especially carbon monoxide concentration and carbon dioxide concentration) and the operating pressure. The critical temperature range defined for the target system is
The gas is 1000 ° C./sec or more, preferably 1500 to
It needs to be heated at a heating rate of 3500 ° C./sec. This is a considerably higher temperature rising rate than the above-mentioned conventional temperature rising rate (at most 300 ° C./second). In the temperature range other than the dangerous temperature range, the heating may be performed at the conventional heating rate.

Generally, the dangerous temperature range is 350 ° C even if it is wide.
(When the temperature is raised from 450 ° C. to 800 ° C.), 0.35 seconds or less at a heating rate of 1000 ° C./second, 1
At the heating rates of 500 ° C./sec and 3500 ° C./sec, the gas passes through the dangerous temperature zone in 0.23 seconds or less and 0.1 seconds or less, respectively. Thus, the residence time of the gas in the dangerous temperature zone becomes extremely short, and the precipitation of carbon due to the Boudard reaction is effectively suppressed. As described above, since the temperature range of 450 ° C to 800 ° C almost covers the dangerous temperature band under normal conditions, the temperature range of 450 ° C to 800 ° C is increased at a rate of 1000 ° C / sec or more. In general, the temperature raising conditions of the present invention are automatically satisfied (without defining a dangerous temperature zone) by designing so.

In order to obtain a large temperature rising rate of 1000 ° C./second or more, it is necessary to increase the heat transfer coefficient as compared with the conventional one. The heat transfer coefficient in this case is related to the heat transfer from the flame to the gas flow.This is the radiative heat transfer from the flame to the tube surface, the heat transfer from the tube outer surface to the tube inner surface, and the tube inner surface. Is the overall heat transfer coefficient, which is the overall contribution of convective heat transfer from the gas to the gas flow. Here, in order to promote convective heat transfer from the inner surface of the tube to the gas flow, it is effective to reduce the thickness of the boundary film (laminar flow) portion formed near the inner surface of the tube. The thickness of the boundary film decreases with increasing gas flow velocity, but since a relatively thick tube having an outer diameter of about 150 to 190 mm was conventionally used, an attempt was made to increase the gas linear velocity in the tube (the number of heating tubes decreases. Due to this, the heat transfer area becomes smaller with respect to the gas flow rate, and if it is attempted to avoid it, the convection time of the gas becomes longer (due to the longer heating tube). On the other hand, in the present invention, as the gas heating pipe of the portion corresponding to the dangerous temperature zone, a thin pipe (outer diameter of 30 to 80 mm) is used as compared with the conventional gas heating pipe, so that the above-mentioned drawback is eliminated. The average gas linear flow velocity in the dangerous temperature zone is preferably about 40 m / sec or more.

Further, conventionally, since a heating tube having a folded shape as shown in FIG. 1 is used, it is thought that gas is accumulated in the U-turn portion, carbon is locally deposited, or erosion in the U-turn portion is considered to be consumed. You can see thinning. In order to solve such a problem, it is preferable not to provide the U-turn part. Therefore, in the present invention, the straight pipe multi-pass system in which the dangerous temperature zone portion is constituted by the straight pipe is adopted. This avoids unnecessary increase in pressure loss, and is therefore considered to bring good results even with an increase in gas flow rate.

Usually, the dangerous temperature zone in which carbon is likely to precipitate is in the first half of the gas heating pipe provided in the combustion chamber (generally, the portion where the temperature rises from about 450 ° C to about 800 ° C). That is, it is not always necessary to increase the linear gas flow velocity by arranging all the gas heating pipes provided in the combustion chamber by the relatively thin straight pipes and reducing the total cross-sectional area. Therefore, the object of the present invention can be achieved by dividing the gas heating pipe in the combustion chamber into the first half and the second half, and adopting the above-mentioned configuration only for the first half of the heating pipe (the part including the dangerous temperature zone).

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Example 1 An experimental heating furnace having a gas heating pipe in which five straight pipes having an outer diameter of 60.5 mm, a wall thickness of 8.5 mm and a length of 13 m were connected in parallel was manufactured. A carbon monoxide-containing reducing gas having a composition was heated.

[Table 6] Gas composition H ₂ 70.7% (volume%, the same applies hereinafter) CO 16.7% CO ₂ 1.0% CH ₄ 3.5% N ₂ 6.8% H ₂ O 1.3%

The gas temperature at the inlet of the gas heating tube was 450 ° C., and the gas temperature at the outlet was 775 ° C.
The gas pressure was 3.5 kg / cm ² G, the gas flow rate and the heating amount were adjusted, and the operation was performed under the following two conditions.

[Table 7] Operating conditions Condition 1 Condition 2 Temperature rising rate 1625 ° C / sec 1080 ° C / sec Gas residence time 0.2 sec 0.3 sec Average gas flow velocity 65 m / sec 43 m / sec

Each of the above two conditions was operated for 6000 hours, and the wall thickness of the heating tube was measured at that time, but thinning was hardly observed.

Embodiment 2 FIGS. 3 and 4 show an example of an apparatus (gas heater) suitable for carrying out the present invention. FIG. 3 is a front view of the device, and FIG. 4 is a side view of the device. The apparatus is roughly composed of a convection heat transfer section A and a radiant heat transfer section B, and the radiant heat transfer section B is composed of a primary heating section B1 and a secondary heating section B2. The convection heat transfer part A is a part which hits the exhaust path of the combustion gas from the radiant heat transfer part B and preheats the carbon monoxide-containing reducing gas by utilizing the heat of the high temperature combustion gas. Further, in the radiant heat transfer section B (primary heating section B1 and secondary heating section B2), the carbon monoxide-containing reducing gas preheated in the convection heat transfer section A is further heated to a predetermined temperature by radiant heat from the flame of the burner. To be done. In the convection heat transfer section A, the primary heating section B1 and the secondary heating section B2, as shown in the figure, respectively, the convection heat transfer section heating pipe 1, the radiant heat transfer section primary heating pipe 2 and the radiant heat transfer section secondary heating Tubes 3 are provided, with crossover tubes 4 and 5 connecting between them. The carbon monoxide-containing reducing gas is heated from room temperature to about 450 ° C. by passing through the convection heat transfer section heating pipe 1, passes through the crossover pipe 4 and the inlet header 6 through the radiant heat transfer unit primary heating pipe 2. Then, it is heated to about 775 ° C, further heated from the distributor 7 through the crossover pipe 5 to the radiant heat transfer section secondary heating pipe 3 to about 920 ° C, and transferred to the target process by the transfer line 8. To be done. Primary heating part B1
A first-stage burner 9 and a second-stage burner 10 are provided on the furnace wall of No. 1, which are ordinary long-flame type burners and heat the radiant heat transfer section primary heating pipe 2 by the radiant heat of the flame. A long flame type first stage burner 11 and a radiant wall burner 12 are provided on the furnace wall of the secondary heating section B2, and these heat the radiant heat transfer section secondary heating pipe 3. The reason why the radiant wall burner is used as the second stage burner of the secondary heating section is that, unlike the long flame type burner, the heating tube is not directly exposed to the flame, and there is no fear of overheating the heating tube. The radiant heat transfer part primary heating pipe is from inlet header 6 to distributor 7
A large number of them run side by side in equilibrium, and each of them rises vertically just below the primary heating part and rises straight up in the primary heating part. The radiant heat transfer section secondary heating pipes are a plurality of crossover pipes 5 horizontally (and a little laterally spreading) protruding from the distributor 7.
It is directly connected to and has a conventional coiled form. Since the gas heated to a high temperature passes through the transfer line 8, the inside thereof is insulated by the refractory heat insulating castable 13.

[0023] Comparative Example Outer diameter 170 mm, wall thickness 10 mm, the heat transfer coil made by connecting straight pipe six 8m long with U-shaped tube and the gas heating tube,
An experimental heating furnace having a gas heating tube of the same type as the conventional heating tube shown in FIG. 1 was manufactured, and a carbon monoxide-containing reducing gas having the same composition as in Example 1 was heated.

The gas temperature at the inlet of the gas heating tube was 450 ° C., and the gas temperature at the outlet was 920 ° C.
The gas pressure was 3.5 kg / cm ² G, the gas flow rate and heating amount were adjusted, and operation was performed under the following conditions.

[Table 8] Operating conditions Temperature rising rate 300 ° C / sec Gas residence time 1.57 sec Average gas flow velocity 31 m / sec

As a result of operating for 6000 hours under the above conditions and measuring the wall thickness of the heating pipe at that time, a maximum thickness reduction of about 5 mm was observed over two straight pipes near the inlet.

[Brief description of the drawings]

FIG. 1 shows a conventional gas heating pipe and a portion where its consumption is reduced.

FIG. 2 shows the temperature dependence of the equilibrium constant of the Boudard reaction and the carbon deposition critical zone.

FIG. 3 is a front view showing an example of an apparatus suitable for implementing the present invention.

FIG. 4 is a side view showing the apparatus of FIG. 3 as viewed in the direction of arrows XX and YY.

[Explanation of symbols]

 A Convection heat transfer part B Radiation heat transfer part B1 Primary heating part B2 Secondary heating part 1 Convection heat transfer part Heating tube 2 Radiation heat transfer part Primary heating tube 3 Radiation heat transfer part Secondary heating tube 4 Crossover tube (Convection transfer 5) Crossover tube (from primary heating part to secondary heating part) 6 Inlet header 7 Distributor 8 Transfer line 9 1st stage burner 10 2nd stage burner 11 1st stage burner (2nd Next heating part) 12 Radiant wall burner 13 Fireproof heat insulating castable 14 Convection heat transfer part Floor surface

Claims (15)

[Claims] 1. A method of heating a carbon monoxide-containing reducing gas via a heat transfer surface, wherein a dangerous temperature zone in which carbon is likely to be precipitated from the gas is determined, and a heating rate of the gas in the dangerous temperature zone is set. A method characterized by rapidly heating to 1000 ° C./second or more. 2. The method according to claim 1, wherein the heating rate is 1500 to 3500 ° C./sec. 3. A method of heating a carbon monoxide-containing reducing gas via a heat transfer surface, wherein a dangerous temperature zone in which carbon is likely to precipitate from the gas is defined, and a residence time of the gas in the dangerous temperature zone is 0. A method characterized in that rapid heating is performed so that the time is not longer than 35 seconds. 4. The method according to claim 3, wherein the residence time is 0.1 to 0.23 seconds. 5. A method of heating a carbon monoxide-containing reducing gas through a heat transfer surface, the method comprising heating the gas at 450 ° C. to 80 ° C.
A method characterized by rapidly heating so that the temperature rising rate up to 0 ° C is 1000 ° C / sec or more. 6. The method according to claim 5, wherein the heating rate is 1500 to 3500 ° C./sec. 7. The method according to claim 1, wherein the gas is a reducing furnace tail gas for direct reduction ironmaking. 8. The method according to claim 1, wherein the carbon monoxide content of the gas is 4% by volume or more (on a dry basis). 9. The method according to claim 1, wherein the Kp (obs) value defined below of the gas is 2.0 or less. Kp (obs) = P _CO2 / (P _CO ) ² P _CO : Carbon monoxide partial pressure (unit: kg / cm ² ) P _CO2 : Carbon dioxide partial pressure (unit: kg / cm ² ) 10. The method according to claim 1, wherein an average value of velocity components parallel to the heat transfer surface in the dangerous temperature zone of the gas is 40 m / sec or more. 11. The method according to claim 5, wherein the average value of velocity components parallel to the heat transfer surface in the region where the gas is heated from 450 ° C. to 800 ° C. is 40 m / sec or more. 12. A combustion unit having a burner and a gas heating pipe arranged in the combustion chamber, wherein the gas heating pipe is heated from the outside by the flame of the burner while the carbon monoxide-containing reducing gas is heated by the gas. In a gas heating furnace for raising the temperature of the carbon monoxide-containing reducing gas by circulating the gas in the tube, the gas heating tube has a multi-pass structure composed of a straight tube having an outer diameter of 30 to 80 mm. heating furnace. 13. The gas heating furnace according to claim 12, wherein the heating section heats the carbon monoxide-containing reducing gas from a temperature of 450 ° C. or lower to a temperature of 800 ° C. or higher. 14. In addition to the heating section, a preheating section for preheating the carbon monoxide-containing reducing gas supplied to the heating section and a carbon monoxide-containing reducing gas flowing out from the heating section are further heated. The gas heating furnace according to claim 12, further comprising a secondary heating unit. 15. The gas heating furnace according to claim 14, wherein the secondary heating section has a radiant wall burner.