阅读说明：本技术 一种利用焦炉气生产还原铁用合成气的方法 (Method for producing synthesis gas for reduced iron by using coke oven gas ) 是由 周红军 吴全贵 周广林 于 2021-08-13 设计创作，主要内容包括：本发明提供了一种利用焦炉气生产还原铁用合成气的方法。该方法包括以下步骤：在反应器的上半段装填CO-(2)甲烷化催化剂,在反应器的下半段装填CH-(4)蒸汽转化催化剂；将原料气输入反应器与催化剂接触进行反应,得到还原铁用合成气；其中,所述原料气为焦炉气,或者还原铁竖炉尾气与焦炉气的混合气。本发明的技术方案是以焦炉气或者焦炉气与竖炉尾气的混合气为原料,采用CO-(2)甲烷化催化剂、CH-(4)蒸汽转化催化剂的组合,并控制两种催化剂床层的反应温度,进而控制原料在反应管内的反应过程、反应产物,实现调整气体组成,使最终得到的合成气能够用于生产还原铁。(The invention provides a method for producing synthesis gas for reduced iron by using coke oven gas. The method comprises the following steps: filling the upper half of the reactor with CO 2 Methanation catalyst, filling CH in the lower half section of reactor 4 A steam reforming catalyst; inputting raw material gas into a reactor to contact with a catalyst for reaction to obtain synthesis gas for reducing iron; wherein the raw material gas is coke oven gas or mixed gas of the tail gas of the reduced iron shaft furnace and the coke oven gas. The technical scheme of the invention is to take coke oven gas or mixed gas of the coke oven gas and tail gas of a shaft furnace as raw materials and adopt CO 2 Methanation catalyst, CH 4 The combination of the steam reforming catalyst and the control of the reaction temperature of the two catalyst bed layers further control the reaction process and reaction products of the raw materials in the reaction tube, realize the adjustment of the gas composition, and ensure that the finally obtained synthesis gas can be used for producing the reduced iron.)

1. A method for producing synthesis gas for reduced iron by using coke oven gas comprises the following steps:

filling the upper half of the reactor with CO₂Methanation catalyst, filling CH in the lower half section of reactor₄A steam reforming catalyst;

inputting raw material gas into a reactor to contact with a catalyst for reaction to obtain synthesis gas for reducing iron;

wherein the raw material gas is coke oven gas or mixed gas of the tail gas of the reduced iron shaft furnace and the coke oven gas.

2. The process of claim 1, wherein the process conditions of the catalyst bed in the reactor are: the temperature is 500-950 ℃, the pressure is normal pressure-1.0 MPa, and the space velocity is 500-3000h^-1。

3. The method of claim 1, wherein the reactor is an isothermal bed reactor and the temperature of the isothermal bed reactor is controlled to be in the range of 500 ℃ to 900 ℃.

4. The method as claimed in claim 1, wherein the reactor is a variable temperature bed reactor, and the inlet temperature of the variable temperature bed reactor is 400-700 ℃ and the outlet temperature is 700-950 ℃.

5. The method of claim 1, wherein the CO is₂The methanation catalyst is a nickel catalyst added with alkali metal or/and alkaline earth metal; preferably, in said CO₂In the methanation catalyst, the addition amount of alkali metal or/and alkaline earth metal is 0.01-8%, the content of nickel is 1-20%, and the balance is carrier alumina;

the CH₄Steam reforming catalyst to Ni/Al₂O₃A conversion catalyst; preferably, in said CH₄In the steam conversion catalyst, the content of nickel is 5-25%, and the rest is carrier alumina.

6. The method of claim 5, wherein the CO is₂Methanation catalyst with said CH₄The mass ratio of the steam reforming catalyst is 1-99:99-1, preferably 1-10: 10-1; more preferably 1 to 3: 3-1.

7. The method of claim 3, wherein the isothermal bed reactor is energized using an induction coil that is uniformly wound around the outside of the reaction tube.

8. The method of claim 4, wherein an induction coil is used for the variable temperature bed reactor function, the induction coil is wound on the outside of the reaction tube, and the number of turns of the induction coil wound on the reaction tube is different at different positions from the inlet to the outlet to control the temperature at the different positions.

9. The method according to claim 7 or 8, wherein the frequency of the current input to the induction coil is a medium frequency or a high frequency, wherein the high frequency is 5-20KHz, preferably 8-16KHz, more preferably 10-15 KHz; the intermediate frequency is 50-3000Hz, preferably 300-2000 Hz;

preferably, the frequency of the current input to the induction coil is adjusted by a power supply and a capacitor;

preferably, the induction coil is connected with the power supply to form a loop, and the power supply is connected with the capacitor in parallel;

more preferably, the power of the power supply is 100-1000KW, and more preferably 200-500 KW;

preferably, the induction coil is selected from one or a combination of more than two of ferrite coil, iron core coil, air core coil and copper core coil.

10. The method of claim 1, wherein the feed gas is a mixture of reduced iron shaft furnace tail gas and coke oven gas in a molar ratio of 0-5: 1; preferably, the molar ratio is 0.1 to 5: 1.

Technical Field

The invention relates to a method for producing synthesis gas for reduced iron by using coke oven gas, belonging to the technical field of synthesis gas production.

Background

Coke oven gas (also called coke oven gas) is a by-product of steel making, the yield is high, but the coke oven gas lacks a good application direction at present, and most of the coke oven gas is burnt as fuel. Apart from complex components, the coke oven gas is mainly characterized by being rich in H₂If the method can be applied to the production of gas-based reduced iron by means of the method, the method can provide a good direction for the application of the coke oven gas. However, the standard of synthesis gas for producing gas-based reduced iron is difficult to satisfy by the general conversion means, and therefore, if coke oven gas is used for producing synthesis gas for reduced iron, innovative technical means are required for removing hydrogen in the raw materials.

Disclosure of Invention

In order to solve the above technical problems, the present invention provides a method for producing a synthesis gas for reduced iron using coke oven gas by CO₂Methanation and CH₄The combination of steam reforming catalysts enables the production of synthesis gas.

In order to achieve the above object, the present invention provides a method for producing a synthesis gas for reduced iron using coke oven gas, comprising the steps of:

filling the upper half of the reactor with CO₂Methanation catalyst, the lower half section of the temperature-changing bed is filled with CH₄A steam reforming catalyst;

inputting raw material gas into a temperature-variable bed to contact with a catalyst for reaction to obtain synthesis gas for reducing iron;

wherein the raw material gas is coke oven gas or mixed gas of the tail gas of the reduced iron shaft furnace and the coke oven gas. The coke oven gas generally comprises hydrogen (55-60%) and methane (23-27%), and also contains a small amount of carbon monoxide (5-8%), unsaturated hydrocarbons above C2 (2-4%), carbon dioxide (1.5-3%), oxygen (0.3-0.8%), and nitrogen (3-7%). Wherein hydrogen, methane, carbon monoxide, C₂The unsaturated hydrocarbon is a combustible component, and the carbon dioxide, the nitrogen and the oxygen are non-combustible components.

In the above method, preferably, the process conditions of the catalyst bed in the reactor are: the temperature is 500-950 ℃, the pressure is normal pressure-1.0 MPa, and the space velocity is 500-3000h^-1。

In the above method, preferably, the reactor is an isothermal bed reactor, and the temperature of the isothermal bed reactor is controlled to be 500 ℃ to 900 ℃.

In the method, preferably, the reactor is a variable temperature bed reactor, and the inlet temperature of the variable temperature bed is 400-. The catalyst bed is generally located in the middle of the reactor and its upper part is at a distance from the inlet of the reactor, so that it is reasonable that the temperature at the inlet of the reactor is lower than the temperature of the catalyst bed.

In the above method, preferably, the CO is₂The methanation catalyst is a nickel catalyst added with alkali metal or/and alkaline earth metal. Wherein, the alkali metal as the auxiliary agent comprises K and the like, and the alkaline earth metal comprises Ca, Mg and the like. Wherein, the active component and the auxiliary agent are both in the form of oxides in the catalyst. Preferably, in said CO₂In the methanation catalyst, the addition amount of alkali metal or/and alkaline earth metal is 0.01-8%, the content of nickel is 1-20%, and the rest is carrier alumina.

In the above method, preferably, the CH₄Steam reforming catalyst to Ni/Al₂O₃A conversion catalyst.

In the above method, preferably, in the CH₄In the steam conversion catalyst, the content of nickel is 5-25%, and the rest is carrier alumina.

In the above method, preferably, the raw material gas is a mixed gas of reduced iron shaft furnace tail gas and coke oven gas in a molar ratio of 0-5: 1. More preferably, the molar ratio is from 0.1 to 5: 1.

In the above method, preferably, the CO is₂Methanation catalyst with said CH₄The mass ratio of the steam reforming catalyst is 1-99:99-1, more preferably 1-1010-1, more preferably 1-3: 3-1.

According to the specific embodiment of the invention, the isothermal bed reactor and the variable temperature bed reactor adopted by the invention can be both tubular, the catalyst is filled in the reaction tube, the induction coil is uniformly wound on the outer wall of the reaction tube, after the induction coil is electrified, electromagnetic induction is generated between the reaction tube and the induction coil, and the reaction tube generates heat, so that the heating of the raw materials in the reaction tube is realized. Wherein, the space between the reaction tube and the induction coil can be filled with heat insulation materials (such as cement, fireproof materials and the like).

According to a specific embodiment of the present invention, when the isothermal bed reactor is energized by using the induction coil, the induction coil is uniformly wound around the outside of the reaction tube. Conventional CO₂Methanation apparatus, CH₄Steam reformer provides the heat through the burning of fuel, gas, burns the heat supply through the nozzle in the combustion chamber, then realizes the heating to the reaction tube through the heat transfer with the reaction tube, and then the raw materials in the heating reaction tube, however because the temperature of different regions is inhomogeneous in the combustion chamber, it is often all inhomogeneous to lead to this kind of heat transfer, and the heat can be concentrated at local region, can't realize that the temperature homoenergetic of each part of catalyst is evenly controlled, and the conversion reaction is also inhomogeneous. The reaction tube is heated by the induction coil, so that the heating efficiency is high, and the induction coil is uniformly distributed in the reaction tube, so that the reaction tube can uniformly generate electromagnetic induction, and isothermal reaction can be really realized.

According to the specific embodiment of the invention, when the induction coil is used for supplying energy to the temperature-variable bed reactor, the induction coil is wound outside the reaction tube, and the number of turns of the induction coil wound at different positions of the reaction tube from the inlet to the outlet is different so as to control the temperature at different positions, the temperature at the inlet is lower, the number of turns of the induction coil can be smaller, and the temperature gradually rises along with the gradual transition from the inlet to the outlet, and the number of turns of the induction coil also gradually increases. Although the conventional apparatus can make different positions of the reaction tube have different temperatures due to the problems of the conventional apparatus, the temperature control of the different positions of the reaction tube cannot be truly realized because the combustion is uncontrollable, and the temperature change degree of the variable temperature bed reactor cannot be controlled. The invention can control the electromagnetic induction heating degree by controlling the winding mode of the induction coil outside the reaction tube, thereby relatively and accurately controlling the temperature of the catalyst at different positions inside the reaction tube and realizing the control of the temperature change degree. Moreover, the technical scheme of the invention can adopt a plurality of reaction tubes simultaneously, and can realize different temperature control for different reaction tubes, thereby controlling the reaction processes and reaction results in different reaction tubes, which cannot be realized by the existing heating equipment.

In the above method, preferably, the frequency of the current input to the induction coil is an intermediate frequency or a high frequency, wherein the high frequency is 5-20KHz, preferably 8-16KHz, more preferably 10-15KHz, further preferably 12-14KHz, and specifically may be 8KHz, 8.5KHz, 9KHz, 9.5KHz, 10KHz, 10.5KHz, 11KHz, 11.5KHz, 12KHz, 12.5KHz, 13KHz, 13.5KHz, 14KHz, 14.5KHz, 15KHz, 15.5KHz, 16KHz, or may be a range obtained by combining the endpoints of the above range and specific frequency values listed, such as 5-16KHz, 5-15KHz, 5-10KHz, 8-20KHz, 8-15KHz, 8-10KHz, 10-20KHz, 10-16KHz, 10-12KHz, 9-20KHz, 9-15KHz, 12-14KHz, 8-15KHz, 8-10KHz, 10-20KHz, 10-16KHz, 10-12KHz, 9-20KHz, 9-15KHz, 10-15KHz, 9-14 KHz, 11KHz, and 10KHz, 12-20 KHz; the intermediate frequency is 50-3000Hz, preferably 300-2000Hz, more preferably 600-1500Hz, and specifically can be 300Hz, 400Hz, 500Hz, 600Hz, 700Hz, 800Hz, 900Hz, 1000Hz, 1100Hz, 1200Hz, 1300Hz, 1400Hz, 1500Hz, 1600Hz, 1700Hz, 1800Hz, 1900Hz, 2000Hz, or can be the end point of the above range and the range obtained by combining the specific frequency values listed, such as 300-3000Hz, 300-1500Hz, 600-3000Hz, 600-2000Hz, 1000-3000Hz, 1000-2000Hz, 1200-2000Hz, 1500-3000Hz, 1500-2000Hz, and the like.

In the above method, preferably, the frequency of the current input to the induction coil is adjusted by a power supply and a capacitor. The induction coil is connected with the power supply to form a loop, and the power supply is connected with the capacitor in parallel, as shown in fig. 1. The power supply used in the present invention may be a common industrial power supply, such as a medium frequency power supply and a high frequency power supply. The specification parameters such as the power of the power supply can be selected according to the frequency adjusted as required, and the rated power of the power supply is preferably 100-1000KW, and more preferably 200-500 KW. The specification of the capacitor can be selected according to the requirement, and the capacitor can be matched with a power supply to meet the frequency control requirement.

The induction coil used in the present invention may be one or a combination of two or more selected from ferrite coil, iron core coil, air core coil, copper core coil, and the like.

According to the specific embodiment of the present invention, the size of the reaction tube used in the present invention can be selected according to the need, wherein the inner diameter of the reaction tube can be 50-250mm, and the length can be selected according to the need of the reaction.

According to embodiments of the present invention, the material of the reaction tubes may be a metal or an alloy, respectively, including but not limited to the materials of reaction tubes generally used for steam reforming, reaction tubes for dry reforming. The metal or alloy is preferably one that can withstand a temperature of 1000 c, more preferably one that can withstand a temperature of 1200 c. The material of the reaction tube can be respectively selected from 316L stainless steel, 304S stainless steel, HK40 high-temperature furnace tube material, HP40 high-temperature furnace tube material, HP Micro Alloy steel or material for a Manaurite XTM steam cracking furnace, and the like.

The technical scheme of the invention is to take coke oven gas or mixed gas of the coke oven gas and tail gas of a shaft furnace as raw materials and adopt CO₂Methanation catalyst, CH₄The combination of the steam reforming catalyst and the control of the reaction temperature of the two catalyst bed layers further control the reaction process of the raw materials in the reaction tube and the reaction products, realize the adjustment of the gas composition, and ensure that the finally obtained synthesis gas meets the synthesis gas quality standard required by the reduced iron. Specifically, the invention makes the raw material gas firstly react with CO₂Controlling CO by controlling proper temperature through contact reaction of methanation catalyst₂+4H₂→CH₄+2H₂Degree of O reaction, enabling CO₂Proper conversion of CO into CH₄(ii) a As the temperature of the catalyst bed increases, the gas and CH₄Steam reforming catalyst contact processCarrying out a reaction on CH₄Conversion to CO and H₂Finally, the synthesis gas obtained by preparation has a proper composition and can be used for producing the reduced iron. In the process, the requirement on temperature control is high, and the control cannot be realized in a conventional oil-burning and gas-burning mode.

Drawings

FIG. 1 is a schematic diagram of an isothermal bed reactor used in examples 1 and 2.

FIG. 2 is a schematic of a variable temperature bed reactor used in example 3.

FIG. 3 is a graph showing the temperature of the upper, middle and lower portions of the catalyst in example 4 as a function of reaction time.

FIG. 4 shows CO and H in the exhaust gas of example 4₂Graph of volume fraction as a function of reaction time.

FIG. 5 is the CO yield, CH, of example 4₄And CO₂Graph of conversion as a function of reaction time.

Detailed Description

The technical solutions of the present invention will be described in detail below in order to clearly understand the technical features, objects, and advantages of the present invention, but the present invention is not limited to the practical scope of the present invention.

Example 1

This example is for CO at different temperatures₂And (3) evaluating the methanation conversion condition, wherein the raw material gas and the catalyst are as follows:

simulating coke oven gas: CH (CH)₄、CO₂、H₂In a molar ratio of 1: 1: 1 (33.3%: 33.3%: 33.3%);

CO₂the methanation catalyst is MC-06-203, the content of active component nickel is 5%, K₂The content of O is 0.5 percent, and the balance is carrier alumina; an isothermal bed reactor was used, as shown in fig. 1.

The reaction process is as follows:

at the pressure of 0.5MPa and the space velocity of 2000h^-1Adding water into the mixed gas under the condition of (1),the molar ratio is controlled to be H₂O/CH₄The conversion performance was evaluated by controlling the reaction temperature to 1.0 and then 500 c, 600 c, 800 c and 850 c, respectively, and the conversion results are shown in table 1, in which the compositions of the gases are all in mole percent.

TABLE 1

H2	CH4	CO2	CO	Isothermal bed
					33.3％	33.3％	33.3％	0	Temperature (. degree.C.)
15.1％	49.2％	31.2％	4.5％	500
					18.6％	43.7％	27％	10.6％	600
49.3％	12.2％	10.2％	28.3％	800
					51.1％	10.3％	7.5％	31.1％	850

From the data in table 1 it can be seen that: at a temperature below 500 ℃, CO₂Start of methanation, CH₄Converting part of steam into CO, and at about 500 deg.C, the CH in the gas₄The content reaches the highest value. Then CH as the temperature rises to 850 deg.C₄With a gradually decreasing content of CH₄Steam reforming and CO₂And CH₄May be produced simultaneously with CH₄Steam is converted to main, thereby making CO₂Is further consumed and produces more H₂And CO.

Example 2

This example is for CO at different temperatures₂And (3) evaluating the methanation conversion condition, wherein the raw material gas and the catalyst are as follows:

raw material gas (mole percentage): CH (CH)₄About 20%, CO₂About 20%, H₂About 60%, H₂S<10ppm；

CO₂The methanation catalyst is AC12-412, the content of active component nickel is 17%, and the rest is carrier alumina; an isothermal bed reactor was used, as shown in fig. 1.

The reaction process is as follows:

at the pressure of 0.5MPa and the space velocity of 2000h^-1Under the condition of (1), adding water into the mixed gas, and controlling the molar ratio to be H₂O/CH₄The conversion performance was evaluated by controlling the reaction temperature at 500 ℃, 600 ℃, 700 ℃, 800 ℃, 850 ℃, 900 ℃, respectively, 1.5, and the conversion results are shown in table 2, wherein the gas compositions are all mole percentages.

TABLE 2

H2	CH4	CO2	CO	Temperature (. degree.C.)
					59.86％	19.96％	20.18％	0	Isothermal bed
34.3％	49.5％	14.3％	1.8％	500
					31.3％	51.8％	14％	2.8％	600
41.4％	38.7％	12.1％	7.8％	699
					52.4％	25.1％	8.4％	14％	799
58.6％	17.9％	5.8％	17.6％	849
					62.4％	12.5％	4.6％	20.5％	900

From the data in table 2 it can be seen that: at lower temperatures, CO₂And H₂Generation of CO₂Methanation reaction to CH₄With increasing temperature, methane and CO₂The water produced by methanation is converted into CO and H₂。

Example 3

The embodiment provides a method for producing synthesis gas for reduced iron by using coke oven gas, and the catalyst and the reaction conditions are as follows:

raw material gas: h₂、57.9％，CH₄、10.1％，CO₂10.2%, CO, about 21%.

CO₂The methanation catalyst is MC-06-207, the content of active components of nickel is 8%, the content of CaO is 1%, and the balance is alumina; a variable temperature bed reactor was used, as shown in fig. 2.

The reaction process is as follows:

at the pressure of 0.3MPa and the airspeed of 1000h^-1Under the condition of (1), adding water into the raw material gas, and controlling the molar ratio to be H₂O/CH₄The temperature at the inlet of the temperature-changing bed is controlled to be 400-450 ℃, the temperature at the outlet is controlled to be 700 ℃, and the conversion results are shown in table 3, wherein the gas compositions are all mole percent.

TABLE 3

From the data in table 3 it can be seen that: at a low temperature of 400 ℃ and 440 ℃, CO₂And methanation of CO, steam reforming of methane with increasing inlet temperature, to produce H₂And CO, so as to adjust the gas composition and meet the requirement of the reduced iron production on the synthesis gas.

Example 4

The embodiment provides a method for producing synthesis gas for reduced iron by using coke oven gas, and the catalyst and the reaction conditions are as follows:

raw material gas: methane: carbon dioxide: carbon monoxide: hydrogen gas: ethane 10:10:20:59:1 (molar ratio); the feed sulfur content was about 20ppmV feed stock feed.

CO₂Methanation catalyst: the active component nickel content is 4%, CaO content is 0.2%, and the balance is alumina;

CH₄steam reforming catalyst: the content of the active component nickel is 10 percent, and the balance is alumina;

adopting a variable temperature bed reactor, specifically as shown in FIG. 2, adding 5cm ceramic ball fragments and 5cm CO from inlet to outlet₂Methanation catalyst and CH₄A steam reforming catalyst.

The reaction process is as follows:

at the pressure of 0.3MPa and the airspeed of 1000h^-1Under the condition of (1), adding water into the raw material gas, and controlling the molar ratio to be H₂O/CH₄The preheating temperature of the temperature changing bed is 400 ℃, the inlet temperature is controlled to be 550 ℃, the outlet temperature is controlled to be 900 ℃, and the conversion results of continuous production for 100 hours are shown in figures 3-5.

FIG. 3 is a graph showing the temperature of the upper, middle and lower portions of the catalyst as a function of reaction time. As can be seen from fig. 3: along with the reaction, the upper part of the catalyst is slowly heated from about 600 ℃ at the beginning to 620 ℃ after the end of the experiment, the middle part of the catalyst, namely the joint of the two catalysts, is slowly heated from about 700 ℃ at the beginning to 710 ℃ after the end of the experiment, and the lower part of the catalyst is slowly cooled from about 900 ℃ at the beginning to 880 ℃ after the end of the experiment. The slow temperature rise at the inlet may be caused by the exotherms of the methanation reaction and the water gas reaction, the middle part is similar, and the slow temperature drop at the outlet may be caused by the endotherms of the dry reforming reaction and the reverse water gas shift reaction.

FIG. 4 and FIG. 5 show CO and H in the tail gas, respectively₂Volume fraction and CO yield, CH₄And CO₂Graph of conversion as a function of reaction time. As can be seen from fig. 4 and 5: CO and H in tail gas in 100H stability experiment process₂The volume ratio is basically kept stable, the CO ratio is slightly reduced along with the prolonging of the reaction time, and is reduced from 27.5 percent to about 26.8 percent, and H₂The ratio of the components is slightly increased from 68.4% to about 68.9%; similarly, the CO yield is reduced from 79 percent at the beginning to about 75 percent; CH (CH)₄The conversion rate is slightly increased from 91% to about 92.5%; CO 2₂The conversion rate decreased from 56.5% to about 54.5% from the beginning, except for CH₄The remaining values outside the conversion rate fluctuate somewhat, but overall a higher conversion rate is achieved and remains essentially constant.

In the experiment of this example, the inlet temperature of the catalyst bed was controlled to 550 deg.C (620 deg.C as the reaction proceeded), and the outlet temperature was controlled to 900 deg.C (880 deg.C as the reaction proceeded), and 5cm pieces of ceramic balls and 5cm of CO were added to the bed, respectively₂Methanation catalyst and 10cm CH₄And (5) carrying out a 100-hour stability experiment under the condition of the steam reforming catalyst, and inspecting the composition condition of the outlet tail gas in the experiment process.

In the experimental process, except that experimental data are unstable due to unstable temperature at the beginning, no water is injected when the reaction time is 4h, and the like, the CO yield is basically stabilized at about 77-78% within 50h, and CH₄The conversion rate is basically stabilized at about 90-91 percent, and CO is₂The conversion rate is basically stabilized at about 56-57%. The CO yield is slightly reduced within 50-100h and fluctuates at about 74% -75%; CH (CH)₄The conversion rate slightly rises and fluctuates between 91.5 percent and 92.5 percent; CO 2₂The conversion rate is slightly reduced and fluctuates between 54 and 55 percent. Wherein CH₄The conversion rate is obviously higher than that of CO₂The conversion rate is due to the competition of the steam reforming reaction and the dry reforming reaction in the system. But the fluctuation is not large on the whole, all the values are kept stable, the pressure is not changed in the experimental process, and the tail gas quantity is relatively stable.