JP7564446B2 - Method for evaluating carbonaceous materials for sintering and method for producing sintered ore - Google Patents

Description

The present invention relates to a method for evaluating carbonaceous materials for sintering and a method for producing sintered ore.

In the sintering process, new raw materials for sintering are made up of iron ore (fine iron ore), miscellaneous iron-containing raw materials such as undersize powder, dust, and mill scale generated inside and outside the sintering plant, and slag-forming materials (auxiliary raw materials) such as limestone. The new raw materials are mixed with sintering carbon (agglomeration material), which is the fuel (solid fuel) that sinters (agglomerates) the sintered ore, and return ore to make a blended raw material. Return ore is sintered ore that does not meet the product particle size requirements, and is circulated in the sintering process to be sintered again.

In the currently commonly used sintering process using the Dwight Lloyd (DL) sintering machine, the pressure below the packed bed of the raw materials is negative, and air is drawn downwards from above to combust the coagulants in the raw materials. The heat of combustion generated by the combustion of the coagulants is used to sinter the raw materials to produce agglomerated sintered ore. This sintered ore is used as the main raw material in the blast furnace.
Conventionally, coke breeze has been mainly used as the coking material. Coke breeze is a coke produced in a coke oven and crushed, which has a certain particle size (e.g., particle size 0.5 to 3 mm) that falls under the sieve. Compared to the granulated coke that is used in blast furnaces, the amount of coke breeze that falls under the sieve is small, and the amount of coke for sintered ore production is insufficient. For this reason, anthracite, which has a low volatile content, has also been used as the carbon material, but the reserves of anthracite are becoming depleted.

As an alternative fuel to coke powder and anthracite, it is possible to use coal (low-grade coal) such as lignite, subbituminous coal, and high-volatile bituminous coal as sintering materials. However, these coals have a high volatile content of 20 to 50%. If used as is, the volatile content in the coal will evaporate and generate gas containing volatile organic compounds (such as tar). The volatile organic compounds cannot be completely burned in the sintering bed and will move and be discharged in the exhaust gas. As a result, there is a risk that the exhaust gas blower will malfunction due to the adhesion of volatile organic compounds, and the exhaust gas discharged from the chimney to the outside of the sintering equipment will pollute the air (not meet the emission standards). Therefore, in order to use these coals as sintering materials, it is necessary to reduce the volatile content by performing a heat treatment (carbonization treatment) in advance to char them.

Since char generally has a fast combustion rate, it is expected to have the effect of accelerating the sintering process and increasing productivity in the sintered ore manufacturing process. On the other hand, it is also necessary to consider the impact on sintering yield of the increased sintering rate.

In addition, in the sintered ore manufacturing process at steelworks, nitrogen oxides (NOx) are generated by the combustion of carbonaceous materials used as fuel, and are discharged as exhaust gas. Reducing NOx emissions is an important issue in improving air pollution. Char generally has a large specific surface area per unit weight, so it is expected to be effective in reducing the amount of NOx generated.

As described above, char is expected to serve as an alternative fuel to coke breeze and anthracite in the sintered ore manufacturing process, but it is necessary to evaluate its impact on productivity and on nitrogen oxides (NOx) in exhaust gases.

Here, in the sintering process, the porosity and specific surface area of the carbonaceous material are important factors related to the combustibility of the carbonaceous material itself and the breathability of the packed bed. There are various methods for measuring porosity, such as a method using a mercury porosimeter and a gas adsorption method that measures the amount of adsorbed gas. The range of pore sizes that can be measured varies depending on the measurement method.

Patent Document 1 describes the limitation of char by a parameter of a certain porosity as one of the indicators of suitable carbonaceous materials to be used as fuel for iron ore sintering. Here, a mercury porosimeter is used to measure the porosity, and the range of porosity is specified, on the basis that pores of 0.1 microns or less are not a factor that determines combustibility.

Non-Patent Documents 1 and 2 report that when measuring the specific surface area of a substance having micropores such as activated carbon by gas adsorption, _N2 may be underestimated, and _CO2 is more suitable as an adsorption gas.

International Publication No. WO 2010/087468

Om P.Mahajan, Carbon,29(6),pp735-742,1991. D. Cazorla-Amoros, Langmuir,1996, 12, 2820-2824, 1996.

Char properties vary greatly depending on the raw materials, the raw material blend ratio, the manufacturing method (manufacturing equipment), and manufacturing conditions (temperature, residence time, atmospheric gas, etc.). As a result, even if part or all of the conventionally used sintering carbonaceous material (coke powder, anthracite) is replaced with char with the same particle size, calorific value, amount of fixed carbon in the carbonaceous material, etc. during the production of sintered ore, the combustibility in the sintering process changes significantly depending on the properties of the char used.

In the sintering process, as described above, porosity and specific surface area per unit weight are important as indicators of char properties. In Patent Document 1, pores of 0.1 microns or less are not a factor that determines combustibility, and porosity is specified based on the pore range measured by a mercury porosimeter. However, since pores of 0.1 microns or less are not considered, there are still problems to be solved in evaluating char used in the sintering process. In addition, Non-Patent Documents 1 and 2 do not evaluate the specific surface area of sintering carbonaceous materials using _CO2 as the adsorbed gas, much less mention the relationship between the specific surface area and sintered ore productivity or NOx conversion rate.

The inventors have conventionally measured the specific surface area by _N2 gas adsorption and evaluated char based on the measured value. However, it is difficult to measure extremely fine pores (micropores) by the _N2 gas adsorption method, and there is a problem in that the specific surface area is underestimated.

This time, the inventors measured the specific surface area of chars produced from lignite, subbituminous coal, and high volatile bituminous coal by a new _CO2 gas adsorption method, and conducted sintering experiments using the produced chars. As a result, they found that it is possible to specify a carbonaceous material (char) that can achieve both "increased sintered ore production" and "low NOx emissions" based on the specific surface area of the char measured by the _CO2 gas adsorption method. Here, the _CO2 gas adsorption method is a method of measuring the specific surface area using _CO2 as the adsorption gas.

The object of the present invention is to provide a method for evaluating new carbonaceous materials for sintering that can increase sintered ore production and reduce NOx emissions.

The gist of the present invention is as follows.
(1) A method for evaluating a carbonaceous material used in a sintered ore manufacturing method, comprising measuring the specific surface area of char used as the carbonaceous material using _CO2 as an adsorbed gas, and evaluating the suitability of the char as a solid fuel based on the specific surface area.
(2) The method for evaluating a carbonaceous material for sintering according to (1), characterized in that, in the evaluation of suitability, the char is evaluated as having suitability as a solid fuel when the specific surface area of the char is within ^the range of 200 to 450 m 2 /g.
(3) A method for producing sintered ore, characterized in that the suitability of char to be used as the carbonaceous material is evaluated by the evaluation method for carbonaceous material for sintering described in (1) or (2), and char having a specific surface area within a predetermined range is used as a solid fuel.

By using a simple and new evaluation method for carbonaceous materials for sintering and using the carbonaceous materials that are deemed suitable as sintering raw materials (solid fuel), it is possible to achieve "increased sintered ore production" and "low NOx emissions" during the sintering manufacturing process.

<Measurement of specific surface area of carbonaceous material>
Lignite, subbituminous coal, and bituminous coal contain high volatile matter. Char produced by heat treating (carbonizing) these coals has fine pores due to the volatilization of the high volatile matter. This tendency is particularly noticeable for char produced from coal with a Roga index of 10 or less. Char with fine pores can be produced by using low fluidity coal with a Roga index of 10 or less as raw coal. Note that the fluidity of coal is a property resulting from the degree of molecular breakdown when heated, and low fluidity coal is coal that is less likely to be broken down into molecules when heated.

The specific surface area measurement method is a method for calculating the specific surface area of a sample (total surface area of a powder per unit mass) by gas adsorption. The specific surface area of a sample is measured by physical adsorption of gas on a solid surface, and is calculated by determining the amount of adsorbed gas equivalent to a monolayer on the surface. Typically, the measurement is performed at the boiling point of liquid nitrogen, and the amount of adsorbed gas is measured by a dynamic flow method or a volumetric method.
Based on the relationship between the gas adsorption amount and the relative pressure, the specific surface area (BET specific surface area) is calculated from the following BET equations (1) and (2).

Here, the elements in the above formula are as follows:
P: Measurement pressure [Pa]
P ₀ : Saturation vapor pressure of adsorbed gas [Pa]
P/P ₀ : Relative pressure of adsorbed gas [-]
Q: Adsorption amount [mol/g]
Qm: Monolayer adsorption amount [mol/g]
C: BET parameters [-]
S: Specific surface area [ ^m2 /g]
s: Molecular occupied cross section [nm ² ]
(N ₂ : 0.162 nm ² , CO ₂ : 0.210 nm ² )
K _A : Avogadro constant [mol ^-1 ]

( _N2 Adsorption Method - Prior Art)
The inventors have conventionally used the _N2 adsorption method to measure the specific surface area of carbonaceous materials. Using a commercially available flow method specific surface area measuring device, _N2 was adsorbed as an adsorption gas at -196°C onto a carbonaceous material sample (0.25-0.5 mm) that had been pretreated (heated at 150°C for 4 hours while evacuating to a vacuum), and the amount of _N2 adsorption at a relative pressure of 0.3 was measured. Here, the relative pressure is P/ _P0 in the above formula (1).

( _CO2 adsorption method - the present invention)
The present inventors considered that when measuring the specific surface area of coal char having micropores, the specific surface area would be underestimated if _N2 was used as the adsorption gas, and that _CO2 would be more suitable.
In the present invention, the carbonaceous material for sintering is evaluated based on the value of the BET specific surface area calculated based on the measured value by the _CO2 adsorption method. Also, the carbonaceous material for sintering whose BET specific surface area measured and calculated by the _CO2 adsorption method is within a predetermined range is used as the solid fuel.

By using the _CO2 adsorption method, it becomes possible to express the extremely fine pores (micropores) of the carbonaceous material, which could not be measured by the conventional _N2 adsorption method, as an index called specific surface area. By using char with a BET specific surface area measured by the _CO2 adsorption method in an appropriate range as the carbonaceous material for sintering (fixed fuel), it becomes possible to maintain an appropriate combustion state in the sintering process, and it becomes possible to realize increased production of sintered ore (increased FFS, and maintained or slightly decreased yield) and low NOx emissions (low NOx conversion rate). In addition, char produced from coal with a Roga index of 10 or less as a raw material has particularly many developed micropores. Therefore, it is preferable to use the _CO2 adsorption method rather than the conventional _N2 adsorption method for measuring the BET specific surface area. The Roga index is calculated by the Roga test method specified in JIS-M8801.

When char having a medium BET specific surface area (200 m ² /g or less) measured by the CO ₂ adsorption method is used as the carbonaceous material for sintering, it is possible to increase the production of sintered ore, but it is not possible to achieve low NOx emissions.
Furthermore, when char with a high BET specific surface area (450 ^m2 /g or more) measured by the _CO2 adsorption method is used as the carbonaceous material for sintering, low NOx emissions can be achieved, but the FFS increases excessively, significantly reducing the yield, and it is not possible to increase sintered ore production.

The inventors produced several chars of different qualities using low-grade coals such as lignite, subbituminous coal, and high-volatility bituminous coal as raw materials, and measured the specific surface area using the _CO2 adsorption method according to the present invention. In addition, using the produced several chars, sintering tests were carried out using a pot test apparatus.
The following examples are provided to illustrate the details of the embodiments of the present invention.

<Char production>
(Char raw material)
Table 1 shows the properties of the coals used as raw materials in the production of char. Table 1 shows the proximate analysis values, elemental analysis values, and Roga index of the coals used as raw materials. Coal A is lignite, coals B and C are subbituminous coal, and coals D to G are bituminous coal.

(Char properties)
Table 2 shows the properties of chars (A1 to A3, B to G) produced from the coals A to G shown in Table 1 as raw materials, and the properties of conventional carbonaceous materials (coke breeze, anthracite) as reference examples. The leftmost column shows the correspondence with the experimental examples (see Table 5) described later.
For chars A1 to A3, B, C, D, E, F, and G, char was produced by filling the reactor with each carbonaceous raw material, and then heating the electric furnace outside the reactor to a predetermined pyrolysis temperature while circulating nitrogen gas. The pyrolysis temperature for the examples (chars A1, B to E) was 700°C, while the comparative examples (chars F, G, and A3) were pyrolyzed at a temperature exceeding 700°C. For char A2, char was produced by filling the reactor with coal A, and then heating the electric furnace outside the reactor to a predetermined pyrolysis temperature while circulating a mixed gas (N ₂ : 70%, CO ₂ : 30%). The volatile matter content of all the produced chars was less than 10%. It is possible to use char having a volatile matter content of 10% or more by lowering the production temperature. However, this is not preferable because, compared with conventional carbonaceous materials (coke powder, anthracite), a larger amount of volatile matter-derived components (tar components) will flow out unburned into the exhaust gas from the sintering machine, which may have an adverse effect on the exhaust gas blower and exhaust gas purification equipment (dust collector, desulfurization equipment, etc.).

The BET specific surface area (CO ₂ ) shown in Table 2 was measured using a commercially available constant volume adsorption measurement device (constant volume specific surface area measurement device) by adsorbing CO ₂ as the adsorption gas at 0°C on a carbonaceous material sample (0.25-0.5 mm) that had been pretreated in advance (heated at 150°C for 4 hours while evacuating to a vacuum) in a relative pressure range (ratio of adsorption equilibrium pressure to saturated vapor pressure) of 0.012 to _0.03 (in increments of 0.002). The rightmost column shows the BET specific surface area measured by the N ₂ adsorption method for reference.

<Sintering experiment>
A sintering pot test was conducted to simulate the sintering process of a packed bed of raw materials in an actual machine, and the effects of improving productivity and reducing NOx emissions in the sintering process were confirmed.
(Ingredients for pot testing)
Table 3 shows the raw material composition used as the base for the pot test (Reference Example 1 in Table 5).
In the case of Reference Example 1 serving as a base, as shown in Table 3 (and Reference Example 1 in Table 5), the raw materials excluding the return ore and the carbonaceous material were taken as 100 mass%, and the blending ratios of the return ore and the carbonaceous material (coke breeze in Table 2) were 15.0 mass% and 4.5 mass%, respectively, with the exception of these.
In the cases of Reference Example 2, Examples 1 to 6, and Comparative Examples 1 to 3, as shown in the upper part of Table 5 (type of carbonaceous material and blending amount of carbonaceous material), one of the chars (A1 to A3, B to G) and anthracite shown in Table 2 was used as the carbonaceous material. In these cases, the blending amount of carbonaceous material was determined so that the amount of fixed carbon input of the char or anthracite used was the same as that of the base (Reference Example 1) using 4.5 mass% (excluding numbers) of coke powder.

The particle size distribution of the carbonaceous materials used is shown in Table 4. The particle size distribution of the carbonaceous materials was the same for all experimental examples (Reference Examples 1 and 2, Examples 1 to 6, and Comparative Examples 1 to 3).
After mixing these blended raw materials, they were granulated for a specified time (5 minutes) using a drum type granulator with a target moisture content of 7.5% by mass to produce blended raw materials (sintered raw materials) for pot sintering tests.

(Pot test firing conditions and evaluation)
The raw materials were charged into a test pot having a diameter of 300 mm so that the layer thickness was 600 mm, and air was sucked downward at a pressure of 1530 kPa.

(Evaluation items for pot testing)
For each case, the combustion rate, product yield, productivity, NOx emissions, and NOx conversion were evaluated. The NOx emissions are the amount of NOx (NO+ _NO2 ) contained in the exhaust gas from the start of ignition to the end of sintering (burn-through), expressed as nitrogen weight (oxygen not included). The NOx conversion is the ratio, expressed as a percentage, of the amount of nitrogen detected as NOx in the exhaust gas to the amount of nitrogen contained in the carbonaceous material used in the pot test.

(Pot test results)
The results of the lower pan test are shown in Table 5.
As a result of the pot test, when chars with BET specific surface areas measured by the _CO2 adsorption method within the appropriate range (chars A1, A2, B, C, D, E) were used as carbonaceous materials (solid fuel), the productivity of sintered ore was improved and NOx emissions were significantly reduced compared to the reference examples (coke breeze, anthracite).
In the case of Comparative Example 1 (Char F) and Comparative Example 2 (Char G), which used char whose BET specific surface area was below the lower limit of the appropriate range, the productivity of sintered ore was higher than that of the Reference Example, but the NOx emissions were at the same level as the Reference Example. This is because the coals (Coal F, Coal G) that are the raw materials for Char F and Char G have a Roga index greater than 10 and have greater caking properties (softening and melting properties) than the other coals used this time, so fewer pores are formed during char production (carbonization), and as a result, the BET specific surface area was below the appropriate range.
Furthermore, in the case of Comparative Example 3 (Char A3), in which char having a BET specific surface area equal to or greater than the upper limit of the appropriate range was used, the NOx emissions were significantly reduced compared to the Reference Example, but the productivity of sintered ore was lower than that of the Reference Example.

Claims (3)

A method for evaluating a carbonaceous material used in a sintered ore manufacturing method, comprising: measuring a specific surface area of char used as the carbonaceous material using _CO2 as an adsorbed gas; and evaluating the suitability of the char as a solid fuel based on the specific surface area. The method for evaluating a ^carbonaceous material for sintering according to claim 1, characterized in that, in the evaluation of suitability, the char is evaluated as having suitability as a solid fuel when the specific surface area of the char is within a range of 200 to 450 m 2 /g. A method for producing sintered ore, characterized in that the suitability of char used as the carbonaceous material is evaluated by the method for evaluating carbonaceous material for sintering described in claim 1 or 2, and char having a specific surface area within a predetermined range is used as solid fuel.