2.1. Raw Materials
The metallurgical properties of coke at high temperatures can be measured by thermal simulation, but it is difficult to simulate the actual reaction conditions of a blast furnace in the laboratory due to the complex conditions such as high temperature, extrusion, and co-existing reduction and oxidation atmosphere in the blast furnace. In order to understand the performance of coke truly and effectively in the tuyere area of a blast furnace at about 1100 °C, an offline sampling method was used to obtain tuyere coke (TC) at various positions in the radial direction of the tuyere plane and corresponding batches of incoming coke (IC). The performance indexes of radial TC and IC were compared and analyzed, which can be helpful for the study of coke degradation in a blast furnace and the real-time analysis of furnace conditions.
The radial TC was fully representative of the research on the deterioration of tuyere plane coke, and the TC sampler was autocratic equipment for this purpose. Before sampling, the blast furnace duct of the tuyere was removed, and the sampling rod was pushed into the tuyere by a hydraulic device of the sampling machine itself. The sampling rod and the upper cover of the sampling rod always provided a high-pressure water flow for cooling throughout the sampling process. The sampling rod extended 6 m from the tuyere to the furnace core and was divided into 12 sections, each length 0.5 m. After the sampling rod was extended into the inside of the tuyere, the upper cover of the sampling rod was pulled out, and the coke in all parts of the tuyere plane fell into the sampling rod in a natural way. Then the sampling rod was pulled out, and the coke sample taken from the tuyere was segmented into the barrel and cooled by nitrogen. The coke used in this study was MG 2# furnace coke, and the hearth radius of the 2# blast furnace was 5.5 m.
Figure 1a,b show the schematic diagram of blast furnace tuyere sampling and the actual diagram of field tuyere sampling.
Under the influence of high-pressure air flow and burden drop, coke was distributed in different forms in the tuyere plane of the blast furnace. The coke taken by the sampling rod at each position of the blast furnace tuyere plane could be divided into four categories. First, the coke was not close to the edge of the furnace shell, which was mainly from the bosh coke that falls here when the blast furnace is off. Further to the direction of the furnace core was the coke that burns violently in the tuyere raceway. The coke in this part had small lumpiness and no edges and corners, and under the action of high temperature, the alkali content in the coke decreased. Then, deep into the furnace core, that is, the melting area, the coke was eroded by slag iron. Finally, it arrived at the furnace core, also known as the dead column area, where the temperature was low, and the coke pulverization was serious. The advantage of the sampling rod was that the coke samples taken included all types of coke in the tuyere area. Through studying and analyzing the metallurgical properties, the deterioration of coke in the tuyere plane of the blast furnace can be understood in detail, providing a theoretical basis for the smooth operation of the blast furnace.
According to GB/T2001-2013 “coke-determination of proximate analysis” [
23] and GB/T1574-2007 “test method for analysis of coal ash” [
24], the industrial characteristic indexes and components of coke were measured, respectively, and the results are shown in
Table 1 and
Table 2. Industrial analysis is a key index to analyze the characteristics of coke, and it is also the basis widely used to evaluate the properties of coke [
25]. At present, the basic properties of coke are mainly evaluated by measuring the industrial, elemental analysis, and cold and hot strength of coke. Among them, industrial analysis includes the calculation of ash (Ad), volatile matter (Vd), moisture, and fixed carbon (FC
d) content of coke, and according to the results of industrial analysis, can preliminarily judge the nature of coke, such as fixed carbon content can determine the skeleton of coke. Reactivity (CRI) and post-reaction strength (CSR) are indicators reflecting the thermal stress and mechanical force that coke can withstand under high-temperature conditions. The basic property parameters of TC and IC involved in this study are shown in
Table 1. By comparing the data in
Table 1, it can be seen that the ash content (Ad) of IC was lower than that of TC, indicating that coke fell from the top of the furnace to the tuyere area, and the carbon content in coke gradually reduced through carbon melting reaction, but the ash content was relatively increased. The fixed carbon content (FC
d) of IC was higher than that of TC, indicating that TC’s skeleton function in the blast furnace decreased. The reactivity (CRI) of TC increased, and post-reaction strength (CSR) decreased because after TC participates in the carbon melting reaction in the furnace, the surface pores expand and the reaction area increases, so the reactivity is high.
The ash content of coke directly affects the amount of blast furnace slag, and too high an ash content will affect the reactivity and thermal properties and reduce the strength of coke [
26]. For example, the defects of coke structure increase, and the differences of pore wall structure also increase. The Na and K alkali metals in the ash can make CO
2 in the blast furnace adhere to the coke surface and generate carbon and alkali compounds through a catalytic reaction, thus promoting a coke melting loss reaction at high temperature.
Table 2 shows the ash content, and the ash composition of IC and TC determined through the X-ray fluorescence (XRF) method. From the variation trend of ash content, the ash content of TC in all radial positions was higher than that of IC, and along the direction of tuyere pointing to the furnace core, the ash content decreased first and then increased. From the furnace to the tuyere, the carbon content in the coke was reduced by the melting loss reaction, and the ash content was relatively increased. In addition, when flux and sinter were inside the blast furnace, their powder was adsorbed on the coke surface, so it was difficult to separate them when preparing samples, resulting in varying degrees of increase in ash content. At the same time, the SiO
2 content in the ash of TC was lower than that of IC, mainly because the SiO
2 attached to the coke surface reacted with C during the melting loss reaction of TC, and the reduced Si quickly melted into the molten iron, becoming the main source of Si in molten iron.
2.2. Research Methods
The specific surface area and micropore parameters of IC and TC were measured by the accelerated surface area, and a porosimetry system with nitrogen as the adsorbent (BET, ASAP 2460, Micromeritics, Norcross, GA, USA) to compare the pore changes of coke before and after a series of reactions in the blast furnace, and the same method was used to analyze the coke after simulating high-temperature melting loss. The X-ray fluorescence (XRF, AXIOS, PANalytical B.V., Almelo, Netherlands) was used to identify the chemical composition of IC and TC. To intuitively understand the gasification performance of IC and TC, the corresponding thermal gravimetric and differential thermal analysis (TG-DTA) curves were measured with a thermogravimetric analyzer (STA 449C, Netzsch, Selb, Germany) under the constant heating rate of 10 °C/min. The microcrystalline structure of coke was determined by an X-ray diffractometer (XRD, D8 ADVANCE, Bruker, Karlsruhe, Germany). According to the data measured by XRD, the lattice parameters of coke samples were accurately deduced. The average distance d
(002) between aromatic layers in the cell was calculated by the Bragg equation, and the microcrystalline size was characterized by stacking height (Lc) and diameter (La) of the aromatic lamellar surface (002), which are generally calculated using Scherer equation, as shown in Equations (1)–(3). In addition, the reactivity (CRI) and post-reaction strength (CSR) of coke were calculated from Equations (4) and (5).
where
λ represents the wavelength of X-ray (
λ = 0.15406 nm).
θ(002) and
θ(100) represent the diffraction angles (°) of the corresponding diffraction peaks (002) and (100).
β(002) and
β(100) represent the half-height width (°) of the corresponding planes (002) and (100). The
k1 and
k2 are correction coefficients (
k1 = 1.84,
k2 = 0.94). The
m and
m1 respectively represent the mass (g) of coke before and after the reaction, and
m2 represents the mass (g) of coke with a diameter greater than 10 mm after the drum.
The corresponding spectra of coke before and after high-temperature melting loss reaction were measured by Raman spectrometer (inVia, Renishaw, Wotton-under-Edge, UK), and the peak shape of the spectrum was fitted by origin software to study the change law of the coke phase structure. A Fourier transform infrared spectrometer (FTIR, Nicolet6700, Thermo Fisher, Waltham, MA, USA) was used to measure the structural changes in coke functional groups under different reaction conditions to analyze the internal molecular structure of coke. The micromorphology and element types of coke before and after reaction were observed by a scanning electron microscope with energy disperse spectroscopy (SEM-EDS, JSM-6510LV, JEOL, Akishima, Tokyo, Japan).