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Understanding the changes in carbon structure during the mid–low-temperature pyrolysis of low-rank coal is important for efficient utilization. Raman spectroscopy is commonly used to analyze the structural order of carbonaceous materials, but traditional methods may overlook the heterogeneity of coal/char. This research explores the heterogeneity of char structure derived from low-rank coal at 700 °C through multi-point micro-Raman analysis. The analysis of parameters such as area (A), intensity (I), full width at half maximum (FWHM/W), and peak position (P) reveals that the carbon structure becomes less ordered as coal transforms into char due to the deposition of small molecules on the surface. The study emphasizes the benefits of multi-point detection for gaining in-depth insights into the structural evolution of carbonaceous materials. The increased standard deviation of Raman parameters indicates diverse structural characteristics resulting from pyrolysis at this temperature, which traditional methods may not capture effectively. The mapping method used in this research visually illustrates the distribution of carbon structures in the region. [ABSTRACT FROM AUTHOR]

The composition and microstructure of charging coke and raceway coke (R-coke) of a blast furnace were studied by various detection methods. The gasification reactivity and reaction activation energy of the two samples were compared by thermogravimetric method under non isothermal and isothermal conditions. The results show that the R-coke level spacing d002 is slightly smaller than that of the coke, and the graphite layer accumulation height Lc and graphite layer size are significantly higher than that of the coke. The coke undergoes chemical erosion and physical wear in the blast furnace with microscopic pore development and increased surface structural irregularity. The high overlap of alkali metal K and Na with Si and Al elemental distribution positions on the surface of R-coke indicates the generation of complex compounds. In the non-isothermal gasification process, the rate-controlling of the gasification process gradually shifts from chemical reaction to diffusion as the conversion of the sample increases. Diffusion becomes the only limiting link of the reaction during isothermal gasification, and the pore structure is the main factor affecting the gasification rate. Since the pore structure of R-coke is more developed than that of coke, the activation energy of the reaction is lower, which is conducive to the diffusion of CO2 molecules to the reaction interface and the diffusion of the product CO to distant places, thus making the reaction rate of R-coke higher than that of coke. [ABSTRACT FROM AUTHOR]

Highlights: Uncertainty in the one-time measurement for chars by Raman spectroscopy technology is evident. Influence of char particle size and objective lens magnification on Raman characteristic parameters was investigated. The number of measurements required to obtain stable characteristic parameters is related to the micro-zone properties of the char surface. To address the problem of variability in in situ Raman spectroscopy for determining the chemical structure of chars, the rationality of Raman's original peak spectrum fitting method, the influence of objective lens magnification, particle size, and number of measurements on peak shape and characteristic parameters were investigated. The results show that the Raman original peak spectrum of char is fitted by five peaks and the goodness of fit is best when the D3 peak is fitted by a Gaussian curve. The intensity of the peak spectrum and stability of the characteristic parameters of the Raman spectroscopy are related to the objective lens magnification. For a relatively large objective lens magnification, the Raman peak spectrum intensity is increased and the coefficient of variation in the characteristic parameters is accordingly reduced. As particle size increases, the characteristic parameters AG/AAll that characterize the perfect graphite structure decrease and the characteristic parameters AD1/AAll, AD3/AAll, and AD1/AG that characterize the defective graphite structure increase. The coefficient of variation for the characteristic parameters is found to be the smallest for the particle size range of 81–96 μm. Micro-Raman imaging reveals that the microscopic characteristic parameters of different micro zones are significantly different and the number of measurements required to achieve reliable Raman characteristic parameters is related to this anisotropy. [ABSTRACT FROM AUTHOR]

The H 2 atmosphere exerts an important influence on both the pyrolysis behavior of low-rank coal and the efficient application of the resultant char. Thermogravimetry-mass spectrometry in conjunction with the Fourier transform infrared was applied to examine the pyrolytic properties of low-rank coal in an H 2 -containing atmosphere. The coal chars' carbon chemical structure, surface morphology, pore structure, and combustion reactivity were subjected to characterization by Raman spectroscopy, scanning electron microscopy, CO 2 adsorption, and thermal analysis, respectively. The results indicate that the H 2 -atmosphere enhanced the volatiles release during the pyrolysis stage of low-rank coal. H 2 -containing atmosphere accelerated the synthesis of H 2 O through the transformation of the hydroxyl group in coal during coal pyrolysis; the H 2 and CH 4 production was increased through the chemical interaction of active hydrogen group with aromatic or aliphatic C–H groups; CO 2 and CO evolution reduced owing to the dissociation inhibition of carbonyl groups and functional groups containing oxygen. H 2 -containing atmosphere enhanced the order of carbon structure (as the proportion (20–60%) of H 2 contained in the pyrolysis atmosphere increases, the I G / I All (12.72, 13.14, and 14.03%, respectively) of char-H 2 increased), which was not conducive to the pore structure (the specific surface area are 373.7, 364.6, and 357.9 m2·g−1, respectively) development of char-H 2. Lower volatile content as well as ordered carbon structure reduced the combustion reactivity of chars-H 2 (S are 2.27 and 2.04, respectively). Similarly, lesser development of pore structure also reduced the combustion reactivity of chars-H 2 (S is 1.81). Char-H 2 (pyrolysis atmosphere with H 2 ratio of 20, 40%) meets the grindability index requirements of blast furnace coal injection and char-H 2 (pyrolysis atmosphere with H 2 ratio of 60%) meets the strength requirements of ferroalloy smelting. • The pyrolysis mechanism of low-rank coal in an H 2 -containing atmosphere was explained. • The release of volatiles of low-rank coal was promoted in an H 2 -containing atmosphere. • H 2 -containing atmosphere promoted the order degree and inhibited pore expansion of char. • The reactivity of char prepared in an H 2 -containing pyrolysis atmosphere was decreased. • The grindability and strength of char were affected by an H 2 -containing atmosphere. [ABSTRACT FROM AUTHOR]

It is of great theoretical significance and application value to study the influence of particle size difference of low‐rank coal on the structure of char and establish a relationship to regulate the pyrolysis process and performance of char. The structure changes of char prepared from raw coal with different particle sizes were analyzed by N2 adsorption, Raman, X‐ray diffraction (XRD), and Fourier transform infrared (FTIR). The results show that with the increase of pyrolysis temperature (500°C–700°C), the specific surface area of char first increases and then decreases and reaches the maximum value at 650°C. When the particle size of raw coal increases gradually, the process of volatile matter removal is limited by mass transfer, and the diffusion pore expansion makes the specific surface area and pore volume of char increase rapidly, and the char degree of ordering increases. The central sample of char prepared from raw coal with the same particle size has a larger specific surface area than the edge sample, which may be due to the restriction of mass transfer resistance in the release process of volatiles, resulting in a larger specific surface area of the central sample and a lower ordering degree of the central sample. The sample with pyrolysis temperature of 550°C was selected for comparison, and its structural parameters were inconsistent, because the volatiles were not removed at this temperature, which had a great impact on the structural evolution. [ABSTRACT FROM AUTHOR]

Utilizing pyrolysis chars as the injection fuel in a blast furnace can reduce the dependence on high-quality pulverized coal injection (PCI) coal. In this study, the physical properties and reaction characteristics of chars prepared from two different industrial-scale pyrolyzers (fluidized bed and moving bed) are investigated. The results reveal remarkable component segregation in the char particles with different particle sizes. The properties of large-size char particles are found to better satisfy the PCI fuel requirement in a blast furnace. Compared with PCI coal, the char particles exhibit a rough surface, well-developed pore structure, and less-ordered chemical structure of carbon. Results from isothermal combustion, isothermal gasification, and drop tube furnace experiments show that both types of chars exhibit a better reactivity than PCI coal. The excellent reactivity of chars is closely related to its high Brunauer–Emmett–Teller surface area and relatively poor chemical ordering during conversion. Based on these characteristics, both fluidized bed char (FBC) and moving bed char (MBC) have a great potential to be used as PCI fuels. The application method of chars for PCI in a blast furnace is proposed finally. [ABSTRACT FROM AUTHOR]

Abstract Pyrolytic chars are a potential high-quality fuel for use in iron ore sintering and pulverized coal injection (PCI) operations of blast furnace. In this study, industrial char produced from a moving-bed pyrolyzer was screened into four particle size ranges and compared with coke used in an industrial sintering process and a PCI coal used in an industrial blast furnace, and their basic compositions, micropore structures, functional group distributions, carbonaceous structures, reactivities, and gas release behaviors during combustion were analyzed. Furthermore, the mechanism underlying the particle size-dependent properties of the char was analyzed by comparing it with a char prepared under laboratory-simulation conditions. Results demonstrate that char particles larger and smaller than 3 mm (the turning point) have different structures and properties. More specifically, particles larger than 3 mm have stable properties, better developed pore structures, carbonaceous structures that are more ordered, superior reactivities, and lower releases of SO 2 and NO x. Therefore, they represent a desirable option for use in fueling sintering and PCI operations. Char particles smaller than 3 mm were further investigated by screening them into particle size ranges, and results showed that they had significantly varied compositions and structures, which further confirmed their unsuitability for use in the two operations. Finally, the particle-size-dependent properties of the char can be explained by the differing residence times, the radial variations in pyrolysis degrees, and the mechanical (or thermal) stresses experienced by the different raw coal particles within the pyrolyzer. Highlights • Char particles larger than 3 mm meet the requirements. • Char releases less gaseous pollutants during combustion. • The size-dependent properties are explained. [ABSTRACT FROM AUTHOR]

The effects of CaO, FeO, and MnO on the combustion reactivity of anthracite and coal char with low combustibility were investigated using thermogravimetric analysis (TG) and a drop tube furnace (DTF) under an oxygen-rich atmosphere. TG results showed that catalysts exhibited different effects on the combustion characteristics of fuels. For ignition temperature, it was found that, with increasing addition, the ignition temperature first decreased and then increased. For combustion rate, the positive effect of the catalyst on combustion rate was significantly enhanced with the increasing amount of additive. For burnout behaviour, at an equal additive dose, the effects of the three catalysts on burnout behaviour of two fuels varied greatly. The DTF results showed that the combustion reactivity of coal char was improved even more than that of anthracite. Eventually, the effects of three factors, including properties of the catalysts, fuel properties, and combustion conditions, on the catalytic combustion, were discussed. [ABSTRACT FROM AUTHOR]

• Char-N2 was more reactive than char−CO. • Char-N2 developed more pores for release of volatiles than char−CO. • Combustion activation energy was dominated first by volatiles, then by char structure. • Results will assist the development of efficient coal-char use in future. The combustion behaviors of chars derived from coal pyrolysis under a CO-containing atmosphere were analyzed by thermogravimetry, and the structural changes of char samples prepared under different pyrolysis conditions (temperature and atmosphere) were characterized by scanning electron microscopy, X-ray diffraction, and CO 2 adsorption methods. The apparent activation energies (Ea) of char-N 2 and char−CO under non-isothermal combustion conditions were obtained by the Flynn–Wall–Ozawa method. Char-N 2 exhibited higher combustion reactivity than char−CO, a more developed micropore structure, a more ordered carbon microcrystalline structure, and a higher Ea at the same pyrolysis temperature. In a CO-containing pyrolysis atmosphere, the carbon particles precipitated by the disproportionation reaction blocked the pores (inhibiting the escape of volatiles), reduced the number of active char sites, and increased the order of the carbon structure. The volatile content significantly influenced the Ea only during the initial conversion rate of char combustion. Based on the high reactivity of the disordered carbon structure, chemical reaction is the likely limiting factor of char-N 2 combustion, whereas diffusion should mainly limit char−CO combustion. [ABSTRACT FROM AUTHOR]

Co-injection of coal and hydrogen-rich gas with oxygen blast at tuyere is a promising technology for low carbon emission blast furnace ironmaking, which favors the development of hydrogen-based reduction and affordable carbon capture. Effects of hydrogen fraction in the co-injection gas on the combustion characteristics in raceway are numerically studied. Results show that increase of hydrogen fraction in the co-injection gas accelerates the preferential combustion of the injected gas, which promotes the preheating and pyrolysis of pulverized coal. This compensates the oxygen-grabbing effect and improves the burnout of coal from 83.2% to 86.8% with the hydrogen fraction in co-injection gas increase from 20 vol% to 80 vol%. With the CO/H 2 molar ratios in the co-injection gas of 8:2, 6:4, 4:6 and 2:8, the outlet CO/H 2 molar ratios are 54:46, 42:52, 32:68 and 24:76 respectively. Combustion, gasification and water-gas shift reactions play conjoint roles in the redistribution of reducing gas species. • EDC model is applied to consider gaseous reaction equilibrium. • The prediction accuracy of temperature fields is improved. • Increase of hydrogen fraction promotes the pulverized coal burnout. • Heat-release effect and oxygen-grabbing effect of gas preferential combustion are evaluated. [ABSTRACT FROM AUTHOR]

A low-nitrogen pyrolysis atmosphere plays a very important role in the clean and efficient application of low rank coal. The characteristics of low-rank coal pyrolyzed in a low-nitrogen atmosphere were studied using the thermogravimetry-mass spectrometry method. The chemical properties and physical structure of char samples produced by low-rank coal pyrolysis under N 2 , H 2 -containing, CH 4 -containing, CO-containing and a low-nitrogen atmosphere were investigated by Raman spectroscopy, X-ray diffraction, and N 2 adsorption. The results showed that low-rank coal pyrolysis under low-nitrogen atmosphere promoted weight loss (16.63% increase) compared with N 2 atmosphere, and the release of gas products H 2 , CH 4 and CO were increased. Physicochemical microstructure analysis showed that low-nitrogen pyrolysis atmosphere effectively promoted the coal molecules into dense, aromatic structures and forms regular and orderly carbon structures at a higher temperature (>450 °C). The ordered carbon structure proportion (A G /A All) of char prepared in low-nitrogen atmosphere (char-LN) was about three times that of char prepared in N 2 (char-N 2). Overall, low-nitrogen pyrolysis atmosphere reduced the combustion and CO 2 gasification reactivity of char. There was a good linear correlation between the volatile content and the combustion reactivity (R 2 = 0.98), and a good correlation between A G /A All and CO 2 gasification reactivity (R 2 = 0.97) of char-LN. [Display omitted] • The pyrolysis mechanism of low-rank coal in a low-nitrogen atmosphere was explained. • The release of volatiles of low-rank coal was promoted in a low-nitrogen atmosphere. • Low-nitrogen atmosphere promoted the order degree and inhibited pore expansion of char. • The volatile content and char structure were the key factors affecting the char reactivity. • The reactivity of char prepared in a low-nitrogen pyrolysis atmosphere was decreased. [ABSTRACT FROM AUTHOR]