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• Effect of the local microenvironment on the nitrogen transformation and NO evolution is revealed. • Targeted regulation of local microenvironment around fuel-N is realized by selective oxidation. • Attack of free radicals on nitrogen-containing heterocycles promotes the conversion of fuel-N to HCN. • PAA selective oxidation can reduce NO emission from air combustion by 31.72% – 34.30%. Developing a novel high-efficiency coal combustion technology with ultra-low NOx emission is urgently needed to sustain the good ecological environment. Here, the targeted regulation of local microenvironment around fuel-N, such as functional groups, radicals, molecular configurations, and reaction atmosphere, is realized by the selective oxidation. The molecular configurations, including pore networks and microcrystalline structure in coal, are well characterized through synchrotron radiation-induced SAXS (small angle X-ray scattering) and WAXS (wide angle X-ray scattering) simultaneously. Furthermore, by combining density functional theory (DFT) and experiments, the effects of the local microenvironment on the nitrogen transformation and NO evolution during the thermal conversion are focused on. The results indicate that for the PPA oxidation, the H radicals attack the adjacent carbon to pyrrole/pyridine nitrogen, promoting the conversion of fuel-N to HCN. On the other hand, for the H 2 O 2 oxidation, disrupting the π bond electron cloud by the C O and C = O on the ortho carbon of pyrrole/pyridine dominates the NH 3 generation. Additionally, the increased L a (average graphene layer extent), a 3 (average interlayer spacing), σ 3 (standard deviation of interlayer spacing) and σ 1 (standard deviation of the first-neighbor distribution) induce massive smaller pores, promoting the generation of abundant reaction defects inside the particles. Importantly, the intensified adsorption on abundant active sites lead to the decreased HCN and increased NH 3 evolution, which is adverse for the interaction between homogeneous and heterogeneous NO reduction. Interestingly, the PAA selective oxidation can reduce NO emission by 31.72 % - 34.30 % during the air combustion, which is far better than the H 2 O 2 oxidation. Overall, the attack of free radicals on nitrogen-containing heterocycles promotes the conversion of fuel-N to HCN, the adsorption of which on char surfaces can further enhance the heterogeneous reduction in a lean oxygen atmosphere. The work here provides a novel route for developing high-efficiency and low-NOx combustion technologies. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Proposed a model to calculate closed porosity, integrating synchrotron radiation SAXS and nitrogen adsorption. • Pore size distribution, transmittance, and closed porosity trends for NMG coal all pivot at a 20 μm turning point. • Decreasing coal particle size shifts mechanical impact from pore destruction to pore formation. • Closed pores, categorized into dislocation, pressed, and plugged types, exhibit varying resistances to mechanical forces. Superfine pulverized coal exhibits extraordinary properties in thermal conversion processes like pyrolysis and combustion. The exploration of closed pores is of great significance for studying of the macromolecular structure, and developing advanced clean combustion technologies. In this work, the influences of coal rank and particle size on the mesopore characteristics (i.e., pore size, volume, and specific surface area), especially closed pores, were obtained by combining nitrogen adsorption and synchrotron-based small-angle X-ray radiation (SAXS). In addition, combined with the closed pore volume measured by He density and SAXS, research focuses on the formation and opening mechanisms of closed pores in different rank coals at a molecular level. The results indicate that both the bituminous (NMG) and anthracite coal (HN) have a few large open pores (>100 nm), with large amount of small mesopores and micropores (<2 nm). The HN coal has more open mesopores which have a smaller pore size than NMG coal. The closed pore proportion of HN coal in the studied pore size range is between 2 and 52 %, while that of NMG coal is between 47 and 67 %. In addition, both coals have a certain amount of closed pores formed by shear force and ash blockage, while the matrix compression also contribute to certain types of closed pores in NMG. These findings contribute to illustrating the closed pore structure in coal at the molecular level, which sheds light on constructing comprehensive macromolecular pore models of coal and thus promoting the development of clean coal utilization technologies. [ABSTRACT FROM AUTHOR]

[Display omitted] • The sulfur transformations were quantitatively characterized combined XANES and XPS. • The mechanochemical activation and H 2 O 2 oxidation was employed for coal modification. • The synergistic mechanisms on the sulfur removal were elucidated. • The optimal particle size for the SO 2 emission was obtained. • A new desulfurization method with the high commercial feasibility was proposed. Exploiting efficient and low-cost oxidative desulfurization methods is urgently needed for promoting the development of novel desulfurization technologies. In this work, the sulfur speciation in the mechanochemically activated and H 2 O 2 oxidized coals was quantitatively characterized through X-ray absorption near-edge structure (XANES) and X-ray photoelectron spectroscopy (XPS). The influences of H 2 O 2 concentration and particle size on the sulfur transformation were discussed. In addition, the synergistic mechanisms of mechanochemical activation and H 2 O 2 oxidation on the removal of organic sulfur were focused. Finally, combined with the sulfur speciation and deconvolution analysis, the main SO 2 precursors during the pyrolysis were elucidated. The results indicate that mechanochemical activation can promote the conversion from organic to inorganic sulfur by reducing the strength of C-S bond and accelerating the oxidation of thiophene and sulfide, which is beneficial to remove organic sulfur. On the other hand, the dissolution amount of sulfate in small particles decreases due to the decreased macropore volumes, complicated pore structure, and increased mass transfer resistance. Then, an optimal particle size around 20 μm (i.e., HN_23.9 and NMG_18.7) is observed for the studied coals, where the reduced amplitude of SO 2 after the chemical oxidation reaches the maximum. Besides, a dilute oxidant concentration is needed for low-rank coal during the chemical removal of sulfur, which is conducive to reduce the cost. The findings shed light on elucidating the origination of sulfur oxides from a molecular level and further exploiting the new efficient and low-cost desulfurization technologies. [ABSTRACT FROM AUTHOR]

[Display omitted] • SS 13C NMR is introduced into the research for all coal extraction products. • Carbon skeleton characteristics are quantitatively analyzed for all extraction products. • Influence of particle size on the evolution of coal microstructure during the extraction is focused on. • The work promotes the thorough elucidation on the carbon skeleton structure for all extraction products at a molecular level. To better understand the molecular structure of superfine pulverized coal and promote its practical utilization, the NMR (nuclear magnetic resonance) technology is adopted to study the structural information of all the products from the solvent extraction process, including raw coal, residue, and extract. The whole set of NMR spectra is resolved into different individual curves with specific linewidths, shapes, and intensities. According to the chemical shift, the carbon skeleton characteristics like the aromaticity, cluster size, and other structural lattice parameters are analyzed in detail. More importantly, the influences of coal maturity, coal size, and solvent on the carbon structural evolution during the extraction process are focused on. The results show that the aromatic ring stacking and aromatic ring exposure and fragmentation occur competitively during the superfine comminution, promoting the structural evolution of the coal. In addition, aliphatic carbon transfers more from raw coal into the extract than aromatic carbon in the whole extraction process, which is accompanied by the disappearance of carbonyl groups. Furthermore, the reduction of particle size can promote the extraction effect of the extractant on the coal powder, and reduce the proportion of aliphatic carbon in the residue. The work introduces the correlations among the raw coal, residue and extract from a carbon skeleton point of view, providing a reference for elucidating the solvent extraction mechanisms. The results shed light on better understanding the coal structure and its further molecular modeling. [ABSTRACT FROM AUTHOR]