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Coal combustion is the primary source of air pollution in China, especially the heavy metals (HMs) concentrated in the submicron particles. This paper discusses the enhancement of HMs absorption efficiencies during coal combustion experiments. By adding the modified Ca-based sorbents, the orthogonal design was used in this experiment. Combining with the analysis of variance and capturing rate of HMs, the optimal combination was arrived at. BP neural network model was established and trained using orthogonal capturing tests data to reach the best compositions. Confirmatory trial was conducted, thus the BP neural network model can be used to solve complex nonlinear optimization problems. [ABSTRACT FROM AUTHOR]

Understanding the structure of coal is crucial to explain its chemical reactivity. In order to study the microstructure characteristics of coal in-depth, ultimate analysis, solid-state 13C nuclear magnetic resonance (13C-NMR), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) are used to analyze the Datong coal. The results show that the aromaticity of Datong coal is 62.60%, and the ratio of aromatic bridge carbon to aromatic peripheral carbon is 0.238, indicating that more naphthalene and anthracene are in the aromatic structure of coal. Hydroxyl (C–O), carbonyl (C=O) and carboxyl (COO–) are the main oxygen-containing groups in coal, and the nitrogen element mainly exists in the form of pyrrole and pyridine. The main substitution modes on the benzene ring are di- and tri-substituted. Based on the above analysis, the 2D and 3D macromolecular models are constructed and optimized by using computer-aided modeling. The model is verified by 13C-NMR and FTIR spectra with density functional theory calculations. The coal molecular formula is defined as C 149 H 112 O 24 N 2. The reaction sites in coal structure are oxygen-containing functional groups and aliphatic structures (–CH, –CH 2). Therefore, the molecular structure of Datong coal has been systematically described and understood at the molecular level based on experimental data and quantum chemical calculations. [ABSTRACT FROM AUTHOR]

Coal plays an important role in the Chinese energy source. The heavy metals released from combustion process stay in the environment and are not easily degraded. This paper examines 8 coal samples from China. It explores the content, characteristics, transformation and control of heavy metals through the microwave digestion, sequential chemical extraction, static burning test and X-ray diffraction (XRD) methods. The zinc (Zn) and cadmium (Cd) in coal mainly exist in exchangeable, sulfide, and residual forms. The arsenic (As) content is mainly present in the sulfide binding state. For these three elements, degrees of organic speciation are not considered to be important and the residue speciation mainly occurs in coal ash. After coal combustion, the heavy metals in coal ash are relatively enriched, and the harm to the environment cannot be ignored. Most of the Cd, As and Zn residues are released into the atmosphere and their volatility rates can be ordered as follows: As > Zn > Cd. There are significant differences in mineral composition between the main phases and mineral carriers of heavy metals in coal and ash. Based on these differences, modified calcium carbonate (CaCO 3 ) is used as an additive. In orthogonal experiment, the optimal parameters for treating heavy metals are the temperature (1000 °C), metal salt (Al 2 (SO 4 ) 3 ), and ion ratio (γ = 15). Under this condition, the volatilization rates of heavy metals (As, Cd and Zn) decreased remarkably. [ABSTRACT FROM AUTHOR]

To study the effect of erosion on coal microstructure and its oxidation characteristics, eroded coals were prepared by eroding coal samples in an acidic aqueous solution for 30 days, 90 days and 180 days, respectively. The microcrystalline structure, pore structure and active groups distribution of coal samples were studied using X-ray diffractometer, low-temperature N 2 adsorption and infrared spectra. Temperature-programmed experiment was used to analyze the oxidation characteristics of raw coal and eroded coals. The results reflected that the number of aromatic layers and coalification degree of eroded coal are reduced compared with raw coal. The average pore diameter increases, which promotes oxygen transportation in coal pores. Under the erosion effects, the content of active groups such as hydroxyl (–OH), methylene (–CH 2), and methyl (–CH 3) in coal increases. Eroded coals, especially the coal eroded for 180-day demonstrate more oxygen consumption and CO production than raw coal during the oxidation process, and their apparent activation energies decrease. Besides, the enhancement mechanism of eroded coal's oxidation activity is expounded from the aspects of the increase of active sites and the improvement of pore structure. In brief, erosion effect increases the spontaneous combustion tendency of coal. • The effect of weak acid erosion with different durations on coal oxidation behavior was studied. • Changes of coal physicochemical structure after erosion effect were studied. • The apparent activation energy of eroded coal decreases during low-temperature oxidation. • Mechanism for easier oxidation of dried eroded coal was expounded. [ABSTRACT FROM AUTHOR]

The inhibitory effects of compound antioxidants (l -ascorbic acid (VC) and tea polyphenols (TP)) on coal oxidation were studied. The characteristics of coal oxidation was investigated by temperature-programmed experiments. The changes of active groups in coal were confirmed by Fourier-transform infrared spectroscopy. Quantum chemical theory was used to explore the reaction mechanism. Experimental results showed that compound antioxidants exhibit significant synergistic inhibition effect on coal oxidation. The apparent activation energy of coal oxidation increases by 16.23 kJ/mol for the addition of 6 wt% compound antioxidants (TP:VC = 2:1). The content of aliphatic hydrocarbons, –OH, C O and COOH in the antioxidant-treated coal decreases, while that of ether bond and aromatic bond increases. H15 of VC or H48 of epigallocatechin gallate (the main component of TP) can readily scavenge oxygen-containing radicals and inhibit the production of active groups in coal. Water-soluble antioxidants can be evenly dispersed on the coal surface, which provides an environment-friendly and efficient approach to preventing coal spontaneous combustion. • The mixture of VC and TP has a synergistic inhibitory effect on coal oxidation. • The content of active functional groups in coal treated with antioxidants is reduced. • The inhibitory mechanism of antioxidants on active radicals in coal is explored by quantum chemical theory. • Inhibitory effect is realized by eliminating oxygen-containing radicals and interrupting the generation of active groups. [ABSTRACT FROM AUTHOR]

• Release of CO and CO 2 at constant temperature with time follows two different rules. • C–H in side chains is oxidized into precursors of carbon oxides, C = O and –COO. • CO and CO 2 can be produced by stepwise decomposition at ambient temperature. • Production of CO and CO 2 by direct decomposition occurs only at high temperatures. Precursors of carbon oxides in coals are considered to be related to aldehyde group(–CHO) and carboxyl group (–COOH), but the way of decomposition of precursors to produce carbon oxides is not clear. Through theoretical analysis, experimental research and quantum chemical calculation at molecular scale, this paper explores the decomposition mode and products of carbon oxide precursors during coal spontaneous combustion (CSC). A programmed heating device was used to investigate the change rule of gas product concentration during the constant temperature oxidation of coals, and the microstructure of the coals was characterized by low-temperature nitrogen adsorption experiments (BET), X-ray Diffraction (XRD) and X-ray photoelectron spectrometer (XPS). The results of the study show that during coal oxidation, the inhomogeneous energy exchange between the coals and the external heat source results in the change of gas product concentration with time following two rules. BET experiments reveal that oxidation leads to the evolution of fine mesopores to macropores and an increase in the number of pores with diameters larger than 20 nm in the coals, which facilitates the transport of oxygen molecules through the pore structure. Within 200 °C, the microcrystalline structure of coals is basically unchanged, and the active structure C–C/C–H on the side chain is converted to –CHO and –COOH and then decomposed to produce carbon oxides. Quantum chemical calculation shows that the reaction energy barrier of carbonyl group and carboxyl group decomposed step by step to form carbon oxides is much smaller than that of direct decomposition, and carbon oxides and alkyl radicals can be produced even at room temperature by step decomposition. The research results provide theoretical guidance for the formation mechanism of carbon oxides during low temperature oxidation of coals. [ABSTRACT FROM AUTHOR]

An environment-friendly intumescent nanogel was developed to prevent spontaneous combustion in coal mines. Acrylamide (AM) was introduced into the lamellae of organic nano-montmorillonite (OMMT) with layered structure, and sodium lignosulfonate was introduced as a carbon source. Ammonium polyphosphate (APP), which integrates acid source and gas source, was added into gel carrier to form an intumescent nanogel (CFR). The effects of gel components on gel properties were optimized by orthogonal experiment using gelation time, yield and permeability as evaluation indexes. Scanning electron microscope (SEM) was used to observe the micro-morphology of the coal samples after calcination. Fourier transform infrared spectroscopy (FTIR) was employed to analyze the evolution of active radical groups. Characteristic temperature points of coal samples were analyzed by thermogravimetric experiments (TGA). The results showed that the calcined coal treated with CFR was covered with a uniform and dense intumescent char layer, and the carbon residue rate was more than 2.9 times that of the raw coal. The content of active carbonyl (C O), hydroxyl (-OH) and alkyl (-CH 3 , –CH 2) in coal samples was effectively reduced by the intumescent nanogel (CFR), and the content of ether bond (C–O–C) was increased. The characteristic temperature points of the coal samples were all increased to different degrees. Therefore, the intumescent nanogel (CFR) can effectively prevent the coal spontaneous combustion. • 1. The intumescent flame retardant is organically combined with the nanogel. • 2. The reduction of active groups in inhibited coal inhibits coal-oxygen chain reaction. • 3. The inhibitor forms a coating on the calcined coal, and a uniform char layer. • 4. The inhibitor can inhibit the whole process of spontaneous combustion of coal. [ABSTRACT FROM AUTHOR]