Boron carbide (B4C) is one of the main products from the primary combustion of boron (B)-based propellants and has a significant influence on the secondary combustion of B. To systematically evaluate its effects on the secondary combustion of B, mixtures of B4C and B in different mass ratios were prepared. To study the ignition temperatures and combustion flames of the samples, a xenon lamp ignition experimental system and a flame shape test system were designed, respectively. A thermogravimetry–differential scanning calorimetry–Fourier transform infrared spectroscopy combined thermal analysis system was used to study the thermal oxidation characteristics and analyze the gaseous products of the samples. The results indicate that B4C reduces the heat absorption at the beginning of the ignition, but subsequently prevents the rapid rise of sample temperature. During the stable combustion stage, the maximum flame length under optical density 10−4 (OD4) filter was 20.4 mm, and the maximum flame length under 580 nm + OD4 filters (represents the combustion of B element) was 16.7 mm. The samples contained a small amount of HBO2 and H3BO3, which led to slight mass loss during the low temperature section of the thermal oxidation process. During the high temperature section, the oxidation of B and B4C caused considerable mass gain. The gaseous products of the thermal oxidation process include CO2, CO, and H2O. In general, the B content of 60% was the most beneficial to decrease the oxidation temperature, increase the combustion intensity, and improve the heat-releasing ability of the samples.

Particle size and oxygen content are two of the key factors that affect the ignition and combustion properties of aluminum particles. In this study, a laser ignition experimental system and flame test system were built to analyze the ignition and combustion characteristics and the flame morphology of aluminum particles. A thermobalance system was used to analyze the thermal oxidation characteristics. In addition, the microstructure of aluminum was analyzed by scanning electron microscopy. It was found that the oxidized products were some of the gas phase products agglomerated. Smaller particle size samples showed better combustion characteristics. The combustion intensity, self-sustaining combustion time and the burn-off rate showed a rising trend with the decrease in the particle size. Increasing the oxygen content in the atmosphere could improve the ignition and combustion characteristics of the samples. Four distinct stages were observed in the process of ignition and combustion. Small particle size samples had a larger flame height and luminance, and the self-sustaining combustion time was much longer. Three distinct stages were observed during the thermal oxidation process. The degree of oxidation for small-sized samples was significantly higher than that for the larger particle size samples. Moreover, it was observed that the higher the oxygen content, the higher the degree of oxidation was.

•The free and bound water in lignite were quantified before and after HTD.•The removal of free water was associated with the collapse of macropore.•The bound water was reduced because of the removal of oxygen functional groups.•DFT calculation on the interaction energy between lignite and water were carried out.High moisture content greatly restricts the large-scale utilization of low-rank coals (LRCs). Hydrothermal dewatering (HTD) is a promising technique for dewatering and upgrading LRCs. Chinese lignite from XiMeng Mine in Inner Mongolia was upgraded by HTD. The effects of pore structure and oxygen functional group content on moisture distribution were investigated. Thermogravimetric analysis, mercury intrusion porosimetry, and chemical titration were conducted to characterize the physicochemical properties of coal samples. Results show that a substantial amount of moisture was removed, the coal composition was modified, and the energy density was significantly improved after HTD. Both free-phase and bound-phase moisture in lignite were significantly removed. Free water removal was associated with the collapse of macropore structures caused by shrinkage forces. The bound water was remarkably reduced as a result of the removal of oxygen functional groups, such as phenolic hydroxyl and carboxyl. During HTD, the oxygen functional groups were decomposed, and the hydrogen bonding between water and hydrophilic sites was destroyed, thereby leading to a weakening of the water-holding capacity of lignite. Four representative model molecules with different polarities were used to study the intermolecular interactions between lignite and water. The interaction energies of model molecule⋯water complexes were determined by density functional theory (DFT) calculation. The types of non-covalent interactions of both hydrophobic and hydrophilic sites in lignite with water were vividly demonstrated using color-mapped reduced density gradient isosurface. The carboxyl and hydroxyl groups are extremely liable to interact with water via hydrogen bonding with large interaction energy and exhibiting strong hydrophilicity. On the other hand, the alkanes and benzene rings interact with water via van der Waals attractions and show hydrophobic effects.

The grinding characteristics of untreated and microwave-treated Ximeng lignites (XLs) were investigated, and the effects of microwave irradiation time, particle size range, and initial moisture were studied. The mass fraction and particle size distribution of the fine ground product with < 0.154 mm particle size range of untreated and microwave-treated XLs were obtained. According to the analysis results of proximate, scanning electron microscopy and nitrogen absorption, the dominant mechanisms that improved the grindability of XL with high moisture under microwave irradiation included the rapid removal of moisture within the XL and the destruction of the pore structure induced by the steam jet flow generated with moisture removal. Microwave irradiation could improve the grindability of XL with high moisture, thus increasing the breakage rate of microwave-treated XL and mass fraction of the fine ground product significantly. The particle size distributions of the fine ground products of untreated and microwave-treated XLs changed slightly. With increasing microwave irradiation time, initial moisture, and particle size range of XL, the increment in the grindability of microwave-treated XL increased. When XL was treated at low microwave power, the microwave irradiation time was extended properly to avoid the accumulation of moisture on the surface. Therefore, the grindability of microwave-treated XL improved significantly.

•D1 describes the surface roughness of meso- and macro-pores in coal.•D2 describes the volumetric roughness of fine mesopores in coal.•D2 increases with increasing specific surface area and total pore volume of coal.•The increase in D2 has a negative effect on the slurry ability of coal.•With increasing coal rank, D2 has a decreased trend.The fractal characteristics of pore structures in 13 different coal specimens were investigated. Insights into the relationship among fractal dimension, pore structure parameter, and slurry ability of coal were provided. N2 adsorption/desorption at 77 K was applied to analyze the pore structure of coal. Two fractal dimensions, D1 and D2, at relative pressures of 0 to 0.45 and 0.45 to 1, respectively, were calculated with the fractal Frenkel–Halsey–Hill model. Results reveal that the value of D1 is mainly affected by the influence of meso- and macro-pores with an average pore size range of 10 nm to 220 nm on the specific surface area; therefore, D1 can be utilized to quantitatively describe the surface roughness of these meso- and macro-pores in coal. Meanwhile, the value of D2 is mainly related to the effects of fine mesopores with an average pore size range of 2 nm to 10 nm on the total pore volume; therefore, D2 can be utilized to quantitatively describe the volumetric roughness of these mesopores in coal. D1 has no apparent linear correlation with the pore structure parameters and maximum solid loading of coal, and D2 has a positive linear correlation with the specific surface area and total pore volume of coal. The increase in specific surface area, total pore volume, and D2 has negative effects on the slurry ability of coal. High-rank coals with high ash content and low volatile matter relatively have higher D1 and lower D2. Meanwhile, with increasing coal rank, D2 has a decreased trend. The fine mesopores with an average pore size range of 2 nm to 10 nm in coal have direct effects on the pore structure parameters and D2 of coal; thus, the slurry ability of coal may be improved if the number of these mesopores in coal is reduced by modification processes, such as microwave irradiation, hydrothermal treatment and so on.

The pore characteristics and slurryability of two coal blends between Shigang anthracite coal and Huangling bituminous coal (SG/HL), and Guizhou anthracite coal and Xiaotun lean coal (GZ/XT), respectively, were investigated. The fractal dimensions of coal were calculated in the two regions of P/P0 < 0.45 and P/P0 > 0.45 and defined as D1 and D2, respectively. Upon an increase in the blending ratio of parent coal with smaller BET surface area (SBET) and total pore volume (TPV), the SBET and TPV of coal blends monotonously decreased. D1 was mainly related to the Smeso/macro(10–220 nm)/Stotal and mineral phase within coal while D2 was closely affected by the Vmeso(2–10 nm)/Vtotal. D1 of SG/HL coal blends had no apparent linear correlation with the pore structure parameters whereas D1 of GZ/XT coal blends changed linearly with the pore structure parameters. Both D2 of SG/HL coal blends and that of GZ/XT coal blends changed linearly with the pore structure parameters. The slurry quality of coal water slurry (CWS) prepared from coal blends is comprehensively affected by the physicochemical properties and blending ratio of parent coals. Therefore, the maximum solid loading (MSL) and water separation ratio (WSR) of CWS prepared from coal blends do not always change linearly with the blending ratio of parent coal.

The high moisture content of low-rank coals (LRCs) is associated with the noncovalent interactions between coal and water, such as hydrogen bonding and van der Waals attraction. In this study, the molecular model of lignite was constructed and its electrostatic potential (ESP) on a van der Waals surface was analyzed. The mechanism of water absorption on the hydrophilic or hydrophobic sites of the lignite surface was investigated based on the atoms in molecules analysis and reduced density gradient analysis of seven typical lignite···water complexes. The regions with the most negative and positive ESP values were associated with oxygen in oxygen functional groups, and hydrogen was associated with smaller electronegativity. The typical hydrogen bonds O–H···O were formed between water and oxygen functional groups, whereas the weaker hydrogen bonds C–H···O were formed between water and the skeleton components of lignite, such as the benzene ring and methyl and methylene groups. Oxygen functional groups were revealed to exhibit more hydrophilic than skeleton structures of lignite, including benzene rings and aliphatic chains. The hydrophilicity sequences for the different oxygen functional groups in the lignite model were determined as carboxyl > phenolic hydroxyl > carbonyl > alcoholic hydroxyl > ether. Furthermore, the carbon bond C···O was also observed in the lignite···water complex via van der Waals interactions, exhibiting a hydrophobic effect. The color-mapped reduced density gradient isosurface accurately demonstrated the noncovalent interactions of lignite···water complexes.

•NH4ClO4, KNO3, KClO4 and HMX coated B were used to prepare propellant samples.•FTIR, XRD and SEM were used for the microstructure analysis of the prepared B.•Thermal oxidation and combustion characteristics of the propellants were studied.•HMX coating was the most beneficial to the energy release of the samples.The energy release properties of a propellant can be improved by coating boron (B) particles with oxidants. In the study, B was coated with four different oxidants, namely, NH4ClO4, KNO3, LiClO4, and cyclotetramethylenetetranitramine (HMX), and the corresponding propellant samples were prepared. First, the structural and morphological analyses of the pretreated B were carried out. Then, the thermal analysis and laser ignition experiments of the propellant samples were carried out. Coating with NH4ClO4 showed a better performance than mechanical mixing with the same component. Coating with KNO3 efficiently improved the ignition characteristics of the samples. Coating with LiClO4 was the most beneficial in reducing the degree of difficulty of B oxidation. Coating with HMX was the most beneficial in the heat release of the samples. The KNO3-coated sample had a very high combustion intensity in the beginning, but then it rapidly became weak. Large amounts of sparks were ejected during the combustion of the LiClO4-coated sample. The HMX-coated sample had the longest self-sustaining combustion time (4332 ms) and the highest average combustion temperature (1163.92 °C).

•The oxygen functional groups in lignite were quantified before and after HTD.•The simplified molecular model of lignite was constructed.•The ESP, bond order, BDE of lignite molecule were studied using DFT method.•The effect of oxygen functional groups on the slurryability of lignite was studied.Hydrothermal dewatering (HTD) is a promising method for in-depth upgrading of lignite because of its comprehensive modifications, including dehydration, deoxygenation, carbonization, and surface modification. The oxygen functional groups in XiMeng lignite before and after HTD was quantitatively determined using the 13C direct polarization/magic angle spinning technique. After HTD, the amounts of carboxyl, alcoholic hydroxyl, ether, and carbonyl groups decreased, whereas those of phenolic hydroxyl were unchanged. The simplified molecular model of lignite was constructed and the electrostatic potential (ESP), bond order, and bond dissociation enthalpy (BDE) of the lignite molecule were investigated using density functional theory. The oxygen functional groups contributed to regions with a large absolute ESP value. These regions also exhibited strong hydrophilicity because of the formation of hydrogen bonds with water. Bond order and BDE analyses are consistent with the experimental results. The hydrothermal treatment began with the cleavage of phOCH3, followed by the cleavage of CC bonds in carbonyl and carboxyl, and ended with the cleavage of the CO bonds in alcoholic hydroxyl (phCH2OH) and CH3OCH3. All these bonds had BDEs lower than 90 kcal/mol. The cleavage of CO bond in phenolic hydroxyl (phOH) was the most difficult because it has the highest BDE value of 113.4 kcal/mol. The raw coal had inferior slurryability with a solid concentration of 48.49%. After HTD at 300 °C, the surface property of lignite was significantly improved and the solid concentration of upgraded coal significantly increased to 59.14%.

The presence of sulfur in coals has raised serious environmental issues, which are obstacles to large-scale utilization of coals. Hydrothermal dewatering (HTD) is a promising upgrading method for low-rank coals (LRCs) to significantly remove oxygen-containing groups and irreversibly decrease the inherent moisture content. To uncouple the complex behavior of sulfur evolution during HTD processing of lignite and to elucidate the main mechanism, this research experimentally studied the characteristics of sulfur transformation in a Chinese lignite from Xiaolongtan coal mine during HTD upgrading. Results reveal that the HTD upgrading of raw coal within the temperature range from 200 to 300 °C can obtain a desirably upgraded coal with higher calorific value and lower inherent moisture. Compared with raw coal, organic sulfur content decreased significantly, whereas sulfate sulfur content gradually increased after HTD. X-ray photoelectron spectroscopy results showed that HTD promoted aliphatic sulfur decomposition and the release of sulfur-containing gases. The released gases, such as H2S, reacted with the organic matrix of coals to form thiophenic sulfur. As a result, thermally stable thiophenic sulfur increased with increasing HTD temperature. The increase of sulfate sulfur content after HTD was attributed to the release of SO2. The calculation of the mass balance on the sulfur revealed that the vast majority of sulfur remained in upgraded coals, and only a minimal amount was released into gaseous and liquid products. The sulfur-containing gases remarkably increased with increasing HTD temperature, whereas the sulfur in the wastewater decreased.

•Zeta potential of sludge is increased after ultrasonic treatment.•The hydrophilic groups of the sludge decrease with increasing ESpec.•The bound water initially decreases and then increases with increasing ESpec.•Ultrasonic pretreatment significantly improves the slurryability of the sludge.•Energy input of 40 kJ/g DS is the optimal value for improving sludge slurryability.The high water content and complex components of municipal waste sludge (or sludge) lead to increased difficulty and cost of handling sludge. Coal sludge slurry (CSS) technology blends sludge into coal water slurry to produce a new slurry fuel that can be combusted or gasified as a substitute for petroleum. Environmental problems caused by sludge can then be solved and combustible matter in sludge can be used. The poor slurryability of raw sludge, which resulted from its high water holding capacity, significantly increases the viscosity of CSS. In this study, sludge was pretreated by ultrasonic energy and then mixed with coal to prepare CSS. After ultrasonic pretreatment, sludge flocs were significantly disrupted and scattered, and their particle size greatly decreased. Thus, the water trapped by flocs was released. When the ultrasonicated sludge was used to prepare CSS, the released water acted as a lubricant to decrease friction and interaction among coal particles. When the specific energy input of ultrasonic increased from 0 to 30 with 75 kJ/g dry sludge at 190 W, the characteristic viscosity of CSS decreased by 27.54% and 41.04%, respectively. Ultrasonic significantly improved the slurryability of sludge, and thus, could enhance sludge disposal scale to a high level.

Municipal wastewater sludge (denoted as sludge hereafter) can be economically and effectively used by mixing it with coal to prepare slurry fuels. However, the abundant extracellular polymeric substances (EPSs) and strong water-holding capacity of raw sludge cause the viscosity of slurry fuel to increase remarkably, which is disadvantageous for industrial applications. The slurrying properties of sludge can be improved by alkaline treatment. This study examined the effects of calcium oxide (CaO) on EPSs, water-holding capacity, floc structure of sludge, and co-slurrying properties of sludge with coal. Results showed that, after CaO modification, the relative amount of EPSs with a number-average molecular weight (Mn) > 5000 Da noticeably decreased, whereas the relative amount of EPSs with Mn < 5000 Da increased. Moreover, the soluble chemical oxygen demand (SCOD) of sludge increased, and its volatile suspended solid (VSS) content decreased. Raw sludge had a SCOD of 16.2 mg of O2/g of dry sludge (DS) and VSS of 58.19%. After treatment with 0.2 g of CaO/g of DS for 24 h, SCOD increased to 153.6 mg of O2/g of DS, VSS decreased to 45.61%, the water-holding capacity of the sludge decreased, and the saturated water content of raw sludge decreased from 84.71 to 79.10%. The characteristic viscosity ηc of the raw sludge–coal slurry was 1635.3 mPa s, which decreased to 1082.2 mPa s after treatment with 0.2 g of CaO/g of DS for 24 h. These findings strongly suggested that sludge solubilization can help improve slurrying.

•Molded B were ignited by a pressurized concentrated ignition experimental system.•Condensed combustion products were analyzed by SEM, EDS, and XRD.•Complete oxidation rates of the samples were detected using ICP.Ignition and combustion characteristics of amorphous boron (B) have received much attention from researchers in recent decades. A pressurized concentrated ignition experimental system was designed to evaluate the ignition and combustion characteristics of molded B samples. The ignition experiments were carried out under different oxygen pressures (1–9 atm). The condensed combustion products were then analyzed using a scanning electron microscope, an X-ray energy dispersive spectrometer, and an X-ray diffractometer. Furthermore, the complete oxidation rates of the samples were detected by inductively coupled plasma chromatography. As the oxygen pressure increased, the combustion intensity of the samples steadily increased, and the ignition delay time and combustion time both decreased. Under the oxygen pressure of 9 atm, the average ignition delay time and combustion time were 2640 ms and 2596 ms, respectively, and the highest combustion temperature reached 1561.5 °C. The initial diffusion flame on the sample surface was green and the brightest, which was produced by an intermediate combustion product, BO2 (corresponding molecular emission spectrum wavelength, 547.3 nm). Emission spectra of another intermediate product, BO (431.9 nm) was also detected. Two different types of structures were found in the condensed combustion products of the samples. The first type was the flaky B2O3 structure, and the second type was the flocculent structure of incomplete combustion products. The B2O3 content in the condensed combustion products increased with the oxygen pressure during combustion. The complete oxidation ratio of the samples also increased with the oxygen pressure, and reached the maximum value of 68.71% under 9 atm. Overall, the samples showed better ignition and combustion characteristics under higher oxygen pressure.

•Effect of microwave on spontaneous combustion propensity of lignite is investigated.•Lignite is upgraded after microwave irradiation.•Pore structures develop first and then decompose with increasing irradiation time.•Moisture and pore structure are dominant factors in spontaneous combustion propensity.•Spontaneous combustion propensity of lignite decreases after long-term irradiation.The propensity for spontaneous combustion of Inner Mongolia lignite irradiated in microwave irradiation was investigated. The functional groups on the lignite surface were obtained by FTIR and XPS spectra, and the pore structures of irradiated lignite were measured using N2 adsorption/desorption method. The propensity for spontaneous combustion of lignite was classified using the crossing-point method. The irradiation treatment in microwave resulted in a prominent decrease in moisture content, decomposition of oxygen-containing functional groups, breakage of long alkyl side-chains, and increase in aromatic carbons. The quality of lignite developed towards high-rank coals after microwave irradiation. Microwave irradiation showed significant effects on the pore structures of lignite; the specific surface area and total pore volume of the irradiated lignite initially increased and then significantly decreased. A short-term microwave irradiation resulted in an increase the spontaneous combustion propensity of lignite. However, when the irradiation time extended, the propensity for spontaneous combustion was reduced because of the significant change in chemical composition and drastic decomposition of the pore structures.

•Comparative investigation of the samples are conducted using laser ignition and TG.•SEM, EDS, XRD, and XPS are used together in the investigation.•CL-20 shows better combustion-supporting effect than KClO4.The microstructure of amorphous boron and the primary combustion products of boron-based fuel-rich propellant (hereafter referred to as primary combustion products) was analyzed by scanning electron microscope. Composition analysis of the primary combustion products was carried out by X-ray diffraction and X-ray photoelectron spectroscopy. The energy release properties of amorphous boron and the primary combustion products were comparatively studied by laser ignition experimental system and thermogravimetry–differential scanning calorimetry. The primary combustion products contain B, C, Mg, Al, B4C, B13C2, BN, B2O3, NH4Cl, H2O, and so on. The energy release properties of primary combustion products are different from amorphous boron, significantly. The full-time spectral intensity of primary combustion products at a wavelength of 580 nm is ~2% lower than that of amorphous boron. The maximum spectral intensity of the former at full wave is ~5% higher than that of the latter. The ignition delay time of primary combustion products is ~150 ms shorter than that of amorphous boron, and the self-sustaining combustion time of the former is ~200 ms longer than that of the latter. The thermal oxidation process of amorphous boron involves water evaporation (weight loss) and boron oxidation (weight gain). The thermal oxidation process of primary combustion products involves two additional steps: NH4Cl decomposition (weight loss) and carbon oxidation (weight loss). CL-20 shows better combustion-supporting effect than KClO4 in both the laser ignition experiments and the thermal oxidation experiments.

•Wasteliquid is used as a substitute for clean water to prepare coal slurry.•The combustion characteristics of CWLS are studied in a pilot scale furnace.•Wasteliquid enhances the slurryability of the coal and saves the additive agent.•Wasteliquid improves the ignition and combustion performances of coal slurry.•The metal ions in the wasteliquid reduce the SO2 and NOx emissions.The large amount of industrial wasteliquid generated during various industrial processes has raised serious environmental issues. A coal–wasteliquid slurry (CWLS) is proposed to dispose such wasteliquids, which are used as a substitute for clean water in the preparation of a coal-based slurry fuel. By the use of this method, a significant amount of clean water is conserved, and the environmental problems caused by wasteliquid discharge are resolved. However, the high content of organic matters, alkaline metal ions, and sulfur and nitro compounds considerably affects the slurrying, combustion, slagging, and pollution emission characteristics of CWLS. In this study, these characteristics are experimentally studied using a pilot-scale furnace. The results reveal that, compared with conventional coal–water slurry (CWS), CWLS exhibits a good performance with respect to slurrying, combustion, and pollution emission, i.e., low viscosity, rapid ignition, high flame temperature, high combustion efficiency, and low pollution emission. CWLS has a relatively low viscosity of 278 and 221  mPa⋅s and exhibits shear-thinning pseudoplastic behavior without the use of any additive agent. In contrast, CWS requires the use of an additive agent to achieve good fluidity, and its viscosity is 309  mPa⋅s. The maximum flame temperature of the two CWLSs (CWLS-A and CWLS-B) is 1309.0 and 1303.1 °C, respectively, and their respective combustion efficiency is 99.61% and 99.42%. The values of both these parameters are greater than those obtained in the case of CWS. However, the alkaline metal ions in the wasteliquid lead to a considerable slagging status. This status improves significantly after turning down the operating load.