Coal Combustion Research Articles

Bioremediation of uranium enriched coal fly ash based on microbially induced calcite precipitation

Coal fly ash (CFA) is an essential raw material in brickmaking industry worldwide. There are some coal mines with a relatively high content of uranium (U) in the Xinjiang region of China that are yet understudied. The CFA from these coal mines poses substantial environmental risks due to the concentrated uranium amount after coal burning. In this paper, we demonstrated a calcifying ureolytic bacterium Halomonas sp. SBC20 for its biocementation of U in CFA based on microbially induced calcite precipitation (MICP). Rectangle-shaped CFA bricks were made from CFA using bacterial cells, and an electric testing machine tested their compressive strength. U distribution pattern and immobility against rainfall runoff were carefully examined by a five-stage U sequential extraction method and a leaching column test. The microstructural changes in CFA bricks were characterized by FTIR and SEM-EDS methods. The results showed that the compressive strength of CFA bricks after being cultivated by bacterial cells increased considerably compared to control specimens. U mobility was significantly decreased in the exchangeable fraction, while the U content was markedly increased in the carbonate-bound fraction after biocementation. Much less U was released in the leaching column test after the treatment with bacterial cells. The FTIR and SEM-EDX methods confirmed the formation of carbonate precipitates and the incorporation of U into the calcite surfaces, obstructing the release of U into the surrounding environments. The technology provides an effective and economical treatment of U-contaminated CFA, which comes from coal mines with high uranium content in the Xinjiang region, even globally.

The development of exothermic surface reaction between coal and oxygen affected by methane during coal spontaneous combustion in gob

Coal spontaneous combustion (CSC) derives from the exothermic coal-oxygen reaction occurring at coal's surface. However, methane's influence on this process is still unclear in gob environments. To this end, non-isothermal experiments were performed to elucidate methane's impact on the exothermic effect, pore structure evolvement, and surface composite variation in CSC. Our findings indicated that methane inhibited the exothermic effect; however, low methane (<42.9 %) did not significantly reduce the danger of CSC, but increased methane explosion risk. Methane's influence on pore structure can be elevated by a higher temperature, leading to larger pores, greater surface area, and higher pore volumes. X-ray photoelectron spectroscopic analysis revealed an increase in C1s, C-C and C-H bonds with rising methane and a decrease in O1s and oxygen-containing groups. C-O groups were the main structures affected by methane. O1s and oxygen-containing groups remained stable at low (<20 %) and high (>50 %) methane concentrations, because coal lay in oxygen-rich and fuel-rich oxidation states, respectively. Methane's influence on exothermic reaction originates from competitive adsorption. Methane competes with oxygen and other gases for adsorption sites. This changes the surface properties and affects the competitive adsorption between methane and other gases. This study can provide valuable insights for mitigating CSC risk.

Modeling, parameter estimation and optimization of fluidized bed-based alternative ironmaking process for CO2 emission reduction

This study presents the development of a comprehensive process model for simulating and optimizing the FINEX process consists of the multi-stage fluidized beds, the melter-gasifier unit, and a gas recycling system. The model is developed using Pyomo, a Python-based open-source software package that provides extensive capabilities for formulating, solving, and analyzing optimization models. It incorporates heat and mass balance equations and captures a wide range of chemical reactions, including the reduction of iron ore by hydrogen and carbon monoxide, the calcination of carbonate materials, the water–gas shift reaction, and coal gasification and combustion. Unknown parameters in the model, such as heat loss in each reactor, the extent of the calcination reaction, and the outlet gas temperature from the melter-gasifier, were estimated to calibrate the model. These parameters were estimated by solving an optimization problem that minimizes the gap between the model and real plant data. The optimized model was employed to investigate various scenarios for minimizing CO2 emissions in the FINEX process. This included assessing the impact of HBI charging rates, iron ore quality, and the integration of a CCUS unit. For each scenario, an optimization problem was formulated to minimize production costs across a range of CO2 tax levels. The optimal solutions revealed the relationships between process economics and CO2 emissions for each variable.

Preparation and flame retardant properties of new mining fireproof gel

Coal spontaneous combustion (CSC) brings great danger to mine safety. Although traditional colloidal fire-fighting materials can reside at high fire source points, their permeability and fluidity are poor, and such materials focus on their water absorption, but pay less attention to water retention and resistance, which makes their fire-fighting effect not good. This paper proposed a new type of mining fire prevention colloid material (CPS-α), which had good solid water gel formation, viscosity, fluidity, and blocking properties to compensate for traditional colloids' shortcomings. According to the single-factor experiment, polymeric polyacrylamide (PAM) as the base material, sodium carboxymethyl cellulose (CMC) as the coagulant, modified starch (SS) as the flame retardant and reduced ascorbic acid were determined. After gel treatment, compared with raw coal, the critical temperature point of spontaneous combustion of coal was increased by about 28 °C, the time to reach the critical temperature point was delayed by 190s, and the characteristic temperature points were significantly increased; the overlap area of coal and water increases by about 20 % after adding gel, and the change slope of the diffusion coefficient of water decreases, which indicated that CPS-α had good flame retardant effect.

Source apportionment of PM2.5 episodes in the Taichung metropolitan area, Taiwan

To analyze the physicochemical mechanisms affecting the variation in fine suspended particulate matter (PM2.5) concentrations in Taichung City, the largest city in central Taiwan (the second largest city in Taiwan), during a high-pollution event from November 3 to 6, 2021, we applied the sulfur tracking method (STM) and integrated source apportionment method (ISAM) of the WRF/CMAQ model to simulate the impacts of various emission sources. The sources of pollution in Taichung City are very similar, which shows that the impacts of point, line, and area sources should not be neglected in addition to the boundary conditions. SO42− is mainly generated from point emissions and the production of H2O2, Fe, Mn, and O3. NO3− is also mainly generated from point sources in Taichung City, with HNO3 being the main source at noon and ANO3 at other times of the day. NH4+ is mainly generated from area sources in Taichung City. OM is more complex, mainly originating from line sources in Taichung City and other sources, such as point/area emissions in Taichung City and other emissions from Changhua County. The most important mechanism is low-volatility/semivolatile oxidized combustion of OC at noon, followed by low-volatility/semivolatile POA, which is produced in the morning or evening. EC mainly originates from line sources in Taichung City and Changhua County. In other nearby counties, EC is dominated by local emission sources. In addition, when the concentration of PM2.5 is high, the Neutralization Ratio (NR) is high and PM2.5 is relatively neutral or slightly alkaline. On the contrary, when the concentration of PM2.5 is low, the NR is lower than 1 and the aerosol is acidic. Besides, this study used positive matrix factorization (PMF), which indicates that the PM2.5 at the UAPRS originated from eight kinds of pollution, namely, windblown dust, oil cracking, iron and steel industry, sea salt, transport, Cl− containing exhaust, biomass burning, fossil combustion containing abundant SO42− and the heavy oil refining and coal combustion industry. The direction of the source of pollution can be traced by a conditional bivariate probability function (CBPF). Overall, fossil fuel combustion, mainly involving sulfate, is the largest source of pollution, with the heavy oil refining and coal combustion industry contributing less, and the remaining factors contribute relatively evenly.

Role of coal ash morphology and composition in delivery and transport of trace metals in the aquatic environment

Fly ash is predominately the inorganic byproduct of coal combustion for electrical power generation. It is composed of aluminosilicates with Fe, Mg, K, and Ca forming submicron to 100 μm spheres and amorphous particles. During combustion trace elements are incorporated into the heterogenous fine particles that can pose risks to the environment and human health. This study combines optical, rock magnetic, and geochemical analyses of fly ash originating from Appalachian coal to develop an integrated suite of environmental coal ash tracers. The non-magnetic portion of power plant fly ash has higher abundance of clear spheres and clear amorphous particles, combined with enrichment of As, B, Th, Ba, Li, Se, Cd, Pb, and Tl. The magnetic fraction is enriched in opaque and orange spheres and Cu, U, V, Mo, Cr, Ni, and Co. Plerospheres occur in either fraction. We investigated ash-bearing fluvial sediment from Emory-Clinch River system that was impacted by the instantaneous TVA spill in 2008 and Hyco Lake in North Carolina that was contaminated by chronic releases of fly ash since 1964. Five years after the TVA spill, most ash in the riverbed reflects one population with trace element concentrations proportional to percent total ash. This relationship does not hold for As and Se, volatile elements associated with the outer surface of clear spheres, which are affected by river transport. At Hyco Lake, small clear and opaque spheres correlate with trace elements released from storage ponds. The combination of trace elements, fly ash morphology and rock magnetism provides a powerful set of tools to assess the distribution of ash and potential impact on the environment. We conclude that dispersal of fly ash to the aquatic environment, especially small clear and opaque spheres, should be avoided in favor of dry landfills.

Assessing the Potential of Rare Earth Elements in Bottom Ash from Coal Combustion in Poland.

The aim of the research was to assess the potential of bottom ash from Polish coal-fired power plants as an alternative source of rare earth elements (REY). The potential of these ashes was compared with fly ash from the same coal combustion cycle. The phase and chemical composition, as well as REY, were determined using: X-ray diffraction and inductively coupled plasma mass spectrometry. The tested ashes were classified as inert-low pozzolanic and inert-medium pozzolanic, as well as sialic and ferrosialic, with enrichment in detrital material. The phase and chemical composition of bottom ash was similar to fly ash from the same fuel combustion cycle. The REY content in the ash was 199-286 ppm and was lower than the average for global deposits, and the threshold value was considered profitable for recovery from coal. Bottom ash's importance as a potential source of REY will increase by recovering these metals from separated amorphous glass and mullite and grains rich in Al, Mg, K, and P. The industrial value of bottom ash as an alternative source of REY was similar to fly ash from the same fuel combustion cycle.

Chronological evaluation of polycyclic aromatic hydrocarbons in sediments of tangxun lake in central China and impacts of human activities.

This study sheds light on the contamination of polycyclic aromatic hydrocarbons (PAHs) in Tangxun Lake sediments, an urban lake reflecting environmental changes in Central China. By analyzing sediment cores from both the inner and outer areas of the lake, we determined the historical trends and sources of PAHs over the past century. The results reveal a significant increase in PAHs concentrations, particularly since the 1980s, coinciding with China's rapid urbanization and industrialization. Using diagnostic ratios and Absolute principal component score-multivariate linear regression (APCS-MLR) methods, we identified petroleum combustion, coal combustion, and biomass combustion as the primary sources of PAHs in the lake sediments. The spatial analysis indicates higher PAHs levels in the inner lake, likely due to its closer proximity to industrial activities. Moreover, by comparing PAH trends in Tangxun Lake with those in other urban, suburban, and remote lakes across China, based on data from 49 sedimentary cores, we highlight the impact of regional socio-economic dynamics on PAH deposition. These insights are crucial for developing effective pollution mitigation strategies and promoting sustainable development in rapidly urbanizing regions.

Facile and sustainable upcycling of fly ash into multifunctional durable superhydrophobic coatings

Fly ash (FA), a hazardous byproduct of coal combustion in power plants, poses significant environmental and health risks due to improper disposal and utilization. This study introduces a facile, sustainable, and cost-effective method for converting FA into a robust superhydrophobic material for various substrates. -FA particles are modified with polydopamine (PD) in water and covalently grafted with octadecylamine (ODA) via the Michael Addition-Schiff Base reactions, resulting in robust superhydrophobic FA (SH-FA) with a water contact angle (WCA) of 163° (±3.1). When applied as a coating to jute, cotton, polyester fibers, PU sponge, and wood, they became superhydrophobic, with WCAs ranging from 154.7 to 161.2° except for the wood substrate, which achieved a WCA of 132° (±3°). The coated polyester fabric exhibited remarkable durability, retaining consistent WCA values after 70 abrasion cycles, 75 adhesive tape peelings, and 20 detergent washing cycles. It also showcased excellent self-cleaning properties, effectively repelling dust and various liquids. Additionally, the coated PU sponge demonstrated exceptional performance in separating oil from different oil/water mixtures, achieving rapid separation of organic solvents within seconds and maintaining a separation efficiency of over 98% even after 12 reuse cycles. These results indicate the potential for transforming FA through effective management.

Synthesis and Characterization of Silicon Nanoparticles from Coal Fly Ash Using Ultrasonication as a Battery Anode

Fly ash, a byproduct of coal combustion, is rich in silica, alumina, and other minerals, making it a valuable resource for extracting high-purity silicon. The synthesis of silicon nanoparticles from coal fly ash involves several critical steps, including the extraction of silica (SiO2) via the sol-gel method, reduction of silica to silicon using the metallothermic method, and subsequent ultrasonication to achieve nanoscale particles. Studies have shown that fly ash can contain up to 49.21% silica, which can be further purified to 93.52% via chemical extraction methods such as acid leaching and alkali dissolution. The reduction of silica to silicon is carried out using the metallothermic method, which involves the use of magnesium-reducing agents to convert SiO2 to elemental silicon. This process produces silicon with a purity of about 61.3%, which can be further increased through ultrasonication. Ultrasonication is a technique that uses high-frequency sound waves to break particles into smaller sizes, resulting in more uniform and homogeneous nanoparticles. In this study, ultrasonication for 60 and 120 min reduced the average particle size of silicon from 208.94 nm to 58.87 nm and 20.13 nm, respectively, and increased the silicon content to 74.6% and 72.7%. X-ray diffraction (XRD) and distribution particle analyses confirmed the particle size reduction and homogeneity of silicon nanoparticles, indicating the effectiveness of ultrasonication in producing high-quality silicon nanoparticles. The synthesized silicon nanoparticles have significant potential applications, particularly as anode materials in lithium-ion batteries, due to their increased surface area and improved electrochemical properties. Furthermore, the use of fly ash as a raw material for the synthesis of silicon nanoparticles not only provides a cost-effective and environmentally friendly alternative to traditional silica sources but also helps in reducing the environmental impact of fly ash disposal. The integration of the methods and findings of this study underscores the feasibility and benefits of using coal fly ash for the sustainable production of silicon nanoparticles, which can be utilized in energy storage as anode materials in lithium-ion batteries.

Inhibitory Kinetics and Mechanism of the Low Temperature Oxidation of Coal by Polyethylene Glycol-Citric Acid

ABSTRACT To address the issues of low inhibition rate, high cost, and insufficient research on physicochemical synergistic inhibition mechanisms, this study examined the inhibitory effect and mechanism of Polyethylene Glycol-Citric Acid (PEG-CA) on low-temperature oxidation of coal. Differential scanning calorimetry (DSC) was used to analyze the inhibition and thermal behavior at various inhibitor ratios. Fourier transform infrared spectroscopy was used to experimentally assess changes in the functional groups of the coal samples pre- and post-inhibition. Principal component analysis identified the key mechanism of PEG-CA in inhibiting the low-temperature oxidation of coal. It showed that PEG-CA increased the heat absorption of the coal and delayed the average thermal equilibrium temperature of the coal oxidation process by more than 70°C. The addition of PEG-CA increased the maximum heat absorption rate of the coal sample by 2.4 times and the activation energy of coal oxidation by 1.6–3.9 times. The primary inhibition mechanism is that PEG-CA-forming chelates with the metal ions of coal, preventing C-H bond activation and reducing the content of this functional group. Additionally, PEG-CA etherifies with the hydroxyl groups of the coal, creating relatively stable ether bonds. This study offers a cost-effective and eco-friendly solution for the coal mining industry and provides a new theoretical foundation for developing composite corrosion inhibitors and controlling spontaneous coal combustion.

Magnetite nanoparticles from representative coal fired power plants in China: Dust removal capture and their final atmospheric emission

Information on the emission of coal combustion-sourced magnetite nanoparticles (MNPs) is lacking, which is critical for their health-related risks. In this study, MNPs in coal fly ashes (CFAs) from various coal-fired power plants (CFPPs) in China equipped with various dust removal devices were extracted and quantified using single particle ICP-MS. The number concentrations of MNPs in CFAs captured by dust removal increased with stage, while their size decreased. Among all the dust removal devices, electrostatic-fabric-integrated precipitators showed the best removal of MNPs. Furthermore, throughout all the coal combustion by-products in a typical CFPP, MNPs in EFA (fly ash escaped from the stack) showed the highest number concentration (1.2 × 107 particles/mg) and lowest size (78 nm). Although the mass of CFA escaping through the stack is extremely low, it still had an emission rate of 1.9 × 1015 particles/h, contributing 3.56 % of the total emissions of MNPs in number. In addition, the purity of MNPs and their associated toxic metals showed a size-dependent variation pattern. As the particle size of MNPs decreased, the proportion of Fe in MNPs increased from 43 % in bottom ash (BA) to 84 % in EFA, while the abundance of trace toxic metals in EFA was 3.3 times higher than that of BA. These MNPs with the highest purity can adsorb elevated concentrations of toxic metals, and can be discharged directly into the atmosphere, posing a risk of synergistic toxicity.

The influence of the composition and origin of ash and slag waste from coal generation on the response of organisms used in biotesting

Introduction. Different conditions of origin of ash and slag wastes has a significant impact on their final properties. Material and methods. The work is devoted to the search and establishment of dependencies of the influence of the origin of ash and slag waste on their chemical and biological parameters by a comprehensive assessment of the results of the impact of ash and slag waste on the vital signs of hydrobionts. Results. The values of the responses of hydrobionts for ash and slag wastes formed from coals of various origins were investigated. The component composition of aqueous extracts from the studied ash slag samples was evaluated. The dependences of the influence of the conditions of origin of ash and slag wastes on their final chemical and biological properties were established. Limitations. An assessment of the biological and chemical properties of ash and slag waste generated during the combustion of coal from six different deposits was carried out. The total volume was 95 samples. Conclusion. Ash and slag wastes formed from the burning of brown coals have the most pronounced toxic effect on hydrobionts, unlike ash and slag from coal.

Design method and experimental study on a novel self-sustaining internal combustion burner

Peak shaving represents a particular challenge with regard to coal-fired power plants, as methods to achieve stable, high-efficiency, and low-nitrogen-oxide combustion at ultralow loads remain unavailable. In this study, based on analysis of the ignition mechanism, heating mode, and theory of preheating method, we proposed a novel pulverized coal full-time self-sustaining internal combustion burner by combining the processes of plasma ignition, reverse jetting, and multi-stage ignition. A flame stability model for the burner's recirculation zone was established using the temperature change rate of the recirculation zone as a criterion. The ignition position of the stable self-sustained combustion of the pulverized coal gas stream was determined under various working conditions, and the length of the ignition core stage, a component of the burner inside which the pulverized coal gas stream achieves stable self-sustained combustion, was determined based on the calculated maximum ignition position; a self-sustaining internal combustion burner was then designed based on the ascertained length. Subsequently, we established an experimental system for the self-sustaining internal combustion burner and conducted tests in the load range of 25 %–100 %. Notably, the results of the pulverized coal self-sustained ignition position experiment were consistent with the computational results of the model. The self-sustaining internal combustion burner offers a quick start–stop feature, with the pulverized coal able to achieve stable self-sustained combustion after the plasma igniter shuts off. The resultant gas-phase products exiting the burner met the boiler's concentration requirements for enhanced nitrogen reduction, and the burner exhibited good anti-interference properties. The influence of the pulverized coal gas stream velocity and concentration on the self-sustained ignition was expounded. Under the experimental conditions, an optimal concentration range was ascertained for stable ignition of the pulverized coal gas stream, with the ignition position gradually advancing backward with increasing gas stream velocity. Together, our findings highlight the potential of the proposed burner design to support clean and efficient pulverized coal combustion in coal-fired power plants.

Pre-combustion mercury removal potential of rapid pyrolysis in high ash coal and mode of occurrence

Mercury is a widely known pollutant, and coal combustion is one of the largest sources of anthropogenic Hg emission. Mercury reduction technologies globally rely on flue gas treatment. However, pre-combustion techniques show immense potential. Mild pyrolysis of coal offers a promising approach for mitigating mercury emissions. This study investigated the mercury distribution and its mode of occurrence in six high ash coal samples from the Jagannath area of eastern India. The findings revealed that Hg content in coal samples ranged from 0.154-0.308 mg/kg. Majority of the mercury was bound to pyrites (60–90 %) and organic matter (5–15 %). Rapid pyrolysis at temperatures ranging from 200 to 700 °C demonstrated significant mercury removal efficiency (up to 95 %). In particular, at 400 °C with minimal loss in calorific value and mass. Though the high-temperature pyrolysis (600 °C) resulted in maximum mercury removal, but with a notable loss in calorific value and product yield. The Hg content in raw coal, initially ranging from 9 to 20 μg Hg/MJ, was reduced to 3 to 11 μg Hg/MJ in char produced at 400 °C. Further reduction was observed in char produced at 600 °C, where the Hg content decreased to between 2–3 μg Hg/MJ. This research emphasizes the effectiveness of mild pyrolysis in reducing mercury content of high ash coal, thereby facilitating the generation of cleaner energy.

Dual isotope analysis reveals the COVID-19 lockdown impact on nitrate aerosol sources and formation pathways in Shanghai

Nitrate (NO3−) is an important contributor to PM2.5 which can adversely affect the environment and human health. A noticeable decrease in NOx concentrations has been reported due to the lockdown measures implemented to curb the spread of Corona Virus Disease 2019 (COVID-19). However, questions remain, regarding the nonlinear relationship between NOx and NO3−. Here, we collected PM2.5 samples in two periods, before and during the lockdown of COVID-19 in Shanghai. Dual isotopes (δ18O-NO3− and δ15N-NO3−) of NO3− were measured to investigate the formation pathways and potential sources of NO3−. The results showed that the concentration of NO3− decreased significantly during the lockdown period compared to the period before the lockdown. Additionally, the hydroxyl pathway was the dominant contributor to NO3− production during the lockdown period, while N2O5 hydrolyses dominated the formation of NO3− before the lockdown. This change is largely attributable to alterations in the oxidative potential of the environment. In comparison to the period preceding the lockdown, the relative contributions of each NOx source remained largely unchanged throughout the lockdown periods. Nevertheless, the concentration of NO3− contributed by each NOx source exhibited a notable decline, particularly the mobile sources and coal combustion. Furthermore, the reduction extent of NO3− due to the lockdown period was also greater than the reduction during the Clean Air Actions (2013–2017). Our findings provide evidence that the COVID-19 lockdown led to a decrease in NO3− concentration due to changes in the formation pathway and reductions in NOx emissions from various sources.

Dynamic evolution of particulate and gaseous emissions for typical residential coal combustion

Residential coal combustion still accounts for half of the heating energy consumption in many developing countries. The dynamic variation during the combustion process importantly determines the combustion facility design and appropriate air quality assessment, which was omitted in conventional studies. This study investigated the emissions of particulate and gaseous pollutants during the combustion process for typical coal types using online monitoring. During the first pyrolysis stage with temperature climbing, the organic aerosols (OA) and gases reached peak concentration. The second fierce combustion stage had the highest temperature and produced the highest cumulative emissions, particularly a substantial amount of black carbon for coals with higher volatile content. Using higher-quality coals will undoubtedly reduce PM emissions, by a factor of 10 from bituminous to anthracite coal. However, more ultrafine particles (d < 0.1 μm) from cleaner coal may pose additional health risks. Anthracite and honeycomb coal had approximately twice the energy content and emitted more CO2 per unit mass of fuel and had more persistent SO2 emissions throughout the burnout stage. The oxygenation of OA and organic gases remained increased during combustion, suggesting the pyrolysis products underwent oxidation before being emitted. The investigation of the coal combustion process suggests the importance of reducing volatiles to control PM emissions, but the potential negative synergistic effects between PM reduction and increased carbon emissions should also be considered.

Unraveling the Role of Intrinsic Carbon Defects for the Heterogeneous Reduction of NO with NH3

ABSTRACT This study investigates the impact of intrinsic carbon defects on the reduction of nitrogen oxides (NOx) during coal combustion, with a focus on the synergistic effect of ammonia (NH3) and pulverized coal. Two representative computational models containing intrinsic carbon defects are described, and their influence on NH3 reduction of NO is studied. Stable species and transition states present in the pathways are calculated using B3LYP/6-31 G(d). Results indicate that exposed edge carbon atoms exhibit higher charge density distribution, affecting NH3 adsorption and NO reduction. The maximum energy barriers for both pathways do not exceed 45 kcal/mol, achievable in practical coal combustion systems. This paper also calculates the heterogeneous reduction process of a model without intrinsic carbon defects (Char model), with the aim of observing the specific effects of intrinsic carbon defects on the processes of migration of H/O atoms, migration and recombination of N atoms, and formation and desorption of N2 molecules. Furthermore, the analyses demonstrated an increase in the rate of heterogeneous reduction of NH3 and NO on defective carbon surfaces in comparison to the Char/NH/NO system, indicating that the intrinsic defects of the carbon have a contributory effect on the catalytic activity. With regard to the magnitude of the increase in reaction rate with increasing temperature, the presence of intrinsic defects in the carbon base serves to reinforce the role of temperature in the rate-determining step. Overall, this study provides insights into the role of intrinsic defects in carbon-based catalysis and offers implications for nitrogen oxide emission control strategies.

Dynamic correlation between surface carbon response and underlying emissions from spontaneous combustion goaf: field study of an abandoned coal mine

Abandoned coal mine goaf is affected by air leakages and prone to spontaneous combustion, resulting in environmental pollution and geological disasters. Haizhou Open-pit Mine adopts both underground and open-pit mining methods. During the long-term mining process, the original stable stratum structure is constantly destroyed, and the slope slides, increasing cracks and severe air leakage around the goaf and roadway. The spontaneous combustion of coal is particularly prominent after the mine shut down. At present, there is no suitable indirect monitoring method to effectively explore the spontaneous combustion area in goaf. The study developed an all-weather monitoring plan and conducted multi-point continuous long-term measurements of the spontaneous combustion state in one abandoned coal mine goaf located in the eastern part of the Haizhou Open-pit Mine. We evaluated the dynamic correlation between surface CO2 flux (SCF) and changes in the underground fire areas, determined the scope and evolution trend of the fire areas, and identified the distribution and change laws of SCF. The results show a significant positive correlation between SCF and soil temperature; moreover, the SCF value was found to reflect the CO2 emission intensity of the goaf. The high SCF in the test area showed month-wise expansion and increase, while the CO2 emission gradually increased monthly, and the calculated annual total emission was approximately 7017 t. Hence, the study can further provide guidance for the monitoring of spontaneous combustion in shallow coal seams, goaf and the assessment of CO2 emissions from underground coal fires through the on-site monitoring and analysis results.

Experimental study and chemical equilibrium calculation on the mineral transformation of high-alkali coal under oxy-fuel condition

The study aimed to study the mineral transformation of high-alkali coal to address the ash-related issues in oxy-fuel combustion. However, the traditional ashing method alone is inappropriate to study the mineral transformation at the initial stage of coal combustion due to the high ashing temperature. Hence, both the plasma ashing method (<200 °C) and traditional ashing method (600–1200 °C) were employed in this research to prepare the ash samples for characterization. The plasma ashing method is an efficient way to determine the original minerals in coal because it can consume the organic matter at low temperature. The recirculation of flue gas in oxy-fuel combustion can lead to high SO2 and H2O concentrations, while their influences on mineral transformation need to be clarified. In this work, SO2 and H2O were injected into CO2 to simulate the recycled flue gas (RFG). Furthermore, the chemical equilibrium calculation was conducted besides multiple experimental tests to acquire more comprehensive information of mineral transformation. The results show that high sodium and calcium contents are determined in these high-alkali coals, and two of them are rich in iron. In O2/CO2 atmosphere, some sodium (e.g. NaCl) volatilizes into the gas phase and the others converts into aluminosilicates (e.g. NaAlSi3O8) during the heating process. The calcium salts (mainly CaCO3 and CaSO4) decompose and produce silicates, aluminosilicates, and magnesium silicates. The major iron-containing minerals are identified as FeS, FeS2, and Fe2O3. Both FeS and FeS2 are oxidized into Fe2O3 below 600 °C. Moreover, the kaolinite (Al2Si2O5(OH)4) in raw coals decomposes at low temperature (<600 °C). In O2/RFG atmosphere, the additions of SO2 and H2O enhance the sulfation of AAEMs (alkali and alkaline earth metals) and delay the decomposition of sulfates. The productions of some silicates, aluminosilicates, and magnesium silicates of AAEMs are inhibited to varying degrees, and there could be complicated salts containing sulfur (e.g. hauyne). The transformations of iron-containing minerals are little related to the presences of SO2 and H2O. This work obtained the transformations of main minerals in high-alkali coal under O2/CO2 and O2/RFG conditions, which helps to address the ash-related issues.