Preliminary study of trace element emissions and control during coal combustion

Hazardous trace element emissions have caused serious harm to human health in China. Several typical high-toxic trace element coals were collected from different districts and were used to investigate the emission characteristics of toxic trace elements (As, Se, Cr, Hg) and to explore preliminary control methods. Coal combustion tests were conducted in several bench-scale furnaces including drop tube furnace (DTF), circulating fluidized bed (CFB) combustion furnace, and fixed-bed combustion furnace. Calcium oxide was used to control the emission of arsenic and selenium. The granular activated carbons (AC) and activated-carbon fibers (ACF) were used to remove mercury in the flue gas from coal combustion. The chemical composition and trace element contents of ash and particulate matter (PM) were determined by X-ray fluorescence (XRF) spectrometry and inductively coupled plasma-atomic emission spectrometry (ICP-AES), respectively. The speciation and concentration of mercury were investigated using the Ontario-Hydro method. X-ray diffraction spectrometry (XRD) was used to determine the mineral composition of production during combustion experiments. With the addition of a calcium-based sorbent, arsenic concentration in PM₁ sharply decreased from 0.25–0.11 mg/m³. In fixed-bed combustion of coal, the retention rates of selenium volatiles were between 11.6% and 50.7% using lime. In the circulating fluidized-bed combustion of coal, the content of selenium in ash from the chimney was reduced to one-fourth of its original value and that in leaching water from the chimney decreased by two orders of magnitude using lime. Calcium-based sorbent is an effective additive to control the emission of As and Se during coal combustion. The emission of chromium is influenced by the occurrence mode of Cr in coal. Chromium emission in PM_2.5 during coal combustion is 55.5 and 34.7 μg/m³ for Shenbei coal and mixed Pingdingshan coal, respectively. The adsorptive capacity of granular activated carbon for Hg⁰ is significantly enhanced through ZnCl₂-impregnation. The activated carbon fibers showed decent efficiency in mercury adsorption, on which surface oxygen complex showed positive effects on mercury adsorption.     

The correlations of chemical property,alkali metal distribution,and fouling evaluation of Zhundong coal

The utilization of Zhundong coal has encountered severe ash-related problems principally due to its high-alkali feature. However, the evaluation of fouling tendency and correlations of chemical properties of Zhundong coals based on a large database has been seldom performed. The present work aimed to reveal the correlations of chemical properties, distributions of alkali and alkaline earth metals (AAEMs) and iron, and the fouling propensity of Zhundong high-alkali coals based on datasets of >250 samples. The fuel ratio of the majority (∼95%) of Zhundong coals lay in the range of 1.0–2.45. A quantified correlation between fuel ratio and volatile matter content was obtained. The ash composition of Zhundong coal gathered in a small area of the ternary diagram of Fe₂O₃-alkali metal-alkaline earth metal oxide components. The principal sodium of water-soluble form in Zhundong coals varied notably from 45% to 95%, more than half of the potassium was insoluble form, the majority (∼90%) of alkaline earth metals were in forms of ammonium acetate-soluble and HCl-soluble, and ∼67% of iron was HCl-soluble form. The quantitative correlations based on plentiful datasets could improve the comprehension of correlations among the coal chemical properties, fouling behaviors and its reaction activity.     

The production of ultra-clean coal from Zonguldak bituminous coal by chemical leaching

From run-of-mine coal with an average ash of 50% being produced at Zonguldak coal basin, clean coal with 7% ash can be produced through physical enrichment methods such as jig and heavy media. However, for the production of coal with an ash content of lower than 7%, it is required for the coal to be first ground and then subjected to chemical leaching operation. In this study, chemical leaching tests were performed with aqueous HF, HCL, HNO₃, and citric acid (CA) in order to produce ultra-clean coal (UCC) from Zonguldak bituminous coal. The effects of type of chemical, grain size, temperature, and leaching time in the leaching process were studied. It was observed that UCC with 0.82% ash could be produced from sample coal with 8.84% ash as the result of leaching operation with aqueous HF.     

煤中钾存在形式研究

煤中钾的存在形式与煤的灰污性质密切相关。通过萃取的方法,分别以水、醋酸铵和稀盐酸为萃取液对钾在我国一些煤中的存在形式进行了研究。结果显示,煤中钾除了以硅铝酸盐形式存在外,还有其它多种存在形式,尤其在低阶煤中,其它形式的钾占有显著份额。钾在煤中存在形式与煤种、煤阶、煤的显微组分和煤粒径有关。     

Elemental characterization of Nigerian coals and their ashes using particle induced X-ray emission (PIXE) spectrometry for environmental pollution assessment

Nigerian coals and their ashes were investigated using PIXE spectrometry to ascertain their elemental composition, characterize and correlate them, and to determine their environmental risk assessment when combusted. Eleven and 18 elements detected in the raw coals and the ashes, respectively, were subjected to appropriate statistical analyses. Mean elemental summation (%) of the coals and ashes were 8.4 and 39.0, respectively, while average % ash content was 29.6, indicating that they were rich in potential toxic elements which could be emitted into the environment during coal combustion. This was corroborated by t-test results. It is found that pollution index (PI) values for V, Co, Mn, Ni, Cu, and Zn in the coal ash are greater than 1.0. This implies that combustion coal/coal ash may serve as a source of toxic element pollution in environmental media which could produce ill-health. Cross-plot analysis results showed moderate positive correlations, suggesting inter-element correlations among the samples, establishing relationships among the coals. This was validated by the clustering analysis results which indicated three major groups which were also fairly correlated with one another, signifying similar genetic origin. Elemental clustering indicated closest inter-element association among transition metals. This was also confirmed by their Pearson correlation matrices results, indicating chemical affinity and/or similar genetic origin.     

Chromium(VI) Adsorption by Brown Coals

The ability of brown coals (leonardites) to remove chromium(VI) from aqueous solutions was studied as a function of pH, contact time, adsorbent nature, and concentration of metal solutions. The adsorption of Cr(VI) was higher between pH 2.0 and 3.0 for all brown coals and maximum sorption was observed at pH 3.0. The Langmuir adsorption isotherm was used to describe observed sorption phenomena, and the constants were evaluated. The maximum adsorption capacity of 0.92 mmol of Cr(VI)/g for YK, 0.98 mmol of Cr(VI)/g for KK was found from experimental data. KK was the best among the selected adsorbents for the sorption of Cr(VI) at pH 3 and the sorption was 81% out of 100 ppm Cr(VI) after 120 min of stirring.     

Cleaning of high sulfur Indian coal by oil agglomeration and leaching

The present paper reports an attempt for cleaning of high sulfur Indian coal with two consecutive steps of oil agglomeration at ambient temperature followed by leaching at various conditions. The physico-chemical characterizations of the raw and treated coal samples were carried out by using Fourier transform infrared (FTIR), FESEM, Thermogravimetric analysis and Differential Thermal Analysis (TGA-DTA), and petrographic techniques to assess the product quality. The petrographic and FTIR spectral analyses reveal reduction in different forms of sulfur contents in coals by the use of oil agglomeration and leaching. Scanning electron microscope morphology of the treated coal samples are attributed to the occurrence of the cavitations in the coal structure.     

Liquefaction/Solubilization of Low-Rank Turkish Coals by White-Rot Fungus (Phanerochaete chrysosporium)

Microbial coal liquefaction/solubilization of three low-rank Turkish coals (Bursa-Kestelek, Kütahya-Seyitömer and Mu?la-Yata?an lignite) was attempted by using a white-rot fungus (Phanerochaete chrysosporium DSM No. 6909); chemical compositions of the products were investigated. The lignite samples were oxidized by nitric acid under moderate conditions and then oxidized samples were placed on the agar medium of Phanerochaete chrysosporium. FTIR spectra of raw lignites, oxidized lignites and liquid products were recorded, and the acetone-soluble fractions of these samples were identified by GC-MS technique. Results show that the fungus affects the nitro and carboxyl/carbonyl groups in oxidized lignite sample, the liquid products obtained by microbial effects are the mixture of water-soluble compounds, and show limited organic solubility.     

Demineralization of Turkish Tosya lignite coal by boric acid leaching

Boric acid is a weak inorganic acid type which is generally being used in glass, ceramics, detergents, agriculture, nuclear energy, and medicine fields, which has white crystal color and which can easily dissolve in water. Boric acid is being produced from colemanite which is a boron mineral having wide reserves in Turkey. In recent years, intense studies have been performed on the alternative usage areas of boric acid in Turkey. In this study, boric acid was used as a leaching reagent for the demineralization of coal with high ash content. For the accurate determination of the success of boric acid in dissolving mineral matter, first a series of leaching tests were performed with boric acid, and then studies for removing mineral matter with strong acids such as hydrofluoric acid (HF), HCl, and H₂SO₄ were performed.     

Temperatures of Coal Particle During Devolatilization in Fluidized Bed Combustion Reactor

The purpose of this study was to investigate the thermal behavior of coal during devolatilization in fluidized bed. Temperatures in the center of single coal particle were measured by thermocouple. Two coals were tested (brown coal Bogovina and lignite Kosovo), using dry coal particle, shaped into spherical form of diameters 7 and 10 mm, in temperature range from 300 to 850°C. Unsteady behavior of coal particle during heating and devolatilization in fluidized bed was described by a model that takes into account heat transfer between bed and particle surface, heat transfer through particle and an endothermic chemical reaction of first-order. Based on the mathematical model analysis and compared with experimental results, values of heat conductivity (λ _c ) and heat capacity (C _p ) of coal were determined. The best agreement was obtained for constant thermal properties, for brown coal λ _c = 0.20 W/mK and C _p = 1200 J/kgK and for lignite λ _c = 0.17 W/mK and C _p = 1100 J/kgK.     

Uptake of Metal Ions on Humic Acids

The kinetics, the sorption capacities, pH and temperature dependence of sorption of humic acids (HAs) of Turkish brown coals with respect to Zn(II), Cu(II), Ni(II), Co(II) and Pb(II) ions were investigated, and the roles of the carboxylic and phenolic groups in the adsorption of metals ion on HAs were searched in this work. These metal ions are able to form complex compounds with carboxylic and phenolic groups of HAs. Adsorption equilibrium was achieved in between 50 and 60 min for all studied cations. HAs extracted from different brown coals have been characterized by chemical and physical methods. The chemical properties of HAs showed differences depending on the source from which they were obtained. The sorption of metals on the surface of HAs depends strongly on the pH, and sorption decreases with decreasing pH. Maximum removal of metal ions was demonstrated at pH values of 4.1–5.0. The Langmuir adsorption isotherm was used to describe observed sorption phenomena. The Δ G ⁰ became negative as the temperature increased, and so the equilibrium constant decreased slightly. The investigation proved that the HAs are suitable materials for the studied heavy metal ion removal from aqueous solution and could be considered as potential material for purification of effluent polluted with toxic metal ions.     

Formation of Carbon Deposits from Coal in an Arc Plasma

Abstract

The issue of deposited carbon (DC) on a reactor wall during the producing of acetylene by the coal/arc plasma process is a potential obstacle for the industrialization process. The formation mechanism of DC is very difficult to reveal because the high complexity of coals and the volatile matter. Combining with quenching technique, the methane, liquid petroleum gas and benzene were employed as the model materials to roughly act as the light gas, chain and aromatic subcomponents of volatile matter, and then the reasonable formation mechanism of DC was subtly speculated accordingly.     

Diagenetic Relationships between Sulphur Species and Formation of Pyrite in Coals and Sediments

Abstract

Based on the abundant minerals in a Miocene freshwater sequence of mixed sediments (pieces of coal in clays, sand-stones and shales) alternating with coal layers, the following main sedimentation environments were distinguished: illite-montmorillonitic (I-M), calcitic (Ct) and coal-forming environment. Applying factor analyses to 16 geochemical parameters, an environmentally-specific early diagenetic connection between the sulphur transformation processes, amount of C_org and precursor material species (C₂₇-C₂₉-steranes) was determined. It was found that the effects of limiting factors on S_pyr formation depended on the hydrogeochemical conditions of individual sedimentation environments. Pyrite formation in coals depended directly on participation of higher plant precursor materials and/or on the total sulphur content in the system. In the MS, pyrite formation directly depended on total sulphur content, hydrogeological conditions and the amount of C_org.     

Influence of Coal Ash and Slag Dumping on Dump Waste Waters of the Kostolac Power Plants (Serbia)

The content of selected trace and major elements in the river water used for transport, as well as in the subcategories of the waste waters (overflow and drainage) were analyzed in order to establish the influence of transport and dumping of coal ash and slag from the “Kostolac A” and “Kostolac B” power plants located 100 km from Belgrade (Serbia). It was found that during transport of coal ash and slag to the dump, the water used for transport becomes enriched with manganese, nickel, zinc, chromium, vanadium, titanium, cobalt, arsenic, aluminum, and silicon, while more calcium, iron, cadmium, and lead are adsorbed by the ash and slag than is released from them. There is also an equilibrium between the release and adsorption processes of copper and magnesium during transport. The vertical penetration of the water used for transport results in a release of calcium, magnesium, manganese, and cadmium to the environment, while iron, nickel, zinc, chromium, copper, lead, vanadium, titanium, cobalt, and arsenic are adsorbed by the fractions of coal ash and slag in the dump.     

煤中易挥发痕量元素赋存形态的分析方法及实验研究

讨论并比较了煤中痕量元素赋存形态的分析方法及其应用范围，应用逐级化学提取法对3种典型煤进行浸提，根据易挥发痕量元素不同形态的溶解度，将煤中砷、汞、硒分为可交换态、硫化物结合态、有机物结合态和残渣态。结果表明，砷在煤中主要以硫化物结合态存在，汞在煤中主要以硫化物结合态和残渣态存在，硒赋存在硫化物、有机物和残渣态中，不同煤种中易挥发痕量元素各种形态的分布不同，应用浮沉法将煤按不同密度分级后，分别测定了各级煤粉中砷、汞含量，表明砷、汞主要存在于煤的矿物质中。     

Studies on the Reaction Engineering of Combustion and Carbonization of Indian Coals,Depolymerized Coals and Residual Coal Obtained After Organo-Refining

Abstract

Kinetics of the combustion of original and depolymerized Assam, Raniganj and Talcher coals and Neyveli lignite has been studied under non-isothermal conditions in a thermogravimetric analysis apparatus by employing Coats and Redfern kinetic modeling. Depolymerization of coals through acidic phenoation was found to improve the reactivity of the coals for combustion. Similarly, depolymerization reaction also led to the improvement in the pyrolysis behavior of coals. The order and activation energy of the reactions have been reported. The kinetics studies of the residual coal obtained after the successive solvent extraction of coal has also been studied. The present studies on the reaction engineering of coal conversion and utilization technologies may help in the reactor design and in the process development engineering.     

Study on the evolution of the chemical structure characteristics of high rank coals by simulating the ScCO2-H2O reaction

ABSTRACT

To study the influence of CO₂ on the structure of a coal reservoir during CO₂ geological storage-enhanced coalbed methane recovery (CO₂-ECBM), two high rank coals from the Qinshui basin were exposed to supercritical CO₂ (ScCO₂) and water for 240 h in a laboratory high-pressure supercritical geochemical reactor to simulate the reaction of ScCO₂-H₂O-coal under three burial depths. The changes in the chemical structure of the coals before and after reaction were evaluated by Fourier transform infrared spectroscopy (FTIR). The results show that the ScCO₂-H₂O treatment has a substantial influence on the coal structure, reducing the aromaticity and the degree of aromatic ring condensation, increasing the hydrocarbon-generating potential and improving the maturation level of the coals. The ScCO₂-H₂O system mainly affects the organic functional groups of coals through the chemical reactions: swelling effect, bond dissociation reaction, hydrolysis and substitution reaction. The results of peak fitting show that the ScCO₂-H₂O treatment can significantly reduce the intensities of the bands associated with in-plane -CH₂- vibrations and oxygenated groups (especially C-O groups), but increase the intensities of the aliphatic hydrocarbon and hydroxyl group bands. The changes in the functional group contents after the ScCO₂-H₂O treatment varied between the different rank coals. The effect of temperature and pressure on the functional group contents after reaction is complicated, and it is affected by the macromolecular structures of the raw coals and the sensitivities of the chemical reactions to temperature and pressure. However, there is a critical depth that provides the optimum influence of temperature and pressure on the structure of the organic matter in coal. This paper aims to provide a theoretical foundation for the study of the changes in the coal matrix structure during the long-term storage of CO₂ in coal seams.     

Ultrasonic and Microwave Desulfurization of Coal in Tetrachloroethylene

Abstract

The extractive desulfurization of four high sulfur coals from China were promoted with ultrasonic and microwave in tetrachloroethylene organic solvents. It was shown that the joint promotion with ultrasonic and microwave had an active effect on extractive desulfurization. The rate of desulfurization increased with the increase of time in ultrasonic field as well as with the power of ultrasonic, but declined with the increase of the particle size of coal. The rate of desulfurization was not identical for different coal, which was perhaps related to the existed forms of organic sulfur in coal.     

Characterization of eluted metal ions by sequential extraction from Krepoljin coal basin,Serbia: Mechanisms of metal interaction

Eight samples of powdered brown coal (Krepoljin coal basin, Miocene age) were successively extracted by two extractants at room temperature: bidistilled water, Me1, and 2 M MgCl₂ solution, Me2. Large amounts of Ni and Zn are leached from the coal deposit by water. On the contrary, no significant amounts of Pb and Cu are washed under normal conditions. Also, the mobility of Zn and Ni especially depends, to a high extent, on ionic strength. It can be assumed that the humic acids co-extracted during water leaching, Me1, and being a highly competitive ligand for metal ions, will act as a disturbing agent in metal speciation procedure in coals. This assumption is supported by the obtained significant correlation between the Me1:Me2 ratio and the humic acid complex stability constants.     

Hybrid clean energy technologies for power generation from sub-bituminous coals: a case of 250 MW unit

Major re-thinking is required on the conventional pulverized fuel conversion route of power generation wherein the ash and mineral burden in coals is transported through the entire flow passage of the boiler. For high-ash fuels, this has to be contained and the boiler must be clear of all mineral matter. The two independent clean coal candidate technologies for efficiency enhancement and emission controls – ultra-supercritical cycle (USC) and integrated gasification with combined cycle (IGCC) – both have limitations in adaptation to high-ash coals. While the USC is limited by the steam temperature up to 600°C (commercial scale) (700°C pilot scale) and boiler tube failure risks, IGCC is limited to high-quality fuels like diesel, naphtha, etc. (commercial scale) and high-grade coals (pre-commercial scale). The hybridization of the two technologies in their current form (ultra-supercritical cycle with gasification conversion) and carbon capture and storage (CCS) together with solar energy (solar thermal and solar photovoltaic) integration presents possibilities for immediate application to low-grade sub-bituminous coals to achieve the clean technology goals. The energy efficiency of the hybrid system is around 44.45%, which is of the order of the USC with pulverized coal combustion. But the predominant benefits of a clean operation override. The benefits are reduction in CO₂ generation from 0.86 to 0.70 kg/kWh and reduction in ash expelled from 0.20–0.24 to 0.12–0.18 kg/kWh besides elimination of dispersion of ash around the power station and facilitating CCS.