Coal Combustion Products Research Articles

A STATE OF ART AND PROSPECTS OF FLY ASH MANAGEMENT

The basic data on the volume of formation and composition of fly ash, which is a product of coal combustion at thermal power stations, are given. The analysis of the current state of ways of managing fly ash makes it possible to distinguish the following main directions: physical-mechanical, physical-chemical, biological and combined methods of handling it, which are used in agriculture (use as fertilizers and pesticides; chemical land reclamation: regulation of the structure and pH of the soil); during earthworks (reinforcement of weak and expansive soils, use as tamponage compositions, arrangement of embankments and dams); road construction (strengthening of the soil for the lower layers of the road surface; production of asphalt, cement-concrete mixtures for arranging the foundations of road clothing, as well as crushed stone-sand and gravel-sand mixtures reinforced with cement); building materials industry (cement, concrete, building solutions, geopolymers, bricks, building blocks, glass, glass ceramics, building ceramics, porous ceramics, rolled waterproofing materials); in protective coatings (paints, enamels, enamel glass, shotcrete, heat-protective and heat-insulating coatings); as sorbents and catalysts and their carriers; in mechanical engineering (abrasive, frictional, filtering and lubricating materials, materials for soldering, foundry sand forms or rods); as other materials (pigments, polymer composites, elements of chemical sources of electric current, accumulators of certain substances, carbon nanoparticles (fullerenes, nanotubes), cosmetics and hygiene products); in pyrometallurgical and hydrometallurgical methods of recovery of chemical elements; in alloys and metal matrix composites; for extracting rare earth elements. Currently, the most appropriate methods of disposal of fly ash are either its direct use with a minimal change in its properties, or its use with minimal processing and minimization of the formation of new waste, in particular in the building materials industry and road construction. Bibl. 164, Fig. 3.

Rare elements and non-ferrous metals in the paleogene brown-coal deposits of the Zeya-Bureya sedimentary basin (Amur Region, Far East): accumulation models, enrichment conditions and criteria for assessment of resource potential (a review)

A model has been developed for deposition of rare earth elements with yttrium, trace elements, and non-ferrous metals in platform and activated structures of the Upper Amur region. Genetic types of lanthanide enrichment are considered; lanthanides are shown to concentrate non-uniformly in coal seams and in coal combustion products. Light rare earth elements predominate in coals. The capacity of peat and coal for metal extraction from aqueous solutions was proven. Criteria for a preliminary assessment of coal as a raw material for lanthanides were considered and prospects for their identification were analyzed.

Expanded PAH analysis of household air pollution in a rural region of China with high lung cancer incidence

The domestic combustion of locally sourced smoky (bituminous) coal in Xuanwei and Fuyuan counties, China, is responsible for some of the highest lung cancer rates in the world. Recent research has pointed to methylated PAHs (mPAHs), particularly 5-methylchrysene (5MC), within coal combustion products as a driving factor. Here we describe measurements of mPAHs in Xuanwei and Fuyuan derived from controlled burnings (i.e., water boiling tests, WBT, n = 27) representing exposures during stove use, and an exposure assessment (EA) study (n = 116) representing 24 h weighted exposures. Using smoky coal has led to significantly higher concentrations of known and likely human carcinogens than using smokeless coal, including 5MC (3.7 ng/m3 vs. 1.0 ng/m3 for EA samples and 100.8 ng/m3 vs. 2.2 ng/m3 for WBT samples), benzo[a]pyrene (38.0 ng/m3 vs. 7.9 ng/m3 for EA samples and 455.3 ng/m3 vs. 12.0 ng/m3 for WBT samples) and 7,12-dimethylbenz[a]anthracene (1.9 ng/m3 vs. 0.2 ng/m3 for EA samples and 47.7 ng/m3 vs. 0.6 ng/m3 for WBT samples). Mixed effect models for both EA samples and WBT samples revealed clear variation in mPAHs concentrations depending on smoky coal source while stove ventilation was consistently found to reduce measured concentrations (by up to nine fold and 65 fold for EA and WBT samples respectively when using smoky coal). Fuel type had a larger influence on mPAHs concentrations than stove type. These findings indicate that users of smoky coal experience exposure to many PAHs, including known and suspected human carcinogens (especially during cooking activities), many of which are not routinely tested for. Collectively, this provides insights into the potential etiologies of lung cancer in the region and further highlights the importance of targeting clean fuel transitions and stove refinements as the final goal for reducing household air pollution and its associated health risks.

Revisiting sustainable resources in the combustion products of alumina-rich coal: Critical metal (Li, Ga, Nb, and REY) potential of ash from the Togtoh Power Plant, Inner Mongolia, China

Coal fly ash has gained much attention as a potential alternative source for extracting critical metals such as Li, Ga, Nb, and lanthanides and yttrium (REY). This study investigates their distribution characteristics and modes of occurrence in alumina-rich fly ashes from the Togtoh Power Plant in Inner Mongolia, using various analytical methods. The objective was to provide a reference for the pre-enrichment of critical metals in fly ash. Lithium is primarily present in the glass phase, and its concentration is extremely low in the crystalline phases. Lithium is mainly concentrated in “pure” aluminosilicate glass, and is present but at a low level in Ca-rich aluminosilicate glass. Gallium is primarily present in the glass phase and in corundum, while Nb mainly exists in submicron zircon particles surrounded by Si-Al-Ca glass. Lanthanides and yttrium primarily occur in the glass phase and in crystalline phases, including an intermediate phase composed of the three end-member minerals of the gorceixite-crandallite-florencite series, as well as in monazite, crystalline forms of iron oxides and REY oxides. The Li concentrations in the alumina-rich fly ashes range from 562 to 894 μg/g for Li2O, from 43.9 to 81.9 μg/g for Ga, from 58.7 to 70.6 μg/g for Nb2O5, and from 258 to 450 μg/g for REY oxides, respectively, indicating their substantial potential for resource recovery. Especially, the 2nd row fly ash has the highest contents of these metals, allowing for direct extraction without the necessity for complex pre-processing. Physical separation can further enrich Li, Ga, Nb, and REY in the fly ash. In particular, particle size separation enriches these elements in the < 20 μm size range and magnetic separation enriches Li, Ga, Nb, and REY (except Ce) in the non-magnetic fraction. However, Ce is significantly enriched in the magnetic fraction compared to the original fly ash.

Hydraulic conductivity of bentonite-polymer geosynthetic clay liners to aggressive solid waste leachates

Hydraulic conductivity of conventional mock sodium bentonite (Na–B) and bentonite-polymer (B–P) geosynthetic clay liners (GCLs) were evaluated with three synthetic leachates that are chemically representative of aggressive leachates from coal combustion product (CCR) (I = 3179 mM), mining waste (MW) (I = 2127 mM, pH = 2.0), and municipal solid waste incineration ash landfill (MSWI) (I = 2590 mM). The mock B–P GCLs were created by dry mixing bentonite with branched, linear, or crosslinked polymer. The polymer loading of mock B–P GCLs ranged from 3 to 15%. Comparative tests were also conducted with Na–B GCLs. The mock Na–B GCLs cannot maintain low hydraulic conductivity to aggressive CCR, MW, and MSWI leachates. Mock B–P GCLs with 10% branched polymer had low hydraulic conductivity (< 1.0 × 10−10 m/s) to synthetic MW and MSWI leachates at 20 kPa effective confining stress, whereas the hydraulic conductivity of mock B–P GCLs with 10% linear or crosslinked polymer ranged from 1.5 × 10−9 to 1.4 × 10−7 m/s. As the effective stress increased, the B–P GCLs branched polymer showed a faster decreasing trend than that of Na–B and B–P GCLs with linear or crosslinked polymer.

Experimental Investigation on Gallium and Germanium Migration in Coal Gangue Combustion

Gallium (Ga) and germanium (Ge) critical elements have a wide range of applications and market value. Extracting critical elements from coal gangue and combustion products can alleviate pressures on primary mining resources. Understanding the transformation behavior of Ga and Ge during coal gangue combustion processes is significant for resource utilization and environmental protection. Coal gangue from Xing’an League, Inner Mongolia, was chosen to explore how combustion temperatures (600 °C to 1000 °C) and particle sizes (50, 80, 10, 140, and 200 mesh) influence Ga and Ge migration during combustion. Techniques such as ICP-MS, XRD, XRF, SEM, TG-DSC, and sequential chemical extraction were employed to analyze the transformation of minerals and to quantify the contents and occurrence forms of Ga and Ge. Smaller gangue particle sizes were associated with higher concentrations of Ga and Ge. Approximately 99.19% of Ga and Ge in coal gangue were found in the residual, organic/sulfide-bound, and metal-oxide-bound modes. High temperatures promoted element volatilization and changed the reactions and interactions between elements and minerals. As combustion temperatures rose from 600 °C to 1000 °C, Ga and Ge contents in the products declined progressively. Under high temperatures, minerals like kaolinite, illite, and pyrite in gangue converted to silicate glass phases, mullite, and hematite. Minerals like kaolinite, calcite, and pyrite melted, leading to increased cohesion and agglomeration in the products. Over 90% of Ga and Ge in the combustion products existed in the residual, organic/sulfide-bound, and metal-oxide-bound forms. Moreover, Ga was enriched in combustion products, with its content exceeding critical extraction levels. The results may provide a useful reference for developing critical elements enrichment, extraction, and separation technologies from coal gangue.

Herbaceous plants as heavy metals accumulators in conditions of ash and slag dumps

Storage of ash and slag waste causes a number of environmental problems, including the transformation of natural geochemical cycles and the change of the natural radioactive background due to the increased concentration of heavy metals and radioactive isotopes in coal combustion products. Plants can provide an effective approach for the analysis and monitoring of heavy metals contamination. Additionally, vegetation plays a significant role in the restoration and remediation of heavy metal-contaminated soils. The application of plants in phytoremediation is a green and sustainable solution for decontaminating polluted sites. Moreover, the key role in the restoration of contaminated sites belongs to native species. Within the presented research, the abilities of native dominant species to accumulate and tolerate heavy metals in conditions of ash and slag dumps were investigated on a limited number of samples. Primary analysis of the bioaccumulation of heavy metals through the bioaccumulation coefficient showed that zinc has the highest accumulation capacity in the studied species. The ability to bioaccumulate the analyzed heavy metals decreases in the following order: Zn &gt; Fe &gt; Cu &gt; Mn &gt; Cd &gt; Ni &gt; Pb. Indexing the biogeochemical activity of the tested species showed higher levels of biochemical activity for Anthemis arvensis L. than for Achillea millefolium L., however, these data need to be confirmed by statistical analysis. Determination of the translocation index of elements reflected the high mobility of zinc and cadmium with a high rate of transferring to above-ground biomass, but the potential for accumulation requires scaling up the experiment with the involvement of a larger number of samples and performing statistical data processing. Due to their ability to accumulate heavy metals in roots, plant species could be used for phytostabilization. Establishing methods of biomonitoring and the development of green reclamation technologies with native species application need further research with both selected species and other local species. The presented research showed primary results of bioindication and references for phytoremediation of heavy metal contamination of the ash and slag dumps, which is relevant for other similar environmental conditions.

Estimation of Valuable Metal Content in Coal Combustion Products of the Russian Far East by INAA with a Radionuclide Neutron Source

Estimation of Valuable Metal Content in Coal Combustion Products of the Russian Far East by INAA with a Radionuclide Neutron Source

Hydraulic conductivity of two geosynthetic clay liners with different bentonite granule sizes

Experiments were conducted to evaluate the hydraulic conductivity of two commercially available geosynthetic clay liners (GCLs) with similar properties but different bentonite granule sizes (D50=1.0 mm or 0.3 mm). A synthetic municipal solid waste (MSW) leachate prepared in the laboratory and four actual coal combustion product (CCP) leachates collected from the field were used as permeant solutions to represent different permeant chemical conditions for waste containment applications. The two GCLs had comparable hydraulic conductivity to dilute or moderate ionic strength (I) leachates (&lt; 0.1 M). With more concentrated leachates (I &gt; 0.1 M), hydraulic conductivity of the GCL containing finer bentonite granules (FG) was 10 to 500 times lower than the hydraulic conductivity of the GCL containing coarser bentonite granules (CG) under the same equilibrium conditions. These results suggest that GCLs containing finer bentonite granules may be less vulnerable to permeant chemistry.

Quantitative and qualitative composition of solid particles released into the atmosphere when burning coal (a case study from a brick factory of Uzbekistan)

The fight against atmospheric air pollution by combustion products of natural coal, in particular coal ash, is an urgent task. The most refined definition of the qualitative and quantitative characteristics of coal ash, taking into account the specifics of the technological process for the production of burnt bricks, will serve to identify and justify the technological process and the main parameters of the dust and gas treatment device (PGOU) for use in the production of building materials that involve the use of natural coal as fuel.

Quantitative and qualitative composition of solid particles released into the atmosphere when burning coal (a case study from a brick factory of Uzbekistan)

The fight against atmospheric air pollution by combustion products of natural coal, in particular coal ash, is an urgent task. The most refined definition of the qualitative and quantitative characteristics of coal ash, taking into account the specifics of the technological process for the production of burnt bricks, will serve to identify and justify the technological process and the main parameters of the dust and gas treatment device (PGOU) for use in the production of building materials that involve the use of natural coal as fuel.

Characterization of Coal Combustion Products from Malaysian Power Plant for Building Materials Applications

Coal combustion by Products (CCPs) are produced during coal combustion in the process of electric generation. Without proper handling system, this will cause serious problems towards human and environmental. Nevertheless, with proper treatment, these ashes are applicable in many areas specially building construction materials. Coal Bottom Ash, Coal Fly Ash and Cenosphere are three main wastes produced from power plant. The aim of this study is to determine the characteristics of samples collected from different parts of Malaysia Power Plant. Characterization testing involved included surface morphology, chemical composition (oxides), chemical components, amorphousness, and particle size distribution testing. From this study, CBA have more angular-shaped particles compared to CFA which is smoother surface morphology. However, Cenosphere shown the most uniform and spherical shaped particles. EDS testing shown Existing CBA have highest Carbon content (46%) while Cenosphere is lowest (11.38%). The percentage oxides contents for all samples demonstrate more than 70% of Total SiO2 + Al2O3 + Fe2O3. Hence, all ashes considered as Class F (pozzolanic). CBA, SFA, CFA and cenosphere has significantly more amorphous phases of silica and alumina, and this will increase the concrete strength. Due to agglomeration of ashes on ash pond, the cumulative PSDs results shown that the particle size of the existing sample obtained larger than the new sample. Minimum particles size of SFA (New Dry) is 2 µm while SFA (Existing 2) is 20 µm. From characteristic determination above, it shown that each type of ashes has its own uniqueness and capability to be applied wide range of applications.

Hydraulic conductivity of geosynthetic clay liners to trona ash leachate: Effects of mass per unit area, bundles of fiber existence, and preydration conditions

The barrier performance of geosynthetic clay liners (GCLs) to coal combustion products (CCPs) is of primary importance. One of the CCPs leachates that has a damaging effect on hydraulic conductivity is trona ash leachate (TAL). In this study, the hydraulic conductivity of sodium GCL (Na-GCL) to TAL was investigated in terms of mass per unit area (MPUA). The hydraulic conductivity of GCLs to TAL was 2.6 × 10-6 and 7.6 × 10-7 m/s when the MPUA was 3.0 kg/m2 (Mb3) and 4.0 kg/m2 (Mb4), respectively. Dye tests conducted on these GCLs showed that flow preferentially occurred through bundles of fibers existing in the GCLs. In contrast, increasing the MPUA to 5.0 kg/m2 (Mb5) led to a decrease in the hydraulic conductivity (i.e. 4.1 × 10-11 m/s). Additional tests were performed on fiber-free GCLs to determine the role of fiber bundles. Regardless of MPUA, the fiber-free GCLs had low hydraulic conductivity (6.7 × 10-11 m/s). Prehydrating Mb3 and Mb4 with deionized water (DIW) before permeation with TAL also decreased the hydraulic conductivity. The hydraulic conductivities of prehydrated Mb3 and Mb4 were 1.6 × 10-10 and 4.8 × 10-11, respectively. Chemical analyses showed that the cation exchange reaction had a negligible influence on the hydraulic conductivity. Because TAL was a potential source of Na+ throughout the tests.

IMPACT OF LEGISLATIVE CHANGE ON THE CLASSIFICATION OF RAW MATERIALS AND BUILDING MATERIALS IN TERMS OF NATURAL RADIOACTIVITY

Natural radioactivity is present in the human environment to a greater or lesser extent, also in varioustypes of raw or building materials. Monitoring of the concentration of natural radioactive isotopesof potassium 40K, radium 226Ra and thorium 228Th or 232Th in samples of raw and/or constructionmaterials allows assessing the exposure of people to ionizing radiation emitted by these materials.Exceeding the limit value of the radioactive concentration index for raw and/or building materials,defined in the Ordinance of the Council of Ministers (Polish Journal of Laws/Dz.U. No. 33/2021),may result in an effective radiation dose greater than 1 mSv/year – the dose limit for the population.In Poland, pertinent legislation requiring the establishing of the natural radioactivity of raw and/orbuilding materials dates back to the 1970s and has been changed several times by now. However, themost significant changes in the legislation have occurred recently. Due to the obligation of imposingthe application of the provisions of the of European Council 2013/59/EURATOM of 5 December2013 laying down basic safety standards for protection against risks arising from exposure to theeffects of ionizing and repealing Directives 89/618/Euratom, 90/641/ Euratom, 96/29/Euratom,97/43/Euratom and 2003/122/Euratom (the so-called BSS version), it was necessary to adopt theamendment to the Atomic Law. The implementation of the BSS Directive in Poland was issued on thebasis of the resolution of the Parliament of June 13, 2019 amending the Atomic Law Act and the FireProtection Act (Polish Journal of Laws/Dz.U. 2019, item 1593). The amendment of the Atomic LawActs led to the harmonisation of regulations on radioactivity in raw and building materials betweenPoland and the Member States. This article presents the impact of an amendment of the AtomicLaw and of the accompanying normative acts on the studies of the natural radioactivity of raw andconstruction materials, using the example of an analysis of the concentration of natural radioactiveisotopes present in the coal combustion products (CCP), i.e. fly ash, slags and ash-slag mixtures.

Influence of method for determining thermal conductivity of coal combustion products on calculation accuracy

The determination of thermal conductivity of the combustion products of coal of various compositions by a simplified method and in accordance with the molecular-kinetic theory according to the Masson – Saxena equation is considered. Calculations were made for hard coal of the Volodarsky mine of grade A, Donetsk basin of grade OS, Kuznetsk basin of grade K, Tallinn section of grade G, Kizelovsky basin of grade G, and Tulganskoye deposit of grade 1B with the content of the combustible part by weight from 70.8% to 25.1%. The temperature of the combustion products varied from 200°C to 400°C in increments of 50°C, and the excess air coefficient varied from α = 1.15 to α = 1.5 in increments of 0.05. The discrepancy between the results of the simplified calculation and the results obtained by the Masson – Saxena equation was estimated using the relative error. Analysis of the calculation results showed that the thermal conductivity coefficient calculated according to the additivity rule using volume and mass fractions of the components of the combustion products is higher than that calculated using the Masson – Saxena equation at any excess air coefficients and temperature of the combustion products. With an increase in the temperature of the combustion products, in both cases, the error in determining the thermal conductivity decreases due to an increase in the value of the thermal conductivity. The use of simplified calculation methods leads to an increase in the thermal conductivity of combustion products by an average of 7.5% – 2% for coal of various compositions, depending on the temperature of the combustion products, compared with the values calculated using the Masson – Saxena equation.

Hydrometallurgical processing of ash and slag waste

This work assesses the possibility of hydrometallurgical processing of ash and slag waste in order to extract rare and rare earth elements. The ash and slag waste from the Chita CHPP-2 combined heat and power plant was used as a research object. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to determine the elemental chemical composition of coal combustion products. To preconcentrate rare and rare earth elements, magnetic separators were used to isolate the -0.5+0.3 mm and -0.3+0.1 mm grain size fractions of ash and slag waste. The leaching of rare and rare earth elements from the studied samples was investigated using sulfuric, hydrochloric, and nitric acids, as well as an aqua regia solution, in combination with simultaneous ultrasonic exposure. It was determined that electromagnetic separation of the -0.5+0.3 mm and -0.3+0.1 mm grain size fractions of ash and slag waste significantly concentrates rare and rare earth elements in the magnetic fraction, including titanium (up to 25%), zircon (up to 33%), yttrium (up to 50%), lanthanum (up to 150%), and cerium (up to 5%). It was determined that an increase in the duration of ultrasonic treatment during the leaching of metals from ash and slag waste with sulfuric acid resulted in a uniform 7.25-fold increase in gallium content (from 0.008 to 0.058 g/dm3). Additionally, when decomposed with aqua regia, a 3-fold concentration of the same element was observed (from 0.008 to 0.024 g/cm3), while ultrasonic treatment offered only a slight increase in concentration. When leaching with sulfuric acid (the duration of ultrasonic exposure is 5 minutes), a 4-fold increase in the rubidium content was observed (from 0.108 to 0.457 mg/dm3). Therefore, the most effective method for extracting rare and rare earth elements from the ash and slag waste of Chita CHPP-2 involves acid leaching combined with electromagnetic separation and ultrasonic pulp leaching.

EVALUATION OF THE STRENGTH CHARACTERISTIC OF SOIL STABILIZED WITH FLY ASH

High domestic consumption of coal, especially in energy sector, has raised concerns on several environmental issues in Indonesia. Large amount of coal combustion product in the form of solid waste, such as fly ash resulting from coal power plant production causes problems in its disposal since its necessity of land occupancy. Over the last decade, many researchers have put effort to solve this disposal concerns by utilizing these materials for various purposes, such as construction materials as part of circular economy. This paper presents the utilization of fly ash for soil stabilization in Surabaya. Firstly, the index properties of soil samples were determined through a series of laboratory soil test including specific gravity, unit weight, moisture content, sieve analysis and Atterberg’s limit tests. Subsequently, standard proctor test was conducted to obtain the maximum dry density and optimum moisture content of the soil samples. Using those parameters, four types of soil stabilized with fly ash samples were prepared with fly ash content based on weight of soil as follows: 0%, 5%, 10%, and 15%. Finally, unconfined compression tests were performed with cylindrical soil samples to assess the unconfined compressive strength of those samples. The results indicate that soil with fly ash content of 10% showing the highest unconfined compressive strength compared to the other variations of 0.90 kg/cm2.

Physicochemical Properties of Fe-Bearing Phases from Commercial Colombian Coal Ash

High amounts of coal combustion products, such as fly ash and bottom ash, are generated every year; however, only 64% are used, which means that a significant part is landfilled despite containing valuable materials such as ferrospheres, which may be used as catalysts, substituting critical raw materials (e.g., platinoids). In commercial coals, pyrite contents are reduced as a pre-combustion S-emissions control measure, so low amounts of ferrospheres are expected in the respective ashes. However, given the large amounts of ash being generated from these coals, it may provide a reliable source of catalysts, with ferrospheres being easily recovered via magnetic separation. Several studies have been conducted regarding these morphotypes; however, there is a lack of investigation considering the ash derived from highly beneficiated coals and the variations with location and time. In this study, bottom ash, economizer grits, and fly ash samples from a Portuguese power plant burning Colombian commercial coal were fractionated using ferrite (Fe-MC fraction) and Nd (Nd-MC fraction) magnets, and a multi-technique approach was used to assess their properties (magnetic parameters, particle size distribution, mineralogy, particle morphology, microtexture, and chemical composition). The Fe-MC presented higher Fe concentrations (up to 44 wt.% Fe2O4) than the Nd-MC (up to 7 wt.% Fe2O4). Once it was a sequential process, Nd magnets essentially collected Fe-bearing aluminosilicate glass, and Fe-bearing minerals were residual when compared to the Fe-MC, where magnetite, magnesioferrite, hematite, and maghemite accounted for up to 30 wt.%. Among the Fe-MC, the sample collected from electrostatic precipitator fly ash (ESP FA), despite having a lower yield, presented higher Fe concentrations than the ones from bottom ash and economizer grits, which was related to the mode of occurrence of Fe-bearing phases: in the Fe-MC from ESP FA, discrete ferrospheres predominated, while in the remaining Fe-bearing phases, they were often embedded in aluminosilicate glass. All Fe-MC samples showed an increase of Fe-substituting elements (e.g., Mn and Ni) and their concentration tended to increase with decreasing particle size along with Fe. The integrated study of cross-sections enabled the identification of oxidation rims, martitization aspects, and the co-existence of hematite and magnesioferrite.

Determination of concentrations of Sr and Ba in coal and coal combustion by-products: A comparison between results by ICP-MS and XRF techniques

Concentrations of trace elements in coal and coal combustion products are commonly analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) due to large number of elements detected and the relatively the low detection limits of this technique. Like other geological samples, complete dissolution of coal and coal combustion products is also essential for accurate ICP-MS results. In this study, Sr and Ba in coal and coal combustion products (fly and bottom ashes) from two coal-combustion power plants from North China analyzed by XRF and ICP-MS were comparatively studied. The concentrations of Sr and Ba analyzed by ICP-MS, when a mixture of acids (2 ml HF + 5 ml HNO3 for each 50 mg coal sample and 5 ml HF + 2 ml HNO3 for each 50 mg ash sample) was used for microwave-assisted digestion, do not fit well with the their relative XRF results. This is most probably due to the formation of the fluorides during microwave digestion, and this assumption is supported by the presence of various fluoride compounds, NaMgAl(F, OH)6·H2O, NH4MgAlF6, AlF3, and K2SiF6, in the residues of all the coal and ash samples in our sequential extraction experiment. Cations of Sr and Ba were probably trapped into the divalent cation sites of the fluorides. Concentrations of Sr and Ba analyzed by ICP-MS using increased HF and HF/HNO3 ratio (7 ml HF and 2 ml HNO3 for each 50 mg coal/ash sample) are in better agreement with the XRF results. Our results indicate that excess amount of HF probably has led to the suppression of these elements due to fluoride precipitation. The results indicate that the modified digestion method is capable of achieving complete digestion of coal samples and results in a reliable analysis of Sr and Ba concentrations in coal samples and most fly ashes by ICP-MS. However, the formation of insoluble fluorides is probably not completely suppressed for some bottom ash samples, which can result in underestimation of Sr and Ba concentrations. Nevertheless, XRF analysis can serve as a reliable cross-check method to assist in the evaluation of the accuracy of ICP-MS results.

Visualizing high dimensional structures in geochemical datasets using a combined compositional data analysis and Databionic swarm approach

Classical tools for exploratory analysis of large geochemical datasets (e.g., cluster analysis, principal component analysis, etc.) have been successfully utilized for decades to understand complex structures and identify natural, multi-dimensional data clusters. More recent development of machine learning algorithms, both supervised and unsupervised, have increasingly been shown to derive deeper insights from complex datasets than older methods. Moreover, geochemists have long recognized that concentration data are compositional and require special mathematical treatment, such as application of Compositional Data Analysis (CoDA) techniques. Proper approaches for linking CoDA and machine learning is an area of active research. Here, we investigate the behavior of trace elements in coal and coal combustion products from a power plant burning Appalachian Basin coals using a combination of CoDa methods and a relatively new tool for visualizing complex multivariate structures, Databionic swarm (DBS). Databionic swarm is an unsupervised method which combines concepts from emergence, self-organization, game theory, and swarm intelligence, and allows for mapping of higher-order structures onto a lower-dimensional output space (2-dimensions in this case). A suggested approach for converting the raw geochemical data to isometric log-ratios is developed for the system and results from DBS and robust PCA are compared and contrasted.Both PCA and DBS results show similar clustering of feed coal (FC) and pulverized coal (PC) samples and bottom ash (BA) with economizer fly ash (EFA) samples. However, within group variation of the samples was relatively difficult to identify in the PCA analysis. Results from DBS show that the BA and EFA samples are geochemically distinguishable based on the relative abundances of As, Cd, and Pb vs. Cr and Ni and As and Cd vs. Pb. Similarly, the PC and FC samples exhibit statistically different Cr vs. Ni ratios, which was not obvious from the PCA scores. Abnormally low Hg to Cl ratios in the fly ash (FA) samples relative to the rest of the samples, provided insight into the preferential removal of Hg vs. Cl, possibly in response to the generation of more gaseous oxidized mercury in the flue gas, owing to elevated Cl concentrations in the coal. Such findings demonstrate that DBS and other machine learning techniques allow for quick visualization of complex geochemical data structures in a way that is both consistent with the principals of CoDA and an improvement over classical methods.