The Environmental Geochemistry Of Trace Elements During The Utilization Of Coal Gangue

The potential environmental hazards posed by trace elements have reached serious proportions due to their toxicity, bioavailability and geochemical behavior. Some new investigations into the environmental geochemistry of trace elements during utilizations of coal gangue (power generation, brick-making) in the Huainan Coalfield, including the physico-chemical properties of coal gangue, concentrations and associations of trace elements, transformation and partitioning behavior of trace elements during various utilization approaches of coal gangue and potential environmental and health impacts, were conducted in this study.Three hundred and ninety-four samples of coal seam parting (30), roof (182) and floor (182) in the Huainan Coalfield were collected to investigate the geochemical characterizations of trace elements in coal gangue. The concentrations of trace elements in coal gangue and combustion ash were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The morphological analysis revealed that mineral matter in coal gangue is lithic fragments with organic and inorganic layering, clay minerals, quartz, pyrite, and chlorite. The trace elements (except boron) from the coal seam roof/floor have much higher concentrations when compared with the corresponding elements from the coal seam parting. In comparison with the background values of black shale, the trace elements (B, Bi, Ga, Sc and W) are much more abundant in the Huainan Coalfield. According to sequential chemical extraction, the selected trace elements (As, Co, Cr, Cu, Mn, Ni, Se, Sn, V and Zn) are predominantly found in silicate-bound, sulfide-bound and carbonate-bound fractions. Correlation analysis between elements and ash yield suggests that most of environmentally sensitive elements (As, Cd, Co, Cr, Cu, Mn, Ni, Pb, Se, Sn V and Zn) are associated with inorganic matter, but boron is mainly associated with organic matter.Lab simulation experiments were performed to determine the transformation behavior of mineral and trace elements during combustion of coal gangue at various temperatures. Coal gangue samples were placed in a muffle roaster and heated up to eight desired temperature points, from500℃to1200℃at100℃intervals. The mineral compositions of coal gangue and its combustion ash were determined by X-ray powder diffraction techniques (XRD). FTIR spectroscopy analysis was conducted to determine the coal gangue and combustion ash functional group, phase transition characteristics and the variation in combustion. The results show that the transformation behavior of the mineral phase mainly depends on the combustion temperature. The volatilization ratios of selected trace elements increase with increasing combustion temperature. Besides,10trace elements studied in the current study can be classified into two categories according to their volatilization tendencies:a) Ni, Cu, Zn, Cd, Sn, Pb and As vaporize at intermediate temperature and have high volatilization ratios; b) V, Cr, and Co are relatively non-volatile.A study of trace element behavior from a330MW coal gangue circulated fluidized bed co-combustion power plant equipped with electrostatic precipitators (ESPs) was performed. Simultaneous sampling of feed coal, bottom ash, fly ash and flue gas were implemented. Sequential chemical extraction was used to investigate the transformation behavior of trace elements. The relative distribution and partitioning behavior of trace elements in the power plants was analyzed systemically. The trace elements can be divided into two groups:Cluster one, with highly volatilization tendencies are represented by As, Cd, Cu, Pb, Se and Sn which have a volatilization ratio more than20%and are mainly enriched in fly ash. These elements are primarily associated with sulfide minerals. Cluster two, represented by Co, Cr, Mn and V, with low volatilization rate (<5%) and is equally distributed between bottom ash and fly ash. In addition, Bi, Ni and Zn may be located between Cluster one and Cluster two. The variation of modes of occurrence of trace elements could lead to the difference of transformation behavior during the combustion of feed coal. Most trace elements transform and partition into other fractions from Fe-Mn oxides during combustion.Due to the shortage of finite non-regenerated energy (petrochemical resources) and the increasing environmental problems (global warming, surface subsidence, heavy metal pollutions, etc.) during the past few decades, the necessity to develop alternative renewable, sustainable and clean energy has become an irresistible trend. The thermochemical behavior during co-combustion of coal gangue (CG), soybean stalk (SS), sawdust (SD) and their blends prepared at different ratios was determined via thermogravimetric analysis. Simulation experiments in a fixed bed reactor were performed to investigate the partition behavior of trace elements during co-combustion. The combustion profiles of biomass were more complicated than that of coal gangue. Ignition properties and thermal reactivity of coal gangue could be enhanced by the addition of biomass. The volatilization ratios of the selected trace elements are located between the individual feedstock. The volatilization ratios of trace elements are decreased by the addition of biomass, which may be attributed to the higher alkali and alkali-earth elements in the blends. Accordingly, the co-combustion of coal gangue with biomass may reduce the emissions of trace elements which is may be useful for environmental protection. Based on the results of heating value, activation energy, base/acid ratio and gaseous pollutant emissions, the blending ratio of20-30%biomass content is regarded as the optimum composition for blending and can be applied directly in current combustion applications with a few modifications.An investigation of the transformation and distribution behavior of trace elements and natural radionuclides around a coal gangue brick plant was conducted. Simultaneous sampling of coal gangue, brick, fly ash and flue gas was implemented. Soil, soybean and earthworm samples around the brick plant were also collected for comprehensive ecological assessment. During the firing process, trace elements are released and redistributed in the brick, fly ash, and the flue gas. Elements can be divided into two groups according to their releasing characteristics:highly volatile elements (release ratio higher than30%) are represented by Cd, Cu, Hg, Pb, Se and Sn, which are emitted mainly in flue gas and travel and deposit at the northeast and southwest direction around the brick plant. Cadmium, Ni and Pb are bio-accumulated in the soybean grown in the study area, which indicates potential health impacts relation to human consumption. The high activity of natural radionuclides in the atmosphere around the plant as well as in the made-up bricks will increase the health risk of human via respiration.A multiple-year emission inventory of As, Cd, Hg, and Pb from brick making plants in China was first established for the period2008-2013by employing the available emission factors and annual activity data. The atmospheric emissions of trace elements were determined by a bottom-up methodology with the provincial-level statistical data on raw materials (coal, coal gangue, coal ash and clay) consumption and the reasonable emission factors of trace elements. The provincial average concentrations of trace elements in different raw materials were elaborately reviewed and calculated with multiple statistical mean calculation methods. Simulation experiments were performed to determine the emission factors of trace elements from different raw materials. The results show that the total national emission of As, Cd, Hg, and Pb from brick-making plants increased to644.05t,94.961,9.71t, and3269.79t in2013, at an annual average growth rate of22.8%,25.6%,19.2%, and24.6%, respectively, due to the lack of atmospheric pollutant control devices, which figures are higher than that of emissions from coal-fired plants (except for Hg) in China. Coal ash is the main source of As, Cd, and Pb emissions, accounting for87.9%,89.5%, and88.4%of the respective total emissions due to the high consumption of coal ash with high concentration and emission factors of trace elements. Shandong, Henan, Hubei, Hunan, Sichuan and Guangxi are the largest emitting provinces. Advanced technologies and integrated countermeasures to control trace elements from brick making plants are of urgently needed.