Coal Quality And Coal Combustion Byproducts From The Junggar Coal Basin And Power Plants In Xinjiang Province Northwest China

The Junggar coal basin is one of the largest coal-bearing basins in Xinjiang Uyghur Autonomous Region, Northwest China, with enormous Jurassic coal reserves. However, due to the poor infrastructure and long distance away from the developed areas, large amounts of Xinjiang coals are being, and will increasingly be, locally consumed for various energy production, such as power generation, coal gasification, and liquefaction. The utilization of coal resources promotes economic development, but on the other hand, large volumes of coal combustion byproducts (CCPs) produced from pulverized coal combustion (PCC) power plants and emissions of gaseous pollutants and particulate matter to the atmosphere may cause serious environmental problems. Furthermore, the disposal of CCPs also involves huge environmental costs.Consequently, in order to reduce the emissions, it is necessary and environmentally relevant to investigate the concentrations and behavior of trace elements (especially those of environmental concern) during coal combustion. Moreover, the potential reuse of the CCPs for various applications is also of high significance from both economic and environmental viewpoints. This may not only result in reducing the landfill space and the environmental risk caused by it, but also give rise to economic values at low cost.This thesis first investigates the quality of Junngar coals with enormous coal reserves through the coal rank, coal petrology, coal mineralogy, and coal geochemistry of coal samples collected from exploration boreholes distributed in different coal exploration areas of the Eastern and Southern Junggar Coalfields. Subsequently, this thesis focuses on the characteristics of the feed coals and the CCPs generated from the WEI and HONG PCC power plants in Xinjiang that are mainly fed with Junggar coals, with major emphasis on environmental geochemistry of the trace elements, especially on partitioning and leaching potential of trace elements of CCPs to evaluate application their potential. Finally, various utilizations of these CCPs are tested and optimized, with either high-added value or large amounts of CCPs consumption, such as zeolite synthesis, fire-resistant panels, acoustic barriers, ceramic bricks, and foam glass. The processes are optimized and the final products are investigated in view of their properties and applications of these products.The main achievements of this thesis are summarized as follows: ●Coal characteristicsThe Junggar coal basin is a super large coal basin with high coal reserves up to0.7trillion tonnes. Very thick single coal seam to dozens of split multilayer workable coal seams is well developed in the Xishanyao Formation of the Middle Jurassic across the whole basin, with the maximum accumulated coal thickness of over100m.Both the Eastern and Southern Junggar coals belong to high volatile bituminous coal rank, and are characterized by high inertinite contents, indicating a relatively oxidizing coal forming environments dominated by shallow lake sedimentary facies.The Eastern and Southern Junggar coals are both characterized by very low ash yields, low S and mineral contents, as well as low trace element concentrations, indicating high coal quality of the Junggar coals from the viewpoint of environmental influence. But it is worth noting that the Junggar coals have relatively high moisture and that Na contents in Eastern Junggar coals are higher compared to other Chinese coals.The low ash yields, low mineral, S and trace element concentrations of the Junggar coals are attributed to very low detrital supply in very stable shallow-lake sedimentary facies during peat accumulation. The coals from the coal exploration areas with numerous and thinner coal seams have higher mineral contents, higher S and trace element concentrations compared to those with single thick coal seams. Slight differences of the basement subsidence rate account for the merging and splitting of coal seams in different coal exploration areas; the water table and oxidizing-reducing condition in different coal exploration areas also lead to the slight differences of mineral, S, ash, and trace element contents in the Junggar coals.●Characteristics of feed coals and CCPsThe feed coals from the WEI and HONG PCC power plants in Xinjiang, fed with Junggar and regional coals, are characterized by low S and ash contents, and medium calorific values. The low ash contents give rise to low concentrations of most trace elements. The good quality feed coals are expected to produce fly ash and slag that pose a reduced threat to the environment.The fly ash and slag samples from these two PCC power plants are characterized by high glass content (73-90%), with low contents of quartz, anorthite, mullite, and traces of hematite, calcite and anhydrite. Composition of fly ash glass matrix shows relatively high SiO2contents (51%-59%) and high SiO2/Al2O3ratios (2.5-2.8), close to the mean values determined for European fly ash. The fly ashes are classified as the F group-type fly ash.The concentrations of most trace elements in the studied fly ashes are low. These are close to or even lower than their minimum and the25th percentile (P25) values in EU fly ashes because of the low element concentrations in the feed coals.The partitioning of the elements during coal combustion at the WEI and HONG PCC plants are similar to that of common PCC power plants. Most elements are retained in high proportions in fly ash (>70%) rather than in slag according to the high fly ash/slag production (80:20). Sulphur and Cl are highly volatile at ESP; F, Hg, B, Se, As, Cu, and Zn are partially volatile at ESP, but they are trapped and retained in a large proportion by the FGD, with very low emissions into the atmosphere.The low trace element content in fly ash and slag results in low leachable potential, and most of the toxic elements fall in the range of inert landfill material according to the limits established by the2003/33/EC Decision. These CCPs are classified as nonhazardous materials. Furthermore, the the FGD gypsum and coal gangue clay also show low trace element concentrations, and very low leaching potential, belonging to the inert landfill materials.The low trace element contents, low leachable potentials enable these CCPs to be reused for various applications with low environmental implications.●Synthesis of zeolites from fly ash and utilization of the zeolite productsZeolite products were successfully synthesized from the studied fly ashes by direct conversion and alkaline fusion methods.By direct conversion with NaOH as the activation agent, good-quality zeolite NaPl and some amounts of Na-chabazite and Na-phillipsite of industrial interest were synthesized under some activation conditions. The synthesis results from direct conversion method are influenced by the fly ash characteristics (the SiO2/Al2O3ratio and the glass content of the fly ash, the SiO2/Al2O3ratio in the glass matrix, as well as the particle size distribution in the fly ash, among others) and various synthesis conditions (the NaOH concentration, the activation temperatures, and the activation time and the NaOH/fly ash ratios). The synthesis efficiency can be increased by increasing the activation temperature, the activation time, the concentration of NaOH solution, as well as the solution/fly ash ratio. However, when increasing the temperatures up to≥200℃, or the NaOH concentrations up to≥3M, or the reaction time up to≥12h, it favors the synthesis of low CEC zeolites rather than high CEC NaP1. Among three fly ashes, high CEC NaP1zeolite products were synthesized from WAf and HA, with synthesis efficiency ranging from30%to50%and the maximum CEC of up to2.4meq/g.By means of alkaline fusion method using NaOH, zeolites X and A with high CEC were synthesized under proper synthesis conditions. The synthesis efficiency of this method also depends on the fly ash characteristics and various synthesis conditions (the NaOH/fly ash ratios, the crystallization temperatures and crystallization time). The high NaOH/fly ash ratios are favourable for the synthesis of high CEC zeolites, especially X zeolite. Zeolite A was also synthesized at high NaOH/fly ash ratio, but the high synthesis efficiency was only obtained from WAf fly ash. However, the higher crystallization temperatures probably favour the crystallization of low CEC zeolites, especially sodalite, and hydroxyl-cancrinite. High CEC A and X zeolites were also obtained from WAf and HA, with purities of A-IQE up to49%for HA and61%for WAf. When compared with commercial X-IQE, the synthetic zeolites show higher purities ranging from40%to77%, with the maximum CEC of3.3meq/g.The CEC values are comparable to the CEC of natural zeolites marketed as ion exchangers. Therfore, the synthesized zeolite products can be directly applied for removal of heavy metals from wastewaters with high efficiency and low environmental threat.When using KOH as the activation agent, high CEC merlinoite zeolitic products were synthesized from the studied fly ashes, with the maximum CEC of3.4meq/g. This K-bearing zeolitic product synthesized from fly ash can be used for land reclamation as slow release K-fertilizer for plant growing. The only limitation is that relatively high KOH concentration (up to5M) is needed to obtain high synthesis efficieny.●Utilization of fly ash/FGD gypsum for fire-resistant panelsThe fire-resistant products have been successfully manufactured from100%FGD gypsum, and the fly ash/FGD gypsum or fly ash/CGp mixtures with different fly ash/gypsum proportions.The panels are classified as high density (1.09-1.27g/cm3) panels with relatively alkaline pH (8.8-10.5) and high moisture contents (up to19.2%in the pure FGD gypsum panel). The high moisture contents, especially the high free water contents in the panels account for the long evaporating plateau of the temperature profiles of the unexposed surfaces, which finally results in the benign insulating property of the panels.Both the compressive (4.2-12.6MPa) and flexural strength (2.3-4.7MPa) of the panels, even made of80%of fly ash, are still higher than the limit value (1MPa) required for gypsum panels by EN13279regulations.The manufactured fire-resistant products show very promising insulating capacities. The tested panels, even made from up to80%fly ash still presents comparable insulating capacity to the pure CGp panel (t180=31.9min), with a delay time against fire of30.5min. The insulating capacity of the100%FGD gypsum panel (t180=52min) is much better than that of the commercial gypsum panel as well as other tested CGp/fly ash panels.From an environmental point of view, the fire-resistant panels show very low leaching potential; they are classified as nonhazardous materials according to the European Council Decision2003/33/EC for land waste disposal; the leachable concentrations of some environmental-concerned elements are far below the DSQ limits for bound or shaped construction materials.●Utilization of fly ash/clay for ceramic bricksThe ceramic brick products with good physic-mechanical properties were produced with30%to50%fly ash at various temperatures. The shrinkage of the all the brick products during firing remained within tolerable limits. The brick products present flexural strength of9.93MPa to10.66MPa, decreasing with the increase of fly ash proportion in the raw materials. The flexural strength values of all tested brick products are2-3times higher than that of the normal clay bricks reported elsewhere.The linear shrinkage of the brick products drastically increased and the apparent density slightly increased with the increase of the firing temperature, whereas the water absorption accordingly decreased due to the decrease of the brick porosity along with the increased shrinkage of the brick products. Therefore, the low-density brick products made at relatively low temperature not only reduce the production cost, but also present porous microstructure, which offer advantages for specific applications, such as insulation behavior or even thermal shock resistance. On the other hand, the brick products of relatively higher densities will be obtained at higher sintering temperatures.From an environmental point of view, the two brick products present low atmospheric emissions, with low CO2emission (90-116g/kg brick) and very low SO2emission. Furthermore, the brick1and brick2products show very low leaching potential according to both European Council Decision2003/33/EC for land waste disposal and DSQ limits for bound or shaped construction materials.●Utilization of fly ash/recycled glass for foam glass Due to the relatively high organic C content in the WAf-1fly ash, it was used as the main raw materials to produce a thermal and acoustic insulation product-foam glass. Na2CO3fluxing agent and SiC foaming agent were used to reduce the firing temperatue (low production cost) and optimize the foam behavior because of the very high fusion temperature of WAf-1.The properties of the fly ash, the fly ash/recycled glass proportion in the raw materials, the Na2CO3fluxing agent, the SiC foaming agent, the firing temperature and the residence time at the peak temperature are the important facors that influence the foaming behavior and the final properties of the foam glass. The foaming behavior depends highly on the properties of the fly ash, especially on its fluxing temperature and organic C content. Due to the very high fluxing temperature of the fly ash, without fluxing agent, foaming was obtained from the fly ash/glass mixture at relatively high firing temperature, and foaming temperature increased with increasing the fly ash proportion in the raw materials. However, with low proportion of fly ash, foaming didn’t occur due to the low organic C content, which didn’t generated enough CO2and/or CO gas bubbles to expand in the melt.The addition of Na2CO3and SiC as the fluxing and foaming agent reduced the foaming temperature and increased the porous structure. The increase of the residence time gave rise to increased pore sizes but decreased pore amounts of the foam glass, which is not good for the mechanical properties of the foam products. Good foaming behavior depends on a balance between the decrease of viscosity and the increase of gas release with the temperature increase. The increase of firing temperature gives rise to the decrease of melt viscosity and the increase of the gas pressure, which favor the expansion of the melt under the internal gas pressure, and the gas release accompanied by the the formation of porous structure in the foam products. However, too high temperature gives rise to porous structure with lower amounts but larger size, which is not good for the final properties of the foam products.Considering the influences of all the factors, with the aid of9-11%Na2CO3fluxing agent and only0.5%SiC foaming agent, good foam glass consuming up to33.3%to43.3%fly ash as the main raw material can be obtained when fired at865-915℃with15-min residence time at the peak temperature. These foam products present comparable compressive strength to the commercial foam glass. The large porosity of the foam glass indicates a high potential of thermal and acoustic insulation.The foam glass present low gaseous emissions, with no SO2emissions and a CO2emession rate of25-77g/kg foam glass. The foam products also show very low leaching potential of most elements, including those of high environmental concern.●Utilization of slag for acoustic barriersThree different noise barrier products containing up to80%slag were respectively manufactured from three fractions of the mixed slag with different partical sizes (>5mm,1.25-5mm and<1.25mm).The three products present similar density and compressive strength to those of the acoustic barriers made of some Spainish bottom ash as well as some conventional or recycled materials. The density and the compressive strength of the products decreased with the increase of their particle sizes.The particle size of the slag has high influence on the acoustic behavior of the products made from it. The best sound absorption coefficient was obtained in products made from the coarse fraction of the slag mixture, attributed to the highest open void ratio of these products, which leads to the reduction of large proportion of noise due to the energy loss by means of the friction in the void. The acoustic barrier products made from the middle and coarse fractions of the slag presented good noise absorption characteristics. Although they show slightly low noise reduction coefficient compared to the reference noise barrier products and the counterpart products made from a Spainish bottom ash, their noise absorption coefficients are similar to that of the coarse porous cement.The leaching tests indicate that these acoustic products show very low leaching potential of most elements compared with both European Council Decision limit values and DSQ limits, including those of high environmental concern, and they belong to nonhazardous materials, which can be utilized with low environmental threat.●Influences of coal and CCP characteristics on the CCP utilizationThe reuse of these CCPs for various applications depends highly on the various properties of different CCPs, whilst the CCP characteristics are influenced by the coal characteristics and the combustion conditions.Generally, the high coal quality (low ash yield, low S and mineral contents, low trace element concentrations) of the Junggar coals accounts for the low element concentrations in the resulting CCPs and the low environmental emissions (CO2or SO2) to the atmosphere during the coal combustion, which give rise to very promising reuse potential of these CCPs from an environmental-friendly point of view.The low mineral contents in the Junggar coals accounts for the low crystalline phases and high glass contents (Si and Al) in the fly ashes, which are favorable for the synthesis of zeolites from the fly ashes. Furthermore, the high glass content, the fine grain size and the low gas emissions are the attractive properties of fly ash for producing ceramic bricks. However, the high Si and Al contents leads to the very high softening and fusion temperature (1255℃and1345℃respectively for WAf-1) of the fly ash, which is too high and makes it not suitable for producing ceramic tiles. Furthermore, the organic C and the fusion temperature of the fly ash are two important properties that affect the foaming behavior of the foam glass made from fly ash and recycled glass. There is a balance between the foaming temperature and the foaming behavior. With the aid of the fluxing and foaming agent, optimal foam products are obtained at relatively low foaming temperature.In addition, the compositions of CCPs also depend on numerous parameters, such as combustion techniques, temperature and period of residence, type of particle filter, and other flue gas clean-up systems, which consequently causes different physical, chemical and toxicological properties of CCPs. These properties to some extent, affect the suitability of CCPs for various applications as well. For instance, apart from the chemical composition of the fly ash, the particle size of the fly ash also has a high impact on the results and efficiency of zeolite synthesis.