Fly Ash Deposition Research Articles

The Phytoremediation Potential and Physiological Adaptive Response of Tamarix tetrandra Pall. Ex M. Bieb. during the Restoration of Chronosequence Fly Ash Deposits

The challenging process of identifying and selecting plant species suited to the phytoremediation of fly ash (FA) dumps involves studying their functional properties and physiological response to a deficit of essential elements and toxicity from heavy metal(loid)-induced oxidative stress. We hypothesised that Tamarix tetrandra has high potential to be used for the phytoremediation of FA deposit sites thanks to its secretion strategy and antioxidative system. In this study, this hypothesis was examined by determining the bioconcentration and translocation factors for As, B, Cr, Cu, Mn, Ni, Se and Zn at the FA disposal lagoons at the ‘Nikola Tesla A’ thermal power plant in Obrenovac, Serbia, three (lagoon L1) and eleven (lagoon L2) years after the phytoremediation process had begun, and by measuring parameters of photosynthetic efficiency and chlorophyll concentration, non-enzymatic antioxidant defence (carotenoids, anthocyanins and phenolics), oxidative stress (concentration of malondialdehyde—MDA) and total antioxidant capacity to neutralise DPPH free radical activity. Tamarisk not only showed the ability to phytostabilise As, Cr and Ni and to accumulate low-availability Mn, Zn and Cu, but also the potential to maintain the structural and functional integrity of cell membranes and stable vitality at L1 under multiple stress conditions due to the high synthesis of phenols and tolerance to increased salinity. However, toxic concentrations of B and Se in leaves induced oxidative stress in tamarisk at L2 (reflected in higher MDA content and lower vitality) and also decreased the synthesis of chlorophyll, carotenoids, anthocyanins and total antioxidant activity. In addition, the prooxidative behaviour of phenols in the presence of spin-stabilising metals from FA could also have resulted in their weaker antioxidant protection at L2. These findings indicate that the choice of tamarisk was justified, but only at the beginning of the phytoremediation process because its presence contributed to an improvement in the harsh conditions at FA deposit sites and the creation of more favourable conditions for new plant species. This knowledge can be of great importance when planning sustainable ash deposit site management worldwide.

Bearing capacity of spatially variable unsaturated fly ash deposit using random field theory

Bearing capacity of spatially variable unsaturated fly ash deposit using random field theory

Shear Strength Testing of Synthetic Ash: The Role of Surface vs Interparticle Adhesions

In thermal power plants, deposition of fly ash on the surfaces of the heat exchanger causes a significant drop in the heat-transfer efficiency and this inhibits effective fuel utilization. It is widely accepted that alkali metals such as sodium and potassium are the trigger for fly ash deposition given that the alkali metals potentially form low-melting-point materials which have adhesive properties and can be deposited on the surface of the exchanger. However, detailed mechanistic studies of this process are challenging due to the chemical complexity and diversity of fly ash samples collected from commercial thermal plants. Herein, it is demonstrated that fly ash deposition triggered by alkali metals can have different mechanisms due to the effects of interparticle and/or surface adhesions. A key enabler for facilitating detailed mechanistic studies is the combined use of tensile and shear strength testers linked to a synthetic ash strategy that utilizes chemically synthesized inorganic particles.

Numerical study of fly ash deposition process in low temperature economizer under SCR conditions

Numerical study of fly ash deposition process in low temperature economizer under SCR conditions

CFD modelling of biomass ash deposition under multiple operation conditions using a 2D mass-conserving dynamic mesh approach

A dynamic CFD model with conjugate heat transfer that considers particle sticking and erosion is developed and used for predicting the effect of multiple operation conditions on ash deposition for a model biomass fly ash species (K2Si4O9) in a lab-scale entrained flow reactor. In order to achieve stable dynamic mesh morphing, a globally mass-conserving smooth method is proposed using a multiple-point weighted moving average algorithm and a growth scaling factor. A new method to estimate particle count for Lagrangian particle tracking is also proposed, which is based on particle impaction efficiency and face count of the mesh of deposition tube. Particle sticking is predicted based on the two-body collision method and particle erosion is evaluated using the empirical model that is dependent on temperature, particle diameter and velocity, and impaction angle. The proposed smoothing method incorporated with the particle count estimated can accomplish stable dynamic mesh morphing for all the 37 deposition cases without the need to loop the smoothing process and sub-group/averaging the growth rate. The prediction results using the proposed ash deposition model agree reasonably with the experimental observation of the effects of flue gas temperature, tube surface temperature, flue gas velocity, fly ash flux and deposition time on deposit formation rate. R2 of the predicted and measured deposit formation rates is approximately 0.68. The results also show that the predicted erosion rate essentially correlates negatively with the experimental deposit formation rate. Interestingly, coarse particles are numerically seen to reduce the deposition rate caused by smaller particles via erosion.

A comprehensive probabilistic investigation on bearing behavior of unsaturated fly ash deposits

A comprehensive probabilistic investigation on bearing behavior of unsaturated fly ash deposits

Coal combustion emissions and ash formation characteristics during oxy-fuel combustion in a 100 kWth pressurized circulating fluidized bed

Pressurised circulating fluidized bed oxy-fuel combustion (PCFB-OFC) is a promising technology for CO2 capture because of its high carbon capture efficiency and net efficiency. However, there is a lack of comprehensive experimental studies on PCFB thermal state experimental devices due to the complexity of their design, construction, and operation. In this study, a 100 kWth PCFB-OFC experimental device was developed, and a series of coal combustion experiments were conducted under 0.1 to 0.6 MPa at an average oxygen concentration of 30% with a fixed peroxygen coefficient β (1.1–1.3). The effect of combustion pressure was investigated to determine the influence of pressure on coal combustion efficiency, furnace temperature distribution, gaseous pollutant emissions, and fly ash chemical composition. The experimental results showed that the increase in pressure increased the particle concentration in the dilute phase zone and increased the temperature at the top of the furnace, with a more uniform temperature distribution. The carbon content in the fly ash and the CO emissions in the flue gas were gradually reduced, and the combustion efficiency increased to 97.3%. The emissions of NO and N2O gradually decreased with the increase in load, and the NO emissions reduced by approximately 70% at 0.6 MPa. The emission of SO2 showed a downward trend as the pressure increases. Affected by the increase in pressure, the self-desulfurization capability of fly ash was improved. In addition, lower K2O and Na2O contents may reduce the deposition of fly ash in the furnace.

Dissolved Metal (Fe, Mn, Zn, Ni, Cu, Co, Cd, Pb) and Metalloid (As, Sb) in Snow Water across a 2800 km Latitudinal Profile of Western Siberia: Impact of Local Pollution and Global Transfer

Snow cover is known to be an efficient and unique natural archive of atmospheric input and an indicator of ecosystem status. In high latitude regions, thawing of snow provides a sizable contribution of dissolved trace metals to the hydrological network. Towards a better understanding of natural and anthropogenic control on heavy metals and metalloid input from the atmosphere to the inland waters of Siberian arctic and subarctic regions, we measured chemical composition of dissolved (<0.22 µm) fractions of snow across a 2800 km south–north gradient in Western Siberia. Iron, Mn, Co, Ni, and Cd demonstrated sizable (by a factor of 4–7) decrease in concentration northward, which can be explained by a decrease in overall population density and the influence of dry aerosol deposition. Many elements (Mn, Ni, Cu, Cd, Pb, As, and Sb) exhibited a prominent local maximum (a factor of 2–3) in the zone of intensive oil and gas extraction (61–62° N latitudinal belt), which can be linked to gas flaring and fly ash deposition. Overall, the snow water chemical composition reflected both local and global (long-range) atmospheric transfer processes. Based on mass balance calculation, we demonstrate that the winter time atmospheric input represents sizable contribution to the riverine export fluxes of dissolved (<0.45 µm) Mn, Co, Zn, Cd, Pb, and Sb during springtime and can appreciably shape the hydrochemical composition of the Ob River main stem and tributaries.

Valuable Secondary Habitats or Hazardous Ecological Traps? Environmental Risk Assessment of Minor and Trace Elements in Fly Ash Deposits across the Czech Republic

Deposits of coal combustion wastes, especially fly ash, are sources of environmental and health risks in industrial regions. Recently, fly ash deposits have been reported as habitat surrogates for some threatened arthropods in Central Europe. However, the potential environmental risks of fly ash have not yet been assessed in the region. We analysed concentrations of 19 minor and trace elements in 19 lignite combustion waste deposits in the Czech Republic. We assessed their environmental risks by comparison with the national and EU legislation limits, and with several commonly used indices. Over 50% of the samples exceeded the Czech national limits for As, Cu, V, or Zn, whilst only V exceeded the EU limits. For some studied elements, the high-risk indices were detected in several localities. Nevertheless, the measured water characteristics, the long-term presence of fly ash, previous leaching by acid rains, and the low amount of organic matter altogether can infer low biological availability of these elements. We presume the revealed high concentrations of some heavy metals at some studied sites can be harmful for some colonising species. Nevertheless, more ecotoxicological research on particular species is needed for final decision on their conservation potential for terrestrial and freshwater biota.

Assessment and analysis of multi-biomass fuels for sustainable electricity generation

Many biomass power plants are not flexible, using only one fuel type, and have high CO2 emissions. In this study, a new methodology to assess the multi-biomass utilization for a power plant is proposed. The results provide that the proportions of mixed biomass residues in different regions of Thailand are appropriate with lower chloride content and slagging inclination than the standard specification. However, high alkali metal oxides in biomass in the southern region lead to high fouling inclination with high fly ash deposition. The electricity production using multi-biomass fuel is more efficient with lower fuel consumption than that with single-type biomass. To generate 103.43 MW electricity production, 107.89 t h−1 consumption of mixed biomass fuels in the central is lower than that with multi-biomass in other regions. However, it generates a maximum of 12.5 mol% CO2 in exhaust gases which can be captured by suitable amine-based absorbents with 90.82% efficiency.

Impact of boiler load and limestone addition on SO3 and corrosive cold-end deposits in a coal-fired CFB boiler

The risk of cold-end corrosion caused by two different phenomena was studied: the well-known sulfuric acid-induced corrosion and corrosion caused by hygroscopic fly ash deposits. Measurements were performed in a full-scale circulating fluidized bed boiler firing bituminous coal with high contents of sulfur and chlorine. The boiler was run both with and without limestone addition to reveal the effects of limestone on corrosion. Furthermore, the impact of boiler load on corrosion and deposit composition was studied. Corrosion probe, SO3, and dew point measurements were performed up- and downstream of the electrostatic precipitator. Ash deposits were collected from the different sides of the corrosion probe and were analyzed. The formation of SO3 was low in all cases (<0.1 ppmv), which was connected to the relatively low furnace temperature in fluidized bed combustion and the efficient SO3 capturing of the fly ash and limestone. The different operational parameters of the boiler had a significant impact on deposit composition and on expected corrosion risk. At full load and without limestone addition, the chlorine of the fuel stayed as gaseous HCl, whereas no Cl was found in the deposits. However, when limestone was added, corrosion was caused by the presence of deliquescent calcium chloride. At low load operation of the boiler, ammonium chloride was formed on the cold-end deposit probe. Ammonium chloride was formed via the reaction between HCl and NH3 in the cooling flue gases. Laboratory studies with NH4Cl was further conducted to assess its corrosivity.

A nongray-wall emissivity model for the full-spectrum correlated k-distribution method based on uniform media assumption

The walls of high-temperature combustion devices are usually assumed to be black or gray in radiative heat transfer calculations to save computing resources; however, this simplification may introduce large errors. To improve the accuracy of the Full-Spectrum Correlated K-distribution (FSCK) method in calculations of nongray-wall radiative heat transfer problems without reducing the computational efficiency, a nongray-wall emissivity model for the FSCK method is proposed based on uniform-medium assumption. The accuracy of this nongray-wall emissivity model and the commonly used Planck-function-weighted gray-wall emissivity model is evaluated in 1D isothermal and homogeneous cases, 1D non-isothermal and inhomogeneous cases, and 3D flames bounded by walls coated with fly-ash deposit or soot deposit. The results show that the nongray-wall emissivity model for FSCK demonstrates much better accuracy than the gray-wall one, especially for cases of low wall temperatures. The nongray-wall emissivity model provides good accuracy over a wide range of optical depth, while the accuracy of the gray-wall emissivity model is highly dependent on the optical depth. The nongray-wall emissivity model based on the uniform-medium assumption can be applied to non-isothermal and inhomogeneous radiative heat transfer problems involving nongray walls.

Adaptive Morphophysiological Features of Neottia ovata (Orchidaceae) Contributing to Its Natural Colonization on Fly Ash Deposits

In previous decades, some species of the Orchidaceae family have been found growing in man-made habitats. Neottia ovata is one of the most widespread orchids in Europe, however it is quite rare in Russia and is included in several regional Red Data Books. The purpose of this study was to compare the chemical composition and morphophysiological parameters of N. ovata from two forest communities of the Middle Urals, Russia: natural and transformed (fly ash dump of Verkhnetagil’skaya Thermal Power Station) for determining orchid adaptive features. The content of most of the studied metals in the underground parts (rhizome + roots) of N. ovata was considerably higher than in the leaves, which diminished the harmful effect of toxic metals on the aboveground organs. The adaptive changes in the leaf mesostructure of N. ovata such as an increase in epidermis thickness, the number of chloroplasts in the cell, and the internal assimilating surface were found for the first time. The orchids from the fly ash deposits were characterized by a higher content of chlorophyll b and carotenoids than plants from the natural forest community that evidenced the compensatory response on the decrease in chlorophyll a. The ability of N. ovata from the transformed habitat to maintain a relatively favorable water balance and stable assimilation indexes further contribute to its high viability. The study of orchid adaptive responses to unfavorable factors is necessary for their successful naturalization and introduction into a new environment.

Kinetic and Mass Transfer Effects of Fly Ash Deposition on the Performance of SCR Monoliths: A Study in Microslab Reactor

Selective Catalytic Reduction (SCR) reactors with ternary vanadium-based catalysts are the state-of-the-art technology for the multipollutant abatement in cofired biomass and coal power plants. Dur...

Direct seeding offers affordable restoration for fly ash deposits

Direct seeding offers affordable restoration for fly ash deposits

Dynamic simulation on high-temperature corrosion behaviour of tube surface with fouling in utility boiler fired by high-chlorine coal

A dynamic model on high-temperature corrosion with fouling-type ash deposition was established to predict the thickness distribution of corrosion layer and the outer/inner layer of ash deposit. It provides an observation on variation of heat transfer characteristics of the alloy test probe over time. This model involves the combustion characteristics of high-chlorine coal, the process of formation, migration, and deposition of fly ash particles, and the corrosion of alloy test probe. In this study, the dynamic model has been integrated into the numerical simulation of high-chlorine coal combustion in a 50 kW long-period corrosion test furnace. The prediction results show that the amount of fly ash deposition on the windward surface is the most while the least amount appears on the side surface. There is a great correlation between the corrosion layer and the inner layer of ash deposit. Because the inner layer of ash deposit and the alloy substrate are spatially adjacent, the high temperature corrosion process is greatly affected by the growth and property of fly ash deposit layer. The simulation results show that the heat flux of probe surface beneath ash deposit is 2.5 kW/m2 which is 50% of the clean surface (5 kW/m2). The changing pattern of thermal resistance versus time at different positions of the alloy surface is consistent with the variation of thickness of the corrosion layer and ash deposit layer. The windward face has the highest thermal resistance and the least heat flux due to the highest thickness of the corrosion layer and ash deposit layer. With the increase of probe wall temperature, the ratio of thermal resistance of corrosion layer to total heat transfer resistance increases, and the ratio at the side face is larger than that at other locations. And the proportion of the corrosion layer in thermal resistance for alloy TP347H reaches 8% at the probe wall temperature of 973 K.

Functional response of Betula species to edaphic and nutrient stress during restoration of fly ash deposits in the Middle Urals (Russia).

We studied the impact of fly ash produced by the thermal power station in the Middle Urals (Russia) on functional traits of two Betula species naturally colonizing ash dump lagoons. The main limiting factors for tree growth on fly ash deposits were nitrogen deficiency, high alkalinity, and unfavorable mechanical composition of substrate. Leaf area ratio (LAR) and leaf mass ratio (LMR) per tree, leaf area (LA), leaf shape coefficient (LSh), leaf thickness (LT), leaf mass per area (LMA), photosynthesis (Amax) and transpiration rates, chlorophyll (Chl), carotenoid (Car), and nitrogen (N) content were measured in Betula pendula Roth and Betula pubescens Ehrh. growing on the ash dump and in the forest near the dump. Both Betula species showed similar functional response to adverse conditions of the fly ash. We found a 1.5-2-fold increase in LAR and LMR in trees growing on fly ash deposits compared with trees in the forest. In both species, the most significant differences across leaf morphological traits were shown for LT. Higher LT provided an increase in Chl and N content per leaf area that caused the rise in Amax and photosynthetic water use efficiency in the trees on the ash deposit. At the same time, Betula species preserved interspecific differences in values of LA and LT which were larger in B. pubescens whiles B. pendula differed by higher LSh. We concluded that the increase in assimilation activity at both whole-plant and leaf levels provides plant adjustment to edaphic and nutrient stress that allow Betula species to colonize technogenic substrates as fly ash deposits.

A nongray-wall emissivity model for the Wide-Band Correlated K-distribution method

The walls of combustion systems are usually assumed to be black or gray in radiative calculations, which may cause large errors. The Planck-function-weighted emissivity is usually used as the gray-wall emissivity when using the Wide-Band Correlated K-distribution method. This approach can demonstrate good accuracy when coupled with the emissivity-based optimized band interval approach proposed by Solovjov et al., 2013. To improve computational efficiency without losing accuracy, a nongray-wall emissivity model and an absorption-coefficient-based method for determining the band interval are proposed. The accuracy of nongray-wall and gray-wall emissivity models along with different approaches to determine the band intervals is evaluated in three 1D isothermal and homogeneous cases bounded by fly-ash deposit, a high-temperature alloy, and soot deposit and a 3D fuel-air flame bounded by fly-ash deposit. The results show that the nongray-wall emissivity model is much more accurate than the gray-wall one when the number of bands is greater than 1. Coupled with the absorption-coefficient-based band interval approach, the nongray-wall emissivity model becomes more accurate when the number of bands is larger than 2, especially for low-temperature walls. It is sufficient to divide the entire spectrum into 4 bands for the cases tested here.

Physiological profiling of invasive plant species for ecological restoration of fly ash deposits

Extensive gaps in invasive plant species research in terms of ecosystem distribution and impact are major barriers faced by global decision making bodies for scientific management of plant species invasions of native flora. Effective strategies need to be worked out which would enable simultaneous management and the use of invasive plant species for ecosystem restoration. For evaluating the photosynthetic potential of invasive plant species in restoration of degraded areas, exhaustive in situ leaf gas exchange and leaf chlorophyll fluorescence were undertaken on four plants found naturally colonizing on the fly ash deposits. The four invasive species screened for their physiological tolerances towards microclimatic and edaphic stresses in harsh fly ash deposits were: Typha latifolia, Saccharum spontaneum, Prosopis juliflora, and Ipomea carnea. Saccharum spontaneum was found to be most efficient in sequestering CO2 emissions which were reflected in its maximum net photosynthetic rate (A, 29.29 ± 0.46 μmolCO2 m−2s−1) and higher below ground biomass. Maximum water use efficiency (WUE) was observed in S. spontaneum (9.3 ± 0.5 mmolCO2 molH2O−1) among all the species owing to high carboxylation, the lowest transpiration rate (E, 3.15 ± 0.13 mmolH2Om−2s−1) and stomatal conductance (gs, 0.2 ± 0.004 molH2Om−2s−1) values supporting it’s dense growth throughout the fly ash dumps in contrast to other three species. The study provides insights into the successful use of invasive plant species for their effective management and phytorestoration of fly ash dumps by transforming them into net carbon sinks which otherwise lack any native flora.

Phytoremediation Potential, Photosynthetic and Antioxidant Response to Arsenic-Induced Stress of Dactylis glomerata L. Sown on Fly Ash Deposits.

Arsenic (As) from coal fly ash can be released into soil/groundwater, presenting a global threat to the environment and human health. To overcome this environmental problem, phytoremediation represents an urgent need, providing ‘green’ cleanup of contaminated lands. The present study focused on As concentrations in fly ash and plants, evaluation of phytoremediation potential of Dactylis glomerata sown on fly ash deposits together with its photosynthetic activity, and oxidative and antioxidative response to As stress. Field research was carried out on fly ash deposits at the thermal power plant “Nikola Tesla”, Obrenovac (TENT-A, Serbia) and the control site. Fly ash is characterized by alkaline pH reactions, small amounts of organic matter, a large amount of available phosphate, and total and available As concentrations. Results in this study indicate that phosphate application can ameliorate As toxicity, uptake and root-shoot transport. Furthermore, D. glomerata can be considered as good As phytostabilizator, because it retains more As in roots than in leaves. Excess As in leaves decreases photosynthetic efficiency (Fv/Fm) and concentrations of chlorophylls, carotenoids, and anthocyanins, whereas high content of malondialdehyde (MDA) can be a signal for biosynthesis phenolics and ascorbic acid, providing cellular redox homeostasis and recovery of photosystem II (PSII) photochemistry. In the roots, low oxidative stress under high concentrations of As is related to intense antioxidant biosynthesis. Taken together, the results in this study indicate a high adaptive potential of D. glomerata to As stress. These findings may suggest that physiological and metabolic tools can be used as a way forward in the ‘real field’ scenario, phytomanagement of fly ash and ecosystem services providing sustainable phytoremediation of As-contaminated sites around the globe.