Coal Slag Research Articles

Removal of phosphate from wastewater using coal slag

ABSTRACT Adsorption is an efficient, cost-effective, and eco-friendly method for the treatment of phosphorus from wastewater. This work presents the adsorptive removal of phosphate ions in aqueous solution using coal slag (CS) from a thermal power plan. The surface morphological analysis showed that the CS particles were around 50 µm with a surface area of 9.20 m2 g−1 and adsorption average pore width of 6.42 nm, offering the high capability for phosphate ions adsorption. The optimising phosphate ions adsorption conditions were investigated based on various parameters, including contact time, pH, and amount of absorbent. The experimental results showed that the maximum loading adsorption capacity was 21.63 mg g−1 in the concentration range of 0–30 mg L−1 under optimising conditions (i.e. pH of 6, adsorbent dosage (0.1 g vol−1 used) and contact time of 45 min). Furthermore, the use of CS offers several benefits like reducing the sample pre-treatment steps for costly, less time-consuming and reliable methods.

Evaluation of solid waste stratified mixtures as constructed wetland fillers under different operation modes

Based on the strategy that all processes can and should be modified to contribute to a circular economy, this work evaluates the recovery of waste solids as filler material in Constructed Wetlands (CWs) used for wastewater treatment. Five sets of lab-scale CWs were assembled with mixtures of five waste solids and operated to evaluate the removal of chemical oxygen demand (COD) and nutrients from urban and industrial-types of wastewater. The adaptation and growth of the macrophyte Phragmites australis in the mixed-filler CWs was also monitored. Although all evaluated waste solids showed to be acceptable substrates for macrophyte development and wastewater treatment, CWs assembled with mixtures of limestone waste and coal slag showed the best plant growth indicators and wastewater treatment efficiencies. The CWs assembled with mixtures of limestone waste and clay brick fragments or cork granulates showed to be suitable alternatives. With exception for CWs filled with mixtures of limestone and snail shells, pollutant removal efficiencies up to 95%, 86% and 83% were obtained respectively for COD, total phosphorous and total nitrogen, depending on the type of wastewater and mode of operation. Removal efficiencies were not significantly affected by increased hydraulic rate. The CWs can be operated to a hydraulic loading rate of 0.056 m/d, which corresponds to a retention time of 1.5 days. The valorisation of solid waste as a filler can contribute to CWs closely participating in the creation of circular flows for the reuse of waste solids.

THE INFLUENCE OF PROCESS CONDITIONS ON GROUND COAL SLAG AND BLAST FURNACE SLAG BASED GEOPOLYMER PROPERTIES

In this study, the material characterization of Vietnamese ground coal slag and ground granulated blast furnace slag (GGBFS), such as particle size distribution, chemical composition, bulk density and particle density are shown. The geopolymer specimens were prepared by mixing an 80 m/m% mass of solid materials (ground coal slag and GGBFS in a different ratio) with 20 m/m % of a 10M NaOH alkaline activator. A systematic experimental series was carried out in order to optimize the preparation process. In that series, the heat curing temperature was 60°C for 6 hours, and then selected specimens were heat treated at a high temperature (1000 °C) for 1 hour. After 7 days of ageing, the physical properties of the geopolymer (compressive strength, specimen density) were measured. Also, after 180 days of ageing, the pH values of water in the geopolymer leaching preparation were determined. The results show that the geopolymer can be used for refractory applications due to its good heat resistance properties. However, geopolymers that were heated at 1000 °C had lower compressive strength, specimen density and pH values of water containing the geopolymer than those that were cured at 60 °C.

Quantitative Study of Si Structural Units in Coal Slags and Their Influence on Viscosity

The properties of coal slags in slagging gasifiers such as viscosity are intrinsically determined by the structure. In this article, the structure of 10 coal slags is investigated by a combination ...

Effect of water vapor on viscosity behavior of coal slags with high silicon-aluminum level under gasification condition

For the slagging gasifiers, the slag viscosity behavior is a critical parameter for the smooth slag tapping and long term operation. Water vapor is an important component of the syngas in the gasifier, especially for coal water slurry gasification. This paper presents the results of a study to investigate the effect of water vapor on viscosity behavior of coal slags with high silicon-aluminum level. The viscosities of three synthetic coal slags with different Si/Al ratios (mass ratio of SiO2 and Al2O3) was measured in argon with 10%, 20% and 30% water vapor as a function of temperature. As the water vapor proportion increased, the viscosity of the slag with high silicon-aluminum level at same temperature increased slightly. Besides, the slag with Si/Al of 2.0 transformed from a glassy slag to a crystalline one when the water vapor proportion was increase up to 30%. The oxidation state of iron of the slags was not affected by the increase in water vapor proportion under Ar with water vapor condition. Analysis of the slag structure demonstrated that water vapor entering the slag was benefit to the formation of SiOSi bridges and [AlO4]5− structure. Meanwhile, activation energy for viscous flow (Eη) increased with the increasing water vapor proportion. Water vapor also promotes crystallization and growth of crystals of the slag with high silicon-aluminum level during cooling, leading to the increase in the temperature of critical viscosity (TCV).

The research on the influence of carbon content in coal slag on heat loss from solid incomplete combustion

The influence of carbon content in coal slag on heat loss from solid incomplete combustion was analyzed by thermal balance calculation of the boiler. The results showed that the heat loss from solid incomplete combustion increased with the increase of carbon content in slag when other operating parameters were constant. The quality of coal, especially the calorific value had an important influence on the carbon content of the slag. The heat loss from solid incomplete combustion decreased with the increase of calorific value. As a result, in order to reduce the heat loss from solid incomplete combustion, the type of coal used should be consistent with the type of coal designed as much as possible.

Relationship between Microstructure and Thermal Conductivity in Coal Slags with Variable Silica and Alumina

For achieving a stable operation of coal gasification, it is crucial to understand the relationship between the microstructure and thermal conductivity of coal slags. Thermal conductivity measurements were carried out on a SiO2–Al2O3–CaO–Fe2O3–MgO slag system with a mass fraction of silica and alumina (SiO2 + Al2O3) ranging from 35 to 85 wt % using a nonstationary hot wire method. The crystallization tendency was obtained by a differential scanning calorimetry signal. Raman spectroscopy and X-ray diffraction (XRD) were combined to investigate the microstructure. The addition of (SiO2 + Al2O3) leads to a significant increase in the thermal conductivity of slags when (SiO2 + Al2O3) is higher than 55 wt %. The XRD results show that a gradual increase in the silica and alumina contents causes crystalline phase transition, resulting in an increase of bridging oxygen in the slag system. The degree of polymerization Q with variable (SiO2 + Al2O3) is quantified by Raman spectroscopy. The positive linear relations...

Application and thermal economic analysis of plate low pressure economizer in flue gas waste heat recovery

Low pressure economizer is an important equipment to reduce exhaust gas temperature and recover waste heat of flue gas in power plants. In this paper, the welded plate heat exchanger is applied to the field of low pressure economizer. The waste heat of coal slag and flue gas is utilized comprehensively to raise the boiler feed water temperature and reduce coal consumption. The thermal economy of plate economizer and finned tube economizer is analyzed by the equivalent enthalpy drop method. The results show that the plate economizer has obvious advantages under the same pressure drop and heat load. Compared with the finned tube economizer, the heat transfer coefficient the of the plate economizer increases by more than 50%, and the volume decreases by about 2/3. After installing the slag cooler, the heat consumption rate decreases by 119.7 KJ/kWh, and the coal consumption decreases by 4.68 g/kWh.

Preparation of mesoporous silica from coal slag and its metal ion adsorption behavior

A novel mesoporous silica (MS) with the high specific surface area up to 1,199 m2/g was prepared from coal slag by SO 3 2− leaching and hydrothermal synthesis. Its application in metal ion removal was explored using Pb2+, Cu2+, Co2+ and Cd2+ as the model ions. The effects of adsorption time, pH, initial metal ion concentration, adsorbent dosage and adsorption temperature on the metal ion adsorption behaviors of the as-prepared MS were systematically, and the maximum adsorption capacities were measured to be 55.76 mg·g−1, 33.49 mg·g−1, 24.29 mg·g−1 and 22.98 mg·g−l for Pb2+, Cu2+, Co2+ and Cd2+, respectively. The metal ion adsorption on the as-prepared MS followed pseudo-second-order kinetics, and the adsorption isotherm fitted the Langmuir isotherm model, indicating that the adsorption was spontaneous, endothermic and enthalpy driven.

In-situ observation of crystallization inside coal slags and influence of crystals on flow behavior

In entrained flow gasifiers, the viscosity of molten slag along the wall is critical for continuous operation. Crystallization inside the slag can cause a sharp increase of viscosity and even blockage of gasifier. Nucleation and crystal growth processes of four coal slags were observed in-situ by an advanced high-resolution confocal scanning laser microscope (CSLM). The crystallization ratio and kinetics of three crystalline slags were calculated. Crystal phases were determined by XRD and morphologies were analyzed by SEM. The effect of crystals on viscosity is also discussed. Anorthite is detected as the primary as well as the main crystalline phase of three slags, although the crystals can be in different shapes such as rod, flate or even needle. Anorthite at high temperature can move around inside the slag because of low viscosity and small crystallization ratio and little influence can be found of the drifting crystals on viscosity. But when the crystallization ratio reaches 15–20%, the viscosity can be severely impacted.

Xinjiang lignite ash slagging and flow under the weak reducing environment at high temperatures – Slag viscosity and its variation with ash type and addition of clay

In this study, five different ashes derived from Xinjiang lignite were tested for slagging at 1300–1400 °C in a weak reducing environment (1% CO in nitrogen), at different exposure time from 10 min to 2 h. The apparent viscosities for different slags were determined by using a modified inclined plane (M-IP) method with an inclination of 25° based on the slag travel length per unit mass. In particular, the variation on slag viscosity upon the blending of additives including clay and MgO was examined in detail. As have been confirmed, the slags derived from the original basic and neutral ashes start to flow from 1400 °C onwards. The viscosities calculated based on the M-IP method at 1400 °C are in the range from ∼1 Pa·s for neutral ash to ∼5 Pa·s for the basic ash slags. For both neutral and acidic slags, their viscosities calculated by the M-IP method show a good agreement with some of the existing empirical models that have been validated for high-rank coal slags. Upon the addition of 10 wt% clay, the viscosity of basic ash at 1300 °C was reduced to 1.53–1.71 Pa·s, demonstrating a comparable flow-ability and viscosity to that of the original ash at 1400 °C. The addition of MgO promotes the slag flow-ability and reduces the slag viscosity to a relatively small extent in comparison to clay. Instead of melting with other elements in the ash matrix to form slag, the discrete Mg grains are observed in the slag matrix. It could even partially react with the alumina plate. The precipitation of Mg may promote the slag flow-ability by alleviating the slag penetration into the alumina plane and the resultant friction between them. Additionally, the viscosities for both the original slags and those blended with clay decrease quickly upon the increase of the exposure time, substantiating a non-linear slagging propensity and hence, probably non-Newtonian property. A minimum exposure time of 40 min is essential to ensure these slags to settle down and reach their equilibrium viscosities in the M-IP method.

A CALPHAD approach to modelling of slag viscosities

A structure-based model recently developed for the fully liquid system SiO2–Al2O3–CaO–MgO–Na2O–K2O–FeO–Fe2O3 in the Newtonian range is presented in this paper and the methodology to modelling of the viscosity is outlined. In the model, the structural treatment of the various oxides in multicomponent oxide systems is discussed on the basis of the underlying non-ideal associate species model that was used to describe the Gibbs energy of the liquid. Both the temperature- and composition-induced structural changes of oxide melts is then described with a set of monomer associate species in combination with some specific larger structural units induced by the self- and inter-polymerisations of the associate species. On the other hand, the overall performance of the viscosity model will be demonstrated and discussed. One of the challenges of the viscosity behaviour in SiO2-based binary systems, the so called lubricant effect, is well described. The local viscosity maximum around the fayalite composition in the system FeO–SiO2 is also sufficiently described. The viscosity behaviour when substituting one network modifier for another at constant SiO2 contents is properly described. Moreover, the Al2O3-induced viscosity maximum is described, in which the position and magnitude of the viscosity maximum as a function of temperature and composition (charge compensation effect) are properly predicted. In addition to a good performance in describing the various challenging viscosity behaviours, the model parameters bear a clear physico-chemical meaning, which ensures a reliable prediction over the whole range of compositions and a broad range of temperatures and oxygen partial pressures (for iron oxide-containing systems). Finally, in combination with phase relations the model is employed to determine the blending proportions for coal slags according to a target viscosity value, as an application case.

Determining the heating characteristics of non-spherical particles in combusting flows

In order to perform efficient numerical calculations of the heating characteristics of inert, non-spherical particles, an Euler – Lagrangian approach was used for this work. The continuous phase was the combusting flow field in a lab-scale furnace. The Steady Laminar Flamelet (SLF) and the Flamelet Generated Manifold (FGM) combustion model were used for simulations of the continuous phase and the corresponding results were evaluated. It was concluded that the FGM combustion model offers superior performance and moderate computational effort in combustion modelling of air and natural gas. The second phase consisted of discrete coal slag particles which were injected into the furnace. The heating characteristics of spherical and non-spherical particles were evaluated and a model for calculating the temperature of non-spherical particles was validated. The application of the FGM in combination with the Euler-Lagrange approach and specially developed multiphase model adaptions led to very precise results. The maximum deviation between experimentally and numerically determined mean temperatures of non-spherical particles was only 6 K at a temperature level of 680 K.

Depolymerization mechanism of CaO on network structure of synthetic coal slags

In entrained flow gasifier, slag properties such as melting and viscosity of molten slag are critical for continuous operation. Molten coal slag belongs to silicate-containing melts and slag behaviors are intrinsically determined by the microstructure transformation. Calcium is believed as an effective ingredient to decrease viscosity as well as reduce the polymerization degree. The structure of coal slags with different calcium ranging from 10% to 25% is investigated by solid-state nuclear magnetic resonance (SS-NMR) and molecular dynamics (MD). 29Si, 27Al spectra were obtained through quenching slags under 1873 K and the distributions of Qn species are analyzed. For simulation part, the radial distribution function, the proportion of bridge oxygen (Ob) and non-bridge oxygen (Onb) as well as the distributions of Qn species were calculated. Both NMR experiment and MD simulation show that increase of CaO can lead to a reduced degree of polymerization. The Ca ions enter into the network formed by Si and Al tetrahedral and bond with oxygen ions, which weakens the network stability. Consequently, the proportion of non-bridge oxygen increases and complex network unit-Q4 breaks into smaller units such as Q3, Q2, Q1, reducing the polymeric network. The depolymerization effect of CaO can explain the change of slag properties such as viscosity or melting temperature.

Deep structure analysis on coal slags with increasing silicon content and correlation with melt viscosity

In an entrained flow gasifier, the viscosity of molten slag along the wall is critical for continuous operation. Molten coal slag belongs to silicate-containing melts and slag properties are intrinsically affected by structure transformation. In this article, the structure of coal slags with different SiO2 compositions ranging from 48% to 63% wt is investigated by solid-state nuclear magnetic resonance (SS-NMR) spectroscopy and molecular dynamics (MD) simulation. 29Si, 27Al spectra were obtained and the contents of Qn species were analyzed. Radial distribution functions, proportions of bridging oxygen (Ob) and non-bridging oxygen (Onb) were calculated. Viscosities of slags were measured by a high temperature viscometer. Both NMR measurement and MD simulation show that an increase of silicon can lead to a high degree of polymerization. Most alumina is in form of the four-coordinate structure like the tetrahedral Si. The chemical bond of Si-O is the most stable compared to other bonds such as Al-O, Ca-O etc. Therefore, an increase of silicon can promote the [SiO4] formation and form more Ob while lowering the amount of Onb. The increased polymerization degree with higher silicon leads to a high slag viscosity and a linear relationship is found between the measured viscosities and fractions of Q4.

Plate load test of base taken from coal ash and slag mixture in experimental tray and on experimental section of embankment

Plate load test of base taken from coal ash and slag mixture in experimental tray and on experimental section of embankment

Research on Optimization Model of Rural Domestic Sewage Treatment Based on Control Unit

Most of the current research on rural domestic sewage is dispersal, while the research on unitized centralized domestic sewage treatment is relatively rare. The paper uses the bio-ecological combination technology to combine the constructed wetland as the control unit to carry out the ecological process ABR composite artificial optimization model research. Firstly, phosphorus removal research on five kinds of matrix fillers such as anthracite, coal slag, ceramicists, sandstone and brick slag is carried out, and the sewage at various levels is studied by adjusting the low, medium and high influenced pollutant loads and controlling the hydraulic retention time of pollutants. The processing efficiency of the processing unit during the filming period and the running period. Finally, the surface flow constructed wetland and the subsurface flow constructed wetland were set up as the secondary treatment unit for rural sewage treatment. The deep identification and extemporization of the sewage was carried out, and finally the effluent quality reached the first-level indicator of the Pollutant Discharge Standard of Urban Sewage Treatment Plant.

Synthesis of mesoporous silica from coal slag and CO2 for phenol removal

Utilization of coal slag, a waste generated from coal combustion, is an attractive, renewable, and environmentally friendly approach to helps ease energy shortage and stop environmental deterioration. Coal slag was used to synthesize mesoporous silica (MS) by reacting with CO2 released from a CO2-storage material (CO2SM) in a hydrothermal process. Five key conditions including initial pH, synthesis temperature and time, cetyltrimethyl ammonium bromide (CTAB) concentration, and CO2SM dosage were systematically investigated to optimize the synthesis of MS. Additionally, kinetic models and thermodynamic analyses for phenol adsorption by MS were systemically explored. Results showed that the MS exhibited a phenol adsorption amount of 106.2 mg/g. The MS could be recycled and reused, showing a good phenol adsorption after eight cycles. The current study opens a new approach to optimize the utilization of coal slag and provides a promising way to mitigate the greenhouse effect by substantially capturing CO2. In addition, the synthesized MS is promising to be used as a substitute commercially available.

Investigation of fluctuation behavior in viscosity of coal slags used in entrained-flow gasifiers

Entrained-flow gasification has attracted extensive attention because of its clean and efficient utilization of coal as well as flexible application in industry for power generation and chemical production. Numerous theoretical and experimental investigations on slag flow in entrained-flow gasifiers have been performed to ensure a smooth operation, but an unscheduled shutdown due to slag tap blockage remains an ongoing challenge. A fluctuation phenomenon of slag viscosity gives a rise in the risk of slag tap blockage. In this work, three Chinese coal slags with low silicon content and high iron content were used to investigate the viscosity fluctuation phenomenon. It was found that the crystalline phases and suspended bubbles can cause the fluctuation phenomenon. The fluctuation degree is related to the volume fraction of crystalline phases and the crystallization rate. In the search for the mechanism of the viscosity fluctuation, an in-situ observation using a high temperature stage microscope system (HTSM) in combination with XRD and SEM analyses was carried out. In addition, the viscosity of investigated slags with the suppression of solid phases as well as the phase distribution at high temperatures was calculated. Based on a comprehensive analysis of the results, a possible mechanism has been proposed that the viscosity fluctuation is attributed to an instant local change in the macrostructure such as the deformation of suspended bubbles, the change of the orientation of acicular crystals and the interaction of the acicular crystals and suspended bubbles.

Screening of Solid Waste as Filler Material for Constructed Wetlands

The reuse of solid waste can contribute to reducing Earth’s resource depletion, directly through use in the original production processes or by valorisation in alternative applications. In the present work, ten solid wastes were evaluated as candidates for filling material in constructed wetlands (CWs). For that purpose, physical characterization, leaching and adsorption tests were conducted. Limestone fragments and brick fragments resulting from construction activities, coal slags resulting from power plants, snail shells resulting from the food and catering industry, and cork granulates resulting from the cork industry have potential for use as CW fillers. These five materials have adequate physical properties and some capacity to adsorb phosphorous and organic compounds from wastewater. On the other hand, crushed eggshells resulting from egg farms, dealcoholized grape pomaces resulting from alcohol distilleries, olive seeds waste from olive-oil mills, and pine bark fragments and wood pellets resulting from forestry cleaning activities, wood mills and pulp mills did not demonstrate sufficient potential to be used as CW fillers, either because they have very low adsorption capacities or leach compounds in contact with water, or because they have less adequate physical properties. None of the tested solid wastes showed the ability to adsorb nitrogen compounds. Although the five selected materials do not present a special capability for adsorption of nitrogen, phosphorous and organic compounds, they can all be valued as CW fillers, representing a way to reduce the amount of solid waste sent to landfills.