Effects of sewage sludge on rheological characteristics of coal-water slurry

The coal-sludge slurry (CSS) containing coal, sewage sludge and water was prepared to study the effects of sewage sludge on rheological characteristics of the CSS. The yield stress, thixotropy and rheological type of CSS were investigated and compared with those of coal-water slurry (CWS). The results showed that the yield stress of CSS appears at the shear rate range from 0.05 to 0.14 s⁻¹. For CSS with the naphthalene sulfonate sodium formaldehyde condensate as dispersant and the sludge/coal mass ratio of 10:100, the yield stress can reach to 22.9 Pa. The thixotropy was quantitatively described by the thixotropy loop area, and sewage sludge can obviously improve the CWS thixotropy. The non-Newtonian behavior of CSS was characterized by a progressive decrease in viscosity with increasing shear rate at the shear rate range from 5 to 180 s⁻¹. By the analysis of FTIR, SEM and optical microscope, hydrophilic functional groups and colloidal structure of sewage sludge play the key roles on the different rheological characteristics of CSS and CWS.     

The resource utilization of algae—Preparing coal slurry with algae

Nowadays, the occurrence of harmful algal blooms is increasing rapidly all over the world. However, the methods of resource utilization of algae are very few. In this study, we propose a new way to dispose algae, which is gasification of coal-algae slurry. Coal slurries prepared with algae were investigated, and gasification reactivity of coal-algae slurry was compared with that of coal-water slurry (CWS). The results showed that, anaerobic fermentation, chemical treatment, high-speed shearing and heating are effective pre-treatment methods on reducing the viscosity of algae, which could obviously increase the maximum solids concentration of coal-algae slurry. When the de-ionized water/algae ratio is 1:1, the maximum solids concentration could get to 62.5 wt.%, which is almost the same as that of CWS. All the coal-algae slurries exhibit pseudo-plastic behavior, and this type of fluid is shear-thinning. Compared with CWS, the stability of coal-algae slurry is much better, which could be no solids deposition after 70 h. The coal-algae slurry displays better gasification reactivity than CWS.     

Study on the ash fusion temperatures of coal and sewage sludge mixtures

The coal, sewage sludge, water and chemical additives are milled to produce coal-sludge slurry as a substitute for coal-water slurry in entrained-flow gasification, co-gasification of coal and sewages sludge can be achieved. The ash fusion temperature is an important factor on the entrained-flow gasifier operation. In this study, the ash fusion temperatures (DT, ST, HT and FT) of three kinds of coals (A, B and C), two kinds of sewage sludges (W1 and W2) and series of coal-sewage blends were determined, and the mineral composition during the ash melting process was analyzed by X-ray diffraction (XRD). The results showed that the ash fusion temperatures of most coal-sewage blends are lower than those of the coals and sewage sludges. The ashes have different mineral composition at different temperature during the heating process. It was found that the mineral composition of AW1 blend ash is located in the low-temperature eutectic region of the ternary phase diagram of SiO₂-Al₂O₃-CaO. The minerals found in BW1 blend ash are almost the same as those in B coal ash. Kyanite is detected in CW1 blend ash, which results in the ash fusion temperatures of CW1 blend ash higher than those of C coal. We found that sodium mineral matters are formed because of NaOH added to W2, which can reduce the ash fusion temperature of coal-sewage blends.     

Wet jet milling of Al2O3 slurries

A wet jet milling process was employed as a novel method to prepare ceramic slurries. The wet jet milling pulverized raw materials to primary particle size within a short time. The wet jet-milled slurries showed very low viscosity, as comparing to ball-milled slurries. Moreover, the viscosity of wet jet-milled slurries was stable for long times, whereas that of ball-milled slurries increased rapidly with increase in the time. Al₂O₃ particles after wet jet milling kept initial surface conditions, though Al₂O₃ particles after ball milling yielded more OH groups on the surface. The relative density of the green bodies prepared from the wet jet-milled slurries was about 65% or more without depending on the solid content of slurry. On the other hand, the relative density of the green bodies fabricated by the ball-milled slurries increased with the slurry solid content, and it reached more than 60% at 50 vol% of solid content.     

Fabrication of stable Al2O3 slurries and dense green bodies using soft-energy milling process

Wet-jet milling and planetary homogenizing processes as soft-energy milling methods were employed as a novel method to prepare stable Al₂O₃ slurries. The viscosity of slurries prepared from the soft-energy milling was constant and stable for long time, as compared to ball-milled slurries. Though Al₂O₃ particles surface after ball milling yielded more hydroxyl groups, Al₂O₃ particles surface after the soft-energy milling was similar state to raw particles surface. Relative density of the green bodies prepared from the wet-milled slurries was about 67% and was independent on the slurry solid content. On the other hand, the relative density of the green bodies prepared from the ball-milled slurries increased with increasing solid content. Linear shrinkage of the sintered bodies prepared from the soft-energy milled slurries was independent of the slurry solid content, whereas that of the sintered bodies prepared from the ball-milled slurries increased with decreasing solid content.     

RHEOLOGICAL BEHAVIOR OF LOW-RANK COAL-WATER SLURRIES

The rheological behavior of low-rank coal-water slurries was studied using a Brookfleld Rheolog viscometer. A Sarpy Creek subbituminous coal from Montana, with particle sizes ranging from 0.044 mm to 0.223 mm, was used. The solids concentration in the slurry varied from 15 to 65 weight % and the temperature was maintained constant at 82°F. The rheological behavior of the slurry made from as received coal was compared with that of the slurry made from hot water dried coal

Coal-water slurries, made from both as received and hot water dried coal exhibited pseudoplastic behavior. The shear stress was found to be related to the shear rate by the power law model. As expected, the apparent viscosity was found to increase sharply with solids loading. It was also observed that the apparent viscosities of the slurries made from hot water dried coal were lower than those made from as received coal.     

Entrained-flow dry-bottom gasification of high-ash coals in coal-water slurries

It was shown that the effective use of dry ash removal during entrained-flow gasification of coal-water slurries consists in simplification of the ash storage system and utilization of coal ash, a decrease in the coal demand, a reduction in the atmospheric emissions of noxious substances and particulate matter, and abandonment of the discharge of water used for ash slurry. According to the results of gasification of coal-water slurries (5–10 μm) in a pilot oxygen-blow unit at a carbon conversion of >91%, synthesis gas containing 28.5% CO, 32.5% H₂, 8.2% CO₂, 1.5% CH₄, the rest being nitrogen, was obtained. The fly ash in its chemical composition, particle size, and density meets the requirements of the European standard EN 450 as a cement additive for concrete manufacture.     

Attribution of sewage sludge and sludge-based char in a fluidized bed reactor

Fluidized bed pyrolysis has been recognized as an innovative technology for sewage sludge treatment. The physical and attrition properties of sewage sludge are changed through the fluidized bed pyrolysis. The minimum fluidization velocities and attribution rate constants for sewage sludge and sludge based-char were obtained from pressure drop and attribution tests. As a result, sewage sludge with 20% moisture content and char were classified as Geldart B solids and the superficial gas velocity for bubbling fluidization was 0.2142-0.8755 m/s. In addition, attribution of the sewage sludge and char was more affected by particle size than by material type. The equations for the overall attrition rate constants are K _a × 10⁵ = 1.09U − 14.82 for sludge and ln k _a = 0.1(U−U _mf )− 13.63 for char, respectively.     

RHEOLOGICAL BEHAVIOR OF LOW-RANK COAL-WATER SLURRIES

The rheological behavior of low-rank coal-water slurries was studied using a Brookfleld Rheolog viscometer. A Sarpy Creek subbituminous coal from Montana, with particle sizes ranging from 0.044 mm to 0.223 mm, was used. The solids concentration in the slurry varied from 15 to 65 weight % and the temperature was maintained constant at 82°F. The rheological behavior of the slurry made from as received coal was compared with that of the slurry made from hot water dried coal

Coal-water slurries, made from both as received and hot water dried coal exhibited pseudoplastic behavior. The shear stress was found to be related to the shear rate by the power law model. As expected, the apparent viscosity was found to increase sharply with solids loading. It was also observed that the apparent viscosities of the slurries made from hot water dried coal were lower than those made from as received coal.     

Separation of Colloidal Particles from Nonaqueous Media by Cross-Flow Electrofiltration

Abstract

Charge characteristics of particles in aqueous or nonaqueous slurries are known to play an important role in solids-liquid separation processes. We have been conducting a fundamental study on filtration of colloidal particles suspended in nonaqueous media, such as coal and tar sand slurries based on their charge characteristics. This paper presents results of such a study involving cross-flow electrofiltration of nonaqueous slurries. Data are reported for α-Al₂O₃ particles suspended in tetralin. The effects of feed rate, driving pressure and electrical field strength on the filtration rate, total deposition rate on the central electrode, and the efficiency of the filter are presented.

The outlet slurry concentrations were measured with a specially built X-ray densitometer. These data are analyzed by a mathematical model using a Graetz-type analysis. The rate of deposition was found to be determined mainly by the electric field. The sludge flow near the central electrode significantly affected the efficiency of separation.     

Hydrothermal dewatering of lower rank coals. 1. Effects of process conditions on the properties of dried product

The hydrothermal dewatering of a Loy Yang Low Ash Victorian brown coal has been studied under a wide range of process conditions. The intra-particle porosity (<1 μm pore radius) determined by mercury porosimetry of the resulting dried products, a proxy for the maximum solids concentration of coal-water slurries, is reported as a function of reaction time, processing temperature, autoclave size and configuration and slurry concentration. Of these variables only the process temperature had a significant influence on the product intra-particle porosity, which decreased with increasing temperature. Other product variables were affected by a wider range of process conditions; increase in temperature and residence time and decrease in slurry concentration led to marked increase in loss of organic material to waste water, and the elemental composition of the product changed significantly with e.g. residence time. Drying of coal samples of particle sizes >50 μm gave products of constant intra-particle porosity but smaller coal particles gave products with higher intra-particle porosities as a result of agglomeration. Milling of the coal reduced the porosity of the dried coal at constant particle size.     

Author index

High-frequency screen tests of a number of different fine slurries are analyzed to determine the effect of slurry feed rate and weight per cent solids on screen size selectivity values. Results of this analysis indicate that for the screen cloths tested, slurry feed rate has a negligible effect on size selectivity values but that increasing the feed solids concentration causes finer but less efficient particle size separation. The size selectivity data are analyzed to give values for the d₅₀, the sharpness index and apparent bypass parameters of a screen size selectivity equation. Approximate empirical relationships between these parameters and screen aperture size and slurry feed solids concentration are given, and an example of the use of the size selectivity equation in grinding-circuit simulations is presented.     

Sulphur retention during co-combustion of coal and sewage sludge

The aim of this work is the study of the S retention in ashes from combustion of three bituminous coals and a sewage sludge, processed with lime from an urban wastewater treatment plant, that uses FeCl₃ as a coagulant. The effect of the sludge addition during co-combustion of sludge-coal blends in the proportions 10 and 50 wt% of sludge is analysed. The combustion was carried out in an electric furnace at different temperatures (800, 900, and 1100 °C).The results have confirmed that sludge addition to coal enhances S retention at any temperature due to the high CaO contents of the sludge and the formation of CaSO₄. However, the addition of FeCl₃ may be prejudicial, since it produces Na and K volatilisation during co-combustion. Moreover, the presence of Fe₂O₃ from FeCl₃ can reduce the amount of retained sulphur due to its reaction with CaO to produce calcium ferrite. Another element with a similar effect to Fe is Si, since it forms larnite (2CaO·SiO₂) in coal-sludge blends with high Si and Ca contents. Lineal relationships have been found between S retention and the above oxide contents.     

Formation of NOx precursors during the pyrolysis of coal and biomass. Part V. Pyrolysis of a sewage sludge

A sewage sludge sample from a wastewater treatment plant in China was pyrolysed in a fluidised-bed/fixed-bed reactor and in a fluidised-bed/tubular reactor. HCN was found to be the main NO_x precursor, representing up to about 80% of the nitrogen present in the sludge. The thermal cracking of volatiles is the main route of HCN formation. NH₃ was also an important NO_x precursor formed during the pyrolysis of the sewage sludge. The experimental results indicate that there are at least two distinctive stages of NH₃ formation during the pyrolysis of the sewage sludge at a fast heating rate. The formation of NH₃ at temperatures lower than 400-500 °C is at least partly due to the amino structures in the sludge. The reactions of volatiles in the gas phase make negligible contributions to the observed NH₃ yield.     

Effect of fine solid particles on absorption rate of gaseous CO2

The influence of the properties of solid particles in slurry on the absorption of CO₂ in the slurry was investigated in a stirred thermostatic reactor. The absorption experiments were carried out in three different slurries consisting of water, cyclohexane and soybean oil, respectively, and three kinds of solid particles (active carbon, active alumina and silica gel) were incorporated into each of the above mentioned slurries separately. The experimental results show that the active carbon particles could enhance the absorption rate of gaseous CO₂ in the aqueous slurry, while in the cyclohexane slurry, active carbon particles indicated no the absorption enhancement effect. However, it was observed that the active alumina and silica gel particles could enhance the absorption rate of CO₂ in the cyclohexane slurry. These phenomena indicate that the solid particles, which could enhance the gaseous CO₂ absorption rate, should possess two properties simultaneously, i.e. they rejected the solvent and had higher adsorption capacity for the solute. The experimental results also show that, as for those solid particles which could enhance the gas absorption rate, the enhancement increased quickly with the increase of solid concentration in slurry at first, and then reached a constant value gradually. It was also found that the enhancement factor was related to the coverage fraction of solid particles on the gas-liquid interface, and due to the reduction of surface fraction with increasing stirred speed, the enhancement factor decreased. __________ Translated from Journal of Chemical Engineering of Chinese Universities, 2008, 22(2): 356–360 [译自: 高校化学工程学报]     

Effects of sintering additives on dispersion properties of Al2O3 slurry containing polyacrylic acid dispersant

The effects of sintering aid adsorption on the dispersion properties of aluminum oxide slurries were investigated. We considered Al₂O₃ slurry without additives and Al₂O₃ slurry with a Mg additive with 0.1 mass% in oxide equivalent as a sintering aid. In this study, we evaluated the adsorption isotherm of polyacrylic acid (PAA) onto Al₂O₃ and the dispersion degree of Al₂O₃ slurries in sedimentation tests under gravity. The adsorption isotherm featured a characteristic adsorption isothermal line with a maximum value when Mg additive was present in Al₂O₃. In addition, the packing fractions did not correspond to the apparent viscosity. However, in slurry that was allowed to settle for several days, both of them agreed.Therefore, the disagreement between the packing fraction and the apparent viscosity immediately after preparation arose from changes of the dispersion state, such as the decrease of the distance between particles with time.     

Instrument for ceramic particle dispersion II: Computational fluid dynamics analysis

A planetary-type mixer using a container equipped with stainless steel mesh has been developed. For various slurries (Al₂O₃ in water), each modeled as Newtonian fluid, the shear stress was calculated using computational fluid dynamics (CFD) under various mixing conditions and with different equipment properties. The meshed-geometry included more than 100,000 nodes with hexahedral cells in one zone, with quadrilateral cells in the remaining zones. The fluid viscosity, rotation rate, and mesh opening affected the maximum shear stress. The shear stress increased concomitantly with increasing fluid viscosity. The container rotation rate and the maximum shear stress share a proportional relation. For a fluid with 9.2 mPa s, the shear stress was 134 Pa or more for a 0.81 mm and larger mesh opening, as observed at the bottom of the container. Mesh having an opening smaller than 0.81 mm generated high shear stress on the mesh surface. The maximum shear stress increased with decreasing mesh opening size. The particle size distributions of the Al₂O₃ particles in the slurries after treatment by the mixer were estimated under conditions similar to those of the calculations. Results show peaks in the particle size distributions of the Al₂O₃ particles in the slurries before treatment at 0.2 and 2-70 μm because of the primary particle size and agglomerates. The amounts of the agglomerate decreased concomitantly with the decreased mesh opening size. When slurries pass through the small mesh openings, high shear stress is generated. That achieves the good dispersion of the sub-micron sized Al₂O₃ particles in the slurry.     

Preparation of high solid loading and low viscosity ceramic slurries for photopolymerization-based 3D printing

In this study, high solid loading and low viscosity cordierite slurries are successfully developed for the first time for photopolymerization-based additive manufacturing. The processability of the slurries is mainly determined by their rheological properties and photocuring parameters. The slurry preparation involves the orthogonal optimization of compositions in order to achieve suitable viscosity, stability and homogeneity. The photocuring parameters of the as-prepared slurries, including penetration depth D_p and critical exposure E_c, are also determined experimentally. Results show that viscosity increases with reduction in particle size. A higher solid-volume fraction also results in an exponential growth in viscosity. As for the dispersant amount, a concentration of 5?wt% leads to the lowest viscosity. Particle size also play an important role in the solid loading capacity of the slurries, as results suggest that smaller particles improve performance. In terms of the photocuring behaviors, the addition of 2?wt% photoinitiator generates an optimal curing process. 40?vol% solid loading leads to the thickest curing depth for all slurries with different types of particle sizes. Finally, a cordierite part with a complex hollow structure and a fine resolution is successfully fabricated. The present study offers a material basis for the polymerization-based 3D printing of porous cordierite structures.     

Flash spray characteristics of a coal-liquid carbon dioxide slurry

Liquid carbon dioxide (LCO₂) could potentially be utilized in coal gasification plants for effectively transporting coal particles, replacing conventional carriers such as water (H₂O), particularly in wet-fed gasifiers. However, it is essential to understand the atomization behavior of LCO₂ leaving an injector nozzle under both coal-free and coalfed conditions. We examined the atomization behavior of a coal-LCO₂ slurry during the throttling process. The injector nozzle was mounted downstream of a high-pressure spray system. The effect of upstream pressure on flash atomization and devolatilization behavior was presented. Compared with the coal-LCO₂ mixture, the spray pattern of the coal-water mixture was significantly different, since it evidenced a Rayleigh-type breakup mode. This difference indicates that the coal-water slurry did not transport the coal as effectively as the coal-LCO₂ slurry.     

The effect of noncondensable gas on heat transfer in the preheater of the sewage sludge drying system

We used a shell-and-tube type preheater to investigate the effect of noncondensable gas on heat transfer. In the preheater of the drying system, heat is exchanged between steam-air mixed gas which is dryer outlet gas and sewage sludge. To evaluate the performances of the preheater, water was first used in the tube-side material instead of sewage sludge and steam-air mixed gas in the shell-side material. The test variables were as follows: mixed gas inlet temperatures range from 95 to 120 °C; inlet air content, m _air /m _steam from 55 to 83%; tube-side water flow rate from 42 to 62 kg/h. The shell-side heat transfer coefficient varied from 150 to 550W/m²K, which corresponds to the amount of noncondensable gas in the steam-air mixed gas and the overall heat transfer coefficient varied from 60 to 210W/m²K. Using sewage sludge as a tube-side material the overall heat transfer coefficient varied from 60 to 130W/m2K and the outlet temperature of sewage sludge was above 90 °C, which is high enough for reducing energy consumption in the dryer by preheating the sewage sludge.