Effects of pore fractal structures of ultrafine coal water slurries on rheological behaviors and combustion dynamics

The ultrafine coal water slurry (CWS) with the particle size of 1-10 μm, ash content of 1-2%, solid concentration of 50% is a promising substitute fuel for diesel oil. The effects of pore fractal structures of three ultrafine CWSs on their rheological behaviors and combustion dynamics were studied in this paper. When the pore fractal dimensions of Yanzhou, Huainan and Shenhua ultrafine CWSs increase, their apparent viscosities all increase and the increase extents gradually enlarge with decreasing shear rates, while their ignition temperatures and apparent activation energies all decrease. For example, when the pore fractal dimension of Yanzhou coal increases from 2.31 to 2.43, the CWS apparent viscosity at a low shear rate of 12 s⁻¹ increases from 75 mPa s to 2400 mPa s, and that at a high shear rate of 100 s⁻¹ increases from 80 mPa s to 820 mPa s. Meanwhile, the ignition temperature of Yanzhou CWS decreases from 445 °C to 417 °C at a heating rate of 12.5 °C/min, and the apparent activation energy decreases from 104 kJ/mol to 32 kJ/mol.     

Viscometry and rheology of coal water slurry

This paper presents the results of a detailed experimental study on rheological behaviour of coal water slurry (CWS). In contrast to earlier rheological studies at shear rates of rather narrow ranges, this study covered a much wider range of shear rates: 0–800 s⁻¹ in a rotational viscometer and 400–10000 s⁻¹ in a capillary viscometer. While the gap width of a rotational viscometer had no effect on the apparent viscosity of CWS, it was found that selection of proper tube dimensions in a capillary viscometer was important, Thus, the viscosities of CWS measured in both viscometers at the overlapping shear rates of 400–800 s⁻¹ were in good agreement. Consequently, it was possible to study the effect of shear rate higher than 800 s⁻¹ on slurry rheology. In addition, the effects of coal content, coal particle size and size distribution, and temperature on the slurry rheology were investigated. The slurry, a dispersion of coal particles in a pseudoplastic fluid, became more viscous and less pseudoplastic as the coal content was increased. The pseudoplasticity of a highly concentrated utility grind CWS approached that of a concentrated suspension of 40–50 μm solids in a Newtonian fluid. Also, the slurry pseudoplasticity was enhanced at a slightly elevated temperature, but significantly reduced as the shear rate exceeded a threshold dependent on coal content and particle size and size distribution.     

Microbial desulphurization of coal containing pyritic sulphur in a continuously operated bench scale coal slurry reactor

Pre-combustion microbial desulphurization of coal containing total sulphur (3.90%) and pyritic sulphur (2.80%) has been evaluated in a coal slurry reactor. The coal slurry reactor operated at hydraulic retention time (HRT) of 96 h with a coal pulp density of 15 percent and remove 79 percent of pyritic sulphur and 76 percent of ash with an increase in the calorific value of coal from 4400 to 6800 kcal kg⁻¹ at a pyritic load of 1.9 kg pyritic sulphur kg⁻¹ MLSS d⁻¹. The treated coal yield is 72 percent. The biochemical kinetic coefficients, viz. yield coefficient (Y) and decay coefficient (K_d) in the coal slurry reactor system are 0.178 and 0.007 d⁻¹, respectively, while maximum growth rate (μ_max) and half saturation rate constant (K_s) are 0.025 h⁻¹ and 0.220 g l⁻¹ as pyrite, respectively.     

An investigation on the rheological and sulfur-retention characteristics of desulfurizing coal water slurry with calcium-based additives

Desulphurizing coal water slurry is a kind of new clean coal water slurry(CWS), which has good performance on SO₂ emission during combustion and gasification process. But, the addition of sulfur-retention agents have some effects on the stability and fluid characters of the coal water slurry. In this paper, the viscosity, stability and rheology of Xinwen coal water slurry have been studied by adding different kinds of calcium-based sulfur-retention agents and different dosage. The results show that the sulfur-retention agents have little effect on rheological nature of CWS, which still presents pseudoplastic fluid. The addition of sulfur-retention agents will increase the viscosity of CWS, but the stability will decrease a little. The results also show that inorganic calcium has less negative effect on the performance of CWS than the organic calcium. The viscosity of the CWS with organic calcium agent keeps 1000–1200 mPa s when Ca/S molar ratio is 2. Sulfur release of the CWS with CaCO₃ reduces to 52% at Ca/S = 2 compared to original of 98%.     

Effect of dispersing and stabilizing additives on rheological characteristics of the upgraded brown coal water mixture

Upgraded brown coal water mixture (UBCWM) preparation by using an Indonesian upgraded coal produced by upgraded brown coal (UBC) process, was carried out to study the effect of dispersing and stabilizing additives on rheological behavior of the UBCWM. Three kinds of anionic dispersing additives, naphthalene sulfonic formaldehyde condensate (NSF), poly (meth) acrylate (PMA) and poly styrene sulfonic acid (PSS) and three kinds of stabilizing additives, carboxyl methyl cellulose (CMC), rhansam gum (S-194) and gellan gum (S-60) were used in this study. Results indicate that the addition of NSF 0.3 wt.% together with S-194 0.01 wt.% is effective in preparing UBCWM with good slurryability and stability, based on its rheological characteristics with the apparent viscosity at shear rate of 100 s^− 1 and yield stress at zero point of shear rate. The rheological behavior of all of the UBCWM that prepared, exhibits non-Newtonian Bingham plastic. From the economical point of view, the price of S-194 is expensive. On the other hand, CMC is cheap and abundant. Therefore, the addition of CMC 0.01 wt.% together with NSF 0.3 wt.% is also effective in preparing UBCWM with good fluidity and stability.     

Rheostructural studies on a synthetic mesophase pitch during transient shear flow

The steady, shear viscosities of a synthetic mesophase pitch (Mitsubishi AR-HP) obtained from rate-sweep experiments at 0.1-10 s⁻¹ exhibited shear-thinning (region I) and plateau responses (region II), but displayed a hysteresis during the decreasing rate sweep. Transient tests revealed that the shear stress (and consequently the shear viscosity) displayed a local maximum and a minimum before approaching a steady state. Following steady flow at 1 s⁻¹, a reversal of flow direction or a very short interruption in flow did not lead to the maxima or minima in the transient shear stress, but the maxima and the minima reappeared in the transient stress after a rest time of ∼1000 s. An experimental protocol was developed to preserve the rheological samples, and their microstructure was characterized in three orthogonal planes for the initial and final states. The initial microstructure was found to have a weak, but preferred, orientation of mesophase layers in the radial direction of the rheometer cone-plate (due to the initial squeezing flow). The initial microstructure changed to a flow-aligned fibrous structure after shearing in the viscometric flow.     

Influence of sewage sludge on the slurryability of coal-water slurry

In this study, the maximum solids loading, stability and rheological properties of coal-sludge slurries were investigated and compared with those of coal-water slurries. The maximum solids content of coal-sludge slurry was much lower than that of coal-water slurry because of the ∼82% water in sewage sludge. NaOH addition as a pre-treatment of sewage sludge improved the maximum solids content to 61% and 60% for dispersants A₁ and A₂, respectively, for a sludge-coal mass ratio of 22.1:100. Compared to coal-water slurries, the stability of coal-sludge slurries was better, with no solid deposition after 96 h. Coal particles and sewage sludge formed a stable medium and settlement of large coal particles was prevented. All the coal-sludge slurries exhibited pseudo-plastic behavior; this type of fluid is shear-thinning and is easy to transport.     

Plastic deformation of polyethylene crystals as a function of crystal thickness and compression rate

Plastic deformation of polyethylene (PE) samples with crystals of various thickness was studied during uniaxial compression with initial compressive strain rates of 5.5×10⁻⁵, 1.1×10⁻³ and 5.5×10⁻³ s⁻¹. Samples with a broad range of crystals thickness, from usual 20 up to 170 nm, were obtained by crystallization under high pressure. The samples underwent recoverable compression below the compression ratio of 1.05-1.07. Following yield, plastic flow sets in above a compression ratio of 1.12. At a compression rate of 5.5×10⁻⁵ s⁻¹ the yield stress increases with the increase of crystal thickness up to 40 nm. For crystals thicker than 40 nm the yield stress levels off and remains constant. This experimental dependence was compared with the model developed on the basis of classical crystal plasticity and the monolithic nucleation of screw dislocations from polymer crystals. In that model contrary to the experimental evidence, the yield stress does not saturate with increase of crystal thickness. The activation volumes determined from strain rate jump experiments and from stress relaxation for crystals thicker than 40 nm are nearly constant at a level of 8.1 nm³. This activation length agrees very well with 40 nm for crystal thickness above which the yield stress levels off. It is proposed, as shown in a companion communication, that for PE crystals thicker than 40 nm two other modes of dislocation emission in the form of half loops of edge and screw dislocations begin to govern the strain rate, which no longer depend on lamella thickness.     

Supercritical water oxidation of coal: Investigation of operating parameters' effects, reaction kinetics and mechanism

Supercritical water oxidation (SCWO) of coal was conducted in a continuous tubular reactor under various reaction conditions. Our experimental results show that the removal rate of chemical oxygen demand (COD) had no significant dependence on the temperature variations. Effect of residence time was less significant as exceeded fixed values. Free radical mechanism of SCWO reaction may be a possible explanation for the relative low conversion rate of coal at the range of tested oxygen excess. Compared with other parameters, effect of pressure was less significant. A global power-law rate expression was regressed from experimental data. The reaction orders for coal slurry and oxidant were 1.79 and 0.28 respectively. The reaction activation energy E_a was determined to be 112.3 kJ mol⁻¹, and the pre-exponential factor k₀ was 412 (mol/L)^−1.07 s⁻¹. The deviation between the model and experimental data was within ± 9%. Free radical mechanism, oxidation and hydrolysis mechanisms and phenolic hydroxyl oxidation mechanism were considered to be the possible mechanisms for the SCWO process of coal.     

Plastic deformation and damage of polyoxymethylene in the large strain range at elevated temperatures

The deformation behaviour of polyoxymethylene has been studied in plane strain compression at temperatures from 120 °C up to 165 °C and in uniaxial tension and simple shear at 160 °C for strain rates from 10⁻⁴ to 1 s⁻¹. In uniaxial tension the stress-strain behaviour was determined by a novel video-controlled testing system. The measurements showed that there was a very significant evolution of volumetric strain, indicating that damage mechanisms play a key role in the plastic deformation behaviour.All tests showed similar deformation stages with a short region of visco-elastic behaviour followed by a rounded yield point. The von Mises equivalent yield stress for these tests showed a linear relationship with logarithmic strain rate, suggestive of an Eyring type thermally activated process. After yielding, all stress-strain curves showed a long plastic deformation regime, which in shear occurred at constant stress. In plane strain compression there was also only a very small increase in stress, in contrast to uniaxial tension where very significant strain hardening was observed at high strains, which is attributed to the onset of structural changes.     

Experimental investigation of the shear dynamic behavior of double-lap adhesively bonded joints on a wide range of strain rates

The main concern of this work is the mechanical characterization of adhesively bonded assemblies under dynamic shear loading ranging from quasi-static (10⁻⁴ s⁻¹) up to high (10⁴ s⁻¹) strain rates. The double-lap shear sample is proposed and a bonding procedure is established. The assemblies are made of steel substrates bonded with an epoxy adhesive. Two surface treatments of the substrates are considered: ethanol and sand shooting. The shear strength and the failure strain are measured by taking into account the testing setups accuracy and the non-uniform distribution of the stress and strain fields in the overlap region. The sensitivity of the strength and the failure strain to the strain rate is highlighted; it is found that the failure strain decreases and the shear strength increases with the strain rate until reaching a maximum value then it drops for very high strain rates.     

Isolating the effects of thixotropy in geopolymer pastes

The rheological properties of potassium-based geopolymers were investigated through a series of experiments intended to isolate the influence of shear rate, recovery time, and shear ramping on thixotropy for a greater understanding of geopolymer thixotropic properties within the context of the geopolymer setting reaction. It is shown that for thixotropic disruption to occur a critical shear rate of 100 s⁻¹ must be reached or surpassed, full thixotropic restructuring occurs at around 90–100 min of total undisturbed rest time, and that reaching a state of full thixotropic disturbance heavily depends on subjected processing parameters. In addition, a consistent crossover between the storage and loss modulus within 1–3 min of oscillation during cyclical oscillatory measurements greatly indicates the repeatability and reversibility of thixotropy in geopolymers and the potential for tailorable viscosity. Overall, it is found that geopolymer pastes exhibit strong evidence of thixotropy, which is favorable for additive manufacturing, and that allotted rest time before shear and shear rate greatly influence the overall rheological properties.     

Influence of mixing sequence and superplasticiser dosage on the rheological response of cement pastes at different temperatures

An experimental methodology to study the rheological response of superplasticised cement pastes subjected to temperatures ranging from 5 to 45 °C is presented. The content of a polycarboxylate-based superplasticiser (PC) and moment of its addition to the mix (direct or delayed) are investigated. A loop of shear rate ramps is applied to each sample in order to obtain information about apparent viscosities and yield stresses, as well as a measure of their thixotropic behaviour. Results from the experimental campaigns indicate PC saturation dosages depend only slightly on temperature. The evolution of yield stress and thixotropy with temperature shows inverted trends under some conditions.     

Development and relaxation of orientation in sheared concentrated lyotropic solutions of hydroxypropylcellulose in m-cresol

Shear-induced orientation and the relaxation of orientation after the cessation of shear in 45 and 50 wt% solutions of cholesteric hydroxypropylcellulose (HPC) in m-cresol have been studied in situ by infrared spectroscopy and polarised microscopy. The shearing experiments were conducted at 30-80 °C at shear rates of 1-300 s⁻¹, which covered the director tumbling, wagging and a small part of the steady-state shear rate regimes. The steady-state order parameter was proportional to the shear rate and the proportionality constant increased with increasing HPC concentration and decreasing temperature. The concentrated solutions studied showed steady-state alignment even in the tumbling regime. Three different shear-rate regions with different behaviours of the solutions after the cessation of shear were found in these concentrated HPC solutions. At low shear rates (1-5 s⁻¹, referring to the 50% HPC solution) the polymer remained isotropic during shear but became gradually more oriented a few minutes after the cessation of shear, an observation that was substantiated by polarized microscopy. The order parameter reached a final plateau value and stayed constant at this level for long periods of time (∼24 h). At intermediate shear rates (from 5 to 50 s⁻¹ for the 50% HPC solution), a detectable order parameter was recorded at steady shear and, after the cessation of shear, the structure returned to an almost isotropic state within a few minutes, after which the orientation gradually started to increase to approach a plateau value after about 150 min. At even higher shear rates (∼100 s⁻¹ for to the 50% HPC solution), the initial steady shear order parameter relaxed to an almost isotropic state and remained constant at this level for time periods extending up to 24 h.     

Studies on slip casting behavior of lanthanum strontium manganite

Dispersion conditions for slip (slurry) formulation of a powder mixture of lanthanum strontium manganite (La_0.84Sr_0.16MnO₃ - LSM) and carbon (pore former) in water was studied through detailed zeta-potential and rheological measurements. The zeta potential variation with pH for the aqueous suspensions of only LSM or carbon exhibited a maximum value in alkaline medium (−40 mV to −50 mV at a pH of 10-11), establishing the pH window for their co-dispersion for slurry formulation. A study of the viscosity variation with shear rate for the slurries with varying solid content (in the range of 45-65 wt.%) exhibited pseudo-plastic flow behavior, indicating presence of flocculates in them. The yield stress values obtained from the Casson equation reduced with decreasing solid content, indicating reduction in the flocculate strength. The slip with solid content of 50 wt.% exhibited optimum flow characteristics to form long tubes with uniform wall thickness (wall thickness 2-4 mm and length of 150-200 mm). The tubular specimens formed after controlled carbon burn out and sintering at 1400 °C for 1 h possessed about 35% open porosity. The porosity remained the same upon further sintering at 1400 °C for 8 h.     

Needle penetrometry with variable force loading for measuring viscosity of pelletized coal upon heating

A needle penetrometry was performed loading steady force in a range from 1×10⁻³ to 2 N to pelletized coal upon heating via a cylindrical needle. From the observed effects of shear rate on apparent viscosity of softening coal pellet, defined as the shear-rate to shear-stress ratio, it was found that the pellet behaved as a Newtonian fluid for shear rates lower than a critical one while as a pseudo-plastic fluid for higher shear rates. The penetrometry was also carried out varying the force with time. The variable force loading enabled to maintain the shear rate well below the critical one, and thereby to measure the apparent viscosity of coal pellets as Newtonian fluids over a temperature range from 600 to 800 K. Upon heating at 10 K min⁻¹, the apparent viscosity of Goonyella coal pellet decreased from about 10¹⁰ Pa s at 640 K down to a minimum of about 10⁴ Pa s at 755 K, and increased up to 10⁹ Pa s at 800 K. In a course of heating as above, the viscosity of Blind Canyon coal pellet decreased above 600 K, underwent a minimum of about 10⁶ Pa s at 715 K, and increased up to 10¹⁰ Pa s at 770 K. Decreasing the heating rate from 10 to 3 K min⁻¹ caused the minimum viscosities of the pellets to increase by 1-2 orders of magnitude.     

超高性能混凝土流变特性及其调控研究

为了实现超高性能混凝土(UHPC)流变特性的高效调控,采用膨润土作为辅助胶凝材料并对制备的UHPC进行了性能评估,包括UHPC浆体的流动度、静态屈服应力、动态屈服应力、塑性黏度和触变性,并系统分析了不同掺量的膨润土对UHPC浆体流变性能的影响。结果表明:随着膨润土掺量增加,UHPC浆体的流动度整体表现为下降趋势,下降幅度逐渐增大;当膨润土掺量由0%增加到15.0%(质量分数)时,UHPC浆体静态屈服应力、动态屈服应力和塑性黏度均显著增大,分别提高了约17.05倍、5.78倍和1.16倍;随着膨润土掺量增加,滞回环面积和触变指数增大,触变性得到明显改善。同时,掺入膨润土后仍然满足UHPC的优异力学性能要求。     

Experimental measurement and numerical simulation of viscosity reduction effects in HMMPE containing a small amount of exfoliated organoclay-modified TLCP composite

A small amount (1 wt%) of organoclay-modified thermotropic liquid crystalline polymer (TLCP) acting as a viscosity reduction agent in high molecular mass polyethylene (HMMPE) was characterized and compared with purified TLCP (1 wt%) in HMMPE at 190 °C and 230 °C, respectively, where the TLCP displayed nematic and nematic-isotropic biphase structures. In the TLCP/PE blend at 190 °C and 230 °C, dramatic reductions in viscosity were observed with significant improvement in extrudate surface smoothness and an enlarged processing window. For the organoclay-modified TLCP in PE, the viscosity reduction ability of TLCP was further enhanced with viscosity dropped by up to >98.5% and >97.4% at 190 °C and 230 °C and processing window enlarged to >700 s⁻¹ and >900 s⁻¹ respectively in comparison to that of PE. Moreover, yielding stress, initial transition shear rate and transition region decreased to lower magnitudes than those of the TLCP/PE blend. A phenomenological model was applied to elucidate the mechanism of organoclay, TLCP and PE conformation before and after yielding in the confined capillary environment. A binary flow pattern model was applied to successfully predict the rheological behavior of the blends at 190 °C.     

Effect of pressure on early hydration of class H and white cement

The change in viscosity of cement slurry with temperature and pressure can be predicted by assuming that hydration can be treated as an activated process and that a given viscosity corresponds to a fixed degree of reaction. For Class H and White cements, chemical shrinkage experiments indicate that the activation energy is 33.8 kJ/mole and rheological measurements yield an activation volume of −30 cm³/mole. With these parameters, it is possible to predict the limit of pumpability of the slurry (which corresponds to a viscosity of about 2.5 Pa s) for arbitrary temperature and pressure cycles. This method of prediction requires that the physics of the process remain the same, but simply change in rate; therefore, the range of applicability is expected to be limited to temperatures below about 100 °C, since new phases occur at higher temperatures.     

Electrorheological fluids based on nano-fibrous polyaniline

Using a modified oxidative polymerization, the nano-fibrous polyaniline with 200 nm diameter and several micrometer lengths was synthesized on a large scale and then was applied as a new electrorheological (ER) fluid. Compared to conventional granular polyaniline ER fluid, the nano-fibrous polyaniline ER fluid exhibited distinctly improved suspended stability. Under electric fields, the nano-fibrous PANI ER fluid also exhibited larger ER effect. Its shear stresses are about 1.2-1.5 times as high as those of the granular PANI ER fluid. At the same time, the shear stress of nano-fibrous PANI ER fluid could maintain a stable level and even an increase for the wide shear rate regions from 0.1 s⁻¹ to 1000 s⁻¹ under various electric fields. In addition, the dynamic experiment showed that the shear modulus of nano-fibrous polyaniline ER fluid under electric field was higher than that of the granular polyaniline fluid, which also confirmed the larger ER effect.