Slurry Droplets Research Articles

Maximum combustion temperature for coal-water slurry containing petrochemicals

This study examines the temperature change of droplets of coal-water slurry containing petrochemicals (CWSP). The slurry consists of coal and oil processing waste. The temperature of oxidant in a modelled combustion chamber is varied between 600 and 1200 K. The initial size (radius) of CWSP droplets varied in the range of 0.5–3 mm. The study identifies typical temperature trends at the center and on the surface of the CWSP droplet. The temperature trends represent the following stages: (i) heating of fuel, (ii) evaporation of water and a liquid combustible component, (iii) thermal decomposition of coal and yield of volatiles, (iv) gas phase ignition of volatiles together with vapor of the combustible liquid, and (v) heterogeneous ignition of carbon and its burnout. Moreover, these trends indicate the maximum combustion temperatures of CWSP that reflect corresponding heat release. The study specifies the parameters which influence the maximum combustion temperature: fuel component composition, properties of components, droplet size, and the oxidant temperature. Finally, the study defines the minimum ignition temperatures and delay times of sustainable combustion initiation that characterize the ignition inertia. The knowledge of influence of these factors will allow one to predict the optimal conditions for the combustion of the CWSP.

Effect of macroscopic porosity onto the ignition of the waste-derived fuel droplets

We have observed the influence of an artificial macroscopic surface modification onto the ignition of droplets of the waste-derived coal water slurry with petrochemicals. Fuel composition was based on filter cake of bituminous gas-coal with inclusion of small amount of the waste oil fuel. The sizes and other parameters of the fuel droplets were chosen very close to used in typical power units. The fuel droplet surface was pierced making conical pores whose depth was more than half of the droplet radius. Changes of the ignition delay time were analysed together with features of the combustion of volatiles in vicinity of introduced macroscopic pores. The advanced mathematical model was used to clarify the contribution of the droplet surface modulation in different effects that present during the fuel ignition. It was shown that ignition delay time is decreased by up to 20% with growth of number of pores and particularly with growth of the specific surface of the droplet (by 10–20% and more). The maximal combustion temperature at the droplet center was slightly decreased after the introduction of the macro-pores. The recommendations for modification of the industrial power units to optimized ignition regime were presented.

Fine Particle Transformation during the Limestone Gypsum Desulfurization Process

Special attention is required on the fine particle control after desulfurization because a large quantity of fine particles are emitted into the atmosphere from the coal-fired power plants. In an attempt to figure out the transformation of fine particles during the limestone gypsum desulfurization, the physical properties of fine particles before and after the desulfurization system were measured in a power plant. Moreover, the investigations on the removal and formation characteristics of fine particles during the desulfurization process were carried out in a pilot plant. The results showed that the particle characteristics changed after desulfurization and new particles of CaSO4 may be generated. The scrubbing of the desulfurization slurry generated large numbers of fine particles, which were mainly located in the sub-micrometer range, and the inlet SO2 concentration had little influence on the fine particle formation. Besides, the desulfurization slurry characteristics were closely related to the concentration and size distribution of fine particles after desulfurization. With the lower proportion of fine particles and the decrease of the solid mass concentration in the desulfurization slurry, the generated fine particles were reduced. The fine particle emission can be controlled by the adjustment of operation parameters. With the decrease of the gas flow velocity and the liquid/gas ratio, fewer droplets of the desulfurization slurry were entrained out of the scrubber, resulting in the corresponding decrease of fine particle concentrations after desulfurization.

The ignition parameters of the coal-water slurry droplets at the different methods of injection into the hot oxidant flow

Two different experimental approaches were realized for investigations of integral characteristics of single droplet ignition of coal-water slurry containing petrochemicals (CWSP) inside the hot flow of the gas oxidant. At first, the droplet was fixed inside the flow by special holder and the second way is free injection of the soaring droplet into the volume of combustion chamber. Research reports an experimental estimation of influence of material holder on ignition characteristics for typical CWSP. This is important, since many researchers use various holders (the ceramic rod, thermocouple junction, metal wire) when inserting droplets of coal-water slurry (CWS) and CWSP into the combustion chamber. There were three types of the droplet holders: the junction of the high-speed thermocouple, metallic wire and the ceramic rod. The basic components of the CWSP were filter-cake (the coal processing waste), the used turbine oil, the plasticizer (wetting agent) and water. Initial droplet radius was 0.5–1mm, and the temperature and the oxidant flow velocity were 400–1000K and 0.5–5m/s correspondingly. The ignition delays and the combustion times for the fuel droplets together with minimal temperatures of the stable ignition (with further combustion) were defined. Comparison of parameters of the flying droplet ignition with corresponding values for droplets that were fixed by holder was done. Soaring droplets have smaller ignition delay times (for 7–25%) relative to the case of droplet fixed inside the oxidant flow by holder (when the all rest parameters are similar). Influence of the holder material onto the values of times of ignition delay and combustion become essentially weaker with oxidant temperature growth. The obtained numerical results show the limitations and advances of the two approaches for laboratory investigations of the CWSP ignition and combustion.

The effect of the petrochemicals at the ignition of the coal-water slurry droplet

We have observed in details the ignition of the wasted-derived composite fuel based on filter cake of bituminous coal mixed with mass 10% of oil fuel. The effect of the petrochemicals is shown by high speed video recording. The 10X zoom allows to see the accumulation of released volatile gases at the droplet surface inside the oil-shell bubbles. The effect of the volatiles was observed by thermal imaging system that have shown the great jump of the temperature around the droplet at the beginning of the ignition. The principal stages of the fuel combustion were shown by simultaneous monitoring of the temperatures at the surface and in the bulk of the fuel droplet.

CFD modeling of a pilot-scale countercurrent spray drying tower for the manufacture of detergent powder

ABSTRACTA steady-state, three-dimensional, multiphase computational fluid dynamics (CFD) modeling of a pilot-plant countercurrent spray drying tower is carried out to study the drying behavior of detergent slurry droplets. The software package ANSYS Fluent is employed to solve the heat, mass, and momentum transfer between the hot gas and the polydispersed droplets/particles using the Eulerian–Lagrangian approach. The continuous-phase turbulence is modeled using the differential Reynolds stress model. The drying kinetics is modeled using a single-droplet drying model, which is incorporated into the CFD code using user-defined functions (UDFs). Heat loss from the insulated tower wall to the surrounding is modeled by considering thermal resistances due to deposits on the inside surface, wall, insulation, and outside convective film. For the particle–wall interaction, the restitution coefficient is specified as a constant value as well as a function of particle moisture content. It is found that the variation in the value of restitution coefficient with moisture causes significant changes in the velocity, temperature, and moisture profiles of the gas as well as the particles. Overall, a reasonably good agreement is obtained between the measured and predicted powder temperature, moisture content, and gas temperature at the bottom and top outlets of the tower; considering the complexity of the spray drying process, simplifying assumptions made in both the CFD and droplet drying models and the errors associated with the measurements.

Optimization of mass-production conditions for tritium breeder pebbles based on slurry droplet wetting method

Lithium metatitanate (Li2TiO3) is being considered as tritium breeding material for solid-type breeding blanket, which are used in pebble-bed form. The total amount of Li2TiO3 pebbles in Helium Cooled Ceramic Reflector (HCCR) Test Blanket Module (TBM) is approximately 80kg. Furthermore, DEMO reactor requires a great deal of breeder pebbles. Therefore, the development of mass-production system for breeder pebbles is necessary. The slurry droplet wetting method was adopted in the mass-production process for Li2TiO3 pebbles, which had been developed in Korea. In this method, an automatic slurry dispensing system is one of the key apparatuses because the uniformity of pebbles and production rate are able to be improved. The system was successfully manufactured, which was consisted of a dispensing unit for instillation of Li2TiO3 slurry, a glycerin bath for hardening of droplets, and an automatic maintaining unit for constant distance between syringe needle and glycerin surface. The production rate of this system for Li2TiO3 pebble was estimated at 50kg/year. In this study, it was investigated that the effect of dispensing and sintering conditions on the mass-production of Li2TiO3 pebbles.

Minimum temperatures for sustainable ignition of coal water slurry containing petrochemicals

This paper presents the conditions and characteristics of the sustainable ignition of coal water slurry (CWS) containing petrochemicals in the air flow. This study may be useful to expand the raw material base for the power industry and the effective recycling of low-grade wastes from coal and oil processing. We use an experimental setup to investigate the ignition of slurry droplets with the radius from 0.5 mm to 1.5 mm when heating in the air flow with the temperature of 500–900 K and rate of 0.5–5 m/s. The present paper also studies necessary and sufficient conditions for the ignition of the different slurry compositions. We reveal the minimum ignition temperatures of slurry droplets in air flow. In addition, we show that these temperatures can be significantly less than the temperatures in combustion chambers of power plants (as a rule, higher than 900 K). Furthermore, we determine the times of ignition delay and complete combustion for slurry droplets, as well as the influence of the air flow temperature and rate, droplet sizes, and slurry composition on these times. Research results show that combustion initiation conditions and characteristics of fuel compositions can vary within wide ranges. This effect contributes to using CWS containing petrochemicals in power engineering, chemical, petrochemical, and transportation industries.

Measurement of slurry droplets by digital holographic microscopy: Fundamental research

Abstract This study was conducted to research on the size distribution and concentration measurements of slurry droplet by DHM (digital holographic microscopy). The slurry from desulfurization tower was atomized into micron droplets. The ability of DHM to detect micron droplets was studied by simulation. Micro-holographic measurement system of spray field was built, with equivalent CCD (charge-coupled device) pixels being 0.375 μm. The size distribution of droplets was measured by DHM, showed unimodal distribution from 0.53 to 5.3 μm, with the peak at about 2.5 μm. The average particle size was 2.09 μm. Number concentration of the slurry droplets was also obtained. The concentration of DHM concentrated in the range of 1.5–3 μm, about 53–1118 mm −3 . Accordingly, the concentration of PDA (phase Doppler anemometer) concentrated in the range of 0.5–5 μm, about 19–610 mm −3 . The experimental results show that method based on DHM for slurry droplets measurement is feasible. Based on the simulation and experimental research, an on-line measurement system for slurry droplets was designed.

Fabrication of the Li2TiO3 tritium breeder pebbles by a capillary-based microfluidic wet process

Li2TiO3 is one of the most promising candidates among tritium breeders. In this work, Li2TiO3 pebbles with a narrow size distribution, high density, small grain size, and good sphericity are synthesized using the commercial Li2TiO3 powder as the raw material by a co-flow capillary-based microfluidic wet process. Highly uniform slurry droplets containing Li2TiO3 powder and polyvinyl alcohol were formed in the microfluidic device and then solidified with the cross-linking reaction between polyvinyl alcohol and borax. Li2TiO3 pebbles were finally obtained after sintering the green bodies. The size of sintered Li2TiO3 pebbles with a good sphericity (Dmax/Dmin < 1.05) can be controlled precisely in a size range of 400–1000 µm by adjusting the flow rate of disperse and continuous phases. The effects of the calcination method, sintering condition, solid content of slurry, and the particle size of Li2TiO3 powder on the relative density of Li2TiO3 pebbles prepared were studied in detail, and 97.16% T.D. (Theoretical Density) maximum relative density of the pebbles with 5 µm grain size were fabricated.

Numerical simulation of spray drying of hydroxyapatite nanoparticles

As a way of improving handling safety of nanoparticles for environmental applications on an industrial scale, we considered the consolidation of nanoparticles into micron-sized spray-dried agglomerates. Spray drying has recently been favored for manufacturing of nanoparticle agglomerates including hydroxyapatite-based composites. This study numerically investigated the drying mechanism of the slurry of hydroxyapatite nanoparticles in an industrial-scale spray dryer. The detailed model of drying kinetics of a slurry droplet was combined with the energy, mass, and momentum balances of drying air and droplets in the dryer chamber. The results were compared to drying of silica nanoparticles. The simulation analysis exposed dissimilarities between drying mechanisms of hydroxyapatite and silica slurry droplets. Hydroxyapatite droplets required longer drying times. Higher drying air temperature and lower humidity were detected in the chamber during drying of hydroxyapatite droplets. Significant temperature differences were observed between the surface and center of agglomerates. We also found lower rates of external heat and mass transfer between drying air and droplets, higher heat transfer resistance, and heat accumulation in the crust layer of hydroxyapatite droplets. We showed that the larger density, lower heat conductivity, and higher heat capacity of hydroxyapatite were responsible for the differences in drying mechanisms of hydroxyapatite and silica. The obtained results are significant for designing an industrial-scale spray dryer capable of producing nanoparticle agglomerates for environmental applications.

Measurement of slurry droplets in coal-fired flue gas after WFGD.

China owns the world's largest capacity of coal-fired power units. By the end of 2012, the capacity of installed national thermal power has been 819.68 million kilowatts. The latest standard requires that newly built power plants emit SO2 in no more than 100 mg/m(3) and the emission of old ones be lower than 200 mg/m(3) while in some key areas the emission should be controlled under 50 mg/m(3). So by the end of 2012, 90% of the active coal-fired units have been equipped with flue gas desulfurization devices. Among the desulfurization methods adopted, limestone-gypsum wet flue gas desulphurization accounts for 92%, causing the problem of fine droplets in the exhaust gas after defogger, which may even form "gypsum rain." At present, sampling methods are widely used at home and abroad, such as magnesium ion tracer method, modified magnesium ion tracer method and chemical analysis. In addition, some scholars use aerodynamic methods, such as ELPI, to measure the diameter distribution and concentration. The methods mentioned above all have their own demerits, such as the inability to on-line, continuous measurements and the need of prolonged measuring time. Thus, in this paper some potential optical on-line methods are presented, such as Fraunhofer diffraction pattern analysis and wavelength-multiplexed laser extinction. Also brought up are their measuring scope and merits. These methods have already been utilized to measure small liquid droplets and their demonstrations and evaluations are as well stated. Finally, a 3D imaging method based on digital holographic microscope is proposed for in-line measurement of size and concentration of slurry droplets. The feasibility of this method is demonstrated by preliminary experimental investigation.

Removal of Coal-Fired Pollutants Using a Novel Composite Collector in a Wet Electrostatic Precipitator

Rigorous new regulations in coal-fired pollutants emissions from power plant strengthen the requirements for electrostatic precipitator (ESP) technology. The study was inspired by the requirement to use the new composite collectors as substitutes for typical steel collector to save energy and structural materials. A bench scale and a pilot scale experimental system have been set up to investigate the performance of the novel wet composite collector. Contrastive collection efficiencies of different assembled modes were investigated. The removals of coal-fired pollutants including acid aerosol, ammonia aerosol, slurry droplets and fine particles were performed in new wet ESP, as well as to quantify the key properties of the new wet ESP. The results show that the collection efficiency of the new composite collector was higher than that by typical collectors using a thimbleful of water penetrating the surface via capillary flow. The new wet ESP device acts in synergy with WFGD and SCR systems for controlling coal-fired pollutants emissions and solves the adverse impacts caused by wet flue gas desulfurization (WFGD) and selective catalytic reduction (SCR) systems at the same time.

Fabrication of Li2TiO3 pebbles using PVA–boric acid reaction for solid breeding materials

Lithium metatitanate (Li2TiO3) is a candidate breeding material of the Helium Cooled Ceramic Reflector (HCCR) Test Blanket Module (TBM). The breeding material is used in pebble-bed form to reduce the uncertainty of the interface thermal conductance. In this study, Li2TiO3 pebbles were successfully fabricated by the slurry droplet wetting method using the cross-linking reaction between polyvinyl alcohol (PVA) and boric acid. The effects of fabrication parameters on the shaping of Li2TiO3 green body were investigated. In addition, the basic characteristics of the sintered pebble were also evaluated. The shape of Li2TiO3 green bodies was affected by slurry viscosity, PVA content and boric acid content. The grain size and average crush load of sintered Li2TiO3 pebble were controlled by the sintering time. The boron was completely removed during the final sintering process.

Disruptive burning of precursor/solvent droplets in flame‐spray synthesis of nanoparticles

Flame spray pyrolysis is an established technique for synthesizing nanoparticles in the gas phase through aerosol combustion of precursor/solvent droplets. The combustion characteristics of isolated micron‐sized precursor/solvent droplets are investigated experimentally. Pure solvent droplets burn uniformly and classically quasisteady, whereas precursor/solvent droplets manifest disruptive combustion behavior. The fast onset of droplet disruption, which occurs only for solutions with dissolved metal precursors, is not due to solid‐particle precipitation within the droplet. Instead, the mechanism of disruptive droplet burning is similar to that of slurry droplets, consisting of three main steps: (1) diffusion‐controlled burning of the high‐volatile solvent, (2) viscous‐shell formation due to decomposition of the low‐volatile metal precursor, and (3) subsequent disruption due to heterogeneous nucleation. The time sequence of the three steps depends on the concentration and decomposition characteristics of the metal precursor, shortening with increased concentration and higher incremental decomposition temperature. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4553–4566, 2013

Analysis of Drying Kinetics of a Slurry Droplet in the Falling Rate Period of Spray Drying

Analysis of Drying Kinetics of a Slurry Droplet in the Falling Rate Period of Spray Drying

A one-dimensional plug-flow model of a counter-current spray drying tower

A one-dimensional numerical model for a detergent slurry drying process in a counter-current spray drying tower is developed for the prediction of the gas and droplet/particle temperature profiles within the tower. The model accommodates droplets/particles over a range of sizes. A semi-empirical slurry droplet drying model is integrated with a counter-current tower simulation based on mass, energy and particulate phase momentum balances in order to calculate the drying rate and the particle residence time within the tower. The coupled first order ordinary differential equations for the two phases are solved numerically using the iterative shooting method in an algorithm developed within MATLAB. The predictions of the numerical model are compared with industrial pilot plant data. The results are found to vary significantly with the specified size distribution of the droplets. Despite the simplicity of the model in ignoring the coalescence, agglomeration, wall deposition and re-entrainment, the model gives reasonable agreement with the experimental data.

Dual-stage ignition of boron particle agglomerates

The ignition characteristics of boron particle agglomerates prepared by drying 2–3mm water slurry droplets of crystalline or amorphous boron powders were investigated by introducing suspended agglomerates into a flow of high temperature oxidizing gases (oxygen/nitrogen/water vapor) obtained by a combination of combustion and electrical heating. Monitoring of the agglomerate luminosity, spectra, and temperature revealed a dual-stage ignition process. The first stage ignition was observed at flow temperatures as low as 715K and was followed by agglomerate quenching if the oxygen concentration in the flow was below some critical value. As the oxygen concentration of the flow was increased beyond this critical value, the second stage ignition leading to full-fledge combustion was observed. The ignition characteristics of boron particle agglomerates were theoretically investigated by developing a numerical model that considers the oxygen concentration gradient inside the agglomerate in order to simulate the ignition process. The model results accurately captured the qualitative behavior of the first stage ignition and quenching as well as the dual-stage ignition processes. The model identified the closure of pores in the agglomerate due to the build-up of the boron oxide layer, which effectively seals off the large interior surface area of the agglomerate from the oxidizer, as being the mechanism responsible for the two-stage ignition phenomenon. The critical flow conditions in terms of flow temperature and ambient oxygen concentration for the second stage ignition were determined by this model. The experimental result for the critical ambient oxygen concentration for the second stage ignition in dry flow was determined to be 70%, which is fairly close to the model prediction of 79%.

Atomization of petroleum-coke sludge slurry using effervescent atomizer

Experimental investigations were carried out on a bench-scale atomization test facility to explore the atomization performance of petroleum coke sludge slurry (PCSS) which comprises pulverized petroleum coke, sewage sludge, water and a certain amount of chemical additives. In the experiments, effervescent atomizer was employed to spray the petroleum coke sludge slurry. And effects of air injection pressure, air-to-liquid mass flow rate ratio (ALR), exit orifice diameter and rheological properties of petroleum coke sludge slurry with different amount of sewage sludge on the Sauter Mean Diameter (SMD or D32) of the atomized droplets were investigated. Results suggest that petroleum coke particles have strong hydrophobic surface. Solid–liquid separation occurs between petroleum coke particles and water film during atomization. Sewage sludge is viscoelastic non-Newtonian fluids, which would influence the solid–liquid separation process when pulverized petroleum coke and sewage sludge are blended to prepare petroleum coke sludge slurry for atomization. SMD of atomized petroleum coke sludge slurry droplets decreases with ALR increase. However, SMD increases with spray axial distance downstream from the nozzle tip primarily and then decreases. Reduction of exit orifice diameter and increase of air injection pressure could improve the atomization performance, while sewage sludge would weaken the influence on variation of the atomized droplets diameters with increase of the spray axial distance and ALR.

Influence of the Lime Slurry Droplet Spectrum on the Efficiency of Semi‐Dry Flue Gas Desulfurization

AbstractA numerical model based on computational fluid dynamics is presented to predict the efficiency of a semi‐dry flue gas desulfurization process. The Euler‐Lagrange particle tracking method is used to predict both evaporation and mass transfer between the flue gas and the lime slurry droplets. The overall desulfurization process is split into two periods, i.e. the constant‐rate and the falling‐rate period, during which the fundamental chemical reaction between sulfur dioxide and calcium hydroxide is modeled. The absorption efficiency is calculated for different spray characteristics, taking into account different size spectra of the lime slurry droplets characterized by different standard deviations for a constant mass mean diameter d50 of 25 μm. The results are compared with published experiments and show excellent agreement as well as a significant role of the spray characteristics in the FGD efficiency.