Slurry Droplets Research Articles

The impact of non-isothermal adsorption of dissolved gas on drying of acoustically levitated slurry droplet

In this study, we developed a transient convective heat and mass transfer model of an acoustically levitated slurry droplet evaporating in an atmosphere of air, water vapor, and soluble gas. The advanced model considers the effects of acoustic streaming, forced convection, non-isothermal gas absorption, and adsorption of dissolved gas on the evaporation rate of a liquid droplet containing small active solid particles. Adsorption of dissolved in a droplet soluble gas by solid particles leads to decreased dissolved gas concentration in a liquid droplet. A reduction in dissolved gas concentration in a liquid droplet causes an increase in the mass flux of active soluble gas from a gaseous phase to a slurry droplet, which causes a rise in the heating effect of absorption and an increase in evaporation rate of a droplet. The heat effect of adsorption also intensifies the evaporation rate. It is shown that the time of porous shell formation is essentially shorter when solid particles in slurry droplets are active compared to inert particles.

Agglomeration of carbonaceous particles in a pendant water slurry droplet

Liquid spraying is used in a number of engineering applications, such as spray cooling, drying, granulation, etc., which mostly deal with colloidal solutions or slurries. Depending on the application, solid particle agglomeration can be considered a positive or negative phenomenon. Therefore, to make engineering processes reliable and stable, it is necessary take account of secondary processes affecting the size distribution of the material involved. The agglomeration of solid particles in a pendant droplet was studied experimentally. Distilled water was used as the base liquid and 60–120-μm coal particles as solid inclusions. The coal concentration in a droplet was varied in the range of 0.5–10 %. The main parameters of particle agglomeration in a droplet were recorded using the shadow method. Particle sedimentation velocities and heights of the agglomerated particle layers in a droplet were recorded as part of the experiments. These characteristics were affected by the initial liquid volume, relative particle concentration and size. Equations were obtained for the relationships determined in the experiments. These equations can help extrapolate the corresponding functions to wider variation ranges of input parameters. Recommendations have been formulated on managing the processes of slurry droplet stratification over time.

Modeling the evaporation and drying of two-component slurry droplets

In this paper, we present a mathematical model and numerical simulation for the evaporation and drying of a liquid droplet containing suspended solids, relevant to processes such as spray drying and spray pyrolysis in the food and pharmaceutical industries. The model comprises three stages: first is the evaporation of the liquid droplet consisting of solid particles, followed by the second stage starting with the formation of a porous crust around a wet-core region and, finally, the third stage with sensible heating of the dry particle. Using a finite difference method with a moving grid, we account for the moving interface between the crust and wet core. Our model incorporates spatial temperature variations and is validated against experimental data on colloidal silica droplet drying, showing good agreement. We examine model assumptions and analyze the impact of drying conditions on drying rate and final particle morphology. Along with the temperature and velocity of the drying gas, we also find that the shape of suspended solid particles inside the droplet and assuming continuum flow of vapor through the crust influence drying quality. Finally, we develop a regime map to predict whether the final particle will be solid or hollow based on operating conditions.

Using methane hydrate to intensify the combustion of composite slurry fuels

Combustion of composite slurry fuels based on industrial and municipal waste is an effective way to both recover the said waste and address the problem of fossil fuel depletion. The flexible composition of such fuels can be adjusted to include raw materials available in any specific region of the world. Another benefit they provide is the opportunity to recover waste in different proportions to conventional fuels. There is, however, a problem of long ignition delay times of composite slurry fuels, especially if they contain a significant proportion of water or high-moisture wastes. Here we suggest intensifying their ignition by involving methane released from gas hydrates. Gas hydrates contain a large amount of water that reduces the concentrations of sulfur, nitrogen, and carbon oxides in flue gases. The experimental findings have shown that the involvement of gas hydrate reduces the ignition delay time of a coal-water slurry droplet by more than four times. Adding gas hydrate also contributes to higher combustion temperature and degree of fuel combustion. On the basis of the results obtained, we propose a coal-water slurry/gas hydrate co-combustion system for energy production facilities.

Multi-scale numerical analysis of Ca-based desulfurizer dynamics characteristics and SO2 absorption processes in industrial-scale SDA-FGD reactor

Spray dry absorption flue gas desulfurization technology is widely employed to control sulfur dioxide emissions from industrial pollutant gases. This study employs the multiphase particle-in-cell method to numerically simulate gas–solid heat transfer, SO2 removal processes, and the effect of operating conditions in the SDA reactor. The flue gas desulfurization model used has been experimentally validated. The results indicate that flue gas and slurry droplets exhibit swirling flow along the reactor walls, forming gas vortices at the top and near the outlet. The temperature and Reynolds number of slurry droplets show minimal variation when particle size is below 43 μm. The heat transfer coefficient of slurry droplets is notably higher at the reactor top and increases with the slip velocity of the droplets. Furthermore, the CaSO3 volume fraction increases with longer residence times of the slurry droplets in the reactor. When the height and diameter of the main reaction area are lower than 14 m and 8.5 m, respectively, the SO2 mass fraction in the central area undergoes significant changes. The SO2 inlet concentration is negatively correlated with desulfurization efficiency, whereas the slurry injection speed, along with the diameter and height of the main reaction region, are positively correlated with desulfurization efficiency. Among these factors, the SO2 inlet concentration and the height of the main reaction region have the most substantial impact on desulfurization efficiency.

Surface deformation of moving droplets of slurry fuels

Experimental research findings are reported on the characteristics of surface transformation of droplets of promising fuel slurries in the air, as they move at subsonic velocities typical of combustion chambers of power plants. The main components of the fuels were water, coal processing waste, and coal. Typical shapes of droplets and the duration of their deformation cycles were identified. Droplets containing more than 70% of the solid phase remained practically undeformed. The lowest relative velocities of droplets leading to their fragmentation were determined. The key characteristics of secondary droplets (their number, sizes, velocities, and surface area of liquid) were calculated on the basis of the experimental findings. These characteristics were compared with those of initial droplets. Disruption conditions in the chosen range of the gas jet pressure (P ≤ 6 bars) can only be provided for fuel slurry droplets containing less than 60% of a coal component. The effect of a group of factors on deformation characteristics was identified. These include air jet and droplet velocities, droplet sizes, temperature, concentration, and type of components and additives. Approximation equations were derived for the mathematical description of the experimental data. Using certain criteria, the conditions necessary and sufficient for the disruption of water–fuel slurries on impact with an air jet were estimated.

Influence of collision conditions between aerosol flows of liquid droplets and solid particles typical for wet vortex dust collectors

Various industrial devices and systems make use of collisions of droplets with solid particles in aerosol flows. The characteristics of these collisions are usually predicted adopting the same approaches as for binary collisions of a liquid droplet with a solid particle. However, the conditions of such binary collisions are different from those in the spray composition. There are very few experimental findings for sprays. The purpose of this research was to experimentally determine the characteristics of interaction of droplets with particles of slurry fuel components in sprays. High-speed three-dimensional video recording made it possible to identify conditions when liquid droplets break up, and droplets and particles agglomerate to form slurry droplets. Droplet-particle interaction regimes were identified. Collective effects in a cascade of droplet-particle interactions were recognized. The frequency of collision regime occurrence was determined, considering the recorded collective effects in sprays. The characteristics of droplet-particle collisions in sprays were compared with those of binary interactions. Conditions were determined when the findings on binary collisions can be adapted to different systems with multi-phase sprays. The findings obtained promote research and development in the field of secondary atomization of composite fuel droplets.

Removal of multiple pollutants in air pollution control devices of a coal-fired boiler installed with a flue gas condensation scrubber

This work was conducted through field tests on a coal-fired circulating fluidized bed boiler with a condensation scrubber (CS) for flue gas waste heat recovery installed behind the wet flue gas desulfurization (WFGD) scrubber. Levels of multiple pollutants were investigated along various flue gas devices of the boiler, with the CS being the main focus. The CS demonstrated an excellent effect in removing filterable particulate matter (FPM), reducing it from 10.63–14.00 mg/Nm3 to less than 1 mg/Nm3, providing an alternative solution for ultra-low emission. The CS had the ability to remove SO2, reducing it from 211.8–272.0 mg/Nm3 to 11.1–13.0 mg/Nm3, which could be an aid to the WFGD scrubber. The CS showed an effect on SO3 removal, reducing it from 17.49–17.53 mg/Nm3 to 1.07–1.13 mg/Nm3, which might address the discharge of sulphate aerosols. The CS exhibited an effect in removing condensable particulate matter (CPM), reducing it from 29.68–38.48 mg/Nm3 to 12.18–22.80 mg/Nm3, providing a potential solution for CPM emission control. In addition, the escape of fine desulfurization slurry droplets from the WFGD scrubber could also be mitigated by the CS with the reduction efficiency of 88.30–90.51%. Overall, the synergistic removal of multiple pollutants including FPM, SO2, SO3, and CPM by the CS was verified in this case study. Considering its original function of waste heat recovery, the adoption of the CS is a promising measure for energy conservation and environmental protection.

Structural optimization and flow field simulation of gas distribution device in desulfurization tower

AbstractIn order to comply with ultra‐low emission requirements and further reduce the concentration of SO2 emissions from coal‐fired power plants, a novel distributor has been proposed to enhance the uniformity of fluid distribution in the desulfurization tower. The distributor ensures that the inlet flue gas comes into even contact with the slurry droplets ejected from the nozzle, thereby improving the removal efficiency of SO2. The research focuses on a circular desulfurization tower using ammonia‐based desulfurization. To study the enhancement of non‐empty tower desulfurization efficiency, a CFD‐DPM model based on the Euler–Lagrange method has been established. The accuracy of the model has been verified through comparison with experimental or validated data, ensuring its reliability in predicting the performance of the proposed distributor in enhancing desulfurization efficiency in the circular desulfurization tower. The simulation results demonstrate that the addition of the new inflow gas distributor leads to a significantly thinner flue gas escape zone in the circular desulfurization tower. The flue gas flow velocity at the wall side of the tower, above the distributor, increases by only about 1 m/s with the rise of the horizontal cross section, and there is hardly any flue gas escape zone observed. The addition of the distributor increases the pressure drop in the local area, while optimizing the flow field in other areas and reducing the turbulent dissipation energy, which in turn reduces the full tower pressure drop from 701 to 474 Pa. Additionally, the flue gas at the outlet of the distributor is uniformly dispersed, with gas velocity distribution unevenness values of 0.558, 0.587, and 0.601, respectively, which are 15.9%, 14.0%, and 14.0% lower than the unevenness value of the empty tower spraying, at a depth of 1 m below each spray layer. These improvements in gas flow distribution and pressure drop contribute to better control of ammonia escape and aerosol phenomenon, while also enhancing the desulfurization efficiency.

Numerical analysis of SO2 removal characteristics in industrial flue gas desulfurization reactor by spray drying adsorption

The flue gas desulfurization reactor by spray drying adsorption plays a crucial role in the process of gas purification. This study employed the multi-phase particle-in-cell method to accurately model the three-dimensional motion of particles of different sizes in the flue gas desulfurization reactor. The double-film theory was utilized to simulate the heat and mass transfer processes of desulfurization slurry droplets. Additionally, this study conducted a comprehensive analysis of various factors affecting gas-solid flow and mass transfer processes, including flue gas inlet velocity and temperature, Ca/S molar ratio, slurry particle size, and slurry temperature. The results demonstrate that the simulated and experimental results are in good agreement, validating the reliability of the developed model. The axial mass fluxes of gas and particles exhibit contrasting distributions in the central part and near the wall, with the turbulent viscosity being particularly pronounced in the top nozzle region. The inlet velocity of flue gas has a limited effect on the axial mass flux of particles, whereas the Ca/S molar ratio exerts a more significant influence. Furthermore, the Ca/S molar ratio has a substantial impact on the spatial distribution of flue gas temperature. The inlet velocity of flue gas and slurry particle size are negatively and positively correlated with particle residence time, respectively. Based on the mass transfer characteristics, the reactor interior can be divided into three zones: a high-efficiency desulfurization zone, a low-efficiency desulfurization zone, and a partial desulfurization zone. For the investigated operating conditions, the desulfurization efficiency is most strongly influenced by the slurry particle size, followed by the flue gas inlet velocity, Ca/S molar ratio, flue gas inlet temperature, and slurry temperature.

Ignition and combustion of a group of waste-derived fuel slurry droplets: Mutual influence upon low-temperature convective heating

Waste-derived slurries can be burned both in a fluidized bed and when injected into a boiler furnace. This paper is the first to analyze the dynamic characteristics of ignition and combustion of a group of three slurry droplets based on coal slime, water, and waste industrial oil in a low-temperature air flow with a temperature of 500–600 °C and velocity of 0.1–1.2 m/s. The distance between the droplet centers varied from 3 to 15 mm. The results of the study can develop innovative technologies for flare and fluidized bed combustion of fuel slurries at the boiler start-up stage. It was found that a distance of 3–5 mm between droplet centers provides almost simultaneous gas-phase ignition of all droplets. A further increase in the distance critically worsened the ignition and combustion and eliminated the synergy. An increase in the air flow velocity contributed to the appearance of a common flame zone but at the same time increased the ignition delay of droplets by a factor of 1.2–1.6. The negative effects of cooling and entrainment of combustion products by a low-temperature flow can be effectively reduced by adding 5 wt% of industrial oil to the slurry. Also, the use of oil reduced the droplet ignition delay time by 15–20%. At the stages of flame and heterogeneous combustion, the differences for droplets in a group increased (the stage durations differed by 1.5–3 times). For the low-temperature start-up of a boiler, it was found advisable to provide a droplet flow with a sufficient density to stabilize their ignition without significant flow deviations (dimensionless distance 1.5–2.5).

Combustion Characteristics of Coal-Water Slurry Droplets in High-Temperature Air with the Addition of Syngas

An experimental study of the ignition and combustion processes of coal-water slurry (CWS) droplets based on coal enrichment waste in a high-temperature oxidizer at 650–850 °C with a syngas addition was carried out. The fuel slurry was a mixture of finely dispersed solid combustible particles (coal sludge, 10–100 µm in size) and water. The syngas was a product of biomass pyrolysis and two waste-derived fuels in a laboratory gasifier. Composition of the syngas was controlled by a precision analytical gas analyzer. The feasibility of co-firing CWS with syngas was experimentally established. Under such conditions, the CWS droplets ignition process was intensified by 15–40%, compared to fuel combustion without the addition of syngas to the combustion chamber. The greatest positive effect was achieved by adding the gas obtained during the biomass pyrolysis. The ignition delay times of CWS droplets are 5.2–12.5 s versus 6.1–20.4 s (lower by 15–39%) when ignited in a high-temperature medium without adding syngas to the combustion chamber. Based on the results obtained, a concept for the practical implementation of the CWS combustion technology in a syngas-modified oxidizer medium is proposed.

Experimental study on the desulfurization and evaporation characteristics of Ca(OH)2 droplets

The experiments were conducted to focus on the desulfurization and evaporation characteristics of lime slurry droplets at 298 K to 383 K. We designed an evaporation-reaction chamber with quartz glass windows. The monodisperse slurry droplet stream was injected into the evaporation reaction chamber, and the inlet gas components (air, air+SO 2 ) were introduced into the reactor. We applied the magnified digital in-line holography to measure the droplet parameters and calculated the evaporation rate. The effects of temperature, droplet concentration, and the effects of SO 2 concentration of the evaporation rate on Ca(OH) 2 droplets were discussed. Moreover, the Ca(OH) 2 droplets under different experimental conditions were sampled, and the droplets were observed and analyzed using an off-line microscope. The evaporation rate of the Ca(OH) 2 droplet increased at first, and then decreased during the falling process, and remained constant at last. The average evaporation rate of the Ca(OH) 2 droplets increased significantly with the temperature increasing.

Effect of the relative arrangement of atomizers on composite liquid fuel spraying characteristics

The paper presents the experimental research findings for the characteristics of promising composite liquid fuel spraying with two atomizers, when varying the scheme of their relative arrangement. The nozzle arrangement was shown to have a significant effect on the spraying characteristics (droplet sizes and velocities), the volume and rate of the combustion chamber filling with aerosol. Following the experimental research, the relative efficiency indicators of spraying water and composite fuels were calculated. The research findings for the spraying of water and composite liquid fuels based on coal and coal processing waste were compared. It was established that in the interaction of sprayed liquid jets, it is possible to control the average size and, thus, number of secondary fragments by intensifying their transformation during coalescence and disruption. A mathematical processing of the experimental results was performed using a set of dimensionless criteria introduced. Schemes were shown that make it possible to provide minimum sizes of slurry droplets and maximum relative efficiency factor of spraying. • One- and two-atomizer spraying characteristics of fuels were compared. • Droplet number, sizes, velocities and trajectories were determined. • Fuel droplet sizes can be changed by the liquid jet interaction with air. • Maximum efficiency factor of fuel spraying was determined. • Spraying schemes for more complete filling of the chamber with fuel aerosol were proposed.

Entrained flow gasification: Impact of fuel spray distribution on reaction zone structure

Entrained flow gasification (EFG) is an important process for generating syngas from biogenic and anthropogenic waste based feedstocks for a future circular economy. The EFG process is characterized by complex interactions between different physical and thermo-chemical sub processes which determine syngas quality and process efficiency. The understanding of these sub processes is essential for the development of validated models, and therefore for design and scale up of EFG reactors. EFG processes using a central jet burner configuration feature flames that can be described as inverse diffusion flames superimposed by a fuel spray. The flames are characterized by (i) the conversion of liquid and slurry droplets and (ii) the oxidation of recirculating synthesis gas with the gasification medium. This work studies the interactions between fuel and oxidizer in the near-flame region of an atmospheric EFG process. The model fuel ethylene glycol was gasified using oxygen-enriched air for two different burner nozzle configurations. Spray imaging, OH-LIF and Fuel Tracer-LIF measurements were carried out in addition to gas temperature measurements to characterize the fuel distribution and the flame structure. The experimental results show that narrower fuel spray distributions result in shorter flames and changes in flame shape from a compact to a hollow cone shape in the downstream flame region. The experiments were accompanied by 2-phase free-jet modeling and RANS based CFD modeling. The models were improved to reflect the experimental findings including the fuel spray distributions. The simulation results predict the observed flame structures well using both models and for both burner nozzle configurations. The changes in flame structure for different spray distributions can be explained by local stoichiometry using the results of the 2-phase free-jet model.

Experimental evaluation of latent heat use of a microencapsulated phase change material slurry in spraying

A spraying system was constructed with a microencapsulated phase change material (MPCM) slurry as the working medium to weaken the adverse effects of the slurry's high viscosity and low thermal conductivity on heat transfer in a wall tube heat exchanger. The temperature distributions of pure water and MPCM slurry were first obtained through infrared thermal imaging in spraying. Furthermore, the latent heat use of MPCM slurry was investigated through a new method based on the temperature properties of the phase change fluid, in which the working and phase change temperature ranges are matched. The heat-mass transfer characteristics are compared and discussed with those of the base fluid, and the latent heat use of 10wt% MPCM slurry containing the core material n-docosane (C22H46). The experimental results indicate that the phase change enables the MPCM slurry droplets to maintain a higher temperature than a fluid with no phase change or base fluid. Different application modes (static and spraying) have correspondingly different phase change temperature ranges owing to supercooling, and the degree of matching between the working and phase change temperature ranges has a critical impact on the latent heat use of the MPCM slurry. Finally, comparing the heat transfer coefficients and Lewis numbers between the base liquid and MPCM slurry reveals that the phase change not only enhances the heat transfer but also slightly reduces water evaporation compared with pure water spraying.

Combined effect of acoustic field and gas absorption on evaporation of slurry droplet

In this study, we developed a model of convective heat and mass transfer of an acoustically levitated slurry droplet that evaporates in an atmosphere of air, water vapor, and soluble gas. The advanced model considered the effects of acoustic streaming, forced convection, and non-isothermal gas absorption on the evaporation rate of a liquid droplet containing small solid particles. It is shown that the time of porous shell formation decreases with the increase of sound pressure level (SPL) and increases with the increase in frequency. Adding soluble gas to the gaseous phase shortens the time required for porous shell formation during slurry droplet drying. As shown by numerical calculations, in a gas mixture containing air with an ammonia content of 30%, at a temperature of 293 K and a humidity of 50%, with an acoustic field frequency of 2π·45 kHz and an SPL of 140 dB, the time of porous shell formation of a silica-water slurry droplet with the initial radius of 250 µm is about half shorter than that in a gas mixture not containing an active gas. The prediction of the developed model was in good agreement with the experimental results available in literature.

Holographic slurry droplet monitor: Design and its application to 1000 MW coal-fired power unit

Wet flue gas desulfurization (WFGD) is a commonly utilized desulfurization technique. A major application of WFGD can be found in the coal-fired power plant, with up to 90% share of its market. Despite the fact that WFGD has been well developed and proven, its negative effects persist, e.g., entrainment pollution stemming from slurry droplet emission. In this study, we demonstrate in situ monitoring of flue gas slurry droplet emission in the coal-fired power unit using a self-made instrument based on digital holography, named the holographic slurry droplet monitor (HSDM), and gain insight into the key characteristics of slurry droplet emission through further statistics. HSDM exhibits superior accuracy in the respect of mass concentration, compared with conventionally accepted Mg2+ tracer method, and the maximum bias of the two is 5.61%. The application of HSDM in the real scenario of a 1000 MW coal-fired power unit is performed. After that, real-time slurry droplet measurement data is analyzed, revealing the key characteristics of slurry droplets, including size distribution and mass concentration, as well as factors associated with emission. Overall, high accuracy, real-time availability with a response time of 10 min, and multi-info acquisition in both size distribution and mass concentration make HSDM a remarkable alternative to the conventional method, enabling it to play a critical role in slurry droplet emission measurement.

Ignition and combustion characteristics of coal-water-oil slurry placed on modified metal surface at mixed heat transfer

We experimentally determined ignition and combustion of a sessile droplet of coal-water-oil slurry on modified steel samples at mixed heat transfer (radiation, conduction, and convection). The surfaces were modified by two different methods: using abradants and nanosecond laser irradiation. It was shown that a texture of steel samples formed by abradants allows changing the spreading diameter of a 50-mg coal-water-oil slurry droplet (0.44 mm in diameter) from 0.628 mm to 0.743 mm, and, thereby, changing the ignition delay time more than 2 times (from 0.108 s to 0.212 s). A multimodal hierarchical texture formed on steel samples after laser texturing allows increasing the effective contact area of a coal-water-oil slurry droplet with a conductive heat source by more than 18% at identical values of the droplet spreading diameter. This intensifies the droplet heating, increases the formation rate of a combustible vapor-gas mixture, and decreases the ignition delay time by 10–20% (to 0.093 s). A texture formed by laser irradiation increases the number of nucleation centers and intensifies puffing. Modification of steel by nanosecond laser irradiation makes it possible to diminish the intensity of ash deposition on the heating surface after the combustion of coal-water-oil slurry.

Combustion stages of waste-derived blends burned as pellets, layers, and droplets of slurry

This paper describes the results of an experimental investigation on the stages of thermochemical transformation of fuels prepared from waste. We studied the combustion of coal slime and its co-combustion with additives (waste turbine oil, sawdust, and rapeseed oil). The mass fraction of the additive was 5%. Fuels in the forms of a pellet, dry loose layer, and slurry were heated in a tubular muffle furnace. The reactivity of the fuels was evaluated, and the duration of the main process stages was recorded. Experiments have shown that it is more expedient to burn coal slime in combination with other components, which improved the ignition and burnout performance of the blends. The addition of turbine oil accelerated the flame combustion by 3–6 times. Rapeseed oil reduced the duration of the endothermic stage by 50–60%. The slurry droplets had the longest pre-flame stage. Their burnout was more intense than that of pellets or a layer and was less dependent on the temperature in the furnace. The longest burnout was typical of pellets. The data obtained are essential in planning and conducting industrial tests on the energy recovery of coal slime, biomass, and refined products.