Heating and Evaporation of Sessile Droplets: Simple and Advanced Models.

New advanced and simple two-dimensional (2D) models of sessile droplet heating and cooling and evaporation are suggested. In contrast to the earlier developed one-dimensional (1D) model, based on the assumption that heat supplied from the supporting surface is homogeneously and instantaneously spread throughout the droplet, both new 2D models consider the spatial distribution of this heat. The advanced 2D model is based on the numerical solution to the equations of conservation of mass, momentum, vapor mass fraction, and energy with standard boundary and initial conditions, using COMSOL Multiphysics code. Simple 2D and 1D models assume that droplets retain their truncated spherical shapes during the evaporation process. In the 1D model, the analytical solution to the 1D heat conduction equation inside the droplet is implemented into a numerical code. In the simple 2D model, the 2D version of this equation is solved numerically using COMSOL Multiphysics code. Droplet deformation, temperature gradients along the droplet surface, and the Marangoni effect are not considered in this model. The predictions of all three models are validated using in-house experimental data obtained from studies of sessile droplets of distilled water with initial volumes of 5.2, 3.2, and 2.2 µL, at an ambient temperature of 298.15 K, and at atmospheric pressure. The observed values of normalized droplet radii squared are shown to be close to those predicted by all three models. This allows us to recommend the application of the simplest 1D model for predicting this parameter. The time dependences of the droplet average surface temperature predicted by the advanced 2D model are shown to be close to those observed experimentally. The simple 2D and 1D models can correctly predict the initial rapid decrease in droplet average surface temperature followed by its gradual increase, in agreement with experimental data.

Hydrophilic and Hydrophobic Surfaces: Features of Interaction with Liquid Drops.

The processes of interaction of liquid droplets with solid surfaces have become of interest to many researchers. The achievements of world science should be used for the development of technologies for spray cooling, metal hardening, inkjet printing, anti-icing surfaces, fire extinguishing, fuel spraying, etc. Collisions of drops with surfaces significantly affect the conditions and characteristics of heat transfer. One of the main areas of research into the interaction of drops with solid surfaces is the modification of the latter. Changes in the hydrophilic and hydrophobic properties of surfaces give the materials various functional properties-increased heat transfer, resistance to corrosion and biofouling, anti-icing, etc. This review paper describes methods for obtaining hydrophilic and hydrophobic surfaces. The features of the interaction of liquid droplets with such surfaces are considered. The existing and possible applications of modified surfaces are discussed, as well as topical areas of research.

Gas composition during thermochemical conversion of dry solid fuels and waste-derived slurries.

Coal has long remained a promising and widely used energy resource all over the world. Special emphasis is usually put on the research and development of environmentally friendly technologies for the use of coal and coal processing waste. The development of slurry fuels based on coal waste is one of the promising ways to use raw materials with energy potential, recover wastes, and reduce the environmental load. However, no combustion technology has yet been created for heterogeneous wastes as water-based slurries. The physical principles and parameters of the corresponding processes have not been studied adequately. In this research, the environmental combustion indicators (CO2, CO, H2, NOx, and SO2 concentrations) of slurries based on water and petrochemical, coal, and plant wastes were analyzed for the first time in a wide range of temperatures covering all the typical stages of thermochemical fuel conversion: pyrolysis (400-700 °C), gasification (700-900 °C), and combustion (700-1000 °C). We established the key patterns and aspects of changes in gas concentrations at all the main stages during the thermal decomposition of fuels. The use of water-based fuels at the pyrolysis stage was notable for up to 96% higher concentrations of the key combustible gases (CO, H2). The temperature extrema were 50-100 °C lower than those of bituminous coal. In terms of the key anthropogenic emissions (CO2, NOx, and SO2), the combustion of slurries also appeared to be 20-77% more environmentally friendly than that of coal depending on the temperature conditions and fuel composition. The maximum positive effect from adding biomass to coal-water slurries was achieved in the temperature range of 850 to 1000 °C. The research findings can be used for developing the technologies for thermal recovery of waste as water slurries, in particular, by intensifying the pyrolysis, gasification, and combustion.

Multiple-criteria decision analysis to substantiate the prospects of industrial and solid municipal wastes as slurry fuel components.

This article investigates the recovery of typical wastes (coal slime, sawdust, cardboard and tire pyrolysis residue) as part of high-moisture slurry fuels. Using a laboratory furnace, the ignition and combustion characteristics of fuels as well as NOx and SOx emissions were determined. Using multiple-criteria decision-making (MCDM) methods and experimental results, we access the performance of four different slurry fuels in comparison with bituminous coal. The novelty of the study is based on the following features: we consider a unique set of parameters of the fuels (economic, environmental, safety and energy indicators), as well as three countries for their potential use (the USA, India and Russia); three different methods for calculating the efficiency indicator of each fuel were used. Despite rather low energy performance, the summarizing efficiency indicator of waste-based slurries was 53-93% higher than that of coal. The use of cardboard in the composition of a fuel blend showed the best complex result (the increase in the efficiency indicator was 80-93% relative to coal). The least promising additive was the pyrolysis residue of automobile tires. Its addition resulted in a 10-15% decrease in overall efficiency relative to a slurry without additives. The research results are useful for optimizing the component composition of waste-based slurries, technical and economic development of projects for the incineration of various wastes in the form of high-moisture fuel slurries.

Emissions from the combustion of high-potential slurry fuels.

Slurry fuels based on wood and coal processing and petroleum refinery waste are an environmentally friendly and economically feasible alternative to the conventional solid fuel-coal. As part of this experimental research, we compared a set of fuels (coal and coal-water slurries with and without petrochemicals) by normalizing and calculating the specific concentrations of pollutants from their combustion. The pollutant concentrations were normalized with respect to the mass of burnt fuel, the thermal energy released by combustion, specific mass emissions per unit time, specific maximum mass emissions, and specific mass emissions per 1 kg of fuel equivalent or 1 MJ of thermal energy. The key objective of this research was to develop a method for comparing composite fuels in terms of their relative environmental friendliness. As part of the research, coal combustion was notable for the peak emissions of gaseous pollutants irrespective of the fuel mass and combustion chamber temperature. When slurries were burnt, CO2, SO2, and NOx concentrations were 12-90% lower as compared to coal. The research findings established that the most promising fuel of all the slurries under study is the one based on coal slime and sawdust due to its high environmental indicators.

Collisions of Two-Phase Liquid Droplets in a Heated Gas Medium.

The paper presents the experimental research findings for the integral characteristics of processes developing when two-phase liquid droplets collide in a heated gas medium. The experiments were conducted in a closed heat exchange chamber space filled with air. The gas medium was heated to 400-500 °C by an induction system. In the experiments, the size of initial droplets, their velocities and impact angles were varied in the ranges typical of industrial applications. The main varied parameter was the percentage of vapor (volume of bubbles) in the droplet (up to 90% of the liquid volume). The droplet collision regimes (coalescence, bounce, breakup, disruption), size and number of secondary fragments, as well as the relative volume fraction of vapor bubbles in them were recorded. Differences in the collision regimes and in the distribution of secondary fragments by size were identified. The areas of liquid surface before and after the initial droplet breakup were determined. Conditions were outlined in which vapor bubbles had a significant and, on the contrary, fairly weak effect on the interaction regimes of two-phase droplets.

Effective incineration of fuel-waste slurries from several related industries.

This study is based on the analysis of a set of industrial sectors (coal processing, wood processing, transport, oil, and water treatment) in order to identify the amount and type of combustible waste suitable for incineration. The main ignition and combustion parameters of these wastes have been experimentally obtained from their direct individual incineration in the original form and as part of a slurry based on wastewater. It has been established that a set of parameters allow waste-derived fuel mixtures to compete with coal dust and fuel oil with an environmental advantage. In particular, the concentration of sulfur and nitrogen oxides in the combustion products of all the tested slurries is 1.5-3 times as low as that of coal dust. Most of the wastes in question do not provide such advantages when burnt individually. We have assessed the fire safety of fuel mixtures and analyzed the prospects of mass waste incineration technologies. The calculations show that about 14-20% of coal and oil can be saved annually by extensively involving industrial waste in the energy sector. The experimental results obtained are the basis for the development of useful technologies for the safe and efficient combustion of waste from different industries.

Municipal solid waste recycling by burning it as part of composite fuel with energy generation.

In this work, it has been shown that the involvement of composite fuels in thermal power engineering will enable to recycle both industrial and municipal combustible wastes while saving fossil fuels. The ignition and combustion stability of composite fuel droplets up to their complete burnout was experimentally substantiated under the conditions typical of boiler furnaces, using the example of several fuel compositions with wood, food waste, plastic, and cardboard, each added separately. The values of the guaranteed delay times for the ignition of droplets with a size of about 1 mm were established for the considered fuel compositions in a wide range of the ambient temperature variation (600-1000 °C). The minimum values of ignition delay times were about 3 s, the maximum values were about 25 s. It was established experimentally that the concentration of nitrogen and sulfur oxides in flue gases was lower for the fuel compositions containing municipal solid waste (MSW) in comparison with those without it. The maximum difference between NOx and SOx concentrations for such fuel compositions was about 60% and 35% (in absolute units about 110 ppm and 45 ppm). As a result of the theoretical analysis, it was found that partial replacement of coal (50% of energy generation) by composite fuel in the amount equivalent in terms of energy generation will save about 1 billion tons/year of high-quality solid fossil fuel in the course of 20 years (the regulated period of safe operation of a boiler in thermal power engineering).

Gas-Vapor Mixture Temperature in the Near-Surface Layer of a Rapidly-Evaporating Water Droplet.

Mathematical modeling of the heat and mass transfer processes in the evaporating droplet-high-temperature gas medium system is difficult due to the need to describe the dynamics of the formation of the quasi-steady temperature field of evaporating droplets, as well as of a gas-vapor buffer layer around them and in their trace during evaporation in high-temperature gas flows. We used planar laser-induced fluorescence (PLIF) and laser-induced phosphorescence (LIP). The experiments were conducted with water droplets (initial radius 1-2 mm) heated in a hot air flow (temperature 20-500 °C, velocity 0.5-6 m/s). Unsteady temperature fields of water droplets and the gas-vapor mixture around them were recorded. High inhomogeneity of temperature fields under study has been validated. To determine the temperature in the so called dead zones, we solved the problem of heat transfer, in which the temperature in boundary conditions was set on the basis of experimental values.

Coal-water slurries containing petrochemicals to solve problems of air pollution by coal thermal power stations and boiler plants: An introductory review.

This introductory study presents the analysis of the environmental, economic and energy performance indicators of burning high-potential coal water slurries containing petrochemicals (CWSP) instead of coal, fuel oil, and natural gas at typical thermal power stations (TPS) and a boiler plant. We focus on the most hazardous anthropogenic emissions of coal power industry: sulfur and nitrogen oxides. The research findings show that these emissions may be several times lower if coal and oil processing wastes are mixed with water as compared to the combustion of traditional pulverized coal, even of high grades. The study focuses on wastes, such as filter cakes, oil sludge, waste industrial oils, heavy coal-tar products, resins, etc., that are produced and stored in abundance. Their deep conversion is very rare due to low economic benefit. Effective ways are necessary to recover such industrial wastes. We present the cost assessment of the changes to the heat and power generation technologies that are required from typical power plants for switching from coal, fuel oil and natural gas to CWSPs based on coal and oil processing wastes. The corresponding technological changes pay off after a short time, ranging from several months to several years. The most promising components for CWSP production have been identified, which provide payback within a year. Among these are filter cakes (coal processing wastes), which are produced as a ready-made coal-water slurry fuel (a mixture of flocculants, water, and fine coal dust). These fuels have the least impact on the environment in terms of the emissions of sulfur and nitrogen oxides as well as fly ash. An important conclusion of the study is that using CWSPs based on filter cakes is worthwhile both as the main fuel for thermal power stations and boiler plants and as starting fuel.

Experimental evaluation of main emissions during coal processing waste combustion.

The total volume of the coal processing wastes (filter cakes) produced by Russia, China, and India is as high as dozens of millions of tons per year. The concentrations of CO and CO2 in the emissions from the combustion of filter cakes have been measured directly for the first time. They are the biggest volume of coal processing wastes. There have been many discussions about using these wastes as primary or secondary components of coal-water slurries (CWS) and coal-water slurries containing petrochemicals (CWSP). Boilers have already been operationally tested in Russia for the combustion of CWSP based on filter cakes. In this work, the concentrations of hazardous emissions have been measured at temperatures ranging from 500 to 1000°Ð¡. The produced CO and CO2 concentrations are shown to be practically constant at high temperatures (over 900°Ð¡) for all the coal processing wastes under study. Experiments have shown the feasibility to lowering the combustion temperatures of coal processing wastes down to 750-850°Ð¡. This provides sustainable combustion and reduces the CO and CO2 emissions 1.2-1.7 times. These relatively low temperatures ensure satisfactory environmental and energy performance of combustion. Using CWS and CWSP instead of conventional solid fuels significantly reduces NOx and SOx emissions but leaves CO and CO2 emissions practically at the same level as coal powder combustion. Therefore, the environmentally friendly future (in terms of all the main atmospheric emissions: CO, CO2, NOx, and SOx) of both CWS and CWSP technologies relies on low-temperature combustion.

Environmental indicators of the combustion of prospective coal water slurry containing petrochemicals.

Negative environmental impact of coal combustion has been known to humankind for a fairly long time. Sulfur and nitrogen oxides are considered the most dangerous anthropogenic emissions. A possible solution to this problem is replacing coal dust combustion with that of coal water slurry containing petrochemicals (CWSP). Coal processing wastes and used combustible liquids (oils, sludge, resins) are promising in terms of their economic and energy yield characteristics. However, no research has yet been conducted on the environmental indicators of fuels based on CWSP. The present work contains the findings of the research of CO, CO2, NOx, SOx emissions from the combustion of coals and CWSPs produced from coal processing waste (filter cakes). It is demonstrated for the first time that the concentrations of dangerous emissions from the combustion of CWSPs (carbon oxide and dioxide), even when combustible heavy liquid fractions are added, are not worse than those of coal. As for the concentration of sulfur and nitrogen oxides, it is significantly lower for CWSPs combustion as compared to coals. The presented research findings illustrate the prospects of the wide use of CWSPs as a fuel that is cheap and beneficial, in terms of both energy output and ecology, as compared to coal.

Environmentally and economically efficient utilization of coal processing waste.

High concentrations of hazardous anthropogenic emissions (sulfur, nitrogen and carbon oxides) from solid fuel combustion in coal burning plants cause environmental problems that have been especially pressing over the last 20-30 years. A promising solution to these problems is a switch from conventional pulverized coal combustion to coal-water slurry fuel. In this paper, we pay special attention to the environmental indicators characterizing the combustion of different coal ranks (gas, flame, coking, low-caking, and nonbaking coals) and coal-water slurry fuels based on the coal processing waste - filter cakes. There have been no consistent data so far on the acceptable intervals for the anthropogenic emissions of sulfur (SOx), nitrogen (NOx) and carbon (CO, CO2) oxides. Using a specialized combustion chamber and gas analyzing system, we have measured the concentrations of typical coal and filter-cake-based CWS combustion products. We have also calculated the typical combustion heat of the fuels under study and measured the ratio between environmental and energy attributes. The research findings show that the use of filter cakes in the form of CWS is even better than coals in terms of environment and economy. Wide utilization of filter cakes solves many environmental problems: the areas of contaminated sites shrink, anthropogenic emissions decrease, and there is no need to develop new coal mines anymore.