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Biomass is considered to be one of the most easily available and high-potential renewable power sources. Pyrolysis and combustion are the most common ways of utilizing biomass. A ...complex multiple-criteria analysis to identify the most promising technology of using various types of biomass has never been previously performed. The study is focused on comparing two typical techniques of utilizing agricultural and forestry biomass for bioenergy and valorization. In this paper, the direct combustion and pyrolysis of biomass were experimentally studied. The environmental and energy performance parameter of the investigated processes were determined for separate biomass components (leaves, straw, sawdust) and a group of mixtures from them. It was established that the biomass mixtures combustion has better environmental characteristics. Thus, the main anthropogenic emissions decreased by 8–66%. In addition to that, the pyrolysis of a biomass mixture from a group of components has a 30% higher yield of combustible gases. The analysis of relative efficiency indicators of two processes where biomass was used as an energy resource showed that a biomass-derived composite fuel is more efficient than its individual components.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This experimental research studies the influence of heating intensity and typical initial size of particles (or droplets in molten state) on the main process parameter – ignition delay time – for a ...group of gel fuel compositions. The first group of fuels is based on oil-filled cryogels: the aqueous solution of polyvinyl alcohol (5 wt%) + 40–60 vol% of oil + 2 vol% of emulsifier. The second one was produced from identical oil-filled cryogels with fine solid combustible particles: the aqueous solution of polyvinyl alcohol (10 wt%) + 35 vol% of oil + 30% of coal (particle size 100 µm) + 2 vol% of emulsifier. Combustion was initiated in a chamber with a motionless air medium at 700–1,000°C, by introducing fuel particles into it at a rate of 0.04–0.10 m/s, their size varying from 2.5 to 3.1 mm. The consistent patterns and characteristics of processes during the induction period were recorded by a high-speed video camera and fast-response thermocouples. It has been established that the ignition delay times change in the range from 0.5 to 15 s, depending on the heating intensity (characterized by the air temperature in the chamber and the rate of introducing fuel particles into it) and component composition of the gel fuel. The calculated minimum density of the heat flux, required for the gel fuel ignition, is 40 kW/m2. At lower values, ignition does not occur even when the fuel is heated for more than 15 s, which is due to the complete evaporation of molten fuel components. The ignition delay times for all the fuel compositions and particles of different sizes differ by less than 10% (random error of measurement) at the heat flux density over 100 kW/m2. The factors under study affect the duration of the induction period only under the near-threshold (minimum) ignition conditions, when the air temperature in the chamber is 700–800°C. The higher the rates of introducing the fuel particles into the heating chamber (in the range of 0.04–0.10 m/s), the higher the oil concentration in the oil-filled cryogels (in the range of 40–60%), and the smaller the size of particles (ranging from 2.5 to 3.1 mm), the shorter the ignition delay times (by 30–40%, 25–35%, and 20–45%, respectively). At air temperatures above 900°C, the initial size of fuel particles does not have a significant effect on the ignition delay times (the difference in the recorded values does not exceed the random error of measurement).
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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•Micro-explosive dispersion of gel fuel particles occurs when they are ignited in high-temperature air.•Gel fuels with fine particles and without them feature different ignition ...mechanisms.•Average velocities of child droplets and size of the burnout area are 2 m/s and 8.0 cm3, respectively.•The heat generation rate of 10 mg gel fuel is about 190 W (vs. about 25 W for liquid fuel in original state)•Flame temperature during gel fuel combustion was for the first time established with two-color pyrometry.
The characteristics of processes developing during the ignition and combustion of single gel fuel particles with a mass of 10 mg in a high-temperature motionless air medium were investigated by means of a hardware and software system. The system featured a high-speed video camera, LED spotlights and a laptop with commercial and custom software. A group of fuel compositions was derived from oil-filled cryogels alone and with the addition of fine solid combustible components. Polyvinyl alcohol with 10 wt% in an aqueous solution was used as an organic polymer thickener. When exposed to rapid heating, fuel compositions (oil + aqueous solution of PVA + emulsifier; oil + coal dust + aqueous solution of PVA + emulsifier) were characterized by the micro-explosive dispersion of particles. The velocities of fine fragments after the micro-explosive dispersion of a fuel melt droplet were established when the air temperature was varied in the range of 700–1000 °C. The gas–vapor mixture temperature in the region of the fuel gas-phase combustion was determined using the original two-color pyrometry technique.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The research is focused on the ignition of solid fuels in a heated air flow. The experimental conditions correspond closely to solid fuel combustion in coal-fired boiler furnaces. Charcoal, wood, ...brown and bituminous coals were used as independent solid fuels and as fuel mixture components. The particle sizes of these fuels were less than 40 μm, 40–140 μm, 140–250 μm, 250–375 μm and 375–800 μm. They were ignited under the conditions of radiant-convective heating as they were moving in a heated air flow with a temperature of 500–800 °C and at a velocity of 5 m/s. The ignition delay time was the main investigated characteristic, recorded by a high-speed video camera. The induction period of bituminous coal shortens by 25–35% when 25–50% brown coal or wood in an air-dry state is added. The lowest ignition delay times were established for the fuel mixtures of bituminous coal with dry charcoal. The experimental curves of the ignition delay times of fuels and their mixtures versus the heated air temperature were presented as approximation equations. Based on the conducted research findings, a muffle burner design with a vortex structure of the fuel-air flow was proposed for the fuel oil-free start-up of coal-fired boilers.
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•Oil-free start-up of coal-fired boilers can be put into practice.•Adding 25–50% of highly reactive components to the main solid fuel reduces ignition time by 25–35%.•Particle size of fuel mixtures should not exceed 250 μm.•They are reliably ignited within 0.1–0.2 s at an oxidizer flow temperature of 500–800 °C.•Design of a burner unit for fuel oil-free start-up of a coal-fired boiler was proposed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•Breakup conditions and characteristics were established for droplets of waste-derived fuels, when ignited.•Droplets breakup enhances burning out of fuel components (fine coal and ...oil).•Both ignition delay time and combustion duration do not exceed 5 s.•A large group of properties was analyzed for separate fuel components.•Hypothesis was formulated about the influence of fuel composition on droplets breakup.
This research is focused on the ignition and combustion mechanisms and characteristics of single droplets of seven different composite liquid fuels, based on wet coal processing waste (fine coal + 50 wt% of water) with 40 wt% of vegetable oils (castor, rapeseed) and used petroleum-based oils (lubricants – motor, compressor, and turbine oils; working fluid – hydraulic oil; insulating fluid – transformer oil). The combustion of waste-derived fuel suspensions is initiated by introducing single droplets into heated motionless air with a temperature of 700–1,000 °C. Threshold conditions (component composition, concentration of combustible liquid, and temperature) were detected, for which micro-explosions during the induction period resulted in the full breakup of droplets, combustion of products (vapors and fine particles) and their subsequent rapid burnout. A high-speed video recording system is used to establish consistent patterns of physical and chemical processes, as well as ignition and combustion characteristics (ignition delay times, burnout time, velocity of breakup products, and size of their burnout area) for a group of fuels under the conditions of droplet breakup. A hypothesis about how the composition of the component affects the breakup of droplets during ignition was formulated based on the results of analyzing the properties (initial boiling point and evaporation rates of oils, their surface tension and surface free energy, as well as wettability of dry coal processing waste) of separate fuel components.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
·High-speed two-color pyrometry was adjusted to recording temperature fields of coal pellets during combustion·Patterns and characteristics of physical and chemical processes during ignition were ...established at microlevel·Heating source temperature ranges were established for three ignition modes·Uniformity of pellet surface temperature field is a criterion of heterogeneous ignition of fuel·Two-color pyrometry helps to determine optimal conditions for steady combustion of fuel pellets with low level of underburning
The temperature evolution of coal pellets, ignited and burned in an initially motionless high-temperature air medium, was analyzed. The fuel pellet surface temperature was recorded using contactless two-color pyrometry with a time increment of 2 ms by means of a high-speed color video camera and an original video frame processing algorithm developed in Wolfram Mathematica. The obtained results made it possible to explore the patterns and characteristics of coal ignition, considering the detected contribution that the gas-phase combustion of released volatiles makes to the pellet heating. The effect of the heating source temperature on the intensity of the fuel surface temperature evolution during the induction period was shown. When the fuel was heated to temperatures above 1073 K, local extrema were identified on the corresponding temperature trend. These characterize the emergence of hot spots – the sites of volatile ignition, preceding the heterogeneous ignition of the solid carbonaceous residue forming as a result of the thermal decomposition of the near-surface layer of the fuel pellet. The time intervals between consecutive local flashes of volatiles in their series from the first recorded occurrence to the last one, corresponding to the moment of the fuel's heterogeneous ignition, reduce five- to sixfold. The uniformity of temperature distribution over the surface of the fuel pellet at the moment of its heterogeneous ignition allows us to determine the moment when the gas-phase spot ignition of volatiles changes to the heterogeneous combustion of the solid carbonaceous residue at heating source temperatures over 1100 K.
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•The ignition delay and burnout times of OCWF can be controlled by surface roughness.•To change the combustion characteristics, steel is processed by 100-μm abrasives.•Laser surface ...processing allows controlling the ignition and combustion characteristics.•Laser processing of steel intensifies the fuel droplet combustion on its surface.
The ignition and combustion of single organic coal-water fuel (OCWF) droplets made of coal (30%), waste engine oil (35%), and water (35%) during conductive heating on the surfaces of the machine steel modified with abrasive materials and laser radiation were studied experimentally. The experimental conditions (material and temperature of the heating surfaces) corresponded to the operating conditions of modern furnace chambers of boilers at thermal power plants. It is found that the ignition delay and burnout times during conductive heating of OCWF droplets on steel surfaces can be controlled by changing the surface roughness parameters. On the example of 7-mg OCWF droplets with a radius of about 1.2 mm, it is shown that for processing of heating surfaces, it is necessary to use the abrasive materials with an average grit size of 100 μm to decrease the ignition and burnout times. The abrasive materials with an average grit size of more than 100 μm have an insignificant effect on the ignition delay and burnout times of OCWF droplets. However, such processing significantly increases the deposition of solid combustion products (ash deposits) with an increase in the surface roughness. Laser processing of steel surfaces is of particular interest in the implementation of passive ways of controlling the ignition and combustion characteristics of OCWF droplets. This way of changing the surface texture allows controlling the ignition delay and burnout times in a wide range of the heating source temperatures, as well as reducing the intensity of the solid product deposition on the heating surface during the OCWF combustion. The positive effect from laser processing of heating surfaces to intensify the fuel droplet combustion consists in increasing the velocity of vapor flows from combustible fuel components due to the droplet dispersion. This, in turn, intensifies the formation of a combustible vapour-gas mixture and increases the velocities of flame propagation together with the dispersion products of fuel droplets along the textured surface.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Methods of preparing heterogeneous gel fuels have been elaborated on the basis of oil-free and oil-filled cryogels, containing coal dust particles with a size no more than 140μm. Mechanical ...properties of 20-mm fuel pellets have been analyzed. The ignition mechanisms of heterogeneous gel fuels have been experimentally discovered in a high-temperature (600–1000°C) oxidizer medium, and the influence of the initial fuel temperature (from −85 to 20°C) on the fuel ignition characteristics has been researched. The multi-component composition of the oil-filled cryogels with coal dust particles is the reason for microexplosions that leads to the fuel sample dispersion and intensification both the ignition and burnout. Such fuel compositions are characterized by 1.5–2-fold lower ignition delay times (1–6s vs. 2–12s) than the compositions based on cryogels containing only coal particles, other things being equal.
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•We prepared gel fuels based on oil-free and oil-filled cryogels containing coal dust particles.•Ignition mechanisms of gel fuels in a high-temperature air medium were established.•It combines processes that occur during metal dust explosion and combustion of single coal particle.•The microexplosions intensify both the ignition and burnout of oil-filled cryogels.•Initial fuel temperature from −85 to 20°С was found to affect ignition characteristics.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
9.
The gel fuel ignition at local conductive heating Glushkov, D.O.; Nigay, A.G.; Yashutina, O.S.
International journal of heat and mass transfer,
December 2018, 2018-12-00, 20181201, Volume:
127
Journal Article
Peer reviewed
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•Hot metal and nonmetal particles ignite gel fuels, including at initial temperatures close to 120 K.•The initial temperature of particle should be not less than 1050 K at its ...characteristic size over 10 mm.•The size and shape of particle do not affect significantly the process characteristics in the case of stable ignition.•The developed mathematical model reliably describes ignition process.
An experimental study of ignition of a gel fuel at local conductive heating has been performed. The initial temperatures of the fuel samples were 120, 260, and 290 K. Metal particles in the shape of a sphere, a disk, and a parallelepiped with a characteristic size of 10 mm were used as ignition sources. The initial temperature of the heating sources varied in the range of 1050–1350 K. The use of software and hardware high-speed video recording (4200 frames per second at a maximum resolution of 1280 × 800 pixels) allowed analyzing the regularities of physical and chemical processes, occurring during the induction period. Limit conditions necessary for stable ignition of a gel fuel and dependences of ignition delay time on parameters of a local heating source have been established. A low-inertia (sampling rate of 100 Hz) measuring system based on miniature thermocouples and an oscilloscope served to establish the velocities of the gel fuel melting front motion from the contact boundary between the metal particle and the fuel in the direction of the thermal wave propagation during induction period and under stationary combustion. The mathematical model of ignition of the gel fuel has been developed within the framework of the mathematical apparatus of continuum mechanics and the theory of chemical kinetics. The results of numerical simulation are in good agreement with the experimental data obtained. The applicability limits of the developed model within which it reliably describes characteristics of ignition have been established. The mathematical model may be used to develop advanced energy-efficient technologies for ignition of new fuels in wide ranges of parameters of the system gel fuel/local heating source.
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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.
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•Abrasive processing of steel changes the ignition delay time more than 2 times;•Laser surface processing intensifies ignition and combustion of CWOS droplet;•Laser processing increases the number of nucleation centers and intensifies puffing;•A texture formed by laser irradiation reduces ash deposition during CWOS combustion.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
