Attomization Low Air Pressure Fault Continuous Coater

Atomization and Granulation

Oleg D. Neikov , in Handbook of Non-Ferrous Metal Powders (Second Edition), 2019

Process Parameters

In conventional gas atomization processes, the atomization pressures are typically in the range 0.5–4   MPa, and gas velocities in the nozzles range from Mach 1 to 3. However, in free-fall atomizers, gas velocities in the impingement zone usually have fallen to 50–150   m/s (for air or nitrogen). Typically, gas-atomized powders are usually spherical with a lognormal size distribution. Average particle sizes are usually in the range 10–300   μm with a standard deviation of about 2; oxygen content is about 100   ppm. Prealloyed alloys are commonly made by inert gas atomization. The worldwide annual tonnage of inert gas-atomized powder is much less than that of water-atomized powders, probably amounting to >   50,000   tons per year. Metal feed rates are lower than in water atomization, and the melt batch is smaller. However, the tonnage of air-atomized powders, especially zinc and aluminum as well as copper, tin, lead, and copper alloys, probably exceeds 400,000 tons per year [29]. Air atomizers operate continuously for many hours or around the clock. Multinozzle units are often used to boost the yield on aluminum and zinc.

In conventional inert gas or air atomization, typical metal flow rates through single orifice nozzles range from about 1 to 90   kg/min. The capability of plants varies from very little laboratory units to immense plant such as the ANVAL Atomizer 1 (Fig. 4.31), which is the biggest inert gas atomizer and was designed for producing large tonnages of superalloys and other alloy powders. Melting takes place in two 5.5-ton induction furnaces. Plasma-heated tundish is used. Due to the height of the tower, powder with up to a 1   mm particle size can de produced. Very fine powder can also be produced for applications such as MIM.

Fig. 4.31. ANVAL atomizer 1.

Courtesy of ANVAL.

In conventional atomizers, the typical gas flow rate ranges from 1 to 50   m³/min at pressure ranges of 350   kPa to 4   MPa. The superheating of molten metal (the temperature differential between the melting point and the temperature at which the molten metal is atomized) is generally about 75–150   K. In gas atomization with inert gas, the cost of gas consumption is significant, and a means of circulation to promote gas reuse is desirable, especially in large-scale facilities.

In practice, for a given gas nozzle design and size, the mean particle size is controlled by the pressure of the atomizing medium and the melt flow rate. For all nozzles, the velocity of the gas usually "chokes" at sonic velocity (about 300   m/s for nitrogen and argon) in the narrowest region of the nozzle if the upstream gas pressure is at least 1.9 times the external pressure [3].

Therefore, the amount of gas flow (Q) depends on gas pressure, temperature, and nozzle area. For ideal conditions and zero velocity on the entrance side of the nozzle, gas flow can be expressed as:

(4.11) $Q=\mathrm{\omega }{\left(\frac{2}{k+1}\right)}^{\frac{k+1}{2\left(k-1\right)}}\frac{p\sqrt{2g}}{\sqrt{RT}}$

where ω is the cross section of gas nozzle at exit; k equals C _p/C _v, is the ratio of specific heat at constant pressure and volume, correspondingly; p is the gas pressure in the reservoir; T is the temperature in the gas reservoir; R is the gas constant; and g is the acceleration due to gravity. For nitrogen k  =   1.4, Eq. (4.11) takes on the form with the dimension of quantity (g/s):

$Q=4\times {10}^6\frac{\mathit{ap}}{\sqrt{T}}$

Comparisons based on how much powder surface is generated per unit volume of gas spent can be used for evaluation of the gas efficiency. This criterion accounts for higher gas consumption requirements when higher gas pressures are applied in producing finer powders. Confined nozzle designs in comparison with consumption ones give higher efficiencies at a comparable gas-to-metal ratio [41]. A simple equation of median particle size dependence on gas/metal ratios can be used:

${\mathrm{\delta }}_{\mathrm{m}}=\frac{k}{\sqrt{G/M}}$

where k is a constant for the process and G/M is the gas/metal ratio, which is variously measured in kg/kg or cubic meters of gas per metal mass (m³/kg). The source [32] involves typical values of k for confined nozzle designs.

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Atomization and Granulation

Oleg D. Neikov , in Handbook of Non-Ferrous Metal Powders, 2009

Process Parameters

In conventional gas-atomization processes, the atomization pressures are typically in the range 0.5 to 4 MPa and gas velocities in the nozzles range from Mach 1 to 3. However, in free-fall atomizers, gas velocities in the impingement zone usually have fallen to 50–150 m/s (for air or nitrogen). Typically, gas-atomized powders are usually spherical with log normal size distribution. Average particle sizes are usually in the range 10 to 300 μm with a standard deviation of about 2; oxygen content is the range of 100 to 1000 ppm. Prealloyed alloys are made commonly by inert gas atomization. Worldwide annual tonnage of inert gas-atomized powder is much less than that of water-atomized powders, probably amounting to more than 50 000 tonnes per year. Metal feed rates are lower than in water atomization and melt batch is smaller. However, total tonnage of air-atomized powders, especially zinc and aluminum, as well as copper, tin, lead and copper alloys, probably exceeds 400 000 tonnes per year [1]. Air atomizers operate continuously for many hours or round-the-clock. Multinozzle units are often used to boost yield on aluminum and zinc.

In conventional inert gas or air atomization, typical metal flow rates through single-orifice nozzles are in the range 1–90 kg/min. Capability of plants varies from very little laboratory units to immense plant such as ANVAL Atomizer 1 (Figure 5.28), which is the biggest inert-gas atomizer and is designed for producing large tonnages of superalloy and other alloy powders. Melting takes place in two5.5-tonne induction furnaces. A plasma-heated tundish is used. Due to the height of the tower, powder with up to 1 mm particle size can de produced. Very fine powder can also be produced for applications such as MIM.

Figure 5.28. ANVAL Atomizer 1.

Courtesy of ANVAL.

In conventional atomizers, typical gas flow rate ranges from 1 to 50 m³/min at a pressure range of 350 kPa to 4 MPa. The superheat of molten metal (the temperature differential between the melting point and the temperature at which the molten metal is atomized) is generally about 75–150°C. In gas atomization with inert gas, the cost of gas consumption is significant, and a means of circulation to promote gas reuse is desirable, especially in large-scale facilities.

In practice, for a given gas nozzle design and size, mean particle size is controlled by the pressure of the atomizing medium and the melt flow rate, which is regulated by nozzle diameter and nozzle suction. For all nozzles, the velocity of the gas usually 'chokes' at sonic velocity (about 300 m/s for nitrogen and argon) in the narrowest region of the nozzle.

Therefore, the amount of gas flow (Q) depends on gas pressure, temperature and nozzle area. For ideal conditions and zero velocity on the entrance side of the nozzle, gas flow can be expressed [3]:

(6) $Q=\text{ω}{\left(\frac{\text{2}}{k+\text{1}}\right)}^{\frac{k+1}{2\left(k-1\right)}}g\frac{p\sqrt{2g}}{\sqrt{RT}}$

where ω is the cross-section of gas nozzle at exit; k equals C _p/C _v, the ratio of specific heat at constant pressure and volume, correspondingly; p is the gas pressure in reservoir; T is the temperature in gas reservoir; R is the gas constant; and g is the acceleration due to gravity. For nitrogen k = 1.4.

As a compressible fluid passes through a nozzle, a drop in pressure and a simultaneous increase in velocity result. If the pressure drops sufficiently, a point is reached where, in order to accommodate the increased volume due to expansion, the nozzle design must diverge. Thus, nozzles for supersonic velocities must converge to a minimum section and diverge again.

A comparison on the basis of how much powder surface is generated per unit of atomization gas spent can be used for evaluation the gas efficiency. This criterion accounts for higher gas consumption requirements when higher gas pressures are applied to produce finer powders. Confined nozzle designs in comparison with consumption ones give higher efficiencies at a comparable gas to metal ratio [37],

A simple equation of median particle size dependence on gas/metal ratios can be used:

(7) ${\text{δ}}_{\text{m}}=\frac{k}{\sqrt{G/M}}$

where k is a constant for the process and G/M is the gas/metal ratio, which is variously measured in kg/kg or cubic meters of gas per metal mass (m³/kg). Source [16] involves typical values of k for confined nozzle designs.

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THE ATOMIZATION AND BURNING OF LIQUID FUEL SPRAYS

NORMAN A. CHIGIER , in Energy and Combustion Science, 1979

10 ATOMIZATION AND EVAPORATION OF FUEL SPRAYS IN DIESEL ENGINES

Diesel fuel is normally injected into diesel engines under pressures of the order of 10⁸ N/m². The mean droplet diameter of a typical diesel spray is about 15μm with maximum diameters of approximately 70μm. Air temperatures in the combustion chamber approach the critical temperature of fuel so that droplets evaporate rapidly and the major portion of the entire spray is in the gaseous form. At low injection pressures, atomization is less effective than at higher pressures and both liquid ligaments and droplets with diameters greater than 100μm have been found in diesel engines.

Because of the very high pressures and temperatures under which diesel engines operate, atomization and evaporation characteristics of diesel sprays can be substantially different from those of sprays burning under lower pressures and temperatures. Radcliffe ⁴⁰ and Hiroyasu ⁴¹ measured droplet sizes and distributions of diesel sprays under low air temperature conditions. Several attempts have been made to study the specific effects of pressure and temperature on spray characteristics. Photographic observations show that in atmospheric conditions long ligaments of liquid fuel are observed in the central core of the spray. The break-up of the liquid can be seen to take place along the entire length of the spray. The break-up of the spray is not completed for some considerable distance downstream from the nozzle and the number density of droplets is smaller at the periphery of the spray compared with that occurring at a higher gas pressure. The high gas pressures result in higher gas densities, which significantly improve the gas entrainment and the atomization. At the high gas pressures used under normal diesel engine operating conditions, the spray usually breaks up into droplets rapidly, soon after the liquid emerges from the nozzle.

Gas temperatures at the high pressures in diesel engines are typically 200°C higher than the boiling point of diesel fuel and, under these conditions, it has been calculated that droplets would completely evaporate within 10mm from the nozzle exit. The precise temperature and pressure conditions within diesel sprays have not yet been clearly established and, in the direct-photography studies which have been made, it has so far not been possible to determine clearly the extent to which the fuel spray remains in the liquid phase.

For a number of diesel injection systems, the injection pressure becomes very low towards the end of the injection period and, under these conditions, large droplets are formed. Poor atomization, due to inadequate injection pressure, results in the formation of long liquid ligaments and droplets of the order of 100μm. This poor, low pressure, atomization is one of the main factors resulting in the formation of smoke and unburned hydrocarbons in diesel engines. Development studies have shown that smoke levels can be reduced by increasing injection pressures and reducing the size of the injector holes—both of which lead to finer spray atomization.

The use of laser optical techniques in diesel engine research has made very substantial progress in recent years and important developments have been made at the Arnold Engineering Development Center. ^{42, 43} By inserting quartz windows in diesel combustion chambers, in order to allow optical access, measurements have been made using the following techniques: high-speed photography, holographic flow visualization, holographic fuel droplet sizing, resonance absorption spectroscopy and laser velocimetry.

High-speed photography, up to 20,000 frame/s, has been used to follow the injection and combustion processes in diesel combustion chambers. The position, time and propagation of combustion have been determined and smoke formation and turbulence in the gas flow have been observed. Recording high resolution images of small fuel droplets by direct photography has not been very successful, except when measurements are made of a very thin plane of droplets.

Holography has become an established technique for the recording and study of three-dimensional particle field distributions. The production of a holograph requires that light reflected from, or scattered by, the object field be mixed with a mutually coherent reference beam and the sum of the two be recorded. The interference patterns resulting from the sum of these two beams constitute the hologram. When the hologram is re-illuminated with the reference beam, the recorded interference fringes scatter it into the form of an image identical to the original object. When pulsed lasers are used in holography, dynamic events in three dimensions can be frozen over very short periods.

Manuscript received, January 1976

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Fuel Chemistry

Sarma V. Pisupadti , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

XI.A Combustion of Liquid Fuels

Fuel oil-fired furnaces, diesel engines, and distillate fuel-fired gas turbines utilize fine liquid sprays to increase the rate of evaporation and combustion rate of the fuel. In general the combustion of a liquid fuel takes place in a series of stages: atomization, vaporization, mixing of the vapor with air, ignition, and maintenance of combustion (flame stabilization). Recent advances have shown the atomization step to be one of the most important stages of liquid fuel combustion. The main purpose of atomization is to increase the surface area to volume ratio of the mixture. For example, breaking up of a 3-mm droplet into 30-   μm drops results in 10⁶ droplets. This increases the burning rate by 10,000 times. The finer the atomization spray the greater the subsequent benefits are in terms of mixing, evaporation, and ignition. The function of an atomizer is twofold: atomizing the oil and matching the momentum of the issuing jet with the aerodynamic flow in the furnace.

The atomizers for larger boiler burners are usually of the swirl pressure jet or internally mixed two fluid types, producing hollow conical sprays. Less common are the externally mixed two fluid types. The principal considerations in selecting an atomizer for a given application are turn-down performance and auxiliary costs.

There are differences in the structures of the sprays between atomizer types which may affect the rate of mixing of fuel droplets with the combustion air and hence the initial development of a flame.

For distillate fuels of moderate viscosity, (30   mm² sec⁻¹) at ordinary temperatures, a simple pressure atomization with some type of spray nozzle is most commonly used. Operating typically with a fuel pressure of 700–1000   kPa (7–10   atm) such a nozzle produces a distribution of droplet diameters from 10 to 150   μm. They range in design capacity of 0.5–10 or more, cm³ sec⁻¹. A typical domestic oil burner nozzle uses about 0.8   cm³ sec⁻¹ of No. 2 fuel oil at the design pressure. Although pressure-atomizing nozzles are usually equipped with filters, the very small internal passages and orifices of the smallest tend to be easily plugged, even with clean fuels. With decreasing fuel pressure the atomization becomes progressively less satisfactory. Much higher pressures often are used, especially in engine applications, to produce a higher velocity of liquid relative to the surrounding air and accordingly smaller droplets and evaporation times. Other mechanical atomization techniques for production of more monodisperse sprays or smaller average droplet size (spinning disk, ultrasonic atomizers, etc.) are sometimes useful in burners for special purposes and may eventually have more general application, especially for small flows.

Conventional spray nozzles are relatively ineffective for atomizing of fuels of high viscosity such as No. 6 or residual oil (Bunker C) and other viscous dirty fuels. In order to transfer and pump No. 6 oil, it must usually be heated to about 373   K, at which its viscosity is typically 40   mm² sec⁻¹. Relatively large nozzle passages and orifices are necessary for the possible suspended solids. Dry steam may also be used in a similar way, as is common practice in the furnaces of power plant boilers using residual oil.

Combustion of fuel oil takes place through a series of steps, namely, vaporization, gasification, ignition, dissociation, and finally attaining the flame temperature. Vaporization or gasification of the fine spray of fuel droplets takes place as a physicochemical process in the combustion chamber. The temperature of vaporization for fuel oil is in the range of 100–500   °F, depending on the grade of the fuel. Gasification takes place at about 800   °F. The final flame temperature attained is between 2000 and 3000   °F. The combustion of an oil droplet takes place in 2–20   msec depending on the size of the droplet. A typical characteristic of an oil flame is its bright luminous nature, which is due to incandescent carbon particles in the fuel-rich zone.

Figure 8 illustrates the combustion of a single liquid droplet. Evaporation of liquid supplies the gaseous fuel that burns in the gas phase. Evaporation is caused by heat transfer to the surface of the droplet. The time required for complete evaporation is given by

FIGURE 8. Evaporation and combustion of a liquid fuel droplet.

$\beta =\frac{8\lambda }{{\rho }_{\text{l}}{\text{c}}_{\text{p}}}\text{ln}\left(1+{\text{B}}_{\text{T}}\right),$

where, B _T is transfer coefficient, lambda is thermal Conductivity, ρ _l =liquid density, and c _p is the heat capacity.

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The use of ultrasonic atomization for encapsulation and other processes in food and pharmaceutical manufacturing

P.R. Gogate , in Power Ultrasonics, 2015

30.1 Introduction

Liquid atomization is the process in which a liquid film coming out on a solid surface is subjected to a sufficient surface disturbance in the normal direction, such that the liquid gets separated from the surface or essentially splits into small droplets like a fog or mist in the gas phase. Atomization of liquids is important in many industrial processes, including spray drying, spray cooling, film coating, preparation of fine (usually nano-size) powders, incineration and combustion of liquid fuel and wastes, making emulsions (sometimes nano-emulsions), etc. In most of these applications, droplets should have a required size distribution and a specific mean size. This is especially important for applications or products where the specific activity is or product characteristics are strongly dependent on the size distribution obtained in the process.

Various types of atomization processes have been practiced in the past and these processes can be classified based on the mode of energy transfer used to disturb the surface of the liquid film so that the liquid droplets break apart. Mechanical atomization processes such as pressure atomization, two-fluid atomization, and spinning disk atomization use mechanical energy to pressurize the liquid film or increase its kinetic energy for possible disintegration in the form of droplets. Though this is the most popular and convenient approach, mechanical atomization processes require more energy (compared to the theoretical energy requirement for additional surface generation) and have no control on the final droplet size or the velocity at which the droplet is ejected from the surface. Thus, it is imperative to develop new processes for atomization that yield the desired droplet size distribution and desired mean size in an energy efficient manner. One of the approaches available for obtaining specific droplet size distribution, and at the same time avoiding coalescence of the droplets so that the mean size is lower, is based on the use of ultrasonic irradiation as a medium of energy transfer for inducing atomization. Unlike conventional atomization, ultrasonic atomization can be more energy efficient (based on proper design and operation), requiring only transmission of electrical energy to a piezoelectrically vibrating disk. There are no moving parts in the design of the atomizer (although a separate pump will be required for circulation of fluid through the atomizer) and only mechanical vibrations generated by the supplied electrical energy to the vibrating disk are used for generation of droplets.

Ultrasonic atomization can be simply achieved by vibrating a liquid layer with a piezoelectric crystal at high frequency (usually in the 50   kHz to 3   MHz range). A thin film of liquid can be formed on the surface using circulation of liquid through the atomizer at the desired flow rate. During the process of atomization, capillary waves will form on the surface of the liquid with a wavelength dependent on the irradiation frequency, power dissipation, and liquid physicochemical properties, most importantly the liquid density and surface tension at the liquid–air boundary. If the liquid is forced to oscillate with sufficient intensity, the tips of capillary waves will begin to pinch off into droplets and atomization is achieved. The size of the generated droplets is independent of the forcing amplitude and hence the mist density. This feature can be useful in designing a system where mist properties should be adjusted independently.

Use of ultrasonic atomization can overcome some of the common disadvantages associated with the conventional atomizers. Rotary, pressure, or two-fluid atomizers use only a fraction of the supplied energy (centrifugal, pressure, or kinetic energy respectively) to shatter the liquid for forming the droplets, while most of the supplied energy finally appears as the kinetic energy of the particles. As a consequence, problems like partial separation of the components in the mixture, the presence of defects on the particle surface, improper coating characteristics, wide distribution of droplet sizes, etc., can occur, depending on the specific application. Also, the dimensions of the equipment as well as the associated cost will increase when the speed of the atomized particles/droplets increases.

Unlike the conventional atomizing nozzles which rely on mechanical energy to shear a fluid into small drops, ultrasonic atomizers use only low vibrational energy for the generation of drops. Moreover, the association of ultrasound-based atomization with spray drying is a powerful tool that can be harnessed for important applications, especially in food and pharmaceutical industries. Spray drying is in principle a continuous process, giving a good reproducibility and having a good potential for scale-up. If spray drying is based on the use of a pneumatic nozzle, it has similar disadvantages, such as lack of control over the mean droplet size, broad droplet distributions, and the risk of clogging in the case of suspensions. These common disadvantages can be overcome by employing ultrasonic energy to obtain droplets with a relatively uniform size distribution. Another important process based on a similar controlling mechanism would be spray congealing, where the use of ultrasound can give spherical microparticles or nano-particles with good encapsulation efficiency and size distribution. Overall, it can be said that ultrasonic atomization is a robust and an innovative single-step procedure with scale-up potential for a variety of applications.

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Electrostatic Powder Coating

John F. Hughes , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

I Introduction

Almost all manufactured products, from paper clips to automobiles, are coated with some form of durable surface film. The purpose of such a film is twofold. Sometimes a surface coating is applied purely for aesthetic purposes, to enhance the appearance of the product, while for other applications a durable protective film is essential, particularly for products that may be used out doors. In some instances, both enhancement of appearance and protection are desirable; a prime example of a product that needs both is the automobile body and components.

Materials used for application as a thin film are usually liquids, commonly known as coatings. Liquid coatings may be water-based or solvent-based and can be applied in a number of ways. Perhaps the most common and the most primitive of application methods is by brush or roller. The product is distributed in a film over the substrate surface, but the finish is not of prime quality, and there is little or no control over film characteristics such as thickness and evenness of deposition.

It was the lack of such control that led to the development of pneumatically atomized sprayers for liquid spraying. High-pressure air is used to fragment the paint into tiny particles, which are subsequently conveyed in an airstream toward the substrate. When the particles alight on the substrate, the flow characteristics of the liquid paint ensure a complete and even coating.

The development of such paint applicators, or guns, revolutionized painting techniques. Very high quality coatings were possible for the first time, and the efficiency of deposition was enhanced. Also, very thin coatings were now made possible, an important economic consideration.

Painting by high-pressure atomization did, nevertheless, give rise to new operational difficulties. As a result of the high forward air velocity that is required for conveying the aerosol to the object to be coated (workpiece), a large proportion of the spray completely misses the workpiece and is lost as overspray. Recovery and recycling of wet paint is not generally possible, especially when rapid color changing is required in a coating booth. Also, evaporation of the solvent excludes easy reclamation once the aerosol has been created and is airborne.

The next major development in painting techniques was the electrically charged aerosol. This is a technique by which each droplet of paint in the aerosol is artificially charged. Charging can be effected in a number of ways, the most widely used technique being charging by corona. This method imparts electrical charge to individual droplets by the attachment of ions, usually created by the application of a high potential to a sharp-pointed electrode. The charging polarity can be negative or positive, with negative being the usual choice.

Some paint-gun systems use induction charging, but this technique is dependent on the paint conductivity for efficient performance. The more conducting water-based paints are more suitable for this method, while the more insulating solvent-based paints lend themselves more to corona-charging techniques.

With the airborne paint aerosol electrically charged, any grounded substrate presented to the aerosol acquires an equal and opposite induced electrical charge. Since opposite charges attract, there is a natural tendency for the paint particles to be attracted to the substrate. Even particles that are on trajectories that would normally miss the substrate are pulled back by the electrostatic attraction forces. In this way, even the gun's shadow area on the substrate is coated. This wrap-around phenomenon is unique to electrostatic painting processes and has contributed to dramatic improvements in coating deposition efficiency. Of course, some paint droplets miss the substrate and are lost on the walls of the coating booth.

Common to all wet-paint coating systems is the fact that oversprayed paint generally is not recoverable. This is wasteful and expensive.

To a large extent, this problem was solved instantaneously by the introduction of dry powdered paint and related electrostatic powder-coating systems. Materials such as epoxy, polyester, and acrylics lend themselves very well to application in dry powder form. Expensive solvents are dispensed with, and any oversprayed particles are easily recoverable.

Electrostatic powder-coating systems are similar to wet-paint guns, with most applicators using a corona discharge to impart electrical charge to each solid particle. Variations on the gun applicator include systems that rely entirely on tribo, or frictional, charging of the powder and fluidized-bed coaters which are now widely used for coating small workpieces.

Like the wet-paint electrostatic spraying systems, the mechanism of particulate deposition relies primarily on the electrostatic attraction between charged particles and their image charge that appears on the grounded substrate. Since most powdered paint is naturally highly insulating, charge relaxation does not occur on impact with the substrate, and particles therefore remain and accumulate on the substrate. Normally, at the completion of coating, the substrate is then subjected to an elevated temperature at which the powdered paint melts and fuses to form a durable continuous coating.

A number of unique and attractive properties of electrostatic powder coating offer clear advantages over conventional wet-paint spraying. These include the ability to recycle oversprayed powder, which offers potential 100% product utilization. Also, the coating layer thickness is automatically controlled, which results in a more even thickness over the entire substrate surface.

It would appear that electrostatic powder coating might be the ideal system for film deposition, eliminating all the traditional operational difficulties associated with wet spraying. However, the system is not without problems, and development is constantly in progress to produce both optimized application equipment and powdered paints.

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Algal biomass harvesting and drying

Kuan-Yeow Show , ... Duu-Jong Lee , in Biofuels from Algae (Second Edition), 2019

3 Algal Drying

After algal biomass concentration and dewatering, the dewatered slurry is dried for stability, end use, extraction, or other further processing. For example, the algae must be bone dry prior to feeding into a press to obtain algal oil, which could then be processed into biodiesel. Notably, the most feasible drying techniques should be designed to eliminate possible deterioration of the delicate algae quality arising from the dehydration process. A number of process options are available for algae drying, as illustrated in Fig. 4.

Fig. 4. Process options for algae drying.

The main consideration in the selection of the drying technology depends on both the production scale and the purpose for which the dried biomass is intended [78,79]. The aim is to produce the product with reasonable cost and simple operation. Major algae drying methods such as rotary drying, spray drying, solar drying, cross-flow drying, vacuum shelf drying, flashing drying, and incinerator drying are discussed in the following sections.

3.1 Rotary Drying

Rotary drying involves the use of a sloped rotating cylinder (or called rotary dryer) to move the algae being dried from one end to the other by gravity. Use of a thin layer drum dryer to dry Scenedesmus algae produced an excellent dried algal product has been tested [80]. Drying the algae on the drum dryer has the dual advantage of sterilizing the samples and breaking the cell wall. In another study, it was reported that using a drum surface area of 2.5   m², the algae slurry could be thickened up to 25% dry solids [51]. A pilot electric drum-dryer was tested for drying wet slurry containing 30% solids of Scenedesmus algae at 120°C for about 10   s [81]. The rate of energy consumption under such operation was 52   kWh. Conversely, replacement of the electrically heated drum dryer by a steam-heated dryer could lower the processing cost by 6.8 times. Operating at a steam pressure of 8   atm, water loss of up to 50   kg could be achieved for every m² of the drum surface [51]. The energy cost can be significantly reduced if the supply of waste steam can be ascertained.

In an assessment on energy requirement for drying algae with a water content of 4%, heat energy of up to 65.7   MJ was consumed for evaporating 18.2   kg of water for every kg of dry algae product [80]. In addition, a supplementary electric energy input of 1.4   kWh was needed to run the dryer. As the energy requirement depends largely on the water content of the final dried algal, it was proposed that an acceptable higher water content should be maintained for the final product in order to lower the energy cost.

3.2 Spray Drying

Spray drying engages liquid atomization, gas/droplet mixing, and drying from liquid droplets [36]. The atomized water droplets are usually sprayed downward into a vertical tower through which hot gases pass downward. Drying is accomplished within a few seconds. The dried product is removed from the bottom of the tower, and the waste gas stream exhausted through a cyclonic dust separator.

Spray drying is deemed an appropriate drying method for production of algae for human consumption [82]. Despite the fact that it is a very efficient drying method, it could rupture intact cells due to its high-pressure atomization process, and generally imparted unacceptable degradation in product quality.

The predominant deficiencies of spray drying are the high operating cost and low digestibility of dried algae. In an examination involving spray drying and drum drying methods for microalgae, the latter was recommended because of better digestibility, lower energy requirements, and lower investments [51].

3.3 Solar Drying

In remote vicinities where a common energy supply such as an energy grid is lacking, solar heat drying appears to be the most feasible means of drying. Algae drying could be accomplished either by direct solar radiation or by solar water heating. Direct sun radiation causes algal chlorophyll to dehydrate and disintegrate, thereby altering the texture and color of the final algal product. Since solar radiation is uncontrollable and unpredictable, the main problem of solar heat drying is associated with overheating and unreliability of the operation, which is highly dependent on the weather.

Overheating of algae biomass could be avoided in a solar water heating system. Solar thermal energy is derived by proprietary designed glass panels or tubes used to heat up the water. With proper system design, algae drying rate was higher and heating of algae biomass could be regulated. Although this method was more commonly used, the final product was less viable because of the higher capital cost.

The feasibility of using a solar drier in comparison with direct sun radiation for drying Spirulina algae was examined [81]. Consisting of a wooden chamber with internal surface painted black and the top covered with glass plate, a solar dryer used in drying for 5–6   h at temperatures between 60 and 65°C was able to dehydrate the final algal product to about 4%–8% water content. Such a drying method appears to be of low technology, but is simple and inexpensive.

Just like the direct solar radiation method, drying by the solar drier method is highly weather dependent and thus unreliable. The method is also subject to a risk of fermentation and spillage under prolonged drying. Solar heat drying is not recommended for preparing an algal product intended for human consumption. The slow solar drying process invariably emits an unpleasant odor, thereby affecting the product quality. In addition, the algal biomass must be subjected to a short duration of high heat (l20°C) in order to maintain the nutritional value and safety of the food product. For the production of animal feed, however, sun drying may be an acceptable solution.

3.4 Cross-Flow Air Drying

Wet slurry of Spirulina algae containing 55%–66% moisture was dried using cross-flow air drying for 14   h at 62°C in a compartment dryer producing a good-quality dried algal product 2–3   mm thick with 4%–8% moisture contents [81]. It was found that the cell wall of Chlorella and Scenedesmus remained intact after drying. Further assessment revealed that the process was cheaper than drum drying and faster than solar heat drying.

3.5 Vacuum-Shelf Drying

A Spirulina algae slurry was dried to 4% moisture content in a vacuum-shelf dryer at a temperature of 50–65°C and 0.06   atm. Pressure [81]. The dried algae indicated a hygroscopic characteristic and porous biomass structure. Higher capital and running costs were also highlighted by the researchers.

3.6 Flash Drying

Flash drying is a common method for wastewater sludge drying developed in the 1930s in the USA. It was devised for algae drying to achieve rapid removal of moisture by spraying or injecting a mixture of dried and wet algae into a hot gas stream [36]. The turbulent hot gases serve as a carrier for mass transfer of moisture from algae slurry to the gases. The cost of drying and the final algal product quality are influenced greatly by the hot gas source. Uncontaminated waste steam could lower the processing cost and ensure a good-quality final product.

3.7 Incinerator Drying

A multiple hearth incinerator designed with a circular steel cylinder containing several hearths arranged in vertical stack is frequently used to dry and burn wastewater sludge. If heat inputs are reduced, the incinerator can be used as a dryer alone. It could thus be improvised for use as algae dryer with provision of hot gases, and both the wet algae slurry and hot gases flow in parallel downward through the furnace [36]. The technique of parallel flow of product and hot gases is frequently employed in drying operations to prevent burning or scorching a heat-sensitive material such as algae.

Another incinerator that was originally used for sludge incineration is the fluidized bed incinerator. This system utilizes a fluidized sand bed as a heat reservoir to promote uniform combustion of algal solids. The fluidized bed is preheated, using fuel oil or gas, before the algal slurry is introduced. The dried algae are separated from the sand by a cyclone separator [36].

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Spray combustion of fast pyrolysis bio-oils: Applications, challenges, and potential solutions

Mohsen Broumand , ... Murray J. Thomson , in Progress in Energy and Combustion Science, 2020

4.2 Common fuel nozzles

The most common nozzle designs for spray combustion applications are based on pressure and twin-fluid atomization [153]. In these nozzles, liquid fuel is injected in the form of a jet or sheet, depending on the configuration of the nozzle. The process of liquid jet disintegration is of great importance for the design of plain-orifice pressure nozzles or plain-jet twin-fluid nozzles, whereas the mechanism of liquid sheet breakup has direct relevance to the performance of simplex pressure nozzles or prefilming twin-fluid nozzles. A schematic view of these nozzles is illustrated in Fig. 13.

Fig. 13. Schematic of simplex pressure nozzle (left), internally-mixed twin-fluid nozzle (middle) and prefilming twin-fluid nozzle (right). Reprinted from [170] with permission of Elsevier.

Pressure atomization is accomplished by forcing liquid through a small orifice (plain-orifice pressure atomizer) which forms a narrow angle, solid-cone spray or the outlet can be preceded by a swirl chamber (pressure-swirl or simplex atomizer), causing the liquid to leave the nozzle as an annular sheet expanding radially outwards as a solid or hollow-cone spray with a comparatively wider cone angle [58]. Solid-cone sprays result in more uniformly sized droplets, but hollow-cone sprays provide finer atomization (this is also the case with twin-fluid nozzles) [171,175]. Higher atomization pressures and wider spray angles result in finer sprays because the liquid sheet thins out and the spray pattern allows for more air exposure [58,92]. Pressure atomization is used extensively in light fuel oil combustion applications due to its simplicity [24,116]. Nonetheless, the high pressures required for this type of atomization can also cause wear/abrasion, erosion and ultimately performance degradation [60,171].

Twin-fluid nozzles induce large interfacial forces, particularly shear [92,176], between high velocity air and low velocity liquid (large relative velocity) to cause the liquid to disintegrate into ligaments and ultimately droplets [58,171,176]. These nozzles are opposite in principle to pressure atomization which injects fast moving liquid into comparatively still air, though the relative fluid velocity is still one of the critical parameters and plays an equally important role [177]. Among twin-fluid atomizers, air-assisted nozzles use small amounts of highly pressurized air at very high velocities for atomization, while air-blast nozzles use much larger amounts of air at lower pressures to form a spray [174]. Twin-fluid nozzles can be further broken down into internally mixed or externally mixed varieties. For internally mixed twin-fluid nozzles, much of their ligament and droplet formation, also classified as primary atomization, occurs within the nozzle [178,179]. Externally mixed ones are of a less efficient atomization scheme, but their design prevents the oxidizer from entering the fuel line in combustion applications, preventing flashback and improving safety. In both cases, secondary atomization occurs exclusively outside of the nozzle.

Compared to pressure atomization, twin-fluid nozzles provide a more even spray dispersion, have larger internal cross-sections to resist clogging, are effective over a wider operating range and allow for independent control of air and liquid flow rates [58,60,92,116,178,180–183]. Twin-fluid nozzles however, are also more complex and are therefore, generally more expensive [24,92,116]. For twin-fluid nozzles, the ALR is typically the most crucial spray parameter affecting SMD and is directly related to the air/liquid relative velocity. Higher ALR values reduce the spray SMD and generally display asymptotic behavior beyond a certain point [60,184–191]. Furthermore, larger ALR values indicate a greater relative velocity between the air and liquid, meaning that more energy is available to break up the liquid fuel. In reacting sprays, a higher atomizing airflow rate also induces stronger turbulence that can enhance mixing, but consequently also increases shear rates, which if too high can cause the flame to extinguish or prevent (re)ignition. Overall, twin-fluid nozzles are relatively insensitive to liquid properties or those of the surrounding gas medium [58]. In particular, twin-fluid nozzles are far less sensitive to liquid viscosity than pressure atomizers, making them good for highly viscous fuels (as viscosity had a minimal effect on atomization in two studies up to 100–120cP [156,182,183]), particularly if internally mixed [58,93,154,156,182,183]. Therefore, twin-fluid atomizers are often used with heavy fuel oils and FPBO [86]. In combustion applications, twin-fluid nozzles also enhance fuel/oxidizer mixing compared to pressure atomization which can increase combustion efficiency and reduce pollutants [58,180,181,192].

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Review of fuel oil quality and combustion of fast pyrolysis bio-oils from lignocellulosic biomass

Jani Lehto , ... David Chiaramonti , in Applied Energy, 2014

7.6 Replacing heavy fuel oil at Fortum's district heating plant

More than 40   tons of bio-oil have been combusted in Fortum Power and Heat's 1.5   MW district heating plant in Masala, Finland, since 2010. The bio-oil was produced at Metso's pilot plant, and it was whole oil including extractive-rich top phase. No additives were used. The existing burner was replaced with a new bio-oil burner consisting of a modified mono block heavy fuel oil burner originally designed for high-pressure atomization. The modifications to the existing burner included, for example, burner head configuration. Also the piping, pumping and valve systems including pre-heating of the oil were specially designed for bio-oil. Two main topics, the overall functionality of the bio-oil receiving, storing, and pumping system and the function of the burner were successfully addressed. The receiving system and oil tank were located outside the boiler building. The system worked well, despite the outside temperatures, which varied from −20 to +10   °C during the test periods. As a result, good reliability and a satisfactory turn-down ratio of 1:3 were achieved. The unit has even been operated unmanned for 1 night. Flue gas emissions were close to those of heavy fuel oil. No odour emissions occurred [57]. Table 5 shows data on emissions from pyrolysis oil boiler combustion from various sources.

Table 5. Boiler emissions for various liquid sources [13].

	Ensyn		Union		Ensyn		Ensyn		Dynamotive	Fortum
			Fenosa a
Feedstock	Hardwood		Eucalyptus		Hardwood		Hardwood		Pine	Spruce
Solids content, wt%)	0.5		0.7				∼0.4		0.17	0.05
Type of boiler analysis	Arimax Eetta		Arimax Eetta		Water-wall utility boiler, 10   MWa		10   MWa boiler, Oilon Lenox GRT-5L		10   MWa boiler, Oilon Lenox GRT-5L	300   MWa, LFO boiler
	200   kW boiler		200   kW boiler
O2 (vol%)	4	6	5	6				3.3–3.6	3.3–3.4	3–4
CO (ppm)	32	28	40	20	32	32	67	1–2	10–25	10–20
NO2	142	137	170	150	195	198	208	159–164	10–25	100–150
THC					0.8	1.0	1.4
Particulate (mg/MJ)					105	144	161	15	92
Bacharach No.	5	5	2.5	2.8				2	2.8	1–2 b

a

With an additional 3   wt% of ethanol and 3   wt% of water, a modified refractory in the boiler to ensure complete combustion.

b

Particulate emissions contain only inorganic materials. No tars found. The amount is dependent on the ash content of the oil.

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Control of combustion-generated nitrogen oxides by selective non-catalytic reduction

M. Tayyeb Javed , ... B.M. Gibbs , in Journal of Environmental Management, 2007

Muzio et al. (1997) discussed use of a pair of retractable in-furnace lances which were designed to provide a high degree of load following flexibility through on-line adjustments of the injection angle. The majority of the test program was conducted using retractable lances provided by NOELL (Advanced retractable Injection lance). Subsequently, an alternative lance design provided by Diamond Power Specialty Company was evaluated. The performance of the urea NO _x reduction process at the two sites was dominated by variables affecting the temperature at the injection location and mixing of urea with the combustion products. Varying operating conditions, such as load and firing configuration, changed temperature distribution in the boilers as well as initial NO _x levels. Such changes affected the relative location of urea injectors within the urea reaction temperature window and, thus, the amount of NO _x reduction achieved. Available injection process variables, including injector design, solution flow and pressure, injector location and spray orientation, were used to optimize the distribution of the urea within the reaction window at varying loads to achieve maximum NO _x reduction. Major variables identified as influencing the urea DeNO _x process performance, in the investigations of Abele et al. (1991), Nylander et al. (1989) and Mansour et al. (1987), Muzio et al. (1997) are briefly described in the following sections:

(i)

Combustion gas temperature

  The field data have shown NH ₃ emission to be a strong function of combustion gas temperature. Injecting urea at temperatures of 1040   °C or above significantly reduced NH ₃ emission. It was suggested that urea should be injected at the higher end (or right half) of the optimum temperature range, i.e. 1040–1200   °C so as to minimise any chances of N₂O production.

(ii)

Carrier gas pressure and atomisation quality

  The effect of carrier gas pressure on atomisation quality is a function of atomiser design. Generally, high atomisation pressure provides improved atomisation, which promotes the early release of urea to react with NO. However, in one of the atomisers used initially in a study, an increase in atomisation pressure did not result in a measurable improvement in atomisation but did increase the discharge momentum of the reagent. This in turn improved the reagent penetration and mixing and hence the performance of the process.

(iii)

Effect of dilution of the urea solution

  For a given NSR, changing urea solution concentration can vary reagent mass flow. Reduced solution concentration increases the mass throughput of the reagent and thus its penetration and mixing. For a twin fluid atomiser, an increase in reagent flow would also result in deterioration in atomisation quality. This leads to increased momentum of the atomised spray and improved mixing.

(iv)

Injector design

  As indicated by most of the studies on the topic, the injector design had a significant effect on process performance. Three different types of atomisers were used in an investigation, which gave performance significantly different from each other. The type and construction details were not mentioned for commercial reasons. A full-scale demonstration of a urea-based SNCR system was performed at Cardinal Plant unit No. 1. The specific operation goal of the demonstration was to reduce NO _x emissions by 30% beyond the level achieved through the use of LNB while minimising the NH₃ slip at or below 5   ppm. CFD and CKM determined the optimum temperature region and injection strategy for distribution of reagent relative to unit load. Two of the six multi-nozzle lances have developed leaks, which were attributed to manufacturing defects rather than operation-related problems. As an overall impression the system has been able to obtain a fair level of NO _x reduction as installed. Overall the unit experienced very few operational problems and it could be inferred that the technology combined with LNB can obtain a significant reduction in emissions at a comparatively low installed cost than for other post-combustion controls (Malone et al., 2000).

(v)

Effects of furnace geometry, flow pattern and injection location

  In one of the studies, the velocity measurements were made at various locations within the furnace to establish the flow pattern of flue gases passing through the furnace. These data supported the possible presence of gas re-circulation fields. It was believed that recirculation was caused by a negative pressure zone in the centre of the furnace. This appeared to create vertical as well as horizontal vortices which carried flue gas near the furnace walls to the centre. This data allowed the selection of the urea injection point in order to take advantage of the furnace recirculation zones and improve reagent mixing with combustion products.

(vi)

Blending of reagents

  The blending of enhancers may also have a positive point in that they reduce NO _x without significant emissions, which arise from the use of NH₃ alone. The enhancer used was HMTA, furfural, furfural derivatives, sugar or a lower carbon alcohol. In one example from their patent, Epperly et al. reported the injection of a treatment agent comprising 10% by weight of urea, 4% by weight of HMTA and 10% by weight of furfural, at a rate of 300   ml/hr and an excess O₂ in the effluent of 3% by volume (Epperly and Sullivan, 1988; Epperly and Broderick, 1988). At about 915   °C NO _x was reduced from 190   ppm to just 80   ppm giving a 57% reduction with only 8   ppm of NH₃ and 15   ppm of CO.

  Based on a review of patent literature mentioned above, a series of tests were conducted by Teixeira and Muzio to evaluate, in detail, the effectiveness of these HMTA/furfural additives (Teixeira and Muzio, 1991a). A temperature of 900   °C was used for these tests. The quantities of additives used in these tests, as estimated from the patents, were HMTA/urea=0.2, and furfural/urea=3.65 (on a molar basis). The results confirmed the meaningful improvement in both NO and NO _x removal. However, the improvement in NO _x removal was considerably lower than the improvement in NO removal. Evaluation of the final NO₂ level showed that a portion of the initial NO was being oxidised to NO₂. Further, it appeared that the improvement in NO _x reduction could be attributed to the increased N/NO _x injection ratio that results from the addition of HMTA.

Robin et al. (1991) reported studies carried out on the efficacy of methanol in modifying the behaviour of the SNCR process. Results showed that, at low methanol/NO _i ratio, methanol does yield some enhancement of the SNCR process but that, at higher ratios, it can actually cause NO _x formation. The addition of methanol at a relatively low temperature (850   °C) has little effect on NO _x emissions but significantly decreases NH₃ slip. By using a methanol to NH₃ ratio of 2.4:1 it was possible to reduce NH₃ slip to around 10   ppm.

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