Spray Drying Equipment

In a spray drying system, there are a number of system components. Three of the major components are the atomizer, spray dryer chamber and particulate control system.

Atomizers

Currently, two types of atomizers are used in spray dryers for acid gas removal: rotary disks or wheels and dual-fluid nozzles. In either case, the purpose of the atomizer is to break the sorbent slurry into a cloud of fine droplets to promote intimate sorbent contact with the acid bases.

In the rotary atomizer, the slurry is fed into the top of the rotating wheel or disk. Centrifugal force causes the slurry to form a thin film on the internal surface of the cavity. As the slurry emerges from the cavity through abrasion-resistant inserts in the side of the wheel, the liquid is atomized into discrete droplets that are propelled radially outward. These droplets, generally 25-150 μm in diameter, dry rapidly in the hot flue gas within the spray dryer. Figure 1  shows an example of a typical atomizer wheel used in spray dryers (Huang 1988).

Figure 1. Example of rotary atomizer used in spray-dryer FGD

systems

For FGD spray dryer applications, atomizer wheels range from 8 to 16 inches in diameter and have rotational speeds from 7,000 to 20,000 revolutions per minute (rpm). Due to the highly abrasive nature of the slurry (which can consist of either slaked lime [Ca(OH)2] or slaked lime plus recycled fly ash/reacted product), the wheels are constructed of corrosion- and abrasion-resistant materials, including ceramic inserts in the vanes or nozzles.

In dual-fluid pneumatic nozzle atomization, the slurry feed is injected into the body of a nozzle and is entrained into a high-velocity, high-pressure air stream as shown in Figures 2 and 3 (Maurin 1983). The high-velocity air impacts on the slurry-feed stream, resulting in the production of fine droplets. The air stream and slurry comprise the two fluids. The size of liquid droplets produced decreases as the compressed air pressure and relative velocity of the liquid to air increases.

Figure 2. Two-fluid nozzle atomizer (nozzle body)

Figure 3. Two-fluid nozzle atomizer (high pressure air stream)

The mean droplet size for both atomizing systems has been shown to be the same, indicating that the systems perform similarly. Likewise, the capacity of a nozzle system for atomization of slurries is the same as that for a rotary atomizer. Nevertheless, rotary atomizers and pneumatic nozzles have somewhat different advantages and disadvantages (Huang 1988):

1. Rotary atomizers, with their higher capacity per unit, will have a simpler piping system. In a rotary-atomizer system, usually only one feed pipe per atomizer is used; whereas in a nozzle-type atomizer, there will be an individual feed pipe (and valve) to each nozzle. In very large installations, this results in a complex piping system.

2. Pneumatic nozzle atomizers are much easier to maintain than rotary atomizers while the system is on-line because the individual feed lines have isolation and control valves. With multiple nozzles, it is possible to isolate an individual nozzle, remove it for cleaning or replacement, and then return the cleaned or new nozzle to service without reducing the gas flow to the system or bypassing the gas flow to another spray dryer.

3. The net-energy requirements of a rotary atomizer and a set of pneumatic nozzles are approximately the same, but the method by which this energy is applied is different. For a rotary atomizer, the atomization energy is supplied via a motor coupled to the atomizing wheel with a gear and/or belt drive. For a pneumatic atomizer, the energy of atomization is produced primarily by the pressure of the atomizing air. Hence, the energy is supplied through an air compressor that may also supply air for instrumentation or other purposes.

4. A spare rotary atomizer is often required as a backup in case of failure. In a pneumatic nozzle system, the required spares consist of nozzles and an extra air compressor. For a smaller single rotary-atomizer unit, the relative cost of a spare atomizer would be substantial

Spray-Dryer Chamber

The atomization method chosen will affect the design of the spray-dryer chamber, including the physical dimensions. For a rotary-atomizer type of spray dryer, which projects the droplets radially outward and perpendicular to the gas flow, the length-to-diameter ratio of the dryer (L/D) is typically 0.8:1. Figure 4(A) illustrates two typical configurations of rotary atomizer spray dryers. The droplets decelerate rapidly due to the drag forces of the downward-moving flue gas and eventually attain the speed and direction of the flue gas. To avoid wall deposition, the designed radial distance between the atomizer and the dryer wall must be sufficient to allow for adequate drying of the largest droplets. This is accomplished by proper choice of the L/D, droplet size, and residence time.

For a two-fluid pneumatic nozzle spray dryer [shown in Figure 4(B)], which atomizes the droplets in the direction of the gas flow (downward), the L/D is typically 2:1. In this case, sidewall deposition is a minor problem.

Typically, industrial boiler spray dryers have diameters of 25-30 ft, whereas utility spray dryers have diameters of 40-50 ft. Currently, the maximum diameter of an installed spray dryer is about 60 ft. In general, if the gas-flow rate is large enough that a single unit greater than 40-50 ft in diameter would be specified, then the installation of multiple spray dryers should be considered. In utility systems where the gas flow can range from 1-2 million acfm, multiple spray dryers are common. Multiple spray dryers are installed for easy maintenance and high reliability.

Flue gas may enter a spray dryer in one of three patterns relative to the slurry direction: cocurrent, countercurrent, or mixed. In cocurrent spray dryers, all of the gas enters through a roof gas disperser in the top of the vessel, where its rotation is controlled by angled vanes that direct the gas around the atomizer [shown in Figure 4 (A)]. This type of gas distribution precisely controls the exit gas temperature since the gas and slurry travel in the same direction. This is the most common flow pattern used in acid gas control systems.

In countercurrent spray dryers, the gas enters from the bottom of the vessel and is directed at the atomized liquid above. Although uncommon in utility or industrial flue-gas control systems, these spray dryers have the advantage of a much higher drying capacity than the cocurrent system.

Another type of spray dryer, the compound-gas disperser or mixed, is offered by one manufacturer as an option in specific applications. This type of spray dryer is sometimes used on very large units as an alternative to multiple rotary atomizers to obtain efficient contact between the hot gas and the liquid droplets.

Figure 4. Two types of spray-dryer chambers

Particulate-Matter Collection

A spray-dryer system is not complete without a means of particulate-matter collection. Not only is a well-designed particulate-matter control system needed to meet emissions requirements, but it also aids in acid-gas removal. Acid gases are removed when the flue gas comes in contact with lime-containing particles in the fabric filter or ESP. Fabric filters have been used on the majority of acid gas control systems, due to their ability to obtain slightly higher residual acid gas removal than ESPs.

Regardless of the type of particulate control device, an important design feature is to minimize potential heat loss in the fly ash collection system. The fly ash contains unreacted alkaline sorbent along with calcium (or sodium) sulfates, and in the case of waste incinerators, it also contains calcium chlorides. These materials are very hygroscopic and can result in corrosion problems or ash plugging of equipment if condensation occurs. Adding insulation, hopper heaters and reducing air in-leakage are essential to prevent operational problems with the ash handling system.