Except for rotary atomizers in spray dryers, dry injectors and spray dryer absorbers are relatively simple devices with few moving parts. The primary maintenance problem associated with any dry scrubbing system is potential plugging in the solid or slurry transport systems. Manufacturers of the various systems provide suggested maintenance and inspection schedules for each component. These schedules should be followed and information recorded to aid in documenting the system operation.
Dry scrubbing systems involve transporting a solid or slurry (which can be 10 to 40% solids) in small pipes; therefore, plugging problems could occur in a number of locations. The most common locations of plugging problems are in "dead" areas of the solid or slurry piping, valves and the atomizer. Dead areas of piping are associated with tees going to spare pumps or a cleanout port. In these areas, flow only occurs occasionally and provides an area for solids to buildup and block transport lines. Eliminating the tees is not practical since redundancy is needed (and often mandated) in order to assure continual operation of the scrubbing system. Also, certain tees are installed specifically to allow quick access to piping internals in order that a specific length of pipe can be flushed with water to dislodge buildup. Flexible rubber hosing and quick-type connectors have been used to try and minimize line plugging. Flexible piping is not as susceptible to plugging as solid pipe, and with the use of quick connectors, the flexible piping can be installed or removed quickly to flush out areas or to connect spare components.
Plugging problems associated with valves and atomizers in slurry systems are minimized by using screens in transport lines to remove solids. However, these screens must be periodically checked and cleaned or else they will cause plugging. Atomizing systems are often designed so that they can be flushed with water during operation (this will temporary reduce potential acid gas removal efficiency). Atomizers should also be designed so that they can be replaced in a short timeframe.
Another area of maintenance with semi-dry scrubbing systems is the lime slaking system. Lime slaking is the process of mixing controlled amounts of water and lime in a mixing vessel (slaker). The lime and water react (an exothermic process) to form the lime slurry which is then screened of grit, stored in agitated mixing tanks and then metered to the atomizer.
With the slaking system, plugging and the quality of slurry produced are the two biggest maintenance concerns. Plugging of dead spots and pumps can occur as already discussed. In addition, the grit screening process is of concern because if the screen is damaged, then large quantities of grit can get into the entire lime-slurry transport system causing extensive plugging and/or abrasion wear problems. Slurry quality is dependent on the quality of lime and slaking water utilized in addition to the mechanical action of the slaker. Both the lime and water should be of high quality (limited contaminants or other chemicals present) to prevent adverse reactions that can result in scaling, plugging or reduced acid gas removal efficiencies. The mechanical action of the slaker will determine how efficiently the slaking reaction occurs. The slaker should be frequently inspected to ensure that it is operating as designed.
Plugging of lime slurry transport components can also occur due to a lack of slurry movement (i.e. during standby periods) when solids could settle out or the calcium could have time to react and form scale. During extended downtimes, lines and storage tanks should be drained and flushed where practical. Also, manufacturers recommend periodic cleaning in acid of screens and other components that are prone to plugging problems.
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