Summary

Dry scrubbing systems are used to control acid gas emissions primarily from combustion sources such as utility and industrial boilers and municipal and medical waste incinerators. Dry scrubbing systems only remove acid gases and therefore must be followed by a particulate control device (ESP or fabric filter) prior to exhausting the gases to the atmosphere.

Dry scrubbing systems can be categorized as dry sorbent injectors (DSI) or as semi-dry scrubbers (also referred to as spray dryer absorbers or spray dryers). Dry sorbent injection involves the addition of a dry alkaline material (usually hydrated lime or soda ash) into the gas stream to react with any acid gases that are present. The sorbent can be injected directly into the flue gas duct ahead of the particulate control device or into an open reaction chamber. The acid gases are adsorbed onto and react with alkaline sorbents to form solid salts which are removed in the particulate control device.

In spray dryer absorbers (SDAs) the flue gases are introduced into an absorbing tower (dryer) where the gases are contacted with a finely atomized alkaline slurry: usually a calcium-based sorbent such as Ca(OH)2 or CaO. Acid gases are absorbed by the slurry droplets and react to form solid salts. The heat of the flue gas is used to evaporate all the water droplets, leaving a non-saturated (i.e. dry) flue gas exiting the absorber tower. The effect of cooling and humidifying the hot gas stream increases collection efficiency over simple dry injection.

The major components of a spray dryer absorber are the atomizer, spray dryer chamber and the particulate control device. Two types of atomizers are currently utilized for acid gas removal: rotary disks (wheel type) and dual-fluid nozzles. In either case, the purpose of the atomizer is to break the sorbent slurry into a cloud of fine droplets. The spray chamber is designed based on the type of atomizer utilized. Spray chambers used with rotary atomizers are shorter but fatter than those used with two-fluid nozzle atomizers. Both ESPs and baghouses have been used with spray dryer absorbers. An important design feature of the particulate control device is to minimize potential heat loss in the fly ash collection system to prevent potential plugging problems.

An important parameter in the operation of a dry scrubbing system is the amount of alkaline material feed into the system. The amount of sorbent required is a function of the type of sorbent used, inlet and outlet (required removal) acid gas levels and the effectiveness of the dry scrubbing system design. The amount of sorbent added is generally reported as the stoichiometric ratio on a molar basis of sorbent to acid gases. A stoichiometric ratio of 1:1 would be used under ideal conditions; in practice more than the theoretical amount must be utilized to assure compliance with required acid gas removal levels.

Except for rotary atomizers in spray dryers, dry scrubbing systems are relatively simple devices with few moving parts. The primary maintenance problem is potential plugging in the solid or slurry transport lines. Plugging can occur whenever there are bends or restrictions in piping.