Gas Removal Mechanisms

In dry scrubbing, acid gas is removed by the mechanisms of adsorption and absorption. In dry injection systems, where adsorption is the primary removal mechanism, pollutant gas molecules adhere to the surface area of the alkaline particles. Thus, the reaction between the acid gas and the alkaline material takes place on the surface of these alkaline particles. The alkaline materials are generally calcium hydroxide or sodium-based reagents that have the consistency of a fine powder. These fine particles have large surface areas to aid in adsorbing the acid gases.

In spray dryer systems, absorption is the predominant collection mechanism. Absorption can occur in conjunction with a chemical reaction if a reagent has been added to the scrubbing liquid. Spray dryer absorbers utilize this principle. First, the acid gas dissolves in the alkaline slurry droplets, then reacts with the alkaline material dissolved therein to form solid salts. Because the acid gases react to form new compounds, additional acid gases can be absorbed by the liquid. Also, when the liquid droplets evaporate, the acid gases continue to react (by adsorption) with the solid alkaline materials remaining in the SDA.

Adsorption and absorption are similar mass transfer processes in that the acid gases must first be brought into contact with the alkaline sorbent material, be provided ample reaction sites and time, and finally, be removed from the gas stream. Intimate contact between the alkaline sorbent and acid gases is important for effective gas removal. With dry injection, solid powder-like sorbent is dispersed in either the furnace area, exhaust duct, or in a reaction chamber. Dispersion is generally provided by injecting the sorbent through a venturi device countercurrent to the flow of the gas stream to create turbulence. In spray dryers, the alkaline sorbent slurry is dispersed as a mist of tiny liquid droplets in the reaction or drying vessel. Due to their fine spray mists, spray dryers provide much more contact area than dry injectors for gas absorption to occur. Also, spray dryer absorbers provide more effective mixing of acid gases with the alkaline sorbent than dry sorbent injectors because it is easier to mix a gas with a liquid than with a solid. Spray dryer absorbers have some disadvantages; the injection (atomization) equipment required by spray dryer absorbers is much more complicated and expensive to operate.

Residence or reaction time can be enhanced in these applications in a number of ways. In dry injectors, the sorbent is often injected directly into the furnace or ductwork. To extend the residence time, reaction or holding vessels can be added to the dry sorbent injection system. Spray dryers always have a reaction or drying chamber to assure a dry gas stream leaving the chamber. Also, in both systems, the particulate control device will provide an additional area for the acid gases to further react with the sorbent.

In addition, both the absorption and adsorption processes are temperature dependent: the cooler the flue gas, the more effectively the acid gases will react with the sorbents. Spray dryer absorbers cool the gas stream and therefore, can achieve higher removal efficiencies than dry injection with no cooling.