Key design and operating parameters that affect spray dryer design and/or performance are:
• Flue gas flow rate and composition
• Temperature of flue gas exiting the boiler and spray dryer
• Alkaline stoichiometric ratio
• Alkaline (lime or sodium) properties
• Required removal efficiency
The most important parameter in sizing the spray dryer is to ensure an adequate gas residence time at maximum gas flow rate to prevent wet solids at the spray dryer outlet. Gas residence times are generally in the range of 10 to 15 seconds for most commercial systems. Due to the large volume of gases that must be treated from utility boilers, these installations often have multiple dryers per boiler.
The spray dryer outlet temperature is controlled by the amount of water injected either with the alkaline slurry or as makeup water. The key to achieving good SO2 removal is to maintain the temperature of the flue gas exiting the spray dryer as close above its dew point (adiabatic saturation) as possible without actually saturating the flue gas. Generally, 20 - 30°F above the adiabatic saturation point is a good target range. This will enhance the reaction yet still prevent condensation. The amount of water that can evaporate in a spray dryer is dependent on the incoming flue gas temperature and to a lesser extent on the moisture content.
The alkaline feed rate is a function of the incoming acid gas levels and the required removal efficiency. The stoichiometric ratio is defined as the molar ratio of alkaline (i.e. calcium) in the spray dryer feed to the amount of acid gases (SO2 and HCl) present. For example, at a ratio of 1.0 the moles of calcium are equal to the moles of incoming HCl and SO2. However, due to inefficiencies in the mixing process, more than the theoretical amount of alkaline material is required to assure compliance with applicable standards. Thus, stoichiometric feed rates of 1.5 to 2.5 have been used to achieve SO2 removal level in the 75 to 85% range and HCl removal efficiencies of 95% on municipal waste combustors. For utility and industrial boilers, sulfur removal guarantees by spray dryer vendors have ranged from 60 to 90%. Table 1 lists information on operating spray dryer systems at utility boilers.
Summary of spray-dryer applications
1. R = rotary; N = nozzle.
2. Residence time.
3. Flue-gas temperature at entrance.
4. Approach to saturation at exit.
5. Varies.
6. Not reported.
Source: Huang et al. 1988.
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