For many utilities and industrial applications, dry sorbent injection (DSI) of hydrated lime offers impressive advantages over the wet or semi-wet flue gas desulfurization systems for controlling the emissions of SO2, HCl, HF, SO3 and even Hg. These potential advantages include ease of retrofit, dry waste and low capital investment.
Those benefits, combined with the ability to effectively and efficiently absorb most pollutants found in flue gas, make hydrated lime the leading choice for DSI.
Injection of calcium reagents can occur anywhere in flue gas path from the furnace to the particulate control equipment but the optimum location depends on the target pollutant. Common injection points for DSI include the upper furnace, pre-SCR, and the inlet or outlet of the air pre-heater. These injection points can be single injection points to focus on a particular pollutant or they can be combined to optimize the capture of multiple pollutants.
The elevated temperatures of both the furnace and boiler economizer is best suited for SO2 control. The temperature of the upper furnace ranges from 1800-2200°F. At this high temperature the hydrated lime is rapidly calcined forming CaO which reacts with SO2 to form CaSO4.
With the boiler economizer process, hydrated lime is injected between the super heater and the air preheater where the temperature ranges between 800-1200°F. Unlike furnace injection, SO2 reacts with the hydrate to form CaSO3.
As the flue gas cools, hydrated lime favors absorption of the other acid gasses; SO3, HCl and HF. Injection of hydrated lime at the inlet of the air pre-heater (600-800°F) is often paired with carbon injection at the air-preheater outlet to provide a two reagent multi-pollutant control.
Carbon injection is more economical when combined with hydrated lime injection. The hydrate is used to reduce SO3 allowing more efficient use of the activated carbon for Hg control.
Lime alone has been shown to reduce mercury emissions in conventional SDA and CDS. The same chemical mechanisms are present when using hydrated lime DSI with a baghouse. If you are considering a mercury specific sorbent with your baghouse, then consider hydrated lime could meet your compliance needs at a fraction of the cost.
In order to maximize the removal efficiency of all pollutants, it is important to allow the sorbents to remain entrained in the flue gas as long as possible and ensure the sorbent is evenly distributed across the flue duct. The injection lance design therefore is very important and should be considered carefully.
A fabric filter located downstream of the sorbent injection location will also improve sorbent utilization as well as further lower acid gas emissions.