Hydrated lime and SO3 :  Hydrated lime makes and effective sorbent for the capture of sulfur trioxide (SO3).  First let us understand howSO3 is formed.  Sulfur trioxide forms in the boiler and Selective Catalyst Reduction (SCR) equipment.  As coal is burned in a coal-fired boiler, sulfur in that coal oxidizes to form sulfur oxides.  A majority is sulfur dioxide (SO2) but a small amount of sulfur trioxide (SO3) is formed.  SCRs for NOx removal will also oxidize a portion of the SO2 in flue gas to SO3


Without hydrated lime injection for the removal of SO3, some removal of SO3 occurs in the system.   As flue gas cools in the Air Preheater (APH), some SO3 will deposit on the internals of the equipment.  The presence of a small amount of SO3 lowers the resistivity of fly ash and is generally beneficial towards capture of ash in an electrostatic precipitator.  For units equipped with a fabric filter/baghouse, a portion of the SO3 will be removed by absorption onto the fly ash.  Depending upon the calcium level in the fly ash, some additional SO3 may also be neutralized on the baghouse solids layer that develops.  Hydrated Lime has been used effectively in this situation both injecting hydrated lime in front of the APH or injection hydrated lime in front of the baghouse.


Remaining SO3 that continues through the APH can pass through dew point and form a sulfuric acid mist.  This mist continues through the post-APH ductwork and particulate collection device.  Plants equipped with a wet flue gas desulfurization (FGD) system will form H2SO4 aerosols as the flue gas is quenched in the scrubber.  This action results in a characteristic blue plume emitted from the stack.   Hydrated lime does an effective job mitigating H2SO4 aerosols and reducing blude plume at the stack.


Risks of  SO3

The presence of SO3/H2SO4 in flue gas necessitates a variety of operation considerations.  Corrosion due to the acidic species degrades ductwork and equipment.  Low (< 1 sec) residence time requires a higher sorbent utilization rate to achieve desired SO3 reduction.  Process temperatures for heat recovery are dictated by acid dew point temperature of the flue gas.  Ammonia slip from SCR operation can form ammonium bisulfate (ABS), a sticky precipitate that clogs air heater internals.  Use of wet flue gas desulfurization can generate a tell-tale blue plume of sulfuric acid mist that is an eyesore at best and environmental risk to the local community at worst.  Dry srobent injection of hydrated lime is a useful strategy to address these corrosion concerns.


The primary risk of a blue plume is a touch down into neighboring areas, causing potential health effects, corrosion of property, damage to vegetation and/or potential negative attention due to the appearance of the stack emission.  Corrosion of process equipment is also a risk.  Equipment that can be affected includes duct work, fans (Air Preheater and ID), and ESP or baghouse internals.  Hydrated lime has been successfully used to address stack emissions for years.


The presence of acid gases in flue gas dictates operational decisions and increase operating costs.  Minimization of SO2 conversion to SO3 may warrant the extra expense of low conversion catalyst in a SCR.  Fear of forming sticky ammonium bisulfate (ABS) particles on APH internals will affect operation of the SCR in order to contain ammonia slip.  The need to operate safely above dew point in the APH increases heat rate and resulting energy costs.  Greater air flow due to a high heat rate translates to additional power required to run ID fans.  Ash release from baghouse bags can be less efficient if the acid gases are untreated.  Hydrated lime injection should be a strategy to be considered when addressing ABS situations.


Many coal-fired power plants are faced with regulations on mercury emissions.  SO3 in flue gas absorbs onto activated carbon, thereby lower its ability to capture mercury.  Units utilizing bituminous coal must remove SO3 before treating with activated carbon, thus the ability to remove SO3 to very low levels is necessary for units facing mercury removal requirements.  There are high capital cost options such as Wet ESPs available, but those may not be capable of reducing high levels of SO3.  In addition, a Wet ESP does not offer any upstream corrosion prevention.  The use of hydrated lime dry sorbent injection would be beneficial in this situation.

For most plants, the introduction of an alkaline species such as hydrated lime (hydrated lime is also known as calcium hydroxide or CaOH2) into the flue gas has become the chosen pathway to eliminate sulfuric acid mist emissions.  Dry sorbent injection of hydrated lime offers many advantages in this application, which is why most Utility boilers have chosen to use hydrated lime in the past several years.