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Previous MACT Sub Categories. EPA has recognized differences in other industry rules by using sub-categorization: Differences in processes Differences in emissions characteristics Differences in control technology applicability and/or effectiveness

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previous mact sub categories
Previous MACT Sub Categories
  • EPA has recognized differences in other industry rules by using sub-categorization:
    • Differences in processes
    • Differences in emissions characteristics
    • Differences in control technology applicability and/or effectiveness
  • To determine whether utilities should be sub-categorized, there needs to be an analysis of the distinctions between different size/type/class of units.
class type and size relative to utility units
Class, Type and Size Relative To Utility Units
  • ICR data includes 1,086 coal fired generating units
  • Units range from 25 MW to 1,400 MW with each unit designed for site specific needs and conditions
  • Wide variety in equipment, process techniques, and raw materials (fuel and other)
  • EPA made 36 bins for Part III of the ICR
  • Reading EPA’s notice, appears that EPA means to sub categorize oil and coal, consistent with other rules (e.g., boiler MACT)
  • Will not discuss coal gasification because manufacturing of fuel gas should not be covered by this rule
are there potential utility subcategories
Are There Potential Utility Subcategories?
  • Different types and classes of generating units result in a wide variation in Hg emissions and speciation.
  • There are many factors that influence the magnitude and speciation of Hg emissions. Many are interrelated.
  • Must look for relevant differences in processes, emissions characteristics, or control technology applicability and/or effectiveness.
are there potential utility subcategories4
Are There Potential Utility Subcategories?
  • Looked at large number of potential factors grouped in four areas.
    • Different types/methods of fuel combustion
    • Coal considerations
    • Overall operating characteristics
    • Process design configurations & equipment applicability
  • Not realistic to have a sub category for each factor
  • Attempted to group related factors together to define a smaller number of potential sub categories.
combustion processes conventional boiler types
Combustion Processes: Conventional Boiler Types
  • Conventional Coal Boilers
    • Pulverized Coal (Wall fired, T-Fired, etc.)
    • Cyclone Fired
    • Stoker Fired
  • Units differ greatly in coal prep and firing technique
  • With all other variables equal, there are no significant differences in emissions or speciation between these types.
  • Sub categorization on this basis not appropriate.
combustion processes fluid bed combustors fbc
Combustion Processes:Fluid Bed Combustors (FBC)
  • Combusts fuel in a fluidized bed of solids at ~1,500 F vs. 2,500 F for conventional boilers
  • Coal prep equipment, fans, ash collection, and steam water/steam tubing are very different than conventional boilers
  • Sulfur in fuel converted to a solid in the furnace
  • FBCs have wider fuel flexibility than other boilers
  • Radically different combustion process
  • A FBC sub category for all fuel types is appropriate
coal considerations rank
Coal Considerations: Rank
  • ASTM ranks coals by different properties
    • Western Bituminous (AZ, CO, NV, NM, UT and WY)
    • Eastern Bituminous
    • Sub-Bituminous
    • Texas Lignite
    • North Dakota Lignite
  • Boilers are constructed to burn specific coals
    • Different coals have different volatilities
    • Lower rank coals require a larger size boiler
    • Coal prep equipment, furnace configuration, and plant aux. equipment designed for specific fuels
fuel considerations coal rank
Fuel Considerations: Coal Rank
  • Flue gas Hg from bituminous coals tends to oxidize more than sub-bituminous or lignite
  • Eastern bituminous coal tends to have higher levels of chlorine, sulfur and Hg than western bituminous, and different ash characteristics. These differences result in different speciation.
  • Lignite units are mostly mine mouth, limiting fuel flexibility.
  • North Dakota and Texas lignite have different Hg concentrations, and due to ash characteristics, different speciation.
fuel considerations coal rank9
Fuel Considerations: Coal Rank
  • Differences in coal properties affect control technology performance and applicability (different ranks with the same controls have different performance)
  • Appropriate to sub categorize by coal rank
    • Western Bituminous
    • Eastern Bituminous
    • Sub-Bituminous
    • North Dakota Lignite
    • Texas Lignite
fuel considerations chlorine content
Fuel Considerations: Chlorine Content
  • EPRI’s work shows that coal chlorine (Cl) is a critical factor that dictates speciation and mercury removal.
  • Much of the difference is captured by coal rank. Lignite and sub bit. have less Cl than bituminous, and western is lower than eastern bituminous.
  • Sub categorizing by chlorine has merit even within eastern bituminous, but would be complicated to administer both for industry and regulators.
fuel considerations other chemical properties
Fuel Considerations: Other Chemical Properties
  • Ash with high alkali content tends to inhibit the oxidation effect of chlorine
  • Iron content tends to increase oxidation
  • Sulfur content, ash type and amount of ash affect the choice of downstream processes for SO2 and particulate removal
  • These are important factors, but would be largely captured by fuel rank and other process types
operating considerations
Operating Considerations
  • Unit size (MWs, steam flow, fuel flow, etc.)
  • Unit load, capacity factor, dispatching (i.e. baseload, load following, peaking)
  • Online cleaning of air heaters, boiler, etc.
  • Longer residence time which allows more oxidation
  • Unburnt carbon (LOI) shown to remove mercury
  • These are important factors and may be a basis for sub categories.
operating considerations temperature
Operating Considerations:Temperature
  • Temperatures can effect speciation and removal efficiency.
  • Mercury removal is more effective at lower temperatures
  • Process configuration affects temperature
    • Hot stack (above acid dew point)
    • Near saturated stack (above saturation temp)
    • Wet stack ( saturated with water vapor)
operating considerations temperature14
Operating Considerations:Temperature
  • Acid dew point – temperature must be kept high enough to prevent SO3/H2SO4 from condensing and corroding all components downstream of the air heater (e.g. cold ESP, FF)
  • Saturation temperature - must keep temperature high enough to prevent water from condensing and solids plugging downstream (e.g. spray dryer)
  • Wet stack – designed for wet and corrosive conditions (e.g. wet FGD)
process design considerations nox controls
Process Design Considerations:NOx Controls
  • SNCR not conclusively shown to affect Hg speciation or capture
  • Low NOx Burners do not directly affect speciation, but increased unburnt carbon may increase mercury removal downstream
  • SCR in some instances promotes oxidation of mercury making it more readily captured downstream
process design considerations particulate controls
Process Design Considerations:Particulate Controls
  • Overall units have displayed good control of particulate mercury
  • Hot ESPs operate at temperatures not favorable to Hg capture.
  • Particulate controls are selected based on fuel, ash loading and ash characteristics
process design considerations wet fgd for so2 control
Process Design Considerations:Wet FGD for SO2 Control
  • Wet FGD efficient at removing oxidized Hg
  • Wet FGD can convert oxidized Hg to elemental
  • Potential co-benefits from SCR
  • Hg removal efficiency different for different coal ranks
  • Results in a water saturated flue gas
  • Placed after particulate controls
process design considerations dry fgd for so2 control
Process Design Considerations:Dry FGD for SO2 Control
  • Dry scrubbers (spray dryers) used on lower sulfur coal as shown by ICR data. Most are on western units
  • Not capable of high SO2 removal on higher sulfur fuels
  • Located prior to particulate control devices
  • Dry scrubbers limited by approach temperature (saturation temperature) to prevent wet solid deposition.
  • Some spray dryers perform better than others for Hg removal, depending on coal rank & variations in rank
recommended modeling sub categories scenario 1
Recommended Modeling Sub- Categories: Scenario #1

Six total Sub Categories

  • Fluidized Bed Combustors (all coal ranks)
  • Conventional boilers using
    • Eastern Bituminous
    • Western Bituminous
    • Sub-Bituminous
    • North Dakota Lignite
    • Texas Lignite
recommended modeling sub categories scenario 2
Recommended Modeling Sub- Categories: Scenario #2

Sixteen total sub categories

  • Fluidized Bed Combustors (all coal ranks)
  • Previous 5 fuel subcategories each sub divided by process outlet temperature
    • Hot (above acid dew point)
    • Near saturated (above saturation temp)
    • Wet (saturated with water vapor)
  • Some sub categories may be a “null set”