Previous MACT Sub CategoriesPrevious 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 Class, Type and Size Relative To Utility UnitsTo 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 Are There Potential Utility Subcategories?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 Are There Potential Utility Subcategories?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: Combustion Processes: Conventional Boiler TypesConventional 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:Combustion Processes:Fluid Bed Combustors (FBC)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: RankCoal 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 RankFuel 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 RankFuel 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: Fuel Considerations: Chlorine ContentChlorine 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: Fuel Considerations: Other Chemical PropertiesOther 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 ConsiderationsOperating 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:Operating Considerations:TemperatureTemperature

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:Operating Considerations:TemperatureTemperature

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:Process Design Considerations:NOx ControlsNOx 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:Process Design Considerations:Particulate ControlsParticulate 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:Process Design Considerations:Wet FGD for SO2 ControlWet 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:Process Design Considerations:Dry FGD for SO2 ControlDry 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

Process ConfigurationProcess ConfigurationConf. #1 Particulate

Controls OnlyHigh

TemperatureAbove Acid Dew Point

Conf. #2 SO2 then particulate controls

Moderate Temperature

Above Saturation

Temperature

Conf. #3 Particulate then SO2 controls

Low Temperature

Fully Saturated

Sub categorization based on process configuration is appropriate because:Temperature affects Hg speciation and Hg removal effectiveness Units are designed to operate at specified temperatures - can not change temperature without changing the process configuration of the plant

Recommended Modeling Sub- Recommended Modeling Sub- Categories: Scenario #1Categories: 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- Recommended Modeling Sub- Categories: Scenario #2Categories: 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”