QUESTION : Is it technically feasible to sample the emission limit post ESPs fromMK2 before the FGD? In other words, is it possible to separate the TSP emission limits sampling for MK1 and MK2? (Anne McWilliams' question from6/21/12 email to Jeff Underhill and Charles Martone.) ANSWER

: There are testing locations (with multiple ports) on both Unit #1 and Unit #2 ducts between the ESPand FGDfor each unit. While theoretically possible to sample emissions fromthose locations, actual emissions fromUnit #2 (the BART unit) to the ambient air are fromthe FGDand are sampled there for compliance purposes. The only way to isolate, for sampling purposes, the actual PM emissions to the ambient air (i .e., post-FGD) fromUnit #2 is to shut down Unit #1, and vice versa. The bubble concept incorporated into New Hampshire's regional haze rule (Env-A 2300) avoids this potential inconvenience while simultaneously strengthening facility-wide PM emission limits. QUESTION

: Did DES assess Advanced Separated Overfire Air (ASOFA) in its NOx BART assessment for MK2? ANSWER

: The BART analyses discussed in the National Park Service's comments addressed ASOFAand Selective Non-Catalytic Reduction (SNCR), and how much further SNCRcould reduce emissions beyond the use ofASOFAalone. Twoofthe EGUs discussed by the NPS were operating without any NOx controls at the time ofthe state's BART analysis, and one had an overfire air system . All three had overfire air systems by the end of 2009, leading North Dakota to consider it in association with other controls. The state determined that Selective Catalytic Reduction (SCR) was too expensive, but that adding SNCRto the overfire air system would reduce emissions to 0 .35 or 0 .36 (depending on the unit) lb/MMBtu (30-day rolling average) for less than $2,000/ton removed. MK2is already equipped with SCR, the control preferred by EPAfor the facilities discussed by NPS and the basis for the presumptive NOx limit for cyclone units. Because MK2is installed at a power plant smaller than 750 MW, it is not subject to the guidelines of 40 CFRPart 51, Appendix Y, including the presumptive NOx limit. As a part ofits BART analysis, the state recognized that SCRis the best available control technology for MK2but lowered the NOx emission limit from0 .92 lb/MMBtu to 0

.30 lb/MMBtu for this unit. Note that this limit is already less than the approved BART limits for the North Dakota EGUs referenced in NPS's comments. It would be infeasible to add ASOFAto MK2, which burns bituminous coal rather than the lignite used at the North Dakota EGUs. Lignite has an ash fusion temperature of around 2240 F while Merrimack's coal has a fusion temperature ofaround 2900 F. Maintaining high combustion temperatures within the MK2cyclone burners is important to ensure that the coal ashwill fluidly exit the boiler. One drawback ofneeding to maintain high combustion temperature is relatively high production of thermal NOx as compared to other coal combustion designs. Additionally, this necessity of having to maintain high combustion temperatures limits the degree ofchanges to the combustion process that can be made to a cyclone boiler to minimize NOx formation. Iftemperatureswithin the cyclone burners are too low to properly melt the ash, the ash will not be rejected fromthe boiler, causing the ash to collect in the boiler and eventually shutting down the process. Applying ASOFAwould reduce the firing temperature below the fusion temperature ofthe Merrimack coal, precluding the removal ofslag and plugging the cyclone, whichwould lead to eventual boiler shutdown.