QUESTION: Is it technically feasible to sample the emission limit post ESPs from MK2 

before the FGD? In other words, is it possible to separate the TSP emission limits 

sampling for MK 1 and MK2? (Anne McWilliams' question from 6/21/12 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 ESP and FGD for each unit . While theoretically possible to sample 

emissions from those locations, actual emissions from Unit #2 (the BART unit) to the 

ambient air are from the FGD and 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) from Unit #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 ASOFA and Selective Non-Catalytic Reduction (SNCR), and how much 

further SNCR could reduce emissions beyond the use of ASOFA alone. Two of the 

EGUs discussed by the NPS were operating without any NOx controls at the time of the 

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 SNCR to 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. 

MK2 is already equipped with SCR, the control preferred by EPA for the facilities 

discussed by NPS and the basis for the presumptive NOx limit for cyclone units . 

Because MK2 is installed at a power plant smaller than 750 MW, it is not subject to the 

guidelines of 40 CFR Part 51, Appendix Y, including the presumptive NOx limit. As a 

part of its BART analysis, the state recognized that SCR is the best available control 

technology for MK2 but lowered the NOx emission limit from 0.92 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 ASOFA to 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 of around 2900 F. 

Maintaining high combustion temperatures within the MK2 cyclone burners is important 

to ensure that the coal ash will fluidly exit the boiler . One drawback of needing 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 of changes to the combustion 

process that can be made to a cyclone boiler to minimize NOx formation. If temperatures 
within the cyclone burners are too low to properly melt the ash, the ash will not be 

rejected from the boiler, causing the ash to collect in the boiler and eventually shutting 

down the process. Applying ASOFA would reduce the firing temperature below the 

fusion temperature of the Merrimack coal, precluding the removal of slag and plugging 

the cyclone, which would lead to eventual boiler shutdown.