Document ID: EPA-R03-OAR-2022-0347-0005
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2022-05-26T04:00Z

MEMO

        TO			JoAnn Truchan 
                    Section Chief, Engineering
                    Allegheny County Health Department

        FROM		Vince Pascucci VMP 11/10/2021	
                    Air Quality Engineer
                    Technical Support Section, Division of Permits

        	                        Naishadh Bhatt  NB 11/10/2021	
                   Environmental Group Manager
                    Technical Support Section, Division of Permits

                                               Viren Trivedi 
                  Environmental Program Manager
				Division of Permits

        THROUGH	Mark Hammond 
                  Director, Bureau of Air Quality
      			
        DATE 		November 10, 2021

        RE	Technical Evaluation for Case-by-Case NOx RACT
                    Cheswick Generating Station
                    Springdale Borough, Allegheny County
                    Main Boiler No. 1, TVOP-0054-I005

MESSAGE:
This technical review memorandum outlines the technical evaluation, rationale and preliminary determination for Oxides of Nitrogen (NOx) and operational requirements for the main power generating coal-fired combustion unit at Cheswick Generating Station to include in the Pennsylvania Department of Environmental Protection (DEP)'s State Implementation Plan (SIP) revision to address Reasonably Available Control Technology (RACT) requirements for the 2008 8-hour National Ambient Air Quality Standard (NAAQS) for ozone. 

BACKGROUND:
On April 23, 2016, the Pennsylvania Department of Environmental Protection (DEP) published 25 Pa. Code §§ 129.96 - 129.100, "Additional Requirements for Major Sources of NOx and VOCs", commonly referred to as RACT II.  20 Pa.B. 2036.  Pursuant to 25 Pa. Code § 129.99, Alternative RACT proposal and petition for alternative compliance schedule, the owner or operator of the Cheswick Generating Station has proposed an alternative RACT emission limitation and RACT requirements for one coal-fired electric generating unit under 25 Pa. Code § 129.99(d).  

FACILITY DESCRIPTION:
Cheswick Generating Station operates one bituminous coal-fired combustion unit (boilers with steam turbine-driven electric generator) that provides electricity to the Pennsylvania-Jersey-Maryland (PJM) regional electric grid.  The unit has a nominal rating of 5,500 MMBtu/hr (6,000 MMBtu/hr max) with a gross electrical rating of 565 MW.  Natural gas is also used as a secondary fuel for start-up, shutdown, and flame stabilization.  The unit is equipped with Low NOx Burners (LNB), and a Selective Catalytic Reduction (SCR) system.  The unit is also equipped with continuous emission monitoring systems (CEMS) for NOx and contain exhaust gas stream flow monitors.  
II.Regulatory Basis
RACT:
In 40 CFR § 51.100, for the purpose of § 51.341(b) - Request for 18-month extension, RACT is defined as devices, systems, process modifications, or other apparatus or techniques that are reasonably available taking into account: (1) The necessity of imposing such controls in order to attain and maintain a national ambient air quality standard; (2) The social, environmental, and economic impact of such controls; and (3) Alternative means of providing for attainment and maintenance of such standard. 

In 40 CFR Part 52, EPA has defined RACT as the lowest emission limitation that a particular source is capable of meeting by the application of control technology that is reasonably available considering technological and economic feasibility. (https://www3.epa.gov/ttn/naaqs/aqmguide/collection/Doc_0084_VOCFR0917791.pdf)

In 25 Pa. Code §121.1, RACT is defined as the lowest emission limit for VOCs or NOx that a particular source is capable of meeting by the application of control technology that is reasonably available considering technological and economic feasibility.

Therefore, a RACT analysis should consider the technological and economic impacts of controls.

RACT ANALYSIS:
DEP used a top-down approach to determine NOx emissions limits for the coal-fired boiler at Cheswick Generating Station.  This included searching and identifying the best methodology, technique, technology, or other means for reducing NOx while factoring environmental, energy and economic considerations into the analysis.  DEP also identified controls installed on similar air contaminant sources in other states.  

DEP estimated the capital, installation and annual operating costs of NOx control using the EPA's OAQPS and Control Cost Manual (Sixth edition) June 12, 2019, and vendor's quotes.  DEP evaluated the cost effectiveness of technically feasible RACT control options and determined that no additional controls are cost effective.

TECHNOLOGY ANALYSIS AND NOX EMISSION RATE DETERMINATION:
Selective Catalytic Reduction (SCR):

DEP first evaluated the most efficient NOx reduction technology SCR that is generally used to reduce NOx emissions from coal-fired boilers.  The unit is already equipped and operating with an SCR system. 

SCR systems typically uses a titanium or vanadium catalyst and injection of ammonia or urea at optimum temperature of flue gas to convert the flue gas NOx to molecular nitrogen (N2) and water with up to 90% NOx reduction efficiency.  As per EPA's Air Pollution Control technology Fact Sheet (https://www3.epa.gov/ttncatc1/dir1/fscr.pdf), the optimum temperature of the flue gas range between 480[o]F and 800[o]F, which EPA further refined in response to comments on the Cost Manual that it "concluded that 480℉ to 800℉ is an ``operating'' range and that 700℉ to 750℉ was an optimum temperature range." (https://www.epa.gov/sites/default/files/2020-07/documents/scr_costmanual_7thed_rtc.pdf ) .  

Flue gas temperature varies at reduced boiler loads.  Cheswick Generating Station is required to provide the minimum electric output called for by PJM, the grid operator.  Failure to provide the minimum load subjects the facility to substantial penalties, while the price paid for providing excess electricity is minimal.  

SCR efficiency decreases at lower than optimum flue gas temperature.  Ammonia injection at flue gas temperatures less than optimum temperature also become problematic because of formation of ammonium bisulfate (NH4HSO4) which deactivates catalyst surface, decreases NOx reduction efficiency and boiler thermal efficiency and may lead to catalyst plugging, fouling and unplanned boiler shutdown.  The original equipment manufacturer ("OEM") of the SCR system established and documented minimum operating parameters for the SCR system, including the inlet temperature to the SCR.  The SCR system was designed to operate with a baseload electric generation unit (EGU); i.e. a system with a capacity load, typically operating near maximum heat input for lengthy stretches of time.  

Based on a review of the ammonia feed rates, fluctuating load and NOx emission rate, any lag in the ammonia feed system is negligible.  Ammonia injection rates very closely track load, and the changes in emission rate as the ammonia injection rate follows boiler load fluctuations is minor.  

NOx emission limits on a daily basis: 

DEP evaluated and analyzed daily NOx emissions rates from EPA's Clean Air Markets Division (CAMD) database at varying operating load conditions for Cheswick Generating Station's main boiler from 2017-2020.  Figure 1 below shows daily operating statistics for Cheswick's main boiler during November of 2017.  The X axis shows the day of the month while the left Y axis correlates with the green line and is in lb/MMBtu of NOx.  The right Y axis is in percentage of the maximum observed input.  The green line is the NOx rate of the unit for each day in lb/MMBtu.  The red line is the percentage of maximum heat rate input for the unit for the day.  The orange line is the percentage of max ammonia input observed.  The blue line is set at 100% as a reference for the max values for the orange and red lines.  Currently Cheswick EGU is subject to a NOx limit of 0.12 lb/MMBtu and Cheswick appears to control their emissions to a level between 0.10 and 0.12 lb NOx/MMBtu.  Examples of this are shown below in Figures 1 and 2 and numerous other examples exist in the four years of data analyzed.

                                   Figure 1

                                   Figure 2

Despite controlling to a NOx emission rate of between 0.10 and 0.12 lb NOx/MMBtu most of the time, Cheswick's data does show evidence that controlling to a baseline of between 0.09 and 0.08 lb NOx/MMBtu is feasible for them.  Figures 3 and 4 show one two-day example of this though there are other examples in the data.  The 4-hour period on December 20th during which this range was not met was clearly due to a lack of ammonia injection into the SCR.

                                   Figure 3

                                   Figure 4

Load and operating variability must be taken into consideration when setting a RACT limit for a unit.  There is some evidence that NOx emission drops into 0.07 to 0.06 lb NOx/MMBtu range for one or a few hours at a time.  In some of these instances, it appears that this occurs during a rapidly increased plant load which results in the SCR controller "over injecting" ammonia, which in turn, causes a significant drop in emissions.  Figures 5, 6, and 7 show several such examples.  
Overall, the relative difference between the ammonia injection rate and the heat input rate is roughly 15% (heat input rate higher) during periods of relative steady-state operation.  A lower set-point for the desired level of emission control will result in a reduction in the relative difference between the two rates, thereby providing increased emission control.    
                                   Figure 5

                                   Figure 6

                                   Figure 7

Despite the low emission levels briefly achieved, varying load conditions, catalyst condition, exhaust temperature and velocity, moisture level, initial NOx levels in the exhaust, and other factors can and do affect SCR performance and therefore, accounting for these circumstances requires a limit above the minimum achievable under ideal circumstances.  Based on the data presented, DEP recommends a permit limit on the NOx emission rate of 0.090 lb/MMBtu on a daily average basis when operating the SCR.  This limit excludes, emissions during start-up, shut-down, and malfunction; operation pursuant to emergency generation required by PJM, including any necessary testing for such emergency operations; and during periods in which compliance with this emission limit would require operation of any equipment in a manner inconsistent with technological limitations, good engineering and maintenance practices, and/or good air pollution control practices for minimizing emissions.  These exclusions are common exclusions and are included in other state's presumptive RACT regulations.  For example, Maryland's regulations for coal-fired electric generating units includes nearly identical provisions, with the exception that the Maryland regulations include an explicit exclusion for low-load operations. [MD COMAR Title 26, Subtitle 11, Chapter 38.04.(4)].  The daily NOx emission rate includes a factor to provide an appropriate compliance margin, fluctuations in load, any lag in the control system as well as to account for other factors in the facility's projected future operation.

DEP also evaluated and analyzed daily NOx emissions rates from EPA's CAMD database at all operating conditions for Cheswick Generating Station's main boiler from 2017-2020. 

                                   Figure 8

As can be seen from Figure 8 above, in 2017, the unit achieved a NOx emission rate from 0.10 lb/MMBtu to as high as 0.30 lb/MMBtu on a daily average under all operating conditions with existing LNB and SCR system.  DEP believes that given Cheswick's clear targeting of an emission level of 0.10-0.12 lb/MMBtu when the SCR is in operation, the achieved NOx emission rate was not representative of an SCR set-point reflective of the equipment's demonstrated capabilities to reduce NOx. 

Low NOx Burner upgrade:

DEP performed a technical and economic analysis, on behalf of Cheswick, on the possibility of upgrading their low NOx burner to improve their low load NOx emission levels. This analysis is presented below.
The basis of the methodology behind this analysis is the use of the quoted cost to replace an LNB on a similar facility, which operates similar sized tangentially fired boilers fueled by bituminous coal, and using factors from EPA's IPM v5.13 to scale the cost for the size of Cheswick's boiler.  Estimated emission reductions from Cheswick is estimated by assuming 8760 hour per year operation at full load at exactly the proposed emission limit of 0.09 lb/MMBtu and then taking 7% of that value.  The assumed 7% NOx emissions reduction was the same as was estimated for the similar boilers mentioned above.  These values are conservative given that operations during the last 4 years show emissions which are at most half that level despite having a higher NOx limit than what is being proposed here.  The results of this analysis are shown below.

LNB burner replacement analysis for two similar boilers

 

The analysis shows that the cost to replace the LNB for Cheswick to be $10.741 dollars per ton of NOx reduced.  For this reason, DEP does not consider a LNB replacement to be RACT.
Boiler tuning parameters adjustments: 
This option involves making a number of adjustments to the boiler operating parameters that affect the generation of NOx in the boiler fire box.  Changes that can be made to affect NOx generation include excess air levels, secondary air biasing, fuel/auxiliary air damper adjustments, burner tilt, fuel flow biasing, and changes to primary air flows. 
Conducting regular inspections, preventive maintenance, tuning, following procedures during shutdown and upset conditions to prevent excess emissions, inspections and testing of Over Fire Air (OFA) components, and adjusting of burner angle to minimize NOx emissions results in lowering NOx emissions by 5-15% or at an average of 10%.  The changes in set-point over time indicate that the boiler has not been tuned to minimize NOx emissions, but rather has been tuned to maximize output.  A boiler cannot be simultaneously tuned to achieve both of those goals. Nonetheless, DEP has selected the midpoint of the range of NOx emission reductions.
Therefore, DEP determined a NOx emission rate of 0.27 lb/MMBtu on a daily average basis at all operating conditions for Cheswick's main boiler as RACT.  This value includes an appropriate compliance margin. 

NOx emission limits on a 30-operational day rolling average basis: 

As previously stated, DEP is proposing a daily limit of 0.090 lb/MMBtu based on the emission reduction potential of the SCR, including an appropriate margin for compliance. Cheswick's main boiler emits about 540 lb NOx per hour assuming an emission level of 0.090 lb/MMBtu and 100% load.  The impact to the environment should never exceed this level on a long-term basis. DEP is proposing these emission levels as limits on a 30 operational day rolling average basis which accounts for all operating scenarios including situations during which the SCR is not able to operate.

Selective Non-Catalytic Reduction (SNCR):
SNCR system converts NOx to its elemental components by injecting either urea or ammonia under high temperature conditions.  In this add on control technology, ammonia or urea is injected into the flue gas where the temperature of the flue gas is about 1800[o]F to 1900[o]F.  At this temperature, NOx and the ammonia or urea react to form nitrogen gas and water.  There is a great deal of temperature sensitivity in this reaction and since the urea or ammonia are often injected as aqueous solutions, there is an energy penalty on the overall boiler efficiency from vaporizing the water.  Relatively small concentrations of ammonia result from the use of this NOx control.  This system typically provides for 20% reduction in NOx emissions.

Cheswick's main boiler already use LNB and SCR.  SNCR efficiency is low when emissions are well controlled by combustion controls.  SNCR may not be technically feasible due to frequent load changes.  SNCR is not likely to provide a significant emission reduction due to frequent load variations and the existing controls.

DEP determines SNCR technology to be technically infeasible option for Cheswick's main boiler.     

Conversion to solar or wind power:
Replacing coal-fired units with solar or wind power units would require acquisition of large
land areas and associated permitting issues.  DEP determines conversion of coal-fired power plan to solar or wind power plant option as technically and economically infeasible.  Also, this would constitute a change in the nature of the source which is beyond the scope of a RACT analysis. 

Oxygen enhanced combustion:
An oxygen enhanced combustion system uses a cryogenic process to supply pure oxygen; atmospheric-pressure combustion for fuel conversion in a conventional supercritical pulverized-coal boiler and substantial flue gas recycle making it cost-prohibitive for large coal units.  This technology hasn't been demonstrated on coal-fired boilers.  DEP determines oxygen enhanced combustion as a technically infeasible option. 

Flue gas recirculation (FGR): 
Flue gas is used as a thermal diluent to reduce combustion temperatures in FGR system.  Flue
gas is withdrawn after the economizer or air heater and re-admitted through the burner windbox.  This technology reduces thermal NOx and is not applied to coal-fired EGU boilers because NOx emissions from coal-fired boilers are primary fuel NOx and the flue gas contains relatively high concentrations of ash.  DEP determined that the FGR system is a technically infeasible option.

Rotating opposed fire air (ROFA):
In the ROFA system, gases are set within the furnace into rotation via an asymmetric boosted over-fire air system which reduces NOx emissions.  Cheswick's main boiler is a tangentially fired boilers with separated overfire air.  Tangential firing involves injecting fuel and air at a certain angle in the corners of the furnace that allows for imparting rotation to the reacting jets of fuel and air.  ROFA is similar to tangentially fired technology that is already used in Cheswick's main boiler.  For this reason, no improvements in NOx emissions would be expected from a ROFA system when compared to the existing SOFA system already in place.  For this reason, the installation of a ROFA system is not considered RACT for the unit at this facility.

SCR optimization:
The owner or operator of Cheswick Generating Station will be required to work closely with the SCR catalyst vendor to monitor SCR performance in accordance with the catalyst management plans (CMPs) developed for the SCR system.

Economizer bypass:
Cheswick did not address the possibility of adding an economizer bypass in their application.  However, an evaluation done at a facility which operates similar sized tangentially fired boilers fueled by bituminous coal found that bypassing the Economizer is a technically infeasible option due to interference from the main coal feed belts as well as many other issues due to space constraints at the facility.  DEP has determined the economizer bypass is a technically infeasible option. 

Flue gas reheat:
Flue gas reheat during low load, startup and shutdown increases the flue gas temperature making operation of SCR technically feasible at low load operations.  The option involves the installation of burners, dilution air fans and ductwork near the economizer exits to reheat the flue gas.  Cheswick's application does not include an analysis of partial flue gas reheat, nor does it analyze other heat inputs.  Clearly, any additional flue gas reheat will have a beneficial effect on NOx emission rates, and Cheswick will be required to submit an engineering analysis, within 180 days of the effective date of this permit, evaluating partial flue gas reheat. 

Dry sorbent injection:
SCR systems cannot be operated at select low loads due to deposition of ammonium sulfate and ammonium bisulfate formed by ammonia reacting with SO3.  Dry sorbent injection before the SCR uses sodium carbonate to reduce SO3 concentrations and prevent the formation of ammonium sulfate and bisulfate.  The presence of SO3 in the flue gas stream is desirable because it results in enhanced particulate capture by the Electrostatic Precipitator (ESP).  Also, it is likely that the increase in particulate loading across the SCR and downstream electrostatic precipitator (ESP) would result in increased induced draft fan blade wear and accelerated blade replacement.  Beyond this, a detailed and costly trial test program would be required to evaluate all additional impacts to the plant which is beyond the scope of a RACT evaluation.

Compliance demonstration, Recordkeeping, Monitoring and Reporting requirements:

The facility shall demonstrate compliance with NOx emissions limits using existing CEMS.  The facility shall comply with recordkeeping, monitoring and reporting requirements as set forth by ACHD in the Title V Operating Permit.  These requirements shall apply to emission limits, the emissions rate and other records as specified by ACHD for the facility.  The records shall be reported to the program on the schedule specified by ACHD in the permit.  §127.12(a)(3), §127.411(a)(4)(i), §127.12b(c), §127.441, §127.442 and §127.511.

NOx RACT emission limits for Cheswick's main boiler:
DEP concludes that the following NOx emissions limits are reasonable and to be incorporated in RACT permit as they reflect control levels achieved by the application of existing control technologies and after considering both the economic and technological analysis of other NOx mitigations measures.  

 Emissions of NOx expressed as NO2 for Cheswick's main boiler are limited to a maximum of 0.090 lb NOx /MMBtu on a daily average basis.  This limit excludes, emissions during start-up, shut-down, and malfunction; operation pursuant to emergency generation required by PJM, including any necessary testing for such emergency operations; and during periods in which compliance with this emission limit would require operation of any equipment in a manner inconsistent with technological limitations, good engineering and maintenance practices, and/or good air pollution control practices for minimizing emissions.
      Startup means: The period in which operation of the EGU is initiated after a shutdown event for any purpose.  Startup ends when any of the steam from the boiler is used to generate electricity for sale over the grid or for any other purpose (including on-site use).  Any fraction of an hour in which startup occurs constitutes a full hour of startup.
      Shutdown means: The period in which cessation of operation of an EGU is initiated for any purpose.  Shutdown begins when the EGU no longer generates electricity or when no fuel is being fired in the EGU, whichever is earlier.  Any fraction of an hour in which shutdown occurs constitutes a full hour of shutdown.
      Daily average means: The total mass for each of the hours during the calendar day divided by the total heat input for each of the hours during the calendar day.  This calculation methodology would also apply to the limit contained in (2), below.
      
 Emissions of NOx expressed as NO2 from Cheswick's main boiler are limited to a maximum of 0.27 lb NOx /MMBtu on a daily average basis under all operating conditions.  

 Emissions of NOx expressed as NO2 from Cheswick's main boiler are individually limited to a maximum 540 lbs NOx/hr on a 30-operating day rolling average basis under all operating conditions.
      
 The owner or operator shall calibrate, operate, and maintain all elements of the SCR system and units in accordance with the manufacturer's specifications, in a manner consistent with good engineering and air pollution control practices when the SCR system is in use.
      
 The owner or operator shall operate and maintain LNB in accordance with the manufacturer's specifications and in a manner consistent with good engineering and air pollution control practices. (State only requirement)
      
 The owner or operator shall maintain NOx controls as effectively as reasonably possible during startups and shutdowns.
      
 The owner or operator shall take steps to bring NOx controls back into full service as quickly as practicable whenever the control equipment experiences a malfunction.
      
 The owner or operator shall document and report to the ACHD information regarding the cause of the malfunction and the steps for bringing the controls back.
      
 All operators of Cheswick's main boiler's SCR, and LNB shall be trained in the operation and maintenance of the unit(s) they are assigned to operate by qualified personnel.

   (10) The owner or operator shall develop, maintain, and implement an operation and maintenance plan (O&M Plan) for Cheswick's main boiler and the SCR.  The O&M Plan shall include, but not be limited to the following: 
      
 Inspection, repairs, and preventive maintenance procedures to be followed to ensure proper operation of Cheswick's main boiler and SCR system and continuing compliance with the applicable emission limits specified in this Permit. 
         
      (b)  	A description of preventive maintenance schedules, spare parts inventories, 
         procedures and protocols for unscheduled outages, and provisions for equipment 
            replacement and measures to be taken to protect SCR system in the event of failure or shutdown.
            
      (c)  	Inspections of duct work and boiler casing and repairs of leaks to maintain flue gas temperature.
       
      (d) 	Details of the practices and procedures to be followed during periods of startup, 
            shutdown and upset conditions in order to prevent emissions in excess of the 
            standards specified in this permit.
       
    The owner or operator shall develop, maintain and implement an operation and maintenance plan (O&M Plan) for Cheswick's main boiler and LNB.  The O&M Plan shall include, but not be limited to the following: 
      
 Inspection, repairs, and preventive maintenance procedures to be followed to ensure proper operation of the Cheswick's main boiler and LNB and continuing compliance with the emission standards specified in this Permit. 
         
       (b)  	A description of preventive maintenance schedules, spare parts inventories, 
         procedures and protocols for unscheduled outages, and provisions for equipment 
         replacement and measures to be taken to protect air pollution control equipment 
         in the event of any control equipment failure or shutdown.
       
       (c) 	Details of the practices and procedures to be followed during periods of startup, 
            shutdown and upset conditions in order to prevent emissions in excess of the 
            standards specified in this permit. 
       
       (e) 	Inspections, repair and testing of Over Fire Air (OFA) components.
       
       (f)	Details of the practices and procedures to be followed to ensure that the boiler is tuned to optimize NOx reduction over combustion efficiency, including but not limited to the properly adjusted burner angle.
       
         The facility shall tune the boiler to minimize NOx emissions within 6 months of the effective date of this permit. (State only requirement)
   
         The facility shall tune the boiler to minimize NOx emissions annually after the initial 
         	boiler tuning. (State only requirement)
   
         Within 3 months of the effective date of this permit, the facility shall set the SCR at a target NOx emission rate of 0.06 lb. NOx per MMBtu. (State only requirement)
   
         After operating the SCR with an outlet NOx emission rate set-point of 0.06 lb per MMBtu for twelve consecutive months, the facility shall submit an engineering study within 180 days that analyzes the overall environmental performance of the system at that set-point. (State only requirement)
   
         During the first 60 days of each calendar year, the facility shall perform a catalyst activity test.
   
         Within 60 days of receiving the results of catalyst activity test, the facility shall consult with the SCR catalyst vendor to monitor SCR performance in accordance the catalyst management plans (CMPs) developed for the SCR systems.  

         Within 180 days of the effective date of this permit, the owner or operator shall provide a full and complete technical and, if applicable, economic evaluation to the ACHD on the possibility of heating the flue gas prior to the SCR inlet to allow the SCR to operate at low load levels. ACDH shall have sole discretion to determine when the evaluation is full and complete, and this must occur within 180 days of the effective date of this permit unless an extension is granted by the ACHD.

         The main boiler at Cheswick Steam Electric Plant may not fire coal after December 31, 2022, unless PJM has declared an Emergency Action as defined in PJM Manual 13 (Emergency Operations) or an equivalent standard. 
   
         If Cheswick Steam electric plant meets condition 19 and ceases burning coal on or before December 31, 2022, then conditions 12, 13, 14, 15, and 18 are not required.  If Cheswick does not cease burning coal by Dec 31, 2022 then conditions 12, 13, 15, and 18 will apply with a deadline of 180 days after December 31[st] 2022 and condition 14 will apply with a deadline of 90 days after December 31[st] 2023.