NC0038377_Modification Additional Request_20210319

Mayo Steam Plant
Duke Energy Progress
C: DUKE 10660 Boston Road
Roxboro,NC 27574
ENERGY .
File: Mayo12520-R March 19, 2021
Sergei Chernikov, PhD.
North Carolina Division of Environmental Quality
•
1617 Mail Service Center
Raleigh, NC 27699-1617
Subject: Duke Energy Progress, LLC.
Mayo Electric Generating Plant
NPDES Permit NC0038377
Permit Modification Request Attachments
Dear Dr. Chernikov:
Attached with this letter are the Mayo Permit Modification Request Attachments that were
inadvertently missed when the Permit Modification Letter was sent on March 18th. The
documents included are:
1. NPDES Process Narratives for the Ammonia Stripping Column & Containment Drains
2. Mayo Ash Basin Groundwater Monitoring Well Results October 2008-August 2017
If you have any questions or require additional information, please contact me at
[e-mail] or Lori Tollie at [e-mail] or call me at
336-597-7324 or Lori at 336-854-4916. Thank you.
Sincerely,
(121,4m.e_ (0:,621„._
Leanne Wilson
Mayo Station EHS
Duke Energy
10660 Boston Rd.
Roxboro, NC 27574
Office: 336-597-7324
Cell: 336-504-2827

NPDES Process Narratives (Rev 1)
NH3 Stripping Column &Containment Drains
Mayo NH3 Conversion Project
March 12, 2021
Normal Operations:
Under normal operations with the Mayo unit online, 19 wt%aqueous ammonia will be introduced into
the top of the ammonia stripping column where it will contact hot air rising through the column in a
counter-current fashion. The resulting overhead vapor stream will be fed to the unit Selective Catalytic
Reactors (SCRs)for NOx control; in the bottom of the column the resulting product will be primarily
water with small amounts of NH3 and Na present. During low-load SCR operation, a portion of the
bottoms water stream is recycled to the column by combining it with the fresh aqueous ammonia in the
feed stream. Periodically during low-load operation a small amount of demineralized water is also
added to maintain proper level. Figure 1 is a general process flow diagram. Since there are two SCR's at
Mayo, there will be one ammonia stripping column for each reactor(A& B sides).
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Figure 1:Ammonia Stripping Column General Flow Diagram

Under all scenarios of operation, a small amount of 50% NaOH is added to the system for controlling the
pH of the bottoms water system in the range of 9.1-9.4. In this pH control range, design modeling
indicates the bottoms water ammonia concentration typically< 1.0 ppmw as NH3 (the equipment
supplier has a contractual requirement of< 3.0 ppmw as NH3 in the bottoms water).
In normal operation, the bottoms water will be sent to the unit wet scrubber area for water reuse. The
bottoms water from each column will be sent to either the FGD Service Water tank or to the Absorber
Area sump for use within normal scrubber operations. The time-averaged recycle flowrates of bottoms
water ranges from 0.38-4.75 gpm (per column) across the SCR In-Service load range.
Maintenance Cycles:
For periodic maintenance and/or unit shutdown periods, the stripper column contents will need to be
drained for clearance purposes. Per the manufacturer information, each column could have a maximum
of 740 gallons of liquid that would need to be drained to a low-volume treatment system. A separate
vessel drain is provided for this purpose, and maintenance drains will be hard piped to the DAW system.
The DAW system conveys the water to our low-volume waste treatment system (Holding Basin and
Lined Retention Basin), which then discharge through NPDES Outfalls 002A and Outfall 002.
Containment Drains:
There will be three process areas that will have containment systems around the process equipment:
A Stripping Column, B Stripping Column, and the Aqueous Ammonia Unloading&Tank Farm area. Each
area will have a lockable containment drain valve that will remain closed and only opened after rain
events once tested to ensure proper quality. All three containment areas will drain into the DAW
system, conveying rainwater into the low-volume treatment system (Holding Basin and Lined Retention
Basin).
NaOH Feed Building (Area Floor Drain):
The 50% NaOH feed system (pumps, tote, eyewash/safety shower) will be housed in a pre-
manufactured building that will be common for both ammonia stripping columns. The building will have
one common floor drain that will be routed to the existing Flash Tank Drain Sump. This sump currently
pumps to the low-volume treatment systems (Holding Basin and Lined Retention Basin) via the DAW
drain system.

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