NC0038377_Process Narratives - NH3 Conversion Project 2021 - Final_20211215

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 NO), 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).
CliTU IRATE
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.