CO2 in Power Plant Waters Review, Trends and Management Kirk Buecher Matarvattenkonferensen 2014

Agenda  Some of the basics - Where does it come from? - What is the impact on corrosion? - What is the impact on analytics?  Industry trends - Cycling Plants - Gas Combined Cycle - "Mega" Coal Plants  Problem with CO2?  Steps to manage CO2

Some of the basics Where does it come from?  Directly or indirectly from raw/source waters - Direct : Some surface water and well water sources have higher levels of carbonates - Indirect : Higher levels of Organics in the source water will break down into organic acids and CO2  From the processes inside the plant - Air Ingress : 21% O2 with.04% CO2 - Water storage : "Hungry water fighting back to equilibrium"  Conductivity increases quickly with air contact from ~0.06 μS/cm to 1.8 – 2.0 μS/cm within 6-12 hours - Boiler Steam Cycle : Breakdown of some treatment chemicals (amines) and any other organics present 2

Impact on Corrosion A supporting not primary actor  IAPWS - It is considered non-corrosive to the steam turbine, when its concentration is within reasonable limits  CO2 (Carbonic Acid) is a volatile weak acid However, if it is present with O2  The combined effect is even higher levels of corrosion at accelerated rates - Example – Acid in the condensate attacks tube walls forming ferrous bicarbonate. With any O2 present in the feedwater, it oxidizes to ferric oxide, liberating the CO2 to repeat the process 3

Relative Strength of CO2 4

Impact on Corrosion 5 Corrosion as a result of CO2 typically involves grooving patterns and general wall thinning

Impact on Analytics Interferes with accuracy and clear decision making, based on the relationship between readings  Misleading high readings on Cation Conductivity masking other more serious problems - Creates doubt around the alerts/alarms for the most important water sample panel parameter  Affects pH leading to overdosing of phosphate or ammonia  But measurement of CO2 is possible using ASTM methods 6

7 Specific, Cation, Degassed Conductivity Cation Exchanger NH 4 +, OH -, Na +, H +, Cl -, HCO 3 - H +, Cl -, CO 2, HCO 3 -, OH - R–H + Degas Unit CO 2 H +, Cl -, OH - Specific Conductivity Cation (Acid) Conductivity Degassed Cation Conductivity R–NH 4 + R–Na Conductivity (µS/cm) Specific Cation Degassed Cation NH 4 + OH - Na + Cl - HCO 3 - H + Cl - HCO3 - H + Cl -

Relative Impact of CO2 to Acid Conductivity 8 Concentration (ug/kg) Acid Conductivity (uS/kg)

9 Computed CO 2 Cation Conductivity minus Degassed Cation Conductivity ASTM D4519

10 Summary of Computed Parameters Specific conductivity at 25°C → Cation conductivity at 25°C → Degassed Cation Conductivity at 25°C → Computed pH at 25°C Computed CO 2 (ASTM D4519)

Industry Trends Cycling Plants  Better accuracy, faster response to get to that critical "go/no go" full load decision Gas Turbine/Combined Cycle Plants  Tend to cycle more AND have more problems with high volume water treatment and CO2 levels "Mega" Coal Plants (2,000MW and up)  With Super and Ultra Critical Boilers  China leading the way Industry Standards & Boiler/Turbine Mfgs 11

Problem with CO2? CO2 + H2O -> H2CO3 -> HCO3- + H+ How do you know?  Increased cation conductivity in condensate and feed water (with no condensate polisher)  Decrease in cation conductivity from main condensate to feed water when increasing deaeration in feed water tank  Acid conductivity of steam at the same level as the feed water  No increasing tendency of acid conductivity (caustic, AVT) in boiler water  With Phosphate, no increasing tendency in specific conductivity 12

Steps to manage CO2 13

Steps to manage CO2  Storage Tanks - CO2 Purging / Scrubbers - Covering the surface with floating balls - Using a floating roof - Nitrogen blanket (also helps reduce O2 absorption/ingress as well)  Forced Draft DeAerators  Membrane Contactors  Measure and control Organic (TOC) contamination (helps manage/reduce overall acid level as well)  DeGas Conductivity System as standard analytic tool 14

Internal usage only References  IAPWS - Volatile treatments for the steam-water circuits of fossil and combined cycle/HRSG power plants, July 2010  IAPWS – Technical Guidance Document: Steam Purity for Turbine Operation, Sept 2013  IAPWS - Technical Guidance Document – 2012 Revision: Instrumentation for monitoring and control of cycle chemistry for the steam-water circuits of fossil- fired and combined-cycle power plants  S N Mookerji and J C Kapur – Some problems of Corrosion in Thermal Power Plants  Brad Buecker – Power Plant Water Chemistry : A Practical Guide 1997  David Addison - High Purity Water for Gas Turbine Injection – Industry Challenges with Storage and Measurement IAPWS PCC Workshop 2014  Karsten Thomsen – Acid conductivity – basics and some experiences, Matarvattenkonferensen