Piperazine Degradation in a Post Combustion Carbon Capture Pilot Plant

Piperazine Degradation in a Post Combustion Carbon Capture Pilot Plant

Dias Kazbekuly, Paul T Nielsen, Gary T Rochelle

Texas Carbon Management Program

McKetta Department of Chemical Engineering

The University of Texas at Austin

 

Abstract

Amine scrubbing is a proven technology used to separate carbon dioxide and other acid gases from natural gas.  It can also potentially be used to capture CO₂ from post combustion flue gas from coal and natural gas fired power plants.  However, post combustion flue gas will contain oxygen and other contaminants that will degrade the solvent.  Thermal degradation of the solvent can also occur in the high-temperature solvent-regenerating stripper.  Concentrated aqueous piperazine has been proposed as an alternative to the baseline monoethanolamine solvent due to its increased resistance to thermal and oxidative degradation.

Liquid solvent samples were collected and analyzed from a post combustion carbon capture pilot plant, operated with a slipstream of flue gas from a coal-fired power plant.  The campaign was performed at stripper operating temperatures of 120 °C and 155 °C and used 40 wt % piperazine solvent. During the period of the campaign held at 120 °C, formate, the most significant liquid phase oxidation product, accumulated at a rate of 56 μmol/(kg*hr).  The rate increased to 170 μmol/(kg*hr) when the temperature was increased.  No significant accumulation of thermal degradation products was observed.  The rate of stainless steel metal ion accumulation due to corrosion increased significantly from 0.14 to 1.0 µmol/(kg*hr) after the temperature of the stripper was increased.  Sulfate and nitrate from the absorption of SO_x and NO_x in the flue gas accumulated at a rate of 25 and 16 μmol/(kg*hr), respectively.

N-nitroso-piperazine (MNPZ) is a carcinogenic byproduct that can form from the reaction of piperazine with NO₂ absorbed from the flue gas.  MNPZ is thermally unstable and will degrade at stripper conditions, limiting the maximum concentration in the solvent.  After a steady state of 7 mmol/kg was achieved at 120 °C, the temperature was raised to 155 °C, and the MNPZ concentration dropped rapidly to a new steady state of 2 mmol/kg. Both these observations comply with the values predicted using a kinetic model developed for MNPZ decomposition.