Oxidation of piperazine in post-combustion carbon capture

Solvent oxidation in amine scrubbing systems for post-combustion CO₂ capture is a significant issue. Piperazine (PZ) is a promising solvent due to its relative stability and performance. PZ oxidation rates and products were thoroughly characterized in the High Temperature Oxidation Reactor (HTOR) bench-scale cyclic degradation apparatus and compared to observed PZ oxidation from campaigns at the UT Austin SRP, CSIRO Tarong, and "Pilot Plant 2" (PP2) pilot-scale facilities. The HTOR simulated solvent conditions cycling between a 40-55 °C absorber and a 120-150 °C stripper. In both the bench and pilot-scale the intermediary degradation products piperazinol, piperazinone, and ethylenediamine were initially the most significant degradation products before reaching steady-state concentrations, with ammonia and formate the most significant final products produced from the decomposition of the intermediates. PZ oxidation increased as the solvent degraded due to the cycling of dissolved iron, aldehydes, and hydroperoxide contaminants, which could be oxidized in the absorber and subsequently oxidize PZ at high temperature. An N₂ sparger was used to selectively remove dissolved oxygen (DO) in the HTOR before heating while still allowing for oxidation due to contaminant cycling. Ammonia was correlated to dissolved iron at 0.72 mmol NH₃/kg PZ/hr/(mmol/kg Fe) [superscript 0.5]. An additional 0.4 mmol NH₃/kg/hr was produced due to direct reaction of PZ with DO regardless of the level of contamination. Dissolved iron was solubility-limited in both the HTOR and pilot plants, but increased as the solvent degraded, resulting in the autocatalytic effect of PZ oxidation. HTOR data was used to model oxidation and solvent management costs for a full-scale amine scrubber. The model matched observed oxidation at SRP and Tarong. Maintaining 0.1 to 0.5 wt % contaminant accumulation optimized amine make-up, solvent reclaiming, and increased energy costs due to changes in solvent viscosity, at a minimum of $2.6/MT CO₂ for PZ treating coal flue gas with a thermal reclaimer to remove contaminants. Feed rate and amine recovery in the reclaimer were the most impactful design variables, followed by operating temperature and hold-up in the stripper, prescrubbing of flue gas contaminants SO₂ and NO₂, and least significantly N₂ sparging to remove DO.