UKCCSRC Call 2 Project: Novel reductive rejuvenation approaches for degraded amine solutions from PCC in power plants

Aqueous amine scrubbing was originally developed for natural gas treatment and is currently considered to be the current best available technology for post-combustion capture (PCC) of CO2 from both pulverised fuel (PF) and natural gas combined cycle (NGCC) power plants. A major issue is the severe thermo-oxidative degradation of alkanomaine solvents that occurs in PCC compared to natural gas processing, with the problem being compounded by the presence of acid gases that lead to the formation of heat stable salts (HSS). The accumulation of degradation products is known to reduce CO2 capture efficiency and cause excessive foaming and fouling and unacceptably high corrosion rates. Current measures to compensate for degradation involves purging spent solvent solution for reclaimation, makeup with fresh amine and the addition of anti-foam and oxidation/corrosion inhibitors. Reclaimer technologies based on distillation, ion-exchange and elecrodialysis have been developed to deal primarily with HSS where distillation has the advantage of removing both the HSS and their anions (i.e. formate and acetate). However, these technologies do not deal with the majority of the other degradation products, particularly those arising from thermal and oxidative degradation. Further, it has generally recognised that MEA forms high boiling polymeric material where N-(2-hydroxyethyl)-ethylenediamine (HEEDA), in particular, may continue to degrade in the presence of CO2 to form longer substituted ethlyenediamines. This proposal has been prompted by our extremely promising preliminary results that the thermal and oxidative degradation of an amine polymer (polyethyleneimine) can largely be reversed using both hydrogenation and hydrothermal (hydrous) treatments. We used non-catalytic hydropyrolysis and hydrous pyrolysis treatments at temperatures below 250oC which were clearly effective in reducing oxygen functionalities without causing any degradation of the polymer chain. The challenge is to partially reduce degraded amines to hydroxyamines and also, for polymeric forms, to induce some hydrogenolysis to reduce chain lengths. Hydrous pyrolysis has the potential advantage of not directly requiring hydrogen with water being the hydrogen source. Judicious choice of catalysts provides selectivity for hydrogenation and hydrogenolysis and research on amine degradation in natural gas sweetening has shown degradation products, such as N,N-bis(2-hydroxy-ethyl)piperazine and N,N,N-tris(2-hydroxyethyl)ethylenediamine, can be converted back to hydroxyamines by a hydrotreating reactions . •Directly targeting a high research priority identified by the RAPID Handbook, the proposed research aims to investigate novel reductive approaches for rejuvenating spent amine solutions from PCC plants, namely selective catalytic hydrotreatments at modest temperatures and H2 pressures and hydrous pyrolysis (hydrothermal conversion). The specific objectives are: 1.To apply the hydrogenation/ hydropyrolysis and hydrothermal treatments to individual compounds, including 1-(2-hydroxyethyl)-2-imidazolidone (HEIA), HEEDA, .N-(2-hydroxyethyl)acetamide and N-methylformamide 2.Based on the model compound results, to conduct experiments on actual fractions from degraded amine solvents, notably the residues from distillation containing HSS and the compounds targeted above; and 3.To use the results to define the overall benefits hydrogenation, hydropyrolysis and hydrothermal treatments in solvent rejuvenation and a basis for planning the subsequent research needed to take forward these new treatments, in terms of identifying how these treatments can best be conducted continuously. Grant number: UKCCSRC-C2-189.