A Comprehensive Evaluation of Off-Gas Emissions From A Catalyst-Aided, Amine-Based, Post Combustion Capture of CO2 From Industrial Exhaust Gas Streams

Abstract

The major focus of all CO2 capture technologies is to reduce emission of CO2 which is undoubtedly one of the major greenhouse gases blamed for global warming. It is imperative to ensure that while we aim to capture CO2 to achieve the production of clean energy, other contaminants are not released into our environment, so as not to defeat the main purpose of ensuring the safety of the environment. In the amine-based, catalyst-aided post combustion capture of CO2 technology, amine degradation occurs, and the degradation products are present in both the liquid and gas phases. It is important to know the composition and quantities of these degradation products to optimize the capturing process and ascertain that the capture process will not have a negative impact on the environment. For the amine solvent selection process two single amines: hexamethylenediamine (HMDA), and 2-amino-2-methyl-1-propanol (AMP), two polyethyleneimine (PEI) blends: HMDA/PEI and AMP/PEI and two 2-amino-2-methyl-1-propanol (AMP) blends: DMAE/AMP and BEA/AMP which served as the benchmark amine blend for this study were screened. After screening and comparing these amines based on their absorption and desorption properties, DMAE/AMP involving a tertiary amine and a sterically hindered amine with a total concentration of 4M was selected as the optimal amine solvent for the CO2 capture process in this study. The next phase of the work was to synthesise, characterise and screen two solid basic absorber catalysts; Activated Carbon Spheres (ACS) and a proprietary absorber catalyst (APC) and one solid acid desorber catalyst, a proprietary desorber catalyst (DPC). The screening results for the absorber catalysts showed that the equilibrium loading, and initial absorption rate were enhanced by 5.6% and 33.3 % respectively for ACS and by 3.9% and 29.4% respectively for APC. For the desorber catalyst, DPC, the lean loading was enhanced by 16%, the initial rate of desorption by 25.6% and the heat duty by 20.3%. The last phase of the work was to apply the selected amine solvent, DMAE-AMP, and a catalyst in a pilot plant set-up in two separate runs which lasted for a total of 15 days (360 hours) each. The desorber catalyst was used due to its high yield and direct involvement in helping to reduce the heat duty required for the regeneration process as observed during the screening tests. For each run, the off gas from the absorber column was trapped and analysed using the relevant EPA methods to access the volatile species qualitatively and quantitatively. The data obtained confirmed the presence of ammonia and four different aldehydes namely formaldehyde, acetaldehyde, acrolein and butyraldehyde in all samples taken. Traces of propionaldehyde and crotonaldehyde were observed in some samples of both runs with traces of valeraldehyde observed in some samples of only the run with the desorber catalyst. Ammonia had the highest rate of emission of 0.43ppmV/h and this was recorded in the blank run. The emissions of all observed compounds were higher in the blank run than the run with desorber catalyst under the same conditions. Finally, the emissions of both runs were found to be within the stipulated occupational limits set by the relevant regulatory board, confirming the overall environmental safety of the emissions of the capture process using DMAE/AMP, with and without the selected catalyst.