Aerosol-based emission, solvent degradation, and corrosion in post combustion CO2 capture

Abstract

Global greenhouse gas emissions, especially of CO2, have been increasing tremendously over the past century. This is known to cause not only an increase of temperature, but also a change in our climate. Along with a shift to renewable sources of energy, Carbon Capture and Storage is necessary to mitigate climate change. Power plants are the largest point source of CO2 emissions and therefore, capture of CO2 from such sources is a must. Post Combustion CO2 Capture (PCCC), and specifically absorption-desorption based technology is the preferred choice of technology for CO2 capture from flue gases. It has been extensively used in the oil and gas industry for gas treatment. Its application for CO2 capture from flue gases is not straightforward, mainly due to different flue gas composition and operating conditions. Other aspects such as solvent degradation, solvent emissions and corrosion become even more critical. In Chapter 1, the state-of-the-art in PCCC is explained with further details on the current knowledge and understanding of these aspects. In Chapter 2, the aspect of solvent ageing is studied over two test campaigns in a CO2 capture pilot plant using 30 wt.% MEA. Solvent degradation occurs via thermal and oxidative routes, with the latter being more prominent. Ammonia is known to be a major oxidative degradation product, while the remaining degradation products are known to be corrosive. Therefore, solvent degradation is expected to have a significant impact on the corrosion in the plant and the resulting emissions of ammonia. The link between these three parameters was confirmed using online monitoring probes. Moreover, an autocatalytic behaviour was observed resulting in an rapid increase of the solvent metal content and ammonia emissions. The solvent iron content was above 500 mg/kg, while the ammonia emissions exceeded 150 mg/m3 STP (STP; 0°C and 101.325 kPa). By correlating the process conditions to the underlying degradation and corrosion mechanisms, online monitoring tools can be used to assess and manage the lifetime of the solvent. Even if the state of the solvent is kept in check by reclaiming methods, there could be instances where ammonia emissions could increase. Therefore, it is necessary to have an end of pipe countermeasure for such emissions. Chapter 3 presents the results from a test using an acid wash scrubber for ammonia emissions in a pilot plant test campaign. Several parametric tests were conducted in order to test the efficiency of the acid wash. Moreover, the ammonia concentration in the gas inlet to the acid wash was increased artificially (~150 mg/m3 STP). The acid wash scrubber reduced ammonia emission to very low levels, below 5 mg/m3 STP and mostly below 1 mg/m3 STP. Moreover, the MEA content was also reduced to mostly below 1 mg/m3 STP. A comparison between a model made in Aspen Plus and the experimental results showed good agreement, with deviations only at pH above 5 to 6. Aerosol based emissions are known to be a concern in PCCC. In Chapter 4, the impact of flue gas particles such as soot and sulphuric acid aerosol droplets on solvent emissions was studied in a mobile CO2 capture plant. These tests confirmed that solvent emissions can be in the order of grams per m3 STP, which is several orders of magnitude higher than volatile emissions. The number concentration of these particles had a direct relation to the extent of emissions. Particle number concentrations in the range of 107-108 per cm3 led to emissions of MEA in the range of 600-1200 mg/m3 STP. In Chapter 5, further tests were performed on the same setup in order to assess the impact of operating conditions of the CO2 capture plant on aerosol based emissions. Increasing the temperature of the lean solvent resulted in lowering of the aerosol based emissions. However, the total solvent emission increased as a result of increased volatile emissions. Aerosol based emissions were observed also for AMP-Pz as the capture solvent. The pH of the lean solvent was decreased by lowering the stripper temperature and thereby, changing the CO2 loading of the solvent. This resulted in an increase in the aerosol based emissions as the activity of the amine increased in the solvent. As the CO2 content in the flue gas was reduced from 12.7 vol.% to 0.7 vol.%, a maximum in the emissions was observed between 6 and 4 vol.%. When a mixture of a slow reacting volatile amine, AMP, with a fast reacting non-volatile, taurate, was used, no aerosol based emissions were observed. This led to the important conclusion that in reactive absorption, along with supersaturation and particle number concentration, the reactivity of the amine plays an important role in aerosol based emissions. A Brownian Demister Unit (BDU), consisting of multiple polypropylene fibre elements, can be potentially used as a countermeasure for aerosol based emissions. This was tested in a pilot plant campaign using MEA and is discussed in Chapter 6. The BDU reduced emissions from about 85–180 mg/m3 STP to about 1–4 mg/m3 STP. A water wash was found to be effective against vapour based emissions, while the BDU was effective against aerosol based emissions. A BDU is not effective against ammonia emissions, as they are present in the vapour form. From the measured nitrosamines, NDELA was found to be in the solvent in the order of 2000 ng/ml, while in the water wash it was ca. 1 ng/ml. Gas phase nitrosamine concentrations were in the range of tens of ng/m3 STP. The BDU results in a significant additional pressure drop of ca. 50 mbar, for the configuration and type of BDU used here. This translated to an additional consumption of electricity by the blower in the range 26–52%. A system containing three distinct phases, gas, liquid and aerosol droplets, are complex to understand and model. In Chapter 7, a methodology is presented with which such a complicated system can be modelled in commercially available software such as Aspen Plus. The mass and energy exchanges are split into two distinct interactions, gas-liquid and gas-aerosol. Aerosol droplets are considered to be as bulk liquid without any direct interaction with the solvent. The different parameters that were varied were the CO2 concentration in the flue gas, temperature of the lean solvent and CO2 loading of the lean solvent. The resulting trends were in good agreement with the experimental results presented in Chapter 5. The model did not predict a maximum in the emissions as the CO2 content in the flue gas was varied. Although absorption desorption based process for CO2 capture is well known, several operating issues needs to be addressed for its application in PCCC, as evident in this thesis. It is important to monitor the degradation of the solvent and deploy appropriate methods at the right time, to minimize its detrimental effect on the corrosion of the plant and avoid high emissions of ammonia. Aerosol based emissions in a PCCC process is a serious issue. The experimental results, proposed mechanism and modelling methodology will enable the design of appropriate counter-measures against aerosol based emissions. It is recommended to devise appropriate strategy and innovative solutions based on the understanding of the various operational aspects of absorption-desorption based PCCC as presented in this thesis. This will increase the confidence level in the technology and lead to its successful deployment for mitigating climate change in the short term.