Date of Award
Doctor of Philosophy (PhD)
Driven by escalating concerns on global warming and climate change, significant efforts have recently been initiated to reduce atmospheric CO2 concentration. Power generation from fossil fuels is the largest source of anthropogenic CO2. The US government has incentivized carbon capture through Section 45Q Tax code, which provides US$50/tCO2 (per metric ton) of qualified carbon dioxide captured and sequestered, as tax credits. The cost for carbon capture, transport and sequestration is US$75 – 150/t CO2, with the state-of-the-art capture technology costing between US$20 – 100/t CO2. Amine based post-combustion CO2 capture is the most economic technology suited for large power plants. Solvent losses pose a primary challenge for amine-based CO2 capture, resulting in higher operational costs and increased environmental impacts.
During amine-based CO2 capture, flue gas containing CO2 is contacted with liquid aqueous amine in a packed bed absorber. There is preferential mass transfer of CO2 into liquid amine producing CO2 rich amine, which is subsequently stripped from amine at higher temperatures in a stripper, producing concentrated CO2. Inside the absorber, the exothermicity of the reactions between CO2 and amine in the liquid phase vaporize amine, resulting in amine loss as vapor or aerosolized emissions to the exiting flue gas. The nanoparticles in the flue gas that escape conventional control equipment, exacerbate solvent losses through complex aerosol mechanisms, primarily condensation. Thorough understanding of the underlying mechanisms of the various physio-chemical processes is lacking and is critical in developing efficient technologies to mitigate solvent losses. Decreasing particulate concentration in the flue gas reduces amine emissions, however conventional Electrostatic Precipitators (ESP), typically installed in power plants are inefficient in capturing particles in the size range 200 – 500 nm, and below 40 nm, due to the minimum in migrational velocity and partial charging. Particle charging is limited by the ion density generated by DC corona and the residence time, among other factors. Introducing a pre-charging stage and a photo-ionization stage into an ESP has shown potential in increasing particle charging and capture efficiency, but has not been evaluated in bench-scale or realistic power plant conditions. Therefore, the dissertation addresses the aforementioned issues in four parts.
Part I: A process engineering coupled aerosol dynamics model was developed to quantify vapor- and aerosol-based amine emissions in realistic absorber conditions. A process simulation model was developed in ASPEN PLUS accounting for the multicomponent heat and mass transfer, solution thermodynamics and reactions in the bulk liquid and gas phases. A multicomponent discrete-sectional model was developed to predict the time evolution of droplet growth inside the absorber by solving the general dynamics equation. The coupled model predictions were compared with experiments reported in literature. The model predictions showed that by decreasing amine inlet temperature to the absorber, the saturation of water in the warm region can be decreased to reduce water and amine condensation to the droplet, thereby reducing amine emissions without affecting CO2 capture efficiency.
Part II: Controlled lab-scale experiments, modelling and simulation was used to illuminate the mechanisms governing aerosol-driven solvent losses in the absorber. A modified growth system was used to investigate condensational growth of non-wetting nanoparticles due to water and Monoethanol amine vapors at conditions relevant to the absorber. It was shown that the difference in mass diffusivity of the vapor species and the heat diffusivity of air determined the supersaturation in the warm and cool regions of the growth system. Consequently, condensational growth was controlled by mole fractions of the species in the droplet surface. Further, the lower surface tension of amine resulted in increased activation of particles.
Part III: A pilot-scale Photoionization Enhanced Two-staged Electrostatic Precipitator (PI-ESP) was designed, fabricated, and characterized in controlled laboratory conditions. The collection efficiency of the PI-ESP at 600 scfm (~17,000 lpm) was between 85 – 99 % for 30 – 2000 nm particles at 5*105 #/cm3 particles. The influence of aerosol concentration, gas flow rate and soft X-Rays on collection efficiency was systematically evaluated. A modified Deutsch-Anderson model was developed for the PI-ESP and the experimental data was used to determine regression coefficients.Part IV: Flue gas aerosol sampling was performed at the CWLP power plant, Springfield and Abbott power plant, Champaign, IL. It was found that the aerosol number concentration of particle sizes lesser than 400 nm was in the order of 107 #/cm3 at the exhaust stack. The PI-ESP was installed in a pilot-test skid at Abbott power plant along with Linde’s high velocity spray tower and the capabilities and limitations of these technologies in capturing nanoparticles from the exhaust flue gas was systematically evaluated. The aerosol removal efficiency of the spray tower was 70 – 95 % and PI-ESP was 60 – 80 % for particles in the size range 50 – 400 nm, respectively. Design improvements to the PI-ESP to accommodate water condensation was suggested.
In essence, with the PI-ESP aimed at reducing the particulate concentration in the flue gas, and the model relating the influence of particulate concentration in the flue gas to amine emissions, this work elucidates the underlying mechanisms governing amine emissions, and demonstrates a solution to tackle the problem. The work is sponsored by US Department of Energy and Linde
Benjamin Kumfer, Richard Axelbaum, Rajan Chakrabarty, Krish Krishnamurthy,