Thermodynamic and Kinetic Study of Carbon Dioxide and Mercury Removal from Flue Gas in Coal Combustion Power Plants

Carbon dioxide and mercury from anthropogenic emissions pose a significant threat to our environment and human health. Removal from their major source – coal-fired power plants – is one of the most effective approaches to control their emissions. Thermodynamics and kinetics are critical to the studies of the removal technologies as they provide fundamental knowledge of the capture process. In this work, the thermodynamics and kinetics of CO₂ and Hg capture through absorption using aqueous amines solutions and adsorption using supported ionic liquid sorbents were investigated.

A vapor-liquid equilibrium (VLE) data reduction method that simplifies experimental measurements while maintaining accuracy was applied for the first time to the thermodynamic study of CO₂ absorption in aqueous amine systems. The method eliminates the measurements of speciation in liquid phase and vapor phase by applying a layer of mass balance iteration in the correlation. Incorporating the electrolyte non-random two liquid (eNRTL) model and the Soave–Redlich–Kwong (SRK) model, the data reduction method was used to correlate VLE and heat of absorption data collected in a modified batch calorimeter for ethanolamine (MEA) - H₂O - CO₂ system and piperazine (PZ) - H₂O - CO₂ systems. The optimized model with the best-fit eNRTL model parameters was used to predict vapor pressures under the conditions reported in the literature; the predicted values were consistent with the independent literature results, indicating successful application of the Barker data reduction method and the mathematical model in the thermodynamic study of CO₂-aqueous amine systems.

With the current technologies, capture of CO₂ and Hg from coal combustion flue gas requires additional air pollution control devices that can only do a single task . To reduce the cost, a new approach to capture both CO₂ and Hg from coal combustion flue gas in an integrated adsorbent system was discovered. In this approach, a task-specific amino acid ionic liquid is supported on silica gel particles with high surface area and pore volume. The CO₂ capacity for was found to be 0.4 mol of CO₂/ mol of ionic liquid. The ionic liquid loading was optimal for CO₂ capture at 40 wt%. Mass transfer in fixed-bed trials was slow at high ionic liquid loadings due to the decreasing in contact surface area. Hg capture performance was assessed for the same material under a nitrogen environment. These sorbent systems had a total Hg uptake of more than 14 mg/g. Slipstream testing of the sorbents, along with other novel Hg sorbents developed previously, using coal combustion flue gas showed promising and competitive results in Hg removal rate and Hg capacity. When both CO₂ and Hg are present in the gas phase, it is expected that Hg accumulates and fixes in the sorbent via strong chemical bonding over an extended time, while CO₂ can reversibly be adsorbed and desorbed on the sorbent. This hypothesis was validated by the experimental evidence that the present of CO₂ has limited effect on the capture of elemental Hg vapor and the theoretical evidence that oxidized Hg has a stronger bonding with the ionic liquid than CO₂.