Achieving Supercritical Fluid CO2 Pressures Directly from Thermal Decomposition of Solid Sodium Bicarbonate

摘要

Compressing carbon dioxide (CO2) to supercritical pipeline pressures is one of the major costs of carbon capture and storage. Many innovative approaches to decreasing this cost have been suggested, in many cases relying on high temperature regeneration schemes utilizing high enthalpy solvents. Solid sorbent systems have not generally been examined for high pressure regeneration. We have now quantified through experimental and modelling studies that it is possible to obtain CO2 pressures of 150 bars or more by thermal decomposition of solid sodium bicarbonate (nahcolite, NaHCO3). In a cyclic operations with CO2 withdrawal, our model of the process predicts a swing capacity of 5% (wt.) when withdrawing CO2 at 20 bars, and 2% when withdrawing CO2 at 80 bars (supercritical fluid). Precipitation of solid sodium bicarbonate ("scaling") has been observed previously when using sodium carbonate solutions in CO2 capture, and was considered an obstacle to the use of sodium carbonate as a capture solution due to potential mineral precipitation on the packing material. The ability to encapsulate the capture solvent overcomes the scaling problem and allows multiple benefits of the sodium carbonate capture solvent to realized [1]. In addition to high pressures of regeneration, these benefits include the low cost and benign nature of sodium carbonate, thermal stability with no thermal degradation, competitive carbon transfer mass when compared with amines, relatively low heat of solution of CO2 in the carbonate solvent, and potentially a reduced mass of water heated during regeneration. Using previous experimental data as well as our own measurements of CO2 pressures in equilibrium with carbonate solutions, we have developed a model of the phase behavior over a range of processing conditions of up to 210 °C and saturation with respect to nahcolite. Using these data to parametrize a Pitzer electrolyte model, we predicted that in the absence of water, nahcolite reaches supercritical CO2 pressures over the solid at about 125 °C. This encouraged us to pursue experimental determination with realistic amounts of water to allow for conversion of carbonate (CO32) to bicarbonate (HCO3~) as loading with CO2 takes place. High CO2 pressures shift to higher temperature as the carbonate/water ratio decreases. However, encapsulated carbonates allow the system to operate at very high carbonate/water ratios such that a solid phase is present throughout the capture process. Each capsule contains mainly a solid carbonate phase with a small mass of saturated solution. Engineering of the capsule material may allow us to control both CO2 loading and water content independently to maximize CO2 carrying capacity and minimize water content, saving process energy by heating only minimal water during regeneration.