Carbon Dioxide Capture from Fuel Gas Streams under Elevated Pressures and Temperatures Using Novel Physical Solvents

摘要

The conventional processes for acid gas removal (AGR), including CO2 in the Integrated Gasification Combined Cycle (IGCC) power generation facilities are: a chemical process, using methyl-diethanolamine (MDEA); a physical process, using chilled methanol (Rectisol) or a physical process, using mixtures of dimethylethers of polyetheleneglycol (Selexol). These conventional processes require cooling of the fuel gas streams for CO2 capture and subsequent reheating before sending to turbines, which decreases the plant thermal efficiency and increases the overall cost. Thus, there is a pressing need for developing an economical process which can capture CO2 from the hot fuel gas stream without significant cooling.udThe overall objective of this study is to investigate the potential use of physical solvents for selective capture of CO2 from post water-gas-shift streams under relatively elevated pressures and temperatures. In order to achieve this objective, a comprehensive literature review was conducted to define an “ideal solvent” for CO2 capture and to identify six different physical solvents which should obey such a definition. udThe first physical solvents identified were perfluorocarbons (PFCs), which are known to have low reactivity, high chemical stability and relatively low vapor pressures. Three different PFCs, known as PP10, PP11, and PP25, were selected as potential candidates for CO2 capture. The equilibrium solubilities of CO2 and N2 were measured in these PFCs under different operating conditions up to 30 bar and 500 K. These PFCs have relatively low viscosity at 500 K, very good thermal and chemical stabilities and showed high CO2 solubilities; hence they were considered as “ideal solvents.” The CO2 solubilities in PP25 were found to be greater than in the other two PFCs. Due to its superior behavior, PP25 was selected for the development of a conceptual process for CO2 capture form Pittsburgh No. 8 shifted fuel gas mixture using Aspen Plus simulator. Unfortunately, during the pressure-swing option for solvent regeneration, the solvent loss was significant due to the fact that the boiling point of PP25 is 533 K which is close to the absorber temperature (500 K). Also, other drawbacks of PFCs include, high cost, and absorption of other gases (light hydrocarbons) along with CO2. udIt was then decided to seek different physical solvents, which have negligible vapor pressure, in addition to the other attractive properties of the “ideal solvent” in order to use in the Aspen Plus simulator. Extensive literature search led to Ionic Liquids (ILs), which are known to have unique properties in addition to extremely low vapor pressures, and therefore they were considered excellent candidates for the CO2 capture from fuel gas streams under elevated pressures and temperatures. Three ILs, namely TEGO IL K5, TEGO IL P9 and TEGO IL P51P, manufactured by Evonik Goldschmidt Chemical Corporation, were selected as potential solvents for CO2 capture. The solubilities of CO2, H2, H2S and N2 were measured in the TEGO IL K5 and the solubilities of CO2 and H2 were measured in the TEGO IL K5 at pressures up to 30 bar and temperatures from 300 to 500 K. Also, the density and viscosity of these three ILs were measured within the same pressure and temperature ranges, and the surface tension for TEGO IL K5 and TEGO IL P51P were measured from 296 to 369 K. Due to their superior performance for CO2 capture, the TEGO IL K5 and the TEGO IL P51P were selected to be used in the Aspen simulator for the conceptual process development. The density and surface tension data for the TEGO IL K5 and the TEGO IL P51P were used in Aspen Plus, employing the Peng-Robinson Equation of state (P-R EOS) to obtain the critical properties of the two ILs; and the measured solubility data were also used to obtain the binary interaction parameters between the shifted gas constituents and two ILs. udThe Aspen Plus simulator was employed to develop a conceptual process for CO2 capture from a shifted fuel gas stream (102.52 kg/s) generated using Pittsburgh # 8 coal for a 400 MWe power plant. The conceptual process developed consisted mainly of 4 adiabatic absorbers (2.4 m ID) arranged in parallel and packed with Plastic Pall Rings of 0.025 m for CO2 capture; 3 flash drums arranged in series for solvent regeneration using the pressure-swing option; and 2 pressure-intercooling systems for separating and pumping CO2 to the sequestration sites. The compositions of all process steams, CO2 capture efficiency, and net power were calculated using Aspen Plus for each solvent. The results indicated that, based on the composition of the inlet gas stream to the absorbers, 87.6 and 81.42 mol% of CO2 were captured and sent to sequestration sites; and 97.69 and 97. 86 mol% of H2 were separated and sent to turbines using the TEGO IL K5 and the TEGO IL P51P, respectively. Also, the two solvents exhibited minimum loss of 0.06 and 0.17 wt% with a net power balance of -26.44 and -14.72 MW for the TEGO IL K5 and the TEGO IL P51P, respectively. Thus, the TEGO IL K5 could be selected as a physical solvent for CO2 capture from shifted hot fuel gas streams since large quantities of CO2 are absorbed.