Thermally Stable Silicone Solvents for the Selective Absorption of CO₂ from Warm Gas Streams That Also Contain H₂ and H₂O

Abstract

Polydimethylsiloxane (PDMS) is a promising hydrophobic, CO 2 -selective solvent for the absorption of CO 2 from a hot or warm water-rich, H 2 -rich, postwater−gas shift reactor (WGSR) stream in an integrated gasification combined cycle (IGCC) power plant. In this work, there are three hydrophobic silicones that are more thermally stable than PDMS, including an iron-stabilized PDMS (PDMS-Fe), poly(dimethyl-co-methylphenyl)siloxane (PDMMPS), and poly(dimethyl-co-diphenyl)-siloxane (PDMDPS). PDMDPS is an extremely poor CO 2 solvent, which is undesirable for the proposed separation, and a poor hydrogen solvent, which is a desirable trait. PDMDPS and PDMMPS absorb about the same amount of H 2 . Although PDMMPS is a much better CO 2 solvent than PDMDPS, PDMMPS is a slightly poorer CO 2 solvent than PDMS or PDMS-Fe. PDMS-Fe and PDMS are comparable hydrophobic solvents that exhibit the greatest solvent strength for CO 2 ; however, PDMS-Fe and PDMS absorb slightly more hydrogen than PDMDPS and PDMMPS. If the absorption/regeneration process is designed such that the solvent is exposed to temperatures no greater than 230°C, PDMS is recommended due to its low cost. For higher temperatures, the extremely low solubility of CO 2 in PDMDPS precludes its use as a CO 2 -selective solvent. The ferrosilicone additive in PDMS-Fe is designed to inhibit polymer degradation in an oxidizing environment, but it offers no additional stability in the oxygen-free closed system associated with the IGCC. PDMMPS absorbs less H 2 than PDMS or PDMS-Fe but is a slightly poorer CO 2 solvent than PDMS or PDMS-Fe. However, PDMMPS is thermally stable in closed systems to 300°C. Therefore, PDMMPS is recommended for prolonged high temperature use as the precombustion carbon capture absorber solvent at absorption/regeneration temperatures above 230°C. Although these hydrophobic silicones exhibit promising attributes for a warm or hot precombustion carbon capture process, the diminishing CO 2 solubility and increasing CO 2 solubility that occur with increasing temperature will challenge the economic viability of this proposed CO 2 -selective absorption process. ■ INTRODUCTIONChemical and physical absorption methods are typically used for the postcombustion and precombustion capture of CO 2 , respectively. 1 At low CO 2 partial pressures of ∼0.010−0.015 MPa associated with postcombustion capture of CO 2 from flue gas, amine solutions are favored because they will react with dilute concentrations of CO 2 to attain significant loadings. A 30 wt % solution of monoethanolamine (MEA) in water, for example, will bind CO 2 as a water-soluble ammonium carbamate at a 2:1 molar ratio of MEA:CO 2 , enabling this solution to absorb about 11 wt % CO 2 . CO 2 release and solvent regeneration is accomplished via heating. 1,2 Precombustion capture of CO 2 is typically accomplished with physical solvents, however, because the high CO 2 partial pressure in the post water−gas shift reactor (WGSR) gas stream is sufficient to cause the dissolution of significant amounts of CO ...

Process Simulation and Economic Analysis of Pre-combustion CO2 Capture With Deep Eutectic Solvents

“…Nowadays green hydrophobic DESs becomes a hot research area (Florindo et al, 2019). An advanced capture process with much higher absorption temperature is suggested for hydrophobic absorbents (Koronaios et al, 2016). According to the process modeling results from National Energy Technology Laboratory (Netl and Parsons, 2010), if CO 2 can be selectively removed from the stream (250°C and 5.5 MPa) coming out of the WGS reactor using physical solvents with little or no cooling and water remains in the gas phase, then the combustion of H 2 −H 2 O gas mixture in the gas turbine system will result in an increase of 2-3 percentage point in IGCC plant thermal efficiency.…”

Molecular Simulations of the Thermophysical Properties of Polyethylene Glycol Siloxane (PEGS) Solvent for Precombustion CO2 Capture

“…Due to rich H 2 O concentration in the hot or warm gas stream corresponding to CO 2 precombustion capture, hydrophobic solvents are preferred for selective CO 2 absorption over H 2 and H 2 O. 3,4 By using a hydrophobic solvent, it is possible to retain moisture with high thermal energy in the H 2 stream that is necessary for combustion in a gas turbine. Note that although polyethylene glycol (PEG) has good CO 2 /H 2 solubility selectivity, which is partly due to strong CO 2 interactions with −O groups of PEG, the PEG solvent itself is very hydrophilic and consequently not appropriate for precombustion carbon capture application at high temperatures, under which conditions valuable water steam in a hot/warm gas stream will be absorbed in PEG.…”

“…Countries across the globe are acting together to reduce greenhouse gas emission because of growing evidence of harm from rising temperatures and rising sea levels. , Chemical and physical absorption methods have been suggested for CO 2 postcombustion and precombustion capture, respectively, from coal-based power generation emissions due to low CO 2 partial pressure (∼0.1 bar) in flue gas and high CO 2 partial pressure (∼25 bar) in fuel gas. Due to rich H 2 O concentration in the hot or warm gas stream corresponding to CO 2 precombustion capture, hydrophobic solvents are preferred for selective CO 2 absorption over H 2 and H 2 O. , By using a hydrophobic solvent, it is possible to retain moisture with high thermal energy in the H 2 stream that is necessary for combustion in a gas turbine. Note that although polyethylene glycol (PEG) has good CO 2 /H 2 solubility selectivity, which is partly due to strong CO 2 interactions with −O groups of PEG, the PEG solvent itself is very hydrophilic and consequently not appropriate for precombustion carbon capture application at high temperatures, under which conditions valuable water steam in a hot/warm gas stream will be absorbed in PEG.…”

Development of Natural Hydrophobic Deep Eutectic Solvents for Precombustion CO2 Capture

“…In fact, before gas enters the Selexol unit, the dehydration of the feed gas is required, because water accumulation in the absorbent can cause a detrimental effect on the absorption capacity and metal corrosion. The use of hydrophobic solvents not only prevents water accumulation but also allows operation at higher absorption temperatures . According to process modeling results by NETL, a typical IGCC fuel gas stream (31 mol % CO 2 , 43% H 2 , 23% H 2 O, and 3% of other gases such CO, COS, H 2 S) leaves the WGS reactor at 250 °C and 5.5 MPa.…”

“…The use of hydrophobic solvents not only prevents water accumulation but also allows operation at higher absorption temperatures. 8 According to process modeling results by NETL, 9 a typical IGCC fuel gas stream (31 mol % CO 2 , 43% H 2 , 23% H 2 O, and 3% of other gases such CO, COS, H 2 S) leaves the WGS reactor at 250 °C and 5.5 MPa. If CO 2 can be selectively removed from the stream using hydrophobic solvents with little or no cooling and water remains in the gas phase, then the combustion of the H 2 −H 2 O gas mixture in the gas turbine system will result in an increase of 2−3 percentage points in the IGCC plant thermal efficiency.…”