Amine scrubbing for CO2 capture Rochelle, Gary T
Science (American Association for the Advancement of Science),
09/2009, Letnik:
325, Številka:
5948
Journal Article
Recenzirano
Amine scrubbing has been used to separate carbon dioxide (CO2) from natural gas and hydrogen since 1930. It is a robust technology and is ready to be tested and used on a larger scale for CO2 capture ...from coal-fired power plants. The minimum work requirement to separate CO2 from coal-fired flue gas and compress CO2 to 150 bar is 0.11 megawatt-hours per metric ton of CO2. Process and solvent improvements should reduce the energy consumption to 0.2 megawatt-hour per ton of CO2. Other advanced technologies will not provide energy-efficient or timely solutions to CO2 emission from conventional coal-fired power plants.
•CO2 absorbs faster into MEA/NMP (or CARBITOL™)/water than MEA (aq) or PZ (aq).•CO2 physical solubility and MEA volatility were measured in CO2-MEA-NMP-water.•A kinetic model was built in MATLAB® to ...explain the rate behavior.•Amine/physical solvent/water is promising for CO2 capture.•The volatility and viscosity of the solvent may limit its usefulness.
Post-combustion CO2 capture using amine scrubbing is the most promising technology to reduce CO2 emissions from coal- or gas-fired power plants. Increasing CO2 absorption rate (kg′) reduces the absorber capital cost, which is the cost center of the capture plant. By partially replacing water with N-methyl-2-pyrrolidone (NMP) in 7 m (30 wt%) aqueous monoethanolamine (MEA), the CO2 absorption rate (kg′) is significantly enhanced because of lower CO2 loading/higher free MEA at the same CO2 partial pressure (P∗CO2), greater CO2 physical solubility, and greater MEA activity. At 40 °C, in the operating range of 100–5000 Pa P∗CO2, the average kg′ of 7 m MEA in 3 water/1 NMP, 1 water/3 NMP, and 1 water/19 NMP is 1.1 times, 2 times, and 5 times that of 7 m aqueous MEA, respectively. CO2 physical solubility, solvent viscosity, and MEA activity were measured. A kinetic model was built in MATLAB® to better understand the mass transfer of CO2 into semi-aqueous MEA (MEA-NMP-water). The model suggests that the diffusion and reaction of CO2 into aqueous MEA can be approximated by pseudo-first-order (PFO) behavior and adding NMP causes deviation from PFO by the depletion of MEA at the surface. The semi-aqueous solvent provides an excellent rate of CO2 absorption, but the increased viscosity reduces normalized capacity and the volatility of the physical solvent must be addressed.
Aqueous piperazine (PZ) blended with a tertiary or hindered amine combines the high CO2 capacity of the tertiary or hindered amine with the fast rate of PZ. For flue gas CO2 capture, the optimum pKa ...(that which offers the highest CO2 cyclic capacity) of a mono-tertiary amine blended with PZ is 9.1. A generic Aspen Plus® model for PZ/tertiary amine was developed to predict the CO2 vapor-liquid-equilibrium (VLE) from the pKa of the tertiary amine. The polarity of the tertiary amine also affects the CO2 solubility of the PZ/tertiary amine. Hindered amines that form little carbamate show similar CO2 VLE to tertiary amines with the same pKa, when blended with PZ. The CO2 absorption rate of most 2.5m PZ/2.5m tertiary amines was slightly slower than 2.5m PZ, probably due to the higher viscosity of the blends.
•21 tertiary and hindered amines blended with PZ have been studied for CO2 capture.•The optimum pKa of a tertiary amine blended with PZ is around 9.1.•A generic Aspen Plus® model for PZ/tertiary amine has been developed.•Hindered amines show similar CO2 VLE to tertiary amines in PZ blends.•PZ/2E-4M-IMI and PZ/HMPD are promising solvents for CO2 capture from flue gas.
► Alkanolamines and ethylenediamines degrade readily by ring formation. ► Tertiary amines degrade slower by transalkylation and elimination. ► Piperazine and its derivatives are resistant to ...degradation by ring opening. ► Amine blends frequently degrade faster than their components.
Amine scrubbing will be an important technology for CO2 capture and storage. The degradation of the amine at 100–150°C limits the maximum T/P and therefore the energy performance of the solvent regeneration. This is a review of the rate, products, and mechanisms of thermal degradation. Primary and secondary ethanolamines and ethylenediamines degrade at 100–130°C as they form cyclic oxazolidinones and ureas. Tertiary amines can be more resistant to degradation if they do not include methyl and ethanol groups. Piperazine structures and long chain diamines such as hexamethylenediamine degrade by ring opening and closing and can be used at 150–160°C. Thermal degradation can produce products that are more volatile than the parent amines.
Amine scrubbing is the most mature technology for postcombustion carbon capture. Much bench- and pilot-scale work has been focused on CO2 capture from coal-fired flue gas. Because natural gas is ...inexpensive and readily available in the United States and other countries, the natural gas combined-cycle (NGCC) has been replacing coal for electricity generation. Carbon capture for NGCC is therefore an important technology for the modern power plants. The Piperazine (PZ) Advanced Stripper (PZAS) technology has been established as a benchmark system for second-generation amine scrubbing for CO2 capture from coal-fired flue gas. It has a fast absorption rate, good energy performance, and strong resistance to thermal degradation and oxidation. PZAS was operated with simulated NGCC flue gas (4.3 mol % (dry) CO2) at the National Carbon Capture Center (NCCC) in Wilsonville, Alabama in 2019. This paper presents the absorber performance and model validation for the campaign. The absorber was tested with in-and-out and with pump-around intercooling. The variable operating conditions included lean loading (0.19–0.25 m CO2/mol alkalinity), gas temperature (40, 78 °C), and intercooling temperature (35, 40 °C). Using 5 m PZ CO2 removal from 82% to 96% was achieved with intercooling and only 12 m of packing. A rigorous, rate-based absorber model accurately predicted the CO2 removal and temperature profile. The model shows that the delta loading of the solvent at NGCC conditions for 90% removal is greater than at coal conditions, but high CO2 removal (99%) is more difficult to achieve with NGCC gas than with coal-fired flue gas. The pump-around intercooling was effective, and the intercooling temperature had a large impact on the absorber performance. With pump-around, the delta loading penalty for hot gas feed into absorber without a direct contact cooler was less than 5%.
•Estimate energy of novel processes from lost work.•2G amine scrubbing processes and water lean solvents both reduce water vapor heat load.•Comparisons of solvent capacity should include viscosity, ...heat capacity, and thermal conductivity.
Energy use is a critical parameter for CO2 capture from flue gas. Proponents of water-lean solvents claim that they will provide better energy performance than aqueous amine solvents, but papers on water-lean solvents frequently fail to compare their energy performance to second-generation (2 G) aqueous amine systems. These papers claim that the weakest point of aqueous amine systems is the energy loss associated with boiling and condensing water. However, unlike first generation processes, 2 G scrubbing processes use innovative stripper configurations to minimize the loss of heat as excess water vapor from the top of the stripper. Proponents of water-lean solvents also claim that these solvents reduce sensible heat losses because of the lower specific heat of the organics that replace the water. In 2 G aqueous systems, the sensible energy of heating the solvent is minimized by high efficiency plate-and-frame exchangers where the optimized energy and capital cost is proportional not only to the specific heat of the solvent, but also to the viscosity and the thermal conductivity:
CAPEX, OPEX,TC∝m ˙kn-12Cp1-n2μm-n2ΔTcrx0.5∝m˙*k-0.325Cp0.825μ0.175*ΔTcrx0.5
Because water-lean solvents typically have higher viscosity and lower thermal conductivity, they do not provide significantly better exchanger performance than 2 G aqueous solvents.
This paper develops a method to estimate the energy use of 2 G aqueous solvents by summing the minimum work of separation and lost work (irreversibility) of each process unit. This method allows the comparison of energy use for different solvents (aqueous or water-lean) on the same basis without rigorous process simulation. The results demonstrate that a representative water-lean solvent does not provide significantly better energy performance than a representative 2 G aqueous solvent with an advanced stripper configuration.
•Carbon capture by amine scrubbing will reduce coal-fired power plant impacts on PM2.5.•Water wash with or without acid should practically eliminate other air quality impacts.•Research will reduce ...uncertainties in the effects of atmospheric reactions.•Gas sampling should demonstrate amine, nitrosamine, and aldehyde emissions less than 100 ppb.
The current political discussion in the United States around carbon capture and storage includes statements that suggest a need for a technical review to clarify the expected air pollution impacts of amine scrubbing. The Center for International Law and 50 other organizations published an open letter claiming that “CCS is not consistent with the principles of environmental justice… CCS makes dirty energy even more dangerous for frontline communities. Facilities equipped with carbon capture technology have to burn more fossil fuel to get the same energy output, resulting in increased emissions of toxic and hazardous pollutants, like fine particulates (PM2.5).”
This paper reviews air pollutants produced by the use of amine scrubbing on coal- and gas-fired power plants in the U.S. and the process features and mitigation strategies that will minimize their impact on air quality. Even with atmospheric reactions, emissions of amine, nitrosamine, and other air toxics are likely to have insignificant environmental and health impacts. Stacks will disperse emissions with a dilution factor greater than 8000. Water wash with or without acid will reduce emissions of amine and nitrosamine that is produced from atmospheric reactions. Nitrosamine emissions will be managed with selective catalytic reduction (SCR) to reduce the NO/NO2 and/or selection of primary or non-volatile amines. With coal-fired power plants, amine aerosols, Hg, SO3, and fine particulate emissions will probably be managed by a fabric filter with alkali addition. Carbon capture by amine scrubbing will reduce significantly the effect of the power plant emissions on ambient levels of PM2.5. With coal-fired power plants, the application of amine scrubbing will significantly reduce SO2 emissions. NOx emissions will usually be minimized by selective catalytic reduction (SCR) in both gas- and coal-fired plants. Ammonia emissions will be minimized by managing amine oxidation, and if necessary, by adding an acid wash or other controls.
The effect of liquid viscosity (μL) on the effective mass transfer area (a e) and liquid film mass transfer coefficient (k L) of packing must be known to determine packing height. Most existing ...correlations are based on water and are not applicable to viscous systems. In this work, a e, k L, and gas film mass transfer coefficient (k G) were measured in a 0.43 m ID packed column with 0.5–3 m of packing. Liquid viscosity was varied from 0.8 to 70 mPa·s by adding glycerol to water. Liquid viscosity has an insignificant effect on a e over the range of μL investigated. The total dependence of k L on μL is −0.75, of which −0.35 is indirect influence through diffusivity, and −0.4 is direct influence through liquid turbulence. Universal mass transfer models for packing were developed as functions of packing geometry, operating condition, and fluid property.
•Irreversibility of conventional stripper was quantified using exergy analysis.•Major lost work comes from the condenser and the lean/rich solvent cross exchanger.•The proposed new stripper design ...reduces reboiler duty by 16%.•Thermodynamic efficiency of the stripping process has reached to 74%.•Further energy improvement is expected to be marginal.
Amine scrubbing is the most mature CO2 capture technology for coal-fired power plants. However, the energy consumption for regeneration incurs a 20–25% penalty on electricity output. This loss can be reduced by alternative stripper design. This work uses exergy analysis to quantify the inefficiencies and demonstrate the effectiveness of a new configuration, the advanced flash stripper, without having to consider numerous alternatives. The lost work from the overhead condenser and the cross exchanger makes the simple stripper inefficient. The advanced flash stripper successfully eliminates the lost work of the condenser and improves the reversibility of the cross exchanger. It reduces the reboiler duty by 16% and the overall work requirement by 11% compared to the simple stripper. The new design when used with piperazine showed a remarkable 74% thermodynamic efficiency. Further improvement of energy efficiency is expected to be marginal. This work not only provides a better design that reduces the energy consumption, but also indicates that the state-of-the-art is approaching the theoretical work limit.
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•Formulation of water-lean solvents with different amines and organic diluents.•Amines tested: monoethanolamine, 2-methylpiperazine, N-methyl diethanolamine.•Five different organic ...diluents tested.•Measurement of vapor-liquid equilibrium and mass transfer rate data.•Changes in intermolecular interactions affect chemical equilibrium.•Comparison of performances between aqueous and water-lean solvents.
Shifting from aqueous to water lean solvents has been evaluated as a mean for increasing the mass transfer rates in regular chemical solvents with amines. An array of amines (monoethanolamine, 2-methylpiperazine and N-methyl diethanolamine) and diluents (sulfolane, ethylene glycol, 1-methylimidazole, dimethyl sulfoxide, N-methyl-2-pyrrolidinone) has been analyzed. Addition of organic diluents seem to generally induce both a shift in chemical equilibrium and an increase in mass transfer rates for a fixed CO2 partial pressure, though not necessarily for a fixed CO2 loading. However, these relative advantages in terms of mass transfer rates decrease the more loaded the water lean solvent is. The equilibrium shift caused by organic diluents has been evaluated in terms of de-stabilization of the species in the solvent, which for monoethanolamine-based solvents can be easily related to a decrease in dielectric permittivity. However, this analysis indicates that such treatment is insufficient for other types of water-lean solvents, suggesting that different kinds of intermolecular interactions should also be considered in future studies.
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