Eirik Falck da Silva

Emissions from postcombustion CO2 capture plants.

P CO2 capture (PCCC) technology has the potential to contribute significantly to the reduction of anthropogenic CO2 emissions. The application of PCCC technology to reduce CO2 emissions at large point sources, such as fossil fuel-fired power plants is particularly promising. Amine-based scrubbing is the most mature PCCC technology, with 2-aminoethanol (MEA) the most widely utilized solvent. Despite the use of water wash systems to treat effluent, small amounts of the solvent and degradation products are present in amine-based PCCC plant emissions. Recently, concerns have arisen that emissions from PCCC plants could be harmful to human health and the environment. Of particular interest are nitrosamine and nitramine degradation products, many of which are known carcinogens. Degradation products, including nitrosamines and nitramines, will form in PCCC plants from reactions between the amine solvent and NOx species in the flue gas. For primary amine solvents (e.g., MEA), nitrosamines can only be formed indirectly from other degradation products. Solvents with secondary and tertiary amine functionalities (e.g., piperazine and N-methyldiethanolamine) can directly form stable nitrosamines. Nitramines can form directly from primary, secondary, or tertiary amines. However, there is currently limited emission data available for PCCC plants. This problem is compounded by the fact that emission composition and levels will depend significantly on the flue gas being processed, choice of solvent, and plant operation conditions. Emission levels in the range of 2−50 ng/Nm have been reported for both nitrosamines and nitramines at a pilot PCCC plant operating with MEA. These results, together with other reports of nitrosamines detected at pilot PCCC plants and in laboratory experiments, suggest low but quantifiable emissions of nitrosamines and nitramines. In addition to the formation of nitrosamines and nitramines within PCCC plants, it is also possible for these compounds to form through the atmospheric photooxidation of the amine solvent present in the emissions. Steady state conversion rates of amines to nitrosamines and nitramines have been reported for a variety of atmospheric conditions. Steady state nitrosamine yields for N-nitrosodimethylamine (NDMA) were reported to be <0.6% of the amine concentration in a rural scenario and <2.3% in an urban scenario. Similar yields were reported for other nitrosamines and nitramines. The formation of nitrosamines and nitramines can therefore occur both in a PCCC plant and in the atmosphere. Relatively little is known about the potential impacts of PCCC-related emissions on human health and the environment. Compounds emitted from the plant or formed in the atmosphere will disperse and deposit to terrestrial and aquatic environments as either wet or dry aerosols. Preliminary studies have indicated that both nitrosamines and nitramines will partition preferentially to the water phase rather than adsorb to soils and sediments. It seems likely that these compounds will therefore reach an aqueous compartment (e.g., drinking water, groundwater). The biodegradability of nitrosamines and nitramines influences their potential for accumulation in aquatic matrices and ultimately their human and environmental exposure. Initial studies indicate biodegradation of nitrosamines and nitramines is complex, differing significantly between compounds and dependent upon environmental conditions and compound concentration. In general neither nitrosamines nor nitramines seem susceptible to rapid aerobic aqueous biodegradation indicating the possibility of accumulation in these environmental matrices. Owing to their hydrophilic nature, we do not expect nitrosamines and nitramines to bioaccumulate significantly in organisms; however, there seems to be potential for chronic exposure. To determine acceptable PCCC plant emission levels it is important to consider both the environmental fate of the emissions and the acceptable exposure levels. The Norwegian Public Health Institute (NPHI) has proposed acceptable exposure levels of 4 ng/L (drinking water) and 0.3 ng/m (air concentration) based on 10−6 lifetime risk of cancer following exposure to the nitrosamine, NDMA. The NPHI assessment assumes all nitrosamines and nitramines are as carcinogenic as NDMA, which is considered one of the most carcinogenic nitrosamines. The carcinogenicity of nitramines is generally much lower, but insufficient data are available to propose reliable exposure limits. Other existing safety limits for nitrosamines and nitramines following exposure by inhalation, drinking water and to the aquatic environment range from 0.02 ng/m (inhalation; monthly average) to 7 ng/L (drinking water) for nitrosamines, and from 200 ng/L (aquatic

Modeling temperature dependency of amine basicity using PCM and SM8T implicit solvation models.

PCM and SM8T continuum solvation models are used to study the temperature dependency of a set of amines in the temperature range 273-393 K using density functional theoretical calculations. Gaseous phase calculations are done using B3LYP and M06 functionals at the 6-311++G(d,p) basis set level. pK(a) values calculated computationally are compared with experimental values in the given temperature region using both continuum solvation models. The continuum solvation models predict the temperature trends of pK(a) compared to experimental trends very nicely. Accurate pK(a) values at 298 K are however required as input to the model. The absolute values of pK(a) values are not reproduced well by these continuum solvation models, and a correction term is therefore introduced. A set of 10 amines, which have potential for CO(2) capture, and where also a large experimental data set of temperature dependent pK(a) values is available, were studied in this work. The temperature dependency of pK(a) values of amines provides a basis for selection for optimum solvents for postcombustion CO(2) capture processes.

Theoretical study of differential enthalpy of absorption of CO2 with MEA and MDEA as a function of temperature.

Temperature dependent correlations for enthalpy of deprotonation, carbamate formation, and heat of absorption of the overall reaction between aqueous MEA and MDEA and gaseous CO2 are calculated on the basis of computational chemistry based ln K values input to the Gibbs-Helmholtz equation. Temperature dependency of reaction equilibrium constants for deprotonation and carbamate formation reactions is calculated with the SM8T continuum solvation model coupled with density functional theoretical calculations at the B3LYP/6-311++G(d,p) level of theory. Calculated reaction equilibrium constants and enthalpies of individual reactions and overall heat of absorption are compared against experimental data in the temperature range 273.15-373 K. Temperature dependent correlations for different reaction equilibrium constants and enthalpies of reactions are given. These correlated results can be used in thermodynamic models such as UNIQUAC and NRTL for better understanding of post-combustion CO2 capture solvent chemistry.

Explicit Solvation Shell Model and Continuum Solvation Models for Solvation Energy and pKa Determination of Amino Acids.

The study of the Explicit Solvation Shell Model (ESS) presented recently [da Silva, E. F.; Svendsen, H. F.; Merz, K. M. J. Phys. Chem. A 2009, 113, 6404.] for calculation of solvation free energy of ions is extended for the study for amino acids. Solvation free energies and pKa of a data set of 10 amino acids is calculated using ESS. The data set of amino acids is selected based on their potential to be regarded as solvents for postcombustion CO2 capture processes. Calculated results are compared against experimental pKa and pKa calculated from PCM, SM8T, and DivCon continuum solvation models. Error estimates of pKa from different models vs experimental pKa data are also given to evaluate the results calculated by different solvation models. This study also involves a comprehensive study of gas phase basicity, proton affinity,ΔGacid0, ΔHacid0, protonation entropy with density functional methods (B3LYP/6-311++G(d,p)) and composite methods (G3MP2B3, G3MP2, CBS-QB3, G4MP2) and their comparison with experimen...

Modeling temperature dependency of ionization constants of amino acids and carboxylic acids.

The temperature sensitivity of pK(a) values for a data set of 10 amino acids and 5 carboxylic acids is studied using PCM and SM8T continuum solvation models coupled with density functional theoretical calculations at the B3LYP/6-311++G(d,p) level of theory. The data set of amino acids was chosen on the basis of their potential to be solvents for postcombustion CO2 capture processes and available literature data. Calculated results are compared with experimental data in a temperature range of 273-393 K. Both solvation models predict temperature sensitivity of pK(a) of the amino group of amino acids very nicely. It is observed that the temperature dependencies of pK(a) of the carboxyl group of amino acids and carboxylic acids predicted by these models do not agree well with experimental temperature dependencies of carboxylic acids. This issue is discussed in the context of the basic parametrization of these continuum solvation models.

CO2 capture into aqueous solutions of the mixed solvent Cesar 1

Publisher Summary This chapter illustrates experimental data and a simplified vapor-liquid equilibrium (VLE) model for the absorption of CO2 into aqueous solutions of the mixed solvent Cesar 1. The Cesar 1 system has about twice the specific cyclic capacity, measured between 40 and 80°C, compared to 5M Monoethanolamine (MEA). It has almost twice the partial CO2 pressure for a maximum loaded solution at 120°C compared to MEA. The heat of absorption of the two systems is comparable. This should imply both significantly reduced stripping steam consumption and sensible heat loss for the Cesar 1 system making this system a very strong contender to 5 M MEA system in exhaust gas absorption plants.

Temperature sensitivity of piperazine and its derivatives using polarizable continuum solvation model

Temperature dependency of piperazine and related amines in the temperature range 298–393 K is studied using density functional theoretical calculations. B3LYP functional and 6-311++G (d, p) basis set was used in all the calculations. From the pKa and carbamate formation reaction energy values it can be seen that the structural changes of amines have a critical effect on their behavior and that it varies significantly with temperature.