Targeted electrochemical reduction of carcinogenic N-nitrosamines from emission control systems within CO2 capture plants.

N-Nitrosamines are one of the environmentally significant byproducts from aqueous amine-based post-combustion carbon capture systems (CCS) due to their potential risk to human health. Safely mitigating nitrosamines before they are emitted from these CO2 capture systems is therefore a key concern before widescale deployment of CCS can be used to address worldwide decarbonization goals. Electrochemical decomposition is one viable route to neutralize these harmful compounds. The circulating emission control waterwash system, commonly installed at the end of the flue gas treatment trains to minimize amine solvent emissions, plays an important role to capture N-nitrosamines and control their emission into the environment. The waterwash solution is the last point where these compounds can be properly neutralized before becoming an environmental hazard. In this study, the decomposition mechanisms of N-nitrosamines in a simulated CCS waterwash with residual alkanolamines was investigated using several laboratory-scale electrolyzers utilizing carbon xerogel (CX) electrodes. H-cell experiments revealed that N-nitrosamines were decomposed through a reduction reaction and are converted into their corresponding secondary amines thereby neutralizing their environmental impact. Batch-cell experiments statistically examined the kinetic models of N-nitrosamine removal by a combined adsorption and decomposition processes. The cathodic reduction of the N-nitrosamines statistically obeyed the first-order reaction model. Finally, a prototype flow-through reactor using an authentic waterwash was used to successfully target and decompose N-nitrosamines to below the detectable level without degrading the amine solvent compounds allowing them to be return to the CCS and lower the system operating costs. The developed electrolyzer was able to efficiently remove greater than 98% of N-nitrosamines from the waterwash solution without producing any additional environmentally harmful compounds and offers an effective and safe route to mitigate these compounds from CO2 capture systems.

Application of a Small Unmanned Aerial System to Measure Ammonia Emissions from a Pilot Amine-CO2 Capture System.

The quantification of atmospheric gases with small unmanned aerial systems (sUAS) is expanding the ability to safely perform environmental monitoring tasks and quickly evaluate the impact of technologies. In this work, a calibrated sUAS is used to quantify the emissions of ammonia (NH3) gas from the exit stack a 0.1 MWth pilot-scale carbon capture system (CCS) employing a 5 M monoethanolamine (MEA) solvent to scrub CO2 from coal combustion flue gas. A comparison of the results using the sUAS against the ion chromatography technique with the EPA CTM-027 method for the standard emission sampling of NH3 shows good agreement. Therefore, the work demonstrates the usefulness of sUAS as an alternative method of emission measurement, supporting its application in lieu of traditional sampling techniques to collect real time emission data.

Selective Removal of Nitrosamines from a Model Amine Carbon-Capture Waterwash Using Low-Cost Activated-Carbon Sorbents.

Nitrosamines generated in the amine solvent loop of postcombustion carbon capture systems are potent carcinogens, and their emission could pose a serious threat to the environment or human health. Nitrosamine emission control strategies are critical for the success of amine-based carbon capture as the technology approaches industrial-scale deployment. Waterwash systems have been used to control volatile and aerosol emissions, including nitrosamines, from carbon-capture plants, but it is still necessary to remove or destroy nitrosamines in the circulating waterwash to prevent their subsequent emission into the environment. In this study, a cost-effective method for selectively removing nitrosamines from the absorber waterwash effluent with activated-carbon sorbents was developed to reduce the environmental impact associated with amine-based carbon capture. The results show that the commercial activated-carbon sorbents tested have a high capacity and selectivity for nitrosamines over the parent solvent amines, with capacities up to 190 mg/g carbon, under simulated amine waterwash conditions. To further reduce costs, an aerobic thermal sorbent regeneration step was also examined due to the low thermal stability of nitrosamines. To model the effect of oxidation on the sorbent performance, thermal- and acid-oxidized sorbents were also prepared from the commercial sorbents and analyzed. The chemical and physical properties of nitrosamines, the parent amine, and the influence of the physical properties of the carbon sorbents on nitrosamine adsorption was examined. Key sorbent properties included the sorbent hydrophilicity and hydrophobicity, surface pKa of the sorbent, and chemical structure of the parent amine and nitrosamine.

Spray-Coated Multiwalled Carbon Nanotube Composite Electrodes for Thermal Energy Scavenging Electrochemical Cells.

Spray-coated multiwalled carbon nanotube/poly(vinylidene fluoride) (MWCNT/PVDF) composite electrodes, scCNTs, with varying CNT compositions (2 to 70 wt %) are presented for use in a simple thermal energy-scavenging cell (thermocell) based on the ferro/ferricyanide redox couple. Their utility for direct thermal-to-electrical energy conversion is explored at various temperature differentials and cell orientations. Performance is compared to that of buckypaper, a 100% CNT sheet material used as a benchmark electrode in thermocell research. The 30 to 70 wt % scCNT composites give the highest power output by electrode area-seven times greater than buckypaper at ΔT = 50 °C. CNT utilization is drastically enhanced in our electrodes, reaching 1 W gCNT(-1) compared to 0.036 W gCNT(-1) for buckypaper. Superior performance of our spray-coated electrodes is attributed to both wettability with better use of a large portion of electrochemically active CNTs and minimization of ohmic and thermal contact resistances. Even composites with as low as 2 wt % CNTs are still competitive with prior art. The MWCNT/PVDF composites developed herein are inexpensive, scalable, and serve a general need for CNT electrode optimization in next-generation devices.

Carbonic anhydrase mimics for enhanced CO2 absorption in an amine-based capture solvent.

Two new small-molecule enzyme mimics of carbonic anhydrase were prepared and characterized. These complexes contain the salen-like ligand bis(hydroxyphenyl)phenanthroline. This ligand is similar to the salen-type ligands previously incorporated into carbonic anhydrase mimics but contains no hydrolyzable imine groups and therefore serves as a promising ligand scaffold for the synthesis of a more robust CO2 hydration catalyst. These homogeneous catalysts were investigated for CO2 hydration in concentrated primary amine solutions through which a dilute CO2 (14%) fluid stream was flowed and showed exceptional activity for increased CO2 absorption rates.

Enhanced Salt Removal in an Inverted Capacitive Deionization Cell Using Amine Modified Microporous Carbon Cathodes.

Microporous SpectraCarb carbon cloth was treated using nitric acid to enhance negative surface charges of COO(-) in a neutral solution. This acid-treated carbon was further modified by ethylenediamine to attach -NH2 surface functional groups, resulting in positive surface charges of -NH3(+) via pronation in a neutral solution. Through multiple characterizations, in comparison to pristine SpectraCarb carbon, amine-treated SpectraCarb carbon displays a decreased potential of zero charge but an increased point of zero charge, which is opposed to the effect obtained for acid-treated SpectraCarb carbon. An inverted capacitive deionization cell was constructed using amine-treated cathodes and acid-treated anodes, where the cathode is the negatively polarized electrode and the anode is the positively polarized electrode. Constant-voltage switching operation using NaCl solution showed that the salt removal capacity was approximately 5.3 mg g(-1) at a maximum working voltage of 1.1/0 V, which is an expansion in both the salt capacity and potential window from previous i-CDI results demonstrated for carbon xerogel materials. This improved performance is accounted for by the enlarged cathodic working voltage window through ethylenediamine-derived functional groups, and the enhanced microporosity of the SpectraCarb electrodes for salt adsorption. These results expand the use of i-CDI for efficient desalination applications.

Continuous operation of membrane capacitive deionization cells assembled with dissimilar potential of zero charge electrode pairs.

The performance of single stack membrane assisted capacitive deionization cells configured with pristine and nitric acid oxidized Zorflex (ZX) electrode pairs was evaluated. The potentials of zero charge for the pristine and oxidized electrodes were respectively -0.2V and 0.2V vs. SCE. Four cell combinations of the electrodes including a pristine anode-pristine cathode, oxidized anode-pristine cathode, pristine anode-oxidized cathode, and oxidized anode-oxidized cathode were investigated. When the PZC was located within the polarization window of the electrode, diminished performance was observed. The cells were operated at 1.2 V and based on potential distribution results, the effective working potentials were ∼0.9, 0.8, 1.2, and 1.1 V for the pristine anode-pristine cathode, oxidized anode-pristine cathode, pristine anode-oxidized cathode, and oxidized anode-oxidized cathode cells, respectively. The highest electrosorption capacity of 17 mg NaCl/g ZX was observed for the pristine anode-oxidized cathode cell, where both PZCs were outside of the polarization window.

Asymmetric electrode configuration for enhanced membrane capacitive deionization.

Long-term performance of capacitive deionization (CDI) and membrane-assisted capacitive deionization (MCDI) single cells equipped with the same pristine carbon xerogel (CX) electrodes configured as the anode and cathode was investigated. Unlike CDI, which was subject to performance degradation in a short period of time, MCDI showed performance preservation during the 50 h of operation due to its ability to mitigate charge leakage from parasitic electrochemical reactions that result in carbon oxidation. Differential capacitance measurements of the used CDI and MCDI electrodes revealed shifting of the potential of zero charge (EPZC) of the CDI anode from -0.1 to 0.4 V but only to 0.1 V for the MCDI anode. CDI and MCDI cells tested with electrodes having EPZCs at -0.1 and 0.5 V showed strongly contrasting results depending on the anode-cathode EPZC configuration. The MCDI cell configured with a 0.5 V EPZC cathode and -0.1 V EPZC anode displayed the best performance of all the tested cells, benefiting from increased counterion excess within the potential window, and the membrane was in-place to reject expelled co-ions from accessing the bulk.

Morpholine nitrosation to better understand potential solvent based CO2 capture process reactions.

The amine assisted CO2 capture process from coal fired power plants strives for the determination of degradation components and its consequences. Among them, nitrosamine formation and their emissions are of particular concern due to their environmental and health effects. The experiments were conducted using morpholine as a representative secondary amine as a potential CO2 capture solvent with 100 ppm standard NO2 gas to better understand the nitrosamine reaction pathways under scrubber and stripper conditions. The role of nitrite in the nitrosation reaction was probed at elevated temperatures. The effects of different concentrations of nitrite on morpholine were evaluated. Formation rate, decomposition rates, activation energy, and the possible reaction pathways are elaborated. Thermal stability tests at 135 °C indicated the decomposition of nitrosamines at the rate of 1 µg/(g h) with activation energy of 131 kJ/mol. The activation energy for the reaction of morpholine with sodium nitrite was found as 101 kJ/mol. Different reaction pathways were noted for lower temperature reactions with NO2 gas and higher temperature reactions with nitrite.

Rapid analysis of PAHs in fly ash using thermal desorption and fast GC-TOF-MS.

Polycyclic aromatic hydrocarbons (PAHs) are semivolatile organic compounds that may form as a result of incomplete combustion of organic materials. After they are produced in combustion systems, this class of chemicals can be emitted with flue gas or adsorbed in combustion residues such as fly ash and bed ash. The purpose of this study is to develop a thermal extraction (TE) method for the determination of the 16 U.S. Environmental Protection Agency specified priority PAH pollutants in fly ash. The commonly used method for determining PAHs in solid wastes is solvent extraction followed by gas chromatography (GC) or GC-mass spectrometry (MS) analysis. This method is work- and time-intensive and produces solvent waste. In this study, the samples are analyzed using TE and fast GC-time-of-flight (TOF)-MS. The complete process from extraction to analysis can be achieved in less than one hour. The results indicate that the TE-GC-TOF-MS method has good linear range from 1.5 to 60 micro g/g for all 16 PAHs. The recoveries for the 16 target PAHs vary between 83% and 94%.