Simulated Flue Gas Stream Research Articles

Development of dual strain microalgae cultivation system for the direct carbon dioxide utilization of power plant flue gas

This study aims to propose a biological system that allows for direct utilization of flue gas for carbon dioxide capture and utilization by microalgae. The strain Chlorella sp. ABC-001 is employed for its high growth rate as well as lipid and carbohydrate content. Toxicity tests showed that cell growth was unaffected by NO, but the presence of SO2 showed critical damage on cell growth. Hence, an extremophile alga, Galdieria sulphuraria 5587.1 was applied to build a dual-strain cultivation system to mitigate the effect of SO2 toxicity and increase CO2 capture efficiency. All SO2 was removed by Galdieria culture and the system exhibited stable growth from a simulated flue gas stream containing CO2, NO and SO2. Combined CO2 biofixation rate of 793 mg/L/d and lipid productivity of 113 mg/L/d was achieved. The results showed that this new cultivation system is a promising alternative for reducing CO2 emissions from power plants.

Impedance-Based Detection of NO2 Using Ni-MOF-74: Influence of Competitive Gas Adsorption.

Chemically robust, low-power sensors are needed for the direct electrical detection of toxic gases. Metal-organic frameworks (MOFs) offer exceptional chemical and structural tunability to meet this challenge, though further understanding is needed regarding how coadsorbed gases influence or interfere with the electrical response. To probe the influence of competitive gases on trace NO2 detection in a simulated flue gas stream, a combined structure-property study integrating synchrotron powder diffraction and pair distribution function analyses was undertaken, to elucidate how structural changes associated with gas binding inside Ni-MOF-74 pores correlate with the electrical response from Ni-MOF-74-based sensors. Data were evaluated for 16 gas combinations of N2, NO2, SO2, CO2, and H2O at 50 °C. Fourier difference maps from a rigid-body Rietveld analysis showed that additional electron density localized around the Ni-MOF-74 lattice correlated with large decreases in Ni-MOF-74 film resistance of up to a factor of 6 × 103, observed only when NO2 was present. These changes in resistance were significantly amplified by the presence of competing gases, except for CO2. Without NO2, H2O rapidly (<120 s) produced small (1-3×) decreases in resistance, though this effect could be differentiated from the slower adsorption of NO2 by the evaluation of the MOF's capacitance. Furthermore, samples exposed to H2O displayed a significant shift in lattice parameters toward a larger lattice and more diffuse charge density in the MOF pore. Evaluating the Ni-MOF-74 impedance in real time, NO2 adsorption was associated with two electrically distinct processes, the faster of which was inhibited by competitive adsorption of CO2. Together, this work points to the unique interaction of NO2 and other specific gases (e.g., H2O, SO2) with the MOF's surface, leading to orders of magnitude decrease in MOF resistance and enhanced NO2 detection. Understanding and leveraging these coadsorbed gases will further improve the gas detection properties of MOF materials.

Life Cycle Assessment of Post-Combustion CO2 Capture and Recovery by Hydrophobic Polypropylene Cross-Flow Hollow Fiber Membrane Contactors with Activated Methyldiethanolamine

The present study evaluated the environmental impacts of post-combustion CO2 capture and recovery via membrane–gas absorption processes. We have used SimaPro v.9 packages with the Ecoinvent v3.5 database employing two different methods, ReCiPe 2016 Endpoint (H) and Midpoint (H), considering a fundamental methodological framework to determine the most environmentally friendly experimental condition. Life cycle impact categories were examined and assessed supposing a functional unit of 1 kgCO2/h recovered. Fourteen environmental impact categories including global warming, ozone depletion, eutrophication, and toxicity potentials have been evaluated within the context of a gate-to-gate approach focusing on only the process stage. Simulation results showed that the maximum liquid flow rate, sweep helium flow rate together with the minimum solvent concentration demonstrated the highest impact on human health, ecosystem, and resources. The usage of pure methyldiethanolamine (MDEA) activated by piperazine as a reactive absorbent provided the lowest environmental impact due to the elimination of the energy needed to heat and evaporate water present in aqueous absorbent solutions and the prevention of the excess water consumption depending on meeting the water needed for reactive absorption of CO2 in tertiary amine MDEA from simulated humidified flue gas stream. The study highlights the importance of LCA in the determination of an environmentally more sustainable condition during the capture and recovery of post-combustion CO2 by gas absorption and stripping using membrane contactors in tertiary amine MDEA.

A Pressure Swing Approach to Selective CO2 Sequestration Using Functionalized Hypercrosslinked Polymers.

Functionalized hypercrosslinked polymers (HCPs) with surface areas between 213 and 1124 m2/g based on a range of monomers containing different chemical moieties were evaluated for CO2 capture using a pressure swing adsorption (PSA) methodology under humid conditions and elevated temperatures. The networks demonstrated rapid CO2 uptake reaching maximum uptakes in under 60 s. The most promising networks demonstrating the best selectivity and highest uptakes were applied to a pressure swing setup using simulated flue gas streams. The carbazole, triphenylmethanol and triphenylamine networks were found to be capable of converting a dilute CO2 stream (>20%) into a concentrated stream (>85%) after only two pressure swing cycles from 20 bar (adsorption) to 1 bar (desorption). This work demonstrates the ease with which readily synthesized functional porous materials can be successfully applied to a pressure swing methodology and used to separate CO2 from N2 from industrially applicable simulated gas streams under more realistic conditions.

Immobilization of a Polar Sulfone Moiety onto the Pore Surface of a Humid-Stable MOF for Highly Efficient CO2 Separation under Dry and Wet Environments through Direct CO2-Sulfone Interactions.

The stability of microporous metal-organic frameworks (MOFs) in moist environments must be taken into consideration for their practical implementations, which has been largely ignored thus far. Herein, we synthesized a new moisture-stable Zn-MOF, {[Zn2(SDB)2(L)2]·2DMA}n, IITKGP-12, by utilizing a bent organic linker 4,4'-sulfonyldibenzoic acid (H2SDB) containing a polar sulfone group (-SO2) and a N, N-donor spacer (L) with a Brunauer-Emmett-Teller surface area of 216 m2 g-1. This material displays greater CO2 adsorption capacity over N2 and CH4 with high IAST selectivity, which is also validated by breakthrough experiments with longer breakthrough times for CO2. Most importantly, the separation performance is largely unaffected in the presence of moisture of simulated flue gas stream. Temperature-programmed desorption (TPD) analysis shows the ease of the regeneration process, and the performance was verified for multiple cycles. In order to understand the structure-function relationship at the atomistic level, grand canonical Monte Carlo (GCMC) calculation was performed, indicating that the primary binding site for CO2 is between the sulfone moieties in IITKGP-12. CO2 is attracted to the bonded structure (V-shape) of the sulfone moieties in a perpendicular fashion, where CCO2 is aligned with S, and the CO2 axis bisects the SO2 axis. Thus, the strategic approach to immobilize the polar sulfone moiety with a high number of inherent stronger M-N coordination and the absence of coordination unsaturation made this MOF potential toward practical CO2 separation applications.

Effect of Extended Aging and Oxidation on Linear Poly(propylenimine)-Mesoporous Silica Composites for CO2 Capture from Simulated Air and Flue Gas Streams.

Physical aging or degradation of amine-containing polymers and supported amine adsorbents is a critical issue that could limit the practical application of such materials for CO2 capture. However, to date, there is a scarcity of studies that evaluate the long-term stability of amine-based sorbents without the exclusive use of accelerated aging tests. Here, we demonstrate that extended aging (∼2 years) of linear poly(propylenimine) (LPPI) confined in mesoporous silica (SBA-15) supports does not drastically impact the CO2 adsorption performance under simulated flue gas (10% CO2) and direct air capture (DAC, 400 ppm CO2) conditions, although the behavior of the aged sorbents and polymers in the two CO2 concentration regimes differs. The sorbents made with aged LPPI have modestly decreased CO2 uptake performance (≲20% lower) compared to the fresh polymers, with overall good CO2 cycling performance. The data indicate that only slow degradation occurs under the deployed ambient storage conditions. Even after extended aging, the LPPI-based sorbents preserved their ability to display stable temperature-swing cycling performance. In parallel, the impact of blending LPPI polymers of different number-average molecular weights, Mn, is evaluated, seeking to understand its impact on adsorbent performance. The results demonstrate that the blends of two Mn aged LPPI give similar CO2 adsorption performance to adsorbents made from a single-Mn LPPI, suggesting that molecular weight will not negatively impact adsorbent performance in the studied Mn range. After an accelerated oxidation experiment, the aged LPPI sorbents retained a larger portion of the samples' original performance when cycling under simulated flue gas conditions than under DAC conditions. However, in each case, the oxidized sorbents could be cycled repeatedly with consistent uptake performance. Overall, these first of their kind extended aging tests suggest that LPPI-based amine adsorbents offer promise for long-term, stable use in carbon capture applications.

Post-combustion CO2 capture and recovery by pure activated methyldiethanolamine in crossflow membrane contactors having coated hollow fibers

Gas absorption and stripping using membrane contactors is one of a number of carbon capture and storage technologies being investigated for separating CO2 from flue gas. Energy consumption in CO2 absorption-stripping employing amine-containing aqueous absorbent solutions is strongly influenced by demanding stripping conditions involving higher temperatures. The utility of using pure methyldiethanolamine (MDEA) activated by piperazine as a reactive absorbent was studied using a compact hollow fiber device for the absorber as well as the stripper. Water needed for reactive absorption of CO2 in tertiary amine MDEA was obtained from simulated humidified flue gas stream which is saturated with moisture in actual practice. This study investigated also the absorption and stripping performances of aqueous absorbent solutions containing 80% and 90% activated MDEA (aMDEA). Considerable CO2 stripping from CO2-loaded pure aMDEA absorbent was achieved at 92 °C while absorption was carried out at 25–46 °C. Further, the CO2 stripping rate was far higher for pure MDEA compared to the other two aMDEA solutions with water. Results are reported for the performance of the absorption-stripping process as a function of humidified simulated flue gas flow rate and the absorbent flow rate. High values of the overall mass transfer coefficients (MTCs) have also been reported for absorption. The highest volumetric gas phase based overall MTC obtained was 0.504 sec−1. This system has a much lower absorbent circulation load, eliminates the energy needed to heat and evaporate water present in aqueous absorbent solutions and benefits from the absence of excess water during stripping.

Sequential Oxygen Reduction and Adsorption for Carbon Dioxide Purification for Flue Gas Applications

Sequestration or utilization of carbon dioxide (CO2) produced from fossil fuels depends on the ability to transform flue gas into purified streams. Recent developments in oxy‐combustion have improved the efficiency of energy generation and carbon capture (&gt;90% CO2). The subsequent removal of oxygen (O2) from this flue gas is critical, but such a process is energy intensive, technologically challenging, and unsolved. Herein, a simulated flue gas stream is purified by the catalytic conversion of oxygen using methane (CH4). The supported palladium (Pd) catalyst selectively reduces oxygen to an effluent gas with 99.7% CO2 and 0.3% O2. Using a higher Pd loading has no impact on the oxygen conversion, whereas feeding excess CH4 decreases the selectivity to CO2. A complete removal of O2 is achieved using a copper‐based oxygen scavenger placed after the Pd catalyst bed, yielding a 100.0% pure CO2 stream.

Heterogeneous Fenton catalysts prepared from modified-fly ash for NOx removal with H2O2

Heterogeneous Fenton catalysts were developed from fly ash by alkali, acid and alkali-acid modification for NOx removal using H2O2.The experimental results showed that the catalyst derived from alkali-acid modification achieved 90% NOx removal efficiency at 140 °C, 0.07 mg/min of H2O2 mass flow rate (2 mol/L H2O2)and 250 mL/min of simulated flue gas stream containing 500 ppm NO. Catalyst characterization revealed that the hematite used contained surface hydroxyl groups, Fe2+, oxygen vacancies and FeOAl, which were considered beneficial to the generation of hydroxyl radicals. This work illustrates potential practical use of fly ash.

Simultaneous absorption of carbon dioxide and nitrogen dioxide from simulated flue gas stream using gas-liquid membrane contacting system

Membrane gas contactors are a promising alternative to conventional post-combustion carbon capture technologies. However, residuals of the other acid gas compounds can exist in the flue gas streams emitted from industrial facilities, having a notable impact on the absorption performance of the membrane system. Simultaneous removal of CO2 and NO2 from a simulated flue gas stream was carried out in a polytetrafluoroethylene (PTFE) hollow fiber gas-liquid membrane contacting (GLMC) system using different scrubbing solutions. A series of experiments were conducted to study the effects of operating conditions such as gas and liquid cross flow velocities, concentration of feed gas, absorbent nature and concentration, and long-term performance of the GLMC system on the removal efficiencies as well as mass transfer rates of CO2 and NO2. Experimental results indicated that simultaneous absorption of CO2 and NO2 were enhanced with increasing the liquid-phase cross flow velocity, decreasing gas-phase cross flow velocity, and using chemical stripping absorbents. Moreover, it was shown the sodium hydroxide to be a superior absorbent as compared to alkanolamine solutions for the co-capture of CO2 and NO2 species. It was observed that low concentrations of NO2 in the feed gas had a minimal impact on the decarbonization of GLMC system. The durability of the membrane system was also evaluated by running the simultaneous gas removal experiments over a 24-h period. The consistency of the absorption efficiency results confirmed the potential of using PTFE membrane system for the simultaneous absorption of CO2 and NO2 gases.

Nanostructured MgO Sorbents Derived from Organometallic Magnesium Precursors for Post-combustion CO2 Capture

Nanostructured MgO sorbents show promise for intermediate-temperature CO2 capture from post-combustion flue gas stream. However, their CO2 capture behaviors can be affected by the magnesium precursors applied for sorbent synthesis. To screen potential precursors for fabricating excellent CO2 trappers, MgO nanoparticles (NPs) were synthesized by calcining several organometallic precursors, including magnesium ethoxide, magnesium acetate tetrahydrate, magnesium oxalate dehydrate, magnesium lactate dihydrate, magnesium citrate nonahydrate, and magnesium gluconate hydrate. The precursors and MgO NPs were characterized by thermogravimetric analysis, X-ray diffraction, N2 adsorption–desorption, field emission scanning electron microscopy, and CO2 temperature-programmed desorption. A fixed-bed reactor was used to evaluate the CO2 capture performances of the MgO NPs in a simulated flue gas stream. The effect of organometallic precursors on CO2 capture behaviors of the sorbents was further demonstrated. Results in...

Facile synthesis of silica aerogel supported K2CO3 sorbents with enhanced CO2 capture capacity for ultra-dilute flue gas treatment

Silica aerogel supported K2CO3 sorbents with different K2CO3 loadings were synthesized by sol-gel and wet impregnation processes. The sorbents were characterized by N2 adsorption-desorption, X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM) techniques. CO2 capture performances of the sorbents were tested in a simulated ultra-dilute flue gas stream of 1.0%CO2 and 2.0%H2O at 20 °C, using a fixed-bed reactor coupled with an online infrared gas analyzer. Sorbent regeneration performances of the samples were evaluated in a pure N2 atmosphere at 200 °C with a heating rate of 10 °C/min. CO2 capture capacity and K2CO3 utilization efficiency increased first and then decreased with the increase in K2CO3 loading. The desired sorbent with 20 wt% K2CO3 was screened for investigating the effects of acid impurities. The presence of 500 ppm SO2 and 500 ppm NO in the simulated flue gas stream was found to adversely affect the CO2 capture, sorbent regeneration and multiple cyclic performances. Detailed mechanisms of the irreversible sorbent deactivation process were discussed. SO2 could be chemically absorbed by K2CO3 under a moist condition to form byproduct of K2SO3·H2O, and the byproduct was stable and would be accumulated in the sorbent during the repeated cycles. This further attenuated the physical properties and K2CO3 utilization efficiency of the sorbent. These results would lay a solid foundation for further application of the sorbent in ultra-dilute flue gas treatment.

Potential of Chlorella vulgaris to Abate Flue Gas

The likely use of continuous cultures of Chlorella vulgaris for flue gas abatement was studied. Firstly, no pH-controlled photo-bioreactors were operated in order to understand the tolerance of this microalgae to high CO2 concentrations in the airstream. Thus, the effect in biomass productivity, CO2 fixation and biochemical composition of different percentage of pure CO2 (ranging 1–12%) in the air supply was investigated. When the inlet CO2 concentration varied from 1 to 10%, no statistical differences were found (ANOVA, p < 0.05) in the rate of carbon assimilation (0.6–0.8 g L−1 d). In all the cases, biomass presented a high content both proteins and lipids (40 and 25% respectively). However, when cultures were supplied with 12% of pure CO2 in the airstream, pH drastically dropped and cultures were not viable. Next, the potential use of CO2 contained in a simulated flue gas as a unique source of carbon was evaluated. Thus a mix of gases mimicking those presented in an exhausted stream of a power plant was used to aerate constantly the cultures. In this condition, cultures were only viable either when the simulated flue gas stream was diluted twelve times with air (resulting a constant supply of 1% CO2 in the airstream) or no diluted but being used by pulse to control the pH of the culture. In both cases, cultures achieved a steady state, rendering 0.7 and 0.9 g CO2 assimilated L−1 d−1 respectively. Biomass presented high content of proteins and lipids (40% respectively) in both conditions. These results suggest that the use of exhausted gases can make more economically feasible the production of microalgae and generate a valuable biomass rich in proteins and lipids.

(Invited) Electrochemical Membrane Technology for Carbon Dioxide Capture from Flue Gas

To address the concerns about climate change resulting from emission of CO2 by power plants and industrial facilities, FuelCell Energy, Inc. (FCE) has developedthe Combined Electric Power and Carbon-dioxide Separation (CEPACS) system. The CEPACS system utilizes Electrochemical Membrane (ECM) technology derived from the Company’s molten carbonate fuel cell-based Direct FuelCell® products. The system separates CO2 from the flue gas of other plants and produces electric power using a supplementary fuel. FCE is currently evaluating the use of ECM to cost effectively separate CO2 from the flue gas of Pulverized Coal (PC) power plants under cooperative agreements with the U.S. Department of Energy (DOE). The overarching objective of the current project is to verify that the ECM can achieve at least 90% CO2 capture from the flue gas with a cost of electricity (COE) 30% lower than baseline approaches. The objectives will be achieved through pilot-scale testing of a MW-class CEPACS system using a slipstream of flue gas from a PC plant. The potential of ECM for separating more than 90% of CO2 from the flue gas of a PC plant has been experimentally verified. ECM materials tolerance levels to various contaminants in the flue gas including SO2, HCl, Hg, and SeO2 have been identified. The contaminant levels expected from the flue gas clean-up subsystem were also estimated and compared with the ECM tolerance levels. The analysis showed that the ECM tolerance levels are well above the contaminant levels expected in the ECM cathode feed gas. Ability of ECM to destroy NOx components in the flue gas was confirmed in the laboratory tests. Testing showed that 70-80% of all NOx was destroyed at high inlet NOx concentrations of over 200 ppm. This significant finding may result in further reduction of the incremental COE associated with the CEPACS system, if credits are given to elimination of the selective catalytic reduction process step for NOx control in the PC plant. The Technical and Economic Feasibility studies were conducted, including assessments of performance and cost of an ECM-based CO2 capture system for retrofitting a 550 MW PC plant with 90% capture. The CEPACS system generates 421 MW of additional power after compensating for the auxiliary power requirements of CO2 capture and compression. The economic analysis included estimation of the CEPACS plant cost and the incremental COE for ECM-based CO2 capture. The CEPACS technology has the potential to meet DOE’s goal of ≤35% increase in COE for post-combustion CO2 capture. The CEPACS system can also meet the DOE’s target cost of <$40/tonne CO2 captured. A bench-scale ECM-based CO2 capture system was designed and fabricated. The objective of the demonstration is to show the capability of full-size ECM cells to separate >90% of CO2 from a simulated PC plant flue gas stream during extended operation. The system utilizes an ECM stack containing 14 full-area cells with a total electrochemical membrane area of 11.7 m2. Peak gross DC output is ~10 kW, with capability for high-purity liquid CO2 production of ~100 tons/year. The stack has operated for more than 10,000 hours and has demonstrated very stable electric power generation while separating 93% of carbon from flue gas at a constant CO2 flux of 116 cc/s/m2. In this presentation, the status of the ECM technology and its application for carbon capture will be reviewed. The results of trace contaminants experiments, characterization of ECM tolerance to key flue gas contaminants, ECM technology NOx removal capabilities, bench scale full-area ECM stack testing, and the techno-economic studies of CEPACS systems will be discussed. Plans for pilot-scale testing of the technology will as part of a recently-awarded DOE cooperative agreement will also be reviewed. Finally, the benefits and commercialization activities related to the ECM technology will be presented.

Amino-impregnated MOF materials for CO2 capture at post-combustion conditions

Three different commercial Metal-Organic Frameworks (HKUST-1, MIL-53(Al) and ZIF-8) were modified by wetness impregnation of tetraethylenepentamine (TEPA) molecules. These amino-containing MOF materials were studied in CO2 adsorption under post-combustion conditions (45°C and 1bar) using a simulated exhausted flue gas stream of power plants (75% inert gas; 5% O2; 14% CO2; 6% H2O; % in vol.). The yield of TEPA incorporation was significantly affected by the physicochemical properties of the MOF precursors. The presence of reactive sites of HKUST-1 (unsaturated copper sites) or MIL 53(Al) (exposed hydroxyl bridges (µ2-OH)) as well as their worst textural properties as compared to ZIF-8,led to lower TEPA loadings and porosities of the resultant amino-impregnated HKUST-1 and MIL 53(Al) materials. The ZIF-8 material showed a better distribution of TEPA since reactive surface sites are not present in this support with a higher superficial development. Amino-impregnated ZIF-8 samples exhibited enhanced adsorption capacities by combination of physical and chemical adsorption of CO2 over the TEPA-impregnated microporous framework. The increase of the TEPA content in the ZIF-8 material led to CO2 uptake up to 67mgCO2/gads using pure CO2 at 0.15bar and 45°C. Under post-combustion CO2 capture conditions, TEPA-impregnated ZIF-8 samples underwent a remarkable increase of the carbon dioxide uptake up to 104mgCO2/gads, due to the presence of moisture, which is responsible of increasing the CO2 capture efficiency of amino groups. After several adsorption/desorption cycles at post-combustion adsorption conditions, TEPA-containing ZIF-8 samples have shown a high structural stability, maintaining the CO2 uptake practically constant.

Mercury Interaction with the Fine Fraction of Coal-Combustion Fly Ash in a Simulated Coal Power Plant Flue Gas Stream

Mercury associated with fly ash is a significant contaminant released in flue gas emissions from coal-fired power plants. This work focuses on the association of Hg with other elements and phases as well as the molecular-level speciation of Hg in bulk and <0.1 μm sized fly ash particles reacted with a Hg-containing simulated flue gas stream. Following reaction under conditions chosen to simulate an electrostatic precipitator operating at 140 °C, fly ash (bulk and ≤0.1 μm) from a Kentucky power plant was analyzed using microscopic and spectroscopic techniques. The ≤0.1 μm fraction dominates Hg uptake, with total Hg concentrations increasing from 100 ppb to 610 ppm after reaction, whereas bulk ash concentrations increase from 11 to 164 ppb. Synchrotron-based micro-X-ray fluorescence mapping of the reacted ≤0.1 μm fraction showed that Hg is present in two major regions: Fe-rich areas and Hg hot spots not associated with Fe. X-ray absorption spectroscopic analysis revealed that Hg is associated with Br and Cl...

Highly optimized CO2 capture by inexpensive nanoporous covalent organic polymers and their amine composites.

Carbon dioxide (CO2) storage and utilization requires effective capture strategies that limit energy penalties. Polyethylenimine (PEI)-impregnated covalent organic polymers (COPs) with a high CO2 adsorption capacity are successfully prepared in this study. A low cost COP with a high specific surface area is suitable for PEI loading to achieve high CO2 adsorption, and the optimal PEI loading is 36 wt%. Though the adsorbed amount of CO2 on amine impregnated COPs slightly decreased with increasing adsorption temperature, CO2/N2 selectivity is significantly improved at higher temperatures. The adsorption of CO2 on the sorbent is very fast, and a sorption equilibrium (10% wt) was achieved within 5 min at 313 K under the flow of simulated flue gas streams. The CO2 capture efficiency of this sorbent is not affected under repetitive adsorption-desorption cycles. The highest CO2 capture capacity of 75 mg g(-1) at 0.15 bar is achieved under dry CO2 capture however it is enhanced to 100 mg g(-1) in the mixed gas flow containing humid 15% CO2. Sorbents were found to be thermally stable up to at least 200 °C. TGA and FTIR studies confirmed the loading of PEIs on COPs. This sorbent with high and fast CO2 sorption exhibits a very promising application in direct CO2 capture from flue gas.

On the Kinetics of the Absorption of Nitric Oxide into Ammoniacal Cobalt(II) Solutions

Experiments were conducted using a custom double-stirred tank reactor to determine the rate constants of reactions between nitric oxide (NO) and both pentaaminecobalt(II) and hexaaminecobalt(II) at temperatures of 298.2 and 303.2 K and pH levels between 8.50 and 9.87 under atmospheric pressure. The NO concentration of simulated flue gas stream ranged from 400 to 1400 ppmv. Ammoniacal cobalt(II) solutions were prepared by adding aqueous ammonia into a cobalt(II) nitrate solution in the presence of concentrated ammonium nitrate. The reaction rate constants were calculated with an enhancement factor for gas absorption associated with parallel chemical reactions. The results showed that the reaction between NO and pentaaminecobalt(II) was first order with respect to both the NO and the pentaamminecobalt(II) ion. Similarly, the reaction between NO and hexaamminecobalt(II) was first order with respect to both the NO and the hexaamminecobalt(II) ion. The forward reaction rate constants of these two reactions were 6.43 × 10(6) and 1.00 × 10(7) L · mol(-1) · s(-1) at 298.2 K, respectively, and increased to 7.57 × 10(6) and 1.12 × 10(7) L · mol(-1) · s(-1) at 303.2 K, respectively. Ammoniacal cobalt(II) solutions also have the potential to simultaneously remove CO2, SO2, and NOx from postcombustion flue gas.

Facile Preparation of Nitrogen-Doped Activated Carbon for Carbon Dioxide Adsorption

Nitrogen-doped activated carbons with high surface areas obtained from resorcinol and formaldehyde resins were evaluated as CO2 adsorbents in a simulated flue gas stream under anhydrous and humid conditions. These carbons were prepared using two approaches, namely ammonia treatment without nitric acid pre-oxidation and amination after pre-oxidation. The pre-oxidation of activated carbons considerably enhanced the nitrogen incorporation during the amination process. The amination temperature affects the content and type of nitrogen incorporated onto the carbon surface, as determined by X-ray photoelectron spectroscopy, which enhances the specific adsorbent-adsorbate interaction for CO2 in humid conditions. The presence of H2O in the feed gas significantly decreased CO2 adsorption for a very low nitrogen content of virgin activated carbon. A sample prepared via the amination of pre-oxidized carbon at 700°C (NORF700) exhibited excellent tolerance to moisture and the highest CO2 capacity of 2.10 mmol/g in a 7% CO2/83% N2/10% H2O wet stream at 50°C and 130 kPa. The high performance of NORF700 was ascribed to its high surface area, adequate micropore volume, and high amounts of pyrindinic-like and pyrrole-like nitrogen species. The results indicate that nitric acid pre-oxidation followed by ammonia treatment at 700°C is an appropriate process for preparing adsorbents for CO2 separation in post-combustion applications.

Electrochemical CO2 Capture Using Resin-Wafer Electrodeionization

Energy-efficient capture of CO2 from power-plant flue gas is one of the grand challenges to reduce greenhouse gas (GHG) emissions. Current CO2-capture technologies are limited by parasitic energy loss, inefficient capture, and unfavorable process economics. We present a novel electrochemical method for CO2 capture from coal-fired power-plant flue gas. The method utilizes in-situ electrochemical pH control with a resin wafer electrodeionization (RW-EDI) device that continuously shifts the pH of the process fluid between basic and acidic in sequential chambers (pH swing). This pH swing enables capture of CO2 from flue gas in the basic chamber followed by release (recovery) of the captured CO2 (purified) in the acidic chamber of the same device. The approach is based on the sensitivity of the thermodynamic equilibrium of CO2 hydration/dehydration reactions over a narrow pH range. The method enables simultaneous absorption (capture) of CO2 from flue gas and desorption (release) at atmospheric pressure without heating, vacuum, or consumptive chemical usage. In other words, the method concentrates CO2 from ∼15% in flue gas to >98% in the recovery stream. To the best of our knowledge, this is the first experimental study focusing on simultaneous capture and release (recovery) of CO2 using an electrochemical method. We describe the method, the role of operating parameters on CO2 recovery, and advancements in process design and engineering for improved efficiency. We report on a method to enhance gas/liquid mixing inside the RW-EDI, which significantly increased CO2 capture rates. We also discuss the importance of using an enzyme/catalyst in enhancing the reaction kinetics. CO2 capture was observed to be a strong function of gas and liquid flow rates and applied electrical field. Up to 80% of the CO2 was captured from a simulated flue gas stream with >98% purity. The results indicate that a narrow pH swing from 8 to 6 (near-neutral pH) could offer a viable pathway for energy-efficient CO2 capture if the reaction kinetics are enhanced. Carbonic anhydrase enzyme enhances the reaction kinetics at near-neutral pH; however, the enzyme lost activity due to the instability at the operating conditions. This observation highlighted the necessity of robust enzymes/catalysts to enhance kinetics of CO2 recovery near-neutral pH.