Shihan Zhang

Activity and stability of immobilized carbonic anhydrase for promoting CO2 absorption into a carbonate solution for post-combustion CO2 capture

An Integrated Vacuum Carbonate Absorption Process (IVCAP) currently under development could significantly reduce the energy consumed when capturing CO2 from the flue gases of coal-fired power plants. The biocatalyst carbonic anhydrase (CA) has been found to effectively promote the absorption of CO2 into the potassium carbonate solution that would be used in the IVCAP. Two CA enzymes were immobilized onto three selected support materials having different pore structures. The thermal stability of the immobilized CA enzymes was significantly greater than their free counterparts. For example, the immobilized enzymes retained at least 60% of their initial activities after 90 days at 50 °C compared to about 30% for their free counterparts under the same conditions. The immobilized CA also had significantly improved resistance to concentrations of sulfate (0.4 M), nitrate (0.05 M) and chloride (0.3 M) typically found in flue gas scrubbing liquids than their free counterparts.

Enhanced Stability and Chemical Resistance of a New Nanoscale Biocatalyst for Accelerating CO2 Absorption into a Carbonate Solution

A novel potassium-carbonate-based absorption process is currently being developed to reduce the energy consumption when capturing CO2 from coal combustion flue gas. The process employs the enzyme carbonic anhydrase (CA) as a catalyst to accelerate the rate of CO2 absorption. This study focused on the immobilization of a new variant of the CA enzyme onto a new group of nonporous nanoparticles to improve the enzymes thermal stability and its chemical resistance to major impurities from the flue gas. The CA enzyme was manufactured at the pilot scale by a leading enzyme company. As carrier materials, two different batches of SiO2-ZrO2 composite nanoparticles and one batch of silica nanoparticle were synthesized using a flame spray pyrolysis method. Classic Danckwerts absorption theory with reaction was applied to determine the kinetics of the immobilized enzymes for CO2 absorption. The immobilized enzymes retained 56-88% of their original activity in a K2CO3/KHCO3 solution over a 60-day test period at 50 °C, compared with a 30% activity retention for their free CA enzyme counterpart. The immobilized CA enzymes also revealed improved chemical stability. The inactivation kinetics of the free and immobilized CA enzymes in the K2CO3/KHCO3 solution were experimentally quantified.

A new approach for Fe(III)EDTA reduction in NOx scrubber solution using bio-electro reactor

A new process for the removal of NO(x) by a combined Fe(II)EDTA absorption and microbial reduction has been demonstrated, in which part of the Fe(II)EDTA will be oxidized by oxygen in the flue gas to form Fe(III)EDTA. In former studies, strain FR-2 has been found to reduce Fe(III)EDTA efficiently. Otherwise, it has been reported that bio-electro reactor could efficiently provide a chance for simultaneous denitrification and metal ion removal. Therefore, a use of bio-electro reactor is suggested to promote the reduction of Fe(III)EDTA by strain FR-2 in this paper. The results showed that the concentration of Fe(III)EDTA decreased rapidly when electric current was applied, and that as the current density rose, the Fe(III)EDTA reduction rate increased while followed by a decrease afterward. The formation of the biofilm on the electrode was observed by ESEM (Environmental Scan Electro-Microscope). In addition, the Fe(III)EDTA reduction rate obviously decreased with the existence of NaNO(2).

Current advances of integrated processes combining chemical absorption and biological reduction for NO x removal from flue gas.

Anthropogenic nitrogen oxides (NOx) emitted from the fossil-fuel-fired power plants cause adverse environmental issues such as acid rain, urban ozone smoke, and photochemical smog. A novel chemical absorption–biological reduction (CABR) integrated process under development is regarded as a promising alternative to the conventional selective catalytic reduction processes for NOx removal from the flue gas because it is economic and environmentally friendly. CABR process employs ferrous ethylenediaminetetraacetate [Fe(II)EDTA] as a solvent to absorb the NOx following microbial denitrification of NOx to harmless nitrogen gas. Meanwhile, the absorbent Fe(II)EDTA is biologically regenerated to sustain the adequate NOx removal. Compared with conventional denitrification process, CABR not only enhances the mass transfer of NO from gas to liquid phase but also minimize the impact of oxygen on the microorganisms. This review provides the current advances of the development of the CABR process for NOx removal from the flue gas.

A green and robust solid catalyst facilitating the magnesium sulfite oxidation in the magnesia desulfurization process

Oxidation of magnesium sulfite is a crucial step in the wet magnesia desulfurization process. In this study, a green and robust solid catalyst, a SBA-15 (SBA, Santa Barbara Amorphous)-supported cobalt catalyst, was developed to promote the oxidation of sulfite. Both Co(III) and Co(II) served as the active sites for the oxidation of sulfite and were mainly located in the inner pore of the SBA-15. The lattice defects formed via the reduction of Co(III) to Co(II) and the dispersion of cobalt played an important role in the catalytic activity. Excess loadings of cobalt resulted in the blockage of the pore and hence decreased the catalytic activity of the catalyst. The catalytic mechanism of sulfite oxidation by this solid catalyst was also proposed. Sulfite was oxidized by the active site, such as Co(III) and Co(II), and the spent catalyst was in situ regenerated by oxygen. Therefore, the diffusion of oxygen into the pore is important to sustain the catalytic activity of the catalyst. Even after four cycles of catalyst reclamation, the catalytic activity was still comparable with that of the aqueous Co2+ counterpart, indicating that the catalyst is robust and efficient. In addition, no leakage of cobalt from the catalyst was detected, implying that it is a green catalyst without risks of secondary pollution. This study is expected to aid in the development of a green and robust catalyst for reclaiming the desulfurization byproduct and downsizing the absorber.

A Biophysicochemical Model for NO Removal by the Chemical Absorption–Biological Reduction Integrated Process

The chemical absorption-biological reduction (CABR) integrated process is regarded as a promising technology for NOx removal from flue gas. To advance the scale-up of the CABR process, a mathematic model based on mass transfer with reaction in the gas, liquid, and biofilm was developed to simulate and predict the NOx removal by the CABR system in a biotrickling filter. The developed model was validated by the experimental results and subsequently was used to predict the system performance under different operating conditions, such as NO and O2 concentration and gas and liquid flow rate. NO distribution in the gas phase along the biotrickling filter was also modeled and predicted. On the basis of the modeling results, the liquid flow rate and total iron concentration were optimized to achieve >90% NO removal efficiency. Furthermore, sensitivity analysis of the model revealed that the performance of the CABR process was controlled by the bioreduction activity of Fe(III)EDTA. This work will provide the guideline for the design and operation of the CABR process in the industrial application.

Bioelectrochemical Reduction of Fe(II)EDTA–NO in a Biofilm Electrode Reactor: Performance, Mechanism, and Kinetics

A biofilm electrode reactor (BER) is proposed to effectively regenerate Fe(II)EDTA, a solvent for NOx removal from flue gas, from Fe(II)EDTA-NO, a spent solution. In this study, the performance, mechanism, and kinetics of the bioelectrochemical reduction of Fe(II)EDTA-NO were investigated. The pathways of Fe(II)EDTA-NO reduction were investigated via determination of nitrogen element balance in the BER and an abiotic electrode reactor. The experimental results indicate that the chelated NO (Fe(II)EDTA-NO) is reduced to N2 with N2O as an intermediate. However, the oxidation of NO occurred in the absence of Fe(II)EDTA in abiotic reactors. Furthermore, the accumulation of N2O was suppressed with the help of electricity. The preponderant electron donor for reduction of Fe(II)EDTA-NO was also confirmed via analysis of the electron conservation. About 87% of Fe(II)EDTA-NO was reduced using Fe(II)EDTA as the electron donor in the presence of both glucose and cathode electrons while the cathode electrons were utilized for the reduction of Fe(III)EDTA to Fe(II)EDTA. Michaelis-Menten kinetic constants of bioelectrochemical reduction of Fe(II)EDTA-NO were also calculated. The maximum reduction rate of Fe(II)EDTA-NO was 13.04 mol m(-3) h(-1), which is 50% higher than that in a conventional biofilter.

Kinetics, Thermodynamics, and Mechanism of a Novel Biphasic Solvent for CO2 Capture from Flue Gas

The main issue related to the deployment of the amine-based absorption process for CO2 capture from flue gas is its intensive energy penalty. Therefore, this study screened a novel biphasic solvent, comprising a primary amine e.g., triethylenetetramine (TETA) and a tertiary amine e.g., N, N-dimethylcyclohexylamine (DMCA), to reduce the energy consumption. The TETA-DMCA blend exhibited high cyclic capacity of CO2 absorption, favorable phase separation behavior, and low regeneration heat. Kinetic analysis showed that the gas- and liquid-side mass transfer resistances were comparable in the lean solution of TETA-DMCA at 40 °C, whereas the liquid-side mass transfer resistance became dominant in the rich solution. The rate of CO2 absorption into TETA-DMCA (4 M, 1:3) solution was comparable to 5 M benchmark monoethanolamine (MEA) solution. Based on a preliminary estimation, the regeneration heat with TETA-DMCA could be reduced by approximately 40% compared with that of MEA. 13C NMR analysis revealed that the CO2 absorption into TETA-DMCA was initiated by the reaction between CO2 and TETA via the zwitterion mechanism, and DMCA served as a CO2 sinker to regenerate TETA, resulting in the transfer of DMCA from the upper to lower phase. The proposed TETA-DMCA solvent may be a suitable candidate for CO2 capture.

Inhibiting Mercury Re-emission and Enhancing Magnesia Recovery by Cobalt-Loaded Carbon Nanotubes in a Novel Magnesia Desulfurization Process

Mercury re-emission, because of the reduction of Hg2+ to form Hg0 by sulfite, has become a great concern in the desulfurization process. Lowering the concentrations of Hg2+ and sulfite in the desulfurization slurry can retard the Hg0 formation and, thus, mitigate mercury re-emission. To that end, cobalt-based carbon nanotubes (Co-CNTs) were developed for the simultaneous Hg2+ removal and sulfite oxidation in this work. Furthermore, the thermodynamics and kinetics of the Hg2+ adsorption and effect of Hg2+ adsorption on catalytic activity of Co-CNTs were investigated. Experimental results revealed that the Co-CNTs not only accelerated sulfite oxidation to enable the recovery of desulfurization by-products but also acted as an effective adsorbent of Hg2+ removal. The Hg2+ adsorption rate mainly depended on the structure of the adsorption material regardless of the cobalt loading and morphological distribution. The catalytic activity of the Co-CNTs for sulfite oxidation was not significantly affected due to the Hg2+ adsorption. Additionally, the isothermal adsorption behavior was well-fitted to the Langmuir model with an adsorption capacity of 166.7 mg/g. The mercury mass balance analysis revealed that the Hg0 re-emission was decreased by 156% by adding 2.0 g/L of Co-CNTs. These results can be used as a reference for the simultaneous removal of multiple pollutants in the wet-desulfurization process.

Pathway of FeEDTA transformation and its impact on performance of NOx removal in a chemical absorption-biological reduction integrated process

A novel chemical absorption-biological reduction (CABR) integrated process, employing ferrous ethylenediaminetetraacetate (Fe(II)EDTA) as a solvent, is deemed as a potential option for NOx removal from the flue gas. Previous work showed that the Fe(II)EDTA concentration was critical for the NOx removal in the CABR process. In this work, the pathway of FeEDTA (Fe(III)/Fe(II)-EDTA) transformation was investigated to assess its impact on the NOx removal in a biofilter. Experimental results revealed that the FeEDTA transformation involved iron precipitation and EDTA degradation. X-ray photoelectron spectroscopy analysis confirmed the iron was precipitated in the form of Fe(OH)3. The iron mass balance analysis showed 44.2% of the added iron was precipitated. The EDTA degradation facilitated the iron precipitation. Besides chemical oxidation, EDTA biodegradation occurred in the biofilter. The addition of extra EDTA helped recover the iron from the precipitation. The transformation of FeEDTA did not retard the NO removal. In addition, EDTA rather than the iron concentration determined the NO removal efficiency.