Alkaline Absorbent Research Articles

Electrolytic Regeneration of Spent Caustic Soda from CO2 Capture Systems

The traditional electrochemical caustic soda recovery system uses the generated pH gradient across the ion exchange membrane for the regeneration of spent alkaline absorbent from CO2 capture. This electrochemical CO2 capture system releases the by-products H2 and O2 at the cathode and anode, respectively. Although effective for capturing CO2, the slow kinetics of the oxygen evolution reaction (OER) limit the energy efficiency of this technique. Hence, this study proposed and validated a hybrid electrochemical cell based on the H2-cycling from the cathode to the anode to eliminate the reliance on anodic oxygen generation. The results show that our lab-scale prototype enabled effective spent caustic soda recovery with an electron utilisation efficiency of 90%, and a relative carbonate/bicarbonate diffusional flux of approximately 40%. The system also enabled the regeneration of spent alkaline absorbent with a minimum electrochemical energy input of 0.19 kWh/kg CO2 at a CO2 recovery rate of 0.7 mol/m2/h, accounting for 30% lower energy demand than a control system without H2-recycling, making this technique a promising alternative to the conventional thermal regeneration technology.

Integrated Absorption–Adsorption Process for Waste-Free Decontamination of Gases from Sulfur Dioxide, Part 2: CFD Modeling and Experimental Investigation of a Bubble-Cap Tray

There are many technologies for removal of sulfur dioxide (SO2) from flue gases. They are intrinsic part of the efforts for sustainability of energy production as they reduce the harmful impact of fossil fuel combustion on the environment by minimizing one of the main air pollutants. A wide range of methods use alkaline absorbents. In most cases, the products obtained from the absorption process have to undergo further oxidation, which increases the cost of carrying out the process. As a final result, the sulfates obtained (Na2SO4 and CaSO4) have limited practical application and there is a problem with their disposal. Scientific and engineering efforts have been directed towards the development of a practically waste-free technology for gas purification from SO2. An absorption–adsorption method is proposed, which comprises absorption of SO2 in water with simultaneous adsorption of the resultant sulfurous acid (H2SO3) from the aqueous solution with a synthetic anion-exchange resin. Regeneration of the adsorbent is accomplished with a dilute solution of ammonia (NH3), followed by decomposition of the resulting ammonium sulfite ((NH4)2SO3) with nitric acid (HNO3). The products of the processes are pure gaseous (liquefied) SO2 and an aqueous solution of ammonium nitrate (NH4NO3). Sulfur dioxide has a wide range of applications in the chemical industry; ammonium nitrate is a product with a variety of commercial uses as well, the most common of which is as a soil fertilizer. The new absorption–adsorption method offers a practically waste-free technology. The basic unit of this technology is a bubble-cap tray column where the absorption–adsorption process is carried out in an aqueous suspension of a synthetic anion-exchange resin. This work presents a CFD simulation of the flow on the bubble-cap tray. A physical model of the column is constructed, which contains a bubble-cap tray fabricated by 3D printing. As a result of this experimental study, new data on the tray pressure drop, gas holdup, and the kinetics of the absorption–adsorption process were obtained.

Hydrogen Depolarized Anode Assisted pH Shift Electrolysis: An Experimental Analysis

Prominent purification challenges such as carbon capture, lithium-ion recovery from aqueous solutions, and isolation of carboxylic acids from fermentation broths are often tackled by downstream process concepts that rely on pH gradients as the driving force for separation. For this purpose, the sole use of acids and bases is only tolerable if the jointly produced co-salts are in demand on the market or are disposable at acceptable costs. However, neither of those alternatives recycles the salt for reuse as acid and base despite several regeneration methods suggested in the literature [1]. Next to thermal salt splitting, the pH shift electrolysis is intensively researched to this end [2,3]. The latter is a particularly attractive technique due to the growing availability of renewable electricity from wind and photovoltaic power plants. Conventional water electrolysis produces oxygen and protons at the anode and hydrogen and hydroxide ions at the cathode. The migration of either ion across the membrane keeps the charge balanced and the pH constant in both chambers. In pH shift electrolysis, a salt is added as an electrolyte providing excess ions for the charge transfer between both chambers. Thus, protons at the anode and hydroxide ions at the cathode are largely retained in their respective chambers generating the intended pH difference.If the added salt behaves electrochemically inert, the minimum voltage for the cell to deliver a faradaic current by splitting water is 1.229 V. A less energy-intensive alternative is offered by incorporating a hydrogen depolarized anode (HDA). It suppresses the oxygen evolution reaction by providing hydrogen as an oxidation reagent. The cell’s open circuit voltage (OCV) can thus be reduced to 0 V. The minimum potential difference in both setups diverges when the anolyte turns acidic and the catholyte alkaline. A pH of 1 in the anolyte and 14 in the catholyte increases the OCV in a conventional pH shift cell to 1.996 V and in a HDA-incorporating cell to 0.767 V. Nevertheless, the energetic advantage of the latter remains apparent.Recently, several journal articles have been published applying the HDA technology to various use cases [4-6]. However, a sole investigation of the cell with an electrochemically inert salt, e.g., sodium sulfate, has not been fully addressed. The presentation demonstrates successful pH shifts with a HDA-incorporating two-chamber cell over a range of stationary operating points. The suppression of the oxygen evolution reaction at the anode is highlighted. Moreover, the overall cell voltage constituents are analyzed using Luggin-Haber capillaries, conductivity probes, and electrochemical impedance spectroscopy.[1] López-Garzón, C. S., & Straathof, A. J. (2014). Recovery of carboxylic acids produced by fermentation. Biotechnol. Adv., 32(5), 873-904. DOI: 10.1016/j.biotechadv.2014.04.002[2] Gausmann, M., Kiefel, R., & Jupke, A. (2023). Modeling of electrochemical pH swing extraction reveals economic potential for closed-loop bio-succinic acid production. Chem Eng Res Des, 190, 590-604. DOI: 10.1016/j.cherd.2022.12.022[3] Kocks, C., Wall, D., & Jupke, A. (2022). Evaluation of a Prototype for Electrochemical pH-Shift Crystallization of Succinic Acid. Materials, 15(23), 8412. DOI: 10.3390/ma15238412[4] Kuntke, P., Rodríguez Arredondo, M., Widyakristi, L., Ter Heijne, A., Sleutels, T. H., Hamelers, H. V., & Buisman, C. J. (2017). Hydrogen gas recycling for energy efficient ammonia recovery in electrochemical systems. Environ. Sci. Technol., 51(5), 3110-3116. DOI: 10.1021/acs.est.6b06097[5] Shu, Q., Legrand, L., Kuntke, P., Tedesco, M., & Hamelers, H. V. (2020). Electrochemical regeneration of spent alkaline absorbent from direct air capture. Environ. Sci. Technol., 54(14), 8990-8998. DOI: 10.1021/acs.est.0c01977[6] Sacré, N., Faral, M., Chenitz, R., Mokrini, A., Huot, J. Y., Bouchard, P., Bibienne, T., Magnan, J.-F. and Laroche, N. & Dollé, M. (2022). Hydrogen depolarized anodes with liquid anolyte: proof of concept. Electrocatalysis, 13, 139–153. DOI: 10.1007/s12678-021-00700-8

Absorption mechanism of SO3 on various alkaline absorbents in the presence of SO2

Sulfur trioxide (SO3) as a condensable particle matter has a significant influence on atmospheric visibility, which easily arouses formation of haze. It is imperative to control the SO3 emission from the industrial flue gas. Three commonly used basic absorbents, including Ca(OH)2, MgO and NaHCO3 were selected to explore the effects of temperature, SO2 concentration on the SO3 absorption, and the reaction mechanism of SO3 absorption was further illustrated. The suitable reaction temperature for various absorbents were proposed, Ca(OH)2 at the high temperatures above 500°C, MgO at the low temperatures below 320°C, and NaHCO3 at the temperature range of 320–500°C. The competitive absorption between SO2 and SO3 was found that the addition of SO2 reduced the SO3 absorption on Ca(OH)2 and NaHCO3, while had no effect on MgO. The order of the absorption selectivity of SO3 follows MgO, NaHCO3 and Ca(OH)2 under the given conditions in this work. The absorption process of SO3 on NaHCO3 follows the shrinking core model, thus the absorption reaction continues until NaHCO3 was exhausted with the utilization rate of nearly 100%. The absorption process of SO3 on Ca(OH)2 and MgO follows the grain model, and the dense product layer hinders the further absorption reaction, resulting in low utilization of about 50% for Ca(OH)2 and MgO. The research provides a favorable support for the selection of alkaline absorbent for SO3 removal in application.

Study on the competitive absorption between complex components in flue gas and SO3 by alkaline absorbent

SO3 in coal-fired flue gas will cause equipment corrosion and the emission of sulfuric acid mist. The complex component in the flue gas will affect the performance of alkaline absorbent. In this study, a fixed bed reactor was used to evaluate the absorption performance of SO3 by a variety of absorbents. Then, on this basis, the competition between SO3 and SO2, SO3 and HCl were studied by the fixed bed reactor. The effect of oxygen on the competitive absorption of SO3 and SO2 by absorbents was also investigated. Further, when SO2, HCl and SO3 existed simultaneously, the competitive reactions between multiple gases were investigated. The influence of SO2 and HCl on SO3 absorption was evaluated. Carbonate absorbents had stronger absorption capacity than other absorbents. In the adsorption process of SO3 by Na2CO3, the presence of SO2 inhibited the absorption of SO3 by Na2CO3, and oxygen oxidized Na2SO3 to produce Na2SO4, which affected the reaction between SO3 and Na2CO3.There was strong competition between SO3 and SO2. In the process of SO3 adsorption by Na2CO3, the presence of HCl had an inhibitory effect on the absorption of SO3, SO3 could significantly inhibit the absorption of HCl. When SO2, HCl and SO3 existed simultaneously, SO2 had a stronger influence on the adsorption of SO3 by Na2CO3 and the presence of HCl had little inhibition on the absorption of SO3 by Na2CO3. The inhibition effect of SO2 on SO3 and the inhibition of HCl on SO3 did not accumulate.

Optimization of an electrochemical direct air capture process with decreased CO2 desorption pressure and addition of background electrolyte

An electrochemical process based on pH-swing has been proposed recently to regenerate spent alkaline absorbent from direct air capture (DAC). In this work, we experimentally investigated and theoretically simulated two optimization strategies to further reduce the energy consumption of such novel electrochemical process. First, partial vacuum was applied to the gas phase during CO2 desorption to increase the gas production rate. The energy consumption of the electrochemical cell decreased by 12 to 15% when the CO2 partial pressure in the gas phase was reduced from 0.9 to 0.3 atm. Second, phosphate and sulphate were tested as background electrolyte to the alkaline absorbent, reducing the energy consumption by minimizing the ohmic losses in the electrochemical cell. The optimal concentration for phosphate was 0.1 M, while the CO2 production rate was limited by either the total carbon feeding rate or the high acidifying solution pH at higher concentrations of phosphate. Moreover, due to the low pKa and high molar conductivity of sulphate compared to phosphate, sulphate addition showed an even lower energy consumption than phosphate addition. Finally, the lowest experimental energy consumption was 247 kJ mol−1 CO2 achieved with CO2 partial pressure of 0.3 atm and 0.1 M of sulphate addition at current density of 150 A m−2 while our mathematical model predicted a theoretical minimum energy consumption of 138 kJ mol−1 under the same condition. Overall, the investigated optimization strategies advanced the development of an energy-efficient electricity-driven process for direct air capture.

A Novel Method Based on Hydrodynamic Cavitation for Improving Nitric Oxide Removal Performance of NaClO2.

In the removal of nitric oxide (NO) by sodium chlorite (NaClO2), the NaClO2 concentration is usually increased, and an alkaline absorbent is added to improve the NO removal efficiency. However, this increases the cost of denitrification. This study is the first to use hydrodynamic cavitation (HC) combined with NaClO2 for wet denitrification. Under optimal experimental conditions, when 3.0 L of NaClO2 with a concentration of 1.00 mmol/L was used to treat NO (concentration: 1000 ppmv and flow rate: 1.0 L/min), 100% of nitrogen oxides (NOx) could be removed in 8.22 min. Furthermore, the NO removal efficiency remained at 100% over the next 6.92 min. Furthermore, the formation of ClO2 by NaClO2 is affected by pH. The initial NOx removal efficiency was 84.8-54.8% for initial pH = 4.00-7.00. The initial NOx removal efficiency increases as the initial pH decreases. When the initial pH was 3.50, the initial NOx removal efficiency reached 100% under the synergistic effect of HC. Therefore, this method enhances the oxidation capacity of NaClO2 through HC, realizes high-efficiency denitrification with low NaClO2 concentration (1.00 mmol/L), and has better practicability for the treatment of NOx from ships.

(Digital Presentation) Electrochemical Regeneration of Spent Alkaline Absorbent from Direct Air Capture

The atmospheric carbon dioxide (CO2) concentration has increased by nearly 49% compared to preindustrial levels. This increase is primarily ascribed to anthropogenic activities and has caused rapid climate change in recent years. Direct air capture (DAC) technologies, aiming to remove CO2 directly from the atmosphere, are indispensable for mitigating the climate risks.One of the most studied DAC technologies is wet scrubbing with alkaline hydroxide solutions (e.g., KOH). Recently, we developed an electrochemical system that can regenerate spent alkaline absorbents from DAC and desorb high-purity CO2 gas under atmospheric pressure. The electrochemical cell consisted of three compartments, i.e., an anode compartment, an “acidifying” compartment, and a cathode compartment (Figure 1). During the operation, protons (H+) produced from the H2 oxidation at the anode are transported to the acidifying compartment where the spent absorbent coming from the air contactor is fed. The decreasing pH of the acidifying solution facilitates the conversion of bicarbonate/carbonate ions to carbonic acid (H2CO3 *, including dissolved CO2). When the chemical potential of oversaturated H2CO3 * is higher than the partial pressure of CO2 in the gas phase, CO2 can be desorbed. In the cathode compartment, the K+ from acidifying compartment combines with the locally produced hydroxides (OH−) to regenerate the alkaline absorbent. Overall, the electrochemical cell creates a pH swing in two adjacent compartments. CO2 gas is desorbed at low pH while the alkaline absorbent is regenerated at high pH.In the acidifying compartment, the cations changed from K+ to H+ that can displace the equilibrium of the carbon species towards the desorption of CO2 gas. However, this phenomenon brings two limitations to large-scale application of the system. Firstly, the depleting of K+ and HCO3 -/CO3 2- leads to the decrease of conductivity of the acidifying solution, which induces the increase of ohmic losses in the cell. Moreover, the desorbed CO2 gas bubbles in the cell also contributes to the electrical resistance of the cell. Therefore, the current work investigates optimization strategies to reduce the energy consumption of the system.The first strategy is to desorb CO2 into a lower CO2 partial pressure gas phase instead of atmospheric pressure. A decrease in the CO2 partial pressure from 0.9 to 0.3 bar displayed a maximum increase of 13% in gas production due to higher driving force for desorption and a maximum decrease of 7% in cell potential due to reduced gas bubble presence in the cell. The second strategy adds background electrolyte (phosphate or sulfate) into the absorbent. The adding of background electrolyte provided extra conductivity in the acidifying solution when most of the K+ and CO2 have been removed. Furthermore, the background electrolyte reduced the pH of the acidifying solution, which benefits the desorption of CO2. The lowest specific energy consumption achieved in this study was 249 kJ mol-1 CO2 captured under 150 A m-2. This low energy consumption makes the technology competitive among its counterparts. Finally, an equilibrium model has been developed that simulates the performance of the system and indicates further improvement potentials.This work demonstrates the proof of concept of a novel electrochemical direct air capture process. Our main conclusions show that the process has a competent energy consumption and potential for upscaling with the tested optimization strategies. Figure 1

Chlorine emission characteristics and control status of coal-fired units

Chlorine is a harmful trace element in coal. During coal combustion, the chlorine undergoes complex changes and transfers to the gas, and forms chlorine-containing substances. This paper summarizes the field test data of chlorine emission from domestic coal-fired units, and analyzes the chlorine emission and control characteristics. The results show that during the combustion process, the chlorine in coal is released into flue gas, mainly HCl, and a small amount of Cl2 and particulate Cl. Conventional pollutant control facilities affect the form and concentration of Cl. The impact of dust removal facilities on Cl is mainly reflected in the coordinated removal of particulate Cl. The impact of wet desulfurization facilities on Cl is mainly reflected in the coordinated removal of HCl. The Cl emission concentration in flue gas of coal-fired units is 0.85 mg/m3, which meets the emission requirements. Chlorine is transferred from coal to fly ash, desulfurization wastewater and desulfurization gypsum, so the secondary pollution of chlorine should be paid much attention. HCl in coal-fired flue gas has an adverse effect on wet desulfurization facilities. Based on the material balance of chloride ions in absorption tower slurry, this paper proposes a technical idea for removing HCl from flue gas with alkaline absorbents.

Removal of sulfur trioxide by wet injection with alkaline absorbent: Model and experimental studies

Sulfur trioxide (SO3) emissions from coal flue gas have become an increasingly serious issue, and alkaline absorbent injection technology has been successfully applied in industry as an effective control method. To accurately predict the SO3 removal effect, a new mathematical model based on the shrinking core model and mass conservation was proposed in this study. In this model, a fixed-bed adsorption experiment was used to estimate the absorption capacity of individual particles, and other model parameters were estimated. The theoretical results of the model were compared with the experimental results of an entrained flow reactor, the fitting parameters were determined, and the model was verified. In addition, the effects of key parameters such as the residence time, molar ratio, flue gas composition, and particle size of absorbers on the SO3 removal efficiency were compared and evaluated. Finally, the Sobol method was used to analyze the parameter sensitivity of the model. The results showed that particle size and porosity have an important influence on the removal of SO3, and thus should be considered when determining the type of absorbent.

Selective Sulfur Dioxide Absorption from Simulated Flue Gas Using Various Aqueous Alkali Solutions in a Polypropylene Hollow Fiber Membrane Contactor: Removal Efficiency and Use of Sulfur Dioxide.

Hollow fiber membrane contactors (HFMCs) provide a large specific surface area. Thus, their significantly reduced volume provides an advantage compared to the conventional gas–liquid contactor. In this study, the selective removal efficiency of flue gas, in which sulfur oxide (SO2) and carbon dioxide (CO2) coexist, was measured using a polypropylene (PP) HFMC with such advantages. To increase the selective removal efficiency of SO2, experiments were conducted using various alkaline absorbents. As a result, with 0.05 M ammonia solution, the removal efficiency of 95% or more was exhibited with continuous operation for 100 h or more. We confirmed that the absorbent saturated by the once-through mode was aqueous ammonium sulfate ((NH4)2SO4) solution and could be used as a fertilizer without additional processing.

Experimental Study on Removal of SO3 from Flue Gas by Na2CO3

The alkaline absorbent spraying technology is an efficient method for accurately removing SO3 in flue gas. By studying the characteristics of the technology, the reaction mechanism, the migration rule of SO3, and the experimental platform by using the real flue gas as the source of the alkaline absorbent jet removal of flue gas SO3. The results of the study show that the efficiency of the alkaline absorbent to remove SO3 is related to n(Na2CO3):n(SO3), flue gas temperature, residence time and other factors; under the test conditions, the flue gas temperature is 320°C, n(Na2CO3): n(SO3) is 1:4, the concentration of Na2CO3 solution after gasification is 52.6 mg/m3, and the concentration of soot is 4680 mg/m3 under standard conditions, which can ensure the removal efficiency of SO3 is above 85%; The effect of the catalyst is small and can be ignored. The above results can provide a reference for the domestic coal-fired power plants to carry out the research and application of alkaline absorbent to remove flue gas SO3 technology.

Performance evaluation and process simulation for synergetic removal of NOx, CO2 and PM using green alkaline solution in a high-gravity rotating packed bed

The high-gravity rotating packed bed (HiGee RPB) developed here can simultaneously remove multiple air pollutants using alkaline solution, thus achieving the flue gas deep purification. The electric-arc-furnace reduction slag (EAFRS) leachate was then used as the alkaline absorbent in HiGee RPB for assessing the removal efficiencies of nitrogen oxide (NOx), carbon dioxide (CO2) and particulate matters (PM) from flue gas. The results indicated the changes to the CO2 capture was enhanced by increasing the initial PH value of the alkaline solution. In contrast, this value has no significant influence on NOx removal. The suitable NOx removal efficiency ranged from 90.9% to 97.09% at the liquid-to-gas (L/G) ratio of 0.01 and high gravity factor of 58.9 to 368. The precipitated calcium carbonate gained from carbonation via HiGee process exhibited similar physical properties to commercial products. In addition, 95.9% of PM removal efficiency was achieved at the L/G ratio and high gravity factor of 0.03 and 235.4, respectively. This treatment furthermore demonstrated a high efficiency for PM collection of over 90% and balanced the energy consumption. Based on the result, an Aspen simulation and a preliminary cost-benefit analysis were carried out to develop a mass-balance model for this system and determine the favorable operating conditions.

Optimization of SO2 and NOx sequential wet absorption in a two-stage bioscrubber for elemental sulphur valorisation.

Flue gases contain SO2 and NOx that can be treated together for elemental sulphur recovery in bioscrubbers, a technology that couples physical-chemical and biological processes for gaseous emissions treatment in a more economic manner than classical absorption. Sequential wet absorption of SO2 and NOx from flue gas is thoroughly studied in this work in a two-stage bioscrubber towards elemental sulphur valorisation pursuing reuse of biological process effluents as absorbents. The optimal operating conditions required for SO2 and NOx absorption in two consecutive spray absorbers were defined using NaOH-based absorbents. Overall, removal efficiencies of 98.9% and 55.9% for SO2 and NOx abatement were obtained in two in-series scrubbers operated under a gas contact time of 1 and 100 s, and a liquid-to-gas ratio of 7.5 and 15 L m-3, respectively. Higher NOx removal efficiency to clean gas emission was obtained by oxidants dosing in the absorber for NOx absorption. High NaHCO3 concentration in a two-stage bioscrubber effluent was exploited as alkaline absorbent for flue gas treatment. The performance of scrubbers using an absorbent mimicking a reused effluent exhibited the same removal efficiencies than those observed using NaOH solutions. In addition, the reuse of bioprocess effluent reduced reagents' consumption by a 63.7%. Thus, the two-stage bioscrubber proposed herein offers an environmentally friendly and economic alternative for flue gas treatment.

Electrochemical Regeneration of Spent Alkaline Absorbent from Direct Air Capture.

CO2 capture from the atmosphere (or direct air capture) is widely recognized as a promising solution to reach negative emissions, and technologies using alkaline solutions as absorbent have already been demonstrated on a full scale. In the conventional temperature swing process, the subsequent regeneration of the alkaline solution is highly energy-demanding. In this study, we experimentally demonstrate simultaneous solvent regeneration and CO2 desorption in a continuous system using a H2-recycling electrochemical cell. A pH gradient is created in the electrochemical cell so that CO2 is desorbed at a low pH, while an alkaline capture solution (NaOH) is regenerated at high pH. By testing the cell under different working conditions, we experimentally achieved CO2 desorption with an energy consumption of 374 kJ·mol–1 CO2 and a CO2 purity higher than 95%. Moreover, our theoretical calculations show that a minimum energy consumption of 164 kJ·mol–1 CO2 could be achieved. Overall, the H2-recycling electrochemical cell allowed us to accomplish the simultaneous desorption of high-purity CO2 stream and regeneration of up to 59% of the CO2 capture capacity of the absorbent. These results are promising toward the upscaling of an energy-effective process for direct air capture.

Selective removal of SO2 from coal-fired flue gas by alkaline solvents using a membrane contactor

The presence of SO2 in coal-fired flue gas has adverse effects on the amine solutions used for CO2 capture. In addition, SO2 emissions in the atmosphere are also a primary source of pollution. In this study, the SO2 removal efficiency of three aqueous alkaline absorbents NaOH, Na2SO3, and CaO was studied using a membrane contactor. The morphology of the used membrane was examined using SEM and contact angle characterization techniques. Gas removal efficiency was studied within a feed gas flow rate of 0.5–5 m3/h, absorbent flow rate of 5–100 mL/min, and absorbent concentration of 0–0.01 mol/L. The gathered results showed that aqueous CaO has superior gas removal performance. It achieved a SO2 removal efficiency of 99.99 % at a concentration of 0.0025 mol/L, while the aqueous NaOH and Na2SO3 absorbents achieved 99.99 % removal efficiency at concentrations of 0.001 mol/L. Through comprehensive experimental work, the optimum operating parameters needed to achieve maximum SO2 removal efficiency were also determined. Furthermore, the durability of the membrane was tested in a four week continuous gas removal test and SEM images were recorded weekly to observe module morphology. The obtained results indicated that the membrane was quite durable and did not deteriorate noticeably.

Green process for supercritical water oxidation of sewage sludge with red mud as CO2 absorbent

To mineralize CO2 produced in situ and other acidic substances originating from organic components in sewage sludge, we developed a green SCWO system involving the treatment of sewage sludge with red mud as an alkaline absorbent. In this new system, two steps of mineralization were covered. Primary mineralization occurred inside the SCWO reaction vessels with part of CO2 produced in situ. By this primary treatment, total inorganic carbon of the solid residue increased rapidly, predominantly in the form of calcite. Moreover, the S and P content increased by more than ten times compared with that of pure sludge. The pH of the drained water increased from 5.16 to about 7.7 with the addition of 2% red mud. The second mineralization step involved the exhausts cleaning reaction using red mud slurry. With 2.5h of cleaning reactions at pressures of 0.1, 1, 3, and 5MPa, approximately 4.49, 5.30, 6.06, and 7.52g, respectively, of CO2 per 100g of red mud were bound in calcite. The high CO2 absorption capacity was ascribed to the high pressure cleaning reaction, which was conveniently achieved in this SCWO system just by a simple adjustment of the back-pressure valve. With this green SCWO disposal, CO2 level in the exhausts were cut to below 800ppm (mg/L); SO2 and nitrogen oxides (NO, NO2) diminished to below 15ppm and 12ppm, respectively. Due to the comparative stability of the solid residues obtained, they can easily be handled using conventional methods or recycled for a range of industrial applications including building materials, water treatment materials, special ceramics, or silicate cements.

The selective removal of H2S over CO2 from biogas in a bubble column using pretreated digester effluent

Raw biogas purification can be achieved using various physicochemical techniques. Unfortunately, techniques for the simultaneous absorption of CO2 and H2S are susceptible to the corrosive nature of H2S. Furthermore, the absorbents utilized are often expensive, thus requiring regeneration. Concurrently, on farm digesters are now moving toward the incorporation of NH3 recovery. One such process, air stripping, requires elevation of pH, which as a result produces an alkaline effluent. Due to this, a physicochemical H2S removal process was investigated in a bubble column reactor using this manure based alkaline absorbent. Selective removal of H2S over CO2 increased nearly 50% by altering the sparger configuration. Additional selectivity was achieved by decreasing the effluent height in the bubble column and increasing the superficial gas velocity. Continuous countercurrent mode of operation was tested and relatively high H2S removal efficiency (84%) was achieved at a biogas-to-effluent ratio (v/v) of 20:1, close to that typically found on dairy digesters.

Comparison of Various Alkaline Solutions for H2S/CO2‐Selective Absorption Applied to Biogas Purification

AbstractBiogas is a common renewable energy resource. A very important stage of biogas upgrading, studied in the present work, is its purification from H2S traces. The selective absorption of H2S and CO2 into oxido‐alkaline solutions containing hydrogen peroxide and into amine solutions was compared by performing absorption test runs in a cables‐bundle scrubber at 293.15 K and atmospheric pressure. The absorption rate and selectivity for H2S over CO2 were determined for various solute partial pressures, different alkaline absorbents and hydrogen peroxide concentrations in the scrubbing liquid, and different pH values. Higher H2S‐selective absorption performances with oxido‐alkaline solutions than with amine solutions could be observed provided that the solution is at a low pH value (9.5) and contains a sufficient hydrogen peroxide concentration.

Low-energy sodium hydroxide recovery for CO 2 capture from atmospheric air—Thermodynamic analysis

To reduce the risks of climate change, atmospheric concentrations of greenhouse gases must be lowered. Direct capture of CO 2 from ambient air, “air capture”, might be one of the few methods capable of systematically managing dispersed emissions. The most commonly proposed method for air capture is a wet scrubbing technique which absorbs CO 2 in an alkaline absorbent, i.e. sodium hydroxide producing an aqueous solution of sodium hydroxide and sodium carbonate. In most of the previous works it was assumed that the absorbent would be regenerated and CO 2 liberated from the alkaline carbonate solution using a lime and calcium carbonate causticization cycle. We describe a novel technique for recovering sodium hydroxide from an aqueous alkaline solution of sodium carbonate and present an end-to-end energy and exergy analysis. In the first step of the recovery process, anhydrous sodium carbonate is separated from the concentrated sodium hydroxide solution using a two-step precipitation and crystallization process. The anhydrous sodium carbonate is then causticized using sodium tri-titanate. The titanate direct causticization process has been of interest for the pulp and paper industry and has been tested at lab- and pilot-scale. In the causticization process, sodium hydroxide is regenerated and carbon dioxide is liberated as a pure stream, which is compressed for use or disposal. The technique requires ∼50% less high-grade heat than conventional causticization and the maximum temperature required is reduced by at least 50 °C. This titanate cycle may allow a substantial reduction in the overall cost of direct air capture.