Feasibility assessment of a spray tower for gas-liquid reactive precipitation in CO2 capture

The issue of carbon dioxide emissions and the disposal of reject brine poses significant environmental threats that have garnered widespread concern. A promising strategy to combat these challenges is the modified Solvay carbonation process, which excels in capturing CO2 while converting brine into valuable sodium bicarbonate (NaHCO3). This research presents a novel method that combines modified Solvay carbonation with bipolar membrane electrodialysis (BMED) to recycle wastewater generated post-Solvay carbonation for producing alkaline solutions. To improve CO2 absorption efficacy, we developed a unique series-connected reactor that works in tandem with the BMED system for efficient alkaline recycling. In the first CO2 utilization using series connected reactors, the pH versus time analysis indicated that the carbonation reactions proceed effectively within a pH range of 10 to 9.2, achieving rapid conversion. In contrast, the bicarbonate-dominant pH regime 9.1 to 8 experienced significantly prolonged reaction times, indicative of sluggish kinetics that aligned with theoretical predictions. The absorption reactor achieved over 70% CO2 capture efficiency due to the expedited rates of carbonation reactions, therefore outpacing the slower bi-carbonation reactions occurring in the precipitation reactor. Unreacted gas from the precipitation reactor was subsequently recycled back to the absorption reactor, facilitating total absorption by the alkaline ammonia brine solution. This strategic recycling maximized the CO2 concentration entering the bicarbonation stage, ensuring efficient conversion into sodium bicarbonate while enhancing overall absorption efficiency, resulting in a total CO2 absorption efficiency is 97.8%, comparatively higher than a carbon capture efficiency of ammoniated brine conducted in a single reactor. In the BMED system, we evaluated how various system parameters impact its performance, achieving maximum acid and base concentrations of 3.87 mol/L and 3.95 mol/L, respectively, alongside current efficiencies of 61.7%. The energy costs were determined to be 4.08 kWh/kg for acid and 4.04 kWh/kg for base production. It was observed that increasing the operating voltage and decreasing the volumes of acid and base could enhance the final concentrations. Remarkably, a recycled base mixture of 0.66 kg (NH3·H2O + NaOH), when combined with saturated NaCl, exhibited exceptional CO2 sequestration capabilities, capturing 0.78 kg of CO2 for every kg of NaHCO3 produced while ensuring a purity level above 95%. The XRD patterns of the solid product obtained from the recycled alkaline solution showed that the diffraction peak positions match the standard NaHCO₃, indicating that the NaHCO₃ synthesized under these conditions has the same crystal structure as the pure NaHCO₃. Similarly, the SEM of the solid produced from the alkali solution in the presence of saturated NaCl showed the sodium bicarbonate crystals exhibited irregular morphology, with smaller and more dispersed particles that were prone to agglomeration. This system provides a good source ion, which significantly promotes the formation of sodium bicarbonate crystals; however, it affects the distribution of crystal size. On the contrary, in the NaHCO3 produced from the alkali solution without NaCl, the crystals were better crystallized and had a more regular structure, showing columnar or prismatic particles with larger crystal grains. The reaction conditions were purer, the nucleation rate was moderate, the crystals could grow sufficiently, and the crystallinity was significantly improved. The environmental evaluation showed that the CO2 emissions associated with the production of the base mixture from Solvay carbonation liquid using diverse electricity sources reflect that the amount of CO2 captured exceeds the CO2 emissions from this process, validating its environmental sustainability and energy efficiency. However, it is essential to note that the net CO2 avoided varies significantly with the energy source; for example, the use of coal results in a substantial negative CO2 avoidance of –1798 g CO2/kg of base, making it a non-renewable option. In contrast, utilizing wind energy contributes positively, yielding +1480 g of avoided CO2. Therefore, to enhance the production of alkaline solution from post-Solvay carbonation wastewater for CO2 capture, it is strongly recommended to source electricity from renewable or clean energy systems. In addition, the CO2 emissions of the proposed modified CO2 utilization process integrated with the BMED system are compared to those of the conventional Solvay process, which involves calcination for ammonia recycling. When accounting for coal-based electricity generation in both processes, the conventional method produces CO2 emissions of 2.74 kg per kg of NaHCO3, while the proposed process emits only 1.18 kg of CO2 per kg of NaHCO3, indicating a 43% reduction in CO2 emissions compared to the conventional process. In summary, this method not only addresses critical environmental concerns but also contributes to the development of more sustainable industrial practices.

Feasibility Assessment of CO2 Capture Retrofitted to an Existing Cement Plant: Post-combustion vs. Oxy-fuel Combustion Technology

Performance of CO2 absorption in a spray tower using blended ammonia and piperazine solution: Experimental studies and comparisons

Mature CO2 capture technologies would reduce the net thermal efficiency of the coal-fired power plant by 7–13% points, leading to an electricity cost increase of at least 60%. To minimise the energy-intensity of CO2 capture, novel technologies and CO2 capture materials are being developed. This study assessed the techno-economic feasibility of the CO2 capture system using acrylamide-based molecularly imprinted polymer (MIP) sorbent in a 580 MWel coal-fired power plant retrofit scenario. Under the initial design basis, the net efficiency penalty and the energy penalty of the MIP retrofit scenario were estimated to be 5.3%HHV points and 14.1%, respectively. Furthermore, the cost of CO2 avoided was estimated to be 29.3 £/tCO2. Such techno-economic performance was found to be superior to the CO2 capture system using chemical solvents. The parametric study revealed that the thermodynamic performance of the MIP retrofit scenario is mainly affected by the sorbent capacity, as the net efficiency penalty was found to increase from 4.4 to 8.9%HHV points on reduction of the sorbent capacity from 1 to 0.2 mmol CO2/g. Moreover, the economic performance was not only found to be affected by sorbent capacity, but primarily on the cyclic performance of the MIP sorbent. It was shown that the cost of CO2 avoided would increase linearly with increase of the MIP sorbent make-up at a rate of 6.8 £/tCO2 per 0.1% of sorbent make-up.

Spray towers are an effective means of capturing condensable or absorbable gases and vapors, and they have long been used in a variety of applications. However, the use of a spray towers in CO2 capture is a recent development, and the limited number of studies, to date, have reported experimental data only on the capture efficiency for systems with a fixed size, flow rate, and CO2 concentration of the gas mixture, as well as a fixed flow rate and absorbent concentration of the liquid mixture. Systematic investigations of the parameter space have not been reported. In this study, the capture of CO2 from a CO2/air mixture using aqueous ammonia as the absorbent medium with a single nozzle was experimentally measured over a wide range of operating conditions. Relationships between the capture efficiency and the operating parameters are reported for the first time. We also report the experimental observation of the optimum tower diameter for a given spray nozzle.

Hydrodynamic mechanism study of the diameter-varying spray tower with atomization impinging spray

Experimental studies of air-blast atomization on the CO2 capture with aqueous alkali solutions

Development of a Spray Scrubbing Process for Post Combustion CO2 Capture with Amine Based Solvents

Transient modeling of electrochemically assisted CO2 capture and release

Sulfur dioxide (SO2) is a major flue gas contaminant that has a direct effect on the performance of amine-based carbon dioxide capture units operating on power plant flue gases. In many countries, flue gas desulfurisation (FGD) is an essential upstream requirement to CO2 capture systems, thereby increasing the overall operational and capital cost of the capture system. In Australia, the efficacy of CO2 capture may be compromised by the accumulation of SO2 in the absorption solvent. CSIRO’s CS-Cap process is designed to capture of both these acidic gases in one absorption column, thereby eliminating the need for a separate FGD unit which could potentially save millions of dollars. Previous research at CSIRO’s post-combustion capture pilot plant at Loy Yang power station has shown that mono-ethanolamine (MEA) solvent absorbs both CO2 and SO2, resulting in a spent amine absorbent rich in sulfates. Further development of the CS-Cap concept requires a deeper understanding of the properties of the sulfate-rich absorbent and the conditions under which it can be effectively regenerated. In the present study, thermal reclamation and reactive crystallisation processes were investigated, allowing the parameters affecting the regeneration of sulfate-loaded amine to be identified. It was found that amine losses were considerably higher in thermal reclamation than in reactive precipitation. During thermal reclamation, vacuum conditions were more effective than atmospheric, and pH of the initial solution played a significant role in recovery of MEA from the sulfate-rich absorbent. Reactive crystallisation could be effectively accomplished with the addition of KOH. An advantage of this process was that high purity K2SO4 crystals (~99%) were formed, despite the presence of degradation products in the solvent.

Numerical simulation of aqueous ammonia-based CO2 absorption in a sprayer tower: An integrated model combining gas-liquid hydrodynamics and chemistry

Experimental and numerical study on CO2 absorption mass transfer enhancement for a diameter-varying spray tower

Many industrial gas separations are traditionally carried out by gas-liquid absorption. The implementation of these separation processes in membrane contactors can overcome several limitations (emulsion formation, flooding, unloading, or foaming) associated with conventional equipment with direct G-L contact such as spray towers or packed columns. For the design of membrane contactors, characterization of the mass transport of the target solute from the gas to the liquid crossing the membrane must be done; this is specially challenging when a reactive carrier is added in the liquid phase. The purpose of this Chapter is to offer insight about the phenomena that control the mass transport of a solute from a gas phase to a liquid phase in membrane contactors. Special attention is paid to the transport mechanism and the fundamental equations that describe the process, providing useful information for the development of process design and optimization tools. The performance of hollow fiber membrane contactors in three major separations such as olefin/paraffin separation, CO2 capture and SO2 removal are quantitatively analyzed and compared to conventional absorption equipment. The analysis reported in this Chapter can be extended to any application of membrane contactors where reactive absorption of a gas in a liquid takes place.

The spray behaviour and droplet trajectories in realistic conditions are of crucial importance in many industrial, agricultural and chemical applications. Droplet characteristics and spray trajectory in chemical applications (e. g. flue gas scrubbing, CO2 capture in spray column) determine the amount of mass involved in the gas scrubbing process, mass trapped by the flow or attached to the walls. Knowledge of the droplet behaviour can improve a nozzle design and scaling, increase the process efficiency, minimize the process liquid and blow away the fraction. In this study, experiments with pressure swirl nozzle in cross–flow of air were performed at one nozzle injection pressure (0.5 MPa) and several cross–flow velocities (8, 16, 32 m/s). The results on droplet trajectories are compared with numerical results obtained by ANSYS Fluent. Two Lagrange approaches for spray modelling were used. Injection of droplet groups and Linearized Instability Sheet Atomization (LISA) model incorporated within ANSYS Fluent were used to represent the spray. The CFD results of spray penetration and droplet trajectories are compared with experimental data. A simple analytical model is able to well predict trajectories of large droplets, but fails to predict trajectories of small droplets. The LISA model yields a better accuracy for spray in cross-flow prediction.

CO2 capture in a spray column using a critical flow atomizer