Integrated Gasification Combined Cycle Research Articles

CO2-selective membrane reactor process for water-gas-shift reaction with CO2 capture in a coal-based IGCC power plant

Integrated gasification combined cycle (IGCC) power plants enable pre-combustion carbon capture to reduce CO2 emissions. Membrane water-gas-shift (WGS) reactors can intensify these processes by converting raw syngas to H2 with simultaneous CO2 capture in one unit. This paper reports process design and techno-economic analysis (TEA) for a membrane reactor (MR) process with a CO2 selective ceramic-carbonate dual-phase (CCDP) membrane for WGS reaction with CO2 capture for a IGCC power plant. The target performance includes CO conversion > 95 %, hydrogen purity > 90 %, CO2 purity > 95 %, and carbon capture > 90 %. Using a commercial catalyst and a CCDP membrane, the MR can achieve the performance target at 750 °C and space velocity of 250 h−1. The outcome of the process design and TEA shows that the CCDP MR has an operating cost of $24 M/year, significantly lower than that for the conventional processes (40 M$/year). However, the MR process has a higher capital cost ($1007 M) than the conventional process ($527 M) because of the higher cost of the CCDP MRs. Modeling analysis shows a MR with higher CO2 permeance can deliver the target at a higher space velocity and lower membrane surface area to catalyst volume ratio, leading to a significantly reduced MR capital costs.

Thermodynamic evaluation of an IGCC power plant utilizing allothermal gasification

The present work conducts a comprehensive thermodynamic analysis of a 150 MWe Integrated Gasification Combined Cycle (IGCC) using Indian coal as the fuel source. The plant layout is modelled and simulated using the “Cycle-Tempo” software. In this study, an innovative approach is employed where the gasifier's bed material is heated by circulating hot water through pipes submerged within the bed. The analysis reveals that increasing the external heat supplied to the gasifier enhances the hydrogen (H2) content in the syngas, improving both its heating value and cold gas efficiency. Additionally, this increase in external heat favourably impacts the Steam-Methane reforming reaction, boosting the H2/CH4 ratio. The thermodynamic results show that the plant achieves an energy efficiency of 44.17% and an exergy efficiency of 40.43%. The study also identifies the condenser as the primary source of energy loss, while the combustor experiences the greatest exergy loss.

Enhanced CO2 capture and stability of MgO modified with alkali metal nitrates and carbonates at moderate temperature

AbstractBACKGROUNDMagnesium oxide (MgO) is favored for solid‐state carbon dioxide (CO2) capture due to its high theoretical adsorption capacity, abundant reserves, low cost, and environmental friendliness. However, its practical application in industry is hindered by low CO2 adsorption capacity under moderate operating conditions. In this work, MgO was modified by a deposition method using LiNO3, NaNO3, KNO3, Na2CO3 and K2CO3 as additives.RESULTSThe study determines optimal ratios within the [(Li, Na, K)x − (Na, K)]y/MgO system, specifically identifying x = 0.5 and y = 0.15 as most effective. At 275 °C under pure CO2 conditions, the adsorption capacity peaks at 0.631 g CO2 g−1 adsorbent. Effective regeneration of the adsorbent occurs at 400 °C under 100% N2 for 15 min. Under Integrated Gasification Combined Cycle (IGCC) conditions, the adsorption capacity stabilizes at 0.462 g g−1 after 20 cycles, representing a 25% decrease from initial capacity.CONCLUSIONExperimental findings demonstrate that the inclusion of alkali metal salts in MgO precursors enhances the adsorbent's microstructure, thereby improving its CO2 capture efficiency and bolstering cycling stability. This research enhances our understanding of the factors influencing CO2 adsorption and cyclic stability in alkali metal salt‐promoted MgO, providing valuable insights for further refinement in the formulation and synthesis protocols of MgO‐based CO2 adsorbents. © 2024 Society of Chemical Industry (SCI).

Exergoeconomic Analysis and Optimization of a Biomass Integrated Gasification Combined Cycle Based on Externally Fired Gas Turbine, Steam Rankine Cycle, Organic Rankine Cycle, and Absorption Refrigeration Cycle.

Adopting biomass energy as an alternative to fossil fuels for electricity production presents a viable strategy to address the prevailing energy deficits and environmental concerns, although it faces challenges related to suboptimal energy efficiency levels. This study introduces a novel combined cooling and power (CCP) system, incorporating an externally fired gas turbine (EFGT), steam Rankine cycle (SRC), absorption refrigeration cycle (ARC), and organic Rankine cycle (ORC), aimed at boosting the efficiency of biomass integrated gasification combined cycle systems. Through the development of mathematical models, this research evaluates the system's performance from both thermodynamic and exergoeconomic perspectives. Results show that the system could achieve the thermal efficiency, exergy efficiency, and levelized cost of exergy (LCOE) of 70.67%, 39.13%, and 11.67 USD/GJ, respectively. The analysis identifies the combustion chamber of the EFGT as the component with the highest rate of exergy destruction. Further analysis on parameters indicates that improvements in thermodynamic performance are achievable with increased air compressor pressure ratio and gas turbine inlet temperature, or reduced pinch point temperature difference, while the LCOE can be minimized through adjustments in these parameters. Optimized operation conditions demonstrate a potential 5.7% reduction in LCOE at the expense of a 2.5% decrease in exergy efficiency when compared to the baseline scenario.

Thermodynamic and economic analysis of a new methanol synthesis system coupled with a biomass integrated gasification combined cycle

An innovative design has been developed to optimize the performance of the methanol synthesis (MS) by integration with a biomass integrated gasification combined cycle (BIGCC) system based on the principle of the cascade utilization of chemical and thermal exergy. In the integrated system, the efficient utilization of the chemical energy present in feedstocks is made possible by combining exothermic MS chemical processes and integrating different grades of thermal energy. The graded conversion of methanol facilitates the manifestation of both its chemical and fuel attributes. By such incorporation, the waste heat and purge gas in MS process are utilized in a more efficient way and the biomass consumption in BIGCC plant is significantly reduced. A thermodynamic analysis conducted on a 100 kt/yr methanol production plant and a 41.8 MW BIGCC power plant reveals an increase in energy efficiency of 3.46 % and an achievement of 61.9 % exergy efficiency. Moreover, this design yields a profit of 72.6 M$ more than that generated by individuals throughout its lifespan. Sensitivity analysis further indicates that the optimal conditions for the polygeneration system are approximately 250 °C and 60 bar, with a preference for a 0.90 MS recycle ratio.

Multiparameter optimization of the acid gas removal process using self-heat recuperation technology in IGCC system based on genetic algorithm

The acid gas removal (AGR) process in the integrated gasification combined cycle (IGCC) system consumes a large amount of heat and reduces the generation efficiency. Process modification of self-heat recuperation (SHR) technology based on vapor compression heat pumps is expected to reduce the energy consumption of the AGR process. On this basis, this paper conducted a comprehensive and parametric analysis of the AGR process integrated with a vapor compression heat pump. Wherein, two key parameters, the stripper pressure (P1) and the heat pump heat exchanger inlet pressure (P2), were analyzed in detail to understand the energy consumption characteristics of each module of the system. By establishing the optimization objective functions of minimizing system operating cost and exergy loss respectively, the system multi-parameter optimization was conducted based on the genetic algorithm by coding and solving in HYSYS and MATLAB. The corresponding parameter variables for the optimal cases of the two optimization objectives are also comparatively derived. The results showed that P1 mainly changes the SHR capacity of the system by changing the allowable flow rate of the SHR mass under different working conditions, while P2 mainly affects the SHR capacity of the system by changing the heat of the fraction at the top of the stripper. The minimum operating cost of the system decreased to 0.0655 $/kgH2S, which is 1.50 % lower compared to the original case. The minimum exergy loss of the system is reduced from 7788.89 kW to 7398.59 MW, which is about 5.01 % lower than that of the original case.

Process simulation of biomass looping gasification with the integrated gasification combined cycle (BCLG-IGCC) system: coupled-parameter effect and thermodynamic analysis

Process simulation of biomass looping gasification with the integrated gasification combined cycle (BCLG-IGCC) system: coupled-parameter effect and thermodynamic analysis

Experimental and ReaxFF MD study on the release characteristics of gaseous nitrogen during pressurized bituminous coal gasification

Coal gasification is the leading technology of clean coal technology such as Integrated Gasification Combined Cycle (IGCC). The release pattern of gaseous nitrogen during coal gasification is crucial for clean coal technology. This paper investigated the gaseous nitrogen emission characteristics of pressurized gasification through experiments and molecular dynamics studies. The experimental results show that the elevated temperatures promoted the release of NO and accelerated the conversion of nitrogen-containing precursors (HCN, NH3). However, high pressure weakened the effect of temperature and resulted in the decreased emissions of HCN, NH3, and NO. Because high pressure facilitated the conversion of fuel nitrogen to nitrogen. The emissions of HCN and NH3 decreased as the CO2 blending ratio increased, while NO emissions increased. The observed difference was primarily attributed to the amounts of free radicals and was also related to gas-phase reactions involving H2O and CO2. Furthermore, as the primary nitrogen-containing structure in bituminous coal, the pyrrole was used in Reactive force field molecular dynamics (ReaxFF MD) to study the release of gas-phase nitrogen during gasification process. The simulation results show that both temperature and pressure promote the migration of pyrrole nitrogen and also provided the main nitrogen migration paths during gasification which can be used to (indirectly) confirm the experimental analysis. The O2/N2/CO2 gasification system supplied additional O and OH radicals to enhance NO emissions, while H2O directly participated in reaction paths such as CN, NCO → NHi or provided H radicals to increase NH3 emissions.

A multi-scale experimental investigation of the feasibility of using integrated coal gasification combined slag as a supplementary cementitious material

As thermal plants are being upgraded, integrated gasification combined cycle (IGCC) slag is emerging as a promising supplementary cementitious material (SCMs). The performance of IGCC slag as SCM was comprehensively compared with blast furnace slag (BFS) and fly ash (FA) in various aspects, including material characterisation, pozzolanic reaction and cement paste performance investigation. Characterisation results show that the IGCC slags comprised of calcium-rich aluminosilicate glass with minimal carbon content. The pozzolanic reactions of the IGCC slags were similar to that of BFS but superior to FA. The IGCC slag-blended cement pastes therefore showed similar compression strength as the BFS-blended cement paste, while showing reduced water loss, shrinkage and chloride diffusion, and were superior to the FA-blended cement paste. Economic and environmental evaluation revealed that substituting cement with IGCC slag is beneficial to reduce the cost, embodied energy and CO2 emissions of concrete.

Zero/negative carbon emission coal and biomass staged co-gasification power generation system via biomass heating

Currently, most coal utilization technologies cannot completely remove CO2, which hinders coal utilization in the context of carbon neutrality. Staged coal-steam gasification has proven to be a potential clean coal utilization method suitable for power generation. However, it cannot achieve zero or negative carbon emissions, and there is the potential to improve energy efficiency further. This study introduces biomass into staged coal gasification and proposes a novel power generation system based on coal- and biomass-staged co-gasification via biomass external combustion heating. Biomass with a lower energy level was combusted to provide heat for gasification, thus improving the energy level matching between the gasification and biomass combustion reactions. The synergistic effect of coal and biomass reduces the tar yield and improves the gasification efficiency. Furthermore, as a carbon-neutral fuel, the introduction of biomass in the staged coal-gasification process can achieve zero or negative carbon utilization of coal. The entire system was simulated using the ASPEN PLUS software, key processes were experimentally validated, and the energy efficiency and carbon emission performance were systematically studied. The results indicated that the improved gasification methods and the synergistic effect between coal and biomass significantly improved the energy performance and emission reduction characteristics. The power efficiency of the novel system was the highest at 40.03% when the biomass blending ratio of the co-gasification sub-process was 0.8, which was 5.27% and 10.46% higher than conventional integrated gasification combined cycle (IGCC) system and biomass direct-fired power plant. The carbon-specific emissions of the novel system could be reduced to −308.1 kgCO2/MWh, achieving negative carbon utilization of fossil fuels. This novel staged coal and biomass co-gasification method heating by biomass can achieve efficient and negative carbon utilization of fossil fuels.

Analyses of hot/warm CO2 removal processes for IGCC power plants

Compared with a pulverized coal power plant, the integrated gasification combined cycle (IGCC) has several advantages, including, among others better environmental performance and low CO2 capture cost. Hot/warm CO2 removal from syngas has also been a subject of research due to its potentially higher thermal efficiency. In this study, we proposed a generic adsorption based hot/warm CO2 removal process for IGCC power plants. Through analyses of the proposed generic process we have demonstrated that higher temperature of the hot/warm CO2 removal process will results in larger heat of adsorption, which in turns may increase energy consumption of the process. Under most of the operating temperature range, hot/warm CO2 removal process will lead to more electricity loss compared to the baseline Selexol process. However, if the adsorption step takes place at a temperature close to or higher than the highest steam temperature in steam cycle, our analysis indicates that the process may lead to minimal electricity loss. The study also provided some other insights into the pathways for hot/warm CO2 removal process to improve its energy performance through process and sorbent designs.Graphical

Exergoeconomic analysis and multi-objective optimization using NSGA-II in a novel dual-stage Selexol process of integrated gasification combined cycle

To alleviate the problem of global climate change and provide a new perspective of carbon recovery, a novel configuration of H2S concentrator adopting pressure drop method and self-stripping method in dual-stage Selexol process of integrated gasification combined cycle is developed in this study. A comprehensive thermodynamic and exergoeconomic analysis of the novel Selexol process is carried out. The total product specific cost and total exergoeconomic factor of the process are 48.54 $/hr and 7.84 %. The ideal condition of components in the context of exergoeconomic analysis is defined, and the concrete improvement directions and methods of components which are not in ideal condition are given. Towards the exergoeconomic analysis, the multi-objective optimization of the model is carried out by using the Non-dominant Sorting Genetic Algorithm-II (NSGA-II) in conjunction with Aspen Plus. Besides, the ideal optimal point method is applied to the Pareto frontier in this study to get the optimal operating conditions according to the three targets for actual operation. The multi-objective optimization results reveal that the optimal point between CO2 capture efficiency and total exergy efficiency is that CO2 capture efficiency is 93.13 % and the total exergy efficiency is 26.09 %. This study provides a reference for academic research and engineering design of novel CO2 capture systems.

Energy, exergy, and exergoeconomic analyses of plastic waste-to-energy integrated gasification combined cycles with and without heat recovery at a gasifier

Energy, exergy, and exergoeconomic analyses were performed for two plastic-integrated gasification combined cycle (plastic-IGCC) systems to evaluate the performance of the plastic waste-to-energy cycles. Plastic waste-to-energy is a promising plastic treatment method that can resolve both plastic waste and environmental issues. Thus, improving the efficiency and economy of plastic-IGCC has become crucial because energy is generated during plastic waste-to-energy treatment while treating waste. The difference between the two modeled cycles is the location of heat recovery from the high-temperature syngas. In Cases 1 and 2, heat from the syngas was recovered with a heat recovery steam generator and a gas heater, respectively, to increase the temperature of the air entering the gasifier. The maximum net efficiency for Case 2 increased by 8.2% (from 35.41% to 43.57%), unlike that of Case 1, without changing exergy destruction. To assess the economic value and market potential, the unit electricity cost was examined for the condition in which the highest efficiency was obtained. The unit exergoeconomic costs for Cases 1 and 2 were 0.141 and 0.108 $/kWh, respectively, which were within the range of those in other energy recovery combined cycles; in addition, using air heaters for heat recovery reduced costs. The use of the air heater for heat recovery benefited energy and economic aspects without significantly changing exergy destruction. These findings have important implications for understanding the impact of gasifier agent conditions and energy recovery methods on the optimum conditions for plastic-IGCC. This study aimed to provide insights into the optimal design and operation of plastic-IGCC systems, considering both energy and economic aspects.

Fuel calorific value control process analysis and research based on IGCC power plant

AbstractThe stability of the equipment has been more and more important with the continuous development of IGCC (Integrated Gasification Combined Cycle) technology. The control of the calorific value of the fuel gas of the gas turbine is one of the most important factors for the stable operation of the gas turbine. There are many variables in the calorific value of IGCC fuels that will cause unstable fuel combustion conditions and low thermal efficiency. In terms of process equipment, process conditions, instrument control calculation, and logic adjustment, this paper studies the automatic adjustment system of the calorific value of syngas for IGCC gas turbine fuel. The calorific value of the syngas can be automatically adjusted under different working conditions of the gas turbine. It helps the stable combustion of the gas turbine to form a perfect, efficient, reliable calorific value control system.

Simulation research on optimization of a 200 MW IGCC system

The integrated gasification combined cycle (IGCC) has increasingly attracted attention as a promising high-efficiency clean coal technology. The oxygen-to-carbon ratio (O/C), nitrogen reinjection coefficient (Xgn), and integration air separation coefficient (Xas) affect system performance greatly. Based on the selected coal type, this paper establishes a 200 MW IGCC system model with coal–water slurry gasification, matches the three-pressure reheating heat recovery steam generator and syngas coolers, simulates and calculates the system performance of O/C, Xas and Xgn using Thermo-flex software. From the perspective of the whole system, the optimal O/C of the system is obtained as 0.91 considering the syngas composition and gasification temperature. From the perspective of system efficiency, the Xgn is obtained as 60 % with the Xas of 20 %. The overall IGCC system model is optimized using the optimized O/C, Xgn, and Xas to obtain higher system power and efficiency, the system power generation efficiency can improve up to 51.52 %. A thermal balance diagram of the IGCC system is drawn using the calculation results and provides a reference for the future design and operation of IGCC systems.

Construction of N-Rich Aminal-Linked Porous Organic Polymers for Outstanding Precombustion CO2 Capture and H2 Purification: A Combined Experimental and Theoretical Study.

A large number of scientific investigations are needed for developing a sustainable solid sorbent material for precombustion CO2 capture in the integrated gasification combined cycle (IGCC) that is accountable for the industrial coproduction of hydrogen and electricity. Keeping in mind the industrially relevant conditions (high pressure, high temperature, and humidity) as well as good CO2/H2 selectivity, we explored a series of sorbent materials. An all-rounder player in this game is the porous organic polymers (POPs) that are thermally and chemically stable, easily scalable, and precisely tunable. In the present investigation, we successfully synthesized two nitrogen-rich POPs by extended Schiff-base condensation reactions. Among these two porous polymers, TBAL-POP-2 exhibits high CO2 uptake capacity at 30 bar pressure (57.2, 18.7, and 15.9 mmol g-1 at 273, 298, and 313 K temperatures, respectively). CO2/H2 selectivities of TBAL-POP-1 and 2 at 25 °C are 434.35 and 477.93, respectively. On the other hand, at 313 K the CO2/H2 selectivities of TBAL-POP-1 and 2 are 296.92 and 421.58, respectively. Another important feature to win the race in the search of good sorbents is CO2 capture capacity at room temperature, which is very high for TBAL-POP-2 (15.61 mmol g-1 at 298 K for 30 to 1 bar pressure swing). High BET surface area and good mesopore volume along with a large nitrogen content in the framework make TBAL-POP-2 an excellent sorbent material for precombustion CO2 capture and H2 purification.

Techno-economic and life cycle assessment of the integration of bioenergy with carbon capture and storage in the polygeneration system (BECCS-PS) for producing green electricity and methanol

Bioenergy with carbon capture and storage (BECCS) has the potential to produce negative emissions. This study assessed the overall energy efficiency and carbon dioxide (CO2) avoidance costs and emission footprint following the integration of BECCS with a polygeneration system (BECCS-PS) for the co-production of green electricity and methanol. The process was simulated in Aspen Plus and Aspen HYSYS v.11. Oil palm empty fruit bunches were used as the feed in a biomass integrated gasification combined cycle power plant. The flue gas, which contained CO2, was captured for methanol synthesis and carbon storage. Green hydrogen for use in methanol synthesis was produced through proton exchange membrane (PEM) electrolysis powered by solar PV (PV-PEM) and geothermal power with double-flash technology (GEO-PEM). The environmental impacts of the process were investigated by a life cycle assessment and the economic aspects were evaluated using the levelized cost method. The overall system efficiency was higher in the PV-PEM scenario than in the GEO-PEM scenario. For any production capacities, the green electricity generated from the BECCS-PS plant resulted in negative emissions. A biomass power plant with a low production capacity generated higher production and CO2 avoidance costs than that with a larger production capacity. The CO2 − eq emissions and costs for methanol production in the PV-PEM scenario were larger than those in the GEO-PEM scenario, with values of -0.83 to -0.70 kg CO2 − eq/kg MeOH and 1,191–1,237 USD/ton, respectively. The corresponding values were − 1.65 to -1.52 kg CO2 − eq/kg MeOH and 918–961 USD/ton, respectively, for the GEO-PEM scenario.Graphical

Process modelling and optimization of a 250 MW IGCC system: ASU optimization and thermodynamic analysis

Integrated gasification combined cycle (IGCC) underpins the advancement of pulverized coal combustion, yet the optimization of the air separation unit (ASU) in the IGCC system is still lacking, resulting in the limited reduction of energy consumption and efficiency improvement. Accordingly, a full-process IGCC model developed in our previous work (Energy, 2023,272: 127,040) is employed to optimize the ASU in a 250 MW IGCC power plant at an industrial scale. The feasibility of the ASU optimization was verified, and thermodynamic analysis evaluated the optimized IGCC system efficiency. The results show that the partial integration of ASU significantly reduces the compressor's energy consumption while the nitrogen re-injection method minimizes the use of saturated steam. Furthermore, reducing oxygen purity decreases the minimum separation work of the system. After optimization, the ASU uses only 53% as much power as the traditional system, and the total energy efficiency (44.9%) and exergy efficiency (43.6%) are improved by 2.2% and 1.8%, respectively. Consequently, the total energy consumption and exergy consumption decrease by 10.8% and 35.9%, respectively. The present work provides a comprehensive framework for optimizing other components of the IGCC system, which is expected to further reduce energy consumption and improve the system's efficiency.

Optimization and Comparison of CO<sub>2</sub> Capture Process Using DEPG and Methanol Solvents

This study focus is on the physical solvent-based CO2 capture process from an integrated Gasification Combined Cycle (IGCC) power plant, followed by a comparison of Dimethyl ether of polyethylene glycol (DEPG) solvent and methanol solvent, which is based on traditional technology and has a 95% CO2 purification requirement. After achieving the optimal condition, we would compare which solvents are suitable for this system. This procedure was simulated in Aspen HYSYS. Additionally, this study examined the effects of factors that provide the CO2 capture cost per ton ($/tCO2 captured). The response surface approach is used to get the optimal result. In the case of the optimal condition of DEPG solvent, the inlet temperature of fuel gas is – 2 °C, the stripper of inlet temperature is at 67 °C, the pressure of the absorber is at 2,994 kPa, and the stage of absorber appears to be 12 stages. As a result, the lowest DEPG solvent process cost is $189.63 per ton of CO2 collected. The minimum cost of CO2 capture under the optimal condition of methanol solvent, where the temperature of the absorbent and fuel gas is – 8 and – 20 °C, the stripper inlet temperature appears to be at – 6 °C, and the concentration of the absorbent is 88.85 wt.% in methanol solvent, is then $19.21 per ton of CO2 captured.

Effects of fluid flow on the rate limiting for CO2 capture by an AEEA containing membrane

The efficiency of CO2 capture using a hollow fiber membrane containing 2-(2-Aminoethylamino) ethanol (AEEA) from the emission gas in the Integrated Gasification Combined Cycle (IGCC) was investigated by density functional theory and fluid dynamics. A reaction path for CO2 sorption to AEEA involving two H2O molecules with three – proton transfer was found to give the low barrier of the reaction due to 6 intermolecular hydrogen bonds which make the transition state structure more stable. The activation energy in the reaction path for CO2 sorption to AEEA was 7.11 kcalmol−1 which is approximately 10 kcalmol−1 lower than that involving one H2O molecule with two – proton transfer discussed in our previous study (Higashino et al., 2023). Since the activation energy is much lower than those in the other reaction paths we have investigated, it can occur much more frequently than others when sufficient moisture is maintained in the membrane. The estimated reaction rate k = 3.407 × 108 s−1 indicates that CO2 becomes carbamate by reaction with AEEA as soon as reaching the surface of the membrane, and thus, CO2 attachment to AEEA cannot be the rate limiting. Since flow in the hollow fiber membrane can be laminar, simulated CO2 permeance for the temperature of 358 K and the pressure of 2.4 MPa has a range from 10−10 to 10−9 m3 (STP) m−2s−1Pa−1, which is a few orders smaller than that for the flat sheet membrane, flow over which is turbulent, i.e.10−8 to 10−7 m3(STP) m−2s−1Pa−1 (Higashino et al., 2023). Overall, the reaction rate of CO2 attachment to AEEA is sufficiently large, and hence, any one of mass transfer of CO2 induced by turbulence (turbulent flow), molecular diffusion of CO2 from feed gas to the membrane surface (laminar flow), and molecular diffusion of bicarbonate (HCO3-) from the feed side to the permeate side in the membrane can be the rate limiting depending on which type of membrane, e.g. a flat sheet membrane module, a spiral membrane module, a hollow fiber membrane contactor, etc., is used for CO2 capture.