CO2 capture and separation technologies for end-of-pipe applications – A review

Carbon capture from point source emissions has been recognized as one of several strategies necessary for mitigating unfettered release of greenhouse gases (GHGs) into the atmosphere. To keep GHGs at manageable levels, large decreases in CO₂ emissions through capturing and separation will be required. This article reviews the possible CO₂ capture and separation technologies for end-of-pipe applications. The three main CO₂ capture technologies discussed include post-combustion, pre-combustion and oxyfuel combustion techniques. Various separation techniques, such as chemical absorption, physical absorption, physical adsorption, cryogenics, membrane technology, membranes in conjunction with chemical absorption and chemical-looping combustion (CLC) are also thoroughly discussed. Future directions are suggested for application by oil and gas industry. Sequestration methods, such as geological, mineral carbonation techniques, and ocean dump are not covered in this review.     

Comparative analysis of performance and techno-economics for a H2O–NH3–H2 absorption refrigerator driven by different energy sources

The objectives of the present work are of two-folds. First, it evaluates the transient temperature performance of the H₂O–NH₃–H₂ absorption cooling machine system’s components under two types of energy sources, i.e. the conventional electric energy from grid (electric) and fuel energy from liquid petroleum gas (LPG). Results obtained have shown that performance of various components under different type of energy sources is almost coherent. For the evaporator, the system with electric supply has shorter starting time, around 6 min earlier than the system run with LPG. Meanwhile, the system powered by LPG produced a lower cooling temperature around −9 °C, compared to the system run with electric which produced temperature at around −7 °C. Economical study had been carried out subsequently, for three different energy sources, i.e. electric, LPG and solar energy (photovoltaic). From the techno-economical analyzes, it was found that the conventional electric from grid is still the best form of energy source for short-term application, as far as the present location and conditions are concerned. LPG is the next attractive energy source, especially at locations with constant LPG supply; the photovoltaic energy from solar is attractive for long term consideration since it has zero fuel cost and environmentally-friendly, but with the highest initial cost.     

The retardation kinetics of magnetite reduction using H2 and H2–H2O mixtures

Cylindrical compacts of magnetite were isothermally reduced at 773–1273 K with pure H₂ or H₂–H₂O mixtures. The initial reduction rates increased with temperature and partial pressures of H₂ in the H₂–H₂O mixtures. However, with progressing reduction, a dense iron layer formed around the wüstite grains and the rate significantly reduced. In this regime, solid state oxygen diffusion through the dense iron layer was rate limiting. This retardation of reduction occurred at degrees of reduction of 51–89%, depending on the temperature and H₂ partial pressure, which has a linear relationship with the dimensionless kinetic parameter, k₁^mixed/k₂^mixed, (k₁^mixed, k₂^mixed: contribution of gaseous mass transport (GMT) and interfacial chemical reaction (ICR) to the reduction rate, respectively) in the reaction-regime controlled by a combination of both mechanisms. However, under certain conditions (H₂, H₂–10%H₂O, 773 K//H₂–10, 20%H₂O, 873 K//H₂–20%H₂O, 973 K) the retardation was absent because of the formation of a microporous iron layer product.     

Liquid–liquid equilibrium tie-line compositions at elevated temperatures and pressures for the I2–H2O and HI–I2–H2O systems of the Sulfur–Iodine Cycle

Liquid–liquid equilibrium (LLE) tie-line compositions were measured for the binary system iodine–water and the ternary system hydrogen iodide–iodine–water (HI–I₂–H₂O) at 199.1 and 252.2 °C, at pressures of 60 bar, conditions of interest for the reactive distillation column of the Sulfur–Iodine Cycle. To keep the HI decomposition reaction negligible, a continuous-flow apparatus was used to minimize residence times at the elevated temperatures. Sixteen equilibrium tie lines were measured, using overall feed compositions ranging from 0.0 to 3.3 mol % HI, and the plait-point composition for 199.1 °C was estimated. Sample compositions for the lighter, water-rich phase and the heavier, I₂-rich phase were both determined via titration of HI and I₂, with water being obtained by difference. The phase behavior indicates that the selectivity of HI for the aqueous phase can be large, ranging from 10 to 100+ for tie lines removed from the critical region.     

Combustion,performance and emissions of a diesel engine with H2, CH4 and H2–CH4 addition

Experiments were conducted to investigate the combustion and emission characteristics of a diesel engine with addition of hydrogen or methane for dual-fuel operation, and mixtures of hydrogen–methane for tri-fuel operation. The in-cylinder pressure and heat release rate change slightly at low to medium loads but increase dramatically at high load owing to the high combustion temperature and high quantity of pilot diesel fuel which contribute to better combustion of the gaseous fuels. The performance of the engine with tri-fuel operation at 30% load improves with the increase of hydrogen fraction in methane and is always higher than that with dual-fuel operations. Compared with ULSD–CH₄ operation, hydrogen addition in methane contributes to a reduction of CO/CO₂/HC emissions without penalty on NO_x emission. Dual-fuel and tri-fuel operations suppress particle emission to the similar extent. All the gaseous fuels reduce the geometry mean diameter and total number concentration of diesel particulate. Tri-fuel operation with 30% hydrogen addition in methane is observed to be the best fuel in reducing particulate and NO_x emissions at 70 and 90% loads.     

Effects of alumina powder characteristics on H2 and H2O2 production yields in γ-radiolysis of water and 0.4 M H2SO4 aqueous solution

The effects of powder characteristics on H₂ and H₂O₂ productions in ⁶⁰Co γ-radiolysis were studied in pure water and in 0.4 M H₂SO₄ aqueous solutions containing alumina powders. In 0.4 M H₂SO₄ solution, the H₂ yields strongly depended on alumina structures and decreased in the order of α > θ > γ-alumina, although the specific surface areas increased as α < θ < γ. The yields increased with increasing specific surface area when compared among α-alumina. In pure water, similar dependence was observed but not as strong as that for 0.4 M H₂SO₄ solution. The H₂O₂ yields were strongly decreased by adding the alumina powders in both water and 0.4 M H₂SO₄ aqueous solution, although the amounts of decrease were almost neither correlated with specific surface areas nor structures. The enhancing H₂ production was discussed in terms of the electron supply from alumina to aqueous solution as well as the adsorption of OH radicals on alumina surfaces.     

A comparison of the emission and impingement heat transfer of LPG–H2 and CH4–H2 premixed flames

Experiments were performed to add hydrogen to liquefied petroleum gas (LPG) and methane (CH₄) to compare the emission and impingement heat transfer behaviors of the resultant LPG–H₂–air and CH₄–H₂–air flames. Results show that as the mole fraction of hydrogen in the fuel mixture was increased from 0% to 50% at equivalence ratio of 1 and Reynolds number of 1500 for both flames, there is an increase in the laminar burning speed, flame temperature and NO_x emission as well as a decrease in the CO emission. Also, as a result of the hydrogen addition and increased flame temperature, impingement heat transfer is enhanced. Comparison shows a more significant change in the laminar burning speed, temperature and CO/NO_x emissions in the CH₄ flames, indicating a stronger effect of hydrogen addition on a lighter hydrocarbon fuel. Comparison also shows that the CH₄ flame at α = 0% has even better heat transfer than the LPG flame at α = 50%, because the longer CH₄ flame configures a wider wall jet layer, which significantly increases the integrated heat transfer rate.     

Revision of the NaBO2–H2O phase diagram for optimized yield in the H2 generation through NaBH4 hydrolysis

The binary phase diagram NaBO₂–H₂O at ambient pressure, which defines the different phase equilibria that could be formed between borates, end-products of NaBH₄ hydrolysis, has been reviewed. Five different solid borates phases have been identified: NaBO₂·4H₂O (Na[B(OH)₄]·2H₂O), NaBO₂·2H₂O (Na[B(OH)₄]), NaBO₂·2/3H₂O (Na₃[B₃O₄(OH)₄]), NaBO₂·1/3H₂O (Na₃[B₃O₅(OH)₂]) and NaBO₂ (Na₃[B₃O₆]), and their thermal stabilities have been studied. The boundaries of the different Liquid + Solid equilibria for the temperature range from −10 to 80 °C have been determined, confirming literature data at low temperature (20–50 °C). Moreover the following eutectic transformation, Liq. → Ice + NaBO₂·4H₂O, occurring at −7 °C, has been determined by DSC. The Liquid–Vapour domain has been studied by ebullioscopy. The invariant transformation Liq. → Vap. + NaBO₂·2/3H₂O has been estimated at 131.6 °C. This knowledge is paramount in the field of hydrogen storage through NaBH₄ hydrolysis, in which borate compounds were obtained as hydrolysis reaction products. As a consequence, the authors propose a comparison with previous NaBO₂–H₂O binary phase diagrams and its consequence related to hydrogen storage through NaBH₄ hydrolysis.     

Hydrogen,nitrogen and carbon dioxide production through chemical looping using iron-based oxygen carrier – A Green plant for H2 and N2 production

In order to simulate the performance of pure methane in chemical looping using iron-based oxygen carrier, simultaneously production of three pure streams of hydrogen, nitrogen and carbon dioxide has been investigated. For this purpose, proper operating conditions have been discussed for maximum production of hydrogen, complete consumption of oxygen of inlet air and complete combustion of methane. Professional software is used to simulate the chemical looping reactors and optimize their output streams. Results show that in this process each mole of methane fuel can produce 2.533, 2.65 and 0.99 mol of pure N₂, H₂ and CO₂, respectively which contributes 80.2% energy conversion of CH₄ to H₂. Moreover, in order to consume the whole input fuel and maximize hydrogen production, it is necessary to use a supportive material to improve mechanical property of oxygen carrier particles and optimize temperature of streams by thermal integration of three reactors. Also, due to controllable temperature of three reactors, more flow rate of oxygen carrier particles can be used instead of supportive material while the air flow rate should be justified to produce pure nitrogen. Hence, three chemical looping reactors, beside hydrogen and CO₂ production, can directly produce nitrogen, by means of a process simpler than the conventional technologies like air separation unit.     

Aliphatic–aromatic polyimide blends for H2 separation

In this work the synthesis of a semi aliphatic BTDA-DAH polyimide and their blends with BTDA-ODA and BTDA-DDS polyimides was carried out in order to improve the H₂ permselective properties of polyimides. The syntheses were made using the well-known two steps method and the silylation method. The prepared films were characterized by FTIR, DSC, thermal stability and fluorescence spectroscopy. Intercatenary distances (d-spacing) and gas separation properties were also investigated. PI blend membranes presented only one glass transition temperature (Tg) intermediate between those of the neat polyimides. Fluorescence spectra were a useful tool to recognize electron-donor and electron-acceptor interactions indicating intermolecular charge-transfer complex (CTC) formation which were confirmed by UV–Vis absorptions. As a result, a decrease in the intercatenary distances and a shift for both IR and fluorescence bands of polyimide blends were measured. PI blend membranes showed a permeability decrease with respect to the neat ones, while the selectivity increased according to X-ray diffraction results. To analyze the polyimide blend permselectivities, H₂/CH₄, H₂/CO₂, H₂/O₂ and H₂/N₂ systems were chosen. As a result, H₂/CH₄ separation factor of PI blends was among the highest reported by other authors using traditional membrane materials.     

Nanosizing ammonia borane with nickel – An all-solid and all-in-one approach for H2 generation by hydrolysis

Ammonia borane NH₃BH₃ (AB) and nickel (Ni) have been considered together as an all-solid and all-in-one material for H₂ generation by hydrolysis at 20–50 °C. Our novel approach, denoted Ni/AB, consists of AB nanoparticles within a Ni matrix. Upon contact with water, Ni/AB readily hydrolyzes and liberates H₂ with a turnover frequency of 13.8 mol(H₂) mol_Ni^?1 min^?1 at 43.3 °C. The apparent activation energy, determined over the temperature range 23.5–50.4 °C, is low, with 19.5 ± 4.1 kJ mol^?1. These results imply that such a Ni matrix embedding AB acts as an effective catalyst. Beyond the catalytic performance, this is the first report of the successful utilization of an all-solid and all-in-one approach for the hydrolysis of AB, and the work brings unique perspectives for one-shot catalytic systems.     

Techno-economic analysis (TEA) for CO2 reforming of methane in a membrane reactor for simultaneous CO2 utilization and ultra-pure H2 production

Techno-economic analysis (TEA) for CO₂ reforming of methane in a membrane reactor (MR) was conducted by using process simulation and economic analysis. Parametric studies for key operating conditions like a H₂ permeance, a H₂O sweep gas flow rate, operating temperature, and a CO₂/CH₄ ratio were carried out for a conventional packed-bed reactor (PBR) and a MR using Aspen HYSYS^®, a commercial process simulator program and some critical design guidelines for a MR in terms of a H₂O sweep gas flow rate and a CO₂/CH₄ ratio were obtained. Further economic analysis based on process simulation results showed about 42% reduction in a unit H₂ production cost in a MR (6.48 $ kgH₂^?1) than a PBR (11.18 $ kgH₂^?1) mostly due to the elimination of a pressure swing adsorption (PSA) system in a MR. In addition, sensitivity analysis (SA) revealed that reactant price and labor were the most influential economic factors to determine a unit H₂ production cost for both a PBR and a MR. Lastly, profitability analysis (PA) from cumulative cash flow diagram (CCFD) in Korea provided positive net present value (NPV) of $443,760～$240,980, discounted payback period (DPBP) of 3.03–3.18 y, and present value ratio (PVR) of 7.51–4.97 for discount rates from 2 to 10% showing economic feasibility of the use of a MR as simultaneous CO₂ utilization and ultra-pure H₂ production.     

Start-up behaviors of a H2SO4–H2O distillation column for the 50 NL H2/H sulfur–iodine cycle

The sulfur–iodine (SI) cycle to produce hydrogen from water requires a multistage distillation column to concentrate a sulfuric acid solution. To design a concentration process of a sulfuric acid solution that can be applied to the cycle, its static and dynamic simulation is essentially demanded. A 50 NL H₂/h scale SI test facility to be operated under a pressurized environment has been constructed in Korea. This study focuses on the sulfuric acid multi-stage distillation column (SAMDC-50L) for the 50 NL H₂/h SI test facility. The SAMDC-50L was designed and installed in 2012. Based on the design specifications and operation method, a start-up behavior of the SAMDC-50L has been analyzed using the simulation code “KAERI-DySCo”. As a result of the start-up dynamic simulation, it is confirmed that the SAMDC-50L will approach to the steady state value within 30,000 s to fulfill the hydrogen production rate of 50 NL H₂/h. On the other hand, it is expected that the operation time approaching a steady state decreases with an increase in the set point of the condenser temperature until a dew point of the top vapor product and the time required for the transition to the complete steady state is increased with an increasing reflux ratio and reboiler hold-up.     

Pd–Ag thin film membrane for H2 separation. Part 2. Carbon and oxygen diffusion in the presence of CO/CO2 in the feed and effect on the H2 permeability

The effect of CO and CO₂ on the performance and stability of Pd–Ag thin film membranes prepared by electroless plating deposition (EPD) was investigated, observing the presence of dissociation to carbon and oxygen which slowly diffuse in the membrane influencing also H₂ permeability. The effect of the two carbon oxides was investigated both separately and combined in the 400–450 °C temperature range over long-term cumulative experiments (up to over 350 h) on a membrane that already worked for over 350 h in H₂ or H₂–N₂ mixtures. An increase of the H₂ permeation flux was observed feeding only CO₂ in the range 10–20%. This effect was interpreted as deriving from the facilitated H₂ flux caused from oxygen diffusion (deriving from CO₂ dissociation) in the membrane. CO induces instead a partial inhibition on the H₂ flux deriving from the negative effect of CO competitive chemisorption as well as C diffusion in the membrane, which overcome the positive effect associated to oxygen diffusion in the membrane. Carbon and oxygen diffuse through the membrane with a rate two order of magnitude lower than hydrogen, and recombinate at the permeate side forming CO, CO₂ and CH₄ which amount increases with time-on-stream. The effect is reversible and not associated with the creation of cracks or defects in the membrane, as supported by leak tests.     

Laminar burning velocities and flame characteristics of CO–H2–CO2–O2 mixtures

Laminar burning velocities of CO–H₂–CO₂–O₂ flames were measured by using the outwardly spherical propagating flame method. The effect of large fraction of hydrogen and CO₂ on flame radiation, chemical reaction, and intrinsic flame instability were investigated. Results show that the laminar burning velocities of CO–H₂–CO₂–O₂ mixtures increase with the increase of hydrogen fraction and decrease with the increase of CO₂ fraction. The effect of hydrogen fraction on laminar burning velocity is weakened with the increase of CO₂ fraction. The Davis et al. syngas mechanism can be used to calculate the syngas oxyfuel combustion at low hydrogen and CO₂ fraction but needs to be revised and validated by additional experimental data for the high hydrogen and CO₂ fraction. The radiation of syngas oxyfuel flame is much stronger than that of syngas–air and hydrocarbons–air flame due to the existence of large amount of CO₂ in the flame. The CO₂ acts as an inhibitor in the reaction process of syngas oxyfuel combustion due to the competition of the reactions of H + O₂ = O + OH, CO + OH = CO₂ + H and H + O₂(+M) = HO₂(+M) on H radical. Flame cellular structure is promoted with the increase of hydrogen fraction and is suppressed with the increase of CO₂ fraction due to the combination effect of hydrodynamic and thermal-diffusive instability.     

Phase equilibrium measurements for the I2–H2O and I2–HI–H2O systems of the sulfur–iodine cycle using a continuous-flow apparatus

Liquid–liquid equilibrium (LLE) phase behavior was investigated for the binary iodine–water (I₂–H₂O) and the ternary iodine–hydroiodic acid–water (I₂–HI–H₂O) at the elevated temperatures and pressures of interest for the reactive distillation column of the Sulfur–Iodine Cycle. A continuous-flow apparatus, with wetted parts fabricated from tantalum-tungsten alloys, was designed and constructed for the highly corrosive conditions of this work. A central feature of the apparatus is the equilibrium view cell, which allows for the observation and discernment of vapor-liquid, liquid-liquid, and liquid–liquid–vapor equilibria for HIx systems.     

A new weighted-sum-of-gray-gases model for CO2–H2O gas mixtures

Decoupled radiative transfer calculations of Sandia Flame D are performed employing experimental measurements of temperature and gas compositions and the discrete ordinates method. Turbulence radiation interactions are accounted for employing temperature variance measurements. The spatial variations in the mean H₂O/CO₂ ratios within the flame are seen to vary widely and sometimes fall outside the range of the weighted-sum-of-gray-gases model (WSGGM) parameters reported in the literature. Therefore, parameters for a new WSGGM are computed from total emissivity correlations encompassing the range of the H₂O/CO₂ ratios encountered within the flame. Predictions from the new model compare favorably against the spectral-line-based WSGGM and existing benchmarks.     

Adiabatic burning velocity of H2–O2 mixtures diluted with CO2/N2/Ar

Global warming due to CO₂ emissions has led to the projection of hydrogen as an important fuel for future. A lot of research has been going on to design combustion appliances for hydrogen as fuel. This has necessitated fundamental research on combustion characteristics of hydrogen fuel. In this work, a combination of experiments and computational simulations was employed to study the effects of diluents (CO₂, N₂, and Ar) on the laminar burning velocity of premixed hydrogen/oxygen flames using the heat flux method. The experiments were conducted to measure laminar burning velocity for a range of equivalence ratios at atmospheric pressure and temperature (300 K) with reactant mixtures containing varying concentrations of CO₂, N₂, and Ar as diluents. Measured burning velocities were compared with computed results obtained from one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics and good agreement was obtained. The effectiveness of diluents in reduction of laminar burning velocity for a given diluent concentration is in the increasing order of argon, nitrogen, carbon dioxide. This may be due to increased capabilities either to quench the reaction zone by increased specific heat or due to reduced transport rates. The lean and stoichiometric H₂/O₂/CO₂ flames with 65% CO₂ dilution exhibited cellular flame structures. Detailed three-dimensional simulation was performed to understand lean H₂/O₂/CO₂ cellular flame structure and cell count from computed flame matched well with the experimental cellular flame.     

Ni–Cu/Ce0.9Zr0.1O2 bimetallic cermets for electrochemical and catalytic applications

The development of an economically feasible Solid Oxide Fuel Cell technology has driven the research activity in this area toward the reduction of their operating temperature. New anode materials, with high electrocatalytic activity and resistance to coke deposition when hydrocarbons are used as fuel, have to be developed. The aim of this paper is to give a contribution in this area by studying the effect of Ni substitution by Cu in the NiO (60 wt.%)/Ce_0.9Zr_0.1O₂ cermet recently proposed as a possible anode material.