Search Results (14)

A multiparametric study was conducted on a hydrogen (H₂) production rig designed to process 0.25 Nm³·h⁻¹ of syngas. The rig consists of two Pd-Ag membrane permeator units and two Pd-Ag membrane reactor units for the water–gas shift (WGS) reaction, enabling a detailed and comprehensive analysis of its performance. The aim was to find the optimal conditions to maximize hydrogen production by WGS and its separation in a pure stream by varying the temperature, pressure, and steam-to-CO ratio (S/CO). Two syngas mixtures obtained from an updraft gasifier using different gasification agents (air–steam and oxy–steam) were used to investigate the effect of gas composition. The performance of the rig was investigated under nine combinations of temperature, pressure, and S/CO in the respective ranges of 300–350 °C, 2–8 bar, and 1.1–2 mol·mol⁻¹, as planned with the help of design of experiment (DOE) software. The three parameters positively affected performance, both in terms of capacity to separate a pure stream of H₂, reported as moles permeated per unit of surface area and time, and in producing new H₂ from WGS, reported as moles of H₂ produced per volume of catalyst unit and time. The highest yields were obtained using syngas from oxy–steam gasification, which had the highest H₂ concentration and was free of N₂. Full article

Purifying biogas can enhance the performance of distributed smart grid systems while potentially yielding clean feedstock for downstream usage such as steam reforming. Recently, a novel anion-pillared metal–organic framework (MOF) was reported in the literature that shows good capacity to separate acetylene from carbon dioxide. The present study assesses the usefulness of this adsorbent for separating a typical biogas mixture (consisting of methane, nitrogen, oxygen, hydrogen, carbon dioxide, and hydrogen sulphide) using a multiscale approach. This approach couples atomistic Monte Carlo simulations in the grand canonical ensemble with the batch equilibrium modelling of a pressure swing adsorption system. The metal–organic framework displays selectivity at low pressures for carbon dioxide and especially hydrogen sulphide. An analysis of adsorption isotherm models coupled with statistical distributions of surface–gas interaction energies determined that both CH₄ and CO₂ exhibited Langmuir-type adsorption, while H₂S displayed Langmuir-type behaviour at low pressures, with increasing adsorption site heterogeneity at high pressures. Batch equilibrium modelling of a vacuum swing adsorption system to purify a CH₄/CO₂ feedstock demonstrated that such a system can be incorporated into a solar biogas reforming process since the target purity of 93–94 mol-% methane for incorporation into the process was readily achievable. Full article

The depletion of fossil-based fuels, fluctuating fuel market, and environmental deterioration demand an aggressive approach towards the advancement of renewable energy technologies. By the time reliable technology for a clean and abundant energy supply is established, existing sources must be economized. Biomass gasification is the way forward in that direction. CFD modeling shows promise in the development of advanced gasification systems. A simplified 3D CFD model of a downdraft gasifier is developed to investigate the effect of gasifying agent composition on the quality of syngas. Simulation results are compared with published experimental data and found to be in reasonably good agreement. Mixing CO₂ with a gasification agent is also investigated as a possible carbon capture and utilization (CCU) strategy. An air-steam mixture is used as a base-case gasification agent. Firstly, the effect of air-to-steam ratio on syngas composition is investigated. Secondly, the effect of oxygen and mixing CO₂ with a gasification agent is investigated in two separate cases. A 50%-50% air-steam mixture is found to produce the best quality syngas. Oxygen is found to have a negligible impact on the quality of syngas. The air-steam-CO₂ = 23%-50%-15% mixture is found to be optimum regarding syngas quality. Full article

Redox materials have been investigated for various thermochemical processing applications including solar fuel production (hydrogen, syngas), ammonia synthesis, thermochemical energy storage, and air separation/oxygen pumping, while involving concentrated solar energy as the high-temperature process heat source for solid–gas reactions. Accordingly, these materials can be processed in two-step redox cycles for thermochemical fuel production from H₂O and CO₂ splitting. In such cycles, the metal oxide is first thermally reduced when heated under concentrated solar energy. Then, the reduced material is re-oxidized with either H₂O or CO₂ to produce H₂ or CO. The mixture forms syngas that can be used for the synthesis of various hydrocarbon fuels. An alternative process involves redox systems of metal oxides/nitrides for ammonia synthesis from N₂ and H₂O based on chemical looping cycles. A metal nitride reacts with steam to form ammonia and the corresponding metal oxide. The latter is then recycled in a nitridation reaction with N₂ and a reducer. In another process, redox systems can be processed in reversible endothermal/exothermal reactions for solar thermochemical energy storage at high temperature. The reduction corresponds to the heat charge while the reverse oxidation with air leads to the heat discharge for supplying process heat to a downstream process. Similar reversible redox reactions can finally be used for oxygen separation from air, which results in separate flows of O₂ and N₂ that can be both valorized, or thermochemical oxygen pumping to absorb residual oxygen. This review deals with the different redox materials involving stoichiometric or non-stoichiometric materials applied to solar fuel production (H₂, syngas, ammonia), thermochemical energy storage, and thermochemical air separation or gas purification. The most relevant chemical looping reactions and the best performing materials acting as the oxygen carriers are identified and described, as well as the chemical reactors suitable for solar energy absorption, conversion, and storage. Full article

In order to successfully study the condensation and separation of a steam–CO₂ mixture, a boundary layer model was applied to the mixture condensation of steam and CO₂ on horizontal and vertical plates. The modified condensation boundary layer model of steam and CO₂, given the CO₂ solubility in the condensate, was established, numerically solved, and verified with existing experimental data. Different condensation data of steam–air and steam–CO₂ mixtures were compared, and the effect of CO₂ solubility on the mixed gas condensation was analyzed under multiple pressure conditions (1 atm–10 MPa). The simulation data show that the presence of CO₂ will deteriorate the condensation heat transfer, just like air. Given that CO₂ is slightly soluble, some CO₂ can pass through the gas–liquid interface to enter the condensate film and reduce the accumulated CO₂ on the gas–liquid interface, which improves the condensation. However, the solubility of CO₂ is only significant under high-pressure conditions, inducing its effects on condensation. A comparison of the condensation coefficients of the steam–CO₂ mixture shows the lower impact of CO₂ condensation on the horizontal plate compared to that on the vertical plate. For most conditions, the steam–CO₂ mixture gas condensation heat transfer coefficient on the vertical plate surface is still larger than that on the horizontal plate surface, and the improvement in the condensation heat transfer coefficient caused by low CO₂ solubility (2 or 10%) at 10 MPa on the vertical plate is also larger than that of the horizontal plate. Full article

Water–gas shift (WGS) is an industrial process to tackle CO abatement and H₂ upgradation. The syngas (CO and H₂ mixture) obtained from steam or dry reformers often has unreacted (from dry reforming) or undesired (from steam reforming) CO₂, which is subsequently sent to downstream WGS reactor for H₂ upgradation. Thus, industrial processes must deal with CO₂ and H₂ in the reformate feed. Achieving high CO₂ or H₂ selectivities become challenging due to possible CO and CO₂ methanation reactions, which further increases the separation costs to produce pure H₂. In this study, M/Co₃O₄-ZrO₂ (M = Ru, Pd and Pt) catalysts were prepared using sonochemical synthesis. The synthesized catalysts were tested for WGS activity under three feed conditions, namely, Feed A (CO and steam), Feed B (CO, H₂ and steam) and Feed C (CO, H₂, CO₂ and steam). All the catalysts gave zero methane selectivity under Feed A conditions, whereas the methane selectivity was significant under Feed B and C conditions. Among all catalysts, PtCZ was found to be the best performing catalyst in terms of CO conversion and CO₂ selectivity. However, it still suffered with low but significant methane selectivity. This best performing catalyst was further modified with an alkali component, potassium to suppress undesirable methane selectivity. All the catalysts were well characterized with BET, SEM, TEM to confirm the structural properties and effective doping of the noble metals. Additionally, the apparent activation energies were obtained to showcase the best catalyst. Full article

Carbon dioxide (CO₂) is generally unavoidable during the production of fuel gases such as hydrogen (H₂) from steam reformation and syngas composed of carbon monoxide (CO) and hydrogen (H₂). Efficient separation of CO₂ from these gases is highly important to improve the energetic utilization efficiency and prevent poisoning during specific applications. Metal–organic frameworks (MOFs), featuring ordered porous frameworks, high surface areas and tunable pore structures, are emerging porous materials utilized as solid adsorbents for efficient CO₂ capture and separation. Furthermore, the construction of hierarchical MOFs with micropores and mesopores could further promote the dynamic separation processes, accelerating the diffusion of gas flow and exposing more adsorptive pore surface. Herein, we report a simple, efficient, one-pot template-mediated strategy to fabricate a hierarchically porous CuBTC (CuBTC-Water, BTC = 1,3,5-benzenetricarboxylate) for CO₂ separation, which demonstrates abundant mesopores and the superb dynamic separation ability of CO₂/N₂. Therefore, CuBTC-Water demonstrated a CO₂ uptake of 180.529 cm³ g⁻¹ at 273 K and 1 bar, and 94.147 cm³ g⁻¹ at 298 K and 1 bar, with selectivity for CO₂/N₂ mixtures as high as 56.547 at 273 K, much higher than microporous CuBTC. This work opens up a novel avenue to facilely fabricate hierarchically porous MOFs through one-pot synthesis for efficient dynamic CO₂ separation. Full article

The purification of hydrogen (H₂) has been a vital step in H₂ production processes such as steam–methane reforming. By first-principle calculations, we revealed the potential applications of holey TMC₆ (TM = Mo and W) membranes in H₂ purification. The adsorption and diffusion behaviors of five gas molecules (including H₂, N₂, CO, CO₂, and CH₄) were compared on TMC₆ membranes with different phases. Though the studied gas molecules show weak physisorption on the TMC₆ membranes, the smaller pore size makes the gas molecules much more difficult to permeate into h-TMC₆ rather than into s-TMC₆. With suitable pore sizes, the s-TMC₆ structures not only show an extremely low diffusion barrier (around 0.1 eV) and acceptable permeance capability for the H₂ but also exhibit considerably high selectivity for both H₂/CH₄ and H₂/CO₂ (>10¹⁵), especially under relatively low temperature (150–250 K). Moreover, classical molecular dynamics simulations on the permeation process of a H₂, CO₂, and CH₄ mixture also validated that s-TMC₆ could effectively separate H₂ from the gas mixture. Hence, the s-MoC₆ and s-WC₆ are predicted to be qualified H₂ purification membranes, especially below room temperature. Full article

Reduction of greenhouse gases emissions is a key challenge for the power generation industry, requiring the implementation of new designs and methods of electricity generation. This article presents a design solution for a novel thermodynamic cycle with two new devices—namely, a wet combustion chamber and a spray-ejector condenser. In the proposed cycle, high temperature occurs in the combustion chamber because of fuel combustion by pure oxygen. As a consequence of the chemical reaction and open water cooling, a mixture of H₂O and CO₂ is produced. The resulting working medium expands in one turbine that combines the advantages of gas turbines (high turbine inlet temperatures) and steam turbines (full expansion to vacuum). Moreover, the main purpose of the spray-ejector condenser is the simultaneous condensation of water vapour and compression of CO₂ from condensing pressure to about 1 bar. The efficiency of the proposed cycle has been estimated at 37.78%. COM-GAS software has been used for computational flow mechanics simulations. The calculation considers the drop in efficiency due to air separation unit, carbon capture, and spray-ejector condenser processes. The advantage of the proposed cycle is its compactness that can be achieved by replacing the largest equipment in the steam unit. The authors make reference to a steam generator, a conventional steam condenser, and the steam-gas turbine. Instead of classical heat exchanger equipment, the authors propose non-standard devices, such as a wet combustion chamber and spray-ejector condenser. Full article

This work recovered Ni or Cu cations from simulated electroplating wastewater to synthesize Ni/Cu nano-catalysts for H₂ generation by ethanol steam reforming (ESR). Aluminum lathe waste was used as a framework to prepare the structured catalyst. Li–Al–CO₃ layered double hydroxide (LDH) was electrodeposited on the surface of the framework. The LDH was in a platelet-like structure, working as a support for the formation of the precursor of the metal catalysts. The catalytic performance and the coke properties of a 6Cu_6Ni two-stage catalyst configuration herein used for ESR catalytic reaction were studied. The Cu–Ni two-stage catalyst configuration (6Cu_6Ni) yielded more H₂ (~10%) than that by using the Ni-based catalyst (6Ni) only. The 6Cu_6Ni catalyst configuration also resulted in a relatively stable H₂ generation rate vs. time, with nearly no decline during the 5-h reaction. Through the pre-reaction of ethanol-steam mixture with Cu/LiAlO₂ catalyst, the Ni/LiAlO₂ catalyst in the 6Cu_6Ni catalyst configuration could steadily decompose acetaldehyde, and rare acetate groups, which would evolve condensed coke, were formed. The Ni nanoparticles were observed to be lifted and separated by the carbon filaments from the support and had no indication of sintering, contributing to the bare deactivation of the Ni/LiAlO₂ catalyst in 6Cu_6Ni. Full article

Biogas is a significant by-product produced in algae processing and may be used for many different applications, not only as a renewable energy carrier but also as a chemical intermediate in integrated algae-based biorefineries. In this work, the reforming of biogas to H₂/CO₂ mixtures (referred to as SynFeed) as feed for the direct hydrogenation of CO₂ to methanol is investigated. Two conventional processes, namely steam methane and autothermal reforming, with upstream CO₂ separation from raw biogas are compared to novel concepts of direct biogas bi- and tri-reforming. In addition, downstream CO₂ separation from SynFeed using the commercial Selexol process to produce pure H₂ and CO₂ is considered. The results show that upstream CO₂ separation with subsequent steam methane reforming is the most economic process, costing 142.48 €/t_SynFeed, and taking into consideration the revenue from excess hydrogen. Bi-reforming is the most expensive process, with a cost of 413.44 €/t_SynFeed, due to the high demand of raw biogas input. Overall, SynFeed from biogas is more economical than SynFeed from CO₂ capture and water electrolysis (464 €/t_SynFeed), but is slightly more expensive than using natural gas as an input (107 €/_SynFeed). Carbon capture using Selexol comes with costs of 22.58–27.19 €/${\mathrm{t}}_{{\mathrm{CO}}_2}$, where approximately 50% of the costs are derived from the final CO₂ compression. Full article

H₂ permeation and separation properties of two Pd-based composite membranes were evaluated and compared at 400 °C and at a pressure range of 150 kPa to 600 kPa. One membrane was characterized by an approximately 8 μm-thick palladium (Pd)-gold (Au) layer deposited on an asymmetric microporous Al₂O₃ substrate; the other membrane consisted of an approximately 11 μm-thick pure palladium layer deposited on a yttria-stabilized zirconia (YSZ) support. At 400 °C and with a trans-membrane pressure of 50 kPa, the membranes showed a H₂ permeance of 8.42 × 10⁻⁴ mol/m²·s·Pa^0.5 and 2.54 × 10⁻⁵ mol/m²·s·Pa^0.7 for Pd-Au and Pd membranes, respectively. Pd-Au membrane showed infinite ideal selectivity to H₂ with respect to He and Ar at 400 °C and a trans-membrane pressure of 50 kPa, while the ideal selectivities for the Pd membrane under the same operating conditions were much lower. Furthermore, the permeation tests for ternary and quaternary mixtures of H₂, CO, CO₂, CH₄, and H₂O were conducted on the Pd/YSZ membrane. The H₂ permeating flux decreased at the conclusion of the permeation tests for all mixtures. This decline however, was not permanent, i.e., H₂ permeation was restored to its initial value after treating the membrane with H₂ for a maximum of 7 h. The effects of gas hourly space velocity (GHSV) and the steam-to-carbon (S/C) ratio on H₂ permeation were also investigated using simulated steam methane reforming mixtures. It was found that H₂ permeation is highest at the greatest GHSV, due to a decline in the concentration polarization effect. Variations in S/C ratio however, showed no significant effect on the H₂ permeation. The permeation characteristics for the Pd/YSZ membrane were also investigated at temperatures ranging from 350 to 400 °C. The pre-exponential factor and apparent activation energy were found to be 5.66 × 10⁻⁴ mol/m²·s·Pa^0.7 and 12.8 kJ/mol, respectively. Scanning Electron Microscope (SEM) and X-ray diffraction (XRD) analyses were performed on both pristine and used membranes, and no strong evidence of the formation of Pd-O or any other undesirable phases was observed. Full article

Autoclaving of food wastes (FW) for the resource recovery and reutilization was studied using the pilot plant scale. Experiments were conducted at various temperatures of 408, 428, and 438 K and times of 15 and 60 min. The in-filled steam to the autoclave was supplied by the incineration plant with a gauge pressure of 7 kg/cm² and a temperature of 443 K or above. The results obtained from the experiments show that the less energy- and time-consuming autoclaving conditions (408 K and 15 min, denoted as Case A408-15) are effective. Comparisons of the properties and characteristics of autoclaved FW (FW_A) of Case A408-15 with those of FW are made. The wet bulk volume and wet bulk density of FW _A are dramatically reduced to 15.64% and increased to 313.37% relative to those of FW, respectively. This makes the subsequent processing and reuse for FW_A more convenient than FW. The autoclaving results in an increase of carbon content and a decrease of nitrogen content, and thus an increase of the C/N ratio of FW_A. The contents of sulfur, hemi-cellulose, and cellulose of FW_A are also reduced. All these fluctuations are beneficial for making compost or other usages from FW_A than FW. The autoclaved liquid product (LA) separated from FW_A and liquid condensate (LC) from the released gas possess high COD and TOC. These two liquids can be mixed for use as liquid fertilizers with proper conditioning. Alternatively, further anaerobic digestion of the mixture of FW_A, LA, and LC can offer enhanced biogas production for power generation. All these thus match the appeal of sustainable materials management and circular economy. The emitted gas from autoclaving contains no CO and some hydrocarbons. Suitable air pollution control is needed. The results and information obtained are useful for the proper recovery and reuse of abundant food wastes from domestic households and food industries. Full article

Chemical looping combustion is considered an indirect method of oxidizing a carbonaceous fuel, utilizing a metal oxide oxygen carrier to provide oxygen to the fuel. The advantage is the significantly reduced energy penalty for separating out the CO₂ for reuse or sequestration in a carbon-constrained world. One of the major issues with chemical looping combustion is the cost of the oxygen carrier. Hematite ore is a proposed oxygen carrier due to its high strength and resistance to mechanical attrition, but its reactivity is rather poor compared to tailored oxygen carriers. This problem is further exacerbated by methane cracking, the subsequent deposition of carbon and the inability to transfer oxygen at a sufficient rate from the core of the particle to the surface for fuel conversion to CO₂. Oxygen needs to be readily available at the surface to prevent methane cracking. The purpose of this work was to demonstrate the use of steam to overcome this issue and improve the conversion of the natural gas to CO₂, as well as to provide data for computational fluid dynamics (CFD) validation. The steam will gasify the deposited carbon to promote the methane conversion. This work studies the performance of hematite ore with methane and steam mixtures in a 5 cm fluidized bed up to approximately 140 kPa. Results show an increased conversion of methane in the presence of steam (from 20–45% without steam to 60–95%) up to a certain point, where performance decreases. Adding steam allows the methane conversion to carbon dioxide to be similar to the overall methane conversion; it also helped to prevent carbon accumulation from occurring on the particle. In general, the addition of steam to the feed gas increased the methane conversion. Furthermore, the addition of steam caused the steam methane reforming reaction to form more hydrogen and carbon monoxide at higher steam and methane concentrations, which was not completely converted at higher concentrations and at these residence times. Full article