CO2 Reduction to Substitute Natural Gas: Toward a Global Low Carbon Energy System

Methane has proven to be an outstanding energy carrier and is the main component of natural gas and substitute natural gas (SNG). SNG may be synthesized from the CO₂ and hydrogen available from various sources and may be introduced into the existing infrastructure used by the natural gas sector for transport and distribution to power plants, industry, and households. Renewable SNG may be generated when H₂ is produced from renewable energy sources, such as solar, wind, and hydro. In parallel, the use of CO₂-containing feed streams from fossil origin or preferably, from biomass, permits the avoidance of CO₂ emissions. In particular, the biomass-to-SNG conversion, combined with the use of renewable H₂ obtained by electrolysis, appears a promising way to reduce CO₂ emissions considerably, while avoiding energy intensive CO₂ separation from the bio feed streams. The existing technologies for the production of SNG are described in this short review, along with the need for renewed research and development efforts to improve the energy efficiency of the renewables-to-SNG conversion chain. Innovative technologies aiming at a more efficient management of the heat delivered in the exothermic methanation process are therefore highly desirable. The production of renewable SNG through the Sabatier process is a key process to the transition towards a global sustainable energy system, and is complementary to other renewable energy carriers such as methanol, dimethyl ether, formic acid, and Fischer-Tropsch fuels.     

Sorption-enhanced dimethyl ether synthesis—Multiscale reactor modeling

As an opportunity for the attenuation of atmospheric CO₂ emissions, conversion of carbon dioxide into valuable oxygenates as fuel additives or fuel surrogates was explored conceptually in terms of a potentially feasible dimethyl ether (DME) conversion process. Incentives for application of conventional CO₂–DME conversion process are insufficient due to low CO₂ conversion, and DME yield and selectivity. In-situ H₂O removal by adsorption (sorption-enhanced reaction process) can lead to the displacement of the water gas shift equilibrium and therefore, the enhancement of CO₂ conversion into methanol and the improvement of DME productivity. A two-scale, isothermal, unsteady-state model has been developed to evaluate the performance of a sorption-enhanced DME synthesis reactor. Modeling results show that under H₂O removal conditions, methanol and DME yields and DME selectivity are favoured and the methanol selectivity decreases. The increase of methanol and DME yields and DME selectivity becomes more important at higher CO₂ feed concentration because a relatively large amount of water is produced followed by a large quantity of water removed from the system. Also, the drop in the fraction of unconverted methanol becomes more important when CO₂ feed concentration is higher and the dehydration reaction is favoured. Therefore, application of the sorption-enhanced reaction concept allows the use of CO₂ as a constituent of the synthesis gas as the in-situ H₂O removal accelerates the reverse water gas shift reaction.     

Investigation of the Phase Equilibria of CO2/CH3OH/H2O and CO2/CH3OH/H2O/H2 Mixtures

The storage of excess electricity from renewable energy sources is nowadays a crucial topic. One promising technology is the methanol (CH₃OH) synthesis from H₂/CO₂ mixtures. The achievable one‐pass conversion is limited within this exothermic equilibrium reaction. A possibility to overcome this limitation would be withdrawing CH₃OH and H₂O from the gas phase through in situ condensation under reaction conditions. In this work, the phase equilibrium for mixtures representative for different degrees of conversion was studied. A view cell was employed to determine systematically the single‐ and two‐phase regimes and obtain phase envelopes for mixtures of H₂, CO₂, CH₃OH, and H₂O from 66 to 305 °C and 61 to 233 bar. Furthermore, the densities in the single‐phase area were determined and quantified by an empirical model.     

Effects of space velocity on methanol synthesis from Co2/Co/H2 over Cu/ZnO/Al2O3 catalyst

The space velocity had profound and complicated effects on methanol synthesis from CO₂/CO/H₂ over Cu/ZnO/Al₂O₃ at 523 K and 3.0MPa. At high space velocities, methanol yields as well as the rate of methanol production increased continuously with increasing CO₂ concentration in the feed. Below a certain space velocity, methanol yields and reaction rates showed a maximum at CO₂ concentration of 5–10%. Different coverages of surface reaction intermediates on copper appeared to be responsible for this phenomenon. The space velocity that gave the maximal rate of methanol production also depended on the feed composition. Higher space velocity yielded higher rates for CO₂/ H₂ and the opposite effect was observed for the CO/H₂ feed. For CO₂/CO/H₂ feed, an optimal space velocity existed for obtaining the maximal rate.     

Methanol Synthesis on Potassium-Modified Cu(100) from CO + H2 and CO + CO2 + H2

Methanol cannot be produced from CO + H₂ on a clean copper surface, but a promotional effect of potassium on methanol synthesis from mixtures of CO + H₂ and CO + CO₂ + H₂ at a total pressure of 1.5 bar on a Cu(100) surface is shown in this work. The experiments are performed in a UHV chamber connected with a high-pressure cell (HPC). The methanol produced is measured with a gas chromatograph and the surface is characterized with surface science techniques. The results show that potassium is a promoter for the methanol synthesis from CO + H₂, and that the influence of CO₂ is negligible. Investigation of the post-reaction surface with TPD indicates that potassium carbonate is present and plays an important role. The activation energy is determined as 42 ± 3 kJ/mol for methanol synthesis on K/Cu(100) from CO + H₂.     

ALTERNATIVE GENERATION OF H2, CO AND H2, CO2 MIXTURES FROM STEAM-CARBON DIOXIDE REFORMING OF METHANE AND THE WATER GAS SHIFT WITH PERMEABLE (MEMBRANE) REACTORS

A new process is proposed which converts CO₂ and CH₄ containing gas streams to synthesis gas, a mixture of CO and H₂ via the catalytic reaction scheme of steam-carbon dioxide reforming of methane or the respective one of only carbon dioxide reforming of methane, in permeable (membrane) reactors. The membrane reformer (permreactor) can be made by reactive or inert materials such as metal alloys, microporous ceramics, glasses and composites which all are hydrogen permselective. The rejected CO reacts with steam and converted catalytically to CO₂ and H₂ via the water gas shift in a consecutive permreactor made by similar to the reformer materials and alternatively by high glass transition temperature polymers. Both permreactors can recover H₂ in permeate by using metal membranes, and H₂ rich mixtures by using ceramic, glass and composite type permselective membranes. H₂ and CO₂ can be recovered simultaneously in water gas shift step after steam condensation by using organic polymer membranes. Product yields are increased through permreactor equilibrium shift and reaction separation process integration.

CO and H₂ can be combined in first step to be used for chemical synthesis or as fuel in power generation cycles. Mixtures of CO₂ and H₂ in second step can be used for synthesis as well (e.g., alternative methanol synthesis) and as direct feed in molten carbonate fuel cells. Pure H₂ from the above processes can be used also for synthesis or as fuel in power systems and fuel cells. The overall process can be considered environmentally benign because it offers an in-situ abatement of the greenhouse CO₂ and CH₄ gases and related hydrocarbon-CO₂ feedstocks (e.g., coal, landfill, natural, flue gases), through chemical reactions, to the upgraded calorific value synthesis gas and H₂, H₂ mixture products.     

Methanol Synthesis from CO2, CO,and H2over Cu(100) and Ni/Cu(100)

The catalytic activity of Cu(100) and Ni/Cu(100) with respect to the methanol synthesis from various mixtures containing CO₂, CO, and H₂have been studied in a combined UHV/high pressure cell apparatus at reaction conditions,P_tot=1.5 bar andT=543 K. For the clean Cu(100) surface it is found that admission of CO to a reaction mixture containing CO₂and H₂does not lead to an increase in the rate of methanol formation, which indirectly suggests that the role of CO in the industrial methanol process relates to the change in reduction potential of the synthesis gas. For the Ni/Cu(100) surface it is found that Ni does not promote the rate of methanol formation from mixtures containing CO₂and H₂. In opposition, admission of CO to the reaction mixture leads to a significant increase in the rate of methanol formation with a turnover frequency/Ni site∼60×the turnover frequency/Cu site at Ni coverages below 0.1 ML making it a rather substantial promoting effect. It is found that the admission of CO to the synthesis gas creates segregation of Ni to the surface, whereas this is not the case for a reaction involving CO₂and H₂. It is suggested that CO acts strictly as a promotor in the system and we ascribe the increase in activity to a promotion through gas phase induced surface segregation of Ni.     

Simultaneous absorption of carbon dioxide and hydrogen sulfide into aqueous blends of 2-amino-2-methyl-1-propanol and diethanolamine

This work presents an experimental and theoretical investigation of the simultaneous absorption of CO₂ and H₂S into aqueous blends of 2-amino-2-methyl-1-propanol (AMP) and diethanolamine (DEA). The effect of contact time, temperature and amine concentration on the rate of absorption and the selectivity were studied by absorption experiments in a wetted wall column at atmospheric pressure and constant feed gas ratio. The diffusion-reaction processes for CO₂ and H₂S mass transfer in blended amines are modeled according to Higbie's penetration theory with the assumption that all reactions are reversible. The blended amine solvent (AMP+DEA+H₂O) has been found to be an efficient mixed solvent for simultaneous absorption of CO₂ and H₂S. By varying the relative amounts of AMP and DEA the blended amine solvent can be used as an H₂S-selective solvent or an efficient solvent for total removal of CO₂ and H₂S from the gas streams. Predicted results, based on the kinetics-equilibrium-mass transfer coupled model developed in this work, are found to be in good agreement with the experimental results of rates of absorption of CO₂ and H₂S into (AMP+DEA+H₂O) of this work.     

CO2 hydrogenation to methanol at pressures up to 950 bar

The methanol synthesis from CO₂ hydrogenation is of great interest because it offers a way to mitigate the anthropogenic CO₂ emissions and gives the opportunity to produce methanol from renewable and recyclable feedstock. Methanol is a key component in the chemical industry and can serve as fuel. In this work the high pressure approach of the transformation of CO₂ to methanol is investigated based on the energy balance for the production of 1 Mt methanol per year from air-captured CO₂ and hydrogen from water electrolysis. The energy efficiency is almost pressure independent and is comparable to literature values. The energy consumption for the compression of CO₂ and H₂ accounts only for 26% of the total energy consumption. Experimental investigations of the CO₂ hydrogenation at 950 bar show up to 15 times larger methanol space time yields (STY_methanol) compared to literature values where CO₂ was hydrogenated to methanol at 30 bar.     

Hydrophobic protic ionic liquids tethered with tertiary amine group for highly efficient and selective absorption of H2S from CO2

Developing absorbents with both high absorption capacity of H₂S and large selectivity of H₂S/CO₂ is very important for natural gas sweetening process. To this end, a class of novel hydrophobic protic ionic liquids (ILs) containing free tertiary amine group as functional site for the absorption of H₂S were designed in this work. They were facilely synthesized through a simple neutralization‐metathesis methodology by utilizing diamine compounds and bis(trifluoromethylsulfonyl)imide as the building blocks for cation and anion, respectively. Impressively, the solubility of H₂S can reach 0.546 mol mol^?1 (1 bar) and 0.225 mol mol^?1 (0.1 bar), and the selectivity of H₂S/CO₂ can reach 37.2 (H₂S solubility at 1 bar vs. CO₂ solubility at 1 bar) and 15.4 (H₂S solubility at 0.1 bar vs. CO₂ solubility at 1 bar) in the hydrophobic protic ILs at 298.2 K. Comparing the hydrophobic protic ILs with other absorbents justifies their superior performance in the selective absorption of H₂S from CO₂. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4480–4490, 2016     

Effects of zirconia promotion on the activity of Cu/SiO2 for methanol synthesis from CO/H2 and CO2/H2

The effect of zirconia promotion on Cu/SiO₂ for the hydrogenation of CO and CO₂ at 0.65 MPa has been investigated at temperatures between 473 and 573 K. With increasing zirconia loading, the rate of methanol synthesis is greatly enhanced for both CO and CO₂ hydrogenation, but more significantly for CO hydrogenation. For example, at 533 K the methanol synthesis activity of 30.5 wt% zirconia-promoted Cu/SiO₂ is 84 and 25 times that of unpromoted Cu/SiO₂ for CO and CO₂ hydrogenation, respectively. For all catalysts, the rate of methanol synthesis from CO₂/H₂ is higher than that from CO/H₂. The apparent activation energy for methanol synthesis from CO decreases from 22.5 to 17.5 kcal/mol with zirconia addition, suggesting that zirconia alters the reaction pathway. For CO₂ hydrogenation, the apparent activation energies (~12 kcal/mol) for methanol synthesis and the reverse water-gas shift (RWGS) reaction are not significantly affected by zirconia addition. While zirconia addition greatly increases the methanol synthesis rate for CO₂ hydrogenation, the effect on the RWGS reaction activity is comparatively small. The observed effects of zirconia are interpreted in terms of a mechanism which zirconia serves to adsorb either CO or CO₂, whereas Cu serves to adsorb H₂. It is proposed that methanol is formed by the hydrogenation of the species adsorbed on zirconia.     

Highly selective absorption separation of H2S and CO2 from CH4 by novel azole-based protic ionic liquids

Recently, the selective removal of H₂S and CO₂ has been highly desired in natural gas sweetening. Herein, four novel azole-based protic ionic liquids (PILs) were designed and prepared through one-step neutralization reaction. The solubility of H₂S (0–1.0 bar), CO₂ (0–1.0 bar), and CH₄ (0–5.0 bar) was systematically measured at temperatures from 298.2 to 333.2 K. NMR and theoretical calculation were used to investigate the reaction mechanism between these PILs and H₂S. Reaction equilibrium thermodynamic model (RETM) was screened to correlate the H₂S solubility. Impressively, 1,5-diazabicyclo[4,3,0] non-5-ene 1,2,4-1H-imidazolide ([DBNH][1,2,4-triaz]) shows the highest H₂S solubility (1.4 mol/mol or 7.3 mol/kg at 298.2 K and 1.0 bar) and superior H₂S/CH₄ (831) and CO₂/CH₄ (199) selectivities compared with literature results. Considering the excellent absorption capacity of H₂S, high H₂S/CH₄, and CO₂/CH₄ selectivity, acceptable reversibility, as well as facile preparation process, it is believed that azole-based PILs provide an attractive alternative in natural gas upgrading process.     

Investigations on the ability of di‐isopropanol amine solution for removal of CO2 from natural gas in porous polymeric membranes

In this study, capture of CO₂ and H₂S from natural gas mixture using porous polymeric membranes has been investigated numerically to assess the capacity of a novel absorbent, di‐isopropanol amine (DIPA), in CO₂ removal. Diffusion of acid gases through porous polymeric membranes was simulated by employing CFD techniques and considering a gas feed stream, a porous membrane and a reaction medium. For solving conservation equations, finite element method was applied to calculate the rate of CO₂ and H₂S absorption in the membrane. The type of membrane in this work is a hollow‐fiber module. According to the modeling results, a high H₂S removal can be achieved by DIPA absorber. Moreover, CO₂ was captured from natural gas in an efficient manner in low gas/liquid flow rates. POLYM. ENG. SCI., 55:598–603, 2015. © 2014 Society of Plastics Engineers     

Modeling of a Methanol Synthesis Reactor for Storage of Renewable Energy and Conversion of CO2 – Comparison of Two Kinetic Models

The storage of renewable energy over a long time period, via methanol synthesis, will become very important to reach a greenhouse gas‐free energy supply. A steady‐state methanol synthesis flowsheet, containing a 2D pseudo‐homogeneous reactor, flash, and recycle, is modeled in MATLAB. With the kinetic models of Graaf and Bussche & Froment, two frequently used kinetic models for conventional methanol synthesis are compared and evaluated for applicability regarding methanol synthesis from CO₂/H₂. The results are presented for different cases of synthesis gas compositions. Both kinetic models produce similar results when the system is limited by thermodynamic equilibrium. However, differences in reaction rates are observable from the reactor axial molar component profiles of the reaction products.     

Location Planning for the Production of CO2-Based Chemicals Using the Example of Olefin Production

A methodology for identifying suitable locations for the CO₂-based production of olefins in Germany is presented. Based on electricity and CO₂ requirements, locations are identified that can provide sufficient CO₂ and renewable energy for the conversion of CO₂ to olefins. In addition, the use of existing infrastructures is taken into account. The regional, technical renewable energy potential in Germany is sufficient to produce ∼ 800 kt of olefins from CO₂-based methanol per year in one plant. But the currently available CO₂ point sources with high CO₂ concentrations of around 100 % are not sufficient to meet the CO₂ requirement of an 800 kt a⁻¹ methanol-to-olefins plant. If existing refineries are preferred due to existing infrastructure services, locations in the north of Cologne, in Lower Saxony, and in Brandenburg are particularly suitable. A full substitution of fossil olefins by CO₂-based olefins is possible in Germany. The challenge is to provide sufficient renewable electricity for the production of H₂ with a low CO₂ intensity.     

In situ IR studies on the mechanism of methanol synthesis from CO/H2 and CO2/H2 over Cu-ZnO-Al2O3 catalyst

Methanol synthesis from CO/H₂ and CO₂/H₂ was carried out at atmospheric pressure over Cu/ZnO/Al₂O₃ catalyst. The formation and variation of surface species were recorded by in situ FT-IR spectroscopy. The result revealed that both CO and CO₂ can serve as the primary carbon source for methanol synthesis. For CO/H₂ feed gas, only HCOO-Zn was detected; however, for CO₂/H₂, both HCOO-Zn and HCOO-Cu were observed, and without CH₃O-Cu. HCOO-Zn was the key intermediate. A scheme of methanol synthesis and reverse water-gas shift (RGWS) reaction was proposed.     

Optimal Design Issues of a Methanol Synthesis Reactor from CO2 Hydrogenation

Methanol production through CO₂ hydrogenation was investigated in a series of fixed-bed reactors. Staging of the reactor into several smaller reactors is considered to enhance methanol production, in addition to maximizing a measure of annual profit. The degrees of freedom of the reactor system are the number of stages, the cooling medium temperature, the heat transfer area, and the volume of the stages. Depending on the objective function (OF) criteria, staging of the reactor increases the OF values to various extents. When the objective is to maximize the methanol production, the OF of a three-stage reactor system with an inlet H₂/CO₂ ratio of 2 is 2.61 % higher than in the single-stage configuration. Staging of the reactor also increases the synthesis gas conversion to methanol. However, if maximizing the annual profit is the objective, the profitability of the two-stage configuration is 2.05 % greater than in the case with one reactor, due to the higher methanol production of the staged reaction system.     

Oxide materials research for global environment and energy with a focus on CO2 fixation into polymers

Representing the gas, liquid, and solid phase materials on the earth under the standard conditions, CO₂, H₂O, and SiO₂ attract my interest for their natural abundance, chemical stability, and yet fundamental roles in energy and environmental problems. Chemical consideration is made on the utilization of these stable resources for renewable clean energy materials and processes based on a thermodynamic approximation. A general scheme is presented to compare the utilization of CO₂, H₂O, and SiO₂ for such purposes as CO₂ fixation into functional polymers, photosynthesis of carbohydrates from CO₂ and H₂O, and solar silicon production from SiO₂. Focused is CO₂ copolymerization, since it is originated from our discovery of alternating CO₂–epoxide (oxirane) copolymer in 1968 and it has been revived by the recent industrialization of this unique bio-degradable polymer in China. New ideas of using combinatorial chemical technology and soft plasma processing for fixing CO₂ into polymers are proposed together with some preliminary experimental results.     

CO2 utilization for methanol production; Optimal pathways with minimum GHG reduction cost

Mitigating CO₂ emissions from industries and other sectors of our economy is a critical component of building a sustainable economy. This paper investigates two different methanol synthesis routes based on CO₂ utilization (CO₂ capture and utilization [CCU], and tri-reforming of methane [TRM]), and compares the results with the conventional methanol production using natural gas as the feedstock (NG-MeOH). A comprehensive techno-economic analysis (TEA) model that includes the findings of the life cycle assessment (LCA) models of methanol production using various CO₂ utilization pathways is conducted. Economic analysis is conducted by developing a cost model that is connected to the simulation models for each production route. Compared to the conventional process (with a GHG emission of 0.6 kg CO₂/kg MeOH), the lifecycle GHG reduction of 1.75 and 0.41 kg CO₂/kg MeOH are achievable in the CCU and TRM pathways, respectively. Furthermore, the results indicate that, under current market conditions and hydrogen production costs, methanol production via CO₂ hydrogenation will result in a cost approximately three times higher than that of the conventional process. The integrated TEA–LCA model shows that this increased cost of production equates to a required life cycle GHG reduction credit of $279 to $422 per tonne of CO₂ utilized, depending on construction material and selected pathway. Additionally, when compared to the CO₂ hydrogenation route, the tri-reforming process (TRM-MeOH) can result in a 42% cost savings. Furthermore, a minimum financial support of $56 per tonne utilized CO₂ will be required to make the TRM-MeOH process economically viable.     

Exclusion of CO and CO2 from ammonia synthesis gas by an electrocatalytic process

A new exclusion process for CO and CO₂ from ammonia synthesis gas has been proposed: this takes place at room temperature and atmospheric pressure. The process is based on the electrochemical reduction of CO and CO₂ to methanol proceeding at a mediated electrode via homogeneous catalysis. The maximum percentages of CO and CO₂ excluded from the initial gas were about 1.5 and 4.1%, respectively, with a mediated electrode of 82.8 cm² area in a reaction time of 5 h. The amount of excluded CO and CO₂ was equivalent to the sum of moles of methanol formed and gases dissolved into the solution alone. The electroreduction of CO and CO₂ was more efficient at three-phase (electrode/solution/gas) and at two-phase (electrode/solution) interfaces, respectively.