Energy consumption estimation for greenhouse gas separation processes by clathrate hydrate formation

The process energy consumption was estimated for gas separation processes by the formation of clathrate hydrates. The separation process is based on the equilibrium partition of the components between the gaseous phase and the hydrate phase. The separation and capturing processes of greenhouse gases were examined in this study. The target components were hydrofluorocarbon (HFC-134a) from air, sulfur hexafluoride (SF₆) from nitrogen, and CO₂ from flue gas. Since these greenhouse gases would form hydrates under much lower pressure and higher temperature conditions than the accompanying components, the effective capturing of the greenhouse gases could be achieved by using hydrate formation. A model separation process for each gaseous mixture was designed from the basis of thermodynamics, and the process energy consumption was estimated. The obtained results were then compared with those for conventional separation processes such as liquefaction separation processes. For the recovery of SF₆, the hydrate process is preferable to liquefaction process in terms of energy consumption. On the other hand, the liquefaction process consumes less energy than the hydrate process for the recovery of HFC-134a. The capturing of CO₂ by the hydrate process from a flue gas will consume a considerable amount of energy; mainly due to the extremely high pressure conditions required for hydrate formation. The influences of the operation conditions on the heat of hydrate formation were elucidated by sensitivity analysis. The hydrate processes for separating these greenhouse gases were evaluated in terms of reduction of global warming potential (GWP).     

High-efficiency separation of a CO2/H2 mixture via hydrate formation in W/O emulsions in the presence of cyclopentane and TBAB

CO₂/H₂ mixtures, such as integrated gasification combined cycle (IGCC) syngas, were separated via hydrate formation in water-in-oil (W/O) emulsions. The oil phase was composed of diesel and cyclopentane (CP). Span 20 was used to disperse the aqueous phase or hydrate in the oil phase, and tetra-n-butyl ammonium bromide (TBAB) was added to produce a synergistic effect with CP. The experimental results show that the presence of TBAB can remarkably increase the separation ability and improve the flow behavior of the hydrate slurry. The most suitable contents of TBAB in the aqueous phase and water in the emulsion were determined to be 0.29 mol% and 35 vol%, respectively. The maximum separation factor of CO₂ over H₂ was 103, which is much higher than the literature values for separating CO₂/H₂ gas mixture via hydrate formation. After a two-stage separation, hydrogen was enriched from 53.2 to 97.8 mol%. The influence of temperature, pressure, and the initial gas–liquid volume ratio on the separation ability and hydrate formation rate were investigated in detail. In addition, a criterion for choosing the suitable operation conditions was suggested based on both phase equilibrium and kinetic factors. Based on this criterion, the suitable operation temperature, pressure, and gas–liquid volume ratio for the separation of CO₂/H₂ are approximately 270.15 K, 3–5 MPa, and 80–100, respectively.     

Hydrate-based pre-combustion carbon dioxide capture process in the system with tetra-n-butyl ammonium bromide solution in the presence of cyclopentane

Effects of 0.29 mol% tetra-n-butyl ammonium bromide (TBAB) solution in conjunction with cyclopentane (CP) on the hydrate-based pre-combustion CO₂ capture are investigated by the measurements of the gas uptakes, CO₂ separation efficiencies and induction time of the hydrate formation at the different temperature-pressure conditions. The results show that the volume of the TBAB has an effect on the CO₂ separation and the induction time, and the addition of the CP into the TBAB solution remarkably enhances the CO₂ separation and shortens the induction time. The system with the CP/TBAB solution volume ratio of 5 vol% and TBAB solution/reactor effective volume ratio of 0.54 is optimum to obtain the largest gas uptake and the highest CO₂ separation efficiency at 274.65 K and 4.0 MPa. Compared to the results with tetrahydrofuran (THF) as an additive [1], the gas uptake is enhanced by at least 2 times and the induction time is shortened at least 10 times at the similar temperature-pressure condition. In addition, the CO₂ concentration in the decomposed gas from the hydrate slurry phase reaches approximately 93 mol% after the first-stage separation at 274.65 K and 2.5 MPa. The gas uptakes of more than 80 mol% are obtained after 400 s at the temperature range of 274.65-277.65 K and the pressure range of 2.5-4.5 MPa.     

Membrane processes for post-combustion carbon dioxide capture: A parametric study

Much of the research in the area of carbon dioxide recovery and storage focuses on minimizing the energy required for CO₂ capture, as this step corresponds to the major cost contribution of the overall process (capture, transportation, injection). Out of the three traditional methods of CO₂ capture (absorption, adsorption and membrane processes), absorption is considered to be the best available technology for post-combustion application. However, amine absorption requires 4–6 GJ/tonne of recovered CO₂, in a large part due to significant energy consumption associated with the regeneration step.In this paper, we perform a systematic analysis of the separation performances and associated energy cost of a single-stage membrane module. First, the operational limits are identified in terms of permeate composition and CO₂ recovery ratio via a systematic parametric study for CO₂/N₂ mixtures. The energy consumption of the capture step is then evaluated and compared with the performance of amine absorption. Next, the search for an optimal strategy in terms of compression energy for a combination of membrane capture and CO₂ injection has been addressed. The results allow the identification of an optimal membrane pressure ratio for a given set of conditions.     

CO2 capture study in advanced integrated gasification combined cycle

This paper presents the results of technical and economic studies in order to evaluate, in the French context, the future production cost of electricity from IGCC coal power plants with CO₂ capture and the resulting cost per tonne of CO₂ avoided. The economic evaluation shows that the total cost of base load electricity produced in France by coal IGCC power plants with CO₂ capture could be increased by 39% for ‘classical’ IGCC and 28% for ‘advanced’ IGCC. The cost per tonne of avoided CO₂ is lower by 18% in ‘advanced’ IGCC relatively to ‘classical’ IGCC. The approach aimed to be as realistic as possible for the evaluation of the energy penalty due to the integration of CO₂ capture in IGCC power plants. Concerning the CO₂ capture, six physical and chemical absorption processes were modeled with the Aspen Plus™ software. After a selection based on energy performance three processes were selected and studied in detail: two physical processes based on methanol and Selexol™ solvents, and a chemical process using activated MDEA. For ‘advanced’ IGCC operating at high-pressure, only one physical process is assessed: methanol.     

Investigations on the formation kinetics of semiclathrate hydrate of methane in an aqueous solution of tetra-n-butyl ammonium bromide and sodium dodecyl sulfate in porous media

Natural gas hydrate is considered to be an attractive sustainable energy resource for the world. Hydrate as a technology can be of immense importance for various industrial processes, such as multicomponent natural gas separation, gas storage and transportation, and carbon dioxide capture from flue gases and sequestration. A variety of hydrate additives, which includes promoters (thermodynamics and kinetics) and porous media, are being researched to improve the hydrate formation kinetics. However, studies involving the combinations of these are rare in the open literature. In this work, the formation kinetics of methane hydrate/semiclathrate hydrate using tetra-n-butyl ammonium bromide (TBAB) and sodium dodecyl sulfate (SDS) aqueous solutions at various concentrations in a porous medium containing silica sand at initial hydrate formation pressures (7.5 and 5.5 MPa) and temperatures (273.65 and 276.15 K) have been investigated. All the experiments were conducted using 75% water saturation. Various kinetics parameters, such as gas uptake, gas-to-hydrate conversion, and induction time, have been reported. It was found that the combination of TBAB+ SDS showed favorable hydrate formation kinetics in porous media than the TBAB system. This work provides information for further studies involving semiclathrate hydrate applications for various industrial processes.     

Tetra-n-butyl ammonium bromide semi-clathrate hydrate process for post-combustion capture of carbon dioxide in the presence of dodecyl trimethyl ammonium chloride

To determine the appropriate operating conditions for separating carbon dioxide from flue gas via the hydrate formation, the effects of the concentrations of dodecyl trimethyl ammonium chloride (DTAC) in 0.29 mol% Tetra-n-butyl ammonium bromide (TBAB) aqueous solution and the initial pressures on the induction time of the hydrate formation and CO₂ separation efficiency are investigated. The experiments are conducted at the DTAC concentration range of 0–0.056 mol%, initial pressures range of 0.66 MPa–2.66 MPa and temperature range of 274.95 K–277.15 K. The results indicate that the initial pressure of 1.66 MPa in conjunction with the concentration of 0.028 mol% DTAC is most favorable for CO₂ separation. At the condition, the induction time of forming the hydrate can be shortened considerably and CO₂ can be purified from 17.0 mol% to 99.4% with the two-stage hydrate separation process. CO₂ split fractions for Stage 1 and Stage 2 are 0.54 and 0.39, respectively, and the separation factors are 9.60 and 62.25, respectively.     

Comparative study of chemical absorbents in postcombustion CO2 capture

In order to reduce CO₂ emissions from a power plant, CO₂ can be captured either from the syngas that is to be burned or from the flue gases exiting the energy conversion process. Postcombustion capture has the advantage that it can be applied to retrofit existing power plants. In this paper the authors compare two primary amines (MEA and DGA) to ammonia with respect to their capability to capture CO₂ from a flue gas stream. The ammonia process captures CO₂ by formation of stable salts, which are separated from the solvent stream by filtration or sedimentation. These salts can be used commercially as fertilizers. Energy requirements are greatly reduced, since no heat is required for solvent regeneration, and no compression of the separated CO₂ is necessary. Energy, however, is required for the reduction of ammonia emissions. In order to obtain the solid ammonia salts, their solubility has to be reduced by modification of the solvent and by lowering absorption temperature. With and without separation of the salt products, ammonia proved to be an alternative solvent with high CO₂ removal efficiency. Simulation of all processes was carried out with Aspen Plus^® and compared to experimental results for CO₂ scrubbing with ammonia.     

Hydrate-based removal of carbon dioxide and hydrogen sulphide from biogas mixtures: Experimental investigation and energy evaluations

This paper presents an experimental study on the application of gas hydrate technology to biogas upgrading. Since CH₄, CO₂ and H₂S form hydrates at quite different thermodynamic conditions, the capture of CO₂ and H₂S by means of gas hydrate crystallization appears to be a viable technological alternative for their removal from biogas streams. Nevertheless, hydrate-based biogas upgrading has been poorly investigated. Works found in literature are mainly at a laboratory scale and concern with thermodynamic and kinetic fundamental studies. The experimental campaign was carried out with an up-scaled apparatus, in which hydrates are produced in a rapid manner, with hydrate formation times of few minutes. Two types of mixtures were used: a CH₄/CO₂ mixture and a CH₄/CO₂/H₂S mixture. The objective of the investigation is to evaluate the selectivity and the separation efficiency of the process and the role of hydrogen sulphide in the hydrate equilibrium. Results show that H₂S can be captured along with CO₂ in the same process. The maximum value of the separation factor, defined as the ratio between the number of moles of CO₂ and the number of moles of CH₄ removed from the gas phase, is 11. In the gas phase, a reduction of CO₂ of 24.5% in volume is achievable in 30 min.Energy costs of a real 30-min separation process, carried out in the experimental campaign, are evaluated and compared with those obtained from theoretical calculations. Some aspects for technology improvement are discussed.     

Gas hydrate formation process for pre-combustion capture of carbon dioxide

In this study, gas hydrate from CO₂/H₂ gas mixtures with the addition of tetrahydrofuran (THF) was formed in a semi-batch stirred vessel at various pressures and temperatures to investigate the CO₂ separation/recovery properties. This mixture is of interest to CO₂ separation and recovery from Integrated Gasification Combine Cycle (IGCC) power plants. During hydrate formation the gas uptake was determined and composition changes in the gas phase were obtained by gas chromatography. The impact of THF on hydrate formation from the CO₂/H₂ was observed. The addition of THF significantly reduced the equilibrium formation conditions. 1.0 mol% THF was found to be the optimum concentration for CO₂ capture based on kinetic experiments. The present study illustrates the concept and provides thermodynamic and kinetic data for the separation/recovery of CO₂ (pre-combustion capture) from a fuel gas (CO₂/H₂) mixture.     

Biomass-fired cogeneration systems with CO2 capture and storage

In this study, we estimate and analyze the CO₂ mitigation costs of large-scale biomass-fired cogeneration technologies with CO₂ capture and storage. The CO₂ mitigation cost indicates the minimum economic incentive required (e.g. in the form of a carbon tax) to make the cost of a less carbon intensive system equal to the cost of a reference system. If carbon (as CO₂) is captured from biomass-fired energy systems, the systems could in principle be negative CO₂ emitting energy systems. CO₂ capture and storage from energy systems however, leads to reduced energy efficiency, higher investment costs, and increased costs of end products compared with energy systems in which CO₂ is vented. Here, we have analyzed biomass-fired cogeneration plants based on steam turbine technology (CHP-BST) and integrated gasification combined cycle technology (CHP-BIGCC). Three different scales were considered to analyze the scale effects. Logging residues was assumed as biomass feedstock. Two methods were used to estimate and compare the CO₂ mitigation cost. In the first method, the cogenerated power was credited based on avoided power production in stand-alone plants and in the second method the same reference output was produced from all systems. Biomass-fired CHP-BIGCC with CO₂ capture and storage was found very energy and emission efficient and cost competitive compared with other conversion systems.     

Study on a novel process for CO2 compression and liquefaction integrated with the refrigeration process

In order to transport and store the captured CO₂ from coal‐fired power plants, it is necessary to compress and liquefy CO₂ first. However, the power consumption of conventional process is enormous. In this paper, a novel process for CO₂ compression and liquefaction based on the analysis of the power consumption of traditional method is proposed. The new process integrates the refrigeration process driven by the lower level heat from the coal‐fired power plant. This paper analyzes and compares the energy consumptions of conventional process and new process for CO₂ compression and liquefaction. The research result indicates that, when CO₂ needs to be compressed and liquefied and an abundant low quality heat is available, the new process has obvious superiority in lowering the energy consumption. The new process for CO₂ compression and liquation integrated with the exhaust heat powered refrigeration can greatly reduce the work consumption of CO₂ compression and liquefaction. The refrigeration temperature has great effects both on the coefficient of performance of refrigeration process and work consumption of compressors. The refrigeration temperature can be selected by optimization. Using refrigerator with double stages of evaporation can further reduce the amount of the extracted steam and lower the total energy consumption for CO₂ compression and liquation. Recovering the cool energy of CO₂ is beneficial to the reduction of the total work consumption. The achievements obtained from this paper will provide a useful reference for CO₂ compression and liquefaction with the lower energy consumption. Copyright © 2012 John Wiley & Sons, Ltd.     

添加剂对CO_2水合物浆流动特性的影响

搭建了研究水合物浆生成、流动、传热特性的实验系统,对添加TBAB、TWEEN两种防聚剂的CO_2水合物浆与纯水系中的CO_2水合物浆进行比较,从而筛选出流动特性最好的防聚剂种类及其对应的质量分数.结果发现,添加TBAB和TWEEN后能有效阻止水合物聚集成团,提高了浆体的稳定性和均匀性.研究表明:当流速为0.2~0.4 m·s~(-1)时,浆体表现为H-B流体,呈剪切变稀特性;当流速为0.4~0.6 m·s-1时,TBAB质量分数分别为0.3%、0.9%和TWEEN质量分数为0.6%时浆体均表现为宾汉姆流体,呈剪切变浓特性,其余质量分数时浆体与低流速区相似,均表现为H-B流体,呈剪切变稀特性.提高TBAB质量分数,对改善CO_2水合物浆流动效果明显,而当TWEEN质量分数为0.6%时浆体的流动特性最好.     

Techno-economic and environmental performances of glycerol reforming for hydrogen and power production with low carbon dioxide emissions

This paper is evaluating from the conceptual design, thermal integration, techno-economic and environmental performances points of view the hydrogen and power generation using glycerol (as a biodiesel by-product) reforming processes at industrial scale with and without carbon capture. The evaluated hydrogen plant concepts produced 100,000 Nm³/h hydrogen (equivalent to 300 MW_th) with negligible net power output for export. The power plant concepts generated about 500 MW net power output. Hydrogen and power co-generation was also assessed. The CO₂ capture concepts used alkanolamine-based gas–liquid absorption. The CO₂ capture rate of the carbon capture unit is at least 90%, the carbon capture rate of the overall reforming process being at least 70%. Similar designs without carbon capture have been developed to quantify the energy and cost penalties for carbon capture. The various glycerol reforming cases were modelled and simulated to produce the mass & energy balances for quantification of key plant performance indicators (e.g. fuel consumption, energy efficiency, ancillary energy consumption, specific CO₂ emissions, capital and operational costs, production costs, cash flow analysis etc.). The evaluations show that glycerol reforming is promising concept for high energy efficiency processes with low CO₂ emissions.     

Impurity impacts on the purification process in oxy-fuel combustion based CO2 capture and storage system

Based on the requirements of CO₂ transportation and storage, non-condensable gases, such as O₂, N₂ and Ar should be removed from the CO₂-stream captured from an oxy-fuel combustion process. For a purification process, impurities have great impacts on the design, operation and optimization through their impacts on the thermodynamic properties of CO₂-streams. Study results show that the increments of impurities will make the energy consumption of purification increase; and make CO₂ purity of separation product and CO₂ recovery rate decrease. In addition, under the same operating conditions, energy consumptions have different sensitivities to the variation of the impurity mole fraction of feed fluids. The isothermal compression work is more sensitive to the variation of SO₂; while the isentropic compression work is more sensitive to the variation of Ar. In the flash system, the energy consumption of condensation in is more sensitive to the variation of Ar; but in the distillation system, the energy consumption of condensation is more sensitive to the variation of SO₂, and CO₂ purity of separation is more sensitive to the variation of SO₂.     

Synergic effect of cyclopentane and tetra-n-butyl ammonium bromide on hydrate-based carbon dioxide separation from fuel gas mixture by measurements of gas uptake and X-ray diffraction patterns

The synergic effect of Cyclopentane (CP) and Tetra-n-butyl Ammonium Bromide (TBAB) on the hydrate-based carbon dioxide (CO₂) separation from IGCC (Integrated Gasification Combined Cycle) syngas is investigated by measuring the gas uptake and the power X-ray diffraction (PXRD) patterns in this work. The CP with CP/TBAB solution ratio of 5 vol% added into the 0.29 mol% TBAB solution can remarkably increase the gas uptake at 4.0 MPa and 274.65 K. The PXRD patterns of the semi-clathrate (sc) hydrate and structure II (sII) hydrate are obtained for the CP/TBAB/gas/H₂O system. The synergic effect of the CP and the TBAB includes two aspects: On one hand, the CP molecules housed in the hollow centers of the large cavities together with TBAB cations (TBA⁺) make the sc hydrate more stable. On the other hand, the TBA⁺ displaced out of the large cavities by the CP molecules make the ionization reaction of TBAB in the solution going toward the reverse direction. Thus, the more TBAB molecules exist in the solution and form the more sc hydrate, resulting in the considerable increase of the gas uptake.     

CO2 sequestration in depleted methane hydrate deposits with excess water

The recent increase in atmospheric CO₂ concentration makes it necessary to investigate new ways to reduce CO₂ emissions. Simultaneously, natural gas hydrate mining technology is developing rapidly. The use of depleted methane hydrate (MH) deposits as potential sites for CO₂ storage is relatively safe and economical. This method can alleviate the shortage of hydrate displacement gas with CO₂. The purpose of this study was to investigate CO₂ hydrate formation characteristics during the seepage process—in reservoirs with excess water—and their effect on CO₂ storage. The experimental process can be divided into 5 parts: MH formation, water injection, MH dissociation, CO₂ hydrate formation, and CO₂ hydrate dissociation. Magnetic resonance imaging was employed to monitor the distribution of liquid water, and the effects of different parameters on the formation and dissociation of CO₂ hydrates were analyzed. It was found that a state of initial water saturation can effectively control hydrate saturation in artificial MH reservoirs for hydrate reservoirs with excess gas. In the process of CO₂ flow, initial water saturation was not the main controlling factor for CO₂ hydrate formation. Increasing the flow pressure and reducing the flow rate were beneficial for CO₂ hydrate formation. This study is of great significance for advancing the science of CO₂ geological storage in the form of deep‐sea hydrates.     

Performance investigation of a new renewable energy-based carbon dioxide capturing system with aqueous ammonia

In this study, a novel wind energy-based carbon dioxide (CO₂) capturing system is developed and investigated for practical applications to reduce environmental emissions. The aqueous ammonia-based capturing technology is utilized. Wind turbines are used to operate the onsite ammonia synthesis as well as hydrogen production. The proton exchange membrane electrolysis system is considered for hydrogen production and the Haber-Bosch ammonia synthesis technique is utilized. The developed system is modeled in Aspen Plus software. The system performance for CO₂ capture is studied through economic, energy, and exergy perspectives. The CO₂ capture cost is evaluated to be between 0.1 and 0.23 $/kg CO₂. Furthermore, the system CO₂ capture rate is determined to be 3.5 kg/s. Moreover, for a unit mass of CO₂ captured, the energy consumption is found to be 640.1 kg CO₂/MWh. Several parametric studies are also conducted to analyze the effects of varying operating conditions on the system performance.     

The environmental impact of post-combustion CO2 capture with MEA, with aqueous ammonia, and with an aqueous ammonia-ethanol mixture for a coal-fired power plant

In this paper the authors compare monoethanolamine (MEA) to aqueous ammonia (AA) and a solvent mixture of aqueous ammonia and ethanol (EAA) with respect to their post-combustion CO₂ capture performance and their environmental impact. Simulation of all processes was carried out with Aspen Plus^® and compared to experimental results for CO₂ scrubbing with ammonia. Of special interest was the formation of stable salts, which could be observed in the experimental CO₂ capture with both ammonia solvents. If CO₂ can be captured in the form of ammonium salts, energy requirements are greatly reduced, since no energy is required for solvent regeneration and CO₂ compression. The environmental impact of CO₂ capture was investigated for a 500 MW pulverised coal power plant employing Life Cycle Assessment (LCA) using the software SimaPro^®. For a comprehensive evaluation of this impact, influencing factors such as solvent production and solvent emissions were included. With kinetics taken into account, no salt formation could be observed in CO₂ removal with aqueous ammonia. The necessary reduction of ammonia emissions leads to further energy requirements, and solvent production as well as the remaining ammonia losses to the environment have a more significant environmental impact than CO₂ removal with MEA.     

A cryogen‐based peak‐shaving technology: systematic approach and techno‐economic analysis

A peak‐shaving technology is recently proposed, which integrates peak‐electricity generation, cryogenic energy storage and CO₂ capture. In such a technology, off‐peak electricity is used to produce liquid nitrogen and oxygen in an air separation and liquefaction unit. At peak hours, natural gas (or alternative gases, e.g. from gasification of coal) is burned by oxygen from the air separation unit (oxy‐fuel combustion) to generate electricity. CO₂ produced is captured in the form of dry ice. Liquid nitrogen produced in the air separation plant not only serves as an energy storage medium but also supplies the low‐grade cold energy for CO₂ separation. In addition, waste heat from the tail gas can be used to superheat nitrogen in the expansion process to further increase the system efficiency. This article reports a systematic approach, with an aim to provide technical information for the system design. Three potential blending gases (helium, oxygen and CO₂) are considered not only for assessing thermodynamic performance but also for techno‐economic analysis. The peak‐shaving systems are also compared with natural gas combined cycle and an oxy–natural gas combined cycle in terms of capital cost and peak electricity production cost. Copyright © 2011 John Wiley & Sons, Ltd.