Recycling a hydrogen rich residual stream to the power and steam plant

The benefits of using a residual hydrogen rich stream as a clean combustion fuel in order to reduce Carbon dioxide emissions and cost is quantified. A residual stream containing 86% of hydrogen, coming from the top of the demethanizer column of the cryogenic separation sector of an ethylene plant, is recycled to be mixed with natural gas and burned in the boilers of the utility plant to generate high pressure steam and power. The main advantage is due to the fact that the hydrogen rich residual gas has a higher heating value and less CO₂ combustion emissions than the natural gas. The residual gas flowrate to be recycled is selected optimally together with other continuous and binary operating variables. A Mixed Integer Non Linear Programming problem is formulated in GAMS to select the operating conditions to minimize life cycle CO₂ emissions.     

Thermoeconomic analysis of pressurized hybrid SOFC systems with CO2 separation

In this paper, the results of the thermodynamic and economic analyses of distributed power generation plants (1.5 MWe) are described and compared. The results of an exergetic analysis are also reported, as well as the thermodynamic details of the most significant streams of the plants. The integration of different hybrid solid oxide fuel cell (SOFC) system CO₂ separation technologies characterizes the power plants proposed. A hybrid system with a tubular SOFC fed with natural gas with internal reforming has been taken as reference plant. Two different technologies have been considered for the same base system to obtain a low CO₂ emission plant. The first technology involved a fuel decarbonization and CO₂ separation process placed before the system feed, while the second integrated the CO₂ separation and the energy cycle. The first option employed fuel processing, a technology (amine chemical absorption) viable for short-term implementation in real installations while the second option provided the CO₂ separation by condensing the steam from the system exhaust. The results obtained, using a Web-based Thermo Economic Modular Program software, developed by the Thermochemical Power Group of the University of Genoa, showed that the thermodynamic and economic impact of the adoption of zero emission cycle layouts based on hybrid systems was relevant.     

Capturing CO2 in flue gas from fossil fuel-fired power plants using dry regenerable alkali metal-based sorbent

CO₂ capture and storage (CCS) has received significant attention recently and is recognized as an important option for reducing CO₂ emissions from fossil fuel combustion. A particularly promising option involves the use of dry alkali metal-based sorbents to capture CO₂ from flue gas. Here, alkali metal carbonates are used to capture CO₂ in the presence of H₂O to form either sodium or potassium bicarbonate at temperatures below 100 °C. A moderate temperature swing of 120–200 °C then causes the bicarbonate to decompose and release a mixture of CO₂/H₂O that can be converted into a “sequestration-ready” CO₂ stream by condensing the steam. This process can be readily used for retrofitting existing facilities and easily integrated with new power generation facilities. It is ideally suited for coal-fired power plants incorporating wet flue gas desulfurization, due to the associated cooling and saturation of the flue gas. It is expected to be both cost effective and energy efficient.     

Review of CO2 recovery methods from the exhaust gas of biomass heating systems for safe enrichment in greenhouses

Novel approaches to practice CO₂ enrichment in greenhouses from the exhaust gas of a biomass heating system are reviewed. General CO₂ enrichment benefits for greenhouse plant production are described along with optimal management strategies to reduce fuel consumption while improving benefits. Alternative and renewable fuels for CO₂ enrichment, landfill biogas and biomass, are compared with traditional methods and fuels. Exhaust gas composition is outlined to address the challenges of CO₂ enrichment from biomass combustion and leads to a comparison between combustion and gasification to improve boiler efficiency. In terms of internal modifications to a biomass heating system, syngas combustion, following biomass gasification, presents good potential to achieve CO₂ enrichment. Regarding external modifications to clean the exhaust gas, CO₂ can be extracted from flue gases via membrane separation that has shown a lot of potential for large industries trying to reduce and isolate CO₂ emissions for sequestration. Other research has optimized wet scrubbing systems by extracting SO₂ and NO emissions from flue gases to form ammonium sulphate as a by-product valuable to fertilizer markets. The potential of these techniques are reviewed while future research directions are suggested.     

PVDF/ionic liquid polymer blends with superior separation performance for removing CO2 from hydrogen and flue gas

We have demonstrated, for the first time, a polymer blend comprising poly(vinylidene fluoride) (PVDF) and a room-temperature ionic liquid (RTIL) that shows a high CO₂ permeability of 1778 Barrer with CO₂/H₂ and CO₂/N₂ selectivity of 12.9 and 41.1, respectively. The low viscosity RTIL, 1-ethyl-3-methylimidazolium tetracyanoborate ([emim][B(CN)₄]) possesses a high CO₂ solubility, and plays a significant role in CO₂ separation, whereas PVDF provides the mechanical strength to the blend membranes. A series of PVDF/[emim][B(CN)₄] polymer blends with different compositions were tested for their gas separation performance involving H₂, N₂ and CO₂ in both pure gas and mixed gas conditions. Both optical observation and Maxwell predictions confirm the heterogeneous nature of the PVDF/[emim][B(CN)₄] system. However, compared to miscible ionic liquid based blends, where molecular level interactions may restrain chain flexibility and reduce gas permeability, heterogeneous PVDF/RTIL blend systems show far superior gas transport properties. Most of these blend membranes outperform most reported materials and their gas transport and separation capabilities fall within the attractive region bound by the “2008 Robeson Upper Limit” for CO₂/H₂ and CO₂/N₂ gas pairs, and are also very stable at trans-membrane pressure up to 5 atm. Therefore, they are potential materials for H₂ purification and CO₂ capture from hydrogen production and flue gas.     

Two novel oxy-fuel power cycles integrated with natural gas reforming and CO2 capture

Two novel system configurations were proposed for oxy-fuel natural gas turbine systems with integrated steam reforming and CO₂ capture and separation. The steam reforming heat is obtained from the available turbine exhaust heat, and the produced syngas is used as fuel with oxygen as the oxidizer. Internal combustion is used, which allows a very high heat input temperature. Moreover, the turbine working fluid can expand down to a vacuum, producing an overall high-pressure ratio. Particular attention was focused on the integration of the turbine exhaust heat recovery with both reforming and steam generation processes, in ways that reduce the heat transfer-related exergy destruction. The systems were thermodynamically simulated, predicting a net energy efficiency of 50–52% (with consideration of the energy needed for oxygen separation), which is higher than the Graz cycle energy efficiency by more than 2 percentage points. The improvement is attributed primarily to a decrease of the exergy change in the combustion and steam generation processes that these novel systems offer. The systems can attain a nearly 100% CO₂ capture.     

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.     

Macrovoid-free high performance polybenzimidazole hollow fiber membranes for elevated temperature H2/CO2 separations

Thermally robust membranes are required for H₂ production and carbon capture from hydrocarbon fuel derived synthesis (syn) gas. Polybenzimidaole (PBI) materials have exceptional thermal, chemical and mechanical characteristics and high H₂ perm-selectivity for efficient syngas separations at process relevant conditions. The large gas volumes processed mandate the use of a high-throughput, small footprint hollow fiber membrane (HFM) platform. In this work, an industrially attractive spinning protocol is developed to fabricate PBI HFMs with unprecedented H₂/CO₂ separation performance. A unique dope composition incorporating an acetonitrile diluent is discovered enabling asymmetric macro-void free PBI HFM fabrication using a water coagulant. The influences of dope viscosity, coagulant chemistry, and air gap on HFM morphology are evaluated. Elevated temperature (up to 350 °C) H₂ permeances of 400 GPU with H₂/CO₂ selectivities > 20 are achieved. This unprecedented separation performance is a ground breaking achievement at temperatures traditionally considered out-of-reach for polymeric membranes.     

Post combustion CO2 capture by carbon fibre monolithic adsorbents

As generation of carbon dioxide (CO₂) greenhouse gas is inherent in the combustion of fossil fuels, effective capture of CO₂ from industrial and commercial operations is viewed as an important strategy which has the potential to achieve a significant reduction in atmospheric CO₂ levels. At present, there are three basic capture methods, i.e. post combustion capture, pre-combustion capture and oxy-fuel combustion. In pre-combustion, the fossil fuel is reacted with air or oxygen and is partially oxidized to form CO and H₂. Then it is reacted with steam to produce a mixture of CO₂ and more H₂. The H₂ can be used as fuel and the carbon dioxide is removed before combustion takes place. Oxy-combustion is when oxygen is used for combustion instead of air, which results in a flue gas that consists mainly of pure CO₂ and is potentially suitable for storage. In post combustion capture, CO₂ is captured from the flue gas obtained after the combustion of fossil fuel. The post combustion capture (PCC) method eliminates the need for substantial modifications to existing combustion processes and facilities; hence, it provides a means for near-term CO₂ capture for new and existing stationary fossil fuel-fired power plants.This paper briefly reviews CO₂ capture methods, classifies existing and emerging post combustion CO₂ capture technologies and compares their features. The paper goes on to investigate relevant studies on carbon fibre composite adsorbents for CO₂ capture, and discusses fabrication parameters of the adsorbents and their CO₂ adsorption performance in detail. The paper then addresses possible future system configurations of this process for commercial applications.Finally, while there are many inherent attractive features of flow-through channelled carbon fibre monolithic adsorbents with very high CO₂ adsorption capabilities, further work is required for them to be fully evaluated for their potential for large scale CO₂ capture from fossil fuel-fired power stations.     

Clean combustion of solid fuels

A chemical-looping process is proposed for the clean combustion of solid fuels for electric power or heat generation. The process is based on coal gasification with CO₂ to produce CO. The CO then reduces CaSO₄, which is used as an oxygen carrier, in a separate reactor to give CaS and CO₂. A portion of the CO₂ is recycled for the gasification stage and the rest can be sent for sequestration. The CaS is sent to another reactor for oxidation with air and to generate heat or power. The overall thermal effect is the same as direct combustion, but separation of CO₂ and other pollutants, such as sulphur, is achieved. In comparison with conventional chemical-looping combustion of natural gas, much less water is present in the CO₂ product, and hence the loss of heat energy and corrosion of the fuel–reactor system can be reduced.     

Materials challenges in CO2 capture and storage

Abstract

To achieve deep reductions in CO₂ emission from power generation, technologies for CO₂ capture and storage are required to complement other approaches such as improved fuel use efficiency, the switch to low carbon fuels, and the use of renewable and nuclear energy. Three main options currently exist for CO₂ capture: removal of CO₂ from the flue gas; removal of carbon from the fuel before combustion; and oxyfuel combustion systems that have CO₂ and water, which can be separated by condensation, as principal combustion products. On the transport and storage side, the materials issues arise from corrosion and may be solved by drying and purification of the CO₂ stream. On the capture side, there are few specific issues regarding the materials used in technologies such as chemical absorption of CO₂ in an appropriate solvent (usually amines). The high temperature membranes used to separate oxygen from nitrogen in oxyfuel combustion systems raise materials issues in relation to ionic conduction, thermal and mechanical stability and lifetime when integrated in boilers, fluidised beds and gas turbine systems. The performance of systems integrating ceramic oxygen separating membranes is largely dependant on operating temperature, so the behaviour of these materials at ever higher temperatures is a real technical challenge. Membranes can also be used instead of chemical absorption for the separation of CO₂ and hydrogen in fuel de-carbonisation.     

Predictions of the impurities in the CO2 stream of an oxy-coal combustion plant

Whilst all three main carbon capture technologies (post-combustion, pre-combustion and oxy-fuel combustion) can produce a CO₂ dominant stream, other impurities are expected to be present in the CO₂ stream. The impurities in the CO₂ stream can adversely affect other processes of the carbon capture and storage (CCS) chain including the purification, compression, transportation and storage of the CO₂ stream. Both the nature and the concentrations of potential impurities expected to be present in the CO₂ stream of a CCS-integrated power plant depend on not only the type of the power plant but also the carbon capture method used. The present paper focuses on the predictions of impurities expected to be present in the CO₂ stream of an oxy-coal combustion plant. The main gaseous impurities of the CO₂ stream of oxy-coal combustion are N₂/Ar, O₂ and H₂O. Even the air ingress to the boiler and its auxiliaries is small enough to be neglected, the N₂/Ar concentration of the CO₂ stream can vary between ca. 1% and 6%, mainly depending on the O₂ purity of the air separation unit, and the O₂ concentration can vary between ca. 3% and 5%, mainly depending on the combustion stoichiometry of the boiler. The H₂O concentration of the CO₂ stream can vary from ca. 10% to over 40%, mainly depending on the fuel moisture and the partitioning of recycling flue gas (RFG) between wet-RFG and dry-RFG. NO_x and SO₂ are the two main polluting impurities of the CO₂ stream of an oxy-coal combustion plant and their concentrations are expected to be well above those found in the flue gas of an air-coal combustion plant. The concentration of NO_x in the flue gas of an oxy-coal combustion plant can be up to ca. two times to that of an equivalent air-coal combustion plant. The amount of NO_x emitted by the oxy-coal combustion plant, however, is expected to be much smaller than that of the air-coal combustion plant. The reductions of the recirculated NO_x within the combustion furnace by the reburning mechanism and the char-NO reactions are the main reason for a smaller amount of NO_x emitted by the oxy-coal combustion plant. The concentration of SO₂ in the flue gas of an oxy-coal combustion plant can be up to six times to that of an equivalent air-coal combustion plant if the recycling flue gas is not desulphurized. The flue gas volume flow rate of an oxy-coal combustion plant is much smaller (<20%) than that of an equivalent air-coal combustion plant, which is a significant advantage for the purification of the flue gas.     

Experimental estimate of the laminar burning velocity of iso-octane in oxygen-enriched and CO2-diluted air

Oxygen-enriched combustion is of great interest for industrial applications, since membrane separation technology can be used. The objective of this work is to provide unique data on laminar burning velocity, a key parameter in real combustion development, for the oxygen-enriched combustion of an iso-octane/air mixture for various dilution (by air or CO₂) cases. Experiments were carried out in a stainless steel combustion chamber at atmospheric pressure and 373 K. The iso-octane was mixed with a mixture of O₂, CO_2, and N₂. The volume fraction of O₂ was varied from 21% to 29% and CO₂ was varied from 0% to 28%. The classical shadowgraphy technique was used to detect the reaction zone in order to deduce the un-stretched burning velocity, using a nonlinear methodology. All the experimental data were compared with the numerical results obtained with chemical kinetic schemes available in the literature. For further experimental investigations, a correlation is proposed to predict laminar burning velocity as a function of the quantity of O₂ and CO₂ in the gas mixture. Finally, analytical and experimental data concerning Markstein length are discussed.     

Effect of ignition timing and hydrogen fraction on combustion and emission characteristics of natural gas direct-injection engine

An experimental study on the combustion and emission characteristics of a direct-injection spark-ignited engine fueled with natural gas/hydrogen blends under various ignition timings was conducted. The results show that ignition timing has a significant influence on engine performance, combustion and emissions. The interval between the end of fuel injection and ignition timing is a very important parameter for direct-injection natural gas engines. The turbulent flow in the combustion chamber generated by the fuel jet remains high and relative strong mixture stratification is introduced when decreasing the angle interval between the end of fuel injection and ignition timing giving fast burning rates and high thermal efficiencies. The maximum cylinder gas pressure, maximum mean gas temperature, maximum rate of pressure rise and maximum heat release rate increase with the advancing of ignition timing. However, these parameters do not vary much with hydrogen addition under specific ignition timing indicating that a small hydrogen fraction addition of less than 20% in the present experiment has little influence on combustion parameters under specific ignition timing. The exhaust HC emission decreases while the exhaust CO₂ concentration increases with the advancing of ignition timing. In the lean combustion condition, the exhaust CO does not vary much with ignition timing. At the same ignition timing, the exhaust HC decreases with hydrogen addition while the exhaust CO and CO₂ do not vary much with hydrogen addition. The exhaust NOx increases with the advancing of ignition timing and the behavior tends to be more obvious at large ignition advance angle. The brake mean effective pressure and the effective thermal efficiency of natural gas/hydrogen mixture combustion increase compared with those of natural gas combustion when the hydrogen fraction is over 10%. __________ Translated from Transactions of CSICE, 2006, 24(5): 394–401 [译自:内燃机学报]     

ANFIS modeling of CO2 separation from natural gas using hollow fiber polymeric membrane

The present study is proposed to develop the Adaptive Neuro-Fuzzy Inference System optimized by genetic algorithm to estimate CO₂ value in permeate stream using a hollow fiber polymeric membrane for separation of binary gas containing CO₂ and CH₄ in natural gas. To that end, a number of 65 samples was gathered from the literature. Results indicated that the proposed ANFIS model has great potential with high correlation (R² = 0.9993) and less error (RMSE = 0.0064) for estimation of aforementioned parameter.     

Cycle analysis of low and high H2 utilization SOFC/gas turbine combined cycle for CO2 recovery

A major factor in global warming is CO₂ emission from thermal power plants, which burn fossil fuels. One technology proposed to prevent global warming is CO₂ recovery from combustion flue gas and the sequestration of CO₂ underground or near the ocean bed. Solid oxide fuel cell (SOFC) can produce highly concentrated CO₂, because the reformed fuel gas reacts with oxygen electrochemically without being mixed with air in the SOFC. We therefore propose to operate multi-staged SOFCs with high utilization of reformed fuel to obtain highly concentrated CO₂. In this study, we estimated the performance of multi-staged SOFCs considering H₂ diffusion and the combined cycle efficiency of a multi-staged SOFC/gas turbine/CO₂ recovery power plant. The power generation efficiency of our CO₂ recovery combined cycle is 68.5%, whereas the efficiency of a conventional SOFC/GT cycle with the CO₂ recovery amine process is 57.8%.     

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.     

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.     

A theoretical analysis of the capture of greenhouse gases by single water droplet at atmospheric and elevated pressures

Gas absorption by droplets is an important route to reduce greenhouse gas emissions, especially for carbon dioxide. To recognize the fundamental absorption processes of greenhouse gases by single droplets, the mass transport phenomena of greenhouse gas uptake by a quiescent water droplet at atmospheric and elevated pressures are analyzed theoretically and four common greenhouse gases of CO₂, N₂O, CH₄ and O₃ are taken into consideration. On account of piecewise function encountered at the droplet surface, it is impossible to obtain a fully analytical solution for describing the mass transfer process. Instead, a semi-analytical method is developed to predict the mass diffusion between the gas phase and the liquid phase. The obtained results indicate that, by virtue of the four greenhouse gases characterized by low mass diffusion number, the entire mass transfer is controlled by the liquid phase. A unified formula has been successfully established to aid in estimating the dimensionless solute uptake process and the dimensionless aqueous diffusion time of 0.45 is sufficiently long the implement the absorption process. For the ambient temperature and pressure in the ranges of 280–350 K and 1–20 atm, respectively, it is found that increasing the two parameters will intensify the solute absorption amount significantly and the absorption process can be accelerated by increasing temperature.     

Injection and boiling of liquid CO2 with a hydrate coating

We elucidated the effect of a hydrate film on the CO₂ droplet size when the droplet was injected into high-pressure water. The factors that influence the droplet size and shape are the injection rate, nozzle diameter, temperature, and also the tension and propagation velocity of the hydrate thin film. We also found that the hydrate film on the droplet will promote the boiling of liquid CO₂ when the pressure decreases. These results can be applied to the release of liquid CO₂ droplets in the ocean for CO₂ sequestration: for example, when the drops rise above 500-m in the ocean, boiling due to decompression should occur.