Explosion behavior of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures

In this work, the explosion behavior of stoichiometric CH₄/O₂/N₂/CO₂ and H₂/O₂/N₂/CO₂ mixtures has been studied both experimentally and theoretically at different CO₂ contents and oxygen air enrichment factors. Peak pressure, maximum rate of pressure rise and laminar burning velocity were measured from pressure time records of explosions occurring in a closed cylindrical vessel. The laminar burning velocity was also computed through CHEMKIN–PREMIX simulations.     

Hydrogen permeation through a Pd-based membrane and RWGS conversion in H2/CO2, H2/N2/CO2 and H2/H2O/CO2 mixtures

This study investigated the effect of gases such as CO₂, N₂, H₂O on hydrogen permeation through a Pd-based membrane −0.012 m² – in a bench-scale reactor. Different mixtures were chosen of H₂/CO₂, H₂/N₂/CO₂ and H₂/H₂O/CO₂ at temperatures of 593–723 K and a hydrogen partial pressure of 150 kPa. Operating conditions were determined to minimize H₂ loss due to the reverse water gas shift (RWGS) reaction. It was found that the feed flow rate had an important effect on hydrogen recovery (HR). Furthermore, an identification of the inhibition factors to permeability was determined. Additionally, under the selected conditions, the maximum hydrogen permeation was determined in pure H₂ and the H₂/CO₂ mixtures. The best operating conditions to separate hydrogen from the mixtures were identified.     

High performance composite hollow fiber membranes for CO2/H2 and CO2/N2 separation

The search for a clean energy source as well as the reduction of CO₂ emissions to the atmosphere are important strategies to resolve the current energy shortage and global warming issues. We have demonstrated, for the first time, a Pebax/poly(dimethylsiloxane)/polyacrylonitrile (Pebax/PDMS/PAN) composite hollow fiber membrane not only can be used for flue gas treatment but also for hydrogen purification. The composite membranes display attractive gas separation performance with a CO₂ permeance of 481.5 GPU, CO₂/H₂ and CO₂/N₂ selectivity of 8.1 and 42.0, respectively. Minimizing the solution intrusion using the PDMS gutter layer is the key to achieving the high gas permeance while the interaction between poly(ethylene oxide) (PEO) and CO₂ accounts for the high selectivity. Effects of coating solution concentration and coating time on gas separation performance have been investigated and the results have been optimized. To the best of our knowledge, this is the first polymeric composite hollow fiber membrane for hydrogen purification. The attractive gas separation performance of the newly developed membranes may indicate good potential for industrial applications.     

Role of CO2 in the CH4 oxidation and H2 formation during fuel-rich combustion in O2/CO2 environments

The effect of CO₂ reactivity on CH₄ oxidation and H₂ formation in fuel-rich O₂/CO₂ combustion where the concentrations of reactants were high was studied by a CH₄ flat flame experiment, detailed chemical analysis, and a pulverized coal combustion experiment. In the CH₄ flat flame experiment, the residual CH₄ and formed H₂ in fuel-rich O₂/CO₂ combustion were significantly lower than those formed in air combustion, whereas the amount of CO formed in fuel-rich O₂/CO₂ combustion was noticeably higher than that in air. In addition to this experiment, calculations were performed using CHEMKIN-PRO. They generally agreed with the experimental results and showed that CO₂ reactivity, mainly expressed by the reaction CO₂ + H → CO + OH (R1), caused the differences between air and O₂/CO₂ combustion under fuel-rich condition. R1 was able to advance without oxygen. And, OH radicals were more active than H radicals in the hydrocarbon oxidation in the specific temperature range. It was shown that the role of CO₂ was to advance CH₄ oxidation during fuel-rich O₂/CO₂ combustion. Under fuel-rich combustion, H₂ was mainly produced when the hydrocarbon reacted with H radicals. However, the hydrocarbon also reacted with the OH radicals, leading to H₂O production. In fact, these hydrocarbon reactions were competitive. With increasing H/OH ratio, H₂ formed more easily; however, CO₂ reactivity reduced the H/OH ratio by converting H to OH. Moreover, the OH radicals reacted with H₂, whereas the H radicals did not reduce H₂. It was shown that OH radicals formed by CO₂ reactivity were not suitable for H₂ formation. As for pulverized coal combustion, the tendencies of CH₄, CO, and H₂ formation in pulverized coal combustion were almost the same as those in the CH₄ flat flame.     

Study on stretch extinction limits of CH4/CO2 versus high temperature O2/CO2 counterflow non-premixed flames

Extinction limits of counterflow non-premixed flames with normal and high temperature oxidizers were studied experimentally and numerically for development of new-type oxygen-enriched mild combustion furnace. Extinction stretch rates of CH₄/CO₂ (at 300 K) versus O₂/CO₂ flames at oxygen mole fractions of 0.35 and 0.40 and oxidizer temperatures of 300 K, 500 K, 700 K and 1000 K were obtained. Investigation was also conducted for CH₄/N₂ (at 300 K) versus air (O₂/N₂) flames at the same oxidizer temperatures. An effect of radiative heat loss on stretch extinction limits of oxygen-enriched flames and air flames was investigated by computations with optical thin model (OTM) and adiabatic flame model (ADI). The results show influence of radiative heat loss on stretch extinction limits was not significant in relative high fuel mole fraction regions. The extinction curve of the oxygen-enriched flames with oxygen mole fraction of 0.35 was close to that of the air flames at the oxidizer temperature of 300 K. However, the extinction curve of air flames with high temperature oxidizer was comparable with that of oxygen-enriched flames with oxygen mole fraction of 0.40. Scaling analysis based on asymptotic solution of stretch extinction was applied and it was found that stretch extinction limits can be expressed by two terms. The first term is total enthalpy flux of fuel stream based on thermo-physical parameters. The second term is a kinetic term which reflects an effect of the chemical reaction rate on stretch extinction limits. OH radicals which play important roles in chain propagating and main endothermic reactions were used to represent the kinetic term of both oxygen-enriched and air flames. The global rates of OH formation in these two cases were compared to understand the contribution of kinetic term to stretch extinction limits. Variation of extinction curves of oxygen-enriched flames and air flames was well explained by the present scaling analysis. This offers an effective approach to estimate stretch extinction limits of oxygen-enriched flames based on those of air flames at the same oxidizer temperature.     

Chemical effects of added CO2 on the extinction characteristics of H2/CO/CO2 syngas diffusion flames

Chemical effects of added CO₂ on flame extinction characteristics are numerically studied in H₂/CO syngas diffusion flames diluted with CO₂. The two representative syngas flames of 80% H₂ + 20% CO and 20% H₂ + 80% CO are inspected according to the composition of fuel mixture diluted with CO₂ and global strain rate. Particular concerns are focused on impact of chemical effects of added CO₂ on flame extinction characteristics through the comparison of the flame characteristics between well-burning flames far from extinction limit and flames at extinction. It is seen that chemical effects of added CO₂ reduce critical CO₂ mole fraction at flame extinction and thus extinguish the flame at higher flame temperature irrespective of global strain rate. This is attributed by the suppression of the reaction rate of the principal chain branching reaction through the augmented consumption of H-atom from the reaction CO₂ + H→CO + OH. As a result the overall reaction rate decreases. These chemical effects of added CO₂ are similar in both well-burning flames far from extinction limit and flames at extinction. There is a mismatching in the behaviors between critical CO₂ mole fraction and maximum flame temperature at extinction. This anomalous phenomenon is also discussed in detail.     

Sequestration of CO2 in geological media in response to climate change: capacity of deep saline aquifers to sequester CO2 in solution

Geological sequestration is a means of reducing anthropogenic atmospheric emissions of CO₂ that is immediately available and technologically feasible. Among various options, CO₂ can be sequestered in deep aquifers by dissolution in the formation water. The ultimate CO₂ sequestration capacity in solution (UCSCS) of an aquifer is the difference between the total capacity for CO₂ at saturation and the total inorganic carbon currently in solution in that aquifer, and depends on the pressure, temperature and salinity of the formation water. Assuming non-reactive aquifer conditions, the current carbon content is calculated using standard chemical analyses of the formation waters collected by the energy industry on the basis of the concentration of carbonate and bicarbonate ions. Formation water analyses performed at laboratory conditions are brought to in situ conditions using a geochemical speciation model to account for dissolved gasses that are lost from the water sample. To account for the decrease in CO₂ solubility with increasing water salinity, the maximum CO₂ content in formation water is calculated by applying an empirical correction to the CO₂ content at saturation in pure water. The UCSCS in an aquifer is calculated by considering the effect of dissolved CO₂ on the formation water density, the aquifer thickness and porosity to account for the volume of water in the aquifer pore space and for the mass of CO₂ dissolved in the water currently and at saturation. The methodology developed for estimating the ultimate CO₂ sequestration capacity in solution in aquifers has been applied to the Viking aquifer in the Alberta basin in western Canada. Considering only the region where the injected CO₂ would be a dense fluid, the capacity of the Viking aquifer to sequester CO₂ in solution in the formation water is calculated to be 100 Gt. Simple estimates then indicate that the capacity of the Alberta basin to sequester CO₂ dissolved in the formation waters at depths greater than 1000 m is on the order of 4000 Gt CO₂. The results also show that using geochemical models to bring the analyses of the formation waters to in situ conditions is not warranted when the current total inorganic carbon (TIC) in the aquifer water is very small by comparison with the CO₂ solubility at saturation. Furthermore, in such cases, the current TIC may even be neglected.     

Enhanced H2 fermentation of organic waste by CO2 sparging

This study aimed to improve the productivity of dark fermentative hydrogen production from organic waste. An anaerobic sequencing batch reactor was used for hydrogen fermentation and it was fed with food waste (VS 4.4 ± 0.2% containing 27 g carbohydrate-COD/L) at various CO₂ sparging rates (40–120 L/L/d), hydraulic retention times (HRTs; 18–42 h), and solid retention times (SRTs; 18–160 h). CO₂ sparging increased the H₂ productivity by 5–36% at all the examined conditions, confirming the benefit of the replacement of headspace gas by CO₂. The maximum H₂ production was obtained by CO₂ sparging at 80 L/L/d, resulting in the H₂ productivity of 3.18 L H₂/L/d and the H₂ yield of 97.3 mL H₂/g VS_added. Increase of n-butyrate and isopropanol yields were concurrent with the enhanced H₂ yield by CO₂ sparging. Acidogenic efficiency, the sum of H₂, organic acid, and alcohol, in the CO₂-sparged reactor ranged from 47.9 to 56.0%, which was comparable to conventional acidogenesis. Thermodynamic analysis confirmed that both CO₂ sparging and CO₂ removal were beneficial for H₂-producing reactions, but CO₂ sparing has more profound effect than CO₂ removal on inhibiting H₂-consuming reactions.     

PVTxy properties of CO2 mixtures relevant for CO2 capture,transport and storage: Review of available experimental data and theoretical models

The knowledge about pressure–volume–temperature–composition (PVTxy) properties plays an important role in the design and operation of many processes involved in CO₂ capture and storage (CCS) systems. A literature survey was conducted on both the available experimental data and the theoretical models associated with the thermodynamic properties of CO₂ mixtures within the operation window of CCS. Some gaps were identified between available experimental data and requirements of the system design and operation. The major concerns are: for the vapour–liquid equilibrium, there are no data about CO₂/COS and few data about the CO₂/N₂O₄ mixture. For the volume property, there are no published experimental data for CO₂/O₂, CO₂/CO, CO₂/N₂O₄, CO₂/COS and CO₂/NH₃ and the liquid volume of CO₂/H₂. The experimental data available for multi-component CO₂ mixtures are also scarce. Many equations of state are available for thermodynamic calculations of CO₂ mixtures. The cubic equations of state have the simplest structure and are capable of giving reasonable results for the PVTxy properties. More complex equations of state such as Lee–Kesler, SAFT and GERG typically give better results for the volume property, but not necessarily for the vapour–liquid equilibrium. None of the equations of state evaluated in the literature show any clear advantage in CCS applications for the calculation of all PVTxy properties. A reference equation of state for CCS should, thus, be a future goal.     

Design of CO2 absorption plant for recovery of CO2 from flue gases of gas turbine

The ongoing human-induced emission of carbon dioxide (CO₂) threatens to change the earth's climate. A major factor in global warming is CO₂ emission from thermal power plants, which burn fossil fuels. One possible way of decreasing CO₂ emissions is to apply CO₂ removal, which involves recovering of CO₂ from energy conversion processes. This study is focused on recovery of CO₂ from gas turbine exhaust of Sarkhun gas refinery power station. The purpose of this study is to recover the CO₂ with minimum energy requirement. Many of CO₂ recovery processes from flue gases have been studied. Among all CO₂ recovery processes which were studied, absorption process was selected as the optimum one, due to low CO₂ concentration in flue gas. The design parameters considered in this regard, are: selection of suitable solvent, solvent concentration, solvent circulation rate, reboiler and condenser duty and number of stages in absorber and stripper columns. In the design of this unit, amine solvent such as, diethanolamine (DEA), diglycolamine (DGA), methyldiethanolamine (MDEA), and monoethanolamine (MEA) were considered and the effect of main parameters on the absorption and stripping columns is presented. Some results with simultaneous changing of the design variables have been obtained. The results show that DGA is the best solvent with minimum energy requirement for recovery of CO₂ from flue gases at atmospheric pressure.     

CO2 reforming of CH4 by binode thermal plasma

A binode thermal plasma is first applied to CO₂ reforming of CH₄ to investigate how to enlarge the process and lower energy consumption. Experimental study is conducted in two modes. One is to introduce feed gases (CH₄ and CO₂) only into discharge region between the first anode and the second anode as plasma-forming gas; the other is to introduce them not only into discharge region but also into the plasma jet from the exit of plasma generator. The experimental results show that, the former brings about higher conversion and selectivity but appreciably lower energy conversion efficiency due to its higher energy utilization, while the latter brings about higher energy conversion efficiency but somewhat lower conversion and selectivity due to its larger feeding of CH₄ and CO₂. Furthermore, during discharge in both modes, the oxidation on cathode and anode, or carbon deposition in plasma generator is not observed.     

Catalytic CO2 reforming of CH4 over Cr-promoted Ni/char for H2 production

The objective of the study is to investigate the catalytic performance of Cr-promoted Ni/char in CO₂ reforming of CH₄ at 850 °C. The char obtained from the pyrolysis of a long-flame coal at 1000 °C was used as the support. The catalysts were prepared by incipient wetness impregnation methods with different metal precursor doping sequence. The characterization of the composite catalysts was evaluated by XRD, XPS, SEM-EDS, TEM, H₂-TPR, CO₂-TPD, CH₄-TPSR, and CO₂-TPO. The results indicate that the catalyst prepared by co-impregnation of Ni and Cr possess higher activity than those by sequential impregnation. The optimal loading of Cr on 5 wt% Ni/char is 7.8 wt‰. Moreover, the molar feed ratio of CH₄/CO₂ has a considerable effect on both the stability and the activity of Cr–Ni/char. The main effect of Cr is the great enhance of the adsorption to CO₂. It is interesting that the conversions of CH₄ and CO₂ over Cr-promoted Ni/char and Ni/char decrease initially, following by a steady rise as the reaction proceeds with time-on-stream (TOS). In addition, cyclic tests were conducted and no distinct deterioration in the catalytic performance of the catalysts was observed. On the basis of the obtained results, nickel carbide was speculated to be the active species which was formed during the CO₂ reforming of CH₄ reaction.     

The marginal cost of CO2 emissions

Haraden's model for estimating the economic cost of global warming is analysed. We change his method of discounting and some of his input parameters in a manner consistent with physical and economic theory as well as empirical data. We then find much higher costs than Haraden found. These costs are compared to the cost of reducing CO₂ emissions and we find that deep cuts of the emissions of CO₂ are preferable. A check of the sensitivity of our results with respect to some crucial parameter values does not alter that conclusion.     

CO2 emissions structure of Indian economy

This paper analyses carbon dioxide (CO₂) emissions of the Indian economy by producing sectors and due to household final consumption. The analysis is based on an Input–Output (IO) table and Social Accounting Matrix (SAM) for the year 2003–04 that distinguishes 25 sectors and 10 household classes. Total emissions of the Indian economy in 2003–04 are estimated to be 1217 million tons (MT) of CO₂, of which 57% is due to the use of coal and lignite. The per capita emissions turn out to be about 1.14 tons. The highest direct emissions are due to electricity sector followed by manufacturing, steel and road transportation. Final demands for construction and manufacturing sectors account for the highest emissions considering both direct and indirect emissions as the outputs from almost all the energy-intensive sectors go into the production process of these two sectors. In terms of life style differences across income classes, the urban top 10% accounts for emissions of 3416 kg per year while rural bottom 10% class accounts for only 141 kg per year. The CO₂ emission embodied in the consumption basket of top 10% of the population in urban India is one-sixth of the per capita emission generated in the US.     

High temperature H2/CO2 separation using cobalt oxide silica membranes

In this work high quality cobalt oxide silica membranes were synthesized on alumina supports using a sol–gel, dip coating method. The membranes were subsequently connected into a steel module using a graphite based proprietary sealing method. The sealed membranes were tested for single gas permeance of He, H₂, N₂ and CO₂ at temperatures up to 600 °C and feed pressures up to 600 kPa. Pressure tests confirmed that the sealing system was effective as no gas leaks were observed during testing. A H₂ permeance of 1.9 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ was measured in conjunction with a H₂/CO₂ permselectivity of more than 1500, suggesting that the membranes had a very narrow pore size distribution and an average pore diameter of approximately 3 Å. The high temperature testing demonstrated that the incorporation of cobalt oxide into the silica matrix produced a structure with a higher thermal stability, able to resist thermally induced densification up to at least 600 °C. Furthermore, the membranes were tested for H₂/CO₂ binary feed mixtures between 400 and 600 °C. At these conditions, the reverse of the water gas shift reaction occurred, inadvertently generating CO and water which increased as a function of CO₂ feed concentration. The purity of H₂ in the permeate stream significantly decreased for CO₂ feed concentrations in excess of 50 vol%. However, the gas mixtures (H₂, CO₂, CO and water) had a more profound effect on the H₂ permeate flow rates which significantly decreased, almost exponentially as the CO₂ feed concentration increased.     

High capacity potassium-promoted hydrotalcite for CO2 capture in H2 production

This paper presents an experimental study for a newly modified K₂CO₃-promoted hydrotalcite material as a novel high capacity sorbent for in-situ CO₂ capture. The sorbent is employed in the sorption enhanced steam reforming process for an efficient H₂ production at low temperature (400–500 ^°C). A new set of adsorption data is reported for CO₂ adsorption over K-hydrotalcite at 400 °C. The equilibrium sorption data obtained from a column apparatus can be adequately described by a Freundlich isotherm. The sorbent shows fast adsorption rates and attains a relatively high sorption capacity of 0.95 mol/kg on the fresh sorbent. CO₂ desorption experiments are conducted to examine the effect of humidity content in the gas purge and the regeneration time on CO₂ desorption rates. A large portion of CO₂ is easily recovered in the first few minutes of a desorption cycle due to a fast desorption step, which is associated with a physi/chemisorption step on the monolayer surface of the fresh sorbent. The complete recovery of CO₂ was then achieved in a slower desorption step associated with a reversible chemisorption in a multi-layer surface of the sorbent. The sorbent shows a loss of 8% of its fresh capacity due to an irreversible chemisorption, however, it preserves a stable working capacity of about 0.89 mol/kg, suggesting a reversible chemisorption process. The sorbent also presents a good cyclic thermal stability in the temperature range of 400–500 °C.     

The Ni/ZrO2 catalyst and the methanation of CO and CO2

The Ni/ZrO₂ catalyst is one of the most active systems for the methanation of CO to be employed in the hydrogen purification for PEMFC. This contribution aims to study the effect of ZrO₂ on the methanation of CO and CO₂. The catalytic behavior of Ni/ZrO₂, Ni/SiO₂, a physical mixture comprising Ni and ZrO_2, and a double-bed reactor were evaluated. The TPD of CO and CO₂, TPSR and the cyclohexane dehydrogenation reaction were carried out to describe the catalysts and the reactions. The high activity of Ni/ZrO₂ catalyst toward the methanation of CO is related to the presence of active sites on the ZrO₂ surface. The methanation of CO occurs on ZrO₂ due to its ability to adsorb CO and also because of the hydrogen spillover phenomenon. Apparently, the effect of ZrO₂ is less relevant for the methanation of CO₂. Ni/ZrO₂ is a very promising system for the purification of hydrogen.     

O2/CO2气氛下石灰石脱硫特性的研究

在沉降炉中进行了石灰石的脱硫特性研究.表明用20%O2+80%CO2的混合气体作助燃气时比用空气作助燃气时有较好的脱硫效果,并能减少NOx的排放,是一种新的能同时控制CO2,SO2和NOx三种污染物的好方法.图8参8     

Effect of CO2 diffusion on wettability for hydrocarbon-water-CO2 systems in capillaries

The importance of wettability in enhanced oil recovery with high pressure CO₂ has been demonstrated by some authors. When CO₂ diffuse in hydrocarbons, it promotes swelling and reduction in contact angle and surface tension, these changes improve the tendency to fill the corners of solid surfaces, therefore filaments arise in corners. In this work, hydrocarbon-water-CO₂ behavior was studied in cylindrical and square capillaries. Square capillaries were used in order to observe the wetting in corners. Cylindrical capillaries were used to determine if there is swelling in hydrocarbons due to the CO₂ mass transfer, first through the water and then through the hydrocarbon. Contact angle and interfacial tension were measured in an attemp to explain the results. It was demonstrated that the CO₂ diffusion occurs first through water and then through hydrocarbons. The CO₂ diffusion is evident due to the contact angle diminution and the displacement of water-hydrocarbon interfaces. In addition, it was found that the behavior of water-hydrocarbon interphase depends on contact angle and surface tension. Finally, when interfacial tension is lower, the displacement is different and contact angle increasing is higher, as it was concluded by Campbell and Orr [1].