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.     

Ni/CeO2 catalysts with high CO2 methanation activity and high CH4 selectivity at low temperatures

CO₂ methanation was performed over 10 wt%Ni/CeO₂, 10 wt%Ni/α-Al₂O₃, 10 wt%Ni/TiO₂, and 10 wt%Ni/MgO, and the effect of support materials on CO₂ conversion and CH₄ selectivity was examined. Catalysts were prepared by a wet impregnation method, and characterized by BET, XRD, H₂-TPR and CO₂-TPD. Ni/CeO₂ showed high CO₂ conversion especially at low temperatures compared to Ni/α-Al₂O₃, and the selectivity to CH₄ was very close to 1. The surface coverage by CO₂-derived species on CeO₂ surface and the partial reduction of CeO₂ surface could result in the high CO₂ conversion over Ni/CeO₂. In addition, superior CO methanation activity over Ni/CeO₂ led to the high CH₄ selectivity.     

Effects of modifying Ni/Al2O3 catalyst with cobalt on the reforming of CH4 with CO2 and cracking of CH4 reactions

Alumina supported nickel (Ni/Al₂O₃), nickel–cobalt (Ni–Co/Al₂O₃) and cobalt (Co/Al₂O₃) catalysts containing 15% metal were synthesized, characterized and tested for the reforming of CH₄ with CO₂ and CH₄ cracking reactions. In the Ni–Co/Al₂O₃ catalysts Ni–Co alloys were detected and the surface metal sites decreased with decrease in Ni:Co ratio. Turnover frequencies of CH₄ were determined for both reactions. The initial turnover frequencies of reforming (TOF_DRM) for Ni–Co/Al₂O₃ were greater than that for Ni/Al₂O₃, which suggested a higher activity of alloy sites. The initial turnover frequencies for cracking (TOF_CRK) did not follow this trend. The highest average TOF_DRM, H₂:CO ratio and TOF_CRK were observed for a catalyst containing a Ni:Co ratio of 3:1. This catalyst also had the maximum carbon deposited during reforming and produced the maximum reactive carbon during cracking. It appeared that carbon was an intermediate product of reforming and the best catalyst was able to most effectively crack CH₄ and oxidize carbon to CO by CO₂.     

Estimating marginal CO2 emissions rates for national electricity systems

The carbon dioxide (CO₂) emissions reduction afforded by a demand-side intervention in the electricity system is typically assessed by means of an assumed grid emissions rate, which measures the CO₂ intensity of electricity not used as a result of the intervention. This emissions rate is called the “marginal emissions factor” (MEF). Accurate estimation of MEFs is crucial for performance assessment because their application leads to decisions regarding the relative merits of CO₂ reduction strategies. This article contributes to formulating the principles by which MEFs are estimated, highlighting the strengths and weaknesses in existing approaches, and presenting an alternative based on the observed behaviour of power stations. The case of Great Britain is considered, demonstrating an MEF of 0.69 kgCO₂/kW h for 2002–2009, with error bars at +/−10%. This value could reduce to 0.6 kgCO₂/kW h over the next decade under planned changes to the underlying generation mix, and could further reduce to approximately 0.51 kgCO₂/kW h before 2025 if all power stations commissioned pre-1970 are replaced by their modern counterparts. Given that these rates are higher than commonly applied system-average or assumed “long term marginal” emissions rates, it is concluded that maintenance of an improved understanding of MEFs is valuable to better inform policy decisions.     

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.     

Catalytic conversion of CH4 and CO2 to synthesis gas on Ni/SiO2 catalysts containing Gd2O3 promoter

A series of Ni/SiO₂ catalysts containing different amounts of Gd₂O₃ promoter was prepared, characterized by H₂-adsorption and XRD, and used for carbon dioxide reforming of methane (CRM) and methane autothermal reforming with CO₂ + O₂ (MATR) in a fluidized-bed reactor. The results of pulse surface reactions showed that Ni/SiO₂ catalysts containing Gd₂O₃ promoter could increase the activity for CH₄ decomposition, and Raman analysis confirmed that reactive carbon species mainly formed on the Ni/SiO₂ catalysts containing Gd₂O₃ promoter. In this work, it was found that methane activation and reforming reactions proceeded according to different mechanisms after Gd₂O₃ addition due to the formation of carbonate species. In addition, Ni/SiO₂ catalysts containing Gd₂O₃ promoter demonstrated higher activity and stability in both CRM and MATR reactions in a fluidized bed reactor than Ni/SiO₂ catalysts without Gd₂O₃ even at a higher space velocity.     

Ni-based catalysts for reforming of methane with CO2

Ni catalysts supported on different carriers like δ,θ-Al₂O₃, MgAl₂O₄, SiO₂–Al₂O₃ and ZrO₂–Al₂O₃ were prepared. The solids were characterized by chemical analysis, N₂ adsorption–desorption isotherms, X-ray powder diffraction, UV–vis diffuse reflectance spectroscopy, temperature-programmed reduction, high-resolution transmission electron microscopy and temperature-programmed oxidation. The catalytic properties of the samples were evaluated in the reaction of reforming of methane with CO₂ at 923 K. It was shown that this kind of support greatly affects the structure and catalytic performance of the catalysts. Ni catalyst supported on MgAl₂O₄ showed the highest activity and stability due to the presence of small well dispersed Ni particles with size of 5.1 nm. It was shown that the lowest activity of Ni catalyst supported on SiO₂–Al₂O₃ oxide was caused by the agglomeration of nickel particles and formation of filamentous carbon under reaction conditions detected by the high resolution transmission electron microscopy.     

Renewable hydrogen to recycle CO2 to methanol

The balance of the natural carbon cycle disrupted by the large consumption of fossil fuels, in particular coal producing electricity, may in principle be restored by using renewable hydrogen. This paper considers the opportunity to recycle the CO₂ produced burning fossil fuels with oxy-fuel combustion using renewable hydrogen as the second feed-stock. The product, methanol, is a transportation fuel having significant advantages over not only over hydrogen, but also gasoline, permitting much better fuel conversion efficiencies than gasoline thanks to the larger heat of vaporisation and the largest resistance to knock that make this fuel the best option for small, high power density, turbocharged, directly injected stoichiometric engines.     

Japanese potential of CO2 sequestration in coal seams

As a reduction strategy for global warming by green-house gases underground storage or sequestration of CO₂ into coal beds or seams has been studied by the Japanese government and some associated organizations.     

Performance and costs of power plants with capture and storage of CO2

This paper assesses the three leading technologies for capture of CO₂ in power generation plants, i.e., post-combustion capture, pre-combustion capture and oxy-fuel combustion. Performance, cost and emissions data for coal and natural gas-fired power plants are presented, based on information from studies carried out recently for the IEA Greenhouse Gas R&D Programme by major engineering contractors and process licensors. Sensitivities to various potentially significant parameters are assessed.     

Decomposing international polarization of per capita CO2 emissions

The aim of this paper is to analyze the international polarization of per capita CO₂ emissions with exogenous groups based on the Z–K measure (Zhang and Kanbur, 2001), whose main differential advantage lies on its factor-decomposability. In particular, we propose to use the factor decomposition based on Kaya (1989) by applying the methodology suggested by Duro and Padilla (2006). The main empirical results derived can be summarized as follows. First, the international polarization of emissions has significantly decreased over time during the period 1971–2006, when regional sets of nations based on the IEA structure are used; secondly, this decrease can be almost exclusively based on the reduction of the average dissimilarities among sets of countries and not due to a within-group cohesion process. Lastly, this reduction can be mainly attributed to the role of the affluence factor, and to a lesser extent, to the energy intensities. Thus, and given the values achieved for the different components, it seems that further reductions in the international polarization will continue be based on the economic convergence among groups.     

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.     

Promotion of CO2 methanation activity and CH4 selectivity at low temperatures over Ru/CeO2/Al2O3 catalysts

The effect of CeO₂ loading amount of Ru/CeO₂/Al₂O₃ on CO₂ methanation activity and CH₄ selectivity was studied. The CO₂ reaction rate was increased by adding CeO₂ to Ru/Al₂O₃, and the order of CO₂ reaction rate at 250 °C is Ru/30%CeO₂/Al₂O₃ > Ru/60%CeO₂/Al₂O₃ > Ru/CeO₂ > Ru/Al₂O₃. With a decrease in CeO₂ loading of Ru/CeO₂/Al₂O₃ from 98% to 30%, partial reduction of CeO₂ surface was promoted and the specific surface area was enlarged. Furthermore, it was observed using FTIR technique that intermediates of CO₂ methanation, such as formate and carbonate species, reacted with H₂ faster over Ru/30%CeO₂/Al₂O₃ and Ru/CeO₂ than over Ru/Al₂O₃. These could result in the high CO₂ reaction rate over CeO₂-containing catalysts. As for the selectivity to CH₄, Ru/30%CeO₂/Al₂O₃ exhibited high CH₄ selectivity compared with Ru/CeO₂, due to prompt CO conversion into CH₄ over Ru/30%CeO₂/Al₂O₃.     

Conventional and advanced CO2 based district energy systems

District energy systems can potentially decrease the CO₂ emissions linked to energy services, thanks to the implementation of large polygeneration energy conversion technologies connected to buildings over a network. To transfer the energy from these large technologies to the users, conventional district energy systems use water with often two independent supply and return piping systems for heat and cold. However, sharing energy or interacting with decentralised heat pump units often results in relatively large heat transfer exergy losses due to the large temperature differences that are economically required from the water network. Besides, the implementation of two independent supply and return piping systems for heat and cold, results in large space requirements in underground technical galleries. Using refrigerants as a district heating or cooling fluid at an intermediate temperature could alleviate some of these drawbacks. A new system has been developed, that requires only two pipes, filled with refrigerant, to meet heating, hot water and cooling requirements. Because of the environmental concerns about conventional refrigerants, CO₂, a natural refrigerant, used under its critical point, is considered an interesting candidate. A comparative analysis shows that both in terms of exergy efficiency and costs the proposed CO₂ network is favourable.     

CO2/O2环境对柴油着火及燃烧特性的影响

为研究CO₂/O₂环境对柴油着火和燃烧特性的影响,以正庚烷为柴油表征燃料,利用CONVERGE计算了不同CO₂/O₂环境下正庚烷的着火和燃烧过程,并搭建了可视化定容燃烧弹试验平台进行了验证。使用高速摄影机记录了初始温度850 K,初始压力3 MPa,CO₂体积分数分别为35%、40%、50%和60%时正庚烷燃烧的自发光强度,利用CHEMKIN中定容均质反应器分析了CO₂物理和化学作用对着火的影响。研究结果表明：在CO₂体积分数35%时存在爆燃的现象,随着CO₂体积分数增长,着火延迟时间增长,着火位置远离喷嘴,稳态燃烧阶段火焰的长度和宽度也增大,CO₂体积分数在50%～60%之间时火焰自发光强度峰值明显下降;CO₂的物理作用抑制了着火,第三体作用对着火的促进作用大于直接参与反应对着火的抑制作用,造成CO₂的化学作用缩短了着火延迟时间,并且随着CO     

Absorption behaviors study on doped Li4SiO4 under a humidified atmosphere with low CO2 concentration

Potassium carbonate was used as a doping agent to improve the absorption properties of Li₄SiO₄ for CO₂ capture during critical sorption-enhanced steam methane reforming (SESMR) under moderate temperature and low CO₂ concentration with moisture.     

Kinetic study and modeling of the high temperature CO2 capture by Na2ZrO3 solid absorbent

Hydrogen production through sorption enhanced reforming (SER) use a solid CO₂ absorbent to increase hydrogen purity (98%) and to perform reforming and WGS reactions in one single step, thus producing high methane conversions and important energy savings. Na₂ZrO₃ is as an alternate synthetic CO₂ solid absorbent for SER applications. The present research is aimed to establish CO₂ sorption kinetics parameters; reaction order, rate constant, apparent, intrinsic and diffusional activation energies. Na₂ZrO₃ sorption kinetics was studied through TGA as a function of CO₂ concentration and temperature. A global reaction rate of first order in CO₂ and a strong dependence in temperature was found. The approximate solution to the shrinking core model was used to fit the data. Modeling results indicated the surface reaction as the main resistance to the reaction rate, controlling reaction kinetics with only a minor contribution of the product layer diffusion resistance toward the end of the reaction.     

Tackling CO2 reduction in India through use of CO2 capture and storage (CCS): Prospects and challenges

CO₂ capture and storage (CCS) is not currently a priority for the Government of India (GOI) because, whilst a signatory to the UNFCCC and Kyoto Protocol, there are no existing greenhouse gas emission reduction targets and most commentators do not envisage compulsory targets for India in the post-2012 phase. The overwhelming priority for the GOI is to sustain a high level of economic growth (8%+) and provision of secure, reliable energy (especially electricity) is one of the widely recognised bottlenecks in maintaining a high growth rate. In such a supply-starved context, it is not easy to envisage adoption of CCS—which increases overall generation capacity and demand for coal without increasing actual electricity supply—as being acceptable. Anything which increases costs—even slightly—is very unlikely to happen, unless it is fully paid for by the international community. The majority viewpoint of the industry and GOI interviewees towards CCS appears to be that it is a frontier technology, which needs to be developed further in the Annex-1 countries to bring down the cost through RD&D and deployment. More RD&D is required to assess in further detail the potential for CO₂ storage in geological reservoirs in India and the international community has an important role to play in cultivating such research.     

Thermoeconomic optimization and exergy analysis of CO2/NH3 cascade refrigeration systems

In this paper, thermoeconomic optimization and exergy analysis are applied to a CO₂/NH₃ cascade refrigeration cycle. Cooling capacity, ambient temperature and cold space temperature are constraints of the optimization procedure. Four parameters including condensing temperature of ammonia, evaporating temperature of carbon dioxide, condensing temperature of carbon dioxide and temperature difference in the cascade condenser are chosen as decision variables. The objective function is the total annual cost of the system which includes costs of input exergy to the system and annualized capital cost of the system. Input exergy to the system is the electricity consumption of compressors and fans, and the capital cost includes purchase costs of components. Results show that, optimum values of decision variables may be found by trade-off between the input exergy cost and capital cost. Results of the exergy analysis for each of the system components in the optimum state are also given.     

Reservoir simulation of CO2 sequestration pilot in Frio brine formation,USA Gulf Coast

Reservoir simulation studies were performed to investigate compositional effects between aquifer fluid (brine) and injected supercritical CO₂ during the sequestration process in the Frio brine formation. Accurate data calibrations of CO₂ solubility and density, as well as brine density and viscosity, were performed. Hysteresis relative permeability was taken into consideration to account for the effect of trapped gas in the aquifer. In addition, real aquifer data obtained from the test site were used in order to characterize the Frio aquifer.