Mechanism of heterogeneous adsorption in the storage of hydrogen in carbon fibers activated with supercritical water and steam

In the present work we study the capacity of storage hydrogen on carbon fibers activated with supercritical water and with steam on the basis of a multisite Langmuir model, with three energetically different adsorption sites that may be associated with pores of different sizes: i) very small micropores, accessible only to hydrogen; ii) micropores detected by the adsorption of CO₂, and iii) micropores detected by the adsorption of N₂. The correlation of the experimental data with the model allowed the amount of hydrogen stored in each of the sites to be quantified and confirmed that hydrogen storage mechanism begins with the filling of the smallest pores. Additionally, from the model it was possible to interpret the dependence of the amount of hydrogen stored with textural parameters such as the micropore volumes. The model also allowed the storage capacity of the fibers to be predicted for pressures higher than those obtained experimentally.     

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.     

Stakeholder perceptions of CO2 capture and storage in Europe: Results from a survey

During 2006, a survey was conducted of European energy stakeholders (industry, government, environmental non-governmental organizations (NGOs), researchers and academicians and parliamentarians). A total of 512 responses was received from 28 countries as follows: industry (28%), research (34%), government (13%), NGOs (5%) and parliamentarians (4%). Three-quarters of the sample thought that widespread use of CO₂ capture and storage (CCS) was ‘definitely’ or ‘probably necessary’ to achieve deep reductions in CO₂ emissions between now and 2050 in their own country. Only one in eight considered that CCS was ‘probably’ or ‘definitely not necessary’. For a range of 12 identified risks, 20–40% thought that they would be ‘moderate’ or ‘very serious’, whilst 60–80% thought that there would be no risks or that the risks would be ‘minimal’. A particular risk identified by nearly half the sample is the additional use of fossil fuels due to the ‘energy penalty’ incurred by CCS. Further concerns are that development of CCS would detract from investment in renewable energy technologies. Half of the respondents thought that incentives for CCS should be set either at the same level as those for renewables or at a higher level. Environmental NGOs were consistently less enthusiastic about CCS than the energy industry.     

Evaluation of geologic storage options of CO2: Applicability,cost, storage capacity and safety

CO₂ emissions in the atmosphere are increasing continually, which are mainly originated from burning of fossil fuels. The fossil fuels are expected to remain a major component of the world’s energy supply in the near future, because of their inherent advantages. Therefore, various measures have to be considered to reduce anthropogenic CO₂ emissions. Increasing the efficiency of energy usage and/or developing lower carbon or non-carbon energies to replace high carbon fuels may bring the result of the reduction of the accumulation of CO₂ in the atmosphere. The other alternative to reduce CO₂ concentrations in atmosphere include gaseous storage in various deep geological formations, liquid storage in the ocean, and solid storage by reaction of CO₂ with metal oxides to produce stable carbonates. In this article, the geological storage options of CO₂ are examined. They are discussed in terms of applicability, cost, storage capacity and safety.     

Solar energy powered Rankine cycle using supercritical CO2

A solar energy powered Rankine cycle using supercritical CO₂ for combined production of electricity and thermal energy is proposed. The proposed system consists of evacuated solar collectors, power generating turbine, high-temperature heat recovery system, low-temperature heat recovery system, and feed pump. The system utilizes evacuated solar collectors to convert CO₂ into high-temperature supercritical state, used to drive a turbine and thereby produce mechanical energy and hence electricity. The system also recovers heat (high-temperature heat and low-temperature heat), which could be used for refrigeration, air conditioning, hot water supply, etc. in domestic or commercial buildings. An experimental prototype has been designed and constructed. The prototype system has been tested under typical summer conditions in Kyoto, Japan; It was found that CO₂ is efficiently converted into high-temperature supercritical state, of while electricity and hot water can be generated. The experimental results show that the solar energy powered Rankine cycle using CO₂ works stably in a trans-critical region. The estimated power generation efficiency is 0.25 and heat recovery efficiency is 0.65. This study shows the potential of the application of the solar-powered Rankine cycle using supercritical CO₂.     

CO2 capture and storage: Another Faustian Bargain?

A quarter-century ago, one of us termed the use of nuclear energy a Faustian Bargain. In this paper, we discuss what a Faustian Bargain means, how the expression has been used in characterizing other technologies, and in what measure CO₂ capture and storage is a Faustian Bargain. If we are about to enter into another Faustian Bargain, we should understand the contract.     

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.     

The public perspective of carbon capture and storage for CO2 emission reductions in China

To explore public awareness of carbon capture and storage (CCS), attitudes towards the use of CCS and the determinants of CCS acceptance in China, a study was conducted in July 2009 based on face-to-face interviews with participants across the country. The result showed that the awareness of CCS was low among the surveyed public in China, compared to other clean energy technologies. Respondents indicated a slightly supportive attitude towards the use of CCS as an alternative technology to CO₂ emission reductions. The regression model revealed that in addition to CCS knowledge, respondents’ understanding of the characteristics of CCS, such as the maturity of the technology, risks, capability of CO2 emission reductions, and CCS policy were all significant factors in predicting the acceptance of CCS. The findings suggest that integrating public education and communication into CCS development policy would be an effective strategy to overcome the barrier of low public acceptance.     

Natural emissions of CO2 from the geosphere and their bearing on the geological storage of carbon dioxide

Carbon dioxide (CO₂) capture and storage has the potential to reduce CO₂ emissions from fossil fuel combustion. Although leakage from monitored CO₂ injection sites has been minimal to non-existent, experience from the natural gas storage industry suggests that, if it becomes a widely deployed technology, leaks may be expected from some storage sites. Natural occurrences of CO₂ in the geosphere, some of which have been exploited, provide insights into the types of emissions that might be expected from anthropogenic CO₂ storage sites. CO₂ emission sites are commonly found in clusters in CO₂-prone geological provinces: the most common natural emissions sites in sedimentary basins consist of carbonated springs and mofettes. These represent at worst only a local hazard. In volcanic and hydrothermal provinces, more energetic emissions may occur due to active supply from degassing magma. These include rare, sudden emissions from fissures and craters that have caused fatalities. It is unlikely that such provinces would be considered for CO₂ storage Major lake overturn events such as occurred at Lake Nyos in 1986 are considered highly unlikely to occur as a result of CO₂ storage, not least because CO₂ levels in lake waters can be monitored and remediated. Natural CO₂ fields indicate that under favourable conditions CO₂ can be retained in the subsurface for millions of years. The main risk from man-made CO₂ storage sites that does not have any close analogy in nature is considered to be a well blowout. A blowout that took place at a natural CO₂ field provides some indication of the likely hazard.     

Experimental study on the performance of solar Rankine system using supercritical CO2

An experimental study is carried out to investigate the performance of a solar Rankine system using supercritical CO₂ as a working fluid. The testing machine of the solar Rankine system consists of an evacuated solar collector, a pressure relief valve, heat exchangers and CO₂ feed pump, etc. The solar energy powered system can provide electricity output as well as heat supply/refrigeration, etc. The system performance is evaluated based on daily, monthly and yearly experiment data. The results obtained show that heat collection efficiency for the CO₂-based solar collector is measured at 65.0–70.0%. The power generation efficiency is found at 8.78–9.45%, which is higher than the value 8.20% of a solar cell. The result presents a potential future for the solar powered CO₂ Rankine system to be used as distributed energy supply system for buildings or others.     

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.     

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.     

Directed technical change and the adoption of CO2 abatement technology: The case of CO2 capture and storage

This paper studies the cost-effectiveness of combining traditional environmental policy, such as CO₂-trading schemes, and technology policy that has aims of reducing the cost and speeding the adoption of CO₂ abatement technology. For this purpose, we develop a dynamic general equilibrium model that captures empirical links between CO₂ emissions associated with energy use, directed technical change and the economy. We specify CO₂ capture and storage (CCS) as a discrete CO₂ abatement technology. We find that combining CO₂-trading schemes with an adoption subsidy is the most effective instrument to induce adoption of the CCS technology. Such a subsidy directly improves the competitiveness of the CCS technology by compensating for its markup over the cost of conventional electricity. Yet, introducing R&D subsidies throughout the entire economy leads to faster adoption of the CCS technology as well and in addition can be cost-effective in achieving the abatement target.     

Sorbent enhanced hydrogen production from steam gasification of coal integrated with CO2 capture

In this work, CO₂ capture and H₂ production during the steam gasification of coal integrated with CO₂ capture sorbent were investigated using a horizontal fixed bed reactor at atmospheric pressure. Four different temperatures (650, 675, 700, and 750 °C) and three sorbent-to-carbon ratios ([Ca]/[C] = 0, 1, 2) were studied. In the absence of sorbent, the maximum molar fraction of H₂ (64.6%) and conversion of coal (71.3%) were exhibited at the highest temperature (750 °C). The experimental results verified that the presence of sorbent in the steam gasification of coal enhanced the molar fraction of H₂ to more than 80%, with almost all CO₂ was fixed into the sorbent structure, and carbon monoxide (CO) was converted to H₂ and CO₂ through the water gas shift reaction. The steam gasification of coal integrated with CO₂ capture largely depended on the reaction temperature and exhibited optimal conditions at 675 °C. The maximum molar fraction of H₂ (81.7%) and minimum CO₂ concentration (almost 0%) were obtained at 675 °C and a sorbent-to-carbon ratio of 2.     

Improved hydrogen storage performance of defected carbon nanotubes with Pd spillover catalysts dispersed using supercritical CO2 fluid

Hydrogen storage properties of carbon nanotubes (CNTs) modified by oxidative etching and decoration of Pd spillover catalysts are investigated. A mixed H₂SO₄/H₂O₂ solution containing ferrous ions (Fe²⁺) is useful to open the caps, to shorten the length, and to generate defects on CNTs. The Pd catalysts are deposited on the CNTs with the aid of supercritical carbon dioxide (scCO₂); as a result, a highly dispersed Pd nanoparticles and an intimate connection between Pd and carbon surface can be obtained. Combination of the two approaches can optimize a hydrogen spillover reaction on CNTs, resulting in a superior hydrogen storage capacity of 1.54 wt% (at 25 °C and 6.89 MPa), which corresponds to an enhancement factor of ∼4.5 as compared to that of pristine CNTs.     

Carbon materials for H2 storage

In this work a series of carbons with different structural and textural properties were characterised and evaluated for their application in hydrogen storage. The materials used were different types of commercial carbons: carbon fibers, carbon cloths, nanotubes, superactivated carbons, and synthetic carbons (carbon nanospheres and carbon xerogels). Their textural properties (i.e., surface area, pore size distribution, etc.) were related to their hydrogen adsorption capacities. These H₂ storage capacities were evaluated by various methods (i.e., volumetric and gravimetric) at different temperatures and pressures. The differences between both methods at various operating conditions were evaluated and related to the textural properties of the carbon-based adsorbents. The results showed that temperature has a greater influence on the storage capacity of carbons than pressure. Furthermore, hydrogen storage capacity seems to be proportional to surface area, especially at 77 K. The micropore size distribution and the presence of narrow micropores also notably influence the H₂ storage capacity of carbons. In contrast, morphological or structural characteristics have no influence on gravimetric storage capacity. If synthetic materials are used, the textural properties of carbon materials can be tailored for hydrogen storage. However, a larger pore volume would be needed in order to increase storage capacity. It seems very difficult approach to attain the DOE and EU targets only by physical adsorption on carbon materials. Chemical modification of carbons would seem to be a promising alternative approach in order to increase the capacities.     

Impacts of equations of state (EOS) and impurities on the volume calculation of CO2 mixtures in the applications of CO2 capture and storage (CCS) processes

Volume property is the necessary thermodynamic property in the design and operation of the CO₂ capture and storage system (CCS). Because of their simple structures, cubic equations of state (EOS) are preferable to be applied in predicting volumes for engineering applications. This paper evaluates the reliabilities of seven cubic EOS, including PR, PT, RK, SRK, MPR, MSRK and ISRK for predicting volumes of binary CO₂ mixtures containing CH₄, H₂S, SO₂, Ar and N₂, based on the comparisons with the collected experimental data. Results show that for calculations on the volume properties of binary CO₂ mixtures, PR and PT are generally superior to others for all of the studied mixtures. In addition, it was found that the binary interaction parameter has clear effects on the calculating accuracy of an EOS in the volume calculations of CO₂ mixtures. In order to improve the accuracy, k_ij was calibrated for all of the EOS regarding the gas and liquid phases of all the studied binary CO₂ mixtures, respectively.     

The environmental impact and risk assessment of CO2 capture, transport and storage - An evaluation of the knowledge base

In this study, we identify and characterize known and new environmental consequences associated with CO₂ capture from power plants, transport by pipeline and storage in geological formations. We have reviewed (analogous) environmental impact assessment procedures and scientific literature on carbon capture and storage (CCS) options. Analogues include the construction of new power plants, transport of natural gas by pipelines, underground natural gas storage (UGS), natural gas production and enhanced oil recovery (EOR) projects. It is investigated whether crucial knowledge on environmental impacts is lacking that may postpone the implementation of CCS projects. This review shows that the capture of CO₂ from power plants results in a change in the environmental profile of the power plant. This change encompasses both increase and reduction of key atmospheric emissions, being: NO_x, SO₂, NH₃, particulate matter, Hg, HF and HCl. The largest trade-offs are found for the emission of NO_x and NH₃ when equipping power plants with post-combustion capture. Synergy is expected for SO₂ emissions, which are low for all power plants with CO₂ capture. An increase in water consumption ranging between 32% and 93% and an increase in waste and by-product creation with tens of kilotonnes annually is expected for a large-scale power plant (1 GW_e), but exact flows and composition are uncertain. The cross-media effects of CO₂ capture are found to be uncertain and to a large extent not quantified. For the assessment of the safety of CO₂ transport by pipeline at high pressure an important knowledge gap is the absence of validated release and dispersion models for CO₂ releases. We also highlight factors that result in some (not major) uncertainties when estimating the failure rates for CO₂ pipelines. Furthermore, uniform CO₂ exposure thresholds, detailed dose-response models and specific CO₂ pipeline regulation are absent. Most gaps in environmental information regarding the CCS chain are identified and characterized for the risk assessment of the underground, non-engineered, part of the storage activity. This uncertainty is considered to be larger for aquifers than for hydrocarbon reservoirs. Failure rates are found to be heavily based on expert opinions and the dose-response models for ecosystems or target species are not yet developed. Integration and validation of various sub-models describing fate and transport of CO₂ in various compartments of the geosphere is at an infant stage. In conclusion, it is not possible to execute a quantitative risk assessment for the non-engineered part of the storage activity with high confidence.     

Economic assessment of biogas-to-electricity generation system with H2S removal by activated carbon in small pig farm

This study was conducted to assess the economic feasibility of electricity generation from biogas in small pig farms with and without the H₂S removal prior to biogas utilisation. The 2% potassium iodide (KI) impregnated activated carbon selected as H₂S adsorbent was introduced to a biogas-to-electricity generation system in a small pig farm in Thailand as a case study. With the average inlet H₂S concentration of about 2400 ppm to the adsorption unit, the H₂S removal efficiency could reach 100% with the adsorption capacity of 0.062 kg of H₂S/kg of adsorbent. Under the reference scenario (i.e., 45% subsidy on digester installation and fixed electricity price at 0.06 Euro/kWh) and based on an assumption that the biogas was fully utilised for electricity generation in the system, the payback period for the system without H₂S removal was about 4 years. With H₂S removal, the payback period was within the economic life of digester but almost twice that of the case without H₂S removal. The impact of electricity price could be clearly seen for the case of treated biogas. At the electricity price fixed at 0.07 Euro/kWh, the payback period for the case of treated biogas was reduced to about 5.5 years, with a trend to decrease at higher electricity prices. For both treated and untreated biogas, the governmental subsidy was the important factor determining the economics of the biogas-to-electricity systems. Without subsidy, the payback period increased to almost 7 years and about 11 years for the case of untreated and treated biogas, respectively, at the reference electricity price. Although the H₂S removal added high operation cost to the system, it is still highly recommended not only for preventing engine corrosion but also for the environment benefit in which air pollution by H₂S/SO₂ emission and impact on human health could be potentially reduced.     

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.