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

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

The energy efficiency of carbon dioxide fixation by a hydrogen-oxidizing bacterium

Fixing carbon dioxide (CO₂) with solar hydrogen (H₂) is a novel alternative to conventional photosynthesis of plants and microalgae. The energy efficiency of CO₂ fixation by a hydrogen-oxidizing bacterium was investigated in a closed reactor system. The molar ratio of consumed H₂ and CO₂ was measured under mass transfer limitation in atmospheres of sufficient H₂, low CO₂, and a broad range of O₂. The energy efficiency, ranging from 10% to 60%, was primarily affected by the oxygen concentration (6–30 mol%). The research revealed a clear trend that a low oxygen concentration gave high energy efficiency, but slow gas consumption. A high energy efficiency of 50% was measured under a moderate oxygen concentration (10 mol%). Based on 10% solar hydrogen efficiency, a 5% overall efficiency from solar energy to biomass can therefore be achieved.     

A comparative study of CdS-based semiconductor photocatalysts for solar hydrogen production from sulphide + sulphite substrates

Different types of CdS-based photocatalysts have been prepared, characterized and tested for hydrogen evolution from a sulphide + sulphite solution. The benefit of the thermal treatment of CdS in air, argon, hydrogen sulphide or hydrogen atmospheres, as well as that of etching its surface, have been shown. The effects of adding to CdS a second semiconductor of smaller bandgap (Cu_xS) or larger bandgap (TiO₂), or/and a catalytic material (Pt or RuO_x), have been studied. The best results have been obtained using particles of CdS of ≈ 1.4 ωm, treated at 400°C in air atmosphere and etched with concentrated HNO₃, and on which 0.4% by weight of Pt was deposited.     

Solar photocatalysts based on titanium dioxide nanotubes for hydrogen evolution from aqueous solutions of ethanol

Titanium foil was anodized to grow titanium dioxide nanotubes (TiO₂ NT) by using an original technique. A solution of ethylene glycol and ammonium fluoride with a concentration of 0.5 wt% was employed as electrolyte, corrosion-resistant steel was chosen as the cathode material. Anodic oxidation was carried out potentiostatically at a voltage of 60 V at ambient temperature for 90 min. Modification of as prepared TiO₂ NT were modified by calcination in air at 350°C for 4 h. Platinization of TiO₂ NT was carried out by chemical reduction of platinum from H₂PtCl₆ solution by NaBH₄ solution. Photocatalyst characterization was performed by using XRD, BET, SEM, XPS techniques. It was found that the calcination of as-prepared TiO₂ NT leads to 6-fold increase of the photocatalytic activity under UV-irradiation. The enhanced activity of TiO₂ NT is associated with the developed specific surface area and unique morphology of TiO₂ NT. This results in high rate of separation of photogenerated excited electrons and holes. Due to high separation rate the catalytic activity increases substantially.     

Integrated biomass energy systems and emissions of carbon dioxide

Electric Power Research Institute (EPRI) and the US Department of Energy (DOE) have been funding a number of case studies under the initiative entitled “Economic Development through Biomass Systems Integration”, with the objective of investigate the feasibility of integrated biomass energy systems, utilizing a dedicated feedstock supply system (DFSS) for energy production. This paper deals with the full fuel cycle for four of these case studies, which have been examined with regard to the emissions of carbon dioxide, CO₂. Although the conversion of biomass to electricity in itself does not emit more CO₂ than is captured by the biomass through photosynthesis, there will be some CO₂ emissions from the DFSS. External energy is required for the production and transportation of the biomass feedstock, and this energy is mainly based on fossil fuels. By using this input energy, CO₂ and other greenhouse gases are emitted. However, by utilizing biomass with fossil fuels as external input fuels, we would get about 10–15 times more electric energy per unit fossil fuel, compared with a 100% coal power system. By introducing a DFSS on former farmland the amount of energy spent for production of crops can be reduced, the amount of fertilizers can be decreased, the soil can be improved and a significant amount of energy will be produced compared with an ordinary farm crop. Compared with traditional coal-based electricity production, the CO₂ emissions are in most cases reduced significantly by as much as 95%. The important conclusion is the great potential for reducing greenhouse gas emissions through the offset of coal by biomass.     

Mitigation of carbon dioxide from Indonesia's energy system

In Indonesia, energy consumption (excluding non-commercial energy) increased from 328 MBOE in 1990 to 478 MBOE in 1995. As a consequence, energy sector CO₂ emissions increased from 150 million tons to over 200 million tons during the same period. The present rapid economic growth Indonesia is experiencing (7–8%) will continue in the future. Based on a BAU scenario, primary energy supply for the year 2020 will be 18,551 PJ, an increase of 5.9% annually from 1990 CO₂ from the energy system will increase from 150 Teragrams in 1990 to 1264 Teragram in 2020. The mitigation scenario would reduce total CO₂ emissions from the BAU scenario by 10% for the year 2000 and 20% by 2020. Some demand side management and energy conservation programs are already included in the BAU scenario. In the mitigation scenario, these programs are expanded, leading to lower final energy demand in the industrial and residential sectors.

Indonesia's total primary energy supply in 2020 is approximately 5% lower for the mitigation scenario than for the BAU scenario. In the BAU scenario, coal and oil have the same contribution (25%). In the mitigation scenario, natural gas and nonfossil fuels such as hydropower, geothermal, and nuclear have higher contributions.     

Use of solar energy to reduce carbon dioxide

It is thermodynamically possible to decompose carbon dioxide to carbon and oxygen by means of thermochemical cycles analogous to cyclic processes for decomposing water. Highly concentrated solar energy can supply the necessary energy at temperatures that permit short, efficient cycles. The reduced carbon can be converted to fuels that are dense, convenient, and require no major changes in the consuming sector of the economy.     

Water splitting and carbon dioxide reduction over alkaline-earth tantalate photocatalysts loaded with metal oxide cocatalysts

Calcium and strontium tantalates with general formula A₂Ta₂O₇ (A = Ca, Sr) were prepared by the solid-state synthesis and evaluated in the photocatalytic water splitting and CO₂ reduction. A predominance in the water splitting processes is observed in both materials obtaining competitive yields for H₂ and O₂ compared to the results reported in the literature. A better performance was observed in the Sr₂Ta₂O₇ sample associated with its crystalline structure and morphology (H₂ = 807 μmol g⁻¹, O₂ = 296 μmol g⁻¹, CO = 5 μmol g⁻¹ for 6 h). Besides, both materials were decorated with Co₃O₄ and CuO particles acting as cocatalysts. The presence of these oxides enhanced the evolution of the products considerably due to the efficient charges transport avoiding their recombination, being the Co₃O₄ loaded sample the most efficient in both cases. The presence of CuO favored the formation of CO in both tantalate materials; however, their rates are not comparable with the obtained for H₂ and O₂, being the water-splitting process also predominant with the use of both cocatalysts.     

Photoreduction of carbon dioxide and water into formaldehyde and methanol on semiconductor materials

Heterogeneous photoassisted reduction of aqueous carbon dioxide was achieved using semiconductor powders, with either high-pressure Hg-lamps or sunlight as energy sources. The products were methanol, formaldehyde and methane. The reaction was carried out either as a gas-solid process, by passing carbon dioxide and water vapor over illuminated semiconductor surfaces, or as a liquid-solid reaction, by illuminating aqueous suspensions of semiconductor powders through which carbon dioxide was bubbled. Best results, under illumination by Hg-lamps, were obtained with aqueous suspensions of strontium titanate, SrTiO₃, tungsten oxide, WO₃, and titanium oxide, TiO₂, resulting in absorbed energy conversion efficiencies of 6, 5.9 and 1.2 per cent, respectively.     

Photoelectrical properties of doped cadmium sulphide powders

Photosensitive powders of CdS were prepared with different concentrations of dopants. Doses of donors (Cl⁻) and acceptors (Cu²⁺, Ag⁺) varied from 0 to 41 and from 0 to 9 mg/g CdS, respectively. Reflectance, absorption coefficient and resistance dependence on illumination intensity and voltage at the wavelength of about λ=630 nm and photovoltage spectra in the range 450–900 nm were measured on layers prepared from the powders. The value of absorption coefficient grew with the increasing dopant concentrations; acceptors appeared more efficient than donors. Reflectance decreased with growing acceptor dose. Using the values of reflectance, absorption coefficient and resistance the corrected photovoltage, as the measure of the concentration photogenerated charge carriers, was calculated. The ratio (σ_IGB) of the corrected photovoltage and photocurrent was used as the criterion of intergrain barrier conductivity. All doped samples exhibited similar value of σ_IGB which was about three orders of magnitude lower than that of undoped sample.     

Photoelectrochemical study of screen-printed cadmium sulphide electrodes

Thin films of CdS have been prepared by the screen-printing technique. Optical absorption studies reveal a band gap of 2.42 eV. Current-voltage studies at the CdS-(1 M NaOH-0.1 M Na₂S-0.1 M S) interface yield an exchange current density of 6 X 10⁻⁶ A CM⁻² and a junction ideality factor of 2.8. Mott-Schottky plots at 0.4, 1.0, 2.0 and 4.0 kHz show a flat-band potential of -1.07 V (SCE). At 84.5 mW cm⁻² tungsten-halogen illumination, the photoelectrochemical cell gives an open-circuit voltage of 0.32 V, a short-circuit current density of 0.23 mA cm⁻², and a fill-factor of 0.42.     

Analysis of energy and carbon dioxide emission caused by power consumption

The main objective of this on‐site study is to use a full‐scale Heating, Ventilating, and Air‐Conditioning (HVAC) system installed in an office building in Taiwan for comparing the power consumption, energy‐saving, and carbon dioxide (CO₂) reduction of two different strategies for controlling the HVAC. These strategies are the Constant Volume (CV) system [Constant Air Volume+Constant‐flow], and the Variable Volume (VV) system [Variable Air Volume +Variable‐flow]. The on‐site experimental results indicate that average power consumptions are 164 kW for the CV system, and 88 kW for the VV system; the average electric current drops from 469 A for the CV system to 258 A for the VV system. Approximately 46% of the average energy‐saving can be achieved if the HVAC system is operated as a VV system. Additionally, the reduced quantity of accumulated CO₂ emission varies from 67 to 3687 kg with 0.637 kg CO₂ kwh^?1 emission factor during the office hours of 08:30 (a.m.)–17:00 (p.m.). The results demonstrate that switching the operation of an office building HVAC system from CV to VV will significantly enhance energy‐savings and CO₂ reduction. This studywill offer useful information for evaluating an indoor environmental policy with respect to energy‐savings and CO₂ emission reduction for office HVACs used in subtropical regions. Copyright © 2010 John Wiley & Sons, Ltd.     

Solar energy storage via a closed-loop chemical heat pipe

The performance of a solar chemical heat pipe was studied using CO₂ reforming of methane as the vehicle for storage and transport of solar energy. The endothermic reforming reaction was carried out with a reactor packed with a supported rhodium catalyst and heated by the concentrated solar flux from the Schaeffer solar furnace at the Weizmann Institute (Rehovot, Israel). The maximum absorbed power was 8.5 kW. The reforming was run under variable insolation conditions, including partly cloudy days. The flux input was regulated by opening the doors of the concentrator building. The product gas temperature followed a predetermined set point that automatically controlled the flow of reactants to ensure constant composition of the reformer products. The exothermic methanation reaction was run in a multistage methanator filled with the same Rh catalyst and fed with the products from the reformer. High conversions were achieved for both reactions. In the closed-loop mode, the products from thereformer and from the methanator were compressed into separate storage tanks. The two reactions were run consecutively, and the whole process was repeated for over 60 cycles. The overall performance of the closed loop was satisfactory; scale-up work is in progress.     

Analytical model of carbon dioxide emission with energy payback effect

An analytical model is proposed to account for carbon emission behaviour during replacement of power source from fossil fuel to renewable energy in which sustainability of energy supply is stressed. Logistic function of time is assumed for producing renewable power sources. Analyses show that energy payback time (EPT) should be much shorter than the doubling time of manufacturing cycle to secure adequate available energy during, as well as after, the replacement. A nuclear plant, small hydropower plant, wind power plant and photovoltaic cell are taken as representative candidates and investigated as options to replace fossil power until toward the end of this century. Nuclear or small hydropower plants are promising candidates but the photovoltaic cell needs further development efforts to reduce EPT and avoid energy expense after the replacement.     

Negative carbon intensity of renewable energy technologies involving biomass or carbon dioxide as inputs

Conventional fossil fuel-based energy technologies can achieve efficiency in energy conversion but they are usually completely inefficient in carbon conversion because they generate significant CO₂ emissions to the atmosphere per unit energy converted. In contrast, some renewable energy technologies characterized by negative carbon intensity can simultaneously achieve efficiency in the conversion of energy and in the conversion of carbon. These carbon negative renewable energy technologies can generate useful energy and remove CO₂ from the atmosphere, either by direct capture and recycling of atmospheric CO₂ or indirectly, by involving biofuels. Interestingly, the deployment of carbon negative renewable energy technologies can offset carbon emissions from conventional fossil fuel-based energy technologies and thus reduce the overall carbon intensity of energy systems.The current review analyzes two groups of renewable energy technologies involving biomass or CO₂ as inputs. The discussions focus on useful techniques which enable to achieve negative carbon intensity of energy while being technologically promising in near-term as well as cost-effective. These analyzes include advanced carbon sequestration concepts such as soil carbon sequestration and CO₂ recycling to useful C-rich products such as fuels and fertilizers. The 'drop-in' of renewable energy is achieved by allowing bioenergy and renewable energies in the form of renewable electricity, renewable thermal energy, solar energy, renewable hydrogen, etc. The carbon negative renewable energy technologies are analyzed and perspectives and constraints of each technology are expounded.     

Study of solar energy powered transcritical cycle using supercritical carbon dioxide

In this paper, the performance of solar energy powered transcritical cycle using supercritical carbon dioxide for a combined electricity and heat generation, is studied experimentally. The experimental set‐up consists of evacuated solar collectors, pressure relief valve, heat exchangers and CO₂ feed pump. The pressure relief valve is used to simulate operation of a turbine and to complete the thermodynamic cycle. A complete effort was carried out to investigate the cycle performances not only in summer, but also in winter conditions. The results show that a reasonable thermodynamic efficiency can be obtained and COP for the overall outputs from the cycle is measured at 0.548 and 0.406, respectively, on a typical summer and winter day. The study shows the potential of the application of the solar energy powered cycle as a green power/heat generation system. Copyright © 2006 John Wiley & Sons, Ltd.     

Performance of nano photocatalysts for the recovery of hydrogen and sulphur from sulphide containing wastewater

Stability of photocatalyst plays an important role in efficient hydrogen recovery from sulphide waste streams. This research focuses on the stability and efficiency of visible light active photocatalysts viz., RuO₂/CuGa_1.6Fe_0.4O₄, ZnFe₂O₃, (CdS + ZnS)/Fe₂O₃ and Ce/TiO₂ for H₂ production. RuO₂/CuGa_1.6Fe_0.4O₄ photocatalyst was found to give maximum hydrogen production of 8370 μmol/h. The reusability of the photocatalysts was tested by multiple cycles of catalyst regeneration along with H₂ production. The result shows that (CdS + ZnS) coated iron oxide core shell particles were found to be stable than other prepared nano photocatalysts. It is also demonstrated that H₂S can be split into hydrogen and sulphur under visible light irradiation using sulphide and sulphite reaction media at room temperature. This research paper will help in search of stable photocatalysts in recovering hydrogen from sulphide wastewater along with sulphur separation.     

Syngas production via plasma photocatalytic reforming of methane with carbon dioxide

Anthropogenic emission of CH₄ and CO₂ contributes for most of global warming. Hence, simultaneous conversion of CH₄ and CO₂ into syngas (dry reforming of methane) can be a promising way to alleviate climate change. In this work, we developed a series of perovskite-type photocatalysts, based on LaFeO₃ with various calcination temperatures to combine with a spark discharge reactor to form a hybrid plasma photocatalysis reactor. The hybrid reactor is applied for dry reforming of methane to investigate the syngas generation rate and to reveal possible interactions between plasma and photocatalyst. Results show that LFO600-packed bed has the best CH₄ and CO₂ conversions and syngas generation efficiency of 53.6%, 40.0% and 18.4 mol/kWh, respectively. The enhancement of syngas generation rate can be attributed to synergies between LFO and plasma. Furthermore, changing calcination temperature of photocatalyst also leads to variable characteristics of photocatalyst and hence plasma photocatalysis performance for syngas production.     

太阳能技术对我国未来减排CO2的贡献

1世界对可再生能源减排作用的估计可再生能源不但是重要的后续能源,而且对未来减排CO2将发挥重要作用。国际上许多组织和国家预测,本世纪中叶可再生能源在一次性能源消耗中将超过50%。最近20年来,各种可再生能源技术的日趋成熟和生产规模的不断扩大,对能源的贡献也日渐增大。可以预料,随着可再生能源的快速发展,对未来CO2减排的贡献会越来越大。为加速发展中国家可再生能源的发展,1996年世界银行通过全球环境基金(GEF)项目,对未来CO2的排放作了如下估计:如果不采取措施,50年后,大气中CO2的含量就是现在的3.5倍,如果积极采取各种清洁能…     

Hydrogen production from solar energy powered supercritical cycle using carbon dioxide

A hydrogen production method is proposed, which utilizes solar energy powered thermodynamic cycle using supercritical carbon dioxide (CO₂) as working fluid for the combined production of hydrogen and thermal energy. The proposed system consists of evacuated solar collectors, power generating turbine, water electrolysis, heat recovery system, and feed pump. In the present study, an experimental prototype has been designed and constructed. The performance of the cycle is tested experimentally under different weather conditions. CO₂ is efficiently converted into supercritical state in the collector, the CO₂ temperature reaches about 190 °C in summer days, and even in winter days it can reach about 80 °C. Such a high-temperature realizes the combined production of electricity and thermal energy. Different from the electrochemical hydrogen production via solar battery-based water splitting on hand, which requires the use of solar batteries with high energy requirements, the generated electricity in the supercritical cycle can be directly used to produce hydrogen gas from water. The amount of hydrogen gas produced by using the electricity generated in the supercritical cycle is about 1035 g per day using an evacuated solar collector of 100.0 m² for per family house in summer conditions, and it is about 568.0 g even in winter days. Additionally, the estimated heat recovery efficiency is about 0.62. Such a high efficiency is sufficient to illustrate the cycle performance.