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1.
Due to the challenges of demands on alternative fuels and CO2 emission, the conversion of CO2 has become a hot spot. Among various methods, two-step conversion of CO2 with catalyst ceria (cerium oxide, CeO2) appears to be a promising way. Solar energy is commonly employed to drive the conversion systems. This article proposes a solar-driven system with fluidized bed reactors (FBR) for CO2/H2O conversions. N2 is used as the gas of the heat carrier. The products of CO/H2 could be further used for syngas. To evaluate the capability of the system for exporting work, the system was analysed on the basis of the Second Law of Thermodynamics and the reaction mechanism of ceria. Heat transfer barriers in practical situations were considered. The lowest solar to chemical efficiency is 4.86% for CO2 conversion, and can be enhanced to 43.2% by recuperating waste heat, raising the N2 temperature, and increasing the concentration ratio. The analysis shows that the method is a promising approach for CO2/H2O conversion to produce syngas as an alternative fuel. 相似文献
2.
In most current fossil-based hydrogen production methods, the thermal energy required by the endothermic processes of hydrogen production cycles is supplied by the combustion of a portion of the same fossil fuel feedstock. This increases the fossil fuel consumption and greenhouse gas emissions. This paper analyzes the thermodynamics of several typical fossil fuel-based hydrogen production methods such as steam methane reforming, coal gasification, methane dissociation, and off-gas reforming, to quantify the potential savings of fossil fuels and CO2 emissions associated with the thermal energy requirement. Then matching the heat quality and quantity by solar thermal energy for different processes is examined. It is concluded that steam generation and superheating by solar energy for the supply of gaseous reactants to the hydrogen production cycles is particularly attractive due to the engineering maturity and simplicity. It is also concluded that steam-methane reforming may have fewer engineering challenges because of its single-phase reaction, if the endothermic reaction enthalpy of syngas production step (CO and H2) of coal gasification and steam methane reforming is provided by solar thermal energy. Various solar thermal energy based reactors are discussed for different types of production cycles as well. 相似文献
3.
A new oxy-fuel H2 generation process with CO2 avoidance is provided. The process utilizes mass recirculation of CO and H2O to the oxyforming reactor. A comparison between non-recirculating and mass-recirculating oxyforming reactor operation is given. Main benefits of mass recirculation are emphasized. The oxyforming reactor is integrated with the H2 and CO2 separators, fuel cell and O2 generator. In the process C/O is equal to 0.5 while C/H determines the temperature level in the reactor. The reaction system includes combustion, steam reforming and water–gas shift reactions. The oxyforming process is found to be mass transport controlled with O2 as the limiting reactant. It is emphasized that under MR conditions the decomposition of H2/CO2 by water–gas shift reaction is suppressed by means of CO/H2O-enrichment and hence MR conditions allow for higher temperatures beneficial to endothermic steam reforming reaction. Under MR conditions the thermodynamic equilibrium limits are overcome and all reactions are forced to proceed to the completion which enables 100% selectivities to H2 and CO2. The effects of operation parameters such as temperature, flow rate, pressure and composition are examined. The derived S-terms enable for the concise interpretation of the effect of pressure on the concentration gradients transverse to the flow. The consistent control algorithm of the oxyforming reactor is provided. 相似文献
4.
The influence of redox-treated Pt/TiO2 photocatalysts on H2 production is investigated. Catalyst characterizations are performed by TEM, XPS, XRD, BET, and UV–vis/DR spectroscopy techniques. In terms of production rate, the oxidation treatment shows higher reactivity than the reduction treatment. The reduction treatment allows the formation of metallic Pt(0), which more easily catalyzes the transition of TiO2 from the anatase to the rutile phases. Reduction-treated Pt/TiO2 photocatalysts have lower SBET values than oxidation-treated Pt/TiO2 photocatalysts due to the higher percentage of TiO2 in the rutile phase. Combining the results of XPS and optical analyses, PtO/TiO2 shows a higher energy band gap than metallic Pt(0)/TiO2, indicating that oxidation-treated Pt/TiO2 is more capable of achieving water splitting for H2 production. According to the results of this study, the oxidation treatment of Pt/TiO2 photocatalysts can significantly enhance the reactivity of photocatalytic H2 production because of their homogenous distribution, lower phase transition, higher SBET, and higher energy band gap. 相似文献
5.
A study on the effect of CO2 and H2O dilution on the laminar burning characteristics of CO/H2/air mixtures was conducted at elevated pressures using spherically expanding flames and CHEMKIN package. Experimental conditions for the CO2 and H2O diluted CO/H2/air/mixtures of hydrogen fraction in syngas from 0.2 to 0.8 are the pressures from 0.1 to 0.3 MPa, initial temperature of 373 K, with CO2 or H2O dilution ratios from 0 to 0.15. Laminar burning velocities of the CO2 and H2O diluted CO/H2/air/mixtures were measured and calculated using the mechanism of Davis et al. and the mechanism of Li et al. Results show that the discrepancy exists between the measured values and the simulated ones using both Davis and Li mechanisms. The discrepancy shows different trends under CO2 and H2O dilution. Chemical kinetics analysis indicates that the elementary reaction corresponding to peak ROP of OH consumption for mixtures with CO/H2 ratio of 20/80 changes from reaction R3 (OH + H2 = H + H2O) to R16 (HO2+H = OH + OH) when CO2 and H2O are added. Sensitivity analysis was conducted to find out the dominant reaction when CO2 and H2O are added. Laminar burning velocities and kinetics analysis indicate that CO2 has a stronger chemical effect than H2O. The intrinsic flame instability is promoted at atmospheric pressure and is suppressed at elevated pressure for the CO2 and H2O diluted mixtures. This phenomenon was interpreted with the parameters of the effective Lewis number, thermal expansion ratio, flame thickness and linear theory. 相似文献
6.
E. Pulido Melián Cristina R. López A. Ortega Méndez O. González Díaz M. Nereida Suárez J.M. Doña Rodríguez J.A. Navío D. Fernández Hevia 《International Journal of Hydrogen Energy》2013
A series of synthesised TiO2-based and commercial photocatalysts were modified by Pt photodeposition and a study made of their photocatalytic activity in hydrogen production. The modified commercial photocatalysts were Evonik P25, Kronos vlp7000 and Hombikat UV-100, and the other modified photocatalysts were synthesised by our group using sol–gel and sol–gel hydrothermal processes (SG400, SG750 and HT). Pt weight percentages used in the study were 0.5, 1.0 and 2.1 wt.% (Pt/TiO2). The photocatalysts were extensively characterised by X-ray diffraction (XRD), UV–vis diffuse reflectance, Brunauer–Emmett–Teller (BET) surface area measurement, transmission electron microscopy (TEM), scanning electron microscopy (SEM–EDX), Fourier transform infrared spectroscopy (FTIR) and laser light dispersion. Methanol (25% vol.) was used as sacrificial agent over the 8 h of the hydrogen production tests and measurements were taken of the final concentrations of formaldehyde and formic acid as well as initial and final TOC. Photoactivity of all photocatalysts increased in the presence of Pt. The most efficient of the synthesised photocatalysts was SG750 and of the commercial photocatalysts P25. Maximum production of SG750 was 1846 μmol h−1 at 1.0 wt.% Pt and its production per surface unit was notably higher than that of P25. 相似文献
7.
A new approach to prepare hierarchical and fibrous meso-macroporous N-doped TiO2 is attempted at room temperature without using templates by the addition of titanium isopropoxide droplets to the ammonia solution. The catalysts are thoroughly characterized by physico-chemical and spectroscopic method to explore the structural, electronic and optical properties. The photocatalytic activities of the catalyst were evaluated with hydrogen generation. NTP catalyst calcined at 400 °C (NTP-400) exhibited 602.7 μmol/3 h H2 generation from 10 vol.% methanol under visible light. The excellent photocatalytic activity for NTP-400 is attributed to the porous networks existing in our system with uniform N dispersion throughout the catalyst. The hierarchical and fibrous structures allow easy channelization of electron as in the case of nanotubes for effective surface charge transfer. Along with macroporosity, nitrogen incorporation and mesoporosity play some important roles for enhanced photoactivities. 相似文献
8.
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. 相似文献
9.
The thermochemical dissociation of CO2 and H2O from reactive SnO nanopowders is studied via thermogravimetry analysis. SnO is first produced by solar thermal dissociation of SnO2 using concentrated solar radiation as the high-temperature energy source. The process targets the production of CO and H2 in separate reactions using SnO as the oxygen carrier and the syngas can be further processed to various synthetic liquid fuels. The global process thus converts and upgrades H2O and captured CO2 feedstock into solar chemical fuels from high-temperature solar heat only, since the intermediate oxide is not consumed but recycled in the overall process. The objective of the study was the kinetic characterization of the H2O and CO2 reduction reactions using reactive SnO nanopowders synthesized in a high-temperature solar chemical reactor. SnO conversion up to 88% was measured during H2O reduction at 973 K and an activation energy of 51 ± 7 kJ/mol was identified in the temperature range of 798-923 K. Regarding CO2 reduction, a higher temperature was required to reach similar SnO conversion (88% at 1073 K) and the activation energy was found to be 88 ± 7 kJ/mol in the range of 973-1173 K with a CO2 reaction order of 0.96. The SnO conversion and the reaction rate were improved when increasing the temperature or the reacting gas mole fraction. Using active SnO nanopowders thus allowed for efficient and rapid fuel production kinetics from H2O and CO2. 相似文献
10.
Naser Alenzi Wei-Ssu Liao Paul S. Cremer Viviana Sanchez-Torres Thomas K. Wood Christine Ehlig-Economides Zhengdong Cheng 《International Journal of Hydrogen Energy》2010
Though less frequently studied for solar-hydrogen production, films are more convenient to use than powders and can be easily recycled. Anatase TiO2 films decorated with Ag nanoparticles are synthesized by a rapid, simple, and inexpensive method. They are used to cleave water to produce H2 under UV light in the presence of methanol as a hole scavenger. A simple and sensitive method is established here to monitor the time course of hydrogen production for ultralow amounts of TiO2. The average hydrogen production rate of Ag/TiO2 anatase films is 147.9 ± 35.5 μmol/h/g. Without silver, it decreases dramatically to 4.65 ± 0.39 μmol/h/g for anatase TiO2 films and to 0.46 ± 0.66 μmol/h/g for amorphous TiO2 films fabricated at room temperature. Our method can be used as a high through-put screening process in search of high efficiency heterogeneous photocatalysts for solar-hydrogen production from water-splitting. 相似文献
11.
The mixed metal oxides NiFe2O4 and CoFe2O4 are candidate materials for the Chemical Looping Hydrogen (CLH) process, which produces pure and separate streams of H2 and CO2 without the use of complicated and expensive separations equipment. In the CLH process, syngas reduces a metal oxide, oxidizing the H2 and CO in the syngas to H2O and CO2, and “stores” the chemical energy of the syngas in the reduced metal oxide. The reduced metal oxide is then oxidized in steam to regenerate the original metal oxide and produce H2. In this study, we report thermodynamic modeling and experimental results regarding the syngas reduction and H2O oxidation of NiFe2O4 and CoFe2O4 to determine the feasibility of their use in the CLH process. Modeling predicts the oxidation of nearly all the CO and H2 in syngas to H2O and CO2 during the reduction step for both materials, and regeneration of the mixed metal spinel phase during oxidation with excess H2O. Laboratory tests in a packed bed reactor confirmed over 99% conversion of H2 and CO to H2O and CO2 during reduction of NiFe2O4 and CoFe2O4. Powder XRD analysis of the reduced materials showed, in accordance with thermodynamic predictions, the presence of a spinel phase and a metallic phase. High reactivity of the reduced NiFe2O4 and CoFe2O4 with H2O was observed, and XRD analysis confirmed re-oxidation to NiFe2O4 and CoFe2O4 under the conditions tested. When compared with a conventional Fe-based CLH material, the mixed metal spinels showed a higher extent of reduction under the same conditions, and produced four times the H2 per mass of active material than the Fe-based material. Analysis of the H2 and CO consumed in the reduction and the H2 produced during the oxidation showed over 90% conversion of the H2 and CO in syngas back to H2 during oxidation. 相似文献
12.
Peter G. LoutzenhiserAldo Steinfeld 《International Journal of Hydrogen Energy》2011,36(19):12141-12147
Solar syngas production from CO2 and H2O is considered in a two-step thermochemical cycle via Zn/ZnO redox reactions, encompassing: 1) the ZnO thermolysis to Zn and O2 using concentrated solar radiation as the source of process heat, and 2) Zn reacting with mixtures of H2O and CO2 yielding high-quality syngas (mainly H2 and CO) and ZnO; the ZnO is recycled to the first, solar step, resulting in net reaction βCO2 + (1 − β)H2O → βCO + (1 − β)H2. Syngas is further processed to liquid hydrocarbon fuels via Fischer-Tropsch or other catalytic processes. Second-law thermodynamic analysis is applied to determine the cycle efficiencies attainable with and without heat recuperation for varying molar fractions of CO2:H2O and solar reactor temperatures in the range 1900-2300 K. Considered is the energy penalty of using Ar dilution in the solar step below 2235 K for shifting the equilibrium to favor Zn production. 相似文献
13.
Rosalia N. Widyaningrum Tamara L. Church Ming ZhaoAndrew T. Harris 《International Journal of Hydrogen Energy》2012
Supported nickel metal can be used as a tar-cracking catalyst in the thermal processing of large organics to give H2. Though porous supports offer the opportunity to disperse a relatively large amount of nickel, catalytic sites within small pores may be inaccessible to large tar molecules. To investigate this, we prepared nickel catalysts supported on γ-Al2O3 and on SBA-15- and mesocellular-foam-(MCF)-type silicas and studied their catalytic activities in the pyrolytic decomposition of cellulose (RT → 800 °C, ∂T/∂t = 40 °C/min). A thermogravimetric analyser-mass spectrometer (TG-MS) was used to study the influence of the Ni catalyst and support materials on the H2 yield and selectivity. The silica-MCF-supported Ni catalyst decreased char residue and enhanced H2 yield, producing 1.6 and 3.5× the H2 yield obtained using SBA-15-supported Ni and γ-Al2O3-supported Ni, respectively. MCF, with ultra-large pores (d ∼ 15−50 nm), was thus identified as the most beneficial catalyst support for this application. 相似文献
14.
Chemical effects of added CO2 on the extinction characteristics of H2/CO/CO2 syngas diffusion flames
Jeong Park Jeong Soo Kim Jin Oh Chung Jin Han Yun Sang In Keel 《International Journal of Hydrogen Energy》2009
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. 相似文献
15.
The coal gasification process is used in commercial production of synthetic gas as a means toward clean use of coal. The conversion of solid coal into a gaseous phase creates opportunities to produce more energy forms than electricity (which is the case in coal combustion systems) and to separate CO2 in an effective manner for sequestration. The current work compares the energy and exergy efficiencies of an integrated coal-gasification combined-cycle power generation system with that of coal gasification-based hydrogen production system which uses water-gas shift and membrane reactors. Results suggest that the syngas-to-hydrogen (H2) system offers 35% higher energy and 17% higher exergy efficiencies than the syngas-to-electricity (IGCC) system. The specific CO2 emission from the hydrogen system was 5% lower than IGCC system. The Brayton cycle in the IGCC system draws much nitrogen after combustion along with CO2. Thus CO2 capture and compression become difficult due to the large volume of gases involved, unlike the hydrogen system which has 80% less nitrogen in its exhaust stream. The extra electrical power consumption for compressing the exhaust gases to store CO2 is above 70% for the IGCC system but is only 4.5% for the H2 system. Overall the syngas-to-hydrogen system appears advantageous to the IGCC system based on the current analysis. 相似文献
16.
A. López Ortiz F.J. Neri Segura R. Sandoval Jabalera M. Marques da Silva Paula E. Arias del Campo J. Salinas Gutiérrez M.A. Escobedo Bretado V. Collins-Martínez 《International Journal of Hydrogen Energy》2013
Biomass pyrolysis offers a fast route to produce elevated yields towards highly valued liquid products. This research aims the determination of optimal experimental conditions for a slow and low temperature pyrolysis to produce the highest yield towards condensable (CVM) and non-condensable (NCVM) volatile matter from Mexican cane bagasse and to quantify and characterize the compounds that constitute CVM and NCVM obtained. Results indicate that yield towards volatiles is strongly dependent on temperature. The highest yield was achieved at temperatures greater than 500 °C at a heating rate of 10 °C/min, residence time of 60 min and a particle size between of 420 and 840 μm. Product quantification under isothermal conditions determined that at 550 °C the NCVM, CVM and solid residue was of 26, 57 and 16%, respectively. Preliminary thermodynamic analysis of steam reforming and CO2 absorption reactions using one of the main CVM products resulted in a potential high hydrogen production yield. 相似文献
17.
M.H. Halabi M.H.J.M. de Croon J. van der Schaaf P.D. Cobden J.C. Schouten 《International Journal of Hydrogen Energy》2012
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. 相似文献
18.
Simultaneous photocatalytic reduction of water to H2 and CO2 to CO was observed over Cu2O photocatalyst under both full arc and visible light irradiation (>420 nm). It was found that the photocatalytic reduction preference shifts from H2 (water splitting) to CO (CO2 reduction) by controlling the exposed facets of Cu2O. More interestingly, the low index facets of Cu2O exhibit higher activity for CO2 photoreduction than high index facets, which is different from the widely-reported in which the facets with high Miller indices would show higher photoactivity. Improved CO conversion yield could be further achieved by coupling the Cu2O with RuOx to form a heterojunction which slows down fast charge recombination and relatively stabilises the Cu2O photocatalyst. The RuOx amount was also optimised to maximise the junction's photoactivity. 相似文献
19.
K. SutthiumpornS. Kawi 《International Journal of Hydrogen Energy》2011,36(22):14435-14446
Alkaline earth elements (Mg, Ca and Sr) on Ni-La2O3 catalyst have been investigated as promoters for syngas production from dry CO2 reforming of methane (DRM). The catalysis results of DRM performance at 600 °C show that the Sr-doped Ni-La2O3 catalyst not only yields the highest CH4 and CO2 conversions (∼78% and ∼60%) and highest H2 production (∼42% by vol.) but also has the lowest carbon deposition over the catalyst surface. The XPS, O2-TPD, H2-TPR and FTIR results show that the excellent performance over the Sr-doped Ni-La2O3 catalyst is attributed to the presence of a high amount of lattice oxygen surface species which promotes C-H activation in DRM reaction, resulting in high H2 production. Moreover, these surface oxygen species on the Ni-SDL catalyst can adsorb CO2 molecules to form bidentate carbonate species, which can then react with the surface carbon species formed during DRM, resulting in higher CO2 conversion and lower carbon formation. 相似文献
20.
Dong-Yeup LeeJunHo Kim 《Solar Energy Materials & Solar Cells》2011,95(1):245-249
We studied CuInS2 (CIS) film growth using high electrostatic field assisted ultrasonic spray (HEFAUS) deposition. CIS films were fabricated with various precursors and substrate temperatures. All the as-sprayed CIS films were observed to be grown with mixed ordering mode, where chalcopyrite (CH) and CuAu (CA) orderings coexisted. It was found that application of additional sulfurization to sprayed CIS films induced re-crystallization of films accompanied by enhancement of CH ordering. After the post-sulfurization, the most improved film showed nearly the same CH-fraction as that for a CIS film which was made by sulfurization of sputtered Cu-In alloy film. These results indicate that our modified spray deposition could be used for fabrication of CIS photoabsorbing layer instead of high-cost vacuum-based process. All fabricated films were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscope and energy dispersive X-ray analysis measurements. 相似文献