Thermochemical production of hydrogen from hydrogen sulfide with iodine thermochemical cycles

With the goal of eventually developing a replacement for the Claus process that also produces $H_2$, we have explored the possibility of decomposing hydrogen sulfide through a thermochemical cycle involving iodine. The thermochemical cycle under investigation leverages differences in temperature and reaction conditions to accomplish the unfavorable hydrogen sulfide decomposition to $H_2$ and elemental sulfur over two reaction steps, creating and then decomposing hydroiodic acid. This proposed process is similar to ideas put forth in the 1980s and 1990s by Kalina, Chakma, and Oosawa, but makes use of thermochemical hydrogen iodide decomposition methods and catalysts rather than electrochemical or photoelectrochemical methods.Process models describing a potential implementation of this thermochemical cycle were created. Motivated by the process model results, experimentation showed the possibility of using alternative solvents to dramatically decrease the energy requirements for the process. Further process modeling incorporated these alternative solvents and suggests that this theoretical hydrogen sulfide processing unit has favorable economic and environmental properties.     

Thermochemical decomposition cycle of H2S with nickel sulfide

The two-step thermochemical decomposition cycle of H₂S was proposed as described below, and experimental studies were made on cycle.

The use of lower sulfide such as Ni₃S₂ was regarded as rather important on the basis of thermodynamic and kinetic investigations. Ni₃S₂ powder was mixed with Al₂O₃ to avoid the sintering associated with the depression of the melting point caused by desulfurization on NiS. In the fundamental experiments, the effects of reaction factors were investigated. The cycle under optimum conditions was scaled up 10 times and the thermal efficiency was estimated to be 43–45%.     

Thermochemical storage of solar energy with high-temperature chemical reactions

A new solar energy facility has been installed on the Chemistry Building of the University of Rome. The focusing system and a specially designed chemical reactor are presented, and the results of the first experiments are discussed.     

Thermal decomposition of limestone and gypsum by solar energy

The thermal decomposition of limestone and gypsum by concentrated solar radiation was studied. A 1.5-kW solar furnace was used to obtain the required reaction temperature. Maximum conversions of 65% and 38% were obtained for CaCO₃ and CaSO₄·H₂O decomposition, respectively.     

太阳能制氢技术及其进展

阐述了光解水制氢的原理,介绍了光解水制氢技术的现状,分析了目前光解水制氢技术存在的问题以及提高光解水效率的有效途径,指出了利用光热化学循环进行光解水制氢的新途径.     

Thermochemical energy transport costs for a distributed solar power plant

Thermochemical energy transport costs are calculated for a solar thermal power plant based on a distributed network of para-boloidal collectors. The optimum pipe size distribution within the fluid transport network has been generated subject to requirements of minimum cost and pressure drop equality across parallel conduction paths. The optimization procedure includes the installed capital cost of pipework together with the effective cost of pumping power. An analytical expression for the overall thermochemical energy transport cost has been derived, based on a Black and Veatch pipe cost survey in which conventional pipelaying technology is assumed. Thermochemical energy transport costs are calculated for systems based on ammonia, methanol, water-methane and sulphur trioxide. The derived costs are dominated by the pipe installation component whereas other parameters such as choice of system, operating pressure, reaction enthalpy and degree of reaction are of secondary importance. Larger collectors favour a lower installed cost per unit energy while increases in network area and hence in plant output capacity lead to slow increases in unit cost. Typical thermochemical energy transport costs for a solar thermal power plant operating only during sunlight hours and based on large collectors are estimated at $20 kWt^?1 (1974 U.S. dollars). It is suggested that there is a need for reduction of this estimate by developments in pipelaying technology tailored to the requirements of solar thermal power plants. Such developments would seem to be feasible for thermochemical energy transfer systems based on small diameter pipes and hence on high system pressures.     

Simulation of solar hydrogen energy systems

Solar hydrogen is a promising long-term global energy option for the post-fossil fuel era. On the other hand, solar hydrogen may have already found an early commercial application in the form of seasonal energy storage for remote stand-alone photovoltaic (PV) applications. In a stand-alone solar hydrogen energy system, the photovoltaic array is coupled with an electrolyser to produce H₂ which is stored to be later converted back to electricity in a fuel cell. The system setup comprises several subsystems which have to be controlled in an optimal way. Numerical simulations are used to get a closer insight into the transient response behavior of these elegant, but rather complicated systems during variable insolation conditions and to estimate the overall system performance accurately over extensive periods of time. The simulations are performed with the H2PHOTO program which has been successfully used for the design of a solar hydrogen pilot plant. It has also shown good accuracy against experimental data.     

太阳能热化学制氢技术研究进展

总结了目前国内外太阳能制氢方法的研究现状,经过分析比较,确定太阳能热化学制氢具有极大的潜在发展空间.同时详细介绍了该方法的最新进展,对该方法研究中存在的问题提出了合理的改进建议.     

The thermal decomposition of hydrogen sulfide over transition metal sulfides

The decomposition of hydrogen sulfide to hydrogen and sulfur on a variety of transition metal sulfides has been studied in a flow system at 400–800°C. Hydrogen yields were measured as a function of temperature in order to compare the effectiveness of the metal sulfides in promoting the decomposition. For the series Cr₂S₃, MoS₂, WS₂ it was found that MoS₂ is the most effective catalyst above 600°C but both Cr₂S₃ and WS₂ gave higher H₂ yields than MoS₂ below 600°C. For the group of metal disulfides FeS₂, CoS₂ and NiS₂, thermal decomposition of MS₂ to non-stoichiometric metal sulfides starts at ca. 550°C. The monosulfides FeS, CoS and NiS produce high yields of hydrogen initially due to sulfidation of the solid phase by H₂S to give the same non-stoichiometric sulfides which, themselves, are not good catalysts for the thermal decomposition of H₂S. The copper sulfides Cu₂S, Cu₉S₅. CuS were not effective catalysts for the thermal decomposition of H₂S.     

High temperature solar thermochemical processing—hydrogen and sulfur from hydrogen sulfide

Sunlight, concentrated to high intensities, has a rarely recognized potential for adding process heat to reactors at high temperatures. Hydrogen sulfide is a by-product of the sweetening of fossil fuels. In this paper, we use, as an example, the production of hydrogen and sulfur from hydrogen sulfide as a device for showing how solar processing might be considered as a successor to a currently used industrial process, the Claus process. We conclude that this and other processes should be explored as means of using as well as storing solar energy.     

Thermochemical analysis of dry methane reforming hydrogen production in biomimetic venous hierarchical porous structure solar reactor for improving energy storage

Due to the high working temperature and solar energy is the sole energy resource, radiative transfer field has a significant influence on the solar driven methane reforming conversion efficiency. In order to improve the conversion efficiency of solar driven methane reforming, the idea of using biomimetic venous hierarchical porous structure as solar thermochemical reactor is proposed to regulate the radiative transfer field, which in turn can optimize the temperature field. Through a Finite Volume Method (FVM) combined with thermochemical kinetics, a numerical analysis model of solar driven dry methane reforming (DMR) is established. The effects of different pore diameter combinations (d₁, d₂, and d₃) and partition positions (L₁ and R₁) on the thermochemical performance of reforming are analyzed. The results show that by introducing a biomimetic venous hierarchical porous structure, the methane conversion can be improved by up to 5.9%, which can provide guidance for the optimal design of the solar thermochemical reactor.     

The case for solar/hydrogen energy

Since sustainable, technologically-converted solar energy is the likely basis for our post-fossil-energy future, there is a basic need for solar-produced fuels. It is noteworthy that heat and electricity, solely, are being developed as solar-energy delivery means, while historically civilizations depend on fuels. Hydrogen, a clean, efficiently-used fuel, can be readily derived from water using any of a number of both proved and prospective solar-energy conversion technologies—both direct and indirect (hydropower, wind, etc.). Solar/hydrogen (and oxygen) can also extend depleting fossil-energy resources while ameliorating environmental degradation. The Hydrogen Energy System concept is overviewed as background.A recent ‘Solar/Hydrogen Systems Assessment’ delineated early-availability systems based on photovoltaic, thermal/heat-engine, wind and hydropower solar conversion, and associated water electrolysis to yield product hydrogen and oxygen as ‘hydrogen energy’. Involved technologies being highly modular, good economics of equipment manufacture and deployment are inherent, as is early availability and as-needed rates of construction (in contrast, e.g., with nuclear-plant experience). Proved technological means exist for transporting, storing and distributing hydrogen energy to end-users.Most significant, both small-scale (local, dispersed) and large-scale (central, remote) solar/hydrogen generation facilities can be established in balance with prevailing societal-selection dictates. Involving a readily storable, transportable ‘energy currency’, then-existing hydrogen-energy systems can be inter-tied as desired, providing load-management-related economic advantages to both the energy-user and the ‘energy utility’ of that era. Future solar/hydrogen-electric residences might, as is illustrated, buy and sell hydrogen and electricity in a ‘grid-cooperative’ arrangement.The salient operative question concerns the efficacy of ‘conventional wisdom’ in the energy free-market decision-making process. Will early-enough, adequate level-of-effort programmes be implemented to ensure non-disruptive meeting of tomorrow's demand worldwide? In an aura of business-as-usual, solar/hydrogen's timely contribution to ‘picking up the load’ from exhaustible fossil fuels in the face of still-escalating world energy demand is judged most problematic. Consequently, an unprecedented cooperative world effort for the research, development, demonstration and deployment of solar hydrogen energy delivery capabilities is suggested.     

Hydrogen production via decomposition of hydrogen sulfide by synergy of non-thermal plasma and semiconductor catalysis

Direct H₂S decomposition induced by plasma with an aid of alumina-supported metal sulfide semiconductors (ZnS/Al₂O₃ and CdS/Al₂O₃) for the production of hydrogen was investigated in a dielectric barrier discharge (DBD) reactor. Effects of specific input energy (SIE), feed flow rate, metal sulfide loading, and added hydrogen on the performance of H₂S decomposition were studied. With the aids of ZnS/Al₂O₃ and CdS/Al₂O₃, full conversion was obtained at reasonably low energy costs. The 100-h test runs indicated that both ZnS/Al₂O₃ and CdS/Al₂O₃ were stable in the course of H₂S decomposition. A supported metal sulfide solid solution (Zn_0.4Cd_0.6S/Al₂O₃) exhibited higher performance than ZnS/Al₂O₃ and CdS/Al₂O₃, achieving full conversion at a reduced energy cost. The mechanism of the plasma-induced H₂S decomposition with an aid of a semiconductor catalyst was tentatively proposed.     

Material and energy requirements of solar hydrogen plants

The material and energy requirements for the construction of a hypothetical photovoltaic solar hydrogen plant have been determined. The plant is located in northern Africa and produces electrolytic hydrogen with an energy content equal to 4% of today's final energy consumption of the F.R.G. It turns out that present raw material and economic resources are sufficient to satisfy the total as well as the annual demand for materials. This is true also if several such plants are constructed. The energy payback time will be about two years. The results are compared with the material and energy requirements of solar thermal power generation. Implications with regard to future research activities are discussed.     

The thermal decomposition of hydrogen sulfide over alkali metal sulfides and polysulfides

The decomposition of hydrogen sulfide to hydrogen and sulfur on alkali metal sulfides M₂S (M = Li, Na and K) and polysulfides M₂S_x (x = 2–4; M = Na and K) has been studied in a flow system at 400–800°C. Hydrogen yields were measured as a function of temperature in order to compare the effectiveness of the alkali metal (poly)sulfides in promoting the decomposition. It was found that both Na₂S and K₂S are rapidly sulfided by H₂S to give the corresponding disulfides M₂S₂, whereas Li₂S acts as a catalyst for the thermal decomposition of H₂S. Although Na₂S₂ was further sulfided by H₂S, sodium polysulfides were not effective in enhancing hydrogen yields and all polysulfides produced an amorphous product of approximate composition Na₂S_2.4–2.6. With potassium polysulfides the final product was K₂S₃ in all cases and the sulfidation of K₂S₂ by hydrogen sulfide resulted in an enhancement of hydrogen yields at 500–700°C.     

A possible mechanism for catalytic decomposition of hydrogen sulfide over molybdenum disulfide

The catalytic decomposition of hydrogen sulfide over molybdenum disulfide was studied by use of a closed circulation system at 500°C. The catalytic activity of MoS₂ was remarkably enhanced by reduction with hydrogen but was not considerably increased by increasing the evacuation temperature. A possible mechanism was proposed for the catalytic decomposition of hydrogen sulfide over MoS₂ where the coordinative unsaturation site of MoS₂ surface formed by the reduction with hydrogen acts as the active site.     

Cobalt sulfide modified graphitic carbon nitride semiconductor for solar hydrogen production

Mesoporous graphitic carbon nitride (mpg-CN) was modified with cobalt sulfide (CoS) by using an impregnation-sulfidation approach. The gas-phase sulfidation of CoS in nanoporous networks at certain temperature allows for the formation of intimate contact between the host carbon nitride and CoS nanoparticle, establishing noble-metal free heterojunctions for charge separation and collection at material interface. The resultant CoS/mpg-CN was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂-sorption, X-ray photoelectron spectroscopy (XPS), UV–Vis diffuse reflectance spectroscopy, photoluminescence (PL) spectroscopy and electrochemical measurement. On the basis of the physicochemical and electrocatalytic characterization results of CoS/mpg-CN, it was revealed that CoS functioned as a cocatalyst to promote the migration of excited electron from mpg-CN toward CoS as well as to provide reactive sites for H₂ production at lower overpotentials. As a result, the rate of H₂ evolution over mpg-CN under visible light illumination (λ > 420 nm) was significantly improved after loading with CoS and the optimal loading amount of CoS was found to be 1.0 at. %.