A new approach for hydrogen production in Claus sulfur recovery process

In this study, the decomposition of hydrogen sulfide and reforming of hydrocarbons were employed for producing hydrogen. Thermolysis of H₂S in the platinum-based catalytic reactor led to the production of hydrogen and sulfur. It is observed that the presence of catalyst increased the conversion of H₂S decomposition up to 99.6%. Also, the hydrocarbon content of acid gas stream (CH₄) was converted to syngas, especially hydrogen in a catalytic reforming process. The produced hydrogen was separated using a Pd/Ag membrane. The simulation results showed that hydrogen production in a sulfur recovery unit provided a green source of energy by incinerating gasses. By hydrogen production during the process, the molar flow rate of incinerating gasses reduced from 2555 to 621?Kmol/h. Moreover, the hazardous sulfur compounds of the stack were removed, while hydrogen was produced by 256?Kmol/hr.     

The Roles of Cullins E3 Ubiquitin Ligases in the Lipid Biosynthesis of the Green Microalgae Chlamydomonas reinhardtii

Microalgae-based biodiesel production has many advantages over crude oil extraction and refinement, thus attracting more and more concern. Protein ubiquitination is a crucial mechanism in eukaryotes to regulate physiological responses and cell development, which is highly related to algal biodiesel production. Cullins as the molecular base of cullin-RING E3 ubiquitin ligases (CRLs), which are the largest known class of ubiquitin ligases, control the life activities of eukaryotic cells. Here, three cullins (CrCULs) in the green microalgae Chlamydomonas reinhardtii were identified and characterized. To investigate the roles of CrCULs in lipid metabolism, the gene expression profiles of CrCULs under nutrition starvation were examined. Except for down-regulation under nitrogen starvation, the CrCUL3 gene was induced by sulfur and iron starvation. CrCUL2 seemed insensitive to nitrogen and sulfur starvation because it only had changes after treatment for eight days. CrCUL4 exhibited an expression peak after nitrogen starvation for two days but this declined with time. All CrCULs expressions significantly increased under iron deficiency at two and four days but decreased thereafter. The silencing of CrCUL2 and CrCUL4 expression using RNAi (RNA interference) resulted in biomass decline and lipids increase but an increase of 20% and 28% in lipid content after growth for 10 days, respectively. In CrCUL2 and CrCUL4 RNAi lines, the content of fatty acids, especially C16:0 and C18:0, notably increased as well. However, the lipid content and fatty acids of the CrCUL3 RNAi strain slightly changed. Moreover, the subcellular localization of CrCUL4 showed a nuclear distribution pattern. These results suggest CrCUL2 and CrCUL4 are regulators for lipid accumulation in C. reinhardtii. This study may offer an important complement of lipid biosynthesis in microalgae.     

Improvement in the Photobiological Hydrogen Production of Aggregated Chlorella by Dimethyl Sulfoxide

Photobiological hydrogen production plays a vital role in generating clean renewable energy owing to its low energy consumption and environmental friendliness. Although materials‐induced Chlorella aggregates have been developed to achieve sustained photobiological hydrogen production under normal aerobic conditions, the yield is relatively low and equals only 0.42 % of the light‐to‐H₂ energy‐conversion efficiency. Herein, we report that only 0.5 vol % dimethyl sulfoxide in an aqueous environment significantly enhances the H₂ yield produced by aggregated Chlorella, reaching 0.69 % of the light‐to‐H₂ energy‐conversion efficiency. This improvement can be attributed to an increase in the cellular respiration rate by dimethyl sulfoxide, which results in a decrease in the oxygen content inside the aggregates and, ultimately, to the activation of more hydrogenases. More generally, this strategy consists of a functional enhancement in organism–material hybrids by using small molecules.     

New thermochemical cycles for hydrogen production

The ease of decomposing some metal sulfates to oxides and sulfur trioxide is employed in the search for efficient closed cycle thermochemical methods for water splitting. The main features of the new processes are the production of O₂ through the decomposition of SO₂ and/or SO₃, the production of hydrogen by the decomposition of H₂S, reaction of H₂S with a metal, reaction of water with a metal, and/or reaction of water with sulfides.     

Effect of cultivation mode on microalgal growth and CO2 fixation

The biofixation of carbon dioxide (CO₂) by marine microalgae cultivation has been regarded as one of the potential to diminish the greenhouse effect and produce the biomass. To compare and select the high efficiency cultivation mode, the effect of two different cultivation modes on the performances of growth and CO₂ biofixation from air for energy marine microalgae Chlorella sp. was determined in this work. In one mode the microalga was cultivated using static (open) method and in the other mode it was cultivated using aerated (closed) method. It was found that, under the experimental conditions, the specific growth rate and CO₂ fixation rate of the aerated (closed) cultivation were 0.5121 (d⁻¹) and 1.3784 g CO₂/l d, and are 1.78 and 5.39 times that of the static (open) cultivation, respectively. In addition, the effects of pH value and dissolved oxygen (DO) concentration of the culture medium were analyzed and compared in two cultivation modes. The result indicates the aerated (closed) mode can effectively enhance the performance on microalgal growth and CO₂ biofixation.     

Characterization of the first hexacoordinate phosphorus compound with S→P←S bonds

The first example of a hexacoordinate phosphorus compound [S{6-t-Bu-4-Me-C₆H₂O}₂]₂P⁺(Cl^?·C₃H₄N₂) with two S→P bonds is reported. This compound can be construed as an oxophosphonium salt with double intramolecular coordination by sulfur atoms. X-ray structure reveals a facial arrangement of the ligands with two coordinating sulfur atoms cis to each other. The S→P distance of 2.334 (1) Å is one among very short coordinate bond distances between sulfur and phosphorus.     

The electrochemistry of the nickel sulfides—2. Ni3S2

The anodic and cathodic behaviour of Ni₃S₂ (synthetic hazelwoodite) in acid solutions is examined. The presence of a sulfur-rich sulfide layer which passivates the surface is demonstrated. This layer exists in the potential region 0 to 0.6 V sce, and itself dissolves slowly in acid. At higher potentials complex, anion dependent dissolution behaviour involving the formation of elemental sulfur and sulfur oxyanions occurs. At cathodic potentials, non-oxidative acid dissolution with the formation of H₂S is observed, concurrently with the evolution of hydrogen and the production of metallic nickel.     

Characteristics of hydrogen production by immobilized cyanobacterium Microcystis aeruginosa through cycles of photosynthesis and anaerobic incubation

The characteristics of hydrogen production using immobilized cyanobacterium Microcystis aeruginosa were studied through a two-stage cyclic process. Immobilized cells were first grown photosynthetically under CO₂ and light, followed by anaerobic H₂ production in the absence of light and sulfur. M. aeruginosa was capable of generating H₂ under immobilized conditions, and the use of immobilized cells allowed the maintenance of stable production and sped up the changes in culture conditions for cyclic two-stage operation. M. aeruginosa was also capable of utilizing exogenous glucose as a substrate to generate hydrogen and 30 mM concentration proved to be optimal. The externally added glucose improved H₂ production rates, total produced volume and the lag time required for cell adaptation prior to H₂ evolution. The rate of hydrogen evolution was increased as temperature increased, and the maximum evolution rate was 48 mL/h/L and 34.0 mL/h/L at 42 °C and 37 °C, respectively. The optimal temperature for hydrogen production was 37–40 °C because temperatures higher than 42 °C resulted in cell death. In order to continue repeated cycles of H₂ production, at least two days of photosynthesis under conditions with light, CO₂, and sulfur should be allowed for cells to recover H₂ production potential and cell viability.     

Assessment of phosphorescent paint effects on microalgae cultivation

Light is the most important factor involved in the growth of photosynthetic organisms, and low efficiency of artificial light systems imposes higher cultivation costs. Phosphorescent paints can emit light for a few hours, and so may be appropriate to accumulate scattered light. In this study, the effects of blue and green phosphorescent paints on growth rate, biomass production and chlorophyll content of three different microalgae, Scenedesmus dimorphus, Chlamydomonas reinhardtii and Chlorella vulgaris were investigated. These strains were cultivated in broth medium in three glass bowls. Half of the first two bowls was stained with blue or green phosphorescent paints (B and G treatment), while the third one was unstained and used as the control (C treatment). All measured parameters were higher in B. The results indicate that partial staining of culturing bowls by blue phosphorescent paint can be an economic approach to increase light efficiency in cultivating microalgae.     

Electrolytic pretreatment of Illinois No. 6 coal

Electrolysis of Illinois No. 6 coal in acidic as well as basic electrolytes accomplishes significant amount of sulfur and ash removal under moderate reaction conditions. Electrolysis at 1.4 V vs. SCE (saturated calomel electrode) in 2 M NaCl + 9% HCl electrolyte accomplishes 62% sulfur removal and leaves behind a clean coal with the sulfur to heating value ratio of 1.44 lb S/million Btu (0.619 kg S/GJ) while coal slurry oxidation at 3.0 V vs. SCE results in 72% ash removal. Coal electrolysis in basic electrolytes accomplishes a clean residue with relatively low oxygen content. The sulfur to heating value ratio of 2.11 lb S/million Btu (0.907 kg S/GJ) is observed for coal electrolyzed in 2 M NaOH at 3.0 V vs. SCE. Impurity removal from coal is simultaneously accompanied by clean hydrogen gas production at the cathode at Faradaic coulometric efficiencies of over 95%. Hydrogen gas is produced by the depolarization of water by mineral impurities present in coal. A relatively small amount of H₂ is produced due to water splitting caused by the carbonaceous part of the coal. Model reaction pathways for coal cleaning are discussed. More work is in progress on the types of sulfur forms removed from coal.     

Recent progress in hydrogenase and its biotechnological application for viable hydrogen technology

Despite increasing interest in hydrogen (H₂) as an alternative energy carrier, the current production of H₂ still depends on fossil fuels. Biotechnological hydrogen production can provide a more sustainable way to generate H₂. Hydrogenases are key enzymes involved in hydrogen metabolism of microorganisms with roles of H₂ oxidation or evolution. They have potential applications in H₂ production in vivo, in vitro and fuel cell. Important achievements have been made over the past decade in our understanding of hydrogenase and its biotechnological application as catalyst for H₂ production and fuel cell. This review summarizes recent progress in the study of hydrogenases, involving strategies for biosynthesis, maturation process, isolation of novel hydrogenases, heterologous expression system, structural feature of oxygen (O₂)-tolerant hydrogenases, and biotechnological applications for viable H₂ technology.     

Occurrence,Evolution and Specificities of Iron-Sulfur Proteins and Maturation Factors in Chloroplasts from Algae

Iron-containing proteins, including iron-sulfur (Fe-S) proteins, are essential for numerous electron transfer and metabolic reactions. They are present in most subcellular compartments. In plastids, in addition to sustaining the linear and cyclic photosynthetic electron transfer chains, Fe-S proteins participate in carbon, nitrogen, and sulfur assimilation, tetrapyrrole and isoprenoid metabolism, and lipoic acid and thiamine synthesis. The synthesis of Fe-S clusters, their trafficking, and their insertion into chloroplastic proteins necessitate the so-called sulfur mobilization (SUF) protein machinery. In the first part, we describe the molecular mechanisms that allow Fe-S cluster synthesis and insertion into acceptor proteins by the SUF machinery and analyze the occurrence of the SUF components in microalgae, focusing in particular on the green alga Chlamydomonas reinhardtii. In the second part, we describe chloroplastic Fe-S protein-dependent pathways that are specific to Chlamydomonas or for which Chlamydomonas presents specificities compared to terrestrial plants, putting notable emphasis on the contribution of Fe-S proteins to chlorophyll synthesis in the dark and to the fermentative metabolism. The occurrence and evolutionary conservation of these enzymes and pathways have been analyzed in all supergroups of microalgae performing oxygenic photosynthesis.     

Catalytic processes and catalysts for production of elemental sulfur from sulfur-containing gases

The modern technologies for production of elemental sulfur are considered. It is demonstrated that along with the further wide application of the conventional Claus process with conventional alumina catalyst in the observable future some new trends which may significantly influence the technological picture of recovered sulfur manufacturing may be formulated: active development of Claus tail gas cleanup processes with the stress on replacement of subdewpoint Sulfreen-type processes by processes of hydrogen sulfide selective oxidation by oxygen; development of novel highly-efficient technologies for hydrogen sulfide decomposition to sulfur and hydrogen; application of new catalysts forms, first of all — at microfiber supports for Claus and H₂S oxidation processes; wider application of titania and vanadia catalysts at the newly constructed Claus units; development of technologies and catalysts for direct purification of H₂S-containing gases and for catalytic reduction of SO₂ for sulfur recovery from smelter gases. All these prospective routes are actively developed by Russian science and some of them are completely based on domestic developments in this area.     

Analysis of sulfur poisoning on a PEM fuel cell electrode

The extent of irreversible deactivation of Pt towards hydrogen oxidation reaction (HOR) due to sulfur adsorption and subsequent electrochemical oxidation is quantified in a functional polymer electrolyte membrane (PEM) fuel cell. At 70 °C, sequential hydrogen sulfide (H₂S) exposure and electrochemical oxidation experiments indicate that as much as 6% of total Pt sites are deactivated per monolayer sulfur adsorption at open-circuit potential of a PEM fuel cell followed by its removal. The extent of such deactivation is much higher when the electrode is exposed to H₂S while the fuel cell is operating at a finite load, and is dependent on the local overpotential as well as the duration of exposure. Regardless of this deactivation, the H₂/O₂ polarization curves obtained on post-recovery electrodes do not show performance losses suggesting that such performance curves alone cannot be used to assess the extent of recovery due to sulfur poisoning. A concise mechanism for the adsorption and electro-oxidation of H₂S on Pt anode is presented. H₂S dissociatively adsorbs onto Pt as two different sulfur species and at intermediate oxidation potentials, undergoes electro-oxidation to sulfur and then to sulfur dioxide. This mechanism is validated by charge balances between hydrogen desorption and sulfur electro-oxidation on Pt. The ignition potential for sulfur oxidation decreases with increase in temperature, which coupled with faster electro-oxidation kinetics result in the easier removal of adsorbed sulfur at higher temperatures. Furthermore, the adsorption potential is found to influence sulfur coverage of an electrode exposed to H₂S. As an implication, the local potential of a PEM fuel cell anode exposed to H₂S contaminated fuel should be kept below the equilibrium potential for sulfur oxidation to prevent irreversible loss of Pt sites.     

Low-temperature catalytic decomposition of hydrogen sulfide on metal catalysts under layer of solvent

When hydrogen sulfide decomposition {2 H₂S ? 2 H₂?+?S₂^(gas)} is carried out in the flow regime at room temperature on metal catalysts placed in a liquid capable of dissolving H₂S and sulfur, the reaction equilibrium can be significantly (up to 100%) shifted to the right yielding the desired product – hydrogen. The process efficiency was demonstrated using aqueous solutions of monoethanolamine (MEA), sodium carbonate, which is widely used in industry for H₂S absorption from tail gases, and aqueous hydrazine as examples. IR and Raman spectroscopy data demonstrated that sulfur obtained in the solutions is in the form of diatomic molecules. DFT calculations showed that diatomic sulfur forms weakly bound coordinative complexes with solvent molecules. Some problems related to sulfur accumulation and recovery from the solvents are discussed.     

Effect of H2/O2 addition in increasing the thermal efficiency of a diesel engine

Using hydrogen as an additive to enhance the conventional diesel engine performance has been investigated by several researchers and the outcomes are very promising. However, the problems associated with the production and storage of pure hydrogen currently limits the application of pure hydrogen in diesel engine operation. On-board hydrogen-oxygen generator, which produces H₂/O₂ mixture through electrolysis of water, has significant potential to overcome these problems. This paper focuses on evaluating the performance enhancement of a conventional diesel engine through the addition of H₂/O₂ mixture, generated through water electrolysis. The experimental works were carried out under constant speed with varying load and amount of H₂/O₂ mixture. Results show that by using 4.84%, 6.06%, and 6.12% total diesel equivalent of H₂/O₂ mixture the brake thermal efficiency increased from 32.0% to 34.6%, 32.9% to 35.8% and 34.7% to 36.3% at 19 kW, 22 kW and 28 kW, respectively. These resulted in 15.07%, 15.16% and 14.96% fuel savings. The emissions of HC, CO₂ and CO decreased, whereas the NO_x emission increased.     

Production of H2 from catalytic partial oxidation of H2S in a short-contact-time reactor

Partial oxidation of H₂S over alumina catalysts in a short-contact-time reactor (SCTR) has been shown to yield hydrogen, sulfur and water as the predominant products. At a set temperature of 400 °C and a contact time of 13 ms, the conversion of H₂S is 64.6% with a H₂ selectivity of 20.8%, while the amount of SO₂ in the products was <0.5% of the input H₂S.     

Optimization of the influential factors for the improvement of CO2 utilization efficiency and CO2 mass transfer rate

Microalgae fix CO₂ as energy source and afford biomass and high valued products such as carotenoids, pigments, proteins, and vitamins that can be used for the production of nutraceuticals, pharmaceuticals, animal feed additives, cosmetics, etc. Carbon dioxide is the sole source of carbon and it is supplied continuously for the microalgal cultivation. But undissolved CO₂ is lost by outgassing and sufficient dissolved CO₂ should be provided to avoid carbon limitation. The effect of CO₂ mass transfer with different CO₂ concentrations, aeration rate of gas, bubble size, baffle type and baffle number on the growth of Chlorella sp. AG10002 was investigated and the optimized conditions for the enhancement of biomass productivity were determined. We confirm that these results can be provided as basic data to improve the CO₂ mass transfer ability for the high density culture of Chlorella sp. and some microalgae having commercial value.     

Oxidation of low concentrations of hydrogen sulfide over activated carbon

The oxidation of low concentrations of hydrogen sulfide with air over activated carbon was studied over the temperature range 24-200°C using both fixed and fluid bed reactors. The predominant reaction, H₂S + ½ O_a → H₂O + S, was found to have an order of 0.5 with respect of H₂S concentration. Activity of the catalyst decreased as the amount of sulfur deposited on it increased. Indirect evidence suggests that adsorption of water by the carbon also decreases its activity as a catalyst at lower temperatures. Values of the activation energy and the frequency factor were determined for various sulfur loadings using the fixed bed reaction system. Regeneration of the carbon loaded with sulfur was studied at temperatures between 150 and 500°C using steam as a carrier gas. Bright yellow sulfur was recovered. The regenerated carbon was shown to have its original activity.