Effect of inhibitors on hydrogen consumption and microbial population dynamics in mixed anaerobic cultures

The impact of different chemical microbial stressors (2-bromoethanesulfonate (BES), furfural, fish oil, lauric acid (LUA) and linoleic acid (LA)) on the inhibition of mesophilic hydrogen (H₂) consumption was examined in this study. Hydrogen consumption half-life values were used to compare the extent of inhibition by the different microbial stressing agents. A statistical analysis of the percent H₂ consumed using Tukey's analysis revealed the following trend: Control > fish oil = linoleic acid (LA (C18:2)) = furfural > BES > lauric acid (LUA (C12:0). The terminal restriction fragment length polymorphism (T-RLFP) results indicated that aceticlastic methanogens (Methanosaeta sp., Methanosarcina sp.) and hydrogenotrophic methanogens (Methanococcus sp.) were inhibited by the different chemical stressing agents. Cultures fed LUA and LA had a high abundance of Clostridium sp., Clostridium propionicum and Propionibacterium acnes. In comparison, BES and furfural fed cultures contained large fractions of Clostridium sp., Eubacteria sp. and Bacteroides sp. while in the fish oil fed cultures, the dominant organism detected was Eubacteria sp. This study indicated that H₂ consumption was affected by the chemical stressing agent concentration.     

Inoculum preparation of anaerobic mixed cultures by electric field for dark fermentative hydrogen production

The applicability of electric field as a pretreatment technique for inoculum preparation was confirmed in a previous study. In the present work, newly adopted electric pretreatment conditions were statistically optimized for the preparation of inoculum from anaerobic mixed cultures in dark fermentative H₂ production via response surface methodology with a Box–Behnken design. Pretreatment conditions of applied voltage, distance of electrode, and reaction time were chosen as independent variables, while H₂ yield was chosen as the response variable. Overall performance revealed that applied voltage and reaction time were slightly interdependent or significantly interactive influence on H₂ yield, and they were more influential compared with electrode distance on H₂ yield. In the confirmation test, H₂ yield of 1.42 mol H₂ per mol hexose_added was recorded, corresponding with 94.7% of the predicted response value, under applied voltage of 11 V, distance of electrode of 5 cm, and reaction time of 23 min. Although the H₂ yield under electric pretreatment was similar with the heat pretreated yield, the energy consumption efficiency was about 40‐fold lower. Therefore, it may be concluded that electric pretreatment is a potent alternate technique for inoculum preparation method in terms of energy consumption and easy applicability in the field with high effectiveness. Copyright © 2014 John Wiley & Sons, Ltd.     

Using a statistical approach to model hydrogen production from a steam exploded corn stalk hydrolysate fed to mixed anaerobic cultures in an ASBR

In this study, hydrogen (H₂) production from the fermentation of steam exploded corn stalk (CS) liquor was statistically optimized using a fractional factorial design approach. The factors under consideration included temperature, pH and hydraulic retention time (HRT). Under optimal conditions at 53 °C, a pH at 4.5 and a 9.5 h HRT, the observed maximum H₂ yield of 98 ± 2 mL g⁻¹ TVS together with negligible CH₄ were similar to the model predicted responses. A flux analysis revealed negligible homoacetogenic activity in cultures at 53 °C and low pH (≤5.5). Both homoacetogenic (R17) and methanogenic (R28 and R29) fluxes accounted for more than 68–90% of H₂ consumption in cultures at low temperatures. The low H₂ yields observed in cultures maintained at 21 °C and 37 °C was associated with high lactate and solvent levels. High H₂ yields in cultures at 53 °C were associated with a higher abundance of Clostrdium sp. and CH₄ production at low temperatures was due to the presence of hydrogenotrophic methanogens (Methanothermobacter marburgensis and Methanobrevibacter ruminatum) and aceticlastic methanogens (Methanosaeta sp. and Methanosarcina sp.). The results obtained from this study (within the factor ranges investigated) indicated that steam exploded CS liquor could be a potential substrate for H₂ production using mixed microbial cultures.     

Measurements and analysis of turbulent consumption speeds of H2/CO mixtures

This paper describes measurements of global turbulent consumption speeds, S_T,GC, of hydrogen/carbon monoxide (H₂/CO) mixtures. The turbulent flame properties of such mixtures are of fundamental interest because of their strong stretch sensitivity, and of practical interest since they are the primary constituents of syngas fuels. Data are reported at mean flow velocities and turbulence intensities of 4 < U₀ < 50 m/s and , respectively, for H₂/CO blends ranging from 30% to 90% H₂ by volume. Two sets of experiments are reported. In the first, fuel blends ranging from 30% to 90% H₂ and mixture equivalence ratio, ?, were adjusted at each fuel composition to have nominally the same un-stretched laminar flame speed, S_L,0. In the second set, equivalence ratios were varied at constant H₂ levels. The data clearly corroborate results from other studies that show significant sensitivity of S_T,GC to fuel composition. In particular, at a fixed and S_L,0, values of S_T,GC increase by a factor of almost 2 when H₂ levels are increased from 30% (at ? = 0.61) to 90% (at ? = 0.48). Moreover, S_T,GC in the 90% H₂ case is three times larger than the ? = 0.9 CH₄/air mixture with the same S_L,0 value. An important conclusion from this work is these fuel effects are not simply a low turbulence intensity phenomenon – they clearly persist over the entire range of turbulence intensities used in the measurements. We also describe physics-based correlations of these data, using leading points concepts and detailed kinetic calculations of the stretch sensitivity of these mixtures. These results are used to develop an inequality for negative Markstein length flames that bounds the turbulent flame speed data and show that the data can be collapsed using the maximum stretched laminar flame speed, S_L,max, rather than S_L,0.     

Fermentative H2 production using a switchgrass steam exploded liquor fed to mixed anaerobic cultures: Effect of hydraulic retention time,linoleic acid and nitrogen sparging

Fermentative hydrogen (H₂) production from a steam exploded switchgrass liquor using inhibited mixed anaerobic microbial communities was studied in upflow anaerobic sludge blanket reactors (UASBRs). Increasing the H₂ yield was accomplished by treating the inoculum with linoleic acid (LA), varying the hydraulic retention time (HRT) and sparging liquid phase with nitrogen (N₂). A maximum H₂ yield of 2.56 ± 0.10 mol mol⁻¹ hexose, was obtained at a 6 h HRT in LA treated cultures sparged with N₂. Sparging or LA treatment alone was able to enhance the H₂ yield by 46 ± 5% and 38 ± 3%, respectively, in comparison to control cultures operating at a 6 h HRT. Of the different methods employed, N₂ sparging in combination with LA treatment proved to be more effective in enriching the H₂ producing bacteria belonging to Clostridium sp. Species belonging to Propionibacterium, Bacteroides and Eubacterium, which were associated with H₂ consumption and reduced byproducts formation, were observed in addition to Clostridium sp. in unsparged control cultures.     

Cogeneration of H2 and CH4 from water hyacinth by two-step anaerobic fermentation

A novel reaction mechanism of H₂ and CH₄ cogeneration from water hyacinth (Eichhornia crassipes) was originally proposed to increase the energy conversion efficiency. The glucose and xylose hydrolysates derived from cellulose and hemicellulose are fermented to cogenerate H₂ and CH₄ by two-step anaerobic fermentation. The total volatile solid of hyacinth leaves can theoretically cogenerate H₂ and CH₄ yields of 303 ml-H₂/g-TVS and 211 ml-CH₄/g-TVS, which dramatically increases the theoretical energy conversion efficiency from 19.1% in only H₂ production to 63.1%. When hyacinth leaves are pretreated with 3 wt% NaOH and cellulase in experiments, the cogeneration of H₂ (51.7 ml-H₂/g-TVS) and CH₄ (143.4 ml-CH₄/g-TVS) markedly increases the energy conversion efficiency from 3.3% in only H₂ production to 33.2%. Hyacinth leaves, which have the most cellulose and hemicellulose and the least lignin and ash, give the highest H₂ and CH₄ yields, while hyacinth roots, which have the most ash and the least cellulose and hemicellulose, give the lowest H₂ and CH₄ yields.     

16S rRNA gene based analysis of the microbial diversity and hydrogen production in three mixed anaerobic cultures

To explore of role of microbial diversity and its functionality in commercial bioreactors, three anaerobic microbial communities from Ontario, Canada were characterized using 16S rRNA gene-based, clone library sequencing and terminal restriction fragment length polymorphism (T-RFLP) and compared with the hydrogen (H₂) and methane yields. The T-RFLP method showed more operational taxonomic units than the clone library sequence analysis; however, the two methods showed similar dominant species and relative diversity while Spearman's Rank correlation coefficient (r) values ranged from 0.82 to 0.91. The Chao 1 and Shannon-Wiener indices revealed that the cultures samples have highly diverse microbial communities. Comparatively, cultures from a municipal wastewater treatment plant (CA) showed more diversity than those from facilities treating effluents from a baby food processor and a brewery. Even though culture CA has the highest microbial diversity, low H₂ and methane production yield was attributed to the presence of sulphate reducers, propionate producers and a low percentage of methanogens. This study confirms that the selection of the source of mixed anaerobic cultures plays an important role in H₂ and methane production.     

Influence of operation conditions on direct NaBH4/H2O2 fuel cell performance

Although hydrogen fuel cells have attracted so much attentions in these years because of the application prospect in electric vehicles, some obstacles have not been solved yet, among which hydrogen storage is one of the biggest. Direct borohydride fuel cell (DBFC) is another choice without hydrogen storage problem because borohydride is used as reactant directly in the fuel cell. In this paper, DBFC performance under different operation conditions was studied including electrolyte membrane type, operation temperature, borohydride concentration, supporting electrolyte and oxidant. Results showed that, with Pt/C and MnO₂ as anode and cathode electrocatalyst, respectively, Nafion^® 117 membrane as electrolyte, 1.0 M, 3.0 M and 6.0 M NaBH₄ and H₂O₂ solution in NaOH as reactant solution, 80 °C operation, the peak power density could reach 130 mW/cm².     

Effect of pH and sulfate concentration on hydrogen production using anaerobic mixed microflora

The effects of varying sulfate concentrations with pH on continuous fermentative hydrogen production were studied using anaerobic mixed cultures growing on a glucose substrate in a chemostat reactor. The maximum hydrogen production rate was 2.8 L/day at pH 5.5 and sulfate concentration of 3000 mg/L. Hydrogen production and residual sulfate level decreased with increasing the pH from 5.5 to 6.2. The volatile fatty acids (VFAs) and ethanol fractions in the effluent were in the order of butyric acid (HBu) > acetic acid (HAc) > ethanol > propionic acid (HPr). Fluorescence In Situ Hybridization (FISH) analysis revealed the presence of hydrogen producing bacteria (HPB) under all pH ranges while sulfate reducing bacteria (SRB) were present at pH 5.8 and 6.2. The inhibition in hydrogen production by SRB at pH 6.2 diminished entirely by lowering to pH 5.5, at which activity of SRB is substantially suppressed.     

Statistical thermodynamic description of H2 molecules in normal ortho/para mixture

In this paper, an expression for the internal partition function of normal hydrogen has been obtained starting from the principles of classical statistical thermodynamics. The calculated contribution applies to ideal gas properties. Particular attention has been devoted to the entropy, which depends on the logarithm of the partition function. Comparison between normal and equilibrium hydrogen together with theoretical and experimental data available in literature has been discussed. Tables of the thermodynamic properties of hydrogen and hydrogen ion are also reported.     

H2 production from CH4 decomposition: Regeneration capability and performance of nickel and rhodium oxide catalysts

Nickel–lanthanum (LaNiO₃) and nickel–rhodium–lanthanum (LaNi_0.95Rh_0.05O₃) perovskite-type oxide precursors were synthesized by different methodologies (co-precipitation, sol–gel and impregnation). They were reduced in an H₂ atmosphere to produce nickel and rhodium nanoparticles on the La₂O₃ substrate. All samples were tested in the catalytic decomposition of CH₄. Methane decomposed into carbon and H₂ at reaction temperatures as low as 450 °C—no other reaction products were observed. Conversions were in the range of 14–28%, and LaNi_0.95Rh_0.05O₃ synthesized by co-precipitation was the most active catalyst. All catalysts maintained sustained activity even after massive carbon deposition indicating that these deposits are of the nanotube-type, as confirmed by transmission electron microscopy (TEM). The reaction seems to occur in a way that a nickel or rhodium crystal face is always clean enough to expose sufficient active sites to make the catalytic process continue. The samples were subjected to a reduction–oxidation–reduction cycle and in situ analyses confirmed the stability of the perovskite structure. All diffraction patterns showed a phase change around 400 °C, due to reduction of LaNiO₃ to an intermediate La₂Ni₂O₅ structure. When the reduction temperatures reach 600 °C, this structure collapses through the formation of Ni⁰ crystallites deposited on the La₂O₃. Under oxidative conditions, the perovskite system is recomposed with nickel re-entering the LaNiO₃ framework structure accounting for the regenerative capability of these solids.     

Effect of ZrO2 addition on Ni/Al2O3 catalyst to produce H2 from glycerol

In this work, ZrO₂ was employed as support and as Al₂O₃ modifier of Ni based catalysts due to its special interesting characteristics. The catalytic activity of these systems was studied in steam reforming of glycerol to produce H₂. As the activity results at 773 K and 873 K showed, the NiZ catalyst allowed low glycerol conversion and H₂ production when compared to the NiγA catalyst. Moreover, the NiZ catalyst was not able to reform intermediate liquid products into gaseous products.     

Investigation of novel mixed metal ferrites for pure H2 and CO2 production using chemical looping

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.     

Determination of laminar burning velocities for lean low calorific H2/N2 and H2/CO/N2 gas mixtures

Combustion of lean and ultra-lean synthetic H₂/CO mixtures that are highly diluted in inert gases is of great importance in several fields of technology, particularly in the field of post combustion for combined heat and power (CHP) systems based on fuel cell technology. In this case H₂/CO mixtures occur via hydrocarbon reforming and their complete conversion requires efficient, compact and low emission combustion systems. In order to design such systems, knowledge of global flame properties like the laminar burning velocity, is essential. Using the heat flux burner method, laminar burning velocities were experimentally determined for highly N₂ diluted synthetic H₂ and H₂/CO mixtures with low calorific value, burning with air, at ambient temperature and atmospheric pressure. Furthermore, numerical 1-D simulations were performed, using a series of different chemical reaction mechanisms. These numerical predictions are analysed and compared with the experimental data.     

Effect of H2S on carbon-catalyzed methane decomposition and CO2 reforming reactions

The effect of H₂S on catalytic processing of methane is of a great practical importance. In this work, the effect of small quantities (0.5–1.0 vol.%) of H₂S present in the feedstock on the methane decomposition and CO₂ reforming reactions over carbon and metal based catalysts was investigated. Activated carbon (FY5), an in-house prepared alumina-supported Ni catalyst (NiA) and the mixture of both (FY5 + NiA) were used as catalysts in this study. It was found that CH₄ and CO₂ conversions were noticeably increased when H₂S was added to the reacting mixture, which points to (i) the tolerance of carbon catalyst to H₂S and (ii) the catalytic effect of H₂S on carbon-catalyzed decomposition and dry reforming of methane. In contrast, NiA catalyst and the mixture FY5 + NiA were deactivated in the presence of H₂S in both reactions. The effect of the heating system (i.e., conventional electric resistance vs microwave heating) on the products yield of the dry reforming reaction in the presence of H₂S is also discussed in this paper.     

Flammability limits,limiting oxygen concentration and minimum inert gas/combustible ratio of H2/CO/N2/air mixtures

The flammability limits, the limiting oxygen concentration (LOC) and the inert gas/combustible ratio (ICR) of hydrogen/carbon monoxide/nitrogen/air mixtures are determined for hydrogen fuel molar fractions of 0.44, 0.62 and 0.71, at atmospheric pressure and initial temperatures up to 200 °C. The experiments are performed in a glass cylindrical tube with an internal diameter of 80 mm. The mixtures are ignited by a spark discharge between two electrodes placed at the bottom of the tube. Flame propagation is said to have occurred if the flame propagates a distance of at least 100 mm. The experimental procedure is based upon EN 1839 and EN 14756. Le Chatelier's law is used to estimate the flammability limits of the hydrogen/carbon monoxide mixtures, while the LOC and ICR are estimated based upon the lower flammability limit. The estimates are compared with the experimental data.     

Effect of CO on hydrogen storage performance of 2LiNH2 + MgH2 system

(2LiNH₂ + MgH₂) system is one of the most promising hydrogen storage materials due to its suitable operation temperature and high reversible hydrogen storage capacity. In studies and applications, impurities such as CO, CO₂, O₂, N₂ and CH₄ are potential factors which may influence its performance. In the present work, hydrogen containing 1 mol% CO is employed as the hydrogenation gas source, and directly participates in the reaction to investigate the effect of CO on the hydrogen sorption properties of (2LiNH₂ + MgH₂) system. The results indicate that the hydrogen capacity of the (Mg(NH₂)₂ + 2LiH) system declines from 5 wt.% to 3.45 wt.% after 6 cycles of hydrogenation and dehydrogenation, and can not restore to its initial level when use purified hydrogen again. The hydrogen desorption kinetics decreases obviously and the dehydrogenation activation energy increases from 133.35 kJ/mol to 153.35 kJ/mol. The main reason for these is that two new products Li₂CN₂ and MgO appear after (2LiNH₂ + MgH₂) react with CO. They are formed on the surface of materials particles, which may not only cause a permanent loss of NH²⁻, but also prevent the substance transmission during the reaction process. After re-mechanically milling, both kinetics and dehydrogenation activation energy can be recovered to the initial level.     

Effect of Si content on H2 production using Al-Si alloy powders

The effect of Si content in Al-Si alloy powder with NaOH on H₂ production was investigated. The total amount of H₂ produced decreased as Si content increased, which is inconsistent with the results predicted by the chemical reaction. Si caused a delay in the rate of H₂ production. Energy dispersive spectrometry showed that a large amount of unreacted Si remained in the matrix, and the unreacted fraction increased as the Si content increased. As the evolution reaction of Al and Al-Si alloys is exothermic, the temperature of all the specimens increased. Si addition reduced the hydroxide removal rate, which decreased the average H₂ production rate. The initiation time for H₂ evolution depends on the elimination rate of the oxide film formed during production of the powder. On increasing the Si content, SiO₂ was formed, which is harder to eliminate than Al₂O₃; this delayed the initiation.     

Effect of additive distribution in H2 absorption and desorption kinetics in MgH2 milled with NbH0.9 or NbF5

This paper presents a comparative study of H₂ absorption and desorption in MgH₂ milled with NbF₅ or NbH_0.9. The addition of NbF₅ or NbH_0.9 greatly improves hydriding and dehydriding kinetics. After 80 h of milling the mixture of MgH₂ with 7 mol.% of NbF₅ absorbs 60% of its hydrogen capacity at 250 °C in 30 s, whereas the mixture with 7 mol.% of NbH_0.9 takes up 48%, and MgH₂ milled without additive only absorbs 2%. At the same temperature, hydrogen desorption in the mixture with NbF₅ finishes in 10 min, whereas the mixture with NbH_0.9 only desorbs 50% of its hydrogen content, and MgH₂ without additive practically does not releases hydrogen. The kinetic improvement is attributed to NbH_0.9, a phase observed in the hydrogen cycled MgH₂ + NbF₅ and MgH₂ + NbH_0.9 materials, either hydrided or dehydrided. The better kinetic performance of the NbF₅-added material is attributed to the combination of smaller size and enhanced distribution of NbH_0.9 with more favorable microstructural characteristics. The addition of NbF₅ also produces the formation of Mg(H_xF_1-x)₂ solid solutions that limit the practically achievable hydrogen storage capacity of the material. These undesired effects are discussed.     

Effect of UV irradiation on PC membrane and use of Pd nanoparticles with/without PVP for H2 selectivity enhancement over CO2 and N2 gases

The hydrogen-based economy is one of the possible approaches toward to eliminate the problem of global warming, which are increases because of the gathering of greenhouse gases. Palladium (Pd) is well-known material having a strong affinity to the hydrogen absorbing property and thus appropriate material to embed in the membrane for the improvement of selective permeation of hydrogen gas. In present work, we have functionalized polycarbonate (PC) membranes with the help of UV irradiation to embed the Pd nanoparticles in pores as well as on the surface of the PC membrane. Use of Pd Nanoparticles is helpful to enhance the H₂ selectivity over other gases (CO₂, N_2, etc.). Also, the UV based modification of membrane increases the attachment of Pd Nanoparticles. Further to enhance the Pd nanoparticles attachment, we used PVP binder with Pd nanoparticles solution. Gas permeability measurements of functionalized PC membranes have been carried out, and better selectivity of hydrogen has been found in the functionalized and Pd nanoparticle binded membrane. PC membrane with 48 h UV irradiated and Pd NPs with PVP have been found to have maximum selectivity and permeability for H₂ gas. All the samples being characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy and UV–Vis spectroscopy for their morphological and structural investigation.