Biological hydrogen production in suspended and attached growth anaerobic reactor systems

Biological production of hydrogen gas has received increasing interest from the international community during the last decade. Most studies on biological fermentative hydrogen production from carbohydrates using mixed cultures have been conducted in conventional continuous stirred tank reactors (CSTR) under mesophilic conditions. Investigations on hydrogen production in reactor systems with attached microbial growth have recently come up as well as investigations on hydrogen production in the thermophilic temperature range. The present study examines and compares the biological fermentative production of hydrogen from glucose in a continuous stirred tank type bioreactor (CSTR) and an upflow anaerobic sludge blanket bioreactor (UASB) at various hydraulic retention times (2–12 h HRT) under mesophilic conditions (35 °C). Also the biohydrogen production from glucose in the CSTR at mesophilic and thermophilic (55 °C) temperature range was studied and compared. From the CSTR experiments it was found that thermophilic conditions combine high hydrogen production rate with low production of microbial mass, thus giving a specific hydrogen production rate as high as 104 mmole _H2/h/l/g${\mathrm{H}}_2/\mathrm{h}/\mathrm{l}/\mathrm{g}$ VSS at 6 h retention time compared to a specific hydrogen production rate of 12 mmole _H2/h/l/g${\mathrm{H}}_2/\mathrm{h}/\mathrm{l}/\mathrm{g}$ VSS under mesophilic conditions. On the other hand, the UASB reactor configuration is more stable than the CSTR regarding hydrogen production, pH, glucose consumption and microbial by-products (e.g. volatile fatty acids, alcohols etc.) at the HRTs tested. Moreover, the hydrogen production rate in the UASB reactor was significantly higher compared to that of the CSTR at low retention times (19.05 and 8.42 mmole _H2/h/l${\mathrm{H}}_2/\mathrm{h}/\mathrm{l}$, respectively at 2 h HRT) while hydrogen yield (mmole _H2/mmole${\mathrm{H}}_2/\mathrm{mmole}$ glucose consumed) was higher in the CSTR reactor at all HRT tested. This implies that there is a trade-off between technical efficiency (based on hydrogen yield) and economic efficiency (based on hydrogen production rate) when the attached (UASB) and suspended (CSTR) growth configurations are compared.     

Parametric studies on a metal hydride based hydrogen storage device

In this paper, a two-dimensional computational investigation of coupled heat and mass transfer process in an annular cylindrical hydrogen storage device filled with _MmNi4.6Al0.4${\mathrm{MmNi}}_{4.6}{\mathrm{Al}}_{0.4}$ is presented using a commercial software FLUENT 6.1.22. Hydrogen storage performance of the device is studied by varying the operating parameters such as hydrogen supply pressure and absorption temperature. Further, the effects of various bed parameters such as hydride bed thickness and overall heat transfer coefficient on the storage performance of the device are also studied. The average temperature of the hydriding bed and hydrogen storage capacity at different supply pressures showed good agreement with the experimental data reported in the literature. It is observed that as the hydriding process is initiated, the absorption of hydrogen increases rapidly and then it slows down after the temperature of the hydride bed increases due to the heat of the reaction. At any given absorption temperature, the hydrogen absorption rate and hydrogen storage capacity are found to increase with the supply pressure. The variation in the hydrogen absorption capacity, rate of reaction and temperature profiles at different supply pressures from 5 to 35 bar in steps of 5 bar are presented. Further, the effects of overall heat transfer coefficients from 750 to 1250 W/m² K and cooling fluid temperatures from 288 to 298 K on hydrogen storage capacity are also investigated. It is shown that the heat transfer rate enhances the hydriding rate by accomplishing a rapid reaction. At the supply condition of 35 bar and 298 K, _MmNi4.6Al0.4${\mathrm{MmNi}}_{4.6}{\mathrm{Al}}_{0.4}$ stores about 13.1 g of hydrogen per kg of alloy.     

Optimizing hydrogen production from organic wastewater treatment in batch reactors through experimental and kinetic analysis

Anaerobic hydrogen production from organic wastewater, an emerging biotechnology to generate clean energy resources from wastewater treatment, is critical for environmental and energy sustainability. In this study, hydrogen production, biomass growth and organic substrate degradation were comprehensively examined at different levels of two critical parameters (chemical oxygen demand (COD) and pH). Hydrogen yields had a reverse correlation with COD concentrations. The highest specific hydrogen yield (SHY) of 2.1 mole H₂/mole glucose was achieved at the lowest COD of 1 g/L and decreased to 0.7 mole H₂/mole glucose at the highest COD of 20 g/L. The pH of 5.5–6.0 was optimal for hydrogen production with the SHY of 1.6 mole H₂/mole glucose, whereas the acidic pH (4.5) and neutral pH (6.0–7.0) lowered the hydrogen yields. Under all operational conditions, acetate and butyrate were the main components in the liquid fermentation products. Additionally, a comprehensive kinetic analysis of biomass growth, substrate degradation and hydrogen production was performed. The maximum rates of microbial growth (μ_m) and substrate utilization (R_su) were 0.03 g biomass/g biomass/day and 0.25 g glucose/g biomass/day, respectively. The optimum pH for the rate of hydrogen production (RH₂$R_{{\text{H}}_2}$) and SHY were 5.89 and 5.74 respectively. Based on the kinetic analysis, the highest RH₂$R_{{\text{H}}_2}$ and SHY for batch-mode anaerobic hydrogen production systems were projected to be 13.7 mL/h and 2.32 mole H₂/mole glucose.     

Phototrophic hydrogen production in photobioreactors coupled with solar-energy-excited optical fibers

A novel solar-energy-excited optical fiber (SEEOF) photobioreactor (PBR) was developed to enhance the phototrophic H₂ production by Rhodopseudomonas palustris WP3-5 using acetate (HAc) as the sole carbon source. The PBR was illuminated by combinative light sources, including an internal illumination with optical fiber excited by solar energy (OF(sunlight)) as well as external irradiation of tungsten filament lamp (TL). The photo-H₂ producing performance of the SEEOF photobioreactor was further improved by using an innovative light dependent resistor (LDR) system, which could maintain sufficient and continual light supply. The results show that combination of OF(sunlight)/TL was more effective than the TL/TL illumination system, leading to a 138% and 136% increase in cumulative H₂ production (VH₂)$\left(V_{{\text{H}}_2}\right)$ and H₂ yield (YH₂)$\left(Y_{{\text{H}}_2}\right)$, respectively. The LDR-coupled SEEOF photobioreactor was able to solve the problems of diurnal variation in solar light intensity, enabling the control of a constant total light irradiation intensity on the PBR surface. Combining OF(sunlight)/TL with LDR, the VH₂$V_{{\text{H}}_2}$ and YH₂$Y_{{\text{H}}_2}$ were nearly 27% higher than without LDR. For bioreactor scale up from 50 to 1800 ml working volume, the LDR-coupled SEEOF photobioreactor worked well during daytime, leading to a marked improvement in phototrophic H₂ production with a VH₂$V_{{\text{H}}_2}$ and YH₂$Y_{{\text{H}}_2}$ of 3606 ml and 2.45 mol H₂/mol HAc, respectively. Moreover, continuous cultures operated at a hydraulic retention time (HRT) of 48 h show a high hydrogen production rate of 32.4 ml/l/h with stable operation for over 15 days. This optimal performance of LDR-coupled SEEOF photobioreactor is superior to most reported results and is a favorable choice of electricity-saving PBR strategy to improve photo-H₂ production efficiency.     

Reducing the idle speed of a spark-ignited gasoline engine with hydrogen addition

Reducing idle speed is an effective way for decreasing engine idle fuel consumption. Unfortunately, due to the increased residual dilution and dropped combustion temperature, spark-ignited (SI) gasoline engines are prone to suffer high cyclic variation and even stall at low idle speeds. This paper investigated the effect of hydrogen addition on the performance of an SI gasoline engine at reduced idle speeds of 600, 700 and 800 rpm. The test results shows that cyclic variation was raised with the decrease of idle speed but reduced obviously with the increase of hydrogen energy fraction (βH₂)$\left({\text{β}}_{{\text{H}}_2}\right)$. Decreasing idle speed and adding hydrogen were effective for reducing engine idle fuel consumption. The total fuel energy flow rate was effectively dropped from 30.8 MJ/h at 800 rpm and βH₂${\text{β}}_{{\text{H}}_2}$ = 0% to 17.6 MJ/h at 600 rpm and βH₂${\text{β}}_{{\text{H}}_2}$ = 19.9%. Because of the dropped fuel energy flow rate causing the reduced combustion temperature, both cooling and exhaust losses were markedly reduced after decreasing idle speed and adding hydrogen. HC and CO emissions were dropped with the increase of βH₂${\text{β}}_{{\text{H}}_2}$, but increased after reducing idle speed. However, NOx emissions were decreased after reducing idle speed and adding hydrogen, due to the dropped peak cylinder temperature.     

Synthesis and hydrogenation properties of Mg–La–Ni–H system by reactive mechanical alloying

We have synthesized Mg–30 mass%LaNi_2.28 composite material and investigated its hydrogenation behaviour. The reactive mechanical alloying process of the mixture of Mg and LaNi_2.28 was studied. It is found that a composite of _MgH2${\mathrm{MgH}}_2$, _La4H12.9${\mathrm{La}}_4{\mathrm{H}}_{12.9}$ and _Mg2NiH4${\mathrm{Mg}}_2{\mathrm{NiH}}_4$ formed after 80 h ball-milling under 3.0 MPa hydrogen. Scanning electron microscopic analysis indicated that these new phases are distributed homogeneously. This composite shows excellent hydriding properties even at moderate temperature. Under 3.0 MPa hydrogen pressure it absorbed more than 80% of its full capacity in the temperature range of 473–553 K within less than 1 min. The maximum hydrogen absorption capacity at 553 K is 5.4 mass%. The enhanced hydriding properties could be attributed to the fine and uniform particles and a synergeticly catalytic effect generated by mechanical milling.     

Demonstration of sustained hydrogen photoproduction by immobilized,sulfur-deprived Chlamydomonas reinhardtii cells

It was demonstrated that immobilized, sulfur-deprived algal cultures can photoproduce H₂${}_2$. After identifying the optimal material and procedures for immobilization of Chlamyodomonas reinhardtii   at high cell density, we examined the effect of liquid mixing, sulfate content, acetate levels and light intensity on the H₂${}_2$-production activity of the culture. Our results indicate that (a) liquid mixing is important to provide homogeneous conditions for the immobilized culture; (b) sulfur deprivation is necessary for hydrogen production by immobilized cultures; and (c) high light intensity decreases H₂${}_2$ production. The maximum total volume of H₂${}_2$ produced by the system (160 ml of reactor volume) was 380 ml over 23 days, and the highest rate of H₂${}_2$ production observed was 45 ml day^-1${}^{-1}$. Cell immobilization significantly increased the duration of the H₂${}_2$-photoproduction phase (up to 4 weeks), maintained specific rates of H₂${}_2$ photoproduction similar to those of suspension cultures and showed potential for large increases in H₂${}_2$ production.     

Hydrogen storage in nickel catalysts supported on activated carbon

We have studied hydrogen storage in a commercial activated carbon impregnated with nickel. High-pressure (20–30 bars) hydrogen uptake at room temperature was assessed using a high-pressure volumetric adsorption–desorption system. The properties of the prepared materials were studied by means of _N2${\mathrm{N}}_2$ physisorption, X-ray diffraction, transmission electron microscopy, metal surface area, hydrogen temperature programmed reduction and hydrogen temperature programmed desorption. Various factors influencing the level of hydrogen uptake (metal precursor, metal content, method of preparation) were examined and discussed. It is concluded that the hydrogen stored is loosely chemisorbed on the carbonaceous material surface as spilt-over species through _H2${\mathrm{H}}_2$ dissociation on the metal phase then migration onto the support. This hydrogen would also be directly adsorbed on carbon acceptor sites induced by _H2${\mathrm{H}}_2$-pretreatment at 623 K. In both cases, the stored hydrogen directly desorbs from the active carbon support.     

Kinetic characteristics of sodium borohydride formation when sodium meta-borate reacts with magnesium and hydrogen

Sodium borohydride is attracting considerable interests as a hydrogen storage medium. In this paper, we investigated the effects of hydrogen pressure, reaction temperature and transition metal addition on sodium borohydride synthesis by the reaction of sodium meta-borate with Mg and _H2${\mathrm{H}}_2$. It was found that higher _H2${\mathrm{H}}_2$ pressure was beneficial to _NaBH4${\mathrm{NaBH}}_4$ formation. The increase in reaction temperature first improved _NaBH4${\mathrm{NaBH}}_4$ formation kinetics but then impeded it when the temperature was raised to near the melting point of Mg. It was also found that some additions of transition metals such as Ni, Fe and Co in the _NaBO2+Mg+_H2${\mathrm{NaBO}}_2+\mathrm{Mg}+{\mathrm{H}}_2$ system promoted the _NaBH4${\mathrm{NaBH}}_4$ formation, but Cu addition showed little effect. The activation energy of the _NaBH4${\mathrm{NaBH}}_4$ formation from Mg, _NaBO2${\mathrm{NaBO}}_2$ and _H2${\mathrm{H}}_2$ was estimated to be 156.3 kJ/mol _NaBH4${\mathrm{NaBH}}_4$ according to Ozawa analysis method.