Experimental study of ignited unsteady hydrogen releases from a high pressure reservoir

In order to simulate an accidental hydrogen release from the high pressure pipe system of a hydrogen facility a systematic study on the nature of transient hydrogen jets into air and their combustion behavior was performed at the KIT hydrogen test site HYKA. Horizontal unsteady hydrogen jets from a reservoir of 0.37 dm³ with initial pressures of up to 200 bar have been investigated. The hydrogen jets released via round nozzles 3, 4, and 10 mm were ignited with different ignition times and positions. The experiments provide new experimental data on pressure loads and heat releases resulting from the deflagration of hydrogen–air clouds formed by unsteady turbulent hydrogen jets released into a free environment. It is shown that the maximum pressure loads occur for ignition in a narrow position and time window. The possible hazard potential arising from an ignited free transient hydrogen jet is described.     

A Calphad-type equation of state for hydrogen gas and its application to the assessment of Rh–H system

A new equation of state for hydrogen gas (0 ≤ P ≤ 10¹¹ Pa, 298 ≤ T ≤ 1000 K) has been derived and is given in an analytical form. It is compatible with the Calphad approach (i.e. it is an analytical equation of the Gibbs energy as a function of pressure). This equation has been applied to the Calphad assessment of the H–Rh system, an example of system forming a metal hydride only at a very high pressure. A set of self-consistent thermodynamic parameters describing the Gibbs energy of each phase has been obtained. The scarce experimental data available for this system, particularly the high formation pressure of the hydride, have been well reproduced which otherwise would have been impossible without considering the non-ideal behaviour of the gas phase.     

Investigation of combustion characteristics and emissions in a spark-ignition engine fuelled with natural gas–hydrogen blends

In this study, an experimental study on the performance and exhaust emissions of a spark-ignition engine fuelled with methane–hydrogen mixtures (100% CH₄, 10% H₂ + 90% CH₄, 20% H₂ + 80% CH₄, and 30% H₂ + 70% CH₄) were performed at different engine speeds and different excessive air ratios. This present work was carried out on a Ford engine. This is a four-stroke cycle four-cylinder spark-ignition engine with a bore of 80.6 mm, a stroke of 88 mm and a compression ratio of 10:1. Experiments were performed at 1500, 2000, 2500 and 3000 rpm and at wide open throttle (WOT). CO, CO₂ and HC emission values and cylinder pressure were measured. The results showed that while the speed and excessive air ratio increase, CO emission values decrease. The reduction of HC and CO emissions could be obtained by adding hydrogen into the natural gas when operating on the lean mixture condition. Increasing the excessive air ratio also decreases the maximum peak cylinder pressure.     

Study on real-gas equations of high pressure hydrogen

High pressure hydrogen storage has become an important transportation channel in areas of new energy development and utilization. Actual hydrogen demonstrations require the exploration of the physical and chemical properties in detail before the practical employment, which is completely different from the ideal gas under high pressure conditions. However, the existing real-gas state equations are not easy to use in calculation of high pressure hydrogen due to its complex behavior, and may lead to an unacceptable error. In this paper, a real-gas state equation of hydrogen in a simplified form is proposed. Compared with the NIST datum, the results obtained from this equation have a maximum error 1.1% and 3.8% respectively within the temperature ranges of 253 K < T< 393 K and 173 K < T< 393 K. Also, the proposed equation exhibits higher precision for the state parameters of hydrogen than existing models. Based on the real-gas state equation of hydrogen, formulas of thermodynamic properties which are necessary for solving the hydraulic and thermal aspects of gas transfer are also proposed.     

Feasibility of biohydrogen production at low temperatures in unbuffered reactors

Feasibility of biohydrogen production by dark fermentation at two temperatures (22 °C and 37 °C) in unbuffered batch reactors was evaluated using heat-treated compost as inocula and sucrose as substrate, without any initial pH adjustment or inorganic nutrient supplements. Gas production was quantified by two different pressure release methods – intermittent pressure release (IPR) and continuous pressure release (CPR). Hydrogen production (47.2 mL/g COD/L) and sucrose-to-hydrogen conversion efficiency (53%) were both found to be highest at the lower temperature and IPR conditions. Hydrogen production was higher at the lower temperature irrespective of the pressure release condition. The high yield of 4.3 mol of hydrogen/mole of sucrose obtained in this study under IPR conditions at 22 °C is equivalent to or better than the literature values reported for buffered reactors. Even though literature reports have implied potential inhibition of hydrogen production at high hydrogen partial pressures resulting from IPR conditions, our results did not show any negative effects at hydrogen partial pressures exceeding 5.0 × 10⁴ Pa. While our findings are contrary to literature reports, they make a strong case for cost-effective hydrogen production by dark fermentation.     

Characteristics of hydrogen production by a plasma–catalyst hybrid converter with energy saving schemes under atmospheric pressure

This paper investigated the production of hydrogen from methane under atmospheric pressure using a plasma–catalyst hybrid converter with emphasis on energy conservation. A spark discharge was used to ionize the hydrocarbon fuel and air mixture with a catalyst to enhance hydrogen production using two energy saving schemes, namely, heat recycling and heat insulation. The experimental results showed that higher methane feeding rate resulted in higher reformate gas temperature and a corresponding increase in methane conversion efficiency. The energy saving systems also enabled the oxygen/carbon ratio to be decreased to reduce oxidation of hydrogen and carbon monoxide and thereby improving the concentrations of hydrogen and carbon monoxide. By heat recycling, a lower methane feeding rate showed an 8.7% improvement in methane conversion efficiency whilst improvement was not apparent with higher methane supply rates due to the already high conversion efficiency. Moreover, it was shown that hydrogen production increased significantly with the reaction from water–gas shifting under the same operation parameters but with high methane selectivity. The best combination resulting in a total thermal efficiency of 77.11% was 10 L/min methane feeding rate and 0.8 O₂/C ratio. With water–gas shifting (S/C ratio=0.5), an 86.26% hydrogen yield, equating to 17.25 L/min hydrogen production rate could be achieved. The equilibrium production rate was calculated using the commercialized HSC Chemistry software (^©ChemSW Software, Inc.). Good correlation was obtained between the calculations and the experimental results.     

Experimental study of ignited unsteady hydrogen jets into air

In order to simulate an accidental hydrogen release from the low pressure pipe system of a hydrogen vehicle a systematic study on the nature of transient hydrogen jets into air and their combustion behaviour was performed at the FZK hydrogen test site HYKA. Horizontal unsteady hydrogen jets with an amount of hydrogen up to 60 STP dm³ and initial pressures of 5 and 16 bar have been investigated. The hydrogen jets were ignited with different ignition times and positions. The experiments provide new experimental data on pressure loads and heat releases resulting from the deflagration of hydrogen-air clouds formed by unsteady turbulent hydrogen jets released into a free environment. It is shown that the maximum pressure loads occur for ignition in a narrow position and time window. The possible hazard potential arising from an ignited free transient hydrogen jet is described.     

Equilibrium,kinetics and enthalpy of hydrogen adsorption in MOF-177

Metal–organic framework (MOF-177) was synthesized, characterized and evaluated for hydrogen adsorption as a potential adsorbent for hydrogen storage. The hydrogen adsorption equilibrium and kinetic data were measured in a volumetric unit at low pressure and in a magnetic suspension balance at hydrogen pressure up to 100 bar. The MOF-177 adsorbent was characterized with nitrogen adsorption for pore textural properties, scanning electron microscopy for morphology and particle size, and X-ray powder diffraction for phase structure. The MOF-177 synthesized in this work was found to have a uniform pore size distribution with median pore size of 12.7 Å, a higher specific surface area (Langmuir: 5994 m²/g; BET: 3275 m²/g), and a higher hydrogen adsorption capacity (11.0 wt.% excess adsorption, 19.67 wt.% absolute adsorption) than previously reported values on MOF-177. Freundlich equation fits well the hydrogen adsorption isotherms at low and high pressures. Diffusivity and isosteric heat of hydrogen adsorption were estimated from the hydrogen adsorption kinetics and equilibrium data measured in this work.     

Gas explosion field test with release of hydrogen from a high pressure reservoir into a channel

Field experiments of a high pressure release of hydrogen gas inside a 6 m long, 0.9 m wide, and 0.8 m high channel have been performed, to validate the Froude scaling and to obtain pressure and flame speed data in an inhomogeneous hydrogen–air cloud. Froude scaling with a length scale corresponding to the height of a 100% hydrogen layer in the channel was used to describe the flow of the hydrogen–air cloud in the channel. The estimated time of ignition based on the Froude scaling for release pressures of 100 bars and 150 bars agreed well with the experiments. At lower release pressures the estimated time was lower, which was most likely caused by dilution of the front of the hydrogen cloud. High speed video was used to record the flame speed. For the present experimental conditions it appeared that the deflagration taking place closer to the jet source determines the maximum explosion pressure.     

Design,fabrication and performance analysis of a 200 W PEM fuel cell short stack

A PEM fuel cell short stack of 200 W capacity, with an active area of 100 cm² has been designed and fabricated in-house. The status of unit cell performance was 0.55 W cm⁻². Based on the unit cell technology, a short stack has been developed. The proper design of uniform flow distribution, cooling plate and compressed end plate were important to achieve the best performance of the short stack. The performance of four cells stack was analyzed in static and dynamic modes. In the static mode of polarization curve, the stack has peak power density of 0.55 W cm⁻² (220 W) at 0.5 V per cell, when the voltage was scanning from low to high voltage (1.5–3.5 V), and resulted in minimum water flooding inside the stack. In this study a series of dynamic loadings were tested to simulate the vehicle acceleration. The fuel cell performances respond to dynamic loading influenced by the hydrogen/air stoichiometric, back pressure, and dynamic-loading time. It was needed high hydrogen stoichiometric and back pressure to maintain high dynamic performance. In the long-time stable power testing, the stack was difficult to maintain at high performance, due to the water flooding at high output power. An adjusting cathode back-pressure method for purging water was proposed to prevent the water flooding at flow channels and maintain the stable output power at 170 W (0.42 W cm⁻²).     

Experimental study of combustion and emission characteristics of ethanol fuelled port injected homogeneous charge compression ignition (HCCI) combustion engine

The homogeneous charge compression ignition (HCCI) is an alternative combustion concept for in reciprocating engines. The HCCI combustion engine offers significant benefits in terms of its high efficiency and ultra low emissions. In this investigation, port injection technique is used for preparing homogeneous charge. The combustion and emission characteristics of a HCCI engine fuelled with ethanol were investigated on a modified two-cylinder, four-stroke engine. The experiment is conducted with varying intake air temperature (120–150 °C) and at different air–fuel ratios, for which stable HCCI combustion is achieved. In-cylinder pressure, heat release analysis and exhaust emission measurements were employed for combustion diagnostics. In this study, effect of intake air temperature on combustion parameters, thermal efficiency, combustion efficiency and emissions in HCCI combustion engine is analyzed and discussed in detail. The experimental results indicate that the air–fuel ratio and intake air temperature have significant effect on the maximum in-cylinder pressure and its position, gas exchange efficiency, thermal efficiency, combustion efficiency, maximum rate of pressure rise and the heat release rate. Results show that for all stable operation points, NO_x emissions are lower than 10 ppm however HC and CO emissions are higher.     

Characteristics of hydrogen-rich gas production of biomass gasification with porous ceramic reforming

In the present study, an updraft biomass gasifier combined with a porous ceramic reformer was used to carry out the gasification reforming experiments for hydrogen-rich gas production. The effects of reactor temperature, equivalence ratio (ER) and gasifying agents on the gas yields were investigated. The results indicated that the ratio of CO/CO₂ presented a clear increasing trend, and hydrogen yield increased from 33.17 to 44.26 g H₂/kg biomass with the reactor temperature increase, The H₂ concentration of production gas in oxygen gasification (oxygen as gasifying agent) was much higher than that in air gasification (air as gasifying agent). The ER values at maximum gas yield were found at ER = 0.22 in air gasification and at 0.05 in oxygen gasification, respectively. The hydrogen yields in air and oxygen gasification varied in the range of 25.05–29.58 and 25.68–51.29 g H₂/kg biomass, respectively. Isothermal standard reduced time plots (RTPs) were employed to determine the best-fit kinetic model of large weight biomass air gasification isothermal thermogravimetric, and the relevant kinetic parameters corresponding to the air gasification were evaluated by isothermal kinetic analysis.     

Effects of hydrogen addition and nitrogen dilution on the laminar flame characteristics of premixed methane–air flames

The effect of hydrogen addition and nitrogen dilution on laminar flame characteristics was investigated. The spherical expanding flame technique, in a constant volume bomb, was employed to extract laminar flame characteristics. The mole fraction of hydrogen in the methane–hydrogen mixture was varied from 0 to 1 and the mole fraction of nitrogen in the total mixture (methane–hydrogen–air–diluent) from 0 to 0.35. Measurements were performed at an initial pressure of 0.1 MPa and an initial temperature of 300 K. The mixtures investigated were under stoichiometric conditions. Based on experimental measurements, a new correlation for calculating the laminar burning velocity of methane–hydrogen–air–nitrogen mixtures is proposed. The laminar burning velocity was found to increase linearly with hydrogen mass fraction for all dilution ratios while the burned gas Markstein length decreases with the increase in hydrogen amount in the mixture except for high hydrogen mole fractions (>0.6). Nitrogen dilution has a nonlinear reducing effect on the laminar burning velocity and an increasing effect on the burned gas Markstein length. The experimental results and the proposed correlation obtained are in good agreement with literature values.     

Permeability,solubility and diffusivity of hydrogen isotopes in stainless steels at high gas pressures

In this report, we provide a framework for describing the permeability, solubility and diffusivity of hydrogen and its isotopes in austenitic stainless steels at temperatures and high gas pressures of engineering interest for hydrogen storage and distribution infrastructure. We demonstrate the importance of using the real gas behavior for modeling permeation and dissolution of hydrogen under these conditions. A simple one-parameter equation of state (the Abel–Noble equation of state) is shown to capture the real gas behavior of hydrogen and its isotopes for pressures less than 200 MPa and temperatures between 223 and 423 K. We use the literature on hydrogen transport in austenitic stainless steels to provide general guidance on and clarification of test procedures, and to provide recommendations for appropriate permeability, diffusivity and solubility relationships for austenitic stainless steels. Hydrogen precharging and concentration measurements for a variety of austenitic stainless steels are described and used to generate more accurate solubility and diffusivity relationships.     

Distributed swirl combustion for gas turbine application

Colorless distributed combustion (CDC) has been shown to provide significant improvement in gas turbine combustor performance. Colorless distributed combustion with swirl is investigated here to develop ultra-low emissions of NO and CO, and significantly improved pattern factor. Experimental investigations have been performed using a cylindrical geometry combustor with swirling air injection and axial hot gas exit stream from the combustor. Air was injected tangentially to impart swirl to the flow inside the combustor. The results obtained from the combustor have demonstrated very low levels of NO (∼3 PPM) and CO (∼70 PPM) emissions at an equivalence ratio of 0.7 and a high heat release intensity of 36 MW/m³-atm under non-premixed combustion. To further simulate gas turbine operating conditions, inlet air to the combustor was preheated to 600 K temperature and the combustor operated at 2 atm pressure. Results showed very low levels of CO (∼10 PPM) but the NO increased somewhat to ∼10 PPM at an equivalence ratio of 0.5 and heat release intensity of 22.5 MW/m³-atm under non-premixed combustion conditions. For premixed combustion, the combustor demonstrated low levels of both NO (5 PPM) and CO (8 PPM) at an equivalence ratio of 0.6 and a heat release intensity of 27 MW/m³-atm. Results are reported at different equivalence ratios on the emission of NO and CO, lean stability limit and OH^* chemiluminescence. These results suggest that further performance improvement can be achieved with improved fuel mixture preparation prior to the ignition of fuel at higher operational pressures using swirling combustor design for our quest to develop ultra low emission high intensity combustor for gas turbine application.     

Development of high intensity CDC combustor for gas turbine engines

Colorless distributed combustion (CDC) has been demonstrated to provide ultra-low emission of NOx and CO, improved pattern factor and reduced combustion noise in high intensity gas turbine combustors. The key feature to achieve CDC is the controlled flow distribution, reduce ignition delay, and high speed injection of air and fuel jets and their controlled mixing to promote distributed reaction zone in the entire combustion volume without any flame stabilizer. Large gas recirculation and high turbulent mixing rates are desirable to achieve distributed reactions thus avoiding hot spot zones in the flame. The high temperature air combustion (HiTAC) technology has been successfully demonstrated in industrial furnaces which inherently possess low heat release intensity. However, gas turbine combustors operate at high heat release intensity and this result in many challenges for combustor design, which include lower residence time, high flow velocity and difficulty to contain the flame within a given volume. The focus here is on colorless distributed combustion for stationary gas turbine applications. In the first part of investigation effect of fuel injection diameter and air injection diameter is investigated in detail to elucidate the effect fuel/air mixing and gas recirculation on characteristics of CDC at relatively lower heat release intensity of 5 MW/m³ atm. Based on favorable conditions at lower heat release intensity the effect of confinement size (reduction in combustor volume at same heat load) is investigated to examine heat release intensity up to 40 MW/m³ atm. Three confinement sizes with same length and different diameters resulting in heat release intensity of 20 MW/m³ atm, 30 MW/m³ atm and 40 MW/m³ atm have been investigated. Both non-premixed and premixed modes were examined for the range of heat release intensities. The heat load for the combustor was 25 kW with methane fuel. The air and fuel injection temperature was at normal 300 K. The combustor was operated at 1 atm pressure. The results were evaluated for flow field, fuel/air mixing and gas recirculation from numerical simulations and global flame images, and emissions of NO, CO from experiments. It was observed that the larger air injection diameter resulted in significantly higher levels of NO and CO whereas increase in fuel injection diameter had minimal effect on the NO and resulted in small increase of CO emissions. Increase in heat release intensity had minimal effect on NO emissions, however it resulted in significantly higher CO emissions. The premixed combustion mode resulted in ultra-low NO levels (<1 ppm) and NO emission as low as 5 ppm was obtained with the non-premixed flame mode.     

Continuous supercritical water gasification of isooctane: A promising reactor design

A new design of supercritical water gasification system was developed to achieve high hydrogen gas yield and good gas–liquid flow stability. The apparatus consisted of a reaction zone, an insulation zone and a cooling zone that were directly connected to the reaction zone. The reactor was set up at an inclination of 75° from vertical position, and feed and water were introduced at the bottom of the reactor. The performances of this new system were investigated with gasification of isooctane at various experimental conditions – reaction temperatures of 601–676 °C, residence times of 6–33 s, isooctane concentrations of 5–33 wt%, and oxidant (hydrogen peroxide) concentrations up to 4507 mmol/L without using catalysts. A significant increase in hydrogen gas yield, almost four times higher than that from the previous up-down gasifier configuration (B. Veriansyah, J. Kim, J.D. Kim, Y.W. Lee, Hydrogen Production by Gasification of Isooctane using Supercritical Water, Int. J. Green Energy. 5 (2008) 322–333) was observed with the present gasifier configuration. High hydrogen gas yield (6.13 mol/mol isooctane) was obtained at high reaction temperature of 637 °C, a low feed concentration of 9.9 wt% and a long residence time of 18 s in the presence of 2701.1 mmol/L hydrogen peroxide. At this condition, the produced gases mainly consisted of hydrogen (59.5 mol%), methane (14.8 mol%) and carbon dioxide (22.0 mol%), and a small amount of carbon monoxide (1.6 mol%) and C₂–C₃ species (2.1 mol%). Reaction mechanisms of supercritical water gasification of isooctane were also presented.     

Evolution of the Wairakei geothermal reservoir during 50 years of production

The Wairakei geothermal field has been under production for more than 50 years. Exploration wells show that the high-temperature and very permeable, productive resource extends over about 12 km² within a greater area of about 25 km² that shows various effects of thermal activity. Up to 2006, 3 km³ of fluid and 2750 PJ of energy had been extracted at an average rate of 5250 t/h and enthalpy of 1130 kJ/kg. Significant production started in 1955 and up to 1978 there was no injection of cooled geothermal fluids. During the first decade of operation a pressure drawdown of up to 20 bars (2 MPa) developed and spread evenly across the reservoir, even though fluid extraction was focused within an area of 1 km² close to the northeastern field boundary. This pressure reduction resulted in widespread boiling and formation of segregated steam zones at the top of the reservoir together with inflow of cooler fluids into its northeastern part via the original natural outflow channels. From 1975 to 1997 pressures in the deep liquid reservoir stabilized at 23–25 bars (2.3–2.5 MPa) below the original pressure, with little change up to the time injection commenced in 1998. This natural pressure support indicates that prior to injection there was substantial recharge, 80% of which is assessed as high-temperature deep inflow. Since 1998 about 30% of the extracted fluids have been injected and reservoir pressures have increased by 3–4 bars (0.3–0.4 MPa). To date, significant returns of injected fluids have not been detected in the production areas. Over the 50 years of operation, temperatures in the main production areas have declined from 250 to 220 °C while deeper production zones toward the western boundary of the reservoir have remained at about 250 °C. A series of deeper makeup wells to maintain future production have been drilled in the high-temperature recharge area. An increasing fraction of injection, both in-field and out-field is planned over the next few years.     

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.     

Update on subsidence at the Wairakei–Tauhara geothermal system,New Zealand

The total subsidence at the Wairakei field as a result of 50 years of geothermal fluid extraction is 15 ± 0.5 m. Subsidence rates in the center of the subsidence bowl have decreased from over 450 mm/year during the 1970s to 80–90 mm/year during 2000–2007. The location of the bowl, adjacent to the original liquid outflow zone of the field, has not changed significantly. Subsidence at the Tauhara field due to Wairakei production was not as well documented in the early years but appeared later and has been less intense than at Wairakei. Total subsidence of 2.6 ± 0.5 m has also occurred close to the original liquid outflow zone of this field, and maximum subsidence rates in this area today are in the 80–100 mm/year range. In the western part of the Wairakei field, near the area of hot upflow, subsidence rates have approximately doubled during the last 20 years to 30–50 mm/year. This increase appears to be have been caused by declining pressure in the underlying steam zone in this area, which is tapped by some production wells. At Tauhara field, two areas of subsidence have developed since the 1990s with rates of 50–65 mm/year. Although less well-determined, this subsidence may also be caused by declining pressure in shallow steam zones. The cause of the main subsidence bowls in the Wairakei–Tauhara geothermal system is locally high-compressibility rocks within the Huka Falls Formation (HFF), which are predominantly lake sediments and an intervening layer of pumice breccia. At Wairakei, casing deformation suggests the greatest compaction is at 150–200 m depth. The cause of the large compressibility is inferred to be higher clay content in the HFF due to intense hydrothermal alteration close to the natural fluid discharge areas. Future subsidence is predicted to add an additional 2–4 m to the Wairakei bowl, and 1–2 m elsewhere, but these estimates depend on the assumed production-injection scenarios.