Noncatalytic hydrothermolysis of ammonia borane

Hydrolysis of ammonia borane (AB) is attractive as a chemical method for hydrogen storage. The use of catalysts is, however, usually required. In the present paper, two new methods for releasing hydrogen from AB and water are investigated which do not involve any catalyst. One method is based on combustion of AB mixtures with nanoscale aluminum powder and gelled water. It is shown experimentally that these mixtures, upon ignition, exhibit self-sustained combustion with hydrogen release from both AB and water. The other method involves external heating of aqueous AB solutions to temperatures 120 °C or higher, under argon pressure to avoid water boiling. To clarify the reaction mechanism, isotopic experiments using D₂O instead of H₂O were conducted. It is shown that heating AB/D₂O solution to temperatures 117–170 °C releases 3 equiv. of hydrogen per mole AB, where 2–2.1 equiv. originate from AB and 0.9–1 equiv. from water. The prospects of both methods for hydrogen storage are discussed.     

Hydrogen storage properties of Mg–30 wt.% LaNi5 composite prepared by hydriding combustion synthesis followed by mechanical milling (HCS + MM)

A Mg–30 wt.% LaNi₅ composite was prepared by hydriding combustion synthesis followed by mechanical milling (HCS + MM), and the hydriding and dehydriding properties of the HCS + MM product were compared with those of the HCS product and the MM product. The dehydriding temperature onsets of the MM and HCS + MM products were both 470 K, which were lower than that of the HCS product by 100 K. Moreover, the HCS + MM product desorbed faster than the MM product, e.g., the former desorbed completely upon heating to 510 K, whereas the latter did not decompose completely until 590 K. Additionally, the HCS + MM product reached a saturated hydrogen absorption capacity of 3.80 wt.% at 373 K in 50 s, but both the HCS product and the MM product absorbed less than 1.50 wt.% of hydrogen at 373 K in 1800 s. These results suggest the potential of the HCS + MM processing in preparing Mg-based hydrogen storage materials.     

Hydrogen storage properties of Mg–Ni–Cu prepared by hydriding combustion synthesis and mechanical milling (HCS + MM)

The effect of Cu-doping on the hydrogen storage properties of Mg₉₅Ni₅Cu_x (x = 0, 0.5, 1, 2) prepared by hydriding combustion synthesis and mechanical milling (HCS + MM) was studied. For dehydriding properties, the dehydriding temperature onset decreases from 450 K for Mg₉₅Ni₅ to 420 K for Mg₉₅Ni₅Cu₂. Additionally, the activation energy for dehydriding decreases from 116 kJ/mol for Mg₉₅Ni₅ to 98 kJ/mol for Mg₉₅Ni₅Cu₂, indicating that the dehydriding reaction is activated by the catalytic effect of Cu. Moreover, the hydrogen absorption capacity of Mg₉₅Ni₅Cu₂ at 373 K in 100 s increases from 0.95 to 4.16 wt.% by MM pretreatment before HCS. The factors for the improvement of the hydrogen storage properties are discussed in this paper.     

Catalytic combustion of alcohols for microburner applications

The combustion of energy dense liquid fuels in a catalytic micro-combustor, whose temperatures can be used in energy conversion devices, is an attractive alternative to cumbersome batteries. To miniaturize the reactor, an evaporation model was developed to calculate the minimum distance required for complete droplet vaporization. By increasing the ambient temperature from 298 to 350 K, the distance required for complete evaporation of a 6.5 μm droplet decreases from 3.5 to 0.15 cm. A platinum mesh acted as a preliminary measurement and demonstrated 75% conversion of ethanol. We then selected a more active rhodium-coated alumina foam with a larger surface area and attained 100% conversion of ethanol and 95% conversion of 1-butanol under fuel lean conditions. Effluent post-combustion gas analysis showed that varying the equivalence ratio results in three possible modes of operation. A regime of high carbon selectivity for CO₂ occurs at low equivalence ratios and corresponds to complete combustion with a typical temperature of 775 K that is ideal for PbTe thermoelectric energy conversion devices. Conversely for equivalence ratios greater than 1, carbon selectivity for CO₂ decreases as hydrogen, olefin and paraffin production increases. By tuning the equivalence ratio, we have shown that a single device can combust completely for thermoelectric applications, operate as a fuel reformer to produce hydrogen gas for fuel cells or perform as a bio-refinery for paraffin and olefin synthesis.     

Method to release hydrogen from ammonia borane for portable fuel cell applications

Ammonia borane (AB, NH₃BH₃) is considered to be a promising hydrogen storage material as it contains 19.6 wt% hydrogen. It is difficult, however, to release hydrogen from AB. Thermolysis, catalytic hydrolysis and heat generated by additional reactive mixtures are usually employed, but these methods have disadvantages that limit their use for portable applications. In this paper, we demonstrate a new approach to release hydrogen, which does not require any catalyst and produces relatively high hydrogen yield and environmentally benign byproducts. It involves nano-aluminum (nAl)/water combustion reaction, which provides heat for AB dehydrogenation and releases additional hydrogen from water. To facilitate higher H₂ yield from thermolysis, as compared to hydrolysis, AB is spatially separated from the nAl/water mixture using a concentric cylindrical container. The effect of the container design on hydrogen generation is studied and optimized. This study also includes transient temperature and pressure measurements, and product characterization using mass spectrometer and ¹¹B NMR. This approach provides H₂ yield up to 9.5 wt% on material basis. Our experimental results and analysis show that a proposed power source based on this method is promising for portable electronic devices.     

Hydrogen production in ultrarich combustion of hydrocarbon fuels in porous media

Rich and ultrarich combustion of methane, ethane, and propane inside inert porous media is studied experimentally and numerically to examine the suitability of the concept for hydrogen production. Temperature, velocities, and chemical products of the combustion waves were recorded experimentally at a range of equivalence ratios from stoichiometry (φ = 1.0) to φ = 2.5, for a filtration velocity of 12 cm/s. Two-temperature numerical model based on comprehensive heat transfer and chemical mechanisms is found to be in a good qualitative agreement with experimental data. Partial oxidation products of methane, ethane, and propane (H₂, CO, and C₂ hydrocarbons) are dominant for ultrarich superadiabatic combustion. The maximum hydrogen yield is close to 50% for all fuels, and carbon monoxide yield is close to 80%.     

Ni/CeO2/ZSM-5 catalysts for the production of hydrogen from the pyrolysis–gasification of polypropylene

The production of hydrogen from the two-stage pyrolysis–gasification of polypropylene using a Ni/CeO₂/ZSM-5 catalyst has been investigated. Experiments were conducted on CeO₂ loading, calcination temperature and Ni loading of the Ni/CeO₂/ZSM-5 catalyst in relation to hydrogen production. The results indicated that with increasing CeO₂ loading from 5 to 30 wt.% for the 10 wt.% Ni/CeO₂/ZSM-5 catalyst calcined at 750 °C, hydrogen concentration in the gas product and the theoretical potential hydrogen production were decreased from 63.0 to 49.8 vol.% and 50.4 to 21.6 wt.%, respectively. In addition, the amount of coke deposited on the catalyst was reduced from 9.5 to 6.2 wt.%. The calcination temperature had little influence on hydrogen production for the catalyst containing 5 wt.% of CeO₂. However, for the 10 wt.% Ni/CeO₂/ZSM-5 catalyst with a CeO₂ content of 10 or 30 wt.%, the catalytic activities reduced when the calcination temperature was increased from 500 to 750 °C. The SEM results showed that large amounts of filamentous carbons were formed on the surface of the catalysts. The investigation of different Ni content indicates that the Ni/CeO₂/ZSM-5 ((2-10)-5-500) catalyst containing 2 wt.% Ni showed poor catalytic activity in relation to the pyrolysis–gasification of polypropylene according to the theoretical potential H₂ production (7.2 wt.%). Increasing the Ni loading to 5 or 10 wt.% in the Ni/CeO₂/ZSM-5 ((2-10)-5-500) catalyst, high potential hydrogen production was obtained.     

Effects of hydrogen preconversion on the homogeneous ignition of fuel-lean H2/O2/N2/CO2 mixtures over platinum at moderate pressures

The impact of fractional hydrogen preconversion on the subsequent homogeneous ignition characteristics of fuel-lean (equivalence ratio φ = 0.30) H₂/O₂/N₂/CO₂ mixtures over platinum was investigated experimentally and numerically at pressures of 1, 5 and 8 bar. Experiments were performed in an optically accessible channel-flow reactor and involved Raman measurements of major species over the catalyst boundary layer and planar laser induced fluorescence (LIF) of the OH radical. Simulations were carried out with a 2-D elliptic code that included detailed hetero-/homogeneous chemistry. The predictions reproduced the LIF-measured onset of homogeneous ignition and the Raman-measured transport-limited catalytic hydrogen consumption. For 0% preconversion and wall temperatures in the range 900 K ? T_w ? 1100 K, homogeneous ignition was largely suppressed for p ? 5 bar due to the combined effects of intrinsic gas-phase hydrogen kinetics and the competition between the catalytic and gas-phase pathways for fuel consumption. A moderate increase of preconversion to 30% restored homogeneous combustion for p ? 5 bar, despite the fact that the water formed due to the upstream preconversion inhibited homogeneous ignition. The catalytically-produced water inhibited gas-phase combustion, particularly at higher pressures, and this kinetic inhibition was exacerbated by the diffusional imbalance of hydrogen that led to over-stoichiometric amounts of water in the near-wall hot ignitable regions. Radical adsorption/desorption reactions hindered the onset of homogeneous ignition and this effect was more pronounced at 1 bar. On the other hand, over the post-ignition reactor length, radical adsorption/desorption reactions significantly suppressed gas-phase combustion at 5 and 8 bar while their impact at 1 bar was weaker. By increasing hydrogen preconversion, the attained superadiabatic surface temperatures could be effectively suppressed. An inverse catalytically stabilized thermal combustion (CST) concept has been proposed, with gas-phase ignition achieved in an upstream porous burner via radiative and heat conduction feedback from a follow-up catalytic reactor. This arrangement moderated the superadiabatic surface temperatures and required modest or no preheat of the incoming mixture.     

Enhanced hydrogen adsorption in ordered mesoporous carbon through clathrate formation

Ordered mesoporous carbons were synthesized with a soft-template approach and modified with a water and tetrahydrofuran mixture having a H₂O/THF molar ratio of 17:1 as potential adsorbent media for hydrogen storage. Hydrogen adsorption equilibrium on the carbon adsorbents was measured gravimetrically at 270 K and hydrogen pressures up to 163 bar. Enhanced hydrogen adsorption was observed on the carbon adsorbents doped with 0.5 wt.% and 0.75 wt.% of H₂O/THF due to the combined effects of hydrogen adsorption on the carbon surface and formation of a binary H₂–H₂O–THF clathrate. Hydrogen adsorption capacities on the carbon adsorbents doped with 0.5 wt.%, 0.75 wt.% of H₂O/THF, and the pure carbon at 270 K and 163 bar are 0.747 wt.%, 0.646 wt.% and 0.585 wt.%, respectively. The hydrogen adsorption isotherms on all the doped carbon adsorbents are of typical Type III and can be well correlated by the Freundlich equation. A desorption hysteresis loop was observed on the carbon adsorbents doped with 0.5 wt.% and 0.75 wt.% of H₂O/THF, which was probably caused by the pore size difference during the adsorption and desorption steps.     

Experimental study of a domestic boiler using hydrogen methane blend and fuel-rich staged combustion

Storing excess wind and solar energy in the form of hydrogen injected into the natural gas grid is one of the main ingredients of the energy transition. This hydrogen injection has an impact on emissions and the performance of user equipment. The present work reports on an experimental study of the combustion of methane-hydrogen mixture with fuel rich transverse staggered injection. The 15 kW domestic boiler used was equipped with 16 burners (2 × 8). The aim of this work is to better understand the simultaneous effects of fuel rich staged combustion (Ø = 2.0 ÷ 4.0), hydrogen blending of methane (0 ÷ 45%) on pollutant emissions and efficiency while maintaining a compromise between high power, fuel economy and low emissions. The results show that NO_x, CO₂ emissions decrease, CO and C_xH_y emission values increase and thermal efficiency values decrease with increasing hydrogen percentage and fuel rich staggering combustion.     

Improved hydrogen storage properties of Mg-V nanoparticles prepared by hydrogen plasma-metal reaction

The Mg-10.2 at.% V nanoparticles are prepared by hydrogen plasma-metal reaction (HPMR) method. These nanoparticles are made of Mg, VH₂ and a small amount of MgH₂. The Mg nanoparticles are hexagonal in shape with the particle size in the range of 50-150 nm. The VH₂ nanoparticles are spherical in shape with the particle size around 10 nm, and disperse on the surface of the Mg nanoparticles. After the hydrogen absorption, the mean particle size of MgH₂ decreases to 60 nm, while the V nanoparticles are still about 10 nm. The Mg-V composite nanoparticles can absorb 3.8 wt.% hydrogen in less than 30 min at 473 K and accomplish a high hydrogen storage capacity of 5.0 wt.% in less than 5 min at 623 K. They can release 4.0 wt.% hydrogen in less than 15 min at 573 K. The catalytic effect of the V nanoparticles and the nanostructure and the low oxide content of the Mg particles promote the hydrogen sorption process with the low hydrogen absorption activation energy of 71.2 kJ mol⁻¹.     

Hydrogen and electricity co-production plant integrating steam-iron process and chemical looping combustion

In this paper, steam-iron process (Fe looping) and NiO-based chemical looping combustion (Ni looping) are integrated for hydrogen production with inherent separation of CO₂. An integrated combined cycle based on the Fe and Ni loopings is proposed and modeled using Aspen Plus software. The simulation results show that at Fe-SR 815 °C, Fe-FR 815 °C, Ni-FR 900 °C and Ni-AR 1050 °C without supplementary firing, the co-production plant has a net power efficiency 14.12%, hydrogen efficiency 33.61% and an equivalent efficiency 57.95% without CO₂ emission. At a supplementary firing temperature of 1350 °C, the net power efficiency, hydrogen efficiency and the equivalent efficiency are 27.47%, 23.39% and 70.75%, respectively; the CO₂ emission is 365.36 g/kWh. The plant is attractive because of high-energy conversion efficiency and relatively low CO₂ emission; moreover, the hydrogen/electricity ratio can be varied in response to demand. The influences of iron oxide recycle rate, supplementary firing temperature, inert support addition and other parameters on the system performance are also investigated in the sensitive analyses.     

Flameless combustion for hydrogen containing fuels

In this paper, flameless combustion was promoted to suppress thermal-NO_x formation in the hydrogen-high-containing fuel combustion. The PSRN model was used to model the flameless combustion in the air for four fuels: H₂/CH₄ 60/40% (by volume), H₂/CH₄ 40/60%, H₂/CH₄ 20/80% and pure hydrogen. The results show that the NO_x emissions below 30 ppmv while CO emissions are under 50 ppmv, which are coincident with the experimental data in the “clean flameless combustion” regime for all the four fuels. The simulation also reveals that CO decreases from 48 ppmv to nearly zero when the hydrogen composition varies from 40% to 100%, but the NO_x emission is not sensitive to the hydrogen composition. In the highly diluted case, the NO_x and CO emissions do not depend on the entrainment ratio.     

Hybrid filtration combustion of natural gas and coal

Rich and ultrarich combustion of natural gas in a porous medium composed of aleatory coal particles and alumina spheres was studied experimentally to evaluate the suitability of the concept for hydrogen and syngas production. Temperature, velocity and chemical products of the combustion waves were recorded experimentally in two stages: (1) natural gas in an inert porous medium at filtration velocities of 12, 15 and 19 cm/s for equivalence ratios (φ) from φ = 1.0 to φ = 3.8; (2) natural gas in a porous medium composed of coal and alumina particles for a range of volume coal fractions from 0 to 75% at φ = 2.3, and a filtration velocity of 15 cm/s. It was observed that the flame temperatures and hydrogen yields were increased with the increase of filtration velocity in inert porous media. In hybrid porous media the flame temperature decreased with an increase of coal fraction, and hydrogen and carbon monoxide were dominant partial oxidation products. Syngas yield in hybrid filtration combustion was found to be essentially higher than for the inert porous medium case. The maximum hydrogen conversion for the hybrid coal and alumina bed was ∼55% for a volumetric coal content of 75%.     

Chemical kinetics of Ru-catalyzed ammonia borane hydrolysis

Ammonia borane (AB) is a candidate material for on-board hydrogen storage, and hydrolysis is one of the potential processes by which the hydrogen may be released. This paper presents hydrogen generation measurements from the hydrolysis of dilute AB aqueous solutions catalyzed by ruthenium supported on carbon. Reaction kinetics necessary for the design of hydrolysis reactors were derived from the measurements. The hydrolysis had reaction orders greater than zero but less than unity in the temperature range from 16 °C to 55 °C. A Langmuir–Hinshelwood kinetic model was adopted to interpret the data with parameters determined by a non-linear conjugate-gradient minimization algorithm. The ruthenium-catalyzed AB hydrolysis was found to have activation energy of 76 ± 0.1 kJ mol⁻¹ and adsorption energy of −42.3 ± 0.33 kJ mol⁻¹. The observed hydrogen release rates were 843 ml H₂ min⁻¹ (g catalyst)⁻¹ and 8327 ml H₂ min⁻¹ (g catalyst)⁻¹ at 25 °C and 55 °C, respectively. The hydrogen release from AB catalyzed by ruthenium supported on carbon is significantly faster than that catalyzed by cobalt supported on alumina. Finally, the kinetic rate of hydrogen release by AB hydrolysis is much faster than that of hydrogen release by base-stabilized sodium borohydride hydrolysis.     

Carbon doped MO–SDC material as an SOFC anode

Oxide mixtures MO–SDC, M = Cu, Ni, Co, SDC = Ce_0.9Sm_0.1O_1.95 were synthesized by employing a citrate/nitrate combustion technique. Two kinds of Carbon materials, activated carbon (AC) and vapor grown carbon fiber (VGCF) were homogeneously dispersed into the MO–SDC. The materials can be used as anodes to fabricate single cells using a uniaxial die-press method. The sintering temperature was studied to optimize cell performance. Experimental results showed that cells sintered at 700 °C had better performance. When the temperature was above 750 °C, the cells were severely distorted, and cannot be tested. Compared with the basic MO–SDC anode, AC and VGCF improve the solid oxide fuel cell (SOFC) anode properties, due to a change of the microstructures of the anode materials which enhance their electron conductivity. Single cell performances were evaluated by I–V measurements, and when 1.25 wt.%VGCF was introduced into the MO–SDC by ball-milling, termed: 1.25 wt.%VGCF–MO–SDC, the 1.25 wt.%VGCF–MO–SDC anode material could achieve the highest power density of up to 0.326 W cm⁻² with H₂ as fuel. The calcination temperature of the MO–SDC dry gel also strongly influenced the electrochemical performance of the 1.25 wt.%VGCF–MO–SDC material. XRD spectra for each calcined temperature and the I–V measurement both suggest that calcinations at 550 °C for 1 h are suitable. 1.0 wt.%AC–MO–SDC and 1.25 wt.%VGCF–MO–SDC have similar performance when the cell was fed in methanol/3%H₂O, and the corresponding power density was up to 0.253 W cm⁻². Traces of carbon were found in the off-gases.     

Hydriding/dehydriding properties of MgH2/5 wt.% Ni coated CNFs composite

In this paper, we reported that the prepared nickel coated carbon nanofibers (NiCNFs) by electroless plating method exhibited superior catalytic effect on hydrogen absorption/desorption of magnesium (Mg). It is demonstrated that the nanocomposites of MgH₂/5 wt.% NiCNFs prepared by ball milling could absorb hydrogen very fast at low temperatures, e.g. absorb ∼6.0 wt.% hydrogen in 5 min at 473 K and ∼5.0 wt.% hydrogen in 10 min even at a temperature as low as 423 K. More importantly, the desorption of hydrogen was also significantly improved with additives of NiCNFs. Diffraction scanning calorimetry (DSC) measurement indicated that the peak desorption temperature decreased 50 K and the on-set temperature for desorption decreased 123 K. The composites also desorbed hydrogen fast, e.g. desorb 5.5 wt.% hydrogen within 20 min at 573 K. It is suggested that the new phase of Mg₂Ni, and the nano-sized dispersed distribution of Ni and carbon contributed to this significant improvement. Johnson–Mehl–Avrami (JMA) analysis illustrated that hydrogen diffusion is the rate-limiting step for hydrogen absorption/desorption.     

Optimization of hydrogen production by filtration combustion of natural gas by water addition

The effect of adding steam during filtration combustion of natural gas–air mixtures was studied with the aim to evaluate the optimization of hydrogen production. Temperature, velocity, chemical products of combustion waves, and conversion from fuel to H₂ and CO were evaluated in the range of equivalence ratio (φ) from stoichiometric (φ = 1.0) to φ = 3.0 and steam content in the mixture from 0% to 39%, at filtration velocities from 12 to 25 cm/s. Numerical simulation was carried out using GRI-MECH 3.0. Results suggest that H₂ and CO concentrations, dominant for rich and ultrarich combustion, are products from partial oxidation and steam natural gas reforming processes. Experimental and numerical results show that hydrogen yield increase with an increase of steam content in the natural gas–air mixtures.     

Hydrogen release by thermolysis of ammonia borane NH3BH3 and then hydrolysis of its by-product [BNHx]

Ammonia borane (AB) is one of the most attractive hydrides owing to its high hydrogen density (19.5 wt%). Stored hydrogen can be released by thermolysis or catalyzed hydrolysis, both routes having advantages and issues. The present study has envisaged for the first time the combination of thermolysis and hydrolysis, AB being first thermolyzed and then the solid by-product believed to be polyaminoborane [NH₂BH₂]_n (PAB) being hydrolyzed. Herein we report that: (i) the combination is feasible, (ii) PAB hydrolyzes in the presence of a metal catalyst (Ru) at 40 °C, (iii) a total of 3 equiv. H₂ is released from AB and PAB-H₂O, (iv) high hydrogen generation rates can be obtained through hydrolysis, and (v) the by-products stemming from the PAB hydrolysis are ammonium borates. The following reactions may be proposed: AB → PAB + H₂ and PAB + xH₂O → 2H₂ + ammonium borates. All of these aspects as well as the advantages and issues of the combination of AB thermolysis and PAB hydrolysis are discussed.