Cyclic properties and ammonia by-product emission of Li/Mg-N-H hydrogen storage material

A magnesium amide-based hydrogen storage material, 3 Mg(NH₂)₂ + 8LiH, was subjected to cycling tests of dehydrogenation and hydrogenation, in which the cyclic trend in the hydrogen storage capacity as well as the amount of the ammonia by-product contained in the desorbed hydrogen gas were recorded. After 300 cycles at 473 K, the initial hydrogen capacity of 4.2 mass% dropped to 3.6 mass%, corresponding to the decay rate of 0.0004 per cycle. The average ammonia concentration through the 300 cycles was determined to be 0.05 ± 0.01 mol%(NH₃/H₂) which is entirely responsible for the hydrogen capacity decay because the ammonia emission leads to the loss of elemental nitrogen from the system. When the dehydrogenation temperature was raised to 573 K, the hydrogen capacity decay became more significant and the ammonia concentration increased to 0.27 ± 0.06 mol%(NH₃/H₂). The reaction kinetics also severely deteriorated during cycling at the higher temperature.     

Hydrogen generation from noncatalytic hydrolysis of LiBH4/NH3BH3 mixture for fuel cell applications

Ammonia borane (NH₃BH₃) and lithium borohydride (LiBH₄) are promising hydrides as they contain 19.6 wt.% and 18.5 wt.% hydrogen respectively. However, hydrolysis of NH₃BH₃ needs catalysts or high temperature to initiate the release of hydrogen. On the other hand, hydrolysis of LiBH₄ is incomplete, because the agglomeration of LiBH₄ and its products limits its full utilization. In the present work, hydrolysis performance of LiBH₄/NH₃BH₃ mixture was investigated. The results show that LiBH₄/NH₃BH₃ mixture can fully release its theoretical amount of hydrogen at room temperature without catalysts. In the presence of LiBH₄, NH₃BH₃ can be fully hydrolyzed at room temperature. In return, in the presence of NH₃BH₃, the agglomeration can be avoided resulting in a complete hydrolysis process. Our results indicate that the improvements are attributed to the intermolecular electron migration between LiBH₄ and NH₃BH₃, which changes the reactivity of these compounds. Hydrolytic heat of LiBH₄ also contributes to the promoted hydrolysis of NH₃BH₃. Our results present a novel strategy for noncatalytic hydrolysis of NH₃BH₃ and LiBH₄ for proton exchange membrane fuel cell applications.     

Hydrogen generation by hydrolysis of ammonia borane with a nanoporous cobalt-tungsten-boron-phosphorus catalyst supported on Ni foam

In this study, quaternary cobalt-tungsten-boron-phosphorus porous particles supported on Ni foam (Co-W-B-P/Ni), which are prepared through ultrasonification-assisted electroless deposition route, have been investigated as the catalyst for hydrogen generation (HG) from hydrolysis of ammonia borane (NH₃BH₃, AB). Compared with Ni-supported binary Co-B and ternary Co-W-B catalysts, the as-synthesized Co-W-B-P/Ni shows a higher HG rate. To optimize the preparation parameters, the molar ratio of NaBH₄/NaH₂PO₂·H₂O (B/P) and the concentration of Na₂WO₄·2H₂O (W) have been investigated and the catalyst prepared with B/P value of 1.5 and W concentration of 5 g L⁻¹ shows the highest activity. The results of kinetic studies show that the catalytic hydrolysis of AB is first order with respect to the catalyst and AB concentrations. By using the quaternary catalyst with a concentration of 0.5 wt % AB, a HG rate of 4.0 L min⁻¹ g⁻¹ is achieved at 30 °C. Moreover, the apparent activation energy for the quaternary catalyst is determined to be 29.0 kJ mol⁻¹, which is comparable to that of noble metal-based catalysts. These results indicate that the Co-W-B-P/Ni is a promising low-cost catalyst for on-board hydrogen generation from hydrolysis of borohydride.     

A micro reforming system integrated with a heat-recirculating micro-combustor to produce hydrogen from ammonia

In order to evaluate the potential of reforming ammonia as a carbon-free fuel in production of hydrogen, a new configuration of a micro reforming system integrated with a micro-combustor is studied experimentally. The micro-combustor as a heat source is a simple cylinder with an annular-type shield that applies a heat-recirculation concept. A micro-reformer to convert ammonia to hydrogen is an annulus, which is effective to transfer heat from the micro-combustor. The annulus-type micro reforming system is designed to produce 1-10 W (based on lower heating value, LHV) of hydrogen using various catalysts. The feed rate of ammonia, the micro-combustor inlet velocity of fuel-air mixtures and the catalyst materials substantially affect the performance of the designed micro reforming system. Under optimized design and operating conditions, the micro reforming system using ruthenium as a catalyst produces 5.4 W (based on LHV) of hydrogen with a conversion rate of 98.0% and an overall system efficiency of 13.7%. Thus, the present configuration can be applied to practical micro reforming systems, supporting the potential of using ammonia as a clean fuel.     

Ammonia chemistry below 1400 K under fuel-rich conditions in a flow reactor

The oxidation of NH₃ under fuel-rich conditions and moderate temperatures has been studied in terms of a chemical kinetic model over a wide range of conditions, based on the measurements of Hasegawa and Sato. Their experiments covered the fuels hydrogen (0 to 80 vol%), carbon monoxide (0 to 95 vol%), and methane (0 to 1.5 vol%), stoichiometries ranging from slightly lean to very fuel rich, temperatures from 300 to 1330 K, and NO levels from 0 to 2500 ppm. A detailed reaction mechanism has been established, based on earlier work on ammonia oxidation in flames and on selective noncatalytic reduction of NO by NH₃. The kinetic model reproduces the experimental trends qualitatively over the full range of conditions covered, and often the predictions are in quantitative agreement with the observations. Using reaction path analysis and sensitivity studies, the major reaction paths have been identified. The comparatively low temperatures in the present study, as well as the presence of NO, promote the reaction path NH₃→NH₂→N₂ (directly or via NNH), rather than the sequence NH₃→NH₂→NH→N important in flames. The major conversion of fuel-N species to N₂ occurs by reaction of amine radicals with NO, in particular NH₂+NO. In the presence of CH₄, NO is partly converted to cyanides by reaction with CH₃. The mechanism is recommended for modeling the reduction of NO by primary measures in the combustion of biomass, since it has been validated under conditions resembling the conversion of early nitrogenous volatile species in a staged combustion process. It is also appropriate for studies of NO formation in the combustion of gas from gasifying coal.     

Ammonia as hydrogen carrier for transportation; investigation of the ammonia exhaust gas fuel reforming

In this paper we show, for the first time, the feasibility of ammonia exhaust gas reforming as a strategy for hydrogen production used in transportation. The application of the reforming process and the impact of the product on diesel combustion and emissions were evaluated. The research was started with an initial study of ammonia autothermal reforming (NH₃ – ATR) that combined selective oxidation of ammonia (into nitrogen and water) and ammonia thermal decomposition over a ruthenium catalyst using air as the oxygen source. The air was later replaced by real diesel engine exhaust gas to provide the oxygen needed for the exothermic reactions to raise the temperature and promote the NH₃ decomposition. The main parameters varied in the reforming experiments are O₂/NH₃ ratios, NH₃ concentration in feed gas and gas – hourly – space – velocity (GHSV). The O₂/NH₃ ratio and NH₃ concentration were the key factors that dominated both the hydrogen production and the reforming process efficiencies: by applying an O₂/NH₃ ratio ranged from 0.04 to 0.175, 2.5–3.2 l/min of gaseous H₂ production was achieved using a fixed NH₃ feed flow of 3 l/min. The reforming reactor products at different concentrations (H₂ and unconverted NH₃) were then added into a diesel engine intake. The addition of considerably small amount of carbon – free reformate, i.e. represented by 5% of primary diesel replacement, reduced quite effectively the engine carbon emissions including CO₂, CO and total hydrocarbons.     

Effect of boric acid on thermal dehydrogenation of ammonia borane: Mechanistic studies

Ammonia borane (AB, NH₃BH₃) is a promising hydrogen storage material for use in proton exchange membrane (PEM) fuel cell applications. In this study, the effect of boric acid on AB dehydrogenation was investigated. Our study shows that boric acid is a promising additive to decrease onset temperature as well as to enhance hydrogen release kinetics for AB thermolysis. With heating, boric acid forms tetrahydroxyborate ion along with some water released from boric acid itself. It is believed that this ion serves as Lewis acid which catalyzes AB dehydrogenation. Using boric acid, we obtained high H₂ yield (11.5 wt% overall H₂ yield, 2.23 H₂ equivalent) at 85 °C, PEM fuel cell operating temperatures, along with rapid kinetics. In addition, only trace amount of NH₃ (20–30 ppm) was detected in the gaseous product. The spent AB solid product was found to be polyborazylene-like species. The results suggest that the addition of boric acid to AB is promising for hydrogen storage, and could be used in PEM fuel cell based vehicles.     

Combustion stability limits and NOx emissions of nonpremixed ammonia-substituted hydrogen–air flames

The combustion stability (extinction) limits and nitrogen oxide (NO_x) emissions of nonpremixed ammonia (NH₃)–hydrogen (H₂)–air flames at normal temperature and pressure are studied to evaluate the potential of partial NH₃ substitution for improving the safety of H₂ use and to provide a database for the nonpremixed NH₃-substituted H₂–air flames. Considering coflow nonpremixed NH₃–H₂–air flames for a wide range of fuel and coflow air injection velocities (V_fuel and V_coflow) and the extent of NH₃ substitution, the effects of NH₃ substitution on the stability limits and NO_x emissions of the NH₃–H₂–air flames are experimentally determined, while the nonpremixed NH₃–H₂–air flame structure is computationally predicted using a detailed reaction mechanism. Results show significant reduction in the stability limits and unremarkable increase in the NO_x emission index for enhanced NH₃ substitution, supporting the potential of NH₃ as an effective, carbon-free additive in nonpremixed H₂–air flames. With increasing V_coflow the NO_x emission index decreases, while with increasing V_fuel it decreases and then increases due to the recirculation of burned gas and the reduced radiant heat losses, respectively. Given V_coflow/V_fuel the flame length increases with enhanced NH₃ substitution since more air is needed for reaction stoichiometry. The predicted flame structure shows that NH₃ is consumed more upstream than H₂ due to the difference between their diffusivities in air.     

Reactor technology options for distributed hydrogen generation via ammonia decomposition: A review

Hydrogen (H₂) fuel obtained via thermo-catalytic ammonia (NH₃) decomposition is rapidly attracting considerable interest for portable and distributed power generation systems. Consequently, a variety of reactor technologies are being developed in view of the current lack of infrastructure to generate H₂ for proton exchange membrane (PEM) fuel cells. This paper provides an extensive review of the state-of-the-art reactor technology (also referred to as reactor infrastructure) for pure NH₃ decomposition. The review strategy is to survey the open literature and present reactor technology developments in a chronological order. The primary objective of this paper is to provide a condensed viewpoint and basis for future advances in reactor technology for generating H₂ via NH₃ decomposition. Also, this review highlights the prominent issues and prevailing challenges that are yet to be overcome for possible market entry and subsequent commercialization of various reactor technologies. To our knowledge, this work presents for the first time a review of reactor infrastructure for distributed H₂ generation via NH₃ decomposition. Despite commendable research and development progress, substantial effort is still required if commercialization of NH₃ decomposition reactor infrastructure is to be realized.     

THEORETICAL EARTH TEMPERATURE PROFILES FOR DIFFERENT SOILS AND SOIL CONDITIONS

The annual temperature variations in earth with depth from the surface offers, by its natural characteristics, a phase lag and reduced variation in the maximum and minimum temperatures. The temperature profiles vary for different soils and soil conditions. Soil temperatures are found to be an explicit function of soil thermal properties, particularly the thermal diffusivity. The earth temperatures stabilize at certain depth, where the annual swings in temperature are negligible and this stabilizing depth varies for different soils. A simple first harmonic approximation method has been used to predict the temperature variations as a function of depth and time for different soils (ordinary light soil, heavy soil, organic soil and sand) and soil conditions (dry, damp and wet). The results are a helpful indicator for deciding external design temperatures for the design of earth sheltered or ground coupled spaces.