Clean Energy Technology

Steam methane reformation is a catalytic reaction in which CH4 reacts with high temperature (800 °C) steam to generate H2 and CO (syngas). This process requires an external input of heat, which leads to lower efficiencies and a diluted CO2 stream which is costly to capture. The reforming process is followed by a water gas shift process in which the CO reacts with water at lower temperatures to generate more H2 and CO2. Then, CO2 is captured and a stream of high-purity H2 is obtained. When capture is applied to both the CO2-concentrated process stream and the diluted stream produced in the reformer, capture rates above 90% can be achieved.

The Earth continuously produces natural H2 (also called Native H2) through several chemical reactions that are primarily related to the oxidation of ferrous iron minerals, radiolysis of water, maturation of organic matter and the outgassing from the Earth's mantle. The exploration strategy for hydrogen should focus on areas where ferrous iron and/or natural radioactivity is present and can react with water. The geological exploration of H2 follows the same approach as for hydrocarbons, starting with the identification of the source rock, followed by the migration pathways, and finally the reservoirs and traps. For the latter, formations such as volcanic sills, clays or salt layers could potentially be capable of trapping hydrogen in crystalline or sedimentary rocks, for example, at the bottom of the sedimentary basins. Some players are also contemplating the co-production of He with natural H2, as they are commonly found together. Geothermal power plants could enhance their value chains by co-producing natural H2 and mineral substances, such as lithium. Coupling H2 production with the storage of CO2 in ultrabasic rocks will add additional benefits to natural H2 production.

Thermochemical water splitting uses thermal energy (i.e. pure thermochemical cycles) or thermal and electrical energy (i.e. hybrid thermochemical cycles) and cycles of chemical reactions to produce hydrogen and oxygen from water. Nuclear and concentrated solar can be used for the generation of these high temperatures. Over 360 thermochemical processes are known and have been described. The down selection from all these processes is done by a reduced number of chemical steps (ideally 2, usually 3 to maximum 4), the use of heat at value that can provide nuclear reactors (850°C is a maximum, 650-550°C is be preferred), and the preference (if possible) to liquid and gas products used in all cycles. Today ten or so cycles are still investigated and being compatible with nuclear heat. The concept of using thermochemical cycles for hydrogen production started with the first patent in the early 1920s on a two-step cycle, but it was not until the early 1960s that several research activities started to tackle the topic.

The density of pure hydrogen is increased via its liquefaction to 70 kg/m3 at 1 bar. Due to the low boiling point of hydrogen (-253 °C) compared to natural gas (-162 °C), the design of cryogenic storage tanks seeks to minimise boil-off gas, preventing heat inleak. If hydrogen is evaporated, it must be vented to avoid an increase in the pressure in the storage tank. Small tanks are usually cylindrical but for larger volumes spherical tanks are used to minimise the surface-to-volume ratio, decreasing heat transfer. Liquid hydrogen storage tanks often feature a double-shell vacuum insulation, which minimises heat transfer via conduction and convection, and the space between the tank walls contains additional insulation materials. Cryogenic tanks are lighter than pressure vessels; however, liquefaction is an energy-intensive process compared to compression. Today, large-scale liquid hydrogen storage technology is relatively similar to that of the 1960s; however, design innovation is still needed to further scale up the tank size.

Steam methane reformation is a catalytic reaction in which CH4 reacts with high temperature (800 °C) steam to generate H2 and CO (syngas). The reforming process is followed by a water gas shift process in which the CO reacts with water at lower temperatures to generate more H2 and CO2. Then, CO2 is captured and a stream of high-purity H2 is obtained. When capture is applied only to this CO2-concentrated process stream, only around 60% of the CO2 produced can be captured. The remaining 40% of the CO2 is produced (diluted with nitrogen) in the reformer since natural gas is combusted to provide the heat needed for the process. This technology is widely used in ammonia plants with co-production of urea.

In partial oxidation, natural gas reacts with a limited amount of oxygen that is not enough to completely oxidise it to carbon dioxide and water. With less than the stoichiometric amount of oxygen available, the reaction products are primarily hydrogen and CO. The process is followed by a water gas shift process in which the CO reacts with water to generate more H2 and CO2. The partial oxidation is an exothermic process, which avoids the need for an external input of heat and, therefore, avoiding the production of a diluted CO2 stream.

Thermochemical water splitting cycles use a sequence of thermochemical reactions in a closed cycle whose net effect is the splitting of water into hydrogen and oxygen; intermediate reactants are recycled in the reaction loop. One or more thermochemical reactions require heat at high temperatures, which, however, are lower than those of direct water splitting (typically

Pressure vessels are the most established hydrogen storage technology and involve the physical storage of compressed hydrogen gas in high-pressure vessels for stationary or mobile (such as tube trailers) applications. The pressure rating and internal volume of the container determines the quantity of hydrogen it can hold, and they are often classified into four types: I) vessel made of metal, usually steel (around 1 wt% hydrogen); II) vessel made of a thick metallic liner hoop wrapped with a fibre-resin composite; III) vessel made of a metallic liner fully-wrapped with a fibre-resin composite; IV) vessel made of polymeric liner fully-wrapped with a fibre-resin composite (around 5.3 wt% hydrogen) and V) fully composite vessel (under consideration). The choice of pressure vessel will depend on the final application, being a compromise between volumetric density and cost. Pressure vessels are already used in the chemicals industry and at hydrogen refuelling stations, mostly all-steel tanks. Trucks that haul gaseous hydrogen compress it to pressures of around 180-250 bar into steel vessels (long tubes) carrying approximately 380 kg onboard and limited by the weight of the vessel. However, recently light-weight composite storage vessels have been developed that have capacities of 560-900 kg of hydrogen per trailer, increasing considerably the hauling efficiency per trip. Types III-IV have gravimetric capacities that exceed four times that of steel vessels working on the same pressure, can endure high pressures and are used in the vehicle industry.

In underground reforming, air or oxygen is pumped into an underground gas reservoir then ignited to set fire to the hydrocarbons. Once the fire reaches 500 °C, the water vapour or injected steam reacts with the hydrocarbons producing syngas. Then, more water is added to the syngas to increase hydrogen production and shift the CO in the syngas to CO2.

Photocatalytic water splitting technology includes several approaches that use solar light to drive catalysts to split water into oxygen and hydrogen. Typically, semiconducting materials are activated by light, which induces a charge separated state that generates a potential difference over two electrodes (or within a particle). This potential difference should be enough (>1.23 V) to drive water splitting catalysis. Typical Si, III-V, and CIGS PV-type semiconductors can be used, but also metal oxides, such as BiVO4, Fe2O3, TiO2, Cu2O, and SrTiO3, among other materials. (Co-)catalyst materials are similar to those used in typical electrolyzers (based on Ni, Pt, Ir), but also novel type of catalyst materials are implemented (based on, e.g., Au, NiMo, MoS, Co). Also, the incorporation of molecular dyes and/or catalysts is investigated.

Ammonia has been stored as a liquid since ammonia production on an industrial scale began about 100 years ago. Ammonia was initially stored in pressurised systems, typically of around 2 000 tonnes. Today, atmospheric ammonia storage tanks are used to store up to 50 000 tonnes. Low-pressure ammonia storage has been widely accepted, as it requires much less capital per unit of volume. There are different types of atmospheric tanks for ammonia operating at -33 °C, but the current practice recommends using double-wall double integrity tanks, which can have insulation in the annular space or on the outer tank. As ammonia may be used as a fuel, new ammonia storage units may function as fuel bunker.

The hydrogen storage capacity of sorbent-based systems is in an intermediate level between compressed gas and intermetallic compounds (metal hydrides), involving the transfer of hydrogen molecules to the surface of the pores of solid materials through physical interaction (van der Waals bonding) and the subsequent release of hydrogen, whenever required, by thermal stimulation or other techniques. Different materials have been investigated for their potential use in the storage of hydrogen through adsorption, among them; metal-organic frameworks (MOFs), carbon-based materials, zeolites and polymers of intrinsic microporosity (PIMs) have been some of the most extensively studied due to their fast kinetics, good reversibility and high stability over many cycles. However, due to the weak interactions between hydrogen molecules and the surface of these solid materials, high hydrogen storage capacities are generally achieved at cryogenic temperatures (around -196 °C) and relatively high pressures. At ambient temperature and pressure conditions, hydrogen adsorption capacities are usually very low (