Tag: hydrogen economy

Naturally occurring hydrogen: an abundant green fuel?

Published on by zooks7772 Comments

Burning hydrogen produces only water vapour, so it is not surprising that it has been touted as the ultimate ‘green’ energy source, and increasingly attracts the view that the ‘Hydrogen Economy’ may replace that based on fossil fuels. It is currently produced from natural gas by ‘steam reforming’ of methane that transforms water vapour and CH4 to hydrogen and carbon monoxide. That clearly doesn’t make use of the hydrogen ‘green’ as the CO becomes carbon dioxide because it reacts with atmospheric oxygen; it is termed ‘grey hydrogen’. But should it prove possible to capture CO and store it permanently underground in some way then that can be touted as ‘blue hydrogen’ thereby covering up the carbon footprint of all the rigmarole in getting the waste CO into a safe reservoir. However, if carbon-free electricity from renewables is used to electrolyse water into H and O the hydrogen aficionados can safely call it ‘green hydrogen’.   It seem there is a bewildering colour coding for hydrogen that depends on the various options for its production: ‘yellow’ if produced using solar energy; ‘red’ if made chemically from biowaste; ‘black’ by coking coal using steam; ‘pink’ is electrolysis using nuclear power; and even ‘turquoise’ hydrogen if methane is somehow turned into hydrogen and solid carbon using renewables – a yet-to-be-developed technology! Very jolly but confusing: almost suspiciously so!

But not to be forgotten is the ‘white’ variety, applied to hydrogen that is emitted by natural processes within the Earth. Eric Hand, the European news editor for the major journal Science has written an excellent Feature article about ‘white’ hydrogen in a recent issue (Hand, E. 2023. Hidden hydrogen. Science, v. 379, article adh1460; DOI: 10.1126/science.adh1460). Hand’s feature is quirky, but well-worth a read. It is based on the proceedings of a Geological Society of America mini-conference about non-petroleum, geological energy resources  held in October 2022. He opens with a bizarre anecdote related by a farmer who lives in rural Mali. The only drilling that ever went on in his village was for water, and many holes were dry. But one attempt resulted in ‘wind coming out of the hole’. When a driller looked in the hole, the ‘wind’ burst into flame – he had a cigarette in his mouth. The fire burned for months. Some 20 years later the story reached a Malian company executive who began prospecting the area’s petroleum potential, believing the drilling had hit natural gas. Analysis of the gas revealed that it was 98% hydrogen – now the village has electricity generated by ‘white’ hydrogen.

Mantle rock in the Oman ophiolite, showing cores of fresh peridotite, surrounded by brownish serpentinite and white magnesium carbonate veins (credit: Juerg Matter, Oman Drilling Project, Southampton University, UK)

So how is hydrogen produced by geological processes? Some springs in the mountains of Oman also release copious amounts of the gas. The springs emerge from ultramafic rocks of the vast ophiolite that was emplaced onto the Arabian continental crust towards the end of the Cretaceous. The lower part of this obducted mass of oceanic lithosphere is mantle rock dominated by iron- and magnesium-rich silicates, mainly olivine [(Mg,Fe)2SiO4 – a solid solution of magnesium and iron end members]. When saturated with groundwater in which CO2 is dissolved olivine breaks down slowly but relentlessly. The hydration reaction is exothermic and generates heat, so is self-sustaining. Olivine’s magnesium end member is hydrated to form the soft ornamental mineral serpentine (Mg3Si2O5(OH)4) and magnesium carbonate. Under reducing conditions the iron end member reacts with water to produce an iron oxide, silica and hydrogen:

3Fe2SiO4 + 2H2O → 2 Fe3O4 + 3SiO­2 +3H2

Gases emanating from mid-ocean ridges contain high amounts of hydrogen produced in this way, for example from Icelandic geothermal wells. But Mali is part of an ancient craton, so similar reactions involving iron-rich ultramafic rocks deep in the continental crust are probably sourcing hydrogen in this way too. Hydrogen production on the scale of that discovered in Mali seems to be widespread, with discoveries in Australia, the US, Brazil and the Spanish Pyrenees that have pilot-scale production plants. The US Geological Survey has estimated that around 1 trillion tonnes of ‘white’ hydrogen may be available for extraction and use

Hydrogen, like other natural gases, may be trapped below the surface in the same ways as in commercial petroleum fields. But petroleum-gas wells emit little if any hydrogen mixed in with methane. That absence is probably because petroleum fields occur in deep sedimentary basins well above any crystalline basement. The geophysical exploration that discovers and defines the traps in petroleum fields has never been deployed over areas of crystalline continental crust because as far as the oil companies are concerned they are barren. That may be about to change. There is another exploration approach: known hydrogen seepage seems to deter vegetation so that the sites are in areas of bare ground, which have been called ‘fairy circles’. These could be detected easily using remote sensing techniques.

Artificially increasing serpentine formation by pumping water into the mantle part of ophiolites, such as that in Oman, and other near-surface ultramafic rocks is also a means of carbon sequestration, which should produce hydrogen as a by-product (see: Global warming: Can mantle rocks reduce the greenhouse effect?, July 2021). A ‘two-for-the-price-of-one’ opportunity?

Is there a future for coal?

Published on by zooks777Leave a comment

Burning coal is far and away the main culprit for elevated atmospheric CO2 and global climatic warming. That is a consequence of the Industrial Revolution and a mode of production that centres on the market value of commodities rather than their intrinsic usefulness and thus on the continual generation of profit. As the main original energy source for this capitalist mode, whose viability depends on incessant economic growth, world coal production grew at an exponential rate through the 19th and 20th centuries, as did its influence on global climate. Beginning in the 1920s, coal was joined by other carbon-based fossil fuels (oil and natural gas) in massively increasing energy and greenhouse-gas production. In the data there is little sign of an appetite to reduce this dependence on carbon burning, renewable energy output accounting today for only a few percent of the energy demands of capital. Although coal’s energy contribution is flattening off to be replaced by those of oil and gas, in terms of CO2 emissions it still dominates. That is because oil and gas are carbon-hydrogen compounds rather than almost pure carbon in the case of coal. Unless burning fossil fuels is outlawed and economic growth is drastically curtailed, we are set to live on a far warmer planet.

Growth in energy supply from different sources since 1800 CE (Credit: ourworldindata.org)

For various reasons coal is unique in a social sense. Every producing area has large communities who depend on mining for income. Britain now produces vastly less coal than it did up to the 1990s, with wholesale closure of underground mines. Since 1960 these communities have lost over half a million jobs in mining, let alone those in related industries and those that served mining communities. Three decades on from the last round of closures, those communities remain socially devastated in many respects. A huge amount of coal still lies beneath them. Can it make a come-back? A recent study suggests that perhaps it can, with diametrically opposite environmental consequences.

It seems that surfaces of coal particles are able to take-up and store gases, increasingly so as pressure increases. Miners faced the consequences of that in the form of adsorbed carbon dioxide and methane (‘choke damp’ and ‘fire damp’) that were released when coal was mined underground. Miners’ safety lamps (invented in 1815 by Humphry Davy) enable them to monitor the risks of suffocation by CO2 or explosion of CH4 when the lamps dim or flare, respectively. Engineers at Edith Cowan University in Perth, Western Australia, experimentally measured the amounts of hydrogen adsorbed by crushed coal at different pressures (Iglauer, S et al. 2021. Hydrogen adsorption on sub-bituminous coal: Implications for hydrogen geostorage. Geophysical Research Letters, v. 48, article 2021GL092976; DOI: 10.1029/2021GL092976). From surface atmospheric pressure to 40 times that, adsorption increases rapidly, especially for hydrogen (from 0.05 to 0.25 grams per kilogram). At higher pressures it rises less rapidly to about 0.6 g kg-1 at 100 times atmospheric pressure, which is equivalent to a depth of about 500 m in a sedimentary rock formation. At deeper levels hydrogen adsorption remains about the same.

The experimental results suggest that large amounts of hydrogen can be stored in coal at quite shallow depths. The potential storage in a ton of fractured coal is about 600 kg, equivalent to about 12 cubic metres of liquid hydrogen, but without the need for containment and refrigeration. In the absence of oxygen, such storage would be safe and long-term. If feasible from an engineering standpoint, underground storage of hydrogen in coal seams to overcome one of the current barriers to a hydrogen-based industrial economy through the storage of energy generated by carbon-free technologies, such as wind, wave, tidal and solar generation that operate at highly variable rates, not suited to energy use patterns. Effectively, coalfields could become giant ‘batteries’ without the need to mine vast amounts of elements, such as lithium, needed for conventional batteries; provided that a sustainable means of repeated hydrogen recovery can be devised. A central technology of a future ‘hydrogen economy’ is that of the fuel cell in which hydrogen and oxygen combine using a catalyst to generate electricity, without any combustion and emitting only water vapour.

Life at the battery terminal

Published on by zooks777Leave a comment

Mussels of species Bathymodiolus childressi (B...
Hydrothermal-vent mussel Bathymodiolus. Image via Wikipedia

Having an interior that is dominated by reducing conditions and oxidising surface environments since free oxygen gradually permeated from its initial build up in the atmosphere to the ocean depths, the Earth has been likened to a massive self-charging battery. Electrons flow continually as a consequence of the nature of the linked oxidation-reduction: in terms of electrons, oxidation involves loss while reduction involves gain (the OILRIG mnemonic). Although there are natural electrical currents, most of the electron flow is in the form of reduced compounds rich in electrons that make their way through the flow of fluids from the deep Earth – effectively an anode – towards the surface  where the reduced compounds lose electrons to create the equivalent of a cathode. Reduction-oxidation (redox) is therefore a power source. Inorganic reactions, such as the precipitation on the sea floor of sulfides from hydrothermal fluids at ‘black smokers’ dissipate energy. Yet the power has considerable potential for organic life. Some bacteria oxidise hydrogen sulfide carried by hydrothermal fluids and others do the same to upwelling methane. In 1977 a teeming biome of worms, molluscs and higher animals was discovered in a totally dark environment around ocean-floor vents. It soon became clear that it could only subsist on chemical energy of this kind, rather than any form of photosynthesis. The key to some metazoans’ success had to be symbiosis with bacteria that could perform the chemical tricks possible in the cathode region of the Earth’s electron flow. There are several candidate compounds: H2S, CH4, NH4, metal ions and even hydrogen gas.

As hydrothermal fluids cycle ocean water into the basaltic crust and underlying peridotite mantle, they not only hydrate the olivines and pyroxenes that dominate the oceanic lithosphere but trigger other reactions one of whose products is hydrogen. As well as a reaction being eyed by those keen on a cheap source of clean fuel, it generates more energy potential for biological metabolism in the guise of hydrogen than those which form other common compound in the returning fluids. Although the nature of hydrogen’s organic use has been elusive, it has now come to light in a surprising guise (Petersen, J.M. and 14 others 2011. Hydrogen is an energy source for hydrothermal vent symbioses. Nature, v. 476, p. 176-180).

One highly successful animal in ocean-floor hot spring systems is a mussel called Bathymodiolus. Genetic experiments by the German-French-US team revealed that a gene known as hupL is present in the mussels’ gill tissue; a gene found in bacteria that use either carbon monoxide or hydrogen as an electron donor. The hupL gene encodes for enzymes known as hydrogenases that are needed to set off the reaction H2 = 2H+ + 2e– that provides electrons needed in bacterial metabolism; a sort of living fuel cell. Hydrogen-using bacteria interact symbiotically with the mussels, which would otherwise be unable to live in the pitch black environment. Genomic sequencing of tube worms and shrimps that occur in the vent communities also contain the bacterial hupL gene. Hydrogenase enzymes are proteins with an iron-nickel core, and probably evolved far back in bacterial evolution around metal-rich hot springs. Interesting as the specific detail of hydrogen-based symbiosis is, the general concept of Earth’s redox systems’ having battery-like behaviour is very useful. On land groundwater sometimes comes into contact with sulfide ore bodies that are oxidised to yield hydrogen and sulfate ions ,while the groundwater is reduced: a battery comes into being with a cathode in the aerated groundwater and electrons flow from the unaltered orebody towards it. Such currents are useful in revealing hidden orebodies using the ‘self-potential’ or SP method. Indeed the downward change from oxidising to reducing groundwater, caused by the redox reactions involved in weathering and soil formation also result in weak negative and positive ‘electrodes’ with a sluggish flow of compounds that bacteria can exploit and thereby encourage metazoan life through symbiosis. In doing so, changes in redox conditions affect the inorganic load of the slowly moving groundwater so that reduced metal ions can be precipitated once they rise into the oxidising horizon. The general enrichment of the upper horizons of soils in iron oxides and hydroxides, and metal depletion in lower horizons probably stem from the ‘Earth battery’ produced by an interplay between inorganic and organic redox reactions. Be on the look-out for more on this topic as the quest for hydrogen fuels becomes more urgent. A former colleague, Gordon Stanger, investigating groundwater in the Semail ophiolite of the Oman for his PhD in the 1970s discovered to his surprise that in outcrops of the mantle sequence there were springs from which hydrogen bubbled freely: fortunately he was not a smoker…

Related articles