L’argomento dell’idrogeno come fonte (o vettore) di energia rinnovabile, recentemente discusso anche su hookii (soprattutto in merito allo stoccaggio sotto forma di idruri di metallo), è tornato di attualità in seguito ad un articolo apparso su Science dal titolo “Hidden hydrogen: does Earth hold vast stores of a renewable, carbon-free fuel?”. Questo articolo è stato riportato in italiano da Today. 

Prima di introdurre questo argomento, è utile fare il punto sulla attuale situazione riguardante l’impego dell’idrogeno, la sua produzione e i suoi usi o potenziali usi. L’idrogeno è classificato in base al metodo di produzione. Ad ogni metodo di produzione corrisponde un colore (arcobaleno dell’idrogeno) e un diverso impatto ambientale:

• Gray hydrogen: made from fossil fuels, which release carbon dioxide and add to global warming.
• Blue hydrogen: same as gray hydrogen, but the carbonis captured and sequestered.
• Green hydrogen: made without carbon emissions by using renewable electricity to split water.
• Gold hydrogen: tapped from natural subsurface accumulations.
• Orange hydrogen: stimulated by pumping water into deep source rocks.

Al momento l’uso principale dell’idrogeno (perlopiù grigio, da metano) è la produzione di fertilizzante tramite processo Haber-Bosch.

La possibilità che la terra abbia abbondanti riserve di idrogeno oro è sostenuta con forza da alcuni ricercatori che portano alcuni ritrovamenti, recenti e meno, come prova della potenziale abbondanza di idrogeno sulla terra (l’idrogeno è l’elemento più abbondante dell’universo, ma sulla terra il gas non è trattenuto nell’atmosfera e l’elemento si trova soprattutto in vari composti, da cui finora doveva essere separato per il possibile sfruttamento): ad esempio, una scoperta casuale in Mali e alcune osservazioni e teorie vecchie di decenni, legate soprattutto ad una teoria ora abbandonata sulle origini dei carburanti fossili (teoria che fu molto popolare in URSS):

The Malian discovery was vivid evidence for what a small group of scientists, studying hints from seeps, mines, and abandoned wells, had been saying for years: Contrary to conventional wisdom, large stores of natural hydrogen may exist all over the world, like oil and gas—but not in the same places. These researchers say water-rock reactions deep within the Earth continuously generate hydrogen, which percolates up through the crust and sometimes accumulates in underground traps. There might be enough natural hydrogen to meet burgeoning global demand for thousands of years, according to a U.S. Geological Survey (USGS) model that presentend in October 2022 at a meeting of the Geological Society of America.

Un precedente conference proceeding (articolo open access) riassume le conoscenze sull’idrogeno naturale e propone nuove direzioni di ricerca e possibile sfruttamento. L’abstract dell’articolo sottolinea che le conoscenze in merito all’idrogeno naturale sono ancora limitate:

Much has been learned about natural hydrogen (H₂) seepages and accumulation, but present knowledge of hydrogen behavior in the crust is so limited that it is not yet possible to consider exploitation of this resources. Hydrogen targeting requires a shift in the long-standing paradigms that drive oil and gas exploration. This paper describes the foundation of an integrated source-to-sink view of the hydrogen cycle, and propose preliminary practical guidelines for hydrogen exploration.

L’articolo di Science, dopo la descrizione del ritrovamento in Mali, prova a rispondere ad un’altra ovvia domanda: se c’è così tanto idrogeno nella terra, perchè queste riserve non sono mai state identificate finora?

The oil and gas industry has punctured Earth with millions of wells. How could it have overlooked hydrogen for so long? One reason is that hydrogen is scarce in the sedimentary rocks that yield oil and gas, such as organic-rich shales or mudstones.

…

And so, historically, when well loggers cataloged their borehole emanations, they rarely bothered to measure for hydrogen. “The bottom line—they weren’t really looking for hydrogen,” says Geoffrey Ellis, an organic geochemist at USGS. “We weren’t looking in the right places with the right tools.”

La risposta è che spesso non veniva cercato nei luoghi giusti, ma ancora più di frequente, non veniva cercato affatto. Il motivo della mancanza di dati è legato alle attrezzature utilizzate per analizzare i gas (cromatografi a gas), che utilizzano spesso idrogeno come gas vettore, rendendo questo gas impossibile da identificare nelle misture di gas naturali. Di questo problema parla una recente review sull’idrogeno naturale:

Therefore, if no one expects to find free hydrogen, no one samples for it. This prejudice influences the way gas samples are analyzed and sampled, but also the way detection systems are designed. The standard analytical approach for gas chromatography often uses hydrogen as a carrier gas (Angino et al., 1984). Because of this, if there is any hydrogen in a gas sample it will not be detected. It was reported that even in the 1990’s, many surveys were not equipped to analyze for hydrogen (Smith, 2002). It still holds true, to this day, that only a few modern portable gas analyzers used in the natural sciences include a hydrogen sensor in their design. It is difficult to estimate how many times hydrogen has not been identified in H₂-rich samples because of the lack of a suitable detection technique to measure hydrogen concentrations.

L’articolo di Science descrive il possibile meccanismo di formazione delle riserve di idrogeno naturale:

The main engine of natural hydrogen production is now thought to be a set of high-temperature reactions between water and iron-rich minerals such as olivine, which dominate Earth’s mantle. One common reaction is called serpentinization, because it converts olivine into another kind of mineral called serpentinite. In the process, the iron oxidizes, grabbing oxygen atoms from water molecules and releasing hydrogen.

Ma le questioni accademiche non devono distogliere l’attenzione dallo sfruttamento dei giacimenti, secondo il geologo Prinzhofer, autore dello studio sul giacimento in Mali:

For Prinzhofer, the question of where natural hydrogen comes from is academic. “Maybe we are all completely wrong,” he says. “It doesn’t matter for the industry.” The oil industry sprang up long before it understood oil’s origins, he says. Similarly, what matters for the natural hydrogen industry is simply whether there is enough of the stuff to go after.

Altri esperti sono però più cauti:

Ellis acknowledges that much of this global resource could end up being too scattered to be captured economically, like the millions of tons of gold that are dissolved in the oceans at parts per trillion levels.

Se davvero tutto questo idrogeno naturale si rivelasse disponibile ed estraibile commercialmente, dovremmo comunque affrontare e risolvere alcuni problemi al momento irrisolti legati alla distribuzione, allo stoccaggio e all’uso dell’idrogeno

Even though it’s carbon-free, hydrogen has its faults as an energy source. One kilogram of hydrogen holds as much energy as a gallon of gasoline (just under 4 liters). But at ambient pressures, that same kilogram of hydrogen occupies more space than the drum of a typical concrete mixing truck. Pressurized tanks can hold more but add weight and costs to vehicles. Liquefying hydrogen requires chilling it to –253°C—usually a disqualifying expense.

These storage issues—along with a lack of pipelines and distribution systems—are the main reasons why, in the race to electrify cars, batteries have won out over fuel cells, which convert hydrogen to electricity. Similarly, for domestic heating, most experts believe electric heat pumps make more sense than hydrogen furnaces.