H2 is produced by steam reforming of methane. This process releases 9 kg CO2 per 1 kg H2. H2 can be produced with basalt and water.
Hydrogen is necessary to obtain ammonia, one the most important fertilizer. It is currently produced by steam reforming of methane. This process releases 9 kg CO2 to atmosphere per 1 kg H2 (1). It is perhaps possible to avoid this CO2 production. I think we can produce hydrogen by reaction between basalt and water. This reaction between basalt and water can be seen as a fossil source of hydrogen without CO2 emission.
What actions do you propose?
The oxidation of Fe2+ by water
The Pourbaix diagram (2) defines the fields of existence of chemical compounds according to the pH and the redox potential of an environment. The lines on a Pourbaix diagram define the reactions of transformation of a compound in the other. An area delimited by these lines thus represents the field of existence of a compound.
The following web page (3) shows the Pourbaix diagram of iron and its hydroxides in water. One can see on this diagram that the field of stability of Fe2+ in basic environment is particularly narrow. In addition, this field overlaps the limit of stability of water. This situation shows that the Fe2+ reduces the water and produces hydrogen in basic environment. The slow kinetic of the decomposition of water moderates this reaction.
The fragility of Fe2+ in basic environment appears in nature by the reaction of serpentinization. This reaction is the oxidation of Fe2+ contained in a rock in contact with water. The peridotite, an igneous rock rare in the upper crust of Earth, is particularly sensitive to this reaction because of its very high content of MgO, about 30 %. This high percentage of Mg2+ makes water in contact with these rocks sufficiently basic to allow the oxidation of Fe2+ and the production of hydrogen.
According to the web page (4), the average content of FeO of the peridotites is slightly lower than that of basalts: 6.6 % for 7.1 %.
One can see on this web page that the composition of basalts is not sufficiently basic to cause the oxidation of ferrous ions. The Fe2+ of basalts is stable in contact with natural water.
Production of hydrogen with basalts
There are many studies about the reaction of serpentinization. Among these studies, the thesis of Benjamin Malvoisin (5) is particularly interesting. It studies the oxidation of Fe2+ by water in peridotite but especially shows the possibility of producing hydrogen with steel slag. Hydrogen is obtained by the oxidation of Fe2+ contained in the slag by water. The high pH necessary to the reaction is due to the high content of ions calcium of the slag.
The production of hydrogen with steel slag is not economically viable despite the great amount of this waste in the world. The process needs a temperature higher than 200°C for a sufficient kinetic and this temperature needs a pressure for the keeping of water in liquid state. The cost of a pressurized furnace is too high for a competitive production of hydrogen. The competitive production of hydrogen by ferrous oxidation needs a great reservoir of Fe2+, a cheap base and a cheap pressurized furnace.
Basalt can be a great reservoir of Fe2+ and the depths of the Earth can be a cheap pressurized furnace.
To exploit Fe2+ in basalts, the sequence can be:
- two vertical drillings to reach an underground layer of basalt,
- horizontal drilling between the vertical wells followed by a hydraulic fracturing,
- injection of a limewater in one of the vertical wells.
The limewater is the cheaper base and confers on water in contact with basalt a pH in the order of 12 (6). It should be a sufficient pH to allow the oxidation of Fe2+ of the rock. Ions hydroxyl brought by the limewater are not consumed, so the reaction should be maintained by a regular addition of water in the well.
The other parameter controlling the reaction, the temperature, is determined by the depth of the well. If the layer of basalt is at depths greater than 5000 meters, the temperature of the rock should be higher than 200°C. According to the kinetic established in the thesis of Mr. Malvoisin, this temperature should allow a sufficient speed of reaction.
Another parameter could be favourable to the reaction in the well: the oxidation of Fe2+ transforms olivine and pyroxene contained in rock into serpentine and magnetite. This passage from one mineral to another produces a swelling and generates a pressure of crystallization of 300 MPa (5). This pressure is higher than the lithostatic pressure at depths of 5000 meters. This situation could make the well autofracturing.
The production curve of hydrogen could be similar to that of a shale gas well and thus last several years.
Who will take these actions?
Companies already engaged in shale gas or geothermal industries.
Where will these actions be taken?
A geothermal plant has three elements: a vertical well for water input, a deep horizontal permeable zone for heat reservoir, a vertical well for water output.
I propose an essay of hydrogen production with a geothermal plant if its heat reservoir is drilled in basalt. The essay is to replace water used for the heat extraction by limewater. The circulation of limewater in the wells continues the extraction of heat but also releases in the output the hydrogen produced by reaction between basalt and water. This experiment with an existing plant can test the feasibility of hydrogen production at the lowest cost.
This essay can take place in an area combining layers of basalt and geothermal plants: Iceland, Hawaii, Western United States, New Zealand.
How much will emissions be reduced or sequestered vs. business as usual levels?
If this reaction between basalt and water can replace the steam reforming of methane, we can avoid 9 kg CO2 per each kg H2.
What are other key benefits?
After the end of hydrogen production, the well remains usable for the CO2 sequestration or geothermal energy.
What are the proposal’s costs?
(1) Environmental Impacts of Hydrogen Plants
(2) Pourbaix diagram
(3) Iron Pourbaix diagram
(4) Composition chimique des roches magmatiques
(5) Conditions réductrices associées à la serpentinisation : suivi de l'hydratation de l'olivine de San Carlos, étude de cas naturels et application à la production industrielle d'H2