“Grey hydrogen” is made by using fossil fuels. It is by far the most widespread method to produce hydrogen today. It is also the process with the worst carbon footprint. Far from achieving its future potential as a source of energy, today hydrogen is mainly used as a raw material in industry. It can be used in oil refining for hydrocracking and desulfurizing fuels (approximately 44% of total demand), in ammonia synthesis for nitrogenous fertilizers (38%), in the production of chemicals (8%), or even in the food industry or other applications (10%). These needs represent 75 million tons of hydrogen per year on a global scale.
Forty-eight percent (48%) of the production is covered via natural gas (methane) steam reforming, 30% is produced from petroleum hydrocarbon and 18% through coal gasification. This production causes one billion tons of CO2 each year. Water electrolysis, with a much lower carbon footprint – although it depends on the energy mix – currently covers less than 5% of the demand.
Why do we still need fossil fuels?
In any case, the production of hydrogen requires splitting water molecules, a process which demands a large amount of energy: more than 40 kWh to make 1 kg of hydrogen. In conventional methods, part of this energy is provided by the reaction of fuel with high-temperature steam. This mixture of fuel and water is then transformed in a mix of carbon monoxide (CO) and hydrogen by the reforming reaction. However, this operation requires an additional energy source, brought about by the combustion of fuel or gas to maintain the reforming reactor at the proper temperature. After this first step, it is necessary to resort to a “water-gas shift” reaction to convert CO, which is very toxic, into CO2 by reaction with medium-temperature steam.
In the end, CO2 is produced in large quantities at the different stages of the process: conversion of the CO produced by the reforming reactor, fuel combustion to produce steam and to supply the reactor with additional energy. For every ton of hydrogen produced, nearly 12 tons of CO2 are released into the atmosphere.
The reason why grey hydrogen is still the most widespread production method despite its appalling carbon footprint, is because it offers a significant advantage in terms of costs. Hydrogen produced by natural gas steam reforming in large volume costs approximately 1.5€/kg. In contrast, hydrogen produced by water electrolysis costs 6€/kg. Nevertheless, it is worth noting that even at the lowest cost, hydrogen is still 3 times more expensive than natural gas and that both require the same amount of energy.
In addition to the issues regarding cost and greenhouse gas emissions, hydrogen suffers from highly insufficient production capabilities. Thus, as of today, it is not a viable solution for the energy transition. Indeed, if it was entirely dedicated to energy conversion, current global hydrogen production would cover roughly 214 Mtoe (million tons of oil equivalent). However, the current global annual energy demand is estimated at 14.5 Gtoe (gigatons of oil equivalent). Thus, hydrogen production would need to increase by a factor of 14 to cover 20% of the global energy consumption. But this would obviously not be possible with grey hydrogen, nor make sense in the current context.
Energy conversion of hydrogen
This article will therefore rather focus on energy conversion with hydrogen of renewable origin. Hydrogen is a very versatile compound, which can be converted into energy in different ways:
- By thermochemical reaction with suitable reagents. The result is potential energy, easy to store over long periods of time and available upon demand. The Sabatier process uses CO2 as a reagent and produces synthetic methane. It can then serve as fuel for industry, transportation, or be injected into grids. This concept is known as “power to gas”, in cases where hydrogen comes from water electrolysis. The Fischer-Tropsch process produces liquid fuel (“power to liquid”). The Haber-Bosch process combines hydrogen with nitrogen from the air and produces ammonia which can easily be stored and also serve as fuel.
- By heat and mechanical work, through combustion in air or with pure oxygen. It is the principle of a rocket engine, used on some stages of Ariane launchers. This solution is one of the means considered to propel future hydrogen fuelled aircraft. Hydrogen can also be added in limited quantities to conventional fuels, in natural gas grids or to power internal combustion engines.
- By heat and electrical work, through controlled oxidation using a fuel cell. Today there is a wide variety of fuel cell technologies. A few are very mature, others have just reached the commercial stage, while some are still under development. One of the major challenges is to optimise the electrical performance of fuel cells, which is limited to about 60–65% at best. This means that only 60–65% of the thermochemical energy is transported by the fuel and is actually converted in electrical work. The rest is lost in the form of heat. If this heat is produced at low or medium temperature (< 500°C for example), the investment is unsatisfactory, whereas heat produced at high temperature (between 700 and 1000°C) can be converted in mechanical work with a good efficiency. Herein lies the challenge of Molten-Carbonate Fuel Cells (MCFC) or Solid Oxide Fuel Cells (SOFC). Even though they hold promise for some applications, these high-temperature technologies are still in their early stages. The most widespread fuel cells are liquid electrolyte fuel cells and Proton Exchange Membrane Fuel Cells (PEMFC) which work at medium temperatures. Fuel cells for mobility mainly use PEMFC, which nevertheless remain expensive due to the used materials (membrane, platinum-based catalyst).
One can only hope that ongoing research and the development of a mass market will allow fuel cells to make progress like batteries in terms of cost and efficiency, and that hydrogen will find its place in the energy transition.
