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Sustainable hydrogen: still a long way to go?

Why hydrogen emits CO2

Didier Dalmazzone, Professor of Chemistry and Processes at ENSTA Paris (IP Paris)
On July 8th, 2021 |
4 mins reading time
Why hydrogen emits CO2
Ddidier Dalmazzone
Didier Dalmazzone
Professor of Chemistry and Processes at ENSTA Paris (IP Paris)
Key takeaways
  • Grey hydrogen, the most widespread method to produce H2, is also the process with the worst carbon footprint.
  • It is produced by natural gas steam reforming costing 1.5€/kg vs 6€/kg for hydrogen produced using electrolysis.
  • Hydrogen production would need to increase by a factor of 14 to cover 20% of the global energy consumption –not currently possible with grey hydrogen.
  • Energy from hydrogen can be used to make other fuels, directly as propellant or in a fuel cell – each with their own challenges to overcome.

“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.


Ddidier Dalmazzone

Didier Dalmazzone

Professor of Chemistry and Processes at ENSTA Paris (IP Paris)

Didier Dalmazzone is a member of the Management Committee of the Interdisciplinary Centre Energy for Climate of the Institut Polytechnique de Paris. He is in charge of the Energy Production and Management course in the 3rd year of the ENSTA Paris engineering curriculum, and is also in charge of the Master's Degree in Energy at IP Paris. His research activities on processes for the energy transition concern the hydrogen sector, CO2 capture and refrigeration.