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Sustainable hydrogen: still a long way to go?

Hydrogen, the future of combustion

Laurent Catoire, Head of Chemistry and Processes Unit (UCP) at ENSTA Paris (IP Paris)
On July 8th, 2021 |
3 mins reading time
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Hydrogen, the future of combustion

    Laurent Catoire
    Laurent Catoire
    Head of Chemistry and Processes Unit (UCP) at ENSTA Paris (IP Paris)
    Key takeaways
    • Today, most of our energy is obtained by burning fossil fuels, which are cheaper than renewables.
    • Green hydrogen (H2) could replace certain fossil fuels, in particular natural gas, in some combustion devices as fuel for gas turbines and industrial processes.
    • Some hydrogen combustion methods produce 90% less pollution in the form of nitrous oxides (NOx).

    The demand for ener­gy in the world has increased con­sid­er­ably as the pop­u­la­tion has grown. This is because, as a tru­ism, ener­gy is need­ed for almost all activ­i­ties: indus­try, domes­tic activ­i­ties and urban, inter-city and inter­con­ti­nen­tal trav­el. Today, most of the ener­gy used is obtained by burn­ing fos­sil fuels, which are of course not renew­able resources because it is cheap­er but also renew­ables face a range of chal­lenges that prob­a­bly do not need to be repeat­ed here.

    Unavoid­able combustion

    The desire to stop burn­ing fos­sil fuels does not nec­es­sar­i­ly mean that com­bus­tion process­es will dis­ap­pear – they very wide­ly spread and have been used inten­sive­ly for ~150 years. More­over, there is no seri­ous rea­son why they should dis­ap­pear. How­ev­er, there are too many of them: gas tur­bines, ther­mal and hybrid engines, burn­ers for heat­ing equip­ment and petro­chem­i­cal ovens, burn­ers for dry­ing oper­a­tions, com­bus­tion sys­tems for indus­tri­al and domes­tic boil­ers, etc. Each devel­oped for spe­cif­ic applications. 

    Alter­na­tives such as fuel cells or bat­ter­ies could be inter­est­ing for some niche uses, but they are nei­ther clean nor safe, and remain very cost­ly, both eco­nom­i­cal­ly and social­ly as well as envi­ron­men­tal­ly. It remains to be dis­cussed which sub­stances could replace fos­sil fuels, in par­tic­u­lar nat­ur­al gas, in these com­bus­tion devices. Glass indus­tries are think­ing about green glass, and one of the poten­tial can­di­dates to meet their con­cerns and needs is green hydro­gen (H2). It is also a per­spec­tive for the cement indus­try and, more gen­er­al­ly, all indus­tries too. 

    Let’s remem­ber that pure hydro­gen in gas or liq­uid form does not exist in nature (the Earth’s atmos­phere con­tains very lit­tle). And even though it can in the­o­ry be obtained from plant mat­ter (bio­mass), it seems that we are mov­ing towards pro­duc­tion by elec­trol­y­sis of water and/or using ther­mal process­es. These issues are not dis­cussed fur­ther here but are cov­ered in oth­er arti­cles in this dossier.

    Hydro­gen, the future of combustion?

    Com­bined with the use of renew­able ener­gy sources for its pro­duc­tion, green hydro­gen rep­re­sents a poten­tial alter­na­tive fuel for gas tur­bines to pro­duce low-emis­sion elec­tric­i­ty as well as the indus­tri­al com­bus­tion process­es list­ed above. How­ev­er, due to the dif­fer­ence in phys­i­cal prop­er­ties between hydro­gen and oth­er fuels such as nat­ur­al gas, well-estab­lished gas tur­bine com­bus­tion sys­tems can­not be con­vert­ed direct­ly to hydro­gen com­bus­tion – a process that has been under devel­op­ment for many years, as it offers the promise of sig­nif­i­cant­ly reduc­ing pol­lu­tion in the form of NOx [nitric oxide (x=1) and nitro­gen diox­ide (x=2)] emis­sions, with­out emit­ting par­tic­u­lates (PM or soot) or CO2.

    Numer­ous fun­da­men­tal stud­ies car­ried out in aca­d­e­m­ic and R&D lab­o­ra­to­ries have enabled the mech­a­nisms of hydro­gen com­bus­tion in oxy­gen or in air to be mas­tered very well. They can be imple­ment­ed in in-house or com­mer­cial CFD (Com­pu­ta­tion­al Flu­id Dynam­ics) codes. They con­sid­er not only flu­id mechan­ics, trans­port prop­er­ties and heat exchange. But also, and this is more recent, the chem­istry of com­bus­tion with the nec­es­sary and suf­fi­cient finesse to know in which zones of the device to act to lim­it the for­ma­tion of the only pol­lu­tants like­ly to be formed dur­ing H2/air com­bus­tion – i.e. nitro­gen oxides or NOx.

    Pol­lu­tant reduction

    It should be point­ed out that indus­tri­al­ists and aca­d­e­mics who have been work­ing on these strate­gies for decades have only now proven how to lim­it the for­ma­tion of nitro­gen oxides dur­ing fos­sil fuel com­bus­tion. These include, but are not lim­it­ed to, EGR (Exhaust Gas Recir­cu­la­tion), SNCR (Selec­tive Non-Cat­alyt­ic “advanced” com­bus­tion tech­nolo­gies that com­bine tech­nolo­gies that exist independently.

    These abate­ment strate­gies can be imple­ment­ed as required for equip­ment in which hydrogen/air mix­tures are burned. Know­ing that some of them allow a 90% reduc­tion of NOx, it is clear that hydro­gen com­bus­tion is a seri­ous, clean and safe alter­na­tive. Of course, the chem­i­cal risks involved in the use of hydro­gen are not the same as for nat­ur­al gas. How­ev­er, these risks are known and per­fect­ly con­trolled, and are exact­ly the same as those of fuel cells, for example.

    For fur­ther reading

    Com­bus­tion, Pol­lu­tion and Envi­ron­men­tal Risks (French Edi­tion), Lau­rent Catoire

    Contributors

    Laurent Catoire
    Laurent Catoire
    Head of Chemistry and Processes Unit (UCP) at ENSTA Paris (IP Paris)

    Laurent Catoire is a professor in applied chemical kinetics, in particular in combustion and in general in all reactive systems. After a DGA thesis, he has been working for 30 years on reactive systems that are little studied, poorly known but with important or potentially important applications (hypergolic systems in space propulsion, civil and military energetic materials (explosives, propellants and gas generators), energetic ionic liquids, nanothermites, aluminium combustion, metal combustion, etc).