Hydrogen, the future of combustion

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