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