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.
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.
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