Quantum corrections could boost combustion models
Nov 7, 2012 3 comments
Hydrogen's ability to burn and explode more readily than expected at high pressures is down to quantum mechanics – that is the conclusion of a new study by physicists in Russia, who claim that their research could lead to fewer hydrogen explosions at nuclear power plants and to the safer industrial production of hydrogen and other gaseous fuels.
The potential for hydrogen explosions at nuclear reactors was graphically illustrated in March last year at the Fukushima Daiichi power plant in Japan. Three of the reactors at the plant melted down after the enormous tsunami that struck the north-east of the country knocked out the reactors' cooling systems. The zirconium alloy cladding around the nuclear fuel in the reactors' cores was then exposed by falling water levels, which reacted with steam to produce hydrogen. Once vented into the surrounding containment vessels, the built-up hydrogen then reacted with oxygen in the air and caused an explosion that blew the vessels apart. Such explosions are dangerous because they can potentially release radioactivity into the environment.
However, it is not known under precisely what conditions such hydrogen explosions occur. The problem lies with determining the time needed for hydrogen gas at a certain temperature and pressure to ignite, as well as the threshold temperature required for that gas to detonate – detonation occurs when combustion reactions propagate more quickly than sound. Theoretical predictions of these quantities do not agree with experimental results, and this disagreement increases as temperature decreases and pressure increases. At temperatures of 700–800 K, the calculated ignition delays can be up to 1000 times longer than those measured in reality.
Scientists have previously tried to explain these discrepancies by arguing that the calculations ignored the effect of certain impurities within the gas or, more simply, that the measurements themselves were inaccurate. But these discrepancies also occur for other gaseous fuels. In the latest research, Vladimir Fortov and colleagues at the Joint Institute for High Temperatures in Moscow, together with two physicists at the Troitsk Institute for Innovation and Thermonuclear Research, located outside the Russian capital, provide a general explanation for the mismatches by incorporating quantum corrections into existing combustion models.
In their new study, Fortov and co-workers considered the classical Maxwell Boltzmann distribution, which describes the spectrum of molecular velocities in a gas at a certain temperature. Theorists have recently shown that quantum-mechanical effects can significantly alter the shape of the higher-energy end of this distribution for gases at relatively low temperatures and high pressures. The modification is a result of Heisenberg's uncertainty principle. The uncertainty lies in the energy of colliding particles increasing the probability of reactions taking place between them, as long as the density of those particles is high enough.
While this modification is insignificant for general thermodynamical phenomena governed by the overwhelming majority of molecules with kinetic energies close to the mean, it is relevant for processes in which the high-energy "tail" of the distribution plays an important role. One such process is nuclear fusion in dense, cool plasmas. The Russian group has now shown that it also holds sway in the chemical reactions involved in combustion, given that the energy needed to activate such reactions is far higher than that of thermal molecules' mean energy. The researchers found that by introducing quantum corrections, they obtained a close agreement between theory and experiment for pressurized argon gas containing 4% molecular hydrogen and 2% molecular oxygen. They also found a good agreement when analysing the detonation of acetylene.
Optimized geometry for safety
According to Fortov, these calculations could help improve safety at nuclear plants by allowing engineers to study the full range of conditions across which hydrogen detonation could occur. As such, he says, it should be possible to reduce the chances of hydrogen explosions by optimizing reactor geometry or by knowing how best to position devices known as "hydrogen recombiners" that eliminate hydrogen from containment vessels by combining it with oxygen to form water vapour. Fortov also claims that his group's work could allow industry to produce and store hydrogen more safely and also to better handle acetylene, which is widely used as a fuel and as a chemical building block, and can ignite and detonate without an oxidant.
However, Tony Roulstone, a nuclear engineer at the University of Cambridge, doubts whether the latest work will have significant practical benefits for nuclear power production. He says that designers of reactor containment vessels assume that hydrogen will always burn if it exceeds the "experimentally determined hydrogen flammability limit". He reasons that it might be possible to reduce the pressure that the containment structure is designed to withstand, if there were better ways of calculating when and at what rate the hydrogen is burnt. But he points out that, in practice, such a reduction may not be possible given that reactor containments are also designed to resist large aircraft crashes. "It is these crashes that are likely to be the controlling case," he says.
Keith Ross of Salford University, meanwhile, is doubtful that the quantum corrections will have any significant impact on the implementation of hydrogen storage. He says that he assumes any calculations by safety engineers are simply based on the shorter ignition time revealed by experiment, although he does add that "there is always the potential advantage of using a theoretical experiment to assess a range of experimental situations".
The research is published in Physical Review Letters.
About the author
Edwin Cartlidge is a science writer based in Rome