This work investigates the explosion and dispersion potential of engineered nanoparticles (ENP). The European Union (EU) sponsored this investigation, firstly to predict or estimate risks posed by the use of engineered nanomaterials (ENM), and secondly to implement procedures for the purpose of risk mitigation. These include establishing exposure control limits and controlling and monitoring exposure, including the accidental explosive or massive release of ENP into the environment.
To this end, the release of ENP originating from specific nanopowders was simulated in a 31 m3 airtight chamber of controllable environment. Their loss and dispersion characteristics were studied under ventilated and unventilated conditions. The explosion characteristics of specific ENP in lean hybrid blends of nanoparticles with methane and air, were studied in a 23 L cylindrical combustion vessel providing the adjustment of isotropic turbulence induced by specially designed fans. The influence of ENP on the explosion severity was evaluated by comparing the results obtained for pure methane explosions. Via a 6-jet Collison nebuliser (CN) combined with a considerably modified preparatory process of the tested nanopowders suspensions in water, ENP of Titanium and Silica Dioxide (TiO2 and SiO2) were injected continuously into the dispersion chamber. A specially designed dust injector was used for the introduction of two types of carbon black (CB) nanopowders (Corax N550 and Printex XE2) into the combustion vessel. An arrangement of particulate instrumentation was applied for tracking the evolution of particle number concentration (PNC) and particle size distribution (PSD) at points near to the source within the dispersion chamber. In addition, PSD measurements were conducted in the dust clouds generated for the explosion tests. Using a Log10-normal modal fitting program, the characteristics of groups present within the PSDs, were mathematically described. An indoor aerosol model for the study of the differential effect of coagulation and deposition on the changes of PNC with time in the dispersion chamber, was applied. Finally, the explosion severity was characterised by measurements of the explosion pressure history and of flame speed derived from high speed Schlieren cine photographs.
Results indicated that by reducing the ventilation rate the leftover PNC of ultrafine particles (diameter ≤ 100 nm) was gradually increased at the end of the evacuation process. In parallel, at the high ventilation rates the spatial ventilation efficiency was shown to be optimal close to the inlet diffuser. However, by decreasing the ventilation rate, ventilation efficiency was shown to be independent of the location in the chamber. The study of particle interactions under unventilated conditions indicated that different growth rates due to agglomeration were induced on the two types of dispersed ENP. For fine particles (diameter > 100 nm) of both materials, the model indicated that their losses were dominated by deposition at high PNC, whereas for ultrafine particles, heterogeneous coagulation was the main removal mechanism. However, the model indicated stronger deposition at low PNC, and weaker homogeneous coagulation at high PNC, for ultrafine and fine SiO2 particles respectively, compared to their TiO2 counterparts. The explosion tests indicated that the addition of variable concentrations of ENP in methane resulted in higher burning rates and flame acceleration than those demonstrated by the respective particle-free methane air mixtures. In addition, the mixture of the highest fraction in ultrafine ENP yielded the most severe explosion, while this mixture was of the lowest dust concentration. Finally, hybrid mixtures with methane below its lower flammability limit (LFL) were shown to be ignitable. Furthermore, the level of this extension below LFL was shown to be dependent to the material as different extensions were performed by Corax N550 and Printex XE2 hybrid mixtures.
The investigation recommends that in order to design efficient ventilation systems for nanotechnology workplace, only specific ventilation rates and arrangements of inlet/outlet diffusers, should be considered. Exposure to accidentally released ENP is expected to be different for different materials and strongly related to their emission profile. Finally, the generation of a dust cloud from a minor amount of a nanopowder combined with a low amount of a flammable gas and an electrostatic spark may result in a severe explosion of higher impacts for human health and installations than those induced by the explosion of a higher dust concentration hybrid mixture. Also this work demonstrated that as the mean particle size in the dust cloud decreases, a hybrid mixture of an extremely low content of gas could become ignitable. The latter could be applicable not only in the field relating to the risk assessment of ENP but also in generic technological applications involving airborne nanoparticles (e.g. soot particles) suspended in flammable gases (e.g. automotive applications that use natural gas).
