2.1.2 Differences between gas and dust explosions 2.2 Dust explosibility parameters and assessment
2.2.1 Explosion characteristics 2.2.2 Flame propagation
2.2.3 Experimental measurement of explosion characteristics 2.2.4 Factors influencing dust explosion test results
2.3 Biomass and torrefied biomass explosibility 2.3.1 Biomass and torrefied biomass powders
2.3.2 Reactivity of biomass, torrefied biomass and coal 2.3.3 Difficulties in measuring biomass explosibility 2.3.4 Explosion characteristics of biomass
2.1. Dust explosions - General
2.1.1. DefinitionsA dust explosion is the rapid combustion of a finely divided combustible solid material with a subsequent increase in temperature and pressure. There is lack of agreement on an exact definition of how fine a material must be to be referred to as a “dust” as opposed to a “powder”, values often quoted in the standards are less than 500 μm [30]. Explosion characteristics are required to be measured for dusts of <60μm by the standards as these will provide the worst case scenarios [31]. However, these sizes are often artificial and not representative of the size distributions used in some industries.
2.1.2. Differences between gas and dust explosions
The main differences between gas and dust explosions are related to the heterogeneous character of dust explosions and the need of a dispersive medium to suspend the powder into the oxidising atmosphere (usually air) and to prevent particles from depositing, before ignition. Dispersion can also be the mechanism for gaseous fuels mixing with oxidants but they can also mix by diffusion and the
mixing is at a molecular level therefore can remain stable (and flammable) even in a quiescent mixture (unlike dust clouds). These differences and the lower frequency of occurrence of dust explosion incidents have resulted in more research in gas explosions and hence better understanding of gas rather than dust explosions. Fundamentally, gas and dust explosions have the same propagation mechanisms (deflagration or detonation). The damage produced is also similar and therefore safety systems use similar principles [32].
Other observed differences between gases and dusts are that when the reactivity of gases is investigated for different mixtures, the most reactive concentration is always found for mixtures slightly richer than stoichiometric. However, the peak reactivity for dusts occurs at much richer mixtures. This fact has been usually overlooked because concentrations of dusts are expressed as grams of dust per m3 of air and gases as the volume %. However, if concentrations are expressed as equivalence ratios (Ø), that is, as the ratio of actual to stoichiometric concentrations these differences become clear. In addition this method of expressing mixture concentrations allows direct comparison between fuels with different stoichiometry. Illustrative examples of gas peak reactivity are methane, propane, ethylene and hydrogen gases, for which their respective most reactive mixtures are found at equivalence ratios of 1.06, 1.13, 1.30, 1.60 [33]. However for dusts, most reactive mixtures are often found for very rich mixtures (Ø~2) [34]. Another difference between gases and dusts is related to the upper flammability limits. UFL of the previously listed gases are measurable at equivalence ratios of 1.7, 2.6, 5.8 and 7.2 [33] whereas for dusts, the reactivity decays very slowly and in many cases the standard methods cannot measure upper explosible limits.
Slatter et al. [35] postulated reasons for dusts having such rich upper limits in comparison to gases: in a closed vessel a fixed mass of air is available, therefore there is a fixed heat release of 3.68 MJ per m3 of air irrespective of the fuel. For rich mixtures of gases air is displaced and therefore the energy available to be released is lower in an equivalent gas system. This is illustrated in Figure 2-1 for methane, propane, ethylene and hydrogen gases and for two different types of dusts, a coal and a biomass dust. Another given explanation for the pressure to remain high at rich concentrations is that although the initial mixture pressure is 1 atm, this increases when the dust particles pyrolyse in the preheat zone of the flame. Very few other hypotheses have been published; however this matter should be taken into account as many processes, such as milling in pulverised fuel power plants, operate with very rich mixtures on the premise that such rich mixtures are not flammable.
Figure 2-1. Comparison of fixed heat release due to the mass of air available for gas and dust fuels
Hertzberg et al. [36] did recognise that measured explosion pressures did not parallel adiabatic predictions for pressures and flame temperatures at rich mixtures. Predictions indicated that pressures and flame temperatures decreased at rich mixtures (see Figure 2-2 and Figure 2-3).
Figure 2-2. Calculated adiabatic flame temperatures for constant volume and pressure combustion compared to measured temperature at constant volume [36]
The unparallel difference between predicted and experimental curves for rich mixtures could not be explained by nonadiabaticities in the system. They instead proposed that the aforementioned differences between predicted and experimental data were due to “limitations on the rate of devolatilisation”. Although rich mixtures generate more volatiles these are emitted too late to dilute the flame front with excess fuel vapour. As the fuel loading is very high it continues to reach high explosion pressures and temperatures. However, due to the presence of excess coal which did not contribute to flame propagation, heat from the flame front is absorbed slowly reducing flame temperatures and explosion pressures.
Figure 2-3. Calculated adiabatic explosion pressure ratio for constant volume compared to experimental result [36]