Dust/gas mixtures explosion regimes

The results presented in this article focus on the importance of the prevention and mitigation of dust and hybrid mixture explosions. The main objective is to demonstrate the use of the inherent safety principle of moderation to achieve a significant reduction of the risk of explosions. Experiments and a companion modeling study were conducted with a test matrix composed of various size fractions of polyethylene powder together with concentrations of hydrocarbon gas (ethylene, hexane, and propane). The results quantitatively show the increased hazard posed by fine particle sizes of dust and the addition of flammable gases. There are clear implications for industry in terms of moderating the risk of an explosion.

Gas concentrations used in this work were all less than the lower flammability limit (LFL) of the particular chemical species and the ratio of gas concentration to LFL concentration was at least 75%. The enhancement of mixture reactivity brought about by a flammable gas admixture could therefore be correlated with the burning velocity of the gas.

This article describes how to predict KSt for hybrid mixtures and includes the concept of using propane as a surrogate for hexane. Additionally, it shows that the avoidance of both fine dust sizes and hybrid mixtures is a beneficial approach in the process industries to reduce the risk arising from the hazards posed by combustible dusts and their mixtures with flammable gases. © 2009 American Institute of Chemical Engineers Process Saf Prog, 2010

Prevention and mitigation of dust and hybrid mixture explosions

The explosion behaviour of heterogeneous/homogeneous fuel–air (hybrid) mixtures is here analysed and compared to the explosion features of heterogeneous fuel–air and homogeneous fuel–air mixtures separately. Experiments are performed to measure the pressure history, deflagration index and flammability limits of nicotinic acid/acetone–air mixtures in a standard 20 L Siwek bomb adapted to vapour–air mixtures. Literature data are also used for comparison. The explosion tests performed on gas–air mixtures in the same conditions as explosion tests of dust–air mixtures, show that the increase in explosion severity of dust/gas–air mixtures has to be addressed to the role of initial level of turbulence prior to ignition. At a fixed value of the equivalence ratio, by substituting the dust to the flammable gas in a dust/gas–air mixture the explosion severity decreases. Furthermore, the most severe conditions of dust–gas/air mixtures is found during explosion of gas–air mixture at stoichiometric concentration.

Study of the severity of hybrid mixture explosions and comparison to pure dust–air and vapour–air explosions

Explosion severity of methane-coal dust hybrid mixtures in a ducted spherical vessel

Ignition sensitivity of gas–vapor/dust hybrid mixtures

To evaluate the hazard of combined hydrogen/dust explosions under severe accident conditions in International Thermonuclear Experimental Reactor (ITER), standard method of 20-L-sphere was used to measure the explosion indices of 4-μm fine graphite dust in lean hydrogen/air mixtures The mixtures were ignited by a weak electric spark The tested fuel concentrations were 8–18 vol% H2 and 25–250 g/m3 dust If the hydrogen content is higher than 10 vol%, the dust constituent can be induced to explode by the hydrogen explosion initiated by a weak electric spark Depending on the fuel component concentrations, the explosions proceed in either one or two stages In two-stage explosions occurring at low hydrogen and dust concentrations, the mixture ignition initiates first a fast hydrogen explosion followed by a slower phase of the dust explosion With increasing dust concentration, the dust explodes faster and can overlap the hydrogen-explosion stage At higher hydrogen concentrations, the hybrid mixtures explode in one stage, with hydrogen and dust reacting at the same time scale Maximum overpressures of hybrid explosions are higher than those observed with hydrogen alone; maximum rates of pressure rise are lower in two-phase explosions and, generally, higher in one-stage explosions, than those characteristic of the corresponding H2/air mixtures

A. Denkevits

Papers

Explosibility of hydrogen–graphite dust hybrid mixtures