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Methods of Charge Reduction. There are a number of methods for reducing electrostatic charging in fluidized beds, including humidification of the fluidizing air, conductive particle coating, reduced gas flow, careful selection of vessel materials, the choice of more conductive particulate, and finally of course the addition of antistatic agents. As already mentioned, the application of humidified air to reduce electrification in fluidized beds dates from some of the earliest investigations (Osberg and Charlesworth, 1951). Many investigators have studied its practical usefulness (Katz, 1957; Davies and Robinson, 1960; Tardos and Pfeffer, 1980; Guardiola et al., 1996). In general, the conclusion is that relative humidities in the 60% to 70% range are required to reduce charge accumulation significantly. Even then, at least one report observed that the electrostatic effects do not correlate very well with measured specific charge values (Wolny and Kazmeirczak, 1989). There is also the problem that high humidities can increase the cohesive properties of some particles. Like humidification, conductive coatings on particles can reduce electrostatic charge by accelerating the rate at which the charge leaks away from the bed to ground (Katz, 1957; Boland and Geldart, 1972). Unfortunately, many coatings are not sufficiently robust to last very long in a vigorously churning, bubbling bed, especially at elevated temperature. The same problem exists with most of the modern antistatic agents, of which there are many types (anon., 1993). These materials, which work by making the particle surface attract and hold moisture, are sufficiently effective to improve flow characteristics of powders but the coatings cannot be expected to stay intact for long in a fluidized bed.

Another method to reduce the effects of charging is to add a small quantity of fines (conductive or nonconductive). For example, graphite, added to beds of glass beads in amounts of ~0.14% by weight, has been found to reduce electrostatic activity (Bafrnec and Beña, 1972). In similar tests, finely ground coal, titanium dioxide, pigment particles, and aluminum powder were introduced into beds of tribo-active polystyrene beads (Wolny and Opalinski, 1983; Wolny and Kazmeirczak, 1989). The mechanism by which the fines reduce tribocharging is believed to be that the smaller particles coat the larger particles and reduce the number of intimate contacts. It is also true that, if the fines are oppositely charged, then the coating will simply shield the particle charge and reduce the effective specific charge q/m. In one interesting experiment, it was shown that the addition of alumina fines to a particle bed of the same material reduces charging, apparently via the same mechanism of the fines coating the larger

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dust explosions occur only under very stringent conditions imposed upon concentration, oxygen content, and ignition source spatial and temporal characteristics.

4.1Basics of Suspended Solids Ignition

Preliminaries. The combustion of suspended dusts and powders is quite complex and only imperfectly understood. The complexity stems from both fundamental and practical considerations. On the fundamental side, the ignition of suspensions of finely divided solids is influenced by hard-to-quantify factors such as the time-varying concentration of solids, the chemical activity and morphology of the particulate, and the degree of confinement provided by the vessel. On the practical side, industrial conditions are seldom sufficiently well-controlled or characterized to justify application of existing theoretical models. For all the above reasons, this chapter can provide only a very abbreviated coverage of ignition basics. The reader is referred to other sources for in-depth treatment of dust and powder explosions (Bodurtha, 1980; Bartknecht, 1981; Bartknecht, 1987).

Fire Triangle. In the context of industrial fire and explosion safety, the universally recognized means to introduce the necessary conditions for fires and explosions is the fire triangle. The fire triangle, shown in Fig. 4, signifies the three essential requirements for a fire or explosion: (i) fuel, (ii) an oxidizing agent, and (iii) an ignition source. The absence of any one of these elements means that no fire or explosion can occur. In dust explosions, the dust is the fuel while the oxidant is almost always atmospheric oxygen. There are certain powder mixtures that can support potentially hazardous exothermic reactions—the thermite reaction in mixtures of cuprous oxide and aluminum powder is one—but these do not fit the conventional model of a dust deflagration and are not of interest here. In addition to electrostatic discharges, many other ignition sources must be acknowledged, including open flames, overheated bearings, and sparks caused by short circuits in electric power equipment. Of all these, ESD is simultaneously the least likely to cause an ignition and the most difficult to control, an irony due to the seemingly capricious nature of electrostatics.

The investigation of an industrial fire or explosion invariably starts with an effort to identify each element in the fire triangle. Quite often, the fuel is known and so either the oxidizer or the ignition source becomes the focus of the investigation. For example, in a blender used to mix a

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years, lower explosive limit data have been obtained for the dusts of many important powders and granular solids (National Fire Protection Association, 1986). Table 5, though only representative, shows that most dusts have a minimum explosion concentration in the range from ~30 to ~100 grams per cubic meter. For a cloud of 10 μm diameter polymer particles, this figure translates to ~10 particles per cubic millimeter. A useful rule of thumb helpful in interpreting this result is that a dust cloud in the explosive range will appear optically opaque. If visibility is heavily obscured by a cloud of suspended dust, then it is best to assume that the concentration is right for a dust explosion.

The particulate concentration levels within an operating fluidized bed will exceed the upper explosive limit so that an ignition starting below the surface of the bed is virtually ruled out. The concentration in the freeboard (above the bed) may be in the explosive range; however, the vigorous flow of air or gas through the bed will tend to quench any ignition before it really gets started. In manufacturing processes involving fluidization, the most serious powder explosion risk is probably going to be associated with filling or emptying operations, or when the bed is started up or shut down. In all these situations, there will exist a transient period when the dust concentration in the vessel—probably in the freeboard region—will be in the explosive range. Numerous occurrences of explosions during filling and emptying of storage silos and hoppers have been reported, and there is every reason to anticipate similar hazards in the filling and emptying of fluidized bed systems. Therefore, measures taken to avoid risks in hoppers and silos are appropriate for fluidized beds. These measures are discussed in Secs. 5.1 and 5.2.

Minimum Ignition Energy of Powders. With dust and oxygen concentrations in the correct range, there is still the requirement of a ignition source before a fire or explosion can occur. The more well-known ignition sources—open flames, overheated bearings, and electrical sparks caused by short-circuits—are examples where the available energy is adequate for igniting virtually any suspended dust. On the other hand, electrostatic sparks usually have a limited pool of available energy and, quite often, the electrostatic energy released in an electrostatic discharge is only just comparable to the ignition requirement of the dust. Given the uncertainty of the estimates for the parameters used, the assessment of ESD hazards is quite problematic. Usually, close attention is paid to the minimum ignition energy (MIE) of the suspended dust.

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Table 5. Representative ignition temperatures, minimum explosion concentration, and minimum ignition energy for selected dusts, from NFPA Fire Protection Handbook (National Fire Protection Association, 1986) and other sources.

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To measure the MIE of a powder, a sample of the material is placed in a vessel called a Hartmann cell, the powder is dispersed by a strong jet of air, and then a capacitive spark of well-characterized energy and time duration is initiated between two electrodes mounted inside the vessel. The spark energy is increased incrementally under ignition occurs. Despite standardized procedures established for these measurements (British Standards Institute, 1991; ISC, 1994), reliable measurement of the minimum ignition energy in dusts is notoriously difficult. There is of course the problem of dispersing the dust reproducibly and uniformly, but there also exists controversy about the nature of the igniting spark. Investigators have found that the wave shape and duration of the current pulse associated with the spark—controlled by the series impedance of the discharge circuit— influence the measured value of the MIE (Eckhoff, 1975; Field, 1982). It is now suspected that the true MIE values of many dusts may be lower by factors from two to five than the accepted data of twenty years ago (Taylor and Secker, 1994). The MIE data in Table 5 must be questioned for this reason. In fact, a much more conservative value of 5 mJ has now been adopted for the MIE of manufactured polymer powders.

Another important discovery is that the measured MIE of powders depends very strongly on particle size. For example, the measured values for polyethylene are 500 mJ and 10 mJ respectively for narrow size cuts centered at 100 μm and 30 μm. This result indicates how strongly fines can influence explosion hazards in powders. In fact, Britton argues that it is the surface-average diameter that provides the best correlation to measured MIE (Britton, 1992). Uncertainties about the accuracy of measured MIE values not withstanding, the data provided in Table 5 do reveal that the dusts of a wide range of commercial and industrial materials—from agricultural grains to powdered metals and polymer plastics—pose an undeniable explosion or fire hazard when dispersed in air.

4.2Types of Discharges

An electrostatic dust ignition can occur when the discharge releases sufficient thermal energy within a sufficiently short period of time and small volume to ignite the suspended dust. Electrostatic ignition is complicated by the fact that there are a number of distinct ESD mechanisms important in electrostatic hazards and hazard abatement (Glor, 1988).

Capacitive Discharges. Capacitive discharges are responsible for at least 90% of all dust and vapor ignitions of ESD origin. The

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Table 6. Representative Capacitance Values for Typical Components in a Plant or Commercial Facility Handling Powders (Eichel, 1967)

Two distinct conditions must be met in order for a capacitive spark to ignite a flammable dust. First, the voltage difference between the object and ground must be sufficiently high to promote a discharge. The sparking potential is a complex function of the capacitor’s geometry and the length of the gap across which the discharge must jump. At standard atmospheric conditions, the minimum sparking potential is Vmin ≈ 350 V and it is achieved at a gap spacing of 6 to 7 μm. According to Paschen’s law, the sparking potential is higher for gaps larger or smaller than this value (Cobine, 1958). Figure 6 shows the dependence of the sparking potential upon the product of pressure and gap spacing. In general, the following condition on voltage is required for a spark to occur

In ordinary practice, the rather conservative value of 100 volts is recommended for Vmin to provide a margin of safety (Gibson, 1979).

The second condition for an ignition is that the energy released in the spark Ue must exceed the minimum ignition energy (or MIE) of the dust.

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pressure times spacing, pd (torr-mm)

Figure 6. Sparking potential in air as a function of pd, the product of pressure and gap spacing. (Adapted from data of Cobine, 1958.)

Two other useful parameters related to capacitive sparks and dust ignitions are (i) the optimum sparking distance is ~10 mm and (ii) the quenching distance is ~7 mm. Refer to Sec. 4.1 and Table 5, where MIE is discussed in more detail.

In a specific situation, determination of the voltage V for use in Eqs. (10) and (11) is based on having reliable estimates for charging current and leakage resistance (cf. Sec. II.B of Jones and King, 1991). Figure 7 contains a convenient nomograph for assessment of capacitive discharge ignition risks. With estimates for the capacitance and voltage of an object, the value of Ue is obtained by drawing a straight line between the points on these two scales and then reading the intercept with the middle scale. For convenience, the MIE values of some important powders are indicated on this scale, including the generally accepted value of 5 mJ for polymer powders. Note that, in general, MIE values for vapors are much lower.