Elsevier

Powder Technology

Volume 157, Issues 1–3, 29 September 2005, Pages 144-148
Powder Technology

Influence of the size distribution and concentration on wood dust explosion: Experiments and reaction modelling

https://doi.org/10.1016/j.powtec.2005.05.021Get rights and content

Abstract

The explosion ability of wood dust was characterized by a 20 L explosion sphere (Kühner). The overpressure inside the sphere is recorded during the explosion. The results show that the violence of the explosion is all the more important that the particle size is low. A model based on balances on chemical reaction, kinetics and thermodynamics leads to the representation of the pressure change during the explosion. There is a good agreement between the calculations and the experiments.

Introduction

A lot of combustible powders can cause explosion when they are mixed with an oxidant like O2 and if the mixture is ignited with an energy source (electrical discharge for example). We can find these powders in the food-processing industry (flour, sugar, etc.), in the mechanical engineering industries (Aluminium, etc.) and in the chemical engineering industries (fertilizer, plastic, pulverulent waste, etc.).

One explosion per day occurs on average in France [1]. The reasons are the increase of storage capacity and of handling flows of powders which present lower and lower sizes [2]. The hazard is then linked to the transport and the storage of pulverulent materials.

Only the knowledge of the parameters which quantify the explosion ability of powders and the control of their environment can help the risk prevention.

Cashdollar [3] said that the form and the size of powders have a strong influence on their explosion. As a matter of fact these physical parameters are linked to the particles volumic surface which plays a role in the combustion kinetics. Moreover large size particles (> 500 μm) do not significantly participate in the flame propagation because of the sedimentation. The fine fraction of the size distribution controls then the explosion due to its large volumic surface and its ability to stay in suspension. These conclusions were supported with experimental results on aluminium and coal powders [4]. In each case maximum overpressure and maximum rate of pressure rise decrease when the particle size increases. This result was also observed by Soundararajan and al. [5] on ferrous materials.

Authors also present results of explosion tests of mixed powders of different sizes which lead to the inhibition of the explosion mechanism. One condition is that one of the two powders is inert and the other is combustible [6], [7]. These works show that the lower the inert particle size is, the lower is the amount of these particles necessary to prevent the explosion.

Finally Sweiss and Sinclair [8] measured the maximum admissible O2 concentration to prevent dust explosion as a function of particle size. They obtained the following result: the lower the particle size, the lower the maximum admissible O2 concentration.

The aim of the present work is first to characterize experimentally the explosion of wood dust of different sizes. Secondly a descriptive model for the increase of the pressure during the explosion is presented. The calculated values are finally compared with the experiments. We will focus on the effects of powder concentration and size in the model and then on the explosion mechanism.

Section snippets

Physical properties of wood dust

Tested wood dusts are produced by sanding a mixture of beech and oak.

Particle density equals 1600 kg/m3.

The large size distribution of the wood dust allows the preparation of four size fractions by sifting (Alpine). The mean volumic diameter was determined for each fraction by Coulter counter. The values obtained are presented in Table 1.

The 20 L explosion sphere

The 20 L explosion sphere was initially developped by Siwek [9] in order to characterize the ability of powders to explode. The parameters measured are the

Results

Fig. 3 shows the pressure evolution inside the sphere during the 25–45 μm particles explosion. The amount of powder which reacts varies between 2.6 and 40.9 g.

Tests show that a maximum overpressure and a maximum rate of pressure rise exist as a function of dust concentration. This concentration corresponds to a mass of 14.6 g in the 20 L of the sphere. The maximum values obtained are Pmax = 7.7 bars (± 0.5 bars) and (dP / dt)max = 500 bars s 1 50 bars s 1). The Kst criterium can be calculated by the

Modelling of the pressure increase during the wood powder explosion

The aim of this modelling is to combine the effects of the exothermic combustion reaction and the containment on the pressure rise in the sphere.

Conclusion

This work presents first results of wood dust explosion of different sizes obtained with the 20 L sphere. Results of the literature were confirmed with a decrease of the violence of the explosion with the increase of the particle size. The way to model the evolution of the pressure in the sphere which is proposed in this paper is satisfactory but the effect of powder mass is not taken into account well. The values obtained for Pmax and (dP / dt)max are close to the experimental ones in spite of

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