Safety data figures of combustible dust - Correlation to heat of combustion and particle surface

Status:

completed 02/2016

Aims:

In the area of explosion protection, flammable dusts are characterized by safety parameters, and the necessary protective measures to be taken in plants planned in consideration of these parameters. The safety parameters of the maximum explosion pressure (Pmax), rate of pressure rise (dp/dt) and lower explosive limit (LEL) are determined in closed laboratory pressure vessels (of 0.02 m³ and 1 m³). The sample quantities required for these analyses are in the order of a few kilogrammes. The values determined are not intrinsic material properties, and vary according to the selected test apparatus. The estimated laboratory time per analysis, including cleaning and maintenance, is approximately 1.5 days. The combustion reaction within the vessels forms a range of reaction products that may be harmful to health.

The calorific value (enthalpy of combustion) of a dust and its surface area are key variables that have been shown to influence the safety parameters. Initial tests in this area were first performed in the 1970s by the German Federal Institute for Materials Research and Testing (BAM). The calorific value and the particle surface area can be determined in approximately 0.5 hours with 1 g of substance. The calorific value and surface area are also intrinsic substance properties and are constant irrespective of the test apparatus employed. The aim of the project was to demonstrate if possible a correlation between the safety parameters on the one hand and the calorific value and surface area of the dusts on the other. This would enable the scope and duration of analysis to be substantially reduced.

Activities/Methods:

Analysis of the approximately 6,000 substance data records in the GESTIS-STAUB-Ex database for empirical relationships between the Kst, Pmax, LEL and median, and comparison with data from the literature.

In addition to the parameters Pmax, Kst and LEL, the surface area of the dust (according to Brunnauer, Emmett and Teller/BET) and the calorific value were determined at the beginning of the project in the IFA's explosion protection laboratory. Approximately 120 substances were analysed in this way over the duration of the project. The data records formed the basis for statistical analyses and searches for possible correlations. In addition, five selected raw substances were divided into fractions of differing total surface area by screening. By systematic analysis and comparison with the determined values of the safety parameters, it was possible for the influence of the surface area and calorific value to be determined more precisely. Analysis of the combustion gases before and after the explosion also enabled the degree of conversion to be determined.

Results:

The relationship between the surface area and the safety parameters, particularly the rate of pressure rise (Kst), was not demonstrated clearly. The studies showed clearly that the explosion reaction was determined not by the measured BET surface area with pores and roughness, but more by the outer surface of the dust particles directly accessible to the atmospheric oxygen. The effect of the heat of combustion upon the safety parameters is influenced by the oxygen concentration during the explosion reaction. The latter is decisive for the speed of the reaction and thus for the Kst value. Bound oxygen leads to higher conversion rates and thus modifies the correlation between the heat of combustion and the safety parameters. A correlation between the heat of combustion and surface area on the one hand and the safety parameters for explosion protection on the other is evident, but is not sufficient for the safety parameters to be calculated with adequate validity by means of a correlation function. Intrinsic substance properties such as chemical composition, melting and boiling points and density must also be considered, in addition to the properties of the apparatus, such as volume, turbulence and ignition energy.

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