Monday, June 19, 2017

Why Dust Makes an Excellent Fuel

There are a lot of factors that affect how quickly a fuel oxidizes, i.e. burns. In the following discussion we will assume everything about the reaction is constant: the reaction rate, the fuel is homogeneous, moisture content is the same, etc. These simplifications will help keep things simple and allow us to concentrate on the major issue dust presents: incredibly large surface areas.

The burning reaction essentially takes place on the surface of the fuel. As more surface is exposed to the oxidizing environment the reaction takes place over a larger area, more material is consumed in a shorter period of time and the burning rate increases. As an example, assume we have a perfectly spherical fuel that has a diameter of 1 (of whatever unit you choose). The surface area of that sphere will be 3.14159 square units of area.

Now assume that we have the same amount of fuel but this time it is made up into four spheres, i.e. each sphere is ¼ the volume of the original sphere. The surface area of the four spheres is 4.99 square units, or an increase of approximately 58%. If the fuel is dispersed with the surrounding air the total surface area exposed to air is 58% more than with the original fuel and there is 58% more reaction locations. This increase in reaction locations has the effect of increasing the reaction rate.

Now assume that we have the same amount of fuel made up into 25 identical spheres. The new surface area is 9.19 square units. This is almost three times (2.92) the original area and will allow the reaction rate to be approximately three times the original reaction rate.

Now assume that the same amount of fuel is divided into 10,000 spherical particles. In this case the original surface area increases approximately 232 times! When these particles are dispersed in the air, all 232 times the surface area is available to the burning process.

This exercise helps to demonstrate why a combustible material which is in the form of dust form is more energetic and presents a danger: the surface area available for the oxidization reaction is much greater than found with a large chunk of fuel.

This greater surface areas isn't of great concern if the dust is in a pile. The particles pack fairly tightly and reduce the amount of surface area exposed to oxygen. The problems arise when the dust particles are suspended in a cloud. These dust clouds can easily be formed from normal operations.It is the responsibility of the owner/operator of a facility to identify the locations where dust clouds are likely to form and to implement the engineering controls and administrative controls to prevent these clouds from forming. This identification of risks and how they will be managed form the basis for the Dust Hazard Analysis (DHA).

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