Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 264

The bulk density is the mass of an assembly of coal particles in a container divided by the volume of the container, assuming that the container is full and represents the volume occupied by the solid. The bulk density depends on true density, particle size and size distribution, particle shape, surface moisture, and degree of compaction. The parameter is often used in the design of coal handling, transportation, and storage systems.

Bulk density is a property of powders, granules, and other “divided” solids, especially used in reference to coal and similar solids. The total volume includes particle volume, inter-particle void volume, and internal pore volume.

The bulk density is not an intrinsic property of a material; it can change depending on how the material is handled. For example, powdered coal poured into a cylinder will have a particular bulk density, but if the cylinder is disturbed, the coal particles will move and usually settle closer together, resulting in a higher bulk density. For this reason, the bulk density of powders is usually reported both as freely settled and tapped density – the tapped density refers to the bulk density of the coal powder after a specified compaction process, usually involving vibration of the container.

The test method (ASTM D29l) for determining bulk density concerns the compaction of crushed coal to determine either its compacted or uncompacted weight, for purposes such as charging coke ovens. In addition to the character of the coal itself, moisture content and size distribution of the coal are the two main factors that affect the cubic foot weight. A moisture determination and sieve analysis of the coal should be reported along with the cubic foot weight for proper interpretation of the cubic foot weight. During the period of collecting the gross sample, the increments of the sample shall be stored in a waterproof container with a tightly fitting cover in order to prevent the loss of moisture.

The bulk density differs widely between different types of biomass. Together with the heating value, it determines the energy density of the gasifier feedstock, i.e., the potential energy available per unit volume of the feedstock. Biomass of low bulk density is expensive to handle, transport, and store. Apart from handling and storing behavior, the bulk density is important for the performance of the biomass as a fuel inside the reactor: a high void space tends to result in channeling, bridging, incomplete conversion and a decrease in the capacity of the gasifier. The bulk density varies widely (100 to 1,000 kg/m3) between different biomass feedstocks not only because of the character of the biomass but also as a result of the way the biomass comes available (chips, loose, baled).