Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 256

The shape and the size of briquettes depend essentially on their application. Briquettes for industrial use are usually cubes or bricks weighing 1 to 4 lbs. The briquettes manufactured for domestic use must be small enough to be loaded conveniently through the appliance filling aperture but not small enough to drop through the grate bars or to give high resistance to air flow, even in deep beds.

There are few, if any, recognized specifications or standard tests available for measuring the mechanical properties of bituminous coal briquettes. The properties currently measured are the breaking strengths and the abrasion or attrition of the briquettes. The attrition testing of briquettes by means of the tumbler tests permits the comparison of briquettes that have the same weight but different shapes (breaking or crushing strengths of briquettes that vary in shape cannot be compared). The test involves placing the briquette between two parallel plates and loading one plate until the briquette breaks. The larger the surface of contact between the testing plate and the briquette, the larger the point compressive strength.

The thermal post-treatment of the briquettes at different temperatures, with or without the influence of oxygen in the air, leads to briquettes which (depending on the feed material, the treatment method, and the treatment temperature) have properties more or less similar to those of coke. The briquettes undergo carbonization in one of several steps. The thermal post-treatment depends on the type of briquettes being treated and the desired properties of the final product. Heating rate and residence time in the hot zone are the deciding factors; however, the composition properties of the raw briquettes also influence the processing steps and the nature of the final product.

If the binder pitch softens, the briquettes may be deformed, or glued together as a result of the exuding of binder from the briquette surface. Such difficulties are avoided by an oxidative hardening pretreatment or by maintaining the briquettes in a quiescent state during the temperature interval used for binder softening.

The most important data used to characterize briquette binders are the softening point, the penetration, the carbon residue, and the plasticity range. These data indicate the thermoplastic behavior of the binder and content of coke-forming components. These two properties are considerably different in bitumens and coal-tar pitches. For example, crude oil-derived materials have a lower content of coke-forming components than coal-tar pitch, and this may be a disadvantage when crude oil-derived materials are used as binders for noncaking coal briquettes, especially of anthracites. Use of crude oil-derived materials having a lower-than-desirable propensity for coke formation, as indicated by a low Conradson carbon residue, produces briquettes that have a low stability during firing.

The use of propane asphalt as a briquette binder has also been investigated; propane asphalt is the propane-insoluble portion of crude oil residua. The asphalt has a lower penetration value than asphalts obtained by distillation or by oxidation. Propane asphalt has a relatively high temperature sensitivity which may cause the briquettes to stick together. It is possible to alter the temperature sensitivity of the propane asphalt by the conventional methods of treatment which are used to alter asphalt properties for highway use.

See also: Briquette, Briquette Binder, Briquette Manufacture, Briquetting Processes.