Influence of Coal Type

Coal briquettes can be a very dangerous fuel if they produce carbon monoxide while burning in a closed room but in normal combustion these conditions will not appear. If a chimney is provided to extract the smoke, briquettes are 100% safe even if the stove is installed in a bedroom. 

If the coal used for briquette is rich in volatile matter, the coal has to be carbonized first. Carbonized briquettes are superior to uncarbonized briquettes for the same reasons that charcoal is preferred to fuel wood. The sulfur content is also reduced during carbonization as the sulfur is converted to hydrogen sulfide gas. 

Feed materials for briquettes include coals of different properties pretreated coals, low-temperature coke, high-temperature coke breeze, or mixtures of these. Various binders are used in manufacturing the briquettes. The binder may be obtained partly during the pretieatment of the coal or during the carbonization of the briquette. The thermal posttreatment of briquettes at different temperatures, with or without the influence of oxygen in the air, leads to briquettes that (depending on the feed material, the treatment method, and the treatment temperature) have properties more or less similar to those of coke. 

If anthracite or low-volatile coals are used as feed materials, the comminuted coal usually mixed with an additional binder such as pitch or sulfite liquor. The briquettes undergo carbonization in one of several steps. The thermal posttreatment 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. 

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