Briquette Properties

      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.

      Briquetting Processes

      The processes available for the production of briquettes are many and, often, not too varied. The production of coal briquettes is closely related to the carbonization process by which fuel-grade coke is produced. In fact, briquettes are manufactured from coals that are normally considered to be of insufficient quality to produce coke. A non-caking coal having a low content of volatile matter might be briquetted with a binder at 750 to 800°C (1,380 to 1,470°F).

      A recent emphasis on the use of coal is the production of added-value products using a variety of processes by which the total coal can be used with little wastage of the products. Such is the character of the mild gasification process and a variety of other clean coal processes which have brought related to renewed interest in briquetting and pelletization of coal.

      The mild gasification process is a modification of the conventional coal gasification and produces gaseous, liquid, and solid products by heating coal in an oxygen-free reactor. The process is less of a gasification process and more of a pyrolysis process insofar as volatile matter is removed by thermal means to leave a carbonaceous char/residue. The char can be upgraded further to remove both ash and pyritic sulfur, mixed back with coal-derived liquids, and burned in both coal- and oil-fired boilers.

      The process is of interest since a slurry of the liquid fuel and the upgraded char has the potential of being a very versatile fuel that can be burned in both coal- and oil-fired boilers. If the char is upgraded to a high degree, even feedstock coal with a high sulfur content can be used without alternating heat rates or capacity factors.

      The production of briquettes (and/or pellets) from coal is a thermal decomposition process, akin to the carbonization process.

      Carbonization is essentially a thermal treatment process for the production of a carbonaceous residue (with the simultaneous removal of distillate) from a variety of organic substances:

The process may also be referred to as destructive distillation and has been applied to a range of organic materials, but more particularly to naturally-occurring materials such as wood, sugar, and vegetable matter. The carbonaceous residue from the thermal decomposition of coal is usually referred to as coke (which is physically dissimilar from charcoal) and has the more familiar honeycomb-type structure.

      The thermal decomposition of the carbonaceous feedstock is a complex sequence of events which can be described in terms of several important physicochemical changes, such as the tendency of the feedstock to soften and flow when heated or the relationship to carbon-type in the feedstock. In fact, some feedstocks become quite fluid at temperatures of the order of 400 to 500°C (750 to 930°F) and there is a considerable variation in the degree of maximum plasticity, the temperature of maximum plasticity, as well as the plasticity temperature range for some feedstocks. Significant changes also occur in the structure of the char during the various stages of devolatilization.

      A briquetting plant typically consists of the mixer-dryer with drive, the fan, and the gas ducts. From the mixing chamber, flue gas at 300 to 500°C (570 to 930°F) passes through the coal-binder mixture, suction being provided by the fan. After cooling, the material enters roll presses from the mixer-dryer.

      Water, among other factors, influences the wetting and adhesion between coal and bituminous binder and on the strength of the resultant briquette. Indeed, interfacial interactions between the coal and the binder are extremely important in determining the strength of the briquette. The moisture content of the feedstock, before the addition of the bituminous binder, should be as low as possible, preferably not exceeding 4% w/w.

      See also: Briquette, Briquette Binder, Briquette Manufacture, Briquette Properties.

      Bromine Number

      The bromine number is used as a method of determining the unsaturation in typical hydrocarbon molecules.

      The bromine number is the number of grams of bromine absorbed by 100 g of oil which indicates the percentage of double bonds in the material. The number is an indicator of the unsaturated constituents in a liquid fuel. In the test method, a known weight of the

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