Encyclopedia of Renewable Energy. James G. Speight

Чтение книги онлайн.

СКАЧАТЬ the properties of the finished product. Coke-oven tar (or the solid pitch) has been, and continues to be, widely used as a binder in the production of briquettes from bituminous coal, although crude oil-derived products are also being used.

      The composition of bituminous coal tar and pitch is complex and consists of a wide variety of components. The coke-forming components of the pitch are of great importance for briquetting, and pitch from low-volatile coal (such as anthracite) has been preferred. These coals do not cake or melt while burning in the furnace; thus, the stability of the briquettes depends entirely on the skeleton of pitch coke formed in the briquette structure. Furthermore, the content of coke-forming components in the tar pitch increases as the rate of pitch distillation is decreased.

      Pitch viscosity also exerts a strong influence on the process as do the coal-binder interactions. During the briquetting process, the properties of the binder pitch are changed by heat, duration of the process, air, and steam. These factors can change pitch composition in the thin layer present in the heated briquetting matrix.

      Crude oil residua used in the briquetting of coals have often been given the collective name asphalt (bitumen in Europe and many other countries). The most important data used to characterize such asphalt is the softening point, the penetration, the Conradson carbon residue, and the plasticity range which indicate the thermoplastic character of the binder and content of coke-forming components.

      These two properties are considerably different in asphalt and coal-tar pitch. 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. 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.

      Propane asphalt (produced in a crude oil refinery) 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 adhere more efficiently. 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.

      Sulfite liquor is a by-product obtained during the treatment of wood with sulfites (calcium, sodium, magnesium, and ammonium) to produce cellulose. The liquor has significant adhesion properties thereby fulfilling one of the binder requirements for use as a briquette binder.

      The sulfite liquor and the sulfite pitch remaining after water removal are water soluble. Briquettes made with sulfite liquor as the binder can be rendered water-resistant (referred to as hardening the briquette) by heating to 220 to 350°C (430 to 660°F).

      Slightly sintering or lightly caking coals can be briquetted with calcium sulfite liquor or with magnesium or sodium sulfite liquor as a binder. Briquettes that are stable during firing can be made from anthracite coals only when using ammonium sulfite liquor. This low-ash binder produces a favorable coke skeleton in the briquette structure during combustion.

      Starch has a substantial adhesive power, and 1 to 3% w/w starch added to the coal, as a solid or in a suspension, can be used as a binder for the production of coal briquettes. Starch is used in the manufacture of charcoal briquettes, which are used in increasing amounts as barbeque briquettes.

See also: Briquette, Briquette Manufacture.

      Briquette Manufacture

      Briquette manufacture (also known as briquetting) is undergoing resurgence, principally due to the convergence of three critical factors. In fact, the recent developments in briquette processing and binding have dramatically changed the economics of using fuel briquettes as an energy resource. Also, a shortage of fuel wood has become increasingly severe in most of the developing countries. Finally, there has been a steady increase by environmental concerns to address the problem of domestic and urban waste disposal, a dilemma that briquetting can help remedy.

      Generally, briquette manufacture (briquetting) involves the collection of combustible materials that are not usable as such because of their low density, and compressing them into a solid fuel product of any convenient shape that can be burned like wood or charcoal. Thus, the material is compressed to form a product of higher bulk density, lower moisture content, and uniform size, shape, and material properties. Briquettes are easier to package and store, cheaper to transport, more convenient to use, and their burning characteristics are better than those of the original organic waste material.

      The raw material of a briquette must bind during compression; otherwise, when the briquette is removed from the mold, it will crumble. Improved cohesion can be obtained with a binder but also without, since under high temperature and pressure, some materials such as wood bind naturally. A binder must not cause smoke or gummy deposits, while the creation of excess dust must also be avoided. Two different sorts of binders may be employed. Combustible binders are prepared from natural or synthetic resins, animal manure or treated, dewatered sewage sludge. Non-combustible binders include clay, cement, and other adhesive minerals. Although combustible binders are preferable, non-combustible binders may be suitable if used in sufficiently low concentrations. For example, if organic waste is mixed with too much clay, the briquettes will not easily ignite or burn uniformly. Suitable binders include starch (5 to 10%) or molasses (15 to 25%), although their use can prove expensive. It is important to identify additional, inexpensive materials to serve as briquette binders and the optimum concentrations of the materials. The exact method of preparation depends upon the material being briquetted but, in general, the overall process can be presented as a step-wise operation: Thus:

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

      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 СКАЧАТЬ