Briquette
A briquette (or briquet) is a block of carbonaceous smokeless fuel (flammable matter) which is used as fuel to start and maintain a fire. Common types of briquettes are charcoal briquettes and biomass briquettes. Briquettes are used for both domestic and industrial use, prepared from bituminous coal using a binder. Briquette production is a mature technology, and the technology associated with coal briquetting has been developed for almost 100 years.
Historically, briquettes (especially coal and coke briquettes) have been used for fuel for approximately 100 years. Traditionally, briquetting technology was established for developing countries to produce briquettes of local residues, for use in household cooking stoves and restaurants. Later, as the capacities of the machines increased, briquettes were used in industrial boilers to create heat, steam, and power for industry and power plants. Within the past three decades, briquetting has also found its way to households in industrialized countries as consumer logs for wood-burning stoves and fireplaces. However, with the advent of modern fuel systems, the use of briquettes has declined and use for these products is found most often in the operation of barbecue units. While still a marketable product for such use, briquettes as fuel (such as smokeless fuel) are not used often for domestic and industrial heating. In more recent years, as the focus on renewable energy has grown, the applications for briquettes have grown concurrently, as have different technologies and new applications.
The briquette has to be ignited from the bottom by burning wood. During ignition, the upper part of briquette, upon heating, emits volatile matter which is a mixture of carbon and hydrogen compounds and is a strong pollutant. The newly developed briquettes can be ignited from the top just by striking a match, and the volatile matter will be burnt away when it has passed the upper flame. The briquette should be coated with a thin layer of combustibles and oxidizing agent and, therefore, should be kept in a dry place lest moisture retards ignition.
See also: Briquette Binder, Briquette Manufacture.
Briquette Binder
The binder used for coal briquetting has considerable influence of 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