Coal Dust: A Superior Choice As A Binder For Charcoal Briquettes
Charcoal briquettes are the economical choice when it comes to domestic solid fuel - and are desirable from a manufacturing point of view too. The use of organic binders such as coal dust can make viable fuels from smaller and lower quality pieces of wood and biomass, compared to the hardwood required for lumpwood charcoal, with little to no difference in performance.
Charcoal briquettes are a popular source of domestic fuel, primarily fulfilling a heating or cooking function. Their high energy density, low weight and low cost mean they are a sensible choice for when a solid fuel is required in the domestic and light industrial setting. Charcoal has been around for many hundreds - if not thousands - of years, being the sole carbon source used to reduce ores in pre-industrial times(1).
Charcoal does not contain coal, despite its name. It is a residue formed when wood is heated to high temperatures in an oxygen deficient atmosphere, removing virtually all of the water and volatile organic compounds present in the process. Lump charcoal (lumpwood) is that which has been produced from lumps of hardwood, whereas briquettes of charcoal are produced using finer charcoal particles. These particles are formed and held together with a binder.
Binders come in a variety of types but, as a general rule, organic binders are preferred. This is due to the increased ash content required if an inorganic binder is used - this means that the briquette will have a lower heating capacity than lumpwood charcoal or organic binded briquettes(2). Anthracite - coal dust - is a binder that is becoming more popular in use.
One of the advantages to briquetting is that it can use lower quality wood sources (i.e. not large pieces of expensive hardwood), meaning that more charcoal can be produced per unit of felled tree area. The core concept behind charcoal is the idea of carbonisation. That is, where organic material (wood) is converted to carbon (charcoal) through destructive distillation (in this case, pyrolysis). During the carbonisation process of wood, other materials are produced, including tar, terpenes and ‘bio oil’. Many of these can also be used as binders in later charcoal making processes(3).
The foundry industry is a carbon emission intensive one. It has been proposed by some that charcoal may be a viable alternative to foundry coke as a fuel source in an effort to reduce emissions(4). Part of the problem is that - compared to coke - virtually all types of charcoal have lower energy densities and relatively high reactivities(5) and therefore cannot be used in the highest temperature furnaces, for example cupolas. Notwithstanding this, research has shown that replacing just some of the coke with anthracite-containing charcoal, the ‘carbon charge’ cost to the cupola was reduced by 6%(6). The ‘carbon charge’ refers to the overall emissions of greenhouse gases per cycle of the furnace. Other viable use cases - especially for the higher temperature furnaces - include those where charcoal is used to heat the blast air going to the furnace, saving coke(7). This process also increased efficiency from 43 to 62%.
The Process Of Pelletisation And Briquetting
In addition to producing a product that is more easily handled and packaged, briquetting increases the bulk density of the material and makes the particle size distribution within much more consistent. Transport and storage costs can be more easily minimised in the briquetted format. Briquetting is a type of pressure compaction, where the material to be formed is allowed to agglomerate and then is pressure treated into the desired density, when mechanical separation of briquettes by size may occur.
As mentioned, briquetting processes typically require a binder for all but the highest purity starting materials, which can take many forms, but the most popular are pyrolysed starchy plants or powdered coal dust(8). The briquetting process when used with a binder is attractive from a safety perspective, too, as fine particles which may be susceptible to explosion are reduced. The manipulation of briquetting process, final particle size and choice of binder can lead to a well tailored charcoal for a specific application, if required. One major advantage in using a binder to produce briquettes is that lower pressure compaction is required. If no binder is used, the strength of the compaction process relies largely on valence or Van der Waals’ forces, or interlocking. The use of a binder adds much more solidity, with highly viscous binders forming adsorption layers under pressure - a great improvement over any strength that could be provided through something like Van der Waals’ alone.
Briquetting typically proceeds via either a traditional press or with a screw extruder(9). Screw extruders are responsible for producing briquettes with a higher degree of homogeneity. They produce a denser briquette but are less tolerant of high moisture content. Both are examples of pressure applications, with internal pressures often reaching around 150 MPa. The application of pressure is crucial in the formation of a viable briquette - regardless of whether a binder is used or not(10). Pelletisation is another method, but this produces a smaller fuel than what would be considered conventional charcoal.
Depending on when the binder is added, it may have an effect on carbonisation. If charcoal is made from already carbonised wood, the binder serves only as a binder. Conversely, if added before carbonisation, an effect may be rendered. Organic binders added to non-coking charcoal prior to carbonisation and strength increases were noted(11).
Coal Dust As A Binder
Coal dust - often and interchangeably known as anthracite - is a material produced by the fine milling of anthracite coal. Anthracite is regarded as the superior form of coal. Coal dust also proves its mettle as a binder in other scenarios.
As binders are added prior to compaction and are responsible for particle agglomeration, it is critical that several factors are considered and an appropriate grading of the coal dust is reached:
- Particle size must be uniform, ensuring a consistency of binding
- Particle size must not be too large, risking flowability into the compaction area
- Material hardness must be appropriate, otherwise agglomeration will be hindered
Thankfully, anthracite/coal dust is easily manipulated and can be readily formed into a highly appropriate binder.
Research and patent literature show that when charcoal briquettes are produced using an organic binder such as anthracite, the binder is present in quantities of between approximately 2 and 8% by weight(12,13), meaning that typically in excess of 90% of the mass of the charcoal is derived from the original wood product.
Anthracite binders in charcoal also add functionality in the form of a heat source - anthracite is in itself combustible, and is regarded as an excellent fuel source(14). This phenomenon is also observed in briquetting when other carbon rich binders, such as lignin, are used(15).
Coal-based binders have been shown to outperform other organic and virtually all inorganic binders in briquetting in terms of compressive strength(16). The interaction of carbonaceous binder and original fuel material is complex, relying on an intricate network of hydrogen bonding and Van der Waals forces; in addition to the physical compressive forces applied onto it by the manufacturing process(17). However, the strength increase is largely explained by the differing wetting properties of coal relative to charcoal; with increasing carbon content (i.e. carbon rich coals such as anthracite), the contact angle of the charcoal and binder decreases, thereby causing the energy of adhesion to increase(18) - therefore increasing strength.
It is widely regarded that anthracite is an easy material with which to work, owing to its free flowing nature when in powdered form, and it is this which makes it even more suited to use as a binder. Reports within patent literature describe the use of more tarry or viscous carbon binders as relatively difficult, especially at moderately high atmospheric temperature(19). This could potentially be a problem for manufacture of charcoal in some warmer climates throughout the developing world.
The Impact Of Water
As with many considerations in the foundry and fuels production environment, the control of moisture is paramount. A wet fuel is a bad fuel. The addition of carbon rich materials such as anthracite can modulate the relative hydrophilicity of a fuel - this is important in terms of storage. It has been shown that when anthracite is heated to extreme temperatures, it becomes more hydrophilic(20). Appropriate care should be taken to use only the appropriate amount of anthracite. Other carbon rich materials, however, including lignite and bituminous coals are consistently outperformed by anthracite in this respect(21).
- Briquettes are a cheap and reliable source of fuel
- Wood is heat treated in a low oxygen environment to produce charcoal
- Binders may be used to adhere smaller wood pieces together, meaning that lower quality wood can be used instead of expensive pieces of large hardwood
- Coal dust (anthracite) is an excellent binder owing to its availability, ease of application and excellent performance
- Charcoal briquettes are being investigated for foundry uses to reduce overall carbon emissions
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