What is Briquetting?
Briquetting process converts low bulk density biomass materials into high density fuel briquettes. In the briquetting plant, ground charcoal burned from sawdust and other wood by-products are compressed into briquettes along with a binder and other additives that helps the briquette to burn. The selection of binder and additives is related to the quality and cost of the briquettes.
Selection of Binder Materials
Charcoal is totally lack of plasticity, thus it needs addition of a binding material to hold the briquette together for transportation, briquette forming and storage. Every particle of char is coated with binder, which enhances charcoal adhesion and produces identical briquettes. After the wet pressed briquettes are dried the binding operation is completed. Starch, clay, molasses and gum Arabic are common types of briquette binders.
Starch is the most common binder though it is usually expensive. It doesn't have to be an food grade. In general, about 4-8% of starch is needed to make the briquettes. Starch sources can be corn starch, wheat starch, maize flour, wheat flour, rice flour, cassava flour, potato starch, etc. To use the starch as a binder, you must first gelatinize the starch, which is added to water and heated to form a sticky consistency, then adding to the mixer to be mixed with the charcoal powder.
Clay is widely available available at almost no cost in many areas. A briquette can contain about 15% of clay. Clay does not add to the heating value of the briquette. If too much clay is added, the briquette will ignite and burn poorly or not at all. Besides, clay will turn into ash after burning, which blocks the passage of radiant heat, resulting in the loss of heating value of the charcoal.
Gum Arabic, also known as acacia gum, is a natural gum harvested from acacia tree, which is very common in Africa Sahel, especially Senegal, Sudan, Somalia, etc. Gum Arabic is successfully being used as binder material for charcoal briquette. It does not emit heavy smoke, nor is thermal treatment needed.
Molasses is a by-product of the sugarcane industry. One tonne of briquettes needs about 20-25% molasses. Briquettes binded by molasses burn well, but have an unpleasant smell during combustion. To avoid this, thermal treatment can be applied before using the briquette, which is also called”curing”.
Wood Tar and Pitch
Wood tar arises during the carbonisation process and are recovered from stationary kilns and retorts. Pitch is a viscous liquid that remains after the distillation of coal tar. Tar is more liquid while pitch is more solid. Both of them require re-carbonisation to avoid the emission of heavy smoke which may generate adverse health.
Besides, cow dung and paper pulp also can be the binding material for briquettes. Cow dung is available mainly in rural areas. Waste papers are torn to small pieces and soaked in water to form a gelatinized paste.
Binder Less Briquetting
Briquetting process can be done without any additional binding material(binderless briquetting) in the briquetting plant. Usually high pressure process will release sufficient lignin which acts as the binder. Medium pressure process may or may not require binders depending on the raw material, while low-pressure process always require binders.
Besides the binding material, other additives are also added during manufacture to aid combustion of briquettes.
Briquettes are not able to absorb sufficient oxygen for faster combustion due to compaction. Sodium nitrate gives out oxygen when heated, so it is used as ignition aid for briquettes, helping the briquettes to light faster. About 3-4% of sodium nitrate is needed for briquetting. Sawdust burns quickly and is also used as ignition aid. The amount of sawdust needed is about 10-20%.
White ash color looks nicer and acts as a signal that the briquettes are ready to cook on. A 2-3% lime, limestone or calcium carbonate is sufficient to make the ashes turn white. They are not heat fuels but can lower the burning rate to make the briquettes burn longer.
Press Release Agent
Borax or sodium borate is used in small amounts to help briquettes release from the manufacturing presses. It is not necessary to use this press release agent if you are using simple press or manual press. It is only necessary when using a high speed and high pressure briquette making machine.
Fillers are added to briquettes to increase their weight, density or volume, which can lower production cost of the manufacturer. Fillers add no energy value. It only adds ash content. Using filler can slow down the burning rate of briquettes, but with too much filler, the briquettes will be of poor quality. Fillers must be cheaper than the charcoal fine. Cement can be used but it is expensive. Clay is cheap and can be used as filler. Sandy soil is ideal as filler for it’s very common in most places.