Briquetting of biomass residues in India using a Beehive Pyrolyser and briquetting machine

Dr Kiran Rawat & Dr Anil P. Joshi, HESCO, 'Vigyanprasth', Gwar-Chauki, Gholtir - 246 430, Chamoli (U.P.), INDIA

Charcoal briquetting is one efficient method of utilizing biowastes. Charcoal has mainly been used as a domestic fuel for cooking and heating, but it is becoming an important industrial fuel. Briquette-making can serve as a cottage industry in areas where the biowaste, wood waste and invasive shrubs are in abundance. Apart from being an energy source, briquetting involves manpower in both the formal and informal sector, thus providing an income-generating activity for the unemployed.

Biomass for charcoal

Though any wood can form the raw material for charcoal, emphasis should be placed on using residues and invasive biomass, which have proved to be an environmental liability. Plants such as Lantana camara, Euphorbia royleana, Parthinium histerophorus etc. are spreading at an alarming rate, encroaching into agricultural lands and destroying the balance of plant growth in the forests. Increased demand for fuel has increased pressure on the forests, resulting in an increased area of deforestation. These deforested lands have been taken over by these invasive plants, due to their fast growth. Another source of biomass is the waste from sawmills, plywood industries, furniture factories and match factories.

The charcoal-making process

Charcoal is a solid residue remaining after the process of carbonization under controlled conditions in a closed space such as a pyrolyser. Air entry is limited, which prevents the wood burning to ash, as it would in a conventional fire. The thermal decomposition of wood starts when the wood is exposed to a temperature of 250-300 °C. Once started, the combustion process generates sufficient heat to continue by itself. The heat causes the substrate to decompose chemically to form charcoal, and also a small amount of tar residue, some chemicals such as acetic acid, methanol, ash and a large amount of water, which is driven off as steam. When the temperature reaches 450-500°C, the process is complete. The pyrolyser is allowed to cool down to ambient temperature without allowing air to get in, so unloading is not done until the pyrolyser is completely cool.

The weight of charcoal produced is nearly one third of the original weight of biomass put into the pyrolyser. The charcoal obtained is either in small lumps or a powder. It would burn rapidly if used in this form, so it is blended with a small quantity of binder and clay to increase the length of time for which it will burn.

The charcoal pyrolyser

The Beehive Pyrolyser has been chosen for the charcoal preparation of biowastes for the following reasons:

· Simple to construct
· Low capital investment
· Suitable for domestic use
· Simple to operate
· Relatively unaffected by thermal stresses when heating and cooling
· Strong enough to withstand mechanical stresses of loading and unloading
· When properly maintained, it is unaffected by weather conditions over its 4-8 years of working life.

Design and construction

The Beehive pyrolyser is shown in Figure 1. It is hemispherical in shape and has one opening at the top and one at the base. Once filled, the lower opening is closed and sealed. The firing is done from the top, which is then also sealed. The seals must be capable of withstanding thermal stresses caused by the combustion inside. The lower door is only opened when pyrolysis is complete. The whole cycle takes between six and eight hours depending on the volume of the pyrolyser. Once emptied, the pyrolyser can be used immediately for the next production cycle.

For industrial use, bricks, fine charcoal and mortar are used. Recently, for making household fuel, a pyrolyser has been designed which can be made from local stones and a mud/dung mixture. This means that the cost of building the pyrolyser is low. The dimensions of the pyrolyser are as follows:

The pyrolyser mainly converts pine cones, Carrisa, Lantana and unutilized agrowastes. The domestic pyrolyser has been readily accepted by the communities as it is cheap to construct, and effective and efficient in use.

Figure 1: A beehive pyrolyser

Briquetting technology

Briquetting fine charcoal requires the addition of a binder. The mix is formed into briquettes in a briquetting machine. These are dried in a dryer or oven, to produce a briquette which is strong enough to use.

Binders

The binders used must have the following properties

· Combustible
· Easily available
· Cheap
· Strong binding capacity

Binders in common use include starch, molasses, resin, clay etc.

The briquetting process

Charcoal from the pyrolysers is collected and ground to a fine powder using an iron or wooden roller as shown in Figure 2.

Figure 2: Charcoal being ground to a fine powder using a roller.

The binder, depending on its binding capacity (6 - 12% by weight), is mixed into the charcoal. A quantity of clay between 4% and 8% is also added to increase the binding and to increase the length of time for which the briquettes will burn.

Once a paste has been formed, dies of different shapes are put into the briquetting machine, shown in Figure 3. This squeezes the paste into the required shape, as shown in Figure 4. The briquettes are damp, so they must be dried either directly in the sun or in a solar drier.

Figure 3: Briquetting machine with circular die.

Figure 4: Charcoal briquettes.

Conclusions

Converting biomass waste into briquettes reduces the amount of smoke when they are burnt, uses residues and invasive plants which would otherwise be considered a nuisance, increases fuel availability and provides employment for those making them.