3.1.4. Charcoal and Briquetting

Charring and briquetting technologies reduce many problems associated with management and utilization of biomass in domestic and industrial sectors. Briquetting of some of crop residues has become cost effective and it is considered as the replacement of firewood in India.
Biomass is widely used for heat and power generation with the latest combustion and gasification technologies. Same combustion technologies can be used to burn domestic and commercial waste to obtain energy, which can present opportunities for improved waste management strategies.
The raw materials needed for making briquettes include biomass derived from agricultural residue and forest products, shrubs, pieces of fuel wood trees, saw dust, etc. and sticky clay soil as binder. When briquettes are produced using charring drum, funnel is inverted first inside the drum; dried materials are then spread over funnel and burnt. As the dried biomass materials start burning a little, dried materials are continued to add and burn. Raw materials should not be burnt completely. Chimney is attached to the top of the inverted funnel through which white smoke is ejected. The process of semi burning of biomass is done layer by layer until the drum is filled two-third. Then the chimney is taken out and drum is covered and water-sealed to completely extinguish the fire. Once the fire is extinguished and cooled down then the coal is taken out, pounded into powder, mixed with the binding sticky clay soil with water all in appropriate ratio (3 parts of coal: 1 part of binding clay soil). Then the well-mixed coal is put in the briquette mould and compressed well with hand or machine. The briquette is then taken out and dried for 2-3 days under the sun. While drying, briquettes should be kept on plane and hard surface and should be covered with plastic during the night to protect from rain and wind. Once the briquette is dried and made hard, it is ready for burning in the briquette stove. When produced manually, one person can make about 30 round beehive briquettes with 19 holes through which blue fire-flame comes out when burnt. Depending on the quality of briquettes, one beehive briquette burns for an hour to two and half hour. If the semi-burnt charcoal is machine pressed, it results better fire efficiency. Cost of the briquette piece ranges from 10-20 rupees (15-30 cents). A normal meal for a nucleus family of 4-5 members can be cooked with one briquette.

Applications:

Advantages:

• Availability is abundant.
• Can be used to burn waste products.
• Continuous planned growing of energy crops absorbs CO2 at same rate as it is produced in the combustion process, thus leading to no net increase in atmospheric CO2.
• Organized tree planting contributes to water management, reduction of heat in arid areas and in prevention of desertification.
• Reforestation schemes improve soil conditions and prevent severe floods etc.
• Easy to convert to high energy portable fuel (e.g. gas).
• Comparatively cheap.

Disadvantages:

• Needs a large area of land with high initial cost of building power stations.
• Burning biomass can result in air-pollution, if not planned properly.

3.1.4.1 Portable Charring Kiln

Portable charring kiln is a simple unit for converting agricultural residues to a charred mass. It consists of a M.S. drum, handle and door. Due to its cylindrical shape, it can be rolled easily to the site of use. Waste agricultural mass such as soybean straw, pigeon pea stalks, cotton stalks and other material can be used. A small quantity of residues is fed into the kiln and ignited. When it gives a white smoke and starts to burn properly, additional material is added to the kiln. By continuing the process, whole of the kiln gets filled. Cover is then closed and the hot mass is allowed to pyrolysis. After 6-8 hours, the unit cools down and the charred mass can be emptied. The char obtained is used for making smoke free kitchen fuel by converting them into briquettes.

3.1.4.2 Honeycomb/Beehive Briquettes

The honeycomb/beehive shaped biomass briquettes is made by using hand mould and it is so simple that it can be fabricated/manufactured by local blacksmiths in rural areas. This hand mould (for honeycomb/beehive briquettes) consists of 3 parts and is manufactured by local blacksmiths from thick steel plate (5 mm) and smooth iron concrete reinforcement bars (12 mm). The mould is 90 mm high and has an internal diameter of 5" (127 mm). 19 holes of 12.5 mm (½ inch) in the bottom allow easy lifting of the pins and briquette out of the mould. Handles are 10 mm thick. This mould costs NR (Nepalese Rupees) 5,000 or INR (Indian Rupees) 2,000. Hand moulding requires 5 kg metal; however, it does not produce a high briquette density. Estimated moulding pressure may vary between 2-3 kg/cm2.
One person can make 30 briquettes per hour with hand moulding provided the charcoal-clay mix/paste is ready.


During the manufacturing of new moulds it is recommended to use a precise welding jig for 19 pins and a drilling jig for the holes. This way the perforated plate can be placed in any position over 19 pins . The weight of hand-made, dried briquette is about ½ kg. Average production cost of dried honeycomb briquettes in Nepal is NRs. 2.50-3.00 per piece, whereas the briquettes market price is NRs. 4.00 per piece. Local cost of pure charcoal used by blacksmiths is about NRs. 8.00 per kg.
Calorie Value: Pure woody biomass charcoal produces about 28 mega Joule/kg. Well-compacted, dried briquette has a weight of about ½ kg. At high altitudes the briquettes should be adequately dry, while at lower altitudes it may contain humidity by 15 % or more. Hardwood biomass charcoal briquettes with 20% clay content produce about 18 MJ/kg or about 9 MJ/briquette. In practice this may heat 2 litres of water in 15-20 minutes using the insulated (one briquette) metal stove (from about 20-98 ºC at 1300 m altitude). Burning duration of this briquette is about 1.5 hour.

Forest and agricultural waste charcoal briquettes, also with 20% clay, produce about 12 MJ/kg or 6 MJ/briquette depending on the composition of charcoal. In practice, this may heat 2 litres of water in 30-45 minutes by using the single briquette stove. Burning period of 1 briquette is about 1 hour.

3.1.4.3 Low Cost Briquetting Machine

The briquetting machine designed for converting charred biomass into cylindrical briquettes is a screw type extruder unit. It has a hopper for feeding the char and cow dung mixed to predetermined proportion of water. Feeding is done slowly. Outlet end has a number of openings forming the die through which the briquettes come out continuously. These are collected separately in trays and left in the sun for drying. The larger unit is operated with 2.25 KW motor and produces 60-75 kg of briquettes per hour. The smaller version produces about 40 kg of briquettes per hour.

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