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Once the drum is full and top layer is partially carbonized, the telescopic
chimney is removed and the drum is covered with its cover. Water is filled in
the channel so that there is no leakage of air through the water sealing
arrangement.
At this stage, smoke will be released through the hole provided at the centre
of the cover. When the smoke ceases to come out, the hole should be
blocked by the stopper. There should be no leakage of air during cooling. The
drum is allowed to cool for 2 to 3 hours before it is opened for removing the
char and starting the next batch. The biomass residues should be sized
properly and sun-dried to only 10-15% moisture (wet base). Depending upon
the type of biomass and its moisture content, char yields of 25-35% can be
obtained. During charring, a large amount of volatiles are released so it is
advisable to use these drums in open spaces. Further these drums can be
easily transported to the areas of biomass availability. To get maximum
production, the drum should be filled up to the top and only then the lid should
be placed and water seal made. To avoid rusting of the drum, it is coated with
coal tar while it is hot. Due to heat, the applied tar will crack leaving behind an
impervious coating of carbon on the surface, which prevents the drum from
rusting.
(iv) Briquette-making. The biomass char is crushed to fine powder form to get
a particle size of not more than 0.8 mm. It is then mixed with 20-30% by
weight of bentonite clay or local potters’ clay which acts as a binder. Molasses
or cooked starch may also be used as binder. The amount of water that is
sufficient can be judged by taking the mixture in hand and pressing it firmly to
form a ball. If a ball cannot be formed, extra water may be added to the
mixture. The mixture thus obtained is covered with wet gunny bags, and is left
for 24 hours for maturing.
For making beehive shaped briquette from the char-clay mixture, different
parts of the briquette mould set are placed one over the other in a sequence,
as shown in For making beehive shaped briquette from the char-clay mixture,
different parts of the briquette mould set are placed one over the other in a
sequence, as shown in Figure 7.5. First, the base plate, part 'A' is placed on a
levelled surface. The outer part 'B' is placed on it. Finally, the plate with 19
holes, 'C' is placed over the bottom plate 'A', in such a way that it rests on pins
supports as shown in the figure.
