Converting Waste Agricultural Biomass into a Resource - UNEP

Detailed Process Description
Drying chamber
The drying chamber, 1.37m wide, 2m tall and 2.9m long, contains two
movable tray carts having 26 trays each. It is made of galvanized iron sheets
framed together by angular steel bars. The trays (of dimensions 0.98m x
0.97m) are made of polyethylene plastic mesh reinforced with aluminum bars.
The total area of the drying trays can accommodate 200 kg of mango slices.
The chamber also has an air distribution system, which ensures distribution of
air into the entire height of the chamber. The air distribution system is
composed of an axial fan (with a 0.35m dia. rotor) and a distributor duct. The
distributor duct has a converging shape from top to bottom, and has thirteen
concentric rings of varying diameter, arranged one above the other, at specific
spacing. This system can deliver an air volume of up to 0.37 m3/sec.
Biomass furnace - Heat exchanger
The rice husk-fuelled furnace is composed of five parts: the combustion
chamber, heat exchanger system, fuel hopper, air inlet duct, and the chimney.
The combustion chamber is made of bricks enclosed in galvanized sheet
metal while the heat exchanger system is made of 27 boiler tubes arranged in
a rectangular array. All other parts of the furnace are made of galvanized iron
sheet. A butterfly valve controlled by a variable speed electric motor (0.5 hp)
controls the feed rate of rice husk; the fuel hopper is inclined at 45º (angle of
repose of rice husk) to allow free flow of rice husk into the combustion
chamber. An auxiliary blower (0.5 hp) was installed in the combustion
chamber to supply the primary air required for complete combustion. The
primary air supply to the furnace can be controlled by adjusting the opening of
the blower air inlet. The main blower installed between the furnace and the
drying chamber draws the ambient air through the furnace-heat exchanger
unit, and supplies hot air to the drying chamber. An S-shaped duct is installed
at the air inlet to the heat exchanger. The mouth of the duct is located away
from the combustion chamber to avoid suction of ashes that may otherwise
contaminate the product to be dried. The fuel hopper has a capacity of three
sacks of rice hull per loading which lasts for about 3 to 4 hours of dryer
operation.
The dryer has been tested with mango and banana. The quality of the dried
banana and mango were found to be very good, with no browning observed.
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