Converting Waste Agricultural Biomass into a Resource - UNEP

Other fuels such as saw dust briquettes and wood twigs can also be used in
the stove. The fuel should be sized before loading into the fuel chamber.
Figure 2.17
illustrates the types of fuels used in the stove. One inch is the average size of
fuel pieces that can be used. Stove ignition: Fuel is first loaded in the fuel
hopper, the lid is closed and the water seal is filled with water. The fuel is then
ignited from below the grate using a flame torch through the ash pit door.
About five minutes later, the torch is removed and the ash pit door is closed.
The ignition builds up slowly, taking about 20 minutes for the combustible
gases (producer gas) to generate at the gas burner side. The gases are then
ignited in the gas burner by introducing a flame through the secondary air
holes in the burner. Once the gas is ignited, the flow of gas becomes smooth.
The stove can operate continuously for several hours, until the fuel in the fuel
chamber is used up.
Additional fuel can be loaded through the top of the fuel chamber to extend
operation.
Flat Plate Solar Collector
The dimensions of the flat plate solar air heater are 2m x 2m. The cover
glazing is 4 mm thick tempered glass. Glass wool insulation is provided below
the absorber sheet. The absorber and insulation materials are placed inside a
mild steel box, which protects the interior from moisture/rain. The air inlet to
the collector is fitted with a fine stainless steel wire mesh. The solar collector
is placed facing south at an angle of 15° to the horizontal for absorbing
maximum solar radiation during summer.
Heat Exchanger
A heat exchanger transfers heat from the flue gas from the gasifier stove to
the drying air entering from the solar collector and bottom air
blowers/openings of the drying chamber. The heat exchanger, fitted above
the level of the solar collector air outlet, is of cross-flow type with hot flue gas
flowing through horizontal pipes and process air flowing up across these
pipes. The central pipe of the heat exchanger (6” diameter) and two side
pipes (4” diameter) are fabricated from mild steel sheet. The center pipe and
side pipes are connected with eighteen 2” diameter steel pipes.
Temperature Controller
A temperature controller using a thermostat is provided to maintain the
temperature of hot air within specified limits. The thermostat controls the
primary air supply to the biomass stove. A car radiator thermostat with a
working temperature of 76.5°C is used as a temperature controller in the
drying system. The thermostat is fixed inside a housing which has eight
copper fins, each of size 7cm x 3cm welded to an annular cylinder. This
housing has been provided to enhance the heat transfer from the hot air to the
sensing element in the thermostat. The thermostat is fixed on the central
heat exchanger pipe near the entrance of flue gas from the gasifier stove. The
temperature of drying air inside of the dryer can be set to a particular value by
adjusting the height of the thermostat expander rod above the heat exchanger
pipe. When the average temperature inside the dryer rises above the desired
level, the thermostatic element will extend and push the short arm of a lever;
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