Rice husk furnace for recirculating type of mechanical paddy dryer

To reduce the fuel cost in operating recirculating dryers, Belonio(CPU) design and evaluated a rice husk furnace. The furnaces major parts are rice husk feed hopper, combustion chamber, heat exchanger, char discharge screw, and chimney. One full load on the hopper can contain about 3 sacks of rice husk. The combustion chamber with 1.03 cum volume is where the rice husks are burned. Its inner wall is made of 9-mm thick mild steel plate enough to resists corrosion due to excessive heat. The air supply needed during combustion is supplied by 2.5-in electric blower. At the top of the chamber is a heat exchanger made of 27 pieces of 3-in (diameter x 2-ft (height) pipe. At 12-in diameter axial fan directly attached to the heat exchanger pushes the heat to the annular space of the chamber until it reaches the inlet section of the dryer. Beneath the chamber is a screw to discharge burnt rice husk. Above the screw is a plate to enlarge or reduce the gap of the opening of the discharge outlet. Hot flue gas from the burnt rice husks passes through the vertical pipes in the heat exchanger, and then goes directly to the chimney located on the top of the heat exchanger, extending through the roof top of the building where the dryer is installed. Burnt rice husks are hauled outside the building, stored then used later as carbonized rice husk. Performance of the furnace was evaluated in collaboration with private millers. Findings of the study revealed the following: The rice husk fuel inside the furnace easily ignited within 5-12 minutes using few pieces of papers. The temperature inside the combustion chamber measured using thermocouple varied from 400 deg C to 700 deg C. The furnace consumed 3-5 sacks in an hour of operation. This volume of rice husks was meant for the production of carbonized rice husk rather than ash. The furnace supplied the air temperature required for drying paddy. A drying temperature ranging from 55 deg C to 80 deg C was achieved. Ambient temperatures during the tests ranged from 29 deg C to 32 deg C. The char/ash produced from the furnace ranged from 28% to 40% of the rice husk fuel. The electric power consumption needed to operate the blower and the fan was 500 watts. Paddy samples with high moisture content of 26-27% were dried down to 14-15% within 8-9 hours. Comparative cost analysis of rice husk furnace and diesel burner showed a lesser total operating cost of P50.39/hour for rice husk furnace compared with P232.80/hour for diesel burner. Using rice husk furnace means generating savings from the cost of diesel fuel. Firing can be done continuously with very little smoke emission. It is thermostatically operated wherein temperature during drying can be controlled by adjusting the amount of air supplied. Burnt rice husk can be converted to char or ash, which can serve as soil conditions or as refractory material when mixed with cement. Materials for fabricating the furnace are locally available. This technology is not advisable for use in areas where rice husks are not available. Loading and unloading of burnt husks are quite inconvenient compared with diesel burner. The energy needed to run the blower and the fan is an added cost in the operation of the furnace.