【摘要】：Paddy rice is normally harvested from the field at moisture content of about 20%-25% (w.b) for maximum field recovery and to improve and maintain the quality of the rice.
However, rough rice is sometimes harvested at high field moisture of over 30%(w.b) especially at the peak of the raining season in most rice producing countries in the tropics.
This high moisture content has to be reduced immediately after harvesting to level of 12-14%(w.b) or lower for safe storage, to reduce and/or prevent quality deterioration, increasing the shelf life of the paddy rice thus, reduce post harvest losses.
In most rice producing countries especially in the tropics, sun drying is still the traditional and dominant method use to dry agricultural crops because it is a low cost and easy method. This method however, results to quality deterioration and quantity loss.
Many drying systems have been designed, constructed and developed for grain drying to reduce the problems associated with sun drying and to improve the quality of the grain. However, most of these drying systems are expensive at farmers' level, use fossil fuels which are very expensive and have negative effect from the environmental point of view due to the emission of green house gases.
This has led to the increase interest and attention for alternative use of fuels which are cheap and environmental friendly in heat and energy production.
Many agricultural wastes have been used to harness the drying of agricultural crops to reduce the impact of fossil fuels to the environment and to reduce the cost of production thereby, increasing the income and well being of the farmer.
Many of these wastes have been used in grain drying however, the use of rice straw and wood in grain drying has not gained momentum and interest in most rice producing countries.
This may be primarily due to the low feedstock of rice straw, the associated problems of collecting, transport, handling and storage, the impact on the environment of using wood respectively and to greater extent the unavailability of drying systems that use this biomass in grain drying.
Researchers have shown that rice straw and wood could be an attractive alternative in heat and energy applications and the low feedstock of rice straw and its associated problems can be improved by pretreatment the material and the effect of using wood can be reduce by replanting.
It is against this background that, this study was geared to conduct experimental study and performance evaluation on continuous grain dryer using rice straw and wood as fuels.
The dryer is of simple design and easy to operate and made from locally available materials. It consists of four main parts and additional accessories:
Six levels of grain flow rate were established and used to determine the optimum grain flow rate with respect to moisture reduction during the drying time. The three best grain flow rate obtained out of the six levels were chosen and used in the subsequent experiments.
Orthogonal experimental design was used to determine the optimum operational conditions of the dryer with respect to moisture reduction during the drying tests. Three elements with three levels for each element were used. The three elements were moisture content, grain low rate and fan velocity and the three levels for each element were moisture contents of approximately 28,30 and 32%(w.b),the fan velocity were 900r/min, 1038r/min and 1170r/min, the grain speed were 2.6kg/h,3.18kg and 3.78kg/h respectively.
Nine experiments with three replications each were conducted using rice straw and wood as fuel.10kg of paddy rice was used in each experiment and fuel consumption was measured prior to each experiment.
A drying temperature of approximately 60 degree Celsius and heating time of 60 minutes were maintained in all the experiments conducted.
The experimental results showed that the optimum drying combination of the dryer with respect to moisture reduction was grain flow rate of 3.78kg/sec for both fuels and fan velocity of 900r/min for rice straw and 1038r/min for wood respectively.
Further examination of the results showed that, a drying time of 60 minutes, drying temperature of approximately 60 degree and fuel consumption of 7-8kg/h and 4-5kg/h of rice straw and wood respectively, seem to be conducive or satisfactory for the general use to dry paddy of high initial moisture of approximate range of 28%-32%(w.b) to final moisture level of approximately 15%-16%(w.b.)
The result also revealed that two-stage drying with tempering period of five hours inserted between the drying cycles, a total drying time of 90 minutes, drying temperature of 60 degree Celsius and fuel consumption of 13.48kg/h and 9.22 of rice straw and wood respectively were found to be satisfactory to dried high moisture paddy to optimum and safe storage and more preferable than continuous drying with respect to moisture reduction under similar drying conductions.
The results further indicated that high percentage of head rice yield (HRY) was obtained from the samples dried under two-stage drying at different temperatures and compared with the samples dried under continuous drying.