Physical Appearance of Rice Hulls under Thermal Decomposition

Abstract

During my first trial runs using rice hulls in standard wood gasifiers of the updraft and downdraft type, it became apparent that the quickly changing flow properties and the unique thermal decomposition characteristics of rice hulls cause severe operational problems in small gasifiers of 15 cm to 30 cm hearth diameter. For instance, common slot grates used in coal and wood gasification can neither remove the rice hull ash continuously nor prevent the rice hulls from falling through. Another extreme example of the variable nature of rice hull flow properties is shown in Figure 7-1. An experimental hopper with 70° and 45° walls was used to feed the rice hulls into a downdraft gas producer. The rice hulls were loosely poured into the hopper, but because of gravity feeding the rice hulls became compacted and formed a vertical wall during the trial run. Additional difficulties were encountered in updraft gasification. Tar vapors condensed in the cooler upper layers and formed a sticky coating of charred rice hulls and tarry liquids, as shown in Figure 7-2. The frequent flow distortion and channel generation within the fuel bed could not he attributed to one particular cause. The next step was to examine closely the physical appearance of rice hulls and rice hull pellets before and after gasification to see if any insights into fuel bed disturbance could be found.