Being the most abundant sources of biomass energy in Vietnam and in Asia, rice husk and wood fuel at present are still not used properly.
They are mainly used by direct burning in the cooking stove in household in remote areas that is very smoky and harmful to the health.
Furthermore, biomass is used for direct burning with a low efficiency in some other heating applications such as in brick kiln and drying processes. A vast amount of rice husk is wasted to the river every year in Mekong Delta River in Vietnam every year. Fluidized beds are one advanced technology that can be used for combustion and gasification processes. However, biomasses such as rice husk and wood pellets have particular characteristics so that they can not be fluidized alone. In order to process rice husk and wood pellets for fluidization applications, another fluidization material must be added to support the fluidization of the bed. Inert materials such as quartz sand, calcite, dolomite, olivine, etc. could be used as such fluidization bed materials. However, the fluidization behavior of the mixture of two different materials will be dependent on the percentage of biomass in the mixture.
Many experiments have been carried out to study the fluidization and mixing behavior of the mixtures of biomass and quartz sand. Furthermore, the knowledge was extended by a comprehensive literature review.
Fluidization behavior becomes more complex and the characteristic values of the mixture (e.g. Umf) are not easy to determine. The standard deviation of pressure drop fluctuation is decreased when adding biomass to the sand bed depending on the percentage of biomass in the mixture. A good mixing is difficult to obtain when biomass and sand is initially in separated layers but becomes easier when biomass was continuously fed into the fluidized bed. Rice husk affected the fluidization behavior of the bed more than wood pellets due to its lower density and different shape.
Experiments were carried out also on gasification of rice husk and wood pellets in a laboratory scale fluidized bed gasifier. Gas product compositions and tar content were measured at different temperatures and equivalent ratios (ER). Temperatures inside reactor, gas compositions, and tar content in the producer gas were recorded or analyzed to understand the gasification process. While the gasification process occurs quite stable with wood pellets, some problems have been recognized when gasifying rice husk. Due to its low bulk density, high ash content and specific characteristic of rice husk ash, the problem of rice husk ash removal becomes significant. Furthermore, also the feeding of rice husk due to its low density and surface characteristics is not easy. This is a quite normal because there will be no existing gasifier that can be operated with every fuel. With the recognized problems in gasifying rice husk, some modifications have been proposed mainly on the feeding system and the ash removal for a properly operation of gasifying rice husk.