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Abstract
This work aims at utilizing rice straw as a source of renewable energy. Direct combustion of rice straw has the highest thermal efficiency compared with other thennal conversion techniques such as pyrolysis and thermo-chemical gasification. The main barriers of this are; low bulk density of the straw, high ash content and low melting temperature of the ash. Fluidized bed, as suitable technique for burning solid fuels with high efficiency and low emissions, can be used to bum rice straw after proper preprocessing. To make rice straw suitable for burning in fluidized bed and overcome the aforementioned barriers, rice straw is converted into pellets using a disc pelletizer and suitable bonding material. The produced pellets are evaluated considering four criteria. These criteria are; mechanical, aerodynamic, combustion and thermal. Regarding the mechanical criterion it is mainly concerned with crushing strength of the pellets which an indicator of suitability for storage and transportation. Aerodynamic criterion is concerned with retaining the spherical shape of the pellet during combustion. Regarding the combustion criterion, it is mainly concerned with reaction rate, combustion efficiency and pellet temperature. The later affects NOx emissions. Regarding the thermal criterion, it is concerned with the thermal stability of the pellet during combustion as well as ash melting temperature. The former ensures the integrity of the pellet during combustion while the later avoids the sintering problem, which related to the agglomeration of the pellets as well as fouling inside the bed and separation equipment.
A test rig was designed and manufactured to test the combustion characteristics of a single pellet in conditions similar to those found in fluidized bed. The test rig equipped with a facility to stop the reaction at any moment to track the evolution of pellet combustion. In addition to the main test rig, separate arrangements were used.