Experimental study on a novel technique for rice syrav burning minimizing emissions

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 used5- Straw particle size has a significant effect on both combustion characteristics and pelletization. The smaller the particle size, the higher the reaction rate and the better pelletization quality. In this study straw particle size of 0.125 mm is suggested to be suitable for using rice straw as a fuel.6- The type of bonding material is an important parameter in both pelletization and combustion. Out of the tested materials starch is the best bonding material regarding mechanical and some combustion characteristics. Sodium silicates have the shortest heatup period while starch behaves slowly at the start of combustion. It may be attributed to that sodium silicates has less moisture content compared with starch. On the other hand starch is an organic material which bum such that starch and rice straw devolatilization may be interfere with each other leading to slower reaction rate. A compromise between strength and reaction rate should be done to choose the best bonding material.7- The pellet strength is directly proportional to the bonding materialconcentration. The reaction rate decreases with the increase in bonding material concentration. Using bonding material at 20 % is optimum from strength and combustion perspective8- Crushing strength and reaction rate increase with the increase in pellet size.The increase of reaction rate with pellet size is attributed to that big pellets are much porous than small pellets which improve oxygen diffusion.9- Adding some chemicals to the pellet such as kaolin at the pellet core improves the crushing strength and reaction rate. Also, it improves the thermal characteristics such as sintering behaviour of the pellets.10- Char bums according to diffusion model at the start of combustion while approach the kinetic model at the end of combustion.11- Reaction rate of char increases with straw particle size up to certain value ( 0.2 mm ) after which it decreases.