Effectiveness of Air and Biomass Heating using Waste Heat from a Gasification System

A downdraft gasifier is a type of gasification system device which is used to generate a synthetic gas (syngas) from organic carbonaceous material through an incomplete combustion process. This is achieved by reacting the material at high temperatures (>700°C), without combustion, with a controlled amount of oxygen and/or steam. Previous studies had proved that by having a lower moisture content of biomass fuel and preheating of gasifying air, it can improve the quality of the syngas produced and increase the efficiency of the gasification. During gasification, the temperature of waste heat produced at the gasifier is high and it can be used to reduce the moisture content of biomass fuel and increase the gasifying air temperature before being fed into gasification system. Therefore, the objective of the project is to evaluate the effectiveness of utilizing waste heat from a gasification system for secondary drying of the biomass fuel and for heating of the gasifying air. The heat exchanger system had been designed to capture the waste heat from the gasifier wall and exhaust pipe for biomass drying and preheating of gasifying air in the drying box. The drying box designed consists of 4 main boxes which can be assembled into one. The boxes are differentiated by its location towards heat exchanger which is located at the middle of the box. The heat exchanger in the drying box was designed to dissipate the heat from flowing hot water to inside the drying box and air tube. Due to time constrain in academic calendar, the experiment was simplified by only simulating the heat exchanging process in the drying box. The experiment was performed to see the effectiveness of the system by using 60°C hot water flowing at 3.5 LPM flow rate and 150 LPM of gasifying air flow rate. The experiment was conducted by using 55% moisture content (dry basis) of oil palm frond that had been cut into block shape. The result shown from the experiments showed that the system designed fails to reduce the biomass moisture content in the drying box. However there is some increment of temperature in gasifying air, especially when the air is heated with biomass. The compactness of biomass in the drying box forces it to act like an insulation barrier around the heat exchanger. For future study, it is recommended that the drying box is designed with large surface area for biomass drying.