Exergy Analysis of a Fixed-Bed, Batch Type Rice Husk Fuelled Downdraft Gasifier

Abstract

This paper treats exergy analysis of rice husk at various loading rates of 1 kg, 1.5 kg and 2 kg of feedstock, using a batch type, fixed-bed gasifier, equipped with a gas filtering system and between the operating temperatures of 690 K and 1100 K, operated for the whole year. Exergy analysis is a thermodynamic technique which uses conservation of mass and energy principles to design thermal systems with maximum efficiency and it is based on First and Second law of thermodynamics. Formulas from literature and previous researches as well as Microsoft Excel 2007 were used to make calculations and graphically interpret the results. The exergoeconomic analysis is based on location specific and national data collected from official sources. A MATLAB based hybrid model developed from a combination of other studies is used to generate the exergoeconomic analysis results. Methane was found with the highest total exergies where values of exergies were varying between 1110126 kJ and 1066279 kJ for 2kg; 1037525 kJ and 1004639 kJ for 1.5 kg and finally, 965559.3kJ and 943635.6kJ for a feedstock mass of 1 kg between 690 K and 960 K respectively. From the overall results obtained, the gasification efficiency can be improved by increasing the temperature with an addition of heat in the process. The R2 values of 0.999 to 1 show the true behavior and similarities of the graphs and highlighting the enormous exergy destruction rate in the process. The CO2, CO and NOx capture/avoidance increases the plant efficiency by producing more electricity thereby increasing the financial potential of the plant. From the total exergies of CO2, CO and NOx, $0.156/kg, $0.20/kg and $12.335/kg could be made when sold respectively. The total exergies of gases considerably assist in the exergoeconomic analysis of the gasification plant, when the environmental aspect is considered.