Experimental Investigation of Woody Biomass in A Double Walled Downdraft Gasifier for Energy Applications

摘要

In a gasifier the main attractive factors are the design and the operation. An optimised gasification process produces a considerably clean gas with out additional energy. So a double walled gasifier combining the features of a closed top and an open top gasifier was developed and the performance behaviour was investigated. The gasifier is made of mild steel construction of 20 cm inner shell diameter, 32 cm outer shell diameter and 100 cm height. The gasifier was tested using silver oak wood chips of size approximately 2.5cm × 2.5cm × 2.5cm. Feeding rates of 2-4.67 kg/hr and airflow rate of 50-130 1pm which correspond to 30 - 42% of the stoichiometric amount of air needed for complete combustion were used and tested for four different modes of operation. Of the 4 modes investigated in the gasifier using wood chips as feedstock and by assessing the main variables namely oxidation zone temperature, combustible contents ( H_2, CO & CH_4), HHV, thermal efficiency and tar content; only mode 4 yielded a relatively tar- free fuel gas acceptable for I.C engines with a reasonably good calorific value of the product gas. The percentage of total combustible components (H_2, CO, CH_4) in the optimum range of air flow (95 - 105 1pm) was found to vary between 28.83 - 30.4% and their average composition was H_2 =17.7-18.69%, CO = 10 - 10.6%, CH_4 =1.08 - 1.13%, N_2 = 57.42 - 58.16%, CO_2 = 11.04-12.97%. Air injection at 30 cm from the bottom of the gasifier enables the gasifier to operate with a good HHV fuel gas and reduced tar content. Also, it was observed that air injection at 10 cm from the bottom of the gasifier resulted in improper burning inside the reactor which resulted in clogging. From the above study, the equivalence ratio and gasification zone length are found to be vital performance parameters for downdraft gasifiers.