Tar Formation and Destruction in a Fixed-Bed Reactor Simulating Downdraft Gasification: Equipment Development and Characterization of Tar-Cracking Products

The aim of the present study is to examine operating parameters that would reduce the residual tar content in the fuel gas in downdraft gasifiers and eventually eliminate it altogether. A two-stage fixed-bed reactor has been employed to simulate elements of tar cracking in a downdraft gasifier. In this reactor, tar is generated by pyrolysis in the first stage and cracking and gasification take place in the second stage. Modifications to a previous configuration of this reactor are described, which have enabled the use of smaller char particle sizes in the second stage and the generation of a more complete inventory of the reaction products. In this work, the effect of the temperature and the presence of char on product distributions are reported. Increasing the temperature from 700 to 1000 degrees C resulted in a decrease in the quantity of tar recovered and an increase in the total amount of CO released. The amount of CH4 released increased between 700 and 800 degrees C before remaining steady up to 1000 degrees C. The CO2 content of the gas was relatively constant between 700 and 800 degrees C and increased as the temperature increased from 800 to 1000 degrees C. The amount of water and light hydrocarbons (C-2-C-5 alkanes and alkenes) sharply decreased at 1000 degrees C. The presence of char in the second stage had significant effects on tar cracking and product distributions. These effects were more obvious on the concentrations of CO, CO2, and H2O, which may be a result of reduction reactions taking place with the carbon in the packed char bed. These reactions appear to be more significant at temperatures between 900 and 1000 degrees C, where the rates of gasification are expected to increase.